Research opportunities and ethical considerations for heart and lung xenotransplantation research: A report from the National Heart, Lung, and Blood Institute workshop

Xenotransplantation offers the potential to meet the critical need for heart and lung transplantation presently constrained by the current human donor organ supply. Much was learned over the past decades regarding gene editing to prevent the immune activation and inflammation that cause early organ injury, and strategies for maintenance of immunosuppression to promote longer-term xenograft survival. However, many scientific questions remain regarding further requirements for genetic modification of donor organs, appropriate contexts for xenotransplantation research (including nonhuman primates, recently deceased humans, and living human recipients), and risk of xenozoonotic disease transmission. Related ethical questions include the appropriate selection of clinical trial participants, challenges with obtaining informed consent, animal rights and welfare considerations, and cost. Research involving recently deceased humans has also emerged as a potentially novel way to understand how xeno-organs will impact the human body. Clinical xenotransplantation and research involving decedents also raise ethical questions and will require consensus regarding regulatory oversight and protocol review. These considerations and the related opportunities for xenotransplantation research were discussed in a workshop sponsored by the National Heart, Lung, and Blood Institute, and are summarized in this meeting report.

Novel factors potentially initiating acute antibody-mediated rejection in pig kidney xenografts despite an efficient immunosuppressive regimen

Antibody-mediated rejection (AMR) is a common cause of graft failure after pig-to-nonhuman primate organ transplantation, even when the graft is from a pig with multiple genetic modifications. The specific factors that initiate AMR are often uncertain. We report two cases of pig kidney transplantation into immunosuppressed baboons in which we identify novel factors associated with the initiation of AMR. In the first, membranous nephropathy was the initiating factor that was then associated with the apparent loss of the therapeutic anti-CD154 monoclonal antibody in the urine when severe proteinuria was present. This observation suggests that proteinuria may be associated with the loss of any therapeutic monoclonal antibody, for example, anti-CD154 or eculizumab, in the urine, resulting in xenograft rejection. In the second case, the sequence of events and histopathology tentatively suggested that pyelonephritis may have initiated acute-onset AMR. The association of a urinary infection with graft rejection has been well-documented in ABO-incompatible kidney allotransplantation based on the expression of an antigen on the invading microorganism shared with the kidney graft, generating an immune response to the graft. To our knowledge, these potential initiating factors of AMR in pig xenografts have not been highlighted previously.

Pediatric Cardiac Xenotransplantation: Recommendations for the Ethical Design of Clinical Trials

For children with complex congenital heart problems, cardiac allotransplantation is sometimes the best therapeutic option. However, availability of hearts for pediatric patients is limited, resulting in a long and growing waitlist, and a high mortality rate while waiting. Cardiac xenotransplantation has been proposed as one therapeutic alternative for neonates and infants, either in lieu of allotransplantation or as a bridge until an allograft becomes available. Scientific and clinical developments in xenotransplantation appear likely to permit cardiac xenotransplantation clinical trials in adults in the coming years. The ethical issues around xenotransplantation of the heart and other organs and tissues have recently been examined, but to date, only limited literature is available on the ethical issues that are attendant with pediatric heart xenotransplantation. Here, we summarize the ethical issues, focusing on (1) whether cardiac xenotransplantation should proceed in adults or children first, (2) pediatric recipient selection for initial xenotransplantation trials, (3) special problems regarding informed consent in this context, and (4) related psychosocial and public perception considerations. We conclude with specific recommendations regarding ethically informed design of pediatric heart xenotransplantation trials.

Expression of human thrombomodulin by GalTKO.hCD46 pigs modulates coagulation cascade activation by endothelial cells and during ex vivo lung perfusion with human blood

Thrombomodulin is important for the production of activated protein C (APC), a molecule with significant regulatory roles in coagulation and inflammation. To address known molecular incompatibilities between pig thrombomodulin and human thrombin that affect the conversion of protein C into APC, GalTKO.hCD46 pigs have been genetically modified to express human thrombomodulin (hTBM). The aim of this study was to evaluate the impact of transgenic hTBM expression on the coagulation dysregulation that is observed in association with lung xenograft injury in an established lung perfusion model, with and without additional blockade of nonphysiologic interactions between pig vWF and human GPIb axis. Expression of hTBM was variable between pigs at the transcriptional and protein level. hTBM increased the activation of human protein C and inhibited thrombosis in an in vitro flow perfusion assay, confirming that the expressed protein was functional. Decreased platelet activation was observed during ex vivo perfusion of GalTKO.hCD46 lungs expressing hTBM and, in conjunction with transgenic hTBM, blockade of the platelet GPIb receptor further inhibited platelets and increased survival time. Altogether, our data indicate that expression of transgenic hTBM partially addresses coagulation pathway dysregulation associated with pig lung xenograft injury and, in combination with vWF-GP1b-directed strategies, is a promising approach to improve the outcomes of lung xenotransplantation.

Shooting for the moon: Genome editing for pig heart xenotransplantation

Gene-edited pigs could eventually provide organs that are safely and effectively protected from the human immune response without exogenous immunosuppression.

Genome editing technology has revolutionized heart xenotransplantation and made transplantation of bioengineered pig hearts into humans a possibility. This first clinical application resulted from a tremendous amount of research. Dramatic early attempts of clinical cardiac xenotransplantation during the last century paved the way to modern xenotransplantation using bioengineered pig hearts. It appears that such genome-edited hearts will be most suitable for neonates and infants because of their immature immune system. The bioengineered pig heart may also be used as a bridge to human heart transplantation, avoiding the risk of thromboembolic events of durable ventricular assist devises in these young children. It is also intriguing to think that bioengineered hearts using pigs as a host may result in a new source of donor hearts that would not evoke the human immune response and minimize, if not eliminate, the need for immunosuppression.

