Osteoclasts are essential for TNF-alpha-mediated joint destruction

Abstract

The detailed cellular and molecular mechanisms leading to joint destruction in rheumatoid arthritis, a disease driven by proinflammatory cytokines, are still unknown. To address the question of whether osteoclasts play a pivotal role in this process, transgenic mice that express human TNF (hTNFtg) and that develop a severe and destructive arthritis were crossed with osteopetrotic, c-fos-deficient mice (c-fos(-/-)) completely lacking osteoclasts. The resulting mutant mice (c-fos(-/-)hTNFtg) developed a TNF-dependent arthritis in the absence of osteoclasts. All clinical features of arthritis, such as paw swelling and reduction of grip strength, progressed equally in both groups. Histological evaluation of joint sections revealed no difference in the extent of synovial inflammation, its cellular composition (except for the lack of osteoclasts), and the expression of matrix metalloprotein-ase-3 (MMP-3) and MMP-13. In addition, cartilage damage, proteoglycan loss, and MMP-3, -9, and -13 expression in chondrocytes were similar in hTNFtg and c-fos(-/-)hTNFtg mice. However, despite the presence of severe inflammatory changes, c-fos(-/-)hTNFtg mice were fully protected against bone destruction. These data reveal that TNF-dependent bone erosion is mediated by osteoclasts and that the absence of osteoclasts alters TNF-mediated arthritis from a destructive to a nondestructive arthritis. Therefore, in addition to the use of anti-inflammatory therapies, osteoclast inhibition could be beneficial for the treatment of rheumatoid arthritis.

Figure 1

There are no differences in the clinical course of arthritis between hTNFtg and c-fos^–/–hTNFtg mice. Clinical course of arthritis indicated by (a) joint swelling and (b) grip strength was assessed in wild-type mice (filled triangles), c-fos^–/– mice (open circles), hTNFtg mice (open diamonds), and c-fos^–/–hTNFtg mice (filled squares). Asterisks indicate a significant (P < 0.01) increase in joint swelling and decrease in grip strength in hTNFtg and c-fos^–/–hTNFtg mice compared with baseline and with wild-type and c-fos^–/– mice.

Figure 2

Severe joint inflammation is seen in hTNFtg and c-fos^–/–hTNFtg mice, but no signs of bone erosions are found in c-fos^–/–hTNFtg mice. H&E-stained sections of a digital joint of an hTNFtg (a), a c-fos^–/–hTNFtg (b), a wild-type (WT) (c), and a c-fos^–/– mouse (d). Inflammatory tissue (indicated by black arrows) and erosion (indicated by a green arrow) are present in the hTNFtg mice. c-fos^–/–hTNFtg mice show only inflammatory tissue (black arrows) and no erosions. Wild-type and c-fos^–/– mice lack any signs of inflammation and served as controls. Magnification: ×50.

Figure 3

No difference is found in the amount of inflammatory tissue between c-fos^–/–hTNFtg and hTNFtg mice, but c-fos^–/–hTNFtg mice show a complete lack of bone erosions. Mean areas of (a) inflammation and (b) erosions are shown for wild-type, c-fos^–/–, hTNFtg, and c-fos^–/–hTNFtg mice. Mice lacking the hTNF transgene (wild-type and c-fos^–/–) showed no signs of inflammation. No erosions were detectable in c-fos^–/–hTNFtg mice in contrast to hTNFtg mice. Mice lacking the hTNF transgene also showed no erosions.

Figure 4

There are abundant osteoclasts at the site of erosions in hTNFtg mice. Histology of a proximal interphalangeal joint of an hTNFtg mouse is shown. Serial sections were stained with either H&E (a), TRAP (b and c), or calcitonin receptor (d). Inflammation (indicated by black arrows) with subsequent subchondral bone erosion (indicated by green arrows) was found only in hTNFtg mice (a). TRAP staining revealed a remarkable number of TRAP-positive multinucleated cells (black arrows) in hTNFtg mice at erosion sites (b and c). These TRAP-positive cells also showed positive staining for the calcitonin receptor (d). Magnification: a, ×100; b, ×200; c and d, ×400.

Figure 5

Size of bone erosions is strongly correlated to number of osteoclasts. Numbers of osteoclasts assessed by histological quantification of TRAP-positive multinucleated cells in all joints at erosion sites are shown for wild-type, c-fos^–/–, hTNFtg, and c-fos^–/–hTNFtg mice (a). The areas of erosions given in mm² were highly correlated to the numbers of osteoclasts per compartment (b).

Figure 6

Proteoglycan content is similarly reduced in cartilage of hTNFtg and c-fos^–/–hTNFtg mice. Toluidine blue staining of articular cartilage was performed in hTNFtg (a), c-fos^–/–hTNFtg (b), wild-type (c), and c-fos^–/– mice (d). A marked reduction of proteoglycan content indicated by decreased staining intensity (green arrows) was found in hTNFtg and c-fos^–/–hTNFtg mice. Asterisks indicate the joint space. AC indicates articular cartilage. Magnification: ×200.