A cytokine, lymphocyte blastogenesis inhibitory factor (LBIF), arrests mitogen-stimulated T lymphocytes at early G1 phase with no influence on interleukin 2 production and interleukin 2 receptor light chain expression.

The function of a human cytokine, lymphocyte blastogenesis inhibitory factor (LBIF), was characterized. To this end, LBIF was purified from crude supernatant of U-937 cells, a human macrophage-like cell line, by using fast protein liquid chromatography (FPLC). We demonstrated here that (a) the LBIF preparation completely inhibited phytohemagglutinin (PHA)-stimulated T cell proliferation; (b) however, in PHA-stimulated T lymphocytes LBIF inhibited neither interleukin (IL)2 production nor the expression of IL2 receptor (IL2R) light chain (CD25) which play a critical role for T cell proliferation; (c) LBIF arrested PHA-stimulated T lymphocytes at the G1 phase of cell cycle and inhibited entry into S phase, thus inhibiting lymphocyte proliferation. Wright-Giemsa's staining of the cells showed that PHA/LBIF-stimulated cells were arrested in early G1. In agreement with this result, LBIF strongly inhibited PHA-induced RNA synthesis. Further, LBIF inhibited the induction of the transferrin receptor which is normally expressed at the late G1 phase of the cell cycle. The inhibitory activity of LBIF was reversible. Thus, this study elucidated that LBIF arrests PHA-stimulated T lymphocytes at a point between the stages of IL2 production or IL2R light chain expression (in early G1) and transferrin receptor expression (in late G1). Taken together, these results suggest that there might be a control system of T cell proliferation distinct from the previously reported mechanisms, such as the inhibition of IL2 production or the inhibition of IL2R light chain (CD25) expression, and that LBIF might be an important molecule in the regulation of normal lymphocyte proliferation.