Macrophage ingestion of Candida albicans requires recognition by multiple receptors and activation of diverse signaling programs. Synthesis of the lipid mediator prostaglandin E2 (PGE2) and generation of cyclic adenosine monophosphate (cAMP) also accompany this process. Here we characterized the mechanisms underlying PGE2-mediated inhibition of phagocytosis and of F-actin polymerization in response to ingestion of C. albicans. PGE2 suppressed macrophage phagocytosis and F-actin content through E-series prostanoid 2 and 4 receptors, cAMP, and activation of types I and II protein kinase A, but not guanine nucleotide exchange protein activated by cAMP. Dephosphorylation and activation of the actin depolymerizing factor cofilin-1 was necessary for these inhibitory effects of PGE2. Unexpectedly, cofilin-1 activation by PGE2 was mediated by the protein phosphatase activity of PTEN (phosphatase and tensin homolog deleted on chromosome 10), with which it directly associated. Because PGE2 overproduction accompanies many immunosuppressed states, the PTEN-dependent pathway described here may contribute to impaired antifungal defenses.

Pattern recognition receptors for fungi include dectin-1 and mannose receptor, and these mediate phagocytosis, as well as production of cytokines, reactive oxygen species, and the lipid mediator leukotriene B4 (LTB4). The influence of G protein-coupled receptor ligands such as LTB4 on fungal pattern recognition receptor expression is unknown. In this study, we investigated the role of LTB4 signaling in dectin-1 expression and responsiveness in macrophages. Genetic and pharmacologic approaches showed that LTB4 production and signaling through its high-affinity G protein-coupled receptor leukotriene B4 receptor 1 (BLT1) direct dectin-1–dependent binding, ingestion, and cytokine production both in vitro and in vivo. Impaired responses to fungal glucans correlated with lower dectin-1 expression in macrophages from leukotriene (LT)- and BLT1-deficent mice than their wild-type counterparts. LTB4 increased the expression of the transcription factor responsible for dectin-1 expression, PU.1, and PU.1 small interfering RNA abolished LTB4-enhanced dectin-1 expression. GM-CSF controls PU.1 expression, and this cytokine was decreased in LT-deficient macrophages. Addition of GM-CSF to LT-deficient cells restored expression of dectin-1 and PU.1, as well as dectin-1 responsiveness. In addition, LTB4 effects on dectin-1, PU.1, and cytokine production were blunted in GM-CSF−/− macrophages. Our results identify LTB4-BLT1 signaling as an unrecognized controller of dectin-1 transcription via GM-CSF and PU.1 that is required for fungi-protective host responses.

Candida albicans is the most common opportunistic fungal pathogen and causes local and systemic disease in immunocompromised patients. Alveolar macrophages (AMs) are pivotal for the clearance of C. albicans from the lung. Activated AMs secrete 5-lipoxygenase-derived leukotrienes (LTs), which in turn enhance phagocytosis and microbicidal activity against a diverse array of pathogens. Our aim was to investigate the role of LTB 4 and LTD 4 in AM antimicrobial functions against C. albicans and the signaling pathways involved. Pharmacologic and genetic inhibition of LT biosynthesis as well as receptor antagonism reduced phagocytosis of C. albicans when compared with untreated or WT controls. Conversely, exogenous LTs of both classes augmented base-line C. albicans phagocytosis by AMs. Although LTB 4 enhanced mainly mannose receptor-dependent fungal ingestion, LTD 4 enhanced mainly dectin-1 receptor-mediated phagocytosis. LT enhancement of yeast ingestion was dependent on protein kinase C-␦ (PKC␦) and PI3K but not PKC␣ and MAPK activation. Both LTs reduced activation of cofilin-1, whereas they enhanced total cellular F-actin; however, LTB 4 accomplished this through the activation of LIM kinases (LIMKs) 1 and 2, whereas LTD 4 did so exclusively via LIMK-2. Finally, both exogenous LTB 4 and LTD 4 enhanced AM fungicidal activity in an NADPH oxidase-dependent manner. Our data identify LTB 4 and LTD 4 as key mediators of innate immunity against C. albicans, which act by both distinct and conserved signaling mechanisms to enhance multiple antimicrobial functions of AMs.

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