Microbial metabolite triggers antimicrobial defense Bacteria can activate innate immune responses by releasing a metabolite that enters host cells By Sky W. Brubaker and Denise M. Monack
ILLUSTRATION: NICOLLE FULLER
I
n general, innate immune responses are beneficial to the host. However, under certain circumstances, such as coinfections, these responses may contribute to disease progression. For instance, an initial infection may induce an immune response that renders the host susceptible to a subsequent infection by a different pathogen. For years, it has been understood that Neisseria gonorrhoeae and HIV coinfection increase viral shedding and transmission (1). However, the molecular mechanisms that control this phenomenon have been unclear. Neisseria spp. release a factor that can activate the transcription factor nuclear factor κB (NF-κB) in host cells and drive HIV gene expression (2). On page 1251 of this issue, Gaudet et al. (3) reveal the identity of this bacterial-derived factor, as well as the signaling axis through which it drives innate immune gene expression. This detection pathway may constitute a previously unknown innate immune signaling response with broader implications in microbial defense, as well as the pathogenesis of HIV. In mammals, host defense and immunity rely heavily on pattern recognition receptors expressed by cells of the innate immune system. When activated, these receptors initiate inflammation at the site of infection and instruct adaptive immune responses (4). The receptors recognize conserved motifs of microbial and viral origin that are often referred to as pathogen-associated molecular patterns (PAMPs). Both transcriptional and nontranscriptional responses can take place following PAMP detection; however, the responses mediated by NF-κB are most notable for the production of proinflammatory cytokines. Importantly, HIV utilizes NF-κB for its own expression (5). Therefore, innate immune signaling can enhance the replication of HIV in some cases. Gaudet et al. took a genetic and biochemical approach to identify the PAMP released by Neisseria spp. as heptose-1,7-bisphosDepartment of Microbiology and Immunology, Stanford School of Medicine, Stanford University, Stanford, CA 94305, USA. E-mail: [email protected]
phate (HBP). Interestingly, this monosaccharide belongs to part of the biosynthetic pathway that produces lipopolysaccharide, a membrane component of Gram-negative bacteria and a potent innate immune agonist (6). Thus, HBP is a common metabolite among many Gram-negative bacterial species. However, the release of HBP into the extracellular environment by Neisseria spp. is unique. Other Gram-negative species require bacterial lysis to liberate HBP and produce an immunostimulatory effect. Gaudet et al. show that either the lysates of Gram-negative bacteria or the supernatants from Neisseria spp. that have not been intentionally lysed can induce the activity of NF-κB in immune cells, resulting in a transcriptional profile similar to those associNeisseria bacterium
ated with other activated innate immune receptors. Importantly, these transcriptional changes depend on HBP. Intracellular delivery of lysates or synthetic HBP into host cells greatly enhanced transcriptional responses, indicating that HBP is detected from within the cell. However, the mechanism by which HBP is transported into the host cell during Neisseria spp. infection remains to be determined. Although HBP is actively released from viable Neisseria spp., its uptake may be controlled by an unidentified host factor or process. To characterize the HBP detection pathway in host cells, Gaudet et al. performed a genome-wide RNA interference screen and identified TRAF-interacting protein with forkhead-associated domain (TIFA). This protein is a constituent of a cellular signaling pathway controlled by the receptor for the cytokine interleukin-1. TIFA interacts with tumor necrosis factor receptor–associated factor 6 (TRAF6) to activate the transcription factors NF-κB and activator protein 1 (7). Treatment of immune or nonimmune cells with HBP triggered several regulatory events that control the activity of TIFA, including tyrosine phosphorylation, the formation of large oligomeric complexes, and the recruitment of phosphorylated oligomers TIFA
?
P
HBP
?
LPS
?
