Pathogenicity of a human thyroglobulin peptide (2340-2359) in mice with high or low genetic susceptibility to thyroiditis

Estrogen-mediated downregulation of AIRE influences sexual dimorphism in autoimmune diseases

Autoimmune diseases affect 5% to 8% of the population, and females are more susceptible to these diseases than males. Here, we analyzed human thymic transcriptome and revealed sex-associated differences in the expression of tissue-specific antigens that are controlled by the autoimmune regulator (AIRE), a key factor in central tolerance. We hypothesized that the level of AIRE is linked to sexual dimorphism susceptibility to autoimmune diseases. In human and mouse thymus, females expressed less AIRE (mRNA and protein) than males after puberty. These results were confirmed in purified murine thymic epithelial cells (TECs). We also demonstrated that AIRE expression is related to sexual hormones, as male castration decreased AIRE thymic expression and estrogen receptor α-deficient mice did not show a sex disparity for AIRE expression. Moreover, estrogen treatment resulted in downregulation of AIRE expression in cultured human TECs, human thymic tissue grafted to immunodeficient mice, and murine fetal thymus organ cultures. AIRE levels in human thymus grafted in immunodeficient mice depended upon the sex of the recipient. Estrogen also upregulated the number of methylated CpG sites in the AIRE promoter. Together, our results indicate that in females, estrogen induces epigenetic changes in the AIRE gene, leading to reduced AIRE expression under a threshold that increases female susceptibility to autoimmune diseases.

A novel pathogenic peptide of thyroglobulin (2208-2227) induces autoreactive T-cell and B-cell responses in both high and low responder mouse strains

Experimental autoimmune thyroiditis (EAT) is commonly induced by thyroglobulin (Tg) or Tg peptides in mice genetically susceptible to thyroiditis. In the present study, we investigated the immunogenic and pathogenic potential of a novel 20mer human Tg peptide, p2208 (amino acids 2208-2227), in mouse strains classified as low (LR) or high (HR) responders in EAT. The peptide was selected for its content in overlapping binding motifs for MHC class II products, associated with either resistance (A(b)), or susceptibility (A(s), E(k)) to EAT. We therefore immunized LR BALB/c (H-2(d)) and C57BL/6 (H-2(b)) strains, as well as HR CBA/J (H-2(k)) and SJL/J (H-2(s)) mice with 100 nmol of p2208 in adjuvant and collected their sera, lymph nodes and thyroid glands for further analysis. The p2208 peptide was found to contain B-cell and cryptic T-cell epitope(s) in two of the four strains examined, one LR and one HR. Specifically, it elicited direct EAT in C57BL/6 mice (two of seven mice, infiltration index 1-3), as well as in SJL/J mice (two of six mice, infiltration index 1-2). Such an EAT model could provide insights into the immunoregulatory cascades taking place in resistant hosts.

Fine epitope mapping within the pathogenic thyroglobulin peptide 2340-2359: minimal epitopes retaining antigenicity across various MHC haplotypes are not necessarily immunogenic

We have previously reported that the 20-mer peptide p2340 (amino acids 2340-2359), of human thyroglobulin (Tg) has the unique feature that it causes experimental autoimmune thyroiditis (EAT) in mouse strains bearing high-responder (HR) or low-responder (LR) MHC haplotypes in Tg-induced EAT. In this study, we have employed fine epitope mapping to examine whether this property of p2340 is the result of recognition of distinct or shared minimal T-cell epitopes in the context of HR or LR MHC class II molecules. Use of overlapping peptides showed that a core minimal 9-mer epitope (LTWVQTHIR, amino acids 2344-2352) was recognized by p2340-primed T cells from both HR (H2(k,s) ) and LR (H2(b,d) ) strains, whereas a second 9-mer epitope (HIRGFGGDP, amino acids 2350-2358) was antigenic only in H2(s) hosts. Truncation analysis of LTWVQTHIR and HIRGFGGDP peptides delineated them as the minimal epitopes recognized by p2340-primed T cells from the above strains. Subcutaneous challenge of all mouse strains with the 9-mer core peptide LTWVQTHIR in adjuvant elicited specific lymph node cell proliferative responses and mild EAT only in HR hosts, highlighting this sequence as a minimal pathogenic Tg peptide in EAT. The 9-mer peptide HIRGFGGDP was not found to be immunogenic in H2(s) hosts. These data demonstrate that minimal T-cell epitopes, defined as autoantigenic in hosts of various MHC haplotypes, are not intrinsically immunogenic. Activation of naive autoreactive T cells may require contributions from flanking residues within longer peptide sequences encompassing these epitopes.

