Origin and differentiation trajectories of fibroblastic reticular cells in the splenic white pulp

Stromal KITL/SCF Maintains Pancreas Tissue Homeostasis and Restrains Tumor Progression

SIGNIFICANCE

By analyzing transcriptional programs in healthy and tumor-associated pancreatic mesenchyme, we find that a subpopulation of mesenchymal cells in healthy pancreas tissue expresses the paracrine signaling factor KITL. The loss of mesenchymal KITL is an accompanying and permissive feature of pancreas tumor evolution, with potential implications for cancer interception. See related article by Dolskii and Cukierman, p. 872.

CXCL12+ fibroblastic reticular cells in lymph nodes facilitate immune tolerance by regulating T cell-mediated alloimmunity

Fibroblastic reticular cells (FRCs) are the master regulators of the lymph node (LN) microenvironment. However, the role of specific FRC subsets in controlling alloimmune responses remains to be studied. Single-cell RNA sequencing (scRNA-Seq) of naive and draining LNs (DLNs) of heart-transplanted mice and human LNs revealed a specific subset of CXCL12hi FRCs that expressed high levels of lymphotoxin-β receptor (LTβR) and are enriched in the expression of immunoregulatory genes. CXCL12hi FRCs had high expression of CCL19, CCL21, indoleamine 2,3-dioxygenase (IDO), IL-10, and TGF-β1. Adoptive transfer of ex vivo-expanded FRCs resulted in their homing to LNs and induced immunosuppressive environments in DLNs to promote heart allograft acceptance. Genetic deletion of LTβR and Cxcl12 in FRCs increased alloreactivity, abrogating the effect of costimulatory blockade in prolonging heart allograft survival. As compared with WT recipients, CXCL12+ FRC-deficient recipients exhibited increased differentiation of CD4+ T cells into Th1 cells. Nano delivery of CXCL12 to DLNs improved allograft survival in heart-transplanted mice. Our study highlights the importance of DLN CXCL12hi FRCs in promoting transplant tolerance.

Fibroblast heterogeneity and its role in generating protective immunity in the secondary lymphoid organs

Fibroblasts are cells of mesenchymal origin with a range of phenotypic diversity and heterogeneity. One of the major functions of fibroblasts is the formation and turnover of the extracellular matrix and establishing a tissue structure by forming a matrisome from embryonic development to the adult stage. It plays an indispensable role in extracellular matrix remodeling during injury, repair, and infection, providing a scaffold for cell-to-cell interaction. Despite their important pathophysiological roles, molecular markers for tissue-resident fibroblasts are only now being identified. Fibroblasts acquire molecular signatures based on anatomical locations, thus impacting their phenotypic heterogeneity despite their overlapping morphology. Fibroblasts are now recognized as key immune sentinel cells, capable of regulating the inflammatory milieu through their distinct functional subsets that are designed to respond differently with unique immune signatures. Fibroblasts can detect pathogenic and danger signals through their diverse pattern recognition receptors (PRRs) and release soluble mediators that can modulate the immune infiltrates at the site of tissue injury and repair. This review discusses the diversity and heterogeneity of fibroblasts in secondary lymphoid organs such as lymph nodes, spleen, and Peyer's patches, and their contributions to a range of pathological and physiological processes. The role of trans-differentiated effector fibroblast phenotypes that modulate the expression and function of various innate immune components (PRRs, cytokines, chemokines, and complement) in maintaining homeostasis has also been discussed.

A Gremlin 1-expressing splenic niche cell population restrains chronic myeloid leukemia by antagonizing the BMP pathway

The spleen plays a critical role in the pathogenesis of leukemia. However, our understanding of the splenic niche is very limited. Herein, we report that induced expression of the secreted protein Gremlin 1 in a mouse model restrains chronic myeloid leukemia (CML) progression and synergizes with tyrosine kinase inhibitor treatment, whereas blockade of Gremlin 1 promotes CML development. Intriguingly, the effect of Gremlin 1 is most evident in the spleen but not in the bone marrow. Gremlin 1 induces apoptosis of leukemic stem cells via antagonizing the BMP pathway. Single-cell RNA sequencing and experimental validation together show that Gremlin 1 marks a unique stromal cell population in the spleens of both mice and humans. Genetic ablation of Gremlin 1+ cells leads to accelerated CML progression. Collectively, Gremlin 1 and Gremlin 1+ cells are key defensive niche components in the spleen that limit CML progression, revealing an unprecedented mechanism for the body to fight off leukemia.

Fibroblastic reticular cells generate protective intratumoral T cell environments in lung cancer

Stringent control of T cell activity in the tumor microenvironment is essential for the generation of protective antitumor immunity. However, the identity, differentiation, and functions of the cells that create critical fibroblastic niches promoting tumor-infiltrating T cells remain elusive. Here, we show that CCL19-expressing fibroblastic reticular cells (FRCs) generate interconnected T cell environments (TEs) in human non-small cell lung cancer, including tertiary lymphoid structures and T cell tracks. Analysis of the FRC-T cell interactome in TEs indicated molecular networks regulating niche-specific differentiation of CCL19-expressing fibroblasts and T cell activation pathways. Single-cell transcriptomics and cell fate-mapping analyses in mice confirmed that FRCs in TEs originate from mural and adventitial progenitors. Ablation of intratumoral FRC precursors decreased antitumor T cell activity, resulting in reduced tumor control during coronavirus vector-based immunotherapy. In summary, specialized FRC niches in the tumor microenvironment govern the quality and extent of antitumor T cell immunity.

Splenic fibroblasts control marginal zone B cell movement and function via two distinct Notch2-dependent regulatory programs

Innate-like splenic marginal zone (MZ) B (MZB) cells play unique roles in immunity due to their rapid responsiveness to blood-borne microbes. How MZB cells integrate cell-extrinsic and -intrinsic processes to achieve accelerated responsiveness is unclear. We found that Delta-like1 (Dll1) Notch ligands in splenic fibroblasts regulated MZB cell pool size, migration, and function. Dll1 could not be replaced by the alternative Notch ligand Dll4. Dll1-Notch2 signaling regulated a Myc-dependent gene expression program fostering cell growth and a Myc-independent program controlling cell-movement regulators such as sphingosine-1 phosphate receptor 1 (S1PR1). S1pr1-deficient B cells experienced Notch signaling within B cell follicles without entering the MZ and were retained in the spleen upon Notch deprivation. Key elements of the mouse B cell Notch regulome were preserved in subsets of human memory B cells and B cell lymphomas. Thus, specialized niches program the poised state and patrolling behavior of MZB cells via conserved Myc-dependent and Myc-independent Notch2-regulated mechanisms.

