An Investigation of Extracellular Histones in Pig-To-Baboon Organ Xenotransplantation

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.

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.

Application of microphysiologic system to assess neutrophil extracellular trap in xenotransplantation

Transplantation of organs, cells, or tissues from one species to another, known as xenotransplantation, has the potential to alleviate organ donor shortages and enhance the success of organ transplantation. However, the possibility of immunological rejection by the recipient is one of the biggest difficulties associated with xenotransplantation. The creation of neutrophil extracellular traps (NETs), also known as NETosis, is hypothesized as a mechanism of rejection. Innovations in microfluidics and co-culturing techniques have provided access to several classes of microengineered model systems in experimental models, connecting animal research and traditional in vitro methods such as organoids, microphysiological systems, and organs-on-chip. To achieve this goal, we established a perfusable 3D Xeno vessel chip using a porcine aortic endothelial cell line and examined how NETs grow when isolated human and primate neutrophils were used. Neutrophils from both humans and monkeys displayed the usual NETosis phases, including nuclear decondensation, enlargement, and rounding of DNA, occupying the entire cytoplasm, and discharge of fragmented DNA after cell membrane rupture. Using confocal fluorescence imaging of DNA and citrullinated histone colocalization and DNA histone complex formation in supernatants from xeno vessel chips, we confirmed NETs generation by human and monkey neutrophils when cocultured in a xeno-vessel chip.

Trichostatin A inhibits dendritic cell maturation through down-regulating NF-κ B (p65) pathway

BACKGROUND

Recent evidence suggested that histone deacetylase inhibitor (HDACi) could inhibit dendritic cell (DC) maturation. However, the mechanism is unclear. Here, we aimed to study whether Trichostatin A (TSA), the most widely studied HDACi, inhibits the maturation of DCs by down-regulating NF-κB (p65) pathway.

METHODS AND RESULTS

Mouse bone marrow-derived DCs were cultured. Lipopolysaccharide (LPS) was applied as stimulation for maturation. Triptolide (TTL) was applied as p65 inhibitor. Microphotography and flow cytometry showed that TSA and p65 inhibitor separately inhibited the maturation of DCs stimulated by LPS from the aspects of cell morphology and cell phenotype. Mixed lymphocyte reaction test and ELISA showed that TSA and p65 inhibitor synergistically inhibited the proliferation of T lymphocytes stimulated by DCs, reduced the secretion of pro-inflammatory cytokine IL-12 and elevated the secretion of anti-inflammatory cytokine IL-10. Western blot and RT-qPCR showed that TSA down-regulated the expression of phosphorylated IκBα, phosphorylated-p65, Ikkβ and Ikkγ, suggesting TSA down-regulates NF-κB (p65) pathway.

CONCLUSIONS

TSA inhibits DC maturation through down-regulating NF-κB (p65) pathway.

Serum Antibody Binding and Cytotoxicity to Pig Cells in Chinese Subjects: Relevance to Clinical Renal Xenotransplantation

