Immunity
Previews they transport bacteria from tissue sites to initiate bubo formation and, via the production of chemokines, recruit monocytes that allow the spread of bacteria to secondary LNs. However, these two roles for DCs might not be unique to this cell type. Monocyte subsets can also traffic to LNs, raising the possibility that they could similarly initiate entry from tissues into dLNs (Jakubzick et al., 2013). The authors’ experiments with an S1P1 inhibitor argue that internodal trafficking is most likely the main contribution of monocytes to disease progression, thus providing a simple two-cell model of DC-mediated initial dLN entry followed by monocytemediated secondary LN spread. The ability to block migration of host cells, either from the periphery to dLNs or between LNs, could be an important therapeutic result. In this study, mice were treated with FTY720 prior to and during infection with Y. pestis, and it would be interesting to see whether administration following inoculation could
also block LN egress. Spread to secondary LNs occurred within 24 hr postinoculation in the model of St. John et al., so it remains unclear whether it is feasible to apply this treatment regimen to prevent disease progression. It is also unclear whether host-cell-dependent egress continued after 24 hr postinoculation because many bacteria were found extracellularly at later time points. Even so, if pharmacological inhibitors can block the spread of the disease within the host, they could be powerful adjuvants used in combination with antimicrobials to limit spread and slow bacterial growth. This mouse model, utilizing an attenuated strain that affects the kinetics of disease progression, will most likely provide a platform for further experiments to address the efficacy of therapeutic treatment options. REFERENCES Bell, E.B. (1979). Immunology 38, 797–808.
Butler, T. (1994). Clin. Infect. Dis. 19, 655–661, quiz 662–663. Cyster, J.G., and Schwab, S.R. (2012). Annu. Rev. Immunol. 30, 69–94. Jakubzick, C., Gautier, E.L., Gibbings, S.L., Sojka, D.K., Schlitzer, A., Johnson, T.E., Ivanov, S., Duan, Q., Bala, S., Condon, T., et al. (2013). Immunity 39, 599–610. Rathinasamy, A., Czeloth, N., Pabst, O., Fo¨rster, R., and Bernhardt, G. (2010). J. Immunol. 185, 4072–4081. St. John, A.L., Ang, W.X.G., Huang, M.-N., Kunder, C., Chan, E.W., Gunn, M.D., and Abraham, S.N. (2014). Immunity 41, this issue, 440–450. Viboud, G.I., and Bliska, J.B. (2005). Annu. Rev. Microbiol. 59, 69–89. Zhang, P., Skurnik, M., Zhang, S.S., Schwartz, O., Kalyanasundaram, R., Bulgheresi, S., He, J.J., Klena, J.D., Hinnebusch, B.J., and Chen, T. (2008a). Infect. Immun. 76, 2070–2079. Zhang, S.S., Park, C.G., Zhang, P., Bartra, S.S., Plano, G.V., Klena, J.D., Skurnik, M., Hinnebusch, B.J., and Chen, T. (2008b). J. Biol. Chem. 283, 31511–31521.