Characterizing cell-type spatial relationships across length scales in spatially resolved omics data

Spatially resolved omics (SRO) technologies enable the identification of cell types while preserving their organization within tissues. Application of such technologies offers the opportunity to delineate cell-type spatial relationships, particularly across different length scales, and enhance our understanding of tissue organization and function. To quantify such multi-scale cell-type spatial relationships, we present CRAWDAD, Cell-type Relationship Analysis Workflow Done Across Distances, as an open-source R package. To demonstrate the utility of such multi-scale characterization, recapitulate expected cell-type spatial relationships, and evaluate against other cell-type spatial analyses, we apply CRAWDAD to various simulated and real SRO datasets of diverse tissues assayed by diverse SRO technologies. We further demonstrate how such multi-scale characterization enabled by CRAWDAD can be used to compare cell-type spatial relationships across multiple samples. Finally, we apply CRAWDAD to SRO datasets of the human spleen to identify consistent as well as patient and sample-specific cell-type spatial relationships. In general, we anticipate such multi-scale analysis of SRO data enabled by CRAWDAD will provide useful quantitative metrics to facilitate the identification, characterization, and comparison of cell-type spatial relationships across axes of interest.

Haemolymphatic tissues of captive boa constrictor (Boa constrictor): Morphological features in healthy individuals and with boid inclusion body disease

Knowledge on the structure and composition of the haematopoietic tissue (HT) is essential to understand the basic immune functions of the immune system in any species. For reptiles, it is extremely limited, hence we undertook an in-depth in situ investigation of the HT (bone marrow, thymus, spleen, lymphatic tissue of the alimentary tract) in the common boa (Boa constrictor). We also assessed age- and disease-related changes, with a special focus on Boid Inclusion Body Disease, a highly relevant reptarenavirus-associated disease in boid snakes. The HT was subjected to gross, histological and ultrastructural examination, including special stains to highlight collagen and reticulin fibers and iron, immunohistochemistry, in situ hybridization and morphometric analyses. In general, the HT was dominated by T cells and lacked a clear structural organization, such as follicle formation. BIBD was associated with significantly higher cellularity and a granulomatous response in the spleen, and the presence of virus-infected haematopoietic cells in the bone marrow, suggesting the latter as a persistent source of viremia.

Artificial Tertiary Lymphoid Structures: Exploring Mesenchymal Stromal Cells as a Platform for Immune Niche Formation

Constructing artificial tertiary lymphoid structures (TLSs) opens new avenues for advancing cancer immunotherapy and personalized medicine by creating controllable immune niches. Mesenchymal stromal cells (MSCs) offer an ideal stromal source for such constructs, given their potent immunomodulatory abilities and accessibility. In this study, we explored the potential of adipose-derived MSCs to adopt TLS-supportive phenotypes and facilitate lymphocyte organization. Single-cell RNA sequencing revealed a distinct subpopulation of MSCs expressing key fibroblastic reticular cell (FRC)-associated markers, including IL-7, PDPN, and IL-15, though lacking follicular dendritic cell (FDC) markers. TNF-α stimulation, but not LTα2β1, further enhanced FRC marker expression (IL-7, PDPN, and ICAM1). Notably, in 3D spheroid co-culture with lymphocytes, MSCs upregulated additional FRC markers, specifically CCL21. Upon implantation into adipose tissue, MSC-lymphocyte organoids maintained structural integrity and showed extensive T-cell infiltration and partial vascularization after 15 days in vivo, although organized B-cell follicles and FDC markers were still lacking. These findings highlight MSCs' intrinsic ability to adopt an FRC-like phenotype that supports T-cell and HEV organization, suggesting that further optimization, including genetic modification, may be needed to achieve an FDC phenotype and replicate the full architectural and functional complexity of TLSs.

Characterizing cell-type spatial relationships across length scales in spatially resolved omics data

Spatially resolved omics (SRO) technologies enable the identification of cell types while preserving their organization within tissues. Application of such technologies offers the opportunity to delineate cell-type spatial relationships, particularly across different length scales, and enhance our understanding of tissue organization and function. To quantify such multi-scale cell-type spatial relationships, we developed CRAWDAD, Cell-type Relationship Analysis Workflow Done Across Distances, as an open-source R package with source code and additional documentation at https://jef.works/CRAWDAD/. To demonstrate the utility of such multi-scale characterization, recapitulate expected cell-type spatial relationships, and evaluate against other cell-type spatial analyses, we applied CRAWDAD to various simulated and real SRO datasets of diverse tissues assayed by diverse SRO technologies. We further demonstrate how such multi-scale characterization enabled by CRAWDAD can be used to compare cell-type spatial relationships across multiple samples. Finally, we applied CRAWDAD to SRO datasets of the human spleen to identify consistent as well as patient and sample-specific cell-type spatial relationships. In general, we anticipate such multi-scale analysis of SRO data enabled by CRAWDAD will provide useful quantitative metrics to facilitate the identification, characterization, and comparison of cell-type spatial relationships across axes of interest.

Essential growth factor receptors for fibroblast homeostasis and activation: Fibroblast Growth Factor Receptor (FGFR), Platelet Derived Growth Factor Receptor (PDGFR), and Transforming Growth Factor β Receptor (TGFβR)

Fibroblasts are cells of mesenchymal origin that are found throughout the body. While these cells have several functions, their integral roles include maintaining tissue architecture through the production of key extracellular matrix components, and participation in wound healing after injury. Fibroblasts are also key mediators in disease progression during fibrosis, cancer, and other inflammatory diseases. Under these perturbed states, fibroblasts can activate into inflammatory fibroblasts or contractile myofibroblasts. Fibroblasts require various growth factors and mitogenic molecules for survival, proliferation, and differentiation. While the activity of mitogenic growth factors on fibroblasts in vitro was characterized as early as the 1970s, the proliferation and differentiation effects of growth factors on these cells in vivo are unclear. Recent work exploring the heterogeneity of fibroblasts raises questions as to whether all fibroblast cell states exhibit the same growth factor requirements. Here, we will examine and review existing studies on the influence of fibroblast growth factor receptors (FGFRs), platelet-derived growth factor receptors (PDGFRs), and transforming growth factor β receptor (TGFβR) on fibroblast cell states.

Activation of TLR9 signaling suppresses the immunomodulating functions of CD55lo fibroblastic reticular cells during bacterial peritonitis

Fibroblastic reticular cells (FRCs) are a subpopulation of stromal cells modulating the immune environments in health and disease. We have previously shown that activation of TLR9 signaling in FRC in fat-associated lymphoid clusters (FALC) regulate peritoneal immunity via suppressing immune cell recruitment and peritoneal resident macrophage (PRM) retention. However, FRCs are heterogeneous across tissues and organs. The functions of each FRC subset and the regulation of TLR9 in distinct FRC subsets are unknown. Here, we confirmed that specific deletion of TLR9 in FRC improved bacterial clearance and survival during peritoneal infection. Furthermore, using single-cell RNA sequencing, we found two subsets of FRCs (CD55hi and CD55lo) in the mesenteric FALC. The CD55hi FRCs were enriched in gene expression related to extracellular matrix formation. The CD55lo FRCs were enriched in gene expression related to immune response. Interestingly, we found that TLR9 is dominantly expressed in the CD55lo subset. Activation of TLR9 signaling suppressed proliferation, cytokine production, and retinoid metabolism in the CD55lo FRC, but not CD55hi FRC. Notably, we found that adoptive transfer of Tlr9 -/-CD55lo FRC from mesenteric FALC more effectively improved the survival during peritonitis compared with WT-FRC or Tlr9 -/-CD55hi FRC. Furthermore, we identified CD55hi and CD55lo subsets in human adipose tissue-derived FRC and confirmed the suppressive effect of TLR9 on the proliferation and cytokine production in the CD55lo subset. Therefore, inhibition of TLR9 in the CD55lo FRCs from adipose tissue could be a useful strategy to improve the therapeutic efficacy of FRC-based therapy for peritonitis.

