In vitro evolution of herpes simplex virus 1 (HSV-1) reveals selection for syncytia and other minor variants in cell culture. Human alphaherpesvirus 1

The large dsDNA virus HSV-1 is considered to be genetically stable, yet it can rapidly evolve in response to strong selective pressures such as antiviral treatment. Deep sequencing has revealed that clinical and laboratory isolates of this virus exist as populations that contain a mixture of minor alleles or variants, similar to many RNA viruses. The classic virology approach of plaque purifying virus creates a genetically homogenous population, but it is not clear how closely this represents the mixed virus populations found in nature. We sought to study the evolution of mixed versus highly purified HSV-1 populations in controlled cell culture conditions, to examine the impact of this genetic diversity on evolution. We found that a mixed population of HSV-1 acquired more genetic diversity and underwent a more dramatic phenotypic shift than a plaque-purified population, producing a viral population that was almost entirely syncytial after just ten passages. At the genomic level, adaptation and genetic diversification occurred at the level of minor alleles or variants in the viral population. Certain genetic variants in the mixed viral population appeared to be positively selected in cell culture, and this shift was also observed in clinical samples during their first passages in vitro. In contrast, the plaque-purified viral population did not appear to change substantially in phenotype or overall quantity of minor allele diversity. These data indicate that HSV-1 is capable of evolving rapidly in a given environment, and that this evolution is facilitated by diversity in the viral population.

大型双链DNA（double-stranded DNA, dsDNA）病毒单纯疱疹病毒1型（HSV-1）通常被认为具有遗传稳定性，但其可在抗病毒治疗等强选择压力下快速进化。深度测序研究显示，该病毒的临床分离株与实验室分离株均以携带次要等位基因或变异体的混合种群形式存在，这一特征与多数RNA病毒相似。经典病毒学空斑纯化方法可获得遗传均一的病毒群体，但目前尚不明确该方法能否如实反映自然界中存在的混合病毒种群。本研究旨在在可控细胞培养条件下，对比混合病毒群体与高度纯化病毒群体的进化过程，以探究该遗传多样性对病毒进化的影响。研究发现，与空斑纯化的病毒群体相比，混合HSV-1种群获得了更多遗传多样性，并经历了更为显著的表型转变：仅经过10次传代，病毒群体便几乎完全形成合胞体（syncytial）。在基因组层面，病毒群体的适应性进化与遗传多样化均发生于次要等位基因或变异体层面。混合病毒群体中的部分遗传变异体在细胞培养过程中受到正向选择，且该演化转变在临床样本的首次体外传代中同样被观测到。与之相对，空斑纯化的病毒群体在表型与次要等位基因多样性的整体水平上均未出现显著变化。上述数据表明，HSV-1能够在特定环境中快速进化，而病毒群体的遗传多样性可促进这一进化过程。