primary prevention of coeliac disease - Associazione Italiana ...

24 A DOMINANT EPITOPE IN GLUTEN PEPTIDES ?
coeliac disease are consistent with A-gliadin 57-73 deamidated by tTG being the
dominant a-, a/b-wheat gliadin T cell epitope. Searches for peptides including the core
sequence PQLPY have not revealed other wheat, rye or barley gluten sequences outside
the a-, a/b-gliadins of wheat. One possibility is that epitope spreading initiated by the
dominant epitope leads to other epitopes in gluten being recognized. Interestingly, B
cell epitopes have been identified that are immediately adjacent or including the C - and
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N-terminal portions of A-gliadin 57-73 . The other HLA-DQ2 restricted epitopes, g-
gliadin GDB2 and A-gliadin 31-49 are also adjacent to or include sequences identical to
or very similar to the same gliadin B cell epitope (QXQPFP). Hence, B cell mediated
epitope spreading may be initiated by B cells given “help” by T cells specific for the
dominant epitope. Alternatively, wheat, rye and barley may contain prolamin
sequences that are cross-reactive with A-gliadin 57-73 but do not include PQLPY, or
there are other unique dominant epitopes.
Fine molecular specificity of A-gliadin 57-73 QE65-specific peripheral blood T
cells
Residues in A-gliadin 57-73 QE65 that determine bioactivity were mapped by
comparing bioactivity of A-gliadin 57-73 QE65 variants substituted with lysine at each
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aminoacid . Bioactivity was abolished if lysine was substituted at position 8-11
(PELP), and substantially reduced with lysine at positions 4-7 (PFPQ) and 12-13 (YP).
Single aminoacid substituted variants of A-gliadin 57-73 QE65 with all naturally
occurring aminoacids except cysteine at positions 4-13 were synthesized and their
bioactivity compared to the parent peptide. Positions 4-7 were highly sensitive to
substitution, no more than three aminoacids at each position conveyed bioactivity
greater than 50% of the parent peptide. Positions 4-7 and 12-13 were less sensitive to
substitution but certain aminoacids such as proline, lysine and arginine tended to
abolish bioactivity. The fine molecular specificity of peripheral blood T cells for A-
gliadin 57-73 QE65 was similar amongst all of the eight coeliac subjects tested.
Antigen-specific therapy using A-gliadin 57-73 Qe65
A-gliadin 57-73 QE65 is clearly “the dominant” or “one of the dominant” gliadin T
cell epitopes in HLA-DQ2-associated coeliac disease. Therefore it is reasonable to
consider peptide therapeutics based on A-gliadin 57-73 QE65. Peptide delivered orally
or nasally would be simple and easy to formulate. Another possibility is design of
altered peptide ligand (APL) antagonists that differ from the parent peptide by one or
more residues and subtly alter T cell receptor signaling. APL antagonsists have the
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potential to “switch off” or “skew” TH1 to TH2 responses in vitro . We have shown
that at least 5 single aminoacid-substituted variants of A-gliadin 57-73 QE65 with weak
agonist properties also significantly reduce IFNgELISpot responses to A-gliadin 57-73
QE65 when incubated in 5-fold excess with A-gliadin 57-73 QE65 (unpublished
observations). It is likely that multiple substitutions or “cocktails” of APL antagonsists
will be required for complete blockade of the polyclonal T cell response to A-gliadin
57-73 QE65.