Youn Soo Choi

STAT5 is a potent negative regulator of TFH cell differentiation

Interleukin 2, STAT5, and Blimp-1 work together to suppress differentiation of follicular helper T cells in mice.

Cutting Edge: STAT1 Is Required for IL-6–Mediated Bcl6 Induction for Early Follicular Helper Cell Differentiation

Bcl6 is required for CD4 T cell differentiation into T follicular helper cells (Tfh). In this study, we examined the role of IL-6 in early processes of in vivo Tfh differentiation, because the timing and mechanism of action of IL-6 in Tfh differentiation have been controversial in vivo. We found that early Bcl6+CXCR5+ Tfh differentiation was severely impaired in the absence of IL-6; however, STAT3 deficiency failed to recapitulate that defect. IL-6R signaling activates the transcription factor STAT1 specifically in CD4 T cells. Strikingly, we found that STAT1 activity was required for Bcl6 induction and early Tfh differentiation in vivo. IL-6 mediated STAT3 activation is important for downregulation of IL-2Rα to limit Th1 cell differentiation in an acute viral infection. Thus, IL-6 signaling is a major early inducer of the Tfh differentiation program unexpectedly mediated by both STAT3 and STAT1 transcription factors.

Bcl6 Expressing Follicular Helper CD4 T Cells Are Fate Committed Early and Have the Capacity To Form Memory

Follicular helper CD4 T (Tfh) cells are a distinct type of differentiated CD4 T cells uniquely specialized for B cell help. In this study, we examined Tfh cell fate commitment, including distinguishing features of Tfh versus Th1 proliferation and survival. Using cell transfer approaches at early time points after an acute viral infection, we demonstrate that early Tfh cells and Th1 cells are already strongly cell fate committed by day 3. Nevertheless, Tfh cell proliferation was tightly regulated in a TCR-dependent manner. The Tfh cells still depend on extrinsic cell fate cues from B cells in their physiological in vivo environment. Unexpectedly, we found that Tfh cells share a number of phenotypic parallels with memory precursor CD8 T cells, including selective upregulation of IL-7Rα and a collection of coregulated genes. As a consequence, the early Tfh cells can progress to robustly form memory cells. These data support the hypothesis that CD4 and CD8 T cells share core aspects of a memory cell precursor gene expression program involving Bcl6, and a strong relationship exists between Tfh cells and memory CD4 T cell development.

LEF-1 and TCF-1 orchestrate TFH differentiation by regulating differentiation circuits upstream of the transcriptional repressor Bcl6

Follicular helper T cells (TFH cells) are specialized effector CD4+ T cells that help B cells develop germinal centers (GCs) and memory. However, the transcription factors that regulate the differentiation of TFH cells remain incompletely understood. Here we report that selective loss of Lef1 or Tcf7 (which encode the transcription factor LEF-1 or TCF-1, respectively) resulted in TFH cell defects, while deletion of both Lef1 and Tcf7 severely impaired the differentiation of TFH cells and the formation of GCs. Forced expression of LEF-1 enhanced TFH differentiation. LEF-1 and TCF-1 coordinated such differentiation by two general mechanisms. First, they established the responsiveness of naive CD4+ T cells to TFH cell signals. Second, they promoted early TFH differentiation via the multipronged approach of sustaining expression of the cytokine receptors IL-6Rα and gp130, enhancing expression of the costimulatory receptor ICOS and promoting expression of the transcriptional repressor Bcl6.T follicular helper (TFH) cells are specialized effector CD4+ T cells that help B cells develop germinal centers and memory. However, the transcription factors that regulate TFH differentiation remain incompletely understood. Here we report that selective loss of either Lef1 (LEF-1) or Tcf7 (TCF-1) resulted in TFH defects, while deletion of Lef1 and Tcf7 severely impaired TFH differentiation and germinal centers. Forced expression of LEF-1 enhanced TFH differentiation. LEF-1 and TCF-1 coordinated TFH differentiation by two general mechanisms. First, they established the responsiveness of naïve CD4+ T cells to TFH signals. Second, they promoted early TFH differentiation via the multipronged approach of sustaining expression of IL-6Rα and gp130, enhancing ICOS expression, and promoting expression of Bcl6.

B Cell-Specific Expression of B7-2 Is Required for Follicular Th Cell Function in Response to Vaccinia Virus

Follicular Th (TFH) cells are specialized in provision of help to B cells that is essential for promoting protective Ab responses. CD28/B7 (B7-1 and B7-2) interactions are required for germinal center (GC) formation, but it is not clear if they simply support activation of naive CD4 T cells during initiation of responses by dendritic cells or if they directly control TFH cells and/or directly influence follicular B cell differentiation. Using a model of vaccinia virus infection, we show that B7-2 but not B7-1 deficiency profoundly impaired TFH cell development but did not affect CD4 T cell priming and Th1 differentiation. Consistent with this, B7-2 but not B7-1 was required for acquisition of GC B cell phenotype, plasma cell generation, and virus-specific neutralizing Ab responses. Mixed adoptive transfer experiments indicated that bidirectional interactions between CD28 expressed on activated T cells and B7-2 expressed on follicular B cells were essential for maintenance of the TFH phenotype and GC B cell development. Our data provide new insight into the source and nature of molecules required for TFH cells to direct GC B cell responses.

