Follicular helper T (Tfh) cells provide selection signals to germinal center B cells, which is essential for long-lived antibody responses. High CXCR5 and low CCR7 expression facilitates their homing to B cell follicles and distinguishes them from T helper 1 (Th1), Th2, and Th17 cells. Here, we showed that Bcl-6 directs Tfh cell differentiation: Bcl-6-deficient T cells failed to develop into Tfh cells and could not sustain germinal center responses, whereas forced expression of Bcl-6 in CD4(+) T cells promoted expression of the hallmark Tfh cell molecules CXCR5, CXCR4, and PD-1. Bcl-6 bound to the promoters of the Th1 and Th17 cell transcriptional regulators T-bet and RORgammat and repressed IFN-gamma and IL-17 production. Bcl-6 also repressed expression of many microRNAs (miRNAs) predicted to control the Tfh cell signature, including miR-17-92, which repressed CXCR5 expression. Thus, Bcl-6 positively directs Tfh cell differentiation, through combined repression of miRNAs and transcription factors.

Recent studies have revealed the importance of multiple microRNAs (miRNAs) in promoting tumorigenesis, among which mir-17-92/Oncomir-1 exhibits potent oncogenic activity. Genomic amplification and elevated expression of mir-17-92 occur in several human B-cell lymphomas, and enforced mir-17-92 expression in mice cooperates with c-myc to promote the formation of B-cell lymphomas. Unlike classic protein-coding oncogenes, mir-17-92 has an unconventional gene structure, where one primary transcript yields six individual miRNAs. Here, we functionally dissected the individual components of mir-17-92 by assaying their tumorigenic potential in vivo. Using the Em-myc model of mouse B-cell lymphoma, we identified miR-19 as the key oncogenic component of mir-17-92, both necessary and sufficient for promoting c-myc-induced lymphomagenesis by repressing apoptosis. The oncogenic activity of miR-19 is at least in part due to its repression of the tumor suppressor Pten. Consistently, miR-19 activates the Akt-mTOR (mammalian target of rapamycin) pathway, thereby functionally antagonizing Pten to promote cell survival. Our findings reveal the essential role of miR-19 in mediating the oncogenic activity of mir-17-92, and implicate the functional diversity of mir-17-92 components as the molecular basis for its pleiotropic effects during tumorigenesis.

IntroductionCD4 ϩ T cells are essential components of the adaptive immune system that regulate immune responses against foreign pathogens and tumors. Upon antigen recognition, naive CD4 ϩ T cells undergo activation and expansion, and then contract via programmed cell death. 1 Specific antigen challenges also induce CD4 ϩ T cells to differentiate into distinct Th cell lineages characterized by unique cytokine production profiles. 2 Among these lineages, Th1 cells, the differentiation of which is controlled by the master transcription factor T-bet, 3 are specialized for the clearance of intracellular infections and are implicated as the major effectors against tumors. 4 In addition, the conversion of effector T cells to Foxp3 ϩ inducible regulatory T cells (iTregs) is an important mechanism used to balance immune responses 5 that is exploited by tumors as a strategy for immune evasion. 6 Whereas the protein-based regulatory machinery that operates during the T-cell response has been vigorously explored, we have recently become aware of a novel and crucial element modulating T-cell function: miRNA. 7,8 miRNAs are 18-to 24-nucleotide noncoding RNAs that regulate gene expression by destabilizing target mRNAs, leading to mRNA degradation and/or translational repression. 9 Recent studies suggest that miRNA-mediated gene regulation represents a fundamental layer of posttranscriptional regulatory programs in metazoan genomes. 10 Global disruption of miRNAs caused by defective biogenesis had profound effects on the development of B cells, 11 Th1/Th2 differentiation, 12,13 and Treg function. 14,15 In addition to these demonstrations of the importance of miRNA biogenesis in general, accumulating evidence shows that many specific miRNAs are differentially regulated in hematopoietic lineages and play important roles in controlling the development and function of immune cells. 7,8,[16][17][18] One such regulator is the miR-17-92 cluster.This cluster of miRNAs is encoded by a polycistronic miRNA gene and generates a single transcript that yields 6 individual mature miRNAs. These miRNAs are categorized into 3 families based on sequence homology: the miR-17 family (miR-17, miR-20, and miR-18a), the miR-19 family (miR-19a and miR-19b), and the miR-25 family (miR-92a; supplemental Figure 1A, available on the Blood Web site; see the Supplemental Materials link at the top of the online article). miR-17-92 is well recognized as an "onco-miR" because of its genomic amplification in certain tumor tissues and its potent acceleration of c-My-induced B-cell lymphoma. 19 Genetic ablation has clearly established the critical roles of miR-17-92 in embryonic development. 20 In immune cells, miR-17-92 plays an integral part in the development of myeloid cells and B cells. 20,21 Mice with germline deletion of miR-17-92 exhibit a severe defect in adult B-cell development with an augmentation of apoptosis in the pro-B-cell fraction and consequently a blockade at the pro-B to pre-B transition. 20 In addition, transgenic mice overexpressing the miR-...

How T cells respond with extraordinary sensitivity to minute amounts of agonist peptide and major histocompatibility complex (pMHC) molecules on the surface of antigen-presenting cells bearing large numbers of endogenous pMHC molecules is not understood. Here we present evidence that CD4 affects the responsiveness of T helper cells by controlling spatial localization of the tyrosine kinase Lck in the synapse. This finding, as well as further in silico and in vitro experiments, led us to develop a molecular model in which endogenous and agonist pMHC molecules act cooperatively to amplify T cell receptor signaling. At the same time, activation due to endogenous pMHC molecules alone is inhibited. A key feature is that the binding of agonist pMHC molecules to the T cell receptor results in CD4-mediated spatial localization of Lck, which in turn enables endogenous pMHC molecules to trigger many T cell receptors. We also discuss broader implications for T cell biology, including thymic selection, diversity of the repertoire of self pMHC molecules and serial triggering.