Virulence determinants of novel influenza A (H7N9) virus in mammals
Tanhuan Gan1, LaiGiea Seng1, ZhiGang Yan2, PeiWen Chen1,2 , XianFan Wu1,2 , RiRong Chen2 ,Meijin Zhou2 , Yi Guan1,2 , HuaChen Zhu1,2* 1State Key Laboratory of Emerging Infectious Diseases/Centre of Influenza Research, School of Public Health, The University of Hong Kong, Hong Kong SAR, China; 2Joint Influenza Research Centre (SUMC/HKU), Shantou University Medical College, Shantou 515041, China *Corresponding Author The novel H7N9 virus infections in humans have various clinical presentations, ranging from asymptomatic to severe and fatal outcomes. Mechanisms that led to the increased mammalian infectivity and pathogenicity remain to be explored. To identify the virulence marker, we generated a series of reassortants derived from two closely related viruses, which are the virulent A/Shanghai/01/2013 (SH1) and the avirulent A/Shanghai/5/2013 (SH5) viruses. Infection in the C57BL/6 mouse model indicated that PB2, PB1, NA and M genes are potential virulence determinants. Further investigations demonstrated that SH1-derived PB2 and PB1 genes play indispensable roles in improving virus fitness. Significantly, PB1-L598M, PB2-E191K and the well-established mammalian marker PB2-E627K mutations could independently lead to increased polymerase activity and virulence of SH5 virus. Conversely, the corresponding reversed mutation restored the virulence of SH1 virus. In addition, PB2-191K was highly conserved in early H7N9 influenza virus isolates, suggesting PB2-191K contributes to the genesis and emergence of this novel H7N9 virus. The biological significances of the virulence markers reported here could serve as indicators for virus surveillance for treatment and prevention of the H7N9 virus.