Dual Role of the Tyrosine Kinase Syk in Regulation of Toll-Like Receptor Signaling in Plasmacytoid Dendritic Cells

Targeted Small Molecules for Systemic Lupus Erythematosus: Drugs in the Pipeline

Despite the uncertainty of the pathogenesis of systemic lupus erythematosus, novel small molecules targeting specific intracellular mechanisms of immune cells are being developed to reverse the pathophysiological processes. These targeted molecules have the advantages of convenient administration, lower production costs, and the lack of immunogenicity. The Janus kinases, Bruton's tyrosine kinases, and spleen tyrosine kinases are important enzymes for activating downstream signals from various receptors on immune cells that include cytokines, growth factor, hormones, Fc, CD40, and B-cell receptors. Suppression of these kinases impairs cellular activation, differentiation, and survival, leading to diminished cytokine actions and autoantibody secretion. Intracellular protein degradation by immunoproteasomes, levered by the cereblon E3 ubiquitin ligase complex, is an essential process for the regulation of cellular functions and survival. Modulation of the immunoproteasomes and cereblon leads to depletion of long-lived plasma cells, reduced plasmablast differentiation, and production of autoantibodies and interferon-α. The sphingosine 1-phosphate/sphingosine 1-phosphate receptor-1 pathway is responsible for lymphocyte trafficking, regulatory T-cell/Th17 cell homeostasis, and vascular permeability. Sphingosine 1-phosphate receptor-1 modulators limit the trafficking of autoreactive lymphocytes across the blood-brain barrier, increase regulatory T-cell function, and decrease production of autoantibodies and type I interferons. This article summarizes the development of these targeted small molecules in the treatment of systemic lupus erythematosus, and the future prospect for precision medicine.

Immune cells surveil aberrantly sialylated O-glycans on megakaryocytes to regulate platelet count

Immune thrombocytopenia (ITP) is a platelet disorder. Pediatric and adult ITP have been associated with sialic acid alterations, but the pathophysiology of ITP remains elusive, and ITP is often a diagnosis of exclusion. Our analysis of pediatric ITP plasma samples showed increased anti-Thomsen-Friedenreich antigen (TF antigen) antibody representation, suggesting increased exposure of the typically sialylated and cryptic TF antigen in these patients. The O-glycan sialyltransferase St3gal1 adds sialic acid specifically on the TF antigen. To understand if TF antigen exposure associates with thrombocytopenia, we generated a mouse model with targeted deletion of St3gal1 in megakaryocytes (MK) (St3gal1MK-/-). TF antigen exposure was restricted to MKs and resulted in thrombocytopenia. Deletion of Jak3 in St3gal1MK-/- mice normalized platelet counts implicating involvement of immune cells. Interferon-producing Siglec H-positive bone marrow (BM) immune cells engaged with O-glycan sialic acid moieties to regulate type I interferon secretion and platelet release (thrombopoiesis), as evidenced by partially normalized platelet count following inhibition of interferon and Siglec H receptors. Single-cell RNA-sequencing determined that TF antigen exposure by MKs primed St3gal1MK-/- BM immune cells to release type I interferon. Single-cell RNA-sequencing further revealed a new population of immune cells with a plasmacytoid dendritic cell-like signature and concomitant upregulation of the immunoglobulin rearrangement gene transcripts Igkc and Ighm, suggesting additional immune regulatory mechanisms. Thus, aberrant TF antigen moieties, often found in pathological conditions, regulate immune cells and thrombopoiesis in the BM, leading to reduced platelet count.

CD303 (BDCA-2) - a potential novel target for therapy in hematologic malignancies

Plasmacytoid dendritic cells (pDCs) serve as immunoregulatory antigen-presenting cells that play a role in various inflammatory, viral, and malignant conditions. Malignant proliferation of pDCs is implicated in the pathogenesis of certain hematologic cancers, specifically blastic plasmacytoid dendritic cell neoplasm (BPDCN) and acute myelogenous leukemia with clonal expansion of pDC (pDC-AML). In recent years, BPDCN and pDC-AML have been successfully treated with targeted therapy of pDC-specific surface marker, CD123. However, relapsed and refractory BPDCN remains an elusive cancer, with limited therapeutic options. CD303 is another specific surface marker of human pDCs, centrally involved in antigen presentation and immune tolerance. Monoclonal antibodies directed against CD303 have been studied in preclinical models and have achieved disease control in patients with cutaneous lupus erythematosus. We performed a comprehensive review of benign and malignant disorders in which CD303 have been studied, as there may be a potential future CD303-directed therapy for many of these conditions.

