ß-Glucan enriched yeast cell wall adjuvants stimulated humoral immune response and protected broilers against viral challenge

Yeast β-glucans consist of a β(1,3)-linked glucose backbone, with β(1,6)- linked side chains at different interval and sizes along the backbone. They are recognized by several receptors present on the surface of innate immune cells, such as the Dectin-1 receptor. Previous research done with crude yeast cell wall (YCW) extracts, have led to the description of β-glucans as immunestimulant molecules resulting in a strong activation of macrophages that secrete high levels of inflammatory cytokines (Goodridge et al., 2009). However, recent research has shown that this macrophage activation is most likely the result of a combined activation of the Dectin-1 receptor with other pattern recognition receptors (PRRs), including TLR-receptors that can interact with other Pathogen Associated Molecular Patterns (PAMPs) in the crude YCW. When YCW are enriched in β-glucan content, the PAMPs are partially or totally removed, while Dectin-1 signaling remains intact. In vitro assays performed with β-glucan-enriched YCW show only poor macrophage activation (Walachowski et al., 2016). Nevertheless, Walachowski et al. (2017) demonstrated that pretreatment of macrophages with these β-glucan-enriched YCWs, showed an exacerbated cytokine production in response to a secondary stimulation with TLR ligands, a concept also known as immune training. The objective of this study was, therefore, to evaluate the effect of β-glucan enriched YCW adjuvants on broiler immunity during a vaccination and challenge trial.

β-glucan enriched YCW were prepared as 2% aqueous solutions, containing killed Newcastle Disease (ND) vaccine and administered by eye-drop to 21-day old specific pathogen free (SPF) broiler chickens. Two control groups received either the normal vaccine control or physiological solution. Fourteen days after vaccination, blood samples were taken to assess antibody titers. Additionally on day 14 after vaccination, animals were challenged with a virulent ND strain after which survival rates were evaluated. Blood analyses clearly showed that β-glucan enriched YCW adjuvant vaccines were able to stimulate the production of ND specific antibodies to a significantly higher level than the normal vaccine control. In challenged birds, vaccination with β-glucan enriched YCW was able to significantly decrease (P < 0.01) the viral loads found in target organs. Finally, a 100% protection against the lethal ND challenge as compared to 80% protection with normal vaccine control or 0% in the negative control birds was observed when using β-glucan enriched YCW as adjuvant.

In conclusion, it was demonstrated that adjuvants based on β-glucan enriched YCW can induce a strong humoral immune response, which we think has greatly contributed to the effective protecting against a virulent ND challenge. Even though, it is imperative to investigate the role of the cell mediated immunity, we believe that it is plausible to hypothesize here that innate immune training achieved with β-glucan enriched YCWs might influence the development of adjuvant strategies as it could affect the antibody titer and the overall protective response.