Association of Blood Concentrations of Complement Split Product iC3b and Serum C3 With Systemic Lupus Erythematosus Disease Activity

Abstract

Objective: To examine correlations between blood levels of complement split product iC3b and serum component C3 with clinically meaningful changes in disease activity in patients with systemic lupus erythematosus (SLE).

Methods: A total of 159 consecutive patients with SLE, diagnosed according to the American College of Rheumatology or Systemic Lupus International Collaborating Clinics classification criteria, were enrolled in CASTLE (Complement Activation Signatures in Systemic Lupus Erythematosus), a prospective observational study. Patients with 1-7 study visits were included in this longitudinal analysis. In addition, 48 healthy volunteers were enrolled to establish a normal reference value for the ratio of blood iC3b to serum C3 concentrations. Serum C3 and C4 levels were measured by nephelometry, and blood iC3b levels were measured by a lateral flow assay. SLE disease activity was monitored with the Responder Index 50 instrument of the SLE Disease Activity Index 2000.

Results: Relative changes in the iC3b:C3 ratio, levels of anti-double-stranded DNA (anti-dsDNA) antibodies, and use of a supraphysiologic dose of prednisone (>7.5 mg/day) each independently correlated with SLE disease activity, as determined in multilevel multiple logistic regression analyses. Only the iC3b:C3 ratio was significantly associated with clinically meaningful improvements in disease activity among patients with SLE who were receiving a supraphysiologic dose of prednisone. The iC3b:C3 ratio outperformed C3 and C4 levels with regard to discriminating active SLE from inactive SLE, and major flares from no disease activity. The iC3:C3 ratio, anti-dsDNA antibody levels, erythrocyte sedimentation rate, and use of a supraphysiologic prednisone dose were each independently associated with the presence of lupus nephritis, whereas none of these measures was associated with SLE rash. The association of the iC3b:C3 ratio with lupus nephritis was independent of other observed clinical manifestations.

Conclusion: The ratio of blood iC3b to serum C3 concentrations correlates with the extent of SLE disease activity and with clinically meaningful changes in disease activity in patients with SLE. Furthermore, the iC3b:C3 ratio may discriminate between active and inactive SLE, and between major flares and no active disease.

Figure 1

Elevated iC3b:C3 ratios are more commonly observed in patients with active systemic lupus erythematosus (SLE). The distributions of iC3b:C3 ratios (A), serum iC3b levels (B), and serum C3 levels (C) were compared between patients with active SLE (n = 229), patients with inactive SLE (n = 250), and healthy controls (n = 48). Values are the mean ± SD. The broken horizontal line indicates the upper limit of normal (ULN). * = P < 0.0001; ** = P < 0.008 by 1‐tailed analysis of variance with Dunn's correction.

Figure 2

The iC3b:C3 ratio is associated with active systemic lupus erythematosus (SLE). A and B, Variables were examined for association with active disease as determined using the SLE Disease Activity Index 2000 (SLEDAI‐2K) scores, in univariate (A) and multivariate (B) logistic regression analyses with generalized estimating equations (GEE). C and D, Variables were examined for association with the serology‐free SLEDAI‐2K Responder Index 50 scores in univariate (C) and multivariate (D) logistic regression analyses with GEE. Whisker plots show the average odds ratios with 95% confidence intervals (95% CIs). Variables that were significantly associated with active SLE are shown in red. ESR = erythrocyte sedimentation rate; dsDNA = anti–double‐stranded DNA; CRP = C‐reactive protein; AA = African American.

Figure 3

The iC3b:C3 ratio is associated with clinically meaningful change in systemic lupus erythematosus (SLE) disease activity in patients with more active disease. Variables were examined in univariate logistic regression analyses with generalized estimating equations for association with clinically meaningful changes in SLE disease activity in all study subjects assessed in longitudinal analyses (A), and in only those taking a supraphysiologic dose of prednisone (daily dose >7.5 mg/day) (B). Whisker plots show the average odds ratios with 95% confidence intervals (95% CIs). The statistically significant association of the iC3b:C3 ratio with clinically meaningful change is shown in red. ESR = erythrocyte sedimentation rate; dsDNA = anti–double‐stranded DNA; CRP = C‐reactive protein; AA = African American.

Figure 4

The iC3b:C3 ratio has the capacity to discriminate active systemic lupus erythematosus (SLE) from inactive SLE, and major flares from no disease activity. Receiver operating characteristic (ROC) curves using the iC3b:C3 ratio and blood levels of iC3b, C3, C4, and anti–double‐stranded DNA (anti‐dsDNA) antibodies were used to discriminate active SLE from inactive SLE using the complete SLE Disease Activity Index 2000 (SLEDAI‐2K) Responder Index 50 (RI‐50) score (A), active SLE from inactive SLE using the serology‐free SLEDAI‐2K RI‐50 score (B), and major flare from no disease activity (C). AUC = area under the ROC curve; 95% CI = 95% confidence interval.

Figure 5

The iC3b:C3 ratio is associated with lupus nephritis, but not rash, in patients with systemic lupus erythematosus. A and B, Variables were examined for association with lupus nephritis using univariate (A) and multivariate (B) logistic regression analyses with generalized estimating equations (GEE). C, Multiple linear regression analysis with GEE was used to assess changes in the urinary protein:creatinine (UPC) ratio for a given unit change in each biomarker. D, Univariate logistic regression analysis was used to examine the association between each variable and lupus rash. Whisker plots show the average odds ratios with 95% confidence intervals (95% CIs). Variables that were significantly associated with lupus nephritis are shown in red. ESR = erythrocyte sedimentation rate; dsDNA = anti–double‐stranded DNA; CRP = C‐reactive protein; AA = African American.