Study of the antigenic structure of human serum albumin with monoclonal antibodies

Non-enzymatic glucosylation induced neo-epitopes on human serum albumin: A concentration based study

Hyperglycaemia induced non enzymatic glycation is accelerated in diabetic patients and aggressively involved in diabetes progression. Human serum albumin (HSA) is the most abundant protein in blood circulation. In hyperglycaemia, it undergoes fast glycation and results in the impairment of structure. Our previous study has demonstrated structural alterations in Amadori-albumin modified with different glucose concentrations from physiological to pathophysiological range. Here, we focused on immunological characterization of Amadori-albumin. Immunogenicity of Amadori-albumin was analysed by direct binding and competitive ELISA. Amadori-albumin was found to be highly immunogenic (expect albumin modified with 5mM) and induced high titre antibodies depending upon the extent of modification. Very high titre antibodies were obtained with albumin modified with 75mM glucose as compared to native albumin. Anti-Amadori-albumin-IgG from rabbit sera exhibited increased recognition of Amadori-albumin than native albumin in competitive immunoassay. Alteration induced in albumin after glucosylation has made it highly immunogenic. Induced antibodies were quite specific for respective immunogens but showed cross-reaction with other Amadori/native proteins. It suggests that glucosylation has generated highly immunogenic epitopes on albumin. Formation of high molecular weight immune complex with retarded mobility further supports specificity of anti-Amadori-albumin-IgG towards Amadori-albumin. It may be concluded that due to early glycation, an array of modification occurred in HSA structure. Such gross structural changes might favour polymerization of most of the native epitopes into potent immunogenic neo-epitopes, but some original epitopes were still active and has contributed in the immunogenicity. It could be concluded that induction of anti-Amadori-albumin antibodies may be due to protection of glucose modified albumin from protiolytic breakdown. We assumed that this type of protein modifications might occur in diabetic patients in hyperglycaemic conditions that may be recognised as foreign molecules and can induce autoantibodies. Increased level of anti-Amadori-albumin autoantibodies may be used as a biomarker in disease diagnosis and its progression.

Albuminuria: what can we expect from the determination of nonimmunoreactive albumin?

Albuminuria is an early marker for diabetic nephropathy in patients with diabetes, and has a clear place in patient care. It also predicts cardiovascular events and mortality in diabetic patients and in the general population, and is slowly becoming accepted in population screening for cardiovascular disease and chronic kidney disease. Recently, investigators found that a considerable amount of albumin in urine is nonimmunoreactive and that classic immunochemical assays do not properly measure all albumin in urine. Assays that detect immunoreactive plus nonimmunoreactive albumin may better predict development of diabetic nephropathy, cardiovascular events, and mortality than assays that only detect immunoreactive albumin. Proof of the existence of nonimmunoreactive albumin emerged from the finding that albumin contains urine fragments. In this review, we critically appraise the presence and relevance of albumin fragments and nonimmunoreactive albumin molecules in urine, and the potential additive value of albuminuria detected by assays that assess nonimmunoreactive plus immunoreactive albumin over albuminuria detected by classic immunochemical assays in predicting end points.

Functional analysis of Bacillus anthracis protective antigen by using neutralizing monoclonal antibodies

Protective antigen (PA) is central to the action of the lethal and edema toxins produced by Bacillus anthracis. It is the common cell-binding component, mediating the translocation of the enzymatic moieties (lethal factor [LF] and edema factor) into the cytoplasm of the host cell. Monoclonal antibodies (MAbs) against PA, able to neutralize the activities of the toxins in vitro and in vivo, were screened. Two such MAbs, named 7.5 and 48.3, were purified and further characterized. MAb 7.5 binds to domain 4 of PA and prevents the binding of PA to its cell receptor. MAb 48.3 binds to domain 2 and blocks the cleavage of PA into PA63, a step necessary for the subsequent interaction with the enzymatic moieties. The epitope recognized by this antibody is in a region involved in the oligomerization of PA63; thus, MAb 48.3 does not recognize the oligomer form. MAbs 7.5 and 48.3 neutralize the activities of anthrax toxins produced by B. anthracis in mice. Also, there is an additive effect between the two MAbs against PA and a MAb against LF, in protecting mice against a lethal challenge by the Sterne strain. This work contributes to the functional analysis of PA and offers immunotherapeutic perspectives for the treatment of anthrax disease.

