A Distinct Subset of Fibroblastic Stromal Cells Constitutes the Cortex-Medulla Boundary Subcompartment of the Lymph Node

Molecular cues for immune cells from small leucine-rich repeat proteoglycans in their extracellular matrix-associated and free forms

In this review we highlight emerging immune regulatory functions of lumican, keratocan, fibromodulin, biglycan and decorin, which are members of the small leucine-rich proteoglycans (SLRP) of the extracellular matrix (ECM). These SLRPs have been studied extensively as collagen-fibril regulatory structural components of the skin, cornea, bone and cartilage in homeostasis. However, SLRPs released from a remodeling ECM, or synthesized by activated fibroblasts and immune cells contribute to an ECM-free pool in tissues and circulation, that may have a significant, but poorly understood foot print in inflammation and disease. Their molecular interactions and the signaling networks they influence also require investigations. Here we present studies on the leucine-rich repeat (LRR) motifs of SLRP core proteins, their evolutionary and functional relationships with other LRR pathogen recognition receptors, such as the toll-like receptors (TLRs) to bring some molecular clarity in the immune regulatory functions of SLRPs. We discuss molecular interactions of fragments and intact SLRPs, and how some of these interactions are likely modulated by glycosaminoglycan side chains. We integrate findings on molecular interactions of these SLRPs together with what is known about their presence in circulation and lymph nodes (LN), which are important sites of immune cell regulation. Recent bulk and single cell RNA sequencing studies have identified subsets of stromal reticular cells that express these SLRPs within LNs. An understanding of the cellular source, molecular interactions and signaling consequences will lead to a fundamental understanding of how SLRPs modulate immune responses, and to therapeutic tools based on these SLRPs in the future.

Lymph node medulla regulates the spatiotemporal unfolding of resident dendritic cell networks

Unlike macrophage networks composed of long-lived tissue-resident cells within specific niches, conventional dendritic cells (cDCs) that generate a 3D network in lymph nodes (LNs) are short lived and continuously replaced by DC precursors (preDCs) from the bone marrow (BM). Here, we examined whether specific anatomical niches exist within which preDCs differentiate toward immature cDCs. In situ photoconversion and Prtn3-based fate-tracking revealed that the LN medullary cords are preferential entry sites for preDCs, serving as specific differentiation niches. Repopulation and fate-tracking approaches demonstrated that the cDC1 network unfolded from the medulla along the vascular tree toward the paracortex. During inflammation, collective maturation and migration of resident cDC1s to the paracortex created discontinuity in the medullary cDC1 network and temporarily impaired responsiveness. The decrease in local cDC1 density resulted in higher Flt3L availability in the medullary niche, which accelerated cDC1 development to restore the network. Thus, the spatiotemporal development of the cDC1 network is locally regulated in dedicated LN niches via sensing of cDC1 densities.

Lymphoid stromal cells - potential implications for the pathogenesis of CVID

Non-hematopoietic lymphoid stromal cells (LSC) maintain lymph node architecture and form niches allowing the migration, activation, and survival of immune cells. Depending on their localization in the lymph node, these cells display heterogeneous properties and secrete various factors supporting the different activities of the adaptive immune response. LSCs participate in the transport of antigen from the afferent lymph as well as in its delivery into the T and B cell zones and organize cell migration via niche-specific chemokines. While marginal reticular cells (MRC) are equipped for initial B-cell priming and T zone reticular cells (TRC) provide the matrix for T cell-dendritic cell interactions within the paracortex, germinal centers (GC) only form when both T- and B cells successfully interact at the T-B border and migrate within the B-cell follicle containing the follicular dendritic cell (FDC) network. Unlike most other LSCs, FDCs are capable of presenting antigen via complement receptors to B cells, which then differentiate within this niche and in proximity to T follicular helper (T_FH) cells into memory and plasma cells. LSCs are also implicated in maintenance of peripheral immune tolerance. In mice, TRCs induce the alternative induction of regulatory T cells instead of T_FH cells by presenting tissue-restricted self-antigens to naïve CD4 T cells via MHC-II expression. This review explores potential implications of our current knowledge of LSC populations regarding the pathogenesis of humoral immunodeficiency and autoimmunity in patients with autoimmune disorders or common variable immunodeficiency (CVID), the most common form of primary immunodeficiency in humans.