Brain-Splenic Immune System Interactions in Hypertension: Cellular and Molecular Mechanisms

Hypertension represents a major worldwide cause of death and disability, and it is becoming increasingly clear that available therapies are not sufficient to reduce the risk of major cardiovascular events. Various mechanisms contribute to blood pressure increase: neurohormonal activation, autonomic nervous system imbalance, and immune activation. Of note, the brain is an important regulator of blood pressure levels; it recognizes the peripheral perturbation and organizes a reflex response by modulating immune system and hormonal release to attempt at restoring the homeostasis. The connection between the brain and peripheral organs is mediated by the autonomic nervous system, which also modulates immune and inflammatory responses. Interestingly, an increased autonomic nervous system activity has been correlated with an altered immune response in cardiovascular diseases. The spleen is the largest immune organ exerting a potent influence on the cardiovascular system during disease and is characterized by a dense noradrenergic innervation. Taken together, these aspects led to hypothesize a key role of neuroimmune mechanisms in the onset and progression of hypertension. This review discusses how the nervous and splenic immune systems interact and how the mechanisms underlying the neuroimmune cross talk influence the disease progression.

Protective fibroblastic niches in secondary lymphoid organs

Fibroblastic reticular cells (FRCs) are specialized fibroblasts of secondary lymphoid organs that provide the structural foundation of the tissue. Moreover, FRCs guide immune cells to dedicated microenvironmental niches where they provide lymphocytes and myeloid cells with homeostatic growth and differentiation factors. Inflammatory processes, including infection with pathogens, induce rapid morphological and functional adaptations that are critical for the priming and regulation of protective immune responses. However, adverse FRC reprogramming can promote immunopathological tissue damage during infection and autoimmune conditions and subvert antitumor immune responses. Here, we review recent findings on molecular pathways that regulate FRC-immune cell crosstalk in specialized niches during the generation of protective immune responses in the course of pathogen encounters. In addition, we discuss how FRCs integrate immune cell-derived signals to ensure protective immunity during infection and how therapies for inflammatory diseases and cancer can be developed through improved understanding of FRC-immune cell interactions.

Crosstalk between fibroblasts and immunocytes in fibrosis: From molecular mechanisms to clinical trials

BACKGROUND

The impact of fibroblasts on the immune system provides insight into the function of fibroblasts. In various tissue microenvironments, multiple fibroblast subtypes interact with immunocytes by secreting growth factors, cytokines, and chemokines, leading to wound healing, fibrosis, and escape of cancer immune surveillance. However, the specific mechanisms involved in the fibroblast-immunocyte interaction network have not yet been fully elucidated.

MAIN BODY AND CONCLUSION

Therefore, we systematically reviewed the molecular mechanisms of fibroblast-immunocyte interactions in fibrosis, from the history of cellular evolution and cell subtype divisions to the regulatory networks between fibroblasts and immunocytes. We also discuss how these communications function in different tissue and organ statuses, as well as potential therapies targeting the reciprocal fibroblast-immunocyte interplay in fibrosis. A comprehensive understanding of these functional cells under pathophysiological conditions and the mechanisms by which they communicate may lead to the development of effective and specific therapies targeting fibrosis.

Splenic stromal niches in homeostasis and immunity

The spleen is a gatekeeper of systemic immunity where immune responses against blood-borne pathogens are initiated and sustained. Non-haematopoietic stromal cells construct microanatomical niches in the spleen that make diverse contributions to physiological spleen functions and regulate the homeostasis of immune cells. Additional signals from spleen autonomic nerves also modify immune responses. Recent insight into the diversity of the splenic fibroblastic stromal cells has revised our understanding of how these cells help to orchestrate splenic responses to infection and contribute to immune responses. In this Review, we examine our current understanding of how stromal niches and neuroimmune circuits direct the immunological functions of the spleen, with a focus on T cell immunity.

Aging of lymphoid stromal architecture impacts immune responses

The secondary lymphoid organs (SLOs) undergo structural changes with age, which correlates with diminishing immune responses against infectious disease. A growing body of research suggests that the aged tissue microenvironment can contribute to decreased immune function, independent of intrinsic changes to hematopoietic cells with age. Stromal cells impart structural integrity, facilitate fluid transport, and provide chemokine and cytokine signals that are essential for immune homeostasis. Mechanisms that drive SLO development have been described, but their roles in SLO maintenance with advanced age are unknown. Disorganization of the fibroblasts of the T cell and B cell zones may reduce the maintenance of naïve lymphocytes and delay immune activation. Reduced lymphatic transport efficiency with age can also delay the onset of the adaptive immune response. This review focuses on recent studies that describe age-associated changes to the stroma of the lymph nodes and spleen. We also review recent investigations into stromal cell biology, which include high-dimensional analysis of the stromal cell transcriptome and viscoelastic testing of lymph node mechanical properties, as they constitute an important framework for understanding aging of the lymphoid tissues.

Features of Spleen Development in Newborn Rats Exposed to Endocrine Disrupter Dichlorodiphenyltrichloroethane during the Prenatal Period

The development of the spleen in newborn male Wistar rats exposed to low-dose endocrine disruptor dichlorodiphenyltrichloroethane during the prenatal period was studied. Histological examination of the spleen revealed a more active development of periarterial lymphoid sheaths and lower granulocyte content in the organ. Cytofluorimetry showed a significantly lower content of B cells. Thus, low-dose exposure of the endocrine disruptor dichlorodiphenyltrichloroethane on the developing organism during the prenatal period induced changes in spleen morphogenesis resulting in extensive development of lymphoid formations and a lower intensity of lymphocyte and granulocyte differentiation.

Disorganization of secondary lymphoid organs and dyscoordination of chemokine secretion as key contributors to immune aging

Aging is characterized by progressive loss of organ and tissue function, and the immune system is no exception to that inevitable principle. Of all the age-related changes in the body, reduction of the size of, and naïve T (Tn) cell output from, the thymus occurs earliest, being prominent already before or by the time of puberty. Therefore, to preserve immunity against new infections, over much of their lives, vertebrates dominantly rely on peripheral maintenance of the Tn cell pool in the secondary lymphoid organs (SLO). However, SLO structure and function subsequently also deteriorate with aging. Several recent studies have made a convincing case that this deterioration is of major importance to the erosion of protective immunity in the last third of life. Specifically, the SLO were found to accumulate multiple degenerative changes with aging. Importantly, the results from adoptive transfer and parabiosis studies teach us that the old microenvironment is the limiting factor for protective immunity in old mice. In this review, we discuss the extent, mechanisms, and potential role of stromal cell aging in the age-related alteration of T cell homeostatic maintenance and immune function decline. We use that discussion to frame the potential strategies to correct the SLO stromal aging defects - in the context of other immune rejuvenation approaches, - to improve functional immune responses and protective immunity in older adults.

