Synergistic effects of CTLA‐4 blockade with tremelimumab and elimination of regulatory T lymphocytes in vitro and in vivo

Anti‐CTLA‐4 monoclonal antibodies (mAb) that block the interaction of CTLA‐4 with CD80 and CD86 such as tremelimumab and ipilimumab are currently being tested in the clinic for cancer treatment exploiting their properties to de‐repress tumor‐specific cellular immunity. Addition of the fully human anti‐CTLA‐4 (tremelimumab) to cultures of human T cells with allogenic dendritic cells (DCs) did not increase proliferation. Magnetic bead‐mediated elimination of CD4⁺ CD25⁺ regulatory T cells (T_reg) before setting up those alloreactive cultures also largely failed to increase primary proliferation. In contrast, predepletion of CD4⁺ CD25⁺ T_reg and culture in the presence of tremelimumab synergistically resulted in increased proliferation and DC:T‐cell aggregation. These effects were much more prominent in CD4 than in CD8 T cells. The synergy mechanism can be traced to enhanced CTLA‐4 expression in effector cells as a result of T_reg elimination, thereby offering more targets to the blocking antibody. Human T cells and allogenic DCs (derived both from healthy donors and advanced cancer patients) were coinjected in the peritoneum of Rag2^?/? IL‐2Rγ^?/? mice. In these conditions, tremelimumab injected intravenously did not significantly enhance alloreactive proliferation unless T_reg cells had been predepleted. Synergistic effects in vivo were again largely restricted to the CD4 T‐cell compartment. In addition, T_reg depletion and CTLA‐4 blockade synergistically enhanced specific cytotoxicity raised in culture against autologous EBV‐transformed cell lines. Taken together, these experiments indicate that tremelimumab therapy may benefit from previous or concomitant T_reg depletion.