ANTIBODY-DEPENDENT ENHANCEMENT OF FELINE INFECTIOUS PERITONITIS VIRUS-INFECTION IN FELINE ALVEOLAR MACROPHAGES AND HUMAN MONOCYTE CELL-LINEU937 BY SERUM OF CATS EXPERIMENTALLY OR NATURALLY INFECTED WITH FELINE CORONAVIRUS

Infection of the type II feline infectious peritonitis virus (FIPV) st rain 79-1146 to primary feline alveolar macrophages and human monocyte cell line U937 was enhanced by the sera of cats experimentally infect ed with the 79-1146 strain, but not those of cats infected with KU-2 o r UCD-1 strain of type I FIPV. The experiments using sera of cats with feline infectious peritonitis (FIP) and of cats naturally infected wi th feline coronavirus (FCoV) revealed that infection of the FIPV 79-11 46 strain to the U937 cells was enhanced only by the sera of cats infe cted with type II FIPV or feline enteric coronavirus. The samples posi tive for antibody-dependent enhancement (ADE) activity had high neutra lizing antibody titers against the FIPV 79-1146 strain and the samples negative for ADE activity had low neutralizing antibody titers. These findings support the previous results where a monoclonal antibody wit h neutralizing activity had high ADE activity, suggesting that there w as a close relationship between the neutralization and enhancement sit es. And then it is also suggested that ADE of infection is likely to b e induced by re-infection with the same serotype of virus in type II F IPV infection. Furthermore, U937 cells are considered useful and can b e substituted for the feline macrophages for determining ADE of FIPV-i nfection.