It this issue of the Journal, a group of experts led by Dr Cooper, whose personal work spans over 50 years of heart transplantation research, outline the current state of the genome editing of bioengineered hearts and discuss the prospects of clinical application.

TNX-1500, a crystallizable fragment-modified anti-CD154 antibody, prolongs nonhuman primate renal allograft survival

The blockade of the CD154-CD40 pathway with anti-CD154 monoclonal antibody has been a promising immunomodulatory approach to prevent allograft rejection. However, clinical trials of immunoglobulin G1 antibodies targeting this pathway revealed thrombogenic properties, which were subsequently shown to be mediated by crystallizable fragment (Fc)-gamma receptor IIa-dependent platelet activation. To prevent thromboembolic complications, an immunoglobulin G4 anti-CD154 monoclonal antibody, TNX-1500, which retains the fragment antigen binding region of ruplizumab (humanized 5c8, BG9588), was modified by protein engineering to decrease Fc binding to Fc-gamma receptor IIa while retaining certain other effector functions and pharmacokinetics comparable with natural antibodies. Here, we report that TNX-1500 treatment is not associated with platelet activation in vitro and consistently inhibits kidney allograft rejection in vivo without clinical or histologic evidence of prothrombotic phenomena. We conclude that TNX-1500 retains efficacy similar to that of 5c8 to prevent kidney allograft rejection while avoiding previously identified pathway-associated thromboembolic complications.

TNX-1500, a crystallizable fragment-modified anti-CD154 antibody, prolongs nonhuman primate cardiac allograft survival

Blockade of the CD40/CD154 T cell costimulation pathway is a promising approach to supplement or replace current clinical immunosuppression in solid organ transplantation. We evaluated the tolerability and activity of a novel humanized anti-CD154 monoclonal antibody, TNX-1500 (TNX), in a nonhuman primate heterotopic cardiac allogeneic (allo) transplant model. TNX-1500 contains a rupluzimab fragment antigen-binding region and an immunoglobin G4 crystallizable fragment region engineered to reduce binding to the crystallizable fragment gamma receptor IIa and associated risks of thrombosis. Recipients were treated for 6 months with standard-dose TNX (sTNX) monotherapy, low-dose TNX monotherapy (loTNX), or loTNX with mycophenolate mofetil (MMF) (loTNX + MMF). Results were compared with historical data using chimeric humanized 5c8 monotherapy dosed as for loTNX but discontinued at 3 months. Median survival time was similar for humanized 5c8 and both loTNX groups, but significantly longer with sTNX (>265 days) than with loTNX (99 days) or loTNX + MMF (88 days) (P < 0.05 for both comparisons against sTNX). Standard-dose TNX prevented antidonor alloantibody elaboration, inhibited chronic rejection, and was associated with a significantly reduced effector T cells/regulatory T cells ratio relative to loTNX with MMF. No thrombotic complications were observed. This study demonstrated that TNX was well tolerated, prolongs allograft survival, and prevents alloantibody production and cardiac allograft vasculopathy in a stringent preclinical nonhuman primate heart allotransplant model.

Recruited macrophages elicit atrial fibrillation

Atrial fibrillation disrupts contraction of the atria, leading to stroke and heart failure. We deciphered how immune and stromal cells contribute to atrial fibrillation. Single-cell transcriptomes from human atria documented inflammatory monocyte and SPP1+ macrophage expansion in atrial fibrillation. Combining hypertension, obesity, and mitral valve regurgitation (HOMER) in mice elicited enlarged, fibrosed, and fibrillation-prone atria. Single-cell transcriptomes from HOMER mouse atria recapitulated cell composition and transcriptome changes observed in patients. Inhibiting monocyte migration reduced arrhythmia in Ccr2-∕- HOMER mice. Cell-cell interaction analysis identified SPP1 as a pleiotropic signal that promotes atrial fibrillation through cross-talk with local immune and stromal cells. Deleting Spp1 reduced atrial fibrillation in HOMER mice. These results identify SPP1+ macrophages as targets for immunotherapy in atrial fibrillation.

Increased human complement pathway regulatory protein gene dose is associated with increased endothelial expression and prolonged survival during ex-vivo perfusion of GTKO pig lungs with human blood

INTRODUCTION

Expression of human complement pathway regulatory proteins (hCPRP's) such as CD46 or CD55 has been associated with improved survival of pig organ xenografts in multiple different models. Here we evaluate the hypothesis that an increased human CD46 gene dose, through homozygosity or additional expression of a second hCPRP, is associated with increased protein expression and with improved protection from injury when GTKO lung xenografts are perfused with human blood.

METHODS

Twenty three GTKO lungs heterozygous for human CD46 (GTKO.heteroCD46), 10 lungs homozygous for hCD46 (GTKO.homoCD46), and six GTKO.homoCD46 lungs also heterozygous for hCD55 (GTKO.homoCD46.hCD55) were perfused with human blood for up to 4 h in an ex vivo circuit.

RESULTS

Relative to GTKO.heteroCD46 (152 min, range 5-240; 6/23 surviving at 4 h), survival was significantly improved for GTKO.homoCD46 (>240 min, range 45-240, p = .034; 7/10 surviving at 4 h) or GTKO.homoCD46.hCD55 lungs (>240 min, p = .001; 6/6 surviving at 4 h). Homozygosity was associated with increased capillary expression of hCD46 (p < .0001). Increased hCD46 expression was associated with significantly prolonged lung survival (p = .048),) but surprisingly not with reduction in measured complement factor C3a. Hematocrit, monocyte count, and pulmonary vascular resistance were not significantly altered in association with increased hCD46 gene dose or protein expression.

