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Clinical use of antibiotics is based on their capacity to inhibit bacterial growth via bacteriostatic or bacteriocidal effects. In this article, we show that the aminoglycoside antibiotic neomycin, the cyclic lipopeptide antibiotic polymyxin B, and the cyclic peptide antibiotics gramicidin and tyrothricin can induce IL-1β secretion in bone marrow dendritic cells and macrophages. LPS priming was required to trigger the transcription and translation of pro–IL-1β but was independent of TNFR or IL-1R signaling. All four antibiotics required the NLRP3 inflammasome, the adaptor ASC, and caspase-1 activation to secrete IL-1β, a process that depended on potassium efflux but was independent of P2X7 receptor. All four antibiotics induced neutrophil influx into the peritoneal cavity of mice, which required NLRP3 only in the case of polymyxin B. Together, certain antibiotics have the potential to directly activate innate immunity of the host.

Inflammasome-mediated IL-1β production is central to the innate immune defects that give rise to certain autoinflammatory diseases and may also be associated with the generation of IL-17–producing CD4+ T (Th17) cells that mediate autoimmunity. However, the role of the inflammasome in driving adaptive immunity to infection has not been addressed. In this article, we demonstrate that inflammasome-mediated IL-1β plays a critical role in promoting Ag-specific Th17 cells and in generating protective immunity against Bordetella pertussis infection. Using a murine respiratory challenge model, we demonstrated that the course of B. pertussis infection was significantly exacerbated in IL-1R type I-defective (IL-1RI−/−) mice. We found that adenylate cyclase toxin (CyaA), a key virulence factor secreted by B. pertussis, induced robust IL-1β production by dendritic cells through activation of caspase-1 and the NALP3-containing inflammasome complex. Using mutant toxins, we demonstrate that CyaA-mediated activation of caspase-1 was not dependent on adenylate cyclase enzyme activity but was dependent on the pore-forming capacity of CyaA. In addition, CyaA promoted the induction of Ag-specific Th17 cells in wild-type but not IL-1RI−/− mice. Furthermore, the bacterial load was enhanced in IL-17–defective mice. Our findings demonstrate that CyaA, a virulence factor from B. pertussis, promotes innate IL-1β production via activation of the NALP3 inflammasome and, thereby, polarizes T cell responses toward the Th17 subtype. In addition to its known role in subverting host immunity, our findings suggest that CyaA can promote IL-1β–mediated Th17 cells, which promote clearance of the bacteria from the respiratory tract.

Vibrio vulnificus and Vibrio cholerae are Gram-negative pathogens that cause serious infectious disease in humans. The β form of pro–IL-1 is thought to be involved in inflammatory responses and disease development during infection with these pathogens, but the mechanism of β form of pro–IL-1 production remains poorly defined. In this study, we demonstrate that infection of mouse macrophages with two pathogenic Vibrio triggers the activation of caspase-1 via the NLRP3 inflammasome. Activation of the NLRP3 inflammasome was mediated by hemolysins and multifunctional repeat-in-toxins produced by the pathogenic bacteria. NLRP3 activation in response to V. vulnificus infection required NF-κB activation, which was mediated via TLR signaling. V. cholerae-induced NLRP3 activation also required NF-κB activation but was independent of TLR stimulation. Studies with purified V. cholerae hemolysin revealed that toxin-stimulated NLRP3 activation was induced by TLR and nucleotide-binding oligomerization domain 1/2 ligand-mediated NF-κB activation. Our results identify the NLRP3 inflammasome as a sensor of Vibrio infections through the action of bacterial cytotoxins and differential activation of innate signaling pathways acting upstream of NF-κB.

Adenovirus is a nonenveloped dsDNA virus that activates intracellular innate immune pathways. In vivo, adenovirus-immunized mice displayed an enhanced innate immune response and diminished virus-mediated gene delivery following challenge with the adenovirus vector AdLacZ suggesting that antiviral Abs modulate viral interactions with innate immune cells. Under naive serum conditions in vitro, adenovirus binding and internalization in macrophages and the subsequent activation of innate immune mechanisms were inefficient. In contrast to the neutralizing effect observed in nonhematopoietic cells, adenovirus infection in the presence of antiviral Abs significantly increased FcR-dependent viral internalization in macrophages. In direct correlation with the increased viral internalization, antiviral Abs amplified the innate immune response to adenovirus as determined by the expression of NF-κB-dependent genes, type I IFNs, and caspase-dependent IL-1β maturation. Immune serum amplified TLR9-independent type I IFN expression and enhanced NLRP3-dependent IL-1β maturation in response to adenovirus, confirming that antiviral Abs specifically amplify intracellular innate pathways. In the presence of Abs, confocal microscopy demonstrated increased targeting of adenovirus to LAMP1-positive phagolysosomes in macrophages but not epithelial cells. These data show that antiviral Abs subvert natural viral tropism and target the adenovirus to phagolysosomes and the intracellular innate immune system in macrophages. Furthermore, these results illustrate a cross-talk where the adaptive immune system positively regulates the innate immune system and the antiviral state.

