Both human herpes viruses and Chlamydia are highly prevalent in the human population and are detected together in different human disorders. Here, we demonstrate that co-infection with human herpes virus 6 (HHV6) interferes with the developmental cycle of C. trachomatis and induces persistence. Induction of chlamydial persistence by HHV6 is independent of productive virus infection, but requires the interaction and uptake of the virus by the host cell. On the other hand, viral uptake is strongly promoted under co-infection conditions. Host cell glutathione reductase activity was suppressed by HHV6 causing NADPH accumulation, decreased formation of reduced glutathione and increased oxidative stress. Prevention of oxidative stress restored infectivity of Chlamydia after HHV6-induced persistence. We show that co-infection with Herpes simplex virus 1 or human Cytomegalovirus also induces chlamydial persistence by a similar mechanism suggesting that Chlamydia -human herpes virus co-infections are evolutionary shaped interactions with a thus far unrecognized broad significance.

The obligate intracellular human pathogenic bacterium Chlamydia trachomatis has evolved multiple mechanisms to circumvent the host immune system. Infected cells exhibit a profound resistance to the induction of apoptosis and downregulate the expression of major histocompatibility complex class I and class II molecules to evade the cytotoxic effect of effector immune cells. Here we demonstrate the down-regulation of tumor necrosis factor receptor 1 (TNFR1) on the surface of infected cells. Interestingly, other members of the TNFR family such as TNFR2 and CD95 (Fas/Apo-1) were not modulated during infection, suggesting a selective mechanism underlying surface reduction of TNFR1. The observed effect was not due to reduced expression since the overall amount of TNFR1 protein was increased in infected cells. TNFR1 accumulated at the chlamydial inclusion and was shed by the infected cell into the culture supernatant. Receptor shedding depended on the infection-induced activation of the MEK-ERK pathway and the metalloproteinase TACE (TNF␣ converting enzyme). Our results point to a new function of TNFR1 modulation by C. trachomatis in controlling inflammatory signals during infection.Chlamydia trachomatis are obligate intracellular bacteria with a unique biphasic developmental cycle. Infectious elementary bodies enter the cell through endocytosis. Inside the cell elementary bodies mature into non-infectious metabolically active reticulate bodies within a vacuole termed the inclusion. At the end of the cycle, reticulate bodies redevelop into elementary bodies that are released from the cell to start a new infection. C. trachomatis is the most common causative agent of sexually transmitted diseases in the western world. In developing countries it is responsible for trachoma, a form of follicular conjunctivitis passed on by smear infection, which is the leading cause of acquired blindness. Chronic infections with C. trachomatis are associated with inflammatory diseases of the joints including reactive or rheumatoid arthritis (1, 2).The details of how diseases caused by C. trachomatis infection develop are still not fully understood, but immunological responses of the host very likely play a major role (for review, see Ref.3). These involve the secretion of cytokines and chemokines by infiltrating immune cells. One of the best-studied cytokines is interferon ␥ produced by activated T-lymphocytes. It limits chlamydial growth by stimulating the synthesis of indoleamine 2,3-dioxygenase, an enzyme that participates in tryptophan catabolism. Thereby, the availability of tryptophan for the bacteria is limited, resulting in reduced growth (4, 5). Tryptophan limitation induced by interferon ␥ can synergistically be enhanced by tumor necrosis factor ␣ (TNF␣) 2 (6) and reversed by supplying additional tryptophan (7). Chlamydial infection activates macrophages to produce TNF␣ and interferon ␥, which is believed to drive an inflammatory loop especially during a persistent infection (8, 9).However, TNF␣ also blocks chlamydial growth on ...

SummaryChlamydiae are obligate intracellular Gramnegative bacteria that cause widespread diseases in humans. Due to the intimate association between bacterium and host, Chlamydia evolved various strategies to protect their host cell against deathinducing stimuli, allowing the bacterium to complete its development cycle. An RNA interference (RNAi)-based screen was used to identify host cell factors required for apoptosis resistance of human epithelial cells infected with Chlamydia trachomatis serovar L2. Among the 32 validated hits, the antiapoptotic Bcl-2 family member Mcl-1 was identified as a target. Protein network analyses implicated the transcription factor hypoxia-induced factor 1 alpha (HIF-1a) to be central to the regulation of many of the identified targets. Further mechanistic investigations showed that HIF-1a was stabilized within the host cell cytoplasm during early infection time points, followed by its translocation to the nucleus and eventual transcriptional activation of Mcl-1. siRNA-mediated depletion of HIF-1a led to a drastic decrease in Mcl-1, rendering the cell sensitive to apoptosis induction. Taken together, our findings identify HIF-1a as responsible for upregulation of Mcl-1 and the maintenance of apoptosis resistance during Chlamydia infection.

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