Replacement of Pre-T Cell Receptor Signaling Functions by the CD4 Coreceptor

An important checkpoint in early thymocyte development ensures that only thymocytes with an in-frame T cell receptor for antigen β (TCR-β) gene rearrangement will continue to mature. Proper assembly of the TCR-β chain into the pre-TCR complex delivers signals through the src-family protein tyrosine kinase p56(lck) that stimulate thymocyte proliferation and differentiation to the CD4(+)CD8(+) stage. However, the biochemical mechanisms governing p56(lck) activation remain poorly understood. In more mature thymocytes, p56(lck) is associated with the cytoplasmic domain of the TCR coreceptors CD4 and CD8, and cross-linking of CD4 leads to p56(lck) activation. To study the effect of synchronously inducing p56(lck) activation in immature CD4(−)CD8(−) thymocytes, we generated mice expressing a CD4 transgene in Rag2(−/−) thymocytes. Remarkably, without further experimental manipulation, the CD4 transgene drives maturation of Rag2(−/−) thymocytes in vivo. We show that this process is dependent upon the ability of the CD4 transgene to bind Lck and on the expression of MHC class II molecules. Together these results indicate that binding of MHC class II molecules to CD4 can deliver a biologically relevant, Lck-dependent activation signal to thymocytes in the absence of the TCR-α or -β chain.