Multiple myeloma (MM) is a paradigm disease in which progression is fueled by intrinsic alterations of myeloma cells and tumor-host interactions in the tumor microenvironment (TME) (1). Disease evolution from monoclonal gammopathy of undetermined significance (MGUS) to smoldering myeloma (SMM), and symptomatic disease is characterized by a progressive increase of myeloma cells associated with co-evolving immunological and metabolic changes making the TME unable to hold the disease in check (1). We and others have shown that immune alterations are already detectable in the very early stage of the disease (2, 3) and that they persist in the remission phase (2). The immune MM TME contexture consists of effector cells (i.e, conventional T cells, unconventional T cells like NKT cells, γδ T cells, NK cells etc), professional suppressor cells [i.e, regulatory T cells (Tregs), regulatory B cells (Bregs), myeloid derived suppressor cells (MDSC)], and cells that are functionally conditioned by the TME and acquire protumoral functions like bone marrow stromal cells (BMSC), endothelial cells, osteoblasts (OB), and osteoclasts (4). Recently, BM-resident neutrophils have also been reported to contribute to the TME-induced suppressive commitment of MM patients (5). Unbalanced distribution of effector and suppressor cells already detectable in MGUS is induced by the progressive accumulation of myeloma cells driven by genetic and epigenetic drivers. The bone marrow (BM), which is where MM originates and propagates, has the capacity to physiologically host around 2-5% polyclonal plasma cells. When myeloma cell infiltration overcomes this threshold, the TME is immunologically and metabolically shaped to support myeloma cell growth, to induce drug resistance, and to suppress immune recognition. MDSC play a major role in the protumoral reset of MM TME.

We have previously shown that MDSC are significantly increased in the BM of MGUS and MM patients: granulocytic/polymorphonuclear MDSC (PMN-MDSC), and not monocytic MDSC (M-MDSC), are responsible for the increase (2). MDSC frequency is very similar in MGUS, MM at diagnosis, and MM in relapse. Unexpectedly, we have found that MDSC frequency is significantly higher in MM in remission (2), indicating that there is no correlation between the proportion of BM myeloma cells and MDSC expansion. Similar data have been reported in mouse models in which MDSC start to accumulate in the TME as early as one week after tumor inoculation when the frequency of myeloma cells is very low (<10%) as in MGUS individuals (6).

Approximately, 20-40% of MDCS express the Programmed Cell Death-Ligand 1+ (PD-L1+) (2) and therefore are very well-suited to engage and suppress immune effector cells like Vγ9Vδ2 cells and NK cells expressing the Programmed Cell Death-1 (PD-1) receptor (2). MDSC are PD-L1+ in MGUS and MM irrespective of the disease stage, including MM in remission when most myeloma cells have been cleared from BM (2). The persistence of PD-L1+ MDSC can hinder the immunomodulatory activity of drugs like bortezomib or lenalidomide after autologous stem cell transplantation.

In conclusion, MDSC play a major role in the establishment of the immune suppressive TME in MM. The aim of this minireview is to discuss the mechanisms exploited by MDSC in cooperation with myeloma cells, professional immune suppressor cells, and other bystander cells to promote myeloma cell growth in the BM of MM patients. We will also discuss possible interventions to dampen the immune suppression operated by MDSC and other suppressor cells to recover the antimyeloma activity of immune effector cells.

MDSC play a major role in the regulation of immune homeostasis in healthy individuals, and the regulation of immune responses in infectious diseases, autoimmunity, aging, pregnancy, transplantation, and obesity (7). In cancer, the immune suppressive activity of MDSC is exploited by tumor cells to evade immune surveillance and support their survival and accumulation (7).

MDSC are derived from bone marrow hematopoietic stem cells (7). There are two major subsets of MDSC in humans: PMN-MDSC and M-MDSC. The first one are phenotypically and morphologically similar to neutrophils (CD15⁺ and/or CD66b⁺), whereas M-MDSC are similar to monocytes (CD14+)(7). More recently, a third subset of phenotypically distinct immature early-MDSC (e-MDSC) has been identified in cancer patients (8). In this review we will use the term MDSC to identify both PMN-MDSC and M-MDSC unless otherwise specified.

MDSC development occurs in two partially overlapping waves (9). The first one is driven by cytokines and soluble factors including granulocyte-macrophage colony-stimulating factor (GM-CSF), macrophage colony-stimulating factor (M-CSF), granulocyte colony-stimulating factor (G-CSF), interleukin 6 (IL-6), and vascular endothelial growth factor (VEGF). These cytokines and soluble factors are produced by tumor cells and/or BMSC in the TME and promote MDSC differentiation from hematopoietic progenitor cells via STAT3 and STAT5 activation (10, 11, 12). Mesenchymal stromal cells (MSC) also induce MDSC expansion via the hepatocyte growth factor (HGF), c-Met, and STAT3 phosphorylation (10). The second wave is driven by a different set of cytokines and inflammatory soluble factors like interleukin 13 (IL-13), toll-like receptor (TLR) ligands, and prostaglandin E2 (PGE2) yielding to the functional MDSC activation via the STAT1 and NF-kB pathways (10–12). The TME is highly predisposed to drive the expansion and activation of MDSC at the expense of other myeloid-derived cells like monocytes, macrophages and dendritic cells (DC) (8).

