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

Immune response to bovine pericardium implanted into α1,3-galactosyltransferase knockout mice: feasibility as an animal model for testing efficacy of anticalcification treatments of xenografts

OBJECTIVES

Glutaraldehyde (GA)-fixed xenografts are prone to calcification after implantation in humans and there is evidence that immune reaction to the Galα1,3-Galβ1,4GlcNAc-R (α-Gal) antigen may play a part in this process. The objectives of this study were to evaluate the immune response of α1,3-galactosyltransferase knockout (α-Gal KO) mice to bovine pericardium and to evaluate the effect of various anticalcification treatments on bovine pericardium using mouse subcutaneous implantation model.

METHODS

Bovine pericardial tissues were divided into eight groups according to the method of anticalcification treatments. Prepared tissues were subcutaneously implanted into the α-Gal KO and wild-type mice for 2 months, and anti-α-Gal antibodies were measured at 2 weeks and 2 months after implantation. Explanted tissues were examined by immunohistochemistry and calcium contents of the explanted tissues were measured.

RESULTS

Titres of IgM and IgG antibodies in the α-Gal KO mice increased significantly according to the duration of implantation, whereas titres of IgM and IgG antibodies in the wild-type mice increased until 2 weeks after implantation without further increase thereafter. Titres of IgG antibodies measured at 2 months after implantation were significantly higher in the α-Gal KO mice than in the wild-type mice. Immunohistochemistry revealed macrophages surrounding the pericardial tissues irrespective of the mouse type into which the tissues implanted, whereas T-cells could only be observed in the tissues implanted into the α-Gal KO mice. Except the high-concentration GA-treated group, calcium contents of anticalcification-treated groups were all significantly lower or tended to be lower than that of the control group, irrespective of the mouse type. Calcium contents of the control group were significantly higher in the α-Gal KO mice than in the wild-type mice.

CONCLUSIONS

Bovine pericardium implanted into the α-Gal KO mice caused significant increase in anti-α-Gal antibodies, showed some histologic evidences of chronic rejection and revealed a potential toward more calcification. These findings suggest a possible role of immune response in calcification of xenografts. High-concentration GA fixation alone did not prove to be an effective anticalcification treatment in mouse subcutaneous implantation model. α-Gal KO mouse subcutaneous implantation model might be a feasible animal model for testing efficacy of anticalcification treatments incorporating immunologic approach.

The Galalpha1,3Galbeta1,4GlcNAc-R (alpha-Gal) epitope: a carbohydrate of unique evolution and clinical relevance

In 1985, we reported that a naturally occurring human antibody (anti-Gal), produced as the most abundant antibody (1% of immunoglobulins) throughout the life of all individuals, recognizes a carbohydrate epitope Galalpha1-3Galbeta1-4GlcNAc-R (the alpha-gal epitope). Since that time, an extensive literature has developed on discoveries related to the alpha-gal epitope and the anti-Gal antibody, including the barrier they form in xenotransplantation and their reciprocity in mammalian evolution. This review covers these topics and new avenues of clinical importance related to this unique antigen/antibody system (alpha-gal epitope/anti-Gal) in improving the efficacy of viral vaccines and in immunotherapy against cancer.

Natural anti-galactose alpha1,3 galactose antibodies delay, but do not prevent the acceptance of extracellular matrix xenografts

Naturally occurring antibodies to the galactose alpha1,3 galactose (alpha gal) epitope expressed on xenogeneic grafts are a major barrier to organ transplantation in humans. Porcine small intestinal submucosa (SIS) expresses the alpha gal epitope and is currently being used as a bioscaffold for tissue remodeling. To examine in detail the potential role of the alpha gal epitope in immune recognition of this acellular, avascular biomaterial, we have used mice which have a genetic disruption in the alpha1,3 galactosyltransferase gene (alpha gal(-/-)mice) and thus express natural anti-alpha gal antibodies in a manner similar to humans. It was found that alpha gal(-/-)mice produced IgM anti-alpha gal antibodies in addition to IgG1 SIS-specific antibodies, which did not bind to the alpha gal epitope. Histological examination of implant sites demonstrated an early inflammatory response that consisted predominantly of neutrophils in both alpha gal(+/+) and alpha gal(-/-)mice. However, while alpha gal(+/+)mice completely remodeled SIS implants by day 25, alpha gal(-/-)mice still exhibited some visible SIS together with inflammatory cellular infiltrates at this time point. Nevertheless, by day 35, the implant site in alpha gal(-/-)mice appeared to be entirely remodeled although a few acute inflammatory cells were still present. Immunization of alpha gal(-/-)mice with sheep erythrocytes to enhance anti-alpha gal antibody levels led to a more robust early inflammatory response following implantation but did not change the ultimate fate of the graft. We conclude that, in contrast to xenotransplantation of whole organs, naturally-occurring anti-alpha gal antibodies do not influence the ability of xenogeneic extracellular matrices to serve as bioscaffolds for tissue remodeling.