Loss of Aire-dependent thymic expression of a peripheral tissue antigen renders it a target of autoimmunity

Abstract

Both humans and mice with a mutation in the autoimmune regulator (aire) gene develop multiorgan autoimmune disease. Aire was shown to exert its critical function in medullary epithelial cells of the thymus by promoting ectopic expression of peripheral tissue antigens. It was hypothesized that the widespread autoimmunity of Aire-deficient individuals reflects a lack of tolerance induction to the repertoire of peripheral tissue antigens expressed in the thymus of normal individuals. Here, we substantiate this hypothesis by identifying Mucin 6 as a stomach-specific antigen targeted by autoantibodies in gastritis-prone mice lacking thymic expression of aire and demonstrate that transcription of the Mucin 6 gene in thymic medullary epithelial cells is indeed Aire-dependent.

Fig. 1.

Stomach autoimmunity in Aire-deficient mice. Chronic inflammation of the stomach in aire KO mice. (A) H&E-stained paraffin stomach sections with normal gastric anatomy from a 20-week-old WT mouse (Left, magnification ×10); inflammatory infiltrates in the stomach mucosa of an age- and sex-matched aire KO mouse (Right). (B) Indirect immunofluorescence staining of frozen sections from a Rag-deficient mouse's stomach with serum from aire KO (Right, ×10) or WT (Left, ×10) mice. Positive staining is indicated by green fluorescence. (C) A stomach antigen of 100 kDa immunoprecipitated by autoantibodies from aire KO/WT chimeras and visualized by silver staining on an SDS-PAGE gel. Immunoprecipitating antibodies were derived from sera of 20-week-old WT (antigen fraction numbers 1–5) (Right) or chimeric Aire-deficient recipients of WT bone marrow (antigen fractions 1–5) (Left).

Fig. 2.

The autoantibody target in the stomach is a mucin. (A) Tandem MS analysis of the gel-purified, trypsin-digested stomach antigen. A characteristic mass spectrum (mass/charge on the x axis versus relative abundance on the y axis) is exemplified for one tryptic peptide, along with the backbone fragmentation pattern, where b and y ions have the charge retained on the N- or C-terminal fragment, respectively. The corresponding amino acid sequence of the peptide AQCPCLLDDYK_761–771, (M + 2H)²⁺ = 691.71, is derived by adding the mass differences between adjacent b (or y) ions, indicative of particular amino acid residues. (B) Summary of the best matches of the peptide mass fingerprint. The Mowse scoring algorithm (33) was used to compare the calculated peptide masses [Mr(calc)] for each entry in the sequence database with the set of experimental data [Mr(expt)]. Each calculated value that falls within a given mass tolerance (Delta) of an experimental value counts as a match. The peptide score is defined as the probability that the observed match is a random event and equals −10·log₁₀(P), where P is the absolute probability. Peptide scores >40 indicate identity. The expectation value is the number of matches with equal or better scores that are expected to occur by chance alone. (C) Microsequenced peptides were found to derive from a gastric, secreted, gel-forming mucin (Mucin 6; GenBank accession no. NP859418). The peptide sequences identified through MS/MS analysis are marked in bold letters within the N terminus of the protein. (D) Schematic representation of the predicted Mucin 6 sequence motifs (34, 35). The Mucin 6 peptides identified by microsequencing cluster within the N-terminal region that is homologous to type D repeat domains of the von Willebrand clotting factor (D1–D3). Threonine-, serine-, and proline-rich domains are composed of either unique sequence (TSP) or tandem repeats (TR). A cystine knot domain (CK) is located at the C terminus of the protein. Predicted regions of N- and O-glycosylation are marked symbolically within protein sequence motifs.

Fig. 3.

Generation of anti-Mucin 6 autoantibodies versus gastric inflammation. (A) Immunoblots of recombinant Mucin 6 with sera from WT (Left) and aire KO littermate mice (Right) on a mixed B6×129 background. The latter, but not the former, develop gastritis. (B) Immunoblots of recombinant Mucin 6 with sera from gastritis-susceptible, Aire-deficient mice on the BALB/c (Left) and NOD (Right) backgrounds, and sera from gastritis-resistant B6 Aire-deficient mice (Center).

Fig. 4.

Aire regulates thymic mucin 6 expression. (A) Mucin 6 expression levels were quantitated by RT-PCR on cDNA prepared from diverse tissues of a 6-week-old mouse. The ratios of muc6 to hprt expression for each tissue are plotted on a logarithmic scale in arbitrary units. (B) RT-PCR assay of mucin 6 expression in thymic epithelia prepared by teasing out the thymocytes from individual thymi of 4-week-old aire KO and WT mice. (C) RT-PCR assay of mucin 6 transcription levels in sorted thymic MECs from WT and aire KO mice. In all cases, results are representative of three independent experiments.