What T cells tell macrophages to do during resistance to listeriosis.

We have presented 5 experimental observations which support a model for resistance to infection by the facultative intracellular bacterium, L. monocytogenes, in which the primary role for listeria-specific effector T cells is to cause recruitment of heightened numbers of macrophages and neutrophils which express potent anti-bacterial activity. These observations are: 1. Both macrophages and neutrophils, from mice and men, are bactericidal for listeria, salmonella, and E. coli. 2. When titrated, inflammatory macrophages and neutrophils have as much bactericidal activity as do immunologically-elicited macrophages and neutrophils. Thus "activated" macrophages are no better than inflammatory macrophages at killing bacteria. 3. Spleen T cells which transfer resistance to listeria also transfer, in a dose-dependent manner, the ability to accumulate enhanced numbers of neutrophils and macrophages following antigenic challenge. Phagocytes which accumulate following injection of immune vs. normal T cells kill bacteria equally well. 4. The major defect in mice which are genetically-susceptible to listeria is that they accumulate fewer macrophages and neutrophils in response to challenge with antigen or with an inflammatory agent than do mice which are genetically-resistant to listeria. 5. Macrophages induced to express bactericidal activity are not necessarily tumoricidal, and macrophages induced to express tumoricidal activity are not necessarily bactericidal. Thus bactericidal and tumoricidal properties of macrophages can be dissociated. Moreover, macrophages stimulated by recombinant IFN gamma to express tumoricidal activity do not express bactericidal activity.