ABSTRACT/PROJECT SUMMARY Acute kidney injury (AKI) occurs at a high rate in both native kidneys and allografts and has no specific therapy available. Prior studies have established T cell activation and trafficking as an important mechanism that modulate ischemia reperfusion (IR) and nephrotoxic AKI, along with other overlapping immune and non- immune mechanisms. Furthermore, AKI is common in patients treated with immune checkpoint inhibitors targeting cytotoxic T lymphocyte-associated antigen 4 (CTLA4) and programmed cell death receptor 1 (PD1) for multiple cancers. Our preliminary data using RNA sequencing and flow cytometry shows significant expression of novel immune checkpoint molecule, T cell immunoreceptor with Ig and ITIM domains (TIGIT) in T cells from post IR mouse kidney and ischemic human kidney. Recently published data suggest that TIGIT co-inhibitory activity modifies Th1 and Th17 responses, plus regulates Treg suppression activity. Our preliminary data shows that TIGIT expressing T cells in mouse kidney are highly activated and produce proinflammatory cytokines after IR. Importantly, mice lacking TIGIT (TIGIT KO) were protected from AKI in IR and Cisplatin AKI models, suggesting detrimental role for TIGIT during AKI. Therefore, understanding TIGIT-mediated inflammatory response during AKI is critical for developing novel AKI therapy and to mitigate kidney adverse effects of immune checkpoint therapies. The central hypothesis of this proposal is that TIGIT promotes proinflammatory functions of kidney T cells and impairs Treg suppression function. To test this hypothesis, we will (Aim 1) investigate phenotypic, functional and transcriptional effects of TIGIT in mouse kidney T cells using in vitro and in vivo approaches. We will further investigate the functional relationship between TIGIT and its co-signaling partners (CD226, CD155) and other co-inhibitory molecules (PD1, CTLA4) in regulating kidney T cell functions at baseline and during AKI. Additionally (Aim 2), we will test the hypothesis that T cell specific TIGIT activity is the major mechanism that drives AKI and impairs repair process using adoptive transfer approaches, in vivo anti-TIGIT agonist/antagonist antibody effects on AKI outcome in WT and TIGIT KO mice and blocking TIGIT signaling in repair phase after established AKI. Finally (Aim 3), we will investigate functional effects of TIGIT expression in human kidney T cells in patients with renal cell carcinoma and live donor biopsies. We will also evaluate TIGIT expression on T cells isolated from ischemic deceased donor kidney samples and transcriptional effects at single cell level in T cells from live donor kidney. Results from these studies will be the first to provide important information on TIGIT mediated effect in kidney T cell functions, therapeutic potential of targeting TIGIT for AKI treatment and set the stage for future pre-clinical and clinical studies.