Prolonged discordant xenograft survival by inhibition of the intrinsic coagulation pathway in complement C6-deficient recipients

Background: Xenotransplantation of vascularized organs between unmodified discordant species results in hyperacute graft rejection within minutes to hours after graft reperfusion. This process is due to the presence of natural xenoreactive antibodies and complement activation, which lead to vessel injury, thrombosis, and hemorrhage. Because multiple components of the coagulation and complement cascades interact with each other, we have investigated the effects of inhibiting these systems together. The recombinant Kunitz type serine protease inhibitor (KPI-BG022) tested in these experiments inhibits factor XIIa, kallikrein, and plasmin. Methods: Cardiac xenografts from male Hartley guinea pigs were heterotopically grafted into male PVG rats that were either sufficient (C6[+]) or deficient (C6[-]) for the complement component C6 and thus formation of the membrane attack complex. Experimental animals received KPI 5 mg/kg intravenously before reperfusion, and control animals received saline placebo. Results: C6(+) recipients rejected their grafts hyperacutely, without a significant difference between KPI-treated (0.12 ± 0.05 hours) and placebo-treated (0.13 ± 0.06 hours) recipients (n = 10). As expected, C6(-) recipients showed prolonged graft survival (17.65 ± 3.45 hours, n = 5). However, a single intravenous bolus of KPI before releasing the clamps further delayed graft rejection in C6(-) recipients (46.2 ± 3.3 hours; n = 5). Histologic examination at 2, 6, and 12 hours after transplantation showed platelet aggregation and inflammatory infiltrates were significantly decreased in KPI-treated (C6[-]) recipients. However, intragraft hemorrhage was apparent at 6 and 12 hours. Conclusions: We conclude that in vivo inhibition of the intrinsic clotting cascade by functional inactivation of factor XIIa has a synergistic effect with inhibition of membrane attack complex formation in preventing hyperacute discordant xenograft rejection.