Peptides of human thyroglobulin reactive with sera of patients with autoimmune thyroid disease

Autoantibodies to thyroglobulin (Tg) are a prominent feature of the two autoimmune thyroid diseases, chronic lymphocytic (Hashimoto's) thyroiditis and Graves' disease. Similar autoantibodies are found in the serum of many normal individuals without evidence of thyroid disease. Previous studies have indicated that patients with autoimmune thyroid disease recognize epitopes of Tg which are not usually recognized by normal individuals. The goal of this investigation was to identify peptide fragments of Tg bearing these disease-associated epitopes. For this purpose, we utilized a panel of mAbs that bind to different epitopes of the Tg molecule. One of these mAbs (137C1) reacted with an epitope that was also recognized by the sera of patients with autoimmune thyroiditis. In the present study, we show that two peptides (15 and 23 kDa) that reacted with mAb 137C1 are located in different parts of the Tg molecule. Each peptide inhibited the binding of mAb 137C1 to the other peptide and to the intact Tg, indicating that the same epitope was represented on the two peptides. Loops and helices of the secondary structure of the two peptides might be involved in the conformational epitope recognized by mAb 137C1. A striking finding of this study is that two apparently unrelated fragments of the Tg molecule bind to the same mAb. These findings may have important ramifications with regard to epitope spread and the progression of the autoimmune response to disease.

Structural analysis of affinity maturation: the three-dimensional structures of complexes of an anti-nitrophenol antibody

Affinity maturation of the immune response to nitrophenol-containing antigens has been extensively investigated. Significant strides made during the past several years with the advent of PCR technology have provided a wealth of biochemical knowledge. Structural investigations of the phenomena have however been limited. We have determined the three-dimensional structure of the Fab fragment of 88C6/12, an anti-4-hydroxy-3-nitrophenyl acetic acid antibody complexed with the immunizing hapten and with a heteroclitic iodinated hapten. The crystallographic structure of the complexes reveals that the binding is stabilized by a number of hydrogen bonds and extensive van der Waals interactions between the hapten and the antibody. In addition, the Fab binding pocket contains a region of positive electrostatic potential well suited for interaction with the predominant resonance form of the nitrophenyl ring system. The observed heteroclicity towards the iodinated hapten is not a direct result of iodine-protein interactions, but results from the enhanced stability in the iodinated ring of the resonance form that binds the antibody. In addition this investigation provides a rationale for the strong preference for the substitution in the heavy chain from the germ-line gene encoded Trp 33 to Leu 33 in the mature anti-nitrophenol response.

Anti-idiotypic antibodies: biological function and structural studies

Under a variety of circumstances antibodies can be elicited against the variable region of other antibody molecules (anti-idotypic antibodies, anti-ids). Some of the antibodies are directed against the binding sites of the eliciting antibodies. Of particular interest are the antibodies that recognize epitopes of the original antibody that are in contact with antigen. Antibodies of this kind have been produced and used in a variety of situations including attempts at using them as therapeutic agents. In recent years structural data at the atomic level have emerged for anti-idiotypic antibodies from X-ray diffraction studies. These studies provided structural basis for molecular mimicry of anti-ids. For a large globular antigen (lysozyme), where epitope is noncontinuguous, molecular mimicry is not present at the atomic level. In this case, idiotopes are largely composed of CDR residues, but framework residues are also used. For an epitope that is sequence-specific (anti-FIPV system), molecular mimicry appears to be present as evidenced by the sequence homology between the CDR loops of the anti-id and the epitope of the original antigen. In the case of a small hormone antigen (angiotensin II), an internal image of the eliciting antigen appears to be represented in a single CDR loop of the antiiodiotypic antibody.

