VISTA-Mediated Immune Evasion in High-Risk Myelodysplastic Syndrome

Project Summary/Abstract Myelodysplastic syndrome (MDS) is a hematologic disorder characterized by ineffective hematopoiesis, cytopenias, and a risk of progression to acute myeloid leukemia (AML). While immune dysregulation is seen almost universally in MDS, studies have observed immunologically distinct phenotypes in low-risk (LR-) or high- risk (HR-) MDS. Inflammation is often seen in LR-MDS while HR-MDS is associated with immune suppression. For example, HR-MDS has been associated with an increased frequency of inhibitory myeloid-derived suppressor cells (MDSCs) and a skewing of macrophages towards anti-inflammatory (M2-like) phenotypes. However, how these contrasting immune profiles influence MDS pathogenesis and progression is not well understood, and efforts to target immune suppression in HR-MDS with available immune checkpoint inhibitors (such as anti-PD-1/PD-L1 therapy) have had limited success, indicating the presence of other immune suppressive mechanisms. VISTA (V-domain Ig suppressor of T cell activation, also known as PD-1H) is a co- inhibitory checkpoint molecule known to suppress the inflammatory activation of both T cells and myeloid cells. In myeloid cells, VISTA can promote M2-like characteristics in macrophages and mediate the immune suppressive functions of MDSCs. Our group recently reported that myeloid-expressed VISTA is a significant contributor to AML immune evasion in vivo. In my preliminary studies, I have found that VISTA is overexpressed in HR-MDS, particularly by monocytes and macrophages, but how this VISTA expression influences the immune microenvironment in HR-MDS and regulates MDS immune evasion is unknown. Therefore, this proposal will test the hypothesis that VISTA expression by host immune cells promotes MDS immune evasion in high- risk MDS. Aim 1 will use spectral flow cytometry (CyTEK) and highly-multiplexed immunofluorescence (CODEX) to profile and spatially resolve the bone marrow microenvironment of LR-MDS and HR-MDS. These studies would help to clarify the role of VISTA in HR-MDS and identify cell-cell interactions mediated by myeloid VISTA. Aim 2 will further investigate the role of myeloid and macrophage VISTA in MDS. In vitro experiments will reveal the VISTA-mediated mechanisms of macrophage anti-MDS immunity while in vivo experiments will assess the impact of myeloid VISTA on MDS progression and immune evasion using a myeloid-specific conditional knockout (LysM-Cre+;VISTAfl/fl). Finally, ex vivo experiments will validate the role of VISTA in human MDS and test the utility of anti-VISTA antibody blockade in the treatment of HR-MDS. Together, the studies in this proposal will deepen our understanding of how immune dysregulation facilitates MDS disease progression and help elucidate the immune suppressive mechanisms present in HR-MDS, providing a preclinical basis for the development of VISTA antagonists to treat high-risk MDS. Furthermore, funding of this fellowship application will provide comprehensive scientific training in basic and translational cancer immunology and outstanding career mentorship for a future hematology-oncology physician-scientist.