Key residues of a major cytochrome P4502D6 epitope are located on the surface of the molecule

Eukaryotically expressed CYP2D6 is the universal target of liver kidney microsomal Ab type 1 (LKM1) in both type 2 autoimmune hepatitis (AIH) and chronic hepatitis C virus (HCV) infection. In contrast, reactivity to prokaryotically expressed CYP2D6 protein and synthetic peptides is significantly lower in HCV infection than in AIH. The aim of the present study was to characterize LKM1 reactivity against a panel of eukaryotically expressed CYP2D6 constructs in the two conditions. LKM1-positive sera obtained from 16 patients with AIH and 16 with HCV infection were used as probes to perform a complete epitope mapping of CYP2D6. Reactivity to the full-length protein and 16 constructs thereof was determined by radioligand assay. We found that antigenicity is confined to the portion of the molecule C-terminal of aa 193, no reactivity being detectable against the aa sequence 1-193. Reactivity increases stepwise toward the C-terminal in both AIH and HCV, but the frequency of reactivity in the two conditions differs significantly between aa 267-337. To further characterize this region, we introduced a five and a three amino acid swap mutation selected from the homologous regions of CYP2C9 and HCV. This maneuver resulted in a substantial loss of LKM1 binding in both conditions, suggesting that this region contains a major epitope. Molecular modeling revealed that CYP2D6(316-327) is exposed on the surface of the protein, and may represent a key target for the autoantibody. These findings provide an initial characterization of the antigenic constitution of the target of LKM1 in AIH and HCV infection.

Induction of autoantibody production is limited in nonautoimmune mice

Many individuals develop a single or a few brief episodes of autoimmunity from which they recover. Mechanisms that quell pathologic autoimmunity following such a breakdown of self-tolerance are not clearly understood. In this study, we show that in nonautoimmune mice, dsDNA-specific autoreactive B cells exist but remain inactive. This state of inactivation in dsDNA-specific B cells could be disrupted by autoreactive Th cells; in this case T cells that react with peptides from the V(H) region of anti-DNA Abs (hereafter called anti-V(H) T cells). Immunization with anti-DNA mAb, its gamma-chain or peptides derived from its V(H) region induced anti-V(H) Th cells, IgG anti-dsDNA Ab, and proteinuria. The breakdown of B cell tolerance in nonautoimmune mice, however, was short-lived: anti-DNA Ab and nephritis subsided despite subsequent immunizations. The recovery from autoimmunity temporally correlated with the appearance of T cells that inhibited anti-DNA Ab production. Such inhibitory T cells secreted TGFbeta; the inhibition of anti-DNA Ab production by these cells was partly abolished by anti-TGFbeta Ab. Even without immunization, nonautoimmune mice possess T cells that can inhibit autoantibody production. Thus, inhibitory T cells in nonautoimmune mice may normally inhibit T-dependent activation of autoreactive B cells and/or reverse such activation following stimulation by Th cells. The induction of such inhibitory T cells may play a role in protecting nonautoimmune mice from developing chronic autoimmunity.

Two distinctly HLA-associated contiguous linear epitopes uniquely expressed within the islet antigen 2 molecule are major autoantibody epitopes of the diabetes-specific tyrosine phosphatase-like protein autoantigens

The related tyrosine phosphatase-like proteins islet Ag (IA)-2 and IA-2beta are autoantigens of type 1 diabetes in humans. Autoantibodies are predominantly against IA-2, and IA-2-specific epitopes are major autoantibody targets. We used the close homology of IA-2 and IA-2beta to design chimeras and mutants to identify humoral IA-2-specific epitopes. Two major IA-2 epitopes that are absent from the related autoantigens IA-2beta and IA-2Delta 13 splice variant ICA512.bdc were found contiguous to each other within IA-2 juxtamembrane amino acids 611-620 (epitope JM1) and 621-630 (epitope JM2). JM1 and JM2 are recognized by sera from 67% of patients with IA-2 Abs, and relatives of patients with type 1 diabetes having Abs to either JM epitope had a >50% risk for developing type 1 diabetes within 6 years, even in the absence of diabetes-associated HLA genotypes. Remarkably, the presence of Abs to one of these two epitopes was mutually exclusive of the other; JM2 Abs and not JM1 Abs were found in relatives with HLA DR3/4, DR4/13, or DR1/4 genotypes; and the binding of autoantibodies to the JM2 epitope, but not the JM1 epitope, markedly affected proteolysis of IA-2. This is a unique demonstration of HLA-associated B cell responses to epitopes within a single autoantigen in humans and is consistent with modification of Ag processing by specific Ab-influencing peptide presentation by HLA molecules.

