Introduction: While nonspecific immunosuppression remains the mainstay of clinical transplantation, more selective immunomodulatory approaches may decrease treatment-related morbidity. Development of less toxic treatment regimens would broaden the indications for nerve allotransplantation. In mice, successful prevention of nerve allograft rejection has been demonstrated through interference with the CD40/CD40 ligand interaction. This study investigated the effectiveness of anti-CD40 ligand monoclonal antibody (anti-CD40L mAb) in supporting nerve regeneration across long nerve allografts in non-human primates.

Methods:Twelve outbred cynomolgus macaques were arranged into 6 genetically mismatched pairs, with each animal receiving a 5cm ulnar nerve allograft in the right arm and a 5cm autograft in the left arm. Mixed lymphocyte reaction assays (MLR) were used to assess resulting donor-specific unresponsiveness. Treated animals (n=10) received anti-CD40L mAb at 10mg/kg (one-time, locally applied) and at 20mg/kg systemically on post-operative days 0, 1, 3, 10, 18, and 28, and then monthly. Untreated animals (n=2) served as the untreated controls. Post-transplant, nerve conduction studies were performed and nerves were harvested for histologic analysis. Four treated animals underwent an additional challenge after cessation of anti-CD40L mAb therapy and nerve graft harvests.    Autogenous and allogeneic skin and nerve inlay grafting was performed to assess the permanence of anti-CD40L mAb-induced immune unresponsiveness.

Results: Animals that received anti-CD40L mAb demonstrated robust regeneration across nerve allografts, similar to that in the autograft control in the contralateral arm, with mean fiber counts of 6581 ± 2456 and 10605 ± 3905, respectively. The histomorphometric analysis of allografts in the untreated animals demonstrated significantly decreased percent nerve tissue, fibers count, and nerve density as compared to their matched autograft controls, with fiber counts of 2374 ± 1185 and 9511 ± 4846, respectively (p< 0.05). Animal that received anti-CD40L mAb with concomitant skin allografts had virtually no nerve regeneration through allografts and extensive scarring.  Allogeneic skin and nerve allografts applied after anti-CD40L mAb was withdrawn were consistently rejected.

Conclusions: Anti-CD40L mAb prevents rejection and allows regeneration of peripheral nerve allografts in non-human primates. However, the effect of anti-CD40L mAb appears to be transient, with restoration of immunocompetence shortly after withdrawal of therapy.The antigen load imposed by addition of a skin graft ablates the effect of anti-CD40L mAb, which has important implications for the use of anti-CD40L mAb in composite tissue allotransplantation.