Overexpression of CD85j in TNBC patients inhibits Cetuximab-mediated NK-cell ADCC but can be restored with CD85j functional blockade

Investigating tumor immunogenicity in breast cancer: deciphering the tumor immune response to enhance therapeutic approaches

The interplay between immune cells and malignant cells represents an essential chapter in the eradication of breast cancer. This widely distributed and diverse form of cancer represents a major threat to women worldwide. The incidence of breast cancer is related to several risk factors, notably genetic predisposition and family antecedents. Despite progress in treatment modalities varying from surgery and chemotherapy to radiotherapy and targeted therapies, persistently high rates of recurrence, metastasis, and treatment resistance underscore the urgent need for new therapeutic approaches. Immunotherapy has gained considerable ground in the treatment of breast cancer, as it takes advantage of the complex interactions within the tumor microenvironment. This dynamic interplay between immune and tumor cells has become a key point of focus in immunological research. This study investigates the role of various cancer markers, such as neoantigens and immune regulatory genes, in the diagnosis and treatment of breast tumors. Moreover, it explores the future potential of immune checkpoint inhibitors as therapeutically effective agents, as well as the challenges that prevent their efficacy, in particular tumor-induced immunosuppression and the difficulty of achieving tumor specificity.

MHC1/LILRB1 axis as an innate immune checkpoint for cancer therapy

Immune checkpoint blockades (ICBs) have revolutionized cancer therapy through unleashing anti-tumor adaptive immunity. Despite that, they are usually effective only in a small subset of patients and relapse can occur in patients who initially respond to the treatment. Recent breakthroughs in this field have identified innate immune checkpoints harnessed by cancer cells to escape immunosurveillance from innate immunity. MHC1 appears to be such a molecule expressed on cancer cells which can transmit a negative signal to innate immune cells through interaction with leukocyte immunoglobulin like receptor B1 (LILRB1). The review aims to summarize the current understanding of MHC1/LILRB1 axis on mediating cancer immune evasion with an emphasis on the therapeutic potential to block this axis for cancer therapy. Nevertheless, one should note that this field is still in its infancy and more studies are warranted to further verify the effectiveness and safety in clinical as well as the potential to combine with existing immune checkpoints.

Immunoglobulin-like transcript 2 as an impaired anti-tumor cytotoxicity marker of natural killer cells in patients with hepatocellular carcinoma

Introduction

Natural killer (NK) cells play a pivotal role in immune surveillance in the liver. We aimed to identify potential targets for NK cell-mediated immune intervention by revealing the functional molecules on NK cells in HCC patients.

Methods

To evaluate the impact of aging on NK cell phenotypes, we examined NK cells from healthy volunteers (HVs) of various ages. Because ILT2 expression on CD56dim NK cells increased with increasing age, we enrolled age-matched HCC patients and HVs. We determined the NK cell phenotypes in blood mononuclear cells (PBMCs) and intrahepatic lymphocytes (IHLs) from cancerous and non-cancerous tissues. We evaluated cytotoxicity and antibody-dependent cellular cytotoxicity (ADCC) of NK cells in vitro.

Results

ILT2-positive CD56dim NK cells in PBMCs were increased in HCC patients compared with HVs. In HCC patients, ILT2-positive CD56dim NK cells were increased in cancerous IHLs compared with non-cancerous IHLs and PBMCs. We examined the impact of macrophage migration inhibitory factor (MIF) on ILT2 expression in co-cultures of HCC cells and NK cells. The enhanced expression of ILT2 on CD56dim NK cells from HCC patients was inhibited by masking antibodies against MIF and CXCR4. ILT2-positive CD56dim NK cells exhibited lower capacities for cytotoxicity and ADCC than ILT2-negative cells, which were partially restored by ILT2 blockade.

Conclusions

In HCC patients, ILT2 is a signature molecule for cancerous CD56dim NK cells with impaired cytolytic capacity. The MIF-CXCR4 interaction is associated with ILT2 induction on CD56dim NK cells and ILT2 serves as a target for functional NK cell restoration.

