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Immune responses induced by DNA vaccines encoding Newcastle virus haemagglutinin and/or fusion proteins in maternal antibody-positive commercial broiler chicken

1. The immune responses induced by recombinant plasmids containing Newcastle disease virus (NDV) F (pVAX.nd.f) or HN (pcDNA.nd.hn) genes separately or in combination in bi-cistronic (pIRES.nd.hn.f) constructs were evaluated in maternal antibody-positive commercial chicks. 2. Immunofluorescence and immunoperoxidase tests demonstrated the expression of both F and HN proteins in Vero cells. Real-time PCR analysis revealed the expression of HN and/or F genes in muscle, peripheral blood mononuclear cells (PBMC), spleen and liver after immunisation. 3. Chicks inoculated intramuscularly thrice (two booster doses) with pVAX.nd.f and pcDNA.nd.hn did not develop detectable haemagglutination inhibiting (HI) antibodies. In contrast, an increase in a NDV-specific cell-mediated immune response was demonstrated. 4. After challenge with virulent NDV, chicks immunised with the recombinant plasmids as well as those in control groups succumbed to Newcastle disease. 5. Based on these results, it is concluded that DNA vaccines containing HN and/or F genes fail to protect commercial chicks, possibly due to interference from maternal antibodies.