Multinucleated Giant Cells: Current Insights in Phenotype, Biological Activities, and Mechanism of Formation

Abstract

Monocytes and macrophages are innate immune cells with diverse functions ranging from phagocytosis of microorganisms to forming a bridge with the adaptive immune system. A lesser-known attribute of macrophages is their ability to fuse with each other to form multinucleated giant cells. Based on their morphology and functional characteristics, there are in general three types of multinucleated giant cells including osteoclasts, foreign body giant cells and Langhans giant cells. Osteoclasts are bone resorbing cells and under physiological conditions they participate in bone remodeling. However, under pathological conditions such as rheumatoid arthritis and osteoporosis, osteoclasts are responsible for bone destruction and bone loss. Foreign body giant cells and Langhans giant cells appear only under pathological conditions. While foreign body giant cells are found in immune reactions against foreign material, including implants, Langhans giant cells are associated with granulomas in infectious and non-infectious diseases. The functionality and fusion mechanism of osteoclasts are being elucidated, however, our knowledge on the functions of foreign body giant cells and Langhans giant cells is limited. In this review, we describe and compare the phenotypic aspects, biological and functional activities of the three types of multinucleated giant cells. Furthermore, we provide an overview of the multinucleation process and highlight key molecules in the different phases of macrophage fusion.

Keywords: Langhans giant cell (LGC); cell fusion; foreign body giant cell (FBGC); macrophage; migration; multinucleated giant cell (MGC); multinucleation; osteoclast.

FIGURE 1

Osteoclasts mediate bone resorption and proper bone replacement by osteoblasts. Bone is a dynamic tissue that is remodeled by interplay of bone-resorbing osteoclasts and bone-generating osteoblasts. During bone resorption, osteoclasts firmly adhere to bone mediated by podosomes. Each podosome is composed of a central core of a dense F-actin network and actin polymerization activators. The core is surrounded by a loose F-actin network interspersed with regulatory proteins and adaptor proteins linking the podosome structure with integrins, referred as the podosome cloud. Podosomes are organized in an extensive circular pattern, the sealing zone, isolating an extracellular compartment in which bone resorbing substances are released. Bone-resorbing osteoclasts are characterized by a ruffled border, essential for bone resorption. The ruffled border is enriched with V-ATPase proton pumps, pumping hydrogen protons into the resorption lacunae, required for dissolution of bone minerals. During proton secretion, electroneutrality is maintained by release of chloride ions through CLC-7. Lysosomal enzymes, including cathepsin K, are released at the ruffled border and mediate the degradation of bone proteins, such as collagen I. Bone remnants are taken up in transcytotic vesicles and transported across the osteoclast cytoplasm towards the functional secretory zone where they are released. In order to ensure proper bone replacement, osteoclasts stimulate bone formation by osteoblasts. During bone resorption, osteoblastic growth factors are released from the bone matrix. Additionally, osteoclasts secrete clastokines, soluble factors that support osteoblast proliferation, differentiation, migration, activity, and survival. EpnB2 is expressed on osteoclast and is a transmembrane protein that stimulates osteoblast differentiation through interaction with EphB4 on osteoblasts. Vasculogenesis is stimulated through the release of VEGF.

FIGURE 2

Immunoregulation by osteoclasts. Under physiological conditions, osteoclasts are considered to be derived from monocyte progenitor cells and exhibit immunomodulatory activities in order to maintain immune tolerance to bone remnants. To do so, osteoclasts induce Treg cells through antigen presentation in MHC-I complexes towards CD8⁺ T cells. Osteoclasts inhibit T cell proliferation, production of inflammatory cytokines, and activation-induced apoptosis through release of soluble mediators. Under inflammatory conditions, osteoclasts are hypothesized to derive from DC and to contribute to immune diseases, including RA. In response to LPS or IFN-γ, osteoclasts produce pro-inflammatory mediators and differentiate naïve T cells into Th1 cells through antigen presentation in MHC-II molecules.

FIGURE 3

Biological activities of foreign body giant cells. FBGCs are formed on the surface of foreign bodies, including implants and protheses, and are thought to contribute to the degradation of foreign particles through phagocytosis or secretion of ROS, MMPs, and hydrogen protons. In case of bone implants, FBGCs are hypothesized to contribute to aseptic loosening, bone destruction around the implant. Hydrogen protons released from FBGCs dissolve bone minerals in proximity to the implant. Additionally, FBGCs recruit and activate osteoclasts through secretion of chemokines and pro-inflammatory cytokines. Finally, FBGCs are thought to recruit and activate T cells, which in turn produce osteoclast-stimulating mediators.

FIGURE 4

Langhans giant cells in tuberculosis. LGCs are a hallmark of M. tuberculosis-induced granulomas. In tuberculosis, LGCs exhibit both protective and disease promoting factors. Clearance of M. tuberculosis is inefficient since LGCs cannot mediate bacterial uptake or sufficient ROS production for pathogen killing. Additionally, LGCs secrete MMPs leading to tissue destruction. On the other hand, LGCs are involved in isolation of bacteria and inflammation from the surrounding tissue. LGCs highly express MHC-II molecules, suggesting that they promote the adaptive immune response through antigen presentation.

FIGURE 5

Different steps in the multinucleation process mainly based on findings in osteoclasts. MGCs originate from macrophage fusion, a multi-step process comprising acquirement of fusion competency, chemotaxis and adhesion, and finally cell fusion resulting into multinucleation. Fusion competency is accomplished through upregulation of fusogens downstream of signaling through so-called prefusion mediators. DAP12 and its associated receptor TREM2 are well-established prefusion mediators. After binding of its unknown ligand, TREM2 associates with DAP12 leading to phosphorylation of DAP12 and downstream signaling via SYK. In IL-4 induced fusion, DAP12-mediated signaling results in activation of STAT6 and subsequent induction of several fusogens, including MMP9, E-cadherin, and DC-STAMP. During osteoclastogenesis, combined action of RANK- and DAP12-mediated signaling leads to Ca²⁺ signaling downstream of PLCγ, resulting in an increase of intracellular Ca²⁺ concentration enabling translocation of NFATc1 to the nucleus. Fusogens are induced by combined action of P2RX7 and A2A receptor. ATP release through P2RX7 provides extracellular ATP for biosynthesis of adenosine, which in turn induces fusion competency by binding to A2A receptor. Fusing macrophages are characterized by exposure of PS, a lipid that is normally localized in the inner membrane leaflet. The mechanism of PS externalization may involve P2RX7 and DC-STAMP. Macrophage chemotaxis is driven by CCL2 and is essential to bring the membranes of two individual cells in close proximity for fusion. Next to chemotaxis, CCL2 is hypothesized to mediate the induction of fusogens, including MMP9 and DC-STAMP. In order to allow cell migration and subsequent cell adhesion, macrophages undergo cytoskeletal arrangements, which are mediated by RAC1, a major regulator of the cytoskeleton. It has been established that MT-MMP1 activates RAC1. Furthermore, CCL2 signaling has also been associated with RAC1 activation. Homotypic cell-cell adhesion is mediated by integrins and E-cadherin. The latter might also be involved in induction of fusogens, such as DC-STAMP. Cell-cell attachment is dependent on interaction between CD47 and MFR. Once macrophages are firmly attached to each other, the real fusion process can proceed. DC-STAMP is considered a main fusion regulator; however, its ligand and mechanism of action remain to be defined. Several putative ligands for DC-STAMP has been proposed, including CCL2, MFR, and CD47.