Mapping Attenuation Determinants in Enterovirus-D68

Immune cells promote paralytic disease in mice infected with enterovirus D68

Enterovirus D68 (EV-D68) is associated with acute flaccid myelitis (AFM), a poliomyelitis-like illness causing paralysis in young children. However, mechanisms of paralysis are unclear, and antiviral therapies are lacking. To better understand EV-D68 disease, we inoculated newborn mice intracranially to assess viral tropism, virulence, and immune responses. Wild-type (WT) mice inoculated intracranially with a neurovirulent strain of EV-D68 showed infection of spinal cord neurons and developed paralysis. Spinal tissue from infected mice revealed increased levels of chemokines, inflammatory monocytes, macrophages, and T cells relative to controls, suggesting that immune cell infiltration influences pathogenesis. To define the contribution of cytokine-mediated immune cell recruitment to disease, we inoculated mice lacking CCR2, a receptor for several EV-D68-upregulated cytokines, or RAG1, which is required for lymphocyte maturation. WT, Ccr2 -/- , and Rag1 -/- mice had comparable viral titers in spinal tissue. However, Ccr2 -/- and Rag1 -/- mice had significantly less paralysis relative to WT mice. Consistent with impaired T cell recruitment to sites of infection in Ccr2 -/- and Rag1 -/- mice, antibody-mediated depletion of CD4 + or CD8 + T cells from WT mice diminished paralysis. These results indicate that immune cell recruitment to the spinal cord promotes EV-D68-associated paralysis and illuminate new targets for therapeutic intervention.

Development of the first officially licensed live attenuated duck hepatitis A virus type 3 vaccine strain HB80 in China and its protective efficacy against DHAV-3 infection in ducks

Duck hepatitis A virus type 3 (DHAV-3) is an infectious virus that is highly fatal to ducklings and causes significant economic losses in the duck industry worldwide. Biosecurity and vaccination are required to control the pathogen. In the present study, we attenuated a lowly pathogenic DHAV-3 clinical isolate, named as HB, by serial passaging in duck embryos, and followed by several adaptive proliferations in specific-pathogen-free (SPF) chicken embryos. The virulence of DHAV-3 at different passages was assessed by infecting 3-day-old ducklings. We found that the HB strain lost pathogenicity to ducklings from the 55th passage onwards. The 80th passage strain (HB80), which achieved good growth capacity in duck embryos with a viral titer of 108.17 50% egg lethal dose per milliliter (ELD50/mL), was selected as a live attenuated vaccine candidate. The HB80 strain did not induce clinical symptoms or pathological lesions in 3-day-old ducklings and showed no virulence reversion after 5 rounds of in vivo back-passage. The minimum effective dose of HB80 was determined to be 104.5 ELD50 by hypodermic inoculation of the neck. Importantly, a single dose of HB80 elicited good immune responses and provided complete protection against challenge with the lethal DHAV-3 strain. Compared with the genomic sequence of the parental HB strain, HB80 had 7 amino acid substitutions, two of them are in the hypervariable region of the VP1 and polymerase-encoding 3D regions, which may play a role in virulence attenuation. Our data suggest that the attenuated HB80 strain is a promising vaccine candidate for the prevention of DHAV-3 infections in China. HB80 has been registered as a New Veterinary Drug Registration Certificate by the Chinese Ministry of Agriculture and Rural Affairs (MARA), and is the first live attenuated DHAV-3 vaccine strain to be officially licensed in China.

Characterization of Effect of Enterovirus D68 in 129/Sv Mice Deficient in IFN-α/β and/or IFN-γ Receptors

