The Cell-Intrinsic Circadian Clock Is Dispensable for Lymphocyte Differentiation and Function

SummaryCircadian rhythms regulate many aspects of physiology, ranging from sleep-wake cycles and metabolic parameters to susceptibility to infection. The molecular clock, with transcription factor BMAL1 at its core, controls both central and cell-intrinsic circadian rhythms. Using a circadian reporter, we observed dynamic regulation of clock activity in lymphocytes. However, its disruption upon conditional Bmal1 ablation did not alter T- or B-cell differentiation or function. Although the magnitude of interleukin 2 (IL-2) production was affected by the time of bacterial infection, it was independent of cell-intrinsic expression of BMAL1. The circadian gating of the IL-2 response was preserved in Bmal1-deficient T cells, despite a slight reduction in cytokine production in a competitive setting. Our results suggest that, contrary to the prevailing view, the adaptive immune response is not affected by the cell-intrinsic clock but is likely influenced by cell-extrinsic circadian cues operating across multiple cell types

In vivo sites and cellular mechanisms of T reg cell–mediated suppression

Regulatory CD4 T (T reg) cells control immune responses to self-antigens and pathogens. However, where T reg cells act to curtail effector T cells in vivo and what stage of effector T cell activation or differentiation is targeted by T reg cells remain unknown. Furthermore, a requirement for direct effector T cell–T reg cell contact in vivo has not been ascertained. Varying answers to these important questions have been provided by several new studies

Cellular Mechanisms of Fatal Early-Onset Autoimmunity in Mice with the T Cell-Specific Targeting of Transforming Growth Factor-β Receptor

SummaryTransforming growth factor-β (TGF-β) has been implicated in the control of differentiation and proliferation of multiple cell types. However, a role for TGF-β in the control of immune homeostasis is not fully understood because of its pleiotropic action. Here we report that complete ablation of the TGF-β signaling in T cells engendered aggressive early-onset, multiorgan, autoimmune-associated lesions with 100% mortality. Peripheral CD4+ and CD8+ T cells with TGF-β-receptor II (TGF-βRII) deficiency activated cytolytic and T helper 1 (Th1) differentiation program in a cell-intrinsic T cell receptor (TCR)-specific fashion. Furthermore, TGF-βRII deficiency blocked the development of canonical CD1d-restricted NKT cells. Instead, it facilitated generation of a highly pathogenic T cell subset exhibiting multiple hallmarks of NK cells and sharply elevated amounts of FasL, perforin, granzymes, and interferon-γ. Thus, TGF-β signaling in peripheral T cells is crucial in restraining TCR activation-dependent Th1, cytotoxic, and NK cell-like differentiation program which, when left unchecked, leads to rapidly progressing fatal autoimmunity

Deficient Positive Selection of CD4 T Cells in Mice Displaying Altered Repertoires of MHC Class II–Bound Self-Peptides

AbstractThe role of self-peptides in positive selection of CD4+ T cells has been controversial. We show that some self-peptides are presented by the MHC class II molecule I-Ab in mice lacking Ii or H-2M but not in mice expressing a transgene-encoded peptide fused to I-Ab. In experiments using specific antibodies to block selection, these low-abundance self-peptides were implicated in the positive selection of some CD4+ T cells in H-2M−/− mice. However, all three mutant backgrounds failed to positively select two class II–restricted transgenic T cell receptors. Our findings suggest that minor components of the self-peptide repertoire can contribute to positive selection of a significant number of CD4+ T cells. In addition, the data suggest that T cell receptor repertoires selected in wild-type mice and in mice displaying limited spectra of self-peptides are distinct

Cathepsin L Regulates CD4+ T Cell Selection Independently of Its Effect on Invariant Chain: A Role in the Generation of Positively Selecting Peptide Ligands

CD4+ T cells are positively selected in the thymus on peptides presented in the context of major histocompatibility complex class II molecules expressed on cortical thymic epithelial cells. Molecules regulating this peptide presentation play a role in determining the outcome of positive selection. Cathepsin L mediates invariant chain processing in cortical thymic epithelial cells, and animals of the I-Ab haplotype deficient in this enzyme exhibit impaired CD4+ T cell selection. To determine whether the selection defect is due solely to the block in invariant chain cleavage we analyzed cathepsin L–deficient mice expressing the I-Aq haplotype which has little dependence upon invariant chain processing for peptide presentation. Our data indicate the cathepsin L defect in CD4+ T cell selection is haplotype independent, and thus imply it is independent of invariant chain degradation. This was confirmed by analysis of I-Ab mice deficient in both cathepsin L and invariant chain. We show that the defect in positive selection in the cathepsin L−/− thymus is specific for CD4+ T cells that can be selected in a wild-type and provide evidence that the repertoire of T cells selected differs from that in wild-type mice, suggesting cortical thymic epithelial cells in cathepsin L knockout mice express an altered peptide repertoire. Thus, we propose a novel role for cathepsin L in regulating positive selection by generating the major histocompatibility complex class II bound peptide ligands presented by cortical thymic epithelial cells

The RNAseIII enzyme Drosha is critical in T cells for preventing lethal inflammatory disease

MicroRNAs (miRNAs) are implicated in the differentiation and function of many cell types. We provide genetic and in vivo evidence that the two RNaseIII enzymes, Drosha and Dicer, do indeed function in the same pathway. These have previously been shown to mediate the stepwise maturation of miRNAs (Lee, Y., C. Ahn, J. Han, H. Choi, J. Kim, J. Yim, J. Lee, P. Provost, O. Radmark, S. Kim, and V.N. Kim. 2003. Nature. 425:415–419), and genetic ablation of either within the T cell compartment, or specifically within Foxp3+ regulatory T (T reg) cells, results in identical phenotypes. We found that miRNA biogenesis is indispensable for the function of T reg cells. Specific deletion of either Drosha or Dicer phenocopies mice lacking a functional Foxp3 gene or Foxp3+ cells, whereas deletion throughout the T cell compartment also results in spontaneous inflammatory disease, but later in life. Thus, miRNA-dependent regulation is critical for preventing spontaneous inflammation and autoimmunity