On integrity in immunity during ontogeny or how thymic regulatory T cells work

Abstract

The Standard model of T cell recognition asserts that T cell receptor (TCR) specificities are positively and negatively selected during ontogeny in the thymus and that peripheral T cell repertoire has mild self‐major histocompatibility complex (MHC) reactivity, known as MHC restriction of foreign antigen. Thus, the TCR must bind both a restrictive molecule (MHC allele) and a peptide reclining in its groove (pMHC ligand) in order to transmit signal into a T cell. The Standard and Cohn's Tritope models suggest contradictory roles for complementarity‐determining regions (CDRs) of the TCRs. Here, I discuss both concepts and propose a different solution to ontogenetic mechanism for TCR‐MHC–conserved interaction. I suggest that double (CD4+CD8+)‐positive (DP) developing thymocytes compete with their αβTCRs for binding to self‐pMHC on cortical thymic epithelial cells (cTECs) that present a selected set of tissue‐restricted antigens. The competition between DPs involves TCR editing and secondary rearrangements, similar to germinal‐centre B cell somatic hypermutation. These processes would generate cells with higher TCR affinity for self‐pMHC, facilitating sufficiently long binding to cTECs to become thymic T regulatory cells (tTregs). Furthermore, CD4+ Foxp3+ tTregs can be generated by mTECs via Aire‐dependent and Aire‐independent pathways, and additionally on thymic bone marrow–derived APCs including thymic Aire‐expressing B cells. Thymic Tregs differ from the induced peripheral Tregs, which comprise the negative feedback loop to restrain immune responses. The implication of thymocytes’ competition for the highest binding to self‐pMHC is the co‐evolution of species‐specific αβTCR V regions with MHC alleles.