Abstract
The intracellular bacterial pathogen, Chlamydia trachomatis, is a tryptophan auxotroph. Therefore, induction of the host tryptophan catabolizing enzyme, indoleamine-2,3-dioxgenase-1 (IDO1), by interferon gamma (IFNγ) is one of the primary protective responses against chlamydial infection. However, despite the presence of a robust IFNγ response, active and replicating C. trachomatis can be detected in cervical secretions of women. We hypothesized that a primary C. trachomatis infection may evade the IFNγ response, and that the protective effect of this cytokine results from its activation of tryptophan catabolism in bystander cells. To test this hypothesis, we developed a novel method to separate a pool of cells exposed to C. trachomatis into pure populations of live infected and bystander cells and applied this technique to distinguish between the effects of IFNγ on infected and bystander cells. Our findings revealed that the protective induction of IDO1 is suppressed specifically within primary infected cells because Chlamydia attenuates the nuclear import of activated STAT1 following IFNγ exposure, without affecting STAT1 levels or phosphorylation. Critically, the IFNγ-mediated induction of IDO1 activity is unhindered in bystander cells. Therefore, the IDO1-mediated tryptophan catabolism is functional in these cells, transforming these bystander cells into inhospitable hosts for a secondary C. trachomatis infection.
Highlights
Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen, causing a myriad of diseases that can impinge severely on female fertility and the health of neonates
cell penetrant peptide (CPP)-labeling of C. trachomatis elementary bodies permits the recovery of pure populations of live infected and bystander cells by flow cytometry
The Cell Penetrant Peptide’s (CPP’s) sequence and the elementary bodies (EBs) labeling procedure used are described in the Methods section
Summary
CPP-labeling of C. trachomatis elementary bodies permits the recovery of pure populations of live infected and bystander cells by flow cytometry. It was anticipated that diminished nuclear accumulation of STAT1 in IFNγ-exposed infected cells would correspond to lower pSTAT1 levels; in contrast, there was no difference in the levels of pSTAT1 in bystander and infected cells after exposure to IFNγ (Fig. 6A). The specificity of this antibody is indicated by the absence of pSTAT1 in cells not exposed to IFNγ (ibid). PSTAT1 was observed to be nuclear if cells were pre-exposed to IFNγ for 24 hours prior to infection and maintained in IFNγ during infection These experiments reveal that the effect of IFNγ on bystander cells is critical to limit spread of a primary infection
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