Increased Sensitivity of Lymphocytes from People over 65 to Cell Cycle Arrest and Chromosomal Damage

Flow cytometry revealed that, in the presence of tritiated thymidine, a greater percentage of phytohemagglutinin-stimulated lymphocytes from old human donors were arrested in the G$_{2}$ or M phase than were cells from young donors. Furthermore, lymphocytes from old donors showed significantly more chromosomal damage than did lymphocytes from young donors. Lymphocyte cultures from old or young donors not exposed to tritiated thymidine had the same percentage of cycling lymphocytes in G$_{2}$ or M, although the number of lymphocytes stimulated by phytohemagglutinin to enter the cell cycle was significantly lower in cultures from old donors. Thus, the impaired incorporation of tritiated thymidine by phytohemagglutinin-exposed lymphocytes from old humans reflects both an impaired proliferative response to phytohemagglutinin and an increased sensitivity to the radiobiological effects of tritiated thymidine.