Engagement of the leukocyte-associated Ig-like receptor-1 induces programmed cell death and prevents NF-kappaB nuclear translocation in human myeloid leukemias

FLT3-Mutated Leukemic Stem Cells: Mechanisms of Resistance and New Therapeutic Targets

Despite the availability of target drugs in the first and second line, only 30% of FLT3mut AMLs are cured. Among the multiple mechanisms of resistance, those of FLT3mut LSC are the most difficult to eradicate because of their metabolic and genomic characteristics. Reactivation of glycogen synthesis, inhibition of the RAS/MAPK pathway, and degradation of FLT3 may be potential aids to fight the resistance of LSC to FLT3i. LSC is also characterized by the expression of a CD34+/CD25+/CD123+/CD99+ immunophenotype. The receptor and ligand of FLT3, the natural killer group 2 member D ligand (NKGD2L), and CD123 are some of the targets of chimeric antigen receptor T cells (CAR-T), bispecific T-cell engager molecules (BiTEs), CAR-NK and nanoparticles recently designed and reported here. The combination of these new therapeutic options, hopefully in a minimal residual disease (MRD)-driven approach, could provide the future answer to the challenge of treating FLT3mut AML.

LAIR-1 agonism as a therapy for acute myeloid leukemia

Effective eradication of leukemic stem cells (LSCs) remains the greatest challenge in treating acute myeloid leukemia (AML). The immune receptor LAIR-1 has been shown to regulate LSC survival; however, the therapeutic potential of this pathway remains unexplored. We developed a therapeutic LAIR-1 agonist antibody, NC525, that induced cell death of LSCs, but not healthy hematopoietic stem cells in vitro, and killed LSCs and AML blasts in both cell- and patient-derived xenograft models. We showed that LAIR-1 agonism drives a unique apoptotic signaling program in leukemic cells that was enhanced in the presence of collagen. NC525 also significantly improved the activity of azacitidine and venetoclax to establish LAIR-1 targeting as a therapeutic strategy for AML that may synergize with standard-of-care therapies.

LAIR1, an ITIM-Containing Receptor Involved in Immune Disorders and in Hematological Neoplasms

Leukocyte-associated immunoglobulin (Ig)-like receptor 1 (LAIR1, CD305) belongs to the family of immune-inhibitory receptors and is widely expressed on hematopoietic mature cells, particularly on immune cells. Four different types of ligands of LAIR1 have been described, including collagens, suggesting a potential immune-regulatory function on the extracellular matrix. By modulating cytokine secretion and cellular functions, LAIR1 displays distinct patterns of expression among NK cell and T/B lymphocyte subsets during their differentiation and cellular activation and plays a major negative immunoregulatory role. Beyond its implications in physiology, the activity of LAIR1 can be inappropriately involved in various autoimmune or inflammatory disorders and has been implicated in cancer physiopathology, including hematological neoplasms. Its action as an inhibitory receptor can result in the dysregulation of immune cellular responses and in immune escape within the tumor microenvironment. Furthermore, when expressed by tumor cells, LAIR1 can modulate their proliferation or invasion properties, with contradictory pro- or anti-tumoral effects depending on tumor type. In this review, we will focus on its role in normal physiological conditions, as well as during pathological situations, including hematological malignancies. We will also discuss potential therapeutic strategies targeting LAIR1 for the treatment of various autoimmune diseases and cancer settings.

Protection against COVID-19 in African population: Immunology, genetics, and malaria clues for therapeutic targets

BACKGROUND

There is a marked discrepancy between SARS-CoV-2 seroprevalence and COVID-19 cases and deaths in Africa. MAIN: SARS-CoV-2 stimulates humoral and cellular immunity systems, as well as mitogen-activated protein kinase (MAPK) and nuclear NF-kB signalling pathways, which regulate inflammatory gene expression and immune cell differentiation. The result is pro-inflammatory cytokines release, hyperinflammatory condition, and cytokine storm, which provoke severe lung alterations that can lead to multi-organ failure in COVID-19. Multiple genetic and immunologic factors may contribute to the severity of COVID-19 in African individuals when compared to the rest of the global population. In this article, the role of malaria, NF-kB and MAPK pathways, caspase-12 expression, high level of LAIR-1-containing antibodies, and differential glycophorins (GYPA/B) expression in COVID-19 are discussed.

CONCLUSION

Understanding pathophysiological mechanisms can help identify target points for drugs and vaccines development against COVID-19. To our knowledge, this is the first study that explores this link and proposes a biological and molecular answer to the epidemiologic discrepancy in COVID-19 in Africa.

