Interelationship between CD3 and CD28 pathways in a murine T cell thymoma

It is well known that the CD28 costimulatory signal is important to complem ent T cell receptor (TCR)/CD3-initiated T cell activation, but the mechanis m by which these two distinct signaling pathways are integrated is not clea rly understood. In our laboratory. we dispose of a murine T cell hybridoma transfected with human CD28 molecule which is able to produce IL-2 in respo nse to stimulation, suggesting that the signal transduction machinery coupl ed to the CD28 molecule is capable of triggering effector functions. Nevert heless, the action of three immunosuppressive agents previously shown in ou r model, suggested an interaction between the CD3 and CD28 pathways. We con firmed here this hypothesis by transfecting the cDNA of the human CD28 mole cule in the BW5147 thymoma which lacks CD3 surface expression. Stimulation of the human CD28 did not lead to IL-2 secretion while the restoration of t he TCR/CD3 complex re-established the functionality of this costimulatory m olecule. These data demonstrate that the IL-2 production induced by the CD2 8 activation pathway is dependent of the TCR/CD3 complex cell surface expre ssion and suggest the formation of a functional membrane complex between th e CD3 and CD28 molecules. The molecular basis of the functional dependence of CD28 signaling on the TCR/CD3 complex is presently unknown. Nonetheless, we showed that some early events induced by CD28 stimulation, such as PI3- kinase association, are independent of the TCR/CD3 complex expression. (C) 2001 Elsevier Science Ltd. All rights reserved.