Association of CD8 with p56lck is required for early T cell signalling events

The human CD8 glycoprotein functions as a co-receptor during T cell activation by both binding to MHC class I and transducing a transmembrane signal. The ability of CD8 to transduce a signal is mediated in part by its association with the protein tyrosine kinase p56lck. Using a panel of human CD8 alpha mutants, we demonstrated that the presence of a functional p56lck binding site is required for the early signalling events transduced by CD8, including increased [Ca2+]i and protein tyrosine phosphorylation. In addition, our results demonstrate that wild-type and all mutant forms of CD8 alpha have an inhibitory effect on signal transduction after CD3-CD3 or CD3-CD4 crosslinking when transfected into the (CD3+, CD4+, CD8-) H9 T cell line, suggesting that intermolecular associations of CD8, independent of its association with p56lck, are responsible for this effect. Signalling through CD4 or CD8 in a double positive thymocyte may therefore be different than in a single positive thymocyte or mature T cell.