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Host alternation is necessary to maintain the genome stability of rift valley fever virus.

Moutailler S, Roche B, Thiberge JM, Caro V, Rougeon F, Failloux AB - PLoS Negl Trop Dis (2011)

Bottom Line: This genetic stability is assumed to result from a fitness trade-off imposed by host alternation, which constrains arbovirus genome evolution.The phosphoprotein NSs is not essential to viral replication allowing clones carrying deletions in NSs to predominate as they replicate slightly more rapidly.Our results strongly support the view that alternating replication is necessary to maintain the virulence factor carried by the NSs phosphoprotein.

View Article: PubMed Central - PubMed

Affiliation: Molecular Genetics of Bunyavirus, Department of Virology, Institut Pasteur, Paris, France.

ABSTRACT

Background: Most arthropod-borne viruses (arboviruses) are RNA viruses, which are maintained in nature by replication cycles that alternate between arthropod and vertebrate hosts. Arboviruses appear to experience lower rates of evolution than RNA viruses that replicate in a single host. This genetic stability is assumed to result from a fitness trade-off imposed by host alternation, which constrains arbovirus genome evolution. To test this hypothesis, we used Rift Valley fever virus (RVFV), an arbovirus that can be transmitted either directly (between vertebrates during the manipulation of infected tissues, and between mosquitoes by vertical transmission) or indirectly (from one vertebrate to another by mosquito-borne transmission).

Methodology/principal findings: RVFV was serially passaged in BHK21 (hamster) or Aag2 (Aedes aegypti) cells, or in alternation between the two cell types. After 30 passages, these single host-passaged viruses lost their virulence and induced protective effects against a challenge with a virulent virus. Large deletions in the NSs gene that encodes the virulence factor were detectable from the 15(th) serial passage onwards in BHK21 cells and from the 10(th) passage in Aag2 cells. The phosphoprotein NSs is not essential to viral replication allowing clones carrying deletions in NSs to predominate as they replicate slightly more rapidly. No genetic changes were found in viruses that were passaged alternately between arthropod and vertebrate cells. Furthermore, alternating passaged viruses presenting complete NSs gene remained virulent after 30 passages.

Conclusions/significance: Our results strongly support the view that alternating replication is necessary to maintain the virulence factor carried by the NSs phosphoprotein.

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Related in: MedlinePlus

Survival of mice inoculated with RVFV strains.Mice were observed for 21 days post-inoculation (A) or 36 days after challenge with the parental P strain given 14 days post-inoculation (B). The first experiment used batches of mice inoculated with 104 PFU of a viral strain and kept for 21 days post-inoculation. The second experiment used batches of mice inoculated with 104 PFU of Z30BC or Z30AC. At day 14 post-inoculation, one half of surviving mice were challenged with 104 PFU of P and the other half were inoculated with medium. Mice were then observed for 21 days after challenge. The control was inoculated with medium before being challenged by 104 PFU of P. The viral strains tested were: P (the parental strain), Z30Alt (from the 30th alternating passage in BHK21 and Aag2 cells), Z30B (from the 30th serial passage in BHK21 cells), Z30A (from the 30th serial passage in Aag2 cells), Z30BC (a clone selected from the 30th serial passage in BHK21 cells) and Z30AC (a clone selected from the 30th serial passage in Aag2 cells).
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pntd-0001156-g003: Survival of mice inoculated with RVFV strains.Mice were observed for 21 days post-inoculation (A) or 36 days after challenge with the parental P strain given 14 days post-inoculation (B). The first experiment used batches of mice inoculated with 104 PFU of a viral strain and kept for 21 days post-inoculation. The second experiment used batches of mice inoculated with 104 PFU of Z30BC or Z30AC. At day 14 post-inoculation, one half of surviving mice were challenged with 104 PFU of P and the other half were inoculated with medium. Mice were then observed for 21 days after challenge. The control was inoculated with medium before being challenged by 104 PFU of P. The viral strains tested were: P (the parental strain), Z30Alt (from the 30th alternating passage in BHK21 and Aag2 cells), Z30B (from the 30th serial passage in BHK21 cells), Z30A (from the 30th serial passage in Aag2 cells), Z30BC (a clone selected from the 30th serial passage in BHK21 cells) and Z30AC (a clone selected from the 30th serial passage in Aag2 cells).

Mentions: We inoculated four-week-old mice intraperitoneally with 104 PFU of one of six viral strains to evaluate their virulence. Strains tested were: P (the parental strain), Z30Alt (isolated at the 30th alternating passage in BHK21 and Aag2 cells), Z30B (pool harvest at the 30th serial passage in BHK21 cells), Z30A (pool harvest at the 30th serial passage in Aag2 cells), Z30BC (clone selected at the 30th serial passage in BHK21 cells) and Z30AC (clone selected at the 30th serial passage in Aag2 cells). Mouse survival rates were recorded over 21 days post-inoculation (Figure 3A). The control, inoculated with Dulbecco's MEM (DMEM) medium, survived 21 days. All batches of mice inoculated with a given viral strain also survived, except for those inoculated with the parental P strain and the 30th alternating passage strain (Figure 3A). All mice inoculated with P died before day 7 post-inoculation, and 4 of 5 treated with Z30Alt died before day 9 post-inoculation. Thus, the 30th alternating passage strain behaved roughly like the parental P strain after 30 passages. All surviving mice were tested for the presence of IgG against RVFV at day 21 post-inoculation, and showed positive compared to the IgG level in non-infected mice (Table S2).


