Identification of a surface epitope of human erythropoietin with anti-peptide antibodies

Antibodies reactive with human erythropoietin were isolated from the serum of rabbits immunized with a twenty-six amino acid synthetic polypeptide corresponding to a proposed NH2-terminal sequence of the hormone. As shown by inhibition with peptide fragments, those antibodies that bound to erythropoietin recognized the (8-15) domain, strongly suggesting tht this region is exposed on the hormone's surface. This was confirmed by affinity purification of these antibodies on immobilized fragment (8-15). These results provide insight into the tertiary structure of human erythropoietin and suggest uses for the sequence-specific antibodies in labeling the hormone.     

N-glycans harboring the Lewis a epitope are expressed at the surface of plant cells

In plants, N -linked glycans are processed in the Golgi apparatus to complex-type N -glycans of limited size containing a β(1,2)-xylose and/or an α(1,3)-fucose residue. Larger mono- and bi-antennary N -linked complex glycans have not often been described. This study has re-examined the structure of such plant N -linked glycans, and, through both immunological and structural data, it is shown that the antennae are composed of Lewis a (Le^a) antigens, comprising the carbohydrate sequence Galβ1-3[Fucα1-4]GlcNAc. Furthermore, a fucosyltransferase activity involved in the biosynthesis of this antigen was detected in sycamore cells. This is the first characterization in plants of a Lewis antigen that is usually found on cell-surface glycoconjugates in mammals and involved in recognition and adhesion processes. Le^a-containing N -linked glycans are widely distributed in plants and highly expressed at the cell surface, which may suggest a putative function in cell/cell communication.     

A panel of monoclonal antibodies against the prion protein proves that there is no prion protein in human pancreatic ductal epithelial cells

Prion diseases are a group of neurodegenerative diseases that are fatal. The study of these unique diseases in China is hampered by a lack of resources. Amongst the most important resources for biological study are monoclonal antibodies. Here, we characterize a panel of monoclonal antibodies specific for cellular prion protein by enzyme-linked immunosorbent assay（ELISA）, immunofluorescent staining, flow cytometry, and western blotting. We identify several antibodies that can be used for specific applications and we demonstrate that there is no prion protein expression in human pancreatic ductal epithelial cells（HPDC）.     

Pan-protective anti-alphavirus human antibodies target a conserved E1 protein epitope

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Anti-idiotype-mediated epitope spreading and diminished phagocytosis by a human monoclonal antibody recognizing late-stage apoptotic cells

Apoptotic cells are considered an important auto-antigenic source in diseases such as systemic lupus erythematosus (SLE). A human monoclonal antibody demonstrating exquisite specificity towards late-stage apoptotic cells was generated from an SLE patient. Polyreactive recognition of ribonucleoproteins Ro52 and Ro60 was observed. The antibody significantly diminished the phagocytosis of apoptotic cells and a concomitant decrease in transforming growth factor-beta (TGF-beta) secretion was observed. Light and heavy chain sequencing revealed the antibody to be in essentially germline configuration. Elicited anti-idiotypic antibodies bound distinct self-antigens and showed augmented reactivity towards apoptotic cells as well. Thus, near-germline encoded antibodies recognizing antigens externalized during the process of apoptosis can mediate a variety of potentially pathogenic effects; decreases in the phagocytic uptake of dying cells would constitute a disease-perpetuating event and stimulation of the idiotypic network could lead to intermolecular epitope spreading, increasing the range of molecular targets..     

Beta-PrP form of human prion protein stimulates production of monoclonal antibodies to epitope 91-110 that recognise native PrPSc

Prion diseases are associated with accumulation of strain-dependent biochemically distinct, disease-related isoforms (PrP(Sc)) of host-encoded prion protein (PrP(C)). PrP(Sc) is characterised by increased beta-sheet content, detergent insolubility and protease resistance. Recombinant alpha-PrP adopts a PrP(C)-like conformation, while beta-PrP conformationally resembles PrP(Sc), to these we raised 81 monoclonal antibodies in Prnp(0/0) mice. The N-terminal residues 91-110 are highly immunogenic in beta-PrP-immunised mice and of (17/41) anti-beta-PrP antibodies that could be epitope-mapped, approximately 70%, recognised this segment. In contrast, only 3/40 anti-alpha-PrP antibodies could be mapped and none interacted with this region, instead recognising residues 131-150, 141-160 and 171-190. Native PrP(C) was recognised by both antibody groups, but only anti-beta-PrP antibodies directed to 91-110 residues recognised native PrP(Sc) with high affinity, where in addition, species heterogeneity was also evident. Within the six anti-beta-PrP antibodies studied, they all recognised PK-treated native human and mouse PrP(Sc), four failed to recognise PK-treated native bovine PrP(Sc), one of which also did not recognise native PK-treated ovine PrP(Sc), showing the epitope becomes exposed on unfolding and disaggregation. These results demonstrate strain-dependent variations in chain conformation and packing within the 91-110 region of PrP(Sc).     

