The gut microbiota prime systemic antiviral immunity via the cGAS-STING-IFN-I axis

The microbiota are vital for immune homeostasis and provide a competitive barrier to bacterial and fungal pathogens. Here, we investigated how gut commensals modulate systemic immunity and response to viral infection. Antibiotic suppression of the gut microbiota reduced systemic tonic type I interferon (IFN-I) and antiviral priming. The microbiota-driven tonic IFN-I-response was dependent on cGAS-STING but not on TLR signaling or direct host-bacteria interactions. Instead, membrane vesicles (MVs) from extracellular bacteria activated the cGAS-STING-IFN-I axis by delivering bacterial DNA into distal host cells. DNA-containing MVs from the gut microbiota were found in circulation and promoted the clearance of both DNA (herpes simplex virus type 1) and RNA (vesicular stomatitis virus) viruses in a cGAS-dependent manner. In summary, this study establishes an important role for the microbiota in peripheral cGAS-STING activation, which promotes host resistance to systemic viral infections. Moreover, it uncovers an underappreciated risk of antibiotic use during viral infections.

Multiple ferritins are vital to successful blood feeding and reproduction of the hard tick Haemaphysalis longicornis

Ticks are obligate hematophagous parasites and important vectors of diseases. The large amount of blood they consume contains great quantities of iron, an essential but also toxic element. The function of ferritin, an iron storage protein, and iron metabolism in ticks need to be further elucidated. Here, we investigated the function a newly identified secreted ferritin from the hard tick Haemaphysalis longicornis (HlFER2), together with the previously identified intracellular ferritin (HlFER1). Recombinant ferritins, expressed in Escherichia coli, were used for anti-serum preparation and were also assayed for iron-binding activity. RT-PCR and western blot analyses of different organs and developmental stages of the tick during blood feeding were performed. The localization of ferritins in different organs was demonstrated through an indirect immunofluorescent antibody test. RNA interference (RNAi) was performed to evaluate the importance of ferritin in blood feeding and reproduction of ticks. The midgut was also examined after RNAi using light and transmission electron microscopy. RT-PCR showed differences in gene expression in some organs and developmental stages. Interestingly, only HlFER2 was detected in the ovary during oviposition and in the egg despite the low mRNA transcript. RNAi induced a reduction in post-blood meal body weight, high mortality and decreased fecundity. The expression of vitellogenin genes was affected by silencing of ferritin. Abnormalities in digestive cells, including disrupted microvilli, and alteration of digestive activity were also observed. Taken altogether, our results show that the iron storage and protective functions of ferritin are crucial to successful blood feeding and reproduction of H. longicornis.

Reversible senescence of human colon cancer cells after blockage of mitosis/cytokinesis caused by the CNF1 cyclomodulin from Escherichia coli

Cytotoxic necrotizing factor 1 (CNF1), a protein toxin produced by extraintestinal pathogenic Escherichia coli, activates the Rho-family small GTPases in eukaryotic cell, thereby perturbing multiple cellular functions. Increasing epidemiological evidence suggests a link between CNF1 and human inflammatory bowel disease and colorectal cancer. At the cellular level, CNF1 has been hypothesized to reprogram cell fate towards survival due to the role in perturbing cell cycle and apoptosis. However, it remains undetermined how cells survive from CNF1 intoxication. In this work, we show that CNF1 treatment blocks mitosis/cytokinesis, elicits endoreplication and polyploidisation in cultured human colon cancer cells, and drives them into reversible senescence, which provides a survival route for cells via depolyploidisation. Senescence in CNF1-treated cells is demonstrated with upregulation of several senescence markers including senescence-associated β-galactosidase activity, p53, p21 and p16, and concomitant inhibition of the retinoblastoma protein phosphorylation. Importantly, progeny derived from CNF1 treatment exhibit genomic instability exemplified by increased aneuploidy and become more resistant to CNF1, but not to 5-fluorouracil and oxaliplatin, the two agents commonly used in chemotherapeutic treatment for colorectal cancer. These observations display survival features of the cell after CNF1 treatment that may have implications for the potential role of CNF1 in carcinogenesis.

