Cytotoxicity of extracellular Legionella pneumophila

Legionella pneumophila mediated activation of dendritic cells involves CD14 and TLR2

In this study, we analyzed the activation of bone-marrow derived dendritic cells (BMDCs) from mice lacking the cd14-gene with purified Legionella pneumophila lipopolysaccharide and with viable or formalin-killed L. pneumophila .We found that low concentrations of LPS and doses of L. pneumophila that are relevant to infection are dependent on CD14 to activate BMDCs. Higher concentrations of LPS are able to overcome the lack of CD14 indicating that other receptors are involved. We, therefore, included studies using BMDCs from mice lacking functional TLR2 and/or TLR4 molecules. We found that purified L. pneumophila LPS as well as L. pneumophila either viable or formalin-killed are able to activate BMDCs from TLR4-deficient C3H/HeJ mice but fail to activate BMDCs from TLR2-knockout mice. Our data show that not only purified LPS from L. pneumophila but also the microorganism itself stimulate BMDCs via TLR2 and that this stimulation is dependent on CD14 in this mouse model.

Two signal models in innate immunity

Two-signal models have a rich history in immunology. In the classic two-signal model of T-cell activation, signal one consists of engagement of the T-cell receptor by antigen/major histocompatibility complex, whereas signal two arises from costimulatory ligands on antigen-presenting cells. A requirement for two independent signals helps to ensure that T-cell responses are initiated only in response to bona fide infectious threats. Our studies have led us to conclude that initiation of innate immune responses to pathogens also often requires two signals: signal one is initiated by a microbe-derived ligand, such as lipopolysaccharide (LPS) or flagellin, whereas signal two conveys additional contextual information that often accompanies infectious microbes. Although signal one alone is sufficient to initiate many innate responses, certain responses-particularly ones with the potential for self-damage-require two signals for activation. Many of our studies have employed the intracellular bacterial pathogen Legionella pneumophila, which has been established as a valuable model for understanding innate immune responses. In this review, we discuss how the innate immune system integrates multiple signals to generate an effective response to L. pneumophila and other bacterial pathogens.

Legionella pneumophila induces cathepsin B-dependent necrotic cell death with releasing high mobility group box1 in macrophages

BACKGROUND

Legionella pneumophila (LPN) can cause a lethal infectious disease with a marked inflammatory response in humans. However, the mechanism of this severe inflammation remains poorly understood. Since necrosis is known to induce inflammation, we investigated whether LPN induces necrosis in macrophages. We also analyzed the involvement of lysosomal cathepsin B in LPN-induced cell death.

METHODS

The human monocytic cell line THP-1 was infected with LPN, NUL1 strain. MG132-treated cells were used as apoptotic control cells. After infection, the type of cell death was analyzed by using microscopy, LDH release and flow cytometry. As a proinflammatory mediator, high-mobility group box 1 (HMGB-1), was measured. Cathepsin B activity was also measured and the inhibitory effects of cathepsin B on LPN-induced cell death were analyzed.

RESULTS

THP-1 cells after treatment with high dose of LPN showed necrotic features with releasing HMGB-1. This necrosis and the HMGB-1 release were inhibited by a specific lysosomal cathepsin B inhibitor and were characterized by a rapid and high activation of cathepsin B that was not observed in apoptotic control cells. The necrosis was also accompanied by cathepsin B-dependent poly(ADP-ribose) polymerase (PARP) cleavage.

CONCLUSIONS

We demonstrate here that L. pneumophila rapidly induces cathepsin B-dependent necrosis in a dose-dependent manner and releases a proinflammatory mediator, HMGB-1, from macrophages. This report describes a novel aspect of the pathogenesis of Legionnaires' disease and provides a possible therapeutic target for the regulation of inflammation.

Molecular pathogenesis of infections caused by Legionella pneumophila

The genus Legionella contains more than 50 species, of which at least 24 have been associated with human infection. The best-characterized member of the genus, Legionella pneumophila, is the major causative agent of Legionnaires' disease, a severe form of acute pneumonia. L. pneumophila is an intracellular pathogen, and as part of its pathogenesis, the bacteria avoid phagolysosome fusion and replicate within alveolar macrophages and epithelial cells in a vacuole that exhibits many characteristics of the endoplasmic reticulum (ER). The formation of the unusual L. pneumophila vacuole is a feature of its interaction with the host, yet the mechanisms by which the bacteria avoid classical endosome fusion and recruit markers of the ER are incompletely understood. Here we review the factors that contribute to the ability of L. pneumophila to infect and replicate in human cells and amoebae with an emphasis on proteins that are secreted by the bacteria into the Legionella vacuole and/or the host cell. Many of these factors undermine eukaryotic trafficking and signaling pathways by acting as functional and, in some cases, structural mimics of eukaryotic proteins. We discuss the consequences of this mimicry for the biology of the infected cell and also for immune responses to L. pneumophila infection.

Suppression of dendritic cell activation by anthrax lethal toxin and edema toxin depends on multiple factors including cell source, stimulus used, and function tested

Bacillus anthracis produces lethal toxin (LT) and edema toxin (ET), and they suppress the function of LPS-stimulated dendritic cells (DCs). Because DCs respond differently to various microbial stimuli, we compared toxin effects in bone marrow DCs stimulated with either LPS or Legionella pneumophila (Lp). LT, not ET, was more toxic for cells from BALB/c than from C57BL/6 (B6) as measured by 7-AAD uptake; however, ET suppressed CD11c expression. LT suppressed IL-12, IL-6, and TNF-alpha in cells from BALB/c and B6 mice but increased IL-1beta in LPS-stimulated cultures. ET also suppressed IL-12 and TNF-alpha, but increased IL-6 and IL-1beta in Lp-stimulated cells from B6. Regarding maturation marker expression, LT increased MHCII and CD86 while suppressing CD40 and CD80; ET generally decreased marker expression across all groups. We conclude that the suppression of cytokine production by anthrax toxins is dependent on variables, including the source of the DCs, the type of stimulus and cytokine measured, and the individual toxin tested. However, LT and ET enhancement or suppression of maturation marker expression is more related to the marker studied than the stimuli or cell source. Anthrax toxins are not uniformly suppressive of DC function but instead can increase function under defined conditions.

