A structural perspective of liver X receptors.

Liver X receptors α and ß are members of the nuclear receptor family, which comprise a flexible N-terminal domain, a DNA binding domain, a hinge linker, and a ligand binding domain. Liver X receptors are important regulators of cholesterol and lipid homeostasis by controlling the transcription of numerous genes. Key to their transcriptional role is synergetic interaction among the domains. DNA binding domain binds on DNA; ligand binding domain is a crucial switch to control the transcription activity through conformational change caused by ligand binding. The Liver X receptors form heterodimers with retinoid X receptor and then the liganded heterodimer may recruit other necessary transcription components to form an active transcription complex.

TRIM18 is a critical regulator of viral myocarditis and organ inflammation.

BACKGROUND: Infections by viruses including severe acute respiratory syndrome coronavirus 2 could cause organ inflammations such as myocarditis, pneumonia and encephalitis. Innate immunity to viral nucleic acids mediates antiviral immunity as well as inflammatory organ injury. However, the innate immune mechanisms that control viral induced organ inflammations are unclear. METHODS: To understand the role of the E3 ligase TRIM18 in controlling viral myocarditis and organ inflammation, wild-type and Trim18 knockout mice were infected with coxsackievirus B3 for inducing viral myocarditis, influenza A virus PR8 strain and human adenovirus for inducing viral pneumonia, and herpes simplex virus type I for inducing herpes simplex encephalitis. Mice survivals were monitored, and heart, lung and brain were harvested for histology and immunohistochemistry analysis. Real-time PCR, co-immunoprecipitation, immunoblot, enzyme-linked immunosorbent assay, luciferase assay, flow cytometry, over-expression and knockdown techniques were used to understand the molecular mechanisms of TRIM18 in regulating type I interferon (IFN) production after virus infection in this study. RESULTS: We find that knockdown or deletion of TRIM18 in human or mouse macrophages enhances production of type I IFN in response to double strand (ds) RNA and dsDNA or RNA and DNA virus infection. Importantly, deletion of TRIM18 protects mice from viral myocarditis, viral pneumonia, and herpes simplex encephalitis due to enhanced type I IFN production in vivo. Mechanistically, we show that TRIM18 recruits protein phosphatase 1A (PPM1A) to dephosphorylate TANK binding kinase 1 (TBK1), which inactivates TBK1 to block TBK1 from interacting with its upstream adaptors, mitochondrial antiviral signaling (MAVS) and stimulator of interferon genes (STING), thereby dampening antiviral signaling during viral infections. Moreover, TRIM18 stabilizes PPM1A by inducing K63-linked ubiquitination of PPM1A. CONCLUSIONS: Our results indicate that TRIM18 serves as a negative regulator of viral myocarditis, lung inflammation and brain damage by downregulating innate immune activation induced by both RNA and DNA viruses. Our data reveal that TRIM18 is a critical regulator of innate immunity in viral induced diseases, thereby identifying a potential therapeutic target for treatment.

Identification of poly(ADP-ribose) polymerase 9 (PARP9) as a noncanonical sensor for RNA virus in dendritic cells.

Innate immune cells are critical in protective immunity against viral infections, involved in sensing foreign viral nucleic acids. Here we report that the poly(ADP-ribose) polymerase 9 (PARP9), a member of PARP family, serves as a non-canonical sensor for RNA virus to initiate and amplify type I interferon (IFN) production. We find knockdown or deletion of PARP9 in human or mouse dendritic cells and macrophages inhibits type I IFN production in response to double strand RNA stimulation or RNA virus infection. Furthermore, mice deficient for PARP9 show enhanced susceptibility to infections with RNA viruses because of the impaired type I IFN production. Mechanistically, we show that PARP9 recognizes and binds viral RNA, with resultant recruitment and activation of the phosphoinositide 3-kinase (PI3K) and AKT3 pathway, independent of mitochondrial antiviral-signaling (MAVS). PI3K/AKT3 then activates the IRF3 and IRF7 by phosphorylating IRF3 at Ser385 and IRF7 at Ser437/438 mediating type I IFN production. Together, we reveal a critical role for PARP9 as a non-canonical RNA sensor that depends on the PI3K/AKT3 pathway to produce type I IFN. These findings may have important clinical implications in controlling viral infections and viral-induced diseases by targeting PARP9.

