Do CD4 and CD8 control T-cell activation via a specific tyrosine protein kinase?

The CD4 and CD8 glycoproteins play an important role in T-cell activation by binding to major histocompatibility complex (MHC) class II or class I molecules, respectively, and stabilizing their interactions with the T-cell receptor-CD3 complex during antigen presentation. Recent evidence suggesting that the cytoplasmic domains of CD4 and CD8 are physically, and perhaps functionally, linked to the T-cell specific tyrosine protein kinase, p56lck, adds a new dimension to our current understanding of their physiological function. Based on these and other recent findings, Tomas Mustelin and Amnon Altman present a working hypothesis that defines a novel role for CD4 or CD8 in regulating T-cell activation, and perhaps other processes, such as thymic repertoire selection and human immunodeficiency virus (HIV)-induced immunosuppression.