Human NK cell-mediated direct and IgG-dependent cytotoxicity against xenogeneic porcine endothelial cells

Cytotoxic Responses Mediated by NK Cells and Cytotoxic T Lymphocytes in Xenotransplantation

Xenotransplantation represents a potential solution to the shortage of organs for transplantation. The recent advancements in porcine genetic modification have addressed hyperacute and acute vascular rejection; however, challenges persist with regard to delayed xenograft rejection. Porcine endothelial cells (pECs) represent a crucial target in the context of xenograft rejection, which is mediated by cytotoxic lymphocytes. It is crucial to comprehend the manner in which human natural killer (NK) cells and cytotoxic CD8+ T lymphocytes (CTL) recognize and target pECs in order to develop efficacious prophylactic strategies against rejection. The objective of the present review is to synthesize the existing knowledge regarding the mechanisms and techniques employed to modulate xenogeneic responses mediated by human NK cells and CTL. We will elucidate recent methodological advancements, debate potential novel strategies, and emphasize the imperative necessity for further research and innovative approaches to enhance graft survival.

Genetic engineering of pigs for xenotransplantation to overcome immune rejection and physiological incompatibilities: The first clinical steps

Xenotransplantation has the potential to solve the shortfall of human organ donors. Genetically modified pigs have been considered as potential animal donors for human xenotransplantation and have been widely used in preclinical research. The genetic modifications aim to prevent the major species-specific barriers, which include humoral and cellular immune responses, and physiological incompatibilities such as complement and coagulation dysfunctions. Genetically modified pigs can be created by deleting several pig genes related to the synthesis of various pig specific antigens or by inserting human complement- and coagulation-regulatory transgenes. Finally, in order to reduce the risk of infection, genes related to porcine endogenous retroviruses can be knocked down. In this review, we focus on genetically modified pigs and comprehensively summarize the immunological mechanism of xenograft rejection and recent progress in preclinical and clinical studies. Overall, both genetically engineered pig-based xenografts and technological breakthroughs in the biomedical field provide a promising foundation for pig-to-human xenotransplantation in the future.

Progress in xenotransplantation: overcoming immune barriers

A major limitation of organ allotransplantation is the insufficient supply of donor organs. Consequently, thousands of patients die every year while waiting for a transplant. Progress in xenotransplantation that has permitted pig organ graft survivals of years in non-human primates has led to renewed excitement about the potential of this approach to alleviate the organ shortage. In 2022, the first pig-to-human heart transplant was performed on a compassionate use basis, and xenotransplantation experiments using pig kidneys in deceased human recipients provided encouraging data. Many advances in xenotransplantation have resulted from improvements in the ability to genetically modify pigs using CRISPR-Cas9 and other methodologies. Gene editing has the capacity to generate pig organs that more closely resemble those of humans and are hence more physiologically compatible and less prone to rejection. Despite such modifications, immune responses to xenografts remain powerful and multi-faceted, involving innate immune components that do not attack allografts. Thus, the induction of innate and adaptive immune tolerance to prevent rejection while preserving the capacity of the immune system to protect the recipient and the graft from infection is desirable to enable clinical xenotransplantation.

Strategies to induce natural killer cell tolerance in xenotransplantation

Eliminating major xenoantigens in pig cells has drastically reduced human antibody-mediated hyperacute xenograft rejection (HXR). Despite these advancements, acute xenograft rejection (AXR) remains one of the major obstacles to clinical xenotransplantation, mediated by innate immune cells, including macrophages, neutrophils, and natural killer (NK) cells. NK cells play an 'effector' role by releasing cytotoxicity granules against xenogeneic cells and an 'affecter' role on other immune cells through cytokine secretion. We highlight the key receptor-ligand interactions that determine the NK cell response to target cells, focusing on the regulation of NK cell activating receptor (NKG2D, DNAM1) and inhibitory receptor (KIR2DL1-4, NKG2A, and LIR-1) signaling pathways. Inhibition of NK cell activity may protect xenografts from cytotoxicity. Recent successful approaches to reducing NK cell-mediated HXR and AXR are reviewed, including genetic modifications of porcine xenografts aimed at improving pig-to-human compatibility. Future directions to promote xenograft acceptance are discussed, including NK cell tolerance in pregnancy and NK cell evasion in viral infection.

Immunogenetics of xenotransplantation

Xenotransplantation may become the highly desired solution to close the gap between the availability of donated organs and number of patients on the waiting list. In recent years, enormous progress has been made in the development of genetically engineered donor pigs. The introduced genetic modifications showed to be efficient in prolonging xenograft survival. In this review, we focus on the type of immune responses that may target xeno-organs after transplantation and promising immunogenetic modifications that show a beneficial effect in ameliorating or eliminating harmful xenogeneic immune responses. Increasing histocompatibility of xenografts by eliminating genetic discrepancies between species will pave their way into clinical application.

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.

Natural anti-carbohydrate antibodies contributing to evolutionary survival of primates in viral epidemics?

