Removal of terminal alpha-galactosyl residues from xenogeneic porcine endothelial cells. Decrease in complement-mediated cytotoxicity but persistence of IgG1-mediated antibody-dependent cell-mediated cytotoxicity

Statistical optimization and sequential scale-up of α-galactosidase production by Actinoplanes utahensis B1 from shake flask to pilot scale

α-Galactosidase (α-GAL) is a class of hydrolase that releases galactose from galacto-oligosaccharides and synthetic substrates such as pNPG. In this study, the production of α-GAL by Actinoplanes utahensis B1 in submerged fermentation was enhanced by using statistical methods. The effects of temperature, pH, and inoculum percentage on enzyme secretion were optimized using BBD of RSM. The optimized process was scaled up from the shake flask to the laboratory scale (5 L) and to pilot scale (30 L) using KLa based scale-up strategy. By using BBD, a maximum yield of 62.5 U/mL was obtained at a temperature of 28 °C, a pH of 6.9, and an inoculum of 6.4%. Scale-up was performed successfully and achieved a yield of 74.4 U/mL and 76.8 U/mL in laboratory scale and pilot scale fermenters. The TOST was performed to validate the scale-up strategy and the results showed a confidence level of 95% for both scales indicating the perfect execution of scale-up procedure. Through the implementation of BBD and scale-up strategy, the overall enzyme yield has been significantly increased to 76%. This is the first article to explore the scale-up of α-GAL from the A. utahensis B1 strain and provide valuable insights for industrial applications.

The Role of NK Cells in Pig-to-Human Xenotransplantation

Recruitment of human NK cells to porcine tissues has been demonstrated in pig organs perfused ex vivo with human blood in the early 1990s. Subsequently, the molecular mechanisms leading to adhesion and cytotoxicity in human NK cell-porcine endothelial cell (pEC) interactions have been elucidated in vitro to identify targets for therapeutic interventions. Specific molecular strategies to overcome human anti-pig NK cell responses include (1) blocking of the molecular events leading to recruitment (chemotaxis, adhesion, and transmigration), (2) expression of human MHC class I molecules on pECs that inhibit NK cells, and (3) elimination or blocking of pig ligands for activating human NK receptors. The potential of cell-based strategies including tolerogenic dendritic cells (DC) and regulatory T cells (Treg) and the latest progress using transgenic pigs genetically modified to reduce xenogeneic NK cell responses are discussed. Finally, we present the status of phenotypic and functional characterization of nonhuman primate (NHP) NK cells, essential for studying their role in xenograft rejection using preclinical pig-to-NHP models, and summarize key advances and important perspectives for future research.

Targeting tumor tolerance: A new hope for pancreatic cancer therapy?

With a 5-year survival rate of just 8%, pancreatic cancer (PC) is projected to be the second leading cause of cancer deaths by 2030. Most PC patients are not eligible for surgery with curative intent upon diagnosis, emphasizing a need for more effective therapies. However, PC is notoriously resistant to chemoradiation regimens. As an alternative, immune modulating strategies have recently achieved success in melanoma, prompting their application to other solid tumors. For such therapeutic approaches to succeed, a state of immunologic tolerance must be reversed in the tumor microenvironment and that has been especially challenging in PC. Nonetheless, knowledge of the PC immune microenvironment has advanced considerably over the past decade, yielding new insights and perspectives to guide multimodal therapies. In this review, we catalog the historical groundwork and discuss the evolution of the cancer immunology field to its present state with a specific focus on PC. Strategies currently employing immune modulation in PC are reviewed, specifically highlighting 66 clinical trials across the United States and Europe.

Transgenic pigs designed to express human α-galactosidase to avoid humoral xenograft rejection

The use of animals as a source of organs and tissues for xenotransplantation can overcome the growing shortage of human organ donors. However, the presence of xenoreactive antibodies in humans directed against swine Gal antigen present on the surface of xenograft donor cells leads to the complement activation and immediate xenograft rejection as a consequence of hyperacute reaction. To prevent hyperacute rejection, it is possible to change the swine genome by a human gene modifying the set of donor’s cell surface proteins. The gene construct pGal-GFPBsd containing the human gene encoding α-galactosidase enzyme under the promoter of EF-1α elongation factor ensuring systemic expression was introduced by microinjection into a male pronucleus of the fertilised porcine oocyte. As a result, the founder male pig was obtained with the transgene mapping to chromosome 11p12. The polymerase chain reaction (PCR) analysis revealed and the Southern analysis confirmed transgene integration estimating the approximate number of transgene copies as 16. Flow cytometry analysis revealed a reduction in the level of epitope Gal on the cell surface of cells isolated from F0 and F1 transgenic animals. The complement-mediated cytotoxicity assay showed increased viability of the transgenic cells in comparison with the wild-type, which confirmed the protective influence of α-galactosidase expression.

Addition of algenpantucel-L immunotherapy to standard adjuvant therapy for pancreatic cancer: a phase 2 study

BACKGROUND

Despite continued investigation, limited progress has been made in the adjuvant treatment of resected pancreatic cancer. Novel or targeted therapies are needed.

METHODS

Multi-institutional, open-label, dose-finding, phase 2 trial evaluating the use of algenpantucel-L (NewLink Genetics Corporation, Ames, IA) immunotherapy in addition to chemotherapy and chemoradiotherapy in the adjuvant setting for resected pancreatic cancer (ClinicalTrials.gov identifier, NCT00569387). The primary outcome was 12-month disease-free survival. Secondary outcomes included overall survival and toxicity.

RESULTS

Seventy patients were treated with gemcitabine and 5-fluorouracil-based chemoradiotherapy as well as algenpantucel-L (mean 12 doses, range 1-14). After a median follow-up of 21 months, the 12-month disease-free survival was 62 %, and the 12-month overall survival was 86 %. The most common adverse events were injection site pain and induration.

CONCLUSIONS

The addition of algenpantucel-L to standard adjuvant therapy for resected pancreatic cancer may improve survival. A multi-institutional, phase 3 study is ongoing (ClinicalTrials.gov identifier, NCT01072981).

Porcine cells express more than one functional ligand for the human lymphocyte activating receptor NKG2D

BACKGROUND

Xenotransplantation could ameliorate the severe shortage of donor organs. The initial results of transplantation from genetically-modified pig donors to primate recipients suggest that hyperacute rejection can be overcome, but thrombotic microangiopathy and the human anti-pig cellular immune response remain as significant impediments to successful clinical xenotransplantation. NKG2D is an activating immunoreceptor found on human natural killer (HuNK) cells, CD8(+) and gammadelta T cells. Signaling through NKG2D mediates cytotoxicity and cytokine secretion by NK cells and co-stimulation of T cells.

METHODS

Chinese hamster ovary P (CHOP) cells were transfected with human NKG2D and used in cell-cell binding studies with porcine epithelial, and endothelial cell lines. Soluble recombinant NKG2D-Fc was used to stain various porcine cells and tissues to indicate ligand expression. Porcine cells were used as targets in cytotoxicity assays with the HuNK cell lines NKL and YT, with and without enzymatic removal of pULBP1 and antibody blockade of NKG2D signaling.

RESULTS AND CONCLUSIONS

In this study, we demonstrate the expression of ligands for human NKG2D on porcine cell lines of endothelial and epithelial origin, islet cell clusters and rejecting kidney. HuNK cells were activated to kill pig cells expressing NKG2D ligands, and cytotoxicity was inhibited by antibody blockade of NKG2D. A previous study identified pULBP1 as the principal ligand for human NKG2D on pig aortic endothelial cells. In the current study, renal epithelial and intestinal endothelial cells each expressed high surface levels of pULBP1, but binding of soluble recombinant NKG2D and NKG2D-dependent cytotoxicity against these cells persisted after the enzymatic removal of pULBP1, strongly suggesting the presence of at least one additional functional ligand for human NKG2D in these cell types.

