Nature Reviews Microbiology | AOP, published online 15 December 2014; doi:10.1038/nrmicro3390

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Enemy attraction: bacterial agonists for leukocyte chemotaxis receptors Dominik Alexander Bloes, Dorothee Kretschmer and Andreas Peschel

Abstract | The innate immune system recognizes conserved microorganism-associated molecular patterns (MAMPs), some of which are sensed by G protein-coupled receptors (GPCRs), and this leads to chemotactic leukocyte influx. Recent studies have indicated that these processes are crucial for host defence and rely on a larger set of chemotactic MAMPs and corresponding GPCRs than was previously thought. Agonists, such as bacterial formyl peptides, enterococcal pheromone peptides, staphylococcal peptide toxins, bacterial fermentation products and the Helicobacter pylori peptide HP(2–20), stimulate specific GPCRs. The importance of leukocyte chemotaxis in host defence is highlighted by the fact that some bacterial pathogens produce chemotaxis inhibitors. How the various chemoattractants, receptors and antagonists shape antibacterial host defence represents an important topic for future research. Complement An innate immune defence system of >25 proteins that recognizes foreign objects and targets them for destruction or phagocytosis.

Leukotrienes A class of eicosanoids that are derived from arachidonic acid and have pro-inflammatory activities.

Phenol-soluble modulins (PSMs). Staphylococcal secreted peptides with a length of 20–44 amino acids that are potent bacterial agonists for formyl peptide receptor 2 (at nanomolar concentrations) and cytolytic toxins (at micromolar concentrations). Cellular and Molecular Microbiology Division, Interfaculty Institute of Microbiology and Infection Medicine, University of Tübingen, Tübingen 72076, Germany. Correspondence to A.P. e-mail: [email protected] uni-tuebingen.de doi:10.1038/nrmicro3390 Published online 15 December 2014

The initial detection and elimination of bacterial infections rely largely on the innate immune system, which depends on pre-formed receptors known as pattern recognition receptors (PRRs). These receptors sense bacterial molecules carrying evolutionarily conserved molecular motifs known as micro organism-ass o ciated molecular patterns (MAMPs)1. Activation of the innate immune system elicits immediate antimicrobial responses or initiates adaptive immunity if the pathogen cannot be easily eliminated. Along with the activation of resident immune cells, the detection of MAMPs causes the recruitment of leukocytes from the bloodstream and leads to local or disseminated inflammation (FIG. 1). MAMPs are also released by commensals but, by contrast, the release of MAMPs by intestinal commensal bacteria governs epithelial homeostasis and repair processes and does not lead to constant inflammation2–4. Several bacterial molecules, including lipopoly­ saccharide, di- or triacylated lipopeptides, peptidoglycan, flagellin and nucleic acids, are agonists for PRRs of the innate immune system1. The Toll-like receptor (TLR) family is responsible for detecting many of these MAMPs. However, although TLRs elicit the activation of, and mediator release by, immune cells, they contribute to leukocyte recruitment only indirectly by, for example, inducing the secretion of chemokines such as interleukin-8 (IL-8) 5. Chemokines, chemotactic complement products (for example, C5a and C3a6) and lipid mediators (for example, leukotrienes or plateletactivating factor 7,8) are sensed by members of the

seven-transmembrane spanning G protein-coupled receptor (GPCR) family that transduce signals via cytoplasmic G proteins to cytoskeletal proteins, which govern the directed movement of leukocytes9,10. The GPCR family includes >800 genes in humans. GPCRs usually have similar trans-membrane topologies but vary in the intracellular responses that are induced following binding of different G proteins. GPCRs have multiple roles ranging from the perception of light by rhodopsin-related proteins and the detection of small molecules by olfactory chemosensors to the detection of larger proteins such as chemokines by chemotaxis receptors11,12. Only some GPCRs are involved in leuko­ cyte chemotaxis. Although most GPCRs are known to bind endogenous ligands, the human formyl peptide receptor 1 (FPR1) and the related FPR2 as well as GPCRs 41 and 43 (GPR41 and GPR43; also known as free fatty acid receptor 3 (FFAR3) and 2 (FFAR2), respectively) can directly detect certain bacterial MAMPs and seem to have crucial roles in the innate immune response to bacterial infections and in immune homeostasis. FPR1 and FPR2 respond to very low concentrations of bacterial formyl peptides13,14 and phenol-soluble modulins (PSMs)15, respectively (FIG. 2); the function of a third FPR, FPR3, has remained largely elusive. GPR41 and GPR43 are activated by the short-chain fatty acids (SCFAs) acetate, propionate and butyrate, which are all products of bacterial fermentation16,17 (FIG. 2). The exact ‘molecular patterns’ that are required for the specific activation of MAMP-sensing GPCRs have