The immune suppressive MDSC activity is dependent on: 1) the depletion of essential CD8⁺ T- cell nutrients in the TME; 2) the production of immune suppressive cytokines and/or soluble factors; 3) the expression of cell surface inhibitory molecules [i.e., (PD-L1)]; 4) the protumoral metabolic TME rewiring at the expense of immune effector cells.

MM is a hematologic cancer characterized by the accumulation of malignant plasma cells (myeloma cells) in the BM. Progressive colonization of BM results in a deep remodelling of the BM niche that becomes committed to support myeloma cell growth, immune evasion, and drug resistance (1).

MDSC play a major role in establishing the protumoral TME commitment. We have shown that MDSC accumulation in the BM is already detectable in MGUS, and their expansion persists throughout the entire period of the disease (2), including the remission phase (2). In our hands, PMN-MDSC was the main subpopulation to be expanded in MGUS and MM (2), while other groups have reported the predominance of M-MDSC in MM at diagnosis and in relapse (36, 37). Immunogenomic characterization identified CD11b+CD13+CD16+ cells as the PMN-MDSC subset with strongest capacity to suppress anti-myeloma activity T-cell immune responses (38). MDSC-like suppressive activity is also exhibited by MM neutrophils (5), suggesting that an accurate characterization of MDSC should be based on phenotypic markers, immunosuppressive potential, and transcriptional network.

Development and suppressor functions of MDSC are supported by myeloma cells and bystander cells via direct and indirect mechanisms. Direct mechanisms operated by myeloma cells include: 1) IL-6 production (39, 40) which prevents MDSC differentiation and promotes MDSC accumulation and activation via the STAT3 signaling pathway (41); 2) the induction of Mcl-1, an anti-apoptotic protein sustaining MDSC survival (42); 3) the secretion of galectin-1 that targets CD304 on MDSC and enhances their immune suppressive capacity (43); 4) the production of chemokine ligand 5 (CCL5) and macrophage migration inhibitory factor (MIF) (44). MIF has also been reported to potentiate the immune suppressive activity of MDSC via CD84-mediated PD-L1 upregulation (45); 5) the release of exosomes that promotes MDSC growth and NO production (46)

Bystander cells in the TME also cooperate with myeloma cells in the induction and activation of immune suppressive MDSC via direct mechanisms including: 1) IL-6 release (47, 48); 2) exosome release by BMSC (49); 3) production and release of immune suppressive molecules [i.e. Prostaglandin-Endoperoxide Synthase 2 (PTGS2), TGF-β, Nitric Oxide Synthase 2 (NOS2), IL-10 and IL-6] by MSC and OB (50, 51).

In addition to the direct mechanisms listed above, myeloma cells and bystander cells promote the accumulation and activation of MDSC via indirect mechanisms. An example is the metabolic rewiring operated by myeloma cells and bystander cells that creates an hypoxic and nutrient-depleted TME that promotes the accumulation and activation of MDSC at the expense of immune effector cells (52–54). Lactate over-production shifts MDSC differentiation toward PMN-MDSC (55), which is the subset that we and others have shown to be increased in the peripheral blood (PB) and BM of MM patients (2, 56).

The accumulation and activation of MDSC is beneficial to myeloma cells creating a very effective protumoral loop (3, 57). MDSC facilitate the self-renewal of myeloma stem-cells, enhance their tumorigenic potential via epigenetic regulation (58), and promote myeloma cell survival via AMPK phosphorylation leading to increase β-oxidation, ATP production, and increased NADPH levels (59). MDSC production of S100A9, a calcium-binding protein that promotes the release of TNF-α, IL-6, and IL-10 in autocrine pathway through TLR4 interaction, attracts myeloma cells in the TME (60) and supports myeloma cell growth via the activation of the canonical NFκB pathway (61).

Indirect mechanisms operated by MDSC to support myeloma cells are deprivation of nutrients, production of soluble factors, and the expression of cell surface inhibitory molecules. The common denominator is the impairment of anti-myeloma immune responses. In addition, PMN-MDSC are educated to express angiogenesis-related proteins to support tumor angiogenesis (62).

