In chronic infection, inflammation and cancer, the tissue microenvironment controls how local immune cells behave, with tissue-resident fibroblasts emerging as a key cell type in regulating ...activation or suppression of an immune response. Fibroblasts are heterogeneous cells, encompassing functionally distinct populations, the phenotypes of which differ according to their tissue of origin and type of inciting disease. Their immunological properties are also diverse, ranging from the maintenance of a potent inflammatory environment in chronic inflammation to promoting immunosuppression in malignancy, and encapsulating and incarcerating infectious agents within tissues. In this Review, we compare the mechanisms by which fibroblasts control local immune responses, as well as the factors regulating their inflammatory and suppressive profiles, in different tissues and pathological settings. This cross-disease perspective highlights the importance of tissue context in determining fibroblast-immune cell interactions, as well as potential therapeutic avenues to exploit this knowledge for the benefit of patients with chronic infection, inflammation and cancer.
Summary
Lymphoid organs guarantee productive immune cell interactions through the establishment of distinct microenvironmental niches that are built by fibroblastic reticular cells (FRC). These ...specialized immune‐interacting fibroblasts coordinate the migration and positioning of lymphoid and myeloid cells in lymphoid organs and provide essential survival and differentiation factors during homeostasis and immune activation. In this review, we will outline the current knowledge on FRC functions in secondary lymphoid organs such as lymph nodes, spleen and Peyer's patches and will discuss how FRCs contribute to the regulation of immune processes in fat‐associated lymphoid clusters. Moreover, recent evidence indicates that FRC critically impact immune regulatory processes, for example, through cytokine deprivation during immune activation or through fostering the induction of regulatory T cells. Finally, we highlight how different FRC subsets integrate innate immunological signals and molecular cues from immune cells to fulfill their function as nexus between innate and adaptive immune responses.
Secondary lymphoid organs (SLO) are underpinned by fibroblastic reticular cells (FRC) that form dedicated microenvironmental niches to secure induction and regulation of innate and adaptive immunity. ...Distinct FRC subsets are strategically positioned in SLOs to provide niche factors and govern efficient immune cell interaction. In recent years, the use of specialized mouse models in combination with single‐cell transcriptomics has facilitated the elaboration of the molecular FRC landscape at an unprecedented resolution. While single‐cell RNA‐sequencing has advanced the resolution of FRC subset characterization and function, the high dimensionality of the generated data necessitates careful analysis and validation. Here, we reviewed novel findings from high‐resolution transcriptomic analyses that refine our understanding of FRC differentiation and activation processes in the context of infection and inflammation. We further discuss concepts, strategies, and limitations for the analysis of single‐cell transcriptome data from FRCs and the wide‐ranging implications for our understanding of stromal cell biology.
Fibroblastic reticular cells (FRCs) form a road-like cellular network in lymph nodes (LNs) that provides essential chemotactic, survival, and regulatory signals for immune cells. While the ...topological characteristics of the FRC network have been elaborated, the network properties of the micro-tubular conduit system generated by FRCs, which drains lymph fluid through a pipeline-like system to distribute small molecules and antigens, has remained unexplored. Here, we quantify the crucial 3D morphometric parameters and determine the topological properties governing the structural organization of the intertwined networks. We find that the conduit system exhibits lesser small-worldness and lower resilience to perturbation compared to the FRC network, while the robust topological organization of both networks is maintained in a lymphotoxin-β-receptor-independent manner. Overall, the high-resolution topological analysis of the “roads-and-pipes” networks highlights essential parameters underlying the functional organization of LN micro-environments and will, hence, advance the development of multi-scale LN models.
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•FRC network and conduit system exhibit distinct small-world topologies•FRC network displays higher robustness to perturbation than the conduit system•LN structural networks are generated and maintained in a LTβR-independent manner
Novkovic et al. elaborate the topological organization of the FRC network and conduit system in lymph nodes and demonstrate their distinct small-world topologies, which are maintained in a lymphotoxin-β-receptor-independent manner. The FRC network exhibited higher tolerance and robustness to perturbation than the conduit system did.
The splenic white pulp is underpinned by poorly characterized stromal cells that demarcate distinct immune cell microenvironments. Here we establish fibroblastic reticular cell (FRC)-specific ...fate-mapping in mice to define their embryonic origin and differentiation trajectories. Our data show that all reticular cell subsets descend from multipotent progenitors emerging at embryonic day 19.5 from periarterial progenitors. Commitment of FRC progenitors is concluded during the first week of postnatal life through occupation of niches along developing central arterioles. Single cell transcriptomic analysis facilitated deconvolution of FRC differentiation trajectories and indicated that perivascular reticular cells function both as adult lymphoid organizer cells and mural cell progenitors. The lymphotoxin-β receptor-independent sustenance of postnatal progenitor stemness unveils that systemic immune surveillance in the splenic white pulp is governed through subset specification of reticular cells from a multipotent periarterial progenitor cell. In sum, the finding that discrete signaling events in perivascular niches determine the differentiation trajectories of reticular cell networks explains the development of distinct microenvironmental niches in secondary and tertiary lymphoid tissues that are crucial for the induction and regulation of innate and adaptive immune processes.
