Optimization of the Irf8 +32-kb enhancer disrupts dendritic cell lineage segregation.

Autoactivation of lineage-determining transcription factors mediates bistable expression, generating distinct cell phenotypes essential for complex body plans. Classical type 1 dendritic cell (cDC1) and type 2 dendritic cell (cDC2) subsets provide nonredundant functions for defense against distinct immune challenges. Interferon regulatory factor 8 (IRF8), the cDC1 lineage-determining transcription factor, undergoes autoactivation in cDC1 progenitors to establish cDC1 identity, yet its expression is downregulated during cDC2 differentiation by an unknown mechanism. This study reveals that the Irf8 +32-kb enhancer, responsible for IRF8 autoactivation, is naturally suboptimized with low-affinity IRF8 binding sites. Introducing multiple high-affinity IRF8 sites into the Irf8 +32-kb enhancer causes a gain-of-function effect, leading to erroneous IRF8 autoactivation in specified cDC2 progenitors, redirecting them toward cDC1 and a novel hybrid DC subset with mixed-lineage phenotypes. Further, this also causes a loss-of-function effect, reducing Irf8 expression in cDC1s. These developmental alterations critically impair both cDC1-dependent and cDC2-dependent arms of immunity. Collectively, our findings underscore the significance of enhancer suboptimization in the developmental segregation of cDCs required for normal immune function.

Mechanisms of CD40-dependent cDC1 licensing beyond costimulation.

CD40 signaling in classical type 1 dendritic cells (cDC1s) is required for CD8 T cell-mediated tumor rejection, but the underlying mechanisms are incompletely understood. Here, we identified CD40-induced genes in cDC1s, including Cd70, Tnfsf9, Ptgs2 and Bcl2l1, and examined their contributions to anti-tumor immunity. cDC1-specific inactivation of CD70 and COX-2, and global CD27 inactivation, only partially impaired tumor rejection or tumor-specific CD8 T cell expansion. Loss of 4-1BB, alone or in Cd27-/- mice, did not further impair anti-tumor immunity. However, cDC1-specific CD40 inactivation reduced cDC1 mitochondrial transmembrane potential and increased caspase activation in tumor-draining lymph nodes, reducing migratory cDC1 numbers in vivo. Similar impairments occurred during in vitro antigen presentation by Cd40-/- cDC1s to CD8+ T cells, which were reversed by re-expression of Bcl2l1. Thus, CD40 signaling in cDC1s not only induces costimulatory ligands for CD8+ T cells but also induces Bcl2l1 that sustains cDC1 survival during priming of anti-tumor responses.

High Amount of Transcription Factor IRF8 Engages AP1-IRF Composite Elements in Enhancers to Direct Type 1 Conventional Dendritic Cell Identity.

Development and function of conventional dendritic cell (cDC) subsets, cDC1 and cDC2, depend on transcription factors (TFs) IRF8 and IRF4, respectively. Since IRF8 and IRF4 can each interact with TF BATF3 at AP1-IRF composite elements (AICEs) and with TF PU.1 at Ets-IRF composite elements (EICEs), it is unclear how these factors exert divergent actions. Here, we determined the basis for distinct effects of IRF8 and IRF4 in cDC development. Genes expressed commonly by cDC1 and cDC2 used EICE-dependent enhancers that were redundantly activated by low amounts of either IRF4 or IRF8. By contrast, cDC1-specific genes relied on AICE-dependent enhancers, which required high IRF concentrations, but were activated by either IRF4 or IRF8. IRF8 was specifically required only by a minority of cDC1-specific genes, such as Xcr1, which could distinguish between IRF8 and IRF4 DNA-binding domains. Thus, these results explain how BATF3-dependent Irf8 autoactivation underlies emergence of the cDC1-specific transcriptional program.

The evolving biology of cross-presentation.

