A cytostatic protein isolated from the conditioned medium of mouse monocytic leukemia WEHI-3 cell cultures.

A cytostatic factor (CF) with a molecular mass of 50 kDa was purified to more than 16,000-fold from the conditioned medium of LPS-treated mouse myelomonocytic leukemia (WEHI-3) cell cultures. The activity of CF was completely destroyed by heating at 70 degrees C for 10 min, 50 degrees C for 30 min, or by the treatment in pH 3 buffer for 2 h. CF showed a strong growth inhibitory effect on CHO cells, as well as several other unrelated cell lines, e.g., K562 and L1210, but not on L929 cells. It also inhibited Con A-induced mitogenesis in mouse and rat spleen cells. The growth inhibitory effect of CF, however, was highly reversible; CHO cells were able to regain the normal growth after removal of the factor from the culture medium. Our results suggest that CF is a protein secreted by WEHI-3 cells, which is distinct from other known macrophage- or tumor-derived cytotoxic proteins.