Immunochemical mapping of a specific domain on human choriogonadotropin using anti-protein and anti-peptide monoclonal antibodies

Investigation of factors influencing the immunogenicity of hCG as a potential cancer vaccine

Human chorionic gonadotrophin (hCG) and its β-subunit (hCGβ) are tumour autocrine growth factors whose presence in the serum of cancer patients has been linked to poorer prognosis. Previous studies have shown that vaccines which target these molecules and/or the 37 amino acid C-terminal hCGβ peptide (hCGβCTP) induce antibody responses in a majority of human recipients. Here we explored whether the immunogenicity of vaccines containing an hCGβ mutant (hCGβR68E, designed to eliminate cross-reactivity with luteinizing hormone) or hCGβCTP could be enhanced by coupling the immunogen to different carriers [keyhole limpet haemocyanin (KLH) or heat shock protein 70 (Hsp70)] using different cross-linkers [1-ethyl-3(3-dimethylaminopropyl)carboiimide (EDC) or glutaraldehyde (GAD)] and formulated with different adjuvants (RIBI or Montanide ISA720). While there was little to choose between KLH and Hsp70 as carriers, their influence on the effectiveness of a vaccine containing the BAChCGβR68E mutant was less marked, presumably because, being a foreign species, this mutant protein itself might provide T helper epitopes. The mutant provided a significantly better vaccine than the hCGβCTP peptide irrespective of the carrier used, how it was cross-linked to the carrier or which adjuvant was used when hCG was the target. Nonetheless, for use in humans where hCG is a tolerated self-protein, the need for a carrier is of fundamental importance. Highest antibody titres were obtained by linking the BAChCGβR68E to Hsp70 as a carrier by GAD and using RIBI as the adjuvant, which also resulted in antibodies with significantly higher affinity than those elicited by hCGβCTP peptide vaccine. This makes this mutant vaccine a promising candidate for therapeutic studies in hCGβ-positive cancer patients.

Candidate epitopes for measurement of hCG and related molecules: the second ISOBM TD-7 workshop

Participants of the Second International Workshop (WS) on human chorionic gonadotropin (hCG) of the International Society of Oncology and Biomarkers Tissue Differentiation 7 (ISOBM TD-7) have characterized in detail a panel of 69 antibodies (Abs) directed against hCG and hCG-related variants that were submitted by eight companies and research groups. Specificities of the Abs were determined using the First WHO International Reference Reagents for six hCG variants, i.e., hCG, hCGn, hCGβ, hCGβn, hCGβcf, and hCGα, which are calibrated in SI units, and hLH. Molecular epitope localizations were assigned to the ISOBM-mAbs by comparing ISOBM-Ab specificity, sandwich compatibility, and mutual inhibition profiles, to those of 17 reference monoclonal (m)Abs of known molecular epitope specificities. It appeared that 48 Abs recognized hCGβ-, 8 hCGα-, and 13 αβ-heterodimer-specific epitopes. Twenty-seven mAbs were of pan hCG specificity, two thereof with no (<0.1 %; epitope β₁), 12 with low (<1.0 %; epitopes β_2/4), and 13 with high (>>1 %; epitopes β_3/5) hLH cross-reactivity. The majority of hCGβ epitopes recognized were located in two major antigenic domains, one on the peptide chain of the tips of β-sheet loops 1 and 3 (epitopes β_2–6; 27 mAbs) and the second around the cystine knot (e.g., epitopes β₁, β₇, and β₁₀; 9 mAbs). Four mAbs recognized epitopes on hCGβcf-only (e.g., epitopes β₁₁ and β₁₃) and six mAbs epitopes on the remote hCGβ-carboxyl-terminal peptide (epitopes β₈ and β₉ corresponding to amino acids 135–144 and 111–116, respectively). For routine diagnostic measurements, methods are used that either detect hCG-only, hCGβ-only, or hCG together with hCGβ or hCG together with hCGβ and hCGβcf. Sandwich assays that measure hCG plus hCGβ and eventually hCGβcf should recognize the protein backbone of the analytes preferably on an equimolar basis, should not cross-react with hLH and not be susceptible to blunting of signal by nonmeasured variants like hCGβcf. Such assays can be constructed using pairs of mAbs directed against the cystine knot-associated epitope β₁ (Asp10, Asp60, and Gln89) in combination with epitopes β₂ or β₄ located at the top of β-sheet loops 1 + 3 of hCGβ involving aa hCGβ20-25 + 68-77. In summary, the results of the First and Second ISOBM TD-7 WSs on hCG provide the basis for harmonization of specificities and epitopes of mAbs to be used in multifunctional and selective diagnostic hCG methods for different clinical purposes.