Cross-priming was first recognized in the context of in vivo cytotoxic T lymphocyte (CTL) responses generated against minor histocompatibility antigens induced by immunization with lymphoid cells. Even though the basis for T cell antigen recognition was still largely unclear at that time, these early studies recognized the implication that such minor histocompatibility antigens were derived from the immunizing cells and were obtained exogenously by the host's antigen presenting cells (APCs) that directly prime the CTL response. As antigen recognition by the T cell receptor became understood to involve peptides derived from antigens processed by the APCs and presented by major histocompatibility molecules, the "cross-priming" phenomenon was subsequently recast as "cross-presentation" and the scope considered for examining this process gradually broadened to include many different forms of antigens, including soluble proteins, and different types of APCs that may not be involved in in vivo CTL priming. Many studies of cross-presentation have relied on in vitro cell models that were recently found to differ from in vivo APCs in particular mechanistic details. A recent trend has focused on the APCs and pathways of cross-presentation used in vivo, especially the type 1 dendritic cells. Current efforts are also being directed towards validating the in vivo role of various putative pathways and gene candidates in cross-presentation garnered from various in vitro studies and to determine the relative contributions they make to CTL responses across various forms of antigens and immunologic settings. Thus, cross-presentation appears to be carried by different pathways in various types of cells for different forms under different physiologic settings, which remain to be evaluated in an in vivo physiologic setting.

WDFY4 deficiency in NOD mice ameliorates autoimmune diabetes and insulitis.

The events that initiate autoimmune diabetes in nonobese diabetic (NOD) mice remain poorly understood. CD4+ and CD8+ T cells are both required to develop disease, but their relative roles in initiating disease are unclear. To test whether CD4+ T cell infiltration into islets requires damage to ß cells induced by autoreactive CD8+ T cells, we inactivated Wdfy4 in nonobese diabetic (NOD) mice (NOD.Wdfy4-/--) using CRISPR/Cas9 targeting to eliminate cross-presentation by type 1 conventional dendritic cells (cDC1s). Similar to C57BL/6 Wdfy4-/- mice, cDC1 in NOD.Wdfy4-/- mice are unable to cross-present cell-associated antigens to prime CD8+ T cells, while cDC1 from heterozygous NOD.Wdfy4+/- mice cross-present normally. Further, NOD.Wdfy4-/- mice fail to develop diabetes while heterozygous NOD.Wdfy4+/- mice develop diabetes similarly to wild-type NOD mice. NOD.Wdfy4-/- mice remain capable of processing and presenting major histocompatibility complex class II (MHC-II)-restricted autoantigens and can activate ß cell-specific CD4+ T cells in lymph nodes. However, disease in these mice does not progress beyond peri-islet inflammation. These results indicate that the priming of autoreactive CD8+ T cells in NOD mice requires cross-presentation by cDC1. Further, autoreactive CD8+ T cells appear to be required not only to develop diabetes, but to recruit autoreactive CD4+ T cells into islets of NOD mice, perhaps in response to progressive ß cell damage.

CD318 is a ligand for CD6.

It has been proposed that CD6, an important regulator of T cells, functions by interacting with its currently identified ligand, CD166, but studies performed during the treatment of autoimmune conditions suggest that the CD6-CD166 interaction might not account for important functions of CD6 in autoimmune diseases. The antigen recognized by mAb 3A11 has been proposed as a new CD6 ligand distinct from CD166, yet the identity of it is hitherto unknown. We have identified this CD6 ligand as CD318, a cell surface protein previously found to be present on various epithelial cells and many tumor cells. We found that, like CD6 knockout (KO) mice, CD318 KO mice are also protected in experimental autoimmune encephalomyelitis. In humans, we found that CD318 is highly expressed in synovial tissues and participates in CD6-dependent adhesion of T cells to synovial fibroblasts. In addition, soluble CD318 is chemoattractive to T cells and levels of soluble CD318 are selectively and significantly elevated in the synovial fluid from patients with rheumatoid arthritis and juvenile inflammatory arthritis. These results establish CD318 as a ligand of CD6 and a potential target for the diagnosis and treatment of autoimmune diseases such as multiple sclerosis and inflammatory arthritis.

