Carbohydrate antigens in pig with special relevance to human xenotransplantation. Aspects on structural characterisation and organ distribution

Abstract

Transplantation between humans is today an accepted treatment for several diseases. The lack of human donors is however the major obstacle against widening the indications for organ transplantation. Transplantation of tissue between different species e.g. xenotransplantation, is considered as one solution to this problem. Different animal species have been considered, but the pig is today considered as the most suitable donor. The primary barrier to pass when transplanting between species is the hyperacute rejection which appears when the organ is connected to the recipient s blood stream. This is caused by preformed natural antibodies in the recipient, which react with carbohydrate antigens exposed on the endothelial cells of blood vessels in the transplanted organ. A galactose linked in an a1,3 linkage to another galactose (Gala1,3Gal) as a terminal carbohydrate sequence is the major target antigen epitope in the pig organ which is recognised by human preformed antibodies leading to rejection of the organ.We have studied and characterised the expression of carbohydrate structures in different porcine organs.. The structural elucidation of the cell surface carbohydrates was made with antibodies and with different mass spectrometric and/or nuclear magnetic resonance spectroscopy methods. The development of improved, more sensitive, methods for carbohydrate analysis have made it possible to analyse carbohydrates in small amounts of tissue. By using the GC/MS technique we were allowed to look for differences in carbohydrate expression in small tissue specimens from pig small intestine, heart, spleen, liver, salivary gland, kidney and lung. The advantages of the technique is the small sample amount needed, the sensitivity and the speed of analysis, and the screening pattern obtained, showing both qualitative and quantitative differences in the analysed mixtures of oligosaccharides. By using the LC on-flow NMR technique, we got the possibility to separate, and at the same time analyse single carbohydrates, e.g. glycolipids, in a mixture from pig lung. This is the first time this method has been used for native glycolipids.The enzymes involved in carbohydrate chain biosynthesis in pig small intestine were also studied. The biosynthetic products from experimental studies with in vitro prepared enzymes were compared to the carbohydrate expression in vivo , produced by the same enzymes. The enzymes were shown to accept a variety of precursor carbohydrate chains in the in vitro situation compared to the in vivo situation where one precursor chain type was dominant.There is still a long way to go before the mysteries of xenotransplantation are solved. By characterising the carbohydrates on the cell surfaces on organs of interest, we have taken a small step on the road towards a complete understanding of the mechanisms of xenotransplant rejection.