Release of growth promotors from mononuclear human blood cells by LPS, lectins and lithium, as tested in the mouse CFU-C assay.

Medium (MCM, 20% human serum), conditioned for 24 h by mononuclear human blood cells, had no colony-forming ability when tested in the mouse CFU-C assay. However, when combined with L-CSF, which predominantly generates macrophage colonies, MCM increased the colony number and size. Even more significant was the increased cellularity with a striking shift towards granulocyte production. Some human sera induced GIF formation without additives, but mostly LPS was required. After heat inactivation, all sera became dependent on LPS to yield an active MCM. Lithium, PHA, Con A and PMW also stimulated GIF formation, which itself depended upon protein synthesis. Heat inactivation of LPS and serum did not reduce their ability, in combination, to yield an active MCM. However, when serum and LPS were mixed before heat treatment, the ability to induce GIF production was abolished, but could be restored by adding intact LPS. This may indicate that LPS exerts its effect by combining with a serum factor yielding a heat-sensitive complex. However, even in the absence of serum, LPS had a stimulatory effect when used in large concentrations, but still the MCM was less active than MCM with serum.