Complement activation and coagulation in 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.

Current status of xenotransplantation research and the strategies for preventing xenograft rejection

Transplantation is often the last resort for end-stage organ failures, e.g., kidney, liver, heart, lung, and pancreas. The shortage of donor organs is the main limiting factor for successful transplantation in humans. Except living donations, other alternatives are needed, e.g., xenotransplantation of pig organs. However, immune rejection remains the major challenge to overcome in xenotransplantation. There are three different xenogeneic types of rejections, based on the responses and mechanisms involved. It includes hyperacute rejection (HAR), delayed xenograft rejection (DXR) and chronic rejection. DXR, sometimes involves acute humoral xenograft rejection (AHR) and cellular xenograft rejection (CXR), which cannot be strictly distinguished from each other in pathological process. In this review, we comprehensively discussed the mechanism of these immunological rejections and summarized the strategies for preventing them, such as generation of gene knock out donors by different genome editing tools and the use of immunosuppressive regimens. We also addressed organ-specific barriers and challenges needed to pave the way for clinical xenotransplantation. Taken together, this information will benefit the current immunological research in the field of xenotransplantation.

Bioinformatic analysis as a first step to predict the compatibility of hematopoiesis and immune system genes between humans and pigs

The shortage of allogeneic donor organs leaves its supply far short of clinical need. There are great expectations on xenotransplantation, especially with pigs' organs. With the genetic modification of donor pigs, the rejection and cross-species transmission issues have now been widely addressed. However, research on the compatibility of genes between humans and pigs was limited. We performed a systematic screening analysis of predicted incompatible genes between humans and pigs, judged by low protein sequence similarities or different predicted protein domain compositions. By combining with gene set enrichment analysis, we screened out several key genes of hematopoiesis and the immune system with possible incompatibilities, which might be important for establishing chimera and xenotransplantation between humans and pigs. There were seven chemokine genes, including CCL1, CCL5, CCL24, CCL25, CCL28, CXCL12, and CXCL16, that exhibited limited similarity between humans and pigs (similarity < 0.8). Among hematopoiesis process-related genes, 15 genes of adhesion molecules, Notch ligands, and cytokine receptors exhibited differences between humans and pigs. In complement and coagulation cascades, 19 genes showed low similarity and 77 genes had different domain compositions between humans and pigs. Our study provides a good reference for further genetic modification of pigs, which might be beneficial for xenotransplantation.

CRISPR/Cas Technology in Pig-to-Human Xenotransplantation Research

CRISPR/Cas (clustered regularly interspaced short palindromic repeats linked to Cas nuclease) technology has revolutionized many aspects of genetic engineering research. Thanks to it, it became possible to study the functions and mechanisms of biology with greater precision, as well as to obtain genetically modified organisms, both prokaryotic and eukaryotic. The changes introduced by the CRISPR/Cas system are based on the repair paths of the single or double strand DNA breaks that cause insertions, deletions, or precise integrations of donor DNA. These changes are crucial for many fields of science, one of which is the use of animals (pigs) as a reservoir of tissues and organs for xenotransplantation into humans. Non-genetically modified animals cannot be used to save human life and health due to acute immunological reactions resulting from the phylogenetic distance of these two species. This review is intended to collect and summarize the advantages as well as achievements of the CRISPR/Cas system in pig-to-human xenotransplantation research. In addition, it demonstrates barriers and limitations that require careful evaluation before attempting to experiment with this technology.

Xenotransplantation: Current Status in Preclinical Research

The increasing life expectancy of humans has led to a growing numbers of patients with chronic diseases and end-stage organ failure. Transplantation is an effective approach for the treatment of end-stage organ failure; however, the imbalance between organ supply and the demand for human organs is a bottleneck for clinical transplantation. Therefore, xenotransplantation might be a promising alternative approach to bridge the gap between the supply and demand of organs, tissues, and cells; however, immunological barriers are limiting factors in clinical xenotransplantation. Thanks to advances in gene-editing tools and immunosuppressive therapy as well as the prolonged xenograft survival time in pig-to-non-human primate models, clinical xenotransplantation has become more viable. In this review, we focus on the evolution and current status of xenotransplantation research, including our current understanding of the immunological mechanisms involved in xenograft rejection, genetically modified pigs used for xenotransplantation, and progress that has been made in developing pig-to-pig-to-non-human primate models. Three main types of rejection can occur after xenotransplantation, which we discuss in detail: (1) hyperacute xenograft rejection, (2) acute humoral xenograft rejection, and (3) acute cellular rejection. Furthermore, in studies on immunological rejection, genetically modified pigs have been generated to bridge cross-species molecular incompatibilities; in the last decade, most advances made in the field of xenotransplantation have resulted from the production of genetically engineered pigs; accordingly, we summarize the genetically modified pigs that are currently available for xenotransplantation. Next, we summarize the longest survival time of solid organs in preclinical models in recent years, including heart, liver, kidney, and lung xenotransplantation. Overall, we conclude that recent achievements and the accumulation of experience in xenotransplantation mean that the first-in-human clinical trial could be possible in the near future. Furthermore, we hope that xenotransplantation and various approaches will be able to collectively solve the problem of human organ shortage.

