Tissue-Specific Function of Lymph Node Fibroblastic Reticulum Cells

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.

Age-Related Changes in HAPLN1 Increase Lymphatic Permeability and Affect Routes of Melanoma Metastasis

Older patients with melanoma have lower rates of sentinel lymph node (LN) metastases yet paradoxically have inferior survival. Patient age correlated with an inability to retain Technetium radiotracer during sentinel LN biopsy in more than 1,000 patients, and high Technetium counts correlated to better survival. We hypothesized that loss of integrity in the lymphatic vasculature due to extracellular matrix (ECM) degradation might play a role. We have implicated HAPLN1 in age-dependent ECM degradation in the dermis. Here, we queried whether HAPLN1 could be altered in the lymphatic ECM. Lymphatic HAPLN1 expression was prognostic of long-term patient survival. Adding recombinant HAPLN1 to aged fibroblast ECMs in vitro reduced endothelial permeability via modulation of VE-cadherin junctions, whereas endothelial permeability was increased following HAPLN1 knockdown in young fibroblasts. In vivo, reconstitution of HAPLN1 in aged mice increased the number of LN metastases, but reduced visceral metastases. These data suggest that age-related changes in ECM can contribute to impaired lymphatics. SIGNIFICANCE: Our studies reveal that changes in the stroma during aging may influence the way tumor cells traffic through the lymphatic vasculature. Aging may dictate the route of metastatic dissemination of tumor cells, and understanding these changes may help to reveal targetable moieties in the aging tumor microenvironment.See related commentary by Marie and Merlino, p. 19.This article is highlighted in the In This Issue feature, p. 1.

Impaired lymph node stromal cell function during the earliest phases of rheumatoid arthritis

Background

Systemic autoimmunity can be present years before clinical onset of rheumatoid arthritis (RA). Adaptive immunity is initiated in lymphoid tissue where lymph node stromal cells (LNSCs) regulate immune responses through their intimate connection with leucocytes. We postulate that malfunctioning of LNSCs creates a microenvironment in which normal immune responses are not properly controlled, possibly leading to autoimmune disease. In this study we established an experimental model for studying the functional capacities of human LNSCs during RA development.

Methods

Twenty-four patients with RA, 23 individuals positive for autoantibodies but without clinical disease (RA risk group) and 14 seronegative healthy control subjects underwent ultrasound-guided inguinal lymph node (LN) biopsy. Human LNSCs were isolated and expanded in vitro for functional analyses. In analogous co-cultures consisting of LNSCs and peripheral blood mononuclear cells, αCD3/αCD28-induced T-cell proliferation was measured using carboxyfluorescein diacetate succinimidyl ester dilution.

Results

Fibroblast-like cells expanded from the LN biopsy comprised of fibroblastic reticular cells (gp38⁺CD31⁻) and double-negative (gp38⁻CD31⁻) cells. Cultured LNSCs stably expressed characteristic adhesion molecules and cytokines. Basal expression of C-X-C motif chemokine ligand 12 (CXCL12) was lower in LNSCs from RA risk individuals than in those from healthy control subjects. Key LN chemokines C-C motif chemokine ligand (CCL19), CCL21 and CXCL13 were induced in LNSCs upon stimulation with tumour necrosis factor-α and lymphotoxin α₁β₂, but to a lesser extent in LNSCs from patients with RA. The effect of human LNSCs on T-cell proliferation was ratio-dependent and altered in RA LNSCs.

Conclusions

Overall, we developed an experimental model to facilitate research on the role of LNSCs during the earliest phases of RA. Using this innovative model, we show, for the first time to our knowledge, that the LN stromal environment is changed during the earliest phases of RA, probably contributing to deregulated immune responses early in disease pathogenesis.

Electronic supplementary material

The online version of this article (10.1186/s13075-018-1529-8) contains supplementary material, which is available to authorized users.

