Fine specificity of antibody recognition of carcinoma-associated epithelial mucins: antibody binding to synthetic peptide epitopes

Peptide Scanning-assisted Identification of a Monoclonal Antibody-recognized Linear B-cell Epitope

The identification of an antigenic epitope by the immune system allows for the understanding of the protective mechanism of neutralizing antibodies that may facilitate the development of vaccines and peptide drugs. Peptide scanning is a simple and efficient method that straightforwardly maps the linear epitope recognized by a monoclonal antibody (mAb). Here, the authors present an epitope determination methodology involving serially truncated recombinant proteins, synthetic peptide design, and dot-blot hybridization for the antigenic recognition of nervous necrosis virus coat protein using a neutralizing mAb. This technique relies on the dot-blot hybridization of synthetic peptides and mAbs on a polyvinylidene fluoride (PVDF) membrane. The minimum antigenic region of a viral coat protein recognized by the RG-M56 mAb can be narrowed down by step-by-step trimmed peptide mapping onto a 6-mer peptide epitope. In addition, alanine scanning mutagenesis and residue substitution can be performed to characterize the binding significance of each amino acid residue making up the epitope. The residues flanking the epitope site were found to play critical roles in peptide conformation regulation. The identified epitope peptide may be used to form crystals of epitope peptide-antibody complexes for an x-ray diffraction study and functional competition, or for therapeutics.

Rise and fall of an anti-MUC1 specific antibody

BACKGROUND

So far, human antibodies with good affinity and specificity for MUC1, a transmembrane protein overexpressed on breast cancers and ovarian carcinomas, and thus a promising target for therapy, were very difficult to generate.

RESULTS

A human scFv antibody was isolated from an immune library derived from breast cancer patients immunised with MUC1. The anti-MUC1 scFv reacted with tumour cells in more than 80% of 228 tissue sections of mamma carcinoma samples, while showing very low reactivity with a large panel of non-tumour tissues. By mutagenesis and phage display, affinity of scFvs was increased up to 500fold to 5,7×10(-10) M. Half-life in serum was improved from below 1 day to more than 4 weeks and was correlated with the dimerisation tendency of the individual scFvs. The scFv bound to T47D and MCF-7 mammalian cancer cell lines were recloned into the scFv-Fc and IgG format resulting in decrease of affinity of one binder. The IgG variants with the highest affinity were tested in mouse xenograft models using MCF-7 and OVCAR tumour cells. However, the experiments showed no significant decrease in tumour growth or increase in the survival rates. To study the reasons for the failure of the xenograft experiments, ADCC was analysed in vitro using MCF-7 and OVCAR3 target cells, revealing a low ADCC, possibly due to internalisation, as detected for MCF-7 cells.

CONCLUSIONS

Antibody phage display starting with immune libraries and followed by affinity maturation is a powerful strategy to generate high affinity human antibodies to difficult targets, in this case shown by the creation of a highly specific antibody with subnanomolar affinity to a very small epitope consisting of four amino acids. Despite these "best in class" binding parameters, the therapeutic success of this antibody was prevented by the target biology.

Selection of peptide ligands for the antimucin core antibody C595 using phage display technology: definition of candidate epitopes for a cancer vaccine

Aims-To further define the specificity of the antimucin core antibody C595 by fitting it with a family of hexapeptide ligands by immunoselection of filamentous bacteriophage from a gene III display library of approximately 6.4 x 10(7) random hexapeptides.Methods-Three rounds of immuno-selection were used to enrich for C595 binding phage. DNA sequencing revealed the hexapeptides expressed. Bacteriophage and corresponding synthetic hexapeptides were used in ELISA assay to determine binding affinities.Results-Twenty nine clones from this selected population were analysed. Seven contained the natural epitope RPAP, encoded by two different DNA sequences; 17/29 contained the motif RLPP. In all, 28/29 clones contained the motif RXXP and one clone (RVRPAP) contained the motif RXXP in two peptidic registers; 24/28 clones (6/8 DNA sequences) contained a hydrophobic residue (V or I) at position 1 relative to the RXXP motif. In addition the proximity of RXXP to glycine (position 5) suggests that this contributes in the natural epitope to antibody/antigen binding, which was not detected by chemical synthetic methods. One clone, KSKAGV, bears no obvious relationship to the natural epitope and therefore qualifies as a weakly binding mimotope.Conclusions-This approach has rapidly defined the specificity of this antibody in unprecedented detail, and provides a more comprehensive molecular basis for exploring the immune recognition of the MUC1 mucin by the C595 antibody. Importantly, the novel but related epitopes seen provide peptide specificities and a strategy which may prove useful in generating cancer vaccine candidates.

