Involved in T-cell activation, the induction of cell proliferation and cytokine production and promotion of T-cell survival. Enhances the production of IL4 and IL10 in T-cells in conjunction with TCR/CD3 ligation and CD40L costimulation (PubMed:8617933). Isoform 3 enhances CD40L-mediated activation of NF-kappa-B and kinases MAPK8 and PAK2 in T-cells (PubMed:15067037). Homodimer; disulfide-linked. Interacts with DUSP14. Binds to CD80/B7-1 and CD86/B7-2/B70. Interacts with GRB2. Isoform 3 interacts with CD40LG (PubMed:15067037). Expressed in T-cells and plasma cells, but not in less mature B-cells. CD40LG induces tyrosine phosphorylation of isoform 3. CD28 entry