Enzymatic remodelling of the carbohydrate surface of a xenogenic cell substantially reduces human antibody binding and complement-mediated cytolysis

Generation of serum-stabilized retroviruses: Reduction of α1,3gal-epitope synthesis in a murine NIH3T3-derived packaging cell line by expression of chimeric glycosyltransferases

Retroviral vectors released from mouse-derived packaging cell lines are inactivated in human sera by naturally occurring antibodies due to the recognition of Galalpha1,3Galbeta1,4GlcNAc (alphagal-epitope) decorated surface proteins. In this study, an extensive analysis of the glycosylation potential of NIH3T3-derived PA317 packaging cells using combined MALDI/TOF-MS and HPAE-PAD reveals that 34% of the N-glycan moiety represents alphagal-epitope containing structures. Stable expression of glycosyltransferases and transport signal chimeras has been demonstrated to represent an efficient tool to alter cell- and species-specific glycosylation (Grabenhorst and Conradt, 1999. J. Biol. Chem. 274, 36107-36116). In order to reduce alphagal-epitope synthesis selected chimeric glycosyltransferases were constructed by fusing Golgi-signal sequences for compartment-specific localization with the catalytic domain of alpha2,3-sialyltransferase (ST3). Stable expression of these constructs in these cells resulted in a significant reduced alphagal-epitope synthesis, and moreover, a release of retroviral vectors showing an up to 3.5-fold increase in serum stability. Thus, our results suggest that the stably transfected cells stably transfected with chimeric glycosyltransferases compete efficiently with endogenous alpha1,3-galactosyltransferase. This approach allows favored glycodesign and we anticipate the applicability of such improved retroviral vectors produced by glycosylation engineered host cells for in vivo gene therapy and, furthermore, suggest the therapeutic benefit of this technology for xenotransplantation.

Effect of RNA interference on Gal alpha 1,3 Gal expression in PIEC cells

Xenotransplantation from pig to human being is viewed as a potential solution for the acute organ shortage. However, consequent xenorejection induced by Gal alpha 1,3 Gal (Gal, Gal antigen) prevents xenotransplantation from clinical application. Thus, the most attracting attempt to prevent xenorejection is the elimination of Gal. Our study suggested that compared with the human alpha 1,2 fucosyltransferase (FT) gene and porcine antisense alpha 1,3 galactosyltransferase gene, sequence-specific siRNA targeting Gal were capable of suppressing Gal expression markedly, and therefore, significantly inhibiting xenoreactivity and the complement activation with human serum in PIEC cells. We also demonstrated the concordant inhibitory effect of siRNA and human FT gene on Gal and corresponding functions, which implied a practical significance of combined transgenic strategy. The successful application of vector-based dsRNA-GT may extend the list of available modalities in the abrogation of xenorejection in xenotransplantation.

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.

A study of the xenoantigenicity of adult pig islets cells

BACKGROUND

The pig pancreas is considered to be the most suitable source of islets for xenotransplantation into patients with type I diabetes. The purpose of this study was to assess the antigenicity of pig islets, including the Galalpha1-3Galbeta1-4GlcNAc-R (the alpha-Gal) and Hanganutziu-Deicher (H-D) antigens, and the pathway involved in human complement activation.

METHODS

The expression of alpha-Gal on islets from adult pigs was investigated by immunohistochemical staining and flowcytometric analysis. The alpha1,3 galactosyltransferase (alpha1,3GT) activity of islets was determined by high-performance liquid chromatography. Antigenicity to human natural antibodies, including the H-D antigen of pig islets was next examined by treatment of pig islets with tunicamycin, D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) and/or neuraminidase. In addition, complement-mediated islets lysis was examined using factor D-deficient and C1-deficient sera.

RESULTS

Adult pig islets expressed negligible amounts of alpha-Gal epitope, and alpha1,3GT activity was also undetectable. However, human natural antibodies, immunoglobulin G and M, and the anti H-D antibody react to the adult islet. Treatment of pig islets with tunicamycin, but not PDMP, led to a drastic reduction in antigenicity to human serum, indicating the importance of N-linked sugars on the islets. Neuraminidase treatment indicated the presence of, not only the H-D antigen, but also other sialic acid antigens that reacted with the human natural antibody. The complement deposition of C4, C3 and factor B on islets was demonstrated. The alternative pathway-mediated pig islet killing accounted for approximately 30% of that by the total complement pathway.

