Diffuse CNS involvement in systemic lupus erythematosus: intrathecal synthesis of the 4th component of complement.

In extracerebral systemic lupus erythematosus (SLE), the complement system plays a prominent pathogenic role, and decreased serum concentration of the 4th component (C4) is a reliable indicator of systemic disease activity. In diffuse CNS-SLE, however, the pathogenic role of complement is less clear. In 12 patients with active diffuse CNS-SLE presenting with delirium (4), organic personality syndrome (3), or generalized seizures (5), we determined the CSF indexes of the complement components C3, C4, and factor B, and of IgG, IgA, and IgM. There was a significant increase of the C4 index in these patients compared with controls and a significantly higher CSF C4 index in patients with an increased IgM index. We conclude that intrathecal C4 is being produced in diffuse CNS-SLE.