The role of cancer-derived microRNAs in cancer immune escape

Regulatory T cell (Treg) is an immunosuppressive class of CD4⁺ T cells [28]. In the tumor microenvironment, hyperactive Tregs could inhibit the tumor-killing activity of effector cells by secreting cytokines such as interleukin-10 (IL-10) and TGF-β [29]. Besides, Tregs promote cancer immune evasion by consuming IL-2 and upregulating the expression of multiple immune checkpoints including PD-L1, CTLA-4, T cell immunoglobulin and mucin domain-containing protein 3 (TIM-3), V-domain Ig suppressor of T cell activation (VISTA), as well as T-cell immunoreceptor with Ig and ITIM domains (TIGIT) [30‐33]. It has been recognized that Tregs plays an indispensable role in ICI resistance at the present stage [34]. Similarly to Tregs, a subset of B cells are identified as immune inhibitory cells which are termed Bregs [35]. Bregs could inhibit inflammation response by increasing the generation of PD-L1 and cytokines such as IL-10 [36, 37].

Myeloid-derived suppressor cells (MDSCs) are a heterogeneous class of myeloid cell precursors which are halted at different stages of differentiation [38]. Abundant MDSCs in tumors induce cell cycle arrest of T cells via upregulating inducible nitric oxide synthase (iNOS) and arginase-1 (Arg1) [39]. Besides, MDSCs participate in oxidative stress and generate peroxynitrite which eventually blocks T cell activation [40]. Some MDSCs-derived materials such as IL-10, Arg1, and TGF-β could modulate the ratio of regulatory immune cells and effector cells [39]. Moreover, a specific phenotype of tumor-associated macrophages (TAMs), M2-type macrophages undermine anti-cancer immune response and promote immune evasion by anti-inflammation cytokines and immune checkpoint-associated pathways [41].

The cytotoxicity of cancer-specific effector cells is highly dependent on antigens expressed on cancer cells. Cancer cells tend to harbor alterations in antigen processing machinery (APM), which result in the loss of tumor-associated antigens (TAAs) and neoantigens [5]. Mutations in major histocompatibility complex class I (MHC-I), proteasome subunits latent membrane protein (LMP), as well as transporter associated with antigen processing (TAP) reduce the presentation of recognizable targets on cancer cells [42]. As a result, cancer cells exhibiting low immunogenicity are prone to survive from immune attack.

Tumor-derived cytokines especially TGF-β remarkably reshape the tumor immune microenvironment. This immunosuppressive cytokine repertoire inhibits the functions of multiple effector cells, induces the differentiation of regulatory cells, and impedes the infiltration of T cells [43, 44]. In addition, overexpressed immune checkpoints or their ligands such as PD-L1 on cancer cells promote the formation of exhausted TILs [45]. Moreover, some cancer cell-derived metabolites including indoleamine 2, 3-dioxygenase (IDO), arginase, and inhibitor of nuclear factor kappa-B kinase are greatly related to immune resistance in tumor as well [46‐48].

Some im-miRNAs disturb antigen processing and presentation by targeting one or multiple components of APM and MHC-I molecules in cancer cells (Table 1). Specifically speaking, in nasopharyngeal cancer cells, the results of microarray profiles indicated that miR-9 could target several APM components including TAP1, LMP8 (also termed PSMB8), LMP9 (PSMB9), LMP10 (PSMB10), and β2-microglobulin [53]. Besides, miR-9 significantly downregulates MHC-I molecules including human leukocyte antigen-B (HLA-B), HLA-C, HLA-F, and HLA-H [53]. The overexpression of miR-9 in multiple cancers such as cervical cancer, non-small cell lung cancer (NSCLC), and glioma might contribute to enhanced immune tolerance in the tumor microenvironment [57‐59]. Meanwhile, some endoplasmic reticulum stress-associated miRNAs such as miR-346 regulates immune response by directly targeting TAP1 or indirectly suppressing the expression of MHC-I molecules and interferon (IFN) signaling pathway [60].

