The immunosuppressive effect of domain-deleted dimer of HLA-G2 isoform in collagen-induced arthritis mice

Enhanced effect of the immunosuppressive soluble HLA-G2 homodimer by site-specific PEGylation

Human leukocyte antigen (HLA)-G is a nonclassical HLA class I molecule that has an immunosuppressive effect mediated by binding to immune inhibitory leukocyte immunoglobulin-like receptors (LILR) B1 and LILRB2. A conventional HLA-G isoform, HLA-G1, forms a heterotrimeric complex composed of a heavy chain (α1-α3 domains), β2-microglobulin (β2m) and a cognate peptide. One of the other isoforms, HLA-G2, lacks a α2 domain or β2m to form a nondisulfide-linked homodimer, and its ectodomain specifically binds to LILRB2 expressed in human monocytes, macrophages, and dendritic cells. The administration of the ectodomain of HLA-G2, designated the soluble HLA-G2 homodimer, showed significant immunosuppressive effects in mouse models of rheumatoid arthritis and systemic lupus erythematosus, presumably by binding to a mouse ortholog of LILRB2, paired immunoglobulin-like receptor B. However, the refolded soluble HLA-G2 homodimer used in these studies tends to aggregate and degrade; thus, its stability for clinical use has been a concern. In the present study, we improved the stability of the refolded soluble HLA-G2 homodimer via a site-directed PEGylation method. PEGylation at an original free cysteine residue, Cys42, resulted in increased lyophilization and thermal and serum stability. Furthermore, the PEGylated soluble HLA-G2 homodimer could better suppress atopic symptoms in mice than the non-PEGylated homodimer. These results suggest that PEGylated soluble HLA-G2 homodimers could be candidates for immunosuppressive biologics that specifically target LILRB2-positive myelomonocytic antigen-presenting cells.

Cell-Int: a cell-cell interaction assay to identify native membrane protein interactions

Intercellular protein-protein interactions (PPIs) have pivotal roles in biological functions and diseases. Membrane proteins are therefore a major class of drug targets. However, studying such intercellular PPIs is challenging because of the properties of membrane proteins. Current methods commonly use purified or modified proteins that are not physiologically relevant and hence might mischaracterize interactions occurring in vivo. Here, we describe Cell-Int: a cell interaction assay for studying plasma membrane PPIs. The interaction signal is measured through conjugate formation between two populations of cells each expressing either a ligand or a receptor. In these settings, membrane proteins are in their native environment thus being physiologically relevant. Cell-Int has been applied to the study of diverse protein partners, and enables to investigate the inhibitory potential of blocking antibodies, as well as the retargeting of fusion proteins for therapeutic development. The assay was also validated for screening applications and could serve as a platform for identifying new protein interactors.

Get Spliced: Uniting Alternative Splicing and Arthritis

Immune responses demand the rapid and precise regulation of gene protein expression. Splicing is a crucial step in this process; ~95% of protein-coding gene transcripts are spliced during mRNA maturation. Alternative splicing allows for distinct functional regulation, as it can affect transcript degradation and can lead to alternative functional protein isoforms. There is increasing evidence that splicing can directly regulate immune responses. For several genes, immune cells display dramatic changes in isoform-level transcript expression patterns upon activation. Recent advances in long-read RNA sequencing assays have enabled an unbiased and complete description of transcript isoform expression patterns. With an increasing amount of cell types and conditions that have been analyzed with such assays, thousands of novel transcript isoforms have been identified. Alternative splicing has been associated with autoimmune diseases, including arthritis. Here, GWASs revealed that SNPs associated with arthritis are enriched in splice sites. In this review, we will discuss how alternative splicing is involved in immune responses and how the dysregulation of alternative splicing can contribute to arthritis pathogenesis. In addition, we will discuss the therapeutic potential of modulating alternative splicing, which includes examples of spliceform-based biomarkers for disease severity or disease subtype, splicing manipulation using antisense oligonucleotides, and the targeting of specific immune-related spliceforms using antibodies.

Dimerization of Transmembrane Proteins in Cancer Immunotherapy

Transmembrane proteins (TMEMs) are integrated membrane proteins that span the entire lipid bilayer and are permanently anchored to it. TMEMs participate in various cellular processes. Some TMEMs usually exist and perform their physiological functions as dimers rather than monomers. TMEM dimerization is associated with various physiological functions, such as the regulation of enzyme activity, signal transduction, and cancer immunotherapy. In this review, we focus on the dimerization of transmembrane proteins in cancer immunotherapy. This review is divided into three parts. First, the structures and functions of several TMEMs related to tumor immunity are introduced. Second, the characteristics and functions of several typical TMEM dimerization processes are analyzed. Finally, the application of the regulation of TMEM dimerization in cancer immunotherapy is introduced.

