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. Homodimer; disulfide-linked. Interacts with DUSP14. Binds to CD80/B7-1 and CD86/B7-2/B70. Interacts with GRB2 (By similarity).