A locally administered single cycle influenza vaccine expressing a non-fusogenic stabilised haemagglutinin stimulates strong T-cell and neutralising antibody immunity

Current influenza vaccination approaches protect against specific viral strains, but do not consistently induce broad and long-lasting protection to the diversity of circulating influenza viruses. Single cycle viruses delivered to the respiratory tract may offer a promising solution as they safely express a diverse array of viral antigens by undergoing just one round of cell infection in their host and stimulate broadly protective resident memory T-cell responses in the lung. We have previously developed a vaccine candidate called S-FLU that is limited to a single cycle of infection by inactivation of the hemagglutinin signal sequence and induces a broadly cross-reactive T-cell response and antibodies to neuraminidase, but fails to induce neutralising antibodies to hemagglutinin after intranasal administration. This study describes the development of CLEARFLU, a derivative of S-FLU that is designed to add a neutralising antibody response to hemagglutinin. In contrast to S-FLU, which does not express a hemagglutinin molecule at the infected cell surface, CLEARFLU viruses express a stabilised non-fusogenic hemagglutinin. They are equally limited to a single cycle of infection, but induce a neutralising antibody response to the expressed hemagglutinin in addition to the cytotoxic T lymphocyte (CTL) responses to internal proteins and antibodies to neuraminidase induced by S-FLU. This represents a notable advantage as CLEARFLU viruses may provide sterile immunity against strain-matched challenge as well as non-sterile protection against a broad range of influenza viruses. ImportanceInfluenza is a serious public health concern, causing seasonal epidemics as well as pandemics in people. Influenza can also cause severe agricultural losses due to its circulation in farmed poultry and swine. A major challenge in the control of influenza is the diversity of circulating viruses. Developing vaccines which stimulate immunity to a wide array of influenza viruses is therefore important for protecting human and animal populations from disease and death. In this study, we describe an approach for developing influenza vaccines which trigger immune mechanisms shown to induce broad protection against a diversity of viruses, while also conserving the strong protection against specific strains observed in existing vaccines.