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

Abstract

The kinetics of phytohemagglutinin (PHA)‐stimulated human peripheral blood lymphocytes in the early phases (G₀, G_1a, G_1b) of the first cell cycle have been analyzed in vitro by cytofluorometry and [3H] thymidine incorporation. Cells were detected in the G_1a G_1b 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 G_1a, 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 G_1a‐G_1b transition took place varied. Lower numbers of G₁ cells and delayed G_1a‐G_1b transition coincided with lower IL 2 titers and delayed occurrence of maximal titers in the supernatants.