Legionella pneumophila immunity and immunomodulation: nature and mechanisms

Invasion of eukaryotic cells by Legionella pneumophila: A common strategy for all hosts?

Legionella pneumophila is an environmental micro-organism capable of producing an acute lobar pneumonia, commonly referred to as Legionnaires' disease, in susceptible humans. Legionellae are ubiquitous in aquatic environments, where they survive in biofilms or intracellularly in various protozoans. Susceptible humans become infected by breathing aerosols laden with the bacteria. The target cell for human infection is the alveolar macrophage, in which the bacteria abrogate phagolysosomal fusion. The remarkable ability of L pneumophila to infect a wide range of eukaryotic cells suggests a common strategy that exploits very fundamental cellular processes. The bacteria enter host cells via coiling phagocytosis and quickly subvert organelle trafficking events, leading to formation of a replicative phagosome in which the bacteria multiply. Vegetative growth continues for 8 to 10 h, after which the bacteria develop into a short, highly motile form called the 'mature form'. The mature form exhibits a thickening of the cell wall, stains red with the Gimenez stain, and is between 10 and 100 times more infectious than agar-grown bacteria. Following host cell lysis, the released bacteria infect other host cells, in which the mature form differentiates into a Gimenez-negative vegetative form, and the cycle begins anew. Virulence of L pneumophila is considered to be multifactorial, and there is growing evidence for both stage specific and sequential gene expression. Thus, L pneumophila may be a good model system for dissecting events associated with the host-parasite interactions.

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.

Community-acquired pneumonia: paving the way towards new vaccination concepts

Despite the availability of antimicrobial agents and vaccines, community-acquired pneumonia remains a serious problem. Severe forms tend to occur in very young children and among the elderly, since their immune competence is eroded by immaturity and immune senescence, respectively. The main etiologic agents differ according to patient age and geographic area. Streptococcus pneumoniae, Haemophilus influenzae, respiratory syncytial virus (RSV) and parainfluenza virus type 3 (PIV-3) are the most important pathogens in children, whereas influenza viruses are the leading cause of fatal pneumonia in the elderly. Effective vaccines are available against some of these organisms. However, there are still many agents against which vaccines are not available or the existent ones are suboptimal. To tackle this problem, empiric approaches are now being systematically replaced by rational vaccine design. This is facilitated by the growing knowledge in the fields of immunology, microbial pathogenesis and host response to infection, as well as by the availability of sophisticated strategies for antigen selection, potent immune modulators and efficient antigen delivery systems. Thus, a new generation of vaccines with improved safety and efficacy profiles compared to old and new agents is emerging. In this chapter, an overview is provided about currently available and new vaccination concepts.

Bronchoalveolar lavage findings in severe community-acquired pneumonia due to Legionella pneumophila serogroup 1

BACKGROUND

No specific data are available in the literature on the bronchoalveolar Lavage (BAL) findings of Legionella pneumophila pneumonia. We report on the cytological and immunophenotypical BAL data of three immunocompetent patients with severe community-acquired pneumonia due to L. pneumophila serogroup 1.

METHODS

Retrospective chart review. The microbiologial diagnosis was obtained by BAL culture or/and urinary antigen assay.

RESULTS

All patients presented with high-grade fever, bilateral chest infiltrates and severe respiratory failure requiring ventilatory support. The cytological BAL pattern at presentation showed in all patients the association of a marked neutrophilia with a variable but remarkable percentage of lymphoblasts. Increased levels of activated T-Lymphocytes (both HLA-DR + and CD25 + cells) and, in 2 out of 3 patients, of T-cells bearing the gamma/delta T-cell receptor were the main immunophenotypical findings on flow cytometric analysis.

CONCLUSIONS

We suggest that the association of lymphoblasts with a marked neutrophilia in BAL fluid of patients with a clinical-radiological setting compatible with acute pneumonia should suggest L. pneumophila as a possible etiologic agent.

