MM is a clonal B cell malignancy, accounting for approximately 1% of all cancers and 10% of hematologic malignancies (127). It is characterized by abnormal accumulation of ≥10% malignant clonal plasma cells in the bone marrow (128–132). MM is commonly diagnosed in elderly individuals, with a median age at diagnosis around 66 years (133). There is a clear sex disparity in MM incidence across all ages and racial/ethnic groups, with males showing higher incidence and mortality rates compared to females. However, the underlying mechanisms and the impact of sex on patient outcomes remain poorly understood and underexplored (134, 135). Data from a recent study suggest that individuals with MM have a median survival rate of about 7.5 years (136). Most individuals with MM exhibit the production of a monoclonal immunoglobulin protein, also termed M-protein, generated by aberrant clonal plasma cells. Nonetheless, in 15–20% of cases, MM cells exclusively release monoclonal free light chains, and in fewer than 3% of patients, these cells do not secrete any monoclonal protein (133, 137).

The malignant clonal plasma cells in MM cause typical end-organ damage, such as hypercalcemia, renal failure, anemia, and lytic bone lesions referred to as “CRAB” symptoms (138–140). The presence of one or more of the CRAB symptoms sets MM apart from other plasma cell disorders, such as smoldering multiple myeloma (SMM) and monoclonal gammopathy of undetermined significance (MGUS) (138). SMM is identified by 10-60% bone marrow plasma cell proliferation and/or elevated M-protein ≥3 g/dL, without CRAB symptoms (141), while MGUS is characterized by a clonal bone marrow plasma cells <10% and a serum M-protein level <3 g/dL in asymptomatic patients (142). Both SMM and MGUS are recognized as precursors to MM, with an annual progression risk of 10% and 1%, respectively (143).

The surface antigens CD138, CD38, CD45, CD19, CD56, CD117, CD20, CD28, CD27, and CD81 are the most widely used markers for characterizing normal and malignant plasma cells (144). BM-derived myeloma cells are typically defined by their high expression of CD38, CD138, and CD56, low expression of CD45, and absence of CD19 (145). In addition to its crucial role in establishing MM diagnosis, numerous studies have reported the prognostic and therapeutic significance of plasma cell immunophenotypic features in MM patients (146). For instance, the upregulation of CD19 and its regulatory partner CD81 are linked to poorly differentiated plasma cell clones associated with poor outcomes in MM patients (147). Similarly, the expression of CD45 on clonal plasma cells is associated with an aggressive phenotype of MM (148). B cell maturation antigen (BCMA/CD269) is another surface marker that is primarily expressed by plasma cells and mature B lymphocytes (149, 150), and is upregulated on malignant plasma cells in MM (151, 152). BCMA is a target antigen for the FDA approved chimeric antigen receptor-T (CAR-T) cell therapies for MM (153).

In breast cancer, Gal-9 overexpression enhances cancer cell adhesion and invasion through Focal Adhesion Kinase activation, which upregulates the pro-invasive protein S100A4, and facilitates immune evasion via the Tim-3-Gal-9 pathway by transferring Gal-9 to the cell surface (85, 154–156). In contrast, recombinant protease-resistant Gal-9 (rGal-9) exerts anti-proliferative effects in gastric cancer by inducing apoptosis, reducing the phosphorylation of VEGFR-3 and IGF-1R, and altering miRNA expression (100). In colorectal cancer, Gal-9 promotes extensive intra-tumoral NK cell infiltration through Rho/ROCK1 signaling, correlating with improved prognosis (93, 157, 158). Similarly, in mice with lung cancer, Gal-9 induces macrophage differentiation into plasmacytoid dendritic cell-like macrophages, potentially enhancing NK cell activation, and extending longevity (118). Gal-9 also inhibits tumor growth in hepatocellular carcinoma by inducing apoptosis via MicroRNAs mediated miR-1246-DYRK1A-caspase-9 axis (121), and suppresses esophageal squamous cell carcinoma proliferation both in vitro and in vivo through Jun NH2-terminal kinase (JNK) and p38 MAPK pathways (113). However, Gal-9 may support tumor immunosuppression in glioblastoma by promoting M2 tumor-associated macrophage activity (88). In contrast, in pancreatic ductal adenocarcinoma, Gal-9’s binding to Dectin-1-expressing macrophages fosters immunosuppression and tumor progression, though exogenous rGal-9 can induce apoptosis by promoting cytochrome release and altering miRNAs expression in this context (66, 159). In cervical cancer, the Tim-3-Gal-9 pathway facilitates immune escape by promoting regulatory T cell activity, with epigenetic regulation through the H3K9me3-specific histone methyltransferase (SUV39H1) - DNA methyltransferase 3 alpha (DNMT3A) axis (86, 109). While serum Gal-9 levels are elevated in patients with advanced cutaneous T-cell lymphoma, the exogenous administration of rGal-9 induces apoptosis by Tim-3 independent activation of caspase-3 and caspase-9, and suppresses in vivo tumor growth (89). Finally, in AML, the Tim-3-Gal-9 autocrine loop promotes leukemic stem cell self-renewal and disease progression through the NF-κB, β-catenin, and PKC/mTOR pathways (91, 111). These findings underscore the complex and context-dependent roles of Gal-9 in cancer biology, highlighting its potential therapeutic importance in MM.