Soluble Galalpha(1,3)Gal conjugate combined with hDAF preserves morphology and improves function of cardiac xenografts

BACKGROUND

Cytotoxic anti-Galalpha(1,3)Gal antibodies play a key role in the rejection of pig organs transplanted into primates. Regimens reducing anti-Galalpha(1,3)Gal antibodies were associated with severe side effects unable to prevent antibody rebound until soluble synthetic oligosaccharides with terminal Galalpha(1,3)Gal inhibiting antigen binding became available. We displayed kinetics of anti-pig and anti-Galalpha(1,3)Gal IgM and IgG antibody levels using GAS914, a Galalpha(1,3)Gal trisaccharide conjugated to poly-l-lysine, and investigated corresponding changes of parameters of heart function.

METHODS

Using a working heart model, hDAF pig hearts were perfused with human blood containing GAS914 (group 1). As controls hDAF pig hearts (group 2) and landrace pig hearts (group 3) were perfused with human blood only. Levels of anti-Galalpha(1,3)Gal (IgM, IgG) and anti-pig antibodies were assessed to prove the effectiveness of GAS914. As parameters of heart function, cardiac output (CO), stroke work index (SWI), coronary blood flow (CBF) and coronary resistance were measured. Creatine phosphokinases, lactate dehydrogenase and aspartate aminotransferase were evaluated as markers of myocardial damage. Histological and immunohistochemical investigations were performed at the end of perfusion.

RESULTS

In group 1 an immediate and extensive reduction in both IgM and IgG anti-Galalpha(1,3)Gal was found. Anti-pig antibodies were eliminated accordingly. Antibody binding to GAS914 was complete before the start of organ perfusion. Corresponding to rapid antibody elimination in group 1 GAS914 not only was able to significantly prolong the beating time of the heart in hDAF pigs, but also to clearly improve functional parameters. When switching to the working heart mode hDAF pig hearts perfused with human blood containing GAS914 (group 1) revealed a CO starting at a significantly higher level than hDAF (group 2) and non-transgenic pig hearts (group 3) perfused with human blood only. Similarly, in group 1 SWI was significantly increased at the beginning of perfusion compared to that of group 2 and group 3. The increase in CBF during perfusion and the corresponding fall of coronary resistance occurred without significant differences between the groups revealing the independence of hDAF and GAS914.

CONCLUSIONS

Due to an immediate and profound reduction in Galalpha(1,3)Gal-specific antibodies, soluble Galalpha(1,3)Gal conjugates not only prolong survival, but also improve the hemodynamic performance of the heart in DAF pigs.

Xenotransfusions, past and present

The first blood transfusions in humans were xenotransfusions, carried out by Jean-Baptiste Denis beginning in 1667. Richard Lower, Matthäus Purmann and Georges Mercklin also experimented with the use of animal blood for transfusion until this practice was forbidden in 1670, after the death of one of Denis's patients. In the middle of the 19th century, xenotransfusion was rescued from oblivion by the work of Pierre Cyprien Oré. Franz Gesellius and Oscar Hasse fervently defended xenotransfusion, but Emil Ponfick and Leonard Landois stressed the potentially harmful effects of inter-species transfusion from 1874 onward. Xenotransfusion was abandoned completely following the discovery of blood groups by Karl Landsteiner in 1900. From 2000, because of progress in xenotransplantation and the need of blood supply, xenotransfusion is again being considered. Pigs are the best potential donors. The development of alpha-1,3-galactosyltransferase gene-knockout pigs has overcome the first hurdle to xenotransfusion. The main obstacle to porcine red blood cell transfusion is now the cellular response involving macrophages or natural killer cells.

Reactivity of human natural antibodies to endothelial cells from Galalpha(1,3)Gal-deficient pigs

BACKGROUND

Xenoreactive human natural antibodies (NAb) are predominantly directed against galactose-alpha(1,3)galactose (Gal). Binding of immunoglobulin (Ig) G and IgM NAb activates porcine endothelial cells (pEC) and triggers complement lysis responsible for hyperacute xenograft rejection. In vitro, IgG NAb induce human natural killer (NK) cell-mediated lysis of pEC by antibody-dependent cell-mediated cytotoxicity (ADCC). The present study examined the levels of anti-porcine NAb in a large number of individuals and addressed the functional role of non-Gal anti-porcine NAb.

METHODS

Sera from 120 healthy human blood donors were analyzed for the presence of anti-porcine NAb by flow cytometry using porcine red blood cells (pRBC), lymphoblastoid cells (pLCL), and pEC derived from control or Gal-deficient pigs. Xenogeneic complement lysis was measured by flow cytometry using human serum and rabbit complement. ADCC was analyzed by chromium-release assays using human serum and freshly isolated NK cells.

RESULTS

Human IgM binding to pRBC was found in 93% and IgG binding in 86% of all samples. Non-Gal NAb comprised 13% of total IgM and 36% of total IgG binding to pEC. NAb/complement-induced lysis and ADCC of Gal-deficient compared to Gal-positive pEC were 21% and 29%, respectively. The majority of anti-Gal and non-Gal IgG NAb were of the IgG2 subclass.

CONCLUSIONS

The generation of Gal-deficient pigs has overcome hyperacute anti-Gal-mediated xenograft rejection in nonhuman primates. Non-Gal anti-porcine NAb represent a potentially relevant immunological hurdle in a subgroup of individuals by inducing endothelial damage in xenografts.

Gal knockout and beyond

Recently, Galalpha1-3Galbeta1-4GlcNAc (Gal) knockout (k/o) pigs have been developed using genetic cloning technologies. This remarkable achievement has generated great enthusiasm in xenotransplantation studies. This review summarizes the current status of nonhuman primate experiments using Gal k/o pig organs. Briefly, when Gal k/o pig organs are transplanted into primates, hyperacute rejection does not occur. Although graft survival has been prolonged up to a few months in some cases, the overall results were not better than those using Gal-positive pig organs with human complement regulatory protein transgenes. Gal k/o pig kidneys rapidly developed rejection which was associated with increased anti-non-Gal antibodies. Although the precise mechanisms of Gal k/o pig organ rejection are not clear, it could result from incomplete deletion of Gal, up-regulation of new antigen (non-Gal antigen) and/or production of non-Gal antibodies. Future work in xenotransplantation should place emphasis on further modification of donors, such as combining human complement regulatory genes with Gal k/o, deleting non-Gal antigens and adding protective/surviving genes or a gene that inhibits coagulation. Induction of donor-specific T- and B-cell tolerance and promotion of accommodation are also warranted.

IgG Fc receptor III homologues in nonhuman primate species: genetic characterization and ligand interactions

Ig Fc receptors bind to immune complexes through interactions with the Fc regions of specific Ab subclasses to initiate or inhibit the defense mechanisms of the leukocytes on which they are expressed. The mechanism of action of IgG-based therapeutic molecules, which are routinely evaluated in nonhuman primate models, involves binding to the low-affinity FcRIII (CD16). The premise that IgG/CD16 interactions in nonhuman primates mimic those present in humans has not been evaluated. Therefore, we have identified and characterized CD16 and associated TCR zeta-chain homologues in rhesus macaques, cynomolgus macaques, baboons, and sooty mangabeys. Similar to humans, CD16 expression was detected on a lymphocyte subpopulation, on monocytes, and on neutrophils of sooty mangabeys. However, CD16 was detected only on a lymphocyte subpopulation and on monocytes in macaques and baboons. A nonhuman primate rCD16 generated in HeLa cells interacted with human IgG1 and IgG2. By contrast, human CD16 binds to IgG1 and IgG3. As shown for humans, the mAb 3G8 was able to block IgG binding to nonhuman primate CD16 and inhibition of nonhuman primate CD16 N-glycosylation enhanced IgG binding. Clearly, differences in interaction with IgG subclasses and in cell-type expression should be considered when using these models for in vivo evaluation of therapeutic Abs.

