The interaction between myeloma cells and various components in the TME often leads to the proliferation, survival, and metastasis of myeloma cells, excessive angiogenesis, bone destruction, drug resistance, and most importantly low anti-tumor immunity, including innate immunity and acquired immunity, in which the immune cells play a major role [
49]. However, TGF-β changes the effect that immune cells should play under normal conditions through various signaling pathways, mainly resulting in the decreased function of effector immune cells and ultimately forming an immunosuppressive microenvironment [
49,
51,
54,
56,
57]. Acquired immune cells involve T cells, including naive, effector, and memory T cells, and subsets like helper, regulatory, and cytotoxic T cells [
51], as well as antigen-presenting cells (APCs), such as dendritic cells (DCs) and B cells. APCs are divided into dedicated APCs and non-dedicated APCs. The former contains MHCII, such as dendritic cells (DCs) and B2 cells and the latter includes vascular endothelial cells, epithelial cells, stromal cells, skin fibroblasts, activated T cells, etc. Innate immune cells include phagocytes, NK cells, and B1 cells [
51,
57]. Phagocytes consist of monocytes that are precursors of macrophages and DCs and macrophages stimulated by special inducible factors. NK cells can directly kill cells without relying on antibodies and complement via the production of IFN-γ [
56].
TGF-β and acquired immune cells in MM
Naïve T cells: They originate and mature in the thymus before migrating to peripheral lymphoid organs, and are unresponsive until activated by antigens to differentiate into various subtypes such as Th cells, Tregs, and CTLs [
51]. TGF-β, however, inhibits their development into effector cells [
60], with research by Rana et al. indicating that naïve T cells downregulate TGF-β receptors to diminish its signaling [
51]. This cytokine skews differentiation, decreasing functional Th1 and Th2 cells while increasing Th17 cells, which secrete tumor-promoting cytokines IL-17 and IL-22 [
49]. Moreover, TGF-β from DCs prompts the development of Foxp3 + Tregs that can suppress antigen-specific T cell expansion and foster the generation of less responsive memory T cells by inhibiting crucial TCR-CD28 signaling required for effector T cell formation [
51,
54,
56].
Effector T cells: They derive from T cells after antigenic stimulation, encompass various types including Th cells, Tregs, and CTLs, and function by secreting lymphokines. During this process, a subset transitions into memory T cells—this is known as the induction stage. The rest then engage with target cells to induce apoptosis through granule exocytosis, with perforin creating pores in the target cell membranes to execute cytotoxicity [
54]. Effector T cells also emit immunomodulatory molecules like interleukins and interferons. Nonetheless, in MM, TGF-β hinders the growth, differentiation, and functional activity of CD4 + and CD8 + effector T cells [
51,
54,
56]. Daniele V. F. Tauriello et al. reported that increased TGF-β in the TME represents a primary mechanism of immune evasion that promotes T-cell exclusion and blocks acquisition of the Th1-effector phenotype [
50]. TGF-β blocks IL-2 production, thereby inhibiting IL-2-dependent effector T cell proliferation and maturation [
60]. It also suppresses Ca2 + influx-triggered TCR signaling, Tec kinase activation, NFAT translocation, and MAPK/ERK pathway activation, impeding effector T cell differentiation [
54,
56,
57,
63]. TGF-β impacts CD4 + effector T cells by inhibiting nuclear κ B family protein, which dampens IFN-γ expression [
51] and reduces key transcription factors like T-bet, GATA-3, and STAT-4 crucial for Th1 and Th2 cells [
56,
57]. For CD8 + effector T cells, TGF-β boosts CD39 and CD73 [
56], while IL-17 receptor signaling monitors TCR signaling duration, affecting CD8 + T cell differentiation [
51,
54]. CD8 + effector T cells employ cytotoxicity via granulocyte exocytosis (perforin, Granzyme A/B) and Fas-Fas ligand pathways, along with TNF-α and IFN-γ secretion for target cell elimination [
64]. However, TGF-β-activated SMADs and ATF1 block various genes crucial for CD8 + T cell cytotoxicity, including perforin, Granzyme A/B, FasL, and IFN-γ [
51,
54,
56,
57]. TGF-β also targets key regulators like T-bet, EOMES, and BLIMP-1, which promote cytotoxic molecule expression. However, TGF-β suppresses T-bet, EOMES, and BLIMP-1, hindering effector T cell functions. Tumor-derived TGF-β induces miR-23α to inhibit BLIMP-1 [
54]. Furthermore, TGF-β boosts Zeb1, supporting memory T cell survival, while suppressing Zeb2, favoring effector T cell differentiation. Low Zeb2 levels result in decreased Bcl-2 expression, an anti-apoptotic molecule in effector T cells [
54].