Gut Microbiota: A Natural Adjuvant for Vaccination Oliver Pabst1,* and Mathias Hornef2,3 1Institute
of Molecular Medicine, RWTH University, 52074 Aachen, Germany of Medical Microbiology and Hospital Epidemiology, Hannover Medical School, 30625 Hannover 3Institute of Medical Microbiology, RWTH University, 52074 Aachen, Germany *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2014.09.002 2Institute
In this issue of Immunity, Oh et al. (2014) reveal an unappreciated facet of how the microbiota influences immune responses. Immunity to nonadjuvanted vaccines depends on Toll-like-receptor-5-mediated sensing of the microbiota. The distal gut is colonized with an astonishing 1012 bacteria per gram of gut content. Other body surfaces are also colonized by microbes, which are collectively referred to as the microbiota. The term ‘‘supraorganism’’ has been coined to describe the fact that we are ‘‘running fermenters,’’ carrying numerically more bacteria than we have cells in our body. However, we are only beginning to understand the impact of the microbiota on health. Perhaps not surprisingly, the microbiota constantly produce and release
potent immunostimulatory molecules, which significantly affect the immune system. Thus, germ-free animals, i.e., animals bred in the absence of any viable microbes, show substantial differences in their intestinal mucosal immune system; such differences include underdeveloped Peyer’s patches, very few plasma cells, and reduced numbers of T cells. An exciting new development comes with the emerging mechanistic understanding of the influence of the microbiota on the host’s immune system. Strikingly, effects
are not restricted to the colonized mucosal tissue but are also observed at systemic body sites. For example, the gut microbiota has been shown to affect neutrophil maturation in the bone marrow, susceptibility to type 1 diabetes, and experimental encephalomyelitis. In this issue of Immunity, Oh et al. (2014) show that the gut microbiota can impact vaccination to flu. Two seasonally administrated flu vaccines are available in the USA: a nonadjuvanted subunit vaccine (trivalent
Immunity 41, September 18, 2014 ª2014 Elsevier Inc. 349
Immunity
Previews
Figure 1. Microbiota-Derived Flagellin Acts as an Adjuvant for Systemic Immunization Data obtained in experimental mouse models suggest that microbiota-released flagellin (red diamonds) reaches systemic sites and provides, via TLR5, the costimulus that macrophages and B cells (green) need to induce antibodies to exogenously administered adjuvant-free TIV influenza vaccine antigen (blue triangles). The critical role of the microbiota-derived flagellin suggests that microbiota alterations might influence the efficacy of adjuvant-free vaccination strategies.
inactivated vaccine, or TIV) and a live attenuated influenza strain. Despite the effectiveness of TIV, it has remained unclear how the vaccine can elicit protective immunity in the absence of an adjuvant or infection. Adjuvants or complete infective microorganisms contain innate immune stimuli and provide the costimulus required for efficient activation of the adaptive immune system. Oh et al. now show that TIV responses are reduced in Toll-like receptor 5 (TLR5)-deficient mice, which are unable to respond to the bacterial innate immune stimulus flagellin, as well as in germ-free mice or mice that have undergone sustained treatment with antibiotics. Colonization of germfree mice with flagellin-expressing but not flagellin-mutated bacteria was able to revert the phenotype. Oh et al. suggest that this effect is mediated by TLR5 expression on immune cells such as B cells and macrophages at systemic sites. It therefore appears that bacterial flagellin derived from the microbiota reaches immune cells at systemic sites and provides the costimulus and adjuvant for the TIV vaccine (Figure 1).
Several members of the mammalian microbiota express flagella, and the structural subunit flagellin is constantly released. But how does flagellin produced in the gut lumen reach systemic sites? Although flagellin is as potent as lipopolysaccharide (LPS) for driving immunoglobulin synthesis in B cells in vitro, Oh et al. demonstrate that the absence of TLR5 is sufficient to impair the vaccine response. Many years ago, flagellin (at the time called proinflammatory factor, PIF) was shown to rapidly cross the polarized intestinal epithelium and concentrate at the basolateral site (Gewirtz et al., 2001). This is in contrast to LPS, which forms a steep gradient between gut lumen and the epithelial surface (Dupont et al., 2014). Nevertheless, several microbial stimuli reach the subepithelial tissue. Gut-derived TLR4 and TLR9 ligands, LPS and bacterial DNA, respectively, were shown to reach the liver via the portal vein and drive nonalcoholic fatty liver disease (NAFLD) in mice (Henao-Mejia et al., 2012). Although the liver efficiently removes exogenous molecules, some might evade this process and reach systemic sites. Indeed, gut-microbiotaderived peptidoglycan fragments can reach the bone marrow. However, the systemic spread of gut-derived flagellin has not been examined. Previous work suggested a differential effect of the skin and gut microbiota on skin- and gut-draining lymph nodes, respectively (Naik et al., 2012). In contrast, the work by Oh et al. suggests a direct effect of the gut microbiota on the differentiation or survival of plasma cells in the peripheral lymph nodes. Thus, depending on their precise anatomical location, lymph nodes might be affected by microbial products coming from different sources, including their respective draining region and the gut. Macrophages might be particularly suited to capture microbial products and thereby set the tone for on-going immune responses. As an alternative to direct spread, phagocytic cells might internalize microbiota constituents at the mucosal surface and transport them to systemic sites. Interestingly, in the gut lamina propria a myeloid TLR5-expressing cell population has been described as regulating IgA-secreting plasma cells (Uematsu et al., 2008). Medullary macrophages situated in close
