Growth characteristics of experimental live influenza vaccine strains with modified NP and NS genes

BACKGROUND: Vaccination is the most effective means of fighting influenza epidemics, but the immunogenicity of licensed influenza vaccines is not always satisfactory. One of the ways to increase the immunogenicity of an attenuated live influenza vaccine is to shorten the open reading frame of the NS1 protein, a modulator of innate antiviral immunity. In addition, the T-cell response to vaccination can be optimized by including the NP gene from the epidemic parental virus into the genome of vaccine strains.

MATERIALS AND METHODS: The open reading frame of the NS1 protein of the master donor virus A/Leningrad/134/17/57 was truncated to 126 amino acids by site-directed mutagenesis. The HA, NA, and NP genes of the model virus A/Anhui/1/2013 (H7N9) were cloned into the pCIPolISapIT vector. The rescue of recombinant influenza viruses was performed by transfection of Vero cells with a desired set of plasmids. The growth properties of the recombinant viruses were determined in embryonated chicken eggs incubated at different temperatures, as well as in the tissues of the respiratory tract of mice (nasal turbinates, lungs).

RESULTS: Experimental live influenza vaccine strains of subtype H7N9 with genome compositions 6:2 and 5:3 and carrying a full-length or truncated NS1 gene were actively replicated in eggs under optimal conditions, while maintaining the temperature-sensitive and cold-adapted phenotypes characteristic of classical live influenza vaccine strains. All viruses lacked the ability to grow in the lungs of C57BL/6J mice, which confirms the attenuated phenotype of the viruses. In the nasal passages of mice, only viruses with the full-length NS1 gene replicated, while viruses expressing the truncated NS1 protein were not detected in the respiratory tract of animals.

CONCLUSIONS: The results indicate that modification of the NS1 gene of the vaccine virus and the inclusion of wild-type NP gene in its genome does not affect its growth characteristics in eggs. A decrease in the activity of viral replication in the upper respiratory tract of mice with a shortening of the NS1 open reading frame indicates an increase in the attenuating properties of modified vaccines, which opens up prospects for the use of new vaccines in children under three years of age.