Epitope Mapping of PR81 anti-MUC1 Monoclonal Antibody Following PEPSCAN and Phage Display Techniques

PR81 is an anti-MUC1 monoclonal antibody (MAb) which was generated against human MUC1 mucin that reacted with breast cancerous tissue, MUC1 positive cell line (MCF-7, BT-20, and T-4 7 D), and synthetic peptide, including the tandem repeat sequence of MUC1. Here we characterized the binding properties of PR81 against the tandem repeat of MUC1 by two different epitope mapping techniques, namely, PEPSCAN and phage display. Epitope mapping of PR81 MAb by PEPSCAN revealed a minimal consensus binding sequence, PDTRP, which is found on MUC1 peptide as the most important epitope. Using the phage display peptide library, we identified the motif PD(T/S/G)RP as an epitope and the motif AVGLSPDGSRGV as a mimotope recognized by PR81. Results of these two methods showed that the two residues, arginine and aspartic acid, have important roles in antibody binding and threonine can be substituted by either glycine or serine. These results may be of importance in tailor making antigens used in immunoassay.

T cells recognize PD(N/T)R motif common in a variable number of tandem repeat and degenerate repeat sequences of MUC1

The tumor-associated antigen MUC1 is a transmembrane glycoprotein, which is overexpressed in human carcinomas. Peptide epitopes, containing the PDTR fragment from the variable number of tandem repeat (VNTR) domains of MUC1 have been found to be immunodominant in T-cell and B-cell responses. However, little is known about the immunogenicity and specificity of T-cell epitopes from other regions of MUC1 that may also participate in immune responses against tumors. In this study, the combination of immunoinformatics, molecular modeling and a vaccine adjuvant strategy were used to predict and describe a novel T-cell epitope, SAPDNRPAL, located within the degenerate tandem repeat of MUC1. This peptide possesses structural similarity to both VNTR-derived SAPDTRPAP and Sendai virus peptide FAPGNYPAL, which are known to induce cytotoxic T lymphocytes (CTL). We found that SAPDNRPAL had a higher affinity for mouse H-D(b), H-2K(b) and human HLA-A2 molecules than SAPDTRPAP. A chimeric peptide (CP) containing SAPDNRPAL and an adjuvant C5a-derived decapeptide induced epitope-specific type 1 T cells in human MUC1 transgenic mice (ELISPOT). Mice that received dendritic cells (DC) pulsed with the CP or a 25-mer peptide containing the SAPDNRPAL sequence showed increased frequencies of SAPDNRPAL- and SAPDTRPAP-specific interferon-gamma producing T cells. PDTR-specific antibody 214D4 reacted with both SAPDNRPAL and SAPDTRPAP (ELISA). Altogether, our data suggest that the degenerate MUC1 repeat sequence contains the immunogenic T-cell epitope SAPDNRPAL, which is cross-reactive with the VNTR-derived peptide SAPDTRPAP. We suggest that the use of immunogenic PDNR-containing epitope(s) in vaccine strategies could be beneficial for developing increased, PD(N/T)R motif-specific T-cell responses against tumors expressing MUC1.

IgE-binding epitopes of the American cockroach Per a 3 allergen

BACKGROUND

The Per a 3 is a species-specific allergen of the American cockroach (Periplaneta americana) related to insect hemolymph proteins and includes four known isoallergens. This study aimed to identify Per a 3 linear IgE-binding epitopes.

