Abstract
Intracellular pathogens have developed various strategies to escape immunity to enable their survival in host cells, and many bacterial pathogens preferentially reside inside macrophages, using diverse mechanisms to penetrate their defenses and to exploit their high degree of metabolic diversity and plasticity. Here, we characterized the interactions of the intracellular pathogen Chlamydia pneumoniae with polarized human macrophages. Primary human monocytes were pre-differentiated with granulocyte macrophage colony-stimulating factor or macrophage colony-stimulating factor for 7 days to yield M1-like and M2-like macrophages, which were further treated with interferon-γ and lipopolysaccharide or with interleukin-4 for 48 h to obtain fully polarized M1 and M2 macrophages. M1 and M2 cells exhibited distinct morphology with round or spindle-shaped appearance for M1 and M2, respectively, distinct surface marker profiles, as well as different cytokine and chemokine secretion. Macrophage polarization did not influence uptake of C. pneumoniae, since comparable copy numbers of chlamydial DNA were detected in M1 and M2 at 6 h post infection, but an increase in chlamydial DNA over time indicating proliferation was only observed in M2. Accordingly, 72±5% of M2 vs. 48±7% of M1 stained positive for chlamydial lipopolysaccharide, with large perinuclear inclusions in M2 and less clearly bordered inclusions for M1. Viable C. pneumoniae was present in lysates from M2, but not from M1 macrophages. The ability of M1 to restrict chlamydial replication was not observed in M1-like macrophages, since chlamydial load showed an equal increase over time for M1-like and M2-like macrophages. Our findings support the importance of macrophage polarization for the control of intracellular infection, and show that M2 are the preferred survival niche for C. pneumoniae. M1 did not allow for chlamydial proliferation, but failed to completely eliminate chlamydial infection, giving further evidence for the ability of C. pneumoniae to evade cellular defense and to persist in human macrophages.
Highlights
Macrophages, the first line of host defense against invading microbes, are frequently targeted by intracellular pathogens that escape eradication by internalization, using antigen-presenting cells as niche for survival and replication
Findings from an in vivo mouse model using C. pneumoniae indicate an impact of macrophage polarization on the course of chronic lung inflammation, suggesting that M1 macrophages result in enhanced inflammation, tissue injury, and fibrosis [29]
To explore the ability of C. pneumoniae to survive in polarized human macrophages and to assess the influence of macrophage polarization on the control of chlamydial infection, we predifferentiated human monocytes with Granulocyte macrophage colony-stimulating factor (GM-CSF) and macrophage colony-stimulating factor (M-CSF) for 7 days, followed by treatment of the resulting M1-like and M2-like macrophages with IFN-γ/LPS or IL-4 for 48 h to yield M1 and M2 macrophages
Summary
Macrophages, the first line of host defense against invading microbes, are frequently targeted by intracellular pathogens that escape eradication by internalization, using antigen-presenting cells as niche for survival and replication. Pathogens have evolved a number of different strategies to survive in macrophages [2, 3], and cross-talk between intracellular pathogens and their host cells occurs within various membrane-bound compartments or within the cytosol, depending on the nature of the invading pathogen. In response to external triggers, C. pneumoniae may enter a state of persistence, allowing the pathogen to ride out hostile conditions while maintaining a long-term, chronic infection within membrane-bound compartments in the cytoplasm of its host cells [4, 8]. There is evidence for the persistence of C. pneumoniae in primary human monocytes [9] and for their ability to replicate in monocyte-derived macrophages [10, 11], but the interdependence of chlamydial infection and macrophage polarization is unknown
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