Micro- and Macro-Anatomical Frameworks of Lymph Nodes Indispensable for the Lymphatic System Filtering Function

In the lymphatic vascular system, lymph nodes (LNs) play a pivotal role in filtering and removing lymph-borne substances. The filtering function of LNs involves resident macrophages tightly associated with unique lymphatic sinus structures. Moreover, an intermittently arranged LN in the lymphatic pathway is considered to cooperatively prevent lymph-borne substances from entering blood circulation. However, the functional significance of tissue microarchitecture, cellular composition, and individual LNs in the “LN chain” system is not fully understood. To explore the mechanistic and histo-anatomical significance of LNs as lymph fluid filters, we subcutaneously injected fluorescent tracers into mice and examined the details of lymphatic transport to the LNs qualitatively and quantitatively. Lymph-borne tracers were selectively accumulated in the MARCO⁺ subcapsular-medullary sinus border (SMB) region of the LN, in which reticular lymphatic endothelial cells and CD169⁺F4/80⁺ medullary sinus macrophages construct a dense meshwork of the physical barrier, forming the main body to capture the tracers. We also demonstrated stepwise filtration via the LN chain in the lymphatic basin, which prevented tracer leakage into the blood. Furthermore, inflammatory responses that induce the remodeling of LN tissue as well as the lymphatic pathway reinforce the overall filtering capacity of the lymphatic basin. Taken together, specialized tissue infrastructure in the LNs and their systematic orchestration constitute an integrated filtering system for lymphatic recirculation.

Extracellular ATP Limits Homeostatic T Cell Migration Within Lymph Nodes

Whereas adenosine 5'-triphosphate (ATP) is the major energy source in cells, extracellular ATP (eATP) released from activated/damaged cells is widely thought to represent a potent damage-associated molecular pattern that promotes inflammatory responses. Here, we provide suggestive evidence that eATP is constitutively produced in the uninflamed lymph node (LN) paracortex by naïve T cells responding to C-C chemokine receptor type 7 (CCR7) ligand chemokines. Consistently, eATP was markedly reduced in naïve T cell-depleted LNs, including those of nude mice, CCR7-deficient mice, and mice subjected to the interruption of the afferent lymphatics in local LNs. Stimulation with a CCR7 ligand chemokine, CCL19, induced ATP release from LN cells, which inhibited CCR7-dependent lymphocyte migration in vitro by a mechanism dependent on the purinoreceptor P2X7 (P2X7R), and P2X7R inhibition enhanced T cell retention in LNs in vivo. These results collectively indicate that paracortical eATP is produced by naïve T cells in response to constitutively expressed chemokines, and that eATP negatively regulates CCR7-mediated lymphocyte migration within LNs via a specific subtype of ATP receptor, demonstrating its fine-tuning role in homeostatic cell migration within LNs.