METHODS

Per a 3 recombinant fragments were generated from the recombinant Per a 3.01 allergen (685 amino acid residues) by using existing restriction sites or by using polymerase chain reaction products, and expressed in Escherichia coli. Antigenicities were assessed by immunoblotting, enzyme-linked immunosorbent assay (ELISA), and binding inhibition with human IgE.

RESULTS

Human IgE recognized recombinant fragments 340-425, 466-579, 502-595, and 595-636 as revealed by immunoblotting and ELISA. On the other hand, the N-terminal fragment 1-399, recombinants 410-443, 472-551, 502-579, 606-636, and the C-terminal fragment 636-685 were unable to bind human IgE. Amino acid sequences 400-409, 466-471, 580-595, and 595-605 were shown to be required for IgE binding to the Per a 3.01 allergen, suggesting that the C-terminus contains most of the IgE-binding sites. Four peptides corresponding to these IgE-binding amino acid sequences were synthesized. These peptides reacted with most sera (62.5-87.5%) tested as revealed by ELISA, demonstrating a heterogeneous IgE-binding response. Moreover, preincubation of IgE-positive recombinant proteins and synthetic peptides with atopic IgE resulted in marked inhibition of the IgE binding to Per a 3.01 allergen. Amino acid sequences 400TVLRDPVFYQ409, 466NNVDQI471, 580VDKGHNYCGYPENLLI595, and 595IPKGKKGGQAY605 of the major recombinant American cockroach Per a 3.01 allergen were involved in IgE binding.

CONCLUSION

These findings will advance our understanding of the antigenic structures responsible for allergenicity to the American cockroach, thereby providing strategies for the development of immunotherapies.

Purification of anti-MUC1 antibodies by peptide mimotope affinity chromatography using peptides derived from a polyvalent phage display library

A polyvalent, lytic phage display system (T7Select415-1b) displaying a random peptide library has been investigated for its ability to discover novel mimotopes reactive with the therapeutic monoclonal antibody C595. Sequence analysis of enriched phage lead to the identification of a predominant sequence RNREAPRGKICS, and two other consensus sequences RXXP and RXP. The novel synthetic peptide RNREAPRGKICS was linked to beaded agarose and the performance as a mimotope affinity chromatography matrix evaluated. Antibody purified using the novel matrix was found to be of higher specific reactivity than antibody purified using the conventional epitope matrix (peptide APDTRPAPG). The RNREAPRGKICS peptide binding to C595 demonstrated a higher equilibrium association constant (K(A)=0.75 x 10(6)) than the epitope peptide (K(A)=0.16 x 10(6)). Circular dichroism showed that the novel peptide had a more highly ordered structure at 4 degrees C and room temperature, than the epitope peptide.

Direct isolation of monoclonal antibodies from tissue culture supernatant using the cation-exchange cellulose Express-Ion S

The chromatography of the murine hybridoma cell C595/102 culture supernatant expressing the therapeutic monoclonal antibody C595, on the cation-exchange cellulose Whatman Express-Ion Exchanger S has been investigated. Initial method scouting studies using purified C595 in 1-ml mini columns demonstrated that binding capacity and binding efficiency were dependent not only on decreasing pH but also on the buffer salts used to prepare the mobile phase. Under optimised conditions of 0.1 M sodium acetate buffer, pH 5.0, we were able to separate purified C595 from BSA, the major contaminant in tissue culture fluid. Under these conditions immunoreactive C595 could be isolated directly from tissue culture supernatant. A scale-down study was carried out using a 25-ml column operated at a flow-rate of 150 cm/h which also yielded purified immunoreactive antibody. This procedure should now be suitable for scale-up.

