An endonuclease allows Streptococcus pneumoniae to escape from neutrophil extracellular traps.

Streptococcus pneumoniae (pneumococcus) is the most common cause of community-acquired pneumonia, with high morbidity and mortality worldwide. A major feature of pneumococcal pneumonia is an abundant neutrophil infiltration . It was recently shown that activated neutrophils release neutrophil extracellular traps (NETs), which contain antimicrobial proteins bound to a DNA scaffold. NETs provide a high local concentration of antimicrobial components and bind, disarm, and kill microbes extracellularly. Here, we show that pneumococci are trapped but, unlike many other pathogens, not killed by NETs. NET trapping in the lungs, however, may allow the host to confine the infection, reducing the likelihood for the pathogen to spread into the bloodstream. DNases are expressed by many Gram-positive bacterial pathogens, but their role in virulence is not clear. Expression of a surface endonuclease encoded by endA is a common feature of many pneumococcal strains. We show that EndA allows pneumococci to degrade the DNA scaffold of NETs and escape. Furthermore, we demonstrate that escaping NETs promotes spreading of pneumococci from the upper airways to the lungs and from the lungs into the bloodstream during pneumonia.

Neutrophil extracellular traps: casting the NET over pathogenesis.

Neutrophil extracellular traps (NETs) are considered to be part of the human innate immunity because they trap and kill pathogens. NETs are formed by activated neutrophils and consist of a DNA backbone with embedded antimicrobial peptides and enzymes. They are involved in host defense during pneumococcal pneumonia, streptococcal necrotizing fasciitis, appendicitis and insemination. Recently, bacterial virulence factors that counteract NETs have been identified. These include the degradation of the NET-backbone by DNases enabling the liberation of bacteria from NETs, as well as capsule formation, which reduces bacterial trapping. Furthermore, pathogens can resist NET-mediated killing by adding positive charge to their cell surface.

The capsule sensitizes Streptococcus pneumoniae to alpha-defensins human neutrophil proteins 1 to 3.

Streptococcus pneumoniae is a major cause of morbidity and mortality worldwide. Its polysaccharide capsule causes resistance to phagocytosis and interferes with the innate immune system's ability to clear infections at an early stage. Nevertheless, we found that encapsulated pneumococci are sensitive to killing by a human neutrophil granule extract. We fractionated the extract by high-performance liquid chromatography and identified alpha-defensins by mass spectrometry as the proteins responsible for killing pneumococci. Analysis of sensitivity to the commercial alpha-defensins human neutrophil proteins 1 to 3 (HNP1-3) confirmed these findings. We analyzed the sensitivities of different pneumococcal strains to HNP1-3 and found that encapsulated strains are efficiently killed at physiological concentrations (7.5 microg/ml). Surprisingly, nonencapsulated, nonvirulent pneumococci were significantly less sensitive to alpha-defensins. The proposed mechanisms of alpha-defensin resistance in nonencapsulated pneumococci is surface charge modification, e.g., by introduction of positive charge by D-alanylation of surface-exposed lipoteichoic acids. These mechanisms are surmounted by the presence of the capsule, which we hypothesize is masking these charge modifications. Hence, alpha-defensins in the phagolysosome of neutrophils possibly contribute to intracellular killing after antibody-mediated opsonophagocytosis of encapsulated pneumococci.

beta-Catenin regulates the expression of tenascin-C in human colorectal tumors.

Tenascin-C (TN-C) is a component of the extracellular matrix (ECM). It is expressed during development and re-expressed in many types of cancers, where it is involved in the modulation of adhesion and proliferation. TN-C expression is especially high at sites of epithelial mesenchymal transition (EMT), which are found frequently at the invasion front of well-differentiated human colorectal adenocarcinomas. Tumor cells in this compartment are characterized by a strong nuclear expression of the oncogenic transcription factor beta-catenin. Here, we demonstrate that TN-C is a beta-catenin target gene in human colorectal tumors. Thus, by far the most common mutations in colorectal tumors, found in the Wnt-signaling pathway and leading to the stabilizing of beta-catenin, might influence invasion by altering adhesive properties and EMT of tumor cells.

Capsule and D-alanylated lipoteichoic acids protect Streptococcus pneumoniae against neutrophil extracellular traps.

Streptococcus pneumoniae is a major cause of morbidity and mortality worldwide. Pneumococci can counteract the action of neutrophils with an antiphagocytic capsule and through electrochemical repulsion of antimicrobial peptides via addition of positive charge to the surface. Pneumococci are captured, but not killed in neutrophil extracellular traps (NETs). Here, we study the role of the polysaccharide capsule and lipoteichoic acid (LTA) modification on pneumococcal interaction with NETs. Expression of capsule (serotypes 1, 2, 4 and 9V) significantly reduced trapping by NETs, but was not required for resistance to NET-mediated killing. Pneumococci contain a dlt operon that mediates the incorporation of d-alanine residues into LTAs, thereby introducing positive charge. Genetic inactivation of dltA in non-encapsulated pneumococci rendered the organism sensitive to killing by antimicrobial components present in NETs. However, the encapsulated dltA mutant remained resistant to NET-mediated killing in vitro. Nevertheless, in a murine model of pneumococcal pneumonia, the encapsulated dltA-mutant strain was outcompeted by the wild-type upon invasion into the lungs and bloodstream. This suggests a non-redundant role for LTA alanylation in pneumococcal virulence at the early stage of invasive disease when capsule expression has been shown to be low.

Toll-like receptor 9 acts at an early stage in host defence against pneumococcal infection.

Toll-like receptor 9 (TLR9) induces an inflammatory response by recognition of unmethylated CpG dinucleotides, mainly present in prokaryotic DNA. So far, TLR9-deficient mice have been shown to be more sensitive than wild-type mice to viral, but not to bacterial infections. Here, we show that mice deficient in TLR9 but not in TLR1, TLR2, TLR4 and TLR6 or IL-1R/IL-18R are more susceptible to a respiratory tract bacterial infection caused by Streptococcus pneumoniae. Intranasal challenge studies revealed that TLR9 plays a protective role in the lungs at an early stage of infection prior to the entry of circulating inflammatory cells. Alveolar as well as bone marrow-derived macrophages deficient in either TLR9 or the myeloid adaptor differentiation protein MyD88 were impaired in pneumococcal uptake and in pneumococcal killing. Our data suggest that in the airways, pneumococcal infection triggers a TLR9 and MyD88-dependent activation of phagocytic activity from resident macrophages leading to an early clearance of bacteria from the lower respiratory tract.

Myeloid differentiation factor 88-dependent signalling controls bacterial growth during colonization and systemic pneumococcal disease in mice.

The Toll-like receptors (TLRs) and the myeloid differentiation factor 88 (MyD88) are key players in the activation of the innate immune defence during microbial infections. Using different murine infection models, we show that MyD88-dependent signalling is crucial for the activation of the innate immune defence against Streptococcus pneumoniae. Our data demonstrate that both local and systemic inflammatory response to S. pneumoniae depends on the presence of MyD88 to clear bacterial colonization of the upper respiratory tract and to prevent pulmonary and systemic infection in mice. Finally, we described a strong correlation between enhanced bacterial growth in the bloodstream of MyD88-deficient mice and the inability to lower the serum iron concentration in response to infection.