Enterovirus D68 (EV-D68), a respiratory RNA virus in the family Picornaviridae, is implicated as a potential etiological agent for acute flaccid myelitis in preteen adolescents. The absence of a specific therapeutic intervention necessitates the development of an effective animal model for EV-D68. The AG129 mouse strain, characterized by the double knockout of IFN-α/β and IFN-γ receptors on the 129 genetic background, has been proposed as a suitable model for EV-D68. The goals of this study were to assess the effect of a nonmouse-adapted EV-D68 strain (US/MO/14-18947, NR-49129) in AG129 (IFN-α/β and IFN-γ receptors null), A129 (IFN-α/β receptor null), G129 (IFN-γ receptor null), and the 129 background strain (129S2/SvPasCrl) when infected intraperitoneally at 10 d of age. Both AG129 and A129 strains demonstrated similar clinical signs (paralysis, paresis, lethargy, dyspnea [characterized by prominent abdominal respiration], and morbidity requiring euthanasia) induced by EV-D68. While G129 and 129S2 strains also exhibited susceptibility to EV-D68, the severity of clinical signs was less than in AG129 and A129 strains, and many survived to the experimental endpoint. Histopathological and immunohistochemical data confirmed EV-D68 tropism for the skeletal muscle and spinal cord and suggest that the dyspnea observed in infected mice could be attributed, in part, to lesions in the diaphragmatic skeletal muscles. These findings contribute valuable insights into the pathogenesis of EV-D68 infection in this mouse model and provide investigators with key information on virus dose and mouse strain selection when using this mouse model to evaluate candidate EV-D68 therapeutics.

VP1 is the primary determinant of neuropathogenesis in a mouse model of enterovirus D68 acute flaccid myelitis

Enterovirus D68 (EV-D68) is an emerging pathogen that can cause severe respiratory and neurologic disease [acute flaccid myelitis (AFM)]. Intramuscular (IM) injection of neonatal Swiss Webster (SW) mice with US/IL/14-18952 (IL52), a clinical isolate from the 2014 EV-D68 epidemic, results in many of the pathogenic features of human AFM, including viral infection of the spinal cord, death of motor neurons, and resultant progressive paralysis. In distinction, CA/14-4231 (CA4231), another clinical isolate from the 2014 EV-D68 outbreak, does not cause paralysis in mice, does not grow in the spinal cord, and does not cause motor neuron loss following IM injection. A panel of chimeric viruses containing sequences from IL52 and CA4231 was used to demonstrate that VP1 is the main determinant of EV-D68 neurovirulence following IM injection of neonatal SW mice. VP1 contains four amino acid differences between IL52 and CA4231. Mutations resulting in substituting these four amino acids (CA4231 residues into the IL52 polyprotein) completely abolished neurovirulence. Conversely, mutations resulting in substituting VP1 IL52 amino acid residues into the CA4231 polyprotein created a virus that induced paralysis to the same degree as IL52. Neurovirulence following infection of neonatal SW mice with parental and chimeric viruses was associated with viral growth in the spinal cord.

IMPORTANCE

Emerging viruses allow us to investigate mutations leading to increased disease severity. Enterovirus D68 (EV-D68), once the cause of rare cases of respiratory illness, recently acquired the ability to cause severe respiratory and neurologic disease. Chimeric viruses were used to demonstrate that viral structural protein VP1 determines growth in the spinal cord, motor neuron loss, and paralysis following intramuscular (IM) injection of neonatal Swiss Webster (SW) mice with EV-D68. These results have relevance for predicting the clinical outcome of future EV-D68 epidemics as well as targeting retrograde transport as a potential strategy for treating virus-induced neurologic disease.

Structure and dynamics of enterovirus genotype networks

Like all biological populations, viral populations exist as networks of genotypes connected through mutation. Mapping the topology of these networks and quantifying population dynamics across them is crucial to understanding how populations adapt to changes in their selective environment. The influence of mutational networks is especially profound in viral populations that rapidly explore their mutational neighborhoods via high mutation rates. Using a single-cell sequencing method, scRNA-seq-enabled acquisition of mRNA and consensus haplotypes linking individual genotypes and host transcriptomes (SEARCHLIGHT), we captured and assembled viral haplotypes from hundreds of individual infected cells, revealing the complexity of viral population structures. We obtained these genotypes in parallel with host cell transcriptome information, enabling us to link host cell transcriptional phenotypes to the genetic structures underlying virus adaptation. Our examination of these structures reveals the common evolutionary dynamics of enterovirus populations and illustrates how viral populations reach through mutational "tunnels" to span evolutionary landscapes and maintain connection with multiple adaptive genotypes simultaneously.

