Differences in the Rate of Intracellular Killing of Catalase‐Negative and Catalase‐Positive Listeria monocytogenes by Normal and Interferon‐Gamma‐Activated Macrophages

Intracellular killing of catalase‐positive bacteria by murine resident macrophages requires the presence of extracellular serum, whereas killing of catalase‐negative bacteria can occur in the absence of serum. To find out whether the intracellular killing of bacteria by rIFN‐γ‐activated macrophages also requires serum stimulation, we investigated the handling of ingested catalase‐negative and ‐positive Listeria monocytogenes by peritoneal macrophages of normal Swiss mice and mice injected i. p. with I × 10 4 U rIFN‐γ 18 h earlier. In the absence of extracellular serum. rlFN‐γ‐activated macrophages killed ingested catalase‐negative Listeria more efficiently ( P <0.01) than normal resident macrophages. Maximal killing of catalase‐negative bacteria by rlFN‐γ‐activated macrophages required an extracellular serum concentration of only 1.0 to 2.5% compared with the 10% needed by normal macrophages. No differences were observed in the rates of intracellular killing of catalase‐positive Listeria by rlFN‐γ‐activated and normal resident macrophages: both populations of macrophages required 10% extracellular serum for maximal killing of these bacteria, and killing was minimal in the absence of scrum. The rIFN‐γ‐activated macrophages displayed enhanced O 2 ‐consumption after stimulation with phorbol myristate acetate and heat‐killed Listeria compared with macrophages from normal mice. These findings indicate that, under suboptimal stimulation by extracellular serum. rlFN‐γ enhances the intracellular killing of catalase‐negative Listeria which lack endogenous catalase acting as a scavenger of reactive oxygen intermediates. The mechanism underlying the enhancement is probably the amplification of the respiratory burst by IFN‐γ.