Complexes in plasma between light chain kappa immunoglobulins and alpha 1-antitrypsin respectively prealbumin

A Review of Alpha-1 Antitrypsin Binding Partners for Immune Regulation and Potential Therapeutic Application

Alpha-1 antitrypsin (AAT) is the canonical serine protease inhibitor of neutrophil-derived proteases and can modulate innate immune mechanisms through its anti-inflammatory activities mediated by a broad spectrum of protein, cytokine, and cell surface interactions. AAT contains a reactive methionine residue that is critical for its protease-specific binding capacity, whereby AAT entraps the protease on cleavage of its reactive centre loop, neutralises its activity by key changes in its tertiary structure, and permits removal of the AAT-protease complex from the circulation. Recently, however, the immunomodulatory role of AAT has come increasingly to the fore with several prominent studies focused on lipid or protein-protein interactions that are predominantly mediated through electrostatic, glycan, or hydrophobic potential binding sites. The aim of this review was to investigate the spectrum of AAT molecular interactions, with newer studies supporting a potential therapeutic paradigm for AAT augmentation therapy in disorders in which a chronic immune response is strongly linked.

IgE-tailpiece associates with α-1-antitrypsin (A1AT) to protect IgE from proteolysis without compromising its ability to interact with FcεRI

Several splice variants of IgE exist in human plasma, including a variant called IgE-tailpiece (IgE-tp) that differs from classical IgE by the replacement of two carboxy-terminal amino acids with eight novel residues that include an ultimate cysteine. To date, the role of the secreted IgE-tp isoform in human immunity is unknown. We show that levels of IgE-tp are raised in helminth-infected donors, and that both the classical form of IgE (IgE-c) and IgE-tp interact with polymers of the serine protease inhibitor alpha-1-antitrypsin (A1AT). The association of IgE-tp with A1AT polymers in plasma protects the antibody from serine protease-mediated degradation, without affecting the functional interaction of IgE-tp with important receptors, including FcεR1. That polymers of A1AT protect IgE from degradation by helminth proteases may explain why these common and normally non-disease causing polymorphic variants of A1AT have been retained by natural selection. The observation that IgE can be complexed with polymeric forms of A1AT may therefore have important consequences for our understanding of the pathophysiology of pulmonary diseases that arise either as a consequence of A1AT-deficiency or through IgE-mediated type 1 hypersensitivity responses.

The discovery of α1-antitrypsin and its role in health and disease

α1-Antitrypsin (AAT) is the archetype member of the serine protease inhibitor (SERPIN) supergene family. The AAT deficiency is most often associated with the Z mutation, which results in abnormal Z AAT folding in the endoplasmic reticulum of hepatocytes during biogenesis. This causes intra-cellular retention of the AAT protein rather than efficient secretion with consequent deficiency of circulating AAT. The reduced serum levels of AAT contribute to the development of chronic obstructive pulmonary disease (COPD) and the accumulation of abnormally folded AAT protein increases risk for liver diseases. In this review we show that with the discovery of AAT deficiency in the early 60s as a genetically determined predisposition to the development of early-onset emphysema, intensive investigations of enzymatic mechanisms that produce lung destruction in COPD were pursued. To date, the role of AAT in other than lung and liver diseases has not been extensively examined. Current findings provide new evidence that, in addition to protease inhibition, AAT expresses anti-inflammatory, immunomodulatory and antimicrobial properties, and highlight the importance of this protein in health and diseases. In this review co-occurrence of several diseases with AAT deficiency is discussed.

The reactivity and oxidation pathway of cysteine 232 in recombinant human alpha 1-antitrypsin

Oxidative damage to the sulfur-containing amino acids, methionine and cysteine, is a major concern in biotechnology and medicine. alpha1-Antitrypsin, which is a metastable and conformationally flexible protein that belongs to the serpin family of protease inhibitors, contains nine methionines and a single cysteine in its primary sequence. Although it is known that methionine oxidation in the protein active site results in a loss of biological activity, there is little specific knowledge regarding the reactivity of its unpaired thiol, Cys-232. In this study, the thiol-modifying reagent NBD-Cl (7-chloro-4-nitrobenz-2-oxa-1,3-diazole) was used to label peroxide-modified alpha1-antitrypsin and demonstrate that the Cys-232 in vitro oxidation pathway begins with a stable sulfenic acid intermediate and is followed by the formation of sulfinic and cysteic acid in successive steps. pH-dependent reactivity with hydrogen peroxide showed that Cys-232 has a pK(a) of 6.86 +/- 0.05, a value that is more than 1.5 pH units lower than that of a typical protein thiol. pH-induced conformational changes in the region surrounding Cys-232 were also examined and indicate that mildly acidic conditions induce a conformation that enhances Cys-232 reactivity. In summary, this work provides new insights into alpha1-antitrypsin reactivity in oxidizing environments and shows that a unique structural environment renders its unpaired thiol, Cys-232, its most reactive amino acid.