Breast Carcinoma-associated Fibroblasts Share Similar Biomarker Profiles in Matched Lymph Node Metastasis

This study sought to understand the role of breast carcinoma-associated fibroblasts in the progression of cancer cells into lymph nodes. We compared fibroblasts of primary tumors and matched the involved lymph nodes to select fibroblast activation markers, namely α-smooth muscle actin (α-SMA), S100A4, and vimentin, as well as to determine the frequency of transforming growth factor β1, a pleiotropic cytokine that induces the differentiation of fibroblasts to myofibroblasts, and its downstream effectors: CXCR4 and p-AKT. We disposed samples of 80 primary invasive ductal carcinomas and matched the involved lymph nodes from 43 cases into 3 tissue microarrays, and analyzed stromal and tumor epithelial cells separately by immunohistochemistry. Control uninvolved lymph nodes were analyzed by whole-tissue sections. Cancer-associated fibroblast in lymph nodes with macrometastasis expressed similar profiles of vimentin, α-SMA, and S100A4 as those found in primary tumors. Cancer-associated fibroblast were uniformly estrogen receptor, progesterone receptor, HER-2, Ki-67, and p53 negative, but expressions of transforming growth factor β1 (TGFβ1), CXCR4, and p-AKT staining (62.3%, 52.4%, 65%, respectively) were equivalent between primary and lymph node metastasis (LNM) fibroblasts. A significant coexpression of TGFβ1 with p-AKT and CXCR4 in LNMs suggested the involvement of these proteins with TGFβ1 signaling. These biomarkers, including α-SMA and S100A4, were negative in fibroblasts of cancer-free lymph nodes, with the exception of vimentin. Our finding that expressions of biological markers were similar in fibroblasts of the primary tumors and in matched LNMs, but were absent in cancer-free lymph nodes, supports the assumption that the lymph node stroma mimics the microenvironment observed in primary tumors.

Human Lymph Node-Derived Fibroblastic and Double-Negative Reticular Cells Alter Their Chemokines and Cytokines Expression Profile Following Inflammatory Stimuli

Lymph node (LN) is a secondary lymphoid organ with highly organized and compartmentalized structure. LNs harbor B, T, and other cells among fibroblastic reticular cells (FRCs). FRCs are characterized by both podoplanin (PDPN/gp38) expression and by the lack of CD31 expression. FRCs are involved in several immune response processes but mechanisms underlying their function are still under investigation. Double-negative cells (DNCs), another cell population within LNs, are even less understood. They do not express PDPN or CD31, their localization within the LN is unknown, and their phenotype and function remain to be elucidated. This study evaluates the gene expression and cytokines and chemokines profile of human LN-derived FRCs and DNCs during homeostasis and following inflammatory stimuli. Cytokines and chemokines secreted by human FRCs and DNCs partially diverged from those identified in murine models that used similar stimulation. Cytokine and chemokine secretion and their receptors expression levels differed between stimulated DNCs and FRCs, with FRCs expressing a broader range of chemokines. Additionally, dendritic cells demonstrated increased migration toward FRCs, possibly due to chemokine-induced chemotaxis since migration was significantly decreased upon neutralization of secreted CCL2 and CCL20. Our study contributes to the understanding of the biology and functions of FRCs and DNCs and, accordingly, of the mechanisms involving them in immune cells activation and migration.

Stromal Fibroblasts in Tertiary Lymphoid Structures: A Novel Target in Chronic Inflammation

Tertiary lymphoid structures (TLS) are organized aggregates of lymphocytes, myeloid, and stromal cells that provide ectopic hubs for acquired immune responses. TLS share phenotypical and functional features with secondary lymphoid organs (SLO); however, they require persistent inflammatory signals to arise and are often observed at target sites of autoimmune disease, chronic infection, cancer, and organ transplantation. Over the past 10 years, important progress has been made in our understanding of the role of stromal fibroblasts in SLO development, organization, and function. A complex and stereotyped series of events regulate fibroblast differentiation from embryonic life in SLOs to lymphoid organ architecture observed in adults. In contrast, TLS-associated fibroblasts differentiate from postnatal, locally activated mesenchyme, predominantly in settings of inflammation and persistent antigen presentation. Therefore, there are critical differences in the cellular and molecular requirements that regulate SLO versus TLS development that ultimately impact on stromal and hematopoietic cell function. These differences may contribute to the pathogenic nature of TLS in the context of chronic inflammation and malignant transformation and offer a window of opportunity for therapeutic interventions in TLS associated pathologies.

