Legionella pneumophila pathogenesis and immunity

Hiding in the yolk: A unique feature of Legionella pneumophila infection of zebrafish

The zebrafish has become a powerful model organism to study host-pathogen interactions. Here, we developed a zebrafish model to dissect the innate immune response to Legionella pneumophila during infection. We show that L. pneumophila cause zebrafish larvae death in a dose dependent manner. Additionally, we show that macrophages are the first line of defence and cooperate with neutrophils to clear the infection. Immunocompromised humans have an increased propensity to develop pneumonia, when either macrophages or neutrophils are depleted, these "immunocompromised" larvae become lethally sensitive to L. pneumophila. Also, as observed in human infections, the adaptor signalling molecule Myd88 is not required to control disease in the larvae. Furthermore, proinflammatory cytokine genes il1β and tnf-α were upregulated during infection, recapitulating key immune responses seen in human infection. Strikingly, we uncovered a previously undescribed infection phenotype in zebrafish larvae, whereby bloodborne, wild type L. pneumophila invade and grow in the larval yolk region, a phenotype not observed with a type IV secretion system deficient mutant that cannot translocate effectors into its host cell. Thus, zebrafish larva represents an innovative L. pneumophila infection model that mimics important aspects of the human immune response to L. pneumophila infection and will allow the elucidation of mechanisms by which type IV secretion effectors allow L. pneumophila to cross host cell membranes and obtain nutrients from nutrient rich environments.

Legionella pneumophila: The Journey from the Environment to the Blood

An outbreak of a potentially fatal form of pneumonia in 1976 and in the annual convention of the American Legion was the first time that Legionella spp. was identified. Thereafter, the term Legionnaires’ disease (LD) was established. The infection in humans is transmitted by the inhalation of aerosols that contain the microorganisms that belong to the Legionellaceae family and the genus Legionella. The genus Legionella contains genetically heterogeneous species and serogroups. The Legionella pneumophila serogroup 1 (Lp1) is the most often detected strain in outbreaks of LD. The pathogenesis of LD infection initiates with the attachment of the bacterial cells to the host cells, and subsequent intracellular replication. Following invasion, Legionella spp. activates its virulence mechanisms: generation of specific compartments of Legionella-containing vacuole (LCV), and expression of genes that encode a type IV secretion system (T4SS) for the translocation of proteins. The ability of L. pneumophila to transmigrate across the lung’s epithelium barrier leads to bacteremia, spread, and invasion of many organs with subsequent manifestations, complications, and septic shock. The clinical manifestations of LD depend on the bacterial load in the aerosol, the virulence factors, and the immune status of the patient. The infection has two distinct forms: the non- pneumatic form or Pontiac fever, which is a milder febrile flu-like illness, and LD, a more severe form, which includes pneumonia. In addition, the extrapulmonary involvement of LD can include heart, brain, abdomen, and joints.

Cavitary Legionella Pneumonia in AIDS: When Intracellular Immunity Failure Leads to Rapid Intrapulmonary Cavitation

Introduction. Legionella is a frequent cause of bacterial pneumonia in patients with AIDS. While multiple organisms have been associated with cavitary pneumonia in this population, Legionella has not. Clinical Case. A middle-aged woman with HIV-AIDS and severely depressed CD-4 count presented with one month of progressively worsening productive cough and dyspnea. Serial imaging showed focal consolidations which multiplied and cavitated over the subsequent days. Legionella urine antigen was positive, and appropriate treatment was continued for 3 weeks total. The patient recovered quickly, and follow-up imaging 8 weeks later showed near-resolution of all lesions. Discussion. Cavitary pneumonia secondary to Legionella has been seldom described, traditionally in the context of immunosuppressive therapy. Patients with AIDS and severely depressed CD4 counts have significantly compromised cell-mediated immunity. This case highlights the importance of consideration for legionellosis in rapidly progressing cavitary pneumonia, especially in patients with severely compromised cell-mediated immunity, including those with HIV-AIDS.

Mechanisms of Effector-Mediated Immunity Revealed by the Accidental Human Pathogen Legionella pneumophila

Many Gram-negative bacterial pathogens employ translocated virulence factors, termed effector proteins, to facilitate their parasitism of host cells and evade host anti-microbial defenses. However, eukaryotes have evolved to detect effector-mediated virulence strategies through a phenomenon termed effector-triggered immunity (ETI). Although ETI was discovered in plants, a growing body of literature demonstrates that metazoans also utilize effector-mediated immunity to detect and clear bacterial pathogens. This mini review is focused on mechanisms of effector-mediated immune responses by the accidental human pathogen Legionella pneumophila. We highlight recent advancements in the field and discuss the future prospects of harnessing effectors for the development of novel therapeutics, a critical need due to the prevalence and rapid spread of antibiotic resistance.

