Novel vaccination strategy shows promise in boosting immunity and controlling influenza spread in pigs

By Dr. Priyom Bose, Ph.D.Reviewed by Benedette Cuffari, M.Sc.Aug 13 2024

A recent PLoS Pathogens study evaluates the efficacy of a newly developed live-attenuated influenza vaccine (LAIV) protocol in eliciting immune responses in a porcine animal model.

Study: Evaluation of a novel intramuscular prime/intranasal boost vaccination strategy against influenza in the pig model. Image Credit: ezps / Shutterstock.com

Live-attenuated influenza vaccines against the influenza virus

LAIVs induce a local immune response directed against several viral antigens. Hemagglutinin (HA), a specific type of immunoglobulin A (IgA) secreted into the respiratory tract, can block the virus at its point of entry and prevent its spread. Unlike traditional influenza vaccines, LAIVs do not require an adjuvant and provide immunity for longer durations.

However, a key challenge regarding LAIV development is adequate replicative capacity, which could lead to over-attenuation of the virus, which subsequently prevents the ability to generate a sustained immune response. Comparatively, an insufficiently attenuated vaccine could lead to viral shedding from the upper respiratory tract, thereby increasing the risk of the emergence of more virulent viruses. Thus, finding an appropriate balance between achieving sufficient immunogenicity while ensuring the safety of this vaccination approach is crucial.

About this study

To date, several LAIVs have been developed. For example, LAIVs based on cold-adapted viruses are recommended for individuals between the ages of two and 49. Other LAIV strategies utilize the recombinant influenza A virus (IAV) with a modified non-structural 1 (NS1) protein.

Previously, the authors of the current study developed the recombinant LAIV candidate NS1(1–126), which encoded a C-terminally truncated NS1 protein and combined it with a mutation in the PA gene (NS1(1–126)-ΔPAX). This modification of the PA gene, which prevented PA-X protein expression, enhanced the safety profile of the NS1(1–126) LAIV candidate.

Recombinant viruses are associated with reduced apoptotic cell death, attenuated viral replication, and improved innate immune response induction in a porcine bronchiolar epithelial cell line. The present study aims to assess the efficacy of these LAIV candidates using an in vivo pig model.

In the first animal study, 15 healthy 10-week-old pigs were assigned to three groups that were intranasally infected with wild-type pH1N1/09, pH1N1/09-NS1(1–126), or VSV-Luc as control.

The second experimental study aimed to evaluate the effects of a novel prime/boost immunization protocol. To this end, 25 healthy 10-week-old specific pathogen-free (SPF) were initially primed intramuscularly with VSV-H1 or the control VSV-Luc and subsequently boosted with nasal immunization of NS1(1–126)-ΔPAX or NS1(1-126) ΔPAX LAIV.

Study findings

NS1(1-126) infection led to viral excretion at significantly lower levels and reduced pH1N1/09-specific antibody levels as compared to wild-type infection. However, virus-specific IgG levels remained similarly high in bronchioalveolar lavage (BAL) samples in both NS1(1-126) and wild-type infected pigs.  

Animals that were initially primed with the control vector VSV-Luc and subsequently immunized with NS1-(1-126) ΔPAX exhibited reduced viral ribonucleic acid (RNA) shedding, whereas no viral shedding was observed in pigs initially primed with VSV-H1 and subsequently boosted with NS1(1-126) ΔPAX after six days.

Serum IgG and IgA levels were not detected following primary immunization with VSV-H1; however, a strong antibody response was observed after boosting with NS1(1–126)-ΔPAX or NS1(1–126) LAIV. Whereas serum IgA levels peaked at two weeks, followed by a sharp decline, serum IgG levels plateaued two weeks following booster immunization and decreased slowly thereafter.

Primary vaccination with VSV-H1 did not produce a high level of virus-neutralizing antibodies. However, boosting with NS1(1–126)-ΔPAX or NS1(1–126) LAIV led to a significant increase in virus-neutralizing antibodies and CD4⁺ T-cells.

CD4⁺ T-cells expressed tumor necrosis factor (TNF) and interferon γ (IFNγ), independently and together, but not interleukin 17 (IL-17). These effects were absent in pigs that were immunized with the control VSV-Luc and subsequently boosted with NS1(1–126)-ΔPAX.

Following their booster vaccination, pigs were intranasally inoculated with pH1N1/09 to determine the efficacy of this vaccination protocol against infection. To this end, no viral RNA was identified in nasal samples obtained from VSV-H1/NS1(1-126) ΔPAX, VSV-H1/NS1(1-126), and VSV-Luc/NS1(1-126)- ΔPAX groups.

Conclusions

The study findings suggest that the proposed vaccine protocol induced systemic IAV-specific immunity in terms of enhanced memory Th1 cells, IgG, and neutralizing antibody titers. Primary vaccination with VSV-H1 and subsequent boosting with NS1(1–126)-ΔPAX LAIV augmented the safety by significantly reducing LAIV shedding and altogether preventing challenge virus shedding.

The novel prime/boost vaccination protocol established in this study will likely support the future development of next-generation vaccination strategies to manage seasonal influenza epidemics more effectively. The mucosal immunity generated by this approach can potentially improve herd immunity, mitigate the spread of IAV, and potentially control future IAV epidemics, as this strategy can be easily adapted and timed to emerging IAV subtypes.