Conformational properties of serine proteinase inhibitors (serpins) confer multiple pathophysiological roles

Serine proteinase inhibitors (Serpins) are irreversible suicide inhibitors of proteases that regulate diverse physiological processes such as coagulation, fibrinolysis, complement activation, angiogenesis, apoptosis, inflammation, neoplasia and viral pathogenesis. The molecular structure and physical properties of serpins permit these proteins to adopt a number of variant conformations under physiological conditions including the native inhibitory form and several inactive, non-inhibitory forms, such as complexes with protease or other ligands, cleaved, polymerised and oxidised. Alterations of a serpin which affect its structure and/or secretion and thus reduce its functional levels may result in pathology. Serpin dysfunction has been implicated in thrombosis, emphysema, liver cirrhosis, immune hypersensitivity and mental disorders. The loss of inhibitory activity of serpins necessarily results in an imbalance between proteases and their inhibitors, but it may also have other physiological effects through the generation of abnormal concentrations of modified, non-inhibitory forms of serpins. Although these forms of inhibitory serpins are detected in tissues and fluids recovered from inflammatory sites, the important questions of which conditions result in generation of different molecular forms of serpins, what biological function these forms have, and which of them are directly linked to pathologies and/or may be useful markers for characterisation of disease states, remain to be answered. Elucidation of the biological activities of non-inhibitory forms of serpins may provide useful insights into the pathogenesis of diseases and suggest new therapeutic strategies.

Role of oxidative mixed-disulfide formation in elastase-serine proteinase inhibitor (serpin) complex

To understand the role of thiol and oxidative mixed-disulfide exchange reaction in serpins, we analyzed the conformation of native and mixed-disulfide forms of alpha 1-proteinase inhibitor (alpha 1-PI), alpha 1-antichymotrypsin (alpha 1-ACT), alpha 2-antiplasmin (alpha 2-AP), angiotensinogen, and ovalbumin. The conformation of native and oxidized mixed-disulfide serpins was measured by transverse urea gradient (TUG) gels. The results suggest that the acute phase proteins alpha 1-PI and alpha 1-ACT undergo conformational changes following oxidative mixed-disulfide formation and that alpha 2-AP and angiotensinogen do not. The kinetics of disulfide formation was followed by measuring changes in absorbance at 412 nm resulting from Ellman's reaction of disulfide exchange. The rate of mixed-disulfide formation in albumin was 10-fold faster than in the serpin tested. The rate of disulfide exchange in alpha 1-PI was 2-fold faster than that of alpha 1-ACT. However, disulfide formation in alpha 1-PI and alpha 1-ACT was much slower than for any other serpin, e.g., alpha 2-AP and angiotensinogen. We present evidence that alpha 1-PI forms a dimer sensitive to thiol reduction, suggesting cysteinyl-mediated dimerization of alpha 1-PI. The alpha 1-PI also demonstrated two types of inter-protein disulfide linkages: one resulting in homodimer and other involving heterodimer formation. TUG-Western immunoblot methodology was developed to identify the conformational changes in serpins. We found that the conformational changes in serpins by mixed-disulfide formation are due to unfolding and not to decomposition or degradation in TUG gels. Using fluorescence measurements with isolated tryptic fragments of fluorescence-labelled elastase, we observed that the cysteinyl232 in alpha 1-PI interacted with the cysteinyl168 of elastase in the proteinase-inhibitor complex. Our data suggests that serpin thiols may play an important role in forming a stable serpin-proteinase complex.

The putative role of alpha-1-antitrypsin in the disaggregation of amyloid lambda fibrils

BACKGROUND AND OBJECTIVES

Interactions between hydrophobic compounds like cholesterol and lithocholic acid and alpha-1-antitrypsin (alpha-1-AT) have previously been described. We studied the putative interaction between alpha-1-AT and the insoluble, hydrophobic, beta-pleated sheet, light-chain-derived fibrils that predominate the tissue deposits in primary immunocytic (AL) related amyloidosis.

