Effectively immunoreactive T cells in tumor microenvironment(TME) are required to successfully eliminate tumor cells. Defects in T cell activity and tissue distribution have been reported in MM patients (37–39). Immunoreactive T cells normally encompass helper T cells and effector T cells. Among the effector T cells, the percentage and absolute number of CD4+ cells were significantly decreased in MM patients. In contrast, the relative number of CD8+ cells increased only slightly (40). Low CD4+ T-cell counts and CD4/CD8 ratios at diagnosis were independent predictors of poor prognosis in MM patients (41). The effector T cell therapies, including CAR-T, bispecific T cell Engager (BiTE), and antibody-drug conjugate(ADC), have been applied in clinical practice. CAR-T therapies targeting BCMA, GP5CRD, Signaling lymphocytic activation molecule family member 7(SLAMF7), CD38 etc. along with BITE and ADC technologies have demonstrated remarkable efficacy in clinical settings, leading to breakthrough therapeutic outcomes (42). T helper cells(Th) mainly comprise Th1, Th2, Th17, regulatory cell(Treg), and follicular helper T (Tfh) cells. Furthermore, elevated peripheral blood Th1/Th2 ratios have also been reported in patients with primary and refractory MM (43). In addition, the equilibrium between Treg and Th17 cells is pivotal for effective anti-tumor immune response. An increased Treg/Th17 ratio results in a bias toward immunosuppression. The experts showed that serum levels of interleukin(IL)-17, IL-22 as well as the frequency of Th17 cells produced by bone marrow and peripheral blood mononuclear cells were significantly increased in myeloma patients compared with healthy donors, suggesting that Th17-associated pro-inflammatory cytokines may be present in MM BM microenvironment and may regulate MM cell growth as well as immune response. Then, they further observed that IL-17 promotes myeloma cell growth, colonization, and adhesion to bone marrow stromal cells (BMSC) through blinding to IL-17 receptor, as well as increased the in vivo growth in murine xenotransplantation model of human MM (44).Likewise, the highly significant statistical relationship between the IL-17 in bone marrow and osteoclast formation also strongly supports the promotion of IL-17 to osteolytic disease (45). Next, it is confirmed that IL-22 promotes signal transducer and activator of transcription 3(STAT-3) phosphorylation, cell growth in IL-22RA1+(IL-22 receptor) MM cells. Moreover, the treatment of IL-22RA1+MM cells with IL-22 increases both cell growth through p38 mitogen activated protein kinase (MAPK)activation and resistance to drug-induced cell death through up-regulation of myeloid leukemia 1 (Mcl-1) expression and inhibition of drug-induced caspase-3 activation (46). The population of CD4+ CD25+ Treg cells is elevated in the peripheral blood of MM patients, and the upregulation of transcription factor 3 (Foxp3) in CD4+ CD25+ Treg cells is correlated with their immunosuppressive activity (47). In addition, increased Treg cell count in MM patients is pertaining to abbreviated disease progression time and diminished survival (48). Treg cells derived from MM patients exhibit potent suppressor activity against antigen-presenting cells (APCs) and T cell proliferation. Detailedly, it can be classified into three basic “modes of action”: inhibition by secretion of suppressive cytokines, inhibition by cytolysis, and inhibition by regulation of dendritic cell (DC) maturation or function (49). Compared to healthy individuals, patients with MM exhibit significantly elevated levels of transforming growth factor-beta (TGF-β) and interleukin-10 (IL-10) secreted by regulatory T cells (Tregs), indicating an enhanced suppressive activity. Notably, Treg cells also secrete granzyme B, resulting in the apoptosis of cytotoxic T lymphocytes (CTLs), B cells, and natural killer cells (NK cells) (50–52) Regarding the immunomodulatory effects mediated by Treg cells on DC maturation and function, the binding of lymphocyte activation gene-3(LAG3) to immature DC-expressing MHC class II molecules induces immunoreceptor tyrosine-activated motifs (ITAM)-mediated inhibitory signaling pathway, thereby inhibiting DC maturation and immune stimulatory capacity (53–56). In addition, CD86 and human leukocyte antigen G (HLA-G) are common antigens acquired by T cells from malignant plasma cells (trogocytosis) and HLA-G+ T cells, which have regulatory functions similar