Targeting immune inhibitory molecule SUSD2 to reverse immunosuppression

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Project Summary/Abstract Sepsis is the most common cause of death in many intensive care units and represents a major burden to the US health care system. Despite advances in intensive care technology and mechanical ventilator support, pharmacological options for sepsis are limited, which reflects an insufficient understanding of host-dependent mechanisms that underlie this pathophysiological disorder. A wealth of evidence from recent clinical and experimental sepsis studies indicates that a prolonged immunosuppressive status, due to profound cell death and dysfunction of lymphocytes, is a critical determinant of sepsis-elicited mortality. Therefore, restoration of lymphocyte cell survival and functions by blocking immune inhibitory molecule(s) may represent a promising therapeutic regimen for treating sepsis. In this Proposal, we aim to study the role and mechanism of a previously unrecognized immune inhibitory molecule called SUSD2 (sushi domain containing 2) in promoting sepsis-induced immunosuppression. Through an unbiased gene profiling assay, our previous study has identified a cell surface molecule SUSD2 whose high expression correlated with an immunosuppressive phenotype in an experimental cancer model. In this Proposal, we observed elevated Susd2 expression in T lymphocytes in experimental septic animals and patients with sepsis compared to non-septic controls. Genetic deletion of SUSD2 (Susd2−/−) resulted in a significantly improved animal survival and attenuated apoptosis of T lymphocytes in the cecal ligation and puncture (CLP)-induced polymicrobial sepsis model. Mechanistically, our preliminary studies discovered an inhibitory effect of SUSD2 on interleukin-2 receptor (IL-2R) signaling, a well- established pathway essential for T cell survival and effector functions. The goal of the proposal is to examine the causal effect of SUSD2 on cell death and dysfunction of T lymphocytes in microbial sepsis. We hypothesize that 1) elevated SUSD2 expression leads to diminished IL-2-dependent cell survival and effector functions in T lymphocytes, resulting in a sustained immunosuppressive state and worse disease outcome in sepsis; 2) enhanced activation of STAT5 (signal transducer and activator of transcription 5) and BATF (basic leucine zipper ATF-like transcription factor) signaling maintains cell survival and effector functions in Susd2−/− T cells post sepsis; 3) SUSD2 blockade reverses sepsis-induced cell death and dysfunction of T lymphocytes. Single-cell RNA sequencing analysis of circulatory immune cells will be performed to examine the inhibitory effect of SUSD2 on T cell response post sepsis at the single-cell level. We will test whether treatment with a neutralizing anti-SUSD2 antibody reverses dysfunctional T cells isolated from septic patients. Results of these studies will provide both experimental and clinical evidence to support a promoting function of SUSD2 on sepsis-induced immunosuppression, which can potentially lead to the development of new approach for sepsis treatment.