Structure, function, and evolutionary relationships of Fc domains of human immunoglobulins A, G, M, and E

A peptide-linked recombinant glucocerebrosidase for targeted neuronal delivery: Design, production, and assessment

Although recombinant glucocerebrosidase (GCase) is the standard therapy for the inherited lysosomal storage disease Gaucher's disease (GD), enzyme replacement is not effective when the central nervous system is affected. We created a series of recombinant genes/proteins where GCase was linked to different membrane binding peptides including the Tat peptide, the rabies glycoprotein derived peptide (RDP), the binding domain from tetanus toxin (TTC), and a tetanus like peptide (Tet1). The majority of these proteins were well-expressed in a mammalian producer cell line (HEK 293F). Purified recombinant Tat-GCase and RDP-GCase showed similar GCase protein delivery to a neuronal cell line that genetically lacks the functional enzyme, and greater delivery than control GCase, Cerezyme (Genzyme). This initial result was unexpected based on observations of superior protein delivery to neurons with RDP as a vector. A recombinant protein where a fragment of the flexible hinge region from IgA (IgAh) was introduced between RDP and GCase showed substantially enhanced GCase neuronal delivery (2.5 times over Tat-GCase), suggesting that the original construct resulted in interference with the capacity of RDP to bind neuronal membranes. Extended treatment of these knockout neuronal cells with either Tat-GCase or RDP-IgAh-GCase resulted in an >90% reduction in the lipid substrate glucosylsphingosine, approaching normal levels. Further in vivo studies of RDP-IgAh-GCase as well as Tat-GCase are warranted to assess their potential as treatments for neuronopathic forms of GD. These peptide vectors are especially attractive as they have the potential to carry a protein across the blood-brain barrier, avoiding invasive direct brain delivery.

IgA, IgA receptors, and their anti-inflammatory properties

Immunoglobulin A (IgA) is the most abundantly produced antibody isotype in mammals. The primary function of IgA is to maintain homeostasis at mucosal surfaces and play a role in immune protection. IgA functions mainly through interaction with multiple receptors including IgA Fc receptor I (FcαRI), transferrin receptor 1 (CD71), asialoglycoprotein receptor (ASGPR), Fcα/μR, FcRL4, and DC-SIGN/SIGNR1. In this review we discuss recent data demonstrating anti-inflammatory functions of IgA through two receptors, the FcαRI and DC-SIGN/SIGNR1 interactions in the regulation of immunity. Serum monomeric IgA is able to mediate an inhibitory signal following the interaction with FcαRI. It results in partial phosphorylation of its FcRγ-ITAM and the recruitment of the tyrosine phosphatase SHP-1, which induces cell inhibition following the formation of intracellular clusters named inhibisomes. In contrast, cross-linking of FcαRI by multimeric ligands induces a full phosphorylation of the FcRγ-ITAM leading to the recruitment of the tyrosine kinase Syk and cell activation. In addition, secretory IgA can mediate a potent anti-inflammatory function following the sugar-dependent interaction with SIGNR1 on dendritic cells which induces an immune tolerance via regulatory T cell expansion. Overall, the anti-inflammatory effect of serum and secretory IgA plays a crucial role in the physiology and in the prevention of tissue damage in multiple autoimmune and inflammatory diseases.

Different phylogenetic profile and reduced mannose-sensitive adherence capacity characterize commensal Escherichia coli in IgA deficient individuals

In IgA deficiency, secretory IgA (S-IgA) is absent from intestinal secretions. S-IgA carbohydrate chains act as receptors for the mannose specific (MS) adhesin fim of Escherichia coli. In IgA deficient (IgAd) individuals, commensal E. coli express less MS adherence to epithelial cells, due both to reduced carriage of the fimH adhesin gene, reduced capacity to switch it on, and reduced adherence of adhesin-expressing bacteria. Here, we show that commensal E. coli microbiota of IgA deficient individuals belong to phylogenetic group A and display low MS adherence. In healthy individuals, group B2 with strong MS adherence dominate.

