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

在发育过程中，次级淋巴器官通过局部间充质淋巴组织组织者细胞(LTO)的相互作用出现在身体的特定位置，这些细胞吸引淋巴组织诱导细胞(LTIs)。胚胎LTO前体向能够吸引LTI细胞的特定组织者细胞承诺所涉及的信号来源和类型仍不清楚。我们发现转录因子Odd跳过相关基因1(Osr1)在胚胎淋巴结原基及其周围间充质细胞中表达。功能丧失研究揭示了Osr1在胚胎发育过程中协调淋巴结启动和淋巴管形成的重要功能。OSCR1KO胚胎不能形成淋巴结原，LTO细胞通过CXCL13对CD4+细胞的吸引作用被取消。淋巴内皮细胞(LECs)的增殖率降低，不能形成淋巴管，这与Osr1缺陷细胞表达血管内皮生长因子C(VEGFC)的减少一致。我们认为，Osr1标志着胚胎LTO祖细胞和功能性LTO，并且在淋巴启动过程中，Osr1对LTO的承诺和功能至关重要。通过谱系示踪实验，我们发现胚胎早期的Osr1+LTO细胞可以分化出相当比例的成人淋巴结成纤维细胞(滤泡网状细胞)，并且这些细胞在整个胚胎发育和成年生活中一直保持。此外，我们发现Osr1的表达持续存在于成人淋巴结髓质区的成纤维细胞网状细胞(FRC)亚群中，这些细胞与淋巴管密切相关。此外，在炎症过程中，OSR1在FRC中的表达被重新激活，预期OSR1+FRCs在炎症过程中可能重新获得胚胎功能。在本研究中，我们旨在阐明导致LTO前体承诺的信号以及吸引第一批CD4+细胞进入淋巴结原基的机制，并确定转录因子Osr1在其中的作用。其次，我们将利用我们的遗传工具来研究Osr1在成人FRC中的作用，从而对其在静息淋巴结以及炎症过程中的功能产生新的见解。

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.