We have examined the growth inhibitory effects of human lymphoblastoid interferon (IFN) on the human hepatocellular carcinoma (HCC) cell line PLC/PRF/5. In vitro, PLC/PRF/5 cells were sensitive to the antiproliferative effects of IFN, growth inhibition being noted at concentrations as low as 1.25 i.u. ml-1. Athymic mice with xenografted tumours derived from the PLC/PRF/5 cell line were treated daily with IFN or a saline control. An IFN dose of 2 × 105 i.u./day was found capable of significantly slowing tumour growth rate and prolonging mouse survival. Further studies to examine the mechanisms involved in growth inhibition in vivo demonstrated that IFN was capable of inducing the activity of the enzyme 2,5-oligoadenylic acid (2,5 A) synthetase, a potent inhibitor of protein synthesis, in tumour xenografts but not in mouse tissue, and that IFN significantly enhanced the membrane display of HLA class I glycoproteins on tumour cells, though histology did not reveal any increase in tumour infiltration by host lymphocytes. We conclude that IFN exerts potent growth inhibitory effects on the HCC cell line PLC/PRF/5 both in vitro and in vivo and its mode of action in this animal model system appears to be predominantly mediated by a direct antiproliferative effect on tumour cells.
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