Kidney transplantation in adults: Xenotransplantation

INTRODUCTION — The transplantation of cells, tissues, and organs between individuals of different species is called xenotransplantation. Xenotransplantation for the treatment of kidney disease has been considered for over 100 years. The first serious attempts at clinical organ transplantation at the beginning of the 20^th century used animals as sources of kidneys because it was not readily apparent that human kidneys could be ethically retrieved [1]. Today, interest in kidney xenotransplantation is driven by the considerable shortage of donated human kidneys, as well as advances in genetic engineering and immunology that have reduced the biological barriers to using animals as sources of organs and tissues.

This topic will review xenotransplantation, its potential clinical applications, and its pertinent challenges.

RATIONALE FOR XENOTRANSPLANTATION — A severe shortage of donated human organs has always motivated interest in xenotransplantation. The shortage of donated human kidneys is enhanced by the increasing prevalence of end-stage kidney disease (ESKD) and diabetes due to aging in populations, mitigated only in part by living donation of kidneys. Other approaches to kidney replacement, such as implantable devices, cell or stem cell therapies, and organogenesis are being explored [2]; however, xenotransplantation could provide the most widely available, quickly implemented, and cost-effective approach [3-5]. Xenotransplantation might also be used as temporary support during the period needed to generate an autologous organ or device from stem cells [6]. It might also be used to avert certain infections (as animal organs are not susceptible to certain viruses, such as Epstein-Barr virus and hepatitis B and C) or to introduce extrinsic genetic material for therapeutic purposes [3].

Despite these possible benefits, xenotransplantation remains controversial because the survival and function of xenografts require intense, potentially toxic immunosuppression, and a xenograft potentially might convey infections that pose risks to public health. However, the implantation of pig kidneys into recently deceased recipients [7,8] and the transplantation of porcine hearts into living recipients with severe heart failure [9] suggest that xenotransplantation of the kidney may be approaching clinical application.

PIGS AS SOURCES OF XENOGRAFTS — Although nonhuman primates (such as chimpanzees and baboons) have been used as sources of xenografts with some clinical success [10,11], they have largely been replaced by pigs, which have greater availability and can be genetically engineered to lower some of the biological hurdles of xenotransplantation. Use of nonhuman primates is also deterred by the risk of transferring a lethal infectious agent to a human recipient and/or the public. (See 'Barriers to xenotransplantation' below.)

Physiologic compatibility — Pigs of certain strains and ages have kidneys and other organs large enough for replacement of human organs. In spite of some molecular incompatibilities, pig kidneys appear capable of providing the life-supporting functions of human kidneys [12,13], although there are some minor physiologic differences. Experimental studies in which pig kidney xenografts were transplanted into nonhuman primates have collectively shown the following:

●Glomerular filtration – Pig kidney xenografts support life and can maintain normal or near-normal serum creatinine levels [12-18], in some cases for more than one year [14].

●Serum electrolytes – Pig kidney xenografts can maintain normal or near-normal serum urea, sodium, chloride, potassium, and chloride levels [12-18]. Serum phosphorus levels increase transiently immediately posttransplant but then decrease to the low to normal range [12,13,15-17], possibly due to the higher glomerular filtration rate in pigs. Some reports have found that nonhuman primates with pig kidney xenografts develop hypophosphatemia [13,19]; the cause for this is not known.

●Proteinuria – Some nonhuman primates with pig kidney xenografts develop mild to severe proteinuria and hypoalbuminemia [13,19,20]. Whether this complication reflects intrinsic limitations of the porcine kidneys or results from immunosuppressive therapy or rejection remains unclear.

●Urine concentrating ability – Pig kidneys produce more dilute urine (ie, lower urine osmolality) than human kidneys, raising the concern that this could lead to the excretion of large amounts of urine and dehydration. However, this has not been observed in nonhuman primates receiving pig kidney xenografts.

●Erythropoietin production – Pig kidneys produce erythropoietin, which has approximately 82 percent similarity with human erythropoietin [21,22]. Monkeys with pig kidney xenografts have been found to develop a normocytic, nonhemolytic anemia, which can be reversed with the administration of recombinant erythropoietin [13]. This is thought to be due to incompatibility of pig erythropoietin with the primate erythropoietin receptor.

●Renin production – Pig renin, which is produced by the kidney, cannot cleave human angiotensinogen. However, nonhuman primates with functioning pig kidney xenografts are able to maintain relatively normal fluid balance and potassium balance [12,13,15-17]. (See "Overview of the renin-angiotensin system".)

●Kidney size and growth – In some but not all studies, pig kidney xenografts have exhibited rapid growth in size following transplantation into nonhuman primates [15-17,23-25]. This growth can be reduced by the use of smaller size pig strains (miniature pigs) or genetic knockout of the pig growth hormone receptor [23,26-29].

