Circulating monoclonal IgM proteins in B cell chronic lymphocytic leukemia: their identification, characterization and relationship to membrane IgM

Accumulated evidence indicates that there is a circulating monoclonal Ig protein related to the leukemic cell-associated Ig in the majority of patients with B cell chronic lymphocytic leukemia (CLL) despite the failure to demonstrate such a protein by conventional serum electrophoresis. Methodology has been developed to reveal these hidden monoclonal bands and to show that they are related to the leukemia-associated membrane Ig (mIg). Of nine CLL cases with stainable mIgM and without discernable plasma Ig bands, marked hypogammaglobulinemia was evident in six. In the other three, a significant amount of protein was present in the gamma region. IgM was isolated from the plasma of these patients by affinity chromatography with Sepharose-4B, conjugated with affinity purified anti-human IgM antibodies. One to 3 mg were isolated from 20 to 40 ml of plasma. Agarose electrophoresis revealed a monoclonal Ig band in the isolated IgM in all cases. Eight of these IgM proteins were analyzed by high-pressure liquid chromatography. Five were found to be pentameric IgM. In the remaining three, various amounts of monomeric IgM were detected. Attempts to make anti-idiotypic antibodies to the isolated proteins have been successful. Thus far, a rabbit anti-idiotypic antiserum was obtained in one case and two mouse monoclonal anti-idiotypic antibodies in two additional cases. Immunofluorescence analysis revealed that plasma IgM and mIgM shared similar idiotypic determinants. One other monoclonal antibody was shown to be specific for a V region marker of a minor Ig population. These findings indicate that B leukemic lymphocytes do secrete a small amount of IgM and lend further support to the thesis that the maturation defect in CLL is incomplete. It is also feasible to isolate the secreted IgM and to produce anti-idiotypic antibodies to them. In view of the potential therapeutic effect of anti-idiotypic antibodies, this may offer an alternative and efficient approach to generate a large panel of anti-idiotypic antibodies for clinical trials. The possibility also exists that this approach is applicable to other B cell proliferative disorders such as the non-Hodgkin B cell lymphomas.