Duration of CTL activation regulates IL-2 production required for autonomous clonal expansion

The extent and magnitude of CD8(+) T lymphocyte (CTL) clonal expansion in vivo depends on the duration of stimulation provided during primary activation, whereas expansion under conventional in vitro conditions fails to reveal this. The molecular details of this differential programming are unclear. We developed a low-density culture system that recapitulates in vivo conditions and enables analysis of both proliferation and survival/apoptosis during primary expansion. We found that CTL given a prolonged stimulus produced more IL-2 and for a longer period than briefly activated CTL, and the IL-2 produced during the expansion phase was essential for cell proliferation and accumulation at low densities. CTL given a short stimulation and plated at low density followed the fate of briefly stimulated cells in vivo and underwent an abortive expansion and apoptosis, unless IL-2 was exogenously provided. Inhibition of apoptosis did not replace the requirement for IL-2 to drive clonal expansion. CTL primed for prolonged periods but in the absence of costimulation similarly behaved as though "partially programmed", undergoing abortive expansion at low densities. These results indicate that the nature and duration of antigenic stimulation regulate the autonomy of CTL expansion via the production of autocrine IL-2.