Antibody-dependent cellular cytotoxicity via humoral immune epitope of Nef protein expressed on cell surface

Antibodies against various proteins of HIV type 1 (HIV-1) can be detected in HIV-1-infected individuals. We previously reported that the level of Ab response against one Nef epitope is correlated with HIV-1 disease progression. To elucidate the mechanism for this correlation, we examined Ab-dependent cellular cytotoxicity (ADCC) against target cells expressing Nef. We observed efficient cytotoxicity against Nef-expressing target cells in the presence of patient plasma and PBMCs. This ADCC activity was correlated with the dilution of plasma from HIV-1-infected patients. Addition of a specific synthetic peptide (peptide 31:FLKEKGGLE) corresponding to the Nef epitope reduced cell lysis to approximately 50%. These results suggest that PBMCs of HIV-1-infected patients may exert ADCC via anti-Nef Abs in the patients' own plasma and serve as a mechanism used by the immune system to regulate HIV-1 replication.     

Normal cellular prion protein is preferentially expressed on subpopulations of murine hemopoietic cells

We studied the expression of normal cellular prion protein (PrP(C)) in mouse lymphoid tissues with newly developed mAbs to PrP(C). Most of the mature T and B cells in the peripheral lymphoid organs do not express PrP(C). In contrast, most thymocytes are PrP(C+). In the bone marrow, erythroid cells and maturing granulocytes are PrP(C+). Approximately 50% of the cells in the region of small lymphocytes and progenitor cells also express PrP(C). Most of these PrP(C+) cells are CD43(+), but B220(-), surface IgM(-) (sIgM(-)), and IL-7R(-), a phenotype that belongs to cells not yet committed to the B cell lineage. Another small group of the PrP(C+) cell are B220(+), and some of these are also sIgM(+). The majority of the B220(+) cells, however, are PrP(C-). Therefore, PrP(C) is preferentially expressed in early bone marrow progenitor cells and subsets of maturing B cells. Supporting this interpretation is our observation that stimulation of bone marrow cells in vitro with PMA results in a decrease in the number of PrP(C+)B220(-) cells with a corresponding increase of sIgM(+)B220(high) mature B cells. This result suggests that the PrP(C+)B220(-) cells are potential progenitors. Furthermore, in the bone marrow of Rag-1(-/-) mice, there are an increased number of PrP(C+)B220(-) cells, and most of the developmentally arrested pro-B cells in these mice are PrP(C+). Collectively, these results suggest that PrP(C) is expressed preferentially in immature T cells in the thymus and early progenitor cells in the bone marrow, and the expression of PrP(C) is regulated during hemopoietic differentiation.     

Chicken antibody against a restrictive epitope of prion protein distinguishes normal and abnormal prion proteins

Recently, we reported the application of a recombinant chicken IgY monoclonal antibody, Ab3-15, against mammalian prion protein (PrP), for the diagnosis of bovine spongiform encephalopathy in cattle. In this study, we have characterized a soluble, single-chain variable fragment (scFv) form of this antibody, sphAb3-15 using brain homogenates from mice. This sphAb3-15 antibody recognized denatured forms of both PrP(C) and PrP(Sc), and PrP(Sc) after PK-treatment, on Western blotting. In sandwich ELISAs, on dot blots and by immunoprecipitation, sphAb3-15 efficiently bound to PrP from normal brain homogenates, but weakly bound PrP from scrapie-infected brain homogenates. These results suggest that sphAb3-15 selectively recognizes PrP(C) under native conditions and that the epitope recognized by sphAb3-15 may undergo conformational changes during the conversion of PrP(C) into PrP(Sc).     

