Repopulation of Irradiated Lymph Nodes by Recirculating Lymphocytes

The effects of local versus total-body irradiation of lymph nodes were compared in rats in which the long-lived lymphocytes had been labeled with tritiated thymidine (TTH) prior to irradiation. Three parameters of the lymph nodes were compared after the irradiation: (1) quantitative cell depletion, (2) histologic evidence of radiation damage, and (3) percentage of lymphocytes labeled with TTH shown in radioautographs. Following equal doses of 300 rads, the cervical lymph nodes given local irradiation were not significantly depleted of cells, while the cervical nodes included in total-body irradiation were severely depleted of cells. Histologic evidence of cell death was seen in both the local and total-body-irradiated nodes. The percentage of lymphocytes labeled with TTH in the locally irradiated nodes, both at 2 days and at 14 days after irradiation, was the same as in the animal's unirradiated mesenteric nodes and in the cervical nodes of the control animals. The steadfast percentage of labeled lymphocytes, observed in the locally irradiated nodes at the same time that cell numbers were unchanged and that cell destruction was evident, demonstrated that cell depletion after local radiation of the lymph nodes was counterbalanced by entry of lymphocytes into the nodes from the recirculating pool. Up to 2 weeks following total-body irradiation there was no evidence of quantitative cell recovery of the long-lived lymphocyte pool.