It is known that host cells can produce type I IFNs (IFN-αβ) after exposure to conserved bacterial products, but the functional consequences of such responses on the outcome of bacterial infections are incompletely understood. We show in this study that IFN-αβ signaling is crucial for host defenses against different bacteria, including group B streptococci (GBS), pneumococci, and Escherichia coli. In response to GBS challenge, most mice lacking either the IFN-αβR or IFN-β died from unrestrained bacteremia, whereas all wild-type controls survived. The effect of IFN-αβR deficiency was marked, with mortality surpassing that seen in IFN-γR-deficient mice. Animals lacking both IFN-αβR and IFN-γR displayed additive lethality, suggesting that the two IFN types have complementary and nonredundant roles in host defenses. Increased production of IFN-αβ was detected in macrophages after exposure to GBS. Moreover, in the absence of IFN-αβ signaling, a marked reduction in macrophage production of IFN-γ, NO, and TNF-α was observed after stimulation with live bacteria or with purified LPS. Collectively, our data document a novel, fundamental function of IFN-αβ in boosting macrophage responses and host resistance against bacterial pathogens. These data may be useful to devise alternative strategies to treat bacterial infections.

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 in this study 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 caspase activation and recruitment domain adaptor. Moreover, activation of the NLRP3 inflammasome required GBS expression of β-hemolysin, an important virulence factor. We further found that mice lacking NLRP3, apoptosis-associated speck-like protein, 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.

Toll-like receptors (TLRs) are involved in pathogen recognition by the innate immune system. Different TLRs and the adaptor molecule myeloid differentiation factor 88 (MyD88) were previously shown to mediate in vitro cell activation induced by group B streptococcus (GBS). The present study examined the potential in vivo roles of TLR2 and MyD88 during infection with GBS. When pups were infected locally with a low bacterial dose, none of the TLR2- or MyD88-deficient mice, but all of the wild-type ones, were able to prevent systemic spread of GBS from the initial focus. Bacterial burden was higher in MyD88- than in TLR2-deficient mice, indicating a more profound defect of host defense in the former animals. In contrast, a high bacterial dose induced high level bacteremia in both mutant and wild-type mice. Under these conditions, however, TLR2 or MyD88 deficiency significantly protected mice from lethality, concomitantly with decreased circulating levels of TNF-α and IL-6. Administration of anti-TNF-α Abs to wild-type mice could mimic the effects of TLR2 or MyD88 deficiency and was detrimental in the low dose model, but protective in the high dose model. In conclusion, these data highlight a dual role of TLR2 and MyD88 in the host defense against GBS sepsis and strongly suggest TNF-α as the molecular mediator of bacterial clearance and septic shock.

Although type I interferons (IFN‐α/β) 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‐α/β production in response to pathogenic yeasts. Here, we found that conventional DCs, but not macrophages nor plasmacytoid DCs, mounted IFN‐β 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‐β response. After intravenous challenge with Candida albicans, most mice lacking the IFN‐α/β 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‐α/β response.

Despite convincing evidence for involvement of members of the Toll-like receptor (TLR) family in fungal recognition, little is known of the functional role of individual TLRs in antifungal defenses. We found here that TLR7 was partially required for the induction of IL-12 (IL-12p70) by Candida albicans or Saccharomyces cerevisiae. Moreover, the IL-12p70 response was completely abrogated in cells from 3d mice, which are unable to mob-ilize TLRs to endosomal compartments, as well as in cells from mice lacking either the TLR adaptor MyD88 or the IRF1 transcription factor. Notably, purified fungal RNA recapitulated IL-12p70 induction by whole yeast. Although RNA could also induce moderate TLR7-dependent IL-23 and tumor necrosis factor-alpha (TNF-α) secretion, TLR7 and other endosomal TLRs were redundant for IL-23 or TNF-α induction by whole fungi. Importantly, mice lacking TLR7 or IRF1 were hypersusceptible to systemic C. albicans infection. Our data suggest that IRF1 is downstream of a novel, nonredundant fungal recognition pathway that has RNA as a major target and requires phagosomal recruitment of intracellular TLRs. This pathway differs from those involved in IL-23 or TNF-α responses, which we show here to be independent from translocation of intracellular TLRs, phagocytosis, or phagosomal acidification.

The antiviral activities of type I IFNs have long been established. However, comparatively little is known of their role in defenses against nonviral pathogens. We examined here the effects of type I IFNs on host resistance against the model pathogenic yeast Cryptococcus neoformans. After intratracheal or i.v. challenge with this fungus, most mice lacking either the IFN-α/β receptor (IFN-α/βR) or IFN-β died from unrestrained pneumonia and encephalitis, while all wild-type controls survived. The pulmonary immune response of IFN-α/βR−/− mice was characterized by increased expression of IL-4, IL-13, and IL-10, decreased expression of TNF-α, IFN-γ, inducible NO synthetase, and CXCL10, and similar levels of IL-12 mRNA, compared with wild-type controls. Histopathological analysis showed eosinophilic infiltrates in the lungs of IFN-α/βR−/− mice, although this change was less extensive than that observed in similarly infected IFN-γR-deficient animals. Type I IFN responses could not be detected in the lung after intratracheal challenge. However, small, but statistically significant, elevations in IFN-β levels were measured in the supernatants of bone marrow-derived macrophages or dendritic cells infected with C. neoformans. Our data demonstrate that type I IFN signaling is required for polarization of cytokine responses toward a protective type I pattern during cryptococcal infection.