Serine 132 is the C3 covalent attachment point on the CH1 domain of human IgG1

Current Understanding of Complement Proteins as Therapeutic Targets for the Treatment of Immunoglobulin A Nephropathy

Immunoglobulin A nephropathy (IgAN) is the most common primary glomerulonephritis worldwide and a frequent cause of kidney failure. Currently, the diagnosis necessitates a kidney biopsy, with routine immunofluorescence microscopy revealing IgA as the dominant or co-dominant immunoglobulin in the glomerular immuno-deposits, often with IgG and sometimes IgM or both. Complement protein C3 is observed in most cases. IgAN leads to kidney failure in 20-40% of patients within 20 years of diagnosis and reduces average life expectancy by about 10 years. There is increasing clinical, biochemical, and genetic evidence that the complement system plays a paramount role in the pathogenesis of IgAN. The presence of C3 in the kidney immuno-deposits differentiates the diagnosis of IgAN from subclinical glomerular mesangial IgA deposition. Markers of complement activation via the lectin and alternative pathways in kidney-biopsy specimens are associated with disease activity and are predictive of poor outcome. Levels of select complement proteins in the circulation have also been assessed in patients with IgAN and found to be of prognostic value. Ongoing genetic studies have identified at least 30 loci associated with IgAN. Genes within some of these loci encode complement-system regulating proteins that can interact with immune complexes. The growing appreciation for the central role of complement components in IgAN pathogenesis highlighted these pathways as potential treatment targets and sparked great interest in pharmacological agents targeting the complement cascade for the treatment of IgAN, as evidenced by the plethora of ongoing clinical trials.

Production of Antigen-Binding Fragment against O,O-Diethyl Organophosphorus Pesticides and Molecular Dynamics Simulations of Antibody Recognition

Immunoassay for pesticides is an emerging analytical method since it is rapid, efficient, sensitive, and inexpensive. In this study, a recombinant antigen-binding fragment (Fab) against a broad set of O,O-diethyl organophosphorus pesticides (DOPs) was produced and characterized. The κ chain and Fd fragment were amplified via PCR and inserted into the vector pComb3XSS and the soluble Fab on phagemid pComb3XSS was induced by isopropyl β-d-thiogalactoside in E. coli TOP 10F’. SDS-PAGE, Western blotting, and indirect competitive ELISA results indicated that Fab maintained the good characteristics of the parental mAb. To better understand antibody recognition, the three-dimensional (3D) model of Fab was built via homologous modeling and the interaction between Fab and DOPs was studied via molecular docking and dynamics simulations. The model clearly explained the interaction manner of Fab and DOPs, and showed that the Arg-L96 and Arg-H52 were mainly responsible for antibody binding. This work provided a foundation for further mutagenesis of Fab to improve its characteristics.

A fully recombinant human IgG1 Fc multimer (GL-2045) inhibits complement-mediated cytotoxicity and induces iC3b

GL-2045 is a recombinant human immunoglobulin G1 (IgG1)-based Fc multimer designed to recapitulate the anti-inflammatory activities of intravenous immunoglobulin (IVIG) on the innate and adaptive immune responses. We used functional in vitro studies to determine if GL-2045 could mimic the modulatory activity of IVIG on complement activation. GL-2045, at log-order lower concentrations than heat-aggregated IgG (HAGG) and IVIG, protected antibody-opsonized cells from complement-dependent cytotoxicity. These protective effects were completely mediated by the higher order multimer fractions of GL-2045 and were partially dependent upon sequestration of C1q. Exposure of serum to GL-2045 and, to a lesser extent, IVIG, resulted in high levels of C4a, limited levels of C3a, and no C5a. In contrast, HAGG induced high levels of C4a, C3a, and C5a. The means by which GL-2045 governed complement activation was dependent on its ability to augment the function of factor H, alone and in combination with factor I, to indirectly limit the alternative form of C3 convertase, with resultant increases in the anti-inflammatory molecule, the "inactive" form of C3b, called iC3b. Although IVIG, like GL-2045, potentiated factor H function, it also directly inhibited the alternative form of C3 convertase. Our findings help elucidate how IVIG, GL-2045, and HAGG regulate complement function. Furthermore, the capacity of GL-2045 to sequester C1q and augment factor H activity, in combination with its ability to generate activation-induced immunomodulatory complement split products, such as iC3b, make it a viable drug candidate for the treatment of diverse complement-mediated diseases.