Characterization of the mechanism of action of lanraplenib, a novel spleen tyrosine kinase inhibitor, in models of lupus nephritis

Background

B cells are critical mediators of systemic lupus erythematosus (SLE) and lupus nephritis (LN), and antinuclear antibodies can be found in the serum of approximately 98% of patients with SLE. Spleen tyrosine kinase (SYK) is a nonreceptor tyrosine kinase that mediates signaling from immunoreceptors, including the B cell receptor. Active, phosphorylated SYK has been observed in tissues from patients with SLE or cutaneous lupus erythematosus, and its inhibition is hypothesized to ameliorate disease pathogenesis. We sought to evaluate the efficacy and characterize the mechanism of action of lanraplenib, a selective oral SYK inhibitor, in the New Zealand black/white (NZB/W) murine model of SLE and LN.

Methods

Lanraplenib was evaluated for inhibition of primary human B cell functions in vitro. Furthermore, the effect of SYK inhibition on ameliorating LN-like disease in vivo was determined by treating NZB/W mice with lanraplenib, cyclophosphamide, or a vehicle control. Glomerulopathy and immunoglobulin G (IgG) deposition were quantified in kidneys. The concentration of proinflammatory cytokines was measured in serum. Splenocytes were analyzed by flow cytometry for B cell maturation and T cell memory maturation, and the presence of T follicular helper and dendritic cells.

Results

In human B cells in vitro, lanraplenib inhibited B cell activating factor-mediated survival as well as activation, maturation, and immunoglobulin M production. Treatment of NZB/W mice with lanraplenib improved overall survival, prevented the development of proteinuria, and reduced blood urea nitrogen concentrations. Kidney morphology was significantly preserved by treatment with lanraplenib as measured by glomerular diameter, protein cast severity, interstitial inflammation, vasculitis, and frequency of glomerular crescents; treatment with lanraplenib reduced glomerular IgG deposition. Mice treated with lanraplenib had reduced concentrations of serum proinflammatory cytokines. Lanraplenib blocked disease-driven B cell maturation and T cell memory maturation in the spleen.

Conclusions

Lanraplenib blocked the progression of LN-like disease in NZB/W mice. Human in vitro and murine in vivo data suggest that lanraplenib may be efficacious in preventing disease progression in patients with LN at least in part by inhibiting B cell maturation. These data provide additional rationale for the use of lanraplenib in the treatment of SLE and LN.

Supplementary Information

The online version contains supplementary material available at 10.1186/s41927-021-00178-3.

Interferon-directed therapies for the treatment of systemic lupus erythematosus: a critical update

The interferon (IFN) pathway, especially type I IFN, plays a critical role in the immunopathogenesis of systemic lupus erythematosus (SLE). We have gained significant insights into this pathway over the past two decades, including a better understanding of the key mediators of inflammation upstream and downstream of type I IFN. This has led to the identification of multiple potential targets for the treatment of SLE, for which a significant unmet need remains due to the failure of many patients to adequately respond to standard-of-care medications. Unfortunately, most new therapies in SLE have disappointed in preclinical or clinical trials to date, including a number that target type I IFN. Nevertheless, several IFN-directed therapies aimed at specific steps within this immunologic pathway have recently shown promise, and additional agents are in the treatment pipeline. In this review, we focus on the results of key therapeutic studies targeting the type I IFN pathway and discuss the future state of IFN-blockade in SLE.

Zika Virus Inhibits IFN-α Response by Human Plasmacytoid Dendritic Cells and Induces NS1-Dependent Triggering of CD303 (BDCA-2) Signaling

Zika virus (ZIKV) dramatically emerged in French Polynesia and subsequently in the Americas where it has been associated with severe neurological complications in adults and newborns, respectively. Although plasmacytoid dendritic cells (pDCs) are a key sensor of viral infection and are critical for initiating an antiviral response, little is known about the impact of ZIKV infection on pDCs. Here, we investigated the susceptibility of human pDCs to infection with multiple strains of ZIKV and further investigated the impact of infection on pDCs functions. We observed that pDCs were refractory to cell-free ZIKV virions but were effectively infected when co-cultured with ZIKV-infected cells. However, exposure of pDCs to ZIKV-infected cells resulted in limited maturation/activation with significant down regulation of CD303 expression, a severe impairment of inflammatory cytokine production, and an inability to mount an IFN-α response. We show that ZIKV developed a strategy to inhibit the IFN-α response in primary human pDCs likely mediated through NS1-dependent CD303 signaling, thus suggesting a new mechanism of immune evasion.