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Bacteria

Gradient of chemotactic MAMPs

Neutrophil Tissue

Bloodstream

Figure 1 | Recruitment of polymorphonuclear Nature Reviews | Microbiology neutrophils from the bloodstream.  Neutrophils sense bacterial chemotactic microorganism-associated molecular patterns (MAMPs) and migrate from the bloodstream across the endothelium towards the origin of the ligands in infected tissues along an increasing ligand gradient.

been only partially defined. Although FPR1 seems to respond almost exclusively to formyl peptides from bacteria, FPR2 also senses endogenous ligands in addition to PSMs, including fragments from amyloidogenic human proteins such as amyloid‑β(1–42) and prion protein fragment PrP(106–126), thus indicating an additional role for FPR2 in endogenous inflammatory processes such as Alzheimer’s disease and prion disease, respectively14. Although many studies over the past 15 years have addressed the roles of TLRs in infections, the roles of MAMP-sensing GPCRs in antibacterial host defence are still not well understood. Several previous reviews have discussed the immunomodulatory and pharmacological properties of synthetic formyl peptides and endogenous human GPCR ligands14,18–20. However, in recent years, some reports have indicated that the nature and diversity of chemotactic MAMPs and their cognate human receptors is more complex than was previously thought. In this Review, we describe the most recent advances and the current state of knowledge on bacterial leukocyteattracting molecules, the corresponding human receptors and their roles in antibacterial host defence. Vomeronasal sensory neurons An olfactory sensory sytem that detects chemical stimuli such as pheromones, which are produced during mating by many animals.

GPCRs with a PRR function FPR1 and FPR2 are expressed by neutrophils, whereas all three FPRs are expressed by macrophages, monocytes and immature dendritic cells14,20. GPR41 and GPR43 are found on neutrophils and monocytes16,17. FPR1, FPR2,

GPR41 and GPR43 are also expressed on epithelial cells, particularly in the gut21–23. Moreover, FPRs have been identified on dendrites of vomeronasal sensory neurons, which suggests that they may have olfactory functions associated with the detection of bacteria24. In humans, the genes for all these receptors are on chromosome 19, which is in close proximity to several genes that encode other GPCRs, such as the complement C5a receptor25–27. FPR1- and FPR2‑deficient mice are more susceptible than wild-type mice to systemic Listeria monocytogenes infection28,29 and to meningitis caused by Streptococcus pneumoniae30, which shows the crucial role of these receptors in bacterial infection. Moreover, human polymorphisms that compromise the function of human FPR1 are associated with severe courses of local perio­ dontitis infections31,32. Mammalian species differ largely with respect to the numbers and specificities of FPR paralogues20, which indicates that they are under strong selective pressure as a result of host–pathogen co-evolution. Therefore, mouse models can only partially reflect the roles of human MAMP-sensing GPCRs. GPCR stimulation leads to chemotaxis of neutrophils and has various additional consequences in these cells, including activation of the respiratory burst, release of granule contents and upregulation of surface molecules (FIG. 3). MAMP-sensing GPCRs are recruited from storage organelles inside the cell33,34 and transmit activation signals into the cell. After ligand recognition, the GPCRs undergo a conformational change that enables them to interact with their cognate G proteins. FPRs and SCFAsensing GPCRs are coupled to G proteins of the Gi subtype35,36, but there is evidence that additional types of G proteins might be involved37. The downstream events are relatively well established for FPR1 and FPR2. The downstream heterotrimeric G protein complex, which consists of Gα, Gβ and Gγ, dissociates into Gα and Gβγ following activation38. As a consequence, phospholipase Cβ2 (PLCβ2), phosphoinositide 3‑kinase (PI3K) and guanine nucleotide exchange factors (GEFs) are activated39,40. Moreover, calcium ions are released from the endoplasmic reticulum41 to elicit cell signalling42. PI3K and PLCβ2‑generated messenger molecules that derive from phosphatidylinositol 4,5‑biphosphate accumulate at the leading edge of neutrophils. Together with GEF-stimulated activation of small G proteins of the RHO family (RHO, RAC and CDC42), this process initiates actin polymerization and directs chemotactic neutrophil migration43. This process is also influenced by the activation of protein kinases, including protein kinase C (PKC), p38 and extracellular signal-regulated kinase (ERK)44–47. Activation of the respiratory burst involves the phosphorylation of subunits of the NADPH oxidase by PKC isoforms48, p38 and ERK49,50 and interactions with RAC49. Activation of the respiratory burst is accompanied by release of neutrophil granule components, such as myeloperoxidase, and the activation of nuclear factor-κB (NF‑κB), which initiates the expression of proinflammatory mediators such as IL‑8 (REF. 51). p38 is also responsible for the regulation of surface markers such as CD11b and CD66b45.

