Corneal Xenotransplantation From Pig to Rhesus Monkey: No Signs of Transmission of Endogenous Porcine Retroviruses

Virus Safety of Xenotransplantation

The practice of xenotransplantation using pig islet cells or organs is under development to alleviate the shortage of human donor islet cells or organs for the treatment of diabetes or organ failure. Multiple genetically modified pigs were generated to prevent rejection. Xenotransplantation may be associated with the transmission of potentially zoonotic porcine viruses. In order to prevent this, we developed highly sensitive PCR-based, immunologicals and other methods for the detection of numerous xenotransplantation-relevant viruses. These methods were used for the screening of donor pigs and xenotransplant recipients. Of special interest are the porcine endogenous retroviruses (PERVs) that are integrated in the genome of all pigs, which are able to infect human cells, and that cannot be eliminated by methods that other viruses can. We showed, using droplet digital PCR, that the number of PERV proviruses is different in different pigs (usually around 60). Furthermore, the copy number is different in different organs of a single pig, indicating that PERVs are active in the living animals. We showed that in the first clinical trials treating diabetic patients with pig islet cells, no porcine viruses were transmitted. However, in preclinical trials transplanting pig hearts orthotopically into baboons, porcine cytomegalovirus (PCMV), a porcine roseolovirus (PCMV/PRV), and porcine circovirus 3 (PCV3), but no PERVs, were transmitted. PCMV/PRV transmission resulted in a significant reduction of the survival time of the xenotransplant. PCMV/PRV was also transmitted in the first pig heart transplantation to a human patient and possibly contributed to the death of the patient. Transmission means that the virus was detected in the recipient, however it remains unclear whether it can infect primate cells, including human cells. We showed previously that PCMV/PRV can be eliminated from donor pigs by early weaning. PERVs were also not transmitted by inoculation of human cell-adapted PERV into small animals, rhesus monkey, baboons and cynomolgus monkeys, even when pharmaceutical immunosuppression was applied. Since PERVs were not transmitted in clinical, preclinical, or infection experiments, it remains unclear whether they should be inactivated in the pig genome by CRISPR/Cas. In summary, by using our sensitive methods, the safety of xenotransplantation can be ensured.

Local immunosuppression in WZS-pig to rhesus monkey Descemet's stripping automated endothelial keratoplasty: An innovative method to promote the survival of xeno-grafts

Corneal xenotransplantation is an effective solution for the shortage of human corneas. We investigated the feasibility and efficacy of different postoperative protocols on xeno-Descemet's stripping automated endothelial keratoplasty (DSAEK) grafts. Thirty rhesus monkeys were randomly divided into three groups: control group (C), only Descemet's membrane (DM) stripping; DSAEK 1 (D1) and DSAEK 2 (D2) groups, DM stripping followed by endothelial keratoplasty. Betamethasone 3.5 mg was subconjunctival injected in groups control and D1 postoperatively, while animals in group D2 were treated with topical 0.1% tacrolimus and topical steroids. All groups were evaluated by slit-lamp microscopy, anterior segment OCT and LSCM for at least nine months. A total of 24 monkeys (24 eyes) met the inclusion criteria. Nine months after DSAEK surgery, all xenografts showed good attachment, and most corneas were transparent. Graft rejection occurred in 25% of the cases in group D1 and 28.57% of those in group D2 (P > 0.05). The corneal endothelium density in the DSAEK groups was 2715.83±516.20/mm² (D1) and 2220.00 ± 565.13/mm² (D2) (P > 0.05). Xenogeneic corneal endothelial grafts can survive and function in rhesus monkey eyes for a long time with subconjunctival steroid or topical tacrolimus and steroid treatment.

Corneal xenotransplantation: Where are we standing?

The search for alternatives to allotransplants is driven by the shortage of corneal donors and is demanding because of the limitations of the alternatives. Indeed, current progress in genetically engineered (GE) pigs, the introduction of gene-editing technology by clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9, and advanced immunosuppressants have made xenotransplantation a possible option for a human trial. Porcine corneal xenotransplantation is considered applicable because the eye is regarded as an immune-privileged site. Furthermore, recent non-human primate studies have shown long-term survival of porcine xenotransplants in keratoplasty. Herein, corneal immune privilege is briefly introduced, and xenogeneic reactions are compared with allogeneic reactions in corneal transplantation. This review describes the current knowledge on special issues of xenotransplantation, xenogeneic rejection mechanisms, current immunosuppressive regimens of corneal xenotransplantation, preclinical efficacy and safety data of corneal xenotransplantation, and updates of the regulatory framework to conduct a clinical trial on corneal xenotransplantation. We also discuss barriers that might prevent xenotransplantation from becoming common practice, such as ethical dilemmas, public concerns on xenotransplantation, and the possible risk of xenozoonosis. Given that the legal definition of decellularized porcine cornea (DPC) lies somewhere between a medical device and a xenotransplant, the preclinical efficacy and clinical trial data using DPC are included. The review finally provides perspectives on the current standpoint of corneal xenotransplantation in the fields of regenerative medicine.

Intrastromal implantation of chicken corneal grafts into the cornea of rabbits for corneal thickening: an experimental study

PURPOSE

To evaluate the feasibility and effects of the intrastromal implantation of chemically modified corneal stroma obtained from chicken into the corneas of rabbits for corneal thickening.

