ANTIGEN-SPECIFICITY OF DUAL REACTIVE T HYBRIDOMAS DETERMINES THE REQUIREMENT FOR CD40-LIGAND-CD40-INTERACTIONS

CD40 ligand (CD40L) expression on T cells is known to play a crucial r ole in B cell responses, Some evidence also supports a role for CD40L- CD40 interactions in T cell responses, at least in vivo. Whether the T cell requirement for these interactions is an invariable finding, how ever, is less clear. Here, we provide evidence that the Ag specificity of T cells influences the requirement for CD40L. T cell hybridomas wi th dual reactivity for two different Ags, allo-H2-A(p) and Mls(a) supe rantigens, display a differential requirement for CD40L expression. Wh ereas the response to splenic APC expressing Mls(a) Ags requires CD40L expression, the response to alloantigen-bearing APC does not. The req uirement for CD40L expression for the Mls(a) response appears to refle ct a strong dependence of this response on ICAM-1 (intercellular adhes ion molecule-1) and the ability of CD40-mediated signals to regulate I CAM-1 expression. These findings demonstrate that CD40L-CD40-mediated cross-talk is important for some but not all T cell responses and is i nfluenced by both the type of Ag recognized and the type of APC.