Sources and biology of regulatory factors active on mouse myeloid leukemic cells

The action of serum or cells in enforcing differentiation in mouse myelomonocytic leukemic cells was monitored in agar cultures of WEHI-3B leukemic cells. The repeated intravenous injection of 5 micrograms endotoxin initially increased serum differentiating activity but after the third injection responses to further injections decreased markedly. Congenitally athymic (nude) mice exhibited normal rises in serum differentiating activity when injected with endotoxin but C3H HeJ mice failed to respond to challenge with purified lipid A. Whole body irradiation up to 1,200 rads did not increase serum differentiating activity but did not suppress responses to challenge injection of endotoxin. Coculture of WEHI-3B cells with peritoneal cells from normal or irradiated BALB/c mice caused marked granulocytic differentiation in WEHI-3B colonies. This effect was not seen if leukemic cells were cultured with thymus, spleen, or bone marrow cells. The serum halflife of the factor in postendotoxin serum enforcing differentiation of WEHI-3B cells was shown to be 1.5-2.3 hr.