Purification and structural analysis of a soluble human chorionogonadotropin hormone-receptor complex

Receptors for the luteotropin/human chorionogonadotropin hormone belong to the G-protein-coupled receptor family by their membrane-anchoring domains. They also possess a large extracellular domain (ECD) responsible for most of the hormone-receptor interactions. Structure-function studies identified several contacts between hormone and receptor ECD, but the precise topology of the complex is still unknown because of the lack of suitable heterologous expression means. Receptor ECDs exhibit leucine repeats and have been modelized on the basis of the three-dimensional structure of the porcine ribonuclease inhibitor, the first structurally known leucine-rich repeats protein. Here we report overexpression (up to 20 mg per liter) and purification to homogeneity of a soluble human chorionogonadotropin-ECD receptor complex secreted by stably cotransfected Chinese hamster ovary cells. Biochemical analysis and surface plasmon resonance data were in favor of a unique dimer with a 1:1 ligand-receptor stoichiometry. Immunopurified complex was submitted to circular dichroism characterization; CD spectra deconvolution indicated more than 25% alpha helices contributed by the receptor, in agreement with the porcine ribonuclease inhibitor-based modelization.

Molecular architecture and biorecognition processes of the cystine knot protein superfamily: part I. The glycoprotein hormones

In this review article, the reader is introduced to recent advances in our knowledge on a subset of the cystine knot superfamily of homo- and hetero-dimeric proteins, from the perspective of the endocrine glycoprotein hormone family of proteins: follitropin (FSH), Iutropin (LH), thyrotropin. (TSH) and chorionic gonadotropin (CG). Subsequent papers will address the structure-function behaviour of other members of this increasingly significant family of proteins, including various members of the transforming growth factor-beta (TGF-beta) family of proteins, the activins, inhibins, bone morphogenic growth factor, platelet derived growth factor-beta, nerve growth factor and more than 35 other proteins with similar topological features. In the present review article, specific emphasis has been placed on advances with the glycoprotein hormones (GPHs) that have facilitated greater insight into their physiological functions, molecular structures and most importantly the basis of the molecular recognition events that lead to the formation of hetero-dimeric structures as well as their specific and selective recognition by their corresponding receptors and antibodies. Thus, this review article focuses on the structural motifs involved in receptor recognition and the current techniques available to identify these regions, including the role of immunological methodology, peptide fragment design and synthesis and mutagenesis to delineate their structure-function relationships and molecular recognition behaviour.

Immunochemical mapping of gonadotropins

As a glycoprotein hormone, human chorionic gonadotropic (hCG) is not a single molecular entity but this term rather comprises an array of molecular variants such as hCG, hCG beta, hCGn, hCG beta n, hCG beta cf, -CTPhCG, hCG beta CTP, deglyhCG, asialohCG, hCGav and the closely related molecules hLH, hLH beta and hLH beta ef. The advent of monoclonal antibodies (MCA), the availability of ultrasensitive detection systems and the recent determination of the crystal structure of hCG, made it possible to design special purpose diagnostic and clinical research immunoassays for hCG-like molecules. For more than a decade we and others have tried to refine epitope maps for hCG and related molecules by means of a large panel of MCA, naturally occurring metabolic variants of hCG (hCGn, hCG beta, hCG alpha, hCG beta cf, hCG beta CTP), homologous hormones and subunits of various species (e.g. hLH, hLH beta, hFSH, hTSH, oLH, rLH beta), chemically modified molecules (deglyhCG, asialohCG, tryptic and chymotryptic hCG beta and hCG alpha fragments) and synthetic peptides (octapeptides and longer). It appeared that all epitopes on molecular hCG-variants recognized by our MCA are determined by the protein backbone. Except for the two major epitopes on hCG beta CTP and parts of two antigenic domains on hCG alpha, epitopes on hCG-derived molecules are determined by the tertiary and quarternary structure. Operationally useful descriptive epitope maps were designed including information on assay suitability of antigenic determinants. On this basis we established ultrasensitive time-resolved fluoroimmuno-assays for hCG, hCG and hCGn, hCG beta and hCG beta n and hCG beta cf, hCG alpha and additional assays recognizing different spectra of hCG-variants. Such assay have been applied by us and others to the detection of pregnancy, early pregnancy loss, choriocarcinoma, testicular cancer, other cancers and prenatal diagnosis. However, as the molecular structure of many epitopes utilized in immunoassays of different laboratories was not resolved, comparability of results was not satisfactory. Consequently, attempts were made to compare schematic epitope maps from different research institutions. The situation has been much improved by solving the three-dimensional (3D) structure of hCG. It has been shown that hCG is a member of the structural superfamily of cystine knot growth factors like NGF, PDGF-B and TGF-beta. Each of its subunits is stabilized in its topology by three disulfide bonds forming a cystine knot. Moreover, it turned out that the disulfide bridges in their majority have previously been wrongly assigned. Computer molecular modeling of crystallographic coordinates of hCG and subsequent selective combined--PCR-based and immunological--mutational analyses of hCG beta expressed via the transmembrane region of a MHC molecule made it possible to more precisely localize epitopes on hCG-derived molecules. Although the entire surface of hCG has to be regarded as potentially immunogenic there seems to be hot spots where epitopes are clustered in antigenic domains. These are located on the first and third loops protuding from the cystine knots of both subunits and are possibly centered around the knot itself. Ultimate answers on epitope localizations will be given by the crystal structure determination of hCG complexed with different Fabs.

