The feline infectious peritonitis virus (FIPV) is a member of the feline 
coronavirus family that causes FIP, which is incurable and fatal in cats. 
Cyclosporin A (CsA), an immunosuppressive agent that targets the nuclear factor 
pathway of activated T-cells (NF-AT) to bind cellular cyclophilins (CyP), 
dose-dependently inhibited FIPV replication in vitro. FK506 (an immunosuppressor 
of the pathway that binds cellular FK506-binding protein (FKBP) but not CyP) did 
not affect FIPV replication. Neither cell growth nor viability changed in the 
presence of either CsA or FK506, and these factors did not affect the NF-AT 
pathway in fcwf-4 cells. Therefore, CsA does not seem to exert inhibitory 
effects via the NF-AT pathway. In conclusion, CsA inhibited FIPV replication in 
vitro and further studies are needed to verify the practical value of CsA as an 
anti-FIPV treatment in vivo.