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

Versatility and stability of melamine foam-based biosensors (f-ELISA) using antibodies, nanobodies, and peptides as sensing probes

Macroporous three-dimensional (3D) framework structured melamine foam-based Enzyme-Linked Immunosorbent Assay (f-ELISA) biosensors were developed for rapid, reliable, sensitive, and on-site detection of trace amount of biomolecules and chemicals. Various ligands can be chemically immobilized onto the melamine foam, which brings in the possibility of working with antibodies, nanobodies, and peptides, respectively, as affinity probes for f-ELISA biosensors with improved stability. Different chemical reagents can be used to modify the foam materials, resulting in varied reactivities with antibodies, nanobodies, and peptides. As a result, the f-ELISA sensors produced from these modified foams exhibit varying levels of sensitivity and performance. This study demonstrated that the chemical reagents used for immobilizing antibodies, nanobodies, and peptides could affect the sensitivities of the f-ELISA sensors, and their storage stabilities under different temperatures varied depending on the sensing probes used, with f-ELISA sensors employing nanobodies as probes exhibiting the highest stability. This study not only showcases the versatility of the f-ELISA system but also opens new avenues for developing cost-effective, portable, and user-friendly diagnostic tools with optimized sensitivity and stability.

Autoantibodies to tumor-associated antigens: reporters from the immune system

Although autoantibodies have been recognized as participants in pathogenesis of tissue injury, the collateral role of autoantibodies as reporters from the immune system identifying cellular participants in tumorigenesis has not been fully appreciated. The immune system appears to be capable of sensing aberrant structure, distribution, and function of certain cellular components involved in tumorigenesis and making autoantibody responses to the tumor-associated antigens (TAAs). Autoantibodies to TAAs can report malignant transformation before standard clinical studies and may be useful as early detection biomarkers. The autoantibody response also provides insights into factors related to how cellular components may be rendered immunogenic. As diagnostic biomarkers, specific TAA miniarrays for identifying autoantibody profiles could have sufficient sensitivity in differentiating between types of tumors. Such anti-TAA profiles could also be used to monitor response to therapy. The immune system of cancer patients reveals the immune interactive sites or the autoepitopes of participants in tumorigenesis, and this information should be used in the design of immunotherapy.

The antigen binding sites of various hCG monoclonal antibodies show homology to different domains of LH receptor

The common feature of receptors and antibodies against the ligand is that both display very specific, high affinity binding towards the ligand. Therefore, it can be hypothesized that the paratope of antibodies may exhibit homology with distinct domains of the receptor. By locating the hormone epitopes and determining the structure of the paratopes, it should be possible to identify the contact points between the ligand and the receptor. This hypothesis has been tested using hCG monoclonal antibodies (MAbs) recognizing different epitopes and having different effects on hormone binding and response. The beta subunit and heterodimer specific antibodies inhibited both hormone binding and response, while the alpha subunit specific antibodies inhibited response without affecting binding. The single chain fragment variables (ScFvs) produced from these antibodies also retained the properties of the parent antibodies. The amino acid sequences of the ScFvs exhibited homology to different regions of the receptor; the beta subunit specific antibody being homologous to the concave surface of the leucine rich repeats (LRR) of the receptor, particularly the concave surface of the LRRs, while the heterodimer specific antibody showed homology to the hinge region. The alpha subunit specific antibody showed homology to the transmembrane domain of the receptor. The exact locations of the epitopes of the monoclonal antibodies in the hormone molecule have also been identified. The data presented here also support the model of glycoprotein hormone-receptor interaction in which the hormone binds to the extracellular domain through the beta subunit and then the alpha subunit is brought in contact with the transmembrane domain leading to signal transduction.

Synthesis and application of peptide immunogens related to the sperm tail protein tpx-1, a member of the CRISP superfamily of proteins

The synthesis of peptides containing 0, 1 and 2 cysteine residues related to the human sperm tail protein, tpx-1, is described. These synthetic peptides, following conjugation to keyhole limpet hemocyanin modified with maleimidobenzoic acid N-hydroxysuccinimide ester, were used as immunogens to generate polyclonal antibodies in female New Zealand white rabbits. The binding characteristics of the derived antipeptide sera were evaluated using indirect and competitive ELISA procedures. Western immunoblot experiments also confirmed that these synthetic peptide immunogens are able to generate high-titer polyclonal antibodies capable of cross-reacting with the mature tpx-1 protein present in crude rat sperm tail/testis preparations as well as in outer dense fiber preparations. Consequently, these synthetic peptides represent promising candidates for investigations into the role of tpx-1 in the immunoregulation of sperm function in the rat and other mammalian models, with the derived antisera also providing an avenue to explore possible sites of expression of tpx-1 proteins in other tissues.