Humans produce multiple natural antibodies against carbohydrate antigens on gastrointestinal bacteria. Two such antibodies appeared in primates in recent geological times. Anti-Gal, abundant in humans, apes and Old-World monkeys, appeared 20-30 million years ago (mya) following inactivation of the α1,3GT gene (GGTA1). This gene encodes in other mammals the enzyme α1,3galactosyltransferase (α1,3GT) that synthesizes α-gal epitopes (Galα1-3Galβ1-4GlcNAc-R) which bind anti-Gal. Anti-Neu5Gc, found only in humans, appeared in hominins <6 mya, following elimination of N-glycolylneuraminic-acid (Neu5Gc) because of inactivation of CMAH, the gene encoding hydroxylase that converts N-acetylneuraminic-acid (Neu5Ac) into Neu5Gc. These antibodies, were initially produced in few individuals that acquired random mutations inactivating the corresponding genes and eliminating α-gal epitopes or Neu5Gc, which became nonself antigens. It is suggested that these evolutionary selection events were induced by epidemics of enveloped viruses, lethal to ancestral Old World primates or hominins. Such viruses presented α-gal epitopes or Neu5Gc, synthesized in primates that conserved active GGTA1 or CMAH, respectively, and were lethal to their hosts. The natural anti-Gal or anti-Neu5Gc antibodies, produced in offspring lacking the corresponding carbohydrate antigens, neutralized and destroyed viruses presenting α-gal epitopes or Neu5Gc. These antibodies further induced rapid, effective immune responses against virus antigens, thus preventing infections from reaching lethal stages. These epidemics ultimately resulted in extinction of primate populations synthesizing these carbohydrate antigens and their replacement with offspring populations lacking the antigens and producing protective antibodies against them. Similar events could mediate the elimination of various carbohydrate antigens, thus preventing the complete extinction of other vertebrate species.

Acceleration of wound healing by α-gal nanoparticles interacting with the natural anti-Gal antibody

Application of α-gal nanoparticles to wounds and burns induces accelerated healing by harnessing the natural anti-Gal antibody which constitutes ~1% of human immunoglobulins. α-gal nanoparticles present multiple α-gal epitopes (Galα1-3Galβ1-4GlcNAc-R), the carbohydrate ligand of anti-Gal. Studied α-gal nanoparticles were comprised of glycolipids with α-gal epitopes, phospholipids, and cholesterol. Binding of anti-Gal to α-gal nanoparticles in wounds activates the complement cascade, resulting in formation of chemotactic complement cleavage peptides that induce rapid recruitment of many macrophages. The Fc/Fcγ receptors interaction between anti-Gal coating α-gal nanoparticles and the recruited macrophages activates macrophages to produce cytokines/growth factors that promote wound healing and recruit stem cells. Studies of wound healing by α-gal nanoparticles were feasible in α1,3galactosyltransferase knockout mice and pigs. In contrast to other nonprimate mammals, these mice and pigs lack the α-gal epitope, and thus they are not immunotolerant to it and produce anti-Gal. Treatment of skin wounds and burns with α-gal nanoparticles resulted in 40-60% decrease in healing time in comparison with control wounds treated with saline. This accelerated healing is associated with increased recruitment of macrophages and extensive angiogenesis in wounds, faster regrowth of epidermis, and regeneration of the dermis. The accelerated healing further decreases and may completely eliminate fibrosis and scar formation in wounds. Since healing of internal injuries is mediated by mechanisms similar to those in external wound healing, it is suggested that α-gal nanoparticles treatment may also improve regeneration and restoration of biological function following internal injuries such as surgical incisions, myocardial ischemia following infarction, and nerve injuries.

Xenotransplantation: immunological hurdles and progress toward tolerance

The discrepancy between organ need and organ availability represents one of the major limitations in the field of transplantation. One possible solution to this problem is xenotransplantation. Research in this field has identified several obstacles that have so far prevented the successful development of clinical xenotransplantation protocols. The main immunologic barriers include strong T-cell and B-cell responses to solid organ and cellular xenografts. In addition, components of the innate immune system can mediate xenograft rejection. Here, we review these immunologic and physiologic barriers and describe some of the strategies that we and others have developed to overcome them. We also describe the development of two strategies to induce tolerance across the xenogeneic barrier, namely thymus transplantation and mixed chimerism, from their inception in rodent models through their current progress in preclinical large animal models. We believe that the addition of further beneficial transgenes to Gal knockout swine, combined with new therapies such as Treg administration, will allow for successful clinical application of xenotransplantation.

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).

Evaluation of novel decellularizing corneal stroma for cornea tissue engineering applications

AIM

To develop a new decellularization method depended upon the natural corneal structure and to harvest an ideal scaffold with good biocompatibilities for corneal reconstruction.

METHODS

The acellular cornea matrix (ACM) were prepared from de-epithelium fresh porcine corneas (DFPCs) by incubation with 100% fresh human sera and additional electrophoresis at 4°C. Human corneal epithelial cells (HCEs) were used for the cytotoxicity tests of ACM. ACM were implanted into the Enhanced Green Fluorecence Protein (eGFP) transgenic mouse anterior chamber for evaluation of histocompatibility.

RESULTS

HE and GSIB4 results showed fresh porcine cornea matrix with 100% human sera and electrophoresis could entirely decellularize stromal cell without reducing its transparency. ACM had no cytotoxic effect ex vivo. Animal test showed there was no rejection for one month after surgery.

CONCLUSION

These results provide a decellularizing approach for the study of corneal tissue engineering and had the broader implications for the field of biological tissue engineering in other engineered organ or tissue matrix.

CD47: a new player in phagocytosis and xenograft rejection

Organ transplantation is limited by the availability of human donor organs. The transplantation of organs and tissues from other species (xenotransplantation) would supply an unlimited number of organs and offer many other advantages for which the pig has been identified as the most suitable source. However, the robust immune responses to xenografts remain a major obstacle to clinical application of xenotransplantation. The more vigorous xenograft rejection relative to allograft rejection is largely accounted for by the extensive genetic disparities between the donor and recipient. Xenografts activate host immunity not only by expressing immunogenic xenoantigens that provide the targets for immune recognition and rejection, but also by lacking ligands for the host immune inhibitory receptors. This review is focused on recent findings regarding the role of CD47, a ligand of an immune inhibitory receptor, signal regulatory protein alpha (SIRPα), in phagocytosis and xenograft rejection.

