Role of farnesoid X receptor in inflammation and resolution.

OBJECTIVE: The aim of this paper is to review the developments of farnesoid X receptor (FXR) biology, its ligands, and various functions, in particular we discuss the anti-inflammatory and anti-fibrotic role in chronic inflammatory diseases. INTRODUCTION: FXR is a ligand-dependent transcription factor belonging to the nuclear hormone receptor superfamily. The accrued data have shown that the FXR plays important roles not only in bile acid, lipid metabolism, and carbohydrate homeostasis, but also in inflammatory responses. The anti-inflammatory and anti-fibrotic effects of FXR on chronic inflammatory diseases are not well documented. METHODS: A literature survey was performed using PubMed database search to gather complete information regarding FXR and its role in inflammation. RESULTS AND DISCUSSION: FXR is highly expressed in liver, intestine, kidney and adrenals, but with lower expression in fat tissue, heart and recently it has been found to express in lungs too. Primary bile acids, cholic acid and chenodeoxycholic acid are the natural endogenous ligands for FXR. GW4064 and 6α-ethyl-chenodeoxycholic acid are the synthetic high-affinity agonists. An exhaustive literature survey revealed that FXR acts as a key metabolic regulator and potential drug target for many metabolic syndromes that include chronic inflammatory diseases.

Plexin-A1 and its interaction with DAP12 in immune responses and bone homeostasis.

Semaphorins and their receptors have diverse functions in axon guidance, organogenesis, vascularization and/or angiogenesis, oncogenesis and regulation of immune responses. The primary receptors for semaphorins are members of the plexin family. In particular, plexin-A1, together with ligand-binding neuropilins, transduces repulsive axon guidance signals for soluble class III semaphorins, whereas plexin-A1 has multiple functions in chick cardiogenesis as a receptor for the transmembrane semaphorin, Sema6D, independent of neuropilins. Additionally, plexin-A1 has been implicated in dendritic cell function in the immune system. However, the role of plexin-A1 in vivo, and the mechanisms underlying its pleiotropic functions, remain unclear. Here, we generated plexin-A1-deficient (plexin-A1(-/-)) mice and identified its important roles, not only in immune responses, but also in bone homeostasis. Furthermore, we show that plexin-A1 associates with the triggering receptor expressed on myeloid cells-2 (Trem-2), linking semaphorin-signalling to the immuno-receptor tyrosine-based activation motif (ITAM)-bearing adaptor protein, DAP12. These findings reveal an unexpected role for plexin-A1 and present a novel signalling mechanism for exerting the pleiotropic functions of semaphorins.

PAD Inhibitors as a Potential Treatment for SARS-CoV-2 Immunothrombosis.

Since the discovery of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in December 2019, the virus's dynamicity has resulted in the evolution of various variants, including the delta variant and the more novel mu variant. With a multitude of mutant strains posing as challenges to vaccine efficacy, it is critical that researchers embrace the development of pharmacotherapeutics specific to SARS-CoV-2 pathophysiology. Neutrophil extracellular traps and their constituents, including citrullinated histones, display a linear connection with thrombotic manifestations in COVID-19 patients. Peptidylarginine deiminases (PADs) are a group of enzymes involved in the modification of histone arginine residues by citrullination, allowing for the formation of NETs. PAD inhibitors, specifically PAD-4 inhibitors, offer extensive pharmacotherapeutic potential across a broad range of inflammatory diseases such as COVID-19, through mediating NETs formation. Although numerous PAD-4 inhibitors exist, current literature has not explored the depth of utilizing these inhibitors clinically to treat thrombotic complications in COVID-19 patients. This review article offers the clinical significance of PAD-4 inhibitors in reducing thrombotic complications across various inflammatory disorders like COVID-19 and suggests that these inhibitors may be valuable in treating the origin of SARS-CoV-2 immunothrombosis.

Methylation regulates HEY1 expression in glioblastoma.

Glioblastoma (GBM) remains one of the most lethal and difficult-to-treat cancers of the central nervous system. The poor prognosis in GBM patients is due in part to its resistance to available treatments, which calls for identifying novel molecular therapeutic targets. In this study, we identified a mediator of Notch signaling, HEY1, whose methylation status contributes to the pathogenesis of GBM. Datamining studies, immunohistochemistry and immunoblot analysis showed that HEY1 is highly expressed in GBM patient specimens. Since methylation status of HEY1 may control its expression, we conducted bisulphite sequencing on patient samples and found that the HEY1 promoter region was hypermethylated in normal brain when compared to GBM specimens. Treatment on 4910 and 5310 xenograft cell lines with sodium butyrate (NaB) significantly decreased HEY1 expression with a concomitant increase in DNMT1 expression, confirming that promoter methylation may regulate HEY1 expression in GBM. NaB treatment also induced apoptosis of GBM cells as measured by flow cytometric analysis. Further, silencing of HEY1 reduced invasion, migration and proliferation in 4910 and 5310 cells. Furthermore, immunoblot and q-PCR analysis showed the existence of a potential positive regulatory loop between HEY1 and p53. Additionally, transcription factor interaction array with HEY1 recombinant protein predicted a correlation with p53 and provided various bonafide targets of HEY1. Collectively, these studies suggest HEY1 may be an important predictive marker for GBM and potential target for future GBM therapy.

