Protection of cats from infectious peritonitis by vaccination with a recombinant raccoon poxvirus expressing the nucleocapsid gene of feline infectious peritonitis virus.

Feline Infectious Peritonitis Virus (FIPV) is a coronavirus that induces an often fatal, systemic infection in cats. Various vaccines designed to prevent FIPV infection have been shown to exacerbate the disease, probably due to immune enhancement mediated by virus-specific immunoglobulins against the outer envelope (S) protein. An effective vaccine would be one that induces cell-mediated immunity without disease enhancing antibodies. In this report, we describe the use of a recombinant raccoon poxvirus that expresses the gene encoding the nucleocapsid protein of FIPV (rRCNV-FIPV N) as an effective vaccine against FIPV-induced disease. Cats were parenterally or orally vaccinated twice, three weeks apart. Cats were then orally challenged with Feline Enteric Coronavirus (FECV), which induces a subclinical infection that can cause enhancement of subsequent FIPV infection. Three weeks later, cats were orally challenged with FIPV. The FIPV challenge induced a fatal infection in 4/5 (80%) of the controls. On the other hand, all five cats vaccinated subcutaneously with rRCNV-FIPV N showed no signs of disease after challenge with FIPV. Four of the five subcutaneous vaccinates survived an additional FIPV challenge. Vaccination with rRCNV-FIPV N induced serum IgG antibody responses to FIPV nucleocapsid protein, but few, if any, FIPV neutralizing antibodies. In contrast to the controls, protected vaccinates maintained low FIPV serum neutralizing antibody titers after FIPV challenge. This suggests that the protective immune response involves a mechanism other than humoral immunity consisting of FIPV neutralizing antibodies.