Alpha-galactosyl epitope-mediated activation of porcine aortic endothelial cells: type I activation

Porcine genome engineering for xenotransplantation

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

Application of cyclophosphamide-induced tolerance in alpha1,3-galactosyltransferase knockout mice presensitized with Gal alpha 1-3Gal beta-4-GlcNAc antigens

PURPOSE

Hyperacute rejection (HAR) mediated by the natural antibody (nAb) against Gal alpha 1-3Gal beta-4-GlcNAc (alpha Gal) is the major obstacle in xenogeneic organ transplantation. Previously, we reported the acceptance of donor heart grafts in anti-alpha Gal nAb-producing galactosyltransferase knockout (GalT KO) mice after cyclophosphamide (CP)-induced tolerance conditioning. In the present study, we applied our tolerance induction conditioning in presensitized recipient mice.

METHODS

GalT KO (alpha Gal(-/-), H-2(b/d)) recipient mice were presensitized with alpha Gal(+) rabbit red blood cells (RRBCs). Presensitized or nonsensitized recipient mice were treated with CP-induced tolerance conditioning, consisting of AKR (alpha Gal(+/+), H-2(k)) spleen cells (SC), CP, busulfan (BU), and AKR bone marrow cells (BMC). We assessed the survival of donor hearts and skin grafts and analyzed the production of anti-alpha Gal Abs by flow cytometry.

RESULTS

Donor mixed chimerism was achieved in the presensitized GalT KO mice treated with CP-induced tolerance conditioning. In parallel with the disappearance of anti-alpha Gal Abs, permanent acceptance of donor heart grafts and skin grafts was observed in presensitized and GalT KO mice treated with CP-induced tolerance conditioning.

CONCLUSIONS

Both B-cell and T-cell tolerance was achieved in the presence of a higher titer of anti-alpha Gal Abs after treatment with CP-induced tolerance conditioning.

Increased immunosuppression, not anticoagulation, extends cardiac xenograft survival

BACKGROUND

Cardiac xenograft function is lost due to delayed xenograft rejection (DXR) characterized by microvascular thrombosis and myocardial necrosis. The cause of DXR is unknown but may result from thrombosis induced by antibody-mediated activation of endothelial cells and/or by incompatibilities in thromboregulatory interactions.

METHODS

To examine these issues, a series (Groups 1-6) of previous transgenic CD46 pig-to-baboon heterotopic cardiac transplants were reanalyzed for baseline immunosuppressive levels, graft survival and infectious complications with and without systemic anticoagulation. Groups 1-4 received low dose tacrolimus and sirolimus maintenance therapy, with splenectomy, anti-CD20 and daily alpha-Gal polymer. Group 1 recipients received no anticoagulation. Groups 2-4 were anticoagulated with aspirin and Plavix, Lovenox, or Coumadin, respectively. Group 5 was treated with Lovenox and high dose tacrolimus and sirolimus maintenance therapy. Group 6 recipients received no postoperative anticoagulation but the same immunosuppression as group 5.

RESULTS

Median survival (15-22 days) within groups 1-4 was not significantly different. At rejection all tissues exhibited microvascular thrombosis, coagulative necrosis and similar levels of platelet and fibrin deposition. Groups 5 and 6 median survival (76 days) was significantly increased compared to groups 1-4. There was no significant difference in median survival between Lovenox treated recipients (68 days) and anticoagulant free recipients (96 days). Rejected tissues showed vascular antibody deposition, microvascular thrombosis, and myocyte necrosis.

CONCLUSION

Significant prolongation in xenograft survival is achieved by improved immunosuppression. These results suggest that ongoing immune responses remain the major stimulus for DXR.

Pathology of renal xenograft rejection in pig to non-human primate transplantation

Xenotransplantation has the potential to alleviate the critical shortage of organs for transplantation in humans. Miniature swine are a promising donor species for xenotransplantation. However, when swine organs are transplanted into primates, hyperacute rejection (HAR), acute humoral xenograft rejection (AHXR), acute cellular xenograft rejection (ACXR), and chronic xenograft rejection prevent successful engraftment. Developing a suitable regimen for preventing xenograft rejection requires the ability to accurately diagnosis the severity and type of rejection in the graft. For this purpose, histopathology remains the most definitive and reliable tool. We discuss here the characteristic features of xenograft rejection in a preclinical pig-to-non-human primate transplantation model. In miniature swine to baboon xenotransplantation, marked interstitial hemorrhage develops in HAR, and renal microvascular injury develops with multiple platelet-fibrin microthrombi in both HAR and AHXR. T-cell-mediated cellular immunity plays an important role in ACXR. Chronic humoral and cellular rejection may induce chronic xenograft rejection, and will be a major cause of graft loss in discordant xenotransplantation.

Sequential analysis of anti-alpha Gal natural antibody-producing B cells in GalT knockout mice in cyclophosphamide-induced tolerance

Previously, we have shown that cyclophosphamide (CP)-induced tolerance, marked by permanent acceptance of donor skin graft and establishment of donor mixed chimerism, was readily induced with treatment with donor spleen cells (SC), CP, busulfan (BU) and donor bone marrow cells (BMC). Here, we investigated the mechanism of anti-donor natural antibody (nAb) producing B-cell tolerance in our CP-induced tolerance systems in alpha1,3-galactosyltransferase-deficient knockout mice (GalT KO; GalT-/-, H-2(b/d)). After induction of tolerance using donor AKR SC and BMC, survival of donor heart and skin grafts and production of anti-Galalpha1-3Galbeta1-4GlcNAc (anti-alphaGal) Ab in recipient GalT KO mice were analyzed. In addition, the production of anti-alphaGal Ab and the presence of Gal-BSA binding B cells in GalT KO mice were analyzed by flow cytometry (FCM) after treatments with rabbit red blood cells (RRBC) and CP. Permanent acceptance of donor skin and heart grafts and abrogation of anti-alphaGal Ab were achieved in GalT KO mice treated with donor SC + CP/BU + BMC. However, in the GalT KO mice treated with donor SC and CP, donor skin grafts were acutely rejected, even though anti-alphaGal Ab was undetectable. Similarly, anti-alphaGal Ab was undetectable in GalT KO mice treated with RRBC and CP. Our data strongly indicated the following mechanisms: the clonal destruction in the early stage and the clonal anergy or ignorance in the late stage after conventional conditioning with RRBC and CP. In conclusion, our drug-induced tolerance protocols are effective to induce tolerance in recipients that produce anti-donor nAb.

