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Avian influenza H7N9 viruses continue to pose a great threat to public health, which is evident by their high case-fatality rates. Although H7N9 was first isolated in humans in China in 2013, to date, there is no commercial vaccine available against this particular strain. Our previous studies developed a replication-defective influenza virus through mutation of the hemagglutinin (HA) cleavage site from a trypsin-sensitive to an elastase-sensitive motif. In this study, we report the development of a reassortant mutant influenza virus derived from the human isolate A/British Columbia/01/2015 (H7N9) [BC15 (H7N9)], which is the QVT virus. The HA gene of this virus possesses three mutations at the cleavage site, Lys-Gly-Arg were mutated to Gln-Thr-Val at amino acid (aa) positions 337, 338, and 339, respectively. We report this virus to rely on elastase in vitro, possess unaltered replication abilities when elastase was provided compared to the wild type virus in vitro, and to be non-virulent and replication-defective in mice. In addition, we report this virus to induce significant levels of antibodies and IFN-γ and IL-5 secreting cells, and to protect mice against a lethal challenge of the BC15 (H7N9) virus. This protection is demonstrated through the lack of body weight loss, 100% survival rate, and the prevention of BC15 (H7N9) viral replication as well as the reduction of proinflammatory cytokines induced in the mouse lung associated with the influenza disease. Therefore, these results provide strong evidence for the use of this reassortant mutant H7N9 virus as a replication-defective virus vaccine candidate against H7N9 viruses.

The recent highly pathogenic avian influenza (HPAI) H5N1 and H7N9 viruses have caused hundreds of human infections with high mortality rates. Although H5N1 and H7N9 viruses have been mainly limited to avian species, there is high potential for these viruses to acquire human-to-human transmission and initiate a pandemic. A highly safe and effective vaccine is needed to protect against a potential H5N1 or H7N9 influenza pandemic. Here, we report the generation and evaluation of two reassortant influenza viruses, PR8-H5-H7NA and PR8-H7-H5NA. These viruses contain six internal segments from A/Puerto Rico/8/1934 (PR8), the HA segment from either A/Alberta/01/2014 (H5N1) [AB14 (H5N1)] or A/British Columbia/01/2015 (H7N9) [BC15 (H7N9)], and a chimeric NA segment with either the BC15 (H7N9) HA gene or the AB14 (H5N1) HA gene flanked by the NA packaging signals of PR8. These viruses expressed both H5 and H7 HAs in infected cells, replicated to high titres when exogenous NA was added to the culture medium in vitro, and were replication-defective and non-virulent when administered intranasally in mice. Moreover, intranasal vaccination with PR8-H5-H7NA elicited robust immune responses to both H5 and H7 viruses, conferring complete protection against both AB14 (H5N1) and BC15 (H7N9) challenges in mice. Conversely, vaccination with PR8-H7-H5NA only elicited robust immune responses towards the H7 virus, which conferred complete protection against BC15 (H7N9) but not against AB14 (H5N1) in mice. Therefore, PR8-H5-H7NA has strong potential to serve as a vaccine candidate against both H5 and H7 subtypes of influenza viruses. Importance Avian influenza H5N1 and H7N9 viruses infected human with high mortality rates. A highly safe and effective vaccine is needed to protect against a potential pandemic. We generated and evaluated two reassortant influenza viruses, PR8-H5-H7NA and PR8-H7-H5NA as vaccine candidates. Each virus contains one type of HA in segment 4 and the other subtype of HA in segment 6, thus expressing both H5 and H7 subtypes of HA molecule. The viruses’ replication is dependent in the addition of exogenous NA in cell culture, and are replication-defective in vivo. Vaccination of PR8-H5-H7NA virus confers protection to both H5N1 and H7N9 virus challenge; conversely, vaccination of PR8-H7-H5NA only provides protection to H7N9 virus challenge. Our data revealed when engineering such virus, the H5 or H7 HA in segment 6 affects the immunogenicity. PR8-H5-H7NA has strong potential to serve as a vaccine candidate against both H5 and H7 subtypes of influenza viruses.

Nucleotide-binding domain and leucine-rich repeat-containing protein 3 (NLRP3) inflammasome-mediated interleukin-1 beta (IL-1β) production is one of the crucial responses in innate immunity upon infection with viruses including influenza A virus (IAV) and is modulated by both viral and host cellular proteins. Among host proteins involved, we identified tripartite motif-containing protein 25 (TRIM25) as a positive regulator of porcine NLRP3 inflammasome-mediated IL-1β production. TRIM25 achieved this function by enhancing the pro-caspase-1 interaction with apoptosis-associated speck-like protein containing caspase recruitment domain (ASC). The N-terminal RING domain, particularly residues predicted to be critical for the E3 ligase activity of TRIM25, was responsible for this enhancement. However, non-structural protein 1 (NS1) C-terminus of 2009 pandemic IAV interfered with this action by interacting with TRIM25, leading to diminished association between pro-caspase-1 and ASC. These findings demonstrate that TRIM25 promotes the IL-1β signaling, while it is repressed by IAV NS1 protein, revealing additional antagonism of the NS1 against host pro-inflammatory responses.

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