Similarities and differences in interactions of thyroid stimulating and blocking autoantibodies with the TSH receptor

Abstract

Binding of a new thyroid-stimulating human monoclonal autoantibody (MAb) K1-18 to the TSH receptor (TSHR) leucinerich domain (LRD) was predicted using charge-charge interaction mapping based on unique complementarities between the TSHR in interactions with the thyroid-stimulating human MAb M22 or the thyroid-blocking human MAb K1-70. The interactions of K1-18 with the TSHR LRD were compared with the interactions in the crystal structures of the M22-TSHR LRD and K1-70-TSHR LRD complexes. Furthermore, the predicted position of K1-18 on the TSHR was validated by the effects of TSHR mutations on the stimulating activity of K1-18. A similar approach was adopted for predicting binding of a mouse thyroid-blocking MAb RSR-B2 to the TSHR. K1-18 is predicted to bind to the TSHR LRD in a similar way as TSH and M22. The binding analysis suggests that K1-18 light chain (LC) mimics binding of the TSH-a chain and the heavy chain (HC) mimics binding of the TSH-b chain. By contrast, M22 HC mimics the interactions of TSH-a while M22 LC mimics TSH-b in interactions with the TSHR. The observed interactions in the M22-TSHR LRD and K1-70-TSHR LRD complexes (crystal structures) with TSH-TSHR LRD (comparative model) and K1-18-TSHR LRD (predictive binding) suggest that K1-18 and M22 interactions with the receptor may reflect interaction of thyroid-stimulating autoantibodies in general. Furthermore, K1-70 and RSR-B2 interactions with the TSHR LRD may reflect binding of TSHR-blocking autoantibodies in general.

Yersinia enterocolitica Provides the Link between Thyroid-Stimulating Antibodies and Their Germline Counterparts in Graves’ Disease

“…Remarkably, identical residues K58, R80, and H105 in the LRR region of TSHR A-subunit, common to the M22 epitopes on the receptor, were identified as important determinants for the murine TSAb, IRI-SAb2 binding and activation of the receptor (8). Thus, although knowledge on the epitopes on TSHR A-subunit of murine TSAbs is limited to a small number of TSAbs (8,16), the fact that KSAb1 and KSAb2 are comparable in their stimulating properties to the murine IRI-SAb2 and human M22 mAbs (52) makes it likely that amino acids R38, K58, R80, H105, and K129 important for M22 interaction are also important for binding and stimulation function KSAb1 and KSAb2 evaluated in this study. The restricted number of the amino acid determinants on the receptor essential for TSAb activity is supported by the finding of restricted germline gene usage of mouse and human TSAbs (Supplemental Table I), which perhaps can only be mediated by a particular conformation of Ab that can be attained by a few combinations of H and L chains.…”

“…Importantly, human monoclonal TSAbs from patients with Graves' disease have also been derived, providing detail into their molecular and biochemical properties, and pathogenicity in vivo (11)(12)(13). A major advance has been the delineation of the atomic structures of immune complex of human monoclonal TSAb, M22 and human monoclonal TSBAb, and K1-70 with human TSHR A-subunit (14)(15)(16). The structures reveal the epitopes for M22 and K1-70 to be dependent on discontinuous determinants on leucine-rich repeat (LRR) regions of TSHR A-subunit (14,15).…”

TSH-receptor-expressing fibrocytes and thyroid-associated ophthalmopathy

“…Highly selective small molecule antagonists of TSHR have been reported by Neumann et al 114 and among the latest is an agent designated ANTAG3 which demonstrates activity in vitro as well as in mice. Alternatively, stimulatory and blocking antibodies targeting TSHR have been developed 115,116 and their divergent interactions with the receptor have been scrutinized in great detail 117 . Either approach might prove effective in reducing the impact of the inflammatory phenotype exhibited by TSH-engaged fibrocytes.…”

Building the Case for Insulin-Like Growth Factor Receptor-I Involvement in Thyroid-Associated Ophthalmopathy

“…Some block binding of TSH to the receptor (33) while others are viewed as “neutral.” The exact mechanisms involved in the activation of TSHR by either TSH or TSIs remain uncertain although the ligand binding epitopes have been localized (33, 34). Interactions between the different classes of anti-TSHR antibodies and the receptor have also been characterized (41). Signaling downstream from TSHR is complex and involves several pathways that cross talk in patterns that determine the ultimate genes targeted for activation (42–44).…”