Comparison of Mechanisms of Interaction between Protein A from Staphylococcus aureus and Human Monoclonal IgG, IgA and IgM in Relation to the Classical Fcγ and the Alternative F(ab')2γ Protein A Interactions

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Abstract
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; most, but not all, myeloma proteins, irrespective of their Ig 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; 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, a common and variably expressed protein A reactivity exists in at least 4 of 5 human Ig. Apparently an interaction with protein A cannot be used as a criterion for subclass differentiation of IgA and IgM.