T-cell responses during pig-to-primate xenotransplantation

Xenotransplantation using porcine organs may resolve a chronic shortage of donor organs for clinical transplantation if significant immunological barriers can be overcome. To determine the potential role of T lymphocytes in Xenograft (Xg) rejection, we transplanted transgenic hCD46 porcine hearts heterotopically into baboon recipients.

METHODS

Recipients were treated to deplete anti-Gal antibody with a non-antigenic alpha-Gal polyethylene glycol polymer (TPC) (n = 2), TPC plus rituximab (anti-CD20) (n = 1) or were untreated (n = 1). None of the recipients received T-cell immunosuppression.

RESULTS

All Xgs failed within 7 days and showed evidence of a mixed humoral and cellular rejection process. Cellular infiltration consisting primarily of CD4+ T cells and few CD8+ T cells. Proliferation and cytotoxicity assays showed sensitization of CD4+ and CD8+ T cells that reacted with porcine IFN-gamma (pIFN-gamma)-stimulated porcine aortic endothelial cells (PAEC). The CD4+ lymphocytes displayed greater cytotoxicity than CD8+ cells. An increased frequency of PAEC-specific interleukin (IL) 2 and IFN-gamma-secreting T cells was observed, suggesting a Th1 cytokine bias. An increase in the percentage of circulating CD4+CD28- cells was observed at the time of rejection and over 50% of the CD4+ cells recovered from residual pig tissue at necropsy lacked CD28 expression.

CONCLUSIONS

These findings show that lymphocytes are efficiently stimulated by PAEC antigens and can mediate direct tissue destruction. These studies (1) provide an insight into the potential of cellular-mediated cardiac Xg rejection, (2) show for the first time the induction of cytotoxic pig-specific CD4+CD28- lymphocytes and (3) provide a rational basis for determining different modes of immunosuppression to treat Xg rejection.

Recognition of a carbohydrate xenoepitope by human NKRP1A (CD161)

BACKGROUND

Many immunologically important interactions are mediated by leukocyte recognition of carbohydrates via cell surface receptors. Uncharacterized receptors on human natural killer (NK) cells interact with ligands containing the terminal Galalpha(1,3)Gal xenoepitope. The aim of this work was to isolate and characterize carbohydrate binding proteins from NK cells that bind alphaGal or other potential xenoepitopes, such as N-acetyllactosamine (NAcLac), created by the deletion of alpha1,3galactosyltransferase (GT) in animals.

METHODS AND RESULTS

Initial analysis suggested the human C-type lectin NKRP1A bound to a pool of glycoconjugates, the majority of which contained the terminal Galalpha(1,3)Gal epitope. This was confirmed by high level binding of cells expressing NKRP1A to mouse laminin, which contains a large number of N-linked oligosaccharides with the Galalpha(1,3)Gal structure. The consequence of removing the terminal alphaGal was then investigated. Elevated NAcLac levels were observed on thymocytes from GT-/- mice. Exposing NAcLac on laminin, by alpha-galactosidase treatment, resulted in a significant increase in NKRP1A binding.

CONCLUSIONS

NKRPIA binds to the alphaGal epitope. Moreover, exposing NAcLac by removal of alphaGal resulted in an increase in binding. This may be relevant in the later phases of xenotransplant rejection if GT-/- pigs, like GT-/- mice, display increased NAcLac expression.

Effective treatment of preexisting melanoma with whole cell vaccines expressing alpha(1,3)-galactosyl epitopes

The hyperacute immune response in humans is a potent mechanism of xenograft rejection mediated by complement-fixing natural antibodies recognizing alpha(1,3)-galactosyl epitopes (alphaGal) not present on human cells. We exploited this immune mechanism to create a whole cell cancer vaccine to treat melanoma tumors. B16 melanoma vaccines genetically engineered to express alphaGal epitopes (B16alphaGal) effectively treated preexisting s.c. and pulmonary alphaGal-negative melanoma (B16Null) tumors in the alpha(1,3)-galactosyltransferase knockout mouse model. T cells from mice vaccinated with B16alphaGal recognized B16Null melanoma cells measured by detection of intracellular tumor necrosis factor-alpha. We showed successful adoptive transfer of immunity to recipient mice bearing lung melanoma metastasis. Mice receiving lymphocytes from donors previously immunized with B16alphaGal had reduced pulmonary metastases. The transfer of lymphocytes from mice vaccinated with control vaccine had no effect in the pulmonary metastasis burden. This study unequivocally establishes for the first time efficacy in the treatment of preexisting melanoma tumors using whole cell vaccines expressing alphaGal epitopes. Vaccination with B16alphagal induced strong long-lasting cell-mediated antitumor immunity extended to B16Null. These data formed the basis for the testing of this therapeutic strategy in human clinical trials currently under way.

α1,3‐Galactosyltransferase knockout pigs are available for xenotransplantation: Are glycosyltransferases still relevant?

In the early 1990s, the Galalpha(1,3)Gal carbohydrate linkage was found to be the major xenoepitope causing hyperacute rejection. This carbohydrate, the antibodies that bind to it, and the enzyme that produces it (alpha1,3-galactosyltransferase) were the foci of research by many groups. Nearly a decade later, alpha1,3-galactosyltransferase knockout pigs were finally produced; hyperacute rejection could be avoided in these pigs. Having achieved this goal, enthusiasm declined for the study of glycosyltransferases and their carbohydrate products. To examine whether this decline was premature, we evaluate whether gene deletion has indeed solved the initial rejection problem or, in fact, created new problems. This review addresses this by examining the impact of the gene deletion on cell surface carbohydrate. Surprisingly, Galalpha(1,3)Gal is still present in alpha1,3-galactosyltransferase knockout animals: it is possibly synthesized on lipid by iGb3 synthase. Furthermore, removal of alphaGal resulted in the exposure of the N-acetyllactosamine epitope. This exposed epitope can bind natural antibodies and perhaps should be capped by transgenic expression of another transferase. We believe the continued study of glycosyltransferases is essential to examine the new issues raised by the deletion of alpha1,3-galactosyltransferase.

Reducing Gal expression on the pig organ - a retrospective review

The rejection caused by the presence of Galalpha1,3Gal (Gal) on the pig vascular endothelium and of natural anti-Gal antibodies in human blood has recently been prevented by the breeding of pigs that do not express Gal, achieved by knocking out the gene for the enzyme, alpha1,3-galactosyltransferase. However, prior to the introduction of nuclear transfer/embryo transfer techniques, a major effort was directed towards reducing Gal expression on pig cells by other methods, such as by cleaving Gal from the underlying substrate, or replacing Gal with an alternative, innocuous oligosaccharide by a process that has been termed 'competitive glycosylation'. Gal has been cleaved by alpha-galactosidase or endo-beta-galactosidase C. Competitive glycosylation has largely targeted replacement of Gal by insertion of a gene for a fucosyltransferase or a sialyltransferase, or by insertions of the gene for N-acetylglucosaminyltransferase III to reduce cell-surface expression of several oligosaccharides. The results of these approaches to render the pig cells less immunogenic to the human immune system are summarized. With regard to the problem provided by Gal expression, the above approaches may be considered by some to be largely obsolete, but the principles underlying them may prove valuable when other antigen targets for human antibodies are definitively identified, if these prove to be carbohydrates.

