Cell surface organization of stress-inducible proteins ULBP and MICA that stimulate human NK cells and T cells via NKG2D.

Cell surface proteins major histocompatibility complex (MHC) class I-related chain A (MICA) and UL16-binding proteins (ULBP) 1, 2, and 3 are up-regulated upon infection or tumor transformation and can activate human natural killer (NK) cells. Patches of cross-linked raft resident ganglioside GM1 colocalized with ULBP1, 2, 3, or MICA, but not CD45. Thus, ULBPs and MICA are expressed in lipid rafts at the cell surface. Western blotting revealed that glycosylphosphatidylinositol (GPI)-anchored ULBP3 but not transmembrane MICA, MHC class I protein, or transferrin receptor, accumulated in detergent-resistant membranes containing GM1. Thus, MICA may have a weaker association with lipid rafts than ULBP3, yet both proteins accumulate at an activating human NK cell immune synapse. Target cell lipid rafts marked by green fluorescent protein-tagged GPI also accumulate with ULBP3 at some synapses. Electron microscopy reveals constitutive clusters of ULBP at the cell surface. Regarding a specific molecular basis for the organization of these proteins, ULBP1, 2, and 3 and MICA are lipid modified. ULBP1, 2, and 3 are GPI anchored, and we demonstrate here that MICA is S-acylated. Finally, expression of a truncated form of MICA that lacks the putative site for S-acylation and the cytoplasmic tail can be expressed at the cell surface, but is unable to activate NK cells.

Activation of a subset of human NK cells upon contact with Plasmodium falciparum-infected erythrocytes.

Human NK cells are the earliest source of the protective cytokine IFN-gamma when PBMC from nonimmune donors are exposed to Plasmodium falciparum-infected RBC (iRBC) in vitro. In this study, we show that human NK cells form stable conjugates with iRBC but not with uninfected RBC and that induction of IFN-gamma synthesis is dependent on direct contact between the NK cell and the iRBC. NK cells respond to iRBC only in the presence of a source of IL-12/IL-18 and the subset of NK cells that preferentially respond to iRBC express high levels of the lectin-like receptor CD94/NKG2A. There is heterogeneity between donors in their ability to respond to iRBC. DNA analysis has revealed considerable heterogeneity of killer Ig-like receptor (KIR) genotype among the donor population and has identified 21 new KIR allelic variants in the donors of African and Asian descent. Importantly, we find evidence for significant associations between KIR genotype and NK responsiveness to iRBC. This emphasizes the need for large-scale population-based studies to address associations between KIR genotype and susceptibility to malaria.

The size of the synaptic cleft and distinct distributions of filamentous actin, ezrin, CD43, and CD45 at activating and inhibitory human NK cell immune synapses.

In this study, we report the organization of cytoskeletal and large transmembrane proteins at the inhibitory and activating NK cell immunological or immune synapse (IS). Filamentous actin accumulates at the activating, but not the inhibitory, NK cell IS. However, surprisingly, ezrin and the associated protein CD43 are excluded from the inhibitory, but not the activating, NK cell IS. This distribution of ezrin and CD43 at the inhibitory NK cell IS is similar to that previously seen at the activating T cell IS. CD45 is also excluded from the inhibitory, but not activating, NK cell IS. In addition, electron microscopy reveals wide and narrow domains across the synaptic cleft. Target cell HLA-C, located by immunogold labeling, clusters where the synaptic cleft spans the size of HLA-C bound to the inhibitory killer Ig-like receptor. These data are consistent with assembly of the NK cell IS involving a combination of cytoskeletal-driven mechanisms and thermodynamics favoring the organization of receptor/ligand pairs according to the size of their extracellular domains.

Imaging immune surveillance by T cells and NK cells.

As T cells and natural killer (NK) cells survey the surface of other cells, cognate receptors and ligands are commonly organized into distinct micrometer-scale domains at the intercellular contact, creating an immune or immunological synapse (IS). We aim to address the still unanswered questions of how this organization of proteins aids immune surveillance and how these domains are biophysically constructed. Molecular mechanisms for the formation of the IS include a role for the cytoskeleton, segregation of proteins according to the size of their extracellular domains, and association of proteins with lipid rafts. Towards understanding the function of the IS, it is instructive to compare and contrast the supramolecular organization of proteins at the inhibitory and activating NK cell IS with that at the activating T cell IS. Finally, it is essential to develop new technologies for probing molecular recognition at cell surfaces. Imaging parameters other than fluorescence intensity, such as the lifetime of the fluorophore's excited state, could be used to report on protein environments.