The isolation of thymopoietin (thymin).

The isolation from bovine thymus of two closely related polypeptides, thymopoietin I and II, is described. These are considered to be thymic hormones, which physiologically induce the differentation of prothymocytes to thymocytes within the thymus. Ths isolation of the thymopoietins was monitored not by their differentiative effects, but by a presumably secondary effect on neuromuscular transmission. This was discerned in experimental studies related to the human disease myasthenia gravis in which it was suggested that autoimmune thymitis was regularly present. In an animal model, experimental autoimmune thymitis, the thymic disease was shown to result in the release of a substance that depressed neuromuscular transmission and this substance was shown to be also secreted in small amounts by the normal thymus. A bioassay was developed, this being the delayed appearance of neuromuscular impairment after in vivo injection of the active material, and this bioassay was used to monitor the fractionation of thymus extracts and isolate thymopoietin. Pure thymopoietin was active at subnanogram concentrations, both in producing its effect on neuromuscualr transmission and in inducing the differentiation of prothymocytes to thymocytes. This potency of activity of the purified polypeptide, as well as its specificity in inducing the differentiation of T-cells and not B-cells, support the consideration that thympoietin is a physiological inducing hormone produced by the thymus. This is further supported by the evidence that thymopoietin is only produced in the thymus: neuromuscular blocking effects are not present in extracts of other tissues and immunofluorescent localization of thymopoietin shows it to be present only in thymic epithelial cells.