Dickkopf-1 is a master regulator of joint remodeling.

Degenerative and inflammatory joint diseases lead to a destruction of the joint architecture. Whereas degenerative osteoarthritis results in the formation of new bone, rheumatoid arthritis leads to bone resorption. The molecular basis of these different patterns of joint disease is unknown. By inhibiting Dickkopf-1 (DKK-1), a regulatory molecule of the Wnt pathway, we were able to reverse the bone-destructive pattern of a mouse model of rheumatoid arthritis to the bone-forming pattern of osteoarthritis. In this way, no overall bone erosion resulted, although bony nodules, so-called osteophytes, did form. We identified tumor necrosis factor-alpha (TNF) as a key inducer of DKK-1 in the mouse inflammatory arthritis model and in human rheumatoid arthritis. These results suggest that the Wnt pathway is a key regulator of joint remodeling.

Proteasome inhibition aggravates tumor necrosis factor-mediated bone resorption in a mouse model of inflammatory arthritis.

OBJECTIVE: The proteasome inhibitor bortezomib has potent anti-myeloma and bone-protective activity. Recently, bortezomib was shown to directly inhibit osteoclastogenesis. The aim of this study was to analyze the influence and therapeutic effect of bortezomib in a mouse model of inflammatory arthritis. METHODS: Heterozygous human tumor necrosis factor α (hTNFα)-transgenic mice and their wild-type (WT) littermates were intravenously injected with 0.75 mg/kg of bortezomib or phosphate buffered saline twice weekly. The mice were assessed for clinical signs of arthritis. After 6 weeks of treatment, mice were analyzed for synovial inflammation, cartilage damage, bone erosions, and systemic bone changes. Osteoclast precursors from WT and hTNF-transgenic mice were isolated from bone marrow, treated with bortezomib, and analyzed for osteoclast differentiation, bone resorption, and expression of osteoclast-specific genes as well as apoptosis and ubiquitination. RESULTS: Bortezomib-treated hTNF-transgenic mice showed moderately increased inflammatory activity and dramatically enhanced bone erosions associated with a significant increase in the number of synovial osteoclasts. Interestingly, bortezomib did not alter systemic bone turnover in either hTNF-transgenic mice or WT mice. In vitro, treatment with therapeutically relevant concentrations of bortezomib resulted in increased differentiation of monocytes into osteoclasts and more resorption pits. Molecularly, bortezomib increased the expression of TNF receptor-associated factor 6, c-Fos, and nuclear factor of activated T cells c1 in osteoclast precursors. CONCLUSION: In TNF-mediated bone destruction, bortezomib treatment increased synovial osteoclastogenesis and bone destruction. Hence, proteasome inhibition may have a direct bone-resorptive effect via stimulation of osteoclastogenesis during chronic arthritis.

Antiinflammatory effects of tumor necrosis factor on hematopoietic cells in a murine model of erosive arthritis.

OBJECTIVE: To investigate the mechanisms leading to the influx of inflammatory hematopoietic cells into the synovial membrane and the role of tumor necrosis factor receptor I (TNFRI) and TNFRII in this process in an animal model of rheumatoid arthritis (RA). METHODS: We performed bone marrow transplantations in human TNF-transgenic mice using hematopoietic cells from wild-type, TNFRI(-/-), TNFRII(-/-), and TNFRI/II(-/-) mice as donors and assessed the severity of arthritis histologically. Generation of osteoclasts from the different genotypes was analyzed in vitro and in vivo. Apoptosis was analyzed using annexin V staining as well as TUNEL assays. RESULTS: Despite lacking responsiveness to TNF in their hematopoietic compartment, mice not only developed full-blown erosive arthritis but even showed increased joint destruction when compared with mice with a TNF-responsive hematopoietic compartment. We demonstrated different roles of the 2 different TNFRs in the regulation of these processes. The absence of TNFRI on hematopoietic cells did not affect joint inflammation but markedly attenuated erosive bone destruction via reduced synovial accumulation of osteoclast precursors. In contrast, the absence of TNFRII on hematopoietic cells increased joint inflammation as well as erosive bone destruction via the regulation of osteoclast precursor apoptosis. CONCLUSION: Our findings indicate that selective blockade of TNFRI, leaving the antiinflammatory effects of TNFRII unaltered instead of unselectively blocking TNF, might be advantageous in patients with RA.

The lonely death: chondrocyte apoptosis in TNF-induced arthritis.

Inflammatory joint disease typically provokes progressive cartilage damage. The proliferative synovial inflammatory tissue directly invades the cartilage and induces the expression and activation of degrading enzymes such as matrix metalloproteases (MMPs) and aggrecanases. However, also chondrocyte apoptosis has been observed in cartilage samples of inflamed joints. It remains unclear whether this is a secondary phenomenon due to cartilage damage or a primary event initiated by the synovial inflammation. To determine the presence or absence of chondrocyte death in experimental arthritis, we longitudinally assessed proteoglycan depletion and chondrocyte apoptosis in paw sections from human tumor necrosis factor transgenic (hTNFtg) mice and wild-type littermates. Whereas, wild-type mice showed no signs of cartilage damage, hTNFtg mice exhibited progressive proteoglycan loss starting at clinical onset of arthritis. However, we already found the first apoptotic chondrocytes well before cartilage matrix breakdown occurred indicating that chondrocyte death can be induced before matrix resorption. Chondrocyte death could constantly be observed until late stages of arthritis causing a continuous increase in the number of empty cartilage lacunae. As apoptotic cells in cartilage cannot be cleared by phagocytes due to their spatial isolation in the avascular lacunae of cartilage, having no contact to professional or amateur phagocytes. The dying cells are compelled to undergo a "lonely death" inevitable ending up in secondary necrosis giving rise to perpetuation of a pro-inflammatory cascade. These data indicate that chondrocyte death may play a primary role in inflammatory arthritis fueling cartilage inflammation and damage due to secondary necrosis.

