hence role in transmissibility
Inhibits RNA and prolein synthesis
Role in persistent infection
Fig. 7-1 Schematic diagram of part of the reovirus outer capsid, showing the location of the polypeptides that play a major role in virulence The irl protein is located at (he vertices of the icosahedron and consists of two components, a globular dimerat the surface, which is responsible for hemagglutination and cell attachment, and an tt-hehcal tegion which anchors the hemagglutinin by interaction with the X2 spike protein The polypeptides (J.1C and cr3 are associated with one another, in the ratio of one molecule of n IC to two molecules of it3, on the surfaces of the icosahedral capsid [Modified from L A Schiff and B N Fields, m "Fields Virology" (B N Fields, D M. Kmpc, R M Chanock, M S Hhsch,) L Melnick, LP Monath, and H Rot/man, eds.), 2nd Ed , p 1277 Raven, New York, 1990.]
lence. Gene SI specifies the hemagglutinin (protein rrl), which is located on the vertices of the icosahedron and is responsible for cellular and tissue trop-ism. With reovirus 3, but not reovirus 1, the crl protein is responsible for binding to neurons, whose sequential infection leads to fatal encephalitis, and it is also responsible for viral spread from footpad to the spinal cord via peripheral nerves. Gene M2 specifies polypeptide p.1C, which determines sensitivity of the virion to chymotrypsin and hence affects the capacity of the virus to grow in the intestine. Thus p,lC of reovirus 3 is protease sensitive, and reovirus 3 is avirulent by mouth; jxlC of reovirus 1 is protease resistant, and the vitus is infectious by mouth. A reassortant with the M2 gene from reovirus 1 and the SI gene from reovirus 3 was infectious orally and caused fatal encephalitis.
Gene 1,2 specifies the A2 spike protein, which is primarily a core protein but surrounds crl; it is a guanylyltransferase and plays a role in RNA transcription. The ability of reovirus to reach high titers in the gut and be efficiently transmitted via feces is associated with the X2 spike protein. Gene S4 specifies polypeptide o-3, which inhibits cellular protein and RNA synthesis in infected cells; mutations in the S4 gene play a role in establishing persistent infection in cultured cells.
The genome of poliovirus is a single molecule of ssRNA, and the entire nucleotide sequence of several strains has been determined. Because vaccine strains are requited to be infectious by mouth but not cause central nervous system disease, the virulence of polioviruses is narrowly defined as the ability to replicate and cause lesions after introduction into the central nervous system of primates.
Comparison of the nucleotide sequence of the poliovirus type 1 vaccine strain with that of the virulent parental strain (Mahoney) from which Sabin derived it shows that there are 55 substitutions in the 7441 bases, scattered along the entire genome. Twenty-one of these substitutions resulted m ammo acid changes, involving several ol the viral proteins. Studies with recombi-
nants showed that mutations in the VI'l capsid protein gene and other genes, as well as in the 5' noncoditig region, contribute to neurovirulence in primates. On the other hand, with pohovirus type 3 vaccine strain, which reverts to virulence with a frequency at least 10 times that of type 1 vaccine, there are only 10 nucleotides different from those of the parental strain, of which only 3 result in amino acid changes, 1 in each of three viral structural proteins
The two most important mutations with respect to neurovirulence are a substitution of a single nucleotide at position 472 in the 5' noncoding region and an amino acid substitution in VP3 which represents a fs mutation, destabilizing the capsid. The importance of the 5' noncoding region in neurovirulence is intriguing; since position 472 also tends to be the first reversion to occur when the vaccine grows in the human gut, it is possible that the wild-type nucleotide at this position allows optimal intestinal replication and that neurovirulence is a consequence of the resultant higher titer of virus in the bloodstream.
Poxviruses are much larger and more complex than any viruses yet discussed, and the genetic control of virulence is correspondingly more complex. Interest in the virulence of poxviruses has been stimulated by proposals to use vaccinia virus as a vector for human vaccines. Recent work has shown that many of the large number of genes encoding virokines that influence virulence have effects on the defense mechanisms of the host (see Table 7-1).
Deletion analysis of vaccinia virus has shown that 56 of a total of 198 open reading frames are not required for replication in cultured cells. Many of these are located at the ends of the genome and affect virulence in animals. The first poxvirus gene to be shown to affect virulence was the gene for vaccinia virus thymidine kinase (TK), which is not required for replication in cultured cells; however, TK~ mutants are much less virulent than the wild type in animals. The envelope of vaccinia virus, which contains at least seven virus-coded polypeptides, is not essential for infection of either cultured cells or animals, but enveloped forms are more virulent because they spread around the body more effectively.
Cowpox virus replicates in a wide range of cells, but three open reading frames have been identified which affect its replication in a nonpermissive cell, namely, Chinese hamster ovary (CHO) cells. In animals these genes are probably important in determining host range and tissue tropisms The vaccinia growth factor (VGF) has homologies with epidermal growth factor and appears to induce localized hyperplasia around foci of infection, thus providing additional metabolically active cells for viral replication. A 35K protein, which has been called the vaccinia virus complement-binding protein, blocks the classical complement pathway and binds to C4b. Then there are at least three distinct serine protease inhibitor (serpin) genes in vaccinia virus. Ser: pins exert a control over a number ol critical events associated with connective tissue turnover, coagulation, complement activation, and inflammatory reactions. The presence of the 38K gene of cowpox virus, which encodes a serpinlike protein, is associated with the bright red pock produced on the chorioallantoic membrane and higher virulence for a number of experimental animals It has been suggested that the serpin delays the onset or decreases the magnitude of the inflammatory response.
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