? TRAF6 Ub NF-κB Lysosome
Infammatory responses
Immune cells
Cytokines
Immune or nonimmune cells
Antimicrobial defense. During Neisseria infection, a bacterial metabolite (HBP) either incorporates into lipopolysaccharide (LPS) or is released. HBP can enter a host cell and initiate TIFA activation, which involves phosphorylation (P), oligomerization, relocalization to lysosomes, and TRAF6 interaction. TRAF6 is activated by ubiquitin (Ub) modification and triggers the canonical NF-κB activation pathway, which drives the expression of innate immune genes (such as cytokines), and HIV replication during coinfection.
SCIENCE sciencemag.org
12 JUNE 2015 • VOL 348 ISSUE 6240
Published by AAAS
1207
Downloaded from www.sciencemag.org on June 11, 2015
IMMUNOLOGY
INSIGHTS | P E R S P E C T I V E S
to lysosomes. There, TIFA interacts with TRAF6, which has been modified with ubiquitin. A canonical NF-κB activation pathway is subsequently triggered by oligomers of ubiquitinated TRAF6, which ultimately induces innate immune gene expression (8) (see the figure). The regulators that control this cascade of events will be of interest in future studies. For instance, the identity of the tyrosine kinase that phosphorylates TIFA remains to be determined. Subcellular relocalization of adapter proteins involved in controlling innate immune responses can also be a point of regulation for gene expression (9). Therefore, identifying the factors that control TIFA relocalization to lysosomes after HBP treatment will be critical for understanding its regulation. It is also unclear whether an upstream HBP receptor exists, or if TIFA itself is the functional HBP receptor. Gaudet et al. further observed that injection of HBP-containing bacterial supernatants into mice elicited a proinflammatory response, and sublethal Neisseria meningitides infection of mice increased antibody titers and immunoglobulin class switching. However, it is unclear whether these responses contribute to the control or elimination of Neisseria spp. during infection. Given that Neisseria spp. replicate primarily outside of mammalian cells, it will be important to determine, in the context of an infection, which host cells respond to HBP and how HBP gains access to the host cell cytosol. The bacterial metabolite HBP appears to fit the classical definition of a PAMP because it is not a product of eukaryotic or mammalian cell biology, it is found in a broad class of prokaryotes (Gram-negative bacteria), and it stimulates the production of cytokines that drive inflammation and adaptive immune responses (10). However, HBP is not released from other Gram-negative bacteria, and it is unclear whether the response to this metabolite is a general mechanism of antimicrobial defense. Regardless, understanding the TIFA-mediated immune response to HBP may provide insight into therapies for patients coinfected with N. gonorrhoeae and HIV. ■ REFERENCES
1. S. R. Galvin, M. S. Cohen, Nat. Rev. Microbiol. 2, 33 (2004). 2. R. J. Malott et al., Proc. Natl. Acad. Sci. U.S.A. 110, 10234 (2013). 3. R. G. Gaudet et al., Science 348, 1251 (2015). 4. N. W. Palm, R. Medzhitov, Immunol. Rev. 227, 221 (2009). 5. J. Hiscott, H. Kwon, P. Génin, J. Clin. Invest. 107, 143 (2001). 6. B. Kneidinger et al., J. Bacteriol. 184, 363 (2002). 7. H. Takatsuna et al., J. Biol. Chem. 278, 12144 (2003). 8. C. K. Ea et al., Proc. Natl. Acad. Sci. U.S.A. 101, 15318 (2004). 9. S. W. Brubaker et al., Annu. Rev. Immunol. 33, 257 (2015). 10. C. A. Janeway Jr., Quant. Biol 54, 1 (1989). 10.1126/science.aac5835
1208
BIOMEDICINE
Bringing PGE2 in from the cold A small molecule that prevents the breakdown of a prostaglandin promotes tissue regeneration By Garret A. FitzGerald
P
rostaglandins (PGs) are evanescent, locally acting lipids. They are not stored within cells, but are generated when their precursor, arachidonic acid, is mobilized from cellular membranes by rather nonspecific activation of phospholipases. Most cells generate one or two dominant PGs, each with a remarkable diversity of effects. Prostaglandin E2 (PGE2) is a vasodilator that acts with other PGs and metabolic products of arachidonic acid to promote pain and inflammation. The development and clinical use of drugs that suppress the production of these PGs has been ongoing for decades, but PGE2 also has been long recognized to have a role in tissue maintenance