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.

Graft-versus-host disease is locally maintained in target tissues by resident progenitor-like T cells

In allogeneic hematopoietic stem cell transplantation, donor αβ T cells attack recipient tissues, causing graft-versus-host disease (GVHD), a major cause of morbidity and mortality. A central question has been how GVHD is sustained despite T cell exhaustion from chronic antigen stimulation. The current model for GVHD holds that disease is maintained through the continued recruitment of alloreactive effectors from blood into affected tissues. Here, we show, using multiple approaches including parabiosis of mice with GVHD, that GVHD is instead primarily maintained locally within diseased tissues. By tracking 1,203 alloreactive T cell clones, we fitted a mathematical model predicting that within each tissue a small number of progenitor T cells maintain a larger effector pool. Consistent with this, we identified a tissue-resident TCF-1+ subpopulation that preferentially engrafted, expanded, and differentiated into effectors upon adoptive transfer. These results suggest that therapies targeting affected tissues and progenitor T cells within them would be effective.

The aging of the immune system and its implications for transplantation

By the last third of life, most mammals, including humans, exhibit a decline in immune cell numbers, immune organ structure, and immune defense of the organism, commonly known as immunosenescence. This decline leads to clinical manifestations of increased susceptibility to infections, particularly those caused by emerging and reemerging microorganisms, which can reach staggering levels-infection with SARS-CoV-2 has been 270-fold more lethal to older adults over 80 years of age, compared to their 18-39-year-old counterparts. However, while this would be expected to be beneficial to situations where hyporeactivity of the immune system may be desirable, this is not always the case. Here, we discuss the cellular and molecular underpinnings of immunosenescence as they pertain to outcomes of solid organ and hematopoietic transplantation.

Lymph Node Fibroblastic Reticular Cells Attenuate Immune Responses Through Induction of Tolerogenic Macrophages at Early Stage of Transplantation

BACKGROUND

Fibroblastic reticular cells (FRCs) are a type of stromal cells located in the T zone in secondary lymphoid organs. Previous studies showed that FRCs possess the potential to promote myeloid differentiation. We aim to investigate whether FRCs in lymph nodes (LNs) could induce tolerogenic macrophage generation and further influence T-cell immunity at an early stage of allogeneic hematopoietic stem cell transplantation (allo-HSCT).

METHODS

LNs were assayed to confirm the existence of proliferating macrophages after allo-HSCT. Ex vivo-expanded FRCs and bone marrow cells were cocultured to verify the generation of macrophages. Real-time quantitative PCR and ELISA assays were performed to observe the cytokines expressed by FRC. Transcriptome sequencing was performed to compare the difference between FRC-induced macrophages (FMs) and conventional macrophages. Mixed lymphocyte reaction and the utilization of FMs in acute graft-versus-host disease (aGVHD) mice were used to test the inhibitory function of FMs in T-cell immunity in vitro and in vivo.

RESULTS

We found a large number of proliferating macrophages near FRCs in LNs with tolerogenic phenotype under allo-HSCT conditions. Neutralizing anti-macrophage colony-stimulating factor receptor antibody abolished FMs generation in vitro. Phenotypic analysis and transcriptome sequencing suggested FMs possessed immunoinhibitory function. Mixed lymphocyte reaction proved that FMs could inhibit T-cell activation and differentiation toward Th1/Tc1 cells. Injection of FMs in aGVHD mice effectively attenuated aGVHD severity and mortality.

CONCLUSIONS

This study has revealed a novel mechanism of immune regulation through the generation of FRC-induced tolerogenic macrophages in LNs at an early stage of allo-HSCT.

