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      Molecular Recognition of Paired Receptors in the Immune System

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          Cell surface receptors are responsible for regulating cellular function on the front line, the cell membrane. Interestingly, accumulating evidence clearly reveals that the members of cell surface receptor families have very similar extracellular ligand-binding regions but opposite signaling systems, either inhibitory or stimulatory. These receptors are designated as paired receptors. Paired receptors often recognize not only physiological ligands but also non-self ligands, such as viral and bacterial products, to fight infections. In this review, we introduce several representative examples of paired receptors, focusing on two major structural superfamilies, the immunoglobulin-like and the C-type lectin-like receptors, and explain how these receptors distinguish self and non-self ligands to maintain homeostasis in the immune system. We further discuss the evolutionary aspects of these receptors as well as the potential drug targets for regulating diseases.

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          Most cited references 90

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          HLA-E binds to natural killer cell receptors CD94/NKG2A, B and C.

          The protein HLA-E is a non-classical major histocompatibility complex (MHC) molecule of limited sequence variability. Its expression on the cell surface is regulated by the binding of peptides derived from the signal sequence of some other MHC class I molecules. Here we report the identification of ligands for HLA-E. We constructed tetramers in which recombinant HLA-E and beta2-microglobulin were refolded with an MHC leader-sequence peptide, biotinylated, and conjugated to phycoerythrin-labelled Extravidin. This HLA-E tetramer bound to natural killer (NK) cells and a small subset of T cells from peripheral blood. On transfectants, the tetramer bound to the CD94/NKG2A, CD94/NKGK2B and CD94/NKG2C NK cell receptors, but did not bind to the immunoglobulin family of NK cell receptors (KIR). Surface expression of HLA-E was enough to protect target cells from lysis by CD94/NKG2A+ NK-cell clones. A subset of HLA class I alleles has been shown to inhibit killing by CD94/NKG2A+ NK-cell clones. Only the HLA alleles that possess a leader peptide capable of upregulating HLA-E surface expression confer resistance to NK-cell-mediated lysis, implying that their action is mediated by HLA-E, the predominant ligand for the NK cell inhibitory receptor CD94/NKG2A.
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            In search of the 'missing self': MHC molecules and NK cell recognition.

            Natural killer (NK) cells can defend an organism against a variety of threats, probably using several different strategies to discriminate between normal and aberrant cells. According to the 'missing self' hypothesis, one function of NK cells is to recognize and eliminate cells that fail to express self major histocompatibility complex (MHC) class I molecules. In this article Hans-Gustaf Ljunggren and Klas Kärre review in vivo studies with H-2-deficient targets that support this hypothesis. In vitro studies, some of which have given conflicting results, are interpreted within a multiple choice model for NK cell recognition. The authors derive testable predictions for how MHC class I molecules act in cases where they control a rate-limiting step in the NK cell-target interaction.
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              Direct recognition of cytomegalovirus by activating and inhibitory NK cell receptors.

              Natural killer (NK) cells express inhibitory receptors for major histocompatibility complex (MHC) class I antigens, preventing attack against healthy cells. Mouse cytomegalovirus (MCMV) encodes an MHC-like protein (m157) that binds to an inhibitory NK cell receptor in certain MCMV-susceptible mice. In MCMV-resistant mice, this viral protein engages a related activating receptor (Ly49H) and confers host protection. These activating and inhibitory receptors are highly homologous, suggesting the possibility that one evolved from the other in response to selective pressure imposed by the pathogen.

                Author and article information

                Front Microbiol
                Front Microbiol
                Front. Microbio.
                Frontiers in Microbiology
                Frontiers Media S.A.
                04 September 2012
                31 December 2012
                : 3
                1Laboratory of Biomolecular Science, Faculty of Pharmaceutical Sciences, Hokkaido University Sapporo, Japan
                2Core Research for Evolutional Sciences and Technology, Japan Science and Technology Agency Saitama, Japan
                Author notes

                Edited by: Hironori Sato, National Institute of Infectious Diseases, Japan

                Reviewed by: Hidekatsu Iha, Oita University, Japan; Takamasa Ueno, Kumamoto University, Japan; Hisashi Arase, Osaka University, Japan

                *Correspondence: Katsumi Maenaka, Laboratory of Biomolecular Science, Faculty of Pharmaceutical Sciences, Hokkaido University, Kita-12, Nishi-6, Kita-ku, Sapporo 060-0812, Japan. e-mail: maenaka@

                This article was submitted to Frontiers in Virology, a specialty of Frontiers in Microbiology.

                Copyright © 2012 Kuroki, Furukawa and Maenaka.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in other forums, provided the original authors and source are credited and subject to any copyright notices concerning any third-party graphics etc.

                Figures: 6, Tables: 2, Equations: 0, References: 88, Pages: 12, Words: 9582
                Review Article


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