Antibody to Epitope Tag Induces Internalization of Human Muscarinic Subtype 1 Receptor

Abstract: We have studied the effect of an antibody against the epitope EYMPME on the internalization of the human muscarinic cholinergic receptor hm1 tagged with the epitope at the amino terminus. The antibody to the tag induces internalization of the hm1 receptor within minutes after exposure of human embryonic kidney 293 cells transfected with the tagged receptor. This antibody-induced internalization is reversible following removal of the antibody. In contrast to hm1 internalization induced by the agonist carbachol, internalization induced by antibody is not blocked by the muscarinic antagonist atropine. The mechanism of antibody-mediated internalization does not appear to involve receptor dimerization by the antibody, as Fab fragments derived from the antibody also induce internalization. The pathway of antibody-induced internalization, similar to the agonist-induced process, is mediated by clathrin-coated vesicles. Furthermore, antibody treatment does not result in any second messenger production, as measured by phosphoinositide accumulation. Our data show that internalization of a G protein-coupled receptor can be triggered by interaction of the amino terminus of the receptor with an exogenous ligand and can occur independently of second messenger production. This result suggests that the receptor can exist in multiple conformations, each mediating distinct downstream events.     

Egr-1 expression in surface Ig-mediated B cell activation. Kinetics and association with protein kinase C activation

We have studied the expression of an immediate/early type gene, Egr-1, in murine B lymphocyte responses to Ag receptor-generated signals. The Egr-1 gene encodes a zinc finger protein with sequence-specific DNA binding activity and is believed to act as an intracellular "third messenger," to couple receptor-generated signals to activation-associated changes in gene expression. We show here that Egr-1 mRNA expression is rapidly and transiently (returning to basal levels by 6 h) induced after receptor crosslinking with anti-receptor antibodies. Egr-1 protein expression is more prolonged, maintaining detectable levels through 12 h. The induction of Egr-1 is a primary response to Ag receptor signaling, as it is independent of new protein synthesis and is inhibited by actinomycin D. We have also examined the linkage of Egr-1 to known signaling pathways associated with G0 to G1 transition by these cells in response to signals generated through the B cell Ag receptor. Egr-1 mRNA was not induced after elevation of intracellular free Ca2+. In contrast, the pharmacologic agents PMA and SC-9, which directly activate protein kinase C, both cause marked increases in Egr-1 mRNA levels with the same kinetics as observed after anti-receptor antibody stimulation. Further, the protein kinase C inhibitors H7, sangivamycin, and staurosporin block anti-receptor antibody-induced expression of Egr-1, thus, B cell Ag receptor-linked Egr-1 expression is likely coupled to the protein kinase C component of transmembrane signaling. Preliminary promoter mapping studies are consistent with this conclusion, because both PMA and anti-receptor antibody act through the same or overlapping cis-regulatory elements.     

Antigenic modulation induced by monoclonal antibodies: antibodies to measles virus hemagglutinin alters expression of other viral polypeptides in infected cells

Polyclonal antibody to measles virus can have profound effects on external (outer plasma membrane) as well as internal (cytoplasmic) viral polypeptides expressed in infected cells. The process, termed "antibody-induced antigenic modulation," was further investigated by using monoclonal antibody to several viral polypeptides. Four monoclonal antibodies against the viral hemagglutinin had the ability to decrease the expression of the phosphoprotein, fusion, and membrane protein. A monoclonal antibody to the nucleocapsid protein did not cause these changes. The observed decreases were not due to preferential degradation of viral polypeptides as determined by pulse-chase experiments. Our results indicate that a specific signal to an epitope on the plasma membrane (monoclonal antibody measles virus hemagglutinin) can alter the expression of measles virus phosphoprotein and membrane protein, both polypeptides present in the cytoplasm of infected cells.     

