T and B cells participate in bone repair by infiltrating the fracture callus in a two-wave fashion.
Fracture healing is a regenerative process in which bone is restored without scar tissue formation. The healing cascade initiates with a cycle of inflammation, cell migration, proliferation and differentiation. Immune cells invade the fracture site immediately upon bone damage and contribute to the initial phase of the healing process by recruiting accessory cells to the injury site. However, little is known about the role of the immune system in the later stages of fracture repair, in particular, whether lymphocytes participate in soft and hard callus formation. In order to answer this question, we analyzed femoral fracture healing in mice by confocal microscopy. Surprisingly, after the initial inflammatory phase, when soft callus developed, T and B cells withdrew from the fracture site and were detectable predominantly at the femoral neck and knee. Thereafter lymphocytes massively infiltrated the callus region (around day 14 after injury), during callus mineralization. Interestingly, lymphocytes were not found within cartilaginous areas of the callus but only nearby the newly forming bone. During healing B cell numbers seemed to exceed those of T cells and B cells progressively underwent effector maturation. Both, osteoblasts and osteoclasts were found to have direct cell-cell contact with lymphocytes, strongly suggesting a regulatory role of the immune cells specifically in the later stages of fracture healing.
Julius Wolff Institut and Center for Musculoskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany; Berlin - Brandenburg Center for Regenerative Therapies, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany. Electronic address: Ireen1907@gmx.de.
Ireen
I
Könnecke
German Arthritis Research Center (DRFZ), Charitéplatz 1, 10117 Berlin, Germany. Electronic address: serra@drfz.de.
Alessandro
A
Serra
Laboratory of Experimental Trauma Surgery, Justus-Liebig University, Kerkraderstr. 9, 35394 Giessen, Germany. Electronic address: Thaqif.ElKhassawna@chiru.med.uni-giessen.de.
Thaqif
T
El Khassawna
Julius Wolff Institut and Center for Musculoskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany; Berlin - Brandenburg Center for Regenerative Therapies, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany. Electronic address: claudia.schlundt@charite.de.
Claudia
C
Schlundt
Julius Wolff Institut and Center for Musculoskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany; Berlin - Brandenburg Center for Regenerative Therapies, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany. Electronic address: hanna.schell@charite.de.
Hanna
H
Schell
German Arthritis Research Center (DRFZ), Charitéplatz 1, 10117 Berlin, Germany. Electronic address: hauser@drfz.de.
Anja
A
Hauser
Julius Wolff Institut and Center for Musculoskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany. Electronic address: agnes.ellinghaus@charite.de.
Agnes
A
Ellinghaus
Berlin - Brandenburg Center for Regenerative Therapies, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany; Institute of Medical Immunology, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany. Electronic address: hans-dieter.volk@charite.de.
Hans-Dieter
HD
Volk
Berlin - Brandenburg Center for Regenerative Therapies, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany; German Arthritis Research Center (DRFZ), Charitéplatz 1, 10117 Berlin, Germany. Electronic address: radbruch@drfz.de.
Andreas
A
Radbruch
Julius Wolff Institut and Center for Musculoskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany; Berlin - Brandenburg Center for Regenerative Therapies, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany. Electronic address: georg.duda@charite.de.
Georg N
GN
Duda
Julius Wolff Institut and Center for Musculoskeletal Surgery, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany; Berlin - Brandenburg Center for Regenerative Therapies, Charité - Universitätsmedizin Berlin, Augustenburger Platz 1, 13353 Berlin, Germany. Electronic address: katharina.schmidt-bleek@charite.de.
Katharina
K
Schmidt-Bleek
D000818:Animals; D001402:B-Lymphocytes / Q000276:immunology*; D001483:Base Sequence; D002146:Bony Callus / Q000276:immunology* / Q000378:metabolism; D017931:DNA Primers; D005434:Flow Cytometry; D017102:Fracture Healing / Q000276:immunology*; D050723:Fractures, Bone / Q000276:immunology*; D051379:Mice; D008810:Mice, Inbred C57BL; D053244:Osteoprotegerin / Q000235:genetics; D016133:Polymerase Chain Reaction; D053245:RANK Ligand / Q000235:genetics; D013601:T-Lymphocytes / Q000276:immunology*
D017931:DNA Primers; D053244:Osteoprotegerin; D053245:RANK Ligand; C506031:Tnfrsf11b protein, mouse; C505689:Tnfsf11 protein, mouse
Fracture healing; Immune cells; Osteoimmunology