Legionella reveal dendritic cell functions that facilitate selection of antigens for MHC class II presentation

To understand how adaptive immune responses are generated against bacteria that avoid being delivered to lysosomes, interactions between professional antigen-presenting cells (APCs) and the intracellular pathogen Legionella pneumophila were examined. In contrast to murine bone marrow-derived macrophages (BMMs), we show that dendritic cells (DCs) restrict the growth of intracellular Legionella. Similar to what has been reported in BMMs, phagosomes containing Legionella matured into endoplasmic reticulum (ER)-derived organelles after DC internalization. Biogenesis of an ER-derived vacuole did not effectively sequester Legionella antigens from presentation on MHC class II molecules (MHC II). It was determined that proteins synthesized after Legionella had established residence in an ER-derived vacuole were presented by infected APCs. These data indicate that the ability of DCs to restrict intracellular growth of Legionella could be an important property that facilitates priming of protective T cell-mediated immune responses to vacuolar pathogens.

Comparison of responses elicited by immunization with a Legionella species common lipoprotein delivered as naked DNA or recombinant protein

To evaluate the peptidoglycan-associated lipoprotein (PAL) antigen of Legionella pneumophila as a vaccine candidate, mice were immunized intramuscularly with pcDNA3-PAL and intraperitoneally with recombinant PAL (t-rPAL), which were compared for their ability to induce PAL-specific immune responses. The t-rPAL protein induced PAL-specific IgG antibody production significantly more than did pcDNA3-PAL. The IgG2a and IgG1 production was predominant after pcDNA3-PAL and t-rPAL administration, respectively. In particular, pcDNA3-PAL induced much higher PAL-specific cytotoxic T-lymphocyte responses than did t-rPAL. Furthermore, in vivo, CD19+ B-cell populations were dramatically increased by t-rPAL vaccination, suggesting a B-cell immunomodulatory activity of the lipoprotein. The PAL antigen was also conserved among Legionella species, as determined by PCR and immunoblot analyses. These results support a potential use of the t-rPAL protein and in particular DNA vaccines against Legionella infections.

Identification of an alpha-D-Manp-(1-->8)-Kdo disaccharide in the inner core region and the structure of the complete core region of the Legionella pneumophila serogroup 1 lipopolysaccharide

A disaccharide alpha-D-mannopyranosyl-(1-->8)-3-deoxy-D-manno-octulosonic acid [alpha-D-Manp-(1-->8)-Kdo] was released by mild acid degradation of Legionella pneumophila serogroup 1 (strain Philadelphia 1) lipopolysaccharide (LPS) and identified using NMR spectroscopy and GLC-MS of derived products. These data, together with methylation analysis of the native LPS and previously reported data [Y.A. Knirel, H. Moll, and U. Zähringer, Carbohydr. Res., 293 (1996) 223-234], allowed elucidation of the complete core region of the LPS as having the following nonasaccharide structure: [Sequence: see text]

Chemical characterization of a new 5,7-diamino-3,5,7,9-tetradeoxynonulosonic acid released by mild acid hydrolysis of the Legionella pneumophila serogroup 1 lipopolysaccharide

A derivative of a new 5,7-diamino-3,5,7,9-tetradeoxynonulosonic acid was released from the lipopolysaccharide of Legionella pneumophila serogroup 1 (strain Philadelphia 1) by mild acid hydrolysis, and identified, using NMR spectroscopy and GLC-MS, as 5,7-diacetamido-8-O-acetyl-3,5,7,9-tetradeoxy-L-glycero-D-talo- nonulosonic acid or its enantiomer.

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

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

The structure of the O-specific chain of Legionella pneumophila serogroup 1 lipopolysaccharide

The O-polysaccharide chain of Legionella pneumophila Philadelphia strain 1 (serogroup 1) lipopolysaccharide was investigated by means of 1H- and 13C-NMR spectroscopy, and chemical analysis. It was found to consist of an alpha-(2-->4) interlinked homopolymer of a 5-acetamidino-7-acetamido-8-O-acetyl-3,5,7,9-tetradeoxy-nonulos onic acid possessing most likely the D-glycero-L-galacto configuration, representing the first example of an acidic homopolymer of a higher sugar of this class. The ladder-like banding pattern exhibiting small distances between individual bands in the SDS/PAGE is compatible with a monosaccharide repeating unit.