The Dot/lcm transporter of Legionella pneumophila: a bacterial conductor of vesicle trafficking that orchestrates the establishment of a replicative organelle in eukaryotic hosts

Legionella pneumophila are gram negative bacteria that parasitize professional phagocytes. When ingested by their eukaryotic hosts, these bacteria have the unique ability to alter maturation of the endocytic vacuoles in which they reside initially and create an organelle that supports replication. This review will focus on several scientific breakthroughs made recently that shed light on the mechanisms by which L. pneumophila are able to establish a niche permissive for intracellular growth.     

Flagellum of Legionella pneumophila positively affects the early phase of infection of eukaryotic host cells

Legionella pneumophila, the etiologic agent of Legionnaires' disease, contains a single, monopolar flagellum which is composed of one major subunit, the FlaA protein. To evaluate the role of the flagellum in the pathogenesis and ecology of Legionella, the flaA gene of L. pneumophila Corby was mutagenized by introduction of a kanamycin resistance cassette. Immunoblots with antiflagellin-specific polyclonal antiserum, electron microscopy, and motility assays confirmed that the specific flagellar mutant L. pneumophila Corby KH3 was nonflagellated. The redelivery of the intact flaA gene into the chromosome (L. pneumophila Corby CD10) completely restored flagellation and motility. Coculture studies showed that the invasion efficiency of the flaA mutant was moderately reduced in amoebae and severely reduced in HL-60 cells. In contrast, adhesion and the intracellular rate of replication remained unaffected. Taking these results together, we have demonstrated that the flagellum of L. pneumophila positively affects the establishment of infection by facilitating the encounter of the host cell as well as by enhancing the invasion capacity.     

Identification of a gene that affects the efficiency of host cell infection by Legionella pneumophila in a temperature-dependent fashion

The ability to infect host cells is critical for the survival and replication of intracellular pathogens in humans. We previously found that many genes involved in the ability of Legionella pneumophila to infect macrophages are not expressed efficiently under standard laboratory growth conditions. We have developed an approach using expression of L. pneumophila genes from an exogenous constitutive promoter on a low-copy-number vector that allows identification of genes involved in host cell infection. Through the use of this strategy, we found that expression of a gene, lvhB2, enhances the efficiency of L. pneumophila infection of mammalian cells. The putative protein encoded by lvhB2 has similarity to structural pilin subunits of type IV secretion systems. We confirmed that this gene plays a role in host cell infection by the construction of an in-frame deletion in the L. pneumophila lvhB2 gene and complementation of this mutant with the wild-type gene. The lvhB2 mutant does not display a very obvious defect in interactions with host cells when the bacteria are grown at 37 degrees C, but it has an approximately 100-fold effect on entry and intracellular replication when grown at 30 degrees C. These data suggest that lvhB2 plays an important role in the efficiency of host cell infection by L. pneumophila grown at lower temperatures.     

In vivo regulation of replicative Legionella pneumophila lung infection by endogenous tumor necrosis factor alpha and nitric oxide.

The in vivo role of endogenous tumor necrosis factor alpha (TNF-alpha) and reactive nitrogen intermediates (RNIs) in modulation of growth of Legionella pneumophila in the lung was assessed using a murine model of replicative L. pneumophila lung infection. Intratracheal inoculation of mice with L. pneumophila resulted in induction of endogenous TNF-alpha, which preceded clearance of L. pneumophila from the lung. Inhibition of endogenous TNF-alpha activity, via in vivo administration of TNF-alpha neutralizing antibody, or inhibition of endogenous RNIs, via administration of the nitric oxide (NO) synthetase inhibitor N-monomethyl-L-arginine (NMMA), resulted in enhanced growth of L. pneumophila in the lung at > or = 3 days postinfection (when compared with untreated L. pneumophila-infected mice). Because of the similar kinetics of enhanced pulmonary growth of L. pneumophila in mice treated in vivo with either anti-TNF-alpha antibody or NMMA, the immunomodulatory effect of NO on endogenous TNF-alpha activity in the lung was assessed. Administration of NMMA to L. pneumophila-infected mice resulted in a significant decrease in endogenous TNF-alpha activity in the lung during replicative L. pneumophila infections in vivo. However, administration of exogenous TNF-alpha to NMMA-treated mice failed to significantly enhance clearance of L. pneumophila from the lung. Results of these studies indicate that both endogenous NO and TNF-alpha facilitate resolution of replicative L. pneumophila lung infections and that regulation of L. pneumophila replication by TNF-alpha is mediated, at least in part, by NO.     

