Multiple basic amino acids in the cleavage site of H7N9 hemagglutinin contribute to high virulence in mice.
Background
Avian influenza A (H7N9) virus has caused more than 1,500 cases of human infection since its emergence in early 2013. Displaying little or no pathogenicity in poultry, but a 40% case-fatality rate in humans, five waves of H7N9 human infections occurred in China during 2013-2017, caused solely by a low pathogenicity strain. However, avian isolates possessing a polybasic connecting peptide in the hemagglutinin (HA) protein were detected in mid-2016, indicating that a highly pathogenic virus had emerged and was co-circulating with the low pathogenicity strains.


Methods
Here we characterize the pathogenicity of a newly emerged human H7N9 variant with a PEVPKRKRTAR/GLF insertion motif at the cleavage site of the HA protein in vitro and in vivo.


Results
This variant replicates in MDCK cells independently of TPCK-trypsin, which is indicative of high pathogenicity in chickens. The 50% mouse lethal dose (MLD50) of this novel isolate was less than 10 plaque forming units (PFU), compared with 3.16×104 for an identical virus lacking the polybasic insertion, indicating a high virulence phenotype.


Conclusions
Our results demonstrate that the multiple basic amino acid insertion in the HA protein of the H7N9 variant confers high virulence in mammals, highlighting a potential risk to humans. Continuous viral surveillance is therefore necessary in the China region to improve pandemic preparedness.
['Song|Wenjun|W|', 'Huang|Xiaofeng|X|', 'Guan|Wenda|W|', 'Chen|Pin|P|', 'Wang|Pui|P|', 'Zheng|Min|M|', 'Li|Zhengtu|Z|', 'Wang|Yutao|Y|', 'Yang|Zifeng|Z|', 'Chen|Honglin|H|', 'Wang|Xinhua|X|']
cleavage site H7N9 polybasic connecting peptide Influenza A virus (IAV) highly pathogenic avian influenza (HPAI)