Expression of galactose-α(1,3)galactose in pig tissue samples
Introduction The use of pig organs for xenotransplantation has been widely studied as a potential option to overcome the current shortage of organs for transplantation. The binding of xenoreactive naturally occurring antibodies, largely directed to galactose-α(1,3)galactose (αGal), results in complement activation and hyperacute rejection of the organ. The use of animals with transgenes for complement regulatory proteins has largely overcome this problem. Transgene expression has been shown to vary in different cell types and in order for the transgene product to maximally protect the tissues from complement mediated damage, the expressed product should be distributed in a similar pattern to the αGal.
Materials and methods In this study, the expression of αGal in potentially transplantable pig tissues was assessed. Samples of heart, lung, liver, kidney, pancreas, small intestine and skin from 10 pigs were processed to paraffin blocks. αGal expression was evaluated using a polyclonal antibody raised in chickens.
Results Expression was consistent between animals, and predominantly localized to vessels of all sizes, particularly the endothelial layer in all tissues studied. In addition to vascular staining in the kidney there was expression in the proximal tubules, and in the lung staining was also seen in the alveolar and bronchial epithelium.
Conclusion The expression of any transgene to inhibit recipient complement activation needs to mirror the natural distribution of αGal in pig tissues. The study is limited as a single antibody may not demonstrate all αGal containing residues. However, the high endothelial expression is in keeping with the patterns of hyperacute and acute vascular rejection. The expression in both epithelial and endothelial layers of the alveolar wall may account for the poor results to date in lungs. The finding of tubular expression in kidneys may contribute to the different pattern of cellular infiltrate, which includes B cells, seen in renal xenografts.