Dual modulating functions of thrombomodulin in the alternative complement pathway

Pathophysiology-Informed Design of Negatively Charged Liposomes for Enhanced Antibiotic Delivery across the Intact Tympanic Membrane in Acute Otitis Media Treatment

Acute otitis media (AOM) is a leading cause of oral antibiotic prescriptions for children in the U.S., often resulting in systemic side effects and contributing to antibiotic resistance. Local delivery of antibiotics across an intact tympanic membrane (TM) to treat the infection in the middle ear is challenging due to the impermeable TM, which blocks most molecules via the outermost stratum corneum layer. Recent research has identified liposomes encapsulating antibiotics as a highly promising approach to overcoming the intact TM during AOM, demonstrating superior delivery efficiency. However, their design principles remain elusive, especially regarding the desirable surface charge. While previous research has identified positive surface charge as being more effective for crossing healthy stratum corneum, this study illustrates the opposite is true during infection. We compared hydrogel formulations containing positively and negatively charged liposomes in terms of their in vitro release, permeation across intact TM ex vivo, in vivo AOM treatment efficacy, and tissue-level biocompatibility using an established chinchilla model. Our results indicate that negatively charged liposomes outperformed positively charged ones, successfully eradicating 100% of AOM cases. We attributed this to interactions between the negatively charged liposomes and the immune response to infection. Specifically, the complement activation, which triggers neutrophils' phagocytosis, is enhanced in response to the negatively charged liposomes. Our findings highlight an opportunity to improve delivery efficiency by considering the pathophysiology more wholistically during the design of drug delivery vehicles.

Immunity and Coagulation in COVID-19

Discovered in late 2019, the SARS-CoV-2 coronavirus has caused the largest pandemic of the 21st century, claiming more than seven million lives. In most cases, the COVID-19 disease caused by the SARS-CoV-2 virus is relatively mild and affects only the upper respiratory tract; it most often manifests itself with fever, chills, cough, and sore throat, but also has less-common mild symptoms. In most cases, patients do not require hospitalization, and fully recover. However, in some cases, infection with the SARS-CoV-2 virus leads to the development of a severe form of COVID-19, which is characterized by the development of life-threatening complications affecting not only the lungs, but also other organs and systems. In particular, various forms of thrombotic complications are common among patients with a severe form of COVID-19. The mechanisms for the development of thrombotic complications in COVID-19 remain unclear. Accumulated data indicate that the pathogenesis of severe COVID-19 is based on disruptions in the functioning of various innate immune systems. The key role in the primary response to a viral infection is assigned to two systems. These are the pattern recognition receptors, primarily members of the toll-like receptor (TLR) family, and the complement system. Both systems are the first to engage in the fight against the virus and launch a whole range of mechanisms aimed at its rapid elimination. Normally, their joint activity leads to the destruction of the pathogen and recovery. However, disruptions in the functioning of these innate immune systems in COVID-19 can cause the development of an excessive inflammatory response that is dangerous for the body. In turn, excessive inflammation entails activation of and damage to the vascular endothelium, as well as the development of the hypercoagulable state observed in patients seriously ill with COVID-19. Activation of the endothelium and hypercoagulation lead to the development of thrombosis and, as a result, damage to organs and tissues. Immune-mediated thrombotic complications are termed "immunothrombosis". In this review, we discuss in detail the features of immunothrombosis associated with SARS-CoV-2 infection and its potential underlying mechanisms.

The Role of the Complement System in the Pathogenesis of Infectious Forms of Hemolytic Uremic Syndrome

Hemolytic uremic syndrome (HUS) is an acute disease and the most common cause of childhood acute renal failure. HUS is characterized by a triad of symptoms: microangiopathic hemolytic anemia, thrombocytopenia, and acute kidney injury. In most of the cases, HUS occurs as a result of infection caused by Shiga toxin-producing microbes: hemorrhagic Escherichia coli and Shigella dysenteriae type 1. They account for up to 90% of all cases of HUS. The remaining 10% of cases grouped under the general term atypical HUS represent a heterogeneous group of diseases with similar clinical signs. Emerging evidence suggests that in addition to E. coli and S. dysenteriae type 1, a variety of bacterial and viral infections can cause the development of HUS. In particular, infectious diseases act as the main cause of aHUS recurrence. The pathogenesis of most cases of atypical HUS is based on congenital or acquired defects of complement system. This review presents summarized data from recent studies, suggesting that complement dysregulation is a key pathogenetic factor in various types of infection-induced HUS. Separate links in the complement system are considered, the damage of which during bacterial and viral infections can lead to complement hyperactivation following by microvascular endothelial injury and development of acute renal failure.