CONCLUSION

Genetic engineering approaches designed to augment hCPRP activity - increasing the expression of hCD46 through homozygosity or co-expressing hCD55 with hCD46 - were associated with prolonged GTKO lung xenograft survival. Increased expression of hCD46 was associated with reduced coagulation cascade activation, but did not further reduce complement activation relative to lungs with relatively low CD46 expression. We conclude that coagulation pathway dysregulation contributes to injury in GTKO pig lung xenografts perfused with human blood, and that the survival advantage for lungs with increased hCPRP expression is likely attributable to improved endothelial thromboregulation.

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.

Guidelines and principles for the care of the cardiothoracic transplant patient in the intensive care unit

Heart and lung transplant recipients require care provided by clinicians from multiple different specialties, each contributing unique expertise and perspective. The period the patient spends in the intensive care unit is one of the most critical times in the perioperative trajectory. Various organizational models of intensive care exist, including those led by intensivists, surgeons, transplant cardiologists, and pulmonologists. Coordinating timely efficient intensive care is an essential and logistically difficult goal. The present work product of the American Society of Transplantation's Thoracic and Critical Care Community of Practice, Critical Care Task Force outlines operational guidelines and principles that may be applied in different organizational models to optimize the delivery of intensive care for the cardiothoracic organ recipient.

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.

Future of Lung Transplantation: Xenotransplantation and Bioengineering Lungs

Xenotransplantation promises to alleviate the issue of donor organ shortages and to decrease waiting times for transplantation. Recent advances in genetic engineering have allowed for the creation of pigs with up to 16 genetic modifications. Several combinations of genetic modifications have been associated with extended graft survival and life-supporting function in experimental heart and kidney xenotransplants. Lung xenotransplantation carries specific challenges related to the large surface area of the lung vascular bed, its innate immune system's intrinsic hyperreactivity to perceived 'danger', and its anatomic vulnerability to airway flooding after even localized loss of alveolocapillary barrier function. This article discusses the current status of lung xenotransplantation, and challenges related to immunology, physiology, anatomy, and infection. Tissue engineering as a feasible alternative to develop a viable lung replacement solution is discussed.

Cardiac xenotransplantation: from concept to clinic

For many patients with terminal/advanced cardiac failure, heart transplantation is the most effective, durable treatment option, and offers the best prospects for a high quality of life. The number of potentially life-saving donated human organs is far fewer than the population who could benefit from a new heart, resulting in increasing numbers of patients awaiting replacement of their failing heart, high waitlist mortality, and frequent reliance on interim mechanical support for many of those deemed among the best candidates but who are deteriorating as they wait. Currently, mechanical assist devices supporting left ventricular or biventricular heart function are the only alternative to heart transplant that is in clinical use. Unfortunately, the complication rate with mechanical assistance remains high despite advances in device design and patient selection and management, and the quality of life of the patients even with good outcomes is only moderately improved. Cardiac xenotransplantation from genetically multi-modified (GM) organ-source pigs is an emerging new option as demonstrated by the consistent long-term success of heterotopic (non-life-supporting) abdominal and life-supporting orthotopic porcine heart transplantation in baboons, and by a recent 'compassionate use' transplant of the heart from a GM pig with 10 modifications into a terminally ill patient who survived for 2 months. In this review, we discuss pig heart xenotransplantation as a concept, including pathobiological aspects related to immune rejection, coagulation dysregulation, and detrimental overgrowth of the heart, as well as GM strategies in pigs to prevent or minimize these problems. Additional topics discussed include relevant results of heterotopic and orthotopic heart transplantation experiments in the pig-to-baboon model, microbiological and virologic safety concepts, and efficacy requirements for initiating formal clinical trials. An adequate regulatory and ethical framework as well as stringent criteria for the selection of patients will be critical for the safe clinical development of cardiac xenotransplantation, which we expect will be clinically tested during the next few years.

The respective relevance of sensitization to alloantigens and xenoantigens in pig organ xenotransplantation

BACKGROUND

Antibody-mediated rejection is a major cause of graft injury and contributes to failure of pig xenografts in nonhuman primates (NHPs). Most 'natural' or elicited antibodies found in humans and NHPs are directed against pig glycan antigens, but antibodies binding to swine leukocyte antigens (SLA) have also been detected. Of clinical importance is (i) whether the presence of high levels of antibodies directed towards human leukocyte antigens (HLA) (i.e., high panel-reactive antibodies) would be detrimental to the outcome of a pig organ xenograft; and (ii) whether, in the event of sensitization to pig antigens, a subsequent allotransplant would be at increased risk of graft failure due to elicited anti-pig antibodies that cross-react with human HLA or other antigens.

SUMMARY

A literature review of pig-to-primate studies indicates that relatively few highly-HLA-sensitized humans have antibodies that cross-react with pigs, predicting that most would not be at increased risk of rejecting an organ xenograft. Furthermore, the existing evidence indicates that sensitization to pig antigens will probably not elicit increased alloantibody titers; if so, 'bridging' with a pig organ could be carried out without increased risk of subsequent antibody-mediated allograft failure.

KEY MESSAGE

These issues have important implications for the design and conduct of clinical xenotransplantation trials.

