Multiple myeloma (MM) is a blood cancer characterized by the unchecked proliferation of neoplastic plasma cells (PCs). These malignant PCs can infiltrate various tissues and organs, and the M-protein they secrete leads to clinical symptoms such as anemia, hypercalcemia, skeletal pain or osteolytic bone lesions, and renal insufficiency. Additionally, immunosuppression within the bone marrow (BM) often results in recurrent infections. Globally, MM accounts for 1 % of all cancers and approximately 10 % of hematologic malignancies, with about 588,000 new cases diagnosed annually.1 The incidence of MM is higher in males over the age of 50 and rare in individuals under 30.2,3 Despite recent advances in treatment, MM continues to carry a poor prognosis, with drug resistance remaining a significant challenge to both traditional and newer therapies.4

MM typically arises from monoclonal gammopathy of undetermined significance (MGUS), a precancerous condition marked by abnormal plasma cell growth within the BM. MGUS is usually asymptomatic and remains stable due to a balance between tumor cells and the immune system, which helps prevent progression to MM.3,5, 6, 7, 8 However, disruption of this balance and the accumulation of various genetic and epigenetic changes can eventually result in progression to more severe forms such as extramedullary myeloma or plasma cell leukemia. Despite extensive research, the exact origin of the malignant PCs remains unclear. Several factors, including genetic predisposition, exposure to ionizing radiation, chemicals, viral infections, and antigenic stimulation, are thought to contribute to MM development.9

Importantly, the bone marrow microenvironment (BMME) and its interaction with myeloma cells are central to the pathophysiology of the disease.10 Research has shown that the BMME plays a pivotal role in supporting the proliferation and survival of MM clones and allows them to evade immune surveillance, highlighting the influence of the surrounding microenvironment in the malignant transformation of plasma cells.11 Furthermore, the BMME has been shown to drive disease progression and significantly contribute to drug resistance.10,12 Understanding the mechanisms underlying this resistance is therefore critical for developing more effective strategies to prevent and treat MM.

This review will explore the complex interactions between MM cells and the BMME, focusing on how both cellular and non-cellular components, along with other specific factors, influence resistance mechanisms. These insights aim to provide a foundation for optimizing treatment strategies and improving patient outcomes. First, we will briefly introduce the role of the BMME in MM. Then, we will discuss the mechanisms of drug resistance in MM and the contributions of the individual niches within the BMME. Finally, we will discuss how the various components of the BMME cooperate to resist the effects of MM therapies.

MM primarily develops within the BM, where signals from the surrounding environment support the growth, dissemination, and survival of PCs.13 The BMME that encases malignant PCs during the active stages of the disease is also referred to as the tumor microenvironment (TME). This is a heterogeneous system, consisting of both a cellular and a non-cellular compartment (Fig. 1). The former includes osteoclasts, myeloid-derived suppressor cells (MDSCs), macrophages, dendritic cells (DCs), neutrophils, megakaryocytes, mast cells, T cells, B cells, natural killer (NK) cells, bone marrow stromal cells (BMSCs), vascular endothelial cells, fibroblasts, osteoblasts, and adipocytes while the latter consists of the extracellular matrix (ECM), extracellular vesicles (EVs), and the fluid environment of the BM.11,14,15

The BMME is organized into a complex three-dimensional architecture composed of various specialized niches. These niches include the 'osteoblast or endosteal niche,' located near the endosteum, and the 'vascular niche,' situated near the BM vasculature.13 The liquid BM milieu, containing soluble factors such as cytokines, growth factors, and chemokines, is produced and/or influenced by the cellular compartment of the BMME. It is known that the myeloma microenvironment is highly hypoxic, which is a necessary condition for myeloma cell survival and proliferation, especially in the early stages of the disease.16 Consequently, the BMME plays a pivotal role in the progression of MM, and its heterogeneity is important for studying tumor cell survival and drug resistance.

The interactions between MM cells and the BMME are closely related to the progression and expansion of myeloma. MM cells reside in specialized niches within the BM, where they engage with components of the TME. This interaction effectively remodels the BMME, creating an optimal niche that supports the migration, proliferation, and survival of MM cells.17,18 In the development of MM, the contact-dependent interactions between tumor cells and the BMME play a crucial role. MM cells engage with mesenchymal stem cells and endothelial cells through various adhesion molecules (such as VLA-4, LFA-1, and MUC1), activating multiple signaling pathways (including NF-κB, MAPK, Notch, and PI3K) that enhance their survival and proliferation.19 Additionally, MM cells promote early adhesion through P-selectin glycoprotein ligand (PSGL-1) binding to selectins and modulating immune responses and drug resistance through interactions with immune cells via CD28 and PD-L1. MM cells also impact bone metabolism by inhibiting the formation and differentiation of osteoblasts and reducing the expression of osteocalcin, alkaline phosphatase, and collagen I. Mitochondrial trafficking, related to CD38 and CXCR4, supports MM cell survival, drug resistance, and ATP production, while increasing oxidative phosphorylation and reducing superoxide levels. These mechanisms collectively sustain MM growth and contribute to treatment resistance.20,21 In addition to contact-dependent interactions, MM cells also exhibit contact-independent interactions through direct ligand-receptor-mediated crosstalk with BM cells. This crosstalk induces the release of soluble factors such as cytokines, growth factors, and chemokines, which bind to their receptors and activate intracellular signaling pathways. These soluble factors stimulate MM cell growth and survival (e.g., Interleukin-6 (IL-6), TNF-α, IGF-1 receptor (IGF-1R)), promote BM angiogenesis (e.g., VEGF, HGF), enhance bone remodeling (e.g., MIP-1α, IL-6), and facilitate immune evasion (e.g., TGF-β, IL-10).22,23 Additionally, EVs from MM cells contribute to these processes by transferring molecular signals that support MM cell growth, survival, and drug resistance, with MM patient-derived BMSCs exosomes promoting disease progression through altered cellular pathways and increased cytokine and microRNA levels.24, 25, 26

In recent years, the treatment landscape for MM has evolved rapidly. While autologous stem cell transplant (ASCT) remains the standard treatment for eligible patients, several new therapies have been introduced into both first-line and relapsed/refractory disease treatment.2 These newer therapies, including third-generation immunomodulatory agents, second-generation proteasome inhibitors, histone deacetylase inhibitors, and monoclonal antibodies, and chimeric antigen receptor T-cell (CAR-T) therapies (Table 1), have significantly improved survival rates in MM patients. However, lasting remission remains challenging and outcomes are poor for high-risk and relapsed/refractory patients.27

