title: Macrophages in bone fracture healing: Their essential role in endochondral ossification.
abstract: In fracture healing, skeletal and immune system are closely interacting through common cell precursors and molecular mediators. It is thought that the initial inflammatory reaction, which involves migration of macrophages into the fracture area, has a major impact on the long term outcome of bone repair. Interestingly, macrophages reside during all stages of fracture healing. Thus, we hypothesized a critical role for macrophages in the subsequent phases of bone regeneration. This study examined the impact of in vivo induced macrophage reduction, using clodronate liposomes, on the different healing phases of bone repair in a murine model of a standard closed femoral fracture. A reduction in macrophages had no obvious effect on the early fracture healing phase, but resulted in a delayed hard callus formation, thus severely altering endochondral ossification. Clodronate treated animals clearly showed delayed bony consolidation of cartilage and enhanced periosteal bone formation. Therefore, we decided to backtrack macrophage distribution during fracture healing in non-treated mice, focusing on the identification of the M1 and M2 subsets. We observed that M2 macrophages were clearly prevalent during the ossification phase. Therefore enhancement of M2 phenotype in macrophages was investigated as a way to further bone healing. Induction of M2 macrophages through interleukin 4 and 13 significantly enhanced bone formation during the 3week investigation period. These cumulative data illustrate their so far unreported highly important role in endochondral ossification and the necessity of a fine balance in M1/M2 macrophage function, which appears mandatory to fracture healing and successful regeneration.
authors.affiliation: Julius Wolff Institute and Center for Muskuloskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany; Berlin Brandenburg Center for Regenerative Therapies (BCRT), Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany. Electronic address: claudia.schlundt@charite.de.
authors.forename: Claudia
authors.initials: C
authors.lastname: Schlundt
authors.affiliation: Laboratory of Experimental Trauma Surgery, Faculty of Medicine, Justus-Liebig University, Schubertstrasse 81, 35392, Giessen, Germany. Electronic address: Thaqif.elkhassawna@chiru.med.uni-giessen.de.
authors.forename: Thaqif
authors.initials: T
authors.lastname: El Khassawna
authors.affiliation: German Arthritis Research Center (DRFZ), Charité - Universitätsmedizin Berlin, Charitestrasse 1, 10117, Berlin, Germany. Electronic address: Serra@drfz.de.
authors.forename: Alessandro
authors.initials: A
authors.lastname: Serra
authors.affiliation: Julius Wolff Institute and Center for Muskuloskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany; Berlin Brandenburg Center for Regenerative Therapies (BCRT), Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany. Electronic address: anke.dienelt@charite.de.
authors.forename: Anke
authors.initials: A
authors.lastname: Dienelt
authors.affiliation: Julius Wolff Institute and Center for Muskuloskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany; Berlin Brandenburg Center for Regenerative Therapies (BCRT), Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany. Electronic address: sebastian.wendler@charite.de.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Wendler
authors.affiliation: Julius Wolff Institute and Center for Muskuloskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany. Electronic address: hanna.schell@charite.de.
authors.forename: Hanna
authors.initials: H
authors.lastname: Schell
authors.affiliation: Vrije Universiteit, VUMC, Department of Molecular Cell Biology, Faculty of Medicine, Van der Boechorststraat 7, 1007 MB, Amsterdam, The Netherlands. Electronic address: nvanrooijen@clodronateliposomes.org.
authors.forename: Nico
authors.initials: N
authors.lastname: van Rooijen
authors.affiliation: German Arthritis Research Center (DRFZ), Charité - Universitätsmedizin Berlin, Charitestrasse 1, 10117, Berlin, Germany. Electronic address: radbruch@drfz.de.
authors.forename: Andreas
authors.initials: A
authors.lastname: Radbruch
authors.affiliation: Department of Molecular Parasitology, Humboldt-University Berlin, Philippstrasse 13, 10115, Berlin, Germany. Electronic address: richard.lucius@rz.hu-berlin.de.
authors.forename: Richard
authors.initials: R
authors.lastname: Lucius
authors.affiliation: Center for Infection Medicine, Institute for Immunology, Freie Universität Berlin, Robert von Ostertag-Strasse 7-13, 14163, Berlin, Germany. Electronic address: susanne.hartmann@fu-berlin.de.
authors.forename: Susanne
authors.initials: S
authors.lastname: Hartmann
authors.affiliation: Julius Wolff Institute and Center for Muskuloskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany; Berlin Brandenburg Center for Regenerative Therapies (BCRT), Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany. Electronic address: georg.duda@charite.de.
authors.forename: Georg N
authors.initials: GN
authors.lastname: Duda
authors.affiliation: Julius Wolff Institute and Center for Muskuloskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany; Berlin Brandenburg Center for Regenerative Therapies (BCRT), Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353, Berlin, Germany. Electronic address: katharina.schmidt-bleek@charite.de.
authors.forename: Katharina
authors.initials: K
authors.lastname: Schmidt-Bleek
mesh_terms: D000818:Animals; D002146:Bony Callus / Q000378:metabolism*; D017102:Fracture Healing / Q000502:physiology*; D005596:Fractures, Closed / Q000378:metabolism*; D008264:Macrophages / Q000378:metabolism* / Q000502:physiology*; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D010012:Osteogenesis / Q000502:physiology*; D014945:Wound Healing / Q000502:physiology*
keywords: Delayed fracture healing; Endochondral ossification; M1 and M2 macrophage phenotype; Macrophage depletion; Osteoimmunology