MHC‐restricted T cell receptor signaling is required for αβ TCR replacement of the pre T cell receptor

Abstract

A developmental block is imposed on CD25 + CD44 -thymocytes at the b-selection checkpoint in the absence of the pre T cell receptor (preTCR) a-chain, pTa. Early surface expression of a transgenic ab TCR has been shown to partially circumvent this block, such that thymocytes progress to the CD4 + CD8 + double-positive stage. We wanted to analyze whether a restricting MHC element is required for ab TCR-expressing doublenegative (DN) thymocytes to overcome the developmental block in pTa-deficient animals. We used the HY-I knock-in model that endows thymocytes with ab TCR expression in the DN compartment but has the advantage of physiological expression levels, in contrast to conventional TCR transgenes. On a pTa-deficient background, this HY-I TCR transgene 'rescued' CD25 + CD44 -thymocytes from apoptosis and enabled progression to later differentiation stages. On a non-selecting MHC background, however, pTa-deficient HY-I mice presented a pronounced reduction in numbers of splenocytes and thymocytes when compared to animals of selecting MHC genotype, showing that MHC restriction is necessary to drive HY-TCR-mediated rescue of pTa-deficient thymocytes.

The thymus chapter in the life of gut-specific intra epithelial lymphocytes

“…Although CD4 À CD8ab À TCRab + IEL display phenotypic and functional similarities with the gd subset, this is likely not because of a shared lineage commitment but rather it coincides with the full TCRbased agonist selection process in the thymus. Some DN thymocytes can commit to the gd-lineage upon expression/ ligation of a full abTCR, as is the case in male H-YTCR transgenic mice [40,56], however the nearly absence of CD4 À CD8ab À TCRab + IEL in pre-Tcr À/À animals [57,58], makes it unlikely that this pre-TCR independent pathway is contributing significantly to the CD4 À CD8ab À TCRab + IEL sublineage in normal mice. Consistent with this, H-YTCR transgenic mice in which the transgenic TCR was driven by the CD4 promoter, do not generate DN H-YTCRab + thymocytes and male mice do not generate significant numbers of CD4 À CD8ab À H-YTCR + IEL [59 ].…”

Bacterial clearance reverses a skewed T‐cell repertoire induced by Salmonella infection

“…Under homeostatic conditions, newly arrived progenitors, termed double‐negative (DN, CD4 − CD8 − ) thymocytes, localize to the cortex region of the thymus to initiate sequential stages of maturation toward a specific T‐cell phenotype. The selection of the beta chain, which forms the pre‐T‐cell receptor (pre‐TCR) by combining with the pTα, is the first checkpoint that inhibits the generation of inappropriately rearranged TCRs . These receptors are subsequently submitted to positive and negative selection processes in the recognition of endogenous specific antigens presented by antigen‐presenting cells (APCs) .…”

Negative selection, not receptor editing, is a physiological response of autoreactive thymocytes

“…These secondary rearrangements, also observed in another Tcra knock-in system (Wang et al, 1998), are thought to be a mechanism that enhances the generation of TCRs suitable for positive selection in WT mice. Unexpectedly, however, the knock-in TCR allele constructed by Buch et al (2002) was expressed prematurely, in CD4/CD8 double-negative 3 (DN3) thymocytes (Croxford et al, 2008), whereas under physiological conditions Tcra is rearranged only at the CD4/CD8 double positive (DP) stage. Thus expression of the TCR in this system could occur too early, at a point in time when editing may not yet be possible.…”