Alternative complement pathway activity in sera from patients with sickle cell disease.

The low molecular weight cobra venom factor (CoVF) was used to activate the terminal sequence of the alternative complement pathway in thirty-one sera from patients with sickle cell disease (SCD). The SCD sera were compared with normal sera as a source of the alternative complement pathway factors C3 proactivator (C3PA) and C3PA convertase. These factors are required for formation of the enzymatically active CoVF-C3PA complex which is capable of cleaving C3 and thus initiating generation of the cytolytic C5b-9 complex. CoVF cofactor activity was significantly less than normal in SCD sera as measured in an indirect lysis assay, indicating reduced C3PA or C3PA convertase activity in these sera. Qualitative (immunoelectrophoresis) and quantitative (radial immunodiffusion) measurement of C3PA showed, however, that this protein is normal or elevated in SCD sera. Taken together, the reduced CoVF cofactor activity and normal or elevated C3PA in SCD sera suggests that sera from patients with sickle cell disease have reduced C3PA convertase activity.