αβT Cell Receptors Expressed by CD4−CD8αβ− Intraepithelial T Cells Drive Their Fate into a Unique Lineage with Unusual MHC Reactivities

Abstract

Summary Coreceptor CD4 and CD8αβ double negative (DN) TCRαβ+ intraepithelial T cells, although numerous, have been greatly overlooked and their contribution to the immune response is not known. Here we used T cell receptor (TCR) sequencing of single cells combined with retrogenic expression of TCRs, to study the fate and the major histocompatibility complex (MHC) restriction of DN TCRαβ+ intraepithelial T cells. The data show that commitment of thymic precursors to the DN TCRαβ+ lineage is imprinted by their TCR specificity. Moreover, the TCRs they express display a diverse and unusual pattern of MHC restriction that is non-overlapping with that of CD4+ or CD8αβ+ T cells, indicating that they sense antigens that are not recognized by the conventional T cell subsets. The new insights indicate that DN TCRαβ+ T cells form a third lineage of TCRαβ T lymphocytes expressing a variable TCR repertoire, which serve non-redundant immune functions.

Intraepithelial TCRαβ T Cells in Health and Disease

“…A major subset is double negative (DN) cells, lacking expression of either CD4 or CD8αβ coreceptor, which acquire their "memory" phenotype during a self-antigen-based selection process (agonist selection) in the thymus (Leishman et al, 2002;Gangadharan et al, 2006;Mayans et al, 2014;McDonald et al, 2014). The typical, MHC class I-restricted CD8αβ-expressing and MHC class II-restricted CD4-expressing Epi-T cells, on the other hand, acquired their activated memory phenotype post-thymically, in response to antigens encountered in the periphery.…”

“…In contrast to conventional selected immature thymocytes, precursor cells of DN TCRαβ Epi-T cells go through a self (agonist)-antigen-based differentiation/selection process in the thymus that instead of deleting these self-reactive progenitor cells, results in their functional education and homing capacity to directly migrate to the epithelium of the intestine (Leishman et al, 2002;Gangadharan et al, 2006;Denning et al, 2007;Mayans et al, 2014;McDonald et al, 2014). Consequently, many DN TCRαβ Epi-T cells express the so-called "forbidden" TCRs that are purged from the TCR repertoire of mainstream T cells (von Boehmer et al, 1991).…”

“…In addition to their functional phenotype, the self-antigeninduced DN TCRαβ precursor cells also acquire gut homing receptors in the thymus, including α E b7 and CCR9 (Gangadharan et al, 2006;Yamagata et al, 2004Yamagata et al, , 2006Mayans et al, 2014;McDonald et al, 2014). The specific expression of their ligands, E-cadherin and CCL25 respectively, by the small intestine IECs recruits the precursors directly to the intestine epithelium.…”

“…In the case of the non-self antigen-driven CD4 and CD8αβ T cell subsets, which increase with age, the specificity and diversity of the (oligoclonal) TCR repertoire is likely a direct reflection of the immediate environmental antigen-platform, whereas the TCR repertoire of the DN subset, although also oligoclonal, is mainly driven by self-antigens encountered during thymic selection and, therefore, shows very little overlap with that of the CD4 and CD8αβ Epi-T cells (Mayans et al, 2014). Furthermore, some Vβ segments, which are deleted from the TCRαβ repertoire of the conventional T cells, are highly enriched in the repertoire of the DN TCRαβ T cells, including the so-called "forbidden Vβ segments" (Rocha et al, 1992).…”

“…The development of these cells is not impaired in CD1d knock-out (KO), TL KO, or K b D b KO mice, but they are drastically reduced in β2-microglobulin (β2m)-deficient mice and present in mice lacking transporter associated with antigen-processing (TAP) (Sydora et al, 1996;Fujiura et al, 1996;Park et al, 1999;Mayans et al, 2014;McDonald et al, 2014). These observations indicate that the self-antigeninduced DN TCRαβ Epi-T cells are restricted to an as of yet unidentified β2m-dependent nonclassical MHC molecule, or else they show specificity to variable nonclassical and TAP-independent MHC I molecules (Mayans et al, 2014).…”