The BM consists of a heterogeneous population of cells, including hematopoietic stem cells and their myeloid and lymphoid progeny, along with endothelial cells and cells derived from mesenchymal stromal cells, such as adipocytes, chondrocytes and osteoblasts, all of which are embedded in the extracellular matrix (ECM) (160). In MM, the crosstalk between MM cells and the surrounding BM microenvironment activates intracellular signaling pathways that impact tumor behavior, including tumor growth, progression, angiogenesis, immune evasion, and drug resistance (161, 162). This crosstalk is facilitated by direct cell-cell contact through surface adhesion molecules, secreted soluble cytokines and growth factors, and exosome-mediated intercellular communication (163).

Different molecular complexes on MM cells interact with ECM proteins, such as syndecan-1 (CD138) with collagen type 1 (164) and very late antigen-4 (VLA-4) with fibronectin (165), or with their binding partners on various non-tumor cells, such as VLA-4 with vascular cell adhesion molecule-1 (VCAM-1) and lymphocyte function-associated antigen-1 (LFA-1) with intercellular adhesion molecule-1 (ICAM-1) (166). These interactions activate various downstream signaling pathways associated with tumor progression and poor outcomes in MM patients such as NF-κB, ERK, and phosphoinositide 3-kinases (PI3K)/Akt pathways (163). Cytokines and growth factors, synthesized and released by bone marrow stromal cells (BMSCs) or MM cells, are also implicated in triggering the major signaling pathways that promote the survival and dissemination of MM cells and mediate drug resistance (166). Key MM-associated cytokines include interleukin-6 (IL-6), IL-10, IL-17a, insulin-like growth factor-1 (IGF1), vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), transforming growth factor-β (TGF-β), and B cell-activating factor (BAFF) (167–169). Cytokines are also involved in disrupting normal bone remodeling in MM, leading to increased bone resorption and formation of osteolytic lesions, which are present in approximately 80% of MM patients (170). Both BMSCs and MM cells contribute to the pathogenesis of myeloma osteolytic lesions through secretion of osteoclast activating factors, such as receptor activator of nuclear factor-κB (RANKL), tumor necrosis factor-α (TNF-α), macrophage inflammatory protein-1α (MIP-1α), macrophage colony-stimulating factor (MCSF), IL-3, and IL-6 (171) as well as osteoblast inhibitory factors such as the Wnt antagonists Dickkopf-1 (DKK1), secreted frizzled-related protein-2 (sFRP-2), Runt-related transcription factor 2 (Runx2), and TGF-β (172).

Characteristically, MM cells foster an immunosuppressive bone marrow environment not only by inhibiting antitumor effector cells and disrupting antigen presentation, but also by encouraging the expansion of regulatory immune cells (173), such as myeloid-derived suppressor cells (174, 175), Tregs (176, 177), and regulatory B cells (178, 179). The myeloma microenvironment also shows a significant increase in Th17 T cells, probably due to the myeloma cell-derived IL-6 and TGF-β production (180). Increased IL-17 production by Th17 T cells has been demonstrated to suppress cytotoxic T cell activity, promote myeloma cell growth in vitro and in vivo through IL-17 receptors (IL17R), and play a crucial role in MM-associated bone disease (180, 181). Moreover, dendritic cells (DCs) exhibit functional impairment in MM patients, characterized by the inability to upregulate CD80 expression in response to stimulation by human CD40LT and IL-2, possibly due to their inhibition by the elevated levels of TGFβ1 and IL-10 in the myeloma microenvironment (182, 183). Importantly, studies have demonstrated an upregulated programmed death ligand-1 (PD-L1) on malignant plasma cells is reported to increase as MGUS progresses to MM (184), combined with elevated expression of its receptor programmed death 1 (PD-1) on several MM-associated immune cell subsets, including T cells, B cells, NK cells, and DCs (185). The interaction between PD-L1 and PD-1 represents a major contributor to the immunosuppressive characteristics observed in the MM microenvironment and is shown to be counteracted by PD-L1 or PD-1 blockade (186, 187).

