ARTICLE ABSTRACTDysregulated expression of S100 protein family members is associated with cancer proliferation, invasion, angiogenesis, and inflammation. S100A9 induces myeloid-derived suppressor cell (MDSC) accumulation and activity. MDSCs, immunosuppressive cells that contribute to tumor immune escape, are the main producers of S100A9. In this study, we evaluated the role of extracellular S100A9 and the therapeutic relevance of S100A9 inhibition in multiple myeloma (MM), using the immunocompetent murine 5T33MM model. We demonstrated the presence of S100A9 and its receptor TLR4 in both monocytic and granulocytic MDSCs in human and mouse samples. We showed that S100A9 acted as a chemoattractant for MM cells and induced MDSCs to express and secrete inflammatory and pro-myeloma cytokines, including TNFα, IL6, and IL10. Blocking S100A9 interactions in vivo with the small molecule ABR-238901 did not directly affect MDSC accumulation but did reduce IL6 and IL10 cytokine expression by MDSC. ABR-238901 treatment in vivo reduced angiogenesis but had only minor effects on tumor load as single agent (6% reduction). However, ABR-238901 treatment in combination with bortezomib resulted in an increased reduction in tumor load compared with single treatments (50% relative reduction compared with bortezomib alone). Our data suggest that extracellular S100A9 promotes MM and that inhibition of S100A9 may have therapeutic benefit. Cancer Immunol Res; 5(10); 839–46. ©2017 AACR.