title: Effect of Continuous-Wave Low-Intensity Ultrasound in Inflammatory Resolution of Arthritis-Associated Synovitis.
abstract: Low-intensity ultrasound (LIUS) can reduce pain and improve function in arthritic joints. Neutrophils are first-line actors in host defense that recruit macrophages. Dead neutrophils are removed during resolution of inflammation. Delayed neutrophil clearance can lead to extended inflammation or even chronic autoimmune disease. Although neutrophil extracellular traps (NETs) in arthritic tissue are involved in the pathogenesis of arthritis, their functional role has not been clarified.
  
  
  
  This study aimed to investigate the effect of LIUS on synovial inflammation and its resolution via neutrophil clearance.
  
  
  
  Synovitis was induced by intra-articular injection of complete Freund's adjuvant (CFA) into the left knee joint of 58 adult male Sprague-Dawley rats. Low-intensity ultrasound (1 MHz, 200 mW/cm(2)) was applied for 10 minutes daily. Neutrophil clearance was assessed with the expression of myeloperoxidase (MPO). In addition, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) staining and NET formation in the synovium were observed. In neutrophil and macrophage cultures from peripheral blood, the effect of NET clearance by LIUS was investigated.
  
  
  
  In CFA-induced synovitis, MPO-positive neutrophils peaked after 2 to 3 days, filling the inflammatory core. Monocytes and macrophages in the periphery later infiltrated the core and were reduced thereafter. Low-intensity ultrasound reduced synovial hyperplasia and induced earlier MPO clearance. Neutrophils in the core of the inflamed synovium exhibited NET formation, which LIUS increased. Low-intensity also induced NETs in peripheral polymorphonuclear cells in an intensity-dependent manner and potentiated phorbol myristate acetate (PMA)-induced NETosis. The PMA-induced NETs were cleared by macrophages; clearance was enhanced by LIUS.
  
  
  
  The effect of LIUS on CFA-induced inflammation was observed only during the acute phase. Although the effect of LIUS on NETosis in the in vitro neutrophil culture system was clear, the in vivo NETosis cannot be quantified.
  
  
  
  Neutrophil extracellular traps act in inflammatory synovitis, and LIUS enhanced the NETs and resulted in neutrophil clearance by enhancing the phagocytosis of macrophages, which might be a factor underlying the therapeutic effect of LIUS in arthritic synovium.
authors.affiliation: J-I. Chung, PhD, Department of Physiology, Ajou University School of Medicine; Neuroscience Graduate Program, Department of Biomedical Science, Ajou University School of Medicine; and Chronic Inflammatory Disease Research Center, Ajou University School of Medicine, Suwon, Republic of Korea.
authors.forename: Jee-In
authors.initials: JI
authors.lastname: Chung
authors.affiliation: B-H. Min, MD, PhD, Department of Orthopedic Surgery, Ajou University School of Medicine.
authors.forename: Byung-Hyun
authors.initials: BH
authors.lastname: Min
authors.affiliation: E.J. Baik, MD, PhD, Department of Physiology, Ajou University School of Medicine, Yeongtong-gu Worldcup-ro 164, Suwon 443-380 Republic of Korea; Neuroscience Graduate Program, Department of Biomedical Science, Ajou University School of Medicine; and Chronic Inflammatory Disease Research Center, Ajou University School of Medicine. eunjoo@ajou.ac.kr.
authors.forename: Eun Joo
authors.initials: EJ
authors.lastname: Baik
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000528:radiation effects; D002478:Cells, Cultured; D065206:Extracellular Traps / Q000528:radiation effects; D006965:Hyperplasia / Q000628:therapy; D008264:Macrophages / Q000502:physiology; D008297:Male; D009504:Neutrophils / Q000201:enzymology; D009195:Peroxidase / Q000378:metabolism; D010587:Phagocytosis / Q000528:radiation effects; D017207:Rats, Sprague-Dawley; D013583:Synovial Membrane / Q000473:pathology*; D013585:Synovitis / Q000139:chemically induced / Q000628:therapy*; D014464:Ultrasonic Therapy / Q000379:methods*; D000069453:Ultrasonic Waves
chemical_list: D009195:Peroxidase