Role of natural killer cells, macrophages, and accessory molecule interactions in the rejection of pig-to-primate xenografts beyond the hyperacute period.

Pig-to-primate cardiac xenografts surviving beyond the period of hyperacute rejection succumb after 3-4 days to a secondary immunologic response characterized by xenograft infiltration with NK cells and macrophages. Circulating baboon mononuclear cells contain NK cell precursors which mediate lysis of porcine endothelium by two distinct mechanisms: antibody-dependent cellular cytotoxicity and lymphokine activation. IL-2 activated NK lysis of porcine endothelium was 2.4-fold stronger than lysis occurring following engagement of FcRIII by xenoreactive IgG. IL-2 augmented NK lysis involved interactions between CD2 and CD49d on baboon NK cells and their respective ligands on porcine endothelium, since NK lysis was reduced either by using Mabs against CD2, CD49d, or porcine VCAM, or by treating endothelial cells with PIPLC to cleave GPI-linked molecules. These results imply that interactions between accessory molecule receptor-ligand pairs on primate NK cells, macrophages and porcine endothelium are of critical importance in delayed xenograft rejection.

Triple immunosuppression reduces mononuclear cell infiltration and prolongs graft life in pig-to-newborn baboon cardiac xenotransplantation.

OBJECTIVE: Pig hearts transplanted into unmedicated newborn baboons do not undergo hyperacute rejection by preformed xenoantibody and complement. These grafts are rejected at days 3 to 4 in association with the infiltration of macrophages and natural killer cells. We investigated whether an immunosuppressive regimen used widely in cardiac allotransplantation could reduce this cellular response and prolong xenograft life. METHODS: Ten newborn baboons underwent heterotopic pig cardiac xenotransplantation. Five baboons were immunosuppressed with mycophenolate mofetil (100 mg/kg), methylprednisolone acetate (0.8 mg/kg), and cyclosporine A (INN: ciclosporin; 10 mg/kg). Xenograft rejection was studied by light microscopy and immunofluorescence. The induced humoral response to porcine xenoantigens was documented by enzyme-linked immunosorbent assay using synthetic alpha-1,3-galactosyl epitopes coupled to bovine serum albumin. RESULTS: Graft life was extended from a mean of 3.6 +/- 0.5 days (n = 5) to a mean of 6.2 +/- 1.1 days (n = 5, p = 0.01). In comparison with controls, explanted grafts from medicated baboons demonstrated reduced infiltration with natural killer cells and macrophages, but increased evidence of complement-mediated rejection substantiated by increased deposition of immunoglobulin M, complement, and fibrin. In all baboons receiving transplants, levels of both immunoglobulin M and immunoglobulin G anti-galactose were significantly increased after transplantation, with immunoglobulin G levels remaining persistently elevated. CONCLUSIONS: These results indicate that cyclosporine-based triple immunosuppression marginally prolonged xenograft survival and appears to have reduced the natural killer cell and macrophage infiltrates. The immunosuppressive protocol, however, was not adequate to prevent the induced immunoglobulin M humoral response and prevent complement-mediated graft injury.

Heparinless cardiopulmonary bypass with active-site blocked factor IXa: a preliminary study on the dog.

OBJECTIVE: Cardiopulmonary bypass is a potent stimulus for activation of procoagulant pathways. Heparin, the traditional antithrombotic agent, however, is often associated with increased perioperative blood loss because of its multiple sites of action in the coagulation cascade and its antiplatelet and profibrinolytic effects. Furthermore, heparin-mediated immunologic reactions (that is, heparin-induced thrombocytopenia) may contraindicate its use. Cardiopulmonary bypass with a selective factor IXa inhibitor was tested to see whether it could effectively limit bypass circuit/intravascular space thrombosis while decreasing extravascular bleeding, thereby providing an alternative anticoagulant strategy when heparin may not be safely administered. METHODS: Active site-blocked factor IXa, a competitive inhibitor of the assembly of factor IXa into the factor X activation complex, was prepared by modification of the enzyme's active site by the use of dansyl glutamic acid-glycine-arginine-chlormethylketone. Twenty mongrel dogs (five were given standard heparin/protamine; 15 were given activated site-blocked factor IXa doses ranging from 300 to 600 microg/kg) underwent 1 hour of hypothermic cardiopulmonary bypass, and blood loss was monitored for 3 hours after the procedure. RESULTS: Use of activated site-blocked factor IXa as an anticoagulant in cardiopulmonary bypass limited fibrin deposition within the extracorporeal circuit as assessed by scanning electron microscopy, comparable with the antithrombotic effect seen with heparin. In contrast to heparin, effective antithrombotic doses of activated site-blocked factor IXa significantly diminished blood loss in the thoracic cavity and in an abdominal incisional bleeding model. CONCLUSION: These initial studies on the dog suggest that administration of activated site-blocked factor IXa may be an effective alternative anticoagulant strategy in cardiopulmonary bypass when heparin is contraindicated, affording inhibition of intravascular/extracorporeal circuit thrombosis with enhanced hemostasis in the surgical wound.

