Alternative immunomodulatory strategies for xenotransplantation: CD40/154 pathway-sparing regimens promote xenograft survival

Immunosuppressive therapies that block the CD40/CD154 costimulatory pathway have proven to be uniquely effective in preclinical xenotransplant models. Given the challenges facing clinical translation of CD40/CD154 pathway blockade, we examined the efficacy and tolerability of CD40/CD154 pathway-sparing immunomodulatory strategies in a pig-to-nonhuman primate islet xenotransplant model. Rhesus macaques were rendered diabetic with streptozocin and given an intraportal infusion of ≈ 50 000 islet equivalents/kg wild-type neonatal porcine islets. Base immunosuppression for all recipients included maintenance therapy with belatacept and mycophenolate mofetil plus induction with basiliximab and LFA-1 blockade. Cohort 1 recipients (n = 3) were treated with the base regimen alone; cohort 2 recipients (n = 5) were additionally treated with tacrolimus induction and cohort 3 recipients (n = 5) were treated with alefacept in place of basiliximab, and more intense LFA-1 blockade. Three of five recipients in both cohorts 2 and 3 achieved sustained insulin-independent normoglycemia (median rejection-free survivals 60 and 111 days, respectively), compared to zero of three recipients in cohort 1. These data show that CD40/CD154 pathway-sparing regimens can promote xenoislet survival. Further optimization of these strategies is warranted to aid the clinical translation of islet xenotransplantation.

Islet xenotransplantation using gal-deficient neonatal donors improves engraftment and function

Significant deficiencies in understanding of xenospecific immunity have impeded the success of preclinical trials in xenoislet transplantation. Although galactose-α1,3-galactose, the gal epitope, has emerged as the principal target of rejection in pig-to-primate models of solid organ transplant, the importance of gal-specific immunity in islet xenotransplant models has yet to be clearly demonstrated. Here, we directly compare the immunogenicity, survival and function of neonatal porcine islets (NPIs) from gal-expressing wild-type (WT) or gal-deficient galactosyl transferase knockout (GTKO) donors. Paired diabetic rhesus macaques were transplanted with either WT (n = 5) or GTKO (n = 5) NPIs. Recipient blood glucose, transaminase and serum xenoantibody levels were used to monitor response to transplant. Four of five GTKO versus one of five WT recipients achieved insulin-independent normoglycemia; transplantation of WT islets resulted in significantly greater transaminitis. The WT NPIs were more susceptible to antibody and complement binding and destruction in vitro. Our results confirm that gal is an important variable in xenoislet transplantation. The GTKO NPI recipients have improved rates of normoglycemia, likely due to decreased susceptibility of xenografts to innate immunity mediated by complement and preformed xenoantibody. Therefore, the use of GTKO donors is an important step toward improved consistency and interpretability of results in future xenoislet studies.

A comparison of the immunosuppressive effects of cyclosporine A and cyclosporine G in vivo and in vitro

In this study we compared the immunosuppressive effects of cyclosporine (Cs) A and G, both in vitro (human and rabbit) and in vivo (rabbit). The 50% inhibitory concentration (IC50) of CsG was up to three times greater than that of CsA for mitogen and alloantigen-induced lymphocyte proliferation (IC50 CsA 1 degree MLC = 19 +/- 4 micrograms/L vs. IC50 CsG = 60 +/- 7 micrograms/L; P < 0.01). Kinetic studies in both human and rabbit systems showed that the effectiveness of both drugs was similarly reduced when added at later times after culture initiation. The effects of CsA and CsG in combination on immune responses appeared to be antagonistic at higher and additive at lower drug doses. We also compared the ability of CsA and CsG to displace 3H-CsA from PBMC. The 50% displacement concentration (DC50) for CsG was up to three times greater than that for CsA (DC50 CsA = 1.44 +/- 2.49 x 10(-7) M vs. DC50 CsG = 3.9 +/- 5.4 x 10(-7) M; P < 0.05), suggesting that CsG does not bind as well to PBMC as CsA. In vivo studies using skin allografts confirmed in vitro findings. Both CsA and CsG at 5 and 10 mg/kg/day significantly (P < 0.01) prolonged graft survival compared with control animals. However, at these doses and even at 15 mg/kg/day CsG, CsA was more efficacious at prolonging skin graft survival in rabbits (P < 0.01), e.g., mean survival time (MST, days) 10 mg/kg/day CsA = > 20.5 vs. MST 15 mg/kg/day CsG = 15.0. These results suggest that both in vitro and in vivo in rabbits CsG is less immunosuppressive than CsA.