Identification of porcine endothelial cell membrane antigens recognized by human xenoreactive natural antibodies.

Rejection of an organ transplanted into a phylogenetically disparate recipient may be initiated by the binding of xenoreactive natural Abs of the recipient to Ags expressed on donor endothelium leading to the activation of the C system. Having previously shown that human xenoreactive Abs bind predominantly to N-linked oligosaccharides on porcine endothelial cell glycoproteins, we sought to identify those glycoproteins to gain insight into the pathogenesis of the rejection reaction and to develop improved methods for depleting those Abs. Based on amino-terminal and internal sequencing of the glycoproteins, reactivity with monospecific rabbit Abs, and functional properties, the porcine endothelial cell targets of human natural Abs were shown to be the von Willebrand factor, integrin alpha 1, alpha v, alpha 3/alpha 5, beta 1, and beta 3 chains and a 95-kDa glycoprotein homologous with chick DM-GRASP. Human and baboon Abs directed against those glycoproteins were shown to be absorbed during perfusion of porcine organs and the binding of human IgM to the isolated glycoproteins to initiate the activation of C leading to formation of iC3b neoantigen. The determinants recognized by human xenoreactive Abs had immunodominant alpha-galactosyl residues based on affinity chromatography, reactivity with anti-Gal alpha(1-3)Gal Abs and sensitivity to alpha-galactosidase. The findings show that members of the integrin family and the von Willebrand factor are major targets of xenoreactive Abs and suggest potential mechanisms by which the binding of Abs to endothelial cells might perturb the physiology of those cells and thus cause aberrant functioning of a tissue or organ.