CD4, CD8 and CD45 regulate the coupling of the T-cell receptor complex (CD3-TCR) to tyrosine kinase activation and phosphorylation of key substrates such as phospholipase C gamma.CD3 (cluster of differentiation 3) is a protein complex and T cell co-receptor that is involved in activating both the cytotoxic T cell (CD8+ naive T cells) and T helper cells (CD4+ naive T cells).