Molecular characterization of weak D phenotypes by site-directed mutagenesis and expression of mutant Rh-green fluorescence protein fusions in K562 cells

BACKGROUND AND OBJECTIVES

Mutations detected in 161 weak D samples from Caucasians have been classified into 16 types. Because flow cytometry using monoclonal anti-D antibodies (mAbs) has shown that weak D red cells display type-specific antigen density, these mutations in transmembranous regions have been assigned weak D phenotypes. The present study attempts to confirm or refute this assignment.

MATERIALS AND METHODS

We amplified DNA from four Japanese weak D samples using the polymerase chain reaction (PCR), and directly sequenced the amplified DNA. Using site-directed mutagenesis, we constructed three vectors expressing mutant RHDs-- G212C, V270G (weak D type 1) and G358A (type 2)--in K562 cells. The expression of RhD antigens was examined by flow cytometry using mAbs.

RESULTS

A new mutation resulting in a conversion at amino acid residue 212 (Gly to Cys) was detected in a Japanese weak D sample. K562 cells transduced with mutant RhD cDNA reacted weakly in a type-specific manner with mAbs.

CONCLUSIONS

The mutations--G212C (new weak D type), V270G (weak D type 1) and G358A (type 2)-- in transmembranous regions had obvious effects on the D epitopes recognized by mAbs. The results of this study provide direct evidence that these mutations can account for weak D phenotypes.

Mutations of an epitope hot-spot residue alter rate limiting steps of antigen-antibody protein-protein associations

The antibodies, HyHEL-10 and HyHEL-26 (H10 and H26, respectively), share over 90% sequence homology and recognize with high affinity the same epitope on hen egg white lysozyme (HEL) but differ in degree of cross-reactivity with mutant lysozymes. The binding kinetics, as measured by BIAcore surface plasmon resonance, of monovalent Fab from both Abs (Fab10 and Fab26) to HEL and mutant lysozymes are best described by a two-step association model consistent with an encounter followed by docking that may include conformational changes. In their complexes with HEL, both Abs make the transition to the docked phase rapidly. For H10, the encounter step is rate limiting, whereas docking is also partially rate limiting for H26. The forward rate constants of H10 are higher than those of H26. The docking equilibrium as well as the overall equilibrium constant are also higher for H10 than for H26. Most of the free energy change of association (Delta G degrees) occurs during the encounter phase (Delta G1) of both Abs. H10 derives a greater amount and proportion of free energy change from the docking phase (Delta G2) than does H26. In the H10--HEL(R21Q) complex, a significant slowing of docking results in lowered affinity, a loss of most of Delta G2, and apparently faster dissociation. Slower encounter and docking cause lowered affinity and a loss of free energy change primarily in the encounter step (Delta G1) of H26 with mutant HEL(R21Q). Overall, in the process of complex formation with lysozyme, the mutations HEL(R21X) affect primarily the docking phase of H10 association and both phases of H26. Our results are consistent with the interpretation that the free energy barriers to conformational rearrangement are highest in H26, especially with mutant antigen.

Anti-subnucleosome reactivities in systemic lupus erythematosus (SLE) patients and their first-degree relatives

Antibodies specific for dsDNA appear to have different genetic origins and pathogenic consequences, compared with histone/dsDNA-specific antibodies, in a recently described murine model. The purpose of this study was to examine if this is also true in human lupus. Sera from 40 SLE families (comprising 40 probands and 153 first-degree relatives), and 45 normal adult controls were assayed for the levels of anti-dsDNA, anti-H1/dsDNA, anti-H2A/H2B/dsDNA, and anti-H3/H4/dsDNA autoantibodies by ELISA. Both the probands and the first-degree relatives exhibited significantly increased levels of antinuclear antibodies (ANA) targeting the different subnucleosomal epitopes. Importantly, probands with anti-dsDNA antibodies had a significantly higher incidence of renal disease compared with those with just anti-H2A/H2B/dsDNA antibodies, in resonance with murine studies. The frequency of anti-dsDNA and anti-H2A/H2B/DNA ANA among the first-degree relatives was 11.8% and 18.3%, respectively. Surprisingly, whereas probands with anti-dsDNA ANA had families with several seropositive members, first-degree relatives of patients with anti-H2A/H2B/DNA ANA (but not anti-dsDNA ANA) were uniformly ANA-free. These findings suggest that anti-dsDNA ANA in lupus may not only have worse disease associations, they may also have very different genetic origins, compared with anti-H2A/H2B/DNA (or anti-nucleosome) ANA.

