IgE structure-function relationships defined by sequence directed antibodies induced by synthetic peptides

Variable Region Identical IgA and IgE to Cryptococcus neoformans Capsular Polysaccharide Manifest Specificity Differences

In recent years several groups have shown that isotype switching from IgM to IgG to IgA can affect the affinity and specificity of antibodies sharing identical variable (V) regions. However, whether the same applies to IgE is unknown. In this study we compared the fine specificity of V region-identical IgE and IgA to Cryptococcus neoformans capsular polysaccharide and found that these differed in specificity from each other. The IgE and IgA paratopes were probed by nuclear magnetic resonance spectroscopy with (15)N-labeled peptide mimetics of cryptococcal polysaccharide antigen (Ag). IgE was found to cleave the peptide at a much faster rate than V region-identical IgG subclasses and IgA, consistent with an altered paratope. Both IgE and IgA were opsonic for C. neoformans and protected against infection in mice. In summary, V-region expression in the context of the ϵ constant (C) region results in specificity changes that are greater than observed for comparable IgG subclasses. These results raise the possibility that expression of certain V regions in the context of α and ϵ C regions affects their function and contributes to the special properties of those isotypes.

Molecular cloning of the chicken oviduct ecto-ATP-diphosphohydrolase

The chicken oviduct ecto-ATP diphosphohydrolase (ATPDase), a member of the ecto-ATPase family, was purified to homogeneity previously (Strobel, R. S., Nagy, A. K., Knowles, A. F., Buegel, J., and Rosenberg, M. O. (1996) J. Biol. Chem. 271, 16323-16331). It is an 80-kDa glycoprotein with high specific activity (approximately 1,000 micromol/min/mg with MgATP as the substrate) and hydrolyzes both nucleoside triphosphates and diphosphates. Using amino acid sequence information obtained from the purified enzyme, two partial cDNA clones were obtained using reverse transcriptase-polymerase chain reaction and library screening. This is the second ecto-ATPase family member and the first ecto-ATPDase to be cloned from information derived from purified proteins. The deduced primary sequence of the chicken oviduct ecto-ATPDase indicates a protein of 493 amino acid residues with a molecular mass of 54 kDa. The predicted orientation shows it to be anchored to the membrane by two transmembranous segments near the NH2 and COOH termini with very short intracytoplasmic peptides at either end. The bulk of the protein is extracellular and contains 12 potential N-glycosylation sites, several potential phosphorylation sites, and five sequences that are conserved in seven other related membrane proteins. Four of the conserved sequences, designated as apyrase conserved regions, are present in both ecto-ATPases and soluble E-type ATPases. The fifth conserved region, which occurs near the COOH terminus of the eight proteins, is observed only in the membrane-bound ecto-ATPases. Unexpectedly, sequence comparison revealed that the chicken oviduct ecto-ATPDase is equally distant from the two ecto-ATPases, which exhibit low activity toward ADP, and the four putative ecto-ATPDases, which are closely related to CD39.

Identification of the high affinity receptor binding region in human immunoglobulin E

We have investigated the capacity of N- and C-terminally truncated and chimeric human (h) IgE-derived peptides to inhibit the binding of 125I-labeled hIgE, and to engage cell lines expressing high and low affinity receptors (Fc-epsilon-RI/II). The peptide sequence Pro343-Ser353 of the hC-epsilon-3 domain is common to all h-epsilon-chain peptides that recognize hFc-epsilon-RI. This region in IgE is homologous to the A loop in C-gamma-2 that engages the rat neonatal IgG receptor. Optimum Fc-epsilon-RI occupancy by hIgE occurs at pH 6.4, with a second peak at 7.4. N- or C-terminal truncation has little effect on the association rate of the ligands with this receptor. Dissociation markedly increases following C-terminal deletion, and hFc-epsilon-RI occupancy at pH 6.4 is diminished. His residue(s) in the C-terminal region of the epsilon-chain may thus contribute to the high affinity of interaction. Grafting the homologus rat epsilon-chain sequence into hIgE maintains hFc-epsilon-RI interaction without conferring binding to rat Fc-epsilon-RI. hFc-epsilon-RII interaction is lost, suggesting that these residues also contribute to hFc-epsilon RII binding. h-epsilon-chain peptides comprising only this sequence do not block hIgE/hFc-epsilon-RI interaction or engage the receptor. Therefore, sequences N- or C-terminal to this core peptide provide structures necessary for receptor recognition.

