Studies on the IgA-independent immunological responses in mice to influenza virus challenge after oral vaccination with irradiated whole virus and anerythrocyte complex

Previous studies have described an oral influenza vaccine comprising whole irradiated virus and an erythrocyte complex (IV-EC), which gave broad-based protection against influenza virus challenge in mice. The present study ex amined the immune responses generated after live virus challenge of vaccina ted mice, particularly to determine whether mice vaccinated with IV-EC had enhanced CTL activity to compensate for the previously reported diminution in lung IgA response. Oral vaccine groups examined were IV-EC, live virus a lone (LV) or live virus-erythrocyte complex (LV-EC), compared with irradiat ed virus and erythrocyte alone controls. The antibody responses of IV-EC an d LV-EC vaccinated mice showed significantly elevated lung and serum IgG2a levels post live virus challenge, with no comparable increases in IgG1 leve ls compared to controls. Spleen cells from IV-EC mice showed an enhanced po st-challenge proliferative response to antigen compared with mice that had received live oral vaccines, indicating enhanced cellular activity post IV- EC immunization. However, CTL activity was not enhanced for IV-EC mice, and live virus-vaccinated mice had reduced CTL activity compared with controls , indicating that CTL were not important for post-vaccine protection. Cytok ine analysis revealed a predominant IFN-gamma response in spleen cells from orally vaccinated mice, whereas IL-4 was not detected in any lung or splee n culture analysed. The results suggest, therefore, that protection from li ve influenza challenge after IV-EC or LV-EC vaccination was due to an IFN-m ediated IgG2a response. Definitive confirmation of the role of these factor s in post-vaccine protection can now be tested in IgG2a-depleted or IFN-gam ma gene knockout mouse models.