Five ETS family members, ELF-1, ETV-4, ETV-3L, ETS-1, and ETS-2 upregulate human leukocyte-associated immunoglobulin-like receptor-1 gene basic promoter activity

Human leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1), an immunoinhibitory receptor, is expressed on most types of hematopoietic cells and some tumor cells. LAIR-1 plays an inhibitory role in immune cell maturation, differentiation, and activation. LAIR-1 is also involved in some autoimmune diseases and tumors. However, the mechanism controlling the regulation of the LAIR-1 gene is still unknown. In order to elucidate the molecular mechanisms involved in LAIR-1 regulation, in the present study, we cloned and characterized the promoter region of LAIR-1 gene using a series of truncated promoter plasmids in luciferase reporter assays. Our results show that the basic core promoter of LAIR-1 is located within the region -256/-8 relative to the translational start site. Our further studies indicate that five ETS transcription factors: ELF-1, ETV-4, ETV-3L, ETS-1 and ETS-2, can up-regulate the LAIR-1 basic promoter activity. Of these, ETS-2 is the most effective transcription factor. Moreover, ETS-2 was confirmed to interact directly with the basic promoter of LAIR-1. This study presents the first description of regions/factors capable of up-regulation the promoter activity of LAIR-1. This new knowledge contributes to understanding of the molecular mechanisms involved in LAIR-1 associated immune regulation and diseases.

Molecular basis of reduced LAIR1 expression in childhood severe malarial anaemia: Implications for leukocyte inhibitory signalling

BACKGROUND

Leukocyte-associated immunoglobulin like receptor-1 (LAIR1) is a transmembrane inhibitory receptor that influences susceptibility to a myriad of inflammatory diseases. Our recent investigations of severe malarial anaemia (SMA) pathogenesis in Kenyan children discovered that novel LAIR1 genetic variants which were associated with decreased LAIR1 transcripts enhanced the longitudinal risk of SMA and all-cause mortality.

METHODS

To characterize the molecular mechanism(s) responsible for altered LAIR1 signalling in severe malaria, we determined LAIR1 transcripts and protein, sLAIR1, sLAIR2, and complement component 1q (C1q) in children with malarial anaemia, followed by a series of in vitro experiments investigating the LAIR1 signalling cascade.

FINDINGS

Kenyan children with SMA had elevated circulating levels of soluble LAIR1 (sLAIR1) relative to non-SMA (1.69-fold P < .0001). The LAIR1 antagonist, sLAIR2, was also elevated in the circulation of children with SMA (1.59 fold-change, P < .0001). There was a positive correlation between sLAIR1 and sLAIR2 (ρ = 0.741, P < .0001). Conversely, circulating levels of complement component 1q (C1q), a LAIR1 natural ligand, were lower in SMA (-1.21-fold P = .048). These in vivo findings suggest that reduced membrane-bound LAIR1 expression in SMA is associated with elevated production of sLAIR1, sLAIR2 (antagonist), and limited C1q (agonist) availability. Since reduced LAIR1 transcripts in SMA were associated with increased acquisition of haemozoin (PfHz) by monocytes (P = .028), we explored the relationship between acquisition of intraleukocytic PfHz, LAIR1 expression, and subsequent impacts on leukocyte signalling in cultured PBMCs from malaria-naïve donors stimulated with physiological concentrations of PfHz (10 μg/mL). Phagocytosis of PfHz reduced LAIR1 transcript and protein expression in a time-dependent manner (P < .050), and inhibited LAIR1 signalling through decreased phosphorylation of LAIR1 (P < .0001) and SH2-domain containing phosphatase-1 (SHP-1) (P < .001). This process was associated with NF-κB activation (P < .0001) and enhanced production of IL-6, IL-1β, and TNF-α (all P < .0001).

INTERPRETATION

Collectively, these findings demonstrate that SMA is characterized by reduced LAIR1 transmembrane expression, reduced C1q, and enhanced production of sLAIR1 and sLAIR2, molecular events which can promote enhanced production of cytokines that contribute to the pathogenesis of SMA. These investigations are important for discovering immune checkpoints that could be future targets of immunotherapy to improve disease outcomes.

