Performance of multistep immunochemical reactions by counterflow isotachophoresis on nitrocellulose membranes--I. Immunoblotting

Urine with trace amounts of different proteins from healthy people or B-lymphoma patients was concentrated and separated simultaneously by counterflow isotachophoresis on cellulose acetate membranes (CAM). The protein zones were blotted onto nitrocellulose membrane (NCM) by direct contact of CAM and NCM. NCM-blots were exposed to second isotachophoresis with the leading electrolyte 0.06 M Tris-HCl and the terminating one, 0.012 M Tris-beta-alanine. Under these conditions the moving boundary formed by Cl-/beta-alanine- migrated towards the anode with decreasing velocity. At a certain point the rate of migration of the moving boundary became completely compensated by the electroendosmotic counterflow. In this steady state position the boundary stopped on the NCM support, while the electroendosmotic rate in the area before the boundary was much higher than the rate of the opposite migration of any protein to the anode. Under these conditions electroendosmosis served as a "conveyer belt" which transferred consecutively the immunoreagents, antibodies, immunoconjugates, or antiperoxidase-peroxidase system through the protein blots "printed" on NCM. The immunoblots obtained in this way were developed by the substrate for the immunoenzyme complex used in the experiment. The technique could be used to characterize light chains present in the urine of normal donors and monoclonal light chains in the urine of patients with B-cell malignancies.     

Immunoenzymatic analysis by monoclonal antibodies of bacterial lipopolysaccharides after transfer to nitrocellulose

A rapid, sensitive immunoenzymatic technique for the analysis of lipopolysaccharide (LPS) from gram-negative bacteria using monoclonal antibodies is described. After separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the LPS was either stained with silver or electrophoretically transferred to nitrocellulose. After reaction with anti-LPS monoclonal antibodies, the transferred antigens were visualized by reaction with alkaline-phosphatase-labelled anti-mouse antibodies and a substrate containing naphthol phosphoric acid and Fast Red.     

Epitope analysis of the oncofetal antigen alphafetoprotein using monoclonal antibodies.

Alphafetoprotein (AFP), an oncofetal antigen, plays very important roles in the early embryonic life and oncogenesis. Under various physiological and pathological conditions AFP exhibits microheterogeneity, probably as a result of differential expression of its epitopes. To analyse the epitopes we have developed a panel of monoclonal antibodies against human AFP purified by a new and efficient method using an immunoadsorbent consisting of polyclonal antibodies immobilized on cyanogen bromide activated Sepharose. Clones producing antibodies of various isotypes, e.g. IgG1, IgG2a, IgG2b, IgA and IgM have been subcloned and characterized. The antibodies showed high avidity for AFP (with half-maximal binding concentrations between 0.012 and 3.87 nM). Mutual inhibition efficiencies of a panel of 14 monoclonal antibodies were determined by RIA. Based on these inhibition data a computer program was used to group these antibodies with respect to their "epitope specificity distance". As a result of this grouping, clones have been identified which can recognize at least five different epitopes on AFP. This panel of antibodies may be very useful for analysis of the epitopic variation of AFP under various physiological and pathological conditions.     

Epitope analysis of aztreonam by antiaztreonam monoclonal antibodies and possible consequences in beta-lactams hypersensitivity.

Three cell lines producing monoclonal antibodies, Az-1 (IgG1), Az-2 (IgG1) and Az-3 (IgM) against aztreonam were established. The epitopes and the cross-reactions of the antibodies with various beta-lactams, which were conjugated with human serum albumin (HSA), were examined by enzyme-linked immunosorbent assay (ELISA) and ELISA inhibition test. In ELISA, Az-1 and Az-2 reacted only with aztreonam and ceftazidime, which have the same acyl side chain. Furthermore, Az-2 showed a strong cross-reaction with carumonam. In the ELISA inhibition test, Az-1 and Az-2 were inhibited from binding to aztreonam-HSA by aztreonam, ceftazidime, aztreonam hydrolysate, aztreonam-epsilon-amino-n-caproic acid (EACA) and ceftazidime-EACA. Az-2 was also inhibited with carumonam. From the above results, it seems that Az-1 can recognize only the degraded structure of monobactam nucleus, and Az-2 can recognize the degraded nucleus moiety and the acyl side chain. On the other hand, Az-3 displayed broad cross-reaction to various beta-lactams in ELISA. Furthermore, the MAb showed no inhibitory reaction with various beta-lactams except aztreonam- and ceftazidime-EACA conjugates in the ELISA inhibition test, suggesting that Az-3 recognize a new antigenic determinant (NAD), which is formed by the conjugation of beta-lactam and carrier protein. The above results indicate that antibodies can recognize at least three epitopes of the degraded product(s) of aztreonam nucleus, acyl side chain and NAD in aztreonam-protein conjugate.     

