Reduction of the major porcine xenoantigen Galoc(1,3)Gal by expression of α(1,2)fucosyltransferase

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.

Xenotransplantation: Past achievements and future promise

Xenotransplantation offers a potential solution to the shortfall in donor organs for human transplantation. This review describes the barriers to xenotransplantation and the progress that has been made towards making it a clinical reality. Data from preclinical pig-to-primate cardiac and pulmonary xenografts are highlighted.

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.

Xenogeneic transplantation

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

Targeted modification of the domestic animal genome

While the technique of homologous recombination, or gene targeting, has led to the generation of transgenic mice of great value to biomedical research, similar approaches are only being developed in other species. With the exception of recent reports on the generation of gene-targeted sheep, the technology in domestic animals is still in its infancy (45). The development of techniques for generating large animals with deleted or modified genes will result in the generation of animals of great value to society. While the technical difficulties to achieve gene targeting in domestic species are significant, they are not insurmountable. Potential applications in both the bovine and porcine species are described with particular emphasis on the generation of cattle resistant to bovine spongiform encephalopathy (BSE) and pigs that can be of use in xenotransplantation.

Xenotransplantation

BACKGROUND

Over the past 10 years xenotransplantation has generated much interest in the hope that it will enable us to overcome the current lack of human organ donors. This review examines the evolution and current therapeutic strategies that have been developed to overcome the predominant problem of graft rejection.

METHODS

A literature review was undertaken using a Medline search from January 1966 to August 1999.

RESULTS AND CONCLUSION

Despite the considerable advances that have been made in molecular biological techniques, xenograft rejection cannot be prevented without significant immunosuppression and toxic side-effects. The problem of delayed rejection, in particular, will probably be very difficult to overcome, although some of the difficulties associated with hyperacute rejection have been resolved. The potential risk of porcine endogenous retrovirus transmission has generated much debate recently, but it is likely that some of the important issues relating to xenotransplantation will never be resolved until carefully regulated clinical trials are allowed to begin.

Potential application of neonatal porcine islets as treatment for type 1 diabetes: a review

Islet transplantation has been shown to be a viable option for treating patients with type 1 diabetes. However, widespread clinical application of this treatment will necessitate an alternative source of insulin-producing tissue. Porcine pancreata may be a potential source of islets since pigs are inexpensive, readily available, and exhibit morphological and physiological characteristics comparable to humans. Recently, we developed a simple, standardized procedure for isolating large numbers of neonatal porcine islets with a reproducible and defined cellular composition. Following nine days of in vitro culture, tissue from one neonatal pig pancreas yielded approximately 50,000 islet cell aggregates, consisting of primarily epithelial cells (57%) and pancreatic endocrine cells (35%). In addition, neonatal porcine islets were responsive to glucose challenge in vitro and were capable of correcting hyperglycemia in alloxan-induced diabetic nude mice. Although neonatal porcine islets constitute an attractive alternative source of insulin-producing tissue for clinical transplantation, many aspects such as the immunological responses to these tissue and the latent period (2 to 8 weeks) between transplantation of these islets and the reversal of hyperglycemia need further investigation. This article discusses these issues and presents possible solutions to problems that may hinder the potential application of neonatal porcine islets for transplantation into patients with type 1 diabetes.

Factors in xenograft rejection

Important mechanisms underlying immediate xenograft loss by hyperacute rejection (HAR), in the pig-to-primate combination, have been recently delineated. There are now several proposed therapies that deal with the problem of complement activation and xenoreactive natural antibody (XNA) binding to the vasculature that have been shown to prevent HAR. However, vascularized xenografts are still lost, typically within days, by delayed xenograft rejection (DXR), alternatively known as acute vascular rejection (AVR). This process is characterized by endothelial cell (EC) perturbation, localization of XNA within the graft vasculature, host NK cell and monocyte activation with platelet sequestration and vascular thrombosis. Alternative immunosuppressive strategies, additive anti-complement therapies with the control of any resulting EC activation processes and induction of protective responses have been proposed to ameliorate this pathological process. In addition, several potentially important molecular incompatibilities between activated human coagulation factors and the natural anticoagulants expressed on porcine EC have been noted. Such incompatibilities may be analogous to cross-species alterations in the function of complement regulatory proteins important in HAR. Disordered thromboregulation is potentially relevant to the progression of inflammatory events in DXR and the disseminated intravascular coagulation seen in primate recipients of porcine renal xenografts. We have recently demonstrated the inability of porcine tissue factor pathway inhibitor (TFPI) to adequately neutralize human factor Xa (FXa), the aberrant activation of both human prothrombin and FXa by porcine EC and the failure of the porcine natural anticoagulant, thrombomodulin to bind human thrombin and hence activate human protein C. The enhanced potential of porcine von Willebrand factor to associate with human platelet GPIb has been demonstrated to be dependent upon the isolated A1 domain of von Willebrand factor. In addition, the loss of TFPI and vascular ATPDase/CD39 activity following EC activation responses would potentiate any procoagulant changes within the xenograft. These developments could exacerbate vascular damage from whatever cause and enhance the activation of platelets and coagulation pathways within xenografts resulting in graft infarction and loss. Analysis of these and the other putative factors underlying DXR should lead to the development and testing of genetic approaches that, in conjunction with selected pharmacological means, may further prolong xenograft survival to a clinically relevant extent.

Serum cytotoxicity to pig cells and anti-alphaGal antibody level and specificity in humans and baboons

BACKGROUND

Removal and/or "neutralization" of anti-Gal alpha1-3Gal (alphaGal) antibodies can prevent or delay the hyperacute rejection of pig organs transplanted into primates.

