Project Summary Cancer immunotherapy has become a standard therapy for many cancers. However, it’s associated with acute kidney injury, resulting in significantly increased mortality. Acute interstitial nephritis is the most common acute kidney injury, affecting 2-3% of the patients receiving immune checkpoint inhibitors (ICIs). In addition, 40% of the kidney transplant patients receiving ICIs suffer from acute rejection, and once rejection occurs, 65% lose allograft and require renal replacement therapy. Thus, understanding the mechanisms of ICI-associated acute kidney injury and finding therapies is critical. My K08 project aims to understand the roles of immune checkpoint molecules (PD-1 and CTLA-4) in kidney inflammation, by using the novel animal models that express neoantigen peptides specifically in kidney proximal tubules and tracking antigen-specific T cell response in the animals by tetramer staining technique. The results showed that the presence of antigen- specific T cells and ICIs trigger the immune cell infiltration to the kidneys, mimicking acute interstitial nephritis seen in the cancer patients treated with ICIs. While animal models are ideal to address the precise molecular mechanisms of the disease, there’s a limitation in the applicability of the findings to the human disease. The largest multicenter clinical study that I led recently found that the ICI-associated kidney injury occurs much faster and more robust in kidney transplant recipients compared to non-kidney transplant patients. The findings led to my central hypothesis that pre-existing donor antigen-specific T cells in the kidney transplant recipients are quickly activated, proliferated in the presence of ICIs, and cause direct kidney injury. In this pilot study, I propose to perform high dimensional analyses using human tissue samples to track antigen-specific T cells by single cell RNA sequencing and T cell receptor (TCR) repertoire analysis. We will analyze the TCR clonotype and phenotype of antigen-specific T cells, using the archived peripheral blood mononuclear cells, tumor and kidney biopsy samples, obtained from the patients who had acute graft rejection after cancer ICI therapy, by collaborating with Center of Immuno-Oncology at Dana Faber Cancer Institute and single cell genomics core at Brigham and Women’s Hospital. This pilot project tests the feasibility of the high dimensional analysis of the patient samples for a future prospective clinical trial in the patients with ICI-associated kidney injury. If feasible, we will incorporate these analyses in the prospective clinical trial, which will eventually help understand the mechanism of acute interstitial nephritis and acute graft rejection in the patients treated with cancer immunotherapy.