Binding of monoclonal antibodies to cerebral cytoplasmic tetrodotoxin-sensitive protein to human lymphocytes

To examine whether lymphocytes express antigenic determinants of brain cytoplasmic tetrodotoxin-sensitive protein (CTSP), anti-CTSP monoclonal antibody (Bab) binding to human peripheral blood lymphocytes was studied using ELISA assay. It was shown that the Mab bound human lymphocytes in a dose-dependent fashion. Halph-maximal binding of this Mab was significantly enhanced if test-cells were pretreated with concanavalin A or with mixed lymphocyte culture supernatant. Results are discussed from the point of view of the hypothesis that CTSP are metabolic precursors of membrane sodium channels.     

Mapping a neutralizing epitope on the SARS coronavirus spike protein: computational prediction based on affinity-selected peptides

Rapid elucidation of neutralizing antibody epitopes on emerging viral pathogens like severe acute respiratory syndrome (SARS) coronavirus (CoV) or highly pathogenic avian influenza H5N1 virus is of great importance for rational design of vaccines against these viruses. Here we combined screening of phage display random peptide libraries with a unique computer algorithm "Mapitope" to identify the discontinuous epitope of 80R, a potent neutralizing human anti-SARS monoclonal antibody against the spike protein. Using two different types of random peptide libraries which display cysteine-constrained loops or linear 13-15-mer peptides, independent panels containing 42 and 18 peptides were isolated, respectively. These peptides, which had no apparent homologous motif within or between the peptide pools and spike protein, were deconvoluted into amino acid pairs (AAPs) by Mapitope and the statistically significant pairs (SSPs) were defined. Mapitope analysis of the peptides was first performed on a theoretical model of the spike and later on the genuine crystal structure. Three clusters (A, B and C) were predicted on both structures with remarkable overlap. Cluster A ranked the highest in the algorithm in both models and coincided well with the sites of spike protein that are in contact with the receptor, consistent with the observation that 80R functions as a potent entry inhibitor. This study demonstrates that by using this novel strategy one can rapidly predict and identify a neutralizing antibody epitope, even in the absence of the crystal structure of its target protein.     

Plasmodium falciparum: sporozoite boosting of immunity due to a T-cell epitope on a sporozoite vaccine

Plasmodium falciparum: Sporozoite boosting of immunity due to a T-cell epitope on a sporozoite vaccine. Experimental Parasitology 64, 64-70. The impact of a malaria sporozoite vaccine may be enhanced if protective immunity elicited by the vaccine is boosted by natural exposure to sporozoites. For this to occur, a helper T lymphocyte epitope present on the vaccine must be shared by sporozoites. These studies show that T cells from mice immunized with R32tet32, the Plasmodium falciparum sporozoite vaccine candidate, recognize an epitope of less than or equal to 7 amino acids derived from the circumsporozoite protein repeat region of R32tet32, as well as an epitope on the tet32 fusion protein tail of R32tet32. Exposure of R32tet32 immunized animals to P. falciparum sporozoites elicits a significant secondary antibody response which suggests that humans who are immunized and respond to this vaccine may be boosted by field exposure to sporozoite infected mosquitoes.     

The CD20 calcium channel is localized to microvilli and constitutively associated with membrane rafts: antibody binding increases the affinity of the association through an epitope-dependent cross-linking-independent mechanism

CD20 is a B cell-specific membrane protein that functions in store-operated calcium entry and serves as a useful target for antibody-mediated therapeutic depletion of B cells. Antibody binding to CD20 induces a diversity of biological effects, some of which are dependent on lipid rafts. Rafts are isolated as low density detergent-resistant membranes, initially characterized using Triton X-100. We have previously reported that CD20 is soluble in 1% Triton but that antibodies induce the association of CD20 with Triton-resistant rafts. However, by using several other detergents to isolate rafts and by microscopic co-localization with a glycosylphosphatidylinositol-linked protein, we show in this report that CD20 is constitutively raft-associated. CD20 was distributed in a punctate pattern on the cell surface as visualized by fluorescence imaging and was also localized to microvilli by electron microscopy. The mechanism underlying antibody-induced association of CD20 with Triton-resistant rafts was investigated and found not to require cellular ATP, kinase activity, actin polymerization, or antibody cross-linking but was dependent on the epitope recognized. Thus, antibody-induced insolubility in 1% Triton most likely reflects a transition from relatively weak to strong raft association that occurs as a result of a conformational change in the CD20 protein.     