Potential Role of Gr-1+ CD8+ T Lymphocytes as a Source of Interferon-γ and M1/M2 Polarization during the Acute Phase of Murine Legionella pneumophila Pneumonia

In this study, we analyzed interferon (IFN)-γ-producing cells and M1/M2 macrophage polarization in Legionella pneumophila pneumonia following anti-Gr-1 antibody treatment. Anti-Gr-1 treatment induced an M1-to-M2 shift of macrophage subtypes in the lungs and weakly in the peripheral blood, which was associated with increased mortality in legionella-infected mice. CD8+ T lymphocytes and natural killer cells were the dominant sources of IFN-γ in the acute phase, and anti-Gr-1 treatment reduced the number of IFN-γ-producing CD8+ T lymphocytes. In the CD3-gated population, most Gr-1-positive cells were CD8+ T lymphocytes in the lungs and lymph nodes (LNs) of infected mice. Additionally, the number of IFN-γ-producing Gr-1+ CD8+ T lymphocytes in the lungs and LNs increased 2 and 4 days after L. pneumophila infection, with anti-Gr-1 treatment attenuating these populations. Antibody staining revealed that Gr-1+ CD8+ T lymphocytes were Ly6C-positive cells rather than Ly6G, a phenotype regarded as memory type cells. Furthermore, the adoptive transfer of Gr-1+ CD8+ T lymphocytes induced increases in IFN-γ, M1 shifting and reduced bacterial number in the Legionella pneumonia model. These data identified Ly6C+ CD8+ T lymphocytes as a source of IFN-γ in innate immunity and partially associated with reduced IFN-γ production, M2 polarization, and high mortality in anti-Gr-1 antibody-treated mice with L. pneumophila pneumonia.

Outside-host phage therapy as a biological control against environmental infectious diseases

BACKGROUND

Environmentally growing pathogens present an increasing threat for human health, wildlife and food production. Treating the hosts with antibiotics or parasitic bacteriophages fail to eliminate diseases that grow also in the outside-host environment. However, bacteriophages could be utilized to suppress the pathogen population sizes in the outside-host environment in order to prevent disease outbreaks. Here, we introduce a novel epidemiological model to assess how the phage infections of the bacterial pathogens affect epidemiological dynamics of the environmentally growing pathogens. We assess whether the phage therapy in the outside-host environment could be utilized as a biological control method against these diseases. We also consider how phage-resistant competitors affect the outcome, a common problem in phage therapy. The models give predictions for the scenarios where the outside-host phage therapy will work and where it will fail to control the disease. Parameterization of the model is based on the fish columnaris disease that causes significant economic losses to aquaculture worldwide. However, the model is also suitable for other environmentally growing bacterial diseases.

RESULTS

Transmission rates of the phage determine the success of infectious disease control, with high-transmission phage enabling the recovery of the host population that would in the absence of the phage go asymptotically extinct due to the disease. In the presence of outside-host bacterial competition between the pathogen and phage-resistant strain, the trade-off between the pathogen infectivity and the phage resistance determines phage therapy outcome from stable coexistence to local host extinction.

CONCLUSIONS

We propose that the success of phage therapy strongly depends on the underlying biology, such as the strength of trade-off between the pathogen infectivity and the phage-resistance, as well as on the rate that the phages infect the bacteria. Our results indicate that phage therapy can fail if there are phage-resistant bacteria and the trade-off between pathogen infectivity and phage resistance does not completely inhibit the pathogen infectivity. Also, the rate that the phages infect the bacteria should be sufficiently high for phage-therapy to succeed.

Metformin Mediates Protection against Legionella Pneumonia through Activation of AMPK and Mitochondrial Reactive Oxygen Species

In Legionella pneumophila infection, macrophages play a critical role in the host defense response. Metformin, an oral drug for type 2 diabetes, is attracting attention as a new supportive therapy against a variety of diseases, such as cancer and infectious diseases. The novel mechanisms for metformin actions include modulation of the effector functions of macrophages and other host immune cells. In this study, we have examined the effects of metformin on L. pneumophila infection in vitro and in vivo. Metformin treatment suppressed growth of L. pneumophila in a time- and concentration-dependent fashion in bone marrow-derived macrophages, RAW cells (mouse), and U937 cells (human). Metformin induced phosphorylation of AMP-activated protein kinase (AMPK) in L. pneumophila-infected bone marrow-derived macrophages, and the AMPK inhibitor Compound C negated metformin-mediated growth suppression. Also, metformin induced mitochondrial reactive oxygen species but not phagosomal NADPH oxidase-derived reactive oxygen species. Metformin-mediated growth suppression was mitigated in the presence of the reactive oxygen species scavenger glutathione. In a murine L. pneumophila pneumonia model, metformin treatment improved survival of mice, which was associated with a significant reduction in bacterial number in the lung. Similar to in vitro observations, induction of AMPK phosphorylation and mitochondrial ROS was demonstrated in the infected lungs of mice treated with metformin. Finally, glutathione treatment abolished metformin effects on lung bacterial clearance. Collectively, these data suggest that metformin promotes mitochondrial ROS production and AMPK signaling and enhances the bactericidal activity of macrophages, which may contribute to improved survival in L. pneumophila pneumonia.