Clustered Regularly Interspaced Short Palindromic Repeats/Cas9 Gene Editing Technique in Xenotransplantation

Genetically modified pigs have been considered favorable resources in xenotransplantation. Microinjection of randomly integrating transgenes into zygotes, somatic cell nuclear transfer, homologous recombination, zinc finger nucleases, transcription activator-like effector nucleases, and most recently, clustered regularly interspaced short palindromic repeats-cas9 (CRISPR/Cas9) are the techniques that have been used to generate these animals. Here, we provide an overview of the CRISPR approaches that have been used to modify genes which are vital in improving xenograft survival rate, including cytidine monophosphate-N-acetylneuraminic acid hydroxylase, B1,4N-acetylgalactosaminyltransferase, isoglobotrihexosylceramide synthase, class I MHC, von Willebrand factor, C3, and porcine endogenous retroviruses. In addition, we will mention the importance of potential candidate genes which could be targeted using CRISPR/Cas9.

Assessment of the Anticoagulant and Anti-inflammatory Properties of Endothelial Cells Using 3D Cell Culture and Non-anticoagulated Whole Blood

In vivo, endothelial cells are crucial for the natural anticoagulation of circulating blood. Consequently, endothelial cell activation leads to blood coagulation. This phenomenon is observed in many clinical situations, like organ transplantation in the presence of pre-formed anti-donor antibodies, including xenotransplantation, as well as in ischemia/reperfusion injury. In order to reduce animal experimentation according to the 3R standards (reduction, replacement and refinement), in vitro models to study the effect of endothelial cell activation on blood coagulation would be highly desirable. However, common flatbed systems of endothelial cell culture provide a surface-to-volume ratio of 1 - 5 cm² of endothelium per mL of blood, which is not sufficient for natural, endothelial-mediated anticoagulation. Culturing endothelial cells on microcarrier beads may increase the surface-to-volume ratio to 40 - 160 cm²/mL. This increased ratio is sufficient to ensure the "natural" anticoagulation of whole blood, so that the use of anticoagulants can be avoided. Here an in vitro microcarrier-based system is described to study the effects of genetic modification of porcine endothelial cells on coagulation of whole, non-anticoagulated human blood. In the described assay, primary porcine aortic endothelial cells, either wild type (WT) or transgenic for human CD46 and thrombomodulin, were grown on microcarrier beads and then exposed to freshly drawn non-anticoagulated human blood. This model allows for the measurement and quantification of cytokine release as well as activation markers of complement and coagulation in the blood plasma. In addition, imaging of activated endothelial cell and deposition of immunoglobulins, complement- and coagulation proteins on the endothelialized beads were performed by confocal microscopy. This assay can also be used to test drugs which are supposed to prevent endothelial cell activation and, thus, coagulation. On top of its potential to reduce the number of animals used for such investigations, the described assay is easy to perform and consistently reproducible.

Production and Breeding of Transgenic Cloned Pigs Expressing Human CD73

One of the reasons to causing blood coagulation in the tissue of xenografted organs was known to incompatibility of the blood coagulation and anti-coagulation regulatory system between TG pigs and primates. Thus, overexpression of human CD73 (hCD73) in the pig endothelial cells is considered as a method to reduce coagulopathy after pig-to-non-human-primate xenotransplantation. This study was performed to produce and breed transgenic pigs expressing hCD73 for the studies immune rejection responses and could provide a successful application of xenotransplantation. The transgenic cells were constructed an hCD73 expression vector under control porcine Icam2 promoter (pIcam2-hCD73) and established donor cell lines expressing hCD73. The numbers of transferred reconstructed embryos were 127 ± 18.9. The pregnancy and delivery rate of surrogates were 8/18 (44%) and 3/18 (16%). The total number of delivered cloned pigs were 10 (2 alive, 7 mummy, and 1 died after birth). Among them, three live hCD73-pigs were successfully delivered by Caesarean section, but one was dead after birth. The two hCD73 TG cloned pigs had normal reproductive ability. They mated with wild type (WT) MGH (Massachusetts General Hospital) female sows and produced totally 16 piglets. Among them, 5 piglets were identified as hCD73 TG pigs. In conclusion, we successfully generated the hCD73 transgenic cloned pigs and produced their litters by natural mating. It can be possible to use a mate for the production of multiple transgenic pigs such as α-1,3-galactosyltransferase knock-out /hCD46 for xenotransplantation.

