Identification of Icm protein complexes that play distinct roles in the biogenesis of an organelle permissive for Legionella pneumophila intracellular growth

The Legionella IcmSW complex directly interacts with DotL to mediate translocation of adaptor-dependent substrates

Legionella pneumophila is a Gram-negative bacterium that replicates within human alveolar macrophages by evasion of the host endocytic pathway through the formation of a replicative vacuole. Generation of this vacuole is dependent upon the secretion of over 275 effector proteins into the host cell via the Dot/Icm type IVB secretion system (T4SS). The type IV coupling protein (T4CP) subcomplex, consisting of DotL, DotM, DotN, IcmS and IcmW, was recently defined. DotL is proposed to be the T4CP of the L. pneumophila T4SS based on its homology to known T4CPs, which function as inner-membrane receptors for substrates. As a result, DotL is hypothesized to play an integral role(s) in the L. pneumophila T4SS for the engagement and translocation of substrates. To elucidate this role, a genetic approach was taken to screen for dotL mutants that were unable to survive inside host cells. One mutant, dotLY725Stop, did not interact with the type IV adaptor proteins IcmS/IcmW (IcmSW) leading to the identification of an IcmSW-binding domain on DotL. Interestingly, the dotLY725Stop mutant was competent for export of one class of secreted effectors, the IcmSW-independent substrates, but exhibited a specific defect in secretion of IcmSW-dependent substrates. This differential secretion illustrates that DotL requires a direct interaction with the type IV adaptor proteins for the secretion of a major class of substrates. Thus, by identifying a new target for IcmSW, we have discovered that the type IV adaptors perform an additional role in the export of substrates by the L. pneumophila Dot/Icm T4SS.

Identification of the DotL coupling protein subcomplex of the Legionella Dot/Icm type IV secretion system

Legionella pneumophila, the causative agent of Legionnaires' disease, survives in macrophages by altering the endocytic pathway of its host cell. To accomplish this, the bacterium utilizes a type IVB secretion system to deliver effector molecules into the host cell cytoplasm. In a previous report, we performed an extensive characterization of the L. pneumophila type IVB secretion system that resulted in the identification of a critical five-protein subcomplex that forms the core of the secretion apparatus. Here we describe a second Dot/Icm protein subassembly composed of the type IV coupling protein DotL, the apparatus proteins DotM and DotN, and the secretion adaptor proteins IcmS and IcmW. In the absence of IcmS or IcmW, DotL becomes destabilized at the transition from the exponential to stationary phases of growth, concurrent with the expression of many secreted substrates. Loss of DotL is dependent on ClpA, a regulator of the cytoplasmic protease ClpP. The resulting decreased levels of DotL in the icmS and icmW mutants exacerbates the intracellular defects of these strains and can be partially suppressed by overproduction of DotL. Thus, in addition to their role as chaperones for Legionella type IV secretion system substrates, IcmS and IcmW perform a second function as part of the Dot/Icm type IV coupling protein subcomplex.

Bacterial Type IV secretion systems: versatile virulence machines

Many bacterial pathogens employ multicomponent protein complexes to deliver macromolecules directly into their eukaryotic host cell to promote infection. Some Gram-negative pathogens use a versatile Type IV secretion system (T4SS) that can translocate DNA or proteins into host cells. T4SSs represent major bacterial virulence determinants and have recently been the focus of intense research efforts designed to better understand and combat infectious diseases. Interestingly, although the two major classes of T4SSs function in a similar manner to secrete proteins, the translocated 'effectors' vary substantially from one organism to another. In fact, differing effector repertoires likely contribute to organism-specific host cell interactions and disease outcomes. In this review, we discuss the current state of T4SS research, with an emphasis on intracellular bacterial pathogens of humans and the diverse array of translocated effectors used to manipulate host cells.

Caspase-11 promotes the fusion of phagosomes harboring pathogenic bacteria with lysosomes by modulating actin polymerization

Inflammasomes are multiprotein complexes that include members of the NLR (nucleotide-binding domain leucine-rich repeat containing) family and caspase-1. Once bacterial molecules are sensed within the macrophage, the inflammasome is assembled, mediating the activation of caspase-1. Caspase-11 mediates caspase-1 activation in response to lipopolysaccharide and bacterial toxins, and yet its role during bacterial infection is unknown. Here, we demonstrated that caspase-11 was dispensable for caspase-1 activation in response to Legionella, Salmonella, Francisella, and Listeria. We also determined that active mouse caspase-11 was required for restriction of L. pneumophila infection. Similarly, human caspase-4 and caspase-5, homologs of mouse caspase-11, cooperated to restrict L. pneumophila infection in human macrophages. Caspase-11 promoted the fusion of the L. pneumophila vacuole with lysosomes by modulating actin polymerization through cofilin. However, caspase-11 was dispensable for the fusion of lysosomes with phagosomes containing nonpathogenic bacteria, uncovering a fundamental difference in the trafficking of phagosomes according to their cargo.

The Legionella pneumophila effector DrrA is sufficient to stimulate SNARE-dependent membrane fusion

The intracellular bacterial pathogen Legionella pneumophila subverts host membrane transport pathways to promote fusion of vesicles exiting the endoplasmic reticulum (ER) with the pathogen-containing vacuole. During infection there is noncanonical pairing of the SNARE protein Sec22b on ER-derived vesicles with plasma membrane (PM)-localized syntaxin proteins on the vacuole. We show that the L. pneumophila Rab1-targeting effector DrrA is sufficient to stimulate this noncanonical SNARE association and promote membrane fusion. DrrA activation of the Rab1 GTPase on PM-derived organelles stimulated the tethering of ER-derived vesicles with the PM-derived organelle, resulting in vesicle fusion through the pairing of Sec22b with the PM syntaxin proteins. Thus, the effector protein DrrA stimulates a host membrane transport pathway that enables ER-derived vesicles to remodel a PM-derived organelle, suggesting that Rab1 activation at the PM is sufficient to promote the recruitment and fusion of ER-derived vesicles.

Coxiella burnetii secretion systems

The ability of bacteria to transport proteins across their membranes is integral for interaction with their environment. Distinct families of secretion systems mediate bacterial protein secretion. The human pathogen, Coxiella burnetii encodes components of the Sec-dependent secretion pathway, an export system used for type IV pilus assembly, and a complete type IV secretion system. The type IVB secretion system in C. burnetii is functionally analogous to the Legionella pneumophila Dot/Icm secretion system. Both L. pneumophila and C. burnetii require the Dot/Icm apparatus for intracellular replication. The Dot/Icm secretion system facilitates the translocation of many bacterial effector proteins across the bacterial and vacuole membranes to enter the host cytoplasm where the effector proteins mediate their specific activities to manipulate a variety of host cell processes. Several studies have identified cohorts of C. burnetii Dot/Icm effector proteins that are predicted to be involved in modulation of host cell functions. This chapter focuses specifically on these secretion systems and the role they may play during C. burnetii replication in eukaryotic host cells.

Dissection of a type I interferon pathway in controlling bacterial intracellular infection in mice

Defence mechanisms against intracellular bacterial pathogens are incompletely understood. Our study characterizes a type I IFN-dependent cell-autonomous defence pathway directed against Legionella pneumophila, an intracellular model organism and frequent cause of pneumonia. We show that macrophages infected with L. pneumophila produced IFNβ in a STING- and IRF3- dependent manner. Paracrine type I IFNs stimulated upregulation of IFN-stimulated genes and a cell-autonomous defence pathway acting on replicating and non-replicating Legionella within their specialized vacuole. Our infection experiments in mice lacking receptors for type I and/or II IFNs show that type I IFNs contribute to expression of IFN-stimulated genes and to bacterial clearance as well as resistance in L. pneumophila pneumonia in addition to type II IFN. Overall, our study shows that paracrine type I IFNs mediate defence against L. pneumophila, and demonstrates a protective role of type I IFNs in in vivo infections with intracellular bacteria.

