Interleukin-22 mediates early host defense against attaching and effacing bacterial pathogens

Infections by attaching and effacing (A/E) bacterial pathogens, such as Escherichia coli O157:H7, pose a serious threat to public health. Using a mouse A/E pathogen, Citrobacter rodentium, we show that interleukin-22 (IL-22) has a crucial role in the early phase of host defense against C. rodentium. Infection of IL-22 knockout mice results in increased intestinal epithelial damage, systemic bacterial burden and mortality. We also find that IL-23 is required for the early induction of IL-22 during C. rodentium infection, and adaptive immunity is not essential for the protective role of IL-22 in this model. Instead, IL-22 is required for the direct induction of the Reg family of antimicrobial proteins, including RegIIIbeta and RegIIIgamma, in colonic epithelial cells. Exogenous mouse or human RegIIIgamma substantially improves survival of IL-22 knockout mice after C. rodentium infection. Together, our data identify a new innate immune function for IL-22 in regulating early defense mechanisms against A/E bacterial pathogens.     

The ubiquitin ligation machinery in the defense against bacterial pathogens

The ubiquitin system is an important part of the host cellular defense program during bacterial infection. This is in particular evident for a number of bacteria including Salmonella Typhimurium and Mycobacterium tuberculosis which—inventively as part of their invasion strategy or accidentally upon rupture of seized host endomembranes—become exposed to the host cytosol. Ubiquitylation is involved in the detection and clearance of these bacteria as well as in the activation of innate immune and inflammatory signaling. Remarkably, all these defense responses seem to emanate from a dense layer of ubiquitin which coats the invading pathogens. In this review, we focus on the diverse group of host cell E3 ubiquitin ligases that help to tailor this ubiquitin coat. In particular, we address how the divergent ubiquitin conjugation mechanisms of these ligases contribute to the complexity of the anti‐bacterial coating and the recruitment of different ubiquitin‐binding effectors. We also discuss the activation and coordination of the different E3 ligases and which strategies bacteria evolved to evade the activities of the host ubiquitin system.     

IL-22 mediates mucosal host defense against Gram-negative bacterial pneumonia

Emerging evidence supports the concept that T helper type 17 (T(H)17) cells, in addition to mediating autoimmunity, have key roles in mucosal immunity against extracellular pathogens. Interleukin-22 (IL-22) and IL-17A are both effector cytokines produced by the T(H)17 lineage, and both were crucial for maintaining local control of the Gram-negative pulmonary pathogen, Klebsiella pneumoniae. Although both cytokines regulated CXC chemokines and granulocyte colony-stimulating factor production in the lung, only IL-22 increased lung epithelial cell proliferation and increased transepithelial resistance to injury. These data support the concept that the T(H)17 cell lineage and its effector molecules have evolved to effect host defense against extracellular pathogens at mucosal sites.     

Intracellular innate resistance to bacterial pathogens

Mammalian innate immunity stimulates antigen-specific immune responses and acts to control infection prior to the onset of adaptive immunity. Some bacterial pathogens replicate within the host cell and are therefore sheltered from some protective aspects of innate immunity such as complement. Here we focus on mechanisms of innate intracellular resistance encountered by bacterial pathogens and how some bacteria can evade destruction by the innate immune system. Major strategies of intracellular antibacterial defence include pathogen compartmentalization and iron limitation. Compartmentalization of pathogens within the host endocytic pathway is critical for generating high local concentrations of antimicrobial molecules, such as reactive oxygen species, and regulating concentrations of divalent cations that are essential for microbial growth. Cytosolic sensing, autophagy, sequestration of essential nutrients and membrane attack by antimicrobial peptides are also discussed.     

