Functional evidence for TCR scanning of MHC.

Abstract

Abstract αβ T cells recognize antigenic peptides within their positively selecting self-MHCI or MHCII. While MHC-restriction is well established, the nature of the TCR-MHC interactions that mediate this phenomenon are intensely debated. One possibility is that clonotypic TCRs are positively selected based on their ability to recognize self-peptides within self-MHC (pMHC) in a lock-and-key fashion. Such TCRs would then be limited to scanning for antigenic peptides among those shapes that resemble the selecting pMHC. Alternatively, every TCR may poses germline-encoded specificity for most if not all allelic variants of both MHCI and MHCII. This would allow most TCRs to scan the contents of most MHC for peptides that could mediate positive selection or T cell activation depending on the contributing co-receptor (i.e. CD4 or CD8). Here we asked if increasing the frequency of Lck-associated CD4 molecules in T cell hybridomas would allow for the detection of sub-threshold TCR-MHC interactions that constitute scanning. The reactivity of ten distinct TCRs was assessed in response to selecting and non-selecting MHCII bearing “shaved” peptides with alanine substitutions at TCR contact residues: three of the TCRs were selected on MHCII, two were selected on MHCI, and five were generated in vitro without defined selecting MHC. IL-2 production was observed when each TCR interacted with selecting or non-selecting MHCII presenting shaved peptides. These data provide evidence of TCR-MHC interactions that are independent of the selecting MHC class, allele, or the sequence of the peptide presented by the MHC. We interpret these data as functional evidence for TCR-intrinsic specificity for MHCII.