Nanoparticles in tumor microenvironment remodeling and cancer immunotherapy

Macrophages, renowned for their phagocytic nature, play a crucial role in the immune system. They participate in various physiological processes, including development and homeostasis. The phenotype and function of macrophages are intricately determined by their origin and polarization [64]. Initially believed to originate from hematopoietic stem cells and circulating monocytes [65], the recent studies reported that macrophages have an embryo-derived lineage, with precursors derived from erythro-myeloid progenitors in yolk sacs and fetal liver [65, 66]. Maintaining or enhancing the macrophage population is essential for these cells to function effectively [67, 68]. There are two strategies for the replenishment of macrophages: monocyte recruitment and increased proliferation in the form of tissue-resident macrophages for elevating self-renewal ability [67, 69].

In the TME, macrophages are referred to as tumor-associated macrophages (TAMs), constituting 50% of the tumor mass [70]. The TAMs engage in intricate interactions not only with cancer cells but also with natural killer (NK) cells, T cells, endothelial cells, and fibroblasts. The roles of TAMs extend to the regulation of cancer proliferation, invasion, and angiogenesis [71‐73]. Macrophages have also been associated with the development of resistance to cancer therapies [74]. The TAMs primarily originate from the bone marrow or the yolk sac [75]. They can be polarized into two phenotypes. The M1 macrophages, induced by lipopolysaccharide and type 1 T helper cell (Th1)-derived cytokines, exhibit pro-inflammatory and anti-cancer functions [76]. The M2 macrophages, induced by Th2-derived cytokines like interleukin-4 (IL-4), IL-10, and IL-13, promote proliferation, invasion, and angiogenesis [76]. A delicate balance exists between M1 and M2 macrophages in vivo, influencing tumorigenesis and treatment outcomes [77, 78]. The anti-inflammatory nature of M2 macrophages accelerates cancer progression. The regulation of TAMs has been of importance for cancer immunotherapy. Currently, the strategies for targeting TMAs are based on controlling the origin, functional polarization, and phagocytic function of TAMs. Moreover, macrophages and monocytes have been engineered to mediate anti-cancer immunity. For this purpose, four distinct strategies have been exploited, including a decrease in TAM population, switching from M2 polarization into M1 phenotypes, controlling macrophage phagocytic signal, and bioengineering of macrophages for increasing phagocytosis [79]. Currently, the nanostructures have been widely applied to re-educate TAMs [80], change phagocytosis ability [81], suppress TAMs [82] and deliver drugs to TAMs [83] for cancer immunotherapy.

Cancer-associated fibroblasts (CAFs) represent a diverse group of cells that infiltrate the TME. The CAFs are distinct from normal fibroblasts [84]. These cells play a pivotal role in tumorigenesis by inducing biochemical alterations and signaling network changes that accelerate tumor development [85]. Under specific conditions, CAFs may exhibit anti-cancer activities, contributing to tumor suppression [86].

The heterogeneity of CAFs arises from their diverse origins, including normal fibroblasts, epithelial cells, endothelial cells, peritumoral adipocytes, pericytes, hematopoietic stem cells, mesenchymal stem cells, and cancer stem cells [87]. Based on their functions, CAFs may be categorized into two groups: carcinogenic and anti-carcinogenic CAFs [88, 89]. Ohlund and colleagues identified two distinct subtypes of CAFs in pancreatic cancer: myofibroblasts (myCAFs) and inflammatory CAFs (iCAFs) [90]. The myCAFs, located near cancer cells, are stimulated by transforming growth factor-beta (TGF-β) and exhibit high levels of alpha-smooth muscle actin (α-SMA). In contrast, iCAFs are positioned further away from cancer cells. They demonstrate elevated α-SMA levels and the ability to secrete IL-6 and leukemia inhibitory factor [91].

Another subclass of CAFs, antigen-presenting CAFs (apCAFs), express biomarkers related to the MHC-II class and CD44, enabling them to stimulate CD⁴⁺ T cells in an antigen-dependent manner [92]. Additionally, there is a subtype known as restraining CAFs (rCAFs). Each of these subpopulations plays a distinct role in cancer. For example, iCAFs and myCAFs contribute to metabolic reprogramming and angiogenesis in cancer, respectively. The iCAFs can secrete growth factors, cytokines, and chemokines, including PD-L1/L2, Fas ligand, and others, that influence the regulation of the immune system. The myCAFs, on the other hand, contribute to extracellular matrix remodeling by enhancing collagen synthesis. The apCAFs are involved in stimulating CD⁴⁺ T cells for immune cell regulation, while rCAFs exhibit the ability to suppress tumorigenesis [93]. Regarding the importance of CAFs in tumorigenesis, targeting CAFs for cancer immunotherapy has been of importance. The nanostructures demonstrate high penetration and permeability in tumor sites, and can be utilized to regulate CAFs [94]. Moreover, nanoparticles can be utilized to engineer CAFs to act as APCs and stimulate antigen-specific CD⁸⁺ T cells in cancer immunotherapy [95]. Nanostructures can trigger clearance of activated and senescent CAFs [96], and regulation of CAFs by nanoparticles can disrupt cancer metastasis and invasion [97].

Up to 70% of circulating leukocytes are comprised of neutrophils [98], and are considered a first-line against pathogens [99]. Neutrophils have a short life and can persist in circulation for five days [100]. When there is tissue damage or infection, the epithelial cells secrete chemokines to recruit neutrophils. Upon this, neutrophils extravasate the blood circulation, enter damaged tissue to secrete a number of inflammatory cytokines, release neutrophil extracellular traps (NETs), and finally, phagocytose the pathogens or invading microorganisms [101]. NETs are vehicles for anti-microbial peptides and toxins [102, 103]. In cancer, there are two categories of tumor-associated neutrophils (TANs) similar to the Th1/Th2 pattern, including N1 and N2 with tumor-suppressor and tumor-promoting function, respectively. Tumor type and stage determine the phenotype of neutrophils in TME. During the first stages of tumorigenesis, neutrophils demonstrate an inflammatory phenotype, and as the cancer advances, the neutrophils achieve an immunosuppression phenotype [104]. Neutrophil-mediated inflammation regulation relies on the secretion of ROS and RNS. Moreover, the extracellular matrix can be re-configured by the neutrophils through the secretion of neutrophil elastase and matrix metalloproteinases. The neutrophils display the ability to stimulate angiogenesis through oncostatin-M, increase carcinogenesis through PGE2, and enhance metastasis of cancer through the release of ROS, RNS, NE and MMP-9. Noteworthy, the NETs have consisted of MMPs, cathepsin G, and NE [105, 106]. The function of these proteases is to mediate pro-inflammatory cytokine degradation and re-accumulate in TME for enhancement in tumorigenesis and metastasis [107]. In cancer patients, the plasticity of circulating neutrophils is of importance, known as high-density neutrophils (HDNs) or low-density neutrophils (LDNs), corresponding to N1 and N2 phenotypes, respectively. LDNs that have an immature phenotype, show prevalence in the circulation of many cancers and participate in carcinogenesis and metastasis [100]. In the field of cancer immunotherapy, the stimulation of N1 neutrophils can mediate toxic impacts on cancer cells [108]. Furthermore, the stimulation of Ly6Ehi neutrophils through the STING pathway can enhance sensitivity to anti-PD-1 therapy, and they can be utilized as predictors of cancer immunotherapy [109]. Therefore, the development of nanoparticles for targeting neutrophils in cancer immunotherapy is important.

As innate lymphocytes, NK cells exhibit a shorter half-life compared to B and T cells, necessitating their replenishment from bone marrow progenitors [110]. The NK cells undergo linear differentiation, with highly proliferative immature NK cells differentiating into fully functional and granular effectors [111‐113]. Enhancing the frequency, infiltration, and function of NK cells contributes to the improved survival of cancer patients [114‐117]. This renders NK cells valuable in cancer immunotherapy. These group I innate lymphoid cells can rapidly target cells without prior sensitization [118], and express T-bet and Th1-related cytokines, including IFN-γ [119‐121].

Upon maturation, NK cells migrate from the bone marrow to the blood and subsequently reside in peripheral tissues. Because of their capacity to move between lymphatic and non-lymphatic tissues, NK cells are distributed in numerous organs and tissues [122‐124]. Mature NK cells acquire the capability to exert cytotoxic impacts on cancer cells or virus-infected cells [125]. Serving as contributors to the adaptive immune system, NK cells interact with other immune cells through the secretion of cytokines, growth factors, and chemokines [125]. These effects position NK cells as effective effectors in diseases such as cancer, infectious diseases, autoimmunity, and chronic inflammation [126‐129].

Moreover, NK cells play a significant role in the innate immune system, providing surveillance in hematological cancers and cancer metastasis [110, 130, 131]. The increased infiltration of NK cells into the TME is positively associated with the prognosis of various cancer types, including melanoma, renal cell cancer, liver tumors, and breast cancer, among others [132‐136].

The adaptive immune system is primarily shaped by T cells, providing effective defense against pathogens and cancers. Upon exposure to cytokines and co-stimulatory signals, naïve T cells undergo proliferation, differentiating into effector cells. Naïve CD⁴⁺ T cells can differentiate into T helper cells, including TH1, TH2, TH17, and TFH cells, to exert immune functions. The differentiation of naïve CD⁸⁺ T cells into effective CD⁸⁺ T cells enables these cells to combat infections and cancers through the release of IFN-γ, TNF-α, and cytotoxic molecules [137].

A challenge in cancer arises from T cell exhaustion. This phenomenon is mediated by various mechanisms, with the PD-1 axis being the most prominent. Upon antigen exposure, naïve T cells transform into effector T cells, with some undergoing cell death and others participating in tumor elimination. Antigen presentation can lead to the formation of stem cell memory T (TSCM) cells, which convert into TCM, TEM, and TRM. The TRM cells reside in the tissue, ready to respond to secondary stimulation, while TSCM and TCM possess self-renewal capacity, generating TEM and TE upon re-stimulation [138].