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REVIEWS Extracellular space Helicobacter pylori

All bacteria

Listeria monocytogenes

Staphylococcus aureus CoNS

Enterococcus faecalis Enterococcus faecium Formyl peptides

SCFAs Sex pheromones



Fermenting bacteria

FPR1

Gβ Gγ

Intracellular space

HP(2–20)

??? ???

PSMs



FPR2

Gβ Gγ



GPR41 or GPR43

Gβ Gγ

Chemotaxis IL-8 release Oxidative burst

Figure 2 | Induction of leukocyte chemotaxis.  Neutrophil chemotaxis is induced by the binding of bacterial Reviews | Microbiology chemoattractants to formyl peptide receptor 1 (FPR1), FPR2, G protein-coupled receptor 41Nature (GPR41) or GPR43. These G protein-coupled receptors are located in the cytoplasmic membrane of neutrophils and other leukocytes. FPR1 detects formyl peptides, which are released by all bacteria, and other ligands, such as sex pheromones from Enterococcus faecalis and Enterococcus faecium. FPR2 responds to certain α-helical, amphiphatic peptides released by pathogenic bacteria, only some of which have been identified so far, including the phenol-soluble modulin (PSM) peptides that are secreted by Staphylococcus aureus and coagulase-negative staphylococci (CoNS), and the HP(2–20) peptide from Helicobacter pylori. FPR2 is also involved in the recognition of unknown ligands from E. faecalis, E. faecium and Listeria monocytogenes. GPR41 and GPR43 detect short-chain fatty acids (SCFAs), which are produced by fermenting bacteria. All four G protein-coupled receptors induce chemotaxis of neutrophils and other leukocytes along increasing ligand gradients and induce activation of these cells to combat infections, including the activation of the oxidative burst and the release of interleukin-8 (IL-8).

After ligand binding and G protein dissociation, GPCRs are internalized and are either recycled and returned to the surface or degraded52. Some GPCRs can be desensitized by repeated exposure to cognate agonists because internalization leads to a strong decrease in the number of receptors at the cell surface. This process depends on GPCR phosphorylation and the protein arrestin53. GPCR desensitization can also lead to crossdesensitization of certain other GPCRs, causing, for example, a reduced response to IL‑8 or C5a following treatment with formyl peptides54. In addition, signal transduction cascades can influence each other on many levels, leading to complex and dynamic changes in the responsiveness of GPCRs and other types of receptors.

Formyl peptides The biosynthesis of proteins differs in bacteria and eukaryotes in one highly specific aspect: bacteria incorporate formylated methionine (fMet) at the amino terminus of all proteins, whereas eukaryotes use unmodified methionine (BOX 1). This characteristic difference enables the innate immune system to specifically detect bacterial molecules by the FPR1 receptor55. In many bacterial proteins the formyl group is post-translationally removed by deformylase56, but several proteins remain formylated.