Adjuvants are vaccine additives that stimulate the immune system without having any specific antigenic effect of itself. In this study we show that alum adjuvant induces the release of IL-1β from macrophages and dendritic cells and that this is abrogated in cells lacking various NALP3 inflammasome components. The NALP3 inflammasome is also required in vivo for the innate immune response to OVA in alum. The early production of IL-1β and the influx of inflammatory cells into the peritoneal cavity is strongly reduced in NALP3-deficient mice. The activation of adaptive cellular immunity to OVA-alum is initiated by monocytic dendritic cell precursors that induce the expansion of Ag-specific T cells in a NALP3-dependent way. We propose that, in addition to TLR stimulators, agonists of the NALP3 inflammasome should also be considered as vaccine adjuvants.

Caspase activity is required not only for the death of T cells, but also for their activation. A delicate balance of caspase activity is thus required during T cell activation at a level that will not drive cell death. How caspase activity is initiated and regulated during T cell activation is not known. One logical candidate for this process is cellular FLIP long form (c-FLIPL), because it can block caspase-8 recruitment after Fas (CD95) ligation as well as directly heterodimerize with and activate caspase-8. The current findings demonstrate that after T cell activation, caspase-8 and c-FLIPL associate in a complex enriched for active caspases. This occurs coincidently with the cleavage of two known caspase-8 substrates, c-FLIPL and receptor interacting protein 1. Caspase activity is higher in wild-type CD8+ than CD4+ effector T cells. Increased expression of c-FLIPL results in augmented caspase activity in resting and effector T cells to levels that provoke cell death, especially of the CD8 subset. c-FLIPL is thus not only an inhibitor of cell death by Fas, it can also act as a principal activator of caspases independently of Fas.

Cellular FLIP long form (c-FLIPL) is a caspase-defective homologue of caspase-8 that blocks apoptosis by death receptors. The expression of c-FLIPL in T cells can also augment extracellular signal-regulated kinase phosphorylation after TCR ligation via the association of c-FLIPL with Raf-1. This contributes to the hyperproliferative capacity of T cells from c-FLIPL-transgenic mice. In this study we show that activated CD4+ T cells from c-FLIPL-transgenic mice produce increased amounts of Th2 cytokines and decreased amounts of Th1 cytokines. This correlates with increased serum concentrations of the Th2-dependent IgG1 and IgE. The Th2 bias of c-FLIPL-transgenic CD4+ T cells parallels impaired NF-κB activity and increased levels of GATA-3, which contribute, respectively, to decreased IFN-γ and increased Th2 cytokines. The Th2 bias of c-FLIPL-transgenic mice extends to an enhanced sensitivity to OVA-induced asthma. Taken together, these results show that c-FLIPL can influence cytokine gene expression to promote Th2-driven allergic reaction, in addition to its traditional role of blocking caspase activation induced by death receptors.

TNF is well characterized as a mediator of inflammatory responses. TNF also facilitates organization of secondary lymphoid organs, particularly B cell follicles and germinal centers, a hallmark of T-dependent Ab responses. TNF also mediates defense against tumors. We examined the role of TNF in the development of inflammatory autoimmune disorders resembling systemic lupus erythematosus and Sjögren’s syndrome induced by excess B cell-activating factor belonging to the TNF family (BAFF), by generating BAFF-transgenic (Tg) mice lacking TNF. TNF−/− BAFF-Tg mice resembled TNF−/− mice, in that they lacked B cell follicles, follicular dendritic cells, and germinal centers, and have impaired responses to T-dependent Ags. Nevertheless, TNF−/− BAFF-Tg mice developed autoimmune disorders similar to that of BAFF-Tg mice. Disease in TNF−/− BAFF-Tg mice correlates with the expansion of transitional type 2 and marginal zone B cell populations and enhanced T-independent immune responses. TNF deficiency in BAFF-Tg mice also led to a surprisingly high incidence of B cell lymphomas (>35%), which most likely resulted from the combined effects of BAFF promotion of neoplastic B cell survival, coupled with lack of protective antitumor defense by TNF. Thus, TNF appears to be dispensable for BAFF-mediated autoimmune disorders and may, in fact, counter any proneoplastic effects of high levels of BAFF in diseases such as Sjögren’s syndrome, systemic lupus erythematosus, and rheumatoid arthritis.