Strategies for epitope analysis using peptide synthesis

Immunological properties of recombinant porin of Haemophilus influenzae type b expressed in Bacillus subtilis

The major surface-located, channel-forming protein in the outer membrane of Haemophilus influenzae type b (Hib) is porin (341 amino acids; M(r), 37,782). In order to generate Hib porin that is devoid of lipooligosaccharides and capsular polysaccharide, the Hib porin gene ompP2 was subcloned into a plasmid vector and recombinant Hib porin was expressed in Bacillus subtilis. Recombinant porin was produced in large quantities in B. subtilis and formed intracellular inclusion bodies. Recombinant porin was extracted from inclusion bodies and shown to be active in forming pores in synthetic black lipid membranes. However, these pores demonstrated different pore characteristics than wild-type Hib porin. Mouse hyperimmune sera against recombinant porin were generated and subjected to epitope scanning with a library of 336 overlapping synthetic hexapeptides that corresponded to the entire sequence of Hib porin. The epitope specificities of the anti-recombinant porin antibodies were similar to those of antibodies against Hib porin: selected regions near the amino terminus which include a buried loop in the native structure of Hib porin were more immunogenic than regions at the carboxy terminus. Although some mouse anti-recombinant porin antibodies mediated complement-dependent binding to Hib by polymorphonuclear leucocytes in opsonophagocytosis assays, the antibodies were not bactericidal, nor did they abrogate bacteremia in the infant rat model of infection. It was concluded that the native state of Hib porin is required for the generation of a protective immune response against the bacterium.

Fine molecular analysis of the antigenicity of the Androctonus australis hector scorpion neurotoxin II: a new antigenic epitope disclosed by the Pepscan method

A set of 58 overlapping rod-bound peptides was used to map the antigenic reactivity pattern of a 64-residue neurotoxin (AaH II) from the venom of the scorpion Androctonus australis hector. Five anti-toxin rabbit antisera were assayed serially for their capacity to bind to each peptide in the set. Six regions of antigenic reactivity were thus identified (sequences: 1-8, 4-12, 27-35, 39-45, 52-58 and 55-61). When positioned on a 3-D model of the toxin, these regions appeared to correspond to either beta-turn or extended parts of the molecule. The antigenic regions revealed by this technique agree fairly well with those previously mapped on the same toxin by different methods. One discrepancy was, however, that the present study shows the N-terminus to be strongly reactive with anti-toxin antibodies. The antigenicity of this region was confirmed, since rabbit antibodies raised against a synthetic peptide mimicking the sequence 1-8 of the toxin were found to bind the toxin with high efficiency. A fine analysis of the recognition of this region was performed. Alanine-containing analogs of the sequence 1-7 and peptides mimicking the N-terminal of the four main toxins of AaH were probed with anti-toxin and anti-peptide antibodies. Lysine 2, aspartic acid 3 and glycine 4 were shown to be key residues in the recognition of the N-terminal region of the AaH II toxin by anti-toxin antibodies. In contrast, a loose specificity of recognition was shown by one anti-peptide serum which was, in addition, able to recognize the N-termini of all four AaH toxins.

Induction of antibody responses to breast carcinoma associated mucins using synthetic peptide constructs as immunogens

A strategy for directing and enhancing B cell immune responses against synthetic peptide determinants has been developed in order to produce antibodies specifically against protein epitopes of clinical relevance. A peptide sequence based upon the MUC-1 mucin protein core was selected for this purpose since anti-MUC-1 antibodies have proven diagnostic application and therapeutic potential in human breast and ovarian cancer. Peptide constructs were synthesised co-linearly linking the immunodominant B cell determinant region, PDTRPAP, in the protein core of the MUC-1 mucin, to sequence 111-120 of influenza haemagglutinin A/X-31, a determinant recognised by T helper cells through association with MHC class II molecules. Induction of anti-MUC-1 antibodies to the B cell determinant region by immunisation with peptide was shown to be dependent upon both the presence and the position of the T cell determinant. In addition, haplotype mismatching with respect to the T cell determinant resulted in a significant lowering of the anti-MUC-1 antibody response in peptide construct immunised mice. These findings are relevant to the design of immunogens to produce antibodies against peptide epitopes of tumour associated proteins and glycoproteins.

Development of an anti-idiotypic antibody reactive with an antibody defining the epitope RPAP in the MUC-1 epithelial mucin core

C595 is a murine IgG3 monoclonal antibody (MAb) raised against human urinary mucin. C595 antibody binds to the protein core of the MUC-1 mucin (the polymorphic epithelial mucin, PEM) which is elevated in tissue and secretions from human breast carcinomas. The antibody defines the tetrameric epitope, RPAP. In the present study, a syngeneic anti-idiotypic MAb, 911, was raised in BALB/c mice against the C595 of C595 to its mucin core antigen. In turn, pre-blocking of C595 with a 20 amino-acid mucin core peptide specifically inhibited binding of 911 antibody to C595. Cross-reactivity of antibody 911 was only observed against the IgM MAb, 789/91, which has the same minimum antibody-binding epitope as C595, namely, RPAP. Syngeneic and xenogeneic anti-sera against 911 antibody displayed increased binding to heptameric peptide sequences containing the motif, RPA(P). The anti-idiotypic antibody 911 therefore appears to recognize a site within or close to the binding site of the C595 antibody and thus carries an internal image of the parental mucin epitope. Consequently, the 911 idiotypic network offers a promising model system to investigate the mechanism of anti-idiotype-induced tumour immunity.