Accordingly, formyl peptides with potent chemotactic activities, such as fMLF, fMIFL, fMIVIL or fMIGWI that result from the degradation of larger formylated proteins, have been detected in culture supernatants of several bacterial species, including Escherichia coli, Staphylococcus aureus, L. monocytogenes and Helicobacter pylori57–61 (TABLE 1). In addition, certain formylated mitochondrial peptides have been found to stimulate leukocytes62, which suggests that FPR1 may have a role in tissue injury as well as infection-associated inflammation63–65. The length and sequence of a formyl peptide has a strong influence on its ability to stimulate FPR1. Although formylated dipeptides have only weak activities, tripeptides or longer peptides can activate leukocytes at picomolar concentrations66. Studies with collections of synthetic peptides have revealed that the second amino acid position requires a hydrophobic side chain (for example, leucine, isoleucine or methionine) to achieve maximal activity. Hydrophobic properties are also important in the third amino acid position, whereas the properties of downstream amino acids seem to be less important66,67. The three-dimensional structure of FPR1 has yet to be determined, but mutational analyses suggest that the highly hydrophobic properties of peptide ligands disrupt a salt bridge in the extracellular loops of

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REVIEWS groups by disrupting pyruvate formate lyase had the opposite effect73. Together, these data confirm a crucial role for formyl peptides and FPR1 in bacterial infection.

Stimulus Extracellular space

Intracellular space



GPCR

Gβ Gγ

Ca2+

PLCβ2–DAG–PKC pathway

Phagolysosome

PI3K–AKT pathway

p38–ERK pathway

RHO pathway

Nucleus Chemotaxis Transcription modulation

Superoxide generation

Figure 3 | Activation of GPCRs on neutrophils.  Activation of G protein-coupled Nature Reviewsthe | Microbiology receptors (GPCRs) activates different intracellular pathways, including phospholipase Cβ2 (PLC β2)–diacylglyerol (DAG)–protein kinase C (PKC) pathway, the phosphoinositide 3‑kinase (PI3K)–AKT pathway, the p38–extracellular signal-regulated kinase (ERK) pathway and the RHO pathway. Activation of these pathways leads to multiple cellular responses, such as chemotaxis, Ca2+ influx, the generation of superoxides and the modulation of the transcription of genes that encode cytokines, which are crucial for host defence against invading bacteria.

FPR1 as a prerequisite for receptor activation68. Notably, human and mouse FPR1 differ with respect to the optimal agonist sequence (fMLF and fMIFL, respectively)69, which contributes to the notion that FPRs and bacterial ligands are under co-evolutionary selective pressure. According to this assumption, human or mouse bacterial pathogens may avoid different N‑terminal protein sequences to escape recognition by the host FPR1, a hypothesis that should be addressed by future studies. FPR2 and FPR3 also respond to formyl peptides, albeit at concentrations that are several magnitudes higher compared with FPR1 (REFS 13, 60), which suggests that these two receptors are not bona fide formyl peptide receptors and may have other principal agonists (see below). Although the chemotactic properties of formyl peptides have been known for decades, their importance in the host response to pathogens and commensals has only recently been addressed. Disruption of formyltransferase, the bacterial enzyme that generates fMet-tRNA in the main human pathogen S. aureus, led to strongly reduced capacities of bacterial culture supernatants to recruit neutrophils in vitro and in a mouse model70. This result demonstrates that S. aureus formyl peptides are indeed crucial for leukocyte influx and inflammation in the context of other MAMPs. Moreover, inhibition of deformylase by the antibiotic actinonin led to increased capacities of E. coli and S. aureus to stimulate neutrophils71,72, whereas a decrease in the number of formyl

