ß-Defensin 1 plays a role in acute mucosal defense against Candida albicans.

Candida is an opportunistic fungal pathogen that colonizes the mucosal tract of humans. Pathogenic infection occurs in the presence of conditions causing perturbations to the commensal microbiota or host immunity. Early innate immune responses by the epithelium, including antimicrobial peptides (AMPs) and cytokines, are critical for protection against overgrowth. Reduced salivary AMP levels are associated with oral Candida infection, and certain AMPs, including human ß-defensins 1-3, have direct fungicidal activity. In this study, we demonstrate that murine ß-defensin 1 (mBD1) is important for control of early mucosal Candida infection and plays a critical role in the induction of innate inflammatory mediators. Mice deficient in mBD1, as compared with wild-type mice, exhibit elevated oral and systemic fungal burdens. Neutrophil infiltration to the sites of mucosal Candida invasion, an important step in limiting fungal infection, is significantly reduced in mBD1-deficient mice. These mice also exhibit defects in the expression of other AMPs, including mBD2 and mBD4, which may have direct anti-Candida activity. We also show that mBD1 deficiency impacts the production of important antifungal inflammatory mediators, including IL-1ß, IL-6, KC, and IL-17. Collectively, these studies demonstrate a role for the mBD1 peptide in early control of Candida infection in a murine model of mucosal candidiasis, as well as in the modulation of host immunity through augmentation of leukocyte infiltration and inflammatory gene regulation.

The protein kinase double-stranded RNA-dependent (PKR) enhances protection against disease cause by a non-viral pathogen.

PKR is well characterized for its function in antiviral immunity. Using Toxoplasma gondii, we examined if PKR promotes resistance to disease caused by a non-viral pathogen. PKR(-/-) mice infected with T. gondii exhibited higher parasite load and worsened histopathology in the eye and brain compared to wild-type controls. Susceptibility to toxoplasmosis was not due to defective expression of IFN-γ, TNF-α, NOS2 or IL-6 in the retina and brain, differences in IL-10 expression in these organs or to impaired induction of T. gondii-reactive T cells. While macrophages/microglia with defective PKR signaling exhibited unimpaired anti-T. gondii activity in response to IFN-γ/TNF-α, these cells were unable to kill the parasite in response to CD40 stimulation. The TRAF6 binding site of CD40, but not the TRAF2,3 binding sites, was required for PKR phosphorylation in response to CD40 ligation in macrophages. TRAF6 co-immunoprecipitated with PKR upon CD40 ligation. TRAF6-PKR interaction appeared to be indirect, since TRAF6 co-immunoprecipitated with TRAF2 and TRAF2 co-immunoprecipitated with PKR, and deficiency of TRAF2 inhibited TRAF6-PKR co-immunoprecipitation as well as PKR phosphorylation induced by CD40 ligation. PKR was required for stimulation of autophagy, accumulation the autophagy molecule LC3 around the parasite, vacuole-lysosomal fusion and killing of T. gondii in CD40-activated macrophages and microglia. Thus, our findings identified PKR as a mediator of anti-microbial activity and promoter of protection against disease caused by a non-viral pathogen, revealed that PKR is activated by CD40 via TRAF6 and TRAF2, and positioned PKR as a link between CD40-TRAF signaling and stimulation of the autophagy pathway.

A novel role for the NLRC4 inflammasome in mucosal defenses against the fungal pathogen Candida albicans.

Candida sp. are opportunistic fungal pathogens that colonize the skin and oral cavity and, when overgrown under permissive conditions, cause inflammation and disease. Previously, we identified a central role for the NLRP3 inflammasome in regulating IL-1ß production and resistance to dissemination from oral infection with Candida albicans. Here we show that mucosal expression of NLRP3 and NLRC4 is induced by Candida infection, and up-regulation of these molecules is impaired in NLRP3 and NLRC4 deficient mice. Additionally, we reveal a role for the NLRC4 inflammasome in anti-fungal defenses. NLRC4 is important for control of mucosal Candida infection and impacts inflammatory cell recruitment to infected tissues, as well as protects against systemic dissemination of infection. Deficiency in either NLRC4 or NLRP3 results in severely attenuated pro-inflammatory and antimicrobial peptide responses in the oral cavity. Using bone marrow chimeric mouse models, we show that, in contrast to NLRP3 which limits the severity of infection when present in either the hematopoietic or stromal compartments, NLRC4 plays an important role in limiting mucosal candidiasis when functioning at the level of the mucosal stroma. Collectively, these studies reveal the tissue specific roles of the NLRP3 and NLRC4 inflammasome in innate immune responses against mucosal Candida infection.

RANKL coordinates cell cycle withdrawal and differentiation in osteoclasts through the cyclin-dependent kinase inhibitors p27KIP1 and p21CIP1.

