The control of T cell activation vs. tolerance.

B7 costimulators and the T cell growth factor IL-2 are important stimuli for the activation of T lymphocytes and the development of effective immune responses. Recent studies show that the same signals promote the development of regulatory T cells or the apoptotic death of activated T cells, and thus, function to terminate immune responses and maintain self-tolerance. The balance between the different outcomes of the same signals may be determined by the magnitude of these signals and the presence or absence of concomitant stimuli, such as those generated during innate immune responses to microbes.