350 Immunity 41, September 18, 2014 ª2014 Elsevier Inc.
proximity to recently generated plasma cells might perform a similar function in peripheral lymph nodes. The observations made by Oh et al. suggest that antibiotic treatment and intestinal dysbiosis might affect vaccination in humans. However, the degree of microbiota alteration in the chosen models most probably exceeds the effects of standard antibiotic regimens. Numerous gut-inhabiting bacteria, including members of the phylum Proteobacteria, can produce flagellae; others, such as the common commensal bacterium Bacteroides fragilis, lack synthesis and motility (Lozupone et al., 2012). Microbiota alterations might thus influence the luminal flagellin concentration and thereby alter the threshold of immune activation, including that at systemic sites. The stimulatory potential of different flagellin molecules produced differs substantially; for example, Campylobacter spp. or Helicobacter spp. (both ε-proteobacteria) produce only weakly stimulatory flagellin, whereas the g-proteobacteria Escherichia coli and Salmonella enterica produce highly potent flagellin (Andersen-Nissen et al., 2005). Antibiotics modulating the abundance of species producing stimulatory flagellin along with changes in overall colonization levels and integrity of the gut barrier could potentially affect the availability of TLR5 ligands in the periphery. Moreover, composition and density of the microbiota is regulated at various levels by innate and adaptive immune responses. TLR5-deficient mice exhibit a significantly altered microbiota (Vijay-Kumar et al., 2010). Also, changes in flagellarelated gene expression, elevated amounts of flagellin in the gut, and flagellated bacteria penetrating intestinal villi have been described in TLR5-deficient mice (Cullender et al., 2013). Thus, the effect of TLR5 deficiency might be more complex and at least in some aspects go beyond the effect of antibiotic treatment. Antibiotics differentially affecting distinct bacterial groups showed comparable effects of TIV antibody responses. This further emphasizes the idea that vaccination efficacy might not rely on distinct bacterial species exclusively but rather also on overall colonization levels, integrity of the gut barrier, and functional levels of microbial products in the periphery.
Immunity
Previews Interestingly, the effect of antibiotic treatment was limited to two nonadjuvated vaccines, TIV and another viral subunit vaccine to polio. In contrast, immunization with alum-absorbed toxoids, the HIVenv protein as well as a live-attenuated yellow fever vaccine, were unaffected. Thus, the microbiota has no detectable effect on the induced humoral response to adjuvanted vaccines or attenuated pathogens. Flagellin has long been known to represent a potent immunostimulatory molecule and useful adjuvant. The presented data now reveal an unexpected role of naturally produced flagellin derived from the microbiota to provide costimulation for immune activation. Thus, the intestinal microbiota might effectively take the function of a natural adjuvant. This view is consistent with lessons learned from gut immunology: the microbiota is an essential driver of ho-
meostasis and a key part of an effective immune system.
Eisenbarth, S.C., Jurczak, M.J., et al. (2012). Nature 482, 179–185, http://dx.doi.org/10.1038/ nature10809. Lozupone, C., Faust, K., Raes, J., Faith, J.J., Frank, D.N., Zaneveld, J., Gordon, J.I., and Knight, R. (2012). Genome Res. 22, 1974–1984.