Conversion of bacterially expressed recombinant prion protein

The infectivity associated with prion disease sets it apart from a large group of late-onset neurodegenerative disorders that shares the characteristics of protein aggregation and neurodegeneration. The unconventional infectious agent, PrP(Sc), is an aberrantly folded form of the normal prion protein (PrP(C)) and the PrP(C)-to-PrP(Sc) conversion is a critical pathogenic step in prion disease. Using the Protein Misfolding Cyclic Amplification technique, we converted folded bacterially expressed recombinant PrP into a proteinase K-resistant and aggregated conformation (rPrP-res) in the presence of anionic lipid and RNA molecules. Moreover, high prion infectivity was demonstrated by intracerebral inoculation of rPrP-res into wild-type mice, which caused prion disease with a short incubation period. The establishment of the in vitro recombinant PrP conversion assay makes it feasible for us to explore the molecular basis behind the intriguing properties associated with prion infectivity.     

Study on the surface polypeptides isolated from human lymphoblastoid cells by ionic shock

Ionic shock treatment in the presence of 10% glycerol is an efficient and selective method for extracting cell surface components from Raji cells and effecting the solubilization of up to 22 polypeptides. The majority of these shock-released polypeptides are accessible to lactoperoxidase radioiodination. Sera from rabbits immunized against these soluble extracts are reactive against Raji cell surface as indicated by indirect membrane immunofluorescence and agglutination assays.     

Staphylococcal surface display in combinatorial protein engineering and epitope mapping of antibodies

The field of combinatorial protein engineering for generation of new affinity proteins started in the mid 80s by the development of phage display. Although phage display is a prime example of a simple yet highly efficient method, manifested by still being the standard technique 25 years later, new alternative technologies are available today. One of the more successful new display technologies is cell display. Here we review the field of cell display for directed evolution purposes, with focus on a recently developed method employing Gram-positive staphylococci as display host. Patents on the most commonly used cell display systems and on different modifications as well as specific applications of these systems are also included. General strategies for selection of new affinity proteins from cell-displayed libraries are discussed, with detailed examples mainly from studies on the staphylococcal display system. In addition, strategies for characterization of recombinant proteins on the staphylococcal cell surface, with an emphasis on an approach for epitope mapping of antibodies, are included.     

A prion protein epitope selective for the pathologically misfolded conformation

Conformational conversion of proteins in disease is likely to be accompanied by molecular surface exposure of previously sequestered amino-acid side chains. We found that induction of beta-sheet structures in recombinant prion proteins is associated with increased solvent accessibility of tyrosine. Antibodies directed against the prion protein repeat motif, tyrosine-tyrosine-arginine, recognize the pathological isoform of the prion protein but not the normal cellular isoform, as assessed by immunoprecipitation, plate capture immunoassay and flow cytometry. Antibody binding to the pathological epitope is saturable and specific, and can be created in vitro by partial denaturation of normal brain prion protein. Conformation-selective exposure of Tyr-Tyr-Arg provides a probe for the distribution and structure of pathologically misfolded prion protein, and may lead to new diagnostics and therapeutics for prion diseases.     

Cellular prion protein is expressed in a subset of neuroendocrine cells of the rat gastrointestinal tract.

Prion diseases are believed to develop from the conformational change of normal cellular prion protein (PrPc) to a pathogenic isoform (PrPsc). PrPc is present in both the central nervous system and many peripheral tissues, although protein concentration is significantly lower in non-neuronal tissues. PrPc expression is essential for internalization and replication of the infectious agent. Several works have pointed to the gastrointestinal (GI) tract as the principal site of entry of PrPsc, but how passage through the GI mucosa occurs is not yet known. Here we studied PrPc expression using Western blot, RT-PCR, and immunohistochemistry in rat GI tract. PrPc mRNA and protein were detected in corpus, antrum, duodenum, and colon. Immunoreactivity was found in scattered cells of the GI epithelium. With double immunofluorescence, these cells have been identified as neuroendocrine cells. PrPc immunostaining was found in subsets of histamine, somatostatin (Som), ghrelin, gastrin (G), and serotonin (5HT) cells in stomach. In small and large bowel, PrPc cells co-localized with subpopulations of 5HT-, Som-, G-, and peptide YY-immunolabeled cells. Our results provide evidence for a possible and important role of endocrine cells in the internalization of PrPsc from gut lumen.     

