In vivo Tumor Growth Enhancement by Granulocyte Colony‐stimulating Factor

The intraperitoneal administration of human recombinant granulocyte colony‐stimulating factor (G‐CSF) enhanced the growth of intradermally inoculated tumor in mice; in a Meth A fibrosarcoma model, G‐CSF administration significantly shortened the latency before tumor appearance, accelerated the increase of tumor size, shortened the survival time of tumor‐bearing mice and increased the incidence of lethal tumor growth. A similar growth‐enhancing effect of G‐CSF was observed in models employing Meth 1 fibrosarcoma, colon carcinoma 26, and L1210 leukemia, although not all the effects were statistically significant. In vitro study showed that G‐CSF did not enhance Meth A growth in suspension culture or in soft agar. These data suggest that G‐CSF enhances the Meth A growth not directly but through the mediation of host factors. The accumulation of neutrophils was histologically observed in the tumor nodule, the blood, and the spleen in mice given G‐CSF repeatedly. The spleen cells and the peripheral blood leukocytes of G‐CSF‐injected mice enhanced Meth A growth in vitro as compared with those of mice injected with physiological saline. These results suggest the possibility that the in vivo growth of tumor cells was enhanced by G‐CSF‐induced overproduction of cells including neutrophils.