Frontier studies have shown that a major factor in MM drug resistance is the inflammatory response and intercellular signal transduction. The former refers to the interaction between mesenchymal stem cells, endothelial cells and immune cells, which activates multiple signaling pathways (such as NF-κB, MAPK, PI3K) through adhesion molecules (such as VLA-4, LFA-1) and cytokines (such as IL-6, BAFF, APRIL) to enhance drug resistance.28,29 The latter refers to the chronic inflammatory state in the BMME promoting the proliferation and survival of myeloma cells through the release of pro-inflammatory cytokines (such as TNF-α, IL-6) and chemokines (such as MIP-1α, MIP-1β). These inflammatory mediators can change the BMME, further inhibit the effectiveness of drugs and promote the occurrence of drug resistance. In addition, inflammation-related immune cells (such as macrophages, regulatory T cells (Tregs) and MDSCs weaken the anti-tumor immune response by regulating the local immune response, thereby enabling myeloma cells to evade the clearance of drug therapy.30, 31, 32

Drug resistance in MM can be categorized into intrinsic and extrinsic mechanisms.33 It is well known that intrinsic mechanisms include genetic alterations (e.g., mutations in IRF-4, KRAS, NRAS, MYC, TP53, BRAF, FGFR3, CCND1, ATM, ATR, and ZFHX4) that drive uncontrolled proliferation, inhibit apoptosis, induce senescence, and result in defects in DNA repair mechanisms. In addition, epigenetic aberrations (e.g., hypomethylation of ABGC2, hypermethylation of H3K27, and dysregulation of miRNAs such as miR125a-5p and miR21) play a central role in the resistance of MM to existing drugs. Extrinsic mechanisms involve interactions between myeloma cells and the BMME, persistence of cancer stem cells (CSCs), and drug resistance mediated by soluble factors and cell adhesion. Soluble factors such as IL-6, IGF-1, VEGF, FGF, TNF-α, SDF1-α, Ang1, and HGF produced by BMSCs and plasma cells contribute to drug resistance, along with overexpression of cell cycle inhibitors, anti-apoptotic Bcl-2 family members, and ABC drug transporters in myeloma cells during direct adhesion to stromal cells.34 Due to their high genetic instability and the support of the BMME, MM cells rapidly develop resistance to most chemotherapeutic treatments developed to date.35, 36, 37 It is thus imperative to understand the drug resistance mechanism in the BMME of MM and to develop more effective treatment strategies.

It is well-established that the aggressive progression of MM is connected to the composition and function of the immune components in the BMME. Disease progression involves numerous mechanisms, with myeloma cells inducing changes in the BMME to trigger immune evasion and suppression, ultimately benefiting the proliferation, survival, migration, and drug resistance of the tumor cells. These changes are shaped by the complex interactions between different types of immune cells (e.g., macrophages, helper T Cells, Tregs, B cells, macrophages, MDSCs, DCs, and NK cells) and between cells and the microenvironment. The resulting protective microenvironment is characterized by high concentrations of immunosuppressive factors, loss of effective antigen presentation, dysfunction of effector cells, and the expansion of immunosuppressive cell populations.8

Studies have found that under certain conditions, the interaction between myeloma cells and the BM immune microenvironment becomes dysregulated. This dysregulation enables MM cells to modulate the activity of surrounding myeloid and lymphoid cells, thereby creating a supportive niche that facilitates disease progression and drug resistance.38, 39, 40, 41 T cells lose their antitumor function, as evidenced by a decrease in stem-like/resident memory T cells and an increase in Tregs and pro-inflammatory Th17 cells. Myeloma cells also produce more IL-10 and have decreased expression of IL12p70, which impairs the T helper (Th) 1 response.42 Both unbalanced CD4 T cells and increased Th17 cells and their cytokines are closely associated with disease progression and bone damage.43 Certain Treg subtypes can inhibit naive and effector T cell responses and hinder the proliferation and function of DCs.44,45 When the expression of HLA-DR, CD40, CD80, and CD86 molecules is deficient and there are defects in antigen presentation, certain immunological characteristics of DCs are impaired that leads to a weakened effective anti-tumor immune response, ultimately resulting in the escape of myeloma cells from immune surveillance.44,46 In addition, monocyte-derived DCs (Mo-DCs) are also associated with differentiation defects.47 MDSCs promote immune suppression by generating reactive oxygen species (ROS), which reduce CD3ζ expression and inhibit antigen-specific T-cell proliferation. Furthermore, various studies indicate that MDSCs stimulate the clonal growth of antigen-specific natural T cells and trigger the transformation of naïve T helper cells into inducible Tregs through the interaction between CD40 and CD40L, along with the release of cytokines such as TGF-β, IL-10, and IFN-γ.48, 49, 50, 51, 52 Plasmacytoid dendritic cells (pDC) support the growth and survival of MM cells through activation of the NF-κB pathway and increase drug resistance through enhanced proteasome activity in the MM cell.53,54

At present, many reviews have comprehensively discussed the role of each immune cell in drug resistance.8,17,55,56 Here, we will focus on the contributions of these immune components to the development of resistance against novel immunotherapy drugs for MM.

IMiDs possess multiple properties, including immunomodulation, anti-angiogenesis, anti-inflammatory, and antiproliferative effects, by modulating the immune response. These drugs both directly cause cell cycle arrest and apoptosis in MM cells and indirectly interfere with the interaction between MM cells and the BMME by affecting immune cells, osteoclasts, and BMSCs.57, 58, 59 In addition, these drugs also suppress the expression of various cytokines, regulate immune cell function, and enhance the proliferation and cytotoxicity of NK cells, thereby playing a crucial role in anti-tumor immunity.60 Combining IMiDs with other therapies, such as monoclonal antibodies and immune checkpoint inhibitors, may yield synergistic effects.61,62