Endothelial cell protection in xenotransplantation: looking after a key player in rejection

The endothelium, as an organ at the interface between the intra- and extravascular space, actively participates in maintaining an anti-inflammatory and anti-coagulant environment under physiological conditions. Severe humoral as well as cellular rejection responses, which accompany cross-species transplantation of vascularized organs as well as ischemia/reperfusion injury, primarily target the endothelium and disrupt this delicate balance. Activation of pro-inflammatory and pro-coagulant pathways often lead to irreversible injury not only of the endothelial layer but also of the entire graft, with ensuing rejection. This review focuses on strategies targeted at protecting the endothelium from such damaging effects, ranging from genetic manipulation of the donor organ to soluble, as well as membrane-targeted, protective strategies.

Endothelial cell protection and complement inhibition in xenotransplantation: a novel in vitro model using whole blood

BACKGROUND

Studying the interactions between xenoreactive antibodies, complement and coagulation factors with the endothelium in hyperacute and acute vascular rejection usually necessitates the use of in vivo models. Conventional in vitro or ex vivo systems require either serum, plasma or anti-coagulated whole blood, making analysis of coagulation-mediated effects difficult. Here a novel in vitro microcarrier-based system for the study of endothelial cell (EC) activation and damage, using non-anticoagulated whole blood is described. Once established, the model was used to study the effect of the characterized complement- and coagulation inhibitor dextran sulfate (DXS, MW 5000) for its EC protective properties in a xenotransplantation setting.

METHODS

Porcine aortic endothelial cells (PAEC), grown to confluence on microcarrier beads, were incubated with non-anticoagulated whole human blood until coagulation occurred or for a maximum of 90 min. PAEC-beads were either pre- or co-incubated with DXS. Phosphate buffered saline (PBS) experiments served as controls. Fluid phase and surface activation markers for complement and coagulation were analyzed as well as binding of DXS to PAEC-beads.

RESULTS

Co- as well as pre-incubation of DXS, followed by washing of the beads, significantly prolonged time to coagulation from 39 +/- 12 min (PBS control) to 74 +/- 23 and 77 +/- 20 min, respectively (P < 0.005 vs. PBS). DXS treatment attenuated surface deposition of C1q, C4b/c, C3b/c and C5b-9 without affecting IgG or IgM deposition. Endothelial integrity, expressed by positivity for von Willebrand Factor, was maintained longer with DXS treatment. Compared with PBS controls, both pre- and co-incubation with DXS significantly prolonged activated partial thromboplastin time (>300 s, P < 0.05) and reduced production of thrombin-antithrombin complexes and fibrinopeptide A. Whilst DXS co-incubation completely blocked classical pathway complement activity (CH50 test) DXS pre-incubation or PBS control experiments showed no inhibition. DXS bound to PAEC-beads as visualized using fluorescein-labeled DXS.

CONCLUSIONS

This novel in vitro microcarrier model can be used to study EC damage and the complex interactions with whole blood as well as screen ''endothelial protective'' substances in a xenotransplantation setting. DXS provides EC protection in this in vitro setting, attenuating damage of ECs as seen in hyperacute xenograft rejection.

Warfarin or low-molecular-weight heparin therapy does not prolong pig-to-primate cardiac xenograft function

Microvascular thrombosis is a prominent feature in cardiac delayed xenograft rejection (DXR). We investigated the impact of warfarin or low-molecular-weight heparin (LMWH) anti-coagulation on xenograft function using a heterotopic pig-to-primate model. Donor hearts were from CD46 transgenic pigs and baboon immunosuppression included tacrolimus, sirolimus, anti-CD20 and TPC, an alpha-galactosyl-polyethylene glycol conjugate. Three groups of animals were studied. Group 1 (n = 9) was treated with warfarin, Group 2 (n = 13) with LMWH and Group 3, received no anti-coagulant drugs. The median duration of xenograft function was 20 days (range 3-62 days), 18 days (range 5-109 days) and 15 days (range 4-53 days) in Groups 1 to 3 respectively. Anti-coagulation achieved the targeted international normalized prothrombin ratio (INR) and anti-factor Xa levels consistent with effective in vivo therapy yet, no significant impact on median xenograft function was observed. At rejection, a similar histology of thrombosis and ischemia was apparent in each group and the levels of fibrin deposition and platelet thrombi in rejected tissue was the same. Anti-coagulation with warfarin or LMWH did not have a significant impact on the onset of DXR and microvascular thrombosis. However, a role for specific anti-coagulant strategies to achieve long-term xenograft function cannot be excluded.

Alpha Gal ligation of pig endothelial cells induces protection from complement and apoptosis independently of NF-kappa B and inflammatory changes

Cytoprotection of endothelial cells (EC) is important in EC biology and pathophysiology, including graft rejection. Using porcine aortic EC and human complement as an in vitro model of xenotransplantation, we have reported that ligation of EC Gal alpha (1-3)Gal epitopes (alpha Gal) with antibodies or lectins BS-I and IB4 induces EC resistance to injury by complement. However, before the protective response is observed, alpha Gal ligation induces an early, proinflammatory response. Using a similar model, we now investigated whether the early inflammatory response, as well as NF-kappa B activation, is required for induction of cytoprotection. Despite up-regulation of EC mRNA for many inflammatory cytokines rapidly after BS-I stimulation, recombinant cytokines or conditioned media from EC incubated with BS-I failed to induce protection when used to stimulate EC. While the lectin-induced inflammatory response was markedly reduced by inhibition of NF-kappa B, the protection from complement and apoptosis was unaffected. The lectins caused up-regulation of mRNA for protective genes A20, porcine inhibitor of apoptosis protein and hemoxygenase-1, which was not modified by NF-kappa B inhibition. These findings suggest that induction of cytoprotection in porcine EC by alpha Gal ligation results from activation of pathways that are largely independent of those that elicit NF-kappaB activation and the inflammatory response.

Fate of alphaGal +/+ pancreatic islet grafts after transplantation into alphaGal knockout mice

BACKGROUND

Important phylogenetic differences between pig and human tissues prevent xenotransplantation from becoming a clinically feasible option. Humans lack the galactose-alpha1,3-galactose (alphaGal) epitope on endothelial cell surfaces and therefore have preformed anti-alphaGal antibodies. The role of these antibodies in rejection of non-vascular xenografts remains controversial. This study investigated the role of anti-alphaGal antibodies in rejection of non-vascularized alphaGal+/+ grafts in alphaGal -/- mice.

METHODS

alphaGal +/+ and alphaGal -/- pancreatic islets were transplanted under the renal capsule of streptozotocin-induced diabetic (1) alphaGal -/- mice and (2) alphaGal +/+ mice. alphaGal -/- recepients were immunized with rabbit red blood cell membranes (RRBCs) to produce elevated anti-alphaGal antibody levels.

