Macrophages polarize to the pro‐inflammatory phenotype and delay neutrophil efferocytosis to augment Aggregatibacter actinomycetemcomitance JP2 clearance

Abstract

AbstractObjectiveThe study examines how neutrophils cross‐talk with macrophages during JP2 Aggregatibacter actinomycetemcomitance infection and factors that are involved in inflammatory resolution and efferocytosis.BackgroundAlthough sub‐gingival bacteria constitute the primary initiating factor in the pathogenesis of molar‐incisor pattern periodontitis (MIPP), the non‐resolved host response has a major role in tissue destruction. While evidence links neutrophils to MIPP pathogenesis, their clearance during inflammatory resolution, governed by macrophages, is poorly understood.MethodsHuman neutrophils (differentiated from HL60 cells) and macrophages (differentiated from THP1 cells) were inoculated with JP2. The supernatants were collected and exposed to naïve neutrophils or macrophages with or without exposure to JP2. Reactive oxygen species (ROS) were measured with 2′‐7′‐dichlorofluorescein‐diacetate and a fluorescent plate reader. Immunofluorescence labeling of CD47 and cell vitality were examined using flow cytometry. Macrophage polarization was tested by immunofluorescence staining for CD163 and CD68 and a fluorescent microscope, and TNFα and IL‐10 secretion was tested using ELISA and RT‐PCR. Efferocytosis was examined by pHrodo and carboxyfluorescein succinimidyl ester staining and fluorescent microscopy. In vivo, macrophages were depleted from C57Bl/6 mice and neutrophil CD47 levels were tested using the subcutaneous chamber model.ResultsNeutrophils exposed to macrophage supernatant show increased ROS, mainly extracellularly, that increased during JP2 infection. Macrophages showed pro‐inflammatory M1 phenotype polarization during JP2 infection, and their supernatants prolonged neutrophil survival by inhibiting CD47 down‐expression and reducing neutrophil necrosis and apoptosis. Also, the macrophages delay neutrophil efferocytosis during JP2 infection which, in turn, enhanced JP2 clearance. Depletion of macrophages in mice mildly prevented neutrophils CD47 reduction and reduced JP2 clearance. The JP2 infection in mice also led to macrophage M1 polarization similar to the in vitro results.ConclusionsAs shown in this study, neutrophil efferocytosis potentially may be reduced during JP2 infection, promoting JP2 clearance, which may contribute to the inflammatory‐mediated periodontal tissue damage.