An RNA virus can adapt to the multiplicity of infection.

We used vesicular stomatitis virus to test the effect of complementation on the relative fitness of a deleterious mutant, monoclonal antibody-resistant mutant (MARM) N, in competition with its wild-type ancestor. We carried out competitions of MARM N and wild-type populations at different multiplicities of infection (MOIs) and initial ratios of the wild type to the mutant and found that the fitness of MARM N relative to that of the wild type is very sensitive to changes in the MOI (i.e., the degree of complementation) but depends little, if at all, on the initial frequencies of MARM N and the wild type. Further, we developed a mathematical model under the assumption that during coinfection both viruses contribute to a common pool of protein products in the infected cell and that they both exploit this common pool equally. Under such conditions, the fitness of all virions that coinfect a cell is the average fitness in the absence of coinfection of that group of virions. In the absence of coinfection, complementation cannot take place and the relative fitness of each competitor is only determined by the selective value of its own products. We found good agreement between our experimental results and the model predictions, which suggests that the wild type and MARM N freely share all of their gene products under coinfection.RNA viruses form highly polymorphic populations called quasispecies (8,9,13). The origin of much of their genetic diversity is high mutational pressure. Most RNA viruses have mutation rates on the order of one miscopied base per genome and generation (10, 11). However, high mutational pressure is not necessarily the only factor promoting genetic diversity. Niche differentiation can allow stable polymorphisms in an infected host. For instance, during a polyclonal immune response, different subpopulations may have different sensitivities to individual antibodies; variation within a host can also be related to differences in the tropism of different viral subpopulations. Virus-virus interactions may also promote stable polymorphisms in an infected host. In cell culture, such interactions readily occur during replication inside a cell and are likely to be an important contributor to the maintenance of variation. When two or more virions coinfect the same cell, complementation can take place. Not every function can be complemented in trans. In positive-stranded viruses, translation and replication are coupled (25); therefore, many functions need to be provided in cis and cannot be rescued by complementation (33, 37). Other proteins can freely interact with heterologous genomes or replicons, even from different viral species (18,21). In the absence of compartmentalization or other limitations to the diffusion of viral products, soluble proteins can interact with any genome inside the cell, potentially changing the phenotype of the virions and masking targets for natural selection to operate. A remarkable example of this phenomenon is phenotypic mixing and hiding, particularly for monoclonal antibody (...

Section: Discussionmentioning

confidence: 46%

mentioning

confidence: 99%

Density-Dependent Selection in Vesicular Stomatitis Virus

Novella

Reissig

Wilke

2004

“…These results have been confirmed with MARM N, another low-fitness VSV strain (18,27), demonstrating that MOI-dependent selection has a strong effect during competitions. MOI-dependent selection has been previously reported for other viruses, including foot-and-mouth disease virus (20) and phage -6 (22-24), that had been selected for improved replication at high MOI. In contrast with the work done with foot-and-mouth disease virus and -6, none of the VSV populations had been previously selected for increased fitness at high MOI.…”

Section: Resultsmentioning

confidence: 99%

Fitness Analyses of Vesicular Stomatitis Strains with Rearranged Genomes Reveal Replicative Disadvantages

Novella

Ball

Wertz

2004

Gene expression of the nonsegmented negative-strand RNA viruses is determined by the position of each gene relative to that the single 3 promoter. The general order of genes among all of the viruses of the order Mononegavirales is highly conserved. In previous work we generated recombinant viruses in which the order of the three central genes of the prototypical rhabdovirus, vesicular stomatitis virus, was rearranged to all six possible permutations. While some of these viruses replicated less well than the wild type when assayed by single-step growth analyses in BSC-1 cells, others replicated as well or slightly better. In the work reported here, we used competition assays to compare the fitness of the viruses with alternative gene orders to that of the wild-type (wt) virus. We found that the relative fitness of these recombinant viruses depended on the multiplicity of infection (MOI) but not on the population size. However, during competitions at low MOI, when complementation cannot compensate for the defects of the populations with rearranged genomes, the virus with the wt gene order was always the most fit.

“…These experiments maintained an MOI of 0.6 as the ratios of the viruses were altered. We speculated the input MOI might influence the outcome of the competitive replication as it has for other viruses (31,32,35,38). Thus, the MOI varied as a 1:1 ratio of the viruses was held constant.…”

Section: Resultsmentioning

confidence: 99%

In Vitro and In Vivo Fitness of Respiratory Syncytial Virus Monoclonal Antibody Escape Mutants

Zhao

Liu

Chen

et al. 2006

Respiratory syncytial virus (RSV) is the only infectious disease for which a monoclonal antibody (MAb) is used in humans. Palivizumab (PZ) is a humanized murine MAb to the F protein of RSV. PZ-resistant viruses appear after in vitro and in vivo growth of RSV in the presence of PZ. Fitness for replication could be a determinant of the likelihood of dissemination of resistant viruses. We assessed the fitness of two PZ-resistant viruses (F212 and MP4). F212 grew less well in cell culture than the parent A2 virus and was predicted to be less fit than A2. Equal amounts of F212 and A2 were mixed and passaged in cell culture. F212 disappeared from the viral population, indicating it was less fit than the A2 virus. The MP4 virus grew as well as A2 in culture and in cotton rats. A2/MP4 virus input ratios of 1:1, 10:1, 100:1, and 1,000:1 were compared in competitive replication. For all input ratios except 1,000:1, the MP4 virus became dominant, supplanting the A2 virus. The MP4 virus also dominated the A2 virus during growth in cotton rats. Thus, the mutant MP4 virus was more fit than A2 virus in both in vitro and in vivo competitive replication. Whether this fitness difference was due to the identified nucleotide substitutions in the F gene or to mutations elsewhere in the genome is unknown. Understanding the mechanisms by which mutant virus fitness increased or decreased could prove useful for consideration in attenuated vaccine design efforts.