Spectrum of pathogen- and model-specific histopathologies in mouse models of acute pneumonia

Pneumonia may be caused by a wide range of pathogens and is considered the most common infectious cause of death in humans. Murine acute lung infection models mirror human pathologies in many aspects and contribute to our understanding of the disease and the development of novel treatment strategies. Despite progress in other fields of tissue imaging, histopathology remains the most conclusive and practical read out tool for the descriptive and semiquantitative evaluation of mouse pneumonia and therapeutic interventions. Here, we systematically describe and compare the distinctive histopathological features of established models of acute pneumonia in mice induced by Streptococcus (S.) pneumoniae, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Legionella pneumophila, Escherichia coli, Middle East respiratory syndrome (MERS) coronavirus, influenza A virus (IAV) and superinfection of IAV-incuced pneumonia with S. pneumoniae. Systematic comparisons of the models revealed striking differences in the distribution of lesions, the characteristics of pneumonia induced, principal inflammatory cell types, lesions in adjacent tissues, and the detectability of the pathogens in histological sections. We therefore identified core criteria for each model suitable for practical semiquantitative scoring systems that take into account the pathogen- and model-specific patterns of pneumonia. Other critical factors that affect experimental pathologies are discussed, including infectious dose, time kinetics, and the genetic background of the mouse strain. The substantial differences between the model-specific pathologies underscore the necessity of pathogen- and model-adapted criteria for the comparative quantification of experimental outcomes. These criteria also allow for the standardized validation and comparison of treatment strategies in preclinical models.

Recombinant flagellin-PAL fusion protein of Legionella pneumophila induced cell-mediated and protective immunity against bacteremia in BALB/c mice

We report a new recombinant fusion protein composed of full-length Legionella pneumophila flagellin A and peptidoglycan-associated lipoprotein (PAL), rFLA-PAL, capable of inducing protective immunity against L. pneumophila. The recombinant protein was over expressed in Escherichia coli strain BL21 (DE3) using pET-28a (+) expression vector (pET28a-flaA-pal) and purified by Ni²⁺ exchange chromatography. Immunological properties of rFLA-PAL were assessed in a mouse model. Female BALB/c mice, immunized with rFLA-PAL, exhibited a rapid increase in serum antibody concentration against each of its protein portions. Furthermore, a strong activation of both innate and adaptive cell-mediated immunity was observed as indicated by antigen-specific splenocyte proliferation, IFN-γ and IL-12 production, and early production of TNF-α in the serum and in splenocyte cultures which were separately assessed against PAL and FLA. BALB/c mice were challenged with a lethal dose of L. pneumophila intravenously. In a 10-days follow-up after intravenous lethal challenge with L. pneumophila, a 100% survival rate was observed for mice immunized with rFLA-PAL, same as for those immunized with a sublethal dose of L. pneumophila. Based on the potent immune responses observed in mice immunized with rFLA-PAL, this recombinant fusion protein could be a potential vaccine candidate against the intracellular pathogen L. pneumophila.

IFN-γ induction by neutrophil-derived IL-17A homodimer augments pulmonary antibacterial defense

The role of interleukin-17A (IL-17A) in host defense against Legionella pneumophila remains elusive. To address this issue, we used Il17a(-/-), Il17f(-/-), and Il17a/Il17f(-/-) mice on a C57Bl/6 (non-permissive) background and IL-17 neutralizing Abs in mice on an A/J (permissive) background. Higher bacterial (L. pneumophila) counts in the lung and blood along with reduced neutrophil recruitment were detected in Il17a(-/-), but not Il17f(-/-), mice. We found that neutrophils produce IL-17A homodimer (IL-17A) during L. pneumophila infection, and hematopoietic cell-derived IL-17A is known to be important for bacterial clearance. Thus, intratracheal administration of wild-type neutrophils or recombinant IL-17A restored bacterial clearance and neutrophil recruitment in Il17a(-/-) mice. Furthermore, neutrophil-depleted Rag2(-/-) and Rag2/Il-2rγ(-/-) mice exhibited increased bacterial burden, reduced neutrophil influx and IL-17A production in the lung. Recombinant IFN-γ administration in Il17a(-/-) mice augmented bacterial elimination, whereas IL-17A administration in Ifnγ(-/-) mice did not augment bacterial clearance. IFN-γ is produced by T cells, but not neutrophils or macrophages, suggesting that neutrophil-derived IL-17A induces IFN-γ in a paracrine fashion. Human pneumonic lungs and human neutrophils challenged with L. pneumophila exhibited increased numbers of IL-17A producing cells. These findings display a novel function of neutrophil-derived IL-17A in antibacterial defense via the induction of IFN-γ in a paracrine manner.

Comparing methods of determining Legionella spp. in complex water matrices

BACKGROUND

Legionella testing conducted at environmental laboratories plays an essential role in assessing the risk of disease transmission associated with water systems. However, drawbacks of culture-based methodology used for Legionella enumeration can have great impact on the results and interpretation which together can lead to underestimation of the actual risk. Up to 20% of the samples analysed by these laboratories produced inconclusive results, making effective risk management impossible. Overgrowth of competing microbiota was reported as an important factor for culture failure. For quantitative polymerase chain reaction (qPCR), the interpretation of the results from the environmental samples still remains a challenge. Inhibitors may cause up to 10% of inconclusive results. This study compared a quantitative method based on immunomagnetic separation (IMS method) with culture and qPCR, as a new approach to routine monitoring of Legionella.