to those of natural Treg cells, providing another novel mechanism for evading effective immune surveillance utilized by MM cells (57). Tregs-related therapy has also made a great breakthrough in MM treatment. For example, in vivo removal of Treg in mice with established multiple myeloma induced effective CD8 T-cell and NK cell-mediated immune responses, leading to complete and stable remission of MM. This preclinical in vivo study suggests that Treg is an attractive target for treating MM (58). In addition, since SLAMF7 is highly expressed on immunosuppressive CD8+CD28-CD57+ Treg cells in patients with MM, anti-SLAMF 7 treatment efficiently cleared suppressive T cells from the peripheral blood of MM patients. It is suggested that anti-SLAMF7 antibodies can enhance the anti-tumor immune responses in MM patients (59). Changes in the Treg compartment identified during MM progression are essential for improving our understanding of clinical stability of MGUS and MM pathology, and further for improving the clinical diagnosis, prognosis, and therapeutic implications of MM with potential implications (60).

B regulatory(Breg) cell is an immunosuppressive cell that supports immune tolerance through the production of interleukin 10 (IL-10), IL-35, and transforming growth factor β (TGF-β). Recent studies have shown that different inflammatory environments formed during the MM progression induce different Breg cell phenotypes (61). For example, in the bone marrow microenvironment of MM patients, CD19+CD24hiCD38hi Bregs expression is increased (62). This type of CD19+CD24hiCD38hi B cells can inhibit CD4+CD25 - T cell proliferation and suppress th1 cell differentiation, thereby inhibiting the release of the pro-inflammatory cytokines, such as IFN-γ and TNF- α by T cells. This inhibition is partially mediated by the production of interleukin-10 (IL-10) (63). The addition of CD80 and CD86 monoclonal antibodies can reverse such an inhibition (64). Furthermore, the bone marrow-derived CD19+CD24highCD38high Breg population abrogates NK cell-mediated antibody-dependent cell mediated cytotoxicity (ADCC) against MM by producing IL-10 (65). Next, Bregs also convert CD4+ T cells into activated Tregs by secreting transforming growth factor-beta (TGF-β), which in turn inhibits T cell proliferation and promotes tumor metastasis (66, 67). Therapeutically, the effective treatment with daratumumab is accompanied by the eradication of Bregs in addition to its anti-myeloma effects (62).

NK cell is a type of lymphocyte characteristic with CD3-CD56+, exerting antitumor effects not only through direct killing and triggering ADCC, but also indirectly through induction of cytokines release, such as granzyme, IFN-γ, and TNF-α (68–70). NK cells express a range of receptors, especially on the cell surface, including receptors for inhibition, activation, adhesion, and cytokines. When in contact with interacting cells via surface-expressed receptors, various signals may be transduced simultaneously, in turn determining whether NK cells will be activated (69, 71–73). Interestingly, the expression of activated receptors such as natural killer group 2D receptor(NKG2D), natural killer p30 (NKp30) and natural killer p46(NKp46) on bone marrow natural killer cells was found to be suppressed in patients with MM compared to healthy volunteers (74, 75). Notably, tumor cells evade immune surveillance by down-regulating NK cells’ sensitivity and upregulating human leukocyte antigen C(HLA-C) and human leukocyte antigen E (HLA-E) expression (69, 72, 76, 77). Proteasome inhibitors (Bortezomib and Carfilzomib) can induce NK cell-mediated killing of MM cells by down-regulating MHC-I expression on the surface of MM cells (78, 79). Also, NK cells from MM patients express programmed cell death protein 1(PD-1), whereas normal NK cells do not. The combination of PD-1 and programmed death-ligand 1(PD- L1) down-regulates the effect of NK cells mediated elimination of MM. It was proved that the novel anti-PD-1 antibody CT-011 enhances the killing of autologous primary MM cells by human NK cells. Notably, a phase 2 clinical trial of CT-011 in combination with lenalidomide for the treatment of MM patients warrants consideration (80). In MM, but not MGUS, myeloma cells shed soluble MHC class I-related chain A (sMICA), resulting in downregulation of NKG2D expression, impaired lymphocytotoxicity, and resistance to NKG2D-mediated killing (81–83). These findings revealed