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

Antibody-mediated rejection in xenotransplantation: Can it be prevented or reversed?

Antibody-mediated rejection (AMR) is the commonest cause of failure of a pig graft after transplantation into an immunosuppressed nonhuman primate (NHP). The incidence of AMR compared to acute cellular rejection is much higher in xenotransplantation (46% vs. 7%) than in allotransplantation (3% vs. 63%) in NHPs. Although AMR in an allograft can often be reversed, to our knowledge there is no report of its successful reversal in a pig xenograft. As there is less experience in preventing or reversing AMR in models of xenotransplantation, the results of studies in patients with allografts provide more information. These include (i) depletion or neutralization of serum anti-donor antibodies, (ii) inhibition of complement activation, (iii) therapies targeting B or plasma cells, and (iv) anti-inflammatory therapy. Depletion or neutralization of anti-pig antibody, for example, by plasmapheresis, is effective in depleting antibodies, but they recover within days. IgG-degrading enzymes do not deplete IgM. Despite the expression of human complement-regulatory proteins on the pig graft, inhibition of systemic complement activation may be necessary, particularly if AMR is to be reversed. Potential therapies include (i) inhibition of complement activation (e.g., by IVIg, C1 INH, or an anti-C5 antibody), but some complement inhibitors are not effective in NHPs, for example, eculizumab. Possible B cell-targeted therapies include (i) B cell depletion, (ii) plasma cell depletion, (iii) modulation of B cell activation, and (iv) enhancing the generation of regulatory B and/or T cells. Among anti-inflammatory agents, anti-IL6R mAb and TNF blockers are increasingly being tested in xenotransplantation models, but with no definitive evidence that they reverse AMR. Increasing attention should be directed toward testing combinations of the above therapies. We suggest that treatment with a systemic complement inhibitor is likely to be most effective, possibly combined with anti-inflammatory agents (if these are not already being administered). Ultimately, it may require further genetic engineering of the organ-source pig to resolve the problem entirely, for example, knockout or knockdown of SLA, and/or expression of PD-L1, HLA E, and/or HLA-G.

α

The xenogeneic decellularized corneal matrix (DCM) was expected to be used in lamellar keratoplasty in clinic as the substitute of allogeneic cornea. After decellularization treatment, the remaining risk of xenograft rejection needed to be assessed. The galactose-α1,3-galactose, as the most abundant and closely rejection-related xenogeneic antigen, should be one of the important factors concerned in immunological evaluation. In this study, residual αGal in the DCM was first determined by an enzyme-linked immunosorbent assay method with qualified accuracy and specificity. Then the DCM was implanted subcutaneously into the α1,3-galactosyltransferase gene-knockout (GTKO) mice, accompanied by the implantation in the wild-type C57BL/6 mice as a comparison. The total serum antibody levels, anti-Gal antibody levels, inflammatory cytokines and ratios of splenic lymphocyte subtypes were detected and the histopathological analysis of implants were performed to systematically evaluate the immune responses. The experimental result showed the fresh porcine corneal matrix samples had (9.90 ± 1.54) × 10¹² αGal epitope per mg while the content of residual αGal in the DCM was (7.90 ± 2.00) × 10¹² epitope per mg. The GTKO mice had similar potential of reaction to immune stimulation to that of wild-type C57BL/6 mice. At 4 weeks after implantation of DCM, in WT mice and GTKO mice there were both innate immunity response to the DCM characterized by macrophage infiltration. But the elevations of anti-Gal IgG level and the percentage of splenic natural killer cells were only detected in GTKO mice. These changes were thought to be pertinent to the residual αGal antigen, which could not be detected in WT mice. No further αGal antibody-mediated cellular immunity and significant changes of serum cytokine contents were found in GTKO mice, which perhaps suggested that the immune reactions to the DCM after 4 weeks of implantation were moderate and had minor effect on the survival of the corneal graft.

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.

Immune modulation in xenotransplantation

The use of animals as donors of tissues and organs for xenotransplantations may help in meeting the increasing demand for organs for human transplantations. Clinical studies indicate that the domestic pig best satisfies the criteria of organ suitability for xenotransplantation. However, the considerable phylogenetic distance between humans and the pig causes tremendous immunological problems after transplantation, thus genetic modifications need to be introduced to the porcine genome, with the aim of reducing xenotransplant immunogenicity. Advances in genetic engineering have facilitated the incorporation of human genes regulating the complement into the porcine genome, knockout of the gene encoding the formation of the Gal antigen (α1,3-galactosyltransferase) or modification of surface proteins in donor cells. The next step is two-fold. Firstly, to inhibit processes of cell-mediated xenograft rejection, involving natural killer cells and macrophages. Secondly, to inhibit rejection caused by the incompatibility of proteins participating in the regulation of the coagulation system, which leads to a disruption of the equilibrium in pro- and anti-coagulant activity. Only a simultaneous incorporation of several gene constructs will make it possible to produce multitransgenic animals whose organs, when transplanted to human recipients, would be resistant to hyperacute and delayed xenograft rejection.

