Differences in human and porcine platelet oligosaccharides may influence phagocytosis by liver sinusoidal cells in vitro

Background: Acute thrombocytopenia was revealed as a limiting factor to porcine liver xenotransplantation from in vitro and in vivo studies using porcine liver in human and baboon transplant models. The asialoglycoprotein receptor 1 (ASGR1) on liver sinusoidal endothelial cells (LSEC) and macrophage antigen complex-1 (Mac-1) on Kupffer cells (KC) mediate platelet phagocytosis and have carbohydrate- binding sites that recognize galactose and N-acetyl glucosamine in the beta conformation. Analysis of these receptor carbohydrate-binding domains and surface carbohydrates on human and porcine platelets may shed light on the mechanism of xenotransplantation-induced thrombocytopenia. Methods: An amino acid sequence comparison of human and porcine ASGR1 lectin-binding domains was performed. Using fluorescent labeled-lectins, human platelets, domestic and a1,3 galactosyltransferase knockout/ human decay accelerating factor, porcine platelets were characterized by flow cytometry and lectin blot analyses. After desialylation, human and porcine platelets were examined by flow cytometry to determine whether sialic acid capping of galactose and N-acetyl glucosamine oligosaccharides in the beta conformation was a factor. Further, desialylated human platelets were studied on primary porcine liver sinusoidal cells with regard to binding and phagocytosis. Results: Human platelets have four times more exposed galactose b1-4 N-acetyl glucosamine (Galb) and N-acetyl glucosamine b1-4 N-acetyl glucosamine (bGlcNAc) than fresh porcine platelets. Galb and bGlcNAc moieties on porcine platelets were not masked by sialic acid. Removal of sialic acid from human platelets increased binding and phagocytosis by LSEC and KC. Conclusions: Differences between human and porcine ASGR1 and Mac-1, in combination with a significantly higher number of galactose and N-acetyl glucosamine-containing oligosaccharides on human platelets contribute, in part, to platelet loss seen in porcine liver xenotransplantation.