A Mouse Thyrotropin Receptor A-Subunit Transgene Expressed in Thyroiditis-Prone Mice May Provide Insight into Why Graves' Disease Only Occurs in Humans

Peptidomic profiling of endogenous peptides in the spleens of mouse models of Graves' disease

Background

Graves' disease (GD) is a common autoimmune thyroid disorder. The pathogenesis of GD involves an autoimmune response to the A subunit of the human thyrotropin receptor (hTSHR), although the specific mechanisms are not fully elucidated.

Methods

This study established a GD model by immunizing BALB/c mice with a recombinant adenovirus expressing the hTSHR A subunit. Spleen tissues were collected and analyzed using liquid chromatography-tandem mass spectrometry (LC-MS/MS) to identify differentially expressed peptides (DEPs). Gene Ontology (GO) analysis and KEGG pathway analysis were further utilized to annotate the functions of these DEPs. Additionally, peptide bioactivity prediction and molecular docking studies were conducted using Alphafold and Pymol software, respectively, to assess the binding affinity of specific peptides to the hTSHR A subunit.

Results

The GD mouse model was successfully established, and 1,428 DEPs were identified in the spleen, with 368 upregulated and 1,060 downregulated. Functional analysis indicated that these DEPs are mainly involved in cellular endocytosis, regulation of gene expression, and nucleocytoplasmic transport. Notably, molecular docking studies revealed that the abnormally highly expressed peptide HG2A-24aa demonstrated potential bioactivity and strong binding affinity with hTSHR-289aa.

Conclusion

The specific bioactive peptides may play key roles in the pathogenesis of GD, particularly HG2A-24aa, which may have a significant role in the MHC II antigen presentation pathway mediated by the hTSHR A subunit.

A transgenic mouse that spontaneously develops pathogenic TSH receptor antibodies will facilitate study of antigen-specific immunotherapy for human Graves' disease

Graves' hyperthyroidism can be treated but not cured. Antigen-specific immunotherapy would accomplish this goal, for which purpose an animal model is an invaluable tool. Two types of animal models are available. First, pathogenic TSHR antibodies (TSHRAb) can be induced by injecting mice with fibroblasts co-expressing the human TSHR (hTSHR) and MHC class II, or in mammals using plasmid or adenovirus vectors encoding the hTSHR or its A-subunit. Second, a mouse model that spontaneously develops pathogenic TSHRAb resembling those in human disease was recently described. This outcome was accomplished by transgenic intrathyroidal expression of the hTSHR A-subunit in NOD.H2^h4 mice that are genetically predisposed to develop thyroiditis but, without the transgene, do not generate TSHRAb. Recently, novel approaches to antigen-specific immunotherapy have been tested, primarily in the induced model, by injecting TSHR A-subunit protein or cyclic TSHR peptides. T-cell tolerance has also been induced in "humanized" HLA-DR3 mice by injecting synthetic peptides predicted in silico to mimic naturally processed TSHR T-cell epitopes. Indeed, a phase 1 study based on the latter approach has been conducted in humans. In the spontaneous model (hTSHR/NOD.H2^h mice), injection of soluble or nanoparticle-bearing hTSHR A-subunits had the unwanted effect of exacerbating pathogenic TSHRAb levels. A promising avenue for tolerance induction, successful in other conditions and yet to be tested with the TSHR, involves encapsulating the antigen. In conclusion, these studies provide insight into the potential outcome of immunotherapeutic approaches and emphasize the importance of a spontaneous model to test future novel, antigen-specific immunotherapies for Graves' disease.