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.

Epitope analysis and molecular modeling reveal the topography of the C-terminal peptide of the beta-subunit of human chorionic gonadotropin

Human chorionic gonadotropin (hCG) belongs to a family of heterodimeric glycoprotein hormones that share a common alpha-subunit and a hormone-specific beta-subunit. Among the gonadotropin beta-subunits, greater than 85% homology exists between lutropin (hLH)beta and hCGbeta in their first 114 amino acid residues. However, unlike hLHbeta, hCGbeta contains a 31-amino acid hydrophilic stretch at its carboxyl end (CTPbeta: C-terminal peptide). Although the crystal structure of deglycosylated hCG has been solved, the topography of CTPbeta remains unknown. In this study, we have attempted to define the topology of CTPbeta using mAb probes. We investigated three epitopes on hCGalpha, which are hidden in the hCGalphabeta dimer. However, these epitopes are not hidden in hLH, which has a similar subunit interface to that of hCG, but lacks CTPbeta. This suggested that these epitopes are not masked at the subunit interface of hLH or hCG. Hence, we hypothesized that, in the case of hCG, these epitopes are masked by the CTPbeta. Consistent with this view, several treatments of hCG that removed CTPbeta unmasked these epitopes and enhanced their reactivity with the corresponding mAbs. In order to localise the position of CTPbeta on the alpha-subunit, we used an epitope-mapping strategy [N. Venkatesh & G. S. Murthy (1997) J. Immunol. Methods 202, 173-182] based on differential susceptibility of epitopes to covalent modifications. This enabled us to predict the possible topography of CTPbeta. Further, we were also able to build a model of CTPbeta, completely independently of the epitope-mapping studies, using a homology-based modeling approach [S. Krishnaswamy, I. Lakshminarayanan & S. Bhattacharya (1995) Protein Sci. 4 (Suppl. 2), 86-97]. Results obtained from these two different approaches (epitope analysis and homology modeling) agree with each other and indicate that portions of CTPbeta are in contact with hCGalpha in the native hCG dimer.

Characterization of epitope regions of thyrotropin beta-subunit recognized by the monoclonal antibodies mAb279 and mAb299: a chimeric peptide approach

This investigation describes the design, synthesis and evaluation of chimeric peptides related to the bovine thyrotropin beta-subunit, bTSHbeta. The structures of these chimeric peptides were derived from investigations with linear peptides and sequence alignment studies, in association with a homology model of TSHbeta developed from the hCG X-ray crystallographic structure. The structures of these chimeric peptides comprised beta-turn regions of loop L1 [bTSHbeta(14-20)] and loop L3 [bTSHbeta(65-72)] held in close proximity by a bis-beta-alanine linker and the disulfide bond bTSHbeta[Cys16-Cys67]. Linear and cyclic chimeric peptides were evaluated in immunochemical assays for their ability to inhibit the binding of radio-iodinated bTSHbeta [125I-bTSHbeta] to the monoclonal antibodies, mAb279 and mAb299. Previously, mAb279 and mAb299 have been shown to recognize epitopes accessible on the surface of TSHbeta that lie in close proximity to the TSH receptor-binding site. The results indicate that these chimeric peptides can specifically inhibit in a dose-dependent manner the binding of 125I-bTSHbeta to mAb299, while having a lesser effect on the binding with mAb279. Based on these results, it can be concluded that the bTSHbeta-epitope recognized by mAb299 involves contributions from amino residues from the beta-turn regions of the L1 and L3 loops of TSHbeta, and that these loop regions flank part of the receptor binding site of the bTSH beta-subunit.

Mapping of the interleukin-10/interleukin-10 receptor combining site

The discontinuous interleukin-10(IL-10)/interleukin-10 receptor (IL-10R) combining site was mapped using sets of overlapping peptides derived from both binding partners bound to continuous cellulose membranes. Low affinity binding of single regions of the discontinuous contact sites on IL-10 and IL-10R could be identified due to (1) high peptide density on the membrane support, (2) incubation with high protein concentrations, (3) indirect immunodetection of the ligates after electrotransfer onto polyvinylene difluoride membranes, and (4) use of highly overlapping peptide scans of different length (6-mers and 15-mers). The single binding regions identified for each protein species are separated in the protein sequences, but form continuous areas on the surface of IL-10 (X-ray structure) and IL-10R (computer model). Furthermore, four epitopes of neutralizing anti-IL-10 and anti-IL-10R antibodies were mapped and overlap with these binding regions. Soluble peptides (15- to 19-mers) each spanning one of the three identified IL-10-derived receptor binding regions displayed no significant affinity to IL-10R as expected, whereas a peptide (35-mer) comprising two of these regions had considerably higher binding activity. The data are consistent with a previously published computer model of the IL-10/IL-10R complex. This approach should be generally applicable for the mapping of non-linear protein-protein contact sites.

