There is a critical shortage in the number of deceased human organs that become available for the purposes of clinical transplantation. This problem might be resolved by the transplantation of organs from pigs genetically engineered to protect them from the human immune response. The pathobiological barriers to successful pig organ transplantation in primates include activation of the innate and adaptive immune systems, coagulation dysregulation and inflammation. Genetic engineering of the pig as an organ source has increased the survival of the transplanted pig heart, kidney, islet and corneal graft in non‐human primates (NHPs) from minutes to months or occasionally years. Genetic engineering may also contribute to any physiological barriers that might be identified, as well as to reducing the risks of transfer of a potentially infectious micro‐organism with the organ. There are now an estimated 40 or more genetic alterations that have been carried out in pigs, with some pigs expressing five or six manipulations. With the new technology now available, it will become increasingly common for a pig to express even more genetic manipulations, and these could be tested in the pig‐to‐NHP models to assess their efficacy and benefit. It is therefore likely that clinical trials of pig kidney, heart and islet transplantation will become feasible in the near future. Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Background: The pig‐to‐non‐human primate model is the standard choice for in vivo studies of organ and cell xenotransplantation. In 1998, Lambrigts and his colleagues surveyed the entire world literature and reported all experimental studies in this model. With the increasing number of genetically engineered pigs that have become available during the past few years, this model is being utilized ever more frequently.Methods: We have now reviewed the literature again and have compiled the data we have been able to find for the period January 1, 1998 to December 31, 2013, a period of 16 yr.Results: The data are presented for transplants of the heart (heterotopic and orthotopic), kidney, liver, lung, islets, neuronal cells, hepatocytes, corneas, artery patches, and skin. Heart, kidney, and, particularly, islet xenograft survival have increased significantly since 1998.Discussion: The reasons for this are briefly discussed. A comment on the limitations of the model has been made, particularly with regard to those that will affect progression of xenotransplantation toward the clinic.

Background: Dysregulation of coagulation is considered a major barrier against successful pig organ xenotransplantation in non‐human primates. Inflammation is known to promote activation of coagulation. The role of pro‐inflammatory factors as well as the relationship between inflammation and activation of coagulation in xenograft recipients is poorly understood.Methods: Baboons received kidney (n = 3), heart (n = 4), or artery patch (n = 8) xenografts from α1,3‐galactosyltransferase gene‐knockout (GTKO) pigs or GTKO pigs additionally transgenic for human complement‐regulatory protein CD46 (GTKO/CD46). Immunosuppression (IS) was based on either CTLA4Ig or anti‐CD154 costimulation blockade. Three artery patch recipients did not receive IS. Pro‐inflammatory cytokines, chemokines, and coagulation parameters were evaluated in the circulation after transplantation. In artery patch recipients, monocytes and dendritic cells (DC) were monitored in peripheral blood. Expression of tissue factor (TF) and CD40 on monocytes and DC were assessed by flow cytometry. C‐reactive protein (C‐RP) levels in the blood and C‐RP deposition in xenografts as well as native organs were evaluated. Baboon and pig C‐RP mRNA in heart and kidney xenografts were evaluated.Results: In heart and kidney xenograft recipients, the levels of INFγ, TNF‐α, IL‐12, and IL‐8 were not significantly higher after transplantation. However, MCP‐1 and IL‐6 levels were significantly higher after transplantation, particularly in kidney recipients. Elevated C‐RP levels preceded activation of coagulation in heart and kidney recipients, where high levels of C‐RP were maintained until the time of euthanasia in both heart and kidney recipients. In artery patch recipients, INFγ, TNF‐α, IL‐12, IL‐8, and MCP‐1 were elevated with no IS, while IL‐6 was not. With IS, INFγ, TNF‐α, IL‐12, IL‐8, and MCP‐1 were reduced, but IL‐6 was elevated. Elevated IL‐6 levels were observed as early as 2 weeks in artery patch recipients. While IS was associated with reduced thrombin activation, fibrinogen and C‐RP levels were increased when IS was given. There was a significant positive correlation between C‐RP, IL‐6, and fibrinogen levels. Additionally, absolute numbers of monocytes were significantly increased when IS was given, but not without IS. This was associated with increased CD40 and TF expression on CD14⁺ monocytes and lineage^neg CD11c⁺ DC, with increased differentiation of the pro‐inflammatory CD14⁺ CD11c⁺ monocyte population. At the time of euthanasia, C‐RP deposition in kidney and heart xenografts, C‐RP positive cells in artery patch xenograft and native lungs were detected. Finally, high levels of both pig and baboon C‐RP mRNA were detected in heart and kidney xenografts.Conclusions: Inflammatory responses precede activation of coagulation after organ xenotransplantation. Early upregulation of C‐RP and IL‐6 levels may amplify activation of coagulation through upregulation of TF on innate immune cells. Prevention of systemic inflammation in xenograft recipients (SIXR) may be required to prevent dysregulation of coagulation and avoid excessive IS after xenotransplantation.

The current supply of acceptable donor livers is not sufficient to meet the demands of listed patients awaiting transplantation resulting in thousands of deaths each year. Increased utilization of marginal livers may help alleviate this supply/demand mismatch by expanding the donor liver pool. The current status of liver transplantation using marginal donor grafts and efforts to optimize usage are discussed with attention to elderly donors, steatotic livers, donors after circulatory death, and split liver grafts.

A lack of deceased human donor livers leads to a significant mortality in patients with acute-on-chronic or acute (fulminant) liver failure or with primary nonfunction of an allograft. Genetically engineered pigs could provide livers that might bridge the patient to allotransplantation. Orthotopic liver transplantation in baboons using livers from α1,3-galactosyltransferase gene-knockout (GTKO) pigs (n = 2) or from GTKO pigs transgenic for CD46 (n = 8) were carried out with a clinically acceptable immunosuppressive regimen. Six of 10 baboons survived for 4–7 days. In all cases, liver function was adequate, as evidenced by tests of detoxification, protein synthesis, complement activity and coagulation parameters. The major problem that prevented more prolonged survival beyond 7 days was a profound thrombocytopenia that developed within 1 h after reperfusion, ultimately resulting in spontaneous hemorrhage at various sites. We postulate that this is associated with the expression of tissue factor on platelets after contact with pig endothelium, resulting in platelet and platelet-peripheral blood mononuclear cell(s) aggregation and deposition of aggregates in the liver graft, though we were unable to confirm this conclusively. If this problem can be resolved, we would anticipate that a pig liver could provide a period during which a patient in liver failure could be successfully bridged to allotransplantation.