Modes of Antigen Presentation by Lymph Node Stromal Cells and Their Immunological Implications

Antigen presentation is no longer the exclusive domain of cells of hematopoietic origin. Recent works have demonstrated that lymph node stromal cell (LNSC) populations, such as fibroblastic reticular cells, lymphatic and blood endothelial cells, not only provide a scaffold for lymphocyte interactions but also exhibit active immunomodulatory roles that are critical to mounting and resolving effective immune responses. Importantly, LNSCs possess the ability to present antigens and establish antigen-specific interactions with T cells. One example is the expression of peripheral tissue antigens, which are presented on major histocompatibility complex (MHC)-I molecules with tolerogenic consequences on T cells. Additionally, exogenous antigens, including self and tumor antigens, can be processed and presented on MHC-I complexes, which result in dysfunctional activation of antigen-specific CD8⁺ T cells. While MHC-I is widely expressed on cells of both hematopoietic and non-hematopoietic origins, antigen presentation via MHC-II is more precisely regulated. Nevertheless, LNSCs are capable of endogenously expressing, or alternatively, acquiring MHC-II molecules. Transfer of antigen between LNSC and dendritic cells in both directions has been recently suggested to promote tolerogenic roles of LNSCs on the CD4⁺ T cell compartment. Thus, antigen presentation by LNSCs is thought to be a mechanism that promotes the maintenance of peripheral tolerance as well as generates a pool of diverse antigen-experienced T cells for protective immunity. This review aims to integrate the current and emerging literature to highlight the importance of LNSCs in immune responses, and emphasize their role in antigen trafficking, retention, and presentation.

Shaping of the tumor microenvironment: Stromal cells and vessels

Lymphomas develop and progress in a specialized tissue microenvironment such as bone marrow as well as secondary lymphoid organs such as lymph node and spleen. The lymphoma microenvironment is characterized by a heterogeneous population of stromal cells, including fibroblastic reticular cells, nurse-like cells, mesenchymal stem cells, follicular dendritic cells, and inflammatory cells such as macrophages, T- and B-cells. These cell populations interact with the lymphoma cells to promote lymphoma growth, survival and drug resistance through multiple mechanisms. Angiogenesis is also recognized as an important factor associated with lymphoma progression. In recent years, we have learned that the interaction between the malignant and non-malignant cells is bidirectional and resembles, at least in part, the pattern seen between non-neoplastic lymphoid cells and the normal microenvironment of lymphoid organs. A summary of the current knowledge of lymphoma microenvironment focusing on the cellular components will be reviewed here.

Transcriptional profile of fibroblasts obtained from the primary site, lymph node and bone marrow of breast cancer patients

Cancer-associated fibroblasts (CAF) influence tumor development at primary as well as in metastatic sites, but there have been no direct comparisons of the transcriptional profiles of stromal cells from different tumor sites. In this study, we used customized cDNA microarrays to compare the gene expression profile of stromal cells from primary tumor (CAF, n = 4), lymph node metastasis (N+, n = 3) and bone marrow (BM, n = 4) obtained from breast cancer patients. Biological validation was done in another 16 samples by RT-qPCR. Differences between CAF vs N+, CAF vs BM and N+ vs BM were represented by 20, 235 and 245 genes, respectively (SAM test, FDR < 0.01). Functional analysis revealed that genes related to development and morphogenesis were overrepresented. In a biological validation set, NOTCH2 was confirmed to be more expressed in N+ (vs CAF) and ADCY2, HECTD1, HNMT, LOX, MACF1, SLC1A3 and USP16 more expressed in BM (vs CAF). Only small differences were observed in the transcriptional profiles of fibroblasts from the primary tumor and lymph node of breast cancer patients, whereas greater differences were observed between bone marrow stromal cells and the other two sites. These differences may reflect the activities of distinct differentiation programs.

Multifunctional roles of reticular fibroblastic cells: more than meets the eye?

Fibroblastic reticular cells (FRCs) are stromal cells found in secondary lymphoid organ. Despite its structural function in the lymph nodes being well established, recent studies indicate that the FRCs also play a key role in immunological processes, associated with cell transit, immune response, and cells activation quality, and contribute to peripheral tolerance. To this end, we focus this review on lymph nodes FRC characterization and discuss functional aspects such as production of cytokines and chemokines and their involvement in the immune response, seeking to establish whether certain subsets have a more functional specialization.

