Strong alloantigenicity of the alpha-helices residues of the MHC class I molecule

To evaluate the role of single residues of a MHC class I molecule in the induction of a primary allogeneic response, we have tested the ability of various point mutants (of the alpha-helices or beta-sheet of the alpha1 and alpha2 domains) of the Kd molecule to induce a primary cytotoxic T cell response in mice carrying the wild-type molecule. For that, we have used an in vivo model in which cells expressing mutant molecules were injected into the hind footpads of mice carrying wild-type Kd, and the recipient graft-draining popliteal lymph nodes were tested for the presence of alloreactive CTL. Under these experimental conditions, only 7 of the 25 mutant Kd molecules induced a primary allogeneic response. All of these mutations (positions 62, 65, 69, 72, 155, 163, 166) concern residues of the alpha-helices, demonstrating that very small variances from self in a class I molecule, located outside the peptide-binding groove, can be antigenic. To determine the peptide requirements for the generation of a primary allogeneic response, we have analyzed the repertoire of peptides selected by individual mutant molecules shown to be able or unable to induce a CTL response. No correlation was observed between the peptidic make-up presented by a given mutant and its capacity to induce a primary allogeneic response. On the whole, our data point to the alloantigenicity of potentially TCR-contacting surface residues of the MHC class I molecules.