Characterization of Leptin Receptor+ Stromal Cells in Lymph Node

Lymph node (LN)-resident stromal cells play an essential role in the proper functioning of LNs. The stromal compartment of the LN undergoes significant compensatory changes to produce a milieu amenable for regulation of the immune response. We have identified a distinct population of leptin receptor-expressing (LepR+) stromal cells, located in the vicinity of the high endothelial venules (HEVs) and lymphatics. These LepR+ stromal cells expressed markers for fibroblastic reticular cells (FRCs), but they lacked markers for follicular dendritic cells (FDCs) and marginal reticular cells (MRCs). Leptin signaling deficiency led to heightened inflammatory responses within the LNs of db/db mice, leakiness of HEVs, and lymphatic fragmentation. Leptin signaling through the JAK/STAT pathway supported LN stromal cell survival and promoted the anti-inflammatory properties of these cells. Conditional knockout of the LepR+ stromal cells in LNs resulted in HEV and extracellular matrix (ECM) abnormalities. Treatment of ob/ob mice with an agonist leptin fusion protein restored the microarchitecture of LNs, reduced intra-LN inflammatory responses, and corrected metabolic abnormalities. Future studies are needed to study the importance of LN stomal cell dysfunction to the pathogenesis of inflammatory responses in type 2 diabetes (T2D) in humans.

Visualization and functional characterization of lymphoid organ fibroblasts

Fibroblastic reticular cells (FRCs) are specialized stromal cells of lymphoid organs that generate the structural foundation of the tissue and actively interact with immune cells. Distinct FRC subsets position lymphocytes and myeloid cells in specialized niches where they present processed or native antigen and provide essential growth factors and cytokines for immune cell activation and differentiation. Niche‐specific functions of FRC subpopulations have been defined using genetic targeting, high‐dimensional transcriptomic analyses, and advanced imaging methods. Here, we review recent findings on FRC‐immune cell interaction and the elaboration of FRC development and differentiation. We discuss how imaging approaches have not only shaped our understanding of FRC biology, but have critically advanced the niche concept of immune cell maintenance and control of immune reactivity.

Lymph Node Stromal Cells: Diverse Meshwork Structures Weave Functionally Subdivided Niches

Lymph nodes (LNs) are secondary lymphoid organs that function as the first line of defense against invasive foreign substances. Within the LNs, different types of immune cells are strategically localized to induce immune responses efficiently. Such a sophisticated tissue structure is a complex of functionally specialized niches, constructed by a variety of fibroblastic stromal cells. Elucidating the characteristics and functions of the niches and stromal cells will facilitate comprehension of the immune response induced in the LNs. Three recent studies offered novel insights into specialized stromal cells. In our discussion of these surprisingly diverse stromal cells, we will integrate information from these studies to improve knowledge about the structure and niches of LN.

Compartmentalized multicellular crosstalk in lymph nodes coordinates the generation of potent cellular and humoral immune responses

Distributed throughout the body, lymph nodes (LNs) constitute an important crossroad where resident and migratory immune cells interact to initiate antigen‐specific immune responses supported by a dynamic 3‐dimensional network of stromal cells, that is, endothelial cells and fibroblastic reticular cells (FRCs). LNs are organized into four major subanatomically separated compartments: the subcapsular sinus (SSC), the paracortex, the cortex, and the medulla. Each compartment is underpinned by particular FRC subsets that physically support LN architecture and delineate functional immune niches by appropriately providing environmental cues, nutrients, and survival factors to the immune cell subsets they interact with. In this review, we discuss how FRCs drive the structural and functional organization of each compartment to give rise to prosperous interactions and coordinate immune cell activities. We also discuss how reciprocal communication makes FRCs and immune cells perfect compatible partners for the generation of potent cellular and humoral immune responses.

Lymph node fibroblastic reticular cells steer immune responses

Lymph nodes (LNs), where immune responses are initiated, are organized into distinctive compartments by fibroblastic reticular cells (FRCs). FRCs imprint immune responses by supporting LN architecture, recruiting immune cells, coordinating immune cell crosstalk, and presenting antigens. Recent high-resolution transcriptional and histological analyses have enriched our knowledge of LN FRC genetic and spatial heterogeneities. Here, we summarize updated anatomic, phenotypic, and functional identities of FRC subsets, delve into topological and transcriptional remodeling of FRCs in inflammation, and illustrate the crosstalk between FRCs and immune cells. Discussing FRC functions in immunity and tolerance, we highlight state-of-the-art FRC-based therapeutic approaches for maintaining physiological homeostasis, steering protective immunity, inducing transplantation tolerance, and treating diverse immune-related diseases.