REFERENCES Andersen-Nissen, E., Smith, K.D., Strobe, K.L., Barrett, S.L., Cookson, B.T., Logan, S.M., and Aderem, A. (2005). Proc. Natl. Acad. Sci. USA 102, 9247–9252. Cullender, T.C., Chassaing, B., Janzon, A., Kumar, K., Muller, C.E., Werner, J.J., Angenent, L.T., Bell, M.E., Hay, A.G., Peterson, D.A., et al. (2013). Cell Host Microbe 14, 571–581. Dupont, A., Kaconis, Y., Yang, I., Albers, T., Woltemate, S., Heinbockel, L., Andersson, M., Suerbaum, S., Brandenburg, K., and Hornef, M.W. (2014). Gut. Published online May 7, 2014. http:// dx.doi.org/10.1136/gutjnl-2014-307150. Gewirtz, A.T., Simon, P.O., Jr., Schmitt, C.K., Taylor, L.J., Hagedorn, C.H., O’Brien, A.D., Neish, A.S., and Madara, J.L. (2001). J. Clin. Invest. 107, 99–109. Henao-Mejia, J., Elinav, E., Jin, C., Hao, L., Mehal, W.Z., Strowig, T., Thaiss, C.A., Kau, A.L.,
Naik, S., Bouladoux, N., Wilhelm, C., Molloy, M.J., Salcedo, R., Kastenmuller, W., Deming, C., Quinones, M., Koo, L., Conlan, S., et al. (2012). Science 337, 1115–1119. Oh, J.Z., Ravindran, R., Chassaing, B., Carvalho, F.A., Maddur, M.S., Bower, M., Hakimpour, P., Gill, K.P., Nakaya, H.I., Yarovinsky, F., et al. (2014). Immunity 41, this issue, 478–492. Uematsu, S., Fujimoto, K., Jang, M.H., Yang, B.G., Jung, Y.J., Nishiyama, M., Sato, S., Tsujimura, T., Yamamoto, M., Yokota, Y., et al. (2008). Nat. Immunol. 9, 769–776. Vijay-Kumar, M., Aitken, J.D., Carvalho, F.A., Cullender, T.C., Mwangi, S., Srinivasan, S., Sitaraman, S.V., Knight, R., Ley, R.E., and Gewirtz, A.T. (2010). Science 328, 228–231. Published online Mar 4, 2010. http://dx.doi.org/10.1126/science. 1179721.
For Macrophages, Ndufs Is Enough Stanley Ching-Cheng Huang1 and Edward J. Pearce1,* 1Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, MO63110-1093, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2014.09.004
Proinflammatory macrophage activation is coupled to a metabolic switch toward glycolysis. In Cell Metabolism, Jin et al. (2014) show that this process is negatively regulated by mitochondrial electron transport chain complex I through both cell intrinsic and extrinsic pathways. There is a link between anti-inflammatory properties and mitochondrial oxidative phosphorylation on the one hand, and classical activation and glycolytic metabolism on the other (O’Neill and Hardie, 2013). In their recent paper in Cell Metabolism, Jin and colleagues highlight the importance of complex I (CI) in regulating macrophage activation by showing that deletion of the CI component Ndufs4 causes a switch in the metabolic balance toward glycolysis and associated changes in the monocyte, macrophage, and osteoclast lineage that result in the development of inflammatory disease symptoms and abnormal bone density (Jin et al., 2014). The electron transport chain (ETC), which links nutrient oxidation to ATP
production by oxidative phosphorylation (OXPHOS), consists of complexes I–V in the mitochondrial inner membrane (Figure 1). CI is composed of >45 subunits and is highly regulated by pathways initiated by various extracellular signals (Papa et al., 2012). Unsurprisingly, mutations in genes encoding components of CI are associated with severe clinical disease (Papa et al., 2012). From an immunologic perspective, CI is interesting not only because of its core function in metabolism but because along with complex III, it is a site of reactive oxygen species (ROS) production (Sena and Chandel, 2012). ROS plays a role in a range of innate cell functions, and indeed Toll-like receptor (TLR) signaling