and regeneration. Its stable analog, dimethyl PGE2, is currently being evaluated for use as an adjunct to hematopoietic stem cell transplantation. On page 1223 of this issue, Zhang et al. (1) report that elevating the capacity of tissues to form PGE2 by inhibiting 15-hydroxyprostaglandin dehydrogenase (15-PGDH), its major inactivating enzyme, augments the capacity for tissue regeneration in mouse models. So, is it time to bring PGE2, which we are so used to suppressing, in from the cold? The biosynthesis of PGs begins with the transformation of arachidonic acid to endoperoxide intermediates by the prostaglandin G/H synthase enzymes, colloquially known as cyclooxygenases (COX-1 and COX-2), the targets of nonsteroidal anti-inflammatory drugs (NSAIDs), including aspirin. These PG endoperoxides are acted on, in turn, by isomerases and synthases to generate PGs of the E, I, D, and F series along with thromboxane A2. Although some PGs can activate nuclear receptors, the evidence that they do so at concentrations actually formed in biological systems is scant. Suppression of PG formation by NSAIDs relieves pain and inflammation but may also result in cardiovascular and gastrointestinal adverse events, whereas the effectiveness of aspirin is well established in the secondary prevention of cardiovascular disease. Downstream of the COXs, microsomal PGE synthase-1 (mPGES-1) is the dominant source of PGE2 biosynthesis. Indeed, recent evidence suggests that in response
to a rise in intracellular calcium, a cytosolic phospholipase A2 translocates to the Golgi, where it associates with COX-2 and mPGES-1 to form a biosynthetic complex that generates PGE2 (2). There has been interest in developing inhibitors of mPGES-1 as an alternative to NSAIDs that might bypass the cardiovascular consequences of NSAIDs, attributable to suppression of COX-2–dependent formation of PGI2 (3). PGE2 activates four E prostanoid (EP) receptors, two of which (EP2 and EP4) are coupled to adenylate cyclase activation. Aside from biosynthesis, PGE2 concentration is determined by the activity of two transporters, multiple drug resistance–as-
“…the hope is that inhibition of 15-PGDH emerges as a safe and effective therapy…” sociated protein 4 and prostaglandin transporter, which export the lipid from the cell. PGE2 is also catabolized by 15-PGDH. Loss-of-function mutations in 15-PGDH have been causally implicated in the rare genetic disorder, familial hypertrophic osteoarthropathy–digital clubbing (4). Zhang et al. report that elevating PGE2, either by deleting or inhibiting 15-PGDH, increases hematopoiesis, liver regeneration, and boosts resistance to colitis (inflammation and ulcerations) in mouse models (see the figure). Indeed, a role for PGE2 in tissue maintenance and expansion was recognized in engraftment studies of epithelial stem cells in zebrafish and mice (5, 6). Administration of PGE2 to sublethally irradiated mice also accelerates hematopoietic recovery and increases the number of the short-term subtype of hematopoietic stem cells (7). Zhang et al. show that injecting normal mice, after hematopoietic stem cell transplantation, with the small molecule SW033291, which inhibits 15-PGDH, enhanced neutrophil count recovery. This inInstitute for Translational Medicine and Therapeutics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104. E-mail: [email protected]
sciencemag.org SCIENCE
12 JUNE 2015 • VOL 348 ISSUE 6240
Published by AAAS
Microbial metabolite triggers antimicrobial defense Sky W. Brubaker and Denise M. Monack Science 348, 1207 (2015); DOI: 10.1126/science.aac5835
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
The following resources related to this article are available online at www.sciencemag.org (this information is current as of June 11, 2015 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/348/6240/1207.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/348/6240/1207.full.html#related This article cites 10 articles, 5 of which can be accessed free: http://www.sciencemag.org/content/348/6240/1207.full.html#ref-list-1 This article appears in the following subject collections: Immunology http://www.sciencemag.org/cgi/collection/immunology
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2015 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on June 11, 2015
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.
