Clustering of class I HLA oligomers with CD8 and TCR: three-dimensional models based on fluorescence resonance energy transfer and crystallographic data.

Fluorescence resonance energy transfer (FRET) data, in accordance with lateral mobility measurements, suggested the existence of class I HLA dimers and oligomers at the surface of live human cells, including the B lymphoblast cell line (JY) used in the present study. Intra- and intermolecular class I HLA epitope distances were measured on JY B cells by FRET using fluorophore-conjugated Ag-binding fragments of mAbs W6/32 and L368 directed against structurally well-characterized heavy and light chain epitopes, respectively. Out-of-plane location of these epitopes relative to the membrane-bound BODIPY-PC (2-(4,4-difluoro-5-(4-phenyl-1,3-butadienyl)-4-bora-3a,4a-diaza-s-indacene-3-pentanoyl)-1-hexadecanoyl-sn-glycero-3-phosphocholine) was also determined by FRET. Computer-simulated docking of crystallographic structures of class I HLA and epitope-specific Ag-binding fragments, with experimentally determined interepitope and epitope to cell surface distances as constraints, revealed several sterically allowed and FRET-compatible class I HLA dimeric and tetrameric arrangements. Extension of the tetrameric class I HLA model with interacting TCR and CD8 resulted in a model of a supramolecular cluster that may exist physiologically and serve as a functionally significant unit for a network of CD8-HLA-I complexes providing enhanced signaling efficiency even at low MHC-peptide concentrations at the interface of effector and APCs.