Some macrophages kill Listeria monocytogenes while others do not

It is not known why some macrophages can kill certain microbes, such as the facultative intracellular bacterium Listeria monocytogenes (L. monocytogenes), while other macrophages cannot. Perhaps listericidal activity is a property of macrophages at specific stages of differentiation; maybe the ability to kill this bacterium is regulated by the microenvironment of the cell; or it is possible that other regulatory forces are important. We describe here three characteristics that distinguish macrophages which can kill L. monocytogenes from those which cannot. First, listericidal macrophages must have neither too much nor too little intracellular iron - they must have an intermediate amount. Second, the receptor a macrophage uses to phagocytose L. monocytogenes seems to influence the intracellular face of this bacterium. And third, macrophages which have cell-surface interleukin-10 (IL-10), a known downregulator of macrophage function, cannot kill L. monocytogenes. These trails of macrophages and their effects on listericidal activity are reviewed here, and the possibility that these properties might interact to control macrophage bactericidal activity is discussed.