Further studies on the relative antiviral efficacies of interferons induced by poly I:C and Mengo virus.

Mouse serum interferons induced by polyI:C, vesicular stomatitis virus (VSV), reovirus, and Mengo virus were assayed in monolayers of mouse L-929 cells by the plaque-reduction method using both VSV and Mengo as challenge viruses. Titers obtained with Mengo virus as challenge were all lower than with VSV. With the interferons induced by VSV, reovirus, and ployI:C, the reductions were of the order of two- to three-fold. With Mengo virus-induced interferon the reduction was much greater (about 17-fold). This offers an explanation for the observation that, unit for unit (measured by the plaque reduction of VSV), Mengo virus-induced interferon is only about 1/10 as effective as polyI:C-induced interferon in protecting mice against lethal infection with Mengo virus. The data are consistent with the hypothesis that an interferon antagonist is produced in the serum of mice infected with Mengo virus. This antagonist, which is not produced in mice inoculated with polyI:C, or reovirus, effectively blocks the antiviral action of interferon during Mengo virus infections, both in vivo and in vitro.     

Structure and Infectivity of Picornaviral RNA Encapsidated by Cowpea Chlorotic Mottle Virus Protein

Poliovirus and Mengo virus RNA were shown to associate efficiently with cowpea chlorotic mottle virus protein to form pseudovirions. The sedimentation coefficient for the pseudovirions was similar to that of poliovirus, and electron microscope observations showed the Mengo pseudovirions to be similar in size to Mengo virus. Such pseudovirions were infectious and were more resistant to ribonuclease than viral RNA; however, under our assay conditions, their infectivity was about equal to that of viral RNA.     

Theiler's virus and Mengo virus induce cross-reactive cytotoxic T lymphocytes restricted to the same immunodominant VP2 epitope in C57BL/6 mice.

C57BL/6 mice develop a virus-specific cytotoxic T-lymphocyte (CTL) response after intraperitoneal inoculation with either the DA strain of Theiler's virus or Mengo virus, two members of the Cardiovirus genus. These CTLs contribute to viral clearance in the case of Theiler's virus but do not protect the mice from the fatal encephalomyelitis caused by Mengo virus. In this study we show that DA and Mengo virus-induced CTLs are cross-reactive. The cross-reactivity is due to a conserved, H-2Db-restricted epitope located between amino acid residues 122 and 130 of the VP2 capsid protein (VP2(122-130)). This epitope is immunodominant in C57BL/6 mice infected with Theiler's virus. The VP2(122-130) epitope, initially identified for Mengo virus, is the first CTL epitope described for Theiler's virus.     

Trichomonas vaginalis resistance to Mengo virus infection.

We have investigated the susceptibility of Trichomonas vaginalis to Mengo virus infection by comparing the outcome of Mengo virus or purified Mengo virus RNA infection in T. vaginalis and in CCL-1 mouse fibroblasts. While the adsorption and entry of Mengo virus into T. vaginalis occurred in the same manner as in fibroblasts, the uncoating was much slower. In addition, Mengo virus infection of T. vaginalis displayed no eclipse nor any subsequent production of infectious virus. Purified RNA failed to initiate productive infection in T. vaginalis, whereas it provoked viral replication in the fibroblast controls. It was shown by assessment of protein synthesis in T. vaginalis and mouse fibroblasts cell-free systems that the protozoan ribosomes were able to translate endogenous mRNA and poly-U, but not viral RNA.     

Trichomonas vaginalis Resistance to Mengo Virus Infection*

We have investigated the susceptibility of Trichomonas vaginalis to Mengo virus infection by comparing the outcome of Mengo virus or purified Mengo virus RNA infection in T. vaginalis and in CCL-1 mouse fibroblasts. While the adsorption and entry of Mengo virus into T. vaginalis occurred in the same manner as in fibroblasts, the uncoating was much slower. In addition, Mengo virus infection of T. vaginalis displayed no eclipse nor any subsequent production of infectious virus. Purified RNA failed to initiate productive infection in T. vaginalis, whereas it provoked viral replication in the fibroblast controls. It was shown by assessment of protein synthesis in T. vaginalis and mouse fibroblasts cell-free systems that the protozoan ribosomes were able to translate endogenous mRNA and poly-U, but not viral RNA.     

The use of a complementary DNA probe to detect accumulation of mengo RNA in infected cells pretreated with interferon.

