α-Galactosyl epitopes on glycoproteins of porcine renal extracellular matrix

Background. The pig is the donor animal of choice for human xenotransplantation. In the most relevant pig-to-baboon model, pig organs transplanted into baboons are hyperacutely rejected by natural xenoantibodies, which mainly bind to alpha-galactosyl (alpha Gal) epitopes expressed at the surface of endothelial cells. Recent advances in controlling hyperacute rejection have led to improved survival of these xenografts, and it is now important to identify alpha Gal binding sites in other cells and tissues that may be subject to immunologic attack. To this end, we have studied whether alpha Gal antibodies bind to glycated proteins of the extracellular matrix in the kidney and other organs most likely to be used for human xenotransplantation. Methods. High-titer anti-alpha Gal antibodies, similar to human natural xenoantibodies, were prepared in baboons, and their reactivity with components of pig extracellular matrix was tested by serology and immunohistology. Results. The antibodies recognized epitopes of immobilized murine, bovine or porcine thyroglobulin, laminin, heparan sulfate proteoglycans, and fibronectin. In sections of pig tissue, the antibodies bound to endothelial and certain epithelial cells, as shown in previous studies, and also to mesenchymal cells, basement membranes, and extracellular matrices, in which they colocalized with matrix glycoproteins, especially laminin and heparan sulfate proteoglycans. Conclusions. These results suggest that when pig xenografts can be made to survive for prolonged periods, the reactivity of alpha Gal antibody with matrix molecules can induce basement membrane and matrix lesions similar to those induced in laboratory animals by antilaminin and antiheparan sulfate proteoglycans antibodies.