Knock-out of N-glycolylneuraminic acid attenuates antibody-mediated rejection in xenogenically perfused porcine lungs

BACKGROUND

Antibody-mediated rejection has long been known to be one of the major organ failure mechanisms in xenotransplantation. In addition to the porcine α1,3-galactose (α1,3Gal) epitope, N-Glycolylneuraminic acid (Neu5Gc), a sialic acid, has been identified as an important porcine antigen against which most humans have pre-formed antibodies. Here we evaluate GalTKO.hCD46 lungs with an additional cytidine monophospho-N-acetylneuraminic acid hydroxylase (CMAH) gene knock-out (Neu5GcKO) in a xenogeneic ex vivo perfusion model METHODS: Eleven GalTKO.hCD46.Neu5GcKO pig lungs were perfused for up to 6 h with fresh heparinized human blood. Six of them were treated with histamine (H) blocker famotidine and 1-thromboxane synthase inhibitor Benzylimidazole (BIA) and five were left untreated. GalTKO.hCD46 lungs without Neu5GcKO (n = 18: eight untreated and 10 BIA+H treated) served as a reference. Functional parameters, blood, and tissue samples were collected at pre-defined time points throughout the perfusion RESULTS: All but one Neu5GcKO organs maintained adequate blood oxygenation and "survived" until elective termination at 6 h whereas two reference lungs failed before elective termination at 4 h. Human anti-Neu5Gc antibody serum levels decreased during the perfusion of GalTKO.hCD46 lungs by flow cytometry (∼40% IgM, 60% IgG), whereas antibody levels in Neu5GcKO lung perfusions did not fall (IgM p = .007; IgG p < .001). Thromboxane elaboration, thrombin generation, and histamine levels were significantly reduced with Neu5GcKO lungs compared to reference in the untreated groups (p = .007, .005, and .037, respectively); treatment with BIA+H masked these changes. Activation of platelets, measured as CD62P expression on circulating platelets, was lower in Neu5GcKO experiments compared to reference lungs (p = .023), whereas complement activation (as C3a rise in plasma) was not altered. MCP-1 and lactotransferin level elevations were blunted in Neu5GcKO lung perfusions (p = .007 and .032, respectively). Pulmonary vascular resistance (PVR) rise was significantly attenuated and delayed in untreated GalTKO.hCD46.Neu5GcKO lungs in comparison to the untreated GalTKO.hCD46 lungs (p = .003) CONCLUSION: Additional Neu5GcKO in GalTKO.hCD46 lungs significantly reduces parameters associated with antibody-mediated inflammation and activation of the coagulation cascade. Knock-out of the Neu5Gc sialic acid should be beneficial to reduce innate immune antigenicity of porcine lungs in future human recipients.

Genetic modifications designed for xenotransplantation attenuate sialoadhesin-dependent binding of human erythrocytes to porcine macrophages

The phenomenon of diminishing hematocrit after in vivo liver and lung xenotransplantation and during ex vivo liver xenoperfusion has largely been attributed to action by resident liver porcine macrophages, which bind and destroy human erythrocytes. Porcine sialoadhesin (siglec-1) was implicated previously in this interaction. This study examines the effect of porcine genetic modifications, including knockout of the CMAH gene responsible for expression of Neu5Gc sialic acid, on the adhesion of human red blood cells (RBCs) to porcine macrophages. Wild-type (WT) porcine macrophages and macrophages from several strains of genetically engineered pigs, including CMAH gene knockout and several human transgenes (TKO+hTg), were incubated with human RBCs and "rosettes" (≥3 erythrocytes bound to one macrophage) were quantified by microscopy. Our results show that TKO+hTg genetic modifications significantly reduced rosette formation. The monoclonal antibody 1F1, which blocks porcine sialoadhesin, significantly reduced rosette formation by WT and TKO+hTg macrophages compared with an isotype control antibody. Further, desialation of human RBCs with neuraminidase before addition to WT or TKO+hTg macrophages resulted in near-complete abrogation of rosette formation, to a level not significantly different from porcine RBC rosette formation on porcine macrophages. These observations are consistent with rosette formation being mediated by binding of sialic acid on human RBCs to sialoadhesin on porcine macrophages. In conclusion, the data predict that TKO+hTg genetic modifications, coupled with targeting of porcine sialoadhesin by the 1F1 mAb, will attenuate erythrocyte sequestration and anemia during ex vivo xenoperfusion and following in vivo liver, lung, and potentially other organ xenotransplantation.

Pig heart and lung xenotransplantation: Present status

The recent pig heart transplant in a patient at the University of Maryland Medical Center has stimulated renewed interest in the xenotransplantation of organs from genetically engineered pigs. The barriers to the use of pigs as sources of organs have largely been overcome by 2 approaches - (1) the deletion of expression of the three known pig carbohydrate xenoantigens against which humans have preformed antibodies, and (2) the transgenic introduction of human 'protective' proteins, such as complement-regulatory proteins. These gene modifications, coupled with immunosuppressive therapy based on blockade of the CD40/CD154 costimulation pathway, have resulted in survival of baboons with life-supporting pig heart grafts for almost 9 months. The initial clinical success at the University of Maryland reinforces encouraging preclinical results. It suggests that pig hearts are likely to provide an effective bridge to an allotransplant, but their utility for destination therapy remains uncertain. Because of additional complex immunobiological problems, the same approach has been less successful in preclinical lung xenograft transplantation, where survival is still measured in days or weeks. The first formal clinical trials of pig heart transplantation may include patients who do not have access to an allotransplant, those with contraindications for mechanical circulatory support, those in need of retransplantation or with a high level of panel-reactive antibodies. Infants with complex congenital heart disease, should also be considered.

