Platelets Lacking Sialic Acid Clear Rapidly from the Circulation Due to Ingestion by Asialoglycoprotein Receptor-Expressing Liver Macrophages and Hepatocytes.

Although surface sialic acid is considered a key determinant for the survival of circulating blood cells and glycoproteins, its role in platelet survival is unclear. We investigated the importance of sialic acid for platelet clearance using mice deficient in the ST3GalIV sialyltransferase gene (ST3GalIV^−/− mice) and identified a novel clearance mechanism previously unrecognized for platelets. ST3GalIV catalyzes the addition of sialic acid onto exposed galactose residues of cell surface glycoproteins. ST3GalIV^−/− mice have increased platelet surface galactose exposure, a 70% reduction in platelet count, and prolonged bleeding times. We report that ST3GalIV^−/− platelets transfused into wild-type C57BL/6J mice exhibit markedly reduced recoveries and shortened survivals respectively compared to littermate wild-type platelets. Infusion of asialofetuin, an antagonist of the asialoglycoprotein-receptor (ASGPR) restored platelet recovery time and initial circulation of ST3GalIV^−/− platelets to normal values. Immunohistochemical studies of organ specimens harvested shortly after transfusion of biotin-labelled platelets demonstrated the predominant clearance of ST3GalIV^−/− platelets by the liver Kupffer cells and, unexpectedly, hepatocytes. Megakaryocytes cultured from ST3GalIV^−/− mice produced proplatelets normally compared to megakaryocytes generated from wild-type littermates, indicating that the thrombocytopenia in the ST3GalIV^−/− mice is not due to reduced platelet production. Comparison of ST3GalIV^+/+ and ST3GalIV^−/− platelet surface receptor expression as evidenced by flow cytometry and preliminary in vitro activation studies did not reveal any significant differences in the two genotypes. We conclude that the absence of terminal sialic acid residues on platelet surfaces exposes galactose residues to the lectin domain of ASGPR on both hepatocytes and liver Kupffer cells, resulting in platelet clearance from the circulation.