IgA nephropathy: molecular mechanisms of the disease

Studies of molecular and cellular interactions involved in the pathogenesis of IgA nephropathy have revealed the autoimmune nature of this most common primary glomerulonephritis. In patients with this disease, altered glycan structures in the unique hinge region of the heavy chains of IgA1 molecules lead to the exposure of antigenic determinants, which are recognized by naturally occurring antiglycan antibodies of the IgG and/or IgA1 isotype. As a result, nephritogenic immune complexes form in the circulation and deposit in the glomerular mesangium. Deposited immune complexes induce proliferation of resident mesangial cells, increased production of extracellular matrix proteins and cytokines, and ultimately loss of glomerular function. Structural elucidation of the nature of these immune complexes and their biological activity should provide a rational basis for an effective, immunologically mediated inhibition of the formation of nephritogenic immune complexes that could be used as a disease-specific therapeutic approach.

Selective IgA deficiency

Introduction

Immunoglobulin A (IgA) deficiency is the most common primary immunodeficiency defined as decreased serum level of IgA in the presence of normal levels of other immunoglobulin isotypes. Most individuals with IgA deficiency are asymptomatic and identified coincidentally. However, some patients may present with recurrent infections of the respiratory and gastrointestinal tracts, allergic disorders, and autoimmune manifestations.

IgA and Its Functions

Although IgA is the most abundant antibody isotype produced in the body, its functions are not clearly understood. Subclass IgA1 in monomeric form is mainly found in the blood circulation, whereas subclass IgA2 in dimeric form is the dominant immunoglobulin in mucosal secretions. Secretory IgA appears to have prime importance in immune exclusion of pathogenic microorganisms and maintenance of intestinal homeostasis. Despite this critical role, there may be some compensatory mechanisms that would prevent disease manifestations in some IgA-deficient individuals.

Pathogenesis

In IgA deficiency, a maturation defect in B cells to produce IgA is commonly observed. Alterations in transmembrane activator and calcium modulator and cyclophilin ligand interactor gene appear to act as disease-modifying mutations in both IgA deficiency and common variable immunodeficiency, two diseases which probably lie in the same spectrum. Certain major histocompatibility complex haplotypes have been associated with susceptibility to IgA deficiency.

Conclusion

The genetic basis of IgA deficiency remains to be clarified. Better understanding of the production and function of IgA is essential in elucidating the disease mechanism in IgA deficiency.

Specific antibody activity, glycan heterogeneity and polyreactivity contribute to the protective activity of S-IgA at mucosal surfaces

An explanation of the principles and mechanisms involved in peaceful co-existence between animals and the huge, diverse, and ever-changing microbiota that resides on their mucosal surfaces represents a challenging puzzle that is fundamental in everyday survival. In addition to mechanical barriers and a variety of innate defense factors, mucosal immunoglobulins (Igs) provide protection by two complementary mechanisms: specific antibody activity and innate, Ig glycan-mediated binding, both of which serve to contain the mucosal microbiota in its physiological niche. Thus, the interaction of bacterial ligands with IgA glycans constitutes a discrete mechanism that is independent of antibody specificity and operates primarily in the intestinal tract. This mucosal site is by far the most heavily colonized with an enormously diverse bacterial population, as well as the most abundant production site for antibodies, predominantly of the IgA isotype, in the entire immune system. In embodying both adaptive and innate immune mechanisms within a single molecule, S-IgA maintains comprehensive protection of mucosal surfaces with economy of structure and function.

Antibody structural variation in rainbow trout fluids

Rainbow trout (Oncorhynchus mykiss) were immunized with trinitrophenylated-keyhole limpet hemocyanin (TNP-KLH) and the redox structure of induced anti-TNP antibodies from the serum, mucus, egg and ovarian fluid was examined. In conducting these studies it was determined that all TNP-specific antibody from each source possessed the mAb-specific H chain (1-14) epitopes, which facilitated the direct structural analysis of the induced antibodies. A protocol was developed which ensured complete adsorption of all specific anti-TNP antibody from each fluid. Together these protocols permitted the unbiased compositional analysis of all redox forms of the anti-TNP antibodies from each source. All antibodies, regardless of source, possessed the same molecular mass, characteristic of the trout tetramer (800 kDa). It was found that specific antibody titers were significantly higher in male than female trout, while the degree of disulfide polymerization was relatively invariant in male antibodies, while being highly variable in female antibodies. Within the females, no distinctively different redox ratios were between antibodies isolated from sera, ovarian fluid or eggs: however, mucus antibodies possessed a unique redox structure consisting of halfmeric constituents that were not observed in antibodies from other fluids.

