PS1354 MYELOMA‐INDUCED MYELOID‐DERIVED SUPPRESSOR CELLS ARE TARGETED BY IMMUNOMODULATORY DRUGS THAT REGULATE THE CCL5/CCR5 AXIS AND MIF AND IRF8 EXPRESSION (2019)

Background: The recent advent of immunochemotherapy using immunomodulatory drugs (IMiDs), such as lenalidomide (LEN) and pomalidomide (POM), has greatly improved treatment outcomes of multiple myeloma (MM); however, generation of immunosuppression in the tumor microenvironment hampers the therapeutic potency of immunochemotherapy. Myeloid‐derived suppressor cells (MDSCs), which are a heterogeneous group of immunosuppressive myeloid polymorphonuclear (PMN‐MDSCs) and monocytic (M‐MDSCs) cells, play critical roles in impairing cancer immunosurveillance, and an increase of MDSCs has been associated with treatment failure, disease progression, and poor prognosis in various cancers, including MM. Aims: In this study, we examined the molecular mechanisms underlying induction of MDSCs in MM, and showed that the inhibitory effects of IMiDs on myeloma‐induced MDSCs occur through novel mechanisms of action. Methods: Six human myeloma‐derived cell lines (HMCLs) (KMS‐12‐BM, KMS‐28‐PE, KMS‐34, AMO‐1, RPMI‐8226, and LP‐1) were utilized. HMCLs were co‐cultured with peripheral blood mononuclear cells (PBMCs) from a healthy donor using a transwell system. After 5–7 days, induction of MDSCs from PBMCs was evaluated by flow cytometry. CD14+/CD33+/HLA‐DR‐ cells and CD14‐/CD15+ cells were regarded as M‐MDSCs and PMN‐MDSCs, respectively. Soluble factors in the conditioned medium (CM) of each HMCL and co‐culture system were measured using a Human Cytokine Array Kit (R&amp;D Systems, Minneapolis, U.S.A) for 36 cytokines/chemokines and ELISA kits. LEN and POM were provided by Celgene Corporation. Results: Two of the six cell lines (KMS‐12‐BM, KMS‐28‐PE) induced M‐MDSCs that strongly expressed indoleamine‐2, 3‐dioxygenase (IDO) and interleukin‐8, but not PMN‐MDSCs, from PBMCs in the co‐culture experiments using a transwell system. The increase of M‐MDSCs was accompanied by an increase of regulatory T cells in our system. Analyses of soluble factors in CM derived from HMCLs that did and did not induce MDSCs showed that CCL5 was secreted only by MDSC‐inducible HMCLs, while MIF was secreted by all HMCLs examined. Moreover, addition of an anti‐CCL5 antibody or a MIF inhibitor partly blocked induction of MDSCs, and the combination of an anti‐CCL5 antibody and a MIF inhibitor showed an additive effect in blocking MDSC induction. Supplementation of CCL5 enabled induction of MDSCs in several HMCLs that were otherwise unable to induce MDSCs, although the effect was minor. Collectively, combined secretion of CCL5 and MIF by MM cells was identified as the prerequisite for induction of M‐MDSCs. Next, among currently available drugs for MM, we found that LEN and POM had marked suppressive effects on induction of MDSCs by co‐existing myeloma cells. Both agents significantly reduced CCL5 and MIF at the transcriptional level in MM cells and at the protein level in CM from MM cells. Moreover, IMiDs reduced expression of CCR5, a receptor of CCL5, and increased transcription of IRF8, which is a negative regulator of MDSC differentiation in PBMCs. Summary/Conclusion: This study identified CCL5 and MIF secreted by tumor cells as proteins required for induction of MDSCs in MM, and showed that IMiDs have suppressive effects on MDSC induction through overlapping molecular mechanisms in myeloma cells and PBMCs. These results provide new insights into monitoring of the effects of IMiD‐containing immunochemotherapy and may permit development of a novel therapeutic approach