Principles and pitfalls in designing site-directed peptide ligands

An immunoglobulin light chain dimer with a large generic binding cavity was used as a host molecule for designing a series of peptide guest ligands. In a screening procedure peptides coupled to solid supports were systematically tested for binding activity by enzyme linked immunosorbent assays (ELISA). Key members of the binding series were synthesized in milligram quantities and diffused into crystals of the host molecule for X-ray analyses. These peptides were incrementally increased in size and affinity until they nearly filled the cavity. Progressive changes in binding patterns were mapped by comparisons of crystallographically refined structures of 14 peptide-protein complexes at 2.7 A resolution. These comparisons led to guidelines for ligand design and also suggested ways to modify previously established binding patterns. By manipulating equilibria involving histidine, for example, it was possible to abolish one important intramolecular interaction of the bound ligand and substitute another. These events triggered a change in conformation of the ligand from a compact to an extended form and a comprehensive change in the mode of binding to the protein. In dipeptides of histidine and proline, protonation of both imidazolium nitrogen atoms was used to program an end-to-end reversal of the direction in which the ligand was inserted into the binding cavity. Peptides cocrystallized with proteins produced complexes somewhat different in structure from those in which ligands were diffused into preexisting crystals. In such a large and malleable cavity, space utilization was thus different when a ligand was introduced before the imposition of crystal packing restraints.

Mapping of immunoreactive sites of pneumococcal pneumolysin by use of synthetic peptides

In the search for better diagnostic tools and improved vaccines against pneumococcal diseases, continuous antigenic sites of the pneumococcal pneumolysin molecule were identified with partially overlapping synthetic peptides. Peptides were derived from the predicted amino acid sequence according to the known DNA sequence of pneumolysin. The immunoreactivities of the peptides were studied by enzyme immunoassay with seven human serum samples and three rabbit hyperimmune serum samples. Two immunoreactive sites in the overlapping peptides at the amino-terminal end of the pneumolysin molecule were identified in this study. Two common immunoreactive sites for both human and rabbit sera were demonstrated.

Epitope specificity and capacity to inhibit parasite growth in vitro of human antibodies to repeat sequences of the Plasmodium falciparum antigen Ag332

It has earlier been shown that the Plasmodium falciparum-reactive human monoclonal antibody 33G2 inhibits parasite growth in vitro as well as cytoadherence of infected red blood cells to melanoma cells in vitro. MoAb 33G2 recognizes an epitope of the P. falciparum antigen Ag332 and cross-reactive determinants in Pf155/RESA and Pf11.1 located in repetitive regions containing sequences of regularly spaced pairs of glutamic acid. To study whether antibodies of this specificity frequently occur in human immune sera and if they could be of importance for protective immunity, antibodies were affinity purified on MoAb 33G2 reactive Ag332 peptides. The epitope specificity of the affinity purified antibodies, determined by the Pepscan method, resembled that of MoAb 33G2, but showed differences in fine specificity. The antibodies cross-reacted to some extent with Pf11.1 and Pf155/RESA repeat peptides as detected by peptide ELISA and Pepscan. In indirect immunofluorescence all purified antibodies displayed a dotted pattern of staining of late stage infected red blood cells of two lines of the P. falciparum strain FCR3, including a Pf155/RESA deficient line. The in vitro growth of these two lines was efficiently inhibited by the affinity purified antibodies, indicating that their inhibitory effect was mainly due to reactivity with antigens other than Pf155/RESA. This, and the fact that Pf11.1 has been shown not to be expressed by the asexual stages suggests that Ag332 may be an important target for potentially protective antibodies in vivo and that Ag332 based immunogens are of interest for development of malaria subunit vaccines.

Epitope mapping: synthetic approaches to the understanding of molecular recognition in the immune system

Progress in the field of immunochemistry is rapidly increasing due to very efficient methods of epitope mapping. Experimental results on the allele-specific sequence motifs of MHC-binding peptides allow the exact forecast of T-cell epitopes and, in combination with B-cell prediction methods and synthetic adjuvant systems, fully synthetic vaccines may be constructed. Methods of multiple peptide synthesis are of particular use for such constructs and for the fine mapping of monoclonal antibodies or sera of patients. Peptide libraries, containing hundred thousands of different oligopeptides are made available for novel screening procedures. These techniques and their applications in various fields are summarized and discussed with respect to efficiency and productivity.

Rheumatoid factors from patients with rheumatoid arthritis react with tryptophan 60 and 95, lysine 58, and arginine 97, on human beta 2-microglobulin

OBJECTIVE

To define precise epitopes on human beta 2-microglobulin (beta 2m) reacting with polyclonal IgM rheumatoid factors (RF) from patients with rheumatoid arthritis (RA).

METHODS

Ten polyclonal RF were tested for their human beta 2m epitope-binding specificities using the entire 99-amino acid sequence synthesized as overlapping 7-mers in an enzyme-linked immunosorbent assay. Glycine substitution for each residue within RF-reacting linear regions was employed to define major reactive sites.

RESULTS AND CONCLUSION

Major beta 2m residues contributing to RF reactivity were tryptophans at positions 60 and 95, lysine at 58, and arginine at position 97.

Multipin peptide synthesis at the micromole scale using 2-hydroxyethyl methacrylate grafted polyethylene supports

The multipin peptide synthesis procedure has been adapted to allow the synthesis of peptides at micromole loadings. The original solid pin support was replaced with a detachable crown-shaped polyethylene support with an increased surface area. In addition, the polyethylene crowns were radiation-grafted with 2-hydroxyethyl methacrylate monomer instead of acrylic acid to yield hydroxy functionalized supports with a larger concentration of polymer and hence a larger peptide capacity. Fmoc-beta-Alanine was directly esterified to the HEMA hydroxy groups with subsequent addition of a diketopiperazine-forming handle for peptide attachment. Peptides varying in length from 10 to 25 residues were assembled at a number of loadings from 1.0 to 2.2 mumol. Purity of peptides at all loadings was equal to, and in some instances superior to, that achieved on conventional solid-phase supports.

