A key pathological change in early stage ankylosing spondylitis: cartilage dyshomeostasis induced by Osteogenic Progenitor Cells

Ankylosing spondylitis (AS) is a chronic inflammatory disease primarily affecting the spine and sacroiliac joints, with symptoms of back pain and stiffness partially explained by pathophysiological events encompassing enthesitis, ectopic bone formation, and cartilage degeneration. Progressive fusion of vertebrae leading to reduced mobility is a prominent character of AS, but there remain many unknowns regarding bone pathology, in particular abnormal bone outgrowth. This study aims to elucidate the cellular and molecular mechanisms that underpin pathological osteogenesis in the AS. We conducted a 3-year longitudinal study on cynomolgus monkeys with spontaneous ankylosing spondylitis, which included performing imaging examinations, H&E, Masson, IHC staining, and bone histomorphometric analysis. These methods were employed to clarify the pathological features of different stages of AS and to explore the pathological mechanisms leading to pathological ossification. In the spines of AS-afflicted cynomolgus monkeys, early pathological features included inflammatory infiltration, invasion of cartilage endplates by fibrous-rich granulation tissue, and erosion cavities in the cartilage endplates, all of which were highly spatiotemporally correlated with the formation of ectopic new bone. After acute inflammation, fibroblasts in the granulation tissue underwent osteoblastic differentiation to form woven bone, which was presumed as the primary process of syndesmophyte formation in AS. The appearance of erosion cavities in cartilage endplates marked the onset of disrupted endplate homeostasis. Pathological reductions were observed in the thickness of the ligaments, subchondral bone plates, hypertrophic cell layers, and intervertebral disc height within the lumbar joints. In the later stages, ectopic cartilage ossification and ectopic fibrous ossification together contributed to syndesmophyte formation, such that the cartilage endplate almost completely disappeared, leading to joint ankylosis. At the cell level, accumulated cells within the early cartilage endplate erosion cavities were confirmed to be bone marrow-derived osteoprogenitor cells, whereas the abnormal expression of BMP-2 signaling in the locality promoted bone remodeling in AS. This study provides a pathological analysis of spinal joints in AS-afflicted cynomolgus monkeys, summarizing the pathological features during the progression of AS. It demonstrates that bone marrow-derived osteoprogenitor cells initiate cartilage ossification and likely revealed an unknown mechanism for AS ossification. These findings offer new insights into the skeletal pathology of AS to inform future direction for devising therapeutic approaches to combat AS. In briefIn crab eating macaques with spontaneous ankylosing spondylitis, spinal rigidity is associated with two osteogenic pathways: endochondral ossification initiated by invasion of blood vessels and bone progenitor cells, and inflammation induced fibrous osteogenesis. • Accumulated cells in early cartilage endplate erosion cavities are bone marrow-derived osteoprogenitor cells in AS. • These bone marrow-derived osteoprogenitor cells initiate cartilage ossification, which revealed a new mechanism for AS ossification. • Abnormal BMP-2 signaling in early cartilage endplate erosion promotes pathological bone remodeling in AS.