Kidney xenotransplantation

Xenotransplantation using pigs as donors offers the possibility of eliminating the chronic shortage of donor kidneys, but there are several obstacles to be overcome before this goal can be achieved. Preclinical studies have shown that, while porcine renal xenografts are broadly compatible physiologically, they provoke a complex rejection process involving preformed and elicited antibodies, heightened innate immune cell reactivity, dysregulated coagulation, and a strong T cell-mediated adaptive response. Furthermore, the susceptibility of the xenograft to proinflammatory and procoagulant stimuli is probably increased by cross-species molecular defects in regulatory pathways. To balance these disadvantages, xenotransplantation has at its disposal a unique tool to address particular rejection mechanisms and incompatibilities: genetic modification of the donor. This review focuses on the pathophysiology of porcine renal xenograft rejection, and on the significant genetic, pharmacological, and technical progress that has been made to prolong xenograft survival.

Tumor-Associated Glycans and Immune Surveillance

Changes in cell surface glycosylation are a hallmark of the transition from normal to inflamed and neoplastic tissue. Tumor-associated carbohydrate antigens (TACAs) challenge our understanding of immune tolerance, while functioning as immune targets that bridge innate immune surveillance and adaptive antitumor immunity in clinical applications. T-cells, being a part of the adaptive immune response, are the most popular component of the immune system considered for targeting tumor cells. However, for TACAs, T-cells take a back seat to antibodies and natural killer cells as first-line innate defense mechanisms. Here, we briefly highlight the rationale associated with the relative importance of the immune surveillance machinery that might be applicable for developing therapeutics.

Carbohydrate antigens

PURPOSE OF REVIEW

To summarize the current knowledge of carbohydrate antigens as related to xenotransplantation. The emphasis is on non-Gal carbohydrate antigens identified in many institutes. In addition, several topics such as glycosyltransferase-transgenic pigs, innate cell receptors and porcine endogenous retrovirus (PERV) will be discussed.

RECENT FINDINGS

Studies related to iGb3 and neoantigens after knocking out GalT (GGTA1) were reviewed. Available data do not support the conclusion that GalT-KO remains iGb3 and/or that neoantigens are produced in the pigs. Concerning non-Gal antigen, in addition to the Hanganutziu-Deicher (H-D) antigen (NeuGc), Forrsman antigen, Galα1-3Lew(x), α-linked or β-linked GalNAc, β3 linked Gal, NeuAc, such as Neu5Acα2-3Galβ1-3GlcNAc, and Sid blood group (Sd(a))-like antigens are candidates. However, to date some of these remain controversial and others need further study to completely identify them. Regarding the H-D antigen, different from the α-Gal, it has a complicated expression system, but has cytotoxic effects toward pig cells. To modify other carbohydrate antigen apart from α-Gal, only the overexpression of GnT-III appears to have an effect on the suppression of the N-linked sugar of non-Gal antigen. Concerning innate cell receptors related to carbohydrates (ligands), the focus turned from natural killer (NK) receptor to others, such as monocytes. Finally, PERV contains a ligand with an N-linked sugar. Modification of the glycosylation pattern appears to be associated with regulating PERV infectivity.

SUMMARY

A considerable amount of data related to carbohydrate antigens is now available. At the same time, however, discrepancies between studies complicate this issue. Further studies will be needed to completely understand this complicated area of interest.

Porcine sialoadhesin: a newly identified xenogeneic innate immune receptor

Extracorporeal porcine liver perfusion is being developed as a bridge to liver allotransplantation for patients with fulminant hepatic failure. This strategy is limited by porcine Kupffer cell destruction of human erythrocytes, mediated by lectin binding of a sialic acid motif in the absence of antibody and complement. Sialoadhesin, a macrophage restricted lectin that binds sialic acid, was originally described as a sheep erythrocyte binding receptor. Given similarities between sialoadhesin and the unidentified macrophage lectin in our model, we hypothesized porcine sialoadhesin contributed to recognition of human erythrocytes. Two additional types of macrophages were identified to bind human erythrocytes-spleen and alveolar. Expression of sialoadhesin was confirmed by immunofluorescence in porcine tissues and by flow cytometry on primary macrophages. A stable transgenic cell line expressing porcine sialoadhesin (pSn CHO) bound human erythrocytes, while a sialoadhesin mutant cell line did not. Porcine macrophage and pSn CHO recognition of human erythrocytes was inhibited approximately 90% by an antiporcine sialoadhesin monoclonal antibody and by human erythrocyte glycoproteins. Furthermore, this binding was substantially reduced by sialidase treatment of erythrocytes. These data support the hypothesis that porcine sialoadhesin is a xenogeneic receptor that mediates porcine macrophage binding of human erythrocytes in a sialic acid-dependent manner.

