Properties of myeloma immunoglobulin E(Yu). Chemical, fluorescence polarisation and spin-labeled studies

Human immunoglobulin E flexes between acutely bent and extended conformations

Crystallographic and solution studies have shown that IgE molecules are acutely bent in their Fc region. Crystal structures reveal the Cɛ2 domain pair folded back onto the Cɛ3-Cɛ4 domains, but is the molecule exclusively bent or can the Cɛ2 domains adopt extended conformations and even 'flip' from one side of the molecule to the other? We report the crystal structure of IgE-Fc captured in a fully extended, symmetrical conformation and show by molecular dynamics, calorimetry, stopped-flow kinetic, surface plasmon resonance (SPR) and Förster resonance energy transfer (FRET) analyses that the antibody can indeed adopt such extended conformations in solution. This diversity of conformational states available to IgE-Fc offers a new perspective on IgE function in allergen recognition, as part of the B-cell receptor and as a therapeutic target in allergic disease.

Comparisons of the Ability of Human IgG3 Hinge Mutants, IgM, IgE, and IgA2, to Form Small Immune Complexes: A Role for Flexibility and Geometry

Various native and hinge-modified forms of Ig with identical Ids were reacted with an anti-Id mAb, and the resultant immune complexes were analyzed by negative stain immunoelectron microscopy. Complexes were scored for their geometry (linear versus ring complexes) and size (dimer, trimer, etc.). Ring dimers are the thermodynamically most favorable configuration, unless inhibited by steric and/or flexibility constraints. We found ring dimerization to correlate with the length of the upper, but not middle or lower, hinge. In contrast, the geometry and size of complexes of those molecules lacking formal hinges were unpredictable. A hingeless IgG mutant and native IgE readily formed ring dimers. Remarkably, monomeric IgM formed more ring dimers than any of the other Igs tested, including IgG3. We also tagged the Fab arms and measured the mean Fab-Fab angles and the degree of angular variation for each type of Ig. Surprisingly, IgM proved the most flexible by this assay. In hinged Igs, there was a correlation between length of the upper hinge and Fab-Fab flexibility. In contrast, we found no correlation between the mean Fab-Fab angle in uncomplexed Igs and their ability to dimerize with anti-Id mAb. These data suggest that the physicochemical methods typically used to evaluate molecular flexibility are often of low predictive value when tested in a functional assay.

The nature and importance of the inter-epsilon chain disulfide bonds in human IgE

IgE antibodies are best known for their pathological role in allergy. The class-specific effector sites are located in the epsilon chains; these form covalent dimers via two cystine residues (Cys241 and Cys328) linking opposite C epsilon 2 domains. The nature and biological significance of the inter-epsilon chain disulfide-bond arrangement is unresolved. For structural and functional analysis site-specific mutations were introduced into the C epsilon 2 domain of recombinant human IgE. The introduction of an additional cyanogen bromide cleavage site (His246----Met) facilitated the identification of parallel disulfide bond pairing. This linkage was also confirmed for myeloma IgE PS by sequence determination of disulfide-linked C epsilon 2 dimers. Substitution of Cys241 and Cys328 by Ser does not destroy receptor binding, but reductive alkylation, or the replacement of Cys328 by Met, leads to loss of activity. This shows that covalent dimerization is not essential for IgE/receptor interaction and points to the importance of the structural integrity of the site surrounding Cys328, visualized in a new model of human Fc epsilon.

Segmental flexibility of receptor-bound immunoglobulin E

The segmental flexibility of mouse immunoglobulin E (IgE) bound to its high-affinity receptor on membrane vesicles from rat basophilic leukemia cells was compared to that of IgE in solution by measuring the steady-state anisotropy as a function of temperature and viscosity. A monoclonal IgE was used to bind the fluorescent probe N-[5-(dimethylamino)naphthalene-1-sulfonyl]-L-lysine (DNS-Lys) rigidly and specifically in the antigen combining site at the tip of the Fab region. The average rotational correlation time, phi, of 74-89 ns for the receptor-bound IgE is only slightly longer than that for IgE in solution where phi = 54 ns. Another mouse monoclonal IgE was covalently labeled in the Fab region with N-(1-pyrenyl)maleimide. Anisotropy measurements with this derivative yielded results that are very similar to those found with anti-DNS IgE and DNS-Lys. These findings are strikingly different from that expected for a rigid IgE bound to its receptor since in this case phi is likely to be very much larger. Evidently, the segmental flexibility of IgE is not greatly altered upon binding to its receptor.

