A role for the nonclassical MHC molecule DO in Treg selection and function

Abstract

Abstract Selection of CD4 T cells into the conventional and regulatory lineages is thought to be dependent on the strength of interaction of rearranged T cell receptors with MHCII-bound peptides displayed on APCs in the thymus. The nonclassical MHC molecule DO inhibits DM-mediated peptide exchange and has been shown to play a role in selection of peptides for presentation to CD4 T cells. In mice deficient in DO (H2-O), alterations in T cell responses and in antigen presentation as well as increased autoimmunity have been described. We show that DO−/− mice have increased numbers of thymic and peripheral Tregs, which also display a more activated phenotype. These DO−/− Tregs additionally exert greater suppressive capacity in vivo as compared to WT Tregs, as demonstrated by reduced T cell activation following Treg transfer into Treg-deficient mice. T cell receptor Vβ chain sequencing indicates the Treg population is less diverse and the conventional CD4 T cell population more diverse in DO−/− mice, suggesting skewing of thymic selection as a result of DO expression. To understand the mechanism behind altered T cell selection due to DO, we eluted peptides from thymic and B cell-derived I-Ab and characterized them by tandem mass spectrometry. Peptide diversity was increased in both cell types isolated from WT as compared to DO−/− mice. We speculate that through control of peptide diversity, DO tunes the number and specificity of thymocytes selected into the Treg lineage. These results highlight a previously unappreciated role for DO in controlling Treg selection and function.