CD47 in xenograft rejection and tolerance induction

Robust immune responses to xenografts remain a major obstacle to clinical translation of xenotransplantation, which could otherwise be a potential solution to the worldwide shortage of organ donors. The more vigorous xenograft rejection relative to allograft rejection is largely accounted for by the extensive genetic disparities between the donor and recipient. Xenografts activate host immunity not only by expressing immunogenic xenoantigens that provide the targets for immune recognition and rejection, but also by lacking ligands for the host immune inhibitory receptors. This review is focused on recent findings regarding the role of CD47, a ligand of an immune inhibitory receptor SIRPalpha, in xenograft rejection and induction of xenotolerance.

Suppression of human anti-porcine natural killer cell xenogeneic responses by combinations of monoclonal antibodies specific to CD2 and NKG2D and extracellular signal-regulated kinase kinase inhibitor

Natural killer (NK) cells can destroy xenogeneic tissues by antibody-dependent cell cytotoxicity (ADCC) and direct lysis. Unlike ADCC, activating interactions between human NK receptors and their cognate ligands in pigs are not fully elucidated. We set up this study to identify human NK activating receptors recognizing porcine cells isolated from distinct organs, e.g., aorta, cornea and liver, and to provide a molecular basis for effective immunosuppressive regimens. Among the array of NK receptors tested, NKp46, 2B4, CD49d, CD48, CD2 and NKG2D, only CD2 and NKG2D were shown to be involved in both cytotoxicity and cytokine (interferon-gamma and tumour necrosis factor-alpha) production against porcine targets. Simultaneous blocking of CD2 and NKG2D by combining its monoclonal antibodies further suppressed xenogeneic NK responses. Moreover, addition of a suboptimal dose of PD98059, an extracellular signal-regulated kinase (ERK) kinase inhibitor, to those cells maximally reduced NK cytotoxicity, suggesting that ERK plays an important role in NK-mediated xenoreactivity. These impairments in NK cells were tightly associated with defective intracellular calcium mobilization and the subsequent degranulation process. Therefore, our data demonstrate a distinct role of CD2 and NKG2D on human NK cells in recognizing porcine grafts and further provide a potentially efficacious combinational regimen using anti-CD2 and anti-NKG2D monoclonal antibodies with PD98059 in a pig-to-human transplantation model.

Allogeneic melanoma vaccine expressing alphaGal epitopes induces antitumor immunity to autologous antigens in mice without signs of toxicity

Owing to the absence of alphaGal epitopes in human cells and constant stimulation of the immune system by the symbiotic bacterial flora, humans develop high titers of natural antibodies against these epitopes. It has been demonstrated that syngeneic whole cell vaccines modified to express alphaGal epitopes could be used to generate a potent anticancer vaccine. In this study, we tested whether allogeneic whole cell cancer vaccines modified to express alphaGal epitopes would be effective for the treatment of murine melanoma. The alpha(1,3)galactosyltransferase (alphaGT) knockout mice (H-2) with preexisting subcutaneous and pulmonary tumors [alphaGal B16, H-2] received therapeutic vaccinations with S91M3alphaGal (H-2) whole cell allogeneic vaccines. These mice had better survival and reduced pulmonary metastasis burden compared with control mice treated with S91M3 vaccine cells. Vaccination with S91M3alphaGal-induced cytotoxic CD8 T cells recognizing the syngeneic alphaGal B16 tumors measured by adoptive transfer to recipients bearing pulmonary metastases. The presence of allo-antigens did not dominate the induction of immunity to "cryptic" tumor antigens and had helped in the generation of a more efficient vaccine to treat preexisting tumors when compared with classic autologous vaccines. Vaccination with allogeneic alphaGal vaccines did not induce signs of toxicity including changes in weight, hematology, chemistry, and histopathology of major perfused organs or autoimmunity in long-term murine models for breast, lung, and melanoma. This study established the safety and efficacy data of allogeneic alphaGal whole cell vaccines and constituted the basis for the initiation of human clinical trials to treat human malignancies.

Lack of evidence for PERV expression after apoptosis-mediated horizontal gene transfer between porcine and human cells

Evidence for porcine endogenous retrovirus (PERV) infection of human cells has provoked a public health debate over the proposed use of porcine xenografts to alleviate the worldwide shortage of human allografts. Nevertheless, the potential relevance of PERV transmission by apoptosis-mediated horizontal DNA transfer, a documented means of infection-independent retrovirus delivery, appears to have been overlooked in this discussion. To examine the hypothesis that apoptotic cell death during porcine xenograft rejection is capable of fostering horizontal DNA transfer, we have now assessed in vitro cocultures, consisting of phagocytic human fibroblasts and apoptotic or necrotic porcine B-lymphoblastoid cells, for evidence of cross-species PERV exchange and eventual replication. Using real-time polymerase chain reaction (PCR) assays, designed to differentiate nuclear and cytoplasmic DNA derived from either porcine or human cells, we now report evidence for the presence of porcine DNA, including PERV, in the nucleus of human fibroblasts exposed to apoptotic porcine cells. This novel demonstration of apoptosis-mediated horizontal PERV transfer is characterized by a low efficiency of transfer and a transient nature, being present in only 0.22% of the cocultured human cells and disappearing to undetectable levels within 4 weeks of exposure to apoptotic porcine cells. In contrast, using PERV-specific real-time reverse-transcriptase PCR (RT-PCR) and ultra-sensitive product-enhanced reverse transcriptase (PERT) assays, we find no evidence for human fibroblast-derived cellular PERV RNA or coculture supernatant-based RT-activity, indicating a lack of subsequent PERV replication. Together, these results suggest that apoptosis-mediated horizontal PERV transfer does not present an overt hazard within the framework of porcine xenotransplantation. However, we also present arguments against extrapolation of these in vitro observations directly to clinical circumstances.