Clinical significance of leukocyte-associated immunoglobulin-like receptor-1 expression in human cervical cancer

Leukocyte-associated immunoglobulin-like receptor-1 (LAIR-1) is broadly expressed on the majority of immune cells; however, the biological role of LAIR in solid tumors has yet to be elucidated. In the present study, using immunohistochemical staining analysis, the expression of LAIR-1 in human cervical cancer (HCC) and nontumor-adjacent tissue specimens was determined, and the results indicated that the expression of LAIR-1 in HCC tissue was higher compared with that in noncancerous tissue. The χ² test was used to analyze the correlation between the expression of LAIR-1 in tumor tissues with clinicopathological parameters. The results showed that the expression of LAIR-1 in the cancer cell nucleus was significantly associated with tumor size, pathological differentiation, T classification and clinical stage. In addition, the expression in the cytoplasm was evidently associated with the number of positive lymph nodes. The HCC cell line, ME-180, which does not express LAIR-1, was stably transfected using LAIR-1 cDNA. Cell Counting Kit-8 and an annexin V assay showed that the overexpression of LAIR-1 in ME-180 cells suppressed the proliferation and anti-apoptosis capacity of the cells. These findings demonstrated that LAIR-1 is markedly overexpressed in HCC tissue, and that its expression status is associated with tumor progression. LAIR-1 may be a biomarker and target in the diagnosis and treatment of patients with HCC.

Inhibitory leukocyte immunoglobulin-like receptors: Immune checkpoint proteins and tumor sustaining factors

Inhibitory leukocyte immunoglobulin-like receptors (LILRBs 1-5) transduce signals via intracellular immunoreceptor tyrosine-based inhibitory motifs (ITIMs) that recruit protein tyrosine phosphatase non-receptor type 6 (PTPN6 or SHP-1), protein tyrosine phosphatase non-receptor type 11 (PTPN11 or SHP-2), or Src homology 2 domain-containing inositol phosphatase (SHIP), leading to negative regulation of immune cell activation. Certain of these receptors also play regulatory roles in neuronal activity and osteoclast development. The activation of LILRBs on immune cells by their ligands may contribute to immune evasion by tumors. Recent studies found that several members of LILRB family are expressed by tumor cells, notably hematopoietic cancer cells, and may directly regulate cancer development and relapse as well as the activity of cancer stem cells. LILRBs thus have dual concordant roles in tumor biology - as immune checkpoint molecules and as tumor-sustaining factors. Importantly, the study of knockout mice indicated that LILRBs do not affect hematopoiesis and normal development. Therefore LILRBs may represent ideal targets for tumor treatment. This review aims to summarize current knowledge on expression patterns, ligands, signaling, and functions of LILRB family members in the context of cancer development.

Inhibitory leukocyte immunoglobulin-like receptors in cancer development

Inhibitory leukocyte immunoglobulin-like receptors (LILRB1-5) signal through immunoreceptor tyrosine-based inhibitory motifs (ITIMs) in their intracellular domains and recruit phosphatases protein tyrosine phosphatase, non-receptor type 6 (PTPN6, SHP-1), protein tyrosine phosphatase, non-receptor type 6 (PTPN6, SHP-2), or Src homology 2 domain containing inositol phosphatase (SHIP) to negatively regulate immune cell activation. These receptors are known to play important regulatory roles in immune and neuronal functions. Recent studies demonstrated that several of these receptors are expressed by cancer cells. Importantly, they may directly regulate development, drug resistance, and relapse of cancer, and the activity of cancer stem cells. Although counterintuitive, these findings are consistent with the generally immune-suppressive and thus tumor-promoting roles of the inhibitory receptors in the immune system. This review focuses on the ligands, expression pattern, signaling, and function of LILRB family in the context of cancer development. Because inhibition of the signaling of certain LILRBs directly blocks cancer growth and stimulates immunity that may suppress tumorigenesis, but does not disturb normal development, LILRB signaling pathways may represent ideal targets for treating hematological malignancies and perhaps other tumors.

Comparison of LAIR-1 genetic pathways in murine vs human internal organs

Growing evidence suggests that defective expression or dysfunction of LAIR-1, a novel immunoinhibitory receptor for collagen, is closely associated with some autoimmune diseases, cancers, as well as viral infections. We analyzed the variation of LAIR-1 genetic pathways in murine versus human internal organs, including the lung and brain. The results showed that, under physiological conditions, LAIR-1 links more closely to the common genes in mouse than in human, which poses tissue specificity. It means that mice experimental data in relation to the role of LAIR-1 immune regulation may be overestimated when applied to assess human conditions. Moreover, we found that the in vivo interaction of LAIR-1 with LAIR-2 rarely occurs, implying that the species difference in LAIR-1 genetic pathways could not be primarily attributed to the existence of human LAIR-2. In summary, this study opens the door for insight into LAIR-1 functions inside the human body, and raises concern as to extrapolative credibility of the murine model in biomedical research.

