Unexpected Help: Follicular Regulatory T Cells in the Germinal Center

Follicular helper T (Tfh) cells are necessary for germinal center (GC) formation and within the GC, provide key signals to B cells for their differentiation into plasmablasts and plasma cells that secrete high-affinity and isotype-switched antibody (Ab). A specialized subset of Foxp3+ T cells termed T follicular regulatory (Tfr) cells, also regulate the differentiation of Ab-secreting cells from the GC. Tfr-cell function in the GC is not well understood, however, the dominant paradigm currently is that Tfr cells repress excessive Tfh and GC B cell proliferation and help promote stringent selection of high-affinity B cells. A mouse model where the Bcl6 gene is specifically deleted in Foxp3+ T cells (Bcl6FC mice) allows the study of Tfr cell function with more precision than other approaches. Studies with this model have shown that Tfr cells play a key role in maintaining GC B cell proliferation and Ab levels. Part of the mechanism for this positive "helper" effect of Tfr cells on the GC is Tfr cell-derived IL-10, which can promote B cell growth and entry into the dark zone of the GC. Recent studies on Tfr cells support a new paradigm for Tfr cell function in the GC reaction. Here, we review studies on Tfr cell functions and discuss the evidence that Tfr cells can have a major helper role in the GC-dependent Ab response

Interleukin-13 Is Unlucky for Allergy Sufferers

Anaphylaxis, a life-threatening allergic reaction, is dependent on high affinity allergen-specific IgE. Gowthaman et al . now show that a new interleukin (IL)-13-expressing T helper cell subset specifically promotes high-affinity IgE responses. The discovery of T follicular helper (Tfh)13 cells defines potential new targets for allergy therapies

Stat3 Is Important for Follicular Regulatory T Cell Differentiation

The production of antibody is precisely controlled during the germinal center (GC) reaction. This process is dependent on the help from follicular T helper (Tfh) cells to germinal center (GC) B cells and is regulated by regulatory follicular T helper (Tfr) cells. How Tfr cells develop and how their suppressive activity functions are not well understood. Here, we found that Stat3 is indispensible for Tfr cell differentiation. After immunization with Sheep Red Blood Cells (SRBC), the loss of Tfr cells caused by deletion of Stat3 in Treg cells does not affect the size of Tfh or GC B cell population, but rather leads to strongly enhanced production of antigen-specific IgG1 and IgG2b. In Peyer's patches (PPs) in the gut, we found that Stat3 expression in Treg cells is also required for Tfr cell formation to commensal organisms. However, loss of Tfr cells in the gut did not affect the numbers of Tfh cells and GC B cells, nor affect IgG1 or IgA switching by GC B cells. Overall, our study has uncovered unique roles of Stat3 in Tfr cell differentiation and the regulation of the antibody response

AMP kinase promotes Bcl6 expression in both mouse and human T cells

The transcription factor Bcl6 is a master regulator of follicular helper T (TFH) cells, and understanding the signaling pathway that induces Bcl6 and TFH cell differentiation is therefore critical. IL-2 produced during T cell activation inhibits Bcl6 expression but how TFH cells evade IL-2 inhibition is not completely understood. Here we show that Bcl6 is highly up-regulated in activated CD4 T cells following glucose deprivation (GD), and this pathway is insensitive to inhibition by IL-2. Similar to GD, the glucose analog 2-deoxyglucose (2DG) inhibits glycolysis, and 2DG induced Bcl6 expression in activated CD4 T cells. The metabolic sensor AMP kinase (AMPK) is activated when glycolysis is decreased, and the induction of Bcl6 by GD was inhibited by the AMPK antagonist compound C. Additionally, activation of AMPK by the drug AICAR caused Bcl6 up-regulation in activated CD4 T cells. When mice were immunized with KLH using AICAR as an adjuvant, there was a strong TFH–dependent enhancement of KLH-specific antibody (Ab) responses, and higher Bcl6 expression in TFH cells in vivo. Activation of AMPK strongly induced BCL6 and the up-regulation of TFH cell marker expression by human CD4 T cells. Our data reveal a major new pathway for TFH cell differentiation, conserved by both mouse and human T cells. Mature TFH cells are reported to have a lower metabolic state compared to TH1 cells. Our data indicates that decreased metabolism may be deterministic for TFH cell differentiation, and not simply a result of TFH cell differentiation