Toll-like receptor dual-acting agonists are potent inducers of PBMC-produced cytokines that inhibit hepatitis B virus production in primary human hepatocytes

Recombinant interferon-α (IFN-α) treatment functionally cures chronic hepatitis B virus (HBV) infection in some individuals and suppresses virus replication in hepatocytes infected in vitro. We studied the antiviral effect of conditioned media (CM) from peripheral blood mononuclear cells (PBMCs) stimulated with agonists of Toll-like receptors (TLRs) 2, 7, 8 and 9. We found that CM from PBMCs stimulated with dual-acting TLR7/8 (R848) and TLR2/7 (CL413) agonists were more potent drivers of inhibition of HBe and HBs antigen secretion from HBV-infected primary human hepatocytes (PHH) than CM from PBMCs stimulated with single-acting TLR7 (CL264) or TLR9 (CpG-B) agonists. Inhibition of HBV in PHH did not correlate with the quantity of PBMC-produced IFN-α, but it was a complex function of multiple secreted cytokines. More importantly, we found that the CM that efficiently inhibited HBV production in freshly isolated PHH via various cytokine repertoires and mechanisms did not reduce covalently closed circular (ccc)DNA levels. We confirmed our data with a cell culture model based on HepG2-NTCP cells and the plasmacytoid dendritic cell line GEN2.2. Collectively, our data show the importance of dual-acting TLR agonists inducing broad cytokine repertoires. The development of poly-specific TLR agonists provides novel opportunities towards functional HBV cure.

The role of Toll-like receptor signaling pathways in cerebrovascular disorders: the impact of spreading depolarization

Cerebral vascular diseases (CVDs) are a group of disorders that affect the blood supply to the brain and lead to the reduction of oxygen and glucose supply to the neurons and the supporting cells. Spreading depolarization (SD), a propagating wave of neuroglial depolarization, occurs in different CVDs. A growing amount of evidence suggests that the inflammatory responses following hypoxic-ischemic insults and after SD plays a double-edged role in brain tissue injury and clinical outcome; a beneficial effect in the acute phase and a destructive role in the late phase. Toll-like receptors (TLRs) play a crucial role in the activation of inflammatory cascades and subsequent neuroprotective or harmful effects after CVDs and SD. Here, we review current data regarding the pathophysiological role of TLR signaling pathways in different CVDs and discuss the role of SD in the potentiation of the inflammatory cascade in CVDs through the modulation of TLRs.

Role of spleen tyrosine kinase in liver diseases

Spleen tyrosine kinase (SYK) is a non-receptor tyrosine kinase expressed in most hematopoietic cells and non-hematopoietic cells and play a crucial role in both immune and non-immune biological responses. SYK mediate diverse cellular responses via an immune-receptor tyrosine-based activation motifs (ITAMs)-dependent signalling pathways, ITAMs-independent and ITAMs-semi-dependent signalling pathways. In liver, SYK expression has been observed in parenchymal (hepatocytes) and non-parenchymal cells (hepatic stellate cells and Kupffer cells), and found to be positively correlated with the disease severity. The implication of SYK pathway has been reported in different liver diseases including liver fibrosis, viral hepatitis, alcoholic liver disease, non-alcoholic steatohepatitis and hepatocellular carcinoma. Antagonism of SYK pathway using kinase inhibitors have shown to attenuate the progression of liver diseases thereby suggesting SYK as a highly promising therapeutic target. This review summarizes the current understanding of SYK and its therapeutic implication in liver diseases.

Endosomal toll-like receptors play a key role in activation of primary human monocytes by cowpea mosaic virus

The plant virus, cowpea mosaic virus (CPMV), has demonstrated a remarkable capacity to induce anti-tumour immune responses following direct administration into solid tumours. The molecular pathways that account for these effects and the capacity of CPMV to activate human cells are not well defined. Here, we examine the ability of CPMV particles to activate human monocytes, dendritic cells (DCs) and macrophages. Monocytes in peripheral blood mononuclear cell cultures and purified CD14+ monocytes were readily activated by CPMV in vitro, leading to induction of HLA-DR, CD86, PD-L1, IL-15R and CXCL10 expression. Monocytes released chemokines, CXCL10, MIP-1α and MIP-1β into cell culture supernatants after incubation with CPMV. DC subsets (pDC and mDC) and monocyte-derived macrophages also demonstrated evidence of activation after incubation with CPMV. Inhibitors of spleen tyrosine kinase (SYK), endocytosis or endocytic acidification impaired the capacity of CPMV to activate monocytes. Furthermore, CPMV activation of monocytes was partially blocked by a TLR7/8 antagonist. These data demonstrate that CPMV activates human monocytes in a manner dependent on SYK signalling, endosomal acidification and with an important contribution from TLR7/8 recognition.