Mapping of HCG-receptor complexes

Molecular forms of the porcine LH/CG receptor (pLHR) and complexes between hCG and either the full-length pLHR or its extracellular domain (ectodomain) have been produced in various recombinant systems. In COS cells and in the baculovirus insect cells system, the co-expression of the ecto- and endo-domains reconstituted a functional receptor where the association of the two domains seems to depend upon the presence of disulfide bridges. According to previous observations [39], synthetic peptides mimicking three regions of the ectodomain (21-38, 100-115, 250-272) were found to inhibit hormone binding and stimulation of cAMP production. Antisera raised against these peptides contained anti-peptide antibodies (Ab) able to interfere with hormone signalling. Moreover, the results of peptide mapping indicated that some peptides stretches may be more involved in signalling rather than in binding. Immunochemical mapping based on monoclonal antibodies (mAbs) was used to probe the hCG-ectodomain complex. It appeared that mAbs directed to epitopes present on the 'beta-tip' of hCG (assembled from the beta subunit loops 3 and 1, and previously designated site IIIb) and on the 'alpha-tip' (alpha subunit loops 1 and 3, site IIIa) bound to hCG-receptor complexes, whereas a conformational epitope (defined by the alpha-beta interface between beta seat belt C-terminus and alpha loop 2, site II) was masked. Interestingly, we and others previously reported that, in the hCG-full length receptor complex, site IIIa was shielded to mAb binding. A peptide mimicking the second extracellular loop (EL2) of the receptor endodomain was found to prevent the binding of a mAb directed to site IIIa, suggesting that this region of the endodomain may be interacting with the 'alpha-tip'. In the full-length, membrane anchored pLHR, the EL2 peptide inhibited hCG-induced cAMP production, but not binding. The possibility of inhibiting stimulation without inhibition of binding gives support to the 'negative specificity' hypothesis [6]. Thus, the ectodomain of the glycoprotein hormone receptors might be considered as a screening device preventing access of any glycoprotein hormone to the signalling peptide keys of the endodomain, which otherwise would be sensitive to any alpha subunit stimulation. Finally, antibody binding to site IIIa on the hCG-ectodomain complex was also hindered by an anti-peptide mAb directed against a peptide encoded by the eighth exon (pE x 8) of the LHR. This suggests that pEx8 is vicinal to the alpha-tip of hCG and to EL2 in the hCG-full length receptor complex. Altogether, these observations help to build up a topological model of the hCG-receptor complex.

A new series of mycobacterial expression vectors for the development of live recombinant vaccines

Recombinant BCG (bacillus Calmette-Guérin) is a promising candidate as a live vaccine delivery system. Thus far, however, only autoreplicative plasmids carrying the heterologous genes to be expressed in BCG, together with antibiotic-resistance genes, have been successfully used. This could potentially lead to the spreading of antibiotic resistance among other bacteria, and might therefore be unsafe for the environment. In this study, we present a series of three Escherichia coli-Mycobacteria shuttle vectors which enable expression and secretion of antigens without the use of antibiotic-resistance markers. All these plasmids confer mercury resistance to the host bacteria as the only selectable marker and contain a unique restriction site to allow for single-step in-frame cloning of open reading frames downstream from the Mycobacterium tuberculosis 85A antigen promoter and export signal. The system was used to express the free beta-subunit of human chorionic gonadotropin (hCG beta), a potential target of an immunotherapeutic vaccine.