Whether all kidney functions can be manifested and correctly regulated by a pig kidney xenograft in humans remains to be determined. Limited studies have shown that pig kidney xenografts transplanted into deceased humans provide glomerular filtration and clearance of water for up to seven days, when experiments were arbitrarily terminated [7,30].

Genetic engineering of pig sources — To facilitate successful transplantation, pigs have been genetically engineered to resist the innate immune, inflammatory, and thrombotic barriers to xenotransplantation in nonhuman primates and humans (see 'Barriers to xenotransplantation' below). Commonly used genetic modifications include the following:

●Deletion of pig xenoantigens – To avert binding of natural (preformed) antibodies to a xenograft, three pig antigens have been modified. These are Gal(alpha1-3)Gal, N-glycolylneuraminic acid (Neu5Gc), and Sda. Of the three, Gal(alpha1-3)Gal is the most significant [31,32]. Recognition of these antigens by natural xenoreactive antibodies can lead to hyperacute rejection and early antibody-mediated rejection (ABMR) of pig organ xenografts.

Genetic engineering is used to disrupt the genes encoding the enzymes that synthesize these xenoantigens. Knocking out one xenoantigen, particularly Gal(alpha1-3)Gal, substantially reduces binding of human antibodies to pig cells in vitro [20,33-36] and enables kidney xenografts to function and survive for months in immunosuppressed nonhuman primates [15,20,37]. Knocking out two (double knockout) or all three (triple knockout) xenoantigens has been shown to further reduce antibody binding in vitro and may prolong kidney xenograft survival [14,38,39].

●Expression of human transgenes – The expression of certain human transgenes has been shown to protect and prolong survival of pig kidney xenografts in nonhuman primates. Expression of these proteins can be achieved by making transgenic animals in which the genes for the human proteins are integrated into the porcine genome. These include the following:

•Human complement regulatory proteins (CRPs), such as decay accelerating factor (hCD55) and/or membrane cofactor protein (hCD46) [35,40-42]. Pig CRPs poorly suppress activation of human complement in pig blood vessels, making pig xenografts particularly susceptible to complement-mediated injury. (See 'Hyperacute rejection' below.)

•Human coagulation regulatory proteins, such as human thrombomodulin (hTBM) and endothelial protein C receptor (hEPCR). This genetic modification addresses molecular incompatibilities between the pig and primate coagulation systems that impair regulation of thrombosis. Expression of one or both of these proteins has been shown to prolong kidney xenograft survival [16,17]. (See 'Hyperacute rejection' below.)

•Human heme oxygenase-1 (hHO1) and human CD47 (hCD47), which are thought to reduce the inflammatory response to a pig xenograft. (See 'Antibody-mediated rejection' below.)

●Inactivation of porcine endogenous retrovirus (PERV) inserts – Most pigs harbor an endogenous retrovirus known as PERV, which is capable of infecting human cells in culture (see 'Infection' below). Genetic engineering has been used to inactivate at least some PERV inserts; whether such engineering is necessary and whether it influences the fate of grafts remains to be determined.

These and other genetic modifications can be introduced into somatic cells, which can be tested in vitro and used as a source of nuclei for transfer to and reprogramming by enucleated pig oocytes in conjunction with reproductive cloning [43].

Availability and optimization of genetically engineered pigs — Pigs considered suitable as sources of organs for xenotransplantation are genetically engineered and raised under highly controlled conditions and screened for specified infectious agents. Genetic engineering of pigs for xenotransplantation currently involves the introduction of 10 or more modifications in somatic cells followed by cloning by nuclear transfer and introduction into pseudopregnant females (see 'Genetic engineering of pig sources' above). Pigs generated in this way are presently scarce (more scarce than human organ donors). Eventually, the supply of engineered and suitably raised pigs can be expanded by breeding, but some strains of pigs and some genetically modified pigs breed poorly. Once the optimal combination of genetic modifications and other traits is known, and suitable facilities are built, selection of background traits or efficiencies of scale or both must be introduced to expand the availability of pig sources. However, this process will take time.

The genetic modifications of pigs and immunosuppression regimens that successfully extend the survival and function of organ xenografts in nonhuman primates will most likely prove suboptimal for long-term application of xenotransplantation. The genetic modifications of pigs presently used in experimental xenotransplantation were selected based upon outcomes of a small number of experimental xenografts in nonhuman primates. Some observed improvements could reflect experience and some chance. Neither the set of genetic modifications introduced nor the genetic background of the pigs have been "proven," nor would that be possible given the limitation on availability of pigs and nonhuman primates. Furthermore, even if the optimal modifications of pigs for xenografts in nonhuman primates was determined, these modifications would likely prove suboptimal for clinical applications given genetic and acquired differences between nonhuman primates and humans. Accordingly, incisive investigation of clinical xenotransplants will be needed to realize the clinical potential of kidney xenotransplantation.

