Systemic inflammation in xenograft recipients precedes activation of coagulation

Inhibition of inflammation by IL-6 blockade in xenotransplantation

The inflammatory cytokine interleukin 6 (IL-6) plays a role in both acute and chronic organ allotransplant rejection. Data suggest that IL-6 inhibition may help prevent or reverse rejection, with large multi-center trials now underway. However, the evidence for the benefit of IL-6 inhibitors in xenotransplantation is limited. IL-6 inhibition has been explored in nonhuman-primate models of xenotransplantation, but no clear consensus exists on its efficacy or the best mode of IL-6 inhibition (anti-IL-6 antibodies, or through IL-6 receptor [IL-6R] blockade). Extra considerations for IL-6 blockade exist in xenotransplantation, as both recipient (human) and xenograft-derived (porcine) IL-6 may play roles. The systemic inflammation seen in xenograft recipients (SIXR) contributes to significant morbidity and mortality for the recipient through coagulation dysfunction and augmentation of the immune response. Anti-IL-6 antibodies (e.g., siltuximab) bind to human IL-6 and prevent IL-6R activation, but do not bind to porcine IL-6, and so have no effect in preventing graft-driven inflammatory processes. In contrast, IL-6R inhibitors (e.g., tocilizumab) inhibit IL-6 activity by blocking binding of human and porcine IL-6 to human IL-6R. Although IL-6R blockade cannot prevent the effect of IL-6 on porcine cells, it probably prevents graft-derived IL-6 from contributing to an inflammatory response in the host. This review outlines the role of IL-6 in xenotransplantation and discusses mechanisms for inhibiting IL-6 to improve recipient survival.

Early Results of an Infant Model of Orthotopic Cardiac Xenotransplantation

BACKGROUND

Genetically engineered porcine hearts may have an application for infants in need of a bridge to cardiac allotransplantation. The current animal model that resulted in 2 human applications has been validated in adult non-human primates only. We sought to create an infant animal model of life sustaining cardiac xenotransplantation to understand limitations specific to this age group.

METHODS

We performed 11 orthotopic cardiac xenotransplants from genetically modified infantile pigs into size-matched baboons (Papio spp). Porcine grafts were preserved using a modified Del Nido solution. Protocolized post-operative care and outcomes were tracked with invasive monitoring, echocardiogram, and serial chemistries (including a 7-cytokine panel).

RESULTS

Mean ischemic time was 52.1 +/- 13.9 min. All porcine hearts separated from bypass in normal sinus rhythm with normal systolic function documented by echocardiogram at chest closure and again at 24 h. In the first 48 post-operative hours, mean vasoactive inotropic score for the recipients was 9.6 +/- 3.5. Survival >3months was achieved in 6 animals. Five animals succumbed early (<7days) either due to errors in care (n=2) or pulmonary complications (n=3) confirmed on chest radiograph and necropsy. Cytokine levels objectively increased following xenograft implant but were not significantly different between survivors and non-survivors.

CONCLUSIONS

In a non-human primate model of infant orthotopic cardiac xenotransplantation, cardiac function does not hinder early peri-operative survival. Instead, pulmonary edema and pleural effusions in the setting of systemic inflammation preclude clinical progression. Targeted therapies are necessary to encourage prolonged survival.

Defect in Sensing Human Thrombin by Porcine Endothelial Protease-Activated Receptor-1: Molecular Incompatibility Between Porcine PAR-1 and Human Thrombin

Xenotransplantation, the transplantation of organs from pigs to humans, presents significant challenges due to immune rejection, which is driven by molecular incompatibilities between species. This study investigates the compatibility between human thrombin and porcine protease-activated receptor-1 (PAR-1), a key regulator of both coagulation and inflammatory responses. Human thrombin activates PAR-1 in human vascular endothelial cells, but our results demonstrate that human thrombin does not effectively activate PAR-1 in porcine vascular endothelial cells due to differences in amino acid sequences, particularly at the thrombin cleavage site and the Hir domain. Protein-protein docking analysis further reveals that porcine PAR-1 forms less stable interactions with human thrombin compared to human PAR-1, resulting in reduced activation. This molecular incompatibility likely contributes to impaired nitric oxide (NO) production, endothelial dysfunction, and increased inflammation, which are critical for the survival of transplanted organs. Additionally, experiments using the PAR-1 inhibitor vorapaxar (Vor) show that inhibiting PAR-1 signaling can suppress inflammatory cytokine and chemokine expression in co-cultures of human macrophages and porcine endothelial cells. These findings suggest that selective PAR-1 inhibitors or targeted therapies regulating thrombin-PAR-1 signaling may improve the success rate of xenotransplantation. However, further in vivo studies are needed to validate these findings and explore therapeutic interventions targeting thrombin-PAR-1 interactions to enhance xenograft survival.

Treatment of Presumptive Rejection After Orthotopic Pig-to-Baboon Cardiac Xenotransplantation

BACKGROUND

Significant progress has been made in the long-term survival of non-human primates after orthotopic gene-edited pig cardiac xenotransplantation. However, to our knowledge, there are no reports of the successful reversal of an acute rejection episode in such an experiment. We present evidence suggesting that rejection can be reversed with corticosteroids and complement inhibition.

METHODS

Orthotopic transplantation of a pig heart (with 69 gene-edits) was carried out in a baboon. The immunosuppressive regimen was based on CD40/CD154 T cell co-stimulation pathway blockade and rapamycin. Cardiac function remained excellent until Day 162, when there were increases in heart rate, ventricular septal wall thickness, left ventricular end-diastolic pressures (LVEDP), and troponin level, which were associated with a low serum level of rapamycin (<4 ng/mL). Anti-rejection treatment was begun with an increase in rapamycin dosage, steroid bolus therapy, two doses of a C1-esterase inhibitor, and an extra dose of the anti-CD154mAb.

RESULTS

There was a rapid correction of all hemodynamic parameters, and the troponin T level (which had risen to 139 ng/L) returned to pre-rejection levels. Ventricular septal thickness and LVEDP returned to pre-rejection levels after treatment. The baboon remains well with normal graft function. Baseline heart rate remains faster than before the rejection episode.

CONCLUSIONS

As we transition to the clinical application of gene-edited pig cardiac xenotransplantation, the ability to treat rejection is of vital importance. The optimal treatment for rejection remains uncertain but we suggest that systemic complement inhibition is important.

The Evolution of Immunosuppressive Therapy in Pig-to-Nonhuman Primate Organ Transplantation

An overview is provided of the evolution of strategies towards xenotransplantation during the past almost 40 years, focusing on advances in gene-editing of the organ-source pigs, pre-transplant treatment of the recipient, immunosuppressive protocols, and adjunctive therapy. Despite initial challenges, including hyperacute rejection resulting from natural (preformed) antibody binding and complement activation, significant progress has been made through gene editing of the organ-source pigs and refinement of immunosuppressive regimens. Major steps were the identification and deletion of expression of the three known glycan xenoantigens on pig vascular endothelial cells, the transgenic expression of human "protective" proteins, e.g., complement-regulatory, coagulation-regulatory, and anti-inflammatory proteins, and the administration of an immunosuppressive regimen based on blockade of the CD40/CD154 T cell co-stimulation pathway. Efforts to address systemic inflammation followed. The synergy between gene editing and judicious immunomodulation appears to largely prevent graft rejection and is associated with a relatively good safety profile. Though there remains an incidence of severe or persistent proteinuria (nephrotic syndrome) in a minority of cases. This progress offers renewed hope for patients in need of life-saving organ transplants.