Induction of tolerance in rodent cardiac allotransplantation using an MHC class I-derived peptide and cyclosporin A.

BACKGROUND: The synthetic peptide corresponding to residues 75-84 of the human major histocompatibility complex class I molecule HLA-B7 (Allotrap 07) has been shown to inhibit differentiation of cytotoxic T lymphocyte precursors. Subsequent treatment of LEW-1A rats with this peptide was associated with a reduction in the level of cytotoxic activity directed to donor alloantigens. This study was undertaken to investigate the effect of Allotrap 07 on rodent heart allograft survival in LEW-1A recipients. METHODS: Heart allografts from Lewis rats were heterotopically transplanted into the infrarenal abdominal aorta of ACI recipients. The treatment groups consisted of different regimens of short-term intravenous Allotrap 07 and oral cyclosporin A. All grafts were palpated daily, with rejection defined as the cessation of palpable contractions. RESULTS: Cardiac allografts transplanted from Lewis to ACI rats survived indefinitely after administration of intravenous Allotrap 07 and oral cyclosporin A. Tolerance induction was donor-specific because third-party Brown-Norway, but not Lewis, grafts were rapidly rejected after implantation into ACI recipients. CONCLUSIONS: Because donor-specific tolerance persisted long after cessation of peptide administration and did not occur when cyclosporin A was omitted from the immunosuppressive regimen, the mechanism may involve induction of clonal anergy.

Immature host for xenotransplantation.

The overall shortage of appropriate donor organs has prompted transplant physicians and surgeons to consider using organs from other nonhuman species. The shortage of appropriate human donor hearts for newborn recipients is especially severe. Presently, the pig appears to be the most appropriate source for organs. Humans and baboons uniformly develop high titers of complement-fixing (IgM) anti-pig xenoantibodies, resulting in complement-mediated hyperacute rejection (HAR) of pig organs transplanted into mature baboons within minutes to hours. In contrast, newborn humans and baboons do not have high titers of anti-pig IgM xenoantibody, and consequently pig cardiac xenografts transplanted into newborn baboons do not undergo HAR. Rather, these organs are rejected at days 3 to 4 by a distinctive immunologic process that involves natural killer cells and macrophages. With the addition of cyclosporine-based triple immunosuppression, this process is reduced and graft life is prolonged to 6 to 7 days. This therapy, however, is not sufficient to prevent the induced humoral response to the graft, and the organs are rejected by antibody- and complement-dependent mechanisms. Future treatment strategies to reduce this humoral response must incorporate immunodepletion columns, soluble complement-inhibiting agents, and additional anti-B lymphocyte agents. These strategies, in conjunction with the use of transgenic pig organs that express human membrane-bound complement regulatory proteins or reduced xenoantigenic epitopes could further prolong graft life. Clinical trials are being formulated that would utilize pig organs as a "bridge," sustaining a newborn human in need of a heart transplant until an appropriate donor is located.

Cardiac xenotransplantation.

The severe shortage of human donor hearts has prompted several investigators to develop alternative strategies using cross-specie organs or xenografts. Unlike human allotransplantation, in which the important antigenic differences between donor and recipient are confined to the major histocompatibility and blood group antigens, xenotransplantation is confronted with the potential for multiple antigenic differences. In the pig-to-primate model of xenotransplantation, the primary obstacle to cross-species transplantation has been hyperacute rejection mediated by complement fixing antibodies directed against galactose alpha 1,3-galactose (Gal alpha 1,3-Gal) epitopes on the pig endothelium. Conventional immunosuppression is unable to overcome hyperacute rejection; however, recent efforts in molecular biology have focused on genetically engineering porcine donors to express human proteins in their tissue. Transgenic pigs that express human complement regulatory proteins on their endothelium have been developed. Heterotopic transplantation of these transgenic donor hearts have had only moderate success. Alternative approaches attempt to eliminate the Gal alpha 1,3-Gal epitopes by genetically "knocking out" the enzyme necessary for its synthesis, or to reduce the expression of Gal alpha 1,3-Gal epitopes by genetically inserting enzymes that redirect precursor molecules into alternative synthetic pathways. The technology to knock out the necessary enzymes in pigs is not yet available; however, pigs expressing the H-transferase gene have been developed and show reduced levels of Gal alpha 1,3-Gal epitopes. Although this "new breed" of transgenic pigs may overcome the barrier of hyperacute rejection, special strategies will need to be developed that target the next barrier of xenograft rejection.