APPLICATIONS OF XENOTRANSPLANTATION — Potential applications of xenotransplantation include the following:

●Organ replacement – Xenotransplantation is considered an approach to the replacement of failing organs (such as the kidney, heart, lung, or liver) or tissues (such as pancreatic islets). Clinical application in this context would be the same or very similar to allogeneic transplantation. However, differences in immunogenicity presently would mandate the use of more intense immunosuppressive regimens (including depletion of T cells and possibly B cells), suppression of complement and inflammation at the time of transplantation, and ongoing disruption of the CD40-CD40 ligand (CD40L) interaction to hinder T cell-dependent B cell responses [44-47]. On the other hand, xenotransplantation makes it possible to select or engineer source animals and to treat recipients in advance of the transplant procedure, potentially allowing application of regimens to induce tolerance or to suppress immune responsiveness. The potentially unlimited availability of source animals (ie, pigs) makes it possible to increase the mass of the transplant (eg, to transplant two kidneys or a larger mass of islets) or to repeat the transplant procedure as needed.

●Organogenesis – Xenotransplantation might be used in conjunction with organogenesis to generate functioning kidneys [48,49]. With this strategy, pluripotent stem cells or fetal cells committed to a kidney lineage from a xenogeneic source are transplanted and thereafter may develop spontaneously in the recipient [50,51]. Limited studies suggest that developing metanephroi surgically placed into the omentum of cross-species hosts (eg, pig to rodent) differentiate, grow, become vascularized, and exhibit excretory behavior [6,52].

●Reverse xenotransplantation – Pluripotent or mature stem cells from a person with kidney failure needing treatment might be introduced as “reverse xenografts” into a fetal animal host, in which the cells grow into kidney primordia [53,54]. After development begins, the primordial kidney could be harvested and transferred back to the person from whom the stem cells were generated [2,54,55]. Alternatively, human stem cells might be seeded on the decellularized matrix of an animal kidney to generate an "engineered" autologous human organ that could be implanted in the person with kidney failure.

BARRIERS TO XENOTRANSPLANTATION — The major hurdles to successful application of xenotransplantation include the following:

●Immunologic responses of the recipient against the graft

●Biochemical incompatibility

●Infection

●Ethical and clinical considerations

Immunologic rejection — Immunologic responses of xenograft recipients lead to rejection, with manifestations partly reflecting the intensity of xenoimmunity and distinguished by the greater vulnerability of xenotransplants to injury by xenogeneic immunity, complement activation, and coagulation. Without genetic manipulation of the source of a xenograft or severe immune modulation of the recipient, a pig organ transplanted into a human would quickly succumb to rejection. Descriptions below emphasize distinct features of rejection of kidney xenografts.

Hyperacute rejection — Hyperacute rejection may arise in transplanted organs, whether human to human or across species. It is not observed with transplanted tissue or cells, such as pancreatic islets or hepatocytes. Without specific measures described below, this severe reaction may occur within minutes to hours and would be observed in nearly all pig kidneys transplanted into human patients. (See "Kidney transplantation in adults: Evaluation and diagnosis of acute kidney allograft dysfunction".)

Hyperacute rejection of xenografts is characterized histologically by the aggregation of platelets, formation of platelet thrombi, and the presence of interstitial hemorrhage in the recently transplanted organ. Accumulation of neutrophils is sometimes (but not invariably) observed.

Hyperacute rejection of pig organs by primates is triggered by the binding to endothelium of xenoreactive natural (preformed) antibodies. These antibodies predominately recognize Gal(alpha1-3)Gal, a sugar expressed by pigs, but not by humans, apes, or Old World monkeys. The anti-Gal(alpha1-3)Gal antibodies are believed to be similar to the isohemagglutinins that recognize blood groups A and B [31]. Binding of natural antibodies to other pig antigens might heighten this severe reaction.

Xenografts appear to be particularly susceptible to complement-mediated injury and hyperacute rejection because porcine complement regulatory proteins poorly suppress activation of human complement in porcine blood vessels. (See "Regulators and receptors of the complement system".)

Most evidence suggests that hyperacute rejection is caused by the rapid accumulation of terminal complement complexes (C5b-9) on endothelial surfaces [56]. Deposition of terminal complement complexes causes an alteration in the shape of endothelium, which brings platelets into contact with underlying matrix, thereby triggering aggregation and thrombosis [57-59].

Although hyperacute rejection is arguably the most severe of all known immune pathologies, it can be prevented by one or more of the following strategies:

●Depleting xenoreactive antibodies from the recipient's circulation [60].

