Autoimmunity is complicated by bone loss. In human rheumatoid arthritis (RA), the most severe inflammatory joint disease, autoantibodies against citrullinated proteins are among the strongest risk factors for bone destruction. We therefore hypothesized that these autoantibodies directly influence bone metabolism. Here, we found a strong and specific association between autoantibodies against citrullinated proteins and serum markers for osteoclast-mediated bone resorption in RA patients. Moreover, human osteoclasts expressed enzymes eliciting protein citrullination, and specific N-terminal citrullination of vimentin was induced during osteoclast differentiation. Affinity-purified human autoantibodies against mutated citrullinated vimentin (MCV) not only bound to osteoclast surfaces, but also led to robust induction of osteoclastogenesis and boneresorptive activity. Adoptive transfer of purified human MCV autoantibodies into mice induced osteopenia and increased osteoclastogenesis. This effect was based on the inducible release of TNF-α from osteoclast precursors and the subsequent increase of osteoclast precursor cell numbers with enhanced expression of activation and growth factor receptors. Our data thus suggest that autoantibody formation in response to citrullinated vimentin directly induces bone loss, providing a link between the adaptive immune system and bone.

Closed circulatory systems (CCS) underlie the function of vertebrate organs, but in long bones their structure is unclear, although they constitute the exit route for bone marrow (BM) leukocytes. To understand neutrophil emigration from BM, we studied the vascular system of murine long bones. Here we show that hundreds of capillaries originate in BM, cross murine cortical bone perpendicularly along the shaft and connect to the periosteal circulation. Structures similar to these trans-cortical-vessels (TCVs) also exist in human limb bones. TCVs express arterial or venous markers and transport neutrophils. Furthermore, over 80% arterial and 59% venous blood passes through TCVs. Genetic and drug-mediated modulation of osteoclast count and activity leads to substantial changes in TCV numbers. In a murine model of chronic arthritic bone inflammation, new TCVs develop within weeks. Our data indicate that TCVs are a central component of the CCS in long bones and may represent an important route for immune cell export from the BM.

Immunglobulin G (IgG) sialylation represents a key checkpoint that determines the engagement of pro- or anti-inflammatory Fcγ receptors (FcγR) and the direction of the immune response. Whether IgG sialylation influences osteoclast differentiation and subsequently bone architecture has not been determined yet, but may represent an important link between immune activation and bone loss. Here we demonstrate that desialylated, but not sialylated, immune complexes enhance osteoclastogenesis in vitro and in vivo. Furthermore, we find that the Fc sialylation state of random IgG and specific IgG autoantibodies determines bone architecture in patients with rheumatoid arthritis. In accordance with these findings, mice treated with the sialic acid precursor N-acetylmannosamine (ManNAc), which results in increased IgG sialylation, are less susceptible to inflammatory bone loss. Taken together, our findings provide a novel mechanism by which immune responses influence the human skeleton and an innovative treatment approach to inhibit immune-mediated bone loss.

These findings indicate that low bone formation and expression of Dkk-1 trigger inflammatory bone loss. Dkk-1 blocks osteoblast differentiation, induces sclerostin expression and leads to osteocyte death. Inhibition of Dkk-1 may thus be considered as a potent strategy to protect bone from inflammatory damage.

Activation of the gp130 signal transducer is mandatory for INA-6 cell growth in vitro and in vivo. Both the MAPK and the Jak/STAT pathway are operative in malignant plasma cells and either one is essential for plasma cell growth. The INA-6 cell line provides a preclinical model to study growth regulation of human plasmacytoma cells and to evaluate novel therapeutic strategies.

Objective. Rheumatoid arthritis therapies that are based on inhibition of a single cytokine, e.g., tumor necrosis factor ␣ (TNF␣) or interleukin-6 (IL-6), produce clinically meaningful responses in only about half of the treated patients. This study was undertaken to investigate whether combined inhibition of TNF␣ and IL-17 has additive or synergistic effects in the suppression of mesenchymal cell activation in vitro and inflammation and tissue destruction in arthritis in vivo. Methods. Cultures of human fibroblast-like synoviocytes (FLS

These data indicate that IL1 is essential for TNF-mediated bone loss. Despite TNF-mediated inflammatory arthritis, systemic bone is fully protected by the absence of IL1, which suggests that IL1 is an essential mediator of inflammatory osteopenia.

Nitrogen‐containing bisphosphonates (N‐BPs) are effective antiosteolytic agents in patients with multiple myeloma. Preclinical studies have also demonstrated that these agents have direct antitumor effects in vitro and can reduce tumor burden in a variety of animal models, although it is not clear whether such effects are caused by direct actions on tumor cells or by inhibition of bone resorption. N‐BPs prevent bone destruction in myeloma by inhibiting the enzyme farnesyl pyrophosphate synthase in osteoclasts, thereby preventing the prenylation of small GTPase signaling proteins. In this study, utilizing a plasmacytoma xenograft model without complicating skeletal lesions, treatment with zoledronic acid (ZOL) led to significant prolongation of survival in severe combined immunodeficiency mice inoculated with human INA‐6 plasma cells. Following treatment with a clinically relevant dose of ZOL, histological analysis of INA‐6 tumors from the peritoneal cavity revealed extensive areas of apoptosis associated with poly (ADP‐ribose) polymerase cleavage. Furthermore, Western blot analysis of tumor homogenates demonstrated the accumulation of unprenylated Rap1A, indicative of the uptake of ZOL by nonskeletal tumors and inhibition of farnesyl pyrophosphate synthase. These studies provide, for the first time, clear evidence that N‐BPs have direct antitumor effects in plasma cell tumors in vivo and this is executed by a molecular mechanism similar to that observed in osteoclasts.