Distinct Pro-Inflammatory Species-Specific Transcriptional Changes in Human T Cells Following Pig Xenogeneic Stimulation

Conventional T cell-directed immunosuppression is the mainstay of standard-of-care therapy to prevent graft rejection in clinical organ transplantation. However, it remains ineffective in preventing experimental and clinical organ xenograft rejection. Here, we explored the impact of allogeneic versus xenogeneic antigen stimulation on human T cell responses and gene profile. A comparable proliferative human T cell response was observed in vitro following stimulation with either human or pig cells. Yet, elevated High mobility group box-1 (HMGB1) levels were following xenogeneic but not allogeneic stimulation, suggesting a pro-inflammatory response. Next, human peripheral blood mononuclear cells (PBMC) were cultured with allogeneic human, "concordant" xenogeneic monkey, or "discordant" xenogeneic pig, intact cells, or cell lysates. Flow-sorted CD3+T cells were analyzed for gene expression using NanoString. A distinct pro-inflammatory gene profile was observed in human CD3+T cells following co-culture with discordant xenogeneic pig cells, but not concordant xenogeneic monkey cells or allogeneic human cells. Uniquely, stimulation with pig cells induced the expression of the transcription factor NCF4, which promotes inflammasome activation. Pig cell lysate, but not intact pig cells, induced high expression of the DNA-binding cytokine interleukin-26 gene. Collectively, these observations highlight the impact of xenogeneic stimulation of human T cells in pig xenograft recipients and concomitant inflammatory responses, which may contribute to immunosuppression-resistant xenograft rejection. Finally, the impact of genetic engineering of donor pigs on human T cell transcriptomic gene profile is yet to be determined.

An Approach to Controlling Inflammation and Coagulation in Pig-to-Baboon Cardiac Xenotransplantation

INTRODUCTION

Inflammatory responses and coagulation disorders are a relevant challenge for successful cardiac xenotransplantation on its way to the clinic. To cope with this, an effective and clinically practicable anti-inflammatory and anti-coagulatory regimen is needed. The inflammatory and coagulatory response can be reduced by genetic engineering of the organ-source pigs. Furthermore, there are several therapeutic strategies to prevent or reduce inflammatory responses and coagulation disorders following xenotransplantation. However, it is still unclear, which combination of drugs should be used in the clinical setting. To elucidate this, we present data from pig-to-baboon orthotopic cardiac xenotransplantation experiments using a combination of several anti-inflammatory drugs.

METHODS

Genetically modified piglets (GGTA1-KO, hCD46/hTBM transgenic) were used for orthotopic cardiac xenotransplantation into captive-bred baboons (n = 14). All animals received an anti-inflammatory drug therapy including a C1 esterase inhibitor, an IL-6 receptor antagonist, a TNF-α inhibitor, and an IL-1 receptor antagonist. As an additive medication, acetylsalicylic acid and unfractionated heparin were administered. The immunosuppressive regimen was based on CD40/CD40L co-stimulation blockade. During the experiments, leukocyte counts, levels of C-reactive protein (CRP) as well as systemic cytokine and chemokine levels and coagulation parameters were assessed at multiple timepoints. Four animals were excluded from further data analyses due to porcine cytomegalovirus/porcine roseolovirus (PCMV/PRV) infections (n = 2) or technical failures (n = 2).

RESULTS

Leukocyte counts showed a relevant perioperative decrease, CRP levels an increase. In the postoperative period, leukocyte counts remained consistently within normal ranges, CRP levels showed three further peaks after about 35, 50, and 80 postoperative days. Analyses of cytokines and chemokines revealed different patterns. Some cytokines, like IL-8, increased about 2-fold in the perioperative period, but then decreased to levels comparable to the preoperative values or even lower. Other cytokines, such as IL-12/IL-23, decreased in the perioperative period and stayed at these levels. Besides perioperative decreases, there were no relevant alterations observed in coagulation parameters. In summary, all parameters showed an unremarkable course with regard to inflammatory responses and coagulation disorders following cardiac xenotransplantation and thus showed the effectiveness of our approach.

CONCLUSION

Our preclinical experience with the anti-inflammatory drug therapy proved that controlling of inflammation and coagulation disorders in xenotransplantation is possible and well-practicable under the condition that transmission of pathogens, especially of PCMV/PRV to the recipient is prevented because PCMV/PRV also induces inflammation and coagulation disorders. Our anti-inflammatory regimen should also be applicable and effective in the clinical setting of cardiac xenotransplantation.

Disseminated Intravascular Coagulation in Anaphylaxis

Anaphylaxis is a life-threatening condition that involves severe cutaneous, respiratory, and cardiovascular symptoms. Disseminated intravascular coagulation (DIC) is an acquired, widespread activation of coagulation that can be caused by infectious conditions (e.g., sepsis) and noninfectious conditions. The onset of DIC following anaphylaxis is not commonly known, and information regarding the pathomechanism linking anaphylaxis to DIC is scarce. Further, demographic and clinical data in anaphylaxis-induced DIC are still missing to this day. Triggered by a case of anaphylaxis-induced DIC that seamlessly transitioned to lethal sepsis-induced DIC, we aimed to characterize the patient population affected by anaphylaxis-induced DIC by performing a review of existing literature and expand the discussion to underlying mechanisms. The overall mortality of the patient cohort (n = 30) identified by the literature review was 50%. All patients that died either suffered a bleeding event or a thrombotic event. The majority of patients (n = 25/30; 83%) had bleeding events; thrombotic events were only reported in nonsurvivors (n = 9/15 or 60% of nonsurvivors; vs. n = 0/15 in survivors; p < 0.001). Nonsurvivors of anaphylaxis-induced DIC were on average 25 years older than survivors (p = 0.068). In conclusion, DIC can complicate anaphylaxis and is expected to contribute to poor microvascular perfusion after anaphylaxis. Particularly, elderly patients with known cardiovascular disease and patients who develop thrombotic events are susceptible to lethal outcomes. As a rare and largely uncharacterized disease entity, further research is needed to investigate the link between DIC and anaphylaxis and to potentially identify better treatment strategies.

Pigs in Transplantation Research and Their Potential as Sources of Organs in Clinical Xenotransplantation

The pig has long been used as a research animal and has now gained importance as a potential source of organs for clinical xenotransplantation. When an organ from a wild-type (i. e., genetically unmodified) pig is transplanted into an immunosuppressed nonhuman primate, a vigorous host immune response causes hyperacute rejection (within minutes or hours). This response has been largely overcome by 1) extensive gene editing of the organ-source pig and 2) the administration to the recipient of novel immunosuppressive therapy based on blockade of the CD40/CD154 T cell costimulation pathway. Gene editing has consisted of 1) deletion of expression of the 3 known carbohydrate xenoantigens against which humans have natural (preformed) antibodies and 2) the introduction of human 'protective' genes. The combination of gene editing and novel immunosuppressive therapy has extended life-supporting pig kidney graft survival to greater than 1 y and of pig heart survival to up to 9 mo. This review briefly describes the techniques of gene editing, the potential risks of transfer of porcine endogenous retroviruses with the organ, and the need for breeding and housing of donor pigs under biosecure conditions.

