Neutrophils are more effective than monocytes at containment and clearance of Listeria monocytogenes

Neutrophils and monocytes are phagocytic cells that have previously been shown to be important for host protection during infection with the intracellular bacteria, Listeria monocytogenes. Previous studies have shown that simultaneous depletion of neutrophils and monocytes with the Gr-1 antibody leads to susceptibility to Listeria infection. However, the literature is divided on the necessity of neutrophils for host protection during infection. The purpose of these studies is to delineate differences in function between neutrophils and monocytes during intracellular bacterial infection. The mean fluorescence intensity (MFI), obtained with a flow cytometer, of the antibody against Listeria was measured as a determinant of the total bacteria phagocytosed by the cells. It was observed that neutrophils obtained from the bone marrow, liver and spleen of C57Bl/6 mice were more effective at phagocytosis of Listeria in comparison to monocytes as they had a higher total bacteria MFI than the monocytes. To determine differences in the ability of the cells to keep bacteria contained in the phagosome, the cells were infected with a strain of Listeria that only expresses GFP when the bacteria escapes out of the phagosome into the cytosol. Comparison of the MFI of total bacteria present vs escaped bacteria showed that monocytes from the bone marrow, liver and spleen of mice phagocytosed less bacteria and allowed for more bacteria to escape in comparison to neutrophils. Therefore, monocytes are less effective at bacterial containment in comparison to neutrophils. To ascertain differences in killing ability, bone marrow neutrophils and monocytes were sorted for a killing assay. Neutrophils were also observed to be more effective than monocytes at bacterial killing. In conclusion, although both cell types are important for protection during Listeria infection, neutrophils appear to be essential for protection as they are more effective at phagocytosis, phagosomal containment and bacteria killing. Future studies, including measurement of ROS/RNS and cytokine production, will aid in further defining specific functional differences between neutrophils and monocytes during intracellular bacterial infection.