Trapping of naive lymphocytes triggers rapid growth and remodeling of the fibroblast network in reactive murine lymph nodes

Adaptive immunity is initiated in T-cell zones of secondary lymphoid organs. These zones are organized in a rigid 3D network of fibroblastic reticular cells (FRCs) that are a rich cytokine source. In response to lymph-borne antigens, draining lymph nodes (LNs) expand several folds in size, but the fate and role of the FRC network during immune response is not fully understood. Here we show that T-cell responses are accompanied by the rapid activation and growth of FRCs, leading to an expanded but similarly organized network of T-zone FRCs that maintains its vital function for lymphocyte trafficking and survival. In addition, new FRC-rich environments were observed in the expanded medullary cords. FRCs are activated within hours after the onset of inflammation in the periphery. Surprisingly, FRC expansion depends mainly on trapping of naïve lymphocytes that is induced by both migratory and resident dendritic cells. Inflammatory signals are not required as homeostatic T-cell proliferation was sufficient to trigger FRC expansion. Activated lymphocytes are also dispensable for this process, but can enhance the later growth phase. Thus, this study documents the surprising plasticity as well as the complex regulation of FRC networks allowing the rapid LN hyperplasia that is critical for mounting efficient adaptive immunity.

Dendritic cell sarcoma: a pooled analysis including 462 cases with presentation of our case series

Dendritic cell tumors are extremely rare and current knowledge on these tumors is limited. The characteristics of three dendritic cell sarcoma subtypes and their optimal treatment approaches are not fully clarified. We aimed to make a systematic review of the literature and enrich the current data with five new cases. Pooled analysis of 462 reported cases revealed that the tumor had no age, gender or racial predilection. Our analysis suggests that the young age, advanced stage, intraabdominal involvement and unfavorable histological features (i.e. large tumor size, absence of lymphoplasmacytic infiltration, coagulative necrosis, high mitotic count) may predict poor prognosis. Subtypes of this tumor have different clinical behaviors with interdigitating dendritic cell sarcoma being the most aggressive form. In general, surgery is the most effective treatment modality and adjuvant radiotherapy has no significant effect on overall survival of patients. The role of chemotherapy for the management of advanced disease is controversial.

Maturation of lymph node fibroblastic reticular cells from myofibroblastic precursors is critical for antiviral immunity

The stromal scaffold of the lymph node (LN) paracortex is built by fibroblastic reticular cells (FRCs). Conditional ablation of lymphotoxin-β receptor (LTβR) expression in LN FRCs and their mesenchymal progenitors in developing LNs revealed that LTβR-signaling in these cells was not essential for the formation of LNs. Although T cell zone reticular cells had lost podoplanin expression, they still formed a functional conduit system and showed enhanced expression of myofibroblastic markers. However, essential immune functions of FRCs, including homeostatic chemokine and interleukin-7 expression, were impaired. These changes in T cell zone reticular cell function were associated with increased susceptibility to viral infection. Thus, myofibroblasic FRC precursors are able to generate the basic T cell zone infrastructure, whereas LTβR-dependent maturation of FRCs guarantees full immunocompetence and hence optimal LN function during infection.

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Novel transgenic mouse model that targets FRCs in adult lymph nodes

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FRC-specific ablation of the LTβR did not abrogate LN development

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Myofibroblastic FRC precursors generate the basic infrastructure of the adult LN

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LTβR-mediated FRC maturation is critical for the maintenance of immunocompentence

COX-2 expression in stromal fibroblasts self-limits their numbers in lymph node inflammatory responses