Xenotransplantation experiments in brain-dead human subjects-A critical appraisal

Brain-dead human subjects (decedents) were recently introduced as a potential preclinical experimental model in xenotransplantation. Brain death is associated with major pathophysiological changes, eg, structural injury and cell infiltration in vital organs, and major hormonal, metabolic, inflammatory, and hemodynamic changes. In 2 of the 3 initial experiments, the design of the experiments resulted in little or no new information becoming available. In the third, the experiment was unfortunately unsuccessful as neither of the 2 pig kidneys transplanted into the decedent functioned adequately. Failure may well have been associated with the effects of brain death, but an immune/inflammatory response to the xenograft could not be excluded. Subsequently, 2 further pig kidney transplants and 2 pig heart transplants have been carried out in human decedents, but again the data obtained do not add much to what is already known. In view of the profound changes that take place during and after brain death, it may prove difficult to determine whether graft failure or dysfunction results from the effects of brain death or from an immune/inflammatory response to the xenograft. A major concern is that, if the results are confusing, they may impact decisions relating to the introduction of clinical xenotransplantation.

A Comparison Between GalT-/-;hCD39;hCD55 and GalT-/-;hCD39;hCD46;hCD55;TBM Pig Kidneys Transplanted in Nonhuman Primates

Because there is a shortage of donor kidneys, researchers are exploring the possibility of using genetically modified pig kidneys for transplantation. Approaches involving knockout of carbohydrate genes or knockin of protective proteins have been attempted to determine the best gene modifications. In this study, we utilized GalT-/-;hCD39;hCD55 and GalT-/-;hCD39;hCD46;hCD55;thrombomodulin (TBM) pigs for transplantation in nonhuman primates (NHPs). The NHPs survived for 4 weeks after kidney transplantation (4 WAT) from the GalT-/-;hCD39;hCD55 pig and for 6 WAT from the GalT-/-;hCD39;hCD46;hCD55;TBM pig. However, messenger RNA (mRNA) sequencing and immunohistochemistry analysis revealed that the 6 WAT kidney exhibited more severe apoptosis, inflammation, loss of renal function, and renal fibrosis than the 4 WAT kidney. These results indicate that additional knockin of complement regulator (hCD46) and coagulation regulator (TBM) is not enough to prevent renal damage, suggesting that improved immune suppression is needed for more prolonged survival.

Regulatory macrophages in solid organ xenotransplantation

Due to a critical organ shortage, pig organs are being explored for use in transplantation. Differences between species, particularly in cell surface glycans, can trigger elevated immune responses in xenotransplantation. To mitigate the risk of hyperacute rejection, genetically modified pigs have been developed that lack certain glycans and express human complement inhibitors. Nevertheless, organs from these pigs may still provoke stronger inflammatory and innate immune reactions than allotransplants. Dysregulation of coagulation and persistent inflammation remain obstacles in the transplantation of pig organs into primates. Regulatory macrophages (Mregs), known for their anti-inflammatory properties, could offer a potential solution. Mregs secrete interleukin 10 and transforming growth factor beta, thereby suppressing immune responses and promoting the development of regulatory T cells. These Mregs are typically induced via the stimulation of monocytes or macrophages with macrophage colony-stimulating factor and interferon gamma, and they conspicuously express the stable marker dehydrogenase/reductase 9. Consequently, understanding the precise mechanisms governing Mreg generation, stability, and immunomodulation could pave the way for the therapeutic use of Mregs generated in vitro. This approach has the potential to reduce the required dosages and durations of anti-inflammatory and immunosuppressive medications in preclinical and clinical settings.

Comparative transcriptome analysis between long- and short-term survival after pig-to-monkey cardiac xenotransplantation reveals differential heart failure development

Cardiac xenotransplantation is the potential treatment for end-stage heart failure, but the allogenic organ supply needs to catch up to clinical demand. Therefore, genetically-modified porcine heart xenotransplantation could be a potential alternative. So far, pig-to-monkey heart xenografts have been studied using multi-transgenic pigs, indicating various survival periods. However, functional mechanisms based on survival period-related gene expression are unclear. This study aimed to identify the differential mechanisms between pig-to-monkey post-xenotransplantation long- and short-term survivals. Heterotopic abdominal transplantation was performed using a donor CD46-expressing GTKO pig and a recipient cynomolgus monkey. RNA-seq was performed using samples from POD60 XH from monkey and NH from age-matched pigs, D35 and D95. Gene-annotated DEGs for POD60 XH were compared with those for POD9 XH (Park et al. 2021). DEGs were identified by comparing gene expression levels in POD60 XH versus either D35 or D95 NH. 1,804 and 1,655 DEGs were identified in POD60 XH versus D35 NH and POD60 XH versus D95 NH, respectively. Overlapped 1,148 DEGs were annotated and compared with 1,348 DEGs for POD9 XH. Transcriptomic features for heart failure and inhibition of T cell activation were observed in both long (POD60)- and short (POD9)-term survived monkeys. Only short-term survived monkey showed heart remodeling and regeneration features, while long-term survived monkey indicated multi-organ failure by neural and hormonal signaling as well as suppression of B cell activation. Our results reveal differential heart failure development and survival at the transcriptome level and suggest candidate genes for specific signals to control adverse cardiac xenotransplantation effects.

The history of cardiac xenotransplantation: early attempts, major advances, and current progress

In light of ongoing shortage of donor organs for transplantation, alternative sources for donor organ sources have been examined to address this supply-demand mismatch. Of these, xenotransplantation, or the transplantation of organs across species, has been considered, with early applications dating back to the 1600s. The purpose of this review is to summarize the early experiences of xenotransplantation, with special focus on heart xenotransplantation. It aims to highlight the important ethical concerns of animal-to-human heart xenotransplantation, identify the key immunological barriers to successful long-term xenograft survival, as well as summarize the progress made in terms of development of pharmacological and genetic engineering strategies to address these barriers. Lastly, we discuss more recent attempts of porcine-to-human heart xenotransplantation, as well as provide some commentary on the current concerns and possible applications for future clinical heart xenotransplantation.

Antibody-mediated rejection in xenotransplantation: Can it be prevented or reversed?

Antibody-mediated rejection (AMR) is the commonest cause of failure of a pig graft after transplantation into an immunosuppressed nonhuman primate (NHP). The incidence of AMR compared to acute cellular rejection is much higher in xenotransplantation (46% vs. 7%) than in allotransplantation (3% vs. 63%) in NHPs. Although AMR in an allograft can often be reversed, to our knowledge there is no report of its successful reversal in a pig xenograft. As there is less experience in preventing or reversing AMR in models of xenotransplantation, the results of studies in patients with allografts provide more information. These include (i) depletion or neutralization of serum anti-donor antibodies, (ii) inhibition of complement activation, (iii) therapies targeting B or plasma cells, and (iv) anti-inflammatory therapy. Depletion or neutralization of anti-pig antibody, for example, by plasmapheresis, is effective in depleting antibodies, but they recover within days. IgG-degrading enzymes do not deplete IgM. Despite the expression of human complement-regulatory proteins on the pig graft, inhibition of systemic complement activation may be necessary, particularly if AMR is to be reversed. Potential therapies include (i) inhibition of complement activation (e.g., by IVIg, C1 INH, or an anti-C5 antibody), but some complement inhibitors are not effective in NHPs, for example, eculizumab. Possible B cell-targeted therapies include (i) B cell depletion, (ii) plasma cell depletion, (iii) modulation of B cell activation, and (iv) enhancing the generation of regulatory B and/or T cells. Among anti-inflammatory agents, anti-IL6R mAb and TNF blockers are increasingly being tested in xenotransplantation models, but with no definitive evidence that they reverse AMR. Increasing attention should be directed toward testing combinations of the above therapies. We suggest that treatment with a systemic complement inhibitor is likely to be most effective, possibly combined with anti-inflammatory agents (if these are not already being administered). Ultimately, it may require further genetic engineering of the organ-source pig to resolve the problem entirely, for example, knockout or knockdown of SLA, and/or expression of PD-L1, HLA E, and/or HLA-G.

