Expression of inhibitory receptor ILT3 on neoplastic B cells is associated with lymphoid tissue involvement in chronic lymphocytic leukemia

Leukocyte immunoglobulin-like receptor subfamily B: A novel immune checkpoint molecule at the maternal-fetal interface

Due to their crucial roles in embryo implantation, maternal-fetal tolerance induction, and pregnancy progression, immune checkpoint molecules (ICMs), such as programmed cell death-1, cytotoxic T-lymphocyte antigen 4, and T cell immunoglobulin mucin 3, are considered potential targets for clinical intervention in pregnancy complications. Despite the considerable progress on these molecules, our understanding of ICMs at the maternal-fetal interface is still limited. Identification of alternative and novel ICMs and the combination of multiple ICMs is urgently needed for deeply understanding the mechanism of maternal-fetal tolerance and to discover the causes of pregnancy complications. Leukocyte immunoglobulin-like receptor subfamily B (LILRB) is a novel class of ICMs with strong negative regulatory effects on the immune response. Recent studies have revealed that LILRB is enriched in decidual immune cells and stromal cells at the maternal-fetal interface, which can modulate the biological behavior of immune cells and promote immune tolerance. In this review, we introduce the structural features, expression profiles, ligands, and orthologs of LILRB. In addition, the potential mechanisms and functions mediated by LILRB for sustaining the maternal-fetal tolerance microenvironment, remodeling the uterine spiral artery, and induction of pregnancy immune memory are summarized. We have also provided new suggestions for further understanding the roles of LILRB and potential therapeutic strategies for pregnancy-related diseases.

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.

RelB contributes to the survival, migration and lymphomagenesis of B cells with constitutively active CD40 signaling

Activation of CD40-signaling contributes to the initiation, progression and drug resistance of B cell lymphomas. We contributed to this knowledge by showing that constitutive CD40-signaling in B cells induces B cell hyperplasia and finally B cell lymphoma development in transgenic mice. CD40 activates, among others, the non-canonical NF-ĸB signaling, which is constitutively activated in several human B cell lymphomas and is therefore presumed to contribute to lymphopathogenesis. This prompted us to study the regulatory role of the non-canonical NF-ĸB transcription factor RelB in lymphomagenesis. To this end, we crossed mice expressing a constitutively active CD40 receptor in B cells with conditional RelB-KO mice. Ablation of RelB attenuated pre-malignant B cell expansion, and resulted in an impaired survival and activation of long-term CD40-stimulated B cells. Furthermore, we found that hyperactivation of non-canonical NF-кB signaling enhances the retention of B cells in the follicles of secondary lymphoid organs. RNA-Seq-analysis revealed that several genes involved in B-cell migration, survival, proliferation and cytokine signaling govern the transcriptional differences modulated by the ablation of RelB in long-term CD40-stimulated B cells. Inactivation of RelB did not abrogate lymphoma development. However, lymphomas occurred with a lower incidence and had a longer latency period. In summary, our data suggest that RelB, although it is not strictly required for malignant transformation, accelerates the lymphomagenesis of long-term CD40-stimulated B cells by regulating genes involved in migration, survival and cytokine signaling.

Co-localization of Fibronectin Receptors LILRB4/gp49B and Integrin on Dendritic Cell Surface

A myeloid immune checkpoint, leukocyte immunoglobulin-like receptor (LILR) B4 (B4, also known as ILT3/CD85k in humans and gp49B in mice) is expressed on dendritic cells (DCs). However, a mode of regulation of DCs by B4/gp49B is not identified yet in relation to the ligand(s) as well as to the counteracting, activation-type receptor. Our recent identification of the physiological/pathological ligand for B4/gp49B as the fibronectin (FN) N-terminal 30-kDa domain poses the question of the relationship between B4/gp49B and a classical FN receptor/cellular activator, integrin, on DCs. Here we showed that FN is not constitutively tethered on the surface of bone marrow-derived cultured DCs (BMDCs) or splenic DCs, even though the FN receptor integrin and gp49B are co-expressed on these cells. Confocal laser scanning microscopic analysis, however, revealed weak correlation of fluorescent signals between gp49B and integrin β₁, suggesting their partial co-localization on the BMDC surface even in the absence of FN. We found that the plating of BMDCs onto immobilized FN induced tyrosine phosphorylation of focal adhesion kinase (FAK) and spleen tyrosine kinase (Syk). In the absence of gp49B, while the FAK phosphorylation level was virtually unchanged, that of phosphorylation of Syk was markedly augmented. These results suggested that the immobilized FN induced a crosstalk between gp49B and integrin in terms of the intracellular signaling of BMDCs, in which gp49B suppressed the integrin-mediated pro-inflammatory cascade. Our observations may provide a clue for elucidating the mechanism of the therapeutic efficacy of B4/gp49B blocking in autoimmune disease and cancer.

Myeloid immune checkpoint ILT3/LILRB4/gp49B can co-tether fibronectin with integrin on macrophages

LILRB4 (B4, also known as ILT3/CD85k) is an immune checkpoint of myeloid-lineage cells, albeit its mode of function remains obscure. Our recent identification of a common ligand for both human B4 and its murine ortholog gp49B as the fibronectin (FN) N-terminal 30-kDa domain poses the question of how B4/gp49B regulate cellular activity upon recognition of FN in the plasma and/or the extracellular matrix. Since FN in the extracellular matrix is tethered by FN-binding integrins, we hypothesized that B4/gp49B would tether FN in cooperation with integrins on the cell surface, thus they should be in close vicinity to integrins spatially. This scenario suggests a mode of function of B4/gp49B by which the FN-induced signal is regulated. FN pull-down complex was found to contain gp49B and integrin β1 in bone marrow-derived macrophages. The confocal fluorescent signals of the three molecules on the intrinsically FN-tethering macrophages were correlated to each other. When FN-poor macrophages adhered to culture plate, the gp49-integrin β1 signal correlation increased at the focal adhesion, supporting the notion that gp49B and integrin β1 become spatially closer to each other there. While adherence of RAW264.7 and THP-1 cells to immobilized FN induced phosphorylation of spleen tyrosine kinase, whose level was augmented under B4/gp49B deficiency. Thus, we concluded that B4/gp49B can co-tether fibronectin in cooperation with integrin in the cis configuration on the same cell, forming a B4/gp49B-FN-integrin triplet as a regulatory unit of focal adhesion-dependent proinflammatory signal in macrophages.

