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The cytophilic activity of human myeloma proteins of different classes and subclasses for lymphocytes, monocytes, and neutrophils was investigated. Binding of both unaggregated immunoglobulins (Ig) and Ig aggregated with rabbit F(ab)2 anti-Fab fragment sera was determined. Lymphocytes bound unaggregated IgG1 and IgG3 proteins, but none of the proteins of the other classes. In contrast, after aggregation, IgG of all subclasses and IgE proteins bound to lymphocytes; aggregated proteins of the other classes did not bind. Monocytes bound unaggregated IgG1 and Ig3 better than Ig4 whereas the binding of proteins of other classes was insignificant. Neutrophils bound unaggregated IgG1 and IgG3 proteins and, in addition, IgA1, IgA2, secretory IgA, and IgG4 proteins. After aggregation, the neutrophils bound more Ig of all classes; however, the differences between the amounts bound remained similar to the amounts of unaggregated proteins. The native structure of the Ig molecule is necessary for the maintenance of complete activity, because Fc fragments bound less than intact Ig, and reduction and alkylation abolished cytophilia. The Fc receptors on all cell types tested showed no specificity for any of the respective cytophilic IgG subclasses; however, neutrophils appear to have separate receptors for IgG and IgA proteins.
Lawrence et al. (Sat,) studied this question.