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Induction of cross-reactive antibodies in mice immunized with conserved influenza A virus neuraminidase-derived linear B-cell epitopes

Introduction. Influenza is a socially significant infection that causes profound damage to populational health and national economy annually. Preventive vaccination is the most effective means to manage influenza and its complications. Diverse influenza vaccines exist, but their common drawback is the narrow specificity, the need for annual renewal of the strain composition, not always satisfactory immunogenicity and effectiveness. In this regard, close attention is paid to developing universal influenza vaccines aimed to induce cross-reactive immune-related cues against most conserved parts of viral proteins. Antibodies against neuraminidase (NA) are able to provide heterosubtypic protection, which is important due to potential threat posed by influenza viruses differed in hemagglutinin and neuraminidase sequence in comparison to currently circulating viruses. The present study is aimed to search for new and analysis of earlier predicted NA linear B-cell epitopes conserved among all influenza A virus subtypes. Results. Eight conserved linear B-cell epitopes were identified around the active site of neuraminidase, three of which (MNPNQKIITIGS, ILRTQESEC, and DNWKGSNRP) were synthesized de novo, conjugated with bovine serum albumin to be next used for mouse immunization. IgG antibodies were detected by ELISA in the sera of immunized mice. Antibodies were found to specifically bind to various influenza A viruses containing NA subtypes N1, N2, N3, and N9. Immunization with NA peptides did not protect mice from substantial body weight loss after infection with lethal H1N1 influenza virus. However, all immunized mice survived during the observation period, whereas in the control group the survival rate was as low as 28.6%. Analyzing lung viral load in the mice infected with the H1N1 virus revealed no differences in virus titers on either day 4 or 8 post-infection. Nevertheless, the protective effect lacked after the mice were challenged with lethal H7N9 influenza virus: Mortality rate, body weight loss, and lung virus titers were comparable in immunized and control mice. Conclusions. The data obtained evidenced about serum cross-reactive anti-NA antibodies induced by immunization with NA peptides, as well as protective efficacy against infection caused by H1N1 virus, but not H7N9 virus. Such results hold promise and indicate that NA linear B-cell epitopes can be used for designing epitope-directed influenza vaccines, but a deeper and more comprehensive study on the specificity of conserved NA epitopes, as well as optimization of immunization schemes for achieving higher protective efficacy are required.