Blockade of checkpoint ILT3/LILRB4/gp49B binding to fibronectin ameliorates autoimmune disease in BXSB/Yaa mice

The extracellular matrix (ECM) is the basis for virtually all cellular processes and is also related to tumor metastasis. Fibronectin (FN), a major ECM macromolecule expressed by different cell types and also present in plasma, consists of multiple functional modules that bind to ECM-associated, plasma, and cell-surface proteins such as integrins and FN itself, thus ensuring its cell-adhesive and modulatory role. Here we show that FN constitutes an immune checkpoint. Thus, FN was identified as a physiological ligand for a tumor/leukemia/lymphoma- as well as autoimmune-associated checkpoint, ILT3/LILRB4 (B4, CD85k). Human B4 and the murine ortholog, gp49B, bound FN with sub-micromolar affinities as assessed by bio-layer interferometry. The major B4-binding site in FN was located at the N-terminal 30-kDa module (FN30), which is apart from the major integrin-binding site present at the middle of the molecule. Blockade of B4-FN binding such as with B4 antibodies or a recombinant FN30-Fc fusion protein paradoxically ameliorated autoimmune disease in lupus-prone BXSB/Yaa mice. The unexpected nature of the B4-FN checkpoint in autoimmunity is discussed, referring to its potential role in tumor immunity.

Expression of leukocyte immunoglobulin-like receptor subfamily B expression on immune cells in hepatocellular carcinoma

BACKGROUND

Leukocyte immunoglobulin-like receptor subfamily B (LILRB) is a group of inhibitory receptors involved in innate immune mainly expressed on lymphoid and myelomonocytic cells. LILRB is proposed to serve as immune checkpoint like PD-1 and CTLA-4 for tumor treatment. We recently reported that the expression of LILRB2 in CD1c⁺ mDC from tumor tissue might suppress immune for HCC patients. However, the expression of all the LILRB family on other immune cells in peripheral blood and tumor microenvironment of HCC patients has not been systematically studied.

METHODS

The expression of LILRB family (LILRB1, LILRB2, LILRB3, LILRB4 and LILRB5) on immune cells, including granulocytes, NK cells, NKT cells, monocyte subsets, TAMs, B cells, γδ T cells, CD4⁺ T cells, CD8⁺ T cells and MDSC subsets, was analyzed by flow cytometry in the peripheral blood of 20 HCC patients and 20 healthy donors as well as in the tumor and tumor free tissues of 10 HCC patients.

RESULTS

LILRB1, LILRB2 and LILRB3 in granulocytes from peripheral blood were expressed increased in HCC patients compared with healthy donors. The expression of LILRB5 in NK cells and NKT cells from HCC blood were higher compared with healthy donors` blood. CD14⁺CD16⁺ monocyte subsets in blood of HCC patients expressed increased LILRB1 and LILRB4 than that in healthy donors. CD14⁺CD16^- monocyte subsets in blood of HCC patients expressed increased LILRB3 than that in healthy donors. Compared to corresponding TFL, LILRB3, LILRB4 and LILRB5 were expressed enhanced in TAMs from HCC tumors. LILRB1 expressed on the B cells both in the blood and tumor had significantly increased compared with healthy donors or corresponding TFL. Different from peripheral blood, in the HCC microenvironment, CD4⁺ T cells expressed lower LILRB2, LILRB3 and LILRB4 than that from TFL and CD8⁺ T cells expressed decreased LILRB2. And γδ T cells expressed LILRB1 in HCC blood and microenvironment. Surprisingly, the percentage of LILRB1 expressed on MDSC from HCC peripheral blood and tumors was lower than that from healthy donors and corresponding TFL.

CONCLUSIONS

This is the first systemically examination of the LILRB family expression on a variety of immune cells from both peripheral blood and microenvironment in HCC patients. The specific increasing expression of LILRB on immune cells may regulate innate and adaptive immune and impact on HCC progression. Our findings justify further investigation of LILRB function in HCC.

Leukocyte immunoglobulin-like receptor subfamily B: therapeutic targets in cancer

Inhibitory leukocyte immunoglobulin-like receptors (LILRBs 1–5) transduce signals via intracellular immunoreceptor tyrosine-based inhibitory motifs that recruit phosphatases to negatively regulate immune activation. The activation of LILRB signaling in immune cells may contribute to immune evasion. In addition, the expression and signaling of LILRBs in cancer cells especially in certain hematologic malignant cells directly support cancer development. Certain LILRBs thus have dual roles in cancer biology—as immune checkpoint molecules and tumor-supporting factors. Here, we review the expression, ligands, signaling, and functions of LILRBs, as well as therapeutic development targeting them. LILRBs may represent attractive targets for cancer treatment, and antagonizing LILRB signaling may prove to be effective anti-cancer strategies.

The association of leukocyte immunoglobulin-like receptor subfamily B-4 expression in acute myeloid leukemia and central nervous system involvement

Central nervous system (CNS) involvement in patients with acute myeloid leukemia (AML) varies, ranging from 0.6%-46%. Leukocyte immunoglobulin-like receptor B4 (LILRB4) has been shown to be critical in orchestration of infiltration of AML cells into the CNS in animal models, however it is unknown if an association exists between LILRB4 and CNS involvement (CNS+) in human patients with AML. LILRB4 was measured by flow cytometry in a heterogeneous population of fifty-six AML patients. Patients were then followed clinically for the development of CNS + . LILRB4 was positive in 91 % of patients with CNS + compared to 38 % without CNS involvement (p < 0.002). In logistic analysis: age, BMI, serum albumin and positive LILRB4 were predictive for CNS+ [OR, 95 % CI, p-value]: 0.95, 0.92-0.99, p < 0.01; 0.85, 0.73-0.998, p < 0.05; 0.23, 0.066-0.78, p < 0.02; 16.46, 1.93-140.2, p < 0.02, respectively. This finding of the association of LILRB4 with CNS + in combination with earlier findings suggests that LILRB4 has a mechanistic role in infiltration of the CNS and may provide insight into the pathogenesis of AML seeding the CNS. Moreover, this proof of concept and the findings in the present study may lead to the development of innovative and novel therapies to improve the lives of patients with AML.

