A dual role for neutrophils in periapical lesion pathogenesis

Objectives: the aim of the present study was to assess the role of neutrophils in the pathogenesis of periapical lesion using normal (C57BL/6), neutropenic, ICAM-1 knockout and CCR5 knockout mice. Methods: Neutropenia was induced by intraperitoneal injection of anti-granulocyte receptor-1 mAb (RB6-8C5), before and after surgery. Periapical lesion was experimentally-induced by pulp exposure to the oral environment. After 7, 21 and 42 days, the animals were killed and the periapical region was evaluated under conventional microscopy (morphologic analysis), fluorescence microscopy (morphometric analysis), immunohistochemistry (neutrophils, macrophages and lymphocytes), immunofluorescence and enzymehistochemistry (osteoclasts). A real time RT- PCR was performed to quantify RNAm expression of IL-1alpha, TNF-alpha, IL-10, IFN-gamma, KC, RANK, RANK-L, OPG and catepsin K. Intact teeth were used as controls. Results: Our results show that, in normal mice, neutrophils were the most prevalent inflammatory cell at acute and chronic stages of the lesion development. A positive correlation between the severity of periapical lesions and the number of neutrophils, osteoclasts, and the levels of IL-1alpha, IL-10, IFN-gamma, KC, RANK, RANKL and catepsin K was observed at 21 and 42 days. There was no significant enhancement in mRNA levels of TNF-alpha during all evaluated periods. At days 21 and 42 neutropenic, ICAM-1 knockout and CCR5 knockout mice presented significantly greater bone resorption than normal mice. In ICAM-1 knockout animals increased tissue destruction associated to impaired neutrophils recruitment, whereas in CCR5 knockout animals increased tissue destruction was associated to enhanced neutrophils recruitment. Conclusion: Our findings suggest that neutrophil regulates both acute and chronic inflammation and immune response, combining a dual, anti-infection and pro-inflammatory, role in the pathogenesis of periapical lesion. In normal conditions, neutrophils plays a protective role against endodontic infection, however its abnormal recruitment may result in enhanced damage to the periapical tissues.