We studied the activation and Cl inactivator-dependent dissociation of the first component of complement, the Clq(Clr-Cls)2 complex, in relation to recruitment of the classical activation pathway in the circulation of 24 patients with systemic lupus erythematosus (SLE). The patients were divided into three groups on a clinical basis, and were investigated during flares of disease activity. Group I had mild symptoms, group II major extra- renal manifestations, and group III manifest renal disease. High serum concentrations of trimer complexes containing Cl inactivator, activated Clr and zymogen Cls(Cl IA-Clr- Cls) were found in the majority of the patients. Some patients with high Cl IA-Clr-Cls concentrations showed no evidence of classical pathway activation, indicating that Cl activation was controlled by the action of Cl IA at the Clr level. By contrast, formation in serum of tetramer complexes in which Cl IA was firmly bound to both Clr and Cls (Cl IA- Clr-Cls-Cl IA) was associated with C2 and C3 cleavage in EDTA plasma, and with manifest hypocomplementemia. Low Cl IA-Clr-Cls-Cl IA values were observed in conjunction with substantial C2 cleavage in a few patients. Thus, Cl IA-Clr-Cls-Cl IA may not always be a sensitive indicator of classical pathway activation. Efficient recruitment of the classical pathway was related to disease severity, with some overlap between the clinical groups. In conclusion, Cl dissociation with formation of Cl IA-containing complexes was consistently found in patients with active SLE. The results suggested that Cl IA-dependent control of Cl activation was of biological significance in the disease.

Keywords:
Classical complement pathway, Systemic lupus erythematosus, Complement activation, Cl inactivator
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1991
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