Transferrin receptor synthesis is an early event in B cell activation

Purified, small resting mouse B cells appear to express low levels of transferrin receptor (TfR) as detected by flow cytometry. Moreover, when such cells are stimulated with LPS or F(ab')2 anti-mu, they show increased expression of TfR as early as 4 h after activation when the cells are at the boundary of G0 and G1 phase by cell cycle analysis. Cells treated with anti-mu increased TfR expression gradually, reaching a plateau after 46 h, whereas cells with LPS reached their plateau by 12 h. The kinetics of induction of increased TfR expression was similar for both small and large B cells. Inhibition of protein synthesis with anisomycin or inhibition of transcription with actinomycin-D blocked the increased expression of TfR at 4 h. Northern blot analysis showed a marked increase of TfR mRNA at 2 and 4 h with either anti-mu or LPS which was then followed by an apparent decline at 16 h. These findings together with other recent studies justify reevaluation of the generally accepted models placing TfR induction late in the G1 phase of the lymphocyte activation sequence. They support the concept that in the B lymphocyte the TfR gene is selectively transcribed earlier than previously thought, before the general increase in RNA synthesis.