CONCLUSION

The origin of antigenicity of pig islets is mainly N-linked sugars including sialic acid antigens, but not the alpha-Gal, and pig islets can be injured by both the classical and the alternative complement pathway in human serum.

Natural antibodies prevent in vivo transmission of porcine islet-derived endogenous retrovirus to human cells

The discovery of porcine endogenous retroviruses (PERV) has raised concerns regarding the safety of porcine xenotransplantation. However, transmission of PERV had not been observed in humans exposed to porcine tissue. We examined whether PERV derived from porcine pancreatic islet cells could infect human cells in vivo and the role of natural antibodies in inhibiting PERV infection. In vivo infective potential of PERV was studied in SCID mice reconstituted with human peripheral blood leucocytes. Porcine islets were transplanted under the kidney capsule. PERV infection was determined by analyzing PERV gene expression in graft infiltrating lymphocytes (GIL) harvested 21 days posttransplantation. Mice were administered normal human serum prior to and 2 days posttransplantation to study their role in protection of human cells against PERV infection. PERV genes were expressed in all porcine tissues examined, including purified porcine islets. PERV expression was detected in GILs from three of five human-SCID mice. Administration of human serum blocked PERV infection in GILs in five of five human-SCID mice. These results indicate that PERV from porcine islets can infect human cells in vivo. Normal human serum blocks transmission of retrovirus in vivo, suggesting that natural xenoreactive antibodies can prevent PERV infection.

DEVELOPING A PORCINE TRANSPLANTATION MODEL: EFFICIENT GENE TRANSFER INTO PORCINE VASCULAR CELLS

BACKGROUND

The pig is generally regarded as likely to be the preferred donor animal in xenotransplantation. Although many hurdles remain to be cleared, it would be useful to be able to manipulate porcine endothelium genetically, among other reasons, to test approaches in the modulation of inflammation. However, as a nondividing cell, it is less easy to manipulate.

METHODS

The authors performed in vivo and in vitro gene transfection experiments using as an adjunct the DNA-binding agent 4',6'-diamidino-2-phenylindole (DAPI), which protects DNA from degradation.

RESULTS

The introduction of DAPI into a liposomal transfection system was able to increase in vitro transfection efficiency of both endothelial and vascular smooth muscle cells from the pig, even in the presence of small amounts of serum. This last observation encouraged the authors to use this system in vivo in porcine carotid arteries. In this model, the authors were also able to demonstrate a high degree of transfection efficiency using DAPI, which seemed to work by protecting DNA from degradation.

CONCLUSIONS

The authors believe this technique may allow them to address many biological questions relating to intervening in vascular disease, inflammation, and immune responses in the context of transplantation and beyond.

ACTIVATION OF PRIMARY PORCINE ENDOTHELIAL CELLS INDUCES RELEASE OF PORCINE ENDOGENOUS RETROVIRUSES

BACKGROUND

Endothelial cells form the interface between the porcine graft and the recipient and frequently become activated after xenotransplantation. To evaluate the safety of xenotransplantation further, we assessed the effect of cellular activation on the expression and release of porcine endogenous retroviruses from primary endothelial cells isolated from transgenic and nontransgenic pigs.

METHODS

Primary porcine endothelial cells, cultured from pigs transgenic for human decay accelerating factor, were treated with human tumor necrosis factor-alpha, porcine interferon-gamma, or lipopolysaccharide. The release of porcine endogenous retroviruses into the supernatant was monitored at 24-hr intervals (up to 72 hr) by polymerase chain reaction-based reverse transcriptase (PBRT) assay. Activated and unactivated endothelial cells were co-cultured with human cells to investigate the capacity of any virus released from the porcine cells to infect human cells.

RESULTS

Virus was not detected in supernatants from quiescent cells by PBRT analysis. The number of viral particles released from endothelial cells was 10 to 5 x 10 viral particles/mL after cellular activation with tumor necrosis factor-alpha, interferon-gamma, or lipopolysaccharide, as shown by PBRT analysis. In contrast, in vitro infection of human cells was observed with unactivated endothelial cells only and was not observed in co-cultures with the activated porcine cells.