Properties of a second epitope of the murine Fc receptor for aggregated IgG

The murine macrophage and lymphocyte Fc receptor for aggregated IgG (Fc gamma R) has previously been characterized by using the anti-Fc gamma R monoclonal antibody (mAb), 2.4G2. In the studies presented here, we describe a new mAb, 6B7C, that defines a second epitope of the Fc gamma R. The tissue distribution of the 6B7C epitope is coincident with the 2.4G2 epitope. However, only the 2.4G2 epitope is accessible to mAb binding on intact primary macrophages or lymphocytes. The 6B7C epitope is not detectable on primary macrophages or lymphocytes but is exposed on a portion of B lymphocyte Fc gamma R after activation by lipopolysaccharide and on some tumor cell lines. The expression of the 6B7C epitope on the surface of B lymphoblasts and tumor cell lines seems to correlate with their ability to release soluble Fc gamma R. The 6B7C mAb has the advantage that it reacts with native as well as denatured receptor and therefore can be used for techniques such as immunoblotting.     

Transformation of the estrogen receptor detected by two monoclonal antibodies

The estrogen receptor from fetal guinea-pig uterus is recognised by two monoclonal antibodies (H222 and H226) developed against the human estrogen receptor but it interacts differently with each of them. The H222 antibody, whose epitope is located in the hormone-binding domain of the receptor, shifts the sedimentation coefficient of the nonactivated oligomeric receptor in low salt sucrose gradients from 9S to 11S. When this oligomeric receptor-H222 complex is centrifuged in high salt gradients, it dissociates to an 8S monomer-H222 complex, indicating that all the estradiol-binding units present in the nonactivated receptor can bind the H222 antibody. In contrast, the H226 antibody, whose epitope is located close to the DNA-binding domain, shifts the sedimentation coefficient of the nonactivated receptor only to 9.4S and when this complex sediments in high salt gradients, it dissociates to a 7S monomer-H226 complex plus a 4.5S monomeric receptor not bound to the antibody. This observation suggests that not all the H226 epitopes are accessible in the nonactivated receptor. On the other hand, the temperature-activated receptor reacts with the H226 antibody to form two complexes sedimenting at 7S and 9S in high salt gradients. This 9S complex indicates the formation of a homodimer that binds two molecules of the H226 antibody. However, only one H222 epitope seems to be accessible in this dimeric form of the receptor, since only one 8S complex is observed when the activated receptor reacts with the H222 antibody. In addition, binding to the H222 antibody before activation prevents the dimerisation. This suggests that the H222 epitope is near or directly involved in the dimerisation domain. Interaction of the H222 and H226 antibodies with the estrogen receptor reveals modifications of its structure during activation, and consequently of the exposure of its functional domains.     

A complex of influenza hemagglutinin with a neutralizing antibody that binds outside the virus receptor binding site.

The structure of a complex of influenza hemagglutinin (HA) with a neutralizing antibody shows that the antibody binds to HA at a distance from the virus receptor binding site. Comparison of the properties of this antibody and its Fab with those of an antibody that recognizes an epitope overlapping the receptor binding site leads to two main conclusions. First, inhibition of receptor binding is an important component of neutralization. Second, the efficiency of neutralization by the antibodies ranks in the same order as their avidities for HA, and their large size makes these antibodies highly efficient at neutralization, regardless of the location of their epitope in relation to the virus receptor binding site. These observations provide rationales for the range of antibody specificities that are detected in immune sera and for the distribution of sequence changes on the membrane-distal surface of influenza HAs that occur during 'antigenic drift.'     

