Stromal and hematopoietic cells in secondary lymphoid organs: partners in immunity

International Society for Cell and Gene Therapy Clinical Translation Committee recommendations on mesenchymal stromal cells in graft-versus-host disease: easy manufacturing is faced with standardizing and commercialization challenges

Mesenchymal stromal cells (MSCs) have been used in multiple clinical trials for steroid-refractory moderate-severe (grade II-IV) acute graft-versus-host disease (aGVHD) across the world over the last two decades. Despite very promising results in a variety of trials, it failed to get widespread approval by regulatory agencies such as the U.S. Food and Drug Administration and the European Medicines Agency. What lessons can we learn from this for future studies on MSCs and other cell therapy products? Broad heterogeneity among published trials using MSCs in aGVHD was likely the core problem. We propose a standardized approach in regards to donor-related factors, MSCs-related characteristics, as well as clinical trial design, to limit heterogeneity in trials for aGVHD and to fulfill the requirements of regulatory agencies. This approach may be expanded beyond MSCs to other Cell and Gene therapy products and trials in other diseases.

The bidirectional relationship of depression and disturbances in B cell homeostasis: Double trouble

Major depressive disorder (MDD) is a recurrent, persistent, and debilitating neuropsychiatric syndrome with an increasing morbidity and mortality, representing the leading cause of disability worldwide. The dysregulation of immune systems (including innate and adaptive immune systems) has been identified as one of the key contributing factors in the progression of MDD. As the main force of the humoral immunity, B cells have an essential role in the defense against infections, antitumor immunity and autoimmune diseases. Several recent studies have suggested an intriguing connection between disturbances in B cell homeostasis and the pathogenesis of MDD, however, the B-cell-dependent mechanism of MDD remains largely unexplored compared to other immune cells. In this review, we provide an overview of how B cell abnormality regulates the progression of MMD and the potential consequence of the disruption of B cell homeostasis in patients with MDD. Abnormalities of B-cell homeostasis not only promote susceptibility to MDD, but also lead to an increased risk of developing infection, malignancy and autoimmune diseases in patients with MDD. A better understanding of the contribution of B cells underlying MDD would provide opportunities for identification of more targeted treatment approaches and might provide an overall therapeutic benefit to improve the long-term outcomes of patients with MDD.

Targeting lymph nodes for enhanced cancer vaccination: From nanotechnology to tissue engineering

Lymph nodes (LNs) occupy a critical position in initiating and augmenting immune responses, both spatially and functionally. In cancer immunotherapy, tumor-specific vaccines are blooming as a powerful tool to suppress the growth of existing tumors, as well as provide preventative efficacy against tumorigenesis. Delivering these vaccines more efficiently to LNs, where antigen-presenting cells (APCs) and T cells abundantly reside, is under extensive exploration. Formulating vaccines into nanomedicines, optimizing their physiochemical properties, and surface modification to specifically bind molecules expressed on LNs or APCs, are common routes and have brought encouraging outcomes. Alternatively, porous scaffolds can be engineered to attract APCs and provide an environment for them to mature, proliferate and migrate to LNs. A relatively new research direction is inducing the formation of LN-like organoids, which have shown positive relevance to tumor prognosis. Cutting-edge advances in these directions and discussions from a future perspective are given here, from which the up-to-date pattern of cancer vaccination will be drawn to hopefully provide basic guidance to future studies.

Colon-Targeted Oral Delivery of Hydroxyethyl Starch-Curcumin Microcapsules Loaded with Multiple Drugs Alleviates DSS-Induced Ulcerative Colitis in Mice

Combination therapy through colon-targeted oral delivery of multiple drugs presents a promising approach for effectively treating ulcerative colitis (UC). However, the codelivery of drugs with diverse physicochemical properties in a single formulation remains a formidable challenge. Here, microcapsules are designed based on hydroxyethyl starch-curcumin (HES─CUR) conjugates to enable the simultaneous delivery of hydrophobic dexamethasone acetate (DA) and hydrophilic cefazolin sodium (CS), yielding multiple drug-loaded microcapsules (CS/DA-loaded HES─CUR microcapsules, CDHC-MCs) tailored for colon-targeted therapy of UC. Thorough characterization confirms the successful synthesis and exceptional biocompatibility of CDHC-MCs. Biodistribution studies demonstrate that the microcapsules exhibit an impressive inflammatory targeting effect, accumulating preferentially in inflamed colons. In vivo experiments employing a dextran-sulfate-sodium-induced UC mouse model reveal that CDHC-MCs not only arrest UC progression but also facilitate the restoration of colon length and alleviate inflammation-related splenomegaly. These findings highlight the potential of colon-targeted delivery of multiple drugs within a single formulation as a promising strategy to enhance UC treatment, and the CDHC-MCs developed in this study hold great potential in developing novel oral formulations for advanced UC therapy.

Single-cell RNA sequencing unveils the hidden powers of zebrafish kidney for generating both hematopoiesis and adaptive antiviral immunity

The vertebrate kidneys play two evolutionary conserved roles in waste excretion and osmoregulation. Besides, the kidney of fish is considered as a functional ortholog of mammalian bone marrow that serves as a hematopoietic hub for generating blood cell lineages and immunological responses. However, knowledge about the properties of kidney hematopoietic cells, and the functionality of the kidney in fish immune systems remains to be elucidated. To this end, our present study generated a comprehensive atlas with 59 hematopoietic stem/progenitor cell (HSPC) and immune-cells types from zebrafish kidneys via single-cell transcriptome profiling analysis. These populations included almost all known cells associated with innate and adaptive immunity, and displayed differential responses to viral infection, indicating their diverse functional roles in antiviral immunity. Remarkably, HSPCs were found to have extensive reactivities to viral infection, and the trained immunity can be effectively induced in certain HSPCs. In addition, the antigen-stimulated adaptive immunity can be fully generated in the kidney, suggesting the kidney acts as a secondary lymphoid organ. These results indicated that the fish kidney is a dual-functional entity with functionalities of both primary and secondary lymphoid organs. Our findings illustrated the unique features of fish immune systems, and highlighted the multifaced biology of kidneys in ancient vertebrates.