Epitope diversity of monoclonal antibodies revealed by cross-species reactivity

The epitope specificity of two monoclonal antibodies (MAb) which have the same functional activity has been studied. These two independently raised rat IgG2b MAb, NIMP-R10 and M1/70 (Springer et al., 1979), blocked the complement (C) receptor on mouse macrophages. Both MAb showed essentially the same binding pattern with mouse cells, binding to the same extent mouse eosinophils, macrophages, neutrophils, a small proportion of spleen and bone marrow cells, but not thymocytes. That both MAb were apparently recognizing the same epitope was suggested from experiments in which MAb M1/70 inhibited the binding of MAb NIMP-R10. In addition, both MAb showed identity at the molecular level, precipitating the same molecules from the surface of mouse cells. However, NIMP-R10 and M1/70 could be shown to recognize different epitopes when they were tested on human cells. Thus, NIMP-R10 was found to bind to neutrophils and to large granular lymphocytes with natural killer cell activity but not to eosinophils or monocytes, while M1/70 bound to all of these cell types. It is suggested that inter-species testing may have general application in the analysis of antibody specificity.     

Epitope analysis of human cytomegalovirus glycoprotein complexes using murine monoclonal antibodies

A panel of 10 monoclonal antibodies reactive with human cytomegalovirus (HCMV) glycoproteins was generated. These antibodies immunoprecipitated disulfide-linked complexes which contained glycoproteins with molecular weights of 130,000, 93,000, and 52,000. These complexes were designated gC-I. Epitope analysis of gC-I was done with a simultaneous two antibody binding assay. A network of epitopes was revealed which clustered in three major domains designated I, II, and III. Antibodies within individual domains I and II showed strong mutual inhibition of each other's binding. However, there were multiple antibody interactions between domains I and II. For example, the binding of most antibodies in domain I was augmented to some extent by antibodies from domain II. However, the binding of only one antibody from domain II was augmented by all antibodies from domain I. The augmentation in binding between two antibodies was dependent on the native structure of gC-I and was sensitive to conformational changes due to nonionic detergent extraction of gC-I and/or disruption of disulfide bonds. A synergistic effect was also observed between antibodies in domains I and II in a virus neutralization assay. A neutralizing antibody had a much greater neutralizing activity in the presence of a nonneutralizing antibody, which also enhanced the binding of the neutralizing antibody in the simultaneous two antibody binding assay. Also, two antibodies which were nonneutralizing individually were neutralizing when used in combination. Such antibodies also augmented each other's binding in the simultaneous two antibody binding assay. Finally, domain III consisted of a nonneutralizing antibody that inhibited the binding of all antibodies in domains I and II. This antibody also inhibited the neutralizing activity of a neutralizing antibody in a virus neutralizing assay.     

Epitope diversity of angiotensin II analysed with monoclonal antibodies.

The antigenic heterogeneity of angiotensin II (AII) was studied with monoclonal antibodies. Twelve antibodies were produced and characterized. Association constants for AII varied from 1.2 X 10(8) to 1.1 X 10(10) M-1. The fine specificity of the Mab was studied by immunoenzymoassay using solid-phase AII. Using AII analogues in binding inhibition experiments, three groups of specificity could be characterized: (1) five antibodies reacted only with peptides in which phenylalanine is the carboxy terminal aminoacid; for two of these antibodies, tyrosine4 is closely associated with the binding site, since iodine labelling suppresses reactivity; (2) two antibodies also required phenylalanine in position 8, but, in addition, reacted with AI, a decapeptide in which phenylalanine is not terminal; (3) five antibodies reacted with analogues in which phenylalanine had been substituted for another amino acid. In addition, studies in which binding of a biotinylated Mab to solid-phase AII was analysed in the presence of various unlabelled Mab suggest further antigenic heterogeneity of AII.     

Epitope mapping of anti-breast and anti-ovarian mucin monoclonal antibodies.

