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 ...

*Nicotinamide phosphoribosyl transferase (Nampt)/pre-B cell colony-enhancing factor (PBEF)/visfatin is a protein displaying multiple functional properties. Originally described as a cytokine-like protein able to regulate B cell development, apoptosis, and glucose metabolism, this protein also plays an important role in NAD biosynthesis. To gain insight into its physiological role, we have generated a mouse strain expressing a conditional Nampt allele. Lack of Nampt expression strongly affects development of both T and B lymphocytes. Analysis of hemizygous cells and in vitro cell lines expressing distinct levels of Nampt illustrates the critical role of this protein in regulating intracellular NAD levels. Consequently, a clear relationship was found between intracellular Nampt levels and cell death in response to the genotoxic agent MNNG (N-methyl-N-nitro-N-nitrosoguanidine), confirming that this enzyme represents a key regulator of cell sensitivity to NAD-consuming stress secondary to poly(ADP-ribose) polymerases overactivation. By using mutant forms of this protein and a well-characterized pharmacological inhibitor (FK866), we unequivocally demonstrate that the ability of the Nampt to regulate cell viability during genotoxic stress requires its enzymatic activity. Collectively, these data demonstrate that Nampt participates in cellular resistance to genotoxic/oxidative stress, and it may confer to cells of the immune system the ability to survive during stressful situations such as inflammation.

AMP-activated protein kinase (AMPK), a phylogenetically conserved serine/threonine protein kinase, represents an energy sensor able to adapt cellular metabolism in response to nutritional environmental variations. TCR stimulation activates AMPK, a regulatory event that is known to stimulate ATP-producing processes, possibly in anticipation of the increased energetic needs associated with cell division and expression of effector function. Taking advantage of the selective expression of the AMPKa1 catalytic subunit in lymphoid cells, we have analyzed the in vitro and in vivo capacity of lymphocytes lacking AMPK activity (AMPKa1-KO cells) to respond to metabolic stress and to initiate and sustain an immune response. AMPKa1-KO cells displayed increasing sensitivity to energetic stress in vitro, and were found unable to maintain adequate ATP levels in response to ATP synthase inhibition. These cells were, however, able to respond to antigen stimulation in vitro, as shown by optimal proliferation and cytokine production. Similarly, AMPKa1-KO mice were fully immunocompetent in vivo and displayed normal cell proliferation, humoral, cytotoxic and delayed-type hypersensitivity (DTH) responses following antigen injection. In conclusion, AMPK represents an important enzyme allowing lymphocytes to resist a mild energy crisis in vitro, but is largely dispensable for activation and expression of effector function in response to antigen stimulation.

Follicular helper T cells (Tfh) have been identified as the primary cell subpopulation regulating B cell responses in germinal centers, thus supporting high-affinity antibody production. Among the transcription factors orchestrating Tfh cell differentiation and function, the role played by the proto-oncogene c-Maf remains poorly characterized. We report herein that selective loss of c-Maf expression in the T cell compartment results in defective development of Tfh cells in response to both antigen/adjuvant vaccinations and commensal intestinal bacteria. Accordingly, c-Maf expression in T cells was essential for the development and high-affinity antibody secretion in vaccinated animals. c-Maf was expressed early, concomitantly to BCL6, in Tfh cell precursors and found to regulate Tfh fate in a cell-autonomous fashion. Altogether, our findings reveal a novel, non-redundant, function for c-Maf in the differentiation of Tfh cells and the regulation of humoral immune responses to T-cell-dependent antigens.

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.