ILT3 promotes tumor cell motility and angiogenesis in non-small cell lung cancer

Immunoglobulin-like transcript (ILT) 3 is an immunosuppressive molecule that negatively regulates myeloid cell activation. ILT3 overexpression in tumor cells induces immune escape of solid tumors and facilitates invasion of monocytic acute myeloid leukemia cells. However, the expression and function of ILT3 in non-small cell lung cancer (NSCLC) cells remain elusive. Herein, we found that ILT3 was enriched in human NSCLC cells, and predicted advanced disease and poor overall survival. ILT3 overexpression enhanced the migration and invasion of NSCLC cells and tubule formation of human umbilical vein endothelial cells by upregulating and interacting with its ligand apolipoprotein E (ApoE) in vitro. Mechanistically, ILT3 recruited SHP2 and SHIP1, and subsequently activated ERK1/2 signaling mediating epithelial-mesenchymal transition (EMT) and increasing vascular endothelial growth factor (VEGF)-A expression in NSCLC cells, which are responsible for tumor cell motility and angiogenesis, respectively. Using murine metastasis models, we further confirmed ILT3 promoted NSCLC metastasis and explored the exact correlation of ILT3 with ApoE, EMT, and VEGF-A in vivo. These results unraveled novel mechanisms for ILT3-induced tumor progression and proposed ILT3 as a potential therapeutic target and prognostic biomarker for NSCLC patients.

Leukocyte immunoglobulin-like receptor B1 and B4 (LILRB1 and LILRB4): Highly sensitive and specific markers of acute myeloid leukemia with monocytic differentiation

BACKGROUND

Acute myeloid leukemia (AML) with monocytic differentiation (M-AML) remains a diagnostic challenge largely due to lack of sensitive and specific markers for immature monocytes. The immunoglobulin-like inhibitory receptors, LILRB1 and LILRB4, are expressed on monocytes but have not yet been systematically evaluated in the clinical setting.

METHODS

We evaluated the diagnostic performance of LILRB1 and LILRB4 as monocytic markers for both immature and mature monocytes in comparison to other myelomonocytic markers including CD14, CD15, CD33, CD36, and CD64 in eight cases of control bone marrow (BM, 5) and peripheral blood (PB, 3), 64 cases of (M-AML), and 57 cases of AML without monocytic differentiation (NM-AML) by flow cytometric immunophenotyping.

RESULTS

In control BM, LILRB1 and LILRB4 were consistently expressed on monocytes at all stages of maturation, from CD34⁺ /CD14^- monocytic precursors to CD14^-/dim+ maturing and CD14⁺ mature monocytes. In M-AML, LILRB1 and LILRB4 were consistently expressed on monocytes, regardless of the degree of maturity, from CD14^-/dim+ monoblasts/promonocytes to CD14⁺ mature monocytes but were not expressed on myeloblasts. The diagnostic performances as a monocytic marker assessed by sensitivity/specificity were 100%/100% for LILRB1/LILRB4, 100%/82% for CD11b, 80%/100% for CD14, 100%/81% for CD64, 100%/58% for CD15/CD33, and 89%/97% for CD36/CD64.

CONCLUSION

The co-expression of LILRB1/LILRB4 outperformed other myelomonocytic markers as a highly sensitive and specific marker for monocytes at all stages of maturation and could reliably distinguish M-AML from NM-AML. LILRB4 additionally represents a novel therapeutic target for treating M-AML.

LILRB4-targeting Antibody-Drug Conjugates for the Treatment of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is the most common and aggressive blood cancer in adults. In particular, significant unmet medical needs exist for effective treatment strategies for acute myelomonocytic leukemia (M4) and acute monocytic leukemia (M5) AML subtypes. Antibody-drug conjugates (ADC) are a promising drug class for AML therapy, as demonstrated by the FDA-approved anti-CD33 ADC, gemtuzumab ozogamicin (Mylotarg). However, CD33 is expressed in normal hematopoietic stem cells, highlighting the critical need to identify AML-specific targets to minimize the risk of potential adverse effects. We have demonstrated that the leukocyte immunoglobulin-like receptor subfamily B4 (LILRB4) is expressed at significantly higher levels on monocytic M4 and M5 AML cells than on normal counterparts. Here, we test whether LILRB4 is a promising ADC target to kill monocytic AML cells while sparing healthy counterparts. To this end, we generated ADCs from a humanized anti-LILRB4 mAb and the antimitotic payload, monomethyl auristatin F. The conjugates constructed were characterized and evaluated for LILRB4-specific cell killing potency, toxicity to progenitor cells, pharmacokinetics, and therapeutic efficacy. Our ADC linker technology platform efficiently generated homogeneous anti-LILRB4 ADCs with defined drug-to-antibody ratios. The homogeneous anti-LILRB4 ADCs demonstrated the capacity for LILRB4-mediated internalization, suitable physicochemical properties, and high cell killing potency against LILRB4-positive AML cells. Importantly, our data indicate that these ADCs spare normal progenitor cells. One of our homogeneous conjugates exerted a remarkable therapeutic effect and no significant toxicity in a xenograft mouse model of disseminated human AML. Our findings highlight the clinical potential of anti-LILRB4 ADCs in monocytic AML therapy.

