Proteinuria and tubulointerstitial lesions in lupus nephritis

Abstract

Background: Response of the renal tubules to proteinuria is implicated in progression of renal disease. Experimentally, proteinuria causes increased tubular synthesis of macrophagic and other chemokines, with increased tubular cellular proliferation and apoptosis, leading to interstitial inflammation and fibrosis. Clinically, diminution of proteinuria leads to the slowing of progression, but whether this leads to reduction in tubular lesions has not been directly demonstrated in humans.

Methods: Initial (Bx1) and systematic six-month biopsies (Bx2) from 71 patients with lupus nephritis were studied, with a subset of 34 biopsies also stained for proliferating cell nuclear antigen (PCNA), the macrophage marker PGM1, and cytokeratins (AE1/AE3), and morphometric cell and tubular profile counts performed.

Results: Positive correlations were found between increasing levels of proteinuria and the following light microscopic parameters: tubular epithelial pyknosis, tubular epithelial nuclear "activation," tubular lumenal macrophages, interstitial inflammation and fibrosis, but not with tubulointerstitial immunofluorescence. Significant positive correlations also were found with the following immunohistochemical parameters: PCNA in epithelial cells (r = 0.74) and tubular luminal cells (r = 0.47); tubular lumenal macrophages (r = 0.63) and tubular epithelial cells with acquired PGM1 staining (r = 0.36); and pyknotic tubular epithelial cells (r = 0.47). All showed strong correlations with serum creatinine (S(Cr)) as well. All were reduced at Bx2, generally in parallel to the reduction in proteinuria. Tubulointerstitial immune deposits appear to play only a minor role in the development of tubular epithelial lesions and the progression of renal disease in lupus. They show only limited correlation with SCr and no correlation with proteinuria. By multiple regression, they are not associated with tubular epithelial lesions, interstitial inflammation or interstitial fibrosis at either biopsy, whereas tubular epithelial lesions are strongly associated with interstitial inflammation at Bx1 and with interstitial fibrosis at Bx2. Cytokeratin correlated strongly with S(Cr) (r = 0.53, P = 0.002) but not with proteinuria (r = 0.27, NS), and was the sole immunohistochemical parameter to increase at Bx2. It appears to be a sensitive marker for tubular atrophy.

Conclusions: In this study both proteinuria and SCr showed a hierarchy of correlations with morphologic variables: Tubular epithelial cell changes> tubular macrophages> interstitial inflammation> interstitial fibrosis, corresponding to current experimental models, but not previously demonstrated in humans.