A non-stimulatory monoclonal antibody against the inhibitory immunoreceptor LAIR-1

The human leukocyte-associated Ig-like receptor (LAIR) family contains two members: LAIR-1 (CD305) and LAIR-2 (CD306). Among them, LAIR-1 is a transmembrane glycoprotein bearing two intracellular immunoreceptor tyrosine-based inhibition motifs (ITIM) and LAIR-2 is soluble. Both molecules bind collagen and LAIR-2 has higher affinity than LAIR-1. LAIR-1 can mediate strong inhibitory signal but the functions of leukocytes expressing LAIR-1 are unclear because of the absence of an effective method to isolate them with resting status. In this study, we generated a monoclonal antibody (MAb) by immunizing BALB/c mice with the recombinant LAIR-2-GST fusion protein, which we termed 3G4. The subclass of 3G4 was identified as IgG1. Specificity analysis by Western blotting demonstrated 3G4 could react with both LAIR-1 and LAIR-2. Unlike another LAIR-1-specific MAb (9.1C3), 3G4 did not inhibit the lysis of target cells P815 by NK cells in a redirected cytotoxicity assay. Preincubation of LAIR-1-transfected K562 cells with 3G4 mildly prevented the binding of LAIR-1 to collagens I and III in a dose-dependent manner. Taken together, the novel MAb 3G4 provides a useful tool to isolate LAIR-1-positive cells without changing their resting state for further application.

Mechanisms of tumor escape from immune system: role of mesenchymal stromal cells

Tumor microenvironment represents the site where the tumor tries to survive and escape from immune system-mediated recognition. Indeed, to proliferate tumor cells can divert the immune response inducing the generation of myeloid derived suppressor cells and regulatory T cells which can limit the efficiency of effector antitumor lymphocytes in eliminating neoplastic cells. Many components of the tumor microenvironment can serve as a double sword for the tumor and the host. Several types of fibroblast-like cells, which herein we define mesenchymal stromal cells (MSC), secrete extracellular matrix components and surrounding the tumor mass can limit the expansion of the tumor. On the other hand, MSC can interfere with the immune recognition of tumor cells producing immunoregulatory cytokines as transforming growth factor (TGF)ß, releasing soluble ligands of the activating receptors expressed on cytolytic effector cells as decoy molecules, affecting the correct interaction among lymphocytes and tumor cells. MSC can also serve as target for the same anti-tumor effector lymphocytes or simply impede the interaction between these lymphocytes and neoplastic cells. Thus, several evidences point out the role of MSC, both in epithelial solid tumors and hematological malignancies, in regulating tumor cell growth and immune response. Herein, we review these evidences and suggest that MSC can be a suitable target for a more efficient anti-tumor therapy.

NK cell autoreactivity and autoimmune diseases

Increasing evidences have pointed out the relevance of natural killer (NK) cells in organ-specific and systemic autoimmune diseases. NK cells bear a plethora of activating and inhibiting receptors that can play a role in regulating reactivity with autologous cells. The activating receptors recognize natural ligands up-regulated on virus-infected or stressed or neoplastic cells. Of note, several autoimmune diseases are thought to be linked to viral infections as one of the first event in inducing autoimmunity. Also, it is conceivable that autoimmunity can be triggered when a dysregulation of innate immunity occurs, activating T and B lymphocytes to react with self-components. This would imply that NK cells can play a regulatory role during adaptive immunity; indeed, innate lymphoid cells (ILCs), comprising the classical CD56⁺ NK cells, have a role in maintaining or alternating tissue homeostasis secreting protective and/or pro-inflammatory cytokines. In addition, NK cells display activating receptors involved in natural cytotoxicity and the activating isoforms of receptors for HLA class I that can interact with healthy host cells and induce damage without any evidence of viral infection or neoplastic-induced alteration. In this context, the interrelationship among ILC, extracellular-matrix components, and mesenchymal stromal cells can be considered a key point for the control of homeostasis. Herein, we summarize evidences for a role of NK cells in autoimmune diseases and will give a point of view of the interplay between NK cells and self-cells in triggering autoimmunity.