Human follitropin heterodimerization and receptor binding structural motifs: identification and analysis by a combination of synthetic peptide and mutagenesis approaches

The family of human glycoprotein hormones, including follitropin (FSH), are heterodimeric proteins, each composed of single alpha- and beta-subunits that are tightly associated but non-covalently linked. To study structure and function relationships of FSH, synthetic peptides were used to inhibit subunit association, to map epitopes of FSH antibodies and as antigens to generate site specific antipeptide antibodies which could be used for topographic analysis. Interpretation of such results are generally more straightforward than when peptides are used with radioreceptor assays or in cell cultures which are complex systems. The data we collected using the synthetic peptide approach suggested that FSH residues homologous to human chorionic gonadotropin (hCG) loops L3 beta and L2 alpha are involved in subunit contact. FSH residues homologous to hCG loops L2 beta and L3 alpha seemed involved in receptor binding. Loop L2 beta also seemed involved in subunit contact. Those data provided a rationale for extensive mutagenesis of the four regions of hFSH. Mutagenesis data provided additional information and higher resolution of function when combined with the three dimensional structure of hCG. In the aggregate, this information has provided a reasonable model of the receptor binding site of hFSH. Our current model of the FSH receptor site is that of a discontinuous functional epitope including L3 beta, L2 alpha and L3 alpha. The juxtaposition of residues beta D93, alpha K5 1, alpha Y88 and of alpha Y89 in the 'binding-facet' of hFSH suggest the feasibility of designing a synthetic peptide mimetic of FSH. Additional residues of the alpha-subunit are involved, along this facet of the molecule. The data collected studying hFSH therefore demonstrates that the alpha-subunit features prominently in the mechanism of FSH binding to and stabilizing the interaction with its receptor. In contrast, the beta-subunit determinant loop serves as discriminator in addition to stabilizing the binding interaction whereas mutagenesis data indicates that L2 beta does neither. Instead, L2 beta appears to stabilize FSH conformation, possibly, the alpha-subunit, required for competent binding. In this regard, synthetic peptides provided data which were a useful guide to plan mutagenesis studies and which contributed to the process of understanding the structure and function of the gonadotropins.

Binding of site-directed monoclonal antibodies to an epitope located in the A/B region (amino acids 140-154) of human estrogen receptor-induced conformational changes in an epitope in the DNA-binding domain

The interactions of estrogen receptor (ER) with monoclonal antibody (Mab) F9, developed against a synthetic 30-mer hybrid oligopeptide, were determined in the presence or absence of Mab NMT-1, raised against 15-mer peptide from the N-terminal A/B region (amino acids 140- 154) or Mab 213, raised to a peptide AT3 in the DNA-binding domain (amino acids 247-263). Mab F9 bound ER and formed a complex sedimenting at the approximately 11S region of the gradients. Mabs 213 and NMT-1 bound ER and formed complexes sedimenting at approximately 7S and 9S, respectively. Preincubation of ER with Mab 213, followed by reincubation with Mab F9, resulted in a complex sedimenting at the approximately 11S region of the gradients. Similarly, preincubation of ER with Mab NMT-1 followed by reincubation with Mab F9 also produced an approximately 11S complex on the gradients. These observations suggest that binding of Mab F9 to ER induced conformational changes causing the release of Mab 213 and Mab NMT-1 from ER. Furthermore, binding of Mab NMT-1 to the A/B region of ER also produced conformational changes causing the release of Mab 213 from its epitope in the DNA-binding region. These results indicate that binding of Mab F9 and Mab NMT-1, with epitopes located within amino acids 140-154 of the A/B region of ER, induced conformational changes in the DNA-binding domain, as determined by the inability of Mab 213 to remain bound to its epitope. These data further suggest that the DNA-binding region is sensitive to conformational changes induced in the native protein.

