Lymphokine regulation of human lymphocyte proliferation: Formation of resting G 0 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 [ 3H]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.