Human complement regulatory proteins protect swine-to-primate cardiac xenografts from humoral injury

Complement membrane attack complex and protectin (CD59) in liver allografts during acute rejection

BACKGROUND/AIMS

The complement system is important in the rejection of xenografts, but very little is known about its activation in the rejection of allografts. Complement lysis is induced by the membrane attack complex (MAC), an aggregate of C5b, C6, C7, C8 and C9 molecules. The main defender against MAC is the CD59 molecule, also called protectin. In this study, the aim was to analyze the possible deposition of MAC and the fate of CD59 on distinct cell populations during liver allograft rejection.

METHODS

Liver allografts were monitored by frequent fine-needle aspiration biopsies (FNAB) to demonstrate the immunoactivation of rejection. To examine MAC and CD59 in the FNAB, in relation to the activation markers of rejection, IL2-receptor, MHC class II and ICAM-1 expression, specific monoclonal antibodies and immunoperoxidase staining were used.

RESULTS

Ten out of 21 consecutive liver transplants underwent a histologically confirmed episode of reversible acute rejection. In the FNAB, a significant increase of the activation markers IL2-receptor, class II and ICAM-1 correlated with the peak of inflammation during the episode. In association with inflammation, a significant deposition of MAC was recorded in neutrophils and lymphocytes infiltrating the graft and in the parenchymal cells. MAC deposition subsided together with the inflammation. A significant decrease in CD59 expression was seen in neutrophils during rejection, but CD59 expression on other inflammatory cells and hepatic tissue cells varied greatly.

CONCLUSIONS

Complement activation was seen in association with acute rejection of liver allografts and it led to MAC assembly on leukocytes and tissue cells. A decrease in CD59 expression was less clear-cut, but it may predispose the cells to complement-mediated elimination.

The role of complement in transplantation

The complement system contributes critically to the barrier to transplantation of cells and organs. In the case of tissues and organs transplanted between individuals of the same species, that is in allotransplantation, the barrier posed by complement is seemingly eclipsed by the barrier posed by cellular immune responses. In the case of cells and organs transplanted between individuals of disparate species, that is xenotransplantation, the complement system has been thought to pose a nearly insurmountable barrier. With our understanding on how the complement system contributes to rejection, it is now clear that the complement system is more important in allotransplantation and more forgiving in xenotransplantation than was previously thought.

Distribution of membrane cofactor protein (MCP/CD46) on pig tissues. Relevance To xenotransplantation

Membrane cofactor protein (MCP; CD46) is a 50-60 000 MW glycoprotein, expressed on a wide variety of cells and tissues in man, which plays an important role in regulating complement activation. Human MCP has also been shown to be the receptor for measles virus. We have recently identified the pig analogue of MCP and demonstrated that pig MCP has cofactor activity for factor I-mediated cleavage of C3b when these components are derived either from pig or human. As a consequence, pig MCP is an efficient regulator of the classic and alternative pathways of human and pig complement. In order to define the potential importance of MCP in protecting against complement activation in the pig, we have conducted a comprehensive survey of its distribution in pig cells and organs. As in humans, MCP in the pig is broadly and abundantly distributed. Pig MCP is highly expressed on all circulating cells, including erythrocytes, in contrast to its absence on human erythrocytes. Multiple isoforms of MCP are found on cells and in tissues, probably representing products of alternative splicing analogous to those found in man. MCP is abundantly expressed throughout all tissues examined with particularly strong staining on the vascular endothelium. Connective tissue elements within liver and testis are also strongly stained by anti-pig MCP antibodies. Pig MCP is expressed only weakly on skeletal muscle cells and expression is absent from smooth muscle cells in the lung and vessel walls, sites at which human MCP is expressed. Of particular note, MCP is not expressed in B-cell areas of the germinal centres of lymph nodes.

Gene therapy strategies to facilitate organ transplantation

Organ transplantation is now the definitive therapy for many forms of end-organ disease, but chronic allograft rejection, the side effects of chronic immunosuppressive therapy and the severe donor organ shortage continue to limit its success. Gene therapy has the potential to prevent graft rejection by manipulating the immune response in the microenvironment of the graft or by facilitating the induction of tolerance. Genetic manipulation of stem cells to create transgenic and/or knockout animals that could serve as organ or cell donors could be combined with gene therapy approaches to overcome the problem of limited allogeneic donor organ supply.

Prospects for xenotransplantation

The major limitation on the application of transplantation for the treatment of human disease is a severe shortage of human donor organs and tissues. One approach to overcoming this problem is xenotransplantation, that is the transplantation of animal organs into humans. The major hurdle to xenotransplantation is the immune response of the recipient against the graft. Recent years have brought new information concerning this hurdle and insights of strategies for overcoming it. Other hurdles include the physiological function of the graft in the foreign environment including the possibility of molecular incompatibilities between the donor and recipient and the possibility of transferring infectious diseases from the graft to the recipient. The current perspective on these issues will be presented in the review that follows.

The problem of anti-pig antibodies in pig-to-primate xenografting: current and novel methods of depletion and/or suppression of production of anti-pig antibodies

The role of antibodies directed against Galalpha1-3Gal (alpha-Gal) epitopes in porcine-to-primate xenotransplantation has been widely studied during the past few years. These antibodies (anti-alpha-Gal) have been associated with both hyperacute rejection and acute vascular rejection of vascularized organs. Depletion and (temporary or permanent) suppression of production of anti-alpha-Gal seem to be essential to the long-term survival of these organs, even when the ultimate aim is accommodation or tolerance. Although more than 95% depletion of anti-alpha-Gal can be achieved by the use of immunoaffinity column technology, to date no regimen has been successful in preventing the return of anti-alpha-Gal (from continuing production). In this review, we discuss current and novel methods for achieving depletion or inhibition (i.e. extracorporeal immunoadsorption, anti-idiotypic antibodies, the intravenous infusion of immunoglobulin or oligosaccharides) and suppression of production (i.e. irradiation, pharmacologic agents, specific monoclonal antibodies, immunotoxins) of anti-alpha-Gal antibodies.

