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Crowd control: quorum sensing in Enterococcus
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TRENDS in Microbiology Vol.10 No.3 March 2002
117
Unraveling the host immune response to vacuolar pathogens Intracellular respiratory pathogens, including Mycobacterium tuberculosis, Legionella pneumophila and Chlamydia pneumoniae, continue to cause much morbidity and mortality worldwide. These bacterial pathogens survive and multiply inside host cells in a non-degradative phagocytic vacuole that does not fuse with lysosomes, and are therefore known as vacuolar pathogens. In the lung, they grow primarily in alveolar macrophages and infection results in pneumonia. A better understanding of the mechanisms underlying the host immune responses to these pathogens is essential to develop more effective treatment and prevention strategies. In this context, pathogeninduced cytokines, their cellular sources and their regulation have been given more attention. The prototypical Th1 cytokine, IFN-γ, which is produced by NK- and T cells, has been recognized as the critical cytokine necessary to clear an infection associated with vacuolar pathogens. IFN-γ controls vacuolar pathogens through the generation of reactive oxygen intermediates (ROIs) and the production of reactive nitrogen intermediates (RNIs). In general, IFN-γ is induced by IL-12 as part of both the innate and adaptive immune responses.
Rothfuchs et al. [1] have now shown that, in vitro, in response to C. pneumoniae, a homogenous population of bone marrowderived macrophages produces significant levels of type II interferon (IFN-γ) and inducible nitric oxide synthase (iNOS) through the type I interferons, IFN-α and -β. As the first step, the authors demonstrated that IFN-γ production is attributable to live C. pneumoniae. Using appropriate controls, they determined that the IFN-γ is produced by macrophages, but not by T cells, NK cells or B cells. IFN-γ production is not regulated by IL-12, but is negatively controlled by a Th2 cytokine, IL-10, indicating that the regulation of IFN-γ in macrophages is different than in other cell types. The authors observed a higher bacterial burden in IFN-γ-knockout mice compared with wild-type or IL-10-knockout mice, implying a direct role for IFN-γ in controlling C. pneumoniae infection. They further delineated the regulatory mechanisms involved in the production of IFN-γ by macrophages and found that C. pneumoniae-induced IFN-α and -β play a crucial role in the production of IFN-γ. The results also show that IFN-α and -β control iNOS generation, which finally leads to the production of RNIs. However, macrophagederived IFN-γ is not required for iNOS
production. Taken together, these results suggest that macrophage-derived IFN-α and -β play a central role in regulating the production of IFN-γ as well as iNOS, and that both IFN-γ and iNOS are important for controlling vacuolar pathogens in macrophages. This is the first study to demonstrate that IFN-α and -β might be involved in controlling infections with a vacuolar bacterial pathogen through the regulation of IFN-γ. In the lung, macrophage-derived IFN-α and -β could play both an autocrine and a paracrine role, as receptors for these cytokines are expressed on most cell types in the alveolar lining. This study also leads to several interesting questions for future studies, including whether macrophagederived IFN-α and -β play a key role in the generation of ROIs and whether production of IFN-α and -β by macrophages is a common mechanism to control infections with other vacuolar bacterial pathogens. 1 Rothfuchs, A.G. et al. (2001) IFN-αβ-dependent, IFN-γ secretion by bone marrow-derived macrophages controls an intracellular bacterial infection. J. Immunol. 167, 6453–6461
Samithamby Jeyaseelan [email protected]
Crowd control: quorum sensing in Enterococcus Cytolysin, an important virulence factor for the opportunistic pathogen Enterococcus faecalis, can target bacteria and mammalian cells. The synthesis of this toxin is elaborate, involving post-translational modification, proteolytic cleavage, secretion and an additional step of extracellular proteolytic degradation of two subunits to produce the mature cytolysin. This complexity and an additional gene product are responsible for protecting cytolysin-producing bacteria. Although its synthesis has been well studied, until now the regulation of toxin production was unknown. Haas et al. [1] noticed that expression from the promoter of the cytolysin structural genes was repressed in the presence of two regulators, CylR1 and CylR2, the genes for which are located upstream from, and orientated in the opposite direction to, this promoter. The http://tim.trends.com
motifs present in these regulatory proteins suggest their function: CylR1 has three membrane-spanning regions, and so is probably a membrane detector, whereas CylR2 has a DNA-binding region, presumably for binding a promoter. Together, these regulators are implicated in repressing the expression of the cytolysin structural genes. The signal for the derepression was found to be one of the fully mature cytolysin subunits, CylLs′′. Addition of the culture supernatants containing only CylLs′′ induced transcription of the structural genes for both cytolysin subunits, but not the regulatory genes. This transcriptional activity was dependent on the amount of CylLs′′ added, suggesting that at a certain bacterial cell density the accumulation of this factor induces additional expression of the cytolysin structural genes.
This mechanism of density-dependent activation represents a type of ‘quorum sensing’ that is distinct from those currently known in Gram-negative bacteria or other Gram-positive bacteria in terms of the signal, the detector and the mechanism of activation. This newly recognized system of cell–cell communication could provide good targets for inhibition of virulence factor expression. Finding such inhibitors is of crucial importance as Enterococcus is one of the main causes of antibioticresistant nosocomial infections. 1 Haas, W. et al. (2002) Two-component regulator of Enterococcus faecalis cytolysin responds to quorum-sensing autoinduction. Nature 415, 84–87
Joanna B. Goldberg [email protected]
0966-842X/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.
