LILRA3 Binds Both Classical and Non-Classical HLA Class I Molecules but with Reduced Affinities Compared to LILRB1/LILRB2: Structural Evidence

Myongchol Ryu,Yi Shi,Hao Cheng,Zheng Fan,George F Gao,Yong Chen,Gol Nam,Jun Liu,Jianxun Qi,Jean Kanellopoulos

doi:10.1371/journal.pone.0019245

Abstract

Structurally, Group 1 LILR (Leukocyte Immunogloblin (Ig)-Like Receptor, also known as Ig-like transcripts, ILT; Leukocyte Ig-like receptor, LIR; and CD85) members are very similar in terms of the HLAIs (human leukocyte antigen class I molecules) binding region and were hypothesized that they all bind to HLAIs. As one of the Group 1 LILRs, LILRA3 is the only secretory LILR and may greatly control the inhibitory immune response induced by LILRB1, LILRB2, and other HLA-binding LILR molecules like LILRA1. Nevertheless, little was known about the binding of LILRA3 to HLAIs. In this report, we present the crystal structure of the LILRA3 domain 1 (D1) and evaluate the D1 and D1D2 (domain 1 and domain 2) binding to classical and non-classical HLAIs using BIAcore® surface plasmon resonance analysis (SPR). We found that LILRA3 binds both classical HLA-A*0201 and non-classical HLA-G1 but with reduced affinities compared to either LILRB1 or LILRB2. The polymorphic amino acids and the LILRA3 D1 structure support this notion.

Highlights

Immune cells express activating and inhibitory receptors on their surfaces to allow an adequate immune response
The full-length sequence encoding LILRA3 was cloned from a human lymphocyte cDNA library (Strata Gene), and the fragments encoding the LILRA3 domain 1 (D1) or D1D2 regions were cloned from the full-length LILRA3 gene
Our studies of LILRA3/HLA class I molecules (HLAIs) binding properties using BIAcoreH surface plasmon resonance analysis (SPR) clearly demonstrated that LILRA3 binds both classical and non-classical HLAIs with reduced affinities compared to LILRB1/human leukocyte antigen (HLA)-A*0201 and LILRB2/G1 binding

Summary

Introduction

Immune cells express activating and inhibitory receptors on their surfaces to allow an adequate immune response. LILRs [4,5,6] are called Ig-like transcript (ILT) or leukocyte Ig-like receptor (LIR) or CD85 and are closely related to KIRs but expressed on the surface of a more broad range of cells (i.e., on lymphoid and/or myeloid cells). There are 13 members (two of them are believed to be pseudogenes) of either activating or inhibitory receptors in the LILR family [7]. With the exception of LILRA3, activating receptors (LILRA1–A6) contain a short cytoplasmic tail after the transmembrane domain. They lack any signaling motif but recruit the c-chain of FceRI through a charged arginine residue in the transmembrane domain and deliver an activating signal through the c-chain cytoplasmic immunoreceptor tyrosine-based activating motif (ITAM). LILRA3 is the only secreted soluble LILR lacking the transmembrane and cytoplasmic domains [8,9]

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Conclusion