Hearts from transgenic pigs constructed with CD59/DAF genomic clones demonstrate improved survival in primates

We have previously created transgenic pigs bearing the human complement regulatory proteins CD59 and decay-accelerating factor (DAF) by either the intercellular transfer or the cDNA transgenic method. To achieve more physiologic protein expression, we constructed a new line of transgenic pigs with CD59 and DAF human genomic clones. We transplanted these CD59/DAF transgenic pig hearts into baboons immunosuppressed with cyclosporine, methylprednisone or leflunomide/mofetil mycophenolate. The four wild-type hearts survived for 20-80 min, whereas the four CD59/DAF hearts functioned for 85-130 h. Immunohistochemical staining showed levels of CD59 and DAF protein expression similar to that in human hearts. Wild-type and transgenic hearts demonstrate a similar level of IgM deposition, although transgenic hearts suffered less hyperacute rejection and thus less membrane attack complex deposition. The histology of the transgenic grafts after explant was consistent with acute vascular rejection, with a high level of IgG deposit compared with wild-type control. We conclude that this new line of CD59/DAF transgenic pigs express high levels of the transgene products, which conferred longer survival because of better protection from hyperacute rejection. Similar to previous transgenic pigs, however, these animals suffered from delayed xenograft rejection.

Potential application of neonatal porcine islets as treatment for type 1 diabetes: a review

Islet transplantation has been shown to be a viable option for treating patients with type 1 diabetes. However, widespread clinical application of this treatment will necessitate an alternative source of insulin-producing tissue. Porcine pancreata may be a potential source of islets since pigs are inexpensive, readily available, and exhibit morphological and physiological characteristics comparable to humans. Recently, we developed a simple, standardized procedure for isolating large numbers of neonatal porcine islets with a reproducible and defined cellular composition. Following nine days of in vitro culture, tissue from one neonatal pig pancreas yielded approximately 50,000 islet cell aggregates, consisting of primarily epithelial cells (57%) and pancreatic endocrine cells (35%). In addition, neonatal porcine islets were responsive to glucose challenge in vitro and were capable of correcting hyperglycemia in alloxan-induced diabetic nude mice. Although neonatal porcine islets constitute an attractive alternative source of insulin-producing tissue for clinical transplantation, many aspects such as the immunological responses to these tissue and the latent period (2 to 8 weeks) between transplantation of these islets and the reversal of hyperglycemia need further investigation. This article discusses these issues and presents possible solutions to problems that may hinder the potential application of neonatal porcine islets for transplantation into patients with type 1 diabetes.

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.

Induction of complement attack on human cells by Gal(alpha1,3)Gal xenoantigen expression as a gene therapy approach to cancer

Galactose(alpha1,3)galactose on the surface of cells of non-primate organs is the major xenoantigen responsible for hyperacute rejection in xenotransplantation. The antigen is synthesised by (alpha1, 3)galactosyl transferase. Humans lack this enzyme and their serum contains high levels of pre-existing natural antibody which recognises the structure and activates complement. We have evaluated in vitro the potential for delivery of this enzyme to sensitise human cells to complement attack as a gene therapy approach to cancer. Retrovirus-mediated delivery of (alpha1,3)galactosyl transferase resulted in high level expression which led to serum-mediated lysis of five human cell targets, including endothelial and primary melanoma cells. Lysis was specific for those cells expressing the antigen in a mixed cell population. The mechanism of cell lysis mimicked that involved in hyperacute rejection: activation of the classical complement pathway by natural antibody specific for galactose(alpha1,3)galactose. The degree of lysis was determined by both the level of specific antibody and the expression of glycophosphatidylinositol-linked complement regulatory proteins. We conclude that expression of (alpha1,3)galactosyl transferase is a promising new therapeutic approach for cancer gene therapy, avoiding toxicity problems associated with application of prodrugs and with the potential to elicit further immunological responses.

The concept of glomerular self-defense

The balance between local offense factors and defense machinery determines the fate of tissue injury: progression or resolution. In glomerular research, the most interest has been on the offensive side, for example, the roles of leukocytes, platelets, complement, cytokines, eicosanoids, and oxygen radical intermediates. There has been little focus on the defensive side, which is responsible for the attenuation and resolution of disease. The aim of this review is to address possible mechanisms of local defense that may be exerted during glomerular injury. Cytokine inhibitors, proteinase inhibitors, complement regulatory proteins, anti-inflammatory cytokines, anti-inflammatory eicosanoids, antithrombotic molecules, and extracellular matrix proteins can participate in the extracellular and/or cell surface defense. Heat shock proteins, antioxidants, protein phosphatases, and cyclin kinase inhibitors may contribute to the intracellular defense. This article outlines how the glomerulus, when faced with injurious cells or exposed to pathogenic mediators, defends itself via the intrinsic machinery that is brought into play in resident glomerular cells.

Renal, central nervous system and pancreatic overexpression of recombinant soluble Crry in transgenic mice. A novel means of protection from complement-mediated injury

Crry is a potent complement regulator that inhibits classical and alternative pathway C3 convertases in rodents. We have produced transgenic animals expressing Crry as a recombinant soluble protein driven by the broadly active metallothionein-I promoter. These animals have high serum and urinary levels of rsCrry leading to inhibition of complement activity. In nephrotoxic serum nephritis (NSN), injected antibodies bind to glomeruli, leading to complement activation and subsequent glomerular injury and albuminuria. We have shown that rsCrry can block such injury and reduce albuminuria by as much as 75%. Corresponding to the reduction in albuminuria was the complete absence of C3 staining in glomeruli by immunofluorescence microscopy in 17/20 transgene positive animals. Support for a local source of protective rsCrry in this model is provided by the demonstration of Crry transgene mRNA in the glomerulus and a very high fractional excretion of rsCrry in the urine. Therefore, rsCrry expression markedly ameliorates an antibody-induced disease model in vivo. In addition, local synthesis of Crry in other organs that are targets of immune injury has been found. For example, Crry transgene mRNA is present throughout the central nervous system and in pancreatic islets. Thus, continuous complement inhibition at the C3 convertase step appears to be feasible and is effective in complement-mediated injury states. A number of disease models affecting these target organs can be tested using these mice.

Molecular remodeling of complement regulatory proteins for xenotransplantation

In pig-to-human discordant xenotransplantation, human complement is a major barrier against long survival of xenografts. Human complement regulatory proteins expressed on xenografts have been adapted as safeguards against host-induced hyperacute rejection of xenografts. For successful xenotransplantation, there have been many attempts to generate molecules with potent human complement regulatory activity but without activities related to harmful functions such as infection, immunosuppression and signal transduction devastating cellular homeostasis. Here, we summarize the strategy by which molecules for xenotransplantation should be designed and propose a GPI-anchored form of monomeric human C4bp as a candidate for efficient protection of swine xenografts from human complement attack.

Transfer of swine major histocompatibility complex class II genes into autologous bone marrow cells of baboons for the induction of tolerance across xenogeneic barriers

BACKGROUND

The present study examined the potential role of gene therapy in the induction of tolerance to anti-porcine major histocompatibility complex (SLA) class II-mediated responses after porcine renal or skin xenografts.

