HLA class I and II antigens are partially co-clustered in the plasma membrane of human lymphoblastoid cells

Abstract

Major histocompatibility complex (MHC) class II molecules displayed clustered 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 antigens. Electron microscopy revealed that a fraction of the MHC class II molecules was also heteroclustered with MHC class I antigens at the same hierarchical level as described by the scanning force microscopy data, after specifically and sequentially labeling the antigens with 30-and 15-nm immunogold beads. On JY cells the estimated fraction of co-clustered HLA II was 0.61, whereas that of the HLA I was 0. Transmembrane receptors, antigens, and proteins have limited mobility in the plasma membrane, which can be described more precisely by a restricted diffusion model in contrast to the Singer-Nicolson fluid mosaic membrane model assuming complete diffusional freedom for membrane-bound molecules (1, 2). The limited mobility of receptor proteins may arise from extracellular, intramembrane, and intracytosolic connections. Membrane-bound molecules have a tendency to form oligomeric clusters. Accordingly, dynamic receptor co-distribution patterns have been determined by a number of biophysical and biochemical techniques-e.g., f luorescence spectroscopy, electron and scanning force microscopies, chemical crosslinking, and co-isolation methods (3-13). Such oligomeric assemblies have been described in different cellular systems at the surface of fixed and living cells in their native environments for a wide variety of receptors-e.g., among others the oligomeric structure of concanavalin A receptors or transferrin receptors; or the colocalization of interleukin 2 with HLA class I, class II, and intracellular adhesion molecule; that of the insulin receptor with major histocompatibility complex (MHC) molecules; or the supramolecular structure of the tetraspan molecules have been shown (4-10, 14-24).Apart from multisubunit receptor structures, where subunits must be together before or upon ligand binding, seemingly unrelated proteins are co-associated in many cases and held together in a dynamic cluster, apparently by weak interactions between proteins, or between proteins and lipid moieties in the plasma membrane. Considering the surface density of these membrane components, the probability of a diffusion driven random encounter for the formation and maintenance of oligomeric homo-and͞or heteroassociations without assuming cohesive forces is very low (13). Earlier theoretical and experimental considerations even suggested a genetic origin of the membrane lipid and protein mosaicism (9,25) The majority of the available experimental methods provides information only on the existence of receptor colocalization, without supplying further details on the degree of hetero-or homoclustering (13,15,26,27). Recently, the combined application of fluorescence spectroscopy with scanning force and electron microscopies has offered a unique method to study ...

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