The cellular composition of the tumor microenvironment is an important marker for predicting therapeutic efficacy in breast cancer

At present, the incidence rate of breast cancer ranks first among new-onset malignant tumors in women. The tumor microenvironment is a hot topic in tumor research. There are abundant cells in the tumor microenvironment that play a protumor or antitumor role in breast cancer. During the treatment of breast cancer, different cells have different influences on the therapeutic response. And after treatment, the cellular composition in the tumor microenvironment will change too. In this review, we summarize the interactions between different cell compositions (such as immune cells, fibroblasts, endothelial cells, and adipocytes) in the tumor microenvironment and the treatment mechanism of breast cancer. We believe that detecting the cellular composition of the tumor microenvironment is able to predict the therapeutic efficacy of treatments for breast cancer and benefit to combination administration of breast cancer.

Harnessing the potential of HLA-G in cancer therapy: advances, challenges, and prospects

Immune checkpoint blockades have been prized in circumventing and ablating the impediments posed by immunosuppressive receptors, reaching an exciting juncture to be an innovator in anticancer therapy beyond traditional therapeutics. Thus far, approved immune checkpoint blockades have principally targeted PD-1/PD-L1 and CTLA-4 with exciting success in a plethora of tumors and yet are still trapped in dilemmas of limited response rates and adverse effects. Hence, unveiling new immunotherapeutic targets has aroused immense scientific interest in the hope of expanding the clinical application of immune checkpoint blockades to scale new heights. Human leukocyte antigen-G (HLA-G), a non-classical major histocompatibility complex (MHC) class I molecule, is enriched on various malignant cells and is involved in the hindrance of immune effector cells and the facilitation of immunosuppressive cells. HLA-G stands out as a crucial next-generation immune checkpoint showing great promise for the benefit of cancer patients. Here, we provide an overview of the current understanding of the expression pattern and immunological functions of HLA-G, as well as its interaction with well-characterized immune checkpoints. Since HLA-G can be shed from the cell surface or released by various cells as free soluble HLA-G (sHLA-G) or as part of extracellular vesicles (EVs), namely HLA-G-bearing EVs (HLA-GEV), we discuss the potential of sHLA-G and HLA-GEV as predictive biomarkers. This review also addresses the advancement of HLA-G-based therapies in preclinical and clinical settings, with a focus on their clinical application in cancer.

Unbiased high-dimensional flow cytometry identified NK and DC immune cell signature in Luminal A-type and triple negative breast cancer

Breast cancer is the most common malignancy in women worldwide and a highly heterogeneous disease. Four different subtypes are described that differ in the expression of hormone receptors as well as the growth factor receptor HER2. Treatment modalities and survival rate depend on the subtype of breast cancer. However, it is still not clear which patients benefit from immunotherapeutic approaches such as checkpoint blockade. Thus, we aimed to decipher the immune cell signature of the different breast cancer subtypes based on high-dimensional flow cytometry followed by unbiased approaches. Here, we show that the frequency of NK cells is reduced in Luminal A and B as well as triple negative breast cancer and that the phenotype of residual NK cells is changed toward regulatory CD11b-CD16- NK cells. Further, we found higher frequencies of PD-1+ CD4+ and CD8+ T cells in triple negative breast cancer. Moreover, while Luminal A-type breast cancer was enriched for CD14+ cDC2 (named type 3 DC (DC3)), CD14- cDC2 (named DC2) were more frequent in triple negative breast cancer. In contrast, HER2-enriched breast cancer did not show major alterations in the composition of the immune cell compartment in the tumor microenvironment. These findings suggest that patients with Luminal A- and B-type as well as triple negative breast cancer might benefit from immunotherapeutic approaches targeting NK cells.

