Fibroblastic reticular cells provide a supportive niche for lymph node-resident macrophages.

The lymph node (LN) is home to resident macrophage populations that are essential for immune function and homeostasis, but key factors controlling this niche are undefined. Here, we show that fibroblastic reticular cells (FRCs) are an essential component of the LN macrophage niche. Genetic ablation of FRCs caused rapid loss of macrophages and monocytes from LNs across two in vivo models. Macrophages co-localized with FRCs in human LNs, and murine single-cell RNA-sequencing revealed that FRC subsets broadly expressed master macrophage regulator CSF1. Functional assays containing purified FRCs and monocytes showed that CSF1R signaling was sufficient to support macrophage development. These effects were conserved between mouse and human systems. These data indicate an important role for FRCs in maintaining the LN parenchymal macrophage niche.

In Vitro Suppression of T Cell Proliferation Is a Conserved Function of Primary and Immortalized Human Cancer-Associated Fibroblasts.

T cell immunotherapy is now a mainstay therapy for several blood-borne cancers as well as metastatic melanoma. Unfortunately, many epithelial tumors respond poorly to immunotherapy, and the reasons for this are not well understood. Cancer-associated fibroblasts (CAFs) are the most frequent non-neoplastic cell type in most solid tumors, and they are emerging as a key player in immunotherapy resistance. A range of immortalized CAF lines will be essential tools that will allow us to understand immune responses against cancer and develop novel strategies for cancer immunotherapy. To study the effect of CAFs on T cell proliferation, we created and characterized a number of novel immortalized human CAFs lines (Im-CAFs) from human breast, colon, and pancreatic carcinomas. Im-CAFs shared similar phenotypes, matrix remodeling and contraction capabilities, and growth and migration rates compared to the primary CAFs. Using primary isolates from breast carcinoma, colorectal carcinoma, and pancreatic ductal adenocarcinoma, we report that CAFs across major tumor types are able to potently suppress T cell proliferation in vitro. Im-CAFs retained this property. Im-CAFs are a key tool that will provide important insights into the mechanisms of CAF-mediated T cell suppression through techniques such as CRISPR-Cas9 modification, molecular screens, and pipeline drug testing.

The fibroblastic T cell niche in lymphoid tissues.

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

The human lymph node microenvironment unilaterally regulates T-cell activation and differentiation.

The microenvironment of lymphoid organs can aid healthy immune function through provision of both structural and molecular support. In mice, fibroblastic reticular cells (FRCs) create an essential T-cell support structure within lymph nodes, while human FRCs are largely unstudied. Here, we show that FRCs create a regulatory checkpoint in human peripheral T-cell activation through 4 mechanisms simultaneously utilised. Human tonsil and lymph node-derived FRCs constrained the proliferation of both naïve and pre-activated T cells, skewing their differentiation away from a central memory T-cell phenotype. FRCs acted unilaterally without requiring T-cell feedback, imposing suppression via indoleamine-2,3-dioxygenase, adenosine 2A Receptor, prostaglandin E2, and transforming growth factor beta receptor (TGFßR). Each mechanistic pathway was druggable, and a cocktail of inhibitors, targeting all 4 mechanisms, entirely reversed the suppressive effect of FRCs. T cells were not permanently anergised by FRCs, and studies using chimeric antigen receptor (CAR) T cells showed that immunotherapeutic T cells retained effector functions in the presence of FRCs. Since mice were not suitable as a proof-of-concept model, we instead developed a novel human tissue-based in situ assay. Human T cells stimulated using standard methods within fresh tonsil slices did not proliferate except in the presence of inhibitors described above. Collectively, we define a 4-part molecular mechanism by which FRCs regulate the T-cell response to strongly activating events in secondary lymphoid organs while permitting activated and CAR T cells to utilise effector functions. Our results define 4 feasible strategies, used alone or in combinations, to boost primary T-cell responses to infection or cancer by pharmacologically targeting FRCs.

FAP Delineates Heterogeneous and Functionally Divergent Stromal Cells in Immune-Excluded Breast Tumors.

