LCK-co-receptor association ensures T cell lineage fidelity and maximizes epitope-specific TCR diversity.

The interaction between the CD4/CD8 co-receptors and LCK (an Src family tyrosine kinase) is thought to augment T cell activation upon recognition of peptide-loaded major histocompatibility complexes (pMHCs). How this interaction influences antigen-specific T cell development is unclear however, as is its impact on naïve and immune antigen-specific T cell repertoires. In mice expressing mutated endogenous LCK unable to bind co-receptors (LCK mice), we show that influenza A virus (IAV)-derived pMHC-specific CD8 and CD4 T cell responses had a significantly narrowed T cell receptor (TCR) repertoire, favoring high-affinity TCRs. This narrowing was established during T cell development and was exacerbated after viral infection. The dissociation of LCK from co-receptors also resulted in the redirection of CD4-fated T cells to the CD8 lineage, with expanded pMHCII-specific cytotoxic CD8 T cells observed after IAV infection. Thus, LCK-co-receptor association is critical for ensuring T cell lineage fidelity and maximizing antigen-specific T cell repertoire diversity.