Kidney xenotransplantation is expected to contribute to resolving the shortage of kidneys from deceased human donors. Although progress in experimental life-supporting pig renal xenotransplantation has been encouraging, there are still issues to be considered before a clinical trial can be initiated. We attempted to clarify some of these by an in vitro study. Blood was drawn from healthy volunteers (Volunteers, n=20), patients with end-stage renal disease (ESRD, n=20) pre-operation (Pre), and on Day 1 (POD 1) and Day 14 (POD 14) after renal allotransplantation, brain-dead organ donors (DBD, n=20), and renal allotransplant recipients who were currently experiencing T cell-mediated rejection (Allo-TCMR, n=20). Serum IgM/IgG binding to, and complement-dependent cytotoxicity (CDC) of, PBMCs and RBCs from (a) wild-type (WT), (b) α1,3-galactosyltransferase gene-knockout (GTKO), (c) GTKO/beta-1,4-N-acety1 galactosaminyltransferase 2-knockout (GTKO/β4GalNT2KO), (d) GTKO/cytidine monophosphate-N-acetylneuraminic acid hydroxylase-knockout (GTKO/CMAHKO), and (e) GTKO/β4GalNT2KO/CMAHKO/hCD55 (TKO/hCD55) pigs were measured by flow cytometry. We obtained the following results: (i) Serum IgM/IgG binding and CDC in Volunteers were significantly greater to WT, GTKO, and GTKO/β4GalNT2KO PBMCs or RBCs than to GTKO/CMAHKO and TKO/hCD55 cells; (ii) ESRD, DBD, and Allo-TCMR serum antibody binding and CDC to WT pig PBMCs were significantly greater than to GTKO, GTKO/β4GalNT2KO, GTKO/CMAHKO, and TKO/hCD55 cells; (iii) antibody binding to GTKO/CMAHKO pig cells was significantly lower in hemodialysis than peritoneal dialysis patients. (iv) Two of twenty allotransplantation recipients' serum IgG binding to GTKO pig PBMCs increased on POD14 compared with Pre, but IgG binding to GTKO pig RBCs did not; (v) In all sera, the lowest antibody binding and CDC were to GTKO/CMAHKO and TKO/CD55 pig cells. We conclude (i) CMAHKO in the pig may be critical to the success of clinical pig kidney xenotransplantation, and may be the most important after GTKO, at least in Chinese patients; (ii) subjects with ESRD, or who are immunosuppressed after kidney allotransplantation, and DBD, have lower levels of antibody binding and CDC to genetically-engineered pig cells than do volunteers; (iii) TKO pigs with selected human 'protective' transgenes, e.g., CD55, are likely to prove to be the optimal sources of kidneys for clinical xenotransplantation.

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.

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.

Combined blockade of complement C5 and TLR co-receptor CD14 synergistically inhibits pig-to-human corneal xenograft induced innate inflammatory responses

Inadequate supplies of donor corneas have evoked an escalating interest in corneal xenotransplantation. However, innate immune responses contribute significantly to the mechanism of xenograft rejection. We hypothesized that complement component C5 and TLR co-receptor CD14 inhibition would inhibit porcine cornea induced innate immune responses. Therefore, we measured cytokine release in human blood, induced by three forms of corneal xenografts with or without inhibitors. Native porcine cornea (NPC) induced interleukins (IL-1β, IL-2, IL-6, IL-8, IL-1ra), chemokines (MCP-1, MIP-1α, MIP-1β) and other cytokines (TNF, G-CSF, INF-γ, FGF-basic). Decellularized (DPC) and gamma-irradiated cornea (g-DPC) elevated the release of those cytokines. C5-blockade by eculizumab inhibited all the cytokines except G-CSF when induced by NPC. However, C5-blockade failed to reduce DPC and g-DPC induced cytokines. Blockade of CD14 inhibited DPC-induced cytokines except for IL-8, MCP-1, MIP-1α, and G-CSF, while it inhibited all of them when induced by g-DPC. Combined blockade of C5 and CD14 inhibited the maximum number of cytokines regardless of the xenograft type. Finally, by using the TLR4 specific inhibitor Eritoran, we showed that TLR4 activation was the basis for the CD14 effect. Thus, blockade of C5, when combined with TLR4 inhibition, may have therapeutic potential in pig-to-human corneal xenotransplantation. STATEMENT OF SIGNIFICANCE: Bio-engineered corneal xenografts are on the verge of becoming a viable alternative to allogenic human-donor-cornea, but the host's innate immune response is still a critical barrier for graft acceptance. By overruling this barrier, limited graft availability would no longer be an issue for treating corneal diseases. We showed that the xenograft induced inflammation is initiated by the complement system and toll-like receptor activation. Intriguingly, the inflammatory response was efficiently blocked by simultaneously targeting bottleneck molecules in the complement system (C5) and the TLR co-receptor CD14 with pharmaceutical inhibitors. We postulate that a combination of C5 and CD14 inhibition could have a great therapeutic potential to overcome the immunologic barrier in pig-to-human corneal 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.