Cancer-associated fibroblasts (CAFs) are generally associated with poor clinical outcome. CAFs support tumor growth in a variety of ways and can suppress antitumor immunity and response to immunotherapy. However, a precise understanding of CAF contributions to tumor growth and therapeutic response is lacking. Discrepancies in this field of study may stem from heterogeneity in the composition and function of fibroblasts in the tumor microenvironment. Furthermore, it remains unclear whether CAFs directly interact with and suppress T cells. Here, mouse and human breast tumors were used to examine stromal cells expressing fibroblast activation protein (FAP), a surface marker for CAFs. Two discrete populations of FAP+ mesenchymal cells were identified on the basis of podoplanin (PDPN) expression: a FAP+PDPN+ population of CAFs and a FAP+PDPN- population of cancer-associated pericytes (CAPs). Although both subsets expressed extracellular matrix molecules, the CAF transcriptome was enriched in genes associated with TGFß signaling and fibrosis compared with CAPs. In addition, CAFs were enriched at the outer edge of the tumor, in close contact with T cells, whereas CAPs were localized around vessels. Finally, FAP+PDPN+ CAFs suppressed the proliferation of T cells in a nitric oxide-dependent manner, whereas FAP+PDPN- pericytes were not immunosuppressive. Collectively, these findings demonstrate that breast tumors contain multiple populations of FAP-expressing stromal cells of dichotomous function, phenotype, and location.

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.

Protein Acyltransferase DHHC3 Regulates Breast Tumor Growth, Oxidative Stress, and Senescence.

DHHC-type protein acyltransferases may regulate the localization, stability, and/or activity of their substrates. In this study, we show that the protein palmitoyltransferase DHHC3 is upregulated in malignant and metastatic human breast cancer. Elevated expression of DHHC3 correlated with diminished patient survival in breast cancer and six other human cancer types. ZDHHC3 ablation in human MDA-MB-231 mammary tumor cell xenografts reduced the sizes of both the primary tumor and metastatic lung colonies. Gene array data and fluorescence dye assays documented increased oxidative stress and senescence in ZDHHC3-ablated cells. ZDHHC3-ablated tumors also showed enhanced recruitment of innate immune cells (antitumor macrophages, natural killer cells) associated with clearance of senescent tumors. These antitumor effects were reversed upon reconstitution with wild-type, but not enzyme-active site-deficient DHHC3. Concomitant ablation of the upregulated oxidative stress protein TXNIP substantially negated the effects of ZDHHC3 depletion on oxidative stress and senescence. Diminished DHHC3-dependent palmitoylation of ERGIC3 protein likely played a key role in TXNIP upregulation. In conclusion, DHHC3-mediated protein palmitoylation supports breast tumor growth by modulating cellular oxidative stress and senescence. Cancer Res; 77(24); 6880-90. ©2017 AACR.

Exploring the in meso crystallization mechanism by characterizing the lipid mesophase microenvironment during the growth of single transmembrane α-helical peptide crystals.

The proposed mechanism for in meso crystallization of transmembrane proteins suggests that a protein or peptide is initially uniformly dispersed in the lipid self-assembly cubic phase but that crystals grow from a local lamellar phase, which acts as a conduit between the crystal and the bulk cubic phase. However, there is very limited experimental evidence for this theory. We have developed protocols to investigate the lipid mesophase microenvironment during crystal growth using standard procedures readily available in crystallography laboratories. This technique was used to characterize the microenvironment during crystal growth of the DAP12-TM peptide using synchrotron small angle X-ray scattering (SAXS) with a micro-sized X-ray beam. Crystal growth was found to occur from the gyroid cubic mesophase. For one in four crystals, a highly oriented local lamellar phase was observed, providing supporting evidence for the proposed mechanism for in meso crystallization. A new observation of this study was that we can differentiate diffraction peaks from crystals grown in meso, from peaks originating from the surrounding lipid matrix, potentially opening up the possibility of high-throughput SAXS analysis of in meso grown crystals.This article is part of the themed issue 'Soft interfacial materials: from fundamentals to formulation'.

Transmembrane Complexes of DAP12 Crystallized in Lipid Membranes Provide Insights into Control of Oligomerization in Immunoreceptor Assembly.