A defined epitope on the human choriogonadotropin alpha-subunit interacts with the second extracellular loop of the transmembrane domain of the lutropin/choriogonadotropin receptor

The monoclonal antibody, HT13 recognizes human choriogonadotropin (CG) bound to the extracellular domain of its receptor, but not to the full-length receptor. The HT13 epitope is located in the regions of residues 15-17 and 73-75 of the human CG alpha-subunit. Only one synthetic peptide, lutropin (LH)/CG-receptor-(481-497)-peptide (EL2 peptide), which spans the second putative extracellular loop of the LH/CG-receptor endodomain, prevents recognition of human CG by HT13 mAb. EL2 peptide decreases hormone-induced cAMP production, but not high-affinity binding. An anti-EL2 serum also displays the capacity to inhibit human CG-stimulated cAMP production. These results suggest that the second extracellular loop of the receptor is in contact with the HT13 epitope of human CG alpha-subunit and is involved in signal transduction. A relative orientation of the hormone versus the endodomain is proposed.

Immunochemical mapping of human lutropin: II. Characterization of two monoclonal antipeptide antibodies reacting with the native beta-subunit

To investigate the epitopes present on the beta-subunit of the human lutropin (hLHbeta) and their topographical relationship at the surface of the molecule, we produced two monoclonal antipeptide antibodies, designated LHP03 and LHP04, capable of binding to the radiolabeled 125I-hLHbeta and directed to the 43-52 and 110-117 regions of the hLHbeta, respectively. Analysis of the accessibility of the epitopes on hLH and on the beta-subunit of human chorionic gonadotropin (hCGbeta), equine LH (eLHbeta) and ovine LH (oLHbeta) indicated that: (i) LHP03 binds to both the free hLHbeta subunit and dimeric hLH whereas LHP04 binds preferentially to the free hLHbeta, (ii) LHP03 recognizes weakly the hCGbeta and oLHbeta in comparison to hLHbeta and (iii) LHP04 binds oLHbeta as well as hLHbeta but does not bind to hCGbeta and eLHbeta. The topographical relationship of epitopes recognized by LHP03 and monoclonal antibodies recognizing dimer specific epitopes on hLH allowed us to localize discontinuous antigenic sites that overlaps or are located outside the hHLbeta(43-52) region. Together, our results demonstrated that the hHLbeta(43-52) portion is accessible on both the free hLHbeta subunit and hLH whereas the COOH-terminal portion, hHLbeta(110-117), is probably buried at the alpha/beta interface of the hormone.

Immunochemical mapping of human lutropin: I. Delineation of a conformational antigenic determinant

Lutropin (LH), follitropin (FSH), thyrotropin (TSH) and choriogonadotropin (CG) are assembled of two non-covalently alpha (alpha) and beta (beta) subunits. We studied the discontinuous antigenic regions recognized by a monoclonal anti-hLH antibody designated as LH05 which binds to hLH, hCG and hTSH and does not cross-react with either the free subunits or hFSH. LH05 and an antibody designated HT13, recognizing an epitope partly comprizing the alpha64-76 region, did not bind simultaneously to hCG. Furthermore, LH05 was unable to combine with an anti-peptide antibody (LHP03) directed to residues 43-52 of hLHbeta. Thus, LH05 recognizes an epitope partly overlapping with those recognized by HT13 and LHP03. Using various hybrid molecules, we showed that the human alpha-subunit plays a critical role in the assembly of the epitope that, in contrast, contains amino acid residues conserved in the various beta-subunit of several species. Together, our results suggest that the amino acids Leu49-Pro50, Tyr59-Arg60 and Leu86-Ser87 in the hLHbeta and the alpha64-76 region are probably included in the epitope recognized by LH05 which appeared to be not accessible on the CG/LH receptor.