Mucosal immunoglobulins

Due to their vast surface area, the mucosal surfaces of the body represent a major site of potential attack by invading pathogens. The secretions that bathe mucosal surfaces contain significant levels of immunoglobulins (Igs), which play key roles in immune defense of these surfaces. IgA is the predominant antibody class in many external secretions and has many functional attributes, both direct and indirect, that serve to prevent infective agents such as bacteria and viruses from breaching the mucosal barrier. This review details current understanding of the structural and functional characteristics of IgA, including interaction with specific receptors (such as Fc(alpha)RI, Fc(alpha)/microR, and CD71) and presents examples of the means by which certain pathogens circumvent the protective properties of this important Ig.

An investigation of oligopeptides linking domains in protein tertiary structures and possible candidates for general gene fusion

Fifty-one examples of oligopeptides linking protein domains were extracted from the Brookhaven database of three-dimensional protein structures. In general, the peptides displayed specific characteristics in composition, conformation, hydrogen bonding, flexibility and the like. The entire database was then searched for pentapeptides that would optimize these natural linker properties. The oligopeptides found are suggested as general candidates to link protein molecules or domains through gene fusion.

Use of synthetic peptides in the production and characterization of antibodies directed against predetermined specificities in rat immunoglobulin E

Two peptides, P123 and P124, representing amino acid sequences His 542-Lys 557 and Tyr 459-Arg 472, respectively, of the CH4 domain of rat IgE and predicted to be located on accessible regions of the protein were synthesized by a solid-phase procedure. Rabbits were immunized with the peptides conjugated to KLH and their antisera were tested for reactivity with free peptide and rat IgE by inhibition-ELISA. Each animal produced antibodies which reacted specifically with its immunizing peptide (titre greater than 1/62,500), but not with other synthetic peptides of similar chain-length and composition. Antisera directed against peptides P123 and P124 specifically bound purified rat IgE (IR 162) and IgE in whole myeloma serum (greater than 1/6400), but showed no reaction with normal rat serum proteins and only very low binding to purified human IgE. In addition the binding of anti-peptide sera to rat IgE could be completely inhibited with either homologous peptide or purified rat IgE, but not by other peptides or purified human IgE. Heating rat IgE for 1 hr at 56 degrees C enhanced its binding to anti-peptide antibodies by between 4- and 60-fold, but markedly reduced its reactivity with a rabbit anti-rat IgE (Fc) serum. These results suggest that antibodies directed against the synthetic peptides employed recognize and specifically bind to sites within the CH4 domain of rat IgE represented by their respective immunizing peptides. Furthermore, these antibodies are capable of detecting subtle alterations in structural conformation resulting from heating at 56 degrees C. Epitopes represented by peptides P123 and P124 may contribute to the heat-sensitive cytophilic region of rat IgE.

Diagnosis of allergy: in vitro testing

Relatively few laboratory tests are of proven value in the differential diagnosis and management of allergic diseases. Immunoassays for IgE and for IgE antibodies are the mainstays. Measurement of IgE in serum is advocated as a first-order laboratory test in the differential diagnosis of allergic disease in children and adults. The usefulness of laboratory tests for IgE antibodies in serum, once a subject of debate in the clinical allergy literature, is now firmly established. Confusion, in respect to the use of these tests, is most evident in clinical situations which have been the subject of limited clinical investigation, e.g., the use of tests for IgE antibodies to screen for allergic disease, the indications for their use in patients treated with allergen immunotherapy, and the diagnostic specificity of IgE antibodies to foods as an indicator of food-induced allergic symptoms. Confusion is also apparent in the interpretation of borderline test results, i.e., results which may indicate the presence of low titers of IgE antibodies, and in defining the optimum format for reporting results to maximize the analytical sensitivity of the test method. This review addresses the ambiguities noted above in the interpretation of results. The paragraphs that follow also consider the possible uses of laboratory tests for inflammatory mediators of immediate hypersensitivity, for IgG antibodies to allergens, and of tests designed to evaluate the in vitro functions of lymphocytes in patients with allergic disease.