Rearrangement of the human heavy chain variable region gene V3-23 in transgenic mice generates antibodies reactive with a range of antigens on the basis of VHCDR3 and residues intrinsic to the heavy chain variable region

To formulate a 'logic' for how a single immunoglobulin variable region gene generates antibodies with different antigen specificity and polyreactivity, we analysed chimeric antibodies produced in transgenic mice carrying the germ-line human V3-23 gene, multiple diversity (D) and joining (J) gene segments. Hybridomas producing antibodies encoded by the V3-23 gene in combination with different mouse Vkappa genes were obtained by fusion of splenocytes from transgenic mice. All antibodies had human mu-chains and mouse light chains, were multimeric in structure and expressed the human V3-23 gene. Nucleotide sequence analyses of genes encoding the heavy and light chains of 12 antibodies in relation to antigen specificity highlighted the importance of heavy chain variable region CDR3 in determining reactivity with different antigens. However, the results also suggest that non-CDR3 sequences intrinsic to the V3-23 gene itself may be involved in, or determine, the binding of the chimeric antibodies to some of the antigens tested in the current study.

Deletions of structural glycoprotein E2 of classical swine fever virus strain alfort/187 resolve a linear epitope of monoclonal antibody WH303 and the minimal N-terminal domain essential for binding immunoglobulin G antibodies of a pig hyperimmune serum

The major structural glycoprotein E2 of classical swine fever virus (CSFV) is responsible for eliciting neutralizing antibodies and conferring protective immunity. The current structural model of this protein predicts its surface-exposed region at the N terminus with a short stretch of the C-terminal residues spanning the membrane envelope. In this study, the N-terminal region of 221 amino acids (aa) covering aa 690 to 910 of the CSFV strain Alfort/187 E2, expressed as a fusion product in Escherichia coli, was shown to contain the epitope recognized by a monoclonal antibody (WH303) with affinity for various CSFV strains but not for the other members of the Pestivirus genus, bovine viral diarrhea virus (BVDV) and border disease virus (BDV). This region also contains the sites recognized by polyclonal immunoglobulin G (IgG) antibodies of a pig hyperimmune serum. Serial deletions of this region precisely defined the epitope recognized by WH303 to be TAVSPTTLR (aa 829 to 837) of E2. Comparison of the sequences around the WH303-binding site among the E2 proteins of pestiviruses indicated that the sequence TAVSPTTLR is strongly conserved in CSFV strains but highly divergent among BVDV and BDV strains. These results provided a structural basis for the reactivity patterns of WH303 and also useful information for the design of a peptide containing this epitope for potential use in the detection and identification of CSFV. By deletion analysis, an antigenic domain capable of reacting with pig polyclonal IgG was found 17 aa from the WH303 epitope within the N-terminal 123 residues (aa 690 to 812). Small N- or C-terminal deletions introduced into the domain disrupt its reactivity with pig polyclonal IgG, suggesting that this is the minimal antigenic domain required for binding to pig antibodies. This domain could have eliminated or reduced the cross-reactivity with other pestiviruses and may thus have an application for the serological detection of CSFV infection; evaluation of this is now possible, since the domain has been expressed in E. coli in large amounts and purified to homogeneity by chromatographic methods.

Conformational epitopes on the diabetes autoantigen GAD65 identified by peptide phage display and molecular modeling

