Reduction in the level of Gal(alpha1,3)Gal in transgenic mice and pigs by the expression of an alpha(1,2)fucosyltransferase

Genetically engineered pigs for xenotransplantation: Hopes and challenges

The shortage of donor resources has greatly limited the application of clinical xenotransplantation. As such, genetically engineered pigs are expected to be an ideal organ source for xenotransplantation. Most current studies mainly focus on genetically modifying organs or tissues from donor pigs to reduce or prevent attack by the human immune system. Another potential organ source is interspecies chimeras. In this paper, we reviewed the progress of the genetically engineered pigs from the view of immunologic barriers and strategies, and discussed the possibility and challenges of the interspecies chimeras.

Humanising and dehumanising pigs in genomic and transplantation research

Biologists who work on the pig (Sus scrofa) take advantage of its similarity to humans by constructing the inferential and material means to traffic data, information and knowledge across the species barrier. Their research has been funded due to its perceived value for agriculture and medicine. Improving selective breeding practices, for instance, has been a driver of genomics research. The pig is also an animal model for biomedical research and practice, and is proposed as a source of organs for cross-species transplantation: xenotransplantation. Genomics research has informed transplantation biology, which has itself motivated developments in genomics. Both have generated models of correspondences between the genomes of pigs and humans. Concerning genomics, I detail how researchers traverse species boundaries to develop representations of the pig genome, alongside ensuring that such representations are sufficiently porcine. In transplantation biology, the representations of the genomes of humans and pigs are used to detect and investigate immunologically-pertinent differences between the two species. These key differences can then be removed, to 'humanise' donor pigs so that they can become a safe and effective source of organs. In both of these endeavours, there is a tension between practices that 'humanise' the pig (or representations thereof) through using resources from human genomics, and the need to 'dehumanise' the pig to maintain distinctions for legal, ethical and scientific reasons. This paper assesses the ways in which this tension has been managed, observing the differences between its realisations across comparative pig genomics and transplantation biology, and considering the consequences of this.

Skin xenotransplantation: technological advances and future directions

PURPOSE OF REVIEW

To summarize the evolution of skin xenotransplantation and contextualize technological advances and the status of clinically applicable large animal research as well as prospects for translation of this work as a viable future treatment option.

RECENT FINDINGS

Porcine xenografts at the start of the millennium were merely biologic dressings subject to rapid rejection. Since then, numerous important advances in swine to nonhuman primate models have yielded xenotransplant products at the point of clinical translation. Critical genetic modifications in swine from a designated pathogen-free donor herd have allowed xenograft survival reaching 30 days without preconditioning or maintenance immunosuppression. Further, xenograft coverage appears not to sensitize the recipient to subsequent allograft placement and vice versa, allowing for temporary coverage times to be doubled using both xeno and allografts.

SUMMARY

Studies in large animal models have led to significant progress in the creation of living, functional skin xenotransplants with clinically relevant shelf-lives to improve the management of patients with extensive burns.

Xenotransplantation: Progress Along Paths Uncertain from Models to Application

For more than a century, transplantation of tissues and organs from animals into man, xenotransplantation, has been viewed as a potential way to treat disease. Ironically, interest in xenotransplantation was fueled especially by successful application of allotransplantation, that is, transplantation of human tissue and organs, as a treatment for a variety of diseases, especially organ failure because scarcity of human tissues limited allotransplantation to a fraction of those who could benefit. In principle, use of animals such as pigs as a source of transplants would allow transplantation to exert a vastly greater impact than allotransplantation on medicine and public health. However, biological barriers to xenotransplantation, including immunity of the recipient, incompatibility of biological systems, and transmission of novel infectious agents, are believed to exceed the barriers to allotransplantation and presently to hinder clinical applications. One way potentially to address the barriers to xenotransplantation is by genetic engineering animal sources. The last 2 decades have brought progressive advances in approaches that can be applied to genetic modification of large animals. Application of these approaches to genetic engineering of pigs has contributed to dramatic improvement in the outcome of experimental xenografts in nonhuman primates and have encouraged the development of a new type of xenograft, a reverse xenograft, in which human stem cells are introduced into pigs under conditions that support differentiation and expansion into functional tissues and potentially organs. These advances make it appropriate to consider the potential limitation of genetic engineering and of current models for advancing the clinical applications of xenotransplantation and reverse xenotransplantation.

Analysis of swine leukocyte antigen class I gene profiles and porcine endogenous retrovirus viremia level in a transgenic porcine herd inbred for xenotransplantation research

Molecular characterization of swine leukocyte antigen (SLA) genes is important for elucidating the immune responses between swine-donor and human-recipient in xenotransplantation. Examination of associations between alleles of SLA class I genes, type of pig genetic modification, porcine endogenous retrovirus (PERV) viral titer, and PERV subtypes may shed light on the nature of xenograft acceptance or rejection and the safety of xenotransplantation. No significant difference in PERV gag RNA level between transgenic and non-transgenic pigs was noted; likewise, the type of applied transgene had no impact on PERV viremia. SLA-1 gene profile type may correspond with PERV level in blood and thereby influence infectiveness. Screening of pigs should provide selection of animals with low PERV expression and exclusion of specimens with PERV-C in the genome due to possible recombination between A and C subtypes, which may lead to autoinfection. Presence of PERV-C integrated in the genome was detected in 31.25% of specimens, but statistically significant increased viremia in specimens with PERV-C was not observed. There is a need for multidirectional molecular characterization (SLA typing, viremia estimation, and PERV subtype screening) of animals intended for xenotransplantation research in the interest of xeno-recipient safety.

