Stably Expressed FIPV Peplomer Protein Induces Cell Fusion and Elicits Neutralizing Antibodies in Mice

We have established bovine papilloma virus (BPV)-transformed mouse C127 cell lines that synthesize the peplomer protein of the feline infectious peritonitis virus (FIPV) strain 79-1146. For this purpose, a new cassette expression vector pHSL, which carries the Drosophila HSp70 promotor and the polyadenylation signal of the Moloney murine leukemia virus long terminal repeat, was constructed. Cocultivation of the BPV-transformed cell lines with FIPV-permissive feline fcwf-D cells resulted in polykaryocyte formation. Since it depended on the presence of fcwf-D cells, binding of E2 to the cell receptor may be required for membrane fusion. E2 was synthesized as a core-glycosylated protein of 180K which was only slowly transported from the endoplasmic reticulum to the medial Golgi: of the E2-molecules labeled during a 1-hr pulse about half was still completely sensitive to endoglycosidase H after a 2-hr chase, while the remaining E2 had been chased into multiple, partially endoglycosidase H-resistant forms. Immunofluorescence studies also indicated that most E2 was retained intracellularly. Mice immunized with whole lysates of the transformed cells produced FIPV-neutralizing antibodies as shown by plaque reduction.