Signs of T cell exhaustion include the expression of inhibitory receptors, reduction in T cell function, and decreased proliferation. Exhausted T cells exhibit a unique epigenetic profile that may result in a differential or poor response to immunotherapy. Additionally, exhausted T cells experience metabolic dysregulation, including mitochondrial suppression and glycolysis inhibition [139]. The challenge in cancer therapy extends beyond T cell exhaustion, as their death and reduced proliferation can impair immune reactions. Targeting NK and T cells with nanoparticles has strengthened cancer immunotherapy. The nanoparticles with high uptake in NK cells, such as lipid-based nanoparticles, can be utilized to engineer NK cells [140]. Furthermore, nanostructures can be utilized for non-invasive tracking of NK cells, including their migration and biodistribution in tumor regions [141]. The expression levels of CCR4 and CXCR4 on the surface of NK cells can be changed by nanoparticles to improve their interaction with cancer cells [142]. Noteworthy, the nanoparticles can be designed to stimulate both NK and CD⁸⁺ T cells in cancer immunotherapy [143].

Endothelial cells form the inner lining of blood vessels. The biological functions of endothelial cells are crucial for preserving normal physiological conditions [144]. These cells play important roles in regulating blood clotting, vessel size, and immune functions to enhance blood fluidity, oxygen distribution, cell transport, and nutrient supply. Endothelial cells continuously secrete anticoagulant proteins to prevent clotting in vascular beds, maintaining homeostasis and ensuring blood flow and pressure at an appropriate level to deliver oxygen and nutrients to tissues [145‐149].

Despite their essential physiological functions, endothelial cells have been implicated in cancer progression. Recent reviews have highlighted the role of endothelial cells in the tumor stroma [150, 151]. In the initial stages, endothelial cells induce angiogenesis to increase the presence of blood vessels in the primary tumor. These cells also function as a platform and site for membrane-bound factors and proteins, creating a TME conducive to cancer progression. These localized functions of endothelial cells also play a role in regulating angiocrine signaling at distant sites, influencing organ function. Furthermore, factors and proteins secreted by tumor cells can extend beyond tumor boundaries, affecting endothelial cells at distant sites and exerting systematic functions [152].

Understanding the functions and regulatory impacts of tumors beyond their sites is crucial, given that most cancer-related deaths result from invasion, thrombosis, and cachexia [153‐155]. Proteins and factors secreted by tumor cells can induce changes in endothelial cells in the pre-metastatic niche, enhancing the dissemination of cancer cells and mediating angiogenesis. Additionally, these factors can lead to thrombosis in distant vasculature [156]. Endothelial pericytes have been recognized more than a century ago as microvasculature-associated mural cells [157]. These perivascularly positioned cells [158‐160] are ubiquitously distributed in all vascularized tissues [161, 162]. Identification of pericytes requires immunostaining and the use of biomarkers and antigens to differentiate them from vascular smooth muscle cells, fibroblasts, and mesenchymal cells [157]. Initially considered inert cells contributing to physical vascular stability [163, 164], recent advances have illuminated their roles in both physiological and pathological conditions.

Pericytes play a crucial role in regulating blood vessel development and modulating blood flow, coagulation, and vascular permeability [165]. The structure of capillaries includes endothelial cells, pericytes, the basement membrane, and vascular smooth muscle cells [166]. The primary function of pericyte function in cancer progression is stimulating angiogenesis in the TME [167]. The CD248 is capable of Wnt upregulation and increasing the levels of OPN and SERPINE1 in pericytes to cause angiogenesis and expedite cancer progression [168]. Additionally, pericyte contractility can be induced by the enzyme hexokinase 2 in glycolysis, leading to abnormalities in tumor blood vessels [169]. When present in the tumor site, RGS5-TGFβ-Smad2/3 creates an anti-apoptotic environment that accelerates cancer cell growth [170]. Figure 2 is a schematic representation of TME components.

The Myeloid-derived suppressor cells (MDSCs) are another type of cell present in TME. There are a number of arguments that MDSCs are a subtype of neutrophils [104] due to the presence of overlapping markers among MDSCs and TANs, making it challenging and problematic to distinguish them. There is still controversy regarding whether MDSCs represent a separate lineage of cells or are polarized immature neutrophils [171]. Overall, MDSCs are considered a heterogeneous population of cells with myeloid origin [172]. In spite of origination from myeloid progenitor cells, MDSCs and TANs are considered different cell types. Furthermore, MDSCs demonstrate several distinct features from neutrophils, including downregulation of CD16 and CD621 and upregulation of Arg1, CD66B, and CD11b [173, 174]. Furthermore, the studies have shown other subtypes of MDSCs including monocytic MDSCs (M-MDSCs), which are distinguished by a CD11b hi, LY6C hi, and LY6G lo phenotype, polymorphonuclear MDSCs (PMN-MDSCs), which display a CD11b hi, LY6C lo, and LY6G hi phenotype, and early stage MDSCs (eMDSCs) which are CD13- and CD14-, and CD33 + in humans [175, 176]. In TME, it is possible to observe both M-MDSCs and PMN-MDSCs, and compared to MDSCs, they demonstrate a suppressive phenotype [177]. The MDSCs suppress T cells and the innate immune system to create an immunosuppressive phenotype in TME [177]. MDSCs also contribute to the formation of pre-metastatic niches, can elevate stemness and angiogenesis, and promote metastasis through EMT induction and enhancing IL-6 secretion [178, 179]. There are also other factors in TME that can affect MDSCs. The HIF-1α, a marker of hypoxic TME, stimulates the differentiation of MDSCs into TAMs with carcinogenic function [180]. The metabolism of MDSCs in TME can be changed towards stimulation of fatty acid oxidation to enhance levels of Arg1 and NOS2 [181]. For cancer immunotherapy, the regulation of MDSCs can provide new insights, such as the downregulation of CCRK that disrupts the immunosuppression activity of MDSCs and promotes the potential of immune-checkpoint blockade therapy [182]. The nanostructures are able to reduce the population and function of MDSCs, impair MDSC-mediated immunosuppression and cause MDSC repolarization [183‐185].

The immune cells present in the TME use the cytokines to send messages to other cells in an endocrine, paracrine or autocrine manner and provide intercellular communication [186]. Cytokines, also known as immunomodulatory agents, can be produced in physiological and pathological status, and various classes of cells, including adipocytes and tumor cells, can secrete them. The cytokines contribute to the cellular (type 1) and antibody-mediated (type 2) immunity as anti/pro-inflammatory and pro/anti-tumorigenic effectors that also rely on the TME. Cytokines can bind to the receptor on the surface of other cells to regulate their action and change the molecular pathways. There are different kinds of cytokines in TME, including chemokines, interleukins, adipokines, transforming growth factors (TGFs), tumor necrosis factor (TNF), colony-stimulating factors (CSFs), and interferons (IFN) that can act alone or in a synergistic way to affect immune system [187]. Chemokines are considered as chemoattractant cytokines for the recruitment of inflammatory cells, including leukocytes (monocytes, neutrophils), along with other kinds of cells, such as endothelial and epithelial cells [188]. Depending on the position of conserved cysteine residues, there are various classes of cytokines including CX3C, CXC, CC, or C chemokines [189]. Moreover, chemokines are able to interact with the G protein-linked transmembrane receptors known as chemokine receptors [190]. A number of chemokines, such as CXCL8 and CCL3, have an inflammatory function, and they recruit the cells via the inflammatory signs or/and homeostatic [191]. Interleukins (ILs) possess a low molecular weight and demonstrate pro- and anti-inflammatory functions. The immunocompetent cells, including T cells, granulocytes, monocytes, macrophages, adipocytes, and endothelial cells, can secrete ILs [192]. The ILs play a critical role in the development, differentiation, induction, maturation, migration, and adhesion of immune cells [193]. Adipokines (also known as adipocytokines) are cytokines that can be secreted by adipose tissue and consist of adipocytes, pre-adipocytes, macrophages, stromal cells, fibroblasts, and endothelial cells [194]. The adipokines are comprised of adipose tissue-specific cytokines (adiponectin, leptin) and other categories, including ILs, TNFs, and chemokines. Moreover, inflammation, energy metabolism, and fat distribution can be controlled by adipokines [195]. The adipokines also contribute to obesity-related inflammation to regulate metabolic diseases [196]. Adipocytes are critical regulators of tumorigenesis and metastasis [197]. According to the impact of adipokines on the immune system, there are two kinds, including pro-inflammatory, such as leptin, TNFα, interleukin-1β (IL-1β), interleukin-6 (IL-6), and interleukin-8 (IL-8), potentially linking adiposity and inflammation, and anti-inflammatory, such as interleukin-10 (IL-10) and adiponectin [197, 198]. A number of adipokines, such as adiponectin, demonstrate anti-carcinogenic function [198], while others, such as leptin, demonstrate carcinogenic function [199]. TGFs are a number of protein hormones that are overexpressed in human cancers and can modulate tumorigenesis and cancer growth. TGFα is a member of the EGF family with the potential to regulate epithelial development and cell proliferation and can modulate carcinogenesis and angiogenesis [200]. M2 macrophages and other kinds of cells, including cancer cells, can secrete TGF-β to modulate the function of T cells, NK cells, and macrophages present in TME, disrupting anti-cancer immunity and enhancing carcinogenesis [201]. IFN was discovered upon its function to interfere with viral growth [202]. The host cells secret IFNs, and they can regulate the immune system. The fibroblasts and monocytes are able to secrete type I IFNs such as IFN-α and IFN-β during the viral attack. Then, the expression of proteins with the ability to impair RNA and DNA replication is upregulated. The type II IFNs, including IFN-γ can be released by CD⁸⁺ T and Th1 cells to induce a number of cells, including NK cells, M1 macrophages, and CD⁸⁺ T cells for enhancing MHC I and II presentation, promoting the anti-cancer immunity [203].

