Ontogeny and functional characterization of splenic fibroblastic reticular cells (FRCs) and their mesenchymal precursors during homeostasis, immune-activation and infection

次级淋巴器官（slo），如脾脏，为免疫反应的发展和控制提供了专门的微环境。被称为纤维母细胞网状细胞（FRCs）的特殊间充质基质细胞产生独特的区室，例如，允许T细胞与树突状细胞高效相互作用。重要的是，FRCs不仅仅是简单的搭建脚手架的细胞；这些细胞积极参与诱导和塑造先天和适应性免疫反应。我们自己的初步数据表明，脾白髓的结构和功能完全依赖于成年期CCL19阳性FRCs和/或其推定的祖细胞，即间充质淋巴组织组织组织者细胞（mLTOs）接受的淋巴毒素β受体（LTbR）信号。在继续我们现有的合作活动中，我们开发了一种新的可诱导谱系追踪模型，该模型允许在体内对脾脏FRCs及其祖细胞进行遗传标记，从而有助于阐明它们在脾脏发育过程中的分化途径。此外，该模型系统为鉴定和表征成人脾脏结构和免疫能力维持的分子机制提供了手段。因此，结合我们已经建立的细胞类型特异性LTbR功能丧失的实验模型，我们将能够解决LTbR信号对成年脾FRCs及其胚胎中假定的祖细胞分化和功能的时间要求。最后，我们将利用一个能够促进细胞类型特异性LTbR表达以及时可控的方式重新激活的实验模型，验证frc限制LTbR表达在特定时间窗内是否足以在胚胎生命期间形成适当的白髓和功能。此外，我们将阐明在成人生活中及时控制LTbR激活的要求，以维持脾白髓的区隔化和功能。我们提出的目标是基于我们个人的专业知识，并呈现我们成功合作活动的延续。更具体地说，它们被设计用于表征脾脏FRCs的分子特征，并解剖脾脏FRCs与其他脾脏基质细胞的谱系关系，例如它们在胚胎发育过程中从mLTO前体分化而来。此外，我们将能够识别和功能表征表达LTbR的FRCs参与维持成人脾脏淋巴器官完整性和免疫能力的细胞和分子机制，即它们在免疫稳态、激活和感染期间在脾脏微环境中的生态作用。

Secondary lymphatic organs (SLOs) such as the spleen provide specialized microenvironmental niches for the development and control of immune responses. Particular mesenchymal stromal cells known as fibroblastic reticular cells (FRCs) generate distinct compartments that permit, for example, highly efficient interaction of T cells with dendritic cells. Importantly, FRCs are more than simple scaffold-building cells; these cells actively participate in inducing and shaping innate and adaptive immune responses. Our own preliminary data demonstrate that the structure and function of the splenic white pulp exclusively depends on lymphotoxin-beta-receptor (LTbR) signaling received by CCL19 positive FRCs during adult life and/or their putative progenitors, i.e. mesenchymal lymphoid tissue organizer cells (mLTOs). In continuation of our existing collaborative activities, we have developed a novel inducible lineage tracing model that permits genetic tagging of splenic FRCs and their progenitors in vivo and will thus facilitate the elucidation of their differentiation pathways throughout spleen development. In addition, this model system provides means for the identification and characterization of molecular mechanisms involved in the maintenance of splenic structure and immune-competence during adult life. Thus, in combination with our already established experimental model of cell type-specific loss of LTbR function, we will be able to resolve the temporal requirement of LTbR signaling for differentiation and function of adult splenic FRCs and their putative progenitors in the embryo. Finally, using an experimental model that facilitates cell type specific re-activation of LTbR expression in a timely controlled manner, we will test the hypothesis whether FRC-restricted LTbR expression within a specific time window is sufficient for proper white pulp formation and function during embryonic life. Furthermore, we will elucidate the requirements for a timely controlled LTbR activation during adult life in order to maintain compartmentalization and function of the splenic white pulp. Our proposed objectives are based on our individual expertise and present the continuation of our successful collaborative activities. More specifically, they are designed to molecularly characterize splenic FRCs and to dissect lineage relationship of splenic FRCs with other splenic stromal cells, e.g. their differentiation from mLTO precursors during embryonic development. Moreover, we will be able to identify and functionally characterize the cellular and molecular mechanisms through which LTbR expressing FRCs are involved in the maintenance of lymphoid organ integrity and immune-competence in the adult spleen, i.e. their ecological role in the splenic micro-environment during immune homeostasis, activation and infection.