TRIM29 Negatively Regulates the Type I IFN Production in Response to RNA Virus.

The innate immunity is critically important in protection against virus infections, and in the case of RNA viral infections, the signaling mechanisms that initiate robust protective innate immunity without triggering autoimmune inflammation remain incompletely defined. In this study, we found the E3 ligase TRIM29 was specifically expressed in poly I:C-stimulated human myeloid dendritic cells. The induced TRIM29 played a negative role in type I IFN production in response to poly I:C or dsRNA virus reovirus infection. Importantly, the challenge of wild-type mice with reovirus led to lethal infection. In contrast, deletion of TRIM29 protected the mice from this developing lethality. Additionally, TRIM29-/- mice have lower titers of reovirus in the heart, intestine, spleen, liver, and brain because of elevated production of type I IFN. Mechanistically, TRIM29 was shown to interact with MAVS and subsequently induce its K11-linked ubiquitination and degradation. Taken together, TRIM29 regulates negatively the host innate immune response to RNA virus, which could be employed by RNA viruses for viral pathogenesis.

Guidance of super-enhancers in regulation of IL-9 induction and airway inflammation.

Th9 cells are prominently featured in allergic lung inflammation, but the mechanism that regulates IL-9 induction in T helper cells remains poorly defined. Here we demonstrate that formation of super-enhancers (SEs) is critical in robust induction of IL-9 and that assembly of the Il9 SEs in Th cells requires OX40-triggered chromatin acetylation. Mechanistically, we found that OX40 costimulation induces RelB expression, which recruits the histone acetyltransferase p300 to the Il9 locus to catalyze H3K27 acetylation. This allows binding of the SE factor Brd4 to organize assembly of the SE complex, which in turn drives robust IL-9 expression and Th9 cell induction. Thus, Th9 cells are strongly induced upon OX40 stimulation, and disruption of SEs abolished Th9 cell induction in vitro and inhibited Th9 cell-mediated allergic airway inflammation in vivo. Together, our data suggest that formation of SEs is essential in IL-9 expression and Th9 cell induction. These findings may have important clinical implications.

The first case of ischemia-free organ transplantation in humans: A proof of concept.

Ischemia and reperfusion injury (IRI) is an inevitable event in conventional organ transplant procedure and is associated with significant mortality and morbidity post-transplantation. We hypothesize that IRI is avoidable if the blood supply for the organ is not stopped, thus resulting in optimal transplant outcomes. Here we described the first case of a novel procedure called ischemia-free organ transplantation (IFOT) for patients with end-stage liver disease. The liver graft with severe macrovesicular steatosis was donated from a 25-year-old man. The recipient was a 51-year-old man with decompensated liver cirrhosis and hepatocellular carcinoma. The graft was procured, preserved, and implanted under continuous normothermic machine perfusion. The recipient did not suffer post-reperfusion syndrome or vasoplegia after revascularization of the allograft. The liver function test and histological study revealed minimal hepatocyte, biliary epithelium and vascular endothelium injury during preservation and post-transplantation. The inflammatory cytokine levels were much lower in IFOT than those in conventional procedure. Key pathways involved in IRI were not activated after allograft revascularization. No rejection, or vascular or biliary complications occurred. The patient was discharged on day 18 post-transplantation. This marks the first case of IFOT in humans, offering opportunities to optimize transplant outcomes and maximize donor organ utilization.

TRIM29 promotes DNA virus infections by inhibiting innate immune response.