Cellular xenotransplantation

PURPOSE OF REVIEW

Expectations are high on cellular therapy. Being fundamental to elucidate organogenesis, it is unlikely that embryonic stem cells will be used for clinical purposes. Postembryonic stage, developing cells are, therefore, the front-runner for regenerative medicine. In addition to autologous cells, both allogeneic and xenogeneic cells are hypothetical candidates to treat specific diseases. This review summarizes the current knowledge on immunological and functional aspects of xeno(allo)-cellular transplantation for cardiomyopathy, diabetes, liver failure, neural diseases, and bone regeneration.

RECENT FINDINGS

Xenocellular transplantation is promising for tissue repair in immunologically privileged sites such as the central nervous system or nonvascularized tissues in which no or moderate immunosuppression is required. In vascularized organs, major immune responses are present when cells are transplanted without additional conditioning. Positive results from encapsulation methods that protect cells from the immune system should further stimulate preclinical research. Also, conditioning immunosuppression could be used to circumvent the initial immune response. Transgenic pigs cells are probably the best xenogeneic substitute for human application, although basic research on innate and noninnate immunity toward pig cells is still required.

SUMMARY

In several fields of medicine, cellular xenotransplantation is slowly emerging as a potential therapeutic tool.

Immune responses to alpha1,3 galactosyltransferase knockout pigs

PURPOSE OF REVIEW

To summarize the current knowledge of the immune response generated against xenografts stemming from alpha1,3-galactosyltransferase knockout (GalT-KO) pigs. In particular, we will address the nature of potentially remaining Gal epitopes, the role of non-Gal xenoantigens, and the cellular response directed against GalT-KO tissues.

RECENT FINDINGS

New findings support the view that porcine cells do not express isoglobotrihexosylceramide 3, and GalT-KO pigs, if at all, express negligible levels of Gal. The anti-non-Gal antibody response to GalT-KO cells allowed the identification of several potentially relevant porcine xenoantigens, mainly carbohydrates. Coculture of wildtype pig aortic endothelial cells but not of GalT-KO pig aortic endothelial cells with whole human blood induces the secretion of porcine and human cytokines and the upregulation of E-selectin; in contrast, the transmigration of human leukocytes across porcine endothelium is not regulated by Gal.

SUMMARY

New immunological problems are arising after the elimination of Gal by the generation of GalT-KO pigs; these include non-Gal antibodies and the identification of their elusive antigens, as well as cellular components of the immune system, including neutrophils, macrophages, natural killer cells, and T cells.

Neutralization of HIV-1 by redirection of natural antibodies

The great variability and high glycosylation of gp120 poses a great challenge for the design of a functional immune therapy. The binding region of the CD4 receptor to gp120, however, is well conserved and may constitute a target to limit viral entry and infectivity. Our strategy consists in using a preexisting pool of natural antibodies directed toward the gal(alpha1,3)gal disaccharide and to redirect it to HIV. We here show that using CD4-derived, gp120-binding, synthetic peptides chemically linked to gal(alpha1,3)gal can redirect these natural antibodies and improve the HIV-1 neutralizing activity of the CD4-derived peptides in vitro. Importantly, the binding of the CD4-gal(alpha1,3)gal peptides to HIV-1-infected cells conferred antibody-dependent cellular cytotoxicity after the addition of human sera. Thus, the temporary redirection of naturally occurring antibodies and their biological activities to a new antigen represents a completely new way of targeting a human disease.

Clustered carbohydrates as a target for natural killer cells: a model system

Membrane-associated oligosaccharides are known to take part in interactions between natural killer (NK) cells and their targets and modulate NK cell activity. A model system was therefore developed using synthetic glycoconjugates as tools to modify the carbohydrate pattern on NK target cell surfaces. NK cells were then assessed for function in response to synthetic glycoconjugates, using both cytolysis-associated caspase 6 activation measured by flow cytometry and IFN-gamma production. Lipophilic neoglycoconjugates were synthesized to provide their easy incorporation into the target cell membranes and to make carbohydrate residues available for cell-cell interactions. While incorporation was successful based on fluorescence monitoring, glycoconjugate incorporation did not evoke artifactual changes in surface antigen expression, and had no negative effect on cell viability. Glycoconjugates contained Le(x), sulfated Le(x), and Le(y) sharing the common structure motif trisaccharide Le(x) were revealed to enhance cytotoxicity mediated specifically by CD16 +CD56+NK cells. The glycoconjugate effects were dependent on saccharide presentation in a polymeric form. Only polymeric, or clustered, but not monomeric glycoconjugates resulted in alteration of cytotoxicity in our system, suggesting that appropriate presentation is critical for carbohydrate recognition and subsequent biological effects.