Altered expression of eNOS, prostacyclin synthase, prostaglandin G/H synthase, and thromboxane synthase in porcine aortic endothelial cells after exposure to human serum-relevance to xenotransplantation

Under normal conditions, the activity of platelets is stringently and precisely balanced between activation and quiescent state. This guarantees rapid hemostasis and avoids uncontrolled thrombosis. However, excessive platelet activation and resulting thrombotic microangiopathy are frequently observed in pig-to-primate xenotransplantation models. Endothelium-derived inhibitory mechanisms play an important role in regulation of platelet activation. These mainly include nitric oxide (NO), prostacyclin PGI₂ , and adenosine, which are synthesized by endothelial NO synthases (eNOS), prostacyclin synthase, and CD39/CD73, respectively. We investigated whether endothelium-derived regulatory mechanisms are affected in porcine aortic endothelial cells (PAECs) after exposure to human serum. In the present study, exposure of PAECs or porcine iliac arteries to human serum suppressed gene expression of eNOS and prostacyclin synthase, while induced gene expression of prostaglandin G/H synthase and thromboxane synthase. Simultaneously, exposure to human serum reduced NO and PGI₂ production in PAEC culture supernatants. Thus, human serum altered the balance of endothelium-derived inhibitory mechanisms in PAECs, which may indicate a regulatory mechanism of excessive platelet activation in pig-to-primate xenotransplantation.

Complement C3 inhibitor Cp40 attenuates xenoreactions in pig hearts perfused with human blood

BACKGROUND

The complement system plays a crucial role in acute xenogeneic reactions after cardiac transplantation. We used an ex vivo perfusion model to investigate the effect of Cp40, a compstatin analog and potent inhibitor of complement at the level of C3.

METHODS

Fifteen wild-type pig hearts were explanted, cardiopleged, and reperfused ex vivo after 150 minutes of cold ischemia. Hearts were challenged in a biventricular working heart mode to evaluate cardiac perfusion and function. In the treatment group (n=5), the complement cascade was blocked at the level of C3 using Cp40, using diluted human blood. Untreated human and porcine blood was used for controls.

RESULTS

Throughout the perfusion, C3 activation was inhibited when Cp40 was used (mean of all time points: 1.11 ± 0.34% vs 3.12 ± 0.48% control activation; P<.01). Compared to xenoperfused controls, the cardiac index improved significantly in the treated group (6.5 ± 4.2 vs 3.5 ± 4.8 mL/min/g; P=.03, 180 minutes perfusion), while the concentration of lactate dehydrogenase as a maker for cell degradation was reduced in the perfusate (583 ± 187 U/mL vs 2108 ± 1145 U/mL, P=.02). Histological examination revealed less hemorrhage and edema, and immunohistochemistry confirmed less complement fragment deposition than in untreated xenoperfused controls.

CONCLUSIONS

Cp40 efficiently prevents C3 activation of the complement system, resulting in reduced cell damage and preserved function in wild-type porcine hearts xenoperfused ex vivo. We suggest that this compstatin analog, which blocks all main pathways of complement activation, could be a beneficial perioperative treatment in preclinical and in future clinical xenotransplantation.

In vitro exposure of pig neonatal isletlike cell clusters to human blood

BACKGROUND

Pig islet grafts have been successful in treating diabetes in animal models. One remaining question is whether neonatal pig isletlike cell clusters (NICC) are resistant to the early loss of islets from the instant blood-mediated inflammatory reaction (IBMIR).

METHODS

Neonatal isletlike cell clusters were harvested from three groups of piglets-(i) wild-type (genetically unmodified), (ii) α1,3-galactosyltransferase gene-knockout (GTKO)/CD46, and (iii) GTKO/CD46/CD39. NICC samples were mixed with human blood in vitro, and the following measurements were made-antibody binding; complement activation; speed of islet-induced coagulation; C-peptide; glutamic acid decarboxylase (GAD65) release; viability.

