Model of an HLA/peptide tetrameric complex and its use to stain specific cytotoxic T cells. (a) Molecular model of an HLA/peptide tetrameric complex. Here HLA B*2705, the molecular subtype of HLA B27, has been refolded with beta 2 microglobulin and the influenza nucleoprotein peptide NP383-391. Four biotinylated complexes are shown together with one streptavidin molecule (here without a fluorochrome). (b) Example of HLA-B27/peptide tetrameric complex tetramer staining. Positive FACS staining (shown infilled) for an HLA B27-restricted Influenza NP-specific CTL line is shown.