Flagella-mediated secretion of a novel Vibrio cholerae cytotoxin affecting both vertebrate and invertebrate hosts

Using Caenorhabditis elegans as an infection host model for Vibrio cholerae predator interactions, we discovered a bacterial cytotoxin, MakA, whose function as a virulence factor relies on secretion via the flagellum channel in a proton motive force-dependent manner. The MakA protein is expressed from the polycistronic makDCBA (motility-associated killing factor) operon. Bacteria expressing makDCBA induced dramatic changes in intestinal morphology leading to a defecation defect, starvation and death in C. elegans. The Mak proteins also promoted V. cholerae colonization of the zebrafish gut causing lethal infection. A structural model of purified MakA at 1.9 Å resolution indicated similarities to members of a superfamily of bacterial toxins with unknown biological roles. Our findings reveal an unrecognized role for V. cholerae flagella in cytotoxin export that may contribute both to environmental spread of the bacteria by promoting survival and proliferation in encounters with predators, and to pathophysiological effects during infections.

Scavenger receptor mediates systemic RNA interference in ticks

RNA interference is an efficient method to silence gene and protein expressions. Here, the class B scavenger receptor CD36 (SRB) mediated the uptake of exogenous dsRNAs in the induction of the RNAi responses in ticks. Unfed female Haemaphysalis longicornis ticks were injected with a single or a combination of H. longicornis SRB (HlSRB) dsRNA, vitellogenin-1 (HlVg-1) dsRNA, and vitellogenin receptor (HlVgR) dsRNA. We found that specific and systemic silencing of the HlSRB, HlVg-1, and HlVgR genes was achieved in ticks injected with a single dsRNA of HlSRB, HlVg-1, and HlVgR. In ticks injected first with HlVg-1 or HlVgR dsRNA followed 96 hours later with HlSRB dsRNA (HlVg-1/HlSRB or HlVgR/HlSRB), gene silencing of HlSRB was achieved in addition to first knockdown in HlVg-1 or HlVgR, and prominent phenotypic changes were observed in engorgement, mortality, and hatchability, indicating that a systemic and specific double knockdown of target genes had been simultaneously attained in these ticks. However, in ticks injected with HlSRB dsRNA followed 96 hours later with HlVg-1 or HlVgR dsRNAs, silencing of HlSRB was achieved, but no subsequent knockdown in HlVgR or HlVg-1 was observed. The Westernblot and immunohistochemical examinations revealed that the endogenous HlSRB protein was fully abolished in midguts of ticks injected with HlSRB/HlVg-1 dsRNAs but HlVg-1 was normally expressed in midguts, suggesting that HlVg-1 dsRNA-mediated RNAi was fully inhibited by the first knockdown of HlSRB. Similarly, the abolished localization of HlSRB protein was recognized in ovaries of ticks injected with HlSRB/HlVgR, while normal localization of HlVgR was observed in ovaries, suggesting that the failure to knock-down HlVgR could be attributed to the first knockdown of HlSRB. In summary, we demonstrated for the first time that SRB may not only mediate the effective knock-down of gene expression by RNAi but also play essential roles for systemic RNAi of ticks.