Tg.2098 is a major human thyroglobulin T-cell epitope

An HLA-DR variant containing Arginine at position 74 of the DRbeta1 chain confers a strong genetic susceptibility to autoimmune thyroid diseases (AITD), Graves' disease (GD) and Hashimoto's thyroiditis (HT), while Glutamine at position DRbeta1-74 is protective. We hypothesized that the DRbeta1-Arg74 variant is able to present pathogenic thyroglobulin (Tg) peptides to T-cells more efficiently, thereby triggering thyroid autoimmunity. Indeed, we have previously identified 4 human Tg (hTg) peptides that bind specifically to DRbeta1-Arg74 with much weaker binding to the protective variant DRbeta1-Gln74. The aim of our study was to examine in vivo whether an hTg peptide that binds strongly and specifically to DRbeta1-Arg74 is capable of stimulating T-cells during the induction of thyroiditis in a "humanized" mouse expressing human DR3, and in patients positive for Tg antibodies. Sequencing of exon 2 of the DR transgene in the DR3 mice, null for endogenous MHC II molecules, confirmed that they expressed the disease-associated DRbeta1-Arg74 variant, thus making them an ideal in vivo model to test the presentation of hTg peptides by DRbeta1-Arg74 HLA-DR. Induction of EAT in the DR3 mice lead to T-cell stimulation and proliferation to Tg.2098, a strong and specific DRbeta1-Arg74 binder, while a non-binding control peptide, Tg.2766 did not induce this response. Moreover, Tg.2098 stimulated T-cells from 4 individuals who were positive for thyroglobulin antibodies, demonstrating that Tg.2098 is an immunogenic peptide capable of being presented in vivo and activating T-cells in EAT and AITD. Energetic analysis of the complex formed by Tg.2098 and DRbeta-Arg74 has shown that the origin of the affinity was determined by residues 1, 7 and 9 in the peptide, while the selectivity of the peptide for the MHC was determined by the Asp in position 4. The disease-protective substitution R74Q, leads to reduction in affinity due to changes in local interaction with D4 as well as non-local interaction with other residues. The electrostatic potential on the surface of the DRbeta-Arg74-Tg.2098 complex has a unique signature which may be recognized by T-cell receptors leading to autoimmune thyroiditis. Taken together these findings suggest that Tg.2098, a strong and specific binder to the disease-associated HLA-DRbeta-Arg74, is a major human T-cell epitope and participant in the pathoetiology of AITD.

H2E-derived Ealpha52-68 peptide presented by H2Ab interferes with clonal deletion of autoreactive T cells in autoimmune thyroiditis

Susceptibility and resistance to experimental autoimmune thyroiditis is encoded by MHC H2A genes. We reported that traditionally resistant B10 (H2(b)) mice permit thyroiditis induction with mouse thyroglobulin (mTg) after depleting regulatory T cells (Tregs), supporting A(b) presentation to thyroiditogenic T cells. Yet, Ea(k) transgenic mice, expressing A(b) and normally absent E(b) molecules (E(+)B10 mice), are susceptible to thyroiditis induction without Treg depletion. To explore the effect of E(b) expression on mTg presentation by A(b), seven putative A(b)-binding, 15-16-mer peptides were synthesized. Five were immunogenic for both B10 and E(+)B10 mice. The effect of E(b) expression was tested by competition with an Ealpha52-68 peptide, because Ealpha52-68 occupies approximately 15% of A(b) molecules in E(+)B10 mice, binding with high affinity. Ealpha52-68 competitively reduced the proliferative response to mTg, mTg1677, and mTg2342 of lymph node cells primed to each Ag. Moreover, mTg1677 induced mild thyroiditis in Treg-depleted B10 mice, and in E(+)B10 mice without the need for Treg depletion. Ealpha52-68 competition with mTg-derived peptides may impede clonal deletion of pathogenic, mTg-specific T cells in the thymus.