The changes in the expression level of enzymes are a feature of TME, and it can be exploited in a rational way to treat cancer [204]. Enzymes are a kind of protein or RNA that can facilitate chemical reactions [205]. The enzymes for catalyzing reactions are highly selective and under mild conditions, demonstrate the specific substrates to modulate biological and metabolic mechanisms [206]. The enzymes display a number of changes in expression in diseases such as TME [207]. The TME shows several enzyme secretions consisting of MMPs, hyaluronidase, γ-glutamyl transpeptidase, and esterase with higher expression in tumors compared to normal tissues [208, 209]. The proteases contribute to the degradation of proteins or peptide substrates. The oxidoreductases can mediate the catalysis of electron transfer from the reductant to the oxidant. Kinases provide phosphorylation to affect protein activity and phosphatases mediate dephosphorylation. A number of enzymes demonstrate upregulation such as MMP-2 [210]. In bladder tumors, the expression of HAse is enhanced compared to the normal tissues [211].

The extracellular matrix (ECM) is comprised of collagen, fibronection, laminin, vitronectin, elastin, and other factors including growth factors, cytokines, and matrix metalloproteinases that contribute to the support of the epithelial cell structure [212, 213]. Various cells have the ability to secrete ECM components but they are mainly secreted by fibroblasts [214]. During cancer progression, ECM can be considered as an initiation factor. The composition of ECM can be different based on the type of cancer, such as gastric tumors, in which a lower degree of differentiation improves the abundance of ECM components, heightens cell metabolism, and increases metabolic reprogramming [215]. According to the proteomic analysis, there is no difference between ECM components in tumor and normal tissues, while their levels demonstrate changes that are manifested by enhancement in ECM proteins and reduction in basement membrane components modulating tumor angiogenesis, metastasis, and invasion [216]. The density of ECM components increases during tumor progression, and a number of factors, such as E-cadherin/β-catenin, demonstrate reduction, enhancing proliferation and metastasis of cancer cells [217]. The increase in matrix density can cause a kind of environmental stress to enhance carcinogenesis. The high-strength ECM can stimulate EMT to increase cancer progression and promote the infiltration of M2 polarized macrophages while it suppresses the function of CD⁸⁺ T cells [218, 219].

The presence of hypoxia is another feature of TME resulting from the high proliferation of tumor cells. The alterations in interstitial fluid pressure, decrease in pH, and enhancement in ROS generation can result from hypoxia [220]. In regions with hypoxia, there is high interstitial fluid pressure due to leaky vasculature and abnormal lymphatic drainage in the tumor [221]. Moreover, the hypoxia in TME can enhance the generation of lactic acid and carbonic acid through glycolysis induction, providing an acidic pH. The hypoxia-inducible factor (HIF) can induce carbonic anhydrase IX or XII to transform carbon dioxide and water into HCO3– that, upon diffusion out of the cell membrane, it enhances HCO3– levels in TME. Furthermore, the endosomal and lysosomal vesicles in tumor cells demonstrate more acidic pH compared to cytosolic pH [222]. The hypoxia TME displays a redox potential difference between intracellular space (reducing) and extracellular space (oxidizing). Such redox potential is vital for the development of smart and selective delivery of therapeutics [223]. The enzymatic reduction during hypoxia in TME can cause the metabolism of chemical factors, including nitro, quinones, aromatic N-oxides, aliphatic N-oxides, and transition metals [224]. Such a feature can be utilized to develop hypoxia-responsive structures for exploiting the hypoxic regions [225].

To address the immunosuppressive role played by M2-polarized macrophages, the stimulation of M1 polarization through nanostructures emerges as a promising avenue for enhancing immunotherapy. A pivotal mechanism involves the development of genetically modified pristine cells, whose extracted cell membrane is utilized to coat and functionalize nanoparticles in cancer therapy. Biomimetic magnetic nanoparticles featuring gene-edited cell membranes demonstrate the capacity to target multiple pathways, thereby regulating macrophage polarization and suppressing tumorigenesis. Specifically, the presence of gene-edited cell membranes suppresses the CD44/SIRPα axis by upregulating SIRPα variants. Magnetic nanoparticles, forming the core, play a crucial role in re-educating and reprogramming macrophages, thereby accelerating cancer immunotherapy [248].

Changes in macrophages extend beyond polarization, and their role in regulating antigen processing is also significant. Certain clinically important pathways, such as STING, pose a challenge for targeting at the clinical level due to a lack of targeted delivery. By functioning as a STING agonist, ZnCDA encapsulates CDA and disrupts the endothelial barrier in cancer vasculature, facilitating penetration into the TME and tumor site. These nanoparticles target macrophages, enhancing antigen processing and expediting T-cell-related responses in cancer immunotherapy [249]. A number of nanoparticles have shown potential in changing the polarization of TAMs. In the context of M1 polarization of macrophages, different mechanisms are available for the induction of polarization of macrophages into the M1 phenotype. Ginseng-derived nanostructures with extracellular vesicle-like properties can stimulate the TLR4/MyD88 axis, resulting in increased M1 polarization of macrophages, elevated ROS levels, and induction of apoptosis in melanoma [250]. In fact, the M1 polarization of macrophages has been accompanied by apoptosis induction.

Although the primary focus of this section is to evaluate the role of nanoparticles in macrophage re-education, studies have demonstrated that membranes can be extracted from macrophages to coat and functionalize nanoparticles. This approach results in the development of biocompatible structures with stealth properties [251]. Such an approach can be used mutually in which nanoparticles are functionalized with macrophage membrane to improve their targeting ability towards macrophages and TME, and on the other hand, they can be designed for re-education of macrophages into M1 phenotype.

Targeting macrophages in cancer treatment is primarily driven by their immunosuppressive function. Despite the development of various immune response regulation strategies, such as phototherapy-induced immunotherapy, concerns persist regarding immunogenicity and inflammation induction. Therefore, it is crucial for nanoparticles to employ safe and biocompatible mechanisms to counteract macrophage-mediated immunosuppression. The biomimetic Fe₃O₄-SAS@PLT nanostructures, derived from sulfasalazine-loaded mesoporous magnetic nanostructures and functionalized with platelets, have been designed to suppress the glutamate-cystine antiporter system Xc-pathway in ferroptosis induction. This ferroptosis induction demonstrates synergistic effects with PD-L1 immune checkpoint immunotherapy, as observed in animal models. Notably, these biomimetic nanostructures induce ferroptosis, promoting M1 polarization of macrophages and disrupting the immunosuppressive TME [252].

When considering nanoparticles for modulating macrophages, especially for potential use in cancer immunotherapy at the clinical level, biocompatibility is as important as functionality. Lipid nanoparticles with cationic features have shown promise as carriers, delivering mRNA to targeted sites. Loading mRNA for re-educating macrophage polarization onto lipid nanoparticles creates safe and biocompatible nanostructures for cancer immunotherapy [253]. A significant advancement in utilizing nanoparticles for macrophage re-education involves functionalizing them with macrophage membranes to enhance efficacy. This hypothesis has been tested in experiments, demonstrating the potential of membranes derived from tumor-associated macrophages with immunomodulatory functions and antigen-homing affinity. These membranes were employed to functionalize upconversion nanostructures loaded with photosensitizers. Notably, tumor-associated macrophage membrane-functionalized nanoparticles suppress CSF1 and interactions between cancer cells and the tumor microenvironment, impairing tumorigenesis. Moreover, these nanoparticles stimulate photodynamic therapy by suppressing the M2 phenotype, enhancing M1 macrophage polarization, inducing immunogenic cell death, and improving the generation of T cells through enhanced antigen presentation [254].

Reorienting macrophages toward the M2 phenotype presents a hurdle in achieving successful immunotherapy. This polarization is chiefly instigated by tumor cell-secreted MCSF, resulting in the elevation of CSF1-R. Moreover, the heightened expression of SIRPα on myeloid cell surfaces activates SHP-1 and SHP-2 in macrophages, impeding immunotherapy by hampering phagocytosis. Moving beyond macrophage polarization, efforts are redirected to address macrophage activity failure. To augment macrophage phagocytosis, promising strategies involve the regulation of CSF1R and SHP2. Nanoparticles laden with CSF1R and SHP2 suppressors induce M1 macrophage polarization, boosting phagocytosis to impede tumorigenesis [255].

After elucidating the key mechanisms governing macrophage polarization and activity, the subsequent focus involves exploring nanoparticles with potential clinical applications. The FDA-approved ferumoxytol, an iron supplement and iron oxide nanostructure, serves dual roles as a drug delivery system and imaging agent. When co-cultured with macrophages for treating lung cancer metastasis, ferumoxytol upregulates caspase-3, inducing macrophages to express mRNAs for pro-inflammatory Th1-related responses. Ferumoxytol effectively suppresses tumor metastasis and proliferation while promoting M1 macrophage polarization to enhance the quality of cancer immunotherapy [256].

A growing body of evidence supports the potential involvement of tumor-associated macrophages in the development of drug resistance [257, 258]. These macrophages play a role beyond immune system regulation, influencing the response to chemotherapy. Furin-based aggregated gold nanostructures capitalize on the “enhanced permeability and retention” effect, aggregating in breast cancer due to furin upregulation. This process suppresses exocytosis, leading to increased preferential accumulation at the tumor site. These nanoparticles also inhibit autophagy, promoting M1 macrophage education to counteract drug resistance [259]. Table 1 provides a concise overview of the applications of nanoparticles in macrophage re-education for cancer immunotherapy. Figure 3 provides an overview of the regulation of tumor-associated macrophages by nanoparticles in cancer immunotherapy.

Nanoparticles play a crucial role in influencing cancer-associated fibroblasts (CAFs) within the cancer treatment landscape. Interactions between cancer cells and CAFs in the TME contribute to tumorigenesis, making it essential to explore nanoparticle applications in suppressing these interactions and impeding cancer progression. In ovarian cancer, ovarian cancer cells and TME cells promote the activation of ovarian CAFs. Gold nanoparticles with a size of 20 nm effectively disrupted this interaction, inhibiting CAF activation and offering potential in the treatment of ovarian cancer [275].