CONCLUSIONS

Cytokine treatment of primary porcine endothelial cells results in an increase in the release of virus into the supernatant, but the observed increase in viral titer was not mirrored by an increase in infectivity toward human cells.

Xenotransplantation and risks of zoonotic infections

The shortage of human organs and tissues for transplantation and the advances in immunology of rejection and in genetic engineering have renewed interest in xenotransplantation--the transplantation of animal organs, tissues or cells to humans. Clinical trials have involved the use of non-human primate, porcine, and bovine cells/tissues/organs. In recent years, research has focused mainly on pigs as donors (especially, pigs genetically engineered to carry some human genes). One of the major concerns in xenotransplantation is the risk of transmission of animal pathogens, particularly viruses, to recipients and the possible adaptation of such pathogens for human-to-human transmission. Porcine endogenous retroviruses (PERVs) have been of special concern because of their ability to infect human cells and because, at present, they cannot be removed from the source animal's genome. To date, retrospective studies of humans exposed to live porcine cells/tissues have not found evidence of infection with PERV but more extensive research is needed. This article reviews infectious disease risks associated with xenotransplantation, some measures for minimizing that risk, and microbiological diagnostic methods that may be used in the follow-up of xenotransplant recipients.

Sensitivity to human serum of gammaretroviruses produced from pig endothelial cells transduced with glycosyltransferase genes

Reduction of pig cell-surface alpha-galactosyl (Gal) epitope, Galalpha1, 3Galbeta1, 4GlcNAc-R, by the introduction of glycosyltransferase genes is effective in suppressing hyperacute rejection (HAR) in pig-to-human xenotransplantation. The transmission of porcine endogenous retroviruses (PERVs) has been recognized as a potential risk factor associated with xenotransplantation. In this study, effects of the introduction of glycosyltransferase genes to pig cells on the sensitivity of gammaretroviruses to human serum were investigated. Pig endothelial cells (PEC), PEC transduced with alpha1,2 fucosyltransferase (FT), alpha2,3 sialyltransferase (ST), or N-acetylglucosaminyltransferase III (GnT-III), and human embryonic kidney (HEK) 293 cells were transduced with the LacZ gene with the packaging signal of murine leukemia virus (MuLV) under the control of the long terminal repeat of MuLV by a pseudotype infection. Then, the cells were further infected with PERV subtype B (PERV-B) or feline leukemia virus subgroup B (FeLV-B). Culture supernatants of the infected cells were mixed with human serum (HS) and then inoculated to HEK293 cells. The inoculated cells were histochemically stained and lacZ-positive blue foci were counted. Glycosyltransferase activity, xenoantigenicity, and alpha-Gal epitope density in the cells were measured at the time of the infection experiments. PERV-B or FeLV-B particles from the parental PEC were efficiently neutralized by HS, while those from PEC transduced with alpha1,2FT, alpha2,3ST or GnT-III were less sensitive to HS. The transduced PEC exhibited high levels of activity of the introduced glycotransferases, and expressed fewer xenoantigens and cell-surface alpha-Gal epitopes. Our results suggest that gammaretroviruses including PERVs produced by transgenic pigs, that are generally modified to reduce the cell-surface alpha-Gal epitope to overcome the HAR in xenotransplantation, are less sensitive to HS.

Molecular cloning and functional characterization of infectious PERV and development of diagnostic tests

Pigs are the donor animals of choice for xenotransplantation (XTx) and xenogeneic cell therapy measurements. Most known porcine pathogens can be controlled by conventional means like vaccination, medication or specific pathogen-free breeding conditions. As pigs have co-evolved very closely with humans for a few millennia it is not very likely that even asymptomatic pathogens have escaped attention. Porcine endogenous retroviruses (PERV) are different from conventional pathogens as they are chromosomally fixed in every cell of the animal, hence PERV cannot be easily controlled. While PERV show no phenotype in the porcine host, recent data demonstrate that some polytropic proviruses can be activated by external stimuli and that those can productively infect human cells in vitro. In evaluation of the retrovirological safety of XTx, we determined the number of replication-competent PERV to be limited and to exhibit a heterogeneous distribution, therefore suggesting that they could be removed by conventional breeding. The transcriptional regulation of some PERV due to repetitive elements in their long terminal repeats enables their adaptation to new host cells. The diagnostic tools available, based on immunological and polymerase chain reaction techniques, were shown to be sensitive in both the animal and in vitro, but must still show their potential in human XTx recipients, where they are confronted with very low antigen expression and the phenomenon of microchimerism.

