Marek’s disease virus (MDV) genome contains a number of uncharacterized long open reading frames (LORF) and their role in viral pathogenesis has not been fully investigated. Among them, LORF9 (MDV069) and LORF10 (MDV071) are locate at the right terminus of the MDV genome unique long region (U_L). To investigate their role in MDV pathogenesis, we generated LORF9 or LORF10 deletion and revertant viruses. In vitro growth kinetics results show that both LORF9 and LORF10 are not essential for virus growth in cell culture. However, LORF9, but not LORF10, is involved in MDV early cytolytic replication in vivo, as evidenced by limited viral antigen expression in lymphoid organs of LORF9 deletion virus inoculated chickens. MDV genome copy number data further confirmed that LORF9 is important for MDV replication in spleen during early cytolytic phase. Deletion of LORF9 also partially impairs the replication of MDV in feather follicle epithelium (FFE); however, it can still establish latency and transformation. In addition, pathogenesis studies show that deletion of LORF9, but not LORF10, result in significant reduction of MDV induced mortality and slightly reduce MDV associated tumors of inoculated chickens. Importantly, we confirmed these results with the generation of LORF9 and LORF10 revertant viruses that fully restore the phenotypes of parental MDV. In conclusion, our results show that deletion of LORF9, but not LORF10, significantly impair viral replication in lymphoid organs during early cytolytic phase and attenuate Marek’s disease virus pathogenesis.

马立克氏病病毒（MDV）基因组包含许多未鉴定的长开放阅读框（LORF），其在病毒发病机理中的作用尚未得到充分研究。其中，LORF9（MDV069）和LORF10（MDV071）位于MDV基因组独特长区（U _L）的右端。为了研究它们在MDV发病机理中的作用，我们产生了LORF9或LORF10缺失和回复病毒。体外生长动力学结果表明，LORF9和LORF10都不是细胞培养中病毒生长所必需的。但是，LORF9而不是LORF10参与了体内MDV的早期细胞溶解复制，如LORF9缺失病毒接种鸡的淋巴器官中有限的病毒抗原表达所证明。MDV基因组拷贝数数据进一步证实，LORF9对于早期溶细胞阶段中脾脏MDV复制很重要。LORF9的缺失也会部分损害MDV在羽毛滤泡上皮（FFE）中的复制；但是，它仍然可以建立延迟和转换。此外，发病机制研究表明，删除LORF9而不是LORF10会导致MDV诱导的死亡率显着降低，并略微降低接种鸡的MDV相关肿瘤。重要的是，我们通过完全恢复父母MDV表型的LORF9和LORF10回复病毒的产生证实了这些结果。总之，我们的结果表明删除了LORF9，但没有删除LORF10，