Humoral immune response of cats to virulent and avirulent feline infectious peritonitis virus isolates.

Isolates of feline infectious peritonitis virus (FIPV) vary in their degree of virulence and have antigenically and functionally different peplomer glycoproteins. We tested the most virulent isolate, FIPV-79-1146 and the avirulent isolate, FIPV-UCD-2 in vivo in order to better understand the pathogenesis of FIP. Specific pathogen free cats were inoculated with either FIPV-79-1146 or FIPV-UCD-2. After 28 days cats which had received FIPV-79-1146 were either dead or showing clinical signs of FIP. The FIPV-UCD-2 inoculated cats seroconverted but remained healthy and were then divided into three groups. One group was not manipulated, one group received a second dose of FIPV-UCD-2, and the third group received FIPV-79-1146. One month later all FIPV-UCD-2 cats which had subsequently received FIPV-79-1146 were dead of FIP while the FIPV-UCD-2 inoculated cats remained healthy. Antibody mediated immune enhancement of FIP disease was not observed. Serum neutralization assays demonstrated that only those cats which received FIPV-79-1146 developed neutralizing antibodies to FIPV-79-1146. However, all cats except uninoculated controls developed neutralizing titers to FIPV-UCD-2. Competition ELISAs were used to analyze the specific humoral immune responses of cats to structural polypeptides of the viruses.