Perspectives of targeting LILRB1 in innate and adaptive immune checkpoint therapy of cancer

Immune checkpoint blockade is a compelling approach in tumor immunotherapy. Blocking inhibitory pathways in T cells has demonstrated clinical efficacy in different types of cancer and may hold potential to also stimulate innate immune responses. A novel emerging potential target for immune checkpoint therapy is leukocyte immunoglobulin-like receptor subfamily B member 1 (LILRB1). LILRB1 belongs to the superfamily of leukocyte immunoglobulin-like receptors and exerts inhibitory functions. The receptor is expressed by a variety of immune cells including macrophages as well as certain cytotoxic lymphocytes and contributes to the regulation of different immune responses by interaction with classical as well as non-classical human leukocyte antigen (HLA) class I molecules. LILRB1 has gained increasing attention as it has been demonstrated to function as a phagocytosis checkpoint on macrophages by recognizing HLA class I, which represents a 'Don't Eat Me!' signal that impairs phagocytic uptake of cancer cells, similar to CD47. The specific blockade of the HLA class I:LILRB1 axis may provide an option to promote phagocytosis by macrophages and also to enhance cytotoxic functions of T cells and natural killer (NK) cells. Currently, LILRB1 specific antibodies are in different stages of pre-clinical and clinical development. In this review, we introduce LILRB1 and highlight the features that make this immune checkpoint a promising target for cancer immunotherapy.

Elevated sHLA-G plasma levels post chemotherapy combined with ILT-2 rs10416697C allele status of the sHLA-G-related receptor predict poorest disease outcome in early triple-negative breast cancer patients

Introduction

Triple negative breast cancer (TNBC) shows an aggressive growing and spreading behavior and has limited treatment options, often leading to inferior disease outcome. Therefore, surrogate markers are urgently needed to identify patients at high risk of recurrence and more importantly, to identify additional therapeutic targets enabling further treatment options. Based on the key role of the non-classical human leukocyte antigen G (HLA-G) and its related receptor immunoglobulin-like transcript receptor-2 (ILT-2) in immune evasion mechanisms of tumors, members of this ligand-receptor axis appear to be promising tool for both, defining risk groups and potential therapeutic targets.

Materials and methods

To follow this, sHLA-G levels before and after chemotherapy (CT), HLA-G 3' UTR haplotypes, and allele variations rs10416697 at the distal gene promoter region of ILT-2 were defined in healthy female controls and early TNBC patients. The results obtained were associated with clinical status, presence of circulating tumor cell (CTC) subtypes, and disease outcome of patients in terms of progression-free or overall survival.

Results

sHLA-G plasma levels were increased in TNBC patients post-CT compared to levels of patients pre-CT or controls. High post-CT sHLA-G levels were associated with the development of distant metastases, the presence of ERCC1 or PIK3CA-CTC subtypes post-CT, and poorer disease outcome in uni- or multivariate analysis. HLA-G 3' UTR genotypes did not influence disease outcome but ILT-2 rs10416697C allele was associated with AURKA-positive CTC and with adverse disease outcome by uni- and multivariate analysis. The prognostic value of the combined risk factors (high sHLA-G levels post-CT and ILT-2 rs10416697C allele carrier status) was an even better independent indicator for disease outcome in TNBC than the lymph nodal status pre-CT. This combination allowed the identification of patients with high risk of early progression/death with positive nodal status pre-CT or with non-pathological complete therapy response.

Conclusion

The results of this study highlight for the first time that the combination of high levels of sHLA-G post-CT with ILT-2 rs10416697C allele receptor status is a promising tool for the risk assessment of TNBC patients and support the concept to use HLA-G/ILT-2 ligand-receptor axis as therapeutic targets.

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.

Immunoglobulin-like transcript 2 blockade restores antitumor immune responses in glioblastoma

Glioblastoma stands as the most frequent primary brain tumor. Despite the multimodal therapy for glioblastoma patients, the survival rate is very low, highlighting the need for novel therapies that improve patient outcomes. Immune checkpoint blockade strategies are achieving promising results in a myriad of tumors and several studies have reported its efficacy in glioblastoma at a preclinical level. ILT2 is a novel immune checkpoint that exerts an inhibitory effect via the interaction with classical and non-classical HLA class-I molecules. Herein, we report that ILT2 blockade promotes antitumor responses against glioblastoma. In silico and immunohistochemical analyses revealed that the expression of ILT2 and its ligands HLA-A, -B, -C, and -E are highly expressed in patients with glioblastoma. Disruption of ILT2 with blocking monoclonal antibodies increased natural killer cell-mediated IFN-γ production and cytotoxicity against glioblastoma, partially reverting the immunosuppression linked to this malignancy. In addition, co-treatment with temozolomide strengthened the antitumor capacity of anti-ILT2-treated immune cells. Collectively, our results establish the basis for future studies regarding the clinical potential of ILT2 blockade alone or in combination regimens in glioblastoma.

