Department of Orthopaedics and Rehabilitation, The Pennsylvania State University College of Medicine, Hershey, PA, United States.
Center for Orthopaedic Research and Translational Science (CORTS), The Pennsylvania State University College of Medicine, Hershey, PA, United States.
Front Immunol. 2023 Oct 3;14:1250309. doi: 10.3389/fimmu.2023.1250309. eCollection 2023.
Bone fractures, the most common musculoskeletal injuries, heal through three main phases: inflammatory, repair, and remodeling. Around 10% of fracture patients suffer from impaired healing that requires surgical intervention, a huge burden on the healthcare system. The rate of impaired healing increases with metabolic diseases such as obesity-associated hyperglycemia/type 2 diabetes (T2D), an increasing concern given the growing incidence of obesity/T2D. Immune cells play pivotal roles in fracture healing, and obesity/T2D is associated with defective immune-cell functions. However, there is a gap in knowledge regarding the stoichiometry of immune cells that populate the callus and how that population changes during different phases of healing. Here, we used complementary global and single-cell techniques to characterize the repertoire of immune cells in the fracture callus and to identify populations specifically enriched in the fracture callus relative to the unfractured bone or bone marrow. Our analyses identified two clear waves of immune-cell infiltration into the callus: the first wave occurs during the early inflammatory phase of fracture healing, while the second takes place during the late repair/early remodeling phase, which is consistent with previous publications. Comprehensive analysis of each wave revealed that innate immune cells were activated during the early inflammatory phase, but in later phases they returned to homeostatic numbers and activation levels. Of the innate immune cells, distinct subsets of activated dendritic cells were particularly enriched in the inflammatory healing hematoma. In contrast to innate cells, lymphocytes, including B and T cells, were enriched and activated in the callus primarily during the late repair phase. The Diet-Induced Obesity (DIO) mouse, an established model of obesity-associated hyperglycemia and insulin resistance, suffers from multiple healing defects. Our data demonstrate that DIO mice exhibit dysregulated innate immune responses during the inflammatory phase, and defects in all lymphocyte compartments during the late repair phase. Taken together, our data characterize, for the first time, immune populations that are enriched/activated in the callus during two distinct phases of fracture healing and identify defects in the healing-associated immune response in DIO mice, which will facilitate future development of immunomodulatory therapeutics for impaired fracture healing.
骨折是最常见的肌肉骨骼损伤，通过三个主要阶段进行愈合：炎症、修复和重塑。大约 10%的骨折患者存在愈合受损的情况，需要手术干预，这给医疗系统带来了巨大的负担。在肥胖相关高血糖/2 型糖尿病（T2D）等代谢性疾病患者中，愈合受损的发生率更高，而肥胖/T2D 的发病率正在不断上升，这一问题越来越受到关注。免疫细胞在骨折愈合中起着关键作用，肥胖/T2D 与免疫细胞功能缺陷有关。然而，人们对在骨折愈合的不同阶段填充骨痂的免疫细胞的化学计量以及该群体如何变化知之甚少。在这里，我们使用互补的全局和单细胞技术来描述骨折骨痂中的免疫细胞谱，并确定相对于未骨折骨或骨髓在骨折骨痂中特异性富集的群体。我们的分析确定了免疫细胞向骨痂浸润的两个明显波：第一波发生在骨折愈合的早期炎症阶段，而第二波发生在晚期修复/早期重塑阶段，这与之前的出版物一致。对每个波的综合分析表明，固有免疫细胞在早期炎症阶段被激活，但在后期阶段它们恢复到稳态数量和激活水平。在固有免疫细胞中，激活的树突状细胞的独特亚群在炎症愈合血肿中特别富集。与固有细胞相反，淋巴细胞，包括 B 和 T 细胞，在晚期修复阶段主要在骨痂中富集和激活。饮食诱导肥胖（DIO）小鼠是肥胖相关高血糖和胰岛素抵抗的一种成熟模型，存在多种愈合缺陷。我们的数据表明，DIO 小鼠在炎症阶段表现出固有免疫反应失调，在晚期修复阶段所有淋巴细胞区室都存在缺陷。总之，我们的数据首次描述了在骨折愈合的两个不同阶段中在骨痂中富集/激活的免疫群体，并确定了 DIO 小鼠在愈合相关免疫反应中的缺陷，这将有助于未来开发用于改善骨折愈合的免疫调节治疗方法。
