Lymphokine regulation of human lymphocyte proliferation: Formation of resting G0 cells by removal of interteukin 2 in cultures of proliferating T lymphocytes☆

Abstract The question of whether lymphocytes which have once been activated and have completed one or several cell cycle(s) can return to the G 0 phase and stay ready for a new activation (G 0 -G 1 transition), rather than simply die, was investigated. To do so interleukin 2 (IL-2) was removed from cultures of continuously proliferating human T lymphocytes and the formation of resting (G 0 ) cells was measured. Kinetic analyses in freshly prepared peripheral blood lymphocytes (PBL) revealed that the onset of detectable RNA synthesis and the appearance of structures binding the anti-Tac antibody occurred simultaneously. This allowed the expansion of the definition of G 0 T lymphocytes as cells having a low RNA (and DNA) content, and no Tac antigen. When cultured human T cells proliferating continuously by means of IL-2 were characterized in terms of their distribution in the cell cycle, 7 days after the initial PHA stimulation, it could be demonstrated that very few cells were in the G 0 phase, supporting the concept of direct S/G 2 /M-G 1 transition. However, when IL-2 was removed from the cultures, the [ 3 H]thymidine incorporation per 10 4 cells and correspondingly the number of cells in the S/G 2 /M and G 1 phases were reduced drastically and during the following 72-hr period, the number of G 0 cells increased markedly. Restimulation of such in vitro formed G 0 cells, under conditions permitting observation of their shift from the G 0 to G 0 phase, demonstrated that most cells could respond normally. Based on these observations, it was concluded that IL-2 not only ensures T-lymphocyte survival and proliferation, but IL-2 starvation induces many continuously proliferating T lymphocytes to stop cycling and to return to the G 0 phase of the cell cycle where they remain functional.