Neurological heterotopic ossification following spinal cord injury is triggered by macrophage‐mediated inflammation in muscle

Abstract

Neurological heterotopic ossification (NHO) is the abnormal formation of bone in soft tissues as a consequence of spinal cord or traumatic brain injury. NHO causes pain, ankyloses, vascular and nerve compression and delays rehabilitation in this high-morbidity patient group. The pathological mechanisms leading to NHO remain unknown and consequently there are no therapeutic options to prevent or reduce NHO. Genetically modified mouse models of rare genetic forms of heterotopic ossification (HO) exist, but their relevance to NHO is questionable. Consequently, we developed the first model of spinal cord injury (SCI)-induced NHO in genetically unmodified mice. Formation of NHO, measured by micro-computed tomography, required the combination of both SCI and localized muscular inflammation. Our NHO model faithfully reproduced many clinical features of NHO in SCI patients and both human and mouse NHO tissues contained macrophages. Muscle-derived mesenchymal progenitors underwent osteoblast differentiation in vitro in response to serum from NHO mice without additional exogenous osteogenic stimuli. Substance P was identified as a candidate NHO systemic neuropeptide, as it was significantly elevated in the serum of NHO patients. However, antagonism of substance P receptor in our NHO model only modestly reduced the volume of NHO. In contrast, ablation of phagocytic macrophages with clodronate-loaded liposomes reduced the size of NHO by 90%, supporting the conclusion that NHO is highly dependent on inflammation and phagocytic macrophages in soft tissues. Overall, we have developed the first clinically relevant model of NHO and demonstrated that a combined insult of neurological injury and soft tissue inflammation drives NHO pathophysiology.

Macrophage-derived oncostatin M contributes to human and mouse neurogenic heterotopic ossifications

“…associated with histologically defined stromal cells has been reported in HOs from severe combat-injured orthopedic patients (10). Likewise, in a mouse model of SCI-induced NHO, which we recently developed, hematopoietic marrow-like tissue was observed in the center of NHOs (11). However, we believe that until now, the presence of a hematopoietic stem cell (HSC) niche, defined by functional HSCs, mesenchymal stromal cells (MSCs), and endothelial cells (12), has never been demonstrated in NHO marrow.…”

“…OSM accumulates at sites of NHO in mice. Finally, to confirm the role of OSM in NHO development, we used our mouse model of NHO, in which mice undergo spinal cord transection between T12-T14, together with muscle damage induced by intramuscular injection of cardiotoxin (CDTX) (11). In this model, NHOs develop in the injured muscle exclusively in mice with SCI and CDTX-induced muscle damage (SCI+CDTX) via macrophages infiltrating the injured muscle (11).…”

“…We have recently described the triggering effect of macrophage-mediated inflammation in muscles in a SCI-induced NHO mouse model (11). Among cytokines produced by macrophages in an inflammatory context, oncostatin M (OSM) was upregulated after SCI in mice and it exhibited a neuroprotective effect against SCI or ischemic stroke (28,29).…”

“…Moreover, we demonstrate that activated macrophages contribute to bone formation through the osteogenic action of OSM on muscle cells within an inflammatory context. Finally, we show in a mouse model of SCI-induced NHO that we have recently developed (11) that SCI increases OSM expression in injured muscle and promotes the osteogenic potential of sorted muscle progenitor cells in vitro and that deletion of the Osmr gene reduces NHO development in vivo.…”

Cell death, clearance and immunity in the skeletal muscle

“…Persisting inflammation, inadequate phagocytosis and bone morphogenetic proteins activation are all thought to contribute. 106,107 A rare genetic disorder, fibrodysplasia ossificans progessiva, associated with the activating mutation in the bone morphogenetic protein type I receptor ALK2 clearly highlights the link. 108 Outcomes of Defective Apoptotic Cell Clearance: Autoimmunity IIM leads to disability, decreased quality of life and increased mortality.…”