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.

Analyses of prevalence and polymorphisms of six replication-competent and chromosomally assigned porcine endogenous retroviruses in individual pigs and pig subspecies

As porcine endogenous retroviruses (PERV) productively infect human cells in vitro, they pose a serious risk in xenotransplantation and xenogeneic cell therapies. We have analyzed the prevalence of six well-characterized full-length PERV, five of them being replication-competent and four of them being chromosomally assigned (J. Virol. 75 (2001) 5465; J. Virol. 76 (2002) 2714). These analyses revealed a heterogeneous distribution of PERV among individuals and, as no PERV is present in every pig, it seems feasible to generate pigs free of functional PERV by conventional breeding. Conversely, as PERV are polymorphic, single proviruses may have escaped detection and this kind of assay must be performed for every herd used in xenotransplantation or xenogeneic cell therapies. In addition, specific proviruses show internal point mutations which significantly affect their replicational capacities. As there are two different types of PERV LTR structures showing varying levels of transcriptional capacity (J. Virol. 75 (2001) 6933), an analysis of 21 distinct chromosomal locations revealed that PERV which harbor highly active LTRs with repeat elements in U3 are dominant.

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.

Xenotransplantation

The continued and growing success of lung allotransplantation has intensified the worldwide shortage of donor organs. Yet, xenotransplantation remains a daunting challenge. Additional molecular incompatibilities and unforeseen complications will continue to be discovered. Progress has been made, notably on the generation of alpha-Gal double knockout pigs. Progressive increases in organ survival times have been seen for most organs after significant investments of time and money. The lung continues to be an organ with the lowest supply of cadaveric donors and the least potential for expanded living donation or mechanical alternatives. As such, the impetus for xenotransplantation is strong. The lung appears to be exquisitely sensitive to xenograft rejection and resistant to strategies that have been moderately successful in other organs. A complex program involving genetically modified donor organs, recipient preparation for antibody removal or tolerance promotion, and multitargeted drug therapy will likely be required for successful clinical application.

Xenoreactive anti-Galalpha(1,3)Gal antibodies prevent porcine endogenous retrovirus infection of human in vivo

The discovery of porcine endogenous retroviruses (PERV) has raised concerns regarding the safety of pig to human xenotransplantation. In this study, we examined PERV infection of human cells in vivo. Furthermore, we examined the effect of human xenoreactive natural antibody on in vivo PERV infection. Human peripheral blood leukocyte reconstituted severe combined immunodeficiency mice were transplanted with porcine aortic endothelial cells (PAEC). PERV gene expression was readily detected in human leukocytes after transplantation. In contrast, human leukocytes harvested from mice treated with human serum or anti-Galalpha(1,3)Gal antibody prevented PERV infection in 6 of 8 mice. These results provide the first evidence that PERV can infect human cells in vivo and that natural xenoreactive antibody can prevent this infection.

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.

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.

Effect of various forms of the C1 esterase inhibitor (C1-INH) and DAF on complement mediated xenogeneic cell lysis

The purpose of the present study was to assess the effect of various forms of the surface-bound form of the C1 esterase inhibitor (C1-INH-PI) and decay accelerating factor (DAF) on xenogenic cells. cDNAs of various deletion mutants of the C1-INH-PI, such as delta-1-99 amino acid (AA), delta-108-183AA loop, delta-whole loop, delta-exon5, delta-exon6 + 7, and delta-exon5 + 6 + 7, and that of DAF, the delta-short consensus repeat (SCR) 1-DAF were established. While all deletion mutants of C1-INH-PI except the delta-1-99AA were expressed in the cytoplasm but not on the cell surface, the delta-1-99AA was clearly expressed on the xenogeneic cell surface. Amelioration of complement-mediated xenogeneic cell lysis by delta-1-99AA was next tested, and compared with delta-SCR1 DAF. Both molecules blocked human complement-mediated cell lysis by approximately 57 to 90 and 93 to 98%, respectively, in Chinese hamster ovarian tumor (CHO) cells and pig endothelial cells (PECs). The CHO cell transfectants were incubated with 20% normal human serum, and the amounts of C4 and C3 deposition on the cell surface were analysed by flow cytometry. The DAF transfectant showed a large amount of C4-deposition and much less C3-deposition than the controls (approximately 85% suppression), whereas the delta-1-99AA showed approximately a 40% suppression in both C4- and C3-deposition. Consequently, both the delta-1-99AA C1-INH-PI and delta-SCR1 DAF molecules are quite effective in down-regulating the xenogeneic cell lysis, but accomplished this in different manners.

