Feline infectious peritonitis (FIP), one of the most important lethal infections 
of cats, is caused by feline infectious peritonitis virus (FIPV), the 
high-virulence biotype of feline coronaviruses (FCoVs). FIPVs are suggested to 
emerge from feline enteric coronaviruses (FECVs) by acquiring mutations in 
specific genes in the course of persistent infections. Although numerous studies 
identified mutations predicted to be responsible for the FECV-FIPV biotype 
switch, the presumed roles of specific genetic changes in FIP pathogenesis have 
not been confirmed experimentally. Reverse genetics systems established 
previously for serotype I and the less common serotype II FCoVs were based on 
cell culture-adapted FIPV strains which, however, were shown to be unsuitable 
for FIP pathogenesis studies in vivo To date, systems to produce and manipulate 
recombinant serotype I field viruses have not been developed, mainly because 
these viruses cannot be grown in vitro Here, we report the first reverse 
genetics system based on a serotype I FECV field isolate that is suitable to 
produce high-titer stocks of recombinant FECVs. We demonstrate that these 
recombinant viruses cause productive persistent infections in cats that are 
similar to what is observed in natural infections. The system provides an 
excellent tool for studying FCoVs that do not grow in standard cell culture 
systems and will greatly facilitate studies into the molecular pathogenesis of 
FIP. Importantly, the system could also be adapted for studies of other RNA 
viruses with large genomes whose production and characterization in vivo are 
currently hampered by the lack of in vitro propagation systems.IMPORTANCE The 
availability of recombinant serotype I FCoV field isolates that are amenable to 
genetic manipulation is key to studying the molecular pathogenesis of FIP, 
especially since previous studies using cell culture-adapted FIPVs had proven 
unsuccessful. To our knowledge, we report the first serotype I FECV field 
isolate-based reverse genetics system that allows the production of high-titer 
recombinant virus stocks that can be used for subsequent in vivo studies in 
cats. The system represents a milestone in FCoV research. It provides an 
essential tool for studying the molecular pathogenesis of FIP and, more 
specifically, the functions of specific gene products in causing a fundamentally 
different progression of disease following acquisition of specific mutations. 
The system developed in this study will also be useful for studying other 
coronaviruses or more distantly related RNA viruses with large genomes for which 
suitable in vitro culture systems are not available.