RESULTS

Time to coagulation and viability were both reduced in all groups compared to freshly drawn non-anticoagulated human blood and autologous combinations, respectively. Antibody binding to the NICC occurred in all groups.

CONCLUSIONS

Neonatal isletlike cell clusters were subject to humoral injury with no difference associated to their genetic characteristics.

Complement inhibition in a xenogeneic model of interactions between human whole blood and porcine endothelium

Xenotransplantation (xeno-Tx) is considered as an alternative solution to overcome the shortage of human donor organs. However, the success of xeno-Tx is hindered by immune reactions against xenogeneic cells (e. g. of porcine origin). More specifically, activation of innate immune mechanisms such as complement and triggering of the coagulation cascade occur shortly after xeno-Tx, and adhesion of human leukocytes to porcine endothelium is another early critical step mediating the immune attack. To investigate the therapeutic potential of complement inhibition in the context of xenogeneic interactions, we have employed a whole-blood model in the present study. Incubation of human blood with porcine endothelial cells (PAECs) led to activation of complement and coagulation as well as to increased leukocyte adhesion. The observed responses can be attributed to the pig-to-human xenogeneicity, since the presence of human endothelium induced a minor cellular and plasmatic inflammatory response. Importantly, complement inhibition using a potent complement C3 inhibitor, compstatin analogue Cp40, abrogated the adhesion of leukocytes and, more specifically, the attachment of neutrophils to porcine endothelium. Moreover, Cp40 inhibited the activation of PAECs and leukocytes, since the levels of the adhesion molecules E-selectin, ICAM-1, ICAM-2, and VCAM-1 on PAECs and the surface expression of integrin CD11b on neutrophils were significantly decreased. Along the same line, inhibition of CD11b resulted in decreased leukocyte adhesion. Taken together, our findings provide a better understanding of the mechanisms regulating the acute innate immune complications in the context of xeno-Tx and could pave the way for complement-targeting therapeutic interventions.

ABO incompatible renal transplantation without antibody removal using conventional immunosuppression alone

ABO incompatible living donor renal transplantation (ABOi) can achieve outcomes comparable to ABO compatible transplantation (ABOc). However, with the exception of blood group A2 kidneys transplanted into recipients with low titer anti-A antibody, regimens generally include antibody removal, intensified immunosuppression and splenectomy or rituximab. We now report a series of 20 successful renal transplants across a range of blood group incompatibilities using conventional immunosuppression alone in recipients with low baseline anti-blood group antibody (ABGAb) titers. Incompatibilities were A1 to O (3), A1 to B (2), A2 to O (2), AB to A (2), AB to B (1), B to A1 (9), B to O (1); titers 1:1 to 1:16 by Ortho. At 36 months, patient and graft survival are 100%. Antibody-mediated rejection (AbMR) occurred in one patient with thrombophilia and low level donor-specific anti-HLA antibody. Four patients experienced cellular rejection (two subclinical), which responded to oral prednisolone. This series demonstrates that selected patients with low titer ABGAb can undergo ABOi with standard immunosuppression alone, suggesting baseline titer as a reliable predictor of AbMR. This reduces morbidity and cost of ABOi for patients with low titer ABGAb and increases the possibility of ABOi from deceased donors.

Systemic inflammation in xenograft recipients precedes activation of coagulation

BACKGROUND

Dysregulation of coagulation is considered a major barrier against successful pig organ xenotransplantation in non-human primates. Inflammation is known to promote activation of coagulation. The role of pro-inflammatory factors as well as the relationship between inflammation and activation of coagulation in xenograft recipients is poorly understood.

METHODS

Baboons received kidney (n=3), heart (n=4), or artery patch (n=8) xenografts from α1,3-galactosyltransferase gene-knockout (GTKO) pigs or GTKO pigs additionally transgenic for human complement-regulatory protein CD46 (GTKO/CD46). Immunosuppression (IS) was based on either CTLA4Ig or anti-CD154 costimulation blockade. Three artery patch recipients did not receive IS. Pro-inflammatory cytokines, chemokines, and coagulation parameters were evaluated in the circulation after transplantation. In artery patch recipients, monocytes and dendritic cells (DC) were monitored in peripheral blood. Expression of tissue factor (TF) and CD40 on monocytes and DC were assessed by flow cytometry. C-reactive protein (C-RP) levels in the blood and C-RP deposition in xenografts as well as native organs were evaluated. Baboon and pig C-RP mRNA in heart and kidney xenografts were evaluated.

