title: Aging Affects Bone Marrow Macrophage Polarization: Relevance to Bone Healing.
abstract: Macrophages are an important component of the inflammatory cascade by initiating and modulating the processes leading to tissue regeneration and bone healing. Depending on the local environment, macrophages can be polarized into M1 (pro-inflammatory) or M2 (anti-inflammatory) phenotypes. In order to assess the effects of aging on macrophage function, bone marrow macrophage polarization using primary bone marrow macrophages (BMMs) from young (8 weeks old) and aged (72 weeks old) wild-type male C57BL/6J mice was analyzed. Fluorescence-activated cell sorting (FACS) analysis (CD11b, iNOS, CD206), qRT-PCR (iNOS, TNF-α, CD206, Arginase 1), and ELISA (TNF-α, IL-1ra) were performed to compare the M1 and M2 phenotypic markers in young and aged mouse macrophages. Once M1 and M2 macrophage phenotypes were confirmed, the results showed that TNF-α mRNA was significantly upregulated in aged M1s after interferon gamma (INF-γ) exposure. Arginase 1 and CD206 mRNA expression were still upregulated with IL4 stimulation in aged macrophages, but to a lesser extend than those from younger animals. TNF-α secretion was also significantly increased in aged M1s compared to young M1s, following lipopolysaccharide (LPS) exposure. However, the IL-1ra secretion did not increase accordingly in aged mice. The results demonstrate that, compared to younger animals, aging of bone marrow derived macrophages increases the resting levels of oxidative stress, and the ratios of pro- to anti-inflammatory markers. These age-related changes in macrophage polarization may explain in part the attenuated response to adverse stimuli and delay in processes such as fracture healing seen in the elderly.
  
  
  BACKGROUND
  Bone healing is a complex process that involves both biological and mechanical factors. Macrophages are key cells that regulate the events involved in bone healing, especially the initial inflammatory phase. In this biological cascade of events, macrophages present as different functional phenotypes including uncommitted (M0), pro-inflammatory (M1), and anti-inflammatory (M2), a process called macrophage polarization. A clear understanding of the effects of aging on macrophage polarization is critical to modulating adverse events such as fractures, atraumatic bone loss, and tissue regeneration in an aging population.
authors.affiliation: Department of Orthopaedic Surgery, Stanford University, R116, 300 Pasteur Drive, Stanford, CA 94305, USA; Laboratoire de Biomécanique et Biomatériaux Ostéo-Articulaires -UMR CNRS 7052, Faculté de Médecine - Université Paris7, 10 avenue de Verdun, 75010 Paris, France; Department of Orthopaedic Surgery, Hopital Cochin, APHP, Université Paris5, 27 rue du Faubourg Saint-Jacques, 75014 Paris, France.
authors.forename: E
authors.initials: E
authors.lastname: Gibon
authors.affiliation: Department of Orthopaedic Surgery, Stanford University, R116, 300 Pasteur Drive, Stanford, CA 94305, USA.
authors.forename: F
authors.initials: F
authors.lastname: Loi
authors.affiliation: Department of Orthopaedic Surgery, Stanford University, R116, 300 Pasteur Drive, Stanford, CA 94305, USA; Department of Oral and Maxillofacial Surgery, University of Chile-Conicyt, Santiago, Chile.
authors.forename: Luis A
authors.initials: LA
authors.lastname: Córdova
authors.affiliation: Department of Orthopaedic Surgery, Stanford University, R116, 300 Pasteur Drive, Stanford, CA 94305, USA.
authors.forename: J
authors.initials: J
authors.lastname: Pajarinen
authors.affiliation: Department of Orthopaedic Surgery, Stanford University, R116, 300 Pasteur Drive, Stanford, CA 94305, USA.
authors.forename: T
authors.initials: T
authors.lastname: Lin
authors.affiliation: Department of Orthopaedic Surgery, Stanford University, R116, 300 Pasteur Drive, Stanford, CA 94305, USA.
authors.forename: L
authors.initials: L
authors.lastname: Lu
authors.affiliation: Department of Orthopaedic Surgery, Stanford University, R116, 300 Pasteur Drive, Stanford, CA 94305, USA.
authors.forename: A
authors.initials: A
authors.lastname: Nabeshima
authors.affiliation: Department of Orthopaedic Surgery, Stanford University, R116, 300 Pasteur Drive, Stanford, CA 94305, USA.
authors.forename: Z
authors.initials: Z
authors.lastname: Yao
authors.affiliation: Department of Orthopaedic Surgery, Stanford University, R116, 300 Pasteur Drive, Stanford, CA 94305, USA; Department of Bioengineering, Stanford University, Stanford, CA, USA; Department of Orthopaedic Surgery and (by courtesy) Bioengineering, Stanford University Medical Center Outpatient Center, 450 Broadway St., M/C 6342, Redwood City, CA 94063, USA.
authors.forename: Stuart B
authors.initials: SB
authors.lastname: Goodman
keywords: Aging; Bone healing; Bone marrowmacrophage; Polarization