Anti-pig IgE and IgA Antibodies in Naive Primates and Nonhuman Primates With Pig Xenografts

BACKGROUND

Natural preformed anti-pig IgM/IgG antibodies in primates play an important role in xenograft rejection. As it is not clear how IgE and IgA engage in the immune system in xenotransplantation, we investigated natural preformed and elicited anti-pig IgE/IgA in naive primates and after xenotransplantation in nonhuman primates.

METHODS

The binding of IgM/IgG/IgE/IgA antibodies to red blood cells (RBCs) from wild-type (WT), α1,3-galactosyltransferase gene-knockout (GTKO), and GTKO/cytidine monophospho-N-acetylneuraminic acid hydroxylase gene-knockout/β-1,4 N-acetylgalactosaminyltransferase 2 gene-knockout (ie, triple-knockout pigs) pigs were measured by flow cytometry in naive human (n = 50) and baboon (n = 14) sera. Antibody binding to WT and GTKO pig RBCs (pRBCs) was also measured in the sera of baboons (nonsensitized n = 7, sensitized n = 2) and rhesus monkeys (nonsensitized n = 2, sensitized n = 11) following WT or GTKO pig organ/tissue xenotransplantation. Deposition of IgM/IgG/IgE/IgA in the grafts was detected by immunohistochemistry.

RESULTS

The majority of humans had natural preformed IgM/IgG/IgE/IgA to WT and GTKO pRBCs. In contrast, IgM/IgG/IgE/IgA to triple-knockout pRBCs were present at lower levels and frequency (P < 0.01). Baboons also had IgM/IgG/IgE/IgA antibodies against WT pRBCs, but fewer to GTKO and triple-knockout (P < 0.01). After xenotransplantation into nonhuman primates, when IgM/IgG increased, IgE/IgA also increased, but to a lesser extent. In addition to IgM/IgG, IgE or IgA deposition was observed in rejected pig xenografts.

CONCLUSIONS

Primates develop serum anti-pig IgE/IgA antibodies both naturally and during xenograft rejection. The pathophysiological role, if any, of anti-pig IgE/IgA antibodies remains unknown.

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.

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.

Differences in platelet aggregometers to study platelet function and coagulation dysregulation in xenotransplantation

Xenotransplantation (ie, cross-species transplantation) using genetically engineered pig organs could be a limitless source to solve the shortage of organs and tissues worldwide. However, despite prolonged survival in preclinical pig-to-nonhuman primate xenotransplantation trials, interspecies coagulation dysregulation remains to be overcome in order to achieve continuous long-term success. Different platelet aggregometry methods have been previously used to study the coagulation dysregulation with wild-type and genetically engineered pig cells, including the impact of possible treatment options. Among these methods, while thromboelastography and rotational thromboelastometry measure the change in viscoelasticity, optical aggregometry measures the change in opacity. Recently, impedance aggregometry has been used to measure changes in platelet aggregation in electrical conductance, providing more information to our understanding of coagulation dysregulation in xenotransplantation compared to previous methods. The present study reviews the merits and differences of the above-mentioned platelet aggregometers in xenotransplantation research.

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.

Introduction: The Present Status of Xenotransplantation Research

There is a well-known worldwide shortage of deceased human donor organs for clinical transplantation. The transplantation of organs from genetically engineered pigs may prove an alternative solution. In the past 5 years, there have been sequential advances that have significantly increased pig graft survival in nonhuman primates. This progress has been associated with (1) the availability of increasingly sophisticated genetically engineered pigs; (2) the introduction of novel immunosuppressive agents, particularly those that block the second T-cell signal (costimulation blockade); (3) a better understanding of the inflammatory response to pig xenografts; and (4) increasing experience in the management of nonhuman primates with pig organ or cell grafts. The range of investigations required in experimental studies has increased. The standard immunologic assays are still carried out, but increasingly investigations aimed toward other pathobiologic barriers (e.g., coagulation dysregulation and inflammation) have become more important in determining injury to the graft.Now that prolonged graft survival, extending to months or even years, is increasingly being obtained, the function of the grafts can be more reliably assessed. If the source pigs are bred and housed under biosecure isolation conditions, and weaned early from the sow, most microorganisms can be eradicated from the herd. The potential risk of porcine endogenous retrovirus (PERV) infection remains unknown, but is probably small. Attention is being directed toward the selection of patients for the first clinical trials of xenotransplantation.

