Early Inhibition of heme oxygenase enzymatic activity precipitates xenograft rejection.

Abstract

514 Introduction: We have recently demonstrated that endothelial cell (EC) expression of heme oxygenase-1 (HO-1) is essential to insure accommodation of mouse hearts transplanted into rats treated with the complement inhibitor cobra venom factor (CVF) and Cyclosporin A (CyA). This study aimed to analyze the mechanism by which HO-1 contributes to suppress xenograft rejection and to investigate at what time after transplantation the expression of HO-1 is imperative to insure xenograft survival Methods: BALB/c mouse hearts were transplanted into Lewis rats treated with CVF (60 U/kg one day before transplantation and 20 U/kg at the time of transplantation) and CyA (15 mg/kg/daily starting at the time of transplantation). HO activity was inhibited by administration of Tin Protoporphyrin (SnPP, 30μM/kg/daily). Iron protoporphyrin (FePP, 30μM/kg/daily) was used as a control treatment Result: All mouse hearts transplanted into rats treated with CVF plus CyA survived long-term. Inhibition of HO activity by SnPP given on day −2, −1 and 0 (the day of transplantation) to the donor, and daily on day 0 to the time of rejection to the recipient, led to xenograft rejection 3 to 7 days after transplantation (Table 1).Table 1HO inhibition by SnPP given daily to the recipient from day 2 resulted in xenograft rejection in 3 of 7 cases, while inhibition of HO activity starting at later time points no longer induced xenograft rejection. Xenograft rejection under SnPP treatment was very similar to that of HO-1 deficient (knock-out) hearts: rejection was characterized by thrombosis of large coronary vessels, extensive myocardial infarction, cardiomyocyte apoptosis and infiltration by host macrophages and neutrophils. Thrombosis was detected in the xenograft aorta by 24 hours after transplantation, indicating that the pathogenesis of rejection may involve thrombosis of large coronary arteries, a decreased blood supply to the heart, anoxia and infarction. Conclusion: Early expression of HO-1 in the xenograft prevents injury leading to thrombosis of large coronary vessels, thus avoiding subsequent myocardial infarction and xenograft rejection. Inhibition of HO activity starting on day 2 following transplantation no longer precipitates rejection in all grafts, and later administration of SnPP does not lead to rejection. It is attractive to think that other mechanisms, such as the expression of other protective genes, exist to promote xenograft survival at those later times.