Reconstructing the cell-cell interaction network among mouse immune cells

Intercellular interactions and cell-cell communication are critical to regulating cell functions, especially in normal immune cells and immunotherapies. Ligand-receptor pairs mediating these cell-cell interactions can be identified using diverse experimental and computational approaches. Here, we reconstructed the intercellular interaction network between Mus musculus immune cells using publicly available receptor-ligand interaction databases and gene expression data from the immunological genome project. This reconstructed network accounts for 50,317 unique interactions between 16 cell types between 731 receptor-ligand pairs. Analysis of this network shows that cells of hematopoietic lineages use fewer communication pathways for interacting with each other, while nonhematopoietic stromal cells use the most network communications. We further observe that the WNT, BMP, and LAMININ pathways are the most significant contributors to the overall number of cell-cell interactions among the various pathways in the reconstructed communication network. This resource will enable the systematic analysis of normal and pathologic immune cell interactions, along with the study of emerging immunotherapies.

FRC transplantation restores lymph node conduit defects in laminin α4-deficient mice

Fibroblastic reticular cells (FRCs) play important roles in tolerance by producing laminin α4 (Lama4) and altering lymph node (LN) structure and function. The present study revealed the specific roles of extracellular matrix Lama4 in regulating LN conduits using FRC-specific KO mouse strains. FRC-derived Lama4 maintained conduit fiber integrity, as its depletion altered conduit morphology and structure and reduced homeostatic conduit flow. Lama4 regulated the lymphotoxin β receptor (LTβR) pathway, which is critical for conduit and LN integrity. Depleting LTβR in FRCs further reduced conduits and impaired reticular fibers. Lama4 was indispensable for FRC generation and survival, as FRCs lacking Lama4 displayed reduced proliferation but upregulated senescence and apoptosis. During acute immunization, FRC Lama4 deficiency increased antigen flow through conduits. Importantly, adoptive transfer of WT FRCs to FRC Lama4-deficient mice rescued conduit structure, ameliorated Treg and chemokine distribution, and restored transplant allograft acceptance, which were all impaired by FRC Lama4 depletion. Single-cell RNA sequencing analysis of LN stromal cells indicated that the laminin and collagen signaling pathways linked crosstalk among FRC subsets and endothelial cells. This study demonstrated that FRC Lama4 is responsible for maintaining conduits by FRCs and can be harnessed to potentiate FRC-based immunomodulation.

Lymph node fibroblastic reticular cells preserve a tolerogenic niche in allograft transplantation through laminin α4

Lymph node (LN) fibroblastic reticular cells (FRCs) define LN niches and regulate lymphocyte homeostasis through producing diverse extracellular matrix (ECM) components. We examined the role of ECM laminin α4 (Lama4) using FRC-Lama4 conditional KO Pdgfrb-Cre-/- × Lama4fl/fl mice. Single-cell RNA-sequencing (scRNA-Seq) data showed the promoter gene Pdgfrb was exclusively expressed in FRCs. Depleting FRC-Lama4 reduced Tregs and dendritic cells, decreased high endothelial venules, impaired the conduit system, and downregulated T cell survival factors in LNs. FRC-Lama4 depletion impaired the homing of lymphocytes to LNs in homeostasis and after allografting. Alloantigen-specific T cells proliferated, were activated to greater degrees in LNs lacking FRC-Lama4, and were more prone to differentiate into effector phenotypes relative to the Treg phenotype. In murine cardiac transplantation, tolerogenic immunosuppression was not effective in FRC-Lama4 recipients, which produced more alloantibodies than WT. After lung transplantation, FRC-Lama4-KO mice had more severe graft rejection with fewer Tregs in their LNs. Overall, FRC-Lama4 critically contributes to a tolerogenic LN niche by supporting T cell migration, constraining T cell activation and proliferation, and promoting Treg differentiation. Hence, it serves as a therapeutic target for immunoengineering.

The Role of Podoplanin in the Immune System and Inflammation

Podoplanin is a small cell-surface mucin-like glycoprotein that participates in multiple physiological and pathological processes. Podoplanin exerts an important function in the immune response and is upregulated in fibroblasts, macrophages, T helper cells, and epithelial cells during inflammation. Herein, we summarize the latest knowledge on the functional expression of podoplanin in the immune system and review the contribution of podoplanin to several inflammatory diseases. Furthermore, we discuss podoplanin as a novel therapeutic target for various inflammatory diseases.

Strong influenza-induced TFH generation requires CD4 effectors to recognize antigen locally and receive signals from continuing infection

Influenza infection elicits strong, long-lived protective antibodies, but most current influenza vaccines give weaker, short-lived protection. We noted that live virus is still replicating, making antigen and causing inflammation at 7 d postinfection (dpi), while an inactivated vaccine provides antigen for at most 4 dpi. We show that the generation of key T follicular helper cells (T_FH) requires they recognize antigen locally at 6 dpi in the presence of ongoing viral infection. This creates a checkpoint that restricts T_FH responses to dangerous infections that persist through the checkpoint. Using a live attenuated vaccine, akin to Flumist, we found that adding a second dose at 6 d generated a strong T_FH response, suggesting an approach to improve vaccine strategies.

While influenza infection induces robust, long-lasting, antibody responses and protection, including the T follicular helper cells (T_FH) required to drive B cell germinal center (GC) responses, most influenza vaccines do not. We investigated the mechanisms that drive strong T_FH responses during infection. Infection induces viral replication and antigen (Ag) presentation lasting through the CD4 effector phase, but Ag and pathogen recognition receptor signals are short-lived after vaccination. We analyzed the need for both infection and Ag presentation at the effector phase, using an in vivo sequential transfer model to time their availability. Differentiation of CD4 effectors into T_FH and GC-T_FH required that they recognize Ag locally in the site of T_FH development, at the effector phase, but did not depend on specific Ag-presenting cells (APCs). In addition, concurrent signals from infection were necessary even when sufficient Ag was presented. Providing these signals with a second dose of live attenuated influenza vaccine at the effector phase drove T_FH and GC-T_FH development equivalent to live infection. The results suggest that vaccine approaches can induce strong T_FH development that supports GC responses akin to infection, if they supply these effector phase signals at the right time and site. We suggest that these requirements create a checkpoint that ensures T_FH only develop fully when infection is still ongoing, thereby avoiding unnecessary, potentially autoimmune, responses.

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.

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.

Combined immunodeficiency with autoimmunity caused by a homozygous missense mutation in inhibitor of nuclear factor 𝛋B kinase alpha (IKKα)

[Figure: see text].