Immune parameters relevant to neural xenograft survival in the primate brain

The lack of supply and access to human tissue has prompted the development of xenotransplantation as a potential clinical modality for neural cell transplantation. The goal of the present study was to achieve a better understanding of the immune factors involved in neural xenograft rejection in primates. Initially, we quantified complement mediated cell lysis of porcine fetal neurons by primate serum and demonstrated that anti-C5 antibody treatment inhibited cell death. We then developed an immunosuppression protocol that included in vivo anti-C5 monoclonal antibody treatment, triple drug therapy (cyclosporine, methylprednisolone, azathioprine) and donor tissue derived from CD59 or H-transferase transgenic pigs and applied it to pig-to-primate neural cell transplant models. Pre-formed alphaGal, induced alphaGal and primate anti-mouse antibody (PAMA) titers were monitored to assess the immune response. Four primates were transplanted. The three CD59 neural cell recipients showed an induced anti-alphaGal response, whereas the H-transferase neural cell recipient exhibited consistently low anti-alphaGal titers. Two of these recipients contained surviving grafts as detected by immunohistochemistry using selected neural markers. Graft survival correlated with high dose cyclosporine treatment, complete complement blockade and the absence of an induced PAMA response to the murine anti-C5 monoclonal antibodies.

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.

Histidine 271 has a functional role in pig alpha-1,3galactosyltransferase enzyme activity

Alpha(1,3)Galactosyltransferase (GT) is a Golgi-localized enzyme that catalyzes the transfer of a terminal galactose to N-acetyllactosamine to create Galalpha(1,3)Gal. This glycosyltransferase has been studied extensively because the Galalpha(1,3)Gal epitope is involved in hyperacute rejection of pig-to-human xenotransplants. The original crystal structure of bovine GT defines the amino acids forming the catalytic pocket; however, those directly involved in the interaction with the donor nucleotide sugars were not characterized. Comparison of amino acid sequences of GT from several species with the human A and B transferases suggest that His271 of pig GT may be critical for recognition of the donor substrate, UDP-Gal. Using pig GT as the representative member of the GT family, we show that replacement of His271 with Ala, Leu, or Gly caused complete loss of function, in contrast to replacement with Arg, another basic charged residue, which did not alter the ability of GT to produce Galalpha(1,3)Gal. Molecular modeling showed that His271 may interact directly with the Gal moiety of UDP-Gal, an interaction possibly retained by replacing His with Arg. However, replacing His271 with amino acids found in alpha(1,3)GalNAc transferases did not change the donor nucleotide specificity. Thus His271 is critical for enzymatic function of pig GT.

Reduction of alpha-Gal expression by relocalizing alpha-galactosidase to the trans-Golgi network and cell surface

Historically, the most effective means of modifying cell surface carbohydrates has required the intracellular overexpression of glycosyltransferases or glycosidases and is dependent on the enzymes occupying a cellular localization close to the carbohydrate structures they modify. We report on relocalizing the lysosomal resident glycosidase human alpha-galactosidase to other regions of the cell, Golgi and cell surface, where it is in closer proximity for cleaving the carbohydrate structure Galalpha(1,3)Gal. Relocalization of alpha-galactosidase was achieved by using the transmembrane and cytoplasmic domains from the human protein furin, which is known to localize in the trans-Golgi network (TGN) and cell surface. Two chimeric forms of alpha-galactosidase were generated, one directing it to the TGN of the cell and the other to the cell surface, as shown by confocal microscopy. The relocalized enzymes have the ability to cleave terminal alpha-galactose as detected by expression on the cell surface. Furthermore, when expressed as a transgene in mice, the TGN form of alpha-galactosidase was more effective at decreasing cell surface terminal alpha-galactose than was the native lysosomal form. When expressed in conjunction with the alpha1,2fucosyltransferase that also decreases Galalpha(1,3)Gal, the reduction was additive. The ability to relocalize enzymes that modify cell surface carbohydrate structures has far-reaching implications in biology and may be useful in such fields as xenotransplantation and treatment of glycosidase disorders.

