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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.

CD14-Dependent Monocyte Isolation Enhances Phagocytosis of Listeria monocytogenes by Proinflammatory, GM-CSF-Derived Macrophages