Microbiological safety of xenotransplantation compared with allotransplantation

Transmission of viral, bacterial, parasitic, and fungal infections via organ allografts is uncommon but may be associated with life‐threatening disease. Internationally, programs for screening of human organ donors for infectious risk are non‐uniform and vary with national standards and the availability of screening assays. Further, the failure to recognize and/or to report transmission events limits the utility of available data regarding the incidence of allograft‐associated disease transmission. Advances in xenotransplantation biology have allowed some limited clinical trials with the prospect for increased opportunities for clinical xenotransplantation. As with human allotransplantation, the examination of infectious risk has been a central theme in these studies. Significant advances have been made in the breeding and screening of swine for preclinical studies including the identification of novel, potential human pathogens derived from source animals. Thus far, “expected” xenograft‐derived pathogens such as porcine cytomegalovirus (PCMV) have become activated in immunosuppressed primates but have not resulted in systemic infection outside the xenograft. PCMV has been bred out of swine herds by early weaning strategies. Conversely, host pathogens such as primate‐derived cytomegalovirus (CMV) have become activated and have produced serious infectious complications. These infections are preventable using antiviral prophylaxis. Xenogeneic tissues appear to be relatively resistant to infection by common human pathogens such as HIV, HTLV and the hepatitis viruses. Concerns regarding the potential activation of latent porcine retroviruses from xenograft tissues have resulted in the development of novel assays for xenotropic porcine endogenous retrovirus (PERV). PERV transmission to primate xenograft recipients or to human cells in in vivo models has not been detected. Multiple intrinsic cellular mechanisms appear to be active in the prevention of infection of human cells by PERV. Further, PERV appears to be susceptible to available antiretroviral agents. Thus, while the absolute risk for such infections remains unknown in the absence of human studies with prolonged graft survival in immunosuppressed xenograft recipients, the risk of transmission to human recipients appears limited. Some general principles have been developed to guide clinical trials: Outcomes of xenotransplantation trials, including any infectious disease transmissions, should be reported in the scientific literature and to appropriate public health authorities. Surveillance programs should be developed to detect known infectious agents as well as previously unknown or unexpected pathogens in the absence of recognizable clinical syndromes. Standardization of procedures and validation by expert and/or reference laboratories are needed for microbiological assays. Such validation may require international collaboration. Repositories of samples from source animals and from recipients prior to, and following xenograft transplantation are essential to the investigation of possible infectious disease events. Infection is common in allograft recipients. Thus, in advance of clinical trials, policies and procedures should be developed to guide the evaluation of any infectious syndromes that may develop. (e.g. fever of unknown origin [FUO], leukocytosis, leukopenia, graft dysfunction, pneumonia, hepatitis, abscess formation) in xenograft recipients. Based on preclinical experience these procedures will include: (i) Exclusion of infectious syndromes commonly associated with allotransplantation (e.g. CMV, bacterial pneumonia); (ii) Evaluation of PERV infection by serologic and NAT testing; (iii) Assessment of other recipients of xenografts derived from the same herd or source of swine; and (iv) Evaluation of social contacts of the recipient. Consideration of investigation of xenograft recipients for unknown pathogens may require application of advanced research technologies, possibly including use of broad‐range molecular probes, microarrays or high throughput pyrosequencing. References: 1. Meije Y, TÖnjes RR, Fishman JA. Retroviral restriction factors and infectious risk in xenotransplantation. Amer J Transplant 2010; 10: 1511–1516. 2. Fishman JA, Scobie L, Takeuchi Y. Xenotransplantation‐associated infectious risk: A WHO consultation. Xenotransplantation 2012; 19: 72–81.     

