Extracellular matrix accumulation in immune-mediated tubulointerstitial injury

Extracellular matrix accumulation in immune-mediated tubulointerstitial injury. The accumulation of excessive extracellular matrix (ECM) following tubular injury likely represents an imbalance between ECM production and degradation. We assessed the temporal relationship between the accumulation of ECM, cell adhesion molecules, matrix degrading proteinases, and their inhibitors in a rat model of anti-tubular basement membrane (TBM) antibody-associated tubulointerstitial nephritis (TIN) by the RNase protection assay and immunohistochemistry. There was an increase in the steady state expression of fibronectin (FN) and α 2 (IV) collagen mRNAs beginning on day 7 with the onset of neutrophil infiltration. An increase in α 1 (III) collagen and α 1 -integrin did not occur until days 9 and 10, respectively, at which time mononuclear leukocytes were the predominant infiltrating cell. Increased levels of FN, α 1 (III), α 2 (IV) and α 1 integrin mRNAs occurred through day 14. By immunohistochemistry, increased accumulation of collagen IV, heparan sulfate proteoglycan, and laminin were detected along the thickened TBM; collagens I and III were immunolocalized within the tubulointerstitium, while FN was present in both the TBM and interstitium in rats with TIN on day 14. The increase in matrix accumulation was associated with little or no increase in proteinases. u-PA transcripts fell beginning on day 8, with recovery to control values by day 12. Transin mRNA was found at low levels only on days 8 and 9, and the protein could not be detected by Western blotting. In contrast, these changes were associated with an increase in proteinase inhibitors, so that TIMP and PAI-1 mRNAs increased beginning on day 7 and persisted through day 14. PAI-1 mRNA correlated with biologic activity, while TIMP was immunolocalized within the peritubular endothelium and infiltrating leukocytes. These data demonstrate a temporal association between ECM accumulation, a minimal change in proteinase, and an increase in proteinase inhibitors.