Currently approved IMiDs include thalidomide and its analogs lenalidomide and pomalidomide. Studies have identified Cereblon (CRBN) as a key mediator of the efficacy of thalidomide analogs in MM. CRBN forms an E3 ligase complex, known as CRL4(CRBN), with DDV1 and Cul4A, promoting the degradation of lymphoid transcription factors Ikaros (IKZF1) and Aiolos (IKZF3) and leading to the anti-cancer effect of the drug. MM cells with reduced CRBN expression or mutations in CRBN thus exhibit intrinsic resistance to lenalidomide and pomalidomide. Silencing of AGO2, a CRBN-binding protein, is associated with high levels of CRBN expression, and is a potential target for overcoming IMiD resistance.63, 64, 65, 66 Additionally, a mechanism of IMiD resistance independent of CRBN-IKZF1/3 was identified by Liu J et al. through in vitro and in vivo experiments showing that the immune microenvironment of MM leads to proteasomal degradation of TRAF2 and activation of the downstream ERK signaling pathway and subsequent IMiD resistance.67

ICIs, including PD-1, CTLA-4, and PD-L1 blockers, enhance the immune system's ability to eliminate cancer cells by preventing the inactivation of T cells in the treatment of MM. PD-1 and CTLA-4 are expressed on activated T cells, and higher levels of these markers, as well as PD-L1 on MM cells, have been observed in the BMME of MM patients.68 The expression of PD-L1 is associated with increased proliferation and enhanced resistance to anti-myeloma treatment. The upregulation of PD-L1 expression on plasma cells in the context of relapsed/refractory disease suggests that it may play a role in the development of clonal resistance.69,70 According to Koyama et al. (2016), the overexpression of other immune checkpoints may be linked to PD-1 inhibitor resistance.71 Clinical studies have shown promising results, with combinations of ICIs and other therapies like lenalidomide and dexamethasone yielding significant objective response rates, particularly in lenalidomide-refractory cases.72 In recent years, other immune checkpoint inhibitors are being studied, such as targeting T cell immunoglobulin 3 (TIM3), T cell immune receptor with immunoglobulin and ITIM domain (TIGIT), and lymphocyte activation gene-3 (LAG3).73

CAR-T therapy has emerged as a revolutionary treatment for MM, offering renewed hope for patients dealing with relapsed or refractory disease, particularly targeting B-cell maturation antigen (BCMA). By modifying host cells to express specific antigen recognition domains, these CAR-T cells can effectively target and eliminate tumor cells, thereby revolutionizing the treatment of hematologic malignancies.74 BCMA, also known as tumor necrosis factor receptor superfamily member 17 (TNFRSF17) or CD269, is a crucial transmembrane protein for maintaining plasma cell homeostasis and regulating B-cell development and differentiation. Its highly selective expression in malignant plasma cells has made it a novel target for MM treatment.75 Additionally, elevated levels of its ligands, such as B-cell activating factor (BAFF) and a proliferation-inducing ligand (APRIL), promote the proliferation and anti-apoptotic behavior of MM cells. In the first-in-human CAR-T cell study targeting BCMA, researchers at the National Cancer Institute reported an 81 % objective response rate in early clinical trials, highlighting its potential effectiveness in treating MM.76,77 This positions BCMA as a significant focus for ongoing and future CAR-T clinical research.78, 79, 80, 81

Non-BCMA targets, including CD138 (syndecan-1), CD38, NY-ESO-1, NKG2D ligands, SLAMF7/CS1, CD56, Lewis Y, and GPRC5D, have demonstrated significant therapeutic benefits when combined with BCMA targeting. Specifically, CD19 CAR-T cells have shown efficacy alongside anti-BCMA therapies, while GPRC5D is identified as a viable target due to its high expression on malignant cells. CD138-directed CAR-T cells effectively eradicate myeloma cells in vivo and in vitro. Additionally, the NY-ESO-1 antigen and SLAMF7 are recognized for their immunogenic potential and therapeutic effectiveness in combination treatments. NKG2D CAR-T therapy has also displayed therapeutic effects against MM and other cancers.82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92

Despite the significant efficacy of CAR-T therapy in MM, resistance remains a major obstacle. The interactions among CAR-T cells, tumor cells, and the immunosuppressive TME are complex.93 Factors such as antigen escape, secretion of immunosuppressive cytokines, and T cell exhaustion collectively impact the activity and long-term effectiveness of CAR-T cells. Specifically, within the BMME, tumor cells and immunosuppressive cells induce CAR-T cell exhaustion and reduce CAR-T cell efficacy through direct cell-to-cell contact mechanisms, such as the PD-1/PD-L1 and Fas/FasL pathways.94 Myeloma cells and stromal cells secrete immunosuppressive cytokines, such as TGF-β and IL-10, and recruit Tregs and MDSCs, impeding CAR-T cell activity and promoting an immunopurified environment. The challenging BMME, along with continuous antigen exposure, can accelerate T cell exhaustion, adversely affecting CAR-T treatment outcomes. Furthermore, BMSCs protect MM cells from CAR-T cell-mediated cytotoxicity by upregulating anti-apoptotic proteins within the MM cells.95, 96, 97

Currently, extensive research is focused on identifying strategies to enhance the efficacy and long-term success of CAR-T therapy for MM. This includes optimizing CAR-T cell design, integrating CAR-T therapy with other agents that influence the immune microenvironment, and employing dual-targeting CAR-T cells to prevent antigen escape. Concurrently, addressing T cell exhaustion remains a research priority, exploring approaches such as checkpoint inhibitors and metabolic reprogramming techniques.96,98

MSCs are an important component of the BMME, and are involved in tumorigenesis, metastasis, epithelial-mesenchymal transition, anti-apoptosis, survival promotion, immunosuppression and drug resistance.99 Not only can MSCs develop drug resistance themselves, they may also confer this resistance to many other types of cancer cells. A growing number of studies have shown that drug resistance in MSCs involves multiple pathways and downstream mechanisms, including the secretion of various cytokines and chemokines, delivery of MSC exosomes, acquisition of gene mutations, formation of direct cell-cell contacts, and differentiation.100

