CD28 co-stimulation stabilizes the expression of the CD40 ligand on T cells

The ligand for CD40 (CD40L) is a protein which is expressed on CD4 T cellsfollowing their activation: CD40-CD40L interactions are absolutely requiredfor the induction of T cell-dependent antibody responses, yet little isknown about the mechanisms whereby CD40L+ primary T cells activate naive Bcells, since the protein is only transiently expressed and is rapidlydown-regulated following T cell-B cell contact. We show here, using avariety of assays, that co- stimulation of primary murine T cells via CD3and CD28 stabilizes the expression of the CD40L protein. Firstly, T cellsstimulated in this manner express higher levels of CD40L when activated inthe presence of B cells, compared to CD3-activated T cells. Secondly, theCD40L expressed on CD28-co-stimulated T cells is more resistant to B cell-induced down-regulation. Finally, CD3/CD28-preactivated, rested T cellsre-express higher levels of CD40L more rapidly following re-stimulation viaCD3 than T cells preactivated via CD3 alone. CD3/CD28-preactivated T cells,but not CD3-activated cells, are competent to induce DNA synthesis in naiveB cells, and this requires re-stimulation via CD3 and prolonged ligation ofCD40. These data therefore reinforce the concept that naive T cells need tobe activated initially by cognate interaction with B7-bearingantigen-presenting cells (such as dendritic cells), before becomingcompetent helper effector cells capable of driving B cells intoproliferation via a CD40-dependent pathway.