Fibroblastic reticular cells mitigate acute GvHD via MHCII-dependent maintenance of regulatory T cells

Acute graft versus host disease (aGvHD) is a life-threatening complication of allogeneic hematopoietic cell transplantation (allo-HCT) inflicted by alloreactive T cells primed in secondary lymphoid organs (SLOs) and subsequent damage to aGvHD target tissues. In recent years, Treg transfer and/or expansion has emerged as a promising therapy to modulate aGvHD. However, cellular niches essential for fostering Tregs to prevent aGvHD have not been explored. Here, we tested whether and to what extent MHC class II (MHCII) expressed on Ccl19+ fibroblastic reticular cells (FRCs) shape the donor CD4+ T cell response during aGvHD. Animals lacking MHCII expression on Ccl19-Cre-expressing FRCs (MHCIIΔCcl19) showed aberrant CD4+ T cell activation in the effector phase, resulting in exacerbated aGvHD that was associated with significantly reduced expansion of Foxp3+ Tregs and invariant NK T (iNKT) cells. Skewed Treg maintenance in MHCIIΔCcl19 mice resulted in loss of protection from aGvHD provided by adoptively transferred donor Tregs. In contrast, although FRCs upregulated costimulatory surface receptors, and although they degraded and processed exogenous antigens after myeloablative irradiation, FRCs were dispensable to activate alloreactive CD4+ T cells in 2 mouse models of aGvHD. In summary, these data reveal an immunoprotective, MHCII-mediated function of FRC niches in secondary lymphoid organs (SLOs) after allo-HCT and highlight a framework of cellular and molecular interactions that regulate CD4+ T cell alloimmunity.

Immune function and dysfunction are determined by lymphoid tissue efficacy

Lymphoid tissue returns to a steady state once each immune response is resolved, and although this occurs multiple times throughout life, its structural integrity and functionality remain unaffected. Stromal cells orchestrate cellular interactions within lymphoid tissue, and any changes to the microenvironment can have detrimental outcomes and drive disease. A breakdown in lymphoid tissue homeostasis can lead to a loss of tissue structure and function that can cause aberrant immune responses. This Review highlights recent advances in our understanding of lymphoid tissue function and remodelling in adaptive immunity and in disease states. We discuss the functional role of lymphoid tissue in disease progression and explore the changes to lymphoid tissue structure and function driven by infection, chronic inflammatory conditions and cancer. Understanding the role of lymphoid tissues in immune responses to a wide range of pathologies allows us to take a fuller systemic view of disease progression.

Graft-versus-host disease: a disorder of tissue regeneration and repair

Regenerative failure at barrier surfaces and maladaptive repair leading to fibrosis are hallmarks of graft-versus-host disease (GVHD). Although immunosuppressive treatment can control inflammation, impaired tissue homeostasis leads to prolonged organ damage and impaired quality of life. In this Spotlight article, we review recent research that addresses the critical failures in tissue regeneration and repair that underpin treatment-resistant GVHD. We highlight current interventions designed to overcome these defects and provide our assessment of the future therapeutic landscape.

Lymph node fibroblastic reticular cells steer immune responses

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

BAFF promotes heightened BCR responsiveness and manifestations of chronic GVHD after allogeneic stem cell transplantation

Patients with chronic graft-versus-host disease (cGVHD) have increased B cell-activating factor (BAFF) levels, but whether BAFF promotes disease after allogeneic bone marrow transplantation (allo-BMT) remains unknown. In a major histocompatibility complex-mismatched model with cGVHD-like manifestations, we first examined B-lymphopenic μMT allo-BMT recipients and found that increased BAFF levels in cGVHD mice were not merely a reflection of B-cell number. Mice that later developed cGVHD had significantly increased numbers of recipient fibroblastic reticular cells with higher BAFF transcript levels. Increased BAFF production by donor cells also likely contributed to cGVHD, because BAFF transcript in CD4+ T cells from diseased mice and patients was increased. cGVHD manifestations in mice were associated with high BAFF/B-cell ratios and persistence of B-cell receptor (BCR)-activated B cells in peripheral blood and lesional tissue. By employing BAFF transgenic (Tg) mice donor cells, we addressed whether high BAFF contributed to BCR activation in cGVHD. BAFF increased NOTCH2 expression on B cells, augmenting BCR responsiveness to surrogate antigen and NOTCH ligand. BAFF Tg B cells had significantly increased protein levels of the proximal BCR signaling molecule SYK, and high SYK protein was maintained by BAFF after in vitro BCR activation or when alloantigen was present in vivo. Using T cell-depleted (BM only) BAFF Tg donors, we found that BAFF promoted cGVHD manifestations, circulating GL7+ B cells, and alloantibody production. We demonstrate that pathologic production of BAFF promotes an altered B-cell compartment and augments BCR responsiveness. Our findings compel studies of therapeutic targeting of BAFF and BCR pathways in patients with cGVHD.