MSC-derived IL-6 plays an important role in resistance by activating the downstream STAT3 cascade in different tumors. This parallels the drug resistance induced by the release of IL-6 from cancer-associated fibroblasts (CAFs). In addition, it has also been reported that other paracrine cytokines secreted by MSC, including IL-6, IL-7, IL-8 and growth factors such as VEGF, are involved in the formation of tumor resistance.101 Some scholars have found in the study of solid tumors that MSC-derived exosomes can deliver content to cancer cells, and this interaction is the cause of drug resistance. This process has also been demonstrated in MM, where BMSC-derived exosomes mediate cell-cell communication through the transfer of their contents in MM cells. These exosomes can activate the survival-related p38, p53, c-Jun N-terminal kinase and AKT pathways, and promote the survival of MM cells and chemotherapy resistance in vivo by increasing the anti-apoptotic protein Bcl-2 and reducing the cleavage of caspase-9 and caspase-3.27 Direct cell-cell interactions between MSCs and the BMME can also confer drug resistance. In hematological malignancies, this crosstalk between cancer cells and MSCs can promote the activation of a series of signal cascades in different tumors. For example, in T-cell acute lymphoblastic leukemia (T-ALL), intercellular adhesion molecule-1 (ICAM-1)-mediated adhesion of MSCs to cancer cells induces mitochondrial transfer between MSCs and T-ALL cells, resulting in a decrease in mitochondrial ROS levels in T-ALL cells.102

Macrophages are a major component of inflammatory cells, and those that infiltrate the TME are referred to as TAMs. TAMs primarily originate from circulating monocytes in the blood, which differentiate into pro-tumor macrophages through activation by cytokines secreted by tumor cells. TAMs are activated through binding of Toll-like receptors (TLR) on their surface to tumor-derived damage-associated molecular patterns (DAMP) and pathogen-associated molecular patterns (PAMP) or through direct contact with tumor cells via ICAM-1 or membrane-bound colony-stimulating factor 1 (CSF-1). Activated macrophages secrete a large number of molecules that affect tumor cells and other stromal cells.103, 104, 105, 106 It is noteworthy that TAMs promote angiogenesis, cell invasion, and proliferation, and enhance the survival of myeloma cells by producing more IL-6.41,107

TAMs also promote the development of drug resistance through several mechanisms. TAMs can inhibit apoptosis by inhibiting the activation of caspase-3 and poly (ADP-ribose) polymerase (PARP) while maintaining Bcl-xL levels, leading to resistance to melphalan.108,109 In the TME, TAMs promote drug efflux through various mechanisms, thereby enhancing chemotherapy resistance in tumor cells. One key mechanism involves TAMs secreting chemokines CCL17 and CCL22, which act through their common receptor CCR4, frequently overexpressed on the surface of tumor cells.110 Similar to the role of CAFs in the development of drug resistance, TAMs can also induce epithelial-mesenchymal transition (EMT) in tumor cells by secreting multiple cytokines and growth factors, such as TGF-β, TNF-α, IL-6, and IL-8, which affect several intracellular signaling pathways, including TGF-β-SMAD, MAPK/ERK, NF-κB, PI3K-AKT, and Wnt-β-catenin pathways.111Chen et al. demonstrated that TAMs can also activate downstream Akt signaling through the secretion of CCL2, a process that promotes the expression and nuclear localization of β-catenin. This ultimately induces EMT and enhances the characteristics of CSCs in solid tumors, thereby increasing drug resistance.112 Moreover, the TME itself has the ability to limit drug penetration by expanding the ECM.113

Additionally, TAMs secrete pro-fibrotic growth factors, inducing the transformation of normal stromal fibroblasts into CAFs, which further stimulate the production and remodeling of ECM proteins in the TME, thereby blocking effective drug penetration. Finally, the dysregulation of drug metabolism and detoxification in tumor cells further exacerbates the development of drug resistance. These combined effects collectively contribute to the tumor's resistance to chemotherapy.114, 115, 116 Recent studies have found that exosomes derived from TAMs play a significant role in the proliferation, invasion, metastasis, angiogenesis, immune response, drug resistance, and metabolic reprogramming of malignant tumor cells.117,118

In the TME, activated fibroblasts typically exhibit characteristics similar to myofibroblasts and are referred to as CAFs. BM-derived fibroblasts and locally infiltrating fibroblasts are considered the primary sources of CAFs.103,105,106,119 In addition, CAFs can be derived from a variety of other cell types, including BM-derived cells, epithelial cells undergoing EMT, and astrocytes and therefore have a high degree of heterogeneity. Transformation to CAFs is mediated by soluble factors, microRNAs, and exosomes.120, 121, 122, 123 CAFs produce several molecules related to stimulating tumor cell growth and progression and exosomes that promote tumor microRNAs.119

CAFs also have a significant impact on the development of drug resistance in MM and the formation of an immunosuppressive microenvironment. CAFs promote drug resistance by secreting soluble factors, such as cytokines like IL-6 and IL-8, which activate the STAT3 and NF-κB signaling pathways. This leads to drug resistance through either the upregulation of C-X-C chemokine receptor 7 (CXCR7) expression or EMT.124, 125, 126 The involvement of the NF-κB and Bcl-xL signaling pathways in tumor chemoresistance is mediated through the negative regulation of SDF1 expression by microRNA miR-1.127

It has been documented that both TAMs and CAFs contribute to the development of tumors. However, they are not always independent of each other.128 In 2016, studies suggested that there is a significant cell-cell interaction between the two.129 TAMs and CAFs penetrate into almost all malignant tumors and can cooperate to activate each other and increase tumor cell invasion and angiogenesis.130

For a long time, adipocytes have been thought to be inert cells in the BMME, acting only as energy storage. Nowadays, bone marrow adipose tissue (BMAT), which is considered an endocrine organ, has attracted more and more attention. Studies have shown that it is closely associated with the occurrence of solid tumors or hematological malignancies.131, 132, 133, 134 It is clear that obesity increases the possibility of MGUS progressing to MM.135 However, in MM, these adipose tissues are affected by the BMME to become myeloma-associated adipocytes, and produce a series of growth factors, adipokines and adipocytokines, which in turn affect the progression of the disease.136, 137, 138, 139, 140 Wei et al. suggested that MM cells may influence MSC differentiation into adipocytes in the BMME in vitro. The increased BM adipocytes can effectively enhance the proliferation, migration, and drug resistance of MM cells through cell-cell contact and/or cytokine release regulated by the PPAR-γ signaling pathway.141 Leptin is one of the adipokines which has been shown to promote the growth of MM cells. Yu et al. not only found that elevated leptin levels can promote the proliferation of myeloma cells by up-regulating AKT and STAT3 signaling pathways, thereby enhancing resistance to chemotherapy, but also observed an additional correlation between the overexpression of Bcl-2 and the inhibition of caspase-3.142 In addition, leptin has been shown to enhance autophagy and prevent caspase cleavage and apoptosis, thereby promoting chemoresistance.143

