The dual role of innate immunity in the antiphospholipid syndrome

From Co-Infections to Autoimmune Disease via Hyperactivated Innate Immunity: COVID-19 Autoimmune Coagulopathies, Autoimmune Myocarditis and Multisystem Inflammatory Syndrome in Children

Neutrophilia and the production of neutrophil extracellular traps (NETs) are two of many measures of increased inflammation in severe COVID-19 that also accompany its autoimmune complications, including coagulopathies, myocarditis and multisystem inflammatory syndrome in children (MIS-C). This paper integrates currently disparate measures of innate hyperactivation in severe COVID-19 and its autoimmune complications, and relates these to SARS-CoV-2 activation of innate immunity. Aggregated data include activation of Toll-like receptors (TLRs), nucleotide-binding oligomerization domain (NOD) receptors, NOD leucine-rich repeat and pyrin-domain-containing receptors (NLRPs), retinoic acid-inducible gene I (RIG-I) and melanoma-differentiation-associated gene 5 (MDA-5). SARS-CoV-2 mainly activates the virus-associated innate receptors TLR3, TLR7, TLR8, NLRP3, RIG-1 and MDA-5. Severe COVID-19, however, is characterized by additional activation of TLR1, TLR2, TLR4, TLR5, TLR6, NOD1 and NOD2, which are primarily responsive to bacterial antigens. The innate activation patterns in autoimmune coagulopathies, myocarditis and Kawasaki disease, or MIS-C, mimic those of severe COVID-19 rather than SARS-CoV-2 alone suggesting that autoimmunity follows combined SARS-CoV-2-bacterial infections. Viral and bacterial receptors are known to synergize to produce the increased inflammation required to support autoimmune disease pathology. Additional studies demonstrate that anti-bacterial antibodies are also required to account for known autoantigen targets in COVID-19 autoimmune complications.

Inflammatory markers in thrombosis associated with primary antiphospholipid syndrome

The role of inflammation in thrombotic complications of primary antiphospholipid syndrome (PAPS) is controversial. The aim of this study was to evaluate levels of inflammation and coagulation markers in patients with thrombotic PAPS (t-PAPS). Patients with t-PAPS and individuals with no history of thrombosis were enrolled. The association of t-PAPS with levels of tumor necrosis factor (TNF)-α, C-reactive protein (hs-CRP), interferon (IFN)-α, interleukins (IL)-6, -8, factor VIII (FVIII), von Willebrand factor (VWF) and tissue factor (TF) was evaluated by regression models. The levels of these markers were also compared between controls and subgroups of t-PAPS patients with triple positivity, recently diagnosed thrombosis, recurrent thrombosis and venous thrombosis. Patients with t-PAPS (n = 101) had a 8.6-fold increased levels of TNF-α, 90% increased levels of hs-CRP, 80% increased levels of IL-6, 30% increased levels of FVIIIAg, 50% increased levels of VWF and 66% increased levels of TF as compared to controls (n = 131), and the differences did not change after adjustments for sex, age and cardiovascular risk factors. Inflammatory markers were elevated in t-PAPS regardless of the aPL profile, number of previous thrombosis or time elapsed since diagnosis. TNF-α and IL-8 levels were higher in t-PAPS patients with venous thrombosis, in comparison with those with arterial thrombosis and controls. Patients with t-PAPS presented with increased levels of inflammatory and coagulation markers, which suggests that t-PAPS is associated not only with hypercoagulability but also with a persistent inflammatory state.

Neutrophil subpopulations and their activation potential in patients with antiphospholipid syndrome and healthy individuals

OBJECTIVES

Patients with APS are at increased risk of thromboembolism. Neutrophils have been shown to play a role in inducing thrombosis. We aimed to investigate differences in neutrophil subpopulations, their potential of activation and neutrophil extracellular trap (NET) formation comparing high and low-density neutrophils (HDNs/LDNs) as well as subpopulations in patients with APS and controls to gain deeper insight into their potential role in thrombotic manifestations in patients with APS.

METHODS

HDNs and LDNs of 20 patients with APS and 20 healthy donors were isolated by density gradient centrifugation and stimulated. Neutrophil subpopulations, their activation and NET release were assessed by flow cytometry.

