Antibody-dependent enhancement occurs upon re-infection with the identical serotype virus in feline infectious peritonitis virus infection.

Feline infectious peritonitis virus (FIPV) is classified into serotype I and serotype II according to the amino acid sequence of its spike(S) protein. Antibody dependent enhancement (ADE) of macrophage infection occurs in the presence of antibodies to FIPV S protein, and a close relationship between ADE and neutralizing epitopes has been reported. The importance of differences in FIPV serotype on the induction of ADE remains unclear. In this study, we investigated whether the same or different serotype of FIPV induces ADE in cats. Specific pathogen-free cats were passively immunized with anti-type I or II FIPV antibodies, and we investigated the induction of ADE following subcutaneous inoculation with type I FIPV. Inoculation using FIPV serotype I enhanced the onset of FIP in cats passively immunized with FIPV serotype I-specific antibodies but not in those passively immunized with antibodies to FIPV serotype II. These data suggest that re-infection with the same serotype induces ADE in cats infected with FIPV.