Rapid translocation of intracellular toll-like receptors depends on endosomal NADPH oxidase.

Endosomal toll-like receptors (TLRs) must be translocated from the endoplasmic reticulum (ER) to the endosome and proteolytically cleaved within the endosome before they can induce cellular signals. As ligands for these TLRs are also liberated from apoptotic or necrotic cells, this process is controlled by several mechanisms which shall ensure that there is no inadvertent activation. We have shown previously that antiphospholipid antibodies induce endosomal NADPH-oxidase (NOX) followed by the translocation of TLR7/8 to the endosome. We show now that endosomal NOX is required for the rapid translocation of TLR3, TLR7/8, and TLR9. Deficiency of gp91phox, the catalytic subunit of NOX2, or inhibition of endosomal NOX by the chloride channel blocker niflumic acid both prevent immediate (i.e., within 30 min) translocation of these TLRs as shown by confocal laser scanning microscopy. Under these conditions, the induction of mRNA synthesis for TNF-α and secretion of TNF-α is delayed by approx. 6-9 h. However, maximal expression of TNF-α mRNA or secretion of TNF-α is not significantly reduced. In conclusion, these data add NOX2 as another component involved in the orchestration of cellular responses to ligands of endosomal TLRs.

Pathogenic lipid-binding antiphospholipid antibodies are associated with severity of COVID-19.

BACKGROUND: Coronavirus disease 19 (COVID-19)-associated coagulopathy is a hallmark of disease severity and poor prognosis. The key manifestations of this prothrombotic syndrome-microvascular thrombosis, stroke, and venous and pulmonary clots-are also observed in severe and catastrophic antiphospholipid syndrome. Antiphospholipid antibodies (aPL) are detectable in COVID-19 patients, but their association with the clinical course of COVID-19 remains unproven. OBJECTIVES: To analyze the presence and relevance of lipid-binding aPL in hospitalized COVID-19 patients. METHODS: Two cohorts of 53 and 121 patients from a single center hospitalized for PCR-proven severe acute respiratory syndrome-coronavirus 2 infection were analyzed for the presence of aPL and clinical severity of COVID-19. RESULTS: We here demonstrate that lipid-binding aPL are common in COVID-19. COVID-19 patients with lipid-binding aPL have higher median concentrations of C-reactive protein and D-dimer, and are more likely to have a critical clinical course and fatal outcome. Lipid-binding aPL isolated from COVID-19 patients target the recently described cell surface complex of lysobisphosphatidic acid (LBPA) with the protein C receptor (EPCR) to induce prothrombotic and inflammatory responses in monocytes and endothelial cells. We show that B1a cells producing lipid-reactive aPL of the IgG isotype circulate in the blood of COVID-19 patients. In vivo, COVID-19 aPL accelerate thrombus formation in an experimental mouse model dependent on the recently delineated signaling pathway involving EPCR-LBPA. CONCLUSIONS: COVID-19 patients rapidly expand B1a cells secreting pathogenic lipid-binding aPL with broad thrombotic and inflammatory effects. The association with markers of inflammation and coagulation, clinical severity, and mortality suggests a causal role of aPL in COVID-19-associated coagulopathy.

Lipid presentation by the protein C receptor links coagulation with autoimmunity.

Antiphospholipid antibodies (aPLs) cause severe autoimmune disease characterized by vascular pathologies and pregnancy complications. Here, we identify endosomal lysobisphosphatidic acid (LBPA) presented by the CD1d-like endothelial protein C receptor (EPCR) as a pathogenic cell surface antigen recognized by aPLs for induction of thrombosis and endosomal inflammatory signaling. The engagement of aPLs with EPCR-LBPA expressed on innate immune cells sustains interferon- and toll-like receptor 7-dependent B1a cell expansion and autoantibody production. Specific pharmacological interruption of EPCR-LBPA signaling attenuates major aPL-elicited pathologies and the development of autoimmunity in a mouse model of systemic lupus erythematosus. Thus, aPLs recognize a single cell surface lipid-protein receptor complex to perpetuate a self-amplifying autoimmune signaling loop dependent on the cooperation with the innate immune complement and coagulation pathways.

Induction of tissue factor expression by anti-ß2-glycoprotein I is mediated by tumor necrosis factor α.

