Regulatory Factors of Lymphocyte‐Lymphocyte Interaction: I. Con A‐Induced Mitogenic Factor Acts on the Late G1 Stage of T‐Cell Proliferation

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Abstract
DNA synthesis in murine lymphocytes was augmented by a soluble factor in the supernatant of serum-free cultures of syngeneic spleen cells activated with concanavalin A (Con A). This so-called mitogenic factor (MF), which is probably identical with interleukin II, partially purified by DEAE-cellulose and Sephadex G-75 chromatography, is a fairly homogeneous molecule of 17-25 X 10(3) daltons. By using partially purified MF, the role of MF in lymphocyte proliferation was investigated. Pretreatment of lymphocytes with Con A for the first 3 hr of culture, which does not in itself induce cell proliferation, markedly augmented the effect of MF. The presence of MF, however, is necessary only in a restricted stage(s) of lymphocyte proliferation. The addition and removal of MF at various times during culture showed that MF exerts its effect on a process which occurs 3-6 hr before the beginning of DNA synthesis. These results strongly suggest that MF regulates the proliferation of mitogen-stimulated lymphocytes by acting on a restricted stage(s) of the cell cycle.