The conditions leading to the activation/ differentiation of T-helper (Th) cells dedicated for B-cell antibody production are still poorly characterized. We now demonstrate that interleukin-6 (IL-6) promotes the differentiation of naive T lymphocytes into helper cells able to promote B-cell activation and antibody secretion. IL-6-driven acquisition of B-cell help capacity requires expression of the signal trans- IntroductionOn activation by antigen-presenting cells (APCs), naive CD4 ϩ T-helper (Th) precursors can differentiate into functionally distinct T-cell lineages, including Th1, Th2, Th17, and regulatory T (Treg) cells. Among the critical signals that direct the induced patterns of gene expression in maturing helper T-cell subsets are cytokine-induced specific transcription factors. Interleukin-12 (IL-12) regulates Th1 differentiation through activation of the transcription factor signal transducer and activator of transcription 4 (STAT4) and T-bet, 1-3 whereas IL-4 drives Th2 differentiation through the actions of STAT6 and GATA-3. 4,5 Transforming growth factor-␤ (TGF-␤)-induced FoxP-3 is a master regulator of Treg induction, 6 and it has been recently demonstrated that development of Th17 is prompted by a combination of IL-6 plus TGF-␤ and requires expression of STAT3 and the retinoic acid-related orphan receptor ␥t (ROR␥t). 7 The help that T cells provide to B cells is a fundamental feature of mammalian immune systems that allow the production of memory B cells and long-lived plasma cells secreting high-affinity antigen-specific immunoglobulins. T-cell help to B cells was long thought to be attributable to the Th2 subset, based on the superior ability of Th2 clones to support in vitro antibody (Ab) production, and the well-documented capacity of Th2-derived cytokines (such as IL-4) to sustain B-cell growth, differentiation, and isotype switch. 8,9 However, an increasing number of experimental observations cannot be easily reconciled with this simple view. Th1 cells have been indeed shown to support B-cell responses in vitro and in vivo, [10][11][12] and mouse strains in which Th2 differentiation is strongly impaired (such as cMAF, IL-4, and STAT6 KO mice) retain the ability to secrete antibodies in response to T cell-dependent antigens. [13][14][15] More recently, T cells capable of providing help for B cells were identified in human lymphoid tissues through expression of the chemokine receptor CXCR5 and termed follicular helper T cells (T FH ) based on their anatomic localization. 16-18 Follicular CXCR5-expressing T lymphocytes appear to be particularly apt as B-cell helpers, as determined by T/B collaboration assays in vitro. These cells fail to secrete large amounts of Th1-or Th2-like cytokines, express a distinct set of genes, and can therefore not be easily classified as either Th1 or Th2. 19 It is noteworthy, however, that most T cells up-regulate CXCR5 expression on activation 20 and that not all CXCR5 ϩ cells display B-cell help capacity, 18 leaving open the question of whether Th cells for ...

CD4+ CD25 + T reg cells are critical for peripheral tolerance and prevention of autoimmunity. Here we show that CD4 + CD25 + T reg also regulate the magnitude of humoral responses against a panel of T-dependent antigens of foreign origin during both primary and secondary immune responses. Depletion of CD4 + CD25 + T cells leads to increased antigen-specific antibody production and affinity maturation but does not affect T-independent B cell responses, suggesting that CD4 + CD25 + T reg exert a feedback mechanism on non-self antigen-specific antibody secretion by dampening the T cell help for B cell activation. Moreover, we show that CD4 + CD25 + T reg also suppress in vitro B cell immunoglobulin production by inhibiting CD4 + CD25 -T cell help delivery, and that blockade of TGF-b activity abolishes this suppression.