Xenotransplantation-associated infectious risk: a WHO consultation

Xenotransplantation carries the potential risk of the transmission of infection with the cells or tissues of the graft. The degree of risk is unknown in the absence of clinical trials. The clinical application of xenotransplantation has important implications for infectious disease surveillance, both at the national and international levels. Preclinical data indicate that infectious disease events associated with clinical xenotransplantation from swine, should they occur, will be rare; data in human trials are limited but have demonstrated no transmission of porcine microorganisms including porcine endogenous retrovirus. Xenotransplantation will necessitate the development of surveillance programs to detect known infectious agents and, potentially, previously unknown or unexpected pathogens. The development of surveillance and safety programs for clinical trials in xenotransplantation is guided by a "Precautionary Principle," with the deployment of appropriate screening procedures and assays for source animals and xenograft recipients even in the absence of data suggesting infectious risk. All assays require training, standardization and validation, and sharing of laboratory methods and expertise to optimize the quality of the surveillance and diagnostic testing. Investigation of suspected xenogeneic infection events (xenosis, xenozoonosis) should be performed in collaboration with an expert data safety review panel and the appropriate public health and competent authorities. It should be considered an obligation of performance of xenotransplantation trials to report outcomes, including any infectious disease transmissions, in the scientific literature. Repositories of samples from source animals and from recipients prior to, and following xenograft transplantation are essential to the investigation of possible infectious disease events. Concerns over any potential hazards associated with xenotransplantation may overshadow potential benefits. Careful microbiological screening of source animals used as xenotransplant donors may enhance the safety of transplantation beyond that of allotransplant procedures. Xenogeneic tissues may be relatively resistant to infection by some human pathogens. Moreover, xenotransplantation may be made available at the time when patients require organ replacement on a clinical basis. Insights gained in studies of the microbiology and immunology of xenotransplantation will benefit transplant recipients in the future. This document summarizes approaches to disease surveillance in individual recipients of nonhuman tissues.     

Prevention of infection in xenotransplantation: Designated pathogen-free swine in the safety equation

Post-transplantation infections are common. In immunosuppressed human xenograft recipients, infection is most likely to be due to the same pathogens seen in human allotransplantation. However, organisms derived from swine and transmitted with xenografts have the potential to cause novel infections in xenograft recipients. The specific organisms likely to cause infection or “xenosis” are unknown but are postulated to be like those causing infection in allograft recipients. On this basis, theoretical exclusion criteria have been developed to guide the development of source animal herds. Herds developed based on the exclusion of potential human pathogens have been termed “designated pathogen-free” (DPF). Lists of potential pathogens will require revision with changing epidemiology of infection in swine worldwide and clinical experience. Development of new microbiological assays is required both for animal screening and in clinical diagnosis should infections occur. Genetic modifications of swine have the potential to eliminate certain infectious agents such as the porcine endogenous retrovirus; infectious complications of such modifications have not been observed. Unexpected, off target effects of genetic modifications require further study. Monitoring for infection in asymptomatic recipients is important to define infectious risks which are unknown in the absence of clinical trials data. Advanced microbiological techniques may be applied to diagnose and prevent infection in xenograft recipients.     

Infectious disease risks in xenotransplantation

Hurdles exist to clinical xenotransplantation including potential infectious transmission from nonhuman species to xenograft recipients. In anticipation of clinical trials of xenotransplantation, the associated infectious risks have been investigated. Swine and immunocompromised humans share some potential pathogens. Swine herpesviruses including porcine cytomegalovirus (PCMV) and porcine lymphotropic herpesvirus (PLHV) are largely species‐specific and do not, generally, infect human cells. Human cellular receptors exist for porcine endogenous retrovirus (PERV), which infects certain human‐derived cell lines in vitro. PERV‐inactivated pigs have been produced recently. Human infection due to PERV has not been described. A screening paradigm can be applied to exclude potential human pathogens from “designated pathogen free” breeding colonies. Various microbiological assays have been developed for screening and diagnosis including antibody‐based tests and qualitative and quantitative molecular assays for viruses. Additional assays may be required to diagnose pig‐specific organisms in human xenograft recipients. Significant progress has been made in the evaluation of the potential infectious risks of clinical xenotransplantation. Infectious risk would be amplified by intensive immunosuppression. The available data suggest that risks of xenotransplant‐associated recipient infection are manageable and that clinical trials can be performed safely. Possible infectious risks of xenotransplantation to the community at large are undefined but merit consideration.     

Is sensitization to pig antigens detrimental to subsequent allotransplantation?

