Transgenic mice were generated to identify the first B cell maturation stage showing expression of an immunoglobulin transcriptional enhancer element (Eμ)-green fluorescent protein (GFP) transgene, and to check the ability of the Eμ element to behave as a locus control region. Flow cytometry experiments indicated that stably transfected 18–81 cells (a murine pre-B cell line) and A20 cells (a murine IgM+ B cell line) maintained a constant GFP expression for several months in culture. Contrasting with in vitro results, flow cytometry experiments did not highlight GFP+ B cells in spleen and bone marrow of Eμ-GFP transgenic mice and no GFP transcripts were detected by Northern blot and reverse transcriptase polymerase chain reaction analysis. In transgenic mice, the lack of GFP expression seemed related to transgene DNA methylation occurring within all organs. Our results show dramatic differences for expression of the Eμ-GFP transgene in vitro and in vivo. Despite that Eμ was reported to efficiently control the in vivo expression of other associated transgenes, it is not sufficient to sustain GFP expression in transgenic mice and to counteract developmental silencing programs that occur in the embryo.
