Distinct roles for CD4 and CD8 as co-receptors in antigen receptor signalling.

The co-ordinated interactions of multiple membrane molecules with the T-cell receptor for antigen (the TCR-CD3) are prerequisite for T-cell activation. In this review we consider the involvement of CD4, CD8, and CD45 on the two lymphocyte lineages. Experiments from many laboratories have provided concordant results leading to the consensus that CD4 and CD8 are functional analogues, providing similar supplementary signals to those generated through the TCR-CD3 complex on MHC class-II- and MHC class-I-restricted T cells, respectively. However, recent results demonstrate striking differences in the coreceptor functions of CD4 and CD8. These differences reflect the distinct properties of the molecules themselves, which in turn are associated with CD45 involvement in the activation of CD4+ and CD8+ T cells.