Identification of an innate T helper type 17 response to intestinal bacterial pathogens

Interleukin 17 (IL-17) is a central cytokine implicated in inflammation and antimicrobial defense. After infection, both innate and adaptive IL-17 responses have been reported, but the type of cells involved in innate IL-17 induction, as well as their contribution to in vivo responses, are poorly understood. Here we found that Citrobacter and Salmonella infection triggered early IL-17 production, which was crucial for host defense and was mediated by CD4(+) T helper cells. Enteric innate T helper type 17 (iT(H)17) responses occurred principally in the cecum, were dependent on the Nod-like receptors Nod1 and Nod2, required IL-6 induction and were associated with a decrease in mucosal CD103(+) dendritic cells. Moreover, imprinting by the intestinal microbiota was fully required for the generation of iT(H)17 responses. Together, these results identify the Nod-iT(H)17 axis as a central element in controlling enteric pathogens, which may implicate Nod-driven iT(H)17 responses in the development of inflammatory bowel diseases.     

Hydathode immunity protects the Arabidopsis leaf vasculature against colonization by bacterial pathogens

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Phytoalexins in defense against pathogens

Plants use an intricate defense system against pests and pathogens, including the production of low molecular mass secondary metabolites with antimicrobial activity, which are synthesized de novo after stress and are collectively known as phytoalexins. In this review, we focus on the biosynthesis and regulation of camalexin, and its role in plant defense. In addition, we detail some of the phytoalexins produced by a range of crop plants from Brassicaceae, Fabaceae, Solanaceae, Vitaceae and Poaceae. This includes the very recently identified kauralexins and zealexins produced by maize, and the biosynthesis and regulation of phytoalexins produced by rice. Molecular approaches are helping to unravel some of the mechanisms and reveal the complexity of these bioactive compounds, including phytoalexin action and metabolism.     

Contribution of bacterial outer membrane vesicles to innate bacterial defense

Background

Microorganisms produce cell-wall-degrading enzymes as part of their strategies for plant invasion/nutrition. Among these, pectin lyases (PNLs) catalyze the depolymerization of esterified pectin by a β-elimination mechanism. PNLs are grouped together with pectate lyases (PL) in Family 1 of the polysaccharide lyases, as they share a conserved structure in a parallel β-helix. The best-characterized fungal pectin lyases are obtained from saprophytic/opportunistic fungi in the genera Aspergillus and Penicillium and from some pathogens such as Colletotrichum gloeosporioides. The organism used in the present study, Colletotrichum lindemuthianum, is a phytopathogenic fungus that can be subdivided into different physiological races with different capacities to infect its host, Phaseolus vulgaris. These include the non-pathogenic and pathogenic strains known as races 0 and 1472, respectively.

Results

Here we report the isolation and sequence analysis of the Clpnl2 gene, which encodes the pectin lyase 2 of C. lindemuthianum, and its expression in pathogenic and non-pathogenic races of C. lindemuthianum grown on different carbon sources. In addition, we performed a phylogenetic analysis of the deduced amino acid sequence of Clpnl2 based on reported sequences of PNLs from other sources and compared the three-dimensional structure of Clpnl2, as predicted by homology modeling, with those of other organisms. Both analyses revealed an early separation of bacterial pectin lyases from those found in fungi and oomycetes. Furthermore, two groups could be distinguished among the enzymes from fungi and oomycetes: one comprising enzymes from mostly saprophytic/opportunistic fungi and the other formed mainly by enzymes from pathogenic fungi and oomycetes. Clpnl2 was found in the latter group and was grouped together with the pectin lyase from C. gloeosporioides.

Conclusions

The Clpnl2 gene of C. lindemuthianum shares the characteristic elements of genes coding for pectin lyases. A time-course analysis revealed significant differences between the two fungal races in terms of the expression of Clpnl2 encoding for pectin lyase 2. According to the results, pectin lyases from bacteria and fungi separated early during evolution. Likewise, the enzymes from fungi and oomycetes diverged in accordance with their differing lifestyles. It is possible that the diversity and nature of the assimilatory carbon substrates processed by these organisms played a determinant role in this phenomenon.     