The first clinical pig heart transplant: Was IVIg or pig cytomegalovirus detrimental to the outcome?

The clinical course of the first patient to receive a gene-edited pig heart transplant was recently reported by the University of Maryland team. Although the pig heart functioned well for >40 days, serum anti-pig antibodies then increased, and the patient sadly died after 60 days. Because of his debilitated pre-transplant state, the patient never thrived despite excellent graft function for several weeks, and the cause of his demise continues to be uncertain. A few days before an increase in anti-pig antibodies was observed, the patient had received intravenous human immunoglobulin (IVIg), and whether this played a role in his cardiac deterioration has been discussed. Furthermore, mcfDNA testing indicated an increase in pig cytomegalovirus (CMV), and its possible role in the development of cardiac dysfunction has also been considered. On the basis of the limited data provided in the publication and on our previous investigations into whether IVIg contains anti-TKO pig antibodies and therefore might be deleterious to TKO pig organ xenografts, we suggest that the steady rise in anti-pig antibody titer was more consistent with the failure of the immunosuppressive regimen to prevent elicited anti-TKO pig antibody production, rather than from the passive transfusion of IVIg or the presence of pig CMV in the graft. Although the outcome of the Maryland experience was disappointing, valuable lessons were learned. Our attention was drawn to the potential risks of heart transplantation in a "deconditioned" patient, the administration of IVIg, the transmission of pig CMV, and of the difficulties in interpreting myocardial biopsy findings.

hEPCR.hTBM.hCD47.hHO-1 with donor clodronate and DDAVP treatment improves perfusion and function of GalTKO.hCD46 porcine livers perfused with human blood

INTRODUCTION

Platelet sequestration, inflammation, and inappropriate coagulation cascade activation are prominent in liver xenotransplant models and are associated with poor outcomes. Here, we evaluate a cassette of six additional genetic modifications to reduce anti-pig antibody binding (α-1,3-galactosyl transferase knockout [GalTKO]) and target coagulation dysregulation (human endothelial protein C receptor [hEPRC] and thrombomodulin [hTBM]), complement pathway regulation (human membrane cofactor protein, hCD46), inflammation heme oxygenase 1 [hHO-1]), and a self-recognition receptor (integrin-associated protein [hCD47]), as well as donor pharmacologic treatments designed to blunt these phenomena.

METHODS

Livers from GaltKO.hCD46 pigs ("2-gene," n = 3) and GalTKO.hCD46 pigs also transgenic for hEPRC, hTBM, hCD47, and hHO-1 ("6-gene," n = 4) were perfused ex vivo with whole human blood. Six-gene pigs were additionally pretreated with desmopressin (DDAVP) and clodronate liposomes to deplete vWF and kupffer cells, respectively.

RESULTS

The average perfusion times increased from 304 (±148) min in the 2-gene group to 856 (±61) min in the 6-gene group (p = .010). The average heparin administration was decreased from 8837 U/h in the 2-gene to 1354 U/h in the 6-gene group (p = .047). Platelet sequestration tended to be delayed in the 6-gene group (p = .070), while thromboxane B2 (TXB2, a platelet activation marker) levels were lower over the first hour (p = .044) (401 ± 124 vs. 2048 ± 712 at 60 min). Thrombin production as measured by F1+2 levels tended to be lower in the 6-gene group (p = .058).

CONCLUSIONS

The combination of the hEPCR.hTBM.hCD47.hHO-1 cassette along with donor pig DDAVP and clodronate liposome pretreatment was associated with prolonged function of xenoperfused livers, reduced coagulation pathway perturbations, and decreased TXB2 elaboration, and reflects significant progress to modulate liver xenograft injury in a pig to human model.

Human erythrocyte fragmentation during ex-vivo pig organ perfusion

Platelet sequestration is a common process during organ reperfusion after transplantation. However, instead of lower platelet counts, when using traditional hemocytometers and light microscopy, we observed physiologically implausible platelet counts in the course of ex-vivo lung and liver xenograft organ perfusion studies. We employed conventional flow cytometry (FC) and imaging FC (AMINS ImageStream X) to investigate the findings and found platelet-sized fragments in the circulation that are mainly derived from red blood cell membranes. We speculate that this erythrocyte fragmentation contributes to anemia during in-vivo organ xenotransplant.

Effects of human TFPI and CD47 expression and selectin and integrin inhibition during GalTKO.hCD46 pig lung perfusion with human blood

BACKGROUND

Loss of barrier function when GalTKO.hCD46 porcine lungs are perfused with human blood is associated with coagulation pathway dysregulation, innate immune system activation, and rapid sequestration of human formed blood elements. Here, we evaluate whether genetic expression of human tissue factor pathway inhibitor (hTFPI) and human CD47 (hCD47), alone or with combined selectin and integrin adhesion pathway inhibitors, delays GalTKO.hCD46 porcine lung injury or modulates neutrophil and platelet sequestration.

METHODS

In a well-established paired ex vivo lung perfusion model, GalTKO.hCD46.hTFPI.hCD47 transgenic porcine lungs (hTFPI.hCD47, n = 7) were compared to GalTKO.hCD46 lungs (reference, n = 5). All lung donor pigs were treated with a thromboxane synthase inhibitor, anti-histamine, and anti-GPIb integrin-blocking Fab, and were pre-treated with Desmopressin. In both genotypes, one lung of each pair was additionally treated with PSGL-1 and GMI-1271 (P- and E-selectin) and IB4 (CD11b/18 integrin) adhesion inhibitors (n = 6 hTFPI.hCD47, n = 3 reference).