Scleroderma dermal microvascular endothelial cells exhibit defective response to pro-angiogenic chemokines.

OBJECTIVES: Angiogenesis plays a critical role in SSc (scleroderma). The aim of this study was to examine the expression of growth-regulated protein-γ (Gro-γ/CXCL3), granulocyte chemotactic protein 2 (GCP-2/CXCL6) and their receptor CXCR2 in endothelial cells (ECs) isolated from SSc skin and determine whether these cells mount an angiogenic response towards pro-angiogenic chemokines. The downstream signalling pathways as well as the pro-angiogenic transcription factor inhibitor of DNA-binding protein 1 (Id-1) were also examined. METHODS: Skin biopsies were obtained from patients with dcSSc. ECs were isolated via magnetic positive selection. Angiogenesis was measured by EC chemotaxis assay. RESULTS: Gro-γ/CXCL3 and GCP-2/CXCL6 were minimally expressed in both skin types but elevated in SSc serum. Pro-angiogenic chemokine mRNA was greater in SSc ECs than in normal ECs. SSc ECs did not migrate to vascular endothelial growth factor (VEGF), Gro-γ/CXCL3, GCP-2/CXCL6 or CXCL16. The signalling pathways stimulated by these chemokines were also dysregulated. Id-1 mRNA in SSc ECs was lower compared with normal ECs, and overexpression of Id-1 in SSc ECs increased their ability to migrate towards VEGF and CXCL16. CONCLUSION: Our results show that SSc ECs are unable to respond to pro-angiogenic chemokines despite their increased expression in serum and ECs. This might be due to the differences in the signalling pathways activated by these chemokines in normal vs SSc ECs. In addition, the lower expression of Id-1 also decreases the angiogenic response. The inability of pro-angiogenic chemokines to promote EC migration provides an additional mechanism for the impaired angiogenesis that characterizes SSc.

A key role for Fut1-regulated angiogenesis and ICAM-1 expression in K/BxN arthritis.

OBJECTIVES: Angiogenesis contributes to the pathogenesis of rheumatoid arthritis. Fucosyltransferases (Futs) are involved in angiogenesis and tumour growth. Here, we examined the role of Fut1 in angiogenesis and K/BxN serum transfer arthritis. METHODS: We examined Fut1 expression in human dermal microvascular endothelial cells (HMVECs) by quantitative PCR. We performed a number of angiogenesis assays to determine the role of Fut1 using HMVECs, Fut1 null (Fut1(-/-)), and wild type (wt) endothelial cells (ECs) and mice. K/BxN serum transfer arthritis was performed to determine the contribution of Fut1-mediated angiogenesis in Fut1(-/-) and wt mice. A static adhesion assay was implemented with RAW264.7 (mouse macrophage cell line) and mouse ECs. Quantitative PCR, immunofluorescence and flow cytometry were performed with Fut1(-/-) and wt ECs for adhesion molecule expression. RESULTS: Tumour necrosis factor-α induced Fut1 mRNA and protein expression in HMVECs. HMVECs transfected with Fut1 antisense oligodeoxynucleotide and Fut1(-/-) ECs formed significantly fewer tubes on Matrigel. Fut1(-/-) mice had reduced angiogenesis in Matrigel plug and sponge granuloma angiogenesis assays compared with wt mice. Fut1(-/-) mice were resistant to K/BxN serum transfer arthritis and had decreased angiogenesis and leucocyte ingress into inflamed joints. Adhesion of RAW264.7 cells to wt mouse ECs was significantly reduced when Fut1 was lacking. Fut1(-/-) ECs had decreased intercellular adhesion molecule-1 (ICAM-1) expression at mRNA and protein levels compared with wt ECs. ICAM-1 was also decreased in Fut1(-/-) arthritic ankle cryosections compared with wt ankles. CONCLUSIONS: Fut1 plays an important role in regulating angiogenesis and ICAM-1 expression in inflammatory arthritis.

