The CTLA4-Ig fusion proteins abatacept and belatacept inhibit CD28-mediated T cell activation by binding CD80 (B7-1) and CD86 (B7-2) costimulatory ligands and are clinically proven immunosuppressants used for rheumatoid arthritis and renal transplantation, respectively. Abatacept and belatacept preferentially bind CD80, yet CD86 has been implicated as the dominant ligand for CD28-mediated costimulation of T cells. We investigated the immunosuppressive effects of ASP2408, a novel CTLA4-Ig with CD86 selectivity and high potency created by directed evolution methods. Here we evaluated the effect of ASP2408 in vitro using cynomolgus monkey and rat T cell proliferation assays and in vivo using cynomolgus monkey tetanus toxoid (TTx) immunization and a rat rheumatoid arthritis model. ASP2408 was 290-fold and 21-fold more potent in suppressing in vitro monkey T cell proliferation than abatacept and belatacept, respectively. ASP2408 inhibited anti-TTx immunological reactions in cynomolgus monkey at a 10-fold lower dose level than belatacept, through complete CD86 and partial CD80 receptor occupancies, and also suppressed inflammation in the rat collagen-induced arthritis model. Overall, improved immunosuppressive potency of ASP2408 relative to abatacept and belatacept correlated well with improved CD86 binding affinity. These results may support the advantage of preferential enhancement of CD86 binding affinity to inhibit T cell-mediated immune response and improved dosing convenience in humans relative to abatacept or belatacept.