SUBJECTS AND METHODS

Amyloid fibrils were isolated from two cases with lambda and two cases with kappa AL amyloidosis.

RESULTS

The lambda fibrils could be completely disaggregated (as shown by light and electron microscopy and Congo red uptake) by alpha-1-AT added in the molar ratio 1:5, whereas fibrils with predominantly kappa chains remained unaffected. The lambda-chain interaction was accompanied by characteristic changes of the physicochemical and biological properties of alpha-1-AT apparent in an increased thermal stability and loss of elastase-inhibitory activity. These findings are compatible with a transition of alpha-1-AT from a native, stressed conformation to a relaxed form.

CONCLUSIONS

Disaggregation of lambda AL amyloid fibrils can be achieved by addition of alpha-1-AT. The findings may have therapeutic implications in primary amyloidosis.

Human alpha 1-proteinase inhibitor binds to extracellular matrix in vitro

alpha 1-Proteinase inhibitor (alpha 1-PI) is the major endogenous inhibitor of human leukocyte elastase (HLE). We have employed two different methods to quantitate the binding of alpha 1-PI to extracellular matrix (ECM), composed of 51% glycoproteins and proteoglycans, 37% types I and III collagen, and 12% elastin, derived from rat heart smooth muscle cells. alpha 1-PI is tightly bound to ECM via a saturable adsorption process; the bound protein fails to dissociate from the matrix after repeated washing. Binding of alpha 1-PI is unaffected by the prior removal of ECM glycoproteins with trypsin. Binding to ECM is not decreased in the presence of high salt but is decreased at low pH. A 40-fold excess of unlabeled alpha 1-PI displaces only 50% of [125I]alpha 1-PI prebound to ECM. A 30% decrease in the levels of alpha 1-PI bound to ECM is observed after DTT washes of ECM preincubated with alpha 1-PI or when alpha 1-PI is modified with iodoacetamide prior to incubation with ECM, implying that a fraction of bound alpha 1-PI is covalently linked to ECM via disulfide bond formation. Moreover, high molecular weight complexes between [125I]alpha 1-PI and ECM components can be visualized by SDS-PAGE under nonreducing conditions but disappear upon reduction. Approximately 50% of the total alpha 1-PI bound covalently or noncovalently to ECM retains the ability to inhibit HLE-mediated ECM proteolysis. alpha 1-PI-HLE complexes bound to ECM can be visualized by SDS-PAGE following the addition of HLE to ECM that was pretreated with [125I]alpha 1-PI. alpha 1-PI from normal plasma or serum also binds to ECM with retention of immunoreactivity and partial retention of inhibitory activity. However, ECM pretreated with alpha 1-PI-deficient serum retains no HLE-inhibitory activity.

Conformational changes of the alpha 1-proteinase inhibitor affecting its cholesterol binding ability

The effect of conformational changes of the alpha 1-proteinase inhibitor (alpha 1PI) on alpha 1PI-cholesterol complex (1:2 mol/mol) formation in vitro was studied with electrophoretic and gel chromatographic methods. Native alpha 1PI was modified by adding free thiol agents such as glutathione, cysteine HCl, or DL-homocysteine, by heating, or by cleavage with pancreatic elastase or trypsin. Conformational changes of the alpha 1PI molecule induced by these procedures were all accompanied by a loss of its ability to bind cholesterol in vitro under standard experimental conditions. The data suggest alpha 1PI-cholesterol binding to be affected by both direct and indirect modifications of the alpha 1PI-reactive center, that is situated on a mobile peptide loop.

Free kappa and lambda light chain levels in the cerebrospinal fluid of patients with multiple sclerosis and other neurological diseases

Free kappa and lambda light chains were assayed by particle-counting immunoassay in cerebrospinal fluid (CSF) from patients with various neurological disorders. Detection limits were 25 and 50 ng/ml, respectively. Values of free kappa chain were higher than 50 ng/ml (upper reference limit) in 155 of 191 (81%) multiple sclerosis (MS) patients, in 100 of 168 (60%) patients with central nervous system (CNS) infections but in 41 of 217 (19%) patients with other neurological disorders. Free kappa chains were also assayed in 273 matched sera. The mean concentration in the control group (1.58 micrograms/ml; SD: 0.41) did not differ significantly from those in MS sera (1.63 micrograms/ml; SD: 0.43). The free kappa chain index was increased in 86% of MS patients and in 40% of patients with CNS infections. Regarding free lambda chains, CSF values were higher than 240 ng/ml (upper reference limit) in most neurological disorders (50-100%). However, the use of a lambda chain index increased the specificity of the assay as this index was higher than the upper reference value in 86% of MS patients and in only 23% of patients with infectious diseases. In MS, high levels of free kappa and lambda indices correlated significantly (P less than 0.01) with either the presence of oligoclonal bands or a high IgG index. Local synthesis of free light chains is an additional marker of an ongoing immune response within the CNS, especially in MS.