CAFs play a supportive role in tumor metastasis. Core-shell nanoparticles, with gold as the core and silver as the shell, were effective in suppressing osteopontin expression in CAFs, hindering cancer progression without impacting CAF biomarker expression [276]. Besides modulating CAF activation and secretions, nanostructures may also be used for targeted CAF destruction. Ultra-small iron oxide nanocarriers (6 nm in diameter) combined with low-frequency rotating magnetic fields induce mechanical forces, leading to CAF death and lysosomal disruption [277]. Targeting CAFs for destruction enhances nanoparticle internalization. Such a strategy addresses the challenge of a dense TME that hinders nanoparticle penetration. Ferritin nanocages loaded with the photosensitizer ZnF16Pc and modified with a single-chain variable fragment that targeted fibroblast activation protein, facilitated phototherapy to reduce CAFs and improve nanoparticle penetration into the tumor site [278].

Nanoparticles can serve dual functions in regulating CAFs and modulating immune responses. Poly(lactic-co-glycolic acid) (PLGA) nanoparticles functionalized with cancer cell membrane not only enhanced cancer cell-CAF interactions, but also increased antigen uptake, stimulating CD⁸⁺ and CD⁴⁺ T cells through MHC-I and MHC-II, thus promoting cancer immunotherapy [279]. The fibroblast activation protein, upregulated on CAF surfaces, represents a promising target in cancer immunotherapy. Nanoparticles functionalized with a single-chain variable fragment for ZnF16Pc delivery in cancer phototherapy lacked systemic toxicity. These functionalized nanoparticles suppressed cancer progression in both primary and distant sites by accelerating immune responses and promoting anti-CAF immunity [280].

Some nanoparticles are designed to respond to fibroblast activation protein as a CAF biomarker. Albumin nanostructures encapsulating paclitaxel and functionalized with CAP showed promise in targeting fibroblast activation protein in CAFs. Incorporation of the photosensitive compound IR-780 further enabled near-infrared laser irradiation for photothermal therapy, resulting in tumor suppression and improved deep tumor penetration [281]. The concept of specifically targeting CAFs using their biomarkers has significant potential in enhancing the fight against cancer.

Nanoparticles, through targeted regulation of T cells, have emerged as a promising avenue for effective cancer immunotherapy [282‐288]. Increasing the infiltration of CD⁸⁺ T cells and T helper cells in the TME is crucial for TME remodeling and activating the immune system against cancer progression. Nanoparticles such as manganese zinc sulfide nanostructures play a pivotal role in mediating this effect [289]. A noteworthy trend in recent years involves the integration of immunotherapy with other therapeutic modalities like chemotherapy or phototherapy. Hybrid prodrug nanocarriers carrying cisplatin and camptothecin, stimulate the cGAS/STING axis and induce DNA damage. Additionally, these prodrug nanocarriers enhance CD⁸⁺ T cell infiltration in the TME, improving immunotherapy outcomes for colorectal cancer. These hybrid nanocarriers possess a responsive feature to reactive oxygen species (ROS) and are constructed from mPEG2k-DSPE and other polymers [290]. The mPEG/PLGA/PLL nanocarriers, delivering CD155-siRNA and modified with PD-L1 antibodies, can simultaneously suppress CD155 and PD-L1, avoiding immune evasion. They enhance CD⁸⁺ T cell infiltration and induce immunogenic cell death in breast cancer therapy [291].

Developing an effective anti-cancer vaccine requires nanoparticles that can induce systemic immunity. MnO2-melittin nanostructures, responsive to changes in the TME, serve as promising vaccines by triggering systemic immune responses. These nanostructures induce cancer cell death through the Fenton reaction in the TME, activate the cGAS/STING axis, and enhance antigen-presenting cell maturation. Furthermore, MnO2-melittin nanoparticles stimulate systemic immune reactions, including the promotion of T cells and increased levels of pro-inflammatory cytokines and chemokines [292].

Combining chemotherapy with phototherapy is another strategy to expedite tumor suppression. Prodrug nanocarriers, developed from hyaluronic acid and adamantine-conjugated heterodimers of PPa and JQ1, target CD44-overexpressed pancreatic cancer cells. This combination of phototherapy and immunotherapy increases T lymphocyte infiltration. Moreover, JQ1 suppresses phototherapy-induced immune evasion by downregulating c-Myc and PD-L1, resulting in significant tumor suppression [293].

As cancer development is a gradual process, effective treatment should focus on providing long-term immunity. The use of cancer vaccines has significantly increased in recent years; however, a major challenge remains in the targeted delivery of cargo, including antigens and adjuvants. To address this issue, glycosylated poly(lactic-co-glycolic acid) (PLGA) nanocarriers have been developed for the delivery of the ovalbumin antigen and CpG as an adjuvant in cancer vaccination. The surface of the nanostructures is modified with galactose or mannose. These nanoparticles possess high loading ability and sustained release, which are key features for the development of cancer vaccines. They stimulate dendritic cell maturation, promote antigen uptake, and enhance CD⁴⁺ T cell levels, leading to increased infiltration of CD⁸⁺ T cells in cancer immunotherapy [294].

An innovative approach in cancer therapy involves developing nanoparticles that mimic pathogens to induce a robust immune response. Saccharomyces cerevisiae (yeast)-based nanocarriers function as nano-pathogen-associated molecular patterns (nano-PAMPs) and, through the induction of Dectin-2 and TLR-4, enhance TH17 responses, contributing to anti-cancer immunity [295]. Stimulation of T helper cells has proven effective in cancer immunotherapy. Chondroitin sulfate-modified nanostructures conjugated with glycolic acid or mannose, along with cationic liposomes loaded with ovalbumin, can stimulate the maturation of dendritic cells and evoke T helper type I and II responses [296]. In many cases, nanoparticles not only stimulate T cell infiltration, but also accelerate the maturation of dendritic cells, contributing to cancer immunotherapy [297]. Recognizing the role of epigenetic changes in immune dysfunction, the delivery of miRNAs has been explored in cancer immunotherapy. Lipid nanoparticles delivering anti-miR-21 have demonstrated the ability to stimulate M1 polarization of macrophages and enhance the infiltration of CD⁸⁺ T cells [298].

Nanoparticles have been employed for targeted regulation of immunosuppressive Treg cells in cancer treatment, aiming to enhance immunotherapy potential. For example, PLGA nanoparticles with antigen-capturing capabilities have been developed for this purpose. These nanoparticles primarily elevate the CD⁸⁺ T cell count, consequently increasing the ratio of cytotoxic T cells to Treg cells [299]. By augmenting this ratio, the negative impact of Treg cells on immune responses can be alleviated. For enhanced cargo delivery, layer-by-layer nanostructures, composed of GITR/PLGA and modified with PLG and PLH that are responsive to the TME pH, have been designed to deliver IR780 dye. Subsequent irradiation with a 808 nm laser promotes the maturation of dendritic cells, thereby increasing the activity of CD⁸⁺ and CD⁴⁺ T cells in cancer immunotherapy. Notably, these nanoparticles exhibit a suppressive effect on Treg cell function, contributing positively to immune reactions [300].

Several widely used chemotherapeutic drugs, including doxorubicin, face limitations such as low tumor site accumulation and the development of drug resistance. Prodrug nanocarriers based on doxorubicin and indoximod have been developed to suppress the IDO pathway. These prodrug nanocarriers induce immunogenic cell death, enhance the infiltration of cytotoxic T cells (CD⁸⁺ T cells), and suppress Treg cells, MDSCs, and TAMs in the TME, thereby effectively promoting T cell/Treg cell ratio for cancer immunotherapy [301].

Co-delivery strategies have been used to improve cancer immunotherapy. Metformin, a compound utilized for cancer immunotherapy, has shown promise in re-educating the TME and enhancing macrophage phagocytosis activity. Co-assembled prodrug nanoparticles, designed with hyaluronic acid-cisplatin/polystyrene-polymetformin, effectively co-deliver metformin and cisplatin. With a size of 166.5 nm and a zeta potential of -17.4 mV, these nanoparticles exhibit high potential in cancer immunotherapy. They induce apoptosis through PARP upregulation, enhance cisplatin sensitivity by suppressing ERCC1, and modulate AMPKα/mTOR pathways to increase CD⁸⁺ and CD⁴⁺ T cells, and reduce Treg cell numbers [302].

Unmodified nanoparticles exhibit poor specific targeting of Treg cells. This prompted the use of nanocarrier functionalization. Hybrid nanocarriers functionalized with tLyp1 peptide have been developed to suppress STAT3 and STAT5, reducing Treg cell numbers and increasing the infiltration of CD⁸⁺ T cells in the TME [303]. The functionalized nanoparticles contribute to tumor suppression by increasing the infiltration of dendritic cells, CD⁸⁺ T, and natural killer cells, while reducing Treg and MDSC cells [304]. Furthermore, polymerosomes have been shown to stimulate the STING axis and enhance the infiltration and proliferation of T cells in cancer immunotherapy [57]. Table 2 summarizes the application of nanoparticles for the regulation of T cells in cancer therapy. Figure 4 demonstrates the role of nanoparticles in the regulation of CAFs, T cells, and Treg cells.

In each tumor, the levels of oxygen are different [318]. The oxygen insufficiency in tumor tissue generally ranges from more or less anoxic state (almost no oxygen) to 60 mm Hg (8% oxygen). In spite of this, the tumor cells demonstrate a specific condition known as hypoxia in which oxygen levels fluctuate from anoxia to 7.5 mm Hg (about 1% oxygen) [319]. Hypoxia can be considered a reliable biomarker, since it promotes the progression of tumor cells and can cause therapy resistance [320]. Along with tumorigenesis, the hypoxia in cancer enhances, and it shows some coordination with angiogenesis, proliferation, and metastasis. Hypoxia is able to enhance the levels of CCL22, CCL28 and increases the accumulation of MDSCs and Tregs to mediate immunosuppressive TME [321‐323]. Furthermore, hypoxia has been shown to be a factor involved in immune resistance [324]. Metformin is able to improve cancer immunotherapy by impairing the function of hypoxia in impairing CD⁸⁺ T cells [325]. Exercise has been shown as a mechanism for apoptosis induction and decreasing the proliferation of cancer cells. Moreover, exercise can ameliorate hypoxia, and enhance the function of T cells and reduces levels of Treg cells in cancer immunotherapy [326]. Hypoxia has been also shown as a mechanism in increasing M2 polarization of macrophages and secretion of factors with immunosuppressive function, including VEGF and TGF-β. Moreover, hypoxia has been suggested to cause therapy resistance, especially during photodynamic therapy and radiation in which oxygen molecules are required for cancer suppression [180, 327, 328].

