ICAM-1 interactions in the renal interstitium: a novel activator of fibroblasts during nephritis

Chronic renal diseases often degenerate towards end-stage failure, requiring replacement renal therapy. The progressive decline of such diseases is a highly complex, multi-factorial process, which is poorly understood. Indeed, not all chronic conditions take on a progressive course, some may recover to regain normal function, while others may remain functionally impaired yet stable. The structural features of progressive decline, however, show common histological features, despite the diverse nature of the primary injury. These aberrant structural alterations are characterised essentially by a dramatic expansion of the tubulointerstitium, with accompanying tubular atrophy, resulting from interstitial fibrosis. These changes are thought to be a uniform response to prolonged inflammation which may originate in the glomerulus, the vasculature or the interstitial space (Strutz et al., 1995). A histomorphometric analysis of renal diseases, initially performed by Risdon et al. (1968), and supported by Bohle et al. (1987) and others (Eknoyan et al., 1990), revealed that the severity of abnormal glomerular pathology did not always correlate directly with impaired renal function. The extent of interstitial inflammation and the degree of interstitial fibrosis, however, were both shown to be more accurate predictors of renal function (Bohle et al., 1992). Furthermore there was a high probability of irreversible functional decline, in the presence of interstitial fibrotic lesions and tubular atrophy. Interstitial fibrosis is therefore considered an important histological marker for end stage renal failure, and is believed to be functionally more significant than primary changes within the glomerulus. In most tissues, resident fibroblasts are believed to be the cells principally responsible for the synthesis and breakdown of extracellular matrix (ECM) within connective tissues. Indeed in fibrotic diseases of lung and skin, the resident fibroblast has been identified as the most important cell responsible for the abnormal deposition of ECM components during the disease process (Phan et al., 1985). In the kidney, there are probably several sources of matrix components during fibrosis including tubular epithelial cells, inflammatory macrophages (Vaage and Linbland, 1990) as well as interstitial fibroblasts. Although the precise cellular source of the bulk of this matrix requires clarification, there is mounting evidence supporting a significant contribution from resident or infiltrating fibroblasts (Rodemann and Muller, 1990, 1991a,b; Strutz and Muller, 1995).