Background The balance between inflammation-driven cartilage and bone erosion and new bone formation is set at different levels in rheumatoid arthritis and in ankylosing spondylitis (AS). Inflammation and tissue damage usually prepare the stage for the later repair process of new tissue formation like osteophytosis and bone fusions in AS ^1,2.

Objectives We were aiming to better characterize the mechanisms of bones remodeling in spine in inflammatory conditions through the development of a murine model for spondylitis induced with heterologous aggrecan immunizations.

Methods Human aggrecan G1 domain (1-235 aa of ACAN-201) tagged with 6xHis was cloned using a baculovirus system, and the fusion protein was isolated using Ni-NTA resin. Protein identity and purity was characterized with SDS-PAGE and immunodetection. BALB/c females were immunized intraperitoneally with rhAG1 emulsified either in Complete Freund’s adjuvant (CFA) or in dimethyldioctadecylammonium bromide (DDA) adjuvant ^3,4. Mice were scored for visual signs of peripheral arthritis. Histopathology sections of paws and spines were stained with hematoxylin & eosin, alcian blue, and for macrophage markers.

Results In mice immunized with rhAG1-DDA or rhAG1-CFA, slowly progressing arthritis reached maximum after four immunizations. Histopathological examination of paw sections confirmed the presence of inflammatory cells, but severity of arthritis was rather mild. Despite the weak peripheral inflammation, histopathological assessment of spine sections revealed massive ventral chondroplasia and osteophytosis of fibrocartilaginous annulus fibrosus-like cells that has been displacing anterior longitudinal ligament. Ventral aspects of enthesis, annulus fibrosus, and growth plate were mainly affected. One to three osteophytes per every spine were observed in approximately 20% of all examined mice. Immense osteophytes were easily detectable using micro-CT and routine X-ray analysis. The most affected regions were the junction of sacrum with lumbar and caudal parts of axial skeleton: S1-L6 and S4-Ca1. We observed consistent but low grade inflammation around afflicted skeletal regions that strongly correlated with spine osteophytosis, r=0.81. Correlation between peripheral joints inflammation and osteophytes formation was much weaker, r=0.29. Surrounding osteophytes fibrous tissue contained IBA-1-positive macrophages.

Conclusions A new murine model for osteophytosis disconnected from peripheral joints inflammation is developed. The balance between peripheral arthritis, spondylitis and osteophytosis appears to be dependent on the biochemical properties and immunogenicity of the immunizing aggrecan antigen.

1. Lories, R. The balance of tissue repair and remodeling in chronic arthritis. Nat Rev Rheumatol 7, 700-707, (2011).

2. Uderhardt, S. et al. Blockade of Dickkopf (DKK)-1 induces fusion of sacroiliac joints. Ann Rheum Dis 69, 592-597, (2010).

3. Vegvari, A. et al. Two major interacting chromosome loci control disease susceptibility in murine model of spondyloarthropathy. J Immunol 175, 2475-2483, (2005).

4. Adarichev, V. A. & Glant, T. T. Experimental spondyloarthropathies: animal models of ankylosing spondylitis. Curr Rheumatol Rep 8, 267-274, (2006).

Disclosure of Interest None Declared