Follicular regulatory T cells inhibit the development of granzyme B-expressing follicular helper T cells

T follicular regulatory (TFR) cells are found in the germinal center (GC) response and help shape the antibody (Ab) response. However, the precise role of TFR cells in the GC is controversial. Here, we addressed TFR cell function using mice with impaired TFR cell development (Bcl6-flox/Foxp3-cre, or Bcl6FC mice), mice with augmented TFR cell development (Blimp1-flox/Foxp3-cre, or Blimp1FC mice), and two different methods of immunization. Unexpectedly, GC B cell levels positively correlated with TFR cell levels. Using a gene profiling approach, we found that TFH cells from TFR-deficient mice showed strong upregulation of granzyme B (Gzmb) and other effector CD8+ T cell genes, many of which were Stat4 dependent. The upregulation of cytotoxic genes was the highest in TFH cells from TFR-deficient mice where Blimp1 was also deleted in Foxp3+ regulatory T cells (Bcl6-flox/Prdm1-flox/Foxp3-cre [DKO] mice). Granzyme B- and Eomesodermin-expressing TFH cells correlated with a higher rate of apoptotic GC B cells. Klrg1+ TFH cells from DKO mice expressed higher levels of Gzmb. Our data show that TFR cells repress the development of abnormal cytotoxic TFH cells, and the presence of cytotoxic TFH cells correlates with a lower GC and Ab response. Our data show what we believe is a novel mechanism of action for TFR cells helping the GC response

Bcl6 promotes follicular helper T-cell differentiation and PD-1 expression in a Blimp1-independent manner in mice

The transcription factors Bcl6 and Blimp1 have opposing roles in the development of the follicular helper T (TFH) cells: Bcl6 promotes and Blimp1 inhibits TFH-cell differentiation. Similarly, Bcl6 activates, while Blimp1 represses, expression of the TFH-cell marker PD-1. However, Bcl6 and Blimp1 repress each other's expression, complicating the interpretation of the regulatory network. Here we sought to clarify the extent to which Bcl6 and Blimp1 independently control TFH-cell differentiation by generating mice with T-cell specific deletion of both Bcl6 and Blimp1 (double conditional KO [dcKO] mice). Our data indicate that Blimp1 does not control TFH-cell differentiation independently of Bcl6. However, a population of T follicular regulatory (TFR) cells developed independently of Bcl6 in dcKO mice. We have also analyzed regulation of IL-10 and PD-1, two genes controlled by both Bcl6 and Blimp1, and observed that Bcl6 regulates both genes independently of Blimp1. We found that Bcl6 positively regulates PD-1 expression by inhibiting the ability of T-bet/Tbx21 to repress Pdcd1 transcription. Our data provide a novel mechanism for positive control of gene expression by Bcl6, and illuminate how Bcl6 and Blimp1 control TFH-cell differentiation

Roles of T follicular helper cells and T follicular regulatory cells in Autoantibody Production in IL-2-deficient mice