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.

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.

Carbohydrates borne on human glycophorin A are recognized by porcine Kupffer cells

BACKGROUND

We have previously shown that when porcine livers are perfused with human blood, porcine Kupffer cells extract up to 3 units of human erythrocytes over the course of a 72 hr perfusion. We have previously hypothesized that the recognition event responsible for this interaction involves a lectin receptor on the surface of the porcine Kupffer cell interacting with a carbohydrate epitope on the surface of the human erythrocytes.

METHODS

Treatments to disrupt the protein core of purified glycoproteins from the surface of human erythrocytes included: pronase, trypsin, beta-mercaptoethanol (2-ME), and heating to 90 degrees . Alternatively, we have removed the carbohydrate residues from purified human red blood cell (RBC) glycoproteins using glycosidases. Erythrocyte binding in the presence or absence of treated glycoproteins was quantified by Chromium-labeled RBC recognition by primary cultures of porcine Kupffer cells.

RESULTS

Human, but not porcine, erythrocytes were bound by in vitro primary cultures of porcine Kupffer cells. Binding was inhibited by preincubation of porcine Kupffer cells with purified human erythrocyte glycoproteins (hEGP) from human erythrocyte membranes. Pretreatment of human EGP with pronase, trypsin, 2-ME, or heating did not interfere with the ability of human EGP to inhibit the binding of human erythrocytes to porcine Kupffer cells. Deglycosylation of the purified hEGP completely disrupted hEGP ability to inhibit the binding of human erythrocytes to porcine Kupffer cells.

CONCLUSION

We conclude that porcine Kupffer cells bind xenogeneic human RBC by recognition of a carbohydrate epitope on the surface of human erythrocytes. We hypothesize that this binding is mediated by a porcine Kupffer cell lectin receptor.

Carbohydrates in xenotransplantation

The success of allotransplantation has led to an increasing shortage of human organs from deceased donors. This crisis could be resolved by the use of organs from an anatomically suitable animal, such as the pig. The pig and human have, however, been evolving differently for approximately 80 million years, and numerous immunological and physiological barriers have developed that need to be overcome. Differences in carbohydrate epitopes on pig and human cells have been found to play a major role in some of the immunological barriers that have been identified to date. The rejection caused by the presence of galactose-alpha1,3-galactose (Gal) on the pig vascular endothelium and of natural anti-Gal antibodies in humans 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, which was made possible by the introduction of nuclear transfer/embryo transfer techniques. N-glycolylneuraminic acid (the so-called Hanganutziu-Deicher antigen) has been identified as another carbohydrate antigen present in pigs that may need to be deleted if xenotransplantation is to be successful, although some doubt remains regarding its importance. There remain other antipig antibodies against hitherto unidentified antigenic targets that may well be involved in graft destruction; their possible carbohydrate target epitopes are discussed.

Endothelial Cells Derived from Pigs Lacking Gal??(1,3)Gal: No Reduction of Human Leukocyte Adhesion and Natural Killer Cell Cytotoxicity

BACKGROUND

The expression of galactose-alpha(1,3)galactose (Gal) on porcine cells represents a major barrier to xenotransplantation. The generation of Gal-/- pigs to overcome this barrier redirected the focus of research to other rejection mechanisms, including cellular immunity. The present in vitro study investigated (1) the adhesive interactions between human leukocyte subsets and primary endothelial cells derived from inbred Gal-/- and Gal+/+ pigs, and (2) the susceptibility of such Gal-/- porcine endothelial cells to human natural killer (NK) cell cytotoxicity.

METHODS

Primary porcine aortic endothelial cells (PAEC) were isolated from Gal-/- (PAEC-Gal-/-) and Gal (PAEC-Gal+/+) pigs. Human peripheral blood mononuclear cells (PBMC), polymorphonuclear neutrophils (PMN), and NK cells were isolated from healthy volunteers and tested in functional adhesion and cytotoxicity assays.

RESULTS

Adhesion of human PBMC, PMN, or purified NK cells on PAEC-Gal-/- cells was not different from that on PAEC-Gal+/+ cells. Comparing the different leukocyte subsets of PBMC, a preferential adhesion of NK and B cells on both PAEC-Gal-/- and PAEC-Gal+/+ was detected. Tumor-necrosis factor-alpha stimulation of PAEC-Gal-/- and PAEC-Gal+/+ induced an increase of CD62E and CD106 expression and increased cellular adhesion, in particular, of PMN. The lack of Gal-/- expression on PAEC-Gal cells did not prevent xenogeneic human NK-cell cytotoxicity mediated by freshly isolated or interleukin-2-activated NK cells.

CONCLUSIONS

Neither human leukocyte adhesion nor xenogeneic NK-cell cytotoxicity against PAEC are impaired by the lack of Gal, indicating that Gal is not a dominant target of cellular rejection.