RESULTS

LDNs of both groups showed higher baseline activation, lower response to stimulation (regulation of activation markers CD11b/CD66b), but higher NET formation compared with HDNs. In patients with APS, the absolute number of LDNs was higher compared with controls. HDNs of APS patients showed higher spontaneous activation [%CD11b high: median (interquartile range): 2.78% (0.58-10.24) vs 0.56% (0.19-1.37)] and response to stimulation with ionomycin compared with HDNs of healthy donors [%CD11b high: 98.20 (61.08-99.13) vs 35.50% (13.50-93.85)], whereas no difference was found in LDNs. NET formation was increased in patients' HDNs upon stimulation.

CONCLUSION

HDNs and LDNs act differently, unstimulated and upon various stimulations in both healthy controls and APS patients. Differences in HDNs and LDNs between patients with APS and healthy controls indicate that neutrophils may enhance the risk of thrombosis in these patients and could thus be a target for prevention of thrombosis in APS.

Hemocoagulative post-operative changes after laparoscopic surgery compared to open surgery: the role of lupus anticoagulant

Although still debated, post-operative modification of hemostasis seems to be less pronounced after laparoscopy compared to open surgery. Antiphospholipid antibodies might play a role in the post-operative thromboembolic risk, although their evaluation in surgical patients has never been performed. Post-operative modification of antiphospholipid antibodies could be related to the surgical approach (laparoscopic or open). In this prospective study, the authors statistically compared the pre-operative values and post-operative modification of antiphospholipid antibodies in two homogeneous groups of patients operated on by laparoscopic and open surgery. No statistical differences within each group and between the two groups were shown comparing mean values of pre-operative and post-operative antiphospholipid antibodies. In the open group, there was a significant difference between pre-operative and post-operative LAC means (P < 0.01). In the laparoscopic group, on the contrary, no significant change in LAC values between pre- and post-operative tests (P = 0.55) was observed. Since LAC could be related to coagulation disorders, this study seems to support that laparoscopic surgery might induce a less risk of post-operative thromboembolic disease.

Saturation of acyl chains converts cardiolipin from an antagonist to an activator of Toll-like receptor-4

Cardiolipins (CLs) are tetra-acylated diphosphatidylglycerols found in bacteria, yeast, plants, and animals. In healthy mammals, CLs are unsaturated, whereas saturated CLs are found in blood cells from Barth syndrome patients and in some Gram-positive bacteria. Here, we show that unsaturated but not saturated CLs block LPS-induced NF-κB activation, TNF-α and IP-10 secretion in human and murine macrophages, as well as LPS-induced TNF-α and IL-1β release in human blood mononuclear cells. Using HEK293 cells transfected with Toll-like receptor 4 (TLR4) and its co-receptor Myeloid Differentiation 2 (MD2), we demonstrate that unsaturated CLs compete with LPS for binding TLR4/MD2 preventing its activation, whereas saturated CLs are TLR4/MD2 agonists. As a consequence, saturated CLs induce a pro-inflammatory response in macrophages characterized by TNF-α and IP-10 secretion, and activate the alternative NLRP3 inflammasome pathway in human blood-derived monocytes. Thus, we identify that double bonds discriminate between anti- and pro-inflammatory properties of tetra-acylated molecules, providing a rationale for the development of TLR4 activators and inhibitors for use as vaccine adjuvants or in the treatment of TLR4-related diseases.

Immunohistochemistry of Human Hsp60 in Health and Disease: From Autoimmunity to Cancer

