Influence of factor D concentrations on fluid phase C3 activation, lysis of rabbit erythrocytes and solubilization of immune complexes.

Factor D depleted serum did not support hemolysis of rabbit erythrocytes and solubilization of performed immune complexes. Fluid phase C3 cleavage increased in a dose dependent manner when D protein was added to normal or to factor D depleted serum. During short incubation factor D concentrations were correlated with the capacity of serum to solubilize immune complexes and to lyse rabbit erythrocytes. With prolonged incubation, the hemolytic activity decreased in a factor D dose dependent manner. This was probably due to fluid phase breakdown of C3 and factor B in the presence of high factor D concentrations. Hypocomplementemic sera from patients with active systemic lupus erythematosus (SLE) did not support solubilization of bovine serum albumin (BSA) anti-BSA complexes when factor D was added in excess. Patients with polycystic kidney disease with reduced renal function and high factor D concentrations showed increased concentrations of circulating C3d/dg fragments. The possibility was considered that high factor D concentrations in uremia might promote fluid phase C3 degradation and thereby limit the in vivo efficiency of alternative pathway activation on target surfaces.