Gene mapping and expression of two immunodominant Epstein-Barr virus capsid proteins

The genomic localization of two immunodominant genes encoding two proteins of the Epstein-Barr virus capsid antigen (VCA) complex, VCA-p18 and VCA-p40, has been identified. For that purpose, lambda gt11-based cDNA libraries were constructed from HH514.c16 cells induced for virus production. The libraries were screened with a monoclonal antibody, EBV.OT41A, directed against VCA-p40 or with affinity-purified human antibodies against VCA-p18. Sequencing of the inserts of positive plaques showed that VCA-p18 and VCA-p40 are encoded within open reading frames (ORFs) BFRF3 and BdRF1, respectively. Peptide scanning analysis of the predicted protein of ORF BdRF1 resulted in defining the epitope of monoclonal antibody EBV.OT41A at the C-terminal region. The dominant VCA-p18 reactivity of human sera can be completely inhibited by preadsorption with Escherichia coli-expressed BFRF3-beta-galactosidase. Serum of a rabbit immunized with BFRF3-beta galactosidase reacts with a VCA-specific protein of 18 kDa. In addition, BFRF3-beta-galactosidase affinity-purified antibodies react with VCA-p18 of virus-producing cells (HH514.c16). Complete inhibition of viral DNA polymerase activity by phosphonoacetic acid is associated with the absence of RNAs and protein products of both ORFs, indicating that VCA-p18 and VCA-p40 are true late antigens.

Antibody epitope mapping of the gastric H+/K(+)-ATPase

Several antibodies against the gastric H+/K(+)-ATPase were analysed for the topological and sequence location of their epitopes. Topological mapping was done by comparing indirect immunofluorescent staining in intact and permeabilised rat parietal cells. Epitope definition was by Western analysis of intact and of trypsin or V8-proteinase-fragmented hog gastric ATPase combined with N-terminal sequencing of the fragments; by Western analysis of fragments of rabbit alpha subunit expressed in Escherichia coli; by analysis of rabbit alpha and beta subunits expressed in baculovirus-transfected SF 9 cells and by ELISA assay of synthetic octamers of one region of the hog alpha subunit. It was confirmed that the monoclonal antibody, mAb 95-111, recognised a cytoplasmic region between M4 and M5, close to the ATP-binding domain. The major epitope for monoclonal antibody mAb 12-18 was also in this region, but a second epitope was confirmed to be present in the M7/M8 region. The monoclonal antibody, mAb 146-14, was shown to recognise an extracytoplasmic epitope dependent on intact disulfide bonds, present in the rat and the rabbit, but absent in the hog beta subunit, due to non-conservative amino-acid substitutions. This antibody also recognised an epitope present in the alpha subunit of the H+/K(+)-ATPase at the M7 extracytoplasmic interface, perhaps indicating structural association of these two regions. The polyclonal antibody, pAb39, raised against the C-terminal portion of the enzyme, reacted only with the cytoplasmic surface of the H+/K(+)-ATPase, showing that the alpha subunit of the enzyme has an even number of membrane spanning segments.

Epitope mapping of histone 5 (H5) with systemic lupus erythematosus, procainamide-induced lupus and hydralazine-induced lupus sera

To define the linear epitopes on H5 that react with systemic lupus erythematosus (SLE) and drug-induced lupus (DIL) sera, concurrent overlapping hexameric peptides corresponding to the sequence of H5 were synthesized by stepwise elongation of the polypeptide chains on polyethylene supports. The hexapeptides were tested for reactivity with 8 SLE and 8 DIL sera using an enzyme linked immunosorbent assay (ELISA). SLE and hydralazine-induced lupus (HIL) antibodies were most reactive with peptide 45 (SSRQSI) and patients with procainamide-induced lupus (PIL) were most reactive with peptide 24 (SHPTYS). The epitopes of highest reactivity were in the globular domain of H5. Low reactivity was observed with carboxyl terminal peptides. These findings differ from immunoblotting studies of protease cleaved peptides which have previously shown that the H5 determinants are in the carboxyl terminus.

Epitope analysis using kinetic measurements of antibody binding to synthetic peptides presenting single amino acid substitutions

The fine modulation of peptide-antibody interactions was investigated with anti-peptide monoclonal antibodies recognizing peptide 125-136 of the coat protein of tobacco mosaic virus. Nine synthetic peptides presenting single amino acid substitutions were selected for detailed analysis on the basis of their reactivity in ELISA. Kinetic measurements of the binding of four antibodies to these peptides performed with a biosensor instrument (BIAcore, Pharmacia) were used to quantify the contribution of individual residues to antibody binding. The results showed that even conservative exchanges of some residues in the epitope resulted in a small but significant decrease of the equilibrium affinity constant due mostly to a higher dissociation rate constant of the monoclonal antibodies. Two amino acid residues directly adjacent to the epitope, which appeared to play no role when tested by ELISA, were shown to influence the kinetics of binding. These data should be useful for computer modelling of the peptide-antibody interactions.

