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Segregation of HLA-C from ICAM-1 at NK Cell Immune Synapses Is Controlled by Its Cell Surface Density¹

Division of Cell and Molecular Biology, Imperial College London, London, United Kingdom

NK cell activity is controlled by the integration of signals from numerous activating and inhibitory receptors at the immunological synapse (IS). However, the importance of segregation and patterning of proteins at the NK cell IS is unknown. In this study, we report that the level of expression of HLA-C on target cells determined its supramolecular organization and segregation from ICAM-1 at the NK cell IS, as well as its capacity to inhibit NK cell cytotoxicity. At YTS NK cell synapses formed with target cells expressing low levels of HLA-C (i.e., 10⁴/cell surface), a multifocal patterning of MHC class I protein predominated, whereas for higher levels of expression (10⁵/cell surface), clusters of HLA-C were more commonly homogeneous, ring-shaped, or containing multiple exclusions. This correlation of protein density with its patterning at the IS was independent of ATP- or actin-driven processes. Importantly, ICAM-1 and HLA-C segregated only at synapses involving target cells expressing high levels of MHC protein. For peripheral blood NK clones, there were specific thresholds in the level of target cell HLA-C needed to inhibit cytotoxicity and to cause segregation of HLA-C from ICAM-1 at the synapse. Thus, the synapse organization of HLA-C, determined by its level of expression, could directly influence NK cell inhibition, e.g., by regulating the proximity of activating and inhibitory receptors. For the first time, this suggests an important function for the assembly of an inhibitory NK cell IS. More broadly, segregation of proteins at intercellular contacts could transmit information about protein expression levels between cells.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by the Fundação para a Ciência e a Tecnologia, the Medical Research Council, the Biotechnology and Biological Science Research Council, and a Lister Institute Research Prize.

² Address correspondence and reprint requests to Dr. Daniel M. Davis, Division of Cell and Molecular Biology, Sir Alexander Fleming Building, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K. E-mail address: d.davis{at}imperial.ac.uk

³ Abbreviations used in this paper: IS, immunological synapse; MFI, median fluorescence intensity.

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