Blockade of the PD-1/PD-1L pathway reverses the protective effect of anti-CD40L therapy in a rat to mouse concordant islet xenotransplantation model.

Background: We have previously demonstrated that costimulatory blockade with anti-CD40L monoclonal antibody (mAb) prolongs the survival of non-vascularized concordant rat to mouse islet xenografts. Here, we examine whether signaling through the PD-1/PD-1L pathway is required for the anti-CD40L therapy to prolong concordant islet graft survival using a novel anti-murine PD-1 mAb (clone 4F10). Methods: C57BL/6 mice received a cellular concordant islet xenograft under the left kidney capsule and four experimental groups were prepared. Group I: untreated control; group II: recipient mice were treated with three doses of 0.5 mg of anti-CD40L mAb (clone MR1) on days 0, 2 and 4; group III: mice were treated with 0.5 mg of anti-PD-1 (CD279) mAb (clone 4F10) every other day for 8 days; and finally group IV: mice received the combined treatment that consisted of anti-CD40L plus anti-PD-1 mAb. Results: Concordant islet xenografts transplanted in control untreated mice showed a median survival time (MST) of 17 +/- 7.43 days, whereas anti-CD40L treatment led to a significant prolongation of graft survival (MST: 154 +/- 65.56, P < 0.0001). The administration of anti-PD-1 alone significantly accelerated graft rejection compared to non-treated controls (MST: 10 +/- 2.24 vs. MST: 17 +/- 7.43, P < 0.0004). Remarkably, the combined administration of anti-CD40L and anti-PD-1 reversed the protective effect obtained with anti-CD40L alone (anti-CD40L, MST: 154 +/- 65.56 vs. anti-CD40L plus anti-PD-1, MST: 10 +/- 7.72, P < 0.0002). Conclusion: Overall, our data indicate that the PD-1/PD-1L pathway is required for the achievement of prolonged graft survival in anti-CD40L-treated mice in a setting of rat to mouse concordant islet xenotransplantation.