Ionizing radiation potentiates the induction of nitric oxide synthase by interferon-γ and/or lipopolysaccharide in murine macrophage cell lines. Role of tumor necrosis factor-α

Macrophages respond to infection or injury by changing from a 'resting' cellular phenotype to an 'activated' state defined by the expression of various cytotoxic effector functions. Regulation of the transition from a resting to an activated state is effected by cytokine and/or pathogenic signals. Some signals do not directly induce activation but instead 'prime' the macrophage to respond more vigorously to a second signal. One example of this priming phenomenon involves induction of nitric oxide (NO) synthesis by the enzyme nitric oxide synthase (NOS2). Our experiments indicate that low doses (1-5 Gy) of ionizing radiation can enhance the induction of enzymatically active NOS2 by IFN-γ or LPS in J774.1 and RAW264.7 murine macrophage cell lines. Radiation alone did not produce this induction, rather, it was effective as a priming signal; cells exposed to radiation produced more NO when a second signal, either IFN-γ or LPS, was applied 24 h later.