Reactivity of natural and induced human antibodies to MUC1 mucin with MUC1 peptides and n-acetylgalactosamine (GalNAc) peptides

Antibodies (Abs) to MUC1 occur naturally in both healthy subjects and cancer patients and can be induced by MUC1 peptide vaccination. We compared the specificity of natural and induced MUC1 Abs with the objective of defining an effective MUC1 vaccine for active immunotherapy of adenocarcinoma patients. Serum samples, selected out of a screened population of 492 subjects for their high levels of IgG and/or IgM MUC1 Abs, were obtained from 55 control subjects and from 26 breast cancer patients before primary treatment, as well as from 19 breast cancer patients immunized with MUC1 peptides coupled to keyhole limpet hemocyanin (KLH) and mixed with QS-21. The samples were tested with enzyme-linked immunoassays for reactivity with (1) overlapping hepta- and 20-mer peptides spanning the MUC1 tandem repeat sequence; (2) two modified 60-mer peptides with substitutions in the PDTR (PDTA) or in the STAPPA (STAAAA) sequence of each tandem repeat; and (3) four 60-mer glycopeptides with each 1, 2, 3 and 5 mol N-acetylgalactosamine (GalNAc) per repeat. More than one minimal epitopic sequence could be defined, indicating that Abs directed to more than one region of the MUC1 peptide core can coexist in one and the same subject. The most frequent minimal epitopic sequence of natural MUC1 IgG and IgM Abs was RPAPGS, followed by PPAHGVT and PDTRP. MUC1 peptide vaccination induced high titers of IgM and IgG Abs predominantly directed, respectively, to the PDTRPAP and the STAPPAHGV sequences of the tandem repeat. Natural MUC1 Abs from breast cancer patients reacted more strongly with the N-acetylgalactosamine (GalNAc) peptides than with the naked 60-mer peptide, while reactivity with the GalNAc-peptides was significantly reduced (2-tailed p < 0.0001) in the MUC1 IgG and IgM Abs induced by MUC1 peptide vaccination. Whereas in cancer patients glycans appear to participate in epitope conformation, the epitope(s) recognized by MUC1 Abs induced by peptide vaccination are already masked by minimal glycosylation. Therefore, our results indicate that a MUC1 glycopeptide would be a better vaccine than a naked peptide.

MUC1 and cancer

The MUC1 membrane mucin was first identified as the molecule recognised by mouse monoclonal antibodies directed to epithelial cells, and the cancers which develop from them. Cloning the gene showed that the extracellular domain is made up of highly conserved repeats of 20 amino acids, the actual number varying between 25 and 100 depending on the allele. Each tandem repeat contains five potential glycosylation sites, and between doublets of threonines and serines lies an immunodominant region which contains the epitopes recognised by most of the mouse monoclonal antibodies. The O-glycans added to the mucin produced by the normal breast are core 2 based and can be complex, while the O-glycans added to the breast cancer mucin are mainly core 1 based. This means that some core protein epitopes in the tandem repeat which are masked in the normal mucin are exposed in the cancer associated mucin. Since novel carbohydrate epitopes are also carried on the breast cancer mucin, the molecule is antigenically distinct from the normal breast mucin. (Changes in glycosylation in other epithelial cancers have been observed but are not so well documented.) Immune responses to MUC1 have been seen in breast and ovarian cancer patients and clinical studies have been initiated to evaluate the use of antibodies to MUC1 and of immunogens based on MUC1 for immunotherapy of these patients. The role of the carbohydrates in the immune response and in other interactions with the effector cells of the immune system is of particular interest and is discussed.

CFTR expression does not influence glycosylation of an epitope-tagged MUC1 mucin in colon carcinoma cell lines

The cause of the mucus clearance problems associated with cystic fibrosis remains poorly understood though it has been suggested that mucin hypersecretion, dehydration of mucins, and biochemical abnormalities in the glycosylation of mucins may be responsible. Since the biochemical and biophysical properties of a mucin are dependent on O-glycosylation, our aim was to evaluate the O-glycosylation of a single mucin gene product in matched pairs of cells that differed with respect to CFTR expression. An epitope-tagged MUC1 mucin cDNA (MUC1F) was used to detect variation in mucin glycosylation in stably transfected colon carcinoma cell lines HT29 and Caco2. The glycosylation of MUC1F mucin was evaluated in matched pairs of Caco2 cell lines that either express wild-type CFTR or have spontaneously lost CFTR expression. The general glycosylation pattern of MUC1F was evaluated by determining its reactivity with a series of monoclonal antibodies against known blood group and tumor-associated carbohydrate antigens. Metabolic labeling experiments were used to estimate the gross levels of glycosylation and sulfation of MUC1F mucin in these matched pairs of cell lines. Expression of CFTR in this experimental system did not affect the gross levels of glycosylation or sulfation of the MUC1F mucin nor the types of carbohydrates structures attached to the MUC1F protein.