In vitro and in vivo models for the study of EV-D68 infection

Enterovirus D68 (EV-D68) is one of hundreds of non-polio enteroviruses that typically cause cold-like respiratory illness. The first EV-D68 outbreak in the United States in 2014 aroused widespread concern among the public and health authorities. The infection was found to be associated with increased surveillance of acute flaccid myelitis, a neurological condition that causes limb paralysis in conjunction with spinal cord inflammation. In vitro studies utilising two-dimensional (2D) and three-dimensional (3D) culture systems have been employed to elucidate the pathogenic mechanism of EV-D68. Various animal models have also been developed to investigate viral tropism and distribution, pathogenesis, and immune responses during EV-D68 infection. EV-D68 infections have primarily been investigated in respiratory, intestinal and neural cell lines/tissues, as well as in small-size immunocompetent rodent models that were limited to a young age. Some studies have implemented strategies to overcome the barriers by using immunodeficient mice or virus adaptation. Although the existing models may not fully recapitulate both respiratory and neurological disease observed in human EV-D68 infection, they have been valuable for studying pathogenesis and evaluating potential vaccine or therapeutic candidates. In this review, we summarise the methodologies and findings from each experimental model and discuss their applications and limitations.

Contemporary enterovirus-D68 isolates infect human spinal cord organoids

Enterovirus D68 (EV-D68) is a nonpolio enterovirus associated with severe respiratory illness and acute flaccid myelitis (AFM), a polio-like illness causing paralysis in children. AFM outbreaks have been associated with increased circulation and genetic diversity of EV-D68 since 2014, although the virus was discovered in the 1960s. The mechanisms by which EV-D68 targets the central nervous system are unknown. Since enteroviruses are human pathogens that do not routinely infect other animal species, establishment of a human model of the central nervous system is essential for understanding pathogenesis. Here, we describe two human spinal cord organoid (hSCO)-based models for EV-D68 infection derived from induced, pluripotent stem cell (iPSC) lines. One hSCO model consists primarily of spinal motor neurons, while the another model comprises multiple neuronal cell lineages, including motor neurons, interneurons, and glial cells. These hSCOs can be productively infected with contemporary strains, but not a historic strain, of EV-D68 and produce extracellular virus for at least 2 weeks without appreciable cytopathic effect. By comparison, infection with hSCO with another enterovirus, echovirus 11, causes significant structural destruction and apoptosis. Together, these findings suggest that EV-D68 infection is not the sole mediator of neuronal cell death in the spinal cord in those with AFM and that secondary injury from the immune response likely contributes to pathogenesis. IMPORTANCE AFM is a rare condition that causes significant morbidity in affected children, often contributing to life-long sequelae. It is unknown how EV-D68 causes paralysis in children, and effective therapeutic and preventative strategies are not available. Mice are not native hosts for EV-D68, and thus, existing mouse models use immunosuppressed or neonatal mice, mouse-adapted viruses, or intracranial inoculations. To complement existing models, we report two hSCO models for EV-D68 infection. These three-dimensional, multicellular models comprised human cells and include multiple neural lineages, including motor neurons, interneurons, and glial cells. These new hSCO models for EV-D68 infection will contribute to understanding how EV-D68 damages the human spinal cord, which could lead to new therapeutic and prophylactic strategies for this virus.

RNA Structure in the 5' Untranslated Region of Enterovirus D68 Strains with Differing Neurovirulence Phenotypes

Enterovirus-D68 (EV-D68) is a positive-sense single-stranded RNA virus within the family Picornaviridae. EV-D68 was initially considered a respiratory virus that primarily affected children. However, in 2014, EV-D68 outbreaks occurred causing the expected increase in respiratory illness cases, but also an increase in acute flaccid myelitis cases (AFM). Sequencing of 2014 outbreak isolates revealed variations in the 5' UTR of the genome compared to the historical Fermon strain. The structure of the 5' UTR RNA contributes to enterovirus virulence, including neurovirulence in poliovirus, and could contribute to neurovirulence in contemporary EV-D68 strains. In this study, the secondary and tertiary structures of 5' UTR RNA from the Fermon strain and 2014 isolate KT347251.1 are analyzed and compared. Secondary structures were determined using SHAPE-MaP and TurboFold II and tertiary structures were predicted using 3dRNAv2.0. Comparison of RNA structures between the EV-D68 strains shows significant remodeling at the secondary and tertiary levels. Notable secondary structure changes occurred in domains II, IV and V. Shifts in the secondary structure changed the tertiary structure of the individual domains and the orientation of the domains. Our comparative structural models for EV-D68 5' UTR RNA highlight regions of the molecule that could be targeted for treatment of neurotropic enteroviruses.