Complementary DNA (cDNA) from Mengo virus RNA has been synthesized and used as a probe to measure the synthesis and accumulation of viral RNA in Mengo infected L cell cultures, treated or untreated with interferon. Under experimental conditions used (200 units interferon/ml and 50 virus plaque-forming units/cell) results show that there is some synthesis of Mengo virus RNA in cells treated with interferon. One hour after infection, treated cells contain three times less viral RNA than untreated cells; five hours after infection, this difference has increased to ten fold. As in the control, no fragmented Mengo virus RNA molecules were found in interferon treated cells. The smaller recovery of infectious particles from interferon treated cells as compared to RNA accumulation suggests that not only RNA accumulation is inhibited but also a step posterior in viral maturation.     

Correlation Between the Antiviral Effect of Interferon Treatment and the Inhibition of In Vitro mRNA Translation in Noninfected L cells

When noninfected L-cell suspension cultures are treated with interferon (specific activities superior to 10(6) reference units per mg of protein), the cell-free cytoplasmic extracts obtained are inactive for the translation of exogenous natural mRNAs. The dose-response curve shows that comparable amounts of interferon are required to produce a 50% reduction of Mengo virus multiplication in vivo and Mengo RNA translation in vitro. With higher doses of interferon, Mengo RNA translation is completely abolished, while poly U translation and endogenous protein synthesis are only slightly affected. The inactivation of Mengo RNA translation is reversible; after removal of interferon, normal translation activity is regained together with the ability to support Mengo virus multiplication. Fractionation of the cell-free extracts shows that the effect is localized in the fraction which can be washed off the ribosomes by high salt. These results establish that interferon induces a block in genetic translation in noninfected L cells.     

Antiviral activity of a thymic factor in experimental viral infections. I. Thymic hormonal effect on survival, interferon production and NK cell activity in Mengo virus-infected mice

A partially purified thymic factor, thymostimulin (TS), significantly increased the survival rate of adult, immune-intact mice infected with the neurotropic Mengo virus. TS treatment was begun after virus inoculation by daily i.p. injections. In untreated C57BL/6 mice, LD50 was reached with 1 X 10(4) PFU, but 10-fold more virus (i.e., 1 X 10(5) PFU) was needed to reach LD50 in TS-treated animals. TS effect on survival, though, could be observed with several virus doses (1 X 10(3) to 1 X 10(6) PFU) (p less than 0.001). A significant effect on survival was also observed with outbred ICR mice (p less than 0.005). Serum interferon (IFN) levels in the Mengo virus-infected mice were relatively low (average peak 300 U/ml), but were significantly increased (two- to ninefold) in the TS-treated mice. Peak serum levels were reached earlier in TS than in control animals (24 hr and 72 hr, respectively). Both acid-labile and acid-stable type I IFN production were augmented by TS in the Mengo virus-infected mice. Natural killer activity was also enhanced by TS, in particular on the second day after virus inoculation. In addition, MP-virus was used as a second, unrelated virus challenge. This virus caused a nonlethal infection, with relatively high levels of serum IFN (average peak 10,000 U/ml). TS increased IFN levels (two- to eight-fold) also in this challenge system. In conclusion, TS causes a nonspecific enhancement of endogenous production of IFN and has a significant effect on the survival of lethally infected mice. The data indicate a potential application of thymic factors for the treatment of viral infections.     

Inhibition of Mengo virus by interferon

Gauntt, Charles J. (The University of Texas, Austin), and Royce Z. Lockart, Jr. Inhibition of Mengo virus by interferon. J. Bacteriol. 91:176-182. 1966.-The inhibition of Mengo virus replication in L cells resulting from interferon was studied quantitatively. Interferon was titrated on L cells with Western equine encephalomyelitis (WEE) virus as the challenge virus. One protective unit (PU) of interferon is the least amount of interferon which prevents cytopathic effects when a large multiplicity of WEE virus is added subsequent to overnight incubation with interferon. Ten PU of interferon reduced the yields of Mengo virus by about 90%. Larger doses of interferon, up to 220 PU, caused no further reduction in the amount of virus produced. Plaque formation by Mengo virus was also reduced in number by about 85 to 90%, but could not be further reduced. The plaques which formed on interferon-treated cells were reduced in size. We were unable to obtain a virus population with increased resistance to interferon action by use of five successive growth cycles in interferon-treated cultures. Analysis of the cell population for the proportion of cells able to act as infectious centers revealed that incubation of cells with 10 PU of interferon decreased the proportion of virus-yielding cells by 80%. The yield of virus per virus-producing cell was decreased by 40 to 60%. Despite the reduction in yields, plaques, and infectious centers resulting from interferon, all doses of interferon failed to prevent the complete destruction of the cells. Experiments with puromycin indicated that the cytopathic effects observed in L cells infected with Mengo virus required that a virus-directed protein be synthesized between 4 and 5 hr postinfection. The evidence suggested, therefore, that the Mengo virus genome was able to code for new protein synthesis in the absence of the production of infectious virus.     