Therapeutic Implication of SOCS1 Modulation in the Treatment of Autoimmunity and Cancer

The suppressor of cytokine signaling (SOCS) family of intracellular proteins has a vital role in the regulation of the immune system and resolution of inflammatory cascades. SOCS1, also called STAT-induced STAT inhibitor (SSI) or JAK-binding protein (JAB), is a member of the SOCS family with actions ranging from immune modulation to cell cycle regulation. Knockout of SOCS1 leads to perinatal lethality in mice and increased vulnerability to cancer, while several SNPs associated with the SOCS1 gene have been implicated in human inflammation-mediated diseases. In this review, we describe the mechanism of action of SOCS1 and its potential therapeutic role in the prevention and treatment of autoimmunity and cancer. We also provide a brief outline of the other JAK inhibitors, both FDA-approved and under investigation.

The pipeline of targeted therapies under clinical development for primary Sjögren's syndrome: A systematic review of trials

To date, no immunomodulatory drug has proved efficacious in primary Sjögren's syndrome (pSS). In pSS, difficulties in drug efficacy assessment is related to the large spectrum of clinical involvements (glandular/extraglandular involvement), to the lack of correlation between symptoms of dryness and glandular function assessed by objective measurements, as well as between symptoms and systemic complications of the disease. Severe organ manifestations are generally treated by off-label therapies in accordance with current practice and guidelines for Systemic Lupus Erythematosus or other connective-tissue diseases. Despite a much greater understanding of the pathogenesis of pSS, modern drug development has resulted in no approval of therapy so far. In this study, we performed a systematic review of all targeted therapies under clinical development in pSS, in 17 main online registries of clinical trials. Our search identified 264 trials, from which 25 targeted therapies for pSS were included. The molecules under current clinical development for pSS target B cells (n = 4), T cells or T/B cells costimulation (n = 5), inflammatory cytokines or chemokines and their receptors (n = 5), intracellular signalling pathways (n = 7) and various other targets identified in pSS (n = 4). The current drug development pipeline in pSS may lead to valuable strategies for the treatment of this currently difficult-to-treat disease.

Effect of in vivo Hydroxychloroquine and ex vivo Anti-BDCA2 mAb Treatment on pDC IFNα Production From Patients Affected With Cutaneous Lupus Erythematosus

Objective: Plasmacytoid dendritic cells (pDCs) are a major source of Type-I Interferon (IFN-I), a key driver in cutaneous lupus erythematosus (CLE). Currently evaluated in Phase II clinical trial, 24F4A (BIIB059) is an antibody targeting BDCA2, an inhibitory receptor expressed on pDCs. Given that Hydroxychloroquine (HCQ), a widely-used CLE therapy, and 24F4A are both able to inhibit pDC-derived IFN-I production; this study aimed to determine whether 24F4A would show an additional inhibitory effect on pDC response after ex vivo or in vivo treatment with HCQ.

Methods: The effect of 24F4A on pDC-derived IFNα was measured from peripheral blood mononuclear cells (PBMC) either from healthy donors in presence or absence of HCQ or from CLE patients clinically exposed to various levels of HCQ. TLR7, TLR7/8, and TLR9 agonists (ssRNA, R848, and CpG-A) were used for pDC stimulation.

Results: PDCs were the only producers of IFNα in response to CpG-A, R848, and ssRNA stimulation in PBMC cultures. CLE patients with higher levels of blood HCQ showed lower ex vivo pDC responses to CpG-A, but not R848 or ssRNA. In contrast, 24F4A reduced the amount of IFNα produced by pDCs from CLE patients in response to all TLR agonists, irrespective of the blood HCQ level.

Conclusion: Our findings reveal that clinically-relevant HCQ concentrations partially inhibit the pDC response to TLR9 and weakly affect the response to TLR7/8 stimulation. 24F4A robustly inhibits pDC responses even in the presence of HCQ, highlighting its unique potential to disrupt pDC disease relevant biology, which could provide additional therapeutic benefit for CLE patients.

Plasmacytoid Dendritic Cells: Development, Regulation, and Function

Plasmacytoid dendritic cells (pDCs) are a unique sentinel cell type that can detect pathogen-derived nucleic acids and respond with rapid and massive production of type I interferon. This review summarizes our current understanding of pDC biology, including transcriptional regulation, heterogeneity, role in antiviral immune responses, and involvement in immune pathology, particularly in autoimmune diseases, immunodeficiency, and cancer. We also highlight the remaining gaps in our knowledge and important questions for the field, such as the molecular basis of unique interferon-producing capacity of pDCs. A better understanding of cell type-specific positive and negative control of pDC function should pave the way for translational applications focused on this immune cell type.