RESULTS

Six of the 18 alphaGal -/- mice rejected the alphaGal +/+ grafts within 68 days whereas indefinite graft survival was achieved in the control groups. Animals with surviving islet grafts were challenged with alphaGal +/+ skin grafts. Although all alphaGal +/+ skin grafts were rejected within 58 days, the islet grafts remained intact. This observation correlated with the level of alphaGal expression (which was very low on islets compared to skin) rather than the actual titre of anti-alphaGal antibody.

DISCUSSION

The results suggest that the level of alphaGal expression plays an important role in graft survival. Therefore, its removal is important in the development of a pig islet donor for future clinical therapy.

Acute vascular rejection of xenografts: roles of natural and elicited xenoreactive antibodies in activation of vascular endothelial cells and induction of procoagulant activity

BACKGROUND

Hyperacute rejection of vascularized discordant xenografts can now be effectively managed. However, acute vascular rejection (AVR) then ensues, resulting in graft destruction, coagulopathy, or both within weeks. The aim of this study was to determine associations between humoral responses to the xenograft and the induction of AVR, coagulopathy, or both.

METHODS

In vitro, heat-inactivated, naive or sensitized baboon sera containing xenoreactive natural or elicited antibodies were used to activate porcine aortic endothelial cells (PAEC) in vitro. Tissue factor expression on PAEC was determined as an index of heightened procoagulant activity. In vivo, porcine renal xenografts were transplanted into immunosuppressed baboons, and at the time of rejection or the development of a consumptive coagulopathy, biopsy specimens were obtained for studies of xenoreactive antibody binding and tissue factor expression.

RESULTS

In vitro, incubation of PAEC with naive baboon sera containing natural anti-Galalpha1,3Gal (Gal) antibodies resulted in minimal tissue factor induction; the addition of complement boosted procoagulant responses. Elicited xenoreactive antibodies, and to non-Gal epitopes alone, induced high amounts of procoagulant activity on PAEC; the addition of complement resulted in overt cytotoxicity. In vivo, AVR was associated with xenoreactive antibody deposition in the graft. When vascular endothelial binding of xenoreactive antibody was combined with the expression of tissue factor, consumptive coagulopathy developed irrespective of histopathologic features of AVR.

CONCLUSIONS

Our in vitro results indicate that elicited antibodies, potentially to non-Gal epitopes, induce endothelial cell activation and tissue factor expression; in vivo, a consumptive coagulopathy occurred when there was xenoreactive antibody deposition and increase of tissue factor.

Porcine endothelium activated by anti-α-GAL antibody binding mediates increased human neutrophil adhesion under flow

BACKGROUND

Neutrophils participate in acute vascular rejection (AVR) of organ xenografts. Induced antibodies (Abs), including anti-Galalpha1,3Gal (alpha-Gal) Abs, have been suggested to cause AVR. We investigated the adhesion of naive human neutrophils to porcine aortic endothelial cells (PAECs) stimulated with anti-alpha-Gal Abs under conditions of flow. In addition, the ability of human neutrophils to adhere to human and porcine endothelium under static and flow conditions was evaluated.

METHODS AND RESULTS

In a flow-adhesion assay, a significant increase in adhesion of human neutrophils to PAECs, but not to human aortic endothelial cells (HAECs), was detected 6 hours after anti-alpha-Gal Ab-binding. After Ab stimulation, PAECs expressed CD62E and increased levels of CD106, indicating an activated endothelial cell (EC) phenotype. In a migration assay, supernatants from Ab-stimulated PAECs induced migration of human neutrophils, which was partially blocked by anti-porcine (p) interleukin (IL)-8 Abs and an antagonist to platelet-activating factor (PAF). In static and flow-adhesion assays, no difference in adhesion of human neutrophils to unstimulated or tumor necrosis factor (TNF)-alpha-stimulated HAECs and PAECs could be detected.

CONCLUSIONS

Our data suggest that anti-alpha-Gal Abs play an important role in the initiation of AVR by mediating adhesion and recruitment of neutrophils within an organ xenograft. In contrast with previous investigations, our data argues against a differential recognition of PAECs and HAECs by human neutrophils. Thus, to prevent AVR and accomplish long-term xenograft survival, it will be important to remove anti-alpha-Gal Abs before and after pig-to-human transplantation.

Characteristics of CD59 up-regulation induced in porcine endothelial cells by alphaGal ligation and its association with protection from complement

BACKGROUND

Activation of endothelial cells may result in proinflammatory and procoagulant changes, or in changes that protect the endothelial cells (EC) from injurious insults. Stimulation of porcine EC with human anti-porcine antibodies, or lectins from Bandeiraea simplicifolia that bind terminal Galalpha(1-3)Gal (abbreviated alphaGal), can induce EC protection from cytotoxicity by human complement. These EC also exhibit up-regulation of CD59 protein and mRNA expression. Porcine CD59 has been reported to protect porcine cells from human complement. Therefore we investigated the specificity requirements and other characteristics of the induced CD59 up-regulation, as well as the role of up-regulated CD59 in lectin-induced protection of EC from human complement.

METHODS

Aortic EC were incubated in vitro with alphaGal-binding lectins B. simplicifolia lectin I isolectin B4 (IB4) and B. simplicifolia lectin I (BS-I) and CD59 expression was assessed by flow cytometry and enzyme linked immunosorbent assay (ELISA). Binding requirement was studied using disaccharides containing either alphagalactosyl or betagalactosyl moieties to inhibit CD59 up-regulation. Protection from complement killing was assessed after incubation of EC with human serum as a source of anti-porcine antibodies and complement. The role of CD59 in lectin-induced protection was studied in the presence of an anti-pig CD59 antibody and after removal of CD59 using phosphatidylinositol (PI)-specific phospholipase C (PI-PLC).

RESULTS

We found that induction of CD59 up-regulation required specific binding of the lectin to terminal alphaGal and was not induced either by soluble factors that may be released from EC by stimulation with the lectin or by TNF-alpha, IFN-gamma, or IL-1alpha. Unstimulated or BS-I-treated EC showed little or no expression of decay accelerator factor (DAF). Removal of membrane-associated CD59 (and other proteins that are associated with the membrane through PI linkage) with PI-PLC from EC that had been exposed to lectin restored their complement sensitivity to various degrees, depending on the extent of lectin-induced protection. Cytotoxicity was completely restored in cells that exhibited partial protection induced with lectin at low doses or for a short period of time. However, EC that were fully resistant to complement did not regain sensitivity to complement after removal of CD59. Changes in CD59 expression did not modify the degree of C9 binding.