Memory T cells: In normal conditions, the body uses memory methods to combat antigens upon subsequent invasions, effectively destroying target cells. However, in the presence of tumors like MM, TGF-β not only hampers Th1 cell cytotoxic activity and biases T cell differentiation towards Th2 phenotype but also fosters hypo-responsive memory T cells by inhibiting TCR-CD28 signaling [
56]. Furthermore, TGF-β decreases levels of T-bet, EOMES, and BLIMP-1 [
54], and can convert effector T cells into tissue-resident memory T cells. TGF-β signaling promotes T cell memory retention in epithelial tissues by increasing αE (CD103) and β7 integrin subunits levels, independently of the TGF-β/SMAD pathway. Studies show TGF-β increases CD8 + CD103 + T cells, expressing immunosuppressive molecules (CTLA-4 and IL-10), aiding tumors in immune evasion [
49,
54]. It also prevents memory T cells from leaving secondary lymphoid organs by downregulating KLF2, causing the decrease of sphingosine-1-phosphate receptor 1 (S1P1) expression and ultimately establishing T-cell retention in tissues [
54].
Th1 cells: They combat intracellular bacteria and protozoa, primarily stimulated by IL-12, and include killer CD8 + T cells, IFN-γ-secreting CD4 + T cells, IgG-producing B cells, and macrophages. However, in the TME, there’s elevated TGF-β, reduced Th1 activity, and diminished immune cytotoxicity [
50]. Studies have confirmed that naïve T cells cultured with TGF-β couldn’t differentiate into the Th1 phenotype [
57]. TGF-β inhibits CD4 + Th1 cell proliferation by suppressing IL-2 expression via the TGF-β/SMAD signaling pathway, mediated by TOB1 [
49,
51,
57]. It also hampers Th1 differentiation by downregulating STAT4 and T-bet, crucial transcription factors, thus hindering IFN-γ production [
49,
57]. Moreover, in MM, TME is immunosuppressive, marked by fewer effective anti-tumor immune cells and more apoptotic cells due to TGF-β. Furthermore, TGF-β upregulates CDNK1A (p21Cip1) and CDNK1B (p27Kip1) but downregulates MYC, which are all downstream cell-cycle regulators, identifying the effect of TGF-β [
49,
57,
61].
Th2 cells: They are a subset of CD4 + T cells, and produce cytokines like IL-4/5/10/13, stimulating Th2 cell growth and suppressing Th1 cell development. Their primary impact lies in bolstering B cell proliferation for humoral immunity [
49]. Th1/Th2 cells maintain equilibrium but may shift, termed “Th1/Th2 drift”, potentially leading to diseases, including cancers [
65]. TGF-β influences Th2 cells by impeding activation through TCR signaling inhibition and downregulating crucial transcription factors like T-bet and GATA3, pivotal for Th2 cell differentiation [
49,
54,
56,
63]. Tauriello et al. found that SOX4, a TGF-β target, blocks Th2 cell transcription factors [
49]. TGF-β also hampers Th2 cell differentiation by inhibiting Tec kinase Itk, reducing Ca2 + influx [
63]. Chen et al. suggest TGF-β inhibits Th2 cell development but not activation [
63], and some tumors show heightened Th2 cell gene activity [
49]. Overall, TGF-β still weakens the role of Th2 cells in anti-tumor immunity [
49,
63].
