Flavivirus assembly is driven by the envelope glycoproteins pre-membrane (prM) and envelope (E) in the neutral pH environment of the endoplasmic reticulum. Newly budded, spiky particles are exported through the Golgi apparatus, where mildly acidic pH induces a major surface rearrangement. The glycoproteins reorganize into (prM/E)\₂ complexes at the surface of smooth particles, with prM trapped at the E dimer interface, thereby exposing a furin cleavage site (FCS) for proteolytic maturation into infectious virions. Here, we show that in the absence of furin, immature tick-borne flavivirus particles-tick-borne encephalitis virus, Langat virus, and Louping ill virus-remain fully infectious and pathogenic in female BALB/c mice, in contrast to mosquito-borne flaviviruses such as Usutu, West Nile, and Zika viruses. We further show that the FCS in tick-borne viruses remains exposed at neutral pH, allowing furin at the surface of target cells to activate viral fusogenicity, while mosquito-borne counterparts require acidic re-exposure. Mutations increasing the dynamic behavior of the E dimer mimic the mosquito-borne phenotype, with retracted FCS at neutral pH and loss of infectivity. Our multidisciplinary approach-combining virological assays, targeted mutagenesis, structural modeling, and molecular dynamics simulations-highlights the role of E dimer dynamics in regulating flavivirus maturation and infectivity.

• Publication type
• Journal Article MeSH

INTRODUCTION: The Pathogen Infection Hypothesis proposes that β-Amyloid (Aβ) functions as an antimicrobial peptide, with pathogen-induced aggregation potentially contributing to Alzheimer's disease (AD) pathology. METHODS: We used human iPSC-derived 2D neurons and 3D cerebral organoids from wild-type and familial AD (PSEN1/2 mutant) lines to model acute infections with HSV-1 and TBEV and Aβ aggregation. Transcriptomic and proteomic analyses were conducted to assess molecular responses. RESULTS: HSV-1, but not TBEV, induced robust Aβ clustering, which was, however, dependent on extracellular amyloid peptides. Transcriptomic profiling revealed widespread HSV-1-induced changes, including activation of neurodegeneration-related pathways. Proteomic profiling confirmed enrichment of neurodegeneration- and senescence-associated secretome signatures. PSEN1/2 mutations did not alter the acute infection response. Reanalysis of independent datasets confirmed our findings and revealed a limited protective effect of acyclovir. DISCUSSION: Results directly support the Pathogen Infection Hypothesis and suggest that preventing viral infections via vaccinations may represent a feasible approach to reducing AD risk.

• Keywords
• Alzheimer’s disease, Cerebral organoids, Herpes virus, Senescence, Tick-borne Encephalitis,
• Publication type
• Journal Article MeSH
• Preprint MeSH

Tick-borne encephalitis (TBE) virus (TBEV) is transmitted to humans via tick bites. Infection is benign in >90% of the cases but can cause mild (<5%), moderate (<4%), or severe (<1%) encephalitis. We show here that ∼10% of patients hospitalized for severe TBE in cohorts from Austria, Czech Republic, and France carry auto-Abs neutralizing IFN-α2, -β, and/or -ω at the onset of disease, contrasting with only ∼1% of patients with moderate and mild TBE. These auto-Abs were found in two of eight patients who died and none of 13 with silent infection. The odds ratios (OR) for severe TBE in individuals with these auto-Abs relative to those without them in the general population were 4.9 (95% CI: 1.5-15.9, P < 0.0001) for the neutralization of only 100 pg/ml IFN-α2 and/or -ω, and 20.8 (95% CI: 4.5-97.4, P < 0.0001) for the neutralization of 10 ng/ml IFN-α2 and -ω. Auto-Abs neutralizing type I IFNs accounted for ∼10% of severe TBE cases in these three European cohorts.

• MeSH
• Autoantibodies * immunology   MeSH
• Adult MeSH
• Interferon Type I * immunology   MeSH
• Encephalitis, Tick-Borne * immunology   MeSH
• Middle Aged MeSH
• Humans MeSH
• Antibodies, Neutralizing * immunology   MeSH
• Aged MeSH
• Encephalitis Viruses, Tick-Borne immunology   MeSH
• Check Tag
• Adult MeSH
• Middle Aged MeSH
• Humans MeSH
• Male MeSH
• Aged MeSH
• Female MeSH
• Publication type
• Journal Article MeSH
• Geographicals
• Czech Republic MeSH
• Austria epidemiology   MeSH
• Names of Substances
• Autoantibodies * MeSH
• Interferon Type I * MeSH
• Antibodies, Neutralizing * MeSH

We present structures of three immature tick-borne encephalitis virus (TBEV) isolates. Our atomic models of the major viral components, the E and prM proteins, indicate that the pr domains of prM have a critical role in holding the heterohexameric prM3E3 spikes in a metastable conformation. Destabilization of the prM furin-sensitive loop at acidic pH facilitates its processing. The prM topology and domain assignment in TBEV is similar to the mosquito-borne Binjari virus, but is in contrast to other immature flavivirus models. These results support that prM cleavage, the collapse of E protein ectodomains onto the virion surface, the large movement of the membrane domains of both E and M, and the release of the pr fragment from the particle render the virus mature and infectious. Our work favors the collapse model of flavivirus maturation warranting further studies of immature flaviviruses to determine the sequence of events and mechanistic details driving flavivirus maturation.

• MeSH
• Flavivirus physiology   MeSH
• Encephalitis, Tick-Borne virology   MeSH
• Humans MeSH
• Models, Molecular MeSH
• Viral Envelope Proteins * chemistry   metabolism   MeSH
• Virion MeSH
• Encephalitis Viruses, Tick-Borne * physiology   MeSH
• Animals MeSH
• Check Tag
• Humans MeSH
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH
• Names of Substances
• Viral Envelope Proteins * MeSH

The tick-borne encephalitis virus (TBEV) causes a most important viral life-threatening illness transmitted by ticks. The interactions between the virus and ticks are largely unexplored, indicating a lack of experimental tools and systematic studies. One such tool is recombinant reporter TBEV, offering antibody-free visualization to facilitate studies of transmission and interactions between a tick vector and a virus. In this paper, we utilized a recently developed recombinant TBEV expressing the reporter gene mCherry to study its fitness in various tick-derived in vitro cell cultures and live unfed nymphal Ixodes ricinus ticks. The reporter virus was successfully replicated in tick cell lines and live ticks as confirmed by the plaque assay and the mCherry-specific polymerase chain reaction (PCR). Although a strong mCherry signal determined by fluorescence microscopy was detected in several tick cell lines, the fluorescence of the reporter was not observed in the live ticks, corroborated also by immunoblotting. Our data indicate that the mCherry reporter TBEV might be an excellent tool for studying TBEV-tick interactions using a tick in vitro model. However, physiological attributes of a live tick, likely contributing to the inactivity of the reporter, warrant further development of reporter-tagged viruses to study TBEV in ticks in vivo.

• Keywords
• Ixodes ricinus, TBEV, mCherry reporter, tick cell culture, tick-borne encephalitis virus, ticks, viral reverse genetics,
• MeSH
• Cell Line MeSH
• Ixodes * MeSH
• Encephalitis, Tick-Borne * MeSH
• Polymerase Chain Reaction MeSH
• Models, Theoretical MeSH
• Encephalitis Viruses, Tick-Borne * genetics   MeSH
• Animals MeSH
• Check Tag
• Animals MeSH
• Publication type
• Journal Article MeSH
• Research Support, Non-U.S. Gov't MeSH