CONCLUSIONS

Induction of CD59 expression required specific binding of the lectin to terminal alphaGal and was not induced by soluble factors that may be released from EC by lectin stimulation. Increased CD59 expression may contribute to this form of protection from complement; however, mechanisms other than CD59 up-regulation appear to be essential for the development of full protection.

Multivalent Galalpha1,3Gal-substitution makes recombinant mucin-immunoglobulins efficient absorbers of anti-pig antibodies

Hyperacute organ xenograft rejection can be prevented by removing anti-pig antibodies by extracorporeal absorption prior to transplantation. A novel recombinant absorber of anti-pig antibodies was developed by fusing the cDNA encoding the extracellular part of a mucin-type protein, P-selectin glycoprotein ligand-1, with an antibody Fc fragment cDNA, which upon coexpression with the porcine alpha1,3 galactosyltransferase carried the xenogeneic epitope, Galalpha1,3Gal (Liu J., Qian Y., Holgersson J., Transplantation 1997, 63, 1673-1682). The biochemical characterization of the mucin/Ig and its absorption efficacy compared with that of porcine thyroglobulin and Galalpha1,3Gal-conjugated beads are reported. The carbohydrate portion of the mucin/Ig constituted 43% of its molecular weight and the majority of the Galalpha1,3Gal epitopes were O-linked as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting following N-glycosidase F digestion. Gas chromatography-mass spectrometry of reduced and acetylated saccharides released by alpha-galactosidase treatment revealed that the fusion protein carried approximately 140 mol of terminal, alpha-linked galactose per mole protein. Based on the reduction in pig aortic endothelial cell cytotoxicity, Galalpha1,3Gal-substituted mucin/Igs on agarose beads were, on a carbohydrate molar basis, shown to be approximately 20 times more efficient than agarose-conjugated pig thyroglobulin, and approximately 5000 and 30,000 times more efficient than Galalpha1,3Gal-substituted agarose and macroporous glass beads, respectively. Structural features of the mucin backbone and its carbohydrate core saccharide chains determine the structural context, spatial orientation and spacing of Galalpha1,3Gal epitopes and are likely to explain the superior absorption efficacy of the recombinant mucin-type chimera.

The in vitro and in vivo effects of anti-galactose antibodies on endothelial cell activation and xenograft rejection

We have previously produced a series of antigalactose (anti-Gal) hybridomas and characterized their heavy chain gene usage. Here we have quantified the affinity of these Abs for the alpha-Gal epitope and characterized their in vitro effects on endothelial cell activation and apoptosis. We report that anti-Gal mAbs derived from Gal(-/-) mice show a range of affinity for the alpha-Gal epitope, and that affinity was generally increased as the V(H) gene usage transitioned from germline sequences to sequences exhibiting somatic maturation. Despite an 85-fold range in affinity, all the anti-Gal mAbs examined induced alpha-Gal-specific endothelial cell activation, and after prolonged exposure induced endothelial cell apoptosis in a complement-independent manner. Only murine anti-Gal mAbs of the IgM or IgG3 subclass, but not IgG1, were effective at initiating complement-dependent cell lysis. Using a novel rat to mouse xenograft model, we examined the in vivo ability of these mAbs to induce xenograft rejection and characterized the rejection using histology and immunohistochemistry. Infusion of complement-fixing IgG3 mAbs resulted in either hyperacute rejection or acute vascular rejection of the xenograft. Surprisingly, infusion of an equal amount of a high affinity anti-Gal IgG1 mAb, that fixed complement poorly also induced a rapid xenograft rejection, which we have labeled very acute rejection. These studies emphasize the importance of in vivo assays, in addition to in vitro assays, in understanding the role of anti-Gal IgG-mediated tissue injury and xenograft rejection.

Are anti-endothelial cell antibodies a pre-requisite for the acute vascular rejection of xenografts?

BACKGROUND

Vascular rejection occurring within the first few weeks after transplantation is still the major immunological barrier to the long term survival of xenografts. Currently there is no consensus about what to call this type of rejection (acute vascular rejection, delayed xenograft rejection or acute humoral xenograft rejection), nor about how to prevent or treat it.

METHODS

A review of published evidence to define the heterogeneity of this phase of rejection and examine the role of antibodies, complement and graft-infiltrating inflammatory cells.

RESULTS

i) antibodies are always involved in acute vascular rejection; ii) this antibody-mediated rejection may be complement-dependent or -independent; iii) inflammatory cells may mediate an antibody- and complement-independent phase of rejection in some small animal models (which, in its pure form cannot be called 'vascular rejection') iv) there remain significant questions about the relevance of 'accommodation' and the importance of coagulation abnormalities.

CONCLUSIONS

Without doubt, future research would be helped by distinguishing between these different forms of delayed xenograft rejection, using terminology to reflect the involvement of specific pathophysiological mechanisms. An updated classification of the stages of xenograft rejection is proposed here.

Clinical xenotransplantation: pigs might fly?

Xenotransplantation has the potential to deliver an unlimited supply of organs for transplantation. However, this promise has yet to translate into clinical application, despite substantial research efforts in the last decade. Although increasing numbers of studies are being performed in relevant pre-clinical (pig-to-primate) transplantation models, so far these have highlighted the apparent elusiveness of long-term xenograft survival. Humoral rejection remains the main obstacle to success, but control of T cell-mediated rejection will be a problem in the future and there are major concerns about the possible transmission of porcine endogenous retroviruses (PERV) and other infectious agents. This article reviews recent advances in the understanding of acute vascular rejection (AVR), acute T cell-mediated rejection and PERV transmission and highlights some of the strategies that may prove successful in overcoming these problems. Although progress has been slow, the promise of an inexhaustible supply of organs is sufficient reason to continue research in these areas. Assuming the specific problem of AVR can be ameliorated by one of a number of strategies currently under investigation, there are grounds to believe that xenotransplantation will become a clinical reality. Pig xenografts, currently grounded, might eventually fly!