Study of the heteromeric structure of the untransformed glucocorticoid receptor using chemical cross-linking and monoclonal antibodies against the 90K heat-shock protein

The untransformed rat glucocorticoid receptor is assumed to be a hetero-oligomeric complex, containing a non-steroid binding component, the 90K heat-shock protein (HSP 90). Direct measurement of its molecular weight by chemical cross-linking provides new evidence for a trimeric structure with a Mr of ca. 270,000. Resorting to an anti HSP 90 probe (AC 88), we show that the native dimeric HSP 90 possess two accessible epitopes for this monoclonal antibody, while when bound to the steroid-binding subunit, only one epitope remains accessible. These data clearly suggest that the untransformed rat glucocorticoid receptor is an asymmetrical hetero-oligomeric complex.     

Isolation and characterization of transferrin receptor from embryonic chicken red cell

Transferrin receptor is isolated from the plasma membrane of chicken embryo red cell by affinity chromatography on transferrin-Sepharose 4B matrix. The molecular weight of the protein is approximately 58,000. The purified antibody to this protein is capable of agglutinating chicken embryo red cells, and the purified Fab fragments derived from this antibody are capable of inhibiting the antibody-induced agglutination, as well as the complement-induced hemolysis of chicken embryo red cells. The Fab fragments also inhibit the transferrin-mediated uptake of iron by chicken embryo red cells.     

T cell receptors are structures capable of initiating signaling in the absence of large conformational rearrangements

Native and non-native ligands of the T cell receptor (TCR), including antibodies, have been proposed to induce signaling in T cells via intra- or intersubunit conformational rearrangements within the extracellular regions of TCR complexes. We have investigated whether any signatures can be found for such postulated structural changes during TCR triggering induced by antibodies, using crystallographic and mutagenesis-based approaches. The crystal structure of murine CD3ε complexed with the mitogenic anti-CD3ε antibody 2C11 enabled the first direct structural comparisons of antibody-liganded and unliganded forms of CD3ε from a single species, which revealed that antibody binding does not induce any substantial rearrangements within CD3ε. Saturation mutagenesis of surface-exposed CD3ε residues, coupled with assays of antibody-induced signaling by the mutated complexes, suggests a new configuration for the complex within which CD3ε is highly exposed and reveals that no large new CD3ε interfaces are required to form during antibody-induced signaling. The TCR complex therefore appears to be a structure that is capable of initiating intracellular signaling in T cells without substantial structural rearrangements within or between the component subunits. Our findings raise the possibility that signaling by native ligands might also be initiated in the absence of large structural rearrangements in the receptor.     

A modular approach to assembly of totally synthetic self-adjuvanting lipopeptide-based vaccines allows conformational epitope building

The technology described here allows the chemical synthesis of vaccines requiring correctly folded epitopes and that contain difficult or long peptide sequences. The final self-adjuvanting product promotes strong humoral and/or cell-mediated immunity. A module containing common components of the vaccine (T helper cell epitope and the adjuvanting lipid moiety S-[2,3-bis(palmitoyloxy)propyl]cysteine) was assembled to enable a plug and play approach to vaccine assembly. The inclusion within the module of a chemical group with chemical properties complementary and orthogonal to a chemical group present in the target epitope allowed chemoselective ligation of the two vaccine components. The heat-stable enterotoxin of enterotoxigenic Escherichia coli that requires strict conformational integrity for biological activity and the reproductive hormone luteinizing hormone-releasing hormone were used as the target epitopes for the antibody vaccines. An epitope from the acid polymerase of influenza virus was used to assemble a CD8(+) T cell vaccine. Evaluation of each vaccine candidate in animals demonstrated the feasibility of the approach and that the type of immune response required, viz. antibody or cytotoxic T lymphocyte, dictates the nature of the chemical linkage between the module and target epitope. The use of a thioether bond between the module and target epitope had little or no adverse effect on antibody responses, whereas the use of a disulfide bond between the module and target epitope almost completely abrogated the antibody response. In contrast, better cytotoxic T lymphocyte responses were obtained when a disulfide bond was used.     