The case for the therapeutic use of mechanistic/mammalian target of rapamycin (mTOR) inhibitors in xenotransplantation

The mechanistic/mammalian target of rapamycin (mTOR) is one of the systems that are necessary to maintain cell homeostasis, such as survival, proliferation, and differentiation. mTOR inhibitors (mTOR-Is) are utilized as immunosuppressants and anti-cancer drugs. In organ allotransplantation, current regimens infrequently include an mTOR-I, which are positioned more commonly as alternative immunosuppressants. In clinical allotransplantation, long-term efficacy has been established, but there is a significant incidence of adverse events, for example, inhibition of wound healing, buccal ulceration, anemia, hyperglycemia, dyslipidemia, and thrombocytopenia, some of which are dose-dependent. mTOR-Is have properties that may be especially beneficial in xenotransplantation. These include suppression of T cell proliferation, increases in the number of T regulatory cells, inhibition of pig graft growth, and anti-inflammatory, anti-viral, and anti-cancer effects. We here review the potential benefits and risks of mTOR-Is in xenotransplantation and suggest that the benefits exceed the adverse effects.

Milestones on the path to clinical pig organ xenotransplantation

Progress in pig organ xenotransplantation has been made largely through (1) genetic engineering of the organ-source pig to protect its tissues from the human innate immune response, and (2) development of an immunosuppressive regimen based on blockade of the CD40/CD154 costimulation pathway to prevent the adaptive immune response. In the 1980s, after transplantation into nonhuman primates (NHPs), wild-type (genetically unmodified) pig organs were rejected within minutes or hours. In the 1990s, organs from pigs expressing a human complement-regulatory protein (CD55) transplanted into NHPs receiving intensive conventional immunosuppressive therapy functioned for days or weeks. When costimulation blockade was introduced in 2000, the adaptive immune response was suppressed more readily. The identification of galactose-α1,3-galactose as the major antigen target for human and NHP anti-pig antibodies in 1991 allowed for deletion of expression of galactose-α1,3-galactose in 2003, extending pig graft survival for up to 6 months. Subsequent gene editing to overcome molecular incompatibilities between the pig and primate coagulation systems proved additionally beneficial. The identification of 2 further pig carbohydrate xenoantigens allowed the production of 'triple-knockout' pigs that are preferred for clinical organ transplantation. These combined advances enabled the first clinical pig heart transplant to be performed and opened the door to formal clinical trials.

A porcine islet-encapsulation device that enables long-term discordant xenotransplantation in immunocompetent diabetic mice

Islet transplantation is an effective treatment for type 1 diabetes (T1D). However, a shortage of donors and the need for immunosuppressants are major issues. The ideal solution is to develop a source of insulin-secreting cells and an immunoprotective method. No bioartificial pancreas (BAP) devices currently meet all of the functions of long-term glycemic control, islet survival, immunoprotection, discordant xenotransplantation feasibility, and biocompatibility. We developed a device in which porcine islets were encapsulated in a highly stable and permeable hydrogel and a biocompatible immunoisolation membrane. Discordant xenotransplantation of the device into diabetic mice improved glycemic control for more than 200 days. Glycemic control was also improved in new diabetic mice "relay-transplanted" with the device after its retrieval. The easily retrieved devices exhibited almost no adhesion or fibrosis and showed sustained insulin secretion even after the two xenotransplantations. This device has the potential to be a useful BAP for T1D.

How, where and when to screen for porcine cytomegalovirus (PCMV) in donor pigs for xenotransplantation

Porcine cytomegalovirus (PCMV), that is actually a porcine roseolovirus (PRV), is a common herpesvirus in domestic pigs and wild boars. In xenotransplantation, PCMV/PRV has been shown to significantly reduce the survival time of pig kidneys and hearts in preclinical trials with different non-human primates. Furthermore, PCMV/PRV has been transmitted in the first pig to human heart xenotransplantation and contributed to the death of the patient. Although transmitted to the recipient, there is no evidence that PCMV/PRV can infect primate cells including human cells. PCMV/PRV is closely related to the human herpesviruses 6 and 7, and only distantly related to the human CMV (HCMV). Antiviral drugs used for the treatment of HCMV are less effective against PCMV/PRV. However, there are well described strategies to eliminate the virus from pig facilities. In order to detect the virus and to eliminate it, highly sensitive detection methods and the knowledge of how, where and when to screen the donor pigs is required. Here, a comparative testing of organs from pigs of different ages using polymerase chain reaction (PCR)-based and immunological methods was performed. Testing young piglets, PCMV/PRV was detected effectively by PCR in blood, bronchoalveolar lavage fluid, tonsils and heart. In adult animals, detection by PCR was not successful in most cases, because the virus load was below the detection limit or the virus was in its latent stage. Therefore, detection of antibodies against selected recombinant proteins corresponding to epitopes detected by nearly all infected animals in a Western blot assay is advantageous. By contrast, immunological testing is not beneficial in young animals as piglets might have PCMV/PRV-specific antibodies obtained from their infected mother via the colostrum. Using a thoughtful combination of PCR-based and immunological methods, detection of PCMV/PRV in donor pigs for xenotransplantation is feasible and a controlled elimination of the virus by early weaning or other methods is possible.

Observations on hydronephrosis after pig kidney transplantation in baboons

We have seen hydronephrosis (obstructive nephropathy) at necropsy in 3 of 11 (21%) genetically-engineered pig kidneys that functioned in baboons for >36 days, even when the clinical and histopathological features of rejection were minimal. We briefly report one such case and illustrate the macroscopic and microscopic appearances of such a kidney and ureter. The causes of the observed changes remain uncertain. In our small experience, there seems to be no correlation between the development of hydronephrosis and (i) the surgical technique, (ii) the genotype of the pig, (iii) the length of the pig ureter, or (iv) the immunosuppressive and anti-inflammatory therapy administered. We suggest that the distal ureteric thickening may be the result of an inflammatory response. In two cases, we resolved the problem by carrying out a secondary side-to-side anastomosis between the proximal pig ureter and the baboon bladder.

