Lymphatic Contribution to the Cellular Niche in Heterotopic Ossification

Abstract

Objective The objective of this study was to determine the contribution of lymphatic tissue to heterotopic ossification. Background Heterotopic ossification (HO) is the pathologic development of ectopic bone within soft tissues often following severe trauma. Characterization of the tissue niche supporting HO is critical to identifying therapies directed against this condition. Lymphangiogenesis is up-regulated during incidents of trauma, thereby co-incident with the niche supportive of HO. We hypothesized that lymphatic tissues play a critical role in HO formation. Methods Mice underwent hindlimb Achilles’ tendon transection and dorsal burn injury(burn/tenotomy) to induce HO. The popliteal and inguinal lymph nodes were excised ipsilateral to the tenotomy site. Flow cytometry and immunostaining were used to quantify and localize lymphoendothelium. MicroCT was used to quantify HO. Results Enrichment of mature lymphatic tissues was noted 2 weeks after injury at the tendon transection sites when compared with the contralateral, intact tendon based on LYVE1+ tubules (10.9% v. 0.8%, p<0.05). Excision of the inguinal and popliteal nodes with draining popliteal lymphatic vessel significantly decreased the presence of mature lymphoendothelium 2 weeks after injury (10.9% v. 3.3%, p<0.05). Bone-cartilage-stromal progenitor cells (CD105+/AlphaV+/Tie2−/CD45−/CD90−/BP1−) were also significantly decreased after lymph node excision (10.2% v. 0.5%, p<0.05). A significant decrease was noted in the volume of de novo HO present within the soft tissues (0.12 mm^3 v. 0.02 mm^3). Conclusions These findings suggest that lymphatic vessels are intimately linked with the formation de novo bone within soft tissues following trauma, and their presence may facilitate bone formation.

Heterotopic Ossification: Clinical Features, Basic Researches, and Mechanical Stimulations

“…Inflammation-associated cells, such as macrophages and mast cells, also accumulate at the site of trauma-induced heterotopic ossification and promote heterotopic ossification ( Convente et al, 2018 ). Lymphoid tissues also contribute to the cellular niche in Heterotopic Ossification ( Loder et al, 2016 ). The main role of inflammation is to turn MSCs, such as normal fibroblast lineage, into the osteogenic lineage, initiating the onset of heterotopic ossification.…”

“…HIFs could increase the production of pro-angiogenic cytokines such as VEGF, facilitating localized pathological bone tissue formation (Dilling et al, 2010;Hwang et al, 2019). The inhibition of HIFs could attenuate HO formation in experimental models (Agarwal et al, 2016).…”

The traumatic bone: trauma-induced heterotopic ossification

“…In addition to cellular adaptive immunity, lymphatic drainage has also been shown to play a central role in trauma-induced HO. 95 Using the burn-tenotomy model, the authors demonstrated that removing the inguinal and popliteal lymph nodes, ipsilateral to the tenotomy site resulted in significant decrease in the volume of HO at the injury site, in addition to a reduction in frequency of CD105 + α V-Integrin + Tie2 − CD45 − CD90 − BP1 − osteogenic progenitor cells.…”

Differential Vascularity in Genetic and Nonhereditary Heterotopic Ossification

“…Of note, experimental models of HO suggest that the lymphatic system may also be an important environmental contributor to the formation of HO. 46 In this context, surgical lymph node excision decreased HO formation in a trauma-induced mouse model. 46 An interesting future study would include the characterization of lymphatic ingrowth and distribution across human HO samples.…”