Data from MM studies on the role of Gal-9 are sparse and controversial, with evidence documenting both pro-tumorigenic and anti-tumorigenic activities ( Figure 3 ) (18, 19). Investigations conducted by Lee et al. on BM aspirate samples obtained from 109 newly diagnosed MM patients indicate that Gal-9 expression is an independent predictor of poor survival in patients who exhibit elevated PD-L1 levels (107). In a recent study performed on blood samples from 60 newly diagnosed MM patients and 40 healthy controls, Zhang et al. have revealed upregulated expression levels of Tim-3 on CD4+ T cell surfaces, elevated Gal-9 mRNA in peripheral blood mononuclear cells, and increased serum levels of Gal-9 in MM patients compared to healthy donors (18). The researchers have demonstrated that Tim-3, upon binding with Gal-9, disrupts the balance between CD4+ T cell subsets (Th1, Th2, Th17, and Treg cells) and impacts secretion of their cytokines, leading to inhibition of the cytotoxic function of Th1 cells and promotion of Th2 and Th17 cells involvement in the immune escape of MM (18).

On the other hand, earlier studies led by Kobayashi et al. report that rGal-9 effectively suppresses the proliferation of various human B-lymphoblast myeloma cell lines (HMCL), namely IM9, AMO-1, KMS-12-BM, NCI-H929, and RPMI8226, and its effectiveness is directly associated with the binding affinity of Gal-9 to each HMCL (19). Their data also demonstrate that Gal-9 exerts a dose-dependent pro-apoptotic effect on primary myeloma cells derived from 10 MM patients, including cells from treatment-resistant patients with poor prognosis and chromosomal abnormalities (19). Mechanistically, the authors suggest that Gal-9 induces apoptosis through the activation of caspase-8, -9, and -3, associated with the activation of JNK and p38 MAPK signaling pathways, while inhibiting JNK or p38 MAPK pathways result in attenuated the anti-proliferative effect of Gal-9, highlighting the crucial role of these pathways in mediating the anti-myeloma cell effect of Gal-9 (19). In contrast, data from another study reveal that Gal-9 induces T cell apoptosis with no effect on MM cells (120). The dual role of Gal-9 extends beyond MM; for instance, in hepatocellular carcinoma, its downregulation in hepatocytes promotes tumor growth and metastasis, while its overexpression in Kupffer cells and endothelial cells suppresses the anti-tumor immune response by inducing T cell apoptosis or senescence (98, 121–123). This duality may be due to the loss of Gal-9 expression during tumor progression after its initial upregulation helps establish a tumorigenic environment (122). In malignant melanoma, high Gal-9 expression suppresses the invasion and metastasis by blocking adhesion to endothelium and ECM (67). However, in metastatic malignant melanoma patients, Gal-9 binding to CD206 expressing M2 macrophages promotes tumor growth by increased angiogenesis and chemokine production (64). Further research is required to explore how Gal-9 contributes to each of the cancer hallmarks in MM patients, considering its cellular and subcellular localizations and its interacting partners within the TME. Additionally, the potential significance of individual Gal-9 splice variants in regulating MM progression needs to be elucidated.

The standard treatment for MM involves a variety of drug combinations aiming to enhance patient survival and quality of life (188, 189). These include alkylating agents (melphalan, cyclophosphamide), corticosteroids (dexamethasone, prednisone), anthracyclines (doxorubicin and liposomal doxorubicin), proteasome inhibitors (PIs; bortezomib, carfilzomib, ixazomib), immunomodulatory drugs (IMiDs; thalidomide, lenalidomide, pomalidomide), monoclonal antibodies (mAbs; Daratumumab, isatuximab, elotuzumab), CAR-T cell therapies, nuclear export inhibitors, histone deacetylase inhibitors (iHDACs), and autologous stem cell transplantation (ASCT) (190–192). The selection of therapy throughout the disease course is influenced by factors such as age, performance status, comorbidities, eligibility for stem cell transplantation, and the risk stratification of MM patients (189). The strategic use of these treatment modalities has significantly enhanced the overall survival rates in MM patients (193–195). However, challenges, such as drug intolerability, drug resistance, and disease-relapse remain prevalent, necessitating the ongoing development of novel therapeutic strategies (196).

