Hyung W. Lim

Cutting Edge: Direct Suppression of B Cells by CD4 + CD25 + Regulatory T Cells

Regulatory T cells (Tregs) can potentially migrate to the B cell areas of secondary lymphoid tissues and suppress T cell-dependent B cell Ig response. T cell-dependent Ig response requires B cell stimulation by Th cells. It has been unknown whether Tregs can directly suppress B cells or whether they must suppress Th cells to suppress B cell response. We report here that Foxp3+ Tregs are found in T-B area borders and within germinal centers of human lymphoid tissues and can directly suppress B cell Ig response. Although Tregs can effectively suppress T cells, they can also directly suppress B cell response without the need to first suppress Th cells. The direct suppression of B cell Ig production by Tregs is accompanied by inhibition of Ig class switch recombination.

Vitamin a metabolites induce gut-homing FoxP3+ regulatory T cells

In this study, we report a novel biological function of vitamin A metabolites in conversion of naive FoxP3− CD4+ T cells into a unique FoxP3+ regulatory T cell subset (termed “retinoid-induced FoxP3+ T cells”) in both human and mouse T cells. We found that the major vitamin A metabolite all-trans-retinoic acid induces histone acetylation at the FoxP3 gene promoter and expression of the FoxP3 protein in CD4+ T cells. The induction of retinoid-induced FoxP3+ T cells is mediated by the nuclear retinoic acid receptor α and involves T cell activation driven by mucosal dendritic cells and costimulation through CD28. Retinoic acid can promote TGF-β1-dependent generation of FoxP3+ regulatory T cells but decrease the TGF-β1- and IL-6-dependent generation of inflammatory Th17 cells in mouse T cells. Retinoid-induced FoxP3+ T cells can efficiently suppress target cells and, thus, have a regulatory function typical for FoxP3+ T cells. A unique cellular feature of these regulatory T cells is their high expression of gut-homing receptors that are important for migration to the mucosal tissues particularly the small intestine. Taken together, these results identify retinoids as positive regulatory factors for generation of gut-homing FoxP3+ T cells.

Regulatory T cells can migrate to follicles upon T cell activation and suppress GC-Th cells and GC-Th cell-driven B cell responses

How Tregs migrate to GCs, and whether they regulate the helper activity of the T cells in GCs (GC-Th cells) remains poorly understood. We found a T cell subset in human tonsils that displays potent suppressive activities toward GC-Th cell-dependent B cell responses. These Tregs with the surface phenotype of CD4+CD25+CD69- migrate well to CCL19, a chemokine expressed in the T cell zone, but poorly to CXCL13, a chemokine expressed in the B cell zone. This migration toward the T cell-rich zone rapidly changes to trafficking toward B cell follicles upon T cell activation. This change in chemotactic behavior upon activation of T cells is consistent with their switch in the expression of the 2 chemokine receptors CXCR5 and CCR7. CD4+CD25+CD69- Tregs suppress GC-Th cells and GC-Th cell-induced B cell responses such as Ig production, survival, and expression of activation-induced cytosine deaminase. Our results have identified a subset of Tregs that is physiologically relevant to GC-Th cell-dependent B cell responses and a potential regulation mechanism for the trafficking of these Tregs to GCs.

Regulation of Trafficking Receptor Expression in Human Forkhead Box P3 + Regulatory T Cells

