Antigens in thymus and muscle effective in inducing experimental autoimmune thymitis and the release of thymin.

Hartley guinea-pigs immunized with homologous thymus or muscle in Freund's complete adjuvant (FCA) developed experimental autoimmune thymitis, with lymphocytic accumulations in the thymic medulla and electromyographic evidence of a block in neuromuscular transmission similar to that of humans with myasthenia gravis. These animals immunized with homologous tissues had a higher incidence of thymitis than animals similarly immunized with bovine tissues; yet, unlike the latter, no serum autoantibodies could be demonstrated by immunofluorescence. Muscle was considered to be thymitogenic because there are common antigens in striated muscle and myoid cells of the thymus. Muscle was thymitogenic in doses down to 50 μg while thymus was thymitogenic in doses down to 5 μg. Since muscle contains far more muscle anitgen than thymus, these results were interpreted as showing that thymus probably contains thymitogen(s) which are distinct from, and even more potent than, muscle thymitogen. Animals with thymitis also had electromyographic evidence of neuromuscular block to single stimuli, increasing with repetitive stimulation. The incidence of neuromuscular block closely paralleled the incidence of thymitis, whether the thymitis was induced by thymus or muscle. Thus the release of thymin, the thymic substance causing neuromuscular block, appeared to be a general consequence of inflammation of the thymus and was not related to the type of antigen initiating thymitis.