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.     

Identification of a novel adhesion molecule involved in the virulence of Legionella pneumophila

Legionella pneumophila is an intracellular bacterium, and its successful parasitism in host cells involves two reciprocal phases: transmission and intracellular replication. In this study, we sought genes that are involved in virulence by screening a genomic DNA library of an L. pneumophila strain, 80-045, with convalescent-phase sera of Legionnaires' disease patients. Three antigens that reacted exclusively with the convalescent-phase sera were isolated. One of them, which shared homology with an integrin analogue of Saccharomyces cerevisiae, was named L. pneumophila adhesion molecule homologous with integrin analogue of S. cerevisiae (LaiA). The laiA gene product was involved in L. pneumophila adhesion to and invasion of the human lung alveolar epithelial cell line A549 during in vitro coculture. However, its presence did not affect multiplication of L. pneumophila within a U937 human macrophage cell line. Furthermore, after intranasal infection of A/J mice, the laiA mutant was eliminated from lungs and caused reduced mortality compared to the wild isolate. Thus, we conclude that the laiA gene encodes a virulence factor that is involved in transmission of L. pneumophila 80-045 and may play a role in Legionnaires' disease in humans.     

Identification of a species-specific antigen in Legionella pneumophila by a monoclonal antibody.

A species-specific antigen in Legionella pneumophila was identified by a monoclonal antibody in enzyme-linked immunosorbent and immunofluorescence assays of serogroups 1 through 8. The species-specific antigen was heat stable, and the molecular weight of the major band was 29,000 by immunoblot analysis. In direct immunofluorescence assays, the antigen was cryptic or only partly exposed on the surface of the cells but was effectively exposed by treating the cells with detergent and EDTA. The monoclonal antibody was utilized in direct immunofluorescence assays to specifically identify multiple cultures of L. pneumophila serogroups.     

SIGIRR-EGFP真核表达载体构建及在H292细胞中的亚细胞定位研究

目的 通过增强型绿色荧光蛋向示踪技术,观察SIGIRR(single Ig IL,1R-related molecule)在人气道上皮细胞株H292中的分布特点及对上皮细胞天然免疫功能的影响.方法 构建SIGIRR-EGFP融和蛋白的真核表达载体,运用脂质体转染的方法转染人气道上皮细胞株H292,再利用激光共聚焦显微镜对其进行亚细胞定位研究.经LPS刺激后,ELISA检测转染前 后上皮细胞TNF-α分泌水平的差异.结果 构建的融和蛋白表达载体SIGIRR-EGFP经EooR Ⅰ和BamH Ⅰ双酶切后,电泳显 示其条带大小为4.7 kh和1.23 kb左右,经测序证实SIGIRR的序列与GenBank公布的人cDNA序列完全一致.激光共聚焦显示pEGFP-N1空载体转染表达的绿色荧光蛋白在H292细胞中均匀分布,而重组载体SIGIRR-ECFP表达的融和蛋白在H292细胞核周(膜)和细胞膜边缘上有连续分布现象.经LPS刺激后,pEGFP-N1转染的细胞和未转染细胞TNF-α水平无显著性差异(P＞0.05),而SIGIRR-EGFP转染细胞与pEGFP-N1转染细胞相比,显著降低了TNF-α的产生(P＜0.01).结论 成功构建了SI-GIRR-EGFP融和蛋白的真核表达载体,并利用绿色荧光蛋白标记的方法显示了SIGIRR分子在H292细胞中细胞膜分布.过表达的SIGIRR降低了上皮细胞IPS诱导的炎症因子TNF-α产生.     