Dynamic regulation of Bcl6 in follicular helper CD4 T (Tfh) cells

Our bodies are continuously exposed to various types of infectious pathogens. Vaccinations are the most cost effective way to protect our bodies against a variety of infectious microbes. The efficacy of most vaccines relies on protective antibody production and generation of memory B cells. These two key components develop mostly from B cells that participate in germinal center reactions. Recent efforts have highlighted the critical role of follicular helper CD4 T (Tfh) cells in the generation of germinal centers. Given that Bcl6 is a major transcription factor for Tfh differentiation, here we review recent developments in the understanding of signaling molecules that regulate Bcl6 expression in CD4 T cells, as a potential target for development of more efficacious vaccines.

Exogenous OX40 Stimulation during Lymphocytic Choriomeningitis Virus Infection Impairs Follicular Th Cell Differentiation and Diverts CD4 T Cells into the Effector Lineage by Upregulating Blimp-1

T cell costimulation is a key component of adaptive immunity to viral infection but has also been associated with pathology because of excessive or altered T cell activity. We recently demonstrated that the TNFR family costimulatory molecule OX40 (CD134) is critically required to sustain antiviral T cell and Ab responses that enable control of viral replication in the context of chronic lymphocytic choriomeningitis virus (LCMV) infection. In this study, we investigated whether reinforcing OX40 stimulation through an agonist Ab had the potential to prevent LCMV persistence. We observed that anti-OX40 injection early after LCMV clone 13 infection increased CD8 T cell–mediated immunopathology. More strikingly, OX40 stimulation of virus-specific CD4 T cells promoted expression of the transcriptional repressor Blimp-1 and diverted the majority of cells away from follicular Th cell differentiation. This occurred in both acute and chronic infections, and resulted in dramatic reductions in germinal center and Ab responses to the viral infection. The effect of the OX40 agonist was dependent on IL-2 signaling and the timing of OX40 stimulation. Collectively, our data demonstrate that excessive OX40 signaling can result in deleterious consequences in the setting of LCMV infection.

A TRAF-like motif of the inducible costimulator ICOS controls development of germinal center TFH cells via the kinase TBK1

Signaling via the inducible costimulator ICOS fuels the stepwise development of follicular helper T cells (TFH cells). However, a signaling pathway unique to ICOS has not been identified. We found here that the kinase TBK1 associated with ICOS via a conserved motif, IProx, that shares homology with the tumor-necrosis-factor receptor (TNFR)-associated factors TRAF2 and TRAF3. Disruption of this motif abolished the association of TBK1 with ICOS, TRAF2 and TRAF3, which identified a TBK1-binding consensus. Alteration of this motif in ICOS or depletion of TBK1 in T cells severely impaired the differentiation of germinal center (GC) TFH cells and the development of GCs, interfered with B cell differentiation and disrupted the development of antibody responses, but the IProx motif and TBK1 were dispensable for the early differentiation of TFH cells. These results reveal a previously unknown ICOS-TBK1 signaling pathway that specifies the commitment of GC TFH cells.Inducible costimulator (ICOS) signaling fuels the stepwise development of T follicular helper (TFH) cells. However, a signaling pathway unique to ICOS has not been identified. We show that TANK-binding kinase 1 (TBK1) associates with ICOS via a conserved motif, IProx, which shares homology with tumor necrosis factor receptor (TNFR)-associated factors, TRAF2 and TRAF3. Disruption of this motif abolishes the association with TBK1, thus identifying a TBK1-binding consensus. Mutation of this motif in ICOS, or depletion of TBK1 in T cells severely impaired the differentiation of germinal center (GC) TFH, B cell and antibody responses, but was dispensable for early TFH differentiation. These results reveal a novel ICOS-TBK1 signaling pathway that specifies GC TFH cell commitment.

Infection-induced type I interferons activate CD11b on B-1 cells for subsequent lymph node accumulation.

Innate-like B-1a lymphocytes rapidly redistribute to regional mediastinal lymph nodes (MedLNs) during influenza infection to generate protective IgM. Here we demonstrate that influenza infection-induced type I interferons directly stimulate body cavity B-1 cells and are a necessary signal required for B-1 cell accumulation in MedLNs. Vascular mimetic flow chamber studies show that type I interferons increase ligand-mediated B-1 cell adhesion under shear stress by inducing high-affinity conformation shifts of surface-expressed integrins. In vivo trafficking experiments identify CD11b as the non-redundant, interferon-activated integrin required for B-1 cell accumulation in MedLNs. Thus, CD11b on B-1 cells senses infection-induced innate signals and facilitates their rapid sequester into secondary lymphoid tissues, thereby regulating the accumulation of polyreactive IgM producers at sites of infection.

Capicua deficiency induces autoimmunity and promotes follicular helper T cell differentiation via derepression of ETV5

High-affinity antibody production through the germinal centre (GC) response is a pivotal process in adaptive immunity. Abnormal development of follicular helper T (TFH) cells can induce the GC response to self-antigens, subsequently leading to autoimmunity. Here we show the transcriptional repressor Capicua/CIC maintains peripheral immune tolerance by suppressing aberrant activation of adaptive immunity. CIC deficiency induces excessive development of TFH cells and GC responses in a T-cell-intrinsic manner. ETV5 expression is derepressed in Cic null TFH cells and knockdown of Etv5 suppresses the enhanced TFH cell differentiation in Cic-deficient CD4+ T cells, suggesting that Etv5 is a critical CIC target gene in TFH cell differentiation. Furthermore, we identify Maf as a downstream target of the CIC–ETV5 axis in this process. These data demonstrate that CIC maintains T-cell homeostasis and negatively regulates TFH cell development and autoimmunity.