Characterization of baboon anti-porcine IgG antibodies during acute vascular rejection of porcine kidney xenograft

In the pig-to-baboon model, the removal of anti-porcine natural antibodies abrogates hyperacute vascular rejection (HAVR), but the xenograft then undergoes an acute vascular rejection (AVR) concomitantly to the appearance of newly formed anti-porcine antibodies. The use of anti-IgM monoclonal antibody (mAb) in baboons allowed to avoid HAVR of pig-to-baboon renal xenografts, but, at post-operative day 6, AVR occurred because of a rapid return of anti-porcine antibodies. The aim of this work was to characterize the anti-porcine antibodies during AVR. Sera from anti-IgM-treated animals were assessed prior to the graft and at the time of AVR by enzyme linked immunosorbent assay (ELISA) to determine anti-porcine antibodies concentration as well as the IgG subtypes. The same sera were tested on confluent cultures of porcine aortic endothelial cells (PAECs) to assess (i) the cytolytic complement-dependent activity and (ii) the E-selectin expression. The K affinity of anti-Gal IgG antibodies was measured by ELISA. Anti-porcine (Gal and non-Gal) IgG antibodies were tested on PAECs by flow cytometry to discriminate the presence of Gal epitopes from the recognition of other porcine epitopes. We found that both anti-porcine IgM and IgG antibodies presented a significantly increased cytolytic activity and E-selectin expression on PAECs during AVR. These characteristics are related to an important increase of the antibody (Ab) titer (especially anti-galactosyl) and a switch to anti-galactosyl IgG1 subclass production, whereas the K affinity remained unchanged. The deleterious effects of both IgM and IgG antibodies observed during AVR showed the crucial need for treatment controlling the cells producing anti-porcine antibodies.

Xenotransplantation: Past achievements and future promise

Xenotransplantation offers a potential solution to the shortfall in donor organs for human transplantation. This review describes the barriers to xenotransplantation and the progress that has been made towards making it a clinical reality. Data from preclinical pig-to-primate cardiac and pulmonary xenografts are highlighted.

Tolerization of Gal alpha 1,3Gal-reactive B cells in pre-sensitized alpha 1,3-galactosyltransferase-deficient mice by nonmyeloablative induction of mixed chimerism

Using a alpha 1,3-galactosyltransferase wild-type (GalT(+/+)) to deficient (GalT(-/-)) mouse bone marrow transplantation model, we have previously demonstrated that a non-myeloablative conditioning regimen is capable of permitting induction of allogeneic and xenogeneic mixed chimerism. Chimerism is associated with the rapid and lasting tolerization of anti-Gal alpha 1,3Gal (Gal) natural antibody (Ab)-producing B cells. However, one limitation of this model is that anti-Gal natural Ab levels are lower in GalT(-/-) mice than in humans and other primates. To overcome this limitation, we have now investigated the possibility of inducing such tolerance in GalT(-/-) mice that produce much higher levels of anti-Gal Abs due to presensitization with Gal-bearing xenogeneic cells. B6 GalT(-/-) mice that were pre-sensitized with rabbit red blood cells received non-myeloablative conditioning with depleting anti-CD4 and CD8 mAbs, 3Gy whole body and 7Gy thymic irradiation, and infusion of BALB/c GalT(+/+) bone marrow cells (BMC). Although engraftment of standard marrow doses was inhibited by the presensitization, long-lasting mixed chimerism could be induced in recipients of a high dose [160 x 10(6)] of allogeneic wild-type BMC. Achievement of persistent chimerism was associated with high levels of anti-Gal IgG(1) pretransplant, suggesting an inhibitory effect of non-complement-fixing IgG(1) Ab on anti-Gal-mediated marrow rejection. Induction of mixed chimerism was associated with a rapid disappearance of serum anti-Gal and tolerization of anti-Gal Ab-producing cells. B cells with anti-Gal receptors became undetectable in mixed chimeras. Mixed chimeras accepted subsequently transplanted donor-type GalT(+/+) hearts (> 140 days), whereas rapid (within 2 days) rejection of GalT(+/+) hearts occurred in conditioned control GalT(-/-) mice. In conclusion, when a high dose of GalT(+/+) BMC was administered to pre-sensitized GalT(-/-) mice, chimerism and tolerance were achieved. The absence of B cells with receptors recognizing Gal in mixed chimeras suggests a role for clonal deletion/receptor editing in the maintenance of B cell tolerance.

Influence of humoral immunoreaction on hepatic nonparenchymal cells in ex situ xenoperfused rat livers

BACKGROUND

The influence of xenogeneic humoral immunoreaction on hepatic nonparenchymal cells (NPCs) was evaluated ex situ in xenoperfused rat livers.

METHODS

Isolated rat livers were perfused via the portal vein (PV) for 240 min. The perfusates consisted of fresh rat blood (group 1), fresh human blood (group 2), and fresh human blood containing 5 microg/mL soluble complement receptor type 1 (Group 3).

RESULTS

Deposition of human IgM and C(5b-9) complement was observed in group 2, although only human IgM deposition was detected in group 3. Portal vein pressure in group 2 rose drastically during the first 10 min. Creatine kinase BB component gradually increased in all groups, followed by an elevation in alanine aminotransferase and both parameters were significantly higher in group 2 than in groups 1 and 3. In group 2, platelet thrombi in the peripheral PVs and periportal hemorrhage were observed after 10 min, and massive necrosis around the central veins after 240 min; these changes were not observed in group 1 or 3. Production of tumor necrosis factor alpha and alpha interferon and expression of intercellular adhesion molecule 1 (ICAM-1) were lower in group 2 than in groups 1 and 3. In group 2, there were negative areas for ICAM-1 and tumor necrosis factor alpha staining around the central veins after 240 min, which were consistent with necrotic areas.

CONCLUSIONS

In xenoperfused rat livers, humoral mediators initially caused the disturbance of microcirculation, which would induce long ischemia in the pericentral areas, resulting in massive necrosis. NPC necrosis may be responsible for less production of cytokines and adhesion molecules in the xenoperfused livers.

The α-Gal epitope (Galα1-3Galβ1-4GlcNAc-R) in xenotransplantation

Many patients with failing organs (e.g., heart, liver or kidneys), do not receive the needed organ because of an insufficient number of organ donors. Pig xenografts have been considered as an alternative source of organs for transplantation. The major obstacle currently known to prevent pig to human xenotransplantation is the interaction between the human natural anti-Gal antibody and the α-gal epitope (Galα1-3Galβ1-4GlcNAc-R), abundantly expressed on pig cells. This short review describes the characteristics of anti-Gal and of the alpha-gal epitope, their role in inducing xenograft rejection and some experimental approaches for preventing this rejection.

Role of platelet-activating factor in functional alterations induced by xenoreactive antibodies in porcine endothelial cells

BACKGROUND

Platelet-activating factor (PAF) is a phospholipid mediator of inflammation which has been implicated in rejection. The interaction of anti-alpha-galactosyl natural antibodies (anti-alpha gal Abs) with endothelial cells is the initial step for the development of xenograft rejection. In our study, we stimulated porcine aortic endothelial cells (PAEC) with anti-alpha gal IgG to investigate the synthesis of PAF from PAEC and its biological consequences.

