Autoantibody specific for a thyroglobulin epitope inducing experimental autoimmune thyroiditis or its anti-idiotype correlates with the disease

Thyroglobulin as an autoantigen: what can we learn about immunopathogenicity from the correlation of antigenic properties with protein structure?

Autoantibodies against human thyroglobulin are a hallmark of autoimmune thyroid disease in humans, and are often found in normal subjects. Their pathogenic significance is debated. Several B-cell epitope-bearing peptides have been identified in thyroglobulin. They are generally located away from the cysteine-rich regions of tandem sequence repetition. It is possible that our current epitopic map is incomplete because of the difficulty that proteolytic and recombinant approaches have in restituting conformational epitopes based upon proper pairing between numerous cysteinyl residues. Furthermore, the homology of cysteine-rich repeats with a motif occurring in several proteins, endowed with antiprotease activity, suggests that these regions may normally escape processing and presentation to the immune system, and brings attention to the mechanisms, such as oxidative cleavage, by which such cryptic epitopes may be exposed. A number of T-cell epitope-bearing peptides, endowed with thyroiditogenic power in susceptible mice, were also identified. None of them was dominant, as none was able to prime in vivo lymph node cells that would proliferate or transfer autoimmune thyroiditis to syngeneic hosts, upon stimulation with intact thyroglobulin in vitro. More than half of them are located within the acetylcholinesterase-homologous domain of thyroglobulin, and overlap B-cell epitopes associated with autoimmune thyroid disease, while the others are located within cysteine-rich repeats. The immunopathogenic, non-dominant character of these epitopes also favours the view that the development of autoimmune thyroid disease may involve the unmasking of cryptic epitopes, whose exposure may cause the breaking of peripheral tolerance to thyroglobulin. Further research in this direction seems warranted.

Self-reactive B cells are not eliminated or inactivated by autoantigen expressed on thyroid epithelial cells

Graves' Disease results from the production of autoantibodies against receptors for thyroid stimulating hormone (TSH) on thyroid epithelial cells, and represents the prototype for numerous autoimmune diseases caused by autoantibodies that bind to organ-specific cell membrane antigens. To study how humoral tolerance is normally maintained to organ-specific membrane antigens, transgenic mice were generated selectively expressing membrane-bound hen egg lysozyme (mHEL) on the thyroid epithelium. In contrast to the deletion of autoreactive B cells triggered by systemic mHEL (Hartley, S.B., J. Crosbie, R. Brink, A.B. Kantor, A. Basten, and C.C. Goodnow. 1991. Nature. 353:765-769), selective expression of mHEL autoantigen on thyroid cells did not trigger elimination or inactivation of circulating HEL-reactive B cells. These results provide evidence that tolerance is not actively acquired to organ-specific antigens in the preimmune B cell repertoire, underscoring the importance of maintaining tolerance to such antigens by other mechanisms. The role of an intact endothelial barrier in sequestering organ-specific antigens from circulating preimmune B cells is discussed.

Mouse thyroglobulin: conservation of sequence homology in C-terminal immunogenic regions of thyroglobulin

cDNA encoding 287 amino acids of the C-terminus of mouse thyroglobulin was cloned and sequenced. The amino acid homology between mouse and rat thyroglobulin was 96%, and was 78% between mouse and human. It was found that mouse thyroglobulin completely shared homology with two thyroiditogenic peptides described by other investigators. These findings are consistent with our hypothesis that in murine experimental thyroiditis, the primary thyroiditogenic epitopes are encoded by mouse-specific regions of thyroglobulin.