Type IVB Secretion Systems of Legionella and Other Gram-Negative Bacteria

Type IV secretion systems (T4SSs) play a central role in the pathogenicity of many important pathogens, including Agrobacterium tumefaciens, Helicobacter pylori, and Legionella pneumophila. The T4SSs are related to bacterial conjugation systems, and are classified into two subgroups, type IVA (T4ASS) and type IVB (T4BSS). The T4BSS, which is closely related to conjugation systems of IncI plasmids, was originally found in human pathogen L. pneumophila; pathogenesis by L. pneumophila infection requires functional Dot/Icm T4BSS. A zoonotic pathogen, Coxiella burnetii, and an arthropod pathogen, Rickettsiella grylli – both of which carry T4BSSs highly similar to the Legionella Dot/Icm system – are evolutionarily closely related and comprise a monophyletic group. A growing body of bacterial genomic information now suggests that T4BSSs are not limited to Legionella and related bacteria and IncI plasmids. Here, we review the current knowledge on T4BSS apparatus and component proteins, gained mainly from studies on L. pneumophila Dot/Icm T4BSS. Recent structural studies, along with previous findings, suggest that the Dot/Icm T4BSS contains components with primary or higher-order structures similar to those in other types of secretion systems – types II, III, IVA, and VI, thus highlighting the mosaic nature of T4BSS architecture.

The pattern recognition receptors Nod1 and Nod2 account for neutrophil recruitment to the lungs of mice infected with Legionella pneumophila

The intracellular bacterium Legionella pneumophila induces a severe form of pneumonia called Legionnaires diseases, which is characterized by a strong neutrophil (NE) infiltrate to the lungs of infected individuals. Although the participation of pattern recognition receptors, such as Toll-like receptors, was recently demonstrated, there is no information on the role of nod-like receptors (NLRs) for bacterial recognition in vivo and for NE recruitment to the lungs. Here, we employed a murine model of Legionnaires disease to evaluate host and bacterial factors involved in NE recruitment to the mice lungs. We found that L. pneumophila type four secretion system, known as Dot/Icm, was required for NE recruitment as dot/icm mutants fail to trigger NE recruitment in a process independent of bacterial multiplication. By using mice deficient for Nod1, Nod2, and Rip2, we found that these receptors accounted for NE recruitment to the lungs of infected mice. In addition, Rip2-dependent responses were important for cytokine production and bacterial clearance. Collectively, these studies show that Nod1, Nod2, and Rip2 account for generation of innate immune responses in vivo, which are important for NE recruitment and bacterial clearance in a murine model of Legionnaires diseases.

Legionella pneumophila promotes functional interactions between plasma membrane syntaxins and Sec22b

Biogenesis of a specialized organelle that supports intracellular replication of Legionella pneumophila involves the fusion of secretory vesicles exiting the endoplasmic reticulum (ER) with phagosomes containing this bacterial pathogen. Here, we investigated host plasma membrane SNARE proteins to determine whether they play a role in trafficking of vacuoles containing L. pneumophila. Depletion of plasma membrane syntaxins by RNA interference resulted in delayed acquisition of the resident ER protein calnexin and enhanced retention of Rab1 on phagosomes containing virulent L. pneumophila, suggesting that these SNARE proteins are involved in vacuole biogenesis. Plasma membrane-localized SNARE proteins syntaxin 2, syntaxin 3, syntaxin 4 and SNAP23 localized to vacuoles containing L. pneumophila. The ER-localized SNARE protein Sec22b was found to interact with plasma membrane SNAREs on vacuoles containing virulent L. pneumophila, but not on vacuoles containing avirulent mutants of L. pneumophila. The addition of alpha-SNAP and N-ethylmaleimide-sensitive factor (NSF) to the plasma membrane SNARE complexes formed by virulent L. pneumophila resulted in the dissociation of Sec22b, indicating functional pairing between these SNAREs. Thus, L. pneumophila stimulates the non-canonical pairing of plasma membrane t-SNAREs with the v-SNARE Sec22b to promote fusion of the phagosome with ER-derived vesicles. The mechanism by which L. pneumophila promotes pairing of plasma membrane syntaxins and Sec22b could provide unique insight into how the secretory vesicles could provide an additional membrane reserve subverted during phagosome maturation.

Pore formation triggered by Legionella spp. is an Nlrc4 inflammasome-dependent host cell response that precedes pyroptosis

Legionella pneumophila, the etiological agent of Legionnaires disease, is known to trigger pore formation in bone marrow-derived macrophages (BMMs) by mechanisms dependent on the type IVB secretion system known as Dot/Icm. Here, we used several mutants of L. pneumophila in combination with knockout mice to assess the host and bacterial factors involved in pore formation in BMMs. We found that regardless of Dot/Icm activity, pore formation does not occur in BMMs deficient in caspase-1 and Nlrc4/Ipaf. Pore formation was temporally associated with interleukin-1beta secretion and preceded host cell lysis and pyroptosis. Pore-forming ability was dependent on bacterial Dot/Icm but independent of several effector proteins, multiplication, and de novo protein synthesis. Flagellin, which is known to trigger the Nlrc4 inflammasome, was required for pore formation as flaA mutant bacteria failed to induce cell permeabilization. Accordingly, transfection of purified flagellin was sufficient to trigger pore formation independent of infection. By using 11 different Legionella species, we found robust pore formation in response to L. micdadei, L. bozemanii, L. gratiana, L. jordanis, and L. rubrilucens, and this trait correlated with flagellin expression by these species. Together, the results suggest that pore formation is neither L. pneumophila specific nor the result of membrane damage induced by Dot/Icm activity; instead, it is a highly coordinated host cell response dependent on host Nlrc4 and caspase-1 and on bacterial flagellin and type IV secretion system.

Modulation of host cell function by Legionella pneumophila type IV effectors

Macrophages and protozoa ingest bacteria by phagocytosis and destroy these microbes using a conserved pathway that mediates fusion of the phagosome with lysosomes. To survive within phagocytic host cells, bacterial pathogens have evolved a variety of strategies to avoid fusion with lysosomes. A virulence strategy used by the intracellular pathogen Legionella pneumophila is to manipulate host cellular processes using bacterial proteins that are delivered into the cytosolic compartment of the host cell by a specialized secretion system called Dot/Icm. The proteins delivered by the Dot/Icm system target host factors that play evolutionarily conserved roles in controlling membrane transport in eukaryotic cells, which enables L. pneumophila to create an endoplasmic reticulum-like vacuole that supports intracellular replication in both protozoan and mammalian host cells. This review focuses on intracellular trafficking of L. pneumophila and describes how bacterial proteins contribute to modulation of host processes required for survival within host cells.

A host type I interferon response is induced by cytosolic sensing of the bacterial second messenger cyclic-di-GMP

The innate immune system responds to unique molecular signatures that are widely conserved among microbes but that are not normally present in host cells. Compounds that stimulate innate immune pathways may be valuable in the design of novel adjuvants, vaccines, and other immunotherapeutics. The cyclic dinucleotide cyclic-di-guanosine monophosphate (c-di-GMP) is a recently appreciated second messenger that plays critical regulatory roles in many species of bacteria but is not produced by eukaryotic cells. In vivo and in vitro studies have previously suggested that c-di-GMP is a potent immunostimulatory compound recognized by mouse and human cells. We provide evidence that c-di-GMP is sensed in the cytosol of mammalian cells via a novel immunosurveillance pathway. The potency of cytosolic signaling induced by c-di-GMP is comparable to that induced by cytosolic delivery of DNA, and both nucleic acids induce a similar transcriptional profile, including triggering of type I interferons and coregulated genes via induction of TBK1, IRF3, nuclear factor kappaB, and MAP kinases. However, the cytosolic pathway that senses c-di-GMP appears to be distinct from all known nucleic acid-sensing pathways. Our results suggest a novel mechanism by which host cells can induce an inflammatory response to a widely produced bacterial ligand.

NOD-like receptors: role in innate immunity and inflammatory disease

The NOD-like receptors (NLRs) are a specialized group of intracellular receptors that represent a key component of the host innate immune system. Since the discovery of the first NLR almost 10 years ago, the study of this special class of microbial sensors has burgeoned; consequently, a better understanding of the mechanism by which these receptors recognize microbes and other danger signals and of how they activate inflammatory signaling pathways has emerged. Moreover, in addition to their primary role in host defense against invading pathogens, their ability to regulate nuclear factor-kappa B (NF-kappaB) signaling, interleukin-1-beta (IL-1beta) production, and cell death indicates that they are crucial to the pathogenesis of a variety of inflammatory human diseases.