●Inhibiting complement activation. This can be achieved by administration of an inhibitory agent [61,62] or intravenous immunoglobulin.

●Expressing the human complement regulatory proteins, decay accelerating factor (hCD55) or membrane cofactor protein (hCD46), in the animal organ. (See 'Genetic engineering of pig sources' above.)

●Eradicating antigenic targets of xenoreactive natural antibodies. (See 'Genetic engineering of pig sources' above.)

●Transgenic expression of human proteins to address interspecies incompatibilities that impair regulation of thrombosis and inflammation in ischemia-reperfusion injury and rejection (eg, thrombomodulin, endothelial protein C receptor, and CD47). (See 'Genetic engineering of pig sources' above.)

Antibody-mediated rejection — Antibody-mediated rejection (ABMR) is recognized as the major hurdle to clinical application of xenotransplantation when hyperacute rejection is averted. Although infrequent in kidney allografts, ABMR is the most common and feared type of rejection observed in organ xenografts. It is histologically characterized by severe endothelial thickening, thrombosis consisting of fibrin and platelets, focal ischemia, and widespread inflammation. Because blood vessels are the most prominent target of immunity in ABMR, we sometimes prefer the original term for this condition [63]: “acute vascular rejection.” (See "Kidney transplantation in adults: Clinical features and diagnosis of acute kidney allograft rejection".)

Although a variety of factors have been implicated, ABMR is triggered by the binding of antibodies to graft blood vessels, leading to activation of complement and attachment of leukocytes. This results in widespread activation of endothelium and a shift from an anticoagulant, antithrombotic, antiinflammatory state to a procoagulant, prothrombotic, proinflammatory state. This pathophysiologic shift involves structural changes to endothelial cells and matrix that make the change more readily prevented than treated.

The antibodies that cause ABMR of organ xenografts during the first few weeks after transplantation are specific for Gal(alpha1-3)Gal [64,65], although other natural antibodies might contribute. Organ xenografts that survive two to four weeks or longer, however, become especially susceptible to antibodies generated by T cell-dependent responses against polypeptide components of xenograft endothelium [45,66].

Prevention of early ABMR of xenografts (within two to three weeks of reperfusion) can be accomplished by averting the binding of xenoreactive antibodies to graft blood vessels and/or by overcoming the propensity of xenografts to promote complement activation, coagulation, thrombosis, and inflammation. Prevention of ABMR caused by T cell-dependent B cell responses presently requires disrupting the CD40-CD40 ligand (CD40L) interaction [45-47,67-69]. (See 'Genetic engineering of pig sources' above.)

Because intense induction and maintenance immunosuppression, including ongoing inhibition of the CD40-CD40L interaction, risks significant toxicity, the widespread clinical application of xenotransplantation may depend on the development of more specific and less toxic approaches to constraining T cell-dependent B cell responses. These include induction of accommodation, xenogeneic tolerance, or genetic engineering to modify antigenic targets and susceptibility to immune-mediated injury.

●Accommodation – When antibodies against a kidney transplant are removed from the circulation or temporarily prevented from causing the demise of the graft, the graft may acquire resistance to immune and inflammatory injury, a process known as accommodation. This was first described clinically in blood-group A- and B-incompatible kidney transplant [70]. However, it is best characterized in xenografts [71,72].

Accommodation is sometimes associated with expression in the transplant of "protective" genes such as heme oxygenase-1 [73]. These genes inhibit cellular cytotoxicity, allowing other changes in cell and tissue physiology to mount broad resistance to injury and noxious substances and cytokines [72,74]. Transgenic expression of these human genes in pigs is among the modifications being introduced in pigs genetically engineered as sources of xenografts. Although expression of protective genes may be essential for development of accommodation, other structural and biochemical changes transpiring over periods of weeks may be needed for accommodation to be fully manifest and sustained [6]. (See 'Genetic engineering of pig sources' above.)

Whether accommodation can make xenotransplantation clinically acceptable is yet to be determined. However, accommodation enables kidney transplants to cross ABO and human leukocyte antigen (HLA) barriers with success of ABO-incompatible kidney transplants approaching ABO-compatible transplants. (See "Kidney transplantation in adults: ABO-incompatible transplantation".)

●Tolerance – Transplantation tolerance, defined as specific immune nonresponsiveness to a foreign cell or organ graft, remains a central goal in every field of transplantation. Safe and reliable means for inducing xenogeneic tolerance could substantially advance clinical application of xenotransplantation [75,76], but widely applicable approaches are still lacking [77,78]. Successful induction of immunological tolerance could potentially avert ABMR and the requirement for intense and toxic regimens of immunosuppression. However, since complement, coagulation, and leukocytes alone sometimes can cause vascular pathology resembling ABMR in xenografts, tolerance alone might not avert graft loss. Thus, effective implementation of tolerance may still require genetic modification of xenograft sources (eg, transgenic expression of complement regulators and CD47) to decrease or eliminate antibody-independent pathways of vascular injury. (See 'Genetic engineering of pig sources' above.)

