Differential nitric oxide (NO) production by macrophages from mice and guinea pigs infected with virulent and avirulent Legionella pneumophila serogroup 1

Modelling Legionnaires' disease: Lessons learned from invertebrate and vertebrate animal models

The study of virulence of Legionella pneumophila and its interactions with its hosts has been predominantly conducted in cellulo in the past decades. Although easy to implement and allowing the dissection of molecular pathways underlying host-pathogen interactions, these cellular models fail to provide conditions of the complex environments encountered by the bacteria during the infection of multicellular organisms. To improve our understanding of human infection, several animal models have been developed. This review provides an overview of the invertebrate and vertebrate models that have been established to study L. pneumophila infection and that are alternatives to the classical mouse model, which does not recall human infection with L. pneumophila well. Finally we provide insight in the main contributions made by these models along with their pros and cons.

Species-Specific Transcriptional Regulation of Genes Involved in Nitric Oxide Production and Arginine Metabolism in Macrophages

Activated mouse macrophages metabolize arginine via NO synthase (NOS2) to produce NO as an antimicrobial effector. Published gene expression datasets provide little support for the activation of this pathway in human macrophages. Generation of NO requires the coordinated regulation of multiple genes. We have generated RNA-sequencing data from bone marrow-derived macrophages from representative rodent (rat), monogastric (pig and horse), and ruminant (sheep, goat, cattle, and water buffalo) species, and analyzed the expression of genes involved in arginine metabolism in response to stimulation with LPS. In rats, as in mice, LPS strongly induced Nos2, the arginine transporter Slc7a2, arginase 1 (Arg1), GTP cyclohydrolase (Gch1), and argininosuccinate synthase (Ass1). None of these responses was conserved across species. Only cattle and water buffalo showed substantial NOS2 induction. The species studied also differed in expression and regulation of arginase (ARG2, rather than ARG1), and amino acid transporters. Variation between species was associated with rapid promoter evolution. Differential induction of NOS2 and ARG2 between the ruminant species was associated with insertions of the Bov-A2 retrotransposon in the promoter region. Bov-A2 was shown to possess LPS-inducible enhancer activity in transfected RAW264.7 macrophages. Consistent with a function in innate immunity, NO production and arginine metabolism vary greatly between species and differences may contribute to pathogen host restriction.

Innate sensing and cell-autonomous resistance pathways in Legionella pneumophila infection

Legionella pneumophila is a facultative intracellular bacterium which can cause a severe pneumonia called Legionnaires' disease after inhalation of contaminated water droplets and replication in alveolar macrophages. The innate immune system is generally able to sense and -in most cases- control L. pneumophila infection. Comorbidities and genetic risk factors, however, can compromise the immune system and high infection doses might overwhelm its capacity, thereby enabling L. pneumophila to grow and disseminate inside the lung. The innate immune system mediates sensing of L. pneumophila by employing e.g. NOD-like receptors (NLRs), Toll-like receptors (TLRs), as well as the cGAS/STING pathway to stimulate death of infected macrophages as well as production of proinflammatory cytokines and interferons (IFNs). Control of pulmonary L. pneumophila infection is largely mediated by inflammasome-, TNFα- and IFN-dependent macrophage-intrinsic resistance mechanisms. This article summarizes the current knowledge of innate immune responses to L. pneumophila infection in general, and of macrophage-intrinsic defense mechanisms in particular.

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.

A quantitative microbial risk assessment model for Legionnaires' disease: animal model selection and dose-response modeling

Legionnaires' disease (LD), first reported in 1976, is an atypical pneumonia caused by bacteria of the genus Legionella, and most frequently by L. pneumophila (Lp). Subsequent research on exposure to the organism employed various animal models, and with quantitative microbial risk assessment (QMRA) techniques, the animal model data may provide insights on human dose-response for LD. This article focuses on the rationale for selection of the guinea pig model, comparison of the dose-response model results, comparison of projected low-dose responses for guinea pigs, and risk estimates for humans. Based on both in vivo and in vitro comparisons, the guinea pig (Cavia porcellus) dose-response data were selected for modeling human risk. We completed dose-response modeling for the beta-Poisson (approximate and exact), exponential, probit, logistic, and Weibull models for Lp inhalation, mortality, and infection (end point elevated body temperature) in guinea pigs. For mechanistic reasons, including low-dose exposure probability, further work on human risk estimates for LD employed the exponential and beta-Poisson models. With an exposure of 10 colony-forming units (CFU) (retained dose), the QMRA model predicted a mild infection risk of 0.4 (as evaluated by seroprevalence) and a clinical severity LD case (e.g., hospitalization and supportive care) risk of 0.0009. The calculated rates based on estimated human exposures for outbreaks used for the QMRA model validation are within an order of magnitude of the reported LD rates. These validation results suggest the LD QMRA animal model selection, dose-response modeling, and extension to human risk projections were appropriate.

