Truncation of the Cytoplasmic Tail of Equine Infectious Anemia Virus Increases Virion Production by Improving Env Cleavage and Plasma Membrane Localization

Adaptive truncation of the S gene in IBV during chicken embryo passaging plays a crucial role in its attenuation

Like all coronaviruses, infectious bronchitis virus, the causative agent of infectious bronchitis in chickens, exhibits a high mutation rate. Adaptive mutations that arise during the production of live attenuated vaccines against IBV often decrease virulence. The specific impact of these mutations on viral pathogenicity, however, has not been fully elucidated. In this study, we identified a mutation at the 3' end of the S gene in an IBV strain that was serially passaged in chicken embryos, and showed that this mutation resulted in a 9-aa truncation of the cytoplasmic tail (CT) of the S protein. This phenomenon of CT truncation has previously been observed in the production of attenuated vaccines against other coronaviruses such as the porcine epidemic diarrhea virus. We next discovered that the 9-aa truncation in the S protein CT resulted in the loss of the endoplasmic-reticulum-retention signal (KKSV). Rescue experiments with recombinant viruses confirmed that the deletion of the KKSV motif impaired the localization of the S protein to the endoplasmic-reticulum-Golgi intermediate compartment (ERGIC) and increased its expression on the cell surface. This significantly reduced the incorporation of the S protein into viral particles, impaired early subgenomic RNA and protein synthesis, and ultimately reduced viral invasion efficiency in CEK cells. In vivo experiments in chickens confirmed the reduced pathogenicity of the mutant IBV strains. Additionally, we showed that the adaptive mutation altered the TRS-B of ORF3 and impacted the transcriptional regulation of this gene. Our findings underscore the significance of this adaptive mutation in the attenuation of IBV infection and provide a novel strategy for the development of live attenuated IBV vaccines.

Host cell restriction factors of equine infectious anemia virus

Equine infectious anemia virus (EIAV) is a member of the lentivirus genus in the Retroviridae family and is considered an animal model for HIV/AIDS research. An attenuated EIAV vaccine, which was successfully developed in the 1970s by classical serial passage techniques, is the first and only lentivirus vaccine that has been widely used to date. Restriction factors are cellular proteins that provide an early line of defense against viral replication and spread by interfering with various critical steps in the viral replication cycle. However, viruses have evolved specific mechanisms to overcome these host barriers through adaptation. The battle between the viruses and restriction factors is actually a natural part of the viral replication process, which has been well studied in human immunodeficiency virus type 1 (HIV-1). EIAV has the simplest genome composition of all lentiviruses, making it an intriguing subject for understanding how the virus employs its limited viral proteins to overcome restriction factors. In this review, we summarize the current literature on the interactions between equine restriction factors and EIAV. The features of equine restriction factors and the mechanisms by which the EIAV counteract the restriction suggest that lentiviruses employ diverse strategies to counteract innate immune restrictions. In addition, we present our insights on whether restriction factors induce alterations in the phenotype of the attenuated EIAV vaccine.

A Novel, Fully Spliced, Accessory Gene in Equine Lentivirus with Distinct Rev-Responsive Element

All lentiviruses encode the accessory protein Rev, whose main biological function is to mediate the nuclear export of unspliced and incompletely spliced viral transcripts by binding to a viral cis-acting element (termed the Rev-responsive element, RRE) within the env-encoding region. Equine infectious anemia virus (EIAV) is a member of the lentivirus genus in the Retroviridae family and is considered an important model for the study of lentivirus pathogenesis. Here, we identified a novel transcript from the EIAV genome that encoded a viral protein, named Mat, with an unknown function. The transcript mat was fully spliced and comprised parts of the coding regions of MA and TM. Interestingly, the expression of Mat depended on Rev and the chromosome region maintenance 1 (CRM1) pathway. Rev could specifically bind to Mat mRNA to promote its nuclear export. We further identified that the first exon of Mat mRNA, which was located within the Gag-encoding region, acted as an unreported RRE. Altogether, we identified a novel fully spliced transcript mat with an unusual RRE, which interacted with Rev for nuclear export through the CRM1 pathway. These findings updated the EIAV genome structure, highlighted the diversification of posttranscriptional regulation patterns in EIAV, and may help to expand the understanding of gene transcription and expression of lentivirus. IMPORTANCE In lentiviruses, the nuclear export of viral transcripts is an important step in controlling viral gene expression. Generally, the unspliced and incompletely spliced transcripts are exported via the CRM1-dependent export pathway in a process mediated by the viral Rev protein by binding to the Rev-responsive element (RRE) located within the Env-coding region. However, the completely spliced transcripts are exported via an endogenous cellular pathway, which was Rev independent. Here, we identified a novel fully spliced transcript from EIAV and demonstrated that it encoded a viral protein, termed Mat. Interestingly, we determined that the expression of Mat depended on Rev and identified that the first exon of Mat mRNA could specifically bind to Rev and be exported to the cytoplasm, which suggested that the first exon of Mat mRNA was a second RRE of EIAV. These findings provided important insights into the Rev-dependent nuclear export of completely spliced transcripts in lentiviruses.