The Effect of Total Body Irradiation on the Labeling of Circulating Mouse Lymphocytes with Tritiated Thymidine

Tritiated thymidine was administered to C3H mice at frequent intervals by intraperitoneal injection over a 2-week period. About one-third of the circulating small lymphocytes were labeled by this procedure. The animals were then exposed to 208 rads of external irradiation which resulted in a fivefold drop in the total circulating lymphocyte count. The labeled newly formed lymphocytes and the unlabeled older lymphocytes proved to be equally sensitive to radiation. The rate of labeling of peripheral blood lymphocytes was unaffected by a single dose of radiation or by a subsequent dose delivered 4 days later. The decline in the curve of labeling, following the discontinuation of labeling with tritiated thymidine, was the same for irradiated and unirradiated mice. These experiments suggest that the output of new lymphocytes is directly related to the level of circulating small lymphocytes and may possibly depend on transformation of small lymphocytes to large proliferating cells.