has been linked to increased production of mitochondrial ROS for bacterial killing (Sena and Chandel, 2012). However, ROS can be detrimental when produced in excess, and this can occur when CI function is compromised (Chen et al., 2007). Ndufs4 / mice have been used to model CI deficiency and have been found to die from encephalomyopathy approximately 7 weeks after birth (Kruse et al., 2008). Using these mice, Jin and colleagues discovered that by 3 weeks after birth, the absence of Ndufs4 led to systemic inflammation, apparent as increased levels of proinflammatory cytokines, systemically increased numbers of Ly6chi monocytes and the development
Immunity 41, September 18, 2014 ª2014 Elsevier Inc. 351
Previews they transport bacteria from tissue sites to initiate bubo formation and, via the production of chemokines, recruit monocytes that allow the spread of bacteria to secondary LNs. However, these two roles for DCs might not be unique to this cell type. Monocyte subsets can also traffic to LNs, raising the possibility that they could similarly initiate entry from tissues into dLNs (Jakubzick et al., 2013). The authors’ experiments with an S1P1 inhibitor argue that internodal trafficking is most likely the main contribution of monocytes to disease progression, thus providing a simple two-cell model of DC-mediated initial dLN entry followed by monocytemediated secondary LN spread. The ability to block migration of host cells, either from the periphery to dLNs or between LNs, could be an important therapeutic result. In this study, mice were treated with FTY720 prior to and during infection with Y. pestis, and it would be interesting to see whether administration following inoculation could
also block LN egress. Spread to secondary LNs occurred within 24 hr postinoculation in the model of St. John et al., so it remains unclear whether it is feasible to apply this treatment regimen to prevent disease progression. It is also unclear whether host-cell-dependent egress continued after 24 hr postinoculation because many bacteria were found extracellularly at later time points. Even so, if pharmacological inhibitors can block the spread of the disease within the host, they could be powerful adjuvants used in combination with antimicrobials to limit spread and slow bacterial growth. This mouse model, utilizing an attenuated strain that affects the kinetics of disease progression, will most likely provide a platform for further experiments to address the efficacy of therapeutic treatment options. REFERENCES Bell, E.B. (1979). Immunology 38, 797–808.
Butler, T. (1994). Clin. Infect. Dis. 19, 655–661, quiz 662–663. Cyster, J.G., and Schwab, S.R. (2012). Annu. Rev. Immunol. 30, 69–94. Jakubzick, C., Gautier, E.L., Gibbings, S.L., Sojka, D.K., Schlitzer, A., Johnson, T.E., Ivanov, S., Duan, Q., Bala, S., Condon, T., et al. (2013). Immunity 39, 599–610. Rathinasamy, A., Czeloth, N., Pabst, O., Fo¨rster, R., and Bernhardt, G. (2010). J. Immunol. 185, 4072–4081. St. John, A.L., Ang, W.X.G., Huang, M.-N., Kunder, C., Chan, E.W., Gunn, M.D., and Abraham, S.N. (2014). Immunity 41, this issue, 440–450. Viboud, G.I., and Bliska, J.B. (2005). Annu. Rev. Microbiol. 59, 69–89. Zhang, P., Skurnik, M., Zhang, S.S., Schwartz, O., Kalyanasundaram, R., Bulgheresi, S., He, J.J., Klena, J.D., Hinnebusch, B.J., and Chen, T. (2008a). Infect. Immun. 76, 2070–2079. Zhang, S.S., Park, C.G., Zhang, P., Bartra, S.S., Plano, G.V., Klena, J.D., Skurnik, M., Hinnebusch, B.J., and Chen, T. (2008b). J. Biol. Chem. 283, 31511–31521.