CSCs are a small population of multifunctional cells within tumor cell populations that possess the ability for self-renewal, initiate tumor growth, and resist drug effects.59,144, 145, 146, 147, 148 These cells can remain in a quiescent state and expel anti-cancer drugs via drug transporters. This reduces the efficacy of cytotoxic agents, ultimately leading to relapse through self-renewal, asymmetric division, and differentiation into mature cancer cells.149 Even after a complete response to chemotherapy, these cells may continue to survive. This phenomenon is also observed in MM. CSCs in MM exhibit frequent activation of the Hedgehog (Hh), Wnt, and Notch pathways, which play roles in their self-renewal capabilities. They display abnormally elevated levels of IL-6, DKK1, IL-1β, IL-3, G-CSF, GM-CSF, stem cell factor, and TNF-α.150, 151, 152, 153 There is evidence indicating that an EMT-like phenotype underlies local invasion, extramedullary involvement, distant metastasis, and drug resistance.154,155

In addition, the BMME supports the survival of hematopoietic stem cells and may provide nourishment for CSCs. CD138 is almost exclusively expressed on mature plasma cells and not on mature B lymphocytes. It is, however, detected in nearly 100 % of MM cells and serves as a diagnostic marker. CSCs in MM are believed to exist in a CD138-negative population that exhibit memory B cell markers (CD138-/CD19+/CD20+/CD27+) and are resistant to dexamethasone, lenalidomide, bortezomib, and carfilzomib due to increased activity of aldehyde dehydrogenase 1 and drug efflux pumps (ABCB1/P-glycoprotein and ABCC3).156,157 The CD138−/CD38+ subset also shows innate resistance to bortezomib because they lack the XBP-1 transcription factor, which is crucial for plasma cell development and endoplasmic reticulum stress-induced apoptosis.158, 159, 160

As mentioned earlier, the non-cellular niche provides important environmental support for the growth and survival of MM. Various cytokines, chemokines, and growth factors are released by the bone marrow stroma (BMS). These factors include IL-6, VEGF, and stromal cell-derived factor-1 (SDF-1), which play a crucial role in supporting the growth and vitality of myeloma cells. However, in the past few decades, a substantial body of evidence has shown that, in addition to the intrinsic resistance mechanisms of cancer cells, there are dynamic de novo mechanisms coordinated by the TME that led to EM-DR, which are closely linked to the BMME.161 ECM components such as fibronectin and collagen provide support for MM cells and promote drug resistance by regulating signal transduction pathways (such as PI3K / Akt pathway). This form of resistance is rapidly induced by signaling events triggered by factors present in the TME and can be divided into two categories: the first is soluble factor-mediated drug resistance (SFM-DR), which is induced by cytokines, chemokines, and growth factors secreted by fibroblast-like tumor stroma; the second is cell adhesion-mediated resistance (CAM-DR), which occurs when tumor cells adhere to stromal fibroblasts or components of the ECM, such as fibronectin, laminin, and collagen. These two resistance mechanisms collectively influence the survival of tumor cells and the effectiveness of treatment (Fig. 2).162, 163, 164 Recently, a large number of studies have focused on the acidic environment and hypoxic conditions in the BM that may affect drug activity and cell response to drugs.165,166 We will discuss these in other specific drug resistance of section.

Cell adhesion molecules (CAMs) are a subclass of adhesion proteins located on the cell surface, involved in cell-matrix and cell-cell interactions. In MM, CAMs not only regulate the interactions between MM cells and osteoblasts and osteoclasts, but also mediate the interactions between MM cells and BMSCs, lymphocytes, and endothelial cells.167 These interactions play a critical role in the proliferation and survival of malignant plasma cells (MPCs). MM cells can circulate, extravasate, and subsequently migrate back to the BM, a process mediated by various chemokines and adhesion molecules. Furthermore, the adhesion of MM cells to BM stromal cells and the composition of the BMME significantly contribute to the drug resistance observed in MM.168,169

CAM-DR is a mechanism by which MM cells evade anticancer therapy through adhesion to BMSCs or ECM components. This process relies on various adhesion molecules, including the VLA-4 (very late antigen 4), VLA-5 (very late antigen 5), αvβ3, β7, CD138 (syndecan-1), CD44, VCAM (vascular cell adhesion molecule), LFA-1 (lymphocyte function-associated antigen 1), MUC1, and ICAM-1 (intercellular adhesion molecule 1). These adhesion molecules play a crucial role in the transport and drug resistance of MM cells.170, 171, 172, 173, 174 Studies have confirmed that cell adhesion-mediated drug resistance can manifest in various epithelial tumors (such as prostate cancer, breast cancer, and small cell lung cancer), where tumor cells utilize specific integrins (such as αvβ3 and β1 integrin) to inhibit drug-induced DNA breaks caused by agents like etoposide and paclitaxel, thereby enhancing drug resistance.175, 176, 177 This process is particularly significant in hematological malignancies. By adhering to components of the ECM or BMS, tumor cells can enter a reversible quiescent state, allowing them to evade the toxicity of drugs. The specific mechanisms include cell cycle arrest and the upregulation of anti-apoptotic proteins (such as p27Kip1, p21Cip1/WAF1, and c-FLIPL), as well as the inhibition of apoptotic signals. Through non-transcriptional regulatory mechanisms, the adhesion of tumor cells enhances their survival, making it difficult to eradicate tumor cells that are attached to the BMME. These findings reveal the complex role of the TME in the development of drug resistance.178 The adhesion of MM cells to BMSCs triggers the secretion of IL-6, activates NF-κB in the stromal cells, and upregulates several signaling pathways, thereby promoting the proliferation and survival of MM cells.179

Cell adhesion affects the drug resistance of myeloma cells by regulating the level of the cell cycle inhibitor p27Kip1. Adhesion to fibronectin leads to the upregulation of p27Kip1, causing cells to enter a quiescent state and thereby reducing sensitivity to chemotherapy. Disruption of fibronectin adhesion lowers p27Kip1 levels and restores drug sensitivity. This indicates a causal relationship between β1-mediated fibronectin adhesion, changes in p27Kip1 levels, and CAM-DR.172