Among the most promising approaches to inducing tolerance for xenotransplantation is the engraftment of donor bone marrow or hematopoietic stem cells into recipients [76,79-81]. These approaches usually require conditioning (ie, depletion of mature T and B cells), which might be accomplished by treatment with lymphoid-depleting antibodies and/or lethal irradiation. Another popular approach is to inhibit "costimulation," leading to the paralysis of mature B and T cells. Whether these or other approaches can enable the development of tolerance to highly disparate transplants is under investigation [77]. (See "Transplantation immunobiology".)

Inducing xenogeneic tolerance appears to be more difficult than inducing allogeneic tolerance, in part because the number of immunogens in xenografts vastly exceeds the number in allografts and because incompatibilities impair survival of the stem cells and grafts [82]. However, human hematopoietic stem cells engrafted in unmodified fetal pigs survive and generate mature lineages for more than two years, and pigs harboring such grafts appear tolerant to the human source [83], raising the possibility that xenogeneic tolerance eventually might be achieved in some individuals without undue risk. These observations also raise the possibility that if a kidney disease were detected in utero, xenogeneic tolerance might be induced to facilitate xenotransplantation early in life [6,84,85].

●Genetic engineering – Recent advances have enabled multiple genetic modifications to be introduced in pigs to suppress complement, coagulation, thrombosis, and inflammation in a xenograft, thereby decreasing but not eliminating susceptibility to ABMR. These modifications not only avert hyperacute rejection but allow prolonged survival and function of xenografts in nonhuman primates and in a few human recipients. (See 'Genetic engineering of pig sources' above.)

Early ABMR caused by increased production of natural and T cell-independent antibodies to saccharides is averted by gene targeting of saccharide biosynthetic enzymes. However, T cell-dependent B cell responses can still target xenotransplants [66], leading to ongoing susceptibility to ABMR and inflammation [45,86]. The preferred solution might involve further engineering or selection of the pig sources, thereby avoiding further increases in immunosuppression of xenograft recipients. However, prolonged survival of organ xenografts still requires treatment with severe regimens of induction and maintenance immunosuppression. Eventually, the acute and chronic problems associated with antibody-mediated injury may also require facilitating repair and regeneration in transplants and optimizing the genetic background of source animals.

Some suggest that known antigens could be modified to eliminate epitopes recognized by individual or subsets of recipients [87]. Susceptibility to ABMR and vascular injury might also be decreased by enhancing expression of protective saccharides or by improving the capacity of the xenograft for repair and regeneration [66].

Cellular rejection — Xenografts, like allografts, are susceptible to cellular rejection. Critical questions include the following:

●Whether the cellular immunity to and rejection of xenografts resembles that of allografts [88]. This question is especially important because T cell-dependent B cell responses represent the most important immunologic hurdle to clinical application.

●Whether existing immunosuppressive therapies and regimens provide optimal control of cell-mediated immunity. Immunosuppression used for allotransplant rejection appears to prevent the cellular rejection of xenotransplants over periods of weeks to months [89] and limits production of large quantities of antibodies against xenoantigens [90]. However, success appears to require more intense induction and maintenance immunosuppression than that used for successful allotransplantation. Some argue that immune responses to xenotransplantation may be so severe that tolerance will be needed [76].

If the mechanisms of cell-mediated immunity to xenotransplants are the same as those to allotransplants but are quantitatively more severe, then induction of tolerance or use of less toxic immunosuppressive agents might be necessary. However, if T cell responses to xenotransplantation differ from those evoked by allografts, then genetic engineering or novel therapeutics might suffice for control [66,87]. In either case, clinical trials will be needed to identify opportunities and optimize regimens and outcomes, and these trials will likely delay realization of the potential impact of xenotransplantation.

Chronic rejection — The bane of kidney allografts is chronic rejection, and it is reasonable to assume chronic rejection will also afflict kidney xenografts. However, whether organ xenografts will indeed suffer chronic rejection and whether the manifestations and severity will resemble those in allografts is unclear. To the extent that chronic rejection reflects a biological response of the graft, the diathesis, manifestations, markers, and outcomes in a xenogeneic kidney transplant might differ from those in an allogeneic kidney transplant. Regardless, the ability to limit ischemic injury and potentially control inflammation by genetic engineering could slow the development of chronic rejection, and the ready availability of animal organs makes it possible to replace a chronically rejected xenograft.

