Study of protection by recombinant fowl poxvirus expressing C-terminal nucleocapsid protein of infectious bronchitis virus against challenge

A stable recombinant fowl poxvirus (rFPV) expressing the C-terminal region (119 amino acids) of the nucleocapsid (N) protein of an infectious bronchit is virus (IBV) strain Ch3 was constructed by inserting the coding sequence within the thymidine kinase gene of fowl poxvirus (FPV) by homologous recom bination. The N protein was expressed under control of the vaccinia virus p romoter P7.5 in chicken embryo fibroblast cell cultures as seen in immunofl uorescence assay and in rFPV-inoculated specific-pathogen-free (SPF) chicke ns by detecting antibodies with enzyme-linked immunosorbent assay (ELISA). A homologous IBV strain (Ch3) and two heterologous IBV strains (Ch5 and H4) were used to inoculate SPF chickens in a challenge to examine the protecti ve efficacy of the rFPV. When the chickens were challenged with IBV Ch3 or Ch5, the control birds had respiratory signs of infections bronchitis, wher eas all the vaccinated birds were clinically normal although low levels of the IBV infection were detected by a differential ELISA. In contrast, in th e chickens challenged with IBV H4, all control birds and vaccinated birds s uffered from the highly lethal IBV H4 infection. Our results suggest chat t he C-terminal 119 amino acid of the nucleocapsid expressed by FPV is a host -protective antigen and may induce cross-protective immunity against illnes s among some IBV