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.

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.

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.

Characterization of chromosomally assigned replication-competent gamma porcine endogenous retroviruses derived from a large white pig and expression in human cells

Vertically transmitted endogenous retroviruses pose an infectious risk in the course of pig-to-human transplantation of cells, tissues, and organs. Two classes of polytropic type C porcine endogenous retroviruses (PERV) productively infect human cells in vitro. The cloning and characterization of replication-competent PERV-B sequences from infected human cells (F. Czauderna, N. Fischer, K. Boller, R. Kurth, and R. R. Tönjes, J. Virol. 74:4028-4038, 2000) as well as the cloning of functional PERV-A and -B sequences from porcine cell line PK15 (U. Krach, N. Fischer, F. Czauderna, and R. R. Tönjes, J. Virol. 75:5465-5472, 2001) have been previously described. Here we report the isolation of four full-length proviral sequences from a porcine bacterial artificial chromosome (BAC) library that comprises chromosomally assigned PERV. Clones Bac-PERV-A(130A12) and Bac-PERV-A(151B10) map to pig chromosome 1 and demonstrate close homology to PK15-PERV-A(58) in env and to PERV-MSL in long terminal repeat (LTR), gag, and pro/pol sequences. Clone Bac-PERV-A(463H12) is located on pig chromosome 3 and demonstrates close homology to PK15-PERV-A(58) in env and to 293-PERV-B(43) in LTR, gag, and pro/pol (Czauderna et al.; R. R. Tönjes, F. Czauderna, N. Fischer, U. Krach, K. Boller, P. Chardon, C. Rogel-Gailard, M. Niebert, G. Scheef, A. Werner, and R. Kurth, Transplant Proc. 32:1158-1161, 2000). Clone Bac-PERV-B(192B9) is located on pig chromosome 7 in the swine leukocyte antigen region and is highly homologous with but distinct from the previously described functional clone 293-PERV-B(43) and bears the number of repeats initially observed in the LTRs of clone 293-PERV-A(42) (Czauderna et al.; Krach et al.). Clones Bac-PERV-A(130A12), Bac-PERV-A(151B10), and Bac-PERV-A(463H12) were replication competent upon transfection into susceptible 293 and HeLa cells. Bac-PERV-B(192B9), however, bears two stop codons in pro/pol preventing this clone from being replication competent in some individual pigs, but initial screenings indicate that this provirus might be intact in others. The data suggest that the porcine genome harbors a limited number of infectious PERV sequences, allowing for specific screening in different pig breeds.

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 expressing human decay-accelerating factor regulated by porcine MCP gene promoter

Porcine membrane cofactor protein (pMCP) is abundantly expressed throughout the body with particularly strong expression on the vascular endothelia. Previous studies demonstrated that the promoter of the pMCP gene induced efficient expression of a human complement regulatory protein, decay-accelerating factor (DAF; CD55), in transgenic mice. In the present study, we tried to produce transgenic pigs with two hybrid genes, 0.9/hDAF and 5.4/hDAF, which were composed of human DAF (hDAF) gene regulated under pMCP promoters of different lengths (0.9 and 5.4 kb). Five live founder transgenic pigs were obtained only with the 0.9/hDAF construct. Although, four founder pigs transmitted the transgene to the second generation, the transmission rates varied among founders. We examined the expression of hDAF in tissues of descendants of two lines (Dm1 and Dm4). Human DAF specific RNAs were confirmed by an RT-PCR analysis in all organs examined. Levels of hDAF protein in the organs from the descendants of Dm1 line were higher than those in the corresponding human organs as determined by enzyme-linked immunosorbent assay. Immunohistochemical studies showed that the tissue distribution of hDAF in the descendants of both lines was similar to that of endogenous pMCP. The expression level of hDAF on the vascular endothelial cells in Dm1 line was twice that on the corresponding human cells. We tested whether proinflammatory cytokines upregulate an efficiency of pMCP promoter on hDAF expression in transgenic pigs. Although the expression of hDAF on the human endothelial cells increased with a combination of cytokines, tumor necrosis factor alpha and interferon-gamma, no cytokine-induced upregulation was seen in the cells of transgenic pigs. The endothelial cells from transgenic pigs exhibited high resistance to the human serum-mediated cytolysis.