that changes in the NKG2D path were associated with progression from MGUS to MM, and increased the likelihood of application of anti-MICA monoclonal antibodies to treat these diseases (83). Experts also found that MM, by promoting the upregulation of CXC-chemokine receptor 3(CXCR3) ligands and downregulating of CXC-chemokine receptor 12 ligands(CXCL12), driven NK cells outside of the bone marrow, thereby impairing the antitumor immune responses at the primary site (84). In terms of therapies, developing CAR-NK cells is an exciting approach for MM treatment. Due to the isolation of NK cells and the absence of MHC restriction for tumor cell eradication, various approaches were also developed for the clinical application of NK cell-based therapies, such as bi-specific antibodies specific to NK cells (85). Moreover, remarkable advancements have been achieved in the management of MM by means of ex vivo expansion and activation of NK cells, followed by autologous and haploidentical infusion. (NCT03019640).

MDSCs are mainly divided into two subtypes: granulocytic myeloid-derived suppressive cells(G-MDSCs): CD11b+CD14−CD15+/CD66b+; monocytic myeloid-derived suppressive cells(M-MDSCs): CD14+CD15−HLA- DRlo/– (86). MDSCs are a heterogeneous group of immature hematopoietic cells that play a role in MM cell survival, tumor progression, and immunosuppression (87). In return, MM can also promote the accumulation and activation of MDSCs, thus forming a vicious cycle (88). The regulatory mechanisms managing MDSCs’ acceleration of myeloma progression are primarily composed of direct and indirect methods. In relation to the direct mechanism, Lisha Ai et al. demonstrated that G-MDSCs induced piRNA-823 expression, which in turn facilitated DNA methylation and incorporated stem cell characteristics into MM cells (89). As observed by experts, the activation of adenosine monophosphateactivated kinase (AMPK) partially mediates the tumor-promoting effect of MDSC in mice (90). Additionally, the production of S100A9, a calcium-binding protein that stimulates the secretion of TNF-α, IL-6, and IL-10 through toll-like receptor 4(TLR4)-mediated autocrine signaling, by MDSCs in the tumor microenvironment (TME) attracts myeloma cells (91) and fosters their growth by activating the canonical nuclear factor-kappa B(NF-κB) pathway (92). MDSCs indirectly affect MM cells’ survival through the modulation of nutrient availability, secretion of soluble factors, and the expression of cell surface inhibitory molecules. MDSCs enhance the expression of enzymes that contribute to the depletion of essential amino acids, such as L-arginine, required for effective TCR-mediated signaling (93, 94). Kavita Ramji et al. investigated that arginase 1(ARG 1) expression in myeloid cells strongly suppressed antigen-induced adoptive transfer of T cell proliferation in a murine MM Vκ*MYC model, accompanied by a systemic decrease in L-arginine levels. Furthermore, they found T cell proliferation was restored and Vκ*MYC tumor progression was significantly delayed when they knocked out ARG1 in MM-bearing mice or applied ARG inhibitors (95). In addition,multiple soluble factors,and cytokines, such as IL-10, IL-6, TGF-β, CD40-CD40 ligand, and IFN-γ, contribute to the immune-suppressive activity of MDSC in the TME, tipping the scales in favor of Tregs (96, 97). Purposefully, the expression of CD38 on MDSCs play a crucial role in the discontinuous multi-cellular pathway of adenosine (Ado) (98), a highly represented immune-suppressive nucleoside in the TME of MM patients (99). The expression of immunecheckpoint (ICP)/ICP-L plays a role in the impairment of anti-myeloma immune responses. As previously demonstrated, PD-L1 is expressed by BM MDSC in all disease states and potentially contributes to the suppression of anti-myeloma activity by PD1+ effector cells (100, 101). To break free from this detrimental cycle, studies have shown that macrophage migration inhibitory factor(MIF) present in the MM microenvironment induces bone marrow cells to express CD84, resulting in upregulation of PD-L1 expression on the surface of MDSCs (102). The presented data demonstrate that galectin-1, derived from MM, could mediate the tumor-promoting effect of M-MDSCs through its interaction with CD304 on M-MDSCs and contribute to tumor progression post-ASCT (103). These newly discovered molecules are expected to be used to as specific markers to target MDSCs for the treatment of MM.