Therefore, the function of hypoxia in cancer immunotherapy is of importance [329]. Hypoxia can be exploited by the nanoparticles for improving their specificity and recently, the hypoxia-responsive nanostructures have been designed for cancer immunotherapy [330‐332]. However, most of the attention has been paid to the regulation of hypoxia in cancer immunotherapy. The biodegradable NIR-II pseudo conjugate polymeric nanostructures can regulate hypoxia in cancer immunotherapy. These nanostructures can deliver regorafenib and respond to 808 nm laser irradiation to release drugs for the reduction of cancer hypoxia through vascular normalization, allowing for oxygen entrance into tumors to increase ROS generation, mediating immunogenic cell death (ICD) for cancer immunotherapy. Moreover, these nanoparticles reprogram macrophages from M2 to M1 [269]. In another effort, albumin-based nanostructures have been developed for the co-delivery of IR780, NLG919, and hypoxia-activated prodrug tirapazamine (TPZ) in synergistic tumor suppression. Exposure to nanoparticles to NIR irradiation mediates the generation of ¹O₂ to trigger the release of ROS-responsive linker for TPZ release, causing chemotherapy through enhancing tumor hypoxia. Moreover, these nanostructures stimulate ICD to enhance the activity of cytotoxicity of T lymphocytes [333]. Doping the nanoparticles with Mn²⁺ can alleviate hypoxia and increase cGAS sensitivity, inducing the cGAS/STING pathway, causing macrophage re-education, and increasing the maturation of dendritic cells [334]. In a number of cases, the hypoxia is boosted in the TME to promote the release of drugs from nanoparticles for cancer immunotherapy [335]. Furthermore, macrophage-mimetic microalgae and liposomes have been conjugated to suppress autophagy and reduce hypoxia in cancer immunotherapy [336]. Regarding autophagy regulation, it should be highlighted that autophagy has a dual function in cancer and can exert carcinogenic and anti-carcinogenic functions, complicating its regulation in cancer therapy [337, 338]. According to these studies, the regulation of hypoxia by nanoparticles can pave the new gate for cancer immunotherapy [339‐342].

The infiltration of MDSCs is against anti-cancer immunity since it impairs T-cell proliferation and enhances the differentiation of Treg cells [343]. MDSCs are consideredimmature myeloid cells with a heterogeneous nature providing, an immunosuppressive TME [172]. Overall, MDSCs utilize three distinct mechanisms to cause immunosuppression. In the first method, the arginase 1 and iNOS undergo upregulation in MDSCs, and they can deplete _L-arginine which is vital for the proliferation of T cells and CD3 ζ-chain formation of TCR. However, enhancement in arginase 1 activity and iNOS can suppress the proliferation and function of T cells [344‐347]. In the second method, the MDSCs can enhance the generation of ROS and RNS to mediate dysfunction in T cells [348‐350]. The ROS and peroxynitrite derived from MDSCs can cause post-transcriptional alterations in TCR and CD8 to interfere with the function of peripheral CD⁸⁺ T cells and cause antigen-specific tolerance in these cells through impairing binding affinity to phosphorylated MHC molecules [348]. In the third and final way, MDSCs are able to enhance Treg cell differentiation to disrupt anti-cancer immunity [172, 351]. Upon that, Treg cells secrete a number of inhibitory cytokines such as IL-10, IL-35, and TGF-β to interfere with the proper function of the immune system [352, 353]. Therefore, targeting MDSCs is critically vital for anti-cancer immunity. The intravenous administration of DNA thioaptamer-functionalized liposomes can cause specific targeting of TME, particularly MDSCs. Moreover, such liposomes provided targeted delivery of doxorubicin in breast cancer animal models to enhance infiltration of CD⁸⁺ T cells and diminish MDSCs [354]. Notably, there are different kinds of immune response-related molecules in TME, including IL-1β [355, 356], IL-6 [357], prostaglandin E2 [358], VEGF [359], and IFN-γ [351] that disrupt the differentiation procedure to increase the accumulation of immature myeloid cells [360]. As a result, one of the promising strategies can be targeting MDSCs for mediating their differentiation into other kinds of immune cells. In line with this, the lipid-coated biodegradable hollow mesoporous silica nanostructures have been introduced to regulate MDSC differentiation. Such nanostructures are able to co-deliver IL-2 and all-trans retinoic acid to trigger the MDSC differentiation into mature dendritic cells, macrophages and granulocytes. These nanoparticles showed significant capacity in enhancing the number of mature dendritic cells and decreasing MDSCs. Furthermore, these nanoparticles stimulated CD⁴⁺ and CD⁸⁺ T cells and increased the secretion of IL-12 and TNF-α as anti-tumor cytokines [361].

One of the prominent problems in cancer eradication using immunotherapy is the lack of effective and proper delivery into APCs and lymph nodes. The nanoparticles have opened a new gate for the delivery of immunotherapeutics into APCs and lymph nodes [362]. Noteworthy, a number of nanostructures based on their design, demonstrate the immunostimulatory impact, and even in lack of delivery of cargo, they can stimulate T and B cell responses [363, 364]. The tumor antigens have been conjugated into nanostructures, and upon injection into OVA-expressing melanoma, thymoma, or lymphoma-bearing mice, they caused significant anti-cancer immunity [365, 366]. Furthermore, such model antigens conjugated into nanostructures triggered T cell and antibody responses against lymphoma or colon tumors to impair cancer growth and enhance the survival of animal models [366‐368]. The particle size has been considered an important factor in this case, as small virus-size particles (≤ 40 nm) can reach the lymph nodes and demonstrate high cellular uptake by dendritic cells. Then, the peptide presentation from coated antigen by dendritic cell-related MHC class I molecules occurs and the stimulation of CD⁸⁺ T cells is observed [369, 370]. The endocytosis of such nanostructures by dendritic cells can stimulate the danger-sensing pathway in dendritic cells and mature them for immunogenic APCs [371]. The tumor antigen should reach the tumor-draining lymph nodes to be absorbed by professional APC cells such as dendritic cells, and then, their presentation to T cells occurs. The tumor-specific T cells have been found in lymph nodes. However, the dendritic cells in tumor-draining lymph nodes demonstrate an immature/inactive form that compromises their ability to induce anti-tumor T cell responses [372‐379]. The nanoparticle-bound cytosine-phosphate-guanine (CpG) oligonucleotides (an adjuvant) have been shown to accumulate in tumor-draining lymph nodes in melanoma to induce dendritic cells [380].

A high number of nanostructure-based methods need tumor infiltration by nanoparticles [381]. The clinical implication of EPR [382, 383] is still under question and a controversial debate, and there is a discussion in which only a small proportion of administered nanostructures can reach the tumor tissues, lacking clinical importance and therapeutic value in the clinical setting [384]. Therefore, significant efforts have been made to improve the ability of nanoparticles to reach tumor tissues and optimize the nanostructures in a way to control biological interactions due to the complicated nature of TME resulting from irregular vascular distribution, high tumor interstitial fluid pressure, low blood flow, dense EZN and high number of stroma cells. Therefore, the strategies should be directed towards enhancing the entry of nanostructures into tumor tissue that can be obtained through improving tumor perfusion, elevating tumor vasculature permeability, and mediating ECM remodeling. As an example, the nanostructures applied for ECM degradation or restoring tumor vasculature into normal condition [385] can mediate TME priming to provide desirable immune reactions, reversing immunosuppressive TME and enhancing anti-cancer immunity [386]. In cases where tumors are accessible, the intratumoral administration of nanoparticles is preferred into systemic injection to enhance accumulation at the tumor region. The proper accumulation of nanoparticles in TME and the release of therapeutic cargo can enhance tumor suppression, while it reduces the adverse impacts. A number of clinical studies have suggested the intratumoral injection of immunotherapeutic compounds [387, 388] which has also been confirmed in pre-clinical studies upon intratumoral injection of immune checkpoint inhibitors [389]. The nanoparticles optimized to bind into ECM or cancer cells can enhance the accumulation of these structures in the tumor region [390] and provide a new insight into the effective delivery of therapeutics into tumor cells or TME.

Biomimetic nanoparticles are characterized by structures whose surfaces are modified or coated with membranes from other cells. The development of biomimetic nanoparticles aims to enhance stealth properties, preventing their identification by the reticuloendothelial system. Biomimetic nanoparticles exhibit good biocompatibility, making them widely applicable in cancer treatment. Recent studies have explored the potential of biomimetic nanoparticles in cancer therapy, demonstrating their versatility when modified with aptamers [410], facilitating chemotherapy through co-delivery of chemotherapy drugs and natural products [411], and showcasing high penetration and targeting features [412]. They have also been utilized for bioimaging and immunotherapy [413, 414], evading the mononuclear phagocyte system [415] and improving blood circulation time [416].

The application of biomimetic nanoparticles in cancer immunotherapy has shown promising results in tumor suppression. In some cases, chemotherapy using biomimetic nanoparticles can activate the immune system. For instance, cancer cell membrane-functionalized phosphorus dendrimer-copper(II) complexes (1G3-Cu) and toyocamycin (Toy)-loaded polymeric nanocarriers with a size of 210 nm can be cleaved in the TME to release cargo and reduce glutathione levels. By causing mitochondrial dysfunction and endoplasmic reticulum stress, these nanocarriers trigger apoptosis and immunogenic cell death. They accelerate the maturation of dendritic cells and increase T lymphocyte infiltration. With the application of PD-L1 antibodies, the nanoparticles can enhance chemotherapy, impair relapse, and prevent the invasion of cancer [417].

Biomimetic nanoparticles also serve as effective carriers for delivering siRNA. Recognizing PD-L1 as an immune evasion factor, its downregulation by siRNA can expedite cancer immunotherapy and prevent T cell exhaustion [418]. These nanoparticles exhibit the capability to effectively suppress cancer progression in vivo, making them promising candidates for the development of cancer vaccines [419].

