OBJECTIVETo investigate the tumorigenic mechanisms of human leukemia cell line HL60-n in nude mice.

METHODSDifferent clone strains of HL60-n cells were established by limited dilution and their biological features were compared with parental HL-60 cells.

RESULTSThe colony yields in soft agar, especially the large colony yields of the high tumorigenic clone strains HL60-n/A, HL60-n/B were significantly higher than that of the HL-60 cells (P < 0.01). There was no significant difference between the low tumorigenic clone strains HL60-n/E, HL60-n/F and the HL-60 cells. Ultrastructurally, the nucleus was highly abnormal, the euchromatic element of nuclear chromatin increased, the heterochromatin sparse, and the microfilaments in cytoplasm increased and disarranged in the high tumorigenic cells as compared with HL-60 cells. Cell cycle analysis by flow cytometer showed higher S phase fractions in the high tumorigenic cells. The killing activities of NK cells to the high tumorigenic clone strains were significant lower than to the contrast (P < 0.01). The histopathological features produced by the low tumorigenic leukemia cells showed that there were many inflammatory cells infiltrated, the majority of them were lymphocytes, and many tumor cells were killed especially in vessel abundant areas. By contrast, there were few inflammatory cells infiltrated in the tumors produced by the high tumorigenic cell strains.

CONCLUSIONThe mechanism of the high tumorigenic activity of the HL60-n cell line involved higher colony yields in soft agar, higher S phase fraction, decreased susceptibility to NK cell killing, and the inhibition of the host immunity.