Inhibition of spleen tyrosine kinase attenuates psoriasis-like inflammation in mice through blockade of dendritic cell-Th17 inflammation axis

Psoriasis is a debilitating autoimmune disease of the skin characterized by acanthosis and hyperkeratosis resulting from excessive growth of keratinocytes in the epidermis and inflammatory infiltrates in the dermis. Innate immune cells such as dendritic cells (DCs), perform a critical role in the pathophysiology of psoriasis by presenting inflammatory/costimulatory signals for differentiation of Th17 cells. Recent studies point to the involvement of spleen tyrosine kinase (SYK) in inflammatory signaling cascade of DCs. However, it is yet to be determined whether SYK inhibition in DCs would lead to diminishment of psoriatic inflammation. Therefore, our study evaluated the effects of SYK inhibitor, R406 on imiquimod (IMQ)-induced psoriasis-like inflammation, expression of costimulatory/inflammatory molecules in DCs and their relationship with Th17/Treg cells. Our data show that R406 causes attenuation of IMQ-induced dermal inflammation as shown by reduction in ear/back skin thickness, acanthosis and myeloperoxidase activity. This was concurrent with reduction in inflammatory cytokines and co-stimulatory molecules in CD11c + DCs such as IL-6, IL-23, MHCII, and CD40. This favoured the suppression of Th17 cells and upregulation of Treg cells in R406-treated mice with psoriasis-like inflammation. Direct activation of TLR7 by IMQ in splenocytic cultures led to increased SYK expression in CD11c + DCs and release of IL-23/IL-6. IMQ-induced IL-6/IL-23 levels were significantly diminished by SYK inhibitor, R406 in splenocytic cultures. In essence, our study shows that SYK inhibition supresses psoriasis-like inflammation by modifying DC function in mice. Further, it implies that SYK inhibition could be a prospective therapeutic approach for the treatment of psoriasis-like inflammation.

Tyrosine kinase SYK is a potential therapeutic target for liver fibrosis

Spleen tyrosine kinase (SYK) plays a critical role in immune cell signaling pathways and has been reported as a biomarker for human hepatocellular carcinoma (HCC). We sought to investigate the mechanism by which SYK promotes liver fibrosis and to evaluate SYK as a therapeutic target for liver fibrosis. We evaluated the cellular localization of SYK and the association between SYK expression and liver fibrogenesis in normal, hepatitis B virus (HBV)-infected, hepatitis C virus (HCV)-infected and non-alcoholic steatohepatitis (NASH) liver tissue (n=36, 127, 22 and 30, respectively). A polymerase chain reaction (PCR) array was used to detect the changes in transcription factor (TF) expression in hepatic stellate cells (HSCs) with SYK knockdown. The effects of SYK antagonism on liver fibrogenesis were studied in LX-2 cells, TWNT-4 cells, primary human HSCs, and three progressive fibrosis/cirrhosis animal models, including a CCL₄ mouse model, and diethylnitrosamine (DEN) and bile duct ligation (BDL) rat models. We found that SYK protein in HSCs and hepatocytes correlated positively with liver fibrosis stage in human liver tissue. HBV or HCV infection significantly increased SYK and cytokine expression in hepatocytes. Increasing cytokine production further induced SYK expression and fibrosis-related gene transcription in HSCs. Up-regulated SYK in HSCs promoted HSC activation by increasing the expression of specific TFs related to activation of HSCs. SYK antagonism effectively suppressed liver fibrosis via inhibition of HSC activation, and decreased obstructive jaundice and reduced HCC development in animal models. Conclusion: SYK promotes liver fibrosis via activation of HSCs and is an attractive potential therapeutic target for liver fibrosis and prevention of HCC development. (Hepatology 2018).