Forkhead Box P3+ (FOXP3+) T cells are regulatory cells important for maintaining immune tolerance. While chemokine- and other homing-receptors are important for T cell migration, it has been unclear how they are regulated in FOXP3+ T cells. We thoroughly investigated, ex vivo and in vitro, the regulation of chemokine receptor expression on human FOXP3+ T cells in neonatal cord blood, adult peripheral blood, and tonsils. We found that human FOXP3+ T cells undergo changes in trafficking receptors according to their stages of activation and differentiation. FOXP3+ T cells are divided into CD45RA+ (naive type) and CD45RO+ (memory type) FOXP3+ T cells in neonatal blood, adult blood, and tonsils. CD45RA+FOXP3+ T cells mainly express lymphoid tissue homing receptors (CD62L, CCR7, and CXCR4), while CD45RO+FOXP3+ T cells highly express both Th1 and Th2-associated trafficking receptors along with the lymphoid tissue homing receptors at reduced frequencies. Up-regulation of Th1/Th2-associated trafficking receptors begins with activation of CD45RA+FOXP3+ T cells and is completed after their differentiation to CD45RO+ cells. Some chemokine receptors such as CXCR5 and CXCR6 are preferentially expressed by many FOXP3+ cells at a specific stage (CD69+CD45RO+) in tonsils. Our in vitro differentiation study demonstrated that CD45RA+FOXP3+ T cells indeed undergo chemokine receptor switch from CD45RA+ (secondary lymphoid tissue homing) to CD45RO+ type (lymphoid and nonlymphoid tissue homing). The orderly regulation of trafficking receptors in FOXP3+ T cells according to stages of differentiation and activation is potentially important for their tissue-specific migration and regulation of immune responses in humans.

Mitochondrial Dysfunction Induces Senescence with a Distinct Secretory Phenotype

Cellular senescence permanently arrests cell proliferation, often accompanied by a multi-faceted senescence-associated secretory phenotype (SASP). Loss of mitochondrial function can drive age-related declines in the function of many post-mitotic tissues, but little is known about how mitochondrial dysfunction affects mitotic tissues. We show here that several manipulations that compromise mitochondrial function in proliferating human cells induce a senescence growth arrest with a modified SASP that lacks the IL-1-dependent inflammatory arm. Cells that underwent mitochondrial dysfunction-associated senescence (MiDAS) had lower NAD+/NADH ratios, which caused both the growth arrest and prevented the IL-1-associated SASP through AMPK-mediated p53 activation. Progeroid mice that rapidly accrue mtDNA mutations accumulated senescent cells with a MiDAS SASP in vivo, which suppressed adipogenesis and stimulated keratinocyte differentiation in cell culture. Our data identify a distinct senescence response and provide a mechanism by which mitochondrial dysfunction can drive aging phenotypes.

MicroRNAs of the miR-17∼92 family are critical regulators of T(FH) differentiation.

Follicular helper T cells (TFH cells) provide critical help to B cells during humoral immune responses. Here we report that mice with T cell–specific deletion of the miR-17∼92 family of microRNAs (miRNAs) had substantially compromised TFH differentiation, germinal-center formation and antibody responses and failed to control chronic viral infection. Conversely, mice with T cell–specific expression of a transgene encoding miR-17∼92 spontaneously accumulated TFH cells and developed a fatal immunopathology. Mechanistically, the miR-17∼92 family controlled the migration of CD4+ T cells into B cell follicles by regulating signaling intensity from the inducible costimulator ICOS and kinase PI(3)K by suppressing expression of the phosphatase PHLPP2. Our findings demonstrate an essential role for the miR-17∼92 family in TFH differentiation and establish PHLPP2 as an important mediator of their function in this process.

Human CD57+ Germinal Center-T Cells are the Major Helpers for GC-B Cells and Induce Class Switch Recombination

BackgroundThe function of CD57+ CD4+ T cells, constituting a major subset of germinal center T (GC-Th) cells in human lymphoid tissues, has been unclear. There have been contradictory reports regarding the B cell helping function of CD57+ GC-Th cells in production of immunoglobulin (Ig). Furthermore, the cytokine and co-stimulation requirement for their helper activity remains largely unknown. To clarify and gain more insight into their function in helping B cells, we systematically investigated the capacity of human tonsil CD57+ GC-Th cells in inducing B cell Ig synthesis.ResultsWe demonstrated that CD57+ GC-Th cells are highly efficient in helping B cell production of all four subsets of Ig (IgM, IgG, IgA and IgE) compared to other T-helper cells located in germinal centers or interfollicular areas. CD57+ GC-Th cells were particularly more efficient than other T cells in helping GC-B cells but not naïve B cells. CD57+ GC-Th cells induced the expression of activation-induced cytosine deaminase (AID) and class switch recombination in developing B cells. IgG1-3 and IgA1 were the major Ig isotypes induced by CD57+ GC-Th cells. CD40L, but not IL-4, IL-10 and IFN-γ, was critical in CD57+ GC-Th cell-driven B cell production of Ig. However, IL-10, when added exogenously, significantly enhanced the helper activity of CD57+ GC-Th cells, while TGF-β1 completely and IFN-γ partially suppressed the CD57+ GC-Th cell-driven Ig production.ConclusionsCD57+CD4+ T cells in the germinal centers of human lymphoid tissues are the major T helper cell subset for GC-B cells in Ig synthesis. Their helper activity is consistent with their capacity to induce AID and class switch recombination, and can be regulated by CD40L, IL-4, IL-10 and TGF-β.