The guinea pig as a model of infectious diseases

The words 'guinea pig' are synonymous with scientific experimentation, but much less is known about this species than many other laboratory animals. This animal model has been used for approximately 200 y and was the first to be used in the study of infectious diseases such as tuberculosis and diphtheria. Today the guinea pig is used as a model for a number of infectious bacterial diseases, including pulmonary, sexually transmitted, ocular and aural, gastrointestinal, and other infections that threaten the lives of humans. Most studies on the immune response to these diseases, with potential therapies and vaccines, have been conducted in animal models (for example, mouse) that may have less similarity to humans because of the large number of immunologic reagents available for these other species. This review presents some of the diseases for which the guinea pig is regarded as the premier model to study infections because of its similarity to humans with regard to symptoms and immune response. Furthermore, for diseases in which guinea pigs share parallel pathogenesis of disease with humans, they are potentially the best animal model for designing treatments and vaccines. Future studies of immune regulation of these diseases, novel therapies, and preventative measures require the development of new immunologic reagents designed specifically for the guinea pig.

Infection of cultured human endothelial cells by Legionella pneumophila

Legionella pneumophila is a gram-negative pathogen that causes a severe pneumonia known as Legionnaires' disease. Here, we demonstrate for the first time that L. pneumophila infects and grows within cultured human endothelial cells. Endothelial infection may contribute to lung damage observed during Legionnaires' disease and to systemic spread of this organism.

Components of the Legionella pneumophila flagellar regulon contribute to multiple virulence traits, including lysosome avoidance and macrophage death

Legionella pneumophila is a motile intracellular pathogen of macrophages and amoebae. When nutrients become scarce, the bacterium induces expression of transmission traits, some of which are dependent on the flagellar sigma factor FliA (sigma(28)). To test how particular components of the L. pneumophila flagellar regulon contribute to virulence, we compared a fliA mutant with strains whose flagellar construction is disrupted at various stages. We find that L. pneumophila requires FliA to avoid lysosomal degradation in murine bone marrow-derived macrophages (BMM), to regulate production of a melanin-like pigment, and to regulate binding to the dye crystal violet, whereas motility, flagellar secretion, and external flagella or flagellin are dispensable for these activities. Thus, in addition to flagellar genes, the FliA sigma factor regulates an effector(s) or regulator(s) that contributes to other transmissive traits, notably inhibition of phagosome maturation. Whether or not the microbes produced flagellin, all nonmotile L. pneumophila mutants bound BMM less efficiently than the wild type, resulting in poor infectivity and a loss of contact-dependent death of BMM. Therefore, bacterial motility increases contact with host cells during infection, but flagellin is not an adhesin. When BMM contact by each nonmotile strain was promoted by centrifugation, all the mutants bound BMM similarly, but only those microbes that synthesized flagellin induced BMM death. Thus, the flagellar regulon equips the aquatic pathogen L. pneumophila to coordinate motility with multiple traits vital to virulence.

Cloning and characterization of the gene encoding the major cell-associated phospholipase A of Legionella pneumophila, plaB, exhibiting hemolytic activity

Legionella pneumophila, the causative agent of Legionnaires' disease, is an intracellular pathogen of amoebae, macrophages, and epithelial cells. The pathology of Legionella infections involves alveolar cell destruction, and several proteins of L. pneumophila are known to contribute to this ability. By screening a genomic library of L. pneumophila, we found an additional L. pneumophila gene, plaB, which coded for a hemolytic activity and contained a lipase consensus motif in its deduced protein sequence. Moreover, Escherichia coli harboring the L. pneumophila plaB gene showed increased activity in releasing fatty acids predominantly from diacylphospho- and lysophospholipids, demonstrating that it encodes a phospholipase A. It has been reported that culture supernatants and cell lysates of L. pneumophila possess phospholipase A activity; however, only the major secreted lysophospholipase A PlaA has been investigated on the molecular level. We therefore generated isogenic L. pneumophila plaB mutants and tested those for hemolysis, lipolytic activities, and intracellular survival in amoebae and macrophages. Compared to wild-type L. pneumophila, the plaB mutant showed reduced hemolysis of human red blood cells and almost completely lost its cell-associated lipolytic activity. We conclude that L. pneumophila plaB is the gene encoding the major cell-associated phospholipase A, possibly contributing to bacterial cytotoxicity due to its hemolytic activity. On the other hand, in view of the fact that the plaB mutant multiplied like the wild type both in U937 macrophages and in Acanthamoeba castellanii amoebae, plaB is not essential for intracellular survival of the pathogen.

Activation of caspase-3 by the Dot/Icm virulence system is essential for arrested biogenesis of the Legionella-containing phagosome

The Dot/Icm type IV secretion system of Legionella pneumophila is essential for evasion of endocytic fusion and for activation of caspase-3 during early stages of infection of macrophages, but the mechanisms of manipulating these host cell processes are not known. Here, we show that caspase-3 activation by L. pneumophila is independent of all the known apoptotic pathways that converge on the activation of caspase-3. The cytoplasmic proteins IcmS, IcmR and IcmQ, which are involved in secretion of Dot/Icm effectors, are required for caspase-3 activation. Pretreatment of U937 macrophages and human peripheral blood monocytes (hPBM) with the capase-3 inhibitor (DEVD-fmk) or the paninhibitor of caspases (Z-VAD-fmk) before infection blocks intracellular replication of L. pneumophila in a dose-dependent manner. Inhibition of caspase-3 results in co-localization of the L. pneumophila-containing phagosome (LCP) with the late endosomal/lysosomal marker Lamp-2, and the LCP contains lysosomal enzymes, similar to the dotA mutant, which is defective in caspase-3 activation. However, activation of caspase-3 before infection does not rescue the replication defect of the dotA mutant. Interestingly, inhibition of caspase-3 after a 15 or 30 min infection period by the parental strain has no detectable effect on the formation of a replicative niche. The Dot/Icm-mediated activation of caspase-3 by L. pneumophila specifically cleaves, in a dose- and time-dependent manner, the Rab5 effector Rabaptin-5, which maintains Rab5-GTP on the endosomal membrane. In addition, PI3 kinase, which is a crucial effector of Rab5 downstream of Rababptin-5, is not required for intracellular replication. Using single-cell analysis, we show that apoptosis is not evident in the infected cell until bacterial replication results in > 20 bacteria per cell. We conclude that activation of caspase-3 by the Dot/Icm virulence system of L. pneumophila is essential for halting biogenesis of the LCP through the endosomal/lysosomal pathway, and that this is associated with the cleavage of Rabpatin-5.