BND-22, a first-in-class humanized ILT2-blocking antibody, promotes antitumor immunity and tumor regression

BACKGROUND

Cancer immunotherapy has revolutionized cancer treatment. However, considering the limited success of immunotherapy to only some cancer types and patient cohorts, there is an unmet need for developing new treatments that will result in higher response rates in patients with cancer. Immunoglobulin-like transcript 2 (ILT2), a LILRB family member, is an inhibitory receptor expressed on a variety of immune cells including T cells, natural killer (NK) cells and different myeloid cells. In the tumor microenvironment, binding of class I MHC (in particular HLA-G) to ILT2 on immune cells mediates a strong inhibitory effect, which manifests in inhibition of antitumor cytotoxicity of T and NK cells, and prevention of phagocytosis of the tumor cells by macrophages.

METHODS

We describe here the development and characteristics of BND-22, a novel, humanized monoclonal antibody that selectively binds to ILT2 and blocks its interaction with classical MHC I and HLA-G. BND-22 was evaluated for its binding and blocking characteristics as well as its ability to increase the antitumor activity of macrophages, T cells and NK cells in various in vitro, ex vivo and in vivo systems.

RESULTS

Collectively, our data suggest that BND-22 enhances activity of both innate and adaptive immune cells, thus generating robust and comprehensive antitumor immunity. In humanized mice models, blocking ILT2 with BND-22 decreased the growth of human tumors, hindered metastatic spread to the lungs, and prolonged survival of the tumor-bearing mice. In addition, BND-22 improved the antitumor immune response of approved therapies such as anti-PD-1 or anti-EGFR antibodies.

CONCLUSIONS

BND-22 is a first-in-human ILT2 blocking antibody which has demonstrated efficient antitumor activity in various preclinical models as well as a favorable safety profile. Clinical evaluation of BND-22 as a monotherapy or in combination with other therapeutics is under way in patients with cancer.

TRIAL REGISTRATION NUMBER

NCT04717375.

The role of the natural killer (NK) cell modulation in breast cancer incidence and progress

The importance of the immune system on tumor surveillance has been investigated for many years, and its impact on controlling tumor progression has been verified. An important subgroup of the innate immune system is natural killer (NK) cells, whose essential function in modulating tumor behavior and suppressing metastasis and tumor growth has been demonstrated. The first idea of NK cells' crucial biological processes was demonstrated through their potent ability to conduct direct cellular cytotoxicity, even without former sensitization. These properties of NK cells allow them to recognize transformed cells that have attenuated self-ligand and express stress-induced ligands. Furthermore, secretion of various cytokines and chemokines after their activation leads to tumor elimination via either direct cytotoxic effect on malignant cells or activation of the adaptive immune system. In addition, novel immunotherapeutic approaches tend to take advantage of NK cells' ability, leading to antibody-based approaches, the formation of engineered CAR-NK cells, and adoptive cell transfer. However, the restricted functionality of NK cells and the inability to infiltrate tumors are its blind spots in breast cancer patients. In this review, we gathered newly acquired data on the biology and functions of NK cells in breast cancer and proposed ways to employ this knowledge for novel therapeutic approaches in cancers, particularly breast cancer.

Mechanisms for Host Immune Evasion Mediated by Plasmodium falciparum-Infected Erythrocyte Surface Antigens

Plasmodium falciparum infection causes the most severe form of malaria. It has been hypothesized that P. falciparum directly suppresses host immune responses because sufficient acquired immunity is often not induced even by repeated P. falciparum infections in malaria-endemic areas. It is known that many kinds of P. falciparum-derived proteins are expressed on the surface of P. falciparum-infected erythrocytes (IEs), and these proteins have long been thought to be a key to the elucidation of the host immune evasion mechanisms. Our recent studies have revealed that the P. falciparum-derived erythrocyte surface antigen, RIFIN, the largest multiple gene family protein in the P. falciparum genome, suppresses host immune cell activation through direct interaction with human inhibitory immune receptors. In this review, we will discuss the molecular mechanisms for host immune evasion by P. falciparum-infected erythrocyte surface antigens. In addition, we will discuss the recently identified host immune response to P. falciparum using specialized antibodies that target host-P. falciparum-derived molecule interactions.