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.

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

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

Genetically modified pigs for biomedical research

During the last two decades, pigs have been used to develop some of the most important large animal models for biomedical research. Advances in pig genome research, genetic modification (GM) of primary pig cells and pig cloning by nuclear transfer, have facilitated the generation of GM pigs for xenotransplantation and various human diseases. This review summarizes the key technologies used for generating GM pigs, including pronuclear microinjection, sperm-mediated gene transfer, somatic cell nuclear transfer by traditional cloning, and somatic cell nuclear transfer by handmade cloning. Broadly used genetic engineering tools for porcine cells are also discussed. We also summarize the GM pig models that have been generated for xenotransplantation and human disease processes, including neurodegenerative diseases, cardiovascular diseases, eye diseases, bone diseases, cancers and epidermal skin diseases, diabetes mellitus, cystic fibrosis, and inherited metabolic diseases. Thus, this review provides an overview of the progress in GM pig research over the last two decades and perspectives for future development.

Determination of the Absolute Number of Transgene Copies in CMVFUT Transgenic Pigs

The aim of this research was to determine the number of transgene copies in the DNA of transgenic pigs. The copy number of the transgene was analysed in the transgenic animals with introduced pCMVFUT genetic construct containing a coding sequence of human H transferase under a control of CMV promoter. The copy number of the transgene that had integrated with the genome of the transgenic animals was analysed by qPCR with SYBR Green dye, which enabled nonspecific double-stranded DNA detection. CMVFT-2F and CMVFT-2R primers were used to amplify a 149 bp fragment of DNA. Forward primer had a sequence complementary to a promoter sequence and reverse primer to a coding sequence of H transferase. The copy number of the transgene in the examined samples was established by plotting the CT values obtained on a standard curve, which had been set by the usage of the CT values for the successive standard dilutions with known copy number (1.438-1.431 copies). As a standard we used pCMVFut genetic construct hydrolyzed with Not I restriction enzyme to a linear form. The real-time PCR results helped to establish the range of 3 - 4 as the number of the transgene copies that had integrated to the swine genome.

Induction of human blood group a antigen expression on mouse cells, using lentiviral gene transduction

The ABO histo-blood group system is the most important antigen system in transplantation medicine, yet no small animal model of the ABO system exists. To determine the feasibility of developing a murine model, we previously subcloned the human alpha-1,2-fucosyltransferase (H-transferase, EC 2.4.1.69) cDNA and the human alpha-1,3-N-acetylgalactosaminyltransferase (A-transferase, EC 2.4.1.40) cDNA into lentiviral vectors to study their ability to induce human histo-blood group A antigen expression on mouse cells. Herein we investigated the optimal conditions for human A and H antigen expression in murine cells. We determined that transduction of a bicistronic lentiviral vector (LvEF1-AH-trs) resulted in the expression of A antigen in a mouse endothelial cell line. We also studied the in vivo utility of this vector to induce human A antigen expression in mouse liver. After intrahepatic injection of LvEF1-AH-trs, A antigen expression was observed on hepatocytes as detected by immunohistochemistry and real-time RT-PCR. In human group A erythrocyte-sensitized mice, A antigen expression in the liver was associated with tissue damage, and deposition of antibody and complement. These results suggest that this gene transfer strategy can be used to simulate the human ABO blood group system in a murine model. This model will facilitate progress in the development of interventions for ABO-incompatible transplantation and transfusion scenarios, which are difficult to develop in clinical or large animal settings.

Transgenic pigs for xenotransplantation: selection of promoter sequences for reliable transgene expression

PURPOSE OF REVIEW

Appropriate expression of immunomodulatory and anticoagulant proteins on endothelial cells is essential to prevent rejection of vascularized porcine organs after transplantation into primates. Here, we review the promoter sequences used for the establishment of transgenic pigs, as organ donors for xenotransplantation.

RECENT FINDINGS

Transgenic pigs were produced using viral, chicken, mouse, human, and porcine promoter sequences with ubiquitous or cell type-specific activity. In addition to the expression of human complement regulatory proteins, which were efficient to prevent hyperacute rejection of pig-to-primate xenografts, novel transgenes, targeting cellular rejection mechanisms, abnormal-blood coagulation, or the risk of viral transmission, have been published or announced in preliminary reports.

SUMMARY

Accurate spatiotemporal expression of immunomodulatory and anticoagulant proteins on the endothelial cells of transgenic pigs is required for the successful xenotransplantation of vascularized organs into primates. Targeting transgene expression specifically to the cells critical for xenograft rejection may eliminate potential side effects of ubiquitous expression. Comparison of regulatory sequences from various species indicates that carefully selected porcine promoter sequences may be beneficial to achieve this aim.

Genetically engineered pig red blood cells for clinical transfusion: initial in vitro studies

BACKGROUND

Pigs are a potential source of red blood cells (RBCs) and could resolve the shortage of human blood for transfusion. This study investigated in vitro the compatibility of genetically engineered pig RBCs (pRBCs) with the human innate immune response.