METHODS

Baboons were treated with a non-myeloablative or a myeloablative preparative regimen before bone marrow transplantation with autologous bone marrow cells retrovirally transduced to express both SLA class II DR and neomycin phosphotransferase (NeoR) genes, or the NeoR gene alone. Four months or more after bone marrow transplantation, the immunological response to a porcine kidney or skin xenograft was examined. Both the renal and skin xenografts were SLA DR-matched to the transgene, and recipients were conditioned by combinations of complement inhibitors, adsorption of natural antibodies, immunosuppressive therapy, and splenectomy.

RESULTS

Although the long-term presence of the SLA transgene was detected in the peripheral blood and/or bone marrow cells of all baboons, the transcription of the transgene was transient. Autopsy tissues were available from one animal and demonstrated expression of the SLA DR transgene in lymphohematopoietic tissues. After kidney and skin transplantation, xenografts were rejected after 8-22 days. Long-term follow-up of control animals demonstrated that high levels of induced IgG antibodies to new non-alphaGal epitopes developed after organ rejection. In contrast, induced non-alphaGal IgG antibody responses were minimal in the SLA DR-transduced baboons.

CONCLUSIONS

Transfer and expression of xenogeneic class II DR transgenes can be achieved in baboons. This therapy may prevent late T cell-dependent responses to porcine xenografts, which include induced non-alphaGal IgG antibody responses.

Identification of the individual residues that determine human CD59 species selective activity

Formation of the cytolytic membrane attack complex of complement on host cells is inhibited by the membrane-bound glycoprotein, CD59. The inhibitory activity of CD59 is species restricted, and human CD59 is not effective against rat complement. Previous functional analysis of chimeric human/rat CD59 proteins indicated that the residues responsible for the species selective function of human CD59 map to a region contained between positions 40 and 66 in the primary structure. By comparative analysis of rat and human CD59 models and by mutational analysis of candidate residues, we now identify the individual residues within the 40-66 region that confer species selective function on human CD59. All nonconserved residues within the 40-66 sequence were substituted from human to rat residues in a series of chimeric human/rat CD59 mutant proteins. Functional analysis revealed that the individual human to rat residue substitutions F47A, T51L, R55E, and K65Q each produced a mutant human CD59 protein with enhanced rat complement inhibitory activity with the single F47A substitution having the most significant effect. Interestingly, the side chains of the residues at positions 47, 51, and 55 are all located on the short single helix (residues 47-55) of CD59 and form an exposed continuous strip parallel to the helix axis. A single human CD59 mutant protein containing rat residue substitutions at all three helix residues produced a protein with species selective activity comparable to that of rat CD59. We further found that synthetic peptides spanning the human CD59 helix sequence were able to inhibit the binding of human CD59 to human C8, but had little effect on the binding of rat CD59 to rat C8.

Epitope mapping of 10 monoclonal antibodies against the pig analogue of human membrane cofactor protein (MCP)

Pig membrane cofactor protein (MCP; CD46) is a 50 000-60 000 MW glycoprotein that is expressed on a wide variety of cells, including erythrocytes. Pig MCP has cofactor activity for factor I-mediated cleavage of C3b and is an efficient regulator of the classical and alternative pathway of human and pig complement. A panel of 10 monoclonal antibodies (mAbs) was collected from two different laboratories; all of these mAbs were raised against pig leucocytes and all recognized the same complex banding pattern on sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) of erythrocyte membranes. All were shown to be reactive with pig MCP and were divided into four groups of mutually competitive antibodies based on competition studies for membrane-bound MCP and for soluble MCP, the latter by surface plasmon resonance (SPR) analysis. The antigenic properties of membrane-bound and soluble MCP were similar, although some interesting differences were revealed. None of the 10 mAbs were cross-reactive with human MCP and only one showed cross-reactivity with leucocytes from a panel of large mammals - a weak cross-reactivity with a subset of dog leucocytes. All antibodies in one of the epitope groups and some in a second epitope group were able to block the functional activity of pig MCP, as measured by inhibition of MCP-catalysed C3 degradation by factor I.

Human complement regulatory proteins protect swine lungs from xenogeneic injury

BACKGROUND

Pulmonary xenotransplantation is not possible because of hyperacute lung injury, the pathogenesis of which is unknown. This study evaluates complement-dependent pathways of pulmonary injury during heterologous perfusion of swine lungs.

METHODS

Lungs from unmodified swine and swine expressing human decay-accelerating factor and human CD59 (hDAF/hCD59 swine) were perfused with either human plasma or baboon blood. Pulmonary vascular resistance and static pulmonary compliance were measured serially, and swine lung tissue were examined by light microscopy. Complement activation was assessed by serial measurements of baboon plasma C3a-desArg concentrations.

RESULTS

Perfusion of unmodified swine lungs with human plasma and baboon blood resulted in hyperacute lung injury within minutes of perfusion. However, function was preserved in swine lungs expressing human decay-accelerating factor and human CD59. In both study groups, xenogeneic perfusion with baboon blood resulted in at least a sevenfold increase in plasma C3a-desArg levels suggesting transient activation of complement.

CONCLUSIONS

Lungs from swine expressing human decay-accelerating factor and human CD59 were resistant to injury during perfusion with human plasma and baboon blood, indicating that complement mediated some of the features of xenogeneic acute lung injury.

Chimerism and xenotransplantation. New concepts

In both transplant and infectious circumstances, the immune response is governed by migration and localization of the antigen. If the antigenic epitopes of transgenic xenografts are sufficiently altered to avoid evoking the destructive force of innate immunity, the mechanisms of engraftment should be the same as those that permit the chimerism-dependent immunologic confrontation and resolution that is the basis of allograft acceptance. In addition to "humanizing" the epitopes, one of the unanswered questions is whether the species restriction of complement described in 1994 by Valdivia and colleagues also necessitates the introduction of human complement regulatory genes in animal donors. Because the liver is the principal or sole source of most complement components, the complement quickly is transformed to that of the donor after hepatic transplantation. Thus, the need for complementary regulatory transgenes may vary according to the kind of xenograft used. Much evidence shows that physiologically important peptides produced by xenografts (e.g., insulin, clotting factors, and enzymes) are incorporated into the metabolic machinery of the recipient body. To the extent that this is not true, xenotransplantation could result in the production of diseases that are analogous to inborn errors of metabolism. In the climate of pessimism that followed the failures of baboon to human liver xenotransplantation in 1992-1993, it seemed inconceivable that the use of even more discordant donors, such as the pig, could ever be seriously entertained; however, this preceded insight into the xenogeneic and allogeneic barriers that has brought transplantation infectious immunity to common ground. With this new insight and the increasing ease of producing transgenic donors, the goal of clinical xenotransplantation may not be so distant.

