Epitope mapping of the indirect T cell response to allogeneic class I MHC:Sequences shared by donor and recipient MHC may prime T cells that providehelp for alloantibody production

Indirect allorecognition occurs when T cells recognize donor MHC presented as peptide epitopes by recipient APC, but the precise nature of the epitope s involved remains unclear. Rejection of rat MHC class I-disparate PVG.R8 ( RT1.A(a)) grafts by PVG.RT1(u) (RT1.A(u)) recipients is mediated by indirec tly restricted CD4 T cells that provide help for the generation of alloanti body. In this study, epitope mapping was performed using a functionally rel evant readout (alloantibody production) to identify key peptides that prime an indirect alloimmune response, leading to graft rejection. PVG.RT1(a) ra ts were immunized with a series of overlapping 15-mer peptides (peptides 1- 18) that spanned the alpha1 and alpha2 domains of the RT1.A(a) molecule. Se veral peptides NN ere able to accelerate both the alloantibody response to the intact RT1.A(a) Ag and PVG.R8 heart graft rejection. An immunodominant epitope was identified within the hypervariable region of the alpha1 domain . Fine mapping of this region with a second series of peptides overlapping by single amino acids confirmed the presence of an eight-amino acid core de terminant. Additional "subdominant" epitopes were identified, two of which were located within regions of amino acid homology between the RT1.A(a) and RT1.A(u) molecules and not, as had been expected, within other hypervariab le regions. The contribution of self-epitopes to indirect allorecognition w as emphasized by the demonstration that i.v. administration of a 15-mer pep tide encompassing one of the subdominant self-determinants diminished the r ecipient's ability to mount an alloantibody response on challenge with inta ct A(a) alloantigen. Our findings suggest that cryptic self-epitopes recogn ized by autoreactive T cells may contribute to allograft rejection and shou ld be considered when designing novel strategies for inducing tolerance to alloantigen.