An important question in xenotransplantation is whether an allotransplant can safely be carried out in a patient who has become sensitized to a pig xenograft. To answer this question, we have searched the literature. We primarily limited our review to the clinically relevant pig‐to‐non‐human primate (NHP) model and found five studies that explored this topic. No NHP that had received a pig graft developed antibodies to alloantigens, and in vitro studies indicated no increased humoral and/or cellular alloreactivity. We carried out a small in vitro study ourselves that confirmed this conclusion. There have been three experiments in which patients undergoing dialysis were exposed to wild‐type pig kidneys and three clinical studies related to bridging a patient in hepatic failure to liver allotransplantation. Despite the development of anti‐pig antibodies, all subsequent organ (kidney or liver) allografts were successful (except possibly in one case). In addition, pig fetal islets were transplanted into patients with kidney allografts; there was no increase in panel‐reactive alloantibodies and the kidney grafts continued to function satisfactorily. In conclusion, the limited data suggest that, after sensitization to pig antigens, there is no evidence of antibody‐mediated or accelerated cellular rejection of a subsequent allograft.     

Potential pathological role of pro‐inflammatory cytokines (IL‐6, TNF‐α, and IL‐17) in xenotransplantation

The major limitation of organ transplantation is the shortage of available organs from deceased human donors which leads to the deaths of thousands of patients each year. Xenotransplantation is considered to be an effective way to resolve the problem. Immune rejection and coagulation dysfunction are two major hurdles for the successful survival of pig xenografts in primate recipients. Pro‐inflammatory cytokines, such as IL‐6, TNF‐α, and IL‐17, play important roles in many diseases and in allotransplantation. However, the pathological roles of these pro‐inflammatory cytokines in xenotransplantation remain unclear. Here, we briefly review the signaling transduction and expression regulation of IL‐6, TNF‐α, and IL‐17 and evaluate their potential pathological roles in in vitro and in vivo models of xenotransplantation. We found that IL‐6, TNF‐α, and IL‐17 were induced in most in vitro or in vivo xenotransplantation model. Blockade of these cytokines using gene modification, antibody, or inhibitor had different effects in xenotransplantation. Inhibition of IL‐6 signaling with tocilizumab decreased CRP but did not increase xenograft survival. The one possible reason is that tocilizumab can not suppress IL‐6 signaling in porcine cells or organs. Other drugs which inhibit IL‐6 signaling need to be investigated in xenotransplantation model. Inhibition of TNF‐α was beneficial for the survival of xenografts in pig‐to‐mouse, rat, or NHP models. Blockade of IL‐17 using a neutralizing antibody also increased xenograft survival in several animal models. However, the role of IL‐17 in the pig‐to‐NHP xenotransplantation model remains unclear and needs to be further investigated. Moreover, blockade of TNF‐α and IL‐6 together has got a better effect in pig‐to‐baboon kidney xenotransplantation. Blockade two or even more cytokines together might get better effect in suppressing xenograft rejection. Better understanding the role of these cytokines in xenotransplantation will be beneficial for choosing better immunosuppressive strategy or producing genetic modification pig.     

Human and non-human primate anti-galactosyl response after injection of rat monoclonal antibody bearing galactosyl epitopes

Abstract: In the case of clinical use of pig‐to‐human xenografting, any exogenous source of α‐galactosyl epitopes will elicit an anti‐galactosyl immune response, which could be deleterious for the xenograft. The presence of Galα(1–3)Gal residues was thus examined by western blotting on various rat monoclonal antibodies (mAb), which are used in clinical trials. In parallel, the anti‐galactosyl humoral response was assessed in the serum of kidney allograft recipients and experimental baboons, which received these mAbs. Galactosyl residues were evidenced on all rat monoclonal antibody tested. The anti‐galactosyl response was weak in kidney allograft recipients receiving a basic immunosuppression (Cyclosporine, Azathioprine, Prednisolone) and iterative injections of rat mAbs. In contrast, untreated or immunosuppressed baboons that received rat mAbs developed a major anti‐galactosyl humoral response. These results suggest that anti‐galactosyl sensitization produced by therapeutic agents will have to be considered in the case of clinical xenotransplantation.     

Chronic dialysis in patients with end‐stage renal disease: Relevance to kidney xenotransplantation

Renal allotransplantation clearly offers better survival and quality of life for end‐stage renal disease (ESRD) patients than chronic dialysis. The median waiting time for a deceased donor kidney in a suitable ESRD patient is 3.9 years. The initial candidates for pig kidney xenotransplantation will be those with ESRD unlikely to receive an allograft within a reasonable period of time. It is thus reasonable to ascertain whether clinical trials of xenotransplantation might likewise offer superior outcomes. Chronic dialysis in patients with ESRD is associated with poor quality of life, significant morbidity, and relatively high mortality, with only 56% surviving 3 years and 42% at 5 years. However, a significant number of these patients, because of comorbidities, frailty, etc, would not be considered for renal allotransplantation and likely not for xenotransplantation. As genetically engineered pig kidneys have satisfactorily supported life in immunosuppressed nonhuman primates for many months or even more than a year, consideration in carefully selected patients could be given to pig kidney xenotransplantation. We suggest that, in order to give a patient the best possible outcome, the pig kidney could be transplanted pre‐emptively (before dialysis is initiated). If it fails at any stage, the patient would then begin chronic dialysis and continue to await an allograft. The present (limited) evidence is that failure of a pig graft would not be detrimental to a subsequent allograft.     