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.

Decrease of human pancreatic cancer cell tumorigenicity by alpha1,3galactosyltransferase gene transfer

The enzyme alpha1,3galactosyltransferase synthesizes the alphaGal epitope, a carbohydrate structure (Galalpha1,3Galbeta1,4GlcNAc-R), on glycoconjugates in lower mammals. The enzyme is absent in humans but large amounts of natural antibodies that recognize alphaGal epitopes are present in human serum. It is likely that these antibodies contribute to the host defense and participate in the hyperacute rejection of xenograft. Previous studies indicated that the glycosyltransferase gene transfer into tumoral cells can modify the structure of glycoconjugates at the cell surface and, as a consequence, modulates the metastatic and tumorigenic behaviors of these cells. The aim of our study was to determine whether the expression of alphaGal epitope can modify the tumorigenicity of human pancreatic cancer cells. The expression of alphaGal epitopes in the human pancreatic cancer cell lines BxPC-3 and Panc-1 was obtained by selecting stable cell clones transfected with murine alpha1,3galactosyltransferase gene. The expression of the enzyme activity in BxPC-3 and Panc-1 cells resulted in the formation at the cell surface of alphaGal epitopes that are recognized by human anti-alphaGal antibodies. alphaGal epitope expression at the surface of pancreatic cancer cells was associated with the fixation of complement 1q to human anti-alphaGal antibodies. The alphaGal epitope expression also resulted in a delay in the tumoral development of BxPC-3 and Panc-1 cells in vivo after xenograft transplantation of nude mice. In addition to the impairment of the metastatic potential of murine tumor cell lines and the activation of immune response, our study provides evidence that the cell surface expression of alphaGal epitopes also modulates the tumorigenic behavior of human pancreatic cancer cells.

Cell-mediated cytotoxicity to porcine aortic endothelial cells is not dependent on galactosyl residues when baboon peripheral blood lymphocytes are previously primed with pig xenoantigens

BACKGROUND

In the pig-to-baboon model, acute vascular rejection remains the main hurdle for successful long-term xenograft survival. The production of galactosyl knockout pigs could solve concomitantly the problem of hyperacute and acute vascular rejection. This work studies in vitro the cell-mediated cytotoxicity of natural killer (NK) and T cells after priming of baboon peripheral blood lymphocytes (PBLs) with pig antigens to evaluate whether cytotoxicity is galactosyl-dependent.

MATERIAL AND METHODS

PBLs from naive and primed baboons were used as effectors on primary porcine aortic endothelial cells (PAECs) to assess cytotoxicity. Untreated or galactosidase-digested PAECs were used to evidence the role of galactosyl residues on cell-mediated cytotoxicity. Two rat-anti baboon monoclonal antibodies were tested to inhibit either T+NK cells (LO-CD2b) or NK cells alone (LO-CD94).

RESULTS

When using PBLs from naive animals, spontaneous lysis occurred and was inhibited by both LOCD-2b and LO-CD94. In comparison, lysis of PAECs was significantly higher when baboon PBLs were first primed in vivo with pig xenoantigens. In this case, cytotoxicity was completely inhibited by LO-CD2b but only partially by LO-CD94. Reduction of galactosyl residues by galactosidase digestion showed that PAEC lysis almost completely disappeared with naive baboon PBLs but not with primed baboon PBLs, thereby indicating that anti-pig T-cell response is not dependent on galactosyl residues.

CONCLUSION

Galactosyl knockout pigs could solve hyperacute rejection and also prevent the activation of NK cells even after xenogeneic priming. T cells will then be the next hurdle for the success of xenografting.

Xenotransplantation in the 21st century

Xenotransplantation, i.e. transplantation across species, is increasingly being viewed a potential solution to the problem of severe shortage of transplant donors. Clinical application of xenotransplantation is, however, limited in large part by the pre-eminent hurdle of the immune response of the recipient against the graft. This immunologic reaction is mediated initially by natural xenoreactive antibodies, complement and natural killer cells, and later by elicited humoral and cellular immune responses that ultimately lead to graft failure. Progress in understanding the cellular and molecular basis of xenograft rejection has characterized the past few years. Additional hurdles to xenotransplantation include physiologic incompatibility of the transplant and the risk of infections. The recent development of novel strategies to overcome xenograft rejection has brought about great optimism that xenotransplantation may be attainable in the near future.

