IFN-GAMMA PRIMING OF MONOCYTES ENHANCES LPS-INDUCED TNF PRODUCTION BY AUGMENTING BOTH TRANSCRIPTION AND MESSENGER-RNA STABILITY

The induction of cytokine expression in monocytes/macrophages by bacterial endotoxin or lipopolysaccharide is a critical, highly regulated host defence response. The augmentation of LPS responses by interferon gamma (IFN-gamma), referred to as priming, is weil established. However, the mechanism(s) by which priming occurs is poorly defined. Using tumour necrosis factor (TNF) induction as a model, experiments were designed to analyse in detail the printing effect on the LPS response in human monocytes. Priming by IFN-gamma was primarily manifested at the level of TNF mRNA accumulation. IFN-gamma pre-treatment affected the magnitude rather than the sensitivity of the LPS response. Priming occurred after several hours of treatment, and the primed state was induced by either IFN-gamma or GM-CSF, but not M-CSF. Primed monocytes transcribed TNF mRNA at a higher rate than freshly isolated monocytes upon activation with LPS. The increased transcriptional rate correlated with a marked increase in nuclear factor-kappa B activity in these cells as determined by electrophoretic mobility shift assay using a consensus NF-kappa B oligonucleotide. An additional significant finding was that TNF mRNA induced in primed cells was much more stable than in unprimed cells (T1/2 increased 6-8-fold). Consistent with the increased mRNA stability, the duration of mRNA accumulation was longer following LPS stimulation in primed monocytes, in addition to being of greater magnitude. Finally, primed and unprimed cells possessed a differential sensitivity to the kinase inhibitor H-89. H-89 substantially suppressed LPS-induced TNF mRNA accumulation in unprimed cells, but had no effect on primed monocytes following LPS stimulation. These results demonstrate that priming amplifies LPS-inducible TNF expression by increasing both transcription and mRNA stability. (C) 1995 Academic Press Limited.