CD154 is a 39 kD TNF superfamily member also known as CD40 ligand, gp39, T-BAM, TRAP, and Ly-62. CD154 is an accessory molecule expressed predominantly on activated CD4+ lymphocytes that bind CD40. CD154 plays an important role in T-B cell costimulation. The MR1 antibody has been reported to inhibit the activation of T and B lymphocytes in vitro and antigen-specific lymphocyte responses in vivo.