The molecular basis underlying T cell cross-reactivity between major histocompatibility complex class I and class II

© 2012 Dr. Sidonia Barbara Guiomar EckleIn the adaptive immune response αβ T cell receptors (TCRs) expressed by T cells recognise foreign peptide antigen in the context of major histocompatibility complex (MHC) molecules displayed by antigen presenting cells. The interaction between TCRs and peptide-MHC (pMHC) molecules involves exquisite fine specificity. There are two classes of MHC, MHC-I and MHC-II, which are homologous but differ in sequence and structure and are differentially recognised by CD8+ cytotoxic and CD4+ helper T cells, respectively. However, there are only minor differences in the TCR usage of cytotoxic versus helper T cells. While T cells are restricted for self-MHC molecules, upon transplantation of MHC mismatched stem cells, tissues or organs, T cells frequently recognise allogeneic MHC molecules, a phenomenon referred to as alloreactivity. The molecular basis of T cell alloreativity has been a longstanding problem in immunology and thought to be explained by the hardwiring of TCRs for MHC recognition. This concept is a controversial explanation for the basis of TCR restriction towards self-MHC allotype and MHC-class specificity. One hallmark of T cell alloreactivity is the high frequency with which microbial peptide specific T cells alloreact on allo-MHC molecules. Alloreactivity is generally restricted to MHC class, although recently rare examples of alloreactive T cells violating MHC class restriction have been described. Given the differences of the two MHC classes, as well as the general restriction of T cells for one or other MHC class, it is of interest to understand how a single TCR can recognise both, MHC-I and MHC-II. In addition studying dual MHC class reactivity mediated by a single TCR provides insight into any hardwiring of TCRs for the two classes of MHC. Previously, the molecular basis of how a TCR reacts with both MHC-I and MHC-II has been published for a TCR that arose in a mouse under severely limited negative selection conditions, imposing degenerate recognition of MHC and peptide [1]. By comparison, the work presented in this thesis represents the first of its kind to evaluate the molecular basis of MHC class cross-reactivity mediated by a conventionally selected human T cell clone (2G8), which is restricted for a human cytomegalovirus derived epitope in the context of MHC-I and specifically recognises both pMHC-I and pMHC-II [2]. The dual MHC class reactivity was first confirmed to be mediated by a single TCR in stimulation assays using T cell lines transduced with the 2G8 TCR. A detailed characterisation of the 2G8 TCR-pMHC-I interaction that involved determining the crystal structure of the ternary complex combined with comprehensive TCR and peptide mutagenesis studies, revealed a conventional TCR recognition of pMHC-I. Whilst the MHC-II restricted allopeptide has not been identified yet, TCR mutagenesis studies, as well as a comparison of the crystal structure of the binary pMHC-I complex with that of a published pMHC-II complex, strongly suggested a disparate docking mode of recognition of the pMHC-I and pMHC-II complexes. In addition, TCR plasticity in the CDR3 loops was identified as one of the molecular mechanisms underlying the 2G8 TCR mediated MHC class cross-reactivity