Cardiac and Pulmonary Histopathology in Baboons Following Genetically-Engineered Pig Orthotopic Heart Transplantation

BACKGROUND Although improving, survival after pig orthotopic heart transplantation (OHTx) in baboons has been mixed and largely poor. The causes for the high incidence of early failure remain uncertain. MATERIAL AND METHODS We have carried out pig OHTx in 4 baboons. Two died or were euthanized within hours, and 2 survived for 3 and 8 months, respectively. There was evidence of a significant 'cytokine storm' in the immediate post-OHTx period with the elevations in IL-6 correlating closely with the final outcome. RESULTS All 4 baboons demonstrated features suggestive of respiratory dysfunction, including increased airway resistance, hypoxia, and tachypnea. Histopathological observations of pulmonary infiltration by neutrophils and, notably, eosinophils within vessels and in the perivascular and peribronchiolar space, with minimal cardiac pathology, suggested a role for early lung acute inflammation. In one, features suggestive of transfusion-related acute lung injury were present. The 2 longer-term survivors died of (i) a cardiac dysrhythmia with cellular infiltration around the conducting tissue (at 3 months), and (ii) mixed cellular and antibody-mediated rejection (at 8 months). CONCLUSIONS These initial findings indicate a potential role of acute lung injury early after OHTx. If this response can be prevented, increased survival may result, providing an opportunity to evaluate the factors affecting long-term survival.

Cardiac Xenotransplantation: Progress in Preclinical Models and Prospects for Clinical Translation

Survival of pig cardiac xenografts in a non-human primate (NHP) model has improved significantly over the last 4 years with the introduction of costimulation blockade based immunosuppression (IS) and genetically engineered (GE) pig donors. The longest survival of a cardiac xenograft in the heterotopic (HHTx) position was almost 3 years and only rejected when IS was stopped. Recent reports of cardiac xenograft survival in a life-sustaining orthotopic (OHTx) position for 6 months is a significant step forward. Despite these achievements, there are still several barriers to the clinical success of xenotransplantation (XTx). This includes the possible transmission of porcine pathogens with pig donors and continued xenograft growth after XTx. Both these concerns, and issues with additional incompatibilities, have been addressed recently with the genetic modification of pigs. This review discusses the spectrum of issues related to cardiac xenotransplantation, recent progress in preclinical models, and its feasibility for clinical translation.

Clinically available immunosuppression averts rejection but not systemic inflammation after porcine islet xenotransplant in cynomolgus macaques

A safe, efficacious, and clinically applicable immunosuppressive regimen is necessary for islet xenotransplantation to become a viable treatment option for diabetes. We performed intraportal transplants of wild-type adult porcine islets in 25 streptozotocin-diabetic cynomolgus monkeys. Islet engraftment was good in 21, partial in 3, and poor in 1 recipient. Median xenograft survival was 25 days with rapamycin and CTLA4Ig immunosuppression. Adding basiliximab induction and maintenance tacrolimus to the base regimen significantly extended median graft survival to 147 days (p < .0001), with three animals maintaining insulin-free xenograft survival for 265, 282, and 288 days. We demonstrate that this regimen suppresses non-Gal anti-pig antibody responses, circulating effector memory T cell expansion, effector function, and infiltration of the graft. However, a chronic systemic inflammatory state manifested in the majority of recipients with long-term graft survival indicated by increased neutrophil to lymphocyte ratio, IL-6, MCP-1, CD40, and CRP expression. This suggests that this immunosuppression regimen fails to regulate innate immunity and resulting inflammation is significantly associated with increased incidence and severity of adverse events making this regimen unacceptable for translation. Additional studies are needed to optimize a maintenance regimen for regulating the innate inflammatory response.

Pig-to-baboon lung xenotransplantation: Extended survival with targeted genetic modifications and pharmacologic treatments

Galactosyl transferase knock-out pig lungs fail rapidly in baboons. Based on previously identified lung xenograft injury mechanisms, additional expression of human complement and coagulation pathway regulatory proteins, anti-inflammatory enzymes and self-recognition receptors, and knock-down of the β4Gal xenoantigen were tested in various combinations. Transient life-supporting GalTKO.hCD46 lung function was consistently observed in association with either hEPCR (n = 15), hTBM (n = 4), or hEPCR.hTFPI (n = 11), but the loss of vascular barrier function in the xenograft and systemic inflammation in the recipient typically occurred within 24 h. Co-expression of hEPCR and hTBM (n = 11) and additionally blocking multiple pro-inflammatory innate and adaptive immune mechanisms was more consistently associated with survival >1 day, with one recipient surviving for 31 days. Combining targeted genetic modifications to the lung xenograft with selective innate and adaptive immune suppression enables prolonged initial life-supporting lung function and extends lung xenograft recipient survival, and illustrates residual barriers and candidate treatment strategies that may enable the clinical application of other organ xenografts.

The Role of Interleukin-6 (IL-6) in the Systemic Inflammatory Response in Xenograft Recipients and in Pig Kidney Xenograft Failure

Background

In pig-to-baboon transplantation models, there is increasing evidence of systemic inflammation in xenograft recipients (SIXR) associated with pig xenograft failure. We evaluated the relationship between systemic inflammatory factors and pig kidney xenograft failure.

Methods

Baboons received kidney transplants from genetically engineered pigs (n=9), and received an anti-CD40mAb-based (n=4) or conventional (n=5) immunosuppressive regimen. The pig kidney grafts were monitored by measurements of serum creatinine, serum amyloid A (SAA), white blood cell (WBC) and platelet counts, plasma fibrinogen, and pro-inflammatory cytokines (baboon and pig IL-6, TNF-α, IL-1β).

Results

Six baboons were euthanized or died from rejection, and 3 were euthanized for infection. Changes in serum creatinine correlated with those of SAA (r=0.56, p<0.01). Serum baboon IL-6 was increased significantly on day 1 after transplantation and at euthanasia (both p<0.05) and correlated with serum creatinine and SAA (r=0.59, p<0.001, r=0.58, p<0.01; respectively). but no difference was observed between rejection and infection. Levels of serum pig IL-6, TNF-α, IL-1β were also significantly increased on day 1 and at euthanasia, and serum pig IL-6 and IL-1β correlated with serum creatinine and SAA. The level of serum baboon IL-6 correlated with the expression of IL-6 and amyloid A in the baboon liver (r=0.93, p<0.01, r=0.79, p<0.05; respectively).

Conclusion

Early upregulation of SAA and serum IL-6 may indicate the development of rejection or infection, and are associated with impaired kidney graft function. Detection and prevention of systemic inflammation may be required to prevent pig kidney xenograft failure after xenotransplantation.

Interleukin-27 in liver xenotransplantation: A rational target to mitigate ischemia reperfusion injury and increase xenograft survival

Transplantation of xenogeneic organs is an attractive solution to the existing organ shortage dilemma, thus, securing a clinically acceptable prolongation of xenograft survival is an important goal. In preclinical transplantation models, recipients of liver, kidney, heart, or lung xenotransplants demonstrate significant graft damages through the release of pro-inflammatory molecules, including the C-reactive protein, cytokines, and histone-DNA complexes that all foster graft rejection. Recent studies have demonstrated that mitigation of ischemia reperfusion injury (IRI) greatly improves xenograft survival. Organ IRI develops primarily on a complex network of cytokines and chemokines responding to molecular cues from the graft milieu. Among these, interleukin 27 (IL-27) plays an immunomodulatory role in IRI onset due to graft environment-dependent pro- and anti- inflammatory activities. This review focuses on the impact of IL-27 on IRI of liver xenotransplants and provides insights on the function of IL-27 that could potentially guide genetic engineering strategies of donor pigs and/or conditioning of organs prior to transplantation.

