H2-O deficiency promotes regulatory T cell differentiation and CD4 hyperactivity.

Regulatory T cells (Treg) are crucial immune modulators, yet the exact mechanism of thymic Treg development remains controversial. Here, we present the first direct evidence for H-2O, a peptide editing molecular chaperon for MHC class II antigen processing, on selection of thymic Tregs. We demonstrate that lack of H2-O in the thymic medulla promotes thymic Treg development and leads to an increased peripheral Treg frequency. Single-cell RNA-sequencing (scRNA-seq) analysis of splenic CD4 T cells revealed an enrichment of both effector-like Tregs and activated CD4 T cells in the absence of H2-O. H2-O has been shown to increase the presentation of high-affinity class II epitopes, thus leading to proper negative selection in the medullary thymus. In spite of this notion, our data support two concepts; a) lack of expression of H2-O in thymic medulla creates an environment permissive to Treg development and, b) that loss of H2-O drives increased basal auto-stimulation of CD4 T cells. These findings can help in better understanding of autoimmunity and design of therapeutics for treatment of autoimmune diseases.