Inhibitory NK Receptor Recognition of HLA-G: Regulation by Contact Residues and by Cell Specific Expression at the Fetal-Maternal Interface

Tsufit Gonen-Gross,Dikla Lankry,Ofer Mandelboim,Simcha Yagel,Yaron Hamani,Shira Natanson-Yaron,Caryn Greenfield,Ronit Gilad,Berthold Huppertz,Debra Goldman-Wohl

doi:10.1371/journal.pone.0008941

Tsufit Gonen-Gross, Dikla Lankry + Show 8 more

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https://doi.org/10.1371/journal.pone.0008941

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Abstract

The non-classical HLA-G protein is distinguished from the classical MHC class I molecules by its expression pattern, low polymorphism and its ability to form complexes on the cell surface. The special role of HLA-G in the maternal-fetal interface has been attributed to its ability to interact with specific receptors found on maternal immune cells. However this interaction is restricted to a limited number of receptors. In this study we elucidate the reason for this phenomenon by comparing the specific contact residues responsible for MHC-KIR interactions. This alignment revealed a marked difference between the HLA-G molecule and other MHC class I molecules. By mutating these residues to the equivalent classical MHC residues, the HLA-G molecule regained an ability of interacting with KIR inhibitory receptors found on NK cells derived either from peripheral blood or from the decidua. Functional NK killing assays further substantiated the binding results. Furthermore, double immunofluorescent staining of placental sections revealed that while the conformed form of HLA-G was expressed in all extravillous trophoblasts, the free heavy chain form of HLA-G was expressed in more distal cells of the column, the invasion front. Overall we suggest that HLA-G protein evolved to interact with only some of the NK inhibitory receptors thus allowing a control of inhibition, while permitting appropriate NK cell cytokine and growth factor production necessary for a viable maternal fetal interface.

Highlights

The immune environment at the maternal fetal interface has seemingly paradoxical roles
The killer Ig-like receptor (KIR)/ HLA-C interface possess more residues, we focused on four contact residues; Met76, Gln79, Asn151 and Thr 80 (Fig. 1b)
Sequence alignment of the HLA-G and selected MHC class I molecules in the binding region of the KIR inhibitory receptors revealed that HLA-G differs from the HLA-C molecules in these specific contact residues.To understand the role of the contact HLA-G residues in HLA-G recognition by NK cells we performed an extensive site-directed mutagenesis which is listed in figure 1B

Summary

Introduction

The immune environment at the maternal fetal interface has seemingly paradoxical roles. The invasive trophoblasts express non-classical MHC class I molecules of which the most extensively studied is HLA-G This molecule displays many unique features such as low polymorphism, a truncated cytoplasmic tail and restricted distribution to the extravillous cytotrophoblasts [4,5,6]. Following implantation, the pregnant uterus is remodeled as a site of innate immunity where specialized NK cells termed decidual NK (dNK) comprise more than 40% of the entire cell population in the decidua [9,10,11] These dNK exhibit different phenotypic characteristics and functional abilities compared with the NK population found in the peripheral blood [12,13] and their number in the decidua is progressively diminished from midgestation onwards [14]

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