Fibroblasts and macrophages: Collaborators in tissue homeostasis

Fibroblasts and macrophages are universal cell types across all mammalian tissues. These cells differ in many ways including their cellular origins; dynamics of renewal, recruitment, and motility within tissues; roles in tissue structure and secretion of signaling molecules; and contributions to the activation and progression of immune responses. However, many of the features that make these two cell types unique are not opposing, but instead complementary. This review will present cell-cell communication in this context and discuss how complementarity makes fibroblasts and macrophages highly compatible partners in the maintenance of tissue homeostasis.

Macrophage-fibroblast circuits in the spleen

Macrophages are an integral part of all organs in the body, where they contribute to immune surveillance, protection, and tissue-specific homeostatic functions. This is facilitated by so-called niches composed of macrophages and their surrounding stroma. These niches structurally anchor macrophages and provide them with survival factors and tissue-specific signals that imprint their functional identity. In turn, macrophages ensure appropriate functioning of the niches they reside in. Macrophages thus form reciprocal, mutually beneficial circuits with their cellular niches. In this review, we explore how this concept applies to the spleen, a large secondary lymphoid organ whose primary functions are to filter the blood and regulate immunity. We first outline the splenic micro-anatomy, the different populations of splenic fibroblasts and macrophages and their respective contribution to protection of and key physiological processes occurring in the spleen. We then discuss firmly established and potential cellular circuits formed by splenic macrophages and fibroblasts, with an emphasis on the molecular cues underlying their crosstalk and their relevance to splenic functionality. Lastly, we conclude by considering how these macrophage-fibroblast circuits might be impaired by aging, and how understanding these changes might help identify novel therapeutic avenues with the potential of restoring splenic functions in the elderly.

Regulation of murine B lymphopoiesis by stromal cells

B lymphocytes are crucial for the body's humoral immune response, secreting antibodies generated against foreign antigens to fight infection. Adult murine B lymphopoiesis is initiated in the bone marrow and additional maturation occurs in the spleen. In both these organs, B lymphopoiesis involves interactions with numerous different non-hematopoietic cells, also known as stromal or microenvironment cells, which provide migratory, maturation, and survival signals. A variety of conditional knockout and transgenic mouse models have been used to identify the roles of distinct microenvironment cell types in the regulation of B lymphopoiesis. These studies have revealed that mesenchymal lineage cells and endothelial cells comprise the non-hematopoietic microenvironment cell types that support B lymphopoiesis in the bone marrow. In the spleen, various types of stromal cells and endothelial cells contribute to B lymphocyte maturation. More recently, comprehensive single cell RNA-seq studies have also been used to identify clusters of stromal cell types in the bone marrow and spleen, which will aid in further identifying key regulators of B lymphopoiesis. Here, we review the different types of microenvironment cells and key extrinsic regulators that are known to be involved in the regulation of murine B lymphopoiesis in the bone marrow and spleen.

LN-Derived Fibroblastic Reticular Cells and Their Impact on T Cell Response—A Systematic Review

Fibroblastic reticular cells (FRCs), usually found and isolated from the T cell zone of lymph nodes, have recently been described as much more than simple structural cells. Originally, these cells were described to form a conduit system called the “reticular fiber network” and for being responsible for transferring the lymph fluid drained from tissues through afferent lymphatic vessels to the T cell zone. However, nowadays, these cells are described as being capable of secreting several cytokines and chemokines and possessing the ability to interfere with the immune response, improving it, and also controlling lymphocyte proliferation. Here, we performed a systematic review of the several methods employed to investigate the mechanisms used by fibroblastic reticular cells to control the immune response, as well as their ability in determining the fate of T cells. We searched articles indexed and published in the last five years, between 2016 and 2020, in PubMed, Scopus, and Cochrane, following the PRISMA guidelines. We found 175 articles published in the literature using our searching strategies, but only 24 articles fulfilled our inclusion criteria and are discussed here. Other articles important in the built knowledge of FRCs were included in the introduction and discussion. The studies selected for this review used different strategies in order to access the contribution of FRCs to different mechanisms involved in the immune response: 21% evaluated viral infection in this context, 13% used a model of autoimmunity, 8% used a model of GvHD or cancer, 4% used a model of Ischemic-reperfusion injury (IRI). Another four studies just targeted a particular signaling pathway, such as MHC II expression, FRC microvesicles, FRC secretion of IL-15, FRC network, or ablation of the lysophosphatidic acid (LPA)-producing ectoenzyme autotaxin. In conclusion, our review shows the strategies used by several studies to isolate and culture fibroblastic reticular cells, the models chosen by each one, and dissects their main findings and implications in homeostasis and disease.

LN-Derived Fibroblastic Reticular Cells and Their Impact on T Cell Response-A Systematic Review

Fibroblastic reticular cells (FRCs), usually found and isolated from the T cell zone of lymph nodes, have recently been described as much more than simple structural cells. Originally, these cells were described to form a conduit system called the “reticular fiber network” and for being responsible for transferring the lymph fluid drained from tissues through afferent lymphatic vessels to the T cell zone. However, nowadays, these cells are described as being capable of secreting several cytokines and chemokines and possessing the ability to interfere with the immune response, improving it, and also controlling lymphocyte proliferation. Here, we performed a systematic review of the several methods employed to investigate the mechanisms used by fibroblastic reticular cells to control the immune response, as well as their ability in determining the fate of T cells. We searched articles indexed and published in the last five years, between 2016 and 2020, in PubMed, Scopus, and Cochrane, following the PRISMA guidelines. We found 175 articles published in the literature using our searching strategies, but only 24 articles fulfilled our inclusion criteria and are discussed here. Other articles important in the built knowledge of FRCs were included in the introduction and discussion. The studies selected for this review used different strategies in order to access the contribution of FRCs to different mechanisms involved in the immune response: 21% evaluated viral infection in this context, 13% used a model of autoimmunity, 8% used a model of GvHD or cancer, 4% used a model of Ischemic-reperfusion injury (IRI). Another four studies just targeted a particular signaling pathway, such as MHC II expression, FRC microvesicles, FRC secretion of IL-15, FRC network, or ablation of the lysophosphatidic acid (LPA)-producing ectoenzyme autotaxin. In conclusion, our review shows the strategies used by several studies to isolate and culture fibroblastic reticular cells, the models chosen by each one, and dissects their main findings and implications in homeostasis and disease.