Design of ligands for the purification of anti-MUC1 antibodies by peptide epitope affinity chromatography

The fine specificity of epitope recognition of the anti-MUC1 mucin monoclonal antibody, C595 has been studied using solid-phase replacement net (RNET) analysis. Two peptides (RAAP and RPPP) showed increased reactivity with C595 antibody compared with the native epitope (RPAP). These were synthesized as integral motifs within MUC1 immunodominant peptides and analyzed by fluorescence quenching (FQ) and circular dichroism (CD). They were also tested as ligands for the purification of C595 antibody using epitope affinity chromatography. Affinity matrices were compared with respect to capacity, affinity, and quality of the purified product. In FQ tests the native epitope peptide (APDTRPAPG) and the alanine substituted peptide had similar association constants when reacting with C595 antibody, whereas the proline substituted peptide (APDTRPPPG) had a higher association constant. This order of affinity for C595 was confirmed in chromatography experiments in which antibody was eluted from the former two peptide matrices at approximately the same point on the NaSCN elution gradient, whereas antibody was desorbed from APDTRPPPG at a higher NaSCN concentration. Circular dichroism analysis showed that the thermodynamically preferred conformation of these peptides in aqueous solution is the P-II extended helix, the conformation preferred for an extended bound form of the peptide held by interactions with the peptide amides. The stronger binding peptide (APDTRPPPG) has the higher population of the P-II helix in solution. In conclusion, RNET analysis is useful in the rational design of peptide ligands so that the performance of affinity matrices may be regulated.

Effects of glycosylation on fragments of tumour associated human epithelial mucin MUC1

The glycodecapeptide AcPAPGS(alpha GalNAc)T(alpha GalNAc)APPA and the C-terminal glycohexapeptide AcS(alpha GalNAc)T(alpha GalNAc)APPA have been synthesized by applying the N-terminal Fmoc group in combination with the heptyl ester cleavable by lipase-catalyzed hydrolysis at pH 7. The solution conformation of these MUC1-related synthetic glycopeptides and the control, non-glycosylated decapeptide AcPAPGSTAPPA have been investigated using NMR spectroscopy. The structural studies indicate that the glycohexapeptide has a folded structure in solution. For this molecule, unrestrained molecular dynamics has been used to confirm the presence of the observed solution through-space connections. The results indicate that the non-globular nature of MUC1 is due to both protein core sequence and the effect of carbohydrate.

A novel monoclonal antibody specific for sialylated MUC1 mucin

Development of a new monoclonal antibody (mAb) MY.1E12 which reacts with sialylated MUC1 mucins is described. The mAb did not react with any component in the lysates of COS-1 cells, whereas it bound to sialylated MUC1 mucins produced by COS-1 cells transiently transfected with MUC1 mucin cDNA, strongly suggesting that the expression of the epitope of mAb MY.1E12 depends on the presence of the MUC1 mucin core peptide. The requirement of sialyl residues for antibody recognition was established by Western blotting analysis of extracts of various carcinoma cells and in situ desialylation. In all cases, the mAb binding of electrophoretically separated MUC1 mucin diminished after desialylation by mild acid hydrolysis. When Capan-1 pancreatic carcinoma cells were pretreated with benzyl-N-acetylgalactosaminide in culture, the MUC1 mucins produced under these conditions, which were detected by core peptide-specific mAbs, did not react with mAb MY.1E12. These results suggest that O-linked carbohydrate chains are important for the mAb binding.