Gut Microbiota: A Natural Adjuvant for Vaccination Oliver Pabst1,* and Mathias Hornef2,3 1Institute
of Molecular Medicine, RWTH University, 52074 Aachen, Germany of Medical Microbiology and Hospital Epidemiology, Hannover Medical School, 30625 Hannover 3Institute of Medical Microbiology, RWTH University, 52074 Aachen, Germany *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2014.09.002 2Institute
In this issue of Immunity, Oh et al. (2014) reveal an unappreciated facet of how the microbiota influences immune responses. Immunity to nonadjuvanted vaccines depends on Toll-like-receptor-5-mediated sensing of the microbiota. The distal gut is colonized with an astonishing 1012 bacteria per gram of gut content. Other body surfaces are also colonized by microbes, which are collectively referred to as the microbiota. The term ‘‘supraorganism’’ has been coined to describe the fact that we are ‘‘running fermenters,’’ carrying numerically more bacteria than we have cells in our body. However, we are only beginning to understand the impact of the microbiota on health. Perhaps not surprisingly, the microbiota constantly produce and release
potent immunostimulatory molecules, which significantly affect the immune system. Thus, germ-free animals, i.e., animals bred in the absence of any viable microbes, show substantial differences in their intestinal mucosal immune system; such differences include underdeveloped Peyer’s patches, very few plasma cells, and reduced numbers of T cells. An exciting new development comes with the emerging mechanistic understanding of the influence of the microbiota on the host’s immune system. Strikingly, effects
are not restricted to the colonized mucosal tissue but are also observed at systemic body sites. For example, the gut microbiota has been shown to affect neutrophil maturation in the bone marrow, susceptibility to type 1 diabetes, and experimental encephalomyelitis. In this issue of Immunity, Oh et al. (2014) show that the gut microbiota can impact vaccination to flu. Two seasonally administrated flu vaccines are available in the USA: a nonadjuvanted subunit vaccine (trivalent
Immunity 41, September 18, 2014 ª2014 Elsevier Inc. 349
Immunity
Previews
Figure 1. Microbiota-Derived Flagellin Acts as an Adjuvant for Systemic Immunization Data obtained in experimental mouse models suggest that microbiota-released flagellin (red diamonds) reaches systemic sites and provides, via TLR5, the costimulus that macrophages and B cells (green) need to induce antibodies to exogenously administered adjuvant-free TIV influenza vaccine antigen (blue triangles). The critical role of the microbiota-derived flagellin suggests that microbiota alterations might influence the efficacy of adjuvant-free vaccination strategies.
inactivated vaccine, or TIV) and a live attenuated influenza strain. Despite the effectiveness of TIV, it has remained unclear how the vaccine can elicit protective immunity in the absence of an adjuvant or infection. Adjuvants or complete infective microorganisms contain innate immune stimuli and provide the costimulus required for efficient activation of the adaptive immune system. Oh et al. now show that TIV responses are reduced in Toll-like receptor 5 (TLR5)-deficient mice, which are unable to respond to the bacterial innate immune stimulus flagellin, as well as in germ-free mice or mice that have undergone sustained treatment with antibiotics. Colonization of germfree mice with flagellin-expressing but not flagellin-mutated bacteria was able to revert the phenotype. Oh et al. suggest that this effect is mediated by TLR5 expression on immune cells such as B cells and macrophages at systemic sites. It therefore appears that bacterial flagellin derived from the microbiota reaches immune cells at systemic sites and provides the costimulus and adjuvant for the TIV vaccine (Figure 1).
Several members of the mammalian microbiota express flagella, and the structural subunit flagellin is constantly released. But how does flagellin produced in the gut lumen reach systemic sites? Although flagellin is as potent as lipopolysaccharide (LPS) for driving immunoglobulin synthesis in B cells in vitro, Oh et al. demonstrate that the absence of TLR5 is sufficient to impair the vaccine response. Many years ago, flagellin (at the time called proinflammatory factor, PIF) was shown to rapidly cross the polarized intestinal epithelium and concentrate at the basolateral site (Gewirtz et al., 2001). This is in contrast to LPS, which forms a steep gradient between gut lumen and the epithelial surface (Dupont et al., 2014). Nevertheless, several microbial stimuli reach the subepithelial tissue. Gut-derived TLR4 and TLR9 ligands, LPS and bacterial DNA, respectively, were shown to reach the liver via the portal vein and drive nonalcoholic fatty liver disease (NAFLD) in mice (Henao-Mejia et al., 2012). Although the liver efficiently removes exogenous molecules, some might evade this process and reach systemic sites. Indeed, gut-microbiotaderived peptidoglycan fragments can reach the bone marrow. However, the systemic spread of gut-derived flagellin has not been examined. Previous work suggested a differential effect of the skin and gut microbiota on skin- and gut-draining lymph nodes, respectively (Naik et al., 2012). In contrast, the work by Oh et al. suggests a direct effect of the gut microbiota on the differentiation or survival of plasma cells in the peripheral lymph nodes. Thus, depending on their precise anatomical location, lymph nodes might be affected by microbial products coming from different sources, including their respective draining region and the gut. Macrophages might be particularly suited to capture microbial products and thereby set the tone for on-going immune responses. As an alternative to direct spread, phagocytic cells might internalize microbiota constituents at the mucosal surface and transport them to systemic sites. Interestingly, in the gut lamina propria a myeloid TLR5-expressing cell population has been described as regulating IgA-secreting plasma cells (Uematsu et al., 2008). Medullary macrophages situated in close