Naturally Occurring IgG Antibodies Provide Innate Protection against Vibrio cholerae Bacteremia by Recognition of the Outer Membrane Protein U

Cholera epidemics are caused by Vibrio cholerae serogroups O1 and O139, whereas strains collectively known as non-O1/non-O139 V. cholerae are found in cases of extraintestinal infections and bacteremia. The mechanisms and factors influencing the occurrence of bacteremia and survival of V. cholerae in normal human serum have remained unclear. We found that naturally occurring IgG recognizing V. cholerae outer membrane protein U (OmpU) mediates a serum-killing effect in a complement C1q-dependent manner. Moreover, outer membrane vesicles (OMVs) containing OmpU caused enhanced survival of highly serum-sensitive classical V. cholerae in a dose-dependent manner. OMVs from wild-type and ompU mutant V. cholerae thereby provided a novel means to verify by extracellular transcomplementation the involvement of OmpU. Our data conclusively indicate that loss, or reduced expression, of OmpU imparts resistance to V. cholerae towards serum killing. We propose that the difference in OmpU protein levels is a plausible reason for differences in serum resistance and the ability to cause bacteremia observed among V. cholerae biotypes. Our findings provide a new perspective on how naturally occurring antibodies, perhaps induced by members of the microbiome, may play a role in the recognition of pathogens and the provocation of innate immune defense against bacteremia.

Identification and characterization of class B scavenger receptor CD36 from the hard tick, Haemaphysalis longicornis

Scavenger receptors (SRs) are cell-surface proteins and exhibit distinctive ligand-binding properties, recognizing a wide range of ligands that include microbial surface constituents and intact microbes. The class B scavenger receptor CD36 (SRB) is predominantly expressed by macrophages and is considered important in innate immunity. We here show the identification and characterization of SRB from the hard ixodid tick, Haemaphysalis longicornis (HlSRB). The full-length cDNA was 2,908 bp, including an ORF encoding of 1,518 amino acids with a pI value of 5.83. H. longicornis SRB contains a hydrophobic SRB domain and four centrally clustered cysteine residues for arrangement of disulfide bridges. Deduced amino acid sequence has an identity of 30-38% with the SRB of other organisms. RT-PCR analysis showed that mRNA transcripts were expressed in multiple organs of adult ticks but with a different transcript level in the developmental stages of H. longicornis ticks. His-tagged recombinant HlSRB was expressed in Escherichia coli with an expected molecular mass of 50 kDa. In Western blot analysis, mouse anti-rHlSRB serum recognized a strong reaction with a 50 kDa protein band in lysates prepared from egg and adult tick but showed a weak reaction with lysates of larva and nymph. In an indirect immunofluorescent antibody test, HlSRB antiserum recognized the protein located on the midgut, salivary glands, and ovary of partially fed H. longicornis females. Silencing of the HlSRB gene by RNAi led to a significant reduction in the engorged female body weight. It is noteworthy that more than a dozen SRB orthologs have been identified in the genomes of insect species with functions related to pheromone signaling, innate immunity, phagocytic clearance of apoptotic cells, and various aspects of the fatty acid metabolism. This is the first report of the identification and characterization of the SRB homologue in Chelicerata, including ticks, horseshoe crabs, scorpions, spiders, and mites.

Role of OmpA1 and OmpA2 in Aggregatibacter actinomycetemcomitans and Aggregatibacter aphrophilus serum resistance

Aggregatibacter actinomycetemcomitans and Aggregatibacter aphrophilus belong to the HACEK group of fastidious Gram-negative organisms, a recognized cause of infective endocarditis. A. actinomycetemcomitans is also implicated in aggressive forms of periodontitis. We demonstrated that A. aphrophilus strains, as A. actinomycetemcomitans are ubiquitously serum resistant. Both species encode two Outer membrane protein A paralogues, here denoted OmpA1 and OmpA2. As their respective pangenomes contain several OmpA1 and OmpA2 alleles, they represent potential genotypic markers. A naturally competent strain of A. actinomycetemcomitans and A. aphrophilus, respectively were used to elucidate if OmpA1 and OmpA2 contribute to serum resistance. Whereas OmpA1 was critical for survival of A. actinomycetemcomitans D7SS in 50% normal human serum (NHS), serum resistant ompA1 mutants were fortuitously obtained, expressing enhanced levels of OmpA2. Similarly, OmpA1 rather than OmpA2 was a major contributor to serum resistance of A. aphrophilus HK83. Far-Western blot revealed that OmpA1^AA, OmpA2^AA, and OmpA1^AP can bind to C4-binding protein, an inhibitor of classical and mannose-binding lectin (MBL) complement activation. Indeed, ompA1 mutants were susceptible to these pathways, but also to alternative complement activation. This may at least partly reflect a compromised outer membrane integrity but is also consistent with alternative mechanisms involved in OmpA-mediated serum resistance.