RESULTS

First, pilot studies evaluated the recovery and detectability of Legionella spp using an IMS method, in the presence of microbiota and biocides. The IMS method results were not affected by microbiota while culture counts were significantly reduced (1.4 log) or negative in the same samples. Damage by biocides of viable Legionella was detected by the IMS method. Secondly, a total of 65 water samples were assayed by all three techniques (culture, qPCR and the IMS method). Of these, 27 (41.5%) were recorded as positive by at least one test. Legionella spp was detected by culture in 7 (25.9%) of the 27 samples. Eighteen (66.7%) of the 27 samples were positive by the IMS method, thirteen of them reporting counts below 10(3) colony forming units per liter (CFU l(-1)), six presented interfering microbiota and three presented PCR inhibition. Of the 65 water samples, 24 presented interfering microbiota by culture and 8 presented partial or complete inhibition of the PCR reaction. So the rate of inconclusive results of culture and PCR was 36.9 and 12.3%, respectively, without any inconclusive results reported for the IMS method.

CONCLUSION

The IMS method generally improved the recovery and detectability of Legionella in environmental matrices, suggesting the possibility to use IMS method as valuable indicator of risk. Thus, this method may significantly improve our knowledge about the exposure risk to these bacteria, allowing us to implement evidence-based monitoring and disinfection strategies.

Invasion ability and disease dynamics of environmentally growing opportunistic pathogens under outside-host competition

Most theories of the evolution of virulence concentrate on obligatory host-pathogen relationship. Yet, many pathogens replicate in the environment outside-host where they compete with non-pathogenic forms. Thus, replication and competition in the outside-host environment may have profound influence on the evolution of virulence and disease dynamics. These environmentally growing opportunistic pathogens are also a logical step towards obligatory pathogenicity. Efficient treatment methods against these diseases, such as columnaris disease in fishes, are lacking because of their opportunist nature. We present a novel epidemiological model in which replication and competition in the outside-host environment influences the invasion ability of a novel pathogen. We also analyze the long-term host-pathogen dynamics. Model parameterization is based on the columnaris disease, a bacterial fresh water fish disease that causes major losses in fish farms worldwide. Our model demonstrates that strong competition in the outside-host environment can prevent the invasion of a new environmentally growing opportunist pathogen and long-term disease outbreaks.

Search for microRNAs expressed by intracellular bacterial pathogens in infected mammalian cells

MicroRNAs are expressed by all multicellular organisms and play a critical role as post-transcriptional regulators of gene expression. Moreover, different microRNA species are known to influence the progression of a range of different diseases, including cancer and microbial infections. A number of different human viruses also encode microRNAs that can attenuate cellular innate immune responses and promote viral replication, and a fungal pathogen that infects plants has recently been shown to express microRNAs in infected cells that repress host cell immune responses and promote fungal pathogenesis. Here, we have used deep sequencing of total expressed small RNAs, as well as small RNAs associated with the cellular RNA-induced silencing complex RISC, to search for microRNAs that are potentially expressed by intracellular bacterial pathogens and translocated into infected animal cells. In the case of Legionella and Chlamydia and the two mycobacterial species M. smegmatis and M. tuberculosis, we failed to detect any bacterial small RNAs that had the characteristics expected for authentic microRNAs, although large numbers of small RNAs of bacterial origin could be recovered. However, a third mycobacterial species, M. marinum, did express an ∼23-nt small RNA that was bound by RISC and derived from an RNA stem-loop with the characteristics expected for a pre-microRNA. While intracellular expression of this candidate bacterial microRNA was too low to effectively repress target mRNA species in infected cultured cells in vitro, artificial overexpression of this potential bacterial pre-microRNA did result in the efficient repression of a target mRNA. This bacterial small RNA therefore represents the first candidate microRNA of bacterial origin.

The frustrated host response to Legionella pneumophila is bypassed by MyD88-dependent translation of pro-inflammatory cytokines

Many pathogens, particularly those that require their host for survival, have devised mechanisms to subvert the host immune response in order to survive and replicate intracellularly. Legionella pneumophila, the causative agent of Legionnaires' disease, promotes intracellular growth by translocating proteins into its host cytosol through its type IV protein secretion machinery. At least 5 of the bacterial translocated effectors interfere with the function of host cell elongation factors, blocking translation and causing the induction of a unique host cell transcriptional profile. In addition, L. pneumophila also interferes with translation initiation, by preventing cap-dependent translation in host cells. We demonstrate here that protein translation inhibition by L. pneumophila leads to a frustrated host MAP kinase response, where genes involved in the pathway are transcribed but fail to be translated due to the bacterium-induced protein synthesis inhibition. Surprisingly, few pro-inflammatory cytokines, such as IL-1α and IL-1β, bypass this inhibition and get synthesized in the presence of Legionella effectors. We show that the selective synthesis of these genes requires MyD88 signaling and takes place in both infected cells that harbor bacteria and neighboring bystander cells. Our findings offer a perspective of how host cells are able to cope with pathogen-encoded activities that disrupt normal cellular process and initiate a successful inflammatory response.

