Local and systemic isotype-specific antibody responses to equine influenza virus infection versus conventional vaccination.

Inactivated alum-adjuvanted conventional equine influenza virus vaccines are of poor efficacy and offer limited short-term protection against infection. In sharp contrast, natural infection with equine influenza virus confers long-term protective immunity. In order to identify the protective immune responses to equine influenza virus, the influenza virus-specific IgA, IgGa, IgGb, IgGc and IgG(T) antibody responses in nasal secretions and serum induced by natural infection and a commercial vaccine were studied by ELISA. Two groups of four influenza-naive ponies were established. In the natural infection group, ponies received 10(8.5) EID50 of A/equine/Ky/1/81 by intranasal instillation, were allowed to recover, and then were rechallenged 100 days later. All four ponies exhibited clinical signs of influenza virus infection and viral shedding following primary infection, but were completely protected from challenge infection. Antibody responses to primary infection were characterized by nasal IgA and serum IgGa and IgGb responses. Ponies in the conventional vaccine group received a commercially available vaccine by intramuscular injection followed by a booster injection 3 weeks later. Challenge infection 100 days after vaccination resulted in clinical signs of infection and viral shedding. Antibody responses to vaccination were restricted to serum IgG(T) responses only. These results demonstrate that the protective immunity generated by natural equine influenza virus infection is associated with a mucosal IgA immune response and humoral IgGa and IgGb sub-isotype responses, and that this pattern of response is not generated by conventional vaccines.