Potentiation of cell-mediated immune responses against recombinant HN protein of Newcastle disease virus by recombinant chicken IL-18

In this study, recombinant fowlpox viruses (rFPV/HN) expressing Newcastle disease virus (NDV) HN protein and rFPV/HN/chIL-18 co-expressing chicken IL-18 (chIL-18) and HN protein have been constructed and characterized. The co-expressed rHN/chIL-18 antigen or rchIL-18, expressed by our previous construct rFPV/chIL-18 and co-administered with NDV rHN, was assessed for its immunostimulatory activities and protection against NDV challenge in 2-week-old chickens. Chickens were vaccinated, intramuscularly, with various amounts of rHN or rHN/chIL-18 mixed with mineral oil. Production of hemagglutination–inhibition (HI) antibody depended on the concentration of the injected rHN or rHN/chIL-18. The lower HI antibody titers were obtained in chickens group rHN/chIL-18/6 and rHN/chIL-18/7, receiving 50 ng rHN/16.5 ng chIL-18 with mineral oil and 20 ng rHN/6.6 ng chIL-18 with mineral oil, respectively, compared to those in chickens rHN/6 and rHN/7, respectively receiving 50 ng and 20 ng rHN with mineral oil alone. However, the same protection rates were obtained from chickens in groups rHN/chIL-18/6 and rHN/6. Chicken groups rHN/chIL-18/7 and rHN/chIL-18/8 showed higher protective achievements than those in groups rHN/7 and rHN/8, respectively. When rchIL-18 was co-injected with 20 ng rHN plus mineral oil, low level of HI antibody titer was produced; whereas, higher level of IFN-γ production and full protection rates were obtained. On the other hand, lower levels of IFN-γ production and lower protection rate (67%) were obtained in chickens injected with the same amount of rHN with mineral oil alone. Similar results were obtained when 10 ng rHN was used. Thus, when the concentration of rHN decreased to 50 ng or less, rchIL-18 reduced HI antibody production. The increase in IFN-γ production suggested that the enhancement of the cell-mediated immunity might confer the protection from NDV challenge, even accompanied with low HI antibody induction.