The major diabetes autoantigen, glutamic acid decarboxylase (GAD65), contains a region of sequence similarity, including six identical residues PEVKEK, to the P2C protein of coxsackie B virus, suggesting that cross-reactivity between coxsackie B virus and GAD65 can initiate autoimmune diabetes. We used the human islet cell mAbs MICA3 and MICA4 to identify the Ab epitopes of GAD65 by screening phage-displayed random peptide libraries. The identified peptide sequences could be mapped to a homology model of the pyridoxal phosphate (PLP) binding domain of GAD65. For MICA3, a surface loop containing the sequence PEVKEK and two adjacent exposed helixes were identified in the PLP binding domain as well as a region of the C terminus of GAD65 that has previously been identified as critical for MICA3 binding. To confirm that the loop containing the PEVKEK sequence contributes to the MICA3 epitope, this loop was deleted by mutagenesis. This reduced binding of MICA3 by 70%. Peptide sequences selected using MICA4 were rich in basic or hydroxyl-containing amino acids, and the surface of the GAD65 PLP-binding domain surrounding Lys358, which is known to be critical for MICA4 binding, was likewise rich in these amino acids. Also, the two phage most reactive with MICA4 encoded the motif VALxG, and the reverse of this sequence, LAV, was located in this same region. Thus, we have defined the MICA3 and MICA4 epitopes on GAD65 using the combination of phage display, molecular modeling, and mutagenesis and have provided compelling evidence for the involvement of the PEVKEK loop in the MICA3 epitope.

The autoantibodies to alpha 6 beta 4 integrin of patients affected by ocular cicatricial pemphigoid recognize predominantly epitopes within the large cytoplasmic domain of human beta 4

This study was undertaken to characterize the antigenic determinants recognized by the autoantibodies of patients with ocular cicatricial pemphigoid (OCP). OCP is a subepithelial, blistering, autoimmune disease that mainly affects the conjunctiva and other mucous membranes. We previously demonstrated that a cDNA clone, isolated from a keratinocyte expression library by using immunoaffinity-purified OCP autoantibody, encoded the cytoplasmic domain of beta 4 integrin subunit. Our subsequent studies showed that sera from all the OCP patients that were tested recognize the human beta 4 integrin subunit. To identify the prevalent epitopes of the anti-beta 4 autoantibodies of OCP, we have used cell lines transfected with vectors encoding a wild-type beta 4 subunit, a tailless beta 4 subunit, or a beta 4 subunit lacking the extracellular domain. Nontransfected cell lines were used as controls. Lysates from these cell lines were analyzed with OCP sera, IgG fractions from OCP sera, and immunoaffinity-purified OCP autoantibodies. Abs to extracellular and cytoplasmic domains of human beta 4 integrin were used as positive controls, whereas normal human sera and normal human IgG fractions were used as negative controls. The reactivity of OCP Abs was determined by using immunoblotting, immunoprecipitation, and FACS analysis. The results of this study indicate that OCP sera, OCP IgG fractions, and immunoaffinity-purified OCP autoantibodies react with the intracellular and not the extracellular domain of human beta 4 integrin subunit. In vitro cell culture experiments demonstrated that OCP autoantibody binds to the cytoplasm of the cells. The relevance of these findings to the pathogenesis of OCP is discussed.

HLA genetics for diagnosis of susceptibility to early-onset periodontitis

Human leukocyte antigens (HLA) are essential in the recognition of foreign antigens in humoral immune response, which is genetically predetermined. Susceptibility to certain diseases that involve the immune response has been studied in relation to distinct HLA types. Although some diseases have been found to correlate to specific HLA loci positively, it has been difficult to isolate HLA types that predispose patients to periodontal destruction. Here, we review the current knowledge and recent advances in HLA genetics and its biology, which determine susceptibility to early-onset periodontitis (EOP). The HLA-DRB1*1501-DQB1*0602 genotype has been found with increasing frequency in EOP patients. This HLA genotype expresses aspartic acid at position 57 and glycine at position 70 on the DQ beta chain, suggesting a capability to bind certain bacterial antigens. The T cell response against the outer membrane protein (Ag53) of Porphyromonas gingivalis was examined via this HLA genotype. Strong T cell response against Ag53 p141-161 was inhibited partially by anti-DR antibody, but not by anti-DQ antibody. Possible host and bacterial peptides capable of binding DRB1*1501 were elucidated when the peptide sequence was compared to gene and protein databases. These results suggest that patients who have the HLA-DRB1*1501-DQB1*0602 genotype may have an accelerated T cell response to certain periodontopathic bacteria such as P. gingivalis in hyperimmune reactions and thus increased susceptibility to EOP.