Functions of Immune Checkpoint Molecules Beyond Immune Evasion

Immune checkpoint molecules, including inhibitory and stimulatory immune checkpoint molecules, are defined as ligand-receptor pairs that exert inhibitory or stimulatory effects on immune responses. Most of the immune checkpoint molecules that have been described so far are expressed on cells of the adaptive immune system, particularly on T cells, and of the innate immune system. They are crucial for maintaining the self-tolerance and modulating the length and magnitude of immune responses of effectors in different tissues to minimize the tissue damage. More and more evidences have shown that inhibitory or stimulatory immune checkpoint molecules are expressed on a sizeable fraction of tumor types. Although the main function of tumor cell-associated immune checkpoint molecules is considered to mediate the immune evasion, it has been reported that the immune checkpoint molecules expressed on tumor cells also play important roles in the maintenance of many malignant behaviors, including self-renewal, epithelial-mesenchymal transition, metastasis, drug resistance, anti-apoptosis, angiogenesis, or enhanced energy metabolisms. In this section, we mainly focus on delineating the roles of the tumor cell-associated immune checkpoint molecules beyond immune evasion, such as PD-L1, PD-1, B7-H3, B7-H4, LILRB1, LILRB2, TIM3, CD47, CD137, and CD70.

Transcriptome Analysis Identifies Immune Markers Related to Visceral Leishmaniasis Establishment in the Experimental Model of BALB/c Mice

Visceral leishmaniasis (VL) caused by Leishmania donovani and L. infantum is a potentially fatal disease. To date there are no registered vaccines for disease prevention despite the fact that several vaccines are in preclinical development. Thus, new strategies are needed to improve vaccine efficacy based on a better understanding of the mechanisms mediating protective immunity and mechanisms of host immune responses subversion by immunopathogenic components of Leishmania. We found that mice vaccinated with CPA_162−189-loaded p8-PLGA nanoparticles, an experimental nanovaccine, induced the differentiation of antigen-specific CD8⁺ T cells in spleen compared to control mice, characterized by increased dynamics of proliferation and high amounts of IFN-γ production after ex vivo re-stimulation with CPA_162−189 antigen. Vaccination with CPA_162−189-loaded p8-PLGA nanoparticles resulted in about 80% lower parasite load in spleen and liver at 4 weeks after challenge with L. infantum promastigotes as compared to control mice. However, 16 weeks after infection the parasite load in spleen was comparable in both mouse groups. Decreased protection levels in vaccinated mice were followed by up-regulation of the anti-inflammatory IL-10 production although at lower levels in comparison to control mice. Microarray analysis in spleen tissue at 4 weeks post challenge revealed different immune-related profiles among the two groups. Specifically, vaccinated mice were characterized by similar profile to naïve mice. On the other hand, the transcriptome of the non-vaccinated mice was dominated by increased expression of genes related to interferon type I, granulocyte chemotaxis, and immune cells suppression. This profile was significantly enriched at 16 weeks post challenge, a time-point which is relative to disease establishment, and was common for both groups, further suggesting that type I signaling and granulocyte influx has a significant role in disease establishment, pathogenesis and eventually in decreased vaccine efficacy for stimulating long-term protection. Overall, we put a spotlight on host immune networks during active VL as potential targets to improve and design more effective vaccines against disease.

LILRB4 ITIMs mediate the T cell suppression and infiltration of acute myeloid leukemia cells

We recently demonstrated that leukocyte Ig-like receptor 4 (LILRB4) expressed by monocytic acute myeloid leukemia (AML) cells mediates T-cell inhibition and leukemia cell infiltration via its intracellular domain. The cytoplasmic domain of LILRB4 contains three immunoreceptor tyrosine-based inhibitory motifs (ITIMs); the tyrosines at positions 360, 412, and 442 are phosphorylation sites. Here, we analyzed how the ITIMs of LILRB4 in AML cells mediate its function. Our in vitro and in vivo data show that Y₄₁₂ and Y₄₄₂, but not Y₃₆₀, of LILRB4 are required for T-cell inhibition, and all three ITIMs are needed for leukemia cell infiltration. We constructed chimeric proteins containing the extracellular domain of LILRB4 and the intracellular domain of LILRB1 and vice versa. The intracellular domain of LILRB4, but not that of LILRB1, mediates T-cell suppression and AML cell migration. Our studies thus defined the unique signaling roles of LILRB4 ITIMs in AML cells.

Augmented ILT3/LILRB4 Expression of Peripheral Blood Antibody Secreting Cells in the Acute Phase of Kawasaki Disease

BACKGROUND

Kawasaki disease (KD) is an acute, systemic vasculitis syndrome that occurs in children. The clinical symptoms and epidemiologic features of KD strongly suggest that KD is triggered by unidentified infectious agents in genetically predisposed patients. In addition, a number of studies have described the role of B cells in the development of KD. To obtain a mechanistic insight into the humoral immune response of B-lineage cells in KD patients, we examined peripheral blood antibody secreting cells (ASCs) and inhibitory immunoreceptors, immunoglobulin-like transcript (ILT)/leukocyte immunoglobulin-like receptor (LILR), on each B cell subpopulation.

METHODS

Eighteen Japanese KD patients and thirteen healthy control subjects were recruited for this study. Their peripheral blood mononuclear cells were examined by flow cytometry for the number of CD19 B cells, the size of each B cell subset and the expression of the inhibitory isoforms of ILT/LILR on the B cell subset.

RESULTS

The frequency of CD19CD27 ASCs was significantly increased in the acute phase of KD and reduced after high-dose intravenous immunoglobulin (IVIG) treatment. Interestingly, while ILT2/LILRB1 expression was ubiquitously observed on every B cell/ASCs subset and the level was not significantly different after IVIG, ILT3/LILRB4 (B4) was uniquely expressed on only ASCs, and its expression was significantly decreased after IVIG.

CONCLUSIONS

In the acute phase of KD, the frequency of ASCs is high with augmented B4 expression, whereas it is lower with decreased B4 expression after IVIG. Further studies of B4 expression on ASCs in autoimmune and infectious diseases will be needed to confirm the significance of our findings.

