Human lymphocyte proliferation. II. Formation of activated (G1) cells

The kinetics of phytohemagglutinin (PHA)-stimulated human peripheral blood lymphocytes in the early phases (G0, G1a, G1b) of the first cell cycle have been analyzed in vitro by cytofluorometry and [3H]thymidine incorporation. Cells were detected in the G1a, G1b and S phases 12-14, 20-22 and 24-26 h after stimulation, respectively. The total number of PHA-induced G1 cells reached a plateau after 32 h of incubation, upon which a second increase followed, 10 h later. The latter increase was considered to be a result of cells initiating their second cell cycle, since it could be abolished by hydroxyurea. By examination of supernatants, interleukin 2 (IL 2) activities could be recovered 6-10 h after PHA stimulation. In contrast, the monokine IL 1 was already detected after 2 h. Both interleukins were present in the culture medium before the major G1a cell formation took place. The titer of free IL 2 increased to a maximum after 18-22 h, whereafter a decline was observed. This decline was less pronounced if cultures were treated with hydroxyurea. Finally, among individual donors, the total number of lymphocytes entering the G1 phase and the time at which the G1a-G1b transition took place varied. Lower numbers of G1 cells and delayed G1a-G1b transition coincided with lower IL 2 titers and delayed occurrence of maximal titers in the supernatants.