Antiphospholipid antibodies (aPL) are heterogeneous and there is evidence that binding specificity determines which cellular effects they can trigger. We have therefore hypothesised that the induction of tissue factor (TF) in monocytes and endothelial cells by aPL depends on their binding specificity. To further investigate this, we have analyzed the ability of three human monoclonal aPL with distinctly different binding specificities to induce transcription and cell surface expression of TF in monocytes and endothelial cells. Results with human monoclonal aPL were validated with IgG-fractions obtained from patients with APS. We confirmed previous results that a lipid reactive human monoclonal aPL rapidly induced TF transcription and cell surface expression in monocytes and endothelial cells. A monoclonal aPL reactive against ß2 glycoprotein I (ß2GPI) induced TF with a delayed time course. This was fully dependent on the induction of tumor necrosis factor alpha (TNFα) secretion as capture of TNFα by adalimumab prevented TF induction. This pattern was confirmed with patient IgG fractions. Both lipid reactive and anti-ß2GPI induced TF transcription. Unexpectedly, this activity of anti-ß2GPI was mediated fully by TNFα which was secreted in response to incubation with anti-ß2GPI. The role of TNFα in mediating TF induction by anti-ß2GPI may have wider implications for APS pathogenesis.

Platelet Activation by Antiphospholipid Antibodies Depends on Epitope Specificity and is Prevented by mTOR Inhibitors.

Antiphospholipid antibodies (aPL) have been reported to activate platelets. This is considered to be one of the pathogenic properties of aPL. Even though aPL heterogeneity is quite well established, little is known, if the ability to activate platelets is common to all aPL or depends on antigen specificity. To further study this issue, we analyzed the ability of three human monoclonal aPL with distinctly different antigenic specificities to activate platelets in vitro. The results obtained with human monoclonal aPL were validated with immunoglobulin G (IgG) fractions obtained from patients with antiphospholipid syndrome (APS). A co-factor-independent human monoclonal anticardiolipin aPL had no discernible effect on human platelets. Two monoclonal aPL reactive against ß2 glycoprotein I (ß2GPI) induced platelet aggregation, integrin αIIbß3 activation and P-selectin surface expression. These data could be confirmed with patient IgG fractions which could only induce aggregation, if they had anti-ß2GPI activity. Anti-ß2GPI aPL-induced platelet activation depended on interaction of aPL with the low affinity Fcγ-receptor IIa on the platelet surface. It was completely abolished by pretreatment of platelet-rich plasma with the mechanistic target of rapamycin (mTOR) inhibitors rapamycin or everolimus. This extends previous data showing that mTOR is involved in signaling of anti-ß2GPI in monocytes and endothelial cells. In conclusion, anti-ß2GPI aPL activate platelets while co-factor-independent anticardiolipin aPL have no effect. mTOR is involved in this signaling process which has implications beyond APS, because so far the role of mTOR signaling in platelets is incompletely explored and requires further study.

Hydroxychloroquine inhibits proinflammatory signalling pathways by targeting endosomal NADPH oxidase.

OBJECTIVES: Hydroxychloroquine (HCQ) has been used for decades to treat patients with rheumatic diseases, for example, systemic lupus erythematosus (SLE), rheumatoid arthritis or the antiphospholipid syndrome (APS). We hypothesise that HCQ might target endosomal NADPH oxidase (NOX), which is involved in the signal transduction of cytokines as well as antiphospholipid antibodies (aPL). METHODS: For in vitro experiments, monocytic cells were stimulated with tumour necrosis factor α (TNFα), interleukin-1ß (IL-1ß) or a human monoclonal aPL and the activity of NOX was determined by flow cytometry. The expression of genes known to be induced by these stimuli was quantified by quantitative reverse transcription PCR. Live cell imaging was performed by confocal laser scanning microscopy. Finally, the effects of HCQ on NOX-induced signal transduction were analysed in an in vivo model of venous thrombosis. RESULTS: HCQ strongly reduces or completely prevents the induction of endosomal NOX by TNFα, IL-1ß and aPL in human monocytes and MonoMac1 cells. As a consequence, induction of downstream genes by these stimuli is reduced or abrogated. This effect of HCQ is not mediated by direct interference with the agonists but by inhibiting the translocation of the catalytic subunit of NOX2 (gp91phox) into the endosome. In vivo, HCQ protects mice from aPL-induced and NOX2-mediated thrombus formation. CONCLUSIONS: We describe here a novel mechanism of action of HCQ, that is, interference with the assembly of endosomal NOX2. Since endosomal NOX2 is involved in many inflammatory and prothrombotic signalling pathways, this activity of HCQ might explain many of its beneficial effects in rheumatic diseases including the APS.

Combined B, T and NK Cell Deficiency Accelerates Atherosclerosis in BALB/c Mice.