RESULTS

In heart and kidney xenograft recipients, the levels of INFγ, TNF-α, IL-12, and IL-8 were not significantly higher after transplantation. However, MCP-1 and IL-6 levels were significantly higher after transplantation, particularly in kidney recipients. Elevated C-RP levels preceded activation of coagulation in heart and kidney recipients, where high levels of C-RP were maintained until the time of euthanasia in both heart and kidney recipients. In artery patch recipients, INFγ, TNF-α, IL-12, IL-8, and MCP-1 were elevated with no IS, while IL-6 was not. With IS, INFγ, TNF-α, IL-12, IL-8, and MCP-1 were reduced, but IL-6 was elevated. Elevated IL-6 levels were observed as early as 2 weeks in artery patch recipients. While IS was associated with reduced thrombin activation, fibrinogen and C-RP levels were increased when IS was given. There was a significant positive correlation between C-RP, IL-6, and fibrinogen levels. Additionally, absolute numbers of monocytes were significantly increased when IS was given, but not without IS. This was associated with increased CD40 and TF expression on CD14+ monocytes and lineage(neg) CD11c+ DC, with increased differentiation of the pro-inflammatory CD14+ CD11c+ monocyte population. At the time of euthanasia, C-RP deposition in kidney and heart xenografts, C-RP positive cells in artery patch xenograft and native lungs were detected. Finally, high levels of both pig and baboon C-RP mRNA were detected in heart and kidney xenografts.

CONCLUSIONS

Inflammatory responses precede activation of coagulation after organ xenotransplantation. Early upregulation of C-RP and IL-6 levels may amplify activation of coagulation through upregulation of TF on innate immune cells. Prevention of systemic inflammation in xenograft recipients (SIXR) may be required to prevent dysregulation of coagulation and avoid excessive IS after xenotransplantation.

Kidney xenotransplantation

Xenotransplantation using pigs as donors offers the possibility of eliminating the chronic shortage of donor kidneys, but there are several obstacles to be overcome before this goal can be achieved. Preclinical studies have shown that, while porcine renal xenografts are broadly compatible physiologically, they provoke a complex rejection process involving preformed and elicited antibodies, heightened innate immune cell reactivity, dysregulated coagulation, and a strong T cell-mediated adaptive response. Furthermore, the susceptibility of the xenograft to proinflammatory and procoagulant stimuli is probably increased by cross-species molecular defects in regulatory pathways. To balance these disadvantages, xenotransplantation has at its disposal a unique tool to address particular rejection mechanisms and incompatibilities: genetic modification of the donor. This review focuses on the pathophysiology of porcine renal xenograft rejection, and on the significant genetic, pharmacological, and technical progress that has been made to prolong xenograft survival.

Transplantation of xenogeneic islets: are we there yet?

Beta cell replacement therapy has been proposed as a novel therapy for the treatment of type 1 diabetes. The proof of concept has been demonstrated with successful islet allotransplantation. Islet xenotransplantation has been proposed as an alternative, more reliable, and infinite source of beta cells. The advantages of islet xenotransplantation are the ability to transplant a well differentiated cell that is responsive to glucose and the potential for genetic modification which focuses the treatment on the donor rather than the recipient. The major hurdle remains overcoming the severe cellular rejection that affects xenografts. This review will focus on the major advances that have occurred with genetic modification and the successful therapeutic strategies that have been demonstrated in nonhuman primates. Novel approaches to overcome cell-mediated rejection including biological agents that target selectively costimulation molecules, the development of local immunosuppression through genetic manipulation, and encapsulation will be discussed. Overall, there has been considerable progress in all these areas, which eventually should lead to clinical trials.

Sustained function of genetically modified porcine lungs in an ex vivo model of pulmonary xenotransplantation

BACKGROUND

Xenotransplantation could provide a solution to the donor shortage that is currently the major barrier to solid-organ transplantation. The ability to breed pigs with multiple genetic modifications provides a unique opportunity to explore the immunologic challenges of pulmonary xenotransplantation.

METHODS

Explanted lungs from wild-type and 3 groups of genetically modified pigs were studied: (i) α1,3-galactosyltransferase gene knockout (GTKO); (ii) GTKO pigs expressing the human complementary regulatory proteins CD55 and CD59 (GTKO/CD55-59); and (iii) GTKO pigs expressing both CD55-59 and CD39 (GTKO/CD55-59/CD39). The physiologic, immunologic and histologic properties of porcine lungs were evaluated on an ex vivo rig after perfusion with human blood.