Ultrastructural localization and protective activity of a high-molecular-weight antigen isolated from Legionella pneumophila.

Immunoperoxidase labeling showed that the F-1 antigen of Legionella pneumophila is located on the bacterial cell surface. Protection against lethal intraperitoneal challenge with serogroup 1 L. pneumophila was induced in guinea pigs by heat-killed cells and F-1 antigen from serogroup 1, but not by heat-killed cells or F-1 antigens from serogroup 2, 3, or 4.     

Coinoculation with Hartmannella vermiformis enhances replicative Legionella pneumophila lung infection in a murine model of Legionnaires' disease.

The effect of inhaled amoebae on the pathogenesis of Legionnaires' disease was investigated in vivo. A/J mice, which are susceptible to replicative Legionella pneumophila infections, were inoculated intratracheally with L. pneumophila (10(6) bacteria per mouse) or were coinoculated with L. pneumophila (10(6) bacteria per mouse) and Hartmannella vermiformis (10(6) amoebae per mouse). The effect of coinoculation with H. vermiformis on bacterial clearance, histopathology, cellular recruitment into the lung, and intrapulmonary levels of cytokines including gamma interferon and tumor necrosis factor alpha was subsequently assessed. Coinoculation with H. vermiformis significantly enhanced intrapulmonary growth of L. pneumophila in A/J mice. Histopathologic and flow cytometric analysis of lung tissue demonstrated that while A/J mice inoculated with L. pneumophila alone develop multifocal pneumonitis which resolves with minimal mortality, mice coinoculated with H. vermiformis develop diffuse pneumonitis which is associated with diminished intrapulmonary recruitment of lymphocytes and mononuclear phagocytic cells and significant mortality. Furthermore, coinoculation of mice with H. vermiformis resulted in a fourfold enhancement in intrapulmonary levels of gamma interferon and tumor necrosis factor alpha compared with mice infected with L. pneumophila alone. The effect of H. vermiformis on intrapulmonary growth of L. pneumophila in a resistant host (i.e., BALB/c mice) was subsequently evaluated. While BALB/c mice do not develop replicative L. pneumophila infections following inoculation with L. pneumophila alone, there was an eightfold increase in intrapulmonary L. pneumophila in BALB/c mice coinoculated with H. vermiformis. These studies, demonstrating that intrapulmonary amoebae potentiate replicative L. pneumophila lung infection in both a susceptible and a resistant host, have significant implications with regard to the potential role of protozoa in the pathogenesis of pulmonary diseases due to inhaled pathogens and in the design of strategies to prevent and/or control legionellosis.     

Shuttle mutagenesis of Legionella pneumophila: identification of a gene associated with host cell cytopathicity.

We performed shuttle mutagenesis of Legionella pneumophila. Mutants were screened for reduced cellular infectivity. Approximately 10% of the mutants had decreased cytopathicity. The DNA sequence of one locus was determined; the inferred amino acid sequence revealed homology with transport proteins including Escherichia coli TolC, Bordetella pertussis CyaE, and Alcaligenes eutrophus CzcC and CnrC.     