Natural killer cell subsets in allograft rejection and tolerance

PURPOSE OF REVIEW

To discuss the role of natural killer cells in regulating the survival of transplanted organs.

RECENT FINDINGS

Natural killer cells have been found to have the dual capacity to promote rejection of transplanted organs and be required for the induction of transplantation tolerance. In murine recipients of bone marrow transplants, or in CD28 recipients of cardiac allografts, different natural killer cell subsets have been shown to promote or delay rejection, depending on their major histocompatibility complex class I specificity. In mouse models of skin and islet allograft acceptance mediated by costimulation-targeting therapies, the presence of natural killer cells was found to be essential for long-term graft acceptance, perhaps due to their ability to eliminate donor or recipient immune cells.

SUMMARY

Natural killer cells can either accelerate or avert rejection in a manner that is influenced by both donor-recipient major histocompatibility complex disparity as well as the milieu created by costimulation-targeting therapies. In clinical settings, alloreactivity by defined natural killer cell subsets may be important in achieving tolerance, and the outcome of natural killer cell activity may be influenced by specific immunosuppressive regimens.

Increased immunogenicity of human immunodeficiency virus gp120 engineered to express Galalpha1-3Galbeta1-4GlcNAc-R epitopes

The glycan shield comprised of multiple carbohydrate chains on the human immunodeficiency virus (HIV) envelope glycoprotein gp120 helps the virus to evade neutralizing antibodies. The present study describes a novel method for increasing immunogenicity of gp120 vaccine by enzymatic replacement of sialic acid on these carbohydrate chains with Galalpha1-3Galbeta1-4GlcNAc-R (alpha-gal) epitopes. These epitopes are ligands for the natural anti-Gal antibody constituting approximately 1% of immunoglobulin G in humans. We hypothesize that vaccination with gp120 expressing alpha-gal epitopes (gp120(alphagal)) results in in vivo formation of immune complexes with anti-Gal, which targets vaccines for effective uptake by antigen-presenting cells (APC), due to interaction between the Fc portion of the antibody and Fcgamma receptors on APC. This in turn results in effective transport of the vaccine to lymph nodes and effective processing and presentation of gp120 immunogenic peptides by APC for eliciting a strong anti-gp120 immune response. This hypothesis was tested in alpha-1,3-galactosyltransferase knockout mice, which produce anti-Gal. Mice immunized with gp120(alphagal) produced anti-gp120 antibodies in titers that were >100-fold higher than those measured in mice immunized with comparable amounts of gp120 and effectively neutralized HIV. T-cell response, measured by ELISPOT, was much higher in mice immunized with gp120(alphagal) than in mice immunized with gp120. It is suggested that gp120(alphagal) can serve as a platform for anti-Gal-mediated targeting of additional vaccinating HIV proteins fused to gp120(alphagal), thereby creating effective prophylactic vaccines.

Xenotransplantation and ABO incompatible transplantation: The similarities they share

Transplantation of kidney allografts across the ABO barrier has been feasible with the development of technologies for removal of anti-blood group antibodies from the circulation of the recipent. The recipients of ABO incompatible grafts display tolerance, accommodation or rejection of the graft. Understanding the factors that determine the outcome of the immune response against incompatible blood group antigens has required the study of an appropriate experimental animal model. The model used is that of knockout (KO) mice for the alpha1,3galactosyltransferase gene, lacking the alpha-gal epitopes and transplanted with wild type mouse heart expressing the alpha-gal epitope. The alpha-gal epitope (Galalpha1-3Galbeta1-(3)4GlcNAc-R) is one of the most abundant carbohydrate epitopes on cells of non-primate mammals and New World monkeys, where it is synthesized by the glycosylation enzyme alpha1,3galactosyltransferase. In humans, apes and Old World monkeys, this epitope is absent due to an evolutionary event that led to the inactivation of the alpha1,3galactosyltransferase gene in ancestral Old World primates. Instead, humans, apes and Old World monkeys produce a natural antibody, the anti-Gal antibody, that is the most abundant natural antibody in humans (approximately 1% of circulating immunoglobulins) and which specifically interacts with alpha-gal epitopes. The interaction between anti-Gal and alpha-gal epitopes is a major immunologic barrier in xenotransplantation, preventing transplantation of pig organs or tissues (i.e. xenografts) into humans. Anti-Gal antibodies also comprise a large proportion of anti-blood group B activity in A and O individuals. Moreover, in recipients of ABO incompatible grafts, much of the elicited anti-A and anti-B antibodies are in fact anti-Gal antibodies capable of binding also to the incompatible blood group antigens. Since the alpha-gal epitope is very similar in its structure to blood groups A and B, understanding anti-Gal response to alpha-gal epitopes is likely to provide information on the immune response to ABO incompatible antigens. Studies on the immune response to alpha-gal epitopes in KO mice have indicated that this epitope can not activate T cells. Anti-Gal B cells engaging alpha-gal epitopes on transplated wild type mouse heart can be activated to produce their antibodies only if they receive help from T cells that are activated by allogeneic or xenogeneic peptides. If T cell help is not available for several days the B cells are induced to differentiate into cells capable of producing accommodating antibodies. Accommodating anti-Gal antibodies bind to the incompatible carbohydrate antigen but do not induce rejection. Prolonged exposure of anti-Gal B cells to the incompatible alpha-gal epitope on the wild type mouse heart graft induces tolerance due to the deletion of these B cells. These studies imply that similar variation in the availability of T cell help in recipients of ABO incompatible grafts result in rejection, accommodation or tolerance, to the blood group antigen. The studies on immune response to incompatible alpha-gal epitopes have further indicated that tolerance to incompatible blood group antigens can be achieved by gene therapy with autologous bone marrow cells or autologous lymphocytes engineered to express the incompatible blood group antigen. Studies in the mouse model suggest that administration into the patient such autologous cells engineered to express the incompatible transplantation carbohydrate antigen induces deletion of anti-blood group B cells and induction of tolerance, provided that the anti-blood group antibodies are removed. Such tolerance is perpetuated indefinitely by the subsequent transplantation of the organ expressing the incompatible blood group antigen.

