Abstract
Purpose of ReviewComplement activation is a key event in the pathogenesis of tissue inflammation and injury in systemic lupus erythematosus (SLE). This review is aimed at comparing the usefulness of measurement of complement proteins in serum/plasma (C3, C4) to complement activation (split) products in plasma and on circulating blood cells for SLE diagnosis, disease monitoring, and prognosis.Recent FindingsComplement split products, C3dg, iC3b, and C4d, are elevated in SLE, and C3dg/C3 and iC3b/C3 ratios correlate with active SLE. C4d also is higher in patients with lupus nephritis. An elevated level of the alternative pathway split product, Bb, in early lupus pregnancy is a predictor of adverse outcomes in SLE patients with antiphospholipid antibodies. Elevated levels of cell-bound complement activation products (CB-CAPs), namely, B cell-bound C4d (BC4d) and erythrocyte-bound C4d (EC4d), within a multiparameter assay panel, may predict transition to SLE more than other lupus biomarkers. EC4d better correlates with lupus disease activity than low plasma complement levels. Elevated platelet-bound C4d (PC4d) correlates with thrombosis in SLE. Both EC4d and PC4d are increased in primary and secondary anti-phospholipid syndrome, and anti-beta2glycoproteinI antibodies may directly activate the complement system.SummaryAbnormal levels of plasma complement split products and CB-CAPs support complement activation as an important pathogenetic mechanism in SLE and the antiphospholipid syndromes. These tests show promise for the diagnosis of SLE and monitoring of disease activity.
Highlights
It has been 120 years since the descriptions of the bactericidal activity in normal serum and the elucidation of the role of complement in normal human physiology and disease [1]
Gleaned from the recent studies described below adds to the accumulating evidence that complement activation in vivo in systemic lupus erythematosus (SLE) is better detected by the presence of split products than by low complement protein levels
A more recent study showed that cell-bound complement activation products (CB-CAPs) (BC4d, erythrocyte-bound C4d (EC4d)) are more prevalent than low complement levels in patients with probable SLE [11] and when used in a multiparameter assay panel are better than other biomarkers at predicting transition to SLE
Summary
It has been 120 years since the descriptions of the bactericidal activity in normal serum and the elucidation of the role of complement in normal human physiology and disease [1]. It is known that the complement system consists of more than 30 plasma proteins and cell surface receptors involved in the activation and regulation of its
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