Macrophages in xenotransplantation

Xenotransplantation refers to organ transplantation across species. Immune rejection of xenografts is stronger and faster than that of allografts because of significant molecular differences between species. Recent studies have revealed the involvement of macrophages in xenograft and allograft rejections. Macrophages have been shown to play a critical role in inflammation, coagulation, and phagocytosis in xenograft rejection. This review presents a recent understanding of the role of macrophages in xenograft rejection and possible strategies to control macrophage-mediated xenograft rejection.

Evidence for the important role of inflammation in xenotransplantation

There is increasing evidence of a sustained state of systemic inflammation after pig-to-nonhuman primate (NHP) xenotransplantation (that has been termed systemic inflammation in xenograft recipients [SIXR]). Increases in inflammatory markers, e.g., C-reactive protein, histones, serum amyloid A, D-dimer, cytokines, chemokines, and a decrease in free triiodothyronine, have been demonstrated in the recipient NHPs. The complex interactions between inflammation, coagulation, and the immune response are well-recognized, but the role of inflammation in xenograft recipients is not fully understood. The evidence suggests that inflammation can promote the activation of coagulation and the adaptive immune response, but the exact mechanisms remain uncertain. If prolonged xenograft survival is to be achieved, anti-inflammatory strategies (e.g., the administration of anti-inflammatory agents, and/or the generation of genetically-engineered organ-source pigs that are protected from the effect of inflammation) may be necessary to prevent, control, or negate the effect of the systemic inflammation that develops in xenograft recipients. This may allow for a reduction in the intensity of exogenous immunosuppressive therapy. If immunological tolerance to a xenograft is to be obtained, then control of inflammation may be essential.

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

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

In Case you Missed It-Basic Science Advances in Transplantation 2017

Developments in organ preservation techniques, novel immunosuppressants and improved diagnostics have made organ transplantation the success it is today. That does not mean that we are not still striving to perfect techniques, or that there are no more problems to solve. New strategies to address the donor organ shortage, prevent and manage antibody-mediated rejection, lower long-term allograft failure rates and reduce the toxicity of lifelong immunosuppressive medication are urgently needed, and are being widely researched. Both fundamental research and preclinical studies aim to solve these problems, and ultimately, benefit organ transplant recipients. This article highlights the latest technical developments and trends in xenotransplantation, tissue injury and regeneration, immunosuppression, and transplantation immunology described in the most viewed and cited articles published in the Basic Sciences section of the Transplantation journal during the year 2017.

Extracellular histones are clinically associated with primary graft dysfunction in human liver transplantation

Extracellular histones have been involved in numerous inflammatory conditions such as ischemia/reperfusion (I/R) injury, trauma, and infection. There is growing evidence of I/R injury associated with primary graft dysfunction (PGD) following organ transplantation. Here we investigated whether extracellular histones are clinically involved with PGD in human liver transplantation. In total 58 patients undergoing liver transplantation were studied. We collected blood samples from the recipients before and serially after transplantation (24 h, 72 h). We measured extracellular histones, myeloperoxidase (MPO), S100A8/A9, and multiple inflammatory cytokines. Additionally, we exposed human L02 hepatocytes or U937 monocytic cells to the recipient's sera overnight, and assessed cellular viability and cytokine production respectively. Lastly, we assessed the effect of histone-targeted interventions by administration of heparin or an anti-histone antibody. It showed that extracellular histones increased immediately after transplantation, peaked within 24 hours and remained at high levels up to 72 hours (all p < 0.01). Notably, extracellular histone levels were significantly higher in recipients with PGD (n = 9) than recipients without PGD (n = 49, p = 0.004). Extracellular histones correlated positively with MPO, S100A8/A9 and most detected cytokines. Ex vivo analysis demonstrated that the patients' sera after graft markedly induced L02 cell death and caused profound cytokine production in cultured U937 cells, which could be abrogated by heparin or an anti-histone antibody. Collectively, extracellular histones were increased significantly after liver transplantation, which may contribute to the occurrence of PGD through direct cytotoxicity and enhancement of systemic inflammation. Targeting extracellular histones may provide a promising approach for preventing PGD or other complications in clinical practice.

