Recognition of allo-peptide is governed by novel anchor imposition and limited variations in TCR contact residues.

Immune specificity of a T cell is determined by the TCR contact residues exposed on the antigenic peptide/MHC complex. Naturally processed, biallelic epitopes from H7 minor histocompatibility (mH) antigen vary in position 7 (p7) from aspartic acid (D) to a glutamic acid (E), which differ by an additional methylene (-CH(2)) in the side chain. Here, we show that this variation generates a strong anti-H7a or anti-H7b cytotoxic T cell responses. Further, the H7 allelic peptides use p6 asparagine as their central anchor residue and amino acid variations in either the canonical p5 or the predicted p6 anchor positions in the antigenic epitope were detrimental for TCR recognition. In addition, introduction of any other amino acids, except asparagine, in the polymorphic p7 significantly abolished the ability of anti-H7b TCR recognition. This demonstrates that only an asparagine with an amine group as a side chain instead of a charged oxygen radical could effectively stimulate the anti-H7b specific T cells. Our findings provide evidence that mH antigen-specific TCRs are highly stringent in recognizing their cognate epitopes.

A single nucleotide polymorphism in the Emp3 gene defines the H4 minor histocompatibility antigen.

Minor histocompatibility antigens (minor H antigen) elicit strong T-cell-mediated responses during both graft rejection and graft versus leukemia (GvL) among MHC-matched individuals (where MHC is major histocompatibility complex). Employing expression-cloning methodology, we have identified a cDNA clone, MI-35, encoding the immunodominant H4b minor H antigen within the classical mouse H4 complex. The minimal antigenic epitope derived from H4b presented on Kb class I MHC is SGIVYIHL (SYL8) and the polymorphism is due to C-->T nucleotide modification in p3 resulting in the change of threonine (ACT) to isoleucine (ATT). The results presented here demonstrate that amino acid variation in the allelic epitopes results in the low abundance of H4a peptide. The differential peptide copy number resulted in an immunodominant cytotoxic T cells (CTL) response directed against H4b while the anti-B6 response directed against H4a was easily dominated. These results provide a molecular mechanism for the H4 minor H antigen and suggest a novel mechanism by which alloantigenic disparity caused by conservative amino acid changes can be augmented by posttranslational antigen processing events.