SIALIC-ACID 9-O-ACETYLATION ON MURINE ERYTHROLEUKEMIA-CELLS AFFECTS COMPLEMENT ACTIVATION, BINDING TO I-TYPE LECTINS, AND TISSUE HOMING

O-Acetylation of the 9-hydroxyl group of sialic acids has been suggest ed to modify various recognition phenomena involving these molecules, but direct proof has been lacking in most situations. In the accompany ing paper (Shi, W.-X., Chammas, R., and Varki, A. (1996) J. Biol. Chem . 261, 31517-31525), we report that the extent of 9-O-acetylation of c ell surface sialic acids on murine erythroleukemia (MEL) cells can be modified by various manipulations, including differentiation, nocodazo le treatment, and 9-O-acetyl esterase treatment. We have used this sys tem to explore the putative roles of 9-O-acetylation in modulating alt ernative pathway complement activation, I-type lectin binding, and tis sue homing, MEL cells are shown to be sensitive to lysis in vitro by t he alternative pathway of human complement. Induced differentiation of the MEL cells causes resistance to lysis, and this correlates directl y with extent of decrease in 9-O-acetylation. A similar resistance to alternative pathway lysis can be obtained by selective enzymatic remov al of 9-O-acetyl groups from sialic acids. Conversely the increase in cell, surface 9-O-acetylation caused by nocodazole treatment correlate s with increased sensitivity to alternative pathway lysis. Thus, a 9-O -acetyl group added to the side chain of cell. surface sialic acids ma y abrogate its normal function in restricting alternative pathway acti vation. Indeed, the binding of human complement factor H, a negative r egulator of the alternative pathway, is shown to be blocked by O-acety lation of the sialic acids on MEL cells. MEL cells are also shown to h ave cell surface ligands for the I-type lectins sialoadhesin and CD22. Sialoadhesin (but not CD22) binding is selectively enhanced by differ entiation-induced loss of cell surface 9-O-acetylation and by direct e nzymatic removal of the ester groups, Thus, some sialoadhesin ligands are masked by 9-O-acetylation, presumably because the side chain is re quired for recognition, Since sialoadhesin is expressed on some macrop hages in vice, we reasoned that tissue homing of MEL cells might be af fected by O-acetylation. Indeed, enzymatic removal of cell surface 9-O -acetyl groups alters the tissue distribution of intravenously injecte d cells. In particular, de-O-acetylation caused significant increase i n homing to the liver and spleen. These data demonstrate that cell sur face 9-O-acetylation can affect a variety of biological recognition ph enomena and provide a system for further exploration of the specific m olecular mechanisms involved.