Complete protection against melanoma in absence of autoimmune depigmentation after rejection of melanoma cells expressing alpha(1,3)galactosyl epitopes

The major barrier for xenotransplantation in humans is the presence of alpha(1-3) Galactosyl epitopes (alphaGal) in xenogeneic tissue and the vast quantities of natural antibodies (Ab) produced by humans against this epitope. The binding of anti-alphaGal Ab to cells expressing alphaGal triggers a complement-mediated hyperacute rejection of target cells. The hyperacute rejection of whole cancer cells, modified to express alphaGal epitopes, could be exploited as a new cancer vaccine to treat human cancers. We tested this hypothesis in alphaGalactosyltransferase knockout (alphaGT KO) mice which, like humans, do not express alphaGal on their cell surfaces and can produce anti-alphaGal Ab. Forty-five percent of mice with preexisting anti-alphaGal Ab rejected alphaGal positive melanoma cells (B16alphaGal). These mice remained tumor-free for more than 90 days. The majority of control mice injected with B16Null, alphaGal negative cells succumbed to melanoma. The rejection of B16alphaGal induced strong long-lasting antitumor immunity against B16Null measured by the expansion of cytotoxic T lymphocytes. In addition, mice rejecting B16alphaGal were protected against melanoma since they survived a second rechallenge with B16Null. Protected mice developed antitumor immunity in the absence of autoimmune depigmentation (vitiligo). These results show that rejection of alphaGal positive melanoma cells can efficiently boost the immune response to other tumor associated antigens present in alphaGal negative melanoma cells. This study supports the concept of a novel anticancer vaccine to treat human malignancies.

Administration of GAS914 in an orthotopic pig-to-baboon heart transplantation model

BACKGROUND

Long-term survival of transgenic cardiac xenografts is currently limited by a form of humoral rejection named acute vascular rejection. Preformed and elicited cytotoxic antibodies against Galalpha(1,3)Gal terminating carbohydrate chains, known as the primary cause of hyperacute rejection, are crucial for this process. We investigated whether GAS914, a soluble, polymeric form of a Galalpha(1,3)Gal trisaccharide would sufficiently minimize xenograft rejection of hDAF-transgenic pig hearts orthotopically transplanted into baboons.

METHODS

Orthotopic heart transplantations were performed using hDAF transgenic piglets as donors and four non-splenectomized baboons as recipients. Baseline immunosuppression consisted of tacrolimus, sirolimus, ATG, steroids. In addition two animals received low-dose GAS914, and two animals high-dose GAS914. One of these baboons received high dose GAS914 and cyclophosphamide induction therapy. Serum levels of anti-Galalpha(1,3)Gal IgM and IgG antibodies, and anti-pig antibodies were controlled daily by anti-Galalpha(1,3)Gal enzyme-linked immunosorbant assay and anti-pig hemolytic assays. Histomorphological (hematoxylin and eosin, elastic van Gieson) and immunohistochemical (IgM, IgG) evaluations were performed on tissue specimens.

RESULTS

Following low-dose GAS914 therapy survival time was 1 and 9 days, respectively. In baboons treated with high dosages of GAS914 a survival of 30 h and 25 days could be obtained. GAS914 caused an immediate and significant reduction of both anti-Galalpha(1,3)Gal IgM and IgG antibodies. However, sufficient antibody reduction was independent of dosage and form of application of GAS914. A pre-transplant GAS914 treatment was not necessary to effectively reduce antibody levels and prevent hyperacute rejection. In the early postoperative period preformed anti-pig antibodies corresponded predominantly to anti-Galalpha(1,3)Gal antibodies making them susceptible to GAS914. Subsequently, while anti-Galalpha(1,3)Gal antibodies remained low, anti-pig antibodies increased despite of GAS914 application. Corresponding to increased anti-pig antibody titers depositions of IgM and IgG immunoglobulins were detected, which were possibly non-Galalpha(1,3)Gal-specific.

CONCLUSIONS

Following orthotopic transplantation of hDAF-transgenic pig hearts into baboons, GAS914 is able to maintain a sufficient reduction of Galalpha(1,3)Gal-specific cytotoxicity to the graft. GAS914 therefore is able to prevent not only hyperacute rejection, but also acute vascular rejection at its beginning, when serum cytotoxicity to the pig heart appears to be predominantly Galalpha(1,3)Gal-specific. A sustained prevention of acute vascular rejection, however, still requires the identification of antibody specificities other than to Galalpha(1,3)Gal.

Biological activity of pig islet-cell reactive IgG antibodies in xenotransplanted diabetic patients

BACKGROUND

The IgG antibody response in type I diabetic patients, transplanted with fetal pig islet-like cell-clusters, was investigated using purified immunoglobulin G (IgG) fractions from sera collected 7 to 9 yr after transplantation. From our earlier studies, we knew that the immunological specificities of xenoreactive IgG1 and IgG2 antibodies are different, and that IgG1 antibodies, in contrast to the IgG2 population, are mainly directed against non-Galalpha1,3Gal epitopes. In this study our goal was to establish whether xenoreactive IgG1 and IgG2 antibodies react with pig islet cells and, if so, to identify the target cell type, the biological function as well as the specificity of such antibodies for islet cell antigens. Sera from xenotransplanted patients were compared with those of patients with diabetes, selected for high titres of islet-cell specific autoantibodies.

METHODS

IgG antibody fractions from patient sera were purified on a protein G column. Surface expression of target antigens was studied using flow cytometry as well as immunofluorescence microscopy. The biological function of islet-cell reactive sera was tested using antibody dependent cellular cytotoxicity with both xenogeneic adult pig islet cells and allogeneic human islet cells as targets. Antibody specificity was assessed using 2D Western blots with both fetal and adult pig islet as well as human islet cell antigenic preparations.

RESULTS

Some of the diabetic patients, who have been transplanted with xenogeneic fetal pig islet cells, continue to produce xenospecific IgG1 and IgG2 antibodies for 7 to 9 yr post-transplantation. A separate analysis of IgG1 and IgG2 antibodies showed that IgG1 antibodies react with pig islet beta cells, whereas IgG2 antibodies mainly react with non-endocrine pig cells. Such antibodies are xenospecific, as they were found to mediate antibody-dependent cellular cytotoxicity of adult pig, but not human islet target cells. The reverse was true for antibodies from non-transplanted diabetic patients with high titres of autoantibodies against beta cells. Fluorescence analysis as well as 2D gel Western blots revealed that the reactivity was variable between patient samples, indicating that target antigens for non-Galalpha1,3Gal-specific antibodies are heterogeneous.

CONCLUSION

Thus, xenotransplantation of diabetic patients induces islet-beta cell reactive xenospecific IgG1 antibodies, which are biologically active and can mediate antibody-dependent cellular cytotoxicity of pig islet cells.

Lack of Galactose-α-1,3-Galactose Expression on Porcine Endothelial Cells Prevents Complement-Induced Lysis but Not Direct Xenogeneic NK Cytotoxicity

The galactose-alpha-1,3-galactose (alphaGal) carbohydrate epitope is expressed on porcine, but not human cells, and therefore represents a major target for preformed human anti-pig natural Abs (NAb). Based on results from pig-to-primate animal models, NAb binding to porcine endothelial cells will likely induce complement activation, lysis, and hyperacute rejection in pig-to-human xenotransplantation. Human NK cells may also contribute to innate immune responses against xenografts, either by direct recognition of activating molecules on target cells or by FcgammaRIII-mediated xenogeneic Ab-dependent cellular cytotoxicity (ADCC). The present study addressed the question as to whether the lack of alphaGal protects porcine endothelial cells from NAb/complement-induced lysis, direct xenogeneic NK lysis, NAb-dependent ADCC, and adhesion of human NK cells under shear stress. Homologous recombination, panning, and limiting dilution cloning were used to generate an alphaGal-negative porcine endothelial cell line, PED2*3.51. NAb/complement-induced xenogeneic lysis of PED2*3.51 was reduced by an average of 86% compared with the alphaGal-positive phenotype. PED2*3.51 resisted NK cell-mediated ADCC with a reduction of lysis ranging from 30 to 70%. However, direct xenogeneic lysis of PED2*3.51, mediated either by freshly isolated or IL-2-activated human NK cells or the NK cell line NK92, was not reduced. Furthermore, adhesion of IL-2-activated human NK cells did not rely on alphaGal expression. In conclusion, removal of alphaGal leads to a clear reduction in complement-induced lysis and ADCC, but does not resolve adhesion of NK cells and direct anti-porcine NK cytotoxicity, indicating that alphaGal is not a dominant target for direct human NK cytotoxicity against porcine cells.

