Accumulation of Myeloid-Derived Suppressor Cells in Bone Marrow in Multiple Myeloma Induces Tumor-Specific Immune Suppression and Promotes Tumor Growth

Abstract 3954

Multiple myeloma (MM) is an incurable malignancy of clonal plasma cells in bone marrow (BM). Accumulation of MM cells in BM is associated with a corresponding dysfunction of local immune responses. The mechanisms involved in the immune suppression remains poorly understood. Myeloid-derived suppressor cells (MDSC) are heterogeneous group of relatively immature myeloid cells characterized by potent immune suppressive activity. However, their role in regulation of immune responses in BM in MM remains unclear.

To determine MDSC accumulation in MM patients, BM and blood samples were obtained from newly diagnosed patients and healthy donors. We observed a significant increase in CD11b⁺ CD33⁺ CD14⁻ HLA-DR^lo MDSCs, but not in CD11b⁺ CD33⁺ CD14⁺ HLA-DR^hi monocytes, in the blood and BM of patients as compared to cells with the same phenotype in healthy donors. MDSC isolated from patient BM suppressed T cell responses as measured by IFN-g Elispot assays of mixed leukocyte reactions. Immature myeloid cells with the same phenotype from BM of healthy donor did not inhibit T cells.

The possible role of MDSC in MM progression was further evaluated in mouse model of MM. Three different MM cell lines (BCM, DP42 and ATLN) obtained from Dr Van Ness (University of Minnesota) were used. MM tumors were established by i.v. injection of syngeneic mice. We observed an expansion of CD11b⁺ GR1⁺ MDSCs (up to 70%) in the BM of tumor-bearing (TB) mice within one week after injection of tumor cells. MDSC purified from BM of TB mice at that time point potently suppressed antigen-specific T cell proliferation and IFN-g production. As tumor progressed, CD138⁺ MM cells expanded (up to 90%) in the BM replacing normal BM cells including MDSC. However, at later time points (2–3 weeks), MDSC expansion was seen in spleen and lymph nodes of TB mice.

To evaluate the possible role of MDSC in MM progression, we used S100A9 knockout (KO) mice. S100A9 plays an important role in MDSC accumulation in cancer and S100A9 KO TB mice had reduced accumulation of these cells. When wild-type (WT) and S100A9 KO mice were inoculated with DP42 cells no differences in tumor growth and survival were observed. S100A9 KO mice had a decrease in MDSC accumulation in the BM of TB mice compared to WT mice. Additionally, MDSC from S100A9 KO mice failed to suppress antigen-specific T cells ex vivo, whereas MDSC from WT TB mice did. To evaluate the development of antigen-specific immune responses in MM, we generated a MM cell line over-expressing the chicken ovalbumin protein (DP42-OVA). S100A9 KO mice injected with the immunogenic DP42-OVA cell line showed significantly delayed tumor growth and improved survival as compared with WT mice. Further analysis of T cell responses in vivo showed increased accumulation of antigen-specific T cells within the BM of S100A9 KO mice, but not WT mice, as early as one week post-tumor inoculation. Antigen-specific T cells in spleens were not detected at that time point. These cells were seen in spleens on S100A9 KO mice only 2–3 weeks after tumor injection.

Thus, our study demonstrated that MM is associated with rapid accumulation of MDSC with potent immune suppressive activity against tumor-specific T cells. Our data directly suggest that targeting MDSC in MM may improve antitumor immune response and clinical outcome of the disease.