Modified technique for heterotopic heart transplantation in small primates.

Cardiac transplantation in small primates represents a unique model for investigating both xenograft and allograft rejection. This report describes our technique for heterotopic transplantation of cardiac grafts into the retroperitoneal iliac vessels of newborn baboons and small primates. Small primates tolerate this position better than either cervical or abdominal placement.

Anti-GaL IgG antibodies in sera of newborn humans and baboons and its significance in pig xenotransplantation.

We have previously demonstrated that hyperacute rejection does not occur in a pig-to-newborn baboon heart transplant model, presumably because of low levels of cytotoxic antipig antibodies present in the serum of newborn baboons. Cytotoxic antipig antibodies are primarily directed to alpha-1,3-galactosyl (alpha Gal) residues on endothelial cell surface structures Twenty-one full-term humans and 5 full-term baboons were tested for complement mediated lysis (CML) of pig kidney (PK-15) cells and anti-alpha Gal activity with an ELISA using BSA-conjugated alpha Gal residues as target. To evaluate the significance of the anti-alpha Gal titers in vivo 5 newborn baboons underwent heterotopic pig cardiac xenotransplantation. Six of 21 human samples and 1 of 5 baboon samples demonstrated significant cytotoxicity to PK-15 cells. Twelve of 21 newborn humans had anti-alpha Gal IgG antibodies at titers of 1:80 or greater. None of the samples had anti-alpha Gal IgM. In newborn baboons, 1 of 5 sera had anti-alpha Gal IgG antibodies at titers greater than 1:80 and none of these samples had anti-alpha Gal IgM. Xenografts survived for an average of 3.6 days, even in the baboon with high anti-alpha Gal IgG titers. Analysis of the explanted grafts showed minimal evidence of complement-mediated hyperacute rejection (HAR), but prominent mononuclear cell infiltrates. In serum tested posttransplant there was an induced anti-alpha Gal response with cytotoxicity against PK-15 cells. These results show that anti-alpha Gal IgM is absent in newborn human and baboon sera, allowing pig grafts to avoid HAR. However, the presence of anti-alpha Gal IgG may be associated with mononuclear cell infiltration of the xenograft and its subsequent rejection.

Lysis of pig endothelium by IL-2 activated human natural killer cells is inhibited by swine and human major histocompatibility complex (MHC) class I gene products.

We have previously described a form of xenograft rejection, mediated by natural killer (NK) cells, occurring in pig-to-primate organ transplants beyond the period of antibody-mediated hyperacute rejection. In this study, two distinct NK activation pathways were identified as mechanisms of pig aortic endotheliual cell (PAEC) lysis by human NK cells. Using an antibody-dependent cellular cytotoxicity (ADCC) assay, a progressive increase in human NK lysis of PAEC was observed following incubation with human IgG at increasing serum titer. In the absence of IgG, a second mechanism of PAEC lysis by human NK cells was observed following activation with IL-2. IL-2 activation of human NK cells increased lysis of PAEC by over 3-fold compared with ADCC. These results indicate that IL-2 activation of human NK cells induces significantly higher levels of lytic activity than does conventional ADCC involving IgG and FcRIII. We next investigated the role of MHC class I molecules in the regulation of NK lysis following IL-2 activation. PAEC expression of SLA class I molecules was increased by up to 75% by treatment with human TNFa. Following treatment with TNFa at 1 u/ml, IL-2 activated human NK lysis of PAEC was inhibited at every effector:target (E:T) ratio tested. Maximal effect occurred at an E:T ratio of 10:1, with TNFa inhibiting specific lysis by 59% (p < 0.01). Incubation with an anti-SLA class I Mab, but not IgG isotype control, abrogated the protective effects of TNFa on NK lysis of PAEC, suggesting direct inhibitory effects of SLA class I molecules on human NK function. To investigate whether human MHC class I molecules might have similar effects on human NK lysis of PAEC, further experiments were performed using a soluble peptide derived from the alpha-helical region of HLA-B7. Incubation with the HLA-B7 derived peptide significantly reduced the IL-2 activated NK lytic activity against PAEC in a dose-dependent fashion. Maximal effect occurred at a concentration of 10 mg/ml, where an 8-fold reduction in IL-2 augmented NK lysis was observed (p < 0.01). These results suggest that IL-2 activated human NK lysis of porcine xenografts may be inhibited by strategies which increase PAEC expression of SLA class I molecules, introduce HLA class I genes into PAEC, or use soluble HLA class I peptides.

