Enhancement of T cell activation by immobilized hu5C8 (anti-CD40L) monoclonal antibody

Background: Soluble monoclonal antibodies (MoAb) targeting CD40L on T cells can partially block T cell alloreactivity by preventing the costimulatory signal of antigen presenting cells through CD40. However, it is not known if these MoAbs can also deliver inhibiting or stimulating signals through the CD40L receptor. Materials and methods: Blood mononuclear cells were stimulated by mitogens or allogeneic stimulator cells in the presence of hu5C8 MoAb, either in soluble form, or immobilized to the culture wells. T cell responses were evaluated by means of primary and secondary mixed lymphocyte culture (MLC), cytotoxic T lymphocyte (CTL) generation, immunophenotype, apoptosis assay and cytokine release. Also, the effect of hu5C8 on cells inhibited by CTLA4‐Ig was tested. Results: While the soluble hu5C8 inhibited T cell proliferation, the immobilized hu5C8 enhanced both mitogen and alloantigen‐induced proliferative and cytotoxic T cell responses, without inducing further apoptotic T cell death. In the presence of CTLA4‐Ig, immobilized hu5C8 increased the residual CD28‐independent proliferation of alloantigen‐specific T cells both in primary and secondary MLC, and prevented the inhibiting effect of CTLA4‐Ig on the generation of CTL. Immobilized hu5C8 MoAb‐stimulated T cells also showed a limited capacity of producing interleukin (IL)‐10, even in the presence of CTLA4‐Ig. Conclusions: We show that the hu5C8 MoAb has a strong mitogenic activity when immobilized, likely due to higher crosslinking capacity as compared to the soluble antibody. Strategies to induce ex‐vivo T cell responses against tumor or viral antigens by means of hu5C8 MoAb antibody will be exploited based on these findings.