Extracellular histones have been involved in numerous inflammatory conditions such as ischemia/reperfusion (I/R) injury, trauma, and infection.

Serum amyloid a as an indicator of impending xenograft failure: Experimental studies

BACKGROUND

There is well-documented systemic inflammatory response in xenograft recipients to the presence of a pig graft. Serum amyloid A (SAA) is an inflammatory marker that is elevated in various pathological states. The assay used to measure it is (i) simple, (ii) relatively inexpensive, and (iii) provides an answer within minutes.

METHOD

The levels of SAA (n = 11) and C-reactive protein (C-RP) (n = 8) were measured retrospectively in the serum of baboons with pig kidney transplants, who received therapy with an IL-6R inhibitor and a TNF-α antagonist. Immunohistochemistry (IHC) was used to identify amyloid A and C-RP expression in the native livers and deposition in the pig kidney grafts.

RESULTS

One kidney graft underwent hyperacute rejection, 6 (55%) underwent acute antibody-mediated rejection, 3 baboons (27%) were euthanized for serious systemic infections, and one was euthanized for acute gastric dilatation. The SAA increased temporarily after kidney transplantation, and increased again by the day of euthanasia, indicating moderate (n = 3) or significant (severe) (n = 8) inflammation. In contrast, as the baboons were receiving tocilizumab, C-RP did not increase. There was greater expression of amyloid A in baboon livers (by IHC) than of C-RP (mean OD 53 vs 1, p < 0.01), and greater deposition of amyloid A than C-RP in the pig kidney grafts (mean OD 24 vs 2, p < 0.001). Plasma fibrinogen negatively correlated with the expression of amyloid A in the liver (r = -0.72, p < 0.05). The results of the SAA assay correlated with amyloid A expression in the liver and deposition in the kidney grafts.

CONCLUSIONS

SAA is a sensitive, but non-specific, marker for inflammation in baboons with pig kidney grafts, and is not affected by therapy that suppresses the response of C-RP. The SAA assay is a rapid, reliable, and relatively inexpensive method of following the inflammatory state of baboons with pig xenografts.

Xenotransplantation: past, present, and future

PURPOSE OF REVIEW

To review the progress in the field of xenotransplantation with special attention to most recent encouraging findings which will eventually bring xenotransplantation to the clinic in the near future.

RECENT FINDINGS

Starting from early 2000, with the introduction of galactose-α1,3-galactose (Gal)-knockout pigs, prolonged survival especially in heart and kidney xenotransplantation was recorded. However, remaining antibody barriers to non-Gal antigens continue to be the hurdle to overcome. The production of genetically engineered pigs was difficult requiring prolonged time. However, advances in gene editing, such as zinc finger nucleases, transcription activator-like effector nucleases, and most recently clustered regularly interspaced short palindromic repeats (CRISPR) technology made the production of genetically engineered pigs easier and available to more researchers. Today, the survival of pig-to-nonhuman primate heterotopic heart, kidney, and islet xenotransplantation reached more than 900, more than 400, and more than 600 days, respectively. The availability of multiple-gene pigs (five or six genetic modifications) and/or newer costimulation blockade agents significantly contributed to this success. Now, the field is getting ready for clinical trials with an international consensus.

SUMMARY

Clinical trials in cellular or solid organ xenotransplantation are getting closer with convincing preclinical data from many centers. The next decade will show us new achievements and additional barriers in clinical xenotransplantation.