Hyperoxia mediates acute lung injury and increased lethality in murine Legionella pneumonia: the role of apoptosis

Legionella pneumophila is a major cause of life-threatening pneumonia, which is characterized by a high incidence of acute lung injury and resultant severe hypoxemia. Mechanical ventilation using high oxygen concentrations is often required in the treatment of patients with L. pneumophila pneumonia. Unfortunately, oxygen itself may propagate various forms of tissue damage, including acute lung injury. The effect of hyperoxia as a cofactor in the course of L. pneumophila pneumonia is poorly understood. In this study, we show that exposure to hyperoxic conditions during the evolution of pneumonia results in a marked increase in lethality in mice with Legionella pneumonia. The enhanced lethality was associated with an increase in lung permeability, but not changes in either lung bacterial burden or leukocyte accumulation. Interestingly, accelerated apoptosis as evidenced by assessment of histone-DNA fragments and caspase-3 activity were noted in the infected lungs of mice exposed to hyperoxia. TUNEL staining of infected lung sections demonstrated increased apoptosis in hyperoxic mice, predominantly in macrophages and alveolar epithelial cells. In vitro exposure of primary murine alveolar epithelial cells to Legionella in conjunction with hyperoxia accelerated apoptosis and loss of barrier function. Fas-deficient mice demonstrated partial resistance to the lethal effects of Legionella infection induced by hyperoxia, which was associated with attenuated apoptosis in the lung. These results demonstrate that hyperoxia serves as an important cofactor for the development of acute lung injury and lethality in L. pneumophila pneumonia. Exaggerated apoptosis, in part through Fas-mediated signaling, may accelerate hyperoxia-induced acute lung injury in Legionella pneumonia.

A mutation in an ompR-like gene on a Legionella longbeachae serogroup 1 plasmid attenuates virulence

Examination of a panel of Legionella longbeachae serogroup 1 strains using a guinea pig model of virulence determined that this clonal species of Legionella shows a remarkable variation in symptoms and disease outcome (Doyle et al., Infect. Immun. 69, 5335-5344, 2001). The presence of plasmids was investigated, as plasmid encoded functions may contribute to the virulence of genetically similar strains. Partial sequence analysis of a large native plasmid (approximately 120 kb), designated pA5H5, from a highly virulent Australian isolate revealed a putative two-component regulatory system with inferred identity to the OmpR family of two-component transcriptional regulatory proteins and EnvZ sensor kinases. An isogenic mutant was constructed in the transcriptional regulatory gene, designated lrpR (L. longbeachae sg 1 regulatory protein) and this strain was tested in Acanthamoeba, U937 cells and in a guinea pig animal model. The mutant was reduced in intracellular multiplication within Acanthamoeba but not U937 macrophage-like cells. However, the lrpR mutant did appear reduced in invasion at the early stages of infection of U937 cells. The lrpR mutant was also attenuated for virulence in a guinea pig animal model of infection. The importance of plasmid-encoded functions for the pathogenicity of Legionella longbeachae serogroup 1 strains is discussed.

How does Legionella pneumophila exit the host cell?

In recent years, tremendous progress has been made in unraveling the elegant mechanisms by which intracellular pathogens invade host cells and establish intracellular infections. By contrast, our knowledge of the mechanisms of host cell cytolysis and the egress of intracellular pathogens is still in its infancy. Temporal pore-formation-mediated lysis of the host and exit by Legionella pneumophila and Leishmania could provide a new model of egress for other intracellular pathogens, many of which exhibit pore-forming or cytolysin activity

The cytochrome c maturation locus of Legionella pneumophila promotes iron assimilation and intracellular infection and contains a strain-specific insertion sequence element

Previously, we obtained a Legionella pneumophila mutant, NU208, that is hypersensitive to iron chelators when grown on standard Legionella media. Here, we demonstrate that NU208 is also impaired for growth in media that simply lack their iron supplement. The mutant was not, however, impaired for the production of legiobactin, the only known L. pneumophila siderophore. Importantly, NU208 was also highly defective for intracellular growth in human U937 cell macrophages and Hartmannella and Acanthamoeba amoebae. The growth defect within macrophages was exacerbated by treatment of the host cells with an iron chelator. Sequence analysis demonstrated that the transposon disruption in NU208 lies within an open reading frame that is highly similar to the cytochrome c maturation gene, ccmC. CcmC is generally recognized for its role in the heme export step of cytochrome biogenesis. Indeed, NU208 lacked cytochrome c. Phenotypic analysis of two additional, independently derived ccmC mutants confirmed that the growth defect in low-iron medium and impaired infectivity were associated with the transposon insertion and not an entirely spontaneous second-site mutation. trans-complementation analysis of NU208 confirmed that L. pneumophila ccmC is required for cytochrome c production, growth under low-iron growth conditions, and at least some forms of intracellular infection. Although ccm genes have recently been implicated in iron assimilation, our data indicate, for the first time, that a ccm gene can be required for bacterial growth in an intracellular niche. Complete sequence analysis of the ccm locus from strain 130b identified the genes ccmA-H. Interestingly, however, we also observed that a 1.8-kb insertion sequence element was positioned between ccmB and ccmC. Southern hybridizations indicated that the open reading frame within this element (ISLp 1) was present in multiple copies in some strains of L. pneumophila but was absent from others. These findings represent the first evidence for a transposable element in Legionella and the first identification of an L. pneumophila strain-specific gene.

A two-component regulator induces the transmission phenotype of stationary-phase Legionella pneumophila

Pathogenic Legionella pneumophila evolved as a parasite of aquatic amoebae. To persist in the environment, the microbe must be proficient at both replication and transmission. In laboratory cultures, as nutrients become scarce a stringent response-like pathway coordinates exit from the exponential growth phase with induction of traits correlated with virulence, including motility. A screen for mutants that express the flagellin gene poorly identified five activators of virulence: LetA/LetS, a two-component regulator homologous to GacA/GacS of Pseudomonas and SirA/BarA of Salmonella; the stationary-phase sigma factor RpoS; the flagellar sigma factor FliA; and a new locus, letE. Unlike wild type, post-exponential-phase letA and letS mutants were not motile, cytotoxic, sodium sensitive or proficient at infecting macrophages. L. pneumophila also required fliA to become motile, cytotoxic and to infect macrophages efficiently and letE to express sodium sensitivity and maximal motility and cytotoxicity. When induced to express RelA, all of the strains exited the exponential phase, but only wild type converted to the fully virulent form. In contrast, intracellular replication was independent of letA, letS, letE or fliA. Together, the data indicate that, as the nutrient supply wanes, ppGpp triggers a regulatory cascade mediated by LetA/ LetS, RpoS, FliA and letE that coordinates differentiation of replicating L. pneumophila to a transmissible form.

