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MB MRCP PhD
Senior Lecturer in Immunology and Honorary Consultant in Renal Medicine
Dept of Immunology
Imperial College London
Du Cane Road
The field of allogeneic organ transplantation has advanced rapidly over the last few decades. It offers the only chance of survival for many patients with endstage organ disease or, where there is an alternative, such as long-term dialysis in patients with endstage kidney disease, transplantation offers a better quality of life(1) and is more cost-effective in the medium term.(2) However, the number of organs available has remained static, or even fallen, over the past 10 years, while transplant waiting lists have continued to grow, meaning that demand for organs now greatly exceeds supply.(3) At present, this severe shortage of organs is limiting the number of transplants performed. Many countries have responded to this by trying to increase the supply of donor organs, using numerous methods, including opt-in schemes, increasing the use of so-called marginal donors and encouraging living donation. However, one study has highlighted that, even if the total number of potential human donors was made available for use, demand would still outstrip supply.(4)
Alternatives to allogeneic transplantation
There are some alternatives to allogeneic transplantation that promise eventually to relieve this shortage (see Table 1).(5–7) Of these, only xenotransplantation offers the chance of an inexhaustible supply of organs in the near future.
Because primates have a similar physiology, anatomy and immune responsiveness to those of humans, they would appear to be ideal donors for clinical xenotransplantation. Indeed, primate organs have been used clinically with some success; in one early experience, a chimpanzee kidney survived for nine months in one patient.(8) However, for a variety of reasons, including difficult ethical problems with the use of primates, the widespread application of clinical xenotransplantation will depend on the use of species such as the pig,(9) which breed rapidly and easily and which provoke fewer ethical concerns.
From a pharmaceutical viewpoint, there are two important issues relating to the clinical application of xenotransplantation. First, many immunosuppressive regimens used routinely in allotransplantation appear ineffective in xenotransplantation. Second, xenotransplant recipients may require routine adjunctive therapy not required in allogeneic transplant recipients.
Problems preventing clinical xenotransplantation
Immunological rejection remains the major barrier to clinical xenotransplantation. The main mechanisms affecting vascularised pig xenografts are illustrated in Figure 1. Unmanipulated porcine organs are rejected rapidly in primate recipients by a humoral process called hyperacute rejection (HAR). This occurs within minutes and is triggered by the binding of IgM xenoreactive natural antibodies (XNA) to the xenograft endothelium, which activates complement and in turn provokes intravascular thrombosis, interstitial haemorrhage and organ infarction. This sequence of events is independent of de-novo gene transcription. For many years, HAR proved resistant to all conventional therapeutic regimens, including those used to treat humoral allograft rejection. However, it has been overcome by the use of organs from transgenic pigs expressing human regulators of complement activity (RCA),(10) which are resistant to the effects of complement activation.
These organs can survive for up to seven months in primate recipients, but, despite intensive immunosuppression, they are eventually rejected by a process called delayed xenograft rejection (DXR), or alternatively acute vascular rejection (AVR). This involves activation of graft endothelium by XNA, de-novo expression of proinflammatory genes, intravascular thrombosis and infiltration of the graft with chronic inflammatory leukocytes such as macrophages and natural killer cells. DXR has proven resolutely resistant to all conventional (and many unconventional) immunosuppressive regimens. Because of this, several novel strategies have been proposed to inhibit DXR, including depletion of XNA,(11) inhibition of the clotting cascade(12) and induction of B-cell tolerance,(13) but so far none has proven effective in pig-to-primate models. The recent generation of a-galactosyltransferase knockout pigs,(13) which lack the major xenogeneic epitope gal alpha(1,3)gal, has provoked a great deal of excitement, as organs from these pigs are expected to be resistant to DXR. At the time of writing, results from studies involving these organs are still awaited.
A significant proportion of RCA-transgenic pig organs also suffer from acute T-lymphocyte-mediated rejection. This is despite the fact that many of the primates in reported preclinical studies appear to suffer the consequences of overimmunosuppression, including severe infection and acute malignancy, implying that T-cell responses to xenografts may be resistant to immunosuppression. Data from small animal models support this; conventional immunosuppressive regimes, used successfully in allogeneic experiments, are usually ineffective after xenotransplantation. Similarly, in-vitro data suggest that T-cell antixenogeneic responses are significantly more vigorous than equivalent alloresponses.(14) Several novel strategies are under investigation to overcome this problem, including generating tolerance through mixed haematopoietic chimerism,(15) induction of donor regulatory cells(16) and thymus grafting.(17)
A further problem with the clinical application of xenotransplantation relates to dissonant physiology. A good example of this is the evidence that porcine erythropoietin (EPO) is inactive in primates, implying that all human recipients of a pig kidney will need regular human recombinant EPO.(18) Alternatively, it may be possible to engineer donor organs to express human EPO. However, if porcine organs are made to survive for longer periods, other examples of similar incompatibilities may become apparent.
Transmission of so-called xenozoonoses(19) is a major concern. Although many infectious agents can be eliminated from donor herds through strict monitoring of food, breeding and environment, it will not be possible to eradicate porcine endogenous retroviruses (PERV) using such techniques, as they are encoded by genomic DNA. Although the largest clinical study in humans exposed to porcine tissue found no evidence of PERV transmission,(20) it is clear that PERV are capable of infecting human cells in vitro.(21) One pragmatic approach to this problem may be to have all xenograft recipients on prophylactic antiretroviral therapy, with significant cost and monitoring implications. Alternatively, it may be possible to isolate animals that have replication-deficient PERV, as has been indicated by recent data from a group in the USA.(22)
Clinical xenotransplantation is still some way off, as current pharmacological therapies do not allow long-term survival of porcine organs in primates. Novel strategies to promote survival are needed, including the induction of tolerance, and these are undergoing preclinical trial. At the same time, physiological differences and worries about infection mean that xenograft recipients are likely to need adjunctive hormonal (eg, EPO) and antiretroviral therapy.