Characterization of a sperm nuclear protein

PROBLEM

The molecular identity of sperm DNA-binding structural proteins contributing to the integrity of a sperm residual high salt/nuclease resistant nuclear structure is studied by cDNA cloning and monoclonal antibodies to the recombinant polypeptide. This structure, which is likely to be transferred unimpaired in the oocyte and is anticipated as a molecular correlate of the nuclear scaffold (nuclear matrix/envelope) in somatic cells, may be essential with respect to its DNA organization for the recovery and assembly of somatic-type chromatin in the zygote. Recently, a cDNA encoding one of these proteins has been cloned and the recombinant polypeptide expressed in E. coli as a beta-galactosidase fusion protein. The main objective of the present study is the identification of the native sperm antigen by monoclonal antibodies raised against the recombinant molecule.

METHODS

We evaluated the possibility of immunizing by direct intrasplenic deposition in BALB/c mice of the recombinant fusion protein available as transblotted on nitrocellulose membrane carriers or as nitrocellulose protein-bearing particles. Isolated sperm DNA/tight binding protein complexes were used in ELISA and Western blotting for selection of monoclonal antibodies specific to self epitopes of the nuclear antigen, as well as immunofluorescence of swollen human spermatozoa subjected to in situ extraction with high salt/beta-mercaptoethanol/DNase I and proteolysis, and of a cultured fibroblast cell line L-929.

RESULTS

A monoclonal antibody, Mab 2C4, was selected which recognized a 52 kDa protein in the fraction of sperm high salt/urea resistant proteins. The target polypeptide was detected on swollen spermatozoa primarily to the post-acrosomal and/or equatorial regions whereas in nonextracted sperm cells the epitope was exceedingly unavailable. The somatic cell location of the cognate epitope was confined to the nuclear envelope displaying a cap-like pattern of staining, and also in a juxtanuclear cytoplasmic randomly coiled filamentous network and in compact bodies.

CONCLUSIONS

A nuclear protein salt-stably bound to the sperm residual structure has been identified. The antigen appears localized in sperm exclusively to perinuclear subacrosomal sites that may be anchored at the male nuclear envelope, given the occurrence of the target epitope in somatic cells as well in nuclear and cytoplasmic sites adjacent to the nuclear membrane.

The suitability of human chorionic gonadotropin (hCG)-based birth-control vaccines

It has been widely hoped that immunological methods of fertility regulation by active immunization against specific antigens of the oocyte, sperm, zygote and early embryo, and the placental pregnancy hormone human chorionic gonadotropin (hCG), will provide a means to control the problem of worldwide population growth. The most advanced candidate vaccines are based on hCG immunogens and have entered clinical trials. However, during the past few years, increasing evidence has emerged that the current approaches using hCG as the target molecule may have some major drawbacks. On the basis of their recent findings, Stephan Dirnhofer and colleagues raise doubts on the suitability, safety and efficacy of gonadotropin-based immunological contraceptive vaccines.

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.

Deficient antigen processing of a protein quaternary structure can be overcome by receptor-mediated uptake

Human chorionic gonadotropin (hCG) is a dimer of non-covalently associated alpha (hCG-alpha) and beta (hCG-beta) subunits. This molecule was used to study whether receptor-mediated uptake influences the presentation of a protein quaternary structure. Unprimed splenocytes and a B cell lymphoma were capable of presenting only the free (hCG-alpha) but not the combined (hCG) alpha subunit to hCG-alpha T cell hybridomas, while hCG-alpha-primed lymph node cells (LNC) responded to both hCG-alpha and hCG. As antigen (Ag)-specific antigen-presenting cells (APC) present in the hCG-alpha-primed LNC population may be potentially effective for presenting hCG, we investigated the role of specific Ag capture, through mIg and Fc gamma R, in the processing and presentation of hCG and hCG-alpha to HAG5, a T cell hybridoma directed against the immunodominant region (amino acids 61-81) of hCG-alpha. Results showed that only B cells bearing membrane immunoglobulin capable of recognizing hCG-alpha and hCG, and present in hCG-alpha-primed mice, were extremely effective in presenting the free as well as the combined alpha subunit. The effect of FcR-mediated uptake was analyzed using a B cell line transfected with the Fc gamma RII-B2 gene to present immune complexes of either hCG-alpha or hCG. We found that hCG-alpha and hCG were presented equally well, whatever the Ag-binding site of each antibody to hCG or its alpha subunit. Using HBG 6, an hCG-beta T cell hybridoma, we performed similar experiments with the Fc gamma RII-B2 cell line and determined that the potentiation of hCG presentation to HBG 6 was similar to that observed with HAG 5. Then kinetic experiments were performed to examine the effect of Ag uptake through FcR on processing. Results demonstrated that the uptake pathway drastically influenced the expression of alpha T cell determinants in the alpha/beta dimer. In addition, treatment with cycloheximide, a protein synthesis inhibitor, only impaired the ability of APC to present specifically captured Ag. Thus, the processing pathway for specifically captured Ag might be different from the pathway used to process nonspecifically captured Ag. This observation might explain why receptor-enhanced uptake bypasses the inefficient processing of the hCG quaternary structure and enables similar efficiency in the presentation of alpha and beta T cell specificities. These findings provide new insight into the antigenicity of oligomeric molecules, which is modified whether antigen capture is specific or not.

