Roles of neutrophil-mediated inflammatory response in the bony repair of injured growth plate cartilage in young rats

Rosa Chung^*^,^,, Johanna C. Cool^*, Michaela A. Scherer^*, Bruce K. Foster^*^, and Cory J. Xian^*^,^,^,^,1

^* Department of Orthopaedic Surgery, Women’s and Children’s Hospital, North Adelaide, South Australia, Australia;
Department of Pharmaceutical Biotechnology, University of South Australia, Adelaide, Australia; and Departments of
Paediatrics and
Physiology, University of Adelaide, South Australia, Australia

¹ Correspondence: Department of Orthopaedic Surgery, Women’s and Children’s Hospital, 72 King William Road, North Adelaide, SA 5006, Australia. E-mail: cory.xian{at}adelaide.edu.au

Injured growth plate cartilage is often repaired by bony tissue,resulting in impaired bone growth in children. Previously, injury-induced,initial inflammatory response was shown to be an acute inflammatoryevent containing predominantly neutrophils. To examine potentialroles of neutrophils in the bony repair, a neutrophil-neutralizingantiserum or control normal serum was administered systemicallyin rats with growth plate injury. The inflammatory responsewas found temporally associated with increased expression ofneutrophil chemotactic chemokine cytokine-induced neutrophilchemoattractant-1 and cytokines TNF- ${alpha}$ and IL-1β. Followingthe inflammatory response, mesenchymal infiltration, chondrogenicand osteogenic responses, and bony repair were observed at theinjury site. Neutrophil reduction did not significantly affectinfiltration of other inflammatory cells and expression of TNF- ${alpha}$ and IL-1β and growth factors, platelet-derived growth factor-Band TGF-β1, at the injured growth plate on Day 1 and hadno effects on mesenchymal infiltration on Day 4. By Day 10,however, there was a significant reduction in proportion ofmesenchymal repair tissue but an increase (although statisticallyinsignificant) in bony trabeculae and a decrease in cartilaginoustissue within the injury site. Consistently, in antiserum-treatedrats, there was an increase in expression of osteoblastic differentiationtranscription factor cbf- ${alpha}$ 1 and bone matrix protein osteocalcinand a decrease in chondrogenic transcription factor Sox-9 andcartilage matrix collagen-II in the injured growth plate. Theseresults suggest that injury-induced, neutrophil-mediated inflammatoryresponse appears to suppress mesenchymal cell osteoblastic differentiationbut enhance chondrogenic differentiation, and thus, it may beinvolved in regulating downstream chondrogenic and osteogenicevents for growth plate bony repair.

Key Words: fracture repair • growth factors and cytokines • bone growth