AIM

To determine variations in (1) cytotoxicity to pig kidney (PK15) cells, (2) anti-alphaGal antibody level, and (3) specificity in adult human (n=46) and baboon (n=38) sera.

METHODS

Cytotoxicity to PK15 cells was determined by adding rabbit complement to heat-inactivated serum, using a two-color fluorescent dye to distinguish live and dead cells. Anti-alphaGal antibody level was determined by ELISA using alphaGal trisaccharide type 2-BSA glycoconjugate as antigen target. Specificity determined by ELISA using four different alphaGal-BSA glycoconjugates: (disaccharide, trisaccharides type 2 and 6, and pentasaccharide).

RESULTS

Cytotoxicity of human AB sera varied from 30-100% PK15 relative cell damage (%RCD), although that of baboon sera of all blood groups varied from 35-100% RCD. In human AB sera, anti-alphaGal antibody level (at a dilution of 1:80) varied from undetectable to 0.75 (OD at 405 nm), although in baboon sera of all blood groups, anti-alphaGal antibody level varied from undetectable to >2.0. There was no correlation between anti-alphaGal antibody level and serum cytotoxicity in either species. Specificity varied among individuals in both human and baboon sera.

CONCLUSIONS

These studies have demonstrated (1) considerable variation in cytotoxicity and anti-alphaGal antibody level in human and baboon sera, but a lack of correlation between these two parameters; (2) considerable variation in the specificity of anti-alphaGal antibodies; (3) blood group B human and baboon sera have lower levels of anti-alphaGal antibodies; (4) no relation between blood group and specificity of anti-alphaGal antibodies. Although there are minor differences in the parameters measured, baboons would appear to be suitable surrogates for humans in the pig-to-primate xenograft model.

Anti-xenograft immune responses in alpha 1,3-galactosyltransferase knock-out mice

Although originally generated to test the effect of eliminating the alpha-Gal epitope on HAR, it is becoming increasingly clear that GalT KO mice offer a convenient and inexpensive model to investigate many aspects of the anti-xenorgraft immune response. Clearly, not all aspects of anti-xenograft rejection responses are identical in mice and primates, which should be kept in mind when interpreting results of GalT KO mouse studies. However, with this and other mouse models it is possible to test a large number of variables, which is impractical for both logistical and financial reasons with primates. Furthermore the short gestation time and large litter size of mice means that genetic strategies targeting different aspects of the anti-xenograft immune response can be combined and subsequently tested to identify the optimal combination of genetic and therapeutic approaches to achieve long term xenograft survival. In this regard the GalT KO mouse has been and will continue to be a valuable small animal model for the study of all facets of xenograft rejection involving anti-Gal antibodies.

Knock out of alpha1,3-galactosyltransferase or expression of alpha1,2-fucosyltransferase further protects CD55- and CD59-expressing mouse hearts in an ex vivo model of xenograft rejection

BACKGROUND

Organs from transgenic animals with high-level endothelial expression of the human complement regulatory factors CD55 and CD59 are significantly protected from human complement-mediated injury. Elimination or reduction of the major xenoepitope alphaGal, achieved by knocking out the alpha1,3-galactosyltransferase gene (Gal KO) or expressing human alpha1,2-fucosyltransferase (H transferase or HTF), also affords protection, although to a lesser degree. In this study, we examined whether the protection provided by strong CD55 and CD59 expression can be augmented by the Gal KO or HTF modifications.

METHODS

Hearts from four groups of mice (wild type, CD55/CD59, CD55/CD59/HTF, and CD55/CD59/Gal KO) were perfused ex vivo with 40% human plasma. Mean heart work for each group was compared over a 60-min period.

RESULTS

Wild-type hearts ceased to function effectively within 15 min of plasma addition. CD55/CD59 hearts displayed prolonged survival and maintained approximately 10% maximum work at the end of perfusion. Introduction of Gal KO or HTF onto the CD55/CD59 background resulted in a further prolongation, with work maintained at 20-30% of the maximum level.

CONCLUSIONS

We used an ex vivo model to demonstrate that eliminating alphaGal expression further prolongs the function of mouse hearts expressing high levels of CD55 and CD59. In addition, we showed that reducing alphaGal by expressing HTF is equally as effective in prolonging CD55/CD59 heart function as knocking out Gal transferase, thus providing a feasible strategy for translating these advances to the pig.

Life-supporting pig-to-primate renal xenotransplantation using genetically modified donors

BACKGROUND

In order to circumvent the complement-mediated hyperacute rejection of discordant xenografts, a colony of pigs transgenic for the human regulator of complement activity, human decay-accelerating factor (hDAF), has been produced.

METHODS

Seven kidneys from hDAF transgenic pigs and six kidneys from nontransgenic control pigs were transplanted into cynomolgus monkeys; both native kidneys were removed during the same operation. The recipient animals were immunosuppressed with cyclosporine, steroids, and cyclophosphamide.

RESULTS

In the transgenic group, the median survival time was 13 days (range, 6-35 days); the median survival time in the control group was 6.5 days (range, 0.3-30 days). There were no cases of hyperacute rejection in the transgenic group, and the two longest-surviving kidneys in this group showed no evidence of rejection on histological examination. In contrast, all control kidneys underwent antibody-mediated rejection, one demonstrating hyperacute rejection and the others acute vascular rejection.

CONCLUSION

This study demonstrates that (i) a kidney from an hDAF transgenic pig can support the life of a primate for up to 35 days (and also shows the basic physiological compatibility between the pig and nonhuman primate); (ii) nontransgenic kidneys are not routinely hyperacutely rejected; and (iii) the presence of hDAF on the kidney confers some protection against acute vascular rejection. Improved immunosuppression and immunological monitoring may enable extended survival.