Icm/dot-dependent upregulation of phagocytosis by Legionella pneumophila

Legionella pneumophila is the causative agent of Legionnaires' disease, a severe pneumonia. Dependent on the icm/dot loci, L. pneumophila survives and replicates in macrophages and amoebae within a specialized phagosome that does not fuse with lysosomes. Here, we report that phagocytosis of wild-type L. pneumophila is more efficient than uptake of icm/dot mutants. Compared with the wild-type strain JR32, about 10 times fewer icm/dot mutant bacteria were recovered from HL-60 macrophages in a gentamicin protection assay. The defect in phagocytosis of the mutants could be complemented by supplying the corresponding genes on a plasmid. Using fluorescence microscopy and green fluorescent protein (GFP)-expressing strains, 10-20 times fewer icm/dot mutant bacteria were found to be internalized by HL-60 cells and human monocyte-derived macrophages (HMMPhi). Compared with icm/dot mutants, wild-type L. pneumophila infected two to three times more macrophages and yielded a population of highly infected host cells (15-70 bacteria per macrophage) that was not observed with icm/dot mutant strains. Wild-type and icmT mutant bacteria were found to adhere similarly and compete for binding to HMMPhi. In addition, wild-type L. pneumophila was also phagocytosed more efficiently by Acanthamoeba castellanii, indicating that the process is independent of adherence receptor(s). Wild-type L. pneumophila enhanced phagocytosis of an icmT mutant strain in a synchronous co-infection, suggesting that increased phagocytosis results from (a) secreted effector(s) acting in trans.

Transient transgenic expression of gamma interferon promotes Legionella pneumophila clearance in immunocompetent hosts

Gamma interferon (IFN-gamma) and T1-phenotype immune responses are important components of host defense against a variety of intracellular pathogens, including Legionella pneumophila. The benefit of intrapulmonary adenovirus-mediated IFN-gamma gene therapy was investigated in a nonlethal murine model of experimental L. pneumophila pneumonia. Intratracheal (i.t.) administration of 10(6) CFU of L. pneumophila induced the expression of T1 phenotype cytokines, such as IFN-gamma and interleukin-12 (IL-12). Natural killer cells were identified as the major cellular source of IFN-gamma. To determine if enhanced expression of IFN-gamma in the lung could promote pulmonary clearance of L. pneumophila, we i.t. administered 5 x 10(8) PFU of a recombinant adenovirus vector containing the murine IFN-gamma cDNA (AdmIFN-gamma) concomitant with L. pneumophila. We observed a 10-fold decrease in lung bacterial CFU at day 2 in the AdmIFN-gamma-treated group compared to controls (P < 0.01). Alveolar macrophages isolated from AdmIFN-gamma-treated animals displayed enhanced killing of intracellular L. pneumophila organisms ex vivo. Similar improvements in bacterial clearance were observed with i.t. recombinant IFN-gamma treatment. The transient transgenic expression of IL-12, a known inducer of IFN-gamma and promoter of T1-type immune responses, resulted in more modest improvement in bacterial clearance (sixfold reduction; P < 0.05). These results demonstrate that, even in immunocompetent hosts, exogenous administration or transient transgenic expression of IFN-gamma, and to a lesser extent IL-12, may be of potential therapeutic benefit in the treatment of patients with Legionella pneumonia.

Legionella pneumophila entry gene rtxA is involved in virulence

Successful parasitism of host cells by intracellular pathogens involves adherence, entry, survival, intracellular replication, and cell-to-cell spread. Our laboratory has been examining the role of early events, adherence and entry, in the pathogenesis of the facultative intracellular pathogen Legionella pneumophila. Currently, the mechanisms used by L. pneumophila to gain access to the intracellular environment are not well understood. We have recently isolated three loci, designated enh1, enh2, and enh3, that are involved in the ability of L. pneumophila to enter host cells. One of the genes present in the enh1 locus, rtxA, is homologous to repeats in structural toxin genes (RTX) found in many bacterial pathogens. RTX proteins from other bacterial species are commonly cytotoxic, and some of them have been shown to bind to beta(2) integrin receptors. In the current study, we demonstrate that the L. pneumophila rtxA gene is involved in adherence, cytotoxicity, and pore formation in addition to its role in entry. Furthermore, an rtxA mutant does not replicate as well as wild-type L. pneumophila in monocytes and is less virulent in mice. Thus, we conclude that the entry gene rtxA is an important virulence determinant in L. pneumophila and is likely to be critical for the production of Legionnaires' disease in humans.

Temporal pore formation-mediated egress from macrophages and alveolar epithelial cells by Legionella pneumophila

Legionella pneumophila does not induce apoptosis in the protozoan host, but induces pore formation-mediated cytolysis after termination of intracellular replication (L.-Y. Gao and Y. Abu Kwaik, Environ. Microbiol. 2:79-90, 2000). In contrast to this single mode of killing of protozoa, we have recently proposed a biphasic model by which L. pneumophila kills macrophages, in which the first phase is manifested through the induction of apoptosis during early stages of the infection, followed by an independent and temporal induction of necrosis during late stages of intracellular replication. Here we show that, similar to the protozoan host, the induction of necrosis and cytolysis of macrophages by L. pneumophila is mediated by the pore-forming toxin or activity. This activity is temporally and maximally expressed only upon termination of bacterial replication and correlates with cytolysis of macrophages and alveolar epithelial cells in vitro. We have identified five L. pneumophila mutants defective in the pore-forming activity. The phagosomes harboring the mutants do not colocalize with the late endosomal or lysosomal marker Lamp-1, and the mutants replicate intracellularly similar to the parental strain. Interestingly, despite their prolific intracellular replication, the mutants are defective in cytotoxicity and are "trapped" within and fail to lyse and egress from macrophages and alveolar epithelial cells upon termination of intracellular replication. However, the mutants are subsequently released from the host cell, most likely due to apoptotic death of the host cell. Data derived from cytotoxicity assays, confocal laser scanning microscopy, and electron microscopy confirm the defect in the mutants to induce necrosis of macrophages and the failure to egress from the host cell. Importantly, the mutants are completely defective in acute lethality (24 to 48 h) to intratracheally inoculated A/J mice. We conclude that the pore-forming activity of L. pneumophila is not required for phagosomal trafficking or for intracellular replication. This activity is expressed upon termination of bacterial replication and is essential to induce cytolysis of infected macrophages to allow egress of intracellular bacteria. In addition, this activity plays a major role in pulmonary immunopathology in vivo.