Change in the accessibility of an epitope of the human granulocyte-colony stimulating factor after binding to receptors

In a previous work we demonstrated that monoclonal antibody (mAb) 8C2 recognized a human granulocyte-colony stimulating factor (hG-CSF) region left unmasked after binding to placenta receptors, whereas mAb 6E3 defined a receptor-buried epitope. Herein we examined the role of these antigenic regions on the proliferative response induced by hG-CSF on a myeloid leukaemia cell line. Both mAbs significantly inhibited the hG-CSF-induced cell growing, although epitope 8C2 but not 6E3 remained exposed in hG-CSF:cell receptor complexes. When cytokine:receptor complexes already formed at 4 degrees C were incubated 1 h at 37 degrees C under conditions preventing the internalization, a significant reduction in the amount of accessible 8C2 epitopes was evident. However, this effect was not observed when mAb 8C2:hG-CSF complexes previously bound to cells were incubated at 37 degrees C. Thus, results suggest that a receptor oligomerization process could account for the temperature-induced epitope 8C2 masking. The identification of epitope 8C2 accomplished by synthesis of overlapping octapeptides, revealed that it is formed by sequences 39-52 and 155-164, both in close proximity in the three-dimensional structure of the hG-CSF molecule. Since part of this region has been proposed as a second binding site to receptors, we infer that the change of epitope 8C2 accessibility could be the result of either receptor aggregation or epitope binding to another receptor. In addition, our data support the hypothesis that a ligand-induced receptor oligomerization is required for transduction of cytokine signals.     

A new method for mapping discontinuous antibody epitopes to reveal structural features of proteins.

Antibodies that bind to protein surfaces of interest can be used to report the three-dimensional structure of the protein as follows: Proteins are composed of linear polypeptide chains that fold together in complex spatial patterns to create the native protein structure. These folded structures form binding sites for antibodies. Antibody binding sites are typically "assembled" on the protein surface from segments that are far apart in the primary amino acid sequence of the target proteins. Short amino acid probe sequences that bind to the active region of each antibody can be used as witnesses to the antibody epitope surface and these probes can be efficiently selected from random sequence peptide libraries. This paper presents a new method to align these antibody epitopes to discontinuous regions of the one-dimensional amino acid sequence of a target protein. Such alignments of the epitopes indicate how segments of the protein sequence must be folded together in space and thus provide long-range constraints for solving the 3-D protein structure. This new antibody-based approach is applicable to the large fraction of proteins that are refractory to current approaches for structure determination and has the additional advantage of requiring very small amounts of the target protein. The binding site of an antibody is a surface, not just a continuous linear sequence, so the epitope mapping alignment problem is outside the scope of classical string alignment algorithms, such as Smith-Waterman. We formalize the alignment problem that is at the heart of this new approach, prove that the epitope mapping alignment problem is NP-complete, and give some initial results using a branch-and-bound algorithm to map two real-life cases. Initial results for two validation cases are presented for a graph-based protein surface neighbor mapping procedure that promises to provide additional spatial proximity information for the amino acid residues on the protein surface.     

Epitope mapping of gibberellin to the anti-gibberellin A(4) monoclonal antibody by saturation transfer difference NMR spectroscopy

Saturation transfer difference (STD) NMR spectroscopy is a promising tool for rapid screening, identifying ligands that interact with a target protein, and characterizing the epitopes of the ligands. Gibberellins (GAs) are a class of plant hormones and form a large family consisting of more than 120 members. A few of them, called "active" GAs, are considered to be perceptible to a receptor that remains unknown. We applied STD NMR spectroscopy to detect the binding activity and identify the binding epitope of gibberellin A(3) (GA(3)) that is recognized by monoclonal antibody 4-B8(8)/E9. This is one of the antibodies that can mimic a GA receptor in the manner of recognition of active GAs. The information on the binding epitope, obtained by STD NMR, was in good agreement with that shown by analyzing the crystal structure of the antibody-GA(4) complex. This suggests that STD NMR spectroscopy would be very useful to characterize the interaction between GAs and such binding proteins as GA-catabolic enzymes and receptors.     