Variant forms of the pig lutropin/choriogonadotropin receptor

The cloning and sequencing of porcine lutropin/choriogonadotropin (LH/hCG) receptor messenger RNAs have shown the presence of a full-length receptor (pLHR-A) and of shorter variants lacking either the transmembrane and the intracellular domains (pLHR-B and pLHR-C) or only the transmembrane domain (pLHR-D). Moreover, immunoblotting of testicular membrane extracts has detected 85-, 68-, and 45-48-kDa proteins reacting with antireceptor antibodies. Transfection experiments were performed to assign the protein species to the various messenger RNAs and to study the function of the various receptor species. COS-7 and L-cells transfected with an expression vector encoding full-length receptor pLHR-A yielded a protein of apparent molecular mass of 105 kDa. This corresponded to the complete receptor which had undergone a different glycosylation pattern to that found in testis, since after digestion with peptide N-glycosidase F both the 105-kDa COS-7 protein and the 85-kDa testicular glycoprotein yielded a holoprotein of approximately 63 kDa. Transfection with pLHR-A also yielded a high proportion of the 68-kDa glycoprotein which was shown by digestion with endoglycosidase H to be a high-mannose precursor of the full-length receptor. The existence of a large pool of precursor species in both transfected cells and Leydig cells evokes possible physiological regulations at the level of receptor maturation.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunochemical study of equine chorionic gonadotropin (eCG/PMSG): antigenic determinants on alpha- and beta-subunits

In the present study we have established an immunochemical mapping of equine Chorionic Gonadotropin (eCG/PMSG) using three monoclonal antibodies (mAbs), namely the antibodies ECG01, E10 and D7, raised against the native hormone. These antibodies do not bind to reduced, alkylated hormone, suggesting that they recognize discontinuous rather than continuous epitopes. We have also assessed the reactivity of mAbs towards human CG, and ovine, porcine, equine and bovine LH and FSH. The antigenic determinant recognized by ECG01 is localized on the alpha-subunit of equine gonadotropins and of human CG and LH. The epitopes recognized by E10 and D7 mAbs appear to be very similar and are present on the beta-subunit of eCG and of LHs from all species tested, except hLH, as well as on porcine and equine FSHs. Attempts to specify the amino-acid residues involved in these epitopes suggest that ECG01 mAb might preferentially bind to residues around position 70 whereas the region around disulfide bridges Cys-88-Cys-90 might be involved in the epitopes recognized by D7 and E10 mAbs. Topographical relationships of epitopes show that ECG01 mAb never binds to eCG simultaneously with either D7 or E10 mAbs. Furthermore, simultaneous binding of D7 and E10 mAbs on eCG could not be achieved. Thus, these three epitopes appear to be closely located on the surface of eCG. Finally, ECG01 mAb inhibits eCG binding to LH and FSH receptors, suggesting that its antigenic site is closely related to hormone-receptor interaction site(s).

Topographic analysis of human follicle-stimulating hormone-beta using anti-peptide antisera

The purpose of this study was to identify peptide sequences of human follicle-stimulating hormone-beta (hFSH beta) which are accessible subsequent to association with hFSH alpha in heterodimeric hFSH. Antisera were raised against synthetic peptides (Abpep) corresponding to hFSH beta sequences 1-20, 16-36, 33-53, 49-67, 66-85, 81-100 and 98-111. The topography of hFSH beta was studied by testing the binding of these antisera to hFSH beta and hFSH captured by monoclonal antibodies (MAb) in an enzyme-linked immunosorbent assay (ELISA). When hFSH and hFSH beta were captured by the same MAb, binding of Ab16-36, Ab33-53, Ab81-100 and Ab98-111 to hFSH was significantly lower compared to hFSH beta. However, compared to other Abpep, binding of Ab35-53 to hFSH was strong. Similar results were obtained when hFSH was captured by an alpha-specific MAb (10.3A6). Using 10.3A6, it was also possible to demonstrate significant binding of Ab49-67 to hFSH. The data suggests that residues in regions 33-53 and 49-67 of hFSH beta appear to be accessible in the heterodimeric hFSH in addition to the glycosylated region of 1-15. Regions 16-36, 33-53, 81-100 and 98-111 of hFSH beta appear to contain subunit contact-associated sequences which are either masked or structurally altered subsequent to association with hFSH alpha in the heterodimeric hFSH.

