Staphylococcal protein A binding to the Fab fragments of mouse monoclonal antibodies.

Two mouse IgG1 monoclonal antibodies specific for the Lewis(a) human blood group antigen were purified on protein A-Sepharose by using buffers of decreasing pH for elution. Unlike other IgG1 antibodies that eluted at pH 7.0 to 6.0, these antibodies could only be eluted at pH 4.0 to 3.0. The Fab and F(ab')2 fragments of these antibodies also eluted at pH 4.0 to 3.0, although the Fc fragment of one eluted at pH 6.0. This interaction of protein A with Fab was not due to anti-protein A antibody activity, because the presence of Lewis(a) trisaccharide did not prevent the binding of Fab to protein A-Sepharose and because Fab that had bound to solid phase hapten could still be recognized by protein A. Thus, certain mouse IgG1 antibodies possess determinants in their Fab portion recognized by protein A, allowing for the purification of such Fab fragments on protein A-Sepharose.