Oncogenic K-Ras and loss of Smad4 mediate invasion by activating an EGFR/NF-κB Axis that induces expression of MMP9 and uPA in human pancreas progenitor cells

Activating K-Ras mutations and inactivating mutations of Smad4 are two common genetic alterations that occur in the development and progression of pancreatic ductal adenocarcinomas (PDAC). To further study the individual and combinatorial roles of these two mutations in the pathogenesis of PDAC, immortalized human pancreas nestin postive cells (HPNE) were genetically modified by either expressing oncogenic K-Ras (HPNE/K-Ras), by shRNA knock down of Smad4 (HPNE/ShSmad4) or by creating both alterations in the same cell line (HPNE/K-Ras/ShSmad4). We previously found that expression of oncogenic K-Ras caused an increase in expression of EGFR and loss of Smad4 further enhanced the up regulation in expression of EGFR and that this increase in EGFR was sufficient to induce invasion. Here we further investigated the mechanism that links mutational alterations and EGFR expression with invasion. The increase in EGFR signaling was associated with up regulation of MMP9 and uPA protein and activity. Moreover, the increase in EGFR signaling promoted a nuclear translocation and binding of RelA (p65), a subunit of NF-κB, to the promoters of both MMP-9 and uPA. Treatment of HPNE/K-Ras/ShSmad4 cells with an inhibitor of EGFR reduced EGF-mediated NF-κB nuclear translocation and inhibitors of either EGFR or NF-κB reduced the increase in MMP-9 or uPA expression. In conclusion, this study provides the mechanism of how a combination of oncogenic K-Ras and loss of Smad4 causes invasion and provides the basis for new strategies to inhibit metastases.

Defective expression and function of the leukocyte associated Ig-like receptor 1 in B lymphocytes from systemic lupus erythematosus patients

Systemic lupus erythematosus (SLE) is characterized by the production of a wide array of autoantibodies and dysregulation of B cell function. The leukocyte associated Immunoglobulin (Ig)-like receptor (LAIR)1 is a transmembrane molecule belonging to Ig superfamily which binds to different types of collagen. Herein, we have determined the expression and function of LAIR1 on B lymphocyte from SLE patients. LAIR1 expression in peripheral blood B lymphocytes from 54 SLE, 24 mixed connective tissue disease (MCTD), 20 systemic sclerosis (SSc) patients, 14 rheumatoid arthritis (RA) and 40 sex and age matched healthy donors (HD) have been analyzed by immunofluorescence. The effect of LAIR1 ligation by specific monoclonal antibodies, collagen or collagen producing mesenchymal stromal cells from reactive lymph nodes or bone marrow on Ig production by pokeweed mitogen and B cell receptor (BCR)-mediated NF-kB activation was assessed by ELISA and TransAM assay. The percentage of CD20⁺ B lymphocytes lacking or showing reduced expression of LAIR1 was markedly increased in SLE and MCTD but not in SSc or RA patients compared to HD. The downregulation of LAIR1 expression was not dependent on corticosteroid therapy. Interestingly, LAIR1 engagement by collagen or collagen-producing mesenchymal stromal cells in SLE patients with low LAIR1 expression on B cells delivered a lower inhibiting signal on Ig production. In addition, NF-kB p65 subunit activation upon BCR and LAIR1 co-engagement was less inhibited in SLE patients than in HD. Our findings indicate defective LAIR1 expression and function in SLE B lymphocytes, possible contributing to an altered control of B lymphocytes behavior.

Leucocyte-associated immunoglobulin-like receptor-1 is an inhibitory regulator of contact hypersensitivity

Leucocyte-associated immunoglobulin-like receptor-1 (LAIR-1) is a membrane receptor of the immunoglobulin (Ig) superfamily that is expressed on most types of haematopoietic cells, and delivers inhibitory signals through interacting with collagens. In order to elucidate the immunological functions of LAIR-1 in vivo, we established transgenic mice expressing a chimeric protein composed of the extracellular domain of LAIR-1 fused with an Ig tag (LAIR-1-Ig), which acts as a decoy by competing with endogenous LAIR-1. The transgenic mice showed an increased susceptibility for development of contact hypersensitivity (CHS), an experimental model of allergic contact dermatitis, in association with enhanced hapten-specific T-cell responses. When T cells from the hapten-sensitized donor mice were transferred into non-sensitized recipients, treatment of either donor mice or recipient mice with LAIR-1-Ig protein accelerated CHS, suggesting a potentially negative role of LAIR-1 in both the sensitization and the elicitation of hapten-reactive T cells. In vitro assays revealed that LAIR-1 decreased the production of interleukin-6 and interleukin-12 in dendritic cells, and inhibited the proliferation and cytokine production of naïve and memory T cells along with G(0)/G(1) cell cycle arrest. Collectively, our findings suggest that LAIR-1 plays a crucial inhibitory role in CHS by regulating antigen-presenting cell and T-cell functions.