Hsp60 (also called Cpn60) is a chaperonin with essential functions for cell physiology and survival. Additionally, its involvement in the pathogenesis of a variety of diseases (e.g., some autoimmune disorders and cancer) is becoming evident with new research. For example, the distribution and levels of Hsp60 in cells and tissues have been found altered in many pathologic conditions, and the significance of these alterations is being investigated in a number of laboratories. The aim of this ongoing research is to determine the meaning of these Hsp60 alterations with regard to pathogenetic mechanisms, diagnosis, classification of lesions, and assessing prognosis and response to treatment.Hsp60 occurs in the mitochondria, i.e., its typical residence according to classic knowledge, and also in other locales, such as the cytosol, the cell membrane, the intercellular space, and biological fluids (e.g., blood and cerebrospinal fluid). Detection and quantitative determinations in all these locations are becoming essential components of laboratory pathology in clinics and research. Consequently, immunohistochemistry targeting Hsp60 is also becoming essential for pathologists and researchers interested in disorders involving this chaperonin.In this chapter, we summarize some recent discoveries on the participation of Hsp60 in the pathogenesis of human diseases, and describe in detail how to perform immunohistochemical reactions for detecting the chaperonin, determining its location, and measuring its quantitative levels.

Phospholipid inhibitors

The antiphospholipid syndrome (APS) is defined by the association of arterial and/or venous thrombosis and/or pregnancy complications with the presence of at least one among the main antiphospholipid antibodies (aPL) (i. e., Lupus anticoagulants, LA, IgG and/ or IgM anticardiolipin antibodies, aCL, IgG and/or IgM antiβ2-glycoprotein I antibodies, aβ2-GPI). Several clinical studies have consistently reported that LA is a stronger risk factor for both arterial and venous thrombosis compared to aCL and aβ2-GPI. In particular, LA activity dependent on the first domain of β2-GPI and triple aPL positivity are associated with the risk of thrombosis and obstetrical complications.

Asymptomatic aPL-positive subjects do not require primary thromboprophylaxis. Venous thromboembolism is the most common initial clinical manifestation of APS. To prevent its recurrence indefinite anticoagulation is recommended. Long duration treatment with warfarin or aspirin is used after a first cerebral arterial thrombosis. Low molecular weight heparin (LMWH) with or without aspirin is recommended to reduce the rate of obstetrical complications of APS pregnant women.

Diagnosis and management of the antiphospholipid syndrome

Antiphospholipid syndrome (APS) is characterized by thrombosis and/or pregnancy complications in the presence of persistent antiphospholipid antibodies (APLA). Laboratory diagnosis of APLA depends upon the detection of a lupus anticoagulant, which prolongs phospholipid-dependent anticoagulation tests, and/or anticardiolipin (aCL) and anti-β2-glycoprotein-1 (β2GPI) antibodies. APLA are primarily directed toward phospholipid binding proteins. Pathophysiologic mechanisms underlying thrombosis and pregnancy loss in APS include APLA induced cellular activation, inhibition of natural anticoagulant and fibrinolytic systems, and complement activation, among others. There is a high rate of recurrent thrombosis in APS, especially in triple positive patients (patients with lupus anticoagulant, aCL and anti-β2GPI antibodies), and indefinite anticoagulation with a vitamin K antagonist is the standard of care for thrombotic APS. There is currently insufficient evidence to recommend the routine use of direct oral anticoagulants (DOAC) in thrombotic APS. Aspirin with low molecular weight or unfractionated heparin may reduce the incidence of pregnancy loss in obstetric APS. Recent insights into the pathogenesis of APS have led to the identification of new potential therapeutic interventions, including anti-inflammatory and immunomodulatory therapies. Additional research is needed to better understand the effects of APLA on activation of signaling pathways in vascular cells, to identify more predictive biomarkers that define patients at greatest risk for a first or recurrent APLA-related clinical event, and to determine the safety and efficacy of DOACs and novel anti-inflammatory and immune-modulatory therapies for refractory APS.

Kidney disease in primary anti-phospholipid antibody syndrome

APS is an autoimmune disease defined by the presence of arterial or venous thrombotic events and/or pregnancy morbidity in patients who test positive for aPL. APS can be isolated (primary APS) or associated with other autoimmune diseases. The kidney is a major target organ in APS, and renal thrombosis can occur at any level within the vasculature of the kidney (renal arteries, intrarenal vasculature and renal veins). Histological findings vary widely, including ischaemic glomeruli and thrombotic lesions without glomerular or arterial immune deposits on immunofluorescence. Renal involvement in patients with definite APS is treated with long-term anticoagulants as warfarin, but new treatments are being tried. The aim of this article is to review the links between primary APS and kidney disease.

