Arthritogenic activity of a synthetic immunoadjuvant, muramyl dipeptide.

A synthetic muramyl dipeptide, N-acetylmuramyl-L-alanyl-D-isoglutamine, dissolved in saline only and applied subcutaneously to rats of the Lewis inbred strain, produced arthritis, clinically manifest by hind feet paresis but without apparent paw swelling in most cases. Histologically, the disease was characterized by edema and hyperemia of connective tissues, joint synovias, and tendon sheaths, with massive accumulation of inflammatory cell infiltrates composed mainly of lymphoplasmocytes and partly of neutrophil leukocytes. Fibrin exudation and fibrinoid necrosis in connective tissues were observed in the most severe cases. Synovial layers of the talocrural joint, especially on their villi, exhibited marked swelling or cell desquamation of the inner zone. Clinical symptoms of the disease disappeared spontaneously within 5 days after cessation of the treatment; also, histological examinations showed that the effects were reversible. Our results prove that (i) muramyl dipeptide is the principal substance involved in the production of arthritis, (ii) there is no necessity for the presence of additional mycobacterial cell wall components, and (iii) the involvement of the oil moiety is not requisite for the production of arthritis.