An in vivo study demonstrated that the expression of ICAM-1, VLA-4, and VCAM is higher in patients prior to chemotherapy compared to those receiving initial chemotherapy, and that the expression levels increase with the number of chemotherapy regimens. The expression levels of VLA-4 and ICAM-1 in major multidrug-resistant patients are significantly higher than in responders.180K Noborio-Hatano and colleagues identified VLA-4 as a key molecule inducing CAM-DR in MM cells through functional screening using a lentiviral short hairpin/small interfering RNA (shRNA/siRNA) system. Bortezomib overcomes CAM-DR by downregulating the expression of VLA-4 on MM cells, thereby inhibiting their adhesion to fibronectin and BMSCs.181Yutaka Hattori and his colleagues experimentally confirmed that long-term exposure to lenalidomide can induce the expression of integrin β5 or β7, which is associated with drug resistance and poor prognosis.182 In addition, research indicates that Reelin promotes the adhesion of MM cells to fibronectin by activating α5β1 integrin. The phosphorylation of focal adhesion kinase (FAK) leads to the activation of Src/Syk/STAT3 and Akt, which are key signaling molecules involved in enhancing cell adhesion and protecting cells from drug-induced apoptosis. Thus, Reelin promotes the adhesion and drug resistance of MM cells through the integrin β1 signaling and STAT3 pathway.183

The interaction between MM cells and BMSCs is also mediated by PSGL-1, ICAM-1, and Notch. Y Zheng et al. found that PSGL-1/selectin and ICAM-1/CD18 play a crucial role in macrophage-mediated drug resistance in MM cells through the analysis of differentially regulated and paired membrane protein genes. Blocking antibodies against these molecules or knocking down PSGL-1 or ICAM-1 in myeloma cells inhibited the protective effects of macrophages. The interaction between macrophages and myeloma cells also activated the Src and Erk 1/2 kinases and the c-myc pathway, while suppressing chemotherapy-induced caspase activation.184 MM cells interact with BMSCs through Notch receptor-ligand binding, activating the Notch signaling pathway and leading to increased secretion of growth factors such as IL-6, IGF-1, and VEGF, which promotes the formation of the BMME. This process also upregulates PD-L1 expression on MM cells, enhancing cell proliferation and resistance to specific drugs (such as dexamethasone and melphalan), while inhibiting anti-tumor T cell responses. In summary, this intercellular interaction promotes MM cell growth and drug resistance through Notch signaling and weakens the immune system's ability to suppress the tumor.69,185

Additionally, Masanobu Tsubaki and colleagues found that ANKL induces resistance in RANK-positive cell lines by increasing the expression of multidrug resistance protein 1 (MDR 1), breast cancer resistance protein (BCRP), and lung resistance protein 1 (LRP 1), while decreasing Bim expression. RNA silencing of Bim induces resistance, but silencing MDR 1, BCRP, and LRP 1 does not overcome this resistance. These findings suggest that the RANK/RANKL system induces chemoresistance by activating multiple signaling pathways and reducing Bim expression and reveal a new role for the RANK/RANKL system in the drug resistance of MM.186

Francesca Fontana et al. synthesized 20 nm VLA-4-targeted micelle nanoparticles (V-NP) to carry DiI for tracing of a novel camptothecin prodrug (V-CP), tracking their delivery and treatment response using human or mouse MMC and immune-active mice. The results showed that V-NPs selectively delivered their payload to MMC, with their uptake enhanced by chemotherapy. V-CP was well-tolerated when administered alone or with melphalan in MM mice, extending survival while reducing the dose requirement for melphalan without increasing toxicity. V-NPs thus exploit CAM-DR to target refractory MM cells and prolong survival.187Abdel Kareem Azab and Samuel Achilefu et al. found that the use of nanoparticles to deliver drugs to enhance efficacy and reduce toxicity, the use of PSGL-1 targeted BMME, combined with the delivery of BTZ and ROCK inhibitors, its therapeutic effect is significantly better than small molecules, non-targeted carriers and single-agent controls, reducing the side effects caused by BTZ, while overcoming drug resistance.188Nasrin Rastgoo et al. found that low levels of miR-155 are associated with advanced disease. Overexpression of miR-155 inhibits the expression of CD47 on the surface of MM cells, promoting macrophage phagocytosis of these cells and suppressing tumor growth. Additionally, miR-155 overexpression restores sensitivity to bortezomib in drug-resistant MM cells, leading to cell death by targeting TNFAIP 8, a negative regulator of apoptosis.189 In addition, through genomics and proteomics research, genes related to cell adhesion (such as CXCR4) are identified for individualized treatment.

At the same time, we must not overlook the application of monoclonal antibodies that interact with CAM-DR in practical treatment. For example, anti-CD38 monoclonal antibodies (such as daratumumab), IMiD (such as lenalidomide), proteasome inhibitors that downregulate α4β1 and α5β3 integrins (such as bortezomib), and α4-VLA-4 integrin inhibitors (such as natalizumab) have all shown good efficacy in clinical settings. Additionally, anti-SLAMF7 drugs, CAR-T cell therapy, and anti-BCMA drugs have emerged as new treatment options. However, the issue of resistance to these emerging therapies remains an important challenge.190

Soluble factor-mediated drug resistance is induced by cytokines, chemokines, and growth factors secreted by fibroblast-like tumor stroma, primarily through gene transcription. This process requires dynamic interactions between tumor cells and their stroma to occur.191 IL-6 and SDF-1 are the most commonly studied mediators. Other soluble factors involved in SFM-DR include IGF-1,192, 193, 194 hepatocyte growth factor (HGF),195 SDF-1, VEGF, epithelial growth factor (EGF),196 IL-8, and tumor necrosis factor-B (TNF-B).197 These factors work together to enhance the function of SFM-DR.