The mechanism of killing and exiting the protozoan host Acanthamoeba polyphaga by Legionella pneumophila

The ability of Legionella pneumophila to cause legionnaires' disease is dependent on its capacity to replicate within cells in the alveolar spaces. The bacteria kill mammalian cells in two phases: induction of apoptosis during the early stages of infection, followed by an independent and rapid necrosis during later stages of the infection, mediated by a pore-forming activity. In the environment, L. pneumophila is a parasite of protozoa. The molecular mechanisms by which L. pneumophila kills the protozoan cells, after their exploitation for intracellular proliferation, are not known. In an effort to decipher these mechanisms, we have examined induction of both apoptosis and necrosis in the protozoan Acanthamoeba polyphaga upon infection by L. pneumophila. Our data show that, although A. polyphaga undergoes apoptosis following treatment with actinomycin D, L. pneumophila does not induce apoptosis in these cells. Instead, intracellular L. pneumophila induces necrotic death in A. polyphaga, which is mediated by the pore-forming activity. Mutants of L. pneumophila defective in expression of the pore-forming activity are indistinguishable from the parental strain in intracellular replication within A. polyphaga. The parental strain bacteria cause necrosis-mediated lysis of all the A. polyphaga cells within 48 h after infection, and all the intracellular bacteria are released into the tissue culture medium. In contrast, all cells infected by the mutants remain intact, and the intracellular bacteria are 'trapped' within A. polyphaga after the termination of intracellular replication. Failure to exit the host cell after termination of intracellular replication results in a gradual decline in the viability of the mutant strain bacteria within A. polyphaga starting 48h after infection. Our data show that the pore-forming activity of L. pneumophila is not required for intracellular bacterial replication within A. polyphaga but is required for killing and exiting the protozoan host upon termination of intracellular replication.

Legionella pneumophila pathogesesis: a fateful journey from amoebae to macrophages

Legionella pneumophila first commanded attention in 1976, when investigators from the Centers for Disease Control and Prevention identified it as the culprit in a massive outbreak of pneumonia that struck individuals attending an American Legion convention (). It is now clear that this gram-negative bacterium flourishes naturally in fresh water as a parasite of amoebae, but it can also replicate within alveolar macrophages. L. pneumophila pathogenesis is discussed using the following model as a framework. When ingested by phagocytes, stationary-phase L. pneumophila bacteria establish phagosomes which are completely isolated from the endosomal pathway but are surrounded by endoplasmic reticulum. Within this protected vacuole, L. pneumophila converts to a replicative form that is acid tolerant but no longer expresses several virulence traits, including factors that block membrane fusion. As a consequence, the pathogen vacuoles merge with lysosomes, which provide a nutrient-rich replication niche. Once the amino acid supply is depleted, progeny accumulate the second messenger guanosine 3',5'-bispyrophosphate (ppGpp), which coordinates entry into the stationary phase with expression of traits that promote transmission to a new phagocyte. A number of factors contribute to L. pneumophila virulence, including type II and type IV secretion systems, a pore-forming toxin, type IV pili, flagella, and numerous other factors currently under investigation. Because of its resemblance to certain aspects of Mycobacterium, Toxoplasma, Leishmania, and Coxiella pathogenesis, a detailed description of the mechanism used by L. pneumophila to manipulate and exploit phagocyte membrane traffic may suggest novel strategies for treating a variety of infectious diseases. Knowledge of L. pneumophila ecology may also inform efforts to combat the emergence of new opportunistic macrophage pathogens.

The induction of apoptosis by bacterial pathogens

Apoptosis is a highly regulated process of cell death that is required for the development and homeostasis of multicellular organisms. In contrast to necrosis, apoptosis eliminates individual cells without inducing an inflammatory response. Activation or prevention of cell death could be a critical factor in the outcome of an infection. Programmed cell death has been observed as a response to infection by a wide range of animal and plant pathogens and is mediated by an array of pathogen-encoded virulence determinants. Pathogen-induced modulation of the host cell-death pathway may serve to eliminate key immune cells or evade host defenses that can act to limit the infection. Alternatively, suppression of the death pathway may facilitate the proliferation of intracellular pathogens.

Heterogeneity in intracellular replication and cytopathogenicity of Legionella pneumophila and Legionella micdadei in mammalian and protozoan cells

In contrast to Legionella pneumophila, little is known about the pathogenesis of other legionellae species that are capable of causing Legionnaires' disease. In this report, we contrast L. pneumophila and L. micdadei for their cytopathogenicity and intracellular replication within mammalian and protozoan cells. We show by transmission electron microscopy that L. micdadei replicates within an endoplasmic reticulum (RER)-free phagosome within human macrophages, alveolar epithelial cells, and within the protozoan Hartmannella vermiformis. In contrast, L. pneumophila replicates within a RER-surrounded phagosome within the same host cells. In contrast to replication of L. pneumophila within Acanthamoebae polyphaga, L. micdadei does not replicate within this protozoan host. Despite the prolific intracellular replication, L. micdadei is less cytopathogenic to all host cells than L. pneumophila. Since both species replicate intracellularly to a similar level, we have examined whether the reduced cytopathogenicity of L. micdadei is due to a reduced capacity to induce apoptosis or pore formation-mediated necrosis, both of which contribute to killing of the host cell by L. pneumophila. The data show that both species induced apoptosis-mediated killing of mammalian cells to a similar level. In contrast to L. pneumophila, expression of the pore-forming toxin by L. micdadei and its necrotic effect on macrophages and alveolar epithelial cells is undetectable. This has been further confirmed showing that L. micdadei is completely defective in contact-dependent haemolysis of RBCs, an activity mediated by the pore-forming toxin. Finally, in contrast to L. pneumophila, there was no significant intrapulmonary replication of L. micdadei in the A/J mice animal model. Our data show dramatic differences between L. pneumophila and L. micdadei in intracellular replication, cytopathogenicity, and infectivity to mammalian and protozoan cells.