RESULTS

All except for two reference lungs did not fail within 480 min when experiments were electively terminated. Selectin and integrin adhesion inhibitors moderately attenuated initial pulmonary vascular resistance (PVR) elevation in hTFPI.hCD47 lungs. Neutrophil sequestration was significantly delayed during the early time points following reperfusion and terminal platelet activation was attenuated in association with lungs expressing hTFPI.hCD47, but additional adhesion pathway inhibitors did not show further effects with either lung genotype.

CONCLUSION

Expression of hTFPI.hCD47 on porcine lung may be useful as part of an integrated strategy to prevent neutrophil adhesion and platelet activation that are associated with xenograft injury. Additionally, targeting canonical selectin and integrin adhesion pathways reduced PVR elevation associated with hTFPI.hCD47 expression, but did not significantly attenuate neutrophil or platelet sequestration. We conclude that other adhesive mechanisms mediate the residual sequestration of human formed blood elements to pig endothelium that occurs even in the context of the multiple genetic modifications and drug treatments tested here.

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.

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.

Organ transplants of the future: planning for innovations including xenotransplantation

The future clinical application of animal-to-human transplantation (xenotransplantation) is of importance to society as a whole. Favourable preclinical data relevant to cell, tissue and solid organ xenotransplants have been obtained from many animal models utilizing genetic engineering and protocols of pathogen-free husbandry. Findings have reached a tipping point, and xenotransplantation of solid organs is approaching clinical evaluation, the process of which now requires close deliberation. Such discussions include considering when there is sufficient evidence from preclinical animal studies to start first-in-human xenotransplantation trials. The present article is based on evidence and opinions formulated by members of the European Society for Organ Transplantation who are involved in the Transplantation Learning Journey project. The article includes a brief overview of preclinical concepts and biology of solid organ xenotransplantation, discusses the selection of candidates for first-in-human studies and considers requirements for study design and conduct. In addition, the paper emphasizes the need for a regulatory framework for xenotransplantation of solid organs and the essential requirement for input from public and patient stakeholders.

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.

Progress Toward Cardiac Xenotransplantation

Consistent survival of life-supporting pig heart xenograft recipients beyond 90 days was recently reported using genetically modified pigs and a clinically applicable drug treatment regimen. If this remarkable achievement proves reproducible, published benchmarks for clinical translation of cardiac xenografts appear to be within reach. Key mechanistic insights are summarized here that informed recent pig design and therapeutic choices, which together appear likely to enable early clinical translation.

Pig-to-human heart transplantation: Who goes first?

Cardiac xenotransplantation has recently taken an important step towards clinical reality. In anticipation of the "first-in-human" heart xenotransplantation trial, we propose a set of patient characteristics that define potential candidates. Our premise is that, to be ethically justified, the risks posed by current state-of-the-art options must outweigh the anticipated risks of a pioneering xenotransplant procedure. Suitable candidates include patients who are at high immunologic risk because of sensitization to alloantigens, including those who have exhibited early onset or accelerated cardiac allograft vasculopathy. In addition, patients should be considered (1) for whom mechanical circulatory support would be prohibitively risky due to a hypercoagulable state, a contraindication to anticoagulation, or restrictive physiology; (2) with severe biventricular dysfunction predicting unsuccessful univentricular left heart support; and (3) adults with complex congenital heart disease. In conclusion, because the published preclinical benchmark for clinical translation of heart xenotransplantation appears within reach, carefully and deliberately defining appropriate trial participants is timely as the basis for ethical clinical trial design.

Thromboxane and histamine mediate PVR elevation during xenogeneic pig lung perfusion with human blood

BACKGROUND

Elevated pulmonary vascular resistance (PVR), platelet adhesion, coagulation activation, and inflammation are prominent features of xenolung rejection. Here, we evaluate the role of thromboxane and histamine on PVR, and their contribution to other lung xenograft injury mechanisms.

METHODS

GalTKO.hCD46 single pig lungs were perfused ex vivo with fresh heparinized human blood: lungs were either treated with 1-Benzylimidazole (1-BIA) combined with histamine receptor blocker famotidine (n = 4) or diphenhydramine (n = 6), 1-BIA alone (n = 6) or were left untreated (n = 9).

RESULTS

Six of the nine control experiments (GalTKO.hCD46 untreated), "survived" until elective termination at 4 hours. Without treatment, initial PVR elevation within the first 30 minutes resolved partially over the following hour, and increased progressively during the final 2 hours of perfusion. In contrast, 1-BIA, alone or in addition to either antihistamine treatment, was associated with low stable PVR. Combined treatments significantly lowered the airway pressure when compared to untreated reference. Although platelet and neutrophil sequestration and coagulation cascade activation were not consistently altered by any intervention, increased terminal wet/dry weight ratio in untreated lungs was significantly blunted by combined treatments.

CONCLUSION

Combined thromboxane and histamine pathway blockade prevents PVR elevation and significantly inhibits loss of vascular barrier function when GalTKO.hCD46 lungs are perfused with human blood. Platelet activation and platelet and neutrophil sequestration persist in all groups despite efficient complement regulation, and appear to occur independent of thromboxane and histamine antagonism. Our work identifies thromboxane and histamine as key mediators of xenolung injury and defines those pathways as therapeutic targets to achieve successful xenolung transplantation.

The Application of Optimal Defaults to Improve Elementary School Lunch Selections: Proof of Concept

BACKGROUND

In this study, we applied behavioral economics to optimize elementary school lunch choices via parent-driven decisions. Specifically, this experiment tested an optimal defaults paradigm, examining whether strategically manipulating the health value of a default menu could be co-opted to improve school-based lunch selections.

