Comparative Response of Normal Human Thymus and Lymph Node Cells to Phytohemagglutinin in Culture

Human thymocytes, in short-term tissue culture, show a proliferative activity distinct from that observed by either lymph node or blood lymphocytes. As expected, the behavior of lymph node lymphocytes in culture is very similar to that of peripheral blood lymphocytes. The only difference between these 2 groups of cells was the finding of spontaneous proliferation by normal lymph node cells after 3 days in culture without phytohemagglutinin (PHA). Whereas blood and lymph node lymphocytes show a negligible uptake of H3T in the basal state, approximately 10 per cent of thymus cells incorporate H3T, indicating significant autonomous proliferation. This is unaffected by PHA and is unassociated with globulin synthesis as judged by immunofluorescent technics. After 3 days in culture, there are significantly more transformed cells and more cells which incorporate H3T into DNA in thymus cell cultures containing PHA than in the control cultures. However, the labeling index of stimulated thymus cultures is less than either the basal rate of proliferation of thymocytes or 3-day cultures of PHA stimulated blood and lymph node lymphocytes. These observations suggest that the normal human thymus contains at least two populations of lymphoid cells: a major component which shows autonomous and unsustained proliferative activity and does not respond to PHA, and a second and probably minor cellular component which transforms and proliferates in response to PHA.