Epitope mapping of the site(s) of binding of Fc epsilon RII/CD23 within human IgE. Determination of the B lymphocyte-binding sites by use of synthetic peptides and anti-peptide antibodies representative of linear Fc sequences

The work undertaken has investigated the structure-function relationship between IgE and its low affinity receptor on B lymphocytes. To identify sites on the IgE molecule which interact with the low affinity receptor (Fc epsilon RII/CD23), 10 different peptide sequences within the CH2, CH3 and CH4 domains of human IgE were selected according to charge, overall hydrophobicity and possible accessibility on native IgE sequences. Peptides representative of these were synthesized by the solid phase procedure; and their cytophilic activities were examined by determining their capacity to inhibit the binding of radiolabelled or erythrocyte-bound IgE to a Fc epsilon RII/CD23 positive B cell line (RPMI-8866). Moreover, these linear sequences were rendered immunogenic by conjugation to a protein carrier (KLH) and used to produced polyclonal antibodies in rabbits. The reactivity of the anti-peptide antibodies with both free peptides and native IgE bound to a solid phase, as well as their capacity to inhibit binding of IgE to a Fc epsilon RII/CD23 positive cell line, were investigated. Results from such use of peptides and anti-peptide antibodies indicate that two sequences, representative of residues 364-383 and 401-415, could be involved in the binding of IgE to both membrane-bound and soluble form Fc epsilon RII/CD23; indicating that the B lymphocyte-binding site on human IgE may be restricted to the CH3 domain.

The human mast cell receptor binding site maps to the third constant domain of immunoglobulin E

The characterization of the site on the IgE molecule which accommodates the high affinity receptor for IgE (Fc epsilon RI) should allow the design of IgE analogues which can be utilized to block allergic responses. Using chimeric human IgE molecules in which different constant region domains were exchanged with their murine homologues, we demonstrate here that the C epsilon 3 in its native configuration is essential for the binding to the alpha subunit of the human Fc epsilon RI. Deletion of the human C epsilon 2 from such chimeric molecules did not impair their ability to interact with the Fc epsilon RI, indicating that C epsilon 2 is not directly involved in the human Fc epsilon RI binding site and that C epsilon 3 alone is necessary and sufficient to account for most of the human Fc epsilon RI-binding capacity.

A 20 amino acid synthetic peptide of a region from the 55 kDa human TNF receptor inhibits cytolytic and binding activities of recombinant human tumour necrosis factor in vitro

Tumour Necrosis Factor (TNF) and Lymphotoxin (LT) can exert a wide range of effects on cells and tissues and they are important effector molecules in cell mediated immunity. All these effects are induced subsequent to the binding of these cytokines to specific membrane receptors. Recently, two of these membrane receptors of 55 and 75 kDa, have been identified which share some amino acid (AA) homology in their N-terminal extracellular domains but differ in their intracellular domains. We synthesized two synthetic 20 AA peptides from hydrophilic regions of the N-terminal extracellular domains of the 55 kDa receptor; peptide A shares homology with both 55 and 75 kDa receptors, peptide B is unique. We found peptide B inhibits both the binding and cytolytic activity of recombinant human TNF when tested on murine L929 cells in vitro. Polyclonal antiserum generated against peptide B will block binding of 125I-labelled TNF to these cells in vitro. However, peptide A and antiserum prepared against peptide A are without effect in these same assay systems. These data suggest that the 20 AA sequences from AA 175 to 194 in the N-terminal extracellular domain of the 55 kDa TNF receptor are expressed on the cell surface and are involved in the binding of TNF.