Regulatory T cells from allo‐ to xenotransplantation: Opportunities and challenges

Regulatory T cells (Treg) are currently being evaluated in clinical allotransplantation for tolerance induction, with proven safety in humans with autoimmune diseases and graft‐versus‐host disease. A considerable amount of recent data suggests that additional factors may need to be validated, including the stability and commitment of newly discovered Treg subsets under inflammatory conditions, to further warrant safe and effective Treg‐based therapeutic approaches. This review explores the opportunities and challenges of Treg‐based cell therapy in xenotransplantation. The emerging new technologies for genetic modifications of the donor pig offer a major advantage for Treg therapy to improve xenograft protection. Particularly, the feasibility of (i) ex vivo expansion of donor (pig)‐specific Treg for infusion, and (ii) development of Treg in situ for the life of the xenograft. Our understanding of the Treg biology and their role in xenograft protection, under the newly developed immunosuppressive protocols remains limited. The incidence of various Treg subpopulations in xenograft recipients and their suppressive efficacy across species barriers are largely unknown. Finally, deciphering the dynamics of Treg function, and their interaction with adaptive and innate immune cells are of critical importance to design safe, effective and clinically relevant Treg‐based therapeutic approaches in xenotransplantation.     

Donor‐Derived Fungal Infections in Organ Transplant Recipients: Guidelines of the American Society of Transplantation,Infectious Diseases Community of Practice†

Donor‐derived fungal infections can be associated with serious complications in transplant recipients. Most cases of donor‐derived candidiasis have occurred in kidney transplant recipients in whom contaminated preservation fluid is a commonly proposed source. Donors with cryptococcal disease, including those with unrecognized cryptococcal meningoencephalitis may transmit the infection with the allograft. Active histoplasmosis or undiagnosed and presumably asymptomatic infection in the donor that had not resolved by the time of death can result in donor‐derived histoplasmosis in the recipient. Potential donors from an endemic area with either active or occult infection can also transmit coccidioidomycosis. Rare instances of aspergillosis and other mycoses, including agents of mucormycosis may also be transmitted from infected donors. Appropriate diagnostic evaluation and prompt initiation of appropriate antifungal therapy are warranted if donor‐derived fungal infections are a consideration. This document discusses the characteristics, evaluation and approach to the management of donor‐derived fungal infections in organ transplant recipients.     

T cell-mediated xenograft rejection: Specific tolerance is probably required for long term xenograft survival

Abstract: T cell-mediated mechanisms of xenograft rejection appear resistant to standard immunosuppression protocols used to prevent allograft rejection and, consequently, higher doses of immunosuppressive drugs are required to promote xenograft compared to allograft survival. Evidence from recent studies suggests that porcine xenografts may be especially immunogenic in humans because of a prominent and vigorous indirect xenoresponse and because of the ability of porcine endothelium to activate human T cells. This has led to an anxiety that systemic immunosuppressives, used as the mainstay of therapy for clinical xenotransplantation, may not allow the long-term survival of porcine organs transplanted into human recipients. This article will review the biology of T cell xenoresponses, present the case for the development of novel graft-specific immunosuppressive regimes in clinical xenotransplantation, and review recent experimental progress in this area.     

