Abstract
Macrophages are an important line of defence against invading pathogens. Human macrophages derived by different methods were tested for their suitability as models to investigate Listeria monocytogenes (Lm) infection and compared to macrophage-like THP-1 cells. Human primary monocytes were isolated by either positive or negative immunomagnetic selection and differentiated in the presence of granulocyte macrophage colony-stimulating factor (GM-CSF) or macrophage colony-stimulating factor (M-CSF) into pro- or anti-inflammatory macrophages, respectively. Regardless of the isolation method, GM-CSF-derived macrophages (GM-Mφ) stained positive for CD206 and M-CSF-derived macrophages (M-Mφ) for CD163. THP-1 cells did not express CD206 or CD163 following incubation with PMA, M- or GM-CSF alone or in combination. Upon infection with Lm, all primary macrophages showed good survival at high multiplicities of infection whereas viability of THP-1 was severely reduced even at lower bacterial numbers. M-Mφ generally showed high phagocytosis of Lm. Strikingly, phagocytosis of Lm by GM-Mφ was markedly influenced by the method used for isolation of monocytes. GM-Mφ derived from negatively isolated monocytes showed low phagocytosis of Lm whereas GM-Mφ generated from positively selected monocytes displayed high phagocytosis of Lm. Moreover, incubation with CD14 antibody was sufficient to enhance phagocytosis of Lm by GM-Mφ generated from negatively isolated monocytes. By contrast, non-specific phagocytosis of latex beads by GM-Mφ was not influenced by treatment with CD14 antibody. Furthermore, phagocytosis of Lactococcus lactis, Escherichia coli, human cytomegalovirus and the protozoan parasite Leishmania major by GM-Mφ was not enhanced upon treatment with CD14 antibody indicating that this effect is specific for Lm. Based on these observations, we propose macrophages derived by ex vivo differentiation of negatively selected human primary monocytes as the most suitable model to study Lm infection of macrophages.
Highlights
Listeria monocytogenes (Lm) is a food-borne Gram-positive obligate intracellular pathogen that is able to cope with a wide range of environmental conditions and is found in various of habitats [1]
Surface marker expression of macrophages Human macrophages were generated from primary human monocytes following positive or negative immunomagnetic selection and differentiated for 7 days in the presence of either GMCSF (GM-MQ) or macrophage colony-stimulating factor (M-CSF) (M-MQ)
GM-MQ displayed a CD14+CD1632CD206high phenotype and M-MQ were CD14+CD163highCD206low irrespective of the method used for monocyte isolation (Fig. 1A and B)
Summary
Listeria monocytogenes (Lm) is a food-borne Gram-positive obligate intracellular pathogen that is able to cope with a wide range of environmental conditions and is found in various of habitats [1]. Lm is able to disrupt the vacuolar membrane by the secretion of two phospholipases, PlcA and PlcB, and the poreforming toxin listeriolysin O (LLO) [5]. This results in the release of Lm into the cytoplasm where it starts to replicate and spread from one cell to another by hijacking the host cell actin cytoskeleton [6]
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