The gain of function experiments in which avian influenza H5N1 virus was provided the ability to transmit by aerosol among ferrets were met with substantial outrage from both the press and even some scientists; scenarios of lethal viruses escaping from the laboratory and killing millions proliferated (see examples here and here). The recent publication of new influenza virus gain of function studies from the laboratories of Kawaoka and Perez have unleashed another barrage of criticism.
The work by Kawaoka and colleagues attempts to answer the question of whether an influenza virus similar to that which killed 50 million people in 1918 could emerge today. First they identified in the avian influenza virus sequence database individual RNA segments that encode proteins that are very similar to the 1918 viral proteins.
Next, an infectious influenza virus was produced with 8 RNA segments that encode proteins highly related to those of the 1918 virus. Each RNA segment originates from a different avian influenza virus, and differs by 8 (PB2), 6 (PB1), 20 (PB1-F2), 9 (PA), 7 (NP), 33 (HA), 31 (NA), 1 (M1), 5 (M2), 4 (NS1), and 0 (NS2) amino acids from the 1918 virus.
The 1918-like avian influenza virus was less pathogenic in mice and ferrets compared with the 1918 virus, and more pathogenic than a duck influenza virus isolated in 1976. Virulence in ferrets increased when the HA or PB2 genes of the 1918-like avian influenza virus were substituted with those from the 1918 virus.
Aerosol transmission among ferrets was determined for the 1918-like avian influenza virus, and reassortants containing 1918 viral genes (these experiments are done by housing infected and uninfected ferrets in neighboring cages). The 1918 influenza virus was transmitted to 2 of 3 ferrets. Neither the 1918-like avian influenza virus, nor the 1976 duck influenza virus transmitted among ferrets. Aerosol transmission among ferrets was observed after infection with two different reassortant viruses of the 1918-avian like influenza virus: one which possesses the 1918 virus PB2, HA, and NA RNAs (1918 PB2:HA:NA/Avian), and one which possesses the 1918 virus PA, PB1, PB2, NP, and HA genes (1918(3P+NP):HA/Avian).
It is known from previous work that amino acid changes in the viral HA and PB2 proteins are important in allowing avian influenza viruses to infect humans. Changes in the viral HA glycoprotein (HA190D/225D) shift receptor specificity from avian to human sialic acids, while a change at amino acid 627 of the PB2 protein to a lysine (627K) allows avian influenza viruses to efficiently replicate in mammalian cells, and at the lower temperatures of the human upper respiratory tract.
These changes were introduced into the genome of the 1918-like avian influenza virus. One of three contact ferrets was infected with 1918-like avian PB2-627K:HA-89ED/190D/225D virus (a mixture of glutamic acid and aspartic acid at amino acid 89 was introduced during propagation of the virus in cell culture). Virus recovered from this animal had three additional mutations: its genotype is 1918-like avian PB2-627K/684D : HA-89ED/113SN/ 190D/225D/265DV : PA-253M (there are mixtures of amino acids at HA89, 113, and 265). This virus was more virulent in ferrets and transmitted by aerosol more efficiently than the 1918-like avian influenza virus. The virus recovered from contact ferrets contained yet another amino acid change, a T-to-I mutation at position 232 of NP. Therefore ten amino acid changes are associated with allowing the 1918-like avian influenza virus to transmit by aerosol among ferrets. Aerosol transmission of these viruses is not associated with lethal disease in ferrets.