ICOS signaling is required for inhibition of the transcription factor Klf2, which controls expression of genes expressed by follicular T helper (Tfh) cells. When ICOS signaling is blocked, Tfh cells lose expression of characteristic Tfh genes and revert to an effector phenotype, resulting in disruption of the germinal center response.

After being activated by antigen, helper T lymphocytes switch from a resting state to clonal expansion. This switch requires inactivation of the transcription factor Foxo1, a suppressor of proliferation expressed in resting helper T lymphocytes. In the early antigen-dependent phase of expansion, Foxo1 is inactivated by antigen receptor-mediated post-translational modifications. Here we show that in the late phase of expansion, Foxo1 was no longer post-translationally regulated but was inhibited post-transcriptionally by the interleukin 2 (IL-2)-induced microRNA miR-182. Specific inhibition of miR-182 in helper T lymphocytes limited their population expansion in vitro and in vivo. Our results demonstrate a central role for miR-182 in the physiological regulation of IL-2-driven helper T cell-mediated immune responses and open new therapeutic possibilities.

T-follicular helper (TFH) cells represent the subpopulation of CD4+ T cells that provides help for antigen-specific B cells in the GC response. They are generated from naïve T cells during an immune response and are imprinted by their master transcription factor Bcl-6. It has been a long-standing question if TFH cells contribute to the CD4 + memory pool after the GC response has been terminated. To answer this question, we sorted antigen-specific TFH and non-TFH effector cells from an ongoing GC response and transferred them into naïve mice. Without further signals via the TCR, transferred cells rapidly contracted with a small population of both TFH and non-TFH cells surviving as memory cells in peripheral lymphoid organs for at least 4 weeks in the absence of antigen. TFH cells strongly downregulated their signature genes Bcl-6, CXCR5, and PD-1 in the memory phase. Upon rechallenge with antigen they rapidly upregulated these markers again. An enhanced potential to produce IL-21, paired with higher expression of CXCR5 and lower expression of CCR7, should enable TFH memory cells to provide more efficient help for antigen-specific B cells than their non-TFH counterparts. Immunol. 2012Immunol. . 42: 1981Immunol. -1988 a critical role for the generation of B-cell memory [3]. However, one fundamental question remains: Can TFH cells themselves survive as long-term memory cells as it has been shown for other T-cell subsets [4] or do they simply die after the GC reaction they supported has come to an end? We addressed this important question by sorting antigenspecific TFH cells directly from an ongoing GC response and transferring them into naïve mice. Memory cells derived from these TFH cells were characterized in the memory phase and after rechallenge with antigen. Keywords Results TFH cells require continuous antigen-stimulation for their maintenanceTo analyze the fate of antigen-specific TFH cells, we used an adoptive transfer system with TCR-transgenic cells from either OT-II mice [5] specific for ovalbumin or Smarta mice [6] that have a TCR recognizing a peptide from lymphocytic choriomeningitis virus (LCMV). Expression of the congenic marker Thy-1.1 on the transgenic T cells allows nonambiguous identification of even very small numbers of antigenspecific T cells and sorting of cells without cross-linking their TCR. C57BL/6 recipients of transgenic T cells were injected s.c. with the cognate antigen in CFA. In this system, upregulation of Bcl-6 within CD4 + T cells in the draining lymph node could be observed as early as 48 h after immunization (as reported by others [7,8]; data not shown). By day 5, a distinct population of cells with very high expression of PD-1 and CXCR5, two markers commonly used for defining TFH cells [9], could be clearly identified. The absolute number of TFH cells in the draining lymph node peaked around day 8 and slowly declined afterwards in parallel with the contraction of the GC response (data not shown). At day 8, the TFH phenotype should be fully imprinted and we therefore used th...

T follicular helper (Tfh) cells are a specialized T cell subset that regulates the long-lived production of highly specific Abs by B cells during the germinal center (GC) reaction. However, the transcriptional network sustaining the Tfh cell phenotype and function is still incompletely understood. In this study, we identify the transcription factor Bach2 as a central negative regulator of Tfh cells. Ectopic overexpression of Bach2 in murine Tfh cells resulted in a rapid loss of their phenotype and subsequent breakdown of the GC response. Low Bach2 expression levels are required to maintain high expression of the signature cytokine IL-21, the coinhibitory receptor TIGIT and the transcriptional repressor Bcl-6. In stark contrast to the regulatory network in GC B cells, Bach2 in Tfh cells is not coexpressed with Bcl-6 at high levels to inhibit the antagonizing factor Blimp-1, but suppresses Bcl-6 by direct binding to the promoter. These data reveal that by replacing an activating complex of Batf and Irf-4 at the Bcl-6 promoter, Bach2 regulates the transcriptional network of Tfh cells in a different way, as in GC B cells.

