The murine IgG1/IgE class switch program (1)
In murine B cells immunoglobulin class switching to IgGl and IgE is controlled by the same cytokine, interleukin-4 (IL-4). Only a small subpopulation of activated B cells may be permitted to switch to IgE. Molecular footprints indicating a sequential switch recombination have been described before in hybridomas and LPS blasts expressing IgG2b, IgG3, IgE, or IgA, for chromosomal DNA.
By day 5, 95 % of the viable cells were B cell plasma cells, i. e. cytoplasmically stainable with anti-x or auntie-lambda immunoglobulin light chain antibodies. For both IgG1 and IgE the frequency reaches a plateau at about 200 units of IL-4, and is not much different at 750 units. For IgE, approximately fivefold higher concentrations of IL-4 are required compared to IgG1 to reach half maximal switch frequency.
Frequencies of IgG: and IgE+cells increase with rising concentrations of IL-4. Up to 35 % of the cells are IgG, while the frequency of IgE+ cells was always below 3%.
Clonal identity and the presence of two lymphocytederived IgH alleles were verified with the ‘TH” probe, since the sequence detected by this probe is rearranged individually upon VDJ recombination and usually not deleted by class switch recombination.
All but one of the eight IgG: clones had Su, recombined to Sg1 on both alleles, i.e. switch recombination had been targeted to S,1, a phenomenon. The majority of IgE+ hybridomas, 9 out of 12, had performed switch recombination of Su to Sg1 on the silent allele. Of the remaining two IgE+ hybridomas L.I.23 showed an Su to Sg3, and L.I.24 most probably an Suto Sa switch recombination on the inactive allele.
The “RH” and “5‘Se” probes detect an Eco RI fragment of 17 kb. ”Sg1” and “Se’ hybridize to a 13- or 12-kb Eco RI fragment in the IgE-expressing hybridomas L.I.6b and D16.3.
The inactive alleles of these cells first underwent a deletional Se to Sg1 recombination and then an inversional recombination between the recombined Se/Sg1 region and Se.
Cross-reactions were below 0.1%, except 3% cross-reactivity of the anti-IgG1 serum with IgG3. These cross-reactions could account for a slight suppression of IgG3-expressing cells from 6.4 % to 4.7 % in LPS and from 0.5%o to 0.3 % in LPS/IL-4 cultures. The appearance of IgG: cells was drastically reduced in LPS/IL-4 cultures by anti-IgG1, from 34.8 +/- 2% to 6.52 +/- 0.23 %. The frequency switching to IgE is reduced from 1.77 +/- 0.05% to 0.52 +/- 0.02 % in LPS/IL-4 cultures.
The extent of direct switching from IgM to IgE may differ according to the conditions of B cell activation. Preferential fusion of day 5 plasma cells may switch directly from IgM to IgE very early in the culture.