Antiphospholipid antibody-mediated effects in an arterial model of thrombosis are dependent on Toll-like receptor 4

Patients with antiphospholipid syndrome (APS) produce antiphospholipid antibodies (aPL) and develop vascular thrombosis that may occur in large or small vessels in the arterial or venous beds. On the other hand, many individuals produce aPL and yet never develop thrombotic events. Toll-like receptor 4 (TLR4) appears to be necessary for aPL-mediated prothrombotic effects in venous and microvascular models of thrombosis, but its role in arterial thrombosis has not been studied. Here, we propose that aPL alone are insufficient to cause thrombotic events in an arterial model of APS, and that a concomitant trigger of innate immunity (e.g. TLR4 activation) is required. We show specifically that anti-β2-glycoprotein I (anti-β2GPI) antibodies, a subset of aPL, accelerated thrombus formation in C57BL/6 wild-type, but not TLR4-deficient, mice in a ferric chloride-induced carotid artery injury model. These aPL bound to arterial and venous endothelial cells, particularly in the presence of β2GPI, and to human TLR4 by enzyme-linked immunoassay. Arterial endothelium from aPL-treated mice had enhanced leukocyte adhesion, compared to control IgG-treated mice. In addition, aPL treatment of mice enhanced expression of tissue factor (TF) in leukocytes induced by the TLR4 ligand lipopolysaccharide (LPS). aPL also enhanced LPS-induced TF expression in human leukocytes in vitro. Our findings support a mechanism in which aPL enhance TF expression by leukocytes, as well as augment adhesion of leukocytes to the arterial endothelium. The activation of TLR4 in aPL-positive individuals may be required to trigger thrombotic events.

Pathogenetic mechanisms of antiphospholipid antibody production in antiphospholipid syndrome

Antiphospholiipid syndrome (APS) is an autoimmune disease characterized by the pathological action of antiphospholipid antibodies (aPL), that leads to recurrent pregnancy loss and thrombosis. Despite limited evidence, it is clear that there are both inherited and acquired components of the ontogeny of these antibodies. Animal genetic studies and human familial and population studies highlight the influence of genetic factors in APS, particularly human leukocyte antigen associations. Similarly, both animal and human studies have reported the importance of acquired factors in APS development and infectious agents in particular have a great impact on aPL production. Bacterial and viral agents have been implicated in the induction of autoimmune responses by various mechanisms including molecular mimicry, cryptic autoantigens exposure and apoptosis. In this review we highlight the latest updates with regards to inherited and acquired factors leading to the manufacturing of pathogenic antibodies and APS.

Recent advances in the antiphospholipid antibody syndrome

PURPOSE OF REVIEW

The antiphospholipid syndrome (APS) is a systemic autoimmune disorder characterized by recurrent thrombosis and/or obstetrical morbidity in the presence of persistently positive antiphospholipid antibodies. Recent insights into the pathogenesis of APS have begun to elucidate pathophysiology and led to the identification of potential therapeutic interventions. The objective of this review is to examine the advances in this field and highlight the areas of further investigation.

RECENT FINDINGS

Several mechanisms of thrombosis and pregnancy loss in APS have been proposed. These include activation of endothelial cells, monocytes, and platelets, and/or inhibition of natural anticoagulant and fibrinolytic systems by antiphospholipid antibodies. However, in many cases the underlying molecular mechanisms and their relevance to the human disorder remain uncertain. New therapeutic agents such as statins, hydroxychloroquine, rituximab, complement inhibitors, and interventions aimed at disruption of intracellular signaling pathways have shown promise in preclinical and clinical studies.

SUMMARY

Indefinite anticoagulation remains the mainstay of treatment for thrombotic APS. Despite advances in diagnostic techniques, it remains difficult to predict thrombotic risk in asymptomatic patients with antiphospholipid antibodies. Further mechanistic and clinical studies are needed to predict thrombotic risk and develop improved therapies for this devastating illness.

