Role of NADPH oxidase in formation and function of multinucleated giant cells

Abstract

Macrophages play essential roles in a wide variety of physiological and pathological processes. One of the unique features of these phagocytic leukocytes is their ability to fuse, forming multinucleated giant cells. Multinucleated giant cells are important mediators of tissue remodeling and repair and are also responsible for removal or sequestration of foreign material, intracellular bacteria and non-phagocytosable pathogens, such as parasites and fungi. Depending on the tissue where fusion occurs and the inflammatory insult, multinucleated giant cells assume distinctly different phenotypes. Nevertheless, the ultimate outcome is the formation of large cells that can resorb bone tissue (osteoclasts) or foreign material and pathogens (giant cells) extracellularly. While progress has been made in recent years, the mechanisms and factors involved in macrophage fusion are still not fully understood. In addition to cytokines and a number of adhesion proteins and receptors, it is becoming increasingly clear that NADPH oxidase-generated reactive oxygen species (ROS) also play an important role in macrophage fusion. In this review, we provide an overview of macrophage multinucleation, with a specific focus on the role of NADPH oxidases and ROS in macrophage fusion and in the function of multinucleated giant cells. In addition, we provide an updated overview of the role of these cells in inflammation and various autoimmune diseases.

Fig. 1

Types of multinucleated giant cells derived from monocyte/macrophage precursors. Pathways leading to formation of the primary types of munlinucleated macrophages are shown. Major cytokines known to be involved in the differentiation/fusion of monocyte/macrophage precursors are indicated. Proposed pathways that are not well defined are indicated by dashed lines. M-CSF = Macrophage colony-stimulating factor; GM-CSF = granulocyte-macrophage colony-stimulating factor; RANKL = receptor activator for nuclear factor-κ B ligand; IL-3 = interleukin 3; IL-4 = interleukin 4; IL-6 = interleukin 6; IL-13 = interleukin 13; IFN-γ = interferon-γ. See text for further details.

Fig. 2

Histological images of multinucleated giant cells. a Langhans giant cells and one foreign-body giant cell (arrow) in a granuloma composed entirely of multinucleated giant cells. b Foreignbody giant cell. c Touton giant cell from a cutaneous juvenile xanthogranuloma. Images provided courtesy of Yale Rosen.

Fig. 3

Molecular mechanisms contributing to macrophage fusion. Schematic representation of the process of monocyte/macrophage fusion indicating factors reported to be involved, signaling events and possible roles of NADPH oxidase-generated ROS. A number of fusogenic proteins are involved, including interactions between CD200 and CD200R; CD47 and signal regulatory protein α (SIRPα); CD36 and phosphatidylserine (PtdS); DCSTAMP and CD44, CD47 (not shown), SIRPα (not shown) and monocyte chemoattractant protein-1 (MCP-1). In addition, β₁ and β₂ integrins play a role in the fusion process by binding to their ligands (example shown is the β₂ ligand CD54 or intercellular adhesion molecule 1, but there are other potential ligands for these integrins involved in macrophage fusion). Macrophage activation and adhesion of cells to each other leads to membranemembrane interactions. The events causing actual membrane fusion are complex, but may be facilitated by action of P2X₇ receptors, which form membrane pores that would allow cell contents to interconnect. The d2 isoform of vacuolar ATPase V₀ domain (Atp6v0d2) may also contribute to these events by regulating organelle pH and somehow facilitating fusion (not shown). In addition to extracellular fusion factors, additional intracellular signaling events also are important, including activation of the receptor activator for nuclear factor κB (RANK) by its ligand (RANKL), which leads to intracellular Ca²⁺ flux, activation of c-Jun N-terminal kinase (JNK) and TNF receptor-associated factor 6 (TRAF-6), and downstream induction of CD200 expression. Activation of TRAF-6 eventually leads to activation of transcription via nuclear factor-κB (NF-κB) and nuclear factor of activated T cells (NFAT). Note that the RANK/RANKL pathways are specific for osteoclast formation and do not participate in formation of other multinucleated giant cells. Cleavage of activated CD44 by presenilin 2 (PS2) also contributed to NF-κB activation via release of CD44 intracellular domain (CD44ID), which moves to the nucleus. NADPH oxidase (Nox)-generated ROS play a role in many of these events by inducing expression of integrins and fusion proteins, inducing RANKL expression in a positive feedback loop, and activating redox-sensitive transcription factors (for example, NF-κB and NFAT). In addition, ligation or activation of fusion factors (such as P2X₇, CD44 and SIRPα) can also induce ROS production, thereby enhancing the positive feedback loop involving ROS (not shown). Intracellular signaling induced by the various ligand-receptor interactions involve additional signaling molecules and transcription factors [activator protein 1 (AP-1), Janus kinase (JAK), Lyn tyrosine kinase, mitogen-activated protein kinases (MAPK), phosphoinositide 3-kinase (PI3K), SH2-containing inositol phosphatase (SHIP), and signal transducers and activator of transcription (STAT)], as indicated. See text for further details.