Stable CHO cells lines were established by a SB transposon system to secrete peptide/MHC molecules into culture media. (A) Evidence of extracellular peptide/MHC protein was first determined from cell-free supernatants by SDS-PAGE and coomassie blue staining. (B) Representative chromatogram of small scale AC purification of peptide/MHC-containing media. Cell-free supernatant was passed through an equilibrated agarose bead column containing the MHC class I-reactive antibody M1/42. After washing away unbound material, peptide/MHC protein was eluted, buffer-exchanged, and concentrated. Protein purity was then assessed using SDS-PAGE and coomassie blue staining. Arrow inset indicating peptide/MHC around the predicted molecular weight. Abbreviation used: protein ladder (L), affinity chromatography (AC), preparation (prep)