Pediatric transplantation

Advances in organ preservation, surgical technique, and postoperative care have permitted the rapid development of liver transplantation in children. Consequently, the applicability of this procedure has gone beyond the treatment of life-threatening complications of chronic liver disease and now includes disabling morbidities and quality-of-life issues. The use of hepatic segments for transplantation with reduced or split cadaveric grafts and living-related donors has decreased the mortality of children awaiting liver transplantation. We are presently armed with a new potent immunosuppressive drug, tacrolimus, and an understanding that the migration and grafting of passenger leukocytes of bone marrow origin is the seminal explanation for allograft acceptance. The next forefront will involve manipulation of the process not only for the transplantation of already successful whole organs--such as the liver, kidney, pancreas, and heart--but also in the development of the intestinal transplantation program. Thus, augmentation of leukocyte traffic in unconditioned recipients of cadaver allografts with concomitant intravenous infusion of donor bone marrow cells under the same immunosuppressive management of tacrolimus-prednisone treatment will be the path into the future.

Comparative analysis of three genetic modifications designed to inhibit human serum-mediated cytolysis

Hyperacute rejection (HAR) remains a critical immunologic hurdle in the development of xenogeneic organs for human transplantation. Strategies that simultaneously eliminate both natural antibody reactivity and complement activation on the xenogeneic cell surface may be the best approach to achieve clinical application of xenogeneic vascularized organ transplantation. We have developed multiple lines of genetically manipulated mice to evaluate the combination of different genetic approaches aimed at inhibiting antibody and complement-mediated cell lysis. We utilized transgenic mice expressing the human complement inhibitor, CD59, the human 1,2-fucosyltransferase (H-transferase, HT) and the alpha1,3-galactosyltransferase (alpha1,3-GT) knock-out mouse line (Gal KO). Our data show that expression of hCD59 in combination with HT expression or the null phenotype of alpha1,3-GT are equally effective at preventing human serum-mediated cytolysis. Interestingly, the triple combination affords no additional protective effect. Therefore, coexpression of HT and a complement inhibitor is the most immediate strategy to genetically engineer transgenic pigs to be used as xenogeneic donors.

High-level porcine endothelial cell expression of alpha(1,2)-fucosyltransferase reduces human monocyte adhesion and activation

BACKGROUND

Monocyte binding to and activation by human endothelium requires a number of interactions, including those involving sialylated endothelial cell ligands. As porcine endothelial cell transfection with alpha(1,2)-fucosyltransferase has been shown to reduce terminal sialylation, we investigated whether high-level expression of alpha(1,2)-fucosyltransferase by porcine endothelium would reduce human monocyte adhesion and functional activation.

METHOD

Purified human monocytes were labeled with 51Cr, and measured for adherence to human or porcine endothelial cell monolayers in the presence of either medium or monoclonal antibodies against monocyte lectins or sialylated endothelial cell ligands. Monocyte production of prostaglandin E2 (PGE2) and interleukin-1beta (IL-1beta) was measured by enzyme-linked immunosorbent assay, using supernatants collected from cultures performed between human monocytes and human or porcine endothelial cell monolayers. Finally, monocyte adhesion and activation were measured after culture with a porcine endothelial cell line transfected with alpha(1,2)-fucosyltransferase, expressing reduced surface expression of terminal Gal alpha(1,3)-Gal and sialic acid residues.

RESULTS

Human monocytes adhered by 50% higher levels to porcine endothelium than to human endothelium. This increased level of adherence was associated with augmented monocyte activation, as defined by 3.3-fold higher levels of PGE2 production and 7.3-fold higher levels of IL-1beta production. Monoclonal antibodies against CD62L (L-selectin) on monocytes or CD15s (sialylated Lewis X) on porcine endothelium reduced monocyte adhesion by 38% and 52%, respectively. Porcine endothelial cell transfection with alpha(1,2)-fucosyltransferase reduced terminal sialic acid expression by 65%, monocyte adherence by 50%, and the production of PGE2 and IL-1beta by 67% and 38%, respectively.

CONCLUSIONS

Together, these results demonstrate that human monocytes use surface lectins to bind to sialylated carbohydrate structures on porcine endothelium, and indicate that reduction in porcine endothelial cell surface expression of terminally sialylated structures by high-level alpha(1,2)-fucosyltransferase activity reduces monocyte adherence and activation.

Role of decay-accelerating factor in regulating complement activation on the erythrocyte surface as revealed by gene targeting

Decay-accelerating factor (DAF) is a glycosylphosphatidylinositol (GPI)-anchored membrane protein that inhibits both the classical and the alternative pathways of complement activation. DAF has been studied extensively in humans under two clinical settings: when absent from the erythrocytes of paroxysmal nocturnal hemoglobinuria (PNH) patients, who suffer from complement-mediated hemolytic anemia, and in transgenic pigs expressing human DAF, which have been developed to help overcome complement-mediated hyperacute rejection in xenotransplantation. Nevertheless, the exact role of DAF in regulating complement activation in vivo on the cell surface and the species specificity of this molecule remain to be fully characterized. To address these issues, we have used gene targeting to produce mice lacking GPI-anchored DAF. We found that erythrocytes from mice deficient in GPI-anchored DAF showed no increase in spontaneous complement activation in vivo but exhibited impaired regulation of zymosan-initiated bystander and antibody-triggered classical pathway complement activation in vitro, resulting in enhanced complement deposition. Despite a high level of C3 fixation, no homologous hemolysis occurred. It is noteworthy that GPI-linked DAF knockout erythrocytes, when tested with human and guinea pig sera, were more susceptible to heterologous complement lysis than were normal erythrocytes. These results suggest that DAF is capable of regulating homologous as well as heterologous complement activation via the alternative or the classical pathway. They also indicate that DAF deficiency alone is not sufficient to cause homologous hemolysis. In contrast, when the assembly of the membrane-attack complex is not properly regulated, as in the case of heterologous complement activation or in PNH patients, impaired erythrocyte DAF activity and enhanced C3 deposition could lead to increased hemolytic reaction.

