Chlamydia persistence -- a tool to dissect chlamydia--host interactions

Abstract

Under stress, chlamydiae can enter a non-infectious but viable state termed persistence. In the absence of a tractable genetic system, persistence induction provides an important experimental tool with which to study these fascinating organisms. This review will discuss examples of: i) persistence studies that have illuminated critical chlamydiae/host interactions; and ii) novel persistence models that will do so in the future.

Fig. 1

General overview of the chlamydial developmental cycle.

Fig. 2

Transmission electron micrographs of Chlamydia muridarum-infected BM1.11 murine oviduct epithelial cells. Panels A and B. BM1.11 cells were infected at 1MOI, refed with culture medium + ddH₂O at 8 hpi and harvested at 30 hpi. Panel C. C. muridarum-infected cultures were re-fed at 8 hpi with medium + 0.605 ug/ml amoxicillin and incubated for an additional 22 h. Infected cells were subjected to TEM as described . RB are indicated by black arrows, EB by white arrows, and AB by “AB”. Arrows in panel C indicate AB membrane blebbing.

Fig. 3

IFN-γ elicits different anti-chlamydial responses in human and murine cells. Panel A. IFN-γ/receptor interaction activates at least 2 distinct pathways in human cells that limit chlamydial development. The first increases indoleamine-2,3-dioxygenase (IDO) activity, activating tryptophan to kynurenine catalysis and reducing host intracellular tryptophan concentration, which induces chlamydial persistence . Genital C. trachomatis serovars can use exogenous indole to synthesize tryptophan, thus evading this response. In contrast, ocular serovars, which lack functional trpA/B genes, cannot utilize indole , . The IFN-γ-inducible 65 kD guanylate binding proteins (GPBs) may also restrict C. trachomatis growth in human epithelial cells by activating autophagosomal destruction of the chlamydiae and/or by blocking chlamydial lipid acquisition . As host sphingomyelin synthesis is required for efficient reactivation of C. trachomatis persistence , the GBPs may also function to keep persistent chlamydiae (induced by tryptophan limitation or other stimuli) from re-entering the productive developmental cycle until they can be disposed of by the host autophagic machinery. Panel B. IFN-γ/receptor interaction on murine cells inhibits chlamydial development primarily by inducing immunity-related GTPases (IRGs). C. trachomatis-infection can also increase IRG accumulation via a cPLA2 and IFN-β-dependent pathway . IRGs do not induce persistence but may restrict both normal chlamydial development and recovery from persistence by inhibiting sphingomyelin transport and/or by increasing host autophagic activity . C. muridarum is more resistant to the anti-chlamydial effects of murine IRGs than are human C. trachomatis serovars due to the production of secreted inhibitory effectors , , one of which may be the cytotoxin . Red lines represent those pathways that restrict chlamydial development; green lines those that promote chlamydial productive replication; arrowheads represent activation events and “T”s indicate inhibition.

Fig. 4

Induction of chlamydial persistence by cigarette smoke and host cellular viral co-infection. Panel A. There are several likely mechanism(s) by which components of cigarette smoke induce C. pneumoniae persistence . ATP_e or Ado released from smoke-exposed cells could induce chlamydial persistence by activating purinergic receptor-linked signaling pathways . Nitrogen radicals present in cigarette smoke could also react with intracellular tryptophan , inducing persistence via tryptophan depravation . As nitrogen radicals also activate intracellular sphingomyelinase (SMase) activity , , subsequent reduction in intracellular sphingomyelin would be predicted to restrict recovery from persistence and/or productive chlamydial development. Finally, because smoke components are concentrated in host lipid droplets , these compounds may directly alter chlamydial development when lipid droplets are brought within close proximity to the developing inclusion . Panel B. Herpes Simplex Virus (HSV) co-infection and HSV glycoprotein D (gD)/host nectin-1 interaction , appears to activate a currently unidentified host cellular signaling pathway that induces chlamydial persistence. Since nectin-1 also interacts with other host ligands on adjacent epithelial cells, remodeling of nectin-1 containing adherens junctions may also regulate chlamydial development in the absence of HSV infection. Porcine Epidemic Diarrhea Virus (PEDV) co-infection also induces persistence and may do so directly by interacting with a host cell receptor (aminopeptidase N), in a manner analogous to that stimulated by HSVgD/nectin-1 interaction. Alternatively, PEDV infection may activate IDO expression and tryptophan limitation through an IFN-γ- or -α-dependent mechanism. Red lines represent those pathways that restrict chlamydial development; green lines those that promote chlamydial productive replication; arrowheads represent activation events and “T”s indicate inhibition.