Reduction of alpha-galactosyl xenoantigen by expression of endo-beta-galactosidase C in pig endothelial cells

BACKGROUND

Elimination of the Galalpha1-3Galbeta1-4GlcNAc (alphaGal) epitope has been considered to be essential for successful pig-to-human xenotransplantation but, unfortunately, has not been achieved. Endo-beta-galactosidase C (EndoGalC) is an endoglycosidase which cleaves the Galbeta1-4GlcNAc linkage in the alphaGal epitope and digests out the Galalpha1-3Gal disaccharide. Because of its potent activity in physiological pH conditions, EndoGalC can remove alphaGal epitopes expressed on the cell surface of pig erythrocytes and vascular endothelial cells almost completely. In vivo or ex vivo administration of EndoGalC successfully reduced alphaGal expression in pig kidneys to an undetectable level, but alphaGal epitopes soon reappeared. Gene expression of EndoGalC in pig cells was attempted to solve this problem. As the terminal alphaGal is transferred in the trans-Golgi network by alpha-1,3-galactosyltransferase (alpha1,3GT), colocalization of the EndoGalC gene with the alpha1,3GT gene was expected to be one of the most reliable ways to eliminate the alphaGal epitope.

METHODS AND RESULTS

The sequence of pig alpha1,3GT, including the cytoplasmic tail, transmembrane domain and stem region, was ligated upstream of EndoGalC, and the conjugated gene was expressed in pig aortic endothelial cells and COS7 cells. Following the introduction of the gene, the alphaGal epitope on pig aortic endothelial cells was effectively reduced. Transfection studies in COS7 cells using EndoGalC combined with alpha1,3GT showed that the expressed EndoGalC was localized not only inside, but also outside, the cells. The expression of EndoGalC conjugated with a murine immunoglobulin (Igkappa)-chain signal sequence also showed a similar effect.

CONCLUSIONS

These results suggest the effectiveness of gene transfer with EndoGalC into pig endothelial cells, and strongly encourage us to produce transgenic animals with the expressed enzyme.

Antibody-mediated activation of the classical complement pathway in xenograft rejection

Transplant rejection is a multifactorial process involving complex interactions between components of the innate and the acquired immune system. In view of the shortage of donor organs available for transplantation, xenotransplantation of pig organs into man has been considered as a potential solution. However, in comparison to allografts, xenografts are subject to extremely potent rejection processes that are currently incompletely defined. Consequently, an appropriate and safe treatment protocol ensuring long-term graft survival is not yet available. The first barrier that has to be taken for a xenograft is hyperacute rejection, a rapid process induced by the binding of pre-formed antibodies from the host to the graft endothelium, followed by activation of the classical complement pathway. The present review concentrates on the role of antibodies and complement in xenograft rejection as well as on the approaches for treatment that target these components. The first part focuses on porcine xenoantigens that are recognized by human xenoreactive antibodies and the different treatment strategies that aim on interference in antibody binding. The second part of the review deals with complement activation by xenoreactive antibodies, and summarizes the role of complement in the induction of endothelial cell damage and cell activation. Finally, various options that are currently under development for complement inhibition are discussed, with special reference to the specific inhibition of the classical complement pathway by soluble complement inhibitors.

Continued production of xenoimmune antibodies 6-8 years after clinical transplantation of fetal pig islet-like cell-clusters

We have monitored the humoral immune responses of 10 type I diabetic patients, xenotransplanted with fetal porcine islet-like cell clusters for up to 8 years after xenotransplantation. We investigated the immunoglobulin subclass distribution as well as specificity differences of xenoreactive antibodies. Hemagglutintion tests, using pig erythrocytes, showed that some patients maintained higher titers of xenoreactive IgM antibodies during the entire follow up period, compared with pretransplant levels. In microcytotoxicity tests all but one patient tested showed higher than pretransplant levels of cytotoxic antibodies against pig peripheral blood mononuclear cells (PBMC) 6-8 years after transplantation. Levels of Gal alpha 1,3Gal specific antibodies, were also high. Antibody dependent cellular cytotoxicity (ADCC) activity against a Gal alpha 1,3Gal expressing human B cell line was detected in four patients while ADCC reactivity against adult pig islet cells was detected in only two patients, 6-8 years after transplantation. Immune sera collected 30 days and 1 year after transplantation showed positive staining of adult pig islet cells in fluoromicroscopy whereas sera from later time points did not. Western blot experiments showed that some patients had IgG1 antibodies reactive against epitopes on pig cells other than Gal alpha 1,3Gal, while xenoreactive IgM and IgG2 antibodies mainly reacted with Gal alpha 1,3Gal-containing epitopes as shown by absorption experiments. These results show that patients continue to produce higher than pretransplant levels of IgM and IgG2 xenospecific antibodies against Gal alpha 1,3Gal for extended time periods following xenotransplantation. Some patients also produce xenoreactive IgG1 antibodies directed against non-Gal alpha 1,3Gal epitopes.

Xenogeneic transplantation

This review summarizes the clinical history and rationale for xenotransplantation; recent progress in understanding the physiologic, immunologic, and infectious obstacles to the procedure's success; and some of the strategies being pursued to overcome these obstacles. The problems of xenotransplantation are complex, and a combination of approaches is required. The earliest and most striking immunologic obstacle, that of hyperacute rejection, appears to be the closest to being solved. This phenomenon depends on the binding of natural antibody to the vascular endothelium, fixation of complement by that antibody, and finally, activation of the endothelium and initiation of coagulation. Therefore, these three pathways have been targeted as sites for intervention in the process. The mechanisms responsible for the next immunologic barrier, that of delayed xenograft/acute vascular rejection, remain to be fully elucidated. They probably also involve multiple pathways, including antibody and/or immune cell binding and endothelial cell activation. The final immunologic barrier, that of the cellular immune response, involves mechanisms that are similar to those involved in allograft rejection. However, the strength of the cellular immune response to xenografts is so great that it is unlikely to be controlled by the types of nonspecific immunosuppression used routinely to prevent allograft rejection. For this reason, it may be essential to induce specific immunologic unresponsiveness to at least some of the most antigenic xenogeneic molecules.

Prospects for xenotransplantation

Pig-to-primate organ survival has been extended from a few minutes to weeks and occasionally months, following the development of transgenic pigs that express human complement-regulatory proteins, efficient antibody removal technologies and immunosuppressive strategies. The current limitation to the clinical application of this technology is acute vascular rejection, and an understanding of the mechanisms of this process and the development of modalities to overcome it are key to making significant progress at solving the critical shortage of organs for transplantation. Approaches that address this issue are underway in a number of laboratories.

Alpha-gal-independent dual recognition and activation of xenogeneic endothelial cells and human naïve natural killer cells

BACKGROUND

Interaction between vascularized xenograft and host immune system is thought to occur via Galactose alpha (1,3) Galactose (Gala 1,3 gal) structures decorating the xenograft.

METHODS

We raised anti-Gala 1,3 gal-BSA polyclonal antibodies in baboons and investigated effect(s) of these antibodies as well as soluble Gala 1,3 gal-BSA on human naive natural killer (NK) cell interactions with porcine aortic endothelial cells.

RESULTS

We demonstrate that human naive (unstimulated) NK cells recognize xenogeneic endothelial cells under conditions where binding to the Gala 1,3 gal structures is minimized by the presence of blocking anti-Gala 1,3 gal IgG or soluble Gala 1-3 gal and in the absence of xenoreactive natural antibodies and complement. After xenogeneic encounter both endothelial cells and human NK cells are activated. Endothelial cell activation is rapid and is manifested initially by an intraendothelial calcium transient and subsequently by expression of P-selectin and vascular endothelial cell adhesion molecule-1 on the xenoendothelium surface. NK cell activation is manifested by increased expression of perforin and increased cytotoxicity towards the xenoendothelium. Neither recognition nor activation of the xenoendothelium was affected by the introduction of either anti-Gala 1,3 gal IgG or soluble Gala 1-3 gal.