Anti-breast cancer antibodies (BC2, HMPV and 4B6) and an anti-ovarian cancer antibody (OM1) were found to react with mucins--indeed with the protein core encoded by the MUC1 gene. This gene contains a VNTR (variable number of tandem repeats) encoding a 60 bp (= 20 amino acids) repeat sequence and within this amino acid sequence SAPDTRPAP was predicted, by hydrophilicity analysis, to be the immunogenic peptide sequence. The four antibodies were shown to react with MUC1 VNTR encoded peptides in direct binding and inhibition studies. The precise reactivity of the 4 mAbs was mapped using ELISA in both solid and liquid phase, and demonstrated the epitopes to be: APDTR (BC2 and HMPV), PDTR (4B6) and DTRPA (OM1). By using the pepscan method, the epitopes were shorter (PDTR, DTR and DTRP). However when these short peptides (except DTR) were synthesized they did not react; flanking amino acids are needed for the epitopes. Clearly several different methods should be used to define the reactive epitope. Within (S)APDTR, major amino acid substitutions could be made--even of three to four amino acids without altering antibody binding, provided that P and R were not substituted. It was of interest that an anti-ovarian cancer antibody gave similar anti-peptide reactions to the anti-breast cancer antibodies; apparently MUC1 peptides in ovarian cancer are the same as in breast cancer.     

Epitope mapping of horseradish peroxidase with use of monoclonal antibodies.

Nine mouse monoclonal antibodies (MAbs) to horseradish peroxidase (HRP) were used to develop an epitope map of the enzyme. The results of a competitive binding assay indicated three distinct patterns of reactivity. Two groups of MAbs (I and III) recognized epitopes located in separate antigenic regions on the molecule; another (II) bound to sites that overlapped with epitopes in either region I or III. Further definition of these regions was obtained by analyzing the MAbs for their binding to isolated heme, other peroxidases and heme-containing proteins, and to denatured and apo-HRP. None of the group I MAbs bound heme, suggesting that this region was removed from the active site of the enzyme. All of the group II and III MAbs bound heme as well as the other peroxidases and heme-containing proteins, indicating that they recognized heme-associated epitopes at or near the active site. Only one MAb (2A2) in groups II and III bound to apo-HRP but not to denatured HRP; it was also the only MAb in the entire panel that inhibited the catalytic activity of HRP. This suggests that the epitope recognized by 2A2 involves both the heme moiety and a conformationally dependent protein determinant near the active site.     

Epitope binning of murine monoclonal antibodies by a multiplexed pairing assay

We describe an assay and data evaluation technique for sorting a panel of murine monoclonal antibodies according to epitope specificities. The assay analyzes the simultaneous binding (pairing) of antibodies to an antigen and groups together antibodies with similar pairing profiles. Similar profiles indicate that the antibodies bind to the same or closely related epitopes. The assay works well with crude hybridoma supernatants and can be multiplexed. These features make the assay particularly suitable for the early phase of hybridoma/antibody screening when antibodies are available only as low volume culture harvest samples.     

Serological and immunochemical characterization of monoclonal antibodies to Toxoplasma gondii.

The fusion of mouse NS-1 myeloma cells with spleen cells from mice chronically infected with Toxoplasma gondii resulted in eight clones of hybridomas producing monospecific antibodies against membrane or cytoplasmic antigens of Toxoplasma tachyzoites. One of the antibodies to a cytoplasmic determinant was an IgM; the others directed to membrane or cytoplasmic antigens belonged to the IgG2 or IgG3 isotypes. Antibodies of clones 1E11 (IgG3), 2G11, and 3E6 (IgG2) directed to membrane antigens, bound complement and were reactive in the complement-dependent cytotoxicity assay of Sabin-Feldman. These IgG2 antibodies were strongly agglutinating to parasites, whereas the IgG3 was relatively weak. Another IgG2 antibody (5B6), possibly recognizing a shared antigen of membrane and cytoplasm, exhibited a low titre in the cytotoxicity assay as well as in the agglutination assay. Two other antibodies to membrane antigens (2B7 and 2F8) as well as an antibody to a cytoplasmic antigen (3G3) did not bind complement and did not cause agglutination. The pattern of parasite staining produced by monoclonal antibodies to membrane antigens in an IFA test was different from that of polyvalent antisera. A strictly localized or 'beaded' staining was observed, as well as a smooth, rim fluorescence. Toxoplasma tachyzoites were surface radio-iodinated and the solubilized membrane proteins were immunoprecipitated with monoclonal antibodies and analysed by two-dimensional polyacrylamide gel electrophoresis. Two independently arising monoclonal antibodies to membrane antigens (2G11 and 3E6) consistently precipitated both the solubilized 35,000 and 14,000 mol. wt proteins, while 1E11 precipitated the 27,000 mol. wt protein.     

