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

Dicer-dependent microRNA pathway safeguards regulatory T cell function

Regulatory T (T reg) cells are indispensable for preventing autoimmunity. Incumbent to this role is the ability of T reg cells to exert their suppressor function under inflammatory conditions. We found that T reg cell–mediated tolerance is critically dependent on the Dicer-controlled microRNA (miRNA) pathway. Depletion of miRNA within the T reg cell lineage resulted in fatal autoimmunity indistinguishable from that in T reg cell–deficient mice. In disease-free mice lacking Dicer in all T cells or harboring both Dicer-deficient and -sufficient T reg cells, Dicer-deficient T reg cells were suppressive, albeit to a lesser degree, whereas their homeostatic potential was diminished as compared with their Dicer-sufficient counterparts. However, in diseased mice, Dicer-deficient T reg cells completely lost suppressor capacity. Thus, miRNA preserve the T reg cell functional program under inflammatory conditions

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

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

Developmental regulation of Foxp3 expression during ontogeny

Thymectomy of neonatal mice can result in the development of autoimmune pathology. It has been proposed that thymic output of regulatory T (T reg) cells is delayed during ontogeny and that the development of autoimmune disease in neonatally thymectomized mice is caused by the escape of self-reactive T cells before thymectomy without accompanying T reg cells. However, the kinetics of T reg cell production within the thymus during ontogeny has not been assessed. We demonstrate that the development of Foxp3-expressing T reg cells is substantially delayed relative to nonregulatory thymocytes during ontogeny. Based on our data, we speculate that induction of Foxp3 in developing thymocytes and, thus, commitment to the T reg cell lineage is facilitated by a signal largely associated with the thymic medulla

Lack of Foxp3 function and expression in the thymic epithelium

Foxp3 is essential for the commitment of differentiating thymocytes to the regulatory CD4+ T (T reg) cell lineage. In humans and mice with a genetic Foxp3 deficiency, absence of this critical T reg cell population was suggested to be responsible for the severe autoimmune lesions. Recently, it has been proposed that in addition to T reg cells, Foxp3 is also expressed in thymic epithelial cells where it is involved in regulation of early thymocyte differentiation and is required to prevent autoimmunity. Here, we used genetic tools to demonstrate that the thymic epithelium does not express Foxp3. Furthermore, we formally showed that genetic abatement of Foxp3 in the hematopoietic compartment, i.e. in T cells, is both necessary and sufficient to induce the autoimmune lesions associated with Foxp3 loss. In contrast, deletion of a conditional Foxp3 allele in thymic epithelial cells did not result in detectable changes in thymocyte differentiation or pathology. Therefore, in mice the only known role for Foxp3 remains promotion of T reg cell differentiation within the T cell lineage, whereas there is no role for Foxp3 in thymic epithelial cells