A spin label study of the ionic strength dependent conformational change in the human Ia molecule

Purified human Ia molecules were labelled with maleimide or isothiocyanate spin labels or by reacting "TEMPAMINE" spin label with the neuraminic acid of their carbohydrate residues. It was found that increasing the ionic strength from 0.05 to 0.75 markedly increased the dipolar interaction between the maleimide-attached labels, but no effect was found of ionic strength or motion on dipolar interaction with the other two labels. The effect of increasing ionic strength could be blocked by the prior addition of Ia-specific antibody, but could not be reversed by the addition of antibody after ionic strength was increased. These findings complement an earlier finding that increasing ionic strength over the range 0.05-0.75 has an inhibitory effect on the combination of Ia with its antibody. Because the maleimide spin labels attach predominantly to SH groups it is suggested that increasing ionic strength causes conformational changes in the immunoglobulin loop region which alter the accessibility of the Ia antibody-binding site.

Lectin-binding ability of immunoglobulin E and its participation in triggering of mast cells

Eleven purified lectins were compared in their IgE-precipitating, mast-cell-degranulating and histamine-releasing activity and in eliciting skin allergic reactions in atopic or normal people. A significantly positive relationship could be proved between their IgE-precipitating activity and degranulation of IgE-sensitized rat mast cells. A similar and often significant relation was observed in skin reactions and histamine released from human leukocytes. Soybean lectin reacted with concanavalin A by a precipitation line in coalescence with the IgE showing the possibility of a common structural component. Correlation tests between different lectins show that all but three elicit the cell activation by similar ways, but different from IgE antiserum. Soybean lectin, Triticum vulgare and Vicia cracca reacted in other ways and correlated better with histamine release caused by the IgE antiserum. A certain degree of the heterogeneity in the IgE sugar component together with the complexity of the lectin structure may be responsible for a wide mode of activation on cell membranes. The significance of this non-immunological binding for sensitization, natural desensitization and cell regulation by physiological histamine release was discussed.

Solid-phase immunoassays using chimera antibodies prepared with monoclonal or polyclonal anti-enzyme and anti-erythrocyte antibodies

The possibility of using covalently prepared chimera antibodies in solid phase immunoassays was tested. The antibody specific for the antigen to be quantified was conjugated with a polyclonal or monoclonal antibody specific for an enzyme or for erythrocytes. Using these conjugates, labelling was done by enzyme/anti-enzyme or erythrocyte/anti-erythrocyte interactions. The chimera antibodies were used for quantitation of human IgE and for titration of human anti-Echinococcus granulosus antibody by solid-phase immunoassays, and were found to possess equal or higher amplification than conventionally prepared enzyme-antibody conjugates. A washing device for erythroadsorption test is also described. It allows such a test to be expressed in quantitative terms, as is the case in enzyme-immunoassays.

Cloning and sequence determination of the gene for the human immunoglobulin epsilon chain expressed in a myeloma cell line

Messenger RNA has been isolated from cells of the human myeloma line 266BL which synthesizes IgE of the myeloma ND. A fraction enriched in mRNA for the epsilon heavy chain was copied into DNA and the DNA was cloned in Escherichia coli. A chemically synthesized oligonucleotide probe, based on the experimentally determined sequence of the specific message, was used to screen colonies. The largest epsilon chain cDNA cloned, 2.0 kilobases, was characterized by restriction endonuclease mapping and DNA sequence analysis. It appears to encode the complete amino acid sequence of the epsilon chain, including a signal peptide at the NH2 terminus as well as untranslated sequences at the 5' and 3' ends of the mRNA. The missing part of the previously published amino acid sequence of the ND epsilon chain was determined from the DNA sequence.

Spin labeling of immunoglobulin M and E carbohydrates

The analysis of ESR spectra of spin-labeled human myeloma immunoglobulins M and E has shown the rotation of spin labels bound to carbohydrates to be restricted most probably due to close attachment of oligosaccharide chains to protein parts of molecules. The values of rotational correlation times for IgM, IgMs and Fc5, spin-labeled at carbohydrates, were found to be equal to 7,7 and 6 ns, respectively. These data pointed to the existence of internal lability of the Fc5 fragment. ESR spectra of IgE spin-labeled at carbohydrates also reflected restricted rotation of the spin label, but the presence of a more than one wide extremum spoke in favour of differences in the degree of immobilization between different oligosaccharide chains. The value of rotational correlation times calculated for IgE, spin-labeled at protein and carbohydrate moieties, were similar (60 and 66 ns). These data confirmed the previously found rigidity of IgE molecule as compared with IgG molecule.