TRENDS in Microbiology Vol.10 No.3 March 2002
117
Unraveling the host immune response to vacuolar pathogens Intracellular respiratory pathogens, including Mycobacterium tuberculosis, Legionella pneumophila and Chlamydia pneumoniae, continue to cause much morbidity and mortality worldwide. These bacterial pathogens survive and multiply inside host cells in a non-degradative phagocytic vacuole that does not fuse with lysosomes, and are therefore known as vacuolar pathogens. In the lung, they grow primarily in alveolar macrophages and infection results in pneumonia. A better understanding of the mechanisms underlying the host immune responses to these pathogens is essential to develop more effective treatment and prevention strategies. In this context, pathogeninduced cytokines, their cellular sources and their regulation have been given more attention. The prototypical Th1 cytokine, IFN-γ, which is produced by NK- and T cells, has been recognized as the critical cytokine necessary to clear an infection associated with vacuolar pathogens. IFN-γ controls vacuolar pathogens through the generation of reactive oxygen intermediates (ROIs) and the production of reactive nitrogen intermediates (RNIs). In general, IFN-γ is induced by IL-12 as part of both the innate and adaptive immune responses.
Rothfuchs et al. [1] have now shown that, in vitro, in response to C. pneumoniae, a homogenous population of bone marrowderived macrophages produces significant levels of type II interferon (IFN-γ) and inducible nitric oxide synthase (iNOS) through the type I interferons, IFN-α and -β. As the first step, the authors demonstrated that IFN-γ production is attributable to live C. pneumoniae. Using appropriate controls, they determined that the IFN-γ is produced by macrophages, but not by T cells, NK cells or B cells. IFN-γ production is not regulated by IL-12, but is negatively controlled by a Th2 cytokine, IL-10, indicating that the regulation of IFN-γ in macrophages is different than in other cell types. The authors observed a higher bacterial burden in IFN-γ-knockout mice compared with wild-type or IL-10-knockout mice, implying a direct role for IFN-γ in controlling C. pneumoniae infection. They further delineated the regulatory mechanisms involved in the production of IFN-γ by macrophages and found that C. pneumoniae-induced IFN-α and -β play a crucial role in the production of IFN-γ. The results also show that IFN-α and -β control iNOS generation, which finally leads to the production of RNIs. However, macrophagederived IFN-γ is not required for iNOS
production. Taken together, these results suggest that macrophage-derived IFN-α and -β play a central role in regulating the production of IFN-γ as well as iNOS, and that both IFN-γ and iNOS are important for controlling vacuolar pathogens in macrophages. This is the first study to demonstrate that IFN-α and -β might be involved in controlling infections with a vacuolar bacterial pathogen through the regulation of IFN-γ. In the lung, macrophage-derived IFN-α and -β could play both an autocrine and a paracrine role, as receptors for these cytokines are expressed on most cell types in the alveolar lining. This study also leads to several interesting questions for future studies, including whether macrophagederived IFN-α and -β play a key role in the generation of ROIs and whether production of IFN-α and -β by macrophages is a common mechanism to control infections with other vacuolar bacterial pathogens. 1 Rothfuchs, A.G. et al. (2001) IFN-αβ-dependent, IFN-γ secretion by bone marrow-derived macrophages controls an intracellular bacterial infection. J. Immunol. 167, 6453–6461
Samithamby Jeyaseelan [email protected]
Crowd control: quorum sensing in Enterococcus Cytolysin, an important virulence factor for the opportunistic pathogen Enterococcus faecalis, can target bacteria and mammalian cells. The synthesis of this toxin is elaborate, involving post-translational modification, proteolytic cleavage, secretion and an additional step of extracellular proteolytic degradation of two subunits to produce the mature cytolysin. This complexity and an additional gene product are responsible for protecting cytolysin-producing bacteria. Although its synthesis has been well studied, until now the regulation of toxin production was unknown. Haas et al. [1] noticed that expression from the promoter of the cytolysin structural genes was repressed in the presence of two regulators, CylR1 and CylR2, the genes for which are located upstream from, and orientated in the opposite direction to, this promoter. The http://tim.trends.com
motifs present in these regulatory proteins suggest their function: CylR1 has three membrane-spanning regions, and so is probably a membrane detector, whereas CylR2 has a DNA-binding region, presumably for binding a promoter. Together, these regulators are implicated in repressing the expression of the cytolysin structural genes. The signal for the derepression was found to be one of the fully mature cytolysin subunits, CylLs′′. Addition of the culture supernatants containing only CylLs′′ induced transcription of the structural genes for both cytolysin subunits, but not the regulatory genes. This transcriptional activity was dependent on the amount of CylLs′′ added, suggesting that at a certain bacterial cell density the accumulation of this factor induces additional expression of the cytolysin structural genes.
This mechanism of density-dependent activation represents a type of ‘quorum sensing’ that is distinct from those currently known in Gram-negative bacteria or other Gram-positive bacteria in terms of the signal, the detector and the mechanism of activation. This newly recognized system of cell–cell communication could provide good targets for inhibition of virulence factor expression. Finding such inhibitors is of crucial importance as Enterococcus is one of the main causes of antibioticresistant nosocomial infections. 1 Haas, W. et al. (2002) Two-component regulator of Enterococcus faecalis cytolysin responds to quorum-sensing autoinduction. Nature 415, 84–87
Joanna B. Goldberg [email protected]
0966-842X/02/$ – see front matter © 2002 Elsevier Science Ltd. All rights reserved.