Early Interferon-Gamma Response in Nonhuman Primate Recipients of Solid-Organ Xenotransplantation

BACKGROUND

To understand changes in biological responses in nonhuman primate (NHP) recipients of xenotransplantation (XTP), we retrospectively investigated chronological changes in cytokine profiles of NHP recipients after solid-organ XTP.

METHODS

Plasma samples were collected from 7 NHP recipients of pig heart or kidney XTP with α-1,3-galactosyltransferase gene knockout (GTKO) under anti-CD154-based immune suppression at the following time points: immediately before; 2 hours, 3 days, and 7 days after XTP; and weekly thereafter until the graft failed. The plasma levels of the following cytokines were measured: interleukin (IL)-1α, IL-1β, IL-6, IL-12p70, IL-8, IL-10, IL-15, tumor necrosis factor, interferon gamma (IFN-γ), D-dimer, C3a, and histone-complexed DNA fragments. For in vitro experiments, human natural killer (NK) cells were cocultured with wild-type porcine endothelial cells (PECs), GTKO-PECs, and human umbilical vein endothelial cells, with or without anti-CD154 antibody. IFN-γ levels in the culture supernatants were compared.

RESULTS

IFN-γ levels peaked on day 7 or 10 of XTP and then decreased to basal levels, whereas proinflammatory cytokine levels increased along with the elevation of histone-complexed DNA fragments and were sustained until xenograft failure. In vitro, human NK cells produced more IFN-γ when in contact with wild-type PECs than with human umbilical vein endothelial cells, which was not reduced by the use of GTKO-PECs or addition of anti-CD154 antibody to the mixture.

CONCLUSIONS

In NHP recipients of XTP, the early peak of IFN-γ priming subsequent inflammatory responses may be attributed to NK cell activation in response to xenografts.

Humanized von Willebrand factor reduces platelet sequestration in ex vivo and in vivo xenotransplant models

The transplantation of organs across species offers the potential to solve the shortage of human organs. While activation of human platelets by human von Willebrand factor (vWF) requires vWF activation by shear stress, contact between human platelets and porcine vWF (pvWF) leads to spontaneous platelet adhesion and activation. This non-physiologic interaction may contribute to the thrombocytopenia and coagulation pathway dysregulation often associated with xenotransplantation of pig organs in nonhuman primates. Pigs genetically modified to decrease antibody and complement-dependent rejection (GTKO.hCD46) were engineered to express humanized pvWF (h*pvWF) by replacing a pvWF gene region that encodes the glycoprotein Ib-binding site with human cDNA orthologs. This modification corrected for non-physiologic human platelet aggregation on exposure to pig plasma, while preserving in vitro platelet activation by collagen. Organs from pigs with h*pvWF demonstrated reduced platelet sequestration during lung (p ≤ .01) and liver (p ≤ .038 within 4 h) perfusion ex vivo with human blood and after pig-to-baboon lung transplantation (p ≤ .007). Residual platelet sequestration and activation were not prevented by the blockade of canonical platelet adhesion pathways. The h*pvWF modification prevents physiologically inappropriate activation of human or baboon platelets by porcine vWF, addressing one cause of the thrombocytopenia and platelet activation observed with xenotransplantation.

Genetically engineered pig kidney transplantation in a brain-dead human subject

In September 2021, a kidney (with donor-specific thymic tissue) from an α1, 3-galactosyltransferase gene-knockout (GTKO) pig was transplanted into the groin (with anastomoses to the femoral vessels) of a brain-dead subject by a surgical team at New York University Langone Health (NYU). It was reported to function immediately, passing urine and excreting creatinine. The experiment was terminated after 54 h and, during this period, the kidney did not show macroscopic features of rejection. Does this experiment provide information not available to us previously and does it move the field forward to clinical trials? The information provided was very limited, but the following points are worthy of note. (i) Numerous in vivo studies in nonhuman primates have predicted that the pig kidney would function immediately. (ii) Numerous in vitro studies have predicted that a GTKO pig kidney would not be rejected within the first few days after transplantation into a human subject. (iii) GTKO kidneys are not optimal for clinical transplantation, and the transplantation of a triple-knockout (TKO) pig kidney would have been more relevant. (iv) There was no purpose in transplanting a "thymokidney" without pre-transplant conditioning therapy and follow-up for several months. (v) Because the native kidneys were retained, it is difficult to determine whether the function of the graft was sufficient to support life. (vi) The experiment was announced to the media rather than published in a peer-reviewed medical journal (although hopefully this will follow), suggesting that it was primarily carried out to gain attention to the great potential of xenotransplantation (and/or possibly to NYU). In this respect the experiment was successful. Because of the very limited period of time for which a brain-dead subject can be maintained in a metabolically and hemodynamically stable state, the value of experiments in such subjects will remain very limited. It is hoped that any future similar experiments will be planned to be more relevant to the clinical situation. Nevertheless, the report has stimulated public attention towards xenotransplantation which, unless there is an adverse response to what some might consider to be a bizarre experiment, should be of significant benefit to future progress.

Minimizing Ischemia Reperfusion Injury in Xenotransplantation

The recent dramatic advances in preventing "initial xenograft dysfunction" in pig-to-non-human primate heart transplantation achieved by minimizing ischemia suggests that ischemia reperfusion injury (IRI) plays an important role in cardiac xenotransplantation. Here we review the molecular, cellular, and immune mechanisms that characterize IRI and associated "primary graft dysfunction" in allotransplantation and consider how they correspond with "xeno-associated" injury mechanisms. Based on this analysis, we describe potential genetic modifications as well as novel technical strategies that may minimize IRI for heart and other organ xenografts and which could facilitate safe and effective clinical xenotransplantation.

"You cannot stay in the laboratory forever"*: Taking pig kidney xenotransplantation from the laboratory to the clinic

Progress in life-supporting kidney transplantation in the genetically-engineered pig-to-nonhuman primate model has been encouraging, with pig kidneys sometimes supporting life for > 1 year. What steps need to be taken by (i) the laboratory team, and (ii) the clinical team to prepare for the first clinical trial? The major topics include (i) what currently-available genetic modifications are optimal to reduce the possibility of graft rejection, (ii) what immunosuppressive therapeutic regimen is optimal, and (iii) what steps need to be taken to minimize the risk of transfer of an infectious microorganism with the graft. We suggest that patients who are unlikely to live long enough to receive a kidney from a deceased human donor would benefit from the opportunity of a period of dialysis-free support by a pig kidney, and the experience gained would enable xenotransplantation to progress much more rapidly than if we remain in the laboratory.

What Therapeutic Regimen Will Be Optimal for Initial Clinical Trials of Pig Organ Transplantation?