A model of the rabies virus glycoprotein active site

The glycoprotein from the neurotropic rabies virus shows a significant homology with the alpha neurotoxin that binds to the nicotinic acetylcholine receptor. The crystal structure of the alpha neurotoxins suggests that the Arg 37 guanidinium group and the Asp 31 side-chain carboxylate of the erabutoxin have stereochemical features resembling those of acetylcholine. Conformational studies on the Asn194-Ser195-Arg196-Gly197 tetrapeptide, an essential part of the binding site of the rabies virus glycoprotein, indicate that the side chains of Asn and Arg could also mimic the acetylcholine structure. This observation is consistent with the recently proposed mechanism of the viral infection.

Rheumatoid factors from patients with rheumatoid arthritis react with Des-Lys58-beta 2m, modified beta 2-microglobulin

Ten polyclonal IgM rheumatoid factor (RF) preparations, affinity-purified from IgG columns, from patients with rheumatoid arthritis were studied for their ELISA reactivity with native beta 2m in parallel with Lys58-cleaved beta 2m and Des-Lys58-beta 2m, the latter representing cleavage products of the native molecule present in some pathologic human sera. Most RF showed positive reactions with the native form of beta 2m but reduced reactivity for the cleaved forms of beta 2m. Reactions between cleaved beta 2m and RF, in solution, were demonstrated by inhibition of RF binding to native beta 2m by preincubation with a range of concentrations of Des-Lys58-beta 2m. By contrast, eight of nine murine MoAbs to human beta 2m showed approximately equivalent binding to native beta 2m, Lys58-cleaved beta 2m, and Des-Lys58-beta 2m. Reactions between individual human RF and the altered forms of beta 2m (Lys58-cleaved beta 2m and Des-Lys58-beta 2m) appeared to parallel the previously determined beta 2m single amino acid specificities, in that RF showing strong reactivity with Lysine 58 also showed a significant diminished reactivity with the Des-Lys58-beta 2m lacking the critical lysine residue. The present studies demonstrate that while human RF react with Lys58-cleaved beta 2m or Des-Lys58-beta 2m, preferential reactivity is observed for native unaltered beta 2m.

The use of T bag synthesis with paper discs as the solid phase in epitope mapping studies

Epitope mapping with synthetic peptides bound to derivatized paper discs has been investigated using the discs both as a solid-phase matrix for peptide synthesis as well as the solid phase in immunologic testing procedures without detachment of the peptides from the paper discs. Using the T bag (tea bag) method the simultaneous synthesis and subsequent immunologic testing of large numbers of peptides was demonstrated for the feline major allergen Fel d I. A total of 15,000 paper disc-bound peptides, comprising the 146 nonapeptides overlapping by eight amino acid residues on both chains, were synthesized simultaneously with 100 paper discs per T bag. Using these paper disc-bound peptides as the solid phase in radioimmunoassays the binding sites found coincided with those detected in the PEPSCAN with the commercially available epitope mapping kit and with the binding sites that had been found with Sepharose-coupled peptides. The signal to background-ratio in the paper disc-RIA was comparable to that in the PEPSCAN and the reproducibility was good. The bound antibodies could be eluted from the paper disc-bound peptides, permitting regeneration and repeated use of the paper discs for immunologic testing. This method was shown to be a useful alternative to the PEPSCAN and to have the major advantage that large numbers of antibodies could be tested with large numbers of peptides simultaneously.

Common peptide epitope in mumps virus nucleocapsid protein and MHC class II-associated invariant chain

The present study describes a 7 amino acid-long sequence (YQQQGRL) which is identical in HLA-associated invariant chain and mumps virus nucleocapsid protein and is additionally followed by one conservative amino acid pair. As such a long amino acid homology is extremely rare in two evolutionarily unrelated proteins the possibility that it could induce immunological cross-reactivity was evaluated. Several antigenicity indices suggested high antigen potential within this region. Synthetic peptides containing this sequence were reactive with 31% of monoclonal antibodies specific for mumps virus nucleocapsid protein in ELISA. High antibody levels against this epitope were found in 7% of mumps-seropositive human sera and antibody levels clearly increased after natural mumps infections and mumps vaccinations. Rabbit antibodies raised against a synthetic invariant chain peptide AYF-LYQQQGRLDKL-C reacted with corresponding nucleocapsid peptide RFAKYQQQGRLEAR-C and antibodies against the nucleocapsid peptide reacted with the invariant chain peptide. Rabbit antibodies against the invariant chain peptide also reacted with nucleocapsid molecules in formaldehyde-fixed mumps virus-infected cells, and antibodies against the nucleocapsid peptide reacted with invariant chains expressed in methanol-fixed cells. One monoclonal antibody specific for the nucleocapsid molecule also reacted with cells expressing invariant chains. In immunoprecipitation rabbit antibodies against the invariant chain peptide bound to invariant chains while antibodies against the nucleocapsid peptide did not. The results suggest that there is antigenic similarity in mumps virus nucleocapsid molecule and HLA-associated invariant chain which may cause immunological cross-reactivity between these molecules.

Differences in the antibody response to human immunodeficiency virus-1 envelope glycoprotein (gp160) in infected laboratory workers and vaccinees

Studies of the immune response to the human immunodeficiency virus (HIV) have been hampered by the antigenic diversity of the HIV envelope protein. In an effort to predict the efficacy of vaccination we have compared the systemic anti-envelope antibody response in seronegative volunteers immunized with recombinant gp160 (either in vaccinia or as soluble protein produced in baculovirus) derived from the HTLV-IIIB strain of HIV-1 and in two laboratory workers accidentally infected with the same strain. 11 of 14 vaccinees responded to immunization by producing anti-gp160 of similar titer and the same isotype as that seen in the laboratory workers. Four vaccinees also had antibody to the principal neutralizing domain (V3 loop) that was comparable in titer with that seen in the laboratory workers, but the fine specificity of anti-V3 antibody was qualitatively different in the two groups. Antibody that can block the interaction between CD4 and gp120 was present at comparable levels in three vaccines and the lab workers. Neutralizing antibody titers were markedly lower in the vaccinees than in the laboratory workers. In seven of the vaccinees, an immunodominant epitope was at amino acid 720-740. Analyses of monoclonal antibodies to this region indicate that they do not neutralize, bind to infected cells, nor function as immunotoxins. Although the anti-gp160 antibody response was of similar magnitude in both infected and vaccinated individuals, there were important qualitative differences.