Generation of anti-p53 Fab fragments from individuals with colorectal cancer using phage display

Although many individuals with malignancy develop Abs against p53, little is currently known of the structural features, V gene usage, and degree of somatic mutation of these Abs. Such information is critical to any meaningful understanding of the nature and significance of this humoral immune response to p53. We have constructed phage display libraries from six individuals with colorectal cancer and a demonstrable serum immune response against p53. Following panning with recombinant p53, a total of 43 binding Fab were identified. Four of these Abs bound with high affinity to wild-type denatured p53 (1.19 x 10-8 - 1.57 x 10-8), as determined by BIAcore analysis, and were highly specific for both recombinant and cell line-derived p53, as determined by ELISA and immunoprecipitation. Epitope mapping showed they were reactive with the N terminus of human p53 between residues 27 and 44. Sequence analysis showed that the heavy chains were derived from the VH1 gene family, and the light chains from VL4. The pattern of replacement and silent mutations in the Fab sequence indicated that negative selection had occurred in the framework regions of all the VH genes. We show that lymphocytes from individuals with cancer represent a valuable source of high affinity human Abs against p53. This approach provides an opportunity to examine the genetic structure of these naturally occurring Abs, and to draw inferences regarding the nature of the immune response that produced them. Abs identified in this way have a number of potential therapeutic applications.

Identification of two amino acids within the EIIIA (ED-A) segment of fibronectin constituting the epitope for two function-blocking monoclonal antibodies

Alternative splicing of the fibronectin gene transcript gives rise to a group of adhesive glycoproteins showing restricted spatial and temporal expression during embryonic development, tumor growth, and tissue repair. Alternative splicing occurs in three segments termed EIIIB, EIIIA, and V. The EIIIA (or ED-A) segment of fibronectin is expressed prominently but transiently in healing wounds coincident with fibroblast expression of an activation marker, smooth muscle cell alpha-actin. A monoclonal antibody (IST-9) to the EIIIA segment blocks transforming growth factor-beta-mediated smooth muscle cell alpha-actin expression by fibroblasts in culture. A second monoclonal antibody (DH1) blocks chondrocyte condensation in chicken embryos. We find that IST-9 and DH1 react with human, rat, and chicken but not with mouse or frog EIIIA, suggesting that His44 may be important for antibody binding. A series of deletion mutants of rat EIIIA, constructed as glutathione S-transferase fusion proteins, do not react with either IST-9, DH1, or a third monoclonal antibody (3E2). Mutations of pairs of amino acids to alanine have little effect, except for either (Val34Thr35) or (Tyr36Ser37), which are located in a beta strand upstream from His44. For these double mutants, the binding to all three monoclonal antibodies is markedly reduced. By contrast, single mutants at Thr35, Tyr36, or Ser37 retain full activity, suggesting that the epitope for these antibodies is determined in part by conformation. Alanine-scanning mutagenesis of rat EIIIA demonstrates the importance of Ile43 and His44 for binding. Mutation of frog EIIIA (normally Val43Lys44) to rat (Ile43His44) is sufficient to restore fully IST-9 binding and much of the activity of DH1 and 3E2. Our findings demonstrate that the function-blocking antibodies, IST-9 and DH1, bind to the Ile43 and His44 residues in a conformationally dependent fashion, implicating the loop region encompassing both residues as critical for mediating EIIIA function.

Novel target antigens of the variant-specific immune response to Plasmodium falciparum identified by differential screening of an expression library

A primary infection by the Plasmodium falciparum Palo Alto O and R antigenic variants induces a variant-specific immunity in the Saimiri sciureus monkey. We have shown that these variants express distinct PfEMP1 antigens and differ in their levels of expression of additional antigens, including two conserved erythrocyte membrane-associated proteins, HRP1 and PfEMP3. To identify the antigens eliciting a variant-specific response, we conducted a differential screening of a lambdagt11 library with variant-specific sera. We report here the analysis of the 46 anti-R-specific clones. Two specific targets of the anti-R response were identified: (i) PfEMP3, suggesting that immunogenicity of this antigen is modulated by its relative abundance in different variants, and (ii) Asn-rich motifs. Most anti-R-specific clones, derived from so-far-undescribed genes, were detected by a cross-reaction on poly(Asn) stretches, as indicated by elimination of the signal after absorption on Asn-rich sequences. Reverse transcription-PCR (RT-PCR) showed that expression of the gene defined by clone 13 was R specific. Pepscan analysis of clone 13 identified three Asn-rich polypeptides and one unique peptide reacting specifically with antibodies eluted from the R-infected erythrocyte surface. Antisera raised to the unique peptide reacted with an R-specific protein. Attempts to demonstrate that clone 13 was derived from a var gene by using PCRs combining clone 13 and var-derived primers were unsuccessful. The var genes expressed by O and R parasites were identified not by this strategy but by RT-PCR with var-specific primers. This work has provided novel insights into immunity to antigenic variants and has identified a novel gene switched on during antigenic variation.