Structure and function of interacting IcmR-IcmQ domains from a type IVb secretion system in Legionella pneumophila

During infection, Legionella pneumophila creates a replication vacuole within eukaryotic cells and this requires a Type IVb secretion system (T4bSS). IcmQ plays a critical role in the translocase and associates with IcmR. In this paper, we show that the N-terminal domain of IcmQ (Qn) mediates self-dimerization, whereas the C-terminal domain with a basic linker promotes membrane association. In addition, the binding of IcmR to IcmQ prevents self-dimerization and also blocks membrane permeabilization. However, IcmR does not completely block membrane binding by IcmQ. We then determined crystal structures of Qn with the interacting region of IcmR. In this complex, each protein forms an alpha-helical hairpin within a parallel four-helix bundle. The amphipathic nature of helices in Qn suggests two possible models for membrane permeabilization by IcmQ. The Rm-Qn structure also suggests how IcmR-like proteins in other L. pneumophila species may interact with their IcmQ partners.

A Legionella pneumophila effector protein encoded in a region of genomic plasticity binds to Dot/Icm-modified vacuoles

Legionella pneumophila is an opportunistic pathogen that can cause a severe pneumonia called Legionnaires' disease. In the environment, L. pneumophila is found in fresh water reservoirs in a large spectrum of environmental conditions, where the bacteria are able to replicate within a variety of protozoan hosts. To survive within eukaryotic cells, L. pneumophila require a type IV secretion system, designated Dot/Icm, that delivers bacterial effector proteins into the host cell cytoplasm. In recent years, a number of Dot/Icm substrate proteins have been identified; however, the function of most of these proteins remains unknown, and it is unclear why the bacterium maintains such a large repertoire of effectors to promote its survival. Here we investigate a region of the L. pneumophila chromosome that displays a high degree of plasticity among four sequenced L. pneumophila strains. Analysis of GC content suggests that several genes encoded in this region were acquired through horizontal gene transfer. Protein translocation studies establish that this region of genomic plasticity encodes for multiple Dot/Icm effectors. Ectopic expression studies in mammalian cells indicate that one of these substrates, a protein called PieA, has unique effector activities. PieA is an effector that can alter lysosome morphology and associates specifically with vacuoles that support L. pneumophila replication. It was determined that the association of PieA with vacuoles containing L. pneumophila requires modifications to the vacuole mediated by other Dot/Icm effectors. Thus, the localization properties of PieA reveal that the Dot/Icm system has the ability to spatially and temporally control the association of an effector with vacuoles containing L. pneumophila through activities mediated by other effector proteins.

The Legionella pneumophila replication vacuole: making a cosy niche inside host cells

The pathogenesis of Legionella pneumophila is derived from its growth within lung macrophages after aerosols are inhaled from contaminated water sources. Interest in this bacterium stems from its ability to manipulate host cell vesicular-trafficking pathways and establish a membrane-bound replication vacuole, making it a model for intravacuolar pathogens. Establishment of the replication compartment requires a specialized translocation system that transports a large cadre of protein substrates across the vacuolar membrane. These substrates regulate vesicle traffic and survival pathways in the host cell. This Review focuses on the strategies that L. pneumophila uses to establish intracellular growth and evaluates why this microorganism has accumulated an unprecedented number of translocated substrates that are targeted at host cells.

Free-living freshwater amoebae differ in their susceptibility to the pathogenic bacterium Legionella pneumophila

Legionella pneumophila is known as a facultative intracellular parasite of free-living soil and freshwater amoebae, of which several species have been shown to support the growth of the pathogenic bacteria. We report for the first time the behaviour of two strains (c2c and Z503) of the amoeba Willaertia magna towards different strains of L. pneumophila serogroup 1 and compared it with Acanthamoeba castellanii and Hartmannella vermiformis, known to be L. pneumophila permissive. In contrast to the results seen with other amoebae, W. magna c2c inhibited the growth of one strain of Legionella (L. pneumophila, Paris), but not of others belonging to the same serogroup (L. pneumophila, Philadelphia and L. pneumophila, Lens). Also, the different L. pneumophila inhibited cell growth and induced cell death in A. castellanii, H. vermiformis and W. magna Z503 within 3-4 days while W. magna c2c strain remained unaffected even up to 7 days. Electron microscopy demonstrated that the formation of numerous replicative phagosomes observed within Acanthamoeba and Hartmannella is rarely seen in W. magna c2c cocultured with L. pneumophila. Moreover, the morphological differences were observed between L. pneumophila cultured either with Willaertia or other amoebae. These observations show that amoebae are not all equally permissive to L. pneumophila and highlight W. magna c2c as particularly resistant towards some strains of this bacterium.

Type IV secretion-dependent activation of host MAP kinases induces an increased proinflammatory cytokine response to Legionella pneumophila

The immune system must discriminate between pathogenic and nonpathogenic microbes in order to initiate an appropriate response. Toll-like receptors (TLRs) detect microbial components common to both pathogenic and nonpathogenic bacteria, whereas Nod-like receptors (NLRs) sense microbial components introduced into the host cytosol by the specialized secretion systems or pore-forming toxins of bacterial pathogens. The host signaling pathways that respond to bacterial secretion systems remain poorly understood. Infection with the pathogen Legionella pneumophila, which utilizes a type IV secretion system (T4SS), induced an increased proinflammatory cytokine response compared to avirulent bacteria in which the T4SS was inactivated. This enhanced response involved NF-kappaB activation by TLR signaling as well as Nod1 and Nod2 detection of type IV secretion. Furthermore, a TLR- and RIP2-independent pathway leading to p38 and SAPK/JNK MAPK activation was found to play an equally important role in the host response to virulent L. pneumophila. Activation of this MAPK pathway was T4SS-dependent and coordinated with TLR signaling to mount a robust proinflammatory cytokine response to virulent L. pneumophila. These findings define a previously uncharacterized host response to bacterial type IV secretion that activates MAPK signaling and demonstrate that coincident detection of multiple bacterial components enables immune discrimination between virulent and avirulent bacteria.

Identification of a hypervariable region containing new Legionella pneumophila Icm/Dot translocated substrates by using the conserved icmQ regulatory signature

Legionella pneumophila is an intracellular pathogen that has been shown to utilize the Icm/Dot type IV secretion system for pathogenesis. This system was shown to be composed of Icm/Dot complex components, accessory proteins, and a large number of translocated substrates. In this study, comparison of the icmQ regulatory regions from many Legionella species revealed a conserved regulatory sequence that includes the icmQ -10 promoter element. Mutagenesis of this conserved regulatory element indicated that each of the nucleotides in it affects the level of expression of the icmQ gene but not in a uniform fashion. A genomic analysis discovered that four additional genes in L. pneumophila contain this conserved regulatory sequence, which was found to function similarly in these genes as well. Examination of these four genes indicated that they are dispensable for intracellular growth, but two of them were found to encode new Icm/Dot translocated substrates (IDTS). Comparison of the genomic regions encoding these two IDTS among the four available L. pneumophila genomic sequences indicated that one of these genes is located in a hypervariable genomic region, which was shown before to contain an IDTS-encoding gene. Translocation analysis that was performed for nine proteins encoded from this hypervariable genomic region indicated that six of them are new IDTS which are translocated into host cells in an Icm/Dot-dependent manner. Furthermore, a bioinformatic analysis indicated that additional L. pneumophila genomic regions that contain several neighboring IDTS-encoding genes are hypervariable in gene content.

