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Genetically modified pig kidney xenografts were transplanted into two brain-dead human recipients. The xenografts functioned immediately and showed no evidence of acute rejection on serial biopsy over a period of 54 hours. The serum creatinine level decreased in both recipients.

Xenotransplantation is a promising strategy to alleviate the shortage of organs for human transplantation. In addition to the concerns about pig-to-human immunological compatibility, the risk of cross-species transmission of porcine endogenous retroviruses (PERVs) has impeded the clinical application of this approach. We previously demonstrated the feasibility of inactivating PERV activity in an immortalized pig cell line. We now confirm that PERVs infect human cells, and we observe the horizontal transfer of PERVs among human cells. Using CRISPR-Cas9, we inactivated all of the PERVs in a porcine primary cell line and generated PERV-inactivated pigs via somatic cell nuclear transfer. Our study highlights the value of PERV inactivation to prevent cross-species viral transmission and demonstrates the successful production of PERV-inactivated animals to address the safety concern in clinical xenotransplantation.

Xenotransplantation is a potential solution to the shortage of transplantable human organs. The author discusses the benefits and infectious risks of xenotransplantation and possible protocols for future use.

In light of the recent clinical cases of pig-to-human kidney and heart transplantation, taking stock of progress in xenotransplantation can help us set realistic expectations for clinical adoption of this promising technology.

Triple-knockout (TKO) pigs (with added protective human transgenes) are likely to be optimal sources of organs for clinical organ xenotransplantation because many humans have minimal or no natural antibody to TKO pig cells. However, Old World monkeys (OWMs) have naturally-existing antibodies directed to TKO cells. We measured anti-pig IgM/IgG binding, and complement-dependent cytotoxicity to wild-type (WT), α1,3-galactosyltransferase gene-knockout (GTKO), and TKO pig peripheral blood mononuclear cells (PBMCs) using sera from humans, several OWMs, and two New World monkeys (NWMs). Furthermore, we compared survival of GTKO (n = 5) and TKO (n = 3) pig kidneys in baboons. OWMs had significantly greater IgM binding and cytotoxicity to TKO PBMCs than humans or NWMs. Mean anti-TKO IgM was significantly higher in OWMs and significantly lower in NWMs than in humans. Cytotoxicity of OWM sera to TKO PBMCs was significantly greater than of human serum, but there was no significant difference between human and NWM sera. The median survival of TKO pig kidneys (4 days) in baboons was significantly shorter than that of GTKO kidneys (136 days) (p < 0.05). Even though considered ideal for clinical xenotransplantation, the presence of naturally-existing antibodies to TKO pig cells in OWMs complicates the transplantation of TKO pig kidneys in OWMs.

Preclinical and clinical xenotransplantation trials have shown that successful outcomes depend on a number of factors including the prevention of xenozoonoses. Preclinical trials involving pig kidneys and hearts transplanted into various non-human primates have revealed the potential impact of pig pathogens being present in the transplanted organ/tissue, mainly viruses. The concept of “designated pathogen-free donor animals” was developed to ensure elimination of pathogens during the breeding of donor animals to mitigate this occurrence. This is a challenging process as confirmation of presence and absence of some pathogen, in particular for latent viruses, requires a validated armamentarium of direct and indirect tests. The importance of using the correct diagnostic regimen was highlighted during the first pig-to-man cardiac transplantation with both porcine cytomegalovirus (PCMV), also known as porcine roseolovirus (PRV), and porcine circovirus (PCV) detected in the transplanted organ and in the patient. To further improve xenotransplantation and to achieve trials in Europe it is important that we use these data to inform process for diagnostics both in donor and recipients before and after xenotransplantation to ensure safety. As part of this sensitive and specific pathogen detection systems should be validated and readily available.

Islet transplantation is a treatment modality for diabetes mellitus that can maintain insulin levels within a physiologically appropriate range. However, wider clinical application is limited by insufficient donor numbers and a need for lifelong immunosuppression. Despite various clinical and preclinical trials, there is no single standard immunosuppressive regimen that can suppress acute and chronic immune reactions with lower toxicity to grafted islets. One of the strategies for overcoming lifelong immunosuppression is the incorporation of encapsulation technology, which can provide a physical immune barrier by keeping out high molecular weight immune system components, while still allowing low molecular weight oxygen, insulin and nutrients to pass through. Encapsulated islet transplantation approaches that have been studied so far include macroencapsulation, microencapsulation, conformal coating and nanoencapsulation. Herein we will review the basic concepts of islet encapsulation technique, earlier works to recent progress related to clinical studies and corporate investigations on encapsulated islet transplantation.