Enhanced antigenicity of a four-contact-residue epitope of the measles virus hemagglutinin protein by phage display libraries: evidence of a helical structure in the putative active site

Antigenicity and conformational propensities of synthetic peptides corresponding to the sequential epitope H236-255 of the measles virus hemagglutinin protein were investigated. This epitope corresponds to the neutralising and protective monoclonal antibody BH129 and includes Arg243, implicated in CD46-down-regulation and Arg253 that has been mapped to the putative enzymatic site. Fine mapping with truncation-, elongation-, Gly- and Ala-substitution analogues defined EL-QL as the critical residues of the minimal epitope S244ELSQL249. CD spectra of peptides, comparison with the 3D-structure of homologous sequences, and prediction algorithms suggested a helical structure with the contact residues E245L-QL249 located on the protein surface. Mimotopes obtained with a 6-mer phage display library contained a consensus Pro (important for binding) instead of Ser247 of the wild-type sequence (irrelevant for binding). The kink induced by Pro seemed to be essential to bring the 4 contact-residues in the mimotopes and in the corresponding short peptides together. CD analysis and prediction algorithms suggested that non-helical conformations of the phage insert and of the peptides may favourably mimic the antigenic helical turns of the wild-type sequence, resulting in an up to 135 times higher antigenicity of the mAb towards the mimotope peptides.

Analysis of the three-dimensional antigenic structure of giant ragweed allergen, Amb t 5

The ragweed allergens Amb t 5 and Amb a 5 are among the smallest inhaled protein allergens known, containing a single, immunodominant T-cell epitope. In this study we analyzed the B-cell epitope structure of Amb t 5. The three-dimensional structures of Amb t 5 and Amb a 5 have been determined by NMR spectroscopy, providing a rare opportunity to analyze three-dimensional antigenic sites. Amb t 5 residues likely to be important for antigenicity were identified by examining the surface area of Amb t 5 accessible to a probe of the size of an antibody molecule. After changing these residues to the corresponding Amb a 5 residues, recombinant proteins were purified and tested for loss of antigenic activity. Inhibition radio-immunoassays, using sera from 8 individuals who had received immunotherapy with giant ragweed extract, allowed the mutations to be divided into three groups: (1) mutations that had little or no effect on antibody binding, (2) mutations that caused a loss of antigenic activity to a different degree in different sera and (3) mutations that drastically reduced antigenic activity in all sera tested. This last set of mutations clustered in the third loop of Amb t 5, suggesting that antibody recognition of Amb t 5, like T-cell recognition, is primarily directed towards a single, immunodominant site.

Cloning and immunological characterization of the allergen Hel a 2 (profilin) from sunflower pollen

Sunflower (Helianthus annuus) sensitization is not always related with occupational allergy. We have isolated the allergen profilin (Hel a 2) from this Compositae plant, cloned and sequenced five cDNAs encoding for full-length or partial Hel a 2. Natural sunflower profilin reacted with specific IgE in the 121 sera tested, at a frequency of 30.5%. Expression of the cDNA encoding Hel a 2 in Escherichia coli and a simple purification procedure by poly-L-proline chromatography allowed immunological characterization of the recombinant allergen. Binding of monoclonal antibodies against sunflower profilin revealed that some epitopes responsible for antigen-specific IgG production were not present in the recombinant allergen. High cross-reactivity has been found between recombinant Hel a 2 and profilins from other Compositae plants and also from botanically distant plants.

Study of antibody-antigen interaction through site-directed mutagenesis of the VH region of a hybrid phage-antibody fragment

An important aspect of the study of antibody structure-function relationships involves analysis of natural or synthetic mutations of antigen-combining sites. The anti-hen egg lysozyme monoclonal antibody HyHEL-10 has been a focus for antibody structure-function studies. We have displayed on bacteriophage of a hybrid single chain Fv, containing the light chain variable region of HyHEL-10 and the heavy chain variable region of a structurally related but functionally distinct antibody, AS32. By using a combination of site-directed mutagenesis, complementary determining region grafting and molecular modeling, we have identified a number of contact and non-contact residues that are important in the affinity of HyHEL-10 for lysozyme. In particular, the heavy chain variable region framework residue at position 94 was shown to be an important determinant of high-affinity binding. The phage display approach eliminates the need for purification of antibodies and, when used in combination with polymerase chain reaction for variable region sequence mutagenesis, facilitates the rapid generation and characterization of mutant antibodies.