The N-terminal sequence of three coat proteins of foot-and-mouth disease virus.

The N-terminal section of the amino acids sequences of three coat proteins of foot-and-mouth disease virus were determined and compared with the first ten amino acids of the corresponding Mengo virus proteins. In the proteins of the two viruses beginning with aspartic acid, five of ten amino acids are in identical positions.     

Destruction of L Cells by Mengo Virus: Use of Interferon to Study the Mechanism

Interferon, when added to L cells, inhibited the synthesis of infectious Mengo viral ribonucleic acid, hemagglutinins, and infectious virus by 85 to 95%. Serum-blocking antigens were also reduced by the action of interferon, but threefold excess amounts of these antigens accumulated in interferon-treated cultures above the amounts expected for the quantity of infectious virus that was produced in these cultures. Radioautographic analysis showed that 28 to 36% of the cells of an interferon-treated population synthesized viral ribonucleic acid and 36 to 47% produced viral antigens as determined by an immunofluorescence technique. Despite the reductions in synthesis of viral components, all cells in an interferon-treated culture underwent cytopathic effects at the same time as cells in infected cultures which had not been treated with interferon. The results are compatible with the hypothesis that the cell destruction which results from the infection of L cells with Mengo virus is due to a protein which is coded for by the virus but is not a component of the mature virion.     

Functional aspects of the capsid structure of Mengo virus

The three-dimensional structure of the Mengo virus capsid has been determined at a resolution of 3.0 A. This achievement is discussed in an historical context, and the general features of picornavirus capsid design are presented. The dynamic functional aspects of the Mengo virus capsid--namely its ability to interact with specific receptors on host cells, to dissociate and release the viral genomic RNA into the cellular cytoplasm, to assemble with progeny RNA molecules and form new virions, and to alter its external surface in order to evade neutralization by circulating antibodies--are discussed. Comparisons with other picornaviruses whose capsid structures have also been elucidated (poliovirus serotype 1 and 3, human rhinovirus types 14 and 1A, and foot-and-mouth disease virus type O) illustrate both the similarities and the distinctive features of capsid design found within this family of mammalian viruses.     

Attenuated Mengo virus: a new vector for live recombinant vaccines.

Several features make Mengo virus an excellent candidate for use as a vaccine vector. The virus has a wide host range, including rodents, pigs, monkeys, and most likely humans, and expresses its genome exclusively in the cytoplasm of the infected cell. Stable attenuated strains exist which are deleted for part of the 5' noncoding region of the genome. Here we report an attenuated Mengo virus recombinant, vLCMG4, that encodes an immunodominant cytotoxic T-lymphocyte epitope of the lymphocytic choriomeningitis virus (LCMV) nucleo-protein. vLCMG4 induced protective immunity against lethal LCMV infection after a single, low-dose immunization in BALB/c mice and elicited an LCMV-specific CD8+ cytotoxic T lymphocyte response. This demonstrates the potential of recombinant Mengo virus vaccines to confer protection against infectious diseases by the induction of cellular immune responses.     

Properties of morphologically variant haploid frog cells formed by combined treatment with Mengo virus and the polyene antibiotic mediocidin.

Comparisons have been made of cell surface glycoproteins, concanavalin A agglutinability, and cloning efficiencies in liquid media of ICR 2A (haploid frog cells), ICR 2A M (three cloned populations of haploid frog cells resistant to 5 microgram per ml of the polyene antibiotic mediocidin), and ICR 2A M/MV cells (five cloned populations of morphologically variant haploid frog cells produced by exposure of the parental cells to the combined effects of mediocidin and an RNA mammalian virus, Mengo virus). Independently isolated ICR 2A M/MV clones exhibited altered cell surface glycoproteins, increased concanavalin A agglutinability, and enhanced cloning efficiency in liquid media when compared with ICR 2A parental cells. In contrast, ICR 2A M cells had properties similar to ICR 2A cells, with the exception of the former's increased resistance to mediocidin. The differences in properties between ICR 2A M/MV and ICR 2A cells suggest that alterations resembling transformation have occurred in ICR 2A M/MV cells as a consequence of combined treatment with mediocidin and Mengo virus.     