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.

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.

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 pig as a source of cardiac xenografts

The inadequate availability of human donor hearts and other organs has inspired interest in the field of xenotransplantation. Historically, ten attempts to transplant animal hearts into human recipients have been reported. Those who received hearts from nonhuman primates (i.e., baboons and chimpanzees) survived rather longer than did those who received hearts from nonprimates (i.e., sheep and pigs). Nevertheless, current opinion is that the pig is the best candidate as a source of hearts for humans despite the considerable immunologic disparity between the two species. Pigs are available in large numbers and can be bred easily and rapidly. They grow to appropriate sizes and their cardiovascular system is similar to that of humans. Substantial knowledge has been accumulated regarding both genetic engineering and tolerance induction in pigs, two strategies that may help to overcome the existing immunologic barriers. Concern has been raised, however, with regard to the potential for the transfer of a porcine infection with the pig organ to the human recipient. This brief review addresses these and other aspects of the use of the pig as a source of hearts for patients with end-stage cardiac disease.

Therapeutic strategies for xenograft rejection

The increasing demand for transplantable organs over the past several decades has stimulated the idea of using animal organs in lieu of cadaveric organs in clinical transplantation. Pigs are now considered to be the most suitable source of organs for transplantation because of their abundant availability, their appropriate size, their relatively short gestation period, and the recent development in the technology to genetically manipulate them. In the past few years, some of the seemingly complex immunologic responses in pig-to-primate transplantation have been elucidated. This progress has allowed us to focus our efforts on devising specific therapeutic strategies to overcome or prevent some of the responses that contribute to rejection of the xenograft. In this article, we review the various approaches that might allow clinical xenotransplantation to come to fruition.

Down-regulation of the alpha-Gal epitope expression in N-glycans of swine endothelial cells by transfection with the N-acetylglucosaminyltransferase III gene. Modulation of the biosynthesis of terminal structures by a bisecting GlcNAc

The down-regulation of the alpha-Gal epitope (Galalpha1,3Galbeta-R) in swine tissues would be highly desirable, in terms of preventing hyperacute rejection in pig-to-human xenotransplantation. In an earlier study, we reported that the introduction of the beta1,4-N-acetylglucosaminyltransferase (GnT) III gene into swine endothelial cells resulted in a substantial reduction in the expression of the alpha-Gal epitope. In this study, we report on the mechanism for this down-regulation of the alpha-Gal epitope by means of structural and kinetic analyses. The structural analyses revealed that the amount of N-linked oligosaccharides bearing the alpha-Gal epitopes in the GnT-III-transfected cells was less than 10% that in parental cells, due to the alteration of the terminal structures as well as a decrease in branch formation. In addition, it appeared that the addition of a bisecting GlcNAc, which is catalyzed by GnT-III, leads to a more efficient sialylation rather than alpha-galactosylation. In vitro kinetic analyses showed that the bisecting GlcNAc has an inhibitory effect on alpha-galactosylation, but does not significantly affect the sialylation. These results suggest that the bisecting GlcNAc in the core is capable of modifying the biosynthesis of the terminal structures via its differential effects on the capping glycosyltransferase reactions. The findings may contribute to the development of a novel strategy to eliminate carbohydrate xenoantigens.