Identification of assembled epitopes on the alpha-subunit of human follicle stimulating hormone

In order to characterize further the antigenic structure of human follitropin (hFSH), BALB/c mice were immunized with hFSH and anti-hFSH monoclonal antibodies (mAbs) were generated. The hFSH subunit specificity of the mAbs was assessed by a solid-phase enzyme-linked immunosorbent assay (ELISA) and a solution-phase radioimmunoassay (RIA), each using hFSH, hFSH alpha, and hFSH beta. Five mAbs bound hFSH and hFSH alpha in the ELISA and the RIA. In addition, some mAbs recognized hFSH beta, albeit to a much lower degree, as demonstrated by displacement of [125I]hFSH binding to the mAbs by hFSH beta, in the solution-phase RIAs. Next, synthetic peptides corresponding to the hFSH alpha-subunit sequence were used to identify sequences specific to the epitopes of each of the five mAbs. Using this epitope mapping strategy, two assembled epitopes were identified. mAbs 3A and 4B distinguish one discontinuous epitope comprised minimally of sequences alpha-16-21 and alpha-66-92, whereas mAbs 5F and 2E distinguish a second discontinuous epitope comprised minimally of sequences alpha-40-50 and alpha-66-72.

Number and topography of epitopes of human chorionic gonadotropin (hCG) are shared by desialylated and deglycosylated hCG

A previously established map of the surface epitopes of human chorionic gonadotropin (hCG) served as template for the present study in which we investigated the antigenic surfaces of two glycosylation variants of hCG, i.e. desialylated hCG (asialo-hCG) and deglycosylated hCG (degly-hCG). This map allocates five epitopes to the alpha subunit, five to the beta subunit and four alpha beta epitopes to structures formed only by the alpha/beta heterodimer holo-hCG (Schwarz et al. (1986) Endocrinology 118, 189-197; Berger et al. (1990) J. Endocrinol. 125, 301-309). Here it is described that both variants complied with this template: each of the 14 distinct monoclonal antibodies with which the epitopes of hCG were defined reacted with radiolabeled asialo-hCG and degly-hCG as well and generally bound degly-hCG with greater affinity than hCG. Moreover, every combination of capture and radiolabeled detection antibody that was either compatible or incompatible on unlabeled hCG was so also on unlabeled asialo-hCG and degly-hCG. It thus appears that alterations of the carbohydrate structure of hCG can be associated with a change in affinity between some antibodies and their respective epitopes but not with a loss of an epitope or with a change in the topographical relationships of the 14 epitopes.

A monoclonal antibody recognizing the C-terminal region of chicken egg white riboflavin carrier protein terminates early pregnancy in mice

In order to identify the functionally relevant epitopes on chicken riboflavin carrier protein, we have raised monoclonal antibodies to the vitamin carrier. One of these, 6B2C12, was found to interact specifically with a synthetic oligopeptide corresponding to the C-terminal 17 amino acid residues of the chicken egg white riboflavin carrier protein, which is missing in part in the egg yolk riboflavin carrier protein. This epitope is conserved through evolution in mammals including humans. Administration of the ascites fluid of 6B2C12 to pregnant mice intraperitoneally, resulted in the termination of pregnancy indicating that this epitope is involved in or closely associated with the transplacental transport of the vitamin from the maternal circulation to the growing fetus.

Relationship of orientation with affinity and activity of receptor-bound glycosylation variants of human chorionic gonadotropin (hCG), as visualized by monoclonal antibodies (MCA)

When hCG was receptor-bound, only 2 epitopes (i.e. beta 3 and beta 5) out of its previously mapped total of 14 surface epitopes (10, 11, 12) could be detected by the respective 125I-MCA. Clearly, this indicates a non-random orientation of hCG in this particular state (1). Now we report that on receptor-bound desialylated (asialo-hCG) as well as on receptor-bound deglycosylated hCG (degly-hCG), the beta 3 and beta 5 epitopes were inaccessible for 125I-MCA as were the remaining epitopes, although both variants, when not receptor-bound, were indistinguishable from native hCG with respect to number and topography of epitopes. Thus, the carbohydrate (CHO) units of hCG neither seem to be part of these 14 antigenic sites nor to contribute to the affinity of receptor binding: both variants had even higher affinities than native hCG. However, since the CHO are known to be obligatory for triggering the postreceptor responses of hCG-stimulated target cells, as seen by the 50% reduced or totally abolished biological potency of asialo-hCG and degly-hCG, respectively, the here demonstrated clear-cut differences in epitope accessibility can be related to differences in receptor-bound orientations which reflect signal transduction-competent and incompetent modes of interaction with the receptor. We believe that the CHO, while not contributing to receptor binding per se, function to assure correct positioning of hCG in the ligand recognition domain to allow for proper protein-protein interactions between ligand and receptor and thus optimal activation and hence transduction of the external signal to the cell's interior. In addition, the fact that most of the surface epitopes were masked on receptor-bound hCG, represents the first experimental support for the sofar unproven hypothesis that this unusually long (i.e. 341 amino acid residues) extracellular N' terminal domain of the recently cloned hCG receptor (hCG-R) (23,24), is indeed involved in ligand recognition.