Activation of caspase 3 during Legionella pneumophila-induced apoptosis

The hallmark of Legionnaires' disease is replication of Legionella pneumophila within cells in the alveolar spaces. The mechanisms by which L. pneumophila replicates intracellularly and kills the host cell are largely not understood. We have recently shown that within 3 h of initiation of the infection and prior to intracellular replication, L. pneumophila induces apoptosis in macrophages, alveolar epithelial cells, and peripheral blood monocytes, which correlates with cytopathogenicity (L.-Y. Gao and Y. Abu Kwaik, Infect. Immun. 67:862-870, 1999). In this report, we show that the ability of L. pneumophila to induce apoptosis is, largely, not growth phase regulated. We demonstrate that the induction of apoptosis by L. pneumophila in macrophages is mediated through the activation of caspase 3. The enzymatic activity of caspase 3 to cleave a specific synthetic substrate in vitro is detected in L. pneumophila-infected macrophages at 2 h after infection and is maximal at 3 h, with over 900% increase in activity. The activity of caspase 3 to cleave a specific substrate [poly(ADP-ribose) polymerase, or PARP] in vivo is also detected at 2 h and is maximal at 3 h postinfection. The activity of caspase 3 to cleave the synthetic substrate in vitro and PARP in vivo is blocked by a specific inhibitor of caspase 3. The kinetics of caspase 3 activation correlates with that of L. pneumophila-induced nuclear apoptosis. Inhibition of caspase 3 activity blocks L. pneumophila-induced nuclear apoptosis and cytopathogenicity during early stages of the infection. Consistent with the ability to induce apoptosis, extracellular L. pneumophila also activates caspase 3. Three dotA/icmWXYZ mutants of L. pneumophila that are defective in inducing apoptosis do not induce caspase 3 activation, suggesting that expression and/or export of the apoptosis-inducing factor(s) is regulated by the dot/icm virulence system. This is the first description of the role of caspase 3 activation in induction of nuclear apoptosis in the host cell infected by a bacterial pathogen.

Comparative analysis of Legionella pneumophila and Legionella micdadei virulence traits

While the majority of Legionnaire's disease has been attributed to Legionella pneumophila, Legionella micdadei can cause a similar infection in immunocompromised people. Consistent with its epidemiological profile, the growth of L. micdadei in cultured macrophages is less robust than that of L. pneumophila. To identify those features of the Legionella spp. which are correlated to efficient growth in macrophages, two approaches were taken. First, a phenotypic analysis compared four clinical isolates of L. micdadei to one well-characterized strain of L. pneumophila. Seven traits previously correlated with the virulence of L. pneumophila were evaluated: infection and replication in cultured macrophages, evasion of phagosome-lysosome fusion, contact-dependent cytotoxicity, sodium sensitivity, osmotic resistance, and conjugal DNA transfer. By nearly every measure, L. micdadei appeared less virulent than L. pneumophila. The surprising exception was L. micdadei 31B, which evaded lysosomes and replicated in macrophages as efficiently as L. pneumophila, despite lacking both contact-dependent cytopathicity and regulated sodium sensitivity. Second, in an attempt to identify virulence factors genetically, an L. pneumophila genomic library was screened for clones which conferred robust intracellular growth on L. micdadei. No such loci were isolated, consistent with the multiple phenotypic differences observed for the two species. Apparently, L. pneumophila and L. micdadei use distinct strategies to colonize alveolar macrophages, causing Legionnaire's disease.

Co-ordination of legionella pneumophila virulence with entry into stationary phase by ppGpp

Legionella pneumophila survives in aquatic environments, but replicates within amoebae or the alveolar macrophages of immunocompromised individuals. Here, the signal transduction pathway that co-ordinates L. pneumophila virulence expression in response to amino acid depletion was investigated. To facilitate kinetic and genetic studies, a phenotypic reporter of virulence was engineered by fusing flaA promoter sequences to a gene encoding green fluorescent protein. When subjected to amino acid depletion, L. pneumophila accumulated ppGpp and converted from a replicative to a virulent state, as judged by motility and sodium sensitivity. ppGpp appeared to initiate this response, as L. pneumophila induced to express the Escherichia coli RelA ppGpp synthetase independently of nutrient depletion accumulated ppGpp, exited the exponential growth phase and expressed flaAgfp, motility, sodium sensitivity, cytotoxicity and infectivity, five traits correlated with virulence. Although coincident with the stationary phase, L. pneumophila virulence expression appeared to require an additional factor: mutant Lp120 accumulated ppGpp and acquired two stationary phase traits but none of six virulence phenotypes analysed. We propose that, when nutrients are limiting, ppGpp acts as an alarmone, triggering the expression of multiple traits that enable L. pneumophila to escape its spent host, to survive and disperse in the environment and to re-establish a protected intracellular replication niche.

Pore-forming activity is not sufficient for Legionella pneumophila phagosome trafficking and intracellular growth

Bacterial pathogens often subvert eukaryotic cellular processes in order to establish a replicative niche and evade host immunity. Inhibition of phagosome lysosome fusion is a strategy used by several intracellular bacteria that grow within mammalian cells. It was shown recently that Legionella pneumophila possesses a cytolytic activity that results from the insertion of pores in the macrophage membrane upon contact, and that this activity requires the dot/icm gene products, which are necessary for intracellular growth and phagosome trafficking. Other bacteria that inhibit phagosome lysosome fusion, such as Mycobacterium tuberculosis, demonstrate similar cytolytic activities, which suggests that formation of pores in the phagosome membrane may account for the defects observed in phagosome trafficking. In this study, we identify a new class of L. pneumophila mutant that retains the pore-forming activity found in virulent bacteria, but is defective in phagosome lysosome fusion inhibition and intracellular growth. These data indicate that cytolytic activity is not sufficient for L. pneumophila-induced alterations in phagosome trafficking. Rather, the pore may be a vehicle that facilitates delivery of bacterial-derived effector molecules to the host cell cytoplasm.

Cell biology of Legionella pneumophila

Legionella pneumophila is the causative agent of a potentially fatal form of pneumonia named Legionnaires' disease. L. pneumophila survives and replicates inside macrophages by preventing phagosome-lysosome fusion. A large number of L. pneumophila genes, called dot or icm, have been identified that are required for intracellular growth. It has recently been shown that the dot/icm genes code for a putative large membrane complex that forms a type IV secretion system used to alter the endocytic pathway.