High concentration of branched-chain amino acids promotes oxidative stress, inflammation and migration of human peripheral blood mononuclear cells via mTORC1 activation

Leucine, isoleucine and valine are essential aminoacids termed branched-chain amino acids (BCAA) due to its aliphatic side-chain. In several pathological and physiological conditions increased BCAA plasma concentrations have been described. Elevated BCAA levels predict insulin resistance development. Moreover, BCAA levels higher than 2mmol/L are neurotoxic by inducing microglial activation in maple syrup urine disease. However, there are no studies about the direct effects of BCAA in circulating cells. We have explored whether BCAA could promote oxidative stress and pro-inflammatory status in peripheral blood mononuclear cells (PBMCs) obtained from healthy donors. In cultured PBMCs, 10mmol/L BCAA increased the production of reactive oxygen species (ROS) via both NADPH oxidase and the mitochondria, and activated Akt-mTOR signalling. By using several inhibitors and activators of these molecular pathways we have described that mTOR activation by BCAA is linked to ROS production and mitochondrial dysfunction. BCAA stimulated the activation of the redox-sensitive transcription factor NF-κB, which resulted in the release of pro-inflammatory molecules, such as interleukin-6, tumor necrosis factor-α, intracellular adhesion molecule-1 or CD40L, and the migration of PBMCs. In conclusion, elevated BCAA blood levels can promote the activation of circulating PBMCs, by a mechanism that involving ROS production and NF-κB pathway activation. These data suggest that high concentrations of BCAA could exert deleterious effects on circulating blood cells and therefore contribute to the pro-inflammatory and oxidative status observed in several pathophysiological conditions.

Molecular mechanisms of action of anti-TNF-α agents - Comparison among therapeutic TNF-α antagonists

Tumor necrosis factor (TNF)-α is a potent pro-inflammatory and pathological cytokines in inflammatory diseases such as rheumatoid arthritis and inflammatory bowel diseases. Anti-TNF-α therapy has been established as an efficacious therapeutic strategy in these diseases. In clinical settings, three monoclonal anti-TNF-α full IgG1 antibodies infliximab, adalimumab, and golimumab, PEGylated Fab' fragment of anti-TNF-α antibody certolizumab pegol, extracellular domain of TNF receptor 2/IgG1-Fc fusion protein etanercept, are almost equally effective for rheumatoid arthritis. Although monoclonal full IgG1 antibodies are able to induce clinical and endoscopic remission in inflammatory bowel diseases, certolizumab pegol without Fc portion has been shown to be less effective for inflammatory bowel diseases compared to full IgG1 antibodies. In addition, there are no evidences that etanercept leads clinical remission in inflammatory bowel diseases. Besides the common effect of anti-TNF-α agents on neutralization of soluble TNF-α, each anti-TNF-α agent has its own distinctive pharmacological properties which cause the difference in clinical efficacies. Here we focus on the distinctions of action of anti-TNF-α agents especially in following points; (1) blocking ability against ligands, transmembrane TNF-α and lymphotoxin, (2) effects toward transmembrane TNF-α-expressing cells, (3) effects toward Fcγ receptor-expressing cells, (4) degradation and distribution in inflamed tissue. Accumulating evidence will give us the idea how to modify anti-TNF-α agents to enhance the clinical efficacy in inflammatory diseases.