A single residue, arginine 65, is critical for the functional interaction of leukocyte-associated inhibitory receptor-1 with collagens

ITIM-containing receptors play an essential role in modulating immune responses. Leukocyte-associated inhibitory receptor (LAIR)-1, also known as CD305, is an ITIM-containing inhibitory receptor, expressed by all leukocytes, that binds collagens. In this article, we investigate the effect of a conservative R65K mutation on LAIR-1 ligand binding and function. Compared with LAIR-1 wild-type (wt)-expressing cells, LAIR-1 R65K cells show markedly reduced binding to collagen, which correlates with a reduced level of LAIR-1 polarization to the site of interaction with collagens. Both LAIR-1 wt and R65K cells can generate intracellular signals when ligated by anti-LAIR-1 mAb, but only LAIR-1 wt cells respond to collagens or matrigel. In agreement, surface plasmon resonance analyses showed that LAIR-1 R65K protein has markedly reduced avidity for collagen type I compared with LAIR-1 wt. Likewise, LAIR-1 R65K protein has decreased avidity for cells expressing transmembrane collagen XVII. Thus, a single residue, Arg65, is critical for the interaction of LAIR-1 with collagens.

Comparative analysis of DNA repair in stem and nonstem glioma cell cultures

It has been reported that cancer stem cells may contribute to glioma radioresistance through preferential activation of the DNA damage checkpoint response and an increase in DNA repair capacity. We have examined DNA repair in five stem and nonstem glioma cell lines. The population doubling time was significantly increased in stem compared with nonstem cells, and enhanced activation of Chk1 and Chk2 kinases was observed in untreated CD133(+) compared with CD133(-) cells. Neither DNA base excision or single-strand break repair nor resolution of pH2AX nuclear foci were increased in CD133(+) compared with CD133(-) cells. We conclude that glioma stem cells display elongated cell cycle and enhanced basal activation of checkpoint proteins that might contribute to their radioresistance, whereas enhanced DNA repair is not a common feature of these cells.

Lack of the leukocyte-associated Ig-like receptor-1 expression in high-risk chronic lymphocytic leukaemia results in the absence of a negative signal regulating kinase activation and cell division

In this study, we analysed 30 patients with B-cell chronic lymphocytic leukaemia (CLL), compared with 10 healthy donors, for the expression and function of the leukocyte-associated Ig-like receptor-1 (LAIR-1). LAIR-1 is an inhibitory receptor containing a cytoplasmic tyrosine-based inhibitory motif (ITIM) that binds to the SH2 domain of phosphatases, leading to dephosphorylation of different kinases. Constitutive activation of c-Jun aminoterminal kinase (JNK), p38 mitogen-activated protein kinase and extracellular signal-regulated kinase, has been reported in CLL. We show that LAIR-1 is absent in high-risk (HR) CLL and differently expressed on intermediate- and low-risk CLL and the intensity of expression, which is always significantly lower than in healthy donors, correlates with disease stage and progression. Interestingly, both constitutive and sIgM-induced phosphorylation of p38 and JNK is inhibited by LAIR-1 through an ITIM-dependent signal, as demonstrated by the use of specific ITIM-binding peptides; importantly, this inhibitory signal is missing when LAIR-1 is not expressed as occurs in HR CLL. Moreover, engagement of LAIR-1 blocks constitutive and sIgM-induced Akt phosphorylation, besides nuclear factor kappa-B nuclear translocation, and prevents CLL division. These results suggest that CLL lacking LAIR-1 may miss one of the molecular mechanisms controlling B-cell activation and proliferation.

Human MICL (CLEC12A) is differentially glycosylated and is down-regulated following cellular activation

C-type lectins are the most diverse and prevalent lectin family in immunity. Particular interest has recently been attracted by the C-type lectin-like receptors on NK cells, which appear to regulate the activation/inhibitory balance of these cells, controlling cytotoxicity and cytokine production. We previously identified a human C-type lectin-like receptor, closely related to both the beta-glucan receptor and the lectin-like receptor for oxidized-LDL, named MICL (myeloid inhibitory C-type lectin-like receptor), which we had shown using chimeric analysis to function as an inhibitory receptor. Using a novel MICL-specific monoclonal antibody, we show here that human MICL is expressed primarily on myeloid cells, including granulocytes, monocytes, macrophages, and dendritic cells. Although MICL was highly N-glycosylated in primary cells, the level of glycosylation was found to vary between cell types. MICL surface expression was down-regulated during inflammatory/activation conditions in vitro, as well as during an in vivo model of acute inflammation, which we characterize here. This suggests that human MICL may be involved in the control of myeloid cell activation during inflammation.

