Experimental studies on xenograft rejection

Abstract

The use of animal organs and tissues may provide a solution to the severe and worsening shortage of human grafts. The aims of the present experimental studies were to provide a better understanding of the rejection mechanisms and give new clues for prevention of xenorejection, thereby hopefully contributing to the development of clinical xenotransplantation.

Guinea pig hearts transplanted into rats undergo hyperacute rejection (HAR) within a few minute& C6rats cannot form the membrane attack complex. Guinea pig hearts transplanted into such rats are not hyperacutely rejected, but instead undergo delayed xenograft rejection (DXR) within 1-2 days. This model thus gives us the opportunity to study the later phases of discordant vascularized xenograft rejection.

In Paper I, we studied the effects of deoxyspergualin (DSG), cyclosporine (CsA) and tacrolimus (FK506) on the survival of guinea pig hearts in C6- rats. Our results showed that DSG moderately prolonged xenograft survival, while CsA and FK506 had no effect. DSG also significantly reduced serum xenoreactive IgM levels. At rejection, xenografts were markedly infiltrated by macrophages, regardless of the therapy used. We concluded that the beneficial effect of DSG was probably related to suppression of induced xenoreactive antibodies (XAb) and that strategies targeting macrophages and XAb may help to overcome DXR.

In Paper II, Lip-C12MDP (liposome-encapsulated dichloromethylene diphosphonate) was used to deplete recipient macrophages. We found that LiP-C12MDP in combination with DSG led to significant prolongation of guinea pig heart survival in C6- rats compared to DSG or Lip-C12MDP alone. The importance of macrophages was also shown in Paper III, where depletion of macrophages dramatically prolonged porcine islet xenograft survival in streptozotocin-diabetic mice, 26 ± 3.8 days in the macrophage-depleted group versus 8 ± 1.2 days in untreated controls. Taken together, our observations suggest that strategies targeting macrophages prolong xenograft survival.

In Paper IV, we showed that the removal of preformed antibodies by plasma exchange and suppression of induced antibodies by DSG increased survival of guinea pig hearts to 6.9 ± 1.1 days versus 2.8 ± 0.5 days in untreated C6- rats. As graft survival was longer, the number of T-cells in the graft increased significantly. This finding suggests that when DXR is prevented by reduction of XAb and macrophage depletion, the cellular infiltrate resembles that seen in allografts. In Paper V, we found that CsA resulted in long-term graft survival in the mouse-to-C6rat heart transplant model, indicating that if complement-mediated injury is prevented, suppression of T-cells is sufficient for long-term graft survival in this concordant model.

In summary, our results show that methods targeting macrophages and XAb prolong discordant vascularized xenograft survival. Control of macrophages is important also in discordant islet xenograffing. The complexity of xenorejection makes it unlikely that any single therapy will be effective. Instead, recipient immunosuppression combined with strategies reducing graft immunogeneicity and improving graft resistance to the rejection response seem to be needed.