Allotypic and isotypic aspects of human immunoglobulin A

The location of isotypic, isoallotypic and allotypic determinants is reviewed in the light of data obtained when specific antisera are tested with proteolytic fragments of IgA molecules or mutants of IgA obtained from patients with alpha-heavy chain disease. Isotypic determinants are distributed throughout the alpha chain constant regions although when intact IgA proteins are used as immunogens the CH3 domain is immunodominant. Alpha 1 subclass specific isotypic determinants are present in both Fab and Fc fragments. Amino acid sequence analysis suggest that alpha 1 subclass isotypic determinants depend on substitution in the CH1, hinge and/or CH2 domain. The isoallotypic determinants nA2m(2) appears to be located on the CH1 domain and appears to require disulphide-linked alpha chains for its expression. The allotypic determinant A2m(2) appears to be located in the CH3 domain involving residues 428, 458 and/or 467. The latter residues are present in both A2m(1) and A1 proteins which indicates that for A2m(1) to be the antithetical determinant of A2m(2), the determinant formed by residues 428, 458 and/or 467 in these proteins must be influenced by subclass differences which allows its expression in A2m(1) proteins and not in A1 proteins.

Structure of the human immunoglobulin C epsilon 2 gene, a truncated pseudogene: implications for its evolutionary origin

Cloning of the overlapping DNA fragments together with Southern hybridization experiments showed the organization of the human C epsilon and C alpha gene cluster as 5'-C epsilon 2-14 kilobases-C alpha 1----C epsilon 1-13 kilobases-C alpha 2-3'. Comparison of the nucleotide sequences of the C epsilon 1 and C epsilon 2 genes revealed that four deletions have taken place in the C epsilon 2 gene and its flanking regions. The three deleted regions in the 5' side of the C epsilon 2 gene are partially filled with shorter inserted sequences. One of them has removed the CH1 and CH2 exons and a portion of the epsilon switch (S epsilon) region. The S epsilon region and the CH4 exon still retain the functional structures, whereas the CH3 exon has been inactivated by deleting its 5' intervening sequence necessary for splicing. The tetranucleotide T-G-G-G (or T-G-G-C), which is usually found in close proximity of the class-switch recombination sites in mouse myelomas, is located 5' to the three deletion sites. The results imply that the mechanism responsible for the heavy chain class-switch recombination might be relevant to the evolutionary mechanism of creation of the truncated C epsilon 2 gene. The other deletion in the 3' flanking region of the C epsilon 2 gene may be due to slipped mispairing of the short direct repeat (C-C-C-C-C) at both ends.

Complete amino acid sequence of the Fc region of a human delta chain

The complete amino acid sequence of an Fc-like fragment designated Fc delta (t) and obtained by limited proteolysis with trypsin of an intact myeloma IgD protein (NIG-65) has been determined. The fragment contains 226 amino acid residues and has a molecular weight of 32,000 per monomeric unit. It has three glucosamine oligosaccharides at asparagine residues 68, 159, and 210. Of these, glucosamine-159 is characteristic of the delta chain and has no counterpart position in any of the other classes. On the other hand, glucosamine-68 is shared by gamma, mu, and epsilon, and glucosamine-210 is shared by alpha and mu. Although the Fc delta (t) has the common framework structure of immunoglobulins, its sequence has many individual characteristics when its two domains are compared separately with the counterpart domain of other heavy chains. Such comparison has shown that the two Fc domains of the delta chain should be placed in an independent branch in topology; for all the other classes, the Fc domains are paired well with their counterparts. The comparison has also shown that there are three prominent gaps by which each domain can be divided into two homologous halves. For each class of immunoglobulin, a moderate degree of internal homology exists between the first half and the second half of each domain of the Fc, suggesting that the primordial gene may have coded for a unit about the size of a half domain. Based on this observation together with sequence comparisons, a possible genetic mechanism is proposed for the origin and evolution of the genes for immunoglobulin domains.

