Chlamydia trachomatis Is Resistant to Inclusion Ubiquitination and Associated Host Defense in Gamma Interferon-Primed Human Epithelial Cells

Abstract

The cytokine gamma interferon (IFN-γ) induces cell-autonomous immunity to combat infections with intracellular pathogens, such as the bacterium Chlamydia trachomatis. The present study demonstrates that IFN-γ-primed human cells ubiquitinate and eliminate intracellular Chlamydia-containing vacuoles, so-called inclusions. We previously described how IFN-γ-inducible immunity-related GTPases (IRGs) employ ubiquitin systems to mark inclusions for destruction in mouse cells and, furthermore, showed that the rodent pathogen Chlamydia muridarum blocks ubiquitination of its inclusions by interfering with mouse IRG function. Here, we report that ubiquitination of inclusions in human cells is independent of IRG and thus distinct from the murine pathway. We show that C. muridarum is susceptible to inclusion ubiquitination in human cells, while the closely related human pathogen C. trachomatis is resistant. C. muridarum, but not C. trachomatis, inclusions attract several markers of cell-autonomous immunity, including the ubiquitin-binding protein p62, the ubiquitin-like protein LC3, and guanylate-binding protein 1. Consequently, we find that IFN-γ priming of human epithelial cells triggers the elimination of C. muridarum, but not C. trachomatis, inclusions. This newly described defense pathway is independent of indole-2,3-dioxygenase, a known IFN-γ-inducible anti-Chlamydia resistance factor. Collectively, our observations indicate that C. trachomatis evolved mechanisms to avoid a human-specific, ubiquitin-mediated response as part of its unique adaptation to its human host.

Innate Immunity to Intracellular Pathogens: Balancing Microbial Elimination and Inflammation

“…Work on human GBPs is still rather slim compared to that of the mouse, so further work is necessary to relate function in the two species. The gap in knowledge regarding the role of GBPs in interferon-regulated restriction in humans is exemplified by the fact that GBP localization around PVs appears to be dependent on the cell type being infected by the pathogen (Johnston et al, 2016, Haldar et al, 2016). Clearly, more work needs to be performed to understand how differences in cell types control the interferon-regulated response as well as to identify evolutionarily-conserved and species-specific responses.…”

Safe haven under constant attack-TheChlamydia-containing vacuole

“…In line with these findings, inactivation of autophagy in human epithelial cells interfered with Ctr replication (Al‐Younes et al, ; Al‐Younes, Brinkmann, & Meyer, ). Supporting the hypothesis of an alternative function of autophagy during Ctr infection in human cells, an ubiquitination of the Ctr inclusion in human epithelial cells as the first step of destructive autophagy activation was never observed (Haldar et al, ). On the other hand, autophagy‐dependent restriction of Ctr growth analysed in murine cells depleted for Atg5 is controversially discussed (Pachikara et al, ; Yasir, Pachikara, Bao, Pan, & Fan, ).…”

“…Moreover, the effectors GBP1 and GBP2 were recruited to the inclusion by p62 upon ubiquitination and induced vacuole rupture to interfere with chlamydial replication (Haldar et al, ). In contrast, human epithelial cells are permissive for Ctr replication but restrict Cmu growth upon IFNγ‐priming (Haldar et al, ). In human cells, similar mechanisms as already described for murine cells are effective (Figure ).…”

Sweet host revenge: Galectins and GBPs join forces at broken membranes

“…Differences in the ubiquitin code—defined by the specific composition of the attached ubiquitin polymers (Komander & Rape, )—may in part determine how alternative host defence programmes selectively identify their microbial targets. Our recent observation that dynamin‐like guanylate‐binding proteins (GBPs) can be delivered to ubiquitinated PVs (Haldar et al, ; Haldar et al, ) may provide one clue as to how mammalian host cells translate different types of ubiquitin codes into distinct types of antimicrobial effector functions.…”

“…One such targeting pathway is dependent on the interaction of mGBP1 and mGBP2 with the ubiquitin‐binding protein p62, which escorts GBPs to ubiquitinated Chlamydia‐ containing inclusions (Haldar et al, ). Host species‐specific pathways in mouse and human cells orchestrate the attachment of ubiquitin to Chlamydia inclusions, yet the full set of ubiquitin E3 ligases involved in this process and the underlying mechanism for their delivery to Chlamydia inclusions has not yet been defined (Haldar et al, ; Haldar et al, ). More recently, we discovered a second, seemingly ubiquitin‐independent pathway, by which host cells can direct GBPs towards PVs formed by Legionella or Yersinia (Feeley et al, ).…”