Epitope recognition by diverse antibodies suggests conformational convergence in an antibody response

The thyrotropin receptor in Graves' disease

The application of molecular biology to the study of the thyrotropin receptor (TSHR) has led to major advances in our understanding of its structure, function, and relationship to the pathogenesis of Graves' disease. This review summarizes many of these features and also provides a personal perspective, questioning some assumptions and general concepts, as well as describing remaining challenges. Among the issues raised are the limits in our understanding of the spatial orientation of the structural domains of the TSHR, including the enigmatic hinge region. We review the phenomenon of TSHR intramolecular cleavage, the shedding of the A-subunit component of the ectodomain, and the importance of the latter in generating thyroid-stimulating antibodies. The epitopes of thyroid-stimulating and -blocking autoantibodies have been a confusing and controversial subject that requires review and evaluation of available data. Finally, we address the potential physiological or pathophysiological significance of TSHR multimerization in TSHR. Taken together, this review will, hopefully, convey the fascination and excitement that molecular biology has contributed to the study of the TSHR, especially as it relates to the pathogenesis of Graves' disease.

Pre-structured motifs in the natively unstructured preS1 surface antigen of hepatitis B virus

The preS1 surface antigen of hepatitis B virus (HBV) is known to play an important role in the initial attachment of HBV to hepatocytes. We have characterized structural features of the full-length preS1 using heteronuclear NMR methods and discovered that this 119-residue protein is inherently unstructured without a unique tertiary structure under a nondenaturing condition. Yet, combination of various NMR parameters shows that the preS1 contains "pre-structured" domains broadly covering its functional domains. The most prominent domain is formed by residues 27-45 and overlaps with the putative hepatocyte-binding domain (HBD) encompassing residues 21-47, within which two well-defined pre-structured motifs, formed by Pro(32)-Ala(36) and Pro(41)-Phe(45) are found. Additional, somewhat less prominent, pre-structured motifs are also formed by residues 11-18, 22-25, 37-40, and 46-50. Overall results suggest that the preS1 is a natively unstructured protein (NUP) whose N-terminal 50 residues, populated with multiple pre-structured motifs, contribute critically to hepatocyte binding.

Solution conformation of an immunodominant epitope in the hepatitis B virus preS2 surface antigen

We have determined the solution conformation of the major B cell epitope (residues 123-145, adrl23 hereafter) in the preS2 region of hepatitis B virus known to be associated with infection neutralization. The adrl23 shows an "L" shaped helix-turn-helix topology with two beta-turns formed by residues Ala(130)-Asp(133) and Asp(133)-Val(136) intervening the N- and C-terminal helices. The N-terminal alpha-helix consists of residues Ser(124)-Gln(129) whereas the C-terminal 3(10) helix is formed by residues Val(136)-Tyr(140). The beta-turns overlap partially with the putative "conformational" epitope. The overall topology of adrl23 is primarily maintained by hydrophobic interactions involving Phe(127), Leu(131), Leu(132), Val(136), and Tyr(140) that are clustered on one side of the molecule. An additional hydrophobic stabilization comes from Phe(141) that is buried inside the concave side of the molecule. A network of hydrogen bonds formed among Thr(125), His(128), and Arg(137) further contribute to the "boomerang-shaped" architecture of adrl23. The N-terminus of adrl23 is immobile due to a hydrogen bond between the N-terminal amide proton of Asn(123) and the hydroxyl oxygen of Thr(126). The side chains of Asp(133), Arg(135), Val(136), Leu(139), and Tyr(140) that were shown to be important for binding to a monoclonal antibody H8 mAb are surface exposed.

Differential epitope positioning within the germline antibody paratope enhances promiscuity in the primary immune response

Correlation between the promiscuity of the primary antibody response and conformational flexibility in a germline antibody was addressed by using germline antibody 36-65. Crystallographic analyses of the 36-65 Fab with three independent dodecapeptides provided mechanistic insights into the generation of antibody diversity. While four antigen-free Fab molecules provided a quantitative description of the conformational repertoire of the antibody CDRs, three Fab molecules bound to structurally diverse peptide epitopes exhibited a common paratope conformation. Each peptide revealed spatially different footprints within the antigen-combining site. However, a conformation-specific lock involving two shared residues, which were also associated with hapten binding, was discernible. Unlike the hapten, the peptides interacted with residues that undergo somatic mutations, suggesting a possible mechanism for excluding "nonspecific" antigens during affinity maturation. The observed multiple binding modes of diverse epitopes within a common paratope conformation of a germline antibody reveal a simple, yet elegant, mechanism for expanding the primary antibody repertoire.

