Effects of macrophage-colony-stimulating factor on cyclophosphamide-injected mouse NK1.1+ cell activity

We injected cyclophosphamide into mice and examined their natural killer (NK) activity both in vitro and in vivo. Cyclophosphamide injection temporarily abrogated the lung clearance activity of Yac-1 lymphoma cells, which is considered to be an index of NK activity in vivo. However, administration of recombinant human macrophage-colony-stimulating-factor (rhM-CSF) to cyclophosphamide-injected mice restored the lung clearance activity. To clarify whether the administration of rhM-CSF activated NK cells, we purified NK1.1+ cells from mice treated with cyclophosphamide and/or rhM-CSF and examined their functions (cytotoxicity, proliferation, and interferon gamma production) in vitro. Cyclophosphamide injection decreased the number, but did not suppress the functions of NK1.1+ cells. The numbers of NK1.1+ cells in cyclophosphamide-injected mice restored by rhM-CSF administration. And the functions of NK1.1+ cells from both saline-injected and cyclophosphamide-injected mice were accelerated by rhM-CSF administration. These results suggested that the temporary abrogation of NK activity in vivo caused by cyclophosphamide injection was due to a decrease in the number and not to suppression of the functions of NK1.1+ cells. The injection of cyclophosphamide into mice increased the number of tumor (B16 melanoma) nodules formed in the lungs and liver. However, treatment with rhM-CSF recovered the anti-metastatic activity in the lungs of cyclophosphamide-injected mice. These results show that administration of rhM-CSF restores NK activity suppressed by cyclophosphamide injection in vivo.