IL-6 plays a critical role in the pathogenesis and function of MM. It promotes the proliferation and survival of myeloma cells and leads to chemotherapy resistance through various mechanisms. IL-6 is primarily secreted by osteoblasts, macrophages, and BMSCs in the BMME.132 It activates downstream signaling pathways such as JAK/STAT3 and HIF-1α, resulting in the upregulation of cell survival-related genes. The activation of STAT3 also helps myeloma cells evade immune surveillance.198 Moreover, IL-6 enhances the resistance of tumor cells to apoptosis by promoting the Stat3 signaling pathway and creates an amplification loop between tumor cells and BMSCs, further promoting the secretion of growth factors. This mechanism allows tumor cells to survive and develop resistance after chemotherapy.199, 200, 201 Furthermore, Fan et al. demonstrated that IL-6 activates the transcription factor JunB through MEK/MAPK and NF-κB-dependent mechanisms, which is associated with the proliferation and drug resistance of myeloma cells.202 Inhibition of IL-6 can restore sensitivity to Bcl-2/Bcl-xL while inducing dependence on Mcl-1. Gupta et al. found that when myeloma cells grow alongside BMSCs, their dependence on Bcl-2/Bcl-xL decreases. However, when IL-6 is inhibited, the dependence of myeloma cells on Bcl-2/Bcl-xL is restored despite the presence of BMSCs. Additionally, IL-6 has the capacity to trigger dependence on Mcl-1. Therefore, the combination of Venetoclax or similar drugs with IL-6 inhibition shows promising prospects for treatment.203,204

Bcl-2 is an important anti-apoptotic protein that enhances the survival of MM cells by inhibiting apoptotic signals. Cytokines and growth factors in the BMME, such as IL-6, upregulate the expression of Bcl-2, thereby promoting the survival and resistance of MM cells. Additionally, the adhesion of MM cells to BMSCs activates the expression of Bcl-2 through integrin signaling pathways, further enhancing drug resistance.205Yuan XZ et al. identified that acquired resistance to venetoclax in MM is linked to the regulation of the BCL-2 family, with upregulation of survival proteins (MCL-1, BCL-XL, BCL-2) and downregulation of the pro-apoptotic protein BIM. Their research suggests that upstream signals from cytokines, growth factors, and receptor tyrosine kinase (RTK) pathways, particularly PI3K, are involved in this resistance. Co-inhibiting MCL-1 or BCL-XL along with these upstream signals can effectively overcome venetoclax resistance.206 At present, domestic and foreign research focuses on overcoming MM resistance, such as inhibitors of anti-apoptotic proteins such as Bcl-2, MCL-1 and BCL-XL. In the phase I study (NCT03314181) conducted by Bahlis NJ et al., the efficacy of venetoclax combined with daratumumab and dexamethasone (VenDd) was evaluated in patients with t (11;14) relapsed or refractory multiple myeloma (RRMM), as well as the efficacy of VenDd combined with bortezomib (VenDVd) in genetically unselected RRMM patients. The results indicated that both treatment regimens achieved high levels of deep and durable responses in RRMM patients.207 A phase Ib/II trial (NCT04973605) presented at the 29th European Hematology Association (EHA) congress indicated that the next-generation BCL-2 inhibitor sonrotoclax (BGB-11417) combined with dexamethasone demonstrated good tolerability in patients with RRMM with t (11;14).

SDF-1 (also called CXCL12) is a crucial factor in the BMME, facilitating the homing and retention of hematopoietic stem cells (HSCs) and the transport of lymphocytes. It also plays a role in recruiting endothelial progenitor cells (EPCs) during angiogenesis. In addition, Research shows that under hypoxic conditions, SDF-1 expression in MM cells increases, with hypoxia-inducible factor-2 (HIF-2) binding to the SDF-1 promoter. Notably, using an SDF-1 antagonist suppresses angiogenesis induced by the overexpression of hypoxia-inducible factors in MM cells, indicating that SDF-1 mediates hypoxia-induced angiogenesis.208 Its activity is primarily mediated by the CXCR4 receptor, which is highly expressed on MM cells and other BM cells. Recently, CXCR7 has also been identified as an SDF-1 binding receptor, showing high expression in malignant hematopoietic cells.209,210 SDF-1 interacts with TAMs and CAFs, collectively promoting the progression and resistance of MM.211,212Liu Y and colleagues found that co-culturing BMSCs with RPMI-8226 cells increased the drug resistance of MM cells and inhibited their apoptosis, with SDF-1α-induced IL-6 upregulation playing an important role in this process. Further analysis indicated that SDF-1α upregulates IL-6 expression by activating the PI3K/AKT signaling pathway. SDF-1/CXCR4 may affect adhesion-mediated chemotherapy resistance of MM cells through a similar mechanism.213

It is noteworthy that soluble factors produced by tumor stimulation enhance EM-DR by not only upregulating anti-apoptotic molecules but also inducing integrin expression and increasing their affinity for ligands on tumor cells. This suggests that CAM-DR and SFM-DR may cooperate in vivo within the context of EM-DR. For example, SDF-1 increases β1 integrin-mediated adhesion of myeloma cells, driving signaling that alters the conformation of the extracellular domain of β-integrin, leading to drug resistance.178,214,215 Studies indicate that SDF-1 promotes the transendothelial migration of MM cells, relying on the upregulation of integrin α4β1.216 Additionally, SDF-1 enhances the expression of α4β1, facilitating the adhesion of MM cells to CS-1/fibronectin and VCAM-1. Sphingosine-1 phosphate works synergistically with SDF-1 to promote this α4β1-dependent cell adhesion mechanism, involving cAMP activity and RhoA activation.214,217 Additionally, integrin-mediated adhesion enhances cytokine signaling pathways, such as IL-6-induced STAT3 signaling. The interaction between tumor cells and stroma also regulates the composition of the ECM, further enhancing CAM-DR.218,219

IGF-1 plays a crucial role in treatment resistance in MM. IGF-1 is synthesized by BMSCs and acts as a potent stimulant and survival factor for myeloma cells. It binds to the IGF-1R on MM cells, activating important signaling pathways such as PI3K/Akt and MAPK. Yue et al.220 confirmed that IGF-1 is a key factor in the development of myeloma, driving the EMT process to promote migration, invasion, and colony formation in MM cells through the PI3K/Akt pathway. These pathways not only promote the proliferation of MM cells but also increase their resistance to chemotherapy drugs, while enhancing the expression of anti-apoptotic proteins like Bcl-2 and Mcl-1. Additionally, IGF-1 stabilizes MAF expression by inhibiting GSK3β, and silencing c-MAF can enhance the response to bortezomib. Therefore, focusing on the IGF-1/IGF-1R pathway as a therapeutic strategy is significant for addressing drug resistance and improving treatment outcomes in patients with MM.220,221

