Retrovirus-expressed hemagglutinin protects against lethal influenza virus infections

An influenza virus hemagglutinin gene, H7, has been expressed in a replication-competent Schmidt-Ruppin Rous sarcoma virus-derived vector. This virus, P1/H7, expressed a glycosylated precursor of the H7 protein which was processed to a mature form and transported to the cell surface. The expressed H7 glycoprotein could not be detected in P1/H7 virus particles. A P1/H7 stock which expressed 5 to 10% of the level of H7 observed in influenza virus-infected chicken embryo fibroblasts was used to immunize 1-month-old chickens. This immunization resulted in low or undetectable levels of hemagglutination-inhibiting and neutralizing antibody. Despite the low serum response, challenge with a highly pathogenic H7N7 virus revealed complete protection against lethal infection.