ILLUSTRATION: NICOLLE FULLER
I
n general, innate immune responses are beneficial to the host. However, under certain circumstances, such as coinfections, these responses may contribute to disease progression. For instance, an initial infection may induce an immune response that renders the host susceptible to a subsequent infection by a different pathogen. For years, it has been understood that Neisseria gonorrhoeae and HIV coinfection increase viral shedding and transmission (1). However, the molecular mechanisms that control this phenomenon have been unclear. Neisseria spp. release a factor that can activate the transcription factor nuclear factor κB (NF-κB) in host cells and drive HIV gene expression (2). On page 1251 of this issue, Gaudet et al. (3) reveal the identity of this bacterial-derived factor, as well as the signaling axis through which it drives innate immune gene expression. This detection pathway may constitute a previously unknown innate immune signaling response with broader implications in microbial defense, as well as the pathogenesis of HIV. In mammals, host defense and immunity rely heavily on pattern recognition receptors expressed by cells of the innate immune system. When activated, these receptors initiate inflammation at the site of infection and instruct adaptive immune responses (4). The receptors recognize conserved motifs of microbial and viral origin that are often referred to as pathogen-associated molecular patterns (PAMPs). Both transcriptional and nontranscriptional responses can take place following PAMP detection; however, the responses mediated by NF-κB are most notable for the production of proinflammatory cytokines. Importantly, HIV utilizes NF-κB for its own expression (5). Therefore, innate immune signaling can enhance the replication of HIV in some cases. Gaudet et al. took a genetic and biochemical approach to identify the PAMP released by Neisseria spp. as heptose-1,7-bisphosDepartment of Microbiology and Immunology, Stanford School of Medicine, Stanford University, Stanford, CA 94305, USA. E-mail: [email protected]
phate (HBP). Interestingly, this monosaccharide belongs to part of the biosynthetic pathway that produces lipopolysaccharide, a membrane component of Gram-negative bacteria and a potent innate immune agonist (6). Thus, HBP is a common metabolite among many Gram-negative bacterial species. However, the release of HBP into the extracellular environment by Neisseria spp. is unique. Other Gram-negative species require bacterial lysis to liberate HBP and produce an immunostimulatory effect. Gaudet et al. show that either the lysates of Gram-negative bacteria or the supernatants from Neisseria spp. that have not been intentionally lysed can induce the activity of NF-κB in immune cells, resulting in a transcriptional profile similar to those associNeisseria bacterium
ated with other activated innate immune receptors. Importantly, these transcriptional changes depend on HBP. Intracellular delivery of lysates or synthetic HBP into host cells greatly enhanced transcriptional responses, indicating that HBP is detected from within the cell. However, the mechanism by which HBP is transported into the host cell during Neisseria spp. infection remains to be determined. Although HBP is actively released from viable Neisseria spp., its uptake may be controlled by an unidentified host factor or process. To characterize the HBP detection pathway in host cells, Gaudet et al. performed a genome-wide RNA interference screen and identified TRAF-interacting protein with forkhead-associated domain (TIFA). This protein is a constituent of a cellular signaling pathway controlled by the receptor for the cytokine interleukin-1. TIFA interacts with tumor necrosis factor receptor–associated factor 6 (TRAF6) to activate the transcription factors NF-κB and activator protein 1 (7). Treatment of immune or nonimmune cells with HBP triggered several regulatory events that control the activity of TIFA, including tyrosine phosphorylation, the formation of large oligomeric complexes, and the recruitment of phosphorylated oligomers TIFA
?
P
HBP
?
LPS
?
? TRAF6 Ub NF-κB Lysosome
Infammatory responses
Immune cells
Cytokines
Immune or nonimmune cells
Antimicrobial defense. During Neisseria infection, a bacterial metabolite (HBP) either incorporates into lipopolysaccharide (LPS) or is released. HBP can enter a host cell and initiate TIFA activation, which involves phosphorylation (P), oligomerization, relocalization to lysosomes, and TRAF6 interaction. TRAF6 is activated by ubiquitin (Ub) modification and triggers the canonical NF-κB activation pathway, which drives the expression of innate immune genes (such as cytokines), and HIV replication during coinfection.
