HLA-DO promotes Treg differentiation by altering antigen presentation

Abstract

Abstract HLA-DO in human (H2-O in mice) (DO) is a highly conserved non-classical MHC class II accessory molecule. Unlike the main Class II peptide editor HLA-DM, expression of DO is restricted to the thymic medulla, B cells and certain dendritic cell subsets. DO forms a stable complex with HLA-DM and requires DM for trafficking to MIIC. Using biochemical experiments, our lab has previously discovered that DO has differential effects on editing peptides of different sequences. DO enhances binding of DM-resistant peptides, and reduces the binding of DM-sensitive peptides to the HLA-DR1 molecules. Changes in the quantities of either DM-resistant of DM-sensitive peptides could therefore lead to alterations in both thymic negative selection, and peripheral activation of CD4 T cells. Using single-cell RNA-sequencing (scRNA-seq) of peripheral CD4 T cells from unimmunized DO knock-out (KO) mice we found an increased frequency of “activated” CD4 T cells, in support of our biochemical model. This increased frequency corresponds with a decrease in “naïve” CD4 T cells in both scRNA-seq and FACs analysis. Interestingly, scRNA-seq analysis confirmed our previous findings of elevated regulatory T cells (Treg) levels in DO-KO mice. Genetic analysis showed that unimmunized DO-KO Tregs also have a more activated phenotype. These findings suggests that absence of DO generates a more stimulatory in vivo cellular environment for CD4 T cells, likely due to altered antigen presentation. Research supported by grants from NIH (R01 AI130210, R01 AI121174 and R37 AI060040)