Influence of the Alternative Protein A Interaction on the Precipitation between Human Monoclonal Immunoglobulins and Protein A from Staphylococcus aureus

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Abstract
A quantitative nephelometric test system was used to evaluate the prerequisites for occurrence of precipitation between human immunoglobulins and protein A from Staphylococcus aureus. Purified human monoclonal IgG, IgA, and IgM with varying expressions of the alternative Fab-related protein A reactivity and F(ab′)2γ, Εcγ, and F(ab′)2α fragments from the myeloma proteins were tested for their precipitation reaction with protein A and for their ability to induce coprecipitation with protein A and to inhibit the precipitation between human polyclonal IgG and protein A. The results indicate that both the classical Fcγ and the alternative F(ab’)2ε equivalent protein A interactions are needed to obtain precipitation between protein A and IgG. Fcγ fragments from three IgG myeloma proteins inhibited the precipitation between human polyclonal IgG and protein A in the same way, indicating that the Fcγ fragment is monovalent in its reaction with protein A. In contrast, polyclonal F(ab′)2α fragments precipitated protein A in the presence of nonprecipitating rabbit IgG, suggesting that the alternative protein A reactivity is bivalently expressed in human immunoglobulins.