STUDY DESIGN AND METHODS

Human volunteers of all ABO blood types were sources of sera and those of O blood type were sources of circulating monocytes/macrophages. RBCs from ABO-compatible (ABO-C) and ABO-incompatible (ABO-I) humans and wild-type (WT) and alpha-1,3-galactosyltransferase gene-knockout (GTKO) pigs were tested for hemagglutination, immunoglobulin (Ig)M/IgG antibody binding, and complement-dependent cytotoxicity (CDC) using human sera. Phagocytosis of RBCs by human monocyte-derived macrophages was measured by coculture in the absence or presence of pooled human O serum.

RESULTS

RBCs showed significant differences (p < 0.01) with regard to hemagglutination, IgM and IgG binding, and CDC (ABO-C < GTKO < ABO-I < WT). In the absence of pooled human O serum (antibodies), there was no phagocytosis of any RBCs; in the presence of serum (antibodies), phagocytosis of ABO-I RBCs was greater than of WT (p < 0.01), which in turn was greater than of GTKO RBCs (p < 0.05).

CONCLUSIONS

GTKO RBCs were significantly more compatible than ABO-I and WT RBCs, but were not comparable to ABO-C combinations. In the presence of antibody, human monocyte-derived macrophages phagocytosed ABO-I RBC/sera combinations more efficiently than pRBCs. These observations contribute to our ultimate goal of using genetically engineered pRBCs for clinical blood transfusion. However, pigs will require other modifications or manipulations if they are to become suitable for human transfusion.

Lessons learned from mouse models of hemolytic transfusion reactions

PURPOSE OF REVIEW

Hemolytic transfusion reactions (HTRs) are potentially fatal complications of blood transfusions. Many studies, primarily performed in vitro, have provided a great deal of insight into the initiating events of HTRs; however, it is not clear how they are modulated and how they combine to lead to one or more of the final common pathways. Recently developed mouse HTR models now make it possible to enhance our understanding of the pathogenesis of HTRs; this will allow for the rational design of specific therapies to prevent or ameliorate this serious complication in transfusion medicine.

RECENT FINDINGS

Mouse models support the hypothesis that 'cytokine storm' plays an important role in the pathogenesis of HTRs. Nitric oxide and endothelial cell dysfunction are also implicated in the pathophysiology of these reactions. In addition, the intriguing phenomenon of 'antigen loss,' in which antigen crosslinking by alloantibody leads to antigen removal rather than red blood cell clearance, has been modeled and explored. Finally, these mouse models were used to evaluate new therapeutic targets employing complement receptor 1 peptide homologues and the antimacrophage agent, liposomal clodronate.

SUMMARY

Models of HTRs are valuable for gaining a better understanding of the pathophysiology of these potentially fatal complications of blood transfusion. The participation of various inflammatory mediators was shown to play a role in these reactions in vivo. This knowledge will lead to novel treatment options.

In vitro investigation of pig cells for resistance to human antibody-mediated rejection

Although human complement-dependent cytotoxicity (CDC) of alpha1,3-galactosyltransferase gene-knockout (GTKO) pig cells is significantly weaker than that of wild-type (WT) cells, successful xenotransplantation will require pigs with multiple genetic modifications. Sera from healthy humans were tested by (i) flow cytometry for binding of IgM/IgG, and (ii) CDC assay against peripheral blood mononuclear cells and porcine aortic endothelial cells from five types of pig - WT, GTKO, GTKO transgenic for H-transferase (GTKO/HT), WT transgenic for human complement regulatory protein CD46 (CD46) and GTKO/CD46. There was significantly higher mean IgM/IgG binding to WT and CD46 cells than to GTKO, GTKO/HT, and GTKO/CD46, but no difference between GTKO, GTKO/HT, and GTKO/CD46 cells. There was significantly higher mean CDC to WT than to GTKO, GTKO/HT, CD46, and GTKO/CD46 cells, but no difference between GTKO and GTKO/HT. Lysis of GTKO/CD46 cells was significantly lower than that of GTKO or CD46 cells. CD46 expression provided partial protection against serum from a baboon sensitized to a GTKO pig heart. GTKO/CD46 cells were significantly resistant to lysis by human serum and sensitized baboon serum. In conclusion, the greatest protection from CDC was obtained by the combination of an absence of Gal expression and the presence of CD46 expression, but the expression of HT appeared to offer no advantage over GTKO. Organs from GTKO/CD46 pigs are likely to be significantly less susceptible to CDC.

Reappraisal of biosafety risks posed by PERVs in xenotransplantation

Donor materials of porcine origin could potentially provide an alternative source of cells, tissues or whole organs for transplantation to humans, but is hampered by the health risk posed by infection with porcine viruses. Although pigs can be bred in such a way that all known exogenous microorganisms are eliminated, this is not feasible for all endogenous pathogens, such as the porcine endogenous retroviruses (PERVs) which are present in the germline of pigs as proviruses. Upon transplantation, PERV proviruses would be transferred to the human recipient along with the xenograft. If xenotransplantation stimulates or facilitates replication of PERVs in the new hosts, a risk exists for adaptation of the virus to humans and subsequent spread of these viruses. In a worst-case scenario, this might result in the emergence of a new viral disease. Although the concerns for disease potential of PERVs are easing, only limited pre-clinical and clinical data are available. Small-scale, well-designed and carefully controlled clinical trials would provide more evidence on the safety of this approach and allow a better appreciation of the risks involved. It is therefore important to have a framework of protective measures and monitoring protocols in place to facilitate such initially small scale clinical trials. This framework will raise ethical and social considerations regarding acceptability.

