Confinement of the obligate intracellular bacterium Chlamydia trachomatis to a membrane-bound vacuole, termed an inclusion, within infected epithelial cells neither prevents secretion of chlamydial antigens into the host cytosol nor protects chlamydiae from innate immune detection. However, the details leading to chlamydial antigen presentation are not clear. By immunoelectron microscopy of infected endometrial epithelial cells and in isolated cell secretory compartments, chlamydial major outer membrane protein (MOMP), lipopolysaccharide (LPS) and the inclusion membrane protein A (IncA) were localized to the endoplasmic reticulum (ER) and co-localized with multiple ER markers, but not with markers of the endosomes, lysosomes, Golgi nor mitochondria. Chlamydial LPS was also co-localized with CD1d in the ER. Since the chlamydial antigens, contained in everted inclusion membrane vesicles, were found within the host cell ER, these data raise additional implications for antigen processing by infected uterine epithelial cells for classical and non-classical T cell antigen presentation.

The cellular innate immune system is essential for recognizing pathogen infection and for establishing effective host defence. But critical molecular determinants responsible for facilitating an appropriate immune response-following infection with DNA and RNA viruses, for example-remain to be identified. Here we report the identification, following expression cloning, of a molecule (STING; stimulator of interferon genes) that appears essential for effective innate immune signalling processes. It comprises five putative transmembrane regions, predominantly resides in the endoplasmic reticulum and is able to activate both NF-kappaB and IRF3 transcription pathways to induce expression of type I interferon (IFN-alpha and IFN-beta ) and exert a potent anti-viral state following expression. In contrast, loss of STING rendered murine embryonic fibroblasts extremely susceptible to negative-stranded virus infection, including vesicular stomatitis virus. Further, STING ablation abrogated the ability of intracellular B-form DNA, as well as members of the herpesvirus family, to induce IFN-beta, but did not significantly affect the Toll-like receptor (TLR) pathway. Yeast two-hybrid and co-immunoprecipitation studies indicated that STING interacts with RIG-I and with SSR2 (also known as TRAPbeta), which is a member of the translocon-associated protein (TRAP) complex required for protein translocation across the endoplasmic reticulum membrane following translation. Ablation by RNA interference of both TRAPbeta and translocon adaptor SEC61beta was subsequently found to inhibit STING's ability to stimulate expression of IFN-beta. Thus, as well as identifying a regulator of innate immune signalling, our results imply a potential role for the translocon in innate signalling pathways activated by select viruses as well as intracellular DNA.

Intestinal epithelial cells interact with both microbes in the gut lumen and host immune cells. In this issue, Kaser et al., 2008 link a key mediator of endoplasmic reticulum stress, the protein XBP1, with survival of intestinal secretory epithelial cells and inflammatory bowel disease.

Inflammatory bowel disease (IBD) has been attributed to aberrant mucosal immunity to the intestinal microbiota. The transcription factor XBP1, a key component of the endoplasmic reticulum (ER) stress response, is required for development and maintenance of secretory cells and linked to JNK activation. We hypothesized that a stressful environmental milieu in a rapidly proliferating tissue might instigate a proinflammatory response. We report that Xbp1 deletion in intestinal epithelial cells (IECs) results in spontaneous enteritis and increased susceptibility to induced colitis secondary to both Paneth cell dysfunction and an epithelium that is overly reactive to inducers of IBD such as bacterial products (flagellin) and TNFα. An association of XBP1 variants with both forms of human IBD (Crohn's disease and ulcerative colitis) was identified and replicated (rs35873774; p value 1.6 × 10−5) with novel, private hypomorphic variants identified as susceptibility factors. Hence, intestinal inflammation can originate solely from XBP1 abnormalities in IECs, thus linking cell-specific ER stress to the induction of organ-specific inflammation.