Swine influenza is an economic burden to pig producers due to decreased feed consumption, weight loss, and increased death loss that often occurs from secondary bacterial infections. In addition, influenza A virus – swine (IAV-S), also known as swine influenza virus (SIV), is a zoonotic pathogen, which means it can be transmitted from pigs to people and causes influenza illness. Susceptible individuals include swine farm employees and their families, as well as youth attending agriculture fairs and in close proximity to infected swine. Therefore, the disease is a health and economic threat to both humans and swine worldwide.
Strict biosecurity, proper pig flow in a production system, and preventing transmission of human IAV to pigs through personal protective equipment have been essential control measures for IAV on swine farms. At the same time, swine producers need effective vaccines and vaccination strategies to help control virus infection, disease, and transmission among pigs and potentially between pigs and people. Unfortunately, influenza viruses in swine continue to evolve rapidly, complicating the ability to effectively control infection and transmission through the use of inactivated vaccines. Current inactivated virus vaccines, however, fail to cross-protect against the massive number of antigenically diverse strains circulating in swine and against the threat of human spillover IAV. Since vaccination still remains one of the most important methods to control IAV in swine, it is imperative the swine industry supports research and development of new vaccine platforms, immunogens, or alternative vaccination protocols.
Recently, our team developed a HA stem-based immunogen designated “HIV6HB-HASTEM” based on the hemagglutinin (HA) of a human H3 virus. It had shown its binding with the monoclonal antibody specific for the most conserved epitope (CR9114) among influenza A and B viruses to date. More importantly, mice immunized with this antigen (10 µg, three times) not only developed a high level of ELISA antibody against the immunogen but also were protected from a lethal challenge of H1 and H3 IAV strains.
Based on these previous observations, an HASTEM-based immunogen was constructed and produced based on the consensus sequence of H3 strains of IAV-S as H1-based immunogen was unstable to use for vaccination. Then we conducted a pigs study to evaluate the efficacy of the H3 HASTEM immunogen in pigs against both H1 and H3 IAV-S simultaneously as a proof-of-concept for the universal vaccine platform. The immunogen was given intramuscularly to pigs with one of the three different adjuvants (Alum, Zn-chitosan, Emulsigen), 3 times at 2-week intervals. A group of pigs was kept unvaccinated as a control (NV group). Two weeks after the last immunization, all pigs were challenged with a mix of H1 and H3 IAV-S at the same titer. Some of the immunized and NV pigs were left unchallenged. All pigs were bled at 0, 14, 28, and 35 days after the first immunization and at 5 days post inoculation (dpi) for antibody tests (ELISAs, VN, HI). Pigs were weighed on days of challenge and at 5 dpi to calculate the average daily gain (ADG). Oral fluids and nasal swabs were collected to assess viral shedding by qPCR at 1, 3, and 5 dpi. All pigs were necropsied at 5 dpi, and lungs were collected for gross and microscopic evaluation of lesions and tested by IHC and qPCR.
No injection site reaction was observed in any of the immunized pigs. Pigs developed antibodies specific for the immunogen but no VN or HI antibodies. In general, no febrile response (>104 ºF) was observed in the vaccinated pigs after challenge except for some of the pigs immunized with Zn-chitosan adjuvant (for one day). Yet, the immunized pigs had a lower ADG than the NV group after the challenge. The immunization could not establish sterile immunity as all pigs had both H1 and H3 IAV-S at a similar level in lung and lung lavage fluid samples collected at 5 dpi. Viral load in the lung was lower in pigs who received the immunogen with Zn-chitosan adjuvant than the NV group, while the other immunized groups had a higher viral load. Likewise, pigs who received the immunogen with Zn-chitosan adjuvant developed gross lung lesion scores similar to those of the NV group but lower than those of other vaccinated pigs. The same group shed significantly less virus in nasal secretion as compared to the NV group.
Overall, the study demonstrated that the H3 HASTEM-based antigen was immunogenic to pigs, even though the vaccine-induced antibodies did not neutralize the virus. The vaccine-induced immunity had the same antiviral impact on both H1 and H3 viruses, i.e. no subtype bias. While no sterile immunity could be conferred by this immunogen, the immunization, particularly when given with H3 HASTEM + Zn-chitosan adjuvant, appears to induce some degree of protective immunity based on lack of fever, lower lung scores, lower lung viral load, and lower viral shedding in nasal secretions after challenge. Therefore, the immunogen used in the study may provide a design concept/platform toward a universal IAV vaccine with further optimization, including vaccination strategies.
- The HASTEM antigen based on the consensus of H3 IAV is immunogenic to pigs.
- Antibodies induced by the immunogen do not neutralize IAV-S.
- The immunity induced by the designed immunogen may have the same antiviral effect on both H1 and H3 IAV-S.
- The immunogen may provide some degree of protective immunity (lower lung lesion score, lower lung viral load, lower viral shedding), particularly when it is given with Zn-chitosan adjuvant.
- The immunogen used in the study could be a new vaccine design platform toward a universal IAV vaccine.
Kyoung-Jin Yoon, DVM, Ph.D.
College of Veterinary Medicine
Iowa State University