Host cell processes that influence the intracellular survival of Legionella pneumophila

Key to the pathogenesis of intracellular pathogens is their ability to manipulate host cell processes, permitting the establishment of an intracellular replicative niche. In turn, the host cell deploys defence mechanisms that limit intracellular infection. The bacterial pathogen Legionella pneumophila, the aetiological agent of Legionnaire's Disease, has evolved virulence mechanisms that allow it to replicate within protozoa, its natural host. Many of these tactics also enable L. pneumophila's survival and replication inside macrophages within a membrane-bound compartment known as the Legionella-containing vacuole. One of the virulence factors indispensable for L. pneumophila's intracellular survival is a type IV secretion system, which translocates a large repertoire of bacterial effectors into the host cell. These effectors modulate multiple host cell processes and in particular, redirect trafficking of the L. pneumophila phagosome and mediate its conversion into an ER-derived organelle competent for intracellular bacterial replication. In this review, we discuss how L. pneumophila manipulates host cells, as well as host cell processes that either facilitate or impede its intracellular survival.

Regulation of type IV secretion apparatus genes during Ehrlichia chaffeensis intracellular development by a previously unidentified protein

The type IV secretion (T4S) system is critical for the virulence of several pathogens. In the rickettsial pathogen Ehrlichia chaffeensis, the virBD genes are split into two operons, the virB3-virB6 (preceded by sodB) and virB8-virD4 operons. Between these two operons, there are duplications of virB4, virB8, and virB9. In this study we found that transcription of all five loci was downregulated prior to the release of E. chaffeensis from host THP-1 cells and was upregulated at the initiation of exponential growth. Electrophoretic mobility shift assays revealed an E. chaffeensis-encoded protein that specifically bound to the promoter regions upstream of the virBD loci. The protein was purified from the bacterial lysate by affinity chromatography using a biotinylated promoter region upstream of sodB. Mass spectrometry identified the protein as an E. chaffeensis 12.3-kDa hypothetical protein, which was designated EcxR. Recombinant EcxR bound to the promoter regions upstream of five individual virBD loci. EcxR also activated transcription of all five virBD loci in lacZ reporter constructs. The expression of ecxR was positively autoregulated by EcxR. These results suggest that the five virBD loci are coordinately regulated by EcxR to allow developmental stage-specific expression of the T4S system in E. chaffeensis.

The Legionella pneumophila IcmSW complex interacts with multiple Dot/Icm effectors to facilitate type IV translocation

Many gram-negative pathogens use a type IV secretion system (T4SS) to deliver effector proteins into eukaryotic host cells. The fidelity of protein translocation depends on the efficient recognition of effector proteins by the T4SS. Legionella pneumophila delivers a large number of effector proteins into eukaryotic cells using the Dot/Icm T4SS. How the Dot/Icm system is able to recognize and control the delivery of effectors is poorly understood. Recent studies suggest that the IcmS and IcmW proteins interact to form a stable complex that facilitates translocation of effector proteins by the Dot/Icm system by an unknown mechanism. Here we demonstrate that the IcmSW complex is necessary for the productive translocation of multiple Dot/Icm effector proteins. Effector proteins that were able to bind IcmSW in vitro required icmS and icmW for efficient translocation into eukaryotic cells during L. pneumophila infection. We identified regions in the effector protein SidG involved in icmSW-dependent translocation. Although the full-length SidG protein was translocated by an icmSW-dependent mechanism, deletion of amino terminal regions in the SidG protein resulted in icmSW-independent translocation, indicating that the IcmSW complex is not contributing directly to recognition of effector proteins by the Dot/Icm system. Biochemical and genetic studies showed that the IcmSW complex interacts with a central region of the SidG protein. The IcmSW interaction resulted in a conformational change in the SidG protein as determined by differences in protease sensitivity in vitro. These data suggest that IcmSW binding to effectors could enhance effector protein delivery by mediating a conformational change that facilitates T4SS recognition of a translocation domain located in the carboxyl region of the effector protein.

Effector proteins translocated by Legionella pneumophila: strength in numbers

The Gram-negative bacterium Legionella pneumophila is a parasite of eukaryotic cells. It has evolved to survive and replicate in a wide range of protozoan hosts and can also infect human alveolar macrophages as an opportunistic pathogen. Crucially for the infection process, L. pneumophila uses a type IV secretion system called Dot/Icm to translocate bacterial proteins into host cells. In recent years a large number of Dot/Icm-translocated proteins have been identified. The study of these proteins, referred to as effectors, is providing valuable insight into the mechanism by which an intracellular pathogen can manipulate eukaryotic cellular processes to traffic and replicate in host cells.

Rapid escape of the dot/icm mutants of Legionella pneumophila into the cytosol of mammalian and protozoan cells

The Legionella pneumophila-containing phagosome evades endocytic fusion and intercepts endoplasmic reticulum (ER)-to-Golgi vesicle traffic, which is believed to be mediated by the Dot/Icm type IV secretion system. Although phagosomes harboring dot/icm mutants are thought to mature through the endosomal-lysosomal pathway, colocalization studies with lysosomal markers have reported contradictory results. In addition, phagosomes harboring the dot/icm mutants do not interact with endocytosed materials, which is inconsistent with maturation of the phagosomes in the endosomal-lysosomal pathway. Using multiple strategies, we show that the dot/icm mutants defective in the Dot/Icm structural apparatus are unable to maintain the integrity of their phagosomes and escape into the cytoplasm within minutes of entry into various mammalian and protozoan cells in a process independent of the type II secretion system. In contrast, mutants defective in cytoplasmic chaperones of Dot/Icm effectors and rpoS, letA/S, and letE regulatory mutants are all localized within intact phagosomes. Importantly, non-dot/icm L. pneumophila mutants whose phagosomes acquire late endosomal-lysosomal markers are all located within intact phagosomes. Using high-resolution electron microscopy, we show that phagosomes harboring the dot/icm transporter mutants do not fuse to lysosomes but are free in the cytoplasm. Inhibition of ER-to-Golgi vesicle traffic by brefeldin A does not affect the integrity of the phagosomes harboring the parental strain of L. pneumophila. We conclude that the Dot/Icm transporter is involved in maintaining the integrity of the L. pneumophila phagosome, independent of interception of ER-to-Golgi vesicle traffic, which is a novel function of type IV secretion systems.

A pair of highly conserved two-component systems participates in the regulation of the hypervariable FIR proteins in different Legionella species

Legionella pneumophila and other pathogenic Legionella species multiply inside protozoa and human macrophages by using the Icm/Dot type IV secretion system. The IcmQ protein, which possesses pore-forming activity, and IcmR, which functions as its chaperone, are two essential components of this system. It was previously shown that in 29 Legionella species, a large hypervariable-gene family (fir genes) is located upstream from a conserved icmQ gene, but although nonhomologous, the FIR proteins were found to function similarly together with their corresponding IcmQ proteins. Alignment of the regulatory regions of 29 fir genes revealed that they can be divided into three regulatory groups; the first group contains a binding site for the CpxR response regulator, which was previously shown to regulate the L. pneumophila fir gene (icmR); the second group, which includes most of the fir genes, contains the CpxR binding site and an additional regulatory element that was identified here as a PmrA binding site; and the third group contains only the PmrA binding site. Analysis of the regulatory region of two fir genes, which included substitutions in the CpxR and PmrA consensus sequences, a controlled expression system, as well as examination of direct binding with mobility shift assays, revealed that both CpxR and PmrA positively regulate the expression of the fir genes that contain both regulatory elements. The change in the regulation of the fir genes that occurred during the course of evolution might be required for the adaptation of the different Legionella species to their specific environmental hosts.

Host cell-dependent secretion and translocation of the LepA and LepB effectors of Legionella pneumophila

Legionella pneumophila is the Gram-negative bacterial agent of Legionnaires' disease, an acute, often fatal pneumonia. L. pneumophila infects alveolar macrophages, evading the antimicrobial defences of the phagocyte by preventing fusion of the phagosome with lysosomes and avoiding phagosome acidification. The bacteria then modulate the composition of the vacuole so that it takes on the characteristics of the endoplasmic reticulum. Similar events occur when the bacteria infect unicellular protozoa. It is thought that replication in fresh water protozoa provides an environmental reservoir for the organism. Several effector proteins are delivered to the host by the Icm/Dot type IV secretion system (TFSS). Some of these have been shown to participate in the trafficking of the Legionella phagosome. Here we describe the ability of the Icm/Dot TFSS to translocate two effectors, LepA and LepB, that play a role in the non-lytic release of Legionella from protozoa. We report that translocation of the Lep proteins is inhibited by agents that depolymerize actin filaments and that effectors may be secreted into the extracellular medium upon cell contact. Depletion of the Lep proteins by deletion of their genes results in increased ability to lyse red blood cells. In contrast, overexpression of Lep-containing hybrid proteins appears to specifically inhibit the activity of the Icm/Dot TFSS and may prevent the delivery of other effectors that are critical for intracellular multiplication.