The field of cancer therapy has undergone a revolutionary transformation with the application of nanogels as drug carriers [420, 421]. Nanogels exhibit favorable physicochemical features. They are potential carriers for delivering both hydrophobic and hydrophilic drugs [422], recombinant proteins [423], and genetic materials [424].

Nanogel-induced immunotherapy has proven effective in impeding cancer progression. Polymeric nanogels, developed from PDEA-co-HP-β-cyclodextrin-co-Pluronic F127 and a charge-reversible polymer named dimethylmaleic anhydride-modified polyethyleneimine, undergo degradation in the TME. These nanogels, functionalized with cancer cell membranes, co-deliver paclitaxel and IL-2, inducing the maturation of dendritic cells and enhancing the infiltration of effector cells [425].

Stimuli-responsive biomimetic nanocarriers have been engineered to optimize cancer immunotherapy. Polydopamine-CaCO₃ nanocarriers, functionalized with cancer cell membranes and featuring pH-responsive characteristics, enable phototherapy and bioimaging. Exposure to the TME triggers the degradation of nanocarriers, releasing CO₂ bubbles that promote phototherapy-mediated immunotherapy. Combining this with checkpoint inhibitors further enhances tumor immunotherapy [426].

In the realm of biomimetic nanovaccines, studies have primarily focused on delivering therapeutics or stimulators to dendritic cells. However, the presence of the endocytosis system and endosomal degradation can hinder the effectiveness of these nanovaccines. To address this issue, biomimetic nanoplatforms have been developed to accelerate the internalization of nanoparticles in dendritic cells. Utilizing ROS-responsive nanoparticulate cores fused with peptides and cell membranes, these nanovaccines induce micropinocytosis, facilitating direct cytosolic delivery of Stimulator of Interferon Genes (STING) agonists. This enhances dendritic cell maturation and T cell priming through STING upregulation in cancer immunotherapy [427].

The stimulation of immunogenic cell death and the promotion of dendritic cell maturation and T cell infiltration represents the primary strategy utilized by biomimetic nanocarriers in cancer immunotherapy. Regulation of metabolites is crucial for achieving better immunotherapy responses and immunogenic cell death. Zinc ions-bonded ZIF-8 frameworks with CuS nanodots, functionalized with cancer cell membranes, have been introduced to amplify photothermal-mediated immunotherapy through Zn²⁺ metabolic modulation. These frameworks induce immunogenic cell death, enhance dendritic cell maturation, and increase T cell infiltration [428]. Although less explored compared to macrophage- and cancer-derived membranes, the membranes of red blood cells may also be utilized for the development of biomimetic nanocarriers [429, 430].

One of the main reasons for the modification of nanoparticles with the cancer cell membrane is to provide an antigen resource [431, 432]. The PLGA structures have been functionalized with the membrane of melanoma cells and then, monophosphoryl lipid A (MPLA) as an adjuvant was embedded into nanoparticles to stimulate the maturation of dendritic cells for enhancing antigen-specific T cell response [433]. Since the expression of MHC-I can be found in all cells, such as tumor cells, the cancer cell membrane-functionalized nanostructures can be considered novel APC to induce T cells, even in the absence of professional APCs. The CD80-expressing cancer cells were utilized to derive cell membranes for coating nanoparticles. These nanostructures can directly induce antigen-specific T cells through the interaction of CD28 with cognate T cell receptors, suppressing tumorigenesis in prophylactic and therapeutic tumor models [288]. Furthermore, the cancer cell-membrane functionalized nanoparticles could be considered as vaccines. Despite significant efforts to highlight the potential of cancer cell membrane-functionalized nanoparticles in cancer immunotherapy, there are several limitations to be considered for future studies. Recent studies have highlighted the potential of ferroptosis in cancer immunotherapy [434‐436]. More effort regarding the application of biomimetic nanoplatforms in the regulation of ferroptosis and related pathways should be performed to facilitate cancer immunotherapy. Moreover, autophagy is another factor in the regulation of cancer immunotherapy [437‐440]. The biomimetic nanoparticles should be designed in a rational way to modulate autophagy for the regulation of T cells and other components of immune systems as well as reprogramming macrophages for effective cancer immunotherapy.

Red blood cells (RBCs) have obtained much attention for the purpose of drug delivery due to a number of characteristics, including biocompatibility, biodegradability, long lifespan, and desirable encapsulation capacity [441]. The OVA-encapsulated RBCs are able to stimulate CD⁴⁺ and CD⁸⁺ T cells after intravenous injection in mice [442]. There is also a promising approach in which RBC membrane is utilized to coat the nanostructures for the development of vaccines with long circulation ability [443]. In an attempt, RBC membrane-functionalized PLGA nanoparticles were designed to deliver both antigen hgp10025 − 33 and adjuvant MPLA. In order to selectively target the dendritic cells, mannose was included in the RBC membrane, and these structures demonstrated high potential in suppression of melanoma [444]. However, the studies are limited, and more experiments regarding the application of other kinds of nanostructures, such as metal and carbon nanomaterials, and their modification with RBC membrane should be performed to improve the potential in cancer therapy.

Platelets are released by megakaryocytes, and they can control homeostasis, tumor invasion, and overall physiological and pathological events. Since the platelets have shown expression of self-recognized proteins, including CD47, they are beneficial for reducing clearance and stimulation of the complement system to enhance the blood circulation time of nanostructures [445, 446]. The platelet membrane-functionalized nanoparticles have been exploited to deliver R848 as a TLR7/8 agonist in enhancing accumulation at the tumor region and promoting the interaction with TME components. Moreover, even at low doses (18 µg vaccine per mouse), they could suppress tumors in 87.5% of mice [447]. Moreover, there is the possibility of embedding metformin and IR780 as photosensitizers in platelet membranes to stimulate ICD and improve the potential in cancer immunotherapy through suppressing MDSCs and Treg cells [448]. However, the potential of these nanocarriers in the regulation of TAMs and CAFs for cancer immunotherapy should be highlighted.

The advantage of pH-sensitive biomimetic nanoparticles lies in their ability to induce cancer immunotherapy in a more specific manner due to targeted decomposition in the TME. Immunotherapy with the use of biomimetic nanocarriers may be enhanced through phototherapy. Photo-responsive nanocarriers, through the acceleration of ROS generation, mediate immunotherapy. Macrophage membrane-based vesicles, functioning as nanoparticles, deliver drugs and photosensitizers (TAPP) to induce pyroptosis. These biomimetic vesicles, releasing copper ions, mediate ROS-induced pyroptosis. Utilized as nanoparticles, they increase ROS levels, inducing pyroptosis through the upregulation of caspase-3-induced gasdermin E, resulting in pyroptosis-induced immunotherapy [449].

The macrophage membrane-functionalized nanostructures have shown high potential in specific tumor targeting [450]. The macrophage membrane-coated nanoparticles have been utilized for the delivery of saikopsaponin D, and the surface has been hybridized by adding T7 peptide to provide macrophage-homing capacity for nanostructures and target the tumor cells upregulating transferrin receptor. These nanoparticles specifically accumulate at the tumor site and can escape phagocytosis by the reticuloendothelial system [451]. The functionalization of MOF-derived mesoporous carbon nanostructures with macrophage membranes has been performed to increase their escape from phagocytosis and improve the potential in cancer therapy [452]. In addition to high specificity in tumor targeting, the macrophage membrane-functionalized nanoparticles demonstrate high biocompatibility that, along with their anti-cancer activity, are promising candidates for tumor eradication [251, 453]. Therefore, increasing evidences highlight the application of macrophage membrane-functionalized nanoparticles in cancer therapy [454‐459]. In order to better highlight their potential, it is suggested to develop biomimetic nanoparticles functionalized with cancer and macrophage membranes to improve potential in cancer immunotherapy.

The biomimetic nanoplatforms are also interesting for the regulation of specific mechanisms such as hypoxia and lipid metabolism in TME. Biomimetic nanoparticles may be designed to present tumor antigens and co-stimulatory molecules simultaneously for cancer immunotherapy [460]. The most effective strategy thus far is centered around the development of biomimetic nanoparticles with pathogen-like features. These biomimetic nanoparticles can elicit significant and long-term immune responses in cancer therapy. One of the key reasons for the application of biomimetic nanoparticles is their ability to enhance the blood circulation time of cargo. Cholesterol reduction in the membrane used for nanoparticle functionalization improves blood circulation time and its effectiveness in inducing cancer immunotherapy [461]. Because of rapid cancer cell proliferation, the hypoxic and nutrient-deficient conditions within the TME hinder the proper functioning of immune cells. The development of a biomimetic platform to increase glucose and glutamine levels required by T cells can reprogram the TME, and accelerate cancer immunotherapy [462]. Table 3; Fig. 5 summarize the application of biomimetic nanoparticles in cancer immunotherapy.

The primary source of exosome secretion includes normal cells, stromal cells, cancer cells, and immune cells. Exosomes secreted by breast cancer cells, for example, play a role in inducing immunotherapy by reprogramming macrophage polarization. These exosomes carry PEDF, promoting M1 macrophage polarization by increasing CD80, IRF5, MCP1, and IL-1β levels, while reducing CD206, Arg, TGM2, and TGF-β levels [473].

Despite the immunotherapeutic potential of exosomes, there are endogenously-secreted exosomes that enhance M2 macrophages, thereby contributing to cancer progression. Inflammation, a risk factor for cancer, can initiate cancer development, and the regulatory functions of macrophages in inflammation can alter the pathogenesis of cancer. Myeloid-derived suppressor cells (MDSCs) in the TME secrete exosomes, transferring miR-93-5p to suppress the STAT3 axis. Enrichment of miR-93-5p in these exosomes, induced by IL-6, leads to MDSC differentiation into M2 macrophages, facilitating colitis-induced cancer development [474].