METHODS AND RESULTS

PAF was extracted and chromatographically purified from cultured PAEC stimulated with baboon anti-alpha gal Abs. The Abs induced a dose-dependent synthesis of PAF peaking after 30 min of incubation, and decreasing thereafter. Concomitant cell shape change, motility, and cytoskeleton redistribution were observed. These events were prevented by addition of a panel of PAF-receptor antagonists. An SV40 T-large antigen-immortalized PAEC line was engineered to express PAF acetyl-hydrolase (PAF-AH) cDNA, the major PAF-inactivating enzyme. These transfected cells exposed to anti-alpha gal Abs showed reduced cell contraction and motility compared with empty vector-transfected cells. Moreover, in PAEC stimulated with anti-alpha gal Abs, the synthesis of PAF promoted the adhesion of a monocytic cell line as shown by the inhibitory effect of PAF-receptor antagonists and of PAF-AH expression. Finally, studies on cell monolayer demonstrated an enhanced permeability 48 hr after exposure to anti-alpha gal Abs, and this increase was prevented by PAF-inactivation and by PAF-receptor blockade.

CONCLUSIONS

These results demonstrate that on stimulation with anti-alpha gal Abs, PAEC synthetize PAF which can contribute to several vascular events involved in xenograft rejection.

Disordered regulation of coagulation and platelet activation in xenotransplantation

Rejection of xenografts is associated with vascular-based inflammation, thrombocytopenia and the consumption of coagulation factors that may evolve into disseminated intravascular coagulation (DIC). Similarly, bone marrow-derived cellular xenotransplantation procedures are associated with endothelial cell activation and thrombotic microangiopathic injury. These complications generally develop despite the best available measures for depletion of xenoreactive natural antibody, inhibition of complement activation and suppression of T- and B-cell mediated immune responses. The mechanisms underlying the DIC and thrombotic microangiopathy associated with xenotransplantation are unclear. A proposed primary biological dysfunction of xenografts with respect to regulation of clotting could amplify vascular injury, promote immunological responses and independently contribute to graft failure. Disordered thromboregulation could have deleterious effects, comparable to unregulated complement activation, in the pathogenesis of xenograft rejection and may therefore represent a substantive barrier to xenotransplantation.

Resistance against the membrane attack complex of complement induced in porcine endothelial cells with a Gal alpha(1-3)Gal binding lectin: up-regulation of CD59 expression

Endothelial cells (EC) play central roles in vascular physiology and pathophysiology. EC activation results in proinflammatory activities with production of cytokines and expression of adhesion molecules. However, we have shown before in a model of xenotransplantation that prolonged stimulation of porcine EC with human anti-porcine IgM natural Abs can activate the cells to become resistant against cytotoxicity by the membrane attack complex of complement (MAC). Now we report the major characteristics of induction and maintenance of resistance elicited in porcine EC with Bandeiraea simplicifolia lectin that binds terminal gal alpha(1-3)gal. Lectin-treated cells underwent little or no cytotoxicity and PGI2 release when exposed to MAC. Induction of resistance required incubation of the EC with lectin for 4 h but was not fully manifested until 16 h later. Most of the initially bound lectin remained on the cell surface for >60 h. EC-bound lectin did not inhibit binding of IgM natural Abs or activation and binding of C components, including C9, but a C-induced permeability channel of reduced size was present. Induction of resistance required protein synthesis, developed slowly, and was associated with up-regulation of expression of mRNA for the MAC inhibitor CD59 and membrane-associated CD59 protein. Resistance lasted at least 3 days, and the cells regained normal morphology and were metabolically active. This induced resistance may have a physiologic counterpart that might be amenable to pharmacologic manipulation in vascular endothelium pathophysiology.

Inhibition of expression of the Galalpha1-3Gal epitope on porcine cells using an intracellular single-chain antibody directed against alpha1,3galactosyltransferase

The carbohydrate epitope Galalpha1-3Gal has been shown to be the major target of natural antibodies responsible for hyperacute rejection of porcine tissues transplanted into primates. We have sought to produce a phenotypic knockout of the alpha1, 3Galactosyltransferase enzyme that is responsible for generating this epitope, using an intracellular antibody approach. We have isolated high affinity anti-alpha1,3Galactosyltransferase single-chain antibodies from a semi-synthetic phage display library. Expression of a KDEL-tagged anti-alpha1,3Galactosyltransferase single-chain antibody in a porcine endothelial cell line resulted in the decreased expression of the Galalpha1-3Gal epitope and increased resistance to lysis by human serum.

Human natural antibodies cytotoxic to pig embryonic brain cells recognize novel non-Galalpha1,3Gal-based xenoantigens

Transplantation of porcine embryonic brain cells, including dopaminergic neurons, from ventral mesencephalon (VM) is considered a potential treatment for patients with Parkinson's disease. In the present study, we characterized the distribution among VM cells of the major porcine endothelial xenoantigen, the Galalpha1,3Gal epitope, and evaluated the cytotoxic effect of anti-Galalpha1,3Gal antibody-depleted and nondepleted human AB serum on VM cells. Overall levels of Galalpha1,3Gal-epitope expression was very low on the VM cell population using Bandeiraea simplicifolia IB(4) lectin staining of resuspended VM cells in flow cytometric analyses or staining of SDS-PAGE-separated, solubilized VM cell membrane proteins in Western blot analyses. Lectin-histochemical staining of sections of pig embryonal VM regions with BSA IB(4) lectin showed staining restricted to endothelial cells and microglia. In the presence of complement, both nondepleted and anti-Galalpha1,3Gal antibody-depleted AB sera were shown to be cytotoxic to VM cells as assessed in microcytotoxicity- and flow cytometry-based cytotoxicity assays. Purified IgM and IgG were both cytotoxic in the presence of complement. Three major VM cell membrane antigens of approximately 210, 105, and 50 kDa were reactive with natural IgM antibodies present in pooled human AB sera. Thus, antibody-dependent cytotoxicity may contribute to pig to human brain cell xenorejection, necessitating donor tissue modifications prior to a more widespread utilization of neural tissue xenografting.

