FcγRI ligation leads to a complex with BLT1 in lipid rafts that enhances rat lung macrophage antimicrobial functions

Abstract

IntroductionMacrophages play a central role in antimicrobial immunity via their capacities for recognition, phagocytosis, and killing of opsonized and nonopsonized microbes. Nowhere are these functions more vigorously tested than in the lung, which comprises the largest internal interface with the outside environment and which is continuously exposed to microbes delivered via inhalation or oropharyngeal aspiration. 1 Particles opsonized by immunoglobulin (Ig)G antibodies bind to receptors recognizing the Fc portion of IgG (Fc␥ receptor [Fc␥R]). 2 There are 3 general classes of Fc␥R (Fc␥RI, II, and III) that vary in their molecular structure, affinity for different IgG isotypes, signal transduction, and phagocyte effector functions. 3 Not all Fc␥R classes are capable of eliciting phagocytosis. 3 In the human pulmonary alveolar macrophage (AM), Fc␥RI is the most abundantly expressed class of phagocytic Fc␥Rs, but low levels of Fc␥RIIa are also detected. 4 On Fc␥R clustering, its immunoreceptor tyrosine-based activation motif (ITAM) is phosphorylated by receptor-associated protein tyrosine kinases (PTKs) of the Src family (Src, Lck, Lyn, Fgr, Hck, Fyn, and Yes). The phosphorylated ITAM then serves as a docking site for Src-homology 2 domain-containing adapter proteins and enzymes, including Syk PTK, phospholipase C, the Ras-activating enzyme Sos, and phosphoinositide 3-kinase. 5 Upon their engagement by IgG-opsonized targets, both Fc␥Rs as well as some of their associated signaling molecules redistribute to lipid rafts (LRs), highly dynamic cholesterol-and sphingolipid-enriched microdomains that can act as platforms on which signaling proteins are assembled and which serve to compartmentalize a variety of cellular processes. 6 In addition to their roles in the membrane reorganization events of phagocytosis, these signaling molecules are also required for the induction of proinflammatory mediators involved in the amplification of phagocyte actions as well as the recruitment of leukocytes to the inflammatory milieu. 7 Among such mediators, the 5-lipoxygenase-derived products of arachidonic acid, leukotriene (LT) B 4 , and the cysteinyl LTs (CysLTs), LTC 4 , LTD 4 , and LTE 4 , enhance a myriad of leukocyte functions. 8 LTB 4 and CysLTs each bind 2 different classes of G protein-coupled receptors (GPCRs), termed leukotriene B 4 receptors 1 and 2 (BLT1 and BLT2) 9,10 and CysLT receptors 1 and 2 (CysLT1 and CysLT2). 11,12 GPCRs signal through the activation of small heterotrimeric G proteins that modulate the generation of second messengers, such as cyclic adenosine 5Ј-monophosphate and Ca 2ϩ . 13 We have shown that exogenously added or endogenously generated LTB 4 and LTD 4 promote Fc␥R-dependent phagocytosis and microbial killing in AMs via BLT1 and CysLT1, respectively; 14,15 however, there are key differences in the intracellular programs they use to enhance AM functions. First, LTB 4 / BLT1 is a more potent enhancer of AM phagocytosis and bacterial killing of IgG-opsonized targets than is LTD 4 /CysLT1. 16 Second,...

Ral Isoforms Are Implicated in FcγR-Mediated Phagocytosis: Activation of Phospholipase D by RalA

“…This report provides evidence that 3-mm IgG-opsonized particles activate the Ral GTPases and that these proteins can modulate phagocytic efficiency. Activation of the Ral proteins is mediated, at least in part, by the engagement of FcgR, although simultaneous cross-activation of other phagocytosis receptors (23,24) cannot be excluded. However, our findings are in line with recent observations indicating that the GTPase Rap1 and Ral guanine-dissociation stimulator, a guanosine exchange factor that can activate Ral proteins, participate in FcgR-dependent phagocytosis (25).…”

Leukotriene B4 amplifies NF-κB activation in mouse macrophages by reducing SOCS1 inhibition of MyD88 expression

“…Macrophages adhered on coverslips were stimulated with LPS or poly(I:C) for 24 hours (42). In other experiments, leukocytes were obtained from the lung lavage fluid of mice infected with S. pneumoniae and subsequently treated with aerosolized vehicle or LTB4 24 hours after infection.…”

Leukotriene B4 Augments and Restores FcγRs-dependent Phagocytosis in Macrophages

“…However, although LTB 4 has been shown to activate macrophage phagocytosis, details of the intracellular mechanism of LTB 4 -dependent activation of phagocytosis remain to be elucidated. Recently, Serezani et al (17) showed that Fc␥RI engagement results in tyrosine phosphorylation of BLT1 by Src and subsequent formation of a molecular complex of Fc␥RI and BLT1 within lipid rafts that drives phagocytic functions in rat alveolar macrophages. However, it remains to be determined whether there is direct cross-talk between LTB 4 -BLT1 and IgG-Fc␥Rs signaling pathways.…”