Conformation of the alpha-subunit of glycoprotein hormones: a study using polyclonal and monoclonal antibodies

The conformation of the common alpha-subunit of human glycoprotein hormones, luteinizing hormone (hLH), follicle-stimulating hormone (hFSH), thyroid-stimulating hormone (hTSH) and chorionic gonadotropin (hCG) was probed using a highly specific polyclonal antiserum against the alpha-subunit of hCG and several monoclonal antibodies (MAbs) produced against hCG which recognized the alpha-subunit in free and combined form. The alpha-subunit was found to be conformationally altered (compared to its conformation in the isolated state) when it was in combination with various beta-subunits as indicated by shifts in the displacement curves of binding of [125I]hCG alpha to the polyclonal antiserum. The extent of the change was dependent on the beta-subunit present with minimum change being observed with hLH beta, intermediate with hCG beta and maximum change with hFSH and TSH beta-subunits. However, the affinity constants of this antiserum for all four hormones were nearly similar. Further, it was also found that binding of any one of the glycoprotein hormones to this antibody could be completely inhibited by any other hormone suggesting that the conformation of the alpha-subunit in all the four hormones is probably very similar. This was further investigated using five hCG MAbs capable of recognizing the alpha-subunit, but with different epitope specificities. All these MAbs could recognize all the four hormones suggesting the presence of the epitopes in these proteins. These epitopes were conformation specific since the MAbs did not bind reduced and carboxymethylated alpha-subunit. Displacement analysis using [125I]hCG as the tracer showed that two epitopes have nearly the same conformation in all the four hormones, while two were partially modified depending on the beta-subunit present. Based on these results, it is concluded that the alpha-subunit of glycoprotein hormones has nearly the same conformation, though subtle differences do exist.

Subunit interaction of human chorionic gonadotropin (hCG) with rat ovarian luteinizing hormone (LH)/CG receptor

The subunit interaction of hCG with its rat ovarian LH/CG receptor was studied by cross-linking the solubilized receptor-hormone complex with glutaraldehyde (GA), disuccinimidyl suberate (DSS) or dithiobis(succinimidyl propionate) (DSP) and analyzing the complexes by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and autoradiography. The hormone was labeled either in its alpha-subunit (125I-hCG) or in its beta-subunit (3H-hCG) or the label (3H) was introduced into the receptor molecule instead of the hormone. All of the labeling procedures led to the detection of only the receptor-(alpha,beta)hCG and receptor-(alpha)hCG complexes on the autoradiograms. The sizes of these complexes were 137,000 and 106,000, respectively, under reducing conditions. These results suggest that the receptor binds one hormone molecule, and that hCG interacts with the receptor mainly through its alpha-subunit. In addition, polyclonal antibodies directed against the LH/CG receptor and the alpha- and beta-subunits of hCG were used to detect the non-reduced receptor-(alpha,beta)hCG complex in immunoblotting. As antibodies directed against both the alpha-subunit and the beta-subunit were able to detect the Mr 130,000 complex, it is conceivable that both of the subunits are at least partially exposed on the receptor-hormone complex. 125I-hCG was also cross-linked to the membrane-bound receptor. The membrane-bound complex had an Mr of 144,000 under reducing conditions, i.e. approximately 7000 higher than that of the solubilized complex (Mr 137,000). This may indicate that the membrane-bound receptor is covalently modified or differs in conformation from the solubilized receptor.

Peptide immunogen mimicry of a protein-specific structural epitope on human choriogonadotropin

It is a challenge to construct synthetic immunogens that elicit antibodies (Abs) both directed to conformational epitopes and specific for a complex protein like human choriogonadotropin (hCG). A monoclonal antibody specific for hCG bound to regions around Lys45 of the alpha subunit (hCG alpha) and Asp112 of the beta subunit (hCG beta). A peptide comprising residues 46 to 55 of hCG alpha and residues 106 to 116 of hCG beta elicited Abs in rabbits that were directed to a discontinuous epitope and were specific for hCG. These Abs inhibited the binding of hCG to its receptor. Thus, a synthetic immunogen can mimic a conformational-specific epitope and can be useful for vaccine development.

