Interaction of thymus lymphocytes with histoincompatible cells. I. Quantitation of the proliferative response of thymus cells

Abstract

A detailed investigation of thymus cell activation by histocompatibility antigens is reported. When CBA thymus cells were injected into heavily irradiated (CBA × C57BL)F 1 mice a graft-versus-host reaction occurred. Within 2 days, large numbers of pyroninophilic blasts were found in the thymus-dependent areas of the spleen and lymph nodes. Autoradiographic examination of sections from mice given a single intravenous pulse of tritiated thymidine ( 3HT) 30 min before showed that the vast majority of these cells were synthesizing DNA. By 3 days, the reaction in the spleen was such that splenic enlargement occurred. Increase in spleen size proved to be a simple parameter for quantitating the intensity of the reaction. When syngeneic thymus cells were injected into irradiated mice only very few blasts and dividing cells were observed and splenomegaly did not occur. Semi-allogeneic thoracic duct cells (TDL) behaved as thymus cells although 50 times as many thymus cells as TDL were required to achieve the same degree of splenomegaly. Neonatal thymus cells behaved essentially as adult thymus cells. The amount of 3HT incorporated in spleen and mesenteric lymph nodes of irradiated recipients of semi-allogeneic cells provided a simple, reproducible and quantitative method of estimating the extent of activation of lymphocytes by histocompatibility antigens. Radioactivity in spleen was measured 30 min after an intravenous pulse of 3HT. Peak incorporation in semi-allogeneic systems occurred 3 days after thymus cell injection, and 4–5 days after injection of TDL. 3HT incorporation in recipients of syngeneic cells was only slightly greater than in mice given saline for the first 4–5 days. Evidence obtained both from autoradiographic examination of the lymphoid tissues and from 3HT incorporation data suggested that activated cells eventually left the spleen and mesenteric lymph nodes. Experiments with thymectomized radiation chimeras subjected to further irradiation before thymus cell injection gave results supporting the notion that the immunogenic target cell in this system was a cell derived from bone marrow. The method described here is considered to be a prototype for the study of activation of thymus cells by antigens other than histocompatibility antigens.