LILRB4 signalling in leukaemia cells mediates T cell suppression and tumour infiltration

Immune checkpoint blockade therapy has been successful in treating some types of cancer but has not shown clinical benefits for treating leukaemia1. This result suggests that leukaemia uses unique mechanisms to evade this therapy. Certain immune inhibitory receptors that are expressed by normal immune cells are also present on leukaemia cells. Whether these receptors can initiate immune-related primary signalling in tumour cells remains unknown. Here we use mouse models and human cells to show that LILRB4, an immunoreceptor tyrosine-based inhibition motif-containing receptor and a marker of monocytic leukaemia, supports tumour cell infiltration into tissues and suppresses T cell activity via a signalling pathway that involves APOE, LILRB4, SHP-2, uPAR and ARG1 in acute myeloid leukaemia (AML) cells. Deletion of LILRB4 or the use of antibodies to block LILRB4 signalling impeded AML development. Thus, LILRB4 orchestrates tumour invasion pathways in monocytic leukaemia cells by creating an immunosuppressive microenvironment. LILRB4 represents a compelling target for the treatment of monocytic AML.

A Novel Anti-LILRB4 CAR-T Cell for the Treatment of Monocytic AML

To effectively improve treatment for acute myeloid leukemia (AML), new molecular targets and therapeutic approaches need to be identified. Chimeric antigen receptor (CAR)-modified T cells targeting tumor-associated antigens have shown promise in the treatment of some malignancies. However, CAR-T cell development for AML has been limited by lack of an antigen with high specificity for AML cells that is not present on normal hematopoietic stem cells, and thus will not result in myelotoxicity. Here we demonstrate that leukocyte immunoglobulin-like receptor-B4 (LILRB4) is a tumor-associated antigen highly expressed on monocytic AML cells. We generated a novel anti-LILRB4 CAR-T cell that displays high antigen affinity and specificity. These CAR-T cells display efficient effector function in vitro and in vivo against LILRB4⁺ AML cells. Furthermore, we demonstrate anti-LILRB4 CAR-T cells are not toxic to normal CD34⁺ umbilical cord blood cells in colony-forming unit assays, nor in a humanized hematopoietic-reconstituted mouse model. Our data demonstrate that anti-LILRB4 CAR-T cells specifically target monocytic AML cells with no toxicity to normal hematopoietic progenitors. This work thus offers a new treatment strategy to improve outcomes for monocytic AML, with the potential for elimination of leukemic disease while minimizing the risk for on-target off-tumor toxicity.

Expression of ILT3 predicts poor prognosis and is inversely associated with infiltration of CD45RO+ T cells in patients with colorectal cancer

Immunoglobulin like transcript 3 (ILT3) was previously identified as an inhibitory receptor to induce T cell anergy in tranplantation, autoimmunity and allergy. Here we aimed to investigate the expression of ILT3 in colorectal cancer, analyze the association between ILT3 expression and clinicopathological variables and prognosis, and evaluate the correlation between the expression of ILT3 and CD45RO+ T cells density. Expression of ILT3 was identified on the cell membrane and/or in the cytoplasm. High expression ILT3 was identified in 55 of 85 (64.7%) tumor specimens, which was significantly higher than that in the adjacent normal tissues(5/30) (P < 0.001). High ILT3 expression was significantly associated with positive lymph node metastasis (N1-2; P = 0.03), advanced disease (stage III-IV; P = 0.03), and reduced OS in patients. The ILT3 expression level was an independent prognostic factor (P = 0.004) and inversely correlated with the number of CD45RO+ T cells (P = 0.019). In the present study, high ILT3 expression was observed in colorectal cancer and inversely associated with CD45RO+ T cells density and prognosis, suggesting that ILT3 played an important role in tumor progression by possible influence on CD45RO+ T cells in the tumor microenvironment.

Ectopic ILT3 controls BCR-dependent activation of Akt in B-cell chronic lymphocytic leukemia

The high proportion of long-term nonprogressors among chronic lymphocytic leukemia (CLL) patients suggests the existence of a regulatory network that restrains the proliferation of tumor B cells. The identification of molecular determinants composing such network is hence fundamental for our understanding of CLL pathogenesis. Based on our previous finding establishing a deficiency in the signaling adaptor p66Shc in CLL cells, we undertook to identify unique phenotypic traits caused by this defect. Here we show that a lack of p66Shc shapes the transcriptional profile of CLL cells and leads to an upregulation of the surface receptor ILT3, the immunoglobulin-like transcript 3 that is normally found on myeloid cells. The ectopic expression of ILT3 in CLL was a distinctive feature of neoplastic B cells and hematopoietic stem cells, thus identifying ILT3 as a selective marker of malignancy in CLL and the first example of phenotypic continuity between mature CLL cells and their progenitors in the bone marrow. ILT3 expression in CLL was found to be driven by Deltex1, a suppressor of antigen receptor signaling in lymphocytes. Triggering of ILT3 inhibited the activation of Akt kinase upon B-cell receptor (BCR) stimulation. This effect was achieved through the dynamic coalescence of ILT3, BCRs, and phosphatidylinositol-3,4,5-trisphosphate 5-phosphatase 1 into inhibitory clusters at the cell surface. Collectively, our findings identify ILT3 as a signature molecule of p66Shc deficiency in CLL and indicate that ILT3 may functionally contribute to a regulatory network controlling tumor progression by suppressing the Akt pathway.

Leukocyte immunoglobulin-like receptors in human diseases: an overview of their distribution, function, and potential application for immunotherapies

Myeloid-derived suppressor cells (MDSCs), a population of immature myeloid cells expanded and accumulated in tumor-bearing mice and in patients with cancer, have been shown to mediate immune suppression and to promote tumor progression, thereby, posing a major hurdle to the success of immune-activating cancer therapies. MDSCs, like their healthy counterparts, such as monocytes/macrophages and granulocytes, express an array of costimulatory and coinhibitory molecules as well as myeloid activators and inhibitory receptors, such as leukocyte immunoglobulin-like receptors (LILR) A and B. This review summarizes current findings on the LILR family members in various diseases, their potential roles in the pathogenesis, and possible strategies to revert or enhance the suppressive function of MDSCs for the benefit of patients by targeting LILRs.