Recent progress in type 1 classical dendritic cell cross-presentation - cytosolic, vacuolar, or both?

Type 1 classical dendritic cells (cDC1s) have emerged as the major antigen-presenting cell performing cross-presentation (XP) in vivo, but the antigen-processing pathway in this cell remains obscure. Two competing models for in vivo XP of cell-associated antigens by cDC1 include a vacuolar pathway and cytosolic pathway. A vacuolar pathway relies on directing antigens captured in vesicles toward a class I major histocompatibility complex loading compartment independently of cytosolic entry. Alternate proposals invoke phagosomal rupture, either constitutive or triggered by spleen tyrosine kinase (SYK) signaling in response to C-type lectin domain family 9 member A (CLEC9A) engagement, that releases antigens into the cytosol for proteasomal degradation. The Beige and Chediak-Higashi (BEACH) protein WD repeat- and FYVE domain-containing protein 4 (WDFY4) is strictly required for XP of cell-associated antigens in vivo. However, the cellular mechanism for WDFY4 activity remains unknown and its requirement in XP in vivo is currently indifferent regarding the vacuolar versus cytosolic pathways. Here, we review the current status of these models and discuss the need for future investigation.

IL-6 selectively suppresses cDC1 specification via C/EBPß.

Cytokines produced in association with tumors can impair antitumor immune responses by reducing the abundance of type 1 conventional dendritic cells (cDC1), but the mechanism remains unclear. Here, we show that tumor-derived IL-6 generally reduces cDC development but selectively impairs cDC1 development in both murine and human systems through the induction of C/EBPß in the common dendritic cell progenitor (CDP). C/EBPß and NFIL3 compete for binding to sites in the Zeb2 -165 kb enhancer and support or repress Zeb2 expression, respectively. At homeostasis, pre-cDC1 specification occurs upon Nfil3 induction and consequent Zeb2 suppression. However, IL-6 strongly induces C/EBPß expression in CDPs. Importantly, the ability of IL-6 to impair cDC development is dependent on the presence of C/EBPß binding sites in the Zeb2 -165 kb enhancer, as this effect is lost in Δ1+2+3 mutant mice in which these binding sites are mutated. These results explain how tumor-associated IL-6 suppresses cDC1 development and suggest therapeutic approaches preventing abnormal C/EBPß induction in CDPs may help reestablish cDC1 development to enhance antitumor immunity.

cDC1 Vaccines Drive Tumor Rejection by Direct Presentation Independently of Host cDC1.

As a cell-based cancer vaccine, dendritic cells (DC), derived from peripheral blood monocytes or bone marrow (BM) treated with GM-CSF (GMDC), were initially thought to induce antitumor immunity by presenting tumor antigens directly to host T cells. Subsequent work revealed that GMDCs do not directly prime tumor-specific T cells, but must transfer their antigens to host DCs. This reduces their advantage over strictly antigen-based strategies proposed as cancer vaccines. Type 1 conventional DCs (cDC1) have been reported to be superior to GMDCs as a cancer vaccine, but whether they act by transferring antigens to host DCs is unknown. To test this, we compared antitumor responses induced by GMDCs and cDC1 in Irf8 +32-/- mice, which lack endogenous cDC1 and cannot reject immunogenic fibrosarcomas. Both GMDCs and cDC1 could cross-present cell-associated antigens to CD8+ T cells in vitro. However, injection of GMDCs into tumors in Irf8 +32-/- mice did not induce antitumor immunity, consistent with their reported dependence on host cDC1. In contrast, injection of cDC1s into tumors in Irf8 +32-/- mice resulted in their migration to tumor-draining lymph nodes, activation of tumor-specific CD8+ T cells, and rejection of the tumors. Tumor rejection did not require the in vitro loading of cDC1 with antigens, indicating that acquisition of antigens in vivo is sufficient to induce antitumor responses. Finally, cDC1 vaccination showed abscopal effects, with rejection of untreated tumors growing concurrently on the opposite flank. These results suggest that cDC1 may be a useful future avenue to explore for antitumor therapy. See related Spotlight by Hubert et al., p. 918.

