Transcriptional basis of lymphocyte tolerance

Signaling through lymphocyte antigen receptors has the potential to initiate several distinct outcomes: proliferation, differentiation, apoptosis, or functional unresponsiveness. Expansion and differentiation of effector T cells is required for defense against foreign antigens, whereas functional unresponsiveness, termed anergy, is a cell-intrinsic mechanism that contributes to peripheral self-tolerance. Other mechanisms of peripheral tolerance include the 'dominant' tolerance imposed by regulatory T cells and immunosuppression mediated by interleukin-10 and transforming growth factor-beta. T- and B-cell antigen receptor ligation induces an increase in intracellular calcium levels as well as activating additional signaling pathways that are further potentiated by costimulatory receptors. In this review, we argue that cell-intrinsic programs of peripheral anergy and tolerance are imposed by sustained calcium signaling in lymphocytes. We address in particular the role of the calcium-dependent transcription factor nuclear factor for activation of T cells, which is activated by antigen receptor stimulation and, depending on the presence or absence of input from its transcriptional partner, activator protein-1, dictates two distinct transcriptional programs: activation or tolerance.