Evaluation of Sindbis-M2e Virus Vector as a Universal Influenza A Vaccine
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Although avian influenza virus (AIV) infections in domestic poultry are uncommon, transmission of avian influenza from wild waterfowl reservoirs does occur. Depopulation of the infected flock is the typical response to AIV outbreaks in domestic chicken production, causing a loss in profits and accumulation of unexpected expenses. Because it is impossible to know which of many virus subtypes will cause an outbreak, it is not feasible for the U.S. to stockpile vaccines against all possible avian influenza threats. Currently, the U.S. does not routinely vaccinate chickens against influenza due to the inability to differentiate infected from vaccinated animals (DIVA), which would place limitations on its trade markets. A Sindbis virus vector expressing the PR8 influenza strain's M2e peptide was developed as a potential universal DIVA vaccine. M2e is a conserved peptide amongst influenza A viruses; M2e-specific antibodies induce antibody-dependent cytotoxicity or phagocytosis of infected cells, reducing production and shedding of AIV during infection. In this study, chickens were vaccinated at one-month-of-age with parental (E2S1) or recombinant Sindbis viruses expressing the PR8 M2e peptide (E2S1-M2e) by subcutaneous or intranasal routes at high (106 pfu) or low (103 pfu) dosages. Chickens were boosted at 2-weeks post-initial vaccination using the same virus, route, and dosage, then challenged with low pathogenic H5N3 AIV at 0.2 mL of 106/mL EID50 2-weeks post-boost. Serum samples were collected at 1-week and 2-weeks post-vaccination, 2-weeks post-boost, and 2-weeks post-challenge and screened for PR8 M2e-specific IgY antibody production by ELISA. Both high and low dose subcutaneously, as well as high dose intranasally vaccinated E2S1-M2e groups produced significantly higher levels of PR8 M2e-specific IgY antibodies as early as 1-week post-vaccination, while the uninoculated control and E2S1 groups remained negative for all pre-challenge time points. M2e-specific IgY antibodies capable of binding the challenge H5N3 M2e peptide were detected in groups with existing vaccine-induced M2e-specific antibodies pre-challenge, suggesting antibody M2e cross-reactivity. After challenge, all groups developed M2e-specific IgY antibodies and high HI titers, verifying successful AIV infection during challenge and production of hemagglutinin-specific antibodies. Viral shedding titers 4-days post-challenge were used to measure vaccine efficacy and were similar amongst all groups. Microneutralization assay results confirmed that post-boost serum samples, containing only M2e-specific antibodies, were unable to neutralize AIV in vitro. Although the E2S1-M2e vaccine was capable of producing high levels of M2e-specific IgY antibodies when inoculated subcutaneously, these antibodies were not able to reduce viral shedding and therefore did not protect chickens from AIV.