High level transcription of the complement regulatory protein CD59 requires an enhancer located in intron 1

CD59 is a complement regulatory protein and may also act as a signal-transducing molecule. CD59 transgenic mice have been generated using a CD59 minigene (CD59 minigene-1). Although this minigene contained a 4.6-kilobase pair 5'-flanking region from the human CD59 gene as a promoter, the expression levels of the CD59 mRNA were substantially lower than those observed in humans, suggesting that CD59 gene expression might also require other transcriptional regulatory elements such as an enhancer. To investigate the transcriptional regulation of the CD59 gene, we used three cell lines that express CD59 at different levels. We have identified DNase I-hypersensitive sites in intron 1 in HeLa cells, which express CD59 at high levels, but not in Jurkat (intermediate level) or Raji cells (low level). Furthermore, cell line-specific enhancer activity was detected in a fragment containing these DNase I-hypersensitive sites. The CD59 enhancer was mapped to between -1155 and -888 upstream of the 5'-end of exon 2. To investigate the enhancer activity in vivo, a new CD59 minigene was constructed by the addition of the enhancer fragment into CD59 minigene-1. High expressor CD59 transgenic mice were generated using the new minigene.

Murine mentors: transgenic and knockout models of surgical disease

OBJECTIVE

Transgenic and knockout technologies have emerged from the "molecular biology revolution" as unprecedented techniques for manipulating gene function in intact mice. The goals of this review are to outline the techniques of creating transgenic and knockout mice, and to demonstrate their use in elucidation of the molecular mechanisms underlying common surgical diseases.

SUMMARY BACKGROUND DATA

Gain of gene function is created by transgenic technology, whereas gene function is ablated using gene knockouts. Each technique has distinctive applications and drawbacks. A unique feature of genetically manipulated mice is that combinatorial genetic experiments can be executed that precisely define the functional contribution of a gene to disease progression. Transgenic and knockout mouse models of wound healing, cardiovascular disease, transplant immunology, gut motility and inflammatory bowel disease, and oncology are beginning to illuminate the precise molecular regulation of these diseases. Transgenic technology has also been extended to larger mammals such as pigs, with the goal of using genetic manipulation of the xenogenic immune response to increase the availability of transplant organs. Continual refinements in gene manipulation technology in mice offer the opportunity to turn genes on or off at precise time intervals and in particular tissues, according to the needs of the investigator. Ultimately, investigation of disease development and progression in genetically manipulated mammals may delineate new molecular targets for drug discovery and provide novel platforms for drug efficacy screens.

CONCLUSIONS

Emulation of human disease and therapy using genetically manipulated mammals fulfills a promise of molecular medicine: fusion of molecular biochemistry with "classical" biology and physiology. Surgeons have unique skills spanning both worlds that can facilitate their success in this expanding arena.

Targeting of functional antibody-CD59 fusion proteins to a cell surface

Complement is involved in the pathogenesis of many diseases, and there is great interest in developing inhibitors of complement for therapeutic application. CD59 is a natural membrane-bound inhibitor of the cytolytic complement membrane attack complex (MAC). In this study, the preparation and characterization of antibody-CD59 (IgG-CD59) chimeric fusion proteins are described. Constructs were composed of soluble CD59 fused to an antibody-combining site at the end of CH1, after the hinge (H), and after CH3 Ig regions. The antigen specificity of each construct was for the hapten 5-dimethylamino-naphthalene-1-sulfonyl (dansyl). Correct folding of each IgG-CD59 fusion partner was indicated by recognition with anti-CD59 antibodies specific for conformational determinants and by IgG-CD59 binding to dansyl. The IgG-CD59 fusion proteins all bound specifically to dansyl-labeled Chinese hamster ovary cells and provided targeted cells, but not untargeted cells, with effective protection from complement-mediated lysis. Data indicate that CD59 must be positioned in close proximity to the site of MAC formation for effective function, and that modes of membrane attachment other than glycophosphatidylinositol linkage can affect CD59 functional activity.

Infection and xenotransplantation. Developing strategies to minimize risk

Infection in transplantation results from interaction between the level of immune suppression and the epidemiologic exposures of the recipient. "Xenosis," infection in xenotransplantation, may be increased beyond that of allotransplantation because: (1) the xenograft may serve as a permissive focus of infection for donor-derived organisms; (2) these organisms may be unknown or xenotropic; (3) microbiologic assays may be unavailable; (4) clinical syndromes due to such novel pathogens may not be recognized; (5) the necessary level of immune suppression may be greater than for allotransplantation; (6) donor-derived organisms may acquire new (e.g., genetic) characteristics in the human host; (7) the presence of immune suppression and the high, intrinsic rate of infection may mask the presence of xenosis; and (8) MHC-incompatibility may reduce the efficacy of the immune response within the xenograft. Because immunocompromised individuals are sentinels for infection by many types of novel infectious agents, and because there is some unknown level of risk that such pathogens will spread to the general population, microbiologic studies must be initiated in tandem with preclinical and clinical studies of xenotransplantation.

The future promises of xenotransplantation

The use of animals as a source of organs and tissues for humans has been an enduring goal of transplantation. Xenotransplantation, as such, would overcome a shortage of human donors and allow for biochemical or genetic approaches to modification of transplants. The use of animal organs and tissue, however, is hindered by an intense immune response of the recipient against the graft. The molecular basis for this immune response has recently been elucidated, at least in part, and specific approaches to therapy, including the genetic engineering of source animals, have been developed. Other hurdles, including the physiologic limitations of the transplant and the possibility of transferring infectious agents from the transplant into the host, may also be important. The development of specific therapies and the application of genetic engineering to overcome these problems can now be envisioned. As the immunologic, physiologic, and infectious hurdles to xenotransplantation are addressed, new efforts will focus on the use of the transplant to impart novel functions to answer the therapeutic needs of the transplant recipient.

The Role of Adhesion Molecules in Human Leukocyte Attachment to Porcine Vascular Endothelium: Implications for Xenotransplantation

Many obstacles still prevent successful xenotransplantation of porcine donor organs. When hyperacute rejection is averted, transplanted pig organs are subject to acute vascular and cellular rejection. In autologous systems, leukocyte recruitment into inflamed tissues involves selectins, integrins, and Ig family members. To determine whether these mechanisms allow human leukocytes to effectively enter porcine grafts, the pathways by which human leukocytes adhere to TNF-α-stimulated porcine aortic endothelium were examined under static and physiologic flow conditions. L-selectin and E-selectin had overlapping functions in neutrophil capture and rolling, whereas Ab blockade of E-selectin and the β2 integrins inhibited firm arrest of rolling neutrophils. Combined blockade of selectins and β2 integrins resulted in negligible human neutrophil attachment to pig endothelium. Lymphocyte attachment to porcine endothelium was primarily L-selectin mediated, whereas β2 integrin and VCAM-1/very late Ag-4 (VLA-4) interactions promoted static adhesion. Concurrent β2 integrin, VLA-4, VCAM-1, and L-selectin blockade completely inhibited lymphocyte attachment. Thus, interactions between leukocyte-endothelial cell adhesion receptor pairs remained remarkably intact across the human-porcine species barrier. Moreover, disrupting the adhesion cascade may impair the ability of human leukocytes to infiltrate a transplanted porcine organ during rejection.

