Sequential switching from mu to epsilon via gamma 4 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 C epsilon germ line transcription. However, hydrocortisone increased the levels of IL-4-induced germ line C epsilon transcripts by twofold and delivered the signal required for transcription of mature C epsilon 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 S mu to S epsilon in eight fragments, one of which exhibited an 876-bp deletion in S mu. The ninth fragment contained a 50-bp insertion at the S mu/S epsilon junction, which was likely to be derived from S gamma 4. The sequence of the 10th fragment was consistent with either a 17-bp insertion at the S mu/S epsilon junction derived from S gamma 4 or with a complex 38-bp deletion within S epsilon. Mapping of the switch junction sites showed "hot spots" for recombination within S mu but not within S epsilon. These findings indicate that hydrocortisone induces S mu-S epsilon deletional switch recombination in IL-4-treated B cells, and support a model of sequential isotype switching from IgM to IgE via IgG4.