Studies on the fate of lymphocytes. II. Intravenous injection of labeled thymic lymphocytes into homologous rats and isologous mice

Suspensions of thymic lymphocytes, 60–80% of which had been labeled in vivo with H3-thymidine, were injected intravenously into 54 homologous rats and 22 isologous mice. Radioautographs2 of blood and of tissues were analyzed. The cells rapidly disappeared from the blood, although less rapidly in the isologous than in the homologous systems. A large fraction was still present in the tissues during the first post-injection day. In the first hour many were arrested in the lung, some of which were later redistributed to other organs. At later intervals the spleen contained the greatest concentration, followed by lymph nodes and bone marrow. Cells were found occasionally in the duodenal mucosa, in mice only, and in no case did they reach the thymus. A few survived in both rats and mice for as long as seven days. Only one labeled cell was seen in mitosis. There was evidence of transformation to erythroblasts in the liver, spleen and bone marrow; to myelocytes in the bone marrow; and to macrophages in the spleen. Label from some of the cells dying in the liver apparently was reutilized by the Kupffer cells.