New antigenic clusters on human thyroglobulin defined by an expanded panel of monoclonal antibodies

Characterization of monoclonal antibodies against Escherichia coli core RNA polymerase

Multiple interactions with DNA, RNA and transcription factors occur in a transcription cycle. To survey the proximity of some of these factors to the Escherichia coli RNA polymerase surface, we produced a set of nine monoclonal antibodies (mAbs) against the enzyme. These mAbs, located at different places on the surface of the enzyme, were used in a co-immunopurification assay to investigate interference with the binding of NusA, sigma70, GreB and HepA to core RNA polymerase. One of these mAbs turned out to be the first antibody inhibitor of the binding of NusA and sigma70; it did not affect GreB and HepA interactions. Its epitope was located on the beta' subunit at the C-terminus of region G. The properties of this mAb reinforce the idea that the mutually exclusive binding of NusA and sigma70 to core RNA polymerase is due to, at least partially, overlapping binding sites, rather than allosteric interaction between two distant binding sites. This mAb is also useful to understand the occupancy of sigma70, NusA and Gre proteins on core RNA polymerase.

Reactivity of anti-thyroid antibodies to thyroglobulin tryptic fragments: comparison of autoimmune and non-autoimmune thyroid diseases

Studies concerning the antigenicity of thyroglobulin fragments allow the characterization of the epitopes but do not consider the role of heavier antigenic fragments that could result in vivo from the action of endoproteases. Here we assess the relative importance of the fragments obtained from thyroglobulin by limited proteolysis with trypsin and compare by immunoblotting their reactivity to serum from patients with autoimmune (Graves' disease and Hashimoto's thyroiditis) and non-autoimmune (subacute thyroiditis) disease. The results showed no difference in frequency of recognition of any peptide by sera from patients with autoimmune thyroiditis. In contrast, sera from patients with subacute thyroiditis reacted more frequently with a peptide of 80 kDa. These results suggest the presence of antibody subpopulations directed at fragments produced in vivo by enzymatic cleavage of thyroglobulin. This fragment and antibodies to it may represent markers for subacute thyroiditis.

Antigenic mapping of human thyroglobulin--topographic relationship between antigenic regions and functional domains

We characterized 26 mAb to human thyroglobulin to obtain a topographic map of the thyroglobulin antigenic surface. Among these mAb, three bind thyroglobulin peptides that are located in the primary sequence of thyroglobulin at either the N terminus or in the middle part of the molecule, three bind thyroglobulin via epitopes comprising the thyroid-hormone moiety, and three bind thyroglobulin through epitopes involved in the recognition of the molecule by its receptor. The 18 remaining mAb bind thyroglobulin through undetermined epitopes; most of these epitopes are resistant to trypsinization. We used two methods to map the antigenic regions of thyroglobulin: all 26 mAb were grouped, by means of cross-inhibition experiments, in 11 clusters corresponding to 11 antigenic regions of the thyroglobulin surface; by means of thyroglobulin peptides of decreasing size, obtained by time-controlled tryptic digestion, we analyzed the relative distance between pairs of epitopes in sandwich immunoassays. By combining these two methods, we organized most of the 11 antigenic regions on a topographic representation of the thyroglobulin surface. This new topographic map of thyroglobulin led us to some unexpected features of the thyroglobulin structure. First, antigenic region 8 located far from the N-terminal region is in close contact with two remote N-terminal antigenic regions (1 and 4), both involved in hormone formation. This antigenic region is likely to play a role in the correct positioning of hormonogenic tyrosines so as to optimize iodination-coupling reactions. Secondly, the domain involved in the binding of thyroglobulin to its receptor, probed by three mAb, is shared by two distinct mid-molecule antigenic regions, one being the main autoantigenic region of thyroglobulin.

Analysis of the individual contributions of immunoglobulin heavy and light chains to the binding of antigen using cell transfection and plasmon resonance analysis

We have cloned the Tg10 murine monoclonal antibody, which is specific for a human thyroglobulin (hTg) epitope targeted by autoantibodies in several thyroid pathologies. Transfection of COS-7 cells with plasmids expressing Tg10H and -kappa chains combined with surface plasmon resonance analysis (BIAcore) of culture supernatants showed that the entire cloned Tg10 antibody displays an affinity comparable to that of the parental antibody. This approach also permitted determination of the probable role of each chain to the recognition of the cognate epitope due to the ability of COS-7 cells to secrete independently each of the two constituting immunoglobulin chains. Tg10 heavy chain recognizes hTg in the absence of the light chain, but with a ten-fold lower affinity mainly due to an increase in kappaoff. In contrast, the light chain is unable to bind hTg on its own. This suggests that the latter is probably involved in stabilization rather than in initiating the formation of the antibody/antigen complex and that the specificity of Tg10 is mostly, if not exclusively, carried by the heavy chain. The potential applications of combined cell transfection and surface plasmon resonance to our understanding of antigen/antibody interactions are discussed.

Idiotypic restriction of murine monoclonal antibodies to a defined antigenic region of human thyroglobulin

In previous studies, we demonstrated that anti-human thyroglobulin (hTg) autoantibodies in patients with thyroid disorders exhibit a restricted epitopic specificity towards antigenic region II defined by its reactivity with four murine monoclonal antibodies (mAb 3, 6, 10, 15). To analyze the relationships between epitopic specificity and idiotypic expression of these mAb, two polyclonal anti-idiotypic sera were generated in rabbits by immunization with F(ab')2 fragments of mAb 3 and mAb 10. These anti-idiotypic preparations (AI 3 and AI 10) were tested against a panel of hTg-mAb produced in different strains of mice (HR BIOZZI and BALB/c). The idiotypic analysis showed that AI 3 and AI 10 specifically recognized framework-associated idiotopes as well as paratope-associated idiotopes shared by region II mAb. These results demonstrate that specificity for region II was strongly associated with a restricted idiotype suggesting a high sequence homology between V regions. In addition, naïve BALB/c mice immunized with AI 3 or AI 10 produced anti-hTg (Ab3) antibodies that recognize region II epitopes. These latter findings reveal that anti-Id contain a population of Ab2 beta carrying the internal image of region II epitopes.

Amino acid sequence of a tryptic peptide of human thyroglobulin reactive with sera of patients with thyroid diseases

Autoantibodies to human thyroglobulin (hTg) are found in the sera of many patients with thyroid diseases. To localize epitopes recognized by these autoantibodies, hTg was incubated with tryspin for 4 hours at 37 degrees C under non-reducing conditions. Releasing peptides from hTg in their natural conformation. These peptides were then analyzed by western immunoblot using either autoantibodies from patients with autoimmune thyroiditis or murine monoclonal antibodies (mAb) produced against hTg. The autoantibodies reacted primarily with two low molecular weight peptides with apparent molecular weights (MWap) of 15 and 20 kDa. The pattern of tryptic peptides recognized by these autoantibodies resembled that of one of the mAbs (137C1), as shown by immunoblots in either one or two dimensional SDS-PAGE. To characterize these peptides further, they were separated by a high performance liquid chromatography (HPLC). The column separated the 4-hour tryptic digest of hTg into multiple peptide peaks. Further analysis by SDS-PAGE showed that one of these peaks contained the 15 kDa peptide. The 15 amino acid sequence at the amino-terminus of this peptide was determined. This amino acid sequence (KVPTFATPWPDFVPR) corresponds to a unique sequence near the carboxyl-terminal end of hTg, starting with amino acid 2657. The size of the peptide indicates that it extends to the carboxyl-terminal end of hTg. This fragment contains one of the antigenic sites of hTg that binds autoantibodies from patients with autoimmune thyroid disease.