Experimental arthritis produced by proteoglycan antigens in rabbits

Spontaneous synovitis developed in the limb joints and rheumatoid factor-like component appeared in the sera of two rabbits from a pool 36 animals in the course of a long-term immunization with bovine nasal cartilage antigens. A single intra-articular injection of proteoglycan antigens regularly provoked a heavy synovitis and cartilage destruction irrespective of whether the booster injections were administered in physiological saline, or in Freund's complete adjuvant. The dose-dependent severity of arthritis suggested that the antibody titre against proteoglycan antigens played an important role in this mechanism. The synovial extract and synovial fluid of knee joints injected with proteoglycan antigens showed an increased enzyme activity concerning the four acid hydrolases (acid phosphatase, cathepsin D, hyaluronidase and beta-glucuronidase). The high activity of lysosomal acid hydrolases which persisted for several months can derange the molecular structure of proteoglycans of cartilage. The degraded proteoglycans may trigger autoimmune reactions, and the process eventually leads to chronic inflammation and joint destruction.