The 2018 pipeline of targeted therapies under clinical development for Systemic Lupus Erythematosus: a systematic review of trials

Currently, Systemic Lupus Erythematosus (SLE) therapies range from antimalarials to glucocorticoids, in addition to immunosupressive agents or biologics such as rituximab or belimumab, when needed. Several unmet needs remain in the treatment SLE and more targeted drugs with improved safety profiles are expected. Based on recent advances in the understanding of the complex pathogenesis of SLE, several targeted treatments are currently assessed in clinical trials. In this study, we performed a systematic review of all targeted therapies under clinical development in SLE in 17 online registries of clinical trials. The search yielded a total of 1140 trials, from which we identified 74 targeted therapies for SLE. Those treatments target inflammatory cytokines, chemokines, or their receptors (n = 17), B cells or plasma cells (n = 17), intracellular signalling pathways (n = 10), T/B cells costimulation molecules (n = 8), interferons (n = 7), plasmacytoid dendritic cells (pDC) (n = 3), as well as various other targets (n = 12). Not all these candidate drugs will reach phase III, but the broad spectrum of drugs being investigated may satisfy the urgent need for improved lupus medications. The identification of biomarkers that would allow adequate prediction of response-to-therapy remains high, but when solved will allow a more rationale selection of the optimal pharmacological agent at the patient level.

Viruses Seen by Our Cells: The Role of Viral RNA Sensors

The role of the innate immune response in detecting RNA viruses is crucial for the establishment of proper inflammatory and antiviral responses. Different receptors, known as pattern recognition receptors (PRRs), are present in the cytoplasm, endosomes, and on the cellular surface. These receptors have the capacity to sense the presence of viral nucleic acids as pathogen-associated molecular patterns (PAMPs). This recognition leads to the induction of type 1 interferons (IFNs) as well as inflammatory cytokines and chemokines. In this review, we provide an overview of the significant involvement of cellular RNA helicases and Toll-like receptors (TLRs) 3, 7, and 8 in antiviral immune defenses.

Expression of TIM-3 on Plasmacytoid Dendritic Cells as a Predictive Biomarker of Decline in HIV-1 RNA Level during ART

Depletion and functional impairment of circulating plasmacytoid dendritic cells (pDCs) are characteristic attributes of HIV-1-infection. The mechanism of dysfunction of pDCs is unclear. Here, we studied the development of phenotype of pDCs in a cohort of HIV-1-infected individuals monitored before the initiation and during a 9-month follow up with antiretroviral therapy (ART). Using polychromatic flow cytometry, we detected significantly higher pDC-surface expression of the HIV-1 receptor CD4, regulatory receptor BDCA-2, Fcγ receptor CD32, pDC dysfunction marker TIM-3, and the marker of killer pDC, TRAIL, in treatment-naïve HIV-1-infected individuals before initiation of ART when compared to healthy donors. After 9 months of ART, all of these markers approached but did not reach the expression levels observed in healthy donors. We found that the rate of decline in HIV-1 RNA level over the first 3 months of ART negatively correlated with the expression of TIM-3 on pDCs. We conclude that immunogenic phenotype of pDCs is not significantly restored after sustained suppression of HIV-1 RNA level in ART-treated patients and that the level of the TIM-3 expressed on pDCs in treatment naïve patients could be a predictive marker of the rate of decline in the HIV-1 RNA level during ART.

The MEK1/2-ERK Pathway Inhibits Type I IFN Production in Plasmacytoid Dendritic Cells

Recent studies have reported that the crosslinking of regulatory receptors (RRs), such as blood dendritic cell antigen 2 (BDCA-2) (CD303) or ILT7 (CD85g), of plasmacytoid dendritic cells (pDCs) efficiently suppresses the production of type I interferons (IFN-I, α/β/ω) and other cytokines in response to toll-like receptor 7 and 9 (TLR7/9) ligands. The exact mechanism of how this B cell receptor (BCR)-like signaling blocks TLR7/9-mediated IFN-I production is unknown. Here, we stimulated BCR-like signaling by ligation of RRs with BDCA-2 and ILT7 mAbs, hepatitis C virus particles, or BST2 expressing cells. We compared BCR-like signaling in proliferating pDC cell line GEN2.2 and in primary pDCs from healthy donors, and addressed the question of whether pharmacological targeting of BCR-like signaling can antagonize RR-induced pDC inhibition. To this end, we tested the TLR9-mediated production of IFN-I and proinflammatory cytokines in pDCs exposed to a panel of inhibitors of signaling molecules involved in BCR-like, MAPK, NF-ĸB, and calcium signaling pathways. We found that MEK1/2 inhibitors, PD0325901 and U0126 potentiated TLR9-mediated production of IFN-I in GEN2.2 cells. More importantly, MEK1/2 inhibitors significantly increased the TLR9-mediated IFN-I production blocked in both GEN2.2 cells and primary pDCs upon stimulation of BCR-like or phorbol 12-myristate 13-acetate-induced protein kinase C (PKC) signaling. Triggering of BCR-like and PKC signaling in pDCs resulted in an upregulation of the expression and phoshorylation of c-FOS, a downstream gene product of the MEK1/2-ERK pathway. We found that the total level of c-FOS was higher in proliferating GEN2.2 cells than in the resting primary pDCs. The PD0325901-facilitated restoration of the TLR9-mediated IFN-I production correlated with the abrogation of MEK1/2-ERK-c-FOS signaling. These results indicate that the MEK1/2-ERK pathway inhibits TLR9-mediated type I IFN production in pDCs and that pharmacological targeting of MEK1/2-ERK signaling could be a strategy to overcome immunotolerance of pDCs and re-establish their immunogenic activity.