The cause of chronic rejection of allografts is often unclear, but antibodies directed against the graft are implicated [91]. To the extent that such antibodies contribute to the rejection of clinical xenografts, the approaches discussed above for ABMR might also be applied to chronic rejection. Whether accommodation can be sufficiently durable and whether it could have a detrimental long-term impact are unknown [66]. (See "Kidney transplantation in adults: Chronic allograft nephropathy".)

Infection — The possibility of transferring an infectious agent from the xenotransplant to the recipient and to others in the community is one of the important hurdles to the clinical application of xenotransplantation [92]. Another concern is the possibility that a microorganism carried by a xenograft could undergo mutation, recombination, and selection leading to generation of a novel infectious agent. As an example, transfer of influenza virus, coronavirus, and type C retroviruses between species is thought to have generated new viral strains that led to widespread infections [93-97]. However, genetic engineering of pig sources does not require passage of organisms by transplantation, and all or nearly all microorganisms infectious for the pig can be excluded from potential sources of xenografts by rigorous approaches to animal raising and isolation. In this way, a xenograft would probably be a less likely source of infection than a human allograft.

What cannot be excluded by these approaches are organisms that are endogenous to the porcine genome:

●Porcine endogenous retrovirus (PERV) – Most pigs harbor an endogenous retrovirus known as PERV, which is capable of infecting human cells in culture and murine cells in vivo [92,98]. Studies of human subjects have not demonstrated transfer of this virus to human cells in vivo, or generation of a novel virus capable of transfer from one individual to another or producing disease [99]. However, PERV can infect human cells that fuse spontaneously with swine cells [93].

Various reasons for the limited infectivity of PERV have been hypothesized, including natural resistance of the human host [94]. Clinical trials are required to determine whether this agent poses any risk for humans. Genetic engineering has been successfully employed to inactivate at least some PERV inserts; whether such engineering is necessary and whether it influences the fate of grafts remain to be determined.

●Porcine cytomegalovirus (PCMV) – PCMV is detected in many strains of pigs [92], and tissue invasive disease and functional compromise caused by this organism have been described in pig-to-primate xenotransplant models. However, PCMV can be excluded from swine herds by early weaning of newborns [92].

Ethical considerations — The ethical aspects of xenotransplantation have been considered by various agencies, such as the Institute of Medicine in the United States [95], the Nuffield Council in Great Britain [96], and the International Xenotransplantation Association [100]. Although there are some individuals who oppose any use of animals for purposes such as xenotransplantation, public agencies have concluded that such opposition should not be public policy in societies that allow the use of animals as a source of food.

Of greater concern is the possibility that a xenograft may carry an infectious agent to the xenograft recipient and more broadly into society, thereby making the xenograft potentially a matter of public health. As indicated previously, there is no evidence that a porcine xenograft would introduce infectious agents distinct from those introduced through animal husbandry, slaughterhouses, and other agricultural activities unrelated to transplantation. (See 'Infection' above.)

One potential exception is PERV, which apparently has not entered human populations [100]. However, there is no evidence that PERV can infect or spread among humans as a result of xenotransplantation or other types of exposure to pigs. Nevertheless, because there remains concern about this issue, agencies in various countries have formulated public policies for the regulation of xenotransplantation and the screening and evaluation of recipients [100].

CLINICAL CONSIDERATIONS — Interest in xenotransplantation of the kidney and other organs is motivated by the profound shortage of donated human organs. Many more donated kidneys would be needed to meet the current demand for kidney transplantation, and that demand appears likely to increase as lifespan and the prevalence of diabetes and cancer increase [48]. Because the supply of pigs may exceed the demand for transplants, and because the immunologic barriers to xenotransplantation may be addressed in part by genetic engineering and in part by modern immune therapeutics, xenotransplantation might enable organ transplantation to achieve its full impact for the first time. However, several issues must be addressed before that can be achieved.

Patient selection — As clinical trials of xenotransplantation are planned, a suitable recipient for "experimental" kidney xenotransplants must be identified. Identification of these recipients is challenging because the availability of dialysis and potentially allotransplantation makes xenotransplantation of the kidney less of a life-saving procedure than xenotransplantation of the heart or liver. Weighing the immediate availability of xenotransplantation against the risk caused by prolonged wait for an allograft and the aggregate risk of dialysis initiation added to allotransplantation might eventually favor xenotransplantation for some individuals.

