Role of complement in renal tubular damage.

Deposition of immunoglobulins, complement proteins C1q, C3c, C3d, C4, C5, C6, C7, C8, C9, and terminal complement complex neoantigens in the renal tubulointerstitium was studied in serial sections by immunofluorescence microscopy. Renal tissue from 45 cases with various glomerular diseases, including 8 controls, was studied. The patients were divided into groups; one with tubulointerstitial lesions (24 cases) and the other without (13 cases). The immunoproteins were deposited mainly in the tubular basement membrane and blood vessels. Compared with controls there was a significantly increased staining score for C5 to C9 in the tubular basement membrane in both disease groups. However, the increase in terminal complement complex neoantigens score was significant only in the disease group with tubulo interstitial lesions. The changes in C3d score were not significant. Serial sections showed consistent and heavy ribbon-like deposits of complement proteins C3d, C5 to C9, and terminal complement complex neoantigens in corresponding locations of the segments of tubular basement membrane, mainly in the disease group with tubulointerstitial lesions and especially in the damaged tubules. These findings suggest that in situ activation of the complement cascade leads to the deposition of terminal complement complex neoantigens. Complement activation in the basal area of the tubules may, therefore, be an important pathogenetic mechanism in tubulointerstitial damage.