Roles of HLA-G/KIR2DL4 in Breast Cancer Immune Microenvironment

Human leukocyte antigen (HLA)-G is a nonclassical MHC Class I molecule, which was initially reported as a mediator of immune tolerance when expressed in extravillous trophoblast cells at the maternal-fetal interface. HLA-G is the only known ligand of killer cell immunoglobulin-like receptor 2DL4 (KIR2DL4), an atypical family molecule that is widely expressed on the surface of NK cells. Unlike other KIR receptors, KIR2DL4 contains both an arginine–tyrosine activation motif in its transmembrane region and an immunoreceptor tyrosine-based inhibitory motif (ITIM) in its cytoplasmic tail, suggesting that KIR2DL4 may function as an activating or inhibitory receptor. The immunosuppressive microenvironment exemplified by a rewired cytokine network and upregulated immune checkpoint proteins is a hallmark of advanced and therapy-refractory tumors. Accumulating evidence has shown that HLA-G is an immune checkpoint molecule with specific relevance in cancer immune escape, although the role of HLA-G/KIR2DL4 in antitumor immunity is still uncharacterized. Our previous study had shown that HLA-G was a pivotal mediator of breast cancer resistance to trastuzumab, and blockade of the HLA-G/KIR2DL4 interaction can resensitize breast cancer to trastuzumab treatment. In this review, we aim to summarize and discuss the role of HLA-G/KIR2DL4 in the immune microenvironment of breast cancer. A better understanding of HLA-G is beneficial to identifying novel biomarker(s) for breast cancer, which is important for precision diagnosis and prognostic assessment. In addition, it is also necessary to unravel the mechanisms underlying HLA-G/KIR2DL4 regulation of the immune microenvironment in breast cancer, hopefully providing a rationale for combined HLA-G and immune checkpoints targeting for the effective treatment of breast cancer.

LIR-1 educates expanded human NK cells and defines a unique antitumor NK cell subset with potent antibody-dependent cellular cytotoxicity

Objective

KIR and NKG2A receptors educate human NK cells to stay responsive to cells with diminished HLA class I. Here, we addressed whether the HLA class I-binding receptor LIR-1 (LILRB1/ILT2/CD85j), which is widely expressed on human NK cells, can mediate education and contribute to antitumor functions of NK cells.

Methods

Healthy donor NK cells either unstimulated, overnight cytokine-activated or ex vivo-expanded were used to target human cell lines. Phenotype and function were analysed using flow cytometry and ⁵¹Cr-release assays.

Results

We found that the inhibitory receptor LIR-1 can mediate NK cell education under specific conditions. This novel finding was exclusive to expanded NK cells and further characterisation of the cells revealed high expression of granzyme B and DNAM-1, which both previously have been linked to NK cell education. Corroborating the rheostat education model, LIR-1 co-expression with an educating KIR further increased the responsiveness of expanded NK cells. Inversely, antibody masking of LIR-1 decreased the responsiveness. LIR-1⁺ expanded NK cells displayed high intrinsic ADCC that, in contrast to KIR and NKG2A, was not inhibited by HLA class I.

Conclusion

These findings identify a unique NK cell subset attractive for adoptive cell therapy to treat cancer. Given that LIR-1 binds most HLA class I molecules, this subset may be explored in both autologous and allogeneic settings to innately reject HLA class I^- tumor cells as well as HLA class I⁺ target cells when combined with antitumor antibodies. Further studies are warranted to address the potential of this subset in vivo.

