A single exogenous stimulus activates resident rat macrophages for nitric oxide production and tumor cytotoxicity

Based on experiments with mouse cells, it appears that macrophage activation for cytotoxicity requires two exogenous signals. One signal primes or sensitizes the cells, while the second activates them for killing. The present studies were designed to analyze the capacity of rat macrophages to be activated for nitric oxide production and for cytotoxicity by different inflammatory stimuli. We found that both resident alveolar macrophages (AMs) and resident peritoneal macrophages (PMs) from Sprague‐Dawley rats produced nitric oxide in response to relatively low doses of a single exogenous activating stimulus [interferon‐γ (IFN‐γ) or lipopolysac‐ charide (LPS)]. Resident PMs treated with either of these agents alone also exhibited nitric oxide‐mediated cytotoxicity toward xenogeneic and allogeneic tumor targets. In contrast to results reported previously with both resident and elicited PMs from mice, tumor necrosis factor‐α (TNF‐α) exerted only a small enhancing effect on IFN‐γ‐induced nitric oxide production by resident rat PMs. In addition, although some level of cooperativity between IFN‐γ and LPS was observed at low concentrations of LPS (< 10 ng/ml), IFN‐γ did not augment the effects of higher concentrations of LPS (≥10 ng/ml) on nitric oxide production by PMs. In contrast to PMs, AMs had a strong synergistic response to combinations of IFN‐ γ and LPS or TNF‐α but also only at relatively low concentrations of IFN‐γ and LPS. Furthermore, maximum nitric oxide production induced by IFN‐γ in these cells could be enhanced by TNF‐α or low doses of LPS. However, as observed with PMs, combinations of IFN‐γ and higher doses of LPS did not significantly augment maximum nitric oxide production induced in AMs by LPS alone. Thus, our data suggest that resident rat PMs and AMs resemble elicited mouse PMs in their ability to respond to a single activating stimulus. However, PMs and in some respects AMs differ from the latter by their reduced responsiveness to combinations of activators. Taken together, our results suggest that two exogenous stimuli are not required for full activation of resident macrophages from Sprague‐Dawley rats.