CD5 is phosphorylated on tyrosine after stimulation of the T-cell antigen receptor complex.

Abstract

When T cells are activated by the T-cell antigen receptor, a number of cellular proteins are phosphorylated on tyrosine. We investigated whether any of these proteins were present on the surface of activated T cells. Using the human leukemic T-cell line Jurkat and normal peripheral blood lymphocytes, we identified a 67-kDa cell surface glycoprotein in anti-phosphotyrosine immunoprecipitates, after treatment of the cells with CD3 antibody. When cell lysates were depleted of CD5 by sequential immunoprecipitation, the 67-kDa phosphotyrosyl polypeptide was no longer precipitated by the phosphotyrosine antibody. Western blot analysis of anti-phosphotyrosine precipitates confirmed that this glycoprotein was CD5. It was possible that CD5 was present in the anti-phosphotyrosine immunoprecipitates due to its physical association with phosphotyrosyl proteins rather than being directly tyrosine-phosphorylated itself. However, Western blot analysis of anti-CD5 immunoprecipitates with phosphotyrosine antibody and phosphoamino acid analysis demonstrated that CD5 was indeed phosphorylated on tyrosine after stimulation of the cells with CD3 antibody and was concomitantly phosphorylated on serine and threonine. Tyrosine phosphorylation of CD5 was maximal 2 min after CD3 stimulation and returned to baseline levels by 60 min. CD5 is expressed on the cell surface of all mature T cells and a small proportion of B lymphocytes and has recently been identified as the ligand for CD72, a receptor present on the surface of all B cells. The present data suggest that tyrosine phosphorylation may be involved in B-cel-T-cell communication.Binding of antigenic peptide fragments, presented by major histocompatibility complex molecules, to the T-cell receptor (TCR)-CD3 complex results in the activation of a number of biochemical pathways, culminating in the expression of interleukin 2 and interleukin 2 receptors and subsequent T-cell proliferation (1,2). Early events in the signal transduction process include the stimulation of both serine and tyrosine kinases, followed by the phosphorylation of many intracellular proteins (3-5). Tyrosine kinase activation is very rapid, being observed 5 sec after engagement of the TCR-CD3 complex (5). Although as many as 20 polypeptides are phosphorylated on tyrosine 1 min after CD3 stimulation, very few of these tyrosine kinase substrates have been identified (6). These include CD3 Cchain (4), [a 70-kDa tyrosine phosphoprotein that is associated with ; chain of the TCR complex (7)], and phospholipase C (PLC)-,yl (8, 9). The tyrosine phosphorylation and activation of PLC-,yl result in inositol phospholipid hydrolysis leading to diacylglycerol generation and inositol phosphate release, which in turn stimulate protein kinase C and elevate intracellular Ca2+ (3,10,11). Activation of PLC can be blocked by genistein (12) and herbimycin A (13), two inhibitors of tyrosine kinases.Thus the tyrosine kinase pathway appears to be able to activate or regulate the PLC pathway.The importance of tyrosine phosphorylat...

Signaling through CD5 Activates a Pathway Involving Phosphatidylinositol 3-Kinase, Vav, and Rac1 in Human Mature T Lymphocytes

“…Once the TCR-CD3 complex is engaged by ligand, the cytoplasmic domain of CD5 becomes rapidly phosphorylated on tyrosine residues, as are the TCR chains (8,17,44). These tyrosines are present in a potential tyrosine kinase phosphorylation motif (Y-X 11 -Y-XX), which is similar to the immunoreceptor tyrosine activation motifs as found in the TCR and CD3 chains (3,44).…”

The T-cell antigen CD5 acts as a receptor and substrate for the protein-tyrosine kinase p56lck.

“…Taken It was next of interest to assess whether CD5 was itself phosphorylated in vivo by p56 ck. Previous work by ourselves (9) and others (14) has shown that CD5 is rapidly phosphorylated on tyrosine residues as a result of TcR;/CD3 ligation.…”

“…In this sense, CD5 shares a number of properties with TcR(. Both can be coprecipitated with TcR4/CD3 in gentle detergents and are rapidly phosphorylated on tyrosine residues in response to TcR(/CD3 ligation (9,14,45). Both TcR~and CD5 possess part of a potential tyrosine * Corresponding author.…”

Fine Tuning of TCR Signaling by CD5

“…The precise mechanism by which CD5 inhibits TCR signaling has not been elucidated. However, CD5 is tyrosine-phosphorylated after TCR engagement and associates with several effector molecules that could potentially influence the TCR signaling response, including Cbl, rasGAP, SHP-1, casein kinase 2, and TCR-/ZAP-70 (11)(12)(13)(14)(15).…”