Xenotransplantation: current status and a perspective on the future

Xenotransplantation using pigs as the transplant source has the potential to resolve the severe shortage of human organ donors. Although the development of relatively non-toxic immunosuppressive or tolerance-inducing regimens will be required to justify clinical trials using pig organs, recent advances in our understanding of the biology of xenograft rejection and zoonotic infections, and the generation of alpha1,3-galactosyltransferase-deficient pigs have moved this approach closer to clinical application. This Review highlights the major obstacles impeding the translation of xenotransplantation into clinical therapies and the potential solutions, providing a perspective on the future of clinical xenotransplantation.

The production of red blood cell alloantibodies in mice transfused with blood from transgenic Fyb-expressing mice

BACKGROUND

A murine model would be useful to identify which immune mechanisms could be manipulated to treat or prevent red blood cell (RBC) alloimmunization in patients who become sensitized to multiple or widely expressed antigens.

STUDY DESIGN AND METHODS

Transgenic mice (B6CBAF1/J-Tg-Fy(b)) expressing the human Fy(b) antigen of the Duffy (Fy) blood group were donors. Recipient B6CBA-F1 mice received four weekly intravenous (IV) transfusions: either 0.3 mL of washed buffy coat-depleted RBCs or 0.3 mL of RBCs with spleen cells. Titers of immunoglobulin M (IgM) and immunoglobulin G (IgG) were measured in recipient serum samples by flow cytometry with RBCs from donor mice as target cells. Recipient serum samples were also tested against human RBCs of various Fy phenotypes. Additionally, RBC survival studies were performed in alloimmunized mice utilizing biotin-labeled Fy(b) transgenic mouse RBCs.

RESULTS

B6CBA-F1 mice receiving washed buffy coat-depleted RBCs first made IgM, followed by IgG alloantibodies to transgenic mouse Fy(b)-positive RBCs. Recipients of Fy(b)-positive RBCs mixed with spleen cells also produced IgM and IgG alloantibodies, but at a slower rate than recipients of washed buffy coat-depleted RBCs. Serum samples showed specificity for Fy3, Fy(b), and Fy6. Decreased survival of transfused RBCs was evident at 24 hours after transfusion.

CONCLUSIONS

It is possible to elicit the formation of anti-Fy alloantibodies by IV transfusion in mice that lack Fy antigens. The transfusion of RBCs alone was adequate to stimulate alloantibody production in B6CBA-F1 recipient mice. The survival of transfused Fy(b)-positive RBCs is diminished in sensitized mice. This model will be useful in further studies of RBC alloimmunization.

The Molecular Basis for Galα(1,3)Gal Expression in Animals with a Deletion of the α1,3Galactosyltransferase Gene

The production of homozygous pigs with a disruption in the GGTA1 gene, which encodes alpha1,3galactosyltransferase (alpha1,3GT), represented a critical step toward the clinical reality of xenotransplantation. Unexpectedly, the predicted complete elimination of the immunogenic Galalpha(1,3)Gal carbohydrate epitope was not observed as Galalpha(1,3)Gal staining was still present in tissues from GGTA1(-/-) animals. This shows that, contrary to previous dogma, alpha1,3GT is not the only enzyme able to synthesize Galalpha(1,3)Gal. As iGb3 synthase (iGb3S) is a candidate glycosyltransferase, we cloned iGb3S cDNA from GGTA1(-/-) mouse thymus and confirmed mRNA expression in both mouse and pig tissues. The mouse iGb3S gene exhibits alternative splicing of exons that results in a markedly different cytoplasmic tail compared with the rat gene. Transfection of iGb3S cDNA resulted in high levels of cell surface Galalpha(1,3)Gal synthesized via the isoglobo series pathway, thus demonstrating that mouse iGb3S is an additional enzyme capable of synthesizing the xenoreactive Galalpha(1,3)Gal epitope. Galalpha(1,3)Gal synthesized by iGb3S, in contrast to alpha1,3GT, was resistant to down-regulation by competition with alpha1,2fucosyltransferase. Moreover, Galalpha(1,3)Gal synthesized by iGb3S was immunogenic and elicited Abs in GGTA1 (-/-) mice. Galalpha(1,3)Gal synthesized by iGb3S may affect survival of pig transplants in humans, and deletion of this gene, or modification of its product, warrants consideration.

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.

Progress in xenotransplantation

Organ transplantation is considered the most effective treatment for end-stage organ failure; currently it is limited by a severe worldwide shortage of human donor organs. This has led to investigation of the potential use of animals as organ donors. For a number of reasons, the pig represents the most likely organ donor candidate. Transplantation of a vascularised porcine organ into a human or non-human primate results in an immediate and dramatic rejection process, known as hyperacute rejection, which is mediated by the binding of pre-existing antibody to the porcine graft and the subsequent activation of host complement. Strategies aimed at preventing this initial rejection have been largely successful in experimental models. This has allowed attention to turn towards an understanding of the immunological barriers comprising the next phase of xenograft rejection, termed acute vascular rejection. This delayed rejection process appears to be a humoral event, and it is likely that the control of antibody synthesis will play a pivotal role in overcoming the current barrier to successful xenotransplantation.