Effects of TLR4 on β2-glycoprotein I-induced bone marrow-derived dendritic cells maturation

Our previous study has demonstrated that Toll-like receptor 4 (TLR4) can contribute to anti-β2-glycoprotein I/β2-glycoprotein I (anti-β2GPI/β2GPI)-induced tissue factor (TF) expression in human acute monocytic leukemia cell line THP-1. However, the role of TLR4 in the activation of autoimmune response in antiphospholipid syndrome (APS) has rarely been reported. In this study, we focused on the role of TLR4 in β2GPI-induced maturation of bone marrow-derived dendritic cells (BMDCs). iDCs from C3H/HeN mice stimulated with β2GPI were more mature than that from C3H/HeJ mice, yields of CD11c(+)MHCII(+)DCs, CD11c(+)CD80(+)DCs and CD11c(+)CD86(+)DCs and production of some pro-inflammatory cytokines in iDCs from C3H/HeN were higher than those from C3H/HeJ (p<0.05). Moreover, the ability of β2GPI-treated iDCs from C3H/HeJ to stimulate proliferation of allogeneic mixed lymphocytes was lower than that of iDCs from C3H/HeN. In conclusion, our results indicate that TLR4 may play a significant role in β2-glycoprotein I-induced BMDCs maturation.

Anti-β2-glycoprotein I antibodies

Anti-β(2)-glycoprotein I (anti-β(2)GPI) antibodies are the main antiphospholipid antibodies, along with anticardiolipin and lupus anticoagulant, that characterize the autoimmune disease antiphospholipid syndrome (APS). While the exact physiological functions of β(2)GPI are unknown, there is overwhelming evidence that anti-β(2)GPI antibodies are pathogenic, contributing to thrombosis, pregnancy morbidity, and accelerated atherosclerosis in APS and systemic lupus erythematosus patients. The revelation that these antibodies play a central role in the pathogenesis and pathophysiology of APS has driven research to characterize the physiology and structure of β(2)GPI as well as the pathogenic effects of anti-β(2)GPI antibodies. It has also resulted in the development of improved testing methodologies for detecting these antibodies. In this review we discuss the characteristics of β(2)GPI; the generation, pathogenic effects, and standardized testing of anti-β(2)GPI antibodies; and the potential use of therapies that target the β(2)GPI/anti-β(2)GPI interaction in the treatment of APS.

Prevalence of anti- beta2GPI antibodies and their isotypes in patients with renal diseases and clinical suspicion of antiphospholipid syndrome

BACKGROUND

Antiphospholipid antibodies (aPL) are autoantibodies that are associated with a clinical state of hypercoagulability and diverse clinical manifestations collectively known as antiphospholipid syndrome (APS).

OBJECTIVES

To investigate the prevalence of anti-beta2glycoproteinI-antibodies (anti-β2GPI) and their isotypes in patients with renal diseases and clinical suspicion of antiphospholipid syndrome (APS).

PATIENTS AND METHODS

This is a retrospective study in which we have analyzed the prevalence of anti-β2GPI and its isotypes in 170 patients on initial testing and in 29 patients repeated after 12 weeks for confirmation of APS.  The clinical information was provided by the treating physicians or retrieved from the clinical records. The tests for anti-β2GPI screening and its isotypes (IgG, IgM and IgA) detection were assessed.

RESULTS

On initial samples, anti-β2GPI was positive in 118patients.  IgA-β2GPI positivity (93; 79%) was significantly higher than IgM and IgG isotypes.  Out of anti-β2GPI positive patients, clinical features in 95 patients were suggestive of APS or had SLE.  Of these, IgA isotypes was found in 66% (P = 0.010), IgM in 31% (P = 0.033), and IgG in 11% (P = 0.033). On repeat testing, anti-β2GPI was persistently found In 22 patients with a continual predominance of IgA-anti-β2GPI over IgM and IgG isotypes (91% vs. 45.5% and 18% respectively).

CONCLUSIONS

  Our results show that IgA-anti-β2GPI antibodies are the most prevalent isotypes in patients with renal disease or on renal replacement therapy in our population.  Thus inclusion of IgA-anti-β2GPI in the testing repertoire may increase the diagnostic sensitivity for APS in patients with renal diseases.