Can immunoglobulin C(H)1 constant region domain modulate antigen binding affinity of antibodies?

Although the switch process is frequently associated with affinity maturation, the constant region is not assumed to play a role in Ag-Ab binding. In the present work, we demonstrate that two clonally related human monoclonal Igs sharing identical V(H) and V(L) sequences, but expressing different isotypes (IgA1kappa(PER) and IgG1kappa(PER)), bind tubulin with significantly different affinities. This difference was mainly accounted for by a disparity in the association rate constants. These results suggest that affinity maturation of this clone could be achieved through class switching in the absence of further somatic mutations. Since the differences observed were found at the Fab level, they also suggest a role for the C(H)1 domain in structuring the Ag-binding site into a more kinetically competent form.

Mapping of epitopes recognized by PM/Scl autoantibodies with gene-fragment phage display libraries

Sera from patients suffering from the polymyositis/scleroderma overlap syndrome (PM/Scl) recognize two antigenically non-related proteins with apparent molecular masses of 100 kDa and 75 kDa respectively. The two proteins are part of a particle termed PM/Scl localized in the granular component of the nucleolus. The predominant immunoreactivity of the PM/Scl sera was shown to be directed against the 100 kDa protein. The cDNA of the 100 kDa protein has been cloned recently and its immunogenic regions have been partially mapped using recombinant proteins. Thus far the localization of antigenic determinants on polypeptides has been done by expressing defined cDNA fragments in bacteria or by synthesizing overlapping short peptides and probing their immunoreactivity with antibodies. Here we present an alternative approach to localize autoimmune epitopes using sera containing polyclonal antibodies and gene-fragment phage display libraries. For epitope fine mapping of the PM/Scl-100 protein random fragments of the corresponding cDNA were cloned into the PIII protein of fUSE-5. These gene-fragment phage display libraries were incubated with affinity purified anti-PM/Scl-100 antibodies to enrich for epitope-displaying phages. All PM/Scl sera tested recognized 23 consecutive amino acids (229-251) encoded by four overlapping fUSE-5 clones, suggesting that a major epitope is contained within the 23 amino acids. In addition a minor epitope was localized in a region of 21 amino acids (775-795) encoded by two overlapping fUSE-5 clones since only three out of the seventeen sera reacted with this amino acid sequence. Additional fine mapping of the major epitope was done using synthetic oligopeptides. Thus, a stretch of 16 amino acids at position 229-244 could be identified as a major epitope on the deduced PM/Scl-100 amino acid sequence.

Reactivity and epitope mapping of single-chain T cell receptors with monoclonal antibodies

To examine further the structure of the T cell receptor (TCR) and the specificity of mAbs generated against the native protein, the TCR was expressed in Escherichia coli as a single chain in which the variable regions of the alpha and beta chains are joined by a 25 amino acid linker. Five single-chain TCR that have different alpha and/or beta variable (V) regions were examined with the anti-V beta 8 region mAbs KJ16 and F23.1 and the anti-V alpha 8 mAbs KT50, KT65 and B21.14. Each of the mAbs reacted with one or more of the single-chain receptors. Western blot analysis demonstrated that the intrachain disulfide bonds were required for proper epitope conformation and recognition of the TCR by the antibodies. KT50, KT65 and B21.14 antibodies distinguished between two related V alpha regions that differed at only six residues. A model of the V regions of the TCR based on immunoglobulin (Ig) structure suggests that three of these six variant residues are in the putative CDR1 of the receptor and possibly accessible to antibody. To test this possibility, site-directed mutagenesis of the unreactive V alpha region demonstrated that the combination of all three residues restored binding by the anti-V alpha 8 antibodies. In addition, these three complimentarity determining regions (CDR) residues are likely to be in close proximity to the putative CDR3 which also influenced binding of the antibodies. The epitopes recognized by the V alpha-specific antibodies are thus predicted to reside closer to the putative binding site than the epitopes previously determined to be recognized by the anti-V beta 8 antibodies, KJ16 and F23.1. Finally, the specificities of KT50 and KT65 as determined with the E. coli expression system suggests an explanation for previous observations about the differences in the T cell populations that are recognized by these antibodies.

