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Molecular basis for a lack of correlation between viral fitness and cell killing capacity.

Herrera M, García-Arriaza J, Pariente N, Escarmís C, Domingo E - PLoS Pathog. (2007)

Bottom Line: The relationship between parasite fitness and virulence has been the object of experimental and theoretical studies often with conflicting conclusions.However, subsequent plaque-to-plaque transfers resulted in profound fitness loss, but only a minimal decrease of virulence.As a consequence, depending on the passage regime, viral fitness and virulence can follow different evolutionary trajectories.

View Article: PubMed Central - PubMed

Affiliation: Centro de Biología Molecular Severo Ochoa (CSIC-UAM), Cantoblanco, Madrid, Spain.

ABSTRACT
The relationship between parasite fitness and virulence has been the object of experimental and theoretical studies often with conflicting conclusions. Here, we provide direct experimental evidence that viral fitness and virulence, both measured in the same biological environment provided by host cells in culture, can be two unrelated traits. A biological clone of foot-and-mouth disease virus acquired high fitness and virulence (cell killing capacity) upon large population passages in cell culture. However, subsequent plaque-to-plaque transfers resulted in profound fitness loss, but only a minimal decrease of virulence. While fitness-decreasing mutations have been mapped throughout the genome, virulence determinants-studied here with mutant and chimeric viruses-were multigenic, but concentrated on some genomic regions. Therefore, we propose a model in which viral virulence is more robust to mutation than viral fitness. As a consequence, depending on the passage regime, viral fitness and virulence can follow different evolutionary trajectories. This lack of correlation is relevant to current models of attenuation and virulence in that virus de-adaptation need not entail a decrease of virulence.

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Killing of BHK-21 Cells by FMDVs(A) Time needed by C-S8c1, REDpt60, 							, C-S8p260p3d, and MARLS to kill 104 BHK-21 cells as a function of the initial number of infectious units (PFU) added. The virulence assay is described in Materials and Methods. Each value represents the mean and standard deviation from triplicate assays. Inset: Relative fitness as a function of relative virulence values of FMDVs. The regression (discontinuous) line defined by C-S8c1, REDpt60, C-S8p260p3d, and MARLS is y = 3.026Ln(x) – 1.1028; R2 = 0.9721.						The regression line including 							 is y = 3.1458Ln(x) – 3.5111; R2 = 0.8507 (not drawn).						(B) Time needed by C-S8c1, C-S8c1p113, 							, and 							 to kill 104 BHK-21 cells as a function of the initial number of PFU. Each value represents the mean and standard deviation from triplicate assays. The viruses analyzed are described in Materials and Methods, and their evolutionary relationship is depicted in Figure 1. Virulence values are given in Tables 1, 2, and S1.
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ppat-0030053-g002: Killing of BHK-21 Cells by FMDVs(A) Time needed by C-S8c1, REDpt60, , C-S8p260p3d, and MARLS to kill 104 BHK-21 cells as a function of the initial number of infectious units (PFU) added. The virulence assay is described in Materials and Methods. Each value represents the mean and standard deviation from triplicate assays. Inset: Relative fitness as a function of relative virulence values of FMDVs. The regression (discontinuous) line defined by C-S8c1, REDpt60, C-S8p260p3d, and MARLS is y = 3.026Ln(x) – 1.1028; R2 = 0.9721. The regression line including is y = 3.1458Ln(x) – 3.5111; R2 = 0.8507 (not drawn). (B) Time needed by C-S8c1, C-S8c1p113, , and to kill 104 BHK-21 cells as a function of the initial number of PFU. Each value represents the mean and standard deviation from triplicate assays. The viruses analyzed are described in Materials and Methods, and their evolutionary relationship is depicted in Figure 1. Virulence values are given in Tables 1, 2, and S1.

Mentions: The capacity of to kill BHK-21 cells despite its low fitness in BHK-21 cells led us to quantitatively examine the relationship between fitness of FMDV and its capacity to kill BHK-21 cells. To this aim, FMDV clones or populations were compared in a cell killing assay, consisting in determining the time required to kill 104 BHK-21 cells as a function of the PFU added (described in Materials and Methods). The results (Figure 2A) indicate that over the time range of 12 h to 48 h postinfection, the number of PFUs needed to kill 104 BHK-21 cells varied logarithmically as a function of time. Similar quantifications of relative virulence were obtained by measuring the PFU needed to kill 104 cells in 24 h, and then by extrapolating the PFU values to 0 h postinfection (Tables 1 and S1). Virulence of was 29 to 35 times higher than virulence of C-S8c1, despite the latter displaying a 9-fold higher fitness (Tables 1 and S1). The high virulence of was not due to the plaque-to-plaque transfers, since a high virulence was also quantitated for its parental clone, , and for population C-S8p113 (Figure 2B; Tables 1, 2, and S1). deviated from a line that correlated relative fitness of FMDV and the logarithm of cell killing capacity, as reflected in the decrease of the regression coefficient (R2) (inset in Figure 2A). Probably, this deviation is due to the fact that lost fitness due to plaque-to-plaque transfers, and the other viruses were not subjected to plaque-to-plaque transfers. On the other hand, virulence determinants were acquired during the large population passages done between C-S8c1 and C-S8c1p113. The 29- to 35-fold higher virulence of with respect to C-S8c1 (Tables 1 and S1), despite its low fitness, indicates that viral fitness and virulence can be two unrelated traits.


