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Chapter 18 : Enteroviruses and Parechoviruses
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Human enteroviruses (EV) are members of the Enterovirus genus of the family Picornaviridae and are among the most common human viral infections. These investigations had implications for all of virology because they indicated, first, that poliovirus (PV) grew in various tissue culture cells that did not correspond to the tissues infected during the human disease and, second, that PV destroyed cells with a specific cytopathic effect (CPE). The infectious virus is relatively resistant to many common laboratory disinfectants, including 70% ethanol, isopropanol, dilute Lysol, and quaternary ammonium compounds. Poliomyelitis should be considered in all cases of pure motor paralysis and is usually associated with a normal or slightly elevated value for protein, normal sugar value, and moderate mononuclear pleocytosis in cerebrospinal fluid (CSF). The innate immune system is especially important because it is the earliest response and, in addition, it regulates the adaptive immune response. All important information about a virus could potentially be obtained directly by PCR in conjunction with nucleic acid sequencing if all the molecular correlates of viral phenotypic determinants were understood. The most common molecular typing system is based on reverse transcription (RT)-PCR and nucleotide sequencing of a portion of the genomic region encoding VP1. The major advantage of the pan-EV PCR is that rapid detection of an EV is possible, even with very small amounts of clinical specimens such as CSF. Like the EV, human parechoviruses were traditionally detected and identified by virus isolation and antigenic typing.
Morphology of PV. (A) Transmission electron micrograph of PV type 1 particles, negatively stained with 0.5% uranyl acetate. Bar, 100 nm. (B) Schematic representation of the three-dimensional structure of a PV particle and the four neutralizing antigenic (N-Ag) sites. X-ray crystallographic structure analysis of PV type 1 (Hogle et al., 1985) has revealed an icosahedral capsid structure typical of EV. The capsid surface is composed of 60 protomers, each consisting of the capsid proteins VP1, VP2, and VP3 (black areas). Each of the 12 fivefold symmetry axes is surrounded by five protomers, forming a pentamer (surrounded by a bold black line). The attachment site for the virus-specific receptor is a depression around the fivefold symmetry axis, also called the canyon (dark gray circles). Each of the three surface-exposed capsid proteins contains immunodominant antigenic sites at which neutralizing antibodies bind, resulting in neutralization of virus infectivity. Four N-Ag sites (white ellipses) have been mapped to surface loop extensions (for a review, see Hogle and Filman, 1989). N-Ag I is a continuous sequence in VP1 mapping to amino acids 95 to 105. N-Ag II is a discontinuous site mapping to amino acids 221 to 226 of VP1 and amino acids 164 to 172 and 270 in VP2. N-Ag III consists of two independent discontinuous sites. N-Ag IIIA is composed of amino acids 58 to 60 and 71 to 73 of VP3, and N-Ag IIIB consists of amino acid 72 of VP2 and amino acids 76 to 79 of VP3. The smallest capsid protein, VP4, lies buried in the capsid shell in close association with the single molecule of viral RNA.
Genome organization of PV type 1. The PV genome is a single-stranded positive-sense RNA of approximately 7,500 nucleotides. Nucleotides 743 to 7370 encode in a single open reading frame the capsid proteins (white boxes in coding regions P1) and functional proteins (gray boxes in coding regions P2 and P3). The 5′ and 3′ NTRs are shown as lines. The IRES is shown schematically with the two-dimensional structure. The virus protein VPg is covalently linked to the terminal uracil of the 5′ NTR. For further details, see “Structure” and “Replication in Cell Culture” in the text.
Replication cycle of PV. Parental PV enters its host cell by receptor-mediated endocytosis and releases its viral RNA from the virus capsid (uncoating) in acidic organelles (endosomes). After release of VPg from the parental viral RNA, protein synthesis of PV starts at the rough endoplasmic reticulum. The viral precursor polyprotein is autocatalytically cleaved by viral proteases, releasing in the viral RNA polymerase, and via several precursor proteins, the virus capsid proteins. At the smooth endoplasmic reticulum, the viral RNA polymerase synthesizes new viral RNA. Positive-sense RNA serves as a template for negative-sense RNA molecules, which themselves are templates for new positive-sense RNA. This positive-sense RNA is released from multistranded replicative intermediates (RI) and used for either further viral transcription and translation or encapsidation into assembling provirus particles. After the capsid protein precursor VP0 has finally been cleaved into VP2 and VP4, the maturation of progeny virus is completed. One cycle of PV reproduction takes about 6 h.
Scanning electron micrograph of HEp-2 cells infected with PV type 1. (A) Infected cells show a severe CPE, characterized by rounded-up cells that are attached to the substratum only by long filopodia. (B) Mock-infected HEp-2 cells are characterized as a monolayer of confluent cells with evenly distributed microvilli at the plasma membrane. Bars, 10 μm.
Course of PV infection.
Temporal prevalence of E9, E30, and CVB3 in the United States from 1970 to 2005. The graph shows the number of isolates (bar graph) and the fraction of all nonpolio EV (line graph) that each of the serotypes represents in each year.
Lim and Benyesh-Melnick antiserum pools for typing of EV. Filled boxes represent neutralization of EV by antiserum pools A to H. The typing of a virus is based on combined neutralization of different antiserum pools. (Adapted from Melnick et al., 1979.)
Global distribution of wild PV and progress in eradication. The maps indicate (shading) which countries had endemic wild PV circulation in 1988 (top) and 2007 (bottom). In 2007, several countries (light shading) that were polio free for at least 3 years had virus circulation following importations of wild PV and circulation was reestablished for a period of at least 12 months.