The ability to rearrange dual TCRs enhances positive selection, leading to increased allo- and autoreactive T cell repertoires

Thymic selection is designed to ensure TCR reactivity to foreign Ags presented by self-MHC while minimizing reactivity to self-Ags. We hypothesized that the repertoire of T cells with unwanted specificities such as alloreactivity or autoreactivity are a consequence of simultaneous rearrangement of both TCRa loci. We hypothesized that this process helps maximize production of thymocytes capable of successfully completing thymic selection, but results in secondary TCRs that escape stringent selection. In T cells expressing two TCRs, one TCR can mediate positive selection and mask secondary TCR from negative selection. Examination of mice heterozygous for TRAC (TCRa ^+/-), capable of only one functional TCRa rearrangement, demonstrated a defect in generating mature T cells attributable to decreased positive selection. Elimination of secondary TCRs did not broadly alter the peripheral T cell compartment, though deep sequencing of TCRa repertoires of dual TCR T cells and TCRa^+/- T cells demonstrated unique TCRs in the presence of secondary rearrangements. The functional impact of secondary TCRs on the naive peripheral repertoire was evidenced by reduced frequencies of T cells responding to autoantigen and alloantigen peptide- MHC tetramers in TCRa^+/- mice. T cell populations with secondary TCRs had significantly increased ability to respond to altered peptide ligands related to their allogeneic ligand as compared with TCRa^+/- cells, suggesting increased breadth in peptide recognition may be a mechanism for their reactivity. Our results imply that the role of secondary TCRs in forming the T cell repertoire is perhaps more significant than what has been assumed.