Recombinant LAB vector engineered to secrete chicken IFN-λ3 induces an early but sustained activation of antiviral host responses against AIV infection.

Avian Influenza virus (AIV) poses great pandemic potential with high mortality rates in poultry. In the absence of effective vaccines for birds and growing resistance to antivirals, the frequent outbreaks of various subtypes of AIV continue to pose a major challenge for the poultry industry, with potential implications for public health and food security. As an alternative to conventional antiviral approaches, cytokine-based host-targeted strategies offer a promising means to inhibit key stages of the AIV life cycle. However, the magnitude of cytokine-driven antiviral host responses remains critical since unregulated immune activation often causes severe tissue damage and other immune pathologies, leading to increased mortality. To minimise the risk of systemic side effects that could compromise poultry health and productivity, careful selection of the cytokine class and precise control of its dosage and administration timing are essential. To this end, type III interferons (IFN-λ), such as IFN-λ3, are known to exert pleiotropic effects on non-immune cells and can effectively induce several innate immune factors, including interferon-stimulated genes (ISGs). In view of the key functional attributes of IFN-λ3, we aimed to investigate whether the exogenous application of chicken IFN-λ3 (ChIFN-λ3) could induce an antiviral immune state, thereby restricting viral replication and subsequent shedding in chickens. Given that IFN-λ3 primarily functions locally in mucosal epithelial cells, we bioengineered a Lactic Acid-producing Bacterium (LAB), Lactococcus lactis (L. lactis), to deliver ChIFN-λ3 directly at the host-pathogen interface within the gut and upper respiratory tract. Our study demonstrated that L. lactis-mediated delivery of ChIFN-λ3 protein can trigger temporal activation of several ISGs and some key pro-inflammatory cytokines in chickens without compromising mucosal tissue integrity. Furthermore, our findings reveal a positive correlation between an enhanced antiviral host immune response and an increased threshold of resistance to LPAIV-H9N2 virus infection in chickens. Together, the present study suggests that the mucosal delivery of L. lactis secreting ChIFN-λ3 can elicit a strong antiviral host defense against AIV infection in chickens.