We discuss what therapeutic regimen might be acceptable/successful in the first clinical trial of genetically engineered pig kidney or heart transplantation. As regimens based on a calcineurin inhibitor or CTLA4-Ig have proved unsuccessful, the regimen we administer to baboons is based on induction therapy with antithymocyte globulin, an anti-CD20 mAb (Rituximab), and cobra venom factor, with maintenance therapy based on blockade of the CD40/CD154 costimulation pathway (with an anti-CD40 mAb), with rapamycin, and a corticosteroid. An anti-inflammatory agent (etanercept) is administered for the first 2 wk, and adjuvant therapy includes prophylaxis against thrombotic complications, anemia, cytomegalovirus, and pneumocystis. Using this regimen, although antibody-mediated rejection certainly can occur, we have documented no definite evidence of an adaptive immune response to the pig xenograft. This regimen could also form the basis for the first clinical trial, except that cobra venom factor will be replaced by a clinically approved agent, for example, a C1-esterase inhibitor. However, none of the agents that block the CD40/CD154 pathway are yet approved for clinical use, and so this hurdle remains to be overcome. The role of anti-inflammatory agents remains unproven. The major difference between this suggested regimen and those used in allotransplantation is the replacement of a calcineurin inhibitor with a costimulation blockade agent, but this does not appear to increase the complications of the regimen.

Stable expression of the human thrombomodulin transgene in pig endothelial cells is associated with a reduction in the inflammatory response

BACKGROUND

Xenotransplantation is associated with an inflammatory response. The proinflammatory cytokine, TNF-α, downregulates the expression of thrombomodulin (TBM), and induces coagulation dysfunction. Although human (h) TBM-transgenic pigs (p) have been developed to reduce coagulation dysfunction, the effect of TNF-α on the expression of hTBM and its functional activity has not been fully investigated. The aims of this study were to investigate (i) whether the expression of hTBM on pig (p) cells is down-regulated during TNF-α stimulation, and (ii) whether cells from hTBM pigs regulate the inflammatory response.

METHODS

TNF-α-producing T, B, and natural killer cells in blood from baboons with pig heart or kidney xenografts were investigated by flow cytometry. TNF-α staining in the grafts was detected by immunohistochemistry. Aortic endothelial cells (AECs) from GTKO/CD46 and GTKO/CD46/hTBM pigs were stimulated by hTNF-α, and the expression of the inflammatory/coagulation regulatory protein, TBM, was investigated.

RESULTS

After pig organ xenotransplantation, there was a trend to increases in TNF-α-producing T and natural killer cells in the blood of baboons. In vitro observations demonstrated that after hTNF-α stimulation, there was a significant reduction in the expression of endogenous pTBM on pAECs, and a significant increase in the expression of inflammatory molecules. Blocking of NF-κB signaling significantly up-regulated pTBM expression, and suppressed the inflammatory response induced by hTNF-α in pAECs. Whereas the expression of pTBM mRNA was significantly reduced by hTNF-α stimulation, hTBM expression on the GTKO/CD46/hTBM pAECs was not affected. Furthermore, after hTNF-α stimulation, there was significant suppression of expression of inflammatory molecules on GTKO/CD46/hTBM pAECs compared to GTKO/CD46 pAECs.

CONCLUSIONS

The stable expression of hTBM in pig cells may locally regulate the inflammatory response. This will help suppress the inflammatory response and prevent coagulation dysregulation after xenotransplantation.

Immunological selection and monitoring of patients undergoing pig kidney transplantation

Pig kidney xenotransplantation has the potential to alleviate the current shortage of deceased and living human organs and provide patients with end-stage renal disease with a greater opportunity for long-term survival and a better quality of life. In recent decades, advances in the genetic engineering of pigs and in immunosuppressive therapy have permitted the resolution of many historical obstacles to the success of pig kidney transplantation in nonhuman primates. Pig kidney xenotransplantation may soon be translated to the clinic. Given the potential risks of kidney xenotransplantation, particularly of immunologic rejection of the graft, potential patients must be carefully screened for inclusion in the initial clinical trials and immunologically monitored diligently post-transplantation. We provide an overview of the immunological methods we believe should be used to (i) screen potential patients for the first clinical trials to exclude those with a higher risk of rejection, and (ii) monitor patients with a pig kidney graft to determine their immunological response to the graft.

A perspective on the potential detrimental role of inflammation in pig orthotopic heart xenotransplantation

There is a critical shortage of deceased human donor organs for transplantation. The need is perhaps most acute in neonates and infants with life-threatening congenital heart disease, in whom mechanical support devices are largely unsuccessful. If orthotopic (life-supporting) heart transplantation (OHTx) were consistently successful in the genetically engineered pig-to-nonhuman primate (NHP) model, a clinical trial of bridging with a pig heart in such patients might be justified. However, the results of pig OHTx in NHPs have been mixed and largely poor. We hypothesise that a factor is the detrimental effects of the inflammatory response that is known to develop (a) during any surgical procedure that requires cardiopulmonary bypass, and (b) immediately after an NHP recipient is exposed to a pig xenograft. We suggest that the combination of these two inflammatory responses has a direct detrimental effect on pig heart graft function, but also, and possibly of more importance, on recipient baboon pulmonary function, which further impacts survival of the pig heart graft. In addition, the inflammatory response almost certainly adversely impacts the immune response to the graft. If our hypothesis is correct, the potential steps that could be taken to reduce the inflammatory response or its effects (with varying degrees of efficacy) include (a) white blood cell filtration, (b) complement depletion or inactivation, (c) immunosuppressive therapy, (d) high-dose corticosteroid therapy, (e) cytokine/chemokine-targeted therapy, (f) ultrafiltration or CytoSorb hemoperfusion, (g) reduction in the levels of endogenous catecholamines, (h) triiodothyronine therapy and (i) genetic engineering of the organ-source pig. Prevention of the inflammatory response, or attenuation of its effects, by judicious anti-inflammatory therapy may contribute not only to early survival of the recipient of a genetically engineered pig OHTx, but also to improved long-term pig heart graft survival. This would open the possibility of initiating a clinical trial of genetically engineered pig OHTx as a bridge to allotransplantation.

Decellularized xenografts in regenerative medicine: From processing to clinical application

Decellularized xenografts are an inherent component of regenerative medicine. Their preserved structure, mechanical integrity and biofunctional composition have well established them in reparative medicine for a diverse range of clinical indications. Nonetheless, their performance is highly influenced by their source (ie species, age, tissue) and processing (ie decellularization, crosslinking, sterilization and preservation), which govern their final characteristics and determine their success or failure for a specific clinical target. In this review, we provide an overview of the different sources and processing methods used in decellularized xenografts fabrication and discuss their effect on the clinical performance of commercially available decellularized xenografts.

Porcine genome engineering for xenotransplantation

The extreme shortage of human donor organs for treatment of patients with end-stage organ failures is well known. Xenotransplantation, which might provide unlimited organ supply, is a most promising strategy to solve this problem. Domestic pigs are regarded as ideal organ-source animals owing to similarity in anatomy, physiology and organ size to humans as well as high reproductive capacity and low maintenance cost. However, several barriers, which include immune rejection, inflammation and coagulative dysfunctions, as well as the cross-species transmission risk of porcine endogenous retrovirus, blocked the pig-to-human xenotransplantation. With the rapid development of genome engineering technologies and the potent immunosuppressive medications in recent years, these barriers could be eliminated through genetic modification of pig genome together with the administration of effective immunosuppressants. A number of candidate genes involved in the regulation of immune response, inflammation and coagulation have been explored to optimize porcine xenograft survival in non-human primate recipients. PERV inactivation in pigs has also been accomplished to firmly address the safety issue in pig-to-human xenotransplantation. Many encouraging preclinical milestones have been achieved with some organs surviving for years. Therefore, the clinical trials of some promising organs, such as islet, kidney and heart, are aimed to be launched in the near future.