Monoclonal antibodies defining epitopes on human IgE

Twelve monoclonal antibodies (mAb) were isolated that bound to six clusters of epitopes on the constant region of the epsilon chain of human IgE. Four of the mAb bound to the C epsilon 1 or early C epsilon 2 regions; three of these bound to the IgE myeloma protein PS and to serum IgE but not to the IgE myeloma protein ND. These mAb probably recognize an allotypic marker. Another mAb reacted with heat-denatured, but not native IgE. Four of the mAb failed to release histamine; the epitopes recognized by these mAb are in the C epsilon 1, C epsilon 2 and C epsilon 3-4 regions of IgE. Three of these non-histamine releasing mAb did not bind to IgE on the basophil surface. These mAb recognize epitopes in C epsilon 2 and C epsilon 3-4 that are not accessible when IgE is bound to its receptor. Four mAb inhibited IgE binding to basophils; two of these did not release histamine, and two others that bind to epitopes in the C epsilon 2-4 domain, released histamine and therefore blocked IgE binding by steric hindrance. Inhibition of IgE binding by different mAb suggest that the Fc epsilon RI and Fc epsilon RII bind to partly overlapping regions of the IgE molecule although the sites do not appear to be identical. A number of sites on C epsilon 1 and C epsilon 3-4 were accessible when IgE is bound to its basophil receptor. The data support the concept that only part of the Fc portion of IgE is hidden in the receptor and that portions of C epsilon 1-4 are accessible on the cell surface. These mAb should be useful in determining the domains of IgE that are critical for its biological activity.

Anti-peptide antibodies to cathepsins B, L and D and type IV collagenase. Specific recognition and inhibition of the enzymes

Anti-peptide antibodies were raised against synthetic peptides selected from the sequences of human cathepsins B and L, porcine cathepsin D and human type IV collagenase. Sequences were selected from the active site clefts of the cathepsins in the expectation that these would elicit immunoinhibitory antibodies. In the case of type IV collagenase a sequence unique to this metalloproteinase subclass and suitable for immunoaffinity purification, was chosen. Antibodies against the chosen cathepsin B sequence were able to recognize the peptide but were apparently unable to recognise the whole enzyme. Antibodies against the chosen cathepsin L sequence were found to recognise and inhibit the native enzyme and were also able to discriminate between denatured cathepsins L and B on Western blots. Antibodies against the chosen cathepsin D sequence recognised native cathepsin D in a competition ELISA, but did not inhibit the enzyme. Native type IV collagenase was purified from human leukocytes by immuno-affinity purification with the corresponding anti-peptide antibodies.

Mapping of murine IgE epitopes involved in IgE-Fc epsilon receptor interactions

The generation of anti-IgE monoclonal antibodies has permitted the identification of various serological epitopes on the IgE molecule. The relationship of the sites on IgE recognized by such antibodies to the Fc epsilon receptor (Fc epsilon R) interaction site has been determined using cross-inhibition studies. However, interpretation of this type of experiment is limited by problems of steric hindrance. Thus, to accomplish precise mapping on the IgE molecule of the Fc epsilon R interaction site and the binding sites of various anti-IgE mAb, we employed site-directed mutagenesis of the IgE heavy chain gene. To this end we have constructed and expressed a recombinant murine constant epsilon heavy chain (C epsilon) gene bearing a (4-hydroxy-3-nitrophenyl)acetic acid (NP)-binding VH region. Several site-specific mutants in the C epsilon 3 and C epsilon 4 domains of this recombinant C epsilon gene were prepared and expressed by transfection into the light chain-producing J558L myeloma cell line. The resulting IgE antibodies were tested for binding to mast cells and to various anti-IgE mAb. The mutants produced include a proline to histidine point mutant at amino acid residue 404 in the C epsilon 3 domain, a mutant with a truncated C epsilon 4 domain, a mutant with a 45 amino acid deletion in the carboxy end of C epsilon 3, and a chimeric human C epsilon in which the human C epsilon 3 was replaced by the homologous mouse C epsilon 3 domain. These mutants have permitted the localization, to the C epsilon 3 domain, of the epitopes recognized by the 84.1C and 95.3 anti-IgE mAb. The 84.1C mAb recognizes a site on IgE which is identical or very close to the Fc epsilon R binding site, and 95.3 recognizes a site on IgE which is related, but not identical to the Fc epsilon R binding site. The antigenic determinant recognized by the 51.3 mAb, which is inefficient at blocking the IgE-Fc epsilon R interaction, has been mapped to the C epsilon 4 domain. When tested for binding to the Fc epsilon R on RBL-2H3 cells, the point mutant bound to the Fc epsilon R with twofold reduced affinity, while the C epsilon 3 deletion mutant and the mutant truncated in C epsilon 4 lost all receptor binding activity.(ABSTRACT TRUNCATED AT 400 WORDS)