The structural basis of antibody-antigen recognition

The function of antibodies (Abs) involves specific binding to antigens (Ags) and activation of other components of the immune system to fight pathogens. The six hypervariable loops within the variable domains of Abs, commonly termed complementarity determining regions (CDRs), are widely assumed to be responsible for Ag recognition, while the constant domains are believed to mediate effector activation. Recent studies and analyses of the growing number of available Ab structures, indicate that this clear functional separation between the two regions may be an oversimplification. Some positions within the CDRs have been shown to never participate in Ag binding and some off-CDRs residues often contribute critically to the interaction with the Ag. Moreover, there is now growing evidence for non-local and even allosteric effects in Ab-Ag interaction in which Ag binding affects the constant region and vice versa. This review summarizes and discusses the structural basis of Ag recognition, elaborating on the contribution of different structural determinants of the Ab to Ag binding and recognition. We discuss the CDRs, the different approaches for their identification and their relationship to the Ag interface. We also review what is currently known about the contribution of non-CDRs regions to Ag recognition, namely the framework regions (FRs) and the constant domains. The suggested mechanisms by which these regions contribute to Ag binding are discussed. On the Ag side of the interaction, we discuss attempts to predict B-cell epitopes and the suggested idea to incorporate Ab information into B-cell epitope prediction schemes. Beyond improving the understanding of immunity, characterization of the functional role of different parts of the Ab molecule may help in Ab engineering, design of CDR-derived peptides, and epitope prediction.

Highly specific off-target binding identified and eliminated during the humanization of an antibody against FGF receptor 4

Off-target binding can significantly affect the pharmacokinetics (PK), tissue distribution, efficacy and toxicity of a therapeutic antibody. Herein we describe the development of a humanized anti- fibroblast growth factor receptor 4 (FGFR4) antibody as a potential therapeutic for hepatocellular carcinoma (HCC). A chimeric anti FGFR4 monoclonal antibody (chLD1) was previously shown to block ligand binding and to inhibit FGFR4 mediated signaling as well as tumor growth in vivo. A humanized version of chLD1, hLD1.vB, had similar binding affinity and in vitro blocking activity, but it exhibited rapid clearance, poor target tissue biodistribution and limited efficacy when compared to chLD1 in a HUH7 human HCC xenograft mouse model. These problems were traced to instability of the molecule in rodent serum. Size exclusion high performance liquid chromatography, immunoprecipitation and mass spectral sequencing identified a specific interaction between hLD1.vB and mouse complement component 3 (C3). A PK study in C3 knock-out mice further confirmed this specific interaction. Subsequently, an affinity-matured variant derived from hLD1.vB (hLD1.v22), specifically selected for its lack of binding to mouse C3 was demonstrated to have a PK profile and in vivo efficacy similar to that of chLD1 in mice. Although reports of non-specific off-target binding have been observed for other antibodies, this represents the first report identifying a specific off-target interaction that affected disposition and biological activity. Screens developed to identify general non-specific interactions are likely to miss the rare and highly specific cross-reactivity identified in this study, thus highlighting the importance of animal models as a proxy for avoiding unexpected clinical outcomes.

Eicosanoids in the innate immune response: TLR and non-TLR routes

The variable array of pattern receptor expression in different cells of the innate immune system explains the induction of distinct patterns of arachidonic acid (AA) metabolism. Peptidoglycan and mannan were strong stimuli in neutrophils, whereas the fungal extract zymosan was the most potent stimulus in monocyte-derived dendritic cells since it induced the production of PGE₂, PGD₂, and several cytokines including a robust IL-10 response. Zymosan activated κB-binding activity, but inhibition of NF-κB was associated with enhanced IL-10 production. In contrast, treatments acting on CREB (CRE binding protein), including PGE₂, showed a direct correlation between CREB activation and IL-10 production. Therefore, in dendritic cells zymosan induces il10 transcription by a CRE-dependent mechanism that involves autocrine secretion of PGE₂, thus unraveling a functional cooperation between eicosanoid production and cytokine production.

Transmembrane TNF-alpha: structure, function and interaction with anti-TNF agents