Insight into Graves' hyperthyroidism from animal models

Graves' hyperthyroidism can be induced in mice or hamsters by novel approaches, namely injecting cells expressing the TSH receptor (TSHR) or vaccination with TSHR-DNA in plasmid or adenoviral vectors. These models provide unique insight into several aspects of Graves' disease: 1) manipulating immunity toward Th1 or Th2 cytokines enhances or suppresses hyperthyroidism in different models, perhaps reflecting human disease heterogeneity; 2) the role of TSHR cleavage and A subunit shedding in immunity leading to thyroid-stimulating antibodies (TSAbs); and 3) epitope spreading away from TSAbs and toward TSH-blocking antibodies in association with increased TSHR antibody titers (as in rare hypothyroid patients). Major developments from the models include the isolation of high-affinity monoclonal TSAbs and analysis of antigen presentation, T cells, and immune tolerance to the TSHR. Studies of inbred mouse strains emphasize the contribution of non-MHC vs. MHC genes, as in humans, supporting the relevance of the models to human disease. Moreover, other findings suggest that the development of Graves' disease is affected by environmental factors, including infectious pathogens, regardless of modifications in the Th1/Th2 balance. Finally, developing immunospecific forms of therapy for Graves' disease will require painstaking dissection of immune recognition and responses to the TSHR.

Differentiation of a passive vaccine and the humoral immune response toward infection: analysis of phage displayed peptides

Antibody-genes undergo molecular events that produce unique binding-sites that recognize specific epitopes, thus, leading to B-cell clonal variation. As a result, different binding-site structures (paratope internal images) are produced even when two distinct B-cells bind one and the same epitope. Paratope structural variation can be exploited to enable one to evaluate antibody-diversity in a single polyclonal serum sample. This is accomplished through the selection of antibody-specific peptides isolated from combinatorial phage displayed peptide libraries. As an example, we demonstrate the analysis of macaque sera containing passively administered antibodies, given as a therapeutic vaccine and antibodies actively produced by the virus-infected monkeys.

Anin silico method using an epitope motif database for predicting the location of antigenic determinants on proteins in a structural context

Presently X-ray crystallography of protein-antibody complexes is still the most direct way of identifying B-cell epitopes. The objective of this study was to assess the potential of a computer-based epitope mapping tool (EMT) using antigenic amino acid motifs as a fast alternative in a number of applications not requiring detailed information, e.g. development of pharmaceutical proteins, vaccines and industrial enzymes. Using Gal d 4 as a model protein, the EMT was capable of identifying, in the context of the folded protein, amino acid positions known to be involved in antibody binding. The high sensitivity and positive predictive value of the EMT as well as the relevance of the structural associations suggested by the EMT indicated the existence of amino acid motifs that are likely to be involved in antigenic determinants. In addition, differential mapping revealed that sensitivity and positive predictive value were dependent on the minimum relative surface accessibility (RSA) of the amino acids included in the mapping, demonstrating that the EMTs accommodated for the fact that epitopes are three-dimensional entities with various degrees of accessibility. The comparison with existing prediction scales demonstrated the superiority of the EMT with respect to physico-chemical scales. The mapping tool also performed better than the available structural scales, but the significance of the differences remains to be established. Thus, the EMT has the potential of becoming a fast and simple alternative to X-ray crystallography for predicting structural antigenic determinants, if detailed epitope information is not required.

Plasticity within the Antigen-Combining Site May Manifest as Molecular Mimicry in the Humoral Immune Response

Structural and physiological facets of carbohydrate-peptide mimicry were addressed by analyzing the Ab response to alpha-d-mannopyranoside. mAbs against alpha-d-mannopyranoside were generated and screened with the carbohydrate-mimicking 12 mer (DVFYPYPYASGS) peptide. Three mAbs, 2D10, 1H11, and 1H7, which were subjected to detailed analysis, exhibit diverse V gene usage, indicating their independent germline origins. Although the mAb 1H7 was specific in binding only to the immunizing Ag, the Abs 2D10 and 1H11 recognize the 12 mer peptide as well as the immunogen, alpha-d-mannopyranoside. The Abs that recognize mimicry appear to bind to a common epitope on the peptide and do not share the mode of peptide binding with Con A. Binding kinetics and thermodynamics of Ag recognition suggest that the Ab that does not recognize peptide-carbohydrate mimicry probably has a predesigned mannopyranoside-complementing site. In contrast, the mimicry-recognizing Abs adopt the Ag-combining site only on exposure to the sugar, exploiting the conformational flexibility in the CDRs. Although the mAb 1H7 showed unique specificity toward mannopyranoside, the mimicry-recognizing Abs 2D10 and 1H11 exhibited degenerate specificities with regard to other sugar moieties. It is proposed that the degeneracy of specificity arising from the plasticity at the Ag-combining site in a subset of the Ab clones may be responsible for exhibiting molecular mimicry in the context of Ab response.

Functional, molecular and structural characterisation of five anti-Brucella LPS mAb

The O-antigen of the gram negative bacteria Brucella is composed of an homopolymer of 4,6-dideoxy-4-formamido-alpha-D-mannopyranosyl (or perosamine). Several mAb interact specifically with only the O-antigen of certain Brucella species. Although, many studies show that this specific recognition results mainly from the ratios of alpha 1-2 and alpha 1-3 link between the different Brucella strain perosamine residues, little is known about the mAb recognising this O-antigen. In this paper, we describe the binding profile of five anti-Brucella O-antigen mAb to the LPS of two Brucella strains and a bacteria possessing a nearly identical O-antigen: Yersinia enterocolitica 0:9. We show that the specificity of these five mAb can be correlated to their germ line gene usage. Besides, their relative affinity to the different LPS is correlated to their ability to protect against Brucella infection by passive transfer in a mouse model. The analysis of their 3D structure gives new hypothesis of the epitopes recognised.