Translation inhibition is a common virulence mechanism used by a number of pathogens (e.g. Diphtheria Toxin, Shiga Toxin and Pseudomonas Exotoxin A). It has been a mystery how host cells mount a pathogen-specific response and clear infection under conditions where protein synthesis is blocked by pathogens. Using Legionella pneumophila as a model, a bacterium that efficiently blocks the host protein translation machinery, we show here that the innate immune system has devised a mechanism to cope with translation inhibition by selectively synthesizing proteins that are required for inflammation.

Guanylate binding proteins promote caspase-11-dependent pyroptosis in response to cytoplasmic LPS

IFN receptor signaling induces cell-autonomous immunity to infections with intracellular bacterial pathogens. Here, we demonstrate that IFN-inducible guanylate binding protein (Gbp) proteins stimulate caspase-11-dependent, cell-autonomous immunity in response to cytoplasmic LPS. Caspase-11-dependent pyroptosis is triggered in IFN-activated macrophages infected with the Gram-negative bacterial pathogen Legionella pneumophila. The rapid induction of pyroptosis in IFN-activated macrophages required a cluster of IFN-inducible Gbp proteins encoded on mouse chromosome 3 (Gbp(chr3)). Induction of pyroptosis in naive macrophages by infections with the cytosol-invading ΔsdhA L. pneumophila mutant was similarly dependent on Gbp(chr3), suggesting that these Gbp proteins play a role in the detection of bacteria accessing the cytosol. Cytoplasmic LPS derived from Salmonella ssp. or Escherichia coli has recently been shown to trigger caspase-11 activation and pyroptosis, but the cytoplasmic sensor for LPS and components of the caspase-11 inflammasome are not yet defined. We found that the induction of caspase-11-dependent pyroptosis by cytoplasmic L. pneumophila-derived LPS required Gbp(chr3) proteins. Similarly, pyroptosis induced by cytoplasmic LPS isolated from Salmonella was diminished in Gbp(chr3)-deficient macrophages. These data suggest a role for Gbp(chr3) proteins in the detection of cytoplasmic LPS and the activation of the noncanonical inflammasome.

Restoration of innate immune activation accelerates Th1-cell priming and protection following pulmonary mycobacterial infection

The immune mechanisms underlying delayed induction of Th1-type immunity in the lungs following pulmonary mycobacterial infection remain poorly understood. We have herein investigated the underlying immune mechanisms for such delayed responses and whether a selected innate immune-modulating strategy can accelerate Th1-type responses. We have found that, in the early stage of pulmonary infection with attenuated Mycobacterium tuberculosis (M.tb H37Ra), the levels of infection in the lung continue to increase logarithmically until days 14 and 21 postinfection in C57BL/6 mice. The activation of innate immune responses, particularly DCs, in the lung is delayed. This results in a delay in the subsequent downstream immune responses including the migration of antigen-bearing DCs to the draining lymph node (dLN), the Th1-cell priming in dLN, and the recruitment of Th1 cells to the lung. However, single lung mucosal exposure to the TLR agonist FimH postinfection is able to accelerate protective Th1-type immunity via facilitating DC migration to the lung and draining lymph nodes, enhancing DC antigen presentation and Th1-cell priming. These findings hold implications for the development of immunotherapeutic and vaccination strategies and suggest that enhancement of early innate immune activation is a viable option for improving Th1-type immunity against pulmonary mycobacterial diseases.

Outside-host growth of pathogens attenuates epidemiological outbreaks

Opportunist saprotrophic pathogens differ from obligatory pathogens due to their capability in host-independent growth in environmental reservoirs. Thus, the outside-host environment potentially influences host-pathogen dynamics. Despite the socio-economical importance of these pathogens, theory on their dynamics is practically missing. We analyzed a novel epidemiological model that couples outside-host density-dependent growth to host-pathogen dynamics. Parameterization was based on columnaris disease, a major hazard in fresh water fish farms caused by saprotrophic Flavobacterium columnare. Stability analysis and numerical simulations revealed that the outside-host growth maintains high proportion of infected individuals, and under some conditions can drive host extinct. The model can show stable or cyclic dynamics, and the outside-host growth regulates the frequency and intensity of outbreaks. This result emerges because the density-dependence stabilizes dynamics. Our analysis demonstrates that coupling of outside-host growth and traditional host-pathogen dynamics has profound influence on disease prevalence and dynamics. This also has implications on the control of these diseases.