National Institutes of Health Consensus Development Project on Criteria for Clinical Trials in Chronic Graft-versus-Host Disease: I. The 2020 Etiology and Prevention Working Group Report

Preventing chronic graft-versus-host disease (GVHD) remains challenging because the unique cellular and molecular pathways that incite chronic GVHD are poorly understood. One major point of intervention for potential prevention of chronic GVHD occurs at the time of transplantation when acute donor anti-recipient immune responses first set the events in motion that result in chronic GVHD. After transplantation, additional insults causing tissue injury can incite aberrant immune responses and loss of tolerance, further contributing to chronic GVHD. Points of intervention are actively being identified so that chronic GVHD initiation pathways can be targeted without affecting immune function. The major objective in the field is to continue basic studies and to translate what is learned about etiopathology to develop targeted prevention strategies that decrease the risk of morbid chronic GVHD without increasing the risks of cancer relapse or infection. Development of strategies to predict the risk of developing debilitating or deadly chronic GVHD is a high research priority. This working group recommends further interrogation into the mechanisms underpinning chronic GVHD development, and we highlight considerations for future trial design in prevention trials.

Graft-versus-host disease reduces lymph node display of tissue-restricted self-antigens and promotes autoimmunity

Acute graft-versus-host disease (GVHD) is initially triggered by alloreactive T cells, which damage peripheral tissues and lymphoid organs. Subsequent transition to chronic GVHD involves the emergence of autoimmunity, although the underlying mechanisms driving this process are unclear. Here, we tested the hypothesis that acute GVHD blocks peripheral tolerance of autoreactive T cells by impairing lymph node (LN) display of peripheral tissue–restricted antigens (PTAs). At the initiation of GVHD, LN fibroblastic reticular cells (FRCs) rapidly reduced expression of genes regulated by DEAF1, an autoimmune regulator-like transcription factor required for intranodal expression of PTAs. Subsequently, GVHD led to the selective elimination of the FRC population, and blocked the repair pathways required for its regeneration. We used a transgenic mouse model to show that the loss of presentation of an intestinal PTA by FRCs during GVHD resulted in the activation of autoaggressive T cells and gut injury. Finally, we show that FRCs normally expressed a unique PTA gene signature that was highly enriched for genes expressed in the target organs affected by chronic GVHD. In conclusion, acute GVHD damages and prevents repair of the FRC network, thus disabling an essential platform for purging autoreactive T cells from the repertoire.

Lymph Node Stromal Cells: Mapmakers of T Cell Immunity

Stromal cells (SCs) are strategically positioned in both lymphoid and nonlymphoid organs to provide a scaffold and orchestrate immunity by modulating immune cell maturation, migration and activation. Recent characterizations of SCs have expanded our understanding of their heterogeneity and suggested a functional specialization of distinct SC subsets, further modulated by the microenvironment. Lymph node SCs (LNSCs) have been shown to be particularly important in maintaining immune homeostasis and T cell tolerance. Under inflammation situations, such as viral infections or tumor development, SCs undergo profound changes in their numbers and phenotype and play important roles in contributing to either the activation or the control of T cell immunity. In this review, we highlight the role of SCs located in LNs in shaping peripheral T cell responses in different immune contexts, such as autoimmunity, viral and cancer immunity.