Antiphospholipid antibodies mediate autoimmunity against dying cells

The Antiphospholipid Syndrome (APS) is characterized by thrombosis and pregnancy loss, clinical events mediated by pathogenic anti-phospholipid autoantibodies (aPL). β2-glycoprotein I (β2GPI) is the major autoantigens recognized by aPL. β2GPI is a cationic protein that binds to negatively charged surfaces such as those of apoptotic cells. This feature may lead to two major events: i) immunization with β2GPI fosters the Fc-receptor-mediated uptake by antigen presenting cells of apoptotic material decorated with β2GPI and the activation of β2GPI-specific T cells which in turn provide help to β2GPI-specific B cells for the production of anti-β2GPI; ii) apoptotic bodies decorated with β2GPI can be opsonized by anti-β2GPI and shifted towards a pro-inflammatory clearance by macrophages; epitope spread can occur with the generation of autoimmunity against nuclear autoantigens. In the presence of a predisposing genetic background and of a particular cytokine environment (type I interferons), the sequential emergence of autoantibodies can evolve into overt clinical disease. The spectrum of clinical phenotypes of the patients can be modulated by several factors affecting the pathogenicity of anti-β2GPI (e.g. domain specificity). We conclude that dying cells may play a dual role in APS: (I) as immunogen for the induction of aPL (etiology) and (II) as targets of aPL for the chronification of inflammation and the development of autoimmune diseases (pathology).

Influenza vaccination can induce new-onset anticardiolipins but not β2-glycoprotein-I antibodies among patients with systemic lupus erythematosus

BACKGROUND

Antiphospholipid syndrome is characterized by autoantibodies against cardiolipins (aCL), lupus anticoagulant, and independent β2-glycoprotein (β2GPI). Controversy exists as to whether vaccination triggers the development of antiphospholipid antibodies (aPL) in patients with systemic lupus erythematosus (SLE).

METHODS

Patients with SLE (101) and matched controls (101) were enrolled from 2005-2009 and received seasonal influenza vaccinations. Sera were tested by ELISA for aCL at baseline, 2, 6, and 12 weeks after vaccination. Vaccine responses were ranked according to an overall anti-influenza antibody response index. Individuals with positive aCL were further tested for β2GPI antibodies.

RESULTS

Patients with SLE and healthy controls can develop new-onset aCL post vaccination, although at rates which do not differ between patients and controls (12/101 cases and 7/101 controls, OR 1.81, p = 0.34). New-onset moderate aCL are slightly enriched in African American SLE patients (5/36 cases; p = 0.094). The optical density measurements for aCL reactivity in patients were significantly higher than baseline at 2 weeks (p < 0.05), 6 weeks (p < 0.05), and 12 weeks (p < 0.05) post vaccination. No new β2GPI antibodies were detected among patients with new aCL reactivity. Vaccine response was not different between patients with and without new-onset aCL reactivity (p = 0.43).

CONCLUSIONS

This study shows transient increases in aCL, but not anti-β2GPI responses, after influenza vaccination.

Interaction of β2-glycoprotein I with lipopolysaccharide leads to Toll-like receptor 4 (TLR4)-dependent activation of macrophages

β(2)-Glycoprotein I (β(2)GPI) is an abundant plasma protein that binds to the surface of cells and particles expressing negatively charged lipids, but its physiological role remains unknown. Antibodies to β(2)GPI are found in patients with anti-phospholipid syndrome, a systemic autoimmune disease associated with vascular thrombosis and pregnancy morbidity. Although it has been suggested that anti-β(2)GPI antibodies activate endothelial cells and monocytes by signaling through TLR4, it is unclear how anti-β(2)GPI antibodies and/or β(2)GPI interact with TLR4. A number of mammalian proteins (termed "endogenous Toll-like receptor (TLR) ligands") have been reported to bind to TLR4, but, in most cases, subsequent studies have shown that LPS interaction with these proteins is responsible for TLR activation. We hypothesized that, like other endogenous TLR ligands, β(2)GPI interacts specifically with LPS and that this interaction is responsible for apparent TLR4 activation by β(2)GPI. Here, we show that both LPS and TLR4 are required for β(2)GPI to bind to and activate macrophages. Untreated β(2)GPI stimulated TNF-α production in TLR4-sufficient (but not TLR4-deficient) macrophages. In contrast, neither polymyxin B-treated nor delipidated β(2)GPI stimulated TNF-α production. Furthermore, β(2)GPI bound to LPS in a specific and dose-dependent manner. Finally, untreated β(2)GPI bound to the surface of TLR4-sufficient (but not TLR4-deficient) macrophages. Polymyxin B treatment of β(2)GPI abolished macrophage binding. Our findings suggest a potential new biological activity for β(2)GPI as a protein that interacts specifically with LPS and point to the need to evaluate newly discovered endogenous TLR ligands for potential interactions with LPS.

