Involvement of NFAT1 in B cell self-tolerance.

B cells from anti-lysozyme Ig/soluble lysozyme double-transgenic mice are chronically exposed to self-Ag in the periphery, resulting in an anergic phenotype. Chronic exposure to self-Ag leads to nuclear translocation of NFAT1 and NFAT2, suggesting that they are involved in anergy. To directly test a role for NFAT1 in B cell anergy, NFAT1-deficient mice were crossed with anti-lysozyme Ig transgenic mice. As expected, B cell anergy was evident in the presence of self-Ag based on reduced serum anti-lysozyme levels, percentage and number of mature B cells, and reduced B cell responsiveness. By contrast, B cell anergy was relieved in NFAT1(-/-) mice expressing soluble self-Ag. Bone marrow development was equivalent in NFAT1-sufficient and -deficient mice, suggesting that loss of anergy in the latter is due to selection later in development. Taken together, these studies provide direct evidence that the transcription factor NFAT1 is involved in B cell anergy.

FOXP3 controls regulatory T cell function through cooperation with NFAT.

Antigen stimulation of immune cells activates the transcription factor NFAT, a key regulator of T cell activation and anergy. NFAT forms cooperative complexes with the AP-1 family of transcription factors and regulates T cell activation-associated genes. Here we show that regulatory T cell (Treg) function is mediated by an analogous cooperative complex of NFAT with the forkhead transcription factor FOXP3, a lineage specification factor for Tregs. The crystal structure of an NFAT:FOXP2:DNA complex reveals an extensive protein-protein interaction interface between NFAT and FOXP2. Structure-guided mutations of FOXP3, predicted to progressively disrupt its interaction with NFAT, interfere in a graded manner with the ability of FOXP3 to repress expression of the cytokine IL2, upregulate expression of the Treg markers CTLA4 and CD25, and confer suppressor function in a murine model of autoimmune diabetes. Thus by switching transcriptional partners, NFAT converts the acute T cell activation program into the suppressor program of Tregs.

Foxp1 is an essential transcriptional regulator of B cell development.

Forkhead transcription factors are key participants in development and immune regulation. Here we demonstrate that absence of the gene encoding the forkhead transcription factor Foxp1 resulted in a profound defect in early B cell development. Foxp1 deficiency was associated with decreased expression of all B lineage genes in B220+ fetal liver cells as well as with a block in the transition from pro-B cell to pre-B cell involving diminished expression of recombination-activating genes 1 and 2. Foxp1 bound to the Erag enhancer and was involved in controlling variable-(diversity)-joining recombination of the gene encoding immunoglobulin heavy chain in a B cell lineage-specific way. Our results identify Foxp1 as an essential participant in the transcriptional regulatory network of B lymphopoiesis.

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.