Predicted amino acid contacts in the KIR3DL2 D1 domain with the B27 heavy chain. the D0 and D1 domains with the 1, 2 and 3 domains of both B27 heavy chains. By contrast, the D2 domain primarily contacts residues in the 2 2 domain of one B27 heavy chain. These findings both provide novel insights about the molecular basis of KIR3DL2 binding to HLA-B27 and other ligands and suggest an important role for KIR3DL2 HLA-B27 interactions in controlling the function of NK cells in HLA-B27+ individuals. Introduction The HLA-class I molecule HLA-B27 is associated with development of a group of inflammatory Phenoxodiol arthritic disorders, collectively known as ATA the spondyloarthritides (SpA)(1). HLA-B27 is also positively associated with more Phenoxodiol favourable outcome with HIV and hepatitis C viral infections (2). HLA-B27 immune receptor interactions, including interactions with members of the killer cell immunoglobulin-like receptor (KIR) family play important roles in determining the strength and quality of immune responses in arthritis and infection (3-5). The KIR family member KIR3DL2 is expressed on natural killer (NK) and minor T cell subsets (6). KIR-HLA interactions have been implicated in immune responses against pathogens and in autoimmunity (7). KIR3DL2 was originally identified as a receptor for HLA-A3 and HLA-A11 (8-10). Subsequent studies have suggested either that HLA-A3 and A11 are weak Phenoxodiol ligands for KIR3DL2 or that their interaction with KIR3DL2 is highly specific. HLA-A3 licenses KIR3DL2-expressing NK cells with Phenoxodiol poor effector function and HLA-A3 binding to KIR3DL2 is only promoted by a limited number of viral peptide epitopes (11, 12). However the fact that KIR3DL2 is a framework gene encoding at least 63 allelic variants suggests that there are other ligands (13). KIR3DL2 also binds to 2 microglobulin-free heavy chain (FHC) forms of HLA-B27 (B27) including B27 dimers (termed B272) and other HLA class I free heavy chains (14, 15). KIR3DL2 and other three domain KIRs comprise three immunoglobulin-like domains (D0, D1 and D2) which together form the ligand binding domain (13). It is unclear exactly how these domains determine KIR3DL2 binding to ligand. Additionally, KIR3DL2 forms a disulphide-bonded dimer, presumably via two unpaired cysteines in the stem region (8). The contribution of KIR3DL2 dimerisation to ligand binding has not yet been studied. The D0 domain of KIR3DL1 enhances ligand interactions by binding common shared features of HLA-class I (16, 17). This manifests in a weak affinity of KIR3DL1 for different HLA-class I in functional studies (18). This suggests that other three domain KIR including KIR3DL2 could bind to shared features of HLA-class I. KIR3DL2 binds more strongly to HLA-B27 (B27) 2m-free heavy chain (FHC) forms including HLA-B27 free heavy chain dimers than other HLA-class I (19). The stronger interactions of B27 FHC forms with KIR3DL2 promote survival of NK and CD4 T cells and could account for the increased proportions of these cells in spondyloarthritis (19-21). Stronger binding of B27 FHC dimer forms to KIR3DL2 could also account for increased proportions of KIR3DL2+ CD4 T cells in healthy B27+ individuals (20). Stronger binding of KIR3DL2 to B27 FHC dimers is dependent on cysteine 67-dependent dimerization (19). KIR3DL2 binding to B27 FHC dimers is inhibited by the HLA-class I heavy chain antibody HC10 and by other B27 heavy chain antibodies (22, 23). We reasoned that the strong binding of KIR3DL2 to B27 FHC dimers reflects an innate ability of KIR3DL2 to bind weakly to other HLA-class I free heavy chains. Thus, we compared the strength of functional interactions of KIR3DL2 with HLA-B27 FHC dimers and other HLA-class I heavy chains. We modeled B27 FHC dimer binding to KIR3DL2 and set out to identify contact residues in KIR3DL2 and HLA-B27 involved in this interaction by targeted mutagenesis and epitope mapping of blocking antibodies. Materials and Methods Antibodies and cell lines used in this study Anti-KIR3DL2 antibody DX31 (IgG2a isotype) was a kind gift from Dr Jo Phillips (DNAX, Palo, Alto, USA). D0- specific (D0A-D0C all IgG1 isotype) and D2A (IgG1) and D1A-specific (IgG1) anti-KIR3DL2 antibodies were produced by Innate Pharma (Marseille, France). HLA-A, B, C negative LCL.721.221 (221) cell lines were transfected with pRSVNeo constructs of HLA-B*3501, HLA-B*0702 and HLA-B*27:05 (24). 221 cells transfected with HLA-G1 in pcDNA3.1 were a gift from Kalle Soderstrom. 221 cells transfected with HLA-*A0301 were a gift from Veronique Braud. Functional grade DX17 (IgG1), IgG1 and IgG2a isotype control MAbs were from Biolegend. Tetramer preparation, eGFP plasmid construct generation and FACS staining B27 dimer and HLA-A3 tetrameric.