Phage-displayed mimotopes elicit monoclonal antibodies specific for a malaria vaccine candidate

The phage-displayed peptide CGRVCLRC (C15) has been isolated from a random library by affinity screening with the D14-3 monoclonal antibody, which was raised to the 42 kDa C-terminal fragment of the major merozoite surface protein 1 of Plasmodium vivax (Pv42). In order to investigate the use of such mimotopes as possible vaccine components, we studied the antibody response in Biozzi mice immunized with C15. High titers of antibodies cross-reacting with Pv42 were generated and the IC50 of all immune sera were in the 5 x 10(-9) M range. Two monoclonal antibodies that specifically bind the Pv42 fragment were isolated. Although these mAbs had a lower affinity for Pv42 when compared to D14-3, they reproduced the cross-reactivity of D14-3 with the equivalent protein in P. cynomolgi, a close relative of P. vivax. DNA sequence analysis showed similarities between the germline genes and the canonical CDR conformations of all three antibodies, but molecular modeling failed to reveal common structural features of their paratopes that could account for their cross-reacting patterns. These data demonstrate that mimotopes selected from random repertoires do not necessarily represent structural equivalents of the original antigen but provide functional images that could replace it for vaccine development.

Biologically active human anti-crotoxin scFv isolated from a semi-synthetic phage library

BACKGROUND

The display of repertoires of antibody fragments on the surface of filamentous bacteriophages offers a new way of making antibodies with predefined binding specificities.

OBJECTIVES

Here we explored the use of this technology to find human antibodies with biological properties. Phage-scFv specific for crotoxin, the main toxic component of the venom of the South-American rattlesnake Crotalus durissus terrificus, were isolated from a 'single pot' repertoire of more than 10(8) clones made in vitro from human V gene segments [1]. The crotoxin molecule is composed of two noncovalently linked subunits: a basic and weakly toxic phospholipase A2 (PLA2) called component B (CB) and an acidic, nonenzymatic and nontoxic subunit called component A (CA). CA is able to increase the toxicity as well as the specificity of action of CB simultaneously reducing its enzymatic activity.

STUDY DESIGN

Two clones were isolated (4-21 and 5-3-1) which are specific of the basic subunit CB, but of a moderate affinity (about 10(-7) M). Clones 4-21 and 5-3-1 have different amino acid sequences and different effects on CB properties suggesting that they are raised against different CB epitopes. Purely cholinergic synaptosomes isolated from Torpedo electric organs provide a suitable model to study the presynaptic effects of crotoxin. In this model, CB was shown to induce a larger acetylcholine release than crotoxin.

RESULTS

A dose-dependent increase of acetylcholine release was observed when crotoxin was incubated with increasing amounts of phage-scFv 4-21. This clone was also shown to increase the enzymatic activity of crotoxin. These observations suggest that phage-scFv might dissociate the complex CA-CB. It could be therefore a neutralizing antibody since CB is much less toxic than crotoxin. This shows that 'single pot' libraries are capable of providing not only immunochemical reagents of high specificity but also biological reagents of high quality. The use of this library appears to open new possibilities for immune passive therapy.

Reversible binding interactions between the tryptophan enantiomers and albumins of different animal species as determined by novel high performance liquid chromatographic methods: an attempt to localize the D- and L-tryptophan binding sites on the human serum albumin polypeptide chain by using protein fragments

The stereoselectivity of the reversible binding interactions between the D- and L-tryptophan enantiomers and serum albumins of different animal species and fragments of human serum albumin (HSA) was investigated by applying three novel high performance liquid chromatographic (HPLC) arrangements. The separations were performed by means of 1) an achiral (diol-bond), 2) a chiral (bovine serum albumin-bond) silica gel sorbent, and 3) a column switching technique which uses both the diol- and HSA-bond HPLC stationary phases. A polarimetric detector and/or an ultraviolet (UV) spectrophotometer were used to monitor the separation process. HPLC arrangement 3 allowed the evaluation of enantioselective binding for D- and L-tryptophan to different albumins and albumin fragments. At present, column switching can be considered the technique of the broadest applicability for investigating the reversible binding interactions between a protein and drug enantiomers.

