Attenuated influenza A viruses with modified cleavage sites in hemagglutinin as live vaccines

Development of in ovo-compatible NS1-truncated live attenuated influenza vaccines by modulation of hemagglutinin cleavage and polymerase acidic X frameshifting sites

Emerging avian influenza viruses pose a high risk to poultry production, necessitating the need for more broadly protective vaccines. Live attenuated influenza vaccines offer excellent protective efficacies but their use in poultry farms is discouraged due to safety concerns related to emergence of reassortant viruses. Vaccination of chicken embryos inside eggs (in ovo) induces early immunity in young chicks while reduces the safety concerns related to the use of live vaccines on farms. However, in ovo vaccination using influenza viruses severely affects the egg hatchability. We previously engineered a high interferon-inducing live attenuated influenza vaccine candidate with an enhanced protective efficacy in chickens. Here, we asked whether we could further modify this high interferon-inducing vaccine candidate to develop an in ovo-compatible live attenuated influenza vaccine. We first showed that the enhanced interferon responses induced by the vaccine is not enough to attenuate the virus in ovo. To reduce the pathogenicity of the virus for chicken embryos, we replaced the hemagglutinin cleavage site of the H7 vaccine virus (PENPKTR/GL) with that of the H6-subtype viruses (PQIETR/GL) and disrupted the ribosomal frameshifting site responsible for viral polymerase acidic X protein expression. In ovo vaccination of chickens with up to 10⁵ median egg infectious dose of the modified vaccine had minimal effects on hatchability while protecting the chickens against a heterologous challenge virus at two weeks of age. This study demonstrates that targeted genetic mutations can be applied to further attenuate and enhance the safety of live attenuated influenza vaccines to develop future in ovo vaccines for poultry.

Targeting an Oncolytic Influenza A Virus to Tumor Tissue by Elastase

Oncolytic viruses are currently established as a novel type of immunotherapy. The challenge is to safely target oncolytic viruses to tumors. Previously, we have generated influenza A viruses (IAVs) containing deletions in the viral interferon antagonist. Those deletions have attenuated the virus in normal tissue but allowed replication in tumor cells. IAV entry is mediated by hemagglutinin (HA), which needs to be activated by a serine protease, for example, through trypsin. To further target the IAV to tumors, we have changed the trypsin cleavage site to an elastase cleavage site. We chose this cleavage site because elastase is expressed in the tumor microenvironment. Moreover, the exchange of the cleavage site previously has been shown to attenuate viral growth in lungs. Newly generated elastase-activated influenza viruses (AE viruses) grew to similar titers in tumor cells as the trypsin-activated counterparts (AT viruses). Intratumoral injection of AE viruses into syngeneic B16f1 melanoma-derived tumors in mice reduced tumor growth similar to AT viruses and had a better therapeutic effect in heterologous human PANC-1-derived tumors. Therefore, the introduction of the attenuation marker “elastase cleavage site” in viral HA allows for safe, effective oncolytic virus therapy.

Development of live-attenuated influenza vaccines against outbreaks of H5N1 influenza

Several global outbreaks of highly pathogenic avian influenza (HPAI) H5N1 virus have increased the urgency of developing effective and safe vaccines against H5N1. Compared with H5N1 inactivated vaccines used widely, H5N1 live-attenuated influenza vaccines (LAIVs) have advantages in vaccine efficacy, dose-saving formula, long-lasting effect, ease of administration and some cross-protective immunity. Furthermore, H5N1 LAIVs induce both humoral and cellular immune responses, especially including improved IgA production at the mucosa. The current trend of H5N1 LAIVs development is toward cold-adapted, temperature-sensitive or replication-defective vaccines, and moreover, H5N1 LAIVs plus mucosal adjuvants are promising candidates. This review provides an update on the advantages and development of H5N1 live-attenuated influenza vaccines.

Rationalizing the development of live attenuated virus vaccines

The design of vaccines against viral disease has evolved considerably over the past 50 years. Live attenuated viruses (LAVs)-those created by passaging a virus in cultured cells-have proven to be an effective means for preventing many viral diseases, including smallpox, polio, measles, mumps and yellow fever. Even so, empirical attenuation is unreliable in some cases and LAVs pose several safety issues. Although inactivated viruses and subunit vaccines alleviate many of these concerns, they have in general been less efficacious than their LAV counterparts. Advances in molecular virology--creating deleterious gene mutations, altering replication fidelity, deoptimizing codons and exerting control by microRNAs or zinc finger nucleases--are providing new ways of controlling viral replication and virulence and renewing interest in LAV vaccines. Whereas these rationally attenuated viruses may lead to a new generation of safer, more widely applicable LAV vaccines, each approach requires further testing before progression to human testing.

