Induction of germ-line and mature C∈ transcripts in human B cells stimulated with rIL-4 and EBV

EBV and rIL-4 induce T cell-independent IgE production by normal human B cells. We demonstrate here that EBV and IL-4 induced the synthesis of IgE by surface IgE-negative B cell precursors isolated by cell sorting. This result suggests that the induction of IgE by EBV and IL-4 results not merely from the expansion of a precommitted surface IgE-positive B cell population but more likely from IL-4-directed switching to IgE. At the molecular level, IL-4 and EBV induced the appearance of 2.0- and of 1.8-kilobase (kb) RNA bands, both of which hybridized with an 0.88-kb HinfI fragment spanning part of the C∈1 exon and the entire C∈2 exon. The 1.8-kb band but not the 2.0-kb band also hybridized with a cloned genomic 0.7-kb SmaI fragment located ≈2 kb upstream of C∈. Thus, EBV and IL-4 induced germ-line (1.8-kb) as well as mature (2.0-kb) C∈ transcripts. IL-4 by itself induced germ-line C∈ transcripts but not mature C∈ transcripts in purified normal B cells. IL-4 failed to induce IgE synthesis in established EBV B cell lines and failed to induce 2.0-kb mature C∈ transcripts but induced 1.8-kb germ-line C∈ transcripts. These data show that IL-4 is sufficient for the induction of C∈ germ-line transcription. In contrast, the transcription of mature ∈ mRNA requires an additional activating signal, provided by infection with EBV. Established EBV transformation results in a dissociation between germ-line C∈ transcription and the ability to undergo IgE switching in response to IL-4.