Th9 cells: Th9 cells, a recently identified group of CD4 + Th cells marked by PU.1 [
54], contribute to various health conditions through IL-9 cytokine expression. The interaction of TGF-β and IL-4 signaling pathways promotes the expression of PU.1 and IL-9 production, resulting in both inflammatory and anti-tumor effects. Moreover, IL-4 suppresses TGF-β-Id3 expression through TAK1 activation, which is crucial for Th9 cell differentiation [
51]. The pre-clinical model found that activating glucocorticoid-induced TNFR-related protein (GITR) on T cells triggers anti-tumor effects via Th9 response. Besides MM resistance, Th9 cells and IL-9 induce myeloma cell apoptosis, support T cell survival, activate mast cells, and boost IFN-γ production by T and NK cells [
54]. Elevated Th9 levels correlate with better prognosis, suggesting potential therapeutic targeting of Th9 and IL-9.
Th17 cells: They are a recent CD4 + T cells subset, that produce IL-17 A and IL-22, crucial in autoimmune diseases and body defense [
49]. TGF-β, IL-6, IL-21, and IL-23 aid Th17 cell formation, while IFN-γ, IL-2, IL-4, and Socs3 inhibit it. TGF-β boosts Th17 differentiation [
51] and steers naïve CD4 + T and Th1 cells towards Th17 [
49]. Non-canonical TGF-β signaling, including AKT, MAPK, and NF-κB pathways, drive Th17 differentiation [
54]. TGF-β, with IL-6 or IL-21, heightens RORγt levels, governed by RORC, crucial for Th17 differentiation [
54]. TGF-β and IL-6-induced Th17 cells, with high aryl hydrocarbon receptor (AhR) levels, secrete IL-10, exhibiting immunoregulation [
54]. TGF-β can regulate Th17 cells by inducing adenosine production, suppressing immune responses, and modulating T-bet and CD39/CD73 expression [
54]. Inhibition of TGF-β reduces Th17 cell formation and promotes IFN-γ production by CD4 + T cells. Additionally, high TGF-β concentrations inhibit IL-23R expression, leading to Foxp3-mediated suppression of RORγc expression, favoring Tregs over Th17 cells [
54].
Regulatory T cells (Tregs): They are a subset of T cells, that regulate the body’s autoimmune response, categorized as natural (n-Tregs) and induced adaptative (i-Tregs) [
51,
54]. In MM’s TME, elevated TGF-β and IL-6 levels prompt CD4 + Tregs to inhibit immune surveillance and reduce effector T cells [
8,
65]. Tregs secrete IL-10, fostering MM development [
53], while IL-10 activates TGF-β [
19]. Higher IL-10 levels and increased Tregs correlate with shorter patient survival [
8,
53]. TGF-β and IL-2 induce Foxp3 expression in CD4 + T cells, suppressing effector T cell expansion [
51,
54,
56,
57,
60,
66], contingent upon stable Foxp3 expression for Tregs’ immunosuppressive function. In the thymic, non-coding sequences-2 (CNS2) region near Foxp3’s transcriptional starting site, significant demethylation ensures stable Foxp3 expression. However, induced Tregs in the periphery exhibit less demethylation and often lose Foxp3 expression. TGF-β-induced Tregs also lose Foxp3 stability, but certain factors like retinoic acid and CDK8/19 inhibitors can promote demethylation, enhancing Foxp3 expression [
51]. Foxp3 enhancer enriches SMAD3, which interacts with NFAT [
51,
57], and P300 acetyltransferases, linked to NF-κB and AP-1, positively regulate Treg generation [
51]. L-TGF-β, associated with GARP on the cell membrane, induces Th17-to-Treg trans-differentiation [
49,
57]. Additionally, TGF-β and prostaglandin E2 trigger this process [
57]. TGF-β protects Tregs and their precursors from apoptosis in the thymic environment, generating more immunosuppressive Tregs in the TME of MM [
51].