Chromatographic kinetic measurements of human serum albumin adsorption on monoclonal antibodies

A chromatographic method was employed to study the kinetics of human serum albumin (HSA) adsorbed on immobilized monoclonal antibodies. The antibodies of various specificities were covalently bound to a high-performance liquid chromatography (HPLC) silica support. For very low desorption rates, successive amounts of the reacting protein were injected until column saturation. The analysis of the increase of the non-retained fraction calculated from peak area measurements gives the capacity of the support and the rate of the biospecific adsorption process. The model is based on a second-order Langmuir kinetic law and assumes a global mass transfer for the adsorption process. The use of a silica support of small pore size permits the reduction of the contribution for mass transfer in the stagnant fluid and the decrease in the column capacity: due to its large size, the reacting molecule is adsorbed on the external surface of the particle. The adsorption rate constants of HSA on five monoclonal anti-HSA antibodies of different specificities were determined. For all the immuno-adsorbents studied, the adsorption rate constant is significantly lower than that found on immobilized polyclonal antibodies. Measurements at different flow rates reveal that the mass transfer due to the transport to the adsorbent surface is small and can be estimated.

Body distribution of fully biodegradable [14C]-poly(lactic acid) nanoparticles coated with albumin after parenteral administration to rats

Fully biodegradable polylactic acid (PLA) nanoparticles (90-250 nm) coated with human serum albumin (HSA) were prepared by high-pressure emulsification and solvent evaporation, using the protein as surfactant. A new analytical tool was developed, based on Mie's law and size exclusion chromatography, to establish that, after evaporation of the solvent, the protein saturates the surface of the nanoparticles, masking the PLA core. According to this technique, no HSA is encapsulated in the polymer matrix. A radiolabelled [14C]-PLA50 was synthesized to follow the fate of this new drug carrier after i.v. administration to rats. The time necessary to clear the albumin-coated nanoparticles from the plasma was significantly longer than for the uncoated ones but not extended enough to target cells other than mononuclear phagocytes. As deduced from whole-body autoradiography and quantitative distribution experiments, the 14C-labelled polymer is rapidly captured by liver, bone marrow, lymph nodes, spleen and peritoneal macrophages. Nanoparticle degradation was addressed following 14C excretion. The elimination of the 14C was quick on the first day (30% of the administered dose) but then slowed down. In fact, if the metabolism of the PLA proceeds to lactic acid which is rapidly converted into CO2 via the Krebs cycle (80% of the total excretion was fulfilled by the lungs), anabolism from the lactic acid may also have taken place leading to long-lasting radioactive remnants, by incorporation of 14C into endogenous compounds.

Immunochemical probes for food proteins after heat processing

While better hygiene controls and vaccinations have diminished the occurrence of infectious diseases in humans, food-borne diseases have increased. Thus sterilization of food products is of prime importance. The introduction of new technologies applied to food has necessitated new methods for the control of food safety and food quality. This review aims to point out the importance of immunochemistry in the identification of structural changes induced in food proteins during food processing. New technologies have introduced the use of additives in food products, therefore it is important to identify and quantify such additives, even after complete hear denaturation. Toxic chemicals, toxins and pesticides which can contaminate food products before or during processing should also be identified. Finally, the use of immunochemical tests as a control of sterilization procedures in heterogeneous foodstuffs is discussed.

Enzyme immunoassay using monoclonal antibodies to study conformational changes of human serum albumin

A method for studying conformational changes induced in the human albumin molecule, either in its purified form or in serum, is described. Plates were coated with albumin or human serum at varying pHs and were reacted with peroxidase-labeled anti-albumin monoclonal antibodies of different specificities. The data showed that albumin molecules were coated in conformations induced as a result of pH changes, allowing us to demonstrate that pH modifications involved the N-terminal portion of the albumin molecule whether in its purified form or in serum. This method should be applicable to the study of conformational modifications in other proteins as well.

Monoclonal antibodies to bovine serum albumin: affinity and specificity determinations

A panel of 12 monoclonal antibodies (MAb) to bovine serum albumin (BSA) was developed and characterized as to their physiochemical and immunological properties. Affinity constants of the MAb varied over a wide range from 10(5) to 10(8) M-1. MAb were assembled into several groups of non- or minimally interacting antibodies by analysis of competitive binding experiments, and BSA domain and subdomain specificities of the MAb were assigned by analysis of results of MAb binding to purified BSA fragments. Further fine specificity delineation was accomplished by examination of cross-reactivity patterns to several mammalian albumins. The data suggest that some of the low affinity MAb recognize sites on different portions of the BSA molecule, indicating that similar epitopes exist on different domains of the BSA molecule.