Group B streptococcus (GBS) is a frequent agent of life-threatening sepsis and meningitis in neonates and adults with predisposing conditions. We tested the hypothesis that activation of the inflammasome, an inflammatory signaling complex, is involved in host defenses against this pathogen. We show here that murine bone marrow-derived conventional dendritic cells responded to GBS by secreting IL-1β and Il-18. IL-1β release required both pro-IL-1β transcription and caspase-1-dependent proteolytic cleavage of intracellular pro-IL-1β. Dendritic cells lacking the TLR adaptor MyD88, but not those lacking TLR2, were unable to produce pro-IL-1β mRNA in response to GBS. Pro-IL-1β cleavage and secretion of the mature IL-1β form depended on the NOD-like receptor family, pyrin domain containing 3 (NLRP3) sensor and the apoptosis-associated speck-like protein containing a CARD (ASC) adaptor. Moreover, activation of the NLRP3 inflammasome required GBS expression of β-hemolysin, an important virulence factor. We further found that mice lacking NLRP3, ASC or caspase-1 were considerably more susceptible to infection than wild-type mice. Our data link the production of a major virulence factor by GBS with the activation of a highly effective anti-GBS response triggered by the NLRP3 inflammasome.

Previous studies have indicated that group B streptococcus (GBS), a frequent human pathogen, potently induces the release of interleukin-1␤ (IL-1␤), an important mediator of inflammatory responses. Since little is known about the role of this cytokine in GBS disease, we analyzed the outcome of infection in IL-1␤-deficient mice. These animals were markedly sensitive to GBS infection, with most of them dying under challenge conditions that caused no deaths in wild-type control mice. Lethality was due to the inability of the IL-1␤-deficient mice to control local GBS replication and dissemination to target organs, such as the brain and the kidneys. Moreover, in a model of inflammation induced by the intraperitoneal injection of killed GBS, a lack of IL-1␤ was associated with selective impairment in the production of the neutrophil chemokines CXCL1 and CXCL2 and in neutrophil recruitment to the peritoneal cavity. Decreased blood neutrophil counts and impaired neutrophil recruitment to the brain and kidneys were also observed during GBS infection in IL-1␤-deficient mice concomitantly with a reduction in CXCL1 and CXCL2 tissue levels. Notably, the hypersusceptibility to GBS infection observed in the immune-deficient animals was recapitulated by neutrophil depletion with anti-Gr1 antibodies. Collectively, our data identify a cytokine circuit that involves IL-1␤-induced production of CXCL1 and CXCL2 and leads the recruitment of neutrophils to GBS infection sites. Moreover, our data point to an essential role of these cells in controlling the progression and outcome of GBS disease.

Although type I interferons (IFN-a/b) have been traditionally associated with antiviral responses, their importance in host defense against bacterial pathogens is being increasingly appreciated. Little is known, however, about the occurrence and functional role of IFN-a/b production in response to pathogenic yeasts. Here, we found that conventional DCs, but not macrophages nor plasmacytoid DCs, mounted IFN-b responses after in vitro stimulation with Candida spp. or Saccharomyces cerevisiae. These responses absolutely required MyD88, a Toll-like receptor (TLR) adaptor molecule, and were partially dependent on TLR9 and TLR7. Moreover, Candida DNA, as well as RNA, could recapitulate the IFN-b response. After intravenous challenge with Candida albicans, most mice lacking the IFN-a/b receptor died from their inability to control fungal growth, whereas all WT controls survived. These data suggest that recognition of yeast nucleic acids by TLR7 and TLR9 triggers a host-protective IFN-a/b response.Key words: Candida albicans . Cytokines . DCs . Fungal infections . Macrophages Supporting Information available online IntroductionCandida albicans (C. albicans) is, among fungi, the most common pathogen of humans. Although they are normally harmless commensals of the intestinal and urinary tract, C. albicans and other Candida species can cause recurrent mucosal infections in otherwise healthy subjects and life-threatening conditions in hospitalized or immunocompromised patients [1]. In recent years, C. albicans has become the fourth most common cause of bloodstream infections, with an incidence of between 1 and 24 cases per 100 000 and a mortality rate of 30-50% [2]. Since the host immune status is the major factor that determines the transition of C. albicans from commensalism to pathogenicity, elucidating the mechanisms underlying immune response initiation, particularly those underlying recognition of fungal components, is crucial to developing new and more effective strategies to combat these infections. In fact, despite the availability of new classes of antifungal drugs, current available therapies are only partially effective and are associated with significant side effects.Antimicrobial responses are initiated by the activation of germ-line-encoded receptors (pattern-recognition receptors, PRRs) expressed by cells of the innate immune system, mainly by macrophages and DCs, which monitor potential portals of pathogen entry. Each PRR binds directly or indirectly to one or more evolutionary conserved microbial molecules (pathogenassociated molecular patterns, PAMPs) and triggers intracellular signal transduction cascades culminating in distinctive transcriptional and nontranscriptional responses. By this mechanism, PRRs link microbial recognition with the selective activation of specific arms of the innate immune system [3].Ã These authors have contributed equally to this study. Eur. J. Immunol. 2011. 41: 1969-1979 DOI 10.1002 Immunity to infection 1969Considerable progress has been recently made in the identification ...

