ARTHRITIS & RHEUMATISM Vol. 52, No. 10, October 2005, pp 3192–3201 DOI 10.1002/art.21343 © 2005, American College of Rheumatology Analysis of the Kinetics of Osteoclastogenesis in Arthritic Rats Georg Schett, 1 Marina Stolina, 2 Brad Bolon, 3 Scot Middleton, 2 Matt Adlam, 2 Heather Brown, 2 Li Zhu, 2 Ulrich Feige, 2 and Debra J. Zack 2 Objective. To analyze the kinetics of osteoclasto- genesis in 2 models of chronic immune-mediated arthri- tis and 1 model of acute arthritis. Methods. Adjuvant-induced arthritis (AIA) and collagen-induced arthritis (CIA) in Lewis rats were used as models of chronic arthritis. Acute arthritis was induced in Lewis rats by injecting carrageenan into the hind paw. Osteoclasts were identified by cathepsin K immunohistochemistry at various time points after the onset of arthritis. The location, size, and nucleation of osteoclasts were also analyzed. Results. In both AIA and CIA, multinucleated and cathepsin K–positive osteoclasts first were observed on the day of disease onset. Initially, osteoclasts were localized at the periosteum next to the synovial mem- brane and in subchondral bone channels. The number, size, and nucleation of osteoclasts rapidly increased, leading to severe bone loss within days after disease onset. In addition, numerous mononucleated cathepsin K–positive osteoclast precursor cells emerged in the synovial membrane. All osteoclasts (cathepsin K–positive, multinucleated, attached to bone) and oste- oclast precursors (cathepsin K–positive, mononucleated or multinucleated, within synovial tissue) were also positive for a macrophage-specific marker. Upon induc- tion of acute arthritis with carrageenan, osteoclasts formed transiently in subchondral bone, but regressed 7 days after disease onset. Conclusion. Functional osteoclasts are generated at the earliest stage of arthritis, and new precursors are continuously formed in the synovial membrane to re- plenish the osteoclast pool. These data indicate that antiresorptive therapies may provide the most effective bone protection, when treatment is started soon after the onset of arthritis. Bone erosion is a typical sign of rheumatoid arthritis (RA) and is still considered the best surrogate marker of joint destruction (1). The appearance of bone erosions thus indicates the ability of synovial inflamma- tion to cause articular damage, which increases the likelihood of a poor functional outcome (2,3). Although skeletal damage increases with disease duration, bone erosion is no longer regarded as an exclusive feature of late-stage disease. Bone damage usually starts early in the course of disease, and can be identified even with comparatively insensitive detection tools such as conven- tional radiography. Approximately one-half of RA pa- tients have visible erosions after only 6 months (4). Thus, subclinical bone damage develops very early in the disease process, possibly even from the start. Osteoclast precursors and mature osteoclasts have been detected in the synovial membranes of ani- mals with various forms of experimental arthritis as well as in humans with RA, whereas normal synovial tissue does not harbor osteoclasts (5–9). Inflamed synovium has a particular capacity to invade bone (10). Current thinking holds that the several cell populations residing in the inflamed synovial membrane provide signals that stimulate osteoclast formation and facilitate bone re- sorption. Synovial fibroblast-like cells and activated T cells produce RANKL, which is a potent stimulator of osteoclastogenesis (5,11–13). RANKL can activate cells of the monocyte/macrophage lineage that exist in the inflamed synovium; these cells constitute a large pool of osteoclast precursors. The colocalization of a potent osteoclast-inducing cytokine and its target population of 1 Georg Schett, MD: Amgen, Inc., Thousand Oaks, California, and Medical University of Vienna, Vienna, Austria; 2 Marina Stolina, PhD, Scot Middleton, MS, Matt Adlam, PhD, Heather Brown, BS, Li Zhu, BS, Ulrich Feige, PhD (current address: ESBATech, Zurich- Schlieren, Switzerland), Debra J. Zack, MD, PhD: Amgen, Inc., Thousand Oaks, California; 3 Brad Bolon, DVM, PhD: Amgen, Inc., Thousand Oaks, California, and GEMpath, Inc., Cedar City, Utah. Dr. Schett was a visiting scientist at Amgen. Drs. Stolina, Bolon, Adlam, Feige, and Zack, and Mr. Middleton, Mr. Zhu, and Ms Brown own stock in Amgen. Address correspondence and reprint requests to Debra J. Zack, MD, PhD, Amgen, Inc., One Amgen Center Drive, B-38-2-B, Thousand Oaks, CA 91320. E-mail: dzack@amgen.com. Submitted for publication February 3, 2005; accepted in revised form June 30, 2005. 3192