Comparison of HAV and HCV infections in vivo and in vitro reveals distinct patterns of innate immune evasion and activation.

BACKGROUND & AIMS: Hepatitis A virus (HAV) infections are considered not to trigger innate immunity in vivo, in contrast to hepatitis C virus (HCV). This lack of induction has been imputed to strong interference by HAV proteases 3CD and 3ABC. We aimed to elucidate the mechanisms of immune activation and counteraction by HAV and HCV in vivo and in vitro. METHODS: Albumin-urokinase-type plasminogen activator/severe combined immunodeficiency (Alb/uPA-SCID) mice with humanised livers were infected with HAV and HCV. Hepatic cell culture models were used to assess HAV and HCV sensing by Toll-like receptor 3 and retinoic acid-inducible gene I/melanoma differentiation-associated protein 5 (RIG-I/MDA5), respectively. Cleavage of the adaptor proteins TIR-domain-containing adapter-inducing interferon-ß (TRIF) and mitochondrial antiviral-signalling protein (MAVS) was analysed by transient and stable expression of HAV and HCV proteases and virus infection. RESULTS: We detected similar levels of interferon-stimulated gene induction in hepatocytes of HAV- and HCV-infected mice with humanised liver. In cell culture, HAV induced interferon-stimulated genes exclusively upon MDA5 sensing and depended on LGP2 (laboratory of genetics and physiology 2). TRIF and MAVS were only partially cleaved by HAV 3ABC and 3CD, not sufficiently to abrogate signalling. In contrast, HCV NS3-4A efficiently degraded MAVS, as previously reported, whereas TRIF cleavage was not detected. CONCLUSIONS: HAV induces an innate immune response in hepatocytes via MDA5/LGP2, with limited control of both pathways by proteolytic cleavage. HCV activates Toll-like receptor 3 and lacks TRIF cleavage, suggesting that this pathway mainly contributes to HCV-induced antiviral responses in hepatocytes. Our results shed new light on the induction of innate immunity and counteraction by HAV and HCV. IMPACT AND IMPLICATIONS: Understanding the mechanisms that determine the differential outcomes of HAV and HCV infections is crucial for the development of effective therapies. Our study provides insights into the interplay between these viruses and the host innate immune response in vitro and in vivo, shedding light on previously controversial or only partially investigated aspects. This knowledge could tailor the development of new strategies to combat HCV persistence, as well as improve our understanding of the factors underlying successful HAV clearance.

Effect of variants in LGP2 on MDA5-mediated activation of interferon response and suppression of hepatitis D virus replication.

BACKGROUND & AIMS: Retinoic acid inducible gene I (RIG-I)-like receptors (RLRs), including RIG-I, melanoma differentiation-associated protein 5 (MDA5), and laboratory of genetics and physiology 2 (LGP2), sense viral RNA to induce the antiviral interferon (IFN) response. LGP2, unable to activate the IFN response itself, modulates RIG-I and MDA5 signalling. HDV, a small RNA virus causing the most severe form of viral hepatitis, is sensed by MDA5. The mechanism underlying IFN induction and its effect on HDV replication is unclear. Here, we aimed to unveil the role of LGP2 and clinically relevant variants thereof in these processes. METHODS: RLRs were depleted in HDV susceptible HepaRGNTCP cells and primary human hepatocytes. Cells were reconstituted to express different LGP2 versions. HDV and IFN markers were quantified in a time-resolved manner. Interaction studies among LGP2, MDA5, and RNA were performed by pull-down assays. RESULTS: LGP2 is essential for the MDA5-mediated IFN response induced upon HDV infection. This induction requires both RNA binding and ATPase activities of LGP2. The IFN response only moderately reduced HDV replication in resting cells but profoundly suppressed cell division-mediated HDV spread. An LGP2 variant (Q425R), predominating in Africans who develop less severe chronic hepatitis D, mediated detectably higher basal and faster HDV-induced IFN response as well as stronger HDV suppression. Mechanistically, LGP2 RNA binding was a prerequisite for the formation of stable MDA5-RNA complexes. MDA5 binding to RNA was enhanced by the Q425R LGP2 variant. CONCLUSIONS: LGP2 is essential to mount an antiviral IFN response induced by HDV and stabilises MDA5-RNA interaction required for downstream signalling. The natural Q425R LGP2 is a gain-of-function variant and might contribute to an attenuated course of hepatitis D. IMPACT AND IMPLICATIONS: HDV is the causative pathogen of chronic hepatitis D, a severe form of viral hepatitis that can lead to cirrhosis and hepatocellular carcinoma. Upon infection, the human immune system senses HDV and mounts an antiviral interferon (IFN) response. Here, we demonstrate that the immune sensor LGP2 cooperates with MDA5 to mount an IFN response that represses HDV replication. We mapped LGP2 determinants required for IFN system activation and characterised several natural genetic variants of LGP2. One of them reported to predominate in sub-Saharan Africans can accelerate HDV-induced IFN responses, arguing that genetic determinants, possibly including LGP2, might contribute to slower disease progression in this population. Our results will hopefully prompt further studies on genetic variations in LGP2 and other components of the innate immune sensing system, including assessments of their possible impact on the course of viral infection.

An RNA-dependent RNA polymerase gene in bat genomes derived from an ancient negative-strand RNA virus.

Endogenous bornavirus-like L (EBLL) elements are inheritable sequences derived from ancient bornavirus L genes that encode a viral RNA-dependent RNA polymerase (RdRp) in many eukaryotic genomes. Here, we demonstrate that bats of the genus Eptesicus have preserved for more than 11.8 million years an EBLL element named eEBLL-1, which has an intact open reading frame of 1,718 codons. The eEBLL-1 coding sequence revealed that functional motifs essential for mononegaviral RdRp activity are well conserved in the EBLL-1 genes. Genetic analyses showed that natural selection operated on eEBLL-1 during the evolution of Eptesicus. Notably, we detected efficient transcription of eEBLL-1 in tissues from Eptesicus bats. To the best of our knowledge, this study is the first report showing that the eukaryotic genome has gained a riboviral polymerase gene from an ancient virus that has the potential to encode a functional RdRp.

Persistent natural infection of a Culex tritaeniorhynchus cell line with a novel Culex tritaeniorhynchus rhabdovirus strain.

Culex tritaeniorhynchus rhabdovirus (CTRV) is a mosquito virus that establishes persistent infection without any obvious cell death. Therefore, occult infection by CTRV can be present in mosquito cell lines. In this study, it is shown that NIID-CTR cells, which were derived from Cx. tritaeniorhynchus, are persistently infected with a novel strain of CTRV. Complete genome sequencing of the infecting strain revealed that it is genetically similar but distinct from the previously isolated CTRV strain, excluding the possibility of contamination. These findings raise the importance of further CTRV studies, such as screening of CTRV in other mosquito cell lines.

Modularized CRISPR/dCas9 effector toolkit for target-specific gene regulation.

The ability to control mammalian genes in a synergistic mode using synthetic transcription factors is highly desirable in fields of tissue engineering, stem cell reprogramming and fundamental research. In this study, we developed a standardized toolkit utilizing an engineered CRISPR/Cas9 system that enables customizable gene regulation in mammalian cells. The RNA-guided dCas9 protein was implemented as a programmable transcriptional activator or repressor device, including targeting of endogenous loci. For facile assembly of single or multiple CRISPR RNAs, our toolkit comprises a modular RNAimer plasmid, which encodes the required noncoding RNA components.