Tertiary Lymphoid Tissues Are Microenvironments with Intensive Interactions between Immune Cells and Proinflammatory Parenchymal Cells in Aged Kidneys

SIGNIFICANCE STATEMENT

Ectopic lymphoid structures called tertiary lymphoid tissues (TLTs) develop in several kidney diseases and are associated with poor renal prognosis. However, the mechanisms underlying TLT expansion and their effect on renal regeneration remain unclear. The authors report that single-nucleus RNA sequencing and validation experiments demonstrate that TLTs potentially amplify inflammation in aged injured kidneys. Lymphocytes within TLTs promote proinflammatory phenotypes of the surrounding proximal tubules and fibroblasts within the TLTs via proinflammatory cytokine production. These proinflammatory parenchymal cells then interact with immune cells by chemokine or cytokine production. Such cell-cell interactions potentially increase inflammation, expand TLTs, and exacerbate kidney injury. These findings help illuminate renal TLT pathology and suggest potential therapeutic targets.

BACKGROUND

Ectopic lymphoid structures called tertiary lymphoid tissues (TLTs) develop in several kidney diseases and are associated with poor renal prognosis. However, the mechanisms that expand TLTs and underlie exacerbation of kidney injury remain unclear.

METHODS

We performed single-nucleus RNA sequencing (snRNA-seq) on aged mouse kidneys with TLTs after ischemia-reperfusion injury. The results were validated using immunostaining, in situ hybridization of murine and human kidneys, and in vitro experiments.

RESULTS

Using snRNA-seq, we identified proinflammatory and profibrotic Vcam1+ injured proximal tubules (PTs) with NF κ B and IFN-inducible transcription factor activation. VCAM1 + PTs were preferentially localized around TLTs and drove inflammation and fibrosis via the production of multiple chemokines or cytokines. Lymphocytes within TLTs expressed Tnf and Ifng at high levels, which synergistically upregulated VCAM1 and chemokine expression in cultured PT cells. In addition, snRNA-seq also identified proinflammatory and profibrotic fibroblasts, which resided within and outside TLTs, respectively. Proinflammatory fibroblasts exhibited STAT1 activation and various chemokine or cytokine production, including CXCL9/CXCL10 and B cell-activating factor, contributing to lymphocyte recruitment and survival. IFN γ upregulated the expression of these molecules in cultured fibroblasts in a STAT1-dependent manner, indicating potential bidirectional interactions between IFN γ -producing CXCR3 + T cells and proinflammatory fibroblasts within TLTs. The cellular and molecular components described in this study were confirmed in human kidneys with TLTs.

CONCLUSIONS

These findings suggest that TLTs potentially amplify inflammation by providing a microenvironment that allows intense interactions between renal parenchymal and immune cells. These interactions may serve as novel therapeutic targets in kidney diseases involving TLT formation.

A novel cryopreservation and biobanking strategy to study lymphoid tissue stromal cells in human disease

Nonhematopoietic lymph node stromal cells (LNSCs) regulate lymphocyte trafficking, survival, and function for key roles in host defense, autoimmunity, alloimmunity, and lymphoproliferative disorders. However, the study of LNSCs in human diseases is complicated by a dependence on viable lymphoid tissues, which are most often excised prior to establishment of a specific diagnosis. Here, we demonstrate that cryopreservation can be used to bank lymphoid tissue for the study of LNSCs in human disease. Using human tonsils and lymph nodes (LN), lymphoid tissue fragments were cryopreserved for subsequent enzymatic digestion and recovery of viable nonhematopoietic cells. Flow cytometry and single-cell transcriptomics identified comparable proportions of LN stromal cell types in fresh and cryopreserved tissue. Moreover, cryopreservation had little effect on transcriptional profiles, which showed significant overlap between tonsils and LN. The presence and spatial distribution of transcriptionally defined cell types were confirmed by in situ analyses. Our broadly applicable approach promises to greatly enable research into the roles of LNSCs in human disease.

Pulmonary inflammation and fibroblast immunoregulation: from bench to bedside

In recent years, there has been an explosion of interest in how fibroblasts initiate, sustain, and resolve inflammation across disease states. Fibroblasts contain heterogeneous subsets with diverse functionality. The phenotypes of these populations vary depending on their spatial distribution within the tissue and the immunopathologic cues contributing to disease progression. In addition to their roles in structurally supporting organs and remodeling tissue, fibroblasts mediate critical interactions with diverse immune cells. These interactions have important implications for defining mechanisms of disease and identifying potential therapeutic targets. Fibroblasts in the respiratory tract, in particular, determine the severity and outcome of numerous acute and chronic lung diseases, including asthma, chronic obstructive pulmonary disease, acute respiratory distress syndrome, and idiopathic pulmonary fibrosis. Here, we review recent studies defining the spatiotemporal identity of the lung-derived fibroblasts and the mechanisms by which these subsets regulate immune responses to insult exposures and highlight past, current, and future therapeutic targets with relevance to fibroblast biology in the context of acute and chronic human respiratory diseases. This perspective highlights the importance of tissue context in defining fibroblast-immune crosstalk and paves the way for identifying therapeutic approaches to benefit patients with acute and chronic pulmonary disorders.

Conserved stromal-immune cell circuits secure B cell homeostasis and function

B cell zone reticular cells (BRCs) form stable microenvironments that direct efficient humoral immunity with B cell priming and memory maintenance being orchestrated across lymphoid organs. However, a comprehensive understanding of systemic humoral immunity is hampered by the lack of knowledge of global BRC sustenance, function and major pathways controlling BRC-immune cell interactions. Here we dissected the BRC landscape and immune cell interactome in human and murine lymphoid organs. In addition to the major BRC subsets underpinning the follicle, including follicular dendritic cells, PI16+ RCs were present across organs and species. As well as BRC-produced niche factors, immune cell-driven BRC differentiation and activation programs governed the convergence of shared BRC subsets, overwriting tissue-specific gene signatures. Our data reveal that a canonical set of immune cell-provided cues enforce bidirectional signaling programs that sustain functional BRC niches across lymphoid organs and species, thereby securing efficient humoral immunity.

Integration of whole transcriptome spatial profiling with protein markers

Spatial transcriptomics and proteomics provide complementary information that independently transformed our understanding of complex biological processes. However, experimental integration of these modalities is limited. To overcome this, we developed Spatial PrOtein and Transcriptome Sequencing (SPOTS) for high-throughput simultaneous spatial transcriptomics and protein profiling. Compared with unimodal measurements, SPOTS substantially improves signal resolution and cell clustering and enhances the discovery power in differential gene expression analysis across tissue regions.

