Nitric oxide-mediated inhibition of the ability of Rickettsia prowazekii to infect mouse fibroblasts and mouse macrophagelike cells

Toxoplasma gondii microneme protein MIC3 induces macrophage TNF-α production and Ly6C expression via TLR11/MyD88 pathway

Toxoplasma gondii is the most successful parasite worldwide. It is of great interest to understand how T. gondii induce different immune responses in different hosts. In this study, we found that a peptide of T. gondii microneme protein MIC3 induced TNF-α production, NF-κB phosphorylation, iNOS transcription and Ly6C expression in mouse macrophage RAW264.7 cells. MyD88 inhibition, small interfering RNA against Tlr11 and CRISPR/Cas9-mediated knock-out of Tlr11 all reduced MIC3-induced TNF-α production, NF-κB phosphorylation, iNOS transcription and Ly6C expression. Additionally, we determined the location of MIC3 peptide in mouse macrophages using immunofluorescence. MIC3 could both adhere to the cell membrane of mouse macrophages and enter the cells. These results suggest that MIC3 triggered the immune responses in mouse macrophages via TLR11/MyD88/NF-κB pathway. It is known that human macrophages lacking TLR11. We predicted that the immune responses induced by MIC3 in human macrophages were significantly different from those in mouse macrophages. As expected, MIC3 peptide failed to induce TNF-α expression, iNOS expression and NF-κB phosphorylation in human THP-1 derived macrophages. MIC3 induced macrophage immune responses via TLR11. Intriguingly, the amino acid sequence of MIC3 is completely different from the well-known TLR11 ligand profilin, which generates a potent IL-12p40, TNF-α and IL-6 response. In marked contrast to profilin, MIC3 could not induce IL-12p40 expression in both mouse RAW264.7 cells and human THP-1 derived macrophages. Furthermore, the simulated tertiary structure of MIC3 peptide shows poor similarity with the crystal structure of profilin, suggesting that MIC3 might be a different ligand from profilin. These findings about MIC3 and TLR11 will provide us with important insights into the pathogenesis of toxoplasmosis and coevolution during host-parasite interaction.

Involvement of Pore Formation and Osmotic Lysis in the Rapid Killing of Gamma Interferon-Pretreated C166 Endothelial Cells by Rickettsia prowazekii

Rickettsia prowazekii, the bacterial cause of epidemic typhus in humans, proliferates mainly within the microvascular endothelial cells. Previous studies have shown that murine macrophage-like RAW264.7 cells are rapidly damaged if they are pretreated with gamma interferon (IFN-γ) and then infected with R. prowazekii. In the present study, the effects of IFN-γ and R. prowazekii on murine C166 endothelial cells were evaluated. In the IFN-γ-pretreated R. prowazekii-infected endothelial cell cultures, evidence of cell damage was observed within several hours after addition of the rickettsiae. Considerable numbers of the cells became permeable to trypan blue dye and ethidium bromide, and substantial amounts of lactate dehydrogenase (LDH) were released from the cells. Such evidence of cellular injury was not observed in the untreated infected cultures or in any of the mock-infected cultures. Polyethylene glycols (PEGs) of different nominal average molecular weights were used to assess the possible involvement of pore formation and osmotic lysis in this cellular injury. PEG 8000 dramatically suppressed LDH release, PEG 4000 partially inhibited it, and PEGs 2000 and 1450 had no effect. Despite its inhibition of LDH release, PEG 8000 did not prevent the staining of the IFN-γ-pretreated infected endothelial cells by ethidium bromide. These findings suggest that the observed cellular injury involves the formation of pores in the endothelial cell membranes, followed by osmotic lysis of the cells.

Nitric Oxide Inhibition of Rickettsia rickettsii

Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever, is an enzootic, obligate, intracellular bacterial pathogen. Nitric oxide (NO) synthesized by the inducible NO synthase (iNOS) is a potent antimicrobial component of innate immunity and has been implicated in the control of virulent Rickettsia spp. in diverse cell types. In this study, we examined the antibacterial role of NO on R. rickettsii. Our results indicate that NO challenge dramatically reduces R. rickettsii adhesion through the disruption of bacterial energetics. Additionally, NO-treated R. rickettsii cells were unable to synthesize protein or replicate in permissive cells. Activated, NO-producing macrophages restricted R. rickettsii infections, but inhibition of iNOS ablated the inhibition of bacterial growth. These data indicate that NO is a potent antirickettsial effector of innate immunity that targets energy generation in these pathogenic bacteria to prevent growth and subversion of infected host cells.