We previously reported the expression of cyclooxygenase (COX)-2 in draining lymph nodes during carrageenin-induced pleurisy of rats. Here, we analyzed histological and immunohistochemical characteristics of COX-2-expressing cells. After carrageenin administration into the pleural cavity of rats, parathymic lymph nodes were enlarged beginning at 8h and peaking from 24 to 48h. Lymphatic follicles disappeared 16h after injection, and numerous macrophages and fibroblasts were observed in the cortical region. COX-2-expressing cells in the cortical region showed characteristic dendritic processes from 16 to 48h and primarily co-localized with stromal fibroblastic reticular cell markers, α-smooth muscle actin (α-SMA), and desmin. Expression of α-SMA increased following COX-2 expression. Nimesulide, a COX-2 inhibitor, increased the dendritic processes of COX-2-expressing cells as well as expression of both COX-2 and α-SMA. These results suggest that COX-2-expressing cells may be stromal fibroblastic cells, which negatively self-regulate their proliferation and modulate tissue remodeling of draining lymph nodes at inflammatory sites.

Transcriptome data analysis for cell culture processes

In the past decade, DNA microarrays have fundamentally changed the way we study complex biological systems. By measuring the expression levels of thousands of transcripts, the paradigm of studying organisms has shifted from focusing on the local phenomena of a few genes to surveying the whole genome. DNA microarrays are used in a variety of ways, from simple comparisons between two samples to more intricate time-series studies. With the large number of genes being studied, the dimensionality of the problem is inevitably high. The analysis of microarray data thus requires specific approaches. In the case of time-series microarray studies, data analysis is further complicated by the correlation between successive time points in a series.In this review, we survey the methodologies used in the analysis of static and time-series microarray data, covering data pre-processing, identification of differentially expressed genes, profile pattern recognition, pathway analysis, and network reconstruction. When available, examples of their use in mammalian cell cultures are presented.

Fibroblastic reticular cells and their role in viral hemorrhagic fevers

Viral hemorrhagic fevers (VHFs) caused by Ebola, Marburg and Lassa viruses often manifest as multiple organ dysfunction and hemorrhagic shock with high mortality. These viruses target numerous cell types, including monocytes and dendritic cells, which are primary early targets that mediate critical pathogenetic processes. This review focuses on fibroblastic reticular cells (FRCs), another prevalent infected cell type that is known as a key regulator of circulatory and immune functions. Viral infection of FRCs could have debilitating effects in secondary lymphoid organs and various other tissues. FRCs may also contribute to the spread of these deadly viruses throughout the body. Here, we review the salient features of these VHFs and the biology of FRCs, emphasizing the potential role of these cells in VHFs and the rapid deterioration of immune and hemovascular sytems that are characteristic of such acute infections.

Fibroblastic reticular cell infection by hemorrhagic fever viruses

Viral hemorrhagic fevers (VHFs) often cause high mortality with high infectivity, multiorgan failure, shock and hemorrhagic diathesis. Fibroblastic reticular cells (FRCs) within secondary lymphoid organs provide a supporting scaffold to T-lymphocyte areas. These cells regulate the movement of various immune cells and soluble molecules that promote T-lymphocyte homeostasis. We previously reported Ebola virus infection of FRCs, but ascribed little significance to this finding. Here, we studied infection of FRCs by Ebola, Marburg and Lassa viruses. We demonstrate that FRCs, or the extracellular ‘conduit’ of the fibroblastic reticulum of nonhuman primates, are targets of Ebola, Marburg and Lassa viruses. Furthermore, we observed that FRC damage correlates temporally and spatially with lymphocyte damage and that FRCs serve as nidi of fibrin deposition. In addition, we show that nonhuman primate FRCs express p75 NGF receptor and tissue transglutaminase. Our data suggest that viral infection of FRCs may be crucial to the immunological dysfunction and coagulopathy characteristic of VHFs. We further propose that p75 NGF receptor and tissue transglutaminase may be involved in FRC-associated dysfunction during the course of infection.

The extracellular matrix of the spleen as a potential organizer of immune cell compartments

Until recently little information was available on the molecular details of the extracellular matrix (ECM) of secondary lymphoid tissues. There is now growing evidence that these ECMs are unique structures, combining characteristics of basement membranes and interstitial or fibrillar matrices, resulting in scaffolds that are strong and highly flexible and, in certain secondary lymphoid compartments, also forming conduit networks for rapid fluid transport. This review will address the structural characteristics of the ECM of the murine spleen and its potential role as an organizer of immune cell compartments, with reference to the lymph node where relevant.