Clonally-Expanded, Thyrotoxic Autoimmune Mediator CD8 +T cells Driven by IL21 Contribute to Checkpoint Inhibitor Thyroiditis

Abstract

Abstract Autoimmune toxicity occurs in up to 60% of patients treated with immune checkpoint inhibitor (ICI) cancer therapy and is an increasing clinical challenge with the expanding use of these treatments. Previous human studies into the cause of immune related adverse events (IRAEs) have often evaluated only circulating immune cells rather than affected tissues. Here, we directly sampled thyroid specimens from subjects with ICI-thyroiditis, one of the most common IRAEs, to determine driving mechanisms. We compared immune infiltrates in ICI-thyroiditis to those in subjects with spontaneous autoimmune Hashimoto’s thyroiditis (HT) or no thyroid disease. Single cell RNA sequencing revealed a dominant, clonally expanded population of thyroid-infiltrating cytotoxic CXCR6 +CD8 +T cells (“CD8 +autoimmune mediators”) present in ICI-thyroiditis, but not HT or healthy subjects. Furthermore, we identified a crucial role for interleukin 21, a cytokine secreted by intrathyroidal T follicular (Tfh) and T peripheral helper (Tph) cells, as a driver of these thyrotoxic CD8 +autoimmune mediators. In the presence of IL21, human CD8 +T cells acquired the autoimmune mediator phenotype with upregulation of cytotoxic molecules (IFNg, granzyme); the chemokine receptor CXCR6; and thyrotoxic capacity. We validated these findings in vivo using a novel mouse model of IRAEs, and further demonstrated that genetic inhibition of IL21 signaling protected ICI-treated mice from thyroid autoimmunity. Taken together, our data reveal novel mechanisms by which IL21 +Tfh/Tph cells drive thyrotoxic CD8 +autoimmune mediators for the development of IRAEs in humans. Furthermore, these studies highlight IL21 as a potential therapeutic target in patients to prevent IRAEs. Supported by grants from the NIH NIDDK (K08 DK129829), the American Thyroid Association, and the Aramont Charitable Foundation.