Dendritic cells(DCs) have an antigen-presenting role and bridge innate and adaptive immunity. It is divided into three main subtypes plasma cell-like DC precursor cells (pDCs), myeloid DCs (mDCs), and monocytic DCs (MoDCs) (104). The absolute numbers of circulating MoDCs, mDCs, and pDCs were significantly lower in MM patients than those in healthy subjects, both at diagnosis and during the malignant process (105). In addition, during MGUS-MM progression, the aggregation of mDCs and pDCs in the bone marrow was correlated with the tumor load (106). Myeloma peripheral blood DCs(PBDCs) mature with reduced expression of leukocyte antigen (HLA-DR), CD40, and CD80 antigens. Reduced expression of MHC class II molecules and costimulatory molecules is responsible for the defective induction of T cell proliferation. In contrast, Mo-DCs and PBDCs from healthy controls are potent T-cell immunostimulatory factors. Both in vitro and in vivo models of human MM have shown that pDC promotes MM cell growth, survival, chemotaxis, and drug resistance (bortezomib, lenalidomide). Their findings identify an important role of pDCs in MM pathogenesis and provide a basis for targeting pDC-MM interactions with cytosine-phosphate-guanosine (CpG) oligonucleotides (ODNs), as a therapeutic strategy to improve MM patient prognosis (107). The effect of DCs-mediated induction of Treg depends on the nature of the DCs maturation stimulus with inflammatory cytokine-treated DCs (Cyt-DCs) being the most effective Treg inducer. DC-induced Treg from both healthy donors and MM patients functionally and effectively suppresses T-cell responses (108). DC vaccination has developed from generation to generation over the years. To maximize the therapeutic potential of DC-based therapies, new approaches for the treatment of MM that combine dendritic cell-based immunotherapy with conventional therapies, including chemotherapy, immunomodulatory drugs (IMiDs; Lenalidomide and pomalidomide), and immune checkpoint inhibitors (ICIs; PD-L1) have been proposed (103). In addition, many cytokines were shown to modulate DCs functions. For example, IL-6 inhibits CD34+ DC progenitor cell colony growth, thus converting CD34+ cells from DCs to CD14+ CD1a+ monocytes that exert powerful phagocytosis capacity, but have low antigen presentation capacity. In neutralization experiments, this effect is reversed by anti-IL-6 antibodies (105). In addition, TGF-β1 and IL-10 secreted by myeloma have a role in inhibiting CD80/CD86 upregulation during DC maturation (105, 109). Anti-TGF-β1 antibody, anti-IL-10 antibody or IL-12, IFN-γ infusion can neutralize such an inhibitory effect due to CD80/CD86 upregulation (110). In a recent randomized phase II trial (NCT02728102), experts evaluated the impact of the DC/MM fusion vaccine in combination with maintenance therapy. Although no significant increase in disease response was observed, a notable association with a substantial rise in circulating multiple myeloma-reactive lymphocytes indicative of tumor-specific immunity was identified. These results also lay the foundation for the future development of immunotherapeutic modalities to restore the immune function of DCs, thereby inhibiting tumor growth in MM patients (111).

