Stimulation of CD5 enhances signal transduction by the T cell antigen receptor.

After the addition of a CD3 monoclonal antibody to peripheral T cells that have been previously stimulated with phytohemagglutinin, inositol phosphates are produced at a rapid rate for 2 min and at a much slower rate thereafter. Stimulation of CD5 allows CD3-mediated production of inositol phosphates to be sustained at a brisk rate for greater than 20 min and augments the initial CD3-mediated increase in inositol trisphosphate and release of intracellular Ca2+. Thus, perturbation of CD5 by monoclonal antibody enhances the ability of the CD3-antigen receptor complex to couple to the inositol phospholipid pathway. This effect of CD5 is independent of any direct effect of the CD5 monoclonal antibody on the levels of inositol phosphates.