The Requirement for Membrane Sialic Acid in the Stimulation of Phagocytosis by the Natural Tetrapeptide, Tuftsin

The presence of sialic acid on the polymorphonuclear (PMN) phagocyte is necessary for maximal stimulation of its phagocytic activity by the tetrapeptide, tuftsin, Thr-Lys-Pro-Arg. Treatment of polymorphonuclear cells with bacterial neuraminidases, purified by affinity chromatography, completely abolishes stimulation of phagocytic activity by free tuftsin or by tuftsin bound to the carrier leukokinin molecule. These enzymes cleave sialic acid at both 2-3' and 2-6' glycosidic bonds. By contrast, treatment of cells with influenza virus neuraminidase, which cleaves predominantly the 2-3' linkage, reduces the tuftsin effect by approximately 50%. Enzyme-treated cells remain viable and capable of phagocytosis. Cells treated with any of the neuraminidases and cells of the untreated controls show comparable levels of phagocytosis in the absence of tuftsin. While membrane sialic acid is necessary for stimulation of phagocytosis, it may function only as a binding site for the three positively charged residues of the tetrapeptide. Neuraminidases from Vibrio cholerae or Clostridium perfringens, in excess amounts, released in 2 hours a maximum of about 135 million molecules of sialic acid per polymorphonuclear cell. Under similar conditions, treatment with influenza virus released approximately 30 million molecules per cell.