HLA-B*1303: a new example of poor correlation between serology and structure

The HLA-B locus discloses a higher number of alleles and more complex serologic patterns than those observed for the HLA-A locus. In this article we describe the molecular structure and serologic details of a novel HLA-B allele, B*1303. B*1303 has been serologically defined as a B21 Bw4-associated molecule, whereas its primary structure is closely related to B13 alleles. In fact, only three nucleotide and two amino acid substitutions were found with respect to the Caucasian B*1302 allele. In contrast, there were 10, 14, and 16 amino acid substitutions when compared to B*4901, B*5001, and B*4005, respectively. Serologic analysis definitively supports the essential role of leucine 145 within the B13-specific epitope. Furthermore, the involvement of glutamic 163 in the definition of both the specific B21 and polyspecific B15,B57 epitopes is suggested. B*1303 constitutes a new example of how serology can give a distorted point of view of the structural relationship among alleles.

A mouse Ig kappa domain of very unusual framework structure loses function when converted to the consensus

Antibody gene sequences, particularly those of kappa light chains, are very well conserved in the framework region, and the variability is concentrated in the complementarity-determining regions (CDR). We now found that the murine antibody 93-6 (Djavadi-Ohaniance, L., Friguet, B., and Goldberg, M. (1984) Biochemistry 23, 97-104) whose Fab fragment binds the beta-subunit of Escherichia coli tryptophan synthase with high affinity (Kd of 6.7.10(-9) M) has a highly unusual kappa light chain framework, which is crucial for the function of this antibody. It carries an insertion of 8 amino acids in a conserved framework loop that faces the antigen, and its framework region 2 (FR2) which precedes CDR2 is shortened by one amino acid, normally leucine and part of an absolutely conserved beta-bulge preceding CDR2. Removal of the insertion to restore the consensus sequence reduced the binding affinity of 93-6 by a factor 3, while insertion of the missing leucine into FR2 completely abolished binding.

Production of monoclonal antibodies specific to the carboxyl terminal region of the 85 kDa subunit of phosphatidylinositol 3-kinase: use of the antibodies in recognition of mutant p85

We have established two hybridomas producing mAb to the carboxyl terminal region of phosphatidylinositol-3 kinase 85 kDa subunit type alpha (p85 alpha). Analysis using deletion mutants of p85 revealed that epitopes for the two mAb were located on the border of the src homology 2 (SH2) sequence located at the carboxyl end of p85. They immunoprecipitated free p85 efficiently, but reactivity to p85 bound to p110 was very weak. Together with the mAb which we have reported previously, a panel of mAb that covered the various parts of p85 alpha was obtained. Using this panel, we characterized two mutants of p85 (70 and 50 kDa) expressed in the human colon carcinoma cell line, HCC2998. No wild-type p85 was detected in these cells. A mAb specific to the carboxyl terminal region detected p70 but not p50, suggesting that this region is missing in p50. The panel of mAb is a useful tool to use to analyse mutant forms of p85.

1D-IEF analysis of BoLA class I expression using allo-antisera reveals additional complexity

The use of the bovine allo-antisera in lymphocyte microcytotoxicity assays suggests that there is a single highly polymorphic class I product expressed by the BoLA system encoded by one locus. In contrast, biochemical techniques, such as 1D-IEF, reveal a complex pattern of bands for BoLA class I molecules from each animal. In order to understand the origins of this heterogeneity bovine allo-antisera were used in the immunoprecipitation step of 1D-IEF and the results compared with those from immunoprecipitation using the monoclonal antibody W6/32. By modifying existing protocols to include Gammabind G a range of bovine allo-antisera were used successfully to immunoprecipitate bovine MHC class I molecules. The results indicate that the bovine allo-antisera do not recognize all molecules previously assigned to BoLA class I serotypes by 1D-IEF. Furthermore, some of the allo-antisera immunoprecipitated molecules are not recognized by W6/32 and vice versa. This suggests that more than one polymorphic locus is expressed from the bovine MHC and that each allo-antiserum recognizes molecules encoded by different loci. Examination of the results also suggests the existence of linkage disequilibrium in the BoLA class I region.