Gremlin 1+ fibroblastic niche maintains dendritic cell homeostasis in lymphoid tissues

Fibroblastic reticular cells (FRCs) are specialized stromal cells that define tissue architecture and regulate lymphocyte compartmentalization, homeostasis, and innate and adaptive immunity in secondary lymphoid organs (SLOs). In the present study, we used single-cell RNA sequencing (scRNA-seq) of human and mouse lymph nodes (LNs) to identify a subset of T cell-zone FRCs defined by the expression of Gremlin1 (Grem1) in both species. Grem1-CreER^T2 knock-in mice enabled localization, multi-omics characterization and genetic depletion of Grem1⁺ FRCs. Grem1⁺ FRCs primarily localize at T-B cell junctions of SLOs, neighboring pre-dendritic cells and conventional dendritic cells (cDCs). As such, their depletion resulted in preferential loss and decreased homeostatic proliferation and survival of resident cDCs and compromised T cell immunity. Trajectory analysis of human LN scRNA-seq data revealed expression similarities to murine FRCs, with GREM1⁺ cells marking the endpoint of both trajectories. These findings illuminate a new Grem1⁺ fibroblastic niche in LNs that functions to maintain the homeostasis of lymphoid tissue-resident cDCs.

The metabolism-modulating activity of IL-17 signaling in health and disease

IL-17 was discovered nearly 30 yr ago, but it has only been recently appreciated that a key function of this cytokine is to orchestrate cellular and organismal metabolism. Indeed, metabolic regulation is integrated into both the physiological and the pathogenic aspects of IL-17 responses. Thus, understanding the interplay between IL-17 and downstream metabolic processes could ultimately inform therapeutic opportunities for diseases involving IL-17, including some not traditionally linked to this cytokine pathway. Here, we discuss the emerging pathophysiological roles of IL-17 related to cellular and organismal metabolism, including metabolic regulation of IL-17 signal transduction.

The Strategies and Challenges of CCR5 Gene Editing in Hematopoietic Stem and Progenitor Cells for the Treatment of HIV

HIV infection continues to be a serious health issue with an alarming global spread, owing to the fact that attempts at developing an effective vaccine or a permanent cure remains futile. So far, the only available treatment for the clinical management of HIV is the combined Anti-Retroviral Therapy (cART), but the long-term cART is associated with metabolic changes, organ damages, and development and transmission of drug resistant HIV strains. Thus, there is a need for the development of one-time curative treatment for HIV infection. The allogeneic transplantation with the Hematopoietic Stem and Progenitor cells (HSPCs) having 32 bp deletion in Chemokine receptor 5 gene (CCR5 Δ32) demonstrated successful HIV remission in the Berlin and London patients, and highlighted that transplantation of CCR5 null HSPCs is a promising approach for a long- term HIV remission. The advent of gene editing technologies offers a new choice of generating ex vivo CCR5 ablated allogeneic or autologous HSPCs for stem cell transplantation into HIV patients. Many groups are attempting CCR5 disruption in HSPCs using various gene-editing strategies. At least two such studies, involving CCR5 gene editing in HSPCs have entered the clinical trials. This review aims to outline the strategies taken for CCR5 gene editing and discuss the challenges associated with the development of CCR5 manipulated HSPCs for the gene therapy of HIV infection.

Structural cells are key regulators of organ-specific immune responses

The mammalian immune system implements a remarkably effective set of mechanisms for fighting pathogens¹. Its main components are haematopoietic immune cells, including myeloid cells that control innate immunity, and lymphoid cells that constitute adaptive immunity². However, immune functions are not unique to haematopoietic cells, and many other cell types display basic mechanisms of pathogen defence^3-5. To advance our understanding of immunology outside the haematopoietic system, here we systematically investigate the regulation of immune genes in the three major types of structural cells: epithelium, endothelium and fibroblasts. We characterize these cell types across twelve organs in mice, using cellular phenotyping, transcriptome sequencing, chromatin accessibility profiling and epigenome mapping. This comprehensive dataset revealed complex immune gene activity and regulation in structural cells. The observed patterns were highly organ-specific and seem to modulate the extensive interactions between structural cells and haematopoietic immune cells. Moreover, we identified an epigenetically encoded immune potential in structural cells under tissue homeostasis, which was triggered in response to systemic viral infection. This study highlights the prevalence and organ-specific complexity of immune gene activity in non-haematopoietic structural cells, and it provides a high-resolution, multi-omics atlas of the epigenetic and transcriptional networks that regulate structural cells in the mouse.

Distinctive Subpopulations of Stromal Cells Are Present in Human Lymph Nodes Infiltrated with Melanoma

Metastasis of human tumors to lymph nodes (LN) is a universally negative prognostic factor. LN stromal cells (SC) play a crucial role in enabling T-cell responses, and because tumor metastases modulate their structure and function, this interaction may suppress immune responses to tumor antigens. The SC subpopulations that respond to infiltration of malignant cells into human LNs have not been defined. Here, we identify distinctive subpopulations of CD90⁺ SCs present in melanoma-infiltrated LNs and compare them with their counterparts in normal LNs. The first population (CD90⁺ podoplanin⁺ CD105⁺ CD146⁺ CD271⁺ VCAM-1⁺ ICAM-1⁺ α-SMA⁺) corresponds to fibroblastic reticular cells that express various T-cell modulating cytokines, chemokines, and adhesion molecules. The second (CD90⁺ CD34⁺ CD105⁺ CD271⁺) represents a novel population of CD34⁺ SCs embedded in collagenous structures, such as the capsule and trabeculae, that predominantly produce extracellular matrix. We also demonstrated that these two SC subpopulations are distinct from two subsets of human LN pericytes, CD90⁺ CD146⁺ CD36⁺ NG2^- pericytes in the walls of high endothelial venules and other small vessels, and CD90⁺ CD146⁺ NG2⁺ CD36^- pericytes in the walls of larger vessels. Distinguishing between these CD90⁺ SC subpopulations in human LNs allows for further study of their respective impact on T-cell responses to tumor antigens and clinical outcomes.

Vascular Endothelial Cells and Innate Immunity

In addition to the roles of endothelial cells (ECs) in physiological processes, ECs actively participate in both innate and adaptive immune responses. We previously reported that, in comparison to macrophages, a prototypic innate immune cell type, ECs have many innate immune functions that macrophages carry out, including cytokine secretion, phagocytic function, antigen presentation, pathogen-associated molecular patterns-, and danger-associated molecular patterns-sensing, proinflammatory, immune-enhancing, anti-inflammatory, immunosuppression, migration, heterogeneity, and plasticity. In this highlight, we introduce recent advances published in both ATVB and many other journals: (1) several significant characters classify ECs as novel immune cells not only in infections and allograft transplantation but also in metabolic diseases; (2) several new receptor systems including conditional danger-associated molecular pattern receptors, nonpattern receptors, and homeostasis associated molecular patterns receptors contribute to innate immune functions of ECs; (3) immunometabolism and innate immune memory determine the innate immune functions of ECs; (4) a great induction of the immune checkpoint receptors in ECs during inflammations suggests the immune tolerogenic functions of ECs; and (5) association of immune checkpoint inhibitors with cardiovascular adverse events and cardio-oncology indicates the potential contributions of ECs as innate immune cells.

