α1,3‐Galactosyltransferase knockout pigs are available for xenotransplantation: Are glycosyltransferases still relevant?

Abstract

SummaryIn the early 1990s, the Gal α (1,3)Gal carbohydrate linkage was found to be the major xenoepitope causing hyperacute rejection. This carbohydrate, the antibodies that bind to it, and the enzyme that produces it ( α 1,3-galactosyltransferase) were the foci of research by many groups. Nearly a decade later, α 1,3-galactosyltransferase knockout pigs were finally produced; hyperacute rejection could be avoided in these pigs. Having achieved this goal, enthusiasm declined for the study of glycosyltransferases and their carbohydrate products. To examine whether this decline was premature, we evaluate whether gene deletion has indeed solved the initial rejection problem or, in fact, created new problems. This review addresses this by examining the impact of the gene deletion on cell surface carbohydrate. Surprisingly, Gal α (1,3)Gal is still present in α 1,3-galactosyltransferase knockout animals: it is possibly synthesized on lipid by iGb3 synthase. Furthermore, removal of α Gal resulted in the exposure of the N-acetyllactosamine epitope. This exposed epitope can bind natural antibodies and perhaps should be capped by transgenic expression of another transferase. We believe the continued study of glycosyltransferases is essential to examine the new issues raised by the deletion of α 1,3-galactosyltransferase.

Implications for invariant natural killer T cell ligands due to the restricted presence of isoglobotrihexosylceramide in mammals

“…However, it is not clear whether iGb3S is silenced in Old World primates and humans. Monoclonal antibodies failed to bind to human tissue samples, and mRNA was not detected by Northern blot or RT-PCR from a range of human tissues (16,41). Humans, therefore, are apparently unable to transcribe a functional iGb3S gene, which would be consistent with our inability to detect iGb3 in any of the human tissue samples analyzed in this study.…”

“…However, it is not clear whether iGb3 is recognized by the natural antiGal␣1-3 antibody repertoire present in humans. To date, this has been studied only by using a limited number of monoclonal antibodies (12,16,19,39).…”

“…Furthermore, the iGb3 synthase (iGb3S) gene in the human genome appears to be a pseudogene (15,16). It therefore remains to be established whether iGb3 is a physiologically relevant selecting ligand in vivo or a molecule serendipitously capable of stimulating iNKT cells in vitro.…”

Role for CD47-SIRPα signaling in xenograft rejection by macrophages

“…Galectin-3 binds to porcine cells much more strongly than to human cells. GalT-deficient pigs may not eliminate macrophagemediated rejection because removal of the GalT enzyme could leave many of the N-acetyllactosamine structures uncapped, as seen in GalT-deficient mice (28). It is not yet clear whether inhibitory signals such as CD47-SIRP␣ will override all activating signals delivered to macrophages by xenoantigens.…”

Human Monocytes Recognize Porcine Endothelium via the Interaction of Galectin 3 and α-GAL

“…This fact could also contribute to the differential binding of Galectin-3 to pig vs human cells even in ␣(1,3)GT knockout animals. As well, ␣(1,3)GT knockout pigs may not eliminate DXR because removal of the GT enzyme could leave many of the LacNAc structures uncapped, as seen in ␣(1,3)GT knockout mice (36). In addition to eliminating the ␣ (1,3)GT enzyme, it may be necessary to substitute other glycosyltransferase genes that could cap the LacNAc and decrease galectin-3 binding.…”