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

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

Comparison of replication-competent molecular clones of porcine endogenous retrovirus class A and class B derived from pig and human cells

Vertically transmitted endogenous retroviruses pose an infectious risk in the course of pig-to-human transplantation of cells, tissues, and organs. Two classes of polytropic type C porcine endogenous retroviruses (PERV) which are infectious for human cells in vitro are known. Recently, we described the cloning and characterization of replication-competent PERV-B sequences from productively infected human cells (F. Czauderna, N. Fischer, K. Boller, R. Kurth, and R. R. Tönjes, J. Virol. 74:4028-4038, 2000). Here, we report the isolation of infectious molecular PERV-A and PERV-B clones from pig cells and compare these proviruses with clones derived from infected human 293 cells. In addition to clone PERV-A(42) derived from 293 cells, four "native" full-length proviral PERV sequences derived from a genomic library of the porcine cell line PK15 were isolated. Three identical class A clones, designated PK15-PERV-A(42), PK15-PERV-A(45), and PK15-PERV-A(58), and one class B clone, PK15-PERV-B(213), were characterized. PK15-PERV-B(213) is highly homologous but distinct from the previously described clone PERV-B(43). PK15-PERV-A(58) demonstrates close homology to PERV-A(42) in env and to PERV-C in long terminal repeat, gag, and pro/pol sequences. All three PERV clones described here were replication competent upon infection of susceptible cell lines. The findings suggest that the pig genome harbors a limited number of infectious PERV-A and -B sequences.

Human T cells infiltrate and injure pig coronary artery grafts with activated but not quiescent endothelium in immunodeficient mouse hosts

BACKGROUND

We have previously demonstrated that human artery grafts transplanted to immunodeficient mice are infiltrated and injured by unsensitized allogeneic human T cells. We extended our investigations to human anti-porcine xenoresponses in this model.

METHODS

Pig coronary artery segments were interposed into the infrarenal aorta of severe combined immunodeficiency/beige mice. After 7 days, certain recipients were reconstituted with human leukocytes and/or treated with proinflammatory cytokines. The grafts were harvested after 1-70 days and examined by histology, immunohistochemistry, and morphometry.

RESULTS

Pig artery grafts from untreated mice had no evidence of injury, leukocytic infiltrate, or endothelial cell activation up to 70 days postoperatively, despite deposition of murine complement. Host reconstitution with human peripheral blood mononuclear cells resulted in a discrete population of circulating T cells that did not infiltrate or injure the grafts up to 28 days after adoptive transfer. Administration of porcine interferon-gamma for up to 28 days sustained the expression of graft vascular cell adhesion molecule-1 and major histocompatibility complex antigens, but did not initiate recruitment of human leukocytes. In contrast, treatment with human tumor necrosis factor for 7 days induced the de novo expression of porcine E-selectin by graft endothelial cells and elicited human T cell infiltration and human peripheral blood mononuclear cell-dependent vascular injury.

CONCLUSIONS

The human peripheral blood mononuclear cell-severe combined immunodeficiency/beige mouse model identifies a significant difference between human T cell allogeneic and xenogeneic responses in vivo. Xenografts with quiescent endothelium are not infiltrated or injured by T cells under the same conditions in which allografts are rejected. Activation of pig coronary artery endothelial cells by human tumor necrosis factor, but not porcine interferon-gamma, elicits cellular xenoresponses.

The cytoplasmic tail of alpha 1,2-fucosyltransferase contains a sequence for golgi localization

The Golgi apparatus has a central role in the glycosylation of proteins and lipids. There is a sequential addition of carbohydrates by glycosyltransferases that are distributed within the Golgi in the order in which the glycosylation occurs. The mechanism of glycosyltransferase retention is considered to involve their transmembrane domains and flanking regions, although we have shown that the cytoplasmic tail of alpha1,2-fucosyltransferase is important for its Golgi localization. Here we show that the removal of the alpha1,2-fucosyltransferase cytoplasmic tail altered its function of fucosylation and its localization site. When the tail was removed, the enzyme moved from the Golgi to the trans Golgi network, suggesting that the transmembrane is responsible for retention and that the cytoplasmic tail is responsible for localization. The cytoplasmic tail of alpha1,2-fucosyltransferase contains 8 amino acids (MWVPSRRH), and mutating these to alanine indicated a role for amino acids 3 to 7 in localization with a particular role of Ser(5). Mutagenesis of Ser(5) to amino acids containing an hydroxyl (Tyr and Thr) demonstrated that the hydroxyl at position 5 is important. Thus, the cytoplasmic tail, and especially a single amino acid, has a predominant role in the localization and thus the function of alpha1,2-fucosyltransferase.