Assessment of five serum marker assays in patients with advanced breast cancer treated with medroxyprogesterone acetate

This study concerns five different tumour marker assays examined in the context of 94 patients with advanced breast cancer treated in a prospectively randomised trial of different doses of medroxyprogesterone acetate (MPA). MPA was administered at doses of 500 or 1000 mg daily and clinical evaluation of patients was carried out according to UICC criteria. Carcinoembryonic antigen (CEA) was selected as a standard marker, with three assays for MUC1 mucins (epithelial mucin core antigens (EMCA), EMCA2 and BR-MA immunoradiometric assay) differing in antibody specificities for different mucin epitopes. An additional novel assay for soluble cytokeratin was also evaluated as an example of an independent marker with a different nature and biology. Sensitivity of individual assays ranged between 44 (EMCA2) and 69% (cytokeratin) and the use of two assays in combination led to sensitivities as high as 84% (cytokeratin+BR-MA). The proportion of patients found to be assessable by each assay ranged between 51 (EMCA2) and 76% (cytokeratin). Of those patients whose marker changes were assessable, those receiving the higher dose of MPA displayed significant falls in marker levels after 12 weeks of treatment. This effect was not observed in patients receiving 500 mg. The change in cytokeratin levels in patients undergoing high dose MPA therapy proved to be most marked. Using the cytokeratin assay, 91% (of 23 patients) of patients with progressive disease showed at least a 25% rise in serum marker levels. Of these, 66% showed increases before disease progression was detected clinically with a mean lead time of 14 weeks. There was very little difference between the responses of the five tumour marker assays in patients with stable or responding disease, the proportion of these patients with stable or falling tumour marker levels ranging between 58% (CEA) and 77% (EMCA). We conclude that the cytokeratin assay has an application in monitoring response to therapy and predicting tumour progression in advanced breast cancer patients with assessable tumour marker profiles, especially if used in combination with a MUC1 mucin assay.

Delineation of the minimal hepatitis B surface antigen-specific B- and T-cell epitope contained within an anti-idiotype-derived pentadecapeptide

A pentadecapeptide (2F10 peptide) is capable of mimicking the group-specific "a" determinant of human hepatitis B surface antigen (HBsAg) at both the B- and the T-cell level. This peptide represents a sequence on the heavy-chain hypervariable region of a monoclonal "internal image" anti-idiotype (anti-id 2F10) that has partial sequence homology to the "a" determinant epitope of HBsAg. To identify the exact location of the B- and T-cell epitopes, four truncated peptides (peptides 1-4) were synthesized. Using these truncated peptides we have identified the minimal sequence (octapeptide 3) that represents a functional B- and T-cell epitope capable of generating HBsAg-specific antibodies and T cells. This to our knowledge represents the first example of a short peptide sequence functioning as both a B- and a T-cell epitope. We have also identified another T-cell epitope (2F10 peptide 4), but this peptide fails to elicit HBsAg-specific B cells and T cells. Thus, the 2F10 pentadecapeptide is composed of two nonoverlapping, functional T-cell epitopes only one of which is HBsAg specific. Since peptide 3 represents the complementarity-determining region and peptide 4 represents the framework region of the anti-id 2F10, we conclude that an 8-aa sequence from the complementarity-determining region of anti-id 2F10 is sufficient for the molecular mimicry of HBsAg. Finally, our experiments suggest that sequences flanking the minimal immunodominant epitope exert a considerable influence on the nature of antigenic processing that occurs and the resultant T-cell reactivity elicited.

Peptides with carboxyl-terminal sequence of alanine-proline: detection by a human monoclonal antibody

A human B lymphoblastoid cell line JWCI-L94 secretes an IgM human monoclonal antibody (HuMAb) that reacts with human melanoma cell lines, M14 and M12. To identify the antigenic epitope of this antibody, we screened lambda gt11 expression libraries constructed from M14 and M12. A total of 12 immunoreactive clones were isolated, and their DNA sequences were determined. The only sequence shared by all these clones was alanine-proline (A-P) at the carboxyl (C) terminal. HuMAb L94 reacted not only with C-terminal A-P-containing fusion proteins, but also with the synthetic dipeptide A-P. None of the peptides containing A-P internally or amino terminally reacted to HuMAb L94. Proline or alanine alone had no ability to bind to HuMAb L94. When alanine was replaced by glycine (G-P) or proline (P-P), the binding activity of these peptides was similar to that of A-P. On the other hand, when alanine was replaced by serine, valine, leucine, glutamine, lysine, methionine, phenylalanine, or hydroxyl proline, the resulting peptide completely lost the antigenic activity of HuMAb L94. These results demonstrate that HuMAb L94 recognizes C-terminal A-P, G-P, or P-P, and that a human antibody can recognize peptides as small as a two-amino acid residue.