Suppression of β-catenin signaling in colon carcinoma cells by a bacterial protein

Colorectal cancer is one of the leading causes of cancer-related death worldwide. The adenomatous polyposis coli (APC) gene is mutated in hereditary colorectal tumors and in more than 80% of sporadic colorectal tumors. APC mutations impair β-catenin degradation, leading to its permanent stabilization and increased transcription of cancer-driving target genes. In colon cancer, impairment of β-catenin degradation leads to its cytoplasmic accumulation, nuclear translocation, and subsequent activation of tumor cell proliferation. Suppressing β-catenin signaling in cancer cells therefore appears to be a promising strategy for new anticancer strategies. Recently, we discovered a novel Vibrio cholerae cytotoxin, motility-associated killing factor A (MakA), that affects both invertebrate and vertebrate hosts. It promotes bacterial survival and proliferation in invertebrate predators but has unknown biological role(s) in mammalian hosts. Here, we report that MakA can cause lethality of tumor cells via induction of apoptosis. Interestingly, MakA exhibited potent cytotoxic activity, in particular against several tested cancer cell lines, while appearing less toxic toward nontransformed cells. MakA bound to the tumor cell surface became internalized into the endolysosomal compartment and induced leakage of endolysosomal membranes, causing cytosolic release of cathepsins and activation of proapoptotic proteins. In addition, MakA altered β-catenin integrity in colon cancer cells, partly through a caspase- and proteasome-dependent mechanism. Importantly, MakA inhibited β-catenin-mediated tumor cell proliferation. Remarkably, intratumor injection of MakA significantly reduced tumor development in a colon cancer murine solid tumor model. These data identify MakA as a novel candidate to be considered in new strategies for development of therapeutic agents against colon cancer.

Inhibitory effect of cyclophilin A from the hard tick Haemaphysalis longicornis on the growth of Babesia bovis and Babesia bigemina

Haemaphysalis longicornis is known as one of the most important ticks transmitting Babesia parasites in East Asian countries, including Babesia ovata and Babesia gibsoni, as well as Theileria parasites. H. longicornis is not the natural vector of Babesia bovis and Babesia bigemina. Vector ticks and transmitted parasites are thought to have established unique host-parasite interaction for their survival, meaning that vector ticks may have defensive molecules for the growth control of parasites in their bodies. However, the precise adaptation mechanism of tick-Babesia parasites is still unknown. Recently, cyclophilin A (CyPA) was reported to be important for the development of Babesia parasites in ticks. To reveal a part of their adaptation mechanism, the current study was conducted. An injection of B. bovis-infected RBCs into adult female H. longicornis ticks was found to upregulate the expression profiles of the gene and protein of CyPA in H. longicornis (HlCyPA). In addition, recombinant HlCyPA (rHlCyPA) purified from Escherichia coli exhibited significant inhibitory growth effects on B. bovis and B. bigemina cultivated in vitro, without any hemolytic effect on bovine RBCs at all concentrations used. In conclusion, our results suggest that HlCyPA might play an important role in the growth regulation of Babesia parasites in H. longicornis ticks, during natural acquisition from an infected host. Furthermore, rHlCyPA may be a potential alternative chemotherapeutic agent against babesiosis.

