Is thymus-derived lymphocyte inhibitor a polyamine?

CELL PROLIFERATION is assumed to be under the control of homeostatic mechanism(s)1. Bullough and Laurence2 introduced the concept of cell-specific endogenous inhibitors of cell proliferation, the so-called chalones. Evidence for the existence in lymphoid tissues of factors meeting the criteria for the definition of a lymphocyte chalone has been reported by several groups3–6. The nature of these substances however is ill-defined, since purification from crude extracts of lymphoid tissue (thymus and spleen) has been only partly successful. A lymphocyte inhibitor isolated from bovine thymus tissue has been suggested to be a spermine protein complex7: it was quoted that the inhibitory activity on lymphocyte proliferation in vitro could only be observed if fetal calf serum (FCS) was present in the culture medium7. In contrast to these findings we now report the existence of a nondialysable, active substance extracted from bovine thymus tissue, that inhibits strongly in vitro proliferation of human and murine lymphoid cells in the absence of FCS.