The formation of insoluble immune complexes between ovalbumin and anti-ovalbumin IgG occurs in at least two distinct phases dependent on reactant concentration and ionic strength

[Role of histidine-rich glycoprotein as anti-DAMPs and therapeutic effects on DAMPs-related diseases]

Histidine-rich glycoprotein (HRG) is a plasma glycoprotein produced mainly in the liver. We have shown that HRG replacement therapy has a marked therapeutic effect on sepsis, in which high mobility group box 1 (HMGB1), one of the representative damage-associated molecular patterns (DAMPs), is known to play an important role in the disease progression. The mechanisms of action are diverse, including inhibition of immune thrombus formation and inhibition of ROS production. In addition, HRG has been shown to neutralize the toxicity of heme, a type of DAMPs, and neutralize the activity of LPS, a type of pathogen-associated molecular patterns (PAMPs), and to inhibit the translocation of HMGB1 from the nucleus of vascular endothelial cells to the extracellular space. Since DAMPs/PAMPs are known to play a central role in the pathogenesis of not only sepsis but also many inflammatory diseases, HRG has wide therapeutic applications and is considered to be a very promising seed for drug discovery.

[Various functions of plasma histidine-rich glycoprotein and its clinical application as the biomarker and therapeutic drug for sepsis]

Histidine-rich glycoprotein (HRG) is a 75 kDa plasma glycoprotein synthesized in liver mainly, which exists at approximately 60-100 μg/ml in human plasma. HRG is known to bind to several ligands and cells, leading to exert coagulation, fibrinolysis, immune and inflammation regulatory activity in septic condition. Thus, decreased plasma HRG level induces the dysregulations of coagulation, fibrinolysis and immune system, resulting in disseminated intravascular coagulation and multiple organ failure. This article focuses on the physiological activity of HRG and the potential of HRG as the biomarker and therapeutic drug for sepsis.

Detection and isolation of cell-derived microparticles are compromised by protein complexes resulting from shared biophysical parameters

Numerous diseases, recently reported to associate with elevated microvesicle/microparticle (MP) counts, have also long been known to be characterized by accelerated immune complex (IC) formation. The goal of this study was to investigate the potential overlap between parameters of protein complexes (eg, ICs or avidin-biotin complexes) and MPs, which might perturb detection and/or isolation of MPs. In this work, after comprehensive characterization of MPs by electron microscopy, atomic force microscopy, dynamic light-scattering analysis, and flow cytometry, for the first time, we drive attention to the fact that protein complexes, especially insoluble ICs, overlap in biophysical properties (size, light scattering, and sedimentation) with MPs. This, in turn, affects MP quantification by flow cytometry and purification by differential centrifugation, especially in diseases in which IC formation is common, including not only autoimmune diseases, but also hematologic disorders, infections, and cancer. These data may necessitate reevaluation of certain published data on patient-derived MPs and contribute to correct the clinical laboratory assessment of the presence and biologic functions of MPs in health and disease.

Contribution of histidine-rich glycoprotein in clearance of immune complexes and apoptotic cells: implications for ameliorating autoimmune diseases

The molecular mechanisms that protect against the harmful properties of immune complexes and dying cells are not well understood. This review focuses on newly discovered mechanisms for the disposal of immune complexes and apoptotic cells by histidine-rich glycoprotein (HRG). Since HRG is abundantly synthesized by the liver and released into the blood stream at basal levels, it is readily available to engage in the removal of circulating modified self (e.g. apoptotic cells) and non-self (e.g. immune complexes) antigens, whereas other known mechanisms, such as the complement system, require pre-activation and are often accompanied by phlogistic events. These findings suggest clearance mechanism hierarchies. Through its interactions with naked DNA and immune complexes, HRG may mask epitopes recognized by autoantibody-producing B cells (e.g. rheumatoid factors and anti-double stranded DNA antibodies). The latter property may regulate adaptive immune system activation and has important implications for the involvement of HRG in ameliorating autoimmune reactions. Properties of HRG and possible protective actions of HRG-dependent clearance mechanisms are discussed.