METHODS

The design was a randomized comparison of optimal versus suboptimal (standard) default lunch menus for all first-graders in a school district for a period of 1 week. We hypothesized that making the default lunch option healthier, while providing parents the opportunity to access and choose from the standard school menu for their child, would yield more frequent selection of healthier items than when the default option was suboptimal.

RESULTS

Overall, 127 (93%) first-grade children's families participated. Among those families randomized to receive the nutritionally optimized default menu, all but one remained with these options; of those parents randomized to the standard menu (suboptimal default), all parents remained with these options (Χ² = 123.06, df = 1, p < .001).

CONCLUSIONS

The mere positioning of choices, without restricting options, significantly affected which menu items the children received during the test period. Results are proof of concept for a strategy to increase health-promoting school lunch content, procedures, and policies.

Interleukin-8 mediates neutrophil-endothelial interactions in pig-to-human xenogeneic models

BACKGROUND

Human neutrophils are sequestered by pig lung xenografts within minutes during ex vivo perfusion. This phenomenon is not prevented by pig genetic modifications that remove xeno-antigens or added human regulatory molecules intended to down-regulate activation of complement and coagulation pathways. This study investigated whether recipient and donor interleukin-8 (IL-8), a chemokine known to attract and activate neutrophils during inflammation, is elaborated in the context of xenogeneic injury, and whether human or pig IL-8 promote the adhesion of human neutrophils in in vitro xenograft models.

METHODS

Plasma levels of pig, human or non-human primate (NHP) IL-8 from ex vivo pig lung perfusion experiments (n = 10) and in vivo pig-to-baboon lung transplantation in baboons (n = 22) were analysed by ELISA or Luminex. Human neutrophils stimulated with human or pig IL-8 were analysed for CD11b expression, CD18 activation, oxidative burst and adhesion to resting or TNF-activated endothelial cells (EC) evaluated under static and flow (Bioflux) conditions. For some experiments, human neutrophils were incubated with Reparixin (IL-8/CXCL8 receptor blocker) and then analysed as in the in vitro experiments mentioned above.

RESULTS

Plasma levels of pig IL-8 (~6113 pg/mL) increased more than human (~1235 pg/mL) between one and four hours after initiation of ex vivo lung perfusion. However, pig IL-8 levels remained consistently low (<60 pg/mL) and NHP IL-8 plasma levels increased by ~2000 pg/mL after four hours in a pig-to-baboon lung xenotransplantation. In vitro, human neutrophils' CD11b expression, CD18 activation and oxidative burst all increased in a dose-dependent manner following exposure to either pig or human IL-8, which also were associated with increased adhesion to EC in both static and flow conditions. Reparixin inhibited human neutrophil activation by both pig and human IL-8 in a dose-dependent fashion. At 0.1 mg/mL, Reparixin inhibited the adhesion of IL-8-activated human neutrophils to pAECs by 84 ± 2.5%.

CONCLUSIONS

Pig IL-8 increased in an ex vivo model of pig-to-human lung xenotransplantation but is not detected in vivo, whereas human or NHP IL-8 is elevated to a similar degree in both models. Both pig and human IL-8 activate human neutrophils and increase their adhesion to pig aortic ECs, a process significantly inhibited by the addition of Reparixin to human neutrophils. This work implicates IL-8, whether of pig or human origin, as a possible factor mediating in lung xenograft inflammation and injury and supports the evaluation of therapeutic targeting of this pathway in the context of xenotransplantation.

P- and E-selectin receptor antagonism prevents human leukocyte adhesion to activated porcine endothelial monolayers and attenuates porcine endothelial damage

BACKGROUND

Alongside the need to develop more effective and less toxic immunosuppression, the shortage of human organs available for organ transplantation is one of the major hurdles facing the field. Research into xenotransplantation, as an alternative source of organs, has unveiled formidable challenges. Porcine lungs perfused with human blood rapidly sequester the majority of circulating neutrophils and platelets, which leads to inflammation and organ failure within hours, and is not significantly attenuated by genetic modifications to the pig targeted to diminish antibody binding and complement and coagulation cascade activation.

METHODS

Here, we model the interaction of freshly isolated human leukocytes with xenotransplanted vasculature under physiologic flow conditions using microfluidic channels coated with porcine endothelial cells. Both isolated human neutrophils and whole human blood were perfused over transgenic pig aortic endothelial cells that had been activated with rhTNF-α or rhIL-4 using the BioFlux system. Novel compounds GMI-1271 and rPSGL1.Fc were tested as E- and P- selectin antagonists, respectively. Cellular adhesion and rolling events were tracked using FIJI (imageJ).

RESULTS

Porcine endothelium activated with either rhTNF-α or rhIL-4 expressed high amounts of selectins, to which isolated human neutrophils readily rolled and tethered. Both E-and P-selectin antagonism significantly reduced the number of neutrophils rolling and rolling distance in a dose-dependent manner, with near total inhibition at higher doses (P < .001). Similarly, with whole human blood, selectin blocking compounds exhibited dose-dependent inhibition of prevalent leukocyte adhesion and severe endothelial injury (Untreated: 394 ± 97 PMNs/hpf, 57 ± 6% loss EC; GMI1271+rPSGL1.Fc: 23 ± 9 PMNs/hpf, 8 ± 6% loss EC P < .01).

CONCLUSIONS

Selectin blockade may be useful as part of an integrated strategy to prevent neutrophil-mediated organ xenograft injury, especially during the early time points following reperfusion.

