Sequential switching from μ to ε via γ4 in human B cells stimulated with IL-4 and hydrocortisone

The molecular events leading to IgE synthesis in human B cells stimulated with IL-4 and hydrocortisone were analyzed. IL-4, but not hydrocortisone, induced Cε germ line transcription. However, hydrocortisone increased the levels of IL-4-induced germ line Cε transcripts by twofold and delivered the signal required for transcription of mature Cε mRNA. Nested primer polymerase chain reaction of high m.w. DNA revealed deletional switch recombination occurring in B cells sorted for lack of expression of surface IgE and stimulated with both IL-4 and hydrocortisone, but not in B cells stimulated with IL-4 alone or hydrocortisone alone. DNA sequence analysis of 10 switch fragments revealed direct joining of Sμ to Sε in eight fragments, one of which exhibited an 876-bp deletion in Sμ. The ninth fragment contained a 50-bp insertion at the Sμ/Sε junction, which was likely to be derived from Sγ4. The sequence of the 10th fragment was consistent with either a 17-bp insertion at the Sμ/Sε junction derived from Sγ4 or with a complex 38-bp deletion within Se. Mapping of the switch junction sites showed "hot spots" for recombination within Sμ but not within Sε. These findings indicate that hydrocortisone induces Sμ-Sε deletional switch recombination in IL-4-treated B cells, and support a model of sequential isotype switching from IgM to IgE via IgG4.