Analysis of early immune responses and the role of mast cells in fracture healing

众所周知，免疫系统在骨折愈合过程中发挥着关键作用，因为骨骼修复过程始于局部炎症反应。骨折后早期的免疫反应被认为是后续骨再生的关键，因为它启动了下游反应，导致组织修复；然而，到目前为止，关于某些免疫细胞的影响的知识还很少。肥大细胞是先天免疫系统强有力的促炎感受器。长期以来，人们一直在讨论它们在骨代谢中的作用，因为系统性肥大细胞增多症与骨量减少有关。然而，肥大细胞在骨折愈合和骨再生中的作用到目前为止还没有研究。很少有研究表明肥大细胞可能存在于炎症阶段和骨愈合的后期阶段。因此，本项目旨在表征骨折愈合的早期炎症阶段，特别是肥大细胞在骨再生过程中的影响。我们将使用一种新产生的基于Cre/lox的结构性肥大细胞缺陷小鼠模型(Mcpt5-Cre R-DTA)，其中其他免疫细胞的细胞数量不受影响。这些小鼠的骨折愈合将使用标准化的股骨截骨术进行研究，并用外固定器固定，与肥大细胞能力强的CRE阴性对照进行比较。在骨折愈合的时间过程中进行广泛的分析将回答以下问题：肥大细胞是否调节早期的免疫反应，以及肥大细胞是否特异性地影响骨折愈合后期的血管形成和骨形成。预期的结果将大大有助于理解免疫系统和骨再生的相互作用。

The immune system is known to play a crucial role in fracture healing, as the bone repair process starts with a local inflammatory response. The early immune response after fracture is regarded to be essential for subsequent bone regeneration by initiating down-stream responses leading to tissue repair; however, the knowledge about the impact of certain immune cells is scarce so far. Mast cells are potent pro-inflammatory sensors of the innate immune system. Their role in bone metabolism has been discussed since a long time, as systemic mastocytosis is connected with a decreased bone mass. The function of mast cells in fracture healing and bone regeneration, however, has not been investigated so far. Few studies indicated that mast cells might be present during the inflammatory phase and at later stages of bone healing. Therefore, this project aims to characterize the early inflammatory phase of fracture healing and specifically the impact of mast cells during the time course of bone regeneration. We will use a newly generated Cre/lox-based mouse model of constitutive mast cell deficiency (Mcpt5-Cre R-DTA), in which cell numbers of other immune cells are not affected. Fracture healing will be investigated in these mice using a standardized osteotomy of the femur, which is stabilized with an external fixator, in comparison to their mast cell competent Cre negative littermate controls. Extensive analysis during the time course of fracture healing will answer the questions, whether mast cells modulate the early immune response, and whether mast cells specifically influence vascularization and bone formation in the fracture callus at later stages. The expected results will considerably contribute to the understanding of the interaction of the immune system and bone regeneration.