Leukocyte Labeling in Rats during and after Continuous Infusion of Tritiated Thymidine: Implications for Lymphocyte Longevity and Dna Reutilization.

By STEPHEN H. ROBINSON, GEORGE BRECHER, IRA S. LoumE AND JAMES E. P ROLONGED SURVIVAL of a significant fraction of small lymphocytes was suggested by Ottesen and by Hamilton.12 Their findings at first seemed incompatible with the observation that lymphocytes leave the thoracic duct at a rate sufficient to replace all circulating forms within hours.3 However, continuous recirculation of lymphocytes between lymph and peripheral blood was then demonstrated by Gowans.4 This observation cxplained how the same population of cells could have a prolonged survival, and yet an apparently rapid turnover in the circulation. Direct measurement of the life span of small lymphocytes by isotopic DNA labeling remained a problem, however, because of the possibility of continued label transfer into newborn cells. This difficulty was overcome by continuous intravenous infusion of H3-thymidine, which insured incorporation of the label into all cells newly formed by division. Under these circumstances, circulating unlabeled small lymphocytes must have originated from mitoses antedating the infusion, and their survival therefore measured lymphocyte life spans. In such studies from this laboratory 50 per cent of lymphocytes in the rat were replaced by labeled forms by the end of 1 month and 10 per cent were still unlabeled after 3 months of infusion.5 This paper reports studies in which H3-th -iidine was given to rats for 6-9 months in an attempt to measure the maximum life span of the small lymphocyte. In additional experiments the disappearance of labeled cells was measured after 2-3 weeks of continuous H3-thymidine infusion. Some of these animals then received infusions of large amounts of nonlabeled (“cold”) thymidine in order to suppress H -thymidine reutilization. It was hoped that infusions of 2-3 weeks duration would label the major portion of the precursors of perpiheral blood cells in marrow and lymph nodes and that the disappearance curve after discontinuation of the radioisotope would reflect the turnover of the precursors, when reutilization was suppressed. The results did not permit such a simple interpretation. However, they provided unexpected insights regarding the distribution of life spans of small lymphocytes.