Unusual manifestations of monoclonal gammopathies. Autoimmune and idiopathic syndromes

Manifestations of monoclonal gammopathies are due to four broad mechanisms: destruction of bones, bone marrow, and normal tissues by an expanding tumor; nonspecific effects of large amounts of circulating paraproteins leading to hyperviscosity; autoantibody specificity of paraproteins that give rise to cryoglobulinemia, cold agglutinin disease, and other autoimmune phenomena, as well as incidental binding to bacterial antigens and haptens; and idiopathic syndromes due to as yet unidentified tumor products or other mechanisms. A review with an emphasis on the autoimmune and idiopathic manifestations of monoclonal gammopathies is presented.

Characteristics of the complex between alkaline phosphatase and immunoglobulin A in human serum

An abnormal band of alkaline phosphatase (ALP) activity was detected by electrophoresis in the serum of a patient with liver cirrhosis, and was shown to be a complex between ALP and immunoglobulin A (IgA) of the lambda type. Physicochemical studies of ALP in the patient's serum showed properties of liver and bone isozymes. The patient's IgA and its F(ab')2 fragment were prepared by column chromatography, and used in in-vitro reconstitution studies with various ALP isozymes. It was found that only the liver and bone ALP attached to the IgA, while the placental and intestinal ALP did not. The ALP was attached to the F(ab')2 fragment of IgA. It is concluded that this complex is the result of an antibody-antigen reaction. Molecular weights of the two complexes, ALP-IgA and ALP-IgA-F(ab')2, suggest that two molecules of monovalent ALP associated with one molecule of divalent IgA.

Chromatographic study of the interrelationships of immunoglobulin A and alpha 1-microglobulin in myelomatosis

The binding of alpha 1-microglobulin (alpha 1-m) to serum immunoglobulin A (IgA) myeloma proteins have been examined by analytical and preparative Superose high-performance gel chromatography. Enzyme immunoassays showed that in the serum alpha 1-m was bound to monomeric IgA, but not to the polymeric IgA, and was also present in a free form. The IgA-alpha 1-m complexes involved covalent and non-covalent bonds. Considerable variation in the ratio of bound to unbound forms of alpha 1-m was observed that appears to be a result of variation of the IgA alpha heavy chains. Reduction of monomeric IgA produced alpha 1-m-heavy chain complexes, free alpha 1-m, light and alpha heavy chains, and traces of alpha 1-m attached to IgA that was resistant to reduction.

Existence of disulfide-bonded IgM-IgA, IgM-IgG and IgM-IgG fragment complexes in one patient

The existence of disulfide-bonded immunoglobulin (Ig) complexes of IgM-IgA, IgM-IgG, and IgM-IgG, in addition to a monoclonal IgM (kappa) paraprotein, free kappa chains and fragments of IgG (FIgG), were found in the plasma of a multiple gammopathy patient. This is the first report on the interclass disulfide-bonded Ig complexes. Upon exposure to a dissociating buffer containing 6 M urea, 0.1 M Tris-HCl, pH 8.0, a small portion of the IgM-IgA complexes and most of the IgM-IgG as well as the IgM-FIgG complexes dissociated, but the majority of the IgM-IgA and small amounts of other complexes remained intact. These intact complexes were not the result of antigen-antibody reactions and were apparently held together by disulfide bonds which could be broken into monomeric Ig basic units (H-L)2 and smaller components by mild reduction and alkylation.

The use of thiol-disulphide exchange chromatography for the automated isolation of alpha 1-antitrypsin and other plasma proteins with reactive thiol groups

A method has been developed for the rapid isolation of alpha 1-antitrypsin and other thiol proteins from plasma by an automated chromatography system. The thiol-proteins are initially bound to matrix-linked activated thiol-compounds by an SH-SS interchange reaction. The mixed disulphides are then reduced in two steps and subfractionated by passage through Blue-Sepharose and AH-Sepharose columns. The rate of the interchange reactions varies with the microenvironment of the reacting thiols. alpha 1-Antitrypsin is recovered with 95% purity in 60% yield within two days from 1 l of plasma.

