An inherited deficiency of the third component of complement, C3, in guinea pigs

Hereditary deficiency of the third component of complement, C3, is found very seldom in the human. C3 deficiency is associated with severe bacterial infections revealing the central role of C3 in complement activation via the classical or alternative pathway. We describe a new hereditary C3 deficiency in strain 2 guinea pigs. Serum from these animals had a markedly reduced lytic activity in a standard assay for complement‐dependent, antibody‐mediated cytotoxicity. In functional assays of individual components, the hemolytic activity of the components C4, C2, C5 and of factors B, D and H was in the normal range. The functional C3 titer, and similarly C3 antigenic activity in the serum of these C3‐deficient animals (C3D) was on average only 5.7% of normal activity. Typing the animals with alloantisera or monoclonal antibodies to guinea pig Ia‐antigens revealed that the C3D animals had the major histocompatibility complex‐haplotype of inbred strain 2 guinea pigs (B.1, Ia. 2,4). The C3 defect is not linked to the major histocompatibility complex and, in addition, is not linked to a C3a receptor deficiency. Macrophages and hepatocytes of the C3D animals have an unimpaired capacity for synthesis and secretion of C3 as measured by enzyme‐linked immunosorbent assay. There was no indication for hypercatabolism of normal C3 by the animals as shown by plasma clearance of 125I‐radiolabeled C3. Thrombocytes of the C3D animals responded normally to stimulation with purified C3a in an ATP‐release assay without an indication for a desensitization in vivo. Possibly the fault resides in an enhanced susceptibility of their own C3 to proteolysis. However, C3 partially purified from the plasma of the C3D animals or secreted by hepatocytes exhibited no obvious structural differences to purified normal C3 in sodium dodecyl sulfate‐polyacrylamide gel electrophoresis or in immunoblotting. The C3D serum had a reduced bactericidal activity compared to normal or to C4‐deficient serum. Nevertheless, the animals are apparently healthy without an indication for increased frequency of bacterial infections. These guinea pigs provide an unique model for analysis of the biological functions of C3 in vivo and in vitro without the need for artificial C3‐depletion procedures with all their known and unknown side‐effects.