If xenotransplantation is eventually made safe and effective, some considerations could make it preferable over allotransplantation, such as lower cost [48,55]. However, for xenotransplantation to be considered comparable to dialysis and allotransplantation, the intensity of immunosuppression must be decreased and prolonged function and survival increased. Until then, clinical trials of kidney xenotransplantation might be undertaken in selected sets of recipients, such as the following:

●Presensitized patients – Kidney xenotransplantation may be initially considered for a limited number of individuals who are presensitized to multiple potential donors and are therefore unlikely to receive an allograft without extended delay. Limited evidence suggests that some antihuman leukocyte antigen (HLA) antibodies do not crossreact with porcine major histocompatibility antigens (SLA) and that xenotransplantation might not sensitize recipients to alloantigens [101,102]. However, if xenotransplantation with less intense immunosuppression sensitizes recipients to HLA, then such sensitization must be counted as a risk.

●Infants – Kidney xenotransplantation might be considered for certain young infants. When dialysis is very difficult to perform and allografts are unavailable because of the mismatch in size between the adult human kidney and the infant, a temporary xenograft might be considered as a bridge to allotransplantation. (See "Kidney transplantation in children: General principles".)

●Patients with primary hyperoxaluria – Xenotransplantation might be explored as an interim procedure for patients with primary hyperoxaluria. Conventional transplant experience in these patients has been relatively disappointing because residual oxalate damages the transplant. A xenograft might be used as a temporary device to clear oxalate from selected patients to avert some damage or to improve the prospects for allotransplantation. (See "Primary hyperoxaluria" and "Kidney transplantation in children: Complications".)

Immunosuppression — The regimens of induction and maintenance immunosuppression used in experimental kidney xenotransplantation and in recipients of clinical cardiac xenografts are much more intense than those used for allotransplantation, including depletion of T and B cells and ongoing disruption of CD40-CD40L binding. While these regimens might be acceptable for some individuals, they may impose risks exceeding those of dialysis. Thus, widespread application of kidney xenotransplantation will require decreasing the intensity of the immunosuppressive regimens. However, finding and testing new regimens and agents will delay widespread clinical application and the impact that xenotransplantation might have on public health.

Other considerations — Experimentation in nonhuman primates has identified biological barriers to xenotransplantation and enabled testing of approaches to overcoming these barriers. However, xenotransplantation in nonhuman primates cannot fully model the clinical conditions in which transplantation is applied. As examples:

●Pig kidneys could exhibit toxicity to drugs that do not cause toxicity in human kidneys.

●Pig kidneys could respond in an unforeseen way to human hormones or other agonists; such responses have been observed in pig hearts [103].

●The requirements for regenerative processes in pig kidneys are uncharacterized. Genetic engineering might hinder genetic changes associated with regeneration and accelerate genetic changes associated with aging [104,105].

CLINICAL STUDIES IN HUMANS

●Kidney xenotransplantation – Kidneys from genetically modified cloned pigs have been engrafted into deceased and living human recipients:

•In one report from New York University Langone Hospital, kidneys from genetically modified Gal(alpha1-3)Gal knockout pigs were implanted in two brain-dead human recipients [7,8]. The xenografted kidneys produced urine within moments of reperfusion, and during the ensuing period (up to 54 hours), the estimated glomerular filtration rate increased from 23 to 62 mL/min/1.73 m² in one recipient and from 55 to 109 mL/min/1.73 m² in the second recipient. Biopsies performed at 6, 24, 48, and 54 hours showed no evidence of hyperacute rejection or early antibody-mediated rejection (ABMR).

•In a second report from the University of Alabama at Birmingham, two kidneys from 10 gene-modified pigs were transplanted into one brain-dead human recipient who underwent bilateral native nephrectomies [8]. The kidneys appeared viable until termination 74 hours later; however, urine production was variable, and creatinine clearance did not recover. Hyperacute rejection was not observed; thrombotic microangiopathy was detected at the endpoint.

•In March 2024, surgeons at Massachusetts General Hospital transplanted a kidney from a pig with 69 genetic modifications into a 62-year-old male with end-stage kidney disease (ESKD) [106]. The recipient had had a kidney allograft that failed and was receiving dialysis, albeit with complications and frequent hospitalizations. He was approved for xenotransplantation under a Food and Drug Administration (FDA) Expanded Access Protocol (ie, compassionate use) because substantial delay in allotransplantation could be anticipated. The recipient was treated with tegoprubart (an anti-CD40 ligand antibody) and ravulizumab (an anticomplement C5 monoclonal antibody). The xenograft was reported to be functioning well one week after surgery.

●Cardiac xenotransplantation – In January, 2022, surgeons at the University of Maryland transplanted a heart from a genetically modified pig into a recipient who was suffering from severe end-stage heart failure but was considered ineligible for a cardiac allograft [9]. The pig used as the source had been engineered to eliminate some antigens recognized by natural antibodies and to control activation of complement, inflammation, and coagulation. The cardiac xenograft functioned for seven weeks posttransplant but failed abruptly, and life support was withdrawn on the 60^th day. The cause of failure remains uncertain, but the findings suggest rejection [86].

