Complement C3 fragments in urine: detection in systemic lupus erythematosus patients by western blotting

We characterized urinary excretion of C3 fragments among patients with systemic lupus erythematosus (SLE) as a possible indicator of renal involvement. 28 patients, representing a broad range of disease activity were admitted to our study. Urinary proteins were separated on 4-20% gradient SDS-PAGE gels, under reducing conditions, and transblotted to nitrocellulose. Western blots were developed with a polyvalent goat-anti-human C3d antiserum, and an alkaline phosphatase-conjugated rabbit anti-goat IgG. Three patterns were obtained: 1) no bands detected; 2) bands suggesting the presence of intact C3; and 3) samples with additional low molecular (< 4 x 10(4)) bands. The 12 patients with no C3 bands had minimal disease activity (e.g. fatigue, arthralgia, arthritis, rash, oral ulcers). The seven patients with intact C3 patterns also had minimally active disease. Their primary clinical findings included fatigue, pleurisy, renal disease which had been treated, hemolytic anemia, and arthritis. Patients with low molecular weight C3 fragments in their urine formed two sub-sets, based upon their presenting features. The first group had severe disease and contained all patients with active lupus nephritis (n = 4), while the second consisted of non-renal patients with primary clinical findings of moderate disease activity (e.g. thrombocytopenia, pneumonitis, arthritis). Our results suggest urinary excretion of low molecular weight C3 fragments correlates with active renal disease, but is a variable finding among SLE patients with non-renal manifestations of disease activity.