Leukocyte-associated Ig-like receptor-1 has SH2 domain-containing phosphatase-independent function and recruits C-terminal Src kinase

Most inhibitory receptors in the immune system contain one or several immunoreceptor tyrosine-based inhibitory motifs (ITIM) and recruit the SH2 domain-containing phosphatases SHP-1, SHP-2 and/or SHIP, which are generally believed to be essential for the inhibitory function. However, it has not been systematically investigated whether ITIM-bearing receptors exert their function through alternative interactions. Here we describe that leukocyte-associated Ig-like receptor (LAIR)-1 has inhibitory function in DT40 chicken B cells that lack both SHP-1 and SHP-2. In addition, we found that LAIR-1 did not recruit SHIP upon phosphorylation. Thus, LAIR-1 can function independently from SH2 domain-containing phosphatases and must recruit at least one other signaling molecule. Using a yeast-tri-hybrid system, we found that phosphorylated LAIR-1 bound the C-terminal Src kinase (Csk). The interaction required the SH2 domain of Csk and phosphorylation of the tyrosine in the N-terminal ITIM of LAIR-1. We propose that Csk is an additional player in the regulation of the immune system by ITIM-bearing receptors.

The mouse homologue of the leukocyte-associated Ig-like receptor-1 is an inhibitory receptor that recruits Src homology region 2-containing protein tyrosine phosphatase (SHP)-2, but not SHP-1

We report the molecular cloning and characterization of the first leukocyte-associated Ig-like receptor 1 (LAIR-1) homologue in mice that we have named mouse LAIR-1 (mLAIR-1). The mLAIR-1 gene maps to the proximal end of mouse chromosome 7 in a region syntenic with human chromosome 19q13.4 where the leukocyte receptor cluster is located. The protein shares 40% sequence identity with human LAIR-1, has a single Ig-like domain, and contains two immunoreceptor tyrosine-based inhibitory motif-like structures in its cytoplasmic tail. Mouse LAIR-1 is broadly expressed on various immune cells, and cross-linking of the molecule on stably transfected RBL-2H3 and YT.2C2 cells results in strong inhibition of their degranulation and cytotoxic activities, respectively. Upon pervanadate stimulation, the mLAIR-1 cytoplasmic tail becomes phosphorylated, thereby recruiting Src homology region 2-containing tyrosine phosphatase-2. Interestingly, unlike human LAIR-1, Src homology region 2-containing tyrosine phosphatase-1 is not recruited to the mLAIR-1 cytoplasmic tail. Screening human and mouse cell lines for mLAIR-1 and human LAIR-1 binding partners identified several lines expressing putative ligand(s) for both receptors.

Identification and characterization of a novel human myeloid inhibitory C-type lectin-like receptor (MICL) that is predominantly expressed on granulocytes and monocytes

Inhibitory and activatory C-type lectin-like receptors play an important role in immunity through the regulation of leukocytes. Here, we report the identification and characterization of a novel myeloid inhibitory C-type lectin-like receptor (MICL) whose expression is primarily restricted to granulocytes and monocytes. This receptor, which contains a single C-type lectin-like domain and a cytoplasmic immunoreceptor tyrosine-based inhibitory motif, is related to LOX-1 (lectin-like receptor for oxidized low density lipoprotein-1) and the beta-glucan receptor (Dectin-1) and is variably spliced and highly N-glycosylated. We demonstrate that it preferentially associates with the signaling phosphatases SHP-1 and SHP-2, but not with SHIP. Novel chimeric analyses with a construct combining MICL and the beta-glucan receptor show that MICL can inhibit cellular activation through its cytoplasmic immunoreceptor tyrosine-based inhibitory motif. These data suggest that MICL is a negative regulator of granulocyte and monocyte function.