One-two punch injury to tolerance mechanisms in graft-versus-host disease

Chronic graft-versus-host disease (GVHD) is a major complication of allogeneic hematopoietic cell transplantation that resembles autoimmunity, with unclear pathogenesis and few effective therapeutic options. In this issue of the JCI, Dertschnig et al. used mouse models to investigate the basis of T cell autoreactivity following GVHD. Notably, GVHD caused irreversible damage to a population of tolerogenic stromal cells that display peripheral tissue-restricted antigens in lymph nodes, which impaired their capacity to purge and suppress autoreactive T cells. Together with damage to central tolerance mechanisms in the thymus, these findings outline a critical one-two punch injury that profoundly disrupts immune tolerance in this devastating disease.

Lymph node stromal cells: cartographers of the immune system

Lymph nodes (LNs) are strategically positioned at dedicated sites throughout the body to facilitate rapid and efficient immunity. Central to the structural integrity and framework of LNs, and the recruitment and positioning of leukocytes therein, are mesenchymal and endothelial lymph node stromal cells (LNSCs). Advances in the last decade have expanded our understanding and appreciation of LNSC heterogeneity, and the role they play in coordinating immunity has grown rapidly. In this review, we will highlight the functional contributions of distinct stromal cell populations during LN development in maintaining immune homeostasis and tolerance and in the activation and control of immune responses.

CD157: From Myeloid Cell Differentiation Marker to Therapeutic Target in Acute Myeloid Leukemia

Human CD157/BST-1 and CD38 are dual receptor-enzymes derived by gene duplication that belong to the ADP ribosyl cyclase gene family. First identified over 30 years ago as Mo5 myeloid differentiation antigen and 10 years later as Bone Marrow Stromal Cell Antigen 1 (BST-1), CD157 proved not to be restricted to the myeloid compartment and to have a diversified functional repertoire ranging from immunity to cancer and metabolism. Despite being a NAD⁺-metabolizing ectoenzyme anchored to the cell surface through a glycosylphosphatidylinositol moiety, the functional significance of human CD157 as an enzyme remains unclear, while its receptor role emerged from its discovery and has been clearly delineated with the identification of its high affinity binding to fibronectin. The aim of this review is to provide an overview of the immunoregulatory functions of human CD157/BST-1 in physiological and pathological conditions. We then focus on CD157 expression in hematological tumors highlighting its emerging role in the interaction between acute myeloid leukemia and extracellular matrix proteins and its potential utility for monoclonal antibody targeted therapy in this disease.

IL-7 Is the Limiting Homeostatic Factor that Constrains Homeostatic Proliferation of CD8+ T Cells after Allogeneic Stem Cell Transplantation and Graft-versus-Host Disease

Immune reconstitution after allogeneic hematopoietic stem cell transplantation relies primarily on homeostatic proliferation (HP) of mature T lymphocytes, but this process is typically impaired during graft-versus-host disease (GVHD). We previously showed that low IL-7 levels combined with lack of dendritic cell (DC) regeneration constrain CD4⁺ T cell HP during GVHD. However, it is not clear whether these alterations to the peripheral CD4⁺ T cell niche also contribute to impair CD8⁺ T cell regeneration during GVHD. We found that IL-7 therapy was sufficient for restoring CD8⁺ T cell HP in GVHD hosts while forcing DC regeneration with Flt3-L had only a modest effect on CD8⁺ T cell HP in IL-7 treated mice. Using bone marrow chimeras, we showed that HP of naïve CD8⁺ T cells is primarily regulated by MHC class I on radio-resistant stromal cells, yet optimal recovery of CD8⁺ T cell counts still requires expression of MHC class I on both radio-resistant and radio-sensitive hematopoietic cells. Thus, IL-7 level is the primary limiting factor that constrains naïve CD8⁺ T cell HP during GVHD, and accessibility of MHC class I on stromal cells explains how IL-7 therapy, as a single agent, can induce robust CD8 ⁺ T cell HP in the absence of DCs.

