Abstract
BackgroundIntracellular pathogens have devised various mechanisms to subvert the host immune response in order to survive and replicate in host cells. Here, we studied the infection of human blood monocytes with the intracellular pathogen C. pneumoniae and the effect on cytokine and chemokine profiles in comparison to stimulation with LPS.ResultsMonocytes purified from peripheral blood mononuclear cells by negative depletion were infected with C. pneumoniae. While immunofluorescence confirmed the presence of chlamydial lipopolysaccharide (LPS) in the cytoplasm of infected monocytes, real-time PCR did not provide evidence for replication of the intracellular pathogen. Complementary to PCR, C. pneumoniae infection was confirmed by an oligonucleotide DNA microarray for the detection of intracellular pathogens. Raman microspectroscopy revealed different molecular fingerprints for infected and non-infected monocytes, which were mainly due to changes in lipid and fatty acid content. Stimulation of monocytes with C. pneumoniae or with LPS induced similar profiles of tumor necrosis factor-alpha (TNF-α) and interleukin (IL)-6, but higher levels of IL-1β, IL-12p40 and IL-12p70 for C. pneumoniae which were statistically significant. C. pneumoniae also induced release of the chemokines MCP-1, MIP-1α and MIP-1β, and CXCL-8, which correlated with TNF-α secretion.ConclusionInfection of human blood monocytes with intracellular pathogens triggers altered cytokine and chemokine pattern as compared to stimulation with extracellular ligands such as LPS. Complementing conventional methods, an oligonucleotide DNA microarray for the detection of intracellular pathogens as well as Raman microspectroscopy provide useful tools to trace monocyte infection.Electronic supplementary materialThe online version of this article (doi:10.1186/s12865-014-0060-1) contains supplementary material, which is available to authorized users.
Highlights
Intracellular pathogens have devised various mechanisms to subvert the host immune response in order to survive and replicate in host cells
We established an in vitro model to study the activation of human blood monocytes with C. pneumoniae and to examine immune mediator profiles secreted by infected monocytes
real-time PCR (RT-PCR) did not reveal significant changes in copy numbers of the intracellular pathogen over time, which indicates persistence but not replication of internalized C. pneumoniae in monocytes under these experimental conditions. In line with this finding, no viable C. pneumoniae could be recovered in HEp-2 cells upon re-infection with lysates from monocytes infected with C. pneumoniae for 48 h. This observation is in accordance with previously published data showing that C. pneumoniae does not replicate in freshly isolated monocytes, while monocytederived macrophages cultured for several days support the growth of chlamydial progeny [26,27]
Summary
Intracellular pathogens have devised various mechanisms to subvert the host immune response in order to survive and replicate in host cells. We studied the infection of human blood monocytes with the intracellular pathogen C. pneumoniae and the effect on cytokine and chemokine profiles in comparison to stimulation with LPS. Transition into a state of persistence can be induced in vitro by factors such as penicillin, starvation, or maturation of the host cell. Among the susceptible host cells are the mucosal and vascular endothelium, smooth muscle cells, circulating monocytes, and tissue-specific macrophages [13]. Human monocyte cell lines have been shown to transfer the pathogen to endothelial cells in vitro [15,16,17], and several lines of evidence propose a role of circulating monocytes as vehicle of its vascular dissemination [18]. Monocytes may traffic C. pneumoniae across the blood-brain-barrier, shed them in the central nervous system, and induce neuroinflammation [19,20]
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