β₂-glycoprotein I: a novel component of innate immunity

Sepsis is a systemic host response to invasive infection by bacteria. Despite treatment with antibiotics, current mortality rates are in the range of 20%-25%, which makes sepsis the most important cause of death in intensive care. Gram-negative bacteria are a prominent cause of sepsis. Lipopolysaccharide (LPS), one of the major constituents of the outer membrane of Gram-negative bacteria, plays a major role in activating the host's immune response by binding to monocytes and other cells. Several proteins are involved in neutralization and clearance of LPS from the bloodstream. Here, we provide evidence that β₂-glycoprotein I (β₂GPI) is a scavenger of LPS. In vitro, β₂GPI inhibited LPS-induced expression of tissue factor and IL-6 from monocytes and endothelial cells. Binding of β₂GPI to LPS caused a conformational change in β₂GPI that led to binding of the β₂GPI-LPS complex to monocytes and ultimately clearance of this complex. Furthermore, plasma levels of β₂GPI were inversely correlated with temperature rise and the response of inflammatory markers after a bolus injection of LPS in healthy individuals. Together, these observations provide evidence that β₂GPI is involved in the neutralization and clearance of LPS and identify β₂GPI as a component of innate immunity.

Toll-like receptor (TLR)-4 mediates anti-β2GPI/β2GPI-induced tissue factor expression in THP-1 cells

Our previous study demonstrated that annexin A2 (ANX2) on cell surface could function as a mediator and stimulate tissue factor (TF) expression of monocytes by anti-β₂-glycoprotein I/β₂-glycoprotein I complex (anti-β₂GPI/β₂GPI). However, ANX2 is not a transmembrane protein and lacks the intracellular signal transduction pathway. Growing evidence suggests that Toll-like receptor 4 (TLR-4) might act as an 'adaptor' for intracellular signal transduction in anti-β₂GPI/β₂GPI-induced TF expressing cells. In the current study, we investigated the roles of TLR-4 and its related molecules, myeloid differentiation protein 2 (MD-2) and myeloid differentiation factor 88 (MyD88), in anti-β₂GPI/β₂GPI-induced TF expressing human monocytic-derived THP-1 (human acute monocytic leukaemia) cells. The relationship of TLR-4 and ANX2 in this process was also explored. Along with TF, expression of TLR-4, MD-2 and MyD88 in THP-1 cells increased significantly when treated by anti-β₂GPI (10 µg/ml)/β₂GPI (100 µg/ml) complex. The addition of paclitaxel, which competes with the MD-2 ligand, could inhibit the effects of anti-β₂GPI/β₂GPI on TLR-4, MD-2, MyD88 and TF expression. Both ANX2 and TLR-4 in THP-1 cell lysates could bind to β₂GPI that had been conjugated to a column (β₂GPI-Affi-Gel). Furthermore, TLR-4, MD-2, MyD88 and TF expression was remarkably diminished in THP-1 cells infected with ANX2-specific RNA interference (RNAi) lentivirus (LV-RNAi-ANX2), in spite of treatment with a similar concentration of anti-β₂GPI/β₂GPI complex. These results indicate that TLR-4 and its signal transduction pathway contribute to anti-β₂GPI/β₂GPI-induced TF expression in THP-1 cells, and the effects of TLR-4 with ANX2 are tightly co-operative.