Many double-stranded DNA viruses, such as Epstein-Barr virus, can establish persistent infection, but the underlying virus-host interactions remain poorly understood. Here we report that in human airway epithelial cells Epstein-Barr virus induces TRIM29, a member of the TRIM family of proteins, to inhibit innate immune activation. Knockdown of TRIM29 in airway epithelial cells enhances type I interferon production, and in human nasopharyngeal carcinoma cells results in almost complete Epstein-Barr virus clearance. TRIM29 is also highly induced by cytosolic double-stranded DNA in myeloid dendritic cells. TRIM29 -/- mice have lower adenovirus titers in the lung, and are resistant to lethal herpes simplex virus-1 infection due to enhanced production of type I interferon. Mechanistically, TRIM29 induces K48-linked ubiquitination of Stimulator of interferon genes, a key adaptor in double-stranded DNA-sensing pathway, followed by its rapid degradation. These data demonstrate that Epstein-Barr virus and possible other double-stranded DNA viruses use TRIM29 to suppress local innate immunity, leading to the persistence of DNA virus infections.Proteins of the TRIM family have regulatory functions in immune signaling, often via ubiquitination of target proteins. Here, the authors show that TRIM29 is induced upon infection with DNA viruses, resulting in degradation of STING, decreased interferon signaling and increased pathogenicity in mice.

A20 Restrains Thymic Regulatory T Cell Development.

Maintaining immune tolerance requires the production of Foxp3-expressing regulatory T (Treg) cells in the thymus. Activation of NF-κB transcription factors is critically required for Treg cell development, partly via initiating Foxp3 expression. NF-κB activation is controlled by a negative feedback regulation through the ubiquitin editing enzyme A20, which reduces proinflammatory signaling in myeloid cells and B cells. In naive CD4+ T cells, A20 prevents kinase RIPK3-dependent necroptosis. Using mice deficient for A20 in T lineage cells, we show that thymic and peripheral Treg cell compartments are quantitatively enlarged because of a cell-intrinsic developmental advantage of A20-deficient thymic Treg differentiation. A20-deficient thymic Treg cells exhibit reduced dependence on IL-2 but unchanged rates of proliferation and apoptosis. Activation of the NF-κB transcription factor RelA was enhanced, whereas nuclear translocation of c-Rel was decreased in A20-deficient thymic Treg cells. Furthermore, we found that the increase in Treg cells in T cell-specific A20-deficient mice was already observed in CD4+ single-positive CD25+ GITR+ Foxp3- thymic Treg cell progenitors. Treg cell precursors expressed high levels of the tumor necrosis factor receptor superfamily molecule GITR, whose stimulation is closely linked to thymic Treg cell development. A20-deficient Treg cells efficiently suppressed effector T cell-mediated graft-versus-host disease after allogeneic hematopoietic stem cell transplantation, suggesting normal suppressive function. Holding thymic production of natural Treg cells in check, A20 thus integrates Treg cell activity and increased effector T cell survival into an efficient CD4+ T cell response.

TNF superfamily receptor OX40 triggers invariant NKT cell pyroptosis and liver injury.

Tissue-resident immune cells play a key role in local and systemic immune responses. The liver, in particular, hosts a large number of invariant natural killer T (iNKT) cells, which are involved in diverse immune responses. However, the mechanisms that regulate survival and homeostasis of liver iNKT cells are poorly defined. Here we have found that liver iNKT cells constitutively express the costimulatory TNF superfamily receptor OX40 and that OX40 stimulation results in massive pyroptotic death of iNKT cells, characterized by the release of potent proinflammatory cytokines that induce liver injury. This OX40/NKT pyroptosis pathway also plays a key role in concanavalin A-induced murine hepatitis. Mechanistically, we demonstrated that liver iNKT cells express high levels of caspase 1 and that OX40 stimulation activates caspase 1 via TNF receptor-associated factor 6-mediated recruitment of the paracaspase MALT1. We also found that activation of caspase 1 in iNKT cells results in processing of pro-IL-1ß to mature IL-1ß as well as cleavage of the pyroptotic protein gasdermin D, which generates a membrane pore-forming fragment to produce pyroptotic cell death. Thus, our study has identified OX40 as a death receptor for iNKT cells and uncovered a molecular mechanism of pyroptotic cell death. These findings may have important clinical implications in the development of OX40-directed therapies.

