C.X. their function in this Senegenin process. MicroRNAs (miRNAs) are endogenously encoded small RNAs of ~22 nucleotides in length that play important roles in a large diversity of biological processes1,2,3. Genetic studies have shown that miRNAs are important regulators in the immune system4,5. However, the functions of individual miRNAs during lymphocyte development and effector cell differentiation remain mainly unfamiliar. miR-17~92, miR-106a~363, and miR-106b~25 are users of a family of highly conserved miRNAs, the miR-17~92 family6. Collectively, these three clusters encode for thirteen unique miRNAs, which belong to four miRNA subfamilies (miR-17, miR-18, miR-19, and miR-92 subfamilies). Users in each subfamily share a common seed region (nucleotides 2-7 of adult miRNAs) and are thought to have similar functions. Germline deletion of miR-17~92 led to perinatal lethality of mutant mice. While ablation of miR-106a~363 or miR-106b~25 experienced no obvious phenotypic consequence, compound mutant embryos lacking both miR-17~92 and miR-106b~25 died before embryonic day time 15, with defective development of lung, heart, central nervous system, and B lymphocytes7. These Senegenin genetic studies exposed essential and overlapping functions of miR-17~92 family miRNAs in many developmental processes. T cell help is essential for humoral immune responses. A distinct CD4+ effector T cell subset, T follicular helper cells (TFH), provides this help to B cells8. However, molecular mechanisms underlying TFH differentiation are still mainly unfamiliar. Bcl-6 was identified as a critical transcription element regulating TFH differentiation9,10,11. A recent study reported that Bcl-6 represses the manifestation of miR-17~92, which focuses on the manifestation of CXCR5, a chemokine receptor essential for CD4+ T cell migration to B cell follicles, and suggested that miR-17~92 functions as a negative regulator of TFH differentiation (the repression of the repressors model)11. Here we explore the part of miR-17~92 family miRNAs in TFH differentiation and germinal center reaction using mice with loss- and gain-of function mutations for those miRNAs. We found that these miRNAs function as essential positive regulators of TFH differentiation by controlling CD4+ T cell migration into B cell follicles, and recognized Phlpp2 as an important mediator of their function in this process. RESULTS The miR-17~92 family regulates TFH differentiation We 1st examined the manifestation of miR-17~92 family miRNAs during TFH differentiation. Consistent with a earlier statement11, their manifestation in TFH cells was lower than in naive CD4+ T cells at day time 7 after OVA+Alum+LPS immunization (Fig. 1a). When naive CD4+ T cells were triggered < 0.05; **, < 0.01. To examine whether the jeopardized TFH differentiation in CD4tKO mice reflected a cell-intrinsic miRNA function, we generated WT:CD4tKO mixed bone marrow chimeras and immunized them with NP-OVA+Alum+LPS. Although WT CD4+ T cells differentiated into TFH cells, CD4tKO CD4+ T cells contributed very little to the TFH cell pool in chimeric mice (Fig. 1h). In contrast, dKO CD4+ T cells and B cells underwent relatively normal TFH and GCB cell differentiation in WT:dKO chimeras (Supplementary Fig. 1d). These results demonstrate that miR-17~92 family miRNAs function as CD4+ T cell-intrinsic positive regulators of TFH cell differentiation. CD4tKO mice do not control chronic viral illness Recent studies suggested that Senegenin TFH cells play important roles in controlling chronic virus illness13,14. Illness of mice with a high dose of lymphocytic choriomeningitis disease IL2RG (LCMV) clone-13 (2 x 106 PFU i.v.) resulted in a chronic illness, with disease persisting in multiple cells for 3C4 weeks15. Illness of CD4tKO mice with LCMV clone-13 resulted in reduced TFH differentiation (Fig. 2a, b), GCB formation (Fig. 2c), and 3~6 fold reduction in production of LCMV-specific IgG antibodies (Fig. 2d and Supplementary Fig. 3a). CD4tKO CD4+ T cells were severely impaired in their ability to create IL-21 (Fig. 2e), a cytokine critical for TFH differentiation, GCB formation, and functional CD8+ T cell reactions during chronic viral illness16,17,18,19,20,21,22. We also investigated CD8+ T cell reactions during LCMV clone-13 illness. No significant difference in the percentage or total numbers of virus-specific GP33- or GP276-CD8+ T cells was observed when comparing CD4tKO to WT mice (Supplementary Fig. 3b, c). However, virus specific CD4tKO.