Cytotoxic T lymphocytes (CTLs): CTLs are specific T cells that secrete cytokines and directly attack tumor and virus-infected cells. They play a crucial role in antiviral and antitumor immunity alongside NK cells. Upon encountering target cells, CTLs release perforin, forming pores in the target cell membrane, and GzmB, which induces apoptosis [
64]. However, immunosuppressive elements like MDSCs and M2-TAMs in the TME of MM often hinder CTLs and NK cells. TGF-β is known to inhibit CTLs [
54,
65]. TGF-β inhibits TCR signaling and suppresses Myc and Jun genes, halting CTL proliferation [
49]. It also hampers CTL differentiation and activation by targeting master regulators like T-bet, EOMES, and BLIMP-1 [
49,
54]. Moreover, TGF-β blocks CTL migration into MM by silencing C-X-C chemokine receptor 3 (CXCR3) genes [
49]. It mainly promotes tumor progression by dampening CTL effector function and downregulating genes encoding effector molecules [
54,
57]. Furthermore, TGF-β suppresses EOMES, which is required to establish the gene program of effector CTLs, causing immune evasion [
57]. The canonical TGF-β/SMAD pathway, along with ATF1, suppresses genes crucial for CTL lytic functions, impairing their ability to kill myeloma cells [
49,
54,
57,
64].
Dendritic cells (DCs): DCs arise from the BM stem cells, differentiating into myeloid DCs (MDCs or DC1) and lymphoid DCs (LDCs or DC2). They serve as potent APCs, pivotal in immune responses. Immature DCs migrate efficiently, while mature DCs activate T cells, orchestrating immune reactions. However, MDSCs and certain cytokines from myeloma cells like IL-6, IL-10, and TGF-β [
19,
65]. TGF-β in the TME induces an immune-suppressive DC phenotype by upregulating ID1 via the TGF-β/SMAD pathway [
49,
57]. Tumor-associated DCs also express αVβ8 integrin, promoting CD4 + T cell transformation into immunosuppressive Treg cells [
49]. TGF-β affects chemokine receptor expression, hindering DCs’ entry into lymph nodes [
49] and suppressing MHCII gene expression, reducing DCs’ antigen-presenting ability and T cell activation [
49,
56,
57]. In MM models, TGF-β upregulates IDO in PC-like DCs (PDCs) and CCL22 chemokine in MDCs, leading to immune evasion [
57].
Cancer-associated fibroblasts (CAFs): CAFs are integral components of the TME in MM, with their abundance increasing in more advanced tumors [
16,
31,
57]. CAFs undergo morphological and functional changes, promoting MM growth and resistance to drug-induced apoptosis [
16,
31,
49,
67]. They contribute to MM development by remodeling the ECM, regulating metabolism and angiogenesis, and interacting with myeloma and immune cells through the secretion of various factors, such as GFs, cytokines, and chemokines [
67]. TGF-β is a key driver of the conversion of resident fibroblasts and BMMSCs in MM to CAFs through processes like mesenchymal transition (MT) [
16]. Notably, endothelial cells, epithelial cells, as well as certain stem and progenitor cells can also transform into CAFs via signaling pathways like endothelial MT (EndMT) or epithelial MT (EMT), mediated by TGF-β [
31]. CAF activity is characterized by heightened cytokine and GF production, notably VEGF, TGF-β, and HGF [
31,
57,
67], in conjunction with αsma, which expedites CAF differentiation [
31]. Interaction of TGF-β signaling with SNAIL and Zeb1/2 regulates EndMT [
31,
67]. Conversely, SMAD7 binding to SMAD2/3 obstructs the EMT pathway, countered by TGF-β inducing miR21 maturation to decrease SMAD7 expression [
31]. CAF-derived TGF-β impedes immune cell migration to tumor centers, fostering immune evasion, tumor progression, and metastasis [
57,
67]. It has been reported that tumors rich in TGF-β-activated CAFs often had a low response to checkpoint immunotherapies along with poor prognosis [
49,
57].