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.

The alpha-gal epitope and the anti-Gal antibody in xenotransplantation and in cancer immunotherapy

The alpha-gal epitope (Galalpha1-3Galbeta1-(3)4GlcNAc-R) is abundantly synthesized on glycolipids and glycoproteins of non-primate mammals and New World monkeys by the glycosylation enzyme alpha1,3galactosyltransferase (alpha1,3GT). In humans, apes and Old World monkeys, this epitope is absent because the alpha1,3GT gene was inactivated in ancestral Old World primates. Instead, humans, apes and Old World monkeys produce the anti-Gal antibody, which specifically interacts with alpha-gal epitopes and which constitutes approximately 1% of circulating immunoglobulins. Anti-Gal has functioned as an immunological barrier, preventing the transplantation of pig organs into humans, because anti-Gal binds to the alpha-gal epitopes expressed on pig cells. The recent generation of alpha1,3GT knockout pigs that lack alpha-gal epitopes has resulted in the elimination of this immunological barrier. Anti-Gal can be exploited for clinical use in cancer immunotherapy by targeting autologous tumour vaccines to APC, thereby increasing their immunogenicity. Autologous intact tumour cells from haematological malignancies, or autologous tumour cell membranes from solid tumours are processed to express alpha-gal epitopes by incubation with neuraminidase, recombinant alpha1,3GT and with uridine diphosphate galactose. Subsequent immunization with such autologous tumour vaccines results in in vivo opsonization by anti-Gal IgG binding to these alpha-gal epitopes. The interaction of the Fc portion of the vaccine-bound anti-Gal with Fcgamma receptors of APC induces effective uptake of the vaccinating tumour cell membranes by the APC, followed by effective transport of the vaccinating tumour membranes to the regional lymph nodes, and processing and presentation of the tumour-associated antigen (TAA) peptides. Activation of tumour-specific T cells within the lymph nodes by autologous TAA peptides may elicit an immune response that in some patients will be potent enough to eradicate the residual tumour cells that remain after completion of standard therapy. A similar expression of alpha-gal epitopes can be achieved by transduction of tumour cells with an adenovirus vector (or other vectors) containing the alpha1,3GT gene, thus enabling anti-Gal-mediated targeting of the vaccinating transduced cells to APC. Intratumoral delivery of the alpha1,3GT gene by various vectors results in the expression of alpha-gal epitopes. Such expression of the xenograft carbohydrate phenotype is likely to induce anti-Gal-mediated destruction of the tumour lesion, similar to rejection of xenografts by this antibody. Opsonization of the destroyed tumour cell membranes by anti-Gal IgG further targets them to APC, thus converting the tumour lesion, treated by the alpha1,3GT gene, into an in situ autologous tumour vaccine.

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.

Expression of human ecto-5'-nucleotidase in pig endothelium increases adenosine production and protects from NK cell-mediated lysis

Ecto-5'-nucleotidase (E5'N) is an endothelial surface enzyme that controls conversion of extracellular nucleotides into immunosuppressive adenosine. We evaluated whether expression of human E5'N on pig endothelial cells (EC) attenuates human NK cell-mediated cytotoxicity. A pig EC line was stably transfected with human E5'N and human NK cell adhesion and cytotoxicity toward pig EC cultures was measured by flow cytometry and intracellular enzyme release. E5'N activity in pig EC lysates increased from 0.68 +/- 0.07 to 1013 +/- 293 nmol/min/mg protein, whilst the rate of AMP to adenosine metabolism by intact cells increased from 0.37 +/- 0.05 to >300 nmol/min/mg protein in non-transfected and transfected cells, respectively. The rate of adenosine production in transfected cells increased also with ATP as the extracellular substrate. Cytotoxicity of human NK cells was reduced from 10.7 +/- 0.4% and 11.1 +/- 1.1% with non-transfected pig EC to 5.2 +/- 0.2% and 5.0 +/- 0.2% in transfected cells with 50 microM and 250 microM AMP, respectively. Reduction of cytotoxicity in E5'N-transfected EC was abolished by the E5'N inhibitor and was mimicked in non-transfected EC by the addition of adenosine, demonstrating the key role of adenosine produced by E5'N in inhibiting NK cell cytotoxicity. We suggest that overexpression of E5'N in EC of transgenic pigs is a possible strategy to ameliorate rejection after xenotransplantation.

Application of xenogeneic stem cells for induction of transplantation tolerance: present state and future directions

Xenotransplantation using pig organs provides a possible solution to the severe shortage of allogeneic organ donors, one of the major limiting factors in clinical transplantation. However, because of the greater antigenic differences that exist between different species than within a species, the immune response to xenografts is much more vigorous than to allografts. Thus, tolerance induction is essential to the success of clinical xenotransplantation. Tolerance induced by mixed hematopoietic chimerism across the MHC barrier is remarkably robust, but its ability to induce tolerance across highly disparate xenogeneic barriers remains poorly studied. None of the current available regimens of host conditioning, which permit hematopoietic stem cell engraftment and chimerism induction in allogeneic or closely related (concordant) xenogeneic combinations, has been demonstrated to be effective in establishing porcine hematopoietic chimerism in a discordant xenogeneic species. Unlike bone marrow transplantation within the same species, the innate immune system and the species specificity of cytokines and adhesion molecules essential to hematopoiesis pose formidable obstacles to the establishment of donor hematopoiesis across discordant xenogeneic barriers. The genetic incompatibility between species may also impede xenograft tolerance induction by mixed chimerism. While we remain far from achieving tolerance in clinical xenotransplantation, recent studies using a transgenic mouse model have proven the principle that mixed hematopoietic chimerism may induce mouse and human T cell tolerance to porcine xenografts. This review article focuses on the barriers to porcine hematopoietic engraftment in highly disparate xenogeneic species and the possible application of mixed hematopoietic chimerism to xenograft tolerance induction.