Leukocyte immunoglobulin-like receptors in human diseases: an overview of their distribution, function, and potential application for immunotherapies.

Myeloid-derived suppressor cells (MDSCs), a population of immature myeloid cells expanded and accumulated in tumor-bearing mice and in patients with cancer, have been shown to mediate immune suppression and to promote tumor progression, thereby, posing a major hurdle to the success of immune-activating cancer therapies. MDSCs, like their healthy counterparts, such as monocytes/macrophages and granulocytes, express an array of costimulatory and coinhibitory molecules as well as myeloid activators and inhibitory receptors, such as leukocyte immunoglobulin-like receptors (LILR) A and B. This review summarizes current findings on the LILR family members in various diseases, their potential roles in the pathogenesis, and possible strategies to revert or enhance the suppressive function of MDSCs for the benefit of patients by targeting LILRs.

Identification of a role for TRIM29 in the control of innate immunity in the respiratory tract.

The respiratory tract is heavily populated with innate immune cells, but the mechanisms that control such cells are poorly defined. Here we found that the E3 ubiquitin ligase TRIM29 was a selective regulator of the activation of alveolar macrophages, the expression of type I interferons and the production of proinflammatory cytokines in the lungs. We found that deletion of TRIM29 enhanced macrophage production of type I interferons and protected mice from infection with influenza virus, while challenge of Trim29-/- mice with Haemophilus influenzae resulted in lethal lung inflammation due to massive production of proinflammatory cytokines by macrophages. Mechanistically, we demonstrated that TRIM29 inhibited interferon-regulatory factors and signaling via the transcription factor NF-κB by degrading the adaptor NEMO and that TRIM29 directly bound NEMO and subsequently induced its ubiquitination and proteolytic degradation. These data identify TRIM29 as a key negative regulator of alveolar macrophages and might have important clinical implications for local immunity and immunopathology.

The Costimulatory Receptor OX40 Inhibits Interleukin-17 Expression through Activation of Repressive Chromatin Remodeling Pathways.

T helper 17 (Th17) cells are prominently featured in multiple autoimmune diseases, but the regulatory mechanisms that control Th17 cell responses are poorly defined. Here we found that stimulation of OX40 triggered a robust chromatin remodeling response and produced a "closed" chromatin structure at interleukin-17 (IL-17) locus to inhibit Th17 cell function. OX40 activated the NF-κB family member RelB, and RelB recruited the histone methyltransferases G9a and SETDB1 to the Il17 locus to deposit "repressive" chromatin marks at H3K9 sites, and consequently repressing IL-17 expression. Unlike its transcriptional activities, RelB acted independently of both p52 and p50 in the suppression of IL-17. In an experimental autoimmune encephalomyelitis (EAE) disease model, we found that OX40 stimulation inhibited IL-17 and reduced EAE. Conversely, RelB-deficient CD4(+) T cells showed enhanced IL-17 induction and exacerbated the disease. Our data uncover a mechanism in the control of Th17 cells that might have important clinic implications.

GITR subverts Foxp3(+) Tregs to boost Th9 immunity through regulation of histone acetylation.