CONCLUSION

Our data provide evidence that innate immune cells, such as NK cells, recognize and activate xenoendothelial cells independently of Gala 1-3 gal structures and raise the possibility of novel interactive sites on both human naive NK cells and discordant xenogeneic endothelium.

Development of an ex vivo model of pig kidney perfused with human lymphocytes. Analysis of xenogeneic cellular reactions

BACKGROUND

Because of the explosive nature and the extremely rapid process of hyperacute rejection (HAR), significant infiltration of the xenograft by immunocompetent cells is not observed, and the role and the mechanism of action of cell-mediated rejection in discordant xenografts are therefore still under discussion.

METHOD

We developed an experimental approach using pig kidneys perfused with human peripheral blood lymphocytes (PBL) in which the immunologic barrier of hyperacute rejection was excluded and which mimics the in vivo situation.

RESULTS

PBL retention in the kidney was evaluated at 20-minute intervals for 3 hours. Retention increased from 30% to 80% with the time of perfusion and was specific because significantly fewer syngeneic lymphocytes were retained. Phenotype analysis of recovered PBL showed a significant decrease in natural killer (NK) cells. Immunohistochemical studies revealed the presence of NK cells and T lymphocytes in the glomerular and interstitial tubular structures of the kidney. Functional studies showed a progressive cessation of diuresis and augmentation of renal vascular resistance when the kidney was perfused with PBL. Electron microscopy examinations of kidney sections perfused with PBL showed swollen endothelial zones, suggesting alterations to and damage of the endothelium.

CONCLUSIONS

This system provides a valuable model for the study of early discordant xenogeneic cellular rejection and demonstrates the predominance of xenograft infiltration by NK cells.

Molecular cloning of endo-beta -galactosidase C and its application in removing alpha -galactosyl xenoantigen from blood vessels in the pig kidney

Galalpha1-3Gal is the major xenoantigenic epitope responsible for hyperacute rejection upon pig to human xenotransplantation. Endo-beta-galactosidase C from Clostridium perfringens destroys the antigenic epitope by cleaving the beta-galactosidic linkage in the Galalpha1-3Galbeta1-4GlcNAc structure. Based on partial peptide sequences of the enzyme, we molecularly cloned the enzyme gene, which encodes a protein with a predicted molecular mass of about 93 kDa. The deduced protein sequence of the enzyme has limited homology in the C-terminal half with endo-beta-galactosidase from Flavobacterium keratolyticus and beta-1,3-glucanases. The enzyme expressed in Escherichia coli removed the alpha-galactosyl epitope nearly completely from pig erythrocytes and from pig aortic endothelial cells. The enzyme-treated endothelial cells in culture were greatly reduced in cell surface antigens, which were recognized by IgM, IgG, or IgA in human sera, and became much less susceptible to complement-mediated cytotoxicity caused by human sera. When the pig kidney was perfused with the enzyme, the vascular endothelial cells became virtually devoid of the alpha-galactosyl epitope, with concomitant decrease in binding to IgM in human plasma. These results demonstrated that the recombinant endo-beta-galactosidase C is a valuable aid in xenotransplantation.

The target antigens of naturally occurring human anti-beta-galactose IgG are cryptic on porcine aortic endothelial cells

The identification of the xeno-antigens/xeno-antibodies combinations involved in pig-to-human xenograft rejection is an essential step for understanding this process and for the development of procedures to prevent it. Although it is widely accepted that the terminal disaccharide Galalpha1,3Gal-R is by far the major epitope recognized by human natural antibodies reactive with pig tissues, there is also evidence that other carbohydrate epitopes might be important in xenograft rejection. In an attempt to further improve our knowledge of the repertoire of human natural antibodies with anti-pig specificity we sought to determine whether naturally occurring human anti-beta-galactose IgG could interact with porcine aortic endothelial cells (PAEC). Histochemical analysis of porcine aorta sections revealed that the carbohydrate structures recognized by the anti-beta-galactose IgG are present on endothelial cells but in a cryptic form that can be unmasked by sialidase treatment. These structures were also found to be cryptic in cultured PAEC. In addition we demonstrated that PAEC may adsorb fetal calf serum (FCS) glycoproteins when cultured in FCS-supplemented medium, a process susceptible to generating artifactual observations in carbohydrate antigens analysis. In conclusion, despite their abundance, human anti-beta-galactose IgG do not represent a primary concern in pig-to-human xenotransplantation as the carbohydrate structures to which they bind are normally masked by sialic acid residues on porcine endothelial cells. However, whether these cryptic epitopes might be exposed on endothelial cells from genetically engineered animals should be further investigated because, if so, additional approaches will be needed to suppress their interaction with human anti-beta-galactose IgG.

Potential use of genetically modified pigs as organ donors for transplantation into humans

1. Transplantation is currently viewed as a successful treatment for end-stage organ failure. Its more widespread use is severely limited by a shortage of human organ donors. This has stimulated the scientific and medical communities to look at the potential use of animals to solve this problem. For a number of reasons, the pig appears to be the preferred species. 2. The initial immunological problem encountered in pig-to-primate transplantation is a process of hyperacute rejection, which is mediated by the binding of antibodies to the graft endothelium followed by the activation of the complement cascade. We have sought to overcome these initial immunological challenges by focusing our attention not only on the recipient of the graft but also on the donor. Therefore, we have generated transgenic animals with specific genetic modifications designed to inhibit the activation of the complement cascade. Upon transplantation to baboons of organs derived from these transgenic pigs, we have been able to demonstrate that hyperacute rejection can be prevented. We will discuss the generation of these and other transgenic animals and their potential role in clinical xenotransplantation.

Pig xenogeneic antigen modification with green coffee bean alpha-galactosidase

Green coffee bean alpha-galactosidase can cleave the terminal alpha-galactose (alphaGal) on oligosaccharides that form the major antigen on pig endothelial cells recognized by primate-specific antibodies. Studies have been made of the conditions under which it is functional (e.g. temperature, pH) and of its biochemical and immunologic effects. Pig-to-rhesus monkey vein transplants were studied to identify the efficiency of the enzyme in delaying hyperacute rejection. When a graft became occluded, biopsies were taken for light microscopy (hematoxylin and eosin), scanning electron microscopy (SEM) and immunostaining with Griffonia simplicifolia IB4 lectin (GSIB4), and for IgM, IgG and C3. alpha-Galactosidase was stable for 72-96 h and was effective at 4 degrees C and pH 6.9 (conditions of human liver graft storage), although better function was obtained at 20 degrees C and pH 6.5. Using the porcine PK15 cell assay, the cytotoxicity of human serum was reduced after treatment of the pig cells with the enzyme. In vitro studies demonstrated that porcine veins treated with alpha-galactosidase lost endothelial expression of the Gal epitope within 30 min. SEM, however, demonstrated endothelial damage beginning within 2 h, probably caused by the alpha-galactosidase, as no damage was found in phosphate-buffered saline-treated veins, where the Gal epitope was preserved for >3 h. No change was found in either group on light microscopy. In vivo studies demonstrated that patency of the alpha-galactosidase-treated veins (mean 2.5 h) was longer than that of untreated veins (0.23 h) (P < 0.01). Biopsies showed no GSIB4 lectin staining for alpha-Gal epitopes and much less IgM and C3 deposition in the treated group. Light microscopy and SEM demonstrated more severe endothelial damage, hemorrhage, and fibrin formation in the untreated group. Galactosidase is effective in removing the terminal alphaGal and delays the onset of hyperacute rejection of pig veins transplanted into monkeys. However, its effect is temporary and, on its own, its use is unlikely to prolong survival of pig organs transplanted into primates sufficiently to be of clinical value.