Staphylococcal PSMs Staphylococci are Gram-positive bacteria that are natural inhabitants of the human skin and nasal cavity. In addition to the aggressive pathogen S. aureus, the genus includes opportunistic species and innocuous commensals. All staphylococci, except for the exclusive commensals, secrete PSM peptides74, which contribute substantially to virulence and inflammation15,75,76. A detailed review that highlights the role of PSMs in staphylococcal pathogenicity has recently been published77. Most strains produce collections of up to eight different PSMs with amino acid lengths that fall into two categories: 20–26 amino acids (α‑type PSMs) or 40–44 amino acids (β‑type PSMs)77 (TABLE 1). PSMs contain few conserved amino acid motifs but share α‑helical, amphiphatic structures74,77; they are secreted by the ATP-binding cassette (ABC) transporter Pmt and retain their N‑terminal methionine78. Although most PSMs are secreted as formyl peptides76, they are very weak FPR1 agonists, probably because their second and third amino acids are unfavourable for FPR1 stimulation15,66. At high (micromolar) concentrations, PSMs are potent leuko­cidins that disrupt the cytoplasmic membranes of eukary­otic cells15,76, and the level of PSM release corresponds to the virulence of a particular strain74,76. PSMs preferentially target apoptotic neutrophils, and the release of myeloperoxidase from dead neutrophils is a defence mechanism that inactivates PSMs79. Furthermore, PSMs not only damage human host cells but also competing bacteria because proteolytic processing generates truncated PSM variants, some of which have antimicrobial activity80,81. At low (nanomolar) concentrations, PSMs are chemo­ attractants and activators for neutrophils, monocytes and immature dendritic cells and are highly potent agonists of FPR2 (REFS 15,76,82). N‑terminal formylation increases PSM activity but is not essential for the ability of the peptides to bind to FPR2 (REF. 15). Disruption of PSM genes in S. aureus or inhibition of FPR2 by the synthetic inhibitor WRW4 strongly reduces the influx of leukocytes in local S. aureus infections, which shows that PSMs have a crucial role in leukocyte chemotaxis in addition to formyl peptides15,76. PSMs represent the first secreted bacterial ligands for FPR2. The fact that PSM levels correspond to staphylococcal virulence74 suggests that PSMs are true PAMPs that enable the innate immune system to initiate appropriate immune responses via FPR2. Mast cell degranulation can be elicited by δ‑toxin, an α‑type PSM83, and polymorphisms in FPR2 are associated with urticaria84 and asthma85,86, which suggests that PSM–FPR2 interactions may also be important co‑triggers of allergies. Enterococcal pheromone peptides Enterococcus faecalis and Enterococcus faecium are opportunistic pathogens that colonize the human intestine and cause infections, particularly in patients

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REVIEWS Box 1 | Formylation and deformylation of bacterial proteins Both eukaryotes and bacteria initiate protein biosynthesis with a dedicated methionine start tRNA, but only bacteria modify the aminoacylated start tRNA with a formyl group via formyl transferase (FMT)137. The formyl group is provided by formyl tetrahydrofolic acid (THF)138 (see the figure). It remains unclear why bacteria depend on formylated methionine. FMT is not essential in several bacteria, but FMT mutants have impaired growth characteristics and metabolic activities139,140 because the functions of certain proteins may depend on neutralization of the amino‑terminal positive charge that occurs as a consequence of methionine formylation. Accordingly, a Staphylococcus aureus FMT mutant had reduced pyruvate dehydrogenase activity despite the levels of expression of the pyruvate dehydrogenase gene being unaltered141. Although proteins such as pyruvate dehydrogenase may depend on the presence of an N‑terminal formyl group, many other proteins must be deformylated to be functional; for example, most Bacillus subtilis proteins are deformylated57. Deformylase is an essential enzyme in many bacteria and represents a promising target for a new class of antibiotics142. However, deformylase inhibition increases the release of formylated proteins and peptides and, consequently, the activation of immune cells via formyl peptide receptor 1 (FPR1)71,72. Met

formyl-Met

FMT

formyl-THF

THF Nature Reviews | Microbiology

Hydrophobic moment A measure of the amphiphilicity of a protein helix.