Molecular heterogeneity of antigen- or idiotype-induced anti-thyroglobulin monoclonal autoantibodies

To define the molecular basis of the cognitive interaction in experimental autoimmune thyroiditis (EAT), we sequenced the variable regions of monoclonal autoantibodies to thyroglobulin (Tg), specific or not for the F40D peptide, a Tg peptide capable of inducing EAT in CBA/J mice. Three MoAbs were obtained by immunization with syngeneic Tg of CBA/J (3B8G9, 2F6F2) or C57Bl/6 (4D11F4) mice. 3B8G9 was specific for F40D peptide, whereas 2F6F2 and 4D11F4 were not. Two others were raised in CBA/J mice by manipulation of idiotypic pathways: B12 resulted from the immunization with one Ab2 beta, bearing the internal image of one F40D epitope, and TA2 from the immunization with F40D-specific cytotoxic HTC2 T cells. B12 and TA2 were both specific for F40D. All hybridomas expressed different members of the J558 VH family, except 3B8G9 which expressed a Q52 VH gene segment. These data led us to hypothesize that regulatory anti-id autoantibodies used members of one VH family located in the 5'-end of the VH locus, whereas EAT-associated autoantibodies used a member of one of the most D-proximal VH family. As expected, no homologies were found when anti-F40D monoclonal autoantibodies were compared with two other monoclonal autoantibodies displaying a different epitopic specificity. Among the anti-F40D monoclonal autoantibodies, one histidine residue located in position 35 of the CDR1 region was constantly found. Moreover, TA2 and B12 exhibited two common amino acids in their CDR3 regions, one glycine and one tyrosine, in positions 98 and 99, respectively. Striking homologies were found between TA2 and one anti-polyGAT MoAb, and between 3B8G9 and some anti-phenyloxazolone (phOx) monoclonal autoantibodies. Lastly, the VK sequence from 4D11F4 was identical at the amino acid level to the VK sequence from another monoclonal autoantibody, 81B1, which was previously raised towards syngeneic Tg in CBA/J mice. Our data imply that anti-idiotypic regulatory circuits in EAT might be generated by a heterogeneous population of B cells rather than obtained by a single dominant B cell population.

Ig VH gene family usage in spleen cells of CBA/J mice immunized with experimental autoimmune thyroiditis (EAT) inducer antigens

Experimental autoimmune thyroiditis (EAT) is an autoimmune disorder of the thyroid gland induced in susceptible strains of mice by thyroglobulin (Tg). We recently showed that low Mr (< 10 kDa) Tg tryptic fragments and a 40 amino-acid peptide (F40D) from Tg could induce EAT as well as native Tg. Because it has been reported that autoantibodies (A-Abs) express VH families preferentially located in the D-proximal VH gene segment, we investigated whether A-Abs specific for one pathogenic peptide from Tg were also skewed towards D proximal VH gene segment. In that respect, we immunized CBA/J mice with EAT inducer antigens of decreasing sizes: Tg (660 M(r)), < 10 kDa Tg trypic fragments or F40D peptide (4.9 kDa M(r)) from Tg. The VH gene segments utilized by immune spleen cells were determined by hybridization to total spleen cell RNA previously deposited onto nylon membranes and densitometric scans. This study was conducted on days 7 and 9 after determination of the maximum amounts of mRNA coding for immunoglobulins and on day 28 when A-Ab levels are the highest. Results were compared to VH gene segment expression both in normal and adjuvant-injected mice. We found that immunization of CBA/J mice with EAT inducer antigens stimulate B cells the restriction of which, in terms of VH family usage, depends on the size of the immunizing antigen: the larger the antigen, the higher the numbers of VH families used. Moreover, we found that B cell stimulation consecutive to immunization with the peptidic antigen inducing EAT occurs in VH Q52 family, a VH encoded by D-proximal gene segment.

The effects of a monoclonal antibody to interferon-gamma on experimental autoimmune thyroiditis (EAT): prevention of disease and decrease of EAT-specific T cells

CBA/J mice immunized with thyroglobulin (Tg) develop an experimental autoimmune thyroiditis (EAT) with lymphocytic infiltration of the thyroid glands, autoantibodies to Tg and occurrence of EAT-specific T cells. When these mice were treated for 4 weeks after immunization with 1 mg/week of a monoclonal antibody (mAb) that neutralizes the activity of interferon-gamma (IFN) a beneficial effect on the onset of EAT was observed. Characteristic features of EAT were significantly reduced, including the lymphocytic infiltrations of the thyroid glands and the serum levels of autoantibodies to Tg. Moreover, in lymphoid organs, mAb to IFN-gamma significantly reduced the percentages of Tg-specific CD8+ cells, labeled by the anti-clonotypic mAb AG7. These Tg-specific T cells seem responsible for thyroid damages and disease development, since EAT was simultaneously abrogated. These results show that IFN-gamma plays an essential role in the pathophysiology of EAT and suggest the possibility to treat autoimmune thyroid diseases with mAb to IFN-gamma or drugs able to antagonize the production and/or the action of this cytokine.

