Studies of human anti-IgM anti-IgG cryoglobulins. I. Patterns of reactivity with autologous and isologous human IgG and its subunits.

Monoclonal human anti-IgG preparations purified from mixed IgM-IgG cryoglobulins were tested for their antigenic specificity by haemagglutination-inhibition assay. A panel of fourteen IgG preparations of the four gamma chain subclasses were prepared from myeloma sera and used as inhibitors of haemagglutination. Each of six IgM anti-globulins demonstrated different reactivity profiles with these IgG preparations. In addition, the fraction of the serum IgG which had bound to and cryoprecipitated with the IgM preparations, termed 'antigen-IgG', was purified and assayed for subclass content. The gamma chain subclasses found in the 'antigen-IgG' fractions showed that each IgM cryoprecipitated an IgG from serum which had different quantities of the subclasses present. These 'autologous' reactivity patterns were in instances different from the specificities expected from the results obtained with the myeloma proteins. When all antigen-IgG pools were tested with each IgM, some antiglobulins showed stronger reactivity with isologous than with their own, antigen-IgG pools. The IgM anti-IgG preparations were also compared in reactivity with IgG and its subunits in order to localize the antigenic determinant(s) with which these autoantibodies react. Heavy chains showed far greater reactivity than Fc fragment for 5/6 IgM preparations. Light chains, F(ab')2, pFc' and Fab were non-reactive. A relationship between the length of papain digestion and Fc reactivity was demonstrated. Based on the data, possible locations for the antigenic determinant(s) were considered.