Proteomic analysis of histologically distinct fibrotic regions in a case of IgG4-related tubulointerstitial nephritis

IgG4-related tubulointerstitial nephritis (IgG4-TIN) is histologically characterized by storiform fibrosis; however, the underlying mechanisms remain unclear. This study provides candidate pathways associated with renal fibrosis in this disease for subsequent validation. Renal biopsy specimens of IgG4-TIN (Patient A) were analyzed. Proteins extracted from laser-microdissected interstitial fibrotic lesions at stages 1 (mild), 2 (moderate), and 3 (severe) were examined using mass spectrometry and their relative abundance was compared between stages. Based on proteomic data from Patient A, an immunohistochemical validation study was performed using biopsy specimens from Patient A and 5 cases each of IgG4-TIN, antineutrophil cytoplasmic antibody-associated glomerulonephritis with tubulointerstitial injury, diabetic kidney disease with tubulointerstitial injury, and thin basement membrane nephropathy without tubulointerstitial injury. More than 2,500 proteins were identified with high confidence. The levels of various members of the following protein groups were higher at stage 2/3 than at stage 1: immunoglobulins, complements, proteins expressed by immune cells, signaling proteins, proteins involved in endoplasmic reticulum (ER) stress responses, metabolic reprogramming, and epithelial-mesenchymal transition (EMT), fibroblast/myofibroblast markers, extracellular matrix (ECM) components, keratins, and corneodesmosome/desmosome-related proteins. Proteomic data from Patient A were supported by immunohistochemical results on the ER proteins hypoxia up-regulated protein 1 (HYOU1) and thioredoxin domain-containing protein 5 (TXNDC5), keratin 80 (KRT80), the EMT markers α-smooth muscle actin (α-SMA) and vimentin (VIM), and the pan-leukocyte marker CD45. EMT-like changes were detected in the IgG4-TIN group and in the groups with tubulointerstitial injury. Most interstitial infiltrating cells were HYOU1 + TXNDC5 + KRT80 + α-SMA + VIM + CD45 – . The results of this exploratory, case-based proteomic investigation and literature review suggest that many proteins involved in inflammation/fibrosis (including immune activation, up-regulated signaling pathways, ER stress responses, metabolic reprogramming, EMT-like changes, and ECM deposition) are related to the IgG4-TIN biology and/or common pathways of chronic TIN.