Streptococcus pyogenes and human neutrophils: a paradigm for evasion of innate host defense by bacterial pathogens

Human polymorphonuclear leukocytes (PMNs) are the first line of defense against invading microorganisms. Although most invading bacteria are eliminated by PMNs, some have evolved complex strategies to prevent normal PMN function. This review focuses on the interaction of human PMNs with Streptococcus pyogenes as a paradigm for successful pathogen evasion mechanisms.     

Airway epithelial versus immune cell Stat1 function for innate defense against respiratory viral infection

The epithelial surface is often proposed to actively participate in host defense, but evidence that this is the case remains circumstantial. Similarly, respiratory paramyxoviral infections are a leading cause of serious respiratory disease, but the basis for host defense against severe illness is uncertain. Here we use a common mouse paramyxovirus (Sendai virus) to show that a prominent early event in respiratory paramyxoviral infection is activation of the IFN-signaling protein Stat1 in airway epithelial cells. Furthermore, Stat1-/- mice developed illness that resembled severe paramyxoviral respiratory infection in humans and was characterized by increased viral replication and neutrophilic inflammation in concert with overproduction of TNF-alpha and neutrophil chemokine CXCL2. Poor control of viral replication as well as TNF-alpha and CXCL2 overproduction were both mimicked by infection of Stat1-/- airway epithelial cells in culture. TNF-alpha drives the CXCL2 response, because it can be reversed by TNF-alpha blockade in vitro and in vivo. These findings pointed to an epithelial defect in Stat1-/- mice. Indeed, we next demonstrated that Stat1-/- mice that were reconstituted with wild-type bone marrow were still susceptible to infection with Sendai virus, whereas wild-type mice that received Stat1-/- bone marrow retained resistance to infection. The susceptible epithelial Stat1-/- chimeric mice also exhibited increased viral replication as well as excessive neutrophils, CXCL2, and TNF-alpha in the airspace. These findings provide some of the most definitive evidence to date for the critical role of barrier epithelial cells in innate immunity to common pathogens, particularly in controlling viral replication.     

Invasive and adherent bacterial pathogens co-Opt host clathrin for infection

Infection by the bacterium Listeria monocytogenes depends on host cell clathrin. To determine whether this requirement is widespread, we analyzed infection models using diverse bacteria. We demonstrated that bacteria that enter cells following binding to cellular receptors (termed "zippering" bacteria) invade in a clathrin-dependent manner. In contrast, bacteria that inject effector proteins into host cells in order to gain entry (termed "triggering" bacteria) invade in a clathrin-independent manner. Strikingly, enteropathogenic Escherichia coli (EPEC) required clathrin to form actin-rich pedestals in host cells beneath adhering bacteria, even though this pathogen remains extracellular. Furthermore, clathrin accumulation preceded the actin rearrangements necessary for Listeria entry. These data provide evidence for a clathrin-based entry pathway allowing internalization of large objects (bacteria and ligand-coated beads) and used by "zippering" bacteria as part of a general mechanism to invade host mammalian cells. We also revealed a nonendocytic role for clathrin required for extracellular EPEC infections.     

Dendritic cells and innate defense against tumor cells

Tumor growth results from a delicate balance between intrinsic dysregulation of oncogenes, tumor suppressor and stability genes counteracted by extrinsic defenses composed of immune cells shaping tumor immunogenicity. Although immune subversion might be the ultimate outcome of this process, a complex network of cellular interactions take place eventually leading to tumor specific cognate immune responses. The links between innate and cognate antitumor immunity eliciting protective T cell responses are instigated by cytokines, chemokines and damage associated molecular patterns. The intricate differentiation pathway whereby dendritic cells could undergo an efficient maturation program in the tumor microenvironment appears crucial. We will discuss the role of innate effectors and cancer therapies in the process of defense against tumor cells.     