PI16+ reticular cells in human palatine tonsils govern T cell activity in distinct subepithelial niches

Fibroblastic reticular cells (FRCs) direct the interaction and activation of immune cells in discrete microenvironments of lymphoid organs. Despite their important role in steering innate and adaptive immunity, the age- and inflammation-associated changes in the molecular identity and functional properties of human FRCs have remained largely unknown. Here, we show that human tonsillar FRCs undergo dynamic reprogramming during life and respond vigorously to inflammatory perturbation in comparison to other stromal cell types. The peptidase inhibitor 16 (PI16)-expressing reticular cell (PI16⁺ RC) subset of adult tonsils exhibited the strongest inflammation-associated structural remodeling. Interactome analysis combined with ex vivo and in vitro validation revealed that T cell activity within subepithelial niches is controlled by distinct molecular pathways during PI16⁺ RC-lymphocyte interaction. In sum, the topological and molecular definition of the human tonsillar stromal cell landscape reveals PI16⁺ RCs as a specialized FRC niche at the core of mucosal immune responses in the oropharynx.

Absence of Nkx2-3 induces ectopic lymphatic endothelial differentiation associated with impaired extramedullary stress hematopoiesis in the spleen

The red and white pulps as two main parts of the spleen are arranged around distinct types of vasculature, and perform significantly different functions in both humans and mice. Previous observations indicated a profound alteration of the local vessel specialization in mice lacking Nkx2-3 homeodomain transcription factor, including contradictory results suggesting presence of an ectopic lymphatic vascular structure. Furthermore, how the absence of Nkx2-3 and the consequential changes in endothelial components affect the extramedullary hematopoietic activity restricted to the splenic red pulp is unknown. In this work, we investigated the role of Nkx2-3 homeodomain transcription factor as a major morphogenic determinant for vascular specification, and its effect in the extramedullary hematopoiesis following acute blood loss and pharmacological stimulation of megakaryocyte differentiation after treatment with thrombopoietin-receptor mimetic Romiplostim. We found that, in mice lacking Nkx2-3, Prox1-positive lymphatic capillaries containing gp38/CD31 double positive lymphatic endothelial cells develop, arranged into an extensive meshwork, while the Clever1-positive venous segments of red pulp blood vasculature are absent. This lymphatic endothelial shift is coupled with a severely compromised splenic erythropoiesis and a significantly reduced splenic megakaryocyte colony formation following Romiplostim treatment in mice lacking Nkx2-3. These findings indicate that the shift of microvascular patterning in the absence of Nkx2-3 includes the emergence of ectopic Prox1-positive lymphatic vessels, and that this pivoting towards lymph node-like vascular patterning is associated with an impaired reserve hematopoietic capacity of the splenic red pulp.

Mature B cells and mesenchymal stem cells control emergency myelopoiesis

Systemic inflammation halts lymphopoiesis and prioritizes myeloid cell production. How blood cell production switches from homeostasis to emergency myelopoiesis is incompletely understood. Here, we show that lymphotoxin-β receptor (LTβR) signaling in combination with TNF and IL-1 receptor signaling in bone marrow mesenchymal stem cells (MSCs) down-regulates Il7 expression to shut down lymphopoiesis during systemic inflammation. LTβR signaling in MSCs also promoted CCL2 production during systemic inflammation. Pharmacological or genetic blocking of LTβR signaling in MSCs partially enabled lymphopoiesis and reduced monocyte numbers in the spleen during systemic inflammation, which correlated with reduced survival during systemic bacterial and viral infections. Interestingly, lymphotoxin-α1β2 delivered by B-lineage cells, and specifically by mature B cells, contributed to promote Il7 down-regulation and reduce MSC lymphopoietic activity. Our studies revealed an unexpected role of LTβR signaling in MSCs and identified recirculating mature B cells as an important regulator of emergency myelopoiesis.

A novel cryopreservation and biobanking strategy to study lymphoid tissue stromal cells in human disease

Non-hematopoietic lymph node stromal cells (LNSCs) regulate lymphocyte trafficking, survival, and function for key roles in host defense, autoimmunity, alloimmunity, and lymphoproliferative disorders. However, study of LNSCs in human diseases is complicated by a dependence on viable lymphoid tissues, which are most often excised prior to establishment of a specific diagnosis. Here, we demonstrate that cryopreservation can be used to bank lymphoid tissue for the study of LNSCs in human disease. Using human tonsils, lymphoid tissue fragments were cryopreserved for subsequent enzymatic digestion and recovery of viable non-hematopoietic cells. Flow cytometry and single-cell transcriptomics identified comparable proportions of LNSC cell types in fresh and cryopreserved tissue. Moreover, cryopreservation had little effect on transcriptional profiles, which showed significant overlap between tonsils and lymph nodes. The presence and spatial distribution of transcriptionally defined cell types was confirmed by in situ analyses. Our broadly applicable approach promises to greatly enable research into the roles of LNSC in human disease.

Fibroblast heterogeneity: Keystone of tissue homeostasis and pathology in inflammation and ageing

Fibroblasts, derived from the embryonic mesenchyme, are a diverse array of cells with roles in development, homeostasis, repair, and disease across tissues. In doing so, fibroblasts maintain micro-environmental homeostasis and create tissue niches by producing a complex extracellular matrix (ECM) including various structural proteins. Although long considered phenotypically homogenous and functionally identical, the emergence of novel technologies such as single cell transcriptomics has allowed the identification of different phenotypic and cellular states to be attributed to fibroblasts, highlighting their role in tissue regulation and inflammation. Therefore, fibroblasts are now recognised as central actors in many diseases, increasing the need to discover new therapies targeting those cells. Herein, we review the phenotypic heterogeneity and functionality of these cells and their roles in health and disease.

Postnatal expansion of mesenteric lymph node stromal cells towards reticular and CD34+ stromal cell subsets

Gut-draining mesenteric lymph nodes (LN) provide the framework to shape intestinal adaptive immune responses. Based on the transcriptional signatures established by our previous work, the composition and immunomodulatory function of LN stromal cells (SC) vary according to location. Here, we describe the single-cell composition and development of the SC compartment within mesenteric LNs derived from postnatal to aged mice. We identify CD34⁺ SC and fibroblastic reticular stromal cell (FRC) progenitors as putative progenitors, both supplying the typical rapid postnatal mesenteric LN expansion. We further establish the location-specific chromatin accessibility and DNA methylation landscape of non-endothelial SCs and identify a microbiota-independent core epigenomic signature, showing characteristic differences between SCs from mesenteric and skin-draining peripheral LNs. The epigenomic landscape of SCs points to dynamic expression of Irf3 along the differentiation trajectories of FRCs. Accordingly, a mesenchymal stem cell line acquires a Cxcl9⁺ FRC molecular phenotype upon lentiviral overexpression of Irf3, and the relevance of Irf3 for SC biology is further underscored by the diminished proportion of Ccl19⁺ and Cxcl9⁺ FRCs in LNs of Irf3^-/- mice. Together, our data constitute a comprehensive transcriptional and epigenomic map of mesenteric LNSC development in early life and dissect location-specific, microbiota-independent properties of non-endothelial SCs.

