Enterovirus D68 molecular and cellular biology and pathogenesis

Abstract

In recent years, enterovirus D68 (EV-D68) has advanced from a rarely detected respiratory virus to a widespread pathogen responsible for increasing rates of severe respiratory illness and acute flaccid myelitis (AFM) in children worldwide. In this review, we discuss the accumulating data on the molecular features of EV-D68 and place these into the context of enterovirus biology in general. We highlight similarities and differences with other enteroviruses and genetic divergence from own historical prototype strains of EV-D68. These include changes in capsid antigens, host cell receptor usage, and viral RNA metabolism collectively leading to increased virulence. Furthermore, we discuss the impact of EV-D68 infection on the biology of its host cells, and how these changes are hypothesized to contribute to motor neuron toxicity in AFM. We highlight areas in need of further research, including the identification of its primary receptor and an understanding of the pathogenic cascade leading to motor neuron injury in AFM. Finally, we discuss the epidemiology of the EV-D68 and potential therapeutic approaches.

Keywords: acute flaccid myelitis; adaptive immunity; enterovirus; innate immunity; virus replication.

Figure 1

Phylogenetic tree of enterovirus D68 clades. Year of detection is represented on the x-axis. The region in which each isolate was detected is signified by color as per the key in the upper left. Clade names are labeled at the branch points. Visualization of phylogenetic data was performed using Nextstrain (230).

Figure 2

Assembly of the enterovirus D68 capsid. A, Structural proteins VP0, VP1, and VP3 self-assemble into a trimeric structure that forms the basic building blocks of the capsid. Five of these units next assemble into a pentamer. Twelve pentameric units assemble into the procapsid, an icosahedral structure with 60 sides. This structure contains alternating vertices of fivefold and threefold symmetry. Surrounding the fivefold vertex lies a canyon. Within each VP1 subunit in the canyon is a host-derived small hydrophobic molecule, known as the pocket factor. The exact identity of the pocket factor in enetrovirus D68 is not known. B, cross section of the virion showing VPg-bound (+)-ssRNA in the interior, which is loaded into the capsid during the assembly process. The orientation of VP peptides with respect to the interior and exterior faces of the capsid is schematically represented. During maturation of the capsid, VP0 is cleaved to generate VP2 and VP4. In the mature virion, VP4 is oriented primarily toward the interior, whereas VP1, VP2, and VP3 are oriented primarily toward the outer face.

Figure 3

Life cycle of enterovirus D68 (EV-D68). The mature EV-D68 virion attaches to the plasma membrane of the host cell and then undergoes receptor-mediated endocytosis. The capsid then undergoes uncoating, in which a rearrangement to create a pore through the endosomal membrane through which viral (+)-ssRNA enters the cytoplasm. From here, the viral genome is translated into a polypeptide that undergoes further proteolytic processing to generate structural and nonstructural proteins. In addition, (−)-ssRNA is generated by RNA replication, which occurs on vesicular structures known as replication organelles. These RNAs become the template for new copies of the (+)-RNA genome. Virions assemble from structural proteins and VPg-linked RNA as detailed for Figure 2. These immature viral particles are largely taken up by autophagosomes, within which the acidic environment stimulates maturation of the capsid. Mature virions are released either by exocytosis of these autophagic vesicles or by cell lysis and release of nonenveloped viral particles. Red text indicates steps of the enterovirus life cycle in which contemporary EV-D68 strains have been shown to differ from other enteroviruses and/or from historical strains of EV-D68.

Figure 4

Hypothesized mechanisms of neuroinvasion. Enterovirus D68 (EV-D68) is primarily transmitted as a respiratory infection. In a subset of patients, EV-D68 may translocate into the blood stream to establish viremia. The pathway might involve direct entry into the circulation or like poliovirus may occur first via the lymphatic system. After establishing systemic circulation, EV-D68 may enter the central nervous system in one of two ways: (1) direct hematogenous seeding or (2) replication in muscle fibers followed by translocation across the neuromuscular junction and retrograde transport within the motor axon. EV-D68 then likely continues to spread within the spinal cord.