Isolation and characterization of feline C3 and evidence for the immune complex pathogenesis of feline infectious peritonitis

Infections of cats with feline peritonitis (FIP) virus are usually inapparent but may lead to fatal polyserositis. We have recently advanced the hypothesis that immune complexes play an essential role in the pathogenesis of the condition. To support this hypothesis, the role of the third component of complement in FIP was investigated. In the present paper, the isolation of C3 from normal cat serum and some of its physical and immunologic properties are described. The final protein had an apparent m.w. of 185,000 and was composed of 2 polypeptide chains with m.w. of 128,000 and 71,000, respectively. When tested against whole cat serum, an antiserum raised in rabbits against purified C3 recognized only 1 protein whose identity with C3 was established. With the aid of this antiserum, depositions of C3 in renal glomeruli of FIP-affected cats were demonstrated by immunofluorescence. Their localizaton coincided with that of deposited IgG, thereby supporting the concept of an immune complex pathogenesis of FIP.