Sulfoglycolipids bind to adhesive protein amphoterin (P30) in the nervous system.

HNK-1 antibody reactive carbohydrate epitope carried by glycolipids and glycoproteins has been shown to be involved in cell to cell interactions. It has been proposed that the HNK-1 reactive 3-sulfoglucuronyl carbohydrate epitope in glycolipids may interact with a cell surface receptor to promote the biological response in the developing nervous system. The possible occurrence of such a receptor was examined in rat nervous system. A specific binding of sulfoglycolipids to a 30 kD protein from adult rat cerebellum is described. Little binding was found with neutral glycolipids and gangliosides. The 30 kD protein from cerebellum was partially purified on a sulfatide-octyl-Sepharose affinity column. Binding of sulfoglucuronyl glycolipids to a similar 30 kD protein from forebrain previously identified as amphoterin is also shown. Amphoterin is developmentally regulated and is involved in neural cell adhesion and neurite extension.     

Antibody-induced receptor loss. Different fates for asialoglycoproteins and the asialoglycoprotein receptor in HepG2 cells

The human asialoglycoprotein receptor (ASGP-R) is a membrane glycoprotein which participates in receptor-mediated endocytosis and delivery of its ligands to lysosomes for degradation. In order to examine the pathways and mechanisms responsible for the turnover and degradation of the ASGP-R we have followed the fate of the ASGP-R in HepG2 cells during exposure to anti-receptor antibody as well as inhibitors of lysosomal processing and receptor recycling. Incubation of cells at 37 degrees C with anti-ASGP-R antibody results in the rapid (t 1/2 30 min) loss of mature 46,000-Da ASGP-R (control, t 1/2 20 h). This process requires whole IgG, since Fab fragments do not induce loss of receptor. Furthermore, this antibody-induced loss is specific, since incubation with antibody to the transferrin receptor does not alter cellular ASGP-R content. Of note, weak bases (e.g. primaquine) abrogate this antibody-induced loss of ASGP-R. Inhibitors of lysosomal proteases (EC64 and leupeptin) do not alter this antibody-mediated loss. Furthermore, this effect occurs at 18 degrees C, a temperature at which delivery of ligand to the lysosome is blocked. Thus, the present observations suggest a unique pathway for antibody-induced ASGP-R loss which is distinct from the pathway of lysosomal delivery of ligand.     

A Linear Epitope in the N-Terminal Domain of CCR5 and Its Interaction with Antibody

The CCR5 receptor plays a role in several key physiological and pathological processes and is an important therapeutic target. Inhibition of the CCR5 axis by passive or active immunisation offers one very selective strategy for intervention. In this study we define a new linear epitope within the extracellular domain of CCR5 recognised by two independently produced monoclonal antibodies. A short peptide encoding the linear epitope can induce antibodies which recognise the intact receptor when administered colinear with a tetanus toxoid helper T cell epitope. The monoclonal antibody RoAb 13 is shown to bind to both cells and peptide with moderate to high affinity (6x10^8 and 1.2x10⁷ M^-1 respectively), and binding to the peptide is enhanced by sulfation of tyrosines at positions 10 and 14. RoAb13, which has previously been shown to block HIV infection, also blocks migration of monocytes in response to CCR5 binding chemokines and to inflammatory macrophage conditioned medium. A Fab fragment of RoAb13 has been crystallised and a structure of the antibody is reported to 2.1 angstrom resolution.