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Resting T cells proliferate in response to B cell stimulatory factor 1 (BSF-1; interleukin 4) plus phorbol myristate acetate (PMA). This response is obtained with highly purified T cells and is density independent, suggesting that accessory cells are not required. Both L3T4+ and Lyt-2+ T cells respond to BSF-1 plus PMA. Although BSF-1 alone does not cause T cell proliferation, it maintains the viability of small, dense T cells, indicating that it acts on resting T cells. Furthermore, BSF-1 is required early in the proliferative response of resting T cells to BSF-1 plus PMA, further supporting the concept that it acts on G0 or early G1 cells. However, BSF-1 is also needed late in the first round of division of T cells stimulated with BSF-1 plus PMA. Removing BSF-1 at 24 h of stimulation prevents entry into S phase. These results indicate that BSF-1 is involved in both the induction of competence and in the progression phases of T cell division.