Heterogeneous expression of Gal alpha1-3Gal xenoantigen in pig kidney: a lectin and immunogold electron microscopic study

BACKGROUND

The Gal alpha1-3Gal antigen (Gal alpha) is the primary target for human natural anti-pig xenoantibodies. The presence of Gal alpha has been shown in porcine endothelial cells (ECs) using light microscopy, whereas the expression of Gal alpha in other cell structures in the porcine kidney is only partially characterized.

METHODS

Immunogold electron microscopy of pig kidney cryosections was performed using Griffonia simplicifolia isolectin B4 and affinity isolated human anti-Gal alpha1-3Gal antibodies.

RESULTS

The most intense expression of Gal alpha was found on the apical and basolateral portions of the plasma membrane of the proximal convoluted tubule segments 1 and 2 cells, whereas segment 3 and 4 cells were negative. A strong staining was found in peritubular capillary ECs and in the inner medullary and papillary collecting duct cells. Moderate labeling of ECs and subendothelium was observed in large blood vessels, whereas glomerular ECs reacted weakly. Additionally, glomerular parietal epithelial cells, connecting tubule cells, and some cortical collecting duct cells were labeled. Among interstitial cells, a part of type-1 cells and all type-2 cells were labeled, whereas others were negative.

CONCLUSIONS

By immune electron microscopy, a detailed information of the Gal alpha antigen distribution in porcine nephrons and blood vessels has been revealed, which clarifies conflicting data obtained by light microscopy. In addition, expression of the Gal alpha antigen in the renal interstitial cells was documented for the first time. These data are of importance for the understanding of xenoantibody-mediated hyperacute rejection, for interpretation of pig kidney xenograft biopsies, and for generating transgenic pigs lacking the Gal alpha epitope.

Expression and functional analysis of glycosyl-phosphatidyl inositol-linked CD46 in transgenic mice

BACKGROUND

Complement activation plays a pivotal role in hyperacute xenograft rejection. In humans, activation of complement is regulated by a number of cell surface regulatory proteins. Membrane cofactor protein (CD46) is one such regulator that protects cells by acting as a cofactor for the factor I-mediated cleavage of C3b and C4b. Transgenic animals expressing human CD46 may provide organs that are resistant to complement attack. However, attempts to generate mice expressing human CD46 using cDNA-based constructs have been largely unsuccessful.

METHODS

Transgenic mice expressing a glycosylphosphatidyl inositol (GPI)-linked form of CD46 were generated by microinjection of a hybrid CD46/CD55 cDNA under the control of the human intercellular adhesion molecule-2 promoter. Expression of CD46-GPI on the vascular endothelium was determined by immunohistochemistry. The ability of CD46-GPI to protect mouse tissues from human complement attack was determined using an ex vivo isolated perfused heart model.

RESULTS

Three founder animals expressing CD46-GPI were identified. Histological analysis showed strong and uniform expression of CD46-GPI on the vascular endothelium of all organs examined. Ex vivo perfusion of transgenic mouse hearts with human plasma showed a reduction in C3c deposition and a slightly prolonged function compared with controls.

CONCLUSIONS

High-level expression of CD46-GPI was achieved in transgenic mice by using a modified cDNA-based construct. The CD46-GPI was functional, providing some protection from complement-mediated damage in the ex vivo model, and may be useful in xenotransplantation if expressed in combination with CD55 and CD59.

Expression of hirudin fusion proteins in mammalian cells: a strategy for prevention of intravascular thrombosis

BACKGROUND

Intravascular thrombosis occurs in disorders of diverse pathogeneses, including allograft and xenograft rejection. In this in vitro study, we describe an approach for tethering the specific thrombin inhibitor hirudin to plasma membranes as part of a genetic strategy for regulating intravascular coagulation.

METHODS AND RESULTS

An HLA class I leader sequence was fused with hirudin linked to domains 3 and 4 of human CD4 and intracytoplasmic sequence from either CD4 or human P-selectin. The constructs were transfected into mouse fibroblasts, Chinese hamster ovary (CHO)-K1 cells, immortalized porcine endothelial cells (IPECs), and a pituitary secretory cell line (D16/16). Thrombin binding to the hirudin fusion proteins expressed on fibroblasts and CHO-K1 cells could be blocked by an anti-hirudin monoclonal antibody and by pretreatment of thrombin with either the synthetic tripeptide thrombin inhibitor PPACK or native hirudin. Hirudin expression significantly modified the procoagulant phenotype of IPECs in human plasma, leading to prolongation of clotting times. Hirudin-CD4-P-selectin fusion proteins accumulated in adrenocorticotropic hormone-containing granules in D16/16 cells, with no cell surface expression except on activation with phorbol ester, when hirudin relocated to the outer membrane.

CONCLUSIONS

Hirudin fusion proteins were expressed on mammalian cells, where they reduced local thrombin levels and inhibited fibrin formation. Regulated expression was achieved on activated cells by use of the cytoplasmic sequence from P-selectin. In vivo, these fusion proteins may prove useful transgenic or gene therapy agents for preventing intravascular thrombosis.

Complement regulatory proteins in glomerular diseases

Complement activation plays a critical role in the pathogenesis of many forms of glomerulonephritis. Complement activation leads to tissue injury through various mechanisms including the generation of chemotactic factors and activation of the resident glomerular cells following C5b-9 insertion. Recent advances have disclosed the mechanisms of regulation of complement activation by discovery of a number of complement regulatory proteins. Decay accelerating factor (DAF), membrane cofactor protein (MCP), and complement receptor type 1 (CR1) act by inactivating C3/C5 convertase. They belong to the gene superfamily known as the regulators of complement activation (RCA), and share a common structural motif called a short consensus repeat (SCR). In contrast, CD59 works by inhibiting formation of C5b-9. The glomerulus is particularly well endowed with these membrane-bound complement regulatory proteins. DAF, MCP, and CD59 are ubiquitously expressed by all three resident glomerular cells, while CR1 is localized exclusively in podocytes. Expression of complement regulatory proteins can be changed by many factors including complement attack itself, and their expression levels are affected in various glomerular disorders. Studies utilizing cultured glomerular cells and animal models of glomerular diseases suggest important protective roles of complement regulatory proteins against immune-mediated renal injury. Recent progress in molecular biological techniques has made new therapeutic strategy feasible. Systemic administration of soluble recombinant complement regulatory proteins and local overexpression of complement regulatory proteins are promising therapeutic approaches.

