The Ability To Rearrange Dual TCRs Enhances Positive Selection, Leading to Increased Allo- and Autoreactive T Cell Repertoires

Abstract

Thymic selection is designed to ensure T cell receptor (TCR) reactivity to foreign antigens presented by self-MHC while minimizing reactivity to self-antigens. We hypothesized that the repertoire of T cells with unwanted specificities such as alloreactivity or autoreactivity are a consequence of simultaneous rearrangement of both TCRα 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 2 TCRs, 1 TCR can mediate positive selection and mask secondary TCR from negative selection. Examination of mice heterozygous for TRAC (TCRα+/−), capable of only one functional TCRα 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 TCRα repertoires of dual TCR T cells and TCRα+/− 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 pMHC tetramers in TCRα+/− mice. T cell populations with secondary TCRs had significantly increased ability to respond to altered peptide ligands related to their allogeneic ligand as compared to TCRα+/− 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.

Proinflammatory Dual Receptor T Cells in Chronic Graft-versus-Host Disease

“…We have previously used deep sequencing of mouse TCRα repertoires to demonstrate that dual receptor T cells contain a unique repertoire of TCRs that are not present among T cells with a single receptor [23]. The TCR repertoire of dual receptor cells from patients with cGVHD demonstrated broad use of TRBV and TRAV segments with clonal distribution frequencies comparable to dual TCR cells from healthy donors (Figure 5B).…”

“…Dual TCR cells are generated in normal physiology from simultaneous rearrangement of both TCRα loci in CD4 + CD8 + double positive thymocytes as a mechanism to maximize positive selection efficiency [23]. However, promotion of positive selection comes with the cost of reduced stringency of thymic selection requirements for dual receptor thymocytes; only 1 of the TCRs on a dual receptor cell is required to mediate positive selection [24,25] and expression of a second receptor can mask TCRs that would otherwise be eliminated by negative selection [26–28].…”

“…However, promotion of positive selection comes with the cost of reduced stringency of thymic selection requirements for dual receptor thymocytes; only 1 of the TCRs on a dual receptor cell is required to mediate positive selection [24,25] and expression of a second receptor can mask TCRs that would otherwise be eliminated by negative selection [26–28]. This results in emergence of populations of dual TCR cells containing unique TCRα clonotypes that would not be present under more stringent selection [23]. The presence of these unique TCRs does not appear to affect responses to foreign antigens but is associated with increased frequency of cells reactive against alloantigens and autoantigens [23,29].…”

“…This results in emergence of populations of dual TCR cells containing unique TCRα clonotypes that would not be present under more stringent selection [23]. The presence of these unique TCRs does not appear to affect responses to foreign antigens but is associated with increased frequency of cells reactive against alloantigens and autoantigens [23,29]. These reactivities underlie the potential for dual TCR T cells to initiate pathologic alloreactive and autoimmune responses observed in patients and animal models [25,28,30,31].…”

Generation of Human Alloantigen-specific T Cells from Peripheral Blood

“…This technique has become a powerful tool for examining antigen-specific T cell responses 12 . Our increasing understanding of the highly peptide-specific nature of T cell recognition of allogeneic pMHC complexes 7,8 has enabled us to adapt this approach to the study of alloreactive T cells in both mice 14 and humans 15 . This approach uniquely enables examination of the composition of the naive T cell populations that comprise the initial oligoclonal alloreactive response.…”