Prior studies have underscored the critical role of Gal-9 and its binding partner Tim-3 on CD4+ T cell surfaces in the development of treatment resistance in both hematologic and solid malignancies (197). In AML, patients who failed to respond to chemotherapy exhibited elevated expression levels of Gal-9 and Tim-3, suggesting that targeting the Gal-9/Tim-3 axis could potentially enhance the effectiveness of induction chemotherapy and improve remission rates in AML patients (198). Furthermore, a growing body of evidence from both preclinical studies and clinical trials indicates that combination therapy targeting Gal-9/Tim-3 alongside anti-PD-1/PD-L1 treatment produces superior outcomes in cancer patients compared to either approach alone (83).

These promising results in various cancer types spark interest in exploring the potential of Gal-9/Tim-3 targeting in MM. While comprehensive studies specifically examining the combination of Gal-9 inhibitors with standard anti-MM therapies are currently lacking, the substantial evidence of Gal-9 contribution to an immunosuppressive TME in MM strongly suggests the potential for synergistic therapeutic effects (18). For instance, blocking Gal-9/TIM-3 interactions could potentially enhance the efficacy of FDA-approved BCMA-targeted CAR-T cell therapies by prolonging their survival and activity within the TME, resulting in a more effective elimination of MM cells (199). This approach could be particularly beneficial in MM, where maintaining CAR-T cell function in the hostile TME remains a significant challenge (200). Likewise, combining Gal-9 inhibition with the lenalidomide, a standard anti-MM therapy that enhances the cytotoxic activities of T-cells and NK-cells against tumor cells, could potentially augment the immune response against MM (201). However, this combination strategy warrants further investigation.

In this regard, blocking Tim-3 using mAbs or small molecules has shown promise in preclinical studies (202–204). Similarly, α-Lactose and its derivatives have been used for targeting Gal-9 (205, 206). However, concerns about their non-selective inhibition of galectins and widespread presence in food and pharmaceutical preparations emphasize the need for alternative Gal-9 inhibitors (206). Recently, researchers have developed novel Gal-9-neutralizing antibodies that protect T cells from Gal-9-induced cell death and enhance T cell-mediated tumor cell killing (207).

On the other hand, considering the pro-apoptotic effect of rGal-9 on MM cells, there is a possibility of targeting MM through the development of stable rGal-9 formulations for delivery to myeloma cells (19). Indeed, Kobayashi et al. have demonstrated significant efficacy and relative safety of Gal-9 as a tumor growth inhibitor in MM xenograft models, suggesting its potential as a therapeutic approach for MM (19). Notably, exogenous administration of Gal-9 has been shown to effectively induce tumor cell apoptosis in other types of cancer, such as esophageal cancer (113), hepatocellular carcinoma (121), gastric cancer (100), chronic myeloid leukemia (208), and CTCL (89). Furthermore, rGal-9 administration has been shown to inhibit tumor progression by impeding the in vivo metastasis of highly metastatic melanoma and colon cancer cells (67). Prior studies have also demonstrated that exogenous Gal-9 treatment significantly increases the expression of the B cell immunoregulatory factor SLAMF7, which represents a promising immunotherapeutic target for MM (17). Accordingly, combining rGal-9 with anti-SLAMF7 therapy holds the potential for synergistically enhancing the targeting of MM cells (209, 210). However, alterations in the cell glycome during the transition from normal plasma cells to myeloma cells likely impact Gal-9 binding affinity and modify intracellular signaling activation, highlighting the necessity for thorough investigation of this hypothesis (211, 212). Additionally, combining rGal-9 with proteasome inhibitors, which disrupts protein degradation in myeloma cells, could yield synergistic effects in inducing apoptosis through the simultaneous targeting of complementary cell death pathways (213). Further research is needed to fully elucidate the potential of Gal-9-based therapy, identify patients who would benefit most, and develop novel combination strategies for MM patients to overcome current treatment challenges and improve patient outcomes.