Elastase-Dependent Live Attenuated Swine Influenza A Viruses Are Immunogenic and Confer Protection against Swine Influenza A Virus Infection in Pigs

Abstract

Influenza A viruses cause significant morbidity in swine, resulting in a substantial economic burden. Swine influenza virus (SIV) infection also poses important human public health concerns. Vaccination is the primary method for the prevention of influenza virus infection. Previously, we generated two elastase-dependent mutant SIVs derived from A/Sw/Saskatchewan/18789/02(H1N1): A/Sw/Sk-R345V (R345V) and A/Sw/Sk-R345A (R345A). These two viruses are highly attenuated in pigs, making them good candidates for a live-virus vaccine. In this study, the immunogenicity and the ability of these candidates to protect against SIV infection were evaluated in pigs. We report that intratracheally administrated R345V and R345A induced antigen-specific humoral and cell-mediated immunity characterized by increased production of immunoglobulin G (IgG) and IgA antibodies in the serum and in bronchoalveolar lavage fluid, high hemagglutination inhibition titers in serum, an enhanced level of lymphocyte proliferation, and higher numbers of gamma interferon-secreting cells at the site of infection. Based on the immunogenicity results, the R345V virus was further tested in a protection trial in which pigs were vaccinated twice with R345V and then challenged with homologous A/Sw/Saskatchewan/ 18789/02, H1N1 antigenic variant A/Sw/Indiana/1726/88 or heterologous subtypic H3N2 A/Sw/Texas/4199-2/9/ 98. Our data showed that two vaccinations with R345V provided pigs with complete protection from homologous H1N1 SIV infection and partial protection from heterologous subtypic H3N2 SIV infection. This protection was characterized by significantly reduced macroscopic and microscopic lung lesions, lower virus titers from the respiratory tract, and lower levels of proinflammatory cytokines. Thus, elastase-dependent SIV mutants can be used as live-virus vaccines against swine influenza in pigs.

An Eight-Segment Swine Influenza Virus Harboring H1 and H3 Hemagglutinins Is Attenuated and Protective against H1N1 and H3N2 Subtypes in Pigs

“…These LAIV are attenuated based on the incorporation of cold-adapted (ca) and/or temperature sensitive (ts) phenotypes into vaccine strains by recombination with a donor strain or by multiple passages of the vaccine strain at low temperatures (47)(48)(49). Several different experimental live attenuated SIV vaccines have been demonstrated to induce strong cell-mediated immunity (CMI) and humoral immunity, as well as cross-protection against homologous heterotypic and heterologous SIV isolates (11,16). Even though these experimental SIV LAIV have advantages over their inactivated counterparts, no LAIV is currently registered for use against SIV infections worldwide.…”

“…Very recently, Pena et al generated a recombinant influenza virus by rearranging the genome of an avian H9N2 virus and expressed the entire H5 HA ORF from the NS segment RNA (54). All these results suggest the possibility and feasibility of rescuing influenza A viruses by replacing the coding region of HA, NA, or other influenza virus Previous studies demonstrated that an elastase-dependent SIV could be used as a LAIV against homologous, homologous heterotypic, and heterologous SIV isolates (11,14). Despite the significant protective efficacy against parental and homotypic SIVs, only partial protection was elicited against the heterologous H3N2 subtype.…”

“…Antigen-specific antibody titers (IgG and IgA) from serum, nasal swabs, and BALF were determined as described previously (11).…”

“…In addition, these vaccines primarily generate antibody responses with limited or no cell-mediated immunity (CMI). In contrast, live attenuated influenza vaccines (LAIV) provide strong, long-lived cell-mediated and humoral immunity against different influenza virus subtypes with no need for perfect antigen matching (8)(9)(10)(11). Previously, it was shown that SIVs generated by genetic modifications within the hemagglutinin (HA) or nonstructural (NS) genes are attenuated in pigs (12,13).…”

“…Previously, it was shown that SIVs generated by genetic modifications within the hemagglutinin (HA) or nonstructural (NS) genes are attenuated in pigs (12,13). These viruses demonstrated the ability to induce strong cell-mediated and humoral immunity and to provide full protection against homologous SIVs and partial protection against heterologous SIVs (11,(14)(15)(16)(17). In this study, we describe a novel approach that generates a LAIV candidate containing an H3 subtype of HA derived from a circulating SIV in the genetic background of H1N1 SIV.…”