Inhibitory activity of alpha-1-antitrypsin bound to human IgA

Complexes between alpha-1-antitrypsin (alpha 1AT) and monoclonal IgA are regularly demonstrable in the plasma of myeloma patients. These alpha 1AT-IgA complexes, free of contamination by unbound alpha 1AT, are purified from 5 myeloma patients sera using salt-mediated hydrophobic chromatography. The complexes have a molecular weight greater than or equal to 400 000: this suggests that alpha 1AT is bound to di- or polymeric IgA. The alpha 1AT bound to IgA constitutes the 3.2, 3.5, 7.2, 8.5, and 24.6 per cent of the total alpha 1AT present in the 5 myeloma serum samples. There is a linear correlation between bound alpha 1AT concentration and IgA level in the range of the IgA concentrations considered (r = 0.988; p less than 0.05). Similar values are obtained quantitating bound alpha 1AT by radioimmunodiffusion technique or by determination of the trypsin-inhibiting capacity; this demonstrates that the bound alpha 1AT fully retains its inhibitory capacity. The biological significant of this binding phenomenon is discussed.

Antigenic analysis of the IgA component of LDH-IgA immunoglobulin complexes

LDH-IgA complexes present in some human sera were purified by using affinity chromatography on 5'-AMP-Sepharose 4-B. The IgA component of the purified complexes was analysed in haemagglutination-inhibition tests. Antigenic determinants specific for alpha1 and alpha2 heavy chains and for kappa and lambda light chains were found. Earlier studies suggested that the IgA component is of kappa light chain type only. These different results are discussed. It is suggested that the LDH association site is located in the Fab fragment of the IgA component.

Mu-chain disease in a case of chronic lymphocytic leukaemia and malignant histiocytoma. II. Immunochemical studies

Agarose gel electrophoresis of serum from the eleventh case of mu-chain disease (mu-CD) showed two distinct abnormal bands due to free light chains and kappa-complexes with other serum proteins. This caused diagnostic difficulties on conventional immunoelectrophoresis, and crossed immunoelectrophoresis with intermediate gel is demonstrated as a superior tool for investigation of mu-CD. A simple rocket immunoelectrophoresis method foe mu-CD screening is also shown. The mu-CD protein had alpha-2-mobility and a part of the protein carried kappa-determinants probably due to secondary binding. Preparative ultracentrifugation and size chromatography on serum demonstrated that the mu-CD protein was present in serum in several sizes, the smallest being a dimer of mu-chains. Due to scarcity of material the abnormal protein could not be characterized further.

Thiol-disulfide interchange in the binding of bence jones proteins to alpha-antitrypsin, prealbumin, and albumin

Native light Ig chains of kappa- but not of lambda-type form -S-S linked complexes with prealbumin, alpha1-AT and albumin in vivo. kappa-chains isolated from urines have cysteinyls which are more promptly reacting with dithionitrobenzoate (DTNB) than lambda-chains. Both are monomerized on this reaction. On addition to plasma mixed disulfides between both types of light chains and DTNB form larger amounts of complexes than the native chains. The lower reactivity of native lambda-chains to the plasma proteins can be explained by their higher dimer stability. From the light chain reactions obtained with isolated alpha1-AT and albumin it is concluded that alpha1-AT has a disulfide which efficiently interchanges with monomeric, light chain thiolate ions released from thionitrobenzoate derivates of light chains and that on interchange with the derivatized light chains albumin releases more free light chains into the solution than are bound to albumin. Addition of derivatized light chains to a mixture of alpha1-AT and albumin increases the yield of alpha1-AT complexes and decreases the amount of albumin complexes formed. The relative amount of the different complexes formed in the latter experiments corresponds to the findings in vivo in patients with Bence Jones proteinemia. Prealbumin and alpha1-AT in plasma have a roughly 10-fold stronger tendency to link the light chains than albumin. The complexes are formed through thiol-disulfide interchange though neither the disulfide of native alpha1-AT nor the thiols of prealbumin is available for reaction with DTNB. The three plasma proteins may together constitute a system for linkage and transport of peptides with reactive thiols or disulfides released into the extracellular fluids. The trypsin and elastase binding and inhibiting capacity of alpha1-AT remains after cleavage of the internal -S-S-bridge of alpha1-AT through interchange with a light chain thiol for which reason an intact internal -S-S-bridge of alpha1-AT is not necessary for inhibition and linkdage of the enzymes.