Discrimination of epitopes identified by monoclonal antibodies by competitive binding to nitrocellulose bound antigens

A new method is described for determining the distribution of epitopes identified by monoclonal antibodies. The method utilizes nitrocellulose membranes as a solid support for antigens which are rapidly adsorbed to nitrocellulose by vacuum-blotting and then used in competitive antibody binding assays. The distribution of epitopes is established by the reciprocal cross-blocking of radiolabeled antibody by increasing concentrations of unlabeled antibody. When unlabeled antibody does not block the binding of labeled antibody to antigen, the 2 antibodies recognize distinct epitopes. When unlabeled antibody blocks the binding of labeled antibody to antigen, the 2 antibodies recognize the same epitope. The method is rapid, sensitive and should be applicable to screening monoclonal antibodies to any epitope.     

Epitope mapping of the HIV-1 gag region with monoclonal antibodies.

A new type of immunochemical mapping of the human immunodeficiency virus type 1 (HIV-1) gag region was performed. By use of native HIV-1 viral lysates or the gag recombinant p24-15 antigen, a new set of monoclonal antibodies (Mabs) to the gag region proteins was generated. Synthetic HIV-1 peptides covering the entire gag region were used to specifically localize the continuous epitopes by direct binding to the Mabs and by blocking the Mab immunoreactivity. The identified immunogenic epitopes were localized between the gag amino acids (aa) 108-127, 203-217, 208-222, 248-282, 273-302, 288-307, 308-322, 331-354 and 408-422. These continuous epitopes formed seven immunogenic regions. One strongly p17-reactive Mab appeared to react with a discontinuous epitope, the components of which were 110 aa distant in the linear sequence: aa 23-27 and 128-132. The synthetic peptides appeared to be more congruent with the Mab-reactive sites in solution than when coated to a solid phase.     

Epitope analysis and utility of monoclonal antibodies to native and recombinant human thymidylate synthase

The expression of thymidylate synthase (TS) in human cancer tissues has been suggested to be a prognostic factor for patients receiving 5-fluorouracil-based chemotherapy. We generated monoclonal antibodies to both recombinant and native TS and analyzed the epitopes on the TS molecule. Two monoclonal antibodies were obtained from recombinant human TS proteins (RTSMA1 and RTSMA2) and two monoclonal antibodies raised to native TS in human cancers (NTSMA1 and NTSMA2) were obtained, and were found to have a high affinity and specificity for TS proteins. To identify the human TS epitope that these monoclonal antibodies recognized, we constructed plasmids to produce full-length and two partially deleted TS proteins fused to glutathion-S-transferase (GST). Western blot analysis showed that RTSMA1 and 2 only reacted with full-length TS and NTSMA1 and 2 reacted with all three recombinant TS proteins, suggesting that the epitopes of the former were located at C-terminal sites (D186-V313), and those of the latter were at N-terminal sites (M1-M61). The RTSMA 1 and 2 epitopes were more accurately mapped using 8 oligopeptides to the region of I267 to L282 which was situated at the surface of the TS protein. Highly sensitive detection of human TS was also possible by sandwich ELISA using a combination of different types of antibody rather than a single type. In conclusion, different type monoclonal antibodies to human TS protein may contribute to the detection of TS in cancer patients.     

Epitope mapping of apamin by means of monoclonal antibodies raised against free or carrier-coupled peptide.

A panel of 20 monoclonal antibodies raised against the bee-venom peptide apamin (18 residues, 2 disulfide bridges) was prepared. Nine monoclonal antibodies (mAb) were obtained from a mouse immunized with free apamin and 11 from a mouse immunized with a mixture of free and carrier-coupled peptide. Using a panel of 11 synthetic apamin analogs, we examined the fine antigenic specificity of each antibody. The mAb generated against free apamin preferentially bound to the central part of the peptide and less frequently recognized the N- and C-terminal regions. However, monoclonal antibodies obtained by immunization with carrier-bound apamin showed a broader range of specificities, consistent with the possibility of the entire surface of this small antigen becoming immunogenic upon coupling to the carrier.     

Epitope mapping of monoclonal antibodies against N-domain of carcinoembryonic antigen

Monoclonal antibodies (MoAbs) against N-domain of carcinoembryonic antigen (CEA), C249, K348, K1338, and K1444, that inhibit CEA-mediated cell adhesion, did not crossreact with nonspecific cross-reacting antigen (NCA). To determine amino acid sequences involved in the adhesion, epitopes of the MoAbs were mapped with recombinant NCAs carrying CEA-NCA chimeric N-domain. The data showed that the epitopes of C249, K1338, K1444 are located within the regions 1-32, 1-32, and 33-59 of CEA, respectively, and that two discrete regions 1-32 and 60-93 may be related to the epitope of K348. Comparison of amino acid sequences between CEA and NCA suggested that four residues (21, 27-29), eight residues (21, 27-29, 66, 78, 79, 89), and three residues (43, 44, 46) are important for recognition by C249 (or K1338), K348, and K1444, respectively. These residues seem to participate in the cell adhesion mediated by CEA.     