The Francisella pathogenicity island protein IglA localizes to the bacterial cytoplasm and is needed for intracellular growth

BACKGROUND

Francisella tularensis is a gram negative, facultative intracellular bacterium that is the etiological agent of tularemia. F. novicida is closely related to F. tularensis but has low virulence for humans while being highly virulent in mice. IglA is a 21 kDa protein encoded by a gene that is part of an iglABCD operon located on the Francisella pathogenicity island (FPI).

RESULTS

Bioinformatics analysis of the FPI suggests that IglA and IglB are components of a newly described type VI secretion system. In this study, we showed that IglA regulation is controlled by the global regulators MglA and MglB. During intracellular growth IglA production reaches a maximum at about 10 hours post infection. Biochemical fractionation showed that IglA is a soluble cytoplasmic protein and immunoprecipitation experiments demonstrate that it interacts with the downstream-encoded IglB. When the iglB gene was disrupted IglA could not be detected in cell extracts of F. novicida, although IglC could be detected. We further demonstrated that IglA is needed for intracellular growth of F. novicida. A non-polar iglA deletion mutant was defective for growth in mouse macrophage-like cells, and in cis complementation largely restored the wild type macrophage growth phenotype.

CONCLUSION

The results of this study demonstrate that IglA and IglB are interacting cytoplasmic proteins that are required for intramacrophage growth. The significance of the interaction may be to secrete effector molecules that affect host cell processes.

The Legionella pneumophila effector SidJ is required for efficient recruitment of endoplasmic reticulum proteins to the bacterial phagosome

The virulence of Legionella pneumophila is dependent on the Dot/Icm type IV protein secretion system, which translocates effectors into infected cells. A large number of such translocated proteins have been identified, but few of these proteins are necessary for intracellular replication of the pathogen, making it difficult to correlate these genes with specific cell-biological events associated with L. pneumophila infection. We report here the identification and characterization of a family of two substrates, SidJ and SdjA, with distinctive phenotypes. In contrast to many Dot/Icm substrates, whose expression levels are elevated when bacteria are grown to postexponential phase, SidJ is produced at a constant rate during the entire bacterial growth cycle. Mutation in sidJ causes a significant growth defect in both macrophage and amoeba hosts, but an sdjA mutant is detectably defective only in protozoan hosts. However, in the amoeba host a mutant lacking both sidJ and sdjA does not display a more severe growth defect than the sidJ mutant. Despite its significant intracellular growth defect, the sidJ mutant is still able to effectively evade fusion with lysosomes. Importantly, recruitment of endoplasmic reticulum (ER) proteins by vacuoles containing the sidJ mutant was considerably delayed in both mammalian and amoeba cells. Our results suggest that SidJ modulates host cellular pathways, contributing to the trafficking or retention of ER-derived vesicles to L. pneumophila vacuoles.

Environmental mimics and the Lvh type IVA secretion system contribute to virulence-related phenotypes of Legionella pneumophila

Legionella pneumophila, the causative organism of Legionnaires' disease, is a fresh-water bacterium and intracellular parasite of amoebae. This study examined the effects of incubation in water and amoeba encystment on L. pneumophila strain JR32 and null mutants in dot/icm genes encoding a type IVB secretion system required for entry, delayed acidification of L. pneumophila-containing phagosomes, and intracellular multiplication when stationary-phase bacteria infect amoebae and macrophages. Following incubation of stationary-phase cultures in water, mutants in dotA and dotB, essential for function of the type IVB secretion system, exhibited entry and delay of phagosome acidification comparable to that of strain JR32. Following encystment in Acanthamoeba castellanii and reversion of cysts to amoeba trophozoites, dotA and dotB mutants exhibited intracellular multiplication in amoebae. The L. pneumophila Lvh locus, encoding a type IVA secretion system homologous to that in Agrobacterium tumefaciens, was required for restoration of entry and intracellular multiplication in dot/icm mutants following incubation in water and amoeba encystment and was required for delay of phagosome acidification in strain JR32. These data support a model in which the Dot/Icm type IVB secretion system is conditionally rather than absolutely required for L. pneumophila virulence-related phenotypes. The data suggest that the Lvh type IVA secretion system, previously thought to be dispensable, is involved in virulence-related phenotypes under conditions mimicking the spread of Legionnaires' disease from environmental niches. Since environmental amoebae are implicated as reservoirs for an increasing number of environmental pathogens and for drug-resistant bacteria, the environmental mimics developed here may be useful in virulence studies of other pathogens.

Identification of the core transmembrane complex of the Legionella Dot/Icm type IV secretion system

Type IV secretion systems (T4SS) are utilized by a wide range of Gram negative bacteria to deliver protein and DNA substrates to recipient cells. The best characterized T4SS are the type IVA systems, which exhibit extensive similarity to the Agrobacterium VirB T4SS. In contrast, type IVB secretion systems share almost no sequence homology to the type IVA systems, are composed of approximately twice as many proteins, and remain largely uncharacterized. Type IVB systems include the Dot/Icm systems found in the pathogens Legionella and Coxiella and the conjugative apparatus of IncI plasmids. Here we report the first extensive characterization of a type IVB system, the Legionella Dot/Icm secretion apparatus. Based on biochemical and genetic analysis, we discerned the existence of a critical five-protein subassembly that spans both bacterial membranes and comprises the core of the secretion complex. This transmembrane connection is mediated by protein dimer pairs consisting of two inner membrane proteins, DotF and DotG, which are able to independently associate with DotH/DotC/DotD in the outer membrane. The Legionella core subcomplex appears to be functionally analogous to the Agrobacterium VirB7-10 subcomplex, suggesting a remarkable conservation of the core subassembly in these evolutionarily distant type IV secretion machines.

Identification of non-dot/icm suppressors of the Legionella pneumophila DeltadotL lethality phenotype

Legionella pneumophila, a causative agent of bacterial pneumonia, survives inside phagocytic cells by avoiding rapid targeting to the lysosome. This bacterium utilizes a type IVB secretion system, encoded by the dot/icm genes, to replicate inside host cells. DotL, a critical component of the Dot/Icm secretion apparatus, functions as the type IV coupling protein. In contrast to most dot/icm genes, which are dispensable for growth on bacteriological media, dotL is required for the viability of wild-type L. pneumophila. Previously we reported that DeltadotL lethality could be suppressed by inactivation of the Dot/Icm complex via mutations in other dot/icm genes. Here we report the isolation of non-dot/icm suppressors of this phenotype. These DeltadotL suppressors include insertions that disrupt the function of the L. pneumophila homologs of cpxR, djlA, lysS, and two novel open reading frames, lpg0742 and lpg1594, that we have named ldsA and ldsB for lethality of DeltadotL suppressor. In addition to suppressing DeltadotL lethality, inactivation of these genes in a wild-type strain background causes a range of defects in L. pneumophila virulence traits, including intracellular growth, implicating these factors in the proper function of the Dot/Icm complex. Consistent with previous data showing a role for the cpx system in regulating expression of several dot/icm genes, the cpxR insertion mutant produced decreased levels of three Dot/Icm proteins, DotA, IcmV, and IcmW. The remaining four suppressors did not affect the steady-state levels of any Dot/Icm protein and are likely to represent the first identified factors necessary for assembly and/or activation of the Dot/Icm secretion complex.

