Intraarticular injection of arthritogenic factor causes mast cell degranulation, inflammation, fat necrosis, and synovial hyperplasia.

Arthritis resembling human rheumatoid arthritis is produced in rats either by immunization with type II collagen or injection of complete Freund's adjuvant. The development of arthritis in both models may be mediated by a T cell-derived, type II collagen-specific protein that has been termed arthritogenic factor. Here, the morphologic changes produced after intraarticular injection of this factor were determined. T cell lines were derived from type II collagen-immunized rats. Arthritogenic factor was isolated from culture supernatants by affinity chromatography on type II collagen-conjugated Sepharose and injected into rat knees. The synovium covering the infrapatellar fat pad was examined by light and electron microscopy at 6 hours to 7 days after injection. By 6 hours, the synovium and fat pad were edematous and heavily infiltrated with neutrophils and a few mononuclear cells. Fibrin was present in the synovium and joint space. Most mast cells had partially degranulated. By 24 hours, the infiltrate became primarily mononuclear and fewer neutrophils were seen. Fat necrosis and edema occurred in the subsynovium. By 48 hours and 7 days, the synovium was hyperplastic, some fibrin persisted, and macrophages were present. Control knees, injected with material obtained from T cell lines established with the antigen, ovalbumin, and subjected to type II collagen affinity chromatography, had less fibrin deposition, milder cellular infiltrates, and less mast cell degranulation than knees injected with arthritogenic factor. These studies suggest that arthritogenic factor stimulates acute cellular infiltration and mast cell secretion which is followed by fat necrosis, synovial hyperplasia, and mononuclear cell infiltration.