Characterization of T helper (Th)1- and Th2-type immune responses caused by baculovirus-expressed protein derived from the S2 domain of feline infectious peritonitis virus, and exploration of the Th1 and Th2 epitopes in a mouse model

Feline infectious peritonitis virus (FIPV) may cause a lethal infection in cats. Antibody-dependent enhancement (ADE) of FIPV infection has been recognized, and cellular immunity is considered to play an important role in preventing the onset of feline infectious peritonitis. In the present study, whether or not the T helper (Th)1 epitope was present in the spike (S)2 domain was investigated, the ADE epitope being thought to be absent from this domain. Three kinds of protein derived from the C-terminal S2 domain of S protein of the FIPV KU-2 strain were developed using a baculovirus expression system. These expressed proteins were the pre-coil region which is the N-terminal side of the putative fusion protein (FP), the region from FP to the heptad repeat (HR)2 (FP-HR2) region, and the inter-helical region which is sandwiched between HR1 and HR2. The ability of three baculovirus-expressed proteins to induce Th1- and Th2-type immune responses was investigated in a mouse model. It was shown that FP-HR2 protein induced marked Th1- and Th2-type immune responses. Furthermore, 30 peptides derived from the FP-HR2 region were synthesized. Five and 16 peptides which included the Th1 and Th2 epitopes, respectively, were identified. Of these, four peptides which included both Th1 and Th2 epitopes were identified. These findings suggest that the identification of Th1 epitopes in the S2 domain of FIPV has important implications in the cat.