The Role of Leukocyte Immunoglobulin-Like Receptors Focusing on the Therapeutic Implications of the Subfamily B2

The leukocyte immunoglobulin (Ig)-like receptors (LILRs) are constituted by five inhibitory subpopulations (LILRB1-5) and six stimulatory subpopulations (LILRA1-6). The LILR populations substantially reside in immune cells, especially myeloid cells, functioning as a regulator in immunosuppressive and immunostimulatory responses, during which the nonclassical major histocompatibility complex (MHC) class I molecules are widely involved. In addition, LILRs are also distributed in certain tumor cells, implicated in the malignancy progression. Collectively, the suppressive Ig-like LILRB2 is relatively well-studied to date. Herein, we summarized the whole family of LILRs and their biologic function in various diseases upon ligation to the critical ligands, therefore providing more information on their potential roles in these pathological processes and giving the clinical significance of strategies targeting LILRs.

Evaluation of the Reactivity and Receptor Competition of HLA-G Isoforms toward Available Antibodies: Implications of Structural Characteristics of HLA-G Isoforms

The human leucocyte antigen (HLA)-G, which consists of seven splice variants, is a tolerogenic immune checkpoint molecule. It plays an important role in the protection of the fetus from the maternal immune response by binding to inhibitory receptors, including leukocyte Ig-like receptors (LILRs). Recent studies have also revealed that HLA-G is involved in the progression of cancer cells and the protection from autoimmune diseases. In contrast to its well characterized isoform, HLA-G1, the binding activities of other major HLA-G isoforms, such as HLA-G2, toward available anti-HLA-G antibodies are only partially understood. Here, we investigate the binding specificities of anti-HLA-G antibodies by using surface plasmon resonance. MEM-G9 and G233 showed strong affinities to HLA-G1, with a nM range for their dissociation constants, but did not show affinities to HLA-G2. The disulfide-linker HLA-G1 dimer further exhibited significant avidity effects. On the other hand, 4H84 and MEM-G1, which can be used for the Western blotting of HLA-G isoforms, can bind to native HLA-G2, while MEM-G9 and G233 cannot. These results reveal that HLA-G2 has a partially intrinsically disordered structure. Furthermore, MEM-G1, but not 4H84, competes with the LILRB2 binding of HLA-G2. These results provide novel insight into the functional characterization of HLA-G isoforms and their detection systems.

Structural and Functional Basis for LILRB Immune Checkpoint Receptor Recognition of HLA-G Isoforms

Human leukocyte Ig-like receptors (LILR) LILRB1 and LILRB2 are immune checkpoint receptors that regulate a wide range of physiological responses by binding to diverse ligands, including HLA-G. HLA-G is exclusively expressed in the placenta, some immunoregulatory cells, and tumors and has several unique isoforms. However, the recognition of HLA-G isoforms by LILRs is poorly understood. In this study, we characterized LILR binding to the β2-microglobulin (β2m)-free HLA-G1 isoform, which is synthesized by placental trophoblast cells and tends to dimerize and multimerize. The multimerized β2m-free HLA-G1 dimer lacked detectable affinity for LILRB1, but bound strongly to LILRB2. We also determined the crystal structure of the LILRB1 and HLA-G1 complex, which adopted the typical structure of a classical HLA class I complex. LILRB1 exhibits flexible binding modes with the α3 domain, but maintains tight contacts with β2m, thus accounting for β2m-dependent binding. Notably, both LILRB1 and B2 are oriented at suitable angles to permit efficient signaling upon complex formation with HLA-G1 dimers. These structural and functional features of ligand recognition by LILRs provide novel insights into their important roles in the biological regulations.

The anti-fibrotic role of mast cells in the liver is mediated by HLA-G and interaction with hepatic stellate cells

BACKGROUND & AIMS

We have reported a significant association between HLA-G expression or the number of hepatic mast cells and liver fibrosis. Here, we investigated the role of HLA-G and mast cells in liver fibrosis, focusing, in particular, on interactions between human mast and stellate cells.

METHODS

Human mast cells (HMC cell line, CD34-derived mast cells, or tissue-derived mast cells) were co-cultured with purified human hepatic stellate cells (HSCs), and collagen I production by HSCs was evaluated. Mast cells and HSCs were characterized by immunocytochemistry. Various conditions were tested: different times in direct or indirect contact, presence or absence of cytokines, addition or not of HLA-G, and presence or absence of specific protease inhibitors.

RESULTS

The reciprocal interaction between HSCs and mast cells led to the attraction of mast cells to HSCs in vivo and in vitro, and to a significant decrease in collagen production, at all times of co-culture, following the direct or indirect contact of mast cells with HSCs alone or in the presence of TGF-β, IFN-α or IL-10. We identified the diffusible factors involved in collagen I degradation as mast cell proteases. Moreover, HLA-G expression increased during the co-culture of HSCs and mast cells, with HLA-G acting on both mast cells and HSCs, to enhance collagen I degradation.