Co-effect of HLA-G1 and glycosyltransferases in reducing NK cell-mediated pig endothelial cell lysis

Natural killer (NK) cells play an important role in xenograft rejection. The aim of this study was to evaluate the co-effect of human leukocyte antigen (HLA)-G1 expression and the remodeling of glycoantigens such as the alpha-Gal epitope, Galalpha1,3Galbeta1,4GlcNAc-R, by the introduction of glycosyltransferase genes related to NK cell-mediated direct cytotoxicity. Human peripheral blood mononuclear cells or an NK-like cell line, YT cells, was used as an effector and pig endothelial cells (PEC) as the target. A PEC transfectant with HLA-G1 was first prepared by the transfection of HLA-G1 and human beta2 microglobulin. Several new transfectants were then established by the transfection of glycosyltransferase to the HLA-G1 transfectant. The effect of HLA-G1 on NK cell-mediated PEC lysis was lower than that by the glycosyltransferases. Therefore, in the case of the co-transfectants except for HLA-G1+alpha2,6sialyltransferase, such as HLA-G1+N-acetylglucosaminyltransferase-III and HLA-G1+alpha1,2fucosyltransferase, the effect of HLA-G1 expression on NK-mediated killing appeared to be accounted for by the transfected glycosyltransferase activities and the reduced alpha-Gal expression on the cell surface. However, these transfectants showed significant reductions in direct NK cell-mediated cytotoxicity, compared with the single HLA-G1 transfectant. The results herein suggest that a combination of HLA-G1 and glycosyltransferases has considerable potential for the downregulation of NK cell-mediated cytolysis.

Genetic modification of xenografts

For nearly a century, xenotransplantation has been seen as a potential approach to replacing organs and tissues damaged by disease. Until recently, however, the application of xenotransplantation has seemed only a remote possibility. What has changed this perspective is the advent of genetic engineering of large animals; that is, the ability to add genes to and remove genes from lines of animals that could provide an enduring source of tissues and organs for clinical application. Genetic engineering could address the immunologic, physiologic and infectious barriers to xenotransplantation, and could allow xenotransplantation to provide a source of cells with defined and even controlled expression of exogenous genes. This communication will consider one perspective on the application of genetic engineering in xenotransplantation.

Modulation of the leader peptide sequence of the HLA-E gene up-regulates its expression and down-regulates natural killer cell-mediated swine endothelial cell lysis

BACKGROUND

The inhibitory function of HLA class I molecules, HLA-G1 and HLA-E, on natural killer (NK) cell-mediated cytolysis has previously been reported. In this study, we report on a study of the effects of the co-expression of these molecules on the inhibition of NK cell-mediated cytolysis, using a newly constructed gene.

METHODS

Complementary DNA (cDNA) of HLA-G (G1 and G3), HLA-E, and human beta2-microglobulin (hbeta2m) were prepared and transfected into swine endothelial cell (SEC) and Chinese hamster ovarian tumor (CHO) cell. The leader peptide sequences of HLA-G1 and HLA -E genes were changed to VMAPRTLFL or VMAPRTLVL, which corresponds to the original HLA-G1 and HLA-A2. The cell surface expression of the modified genes was evaluated by flow cytometry, and NK cell-mediated cytolysis by human peripheral blood mononuclear cells (PBMC) was assessed.

RESULTS

The transfectant with the hbeta2m and HLA-G1 genes showed a clear expression of the HLA-G1 molecule and had an inhibitory effect on NK cell-mediated SEC lysis. Whereas neither the transfectant with the hbeta2m and HLA-E genes, nor that with the hbeta2m and HLA-G3 genes, expressed the HLA molecule on SEC, the transfectant with triple genes, hbeta2m, HLA-E, and HLA-G3, expressed the HLA-E molecule and also inhibited NK-mediated SEC lysis. Conversely, the modification of the leader sequence of the HLA-E gene successfully induced the expression of the HLA-E molecule on the SEC surface. Furthermore, the transfectant expressed both HLA-G1 and HLA-E molecules, thus efficiently enhancing the inhibition of NK-mediated SEC lysis.

CONCLUSION

The co-expression of HLA-G1 and HLA-E molecules with the modified genes has potential for use in preventing xenograft rejection, as mediated by human NK cells.

Human NK cell-mediated cytotoxicity triggered by CD86 and Gal alpha 1,3-Gal is inhibited in genetically modified porcine cells

Delayed xenograft rejection is a major hurdle that needs to be addressed to prolong graft survival in pig-to-primate xenotransplantation. NK cell activation has been implicated in delayed xenograft rejection. Both Ab-dependent and independent mechanisms are responsible for the high susceptibility of porcine cells to human NK cell-mediated cytotoxicity. Previous reports demonstrated a role of Galalpha1,3-Gal Ag in triggering the Ab-independent responses. We hypothesize that expression of CD80 and/or CD86 on porcine cells may also play a role in NK cell activation as human NK cells express a variant of CD28. Our initial analysis showed that porcine endothelial cells and fibroblasts express CD86, but not CD80. Genetic engineering of these cells to express hCD152-hCD59, a chimeric molecule designed to block CD86 in cis, was accompanied by a reduction in susceptibility to human NK cell-mediated cytotoxicity. The use of a specific anti-porcine CD86-blocking Ab and the NK92 and YTS cell lines further confirmed the involvement of CD86 in triggering NK cell-mediated lysis of porcine cells. Maximal protection was achieved when hCD152-hCD59 was expressed in H transferase-transgenic cells, which show reduced Galalpha1,3-Gal expression. In this work, we describe two mechanisms of human NK cell-mediated rejection of porcine cells and demonstrate that genetically modified cells resist Ab-independent NK cell-mediated cytotoxicity.

