Abstract 2127: Efficacy of monoclonal antibodies against pathogenic S100A8/9 in myelodysplasia and leukemia models

Abstract Introduction: Myelodysplastic syndromes (MDS); chronic myelomonocytic leukemias (CMML) and acute myeloid leukemia (AML) are characterized by a block in normal differentiation of stem and progenitor cells that leads to cytopenias. The diseased marrow microenvironment is characterized by expansion of immunosuppressive myeloid derived suppressor cells (MDSCs) that produce S100A8 and S100A9 proteins. These proteins form heterodimers that bind multiple receptors including TLR4/MD2, CD33 and RAGE, and lead to leukocyte recruitment, inflammatory cytokine secretion and formation of MDSCs. Even though S100A8/9 heterodimers (also known as DAMPs, alarmins, calprotectin) are known drivers of disease pathogenesis, no specific targeting therapeutic approaches have been tested in MDS/CMML. Results: Analysis of a large dataset of MDS patient derived CD34+ hematopoietic stem and progenitors (N=183) showed increased expression of S100A8/9 receptors TLR4/MD2, CD33 and RAGE when compared to healthy controls. Furthermore, within the MDS patient cohort, those with higher S100A8/9 expression had more severe anemia and required more RBC transfusions. Additionally, in a CMML dataset (N=39), scRNA-seq profiles showed significantly higher expression of both S100A8 and S100A9 in the disease initiating, inflammatory (cluster 2) cells. Distinct neutralizing monoclonal antibodies (mAbs) targeting human S100A8/9 homo/heterodimers were generated by single B cell cloning from immunized rabbits. Selected rabbit derived, chimeric and subsequently humanized antibodies demonstrated nanomolar neutralizing activity, effectively inhibiting interaction of S100A8/A9 with RAGE, Siglec-3, Siglec-9, CD147, CD146, CD300ld and CD69. In functional assays, these neutralizing mAbs also suppressed the formation of MDSCs and T-regulatory cells from human PBMC cultures. Primary MDS, CMML and AML mononuclear cells (N=11, with numerous mutations) were treated with monoclonal S100A8/9 neutralizing antibodies and grown in clonogenic assays. Neutralization of S100A8/9 led to increased erythroid colony formation and increased erythroid differentiation as evident from Glycophorin A and CD71 expression by FACS analysis. Conclusion: We demonstrate overexpression of S100A8/9 and its receptors in MDS and leukemia samples and have developed potent neutralizing monoclonal antibodies against the S100A8/9 heterodimer that can deplete MDSCs and reduce T-reg cell formation. Additionally, antibody neutralization of S100A8/9 stimulates erythropoiesis in a broad range of primary MDS and myeloid leukemia patient specimens. These results provide compelling pre-clinical rationale for clinical development of S100A8/9 antibodies for the treatment of myeloid malignancies. Citation Format: Rahul Sanawar, Ariel Fromowitz, Srinivas Aluri, Sarah Aminov, Srabani Sahu, Leah Kravets, Shanisha Gordon Mitchell, Mani Mohindru, Aditi Shastri, Alan List, Eric Padron, Rongbao Zhao, Amit Verma. Efficacy of monoclonal antibodies against pathogenic S100A8/9 in myelodysplasia and leukemia models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2127.