Transmembrane TNF-α, a precursor of the soluble form of TNF-α, is expressed on activated macrophages and lymphocytes as well as other cell types. After processing by TNF-α-converting enzyme (TACE), the soluble form of TNF-α is cleaved from transmembrane TNF-α and mediates its biological activities through binding to Types 1 and 2 TNF receptors (TNF-R1 and -R2) of remote tissues. Accumulating evidence suggests that not only soluble TNF-α, but also transmembrane TNF-α is involved in the inflammatory response. Transmembrane TNF-α acts as a bipolar molecule that transmits signals both as a ligand and as a receptor in a cell-to-cell contact fashion. Transmembrane TNF-α on TNF-α-producing cells binds to TNF-R1 and -R2, and transmits signals to the target cells as a ligand, whereas transmembrane TNF-α also acts as a receptor that transmits outside-to-inside (reverse) signals back to the cells after binding to its native receptors. Anti-TNF agents infliximab, adalimumab and etanercept bind to and neutralize soluble TNF-α, but exert different effects on transmembrane TNF-α-expressing cells (TNF-α-producing cells). In the clinical settings, these three anti-TNF agents are equally effective for RA, but etanercept is not effective for granulomatous diseases. Moreover, infliximab induces granulomatous infections more frequently than etanercept. Considering the important role of transmembrane TNF-α in granulomatous inflammation, reviewing the biology of transmembrane TNF-α and its interaction with anti-TNF agents will contribute to understanding the bases of differential clinical efficacy of these promising treatment modalities.

Opinion paper: Stimulation of complement amplification or activation of the alternative pathway of complement?

In this opinion paper, we suggest that the scheme of the complement system should be redrawn in order to better illustrate its potencies. This can be achieved by putting the amplification loop of the alternative complement pathway at the center of the complement system. This arrangement emphasizes that C3b molecules, generated by any pathway, can stimulate complement amplification. Furthermore, it allows one to differentiate between this type of stimulation of amplification and that driven by those immune complexes that capture dimeric C3b molecules, which are more potent C3 convertase precursors than C3b. Schemes similar to the one drawn may help to better illustrate the interplay of the pathways and convey a clearer comprehension of the mechanics of the complement system.

Physical mapping of the giant panda immunoglobulin heavy chain constant region genes

We constructed a giant panda bacterial artificial chromosome (BAC) genomic library and isolated a BAC clone containing all the immunoglobulin heavy chain genes. The sequences of these genes were determined, revealing that the deduced panda immunoglobulin heavy chain constant regions (IGHC) are structurally similar to their counterparts in other mammalian species and phylogenetic analysis suggests that the panda immunoglobulins are evolutionarily more close to their counterparts in dogs. Both the panda IgG1 and IgG2 show a unique four amino acids-deletion that has not been observed in any other species to date. The entire panda IGHC gene locus shows an mu-4 kb-delta-39 kb-gamma1-27 kb-gamma2-13 kb-epsilon-14 kb-alpha gene order. It is the most compact mammalian IGHC locus known thus far, encompassing only 120 kb of DNA. Fluorescent in situ hybridization showed that the panda locus is localized on tip of the short arm of chromosome 12.

Complement amplification revisited

Complement amplification in blood takes place not only on activating surfaces, but in plasma as well, where it is maintained primarily by C3b2-IgG complexes. Regular products of C3 activation in serum, these complexes are inherently very efficient precursors of the alternative pathway C3 convertase. Moreover, they can bind properdin bivalently, thus creating preferred sites for convertase formation. C3b2-IgG complexes have a half-life that is substantially longer than that of free C3b, since both C3b molecules are partially protected from inactivation by factor H and I. These complexes are preferentially generated on certain naturally occurring and induced antibodies that exhibit a paratope-independent affinity for C3/C3b. Such antibodies are known to stimulate alternative complement pathway activation. We have assembled the evidence for the generation and the functional potency of the C3b2-IgG complexes, which have been studied during the last two decades. We illustrate their roles in immune complex solubilization, phagocytosis, immune response, and their ability to initiate devastating effects in ischemia/reperfusion and in aggravating inflammation.

Intravenously applied IgG stimulates complement attenuation in a complement-dependent autoimmune disease at the amplifying C3 convertase level

