Scanning a DRB3*0101 (DR52a)-restricted epitope cross-presented by DR3: Overlapping natural and artificial determinants in the human acetylcholine receptor

A recurring epitope in the human acetylcholine receptor (AChR) cu subunit ( alpha 146-160) is presented to specific T cells from myasthenia gravis pati ents by HLA-DRB3*0101-"DR52a"- or by DR4, Here we first map residues critic al for DR52a in this epitope by serial Ala substitution. For two somewhat s imilar T cells, this confirms the recently deduced importance of hydrophobi c "anchor" residues at peptide pi and p9; also of Asp at p4, which compleme nts this allele's distinctive Arg(74) in DR beta, Surprisingly, despite the 9 sequence differences in DR beta between DR52a and DR3, merely reducing t he bulk of the peptide's pi anchor residue (Trp(149)-->Phe) allowed maximal cross-presentation to both T cells by DR3 (which has Val(86) instead of Gl y). The shared K(71)G(73)R(74)N(77) moth in the alpha helices of DR52a and DR3 thus outweighs the five differences in the floor of the peptide-binding groove. A second issue is that T cells selected in vitro with synthetic AC hR peptides rarely respond to longer Ag preparations, whereas those raised with recombinant subunits consistently recognize epitopes processed natural ly even from whole AChR. Here we compared one T cell of each kind, which bo th respond to many overlapping (alpha 140-160 region peptides tin prolifera tion assays). Even though both use V beta 2 to recognize peptides bound to the same HLA-DR52a in the same register, the peptide-selected line neverthe less proved to depend on a recurring synthetic artifact-a widely underestim ated problem, Unlike these contaminant-responsive T cells, those that are t ruly specific for natural AChR epitopes appear less heterogeneous and there fore more suitable targets for selective immunotherapy.