Influence of the alternative protein A interaction on the precipitation between human monoclonal immunoglobulins and protein A from Staphylococcus aureus

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')2 gamma, Fc gamma, and F(ab')2 alpha 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 Fc gamma and the alternative F(ab')2 epsilon equivalent protein A interactions are needed to obtain precipitation between protein A and IgG. Fc gamma fragments from three IgG myeloma proteins inhibited the precipitation between human polyclonal IgG and protein A in the same way, indicating that the Fc gamma fragment is monovalent in its reaction with protein A. In contrast, polyclonal F(ab')2 alpha fragments precipitated protein A in the presence of nonprecipitating rabbit IgG, suggesting that the alternative protein A reactivity is bivalently expressed in human immunoglobulins.