Excretion of complement proteins and its activation marker C5b-9 in IgA nephropathy in relation to renal function

Abstract

Background: Glomerular damage in IgA nephropathy (IgAN) is mediated by complement activation via the alternative and lectin pathways. Therefore, we focused on molecules stabilizing and regulating the alternative pathway C3 convertase in urine which might be associated with IgAN pathogenesis.

Methods: Membrane attack complex (MAC), properdin (P), factor H (fH) and Complement receptor type 1 (CR1) were quantified in urine samples from 71 patients with IgAN and 72 healthy controls. Glomerular deposition of C5, fH and P was assessed using an immunofluorescence technique and correlated with histological severity of IgAN and clinical parameters. Fibrotic changes and glomerular sclerosis were evaluated in renal biopsy specimens.

Results: Immunofluorescence studies revealed glomerular depositions of C5, fH and P in patients with IgAN. Urinary MAC, fH and P levels in IgAN patients were significantly higher than those in healthy controls (p < 0.001), but CR1 was significantly lower than that in healthy controls (p < 0.001). Urinary MAC and fH levels were positively correlated with serum creatinine (sCr), urinary N-acetyl-β-D-glucosaminidase (u-NAG), urinary β2 microglobulin (u-Bm), urinary protein (p < 0.001), interstitial fibrosis (MAC: p < 0.01, fH: p < 0.05) and the percentage of global glomerular sclerosis (p < 0.01). Urinary P was positively correlated with u-NAG, u-Bm, and urinary protein (p < 0.01).

Conclusions: Complement activation occurs in the urinary space in IgAN and the measurement of levels of MAC and fH in the urine could be a useful indicator of renal injury in patients with IgAN.

Figure 1

Glomerular deposition of C3, C5, fH and P were assessed by immunofluorescence technique. Glomerular deposition of C3, C5, fH and P shows the same mesangial pattern.

Figure 2

Urinary MAC and complement regulatory protein levels differ between patients with IgA nephropathy and healthy controls. Levels of urinary MAC, fH, and P are significantly higher in IgAN patients than those in healthy controls, but CR1 is significantly lower.

Figure 3

Levels of urinary MAC and complement regulatory proteins in IgA nephropathy patients grouped according to disease severity. Urinary MAC, fH and P tends to increase with worsening of prognosis, in particular, MAC and fH significantly fluctuate with disease prognosis (p < 0.001).

Figure 4

Detection of fH in urine and serum samples by Western blotting. Representative results show molecular weight assessments for fH. Lane 1: purified fH, lane 2: urine a from patient, 3: serum from the same patient, 4: urine from a healthy control, 5: serum from a healthy control, 6: urine from the patient with the highest level of urinary fH.