呼吸道合胞病毒F蛋白基因片段真核细胞表达纯化及其ELISA建立的研究

目的表达纯化人呼吸道合胞病毒(HRSV)F蛋白基因片段,建立间接夹心酶联免疫吸附分析法(ELISA),为进一步大量表达F基因、构建基因工程疫苗和快速临床检测奠定基础。方法逆转录并扩增F基因中的F1片段,连接到载体中,转染真核细胞,诱导并纯化重组F蛋白。将纯化重组F蛋白免疫小鼠制备抗体血清,并建立间接夹心ELISA。通过100份标本应用"金标法"对所建立的方法进行验证。结果成功获得F基因中F1片段的扩增产物,筛选阳性克隆,得到相对分子质量为45000的大量表达蛋白。确定了间接夹心ELISA的最佳反应条件和工作浓度;鼠抗F1IgG最佳质量浓度为3.2μg/mL,样品最佳反应时间为37℃70 min,酶标兔抗鼠IgG最佳工作浓度为1∶6 000。与"金标法"对比,阳性样品的变异系数为3.2%～8.6%,阴性样品的变异系数为5.1%～8.3%,均小于10.0%,表明该方法有良好的重复性。结论构建的重组真核细胞能够大量表达F蛋白,纯化的F蛋白具有很好的免疫原性,制备的抗体血清用于ELISA能够得到准确的检测结果。     

Clinical Application of a Multiplex Real-Time PCR Assay for Simultaneous Detection of Legionella Species,Legionella pneumophila,and Legionella pneumophila Serogroup 1

We developed a single-tube multiplex real-time PCR assay capable of simultaneously detecting and discriminating Legionella spp., Legionella pneumophila, and Legionella pneumophila serogroup 1 in primary specimens. Evaluation of 21 clinical specimens and 115 clinical isolates demonstrated this assay to be a rapid, high-throughput diagnostic test with 100% specificity that may aid during legionellosis outbreaks and epidemiologic investigations.     

Identification and subcellular localization of a putative cell-to-cell transport protein from red clover mottle virus

To investigate the mode of gene expression of red clover mottle virus (RCMV) middle component (M) RNA, we have synthesized an oligopeptide corresponding to the predicted carboxy-terminus of the RCMV counterparts of the cowpea mosaic virus (CPMV) 48K and 58K proteins. Using an antiserum raised against this synthetic oligopeptide, we have detected a 43-kDa protein in the 30,000 g pellet from extracts of RCMV-infected cowpea protoplasts. Immunogold cytochemistry further localized this protein to the plasmodesmata of RCMV-infected pea tissue. This subcellular location, taken together with other evidence, suggests that this 43-kDa protein has a role in the cell-to-cell spread of RCMV.     

Evaluation of Legionella V-TesT for the detection of Legionella pneumophila antigen in urine samples

We evaluated a new immunochromatographic assay (Legionella V-TesT, Coris BioConcept, Gembloux, Belgium) for its ability to detect Legionella pneumophila serogroup 1 antigen in urine. Test devices were read at various time points to determine the optimum incubation time regarding performance. The results were compared with those obtained with the BinaxNOW urinary antigen test. The sensitivity and specificity were 82.2% and 98.6%, respectively, for the Legionella V-TesT and 83.9% and 100%, respectively, for the BinaxNOW urinary antigen test after 15 min of incubation. When tests were examined after 60 min, the sensitivity for both tests increased to 91.5%.     

Characterization of the icmH and icmF genes required for Legionella pneumophila intracellular growth, genes that are present in many bacteria associated with eukaryotic cells

Legionella pneumophila, the causative agent of Legionnaires' disease, replicates intracellularly within a specialized phagosome of mammalian and protozoan host cells, and the Icm/Dot type IV secretion system has been shown to be essential for this process. Unlike all the other known Icm/Dot proteins, the IcmF protein, which was described before, and the IcmH protein, which is characterized here, have homologous proteins in many bacteria (such as Yersinia pestis, Salmonella enterica, Rhizobium leguminosarum, and Vibrio cholerae), all of which associate with eukaryotic cells. Here, we have characterized the L. pneumophila icmH and icmF genes and found that both genes are present in 16 different Legionella species examined. The icmH and icmF genes were found to be absolutely required for intracellular multiplication in Acanthamoeba castellanii and partially required for intracellular growth in HL-60-derived human macrophages, for immediate cytotoxicity, and for salt sensitivity. Mutagenesis of the predicted ATP/GTP binding site of IcmF revealed that the site is partially required for intracellular growth in A. castellanii. Analysis of the regulatory region of the icmH and icmF genes, which were found to be cotranscribed, revealed that it contains at least two regulatory elements. In addition, an icmH::lacZ fusion was shown to be activated during stationary phase in a LetA- and RelA-dependent manner. Our results indicate that although the icmH and icmF genes probably have a different evolutionary origin than the rest of the icm/dot genes, they are part of the icm/dot system and are required for L. pneumophila pathogenesis.     

