Neurotropic alphaviruses can propagate without capsid

Alphaviruses are a group of enveloped viruses transmitted by mosquitoes. Many alphaviruses are neurotropic, being able to produce encephalitis in humans and animals. Among alphaviruses, both Semliki Forest (SFV) and Sindbis virus (SIN) have been extensively studied as models of viral pathogenicity. In mice, SFV and SIN can infect neurons in the central nervous system and virulent strains induce lethal encephalitis, while avirulent SFV strains induce demyelination [1]. These viruses contain an icosahedral nucleocapsid formed by 240 capsid monomers packaging a positive-strand RNA genome of around 12 kb. The nucleocapsid is surrounded by a lipid envelope coated by viral envelope proteins E1 and E2, or spikes, required for infection. SFV has been used as a model enveloped virus to study different steps in viral infection, including entry, endosomal release, and budding. In fact, SFV was used by Ari Helenius (ETH Zurich, Switzerland) to describe the mechanism by which most enveloped viruses are released from endosomes in infected cells [2]. This process, essential for many human pathogenic viruses like hepatitis C virus (genus Hepacivirus) or Ebola virus (genus Filovirus), is mediated by conformational changes in viral envelope proteins induced by acidic endosomal pH, leading to fusion of viral and endosome membranes, and resulting in cytosolic release of the viral genome [3]. As described for other positive-strand RNA viruses, alphavirus genomes replicate in association with intracellular membranes, leading to very high expression of capsid and spike proteins. Viral budding requires the interaction of pre-assembled nucleocapsids with cytoplasmic domains of viral spike proteins [4]. Although this last process is considered essential in the virus life cycle, we have recently demonstrated that alphavirus budding and propagation is also possible in the absence of capsid protein [5]. This phenomenon, observed for both SFV and SIN, takes place because cells in which capsid-less alphavirus genomes are replicating generate, at the plasma membrane, microvesicles (iMVs) coated by spike proteins capable to uptake viral genomic RNAs, rendering them infectious (Figure 1, left). Interestingly, iMVs use the endocytic pathway described by A. Helenius for wild type (wt) SFV to infect cells (Figure 1, right). A detailed characterization of iMVs showed that they are pleomorphic, possess a