CONCLUSIONS

Mast cells play a beneficial, anti-fibrotic role in liver fibrosis, via the HLA-G-mediated decrease of collagen I. These findings are consistent with high levels of cross-communication between mast cells and hepatic stellate cells and the role of HLA-G.

Tropheryma whipplei Increases Expression of Human Leukocyte Antigen-G on Monocytes to Reduce Tumor Necrosis Factor and Promote Bacterial Replication

BACKGROUND & AIMS

Infection with Tropheryma whipplei has a range of effects-some patients can be chronic carriers without developing any symptoms, whereas others can develop systemic Whipple disease, characterized by a lack a protective inflammatory immune response. Alterations in HLA-G function have been associated with several diseases. We investigated the role of HLA-G during T whipplei infection.

METHODS

Sera, total RNA, and genomic DNA were collected from peripheral blood from 22 patients with classic Whipple's disease, 19 patients with localized T whipplei infections, and 21 asymptomatic carriers. Levels of soluble HLA-G in sera were measured by enzyme-linked immuosorbent assay, and expressions of HLA-G and its isoforms were monitored by real-time polymerase chain reaction. HLA-G alleles were identified and compared with a population of voluntary bone marrow donors. Additionally, monocytes from healthy subjects were stimulated with T whipplei, and HLA-G expression was monitored by real-time polymerase chain reaction and flow cytometry. Bacterial replication was assessed by polymerase chain reaction in the presence of HLA-G or inhibitor of tumor necrosis factor (TNF) (etanercept).

RESULTS

HLA-G mRNAs and levels of soluble HLA-G were significantly increased in sera from patients with chronic T whipplei infection compared with sera from asymptomatic carriers and control individuals. No specific HLA-G haplotypes were associated with disease or T whipplei infection. However, T whipplei infection of monocytes induced expression of HLA-G, which was associated with reduced secretion of TNF compared with noninfected monocytes. A neutralizing antibody against HLA-G increased TNF secretion by monocytes in response to T whipplei, and a TNF inhibitor promoted bacteria replication.

CONCLUSIONS

Levels of HLA-G are increased in sera from patients with T whipplei tissue infections, associated with reduced production of TNF by monocytes. This might promote bacteria colonization in patients.

Chicken novel leukocyte immunoglobulin-like receptor subfamilies B1 and B3 are transcriptional regulators of major histocompatibility complex class I genes and signaling pathways

Objective

The inhibitory leukocyte immunoglobulin-like receptors (LILRBs) play an important role in innate immunity. The present study represents the first description of the cloning and structural and functional analysis of LILRB1 and LILRB3 isolated from two genetically disparate chicken lines.

Methods

Chicken LILRB1-3 genes were identified by bioinformatics approach. Expression studies were performed by transfection, quantitative polymerase chain reaction. Signal transduction was analyzed by western blots, immunoprecipitation and flow cytometric. Cytokine levels were determined by enzyme-linked immunosorbent assay.

Results

Amino acid homology and phylogenetic analyses showed that the homologies of LILRB1 and LILRB3 in the chicken line 6.3 to those proteins in the chicken line 7.2 ranged between 97%–99%, while homologies between chicken and mammal proteins ranged between 13%–19%, and 13%–69%, respectively. Our findings indicate that LILRB1 and LILRB3 subdivided into two groups based on the immunoreceptor tyrosine-based inhibitory motifs (ITIM) present in the transmembrane domain. Chicken line 6.3 has two ITIM motifs of the sequence LxYxxL and SxYxxV while line 7.2 has two ITIM motifs of the sequences LxYxxL and LxYxxV. These motifs bind to SHP-2 (protein tyrosine phosphatase, non-receptor type 11) that plays a regulatory role in immune functions. Moreover, our data indicate that LILRB1 and LILRB3 associated with and activated major histocompatibility complex (MHC) class I and β2-microglobulin and induced the expression of transporters associated with antigen processing, which are essential for MHC class I antigen presentation. This suggests that LILRB1 and LILRB3 are transcriptional regulators, modulating the expression of components in the MHC class I pathway and thereby regulating immune responses. Furthermore, LILRB1 and LILRB3 activated Janus kinase2/tyrosine kinase 2 (JAK2/TYK2); signal transducer and activator of transcription1/3 (STAT1/3), and suppressor of cytokine signaling 1 genes expressed in Macrophage (HD11) cells, which induced Th1, Th2, and Th17 cytokines.

Conclusion

These data indicate that LILRB1 and LILRB3 are innate immune receptors associated with SHP-2, MHC class I, β2-microglobulin, and they activate the Janus kinase/signal transducer and activator of transcription signaling pathway. Thus, our study provides novel insights into the regulation of immunity and immunopathology.

