Mitogenic stimulation of malignant B cells Waldenstrøm's macroglobulinaemia: secretion of monoclonal IgM by in vitro-induced plasmablasts.

The monoclonal B cells present in the blood of three patients with Waldenstrøm's macroglobulinaemia were analysed according to phenotype and function. As a marker for the monoclonal nature of the detected immunoglobulin (Ig) molecules, the kappa/lambda-ratio (two patients) or the presence of idiotypic (Id) determinants (one patient) were used. With double immunofluorescence it was shown that the neoplastic blood B cells were heterogeneous in respect to maturation stage. In addition to B cells carrying membrane bound IgM and IgD (mIgM+/IgD+), both mIgM+/IgD- and cytoplasmic IgM+ blastoid cells could be detected in the blood of the patients. This heterogeneity was also reflected in the responses of the monoclonal B cells upon stimulation with mitogens and T cell factors. B blastoid cells and IgM secretion could be induced after in vitro culture in the presence of Staphylococcus aureus, Pokeweed mitogen or a combination of both mitogens. It was shown that also T cell factors were effective in inducing monoclonal B cell differentiation.