Immune recognition of a molecule naturally presented as a monomeric or an oligomeric structure: the model of the human chorionic gonadotropin alpha subunit

The immune recognition of a molecule naturally presented as a monomeric or an oligomeric structure is analyzed using the human chorionic gonadotropin alpha subunit (hCG-alpha) as a model. Indeed, hCG-alpha circulates as either a free subunit or combined to the beta subunit (hCG-beta) to form the dimeric hCG hormone. A T cell study was performed in BALB/c (H-2d) mice which were found to be high responders to hCG-alpha. Mice were immunized with the free hCG-alpha or the dimeric hCG alpha/beta, and their lymph node cells were challenged in vitro with either alpha subunits from different species, hCG or peptides spanning the entire primary structure of hCG-alpha. Proliferation and IL-2 assays demonstrated that hCG-alpha-primed lymph node cells responded equally well to hCG-alpha and hCG alpha/beta, suggesting that both the free and combined hCG-alpha subunits are processed in a similar way. Among the various synthetic peptides used, only those mimicking the hCG-alpha(59-92) C-terminus portion were able to stimulate hCG-alpha-primed lymph node cells, demonstrating that this region contains immunodominant T cell recognition site(s). The hCG-alpha(23-43) and (32-59) peptides, although incapable of stimulating T cells primed with hCG-alpha, elicited a T cell response when used as immunogens. These regions encompassed cryptic epitopes which were not generated during hCG-alpha processing in H-2d mice. The T cell epitopes of hCG-alpha above described as immunodominant or cryptic on the free alpha subunit, had similar characteristics when the alpha/beta dimer was used as the immunogen. In contrast, T cells primed with peptides mimicking immunodominant sites recognized differently the hCG-alpha and the hCG alpha/beta antigens. Moreover, the analysis of the B cell response to all the immunogenic hCG-alpha peptides indicated that they bear B and T cell epitopes as well. Antibodies elicited against the hCG-alpha(59-92) or (32-59) peptide were capable of recognizing the alpha subunit in its free form but not in the alpha/beta hCG dimer. Such study deserves attention for the comprehensive mechanisms of the immune response to hCG as well as for the design of anti-hCG vaccines.

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.

Caveats for the use of surface-adsorbed protein antigen to test the specificity of antibodies

Rabbit antisera against apo-cytochrome c, which was prepared by removal of the covalently bound heme prosthetic group from yeast iso-1 cytochrome c, were tested for reactivity against native yeast iso-1-cytochrome c. When the antigen was adsorbed to a microtiter plate in a conventional enzyme-linked immunosorbent assay (ELISA), the antisera were unable to distinguish between their cognate antigen apo-cytochrome c, a random coil protein, and native cytochrome c, a small globular protein of remarkable conformational stability in solution. However, when the assay was conducted under conditions where antigen and antibody were free to associate in solution, that is in a solution-phase radioimmunoassay (RIA), the antisera were highly specific for apo-cytochrome c. Similarly, antibodies induced by native cytochrome c and discriminating strongly between native and apo-cytochrome c in a solution-phase RIA, did not distinguish between native and apo-cytochrome c in a solid-phase ELISA. This discrepancy of results obtained by different immuno assay procedures clearly indicates that adsorption to plastic alters the antigenic structure of even a conformationally stable protein such as cytochrome c. A conventional solid-phase ELISA strongly selects for those antibodies that recognize the unfolded antigen. The results presented warrant serious thoughts about previous reports on anti-peptide antibodies reacting with native whole protein molecules, as tested by those ELISA procedures that have the protein antigen adsorbed to plastic.