The membrane-spanning α helices of single-pass receptors play crucial roles in stabilizing oligomeric structures and transducing biochemical signals across the membrane. Probing intermolecular transmembrane interactions in single-pass receptors presents unique challenges, reflected in a gross underrepresentation of their membrane-embedded domains in structural databases. Here, we present two high-resolution structures of transmembrane assemblies from a eukaryotic single-pass protein crystallized in a lipidic membrane environment. Trimeric and tetrameric structures of the immunoreceptor signaling module DAP12, determined to 1.77-Å and 2.14-Å resolution, respectively, are organized by the same polar surfaces that govern intramembrane assembly with client receptors. We demonstrate that, in addition to the well-studied dimeric form, these trimeric and tetrameric structures are made in cells, and their formation is competitive with receptor association in the ER. The polar transmembrane sequences therefore act as primary determinants of oligomerization specificity through interplay between charge shielding and sequestration of polar surfaces within helix interfaces.

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.

EWI-2 negatively regulates TGF-ß signaling leading to altered melanoma growth and metastasis.

In normal melanocytes, TGF-ß signaling has a cytostatic effect. However, in primary melanoma cells, TGF-ß-induced cytostasis is diminished, thus allowing melanoma growth. Later, a second phase of TGF-ß signaling supports melanoma EMT-like changes, invasion and metastasis. In parallel with these "present-absent-present" TGF-ß signaling phases, cell surface protein EWI motif-containing protein 2 (EWI-2 or IgSF8) is "absent-present-absent" in melanocytes, primary melanoma, and metastatic melanoma, respectively, suggesting that EWI-2 may serve as a negative regulator of TGF-ß signaling. Using melanoma cell lines and melanoma short-term cultures, we performed RNAi and overexpression experiments and found that EWI-2 negatively regulates TGF-ß signaling and its downstream events including cytostasis (in vitro and in vivo), EMT-like changes, cell migration, CD271-dependent invasion, and lung metastasis (in vivo). When EWI-2 is present, it associates with cell surface tetraspanin proteins CD9 and CD81 - molecules not previously linked to TGF-ß signaling. Indeed, when associated with EWI-2, CD9 and CD81 are sequestered and have no impact on TßR2-TßR1 association or TGF-ß signaling. However, when EWI-2 is knocked down, CD9 and CD81 become available to provide critical support for TßR2-TßR1 association, thus markedly elevating TGF-ß signaling. Consequently, all of those TGF-ß-dependent functions specifically arising due to EWI-2 depletion are reversed by blocking or depleting cell surface tetraspanin proteins CD9 or CD81. These results provide new insights into regulation of TGF-ß signaling in melanoma, uncover new roles for tetraspanins CD9 and CD81, and strongly suggest that EWI-2 could serve as a favorable prognosis indicator for melanoma patients.

Lymph node fibroblastic reticular cell transplants show robust therapeutic efficacy in high-mortality murine sepsis.

Sepsis is an aggressive inflammatory syndrome and a global health burden estimated to kill 7.3 million people annually. Single-target molecular therapies have not addressed the multiple disease pathways triggered by septic injury. Cell therapies might offer a broader set of mechanisms of action that benefit complex, multifocal disease processes. We describe a population of immune-specialized myofibroblasts derived from lymph node tissue, termed fibroblastic reticular cells (FRCs). Because FRCs have an immunoregulatory function in lymph nodes, we hypothesized that ex vivo-expanded FRCs would control inflammation when administered therapeutically. Indeed, a single injection of ex vivo-expanded allogeneic FRCs reduced mortality in mouse models of sepsis when administered at early or late time points after septic onset. Mice treated with FRCs exhibited lower local and systemic concentrations of proinflammatory cytokines and reduced bacteremia. When administered 4 hours after induction of lipopolysaccharide endotoxemia, or cecal ligation and puncture (CLP) sepsis in mice, FRCs reduced deaths by at least 70%. When administered late in disease (16 hours after CLP), FRCs still conveyed a robust survival advantage (44% survival compared to 0% for controls). FRC therapy was dependent on the metabolic activity of nitric oxide synthase 2 (NOS2) as the primary molecular mechanism of drug action in the mice. Together, these data describe a new anti-inflammatory cell type and provide preclinical evidence for therapeutic efficacy in severe sepsis that warrants further translational study.