Histidine‐rich glycoprotein: A novel adaptor protein in plasma that modulates the immune, vascular and coagulation systems

Histidine-rich glycoprotein (HRG) is an abundant plasma glycoprotein that has a multidomain structure, interacts with many ligands, and has been shown to regulate a number of important biological processes. HRG ligands include Zn(2+) and haem, tropomyosin, heparin and heparan sulphate, plasminogen, plasmin, fibrinogen, thrombospondin, IgG, FcgammaR and complement. In many cases, the histidine-rich region of the molecule enhances ligand binding following interaction with Zn(2+) or exposure to low pH, conditions associated with sites of tissue injury or tumour growth. The multidomain nature of HRG indicates that it can act as an extracellular adaptor protein, bringing together disparate ligands, particularly on cell surfaces. HRG binds to most cells primarily via heparan sulphate proteoglycans, binding which is also potentiated by elevated free Zn(2+) levels and low pH. Recent reports have shown that HRG can modulate angiogenesis and additional studies have shown that it may regulate other physiological processes such as cell adhesion and migration, fibrinolysis and coagulation, complement activation, immune complex clearance and phagocytosis of apoptotic cells. This review outlines the molecular, structural, biological and clinical properties of HRG as well as describing the role of HRG in various physiological processes.

Differential binding of histidine-rich glycoprotein (HRG) to human IgG subclasses and IgG molecules containing kappa and lambda light chains

In previous studies we showed that the plasma protein histidine-rich glycoprotein (HRG) binds strongly to pooled human IgG. In the present work myeloma proteins consisting of different human IgG subclasses were examined for their ability to interact with human HRG. Using an IAsys optical biosensor we found initially that IgG subclasses differ substantially in their affinity of interaction with HRG. However, the most striking finding was the observation that the kinetics of the HRG interaction was dramatically affected by whether the IgG subclasses contained the kappa or lambda light (L)-chains. Thus, the on-rate for the binding of HRG to the kappa L-chain containing IgG1 and IgG2 (IgG1kappa and IgG2kappa) was approximately 4- and approximately 10-fold faster than that for the binding of HRG to lambda L-chain containing IgG1 and IgG2 (IgG1lambda and IgG2lambda), respectively, with the dissociation constants (K(d)) in the range 3-5 nM and 112-189 nM for the kappa and lambda isoforms, respectively. In contrast, the on-rate for the binding of HRG to IgG3kappa and IgG4kappa was found to be 9- and 20-fold slower than that for the binding of HRG to IgG3lambda and IgG4lambda, respectively, with the K(d) in the range 147-268 nM and 96-109 nM for the kappa and lambda isoforms, respectively. The binding of HRG to immunoglobulins containing the kappa L-chain (particularly IgG1kappa) was generally potentiated in the presence of a physiological concentration (20 microM) of Zn(2+) (K(d) decreased to 0.60 +/- 0.01 for IgG1kappa), but Zn(2+) had no effect or slightly inhibited the binding of HRG to immobilized IgG subclasses possessing the lambda L-chain. Interestingly, HRG also bound differentially to Bence Jones (BJ) proteins containing kappa and lambda L-chains, with HRG having a 14-fold lower K(d) for BJkappa than for BJlambda when 20 microM Zn(2+) was present. HRG also bound to IgM (IgMkappa), but the affinity of this interaction (K(d) approximately 1.99 +/- 0.05 microM) was markedly lower than the interaction with IgG, and the affinity was actually decreased 4-fold in the presence of Zn(2+). The results demonstrate that both the heavy (H)- and L-chain type have a profound effect on the binding of HRG to different IgG subclasses and provide the first evidence of a functional difference between the kappa and lambda L-chains of immunoglobulins.