Osr1 orchestrates lymph node initiation and vasculature formation during embryogenesis and its role in adult lymph nodes

在发育过程中，通过吸引淋巴组织诱导细胞（LTis）的局部间充质淋巴组织组织形成细胞（LTos）的相互作用，次级淋巴器官出现在身体的特定部位。参与胚胎LTo前体向能够吸引LTi细胞的特定组织者细胞的承诺的信号的来源和类型仍然不清楚。我们发现，转录因子Odd-skipped related gene 1（Osr 1）在胚胎淋巴结原基的间充质细胞和周围间充质中表达。功能丧失研究揭示了Osr 1在胚胎发育过程中协调淋巴结起始和淋巴管形成的重要功能。Osr 1 KO胚胎不能形成淋巴结原基，LTo细胞通过CXCL 13对CD 4+细胞的吸引被取消。淋巴管内皮细胞（LEC）表现出增殖率下降，未能形成淋巴管，与Osr 1缺陷细胞表达的血管内皮生长因子C（VEGFC）减少一致。我们建议，Osr 1标志着胚胎LT 0祖细胞和功能LT 0，和Osr 1是必不可少的承诺和功能的LT 0在淋巴结的启动。使用谱系追踪实验，我们发现，早期胚胎Osr 1 + LTo细胞引起成人淋巴结成纤维细胞（滤泡网状细胞，FRC）的显着比例，这些细胞在整个胚胎发育和成人生活中保持。此外，我们发现，Osr 1的表达持续存在于成纤维网状细胞（FRC）在成人淋巴结的髓质区，与淋巴管密切相关的亚群。此外，在炎症过程中，Osr 1表达在FRC中被重新激活，预期Osr 1 + FRC可能在炎症过程中重新获得胚胎功能的重要作用。我们的目标是阐明导致LTo前体承诺的信号和吸引第一个CD 4+的机制。细胞的淋巴结原基，并确定转录因子Osr 1在其中的作用。其次，我们将利用我们的遗传工具来研究Osr 1在成人FRC中的作用，从而对它们在静息淋巴结以及炎症过程中的功能产生新的认识。

Secondary lymph organs emerge at specific sites of the body during development by the interaction of local mesenchymal lymphoid tissue organizer cells (LTos) that attract lymphoid tissue inducer cells (LTis). The source and type of signals involved in the commitment of embryonic LTo precursors to specified organizer cells able to attract LTi cells remain obscure. We have found that the transcription factor Odd-skipped related gene 1 (Osr1) is expressed in mesenchymal cells of embryonic lymph node primordia and the surrounding mesenchyme. Loss of function studies revealed an essential function of Osr1 orchestrating lymph node initiation and the formation of lymph vasculature during embryogenesis. Osr1 KO embryos failed to form lymph node anlagen, the attraction of CD4+ cells by LTo cells via CXCL13 was abrogated. Lymph endothelial cells (LECs) showed a decreased proliferation rate and failed to form lymph vessels, in line with reduced expression of Vascular Endothelial Growth Factor C (VEGFC) by Osr1-deficient cells. We propose that Osr1 marks embryonic LTo progenitors and functional LTOs, and that Osr1 is essential for the commitment and function of LTos during lymph node initiation. Using lineage tracing experiments we revealed that early embryonic Osr1+ LTo cells give rise to a significant proportion of adult lymph node fibroblasts (follicular reticular cells, FRCs) and that these cells are maintained throughout embryogenesis and adult life. Furthermore, we showed that Osr1 expression persists in a subpopulation of fibroblastic reticular cells (FRCs) found in the medulla zone of adult lymph nodes that are closely associated with lymph vessels. In addition, Osr1 expression is re-activated in FRCs during the process of inflammation anticipating an important role of Osr1+ FRCs that might reacquire embryonic functions during inflammatory processes.In this proposal, we aim to shed light on the signals that lead to the commitment of LTo precursors and the mechanisms to attract of the first CD4+ cells to the lymph node primordium and to define the role of the transcription factor Osr1 therein. Secondly, we will exploit our genetic tools to investigate the role of Osr1 in adult FRCs generating new insight into their function in resting lymph nodes as well during inflammation.