Measurement of erythrocyte C4d and complement receptor 1 in systemic lupus erythematosus

Abstract

Objective. C4-derived activation fragments are the only complement ligands present on the surfaces of normal erythrocytes. The significance of this observation is unknown, and the role of erythrocyte-bound C4 (E-C4) in human disease has not been explored. More than any other human disease, the pathogenesis of systemic lupus erythematosus (SLE) has been characterized by defects in clearance of complement-bearing immune complexes via erythrocytes expressing complement receptor 1 (CR1). This study was undertaken to determine whether these functional defects might be reflected by abnormal patterns of E-C4 and E-CR1 expression on erythrocytes of patients with SLE.Methods. We conducted a cross-sectional study of 100 patients with SLE, 133 patients with other diseases, and 84 healthy controls. Erythrocytes were characterized by indirect immunofluorescence and by flow cytometry for determination of levels of C4d and CR1.Results. Patients with SLE had higher levels of E-C4d and lower levels of E-CR1 than did patients with other diseases (P < 0.001) or healthy controls (P < 0.001). The test was 81% sensitive and 91% specific for SLE versus healthy controls and 72% sensitive and 79% specific for SLE versus other diseases, and it had an overall negative predictive value of 92%.Conclusion. This is the first report of abnormal levels of E-C4d in human disease. We found that abnormally high levels of E-C4d and low levels of E-CR1 are characteristic of SLE, and combined measurement of the 2 molecules has high diagnostic sensitivity and specificity for lupus. Determination of E-C4d/E-CR1 levels may be a useful addition to current tests and criteria for SLE diagnosis.It has been known for decades that proteolytic fragments of complement component C4, generated during activation of the classical pathway, are present on surfaces of normal erythrocytes. C4 is a class III HLA molecule, and its polymorphism is responsible for the Rogers and Chido blood group antigens; however, the physiologic significance of erythrocyte-bound C4 (E-C4) is entirely unknown. Although there has been extensive study of serum C4 levels in human disease, abnormalities of E-C4 and the potential role of E-C4 molecules during inflammatory and immune responses have not been investigated.A large body of evidence accumulated over several decades has demonstrated that abnormalities in complement activation and clearance of immune complexes by erythrocytes are fundamental to the pathogenesis of systemic lupus erythematosus (SLE) (1,2). These include reduced levels of CR1 on erythrocytes, deficiencies in components of the classical pathway including C4, and saturation of CR1 by preexisting immune complexes (3-12). These observations led to our hypothesis that abnormalities in complement activation specific to SLE may be reflected by molecular events involving both receptors and ligands on erythrocyte surfaces and that

Reticulocytes bearing C4d as biomarkers of disease activity for systemic lupus erythematosus

“…Previous studies conducted by us (15) and by other investigators (16,17) have shown the presence of C4d, a complement activation product, on the surface of erythrocytes. To evaluate the potential of reticulocyte C4d levels as a biomarker of disease activity, we conducted a cross-sectional study to examine and compare the presence of C4d on reticulocytes from healthy subjects, patients with SLE, and patients with …”

“…We recently demonstrated that erythrocytes in patients with SLE are characterized specifically by abnormally high levels of erythrocyte C4d and abnormally low levels of erythrocyte CR1 (15). The capacity of C4-derived complement activation products to attach covalently to cell surfaces via thioester bonds suggests that erythrocytes, once tagged with C4d, may carry the molecule during the remainder of their lifespan (40,41).…”

“…Although serum levels of C3, C4, and their activation-derived products are frequently used to monitor disease activity in SLE, their value as biomark-ers is a subject of controversy (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13). These observations have led us to consider measurement of complement activation products bound to circulating blood cells (14,15).…”

“…Erythrocytes, the most abundant blood cells, have immediate access to complement activation products generated systemically or locally. We therefore postulated that complement-bearing erythrocytes may serve as biologic beacons of inflammation in vivo, reflecting disease activity in patients with SLE (14,15).…”

Platelet C4d is highly specific for systemic lupus erythematosus

“…We recently identified abnormal levels of the complement-activation product C4d on erythrocytes from patients with SLE (10). Erythrocyte C4d appears to be a useful lupus biomarker for diagnosis and monitoring of disease activity over time.…”