Homologous disease in the adult rat, a model for autoimmune disease. II. Findings in the joints, heart, and other tissues.

Polyarthritis and cardiac lesions have been observed in the adult rat with homologous disease. Changes in the lymphoid tissue and kidney have also been noted. A migratory polyarthritis occurred in over half of the affected animals, and cardiac involvement of varying intensity was present in all. Histologically, the arthritis was characterized by a mononuclear synovial inflammatory reaction. In the heart, the valves and myocardium demonstrated a similar type of response. Cultures of involved joints and hearts for pyogens and pleuropneumonia-like organisms were negative. The lymphoid tissues showed progressive changes in three stages: (a) disappearance of follicles; (b) increase in plasma-like cells and histiocytes; and (c) fibrosis. The second stage tended to coincide with the development of the polyarthritis. The kidney showed mild interstitial mononuclear infiltration. The pathologic changes described appear to be a consequence of reaction between foreign immunologically competent cells and host antigens, but the possibility that these lesions represent a response to antigens derived from unknown infectious agents is, however, not ruled out.