Cells from the monoclonal B cell lymphoma I.29 expressing surface IgM (mu +) are capable of differentiating in vitro to IgM secretion and of switching to IgA or IgE production in response to lipopolysaccharide (LPS) stimulation. To determine whether a single mu + B cell is capable of undertaking both differentiative pathways (isotype switch and plasma cell differentiation) I.29 mu + cells were cloned by limiting dilution and a panel of clones were analyzed by immunofluorescence, endogenous labeling and Northern blotting. While 100% of the clones could differentiate toward IgM secretion, only a proportion of them (greater than 70%) also switched to IgA and/or IgE production. Certain clones switched preferentially to a specific isotype. Taken together with the observation that C gamma genes were never the target of switching in our experiments, these data suggest that individual mu + clones from the I.29 lymphoma are "precommitted" as for their switching potentials. The subclones that showed a high frequency of switching to IgA transcribed the germ line C alpha gene(s), suggesting a role for chromatin structure in determining the isotype switch specificity. Switch variant clones expressing either IgA or IgE on the cell surface were isolated and found capable of further differentiating toward Ig secretion in response to LPS. On the contrary, we could not induce switch to IgA in IgE-producing cells. Unlike mu + and alpha + cells, all the switch variant clones expressing IgE tested by endogenous labeling constitutively secreted large amounts of IgE in the supernatants even in the absence of LPS stimulation.

Cells from the murine B lymphoma I.29, expressing IgM or IgA of identical idiotype, were found inducible by lipopolysaccharide to differentiate into plasma cells. Within 3 days, differentiating cells lost membrane-bound immunoglobulin (Ig) and accumulated large quantities of intracytoplasmic Ig. At day 6 of culture, IgA secretion increased 50-100-fold, as determined by enzyme-linked immunoassay. Proliferation increased for the first days of culture but decreased thereafter; by day 10 very few viable cells were present in lipopolysaccharide-stimulated cultures. Similar results were obtained by culturing I.29 cells in the presence of supernatants of certain B cell lines (e.g. BFO.3). The finding of a strict correlation between the inductive activity and presence of contaminating Mycoplasma fermentans suggested that factor(s) released by mycoplasma were responsible for the mitogenic activities. This was further indicated by the findings that: the supernatants of BFO.3 that were rendered free of mycoplasma were not inductive, and a nonactive cell line could be made active by infection with supernatants of BFO.3 cells containing viable microorganisms. Thus, supernatants of mycoplasma-infected cell lines may act as potent polyclonal activators on both normal and malignant B lymphocytes. The ability to induce membrane Ig on 70Z/3 cells indicates that mycoplasma-related mitogens are also active on pre-B cells. The possibility of mycoplasma contamination should thus be carefully excluded when presumptive factors of cloned cell lines are being evaluated.

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