CD14+ monocytes in the MM microenvironment present defective antigen presentation ability due to intracellular HLA-DR accumulation, thus leading to the accumulation of MM and the suppression of T-cell activation (112). Tumor-associated macrophages (TAMs) are derived from circulating monocytes, and their high plasticity and heterogeneity depend on the microenvironment (113, 114). TAMs can be classified into anti-tumor type M1 and tumor-promoting type M2. Recently, CD169+ tissue-resident macrophages were reported to promote early dissemination of MM by secreting IL-6 and TNF-α (115, 116). M2-type components can promote MM proliferation, survival, angiogenesis, as well as immunosuppression, and drug tolerance (117–122). Mechanically, TAMs interact with bone marrow stromal cells(BMSCs) to acquire the function of secreting IL-6 and IL-10, poorly secreting IL-12 and TNF-α, which are suitable for tumor growth (123). In addition, TAMs are not only a rich source of pro-angiogenic cytokines and growth factors’ production such as vascular endothelial growth factor(VEGF), TNF-a, IL-8, and fibroblast growth factor 2(FGF-2), but also assume a vascular endothelial cell-like phenotype after structural activation by VEGF and an essential fibroblast growth factor (bFGF) through “ angiogenic mimicry” contribution to the myeloma vascular network (124). Unlike healthy tissue, macrophages from MM patients cannot phagocytose tumor cells, deliver antigens, and stimulate adaptive immune responses (125, 126). Notably, in MM the expression of IL-10 by TMAs inhibits the activation of cytotoxic T cells and downregulates the expression of crucial cytotoxic T cell factors, including granzyme B and IFN-γ (125–128). Anti-CD47 antibodies have now been found to significantly enhance the ability of macrophages to phagocytose human MM cell lines in vitro (129). Finally, TAMs protect myeloma cell lines and primary myeloma cells from spontaneous and chemotherapeutic drug-induced apoptosis by attenuating caspase-dependent activation and cleavage of apoptotic signals as well as P-selectin glycoprotein ligand 1(PSGL-1)/selectins and intercellular adhesion molecule 1(ICAM-1)/CD18, a pattern of cell-cell contact (117, 130). Macrophage treatments are in infancy, aiming for removing and reprogramming of these invalid TAMs. Wang et al. showed that MM tumor load was significantly reduced in vivo after removing macrophages in optimally selected transgenic mice (126). Similarly, the IKAROS family zinc finger 1 (IKZF1)- interferon regulatory factor 4(IRF4)/interferon regulatory factor 5(IRF5) axis and the Regulator of G Protein Signaling 12 (RGS12) protein induce myeloma-associated macrophage phenotype switch and functionally skew towards the M1 phenotype, thereby resulting in inhibition of MM growth (131, 132). These results suggest that targeting IKZF1 and RGS12 may provide a novel strategy for reprogramming myeloma-associated macrophages for better MM treatment (133, 134). Moreover, cellular inhibitors of apoptosis proteins (cIAP) 1 and cIAP 2 are amplified in approximately 3% of cancers, rendering them potential therapeutic targets across various malignancies. Small molecule IAP antagonists, such as LCL161, have entered clinical trials due to their capacity to induce TNF-mediated cancer cell apoptosis. Remarkably, we observed robust in vivo anti-myeloma activity of LCL161 in a transgenic mouse model of bone marrow cancer and RRMM patients. This effect was not solely attributed to direct induction of tumor cell death but rather stemmed from the upregulation of interferon(IFN) signaling by the tumor cells themselves, leading to potent inflammatory responses that activated macrophages and resulted in phagocytosis of the tumor cells. Treatment with LCL161 conferred long-term protection against tumors in the mouse model and induced regression of tumors in select mice. These findings underscore the profound preclinical anti-myeloma effects exerted by LCL161 (135).

