Frank Dahlstroem

The live-attenuated yellow fever vaccine 17D-204 (YF17D) activates robust innate immune responses followed by rapid induction of adaptive immunity resulting in long-lasting protection. YF17D triggers the production of type I interferons (IFNs) which have a dual role in antigen presenting cells regulating their infection and contributing to their activation. Infection with YF17D was detected in primary human blood monocytes and conventional dendritic cells (DCs) and in monocyte-derived DCs but was highly restricted by type I IFN. Blocking IFNAR signaling in YF17D-infected PBMC from vaccinated donors resulted in increased activation of YF17D-specific CD8 ⁺ T cells. Consistently, peak IFN-alpha plasma levels correlated inversely with the CD8 ⁺ T cells response in YF17D vaccinees. Loss of function experiments demonstrated a dominant role of retinoic acid inducible gene I (RIG-I)-like receptors (RLRs) and mitochondrial antiviral signaling protein (MAVS) for type I IFN induction and restriction of YF17D. The type I IFN response was mediated by 5’ tri- or diphosphate dsRNA intermediates that are formed during YF17D infection. In vivo proximity labelling (IPL) of RIG-I and next-generation sequencing confirmed interaction of RIG-I with YF17D-dsRNA in infected cells. Thus, YF17D-triggered RLR-signaling restricts viral replication through type I IFN and thus limits the production of viral antigens that can be presented to T cells.

T follicular helper (T _FH ) cells are essential for effective antibody responses, but deciphering the intrinsic wiring of mouse T _FH cells has long been hampered by the lack of a reliable protocol for their generation in vitro. We report that transforming growth factor–β (TGF-β) induces robust expression of T _FH hallmark molecules CXCR5 and Bcl6 in activated mouse CD4 ⁺ T cells in vitro. TGF-β–induced mouse CXCR5 ⁺ T _FH cells are phenotypically, transcriptionally, and functionally similar to in vivo–generated T _FH cells and provide critical help to B cells. The study further reveals that TGF-β–induced CXCR5 expression is independent of Bcl6 but requires the transcription factor c-Maf. Classical TGF-β–containing T helper 17 (T _H 17)–inducing conditions also yield separate CXCR5 ⁺ and IL-17A–producing cells, highlighting shared and distinct cell fate trajectories of T _FH and T _H 17 cells. We demonstrate that excess IL-2 in high-density T cell cultures interferes with the TGF-β–induced T _FH cell program, that T _FH and T _H 17 cells share a common developmental stage, and that c-Maf acts as a switch factor for T _FH versus T _H 17 cell fates in TGF-β–rich environments in vitro and in vivo.

Human pathogenic flaviviruses pose a significant health concern and vaccination is the most effective instrument to control their circulation. How pre-existing immunity to antigenically related viruses modulates immunization outcome remains poorly understood. In this study, we evaluated the effect of vaccination against tick-borne encephalitis virus (TBEV) on the epitope immunodominance and immunogenicity of the yellow fever 17D vaccine (YF17D) in a cohort of 250 human vaccinees. Following YF17D vaccination, all study participants seroconverted and generated protective neutralizing antibody titers. At day 28, TBEV pre-immunity did not affect the polyclonal neutralizing response which largely depended on the IgM fraction. We found that sera from TBEV-immunized individuals enhanced YF17D vaccine virus infection via antibody-dependent enhancement (ADE). Upon vaccination, individuals with TBEV pre-immunity had higher concentrations of cross-reactive IgG antibodies with limited neutralizing capacity against YF17D whereas vaccinees without prior flavivirus exposure showed a non-cross-reacting response. Using a set of recombinant YF17D envelope protein mutants displaying different epitopes, we identified quaternary epitopes as the primary target of neutralizing antibodies. Sequential immunizations redirected the IgG response towards the pan-flavivirus fusion loop epitope (FLE) with the potential to mediate enhancement of dengue and Zika virus infections whereas TBEV naïve individuals elicited an IgG response directed towards neutralizing epitopes without an enhancing effect. We propose that the YF17D vaccine effectively conceals the FLE and primes a neutralizing IgG response in individuals with no prior flavivirus exposure. In contrast, the response in TBEV-experienced recipients favors weakly-neutralizing, cross-reactive epitopes potentially increasing the risk of severe dengue and Zika disease due to ADE.

Constitutive T cell-intrinsic miRNA expression is required for the differentiation of naive CD4(+) T cells into Tfh cells, thus making it difficult to study the role of miRNAs in the maintenance of already established Tfh cells and ongoing germinal center (GC) responses. To overcome this problem, we here used temporally controlled ablation of mature miRNAs specifically in CD4(+) T cells during acute LCMV infection in mice. T cell-intrinsic miRNA expression was not only critical at early stages of Tfh cell differentiation, but also important for the maintenance of already established Tfh cells. In addition, CD4(+) T cell-specific ablation of miRNAs resulted in impaired GC B cell responses. Notably, miRNA deficiency also compromised the antigen-specific CD4(+) T cell compartment, Th1 cells, Treg cells, and Tfr cells. In conclusion, our results highlight miRNAs as important regulators of Tfh cells, thus providing novel insights into the molecular events that govern T cell-B cell interactions and Th cell identity.