Programmed death-1 regulates islet-specific lymphocytes in type 1 diabetes

Abstract

Programmed death-1 (PD-1) is a T cell inhibitory receptor important for tolerance maintenance. PD-1 is highly expressed on chronically stimulated T cells, such as those specific for persistent viral or tumor antigens. PD-1 pathway blockade revolutionized cancer therapy in recent years. While response rates are higher than with chemotherapy, not all patients respond, and some develop autoimmune-like symptoms, or even overt autoimmunity. Herein, I sought to understand how the PD-1 pathway regulated islet-specific CD4+ T cells during type 1 diabetes (T1D) progression in non-obese diabetic (NOD) mice. Since insulin itself is one of the main antigens driving T1D, we developed insulin peptide:MHCII tetramer reagents to track insulin-reactive CD4+ T cells. Insulin-specific CD4+ T cells that expressed the most PD-1 also had the highest affinity for self, suggesting that PD-1 preferentially regulated those cells with the highest autoimmune potential. In NOD mice, the majority of insulin-specific CD4+ T cells had an anergic (tolerant) phenotype, but surprisingly, PD-1 blockade did not override the anergy program. These findings suggested that the differentiation state of the CD4+ T cell pre-determine its susceptibility to PD-1 blockade. Autoantibody production is a hallmark of autoimmunity, and has also been reported in patients treated with PD-1 blockade, suggesting that PD-1 might regulate this process. Autoantibody production results from B cell:CD4+ T cell interactions in the germinal center of the lymph node. The dynamics and regulation of the germinal center in spontaneous autoimmunity and after PD-1 blockade are not well understood, primarily due to an inability to track self-specific lymphocytes. To bridge this knowledge gap, we used tetramers to phenotype islet-specific CD4+ T cells and B cells in mice. PD-1- or PD-L1-deficient mice, as well as NOD mice treated with anti-PD-1, had increased insulin autoantibodies, as well as increased insulin-specific T follicular helper CD4+ T cells and germinal center B cells compared to controls. This increase was dependent on CD4+ T cell-intrinsic PD-1 signaling and relied on peptide:MHCII recognition. Taken together, my thesis work provides a mechanistic explanation for autoantibody onset following PD-1 blockade in the clinic, and has important implications for cancer immunotherapy and autoimmunity.