Similarly, in esophageal adenocarcinoma cells, it has been verified that miR-125a-5p could bind to the 3′-UTR of TAP2 mRNA [54]. Also, it was detected that the 3′-UTR of HLA-A, HLA-B, and HLA-C mRNAs had the binding site for miR-148a-3p [54]. Meantime, the results of proteomic screening suggested that miR-27a promoted cancer progression by decreasing MHC-I expression on cell surface, inhibiting T cell infiltration and cytotoxic activities [56]. This miR-27a-induced MHC-I downregulation was dependent on calreticulin suppression [56].

As a non-classic MHC-I molecule, HLA-G was initially found to maintain fetal-maternal tolerance [61]. This immune inhibitory function of HLA-G could be hijacked by cancer cells to escape immune attack [62]. Overexpressed HLA-G on cancer cells binds to the inhibitory receptors on effector cells such as CTLs and NKs, leading to the suppression of the cytotoxic activities of these effector cells [62]. Actually, it has been found that HLA-G expression was aberrantly elevated in multiple cancers including melanoma, breast cancer, colorectal cancer, lung cancer, gastric cancer, hepatocellular carcinoma, and endometrial carcinoma [63, 64].

In cancer cells, the increased HLA-G expression is closely correlated with the loss of some HLA-G-targeting miRNAs. Specifically, several members of miR-148 family such as miR-148a, miR-148b, and miR-152 could downregulate HLA-G expression [65‐67]. In breast cancer cells, it was observed that oncogenic estrogenic G-protein-coupled estrogen receptor-1 (GPER) signaling pathway decreased downstream miR-148a level, further contributing to cancer immune evasion [68]. Additionally, miR-133a was verified as a vital mediator in maintaining peripheral immune tolerance by targeting HLA-G [69].

The oncogenically transformed cells are susceptible to expressing a series of stress-induced ligands including MICA, MICB, and UL16-binding protein (ULBP) [70‐72]. These ligands could be recognized by NKG2D on NKs and CTLs [70]. Intact NKG2D axis is an important signaling pathway to maintain cancer immune surveillance [73]. However, cancer cells could escape immune attack by downregulating NKG2D ligands in post-transcriptional level. So far, numerous miRNAs are portrayed as the modulators of NKG2D ligands (Table 2).

Previous studies demonstrated that overexpressed miR-20a in colorectal cancer cells, breast cancer cells, and ovarian cancer cells decreases MICA level and sensitivity to immune effector cells [74, 77‐79]. Besides, it was found that miR-519a-3p undermined the tumor-killing effect of NKs by decreasing MICA and ULBP2 on breast cancer cells [81]. In breast cancer patients, high miR-519a-3p expression could be deemed as a predictive biomarker for poor prognosis [81]. In addition, Abruzzese et al. noticed that bromodomain and extra-terminal (BET) inhibitor could remarkably increase MICA expression on multiple myeloma cells [82]. This BETi-induced MICA elevation was mediated by miR-125b, which targeted the transcription suppressor of MICA (IRF4) [82]. Moreover, Kishikawa et al. found that miR-93 and miR-106b targeted the 3′-UTR of MICA mRNA [80]. Genes coding miR-93 and miR-106b are both located in human chromosome 7q22.1 (termed miR25-93-106b cluster) [80]. Silencing miR25-93-106b cluster significantly increased MICA expression and decreased the susceptibility of hepatocellular carcinoma cells to NKs [80]. Similarly to the observations of Kishikawa et al., Wu et al. noticed that a panel of miRNAs downregulated MHC-I chain-related molecules A/B (MICA/B) in hepatocellular carcinoma cells including miR-373, miR-29b, miR-15b, miR-195, miR-16, miR-424, miR-29c, miR-106a, miR-107, miR-20a, miR-29a, as well as miR-17 [75]. Notably, 1, 25-(OH)2-D3 treatment could promote cancer immune surveillance by counteracting miR-302c/miR-520c-induced downregulation of MICA/B and ULBP2 downregulation [83].

On the contrary to MICA expressed on cancer cells (also known as membrane MICA), soluble MICA (sMICA) is an unfavorable factor for anti-cancer immunity [84]. In pancreatic cancer cells, hypoxia-associated pathways conspicuously downregulated membrane MICA while simultaneously increased sMICA expression [84]. This hypoxia-induced sMICA upregulation was attributed to the dysregulated balance between circ_0000977 and miR-153 [84].