Structural refinement and analysis of Mengo virus

The structure of Mengo encephalomyelitis virus was refined at 3 A resolution with a final R-factor of 0.221 and a root-mean-square deviation from idealized bond lengths of 0.019 A for 10 A to 3 A data with F greater than or equal to 3 sigma(F). The Hendrickson-Konnert refinement was restrained by the phases derived from the molecular replacement averaging procedure and constrained by the icosahedral symmetry of the virus. The virus consists of 60 protomers each having three major subunits, VP1, VP2 and VP3, along with one smaller internal protein, VP4. The three major subunits form similar eight-stranded beta-barrel structures. Alterations in the original sequence were found at position 45 in VP1 (Arg to Ala) and at position 58 in VP3 (Met to Val). The residues in loops I and II of VP1 (82 to 102), the "FMDV loop" in VP1 (205 to 213), the flexible loop of VP3 in the putative receptor attachment site (175 to 185) as well as the terminal regions 260 to 268 in VP1, 253 to 256 in VP2 and 13 to 15 in VP4 were built or modified in regions of weak density. The variation in temperature factors at the end of the refinement is over a wide range (from 2 to 80 A2), with the disordered outer and inner regions showing high mobility. Four cis proline residues, 105 in VP1, 85 and 152 in VP2 and 59 in VP3, have been identified. The disulfide bridge Cys86 to Cys88 in VP3 has been characterized. One phosphate ion and 233 water positions were included in the refinement. It is suggested that this phosphate is associated with the receptor attachment site. There are two major hydrogen-bonding networks involving solvent atoms; one involving only the subunits of a protomer, and the other connecting the protomers in a pentamer. The distribution of atom types around the icosahedral symmetry axes shows that the 5-fold channel is more hydrophobic than that along the 3-fold axis and that there are more charged residues around the 2-fold axis. The analysis of contacts between the different subunits supports the assignment of the protomeric unit. The five protomers that form the pentameric unit are held together by interactions involving the smaller VP4 protein and the amino termini of VP1 and VP3. The pentamers are associated by means of the amino-terminal region of the VP2 subunits, the beta F strand of the VP3 subunits, the C terminus of the VP4 subunits and the electrostatic helical (alpha A) interactions of VP2 subunits across the icosahedral 2-fold axes. The superposition of the corresponding subunits of Mengo virus, human rhinovirus 14 and southern bean mosaic virus has provided an improved sequence alignment. The largest structural similarity is between the VP3 subunits of Mengo virus and rhinovirus, while the least similarity is between the VP1 subunits. The various specialized insertions in the different subunits can be associated with specific functional requirements.     

Studies of two temperature-sensitive mutants of Mengo virus.

Studies of the synthesis of viral ribonucleates and polypeptides in cells infected with two RNA- ts mutants of Mengo virus (ts 135 and ts 520) have shown that when ts 135 infected cells are shifted from the permissive (33 degrees C) to the nonpermissive (39 degrees C) temperature: (i) the synthesis of all three species of viral RNA (single stranded, replicative form, and replicative intermediate) is inhibited to about the same extent, and (ii) the posttranslational cleavage of structural polypeptide precursors A and B is partially blocked. Investigations of the in vivo and in vitro stability of the viral RNA replicase suggest that the RNA- phentotype reflects a temperature-sensitive defect in the enzyme. The second defect does not appear to result from the inhibition of viral RNA synthesis at 39 degrees C, since normal cleavage of polypeptides A and B occurs in wt Mengo-infected cells in which viral RNA synthesis is blocked by cordycepin, and at the nonpermissive temperature in ts 520 infected cells. Considered in toto, the evidence suggests that ts 135 is a double mutant. Subviral (53S) particles have been shown to accumulate in ts 520 (but not ts 135) infected cells when cultures are shifted from 33 to 39 degrees C. This observation provides supporting evidence for the proposal that this recently discovered particle is an intermediate in the assembly pathway of Mengo virions.     

Properties of morphologically variant haploid prog cells formed by combined treatment with mengo virus and the polyene antibiotic mediocidin

Summary Comparison have been made of cell surface glycoproteins, concanavalin A agglutinability, and cloning efficiencies in liquid media of ICR 2A (haploid frog cells), ICR 2A M (three cloned populations of haploid frog cells resistant to 5 μg per ml of the polyene antibiotic mediocidin), and ICR 2A M/MV cells (five cloned populations of morphologically variant haploid frog cells produced by exposure of the parental cells to the combined effects of mediocidin and an RNA mammalian virus, Mengo virus). Independently isolated ICR 2A M/MV clones exhibited altered cell surface glycoproteins, increased concanavalin A agglutinability, and enhanced cloning efficiency in liquid media when compared with ICR 2A parental cells. In contrast, ICR 2A M cells had properties similar to ICR 2A cells, with the exception of the former's increased resistance to mediocidin. The differences in properties between ICR 2 M/MV and ICR 2 cells suggest that alterations resembling transformation have occurred in ICR 2A M/MV cells as a consequence of combined treatment with mediocidin and Mengo virus. This investigation was supported in part by Contract NIH 69-2161, NIH Grant AI-2095, and NIH Training Grant No. GM 507 from the National Institute of General Medical Sciences.     