Roles of interleukin-17 in an experimental Legionella pneumophila pneumonia model

Interleukin-17 (IL-17) is a key factor in T helper type 17 (Th17) lineage host responses and plays critical roles in immunological control of a variety of infectious diseases. Although Legionella pneumophila, an intracellular bacterium found widely in the environment, often causes a serious and life-threatening pneumonia in humans, the contribution of IL-17 to immune function during Legionella pneumonia is unknown. In the present study, we used an experimental Legionella pneumonia infection to clarify the role of IL-17 in the resulting immune response. We observed robust production of pulmonary IL-17A and IL-17F (IL-17A/F), peaking on day 1 and declining thereafter. Upregulated production of tumor necrosis factor alpha (TNF-α), IL-6, and IL-1β, but not monocyte chemotactic protein 1 (MCP-1), was observed in Legionella-infected bone marrow-derived macrophages from BALB/c mice that had been stimulated with IL-17A or IL-17F. A significant decrease in the production of proinflammatory cytokines IL-6 and TNF-α was observed in IL-17A/F-deficient mice (BALB/c background) infected with L. pneumophila. Moreover, we found impaired neutrophil migration and lower numbers of chemokines (KC, LIX, and MIP-2) in IL-17A/F-deficient mice. IL-17A/F-deficient mice also eliminated L. pneumophila more slowly and were less likely to survive a lethal challenge. These results demonstrate that IL-17A/F plays a critical role in L. pneumophila pneumonia, probably through induction of proinflammatory cytokines and accumulation of neutrophils at the infection site.

In vivo efficacy of sivelestat in combination with pazufloxacin against Legionella pneumonia

It is important to regulate excessive inflammation when patients with severe infectious disease are treated. Sivelestat sodium hydrate (sivelestat), a neutrophil elastase inhibitor, is used in the treatment of lung injury but its effect on bacterial pneumonia is unknown. The authors examined the efficacy of sivelestat in combination with a fluoroquinolone in a Legionella pneumophila pneumonia mouse model. The combination therapy did not show a significant survival improvement compared to the treatment with fluoroquinolone alone, but reduced bacteria number and inflammatory cells in the early phase. The combination therapy can contribute to treatment of L. pneumophila pneumonia with protecting lungs.

Replication of Legionella Pneumophila in Human Cells: Why are We Susceptible?

Legionella pneumophila is the causative agent of Legionnaires’ disease, a serious and often fatal form of pneumonia. The susceptibility to L. pneumophila arises from the ability of this intracellular pathogen to multiply in human alveolar macrophages and monocytes. L. pneumophila also replicates in several professional and non-professional phagocytic human-derived cell lines. With the exception of the A/J mouse strain, most mice strains are restrictive, thus they do not support L. pneumophila replication. Mice lacking the NOD-like receptor Nlrc4 or caspase-1 are also susceptible to L. pneumophila. On the other hand, in the susceptible human hosts, L. pneumophila utilizes several strategies to ensure intracellular replication and protect itself against the host immune system. Most of these strategies converge to prevent the fusion of the L. pneumophila phagosome with the lysosome, inhibiting host cell apoptosis, activating survival pathways, and sequestering essential nutrients for replication and pathogenesis. In this review, we summarize survival mechanisms employed by L. pneumophila to maintain its replication in human cells. In addition, we highlight different human-derived cell lines that support the multiplication of this intracellular bacterium. Therefore, these in vitro models can be applicable and are reproducible when investigating L. pneumophila/phagocyte interactions at the molecular and cellular levels in the human host.

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.

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.

Fulminant legionellosis in two patients treated with infliximab for Crohn's disease: case series and literature review

Two cases of fulminant pulmonary legionellosis, complicated by prolonged intensive care unit stays and acute respiratory distress syndrome, and who were recently treated with infliximab for Crohn's disease, are described. A review of the literature revealed three additional cases in patients with inflammatory bowel disease, and a total of 22 cases of Legionella pneumophila pneumonia in the context of treatment with antitumour necrosis (TNF)-alpha medications. The median age of the patients was 49 years, and men and women were affected equally. The case fatality rate was 14% (three of 22). Early recognition and treatment of this anti-TNF-alpha-related complication would likely result in reduced mortality and morbidity. Physicians prescribing anti-TNF-alpha drugs should be aware of this association.

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.

TNF receptor 1 and 2 contribute in different ways to resistance to Legionella pneumophila-induced mortality in mice

Legionella pneumophila is one of the most important pathogens which cause community-acquired pneumonia. Although TNF-alpha is considered to play an important role in response to bacteria, the role of the TNF-alpha receptor on L. pneumophila infection remains to be elucidated. To investigate this, we infected TNF receptor deficient mice with L. pneumophila. L. pneumophila was inoculated intranasally into TNF receptor (TNFR)-1-knock-out mice or TNFR2-knock-out mice. The mortality rate, histology of the lung, bacterial growth in the lung, and bronchoalveolar lavage (BAL) fluids were investigated. The bacterial growth of L. pneumophila in the macrophages was also studied. Almost all the mice survived after an intranasal inoculation of 1x10(6)CFU/head of L. pneumophila, but more than 90% mice were killed after inoculation of 1x10(8)CFU/head of L. pneumophila. In the case of TNFR1-knock-out mice and TNFR2-knock-out mice, a high mortality rate was observed after inoculation of 1x10(7)CFU/head of L. pneumophila in comparison to wild-type mice. The lung histology from both the TNFR1-knock-out mice documented severe lung injury at day 3 after inoculation. The clearance of L. pneumophila in the lung of the TNFR1-knock-out mice was slower than those from both the TNFR2-knock-out mice and the wild-type mice. Moreover, L. pneumophila growth in the peritoneal macrophages from the TNFR1-knock-out mice was observed. Interestingly, a lack of neutrophils accumulation in the BAL fluids and a dysregulation of cytokines (IFN-gamma, interleukin-12, and TNF-alpha) were observed in the TNFR1-knock-out mice. On the contrary, large accumulation of neutrophils in BAL fluids was observed in TNFR2-knock-out mice. These data suggested that a TNFR1 deficiency led to a compromise of the innate immunity against L. pneumophila, while a TNFR2 deficiency induced an excessive inflammatory response and resulted in death. The present study confirmed that TNFR1 and TNFR2 play a crucial, but different role in the control of L. pneumophila-induced mortality.

