The type I IFN response is critical in host defense against GBS (Mancuso et al., 2007
). Mice unable to mount a type I IFN response are hypersusceptible to lethality induced by GBS, with a mortality rate surpassing that seen in the absence of IFN-γ signaling, a well-established mediator of host defenses against bacteria (Schroder et al., 2004
). The inability of IFN-α/-β R-deficient mice to control GBS infection was partially accounted for by defective IFN-β signaling, however, a role for IFN-α and/or -ω subtypes is also a possibility.
Initially, TLR4 was thought to be the sole bacterial recognition receptor leading to type I IFN induction. Gram-positive bacteria were initially believed to be sensed entirely by TLR2 (Takeuchi et al., 1999
; Yoshimura et al., 1999
), which does not induce an IFN response. Several families of germline-encoded PRRs distributed in different subcellular compartments have since been identified, including the complement system in the extracellular environment, the TLRs and dectin-1 on the plasma membrane, TLRs in the endosomal compartment, and finally RLRs, NLRs, and DAI in the cytosol. In our study, we have examined the role of many of these sensors in regulating the IFN-β response to GBS. We have shown that, upon GBS infection, the induction of the type I IFN-β response proceeds via a pathway dependent on TBK1 and IRF3, a pathway activated by many pathogens. However, unlike many other pathogens, the GBS-induced type I IFN response appears to be independent of all currently known mediators of the IFN pathway. This TLR/NOD/RLR-independent pathway is, to the best of our knowledge, unique for a bacterium that does not have an intracellular lifestyle like Listeria monocytogenes
, although further studies may establish that other bacteria are capable of activating this response through similar mechanisms. GBS is able to survive inside some types of macrophages for extended periods, but this has never been associated with enhanced virulence, an ability to replicate intracellularly, or escape from the phagosomal compartment. Nevertheless, our study demonstrates that live GBS can be found in the cytosol and provides evidence that various bacterial products, such as DNA, escape the phagolysosomal compartment. The translocation of whole bacteria and bacterial products appears to be due to the degradation of the phagosomal membrane by secreted GBS toxins, which form pores in the membrane, thereby enabling the release of microbial components outside of the phagosomal compartment.
The multitude of PRRs in the cytosol highlights how active this environment is in terms of immune surveillance, not only for viruses, but also for other pathogens. Our findings are consistent with the hypothesis that an as yet unidentified PRR exists in the cytosol, poised to respond to GBS infection. We believe that GBS DNA is responsible for triggering this IFN response, similar to that postulated for sensing of Listeria monocytogenes
(Leber et al., 2008
; Stetson and Medzhitov, 2006
). Several PRRs have been identified as sensors for both microbial as well as self DNA. These include TLR9 in the endosome and DAI or NALP3 (Muruve et al., 2008
) in the cytosol. Both TLR9 and DAI have been implicated in IFN responses to other pathogens; however, our studies clearly demonstrate that neither TLR9 nor DAI are responsible for IFN induction in response to GBS.
DNA from GBS found in the cytosol could originate as a result of bacteria being degraded in the phagolysosome. Since the sensor for Listeria monocytogenes
also remains unknown at this point, it is possible that the same molecule(s) could sense both organisms. Should this be the case, it would be of interest to understand why the IFN response is protective in the case of GBS, yet detrimental in the case of Listeria, despite the similarities in the sensing and signaling pathways engaged by both organisms. One partial explanation might be the difference in the amplitude of the type I IFN response upon infection with extracellular or intracellular organisms. Indeed, recent work from Leber and colleagues demonstrated that in the case of Listeria infection, NOD2 signaling appears to cooperate with the cytosolic DNA-sensing pathway for optimal triggering of the cytosolic IFN responses (Leber et al., 2008
). In our study, however, we found no evidence that NOD2 plays such a role, since macrophages lacking RIP2, the downstream transducer of the NOD2 pathway, were not compromised in their ability to trigger IFN production upon GBS infection. As a consequence, the amplitude of the GBS response was lower than that triggered by Listeria.
GBS is a life-threatening infection in newborns and a common cause of disease in pregnant women, the elderly, and the immunocompromised. Given the importance of the type I IFN response in protecting the host against GBS, it is clear that understanding the molecular mechanisms responsible for triggering the IFN-β as well as IFN-α and -ω responses has both clinical and therapeutic potential, particularly in terms of vaccine development.