Immune regulation by novel costimulatory molecules.

CD4 helper T (Th)-cells and the cytokines that they produce play essential regulatory roles in immune and autoimmune responses. Th activation and differentiation is regulated by costimulatory receptors. CD28 and CTLA-4 are important in maintaining the threshold of T-cell activation. ICOS and PD-1 are novel costimulatory receptors expressed on activated T-cells. B7-H3 recognizes a putative costimulatory receptor on activated T-cells. Here we summarize the latest developments in the novel costimulatory molecules and their roles in regulating Th activation, differentiation, and function.

Requirement for CD100-CD72 interactions in fine-tuning of B-cell antigen receptor signaling and homeostatic maintenance of the B-cell compartment.

Co-receptors on the B-cell surface regulate B-cell antigen receptor (BCR) signaling; however, it remains unclear how BCR signals are coordinated to maintain immune homeostasis. CD72, a negative regulator of B-cell responses, has immunoreceptor tyrosine-based inhibitory motifs within its cytoplasmic region, and the tyrosine phosphatase SHP-1 binds these sites. The natural ligand of CD72, CD100/Sema4D, belongs to the semaphorin family and induces the dissociation of SHP-1 from CD72, thereby switching off the negative signals of CD72. In the absence of CD100, BCR signals are significantly suppressed due to the constitutive association of SHP-1 with CD72, resulting in B-cell hyporesponsiveness. Here we show that CD100 regulates the sensitivity of the BCR by preventing the association of the CD72 with BCR, and this interaction is required for proper B-cell homeostasis. Consequently, as CD100-deficient mice age, they accumulate marginal zone B cells and develop high auto-antibody levels and autoimmunity. Collectively, our findings indicate that the strength of BCR signals is strictly tuned by the interaction of CD100 with CD72, and this interaction is essential for maintaining immunological homeostasis as well as generating a proper immune response.

Nonredundant roles of Sema4A in the immune system: defective T cell priming and Th1/Th2 regulation in Sema4A-deficient mice.

The class IV semaphorin Sema4A provides a costimulatory signal to T cells. To investigate the possible developmental and regulatory roles of Sema4A in vivo, we generated Sema4A-deficient mice. Although Sema4A-deficient mice develop normally, DCs and T cells from knockout mice display poor allostimulatory activities and T helper cell (Th) differentiation, respectively. Interestingly, in addition to its expression on DCs, Sema4A is upregulated on Th1-differentiating cells, and it is necessary for in vitro Th1 differentiation and T-bet expression. Consequently, in vivo antigen-specific T cell priming and antibody responses against T cell-dependent antigens are impaired in the mutant mice. Additionally, Sema4A-deficient mice exhibit defective Th1 responses. Furthermore, reconstitution studies with antigen-pulsed DCs reveal that DC-derived Sema4A is important for T cell priming, while T cell-derived Sema4A is involved in developing Th1 responses. Collectively, these results indicate a nonredundant role of Sema4A not only in T cell priming, but also in the regulation of Th1/Th2 responses.

B7S1, a novel B7 family member that negatively regulates T cell activation.

T cell activation by antigen-presenting cells (APC) is regulated by positive and negative costimulatory molecules in the B7 family. Here we describe a novel addition in this family, designated as B7S1, which is uniquely anchored to the cell membrane via a GPI linkage. B7S1 is expressed on professional APC and widely distributed in nonlymphoid tissues. A soluble B7S1-Ig fusion protein binds to activated but not naive T cells. B7S1-Ig inhibits T cell activation and IL-2 production. A monoclonal antibody that blocks binding of B7S1 to its receptor enhances T cell proliferation in vitro and exacerbates experimental autoimmune encephalomyelitis in vivo. This study identifies a novel negative regulator of T cell activation and further reveals complex costimulatory regulation of immune responses.

Characterization of mouse and human B7-H3 genes.

T cell activation and immune function are regulated by costimulatory molecules of the B7 superfamily. Human B7-H3 is a recent addition to this family and has been shown to mediate T cell proliferation and IFN-gamma production. In this work we describe the identification of the mouse B7-H3 homolog, which is ubiquitously expressed in a variety of tissues. Activated CD4 and CD8 T cells express a putative receptor that can be recognized by soluble mouse B7-H3-Ig molecules. While the mouse B7-H3 gene was found to contain a single copy, we discovered a novel isoform of human B7-H3 (named as B7-H3b hereafter) with four Ig-like domains that results from gene duplication and differential splicing. B7-H3b is the major isoform expressed in several tissues. This structural information suggests a genetic variation of the B7-H3 gene in mammalian species.