Three-dimensional structure of a human immunoglobulin with a hinge deletion

X-ray analysis at 3.2-A resolution revealed that the Mcg IgG1 (lambda chain) immunoglobulin is a compact T-shaped molecule. Because of the hinge deletion, the Fc fragment lobe is pulled tightly upward into the junction of the Fab arms. Along the molecular twofold axis, the Fab arms are joined by an interchain disulfide bond between the two light chains. The antigen combining sites consist of large irregular cavities at the tips of the Fab regions. Potential complement (C1q) binding sites on Fc are sterically shielded by the Fab arms, but putative attachment sites are accessible for docking with the FcRI receptor on human monocytes and with protein A of Staphylococcus aureus.

Identification of antigenic epitopes in an alanine-rich repeating region of a surface protein antigen of Streptococcus mutants

A surface protein antigen (PAc) of Streptococcus mutans with a molecular mass of 190 kDa is considered to play an important role in the initial attachment of this streptococcus to the tooth surface. Two internal repeating amino acid sequences are present in the PAc molecule. One repeating region located in the N-terminal region is rich in alanine (A-region), and the other, located in the central region, is rich in proline (P-region). To identify antigenic epitopes on the A-region of the PAc protein, 82 sequential overlapping synthetic decapeptides covering one of the repetitive units of the A-region were synthesized. In the epitope scanning analyses using murine antisera raised against recombinant PAc (rPAc), multiple antigenic epitopes were found in the repetitive unit of the A-region, and some of them reacted with antisera to rPAc from BALB/c, B10, B10.D2, and B10.BR mice. In particular, a peptide YEAALKQY (residues 366 to 373) was recognized by anti-rPAc sera from all four strains of mice. The reactivities of anti-rPAc sera in the epitope scanning were confirmed by using a purified synthetic peptide, NAKATYEAALKQYEADLAA (corresponding to residues 361 to 379). Furthermore, antisera against a surface protein antigen PAg (SpaA) of Streptococcus sobrinus from BALB/c mice reacted strongly to residues 330 to 337, 362 to 369, and 366 to 373 of the PAc protein by the epitope scanning analysis. An AKATYEAALKQY (residues 362 to 373 of the PAc protein)-like sequence, AKANYEAKLAQY, was found within the A-region of S. sobrinus PAg, suggesting that the amino acid sequences AKA-YEA and YEA-L-QY may be major cross-reactive epitopes of the S. mutans PAc protein and the S. sobrinus PAg protein.

Functional importance of amino acid residues making up peptide antigenic determinants

The functional importance of each amino acid residue making up the antigenic determinants of three different peptide-mAb interactions was determined using complete series of substitution analogs of the three immunizing synthetic peptides. Fingerprint substitution profiles for the three different antigenic determinants were obtained separately by direct and competitive ELISA. Competitive ELISA was found to offer the advantage of being able to measure the concn of each peptide substitution analog necessary to inhibit antibody binding to the original peptide. In this manner, the relative functional contribution to antibody binding of each amino acid residue making up the antigenic determinant was determined and termed the relative positional importance factor (RPIF). Each antigenic determinant was found to contain one very highly specific residue (i.e., highest RPIF) that was, on average, the least replaceable with any of the natural L-amino acids (the average decrease in recognition ranged 250- to 28,000-fold). At the other extreme, two or three positions in each antigenic determinant were found to be only weakly involved in recognition. These positions were considered redundant since the average decrease in recognition of the substitution analogs for these positions was found to be 20-fold or less. The remaining antigenic determinant residues exhibited the fine specificity common to antigen-antibody interactions in that only relatively conservative substitutions for these residues were recognized by their respective antibodies. It is of interest that the positional arrangement of specific and nonspecific residues were different for each of the three antigenic determinants examined.

Characterization of a neutralizing monoclonal antibody directed at variable domain I of the major outer membrane protein of Chlamydia trachomatis C-complex serovars

A monoclonal antibody (MAb), C10, that neutralized in vitro the infectivity of serovars C, I, J, and L3 (members of the C and C-related complexes) of Chlamydia trachomatis was identified. Of the 15 major serovars and the mouse pneumonitis strain of C. trachomatis, Chlamydia psittaci, and Chlamydia pneumoniae, which were used as nontreated and heat-treated (56 degrees C, 30 min) antigens in a dot blot assay, only serovars C, I, J, and L3 were recognized with both the native and treated antigens. Western blot (immunoblot) results showed that MAb C10 recognized the major outer membrane protein of these four serovars. Overlapping hexameric peptides corresponding to variable domains (VDs) I, II, III, and IV of the major outer membrane protein of C. trachomatis serovar C were synthesized, and peptide screening showed that MAb C10 mapped to the VD I amino acid sequence VAGLQNDPT. Results of an in vitro neutralization assay correlated with those of the indirect immunofluorescence assay, Western blot, and dot blot assay in that only serovars C, I, J, and L3 were neutralized by MAb C10. In vitro competitive neutralization experiments, using a peptide representing VD I of serovar C to compete with C. trachomatis serovar C for MAb C10 binding, revealed that both serological and neutralizing activities of MAb C10 were inhibited by the VD I peptide. In an in vivo toxicity/infectivity assay using serovar L3 pretreated with MAb C10, there was 100% survival of mice infected with a lethal dose at 48 h. In contrast, the control group, consisting of mice injected with the same dose of L3 pretreated with a MAb that does not recognize L3, had no survivors during a 48-h observation period. In summary, since the surface-exposed contiguous epitope recognized by MAb C10 binds neutralizing antibodies that are subspecies specific for the C and C-related complexes, it should be considered for inclusion in the development of a chlamydial vaccine.