Small Molecule Inhibitors of Mucin-Type O-Linked Glycosylation from a Uridine-Based Library

The polypeptide N-acetyl-alpha-galactosaminyltransferases (ppGalNAcTs, also abbreviated ppGaNTases) initiate mucin-type O-linked glycosylation and therefore play pivotal roles in cell-cell communication and protection of tissues. In order to develop new tools for studying mucin-type O-linked glycosylation, we screened a 1338 member uridine-based library to identify small molecule inhibitors of ppGalNAcTs. Using a high-throughput enzyme-linked lectin assay (ELLA), two inhibitors of murine ppGalNAcT-1 (K(I) approximately 8 microM) were identified that also inhibit several other members of the family. The compounds did not inhibit other mammalian glycosyltransferases or nucleotide sugar utilizing enzymes, suggesting selectivity for the ppGalNAcTs. Treatment of cells with the compounds abrogated mucin-type O-linked glycosylation but not N-linked glycosylation and also induced apoptosis. These uridine analogs represent the first generation of chemical tools to study the functions of mucin-type O-linked glycosylation.

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.

Hyperacute rejection of mouse lung by human blood: characterization of the model and the role of complement

BACKGROUND

The pathophysiology of hyperacute lung rejection (HALR) is not fully understood. A mouse model of HALR by human blood would be valuable to efficiently dissect the molecular mechanisms underlying this complex process, but it has not been described.

METHODS

We developed a xenogenic mouse lung-perfusion model. Perfusion with heparinized autologous blood (n=3) was compared with human blood unmodified (n=7) or pretreated with C1 inhibitor (n=5) or soluble complement receptor type 1 (n=6) at unchanged flow conditions.

RESULTS

Perfusion with autologous blood was associated with stable physiologic parameters and no overt evidence of lung injury for up to 2 hr. Pulmonary artery perfusion pressure increased rapidly after introduction of unmodified human blood, plasma anti-Gal(alpha)1,3Gal antibodies declined (90% immunoglobulin [Ig]M, 80% IgG), and lungs reliably met survival endpoints within 11 min (median 10 min, confidence interval [CI]: 9-11). Human Ig and neutrophils were rapidly sequestered in the lung. Survival was significantly prolonged in the soluble complement receptor type 1 group (36 min, CI: 26-46) (P<0.01) and in the C1 inhibitor group (23 min, CI: 21-25) (P<0.05), and pulmonary vascular resistance elevation and complement activation were significantly attenuated but not prevented.

CONCLUSIONS

Hyperacute rejection of mouse lung by human blood occurs with kinetics, physiology, and histology closely analogous to the pig-to-human model. In addition, as in that model, neither of two potent soluble-phase complement inhibitors prevented complement activation or HALR. We conclude that the mouse lung model is relevant to dissect the cellular and molecular mechanisms governing HALR.

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.

Production of alpha 1,3-galactosyltransferase-knockout cloned pigs expressing human alpha 1,2-fucosylosyltransferase

The production of genetically engineered pigs as xenotransplant donors aims to solve the severe shortage of organs for transplantation in humans. The first barrier to successful xenotransplantation is hyperacute rejection (HAR). HAR is a rapid and massive humoral immune response directed against the pig carbohydrate Galalpha 1,3-Gal epitope, which is synthesized by alpha 1,3-galactosyltransferase (alpha1,3-GT). The Galalpha 1,3-Gal antigen also contributes to subsequent acute vascular rejection events. Genetic modifications of donor pigs transgenic for human complement regulatory proteins or different glycosyltransferases to downregulate Galalpha 1,3-Gal expression have been shown to significantly delay xenograft rejection. However, the complete removal of the Galalpha 1,3-Gal antigen is the most attractive option. In this study, the 5' end of the alpha 1,3-GT gene was efficiently targeted with a nonisogenic DNA construct containing predominantly intron sequences and a Kozak translation initiation site to initiate translation of the neomycin resistance reporter gene. We developed two novel polymerase chain reaction screening methods to detect and confirm the targeted G418-resistant clones. This is the first study to use Southern blot analysis to demonstrate the disruption of the alpha 1,3-GT gene in somatic HT-transgenic pig cells before they were used for nuclear transfer. Transgenic male pigs were produced that possess an alpha 1,3-GT knockout allele and express a randomly inserted human alpha 1,2-fucosylosyltransferase (HT) transgene. The generation of homozygous alpha 1,3-GT knockout pigs with the HT-transgenic background is underway and will be unique. This approach intends to combine the alpha 1,3-GT knockout genotype with a ubiquitously expressed fucosyltransferase transgene producing the universally tolerated H antigen. This approach may prove to be more effective than the null phenotype alone in overcoming HAR and delayed xenograft rejection.

Targeting gene expression to endothelium in transgenic animals: a comparison of the human ICAM-2, PECAM-1 and endoglin promoters

