| Fallopian tube pathology resulting from infection with the obligate intracellular human pathogen Chlamydia trachomatis has been proposed to be caused by excessive host cytokine production. Using the mouse chlamydiae Chlamydia muridarum, this study was undertaken to better understand the mechanistic basis for production of the pathology associated cytokines IFN-P and IL-1 p. Although cytosolic pathogen recognition receptors are crucial for expression of both cytokines, the chlamydial contributions and host proteins involved are distinct. Using a combination of knockout macrophages and RNAi mediated gene silencing, it was determined that IFN-beta expression was TLR independent, requiring chlamydial growth, signaling through the ER protein STING, and the transcription factor IRF3. Cytosolic NOD1 also contributed to maximal IFN-G3, likely by activation of the transcription factor NF-kappaB and p38 MAPK signaling. The role of IRF3, the key transcription factor in IFN-beta induction, was further examined during an in vivo genital infection. In contrast to IFN-beta, activation of IRF3 is ultimately beneficial to the host as its absence leads to exacerbated uterine horn pathology during infection of the mouse female genital tract, suggesting an IFN-beta independent role for IRF3. In contrast to IFN-beta, IL-1beta secretion in primed macrophages was independent of chlamydial entry and growth. Using gene knockout mouse macrophages, it was determined that C. muridarum was capable of activating the inflammasome for subsequent IL-1beta secretion via multiple pathways utilizing the adaptor protein ASC, including cryopyrin and a putative pyrin domain containing protein(s). To further understand the bacterial contribution in host cytokine induction, the role of the chlamydial Type III secretion (T3S) apparatus in the expression of IL-1beta and IFN-beta was addressed. Administration of a T3S antagonist during infection decreased expression of IFN-beta, cxc110, and IL-6. In contrast, IL-1beta secretion occurred independently of the T3S rod sensing inflammasome protein IPAF. However the fact that T3S is present on the chlamydial elementary body in the absence of de novo protein synthesis, suggests it could still play a role, possibly by an alternate T3S dependent inflammasome. These latter studies illustrate that T3S could be a unifying concept in activation of the cytosolic pathways required for IFN-beta and IL-1beta expression. |