Lymphatic endothelial cells prime naïve CD8+ T cells into memory cells under steady-state conditions

Lymphatic endothelial cells (LECs) chemoattract naïve T cells and promote their survival in the lymph nodes, and can cross-present antigens to naïve CD8+ T cells to drive their proliferation despite lacking key costimulatory molecules. However, the functional consequence of LEC priming of CD8+ T cells is unknown. Here, we show that while many proliferating LEC-educated T cells enter early apoptosis, the remainders comprise a long-lived memory subset, with transcriptional, metabolic, and phenotypic features of central memory and stem cell-like memory T cells. In vivo, these memory cells preferentially home to lymph nodes and display rapid proliferation and effector differentiation following memory recall, and can protect mice against a subsequent bacterial infection. These findings introduce a new immunomodulatory role for LECs in directly generating a memory-like subset of quiescent yet antigen-experienced CD8+ T cells that are long-lived and can rapidly differentiate into effector cells upon inflammatory antigenic challenge.

Lymphocyte perform reverse adhesive haptotaxis mediated by integrins LFA-1

Cell Guidance by anchored molecules, or haptotaxis, is crucial in development, immunology and cancer. Adhesive haptotaxis, or guidance by adhesion molecules, is well established for mesenchymal cells like fibroblasts, whereas its existence remains unreported for amoeboid cells that require less or no adhesion to migrate. We show here in vitro that amoeboid human T lymphocytes develop adhesive haptotaxis versus densities of integrin ligands expressed by high endothelial venules. Moreover, lymphocytes orient towards increasing adhesion with VLA-4 integrins, like all mesenchymal cells, but towards decreasing adhesion with LFA-1 integrins, which has never been observed. This counterintuitive ‘reverse haptotaxis’ cannot be explained with the existing mesenchymal mechanisms of competition between cells’ pulling edges or of lamellipodia growth activated by integrins, which favor orientation towards increasing adhesion. Mechanisms and functions of amoeboid adhesive haptotaxis remain unclear, however multidirectional integrin-mediated haptotaxis may operate around transmigration ports on endothelium, stromal cells in lymph nodes, and inflamed tissue where integrin ligands are spatially modulated.

Pathobiological Interactions of Local Bone Marrow Renin-Angiotensin System and Central Nervous System in Systemic Arterial Hypertension

Circulating renin-angiotensin system (RAS) and local paracrin-autocrin-intracrin tissue-based RAS participate in numerous pathobiological events. Pro-inflammatory, pro-fibrotic, and pro-thrombotic consequences associated with local RAS activation have been detected at cellular and molecular level. Regenerative progenitor cell therapy in response to RAS modulating pharmacotherapy has emerged as an adjunct in the context of endothelial cell injury and regeneration to improve regeneration of the vascular endothelium. Local hematopoietic bone marrow (BM) RAS symbolizes the place of cross-interaction between vascular biology and cellular events from embryogenesis to definitive hematopoiesis underlying vascular atherosclerosis. The BM microenvironment also contains Mas receptors, which control the proliferative role of Ang 1-7 on hematopoietic stem cells. Ang 1-7 is produced from Ang-II or Ang-I with the help of ACE2. Various tissues and organs also have an effect on the RAS system. The leukocytes contain and synthesize immunoreactive angiotensinogen species capable of producing angiotensin in the basal state or after incubation with renin. The significance of RAS employment in atherosclerosis and hypertension was indicated by novel bidirectional Central Nervous System (CNS) RAS-BM RAS communications. Myeloid cells generated within the context of hematopoietic BM RAS are considered as the initiators and decision shapers in atherosclerosis. Macrophages in the atherosclerotic lesions contain angiotensin peptides by which RAS blockers inhibit monocyte activation and adherence. Furthermore, vascular biology in relation to inflammation and neoplasia is also affected by local tissue RAS. The purpose of this article is to outline interactions of circulating and local angiotensin systems, especially local bone marrow RAS, in the vascular pathobiological microenvironment of CNS.

The Role of Endothelial Cells and TNF-Receptor Superfamily Members in Lymphoid Organogenesis and Function During Health and Inflammation

Lymph nodes (LNs) are crucial for the orchestration of immune responses. LN reactions depend on interactions between incoming and local immune cells, and stromal cells. To mediate these cellular interactions an organized vascular network within the LN exists. In general, the LN vasculature can be divided into two components: blood vessels, which include the specialized high endothelial venules that recruit lymphocytes from the bloodstream, and lymphatic vessels. Signaling via TNF receptor (R) superfamily (SF) members has been implicated as crucial for the development and function of LNs and the LN vasculature. In recent years the role of cell-specific signaling of TNFRSF members in different endothelial cell (EC) subsets and their roles in development and maintenance of lymphoid organs has been elucidated. Here, we discuss recent insights into EC-specific TNFRSF member signaling and highlight its importance in different EC subsets in LN organogenesis and function during health, and in lymphocyte activation and tertiary lymphoid structure formation during inflammation.

Fibroblastic reticular cells enhance T cell metabolism and survival via epigenetic remodeling

Lymph node fibroblastic reticular cells (FRCs) respond to signals from activated T cells by releasing nitric oxide, which inhibits T cell proliferation and restricts the size of the expanding T cell pool. Whether interactions with FRCs also support the function or differentiation of activated CD8⁺ T cells is not known. Here we report that encounters with FRCs enhanced cytokine production and remodeled chromatin accessibility in newly activated CD8⁺ T cells via interleukin-6. These epigenetic changes facilitated metabolic reprogramming and amplified the activity of pro-survival pathways through differential transcription factor activity. Accordingly, FRC conditioning significantly enhanced the persistence of virus-specific CD8⁺ T cells in vivo and augmented their differentiation into tissue-resident memory T cells. Our study demonstrates that FRCs play a role beyond restricting T cell expansion-they can also shape the fate and function of CD8⁺ T cells.