Citrullinated Inhibitor of DNA Binding 1 Is a Novel Autoantigen in Rheumatoid Arthritis.

OBJECTIVE: To explore the intrinsic role of inhibitor of DNA binding 1 (ID-1) in rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLS) and to investigate whether ID-1 is citrullinated and autoantigenic in RA. METHODS: RA patient serum ID-1 levels were measured before and after infliximab treatment. RA FLS were transfected with a clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated protein 9 construct targeting ID-1 to examine the effects of ID-1 deletion. RA synovial fluid (SF) and homogenized synovial tissue (ST) were immunoprecipitated for ID-1 and measured for citrullinated residues using an enzyme-linked immunosorbent assay and Western blotting. Liquid chromatography tandem mass spectrometry (LC-MS/MS) was performed on in vitro-citrullinated recombinant human ID-1 (cit-ID-1) to localize the sites of citrullination. Normal and RA sera and SF were analyzed by immunodot blotting for anti-citrullinated protein antibodies (ACPAs) to cit-ID-1. RESULTS: RA patient serum ID-1 levels positively correlated with several disease parameters and were reduced after infliximab treatment. RA FLS displayed reduced growth and a robust increase in interleukin-6 (IL-6) and IL-8 production upon deletion of ID-1. ID-1 immunodepletion significantly reduced the levels of citrullinated residues in RA SF, and citrullinated ID-1 was detected in homogenized RA ST (n = 5 samples; P < 0.05). Immunodot blot analyses revealed ACPAs to cit-ID-1 but not to native ID-1, in RA peripheral blood (PB) sera (n = 30 samples; P < 0.001) and SF (n = 18 samples; P < 0.05) but not in normal PB sera. Following analyses of LC-MS/MS results for citrullination sites and corresponding reactivity in immunodot assays, we determined the critical arginines in ID-1 for autoantigenicity: R33, R52, and R121. CONCLUSION: Novel roles of ID-1 in RA include regulation of FLS proliferation and cytokine secretion as well as autoantigenicity following citrullination.

A unique role for galectin-9 in angiogenesis and inflammatory arthritis.

BACKGROUND: Galectin-9 (Gal-9) is a mammalian lectin secreted by endothelial cells that is highly expressed in rheumatoid arthritis synovial tissues and synovial fluid. Roles have been proposed for galectins in the regulation of inflammation and angiogenesis. Therefore, we examined the contribution of Gal-9 to angiogenesis and inflammation in arthritis. METHODS: To determine the role of Gal-9 in angiogenesis, we performed human dermal microvascular endothelial cell (HMVEC) chemotaxis, Matrigel tube formation, and mouse Matrigel plug angiogenesis assays. We also examined the role of signaling molecules in Gal-9-induced angiogenesis by using signaling inhibitors and small interfering RNA (siRNA). We performed monocyte (MN) migration assays in a modified Boyden chamber and assessed the arthritogenicity of Gal-9 by injecting Gal-9 into mouse knees. RESULTS: Gal-9 significantly increased HMVEC migration, which was decreased by inhibitors of extracellular signal-regulating kinases 1/2 (Erk1/2), p38, Janus kinase (Jnk), and phosphatidylinositol 3-kinase. Gal-9 HMVEC-induced tube formation was reduced by Erk1/2, p38, and Jnk inhibitors, and this was confirmed by siRNA knockdown. In mouse Matrigel plug assays, plugs containing Gal-9 induced significantly higher angiogenesis, which was attenuated by a Jnk inhibitor. Gal-9 also induced MN migration, and there was a marked increase in MN ingress when C57BL/6 mouse knees were injected with Gal-9 compared with the control, pointing to a proinflammatory role for Gal-9. CONCLUSIONS: Gal-9 mediates angiogenesis, increases MN migration in vitro, and induces acute inflammatory arthritis in mice, suggesting a novel role for Gal-9 in angiogenesis, joint inflammation, and possibly other inflammatory diseases.

