Lysis of measles virus-infected cells by human serum is dependent on IgG antibody and an intact alternative complement pathway. The binding sites of IgG measles virus antibody, or its fragments, on the surface of measles virus-infected cells were determined by cell surface radioiodination and immunoprecipitation. It was found that IgG bound to either of the two measles virus surface glycoproteins was effective in inducing cell lysis by the alternative pathway. Thus, IgG bound only to the viral hemagglutinin (HA) or only to the fusion protein (F) could induced lysis in C4-depleted human serum; the same number of IgG molecules bound to either HA or F induced equivalent dose-related lysis. Divalent antibody bound preferentially to HA on the surface of intact cells; this enabled antibody to F to be obtained free of antibody to HA by adsorption. Quantitative studies of binding and lysis showed that lysis of measles virus-infected cells by whole or C4-depleted serum required binding of at least 10 times more Fab′ than F(ab′)2 or IgG molecules per cell. In these experiments, C4-depleted serum was quantitatively and kinetically equivalent to whole serum in mediating lysis of measles-infected cells. Hence, the binding of IgG to either of the two measles viral polypeptides (HA, F) expressed on the surface of infected cells induces lysis by the alternative pathway, and there is a clear requirement for divalency for effective induction of lysis.

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This is Publication No. 1824 from the Scripps Clinic and Research Foundation. This work was supported by United States Public Health Service Grants NS-12428 and AI-7007.

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