Study on acquisition of pathogenicity of a virulent avian influenza virus isolates

虽然家禽中的高毒性禽流感病毒被认为是来自野鸟中的无毒病毒，但没有直接证据支持这一观点。此外，病毒致病的确切机制仍不清楚。这种变化最可能的机制是通过易错RNA聚合酶引入突变，然后选择毒性病毒。为了研究这种选择性机制是否存在于鸡中，我们在鸡中传代了一种来自野生水禽的无毒病毒分离株，该病毒在鸡中复制较差。经气囊接种连续传24代，再经鸡脑内传5代，病毒对鸡的致病性达到高致病性，死亡率达100%。在血凝素（HA）切割位点的序列分析表明，原始分离物含有典型的无毒型序列R-E-T-R，并且在鸡中传代期间，该序列逐步改变为典型的毒性型，即，R-R-K-K-R这些结果表明，在野生水禽自然界中保持的无毒病毒，具有一致的无毒型序列，R-E-T-R，有可能成为高致病性病毒。这些发现还表明，在鸡中存在从非致病性前体中选择高致病性衍生物的机制。

Although highly virulent avian influenza viruses in poultry are thought to arise from avirulent viruses maintained in wild birds, there is no direct evidence supporting this notion. In addition, the precise mechanism by which virus become pathogenic remains unknown. Most possible mechanism for this change is the introduction of mutation by error-prone RNA polymerase, followed by the selection of virulent viruses. To investigate whether this selective mechanism exists in chickens, we passaged in chicken an avirulent virus isolate from wild waterfowl, that poorly replicate in chickens. After 24 consecutive passages by air sac inoculation, followed by 5 passages in brain of chicks, the avirulent virus became highly pathogenic in chickens with 100% mortality rate. Sequence analysis at the hemmaglutinin (HA) cleavage site showed that the original isolate contained the typical avirulent type sequence, R-E-T-R, and during passages in chickens, the sequence step wisely changed to a typical virulent type, i.e., R-R-K-K-R. These results demonstrated that avirulent viruses maintained in wild waterfowl in nature, bearing the consensus avirulent type sequence, R-E-T-R, have a potential to become highly pathogenic viruses. These findings also indicated that a mechanism exist in chickens that select highly pathogenic derivatives from non-pathogenic precursors.