Detection of inhibition of antimicrobial activity by mycobacterial lysates in human monocytes infected with Legionella pneumophila

Antimicrobial activity in human monocytes infected with Mycobacterium tuberculosis has been difficult to demonstrate in vitro, and the molecular mechanisms allowing the bacteria to survive intracellularly are unknown. As a means to test the influence of bacterial products in the microbicidal activity of monocytes we have developed an infection model with Legionella pneumophila, which is killed by interferon gamma activated cells. We demonstrate that this model is useful because M. tuberculosis lysates inhibit one hundred fold the interferon gamma induced activity against L. pneumophila. Comparable degrees of inhibition are also detected when we use lysates from the less pathogenic Mycobacterium gordonae and the pathogenic Staphylococcus aureus, suggesting the participation of a common mechanism. This hypothesis is supported by the fact that the pattern of cytokine secretion is similar in all cases. A significant difference is, however, observed when we used lysates from the non-pathogenic Escherichia coli, which resulted in the recovery of low numbers of bacteria, probably because they induce the cell death of infected monocytes.

Low mannose-binding lectin complement activation function is associated with predisposition to Legionnaires' disease

Innate immune system deficiency may predispose to severe infections such as Legionnaires' disease. We have investigated the role of mannose-binding lectin (MBL) deficiency in the Melbourne Aquarium Legionnaires' disease outbreak. Serum samples from patients and controls that were exposed but shown to be uninfected from the Melbourne Aquarium Legionnaires' disease outbreak were tested for MBL function (C4 deposition) and level (mannan-binding). MBL function was lower in Legionnaires' disease cases than in age- and sex-matched uninfected, exposed controls. The frequency of MBL deficiency with C4 deposition < 0.2 U/microl was significantly higher in Legionnaires' disease cases than in controls. This also applied to Legionnaires' disease cases requiring hospital care. There was no difference in MBL mannan-binding levels between Legionnaires' disease patients and controls. There was no significant interval change in MBL function or level after a mean of 46 days. MBL complement activation functional deficiency appears to predispose to Legionnaires' disease.

Anti-apoptotic signalling by the Dot/Icm secretion system of L. pneumophila

The Dot/Icm type IV secretion system of Legionella pneumophila triggers robust activation of caspase-3 during early and exponential stages of proliferation within human macrophages, but apoptosis is delayed till late stages of infection, which is novel. As caspase-3 is the executioner of the cell, we tested the hypothesis that L. pneumophila triggers anti-apoptotic signalling within the infected human macrophages to halt caspase-3 from dismantling the cells. Here we show that during early and exponential replication, L. pneumophila-infected human monocyte-derived macrophages (hMDMs) exhibit a remarkable resistance to induction of apoptosis, in a Dot/Icm-dependent manner. Microarray analyses and real-time PCR reveal that during exponential intracellular replication, L. pneumophila triggers upregulation of 12 anti-apoptotic genes that are linked to activation of the nuclear transcription factor kappa-B (NF-kappaB). Our data show that L. pneumophila induces a Dot/Icm-dependent sustained nuclear translocation of the p50 and p65 subunits of NF-kappaB during exponential intracellular replication. Bacterial entry is essential both for the anti-apoptotic phenotype of infected hMDMs and for nuclear translocation of the p65. Using p65-/- and IKKalpha-/- beta-/- double knockout mouse embryonic fibroblast cell lines, we show that nuclear translocation of NF-kappaB is required for the resistance of L. pneumophila-infected cells to apoptosis-inducing agents. In addition, the L. pneumophila-induced nuclear translocation of NF-kappaB requires the activity of IKKalpha and/or IKKbeta. We conclude that although the Dot/Icm secretion system of L. pneumophila elicits an early robust activation of caspase-3 in human macrophages, it triggers a strong anti-apoptotic signalling cascade mediated, at least in part by NF-kappaB, which renders the cells refractory to external potent apoptotic stimuli.

