The switch from IgM to IgG secretion in single mitogen-stimulated B-cell clones.

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Abstract
The frequency of mitogen-reactive B [bone marrow-derived] cells yielding an Ig[immunoglobulin]G plaque-forming cell (PFC) response was determined in vitro by limiting dilution analysis under culture conditions which allow every growth-induced B cell to grow and mature into a clone of Ig-secreting cells. The frequencies of lipopolysaccharide (LPS) and lipoprotein-reactive precursors for IgG-secreting cells in the spleen of 6-8 wk old C3H/Tif and C57BL/67 mice were between 1 in 30 and 1 in 40 B cells and, therefore, only 1/10 of the frequencies of mitogen-reactive precursors of clones secreting IgM. All IgG-secreting cells developed by switching in clones which previously contained IgM-secreting cells. This was shown in 2 experiments where the total number of mitogen-reactive precursor yielding IgM-secreting cell clones was limited such that 82 or 90% of all responding cultures originated from 1 precursor. Thus, of 480 cultures in the 1st and 720 cultures in the 2nd experiment, 86 and 98 cultures were positive, yielding IgM-secreting cells at day 5. When the same cultures were assayed at day 7 for IgG-secreting cells 9 and 10 cultures were positive. All 19 cultures with IgG-secreting cells previously contained IgM-secreting cells. The probability that IgG-secreting cells and IgM-secreting cells arose from independent precursors was calculated using Fisher''s exact test of independence. For the 2 experiments those probabilities are 3.4 .times. 10-7 and 4.0 .times. 10-9. Each cell in a mitogen-stimulated, growing B-cell clone divides and each dividing cell secretes Ig, therefore the large majority of the IgG-secreting cells in mitogen-stimulated B-cell clones probably develop by switch from IgM-secreting cells. IgG-secreting cells develop early or late during growth of a single IgM-secreting cell clone. The switch to IgG secretion, therefore, is not fixed in the time of clonal growth after mitogenic stimulation.

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