Cross-lineage antibody responses and protection upon vaccination with inactivated swine influenza virus vaccines

Commercial, inactivated swine influenza virus (SIV) vaccines offer a solid protection against the homologous virus and its drift variants. However, they offer limited cross-protection against distinct H1 or H3 lineages. Indeed, vaccines based on European H3N2 SIVs do not protect against North American H3N2 SIVs and vice versa. Given the continuous increase in the number of novel SIV lineages, there is a need for SIV vaccines that offer a broader protection. According to recent studies in the mouse and ferret model, consecutive vaccinations with antigenically distinct influenza viruses of a given subtype may elicit a broader antibody response and protection. This has led us to investigate whether this approach may also work for SIV vaccines. Two monovalent vaccines were prepared: one vaccine contained the European H3N2 SIV sw/Gent/172/08 (Eu H3N2), the other vaccine contained the North American H3N2 SIV sw/Pennsylvania/62170-1/10 (US H3N2). These viruses show only 80,2% amino acid identity in their HA1. An oil-in-water adjuvant was added to UV-inactivated vaccine viruses. Pigs were allocated to 4 groups that were vaccinated twice with one month interval: a challenge control group, two homologous prime-boost vaccination groups (Eu H3N2 - Eu H3N2 and US H3N2 - US H3N2), and a heterologous prime-boost vaccination group (Eu H3N2 - US H3N2). Four weeks after the second vaccination they were challenged intranasally with either the European or North American H3N2 SIV. Serum antibody titers were determined in hemagglutination-inhibition, virus-neutralization and neuraminidase-inhibition assays. Peripheral blood mononuclear cells were isolated to quantify numbers of IFN-γ and IgG secreting cells by ELISPOT assays. All pigs were euthanized at 3 days post challenge and tissues of the respiratory tract were collected for virus titrations and (histo)pathology. Both homologous prime-boost vaccination groups had high serum antibody titers against the vaccine virus and a complete protection against replication of that virus in the lungs. However, antibody titers and protection against the other H3N2 SIV lineage were lacking. Heterologous prime-boost vaccination, in contrast, induced high serum antibody titers against both virus lineages, which correlated with an increased number of IgG secreting cells. A complete protection against virus replication in the lungs was observed in all pigs challenged with the Eu H3N2 and in 3 of 5 pigs challenged with US H3N2. Virus titrations of other tissues are still pending. Our data suggest that it may be possible to protect pigs against multiple lineages of H3N2 SIV using a heterologous prime-boost vaccination strategy.