Three Novel Acetylation Sites in the Foxp3 Transcription Factor Regulate the Suppressive Activity of Regulatory T Cells

The Foxp3 transcription factor is the master regulator of regulatory T cell (Treg) differentiation and function. Its activity is regulated by reversible acetylation. Using mass spectrometry of immunoprecipitated proteins, we identify three novel acetylation sites in murine Foxp3 (K31, K262, and K267) and the corresponding sites in human FOXP3 proteins. Newly raised modification-specific Abs against acetylated K31 and K267 confirm acetylation of these residues in murine Tregs. Mutant Foxp3 proteins carrying arginine substitutions at the three acetylation sites (3KR) accumulate in T cells to higher levels than wild-type Foxp3 and exert better suppressive activity in coculture experiments. Acetylation and stability of wild-type, but not mutant, Foxp3 is enhanced when cells are treated with Ex-527, an inhibitor of the NAD+-dependent deacetylase SIRT1. Treatment with Ex-527 promotes Foxp3 expression during induced Treg differentiation, enhances Foxp3 levels in natural Tregs, and prevents loss of Foxp3 expression in adoptively transferred Tregs in mice. Our data identify SIRT1 as a negative regulator of Treg function via deacetylation of three novel target sites in Foxp3. SIRT1 inhibitors strengthen the suppressive activity of Tregs and may be useful in enhancing Treg-based therapeutic approaches to autoimmune diseases or graft rejections.

SIRT1 deacetylates RORγt and enhances Th17 cell generation

Lim et al. demonstrate that protein deacetylase, Sirtuin 1, promotes autoimmunity by deacetylating RORγt increasing its transcriptional activity and promoting Th17 differentiation and function. Blockade or loss of Sirtuin 1 results in protection from multiple sclerosis-like disease in mice.

Histone Deacetylase 7 Regulates Cell Survival and TCR Signaling in CD4/CD8 Double-Positive Thymocytes

CD4/CD8 double-positive thymocytes express the transcriptional repressor histone deacetylase (HDAC)7, a class IIa HDAC that is exported from the cell nucleus after TCR engagement. Through signal-dependent nuclear export, class IIa HDACs such as HDAC7 mediate signal-dependent changes in gene expression that are important to developmental fate decisions in multiple tissues. We report that HDAC7 is exported from the cell nucleus during positive selection in mouse thymocytes and that it regulates genes mediating the coupling between TCR engagement and downstream events that determine cell survival. Thymocytes lacking HDAC7 are inefficiently positively selected due to a severely shortened lifespan and exhibit a truncated repertoire of TCR Jα segments. The expression of multiple important mediators and modulators of the response to TCR engagement is altered in HDAC7-deficient thymocytes, resulting in increased tonic MAPK activity that contributes to the observed loss of viability. Remarkably, the activity of protein kinase D, the kinase that mediates nuclear export of HDAC7 in response to TCR signaling, is also increased in HDAC7-deficient thymocytes, suggesting that HDAC7 nuclear export governs a self-sustaining autoexcitatory loop. These experiments add to the understanding of the life/death decision in thymic T cell development, define a novel function for class IIa HDACs, and point to a novel feed-forward mechanism whereby these molecules regulate their own state and mediate stable developmental transitions.