350 Immunity 41, September 18, 2014 ª2014 Elsevier Inc.
proximity to recently generated plasma cells might perform a similar function in peripheral lymph nodes. The observations made by Oh et al. suggest that antibiotic treatment and intestinal dysbiosis might affect vaccination in humans. However, the degree of microbiota alteration in the chosen models most probably exceeds the effects of standard antibiotic regimens. Numerous gut-inhabiting bacteria, including members of the phylum Proteobacteria, can produce flagellae; others, such as the common commensal bacterium Bacteroides fragilis, lack synthesis and motility (Lozupone et al., 2012). Microbiota alterations might thus influence the luminal flagellin concentration and thereby alter the threshold of immune activation, including that at systemic sites. The stimulatory potential of different flagellin molecules produced differs substantially; for example, Campylobacter spp. or Helicobacter spp. (both ε-proteobacteria) produce only weakly stimulatory flagellin, whereas the g-proteobacteria Escherichia coli and Salmonella enterica produce highly potent flagellin (Andersen-Nissen et al., 2005). Antibiotics modulating the abundance of species producing stimulatory flagellin along with changes in overall colonization levels and integrity of the gut barrier could potentially affect the availability of TLR5 ligands in the periphery. Moreover, composition and density of the microbiota is regulated at various levels by innate and adaptive immune responses. TLR5-deficient mice exhibit a significantly altered microbiota (Vijay-Kumar et al., 2010). Also, changes in flagellarelated gene expression, elevated amounts of flagellin in the gut, and flagellated bacteria penetrating intestinal villi have been described in TLR5-deficient mice (Cullender et al., 2013). Thus, the effect of TLR5 deficiency might be more complex and at least in some aspects go beyond the effect of antibiotic treatment. Antibiotics differentially affecting distinct bacterial groups showed comparable effects of TIV antibody responses. This further emphasizes the idea that vaccination efficacy might not rely on distinct bacterial species exclusively but rather also on overall colonization levels, integrity of the gut barrier, and functional levels of microbial products in the periphery.
Immunity
Previews Interestingly, the effect of antibiotic treatment was limited to two nonadjuvated vaccines, TIV and another viral subunit vaccine to polio. In contrast, immunization with alum-absorbed toxoids, the HIVenv protein as well as a live-attenuated yellow fever vaccine, were unaffected. Thus, the microbiota has no detectable effect on the induced humoral response to adjuvanted vaccines or attenuated pathogens. Flagellin has long been known to represent a potent immunostimulatory molecule and useful adjuvant. The presented data now reveal an unexpected role of naturally produced flagellin derived from the microbiota to provide costimulation for immune activation. Thus, the intestinal microbiota might effectively take the function of a natural adjuvant. This view is consistent with lessons learned from gut immunology: the microbiota is an essential driver of ho-
meostasis and a key part of an effective immune system.
Eisenbarth, S.C., Jurczak, M.J., et al. (2012). Nature 482, 179–185, http://dx.doi.org/10.1038/ nature10809. Lozupone, C., Faust, K., Raes, J., Faith, J.J., Frank, D.N., Zaneveld, J., Gordon, J.I., and Knight, R. (2012). Genome Res. 22, 1974–1984.
