HLA CLASS-I AND CLASS-II ANTIGENS ARE PARTIALLY CO-CLUSTERED IN THE PLASMA-MEMBRANE OF HUMAN LYMPHOBLASTOID-CELLS

Major histocompatibility complex (MHC) class II molecules displayed cl ustered patterns at the surfaces of T (HUT-102B2) and B (JY) lymphoma cells characterized by interreceptor distances in the micrometer range as detected by scanning force microscopy of immunogold-labeled antige ns. Electron microscopy revealed that a fraction of the MHC class II m olecules was also heteroclustered with MHC class I antigens at the sam e hierarchical level as described by the scanning force microscopy dat a, after specifically and sequentially labeling the antigens with 30- and 15-nm immunogold beads. On JY cells the estimated fraction of co-c lustered HLA II was 0.61, whereas that of the HLA I was 0.24. Clusteri zation of the antigens was detected by the deviation of their spatial distribution from the Poissonian distribution representing the random case. Fluorescence resonance energy transfer measurements also confirm ed partial co-clustering of the HLA class I and II molecules at anothe r hierarchical level characterized by the 2- to 10-nm Forster distance range and providing fine details of the molecular organization of rec eptors. The larger-scale topological organization of the MHC class I a nd II antigens may reflect underlying membrane lipid domains and may f ulfill significant functions in cell-to-cell contacts and signal trans duction.