Circulating human mononuclear cells exhibit augmented lysis of pig endothelium after activation with interleukin 2.

In immunohistochemical studies investigating the cellular infiltrates in pig xenografts undergoing delayed rejection by newborn and adult primate recipients, we observed extensive infiltration with primate macrophages and natural killer (NK) cells. To extend these studies in vitro, we investigated the functional properties of human NK cell precursors with respect to their potential interactions with pig aortic endothelial cells (PAEC). Using a short-term 51Cr release assay, human peripheral blood mononuclear cells (PBMC) demonstrated spontaneous and interleukin (IL) 2 augmented lytic activity against PAEC which increased with increasing effector to target cell ratio. Treatment of human PBMC with anti-CD2 significantly reduced this NK lytic activity by IL-2-activated PBMC. Finally, we investigated the effects of PAEC treatment with certain macrophage-derived human cytokines on adhesion of IL-2-activated human PBMC. Treatment of PAEC with IL-1 and tumor necrosis factor-alpha, in a dose-dependent manner, increased adherence of IL-2-activated human PBMC. These results demonstrate that humans contain circulating NK cells capable of lysing PAEC after activation with IL-2, that the mechanism involves interactions between CD2 and its ligand on porcine endothelium, and that these interactions may be influenced by macrophage-derived cytokines produced at the site of xenograft rejection.

Apoptosis of cardiac myocytes during cardiac allograft rejection. Relation to induction of nitric oxide synthase.

BACKGROUND: Apoptosis is a distinct form of programmed cell death characterized by activation of endonucleases that cleave nuclear DNA, condensation and fragmentation of nuclear chromatin, blebbing of intact membranes, and cell shrinkage and fragmentation. The mechanisms responsible are unclear, but nitric oxide (NO) generated by inducible NO synthase (iNOS) has been demonstrated to induce apoptosis in macrophages in vitro. This study investigated whether apoptosis occurs during cardiac allograft rejection and examined the relationship of apoptosis to iNOS expression. METHODS AND RESULTS: Heterotopic abdominal transplantation from Lewis to Wistar-Furth rats was used as a model of cardiac allograft rejection; Lewis-to-Lewis grafts served as controls. Apoptosis was identified by DNA ladders after electrophoresis on agarose gels and by in situ labeling of DNA fragments; cell types were determined by immunohistochemistry. The number of apoptotic cardiac myocytes increased sharply from day 3 (0.31/mm2 ventricular tissue) to day 5 (1.27/mm2) after transplantation. At day 5, allografts showed a significant increase (P < .01) in apoptotic cardiac myocytes, macrophages, and endothelial cells compared with syngeneic grafts. The expression of iNOS mRNA, protein, and enzyme activity paralleled in time and extent the apoptosis of cardiac myocytes. iNOS immunostaining of infiltrating macrophages and cardiac muscle fibers increased significantly in the allografts at days 3 to 5 and was accompanied by immunostaining of both cell types for nitrotyrosine, which is indicative of peroxynitrite formation. CONCLUSIONS: Apoptosis of myocardial cells occurs during cardiac allograft rejection. Apoptosis during rejection parallels the expression of iNOS, which suggests that apoptosis may be triggered by NO and peroxynitrite.

Recent advances and the potential for clinical use of xenotransplantation.

The success of allotransplantation has paradoxically led to a shortage in the supply of organs required to meet the increasing demand. Xenografts represent a potentially infinite supply of good quality organs. Recent advances in the understanding of the immunobiology of xenograft rejection has spurred experimental approaches aimed at abrogating or suppressing human xenospecific immune responses. The limited supply of colony-bred nonhuman primates (baboons) makes it highly unlikely that this species will be of widespread use as a donor source. Therefore, the pig has been proposed as a realistic alternative donor. Delineation of the major pig xenoantigens recognized by natural human xenoreactive antibodies has led to the development of strategies aimed at depletion of antibodies or the prevention of their binding to xenogeneic endothelium. Recognition that antibody binding leads to endothelial cell activation and complement-mediated injury has led to the development of genetically modified pigs whose organs are potentially resistant to damage by human complement by virtue of the fact that their endothelial cells express membranebound human complement regulatory proteins. Finally, dissection of the relative contributions of humoral and cellular immune responses directed against xenografts from closely or distantly related species has permitted rational recommendations for the administration of immunosuppressive regimens.