Kidney xenotransplantation: Recent progress in preclinical research

Kidney transplantation is the most effective treatment for end-stage renal disease, but is limited by the increasing shortage of deceased and living human donor kidneys. Xenotransplantation using pig organs provides the possibility to resolve the issue of organ supply shortage and is regarded as the next great medical revolution. In the past five years, there have been sequential advances toward the prolongation of life-supporting pig kidney xenograft survival in non-human primates, with the longest survival being 499 days. This progress is due to the growing availability of pigs with multi-layered genetic modifications to overcome the pathobiological barriers and the application of a costimulation blockade-based immunosuppressive regimen. These encouraging results bring the hope to initiate the clinical trials of pig kidney transplantation in the near future. In this review, we summarized the latest advances regarding pig kidney xenotransplantation in preclinical models to provide a basis for future investigation and potential clinical translation.

Pig kidney xenotransplantation: Progress toward clinical trials

Pig organ xenotransplantation offers a solution to the shortage of deceased human organs for transplantation. The pathobiological response to a pig xenograft is complex, involving antibody, complement, coagulation, inflammatory, and cellular responses. To overcome these barriers, genetic manipulation of the organ-source pigs has largely been directed to two major aims-(a) deletion of expression of the known carbohydrate xenoantigens against which humans have natural (preformed) antibodies, and (b) transgenic expression of human protective proteins, for example, complement- and coagulation-regulatory proteins. Conventional (FDA-approved) immunosuppressive therapy is unsuccessful in preventing an adaptive immune response to pig cells, but blockade of the CD40:CD154 costimulation pathway is successful. Survival of genetically engineered pig kidneys in immunosuppressed nonhuman primates can now be measured in months. Non-immunological aspects, for example, pig renal function, a hypovolemia syndrome, and rapid growth of the pig kidney after transplantation, are briefly discussed. We suggest that patients on the wait-list for a deceased human kidney graft who are unlikely to receive one due to long waiting times are those for whom kidney xenotransplantation might first be considered. The potential risk of infection, public attitudes to xenotransplantation, and ethical, regulatory, and financial aspects are briefly addressed.

Impact of porcine cytomegalovirus on long-term orthotopic cardiac xenotransplant survival

Xenotransplantation using pig organs has achieved survival times up to 195 days in pig orthotopic heart transplantation into baboons. Here we demonstrate that in addition to an improved immunosuppressive regimen, non-ischaemic preservation with continuous perfusion and control of post-transplantation growth of the transplant, prevention of transmission of the porcine cytomegalovirus (PCMV) plays an important role in achieving long survival times. For the first time we demonstrate that PCMV transmission in orthotopic pig heart xenotransplantation was associated with a reduced survival time of the transplant and increased levels of IL-6 and TNFα were found in the transplanted baboon. Furthermore, high levels of tPA-PAI-1 complexes were found, suggesting a complete loss of the pro-fibrinolytic properties of the endothelial cells. These data show that PCMV has an important impact on transplant survival and call for elimination of PCMV from donor pigs.

Tissue Factor and Risk of Complications After Kidney Transplantation

OBJECTIVE

Tissue factor (TF) is a membrane component of many cells and a strong activator of blood coagulation. Damage to the cells induces an increase in its expression and concentration in blood plasma. The injury and breakdown of the cells is inseparably connected with the harvesting and preservation of the kidney.

PURPOSE

The aim of the study was an analysis of TF in the renal vein after of restoration of circulation in the transplanted kidney. An additional goal was to investigate the impact of warm ischemia on TF.

MATERIALS AND METHODS

The examined group included 61 kidney recipients. Blood was taken from the renal vein in the first minute during reperfusion. Simultaneously, blood from a peripheral vein was also drawn. Apart from tissue factor (TF), I also examined thrombin/antithrombin complexes and fragments 1+2 of prothrombin.

RESULTS

In blood from renal veins, I noticed higher level of TF, thrombin/antithrombin complexes and fragments 1+2 of prothrombin in comparison with blood from peripheral veins (P < .0048, P < .016, P < .046, respectively). The 29 recipients (47% of the total) with postoperative complications had much higher concentrations of TF than others (P < .019). TF showed a strong positive correlation with the time of warm ischemia (r = 0.53864, P < .05).

CONCLUSIONS

The donor kidney appeared to be one of the main sources of TF in the blood of recipients. Warm ischemia significantly increased its concentration in renal vein blood. This concentration of TF may be associated with damage to the kidney. TF significantly increased the risk of postoperative complications.

Update and breakthrough in cardiac xenotransplantation

PURPOSE OF REVIEW

Considerable advancements have been made in the field of cardiac xenotransplantation in the recent years, achieving prolonged survival of the life-supporting cardiac xenograft and paving the way toward first clinical implications.

RECENT FINDINGS

The combination of genetic modifications and novel immunosuppression with costimulation blockade, as well as supporting therapy with antiinflammatory treatment, growth prevention, and adaptation of the heart procurement system to reduce myocardial ischemia and reperfusion injury improves the overall cardiac xenograft function and overall survival in nonhuman primates. Through the newly identified xenoantigens and novel gene-editing techniques, further genetic modification of the porcine xenografts should be explored, to ensure clinical safety.

SUMMARY

With continuous progress in all fields of cardiac xenotransplantation, first clinical use in humans seems accomplishable. To ensure the clinical safety and to conform to the ethical regulations, further investigation of the infectious and immunological implications on humans should be explored prior to first clinical use. The first clinical use of cardiac xenotransplantation will be limited to only highly selected patients.

Is interleukin-6 receptor blockade (tocilizumab) beneficial or detrimental to pig-to-baboon organ xenotransplantation?

The interleukin (IL)-6/IL-6 receptor-α (IL-6Rα)/signal transduction and activation of the transcription 3 (STAT3) pathway plays an important role in inflammation. Anti-human IL-6Rα blockade by tocilizumab (TCZ) has been used in pig-to-baboon organ xenotransplant models, but whether it is beneficial remains uncertain. After xenotransplant, there were significant increases in both baboon and pig IL-6 in the baboon serum, especially in baboons that received TCZ before xenotransplant. In vitro observations demonstrated that human, baboon, and pig IL-6 can activate the IL-6/IL-6Rα/STAT3 pathway in human, baboon, and pig cells, respectively. Activation of the IL-6/IL-6Rα/STAT3 pathway was blocked by TCZ in human and baboon cells but not in pig cells (ie, pig IL-6R). Siltuximab (human IL-6 inhibitor) bound to both human and baboon, but not pig, IL-6 and suppressed activation of the IL-6/IL-6Rα/STAT3 pathway. These results indicate that TCZ and siltuximab do not cross-react with pig IL-6R and pig IL-6, respectively. Rapamycin partially inhibited human, baboon, and pig IL-6/IL-6Rα/STAT3 pathways and suppressed inflammatory gene expression. TCZ treatment increased serum IL-6 because it could no longer bind to baboon IL-6Rα. We suggest that increased serum IL-6 may be detrimental to the pig xenograft because it is likely to bind to pig IL-6R, resulting in activation of pig cells.