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.

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.

CD157: From immunoregulatory protein to potential therapeutic target

CD157/BST1 glycosylphosphatidylinositol-anchored glycoprotein is an evolutionary conserved dual-function receptor and β-NAD+-metabolizing ectoenzyme of the ADP-ribosyl cyclases gene family. Identified as bone marrow stromal cell and myeloid cell differentiation antigen, CD157 turned out to have a wider expression than originally assumed. The functional significance of human CD157 as an enzyme remains unclear, while it was well established in mouse models. Conversely, the receptor role of CD157 has been clearly delineated. In physiological conditions, CD157 is a key player in regulating leukocyte adhesion, migration and diapedesis. Underlying these functional roles is the ability of CD157 to bind with high affinity selected extracellular matrix components within their heparin-binding domains. CD157 binding to extracellular matrix promotes its interaction with β1 and β2-integrins and induces the organization of a multimolecular complex that is instrumental to the delivery of synergistic outside-in signals leading to optimal cell adhesion and migration, both in physiological and in pathological situations. CD157 also regulates cell adhesion and migration and is a marker of adverse prognosis in epithelial ovarian cancer and pleural mesothelioma. This review focuses on human CD157 expression and functions and provides an overview on its role in human pathology and its emerging potential as target for antibody-mediated immunotherapy.

Long-Lasting Graft-Derived Donor T Cells Contribute to the Pathogenesis of Chronic Graft-versus-Host Disease in Mice

Chronic graft-versus-host disease (cGVHD) is a major complication in long-term survivors of allogeneic hematopoietic stem cell transplantation (allo-HSCT). Graft-derived T cells (T_G) have been implicated in the induction of cGVHD; however, the extent of their contribution to the pathogenesis of cGVHD remains unclear. Using a mouse model of cGVHD, we demonstrate that T_G predominate over hematopoietic stem cell-derived T cells generated de novo (T_HSC) in cGVHD-affected organs such as the liver and lung even at day 63 after allo-HSCT. Persisting T_G, in particular CD8⁺ T_G, not only displayed an exhausted or senescent phenotype but also contained a substantial proportion of cells that had the potential to proliferate and produce inflammatory cytokines. Host antigens indirectly presented by donor HSC-derived hematopoietic cells were involved in the maintenance of T_G in the reconstituted host. Selective depletion of T_G in the chronic phase of disease resulted in the expansion of T_HSC and thus neither the survival nor histopathology of cGVHD was ameliorated. On the other hand, T_HSC depletion caused activation of T_G and resulted in a lethal T_G-mediated exacerbation of GVHD. The findings presented here clarify the pathological role of long-lasting T_G in cGVHD.

Leukocyte-Stromal Interactions Within Lymph Nodes

Lymph nodes play a crucial role in the formation and initiation of immune responses, allowing lymphocytes to efficiently scan for foreign antigens and serving as rendezvous points for leukocyte-antigen interactions. Here we describe the major stromal subsets found in lymph nodes, including fibroblastic reticular cells, lymphatic endothelial cells, blood endothelial cells, marginal reticular cells, follicular dendritic cells and other poorly defined subsets such as integrin alpha-7+ pericytes. We focus on biomedically relevant interactions with T cells, B cells and dendritic cells, describing pro-survival mechanisms of support for these cells, promotion of their migration and tolerance-inducing mechanisms that help keep the body free of autoimmune-mediated damage.

New Insights into Graft-Versus-Host Disease and Graft Rejection

Allogeneic transplantation of foreign organs or tissues has lifesaving potential, but can lead to serious complications. After solid organ transplantation, immune-mediated rejection mandates the use of prolonged global immunosuppression and limits the life span of transplanted allografts. After bone marrow transplantation, donor-derived immune cells can trigger life-threatening graft-versus-host disease. T cells are central mediators of alloimmune complications and the target of most existing therapeutic interventions. We review recent progress in identifying multiple cell types in addition to T cells and new molecular pathways that regulate pathogenic alloreactivity. Key discoveries include the cellular subsets that function as potential sources of alloantigens, the cross talk of innate lymphoid cells with damaged epithelia and with the recipient microbiome, the impact of the alarmin interleukin-33 on alloreactivity, and the role of Notch ligands expressed by fibroblastic stromal cells in alloimmunity. While refining our understanding of transplantation immunobiology, these findings identify new therapeutic targets and new areas of investigation.

