Infections of the lung have a greater impact on global health than any other category of disease, and there is an urgent need to better understand antimicrobial defense of the lung. Alveolar macrophages (AMs) are the resident innate immune defenders of the distal lung, and the immunologic repertoire of these cells often differs substantially from that of other phagocytic cells. Among the substances elaborated during infection, lipid mediators derived from arachidonate (eicosanoids), including leukotrienes (LTs) B4 (LTB4) and D4 (LTD4) as well as prostaglandin E2 (PGE2), have emerged as important modulators of innate immune function. Eicosanoids act by ligating specific G protein-coupled receptors on the cell surface and initiating signaling events. We have shown that both LTBB4 (via B LT receptor 1 [BLT1]) and LTD4 (via cysteinyl LT receptor 1 [cysLT1]) promote, while PGE2 (via E prostanoid receptors 2 and 4 [EP2 and EP4]) inhibits, AM capacity for phagocytosis and killing of IgG-opsonized microbes. However, the intracellular mechanisms by which these eicosanoids act are incompletely understood. Although both LT classes promote innate immune functions in AMs, they act via distinct signaling pathways. Likewise, suppression of AM innate immune functions following ligation of EP2 and EP4 is mediated by distinct pathways. As development of pharmacologic agents targeting specific eicosanoid receptors is proceeding rapidly, it is important that the effects of such targeted therapies on antimicrobial defenses of the lung are understood. This project seeks to understand the mechanisms by which these eicosanoids influence key events in the AM signaling pathway triggered by the receptor for opsonic IgG - the Fc3 receptor (FcR). For comparison, events associated with ingestion of the yeast Candida albicans via the mannose receptor (MR) will be studied. The central signaling events we will focus on are activation of: 1) phosphoinositide 3-kinase (PI3K)/phosphatase and tensin homolog deleted on chromosome 10 (PTEN);2) isoforms of protein kinase C (PKC);and 3) small GTPases. The hypothesis is that divergent effects on these phagocytic receptor-triggered signaling components by ligation of BLT1 vs. cysLT1 and EP2 vs. EP4 reflect the differential localization of these receptors to lipid raft membrane microdomains and their differential coupling to two distinct cyclic AMP effectors - protein kinase A (PKA) and exchange protein activated by cyclic AMP (Epac-1). Employing primary rat AMs, we will address specific aims to determine: 1) regulation by PKA and Epac-1 of FcR- and MR-induced signaling events;2) localization to lipid rafts of FcR, MR, and their downstream signaling components and the role of rafts in phagocytosis and killing;3) the roles of PKA vs. Epac-1 and of lipid raft localization in explaining divergent modulation of FcR and MR signaling by BLT1 vs. cysLT1 ligation;4) the roles of PKA vs Epac-1 and of lipid raft localization in explaining divergent modulation of FcR and MR signaling by EP2 vs. EP4 ligation. These proposed studies will provide fundamental insights into AM antimicrobial function, and a clinically relevant framework for understanding and modulating innate immunity in the lung.
Pneumonia has more of an impact on global health than any other category of disease, making it imperative to understand how the lung defends itself from infection. Lipid mediators termed eicosanoids and including leukotrienes and prostaglandins exert potent and opposing effects on the antimicrobial defense functions of the key immune cell in the lung, the alveolar macrophage. The proposed research aims to understand the intracellular events that underlie these eicosanoid effects on alveolar macrophages, thereby providing new clinically relevant insights into regulation of lung defense mechanisms.
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