Murine B7-H3 is a negative regulator of T cells.

T cell activation is regulated by the innate immune system through positive and negative costimulatory molecules. B7-H3 is a novel B7-like molecule with a putative receptor on activated T cells. Human B7-H3 was first described as a positive costimulator, most potently inducing IFN-gamma production and cellular immunity. In this study we examined the expression and function of mouse B7-H3. B7-H3 is mostly expressed on professional APCs; its expression on dendritic cells appears to be up-regulated by LPS. In contrast to human B7-H3, we found that mouse B7-H3 protein inhibited T cell activation and effector cytokine production. An antagonistic mAb to B7-H3 enhanced T cell proliferation in vitro and led to exacerbated experimental autoimmune encephalomyelitis in vivo. Therefore, mouse B7-H3 serves as a negative regulator of T cell activation and function.

B cells activated by lipopolysaccharide, but not by anti-Ig and anti-CD40 antibody, induce anergy in CD8+ T cells: role of TGF-beta 1.

B cells recognize Ag through their surface IgRs and present it in the context of MHC class II molecules to CD4(+) T cells. Recent evidence indicates that B cells also present exogenous Ags in the context of MHC class I to CD8(+) T cells and thus may play an important role in the modulation of CTL responses. However, in this regard, conflicting reports are available. One group of studies suggests that the interaction between B cells and CD8(+) T cells leads to the activation of the T cells, whereas other studies propose that it induces T cell tolerance. For discerning this dichotomy, we used B cells that were activated with either LPS or anti-Ig plus anti-CD40 Ab, which mimic the T-independent and T-dependent modes of B cell activation, respectively, to provide accessory signals to resting CD8(+) T cells. Our results show that, in comparison with anti-Ig plus anti-CD40 Ab-activated B cells, the LPS-activated B cells (LPS-B) failed to induce significant levels of proliferation, cytokine secretion, and cytotoxic ability of CD8(+) T cells. This hyporesponsiveness of CD8(+) T cells activated with LPS-B was significantly rescued by anti-TGF-beta1 Ab. Moreover, it was found that such hyporesponsive CD8(+) T cells activated with LPS-B had entered a state of anergy. Furthermore, LPS-B expresses a significantly higher level of TGF-beta1 on the surface, which caused the observed hyporesponsiveness of CD8(+) T cells. Therefore, this study, for the first time, provides a novel mechanism of B cell surface TGF-beta1-mediated hyporesponsiveness leading to anergy of CD8(+) T cells.

Evidence that glycoprotein 96 (B2), a stress protein, functions as a Th2-specific costimulatory molecule.

After the engagement of Ag receptor, most of the Th cells for their optimal activation require a second (costimulatory) signal provided by the APCs. We demonstrate the isolation and characterization of a 99- to 105-kDa protein (B2), from LPS-activated B cell surface, and its function as a Th2-specific costimulatory molecule. Appearance of B2 as a single entity on two-dimensional gel electrophoresis and as a distinct peak in reverse-phase HPLC ascertains the fact that B2 is homogeneous in preparation. Electron microscopy as well as competitive binding studies reveal that (125)I-labeled B2 specifically binds anti-CD3-activated T cell surface and also competes with its unlabeled form. Internal amino acid sequences of B2 are found to be identical with stress protein gp96. The identity of B2 as gp96 is also revealed by immunological characterization and by confocal microscopic colocalization studies of B2 and gp96 on LPS-activated B cells. Confocal imaging studies also demonstrate that gp96 can be induced on B cell surface without association of MHC molecules. Furthermore, the novel role of gp96 in Th cell proliferation skewing its differentiation toward Th2 phenotype has also been established. Ab-mediated blocking of gp96-induced signaling not only abrogates in vitro proliferation of CD4(+) T cells, but also diminishes the secretion of Th2-specific cytokines. Notably, the expression of CD91 (receptor of gp96/B2) is up-regulated on anti-CD3-activated Th cells and also found to be present on Th1 and Th2 subsets.

The Th1-specific costimulatory molecule, m150, is a posttranslational isoform of lysosome-associated membrane protein-1.

In an earlier report, we had shown a 150-kDa protein termed as M150, isolated from the surface of activated macrophages, to possess costimulatory activity for CD4(+) T cells. Significantly, this protein was found to specifically elicit Th1 responses. In this study, we characterize M150, which belongs to a unique subset of the lysosome-associated membrane protein-1 glycoprotein. Interestingly, the costimulatory activity of M150 depends on its posttranslational modification, which has a distinct glycosylation pattern restricted to macrophages. Furthermore, it has been demonstrated that in addition to stimulating Th1-specific responses, M150 is also capable of driving differentiation of naive CD4(+) T cells into the Th1 subset. This altered posttranslational modification of housekeeping protein appears to represent a novel pathway by which APCs can additionally regulate T cell responses.