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.

Immunoglobulin-like transcript 4 promotes tumor progression and metastasis and up-regulates VEGF-C expression via ERK signaling pathway in non-small cell lung cancer

Immunoglobulin-like transcript (ILT) 4 has long been thought to be cell-surface molecule in certain immune cells and negatively regulates immune response. Recently, overexpression of ILT4 has been observed in a few cancers with unknown function. Here, we showed manipulation of ILT4 affected non-small cell lung cancer (NSCLC) cell proliferation, migration and invasion in vitro analyses. In vivo, ILT4 promoted the tumor growth and metastasis. Furthermore, the phosphorylation of extracellular regulated protein kinases (ERK1/2) was enhanced in ILT4 overexpressing NSCLC cells. ERK1/2 specific inhibitor U0126 suppressed the proliferation, migration and invasion of those cells. Stepwise investigations demonstrated that vascular endothelial growth factor C (VEGF-C) was the downstream effector of ILT4 and ERK1/2. Silence of VEGF-C attenuated the migration and invasion activity of ILT4 overexpressing cells. Moreover, Kaplan-Meier survival analysis indicated that NSCLC patients with ILT4 positive expression had a poor patient survival. ILT4 and VEGF-C expression had notable positive correlation in cancer cells, and their co-expression was significantly associated with adverse prognostic factors. Our findings suggest that ILT4 drives NSCLC development in part on activation of ERK signaling which in turn upregulates VEGF-C. ILT4 could be a novel cancer therapeutic target for NSCLC.

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.

ILT4 drives B7-H3 expression via PI3K/AKT/mTOR signalling and ILT4/B7-H3 co-expression correlates with poor prognosis in non-small cell lung cancer

Immunoglobulin-like transcript (ILT) 4 is critical for the inhibitory function of certain immune cells. We previously demonstrated that ILT4 is over-expressed in human non-small cell lung cancer (NSCLC) cells and is involved in tumour evasion via an unknown mechanism. In this report, we demonstrate that ILT4 increases the expression of the co-inhibitory molecule B7-H3 through PI3K/AKT/mTOR signalling. In primary human NSCLC tissues, a significant positive relationship is observed between ILT4 and B7-H3 expression. ILT4/B7-H3 co-expression is significantly associated with a reduction in T infiltrating lymphoid cells and lower overall survival. In summary, ILT4 increases B7-H3 expression and ILT4/B7-H3 co-expression may be involved in NSCLC progression.

Immunoglobulin-like transcript 3 is expressed by myeloid-derived suppressor cells and correlates with survival in patients with non-small cell lung cancer

Myeloid-derived suppressor cells (MDSCs) play an important role in immune suppression and accumulate under pathologic conditions such as cancer and chronic inflammation. They comprise a heterogeneous population of immature myeloid cells that exert their immunosuppressive function via a variety of mechanisms. Immunoglobulin-like transcript 3 (ILT3) is a receptor containing immunoreceptor tyrosine-based inhibition motifs (ITIMs) that can be expressed on antigen-presenting cells and is an important regulator of dendritic cell tolerance. ILT3 exists in a membrane-bound and a soluble form and can interact with a yet unidentified ligand on T cells and thereby induce T-cell anergy, regulatory T cells, or T suppressor cells. In this study, we analyzed freshly isolated peripheral blood mononuclear cells (PBMCs) of 105 patients with non-small cell lung cancer and 20 healthy controls and demonstrated for the first time that ILT3 is expressed on MDSCs. We show that increased levels of circulating MDSCs correlate with reduced survival. On the basis of ILT3 cell surface expression, an ILT3low and ILT3high population of polymorphonuclear (PMN)-MDSCs could be distinguished. Interestingly, in line with the immunosuppressive function of ILT3 on dendritic cells, patients with an increased proportion of PMN-MDSCs and an increased fraction of the ILT3high subset had a shorter median survival than patients with elevated PMN-MDSC and a smaller ILT3high fraction. No correlation between the ILT3high subset and other immune variables was found. ILT3 expressed on MDSCs might reflect a previously unknown mechanism by which this cell population induces immune suppression and could therefore be an attractive target for immune intervention.

Inhibitory receptor immunoglobulin-like transcript 4 was highly expressed in primary ductal and lobular breast cancer and significantly correlated with IL-10

BACKGROUND

Immunoglobulin-like transcript 4 (ILT4) is an inhibitory molecule involved in immune response and has recently been identified to be strongly inducible by IL-10. The aim of the present study was to examine the associations of ILT4 expression with clinicopathological characteristics and IL-10 expression in primary ductal and lobular breast cancer.

METHODS

We studied the expression of ILT4 in 4 cancer cell lines, 117 primary tumor tissues and 97 metastatic lymph nodes from patients with primary ductal and lobular breast cancer by reverse transcription-polymerase chain reaction, western blot or immunohistochemistry analysis. Additionally, IL-10 expression was also investigated using immunohistochemistry in primary tumor tissues. Then the relationship between ILT4 expression and clinicopathological characteristics/IL-10 expression was evaluated.

RESULTS

ILT4 was highly expressed in all 4 human breast cancer cell lines on both mRNA and protein levels. In primary tumor tissues, ILT4 or IL-10 was expressed in the cell membrane, cytoplasm, or both; the positive rate of ILT4 and IL-10 expression was 60.7% (71/117) and 80.34% (94/117), respectively. ILT4 level was significantly correlated with IL-10 (r =0.577; p<0.01). Furthermore, the expression of ILT4 or IL-10 was associated with less number of Tumor Infiltrating Lymphocytes (TILs) (p=0.004 and 0.018, respectively) and more lymph node metastasis (p=0.046 and 0.035, respectively).

CONCLUSION

Our data demonstrated the association of ILT4 and IL-10 expression in human breast cancer, suggesting their important roles in immune dysfunction and lymph node metastases.