This study focused on the unique properties of both the Ldlr knockout defect (closely mimicking the human situation) and the BALB/c (C) inbred mouse strain (Th-2 slanted immune response). We generated two immunodeficient strains with severe combined B- and T-cell immunodeficiency with or without a complete lack of natural killer cells to revisit the role of adaptive immune responses on atherogenesis. C-Ldlr-/- Rag1-/- mice, which show severe combined B- and T-cell immunodeficiency and C-Ldlr-/- Rag1-/- Il2rg-/- mice, which combine the T- and B-cell defect with a complete lack of natural killer cells and inactivation of multiple cytokine signalling pathways were fed an atherogenic Western type diet (WTD). Both B6-Ldlr-/- and C-Ldlr-/- immunocompetent mice were used as controls. Body weights and serum cholesterol levels of both immunodeficient strains were significantly increased compared to C-Ldlr-/- controls, except for cholesterol levels of C-Ldlr-/- Rag1-/- double mutants after 12 weeks on the WTD. Quantification of the aortic sinus plaque area revealed that both strains of immunodeficient mice developed significantly more atherosclerosis compared to C-Ldlr-/- controls after 24 weeks on the WTD. Increased atherosclerotic lesion development in C-Ldlr-/- Rag1-/- Il2rg-/- triple mutants was associated with significantly increased numbers of macrophages and significantly decreased numbers of smooth muscle cells compared to both C-Ldlr-/- wild type and C-Ldlr-/- Rag1-/- double mutants pointing to a plaque destabilizing effect of NK cell loss. Collectively, the present study reveals a previously unappreciated complexity with regard to the impact of lymphocytes on lipoprotein metabolism and the role of lymphocyte subsets in plaque composition.

Cofactor-independent antiphospholipid antibodies activate the NLRP3-inflammasome via endosomal NADPH-oxidase: implications for the antiphospholipid syndrome.

The antiphospholipid syndrome (APS) is an autoimmune disease characterised by thromboembolic events and/or pregnancy morbidity in the presence of antiphospholipid antibodies (aPL). Here we show that three cofactor independent human monoclonal aPL can induce transcription of NLRP3 and caspase-1 resulting in inflammasome activation specific for NLRP3. This depends fully on activation of endosomal NADPH-oxidase-2 (NOX2) by aPL. Activation of NOX2 and subsequent inflammasome activation by aPL are independent from TLR2 or TLR4. While endosomal superoxide production induces caspase-1 and NLRP3 transcription, it does not affect prae-IL-1ß transcription. Therefore, release of IL-1ß occurs only after activation of additional pathways like TLR7/8 or TLR2. All effects exerted by the monoclonal aPL can be reproduced with IgG fractions of APS patients proving that the monoclonal aPL are representative for the APS. IgG fractions of healthy controls or patients suffering from systemic lupus erythematosus have no effect. In a mouse model of the APS we can show inflammasome activation in vivo. Furthermore, mononuclear cells isolated from patients with the APS show an increased expression of caspase-1 and NLRP3 which is accompanied by a three-fold increased serum concentration of IL-1ß suggesting chronic inflammasome activation in APS patients. In summary, we provide further evidence that endosomal NOX2 can be activated by cofactor independent aPL. This leads to induction of the NLRP3 inflammasome. Our data indicate that cofactor independent aPL might contribute significantly to the pathogenesis of the APS.

Impact of glutathione peroxidase-1 deficiency on macrophage foam cell formation and proliferation: implications for atherogenesis.

