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.

We investigated here the potential role of Toll‐like receptors (TLR) and the adaptor protein MyD88 in innate immunity responses to Cryptococcus neoformans, a pathogenic encapsulated yeast. Peritoneal macrophages from MyD88^–/– or TLR2^–/– mice released significantly less TNF‐α, compared with wild‐type controls, after in vitro stimulation with whole yeasts. In contrast, no differences in TNF‐α release were noted between macrophages from C3H/HeJ mice, which have a loss of function mutation in TLR4, relative to C3H/HeN controls. When MyD88‐ or TLR2‐deficient mice were infected with low doses of the H99 serotype A strain, all of the control animals, but none of MyD88^–/– and only 38% of the TLR2^–/– animals survived, in association with higher fungal burden in the mutant mice. Both MyD88^–/– and TLR2^–/– animals showed decreased TNF‐α, IL‐12p40 and/or IFN‐γ expression in various organs during infection. No difference in susceptibility to experimental cryptococcosis was found between C3H/HeJ mice and C3H/HeN controls. In conclusion, our data indicate that TLR2 and MyD88, but not TLR4, critically contribute to anti‐cryptococcal defenses through the induction of increased TNF‐α, IL‐12 and IFN‐γ expression.

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.