Comparison of mechanisms of interaction between protein A from Staphylococcus aureus and human monoclonal IgG, IgA and IgM in relation to the classical FC gamma and the alternative F(ab')2 epsilon protein A interactions.

Four purified human monoclonal IgG, IgA and IgM proteins were tested for their inhibitory effect on the binding of protein-A-reactive 125I-IgE and 125I-Fc gamma, respectively, to protein-A-Sepharose. Only IgG myeloma proteins significantly inhibited the binding of 125I-Fc gamma to protein-A-Sepharose, whereas most, but not all, myeloma proteins, irrespective of their immunoglobulin class and with varying efficiency, inhibited the binding of protein-A-reactive 125I-IgE to protein-A-Sepharose. The inhibitory effect of IgG and IgA proteins on the binding of protein-A-reactive 125I-IgE was retained in the respective F(ab')2 fragments, whereas the inhibitory effect of IgG proteins on the binding of 125I-Fc gamma to protein-A-Sepharose was exclusively expressed in the Fc gamma fragment. In addition to the classical Fc gamma-protein A interaction, the results indicate the existence of a common and variably expressed protein A reactivity in at least four of five human immunoglobulins. The data suggest that an interaction with protein A cannot be used as a criterion for subclass differentiation of IgA and IgM.