It is highly likely that successful pig-to-human xenotransplantation of vascularized organs will require genetic modification of the donor pig, and in particular of donor vascular endothelium. Promoters are generally tested in transgenic mice before generating transgenic pigs. Several promoters have been used to drive endothelial cell-specific expression in mice but none have yet been tested in pigs. We compared the promoters of three human genes that are predominantly expressed in vascular endothelium: intercellular adhesion molecule 2 (ICAM-2), platelet endothelial cell adhesion molecule 1 (PECAM-1) and endoglin. Expression of human complement regulatory proteins (hCRPs), directed by each of the promoters in mice, was largely restricted to vascular endothelium and leukocyte subpopulations. However, expression from the PECAM-1 promoter was weak in liver and non-uniform in the small vessels of heart, kidney, and lung. Conversely, expression from the endoglin promoter was consistently strong in the small vessels of these organs but was absent in larger vessels. The ICAM-2 promoter, which produced strong and uniform endothelial expression in all organs examined, was therefore used to generate hCRP transgenic pigs. Leukocytes from 57 pigs containing at least one intact transgene were tested for transgene expression by flow cytometry. Forty-seven of these transgenic pigs were further analyzed by immunohistochemical staining of liver biopsies, and 18 by staining of heart and kidney sections. Only two of the pigs showed expression, which appeared to be restricted to vascular endothelium in heart and kidney but was markedly weaker than in transgenic mice produced with the same batch of DNA. Thus, in this case, promoter performance in mice and pigs was not equivalent. The weak expression driven by the human ICAM-2 promoter in pigs relative to mice suggests the need for additional regulatory elements to achieve species-specific gene expression in pigs.

Outline of a Risk Assessment: The Welfare of Future Xeno-Donor Pigs

The welfare of transgenic animals is often not considered prior to their generation. However, we demonstrate here how a welfare risk assessment can be carried out before transgenic animals are created. We describe a risk assessment identifying potential welfare problems in transgenic pigs generated for future xeno-donation of organs. This assessment is based on currently available information concerning transgenic animal models in which one or more transgenes relevant to future xeno-donation have been inserted. The welfare risk assessment reveals that future xeno-donor pigs may have an increased tendency toward septicaemias, reduced fertility and/or impaired vision. The transgenic animal models used in generating hypotheses about the welfare of xeno-donor pigs can also assist in the testing of these hypotheses. To ensure high levels of welfare of transgenic animals, analogous risk assessments can be used to identify potential welfare problems during the early stages of the generation of new transgenic animals. Such assessments may form part of the basis on which licenses to generate new transgenic animals are granted to research groups.

Presence of Gal-alpha1,3Gal epitope on xenogeneic lines: implications for cellular gene therapy based on the encapsulation technology

Exposure to human serum induces the lysis of xenogeneic cells through natural antibodies and complement activation. The carbohydrate Galactose-alpha1,3-Galactose (Gal-alpha1,3-Gal) epitope, has been shown to be the principal antigenic determinant on target cells. This reaction is, therefore, particularly important for xenogeneic cell-based therapy. As a first step toward the evaluation of the impact of this phenomenon for encapsulated xenogeneic cells, we have evaluated the presence of the Gal-alpha1,3Gal epitope on two cell lines currently being used for the systemic delivery of protein in the periphery or the treatment of neurodegenerative diseases. In the second part of the study, we have tested and compared human serum and cerebrospinal fluid (CSF) for the presence of xenoreactive natural antibodies (XNAs) and their potential impact on the survival of xenogeneic cells. Fluorescence-activated cell sorting analysis indicated that baby hamster kidney (BHK) cells expressed low levels of the alpha-Gal epitope, whereas mouse myoblast C2C12 cells were extensively stained with the specific IB4-lectin. There was a direct correlation between serum killing and the level of Gal-alpha1,3-Gal epitope expression on these cells. Importantly, we showed that CSF did not lyse BHK and C2C12 cells as determined by cytotoxic crossmatch assays. The reaction was specific as the addition of soluble Gal-alpha1,3-Gal sugar to human serum effectively reduced cell killing, and the overproduction of alpha-1,3-galactosyltransferase in BHK cells significantly increased inactivation by human serum. To interfere with this antibody-antigen reaction and develop cell lines particularly suitable for cell-based therapy, we either selected C2C12 clones expressing low levels of Gal-alpha1,3-Gal or high levels of alpha-1,2-fucosyltransferase. These cells were found to be resistant to complement-mediated cytolysis. These strategies may, therefore, protect encapsulated xenogeneic cells transplanted in the periphery or the central nervous system even in an unlikely event of a blood-brain barrier breakage and the post-transplantation development of an antibody response.

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.

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.

Delayed rejection of porcine cartilage is averted by transgenic expression of alpha1,2-fucosyltransferase

The use of xenogeneic cells or tissues for tissue engineering applications may lead to advances in biomedical research. Hyperacute and delayed rejection are immunologic hurdles that must be addressed to achieve xenograft survival in the pig-to-primate setting. Expression of human alpha1,2-fucosyltransferase (HT) in the donor cell or tissue protects from hyperacute rejection (HAR) by reducing expression of Galalpha1,3-Gal epitope, the major xenoantigen recognized by human natural antibodies. We hypothesized that Galalpha1,3-Gal antigen contributes to delayed tissue rejection. To test this hypothesis, we transplanted control or HT-transgenic engineered porcine cartilage s.c. into alpha1,3-galactosyltransferase knockout (Gal KO) mice. Control porcine cartilage grafted in Gal KO mice was not susceptible to HAR but was rejected in several wk by a prominent cellular immune infiltrate and elevated antibody titers. In contrast, Gal KO mice receiving the HT engineered cartilage showed a markedly reduced anti-pig antibody response and no anti-Galalpha1,3-Gal-elicited antibody response. The HT implants had a mild cellular infiltrate that was confined to the graft periphery. Our study demonstrates that a marked reduction of Galalpha1,3-Gal antigen in HT-transgenic porcine cartilage confers resistance to a process of delayed rejection. Further development of tissue engineering applications that use genetically modified porcine tissues is encouraged.