METHODS

Chicken corneas were cut, debrided, treated with cross-linking and implanted in an intrastromal pouch created in the cornea of 10 white New Zealand rabbits with femtosecond laser. Slit-lamp biomicroscopy and optical coherence tomography were performed immediately, 7, 30 and 90 days postoperatively. Corneas were removed at 90 days and cut in two halves. One half was sent to histological analysis for the presence of necrosis, polymorphonuclear inflammatory cells, blood vessels and fibrosis, while the other half was evaluated with transmission electron microscopy to verify tissue organization and the presence of keratocytes and inflammatory cells. Corneal thicknesses were comparatively analyzed over time with Wilcoxon test (p ≤ 0.05).

RESULTS

The chicken grafts were incorporated into the cornea of all animals over time. Mean rabbit cornea thickness increased from 338 µm preoperatively to 538 µm (p < 0.0077) at 90 days, while mean chicken graft thickness decreased from 350 to 215 µm (p < 0.0077). No clear signs of rejection attributable to the xenograft were observed in any of the implanted eyes. However, some macroscopic and histological events were observed in some of the eyes, probably due to procedural issues during implantation.

CONCLUSION

The intrastromal implantation of chicken grafts was shown to be feasible and predictable to thicken the recipient rabbit cornea without apparent rejection. However, before being considered in humans, further meticulous clinical trials are required to establish the clinical utility, safety and efficacy of xenografts for the treatment of patients with advanced keratoconus.

Reducing porcine corneal graft rejection, with an emphasis on porcine endogenous retrovirus transmission safety: a review

Donor cornea shortage is a primary hurdle in the development of corneal transplantation. Of all species, porcine corneas are the ideal transplantation material for humans. However, the xenoimmune rejection induced by porcine corneal xenotransplantation compromises surgical efficacy. Although the binding of IgM/IgG in human serum to a genetically modified porcine cornea is significantly weaker than that of the wild type (WT), genetically modified porcine corneas do not display a prolonged graft survival time in vivo. Conversely, costimulatory blockade drugs, such as anti-CD40 antibodies, can reduce the xenoimmune response and prolong graft survival time in animal experiments. Moreover, porcine endothelial grafts can survive for more than 6mo with only the subconjunctival injection of a steroid-based immunosuppressants regime; therefore, they show great value for treating corneal endothelial disease. In addition, zoonotic transmission is a primary concern of xenotransplantation. Porcine endogenous retrovirus (PERV) is the most significant virus assessed by ophthalmologists. PERV integrates into the porcine genome and infects human cells in vitro. Fortunately, no evidence from in vivo studies has yet shown that PERV can be transmitted to hosts.

Xenotransplantation in China: Present status

The main obstacle to organ transplantation is the shortage of organs from deceased individuals. Especially in China, the ratio of patients on the waiting list versus the transplant recipients is 30:1. Therefore, there is an urgent need for organ donors. Genetically modified pig organs have proved to be a new source for xenotransplantation, and Chinese scientists have made considerable progress in this area during recent years. In this paper, we review four important aspects of the xenotransplantation field in China. First, a large variety of genetically modified pigs have been generated by Chinese scientists: all these genetically modified pigs and the purpose of these modifications will be summarized. Second, the preclinical research in pig-to-nonhuman primate xenotransplantation is outlined. The survival time and major biochemical parameters for the xenografts are summarized. Third, regarding the bench-to-bed approach, more suitable organs have been developed for xenotransplantation in humans, and in particular, pig islet transplantation into diabetic patients as well as pig-to-human cornea and skin transplantation. Fourth, we briefly address the regulations and prospects for recruiting xenotransplantation experts in China. Based on recent progress, we anticipate that genetically modified pigs will offer suitable organs for the treatment of end-stage organ diseases in humans in the near future. Given the recent influx of world-renowned scientists in xenotransplantation to China, our country will definitely become one of the major centers of xenotransplantation research and development in the world.

Long-term safety outcome of systemic immunosuppression in pig-to-nonhuman primate corneal xenotransplantation

BACKGROUND

Safety concerns exist for corneal recipients under immunosuppression. We report long-term safety results of porcine corneal xenotransplantation under immunosuppression in nonhuman primates.

METHODS

Systemic monitoring data from 49 Chinese rhesus macaques that received pig corneal transplant between 2009 and 2018 were retrospectively reviewed. The recipients were divided into 4 groups depending on the systemic immunosuppressants used: (a) conventional steroid group; costimulation blockade groups ([b] anti-CD154 antibody, [c] anti-CD40 antibody); and (d) commercially available immunosuppressants (anti-CD20 antibody, tacrolimus, basiliximab) group. We compared results of general condition monitoring; hematologic, biochemical, and electrolyte tests; and Rhesus Cytomegalovirus infection monitoring.

RESULTS

All recipients recovered from early weight loss. White blood cell counts significantly decreased at 6 months in the steroid and anti-CD154 groups. Abnormal liver and kidney function and electrolyte imbalance were not observed in all groups. The mean value of Rhesus Cytomegalovirus DNA copies was consistently lower than 200 copies/mL, and antibody titers did not change over time in all groups. Tacrolimus-associated thrombotic microangiopathy was developed in one case, which resolved after discontinuation of tacrolimus. In 2017, a simian varicella virus outbreak led to clinical signs in 5 that received immunosuppressive therapies, of which 3 died.

CONCLUSION

Costimulatory blockade-based and anti-CD20 antibody/tacrolimus-based immunosuppressive therapies seem to be comparably safe with steroid therapy in nonhuman primates receiving corneal xenotransplantation, as they did not reactivate Rhesus Cytomegalovirus and maintained manageable systemic status. Although reactivation is rare, antiviral prophylaxis for simian varicella virus should be considered in immunocompromised hosts.