Regulation of immune and neural function via leukocyte Ig-like receptors

Leukocyte Ig-like receptors (LILRs)/Ig-like transcripts (ILTs) are expressed on innate and adaptive immune cells and maintain immune homeostasis. LILRs consist of activating and inhibitory-type receptors that regulate adequate cellular functions. LILRs were firstly identified as MHC class I receptors, therefore expression and/or polymorphisms of LILRs are reported to associate with autoimmune disorders and transplant rejection; however, recent accumulating evidences have revealed that LILRs recognize with diverse ligands including bacteria and virus. In addition, inhibitory LILRB2 (ILT4) and murine relative paired Ig-like receptor (PIR)-B are expressed on neuron and is involved in the dysregulation of central nervous system via interaction with neuronal ligands including amyloid β-protein. In this review, we summarize recent discoveries on the functions of inhibitory MHC class I receptors, and discuss their regulatory roles in immune responses and neural functions.

Therapeutic application of human leukocyte antigen-G1 improves atopic dermatitis-like skin lesions in mice

Human leukocyte antigen (HLA)-G is an immune checkpoint molecule that plays critical roles in immune response and in triggering inhibitory signaling to immune cells such as T cells, natural killer cells, and antigen-presenting cells. Thus, the application of HLA-G can be considered for treating immune response-related inflammatory disorders. We have previously reported that treatment with HLA-G1 and HLA-G2 ameliorates the joint swelling associated with collagen-induced arthritis of DBA/1 mice, an animal model for rheumatoid arthritis. In this study, we further investigated the effects of HLA-G1 on atopic dermatitis (AD), the most common inflammatory skin disorder. AD-like lesions were induced with the extract of the house dust mite Dermatophagoides farinae in NC/Nga mice. Continuous administration of HLA-G1 ameliorated the AD-like skin lesions in the mice. Furthermore, production of immunoglobulin E, interleukin (IL)-13, and IL-17A was significantly reduced in HLA-G1-treated mice, suggesting a Th2/Th17-mediated immune-inhibitory function of HLA-G1 in vivo. Our studies shed light on novel therapeutic strategies with recombinant HLA-G proteins for immune reaction-mediated chronic inflammatory disorders.

A soluble recombinant form of human leucocyte antigen-G 6 (srHLA-G6)

Human Leucocyte Antigen-G (HLA-G) is a non classical major histocompatibility complex (MHC) molecule that through RNA splicing can encode seven isoforms which are membrane bound (-G1, -G2, -G3 and -G4) and soluble (-G5, -G6 and -G7). HLA-G is described as important immune suppressor endogenous molecule to favor maternal-fetal tolerance, transplant survival and tumor immune scape. HLA-G shows low protein variability and a unique structural complexity that is related with the expression of different isoforms followed by biochemical processes, such as, proteolytic cleavage, molecular interactions, and protein ubiquitination. Studies with HLA-G have shown difficult to assess the role of the individual isoforms. Thus, the aim of this work was to obtain a HLA-G6 recombinant form. The results indicated the production of high homogeneous preparations of soluble recombinant HLA-G6 (srHLA-G6) with molecular mass 23,603.76 Da, determined by MALD-TOF/TOF. In addition, native and denatured srHLA-G6 were detected by ELISA, using commercial monoclonal antibodies. Finally, we developed a suitable methodology to express srHLA-G6 that could contribute in structural and functional studies involving specific isoforms.

Cutting Edge: Class II-like Structural Features and Strong Receptor Binding of the Nonclassical HLA-G2 Isoform Homodimer

HLA-G is a natural tolerogenic molecule and has the following unique features: seven isoforms (HLA-G1 to HLA-G7), formation of disulfide-linked homodimers, and β2-microglobulin (β2m)-free forms. Interestingly, individuals null for the major isoform, HLA-G1, are healthy and expressed the α2 domain-deleted isoform, HLA-G2, which presumably compensates for HLA-G1 function. However, the molecular characteristics of HLA-G2 are largely unknown. In this study, we unexpectedly found that HLA-G2 naturally forms a β2m-free and nondisulfide-linked homodimer, which is in contrast to the disulfide-bonded β2m-associated HLA-G1 homodimer. Furthermore, single-particle analysis, using electron microscopy, revealed that the overall structure and domain organization of the HLA-G2 homodimer resemble those of the HLA class II heterodimer. The HLA-G2 homodimer binds to leukocyte Ig-like receptor B2 with slow dissociation and a significant avidity effect. These findings provide novel insights into leukocyte Ig-like receptor B2-mediated immune regulation by the HLA-G2 isoform, as well as the gene evolution of HLA classes.