MDSC upregulate enzymes that contribute to the shortage of amino acids essential for immune effector T cells. Arginase 1 (Arg-1) expression and NO production by MDSC limit the availability of L-Arg needed for effective TCR-mediated signaling (63, 64). MDSC can utilize glutamine for anaplerosis like myeloma cells (65, 66), exacerbating glutamine deprivation in the TME (54).

Several soluble factors and cytokines contribute to the immune suppressor activity of MDSC in the TME, like IL-10, IL-6, TGF-β, CD40-CD40 Ligand, and IFN-γ. These cytokines tip the scales in favor of Tregs (44, 67), whose number is directly correlated with MDSC expansion (56). Lastly, CD38 expression on MDSC (68) contributes to the discontinuous multicellular pathway of adenosine (Ado), an immune suppressive nucleoside highly represented in the TME of MM patients (69).

The expression of immune checkpoint (ICP)/ICP-L contributes to the impairment of anti-myeloma immune responses. We have previously demonstrated that PD-L1 is expressed by BM MDSC in all disease states (2) and can contribute to hold in check anti-myeloma activity of PD1+ effector cells such as T cells, NK cells, and Vγ9Vδ2 T cells. Recently, it has been reported in solid tumors that MDSC can boost the immune suppressive activity of Bregs against T cells via the PD-1/PD-L1 axis (70, 71).

Lastly, MDSC can trans-differentiate into functional osteoclasts (72) to combine immune suppressive functions and direct protumoral functions (73). In mice models, G-MDSC have also been shown to promote angiogenesis (62), another major protumoral TME disruption occurring in human MM (62).

The direct and indirect mechanisms involved in the cross-talk between MDSC, myeloma cells, immune effector, immune suppressor cells, and other bystander cells in the TME of MM patients are shown in Figure 1 .

The correlation between the frequency of MDSC and the clinical outcome identifies these cells as potential targets of immune-based therapeutic interventions (74). However, the therapeutic targeting of MDSC is not easy given their multifaceted biological functions and multiple interactions in the TME. Possible strategies are: 1) to restrain their accumulation in the PB and TME; 2) to prevent their functional activation in the TME; 3) to block their protumoral interactions with myeloma cells and bystander cells.

MDSC accumulation can be restrained by immunomodulatory drugs (IMiDs) (44) and proteasome inhibitors (PI) (59). A cereblon (CRBN)-dependent and -independent down-regulation of CCL5 and MIF is a possible mechanism of IMiDs activity on MDSC (44) that can be improved by Arg-1 inhibitors (75). Clinical data confirm the capacity of IMiDs to restrain MDSC in vivo as shown by the decrease of PB MDSC in MM patients treated with pomalidomide, dexamethasone, and daratumumab (76). Daratumumab can also exert a favourable immune modulatory activity in the TME of MM patients by depleting CD38+ MDSC via antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytoxicity (CDC) (68). Data from mice models indicate that demethylating agents like decitabine (DAC), IL-18, and zoledronic acid (ZA) can also affect MDSC survival in the TME (72, 77, 78). ZA is currently used in MM and other solid cancers to prevent osteoclast activation and bone lesions, but this molecule is also endowed with pleiotropic immune modulatory activity (79), including the capacity in murine models to reduce the numbers of MDSC and prevent their differentiation into osteoclasts (72). Targeting CD84 and the CD304-Gal1 axis are other strategies used in myeloma mouse models to restore anti-myeloma T-cell responses by reducing MDSC accumulation and PD-L1 expression (45).

The immune suppressive activity of MM MDSC has also been inhibited in vitro using ABR-238901, a small molecule inhibiting S100A9 interactions (60), and tasquinimod (74). Anti-estrogen therapy may also restrain MDSC suppressive activity, since 17β-estradiol increases their ability to suppress T-cell proliferation (80). iNOS and Arg-1 activities have been down-modulated in mice models with tadalafil (81), a PDE5 inhibitor that has been used with some evidence of clinical efficacy in relapsed/refractory MM patients in combination with lenalidomide (82). Protumoral MDSC cellular interactions in the TME can also be limited by interrupting ICP/ICP-L interactions (2). Daratumumab in combination with the anti-PD1 monoclonal antibody cetrelimab has been reported to decrease the number of circulating MDSC and increase that of CD8+ T cells in the PB of MM patients in relapse (83). In acute myeloid leukemia (AML), knockdown of VISTA, a negative checkpoint regulator in the B7 family, reduced the MDSC-mediated inhibition of T cells (84). Data are not available in MM yet, but VISTA up-regulation is also expected in the BM of MM given the hypoxia and low pH as reported in solid cancer (85).

In conclusion, understanding the mechanisms underlying the immune suppressive activity of MDSC in MM will pave the ground to the therapeutic targeting of these cells to improve the efficacy of immune-based treatments in MM.

CG, FA and MM contributed to the writing of the manuscript, CG designed the figure, MM revised the manuscript.