Reduced Efficacy of Ganciclovir Against Porcine and Baboon Cytomegalovirus in Pig-to-Baboon Xenotransplantation

In pig-to-baboon xenotransplantation, porcine cytomegalovirus (PCMV) causes viremia, consumptive coagulopathy, and tissue-invasive disease. Baboon cytomegalovirus (BCMV) is associated with invasive disease in xenograft recipients. The efficacy of prophylaxis with intravenous ganciclovir (GCV) was studied for prevention of PCMV and BCMV infections in pig-to baboon xenotransplantation. GCV prophylaxis did not alter the incidence of BCMV activation in recipients, but reduced the amount of virus in tissues (mean 8.38 x 10(2) vs. 3.24 x 10(5) copies/ micro g DNA without treatment) and prevented tissue-invasive infections. PCMV viral loads were unaltered by GCV prophylaxis (8.36 x 10(8) copies/ micro g DNA without prophylaxis vs. 1.20 x 10(9) copies/ micro g DNA with prophylaxis). In vitro, PCMV was relatively resistant to GCV [90% inhibitory concentration (IC90) of 10 micro m, IC50 = 3 micro m], acyclovir (100 micro m), and leflunomide (not achievable). Only cidofovir (IC90 1 micro m) and foscarnet (IC90 100 micro m) might have therapeutic efficacy for PCMV in vivo in achievable concentrations, although these agents often carry significant toxicity in transplant recipients. GCV has limited activity against BCMV and no therapeutic efficacy against PCMV at standard doses in vivo. GCV and other antiviral agents have limited activities against PCMV in vitro. Breeding of PCMV-free xenograft donors may be necessary to prevent PCMV infections in clinical trials.     

Another Look at Cardiac Xenotransplantation

Clinical incentive for development of neonatal cardiac xenotransplantation is clear. Approximately 10% of babies born with congenital heart disease might benefit from transplantation therapy, but human donor resources in this age group are limited. A scientific foundation upon which to base clinical trials of neonatal cardiac xenotransplantation is presented. A single clinical case and its outcome is reviewed. What was learned from this case supports continued clinical trials in the form of xenograft "bridging" to allotransplantation. There are perhaps three potential impediments to progress in xenotransplantation. These restrictions may be broadly categorized as: (1) immunological; (2) philosophical; and (3) sociological. None of these potential "barriers" is absolute and incentive is strong to continue exploration of neonatal cardiac xenotransplantation.     

Donor‐derived infections: Guidelines from the American Society of Transplantation Infectious Diseases Community of Practice

These updated guidelines from the Infectious Diseases Community of Practice of the American Society of Transplantation will review the current state of the art of donor‐derived infections. Specifically, the guideline will summarize standardized definitions and approaches to defining imputability, updated data on the epidemiology of donor‐derived infections, and approaches to risk mitigation against transmission of infections. This update will additionally provide guidance on the use of HIV+ donors in HIV+ recipients, the use of HCV‐viremic donors in non‐viremic recipients, donors with endemic infections, and donors with bacteremia, meningitis, and encephalitis. Lastly, the guidance will summarize an approach to recipients with a suspected donor‐derived infection.     

Organ Donor Screening Practices for Trypanosoma cruzi Infection among US Organ Procurement Organizations

Donor‐derived Trypanosoma cruzi infection in solid organ transplant recipients is associated with significant morbidity and mortality. Little is known about T. cruzi screening practices among U.S. organ procurement organizations (OPOs). We distributed a questionnaire to all U.S. OPO directors, requesting data on T. cruzi screening strategies, laboratory methods, number of donors screened, disposition of organs from positive donors and attitudes toward screening. Fifty‐eight (100%) U.S. OPOs responded to the survey. Donor screening began in 2002 and is presently performed by 11 (19%) OPOs. Among screening OPOs, four screen all donors and seven use a risk‐based strategy. Three different T. cruzi serology tests are used for donor screening. During 2008, 9/993 (0.9%) donors screened positive by a T. cruzi screening test, 6/9 (66%) had confirmatory tests performed and 4/6 (66%) had positive confirmatory tests. These results led to the nonuse of five donors and 17 organs. Five organs from three seropositive donors were transplanted in 2008 without recognized disease transmission. Variability of T. cruzi donor screening strategies, laboratory methods and disposition of organs from positive donors currently exists. Further research is needed to identify the risk of donor‐derived T. cruzi infections to help inform the best screening strategy.     

Lung transplantation. Xenotransplantation

The continued and growing success of lung allotransplantation has intensified the worldwide shortage of donor organs. Yet, xenotransplantation remains a daunting challenge. Additional molecular incompatibilities and unforeseen complications will continue to be discovered. Progress has been made, notably on the generation of alpha-Gal double knockout pigs. Progressive increases in organ survival times have been seen for most organs after significant investments of time and money. The lung continues to be an organ with the lowest supply of cadaveric donors and the least potential for expanded living donation or mechanical alternatives. As such, the impetus for xenotransplantation is strong. The lung appears to be exquisitely sensitive to xenograft rejection and resistant to strategies that have been moderately successful in other organs. A complex program involving genetically modified donor organs, recipient preparation for antibody removal or tolerance promotion, and multitargeted drug therapy will likely be required for successful clinical application.     