When macrophages polarize into the M2 phenotype, they release exosomes enriched with apolipoprotein E that reduces MHC-I expression, resulting in decreased immunogenicity and induced immune resistance [475]. Detecting exosomes with immunosuppressive/inductive functions involves recognizing their surface biomarkers. Exosomes positive for PD-L1 are implicated in suppressing immune reactions during cancer progression. PD-L1 + exosomes stimulate CD⁸⁺ T cell exhaustion, enhancing tumorigenesis during cancer metastasis [476]. The secretion of PD-L1 + exosomes involves intricate molecular pathways, where HMGB1 upregulates RICTOR mRNA levels through miR-200 family downregulation, particularly miR-429. This HMGB1 and RICTOR crosstalk increases PD-L1 generation, promoting the activity of PD-L1 + exosomes in cancer immunotherapy [477].

Although PD-L1 + exosomes have carcinogenic functions, other exosomes can suppress PD-L1 expression. Considering the ability of exosomes to induce cancer immunotherapy, they may be utilized as potential vaccines in the future. Cancer-derived exosomes exhibit superior capabilities in dendritic cell maturation and MHC cross-presentation, compared to cytotoxic T lymphocytes. Furthermore, exosomes can reduce Treg cell numbers in cancer immunotherapy by suppressing PD-L1 expression [478]. This insight suggests that bioengineered exosomes, specifically designed to target dendritic cells, may serve as effective vaccines in cancer therapy. Although the discussion on bioengineered exosomes is reserved for the next section, it is noteworthy that cells can be engineered to secrete exosomes for cancer immunotherapy. For example, the nuclei of cancer cells introduced to M1 macrophages can lead to the secretion of chimeric exosomes, selectively accumulating in lymph nodes and priming T cells to stimulate immune reactions against cancer cells [479]. Therefore, exosomes are potential regulators of the immune system in cancer [480]. Exploring the regulation of exosome secretion and biogenesis from cells opens new avenues for controlling cancer immunotherapy.

Exosomes are used in cancer therapy because they are naturally present in the body, making it less likely for them to be identified as foreign and cleared. Their high biocompatibility allows effective cargo delivery and testing in clinical trials. Dying cancer cell-derived exosomes, modified with MART-1 peptide and CCL22-siRNA, induce T cell-mediated responses and suppress Treg expansion [481]. Bioengineered exosomes can produce persistent immunity against cancer cells, and this paves the way for the development of bio-based vaccines. Bioengineered exosomes, painted with HMGB1, stimulate dendritic cells, enhance immunogenicity, and induce long-term immunity against cancer. These exosomes accumulate in lymphoid tissues and enhance T cell function, inducing long-term immunity and suppressing cancer progression [482]. Another approach involves functionalizing exosomes via CpG DNA to stimulate dendritic cells and enhance antigen presentation, showcasing their co-delivery ability in synergistic cancer immunotherapy [483, 484].

Fusogenic exosomes have been synthesized to address cancer cells escaping the immune system due to self-recognition. These exosomes introduce viral antigens, stimulate dendritic cells, and present antigens to T lymphocytes for CD⁸⁺ T cell cross-priming [485]. Exosomes from bone marrow-mesenchymal stem cells, loaded with galectin-9 siRNA and oxaliplatin, induce immunogenic cell death, recruit T lymphocytes, reduce Treg cells, and promote M1 polarization of macrophages, contributing to cancer immunotherapy [486]. Engineered M1 macrophage-derived exosomes, targeting TAMs with IL4RPep-1, NF-κB p50 siRNA, and miR-511-3p, induce cancer immunotherapy by reprogramming macrophages and restricting cancer proliferation [487]. Considered as biocompatible carriers, exosomes may be used to modify other nanoparticles. Combining exosomes and liposomes enhances gene delivery, suppresses CD47 signal, and increases CD⁸⁺ T cell potential. This strategy has the potential for clinical applications in cancer immunotherapy [488]. Table 4 is a summary of the use of exosomes in cancer immunotherapy.

Smart nanoparticles responsive to TME stimuli may be used to generate targeted delivery systems. Their multifunctionality stems from changing structures in response to TME stimuli, facilitating cargo release at the tumor site. The TME, because of its acidic pH, is distinctive from normal cells, making pH-sensitive nanoparticles ideal for cancer immunotherapy. Nanoparticles have been synthesized using a PLA-b-PEG core and a cytosine (C)-rich i-motif DNA surface to produce the so-called nanovaccines. They are used to deliver cyclic dinucleotides such as cyclic dimeric guanosine monophosphate (CDG) to endosomes, accelerating anti-tumor immunity and preventing TME immunosuppression. In TME’s mild acidic pH, a conformational switch releases cargo, while physiological pH minimally increases CDG release. These pH-sensitive nanoparticles protect CDG from enzymatic degradation, releasing the cargo into the cytoplasm, and stimulating polarization of macrophages into the M1 phenotype [504].

A groundbreaking achievement in cancer immunotherapy involves the creation of prodrug nanoparticles for cargo delivery. These nanoparticles release cargo in response to the pH of the TME. An innovative self-cascaded unimolecular prodrug was designed, comprising an acidic pH-activatable doxorubicin, and an aggregation-induced emission luminogen (AIEgen) photosensitizer linked to a caspase-3-responsive peptide. This engineered prodrug exhibits dual functionality- it can actively release doxorubicin in the acidic TME and initiate apoptosis-related caspase-3 activation. Furthermore, the activated caspase-3 can induce the release and in-situ aggregation of photosensitizers. This process enhances dendritic cell maturation, increases CD⁸⁺ T cell numbers, and prevents metastasis to lung tissue [505].

Mounting evidence supports the combination of chemotherapy and immunotherapy using pH-sensitive nanoparticles for cancer suppression. For example, poly(L-histidine) (PHIS) has been used to encapsulate R848, forming pH-sensitive PHIS/R848 nanocores. Conjugating doxorubicin to hyaluronic acid creates prodrug nanoparticles that coat PHIS/R848. These nanocarriers undergo a switch from hydrophobic to hydrophilic in response to pH change, releasing R848 to modulate the immune system. At pH 5.5, the hydrazone bond cleaves, releasing doxorubicin. The latter is internalized into tumor cells via endocytosis. This activates the immune system and suppresses cancer proliferation [506].

Metal-based nanoparticles can also be designed to decompose in response to TME pH. A human serum albumin-coated zeolite imidazolate framework-8 has been developed for the delivery of a photosensitizer (HPZ), for selective recognition of the TME [507]. This framework enables rapid elevation of zinc ion concentrations while ensuring controlled release of the encapsulated photosensitizer. Under physiological pH (7.4), HPZ exhibits a size of approximately 170 nm, decreasing significantly to less than 10 nm in acidic conditions (pH 6.5). The acid-induced decomposition of HPZ prompts a swift increase in zinc ion concentration, exerting potent cytotoxic effects against colorectal, breast, and pancreatic cancers. Intravenous administration of HPZ in a CT26 tumor-bearing mouse model results in the expansion of T helper and cytotoxic T cells, and reduction in the Treg cell population. These changes lead to a significant inhibition of tumor growth.

While the primary focus of this section is to assess the role of nanoparticles in cancer immunotherapy, it is worth mentioning that microneedle arrays, composed of tiny needles, can also be used for sustained cargo delivery. pH-sensitive nanoparticles can be incorporated into microneedles, facilitating sustained release for hyperactivating the immune system and creating nanovaccines [508]. Hydrogels are also available for sustained cargo delivery. For example, nanoparticles can be embedded in pH-sensitive hydrogels for delivering doxorubicin and JQ1 (a small molecular inhibitor) to induce immunogenic cell death [509]. Of note, pH-sensitive biomimetic nanocarriers, crafted from PLGA and coated with membranes from macrophages and cancer cells, can deliver FGL1-siRNA and metformin for cancer immunotherapy. These biomimetic nanoparticles enhance cargo endosomal escape, with metformin suppressing PD-L1 through AMPK upregulation, and siRNA reducing FGL1 expression to boost anti-cancer immunity through T cell induction [510]. Given the impact of TME pH on tumorigenesis, pH regulators have been developed. Calcium carbonate nanostructures neutralize TME pH by consuming lactate, promoting M1 macrophage polarization, immune cell infiltration, dendritic cell stimulation, and T cell recruitment for cancer immunotherapy [511]. Table 5 summarizes the application of pH-sensitive nanoparticles in cancer immunotherapy.

Redox-sensitive nanoparticles are carriers that release cargo in response to redox imbalances in the TME. The development of these nanoparticles involves introducing redox-sensitive bonds, such as disulfide bonds. They have found application in cancer immunotherapy, capable of stimulating various immune cells.

Multifunctional nanocarriers with redox-sensitive features, comprising toll-like receptor agonists, catalase, and PD-L1-siRNA, have been designed for cancer therapy. These nanoparticles induce M1 polarization of macrophages, increase ROS production, downregulate PD-L1 expression, and enhance CD⁸⁺ T cell functions by suppressing Treg cells [525].

Clinical application of immunotherapy hinges on using biocompatible and long-term safe nanostructures like micelles. Micelles containing doxorubicin and R848, a TLR7/8 agonist, were experimentally used as nanovaccines. Elevated glutathione levels in the TME trigger micelle decomposition, releasing cargo and inducing immunogenic cell death, dendritic cell stimulation, and accelerated immune responses. Additionally, redox-responsive polymers and an A2AR antagonist within micelles suppress adenosinergic signaling to activate NK and CD⁸⁺ T cells in cancer immunotherapy [526].

Cationic polymer dots, known for their small particle size, imaging capabilities, and drug delivery potential, have seen extensive use in biomedicine. PEI600-modified redox-sensitive hyperbranched poly(amido amine) nanostructures, partially carbonized with polymer dots, were used for carrying ovalbumin. These structures enhance splenocyte proliferation, elevate cytokine levels (including IL-12 and IFN-γ), promote dendritic cell maturation, and increase CD⁴⁺ and CD⁸⁺ T cell counts, as well as T lymphocytes. Ovalbumin release from these structures is redox-responsive [527].