Molecular basis for a lack of correlation between viral fitness and cell killing capacity.

Herrera M, García-Arriaza J, Pariente N, Escarmís C, Domingo E - PLoS Pathog. (2007)

Killing of BHK-21 Cells by FMDVs(A) Time needed by C-S8c1, REDpt60, 							, C-S8p260p3d, and MARLS to kill 104 BHK-21 cells as a function of the initial number of infectious units (PFU) added. The virulence assay is described in Materials and Methods. Each value represents the mean and standard deviation from triplicate assays. Inset: Relative fitness as a function of relative virulence values of FMDVs. The regression (discontinuous) line defined by C-S8c1, REDpt60, C-S8p260p3d, and MARLS is y = 3.026Ln(x) – 1.1028; R2 = 0.9721.						The regression line including 							 is y = 3.1458Ln(x) – 3.5111; R2 = 0.8507 (not drawn).						(B) Time needed by C-S8c1, C-S8c1p113, 							, and 							 to kill 104 BHK-21 cells as a function of the initial number of PFU. Each value represents the mean and standard deviation from triplicate assays. The viruses analyzed are described in Materials and Methods, and their evolutionary relationship is depicted in Figure 1. Virulence values are given in Tables 1, 2, and S1.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC1851977&req=5

ppat-0030053-g002: Killing of BHK-21 Cells by FMDVs(A) Time needed by C-S8c1, REDpt60, , C-S8p260p3d, and MARLS to kill 104 BHK-21 cells as a function of the initial number of infectious units (PFU) added. The virulence assay is described in Materials and Methods. Each value represents the mean and standard deviation from triplicate assays. Inset: Relative fitness as a function of relative virulence values of FMDVs. The regression (discontinuous) line defined by C-S8c1, REDpt60, C-S8p260p3d, and MARLS is y = 3.026Ln(x) – 1.1028; R2 = 0.9721. The regression line including is y = 3.1458Ln(x) – 3.5111; R2 = 0.8507 (not drawn). (B) Time needed by C-S8c1, C-S8c1p113, , and to kill 104 BHK-21 cells as a function of the initial number of PFU. Each value represents the mean and standard deviation from triplicate assays. The viruses analyzed are described in Materials and Methods, and their evolutionary relationship is depicted in Figure 1. Virulence values are given in Tables 1, 2, and S1.
Mentions: The capacity of to kill BHK-21 cells despite its low fitness in BHK-21 cells led us to quantitatively examine the relationship between fitness of FMDV and its capacity to kill BHK-21 cells. To this aim, FMDV clones or populations were compared in a cell killing assay, consisting in determining the time required to kill 104 BHK-21 cells as a function of the PFU added (described in Materials and Methods). The results (Figure 2A) indicate that over the time range of 12 h to 48 h postinfection, the number of PFUs needed to kill 104 BHK-21 cells varied logarithmically as a function of time. Similar quantifications of relative virulence were obtained by measuring the PFU needed to kill 104 cells in 24 h, and then by extrapolating the PFU values to 0 h postinfection (Tables 1 and S1). Virulence of was 29 to 35 times higher than virulence of C-S8c1, despite the latter displaying a 9-fold higher fitness (Tables 1 and S1). The high virulence of was not due to the plaque-to-plaque transfers, since a high virulence was also quantitated for its parental clone, , and for population C-S8p113 (Figure 2B; Tables 1, 2, and S1). deviated from a line that correlated relative fitness of FMDV and the logarithm of cell killing capacity, as reflected in the decrease of the regression coefficient (R2) (inset in Figure 2A). Probably, this deviation is due to the fact that lost fitness due to plaque-to-plaque transfers, and the other viruses were not subjected to plaque-to-plaque transfers. On the other hand, virulence determinants were acquired during the large population passages done between C-S8c1 and C-S8c1p113. The 29- to 35-fold higher virulence of with respect to C-S8c1 (Tables 1 and S1), despite its low fitness, indicates that viral fitness and virulence can be two unrelated traits.

Bottom Line: The relationship between parasite fitness and virulence has been the object of experimental and theoretical studies often with conflicting conclusions.However, subsequent plaque-to-plaque transfers resulted in profound fitness loss, but only a minimal decrease of virulence.As a consequence, depending on the passage regime, viral fitness and virulence can follow different evolutionary trajectories.

View Article: PubMed Central - PubMed

Affiliation: Centro de Biología Molecular Severo Ochoa (CSIC-UAM), Cantoblanco, Madrid, Spain.

ABSTRACT
The relationship between parasite fitness and virulence has been the object of experimental and theoretical studies often with conflicting conclusions. Here, we provide direct experimental evidence that viral fitness and virulence, both measured in the same biological environment provided by host cells in culture, can be two unrelated traits. A biological clone of foot-and-mouth disease virus acquired high fitness and virulence (cell killing capacity) upon large population passages in cell culture. However, subsequent plaque-to-plaque transfers resulted in profound fitness loss, but only a minimal decrease of virulence. While fitness-decreasing mutations have been mapped throughout the genome, virulence determinants-studied here with mutant and chimeric viruses-were multigenic, but concentrated on some genomic regions. Therefore, we propose a model in which viral virulence is more robust to mutation than viral fitness. As a consequence, depending on the passage regime, viral fitness and virulence can follow different evolutionary trajectories. This lack of correlation is relevant to current models of attenuation and virulence in that virus de-adaptation need not entail a decrease of virulence.

Show MeSH
Related in: MedlinePlus