VIRTUAL SLIDES

The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1692652692107916.

Expression of immune inhibitory receptor ILT3 in acute myeloid leukemia with monocytic differentiation

BACKGROUND

The diagnosis of AML with monocytic differentiation is limited by the lack of highly sensitive and specific monocytic markers. Immunoglobulin-like transcript 3 (ILT3) is an immune inhibitory receptor expressed by myelomonocytic cells and at high levels by tolerogenic dendritic cells.

METHODS

Using flow cytometry, we analyzed the expression of ILT3 in 37 patients with AML and 20 patients with no detectable disease.

RESULTS

We showed that ILT3 was expressed in all cases of AML displaying monocytic differentiation (FAB M4/M5; N = 18), but not in AML M1/M2 and M3 (N = 19; P < 0.0001). Co-expression of ILT3 and immature cell markers, such as CD34 and CD117, was observed in monoblastic leukemia. ILT3 expression was preserved after treatment in M4/M5 patients with refractory or relapsed disease. ILT3 expression was associated with the presence of cytogenetic abnormalities linked to an intermediate prognosis (P = 0.001). Rare CD45dimCD34+CD117+ILT3+ cells were identified in noninvolved bone marrow, suggesting that ILT3 expression is acquired at an early stage by normal myelomonocytic precursors.

CONCLUSIONS

ILT3 is a highly sensitive and specific marker which distinguishes AML with monocytic differentiation from other types of AML. Testing of ILT3 expression should be incorporated into the initial diagnostic work-up and monitoring of patients with AML.

Cyclosporine induces up-regulation of immunoglobulin-like transcripts 3 and 4 expression on and activity of NKL cells

BACKGROUND

Immunoglobulin-like transcripts (ILTs), which belong to a kind of receptor family discovered recently, are differentially expressed on myeloid and lymphoid cells. Most of them play important roles to regulate human immune responses by interacting with ligands. Cyclosporine (CsA) is frequently used to prevent graft-versus-host disease and treat autoimmune diseases. There are some studies about the effects of CsA on various human immunologic reactions, but its impact on ILT3 and ILT4 expression on natural killer (NK) cells is less well understood.

METHODS

An NKL cell line was exposed to CsA (5, 10, 15, or 20 mg/L) for 12, 24, or 36 hours before real-time quantitative polymerase chain reaction and flow cytometry were used to detect alterations in ILT3 and ILT4 mRNA and protein expressions. NKL cells treated for 36 hours with or without CsA (15 mg/L) and then coincubated with BGC-823 or JEG-3 cells, in cytolytic and proliferative systems measured by Thiazoyl blue tetrazolium bromide assays.

RESULTS

After CsA treatment both RNA and protein levels of ILT3 and ILT4 on NKL cells were increased for 12, 24, or 36 hours. CsA at various concentrations inhibited the proliferation of NKL cells to varying degrees; at 36 hours CsA (15 mg/L) showed greater effects on ILT3 and ILT4 expression and less influence on NKL growth. The ability of NKL cells primed with CsA (15 mg/L) for 36 hours to kill tumor cells was decreased markedly.

CONCLUSIONS

CsA up-regulated the expression of ILT3 and ILT4 on NKL cells, which influenced their cytotoxicity against tumor cells with different expression of HLA-G and proliferation of NKL cells.

Expression of Leukocyte Inhibitory Immunoglobulin-like Transcript 3 Receptors by Ovarian Tumors in Laying Hen Model of Spontaneous Ovarian Cancer

Attempts to enhance a patient's immune response and ameliorate the poor prognosis of ovarian cancer (OVCA) have largely been unsuccessful owing to the suppressive tumor microenvironment. Leukocyte immunoglobulin-like transcript 3 (ILT3) inhibitory receptors have been implicated in immunosuppression in several malignancies. The expression and role of ILT3 in the progression of ovarian tumors are unknown. This study examined the expression and association of ILT3 in ovarian tumors in laying hens, a spontaneous preclinical model of human OVCA. White Leghorn laying hens were selected by transvaginal ultrasound scanning. Serum and normal ovaries or ovarian tumors were collected. The presence of tumors and the expression of ILT3 were examined by routine histology, immunohistochemistry, Western blot analysis, and reverse transcription-polymerase chain reaction. In addition to stromal immune cell-like cells, the epithelium of the ovarian tumors also expressed ILT3 with significantly high intensity than normal ovaries. Among different subtypes of ovarian carcinomas, serous OVCA showed the highest ILT3 staining intensity, whereas endometrioid OVCA had the lowest intensity. Similar to humans, an immunoreactive protein band of approximately 55 kDa for ILT3 was detected in the ovarian tumors in hens. The patterns of ILT3 protein and messenger RNA expression by ovarian tumors in different subtypes and stages were similar to those of immunohistochemical staining. The results of this study suggest that laying hens may be useful to generate information on ILT3-associated immunosuppression in OVCA. This animal model also offers the opportunity to develop and test anti-ILT3 immunotherapy to enhance antitumor immunity against OVCA in humans.

Expression of immunoglobulin-like transcript (ILT)2 and ILT3 in human gastric cancer and its clinical significance

Immune inhibitory receptors play an important role in organ transplantation, autoimmune diseases and cancers. Immunoglobulin-like transcript (ILT)2 and ILT3 belong to the inhibitory receptors of the ILT family, which have been reported to regulate a broad range of cellular functions involved in the immune response. They contain immunoreceptor tyrosine-based inhibitory motifs (ITIMs), which are related to immune regulation. Although ILT receptors have been studied in dendritic cells (DCs), T cells, NK cells and other cell types, the expression and clinical significance of ILT2 and ILT3 in gastric cancer have yet to be elucidated. Here, the expression of ILT2 and ILT3 in gastric cancer cell lines and pathologic tissues, as well as their effects on the cytotoxicity of NK92MI against the gastric cancer cell lines MKNI with ILT2^lowILT3^low and HGC-27 with ILT2^highILT3^high were detected. The results suggest that ILT2 and ILT3 are expressed with diverse degrees in gastric cancer cells and tissues, and the expression of ILT2 is related with differentiation and size of tumors. Furthermore, the cytotoxic activity of NK92MI against the MKNI cell line was stronger than that against HGC-27. This study indicates that ILT2 and ILT3 play a key role in gastric cancer immune escape, and ILT2 may be a new target in the clinical diagnosis and treatment of gastric cancer.