Thrombin cleaves recombinant soluble thrombomodulin into a lectin-like domain fragment and a fragment with protein C-activating cofactor activity

Thrombomodulin (TM) is a transmembrane protein that plays an important role in regulating the coagulation system by acting as a cofactor for thrombin in protein C activation. Additionally, TM is involved in inflammation. Previous studies have shown that soluble fragments of TM of varying sizes, which are derived from membrane-bound TM, are present in plasma and urine. Soluble fragments of TM are speculated to exhibit biological activity. Among these, a lectin-like domain fragment (TMD1) is of particular importance. Recombinant TMD1 has previously been shown to attenuate lipopolysaccharide-induced inflammation. Here, we report that thrombin cleaves recombinant soluble TM, which is used for the treatment of disseminated intravascular coagulation associated with sepsis, into TMD1 and a fragment comprising the C-terminal portion of TM (TMD23), the latter of which retains the cofactor activity for activating protein C. Our findings suggest that thrombin not only activates protein C on membrane-bound TM but may also cleave TM to generate TMD1.

Coagulation and complement: Key innate defense participants in a seamless web

In 1969, Dr. Oscar Ratnoff, a pioneer in delineating the mechanisms by which coagulation is activated and complement is regulated, wrote, “In the study of biological processes, the accumulation of information is often accelerated by a narrow point of view. The fastest way to investigate the body’s defenses against injury is to look individually at such isolated questions as how the blood clots or how complement works. We must constantly remind ourselves that such distinctions are man-made. In life, as in the legal cliché, the devices through which the body protects itself form a seamless web, unwrinkled by our artificialities.” Our aim in this review, is to highlight the critical molecular and cellular interactions between coagulation and complement, and how these two major component proteolytic pathways contribute to the seamless web of innate mechanisms that the body uses to protect itself from injury, invading pathogens and foreign surfaces.

Immunogenetics of xenotransplantation

Xenotransplantation may become the highly desired solution to close the gap between the availability of donated organs and number of patients on the waiting list. In recent years, enormous progress has been made in the development of genetically engineered donor pigs. The introduced genetic modifications showed to be efficient in prolonging xenograft survival. In this review, we focus on the type of immune responses that may target xeno-organs after transplantation and promising immunogenetic modifications that show a beneficial effect in ameliorating or eliminating harmful xenogeneic immune responses. Increasing histocompatibility of xenografts by eliminating genetic discrepancies between species will pave their way into clinical application.

Thrombomodulin ameliorates transforming growth factor-β1-mediated chronic kidney disease via the G-protein coupled receptor 15/Akt signal pathway

Kidney fibrosis is the common consequence of chronic kidney diseases that inexorably progresses to end-stage kidney disease with organ failure treatable only with replacement therapy. Since transforming growth factor-β1 is the main player in the pathogenesis of kidney fibrosis, we posed the hypothesis that recombinant thrombomodulin can ameliorate transforming growth factor-β1-mediated progressive kidney fibrosis and failure. To interrogate our hypothesis, we generated a novel glomerulus-specific human transforming growth factor-β1 transgenic mouse to evaluate the therapeutic effect of recombinant thrombomodulin. This transgenic mouse developed progressive glomerular sclerosis and tubulointerstitial fibrosis with kidney failure. Therapy with recombinant thrombomodulin for four weeks significantly inhibited kidney fibrosis and improved organ function compared to untreated transgenic mice. Treatment with recombinant thrombomodulin significantly inhibited apoptosis and mesenchymal differentiation of podocytes by interacting with the G-protein coupled receptor 15 to activate the Akt signaling pathway and to upregulate the expression of anti-apoptotic proteins including survivin. Thus, our study strongly suggests the potential therapeutic efficacy of recombinant thrombomodulin for the treatment of chronic kidney disease and subsequent organ failure.

Therapeutic Role of Recombinant Human Soluble Thrombomodulin for Acute Exacerbation of Idiopathic Pulmonary Fibrosis

Acute exacerbation of idiopathic pulmonary fibrosis (AE-IPF) is an acute respiratory worsening of unidentifiable cause that sometimes develops during the clinical course of IPF. Although the incidence of AE-IPF is not high, prognosis is poor. The pathogenesis of AE-IPF is not well understood; however, evidence suggests that coagulation abnormalities and inflammation are involved. Thrombomodulin is a transmembranous glycoprotein found on the cell surface of vascular endothelial cells. Thrombomodulin combines with thrombin, regulates coagulation/fibrinolysis balance, and has a pivotal role in suppressing excess inflammation through its inhibition of high-mobility group box 1 protein and the complement system. Thus, thrombomodulin might be effective in the treatment of AE-IPF, and we and other groups found that recombinant human soluble thrombomodulin improved survival in patients with AE-IPF. This review summarizes the existing evidence and considers the therapeutic role of thrombomodulin in AE-IPF.