Primary sequence determination and molecular modelling of the variable region of an antiMUC1 mucin monoclonal antibody

Polymerase chain reaction (PCR) products representative of the DNA sequence coding for the variable heavy (VH) and the variable light (VL) chains of an antiMUC1 mucin monoclonal antibody, C595, have been produced. These products were cloned, sequenced, and the primary amino acid sequences of the VH and VL regions deduced. The hypervariable complementarity determining regions (CDRs) and framework regions in the heavy and light chains were located, and homologies with canonical forms for the CDR loops L1, L2, L3, H1 and H2 were identified by database searching. The structure for the H3 loop was calculated directly. Computational molecular modelling was accomplished using the fully automated AbM package (Oxford Molecular, Oxford, U.K.). Energy minimisation was performed using the program InsightII (Biosym, San Diego, California, U.S.A.). The investigation provides a basis for the molecular analysis of the antigen binding site of the C595 antibody with the aim to identify key residues and interactions involved in the immune recognition of the C595 antibody defined epitope, which is expressed in the majority of breast and ovarian carcinomas.

Analysis of B-cell epitopes in the N-terminal region of Chi t I component III using monoclonal antibodies

The hemoglobins of the midge Chironomus thummi thummi (Chi t I) are known to cause immediate-type hypersensitivity reactions in humans. Further knowledge of the antigenic sites of such allergens will provide new therapeutic approaches. The aim of our study was to identify and characterize linear B-cell epitopes of the hemoglobin component III of Chi t I (136 amino acid residues). Using the antigenic index algorithm of Jameson and Wolf (Jameson and Wolf (1988) Comput. Appl. Biosci. 4, 181-186), three linear binding sequences of this allergen molecule were predicted. Two mouse monoclonal antibodies (mAbs 3 and 6) raised against purified Chi t I component III were investigated by ELISA for their binding to nine synthetic peptides 19-21 residues in length, covering nearly the whole sequence of component III. MAb 6 recognized only one peptide (11-30) while mAb 3 bound to both N-terminal peptides (1-19 and 11-30), suggesting that the antibody binding site is located in the overlapping region. This assumption could be confirmed in ELISA with solid phase-bound recombinant peptides (RP) as well as in inhibition studies with free tryptic peptides indicating that identification of these linear B-cell epitopes is neither influenced by the method of peptide production nor by the kind of used immunoassay. To define the essential amino acid residues we investigated mAbs with solid phase-bound overlapping octamers. In the case of mAb 3, amino acids experimentally identified as essential for antibody binding (aa 13-17) are identical with those residues predicted as a B-cell epitope with the antigenic index of Jameson and Wolf.

Induction of antibody responses to breast carcinoma associated mucins using synthetic peptide constructs as immunogens

A strategy for directing and enhancing B cell immune responses against synthetic peptide determinants has been developed in order to produce antibodies specifically against protein epitopes of clinical relevance. A peptide sequence based upon the MUC-1 mucin protein core was selected for this purpose since anti-MUC-1 antibodies have proven diagnostic application and therapeutic potential in human breast and ovarian cancer. Peptide constructs were synthesised co-linearly linking the immunodominant B cell determinant region, PDTRPAP, in the protein core of the MUC-1 mucin, to sequence 111-120 of influenza haemagglutinin A/X-31, a determinant recognised by T helper cells through association with MHC class II molecules. Induction of anti-MUC-1 antibodies to the B cell determinant region by immunisation with peptide was shown to be dependent upon both the presence and the position of the T cell determinant. In addition, haplotype mismatching with respect to the T cell determinant resulted in a significant lowering of the anti-MUC-1 antibody response in peptide construct immunised mice. These findings are relevant to the design of immunogens to produce antibodies against peptide epitopes of tumour associated proteins and glycoproteins.