Is there a circulating human thymic factor that induces cyclic AMP synthesis?

Abstract

THE action of factors which have been extracted from the thymus or found circulating in peripheral blood and which are active in in vitro lymphocyte differentiation assays¹ has been extensively studied. Astaldi et al.² have described a factor (SF) of molecular weight (MW) less than 50,000 present in the serum of normal donors; a thymic site of production was suggested because it was not found in serum from athymic donors. It also significantly elevated cyclic AMP levels in mouse thymocytes. Because of the potential significance of this finding to the study of thymic factor action, we have attempted to corroborate these findings but, as reported here, we have observed that a serum fraction from both normal and athymic individuals can raise cyclic AMP levels in mouse thymocytes. Neither serum fraction, however, produced the degree of stimulation of cyclic AMP levels described by Astaldi et al.². Increases in cyclic AMP were much lower than those observed using the β-adrenergic agonist isoproterenol. We are therefore unable to confirm the existence of a circulating thymic factor in humans which raises cyclic AMP levels in mouse thymocytes.