Transcriptional regulation of porcine endogenous retroviruses released from porcine and infected human cells by heterotrimeric protein complex NF-Y and impact of immunosuppressive drugs

Recent studies revealed a significant promoter activity of porcine endogenous retrovirus (PERV) long terminal repeats (LTRs) in different human and mammalian cell lines, which is mediated by a 39-bp repeat located in the U3 region in different numbers, representing an enhancer (G. Scheef, N. Fischer, U. Krach, and R. R. Tönjes, J. Virol. 75:6933-6940, 2001). A statistical transcription factor analysis revealed putative binding sites for the CCAAT-binding transcription factor NF-Y inside the 39-bp repeat. Specific binding of NF-Y to the repeat sequence was demonstrated by electrophoretic mobility shift assays and supershift assays with specific antibodies directed against the three subunits of NF-Y. To identify further transcription-regulating elements, genetically modified LTRs lacking the repeat box, U3, R, or U5 were investigated. The results indicated a strong inhibitory element in the R region, as the deletion of R caused a significantly increased promoter activity. Since PERV might play a potential role in the application of xenogeneic cell therapy and xenotransplantation techniques, we have investigated whether immunosuppressive drugs that are routinely used in transplantation medicine have an impact on the promoter activity. Neither cyclosporine nor prednisolone had any influence on the promoter strength of the PERV LTRs. By performing a real-time PCR we were able to compare the proviral loads of porcine and infected human cells as well as the amount of released virions, which revealed a direct link between LTR activity and the number of released retroviruses.

The use of nuclear transfer to produce transgenic pigs

Manipulation of the pig genome has the potential to improve pig production and offers powerful biomedical applications. Genetic manipulation of mammals has been possible for over two decades, but the technology available has proven both difficult and inefficient. The development of new techniques to enhance efficiency and overcome the complications of random insertion is of importance. Nuclear transfer combined with homologous recombination provides a possible solution: precise genetic modifications in the pig genome may be induced via homologous recombination, and viable offspring can be produced by nuclear transfer using cultured transfected cell lines. The technique is still ineffective, but it is believed to have immense potential. One area that would benefit from the technology is that of xenotransplantation: transgenic pigs are expected to be available as organ donors in the foreseeable future.

Characterizing and mapping porcine endogenous retroviruses in Westran pigs

Since porcine endogenous retroviruses (PERVs) can infect cultured human cells, they are a potential hazard to xenotransplantation. For this reason, endogenous retroviruses from the Westran (Westmead Hospital transplantation) inbred line of pigs were analyzed by using consensus primers for the type A and type B viruses to amplify 1.8-kb envelope gene fragments. After preliminary analysis with restriction enzymes KpnI and MboI, 31 clones were sequenced. Between types A and B, five recombinant clones were identified. Fifty-five percent of clones (17 of 31) had premature stop codons within the envelope protein-encoding region. Endogenous retroviruses in Westran pigs were physically mapped by fluorescence in situ hybridization (FISH) using PERV-A and PERV-B envelope clones as probes to identify at least 32 integration sites (19 PERV-A sites and 13 PERV-B sites). The chromosomal sites of integration in the Westran strain are quite different from those in the European Large White pig. The recombinant clones suggest that defective PERVs could become infective through recombination and further that PERVs might recombine with human endogenous retroviruses in xenotransplants.

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

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

Genetic therapies and xenotransplantation

The number of patients in need of an organ transplant is increasing, while the number of satisfactory sources of organs has declined in many countries [101]. The resulting shortage of human organs has spurred an urgent effort to investigate alternative therapies, including the use of animal organs, tissues and cells (i.e., xenotransplantation). Advances in genetic engineering have provided essential tools for the development of practical solutions to human disease. The area of xenotransplantation is no exception. In fact, the use of genetic therapies is especially attractive in the transplant setting as it offers an opportunity to manipulate the donor tissue rather than the recipient. This review will describe the obstacles in the clinical application of xenotransplantation and how genetic engineering might be used to address them.

Transgenic pigs designed to express human CD59 and H-transferase to avoid humoral xenograft rejection

Research in pig-to-primate xenotransplantation aims to solve the increasing shortage of organs for human allotransplantation and develop new cell- and tissue-based therapies. Progress towards its clinical application has been hampered by the presence of xenoreactive natural antibodies that bind to the foreign cell surface and activate complement, causing humoral graft rejection. Genetic engineering of donor cells and animals to express human complement inhibitors such as hCD59 significantly prolonged graft survival. Strategies to decrease the deposition of natural antibodies were also developed. Expression of human alpha1,2-fucosyltransferase (H transferase, HT) in pigs modifies the cell-surface carbohydrate phenotype resulting in reduced Galalpha1,3-Gal expression and decreased antibody binding. We have developed transgenic pigs that coexpress hCD59 and HT in various cells and tissues to address both natural antibody binding and complement activation. Functional studies with peripheral blood mononuclear cells and aortic endothelial cells isolated from the double transgenic pigs showed that coexpression of hCD59 and HT markedly increased their resistance to human serum-mediated lysis. This resistance was greater than with cells transgenic for either hCD59 or HT alone. Moreover, transgene expression was enhanced and protection maintained in pig endothelial cells that were exposed for 24 h to pro-inflammatory cytokines. These studies suggest that engineering donor pigs to express multiple molecules that address different humoral components of xenograft rejection represents an important step toward enhancing xenograft survival and improving the prospect of clinical xenotransplantation.

