Immunoglobulin isotype expression II. Frequency analysis in mitogen‐reactive B cells

The frequency of lipopolysaccharide (LPS)‐reactive B cells developing into clones that secrete the various immunoglobulin (Ig) classes has been determined in vitro, in cells from BALB/c mice, under culture conditions which detect all growth‐inducible cells. Secretion of the different Ig classes was assessed in the protein A plaque assay for Ig‐secreting, plaque‐forming cells by using developing antisera specific for either IgM, IgG1, IgG2a, IgG2b, IgG3 or IgA. In all lymphoid organs tested (spleen, bone marrow, mesenteric lymph nodes and thoracic duct), a considerable proportion of all B cells (5–20%) was induced by LPS to yield a clone of IgM‐secreting cells. Frequency determinations of LPS‐reactive cells giving rise to descendants secreting other Ig isotypes revealed that, on an average, and irrespective of the origin of the cells, 7% of all IgM‐secreting clones switched to the synthesis of IgG1, 39% to IgG2, 41% to IgG3 and 1% to IgA. Roughly the same frequencies of B cells switching CH gene expression were found among spleen cells of athymic nude mice. No correlation was found between the clonal frequencies of CH gene expression in polyclonally activated B cells and the in vivo “background” Ig‐secreting cells suggesting that the CH gene expression in B cells is influenced by the quality of stimulation and other regulating influences.