Targeting interferons and their pathways in systemic lupus erythematosus

Significant advances in the understanding of the molecular basis of innate immunity have led to the identification of interferons (IFNs), particularly IFN-α, as central mediators in the pathogenesis of Systemic Lupus Erythematosus. Therefore, targeting of IFNs and of their downstream pathways has emerged as important developments for novel drug research in SLE. Based on this, several specific interferon blocking strategies using anti-IFN-α antibodies, anti-type I interferon receptor antibodies, Interferon-α-kinoid, or anti-IFN-γ antibodies have all been assessed in recent clinical trials. Alternative strategies targeting the plasmacytoid dendritic cells (pDCs), Toll-Like Receptors (TLRs)-7/9 or their downstream pathways such as the myeloid differentiation primary-response protein 88 (MYD88), spleen tyrosine kinase (Syk), Janus-kinases (JAKs), interleukin-1 receptor-associated kinase 4 (IRAK4), or the Tyrosine Kinase 2 (TYK2) are also investigated actively in SLE, at more preliminary clinical development stages, except for JAK inhibitors which have reached phase 2 studies. In a near future, in-depth and personalized functional characterization of IFN pathways may provide further guidance for the selection of the most relevant therapeutic strategy in SLE, tailored at the patient-level.

Syk inhibitor R406 downregulates inflammation in an in vitro model of Pseudomonas aeruginosa infection

As Pseudomonas aeruginosa infections are characterized by strong inflammation of infected tissues, anti-inflammatory therapies in combination with antibiotics have been considered for the treatment of associated diseases. Syk tyrosine kinase is an important regulator of inflammatory responses, and its specific inhibition was explored as a therapeutic option in several inflammatory conditions; however, this has not been studied in bacterial infections. We used a model of in vitro infection of human monocytic cell line THP-1 and lung epithelial cell line H292 with both wild-type and flagella-deficient mutant of P. aeruginosa strain K, as well as with clinical isolates from cystic fibrosis patients, to study the effect of a small molecule Syk inhibitor R406 on inflammatory responses induced by this pathogen. One-hour pretreatment of THP-1 cells with 10 μmol/L R406 resulted in a significant downregulation of the expression of the adhesion molecule ICAM-1, pro-inflammatory cytokines TNF-α and IL-1β, and phosphorylated signaling proteins ERK2, JNK, p-38, and IκBα, as well as significantly decreased TNF-α release by infected H292 cells. The results suggest that Syk is involved in the regulation of inflammatory responses to P. aeruginosa, and R406 may potentially be useful in dampening the damage caused by severe inflammation associated with this infection.

A human tissue-based functional assay platform to evaluate the immune function impact of small molecule inhibitors that target the immune system

While the immune system is essential for the maintenance of the homeostasis, health and survival of humans, aberrant immune responses can lead to chronic inflammatory and autoimmune disorders. Pharmacological modulation of drug targets in the immune system to ameliorate disease also carry a risk of immunosuppression that could lead to adverse outcomes. Therefore, it is important to understand the 'immune fingerprint' of novel therapeutics as they relate to current and, clinically used immunological therapies to better understand their potential therapeutic benefit as well as immunosuppressive ability that might lead to adverse events such as infection risks and cancer. Since the mechanistic investigation of pharmacological modulators in a drug discovery setting is largely compound- and mechanism-centric but not comprehensive in terms of immune system impact, we developed a human tissue based functional assay platform to evaluate the impact of pharmacological modulators on a range of innate and adaptive immune functions. Here, we demonstrate that it is possible to generate a qualitative and quantitative immune system impact of pharmacological modulators, which might help better understand and predict the benefit-risk profiles of these compounds in the treatment of immune disorders.