who are hospitalized and immunocompromised87. The enterococcal sex pheromone peptides that govern the conjugation process between plasmid-free and plasmidbearing strains88,89 are released by recipient strains to initiate DNA transfer by plasmid donor strains88. These pheromone peptides are seven to eight amino acids in length and arise from the processing of larger precursors, such as lipoprotein signal peptide (cAM373 phero­ mone90; TABLE 1). Enterococcal peptide pheromones do

not contain an N‑terminal methionine or a formyl group but they can stimulate FPR1 in micromolar concentrations, which results in the directed influx of neutrophils91,92. Two pheromones, cPD1 and cAM373, have chemoattractant activities, which are, however, much weaker than those of many formyl peptides91. Conjugative genetic elements that encode cAM373 are also found in certain strains of S. aureus90, but it is not clear whether these elements also produce pheromone peptides with chemotactic activity. How important the pheromone peptides may be in addition to formyl peptides in the stimulation of FPR1, and how this process may affect human colonization or infection by enterococci remains to be explored.

H. pylori chemotactic peptide HP(2–20) H. pylori is a Gram-negative gastric colonizer with an ability to cause inflammation that contributes to the development of gastric ulcers and gastric cancer93,94. It is possible that the bacteria may benefit from the induction of the host pro-inflammatory responses95. The HP(2–20) peptide, which corresponds to the N‑terminal sequence of the ribosomal protein L1 without methionine at the first amino acid position, can trigger chemotaxis of human neutrophils, basophils and monocytes96,97 by interacting with FPR2 at micromolar concentrations and, much less efficiently, with FPR3 (REFS 96,97). Interestingly, HP(2–20) resembles PSMs in terms of length, activity and hydrophobic moment (TABLE 1). It is possible that HP(2–20) reaches the extracellular space by auto­ lysis, which is a process that leads to the release of many cytoplasmic proteins in H. pylori98,99. Proteins released by autolysis can retain their formylated N‑termini, which is a probable reason for previous reports on chemo­attractant activities of H. pylori proteins, such as urease100 and an intracellular iron storage protein, the neutrophil-activating protein (HP‑NAP)101.

Table 1 | Bacteria-derived chemotactic peptide ligands Producing organism Name All bacteria

Amino acid sequence

Formylated peptides fMxx…‡

Hydrophobic Principal moment (µH)* receptor

Refs

N.D.

FPR1

67

Staphylococcus aureus PSMα1

fMGIIAGIIKVIKSLIEQFTGK

0.661

FPR2

15

PSMα2

fMGIIAGIIKFIKGLIEKFTGK

0.676

FPR2

15

PSMα3

fMEFVAKLFKFFKDLLGKFLGNN

0.715

FPR2

15

PSMα4

fMAIVGTIIKIIKAIIDIFAK

0.624

FPR2

15

δ-toxin

fMAQDIISTISDLVKWIIDTVNKFTKK

0.584

FPR2

15

PSMβ1

fMEGLFNAIKDTVTAAINNDGAKLGTSIVSIVENGVGLLGKLFGF

0.493

FPR2

15

PSMβ2

fMTGLAEAIANTVQAAQQHDSVKLGTSIVDIVANGVGLLGKLFGF 0.385

FPR2

15

PSMmec

fMDFTGVITSIIDLIKTCIQAFG

0.543

FPR2

75

cAM373

AIFILAS

ND

FPR1

91

cPD1

FLVMFLSG

ND

FPR1

91

HP(2–20)

AKKVFKRLEKLFSKIQNDK

0.676

FPR2

96

Enterococcus faecalis Helicobacter pylori

FPR, formyl peptide receptor; ND, not determined ; PSM, phenol-soluble modulin. *Predicted using the online application HeliQuest. Only peptides with at least 18 amino acids were considered. ‡Highest activity was demonstrated with peptides with hydrophobic amino acids (x) at positions two and three; for example, in fMLF, fMIVIL or fMIVTLF.