Clinical and experimental evidence suggests a protective role for the antioxidant enzyme glutathione peroxidase-1 (GPx-1) in the atherogenic process. GPx-1 deficiency accelerates atherosclerosis and increases lesion cellularity in ApoE(-/-) mice. However, the distribution of GPx-1 within the atherosclerotic lesion as well as the mechanisms leading to increased macrophage numbers in lesions is still unknown. Accordingly, the aims of the present study were (1) to analyze which cells express GPx-1 within atherosclerotic lesions and (2) to determine whether a lack of GPx-1 affects macrophage foam cell formation and cellular proliferation. Both in situ-hybridization and immunohistochemistry of lesions of the aortic sinus of ApoE(-/-) mice after 12 weeks on a Western type diet revealed that both macrophages and - even though to a less extent - smooth muscle cells contribute to GPx-1 expression within atherosclerotic lesions. In isolated mouse peritoneal macrophages differentiated for 3 days with macrophage-colony-stimulating factor (MCSF), GPx-1 deficiency increased oxidized low density-lipoprotein (oxLDL) induced foam cell formation and led to increased proliferative activity of peritoneal macrophages. The MCSF- and oxLDL-induced proliferation of peritoneal macrophages from GPx-1(-/-)ApoE(-/-) mice was mediated by the p44/42 MAPK (p44/42 mitogen-activated protein kinase), namely ERK1/2 (extracellular-signal regulated kinase 1/2), signaling pathway as demonstrated by ERK1/2 signaling pathways inhibitors, Western blots on cell lysates with primary antibodies against total and phosphorylated ERK1/2, MEK1/2 (mitogen-activated protein kinase kinase 1/2), p90RSK (p90 ribosomal s6 kinase), p38 MAPK and SAPK/JNK (stress-activated protein kinase/c-Jun N-terminal kinase), and immunohistochemistry of mice atherosclerotic lesions with antibodies against phosphorylated ERK1/2, MEK1/2 and p90RSK. Representative effects of GPx-1 deficiency on both macrophage proliferation and MAPK phosphorylation could be abolished by the GPx mimic ebselen. The present study demonstrates that GPx-1 deficiency has a significant impact on macrophage foam cell formation and proliferation via the p44/42 MAPK (ERK1/2) pathway encouraging further studies on new therapeutic strategies against atherosclerosis.

Endosomal NADPH-oxidase is critical for induction of the tissue factor gene in monocytes and endothelial cells. Lessons from the antiphospholipid syndrome.

Antiphospholipid antibodies (aPL) have been shown to induce tissue factor (TF) expression in monocytes and endothelial cells. However, the underlying signal transduction has been more or less elusive in the past. We have recently shown that aPL enter the lysosomal route in monocytes and dendritic cells, and subsequently activate endosomal NADPH-oxidase (NOX). The generation of superoxide which is dismutated to hydrogen peroxide upregulates the intracellular toll like receptors (TLR) 7 and 8, and leads to robust production of inflammatory cytokines. Here we show that induction of TF by aPL follows the same signaling pathway. Inhibition of endosomal NOX by the anion channel blocker niflumic acid or capture of superoxide by the radical scavenger N-acetylcysteine blocks TF induction by aPL. Furthermore, monocytes from mice deficient in NOX2 do not increase TF surface expression in response to aPL, while cells from mice deficient in glutathione peroxidase-1 (GPx-1) show an increased response. Unexpectedly, also induction of TF by tumour necrosis factor (TNF)α and lipopolysaccharide (LPS) was strongly dependent on the activation of endosomal NOX. While TNFα apparently depends alm ost fully on endosomal NOX, signalling of LPS is only partially dependent on this pathway. These data provide further insight into the well-known role of reactive oxygen species in the induction of TF expression and suggest that endosomal signalling may represent a central coordinating point in this process.

Investigation of Sudan IV staining areas in aortas of infants and children: possible prelesional stages of atherogenesis.

OBJECTIVES/METHODS: Although atherosclerosis in infants and children is generally acknowledged, the temporal and spatial sequence of LDL insudation, modification and intimal monocyte accumulation has not been systematically studied. We have investigated herein very early stages of lesion formation in human aortas of individuals up to the age of 15 years. Aortic specimens from 61 cases (37 male, 24 female) were examined. 34 cases were <1 year old, 16 cases were between 1 and 5 years old, and 11 cases were between 6 and 15 years old. Areas preselected under a dissection microscope after Sudan IV staining were investigated in depth by immunohistochemical staining for apolipoprotein B, monocytes/macrophages, smooth muscle cells (SMCs), enzymatically and oxidatively modified lipoproteins, C-reactive protein and complement components. RESULTS: (i) Lipoprotein accumulation in the intima before macrophage infiltration, (ii) virtually no extracellular lipoprotein modification, either enzymatic or oxidative, within intimal lesions in infancy (<1 year), (iii) onset of extracellular enzymatic modification of low-density lipoprotein (LDL) in the age group between 6 and 15 years and (iv) no coincidence of lipoprotein accumulation in the intima with activation of the terminal complement cascade as known from early atherosclerotic lesions in adults. CONCLUSIONS: The present study indicates the existence of hitherto undescribed prelesional stages in atherogenesis characterized by 'inert' lipoprotein insudation in individuals <1 year of age without lipoprotein modification, monocyte/macrophage infiltration and/or inflammation on the one hand and the onset of extracellular enzymatic rather than oxidative lipoprotein modification in individuals between 6 and 15 years of age on the other hand. Further investigations of these stages should advance understanding of events underlying initiation, progression and regression of intimal lesions developing in early atherogenesis.