Bioinformatics Support the Possible Triggering of Autoimmune Thyroid Diseases by Yersinia enterocolitica Outer Membrane Proteins Homologous to the Human Thyrotropin Receptor

Dear Editor:

Several years ago some of us (1,2) provided in silico evidence in this journal that Yersinia enterocolitis (YE) and Borrelia burgdorferi may trigger autoimmune thyroid disease (AITD) via molecular mimicry, particularly with the thyrotropin receptor (TSHR). Indeed, homologies concerning TSHR outnumber those concerning thyroglobulin (Tg) or thyroperoxidase (TPO). Ten Borrelia proteins and eight YE proteins share homology with autoepitope-containing segments of the TSHR (nine cases), of Tg (four cases), and of TPO (five cases). The match with the greatest span was in amino acid (aa) 81–322 of the Yersinia outer membrane protein M (ompM), sharing 26% identity and 43% similarity with aa 22–272 of TSHR (a region enriched in T-cell and B-cell epitopes). In homologous sequences, particularly those concerning the TSHR, binding motifs for AITD-associated human leukocyte antigen (HLA)-DR alleles outnumbered those for non-AITD-associated HLA-DR alleles (see Supplementary Data, available online at www.liebertonline.com/thy).

Recently, Wang (3) reported that (i) positive rates of anti-YE antibodies (YEAb) in Graves' disease (GD) and Hashimoto's thyroiditis patients were significantly higher than in controls and other thyroid disease patients (p<0.01), (ii) within GD, the prevalence of YEAb in relapsing (78%) or de novo (69%) patients was significantly greater than in remission patients (25.5%, p<0.01), (iii) TSHR antibodies (TRAb) correlated significantly with YEAb (r=0.403, p<0.01), but YEAb did not correlate with thyroid microsomal Ab, TgAb, triiodothyronine, or thyroxine, (iv) TRAb cross-reacted with YE proteins and YEAb cross-reacted with TSHR, (v) at least one cross-reactive protein was outer membrane protein F (ompF), and (vi) ompF-Ab share with TRAb TSHR-stimulating activity, though of lower magnitude. Finally, they reported a 10.6% amino acid sequence identity between ompF and the extracellular segment of TSHR, and noted that the region 198–205 of TSHR (DALGNVTS), likely to be epitopic according to the algorithm by Larsen et al. (4), is similar to region 190–197 ( DA F G G V Y S ) [similar residues are presented in boldface; identical residues are presented in boldface with underline] of ompF. They concluded that multiple proteins of YE could induce TRAb, and that their results “suggest that YE ompF is involved in the roduction of TRAb and the pathogenesis of GD through molecular mimicry.”

This Chinese article (3) independently confirms our aforementioned data. We show here that, based on a homogeneous bioinformatic comparison (not possible in our 2006 article, because ompF was added to Entrez Protein in 2007), our ompM protein is numerically superior to ompF.

Using the ClustalW2 software, we found that aa 22–272 of TSHR are homologous to aa 81–322 of ompM (identity 22%, similarity 44%) and to aa 1–242 of ompF (identity 13%, similarity 31%). The predicted TSHR epitope DALGNVTS (aa 198–205) was conserved in ompM (aa 181–189, DA SSN GIR S ) and in ompF (aa 190–197, DA F G G V Y S ).

Most importantly, we searched for HLA-DR binding motifs within the shared segments. The number of HLA-DR binding motifs in the segment of TSHR homologous to ompM and ompF ranges from 3 (HLA-DR6) to 20 (HLA-DR4); in bacterial counterparts, the range is between 0 (HLA-DR6) and 28 (HLA-DR4) for ompM, and between 0 (HLA-DR6) and 16 (HLA-DR4 and DR7) for ompF. The occurrence of binding motifs of AITD-associated HLA alleles was more frequent in the ompM segment than in the ompF segment in the case of DR4 and DR9, less frequent in the case of DR5 and DR8, and identical in the case of DR3 (see Supplementary Data).

Our bioinformatics analysis on ompF, whose cross-reactivity with TSHR is experimentally verified, agrees with results and hypotheses presented in our 2006 article (1). We confirm and explain the neutral role of HLA-DR6 in both AITD and Yersinia infection, because there are very few copies of the HLA-DR6 motif in the relevant segment of TSHR and none in its ompF homolog, and reinforce the idea that Yersinia proteins probably are not a trigger of AITD in HLA-DR5-positive persons.

Bioinformatics data suggest that both ompF and ompM could trigger AITD in Caucasians or Asians, because of molecular mimicry with an immunologically relevant and disease-related part of TSHR. In detail, ompF could trigger anti-TSHR autoimmunity more frequently than ompM in HLA-DR8-positive persons, whereas the opposite should occur in HLA-DR4 and HLA-DR9-positive persons, and the probability should be equal in HLA-DR3-positive subjects.

Wang (3) conclude that “other sequences of Yersinia might also share cross-immunogenicity with hTSHR, and this warrants further study.” Based on the consistency between our data and theirs (3), we believe that the omp family contains AITD triggers. Whether this family is involved in eliciting other autoimmune diseases remains to be explored.