A putative G protein-coupled receptor involved in innate immune defense of Procambarus clarkii against bacterial infection

The immune functions of G protein-coupled receptor (GPCR) were widely investigated in mammals. However, limited researches on immune function of GPCRs were reported in invertebrates. In the present study, the immune functions of HP1R gene, a putative GPCR identified from red swamp crayfish Procambarus clarkii were reported. Expression of HP1R gene was significant up-regulated in response to heat-killed Aeromonas hydrophila challenge. HP1R gene silencing mediated by RNA interference significantly enhanced the susceptibility of red swamp crayfish to A. hydrophila and Vibrio alginolyticus, indicating that HP1R was required for red swamp crayfish to defend against bacterial challenge. In HP1R-silenced crayfish, increased bacterial burden and decreased THC in response to bacterial challenge were observed when compared with control crayfish. No significant difference of proPO gene expression was observed between HP1R-silenced and control crayfish after challenge with heat-killed A. hydrophila. However, PO activity in response to bacterial challenge was significantly reduced in HP1R-silenced crayfish. The results collectively indicated that HP1R was an important immune molecule which was required for red swamp crayfish to defend against bacterial infection.     

In vivo function of airway epithelial TLR2 in host defense against bacterial infection

Decreased Toll-like receptor 2 (TLR2) expression has been reported in patients with chronic obstructive pulmonary disease and in a murine asthma model, which may predispose the hosts to bacterial infections, leading to disease exacerbations. Since airway epithelial cells serve as the first line of respiratory mucosal defense, the present study aimed to reveal the role of airway epithelial TLR2 signaling to lung bacterial [i.e., Mycoplasma pneumoniae (Mp)] clearance. In vivo TLR2 gene transfer via intranasal inoculation of adenoviral vector was performed to reconstitute TLR2 expression in airway epithelium of TLR2(-/-) BALB/c mice, with or without ensuing Mp infection. TLR2 and lactotransferrin (LTF) expression in airway epithelial cells and lung Mp load were assessed. Adenovirus-mediated TLR2 gene transfer to airway epithelial cells of TLR2(-/-) mice reconstituted 30-40% TLR2 expression compared with TLR2(+/+) cells. Such airway epithelial TLR2 reconstitution in TLR2(-/-) mice significantly reduced lung Mp load (an appropriate 45% reduction), coupled with elevated LTF expression. LTF expression in mice was shown to be mainly dependent on TLR2 signaling in response to Mp infection. Exogenous human LTF protein dose-dependently decreased lung bacterial load in Mp-infected TLR2(-/-) mice. In addition, human LTF protein directly dose-dependently decreased Mp levels in vitro. These data indicate that reconstitution of airway epithelial TLR2 signaling in TLR2(-/-) mice significantly restores lung defense against bacteria (e.g., Mp) via increased lung antimicrobial protein LTF production. Our findings may offer a deliverable approach to attenuate bacterial infections in airways of asthma or chronic obstructive pulmonary disease patients with impaired TLR2 function.     

Secretory monoclonal IgA class-switch variants against bacterial enteric pathogens in bile and intestinal secretions

Abstract In a previous study we analyzed the molecular forms of monoclonal IgA class-switch variants (moIgA variants) and their transport into murine respiratory secretions. The aim of the present study is to characterize the transport of moIgA variants into bile and intestinal secretions so that their applicability in a passive immunization model of the gut can be evaluated. Different moIgA variants were directly isolated from IgG1 and IgG2a producing hybridoma clones specific for the same surface determinants of bacterial enteric pathogens ( Salmonella typhimurium and Campylobacter jejuni ) as their respective parent IgG clones. Hepatobiliary transport experiments clearly revealed the selective transport of biologically active polymeric forms of the IgA variants into the murine and rat bile after intravenous injection. Biotinylation of polymeric IgA variants prior to intravenous injection resulted in the recovery of functional, labeled SIgA. Moreover biotin-labeled polymeric IgA variant was recovered in bile with an increased molecular weight, suggesting that the secretory component had been added during passage through the liver. When IgA variant and IgG parent clones were both used in a murine backpack tumor model for passive immunization, IgA variant was selectively transported into intestinal secretions in comparison to IgG. The experimental model described here is suitable for use in comparative studies on the role of IgA and IgG with identical specificity in invasive infections of the intestinal tract.