A second cardiac xenograft at the University of Maryland was performed in 2023 in a recipient that could not receive an allograft or effective mechanical support. The xenograft from a genetically modified pig functioned for six weeks but ultimately failed from what was considered rejection.

ALTERNATIVE AVENUES — Xenotransplantation could enter the clinic through various avenues besides organ transplantation. In the past:

●Porcine livers have been used as devices for the treatment of fulminant hepatic failure.

●Porcine skin has been used for the treatment of burns.

●Porcine cells have been transplanted into patients with Parkinson disease and unremitting pain.

●Reverse xenografts wherein human cells are differentiated or expanded in animals before the cells or cell product are used in a patient.

Cell and tissue transplants are less susceptible to antibody-mediated injury because the vascular supply grows into the graft from the recipient, and only a small fraction of antibodies pass through blood vessels to reach foreign cells [48,66,82]. Unmodified porcine hepatocytes can survive for extended periods of time in nonhuman primates provided that T cell responses are adequately controlled. Another advantage of cell and tissue transplantation is that removal of or full replacement of function of the autogenous organ is not required [107]. Key functions of the kidneys (filtration and regulated reabsorption of water and solutes) are mediated by the three-dimensional structure of the kidney, and replacement presently requires transplantation of whole kidneys. However, in the future, stem cells from a patient or primitive kidney might be coaxed to undergo organogenesis in an animal host to generate a kidney or complex tissue for transplantation into a patient. Reverse xenotransplants, as such, perhaps facilitated by inducible expression of transgenic growth factors, could offer a way to generate "autologous" kidneys [6] or autologous human cells or proteins for therapeutic use.

SUMMARY

●Overview and rationale – Xenotransplantation is the transplantation of cells, tissues, and organs between individuals of different species. Interest in kidney xenotransplantation is driven by the considerable shortage of donated human kidneys, as well as advances in genetic engineering and immunology that have reduced the biological barriers to using animals as sources of organs and tissues. However, xenotransplantation remains controversial because the survival and function of xenografts might prove inferior to allografts; may require intense, potentially toxic, immunosuppression; and could potentially convey infections that pose risks to the recipient and to public health. (See 'Rationale for xenotransplantation' above.)

●Pigs as sources of xenografts – The preferred sources of xenografts are pigs genetically engineered to resist the innate immune, inflammatory, and thrombotic barriers to xenotransplantation. Pig kidneys appear capable of providing the life-supporting functions of human kidneys, although there are some minor physiologic differences. (See 'Pigs as sources of xenografts' above.)

●Applications – Xenotransplantation is considered an approach to the replacement of failing organs (such as the kidney, heart, lung, or liver) or tissues (such as pancreatic islets). Clinical application in this context would be the same or very similar to allogeneic transplantation. Other applications of xenotransplantation, such as bridge xenografts and reverse xenografts, also could impact medicine and health. (See 'Applications of xenotransplantation' above.)

●Barriers – Clinical application of xenotransplantation of the kidney still must overcome some difficult barriers. The most vexing hurdle remains the immune response of the recipient against the graft. The powerful immunologic reactions to xenografts impel use of intensive regimens of immunosuppression, and whether these can be made fully acceptable for widespread clinical application remains unclear. It is also uncertain whether porcine kidney xenografts can provide all of the functions a human kidney allograft can provide and whether yet unappreciated incompatibilities can be overcome by genetic engineering of the source or by medical treatment of the recipient. Although risks of infection associated with xenotransplantation appear manageable, uncertainty about these risks and other ethical considerations will likely remain unsettled until xenotransplantation achieves regular clinical application. (See 'Barriers to xenotransplantation' above.)

●Clinical considerations

•Patient selection – Identifying recipients suitable for experimental kidney xenotransplantation is challenging because the availability of dialysis and potentially allotransplantation makes xenotransplantation less of a life-saving procedure than xenotransplantation of the heart or liver. Weighing the immediate availability of xenotransplantation against the risk caused by prolonged wait for an allograft and the aggregate risk of dialysis added to allotransplantation might eventually favor xenotransplantation for some individuals. (See 'Patient selection' above.)

•Immunosuppression – The regimens of induction and maintenance immunosuppression used in experimental kidney xenotransplantation and in recipients of clinical cardiac xenografts are much more intense than those used for allotransplantation, including depletion of T and B cells and ongoing disruption of CD40-CD40L (CD40 ligand) binding. While these regimens might be acceptable for some individuals, they may impose risks exceeding those of dialysis. Thus, widespread application of kidney xenotransplantation will require decreasing the risks of the immunosuppressive regimens. (See 'Immunosuppression' above.)