Replacement of germ-line ε promoter by gene targeting alters control of immunoglobulin heavy chain class switching

Recent work has shown that the ability of cytokines to direct immunoglobulin heavy chain class-switch recombination to particular heavy chain constant (C) region (CH) genes correlates with the induction of specific germ-line CH transcripts. To test the role of germ-line transcripts in class switching, we have used homologous recombination to mutate the immunoglobulin heavy chain locus of the 18.81A20 murine pre-B-cell line. In the parent cell line, the combination of interleukin-4 (IL-4) and lipopolysaccharide (LPS) induces germ-line ε locus transcription prior to class switching to ε. The heavy chain locus of the mutated cell line contains the immunoglobulin heavy chain enhancer and variable region gene promoter in place of the LPS/IL-4-responsive germ-line ε promoter. The mutant cell line constitutively transcribes the ε locus in the absence of IL-4. Strikingly, the mutant cell line also switches to ε in the absence of IL-4. This result demonstrates that, at least in the 18.81A20 cell line, germ-line ε transcription plays a direct role in class switching to the ε locus. In addition, the ability to change the pattern of class switching by altering transcriptional activity indicates that transcription of germ-line CH is mechanistically important in regulation of class switching.