Apoptosis in macrophages and alveolar epithelial cells during early stages of infection by Legionella pneumophila and its role in cytopathogenicity

The hallmark of Legionnaires' disease is intracellular replication of Legionella pneumophila within cells in the alveolar spaces. Cytopathogenicity of this bacterium to the host cell has been well demonstrated, but the mechanisms of host cell death due to infection by L. pneumophila are not well understood. In this study, induction of apoptosis in macrophages and alveolar epithelial cells by L. pneumophila during early stages of infection was confirmed by using multiple criteria, including DNA fragmentation by agarose gel electrophoresis, terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling, surface exposure of phosphatidylserine, and cellular morphology by transmission electron microscopy. Induction of nuclear apoptosis in L. pneumophila-infected macrophages is mediated by activation of the caspase cascade death machinery. We provide genetic and biochemical evidence that L. pneumophila-induced apoptosis in macrophages and alveolar epithelial cells does not require intracellular bacterial replication or new protein synthesis. In addition, extracellular L. pneumophila is capable of inducing apoptosis. Furthermore, induction of apoptosis by L. pneumophila correlates with cytopathogenicity. We conclude that L. pneumophila-induced apoptosis in macrophages and alveolar epithelial cells plays an important role in cytopathogenicity to the host cell during early stages of infection.

Legionella pneumophila kills human phagocytes but not protozoan host cells by inducing apoptotic cell death

Legionella pneumophila is a facultative intracellular parasite able to replicate within and to kill a variety of eukaryotic cells. One possible killing mechanism is the induction of programmed cell death. Based on electron microscopy and flow cytometry studies using the phosphatidylserine binding protein annexin V, we could demonstrate that L. pneumophila is able to induce apoptosis in human monocytes which was clearly dependent on the multiplicity of infection, the time postinfection and the intracellular location of the bacteria. Furthermore, it became evident that Legionella-induced apoptosis does not require the TNF-alpha mediated signal-transduction pathway. By studying infection in Acanthamoeba castellanii, we found that L. pneumophila is not able to induce programmed cell death in their natural host cells indicating that different mechanisms are responsible for host cell killing in protozoan and human cells.

Early events in phagosome establishment are required for intracellular survival of Legionella pneumophila

During infection, the Legionnaires' disease bacterium, Legionella pneumophila, survives and multiplies within a specialized phagosome that is near neutral pH and does not fuse with host lysosomes. In order to understand the molecular basis of this organism's ability to control its intracellular fate, we have isolated and characterized a group of transposon-generated mutants which were unable to kill macrophages and were subsequently found to be defective in intracellular multiplication. These mutations define a set of 20 genes (19 icm [for intracellular multiplication] genes and dotA [for defect in organelle trafficking]). In this report, we describe a quantitative assay for phagosome-lysosome fusion (PLF) and its use to measure the levels of PLF in cells that have been infected with either wild-type L. pneumophila or one of several mutants defective in different icm genes or dotA. By using quantitative confocal fluorescence microscopy, PLF could be scored on a per-bacterium basis by determining the extent to which fluorescein-labeled L. pneumophila colocalized with host lysosomes prelabeled with rhodamine-dextran. Remarkably, mutations in the six genes that were studied resulted in maximal levels of PLF as quickly as 30 min following infection. These results indicate that several, and possibly all, of the icm and dotA gene products act at an early step during phagosome establishment to determine whether L. pneumophila-containing phagosomes will fuse with lysosomes. Although not ruled out, subsequent activity of these gene products may not be necessary for successful intracellular replication.

How is the intracellular fate of the Legionella pneumophila phagosome determined?

The following pair of articles, the first by Gil Segal and Howard Shuman, and the second by James Kirby and Ralph Isberg (Trends Microbiol. 6, 256-258), explore the genetics and function of the icm/dot genes of Legionella pneumophila. This gene family is implicated in several aspects of virulence and appears to constitute components of a conjugal transfer system that has been adopted to prevent phagosome-lysosome fusion in the host cell and to mediate host cytotoxicity by pore formation. Whether these functions are natural consequences or operate in parallel remains to be discovered.

Expression of Legionella pneumophila virulence traits in response to growth conditions

In nature, Legionella pneumophila replicates exclusively as an intracellular parasite of amoebae, but it also persists in the environment as a free-living microbe. Studies of how this opportunistic pathogen recognizes and responds to distinct extracellular and intracellular environments identified a link between the growth phase and expression of traits previously correlated with virulence. When cultured in broth, only post-exponential-phase L. pneumophila was sodium sensitive, cytotoxic, osmotically resistant, competent to evade macrophage lysosomes, infectious, and motile. Likewise, the L. pneumophila phenotype changed during growth in macrophages. During the intracellular replication period, this bacterium was sodium resistant and lacked flagella; concomitant with macrophage lysis, L. pneumophila became sodium sensitive and flagellated. Expression of the virulent phenotype was a response to starvation, since exponential-phase L. pneumophila became cytotoxic, sodium sensitive, and motile after incubation in broth from stationary-phase cultures, except when it was supplemented with amino acids. Together, these data indicate that while nutrients are plentiful, intracellular L. pneumophila organisms are dedicated to replication; when amino acids become limiting, the progeny express virulence factors to escape the spent host, to disperse and survive in the aquatic environment, and to reestablish a protected intracellular niche favorable for growth.

Multiplication of Legionella pneumophila in HeLa cells in the presence of cytoskeleton and metabolic inhibitors

A study has been carried out on the action of cytoskeleton and metabolic inhibitors on intracellular multiplication in HeLa cells of a virulent strain of Legionella pneumophila serogroup 6. The effects of the substances were separately tested on both penetration and intracellular multiplication of L. pneumophila. Only cytochalasin A and 2-deoxy-D-glucose (2dG) affected bacterial internalisation, whereas intracellular multiplication was inhibited by cytochalasins A, B, C, D and J (D being the most active) and by 2dG with a dose-response effect. The action of 2dG was counteracted by 50 mM glucose. Experiments carried out with cytochalasin D and a rhodamine-phalloidin conjugate showed the involvement of cytoskeletal elements in intracellular multiplication of Legionella; compounds acting on microtubules had no effect.

