The Anti-recognition Function of Sialic Acids: Studies with Erythrocytes and Macrophages

Sialic acids have been found to mask antigens, receptors and other recognition sites on molecules and cells. This has been investigated best with hepatocyte and macrophage lectins which recognize 13-galactose and N-acetyl-13-galactosamine residues on molecules and cells exposed by sialidase treatment. The lectin on macrophages leads to the binding and phagocytosis of erythrocytes after the enzymic removal of a critical amount of sialic acids. New techniques (colorimetry, high-performance liquid chromatography, gas-liquid chromatographymass spectrometry) for the determination of the amount and nature of these sialic acids were developed. The interaction of partially desialylated rat erythrocytes with homologous peritoneal macrophages can be inhibited by 13-galactosyl glycosides in a degree depending on the structure of the oligosaccharides and glycoconjugates tested. Although homologous serum stimulates binding and phagocytosis, immunoglobulins and complement factors were found to play no role in this process. The main factor responsible for the masking effect of sialic acids on subterminal 13-galactosyl groups seems to be the carboxyl group of neuraminic acid, as reduction of this group to an alcohol largely abolishes this important function of sialic acids in the macrophage and hepatocyte systems studied. The anti-recognition function of sialic acids illuminates the pathophysiological role of serum sialidases in infectious and other diseases.