Epitope mapping using bacteriophage peptide libraries

Vast libraries of random peptides displayed on the surface of bacteriophage potentially allow identification of specific ligands for any peptide receptor of interest. The sequence specificity of antibody-peptide interactions allows these libraries to be used to define and localize continuous epitopes. Remarkably clear-cut results have been obtained with some antibodies; however, the technique has not proved to be generally applicable.

Using a neural network to identify potential HLA-DR1 binding sites within proteins

The presentation by antigen-presenting cells of immunodominant peptide segments in association with major histocompatibility complex (MHC) encoded proteins is fundamental to the efficacy of a specific immune response. One approach used to identify immunodominant segments within proteins has involved the development of predictive algorithms which utilize amino acid sequence data to identify structural characteristics or motifs associated with in vivo antigenicity. The parallel-computing technique termed 'neural networking' has recently been shown to be remarkably efficient at addressing the problem of pattern recognition and can be applied to predict protein secondary structure attributes directly from amino acid sequence data. In order to examine the potential of a neural network to generalize peptide structural features related to binding within class II MHC-encoded proteins, we have trained a neural network to determine whether or not any given amino acid of a protein is part of a peptide segment capable of binding to HLA-DR1. We report that a neural network trained on a data base consisting of peptide segments known to bind to HLA-DR1 is able to generalize features relating to HLA-DR1-binding capacity (r = 0.17 and p = 0.0001).

Continuous B-cell epitopes in Chlamydia trachomatis heat shock protein 60

B-cell peptide epitopes in chlamydial heat shock protein 60 (hsp60) were elucidated with antisera from 13 rabbits immunized with Chlamydia trachomatis serovars B, C, and L2 and antisera from eight women with C. trachomatis-associated ectopic pregnancies. Thirteen major epitopes were identified with the human sera, 10 of which were also observed with rabbit antisera. Seven of the 13 epitopes recognized by human antisera exhibited cross-reactive antibody binding to homologous peptide sequences in human hsp60. Self-reactive B-cell immunity to hsp60 may contribute to chlamydial disease pathogenesis.

In vitro gene expression for the localization of antigenic determinants: application to the E. coli tryptophan synthase beta 2 subunit

Gene expression of the beta subunit of E. coli tryptophan synthase in an E. coli cell-free transcription-translation system proceeds by pauses and produces a discrete but quite continuous pattern of nascent chains starting from the N terminus and ranging in size up to the 44 kDa end product corresponding to the completed beta chains. Using specific immunoadsorption of [35S]Met radiolabelled nascent chains by different monoclonal antibodies directed against the beta 2 subunit of E. coli tryptophan synthase, the size of the smallest N-terminal fragment reacting with each antibody has been determined by SDS electrophoretic analysis of the immunoadsorbed polypeptides. The immunoadsorption assay is performed in solution under conditions avoiding the usual drawbacks of solid phase immunoassay. This approach, in combination with the results obtained with a DNA fragment library permitted us to localize the antigenic determinants recognized by the monoclonal antibodies. The proposed method could help to localize rapidly the C-terminal boundary of an epitope, before starting systematic and precise mapping by other approaches. Moreover, the method described may be of general interest for the rapid production of a large set of C-terminal truncated polypeptides for studies of antigen-antibody recognition.

Mapping antibody binding sites on cytochrome c with synthetic peptides: are results representative of the antigenic structure of proteins?

Crystallographic work on antigen-antibody complexes has revealed that extensive surface areas of proteins may interact with antibodies. On the other hand, most experimental approaches to locate and define antigenic determinants of protein antigens rely on the linear sequence of the polypeptide chain. Hence the question arises whether mapping of antibody binding sites by analysis of the reactivity of anti-protein antibodies with synthetic peptides can provide a representative picture of the antigenic structure of a protein antigen. We have addressed this question using yeast iso-1 cytochrome c as a protein antigen against which antisera were raised in rabbits. The reaction of the antisera with 103 synthetic hexapeptides covering the entire sequence of cytochrome c was tested by the pepscan procedure in which peptides are coupled to polyethylene rods and tested by ELISA. For the assay, anti-cytochrome c antibodies were fractionated by affinity chromatography on native yeast iso-1 cytochrome c and on apo-cytochrome c; the latter is a random coil. It was found that only antibodies retained by the apo-cytochrome c affinity column react with synthetic peptides. These antibodies comprise a small fraction, probably less than 2%, of all cytochrome c-specific antibodies. The majority of antigenic determinants, which seem to consist of strongly conformation-dependent topographic epitopes, could not be uncovered by the peptide approach. Epitope mapping with short peptides seems of limited usefulness in the case of small, globular, and conformationally stable proteins like cytochrome c.