Cell surface fucose ablation as a therapeutic strategy for malignant neoplasms

The sugar alpha-L-fucose is overexpressed in many human malignancies, especially on specific glycoproteins, glycolipids, certain mucins, and putative cell adhesion ligands found on cancer cell surfaces. Many of these molecules are known or suspected mediators of cell-cell adhesion, cell signaling, motility, or invasion. As knowledge of fucose metabolism evolves and specific mechanisms of its distribution and incorporation are more exactly documented, modulation of fucose expression in cancer is becoming increasingly more feasible. The authors propose that cancer cell surface alpha-L-fucose is a logical target for selective therapeutic ablation. Reduction of fucose content on the surfaces of malignant cells should effectively cripple the cells' physiologic functions by altering or dysregulating cell-cell or cell-matrix interactions, critical for maintaining the malignant phenotype. Significant therapeutic benefits might include modulation of adhesion abnormalities in the cancer cells, reduction of cancer cell motility or invasiveness, reexposure to immune surveillance, or a combination of these events.

Genetic engineering for xenotransplantation

Xenotransplantation is being pursued vigorously to solve the shortage of allogeneic donor organs. Experimental studies of the major xenoantigen (Gal) and of complement regulation enable model xenografts to survive hyperacute rejection. When the Gal antigen is removed or reduced and complement activation is controlled, the major barriers to xenograft survival include unregulated coagulation within the graft and cellular reactions involving macrophages, neutrophils, natural killer (NK) cells, and T lymphocytes. Unlike allografts, where specific immune responses are the sole barrier to graft survival, molecular differences between xenograft and recipient that affect normal receptor-ligand interactions (largely active at the cell surface and which may not be immunogenic), are also involved in xenograft failure. Transgenic strategies provide the best options to control antigen expression, complement activation, and coagulation. Although the Gal antigen can be eliminated by gene knockout in mice, that outcome has only become a possibility in pigs due to the recent cloning of pigs after nuclear transfer. Instead, the use of transgenic glycosyl transferase enzymes and glycosidases, which generate alternative terminal carbohydrates on glycolipids and glycoproteins, has reduced antigen in experimental models. As a result, novel strategies are being tested to seek the most effective solution. Transgenic pigs expressing human complement-regulating proteins (DAF/CD55, MCP/CD46, or CD59) have revealed that disordered regulation of the coagulation system requires attention. There will undoubtedly be other molecular incompatibilities that need addressing. Xenotransplantation, however, offers hope as a therapeutic solution and provides much information about homeostatic mechanisms.

The immunological hurdles to cardiac xenotransplantation

The main hurdle to clinical application of cardiac xenotransplantation is the immune response of the recipient against the graft. Although all xenografts arouse an intense immune response, the effect of that response depends very much on whether the graft consists of isolated cells or an intact organ, such as the heart. Intact organs, which are transplanted by primary vascular anastomosis, are subject to severe vascular injury owing to the reaction of immune elements with the endothelial lining of donor blood vessels. Vascular injury leads to hyperacute rejection, acute vascular rejection, and chronic rejection. The immunological basis for these types of rejection and potential therapies, which might be used to avert them, are discussed.

Remodeling of the major pig xenoantigen by N-acetylglucosaminyltransferase III in transgenic pig

We have been successful in generating several lines of transgenic mice and pigs that contain the human beta-d-mannoside beta-1,4-N-acetylglucosaminyltransferase III (GnT-III) gene. The overexpression of the GnT-III gene in mice and pigs reduced their antigenicity to human natural antibodies, especially the Galalpha1-3Galbeta1-4GlcNAc-R, as evidenced by immunohistochemical analysis. Endothelial cell studies from the GnT-III transgenic pigs also revealed a significant down-regulation in antigenicity, including Hanganutziu-Deicher antigen, and dramatic reductions in both the complement- and natural killer cell-mediated pig cell lyses. Changes in the enzymatic activities of other glycosyltransferases, such as alpha1,3-galactosyltransferase, GnT-IV, and GnT-V, did not support cross-talk between GnT-III and these enzymes in the transgenic animals. In addition, we demonstrated the effect of GnT-III in down-regulating the xenoantigen of pig heart grafts, using a pig to cynomolgus monkey transplantation model, suggesting that this approach may be useful in clinical xenotransplantation in the future.