CD8-interaction mutant HLA-Cw3 molecules protect porcine cells from human natural killer cell-mediated antibody-dependent cellular cytotoxicity without stimulating cytotoxic T lymphocytes1

BACKGROUND

CD56+ human natural killer (NK) cells are the principal anti-pig cytotoxic effectors in vitro. Expression of certain human leukocyte antigen (HLA) class I molecules in porcine cells can inhibit NK cell-mediated natural cytotoxicity in serum-free medium, but had not been shown to inhibit antibody-dependent cellular cytotoxicity (ADCC) by CD16+ NK cells in the presence of human xenoreactive immunoglobulin G. Moreover, expression of HLA molecules might amplify the previously weak CD8+ cytotoxic T-lymphocyte (CTL) response against porcine cells.

METHODS

A novel porcine B-lymphoblastoid cell line (13271) was stably transfected with HLA-Cw*0304 gene constructs encoding wild-type (wt) Cw3 or genetically modified Cw3 unable to interact with CD8 (Cw3-D227K). The Cw3 transfectants were used in limiting dilution assays to estimate the CTL precursor frequency in CD56-depleted human peripheral blood mononuclear cells (PBMC) obtained from eight unrelated donors. The 13271 transfectants were also used as targets for clonal and polyclonal NK cells in the presence and absence of human serum, to measure inhibition of ADCC.

RESULTS

Expression of Cw3-wt in 13271 cells significantly increased the human CTL response compared with the empty-vector control transfectant, whereas no significant increase resulted from expression of CD8-interaction mutant Cw3-D227K molecules. The Cw3-D227K mutant was indistinguishable from Cw3-wt in its ability to inhibit both natural cytotoxicity and ADCC mediated by human NK clones that have the appropriate CD158b inhibitory receptor.

CONCLUSIONS

Transgenic expression of HLA molecules in pig cells will likely amplify the CD8+ CTL response against the xenograft. Disruption of HLA-CD8 interaction could minimize this amplification without compromising NK-cell inhibition.

Xenotransplantation and pig endogenous retroviruses

Xenotransplantation, in particular transplantation of pig cells, tissues and organs into human patients, may alleviate the current shortage of suitable allografts available for human transplantation. This overview addresses the physiological, immunological and virological factors considered with regard to xenotransplantation. Among the issues reviewed are the merits of using pigs as xenograft source species, the compatibility of pig and human organ physiology and the immunological hindrances with regard to the various types of rejection and attempts at abrogating rejection. Advances in the prevention of pig organ rejection by creating genetically modified pigs that are more suited to the human microenvironment are also discussed. Finally, with regard to virology, possible zoonotic infections emanating from pigs are reviewed, with special emphasis on the pig endogenous retrovirus (PERV). An in depth account of PERV studies, comprising their discovery as well as recent knowledge of the virus, is given. To date, all retrospective studies on patients with pig xenografts have shown no evidence of PERV transmission, however, many factors make us interpret these results with caution. Although the lack of PERV infection in xenograft recipients up to now is encouraging, more basic research and controlled animal studies that mimic the pig to human xenotransplantation setting more closely are required for safety assessment.

Direct killing of xenograft cells by CD8+ T cells of discordant xenograft recipients

BACKGROUND

Long-term pig xenografts in monkeys demonstrated the infiltration of CD8 T cells into pig cartilage xenografts, transplanted into monkeys. The objective of the present study was to determine in an experimental animal model whether CD8 T cells in pig xenograft recipients exert any direct cytotoxic effect on pig cells.

METHODS

The killing of xenograft cells by CD8 T cells, obtained from xenograft recipients, was studied in alpha1,3galactosyltransferase knockout mice that were repeatedly injected intraperitoneally with pig kidney membranes. The pig kidney cell line PK15, which shares many antigens with pig kidney membranes, served as a model for xenograft target cells in cytotoxicity assays. Cell lines from other species were also studied as target cells.

RESULTS

Lymphocytes obtained freshly from spleens of mice immunized with pig kidney membranes failed to display significant cytotoxic activity against pig cells. However, incubation of these lymphocytes with irradiated PK15 cells and addition of recombinant interleukin (IL)-2 (100 U/mL), on the third day of incubation, resulted in extensive proliferation and expansion of CD8 cytotoxic T lymphocytes (CTL). These CTL, obtained after 12 days of incubation, killed nonspecifically pig, human, and mouse normal and malignant cells. These CTL were not generated in cultures in the absence of stimulatory pig cells or in the absence of IL-2. These CTL could not be generated in cultures of lymphocytes from naive mice that were incubated with PK15 cells and IL-2.

CONCLUSIONS

The data obtained imply that CD8 T cells from xenograft recipients can be stimulated in vitro by xenoantigens and IL-2 to differentiate into highly reactive nonspecific CTL that are capable of killing a large variety of xenogeneic and syngeneic cells. Similar in vivo microenvironmental conditions within the xenograft may induce the local differentiation of infiltrating CD8 T cells into CTL that can destroy nonspecifically adjacent xenograft cells. Such cells may not be active outside the xenograft because of the absence of IL-2 in sufficiently high concentrations.