Monoclonal antipeptide antibodies: affinity and kinetic rate constants measured for the peptide and the cognate protein using a biosensor technology

The interaction of antipeptide antibodies with the corresponding peptide and the cognate protein has been compared using a novel biosensor technology (BIAcore, Pharmacia). The peptide corresponds to residues 110-135 of the coat protein of tobacco mosaic virus, known to encompass an alpha-helical region reactive with antiprotein antibodies. A panel of 33 monoclonal antibodies raised against the peptide was studied and the epitope recognized by these antibodies was determined by the pepscan method. Further discrimination between the antibodies was performed by measurements of association and dissociation kinetic constants. Several antibodies showed an heterogeneous binding profile when reacting with the 25 residue long peptide but not with a shorter 10 residue peptide suggesting that they recognized different conformational states in the longer peptide. Equilibrium affinity constants were calculated for five of the antibodies and were found to be 10-50 times higher for the peptide than for the protein, the difference being caused mainly by a lower association rate constant.

Effects of antibody isotype and host cell type on in vitro neutralization of Chlamydia trachomatis

Monoclonal antibodies (MAbs) E-4, E-21, and DIII A3, which recognize the same or similar overlapping peptides in the variable domain IV of the major outer membrane protein of Chlamydia trachomatis but differ in isotype, were used in a complement-independent (CI) in vitro neutralization assay. These MAbs had previously been shown to neutralize chlamydial infectivity in HeLa 229 cells in a complement-dependent assay. In this report, all three MAbs neutralized chlamydial infectivity in HaK cells in a CI assay. However, when HeLa cells were used as the host cell, MAb E-4 (immunoglobulin G2b [IgG2b]) and MAb DIII A3 (IgG2b) failed to neutralize infectivity, while MAb E-21 (IgG1) neutralized chlamydial infectivity. These findings are consistent with the proposal that because of the presence of Fc gamma RIII receptors, HeLa cells facilitate infectivity and thus block neutralization through the uptake of an IgG2b-chlamydia complex. Since Fc gamma RIII receptors do not bind or bind poorly to IgG1, neutralization of C. trachomatis by MAb E-21 in HeLa cells is also corroborative evidence for the role of Fc gamma RIII receptors in this interaction. A fivefold enhancement of infectivity was seen when 10 and 1 micrograms of MAb E-4 per ml were tested in a CI assay with HeLa cells. In performing CI neutralization synergy studies in HeLa cells with MAbs E-4 and E-21, antagonism between MAbs E-4 and E-21 was observed at MAb E-4 concentrations of 10 and 1 micrograms/ml for all concentrations of MAb E-21 tested (10 to 0.1 micrograms/ml). When HaK cells were used in the same studies, no antagonism between the MAbs was found. In addition, when HeLa cells were used in a CI assay, polyclonal serum raised to a peptide representing variable domain IV of the major outer membrane protein inhibited the neutralizing ability of MAb E-21. The blocking of neutralization and the enhancement of infectivity by chlamydia-specific antibodies seen in this investigation with HeLa cells may have important clinical implications for developing preventive strategies for chlamydial infections.

Immunogenicity of a synthetic oligopeptide corresponding to antigenically common T-helper and B-cell neutralizing epitopes of the major outer membrane protein of Chlamydia trachomatis

Sexually transmitted diseases (STDs) caused by Chlamydia trachomatis are an important public health problem and a vaccine to prevent or control these diseases is badly needed. The major outer membrane protein (MOMP) is the principal candidate antigen for the development of subunit vaccine against chlamydial STDs. The immunogenicity of a synthetic oligopeptide, termed A8-VDIV, corresponding to MOMP sequences containing both C. trachomatis species common T-helper (A8) and B-cell (VDIV) epitopes was studied in mice and non-human primates. Six of eight H-2 congenic mouse strains immunized with peptide A8-VDIV produced high-titre IgG antibodies against the VDIV B-cell portion of the oligopeptide. Fine mapping of the anti-peptide antibodies by pepscan ELISA showed that each of the responding mouse strains made antibodies reactive with a species-common septmeric neutralizing epitope 298LNPTIAG304 contained in the VDIV sequence. The mouse anti-peptide antibodies reacted with intact C. trachomatis elementary bodies (EBs) by ELISA and neutralized chlamydial infectivity for cultured eukaryotic cells with sub-species specificity. Three cynomolgus monkeys were immunized with peptide A8-VDIV and their IgG antibody responses were similarly studied. All three monkeys produced IgG antibodies which reacted with the VDIV peptide and which recognized the species-common LNPTIAG neutralizing site within the VDIV sequence. Monkey anti-peptide antibodies bound to intact C. trachomatis EBs and were neutralizing in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Monoclonal IgM rheumatoid factors generated from synovial B cells of rheumatoid arthritis patients react with beta 2-microglobulin. Monoclonal RF react with beta 2m

Four of 15 monoclonal human IgM rheumatoid factors (RF) derived from synovial B cells of patients with rheumatoid arthritis showed positive ELISA reactions with human beta 2-microglobulin. These findings were different from those previously noted using IgM RF derived from monoclonal Waldenstrom's paraproteins or the IgM components of mixed cryoglobulins, and resembled the anti-beta 2 microglobulin specificity of polyclonal IgM RF from patients with rheumatoid arthritis. Reactions of monoclonal IgM synovial RF with overlapping 7-mers of beta 2m sequence indicated major regions of positive reactivity at positions 57-64 and 89-95 which were maintained in the presence of high salt (300 mM NaCl) conditions. Glycine substitution of each residue within RF-reactive beta 2m regions indicated that tryptophanes at position 60 and 95, lysine at 58, phenylalanine at 62, valine at 93 and arginine at 97 constituted important single amino acids for the reactive epitopes. These findings indicate that clonally restricted human IgM RF derived from diseased tissues of patients with RA show anti-beta 2m reactivity similar to polyclonal RF from the same patients. This particular fine specificity is not present in monoclonal RF derived from patients with Waldenstrom's or mixed cryoglobulins showing anti-gamma-globulin activity.