As a hallmark of cancer, upregulated immune checkpoint signal is determined by multiple factors, including previously existing inflammation and some oncogenic signaling pathways [85]. Increased immune checkpoint ligands especially PD-L1 is closely related with cancer-associated miRNA expression pattern. To be more specific, previous studies have indicated that the loss of miR-3609, miR-195-5p, miR-148a-3p, miR-873, miRNA-497-5p, miR-191-5p, miR-34a, and miR-138 closely correlated with the increased PD-L1 expression on numerous cancer cells [86‐94]. In addition, Dong et al. found that decreased miR-140, miR-142, miR-340, and miR-383 enormously elevated PD-L1 expression on cervical cancer cells [95]. The results of a respective study demonstrated that in malignant pleural mesothelioma samples, the abundance of PD-L1 was negatively correlated with the levels of multiple cancer suppressive miRNAs including miR-15b, miR-16, miR-193a-3p, miR-195, and miR-200c [96]. Further investigation in cancer cell lines identified that miR-15b, miR-16, and miR-193a-3p could target PD-L1 mRNA [96]. Notably, some other long noncoding RNAs involved in the reduction of these PD-L1-targeting miRNAs [87, 94].

Contrarily, miR-18a promotes PD-L1 expression by targeting PTEN, WNK2, and SOX6 [95]. Then, PI3K-AKT, MEK-ERK, and Wnt/β-catenin pathways are activated and the transcription activity of PD-L1 is upregulated [95]. Similarly, Tang et al. observed miR-3127-5p induced PD-L1 expression by promoting STAT3 phosphorylation in NSCLC cells [97]. The miRNAs associated with PD-L1 expression on cancer cells were summarized in Table 3.

Fas-FasL pathway has a great impact on immune tolerance in the tumor microenvironment [113]. Increased FasL on cancer cells counterattacks immune cells, induces immune cell death, and eventually promotes cancer immune privilege [114]. In T cell large granular lymphocyte leukemia, it was found that STAT3-induced miR-146b loss led to increased FasL expression and potential neutropenia [115]. Besides, the miR-768-3p mimic treatment in NSCLC cell immensely increased FasL level but decreased Fas expression on cancer cells [116]. Moreover, Wu et al. verified that miR-21 could target FasL in breast cancer cells. In the co-culture experiment, ectopically expressed miR-21 in MCF7 cells could remarkably reduce the apoptosis rate of Jurkat T cells [117]. Actually, FasL has been accepted as the post-transcriptional regulatory target of miR-21 in numerous cancer cell types including esophageal carcinoma and pancreatic cancer [118‐120]. Notably, in spite of inhibiting immune evasion, highly expressed miR-21 was connected with cancer development, treatment resistance, and poor prognosis [118, 119].

Some cell metabolites such as tryptophan are essential to maintain the functions of TILs. IDO1 is a rate-limiting enzyme for tryptophan metabolism, which could convert tryptophan to kynurenine and 3-hydroxyanthranilic acid [121]. Increased IDO1 expression and decreased tryptophan lead to dysfunctional effector T cells and cancer immune evasion [122]. It was reported that downregulated miR-218 protected cervical cancer cells from immune attack via elevating IDO1 level [123]. Besides, Lou et al. found that miR-448 acted as a tumor suppressive factor by targeting downstream IDO1 in colon cancer cells. The results of in vitro study showed that the ectopic expression of miR-448 was beneficial to giving full play to the functions of TILs [124]. Moreover, Huang et al. reported that miR-153 level was a core factor determining the efficacy of chimeric antigen receptor (CAR) T cells treatment in colon cancer models. In colon cancer, miR-153 directly targeted IDO1, enhancing cytotoxic activity of CAR T cells and inhibiting tumor growth [46].

Cancer-derived miRNAs not only modulate the expression profile within cancer cells but also exhibit extracellular bioactivities by exosomes or microvesicles (Table 4) [24]. Cancer-derived miRNAs could be packed into exosomes or microvesicles, which are transferred to numerous TILs and shape an immunosuppressive microenvironment (Fig. 3) [141].