Protein synthesis in extracts from interferon-treated Mengo virus infected cells

We previously showed in intact L cells that interferon treatment did not modify the shut-off of cellular RNA and protein synthesis induced by infection with Mengo virus although viral replication is inhibited (1,2). We have also demonstrated that inhibition of host protein synthesis was not due to degradation of messengers since cellular mRNA could be extracted from interferon-treated infected cells and efficiently translated in a reticulocyte lysate(2). Cellular mRNA was not degraded although 2–5A was present as reported here. We prepared cell-free systems from such cells at a time when cellular shut-off was fully established. The undegraded messengers remained untranslated under cell-free protein synthesis conditions and almost no polysomes were detected. The decreased amount of [³⁵S]Met-tRNA-40S complex observed in these lysates might account for the inhibition of protein synthesis at the level of initiation.     

Crystallization and preliminary X-ray diffraction analysis of foot-and-mouth disease virus

Foot-and-mouth disease virus has been crystallized with the objectives of (1) determining the composition and conformation of the major immunogenic site(s) and (2) comparing its structure with those of the related polio, rhino and Mengo viruses, representing the other three genera of the picornaviruses. Most of the work has been done with virus strain O1BFS 1860, which crystallized as small rhombic dodecahedra of maximum dimension 0.3 mm. Virus recovered from crystals was infectious, and was indistinguishable from native virus both in protein composition and buoyant density. The stability of the crystals in the X-ray beam was comparable with that of other picornavirus crystals and they diffracted to a resolution of better than 2.3 A. Initial analysis of the X-ray diffraction data shows the virus to be positioned on a point of 23 symmetry in a close-packed array so that examples of all the icosahedral symmetry elements, except the 5-fold axes, are expressed crystallographically. The cell dimensions are a = b = c = 345 A, alpha = beta = gamma = 90 degrees, with a space group of I23. The diameter of the virus particle is 300 A. Despite the small size of the crystals, diffraction data have been collected to a reasonable resolution using a synchrotron source. Phasing of the diffraction data will be attempted using the methods of molecular replacement.     

A wild-type porcine encephalomyocarditis virus containing a short poly(C) tract is pathogenic to mice,pigs, and cynomolgus macaques

Previous studies using wild-type Encephalomyocarditis virus (EMCV) and Mengo virus, which have long poly(C) tracts (61 to 146 C's) at the 5' nontranslated region of the genome, and variants of these viruses genetically engineered to truncate or substitute the poly(C) tracts have produced conflicting data on the role of the poly(C) tract in the virulence of these viruses. Analysis of the nucleotide sequence of an EMCV strain isolated from an aborted swine fetus (EMCV 30/87) revealed that the virus had a poly(C) tract that was 7- to 10-fold shorter than the poly(C) tracts of other EMCV strains and 4-fold shorter than that of Mengo virus. Subsequently, we investigated the virulence and pathogenesis of this naturally occurring short-poly(C)-tract-containing virus in rodents, pigs, and nonhuman primates. Infection of C57BL/6 mice, pigs, and cynomolgus macaques resulted in similar EMCV 30/87 pathogenesis, with the heart and brain as the primary sites of infections in all three animals, but with different disease phenotypes. Sixteen percent of EMCV 30/87-infected pigs developed acute fatal cardiac failure, whereas the rest of the pigs were overtly asymptomatic for as long as 90 days postinfection (p.i.), despite extensive myocardial and central nervous system (CNS) pathological changes. In contrast, mice infected with >/==" BORDER="0">4 PFU of EMCV 30/87 developed acute encephalitis that resulted in the death of all animals (n = 25) between days 2 and 7 p.i. EMCV 30/87-infected macaques remained overtly asymptomatic for 45 days, despite extensive myocardial and CNS pathological changes and viral persistence in more than 50% of the animals. The short poly(C) tract in EMCV 30/87 (CUC(5)UC(8)) was comparable to that of strain 2887A/91 (C(10)UCUC(3)UC(10)), another recent porcine isolate.