IL-6 blockade prevents costimulation blockade-resistant allograft rejection by promoting intragraft regulation in T cell-depleted recipients [GIL]

The efficacy of costimulation blockade (CTLA4Ig/belatacept) in transplantation is reduced by an increased incidence of T cell-mediated rejection, which also persists after induction therapy with anti-thymocyte globulin (ATG). Herein, we investigate why ATG fails to prevent costimulation blockade-resistant rejection and how this barrier can be overcome. ATG did not prevent graft rejection in a murine heart transplant model of CTLA4Ig therapy and induced a pro-inflammatory cytokine environment. While ATG improved the balance between Tregs and effector T cells in the peripheral compartment, it had no such effect within cardiac allografts, which showed signs of inflammation. Neutralizing IL-6 alleviated inflammation, increased intragraft Treg frequencies long-term, and enhanced intragraft IL-10 and Th2 cytokine expression. IL-6 blockade together with ATG led to long-term, rejection-free heart graft survival under CTLA4Ig therapy. Combining ATG with IL-6 blockade prevents costimulation blockade-resistant rejection, thereby eliminating a major impediment to clinical use of costimulation blockers in transplantation.