Heme Drives Susceptibility of Glomerular Endothelium to Complement Overactivation Due to Inefficient Upregulation of Heme Oxygenase-1

Atypical hemolytic uremic syndrome (aHUS) is a severe disease characterized by microvascular endothelial cell (EC) lesions leading to thrombi formation, mechanical hemolysis and organ failure, predominantly renal. Complement system overactivation is a hallmark of aHUS. To investigate this selective susceptibility of the microvascular renal endothelium to complement attack and thrombotic microangiopathic lesions, we compared complement and cyto-protection markers on EC, from different vascular beds, in in vitro and in vivo models as well as in patients. No difference was observed for complement deposits or expression of complement and coagulation regulators between macrovascular and microvascular EC, either at resting state or after inflammatory challenge. After prolonged exposure to hemolysis-derived heme, higher C3 deposits were found on glomerular EC, in vitro and in vivo, compared with other EC in culture and in mice organs (liver, skin, brain, lungs and heart). This could be explained by a reduced complement regulation capacity due to weaker binding of Factor H and inefficient upregulation of thrombomodulin (TM). Microvascular EC also failed to upregulate the cytoprotective heme-degrading enzyme heme-oxygenase 1 (HO-1), normally induced by hemolysis products. Only HUVEC (Human Umbilical Vein EC) developed adaptation to heme, which was lost after inhibition of HO-1 activity. Interestingly, the expression of KLF2 and KLF4—known transcription factors of TM, also described as possible transcription modulators of HO-1- was weaker in micro than macrovascular EC under hemolytic conditions. Our results show that the microvascular EC, and especially glomerular EC, fail to adapt to the stress imposed by hemolysis and acquire a pro-coagulant and complement-activating phenotype. Together, these findings indicate that the vulnerability of glomerular EC to hemolysis is a key factor in aHUS, amplifying complement overactivation and thrombotic microangiopathic lesions.

Self-Damage Caused by Dysregulation of the Complement Alternative Pathway: Relevance of the Factor H Protein Family

The alternative pathway is a continuously active surveillance arm of the complement system, and it can also enhance complement activation initiated by the classical and the lectin pathways. Various membrane-bound and plasma regulatory proteins control the activation of the potentially deleterious complement system. Among the regulators, the plasma glycoprotein factor H (FH) is the main inhibitor of the alternative pathway and its powerful amplification loop. FH belongs to a protein family that also includes FH-like protein 1 and five factor H-related (FHR-1 to FHR-5) proteins. Genetic variants and abnormal rearrangements involving the FH protein family have been linked to numerous systemic and organ-specific diseases, including age-related macular degeneration, and the renal pathologies atypical hemolytic uremic syndrome, C3 glomerulopathies, and IgA nephropathy. This review covers the known and recently emerged ligands and interactions of the human FH family proteins associated with disease and discuss the very recent experimental data that suggest FH-antagonistic and complement-activating functions for the FHR proteins.

IRF2BP2-deficient microglia block the anxiolytic effect of enhanced postnatal care

Enhanced postnatal care (EPC) increases resilience to adversity in adulthood. Since microglia participate in shaping neural circuits, we asked how ablation of an inflammation-suppressing factor IRF2BP2 (Interferon Regulatory Factor 2 Binding Protein 2) in microglia would affect the responses to EPC. Mice lacking IRF2BP2 in microglia (KO) and littermate controls (WT) were subjected to EPC during the first 3 weeks after birth. EPC reduced anxiety in WT but not KO mice. This was associated with reduced inflammatory cytokine expression in the hypothalamus. Whole genome RNAseq profiling of the hypothalamus identified 101 genes whose expression was altered by EPC: 95 in WT, 11 in KO, with 5 in common that changed in opposite directions. Proteoglycan 4 (Prg4), prostaglandin D2 synthase (Ptgds) and extracellular matrix protease inhibitor Itih2 were suppressed by EPC in WT but elevated in KO mice. On the other hand, the glutamate transporter VGLUT1 (Slc17a7) was increased by EPC in WT but not KO mice. Prostaglandin D2 (PGD2) is known to enhance microglial inflammation and promote Gfap expression. ELISA confirmed reduced PGD2 in the hypothalamus of WT mice after EPC, associated with reduced Gfap expression. Our study suggests that the anxiety-reducing effect of EPC operates by suppressing microglial inflammation, likely by reducing neuronal prostaglandin D2 production.