Hyperglycosylated mutants of human immunodeficiency virus (HIV) type 1 monomeric gp120 as novel antigens for HIV vaccine design

The ability to induce broadly neutralizing antibodies should be a key component of any forthcoming vaccine against human immunodeficiency virus type 1. One potential vaccine candidate, monomeric gp120, has generally failed to elicit such antibodies. We postulated that gp120 might be a better immunogen if it could be engineered to preferentially bind known broadly neutralizing antibodies. In a first study, we found that four alanine substitutions on the perimeter of the so-called Phe-43 cavity of gp120 could reduce binding of weakly neutralizing CD4-binding site antibodies (R. Pantophlet, E. O. Saphire, P. Poignard, P. W. H. I. Parren, I. A. Wilson, and D. R. Burton, J. Virol. 77:642-658, 2003), while slightly enhancing binding of the potent, broadly neutralizing antibody b12. In the present study, we sought to reduce or abolish the binding of a wider range of nonneutralizing antibodies, by incorporating extra N-glycosylation motifs at select positions into the hypervariable loops and the gp120 core. A hyperglycosylated mutant containing seven extra glycosylation sequons (consensus sequences) and the four alanine substitutions described above did not bind an extensive panel of nonneutralizing and weakly neutralizing antibodies, including a polyclonal immunoglobulin preparation (HIVIG) of low neutralizing potency. Binding of b12, at lowered affinity, and of four antibodies to the C1 and C5 regions was maintained. Removal of N- and C-terminal residues in the C1 and C5 regions, respectively, reduced or abolished binding of the four antibodies, but this also adversely affected b12 binding. The hyperglycosylated mutant and its analogues described here are novel antigens that may provide a new approach to eliciting antibodies with b12-like neutralizing properties.

Mimicry of native peptide antigens by the corresponding retro-inverso analogs is dependent on their intrinsic structure and interaction propensities

Retro-inverso (ri) analogs of model T cell and B cell epitopes were predictively designed as mimics and then assayed for activity to understand the basis of functional ri-antigenic peptide mimicry. ri versions of two MHC class I binding peptide epitopes, one from a vesicular stomatitis virus glycoprotein (VSV(p)) and another from OVA (OVAp), exhibit structural as well as functional mimicry of their native counterparts. The two ri peptides exhibit conformational plasticity and they bind to MHC class I (H-2K(b)) similar to their native counterparts both in silico and in vivo. In fact, ri-OVAp is also presented to an OVAp-specific T cell line in a mode similar to native OVAp. In contrast, the ri version of an immunodominant B cell peptide epitope from a hepatitis B virus protein, PS1, exhibits no structural or functional correlation with its native counterpart. PS1 and its ri analog do not exhibit similar conformational propensities. PS1 is less flexible relative to its ri version. These observed structure-function relationships of the ri-peptide epitopes are consistent with the differences in recognition properties between peptide-MHC vs peptide-Ab binding where, while the recognition of the epitope by MHC is pattern based, the exquisitely specific recognition of Ag by Ab arises from the high complementarity between the Ag and the binding site of the Ab. It is evident that the correlation of conformational and interaction propensities of native L-peptides and their ri counterparts depends both on their inherent structural properties and on their mode of recognition.

Thyroid-stimulating autoantibodies in Graves disease preferentially recognize the free A subunit, not the thyrotropin holoreceptor

Graves disease is directly caused by thyroid-stimulating autoantibodies (TSAb's) that activate the thyrotropin receptor (TSHR). We observed upon flow cytometry using intact cells that a mouse mAb (3BD10) recognized the TSHR ectodomain with a glycosidylphosphatidylinositol (ECD-GPI) anchor approximately tenfold better than the same ectodomain on the wild-type TSHR, despite the far higher level of expression of the latter. The 3BD10 epitope contains the N-terminal cysteine cluster critical for TSAb action. Consequently, we hypothesized and confirmed that TSAb (but not thyrotropin-blocking autoantibodies [TBAb's]) also poorly recognize the wild-type TSHR relative to the ECD-GPI. Despite poor recognition by TSAb of the holoreceptor, soluble TSHR A subunits (known to be shed from surface TSHR) fully neutralized autoantibody-binding activity. These data indicate that the epitope(s) for TSAb's, but not for TBAb's, are partially sterically hindered on the holoreceptor by the plasma membrane, the serpentine region of the TSHR, or by TSHR dimerization. However, the TSAb epitope on the soluble A subunit is freely accessible. This observation, as well as other evidence, supports the concept that A subunit shedding either initiates or amplifies the autoimmune response to the TSHR, thereby causing Graves disease in genetically susceptible individuals.