Intravenously applied normal human immunoglobulin G (IgG) has anti-inflammatory effects in the treatment of autoimmune diseases. Systemic inflammation can originate from an overreacting amplification loop of the complement system. In blood, C3b2-containing complexes maintain complement amplification much better than the extremely short-lived C3b. Therefore, in patients with the complement-dependent autoimmune disease, dermatomyositis, we studied whether intravenously applied normal human IgG (IVIG) stimulated in vivo inactivation of these complexes. In the course of IVIG treatment, clinically effective in 6 of 8 patients, the concentration of C3b2-containing complexes dropped to 37% ± 14% (n = 6) of the pretreatment level when having infused 0.5 g IgG/kg body weight, increased marginally and in parallel to factor Bb thereafter until full-dose IgG was infused. By day 14 following infusion of 2 g IgG/kg body weight the concentration of C3b2-containing complexes was 66% ± 19%. The plasma concentration of C3 remained constant in myopathic or increased by 15% to 20% in amyopathic patients. In contrast to this, IVIG infusion was associated with consumption of up to 40% of plasma C4 at day 1 to 2 after completion of IVIG infusion. Thus, IVIG had an immediate and long-lasting attenuating effect on complement amplification in vivo, despite the fact that it induced classical complement pathway activation.

Release of arachidonic acid by stimulation of opsonic receptors in human monocytes: the FcgammaR and the complement receptor 3 pathways

The role of the opsonic receptors FcgammaR and CR3 on the release of arachidonic acid (AA) by human monocytes was studied using IgG-ovalbumin (OVA) equivalence immune complexes (IC), anti-OVA IgG bound to OVA-coupled latex beads, and C3bi-bound IC. Release of AA was produced by IC and latex-OVA beads bound to IgG, whereas binding of C3bi to IC inhibited the ability of IC to release AA. In contrast, coating of zymosan particles with C3bi enhanced AA release as compared with that produced by non-coated particles. Masking of C3bi on C3bi-bound IC by incubation with anti-C3 IgG resulted in the recovery of their ability to release AA, thereby suggesting that binding of C3b by IC reduces their flogogenic effects, whereas opsonization of microbial walls by complement may enhance their proinflammatory potential. The binding/uptake of opsonized zymosan particles was inhibited by anti-CR3 Ab and C3bi-bound IC, but not by beta-glucan, mannan, and anti-Toll-like receptor 2 Ab. These findings show that cooperative engagement of CR3 on both the lectin-like site involved in beta-glucan binding and the I-domain involved in C3bi binding, as it can be observed in the innate immune response, produces AA release, whereas the unique interaction of C3bi-bound IC with the I-domain of CR3, as it may occur in the adaptive immune response, diverts the IC lattice from a productive interaction with FcgammaR linked to AA release.

An anti-C3b(i) mAb enhances complement activation, C3b(i) deposition, and killing of CD20+ cells by rituximab

We investigated deposition of the complement protein fragment C3b and its breakdown products (collectively designated as C3b(i)) on CD20-positive cells treated with rituximab (RTX) in the presence of normal human serum (NHS). Radioimmunoassay (RIA) demonstrates that about 500 000 C3b(i) molecules deposit per cell, and fluorescence microscopy reveals that C3b(i) colocalizes with bound RTX. Use of mAb 3E7, specific for C3b(i) bound to substrates, enhances C3b(i) deposition; > 1 million C3b(i) deposit when cells are incubated with NHS, RTX and mAb 3E7. Treatment of Raji cells in NHS plus RTX leads to robust cell killing (95%) after 24 to 48 hours, and mAb 3E7 significantly enhances RTX-mediated killing of Raji and DB cells. A cynomolgus monkey model based on intravenous infusion of RTX followed by mAb 3E7 demonstrated that RTX rapidly binds to B cells and promotes complement activation and C3b(i) deposition; fluorescence microscopy analyses revealed the same pattern of colocalization of C3b(i) on cell-bound RTX in vivo as observed in vitro. Preliminary in vitro studies with blood samples from patients with chronic lymphocytic leukemia lead to similar findings. These experiments suggest that complement plays a key role in the mechanism of action of RTX; moreover, the in vivo molecular form of RTX (and possibly other antitumor mAbs) in the circulation or in tissues may include C3b(i) molecules covalently bound to the therapeutic mAb, thus allowing it to interact with cells containing both Fc and complement receptors.

Survey of the year 2001 commercial optical biosensor literature

We have assembled references of 700 articles published in 2001 that describe work performed using commercially available optical biosensors. To illustrate the technology's diversity, the citation list is divided into reviews, methods and specific applications, as well as instrument type. We noted marked improvements in the utilization of biosensors and the presentation of kinetic data over previous years. These advances reflect a maturing of the technology, which has become a standard method for characterizing biomolecular interactions.