Regulation of Legionella phagosome maturation and infection through flagellin and host Ipaf

Legionella pneumophila is an intracellular bacterium that causes an acute form of pneumonia called Legionnaires' disease. After infection of human macrophages, the Legionella-containing phagosome (LCP) avoids fusion with the lysosome allowing intracellular replication of the bacterium. In macrophages derived from most mouse strains, the LCP is delivered to the lysosome resulting in Legionella degradation and restricted bacterial growth. Mouse macrophages lacking the NLR protein Ipaf or its downstream effector caspase-1 are permissive to intracellular Legionella replication. However, the mechanism by which Ipaf restricts Legionella replication is not well understood. Here we demonstrate that the presence of flagellin and a competent type IV secretion system are critical for Legionella to activate caspase-1 in macrophages. Activation of caspase-1 in response to Legionella infection also required host Ipaf, but not TLR5. In the absence of Ipaf or caspase-1 activation, the LCP acquired endoplasmic reticulum-derived vesicles, avoided fusion with the lysosome, and allowed Legionella replication. Accordingly a Legionella mutant lacking flagellin did not activate caspase-1, avoided degradation, and replicated in wild-type macrophages. The regulation of phagosome maturation by Ipaf occurred within 2 h after infection and was independent of macrophage cell death. In vivo studies confirmed that flagellin and Ipaf play an important role in the control of Legionella clearance. These results reveal that Ipaf restricts Legionella replication through the regulation of phagosome maturation, providing a novel function for NLR proteins in host defense against an intracellular bacterium.

NF-kappaB translocation prevents host cell death after low-dose challenge by Legionella pneumophila

Legionella pneumophila, the causative agent of Legionnaires' disease, grows within macrophages and manipulates target cell signaling. Formation of a Legionella-containing replication vacuole requires the function of the bacterial type IV secretion system (Dot/Icm), which transfers protein substrates into the host cell cytoplasm. A global microarray analysis was used to examine the response of human macrophage-like U937 cells to low-dose infections with L. pneumophila. The most striking change in expression was the Dot/Icm-dependent up-regulation of antiapoptotic genes positively controlled by the transcriptional regulator nuclear factor κB (NF-κB). Consistent with this finding, L. pneumophila triggered nuclear localization of NF-κB in human and mouse macrophages in a Dot/Icm-dependent manner. The mechanism of activation at low-dose infections involved a signaling pathway that occurred independently of the Toll-like receptor adaptor MyD88 and the cytoplasmic sensor Nod1. In contrast, high multiplicity of infection conditions caused a host cell response that masked the unique Dot/Icm-dependent activation of NF-κB. Inhibition of NF-κB translocation into the nucleus resulted in premature host cell death and termination of bacterial replication. In the absence of one antiapoptotic protein, plasminogen activator inhibitor–2, host cell death increased in response to L. pneumophila infection, indicating that induction of antiapoptotic genes is critical for host cell survival.

The Legionella pneumophila IcmS-LvgA protein complex is important for Dot/Icm-dependent intracellular growth

Many bacterial pathogens require a functional type IV secretion system (T4SS) for virulence. Legionella pneumophila, the causative agent of Legionnaires' disease, employs the Dot/Icm T4SS to inject a large number of protein substrates into its host, thereby altering phagosome trafficking. The L. pneumophila T4SS substrate SdeA has been shown to require the accessory factor IcmS for its export. IcmS, defined as a type IV adaptor, exists as a heterodimer with IcmW and this complex functions in a manner similar to a type III secretion chaperone. Here we report an interaction between IcmS and the previously identified virulence factor LvgA. Similar to the icmS mutant, the lvgA mutant appears to assemble a fully functional Dot/Icm complex. Both LvgA and IcmS are small, acidic proteins localized to the cytoplasm and are not exported by the Dot/Icm system, suggesting they form a novel type IV adaptor complex. Inactivation of lvgA causes a minimal defect in growth in the human monocytic cell line U937 and the environmental host Acanthamoeba castellanii. However, the lvgA mutant was severely attenuated for intracellular growth of L. pneumophila in mouse macrophages, suggesting LvgA may be a critical factor that confers host specificity.

Recognition and delivery of effector proteins into eukaryotic cells by bacterial secretion systems

The direct transport of virulence proteins from bacterium to host has emerged as a common strategy employed by Gram-negative pathogens to establish infections. Specialized secretion systems function to facilitate this process. The delivery of 'effector' proteins by these secretion systems is currently confined to two functionally similar but mechanistically distinct pathways, termed type III and type IV secretion. The type III secretion pathway is ancestrally related to the multiprotein complexes that assemble flagella, whereas the type IV mechanism probably emerged from the protein complexes that support conjugal transfer of DNA. Although both pathways serve to transport proteins from the bacterium to host, the recognition of the effector protein substrates and the secretion information contained in these proteins appear highly distinct. Here, we review the mechanisms involved in the selection of substrates by each of these transport systems and secretion signal information required for substrate transport.

Attachment and fusion of endoplasmic reticulum with vacuoles containing Legionella pneumophila

Legionella pneumophila is an intracellular pathogen that replicates in a unique vacuole that avoids endocytic maturation. Previous studies have shown host vesicles attached to the L. pneumophila-containing vacuole (LCV) minutes after uptake. Here we examine the origin and content of these vesicles by electron microscopy (EM). Our data demonstrate that the attached vesicles are derived from endoplasmic reticulum (ER) based the presence of the resident ER proteins glucose-6-phosphatase, protein disulphide isomerase (PDI) and proteins having the ER-retention signal lysine-aspartic acid-glutamic acid-leucine (KDEL). After tethering occurred, ER markers inside of attached vesicles were delivered into the lumen of the LCV, indicating ER fusion. Treatment of cells with brefeldin A did not interfere with the attachment of ER vesicles with the LCV, suggesting that tethering of these vesicles does not require activities mediated by ADP-ribosylation factor (ARF). ER vesicles were not tethered to the LCV in cells producing the Sar1H79G protein, indicating that vesicles produced by the Sar1/CopII system are necessary for vesicle attachment. From these data we conclude that formation of the organelle that supports L. pneumophila replication is a two-stage process that involves remodelling of the LCV by early secretory vesicles produced by the Sar1/CopII system, followed by attachment and fusion of ER.

Pathogenese, Diagnostik und Therapie der Legionella-Infektion

Legionella species are ubiquitous in aquatic environments. About 50 years ago they entered the engineered (technical) environment, i.e. warm water systems with zones of stagnation. Since that time they represent a hygienic problem. After transmission to humans via aerosols legionellae might cause Legionella pneumonia (legionnaires' disease) or influenza-like respiratory infections (Pontiac fever). Epidemiological data suggest that Legionella strains might differ substantially in their virulence properties. Although the molecular basis is not understood L. pneumophila serogroup 1 especially MAb 3/1-positive strains cause the majority of infections. The main virulence feature is the ability to multiply intracellularly. After uptake into macrophages legionellae multiply in a specialized vacuole and finally lyse their host cells. Several bacterial factors like surface components, secretion systems and iron uptake systems are involved in this process. Since the clinical picture of Legionella pneumonia does not allow differentiation from pneumoniae caused by other pathogens, microbiological diagnostic methods are needed to establish the diagnosis. Cultivation of legionellae from clinical specimens, detection of antigens and DNA in patients' samples and detection of antibodies in serum samples are suitable methods. However, none of the diagnostic tests presently available offers the desired quality with respect to sensitivity and specificity. Therefore, the standard technique is to use several diagnostic tests in parallel. Advantages and disadvantages of the diagnostic procedures are discussed. Therapeutic options for Legionella infections are newer macrolides like azithromycin and chinolones (ciprofloxacin, levofloxacin and moxifloxacin).

Pathogen-endoplasmic-reticulum interactions: in through the out door

The endoplasmic reticulum (ER) is a crucial intracellular organelle that is involved in the biosynthesis of cellular molecules, including proteins, carbohydrates and lipids. These become constituents of the ER or are transported to the Golgi, plasma membrane, endosomes and lysosomes.

The ER is involved in the transport of both MHC class I and class II complexes to the cell surface. It is the site of peptide loading onto MHC class I molecules, and it is important for regulation of the secretion of soluble immune mediators.

Perturbation in ER–Golgi biogenesis affects antigen presentation and immune effector functions, so it is not surprising that the ER is a target for some pathogens.

Several pathogens have developed strategies to modulate phagosome maturation and therefore avoid phagosome–lysosome fusion and the subsequent degradation of the pathogen: for example, by generating an ER-derived vacuole. In some cases, this has the added benefit of disrupting ER function.