Epitope mapping of four monoclonal antibodies specific for the human RhD antigen.

RhD is a highly immunogenic erythrocyte membrane protein, implicated in hemolytic disease of the newborn and other hemolytic disorders. Anti-RhD antibodies are used in the treatment of such disease states. Six mutant forms of recombinant RhD were stably expressed in K562 cells, and these cells were used to investigate epitope specificities of four anti-RhD monoclonal antibodies (mAbs). Amino acid substitutions were made in the exofacial loops of RhD to the corresponding residues found in the related RhCE polypeptide; M169L/M170R and I172F in the third loop, F223V and E233Q in the fourth loop, and D350H and G353W/A354N in the sixth loop. Each mAb was found to have a unique fine specificity and recognized multiple distant sites within RhD. The mAbs also differed in how they recognized individual amino acids in the exofacial loops of RhD.     

Epitope mapping of recombinant human gamma interferon using monoclonal antibodies

Five monoclonal antibodies (MAbs B22, B27, 3-6, 32 and 35) specific for human recombinant IFN-gamma were characterized. These MAbs were used to set up quantitative sandwich ELISAs which allowed the detection of 1.25 ng/ml of IFN-gamma when diluted in normal human serum. Epitope mapping of the IFN-gamma molecule using these MAbs demonstrated that antibodies 3-6 and 32 which did not inhibit the biological activity of IFN-gamma recognized an epitope localized on the 15 C-terminal amino acids, suggesting that this portion of the molecule was not implicated in the biological activity of IFN-gamma. Sandwich ELISAs were performed using various pairs of MAbs. The level of reactivity obtained when antibodies B22 and B27 were used simultaneously as catcher and tracer was similar to the result obtained with two antibodies recognizing different epitopes. These results confirm that the IFN-gamma molecule is a dimer in solution and indicate that the two sites of the IFN-gamma dimeric molecule which are associated with the biological activity (epitope B22/B27) are fully exposed. In contrast, the C-terminus is only partially accessible to the antibodies 3-6/32, suggesting that the dimerization of IFN-gamma molecule results in the interaction of regions of the monomers that are homologous and adjacent to the C-terminus.     

Epitope analysis of the allergen ovalbumin (Gal d II) with monoclonal antibodies and patients' IgE.

Ovalbumin (OVA) is a major allergen (Gal d II) of hen egg white and is often the cause of hypersensitivity reactions to food. Further knowledge of the antigenic and allergenic epitopes of allergens will provide better treatment of this disease. To analyse these epitopes we produced a panel of monoclonal antibodies (mAbs) against native OVA. The initial information about the epitopes was obtained with the binding patterns of these mAbs in IEF-immunoprints and western blots of OVA under reducing and non-reducing conditions. It was possible to demonstrate that the different conformations of OVA exhibit different epitopes, and that there are other epitopes which are shared by each conformation. Seven different, although sometimes overlapping epitopes, could be determined on native OVA; four different epitopes on denaturated non-reduced OVA by means of immunoblots of the intact molecule. The number of epitopes which could be differentiated by the mAbs was increased by the use of peptide blots after CNBr fragmentation of the molecule. IgE binding to different OVA conformations and to CNBr-fragments of OVA was also detectable and appears in the same regions as the reactivity of some mAbs. Western blots of OVA and CNBr-peptides demonstrate that some antigenic/allergenic binding sites seem at least partly to be continuous epitopes. The identification of the CNBr-fragments was performed by a microsequence analysis of blotted CNBr-fragments after a 2-dimensional electrophoresis. IgE was found to bind the two largest CNBr-fragments (residues 41-172 and 301-385), but not the fragment corresponding to residues 173-196. A number of monoclonal antibodies also reacted with the two large fragments, especially with fragment 301-385, and some bind also to shorter peptides, such as fragment 173-196, which were not reactive to patients' IgE. Most of the monoclonal antibodies and patients' IgE bind to the fragments 41-172 and 301-385 in 2D-PAGE blots suggesting that these fragments are involved in an immunogenic structure.