Characterization of a unique glycoprotein antigen expressed on the surface of human neuroblastoma cells

In order to develop a molecular probe to delineate chemical and biological characteristics of human neuroblastoma cells, a murine monoclonal antibody (Mab 5G3) was produced that is directed to a glycoprotein, preferentially expressed on the surface of such cells. This antibody is of IgG2a isotype, has an association constant of 8 X 10(9) M-1, and reacts preferentially with human neuroblastoma cell lines and fresh frozen tissue sections in enzyme-linked immunosorbent assay and immunoperoxidase assays, respectively. Minimal reactivity is observed with a variety of lymphoblastoid cell lines and normal fetal and adult tissues. Mab 5G3 specifically recognizes a neuroblastoma target glycoprotein antigen of 215 kDa that is derived from a 200-kDa precursor, as evident from pulse-chase biosynthetic studies. Treatment with tunicamycin revealed that both molecules contain N-asparagine-linked oligosaccharides; however, only the 215-kDa species is resistant to treatment with endo-beta-N-acetylglucosaminidase H and sensitive to neuraminidase, indicating that it contains trimmed and terminally sialylated oligosaccharides of the "complex" type. In contrast, the 200-kDa precursor is sensitive to endo-beta-N-acetylglucosaminidase H and resistant to neuraminidase treatment indicating that it contains high-mannose non-processed oligosaccharides. The 215-kDa molecule is sulfated, phosphorylated at serine residues, and expressed on the cell surface. A molecule of 200 kDa is detected by Mab 5G3 in spent culture medium of human neuroblastoma cells which is neither sulfated nor phosphorylated.     

Generation of anti-idiotypic monoclonal antibodies recognizing vasopressin receptors in cultured cells and kidney sections.

To produce anti-idiotypic antibodies against receptors for the neurohypophyseal hormone vasopressin, an anti-vasopressin monoclonal antibody with a ligand specificity similar to that of vasopressin receptors was employed for immunization. Three anti-idiotypic monoclonal antibodies were obtained which induced, like vasopressin, plasminogen activator production in the renal epithelial cell line LLC-PK1 (expressing V2-receptors). Induction of plasminogen activator synthesis by the anti-idiotypic antibodies could be inhibited by coincubation with a vasopressin antagonist. In a fashion similar to that of vasopressin itself, the anti-idiotypic antibodies induced receptor down-regulation. The anti-idiotypic antibodies were employed to visualize vasopressin receptors on LLC-PK1 and A7r5 (V1-receptor-expressing) smooth muscle cells by immunofluorescence. Antibody-mediated fluorescence was not observed in receptor-deficient mutant cell lines or vasopressin-receptor-down-regulated cells. Furthermore, these antibodies were used for immunohistochemical localization of vasopressin receptors in rat and bovine kidney preparations. In accordance with earlier physiological and biochemical observations, vasopressin receptors were detected predominantly in collecting ducts in cortex and medulla. On the cellular level, a differential staining pattern was observed.     

Multiple tandem epitope tagging for enhanced detection of protein expressed in mammalian cells

Epitope tagging is a valuable tool for quick detection, isolation, and analysis of protein-protein interaction, without prior knowledge of the target protein. The FLAG epitope tag, one of the most widely used tags, is an eight amino acid peptide that can be detected by anti-FLAG monoclonal antibody. In the present study, we have examined the detection sensitivity of a protein fused to three tandem FLAG epitopes by Western blot analysis, immunoprecipitation, and immunohistochemical analysis using anti-FLAG® M2 antibody. We find that the triple FLAG epitope significantly enhances the sensitivity of detection of fusion protein expressed in mammalian cells.     

Concealment of epitope by reduction and alkylation in prion protein

Conversion of the cellular prion protein (PrP(C)) into its pathological isoform (PrP(Sc)), the key molecular event in the pathogenesis of prion diseases, is accompanied by a conformational transition of alpha-helix into beta-sheet structures involving alpha-helix 1 (alpha1) domain from residues 144 to 154 of the protein. Reduction and alkylation of PrP(C) have been found to inhibit the conversion of PrP(C) into PrP(Sc) in vitro. Here we report that while antibody affinity of epitopes in the N- and C-terminal domains remained unchanged, reduction and alkylation of the PrP molecule induced complete concealment of an epitope in alpha1 for anti-PrP antibody 6H4 that is able to cure prion infection in the cell model. Mass spectrometric analysis of recombinant PrP showed that the alkylation reaction takes place at reduced cysteines but no modification was observed in this cryptic epitope. Our study suggests that reduction and alkylation result in local or global rearrangement of PrP tertiary structure that is maintained in both liquid and solid phases. The implications in the conversion of PrP(C) into PrP(Sc) and the therapeutics of prion diseases are discussed.