Usefulness of erythrocyte-bound C4d as a biomarker to predict disease activity in patients with systemic lupus erythematosus

Objective. SLE is an autoimmune disorder characterized by abnormal complement activation. Numerous new biomarkers have recently been used to diagnose or monitor disease activity in patients with SLE. We checked the levels of erythrocyte-bound Cod (E-C4d), an activation-derived fragment of C4 that is deposited on the erythrocytes, under different conditions of SLE in order to correlate these levels with disease activity. Methods. We conducted a cross-sectional investigation of three groups of patients: (i) 63 patients with SLE; (ii) 43 patients with other diseases; and (iii) 26 healthy controls. Erythrocytes were analysed by flow cytometry to determine the levels of E-Cod. Results. We found a significant elevation in the mean levels of E-Cod in SLE patients compared with patients with other diseases or healthy controls. In SLE patients, the levels of E-Cod were correlated with the SLEDAI and inversely correlated with serum C3/C4 levels. In the subgroup of SLE patients with haemolytic anaemia (HA), a significantly higher level of E-Cod was observed than that in SLE patients without HA. However, in SLE patients with HA, there was no correlation between the levels of E-Cod and other markers of disease activity, including SLEDAI and levels of anti-dsDNA, C3 and C4. Conclusion. E-Cod levels are useful diagnostic markers for SLE and can serve as biomarkers of disease activity in patients with SLE. However, E-Cod is of limited value in monitoring disease activity in SLE patients with HA.