Nestin+ Mesenchymal Precursors Generate Distinct Spleen Stromal Cell Subsets and Have Immunomodulatory Function

Mesenchymal stromal cells (MSCs) are known to be widespread in many tissues and possess a broad spectrum of immunoregulatory properties. They have been used in the treatment of a variety of inflammatory diseases; however, the therapeutic effects are still inconsistent owing to their heterogeneity. Spleen stromal cells have evolved to regulate the immune response at many levels as they are bathed in a complex inflammatory milieu during infection. Therefore, it is unknown whether they have stronger immunomodulatory effects than their counterparts derived from other tissues. Here, using a transgenic mouse model expressing GFP driven by the Nestin (Nes) promoter, Nes-GFP⁺ cells from bone marrow and spleen were collected. Artificial lymphoid reconstruction in vivo was performed. Cell phenotype, inhibition of T cell inflammatory cytokines, and in vivo therapeutic effects were assessed. We observed Nes-GFP⁺ cells colocalized with splenic stromal cells and further demonstrated that these Nes-GFP⁺ cells had the ability to establish ectopic lymphoid-like structures in vivo. Moreover, we showed that the Nes-GFP⁺ cells possessed the characteristics of MSCs. Spleen-derived Nes-GFP⁺ cells exhibited greater immunomodulatory ability in vitro and more remarkable therapeutic efficacy in inflammatory diseases, especially inflammatory bowel disease (IBD) than bone marrow-derived Nes-GFP⁺ cells. Overall, our data showed that Nes-GFP⁺ cells contributed to subsets of spleen stromal populations and possessed the biological characteristics of MSCs with a stronger immunoregulatory function and therapeutic potential than bone marrow-derived Nes-GFP⁺ cells.

Inducing Ectopic T Cell Clusters Using Stromal Vascular Fraction Spheroid-Based Immunotherapy to Enhance Anti-Tumor Immunity

Tertiary lymphoid structures (TLSs) provide specialized niches for immune cells, resulting in improved prognoses for patients undergoing cancer immunotherapy. Shaping TLS-like niches may improve anti-cancer immunity and overcome the current limitations of immune cell-based immunotherapy. Here, it is shown that stromal vascular fraction (SVF) from adipose tissues can enhance dendritic cell (DC)-mediated T cell immunity by inducing ectopic T lymphocyte clusters. SVF cells expanded ex vivo have phenotypes and functions similar to those of fibroblastic reticular cells in a secondary lymphoid organ, and their properties can be modulated using three-dimensional spheroid culture and coculture with DCs spiked with antigen-loaded iron oxide-zinc oxide core-shell nanoparticles. Thereby, the combination of SVF spheroids and mature DCs significantly augments T cell recruitment and retention at the injection site. This strategy elicits enhanced antigen-specific immune response and anti-tumoral immunity in mice, illustrating the potential for a novel immunotherapeutic design using SVF as a structural scaffold for TLS.

Dysregulated stem cell niches and altered lymphocyte recirculation cause B and T cell lymphopenia in WHIM syndrome

Gain-of-function (GOF) mutations in CXCR4 cause WHIM (warts, hypogammaglobulinemia, infections, and myelokathexis) syndrome, characterized by infections, leukocyte retention in bone marrow (BM), and blood leukopenias. B lymphopenia is evident at early progenitor stages, yet why do CXCR4 GOF mutations that cause B (and T) lymphopenia remain obscure? Using a CXCR4 R334X GOF mouse model of WHIM syndrome, we showed that lymphopoiesis is reduced because of a dysregulated mesenchymal stem cell (MSC) transcriptome characterized by a switch from an adipogenic to an osteolineage-prone program with limited lymphopoietic activity. We identify lymphotoxin beta receptor (LTβR) as a critical pathway promoting interleukin-7 (IL-7) down-regulation in MSCs. Blocking LTβR or CXCR4 signaling restored IL-7 production and B cell development in WHIM mice. LTβR blocking also increased production of IL-7 and B cell activating factor (BAFF) in secondary lymphoid organs (SLOs), increasing B and T cell numbers in the periphery. These studies revealed that LTβR signaling in BM MSCs and SLO stromal cells limits the lymphocyte compartment size.

Age-Associated Changes to Lymph Node Fibroblastic Reticular Cells

The decreased proportion of antigen-inexperienced, naïve T cells is a hallmark of aging in both humans and mice, and contributes to reduced immune responses, particularly against novel and re-emerging pathogens. Naïve T cells depend on survival signals received during their circulation among the lymph nodes by direct contacts with stroma, in particular fibroblastic reticular cells. Macroscopic changes to the architecture of the lymph nodes have been described, but it is unclear how lymph node stroma are altered with age, and whether these changes contribute to reduced naïve T cell maintenance. Here, using 2-photon microscopy, we determined that the aged lymph node displayed increased fibrosis and correspondingly, that naïve T-cell motility was impaired in the aged lymph node, especially in proximity to fibrotic deposition. Functionally, adoptively transferred young naïve T-cells exhibited reduced homeostatic turnover in aged hosts, supporting the role of T cell-extrinsic mechanisms that regulate their survival. Further, we determined that early development of resident fibroblastic reticular cells was impaired, which may correlate to the declining levels of naïve T-cell homeostatic factors observed in aged lymph nodes. Thus, our study addresses the controversy as to whether aging impacts the composition lymph node stroma and supports a model in which impaired differentiation of lymph node fibroblasts and increased fibrosis inhibits the interactions necessary for naïve T cell homeostasis.

Multitier mechanics control stromal adaptations in the swelling lymph node

Lymph nodes (LNs) comprise two main structural elements: fibroblastic reticular cells that form dedicated niches for immune cell interaction and capsular fibroblasts that build a shell around the organ. Immunological challenge causes LNs to increase more than tenfold in size within a few days. Here, we characterized the biomechanics of LN swelling on the cellular and organ scale. We identified lymphocyte trapping by influx and proliferation as drivers of an outward pressure force, causing fibroblastic reticular cells of the T-zone (TRCs) and their associated conduits to stretch. After an initial phase of relaxation, TRCs sensed the resulting strain through cell matrix adhesions, which coordinated local growth and remodeling of the stromal network. While the expanded TRC network readopted its typical configuration, a massive fibrotic reaction of the organ capsule set in and countered further organ expansion. Thus, different fibroblast populations mechanically control LN swelling in a multitier fashion.

Sixt and colleagues show that different fibroblast populations in the lymph node mechanically control its swelling in a multitier fashion.

IL-33 acts as a costimulatory signal to generate alloreactive Th1 cells in graft-versus-host disease

Antigen-presenting cells (APCs) integrate signals emanating from local pathology and program appropriate T cell responses. In allogeneic hematopoietic stem cell transplantation (alloHCT), recipient conditioning releases damage-associated molecular patterns (DAMPs) that generate proinflammatory APCs that secrete IL-12, which is a driver of donor Th1 responses, causing graft-versus-host disease (GVHD). Nevertheless, other mechanisms exist to initiate alloreactive T cell responses, as recipients with disrupted DAMP signaling or lacking IL-12 develop GVHD. We established that tissue damage signals are perceived directly by donor CD4+ T cells and promoted T cell expansion and differentiation. Specifically, the fibroblastic reticular cell-derived DAMP IL-33 is increased by recipient conditioning and is critical for the initial activation, proliferation, and differentiation of alloreactive Th1 cells. IL-33 stimulation of CD4+ T cells was not required for lymphopenia-induced expansion, however. IL-33 promoted IL-12-independent expression of Tbet and generation of Th1 cells that infiltrated GVHD target tissues. Mechanistically, IL-33 augmented CD4+ T cell TCR-associated signaling pathways in response to alloantigen. This enhanced T cell expansion and Th1 polarization, but inhibited the expression of regulatory molecules such as IL-10 and Foxp3. These data establish an unappreciated role for IL-33 as a costimulatory signal for donor Th1 generation after alloHCT.