Vibrio cholerae cytotoxin MakA induces noncanonical autophagy resulting in the spatial inhibition of canonical autophagy

Autophagy plays an essential role in the defense against many microbial pathogens as a regulator of both innate and adaptive immunity. Some pathogens have evolved sophisticated mechanisms that promote their ability to evade or subvert host autophagy. Here, we describe a novel mechanism of autophagy modulation mediated by the recently discovered Vibrio cholerae cytotoxin, motility-associated killing factor A (MakA). pH-dependent endocytosis of MakA by host cells resulted in the formation of a cholesterol-rich endolysosomal membrane aggregate in the perinuclear region. Aggregate formation induced the noncanonical autophagy pathway driving unconventional LC3 (herein referring to MAP1LC3B) lipidation on endolysosomal membranes. Subsequent sequestration of the ATG12-ATG5-ATG16L1 E3-like enzyme complex, required for LC3 lipidation at the membranous aggregate, resulted in an inhibition of both canonical autophagy and autophagy-related processes, including the unconventional secretion of interleukin-1β (IL-1β). These findings identify a novel mechanism of host autophagy modulation and immune modulation employed by V. cholerae during bacterial infection.

Elevated levels of VCA0117 (VasH) in response to external signals activate the type VI secretion system of Vibrio cholerae O1 El Tor A1552

The type VI nanomachine is critical for Vibrio cholerae to establish infections and to thrive in niches co-occupied by competing bacteria. The genes for the type VI structural proteins are encoded in one large and two small auxiliary gene clusters. VCA0117 (VasH) - a σ⁵⁴ -transcriptional activator - is strictly required for functionality of the type VI secretion system since it controls production of the structural protein Hcp. While some strains constitutively produce a functional system, others do not and require specific growth conditions of low temperature and high osmolarity for expression of the type VI machinery. Here, we trace integration of these regulatory signals to the promoter activity of the large gene cluster in which many components of the machinery and VCA0117 itself are encoded. Using in vivo and in vitro assays and variants of VCA0117, we show that activation of the σ⁵⁴ -promoters of the auxiliary gene clusters by elevated VCA0117 levels are all that is required to overcome the need for specialized growth conditions. We propose a model in which signal integration via the large operon promoter directs otherwise restrictive levels of VCA0117 that ultimately dictates a sufficient supply of Hcp for completion of a functional type VI secretion system.

Genotoxin-producing Salmonella enterica induces tissue-specific types of DNA damage and DNA damage response outcomes

Introduction

Typhoid toxin-expressing Salmonella enterica causes DNA damage in the intestinal mucosa in vivo, activating the DNA damage response (DDR) in the absence of inflammation. To understand whether the tissue microenvironment constrains the infection outcome, we compared the immune response and DDR patterns in the colon and liver of mice infected with a genotoxigenic strain or its isogenic control strain.

Methods

In situ spatial transcriptomic and immunofluorescence have been used to assess DNA damage makers, activation of the DDR, innate immunity markers in a multiparametric analysis.

Result

The presence of the typhoid toxin protected from colonic bacteria-induced inflammation, despite nuclear localization of p53, enhanced co-expression of type-I interferons (IfnbI) and the inflammasome sensor Aim2, both classic features of DNA-break-induced DDR activation. These effects were not observed in the livers of either infected group. Instead, in this tissue, the inflammatory response and DDR were associated with high oxidative stress-induced DNA damage.

Conclusions

Our work highlights the relevance of the tissue microenvironment in enabling the typhoid toxin to suppress the host inflammatory response in vivo.

Ecotin and LamB in Escherichia coli influence the susceptibility to Type VI secretion-mediated interbacterial competition and killing by Vibrio cholerae

BACKGROUND

A prevailing action of the Type VI secretion system (T6SS) in several Gram-negative bacterial species is inter-bacterial competition. In the past several years, many effectors of T6SS were identified in different bacterial species and their involvement in inter-bacterial interactions were described. However, possible defence mechanisms against T6SS attack among prey bacteria were not well clarified yet.

METHODS

Escherichia coli was assessed for susceptibility to T6SS-mediated killing by Vibrio cholerae. TheT6SS-mediated bacterial killing assays were performed in absence or presence of different protease inhibitors and with different mutant E. coli strains. Expression levels of selected proteins were monitored using SDS-PAGE and immunoblot analyses.