RESULTS

Lungs from genetically modified pigs demonstrated stable pulmonary vascular resistance and better oxygenation of the perfusate, and survived longer than wild-type lungs. Physiologic function was inversely correlated with the degree of platelet sequestration into the xenograft. Despite superior physiologic profiles, lungs from genetically modified pigs still showed evidence of intravascular thrombosis and coagulopathy after perfusion with human blood.

CONCLUSIONS

The ability to breed pigs with multiple genetic modifications, and to evaluate lung physiology and histology in real-time on an ex vivo rig, represent significant advances toward better understanding the challenges inherent to pulmonary xenotransplantation.

Protective effects of transgenic human endothelial protein C receptor expression in murine models of transplantation

Thrombosis and inflammation are major obstacles to successful pig-to-human solid organ xenotransplantation. A potential solution is genetic modification of the donor pig to overexpress molecules such as the endothelial protein C receptor (EPCR), which has anticoagulant, anti-inflammatory and cytoprotective signaling properties. Transgenic mice expressing human EPCR (hEPCR) were generated and characterized to test this approach. hEPCR was expressed widely and its compatibility with the mouse protein C pathway was evident from the anticoagulant phenotype of the transgenic mice, which exhibited a prolonged tail bleeding time and resistance to collagen-induced thrombosis. hEPCR mice were protected in a model of warm renal ischemia reperfusion injury compared to wild type (WT) littermates (mean serum creatinine 39.0 ± 2.3 μmol/L vs. 78.5 ± 10.0 μmol/L, p < 0.05; mean injury score 31 ± 7% vs. 56 ± 5%, p < 0.05). Heterotopic cardiac xenografts from hEPCR mice showed a small but significant prolongation of survival in C6-deficient PVG rat recipients compared to WT grafts (median graft survival 6 vs. 5 days, p < 0.05), with less hemorrhage and edema in rejected transgenic grafts. These data indicate that it is possible to overexpress EPCR at a sufficient level to provide protection against transplant-related thrombotic and inflammatory injury, without detrimental effects in the donor animal.

Modifiers of complement activation for prevention of antibody-mediated injury to allografts

PURPOSE OF REVIEW

Improvements in prevention and management of cellular rejection of solid organ transplants, coupled with increasing numbers of sensitized patients, have focused attention on antibody-mediated rejection (AbMR). Complement is a critical component of AbMR, in addition to interfacing between innate and adaptive immunity and the coagulation cascade. This article reviews complement biology and strategies to overcome complement in AbMR, cognisant that antibody can act independently of complement.

RECENT FINDINGS

The past decade has witnessed an improvement in the prevention and treatment of AbMR as a result of solid-phase assays to determine antibody specificity, definition of histopathological criteria, and use of plasmapheresis and/or intravenous immunoglobulin (IVIG). Nonetheless, AbMR continues to impact adversely on short- and long-term graft survival. Use of B and/or T-lymphocyte-depleting therapies has not shown measurable benefit, and the need remains for therapies that deplete antibody, or provide better protection from complement-mediated damage. Disordered complement activity in human diseases such as paroxysmal nocturnal haemoglobinuria, has provided additional impetus to pursuing therapeutic complement inhibition. Preliminary data from C5 inhibition with eculizumab in the treatment and prevention of AbMR have shown promise. Trials with recombinant human inhibitors of C1 (effective in angioedema) to prevent or treat AbMR are beginning.

SUMMARY

Despite current limitations, 'protection' of the transplant through plasmapheresis and/or IVIG enables many allografts to survive in sensitized recipients. Elucidating the pathways mediating graft acceptance, by constitutive antibody deletion, or 'accommodation' (wherein donor organ remains uninjured despite antibody binding), or other local protective mechanism(s), is an equally important challenge in the quest to overcome AbMR.

Absence of Gal epitope prolongs survival of swine lungs in an ex vivo model of hyperacute rejection

BACKGROUND

Galactosyl transferase gene knock-out (GalTKO) swine offer a unique tool to evaluate the role of the Gal antigen in xenogenic lung hyperacute rejection.

METHODS

We perfused GalTKO miniature swine lungs with human blood. Results were compared with those from previous studies using wild-type and human decay-accelerating factor-transgenic (hDAF(+/+) ) pig lungs.