Through the formation of concentration gradients, morphogens drive graded responses to extracellular signals, thereby fine-tuning cell behaviors in complex tissues. Here we show that the chemokine ...CXCL13 forms both soluble and immobilized gradients. Specifically, CXCL13
follicular reticular cells form a small-world network of guidance structures, with computer simulations and optimization analysis predicting that immobilized gradients created by this network promote B cell trafficking. Consistent with this prediction, imaging analysis show that CXCL13 binds to extracellular matrix components in situ, constraining its diffusion. CXCL13 solubilization requires the protease cathepsin B that cleaves CXCL13 into a stable product. Mice lacking cathepsin B display aberrant follicular architecture, a phenotype associated with effective B cell homing to but not within lymph nodes. Our data thus suggest that reticular cells of the B cell zone generate microenvironments that shape both immobilized and soluble CXCL13 gradients.
Efficient generation of germinal center (GC) responses requires directed movement of B cells between distinct microenvironments underpinned by specialized B cell-interacting reticular cells (BRCs). ...How BRCs are reprogrammed to cater to the developing GC remains unclear, and studying this process is largely hindered by incomplete resolution of the cellular composition of the B cell follicle. Here we used genetic targeting of Cxcl13-expressing cells to define the molecular identity of the BRC landscape. Single-cell transcriptomic analysis revealed that BRC subset specification was predetermined in the primary B cell follicle. Further topological remodeling of light and dark zone follicular dendritic cells required CXCL12-dependent crosstalk with B cells and dictated GC output by retaining B cells in the follicle and steering their interaction with follicular helper T cells. Together, our results reveal that poised BRC-defined microenvironments establish a feed-forward system that determines the efficacy of the GC reaction.
Lymph nodes (LNs) are strategically situated throughout the body at junctures of the blood vascular and lymphatic systems to direct immune responses against antigens draining from peripheral tissues. ...The current paradigm describes LN development as a programmed process that is governed through the interaction between mesenchymal lymphoid tissue organizer (LTo) cells and hematopoietic lymphoid tissue inducer (LTi) cells. Using cell-type-specific ablation of key molecules involved in lymphoid organogenesis, we found that initiation of LN development is dependent on LTi-cell-mediated activation of lymphatic endothelial cells (LECs) and that engagement of mesenchymal stromal cells is a succeeding event. LEC activation was mediated mainly by signaling through receptor activator of NF-κB (RANK) and the non-canonical NF-κB pathway and was steered by sphingosine-1-phosphate-receptor-dependent retention of LTi cells in the LN anlage. Finally, the finding that pharmacologically enforced interaction between LTi cells and LECs promotes ectopic LN formation underscores the central LTo function of LECs.
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•LTβR signals in mesenchymal LTo cells are not essential for LN organogenesis•RANK-mediated signaling in lymphatic endothelial cells (LECs) drives LN development•LECs control retention of LTi cells in embryonic LN anlagen•Enforced LTi cell retention in lymphatics induces formation of ectopic LNs
Lymph node (LN) formation is thought to rely mainly on interactions between mesenchymal lymphoid tissue organizer cells and lymphoid tissue inducer cells. Onder et al. now show that LN formation is governed by RANK-dependent activation of lymphatic endothelial cells that control retention of lymphoid tissue inducer cells in embryonic LN anlagen.
Fibroblastic reticular cells (FRCs) are a crucial part of the stromal cell infrastructure of secondary lymphoid organs (SLOs). Lymphoid organ fibroblasts form specialized niches for immune cell ...interactions and thereby govern lymphocyte activation and differentiation. Moreover, FRCs produce and ensheath a network of extracellular matrix (ECM) microfibers called the conduit system. FRC-generated conduits contribute to fluid and immune cell control by funneling fluids containing antigens and inflammatory mediators through the SLOs. We review recent progress in FRC biology that has advanced our understanding of immune cell functions and interactions. We discuss the intricate relationships between the cellular FRC and the fibrillar conduit networks, which together form the basis for efficient communication between immune cells and the tissues they survey.
FRCs use polarized microtubule networks to direct the secretion of ECM components and construct the mammalian conduit network.The conduit network functions as a filter that monitors fluid draining peripheral tissues, and as an export system for the systemic dissemination of locally produced molecules including antibodies.Emerging evidence indicates that there are mechanosensitive mechanisms that control fluid flow through secondary lymphoid organs, and that are important for tissue homeostasis and immune function.Fluid flow through conduit systems is robust, and the conduit network remains functional throughout the perturbation of lymph node expansion in response to immunogenic challenges.
Innate lymphoid cells (ILCs) govern immune cell homeostasis in the intestine and protect the host against microbial pathogens. Various cell-intrinsic pathways have been identified that determine ILC ...development and differentiation. However, the cellular components that regulate ILC sustenance and function in the intestinal lamina propria are less known. Using single-cell transcriptomic analysis of lamina propria fibroblasts, we identify fibroblastic reticular cells (FRCs) that underpin cryptopatches (CPs) and isolated lymphoid follicles (ILFs). Genetic ablation of lymphotoxin-β receptor expression in Ccl19-expressing FRCs blocks the maturation of CPs into mature ILFs. Interactome analysis shows the major niche factors and processes underlying FRC-ILC crosstalk. In vivo validation confirms that a sustained lymphotoxin-driven feedforward loop of FRC activation including IL-7 generation is critical for the maintenance of functional ILC populations. In sum, our study indicates critical fibroblastic niches within the intestinal lamina propria that control ILC homeostasis and functionality and thereby secure protective gut immunity.
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