Potential impact of the non‐human sialic acid N‐glycolylneuraminic acid on transplant rejection risk

A major obstacle to clinical applications of xenotransplantation is the expression of immunogenic xenoantigens that provide targets for immune recognition of xenografts, leading to activation of host immunity and consequent rejection or poor engraftment.Among the best known xenoantigens is the "αGal" epitope (Galα1-3Galβ1-(3)4GlcNAc-R, where R is an underlying glycoconjugate) characterized by Galili and colleagues.This epitope is widely expressed by most mammals other than old world primates and recognized by abundant circulating human anti-α-Gal antibodies (1).Such antibodies are universally induced after birth in humans via exposure to gut bacteria bearing similar epitopes (2), and the resulting difficulties in xenotransplantation (3) have even encouraged production of alpha1,3-galactosyltransferase gene-knockout pigs (4) as a potential solution.However, a second class of anti-carbohydrate xenoantibody recognize glycans carrying the non-human sialic acid N-glycolylneuraminic acid (Neu5Gc) (5-9).Unlike the case with αGal, the potential impact of Neu5Gc-glycans as xenoantigens is less well recognized.Here we address the similarities and differences between these two xenoantigens (see Table 1 for a summary and comparison), concluding that anti-Neu5Gc antibodies are of potential relevance not only to xenotransplantation, but also to allotransplantation and even to autotransplantation if methods are incorporated that require an ex-vivo culturing phase using animal materials, including stem-cell based therapies.Sialic acids (Sias) are 9-carbon backbone acidic sugars terminating the glycan chains of various glycoproteins and glycolipids at vertebrate cell surfaces (10).N-Acetylneuraminic acid (Neu5Ac) and its hydroxylated form Neu5Gc are the two major Sias in mammals, with the activated form CMP-Neu5Ac serving as the precursor for synthesis of CMP-Neu5Gc, catalyzed by the enzyme CMP-Neu5Ac hydroxylase (CMAH).This enzyme is specifically inactivated in humans, in contrast to other mammals studied to date, including old world primates and pigs (10).Despite inactive CMAH in humans and lack of an alternative pathway for Neu5Gc synthesis (11), Neu5Gc is found at low levels on human epithelia and endothelia and is especially enriched in tumors, apparently originating from dietary Neu5Gc-rich foods, e.g., red meat and milk products (Neu5Gc is low or undetected in fish and poultry) (12, 13).In this regard, Neu5Gc can be taken up by cultured human cells in vitro from Neu5Gc-containing media supplements like fetal calf serum (FCS), and, being compatible with intrinsic human biochemical pathways, it is metabolically incorporated, resulting in its cell surface expression (13).However, unlike the case with these biochemical pathways, the human immune system recognizes Neu5Gc as foreign, resulting in a humoral response involving a polyclonal highly diverse antibody profile in all humans (14, 15).This unique combination