The complex nature of serum C3 and C4 as biomarkers of lupus renal flare

Abstract

To assess the relationship between serum C3 or C4 levels and lupus renal flare, C3 and C4 levels were measured bimonthly in 71 lupus nephritis patients for a mean of 35 months, during which time 70 renal flares were identified. Comparing baseline, pre-flare, and at-flare values indicated that neither C3 nor C4 levels decreased pre-flare, but both decreased on average significantly at flare. However, sensitivity/specificity for C3 (75%/71%) and C4 (48%/71%) were low. To account for other influencing factors, multiple regression was performed that included bimonthly values of C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR), and genotype data on C3 (S/F), CRP (1846G > A), and the complement regulator factor H (Y402H). This analysis revealed that reduced levels of C4, but not C3, were independently associated with the two-month pre-flare period. Conversely, reduced levels of C3, but not C4, were independently associated with the flare visit. Significant pro-flare interactions included low C3 levels with the factor H 402HH-encoding genotype, and low CRP levels with the C3 F allele. Together these data suggest that C4 activation is critical for initiating renal flare while C3 activation is involved in the actual tissue damage, and that these effects are influenced by genetic variability in complement activation and regulation.

Figure 1

Complement values over time in a cohort of renal systemic lupus erythematosus (SLE) patients. (A) The average C3 and C4 values at baseline, pre-flare, and flare are shown for a subset of 33 well-characterized moderate and severe renal flares. Baseline values were taken as the average of two or three measurements obtained during non-flare periods and 6 or more months removed from any flare activity. C3 and C4 are significantly lower than baseline at flare (*p = 0.01 and 0.002, respectively, based on a repeated measures analysis of variance (ANOVA)). (B) and (C) Complement values over time for individual patients, showing changes in C3 or C4 levels from baseline, 4 and 2 months pre-flare, at flare, and 2 and 4 months postflare. The data are stratified in each panel according to whether or not C3 or C4 levels fell at flare, compared with baseline.

Figure 2

Receiver-operating characteristic (ROC) curves of (A) C3 and (B) C4 sensitivity and specificity to mark concurrent renal flare. The dashed line is the 45° angle tangent to each ROC curve. The area under the curve (AUC) is 0.74 for the C3 curve, and 0.65 for C4 curve.

Figure 3

Risk curves derived from prediction equations, using (A) C4 levels as forecasters of flare or (B) C3 levels as markers of renal flare. Each risk curve is generated for the significant genotype combinations identified in the generalized estimating equation (GEE) analysis. Genotypes that are analyzed as carriers include C3 F carriers (C3F+ versus homozygous wildtype C3SS) and C-reactive protein (CRP) 1846A carriers (CRPA+ versus homozygous wildtype CRPGG). The factor H (FH) polymorphism is noted as homozygous wildtype 402YY (FHYY), heterozygous 402YH (FHYH), or homozygous variant 402HH (FHHH). The X-axis ranges represent the middle 90% of the Ohio SLE Study (OSS) cohort during no-flare visits. The vertical lines identify the median no-flare, pre-flare, or at-flare values for C4 or C3 in the OSS cohort. Risks are calculated from the estimates of the log odds obtained from the respective prediction equation. Note in (A) that the two risk curves for patients who were C3F+ are negligible, and appear superimposed.