Fine specificity of antibody recognition of carcinoma-associated epithelial mucins: antibody binding to synthetic peptide epitopes

The protein core of polymorphic epithelial mucins consists predominantly of a repeating 20 amino acid peptide motif. Many monoclonal antibodies reactive with breast carcinomas recognise determinants located within the mucin protein core, and epitope mapping techniques have demonstrated that these antibodies bind to epitopes of three, four or five amino acids within the hydrophilic sequence, P D T R P A P. Each of these mucin core-reactive antibodies map to epitopes containing the central arginine residue. The fine specificity of a panel of anti-mucin antibodies binding to the tetrameric peptides P D T R or R P A P (synthesised on the heads of polyethylene pins) was examined by systematically replacing each amino acid in turn with all other 19 natural amino acids, and then testing these analogues for antibody binding. We have (i) identified those amino acids in epitopes which are essential for antibody binding, (ii) shown that for each epitope there is a hierarchy of residues required for immune recognition--certain amino acids may be replaced with little or no loss of antibody binding, while the presence of others is essential, and (iii) concluded that antibody specificity is further regulated by the residue(s) flanking an epitope motif which may impose conformational constraints upon the presentation of the epitope to an antibody.

Cross-reactivity between antibodies to thyroid microsomal antigens and myeloperoxidase

Antibodies against thyroid microsomal antigen (thyroid peroxidase, TMA/TPO) and myeloperoxidase (MPO) were measured from 115 patients with vasculitic disorders and 144 patients with suspected thyroid disorders. Nineteen patients, three with vasculitis and 16 with thyroid disorders, were shown to have both TPO and MPO antibodies, suggesting cross-reactivity of these antibodies. Their cross-reactivity was further strengthened by studying the capacity of antibodies to tolerate dilution in enzyme immunoassay and reactivity with synthetic TPO/MPO peptides.

A monoclonal anti-peptide antibody reacting with the insulin receptor beta-subunit. Characterization of the antibody and its epitope and use in immunoaffinity purification of intact receptors

A mouse monoclonal antibody (CT-1) was prepared against the C-terminal peptide sequence of the human insulin receptor beta-subunit (KKNGRILTLPRSNPS). The antibody reacted with native human and rat insulin receptors in solution, whether or not insulin was bound and whether or not the receptor had undergone prior tyrosine autophosphorylation. The antibody also reacted specifically with the receptor beta-subunit on blots of SDS/polyacrylamide gels. Preincubation of soluble receptors with antibody increased the binding of 125I-insulin approx. 2-fold. The antibody did not affect insulin-stimulated autophosphorylation, but increased the basal autophosphorylation rate approx. 2-fold. The amino acid residues contributing to the epitope for CT-1 were defined by construction and screening of an epitope library. Oligonucleotides containing 23 random bases were synthesized and ligated into the vector pCL627, and the corresponding peptide sequences expressed as fusion proteins in Escherichia coli were screened by colony blotting. Reactive peptides were identified by sequencing the oligonucleotide inserts in plasmids purified from positive colonies. Six different positive sequences were found after 900,000 colonies had been screened, and the consensus epitope was identified as GRVLTLPRS. Phosphorylation of the threonine residue within this sequence (corresponding to the known phosphorylation site Thr-1348 in the insulin receptor) decreased the affinity of antibody binding approx. 100-fold, as measured by competition in an e.l.i.s.a. Antibody CT-1 was used for immunoaffinity isolation of insulin receptor from detergent-solubilized human placental or rat liver microsomal membranes. Highly purified receptor was obtained in 60% yield by binding to CT-1-Sepharose immunoadsorbent and specific elution with a solution of peptide corresponding to the known epitope. This approach to purification under very mild conditions may in principle be used with any protein for which an antibody is available and for which a peptide epitope or 'mimotope' can be identified.

Three-dimensional structure of an Fv from a human IgM immunoglobulin

An IgM(kappa) immunoglobulin from a patient (Pot) with Waldenstrom's macroglobulinemia was hydrolyzed with pepsin to release a fragment consisting of the 'variable' (V) domains of the light and heavy chains plus eight residue 'tails' from the 'constant' (C) domains. The crystal structure of this fragment was determined at 2.3 A resolution by molecular replacement and crystallographic refinement methods. When examined separately, the light chain component closely resembles another human kappa chain (Rei) in both the beta-pleated sheet regions and the 'hypervariable' loops. The conserved pleated sheets in the heavy chain are similar to those in the human Kol IgG1 protein, but the third hypervariable loop in particular is different from that in any immunoglobulin structure described to date. As in the Kol protein, this loop blocks the access to any internal active site along the light-heavy chain interface. Unlike the Kol protein, however, the loop does not protrude beyond the boundaries of a conventional antigen combining site. Instead, it forms a very compact structure, which fills almost all residual space between the domains. This is an example of one dominant complementarity-determining region (CDR) essentially negating the diversity possible with five other CDRs in the two chains. Ordered water molecules are associated with light chain constituents along the interface, but not with CDR3 of the heavy chain. In screening exercises the Pot IgM failed to bind a wide variety of peptides. Together, the results suggest that ligand binding can only occur on external surfaces of the protein. These surfaces carry a limited number of side chains usually assigned to CDRs in more typical antibodies.