Emerging evidence has indicated a role of the complement system in the pathogenesis of diabetic nephropathy (DN), although the pathways of complement activation and their clinicopathological relevance in DN are as yet unclear. The present study aimed to investigate levels of various complement components in plasma and urine of DN patients, and their correlation with clinicopathological parameters.

A total of 68 biopsy-proven DN patients with plasma samples were recruited, including 50 patients who also had urine samples available. Seven complement components (C1q, MBL, Bb, C4d, C3a, C5a, soluble C5b-9) were measured by enzyme-linked immunosorbent assay (Elisa), and any associations between their levels and clinicopathological parameters were then investigated.

In DN patients, plasma levels of C1q, MBL, Bb, C4d, C3a, C5a and sC5b-9 were significantly higher than in diabetes patients without renal involvement, as were also urinary levels except for C1q, which showed no significant differences between the two groups. Also, urinary levels of C3a and C5a were significantly correlated with serum creatinine, urinary protein and estimated glomerular filtration rate, whereas urinary sC5b-9 was significantly correlated with the latter two (and not serum creatinine). In addition, urinary levels of MBL, Bb and C4d were significantly correlated with urinary protein, while C3a, C4d and Bb significantly correlated with the classification of glomerular lesions in DN.

In DN patients, the complement system is activated and, of the three possible complement pathways, activation of the lectin and alternative pathways is associated with renal damage.