Analysis of the protective effect of the haemagglutinin-neuraminidase protein in Newcastle disease virus infection.

The role of immune responses to haemagglutinin-neuraminidase (HN) protein in protection against a Newcastle disease virus (NDV) infection was investigated using a recombinant vaccinia virus expressing HN (HN-RVV). Live HN-RVV replicated in chickens and completely protected them from lethal infection with virulent NDV. Inactivated HN-RVV also protected chickens when administered with adjuvant but not when administered without adjuvant. However, large amounts of the inactivated HN-RVV (100-fold excess) without adjuvant provided protection. Specific antibodies against the HN protein of NDV were detected in sera from survivors but not from dying birds. However, the kinetics of antibody responses in chickens inoculated with live HN-RVV and inactivated HN-RVV were considerably different. These results clearly confirm that immune response(s) solely to the HN protein of NDV can provide chickens with protection against NDV challenge, and show that the presence of antibodies to the HN protein correlates significantly with the protection from NDV infection at least in HN-immunized chickens.