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 stimula ted with IL-4 and hydrocortisone were analyzed. IL-4, but not hydrocor tisone, induced Cepsilon germ line transcription. However, hydrocortis one increased the levels of IL-4-induced germ line Cepsilon transcript s by twofold and delivered the signal required for transcription of ma ture Cepsilon mRNA. Nested primer polymerase chain reaction of high m. w. DNA revealed deletional switch recombination occurring in B cells s orted for lack of expression of surface IgE and stimulated with both I L-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 Smu to Sepsilon in eight fragments, one of which exhibited an 876-bp deletion in Smu. The ninth fragment containe d a 50-bp insertion at the Smu/Sepsilon junction, which was likely to be derived from Sgamma4. The sequence of the 10th fragment was consist ent with either a 17-bp insertion at the Smu/Sepsilon junction derived from Sgamma4 or with a complex 38-bp deletion within Sepsilon. Mappin g of the switch junction sites showed ''hot spots'' for recombination within Smu but not within Sepsilon. These findings indicate that hydro cortisone induces Smu-Sepsilon deletional switch recombination in IL-4 -treated B cells, and support a model of sequential isotype switching from IgM to IgE via IgG4.