Search for peptide immunogens of the beta-subunit of human chorionic gonadotropin (hCG) capable of eliciting hormone specific and neutralizing antisera. Identification of an undecapeptide eliciting hCG-specific antisera

The work reported herein describes our attempts to identify peptide immunogens of the beta-subunit of the pregnancy-specific placental hormone, human chorionic gonadotropin (hCG), capable of eliciting hormone specific and neutralizing antisera. Hydrophilicity profiles of the beta-subunits of hCG and the homologous pituitary hormone, human luteinizing hormone (hLH) were compared and two sequences which are hydrophilic but unique to hCG-beta were identified. They are 6-12 and 109-119 of hCG-beta. Results of the studies with the undecapeptide 109-119 of hCG-beta are reported in this paper. The undecapeptide amide was synthesized using the p-(acyloxy) benzhydrylamine resin and antisera to the peptide were elicited in rabbits using the peptide-diphtheria toxoid conjugate. The peptide is highly immunogenic as both the rabbits responded with high titers of antibodies to the peptide. The antipeptide antibodies bound to hCG but not to hLH showing thereby that the region 109-119 of hCG-beta is a unique determinant of hCG. However, the antibodies were found not to neutralize the biological activity of hCG.

Progress and approaches in mapping the surfaces of human follicle-stimulating hormone: comparison with the other human pituitary glycoprotein hormones

The pituitary glycoprotein hormones, including human follicle-stimulating hormone (hFSH), are involved in the physiologic functions of receptor binding, in subunit association during assembly and processing, and in untoward effects such as antibody formation during clinical intervention. These activities derive from specific macromolecular associations; a major research trend has been to map the surfaces of these glycoprotein hormones (hFSH, hLH, hCG, and hTSH) into discrete areas responsible for each activity by using a variety of biochemical approaches. Each surface domain or determinant of the hormone is likely to comprise discontinuous amino acid sequences, from one or both subunits, assembled into a precise, unique, macromolecular surface. The paradigm of antigen-antibody interaction may help to explain how the surfaces are assembled, how the common alpha-subunit combines with the unique beta-subunit of each, and how the receptor interacts with heterodimeric hormone.

Brain-derived neurotrophic factor: subcellular compartmentalization and interneuronal transfer as visualized with anti-peptide antibodies

The recent cloning of a second member of the nerve growth factor family, brain-derived neurotrophic factor (BDNF), has prompted investigation into the cells that express this factor's mRNA and protein. In the present study, antibodies raised against unique peptide sequences within the porcine BDNF protein detect BDNF-like immunoreactivity in neurons in rat hippocampal and cortical areas consistent with the distribution of BDNF mRNA as detected with in situ hybridization. Within these neurons, BDNF-like immunoreactivity was observed in the cytoplasm, dendrites, and nuclei. In addition, BDNF immunoreactivity was observed in the cytoplasm of cholinergic neurons that do not express detectable levels of BDNF mRNA. Thus, anti-peptide antibodies can be used to detect this neurotrophic factor protein in cytoplasmic sites of synthesis and in areas of probable action. We propose that one form of the BDNF protein enters the nucleus and may directly influence transcription, while another fraction of the protein is transported out of the synthesizing cell and can be detected, after retrograde axonal transport, in cytoplasmic granules in the perikarya of cholinergic neurons. These basal forebrain cholinergic neurons project to regions enriched in BDNF-synthesizing cells and are known to be responsive to BDNF in vitro. Our data provide information regarding the cellular distribution of BDNF protein in vivo and suggest a dendro-axonic interneuronal transfer of BDNF as well as an additional, intracellular signaling pathway not previously thought to occur in postmitotic neurons in brain.

Antigen mobility in the combining site of an anti-peptide antibody

The interaction between a high-affinity antibody, raised against a peptide incorporating the loop region of hen egg lysozyme (residues 57-84), and a peptide antigen corresponding to this sequence, has been probed by proton NMR. The two-dimensional correlated spectroscopy spectrum of the antibody-antigen complex shows sharp, well-resolved resonances from at least half of the bound peptide residues, indicating that the peptide retains considerable mobility when bound to the antibody. The strongly immobilized residues (which include Arg-61, Trp-62, Trp-63, and Ile-78) do not correspond to a contiguous region in the sequence of the peptide. Examination of the crystal structure of the protein shows that these residues, although remote in sequence, are grouped together in the protein structure, forming a hydrophobic projection on the surface of the molecule. The antibody binds hen egg lysozyme with only a 10-fold lower affinity than the peptide antigen. We propose that the peptide could bind to the antibody in a conformation that brings these groups together in a manner related to that found in the native protein, accounting for the high crossreactivity.

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.