Simplified quantitative assay system for measuring activities of drugs against intracellular Legionella pneumophila

We developed a new simple assay for the quantitation of the activities of drugs against intracellular Legionella pneumophila. The cells of a murine macrophage-like cell line (J774.1 cells) allowed the intracellular growth and replication of the bacteria, which ultimately resulted in cell death. The infected J774.1 cell monolayers in 96-well microplates were first treated with antibiotics and were further cultured for 72 h. The number of viable J774.1 cells in each well was quantified by a colorimetric assay with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and an enzyme-linked immunosorbent assay reader. The number of growing bacteria in each well was also determined by counting the numbers of CFU on buffered charcoal yeast extract-alpha agar plates. Viable J774.1 cell counts, determined by the colorimetric assay, were inversely proportional to the number of intracellular replicating bacteria. The minimum extracellular concentrations (MIECs) of 24 antibiotics causing inhibition of intracellular growth of L. pneumophila were determined by the colorimetric assay system. The MIECs of beta-lactams and aminoglycosides were markedly higher than the MICs in buffered yeast extract-alpha broth. The MIECs of macrolides, fluoroquinolones, rifampin, and minocycline were similar to the respective MICs. According to their intracellular activities, clarithromycin and sparfloxacin were the most potent among the macrolides or fluoroquinolones tested in this study. Our results indicated that the MTT assay system allows comparative and quantitative evaluations of the intracellular activities of antibiotics and efficient processing of a large number of samples.

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 delta dotI 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.

Evidence for pore-forming ability by Legionella pneumophila

Legionella pneumophila is the cause of Legionnaires' pneumonia. After Internalization by macrophages, it bypasses the normal endocytic pathway and occupies a replicative phagosome bound by endoplasmic reticulum. Here, we show that lysis of macrophages and red blood cells by L. pneumophila was dependent on dotA and other loci known to be required for proper targeting of the phagosome and replication within the host cell. Cytotoxicity occurred rapidly during a high-multiplicity infection, required close association of the bacteria with the eukaryotic cell and was a form of necrotic cell death accompanied by osmotic lysis. The differential cytoprotective ability of high-molecular-weight polyethylene glycols suggested that osmotic lysis resulted from insertion of a pore less than 3 nm in diameter into the plasma membrane. Results concerning the uptake of membrane-impermeant fluorescent compounds of various sizes are consistent with the osmoprotection analysis. Therefore, kinetic and genetic evidence suggested that the apparent ability of L. pneumophila to insert a pore into eukaryotic membranes on initial contact may play a role in altering endocytic trafficking events within the host cell and in the establishment of a replicative vacuole.

Excellent activity of newer quinolones on Legionella pneumophila in J774 macrophages

The activity of six antibiotics directed against intracellularly multiplying Legionella pneumophilia was examined in tissue cultures with J774 macrophages. The drugs tested were the new quinolones, BAY Y 3118 and clinafloxacin, and ciprofloxacin, erythromycin, gentamicin and ampicillin served as reference drugs. Additionally, the MICs of these drugs against L. pneumophila were determined in vitro by broth microdilution. Despite their low MIC values, ampicillin and gentamicin did not inhibit intracellular multiplication of L. pneumophila in J774 macrophages. In contrast, an inhibition of intracellular growth could be demonstrated for the four other antibiotics. The new quinolones BAY Y 3118 and clinafloxacin showed the highest activity against intracellular L. pneumophila. At a concentration of 0.00078 mg/L already, a marked reduction in bacterial counts was seen for both drugs in comparison to the growth control without antibiotics. The corresponding effective concentrations were 0.0125 mg/L for ciprofloxacin and 0.2 mg/L for erythromycin. It may be concluded that new quinolone derivatives might become an alternative to erythromycin and rifampicin which at present are the drugs of primary choice for the treatment of legionnaires' disease.

Evidence for apoptosis of human macrophage-like HL-60 cells by Legionella pneumophila infection

Legionella pneumophila, the causative agent of Legionnaires' disease and Pontiac fever, replicates within and eventually kills human macrophages. In this study, we show that L. pneumophila is cytotoxic to HL-60 cells, a macrophage-like cell line. We demonstrate that cell death mediated by L. pneumophila occurred at least in part through apoptosis, as shown by changes in nuclear morphology, an increase in the proportion of fragmented host cell DNA, and the typical ladder pattern of DNA fragmentation indicative of apoptosis. We further sought to determine whether potential virulence factors like the metalloprotease and the macrophage infectivity potentiator of L. pneumophila are involved in the induction of apoptosis. None of these factors are essential for the induction of apoptosis in HL-60 cells but may be involved in other cytotoxic mechanisms that lead to accidental cell death (necrosis). The ability of L. pneumophila to promote cell death may be important for the initiation of infection, bacterial survival, and escape from the host immune response. Alternatively, the triggering of apoptosis in response to bacterial infection may have evolved as a means of the host immune system to reduce or inhibit bacterial replication.

Legionella pneumophila mutants that are defective for iron acquisition and assimilation and intracellular infection

Legionella pneumophila, a parasite of macrophages and protozoa, requires iron for optimal extracellular and intracellular growth. However, its mechanisms of iron acquisition remain uncharacterized. Using mini-Tn10 mutagenesis, we isolated 17 unique L. pneumophila strains which appeared to be defective for iron acquisition and assimilation. Eleven of these mutants were both sensitive to the iron chelator ethylenediamine di(o-hydroxyphenylacetic acid) and resistant to streptonigrin, an antibiotic whose lethal effect requires high levels of intracellular iron. Six mutants were also defective for the infection of macrophage-like U937 cells. Although none were altered in entry, mutants generally exhibited prolonged lag phases and in some cases replicated at slower rates. Overall, the reduced recoveries of mutants, relative to that of the wild type, ranged from 3- to 1,000-fold. Strain NU216, the mutant displaying the most severe lag phase and the slowest rate of replication, was studied further. Importantly, within U937 cells, NU216 was approximately 100-fold more sensitive than the wild type was to treatment with the Fe3+ chelator deferoxamine, indicating that it is defective for intracellular iron acquisition and assimilation. Furthermore, this strain was unable to mediate any cytopathic effect and was impaired for infectivity of an amoebal host. Taken together, the isolation of these mutants offers genetic proof that iron acquisition and assimilation are critical for intracellular infection by L. pneumophila.