Alpha-galactosyl-mediated activation of porcine endothelial cells: studies on CD31 and VE-cadherin in adhesion and signaling

BACKGROUND

Ligation of alpha-galactosyl epitopes on endothelial cells by naturally occurring human antibodies causes hyperacute rejection in porcine-to-human xenotransplantation. The alpha-galactosyl-specific lectin Bandeiraea simplicifolia isolectin B4 (IB4) has been reported to trigger endothelial "gap" formation and tyrosine phosphorylation of an unidentified 130-kDa protein. We have studied two 130-kDa junctional adhesion molecules, CD31 and VE-cadherin, in porcine aortic endothelial cells (PAECs) during IB4-mediated activation. The cellular distribution of these molecules, their susceptibility to tyrosine phosphorylation, and their capacity to bind IB4 or natural human antibodies have been determined.

METHODS

Porcine CD31 and VE-cadherin were cloned. Recombinant proteins and monoclonal antibodies were prepared. The distribution and phosphorylation of CD31 and VE-cadherin in confluent PAECs activated with IB4 or human serum were studied by confocal microscopy and Western blotting, respectively.

RESULTS

IB4 caused rapid redistribution of CD31 and VE-cadherin away from cell junctions and tyrosine-phosphorylation of CD31 but not VE-cadherin. A monoclonal antibody to CD31 also triggered tyrosine phosphorylation of this molecule, but brief exposure of PAECs to normal human serum did not. Tyrosine-phosphorylated CD31 complexed with SHP2 and other unidentified phosphoproteins. Both IB4 and natural human antibodies bound to porcine CD31 but not to VE-cadherin. Cell adhesion tests showed that porcine and human CD31 are functionally incompatible.

CONCLUSIONS

Endothelial cell retraction during IB4-mediated activation of PAECs is associated with rapid loss of CD31 and VE-cadherin from cell junctions. CD31 becomes strongly tyrosine-phosphorylated and forms a cell signaling complex, which may have a significant role in the response of the xenograft vascular endothelium.

Mixed chimerism induced without lethal conditioning prevents T cell- and anti-Gal alpha 1,3Gal-mediated graft rejection

Gal alpha 1,3Gal-reactive (Gal-reactive) antibodies are a major impediment to pig-to-human xenotransplantation. We investigated the potential to induce tolerance of anti-Gal-producing cells and prevent rejection of vascularized grafts in the combination of alpha 1,3-galactosyltransferase wild-type (GalT(+/+)) and deficient (GalT(-/-)) mice. Allogeneic (H-2 mismatched) GalT(+/+) bone marrow transplantation (BMT) to GalT(-/-) mice conditioned with a nonmyeloablative regimen, consisting of depleting CD4 and CD8 mAb's and 3 Gy whole-body irradiation and 7 Gy thymic irradiation, led to lasting multilineage H-2(bxd) GalT(+/+) + H-2(d) GalT(-/-) mixed chimerism. Induction of mixed chimerism was associated with a rapid reduction of serum anti-Gal naturally occurring antibody levels. Anti-Gal-producing cells were undetectable by 2 weeks after BMT, suggesting that anti-Gal-producing cells preexisting at the time of BMT are rapidly tolerized. Even after immunization with Gal-bearing xenogeneic cells, mixed chimeras were devoid of anti-Gal-producing cells and permanently accepted donor-type GalT(+/+) heart grafts (>150 days), whereas non-BMT control animals rejected these hearts within 1-7 days. B cells bearing receptors for Gal were completely absent from the spleens of mixed chimeras, suggesting that clonal deletion and/or receptor editing may maintain B-cell tolerance to Gal. These findings demonstrate the principle that induction of mixed hematopoietic chimerism with a potentially relevant nonmyeloablative regimen can simultaneously lead to tolerance among both T cells and Gal-reactive B cells, thus preventing vascularized xenograft rejection.

Factors in xenograft rejection

Important mechanisms underlying immediate xenograft loss by hyperacute rejection (HAR), in the pig-to-primate combination, have been recently delineated. There are now several proposed therapies that deal with the problem of complement activation and xenoreactive natural antibody (XNA) binding to the vasculature that have been shown to prevent HAR. However, vascularized xenografts are still lost, typically within days, by delayed xenograft rejection (DXR), alternatively known as acute vascular rejection (AVR). This process is characterized by endothelial cell (EC) perturbation, localization of XNA within the graft vasculature, host NK cell and monocyte activation with platelet sequestration and vascular thrombosis. Alternative immunosuppressive strategies, additive anti-complement therapies with the control of any resulting EC activation processes and induction of protective responses have been proposed to ameliorate this pathological process. In addition, several potentially important molecular incompatibilities between activated human coagulation factors and the natural anticoagulants expressed on porcine EC have been noted. Such incompatibilities may be analogous to cross-species alterations in the function of complement regulatory proteins important in HAR. Disordered thromboregulation is potentially relevant to the progression of inflammatory events in DXR and the disseminated intravascular coagulation seen in primate recipients of porcine renal xenografts. We have recently demonstrated the inability of porcine tissue factor pathway inhibitor (TFPI) to adequately neutralize human factor Xa (FXa), the aberrant activation of both human prothrombin and FXa by porcine EC and the failure of the porcine natural anticoagulant, thrombomodulin to bind human thrombin and hence activate human protein C. The enhanced potential of porcine von Willebrand factor to associate with human platelet GPIb has been demonstrated to be dependent upon the isolated A1 domain of von Willebrand factor. In addition, the loss of TFPI and vascular ATPDase/CD39 activity following EC activation responses would potentiate any procoagulant changes within the xenograft. These developments could exacerbate vascular damage from whatever cause and enhance the activation of platelets and coagulation pathways within xenografts resulting in graft infarction and loss. Analysis of these and the other putative factors underlying DXR should lead to the development and testing of genetic approaches that, in conjunction with selected pharmacological means, may further prolong xenograft survival to a clinically relevant extent.

IgY antiporcine endothelial cell antibodies effectively block human antiporcine xenoantibody binding

Avian IgY antibodies are structurally different from mammalian IgGs and do not fix mammalian complement components or bind human Fc receptors. As these antibody-mediated interactions are believed to play significant roles in both hyperacute rejection (HAR) and acute vascular xenograft rejection (AVXR), IgY antibodies to xenoantigen target epitopes may inhibit these rejection processes. In this report, we show that chicken IgY antibodies to alpha-Gal antigen epitopes and to other porcine aortic endothelial cell (PAEC) antigens block human xenoreactive natural antibody binding to both porcine and rat cardiac tissues and porcine kidney tissues. Chicken IgY antibodies blocked complement-mediated lysis of PAECs by human serum, and inhibited antibody-dependent cell-mediated lysis of PAECs by heat-inactivated human serum plus peripheral blood leukocytes. Binding of IgY to porcine endothelial cells did not affect cell morphology nor expression of E-selectin. These results suggest that avian IgYs could be of potential use in inhibiting pig-to-human xenograft rejection.