RESULTS

GalTKO lungs survived 132 ± 52 min compared to 10 ± 9 min for wild-type lungs (P = 0.001) and 45 ± 60 min for hDAF(+/+) lungs (P = 0.18). GalTKO lungs displayed stable physiologic flow and pulmonary vascular resistance (PVR) until shortly before graft demise, similar to autologous perfusion, and unlike wild-type or hDAF(+/+) lungs. Early (15 and 60 min) complement (C3a) and platelet activation and intrapulmonary platelet deposition were significantly diminished in GalTKO lungs relative to wild-type or hDAF(+/+) lungs. However, GalTKO lungs adsorbed cytotoxic anti-non-Gal antibody and elaborated high levels of thrombin; their demise was associated with increased PVR, capillary congestion, intravascular thrombi and strong CD41 deposition not seen at earlier time points.

CONCLUSIONS

In summary, GalTKO lungs are substantially protected from injury but, in addition to anti-non-Gal antibody and complement, platelet adhesion and non-physiologic intravascular coagulation contribute to Gal-independent lung injury mechanisms.

Controlling coagulation dysregulation in xenotransplantation

PURPOSE OF REVIEW

Deletion of the α1,3-galactosyltransferase (GalT) gene in pigs has removed a major xenoantigen but has not eliminated the problem of dysregulated coagulation and vascular injury. Rejecting GalT knockout organ xenografts almost invariably show evidence of thrombosis and platelet sequestration, and primate recipients frequently develop consumptive coagulopathy. This review examines recent findings that illuminate potential mechanisms of this current barrier to successful xenotransplantation.

RECENT FINDINGS

The coagulation response to xenotransplantation differs depending on the type of organ and quite likely the distinct vasculatures. Renal xenografts appear more likely to initiate consumptive coagulopathy than cardiac xenografts, possibly reflecting differential transcriptional responses. Liver xenografts induce rapid and profound thrombocytopenia resulting in recipient death within days due to bleeding; ex-vivo data suggest that liver endothelial cells and hepatocytes are responsible for platelet consumption by a coagulation-independent process.It has been proposed that expression of recipient tissue factor on platelets and monocytes is an important trigger of consumptive coagulopathy. Finally, pigs transgenic for human anticoagulants and antithrombotics are slowly but surely coming on line, but have not yet been rigorously tested to date.

SUMMARY

Successful control of coagulation dysregulation in xenotransplantation may require different combinatorial pharmacological and genetic strategies for different organs.

Therapeutic issues in the treatment of vascularized xenotransplants using gal-knockout donors in nonhuman primates

PURPOSE OF REVIEW

Solid organ xenotransplantation could be the future of transplantation, but improved outcomes are required in experimental models before clinical trials are justified. This review summarizes recent advances in solid organ xenotransplantation using organs from α1,3-galactosyltransferase gene-knockout (GTKO) pigs (with or without other genetic modifications) and novel therapeutic approaches.

RECENT FINDINGS

Work on the development of genetically engineered pigs has been considerable during the past few years, with many research institutes reporting the outcomes of research. Multiple gene modifications on a GTKO background have been reported, and the results of transplantation using organs from these pigs have been published. Progress, however, has been variable, and several obstacles, for example, coagulation dysregulation, have been identified. Heterotopic pig heart xenotransplantation has been associated with graft survival up to 8 months, but kidney graft survival has not improved significantly.

SUMMARY

The availability of GTKO pigs with additional genetic modifications aimed toward expression of multiple complement-regulatory proteins and/or human thromboregulatory genes, combined with novel immunosuppressive regimens, for example, the inclusion of B cell-depleting agents, should improve pig organ survival in the near future.

Effects of long-term administration of recombinant human protein C in xenografted primates

BACKGROUND

The role potential of recombinant human activated protein C (rhaPC), a recently developed molecule with anticoagulant and antiinflammatory properties, in prolonging survival in immunosuppressed primate recipients of porcine renal xenografts has been evaluated.

METHODS

rhaPC was administered daily for 5 days (24 μg/kg/hr; group A; n = 3) or throughout the postoperative period (8-24 μg/kg/hr; group B; n = 2; or 24-48 μg/kg/hr; group C; n = 4). Animals in group D (n = 2) received rhaPC daily (24 μg/kg/hr) combined with recombinant human antithrombin (84 U/kg every 8 hr). Two animals served as control (group E).

