Co-administration of a chicken interleukin-2-interferon-α fusion protein enhances immune responses and protective efficacy of an inactivated fowl adenovirus serotype 4 vaccine.

Hydropericardium syndrome (HHS) caused by fowl adenovirus serotype 4 (FAdV-4) poses a severe threat to global poultry production. Although inactivated vaccines are widely used, they may elicit suboptimal cellular immune responses and provide incomplete protection. In this study, six candidate immunostimulants were compared for their ability to enhance the immunogenicity and protective efficacy of an inactivated FAdV-4 vaccine. Among the formulations and doses evaluated, the chicken interleukin-2-interferon-α fusion protein (chIL2-IFNα) produced the strongest overall response. At 21 dpi, the chIL2-IFNα-supplemented group reached a mean peak neutralizing-antibody titer of 9.28 log2, significantly exceeding the 4.19 log2 observed in the vaccine-only group (P < 0.05). This formulation also increased the frequencies of phenotypically defined natural killer cells and cytotoxic T lymphocytes and upregulated cytotoxic-effector-gene expression. Following FAdV-4 challenge, 9 of 10 chickens (90%) in the chIL2-IFNα-supplemented group survived to 14 dpc, compared with 3 of 10 chickens (30%) in the vaccine-only group. The mean liver viral load was 5.85 log10 copies/μL lower than that in the vaccine-only group, accompanied by reduced viral shedding and less severe tissue lesions. These findings support further evaluation of chIL2-IFNα as an immunostimulant for inactivated FAdV-4 vaccines.