Further Characterization of the Alternative Protein‐A Interaction of Immunoglobulins: Demonstration of an Fc‐binding Fragment of Protein A Expressing the Alternative Reactivity

Intact IgG and fragments F(ab')2 gamma, Fab gamma and Fc gamma from a rabbit anti-protein-A serum (RapA) and corresponding preparations from normal rabbit IgG (NRG) were tested for their inhibitory effect on the binding of protein-A-reactive 125I-IgE and 125I-Fc gamma, respectively, to protein-A-Sepharose. Intact IgG, F(ab')2 gamma and Fab gamma of RapA inhibited the binding of protein-A-reactive 125I-IgE, whereas only intact IgG and Fc gamma fragments from both RapA and NRG inhibited the binding of 125I-Fc gamma to protein A-Sepharose. Further, the functional relationship between RapA and human polyclonal IgG was studied in a nephelometric test system. Intact IgG or fragments of IgG from human polyclonal IgG and rabbit antiprotein-A were found to affect the precipitation between human IgG and protein A in a similar way. Thus F(ab')2 gamma fragments and intact IgG enhanced the precipitation, whereas Fab gamma and Fc gamma fragments inhibited the precipitation. Protein A and an Fc-binding fragment of protein A (fragment B) were tested for their abilities to link different radiolabelled immunoglobulin preparations expressing the alternative and the classical protein-A reactivity to immobilized Fc fragments. All proteins expressing the alternative reactivity were efficiently bound both by fragment B and by protein A, indicating that fragment B, in addition to its classical Fc-binding activity, also expresses the alternative protein-A reactivity.