Lymphotoxin-Beta Receptor Signaling Is Crucial for the Vascularization of Transplanted Metanephros

This commentary highlights the article by Francipane et al that studied the molecular signals supporting kidney vascularization in host lymphoid sites and omenta.

Role of Cell-Intrinsic and Environmental Age-Related Changes in Altered Maintenance of Murine T Cells in Lymphoid Organs

Age-related changes in primary lymphoid organs are well described. Less is known about age-related changes affecting peripheral lymphoid organs, although defects in old peripheral lymph nodes (pLNs) were recently described in both steady state and during viral infection. To address whether such pLN defects were intrinsic to old T cells or extrinsic (due to aging microenvironment), we employed heterochronic parabiosis. We found no age-related intrinsic or extrinsic barriers to T cell circulation and seeding of pLN, spleen, and bone marrow. However, heterochronic parabiosis failed to improve cellularity of old pLN, suggesting an environment-based limit on pLN cellularity. Furthermore, upon parabiosis, pLN of the adult partner exhibited reduced, old-like stromal and T cell cellularity, which was restored following separation of parabionts. Decay measurement of adult and old T cell subsets following separation of heterochronic parabionts delineated both T cell-intrinsic and environmental changes in T cell maintenance. Moreover, parabiotic separation revealed differences between CD4 and CD8 T cell subset maintenance with aging, the basis of which will require further investigation. Reasons for this asymmetric and subset-specific pattern of differential maintenance are discussed in light of possible age-related changes in lymph nodes as the key sites for peripheral T cell maintenance.

Attenuation of chronic antiviral T-cell responses through constitutive COX2-dependent prostanoid synthesis by lymph node fibroblasts

Lymphoid T-zone fibroblastic reticular cells (FRCs) actively promote T-cell trafficking, homeostasis, and expansion but can also attenuate excessive T-cell responses via inducible nitric oxide (NO) and constitutive prostanoid release. It remains unclear how these FRC-derived mediators dampen T-cell responses and whether this occurs in vivo. Here, we confirm that murine lymph node (LN) FRCs produce prostaglandin E₂ (PGE₂) in a cyclooxygenase-2 (COX2)-dependent and inflammation-independent fashion. We show that this COX2/PGE₂ pathway is active during both strong and weak T-cell responses, in contrast to NO, which only comes into play during strong T-cell responses. During chronic infections in vivo, PGE₂-receptor signaling in virus-specific cluster of differentiation (CD)8 cytotoxic T cells was shown by others to suppress T-cell survival and function. Using COX2^flox/flox mice crossed to mice expressing Cre recombinase expression under control of the CC chemokine ligand (CCL19) promoter (CCL19cre), we now identify CCL19⁺ FRC as the critical source of this COX2-dependent suppressive factor, suggesting PGE₂-expressing FRCs within lymphoid tissues are an interesting therapeutic target to improve T-cell–mediated pathogen control during chronic infection.

Fibroblasts in secondary lymphoid organs can be active participants in adaptive immunity, often enhancing T-cell responses. This study shows how these fibroblasts dampen T-cell responses via the constitutive production of the COX2-dependent prostaglandin PGE2, including during persistent viral infection.

SIRPα+ dendritic cells promote the development of fibroblastic reticular cells in murine peripheral lymph nodes

Nonhematopoietic stromal cells contribute to the organization and homeostasis of secondary lymphoid organs by producing cytokines and chemokines. The development and maintenance of these stromal cells are thought to be regulated by innate immune cells. Indeed, we recently showed that signal regulatory protein α (SIRPα)-positive dendritic cells (DCs) are essential for the proliferation and survival of podoplanin (Pdpn)-positive fibroblastic reticular cells (FRCs) in mouse spleen. We have now established an in vitro culture system for lymph node stromal cells (LNSCs) isolated from mouse peripheral LNs. Activated DCs and TNF-α each promoted the proliferation of cultured LNSCs, most of which were found to be Pdpn⁺ FRCs. Furthermore, ablation of SIRPα in CD11c⁺ cells attenuated this effect of DCs on LNSC proliferation. Transplantation of activated DCs together with cultured LNSCs into the renal subcapsular space markedly increased the number of ER-TR7⁺ stromal cells as well as induced the accumulation of T cells and increased the expression of Ccl19 in the transplants. Ablation of SIRPα in CD11c⁺ cells greatly impaired the development of LN-like structure in the transplants. Our findings thus suggest that SIRPα⁺ DCs are important for the proliferation and differentiation of Pdpn⁺ FRCs in peripheral LNs.

Podoplanin in Inflammation and Cancer

Podoplanin is a small cell-surface mucin-like glycoprotein that plays a crucial role in the development of the alveoli, heart, and lymphatic vascular system. Emerging evidence indicates that it is also involved in the control of mammary stem-cell activity and biogenesis of platelets in the bone marrow, and exerts an important function in the immune response. Podoplanin expression is upregulated in different cell types, including fibroblasts, macrophages, T helper cells, and epithelial cells, during inflammation and cancer, where it plays important roles. Podoplanin is implicated in chronic inflammatory diseases, such as psoriasis, multiple sclerosis, and rheumatoid arthritis, promotes inflammation-driven and cancer-associated thrombosis, and stimulates cancer cell invasion and metastasis through a variety of strategies. To accomplish its biological functions, podoplanin must interact with other proteins located in the same cell or in neighbor cells. The binding of podoplanin to its ligands leads to modulation of signaling pathways that regulate proliferation, contractility, migration, epithelial⁻mesenchymal transition, and remodeling of the extracellular matrix. In this review, we describe the diverse roles of podoplanin in inflammation and cancer, depict the protein ligands of podoplanin identified so far, and discuss the mechanistic basis for the involvement of podoplanin in all these processes.

Identification of Stromal Cells in Spleen Which Support Myelopoiesis

Stromal cells in spleen organize tissue into red pulp, white pulp and marginal zone, and also interact with hematopoietic cells to regulate immune responses. This study has used phenotypic information of a previously described spleen stromal cell line called 5G3, which supports restricted hematopoiesis in vitro, to identify an equivalent stromal cell subset in vivo and to test its capacity to support hematopoiesis. Using stromal cell fractionation, phenotypic analysis, as well as cell growth and hematopoietic support assays, the Sca-1⁺gp38⁺Thy1.2⁺CD29⁺CD51⁺ fraction of spleen stroma has been identified as an equivalent stromal subset resembling the 5G3 cell counterpart. While heterogeneity may still exist within that subset, it has been shown to have superior hematopoietic support capacity compared with the 5G3 cell line, and all other spleen stromal cell fractions tested.

