Macrophages, Myofibroblasts, and Extracellular Matrix Accumulation in Interstitial Fibrosis of Chronic Progressive Nephropathy in Aged Rats

Progressive renal fibrosis is considered to be the final common pathway leading to chronic renal failure. Macrophages are thought to play a role in the induction of the myofibroblasts that produce extracellular matrix (ECM) proteins in renal interstitial fibrosis. We immunohistochemically investigated the relationship between infiltrating macrophages and myofibroblast development in chronic progressive nephropathy (CPN) in 24 month-old male F344 rats, and we also analyzed components of ECM proteins using immunofluorescence microscopy. According to histomorphologic criteria for severity, described elsewhere, rats with CPN were divided into grade 1 (n = 20), grade 2 (n = 34), grade 3 (n = 10), and grade 4 (n = 6). The ratio of fibrotic tissues per unit area, determined by morphometric analysis, was increased with advancing grade of nephropathy. The number of interstitial macrophages continued to be increased gradually, with a peak in grade 4. α-smooth muscle actin-positive myofibroblasts developed, surrounding the regenerating renal tubules in conjunction with the fibrotic areas. The number of the myofibroblasts was also increased, with a peak in grade 3, but in grade 4, it was slightly decreased. There was a significant relationship between the number of infiltrating macrophages and the degree of interstitial fibrosis (r = 0.802; P < 0.05). These observations suggest that macrophages and myofibroblasts might be key cells in fibrogenesis in CPN. However, there was no significant correlation between the numbers of macrophages and myofibroblasts (r = 0.198; P>0.05), although a significant relation between these cells has been reported in the early stages of experimental rat renal fibrosis. Immunostaining for collagen type IV demonstrated increased expression in thickened tubular basement membranes. Abnormal depositions of collagen types I and III, fibronectin, and tenascin were also observed in fibrotic areas adjacent to dilated or atrophic tubules with thickened basement membranes. These ECM proteins were increased in conjunction with the grade of nephropathy, suggesting that ECM accumulation might contribute to progression of renal interstitial fibrosis.