Clonal relationships between thyroid-stimulating hormone receptor-stimulating antibodies illustrate the effect of hypermutation on antibody function

Longitudinal Characterization of Autoantibodies to the Thyrotropin Receptor (TRAb) During Alemtuzumab Therapy: Evidence that TRAb May Precede Thyroid Dysfunction by Many Years

BACKGROUND

Thyroid autoimmunity, especially Graves' disease or hypothyroidism with positive autoantibodies (TRAb) to the thyrotropin receptor (TSHR), occurs in 30-40% of patients with relapsing multiple sclerosis following treatment with alemtuzumab (ALTZ). ALTZ therapy therefore provides a unique opportunity to study the evolution of TRAb prior to clinical presentation. TRAb can stimulate (TSAb), block (TBAb), or not affect ("neutral") the TSHR function, causing hyperthyroidism, hypothyroidism, or euthyroidism, respectively.

METHODS

A longitudinal retrospective analysis was conducted of TRAb bioactivity over a period of nine years in 45 multiple sclerosis patients receiving ALTZ using available stored serum. Of these 45 patients, 31 developed thyroid dysfunction (TD) and 14 remained euthyroid despite being followed for a minimum of five years (NO-TD). The presence of TRAb was evaluated at standardized time points: (i) before ALTZ, (ii) latest time available following ALTZ and before TD onset, and (iii) following ALTZ during/after TD onset. Serum TRAb were detected by published in-house assays (ihTRAb): flow cytometry detecting any TSHR-binding TRAb, and luciferase bioassays detecting TSAb/TBAb bioactivity. Purified immunoglobulin G was used to verify TSAb/TBAb in selected hypothyroid cases. Standard clinical automated measurements of TRAb, antithyroid peroxidase autoantibodies (TPOAb), thyrotropin, free thyroxine, and free triiodothyronine were also collected.

RESULTS

Before ALTZ, combined ihTRAb (positive with flow cytometry and/or luciferase bioassay) but not automated TRAb were present in 5/16 (31.2%) TD versus 0/14 (0%) NO-TD (p = 0.017). Detectable ihTRAb preceded TD development in 9/28 (32.1%) and by a median of 1.2 years (range 28 days-7.3 years). Combination testing of ihTRAb and TPOAb at baseline predicted 20% of subsequent cases of hyperthyroidism and 83% of hypothyroidism.

CONCLUSIONS

Evidence is presented that TRAb measured with custom-made assays can be detected prior to any change in thyroid function in up to a third of cases of ALTZ-related TD. Furthermore, the presence of ihTRAb prior to ALTZ treatment was strongly predictive of subsequent TD. The findings suggest that a period of affinity maturation of TRAb may precede clinical disease onset in some cases. Combined testing of TPOAb and ihTRAb may increase the ability to predict those who will develop TD following ALTZ.

Yersinia enterocolitica provides the link between thyroid-stimulating antibodies and their germline counterparts in Graves' disease

Graves' disease results from thyroid-stimulating Abs (TSAbs) activating the thyrotropin receptor (TSHR). How TSAbs arise from early precursor B cells has not been established. Genetic and environmental factors may contribute to pathogenesis, including the bacterium Yersinia enterocolitica. We developed two pathogenic monoclonal TSAbs from a single experimental mouse undergoing Graves' disease, which shared the same H and L chain germline gene rearrangements and then diversified by numerous somatic hypermutations. To address the Ag specificity of the shared germline precursor of the monoclonal TSAbs, we prepared rFab germline, which showed negligible binding to TSHR, indicating importance of somatic hypermutation in acquiring TSAb activity. Using rFab chimeras, we demonstrate the dominant role of the H chain V region in TSHR recognition. The role of microbial Ags was tested with Y. enterocolitica proteins. The monoclonal TSAbs recognize 37-kDa envelope proteins, also recognized by rFab germline. MALDI-TOF identified the proteins as outer membrane porin (Omp) A and OmpC. Using recombinant OmpA, OmpC, and related OmpF, we demonstrate cross-reactivity of monoclonal TSAbs with the heterogeneous porins. Importantly, rFab germline binds recombinant OmpA, OmpC, and OmpF confirming reactivity with Y. enterocolitica. A human monoclonal TSAb, M22 with similar properties to murine TSAbs, also binds recombinant porins, showing cross-reactivity of a spontaneously arising pathogenic Ab with Y. enterocolitica. The data provide a mechanistic framework for molecular mimicry in Graves' disease, where early precursor B cells are expanded by Y. enterocolitica porins to undergo somatic hypermutation to acquire a cross-reactive pathogenic response to TSHR.