Tumor microenvironment(TME) is a sophisticated network containing hematopoietic or mesenchymal origin cells, extracellular matrix(ECM), and chemokines, growth factors as well as other soluble molecules, which are produced or influenced by the intercellular compartments (136). TME plays an important role in MM development and progression, triggering bone destruction, angiogenesis, immunosuppression, and drug resistance through cell-cell interactions and soluble molecules (137). ( Figure 4 ) For example, osteoprotegerin (OPG) and its ligand (OPGL) are main factors in bone resorption. OPGL stimulates osteoclast differentiation and activation, whereas OPG inhibits these processes. The experts found that myeloma cells react with bone marrow stromal cells in a cell-to-cell interaction, resulting in overexpression of OPGL and downregulation of OPG expression in myeloma patients, which is in turn involved in the pathogenesis of MM-induced bone disease (138). In addition, the chemokine macrophage inflammatory protein-1α (MIP-1α) also promotes osteolytic lesions’ development (139). The process of angiogenesis is primarily triggered by factors released by bone marrow stromal cells and myeloma cells. Among them, vascular endothelial growth factor(VEGF) is the main molecule that promotes angiogenesis (140, 141). VEGF-a is an important isoform of VEGF because its receptor VRGFR2 is overexpressed on endothelial and plasma cells of patients with MM (142, 143). They interact with each other to induce not only VEGF but also other pro-angiogenic factors such as basic fibroblast growth factor(FGF-β), TGF-β (144). Among the soluble molecules, IL-6 is the most important cytokine involved in the pathogenesis and disease progression of MM. It was reported that IL-6 promoted myeloma proliferation, prevented cells from apoptosis, enhanced bone destruction, most importantly, triggered immunosuppression (45, 145, 146). The immunosuppressive effect is mainly reflected in that IL-6 can reduce the ability of DC cells to present specific antigens to autologous T cells, thus inhibiting T activation (105). Recent evidence suggested that several other molecules, including HGF, membrane-bound mucin glycoprotein (Muc-1), indoleamine 2,3-dioxygenase (IDO), as well as soluble primary histocompatibility complex class I -related chain A (sMICA), can also be found in the tumor microenvironment. High levels of hepatocyte growth factor (HGF) in the serum and bone marrow of MM patients correlate with a poor prognosis and HGF also indirectly inhibits antigen-induced T-cell activation (147, 148). Meanwhile, the membrane-bound mucin glycoprotein (Muc-1), indoleamine 2,3-dioxygenase (IDO) and soluble primary histocompatibility complex class I -related chain A (sMICA) suppress innate and adaptive immunity and play an essential role in tumor prognosis (149–151). Drug-resistance induced by the microenvironment can be seen as the sum of resistance caused by direct cell adhesion(CAM-DR) and resistance caused by soluble molecules(SM-DR) (152). As for SM-DR, both TNF superfamily members BAFF (B cell-activating factor of the TNF family) and APRIL (a proliferation-inducing ligand) stimulate the survival of tumor cells, thus conferring their role in drug resistance (153). Besides, it is confirmed that high levels of IGF-1 cytokine are associated with bortezomib resistance (154). Compared with SM-DR, CAM-DR is more important (155). MM connects to the extracellular matrix through activated VLA-4(known as integrin α4β1), while VLA-4 binds to fibronectin to stimulate NF-κB signaling pathway activation, promoting pro-survival signaling and CAM-DR (156–158). Emerging reports have shown that solid interactions between clonal plasma cells and the surrounding bone marrow microenvironment contribute to the pathophysiology of MM. Based on these resistance strategies identified in MM, one can have the rationale for the preclinical evaluation of new therapeutic approaches to target MM clones and the BM environments for better suppression and prevention of MM disease progression (159).