Target organ susceptibility and autoantibody production in an animal model of spontaneous autoimmune thyroiditis

F1-hybrids of Obese strain (OS) chickens, afflicted with spontaneous autoimmune thyroiditis (SAT), and normal, inbred CB chickens, do not develop severe thyroiditis. About 50% of these crosses show circulating autoantibodies to thyroglobulin (TgAAb), but the thyroid glands are only slightly infiltrated, suggesting that the target organ is not susceptible to autoimmune attack. In the present study we show that despite this mild infiltration TgAAb are only synthesized by lymphoid cells within the thyroid gland. Furthermore, we demonstrate that immunization with chicken thyroglobulin (Tg) in complete Freund's adjuvant causes severe experimental autoimmune thyroiditis (EAT) in F1(OSxCB) hybrids.

Synergism between mouse thyroglobulin- and vaccination-induced suppressor mechanisms in murine experimental autoimmune thyroiditis

Previous studies have shown that genetically susceptible mice can be rendered resistant to the induction of experimental autoimmune thyroiditis (EAT) by pretreatment with deaggregated mouse thyroglobulin (dMTg). This resistance is mediated by CD4+ suppressor T cells (Ts) which suppress the afferent/inductive phase of EAT. Recent work has also shown that resistance to EAT can be achieved by vaccination with irradiated spleen cells previously primed in vivo with MTg and cultured in vitro with MTg (gamma SC). The gamma SC-induced resistance also inhibits the afferent phase of EAT but is mediated by both CD4+ and CD8+ Ts. To determine if dMTg- and gamma SC-induced suppression can cooperate to prevent EAT, we pretreated mice with suboptimal doses of dMTg and gamma SC before challenge with MTg and adjuvant. Mice receiving dMTg or gamma SC only showed suppressed in vitro response to MTg, but the development of thyroid lesions was unaltered. However, mice given one or two subtolerogenic doses of dMTg followed by gamma SC not only showed suppressed in vitro response to MTg, but also little or no thyroiditis, indicating cooperation between these two mechanisms. The cooperation was not reciprocal since reversing the order, giving gamma SC first followed by dMTg, was not effective in suppressing EAT. Thus, suppressor mechanisms activated by pretreatment with dMTg and gamma SC can act synergistically to suppress EAT induction; the two mechanisms may cooperate in vivo to maintain self-tolerance provided that MTg-specific CD4+ Ts are initially activated.

In vivo evidence for CD4+ and CD8+ suppressor T cells in vaccination-induced suppression of murine experimental autoimmune thyroiditis

In several experimental autoimmune diseases, including experimental autoimmune thyroiditis (EAT), vaccination with attenuated autoantigen-specific T cells has provided protection against subsequent induction of disease. However, the mechanism(s) of vaccination-induced suppression remains to be clarified. Since we have previously shown that suppression generated by pretreatment with mouse thyroglobulin (MTg) or thyroid-stimulating hormone in EAT is mediated by CD4+, not CD8+, suppressor T cells, we examined the role of T cell subsets in vaccination-induced suppression of EAT. Mice were vaccinated with irradiated, MTg-primed, and MTg-activated spleen cells and then challenged. Pretreatment with these cells suppressed EAT induced by immunization with MTg and adjuvant, but not by adoptive transfer of thyroiditogenic cells, suggesting a mechanism of afferent suppression. The activation of suppressor mechanisms did not require CD8+ cells, since mice depleted of CD8+ cells before vaccination showed reduced EAT comparable to control vaccinated mice. Furthermore, depletion of either the CD4+ or the CD8+ subset after vaccination did not significantly abrogate suppression. However, suppression was eliminated by the depletion of both CD4+ and CD8+ cells in vaccinated mice. These results provide evidence for the cooperative effects of CD4+ and CD8+ T cells in vaccination-induced suppression of EAT.