SCIENCE sciencemag.org
12 JUNE 2015 • VOL 348 ISSUE 6240
Published by AAAS
1207
Downloaded from www.sciencemag.org on June 11, 2015
IMMUNOLOGY
INSIGHTS | P E R S P E C T I V E S
to lysosomes. There, TIFA interacts with TRAF6, which has been modified with ubiquitin. A canonical NF-κB activation pathway is subsequently triggered by oligomers of ubiquitinated TRAF6, which ultimately induces innate immune gene expression (8) (see the figure). The regulators that control this cascade of events will be of interest in future studies. For instance, the identity of the tyrosine kinase that phosphorylates TIFA remains to be determined. Subcellular relocalization of adapter proteins involved in controlling innate immune responses can also be a point of regulation for gene expression (9). Therefore, identifying the factors that control TIFA relocalization to lysosomes after HBP treatment will be critical for understanding its regulation. It is also unclear whether an upstream HBP receptor exists, or if TIFA itself is the functional HBP receptor. Gaudet et al. further observed that injection of HBP-containing bacterial supernatants into mice elicited a proinflammatory response, and sublethal Neisseria meningitides infection of mice increased antibody titers and immunoglobulin class switching. However, it is unclear whether these responses contribute to the control or elimination of Neisseria spp. during infection. Given that Neisseria spp. replicate primarily outside of mammalian cells, it will be important to determine, in the context of an infection, which host cells respond to HBP and how HBP gains access to the host cell cytosol. The bacterial metabolite HBP appears to fit the classical definition of a PAMP because it is not a product of eukaryotic or mammalian cell biology, it is found in a broad class of prokaryotes (Gram-negative bacteria), and it stimulates the production of cytokines that drive inflammation and adaptive immune responses (10). However, HBP is not released from other Gram-negative bacteria, and it is unclear whether the response to this metabolite is a general mechanism of antimicrobial defense. Regardless, understanding the TIFA-mediated immune response to HBP may provide insight into therapies for patients coinfected with N. gonorrhoeae and HIV. ■ REFERENCES
1. S. R. Galvin, M. S. Cohen, Nat. Rev. Microbiol. 2, 33 (2004). 2. R. J. Malott et al., Proc. Natl. Acad. Sci. U.S.A. 110, 10234 (2013). 3. R. G. Gaudet et al., Science 348, 1251 (2015). 4. N. W. Palm, R. Medzhitov, Immunol. Rev. 227, 221 (2009). 5. J. Hiscott, H. Kwon, P. Génin, J. Clin. Invest. 107, 143 (2001). 6. B. Kneidinger et al., J. Bacteriol. 184, 363 (2002). 7. H. Takatsuna et al., J. Biol. Chem. 278, 12144 (2003). 8. C. K. Ea et al., Proc. Natl. Acad. Sci. U.S.A. 101, 15318 (2004). 9. S. W. Brubaker et al., Annu. Rev. Immunol. 33, 257 (2015). 10. C. A. Janeway Jr., Quant. Biol 54, 1 (1989). 10.1126/science.aac5835
1208
BIOMEDICINE
Bringing PGE2 in from the cold A small molecule that prevents the breakdown of a prostaglandin promotes tissue regeneration By Garret A. FitzGerald
P
rostaglandins (PGs) are evanescent, locally acting lipids. They are not stored within cells, but are generated when their precursor, arachidonic acid, is mobilized from cellular membranes by rather nonspecific activation of phospholipases. Most cells generate one or two dominant PGs, each with a remarkable diversity of effects. Prostaglandin E2 (PGE2) is a vasodilator that acts with other PGs and metabolic products of arachidonic acid to promote pain and inflammation. The development and clinical use of drugs that suppress the production of these PGs has been ongoing for decades, but PGE2 also has been long recognized to have a role in tissue maintenance and regeneration. Its stable analog, dimethyl PGE2, is currently being evaluated for use as an adjunct to hematopoietic stem cell transplantation. On page 1223 of this issue, Zhang et al. (1) report that elevating the capacity of tissues to form PGE2 by inhibiting 15-hydroxyprostaglandin dehydrogenase (15-PGDH), its major inactivating