Delayed Xenograft Rejection

Abstract. The triumph of genetic engineering in overcoming hyperacute rejection (HAR) of a discordant organ xenograft is clear, but the promise of clinical application of xenotransplantation remains unfulfilled as further immunologic barriers are defined that lead to rejection of a vascularized xenograft within days of transplantation. This report describes the features of this second set of immunologic responses, collectively termed delayed xenograft rejection (DXR). DXR is a syndrome seen in xenograft recipients in which HAR has been avoided or suppressed by antibody depletion or blockade of complement activation. DXR may result, at least in part, from the persisting activation of those pathways first encountered during the HAR phase. Serial studies over several days after transplant show that, histologically, xenografts undergoing DXR demonstrate varying combinations of (1) progressive infiltration by activated macrophages and natural killer (NK) cells, (2) platelet aggregation and fibrin deposition throughout the microvasculature, and (3) endothelial activation. In various experimental models, DXR is T cell-independent and can occur in the absence of demonstrable xenoreactive antibodies. Hence DXR is probably best regarded as arising from the activation of innate host defense mechanisms coupled with failure of normal regulatory mechanisms due to manifold molecular incompatibilities. Although DXR-like features can be seen in concordant models, T cell involvement in the latter is probably requisite. Similarly, in a much muted form, aspects of a DXR-like process may contribute to numerous inflammatory processes, including allograft rejection. The importance of DXR in xenotransplantation is that its development appears resistant to all but the most dense and toxic forms of immunosuppression, which prolong xenograft survival at the expense of inducing host leukopenia, thrombocytopenia, and coagulopathies. It is likely that until the basis of DXR is more clearly understood there can be no further significant progress toward clinical xenotransplantation. However, as the mechanisms responsible for DXR are dissected and understood, still further genetic engineering of donor pigs, involving the introduction of additional or multiple genes to regulate macrophage and NK cell responses, local coagulation, and endothelial cell activation, may once again prove to be an attractive, practical, powerful therapeutic option.