FcγRIIIa Signaling Modulates Endosomal TLR Responses in Human CD4+ T Cells

Recognition of Ab-opsonized pathogens by immune cells triggers both TLR and Fc receptor signaling. Fc receptors endocytose modified nucleic acids bound to Abs and deliver them to endosomes, where they are recognized by nucleic acid-sensing TLRs (NA-TLRs). We show that in CD4+ T cells, NA-TLRs, TLR3, TLR8, and TLR9 are upregulated by FcγRIIIa-pSyk cosignaling and localize with FcγRIIIa on the cell surface. TLR9 accumulates on the cell surface, where it recognizes CpG oligonucleotide 2006. Subcellular location of NA-TLRs is a key determinant in discriminating self versus viral nucleic acid. Hydroxychloroquine used for treating systemic lupus erythematosus and a Syk inhibitor blocked NA-TLR localization with FcγRIIIa. Engaging TLR9 with CpG oligonucleotide contributes to the development of IL17A+ and IL-21+ populations. RNA-sequencing analysis showed upregulation of proinflammatory cytokines, NF-κB signaling, and heat shock protein pathway RNA transcripts. These data suggest a role for FcγRIIIa-pSyk cosignaling in modulating NA-TLR responses in human CD4+ T cells by affecting the amounts and cellular distribution. These events are important for understanding of autoimmune pathology.

Cross Talk between Inhibitory Immunoreceptor Tyrosine-Based Activation Motif-Signaling and Toll-Like Receptor Pathways in Macrophages and Dendritic Cells

The innate immune cells sense microbial infection and self-ligands by pathogen recognition receptors (PRRs), such as toll-like receptors (TLRs) and regulatory receptors (RRs), associated with immunoreceptor tyrosine-based activation motif (ITAM). Rapid activation and concerted action of PRRs signaling and feedback inhibitory mechanisms must be engaged to ensure the host defense functions and to prevent cytotoxicity associated with excessive activation. ITAM-associated RRs can generate stimulatory or, paradoxically, inhibitory signals. The network of ITAM-associated RR, together with TLR-signaling pathways, are responsible for immunogenic or tolerogenic responses of macrophages and dendritic cells to their microenvironment. In macrophages, TLR4 signaling is inhibited by low-avidity ligation of ITAM-associated receptors, while high-avidity ligation of ITAM-associated receptors results in potentiation of TLR4 signaling together with resistance to extracellular cytokine microenvironment signals. In contrast to macrophages, TLR7/9 signaling in plasmacytoid DCs (pDCs) is inhibited by high-avidity ligation of ITAM-associated RR, while low-avidity ligation does not show any effect. Surprisingly, interference of ITAM-associated receptor signaling with TLR pathways has not been reported in conventional dendritic cells. Here, we present an overview of molecular mechanisms acting at the crossroads of TLR and ITAM-signaling pathways and address the question of how the high-avidity engagement of the ITAM-associated receptors in pDCs inhibits TLR7/9 signaling. Cellular context and spatiotemporal engagement of ITAM- and TLR-signaling pathways are responsible for different outcomes of macrophage versus pDC activation. While the cross-regulation of cytokine and TLR signaling, together with antigen presentation, are the principal functions of ITAM-associated RR in macrophages, the major role of these receptors in pDCs seems to be related to inhibition of cytokine production and reestablishment of a tolerogenic state following pDC activation. Pharmacologic targeting of TLR and ITAM signaling could be an attractive new therapeutic approach for treatment of chronic infections, cancer, and autoimmune and inflammatory diseases related to pDCs.

Proteomic Analysis of HDAC3 Selective Inhibitor in the Regulation of Inflammatory Response of Primary Microglia

HDAC3 has been shown to regulate inflammation. However, the role of HDAC3 in primary microglia is largely unknown. RGFP966 is a newly discovered selective HDAC3 inhibitor. In this study, we used protein mass spectrometry to analyze protein alterations in LPS-treated primary microglia with the application of RGFP966. Generally, about 2000 proteins were studied. 168 of 444 (37.8%) LPS-induced proteins were significantly reduced with the treatment of RGFP966, which mainly concentrated on Toll-like receptor signaling pathway. In this regard, we selected Toll-like receptor 2 (TLR2), TLR3, TLR6, MAPK p38, CD36, and spleen tyrosine kinase (SYK) for further validation and found that they were all significantly upregulated after LPS stimulation and downregulated in the presence of RGFP966. Additionally, RGFP966 inhibited supernatant tumor necrosis factor (TNF)-α and Interleukin 6 (IL-6) concentrations. Activation of STAT3 and STAT5 was partially blocked by RGFP966 at 2 h after LPS-stimulation. The fluorescence intensity of CD16/32 was significantly decreased in LPS + RGFP966-treated group. In conclusion, our data provided a hint that RGFP966 may be a potential therapeutic medication combating microglia activation and inflammatory response in central nervous system, which was probably related to its repressive impacts on TLR signaling pathways and STAT3/STAT5 pathways.