IgM's exit route

In this issue of JEM, Thierry et al. (https://doi.org/10.1084/jem.20180344) demonstrate that, once secreted by freshly activated plasmablasts, IgM leaves the lymph node via the microarchitecture of the fibroblastic reticular cell conduit. This work demonstrates how the very peculiar stromal compartment of lymphatic organs optimizes the systemic distribution of immune effectors.

Integrative Computational Modeling of the Lymph Node Stromal Cell Landscape

Adaptive immune responses develop in secondary lymphoid organs such as lymph nodes (LNs) in a well-coordinated series of interactions between migrating immune cells and resident stromal cells. Although many processes that occur in LNs are well understood from an immunological point of view, our understanding of the fundamental organization and mechanisms that drive these processes is still incomplete. The aim of systems biology approaches is to unravel the complexity of biological systems and describe emergent properties that arise from interactions between individual constituents of the system. The immune system is greater than the sum of its parts, as is the case with any sufficiently complex system. Here, we review recent work and developments of computational LN models with focus on the structure and organization of the stromal cells. We explore various mathematical studies of intranodal T cell motility and migration, their interactions with the LN-resident stromal cells, and computational models of functional chemokine gradient fields and lymph flow dynamics. Lastly, we discuss briefly the importance of hybrid and multi-scale modeling approaches in immunology and the technical challenges involved.

Tertiary Lymphoid Structures: Autoimmunity Goes Local

Tertiary lymphoid structures (TLS) are frequently observed in target organs of autoimmune diseases. TLS present features of secondary lymphoid organs such as segregated T and B cell zones, presence of follicular dendritic cell networks, high endothelial venules and specialized lymphoid fibroblasts and display the mechanisms to support local adaptive immune responses toward locally displayed antigens. TLS detection in the tissue is often associated with poor prognosis of disease, auto-antibody production and malignancy development. This review focuses on the contribution of TLS toward the persistence of the inflammatory drive, the survival of autoreactive lymphocyte clones and post-translational modifications, responsible for the pathogenicity of locally formed autoantibodies, during autoimmune disease development.

Cytotoxic chemotherapy reduces T cell trafficking to the spleen by downregulating the expression of C-C motif chemokine ligand 21 and C-C motif chemokine ligand 19

T cells serve an important role in the destruction of tumor cells and clearing of foreign pathogens. Previous studies have suggested that the T cell immune response of tumor-bearing patients is significantly lower than that of healthy people, and the principal reason for this is lymphocytopenia, which is caused by repeated cycles of chemotherapy. In addition to lymphocytopenia, the present study revealed that cytotoxic chemotherapy also weakens the homing ability of T cells to the T-cell zone of the spleen, which decreases the possibility of encounters between antigen-specific T cells and dendritic cells presenting the appropriate antigen, thereby weakening the immune response of T cells. These changes are attributed to the lower expression of C-C motif chemokine ligand 21 (CCL21) and C-C motif chemokine ligand 19 (CCL19) in the spleen of secondary lymphoid organs (SLOs). Finally, the present study identified that chemotherapy affects the function and survival of fibroblastic reticular cells in SLOs, which are the main source of CCL21 and CCL19. These observations aid us in further understanding the mechanism that is responsible for the decreased T cell immune response following repeated cycles of chemotherapy.

Exploring the role of stromal osmoregulation in cancer and disease using executable modelling

Osmotic regulation is a vital homoeostatic process in all cells and tissues. Cells initially respond to osmotic stresses by activating transmembrane transport proteins to move osmotically active ions. Disruption of ion and water transport is frequently observed in cellular transformations such as cancer. We report that genes involved in membrane transport are significantly deregulated in many cancers, and that their expression can distinguish cancer cells from normal cells with a high degree of accuracy. We present an executable model of osmotic regulation and membrane transport in mammalian cells, providing a mechanistic explanation for phenotype change in varied disease states, and accurately predicting behaviour from single cell expression data. We also predict key proteins involved in cellular transformation, SLC4A3 (AE3), and SLC9A1 (NHE1). Furthermore, we predict and verify a synergistic drug combination in vitro, of sodium and chloride channel inhibitors, which target the osmoregulatory network to reduce cancer-associated phenotypes in fibroblasts.

Repetitive ischemic injuries to the kidneys result in lymph node fibrosis and impaired healing

The contribution of the kidney-draining lymph node (KLN) to the pathogenesis of ischemia-reperfusion injury (IRI) of the kidney and its subsequent recovery has not been explored in depth. In addition, the mechanism by which repetitive IRI contributes to renal fibrosis remains poorly understood. Herein, we have found that IRI of the kidney is associated with expansion of high endothelial venules (HEVs) and activation of fibroblastic reticular cells (FRCs) in the KLN, as demonstrated by significant expansion in the extracellular matrix. The lymphotoxin α signaling pathway mediates activation of FRCs, and chronic treatment with lymphotoxin β receptor-immunoglobulin fusion protein (LTβr-Ig) resulted in marked alteration of the KLN as well as augmentation of renal fibrosis. Depletion of FRCs reduced T cell activation in the KLN and ameliorated renal injury in acute IRI. Repetitive renal IRI was associated with senescence of FRCs, fibrosis of the KLN, and renal scarring, which were ameliorated by FRC administration. Therefore, our study emphasizes the critical role of FRCs in both the initiation and repair phases of injury following IRI of the kidney.

Designed Surface Topographies Control ICAM-1 Expression in Tonsil-Derived Human Stromal Cells

Fibroblastic reticular cells (FRCs), the T-cell zone stromal cell subtype in the lymph nodes, create a scaffold for adhesion and migration of immune cells, thus allowing them to communicate. Although known to be important for the initiation of immune responses, studies about FRCs and their interactions have been impeded because FRCs are limited in availability and lose their function upon culture expansion. To circumvent these limitations, stromal cell precursors can be mechanotranduced to form mature FRCs. Here, we used a library of designed surface topographies to trigger FRC differentiation from tonsil-derived stromal cells (TSCs). Undifferentiated TSCs were seeded on a TopoChip containing 2176 different topographies in culture medium without differentiation factors, then monitored cell morphology and the levels of ICAM-1, a marker of FRC differentiation. We identified 112 and 72 surfaces that upregulated and downregulated, respectively, ICAM-1 expression. By monitoring cell morphology, and expression of the FRC differentiation marker ICAM-1 via image analysis and machine learning, we discovered correlations between ICAM-1 expression, cell shape and design of surface topographies and confirmed our findings by using flow cytometry. Our findings confirmed that TSCs are mechano-responsive cells and identified particular topographies that can be used to improve FRC differentiation protocols.