Streptococcus pneumoniae is a major cause of morbidity and mortality worldwide. The ability of this bacterium to adhere to epithelial cells is considered as an essential early step in colonization and infection. By screening a whole genome phage display library with sera from infected patients, we previously identified three antigenic fragments matching open reading frame spr0075 of the strain R6 genome. This locus encodes for an ϳ120-kDa protein, herein referred to as plasminogen-and fibronectin-binding protein B (PfbB), which displays an LPXTG cell wall anchoring motif and six repetitive domains. In this study, by using isogenic pfbB-deleted mutants of the encapsulated D39 and of the unencapsulated DP1004 type 2 pneumococcal strains, we show that PfbB is involved in S. pneumoniae adherence to various epithelial respiratory tract cell lines. Our data suggest that PfbB directly mediates bacterial adhesion, because fluorescent beads coated with the recombinant PfbB sp17 fragment (encompassing one of the six repetitive domains and the C-terminal region) efficiently bound to epithelial cells. Mutants lacking PfbB bound to fibronectin and plasminogen considerably less efficiently than wild type bacteria, whereas sp17-coated beads specifically bound to both of these substrates. Taken together, our data suggest that, by directly interacting with fibronectin, PfbB significantly increases the ability of S. pneumoniae to adhere to human epithelial cells.

Signal transduction via MyD88, an adaptor protein engaged by the Toll-like receptor (TLR) and interleukin-1 receptor (IL-1R) family receptors, has a crucial role in host defenses against group B streptococcus (GBS). To examine the contribution of IL-1R signaling to MyD88-dependent host defenses, we analyzed GBS infection in type I IL-1R (IL-1RI)-deficient mice. Most of these animals displayed clinical signs of sepsis and neurological disease and died after a challenge with a bacterial dose that did not cause illness or death in any of the wild-type animals. Moreover, bacterial numbers in the blood and brains of the immunodefective mice were considerably increased. The ability of blood leukocytes or bone marrow-derived macrophages to kill GBS in vitro was not affected by a lack of IL-1RI. However, it was found in a newly developed model of GBS-induced peritoneal inflammation that IL-1 signaling selectively promoted the production of the chemokines KC and MIP-1α and neutrophil recruitment. Moreover, the secretion of KC and MIP-1α, but not tumor necrosis factor alpha, by peritoneal macrophages stimulated with GBS was significantly decreased in the absence of IL-1RI. Accordingly, the number of neutrophils in the blood and the concentration of myeloperoxidase, a neutrophil marker, in infected organs were severely reduced in the immunodefective mice during GBS disease, concomitantly with a reduction in tissue KC and MIP-1α levels. In conclusion, IL-1RI plays a crucial role in host defenses against GBS by inducing the high-level production of chemokines and the subsequent recruitment of neutrophilic polymorphonuclear leukocytes to infection sites.