Crystallization and preliminary X-ray diffraction studies of an anti-4-hydroxy-3-nitrophenylacetic acid monoclonal antibody Fab fragment complexed with immunizing and heteroclitic haptens

The Fab fragment of the anti-4-hydroxy-3-nitrophenylacetic acid monoclonal antibody, 88C6/12 has been crystallized in the presence of the eliciting hapten, 4-hydroxy-3-nitrophenacetyl-epsilon-aminocaproic acid (NP-aminocap) and the heteroclitic iodinated analog, 4-hydroxy-3-iodo-5-nitrophenylacetyl-epsilon-aminocaproic acid (NIP-aminocap). Crystals obtained by precipitation with 32% (w/v) polyethylene glycol 3400 in the presence of 40 to 400 microM of either NP-aminocap or NIP-aminocap, belong to the orthorhombic space group P2(1)2(1)2(1) with cell dimensions a = 81.2 A, b = 86.9 A, c = 131.1 A. The cell volume suggests the presence of two molecules of the complex per asymmetric unit. Analysis of the Patterson function indicates that these two molecules are related by a local 2-fold axis parallel to the crystallographic b axis located at x = 0.218 and z = 0.25.

Epitopic discrimination by monoclonal antibodies directed against the same alkylated nucleoside

Epitopic specificity of three monoclonal antibodies (mAb's) (coded as ER-6, ER-3, and EM-1) was examined through the utilization of haptenic structural analogs. The binding affinity expressed by the microscopic equilibrium constant (Ki) (Yuhasz, et al., Biochemistry 26, 2334-2342 (1987] of the immunizing hapten, O6-ethyl-2'-deoxy-guanosine (*G) and eight structural analogs, were analyzed by a nitrocellulose affinity filter assay (NAFA) and radioimmunoassay (RIA) for each mAb to determine the protein-hapten interaction between the epitope and the binding cavity. Several components of the *G hapten were determined to be critical for each mAb recognition, while all three mAb's were found to require the O6-ethyl moiety, conjugated guanine base ring, the glycosyl bond and the sugar ring C [1'] and C [2'] position. This investigation further probes and categorizes the binding specificity of the monoclonal antibodies after incorporation of the *G monomer into three short deoxyribooligomeric haptens: O6-ethyl-2'-deoxyguanylyl 3',5' deoxyadenosine (*GA), 2'-deoxyadenylyl 3',5' O6-ethyl-2'-deoxyguanylyl 3',5' 2'-deoxyadenosine (A*GA), and O6-ethyl-2'-deoxyguanylyl 3',5' 2'-deoxyadenylyl 3',5'-2'-deoxyadenylyl 3',5' 2'-deoxycytosine (*GAAC). Unlike the similar binding profiles for the monoclonal antibodies and the haptenic structural analogs, the binding profiles for the deoxyribooligomeric haptens were found to differ in their modes of recognition. These results will be compared to ascertain the key components of monomer and oligomer interaction of the binding cavity. It is important for investigations where monoclonal antibodies derived from small haptens are utilized in recognition of larger antigens containing those haptens.

Thermodynamic investigation of monoclonal antibody alkylated nucleoside interaction as a model for epitope recognition on nucleic acids

The objective of this investigation is examination of the dominant forces that govern complex formation between a series of monoclonal antibodies directed against O6-ethyl-2'-deoxyguanosine. These monoclonal antibodies (coded as ER-6, ER-3, and EM-1) provide the basis for a thermodynamic comparative evaluation of the potentially different forces that stabilize the various monoclonal antibody (mAb) alkylated nucleoside complexes. The binding affinities of ER-6, ER-3, and EM-1 are measured in terms of specific (O6-ethyl-2'-deoxyguanosine, or O6-EtdGuo) and nonspecific (O6-methyl-2'-deoxyguanosine, or O6-MedGuo) antigens, under a variety of experimental conditions, including pH, sodium chloride addition, 1-propanol addition, and temperature, via a nitrocellulose affinity filter assay. The binding isotherms were analyzed via a least-squares routine fit to a two independent binding sites model. The temperature dependence of the van't Hoff enthalpies for the specific O6-EtdGuo interaction ranges from -15.18 to -18.60 kcal mol-1, while for O6-MedGuo the range was extended from -2.72 to -20.66 kcal mol-1. The standard and unitary entropies were negative for those mAb interactions with O6-EtdGuo as well as for ER-6/O6-MedGuo complex formation. However, it was found that the interactions between ER-3 and EM-1 with O6-MedGuo led to decidedly positive entropic values. These results indicate two different dominant forces at work in complex stabilization. The interaction of the three mAb's with their specific antigen, as well as ER-6/O6-MedGuo interaction (nonspecific), may well be controlled by van der Waals type forces, while ER-3 and EM-1 interactions with nonspecific antigen imply formal charge neutralization electrostatics as the dominant force.