Origins of immunoglobulin heavy chain domains

Using computer programs that analyze the evolutionary history and probability of relationship of protein sequences, we have investigated the gene duplication events that led to the present configuration of immunoglobulin C regions, with particular attention to the origins of the homology regions (domains) of the heavy chains. We conclude that all of the sequenced heavy chains share a common ancestor consisting of four domains and that the two shorter heavy chains, alpha and gamma, have independently lost most of the second domain. These conclusions allow us to align corresponding regions of these sequences for the purpose of deriving evolutionary trees. Three independent internal gene duplications are postulated to explain the observed pattern of relationships among the four domains: first a duplication of the ancestral single domain C region, followed by independent duplications of the resulting first and last domains. In these studies there was no evidence of crossing-over and recombination between ancestral chains of different classes; however, certain types of recombinations would not be detectable from the available sequence data.

Investigations of the molecular basis for the temperature-dependent insolubility of cryoglobulins. VI. Quenching by acrylamide of the intrinsic tryptophan fluorescence of cryoglobulin and non-cryoglobulin IgM proteins

The acrylamide-quenching patterns of the intrinsic tryptophan fluorescence of six cold-soluble monoclonal immunoglobulin M (IgM) and two monoclonal IgM proteins possessing cryoglobulin properties (abnormal cold insolubility) have been compared. Static and dynamic components of quenching have been resolved by a modified form of the Stern-Volmer relationship. The unusual observation of static quenching seen with the multitryptophan containing IgM is determined to be a consequence of essentially homogeneous indole fluorescence arising from conserved tryptophan residues within each homologous immunoglobulin domain. Although the static component of the quenching of the two IgM cryoimmunoglobulins examined is similar to that of the non-cryoimmunoglobulin, IgM, some of the cryoglobulin's tryptophan residues appear to be more kinetically exposed to acrylamide than the tryptophans in the non-cryoglobulin IgM. An unusually large negative entropy of activation observed for the quenching process of both cryoimmunoglobulins suggests some abnormality in the dynamic (flexibility) properties of these proteins.

Structural basis for the specificity of antibody-antigen reactions and structural mechanisms for the diversification of antigen-binding specificities

The ability of an organism to cope with foreign substances (antigens) depends in part on its capacity to synthesize antibodies (immunoglobulins) of the proper binding specificity to recognize and combine with these antigens. In view of the great variety of possible antigens, antibodies, or more specifically their combining sites, display considerable variation and possess structural properties such as to enable them to bind the antigenic determinants. A vast amount of immunoglobulin sequence data has become available and the three-dimensional structures of a number of immunoglobulin fragments have been elucidated. With these results we can now begin to understand the structural aspects of antibody–antigen reactions. The crystallographic results and the sequence data have been reviewed elsewhere (Capra & Kehoe, 1975; Davies, Padlan & Segal 1975a, b; Gally, 1973; Kabat, 1976; Nisonoff, Hopper & Spring, 1975; Poljak, 1975a, b). Here, an attempt is made to interpret these structural data in terms of the structural evolution of the antibody combining site and the structural basis for the specificity of the binding of antibody to antigen. First, the pertinent crystallographic and sequence data will be presented. Next, the structural studies which reveal the complementarity between antibody and ligand will be described. Then, structural mechanisms by which different combining site structures could be generated will be reviewed.

Complete covalent structure of a human IgA1 immunoglobulin

The complete covalent structure has been determined for a human myeloma IgA1 immunoglobulin. This protein has unique features in the amino acid sequence and disulfide bridge structure of the variable (V) and constant (C) regions of both the alpha heavy and the lambda light chains, and in the number and loci of oligosaccharides. Whereas C region domains of heavy chains have evolved independently over eons, recent isotypic variations have occured in lambda light chains and possibly in alpha heavy chains.

Structure and function of the constant regions of immunoglobulins

Immunoglobulins are somewhat unusual in that they are biologically active proteins capable of carrying out several different functions. Thus, they can bind specifically with antigen, activate the complement system, mediate many cytotropic reactions, and act as antigen receptors on lymphocyte membranes.