Combination of anti-CD4 antibody treatment and donor lymphocyte infusion ameliorates graft-versus-host disease while preserving graft-versus-tumor effects in murine allogeneic hematopoietic stem cell transplantation

Allogeneic hematopoietic stem cell transplantation (allo‐HSCT) is not only a well‐established immunotherapy for hematologic malignancies, but is potentially useful for treating solid tumors refractory to available therapies. However, application of allo‐HSCT to solid tumors is limited, despite the beneficial antitumor effects, by the risk of graft‐versus‐host disease (GVHD). CD4⁺ T cells have been implicated in several aspects of GVHD, and suppress antitumor CD8⁺ T‐cell responses. In the present study, we investigated clinically applicable allo‐HSCT protocols designed to maximize antitumor effects while reducing the risk of GVHD. We used a mouse model of allo‐HSCT with s.c. tumors. We found that myeloablative conditioning was associated with better inhibition of tumor growth but with severe acute GVHD. Early treatment with anti‐CD4 mAb substantially ameliorated GVHD while preserving antitumor effects, leading to improved survival in myeloablative allo‐HSCT. Late treatment with anti‐CD4 mAb also ameliorated GVHD to some extent. Donor lymphocyte infusion in GVHD mice treated with anti‐CD4 mAb further suppressed tumor growth without exacerbating GVHD. Collectively, our results suggest that myeloablative allo‐HSCT followed by anti‐CD4 mAb treatment and donor lymphocyte infusion could be a potent and safe immunotherapy for patients with cancers refractory to available therapies.

Lymph node fibroblastic reticular cells in health and disease

Over the past decade, a series of discoveries relating to fibroblastic reticular cells (FRCs) — immunologically specialized myofibroblasts found in lymphoid tissue — has promoted these cells from benign bystanders to major players in the immune response. In this Review, we focus on recent advances regarding the immunobiology of lymph node-derived FRCs, presenting an updated view of crucial checkpoints during their development and their dynamic control of lymph node expansion and contraction during infection. We highlight the robust effects of FRCs on systemic B cell and T cell responses, and we present an emerging view of FRCs as drivers of pathology following acute and chronic viral infections. Lastly, we review emerging therapeutic advances that harness the immunoregulatory properties of FRCs.

Intestinal microbiota-related effects on graft-versus-host disease

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an increasingly important treatment for conditions including hematopoietic malignancies and inherited hematopoietic disorders, and is considered to be the most effective form of tumor immunotherapy available to date. However, graft-versus-host disease (GVHD) remains a major source of morbidity and mortality following allo-HSCT, and understanding the mechanisms of GVHD has been highlighted as a key research priority. During development of GVHD, activation of various immune cells, especially donor T cells, leads to damage of target organs including skin, liver, hematopoietic system, and of particular clinical importance, gut. In addition to histocompatibility complex differences between the donor and recipient, pretransplant conditioning with chemotherapy and irradiation also contributes to GVHD by damaging the gut, resulting in systemic exposure to microbial products normally confined to the intestinal lumen. The intestinal microbiota is a modulator of gastrointestinal immune homeostasis. It also promotes the maintenance of epithelial cells. Recent reports provide growing evidence of the impact of intestinal microbiota on GVHD pathophysiology. This review summarizes current knowledge of changes and effects of intestinal microbiota in the setting of allo-HSCT. We will also discuss potential future strategies of intestinal microbiota manipulation that might be advantageous in decreasing allo-HSCT-related morbidity and mortality.