Targeted Expression of the Human Thyrotropin Receptor A-Subunit to the Mouse Thyroid: Insight into Overcoming the Lack of Response to A-Subunit Adenovirus Immunization

• Published in: The Journal of immunology (1950) 2006-01, Vol.176 (1), p.668-676
• Main Authors: Pichurin, Pavel N, Chen, Chun-Rong, Chazenbalk, Gregorio D, Aliesky, Holly, Pham, Nancy, Rapoport, Basil, McLachlan, Sandra M
• Format: Article
• Language: English
• Subjects: Adenoviridae - genetics; Adenovirus; Amino Acid Sequence; Animals; Autoantibodies - immunology; Autoantigens - immunology; Autoimmunity; Enzyme-Linked Immunosorbent Assay; Genetic Vectors; Graves Disease - immunology; Humans; Mice; Mice, Transgenic; Molecular Sequence Data; Protein Subunits - immunology; Receptors, Thyrotropin - biosynthesis; Receptors, Thyrotropin - genetics; Receptors, Thyrotropin - immunology; Sequence Homology, Amino Acid; Thyroid Gland - immunology; Thyroid Gland - pathology
• ISSN: 0022-1767; 1550-6606
• DOI: 10.4049/jimmunol.176.1.668

The thyrotropin receptor (TSHR), the major autoantigen in Graves' disease, is posttranslationally modified by intramolecular cleavage to form disulfide-linked A- and B-subunits. Because Graves' hyperthyroidism is preferentially induced in BALB/c mice using adenovirus encoding the free A-subunit rather than full-length human TSHR, the shed A-subunit appears to drive the disease-associated autoimmune response. To further investigate this phenomenon, we generated transgenic mice with the human A-subunit targeted to the thyroid. Founder transgenic mice had normal thyroid function and were backcrossed to BALB/c. The A-subunit mRNA expression was confirmed in thyroid tissue. Unlike wild-type littermates, transgenic mice immunized with low-dose A-subunit adenovirus failed to develop TSHR Abs, hyperthyroidism, or splenocyte responses to TSHR Ag. Conventional immunization with A-subunit protein and adjuvants induced TSHR Abs lacking the characteristics of human autoantibodies. Unresponsiveness was partially overcome using high-dose, full-length human TSHR adenovirus. Although of low titer, these induced Abs recognized the N terminus of the A-subunit, and splenocytes responded to A-subunit peptides. Therefore, "non-self" regions in the B-subunit did not contribute to inducing responses. Indeed, transgenic mice immunized with high-dose A-subunit adenovirus developed TSHR Abs with thyrotropin-binding inhibitory activity, although at lower titers than wild-type littermates, suggesting down-regulation in the transgenic mice. In conclusion, in mice expressing a human A-subunit transgene in the thyroid, non-self human B-subunit epitopes are not necessary to induce responses to the A-subunit. Our findings raise the possibility that autoimmunity to the TSHR in humans may not involve epitopes on a cross-reacting protein, but rather, strong adjuvant signals provided in bystander immune responses.