Genetically modified HLA class I molecules able to inhibit human NK cells without provoking alloreactive CD8+ CTLs

Human NK cells are likely to be important effectors of xenograft rejection. Expression of HLA class I molecules by transfected porcine cells can protect them from human NK cell-mediated lysis; however, this strategy has the potential to augment the anti-graft response by recipient CD8(+) T cells recognizing foreign pig peptides presented by HLA. In this study we show that the introduction of a mutation (D227K) in the alpha(3) domain of HLA-Cw3 abrogates its recognition by CD8-dependent T cells but leaves intact its ability to function as an inhibitory ligand for NK cells. Such genetically modified molecules may have potential therapeutic applications in the prevention of delayed xenograft rejection and in the facilitation of allogeneic and xenogeneic bone marrow engraftment.

Activation of human dendritic cells by porcine aortic endothelial cells: transactivation of naïve T cells through costimulation and cytokine generation

BACKGROUND

Dendritic cells (DC) are the most potent antigen-presenting cells in the immune system. To define the role of human DC in human anti-porcine immune responses, we defined the interaction of human DC with porcine aortic endothelial cells (PAEC).

METHODS

To determine the immune responses, both monocyte-derived and peripheral blood DC were cultured with porcine and human endothelial cells. We analyzed the role of CD11a, CD11b, and CD54 in a cell-to-cell adhesion assay using antibodies against these molecules. The expression pattern of costimulatory molecules (CD40, CD80, CD86), adhesion molecules (CD54), and intracellular cytokines (interleukin-12p70 and tumor necrosis factor [TNF]-alpha) in DC after interaction with endothelial cells was determined by immunofluorescence.

RESULTS

Human DC significantly adhered to PAEC (38-40%), and this adhesion was augmented (>50%) upon treatment with either recombinant swine interferon-gamma or recombinant human TNF-alpha. Addition of human DC to PAEC was blocked by pretreatment of DC with antibodies specific to human leukocyte function-associated antigen-1 or CD54. Adhesion of DC to PAEC also resulted in the activation of DC, which was manifested by up-regulation of costimulatory molecules (CD40, CD80, CD86), adhesion molecules (CD54), and HLA-DR. PAEC-activated human DC provided proliferative signals to the naïve autologous CD4+ T cells and synthesized interleukin-12p70 and TNF-alpha. However, activated DCs failed to lyse PAEC in such interaction.

CONCLUSION

Human DC effectively adhered to PAEC and were activated by xenoantigen, resulting in highly efficient antigen presentation and proliferation of CD4+ T cells. Further, this interaction of human DC to PAEC is regulated by the participation of costimulatory and adherence molecules and cytokines.

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

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

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.

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.

Immunobiology of xenogeneic cornea grafts in mouse eyes. II. Immunogenicity of xenogeneic cornea tissue grafts implanted in anterior chamber of mouse eyes

BACKGROUND

Xenogeneic corneal fragments (guinea pig) are highly resistant to immune rejection in the anterior chamber of mouse eyes. Because guinea pigs and mice are discordant (i.e., mouse serum normally contains guinea pig reactive xenoantibodies), we wished to determine the extent to which xenogeneic corneal fragments placed intraocularly in normal and specifically sensitized mice activated xenoreactive T and B cells.

METHODS

Guinea pig corneas, deprived surgically of epithelium, were cut into fragments and inserted into the anterior chamber of eyes of BALB/c mice, adjacent to the central cornea of the recipient. Antibody (immunoglobulin [Ig]M, IgG) and delayed type hypersensitivity (DTH) immune responses of recipient mice to guinea pig xenoantigens were assessed. The fate of xenogeneic cornea implants was assessed in mice immunized systemically to guinea pig antigens.

RESULTS

Guinea pig spleen cells and corneal fragments implanted s.c. induced within 2 weeks of immunization both DTH and IgG antibodies to guinea pig xenoantigens. By contrast, xenogeneic corneal fragments implanted in the anterior chamber of mouse eyes evoked no change in recipient humoral immune status and induced mild guinea pig-specific DTH only after 5 weeks. Presensitization of mice to guinea pig xenoantigens failed to increase the proportion of grafts that were regarded as rejected, but the onset of rejection in failed grafts occurred earlier than in unsensitized recipients. Active systemic immunization of mice bearing intracameral guinea pig corneal fragments failed to curtail the grafts' survival. Guinea pig corneal fragments implanted in the anterior chamber of normal mice failed to induce anterior chamber-associated immune deviation.

CONCLUSIONS

Xenogeneic corneal fragments implanted in the anterior chamber of eyes of normal mice display strikingly reduced immunogenicity, and an inability to induce anterior chamber-associated immune deviation. These properties are discussed in terms of the vulnerability of guinea pig corneal tissue to immune rejection within the eye.

