Increased Susceptibility to Infection in a Patient with Type II Essential Hypercatabolism of C3

Abstract

In a patient with partial lipodystrophy and increased susceptibility to infection, no abnormalities were found in humoral antibody, cellular immunity or leukocyte function. In contrast, the patient's serum complement-mediated functions were grossly deficient. The concentrations of the components of the serum complement and properdin systems were normal, except for that of C3, which was markedly reduced and consisted of native C3 and an inactive conversion product, C3c. The patient's serum contained an enzyme, C3ase, capable of cleaving C3 in vitro and in vivo. Addition of purified C3 to the patient's serum improved complement-mediated phagocytosis and killing of bacteria. In this Type II disorder, low serum C3 levels are ascribable to both hyposynthesis and hypercatabolism of this component of complement.