Immune responses in the gastrointestinal tract of chickens infected with low pathogenic influenza virus subtype H9N2 and the role of probiotic lactobacilli in antiviral immunity and vaccine immunogenicity

Avian influenza viruses (AIVs) pose an economic burden on the global poultry industry and a public health concern due to their zoonotic nature. These concerns have prompted the development of control measures against AIV. AIVs replicate in mucosal tissues of chickens, including the respiratory and gastrointestinal systems. The gastrointestinal tract (GIT) plays an essential task of immune homeostasis, balancing between protecting the host against invading pathogens and interacting with commensal microbes. Control measures may target the GIT because of its relevance as a site of virus replication and a part of the well-developed mucosal immune system of birds. Therefore, understanding immune responses to AIVs in the GIT may prove useful in designing prophylactic or therapeutic measures against AIVs. This thesis is an investigation of host responses in the chicken GIT to infection with AIVs subtype H9N2 and the role of probiotic lactobacilli in antiviral immunity and vaccine immunogenicity. The findings presented in this thesis confirmed the replication of AIVs H9N2 subtype in cecal tonsils (CTs) mononuclear cells and showed that probiotic lactobacilli could reduce AIV replication in these cells. Moreover, probiotic lactobacilli induced antiviral responses in chicken CTs cells, as demonstrated by an increase in the mRNA expression of proinflammatory cytokines and some interferon-stimulated genes. Next, probiotic lactobacilli were used as a co-treatment with inactivated H9N2 vaccine adjuvanted with the Toll-like receptor-21 ligand, CpG oligodeoxynucleotides (CpG), to assess the combination on vaccine immunogenicity. Hemagglutination inhibition titers were higher in sera of vaccinated chickens that received both lactobacilli and CpG than chickens that received either lactobacilli or CpG. Probiotic treatment combined with the CpG adjuvant also enhanced interferon-gamma gene expression and significantly increased interferon-gamma protein secretion by splenocytes of vaccinated chickens. Finally, we assessed global expression changes in the ileum and CTs of H9N2-infected chickens using high-throughput RNA sequencing. The results revealed an increase in interferon-inducible and interferon-stimulated gene expression, followed by continued expression of various heat shock proteins. Overall, the results of this thesis illustrated some of the antiviral immune responses in the chicken GIT and the role that probiotic lactobacilli may play in enhancing antiviral responses and vaccine-mediated immunogenicity.