Tetraspanin TSPAN12 regulates tumor growth and metastasis and inhibits ß-catenin degradation.

Ablation of tetraspanin protein TSPAN12 from human MDA-MB-231 cells significantly decreased primary tumor xenograft growth, while increasing tumor apoptosis. Furthermore, TSPAN12 removal markedly enhanced tumor-endothelial interactions and increased metastasis to mouse lungs. TSPAN12 removal from human MDA-MB-231 cells also caused diminished association between FZD4 (a key canonical Wnt pathway receptor) and its co-receptor LRP5. The result likely explains substantially enhanced proteosomal degradation of ß-catenin, a key effecter of canonical Wnt signaling. Consistent with disrupted canonical Wnt signaling, TSPAN12 ablation altered expression of LRP5, Naked 1 and 2, DVL2, DVL3, Axin 1, and GSKß3 proteins. TSPAN12 ablation also altered expression of several genes regulated by ß-catenin (e.g. CCNA1, CCNE2, WISP1, ID4, SFN, ME1) that may help to explain altered tumor growth and metastasis. In conclusion, these results provide the first evidence for TSPAN12 playing a role in supporting primary tumor growth and suppressing metastasis. TSPAN12 appears to function by stabilizing FZD4-LRP5 association, in support of canonical Wnt-pathway signaling, leading to enhanced ß-catenin expression and function.

Transcriptional profiling of stroma from inflamed and resting lymph nodes defines immunological hallmarks.

Lymph node stromal cells (LNSCs) closely regulate immunity and self-tolerance, yet key aspects of their biology remain poorly elucidated. Here, comparative transcriptomic analyses of mouse LNSC subsets demonstrated the expression of important immune mediators, growth factors and previously unknown structural components. Pairwise analyses of ligands and cognate receptors across hematopoietic and stromal subsets suggested a complex web of crosstalk. Fibroblastic reticular cells (FRCs) showed enrichment for higher expression of genes relevant to cytokine signaling, relative to their expression in skin and thymic fibroblasts. LNSCs from inflamed lymph nodes upregulated expression of genes encoding chemokines and molecules involved in the acute-phase response and the antigen-processing and antigen-presentation machinery. Poorly studied podoplanin (gp38)-negative CD31(-) LNSCs showed similarities to FRCs but lacked expression of interleukin 7 (IL-7) and were identified as myofibroblastic pericytes that expressed integrin α(7). Together our data comprehensively describe the transcriptional characteristics of LNSC subsets.

Tetraspanin protein contributions to cancer.

Among the 33 human tetraspanin proteins, CD151, CD9 and Tspan12 play particularly important roles in cancer. Tetraspanin CD151, in partnership with integrins α6ß1 and α6ß4, modulates tumour cell growth, invasion, migration, metastasis, signalling and drug sensitivity. Tetraspanin CD9 has suppressor functions in multiple tumour cell types. Major CD9 partner proteins, such as EWI-2 and EWI-F, may modulate these tumour-suppressor functions. Tetraspanin Tspan12 mutations are linked to a human disease called familial exudative vitreoretinopathy. In addition, as a regulator of the metalloprotease ADAM10 (a disintegrin and metalloprotease 10) maturation and function, Tspan12 probably contributes to the pro-tumorigenic functions of ADAM10.

Backbone assignment of the N-terminal polyomavirus large T antigen.

Polyoma Large T antigen (PyLT) is a viral oncoprotein that targets cell proteins important for growth regulation. PyLT has two functional domains. Here we report (1)H, (15)N, (13)C backbone and (13)C beta assignments of 76% of the residues of the polyomavirus large T antigen N-terminal domain (PyLTNT) that is sufficient to regulate cell phenotype. PyLTNT is substantially unfolded even in regions known to be critical for its biological function. The protein also includes a previously characterised J domain that although conformationally influenced by the residue extension, retains its folded state unlike the majority of the protein sequence.