Protein profiles of Legionella pneumophila Philadelphia-1 grown in macrophages and characterization of a gene encoding a novel 24 kDa Legionella protein

Legionella pneumophila Philadelphia-1 strain was grown in cultured macrophages of guinea pigs, hamsters, and A/J mice and the bacteria were purified by Percoll density gradient centrifugation without any detergent. Patterns of the bacterial proteins were compared by SDS-PAGE with those of organisms cultured in vitro. A 24 kDa protein was a major protein of intracellularly grown bacteria: its expression was about four times as much as a 24 kDa protein of agar-grown bacteria. At least three novel proteins (100, 65, and 16 kDa) that were not found on agar-grown bacteria were also observed. In this paper, we focused on the 24 kDa major Legionella protein expressed within macrophages. Western blot and N-terminal amino acid analysis revealed that this protein is a novel protein different from Legionella proteins previously reported, including a 24 kDa macrophage infectivity potentiator protein (Mip). On the basis of amino acid sequence (MQRIKKI and IANAQGK), two kinds of oligonucleotides were synthesized and radiolabeled. Using these oligonucleotides as DNA probes, a 7.2 kb EcoRI-digested DNA fragment encoding the 24 kDa protein was cloned into λZAP II phage vector in Escherichia coli XL1-Blue. A 0.9 kb DNA fragment from the 7.2 kb fragment was further subcloned into pUC118 in E. coli CSR603 for maxicell analysis or XL1-Blue for DNA sequencing. Maxicells which carry recombinant plasmids consisting of the 0.9 kb DNA fragment and vector plasmid pUC118 expressed the 24 kDa protein. When the DNA fragment encoding the protein was sequenced, an open reading frame of 555 base pairs was identified. The inferred polypeptide had a molecular weight of 20 kDa and an estimated isoelectric point of 8.14. Both the nucleotide sequence and the deduced amino acid sequence were distinct from those of bacterial proteins reported previously, suggesting that the protein is a novel Legionella protein.     

Identification of Linked Legionella pneumophila Genes Essential for Intracellular Growth and Evasion of the Endocytic Pathway

Legionella pneumophila replicates within a specialized phagosome in cultured cells, a function necessary for its pathogenicity. The replicative phagosome lacks membrane marker proteins, such as the glycoprotein LAMP-1, that are indicators of the normal endocytic pathway. We describe the isolation of several Legionella genes essential for intracellular growth and evasion of the endocytic pathway, using a genetic and cell biological approach. We screened 4,960 ethyl methanesulfonate-mutagenized colonies for defects in intracellular growth and trafficking to the replicative phagosome. Six mutant strains of L. pneumophila that had severe intracellular growth defects in mouse bone marrow-derived macrophages were identified. All six mutants were found in phagosomes that colocalized with LAMP-1, indicating defects in intracellular trafficking. The growth defects of two of these strains were complemented by molecular clones from a bank constructed from a wild-type L. pneumophila strain. The inserts from these clones are located in a region of the chromosome contiguous with several other genes essential for intracellular growth. Three mutants could be complemented by single open reading frames placed in trans, one mutant by a gene termed dotH and two additional mutants by a gene termed dotO. A deletion mutation was created in a third gene, dotI, which is located directly upstream of dotH. The ΔdotI strain was also defective for intracellular growth in macrophages, and this defect was complemented by a single open reading frame in trans. Based on sequence analysis and structural predictions, possible roles of dotH, dotI, and dotO in intracellular growth are discussed.