Abstract
Chlamydia trachomatis is an obligate intracellular bacterial pathogen that cannot synthesize several amino acids, including tryptophan. Rather, C. trachomatis acquires these essential metabolites from its human host cell. Chlamydial dependence on host-provided tryptophan underlies a major host defense mechanism against the bacterium; namely, the induction of the host tryptophan-catabolizing enzyme, indoleamine 2,3- dioxygenase (IDO1) by interferon gamma (IFNγ), which leads to eradication of C. trachomatis by tryptophan starvation. For this reason, IFNγ is proposed to be the major host protective cytokine against genital C. trachomatis infections. The protective effect of IFNγ against C. trachomatis can be recapitulated in vitro using epithelial cell-lines such as the cervical carcinoma derived cell-line Hela, the Hela subclone HEp-2, and the cervical carcinoma derived cell-line ME180. Addition of IFNγ to these cells infected with C. trachomatis results in a strong bactericidal or bacteriostatic effect dependent on the concentration of IFNγ administered. Unlike Hela, HEp-2, and ME180, there are other human epithelial, or epithelial-like cell-lines where administration of IFNγ does not affect chlamydial replication, although they express the IFNγ receptor (IFNGR). In this report, we have characterized the mechanisms that underlie this dichotomy using the cell-lines C33A and 293. Akin to Hela, C33A is derived from a human cervical carcinoma, while 293 cells were produced by transfection of adenovirus type 5 DNA into embryonic kidney cells. We demonstrate that although IFNGR is expressed at high levels in C33A cells, its ligation by IFNγ does not result in STAT1 phosphorylation, an essential step for activation of the IDO1 promoter. Our results indicate that although the IFNγ-dependent signaling cascade is intact in 293 cells; the IDO1 promoter is not activated in these cells because it is epigenetically silenced, most likely by DNA methylation. Because polymorphisms in IFNγ, IFNGR, and the IDO1 promoter are known to affect other human infections or diseased states, our results indicate that the effect of allelic differences in these genes and the pathways they activate should be evaluated for their effect on C. trachomatis pathology.
Highlights
Chlamydia trachomatis is an obligate intracellular bacterium [1] associated with ocular and genital infections of columnar epithelial cells in humans
While the number and size of chlamydial inclusions were dramatically reduced in Hela and A2EN cells grown in the presence of IFNγ, no such reduction was observed C33A and 293 cells (Figure 1A)
Inclusion forming units (IFU) recovery at 42 hours post-infection (h.p.i) was reduced significantly in Hela and A2EN cells when grown in presence of 300 U/ml of IFNγ, but not in C33A and 293 cells (Figure 1B)
Summary
Chlamydia trachomatis is an obligate intracellular bacterium [1] associated with ocular and genital infections of columnar epithelial cells in humans. The inability of C. trachomatis and its host epithelial cell to synthesize tryptophan has rendered the bacterium highly susceptible to the host cytokine IFNγ, which induces expression of the tryptophan catabolizing enzyme indoleamine 2,3dioxygenase 1 (IDO1) [14]. The IDO1 enzyme irreversibly catabolizes tryptophan to kynurenine [16], thereby starving C. trachomatis of this essential amino acid. For this reason, IFNγ is considered to be the major protective host cytokine against C. trachomatis infections [10,13,17]
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