UNLABELLED: The coordination of cell cycle progression and osteoclast differentiation by RANKL signaling was studied. Experiments with mouse genetic models revealed that RANKL promoted cell cycle withdrawal of osteoclast precursors dependent on the cyclin kinase inhibitor p27-KIP1, but that both p27-KIP1 and p21-CIP1 were required for osteoclast differentiation. These cyclin inhibitors may directly regulate osteoclast differentiation in addition to regulating cell cycle withdrawal. INTRODUCTION: RANKL stimulates mononuclear precursor cells of the myeloid lineage to differentiate into multinuclear osteoclasts, thus providing a system to study the fundamental problem of coordination of cell cycle progression with cell differentiation. MATERIALS AND METHODS: Mice that lack expression of functional cyclin inhibitors p27KIP1and p21CIP1 were used to study cell cycle progression and differentiation of osteoclast precursors in vitro and in vivo. RESULTS AND CONCLUSIONS: Experiments with cells derived from p27KIP1- and p21CIP1-deficient mice indicated that p27KIP1 function alone was necessary for RANKL-mediated cell cycle withdrawal by osteoclast precursors, but osteoclasts from mice with single mutations in either of these two genes differentiated normally. In contrast, p21/p27 double knockout mice developed osteopetrosis, with fewer osteoclasts that exhibited lower TRACP activity and abnormal cell morphology present in long bone. Moreover, isolated osteoclast progenitors from p21/p27 double knockout mice were defective in RANKL-mediated differentiation in vitro, expressing low levels of osteoclast-specific genes like TRACP and cathepsin K. Taken together, these data suggest p27KIP1 and p21CIP1 play roles in osteoclast differentiation in response to RANKL signaling distinct from their roles in promoting cell cycle withdrawal.

Rift Valley fever virus infection induces activation of the NLRP3 inflammasome.

Inflammasome activation is gaining recognition as an important mechanism for protection during viral infection. Here, we investigate whether Rift Valley fever virus, a negative-strand RNA virus, can induce inflammasome responses and IL-1ß processing in immune cells. We have determined that RVFV induces NLRP3 inflammasome activation in murine dendritic cells, and that this process is dependent upon ASC and caspase-1. Furthermore, absence of the cellular RNA helicase adaptor protein MAVS/IPS-1 significantly reduces extracellular IL-1ß during infection. Finally, direct imaging using confocal microscopy shows that the MAVS protein co-localizes with NLRP3 in the cytoplasm of RVFV infected cells.

An essential role for the NLRP3 inflammasome in host defense against the human fungal pathogen Candida albicans.

Candida albicans is an opportunistic fungal pathogen causing life-threatening mucosal and systemic infections in immunocompromised humans. Using a murine model of mucosal Candida infection, we investigated the role of the proinflammatory cytokine IL-1beta in host defense to Candida albicans. We find that the synthesis, processing, and release of IL-1beta in response to Candida are tightly controlled and first require transcriptional induction, followed by a second signal leading to caspase-1-mediated cleavage of the pro-IL-1beta cytokine. The known fungal pattern recognition receptors TLR2 and Dectin-1 regulate IL-1beta gene transcription, whereas the NLRP3-containing proinflammatory multiprotein complex, the NLRP3 inflammasome, controls caspase-1-mediated cleavage of pro-IL-1beta. Furthermore, we show that TLR2, Dectin-1, and NLRP3 are essential for defense against dissemination of mucosal infection and mortality in vivo. Therefore, in addition to sensing bacterial and viral pathogens, the NLRP3 inflammasome senses fungal pathogens and is critical in host defense against Candida.

MITF and PU.1 recruit p38 MAPK and NFATc1 to target genes during osteoclast differentiation.

Transcription factors NFATc1, PU.1, and MITF collaborate to regulate specific genes in response to colony-stimulating factor-1 (CSF-1) and receptor activator of NF-kappaB ligand (RANKL) signaling during osteoclast differentiation. However, molecular details concerning timing and mechanism of specific events remain ill-defined. In bone marrow-derived precursors, CSF-1 alone promoted assembly of MITF-PU.1 complexes at osteoclast target gene promoters like cathepsin K and acid 5 phosphatase without increasing gene expression. The combination of RANKL and CSF-1 concurrently increased the levels of MAPK-phosphorylated forms of MITF, p38 MAPK, and SWI/SNF chromatin-remodeling complexes bound to these target promoters and markedly increased expression of the genes. NFATc1 was subsequently recruited to complexes at the promoters during terminal stages of osteoclast differentiation. Genetic analysis of Mitf and Pu.1 in mouse models supported the critical interaction of these genes in osteoclast differentiation. The results define MITF and PU.1 as nuclear effectors that integrate CSF-1/RANKL signals during osteoclast differentiation to initiate expression of target genes, whereas a complex that includes NFATc1 may act to maintain target gene expression in differentiated cells.