Neonatal porcine pancreatic cell clusters as a potential source for transplantation in humans: characterization of proliferation, apoptosis, xenoantigen expression and gene delivery with recombinant AAV

Neonatal porcine islets are characterized by reproducible isolation success and high yields, sizable advantages over adult islets. In this work we have analyzed selected phenotypic and functional characteristics of porcine neonatal islets relevant to their possible use for transplant in humans. We show that porcine islet cells proliferate in culture, and synthesize and store islet-specific hormones. Proliferating beta cells can be easily identified. Implant of cultured neonatal islets in immunodeficient rodents results in the reversal of diabetes, albeit with delay. We also show that measurable apoptosis occurs in cultured neonatal porcine islets. Further, antigens recognized by human natural antibodies are expressed in a dynamic fashion over the culture period analyzed and are not limited to the alpha-Gal epitope. Lastly, we demonstrate that a recombinant Adeno-Associated virus can be used to efficiently deliver a reporter gene in porcine islets. This characterization might be helpful in the definition of the potential use of neonatal porcine islets for human transplantation.

The expression of human FUT1 in HT-29/M3 colon cancer cells instructs the glycosylation of MUC1 and MUC5AC apomucins

Recently, we have reported that in normal gastric epithelium, the expression of gastric apomucins MUC5AC and MUC6 is associated with the specific expression of type 1 and type 2 Lewis antigens, and FUT2 and FUT1 fucosyltransferases, respectively. Until now, there are no data demonstrating the direct implication of specific glycosyltransferases in the specific patterns of apomucin glycosylation. HT29/M3 colon cancer cell line express MUC1, MUC5AC, type 1 Lewis antigens and FUT2 but not type 2 structures and FUT1, as it occurs in the epithelial cells of the gastric superficial epithelium. These cells were transfected with the cDNA of human FUT1, the alpha-1,2-fucosyltransferase responsible for the synthesis of type 2 Lewis antigens, to assess the implication of FUT1 in the glycosylation of MUC1 and MUC5AC. The M3-FUT1 clones obtained express high levels of type 2 Lewis antigens: H type 2 and Ley antigens. Immunoprecipitation of MUC1 and MUC5AC apomucins gives the direct evidence that FUT1 catalyses the addition of alpha-1,2-fucose to these apomucins, supporting the hypothesis that the pattern of apomucin glycosylation is not only instructed by the mucin primary sequence but also by the set of glycosyltransferases expressed in each specific cell type.

Genetic modification of alphaGal expression in xenogeneic endothelial cells yields a complex immunological response

The source of cells for tissue engineering applications remains a hurdle, predominantly for procedures in which there is insufficient time to harvest a patient's own cells. Animal cells are readily available, but undergo immune rejection. Rejection of animal (i.e., xenogeneic) tissue involves practically every component of the immune system. The initial phase, hyperacute rejection (HAR), involves natural xenoreactive antibodies and the complement system, and leads to endothelial cell lysis and rapid tissue destruction. The cell-surface epitope, galactose-alpha(1,3)-galactose (alphaGal), is presumed to play a key role in HAR. The later stage of immune response (delayed xenograft rejection or DXR), is mediated by immune cells such as monocytes. Carbohydrates are likely also involved in DXR, but their role in this phase of the immune response is less clear. A better understanding of all stages of xenogeneic immune rejection may make it feasible to create cell lines that are immune tolerant. In these studies, we have genetically modified bovine endothelial cells to study the roles of carbohydrates in immune rejection. Our studies suggest that one or more epitopes other than alphaGal may influence complement-mediated lysis. Furthermore, antibodies, as instigators in the complement response, and monocytes appear to recognize different cell surface epitopes.

Combinatorial library of five-membered iminocyclitol and the inhibitory activities against glyco-enzymes

BACKGROUND

Oligosaccharide processing enzymes are important classes of catalysts involved in synthesizing specific oligosaccharide structures on proteins and sphingolipids. Development of specific inhibitors of such enzymes is of current interest as these inhibitors may be used to control cellular functions. Five-membered iminocyclitols have been shown to be potent inhibitors of such enzymes. Since a rational design and synthesis of inhibitors is often extremely difficult due to the limited information regarding the structure of the active site, we carried out a combinatorial library approach.

RESULTS

To create diversity, we decided to use an aldehyde group of a protected iminocyclitol for reductive amination and the Strecker reaction. After transformation of the nitrile group introduced by the Strecker reaction into an amine and amide and complete deprotection, a small library of five-membered iminocyclitols consisting of 27 compounds was synthesized. A series of compounds obtained by reductive amination was first screened as potential inhibitors of glycosidases and glycosyltransferases. Among them, compounds carrying a C(10)-alkyl group showed marked enhancement of inhibitory activity against alpha-mannosidase at 10 microM concentration when compared with its parent compound and deoxymannojirimycin. Furthermore, compounds having the phenylethyl group showed an extremely strong inhibitory effect against alpha-galactosaminidase at a K(i) value of 29.4 nM. Compounds with an aminomethyl and amide group at the C-1' position of these two molecules showed a decrease in inhibitory activities.

CONCLUSIONS

A combinatorial approach based on five-membered iminocyclitols with a galacto-configuration was exploited. The potential usefulness of the library as a source of inhibitors of glycoenzymes is clearly shown in this study.

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.