Toxoplasma gondii, Brucella spp. and Legionella pneumophila depend on ER-derived niches for growth, but the precise effects of these pathogen–ER interactions on the immune response are still unclear.

Several viruses target the ER and interfere with antigen presentation by MHC class I molecules: for example, hepatitis C virus and human cytomegalovirus.

Despite this, the ER contributes to host defence against microorganisms, through its role in autophagy, as well as through the ER stress response and the interferon-induced ER-resident proteins (such as viperin and the p47 GTPases).

Different pathogens have evolved distinct strategies to promote their survival in host cells. This Review describes the contribution of the endoplasmic reticulum to host defence and the mechanisms by which pathogens interacting with the endoplasmic reticulum subvert the host immune response.

A key determinant for the survival of intracellular pathogens is their ability to subvert the cellular processes of the host to establish a compartment that allows replication. Although most microorganisms internalized by host cells are efficiently cleared following fusion with lysosomes, many pathogens have evolved mechanisms to escape this degradation. In this Review, we provide insight into the molecular processes that are targeted by pathogens that interact with the endoplasmic reticulum and thereby subvert the immune response, ensure their survival intracellularly and cause disease. We also discuss how the endoplasmic reticulum 'strikes back' and controls microbial growth.

The Birc1e cytosolic pattern-recognition receptor contributes to the detection and control of Legionella pneumophila infection

Baculovirus inhibitor of apoptosis repeat-containing 1 (Birc1) proteins have homology to several germline-encoded receptors of the innate immune system. However, their function in immune surveillance is not clear. Here we describe a Birc1e-dependent signaling pathway that restricted replication of the intracellular pathogen Legionella pneumophila in mouse macrophages. Translocation of bacterial products into host-cell cytosol was essential for Birc1e-mediated control of bacterial replication. Caspase-1 was required for Birc1e-dependent antibacterial responses ex vivo in macrophages and in a mouse model of Legionnaires' disease. The interleukin 1beta converting enzyme-protease-activating factor was necessary for L. pneumophila growth restriction, but interleukin 1beta was not required. These results establish Birc1e as a nucleotide-binding oligomerization-leucine-rich repeat protein involved in the detection and control of intracellular L. pneumophila.

1. Membrane origin for autophagy

Autophagy is a degradative transport route conserved among all eukaryotic organisms. During starvation, cytoplasmic components are randomly sequestered into large double-membrane vesicles called autophagosomes and delivered into the lysosome/vacuole where they are destroyed. Cells are able to modulate autophagy in response to their needs, and under certain circumstances, cargoes, such as aberrant protein aggregates, organelles, and bacteria can be selectively and exclusively incorporated into autophagosomes. As a result, this pathway plays an active role in many physiological processes, and it is induced in numerous pathological situations because of its ability to rapidly eliminate unwanted structures. Despite the advances in understanding the functions of autophagy and the identification of several factors, named Atg proteins that mediate it, the mechanism that leads to autophagosome formation is still a mystery. A major challenge in unveiling this process arises from the fact that the origin and the transport mode of the lipids employed to compose these structures is unknown. This compendium will review and analyze the current data about the possible membrane source(s) with a particular emphasis on the yeast Saccharomyces cerevisiae, the leading model organism for the study of autophagosome biogenesis, and on mammalian cells. The information acquired investigating the pathogens that subvert autophagy in order to replicate in the host cells will also be discussed because it could provide important hints for solving this mystery.

Additive effect on intracellular growth by Legionella pneumophila Icm/Dot proteins containing a lipobox motif

Legionella pneumophila, the causative agent of Legionnaires' disease, utilizes a type IVB secretion system to subvert its host cells and grow intracellularly. This type IV secretion system is composed of 25 icm (or dot) genes that probably constitute parts of a secretion complex as well as more than 30 proteins that are translocated via this system into the host cells. Three of the Icm/Dot proteins (DotD, DotC, and IcmN) contain a lipobox motif at their N terminals and are predicted to be lipoproteins. Two of these lipoproteins (DotD and DotC) were found to be essential for intracellular growth in both HL-60-derived human macrophages and in the protozoan host Acanthamoeba castellanii, while the third lipoprotein (IcmN) was found to be partially required for intracellular growth only in A. castellanii. Mutation analysis of the lipobox cysteine residue, which was shown previously to be indispensable for the lipobox function, indicated that both DotC and DotD are partially functional without this conserved residue. Cysteine mutations in both DotC and DotD or in DotC together with an icmN deletion or in DotD together with an icmN deletion were found to be additive, indicating that each of these lipoproteins performs its function independently from the others. Analysis of the transcriptional regulation of both the dotDC operon and the icmN gene revealed that both had higher levels of expression at stationary phase which were partially dependent on the LetA regulator. Our results indicate that the lipoproteins of the L. pneumophila icm (or dot) system are essential components of the secretion system and that they perform their functions independently.

Autophagy

Autophagy is a major intracellular pathway for the degradation and recycling of long-lived proteins and cytoplasmic organelles. Like apoptotic programmed cell death, autophagy is an essential part of growth regulation and maintenance of homeostasis in multicellular organisms. Autophagic vacuole formation is also activated as an adaptive response to a variety of extracellular and intracellular stimuli, including nutrient deprivation, hormonal or therapeutic treatment, bacterial infection, aggregated and misfolded proteins and damaged organelles. Mediators of class I and class III PI3 kinase signaling pathways and trimeric G proteins play major roles in regulating autophagosome formation during the stress response. Defective autophagy is the underlying cause of a number of pathological conditions, including vacuolar myopathies, neurodegenerative diseases, liver disease, and some forms of cancer. This chapter provides an overview of the morphology and molecular basis of autophagosome formation and offers a glimpse into the role of autophagy in normal growth and development, while discussing the pathological implications of its deregulation.

Characterization of Legionella pneumophila pmiA, a gene essential for infectivity of protozoa and macrophages

The ability of Legionella pneumophila to cause pneumonia is dependent on intracellular replication within alveolar macrophages. The Icm/Dot secretion apparatus is essential for the ability of L. pneumophila to evade endocytic fusion, to remodel the phagosome by the endoplasmic reticulum (ER), and to replicate intracellularly. Protozoan and macrophage infectivity (pmi) mutants of L. pneumophila, which include 11 dot/icm mutants, exhibit defects in intracellular growth and replication within both protozoa and macrophages. In this study we characterized one of the pmi loci, pmiA. In contrast to the parental strain, the pmiA mutant is defective in cytopathogenicity for protozoa and macrophages. This is a novel mutant that exhibits a partial defect in survival within U937 human macrophage-like cells but exhibits a severe growth defect within Acanthamoeba polyphaga, which results in elimination from this host. The intracellular defects of this mutant are complemented by the wild-type pmiA gene on a plasmid. In contrast to phagosomes harboring the wild-type strain, which exclude endosomal-lysosomal markers, the pmiA mutant-containing phagosomes acquire the late endosomal-lysosomal markers LAMP-1 and LAMP-2. In contrast to the parental strain-containing phagosomes that are remodeled by the ER, there was a decrease in the number of ER-remodeled phagosomes harboring the pmiA mutant. Among several Legionella species examined, the pmiA gene is specific for L. pneumophila. The predicted amino acid sequence of the PmiA protein suggests that it is a transmembrane protein with three membrane-spanning regions. PmiA is similar to several hypothetical proteins produced by bacteria with a type IV secretion apparatus. Importantly, the defect in pmiA abolishes the pore-forming activity, which has been attributed to the Icm/Dot type IV secretion system. However, the mutant is sensitive to NaCl, and this sensitivity is abrogated in the icm/dot mutants. These results suggest that PmiA is a novel virulence factor that is involved in intracellular survival and replication of L. pneumophila in macrophages and protozoan cells.