Human B1 cells in umbilical cord and adult peripheral blood express the novel phenotype CD20+ CD27+ CD43+ CD70-

Human B1 cells consist of CD20⁺CD27⁺CD43⁺CD70⁻ cells bearing a skewed B cell receptor repertoire, and are present in umbilical cord and adult peripheral blood.

B1 cells differ in many ways from conventional B cells, most prominently in the production of natural immunoglobulin, which is vitally important for protection against pathogens. B1 cells have also been implicated in the pathogenesis of autoimmune dyscrasias and malignant diseases. It has been impossible to accurately study B1 cells during health and illness because the nature of human B1 cells has not been successfully defined. This has produced controversy regarding the existence of human B1 cells. Here, we determined the phenotype of human B1 cells by testing sort-purified B cell fractions for three fundamental B1 cell functions based on mouse studies: spontaneous IgM secretion, efficient T cell stimulation, and tonic intracellular signaling. We found that a small population of CD20⁺CD27⁺CD43⁺ cells present in both umbilical cord and adult peripheral blood fulfilled these criteria and expressed a skewed B cell receptor repertoire. These B cells express little or no surface CD69 and CD70, both of which are markedly up-regulated after activation of CD20⁺CD27⁻CD43⁻ (naive) and CD20⁺CD27⁺CD43⁻ (memory) B cells. This work identifies human B1 cells as CD20⁺CD27⁺CD43⁺CD70⁻. We determined that the proportion of B1 cells declines with age, which may contribute to disease susceptibility. Identification of human B1 cells provides a foundation for future studies on the nature and role of these cells in human disease.

Membrane and soluble ILT3 are critical to the generation of T suppressor cells and induction of immunological tolerance

The tolerogenic phenotype of human dendritic cells is characterized by high cell surface expression of the inhibitory receptor ILT3. ILT3 signals both intracellularly inhibiting tyrosine phosphorylation, NF-kappaB and MAPK p38 activity, transcription of certain co-stimulatory molecules, secretion of cytokines and chemokines, and extracellularly into the T cells with which the dendritic cells interact. Both ILT3(high) tolerogenic dendritic cells and soluble ILT3 induce CD4 Th anergy and differentiation of antigen specific CD8 T suppressor cells. Recombinant ILT3-Fc protein has important immunotherapeutic potential acting directly on activated T cells and promoting the induction of immunological tolerance.

Regulation of T-cell immunity by leucocyte immunoglobulin-like receptors: innate immune receptors for self on antigen-presenting cells

Following recognition of microbial patterns, innate immune receptors provide a rapid innate response and trigger antigen-presenting cell maturation to instruct adaptive immune responses. Here we discuss a family of innate immune receptors for self - the leucocyte immunoglobulin-like receptors (LILRs). These LILRs exert powerful inhibitory effects on antigen-presenting cell phenotype and subsequent T-cell responses, and may act to constrain the effects of Toll-like receptor signalling. Despite their broad ligand specificity, differing affinities of LILRs for individual complexes of peptide-major histocompatibility complex can determine the nature of their effect on downstream immune responses. Expression and function of LILRs may be skewed in certain conditions such as cancer or human immunodeficiency virus infection, particularly by ectopic expression of human leucocyte antigen-G, a high-affinity LILR ligand. We discuss the relevance of LILR-mediated immune regulation across a range of scenarios from autoimmunity to transplant medicine, infection and cancer.

Central role of ILT3 in the T suppressor cell cascade

CD8+ T suppressor cells differentiate both in vivo and in vitro upon chronic exposure of responding T cells to allogeneic APC. These Ts are allospecific and exhibit their function interacting directly with priming APC which they render tolerogenic. Tolerogenicity of professional and non-professional human APC, such as dendritic cells and endothelial cells, respectively is due to the upregulation of the inhibitory receptors ILT3 and ILT4. ILT3 signals both intracellularly, inhibiting NF-kappaB activation, and transcription of costimulatory molecules, and extracellularly, inducing anergy and regulatory function in T cells with cognate specificity. Both membrane and soluble ILT3 are proteins with potent immunosuppressive activity which are of importance for treatment of rejection, autoimmunity and cancer.

Soluble Ig-like transcript 3 inhibits tumor allograft rejection in humanized SCID mice and T cell responses in cancer patients

Attempts to enhance patients' immune responses to malignancies have been largely unsuccessful. We now describe an immune-escape mechanism mediated by the inhibitory receptor Ig-like transcript 3 (ILT3) that may be responsible for such failures. Using a humanized SCID mouse model, we demonstrate that soluble and membrane ILT3 induce CD8(+) T suppressor cells and prevent rejection of allogeneic tumor transplants. Furthermore, we found that patients with melanoma, and carcinomas of the colon, rectum, and pancreas produce the soluble ILT3 protein, which induces the differentiation of CD8(+) T suppressor cells and impairs T cell responses in MLC. These responses are restored by anti-ILT3 mAb or by depletion of soluble ILT3 from the serum. Immunohistochemical staining of biopsies from the tumors and metastatic lymph nodes suggests that CD68(+) tumor-associated macrophages represent the major source of soluble ILT3. Alternative splicing, resulting in the loss of the ILT3 transmembrane domain, may contribute to the release of ILT3 in the circulation. These data suggest that ILT3 depletion or blockade is crucial to the success of immunotherapy in cancer. In contrast, the inhibitory activity of soluble ILT3 on T cell alloreactivity in vitro and in vivo suggests the potential usefulness of rILT3 for immunosuppressive treatment of allograft recipients or patients with autoimmune diseases.