Evidence for Thymic Dependence of Pha-Reactive Cells in Spleen and Lymph Nodes and Independence in Bone Marrow

Abstract

Abstract The influence of the thymus on the in vitro phytohemagglutinin (PHA) response of spleen, lymph node and bone marrow cells was investigated in lethally irradiated, bone marrow-protected (800 R:BM) mice. Reactivity of the cells was measured by studying their incorporation of 3H-thymidine in vitro. Nucleated cell content of the various sites during recovery was also obtained. In 800 R:BM mice, the PHA-response first appeared in cells of the bone marrow and later in those of the thymus, lymph nodes and spleen. It was markedly impaired in lymph node and spleen cells, but not in bone marrow cells during recovery (25 to 35 days after irradiation) when the mice were thymectomized 3 to 4 weeks prior to irradiation and protection by BM cells or fetal liver cells, as compared to similar cells during similar recovery in sham-thymectomized mice or in 800 R:BM mice given thymic cells with the bone marrow or fetal liver cells. Moreover, treatment of lymph node or bone marrow cells with an antiserum directed against an antigen specific for thymic lymphocytes or their descendants reduced the PHA-response of lymph node cells but not that of the bone marrow cells. These results suggest that the PHA-responses of lymph node and spleen cells are thymus-dependent while that of the bone marrow cells is thymus-independent.