Differentiation in the murine B cell lymphoma I.29: individual μ⁺ clones may be induced by lipopolysaccharide to both IgM secretion and isotype switching

Cells from the monoclonal B cell lymphoma I.29 expressing surface IgM (μ⁺) 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 μ⁺ B cell is capable of undertaking both differentiative pathways (isotype switch and plasma cell differentiation) I.29 μ⁺ cells were cloned by limiting dilution and a panel of clones were analyzed by immunofluorescence, endogeneous labeling and Northern blotting. While 100% of the clones could differentiate toward IgM secretion, only a proportion of them (> 70%) also switched to IgA and/or IgE production. Certain clones switched preferentially to a specific isotype. Taken together with the observation that Cγ genes were never the target of switching in our experiments, these data suggest that individual μ⁺ 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 Cα 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 μ⁺ and α⁺ 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.