Effects of Antithymus Serum on Thymic Lymphopoiesis: A Radioautographic Study

Abstract

Abstract In an attempt to investigate immunosuppressive mechanisms of antithymus serum, especially its superiority to antilymphocyte serum as immunosuppressive agents, the effects of these antisera on thymic lymphopoiesis were studied by radioautography. In rats treated with antithymus serum, thymidine incorporation by the thymic cortical lymphocytes was markedly suppressed although the number of thymidinelabeled lymphocytes appeared to be increased in the medulla and in the perivascular sheaths. In rats treated with antilymphocyte serum, on the other hand, there was increased thymidine incorporation by the cortical lymphocytes as indicated by the higher percentage of thymidine-labeled cells, the greater mean thymus weight and the histologic evidence. Despite lowered thymidine uptake, uridine uptake by the thymus cells of rats treated with antithymus serum was increased, suggesting an increased RNA metabolism in these cells. Histologically the majority of uridinelabeled cells were reticulo-epithelial cells and were mostly located in the medulla and in the perivascular sheaths, but some of them were seen in the subcapsular regions of the cortex showing positive periodic acid Schiff (PAS) stain. These findings suggest that the greater immunosuppressive potency of antithymus serum than that of antilymphocyte serum is related to the former's ability to suppress proliferation of the thymic cortical lymphocytes, possibly by inactivating a thymic humoral factor, whereas the latter serum has no such suppressive effect. Increased uridine uptake by the thymus of the animals treated with antithymus serum may indicate a compensatory increase in the production of a thymic humoral factor by the reticulo-epithelial cells.