The Interaction between Interstitial Inflammatory Cells and Fibrosis-Repair Mechanisms in Tubuloglomerular Diseases

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Abstract
Chronic kidney disease progression is closely linked to tubulointerstitial inflammation, fibrosis, and tubular atrophy, which often predict renal outcomes better than the initiating glomerular lesion. This chapter reviews the cellular, molecular, histopathological, and biomarker evidence connecting interstitial injury with kidney disease. Fibroblasts derived from metanephric stromal progenitors retain substantial phenotypic plasticity. After injury, inflammatory mediators, TGF-β, platelet-derived growth factor, Toll-like receptor signaling, and mechanotransduction induce myofibroblast transition, extracellular matrix deposition, and loss of erythropoietin production. Chronic inflammation may also generate tertiary lymphoid structures that sustain local T- and B-cell activation, autoantibody production, and tissue damage. Intrarenal renin–angiotensin–aldosterone system activation, proteinuria-induced lysosomal and mitochondrial stress, cellular senescence, and alternative complement pathway activation further amplify tubular inflammation and fibrosis. In IgA nephropathy, lupus nephritis, ANCA-associated vasculitis, monoclonal gammopathy of renal significance, and kidney transplantation, interstitial fibrosis and tubular atrophy consistently predict treatment resistance, declining estimated glomerular filtration rate, kidney failure, and reduced graft survival. Urinary biomarkers may noninvasively capture tubular injury or dysfunction (KIM-1, NGAL, NAG, α1- and β2-microglobulin), inflammation (MCP-1/CCL2, CXCL9), stress and fibrosis (DKK-3), and functional tubular mass and repair (uEGF). However, assay heterogeneity, disease-specific performance, age-related variation, and absent universal thresholds limit routine use. Future studies should test whether longitudinal biomarker trajectories detect reversible inflammation before fixed scarring and whether biomarker-guided treatment improves outcomes. Integrating histopathology, clinical variables, and validated biomarker panels may refine prognosis and enable earlier, mechanism-directed interventions.

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