HLA antibodies present in the sera of sensitized patients awaiting renal transplant are also reactive to swine leukocyte antigens

BACKGROUND

To determine whether preformed HLA alloantibodies present in the sera of patients awaiting kidney transplantation will be detrimental to a potential porcine xenograft, we tested their cross-reactivity to swine leukocyte antigens (SLA).

METHODS

Sera obtained from patients with varying levels of HLA sensitization (high panel-reactive antibodies > 70%, n= 7; moderate panel-reactive antibodies 30-40%, n=2) were analyzed. Pooled normal human AB sera and sera from nonsensitized patients (n=3) served as negative control. IgG was purified by protein-G chromatography, and xenoreactive natural antibodies (XNA) were depleted by passing the IgG through a series of melibiose and thyroglobulin-agarose columns. The elimination of XNA from HLA IgG preparations was confirmed by GS-IB4 lectin blocking assay and by an ELISA.

RESULTS

IgG isolated from normal AB serum and three nonsensitized patients, which was depleted of XNA (HLA-IgG), did not react to human or porcine lymphocytes (peripheral blood mononuclear cells; PBMC) either by flow cytometry or by complement-dependent microcytotoxicity assays. However, HLA-IgG isolated from nine sensitized patients were reactive to a panel of porcine peripheral blood lymphocytes (n=6) by flow cytometry (>50 mean channel shift) and in complement-dependent microcytotoxicity assays in addition to their reactivity to human PBMC. The binding of HLA-IgG to porcine PBMC was significantly reduced by preabsorption with pooled human platelet concentrate. Further, the HLA IgG showed recognition of 45-kDa affinity-purified SLA class I on Western blots.

CONCLUSIONS

This study demonstrates that HLA antibodies present in the sera of sensitized individuals can cross-react with SLA. Thus, xenotransplantation of porcine organs into HLA-sensitized patients has the potential to be rejected by humoral mechanisms. Testing to avoid such cross-reactive antibodies should be considered.

The promise of xenografting

Advances in understanding the rejection of foreign tissues has renewed enthusiasm about the possibility of overcoming the present organ shortage by transplanting porcine kidneys into humans. Currently, three known forms of organ rejection stand as obstacles to xenotransplantation as a clinical reality--hyperacute rejection, acute vascular rejection, and cellular rejection. Progress in the knowledge of xenoreactive antibodies and of complement, along with new transgenic technologies, have enabled researchers to overcome hyperacute rejection in xenografts. The advances have brought into focus such issues as the ability of the porcine kidney to replace the physiologic functions of the human kidney and the risk associated with the potential transmission of infectious agents from animals to humans. Despite the remaining hurdles to clinical application of xenotransplantation, the rapid pace of research and emerging technologies would seem to make xenotransplantation a renal replacement therapy of great promise.

Expression of porcine major histocompatibility antigens in cardiac tissue

In this study we have used monoclonal antibodies directed against antigens of the major histocompatibility complex (MHC) class I and class II to reveal the detailed cellular distribution of swine lymphocyte alloantigens (SLA) in cardiac tissue. By applying a sensitive immunophosphatase staining reaction we detected the ubiquitous expression of SLA class I and class II on the vascular endothelium. Endothelial cells of capillaries and blood vessels were intensely stained structures in the examined swine hearts. Similar staining patterns were observed in control experiments with anti-von Willebrand factor serum and Dolichus biflorus lectin used as immunohistochemical markers for endothelial cells. The luminal layer of studied pulmonary valves exhibited a strong staining reaction with anti-SLA class I and class II antibodies. In contrast, normal cardiac myocytes failed to express immunodetectable amounts of either of the SLA determinants. Intercalated discs in porcine heart tissue did not react with either anti-SLA class I or class II antibodies. Our data showing the abundant expression of SLA molecules on endothelial cells in normal swine heart may have physiological implications in cell-mediated rejection occurring in xenotransplanted hearts.

Discordant organ xenotransplantation in primates: world experience and current status

The pig-to-primate model is increasingly being utilized as the final preclinical means of assessing therapeutic strategies aimed at allowing discordant xenotransplantation. We review here the world experience of both pig-to-human and pig-to-nonhuman primate organ transplantation. Eight whole organ transplants using discordant mammalian donors have been carried out in human recipients; only one patient was reported (in 1923) to have survived for longer than 72 hr. Therapeutic approaches in the experimental laboratory setting have included pharmacologic immunosuppression, antibody and/or complement depletion or inhibition, the use of pig organs transgenic for human complement regulatory proteins, and conditioning regimens aimed at inducing a state of tolerance or specific immunologic hyporesponsiveness. The greatest success to date has been obtained with methods that inhibit complement-mediated injury, either by the administration of cobra venom factor or soluble complement receptor I to the recipient (with organ survival up to 6 weeks) or by the use of donor organs transgenic for human decay-accelerating factor (with organ survival up to 2 months). The future of xenotransplantation may lie in the judicious combination of current approaches.

Induction of Species-Specific Host Accommodation in the Hamster-to-Rat Xenotransplantation Model

The combination of two immunosuppressants, leflunomide and cyclosporin A (CsA), completely inhibits immune xenoreactions in the hamster-to-Lewis rat xenotransplantation model. In addition, the control of acute xenograft rejection with this combination of immunosuppressants subdues early T-independent xenoreactivity and uncovers a late immune response that can be controlled by CsA alone. We attribute this acquired responsiveness to CsA to a modification in the recipient’s humoral response to the xenograft, and refer to this change as host accommodation. Host accommodation can be induced in Lewis rats receiving hamster hearts by the combination of leflunomide and CsA. A 7-day treatment with leflunomide and CsA was able to convert xenoreactivity from one that was resistant to CsA treatment into one that was controlled by CsA. The presence of the hamster xenograft was critical for the induction of host accommodation since the immunosuppressive regimen, either alone or in combination with a transfusion with donor-specific spleen cells, was unable to modify the anti-hamster reactivity in Lewis rats. When accommodation was induced in the presence of hamster hearts, these accommodated rats were able to acutely reject third-party mouse hearts while under CsA therapy, thus indicating that the host accommodation is species specific. Finally, we demonstrate that host accommodation is associated with a loss in the ability to produce species-specific, T-independent xenoantibodies. These novel observations suggest that xenoreactive T-independent humoral responses can be deleted selectively without significant loss of other innate, Ag-specific T-independent humoral responses.