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REVIEWS Box 2 | MAMP-sensing GPCRs on intestinal epithelial cells Microorganism-associated molecular pattern (MAMP)-sensing G protein-coupled receptors (GPCRs) such as formyl peptide receptor 1 (FPR1), FPR2, GPR41 and GPR43 are not only found on leukocytes but are also on the apical or lateral sides of intestinal epithelial cells21,143,144, which indicates that these receptors have other roles in addition to the recruitment and activation of immune cells. Intestinal epithelial cells are constantly exposed to MAMPs released by commensal bacteria of the intestinal microbiota and can sense changes in MAMPs. The intestinal microbiota has crucial metabolic and immunomodulatory functions, and several chronic disorders ranging from obesity to inflammatory diseases such as Crohn’s disease145 or ulcerative colitis146, and cancer are influenced by changes in microbiota composition. Invasion of pathogens can drive the microbiota into an imbalanced state known as dysbiosis, which is linked to gastrointestinal inflammation147. Interestingly, non-pathogenic bacteria are not only involved but are necessary for maintenance of gut homeostasis, as FPR1‑induced extracellular signal-regulated kinase (ERK) signalling is believed to influence epithelial cell physiology3,4. By contrast, pro-apoptotic pathways, such as c‑Jun N-terminal kinase (JNK) or nuclear factor‑κB (NF‑κB), do not seem to be initiated by commensals but by pathogens4,148,149. Recent studies have indicated how subtle changes in the diverse collections of MAMPs may contribute to microbiota-associated diseases. For example, FPR1, FPR2 and GPR43 influence epithelial barrier function and repair21,150,151. Their influence on epithelial cell proliferation implicates these receptors in the development of intestinal tumours. Indeed, deletion of Fpr2 in mice leads to increased colonic tumorigenesis152. Moreover, GPR43 (the interaction of which with short-chain fatty acids (SCFAs) is tumour-suppressive) is lost in colon cancer cells153,154. High expression of FPR1 was recently associated with poor overall survival in human patients with gastric cancer155. Stimulation of MAMP-sensing GPCRs on intestinal epithelial cells leads to release of pro- and anti-inflammatory mediators that contribute to inflammatory disorders3. Moreover, GPR43 can trigger the secretion of hormones in epithelial cells that govern obesity and diabetes156.

Bacterial SCFAs Most bacterial commensals and pathogens are facul­ tative anaerobes that produce a range of fermentation products during growth in the absence of oxygen. Although bacterial fermentation pathways are diverse, many lead to the production of SCFAs, such as acetate, propionate and butyrate102, which can reach concentrations above 100 mM in the gastrointestinal tract103. SCFAs are agonists for GPR41 and GPR43 and attract neutrophils16,17,104. GPR41 and GPR43 are related to other GPCRs that sense nutritional medium- and longchain fatty acids and govern metabolic activities in the gut105. It is probable that GPR41 and GPR43 evolved from such receptors to adopt crucial functions in innate immunity. Monocytes respond to SCFAs by the release of prostaglandin E2, which has anti-inflammatory effects106. When produced in the gut lumen, SCFAs are small enough to diffuse easily through tissues and reach the bloodstream. Subsequently, they seem to have systemic effects and influence haematopoiesis in the bone marrow107, thereby influencing the course of chronic inflammatory diseases, such as colitis, arthritis and asthma3. GPR43 seems to be the dominant SCFA receptor on neutrophils, and GPR43‑deficient mice exhibit diminished intestinal neutrophil invasion and increased mortality owing to septic complications in acute colitis, which indicates a crucial role for GPR43 in intestinal infection and inflammation108 (BOX 2). Whether GPR43 is crucial in infection of other organs and in defence against non-intestinal pathogens remains to be investigated.

Bacterial inhibitors of chemotaxis The fact that bacterial pathogens produce potent inhibitors of leukocyte chemotaxis highlights the importance of chemotactic bacterial ligands in antibacterial host defence and the need of the pathogens to interfere with this process. For instance, Haemophilus influenzae releases a small (

Enemy attraction: bacterial agonists for leukocyte chemotaxis receptors.

The innate immune system recognizes conserved microorganism-associated molecular patterns (MAMPs), some of which are sensed by G protein-coupled recep...
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