RESULTS

The results indicate that rhaPC is protective against fibrin deposition early after transplantation but does not prevent fibrin deposition and the occurrence of acute humoral xenograft rejection (AHXR) later on. Animals in the study survived between 8 and 55 days. At the dose used, rhaPC is able to prevent fibrin deposition in the graft in the first 2 weeks after xenotransplantation, except when it is administered in conjunction with antithrombin. However, rhaPC did not prevent the eventual occurrence of AHXR in primate recipients of porcine xenografts.

CONCLUSIONS

In this pig to primate model, rhaPC confers a short advantage in the prevention of early perioperative xenograft damage but does not represent an effective strategy for preventing AHXR.

The anti-nonGal xenoantibody response to alpha1,3-galactosyltransferase gene knockout pig xenografts

PURPOSE OF REVIEW

Anti-nonGal xenoantibodies are a major barrier to the survival of genetically modified porcine xenografts. This review summarizes the contribution of anti-nonGal xenoantibodies to the activation of porcine endothelial cells and graft rejection, and further provides an update on recent advancements in defining the unique features of anti-nonGal xenoantibody structure.

RECENT FINDINGS

Anti-nonGal xenoantibodies pre-exist at low levels in humans and nonhuman primates, and are notably absent in neonates. Exposure of nonhuman primates to alpha1,3-galactosyltransferase gene knockout endothelial cells initiates an induced xenoantibody response that is restricted and encoded by the germline immunoglobulin heavy chain gene IGHV3-21. The target xenoantigen remains undetermined, but several candidate targets have been proposed, including carbohydrate xenoantigens. New advancements in molecular modeling provide insight on the mechanism by which xenoantibodies bind to structurally related carbohydrates.

SUMMARY

Genetic manipulation of porcine donors has significantly prolonged the survival of grafts placed into nonhuman primate recipients, but anti-nonGal xenoantibodies and thrombosis limit the ability of these grafts to function on a long-term basis. Recent developments defining pre-existing anti-nonGal xenoantibody levels, the restriction in the anti-nonGal xenoantibody response and the identification of key sites defining xenoantibody-carbohydrate interactions now provide the information necessary to develop new approaches to preventing xenoantibody-mediated rejection.

Anti-inflammatory and anticoagulant effects of transgenic expression of human thrombomodulin in mice

Thrombomodulin (TBM) is an important vascular anticoagulant that has species specific effects. When expressed as a transgene in pigs, human (h)TBM might abrogate thrombotic manifestations of acute vascular rejection (AVR) that occur when GalT-KO and/or complement regulator transgenic pig organs are transplanted to primates. hTBM transgenic mice were generated and characterized to determine whether this approach might show benefit without the development of deleterious hemorrhagic phenotypes. hTBM mice are viable and are not subject to spontaneous hemorrhage, although they have a prolonged bleeding time. They are resistant to intravenous collagen-induced pulmonary thromboembolism, stasis-induced venous thrombosis and pulmonary embolism. Cardiac grafts from hTBM mice to rats treated with cyclosporine in a model of AVR have prolonged survival compared to controls. hTBM reduced the inflammatory reaction in the vein wall in the stasis-induced thrombosis and mouse-to-rat xenograft models and reduced HMGB1 levels in LPS-treated mice. These results indicate that transgenic expression of hTBM has anticoagulant and antiinflammatory effects that are graft-protective in murine models.

Structural biology of carbohydrate xenoantigens

Transplantation of organs across species (xenotransplantation) is being considered to overcome the shortage of human donor organs. However, unmodified pig organs undergo an antibody-mediated hyperacute rejection that is brought about by the presence of natural antibodies to Galalpha(1,3)Gal, which is the major carbohydrate xenoantigen. Genetic modification of pig organs to remove most of the Galalpha(1,3)Gal epitopes has been achieved, but the human immune system may still recognize residual lipid-linked Galalpha(1,3)Gal carbohydrates, new (cryptic) carbohydrates or additional non-Galalpha(1,3)Gal carbohydrate xenoantigens. The structural basis for lectin and antibody recognition of Galalpha(1,3)Gal carbohydrates is starting to be understood and is discussed in this review. Antibody binding to Galalpha(1,3)Gal carbohydrates is predicted to primarily involve end-on insertion of the terminal alphaGal residue, but it is possible that groove-type binding can occur, as for some lectins. It is likely that similar antibody and lectin recognition will occur with other non-Galalpha(1,3)Gal xenoantigens, which potentially represent new barriers for pig-to-human xenotransplantation.