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.

Target cell susceptibility to lysis by human natural killer cells is augmented by alpha(1,3)-galactosyltransferase and reduced by alpha(1, 2)-fucosyltransferase

Susceptibility of porcine endothelial cells to human natural killer (NK) cell lysis was found to reflect surface expression of ligands containing Gal alpha(1,3)Gal beta(1,4)GlcNAc [corrected], the principal antigen on porcine endothelium recognized by xenoreactive human antibodies. Genetically modifying expression of this epitope on porcine endothelium by transfection with the alpha(1,2)-fucosyltransferase gene reduced susceptibility to human NK lysis. These results indicate that surface carbohydrate remodeling profoundly affects target cell susceptibility to NK lysis, and suggest that successful transgenic strategies to limit xenograft rejection by NK cells and xenoreactive antibodies will need to incorporate carbohydrate remodeling.

Physiologic incompatibilities in discordant xenotransplantation

At present, several incompatibilities between pig and human, not only from the immunologic point of view but also regarding physiological and molecular systems, have been identified. It is anticipated that this is only the tip of the iceberg regarding this topic. However, there are also many systems that are compatible. Strategies have been outlined, and many tools, such as gene modification, are available to solve the problems. Therefore, the statement by Keith Reemtsma in the foreward to the latest issue of the monography Xenotransplantation "The important question is not whether xenotransplantation will succeed, but rather how and under what circumstances xenografts will provide predictable enough results to warrent clinical application" will hopefully be a reality in 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.

Induction of swine major histocompatibility complex class I molecules on porcine endothelium by tumor necrosis factor-alpha reduces lysis by human natural killer cells

BACKGROUND

Natural killer (NK) cells have been implicated in a process of delayed xenograft rejection occurring in pig-to-primate organ transplants. As tumor necrosis factor-a (TNF-a) induces expression of both adhesion receptors and major histocompatibility complex class I molecules on porcine endothelium, we investigated the effects of TNF-alpha on human NK cell adherence to and cytotoxicity of porcine aortic endothelial cell (PAEC) monolayers.

METHODS

Adherence of human NK cells was measured after PAEC treatment with increasing concentrations of TNF-alpha. Monoclonal antibodies (mAbs) against adhesion molecules on NK cells and PAEC were used in inhibition studies. Resting or TNF-alpha-treated PAEC were used as targets for NK lysis. Increasing titers of anti-swine leukocyte antigen (SLA) class I antibodies or pooled human immune globulin (IVIg) were used to reverse the effects of TNF-alpha on NK lysis.

RESULTS

NK cell adhesion to TNF-a-treated PAEC increased in a dose-dependent manner by a maximum of 44%, and was inhibited by mAbs against CD49d, CD11a, CD11b, CD18, and CD2, as well as porcine vascular cell adhesion molecules. In contrast, TNF-alpha treatment of PAEC reduced human NK lysis in a dose-dependent manner. Preincubation of TNF-a-treated PAEC with increasing concentrations of anti-SLA class I mAb increased NK lysis in a titer-dependent manner, and reversed the protective effect on human NK lysis by 77%. Treatment with IVIg, containing antibodies against an a-helical region of HLA class I molecules, had a similar effect.

CONCLUSIONS

These results imply that SLA class I molecules can bind to inhibitory receptors on human NK cells, and that these interactions can be augmented by increasing the level of SLA class I molecule expression on porcine endothelium. Strategies that can increase porcine endothelial cell expression of either swine or human major histocompatibility complex class I molecules may reduce human NK activity against porcine xenografts.

Characterization of human xenoreactive antibodies in liver failure patients exposed to pig hepatocytes after bioartificial liver treatment: an ex vivo model of pig to human xenotransplantation

BACKGROUND

There are limited experimental data on the nature of the humoral response elicited in humans against pig antigens. In this study, we have examined the xenoantibody (XAb) response in eight patients with acute liver failure exposed to pig hepatocytes after treatment with the bioartificial liver (BAL).

METHODS

Patients' plasma samples obtained before and after BAL treatment were tested for IgM and IgG XAbs, IgG subclasses, and XAb cytotoxicity, using enzyme-linked immunosorbent assay and flow-cytometric assays. The characterization of pig aortic endothelial cell (PAEC) surface xenoantigens was analyzed by immunoprecipitation.

RESULTS

We observed by day 10, a strong anti-pig IgG and IgM XAb response in patients undergoing two or more BAL treatments, with a significant increase in all the IgG subclasses; in contrast, XAb titers did not change if the patients received only one BAL treatment. The majority of the XAbs produced to porcine antigens were primarily specific for the alphaGal epitope. Both IgG and IgM XAbs were cytotoxic to PAECs, and the cytotoxic activity of IgG was associated with high levels of IgG1 and IgG3 subclasses, known to be efficient on complement activation. The characterization of porcine surface antigens demonstrated that IgM human XAbs, before and after BAL exposure, recognized xenoantigens on PAECs with similar molecular weights, suggesting that the same population of XAbs were present in the patients before and after exposure to pig antigens.

CONCLUSIONS

Repetitive exposure of humans to porcine antigens after BAL treatment, results in a strong IgG and IgM XAb responses that are primarily directed against the alphaGal epitope. These XAbs are cytotoxic to PAECs and the IgG toxicity correlates with high IgG1 and IgG3 levels. Our data also suggest that no new XAb specificity emerges after porcine exposure.

A reliable, rapid and inexpensive two-color fluorescence assay to monitor serum cytotoxicity in xenotransplantation

Removal and/or neutralization of preformed anti-pig antibodies in non-human primate blood have been shown to prevent the hyperacute rejection of transplanted pig organs. The purpose of this study was to establish a suitable in vitro method that would allow for screening and comparison of various agents and methods potentially useful in the prevention of hyperacute rejection. The pig kidney cell line (PK15), pig aortic endothelial cell line (AG08472), and a primary culture of endothelial cells explanted from a pig aorta were incubated with either human or baboon sera. Complement-dependent cytotoxic activity of human and baboon sera was determined on all three types of pig cells using a two-color fluorescence assay and compared with the conventional 51Chromium (51Cr)-release assay. The assay was also performed on PK15 cells as a 2-chambered slide assay and compared with a microcytotoxicity assay performed in Terasaki trays. Using the microcytotoxicity assay, a 1-step assay utilizing endogenous complement was compared with a 2-step assay where rabbit complement was added. Of the three types of cells studied, PK15 cells were the most sensitive to cytotoxic injury, followed by AG cells and the primary endothelial culture. Good correlation between the 51Cr-release and the two-color fluorescence method was documented. There was good agreement between the results obtained using the 2-chambered slide method and the microcytotoxicity assay, as there was between the 1- and the 2-step assays. The 1- and 2-step assays provided information on the level and efficacy of endogenous complement. We conclude that the two-color fluorescence assay is suitable for the rapid and inexpensive screening of therapeutic interventions that might be useful in the prevention of hyperacute xenograft rejection, and that PK15 cells are suitable for use in this assay.

Human preformed IgG combining with membrane-bound porcine serotransferrin lyse porcine endothelial cells through antibody-dependent cellular cytotoxicity

Preformed antibodies are involved in xenograft rejection. The purpose of this work was to characterize porcine xenoantigens recognized by human preformed IgG (hpIgG), and to investigate the role of hpIgG in xenogeneic rejection. IgG eluted from porcine livers perfused with human plasma, human sera and total human IgG were immunoblotted on porcine aortic endothelial cell extracts. The amino acid sequence of a 76-kDa antigen constantly revealed was 100% homologous with porcine serotransferrin (psTf). hpIgG from human sera, human IgG1 and IgG2 and F(ab')2gamma specifically bound to psTf. Neutralization by psTf abolished that binding. Although alpha1,3-linked galactose residues (Gal(alpha)1,3Gal) is the dominant epitope recognized by preformed antibodies in the swine-to-human combination, the analysis of carbohydrate composition of psTf showed that the molecule was devoid of Gal(alpha)1,3Gal moieties and that preformed anti-psTf IgG bound to epitopes localized on the peptide core of the molecule. Purified human anti-psTf IgG antibodies were able to bind to psTf linked to its receptor on porcine endothelial cells, and to kill those cells through antibody-dependent cellular cytotoxicity.

