Induced expression of Toll-like receptor 9 in peritubular capillary endothelium correlates with the progression of tubulointerstitial lesions in autoimmune disease-prone mice

Background Toll-like receptor (Tlr) 9 is capable of recognizing exogenous and/or endogenous nucleic acids and plays a crucial role in innate and adaptive immunity. Recently, we showed that Tlr9 is overexpressed in podocytes, a component of the blood–urine barrier (BUB), in glomeruli of autoimmune glomerulonephritis (AGN) model mice. This study investigated the activation of peritubular capillary (PTC) endothelial cells (ECs), a component of the BUB in the tubulointerstitium, through overexpressing Tlr9, and the subsequent development of tubulointerstitial lesions (TILs) in AGN model mice. Methods Lupus-prone BXSB/MpJ-Yaa (Yaa) and BXSB/MpJ (BXSB) mice were used as an AGN model and control, respectively. In addition to histopathological and ultrastructural techniques, protein and mRNA levels were also evaluated. The relationship between Tlr9 and TIL indices was analyzed by statistical correlation analysis. Results Yaa mice developed TILs and showed strong Tlr9 mRNA expression in PTC ECs at 24 weeks (wks) of age. However, BXSB mice showed no TIL but faint expression of Tlr9 mRNA at 8 and 24 wks of age. Tlr9 protein localization on PTC was almost absent in BXSB mice at both ages but intense expression was found in Yaa mice only at 24 wks of age. Relative mRNA expression of Tlr9 and its putative downstream cytokines, including interleukin 1 beta (Il1b), Il6, interferon gamma (Ifng), and tumor necrosis factor alpha (Tnf) was markedly increased in isolated tubulointerstitium from Yaa mice at 24 wks of age. Furthermore, electron microscopy examination revealed PTC injury and TIL in Yaa mice at 24 wks. The expression level of Tlr9 in the tubulointerstitium was correlated with inflammatory cells in TILs, injured PTC, Ilb and Tnf expression, and damaged tubules (P