Differentiation and activation of fibroblastic reticular cells

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.

Lymphoid stromal cells-more than just a highway to humoral immunity

The generation of high-affinity long-lived antibody responses is dependent on the differentiation of plasma cells and memory B cells, which are themselves the product of the germinal centre (GC) response. The GC forms in secondary lymphoid organs in response to antigenic stimulation and is dependent on the coordinated interactions between many types of leucocytes. These leucocytes are brought together on an interconnected network of specialized lymphoid stromal cells, which provide physical and chemical guidance to immune cells that are essential for the GC response. In this review we will highlight recent advancements in lymphoid stromal cell immunobiology and their role in regulating the GC, and discuss the contribution of lymphoid stromal cells to age-associated immunosenescence.

Lymph node stromal cell subsets-Emerging specialists for tailored tissue-specific immune responses

The effective priming of adaptive immune responses depends on the precise dispatching of lymphocytes and antigens into and within lymph nodes (LNs), which are strategically dispersed throughout the body. Over the past decade, a growing body of evidence has advanced our understanding of lymph node stromal cells (LNSCs) from viewing them as mere accessory cells to seeing them as critical cellular players for the modulation of adaptive immune responses. In this review, we summarize current advances on the pivotal roles that LNSCs play in orchestrating adaptive immune responses during homeostasis and infection, and highlight the imprinting of location-specific information by micro-environmental cues into LNSCs, thereby tailoring tissue-specific immune responses.

Development and Immunological Function of Lymph Node Stromal Cells

Stromal cells have for a long time been viewed as structural cells that support distinct compartments within lymphoid tissues and little more. Instead, an active cross-talk between endothelial and fibroblastic stromal cells drives the maturation of lymphoid niches, a relationship that is recapitulated during lymph node organogenesis, steady-state conditions, and following inflammation. In this review, we go over recent advances in genetic models and high-resolution transcriptomic analyses that have propelled the finer resolution of the stromal cell infrastructure of lymph nodes, revealing that the distinct subsets are strategically positioned to deliver a catered mixture of niche factors to interacting immune cell populations. Moreover, we discuss how changes in the activation state of poised stromal cell-underpinned niches rather than on-demand differentiation of new stromal cell subsets govern the efficient interaction of Ag, APC, and cognate B and T lymphocytes during adaptive immune responses.

Type I interferon signaling in fibroblastic reticular cells prevents exhaustive activation of antiviral CD8+ T cells

Fibroblastic reticular cells (FRCs) are stromal cells that actively promote the induction of immune responses by coordinating the interaction of innate and adaptive immune cells. However, whether and to which extent immune cell activation is determined by lymph node FRC reprogramming during acute viral infection has remained unexplored. Here, we genetically ablated expression of the type I interferon-α receptor (Ifnar) in Ccl19-Cre⁺ cells and found that sensing of type I interferon imprints an antiviral state in FRCs and thereby preserves myeloid cell composition in lymph nodes of naive mice. During localized lymphocytic choriomeningitis virus infection, IFNAR signaling precipitated profound phenotypic adaptation of all FRC subsets enhancing antigen presentation, chemokine-driven immune cell recruitment, and immune regulation. The IFNAR-dependent shift of all FRC subsets toward an immunostimulatory state reduced exhaustive CD8⁺ T cell activation. In sum, these results unveil intricate circuits underlying type I IFN sensing in lymph node FRCs that enable protective antiviral immunity.