Inflammasome-mediated antagonism of type I interferon enhances Rickettsia pathogenesis

The innate immune system fights infection with inflammasomes and interferons. Facultative bacterial pathogens that inhabit the host cytosol avoid inflammasomes^1–6 and are often insensitive to type I interferons (IFN-I), but are restricted by IFN-γ^7–11. However, it remains unclear how obligate cytosolic bacterial pathogens, including Rickettsia species, interact with innate immunity. Here, we report that the human pathogen Rickettsia parkeri is sensitive to IFN-I and benefits from inflammasome-mediated host cell death that antagonizes IFN-I. R. parkeri-induced cell death requires the cytosolic lipopolysaccharide (LPS) receptor caspase-11 and antagonizes IFN-I production mediated by the DNA sensor cGAS. The restrictive effects of IFN-I require the interferon regulatory factor IRF5, which upregulates genes encoding guanylate binding proteins (GBPs) and inducible nitric oxide synthase (iNOS), which we found to inhibit R. parkeri. Mice lacking both IFN-I and IFN-γ receptors succumb to R. parkeri, revealing critical and overlapping roles for these cytokines in vivo. The interactions of R. parkeri with inflammasomes and interferons are similar to those of viruses, which can exploit the inflammasome to avoid IFN-I¹², are restricted by IFN-I via IRF5^13,14, and are controlled by IFN-I and IFN-γ in vivo^15–17. Our results suggest that the innate immune response to an obligate cytosolic pathogen lies at the intersection of anti-bacterial and anti-viral responses.

Xenophagy in Helicobacter pylori- and Epstein-Barr virus-induced gastric cancer

Helicobacter pylori and Epstein-Barr virus (EBV) account for roughly 80% and 10%, respectively, of gastric carcinomas worldwide. Autophagy is an evolutionarily conserved and intricately regulated cellular process that involves the sequestration of cytoplasmic proteins and organelles into double-membrane autophagosomes that eventually fuse with lysosomes for degradation of the engulfed content. Emerging evidence indicates that xenophagy, a form of selective autophagy, plays a crucial role in the pathogenesis of H. pylori- and EBV-induced gastric cancer. Xenophagy specifically recognizes intracellular H. pylori and EBV and physically targets these pathogens to the autophagosomal-lysosomal pathway for degradation. In this connection, H. pylori or EBV-induced dysregulation of autophagy may be causally linked to gastric tumourigenesis and therefore can be exploited as therapeutic targets. This review will discuss how H. pylori and EBV infection activate autophagy and how these pathogens evade recognition and degradation by the autophagic pathway. Elucidating the molecular aspects of H. pylori- and EBV-induced autophagy will help us better understand the pathogenesis of gastric cancer and promote the development of autophagy modulators as antimicrobial agents.

Beta interferon-mediated activation of signal transducer and activator of transcription protein 1 interferes with Rickettsia conorii replication in human endothelial cells

Infection of the endothelial cell lining of blood vessels with Rickettsia conorii, the causative agent of Mediterranean spotted fever, results in endothelial activation. We investigated the effects of R. conorii infection on the status of the Janus kinase (JAK)-signal transducer and activator of transcription protein (STAT) signaling pathway in human microvascular endothelial cells (HMECs), the most relevant host cell type, in light of rickettsial tropism for microvascular endothelium in vivo. R. conorii infection induced phosphorylation of STAT1 on tyrosine 701 and serine 727 at 24, 48, and 72 h postinfection in HMECs. Employing transcription profile analysis and neutralizing antibodies, we further determined that beta interferon (IFN-β) production and secretion are critical for STAT1 activation. Secreted IFN-β further amplified its own expression via a positive-feedback mechanism, while expression of transcription factors interferon regulatory factor 7 (IRF7) and IRF9, implicated in the IFN-β-STAT1 feedback loop, was also induced. Metabolic activity of rickettsiae was essential for the IFN-β-mediated response(s) because tetracycline treatment inhibited R. conorii replication, IFN-β expression, and STAT1 phosphorylation. Inclusion of IFN-β-neutralizing antibody during infection resulted in significantly enhanced R. conorii replication, whereas addition of exogenous IFN-β had the opposite inhibitory effect. Finally, small interfering RNA-mediated knockdown further confirmed a protective role for STAT1 against intracellular R. conorii replication. In concert, these findings implicate an important role for IFN-β-mediated STAT1 activation in innate immune responses of vascular endothelium to R. conorii infection.