REFERENCES Andersen-Nissen, E., Smith, K.D., Strobe, K.L., Barrett, S.L., Cookson, B.T., Logan, S.M., and Aderem, A. (2005). Proc. Natl. Acad. Sci. USA 102, 9247–9252. Cullender, T.C., Chassaing, B., Janzon, A., Kumar, K., Muller, C.E., Werner, J.J., Angenent, L.T., Bell, M.E., Hay, A.G., Peterson, D.A., et al. (2013). Cell Host Microbe 14, 571–581. Dupont, A., Kaconis, Y., Yang, I., Albers, T., Woltemate, S., Heinbockel, L., Andersson, M., Suerbaum, S., Brandenburg, K., and Hornef, M.W. (2014). Gut. Published online May 7, 2014. http:// dx.doi.org/10.1136/gutjnl-2014-307150. Gewirtz, A.T., Simon, P.O., Jr., Schmitt, C.K., Taylor, L.J., Hagedorn, C.H., O’Brien, A.D., Neish, A.S., and Madara, J.L. (2001). J. Clin. Invest. 107, 99–109. Henao-Mejia, J., Elinav, E., Jin, C., Hao, L., Mehal, W.Z., Strowig, T., Thaiss, C.A., Kau, A.L.,
Naik, S., Bouladoux, N., Wilhelm, C., Molloy, M.J., Salcedo, R., Kastenmuller, W., Deming, C., Quinones, M., Koo, L., Conlan, S., et al. (2012). Science 337, 1115–1119. Oh, J.Z., Ravindran, R., Chassaing, B., Carvalho, F.A., Maddur, M.S., Bower, M., Hakimpour, P., Gill, K.P., Nakaya, H.I., Yarovinsky, F., et al. (2014). Immunity 41, this issue, 478–492. Uematsu, S., Fujimoto, K., Jang, M.H., Yang, B.G., Jung, Y.J., Nishiyama, M., Sato, S., Tsujimura, T., Yamamoto, M., Yokota, Y., et al. (2008). Nat. Immunol. 9, 769–776. Vijay-Kumar, M., Aitken, J.D., Carvalho, F.A., Cullender, T.C., Mwangi, S., Srinivasan, S., Sitaraman, S.V., Knight, R., Ley, R.E., and Gewirtz, A.T. (2010). Science 328, 228–231. Published online Mar 4, 2010. http://dx.doi.org/10.1126/science. 1179721.
For Macrophages, Ndufs Is Enough Stanley Ching-Cheng Huang1 and Edward J. Pearce1,* 1Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, MO63110-1093, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.immuni.2014.09.004
Proinflammatory macrophage activation is coupled to a metabolic switch toward glycolysis. In Cell Metabolism, Jin et al. (2014) show that this process is negatively regulated by mitochondrial electron transport chain complex I through both cell intrinsic and extrinsic pathways. There is a link between anti-inflammatory properties and mitochondrial oxidative phosphorylation on the one hand, and classical activation and glycolytic metabolism on the other (O’Neill and Hardie, 2013). In their recent paper in Cell Metabolism, Jin and colleagues highlight the importance of complex I (CI) in regulating macrophage activation by showing that deletion of the CI component Ndufs4 causes a switch in the metabolic balance toward glycolysis and associated changes in the monocyte, macrophage, and osteoclast lineage that result in the development of inflammatory disease symptoms and abnormal bone density (Jin et al., 2014). The electron transport chain (ETC), which links nutrient oxidation to ATP
production by oxidative phosphorylation (OXPHOS), consists of complexes I–V in the mitochondrial inner membrane (Figure 1). CI is composed of >45 subunits and is highly regulated by pathways initiated by various extracellular signals (Papa et al., 2012). Unsurprisingly, mutations in genes encoding components of CI are associated with severe clinical disease (Papa et al., 2012). From an immunologic perspective, CI is interesting not only because of its core function in metabolism but because along with complex III, it is a site of reactive oxygen species (ROS) production (Sena and Chandel, 2012). ROS plays a role in a range of innate cell functions, and indeed Toll-like receptor (TLR) signaling
has been linked to increased production of mitochondrial ROS for bacterial killing (Sena and Chandel, 2012). However, ROS can be detrimental when produced in excess, and this can occur when CI function is compromised (Chen et al., 2007). Ndufs4 / mice have been used to model CI deficiency and have been found to die from encephalomyopathy approximately 7 weeks after birth (Kruse et al., 2008). Using these mice, Jin and colleagues discovered that by 3 weeks after birth, the absence of Ndufs4 led to systemic inflammation, apparent as increased levels of proinflammatory cytokines, systemically increased numbers of Ly6chi monocytes and the development
Immunity 41, September 18, 2014 ª2014 Elsevier Inc. 351
