Augmentation of mitogen responsiveness in human lymphocytes by a humoral factor obtained from thymic epithelial cultures

Supernatants derived from thymic epithelial cultures were studied for their effect in augmenting mitogen responsiveness in human thymocytes and lymphocytes. Incubation of these cells for 20 hr in diluted supernatants obtained from 14 to 25 day old cultures of thymic epithelium resulted in a significant increase in the response to Con A. The epithelial nature of the cells was confirmed by electron microscopy. Supernatants from fibroblast cultures or thymic epithelial cultures overgrown by fibroblasts were not effective, nor were supernatants from secondary epithelial outgrowths. The molecular weight of the active fraction appeared to be between 17,000 and 45,000 daltons. The data indicated that human thymus epithelium produced one or more humoral factors which were identical to, or shared properties with, thymic hormones.