B-cell reconstitution and xenoreactive anti-pig natural antibody production in severe combined immunodeficient mice reconstituted with immunocompetent B cells from varying sources

BACKGROUND

Little is known about the B-cell subsets that produce xenoreactive natural antibodies (NAb). This study was undertaken to investigate the potential role of varying B-cell populations in anti-pig NAb production in mice.

METHODS

Severe combined immunodeficient (scid) mice were reconstituted with bone marrow or splenic or peritoneal B cells from immunocompetent mice. B-cell reconstitution and anti-pig NAb were evaluated by flow cytometric analysis.

RESULTS

Adult marrow failed to reconstitute normal numbers of CD5+ B1a cells, but fully reconstituted CD5- Mac1- B2 and CD5- Mac1+ B1b cell populations in scid mice. Recipients of peritoneal B cells showed poor reconstitution of B2 cells, and an overshoot of B1 cells in the peritoneal cavity. Although B2 cells predominate in the adult spleen, splenic B cells from immunocompetent mice preferentially reconstituted B cells, including B1 cells, in the peritoneal cavity, but did not reconstitute splenic B2 cells. Therefore, neither adult marrow, splenocytes nor peritoneal cells can fully reconstitute scid mice with all B-cell subpopulations. Nevertheless, serum anti-pig NAb in marrow-reconstituted mice recovered to normal levels by 3 weeks, and were maintained for at least 30 weeks. Serum NAb in scid mice receiving peritoneal B cells reached normal levels by 4-7 weeks after transfer. However, NAb in sera of scid mice receiving splenic B cells took longer (>25 weeks) to reach normal levels.

CONCLUSIONS

These results indicate that adult marrow-derived B cells can efficiently produce anti-pig NAb, and that peritoneal B cells have greater NAb-producing ability than splenic B cells or their immediate progeny.

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.

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.

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

BACKGROUND

The galactose alpha(1-3)galactose (alpha-gal) epitope associated with membrane glycoproteins and glycolipids represents a major determinant recognized on porcine cells by human xenoreactive natural antibodies (XNA). Together, bound XNA and complement rapidly induce porcine aortic endothelial cell (PAEC) activation; this process is associated with cellular shape changes, transient development of intercellular gaps, and loss of ATDPase and thrombomodulin, with release of heparan sulfate. The aim of this study was to evaluate patterns of type I endothelial cell activation (i.e., activation that does not require protein synthesis) following ligation of alpha-gal epitopes with anti-Gal antibodies and alpha-gal-specific lectins.

METHODS AND RESULTS

PAEC incubated in the presence of the alpha-gal binding, Bandeiraea simplicifolia lectin (BS-I) underwent cellular shape changes associated with the formation of intercellular gaps. PAEC exposure to BS-I was also associated with the tyrosine phosphorylation of a protein (apparent molecular mass of approximately 130 kDa), not observed following lipopolysaccharide, tumor necrosis factor, or XNA stimulation. This lectin-induced tyrosine phosphorylation was not affected by cytochalasin D (inhibitor of actin filament polymerization), by genistein (inhibitor of tyrosine kinases), or by staurosporine (inhibitor of tyrosine phosphorylation and protein kinase C). In addition, incubation of PAEC with BS-I and monoclonal anti-Gal IgM induced p42/44 map kinase and activated the transcription factor NF-kappaB.

CONCLUSIONS

Agonist binding of alpha-gal can evoke endothelial cell activation independently of complement activation. These observations have implications for the survival of xenografts.

Removal of primate xenoreactive natural antibodies by extracorporeal perfusion of pig kidneys and livers

Organ perfusion is one of the possible strategies to attenuate rejection of discordant xenografts by reducing the levels of the recipient's xenoreactive natural antibodies (XNA). Its efficacy in terms of XNA removal was studied in models of primate blood or plasma perfusion through porcine kidneys or livers, with special attention to haematological consequences and potential side-effects. We first perfused the blood of rhesus monkeys through pig kidneys and livers, and demonstrated that the perfusion of a pig liver resulted in higher XNA adsorption (72 +/- 13%) than the perfusion of a pig kidney (51 +/- 25%). However, when we normalized for the weight of the perfused organs and for levels of natural antibodies in individual monkeys, livers adsorbed less antibody (1.4 +/- 0.9 U antibody/g) than kidneys (7.2 +/- 7 U antibody/g). Histological signs of rejection were observed in perfused kidneys, but not in perfused livers. A major drawback of the perfusion of blood through livers was a considerable decrease in the primates' haemoglobin and platelet levels. To avoid this, we developed a plasma liver perfusion device. This method allowed a significant improvement in the haemodynamic state of primates and was particularly effective in preventing anaemia. Moreover, plasma liver perfusion was as effective as blood liver perfusion to remove natural antibodies and, resulted in a marked decrease in their functional activity as assessed by complement-dependent cytotoxicity (CDC) and antibody-dependent cell-mediated cytotoxicity (ADCC). The level of other plasma proteins was not significantly affected, apart from a dilution effect. After xenoperfusion a strong antibody response was evidenced by ELISA, CDC and ADCC between days 7 and 14 and then decreased progressively. We conclude that the separation of blood to allow the perfusion of plasma through a pig organ is safer than the perfusion of unseparated blood and is associated with efficient natural antibody removal. However, organ perfusion is limited by a rebound in antibody levels after a few days, and thus will have to be associated with anti-B cell immunosuppressive therapy for long-term or repeated applications.

Combined transgenic expression of alpha-galactosidase and alpha1,2-fucosyltransferase leads to optimal reduction in the major xenoepitope Galalpha(1,3)Gal

Hyperacute rejection of pig organs by humans involves the interaction of Galalpha(1,3)Gal with antibodies and complement. Strategies to reduce the amount of xenoantigen Galalpha(1,3)Gal were investigated by overexpression of human lysosomal alpha-galactosidase in cultured porcine cells and transgenic mice. The overexpression of human alpha-galactosidase in cultured porcine endothelial cells and COS cells resulted in a 30-fold reduction of cell surface Galalpha(1,3)Gal and a 10-fold reduction in cell reactivity with natural human antibodies. Splenocytes from transgenic mice overexpressing human alpha-galactosidase showed only a 15-25% reduction in binding to natural human anti-Galalpha(1,3)Gal antibodies; however, this decrease was functionally significant as demonstrated by reduced susceptibility to human antibody-mediated lysis. However, because there is residual Galalpha(1,3)Gal and degalactosylation results in the exposure of N-acetyllactosamine residues and potential new xenoepitopes, using alpha-galactosidase alone is unlikely to overcome hyperacute rejection. We previously reported that mice overexpressing human alpha1,2-fucosyltransferase as a transgene had approximately 90% reduced Galalpha(1,3)Gal levels due to masking of the xenoantigen by fucosylation; we evaluated the effect of overexpressing alpha-galactosidase and alpha1,2-fucosyltransferase on Galalpha(1,3)Gal levels. Galalpha(1, 3)Gal-positive COS cells expressing alpha1,3-galactosyltransferase, alpha1,2-fucosyltransferase, and alpha-galactosidase showed negligible cell surface staining and were not susceptible to lysis by human serum containing antibody and complement. Thus, alpha1, 2-fucosyltransferase and alpha-galactosidase effectively reduced the expression of Galalpha(1,3)Gal on the cell surface and could be used to produce transgenic pigs with negligible levels of cell surface Galalpha(1,3)Gal, thereby having no reactivity with human serum and improving graft survival.