Incomplete activation of macrophage apoptosis during intracellular replication of Legionella pneumophila

The ability of the intracellular bacterium Legionella pneumophila to cause disease is totally dependent on its ability to modulate the biogenesis of its phagosome and to replicate within alveolar cells. Upon invasion, L. pneumophila activates caspase-3 in macrophages, monocytes, and alveolar epithelial cells in a Dot/Icm-dependent manner that is independent of the extrinsic or intrinsic pathway of apoptosis, suggesting a novel mechanism of caspase-3 activation by this intracellular pathogen. We have shown that the inhibition of caspase-3 prior to infection results in altered biogenesis of the L. pneumophila-containing phagosome and in an inhibition of intracellular replication. In this report, we show that the preactivation of caspase-3 prior to infection does not rescue the intracellular replication of L. pneumophila icmS, icmR, and icmQ mutant strains. Interestingly, preactivation of caspase-3 through the intrinsic and extrinsic pathways of apoptosis in both human and mouse macrophages inhibits intracellular replication of the parental stain of L. pneumophila. Using single-cell analysis, we show that intracellular L. pneumophila induces a robust activation of caspase-3 during exponential replication. Surprisingly, despite this robust activation of caspase-3 in the infected cell, the host cell does not undergo apoptosis until late stages of infection. In sharp contrast, the activation of caspase-3 by apoptosis-inducing agents occurs concomitantly with the apoptotic death of all cells that exhibit caspase-3 activation. It is only at a later stage of infection, and concomitant with the termination of intracellular replication, that the L. pneumophila-infected cells undergo apoptotic death. We conclude that although a robust activation of caspase-3 is exhibited throughout the exponential intracellular replication of L. pneumophila, apoptotic cell death is not executed until late stages of the infection, concomitant with the termination of intracellular replication.

Genetic analysis of the Legionella pneumophila DotB ATPase reveals a role in type IV secretion system protein export

The pulmonary pathogen Legionella pneumophila uses the Dot/Icm type IV secretion system (T4SS) to replicate inside host cells. This apparatus translocates proteins into macrophages to alter their endocytic pathway and enable bacterial growth. Although the secretion ATPase DotB is critical for T4SS function, its specific role in type IV secretion remains undefined. Due to similarity to the VirB11 and PilT ATPases, DotB has been proposed to play a role in assembly of the T4SS, retraction of pili and/or export of substrates. With the goal of understanding the protein's function(s), we isolated and characterized 30 dotB alleles using a variety of phenotypic and biochemical assays. Twenty-four of these alleles possess several dot/icm mutant phenotypes, including a complete lack of intracellular replication, plasmid mobilization and contact-dependent cytotoxicity. These 24 non-functional alleles fall into three classes: those with a known biochemical defect, those with a predicted enzymatic defect and those with an unknown defect. Six other alleles display partial activity in dot/icm phenotypic assays, thus constituting a fourth class. Two mutants in this class are unable to export a subset of T4SS substrates, providing the first evidence for a DotB function in substrate export and suggesting a possible role in substrate selection.

Coevolution between nonhomologous but functionally similar proteins and their conserved partners in the Legionella pathogenesis system

Legionella pneumophila, the causative agent of Legionnaires' disease, and other pathogenic Legionella species multiply inside protozoa and human macrophages by using the intracellular multiplication (Icm)/defect in organelle trafficking (Dot) type-IV secretion system. The IcmQ protein, which possesses pore-forming activity, and IcmR, which regulates the IcmQ activity, are two essential components of this system. Analysis of the region expected to contain these two genes from 29 Legionella species revealed the presence of a conserved icmQ gene and a large hypervariable gene family [functional homologues of icmR (fir) genes], located at the icmR genomic position. Although hypervariable in their sequence, the fir genes from all 29 Legionella species were found, together with their corresponding icmQ genes, to function similarly during infection. In addition, all FIR proteins we examined were found to interact with their corresponding IcmQ proteins. Detailed bioinformatic, biochemical, and genetic analysis of the interaction between the variable FIR proteins and conserved IcmQ proteins revealed that their interaction depends on a variable region located between two conserved domains of IcmQ. This variable region was also found to be critical for IcmQ self-interaction, and the region probably coevolved with the corresponding FIR protein. A FIR-IcmQ pair was also found in Coxiella burnetii, the only known non-Legionella bacterium that contains an Icm/Dot system, indicating the significance of this protein pair for the function of this type-IV secretion system. We hypothesize that this gene variation, which is probably mediated by positive selection, plays an important role in the evolutionary arms race between the protozoan host cell and the pathogen.

The DotL protein, a member of the TraG-coupling protein family, is essential for Viability of Legionella pneumophila strain Lp02

Legionella pneumophila is able to survive inside phagocytic cells by an internalization route that bypasses fusion of the nascent phagosome with the endocytic pathway to allow formation of a replicative phagosome. The dot/icm genes, a major virulence system of L. pneumophila, encode a type IVB secretion system that is required for intracellular growth. One Dot protein, DotL, has sequence similarity to type IV secretion system coupling proteins (T4CPs). In other systems, coupling proteins are not required for viability of the organism. Here we report the first example of a strain, L. pneumophila Lp02, in which a putative T4CP is essential for viability of the organism on bacteriological media. This result is particularly surprising since the majority of the dot/icm genes in Lp02 are dispensable for growth outside of a host cell, a condition that does not require a functional Dot/Icm secretion complex. We were able to isolate suppressors of the Delta dotL lethality and found that many contained mutations in other components of the Dot/Icm secretion system. A systematic analysis of dot/icm deletion mutants revealed that the majority of them (20 of 26) suppressed the lethality phenotype, indicating a partially assembled secretion system may be the source of Delta dotL toxicity in the wild-type strain. These results are consistent with a model in which the DotL protein plays a role in regulating the activity of the L. pneumophila type IV secretion apparatus.

Processing and major histocompatibility complex class II presentation of Legionella pneumophila antigens by infected macrophages

To better understand interactions between the intracellular pathogen Legionella pneumophila and macrophages (Mphis), host and bacterial determinants important for presentation of antigens on major histocompatibility complex class II molecules (MHC-II) were investigated. It was determined that immune CD4 T-cell responses to murine bone marrow-derived Mphis (BMphis) infected with wild-type L. pneumophila were higher than the responses to avirulent dotA mutant bacteria. Although this enhanced response by immune T cells required modulation of vacuole transport mediated by the Dot/Icm system, it did not require intracellular replication of L. pneumophila. Intracellular cytokine staining identified a population of immune CD4 T cells that produced gamma interferon upon incubation with BMphis infected with wild-type L. pneumophila that did not respond to Mphi infection with dotA mutant bacteria. Endocytic processing was required for presentation of L. pneumophila antigens on MHC-II as determined by a defect in CD4 T-cell responses when the pH of BMphi endosomes was neutralized with chloroquine. Investigation of MHC-II presentation of antigens by BMphis infected with L. pneumophila icmR, icmW, and icmS mutants indicated that these mutants have an intermediate presentation phenotype relative to those of wild-type and dotA mutant bacteria. In addition, it was found that antigens from dot and icm mutants are presented earlier than antigens from wild-type L. pneumophila. Although immune CD4 T-cell responses to proteins secreted by the L. pneumophila Lsp system were not detected, it was found that the Lsp system is important for priming L. pneumophila-specific T cells in vivo. These data indicate that optimal antigen processing and MHC-II presentation to immune CD4 T cells involves synthesis of L. pneumophila proteins in an endoplasmic reticulum-derived compartment followed by transport to lysosomes.

The Icm/Dot type-IV secretion systems of Legionella pneumophila and Coxiella burnetii

Type-IV secretion systems are devices present in a wide range of bacteria (including bacterial pathogens) that deliver macromolecules (proteins and single-strand-DNA) across kingdom barriers (as well as between bacteria and into the surroundings). The type-IV secretion systems were divided into two subgroups and Legionella pneumophila and Coxiella burnetii are the only two bacteria known today to utilize a type-IVB secretion system for pathogenesis. In this review we summarized the available information concerning the icm/dot type-IVB secretion systems by comparing the two bacteria that possess this system, the proteins components of their systems as well as the homology of proteins from type-IVB secretion systems to proteins from type-IVA secretion systems. In addition, the phenotypes associated with mutants in the L. pneumophila icm/dot genes, their relations to properties of specific Icm/Dot proteins as well as the protein substrates delivered by this system are described.