ERα signaling in a subset of CXCL12‐abundant reticular cells regulates trabecular bone in mice

Estrogen has pronounced effects on the immune system, which also influences bone homeostasis. In recent years, stromal cells in lymphoid organs have gained increasing attention as they not only support the regulation of immune responses but also affect bone remodeling. A conditional knockout mouse model where estrogen receptor alpha (ERα) is deleted in CCL19‐expressing stromal cells (Ccl19‐Cre ERα^fl/fl mice) was generated and bone densitometry was performed to analyze the importance of stromal cell–specific ERα signaling on the skeleton. Results showed that female Ccl19‐Cre ERα^fl/fl mice display reduced total bone mineral density and detailed X‐ray analyses revealed that ERα expression in CCL19‐expressing stromal cells is important for trabecular but not cortical bone homeostasis. Further analysis showed that the trabecular bone loss is caused by increased osteoclastogenesis. Additionally, the bone formation rate was reduced; however, the expression of osteoprogenitor genes was not altered. Analysis of the bone marrow stromal cell compartment revealed a deletion of ERα in a subgroup of CXCL12‐abundant reticular (CAR) cells resulting in increased secretion of the pro‐osteoclastogenic chemokine CXCL12. In conclusion, this study reveals the importance of ERα signaling in CAR cells for bone health. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Intestinal fibroblastic reticular cell niches control innate lymphoid cell homeostasis and function

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.

Fibroblastic reticular cells (FRCs) support localisation of immune cells in secondary lymphoid tissues but less is known about the lamina propria. Here the authors use scRNA-seq and intestinal infection to characterise FRCs in the intestinal lamina propria and show specialised niches that foster innate lymphoid cells during homeostasis and infection.

A single-cell atlas of non-haematopoietic cells in human lymph nodes and lymphoma reveals a landscape of stromal remodelling

The activities of non-haematopoietic cells (NHCs), including mesenchymal stromal cells and endothelial cells, in lymphomas are reported to underlie lymphomagenesis. However, our understanding of lymphoma NHCs has been hampered by unexplained NHC heterogeneity, even in normal human lymph nodes (LNs). Here we constructed a single-cell transcriptome atlas of more than 100,000 NHCs collected from 27 human samples, including LNs and various nodal lymphomas, and it revealed 30 distinct subclusters, including some that were previously unrecognized. Notably, this atlas was useful for comparative analyses with lymphoma NHCs, which revealed an unanticipated landscape of subcluster-specific changes in gene expression and interaction with malignant cells in follicular lymphoma NHCs. This facilitates our understanding of stromal remodelling in lymphoma and highlights potential clinical biomarkers. Our study largely updates NHC taxonomy in human LNs and analysis of disease status, and provides a rich resource and deeper insights into LN and lymphoma biology to advance lymphoma management and therapy.

Abe et al. profile, characterize and compare non-haematopoietic cells in normal human lymph nodes versus nodal lymphomas from patients, providing insights into stromal modelling in health and disease.

Chemo- and mechanosensing by dendritic cells facilitate antigen surveillance in the spleen

Spleen dendritic cells (DC) are critical for initiation of adaptive immune responses against blood-borne invaders. Key to DC function is their positioning at sites of pathogen entry, and their abilities to selectively capture foreign antigens and promptly engage T cells. Focusing on conventional DC2 (cDC2), we discuss the contribution of chemoattractant receptors (EBI2 or GPR183, S1PR1, and CCR7) and integrins to cDC2 positioning and function. We give particular attention to a newly identified role in cDC2 for adhesion G-protein coupled receptor E5 (Adgre5 or CD97) and its ligand CD55, detailing how this mechanosensing system contributes to splenic cDC2 positioning and homeostasis. Additional roles of CD97 in the immune system are reviewed. The ability of cDC2 to be activated by circulating missing self-CD47 cells and to integrate multiple red blood cell (RBC)-derived inputs is discussed. Finally, we describe the process of activated cDC2 migration to engage and prime helper T cells. Throughout the review, we consider the insights into cDC function in the spleen that have emerged from imaging studies.

CD169+ macrophages in lymph node and spleen critically depend on dual RANK and LTbetaR signaling

The CD169⁺ macrophages that play an important role in the fight against infections and cancer are receptive to environmental signals for their differentiation. We show that lymph node and splenic CD169⁺ macrophages require both LTβR and RANK signaling since the conditional deficiency of either receptor results in their disappearance. Using a reporter mouse, we observe RANKL expression by a splenic mesenchymal cell subset and show that it participates in CD169⁺ macrophage differentiation. Their absence leads to a reduced viral capture and a greatly attenuated virus-specific CD8⁺ T cell expansion. Thus, tight control mechanisms operate for the precise positioning of these macrophages at sites where numerous immune-stimulatory forces converge.

CD169⁺ macrophages reside in lymph node (LN) and spleen and play an important role in the immune defense against pathogens. As resident macrophages, they are responsive to environmental cues to shape their tissue-specific identity. We have previously shown that LN CD169⁺ macrophages require RANKL for formation of their niche and their differentiation. Here, we demonstrate that they are also dependent on direct lymphotoxin beta (LTβ) receptor (R) signaling. In the absence or the reduced expression of either RANK or LTβR, their differentiation is perturbed, generating myeloid cells expressing SIGN-R1 in LNs. Conditions of combined haploinsufficiencies of RANK and LTβR revealed that both receptors contribute equally to LN CD169⁺ macrophage differentiation. In the spleen, the Cd169-directed ablation of either receptor results in a selective loss of marginal metallophilic macrophages (MMMs). Using a RANKL reporter mouse, we identify splenic marginal zone stromal cells as a source of RANKL and demonstrate that it participates in MMM differentiation. The loss of MMMs had no effect on the splenic B cell compartments but compromised viral capture and the expansion of virus-specific CD8⁺ T cells. Taken together, the data provide evidence that CD169⁺ macrophage differentiation in LN and spleen requires dual signals from LTβR and RANK with implications for the immune response.

A diverse fibroblastic stromal cell landscape in the spleen directs tissue homeostasis and immunity

[Figure: see text].

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.

