Dectin-1 Stimulation by Candida albicans Yeast or Zymosan Triggers NFAT Activation in Macrophages and Dendritic Cells

Abstract

A recent report also implicated Dectin-1 in macrophage activation by mycobacteria (9). Together these reports demonstrate that Dectin-1 collaborates with TLRs in the induction of proinflammatory cytokines. We have previously reported collaboration between Dectin-1 and TLR2 in the activation of NF-B (2). Additionally, a recent report has suggested that Dectin-1 can directly activate NF-B in dendritic cells via the signaling adaptor molecule CARD9 (10). Recognition of zymosan and live C. albicans yeast by Dectin-1 also triggers phagocytosis and the production of reactive oxygen species (ROS) 3 by mechanisms independent of TLR signaling (1-3, 11). Thus, Dectin-1 is a key coordinator of macrophage/dendritic cell antimicrobial responses.Dectin-1 has an ITAM-like motif in its intracellular tail and mutation of this motif results in loss of its signaling activity (1-3, 11, 12). ITAM-based signaling is classically associated with lymphocyte Ag receptors (TCR and BCR) ligation, resulting in activation of Src and Syk kinases. We recently demonstrated that Dectin-1 ligation triggers Src-Syk activation in macrophages and that Syk signaling is required for ROS induction (11). Rogers et al. also observed zymosan-stimulated Syk activation in their recent investigation of Dectin-1 signaling in dendritic cells (3). Dectin-1 ligation triggered Syk-dependent induction of IL-2 and IL-10.We now report that recognition of zymosan and live C. albicans yeast by Dectin-1 triggers activation of NFAT in macrophages and dendritic cells. The NFAT family of transcription factors comprises 4 closely related members with overlapping functions (NFATc1 through NFATc4) and the distantly related NFAT5, which is important for cellular responses to hypertonic stress (reviewed in Ref. 13). The role of NFAT activation has been defined most comprehensively in T cells: thymocyte development, T cell differentiation, T cell activation, and anergy are all regulated by NFAT (reviewed in Ref. 13). In T cells, NFAT proteins are normally inactive and highly phosphorylated, and TCR stimulation triggers dephosphorylation by the calcium-dependent phosphatase calcineurin. Dephosphorylated NFAT proteins enter the nucleus where they form strong collaborative complexes on DNA with a variety of transcription factors to integrate signaling pathways. In addition, NFAT transcription factors also regulate cell differentiation outside the immune system, with established roles in processes such as cardiac valve development, skeletal muscle differentiation, and osteoclastogenesis (reviewed in Ref. 14).A role for NFAT activation in the innate antimicrobial response has not previously been demonstrated. In this study we show that Dectin-1-mediated NFAT activation in macrophages and dendritic cells regulates the induction of early growth response (Egr) family transcription factors Egr2 and Egr3, as well as key inflammatory mediators cyclooxygenase-2 (COX-2), IL-2, IL-10, and IL-12 p70.

CLEC‐2 signaling via Syk in myeloid cells can regulate inflammatory responses

“…Consistent with NFAT signaling, transcript profiling revealed immune response genes related to NFAT and PGE2 signaling as the signature of CLEC-2-induced modulation of TLR responses. Interestingly, Dectin-1 signaling in MØs also induces several NFAT-dependent genes, including cyclooxygenase-2 (COX-2) and increased production of PGE2 [46]. An intriguing question for further study is why Syk-mediated signaling by Dectin-1 can couple to the CARD9/NF-kB pathway and autonomously induce the expression of proinflammatory cytokines in DCs whereas crosslinking of CLEC-2 by mAb does not appear to do the same and primarily induces NFAT activation.…”

Requirement of phospholipase C‐γ2 (PLCγ2) for Dectin‐1‐induced antigen presentation and induction of TH1/TH17 polarization

“…Egr1 is induced in response to a variety of proinflammatory conditions [31] and its up-regulation leads to sustained expression of other inflammatory mediators, such as IL-6 and TNF-a [32][33][34]. Egr1 induction by Dectin-1 is regulated by ERK-and JNK-dependent signals, whereas Egr2 and Egr3 expression is controlled by the NFAT-dependent pathway [11].…”

“…Upon ligand binding, this motif is phosphorylated by src family kinases [8] and recruits the tyrosine kinase Syk [9], which initiates a signaling cascade leading to NF-kB [10], NFAT [11] and MAPK [12] activation. Along with Bcl-10 and MALT1, CARD9 is a crucial mediator in the ITAM-signaling pathway that links recognition of b-glucan by Dectin-1 to NF-kB and MAPK activation [13,14].…”

“…7B and C). We conclude that in DC PLCg2 plays a crucial role in Dectin-1-mediated inflammatory cytokine production by controlling AP-1 and NF-kB activation.It has been proposed that Dectin-1 signaling regulates expression of a selective group of genes via activation of NFAT [11]. Indeed, Dectin-1 induces the expression of genes such as IL-2 and COX-2 in an NFAT-dependent fashion [28,29].…”

“…8B). Thus, we conclude that PLCg2 in DC is required for Dectin-1-induced NFAT activation and consequent IL-2 and COX-2 transcription.Dectin-1 also controls the induction of the Egr1, Egr2 and Egr3 transcription factors in an NF-kB and NFAT-mediated manner [11]. Egr1 is induced in response to a variety of proinflammatory conditions [31] and its up-regulation leads to sustained expression of other inflammatory mediators, such as IL-6 and TNF-a [32][33][34].…”