Anti-viral immunity induced by recombinant nucleoprotein of influenza A virus. III. Delivery of recombinant nucleoprotein to the immune system using attenuated Salmonella typhimurium as a live carrier.

A plasmid encoding the influenza nucleoprotein gene from A/NT/60/68 virus was transduced into the attenuated Salmonella typhimurium aroA- strain SL3261. The bacterial vector expressing the viral gene product was able to induce both humoral and cell-mediated immune responses to the nucleoprotein antigen. CD4+ virus-specific T cells capable of proliferation were readily induced and, in some circumstances, class II major histocompatibility complex (MHC)-restricted cytotoxicity was detected. However, virus-specific class I MHC-restricted cytotoxic T lymphocytes (CTL) were not detected after such immunization. Mice immunized orally with the nucleoprotein-expressing bacteria mounted a strong anti-viral antibody response and spleen cells from such mice proliferated specifically to virus challenge in vitro, producing interferon-gamma (IFN-gamma) and interleukin-2 (IL-2). Orally immunized mice showed significant protection from challenge infection with influenza virus if the mice were also boosted intranasally before infection.