The effects of purified recombinant murine interleukin-3 and/or purified natural murine CSF-1 in vivo on the proliferation of murine high- and low-proliferative potential colony-forming cells: demonstration of in vivo synergism.

Purified natural murine L cell (macrophage) colony-stimulating factor (nCSF-1) and purified recombinant murine interleukin-3 (rIL-3) were administered to normal or lactoferrin-pretreated mice 20 to 24 hours before sacrifice. rIL-3 and nCSF-1 administered separately increased the percentage of macrophage high-proliferative potential colony-forming cells (HPP-CFC) and low-proliferative potential colony-forming cells (LPP-CFC) in active cell cycle. Endotoxin was not detected in the samples of nCSF-1 or rIL-3 with the Limulus lysate test, and the in vitro and in vivo hematopoietic stimulatory effects of both molecules were abolished or markedly reduced by 30 minutes' treatment at 100 degrees C, which demonstrates that the effects noted in vivo were not due to endotoxin. Combinations of low concentrations of rIL-3 and nCSF-1, which by themselves were inactive, increased the percentage of HPP-CFC and LPP-CFC in active cell cycle in a synergistic fashion. No significant change in the number of HPP-CFC or LPP-CFC per femur or femoral nucleated cellularity was observed. Thus, rIL-3 and nCSF-1 can synergize to effect the proliferation of the same cell populations in vivo.