9.1C3 is identical to LAIR-1, which is expressed on hematopoietic progenitors

The leukocyte-associated Ig-like receptor-1 (LAIR-1) is a negative regulator of natural killer (NK) cells, its encoding gene belonging to the leukocyte receptor complex (LRC). Antibody to LAIR-1 can inhibit Ab-induced redirected lysis and TNF-alpha release of effector cells. LAIR-1 contains 2 immunoreceptor tyrosine-based inhibitory motifs (ITIM) in its cytoplasmic region that have been shown to bind constitutively and presumably regulate the tyrosine phosphatase SHP-1 in hematopoietic cells. SHP-1 mutation in mice results in abnormal lymphoproliferation, suggesting that LAIR-1 may also be implicated in regulating hematopoiesis. Here we investigated a monoclonal antibody, 9.1C3, against a NK cell antigen previously described as inducing increased colony formation in in vitro assays of human bone marrow cells. We found that 9.1C3 was expressed on CD34 positive hematopoietic progenitors for the first time. In functional assays, 9.1C3 MAb was able to inhibit Ab-induced redirected lysis and TNF-alpha secretion of NK cells. We proved that 9.1C3 is identical to LAIR-1, based on the fact that not only the antigen precipitated by 9.1C3 MAb was of 40kDa but also 9.1C3 MAb bound specifically to LAIR-1 cDNA transfected COS7 cells as well as recognized LAIR-1 fusion protein in ELISA. This finding provided the first evidence that LAIR-1 expresses on hematopoietic progenitor, implicating its role in the regulation of hematopoiesis at early stage.

Soluble HLA class I molecules induce natural killer cell apoptosis through the engagement of CD8: evidence for a negative regulation exerted by members of the inhibitory receptor superfamily

Herein, we show that CD8(dull), CD8(intermediate), and CD8(bright) natural killer (NK) cell clones can be identified. Triggering of CD8 with its natural ligand(s), represented by soluble HLA class I (sHLA-I), isolated either from serum of healthy donors or from HLA-I(-) 721.221 lymphoblastoid cell line transfected with HLA-A2, -Cw4, and -Bw46 alleles, or HLA-G1 leads to NK cell apoptosis. The magnitude of this effect directly correlated with the level of CD8 expression. sHLA-I-induced apoptosis depends on the interaction with CD8, as it was inhibited by masking this molecule with specific monoclonal antibodies (mAbs). Moreover, sHLA-I or CD8 cross-linking with specific mAbs elicited intracellular calcium increases, Fas ligand (FasL) messenger RNA transcription, and FasL secretion, which were needed for delivering the death signal. Indeed, this apoptosis was inhibited by preincubation of NK cell clones with Fas or FasL antagonist mAbs, indicating that the Fas/FasL pathway is involved. Furthermore, members of the inhibitory receptor superfamily, such as CD94/NKG2 complex or killer inhibitory receptors, were shown to exert an inhibitory effect on sHLA-I-mediated apoptosis and secretion of FasL. These findings suggest that interaction between sHLA-I and CD8 evokes an apoptotic signal that is down-regulated by inhibitory receptor superfamily that function as survival receptors in NK cells.

Leukocyte-associated Ig-like receptor-1 prevents granulocyte-monocyte colony stimulating factor-dependent proliferation and Akt1/PKB alpha activation in primary acute myeloid leukemia cells

The leukocyte-associated Ig-like receptor-1 (LAIR-1), a surface leukocyte receptor containing two immune receptor tyrosine-based inhibitory motif (ITIM) is expressed on acute myeloid leukemia (AML) blasts isolated from peripheral blood or bone marrow of 17 patients (2 M0, 3 M1, 5 M2, 2 M4 and 5 M5 according to French, American and British classification). Further, we provide evidence thatLAIR-1 engagement inhibits granulocyte-monocyte colony-stimulating factor (GM-CSF)-induced proliferation of AML blasts. Indeed, leukemia cells stimulated with GM-CSF were blocked in the G0/G1 phase of the cell cycle and underwent apoptosis within 4 days after the engagement of LAIR-1. Remarkably, LAIR-1 was functional also in AML blasts which do not express CD33, mainly M4 and M5. Importantly, the LAIR-1 ligation led to a strong inhibition of both GM-CSF receptor-mediated intracellular calcium increases, phosphorylation and activation of Akt1/protein kinase B alpha, a substrate of the phosphatidylinositol-3 kinase. This last inhibitory effect was prevented by a synthetic peptide spanning the ITIM portion of LAIR-1, suggesting the involvement of SHP-1 phosphatase in LAIR-1-mediated inhibitory signal. Altogether, these findings indicate that the engagement of LAIR-1 can down-regulate GM-CSF-mediated survival and proliferation of AML blasts, suggesting an additional therapeutic approach to the treatment of AML patients.