Definition and characterization of chicken Gal alpha(1,3)Gal antibodies

BACKGROUND

The Gal alpha(1,3)Gal epitope is of interest as, in pig-to-primate xenotransplantation, it is the major target of naturally occurring human IgM and IgG antibodies, leading to hyperacute rejection. Human and Old World monkeys make anti-Gal alpha(1,3)Gal antibodies as they lack a functional gene and do not express Gal alpha(1,3)Gal. Interestingly, the cultured fibroblasts of some other species, such as chickens, have been reported also not to express Gal alpha(1,3)Gal--if this is true for other tissues, and chickens do not express Gal alpha(1,3)Gal antigen, then they would have anti-Gal antibodies--which could have diagnostic and therapeutic value, particularly as chicken antibodies do not fix mammalian complement.

METHODS

Standard serological methods were used to characterize the antibodies. Several baboons received pig kidney xenografts that had been perfused with hyperimmune chicken anti-Gal antibodies.

RESULTS AND CONCLUSIONS

We now demonstrate that chickens do not express Gal alpha(1,3)Gal on their red cells, leukocytes, or tissues, and that their serum contains large amounts of anti-Gal alpha(1,3)Gal antibodies. In addition, chickens could be immunized to produce high-titer, high-avidity antibodies (9.5x10(9) M(-1))--an avidity considerably greater than that of the Gal alpha(1,3)Gal binding lectin IB4 (2.9x10(8) M(-1)) or Gal antibodies in human serum (2.2x10(5) M(-1)). Chicken antibodies, obtained from both normal and immunized chickens, could block the in vitro cytolysis of pig endothelial cells or lymphocytes by human or baboon antibodies. However, such antibodies tested in vivo in pig-to-baboon xenotransplantation failed to block hyperacute rejection and, indeed, may have accelerated this.

Immunochemical properties of anti-Gal alpha 1-3Gal antibodies after sensitization with xenogeneic tissues

In antigen-driven immune responses to proteins, antibodies of low avidity and limited complement fixing capacity undergo affinity maturation to yield antibodies of higher avidity which fix complement to a greater extent. The products of antigen-driven responses to carbohydrates are less defined. To investigate the evolution of natural antibodies against carbohydrates following sensitization, we studied natural antibodies specific for Gal alpha 1-3Gal in patients sensitized to that antigen as a result of perfusion of their blood through porcine livers for the treatment of hepatic failure. The natural antibodies against Gal alpha 1-3Gal, which occur in all unsensitized individuals, were predominantly IgM and IgG2, with average functional avidities of 5 x 10(-9) and 2 x 10(-8) M, respectively. After sensitization, the classes of anti-Gal alpha 1-3Gal included IgM, IgG2, and IgG1, and the average functional avidities of IgM and IgG were 3 x 10(-9) and 2 x 10(-9) M, respectively. The activation of complement by anti-Gal alpha 1-3Gal per microgram of Ab, measured by the fixation of C3bi on porcine cells, increased after sensitization owing to changes in subclass and avidity. Deposition of C3bi correlated with the concentrations of IgG1 and IgM but not IgG2 against Gal alpha 1-3Gal. Consistent with this finding, purified IgG1, but not IgG2, anti-Gal alpha 1-3Gal fixed complement on porcine cells. These results demonstrate that the properties of anticarbohydrate antibodies evolve after sensitization to increase complement fixation on potential targets. These properties may result from the altered costimulation of the humoral response to Gal alpha 1-3Gal due to sensitization.

Extracellular ATP and ADP activate transcription factor NF-kappa B and induce endothelial cell apoptosis

Inflammation within the vasculature is associated with endothelial cell (EC) perturbation, loss of vascular ATP-diphosphohydrolase activity, and platelet microthrombus formation with release of ATP and ADP into the micro-environment. The nature and effects of purinergic stimulation of EC under these circumstances remain largely undetermined. ATP and ADP activated EC transcribed mRNA from certain transcription factor NF-kappa B target genes and expressed E-selectin protein on cell membranes. Band shift analysis and reporter assays confirmed the activation of NF-kappa B in response to both ATP and ADP. Apoptosis was shown to occur in response to purinergic signaling, potentially through the activation of P2z/P2x7 receptors. Induction of EC activation responses and apoptosis in response to stimulation with ATP and ADP is associated with activation of NF-kappa B.

Alpha-galactosyl epitope-mediated activation of porcine aortic endothelial cells: type II activation

BACKGROUND

Xenoreactive natural antibodies (XNAs) and complement mediate hyperacute rejection of discordant xenografts. Inhibition of complement alone results in some prolongation of graft survival, but delayed xenograft rejection still precludes long-term graft survival. In vitro data provide evidence for the direct proinflammatory activation of endothelial cells (ECs) by XNAs. These antibodies are primarily directed against galactose alpha(1-3)-galactose (alpha-gal), the major xenoantigen in the pig to primate xenotransplant model. Previous studies have shown EC activation by XNAs but failed to address the question of whether alpha-gal-specific ligands can induce EC activation. The aim of this study was to investigate whether agonist binding to the alpha-gal epitope by alpha-gal-specific lectins as compared with XNAs or elicited xenoreactive antibodies can directly elicit type II porcine aortic EC (PAEC) activation (i.e., activation that requires protein synthesis).

METHODS AND RESULTS

The tetravalent, alpha-gal-binding Bandeiraea simplicifolia lectin I (BS-I), the wholly alpha-gal-specific BS-I isolectin B4, and elicited primate anti-pig xenoreactive antibodies (decomplemented cynomolgus monkey anti-porcine serum) induced E-selectin protein expression in PAECs. This induction was alpha-gal-specific, as preincubation with synthetic alpha-gal carbohydrate or adsorption of lectin or serum to rabbit, but not human, red blood cells removed the activating component. E-selectin expression, induced by BS-I, was inhibited in the presence of genistein, a tyrosine kinase inhibitor, and by mepacrine, an inhibitor of phospholipase A2. Human and primate XNAs lacked this activity when tested at relevant concentrations; however, stimulation of PAECs with affinity-purified human XNA (IgM and IgG) resulted in slightly increased interleukin-8 and P-selectin mRNA levels but had no apparent effects on E-selectin transcription. BS-I strongly induced E-selectin, P-selectin, intercellular adhesion molecule-1, and interleukin-8 mRNA in an NF-kappaB-dependent manner.

CONCLUSIONS

Several agonists that specifically bind to alpha-gal can evoke type II EC activation. Hence, anti-Gal antibodies may contribute directly to xenograft rejection in the absence of complement activation.