Accommodated Xenografts Survive in the Presence of Anti-Donor Antibodies and Complement That Precipitate Rejection of Naive Xenografts

Hamster hearts transplanted into transiently complement-depleted and continuously cyclosporin A (CyA)-immunosuppressed rats survive long-term despite deposition of anti-donor IgM Abs and complement on the graft vascular endothelium. This phenomenon is referred to as “accommodation.” The hypothesis tested here is that accommodated xenografts are resistant to IgM Abs and complement that could result in rejection of naive xenografts. After first hamster hearts had been surviving in cobra venom factor (CVF) + CyA-treated rats for 10 days, a time when the anti-donor IgM Ab level was maximal and complement activity had returned to approximately 50% of pretreatment levels, naive hamster hearts or hamster hearts that had been accommodating in another rat for 14 days were transplanted into those rats carrying the surviving first graft. The naive hearts were all hyperacutely rejected. In contrast, a majority of regrafted accommodating hearts survived long-term. There was widespread Ab and activated complement deposition on the vascular endothelium of accommodating first hearts, second accommodating hearts, and rejected second naive hearts. However, only the rejected naive hearts showed extensive endothelial cell damage, myocardial necrosis, fibrin deposition, and other signs of inflammation. Accommodating first and second hearts but not rejected second naive hearts expressed high levels of the protective genes A20, heme oxygenase-1 (HO-1), bcl-2, and bcl-xL. These data demonstrate that accommodated xenografts become resistant to effects of anti-donor IgM Abs and complement that normally mediate rejection of xenografts. We hypothesize that this resistance involves expression by accommodated xenografts of protective genes.

Rejection of Cardiac Xenografts by CD4+ or CD8+ T Cells

We recently showed that brief complement inhibition induces accommodation of hamster cardiac transplants in nude rats. We have reconstituted nude rats carrying an accommodated xenograft with syngeneic CD4+ or CD8+ T cells to investigate the cellular mechanism of xenograft rejection. We show that CD4+ T cells can initiate xenograft rejection (10 ± 1.7 days) by promoting production of IgG xenoreactive Abs (XAb). These XAb are able to activate complement as well as to mediate Ab-dependent cell-mediated cytotoxicity. Adoptive transfer of these XAb into naive nude rats provoked hyperacute xenograft rejection (38 ± 13 min). The rejection was significantly (p &lt; 0.001) delayed by cobra venom factor (CVF; 11 ± 8 h in four of five cases) but was still more rapid than in control nude rats (3.3 ± 0.5 days). CVF plus NK cell depletion further prolonged survival (&gt;7 days in four of five cases; p &lt; 0.01 vs CVF only). CD8+ T cell-reconstituted nude rats rejected their grafts later (19.4 ± 5.8 days) and required a larger number of cells for transfer as compared with CD4+ T cell-reconstituted nude rats. However, second xenografts were rejected more rapidly than first xenografts in CD8+ T cell-reconstituted nude rats (9 ± 2 days), indicating that the CD8+ T cells had been activated. This study demonstrates that CD4+ and CD8+ T cells can both reject xenografts. The CD4+ cells do so at least in part by generation of helper-dependent XAb that act by both complement-dependent and Ab-dependent cell-mediated cytotoxicity mechanisms; the CD8+ cells do so as helper-independent cytotoxic T cells.

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.

Adult and neonatal anti-Gal response in knock-out mice for alpha1,3galactosyltransferase

The knockout mouse to alpha1,3galactosyltransferase (alpha1,3GT KO) lacks the ability to synthesize alpha-gal epitopes (Galalpha1,3Galbeta1,4GlcNAc-R) and is capable of producing low amounts of the natural anti-Gal antibody. The present study indicates that repeated immunization of these mice with rabbit red blood cell (RRBC) membranes results in production of anti-Gal in titers and specificity similar to those in humans. In contrast, immunized wild-type mice completely lack anti-Gal. Anti-Gal in the alpha1,3GT KO mice is produced in the circulation as the various IgG subclasses and as IgM isotype, but not IgA. In view of previous reports on the possible induction of T cell tolerance by immunization of mice with large amounts of antigen up to 24 days of age, we assayed possible induction of neonatal B cell tolerance toward the alpha-gal epitope. Eight-day-old neonates were subjected to immunization with 1 x 10(8) RRBC membranes, or 30 x 10(6) wild type mouse splenocytes, both of which express an abundance of alpha-gal epitopes. These neonatal exposures to alpha-gal epitopes did not prevent subsequent production of anti-Gal. Thus, the tolerance induction to this carbohydrate epitope is likely to be mediated by mechanisms other than those inducing neonatal T cell tolerance.

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.

Human lymphocyte adhesion to xenogeneic porcine endothelial cells: modulation by human TNF-alpha and involvement of VLA-4 and LFA-1

Considering that in the allogeneic situation the adhesion of recipient lymphocytes to donor endothelial cells initiates the cellular rejection, we questioned the possible occurrence of a similar process in the xenogeneic situation. The adhesion of human peripheral blood lymphocytes (PBL) to porcine aortic endothelial cells (PAEC) was thus studied in an in vitro porcine-to-human xenogeneic model. It was found that 25.9% of human PBL adhered to resting PAEC. Furthermore, this adhesion increased significantly when the PAEC were stimulated by the human cytokine TNF-alpha (tumor necrosis factor-alpha). The effect of human TNF-alpha was concentration- and time-dependent and was maximal (from 25.9% to 35.6%) with 100 U/ml during 6 h. Moreover, blocking experiments with monoclonal antibody (mAb) demonstrated the role of the PBL adhesion molecules LFA-1 and especially VLA-4. Indeed, an anti-CD11a mAb decreased PBL adhesion to resting PAEC by 17.1% and to TNF-alpha stimulated PAEC by 16.9%, whereas an anti-CD49d mAb decreased dramatically PBL adhesion to resting PAEC by 53.1% and to TNF-alpha stimulated PAEC by 41.0%. Finally, phenotypic analysis of the adherent PBL showed that 50.5% of adherent cells to resting PAEC were NK (natural killer) cells, whereas 50.7% of adherent cells to TNF-alpha stimulated PAEC were T lymphocytes, showing the preferential adhesion of NK cells to resting PAEC, and that the stimulation of the PAEC with human TNF-alpha affects predominantly T lymphocyte adhesion. These results indicate that human PBL could bind to xenogeneic PAEC and that this interaction could be a first step of a xenogeneic cellular rejection.