Biologic activity of guinea pig IFN-gamma in vitro

Interferon-gamma (IFN-gamma) plays a key role in the induction and maintenance of immunity against intracellular infectious agents. Compared to other species, little is known about the biology of this cytokine in the guinea pig (Cavia porcellus). We found that in contrast to humans and mice, IFN-gamma in the guinea pig did not induce the antiviral state, which in other species leads to protection of IFN-gamma -stimulated fibroblasts from the cytopathic effect (CPE) of subsequent viral infections. As an alternative strategy to detect and quantify guinea pig IFN-gamma activity in vitro, a reporter system using guinea pig fibroblasts transfected with a luciferase gene, which is regulated by an IFN-stimulated response element (ISRE), was established. With the help of the highly sensitive reporter assay system, the biologic activity of recombinant guinea pig IFN-gamma (GpIFN-gamma, from prokaryotic and eukaryotic expression systems was detected. The response to both native and recombinant GpIFN-gamma was inhibited by a rabbit antiserum directed against the recombinant cytokine expressed in Escherichia coli, demonstrating structural and functional homology of native and recombinant GpIFN-gamma. Stimulation with GpIFN-gamma, obtained from transfected cells, induced upregulation of MHC class I expression in a guinea pig fibroblast line. The restricted activity of GpIFN-gamma might have implications for this species' ability to control infections with intracellular pathogens.

Production and characterization of guinea pig recombinant gamma interferon and its effect on macrophage activation

Gamma interferon (IFN-gamma) plays a critical role in the protective immune responses against mycobacteria. We previously cloned a cDNA coding for guinea pig IFN-gamma (gpIFN-gamma) and reported that BCG vaccination induced a significant increase in the IFN-gamma mRNA expression in guinea pig cells in response to living mycobacteria and that the virulent H37Rv strain of Mycobacterium tuberculosis stimulated less IFN-gamma mRNA than did the attenuated H37Ra strain. In this study, we successfully expressed and characterized recombinant gpIFN-gamma with a histidine tag at the N terminus (His-tagged rgpIFN-gamma) in Escherichia coli. rgpIFN-gamma was identified as an 18-kDa band in the insoluble fraction; therefore, the protein was purified under denaturing conditions and renatured. N-terminal amino acid sequencing of the recombinant protein yielded the sequence corresponding to the N terminus of His-tagged gpIFN-gamma. The recombinant protein upregulated major histocompatibility complex class II expression in peritoneal macrophages. The antiviral activity of rgpIFN-gamma was demonstrated with a guinea pig fibroblast cell line (104C1) infected with encephalomyocarditis virus. Interestingly, peritoneal macrophages treated with rgpIFN-gamma did not produce any nitric oxide but did produce hydrogen peroxide and suppressed the intracellular growth of mycobacteria. Furthermore, rgpIFN-gamma induced morphological alterations in cultured macrophages. Thus, biologically active rgpIFN-gamma has been successfully produced and characterized in our laboratory. The study of rgpIFN-gamma will further increase our understanding of the cellular and molecular responses induced by BCG vaccination in the guinea pig model of pulmonary tuberculosis.