enzyme, augments the capacity for tissue regeneration in mouse models. So, is it time to bring PGE2, which we are so used to suppressing, in from the cold? The biosynthesis of PGs begins with the transformation of arachidonic acid to endoperoxide intermediates by the prostaglandin G/H synthase enzymes, colloquially known as cyclooxygenases (COX-1 and COX-2), the targets of nonsteroidal anti-inflammatory drugs (NSAIDs), including aspirin. These PG endoperoxides are acted on, in turn, by isomerases and synthases to generate PGs of the E, I, D, and F series along with thromboxane A2. Although some PGs can activate nuclear receptors, the evidence that they do so at concentrations actually formed in biological systems is scant. Suppression of PG formation by NSAIDs relieves pain and inflammation but may also result in cardiovascular and gastrointestinal adverse events, whereas the effectiveness of aspirin is well established in the secondary prevention of cardiovascular disease. Downstream of the COXs, microsomal PGE synthase-1 (mPGES-1) is the dominant source of PGE2 biosynthesis. Indeed, recent evidence suggests that in response
to a rise in intracellular calcium, a cytosolic phospholipase A2 translocates to the Golgi, where it associates with COX-2 and mPGES-1 to form a biosynthetic complex that generates PGE2 (2). There has been interest in developing inhibitors of mPGES-1 as an alternative to NSAIDs that might bypass the cardiovascular consequences of NSAIDs, attributable to suppression of COX-2–dependent formation of PGI2 (3). PGE2 activates four E prostanoid (EP) receptors, two of which (EP2 and EP4) are coupled to adenylate cyclase activation. Aside from biosynthesis, PGE2 concentration is determined by the activity of two transporters, multiple drug resistance–as-
“…the hope is that inhibition of 15-PGDH emerges as a safe and effective therapy…” sociated protein 4 and prostaglandin transporter, which export the lipid from the cell. PGE2 is also catabolized by 15-PGDH. Loss-of-function mutations in 15-PGDH have been causally implicated in the rare genetic disorder, familial hypertrophic osteoarthropathy–digital clubbing (4). Zhang et al. report that elevating PGE2, either by deleting or inhibiting 15-PGDH, increases hematopoiesis, liver regeneration, and boosts resistance to colitis (inflammation and ulcerations) in mouse models (see the figure). Indeed, a role for PGE2 in tissue maintenance and expansion was recognized in engraftment studies of epithelial stem cells in zebrafish and mice (5, 6). Administration of PGE2 to sublethally irradiated mice also accelerates hematopoietic recovery and increases the number of the short-term subtype of hematopoietic stem cells (7). Zhang et al. show that injecting normal mice, after hematopoietic stem cell transplantation, with the small molecule SW033291, which inhibits 15-PGDH, enhanced neutrophil count recovery. This inInstitute for Translational Medicine and Therapeutics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA 19104. E-mail: [email protected]
sciencemag.org SCIENCE
12 JUNE 2015 • VOL 348 ISSUE 6240
Published by AAAS
Microbial metabolite triggers antimicrobial defense Sky W. Brubaker and Denise M. Monack Science 348, 1207 (2015); DOI: 10.1126/science.aac5835
This copy is for your personal, non-commercial use only.
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
The following resources related to this article are available online at www.sciencemag.org (this information is current as of June 11, 2015 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/348/6240/1207.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/348/6240/1207.full.html#related This article cites 10 articles, 5 of which can be accessed free: http://www.sciencemag.org/content/348/6240/1207.full.html#ref-list-1 This article appears in the following subject collections: Immunology http://www.sciencemag.org/cgi/collection/immunology
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2015 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on June 11, 2015
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here.