Glucocorticoid-induced TNFR-related protein (GITR) is a costimulatory molecule with diverse effects on effector T cells and regulatory T cells (Tregs), but the underlying mechanism remains poorly defined. Here we demonstrate that GITR ligation subverts the induction of Foxp3(+) Tregs and directs the activated CD4(+) T cells to Th9 cells. Such GITR-mediated iTreg to Th9 induction enhances anti-tumour immunity in vivo. Mechanistically, GITR upregulates the NF-κB family member p50, which recruits histone deacetylases to the Foxp3 locus to produce a 'closed' chromatin structure. Furthermore, GITR ligation also activates STAT6, and STAT6 renders Il9 locus accessible via recruitment of histone acetyltransferase p300, and together with inhibition of Foxp3, GITR induces strong Th9 responses. Thus, Th9 cells and iTregs are developmentally linked and GITR can subvert tolerogenic conditions to boost Th9 immunity.

Porous silicon microparticle potentiates anti-tumor immunity by enhancing cross-presentation and inducing type I interferon response.

Micro- and nanometer-size particles have become popular candidates for cancer vaccine adjuvants. However, the mechanism by which such particles enhance immune responses remains unclear. Here, we report a porous silicon microparticle (PSM)-based cancer vaccine that greatly enhances cross-presentation and activates type I interferon (IFN-I) response in dendritic cells (DCs). PSM-loaded antigen exhibited prolonged early endosome localization and enhanced cross-presentation through both proteasome- and lysosome-dependent pathways. Phagocytosis of PSM by DCs induced IFN-I responses through a TRIF- and MAVS-dependent pathway. DCs primed with PSM-loaded HER2 antigen produced robust CD8 T cell-dependent anti-tumor immunity in mice bearing HER2+ mammary gland tumors. Importantly, this vaccination activated the tumor immune microenvironment with elevated levels of intra-tumor IFN-I and MHCII expression, abundant CD11c+ DC infiltration, and tumor-specific cytotoxic T cell responses. These findings highlight the potential of PSM as an immune adjuvant to potentiate DC-based cancer immunotherapy.

OX40 signaling favors the induction of T(H)9 cells and airway inflammation.

The mechanisms that regulate the T(H)9 subset of helper T cells and diseases mediated by T(H)9 cells remain poorly defined. Here we found that the costimulatory receptor OX40 was a powerful inducer of T(H)9 cells in vitro and T(H)9 cell-dependent airway inflammation in vivo. In polarizing conditions based on transforming growth factor-ß (TGF-ß), ligation of OX40 inhibited the production of induced regulatory T cells and the T(H)17 subset of helper T cells and diverted CD4(+)Foxp3(-) T cells to a T(H)9 phenotype. Mechanistically, OX40 activated the ubiquitin ligase TRAF6, which triggered induction of the kinase NIK in CD4(+) T cells and the noncanonical transcription factor NF-κB pathway; this subsequently led to the generation of T(H)9 cells. Thus, our study identifies a previously unknown mechanism for the induction of T(H)9 cells and may have important clinical implications in allergic inflammation.

The significance of non-T-cell pathways in graft rejection: implications for transplant tolerance.

Both innate and adaptive immune cells are actively involved in the initiation and destruction of allotransplants, and there is a true need now to look beyond T cells in the allograft response, examining various non-T-cell types in transplant models and how such cell types interact with T cells in determining the fate of an allograft. Studies in this area may lead to further improvement in transplant outcomes.

An update on regulatory T cells in transplant tolerance and rejection.

Several types of T cells with immunosuppressive properties have been identified, but FOXP3(+) regulatory T (T(REG)) cells have emerged as a dominant cell type; they are critically involved in the induction and maintenance of immune tolerance. Manipulation of this cell type for the induction of transplant tolerance including renal transplant tolerance has attracted considerable attention. Studies in this area have demonstrated unexpected complexities, and attempts to translate T(REG) cells towards clinical utility have met with unanticipated difficulties. In this Review, a broad overview is provided on recent progress in the study of T(REG) cells, focusing on challenges, opportunities, and emerging approaches in exploiting T(REG) cells for the induction of transplant tolerance.