Lung xenotransplantation: a review

PURPOSE OF REVIEW

This article reviews recent progress in the field of lung xenotransplantation, including mechanisms of xenograft injury, and the influence of mechanism-directed genetic modifications and other interventions that may soon enable therapeutic use of pig lungs in humans.

RECENT FINDINGS

An extensive series of lung xenotransplantation experiments demonstrates that multiple genetic modifications targeting known xenogeneic lung injury mechanisms are associated with incremental improvements in lung survival or function. Addition of human complement (hCD46, hCD55), coagulation (hEPCR, hTBM, hTFPI, hCD39), or anti-inflammatory pathway regulatory genes (HO-1, HLA-E), and GalT and Neu5Gc gene knockout has each demonstrated protective effects on lung survival or function. In addition, drug treatments targeting key inflammatory and clotting pathways have been shown to attenuate residual mechanisms of lung injury. Work with other pig organs in primate models show that regimens based on costimulatory pathway blocking antibodies prolong xenograft function for months to years, suggesting that once initial lung inflammation mechanisms are fully controlled, clinically useful application of pig lung xenografts may soon be feasible.

SUMMARY

Genetic modification of pigs coupled with drugs targeting complement activation, coagulation, and inflammation have significantly increased duration of pig lung function in ex-vivo human blood perfusion models, and life-supporting lung xenograft survival in vivo.

Synthetic liver function is detectable in transgenic porcine livers perfused with human blood

In addition to immune barriers, molecular incompatibilities between species are predicted to limit pig liver survival in primate xenotransplantation models. Assessment and measurement of synthetic function of genetically modified porcine livers after ex vivo perfusion with human blood have not previously been described. Eight porcine livers from α1,3-galactosyl transferase knockout and human membrane cofactor (GalTKO.hCD46), six livers from GalTKO.hCD46 and N-glycolylneuraminic acid knockout (GalTKO.hCD46.Neu5GcKO), and six livers from GalTKO.hCD46 with humanized decay-accelerating factor (hCD55), endothelial protein C receptor (hEPCR), tissue factor pathway inhibitor (hTFPI), and integrin-associated protein (hCD47) (GalTKO.hCD46.hCD55.hEPCR.hTFPI.hCD47) pigs were perfused with human blood under physiologic conditions. Timed blood samples were tested for liver enzymes and for pig-specific albumin production via Western blot. Porcine albumin levels increased with time in all experiments. By densitometry, GalTKO.hCD46.Neu5GcKO livers had the highest albumin levels, measured both as total produced, and when controlled for perfusion duration, compared to GalTKO.hCD46 (P = .068) and GalTKO.hCD46.hCD55.hEPCR.hTFPI.hCD47 livers (P = .04). Porcine livers perfused with human blood demonstrated the synthetic ability to produce albumin in all cases. GalTKO.hCD46.Neu5GcKO pig livers demonstrated the most robust albumin production. This suggests that the Neu5GcKO phenotype provides a protective effect on the graft due to decreased human antibody recognition and graft injury.

The role of sialic acids in the immune recognition of xenografts

Presentation of sialic acid (Sia) varies among different tissues and organs within each species, and between species. This diversity has biologically important consequences regarding the recognition of cells by "xeno" antibodies (Neu5Gc vs Neu5Ac). Sia also plays a central role in inflammation by influencing binding of the asialoglycoprotein receptor 1 (ASGR-1), Siglec-1 (Sialoadhesin), and cellular interactions mediated by the selectin, integrin, and galectin receptor families. This review will focus on what is known about basic Sia structure and function in association with xenotransplantation, how changes in sialylation may occur in this context (through desialylation or changes in sialyltransferases), and how this fundamental pathway modulates adhesive and cell activation pathways that appear to be particularly crucial to homeostasis and inflammation for xenografts.

N-glycolylneuraminic acid knockout reduces erythrocyte sequestration and thromboxane elaboration in an ex vivo pig-to-human xenoperfusion model

BACKGROUND

Wild-type pigs express several carbohydrate moieties on their cell surfaces that differ from those expressed by humans. This difference in profile leads to pig tissue cell recognition of human blood cells causing sequestration, in addition to antibody-mediated xenograft injury. One such carbohydrate is N-glycolylneuraminic acid (Neu5Gc), a sialic acid molecule synthesized in pigs but not in humans. Here, we evaluate livers with and without Neu5Gc in an ex vivo liver xeno perfusion model.

METHODS

Livers from pigs with an α1,3-galactosyl transferase gene knockout (GalTKO) and transgenic for human membrane cofactor (hCD46) with (n = 5) or without (n = 7) an additional Neu5Gc gene knock out (Neu5GcKO) were perfused ex vivo with heparinized whole human blood. A drug regimen consisting of a histamine inhibitor, thromboxane synthase inhibitor, and a murine anti-human GPIb-blocking antibody fragment was given to half of the experiments in each group.

RESULTS

Liver function tests (AST and ALT) were not significantly different between livers with and without the Neu5GcKO. GalTKO.hCD46.Neu5GcKO livers had less erythrocyte sequestration as evidenced by a higher mean hematocrit over time compared to GalTKO.hCD46 livers (P = .0003). The addition of Neu5GcKO did not ameliorate profound thrombocytopenia seen within the first 15 minutes of perfusion. TXB2 was significantly less with the added drug regimen (P = .006) or the presence of Neu5GcKO (P = .017).

CONCLUSIONS

The lack of Neu5Gc expression attenuated erythrocyte loss but did not prevent profound early onset thrombocytopenia or platelet activation, although TXB2 levels were decreased in the presence of Neu5GcKO.