Extracorporeal ("ex vivo") connection of pig kidneys to humans. III. Studies of plasma complement activation and complement deposition in the kidney tissue

The complement system is one of the important factors involved in the hyperacute rejection of xenografts. This report deals with the activation of the complement system in a clinical trial where pig kidneys were extracorporeally connected to two volunteer dialysis patients who were pretreated with plasmapheresis in order to substantially reduce anti-pig xenoantibodies. The clinical data of the perfusion experiments and the patients humoral immune response to pig xenoantigens have been reported in detail (Xenotransplantation 1996; 3:328-339, 340-353). Three consecutive daily plasmapheresis treatments of the patients reduced the plasma complement protein (C3, C4, and C5) concentrations to 8-27% of the baseline values. The perfusion of the pig kidney connected to patient 1 was terminated at 65 min due to graft rejection and this patient was not hemodynamically affected by the experiment. The second experiment was terminated at 15 min due to an anaphylactic like reaction of the patient. In patient 1 a slight reduction of plasma C3, C4, and C5 and an increase of C5a and SC5b-9 occurred, while C3a decreased during the perfusion. Patient 2 had an increase of all complement parameters, most prominent for C4d and SC5b-9, which occurred concomitant with the appearance of the anaphylactic like side effects. In general, plasma levels of PMN elastase, IL6 and IL8 increased in both patients during the perfusion. Immunohistochemical investigation of the kidney tissues revealed deposition of human complement factors C1q, C4c, and C3c in a congruent pattern with the vasculature of the kidney in patient 1. In kidney 2 only trace amounts of C1q and C3c were found. Both kidneys were negative for properdin. Therefore, in this experimental set up with extracorporeal connection of pig kidneys to the human circulation the human complement cascade is activated mainly through the classical pathway.

Reduction of the major swine xenoantigen, the alpha-galactosyl epitope by transfection of the alpha2,3-sialyltransferase gene

alpha2,3-Sialyltransferase represents a putative enzyme that reduces the Galalpha1-3Gal beta1-4GlcNAc-R (the alpha-galactosyl epitope) by intracellular competition with alpha1,3-galactosyltransferase for a common acceptor substrate. This study demonstrates that the overexpression of the alpha2,3-sialyltransferase gene suppresses the antigenicity of swine endothelial cells to human natural antibodies by 77% relative to control cells and by 30% relative to cells transfected with alpha1,2-fucosyltransferase, and in addition, it reduces the complement-mediated cell lysis by 75% compared with control cells and by 22% compared with cells transfected with alpha1, 2-fucosyltransferase. The mechanism by which the alpha-galactosyl epitope was reduced was also studied. Suppression of alpha1, 3-galactosyltransferase activity by 30-63% was observed in the transfectants with alpha2,3-sialyltransferase, and mRNA expression of the alpha1,3-galactosyltransferase gene was reduced as well. The data suggest that the alpha2,3-sialyltransferase effectively reduced the alpha-galactosyl epitope as well as or better than the alpha1, 2-fucosyltransferase did and that the reduction of the alpha-galactosyl epitope is due not only to substrate competition but also to an overall reduction of endogenous alpha1, 3-galactosyltransferase enzyme activity.

Complement-mediated pulmonary xenograft injury: studies in swine-to-primate orthotopic single lung transplant models

BACKGROUND

The pathogenesis of acute pulmonary xenograft injury has not yet been determined. The present study evaluates the role of complement in mediating pulmonary xenograft dysfunction by using cobra venom factor (CVF) to deplete recipient complement and transgenic swine, which express human regulators of complement activation (human decay-accelerating factor [hDAF] and hCD59).

METHODS

Fifteen orthotopic lung transplants were performed as follows: group I, swine-to-swine (n=5); group II, unmodified swine-to-baboon (n=3); group III, unmodified swine-to-(CVF treated) baboon (n=3); and group IV, hCD59/hDAF swine-to-baboon (n=4). Left pulmonary artery flow and pulmonary vascular resistance were measured at 30-min intervals. Serial lung biopsies were examined by light microscopy and immunofluorescence. The activation of complement was quantified by measurement of baboon plasma CH50 and C4 functional activity.

RESULTS

Group II xenotransplants ceased functioning within 30 min of reperfusion. Histopathologic ab normalities included erythrocyte/platelet aggregates and hemorrhagic pulmonary edema. Groups I and IV showed excellent function throughout. hDAF/hCD59 lungs (group IV) showed trace venular fibrin plugs and moderate loss of alveolar architecture. Pretreatment with CVF (group III) was ineffective in preventing xenograft injury.

CONCLUSIONS

These results characterize the fundamental features of discordant pulmonary xenotransplantation. Correction of the known defects in the regulation of heterologous complement activation was partially effective in preventing pulmonary xenograft dysfunction, suggesting that complement mediates, in part, some of the features of acute lung injury after discordant lung xenotransplantation.

Identity of the residues responsible for the species-restricted complement inhibitory function of human CD59

The membrane-anchored glycoprotein CD59 inhibits assembly of the C5b-9 membrane attack complex (MAC) of human complement. This inhibitory function of CD59 is markedly selective for MAC assembled from human complement components C8 and C9, and CD59 shows little inhibitory function toward MAC assembled from rabbit and many other non-primate species. We have used this species selectivity of CD59 to identify the residues regulating its complement inhibitory function: cDNA of rabbit CD59 was cloned and used to express human/rabbit CD59 chimeras in murine SV-T2 cells. Plasma membrane expression of each CD59 chimera was quantified by use of a 5'-TAG peptide epitope, and each construct was tested for its ability to inhibit assembly of functional MAC from human versus rabbit C8 and C9. These experiments revealed that the species selectivity of CD59 is entirely determined by sequence contained between residues 42 and 58 of the human CD59 polypeptide, whereas chimeric substitution outside this peptide segment has little effect on the MAC inhibitory function of CD59. Substitution of human CD59 residues 42-58 into rabbit CD59 resulted in a molecule that was functionally indistinguishable from native human CD59, whereas the complementary construct (corresponding residues of rabbit CD59 substituted into human CD59) was functionally indistinguishable from rabbit CD59. Based on the solved solution structure of CD59, these data suggest that selectivity for human C8 and C9 resides in a cluster of closely spaced side chains on the surface of CD59 contributed by His44, Asn48, Asp49, Thr51, Thr52, Arg55, and Glu58 of the polypeptide.