The target antigens of naturally occurring human anti-beta-galactose IgG are cryptic on porcine aortic endothelial cells

The identification of the xeno-antigens/xeno-antibodies combinations involv ed in pig-to-human xenograft rejection is an essential step for understandi ng this process and for the development of procedures to prevent it. Althou gh it is widely accepted that the terminal disaccharide Gal alpha 1,3Gal-R is by far the major epitope recognized by human natural antibodies reactive with pig tissues, there is also evidence that other carbohydrate epitopes might be important in xenograft rejection. In an attempt to further improve our knowledge of the repertoire of human n atural antibodies with anti-pig specificity we sought to determine whether naturally occurring human anti-beta-galactose IgG could interact with porci ne aortic endothelial cells (PAEC). Histochemical analysis of porcine aorta sections revealed that the carbohyd rate structures recognized by the anti-beta-galalactose IgG are present on endothelial cells but in a cryptic form that can be unmasked by sialidase t reatment. These structures were also found to be cryptic in cultured PAEC. In addition we demonstrated that PAEC may adsorb fetal calf serum (FCS) gly coproteins when cultured in FCS-supplemented medium, a process susceptible to generating artifactual observations in carbohydrate antigens analysis. In conclusion, despite their abundance, human anti-beta-galactose IgG do no t represent a primary concern in pig-to-human xenotransplantation as the ca rbohydrate structures to which they bind are normally masked by sialic acid residues on porcine endothelial cells. However, whether these cryptic epit opes might be exposed on endothelial cells from genetically engineered anim als should be further investigated because, if so, additional approaches wi ll be needed to suppress their interaction with human anti-beta-galactose I gG.