SIGNALING THROUGH CD5 ACTIVATES A PATHWAY INVOLVING PHOSPHATIDYLINOSITOL 3-KINASE, VAV, AND RAC1 IN HUMAN MATURE T-LYMPHOCYTES

CD5 acts as a coreceptor on T lymphocytes and plays an important role in T-cell signaling and T-cell-B-cell interactions. Costimulation of T lymphocytes with anti-CD5 antibodies results in an increase of the in tracellular Ca2+ levels, and subsequently in the activation of Ca2+/ca lmodulin-dependent (CaM) kinase type IV. In the present study, we have characterized the initial signaling pathway induced by anti-CD5 costi mulation. The activation of phosphatidylinositol (PI) 3-kinase through tyrosine phosphorylation of its p85 subunit is a proximal event in th e CD5-signaling pathway and leads to the activation of the lipid kinas e activity of the p110 subunit. The PI 3-kinase inhibitors wortmannin and LY294002 inhibit the CDS-induced response as assessed in interleuk in-2 (IL-2) secretion experiments. The expression of an inactivated; R ac1 mutant (Rac1 . N17) in T lymphocytes transfected with an IL-2 prom oter-driven reporter construct also abrogates the response to CD5 cost imulation, while the expression of a constitutively active Rac1 mutant (Rac1-V12) completely replaces the CD5 costimulatory signal. The Rac1 -specific guanine nucleotide exchange factor Vav is heavily phosphoryl ated on tyrosine residues upon CD5 costimulation, which is a prerequis ite for its activation. A role for Vav in the CD5-induced signaling pa thway is further supported by the findings that the expression of a do minant negative Vav mutant (Vav-C) completely abolishes the response t o CD5 costimulation while the expression of a constitutively active Va v mutant [Vav(Delta 1-65)] makes the CD5 costimulation signal superflu ous. Wortmannin is unable to block the Vav(Delta 1-65)- or Rac1 . V12- induced signals, indicating that both Vav and Rac1 function downstream from PI 3-kinase. Vav and Rac1 both act upstream from the CD5-induced activation df CaM kinase IV, since KN-62, an inhibitor of CaM kinases , and a dominant negative CaM kinase IV mutant block the Vav(Delta 1-6 5)- and Rac1 . V12-mediated signals. We propose a model for the CDS-in duced signaling pathway in which the PI 3-kinase lipid products, toget her with tyrosine phosphorylation, activate Vav, resulting in the acti vation of Rac1 by the Vav-mediated exchange of GDP for GTP.