Nitric oxide inhibits Coxiella burnetii replication and parasitophorous vacuole maturation

Nitric oxide is a recognized cytotoxic effector against facultative and obligate intracellular bacteria. This study examined the effect of nitric oxide produced by inducible nitric oxide synthase (iNOS) up-regulated in response to cytokine stimulation, or by a synthetic nitric oxide donor, on replication of obligately intracellular Coxiella burnetii in murine L-929 cells. Immunoblotting and nitrite assays revealed that C. burnetii infection of L-929 cells augments expression of iNOS up-regulated in response to gamma interferon (IFN-gamma) and tumor necrosis factor alpha (TNF-alpha). Infection in the absence of cytokine stimulation did not result in demonstrable up-regulation of iNOS expression or in increased nitrite production. Nitrite production by cytokine-treated cells was significantly inhibited by the iNOS inhibitor S-methylisothiourea (SMT). Treatment of infected cells with IFN-gamma and TNF-alpha or the synthetic nitric oxide donor 2,2'-(hydroxynitrosohydrazino)bis-ethanamine (DETA/NONOate) had a bacteriostatic effect on C. burnetii replication. Inhibition of replication was reversed upon addition of SMT to the culture medium of cytokine-treated cells. Microscopic analysis of infected cells revealed that nitric oxide (either cytokine induced or donor derived) inhibited formation of the mature (large) parasitophorous vacuole that is characteristic of C. burnetii infection of host cells. Instead, exposure of infected cells to nitric oxide resulted in the formation of multiple small, acidic vacuoles usually containing one C. burnetii cell. Removal of nitrosative stress resulted in the coalescence of small vacuoles to form a large vacuole harboring multiple C. burnetii cells. These experiments demonstrate that nitric oxide reversibly inhibits replication of C. burnetii and formation of the parasitophorous vacuole.

Induction of iNOS in human monocytes infected with different Legionella species

The contribution of nitric oxide (NO) radicals to the suppression of intracellular replication of Legionella has been well established in rodents but remained questionable in humans. Considering the fact that human monocytes do not exhibit a high-output NO production, we used sensitive methods such as detection of inducible NO synthase (iNOS) mRNA by reverse transcription-PCR and demonstration of iNOS protein expression by means of flow cytometry and Western blot to compare the levels of iNOS induced by Legionella species which, in accordance to their human prevalence, show different multiplication rates within human monocytic cells. The expression of iNOS in Mono Mac 6 (MM6) cells showed an only moderate inverse correlation to the intracellular replication rate of a given Legionella species in the protein expression assays. However, stimulation of host cells with 1,25-dihydroxyvitamin D(3) to enhance NO production and inhibition of NO production by treatment of host cells with N(G)-methyl-L-arginine were not able to modify the intracellular multiplication of legionellae within MM6 cells. Therefore, NO production does not seem to play a crucial role for the restriction of intracellular replication of Legionella bacteria within human monocytic cells. Rodent models in investigations which are supposed to clarify the involvement of NO radicals in defense mechanisms against Legionella infections in humans are of doubtful significance.

Does inhibition of tumor necrosis factor alpha affect chlamydial genital tract infection in mice and guinea pigs?

The role of tumor necrosis factor alpha (TNF-alpha) in host defense against chlamydial infection remains unclear. In order to further evaluate the relevance of TNF-alpha to host resistance in chlamydial genital tract infection, we examined the effect of local inhibition of the TNF-alpha response in normal C57 mice and in interferon gamma gene-deficient C57 mice infected intravaginally with the mouse pneumonitis agent of Chlamydia trachomatis. Since the guinea pig model of female genital tract infection more closely approximates the human in terms of ascending infection and development of pathology, we also examined the effect of local inhibition of the TNF-alpha response in guinea pigs infected intravaginally with the guinea pig strain of Chlamydia psittaci. We successfully blocked the early TNF-alpha response in the respective animal models. This blockade had no effect on the numbers of organisms isolated from the genital tract during the time of TNF-alpha inhibition in mice or guinea pigs. Analysis of interleukin-1beta, macrophage inflammatory protein-2, and granulocyte macrophage-colony stimulating factor in the mouse model revealed that blockade of the TNF-alpha response did not alter the release of these proinflammatory proteins. Yet, in TNF-alpha-depleted mice, increased numbers of neutrophils were detected in the genital tract, and, in TNF-alpha-depleted guinea pigs, increased numbers of neutrophils as well as infiltrating lymphocytes were seen in the endocervix. Blockade of TNF-alpha does not affect the level of infection in mice or guinea pigs, but it may decrease TNF-alpha-induced apoptosis of infiltrating inflammatory cells.