Participation of factor B in residual immune complex red cell binding activity observed in serum from a C2-deficient systemic lupus erythematosus patient may delay the appearance of clinical symptoms.

Objective: To investigate whether participation of factor B (FB) in immune complex transport might explain long periods of clinical remissions in a homozygous C2-deficient patient with systemic lupus erythematosus (SLE) treated regularly with plasma infusions.
Methods: Immune complex red cell binding (ICRB) was assayed as enzyme activity, C3d by enzyme-linked immunosorbent assay, and FB by immunoelectrophoresis.
Results: C2-deficient sera showed low-grade ICRB, which correlated with levels of FB. This activity could be blocked with antibodies to C1q, C4, or FB, but not by antibodies to C2. C3d levels in the patient's plasma changed during infusion, followed by a gradient increase during remission. Comparison of ICRB, C3d, and FB suggested an inverse relationship between FB levels and clinical symptoms.
Conclusion: In C2 deficiency, FB may interact with C4 to provide a low-grade ICRB. This activity could be clinically significant in patients with C2 deficiency and explain why they are less prone to SLE than patients with C1q or C4 deficiency.