异种移植免疫排斥的研究进展

异种移植是解决人体器官严重短缺的重要思路.随着对异种移植排斥和人畜共患感染性疾病的深入研究,以及α-1,3-半乳糖苷转移酶基因敲除猪的成功构建,以猪为供体的异种移植与临床应用之间的距离正在逐渐缩短.阻碍异种移植发展的主要障碍仍是免疫排斥反应.本文试就目前异种免疫排斥的研究进展进行综述,希望对未来的临床异种移植研究提供参...     

Cardiac xenotransplantation—state of the art 2010

The availability of double transgenic pigs (Gal‐KO, CD46) is a major step towards clinical xenogenic heart transplantation. The addition of further transgenes (e.g. with human thrombomodulin/hemoxygenase‐1) will further advance the field. An expanded predictable breeding program to allow adequate numbers of donors for preclinical trials is now a priority. It is now an appropriate time to consider potential indications for clinical application. Clearly, there are patients who have difficulties in finding appropriate donor allotransplants such as those with low body surface areas <1.5m², the obese and the hypersensitized (HLA does not crossreact with SLA). Also terminally ill patients needing biventricular assist devices would be potential recipients, since the mid‐ and long‐term results of total artificial hearts or biventricular devices are significantly worse when compared to those patients with left ventricular assist devices alone. At least at the beginning of the clinical experience, heterotopic thoracic heart transplantations (according to the techniques of Losman and Barnard) should be considered with both donor and recipient organs contributing to the cardiac output. Thereafter an allotransplantation could be performed leaving the xenotransplant in place while replacing the (diseased) recipient heart. This strategy had been proven successful in the past and in the clinical setting of allotransplant vasculopathy. Finally, in regard to safety issues: Initially the few worldwide available designated pathogen free units would be sufficient to supply appropriate animals. A prerequisite for this would be a change in the European legislation allowing the import of live animal stock from other countries.     

Availability,Utilization and Outcomes of Deceased Diabetic Donor Kidneys; Analysis Based on the UNOS Registry

The number of kidneys obtained from deceased diabetic donors available for transplantation has increased >eightfold increase in the past 15 years. We assessed allograft outcomes associated with deceased diabetic donors and compared them with that of standard and extended criteria donors (ECD) in the UNOS data registry. We identified 1982 recipients of diabetic standard criteria donors over a 10‐year period from 1995 through 2004. Both overall and death‐censored survival of organs from diabetic standard criteria donors was significantly better than that of organs obtained from nondiabetic ECD while inferior to that from nondiabetic standard criteria donors. Compared with ECD donors, diabetic donors had lower serum creatinine, less cold ischemia and these kidneys were less likely to be pump‐perfused. Recipients of diabetic kidneys were younger and less likely to experience delayed graft function compared with recipient of ECD kidneys. More recently, many diabetic donor kidneys have been given to diabetic recipients with early graft survival being similar to that among nondiabetic recipients. These findings demonstrate the potential to expand and to improve utilization of this resource without compromising outcomes for recipients. Improved, evidence‐based evaluation and allocation of deceased diabetic donor kidneys is needed to optimize their use.     

Review Article: CD47 in xenograft rejection and tolerance induction

Yang Y‐G. CD47 in xenograft rejection and tolerance induction. Xenotransplantation 2010; 17: 267–273. © 2010 John Wiley & Sons A/S. Abstract: Robust immune responses to xenografts remain a major obstacle to clinical translation of xenotransplantation, which could otherwise be a potential solution to the worldwide shortage of organ donors. The more vigorous xenograft rejection relative to allograft rejection is largely accounted for by the extensive genetic disparities between the donor and recipient. Xenografts activate host immunity not only by expressing immunogenic xenoantigens that provide the targets for immune recognition and rejection, but also by lacking ligands for the host immune inhibitory receptors. This review is focused on recent findings regarding the role of CD47, a ligand of an immune inhibitory receptor SIRPα, in xenograft rejection and induction of xenotolerance.