Host alternation is necessary to maintain the genome stability of rift valley fever virus.

Moutailler S, Roche B, Thiberge JM, Caro V, Rougeon F, Failloux AB - PLoS Negl Trop Dis (2011)

Survival of mice inoculated with RVFV strains.Mice were observed for 21 days post-inoculation (A) or 36 days after challenge with the parental P strain given 14 days post-inoculation (B). The first experiment used batches of mice inoculated with 104 PFU of a viral strain and kept for 21 days post-inoculation. The second experiment used batches of mice inoculated with 104 PFU of Z30BC or Z30AC. At day 14 post-inoculation, one half of surviving mice were challenged with 104 PFU of P and the other half were inoculated with medium. Mice were then observed for 21 days after challenge. The control was inoculated with medium before being challenged by 104 PFU of P. The viral strains tested were: P (the parental strain), Z30Alt (from the 30th alternating passage in BHK21 and Aag2 cells), Z30B (from the 30th serial passage in BHK21 cells), Z30A (from the 30th serial passage in Aag2 cells), Z30BC (a clone selected from the 30th serial passage in BHK21 cells) and Z30AC (a clone selected from the 30th serial passage in Aag2 cells).
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3101185&req=5

pntd-0001156-g003: Survival of mice inoculated with RVFV strains.Mice were observed for 21 days post-inoculation (A) or 36 days after challenge with the parental P strain given 14 days post-inoculation (B). The first experiment used batches of mice inoculated with 104 PFU of a viral strain and kept for 21 days post-inoculation. The second experiment used batches of mice inoculated with 104 PFU of Z30BC or Z30AC. At day 14 post-inoculation, one half of surviving mice were challenged with 104 PFU of P and the other half were inoculated with medium. Mice were then observed for 21 days after challenge. The control was inoculated with medium before being challenged by 104 PFU of P. The viral strains tested were: P (the parental strain), Z30Alt (from the 30th alternating passage in BHK21 and Aag2 cells), Z30B (from the 30th serial passage in BHK21 cells), Z30A (from the 30th serial passage in Aag2 cells), Z30BC (a clone selected from the 30th serial passage in BHK21 cells) and Z30AC (a clone selected from the 30th serial passage in Aag2 cells).
Mentions: We inoculated four-week-old mice intraperitoneally with 104 PFU of one of six viral strains to evaluate their virulence. Strains tested were: P (the parental strain), Z30Alt (isolated at the 30th alternating passage in BHK21 and Aag2 cells), Z30B (pool harvest at the 30th serial passage in BHK21 cells), Z30A (pool harvest at the 30th serial passage in Aag2 cells), Z30BC (clone selected at the 30th serial passage in BHK21 cells) and Z30AC (clone selected at the 30th serial passage in Aag2 cells). Mouse survival rates were recorded over 21 days post-inoculation (Figure 3A). The control, inoculated with Dulbecco's MEM (DMEM) medium, survived 21 days. All batches of mice inoculated with a given viral strain also survived, except for those inoculated with the parental P strain and the 30th alternating passage strain (Figure 3A). All mice inoculated with P died before day 7 post-inoculation, and 4 of 5 treated with Z30Alt died before day 9 post-inoculation. Thus, the 30th alternating passage strain behaved roughly like the parental P strain after 30 passages. All surviving mice were tested for the presence of IgG against RVFV at day 21 post-inoculation, and showed positive compared to the IgG level in non-infected mice (Table S2).

Bottom Line: This genetic stability is assumed to result from a fitness trade-off imposed by host alternation, which constrains arbovirus genome evolution.The phosphoprotein NSs is not essential to viral replication allowing clones carrying deletions in NSs to predominate as they replicate slightly more rapidly.Our results strongly support the view that alternating replication is necessary to maintain the virulence factor carried by the NSs phosphoprotein.

View Article: PubMed Central - PubMed

Affiliation: Molecular Genetics of Bunyavirus, Department of Virology, Institut Pasteur, Paris, France.

ABSTRACT

Background: Most arthropod-borne viruses (arboviruses) are RNA viruses, which are maintained in nature by replication cycles that alternate between arthropod and vertebrate hosts. Arboviruses appear to experience lower rates of evolution than RNA viruses that replicate in a single host. This genetic stability is assumed to result from a fitness trade-off imposed by host alternation, which constrains arbovirus genome evolution. To test this hypothesis, we used Rift Valley fever virus (RVFV), an arbovirus that can be transmitted either directly (between vertebrates during the manipulation of infected tissues, and between mosquitoes by vertical transmission) or indirectly (from one vertebrate to another by mosquito-borne transmission).

Methodology/principal findings: RVFV was serially passaged in BHK21 (hamster) or Aag2 (Aedes aegypti) cells, or in alternation between the two cell types. After 30 passages, these single host-passaged viruses lost their virulence and induced protective effects against a challenge with a virulent virus. Large deletions in the NSs gene that encodes the virulence factor were detectable from the 15(th) serial passage onwards in BHK21 cells and from the 10(th) passage in Aag2 cells. The phosphoprotein NSs is not essential to viral replication allowing clones carrying deletions in NSs to predominate as they replicate slightly more rapidly. No genetic changes were found in viruses that were passaged alternately between arthropod and vertebrate cells. Furthermore, alternating passaged viruses presenting complete NSs gene remained virulent after 30 passages.

Conclusions/significance: Our results strongly support the view that alternating replication is necessary to maintain the virulence factor carried by the NSs phosphoprotein.

Show MeSH
Related in: MedlinePlus