Transdermal entry of yeast components elicits transient B cell-associated responses in skin-draining lymph nodes

Immune responses to non-pathogenic yeasts induced within the draining lymph node remain to be understood. In this study, we have investigated the changes in lymphocytes and their activity in skin-draining lymph nodes in response to transdermally injected zymosan (component of the yeast cell wall). Zymosan elicited the transient increase of B cell number and activation status without affecting the capacity for proliferation. The increased B cell content in the regional lymph nodes was likely due to the reduction of B cell egress from the tissue and in part the increase of homing from the circulation. Zymosan also upregulated the inflammatory cytokines, such as IL-1β, IL-6, IL-12, and IFNγ, regulatory cytokines IL-10 and TGFβ, and lymphoid chemokine CXCL13. Among these, the expression of IL-12 and IL-10 was markedly high in B cells. Altogether, these findings demonstrate a unique B cell-associated response to non-pathogenic yeast component in the draining lymph nodes. This will provide insights into the clinical and healthcare applications of non-pathogenic beneficial microbes.

The fibroblastic T cell niche in lymphoid tissues

Fibroblastic reticular cells (FRCs) are a necessary immunological component for T cell health. These myofibroblasts are specialized for immune cell support and develop in locations where T and B lymphocyte priming occurs, usually secondary lymphoid organs, but also tertiary lymphoid structures and sites of chronic inflammation. This review describes their dual supportive and suppressive functions and emerging evidence on the co-ordination required to balance these competing roles.

Dietary lipids accumulate in macrophages and stromal cells and change the microarchitecture of mesenteric lymph nodes

In obesity, increased dietary lipids are taken up and transported by the lymphatic systems into the circulatory system. Increased fat accumulation results in impairments in the lymph fluid and lymph node (LN) atrophy. LNs filter the lymph fluid for foreign antigens to induce and control immune responses, and the alteration of this function during obesity remains underexplored. Here, the changes within the microarchitecture of mesenteric LNs (mLNs) during high levels of lipid transport were investigated, and the role of stromal cells in mice fed a high-fat diet for 10 weeks was assessed. Microarray experiments revealed that gene probes involved in lipid metabolism are expressed by mLN stromal cells. Transmission electron microscopy enabled the identification of lipid droplets in lymphatic endothelial cells, different reticulum cells, and macrophages, and the lipid droplet sizes as well as their numbers and intercellular distances increased after 10 weeks of high-fat diet feeding. The results indicate that changes in the microarchitecture and increased accumulation of lipid droplets in stromal cells and macrophages influence the immunological function of mLNs.

Stromal cells and immune cells involved in formation of lymph nodes and their niches

Secondary lymphoid organs are critical for efficient interaction between innate antigen presenting cells and adaptive lymphocytes in order to start adaptive immune responses. The efficiency by which these cellular subsets meet is highly increased by the orchestrating role of stromal cells within the secondary lymphoid organs. These cells provide cytokines, chemokines and cell surface receptors necessary for survival and guided migration. This increases the likelihood that antigen specific adaptive immune responses occur. Already from initial formation of secondary lymphoid organs, the interaction of immune cells with stromal cells is crucial and this interaction continues during immune activation. With the recent discovery of many stromal cell subsets new immune micro-niches with specific functions that are orchestrated by stromal cells will be discovered. Here, we will discuss how the development of lymph nodes as well as their specific niches is guided by the interaction of immune cells and stromal cells.

Breadth of Antibody Responses during Influenza Virus Infection and Vaccination

Influenza viruses are a major public health problem, causing severe respiratory diseases. Vaccines offer the effective protective strategy against influenza virus infection. However, the systemic and adaptive immune responses to infection and vaccination are quite different. Inactivated vaccines are the best available countermeasure to induce effective antibodies against the emerged virus, but the response is narrow compared with potential breadth of virus infection. There is solid evidence to indicate that antibody responses to natural infection are relatively broad and exhibit quite different immunodominance patterns. Furthermore, T follicular helper cells (T_FH) and germinal center (GC) responses play a central role in generating broad protective antibodies. In this review, we discuss recent advances on the contribution of T_FH and GC responses to the breadth of antibody responses.

Fibroblastic reticular cells at the nexus of innate and adaptive immune responses

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.