Dual T cell receptor expression promotes agonist selection of regulatory T cells

Abstract

Abstract Generation of CD4+FoxP3+ regulatory T cells (Treg) in the thymus is essential for maintenance of self-tolerance. Treg lineage commitment occurs via agonist selection, via TCR high-affinity interaction with self-peptide-MHC (pMHC) ligand. We have previously demonstrated that natural co-expression of dual TCRs, occurring in ~10% of T cells, results in increased ability to recognize self-pMHC ligands and promote positive selection. We hypothesized that dual TCR expression could similarly promote agonist Treg selection. Examination of Treg development in B6.TCRa+/− mice, heterozygous for disruptive mutation of TCRAC and unable to express dual TCRs, demonstrated decreased frequency of Tregs in the thymus (2.76+0.08% of CD4 single-positive thymocytes) compared to age-matched wild-type B6 mice (3.26+0.09%, P < 0.001). To more precisely compare Treg agonist selection, we co-transferred 1:1 ratios of T cell-depleted B6.TCRa+/−.Thy1.1 and B6.Ly5.1 bone marrow to lethally-irradiated B6 recipients and monitored thymic selection. TCRa+/− cells were deficient in Treg selection compared to co-transferred wild-type cells; TCRa+/− cells produced 37.57+4.16% of donor-derived thymic Tregs (P = 0.003). This defect was specific for Treg selection, as TCRa+/− cells comprised 44.68+6.33% of double-positive thymocytes and 45.50+5.77% of CD4 single-positive thymocytes. Tregs generated from TCRa+/− or wild-type cells were phenotypically similar, with no differences in expression of FoxP3 or CD5. We propose that the difference in agonist selection of dual TCR and single TCR Tregs could have important consequences in the TCR repertoire and antigenic specificity of peripheral Tregs, impacting immunologic tolerance.