TGF-β and innate immune cells in MM
Macrophages: Macrophages are highly adaptable cells that change phenotype based on environmental cues. They exist primarily as M1 (classical activated or inflammatory) and M2 (alternating or anti-inflammatory) subtypes [
49]. In MM, the TME tends to steer macrophages towards the M2 subtype, known for its immunosuppressive role [
57]. There are various connections between TGF-β and macrophages in the TME in MM. M2-like macrophages not only secrete TGF-β through integrin αVβ8 and MMP14 [
56,
57] but also release TGF-β, inducing fibroblast activation indirectly [
31,
55]. It has been reported that IL-10 produced by MDSCs could activate M2-like macrophages [
19]. Besides, TGF-β can recruit M2-like macrophages, which can secrete immunosuppressive factors, such as IL-10 and TGF-β via the TGF-β/SMAD signaling pathway together with SNAIL [
49,
51,
57]. What’s more, TGF-β/SMAD6/7 in macrophages suppress anti-inflammatory reactions alongside NF-κB. SMAD6, acting as an inhibitory factor, recruits E3-ubiquitin ligases, causing MYD88 polyubiquitination and sequestering adapter protein spelling-1, thus fostering inflammatory responses. This inhibition is crucial for immune evasion, MM growth, and drug resistance [
19,
57]. Moreover, TGF-β boosts PD-L1 expression on M2-like macrophages, aiding immune evasion [
49]. It also upregulates CXCR4, facilitating monocyte migration to tumor sites by inducing blood vessel permeability [
57].
Natural killer cells (NK cells): NK cells play vital roles in immune responses, including combating tumors and viruses. However, in MM, the TME often impairs NK cell function, like NK cell exhaustion, desensitization, and exclusion through interactions with myeloma cells and molecules like TGF-β [
19,
49,
65,
68]. NK cells mainly rely on their receptors, such as NKG2D, NKp30, and DNAM-1 to recognize tumors, but TGF-β downregulates or even impairs expression of these receptors, decreasing MM surface ligands recognition, cytotoxicity, and immunosurveillance [
51,
56,
57,
69,
70]. Additionally, TGF-β can also decrease the expression of an adaptor of NKG2D, such as DAP12 by upregulating miR-183 [
57]. Expect for the effect on receptors, the TGF-β /SMAD signaling pathway controls the production of IFN-γ and GzmB from NK cells in response to antibody-dependent cell-mediated cytotoxicity(ADCC) together with T-bet, impeding Th1 response, and which could be counteracted by inflammatory signals [
49,
51,
56,
57,
69]. The TGF-β pathway impacts murine NK cell function and metabolism by inhibiting mTOR, independent of the canonical pathway [
57]. However, Vanessa Zaiatz-Bittencourt et al. reported that in human NK cells, the canonical TGF-β pathway prevails in NK cells [
71]. Finally, TGF-β can transform NK cells into type I innate lymphoid cells (ILCs), reducing immunosurveillance and upregulating CTLA-4 but downregulating IFN-γ [
51]. It can also convert type II ILCs to type III, which produces IL-17, an immunosuppressive cytokine [
49].
Neutrophils and myeloid-derived suppressor cells (MDSCs): Neutrophils, known as the natural candidates to perform in vivo medical tasks, are the most abundant and studied type of granulocyte [
49]. As the first line of defense for the host to resist invading pathogens, it not only has the inherent phagocytic ability, which can absorb nanoparticles and phagocytic dead red blood cells, and after activation, clear foreign pathogens and load targets, but also can migrate across blood vessels to adjacent tissues. However, various tumors including MM exhibit increased numbers of neutrophils expressing metalloproteinases associated with poor outcomes [
49,
57]. Also, Jackstadt, R. et al. reported that TGF-β2 enabled to recruit the of neutrophils. Similar to TAMs, neutrophils can also be divided into two phenotypes called N1 and N2. TGF-β often transforms neutrophils into the pro-tumorigenic N2 phenotype with decreased cytolytic activity and expression of pro-inflammatory cytokines, suggesting the failure of immune surveillance [
49]. Except for neutrophils derived from the myeloid system, MDSCs also play an important regulatory role in tumor immunity. They are pathologically activated neutrophils and monocytes with strong immunosuppressive activity and adverse clinical outcomes. In the TME of MM, there are high levels of MDSCs [
65]. They have significant effects on cancer immune invasion through increasing angiogenesis and formation of OCs, impairing DCs maturation, and suppressing the proliferation of CD4 + T cells [
19,
53,
65]. TGF-β controls MDSCs differentiation and immunoregulatory activity, promoting MM progression and metastasis [
49,
56]. The latest research describes some new genomic and metabolic features of MDSCs, which shape the specific functions of MDSCs and contribute to targeted therapies based on these cells, especially in cancers and autoimmune diseases.