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.

Rickettsia rickettsii infection of cultured human endothelial cells induces heme oxygenase 1 expression

Existing evidence suggests that oxidative insults and antioxidant defense mechanisms play a critical role in the host cell response during infection of endothelial cells by Rickettsia rickettsii, the causative agent of Rocky Mountain spotted fever. Heme oxygenase (HO), a rate-limiting enzyme in the pathway for heme catabolism, protects against oxidant damage in a variety of stress situations. Here, we report on the expression of the inducible and constitutive HO isozymes, HO-1 and HO-2, during R. rickettsii infection of endothelial cells. Steady-state levels for HO-1 mRNA were increased two- to threefold, as early as 4 h postinfection, whereas HO-2 mRNA was not affected. Induction of HO-1 mRNA was dependent on the dose of infection and occurred in a time-dependent manner, reaching maximal levels at 4 to 7 h. The increase in HO-1 mRNA occurred at the level of trancription as it was blocked by the transcriptional inhibitors, actinomycin D and alpha-amanitin. The eukaryotic protein synthesis inhibitor, cycloheximide, caused a >50% reduction in the infection-induced increase in HO-1 mRNA level, suggesting its dependence on de novo protein synthesis of host cell. The uptake of viable organisms appeared to be necessary, since inactivation of R. rickettsii by heat or formalin fixation, or incubation of cells with cytochalasin B to prevent entry resulted in marked inhibition of HO-1 response. N-Acetyl-L-cysteine, a known oxidant scavenger, inhibited the HO-1 induction by R. rickettsii. Finally, Western analysis with a specific monoclonal antibody revealed higher levels of HO-1 protein ( approximately 32 kDa), confirming that changes in HO-1 mRNA levels were followed by increases in the levels of protein. The findings indicate that R. rickettsii infection induces HO-1 expression in host endothelial cells and suggest an important role for this enzyme in cellular response to infection, possibly by serving a protective function against oxidative injury.

Interaction of Bartonella henselae with the murine macrophage cell line J774: infection and proinflammatory response

Bartonella henselae is the causative agent of cat scratch disease (CSD), a self-limiting condition characterized by a subacute regional lymphadenopathy that may develop into disseminated bartonellosis in immunocompromised subjects. Mice experimentally infected with B. henselae display typical liver and spleen granulomas rich in T cells and macrophages. So far there are no data on the interaction between bartonellae and macrophages. In order to clarify this topic, we investigated the interaction of B. henselae with J774, a mouse macrophage cell line. Analysis of bacterial uptake by functional assays and transmission electron microscopy indicates that bartonellae can enter and survive inside J774. Entry occurred within 30 min postinfection and reached a plateau at 160 min. Infection of J774 was followed by a dose-dependent release of the proinflammatory cytokines tumor necrosis factor alpha, interleukin 1beta (IL-1beta), and IL-6. Bartonellae persisted intracellularly without loss of viability for at least 8 h, and their number slightly decreased 24 h postinfection. Gamma interferon (IFN-gamma) treatment of J774 significantly decreased the number of recoverable bacteria at 8 and 24 h. This enhancement of macrophage bactericidal activity was associated with nitric oxide (NO) release and was prevented by the addition of the competitive inhibitor of NO synthesis N(G)-monomethyl L-arginine. These findings suggest that IFN-gamma-mediated activation of macrophages may be important for the clearing of B. henselae infection and that anti-B. henselae microbicidal activity of IFN-gamma-activated macrophages is mediated to a large extent by NO production.

Phagocytosis and killing of intracellular pathogens: interaction between cytokines and antibiotics

Phagocytosis and bacterial killing are the primary functions of macrophages. Among the mechanisms involved in the phagocytic process, cytokines, especially those of T-helper 1 profile, appear to influence considerably the internalization and the intracellular fate of the pathogen within the macrophage. In particular, the evidence for a cooperation of cytokines with antibiotics in intracellular infection could provide new therapeutic approaches to intracellular infectious diseases in the future.