Single-cell transcriptional profiling of splenic fibroblasts reveals subset-specific innate immune signatures in homeostasis and during viral infection

Our understanding of the composition and functions of splenic stromal cells remains incomplete. Here, based on analysis of over 20,000 single cell transcriptomes of splenic fibroblasts, we characterized the phenotypic and functional heterogeneity of these cells in healthy state and during virus infection. We describe eleven transcriptionally distinct fibroblastic cell clusters, reassuring known subsets and revealing yet unascertained heterogeneity amongst fibroblasts occupying diverse splenic niches. We further identify striking differences in innate immune signatures of distinct stromal compartments in vivo. Compared to other fibroblasts and to endothelial cells, Ly6C⁺ fibroblasts of the red pulp were selectively endowed with enhanced interferon-stimulated gene expression in homeostasis, upon systemic interferon stimulation and during virus infection in vivo. Collectively, we provide an updated map of fibroblastic cell diversity in the spleen that suggests a specialized innate immune function for splenic red pulp fibroblasts.

Anatomical structures, cell types and biomarkers of the Human Reference Atlas

The Human Reference Atlas (HRA) aims to map all of the cells of the human body to advance biomedical research and clinical practice. This Perspective presents collaborative work by members of 16 international consortia on two essential and interlinked parts of the HRA: (1) three-dimensional representations of anatomy that are linked to (2) tables that name and interlink major anatomical structures, cell types, plus biomarkers (ASCT+B). We discuss four examples that demonstrate the practical utility of the HRA.

Lymphocyte access to lymphoma is impaired by high endothelial venule regression

Blood endothelial cells display remarkable plasticity depending on the demands of a malignant microenvironment. While studies in solid tumors focus on their role in metabolic adaptations, formation of high endothelial venules (HEVs) in lymph nodes extends their role to the organization of immune cell interactions. As a response to lymphoma growth, blood vessel density increases; however, the fate of HEVs remains elusive. Here, we report that lymphoma causes severe HEV regression in mouse models that phenocopies aggressive human B cell lymphomas. HEV dedifferentiation occurrs as a consequence of a disrupted lymph-carrying conduit system. Mechanosensitive fibroblastic reticular cells then deregulate CCL21 migration paths, followed by deterioration of dendritic cell proximity to HEVs. Loss of this crosstalk deprives HEVs of lymphotoxin-β-receptor (LTβR) signaling, which is indispensable for their differentiation and lymphocyte transmigration. Collectively, this study reveals a remodeling cascade of the lymph node microenvironment that is detrimental for immune cell trafficking in lymphoma.

Canine Non-Angiogenic, Non-Myogenic Splenic Stromal Sarcoma: a Retrospective Clinicopathological Analysis and Investigation of Podoplanin as a Marker of Tumour Histogenesis

Splenic stromal sarcomas are rarely reported tumours that were previously grouped as non-angiomatous, non-lymphomatous mesenchymal neoplasms of the canine spleen. Highly variable survival times have been reported probably due to their heterogeneous nature. The purpose of this study was to assess the outcome and prognostic factors in dogs with splenic stromal sarcoma after treatment by splenectomy. Clinical data were collected retrospectively and histopathology was reviewed for 47 patients. Histological classification, based on morphology in haematoxylin and eosin-stained sections, in conjunction with immunolabelling of macrophage scavenger receptor-A (CD204), desmin, factor VIII-related antigen and smooth muscle actin ,yielded diagnoses of undifferentiated stromal sarcoma (n = 22), complex nodular hyperplasia (CNH, n = 9), sarcoma arising from benign complex nodular hyperplasia (n = 3), histiocytic sarcoma (n = 3), haemangiosarcoma (n = 1) and leiomyosarcoma (n = 1). Four samples were excluded from analysis due to extensive necrosis. An anti-podoplanin (PDPN) antibody was validated on canine tissue and used to assess expression of this protein as a potential indicator of the tissue of origin of the neoplasms (28/42 tumours were positive). There was a statistically significant difference in survival time between patients with stromal sarcoma (sarcoma from benign CNH and undifferentiated stromal sarcoma) and CNH (178 d versus 637 d, respectively; P = 0.027). Dogs with stromal sarcomas and high mitotic count (≥9 per 10 high-power fields) had a significantly shorter survival time (67 d versus 439 d; P = 0.01). Clinical diagnosis of splenic tumours should include evaluation for the presence of benign nodular hyperplasia morphology and immunohistochemistry to exclude more aggressive malignancies where adjuvant therapy is recommended. As in humans, PDPN may be an effective marker for stromal sarcomas of the canine spleen and immunopositivity suggests a fibroblastic reticular or follicular dendritic cell origin.

Follicular dendritic cells restrict interleukin-4 availability in germinal centers and foster memory B cell generation

B cells within germinal centers (GCs) enter cycles of antibody affinity maturation or exit the GC as memory cells or plasma cells. Here, we examined the contribution of interleukin (IL)-4 on B cell fate decisions in the GC. Single-cell RNA-sequencing identified a subset of light zone GC B cells expressing high IL-4 receptor-a (IL4Ra) and CD23 and lacking a Myc-associated signature. These cells could differentiate into pre-memory cells. B cell-specific deletion of IL4Ra or STAT6 favored the pre-memory cell trajectory, and provision of exogenous IL-4 in a wild-type context reduced pre-memory cell frequencies. IL-4 acted during antigen-specific interactions but also influenced bystander cells. Deletion of IL4Ra from follicular dendritic cells (FDCs) increased the availability of IL-4 in the GC, impaired the selection of affinity-matured B cells, and reduced memory cell generation. We propose that GC FDCs establish a niche that limits bystander IL-4 activity, focusing IL-4 action on B cells undergoing selection and enhancing memory cell differentiation.

Characterization of human FDCs reveals regulation of T cells and antigen presentation to B cells

Stromal-derived follicular dendritic cells (FDCs) are essential for germinal centers (GCs), the site where B cells maturate their antibodies. FDCs present native antigen to B cells and maintain a CXCL13 gradient to form the B cell follicle. Yet despite their essential role, the transcriptome of human FDCs remains undefined. Using single-cell RNA sequencing and microarray, we provided the transcriptome of these enigmatic cells as a comprehensive resource. Key genes were validated by flow cytometry and microscopy. Surprisingly, marginal reticular cells (MRCs) rather than FDCs expressed B cell activating factor (BAFF). Furthermore, we found that human FDCs expressed TLR4 and can alter antigen availability in response to pathogen-associated molecular patterns (PAMPs). High expression of PD-L1 and PD-L2 on FDCs activated PD1 on T cells. In addition, we found expression of genes related to T cell regulation, such as HLA-DRA, CD40, and others. These data suggest intimate contact between human FDCs and T cells.

Lymph node stromal cells: subsets and functions in health and disease

Lymph nodes (LNs) aid the interaction between lymphocytes and antigen-presenting cells, resulting in adequate and prolonged adaptive immune responses. LN stromal cells (LNSCs) are crucially involved in steering adaptive immune responses at different levels. Most knowledge on LNSCs has been obtained from mouse studies, and few studies indicate similarities with their human counterparts. Recent advances in single-cell technologies have revealed significant LNSC heterogeneity among different subsets with potential selective functions in immunity. This review provides an overview of current knowledge of LNSCs based on human and murine studies describing the role of these cells in health and disease.