Abstract The CD4 and CD8 coreceptors of T cells are essential for TCR signalling. The current knowledge describes that the coreceptors serve to deliver the tyrosine kinase LCK to the CD3 complex to phosphorylate immunoreceptor tyrosine activation motifs (ITAMs) to initiate TCR signalling. Recently, CD8 was shown to prevent signalling of reverse docking TCRs by sequestering LCK away from CD3, thus restricting TCR docking orientation to peptide MHC complexes and explaining the inability of these unconventional TCRs to signal during immune responses. However, how the association of LCK with the coreceptor shapes antigen-specific TCR repertoires and affects T cell activation in vivo has not been demonstrated. We utilise LCK freemice (in which LCK cannot bind the CD4 or CD8 coreceptors) to study the role of coreceptor-associated LCK in TCR signalling. We found that the LCK freemutation specifically reduced CD4 T cell development while not affecting CD8 T cell output. After influenza A infection, epitope specific CD8 T cell reponses in LCK freemice are numerically and functionally intact, whereas CD4 T cell responses are substantially reduced. Single-cell RNA sequencing data showed minor gene expression differences between WT and LCK freeactivated CD8 and CD4 T cells. Contrary to our hypothesis, epitope-specific TCR analysis revealed a decreased TCR diversity in CD8 TCR repertoires in LCK freemice, biased towards the dominant clones. In contrast, CD4 repertoire show distinct TCR usage compared to WT. In summary, we found that coreceptor: LCK association is not necessary for the generation of functional MHC restricted TCR reperotires, but it is important for the development, functions, and TCR repertoire of CD4 T cells. Supported by grants from ARC (M07001-3237405)