Given that both pH and redox serve as endogenous stimuli, new nanocarriers have been developed that exhibit dual responsiveness. pH- and redox-sensitive micelles containing ovalbumin, modified with PLH-PEG, are used as cancer vaccines. These micellar nanostructures enable cytosolic delivery of ovalbumin through redox release, proton influx, micelle disassembly, and ultimately, a proton sponge effect and lysosome break. These micelles enhance MHC-I rates, antigen presentation, lymph node accumulation, and improve immune reactions [528].

Light serves as an exogenous stimulus in nanoparticle development, where laser irradiation induces bond cleavage to release the loaded cargo. This forms the basis of photodynamic therapy (PDT) and photothermal therapy (PTT) for cancer ablation. In PDT, ROS production induces cell death, while PTT causes cell death through hyperthermia. Combining phototherapy with immunotherapy can enhance cancer suppression. Two methods are utilized, including a combination of phototherapy and immunotherapy or stimulation of phototherapy-mediated immunotherapy. Both methods offer a high likelihood of tumor suppression, improving the fight against cancer. PLGA nanoparticles loaded with R837, docetaxel, and PB agents are used for immunotherapy and PTT. These nanoparticles are modified with cancer cell membranes for enhanced effectiveness. R837 stimulates dendritic cell maturation, and docetaxel increases M1 polarization of macrophages, impairing the immunosuppressive TME. This leads to increased infiltration of cytotoxic T lymphocytes in the TME for effective cancer immunotherapy [529].

Similar to doxorubicin, which induces immunogenic cell death, docetaxel has shown potential in cancer immunotherapy by regulating TAM polarization. CuS nanoparticles with NH₂ functional groups, functionalized with folic acid and conjugated to PEI-PpIX for enhanced solubility, deliver docetaxel and CpG, inducing PTT and immunotherapy. These nanoparticles exhibit excellent photothermal conversion ability upon exposure to 650 and 808 nm laser irradiation and induce M1 polarization of macrophages through the function of docetaxel in cancer immunotherapy [530].

Beyond combining immunotherapy and PTT, it is possible to induce immune reactions resulting from PTT. Tumor cell killing and antigen release can accelerate immune reactions. A peptide-photosensitizer conjugate, developed from anti-PD-L1 peptide, cleavable by MMP-2, and purpurin 18 as a photosensitizer, accumulates in the tumor site. MMP-2 enzyme degradation releases the peptide, causing antigen release. Irradiation induces dendritic cell maturation, migrating into lymph nodes, increasing T cell infiltration, suppressing metastasis to lung tissue, and eliciting anti-tumor immune responses [531].

Photo-responsive structures can also function as vaccines. Chelation of Fe³⁺ ions with ovalbumin leads to biomineralization into nanovaccines, embedding IR820 as a photosensitizer through electrostatic incorporation. The presence of iron induces ferroptosis, mediating immunogenic cell death. Immunogenic cell death stimulates neoantigens and DAMPs, synergizing with ovalbumin in cancer immunotherapy. Near-infrared irradiation induces PTT, enhancing immunotherapy. These nanostructures enhance T cell infiltration, inhibit the primary tumor, and show promising impacts in combination with checkpoint inhibitors [532].

The combination of PDT and immunotherapy has shown promising results in cancer immunotherapy. Although PDT and PTT have different mechanisms of action, both can be combined with immunotherapy to expedite the immunotherapeutic process by exposing antigens or inducing cell death to stimulate immune responses. Photodynamic therapy is preferred over PTT due to potential hyperthermia effects on normal tissues. However, when tumor-targeted nanoparticles are developed, they can precisely execute PDT in the tumor site, effectively killing cancer cells. Various nanoenzymes with cancer cell accumulation can be developed to induce PDT and enhance cancer immunotherapy.

A nanoparticle core was created by connecting paclitaxel drugs through a disulfide bond (PTX-SS-PTX), with P18 as a photosensitizer connected to MPEG-CPPA-b-P(M4). In response to glutathione, the disulfide bond degrades, and laser irradiation in response to ROS increases singlet oxygen generation, releasing high mobility group box 1 (HMGB1) due to GSDME activation. HMGB1 releases and pyroptosis induction leads to dendritic cell maturation, educating naïve T cells in lymph nodes, expanding T cells, and developing memory T cells for cancer immunotherapy [533].

For PDT stimulation, the photosensitizer is loaded into nanoparticles. Recent studies have shown the potential of herbal medicine in cancer treatment. Mesoporous silica nanoparticles modified with PEG can carry both chlorin e6 and astragaloside III, stimulating NK cells and suppressing cancer cell growth. These nanoparticles accumulate in the tumor site, increasing immune cell infiltration, promoting the activity and cytotoxicity of CD⁸⁺ T cells and NK cells, and impairing tumorigenesis [534].

Liposomes are also potential nanocarriers in cancer immunotherapy. Liposomes carrying IPI-549 as a PI3Kγ inhibitor and chlorin e6 as a photosensitizer induce ROS generation after irradiation, facilitating immunogenic cell death and improving the potential of T lymphocytes in eliminating cancer cells. IPI-549 delivery by nanoparticles can decrease arginase-1 (Arg-1) and ROS levels, increasing apoptosis in MDSCs, and preventing their suppressive function on T cells. In addition, these nanoparticles decrease the infiltration of M2-polarized macrophages and mature dendritic cells in cancer immunotherapy [535].

In both nanoparticle-mediated PDT and PTT, the most prominent mechanism is the stimulation of immunogenic cell death to accelerate cancer immunotherapy. Increasing evidence highlights the application of nanoparticle-mediated phototherapy and immunotherapy in cancer suppression [536‐540]. Table 6 summarizes the nanoparticles causing PDT and PTT, and their combination and relationship with immunotherapy. Figure 6 shows the application of stimuli-responsive nanocarriers in cancer immunotherapy. Since the pre-clinical studies demonstrate the function of nanoparticles for cancer immunotherapy and tumor suppression, the clinical application of nanostructures in cancer immunotherapy has been performed to evaluate their potential [541]. Table 7 and table 8 summarize the clinical application of nanoparticles in cancer immunotherapy.

Polymeric nanoparticles are among the most commonly applied structures in cancer immunotherapy owing to their biocompatibility, biodegradability, chemical stability, water solubility, and high drug loading [564, 565]. The PLGA, PGA, PLG, PEG, PEI and chitosan nanoparticles have been widely used in cancer immunotherapy [566]. Moreover, such nanostructures have shown potential as immunostimulatory adjuvants in cancer immunotherapy [567‐569]. Loading TLR7/8 agonists in PLGA nanostructures enhanced levels of CD40, CD80, and CD86 through stimulation of bone marrow-derived dendritic cells and subcutaneous administration of such nanoparticles stimulated dendritic cells and CD⁸⁺ T cells [570].

Liposomes are among the promising nanocarriers for drugs, genes and vaccines [571]. Until now, multiple kinds of liposomal nanostructures including 1,2-dioleoyl-3-trimethylammonium-propane (DOTAP), 3β- (N- [N’, N’-dimethyl aminoethane] - carbamoyl) cholesterol (DC-Chol), and dimethyl diocta decylammonium (DDA) [572, 573] have been significantly applied for the antigen exposure to APCs and providing vaccine adjuvants to increase antigen-specific immune reactions [574, 575]. The dextran-functionalized liposomes with pH-sensitivity activity have shown high uptake by dendritic cells and they can provide cytosol delivery of ovalbumin to accelerate antigen-specific immune reactions and impair cancer progression [576]. Furthermore, loading CpG-ODNs as TLR9 agonist and 3,5-didodecyloxybenzamidine as an adjuvant into liposomes can enhance dendritic cell-mediated cytokine production to enhance antigen-specific immunity [521]. Micelles are among another kind of lipid nanoparticles that can function as delivery systems for antigen/adjuvant to improve potential of vaccines. The polymeric hybrid micelles have been utilized for the delivery of CpG-ODN and Trp2 to create a nanovaccine for targeting lymph nodes and enhancing the accumulation of cargo in dendritic cells, triggering CD⁸⁺ T cell-mediated immune responses and enhancing cancer suppression (melanoma) [577].

Carbon nanotubes (CNTs) have been shown to mediate immunostimulatory impacts in vitro and in vivo. The oxidized multiwalled carbon nanotubes (MWCNT) have shown the ability for the delivery of cancer-testis antigen, known as NY-ESO-1 and CpG-ODNs. These structures showed uptake by dendritic cells and mediated powerful CD⁴⁺ and CD⁸⁺ T cell-driven immune reactions to impair melanoma progression [578]. Furthermore, the co-delivery of ovalbumin and CpG-ODN, as well as anti-CD40 Ig as immunoadjuvants by MWCNTs has shown potential in accelerating T cell responses and suppressing melanoma progression [579].,

Gold nanostructures are promising factors in cancer immunotherapy owing to their characteristics, including biocompatibility, adjustable surface chemistry, and ease of controlling size and shape [580]. The gold nanostructures have shown potential in inducing differentiation of macrophages into dendritic-like cells to enhance T cell proliferation and promote cytokine release [581]. Furthermore, the gold nanostructures have shown incredible potential to act as adjuvants for enhancing antibody generation [582]. The potential of gold nanoparticles in cancer therapy via TME modulation has been revealed [583, 584]. The surface of hollow gold nanostructures has been functionalized with CpG-ODNs to enhance their cellular uptake and enhance function in the induction of immune responses, including enhancing TNF-α secretion [585]. The silica nanoparticles have also been applied widely in biomedicine for imaging [586], specific targeting of cancer [587], and delivery of drugs and genes [588]. The mesoporous silica nanoparticles have been applied for antigen delivery and acted as vaccines to induce humoral- and cell-driven immune reactions while having high biocompatibility and lacking toxicity [589]. The hollow mesoporous silica nanoparticles have been shown to be biodegradable and are capable of TME remodeling in cancer immunotherapy [361]. According to these studies, various categories of nanostructures demonstrate promising characteristics in cancer immunotherapy, and all of them have shown potential in TME remodeling. The next step for the clinical application of these nanocarriers depends on their biocompatibility and long-term safety that lipid nanoparticles are at the front line.