Background: Microvascular injury and mesangial dysfunction contribute to the pathogenesis of diabetic glomerulosclerosis. We investigated the extent of microvascular injury characterized by fragmented red blood cells (RBCs) in the mesangium of glomeruli in diabetic nephropathy, and its clinicopathologic significance. We also investigated the possible contributions of plasminogen activator inhibitor-1 (PAI-1), which has been implicated in thrombosis and sclerosis, and the novel steroid receptor superfamily member, peroxisome proliferator-activated receptorgamma (PPARgamma), implicated in monocyte-foamy macrophage transformation in atherosclerosis and improved insulin responsiveness in diabetes.
Methods: Sixty-four diabetic nephropathy (DN) cases in our renal biopsy files at VUMC, diagnosed between 1997 and 1999, were reviewed. Patients were classified based on the presence or absence of fragmented RBCs in the mesangium (M+, M-). PAI-1 and PPARgamma immunostaining was performed with double staining for the macrophage marker CD68.
Results: M+ lesions were present in 21.9% of cases, and in positive cases, involved on average 10.2 +/- 2.1% of glomeruli. M+ patients were 40- to 78-years-old (mean +/- SD, 60.4 +/- 9.8), the female/male ratio was 2.5, and the white/black ratio was 6. In M-, the patients' ages ranged from 29 to 81 years (57.6 +/- 13.3, P = NS vs. M+), the female/male ratio was 0.5 (P < 0.05 vs. M+), and the white/black ratio was 2.3 (P = 0.1 vs. M+). Mean 24-hour urine protein in M+ was 9.9 +/- 13.6 g/24 h, versus 4.0 +/- 2.8 g/24 h in M- (P < 0.05). The fragmented RBCs in M+ cases localized exclusively within Kimmelstiel-Wilson nodules. PAI-1 and PPARgamma immunostaining was increased in areas of sclerosis in arteries and glomeruli, with expression of both in glomerular mesangial, parietal and visceral epithelial cells. Infiltrating macrophages in glomeruli were PPARgamma negative, contrasting positivity in macrophages in control cases of carotid artery plaque and in renal interstitial macrophages. The Kimmelstiel-Wilson nodules in M+ patients showed increased PAI-1 staining.
Conclusions: Mesangial RBC fragments are indicative of microvascular injury and mesangiolysis in DN and are associated with worse proteinuria, and possible worse prognosis. Possible pathogenic mechanisms involve the fibrinolytic/proteolytic system and locally activated PAI-1.