The amino acid sequence of human thyroglobulin is known to enclose cysteine-rich repetitive regions. In this study, we report the existence of an eleventh type-I repeat within the human thyroglobulin sequence, and we characterize the thyroglobulin type-I repeat as a protein module. The l l thyroglobulin type-I repeats possessed the same number of cysteine residues (six in type A, four in the two type B repeats), a fairly constant number of residues between cysteines and a conserved sequence pattern. By scanning protein sequence databases, 29 proteins belonging to six different families were found to enclose at least one, and up to three, thyroglobulin type-1 repeats in their sequence. Although the repeat was present in numerous proteins possessing binding properties, an examination of the information available in the literature showed that a direct role of the repeat in protein-protein interaction has rarely been assessed. A distance analysis of the sequences indicated that all repeats segregate into four clusters of phylogenically close sequences. A consensus sequence of type-1 repeats was derived from sequence similarity analysis ; it comprised a central core of conserved residues including two highly conserved motifs, QC and CWCV. The type-1 repeat from thyroglobulin was found to differ from several previously described cysteine-rich modules, in particular from the epidermal-growth-factor-like module with which it has sometimes been confused. Therefore, our results provide a complete characterization of the repeats which will help in the detection of these repeats in newly characterized proteins, a necessary step for understanding the structuralhiological role of this module.

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In an attempt to explore the natural variable heavy and light chain (VH/VL) pairing of autoantibodies involved in Graves’ disease, we constructed a phage-displayed Ab library obtained by in-cell PCR of thyroid-infiltrating cells. We report here the molecular cloning and characterization of human single-chain fragment variable regions (scFv) specific for thyroid peroxidase (TPO) generated from this library. On the basis of the nucleotide sequences, three different scFvs were obtained (ICA1, ICB7, and ICA5). All were encoded by genes derived from the VH1 and Vλ1 gene families. Using BIACORE for epitope mapping and kinetic analysis, we showed that these scFvs exhibited high affinity (Kd = 1 nM) for TPO and recognized three different epitopes. The biological relevance of these scFvs as compared with serum anti-TPO autoantibodies was assessed by competition studies. Sera from all the 29 Graves’ disease patients tested were able to strongly inhibit (60–100%) the binding of the 3 scFvs to TPO. These data demonstrate that the in-cell PCR library generated human anti-TPO scFvs that retained the VH/VL pairing found in vivo and that the different epitope specificities defined by these scFvs overlapped with those found in the sera of patients with autoimmune thyroid disease.

In the present study we have analyzed the changes in the expressed antibody repertoire and in temporal fluctuations of antibody levels in serum that followed infusion of normal IgG (IVIg) in a patient with autoimmune thyroiditis. Administration of IVIg resulted in the stimulation of IgM production, in alterations of expressed antibody activity in serum that could not merely be accounted for by the passive transfer of antibody specificities contained in IVIg, in transient down-regulation of B cells clones expressing a specific disease-related idiotype and in the increase in serum in recipient's autoantibodies specifically reactive with F(ab')2 fragments of IVIg. In addition, infusion of IVIg shifted the pattern of spontaneous fluctuations of autoantibody activities in the patient's serum from a pattern indicative of disconnected events in the immune network to a pattern similar to that which is consistently observed in healthy controls. These results suggest that normal IgG may modulate autoreactivity by selecting expressed antibody repertoire through V region-dependent interactions with antibodies.