No abstract

The pattern recognition receptors TLR2 and Dectin-1 play key roles in coordinating the responses of macrophages and dendritic cells (DC) to fungi. Induction of proinflammatory cytokines is instructed by signals from both TLR2 and Dectin-1. A recent report identified a role for CARD9 in innate anti-fungal responses, demonstrating CARD9-Bcl10-mediated activation of NF-κB and proinflammatory cytokine induction in murine bone marrow-derived DC stimulated via Dectin-1. We now report that Dectin-1-CARD9 signals fail to activate NF-κB and drive TNF-α induction in murine bone marrow-derived macrophages. However, priming of bone marrow-derived macrophages with GM-CSF or IFN-γ permits Dectin-1-CARD9-mediated TNF-α induction. Analysis of other macrophage/DC populations revealed further variation in the ability of Dectin-1-CARD9 signaling to drive TNF-α production. Resident peritoneal cells and alveolar macrophages produce TNF-α upon Dectin-1 ligation, while thioglycollate-elicited peritoneal macrophages and Flt3L-derived DC do not. We present data demonstrating that CARD9 is recruited to phagosomes via its CARD domain where it enhances TLR-induced cytokine production even in cells in which Dectin-1 is insufficient to drive cytokine production. In such cells, Dectin-1, CARD9, and Bcl10 levels are not limiting, and data indicate that these cells express additional factors that restrict Dectin-1-CARD9 signaling for TNF-α induction.

CARD9 is dispensable for NF-κB activation induced by Dectin-1 ligands in mice. However, Dectin-1–induced H-Ras activation is mediated by a complex with CARD9, which leads to ERK activation for host innate immune responses to Candida albicans infection.

The scaffold protein CARD9 plays an essential role in antifungus immunity and is implicated in mediating Dectin-1/Sykinduced NF-B activation in response to Candida albicans infection. However, the molecular mechanism by which CARD9 mediates C. albicans-induced NF-B activation is not fully characterized. Here we demonstrate that CARD9 is involved in mediating NF-B activation induced by the hyphal form of C. albicans hyphae (Hyphae) but not by its heat-inactivated unicellular form. Our data show that inhibiting Dectin-2 expression selectively blocked Hyphae-induced NF-B, whereas inhibiting Dectin-1 mainly suppressed zymosan-induced NF-B, indicating that Hyphae-induced NF-B activation is mainly through Dectin-2 and not Dectin-1. Consistently, we find that the hyphae stimulation induces CARD9 association with Bcl10, an adaptor protein that functions downstream of CARD9 and is also involved in C. albicans-induced NF-B activation. This association is dependent on Dectin-2 but not Dectin-1 following the hyphae stimulation. Finally, we find that although both CARD9 and Syk are required for Hyphae-induced NF-B activation, they regulate different signaling events in which CARD9 mediates IB␣ kinase ubiquitination, whereas Syk regulates IB␣ kinase phosphorylation. Together, our data demonstrated that CARD9 is selectively involved in Dectin-2-induced NF-B activation in response to C. albicans hyphae challenging.Candida albicans is a major opportunistic fungal pathogen that predominantly causes infection to cancer patients and immunocompromised individuals. During C. albicans infection, macrophages and dendritic cells recognize components from the fungal cell wall through their pattern recognition receptors (1, 2), which triggers a series of signaling cascades leading to activation of various transcription factors including NF-B (1). The activation of NF-B and other transcription factors further induce the expression of various cytokines and chemokines and inflammatory responses. However, the pattern recognition receptors that recognize fungal cell wall components are not fully defined (3).NF-B is a family of transcription factors that control the expression of pro-inflammatory genes in immune cells (4). In resting cells, the activity of NF-B is tightly controlled by the IB family of proteins, which bind to NF-B dimers and keep these dimers in the cytoplasm. The canonical NF-B activation pathway by most of NF-B-inducing stimuli activates the IB␣ kinase (IKK) 2 complex. The IKK complex is controlled by signal-induced phosphorylation of IKK␣ and IKK␤ subunits (5) and signal-induced K63-linked ubiquitination of the regulatory subunit NEMO (6). The activated IKK complex in turn phosphorylates IB␣ proteins on N-terminal conserved serine residues to target them for ubiquitination-dependent degradation (5). This process releases NF-B and allows its translocation into the nucleus for the activation of its target genes (4). Although it has been shown that bacterial and viral infections induce IKK activation by Toll-like receptors (TLR...