Commercially available IgG was fractionated into different subclasses a) by partial papain-digestion and exclusion chromatography, separating intact IgG2 and IgG4 from F(ab) and Fc of IgG1 and IgG3 and b) by passage through a staphylococcus protein A-coated Sepharose column to yield IgG3 separate from IgG1, IgG2, and IgG4. Measles antibody titers were analyzed by hemagglutination-inhibition (HI), hemolysis-inhibition (HLI), and by cytotoxicity assays. Diluted ordinary rabbit complement (C) was employed to exclude activation via the alternative pathway in studies of the classical pathway. C-4 deficient guinea pig C and rabbit C in ethyleneglycoltetraacetic acid (EGTA), and as a control, ethylenediaminetetraacetic acid (EDTA), were employed for studies of the alternative pathway. The discrepancy in that HI but not hemolysin antibodies activate C by the alternative pathway but both may activate C by the classical way was then further investigated.

Measles virus HI and HLI antibody activity per milligram protein was highest in fractions containing IgG1. The titers in the IgG3 and IgG2 ± 4 materials were low and might result from contamination by IgG1. This demonstrated a segregation of measles virus HI and HLI antibodies into IgG1 in normal individuals. Cytotoxicity by both pathways of complement activation was most pronounced in the presence of IgG1. Since both HI and hemolysin antibodies were mainly of the IgG1 subclass, their separate pathways of C activation could not depend on subclass differences. F(ab) fragments of human and rabbit IgG and human F(ab′) molecules, carrying measles virus HI activity, were as efficient in cytotoxicity tests via the alternative pathway as intact IgG with a corresponding HI titer. It is suggested that characteristics of the surface antigens might determine the pathway of C activation.

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