RESULTS

The T6SS-mediated killing of E. coli by V. cholerae was partly blocked when the serine protease inhibitor Pefabloc was present. E. coli lacking the periplasmic protease inhibitor Ecotin showed enhanced susceptibility to killing by V. cholerae. Mutations affecting E. coli membrane stability also caused increased susceptibility to killing by V. cholerae. E. coli lacking the maltodextrin porin protein LamB showed reduced susceptibility to killing by V. cholerae whereas E. coli with induced high levels of LamB showed reduced survival in inter-bacterial competition.

CONCLUSIONS

Our study identified two proteins in E. coli, the intrinsic protease inhibitor Ecotin and the outer membrane porin LamB, that influenced E. coli susceptibility to T6SS-mediated killing by V. cholerae.

GENERAL SIGNIFICANCE

We envision that it is feasible to explore these findings to target and modulate their expression to obtain desired changes in inter-bacterial competition in vivo, e.g. in the gastrointestinal microbiome.

Retracted: Risks of avian influenza (H5) in duck farms in the Ayeyarwaddy Delta region, Myanmar

The Ayeyarwaddy delta region in the south-west of Myanmar is the main agricultural and rice-growing area. The region has a high density of duck and backyard chicken populations with low biosecurity. The objective of this study was to analyse risk factors for avian influenza (H5) in the Ayeyarwaddy delta region, Myanmar. A case–control risk factor study was conducted from April to June 2010 by individual interviews including risk factor questionnaires given to duck farmers (n = 50) in five townships in the Ayeyarwaddy delta region, Myanmar. Risk factor analyses were conducted using univariate analysis and multivariate logistic regression model with backward stepwise (wald) method. The results showed significant risk factors for AI (H5) sero-positivity in ducks were wooden egg box containers (OR = 52.7, 95% CI = 2.34-1188, P = 0.013) and water sourced from wetlands (OR = 30.7, 95% CI = 1.96-481.6, P = 0.015). Conversely, the cleaning of reusable egg containers was determined as a protective factor (OR = 0.03, 95% CI = 0.00-0.42, P = 0.01). In conclusion, this study identified risk factors for AI (H5) in duck farms and the importance of avian influenza prevention and control.

Protocol for analyzing the function of the type VI secretion system of the oral symbiont Aggregatibacter aphrophilus in targeting pathobionts

Here, we present a protocol for evaluating type VI secretion system (T6SS)-dependent fitness of the oral symbiont A. aphrophilus using biofilm competition assays and metaproteomics. We describe steps for designing T6SS-specific mutants. We then detail procedures for using them in competition assays with the pathobiont A. actinomycetemcomitans and in biofilm models, analyzing metaproteomes to assess the impact of the T6SS on multiple pathobionts. The biofilm model is designed to mimic the oral plaque ecosystem and includes seven species. For complete details on the use and execution of this protocol, please refer to Oscarsson et al.1.

Bacterial symbionts in oral niche use type VI secretion nanomachinery for fitness increase against pathobionts

Microbial ecosystems experience spatial and nutrient restrictions leading to the coevolution of cooperation and competition among cohabiting species. To increase their fitness for survival, bacteria exploit machinery to antagonizing rival species upon close contact. As such, the bacterial type VI secretion system (T6SS) nanomachinery, typically expressed by pathobionts, can transport proteins directly into eukaryotic or prokaryotic cells, consequently killing cohabiting competitors. Here, we demonstrate for the first time that oral symbiont Aggregatibacter aphrophilus possesses a T6SS and can eliminate its close relative oral pathobiont Aggregatibacter actinomycetemcomitans using its T6SS. These findings bring nearer the anti-bacterial prospects of symbionts against cohabiting pathobionts while introducing the presence of an active T6SS in the oral cavity.