Involvement of fractalkine/CX3CL1 expression by dendritic cells in the enhancement of host immunity against Legionella pneumophila

Legionnaires' disease is clinically manifested as severe pneumonia caused by Legionella pneumophila. However, the dendritic cell (DC)-centered immunological framework of the host defense against L. pneumophila has not been fully delineated. For this study, we focused on a potent chemoattractant for lymphocytes, fractalkine/CX3CL1, and observed that the fractalkine expression of DCs was somewhat up-regulated when they encountered L. pneumophila. We therefore hypothesized that fractalkine expressed by Legionella-capturing DCs is involved in the induction of T-cell-mediated immune responses against Legionella, which would be enhanced by a genetic modulation of DCs to overexpress fractalkine. In vivo immunization-challenge experiments demonstrated that DCs modified with a recombinant adenovirus vector to overexpress fractalkine (AdFKN) and pulsed with heat-killed Legionella protected immunized mice from a lethal Legionella infection and that the generation of in vivo protective immunity depended on the host lymphocyte subsets, including CD4(+) T cells, CD8(+) T cells, and B cells. Consistent with this, immunization with AdFKN/Legionella/DC induced significantly higher levels of serum anti-Legionella antibodies of several isotypes than those induced by control immunizations. Further analysis of spleen cells from the immunized mice indicated that the AdFKN/Legionella/DC immunization elicited Th1-dominated immune responses to L. pneumophila. These observations suggest that fractalkine may play an important role in the DC-mediated host defense against intracellular pathogens such as L. pneumophila.

Treatment of Legionnaires??? Disease

Legionnaires' disease is pneumonia, usually caused by Legionella pneumophila, which can range in severity from mild to quite severe. While it is commonly acquired in the community, it can just as easily be acquired nosocomially from water sources that have not been appropriately decontaminated. While historically initial treatment was always with erythromycin, current case series and treatment recommendations suggest that outpatients receive immediate treatment with one of the following antibacterials: azithromycin, erythromycin, clarithromycin, telithromycin, doxycycline or an extended-spectrum fluoroquinolone. If the symptoms are severe enough to warrant hospitalisation then the patient should receive treatment with parenteral azithromycin or extended-spectrum fluoroquinolones followed by step-down to oral formulations to complete the regimens. While a shorter course of 7-10 days for more severe infections may be possible for intravenous/oral azithromycin, other antibacterials should be administered for a total of 10-21 days and started as soon as possible upon presentation to optimise outcomes.

Type I IFN protects permissive macrophages from Legionella pneumophila infection through an IFN-gamma-independent pathway

Legionella pneumophila is an intracellular pathogen whose replication in macrophages is mainly controlled by IFN-gamma. Freshly isolated peritoneal macrophages elicited in vivo with thioglycolate (TG) from A/J mice are highly permissive to L. pneumophila growth in vitro, while TG-elicited macrophages from CD1 mice are resistant. In this study, we show that when CD1 TG-macrophages are cultured for 7 days, they become permissive to Legionella infection. We demonstrate that treatment with type I IFN (IFN-alphabeta) totally inhibits the growth of L. pneumophila in both freshly isolated A/J and in vitro-aged CD1 TG-macrophages. IFN-alphabeta protective effect on permissive macrophages was comparable to that induced by IFN-gamma. Even low doses of either IFN-alpha or IFN-beta alone were effective in inhibiting L. pneumophila multiplication in macrophage cultures. Notably, treatment of resistant, freshly isolated CD1 TG-macrophages with Ab to mouse IFN-alphabeta significantly enhanced their susceptibility to Legionella infection in vitro, thus implying a role of endogenous IFN-alphabeta in mediating the natural resistance of macrophages to L. pneumophila infection. Finally, addition of anti-IFN-gamma-neutralizing Ab did not restore Legionella growth in IFN-alpha- or IFN-beta-treated A/J or CD1 permissive macrophages, indicating that IFN-alphabeta effect was not mediated by IFN-gamma. This observation was further confirmed by the finding that IFN-alphabeta was effective in inhibiting L. pneumophila replication in macrophages from IFN-gamma receptor-deficient mice. Taken together, our results provide the first evidence for a role of IFN-alphabeta in the control of L. pneumophila infection in mouse models of susceptible macrophages and suggest the existence of different pathways for the control of intracellular bacteria in macrophages.

Dendritic cells pulsed with live and dead Legionella pneumophila elicit distinct immune responses

Legionella pneumophila is the causative pathogen of Legionnaires' disease, which is characterized by severe pneumonia. In regard to the pathophysiology of Legionella infection, the role of inflammatory phagocytes such as macrophages has been well documented, but the involvement of dendritic cells (DCs) has not been clarified. In this study, we have investigated the immune responses that DCs generate in vitro and in vivo after contact with L. pneumophila. Heat- and formalin-killed L. pneumophila, but not live L. pneumophila, induced immature DCs to undergo similar phenotypic maturation, but the secreted proinflammatory cytokines showed different patterns. The mechanisms of the DC maturation by heat- or formalin-killed L. pneumophila depended, at least in part, on Toll-like receptor 4 signaling or on Legionella LPS, respectively. After transfer to naive mice, DCs pulsed with dead Legionella produced serum Ig isotype responses specific for Legionella, leading to protective immunity against an otherwise lethal respiratory challenge with L. pneumophila. The in vivo immune responses required the Ag presentation of DCs, especially that on MHC class II molecules, and the immunity yielded cross-protection between clinical and environmental strains of L. pneumophila. Although the DC maturation was impaired by live Legionella, macrophages were activated by live as well as dead L. pneumophila, as evidenced by the up-regulation of MHC class II. Finally, DCs, but not macrophages, exhibited a proliferative response to live L. pneumophila that was consistent with their cell cycle progression. These findings provide a better understanding of the role of DCs in adaptive immunity to Legionella infection.