Structural probing of human lutropin using antibodies raised against synthetic peptides constructed by classical and multiple antigen peptide system approaches

Antibodies were elicited against a synthetic peptide which encompassed two different regions of the human lutropin beta-subunit (hLH-beta). These antibodies were raised against either the peptide which was assembled using a conventional approach and conjugated to the tetanus toxoid, or with the peptide assembled using the multiple antigen peptide system approach. Automated simultaneous synthesis of the two forms of the immunizing peptide was successfully achieved. Animal injected with the peptide conjugated to tetanus toxoid produced high titers of antibodies to the synthetic peptide, but did not bind to the native hLH-beta subunit. In contrast, antisera induced by the peptide in its MAP form displayed reactivity with both the peptide and the native hLH-beta subunit; these latter antisera appeared to preferentially recognize the beta 47-55 portion of the molecule and were able to bind to the beta-subunit of human choriogonadotropin. Present results demonstrate that the beta 47-55 region is accessible to antibody binding and appears to be located at the surface of both hLH-beta and hLH. Moreover, this study confirms that the MAP approach provides a chemically unambiguous method for obtaining antibodies of predetermined specificity, capable of recognizing cognate sequences of various native proteins.

Multiple epitope specificity of monoclonal antibodies to a single synthetic peptide: use in the characterization of the GP IIb-IIIa binding domain of von Willebrand factor

Monoclonal antibodies have been induced against the synthetic peptide with sequence Tyr-Glu-Val-Val-Thr-Gly-Ser-Pro-Arg-Gly-Asp-Ser-Gln-Ser-Ser. This peptide represents residues Glu1737-Ser1750 of the mature von Willebrand factor (vWF) subunit and contains the sequence Arg-Gly-Asp, thought to be important in mediating binding to the platelet receptor glycoprotein (GP) IIb-IIIa complex. Twelve antibodies were obtained, eight of which bound to native vWF as well as to the peptide immunogen insolubilized onto agarose beads. These antibodies defined at least three distinct epitopes, as demonstrated by antibody interaction with peptides having a single phenylalanine substitution at each position in the sequence. In particular, two antibodies bound to epitopes on vWF that included one or more of the three residues (arginine, glycine, aspartic acid) thought to be involved in binding to GP IIb-IIIa, whereas one antibody bound to an epitope that did not include any of those residues. Nevertheless, the three antibodies cross-reacted with each other, a finding explained by the fact that the corresponding epitopes had at least two residues in common, namely Gly1741 and Ser1742. In spite of the cross-reactivity for binding to vWF, only the two antibodies whose epitopes included residues in the Arg-Gly-Asp sequence inhibited vWF interaction with GP IIb-IIIa. The third antibody had no inhibitory effect even though it was bound to an epitope located at a distance of only few residues on the amino terminal side of Arg-Gly-Asp. These results demonstrate that monoclonal antibodies raised against a single synthetic peptide with sequence limited to fifteen residues may exhibit distinct epitope specificity and may be used to define functional domains in macromolecules with a high degree of resolution.

Phase I clinical trial of monoclonal anti-human chorionic gonadotropin antibody in women with an ectopic pregnancy

A phase-I clinical trial of a mouse monoclonal anti-human chorionic gonadotropin (hCG) antibody designated as HT13 was conducted in three patients treated for ectopic pregnancy. Doses of 5 and 25 mg of purified antibody were injected into patients. Monoclonal antibody serum levels dropped to 0 within 1 to 5 days. Human antibodies directed against mouse immunoglobulins were not detected at up to 41 days after injection. In one patient, the tubal pregnancy resolved, as confirmed by hCG levels of less than 10 mUl/mL and by tubal patency at hysterosalpingography. The time of resolution was 30 days. In two patients, salpingectomy was performed because of persistence of elevated hCG levels, whereas HT13 had a striking effect on progesterone (P) and estradiol (E2) serum levels. The injection of anti-hCG antibody did not appear to interfere with the subsequent fertility of patients, and two out of three patients later developed a successful pregnancy. While the precise role of antibody injection in the interruption of ectopic pregnancy remains speculative, the injection of monoclonal anti-hCG antibody appears to induce a dramatic decrease in the production of both P and E2.