The identification of DC-derived signals orchestrating activation of Th1 and Th17 immune responses has advanced our understanding on how these inflammatory responses develop. However, whether specific signals delivered by DCs also participate in the regulation of Th2 immune responses remains largely unknown. In this study, we show that administration of antigen-loaded, IL-6-deficient DCs to naïve mice induced an exacerbated Th2 response, characterized by the differentiation of GATA-3-expressing T lymphocytes secreting high levels of IL-4, IL-5, and IL-13. Coinjection of wild type and IL-6-deficient bone marrow-derived dendritic cells (BMDCs) confirmed that IL-6 exerted a dominant, negative influence on Th2-cell development. This finding was confirmed in vitro, where exogenously added IL-6 was found to limit IL-4-induced Th2-cell differentiation. iNKT cells were required for optimal Th2-cell differentiation in vivo although their activation occurred independently of IL-6 secretion by the BMDCs. Collectively, these observations identify IL-6 secretion as a major, unsuspected, mechanism whereby DCs control the magnitude of Th2 immunity.Keywords: Airway allergy r DCs r IL-6 r iNKT cells r Th2-cell differentiation Additional supporting information may be found in the online version of this article at the publisher's web-site IntroductionThe role of APCs, and in particular DCs, in regulating naïve Th-cell differentiation into the Th2 lineage remains poorly understood. Indeed, although DCs represent the major source of Th1-and Th17-polarizing cytokines (such as [1], these cells do not appear to produce the most efficient and best characterized pro-Th2 soluble factors (such as Correspondence: Dr. Fabienne Andris e-mail: fandris@ulb.ac.be or thymic stromal lymphopoietin) identified to date. Thus, although numerous studies have illustrated the role of several innate cell types, including basophils and DC subtypes (conventional CD8α − DCs, inflammatory monocyte-derived FcεRI + DCs, CD301b + , PDL2 + , and/or MGL2 + dermal DCs) in driving Th2-cell differentiation in vivo [2][3][4][5][6][7][8][9][10][11][12], the mechanisms by which APCs promote Th2-type responses in vivo remain ill-defined. * These authors contributed equally to this work as first authors. * * These authors contributed equally to this work as senior authors.C 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.eji-journal.eu Eur. J. Immunol. 2014. 44: 3252-3262 Cellular immune response 3253The Th2 pathway has therefore been often considered as a "default pathway," mostly based on the observation that stimulation of naïve T cells in the absence of strong Th1-polarizing conditions (such as lack of APC-derived IL-12 [2,13]) favors the acquisition of a Th2 phenotype in several experimental models. This simple concept has been recently challenged, since several APC-derived, pro-Th2 instructing signals have been identified (OX40L, ICOS-L, the Notch family member Jagged-1, or the T-cell immunoglobulin and mucin domain molecule-4 [14-17]), although their relat...

The generation of high-affinity antibodies and the development of B cell memory are dependent on the help provided by CD4 T cells. Mouse studies indicate that STAT3 signaling in CD4 T cells promotes the acquisition of the B cell help function. However, the role of STAT3 in humans has been controversial. In this study, we show that IL-6 and other STAT3 activating cytokines (IL-21 and IL-27) induce the differentiation of CD4 T cells promoting antibody production by B cells. The acquisition of B cell stimulating properties by naive cord blood CD4 T cells required the STAT3-dependent expression of ICOS and IL-21. Gene reporter and ChIP experiments unambiguously demonstrated that upon IL-6 stimulation, STAT3 induces the transcription of the ICOS gene through direct recruitment to the proximal promoter region indicating that STAT3 acts in part through the direct activation of the ICOS gene.

Following antigen inoculation in naïve transgenic mice, we identified an antigen-specific CD4+ T cell subset expressing high level of both CD62L and CD44 that differ from the well characterized inflammatory cytokine producer CD62Llo CD44hi T cells. Antigen-primed CD62LhiCD44hi T cells induced extensive antibody secretion in B cell cooperation tests despite defective IL-4 and IFN-γ production, while CD62Llo CD44hi T cells secreted high amount of IL-4 and IFN-γ but did not induce optimal IgG secretion by cocultured B cells, suggesting that T cell-dependent antibody secretion by B lymphocytes (Th-B function) could be dissociated from elevated Th1 and Th2 cytokine production. We therefore studied the stimulation requirements leading to humoral and inflammatory immune responses following a dendritic cell-based immunisation protocol. The data obtained suggested that injection of antigen-pulsed mature DC primed naïve T cells for IFN-γ secretion in vivo whereas injection of antigen-pulsed immature DC did not. However, immature DC were fully capable of promoting antigen specific antibody secretion, suggesting that activation protocols described as “suboptimal” for Th cell cytokine secretion induced optimal Th-B capacities. Together, these studies should help in the development of novel vaccination strategies inducing optimal antibody responses while minimizing local inflammatory reactions.

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