Induction of an endogenous tumor necrosis factor in mice by murine recombinant interferon-γ combined with a lipid A subunit analog (GLA-60) of low toxicity

We have investigated the endogenous production of a serum cytotoxic factor when recombinant interferon-γ (rIFN-γ) is combined with synthetic lipid A subunit analogs of low toxicity (GLA compounds). The cytotoxic activity of the serum was measured by the crystal violet staining method with L929 cells as a target. Intravenous administration of rIFN-γ followed by intravenous administration of lipopolysaccharide induced the endogenous production of a cytotoxic factor in the serum. The priming effect of rIFN-γ appeared immediately and persisted for approximately 20 h after the injection. Administration of lipopolysaccharide as a trigger enhanced the production of the cytotoxic factor in the serum maximally 2 h after the injection. The cytotoxic activity in the serum was completely inhibited by anti-(mouse tumor necrosis factor) (TNF) antibody. A synthetic lipid A subunit analog (GLA-60), which is much less toxic in its endotoxin activities than lipopolysaccharide or synthetic lipid A (compound 506), induced the endogenous production of serum TNF in rIFN-γ-primed mice. GLA-60 entrapped within liposomes induced the production of serum TNF in rIFN-γ-primed mice more effectively than GLA-60 solubilized in phosphate-buffered saline. Intravenous or intranasal administrations of rIFN-γ followed by intranasal administration of GLA-60 produced TNF in the lung washing fluid but not in the serum, indicating that TNF production can be induced locally rather than systemically by the alteration of the administration route of the primer and trigger. These results indicate that GLA-60, a lipid A subunit analog of low toxicity, is a beneficial triggering agent in the production of endogenous TNF, as well as having other immunopharmacological properties, and may provide a basis for cancer (metastases) treatment as a result of its ability to induce endogenous TNF.