CD28 induces cell cycle progression by IL-2-independent down-regulation ofp27(kip1) expression in human peripheral T lymphocytes

CD28 is the primary T cell costimulatory receptor, and upon ligation with i ts ligands, it enhances T cell proliferation and IL-2 synthesis. In this st udy we examined the role of CD28 in the initial proliferative response and cell cycle entry of T lymphocytes. Stimulation through CD3 alone resulted i n a poor proliferative response, while in the presence of CD28 costimulatio n a strong increase in the number of cells in S-phase could be detected aft er 48 h of stimulation. CD28 costimulation enhanced expression of cyclin D3 and induced downregulation of p27(kip1) expression. Cross-linking CD28 was much more effective in inducing cyclin D3 expression and in down-regulatin g p27(kip1) expression than addition of IL-2, Blocking experiments, using a ntibodies that neutralize IL-2 or the IL-2 receptor, showed that the effect s induced by CD28 are independent of endogenous IL-2. Moreover, using a var iety of immunosuppressants that interfere with IL-2 signaling pathways, we were able to show that IL-2 is not required for cell cycle entry induced by CD28 costimulation. From these experiments it can be concluded that CD28 a nd IL-2 use different signaling pathways for downregulation of p27(kip1) ex pression. We hypothesize that costimulation through CD28 is responsible for initial cell cycle entry of T lymphocytes, while IL-2, which is produced a fter costimulation, might be involved in sustaining proliferation.