The possible use of HLA-G1 and G3 in the inhibition of NK cell-mediated swine endothelial cell lysis

The splicing isoform of HLA-G that is expressed in xenogeneic cells, and its effect on NK-mediated direct cytotoxicity was examined, using stable Chinese hamster ovary (CHO) cell or swine endothelial cell (SEC) transfectants. cDNAs of HLA-G (G1 and G3) and human beta2-microglobulin were prepared and subcloned into the expression vector, pCXN. The transfected HLA-G1 was easily expressed on SEC, and co-transfection with human beta2-microglobulin led to an enhanced level of HLA-G1 expression, as evidenced by flow cytometry. The expressed HLA-G1 significantly suppressed NK-mediated SEC cell lysis, which is an in vitro delayed-type rejection model of a xenograft. On the other hand, the swine leucocyte antigen (SLA) class I molecules could be up-regulated as the result of the transfection of human beta2-microglobulin, but did not down-regulate human NK-mediated SEC lysis. The HLA-G3 was not expressed on CHO and SEC in contrast to HLA-G1, as the result of the transfection. The gene introduction of HLA-G3 in SEC showed no protective effect from human NK cells. However, indirect evidence demonstrated that HLA-G3 transfection resulted in HLA-E expression, but not itself, when transfected to the human cell line, 721.221, thus providing some insight into its natural function in human cells. The present findings suggest that the expression of HLA-G1 on the cell surface could serve as a new approach to overcoming NK-mediated immunity to xenografts.

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.

Susceptibility of rat colon carcinoma cells to lymphokine activated killer-mediated cytotoxicity is decreased by alpha1,2-fucosylation

The presence of alpha1,2-fucosylated glycans at the surface of rat colon carcinoma cells has been associated with an increased tumorigenicity and resistance to natural killer/lymphokine activated killer (NK/LAK) cytotoxicity. We now report that transfection of rat alpha1,2-fucosyltransferases cDNA (FTA and FTB) into REG cells, which are spontaneously devoid of this enzymatic activity, allows expression of histo-blood group H antigen and increases their resistance to LAK, but not NK cell lysis. Conversely, transfection of PRO cells, which spontaneously express alpha1, 2-fucosyltransferase activity, with the FTA cDNA in the antisense orientation decreases expression of the H antigen together with their resistance to LAK cell lysis, but again, not to NK cell lysis. Furthermore, REG cells that are rejected by immunocompetent syngeneic rats are similarly rejected by rats depleted of NK cells by antibody 3.2.3, directed against the NKR-P1 molecule. Thus, the rejection of REG cells by immunocompetent rats and their earlier reported increased tumorigenicity after transfection with an alpha1, 2-fucosyltransferase cDNA cannot be ascribed to NK cell sensitivity or resistance, respectively. The increased resistance to LAK cell lysis, however, may be relevant to tumor progression.

alpha(1,2)-fucosylation prevents sialyl Lewis x expression and E-selectin-mediated adhesion of fucosyltransferase VII-transfected cells

E-selectin is a cytokine-inducible, calcium-dependent endothelial cell adhesion molecule that plays a critical role in the leucocyte-endothelium interaction during inflammation and is thought to contribute to the metastatic dissemination of tumour cells. Like the other selectins, E-selectin binds to ligands carrying the tetrasaccharide sialyl-Lewis x (NeuAcalpha2,3Galbeta1,4[Fucalpha1, 3]GlcNAc)1 or its isomer sialyl-Lewis a (NeuAcalpha2, 3Galbeta1, 3[Fucalpha1,4]GlcNAc). We examined the effect of expressing the H-type alpha(1,2)-fucosyltransferase or the alpha(2, 6)-sialyltransferase on the synthesis of sialyl-Lewis x by alpha(1, 3)fucosyltransferase. We found that H-type alpha(1, 2)-fucosyltransferase but not alpha(2,6)-sialyltransferase, strongly inhibited sialyl-Lewis x expression and E-selectin adhesion. We assume that H-type alpha(1,2)-fucosyltransferase competes with the endogenous alpha(2,3)-sialyltransferase for the N-acetyllactosamine structures assigned to further serve as acceptors for alpha(1, 3)fucosyltransferase.

Recent advances in xenotransplantation

The major barrier to clinically successful pig-to-human xenotransplantation is antibody- and complement-dependent hyperacute rejection, known to be due to host anti-Galalpha(1,3)Gal antibodies. Strategies aimed at eliminating hyperacute rejection involve transgenic approaches to eliminate or reduce expression of Galalpha(1,3)Gal or to reduce complement activation; some of these are now in clinical trials in primates. Another important role of Galalpha(1,3)Gal that is becoming more evident is in antibody-dependent and -independent xenograft rejection that is mediated by natural killer cells and monocytes.