Identification of the α-γ subunit of the eighth component of complement (C8) in a patient with systemic lupus erythematosus and absent C8 activity: Patient and family studies☆

Abstract The serum of a patient with systemic lupus erythematosus (SLE) and no known history of Neisseria infections lacked the activity of the eighth component of complement (C8). Antisera specific for human C8 identified in the serum of the patient and four relatives, whose C8 activity was well below normal, cross-reacting material which differed from normal C8 in electrophoretic mobility and molecular size. Electrophoresis of this C8-related material eluted from a F(ab′) 2 anti-C8 immunoadsorbent column revealed the presence of the α-γ subunit of C8. That this material was intact α-γ subunit was shown by successful reconstitution of C8 hemolytic activity in the proband's serum by purified β subunit. We conclude that the C8-related antigen in the serum of the proband and four apparently heterozygous relatives is the α-γ subunit of C8. The presence or absence of the α-γ subunit in C8-deficient individuals does not appear to influence the type of disease to which they are predisposed since we were unable to find similar C8 antigen in another C8-deficient patient with SLE. Similarly, susceptibility to Neisseria infections has been reported in individuals with and without functionless C8 antigen. Histocompatibility (HLA) studies showed that C8 deficiency in this family is not inherited in association with HLA antigens and the patient does not have the DR antigens commonly associated with SLE.