Increased toll-like receptor 9 expression is associated with the severity of paraquat-induced lung injury in mice

Background: Toll-like receptor 9 (TLR9), an important component of the innate immune system, contributes to the pathogenesis of lung injury. However, its role in paraquat (PQ)-induced lung injury has not been studied. Method: Mice were divided into PQ group (n = 24) and control group (n = 12). Mice in both groups were killed on either day 7 or day 28 after PQ (40 mg/kg intraperitoneally) or saline administration. TLR9 expression was evaluated through real-time polymerase chain reaction and immunohistochemistry. Concurrently, histopathological examinations of lung tissues were performed. On day 7, the lung weight to body weight ratios (LW/BW), total protein content, and inflammatory cytokine levels in bronchoalveolar lavage fluid (BALF) were measured. On day 28, fibrogenic cytokine expressions in lung tissues were measured. The correlations between TLR9 messenger RNA (mRNA) expression and different indicators of lung injury were then evaluated. Results: TLR9 expression was significantly increased over time in lung tissues after PQ poisoning. On day 7, TLR9 expression increased in parallel with LW/BW ratio (r = 0.403, p < 0.05), BALF protein content (r = 0.706, p<0.01), and BALF inflammatory cytokine levels (interleukin 6 (IL-6): r = 0.619, IL-1 beta: r = 0.930, tumor necrosis factor alpha: r = 0.589, all p < 0.05). On day 28, elevated TLR9 expression was closely correlated with Ashcroft score (r = 0.726, p < 0.01), mRNA expressions of alpha-smooth muscle actin, type I collagen and type III collagen (r = 0.926, 0.957, and 0.924, respectively; all p <0.01). Conclusions: The TLR9 expression in lung tissue is markedly elevated during PQ-induced acute lung injury and pulmonary fibrosis and positively correlated with the severity of lung injury in mice.