Complement C3a and C5a modulate osteoclast formation and inflammatory response of osteoblasts in synergism with IL‐1β

Abstract

There is a tight interaction of the bone and the immune system. However, little is known about the relevance of the complement system, an important part of innate immunity and a crucial trigger for inflammation. The aim of this study was, therefore, to investigate the presence and function of complement in bone cells including osteoblasts, MSC and osteoclasts. qRT-PCR and immunostaining revealed that the central complement receptors C3aR and C5aR, complement C3 and C5, and membrane-bound regulatory proteins CD46, CD55, and CD59 were expressed in human mesenchymal stem cells, osteoblasts, and osteoclasts. Furthermore, osteoblasts and particularly osteoclasts were able to activate complement by cleaving C5 to its active form C5a as measured by ELISA. Both C3a and C5a alone were unable to trigger the release of inflammatory cytokines interleukin (IL)-6 and IL-8 from osteoblasts. However, co-stimulation with the proinflammatory cytokine IL-1β significantly induced IL-6 and IL-8 expression as well as the expression of receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) indicating that complement may modulate the inflammatory response of osteoblastic cells in a pro-inflammatory environment as well as osteoblast-osteoclast interaction. While C3a and C5a did not affect osteogenic differentiation, osteoclastogenesis was significantly induced even in the absence of RANKL and macrophage-colony stimulating factor (M-CSF) suggesting that complement could directly regulate osteoclast formation. It can therefore be proposed that complement may enhance the inflammatory response of osteoblasts and increase osteoclast formation, particularly in a pro-inflammatory environment, for example during bone healing or in inflammatory bone disorders.

Urokinase Receptor Mediates Osteogenic Differentiation of Mesenchymal Stem Cells and Vascular Calcification via the Complement C5a Receptor

“…The key central complement receptors C5aR and C3aR were identified in bone cells, including osteoblasts, MSC, and osteoclasts [4]. To examine whether C5aR is expressed in MSC under our experimental conditions and might undergo changes upon osteogenic differentiation, C5aR expression was analyzed.…”

“…Recent studies suggest that components of the complement cascade may enhance the inflammatory response of osteoblasts and modulate their interaction with osteoclasts, especially in a proinflammatory environment, such as inflammatory bone disorders and vascular calcification [1]. Several complement components are expressed in bone cells and during osteogenic differentiation in vitro [2][3][4]. In particular, the key anaphylatoxin receptor C5aR was found on osteoblasts, osteoclasts, and chondroblasts during fracture healing in rats and was strongly upregulated during osteogenic differentiation [5].…”

“…MSC contribution to vascular calcification processes has been suggested [11][12][13]. Recent studies provide evidence that members of the complement cascade, in particular C5a/C5aR and C3a/C3aR, modulate MSC osteogenic program and chemotactic responses [4,5]. Whether or not uPAR is involved in complement regulation and in induction and propagation of osteogenic program in MSC is not known.…”

Treatment with anti-C5aR mAb leads to early-onset clinical and mechanistic effects in the murine delayed-type hypersensitivity arthritis model

“…Levels of C5a protein have been found to be increased in the synovial fluid of RA patients [4], and expression levels of C5a receptor (C5aR) on macrophages and fibroblasts in the synovium of RA patients have shown to correlate with the number of swollen joints [5]. C5aR is expressed on many cell types, including granulocytes, monocytes, macrophages, dendritic cells, mast cells, osteoblasts and osteoclasts [6][7][8]. Expression on T cells, and epithelial end endothelial cells [9,10], has also been reported.…”

“…Dependent on the cell type, ligation of C5a to C5aR leads to endothelial adhesion, transendothelial migration and chemotaxis of cells [6,11,12] and results in cell activation, evident by oxidative burst, granule release and release of inflammatory mediators such as TNFa, IL-6, IL-1b, vasoactive amines, matrix metalloproteinases [11,13,14] and decreased apoptosis of neutrophils [15]. C5a can also induce osteoclast differentiation from peripheral blood mononuclear cells (PBMCs) [8]. All these events are thought to contribute to the tissue inflammation and destruction seen in RA.…”