Inflammatory properties of inhibitor of DNA binding 1 secreted by synovial fibroblasts in rheumatoid arthritis.

BACKGROUND: Inhibitor of DNA binding 1 (Id1) is a nuclear protein containing a basic helix-loop-helix (bHLH) domain that regulates cell growth by selective binding and prevention of gene transcription. Sources of Id1 production in rheumatoid arthritis synovial tissue (RA ST) and its range of functional effects in RA remain to be clarified. METHODS: We analyzed Id1 produced from synovial fibroblasts and endothelial cells (ECs) with histology and real-time polymerase chain reaction (RT-PCR). Fibroblast supernatants subjected to differential centrifugation to isolate and purify exosomes were measured for Id1 by enzyme-linked immunosorbent assay (ELISA). Western blotting of Id1-stimulated ECs was performed to determine the kinetics of intracellular protein phosphorylation. EC intracellular signaling pathways induced by Id1 were subsequently targeted with silencing RNA (siRNA) for angiogenesis inhibition. RESULTS: By PCR and histologic analysis, we found that the primary source of Id1 in STs is from activated fibroblasts that correlate with inflammatory scores in human RA ST and in joints from K/BxN serum-induced mice. Normal (NL) and RA synovial fibroblasts increase Id1 production with stimulation by transforming growth factor beta (TGF-ß). Most of the Id1 released by RA synovial fibroblasts is contained within exosomes. Endothelial progenitor cells (EPCs) and human dermal microvascular ECs (HMVECs) activate the Jnk signaling pathway in response to Id1, and Jnk siRNA reverses Id1-induced HMVEC vessel formation in Matrigel plugs in vivo. CONCLUSIONS: Id1 is a pleotropic molecule affecting angiogenesis, vasculogenesis, and fibrosis. Our data shows that Id1 is not only an important nuclear protein, but also can be released from fibroblasts via exosomes. The ability of extracellular Id1 to activate signaling pathways expands the role of Id1 in the orchestration of tissue inflammation.

Activation of the Thromboxane A2 Receptor by 8-Isoprostane Inhibits the Pro-Angiogenic Effect of Vascular Endothelial Growth Factor in Scleroderma.

The pathogenesis of scleroderma (SSc) includes components of autoimmunity, vascular dysfunction, and accumulation of extracellular matrix. 8-isoprostane, an oxidized lipid created by oxidative stress, activates the thromboxane A2 receptor (TXAR) and the Rho-associated kinase (ROCK) pathway. In this study, we determined whether the TXAR was activated by 8-isoprostane in SSc endothelial cells (ECs) and whether this pathway inhibited VEGF-induced angiogenesis. Elevated 8-isoprostane was observed in plasma and conditioned media from SSc patients. SSc-conditioned media inhibited EC tube formation, whereas addition of vitamin E, by reducing 8-isoprostane, increased tube formation. VEGF did not induce angiogenesis in SSc ECs, but vitamin E or TXAR inhibition restored its effect. The expression of TXAR, RhoA, and ROCK1/2 was elevated in SSc ECs. ROCK activity and 8-isoprostane-induced ROCK activation were significantly higher in SSc ECs, whereas VEGF had no effect. The hyper-activation of the TXAR leads to inhibition of VEGF-induced angiogenesis, as inhibition of the TXAR pathway results in a blockade of 8-isoprostane-induced ROCK activation and restoration of VEGF activity. These results suggest that the TXAR pathway has a crucial role in angiogenesis and that 8-isoprostane is not just a by-product of oxidative stress but also has a significant role in the impaired angiogenesis that characterizes SSc.