The IL-2/IL-2Ralpha (CD25) axis is of central importance for the interplay of effector and regulatory T cells. Nevertheless, the question how different antigen loads are translated into appropriate IL-2 production to ensure adequate responses against pathogens remains largely unexplored. Here we find that at single cell level, IL-2 is binary (digital) and CD25 is graded expressed whereas at population level both parameters show graded expression correlating with the antigen amount. Combining in vivo data with a mathematical model we demonstrate that only this binary IL-2 expression ensures a wide linear antigen response range for Teff and Treg cells under real spatiotemporal conditions. Furthermore, at low antigen concentrations binary IL-2 expression safeguards by its spatial distribution selective STAT5 activation only of closely adjacent Treg cells regardless of their antigen specificity. These data show that the mode of IL-2 secretion is critical to tailor the adaptive immune response to the antigen amount.DOI: http://dx.doi.org/10.7554/eLife.20616.001

Upon activation by antigen, T lymphocytes switch from a resting state to clonal expansion. This switch requires inactivation of the forkhead transcription factor Foxo1, which suppresses proliferation in resting T helper lymphocytes. In the initial antigen-dependent phase of clonal expansion, Foxo1 is inactivated by antigen receptor-dependent post-translational modifi cations. However, the late antigen-independent phase of clonal expansion is controlled by interleukin 2 (IL2), and it has not been clear how Foxo1 is inactivated in this phase. Here the authors show that in the IL2-dependent late phase of clonal expansion, expression of Foxo1 is downregulated by microRNA-182. Expression of microRNA-182 in activated T lymphocytes is induced by IL2. MicroRNA-182 is expressed between 2 and 4 days after onset of T cell activation. In this time, Foxo1 mRNA expression is reduced by 90%. Specifi c inhibition of microRNA-182 by locked nucleic acids inhibits clonal expansion of activated T lymphocytes in vitro. Ectopic constitutive expression of microRNA-182 results in a ninefold enhanced clonal expansion in vivo. Our results identify IL2-induced expression of microRNA-182 as a critical step regulating the late phase of clonal expansion of activated T helper lymphocytes, and presumably also the transition from expansion to contraction, as future research may show. The central role of a microRNA, here microRNA-182, in the regulation of T cell expansion provides entirely new options for therapeutic interference, namely the use of specifi c antagomiRs to limit T cell expansion in vivo. This may be of relevance in immune-mediated diseases.on 7 May 2018 by guest. Protected by copyright.

The IL-2/IL-2Ralpha (CD25) axis is of central importance for the interplay of effector and regulatory T cells. Nevertheless, the question how different antigen loads are translated into appropriate IL-2 production to ensure adequate responses against pathogens remains largely unexplored. Here we find that at single cell level, IL-2 is binary (digital) and CD25 is graded expressed whereas at population level both parameters show graded expression correlating with the antigen amount. Combining in vivo data with a mathematical model we demonstrate that only this binary IL-2 expression ensures a wide linear antigen response range for Teff and Treg cells under real spatiotemporal conditions. Furthermore, at low antigen concentrations binary IL-2 expression safeguards by its spatial distribution selective STAT5 activation only of closely adjacent Treg cells regardless of their antigen specificity. These data show that the mode of IL-2 secretion is critical to tailor the adaptive immune response to the antigen amount.

Background Autoreactive T cells play a central role for the pathogenesis of autoimmune diseases. They recruit proinflammatory cells of the innate immune system to sites of inflammation and cause tissue destruction through the secretion of cytokines. Even more important, the special subpopulation of T follicular helper cells (TFH) provides help for B cells which is a prerequisite for the production of autoantibodies. The inducible T cell costimulator ICOS is an important regulator of T cell responses. We previously showed that T cells in SLE patients express enhanced levels of ICOS and interact directly with B cells in kidney infiltrates. Objectives To analyze the role of ICOS in the cooperation of T and B cells in detail we moved to an in vivo mouse model which would allow to track antigen-specific T and B cells in all phases of the immune response. Methods Our in vivo T/B cooperation system is based on the adoptive transfer of ovalbumin- (OVA-) specific T cells and nitrophenol- (NP-) specific B cells into syngeneic immunocompetent B6 mice which are immunized subcutaneously with NP coupled to OVA. Antigen-specific T and B cells can be tracked and characterized by flow cytometry and immunohistology. Results Using this system and ICOS-deficient mice we could define two important functions of ICOS. First, the early expansion of antigen-specific T and B cells was significantly reduced in the absence of ICOS costimulation. Second, the differentiation of antigen-specific T cells towards a TFH phenotype was strongly impaired. As a direct consequence the development of germinal center B cells was also diminished. Interestingly, at early time points (day 1-4) T cell activation and TFH differentiation was not influenced by the absence of ICOS signaling. In particular, ICOS-deficient T cells upregulated the master transcription factor Bcl-6 to a normal extent. In addition, expression of other TFH signature molecules like production of IL-21 and downregulation of CCR7 was comparable between wild-type and ICOS knock-out T cells. Antigen-specific ICOS-deficient T cells also migrated normally into B cell follicles. Only at later time points (day 7-10) we observed a dramatic reduction in the number of TFH cells. For this reason, we analyzed the effects of missing ICOS costimulation specifically at late phases of the immune response by applying an anti-ICOS-L monoclonal antibody. Late ICOS-L blockade led to an almost complete disappearance of already differentiated TFH cells within 48 h. However, the absolute number of antigen specific T cells remained constant, indicating that they reversed to a non-TFH effector phenotype. Conclusions Our results clearly show that ICOS is not important for the generation of TFH cells. Instead, continuous ICOS costimulation is required to maintain the TFH phenotype at later phases. In addition, on a more general basis, our data show that the TFH phenotype is not a stable endpoint of T cell differentiation. Disclosure of Interest None Declared