Proximal Impact of Transplant Tolerance-Promoting Antibody Therapies on Antigen-Specific T Cell Reactivity

The development of a transient, tolerance-promoting therapy is a critical goal in transplantation. Antibody-perturbation of T cell activation signals is considered a promising candidate. However, the mechanisms of such therapies remain vague. The lack of cohesive and systematic knowledge in the requirements of generating long-term transplantation tolerance using short-term antibody treatments impedes the rational design of tolerance-promoting therapies. Therefore, using anti-LFA-1 and anti-CD154 as representatives of efficacious tolerance-inducing therapeutics, I interrogated their proximal impact on naive antigen-specific T cells during initial antigen encounter.;Using both monoclonal TCR-transgenic and polyclonal T cells in an in vivo adoptive transfer model, I tracked T cell activation and differentiation in the presence of anti-LFA-1 and/or anti-CD154. The antibody therapies markedly reduced the number of T cells in the draining lymph nodes. However, those remaining in the nodes vigorously proliferated. This paradoxical decrease in cell number despite intact proliferative capability was not due to deletion, as the adoptively transferred T cells persisted past primary activation and responded productively to secondary antigen exposure. Surprisingly, while anti-LFA-1 and/or anti-CD154 partially inhibited effector cytokine production, they did not specifically induce differentiation of alternate, tolerogenic phenotypes. However, while the antibody therapies mediated neither complete suppression of T cell reactivity nor T cell tolerance, anti-LFA-1- and anti-CD154-mediated skin graft prolongation was maintained by a dominant regulatory mechanism that allowed naive graft-specific T cell activation and proliferation but inhibited their differentiation.;Taken together, anti-LFA-1 and anti-CD154 appeared to partially inhibit T cell reactivity without inducing early T cell tolerance. However, long-term graft survival and tolerance generated by these antibodies were maintained by an active regulatory mechanism protecting the graft from naive T cells. I therefore hypothesize that, instead of immediately generating tolerance upon interaction with responding T cells, antibodies targeting T cell activation signals dampen initial T cell reactivity to allow early transplant survival in an immunologically quiescent microenvironment, which allows the transplant itself to then gradually tolerize graft-specific T cells and generate donor-specific tolerance in a time-dependent manner. In other words, the transplant, and not the therapeutic antibodies, is the key tolerogen for successful generation of tolerance.