Studies have found that the hypoxic environment within the BM not only enhances the adaptability of myeloma cells but also promotes resistance to novel drugs. Hypoxia helps maintain the homeostasis of various cells, including tumor cells. However, the rapid growth of cancer cells and abnormal angiogenesis in tumors expose cells to low blood flow and consume large amounts of oxygen. HIF-α, as a key regulatory factor in the cellular response to low oxygen levels, is the most important controller of the hypoxic response.222

A large number of studies have demonstrated the important role of hypoxia inducible or inhibitory factors in MM. Under hypoxic conditions, genes related to plasma cell differentiation are downregulated,223, 224, 225, 226, 227 while genes associated with stemness, glycolysis, and drug resistance (such as LDHA and HK2) are upregulated.226,228,229 Additionally, hypoxia promotes angiogenesis, tumor dissemination, and bone disease by upregulating genes like VEGFA, CXCR4, and DKK1.224,228,230, 231, 232, 233 Transcription factors such as CREB and MMSET are also upregulated, while tumor suppressor genes are downregulated.234 Noncoding RNAs regulated by hypoxia, such as miR-210, contribute to MM progression and drug resistance.235 Additionally, hypoxia can induce dissemination and homing, neovascularization, inhibition of osteoblasts, and activation of osteoclasts. Under low oxygen conditions, MM cells upregulate CXCR4 and CCR1 through the expression of HIF-1α and HIF-2α, allowing them to migrate to different BM regions according to the concentration gradients of SDF-1 and CCL5.236,237 HIF also regulates various pro-angiogenic factors (including VEGFA, bFGF, and HGF) and releases fluid factors, altering the BMME to promote neovascularization.230,232 HIF-1α stimulates osteoclast differentiation by regulating IL-3 and induces the MMSET and p38-CREB-DKK1 axis to inhibit osteoblast function, leading to bone disease.234,238 Many studies have shown that the expression of noncoding RNAs is influenced by differences in oxygen partial pressure, especially in hypoxic environments.233 Small functional noncoding RNAs, such as microRNAs, play an important role in maintaining normal cells and in the molecular pathogenesis of cancer by regulating specific target messenger RNAs.239 For example, hypoxia-inducible miR-210 regulates the expression of ribosomal RNA methyltransferase DIMT1, leading to the suppression of IRF4.223

Additionally, miR-135b is transported from hypoxic myeloma cells to vascular endothelial cells via exosomes, promoting angiogenesis. Furthermore, long noncoding RNAs (lncRNAs) also have diverse functions in cell survival. For example, studies have shown that disruption of the lncRNA PVT1 confers a pro-growth phenotype and may be associated with MYC overexpression myeloma cells.240,241 HIF-inducible lncRNAs like DARS-AS1, H19, and MALAT1 contribute to tumorigenesis and cell survival through various mechanisms.231,242 Therefore, exposure to hypoxic conditions in myeloma cells may lead to resistance to proteasome inhibitors, IMiDs, and monoclonal antibodies through mechanisms such as autophagy, reduced expression of IRF4, and activation of glycolysis. However, hypoxia-induced dedifferentiation may partially enhance the antitumor effects of IMiDs.

Exosomes are nanosized membrane vesicles constitutively released by almost all types of cells and are indispensable in both physiological and pathological processes. In addition to mediating local intercellular communication by transferring proteins, lipids, and nucleic acids to recipient cells, exosomes can also induce the activation, proliferation, differentiation, and death of target cells.243 The BMME is closely related to the development, progression, immunosuppression, and drug resistance of MM. Exosomes derived from BMSCs can increase the viability and progression of MM cells through cytokines and chemokines such as IL-6, IP10, MCP-1, and CCL1, and downregulate miR-15a. They also enhance homing by upregulating fibronectin, SDF-1, and MCP-1, and promote resistance by increasing full-length caspase-8, caspase-9, caspase-3, and PARP.25,26 Additionally, it can induce drug efflux by transferring proteins such as P-glycoprotein (P-gp), annexin A3, ATPase copper transporter alpha (ATP7A), and ATPase copper transporter beta (ATP7B).244, 245, 246, 247, 248 MM-derived exosomes influence cell activity in the BMME by transferring specific miRNAs (such as miRNA-146a and miRNA-135b) and other factors, promoting tumor growth and progression. These exosomes upregulate various cytokines in BMSCs and osteoclasts, enhancing angiogenesis and immune suppression, ultimately creating a favorable microenvironment for MM growth and migration.185,235,249, 250, 251, 252

While exosomes have a variety of biological activities and are biomarkers and potential therapeutic targets, their role in the generation of drug resistance is still one of the factors contributing to the poor efficacy of MM, which cannot be overlooked. Exosomes derived from BMSCs have been found to play a key role in facilitating the development of drug resistance MM cells.26 Gao et al. demonstrated that BMSC-derived exosomes carrying miR-155 inhibit apoptosis, promote cell division, and upregulate the expression of drug resistance-related proteins.253 Exosomes derived from adipocytes protect MM cells from chemotherapy-induced apoptosis by upregulating two lncRNAs (LOC606724 and SNHG1), and are associated with poor prognosis in patients. This resistance mechanism involves the methylation of lncRNAs in adipocytes by METTL7A, the activity of which is enhanced by exposure to MM cells. Methylation of these lncRNAs, in turn, promotes their packaging into adipocyte exosomes which then protect MM cells from apoptosis.254 In 2018, research showed that MM-derived exosomes not only enhanced osteoclast activity but also inhibited osteoblast differentiation and function, particularly by suppressing key factor expression in osteoblasts through DKK-1. Blocking exosome secretion increased bone mass and enhanced MM cell resistance to bortezomib.255 BMSC exosomes can induce bortezomib resistance by increasing Bcl-2 levels.26 Exosomes from MSCs of proteasome-resistant patients induce drug resistance in MM cells by transferring PSMA3 mRNA and PSMA3-AS1 lncRNA, thereby increasing proteasome activity in the cells.256 BMSC-derived exosomes from MM contain various miRNAs, including miR-23b-3p, miR-27b-3p, miR-125b-5p, miR-214-3p, and miR-5100. However, MM cells predominantly amplify miR-214-3p and miR-5100, which promote MM cell proliferation and drug resistance by downregulating PTEN.257

The complex interaction between exosomes and MM cells highlights the need to target these pathways to improve the therapeutic effect. By elucidating how exosomes promote drug resistance, researchers can identify new therapeutic targets and potentially improve patient outcomes.