Inhibition of Cell Apoptosis and Amelioration of Pulmonary Fibrosis by Thrombomodulin

Pulmonary fibrosis is the terminal stage of a group of idiopathic interstitial pneumonias, of which idiopathic pulmonary fibrosis is the most frequent and fatal form. Recent studies have shown that recombinant human thrombomodulin (rhTM) improves exacerbation and clinical outcome of idiopathic pulmonary fibrosis, but the mechanism remains unknown. This study evaluated the mechanistic pathways of the inhibitory activity of rhTM in pulmonary fibrosis. Transgenic mice overexpressing human transforming growth factor-β1 that develop spontaneously pulmonary fibrosis, and wild-type mice treated with bleomycin were used as models of lung fibrosis. rhTM was administered to mice by i.p. injection or by the intranasal route. Therapy with rhTM significantly decreased the concentration of high mobility group box1, interferon-γ, and fibrinolytic markers, the expression of growth factors including transforming growth factor-β1, and the degree of lung fibrosis. rhTM significantly suppressed apoptosis of lung epithelial cells in in vivo and in vitro experiments. The results of the present study demonstrated that rhTM can inhibit bleomycin-induced pulmonary fibrosis and transforming growth factor-β1-driven exacerbation and progression of pulmonary fibrosis, and that apart from its well-recognized anticoagulant and anti-inflammatory properties, rhTM can also suppress apoptosis of lung epithelial cells.

Potential influences of complement factor H in autoimmune inflammatory and thrombotic disorders

Complement system homeostasis is important for host self-protection and anti-microbial immune surveillance, and recent research indicates roles in tissue development and remodelling. Complement also appears to have several points of interaction with the blood coagulation system. Deficiency and altered function due to gene mutations and polymorphisms in complement effectors and regulators, including Factor H, have been associated with familial and sporadic autoimmune inflammatory - thrombotic disorders, in which autoantibodies play a part. These include systemic lupus erythematosus, rheumatoid arthritis, atypical haemolytic uremic syndrome, anti-phospholipid syndrome and age-related macular degeneration. Such diseases are generally complex - multigenic and heterogeneous in their symptoms and predisposition/susceptibility. They usually need to be triggered by vascular trauma, drugs or infection and non-complement genetic factors also play a part. Underlying events seem to include decline in peripheral regulatory T cells, dendritic cell, and B cell tolerance, associated with alterations in lymphoid organ microenvironment. Factor H is an abundant protein, synthesised in many cell types, and its reported binding to many different ligands, even if not of high affinity, may influence a large number of molecular interactions, together with the accepted role of Factor H within the complement system. Factor H is involved in mesenchymal stem cell mediated tolerance and also contributes to self-tolerance by augmenting iC3b production and opsonisation of apoptotic cells for their silent dendritic cell engulfment via complement receptor CR3, which mediates anti-inflammatory-tolerogenic effects in the apoptotic cell context. There may be co-operation with other phagocytic receptors, such as complement C1q receptors, and the Tim glycoprotein family, which specifically bind phosphatidylserine expressed on the apoptotic cell surface. Factor H is able to discriminate between self and nonself surfaces for self-protection and anti-microbe defence. Factor H, particularly as an abundant platelet protein, may also modulate blood coagulation, having an anti-thrombotic role. Here, we review a number of interaction pathways in coagulation and in immunity, together with associated diseases, and indicate where Factor H may be expected to exert an influence, based on reports of the diversity of ligands for Factor H.

Thrombomodulin enhances complement regulation through strong affinity interactions with factor H and C3b-Factor H complex

INTRODUCTION

Coagulation and complement systems are simultaneously activated at sites of tissue injury, leading to thrombin generation and opsonisation with C3b. Thrombomodulin (TM) is a cell-bound regulator of thrombin activation, but can also enhance the regulatory activity of complement factor H (FH), thus accelerating the degradation of C3b into inactive iC3b.

OBJECTIVES

This study sought to determine the biophysical interaction affinities of two recombinant TM analogs with thrombin, FH and C3b in order to analyze their ability to regulate serum complement activity.

METHODS

Surface plasmon resonance (SPR) analysis was used to determine binding affinities of TM analogs with FH and C3b, and compared to thrombin as positive control. The capacity of the two recombinant TM analogs to regulate complement in serum was tested in standard complement hemolytic activity assays.

RESULTS

SPR analysis showed that both TM analogs bind FH and C3b-Factor H with nanomolar and C3b with micromolar affinity; binding affinity for its natural ligand thrombin was several fold higher than for FH. At a physiological relevant concentration, TM inhibits complement hemolytic activity in serum via FH dependent and independent mechanisms.

CONCLUSIONS

TM exhibits significant binding affinity for complement protein FH and C3b-FH complex and its soluble form is capable at physiologically relevant concentrations of inhibiting complement activation in serum.