Myeloid Derived Suppressor Cells (MDSCs) Regulate Tumor Growth, Immune Response and Regulatory T Cell (Treg) Development in the Multiple Myeloma Bone Marrow Microenvironment

The interaction of myeloma (MM) cells with bone marrow accessory cells induces genomic, epigenomic and functional changes which promote tumor development, progression, cell adhesion mediated-drug resistance (CAM-DR), and immune suppression. As in other cancers, bidirectional interaction between MM cells and surrounding cells regulates tumor development on the one hand, while transforming the BM microenvironment into a tumor promoting and immune suppressive milieu on the other. Recent developments in targeted therapies have indicated that generation of the most effective therapeutic strategies requires not only targeting tumor or stroma cells, but also methods to overcome blockade of anti-tumor immune responses. In addition to lymphoid immune suppressor cells such as regulatory T cells (Tregs), distinct populations of myeloid cells such as myeloid derived suppressor cells (MDSCs) can effectively block anti-tumor immune responses, thereby representing an important obstacle for immunotherapy. While MDSCs are rare or absent in healthy individuals, increased numbers of MDSCs have been identified in tumor sites and peripheral circulation. Recent studies have in particular focused on MDSCs in the context of tumor promoting, immune suppressing, stroma in solid tumors. However, their presence and role in the tumor promoting, immune suppressive microenvironment in MM remains unclear.

Here we assessed the presence, frequency, and functional characteristics of MDSCs in patients with newly diagnosed or relapsed MM compared to MM patients with response and healthy donors. We first identified a distinct MDSC population (CD11b⁺CD14⁻HLA-DR^-/lowCD33⁺CD15⁺) with tumor promoting and immune suppressive activity in both PB and BM of MM patients. Moreover, we determined the immunomodulatory effects of lenalidomide and bortezomib on induction of MDSCs by MM cells, as well as on MDSC function.

MDSCs were significantly increased in both PB and BM of patients with active MM compared to healthy donors and MM in response (p<0.01). To determine whether the CD11b⁺CD14⁻HLA-DR^-/lowCD33⁺CD15⁺ myeloid cell population represents functional MDSCs, we first assessed tumor promoting role of MDSCs in the MM microenvironment by culturing MM cell lines with MM patient bone marrow stroma cells (BMSC), with or without depletion of MDSCs. Importantly, BMSC-mediated MM growth decreased to baseline levels of MM cells alone when MDSCs were removed from the BMSC microenvironment. Moreover, MDSCs isolated from MM-BM using magnetic-Ab and/or FACS sorting cell separation, directly induced MM cell growth and survival, evidenced by 3H-thymidine incorporation and MTT assays. Since the interaction between tumor and stromal accessory cells is bidirectional, we next analysed the impact of MM cells on MDSC development. Importantly, MM cell lines cultured with PBMCs from healthy donors induced a 7 fold increase in MDSCs. We also examined the immune suppressive functions of MDSCs in cultures of autologous T cells with T cell stimulators, in the presence and absence of MDSCs from MM-PB or MM-BM. Freshly isolated MDSCs from both MM-PB and MM-BM induced significant inhibition of autologous T cell proliferation. Moreover, MDSC-associated immune inhibitory molecules arginase-1 (ARG-1) and reactive oxygen species (ROS), as well as inhibitory cytokines IL-6 and IL-10, were significantly increased in BM MDSCs, evidenced by intracellular flow cytometry analysis. In addition, MM BM MDSCs induced development of Treg from autologous naïve CD4+T cells. Finally, we analysed whether MDSCs impacted response to bortezomib and lenalidomide. Culture of MDSCs with MM cell lines, with or without bortezomib (5nM) and lenalidomide (1uM), demonstrated that less MM cell cytotoxicity in the presence of MDSCs.

Our data show that MDSCs are increased in the MM microenvironment and mediate tumor growth and drug resistance, as well as immune suppression. Therefore targeting MDSCs represents a promising novel immune-based therapeutic strategy to both inhibit tumor cell growth and restore host immune function in MM.

Raje:Onyx: Consultancy; Celgene: Consultancy; Millennium: Consultancy; Acetylon: Research Funding; Amgen: Research Funding; Eli-Lilly: Research Funding. Munshi:Celgene: Consultancy; Millenium: Consultancy; Merck: Consultancy; Onyx: Consultancy. Richardson:Novartis: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Membership on an entity's Board of Directors or advisory committees; Johnson & Johnson: Membership on an entity's Board of Directors or advisory committees. Anderson:Celgene: Membership on an entity's Board of Directors or advisory committees; Millennium: Membership on an entity's Board of Directors or advisory committees; Onyx: Membership on an entity's Board of Directors or advisory committees.