Rationalizing the development of live attenuated virus vaccines

The design of vaccines against viral disease has evolved considerably over the past 50 years. Live attenuated viruses (LAVs)-those created by passaging a virus in cultured cells-have proven to be an effective means for preventing many viral diseases, including smallpox, polio, measles, mumps and yellow fever. Even so, empirical attenuation is unreliable in some cases and LAVs pose several safety issues. Although inactivated viruses and subunit vaccines alleviate many of these concerns, they have in general been less efficacious than their LAV counterparts. Advances in molecular virology--creating deleterious gene mutations, altering replication fidelity, deoptimizing codons and exerting control by microRNAs or zinc finger nucleases--are providing new ways of controlling viral replication and virulence and renewing interest in LAV vaccines. Whereas these rationally attenuated viruses may lead to a new generation of safer, more widely applicable LAV vaccines, each approach requires further testing before progression to human testing.

Report of the fourth meeting on 'Influenza vaccines that induce broad spectrum and long-lasting immune responses', World Health Organization and Wellcome Trust, London, United Kingdom, 9-10 November 2009

Current influenza vaccines are limited by the need for annual immunisation, frequent antigenic updating to match the evolution of circulating influenza virus strains, and reduced efficacy in elderly persons. On 9-10 November 2009, the Initiative for Vaccine Research of the World Health Organization convened jointly with the Wellcome Trust in London, United Kingdom, the fourth meeting on 'Influenza vaccines that induce broad spectrum and long-lasting immune responses'. Presentations were made by representatives from industry, academia, governmental and non-governmental organisations. The objectives of the meeting were to update the progress of research in the field of influenza vaccine strategies able to generate cross protection against divergent influenza virus strains. Improvements in existing strategies including live attenuated influenza vaccines and adjuvantation of inactivated vaccines were summarised. Developments in novel antigen production methods, new routes of vaccine delivery and administration, and vaccine approaches based on conserved virus antigens were explored. In addition, correlates of immune protection and regulatory issues for the evaluation and approval of future novel vaccine strategies were discussed.

Developing live attenuated avian influenza virus in ovo vaccines for poultry

Live attenuated vaccines can mimic natural infection and induce humoral and cellular immune response. However, the possibility of reassortment between vaccine viruses and field isolates and of mutations from low-pathogenic to highly pathogenic viruses has prevented the use of live attenuated strains as poultry vaccines. In ovo vaccination using live attenuated strains that can undergo limited replication cycles would be a better option, because these strains can be used for mass vaccination without spreading or reassorting with other viruses. Our previous study demonstrated that two influenza nonstructural (NS) variant viruses are highly attenuated and immunogenic in chickens, making them potential live vaccine candidates. In this study, we tested whether NS variants could be used as in ovo vaccines alone or in combination with temperature-sensitive (ts) mutations. In addition, we also tested the effect of different hemagglutinin (HA) subtypes on in ovo vaccination of NS variants. Our results demonstrated that NS variants alone or in combination with ts mutations were not attenuated enough to be used for in ovo vaccination. We also observed variable effects of different HA subtypes in the same NS deletion variant backbone on hatchability. However, even with substitution of HA subtypes, NS variant-inoculated eggs still had lower hatchability compared to the mock control group, indicating that the high virulence of NS variant backbone strain in eggs might have affected the results.

Several hemagglutinins of same serotype for induction of broad immunity against influenza A virus antigenic drift variants: WO2008048984

The respiratory disease influenza gives rise to severe public health concerns. During inter-pandemic periods, the constant problem of the annually recurring seasonal influenza is perpetuated by the ability of influenza viruses to alter their surface antigens continuously (antigenic drift). Therefore, vaccines eliciting broad immunity against drift variants still remain a major objective in vaccine development. The patent WO2008048984 evaluated in this article claims an approach which aims to elicit homosubtypic protection against drift variants by simultaneous vaccination with several hemagglutinins (HAs) of the same serotype. The proposed multivalent vaccine based on simultaneous administration of several HAs, the results obtained from mice immunization studies and the implications of this concept are discussed in light of their relevance to application in humans. This proof-of-principle study suggests that a multivalent HA vaccine could elicit broad protection against drifted virus variants of one HA subtype. In the future, the dependence of broad efficacy on large antigenic distances among the HAs used for immunization as well as the antigenic distance between the HAs administered to that of the challenge virus, the immunological correlates of broad efficacy, and the suitability of this concept for domestic animals and humans remain to be investigated.