Autoantibodies can result from excessive T follicular helper (Tfh) cell activity, whereas T follicular regulatory (Tfr) cells negatively regulate autoantibody production. IL-2 knockout (KO) mice on the BALB/c background have elevated Tfh responses, produce autoantibodies, and develop lethal autoimmunity. We analyzed Tfh and Tfr cells in IL-2 KO mice on the C57BL/6 (B6) genetic background. In B6 IL-2 KO mice, the spontaneous formation of Tfh cells and germinal center B cells was greatly enhanced, along with production of anti-DNA autoantibodies. IL-2 has been reported to repress Tfr cell differentiation; however, Tfr cells were not increased over wild-type levels in the B6 IL-2 KO mice. To assess Tfh and Tfr cell regulation of autoantibody production in IL-2 KO mice, we generated IL-2 KO mice with a T cell-specific deletion of the master Tfh cell transcription factor Bcl6. In IL-2 KO Bcl6 conditional KO (2KO-Bcl6TC) mice, Tfh cells, Tfr cells, and germinal center B cells were ablated. In contrast to expectations, autoantibody IgG titers in 2KO-Bcl6TC mice were significantly elevated over autoantibody IgG titers in IL-2 KO mice. Specific deletion of Tfr cells with Foxp3-cre Bcl6-flox alleles in IL-2 KO mice led to early lethality, before high levels of autoantibodies could develop. We found IL-2+/+ Tfr cell-deficient mice produce significant levels of autoantibodies. Our overall findings provide evidence that Tfh cells are dispensable for high-level production of autoantibodies and also reveal a complex interplay between Tfh and Tfr cells in autoantibody production and autoimmune disease

Determination of the Carrier-Envelope Phase of Few-Cycle Laser Pulses with Terahertz-Emission Spectroscopy

The availability of few-cycle optical pulses opens a window to physical phenomena occurring on the attosecond time scale. In order to take full advantage of such pulses, it is crucial to measure and stabilise their carrier-envelope (CE) phase, i.e., the phase difference between the carrier wave and the envelope function. We introduce a novel approach to determine the CE phase by down-conversion of the laser light to the terahertz (THz) frequency range via plasma generation in ambient air, an isotropic medium where optical rectification (down-conversion) in the forward direction is only possible if the inversion symmetry is broken by electrical or optical means. We show that few-cycle pulses directly produce a spatial charge asymmetry in the plasma. The asymmetry, associated with THz emission, depends on the CE phase, which allows for a determination of the phase by measurement of the amplitude and polarity of the THz pulse

Blimp-1 is essential for allergen-induced asthma and Th2 cell development in the lung

A Th2 immune response is central to allergic airway inflammation, which afflicts millions worldwide. However, the mechanisms that augment GATA3 expression in an antigen-primed developing Th2 cell are not well understood. Here, we describe an unexpected role for Blimp-1, a transcriptional repressor that constrains autoimmunity, as an upstream promoter of GATA3 expression that is critical for Th2 cell development in the lung to inhaled but not systemically delivered allergens but is dispensable for TFH function and IgE production. Mechanistically, Blimp-1 acts through Bcl6, leading to increased GATA3 expression in lung Th2 cells. Surprisingly, the anti-inflammatory cytokine IL-10, but not the pro-inflammatory cytokines IL-6 or IL-21, is required via STAT3 activation to up-regulate Blimp-1 and promote Th2 cell development. These data reveal a hitherto unappreciated role for an IL-10-STAT3-Blimp-1 circuit as an initiator of an inflammatory Th2 response in the lung to allergens. Thus, Blimp-1 in a context-dependent fashion can drive inflammation by promoting rather than terminating effector T cell responses

Search for supersymmetry in events with b-quark jets and missing transverse energy in pp collisions at 7 TeV

Results are presented from a search for physics beyond the standard model based on events with large missing transverse energy, at least three jets, and at least one, two, or three b-quark jets. The study is performed using a sample of proton-proton collision data collected at sqrt(s) = 7 TeV with the CMS detector at the LHC in 2011. The integrated luminosity of the sample is 4.98 inverse femtobarns. The observed number of events is found to be consistent with the standard model expectation, which is evaluated using control samples in the data. The results are used to constrain cross sections for the production of supersymmetric particles decaying to b-quark-enriched final states in the context of simplified model spectra.Comment: Submitted to Physical Review