Viral cell-to-cell spread: Conventional and non-conventional ways

Abstract

A critical step in the life cycle of a virus is spread to a new target cell, which generally involves the release of new viral particles from the infected cell which can then initiate infection in the next target cell. While cell-free viral particles released into the extracellular environment are necessary for long distance spread, there are disadvantages to this mechanism. These include the presence of immune system components, the low success rate of infection by single particles, and the relative fragility of viral particles in the environment. Several mechanisms of direct cell-to-cell spread have been reported for animal viruses which would avoid the issues associated with cell-free particles. A number of viruses can utilize several different mechanisms of direct cell-to-cell spread, but our understanding of the differential usage by these pathogens is modest. Although the mechanisms of cell-to-cell spread differ among viruses, there is a common exploitation of key pathways and components of the cellular cytoskeleton. Remarkably, some of the viral mechanisms of cell-to-cell spread are surprisingly similar to those used by bacteria. Here we summarize the current knowledge of the conventional and non-conventional mechanisms of viral spread, the common methods used to detect viral spread, and the impact that these mechanisms can have on viral pathogenesis.

Keywords: Cell-to-cell spread; Filopodia; Nanotubes; Syncytia; Virus.

Fig. 1

Formation of actin tails. Vaccinia virus (VACV) can spread from cell-to-cell through different mechanisms. In VACV infected cells, intracellular enveloped particles are transported to budding sites, where the outer viral membrane fuses with the plasma membrane (A). Extracellular enveloped viral particles remain attached to the plasma membrane and several cellular factors are recruited to the site through a cascade of events initiated by phosphorylation of the A36R protein cytosolic tail (B). Polymerization of F-actin underneath the plasma membrane occurs through activation of the cellular pathways N-WASP/Arp2/3 and FHOD1/Rac1, leading to elongation of actin tails (C). Viral particles can thus reach adjacent cells for rapid direct cell-to-cell spread.

Fig. 2

Syncytia formation. Infected cells (blue) display a high concentration of the viral fusion protein (shown as blue spikes) on cell surfaces. When infected cells come into proximity with an uninfected cell (purple), several sequential steps lead to syncytia formation and spread of infection. (A) Initially, the fusion protein is in an inactive prefusion state; (B) upon the proper stimulus (such as binding a receptor shown in purple on the target cell), the fusion protein is triggered, which induces conformational changes, insertion of a fusion peptide into the adjacent membrane, and conformational changes that bring both membranes, viral and cellular into close proximity, followed by a partial merging known as hemifusion (C). Full merger of both membranes results in the opening of a fusion pore (D) that expands allowing the mixing of cytoplasmic material and spread of viral components between cells (E). Iterations of the fusion process result in formation of large multinucleated syncytia.

Fig. 3

Different strategies of cell-to-cell spread by filopodia. Filopodial bridges have been described as a mechanism for direct cell-to-cell transmission by different viruses. (A) Cells infected with some retroviruses including MLV and avian leukemia virus (ALV) can establish filopodial bridges with uninfected cells, with viral particles surfing along the surface of filopodia toward the cell body to initiate infection. Single as farvirus particles have been observed projected at the tip of filopodial structures (B), contrasting with alphaviruses (C), where multiple particles budding from filopodial extensions facilitate direct cell-to-cell spread. (D) Filamentous pneumovirus particles, including respiratory syncytial virus (RSV) and human metapneumovirus (HMPV), bud from filopodia reaching the surface of adjacent cells for direct cell-to-cell spread. Viral proteins and genome have been detected in filopodia induced by pneumoviruses.

Fig. 4

Formation of intercellular membrane pores. Infection of polarized epithelial cells by measles virus results in the opening of membrane pores between adjacent cells. Intercellular pores are stabilized by interactions between the viral glycoprotein H and the cellular receptor Nectin 4, which is anchored to the cell cytoskeleton through the Afadin protein. It has been suggested that physical constraints due to the presence of the circumapical F-actin ring thwart expansion of the intercellular pores, in contrast to what is observed during syncytia formation. However, the pore has a size that permits direct spread of ribonucleoprotein complexes that can initiate infection in the adjacent cell. Blue dots represent polymerase complexes bound to viral RNA.

Fig. 5

Advantages of cell-to-cell spread. (A) Cell-free viral particles released by budding can be affected by the presence of neutralizing antibodies in the extracellular environment. In contrast, viral particles that are disseminated from cell-to-cell at cell junction-specialized membrane contacts (as one example of cell-to-cell spread) are not accessible to neutralizing antibodies (B). Once they enter a cell, individual viral particles can be targeted by intracellular restriction factors during uncoating. In contrast, multiple viral particles entering a target cell can saturate restriction factors available in the cell and proceed with infection. After successful uncoating, individual viral particles will initiate genome replication starting from single genomes (D), a highly inefficient process. In contrast, several viral genomes coming from particles will be more efficient in replication, leading to a rapid infection. Alternatively, genomes can be transmitted from cell-to-cell directly through intercellular pores (as an example), which can bypass the uncoating step and inhibition by restriction factors, proceeding more efficiently with replication (C).