Evaluation of the Humoral and Cellular Immune Responses Elicited by the Live Attenuated and Inactivated Influenza Vaccines and Their Roles in Heterologous Protection in Ferrets

The humoral and cellular immune responses elicited by the trivalent live attenuated influenza vaccine (LAIV) and the trivalent inactivated influenza vaccine (TIV) were evaluated in the ferret model, using newly developed ferret immunological reagents and assays. In contrast to the TIV, which only induced immune responses in primed animals, LAIV induced strong influenza virus-specific serum antibody and T-cell responses in both naive and influenza-seropositive animals. The LAIV offered significant protection against a heterologous H1N1 virus challenge infection in the upper respiratory tract. Influenza virus-specific immunoglobulin A (IgA) and immunoglobulin G (IgG) antibody-secreting cells (ASCs) and influenza virus-specific CD4(+) and CD8(+) T cells were detected in the circulation and local paratracheal draining lymph nodes. The frequency of the influenza-specific ASCs in the local lymph nodes appeared to correlate with the degree of protection in the upper respiratory tract. The protection conferred by the LAIV could be attributed not only to the antibody response but also to the cell-mediated and local mucosal immune responses, particularly in naive ferrets. These findings may explain why the LAIV is immunologically superior and offers immediate protection after a single dose in children.