Monocytes of individual human subjects display heterogeneous bacterial uptake and antilisterial activity.

Peripheral blood monocytes (Mo) of normal human donors simultaneously exhibit two subsets differing in their functional activity towards the facultative intracellular bacterium Listeria monocytogenes. One subset (on average, 25% of total Mo) was characteristically able to ingest a large number of L. monocytogenes bacteria and permitted intracellular growth of these bacteria. The other Mo subpopulation (on average, 75% of total Mo) was far less active in phagocytosing L. monocytogenes and restricted intracellular L. monocytogenes growth. Electron microscopy revealed that the Listeria-permissive Mo subset allowed the bacteria to escape to the cytosol, a mechanism by which these bacteria evade the lethal attack of phagocytes. The Listeria-restrictive Mo subset, on the other hand, confined the bacteria to the phagolysosomes, where they were exposed to the killing mechanisms of the Mo. Permissiveness for L. monocytogenes growth was further associated with differences in the capacity of the Mo subsets to synthesize tumor necrosis factor alpha TNF-alpha), an important mediator in the defense against intracellular bacteria. Following challenge with L. monocytogenes, the Listeria-restrictive Mo subset secreted two to six times more TNF-alpha than did the Listeria-permissive Mo subset. Enhanced TNF-alpha secretion was paralleled by increased accumulation of TNF-alpha mRNA as assessed by quantitative PCR. Despite these functional differences, the two Mo subsets were indistinguishable with respect to expression of cell surface markers known to be involved in adherence and phagocytosis of microbes. A speculative physiological role of the two Mo subsets may lie in the dual function of Mo as microbicidal effector cells and accessory cells for antigen-specific immune reactions.