Pathogenesis of Churg-Strauss syndrome: recent insights.

Churg-Strauss syndrome (CSS) is a rare systemic necrotizing vasculitis associated with granuloma formation and severe blood and tissue eosinophilia. CSS occurs almost exclusively in patients with asthma. Its pathogenesis remains largely unknown, as triggering factors for CSS development have not been identified so far. AAb, such as anti-neutrophil cytoplasmic autoantibodies, are found in less than half of patients and possibly constitute a subtype of CSS with different clinical behaviour. On a cellular level, CSS is characterized by a strong Th2-type immune response. Th2-associated cytokines such as IL-4, IL-13 and IL-5 may precipitate the severe eosinophilia in CSS, while migration of Eos to inflammatory sites is possibly mediated by eotaxin-3. This review summarizes recent advances in the knowledge on epidemiology, clinical features, and pathogenesis of CSS.

B-cell infiltrates induce endosteal bone formation in inflammatory arthritis.

The objective of this study was to investigate the function of inflammatory bone marrow infiltrates found in vicinity to joints affected by inflammatory arthritis. These bone marrow infiltrates are rich in B cells and emerge at the interphase between bone marrow and synovial inflammatory tissue, where cortical bone has been broken. We deleted an essential molecule of B-cell development, Brutons tyrosine kinase (Btk), in arthritic TNF-transgenic mice and studied its effect on bone marrow inflammation. Although antigen responses, immunoglobulin levels, and autoantibody production were diminished in Btk(-/-)hTNFtg mice, synovial inflammation developed normally. However, bone marrow infiltrates were significantly diminished in Btk(-/-)hTNFtg mice, which lead to impaired bone formation at endosteal sites underneath bone erosions and an increased invasion of synovial inflammatory cells into the bone marrow. Expression of bone morphogenic protein-7 was dramatically decreased in Btk(-/-)hTNFtg mice. These results do not only indicate that bone formation at endosteal regions next to bone marrow infiltrates is driven by B cells but also show that bone marrow aggregates in the vicinity of inflamed joint appear as an attempt to counter the invasion of inflammatory tissue into the bone marrow.

Treg cells suppress osteoclast formation: a new link between the immune system and bone.

OBJECTIVE: To investigate whether Treg cells can suppress osteoclast differentiation, and to define a new potential link between the immune system and the skeleton. METHODS: Regulatory CD4+,CD25+,Foxp3+ T cells were isolated and purified from the spleen and cocultured with CD11b+ osteoclast precursor cells isolated from bone marrow. Osteoclastogenesis and bone erosion were assessed by tartrate-resistant acid phosphatase staining and pit resorption assay, respectively. In addition, Transwell experiments and cytokine-blocking experiments were performed to define the mechanisms of interaction between Treg cells and osteoclasts. RESULTS: CD4+,CD25+,Foxp3+ T cells, but not CD4+,CD25- T cells, dose dependently inhibited macrophage colony-stimulating factor- and RANKL-dependent osteoclast formation. Pit formation was inhibited by up to 80% when Treg cells were added. The blockade of osteoclast formation was not based on the alteration of RANKL/osteoprotegerin balance but was essentially dependent on direct cell-cell contact via CTLA-4. Treg cell-mediated expression of transforming growth factor beta, interleukin-4 (IL-4), and IL-10 contributed but was not essential to the inhibitory effect on osteoclastogenesis. CONCLUSION: These data show that CD4+,CD25+,Foxp3+ Treg cells suppress osteoclast formation, provide a new link between the immune system and bone, and extend our knowledge on regulation of bone homeostasis by the immune system.

TNF-induced structural joint damage is mediated by IL-1.

Blocking TNF effectively inhibits inflammation and structural damage in human rheumatoid arthritis (RA). However, so far it is unclear whether the effect of TNF is a direct one or indirect on up-regulation of other mediators. IL-1 may be one of these candidates because it has a central role in animal models of arthritis, and inhibition of IL-1 is used as a therapy of human RA. We removed the effects of IL-1 from a TNF-mediated inflammatory joint disease by crossing IL-1alpha and beta-deficient mice (IL-1-/-) with arthritic human TNF-transgenic (hTNFtg) mice. Development of synovial inflammation was almost unaffected on IL-1 deficiency, but bone erosion and osteoclast formation were significantly reduced in IL-1-/-hTNFtg mice, compared with hTNFtg mice based on an intrinsic differentiation defect of IL-1-deficient monocytes. Most dramatically, however, cartilage damage was absent in IL-1-/-hTNFtg mice. Chimera studies revealed that protection of cartilage is based on the loss of IL-1 on hematopoietic, but not mesenchymal, cells, leading to decreased expression of ADAMTS-5 and MMP-3. These data show that TNF-mediated cartilage damage is completely and TNF-mediated bone damage is partially dependent on IL-1, suggesting that IL-1 is a crucial mediator for inflammatory cartilage and bone degradation.