Inflammation induces two types of inflammatory dendritic cells in inflamed lymph nodes

The spatiotemporal regulation of immune cells in lymph nodes (LNs) is crucial for mounting protective T-cell responses, which are orchestrated by dendritic cells (DCs). However, it is unclear how the DC subsets are altered by the inflammatory milieu of LNs. Here, we show that the inflamed LNs of Listeria-infected mice are characterized by the clustering of neutrophils and monocytes and IFN-γ production. Significantly, the early inflammatory responses are coupled with the differentiation of not one, but two types of CD64⁺CD11c⁺MHCII⁺ inflammatory DCs. Through the assessment of chemokine receptor dependency, gene expression profiles, growth factor requirements and DC-specific lineage mapping, we herein unveil a novel inflammatory DC population (we termed ‘CD64⁺ cDCs’) that arises from conventional DCs (cDCs), distinguishable from CD64⁺ monocyte-derived DCs (moDCs) in inflamed LNs. We determined that Listeria-induced type I IFN is a critical inflammatory cue for the development of CD64⁺ cDCs but not CD64⁺ moDCs. Importantly, CD64⁺ cDCs displayed a higher potential to activate T cells than CD64⁺ moDCs, whereas the latter showed more robust expression of inflammatory genes. Although CD64⁺ and CD64⁻ cDCs were able to cross-present soluble antigens at a high dose to CD8⁺ T cells, CD64⁺ cDCs concentrated and cross-presented a minute amount of soluble antigens delivered via CD64 (FcγRI) as immune complexes. These findings reveal the role of early inflammatory responses in driving the differentiation of two inflammatory DC subsets empowered with distinct competencies.

Challenges for Integrating Immunotoxicology into the Twenty-First-Century Toxicology Testing Paradigm

An emerging emphasis on mechanism-focused and human-relevant alternatives to animal use in toxicology underlies the toxicology testing in the twenty-first-century initiative. Herein we describe in vitro high-throughput screening programs seeking to address this goal, as well as strategies established to integrate assay results to build weight of evidence in support of hazard assessment. Furthermore, we discuss unique challenges facing the application of such alternatives for assessing immunotoxicity given the complexity of immune responses. Addressing these challenges will require the development of novel in vitro assays that evaluate well-characterized biochemical processes involved in immune response to help inform on putative adverse outcomes in vivo.

Non-neoplastic histiocytic and dendritic cell disorders in lymph nodes

Benign and malignant proliferations of histiocytes and dendritic cells may be encountered in lymph nodes. Reactive histiocytic and dendritic cell infiltrates occur in response to diverse stimuli and in addition to causing lymphadenopathy, may be present unexpectedly in lymph nodes excised for other indications. This review summarizes the pathogenesis and histopathological features of the various non-neoplastic histiocytic and dendritic cell infiltrates that can occur in lymph nodes.

A short field guide to fibroblast function in immunity

Fibroblasts in secondary lymphoid organs, or fibroblastic reticular cells (FRC), are gate-keepers of immune responses. Here, we frame how these cells regulate immune responses via a three-part scheme in which FRC can setup, support or suppress immune responses. We also review how fibroblasts from non-lymphoid tissues influence immunity and highlight how they resemble and differ from FRC. Overall, we aim to focus attention on the emerging roles of lymphoid tissue and non-lymphoid tissue fibroblasts in control of innate and adaptive immunity.

SIRPα+ dendritic cells regulate homeostasis of fibroblastic reticular cells via TNF receptor ligands in the adult spleen

In secondary lymphoid organs, development and homeostasis of stromal cells such as podoplanin (Pdpn)-positive fibroblastic reticular cells (FRCs) are regulated by hematopoietic cells, but the cellular and molecular mechanisms of such regulation have remained unclear. Here we show that ablation of either signal regulatory protein α (SIRPα), an Ig superfamily protein, or its ligand CD47 in conventional dendritic cells (cDCs) markedly reduced the number of CD4⁺ cDCs as well as that of Pdpn⁺ FRCs and T cells in the adult mouse spleen. Such ablation also impaired the survival of FRCs as well as the production by CD4⁺ cDCs of tumor necrosis factor receptor (TNFR) ligands, including TNF-α, which was shown to promote the proliferation and survival of Pdpn⁺ FRCs. CD4⁺ cDCs thus regulate the steady-state homeostasis of FRCs in the adult spleen via the production of TNFR ligands, with the CD47-SIRPα interaction in cDCs likely being indispensable for such regulation.

Tumor Regulation of Lymph Node Lymphatic Sinus Growth and Lymph Flow in Mice and in Humans

The lymphatic vasculature collects and drains fluid and cells from the periphery through lymph nodes (LNs) for immune monitoring, and then returns lymph to the bloodstream. During immune responses LNs enlarge and remodel, featuring extensive growth of lymphatic sinuses (lymphangiogenesis). This LN lymphangiogenesis also arises in cancer, and is associated with altered lymph drainage through LNs. Studies of mouse solid tumor models identified lymphatic sinus growth throughout tumor-draining LNs (TDLNs), and increased lymph flow through the expanded sinuses. Mice developing B cell lymphomas also feature LN lymphangiogenesis and increased lymph flow, indicating that these changes occur in lymphoma as well as in solid tumors. These LN alterations may be key to promote tumor growth and metastasis to draining LNs and distant organs. Lymphatic sinus growth within the TDLN may suppress anti-tumor-immune responses, and/or the increased lymph drainage could promote metastasis to draining LNs and distant organs. Investigations of human cancers and lymphomas are now identifying TDLN lymphatic sinus growth and increased lymph flow, that correlate with metastasis and poor prognosis. Pathology assessment of TDLN lymphangiogenesis or noninvasive imaging of tumor lymph drainage thus could potentially be useful to assist with diagnosis and treatment decisions. Moreover, the expanded lymphatic sinuses and increased lymph flow could facilitate vaccine or drug delivery, to manipulate TDLN immune functioning or to treat metastases. The insights obtained thus far should encourage further investigation of the mechanisms and consequences of TDLN lymphatic sinus growth and lymph flow alterations in mouse cancer models, and in human cancer patients.