The Right Partner in Crime: Unlocking the Potential of the Anti-EGFR Antibody Cetuximab via Combination With Natural Killer Cell Chartering Immunotherapeutic Strategies

Cetuximab has an established role in the treatment of patients with recurrent/metastatic colorectal cancer and head and neck squamous cell cancer (HNSCC). However, the long-term effectiveness of cetuximab has been limited by the development of acquired resistance, leading to tumor relapse. By contrast, immunotherapies can elicit long-term tumor regression, but the overall response rates are much more limited. In addition to epidermal growth factor (EGFR) inhibition, cetuximab can activate natural killer (NK) cells to induce antibody-dependent cellular cytotoxicity (ADCC). In view of the above, there is an unmet need for the majority of patients that are treated with both monotherapy cetuximab and immunotherapy. Accumulated evidence from (pre-)clinical studies suggests that targeted therapies can have synergistic antitumor effects through combination with immunotherapy. However, further optimizations, aimed towards illuminating the multifaceted interplay, are required to avoid toxicity and to achieve better therapeutic effectiveness. The current review summarizes existing (pre-)clinical evidence to provide a rationale supporting the use of combined cetuximab and immunotherapy approaches in patients with different types of cancer.

Antagonistic anti-LILRB1 monoclonal antibody regulates antitumor functions of natural killer cells

BACKGROUND

Current immune checkpoint blockade strategies have been successful in treating certain types of solid cancer. However, checkpoint blockade monotherapies have not been successful against most hematological malignancies including multiple myeloma and leukemia. There is an urgent need to identify new targets for development of cancer immunotherapy. LILRB1, an immunoreceptor tyrosine-based inhibitory motif-containing receptor, is widely expressed on human immune cells, including B cells, monocytes and macrophages, dendritic cells and subsets of natural killer (NK) cells and T cells. The ligands of LILRB1, such as major histocompatibility complex (MHC) class I molecules, activate LILRB1 and transduce a suppressive signal, which inhibits the immune responses. However, it is not clear whether LILRB1 blockade can be effectively used for cancer treatment.

METHODS

First, we measured the LILRB1 expression on NK cells from cancer patients to determine whether LILRB1 upregulated on NK cells from patients with cancer, compared with NK cells from healthy donors. Then, we developed specific antagonistic anti-LILRB1 monoclonal antibodies and studied the effects of LILRB1 blockade on the antitumor immune function of NK cells, especially in multiple myeloma models, in vitro and in vivo xenograft model using non-obese diabetic (NOD)-SCID interleukin-2Rγ-null mice.

RESULTS

We demonstrate that percentage of LILRB1⁺ NK cells is significantly higher in patients with persistent multiple myeloma after treatment than that in healthy donors. Further, the percentage of LILRB1⁺ NK cells is also significantly higher in patients with late-stage prostate cancer than that in healthy donors. Significantly, we showed that LILRB1 blockade by our antagonistic LILRB1 antibody increased the tumoricidal activity of NK cells against several types of cancer cells, including multiple myeloma, leukemia, lymphoma and solid tumors, in vitro and in vivo.

CONCLUSIONS

Our results indicate that blocking LILRB1 signaling on immune effector cells such as NK cells may represent a novel strategy for the development of anticancer immunotherapy.

Tumor Microenvironment: Key Players in Triple Negative Breast Cancer Immunomodulation

Simple Summary

The tumor microenvironment (TME) is a complicated network composed of various cells, signaling molecules, and extra cellular matrix. TME plays a crucial role in triple negative breast cancer (TNBC) immunomodulation and tumor progression, paradoxically, acting as an immunosuppressive as well as immunoreactive factor. Research regarding tumor immune microenvironment has contributed to a better understanding of TNBC subtype classification. Shall we treat patients precisely according to specific subtype classification? Moving beyond traditional chemotherapy, multiple clinical trials have recently implied the potential benefits of immunotherapy combined with chemotherapy. In this review, we aimed to elucidate the paradoxical role of TME in TNBC immunomodulation, summarize the subtype classification methods for TNBC, and explore the synergistic mechanism of chemotherapy plus immunotherapy. Our study may provide a new direction for the development of combined treatment strategies for TNBC.

Abstract

Triple negative breast cancer (TNBC) is a heterogeneous disease and is highly related to immunomodulation. As we know, the most effective approach to treat TNBC so far is still chemotherapy. Chemotherapy can induce immunogenic cell death, release of damage-associated molecular patterns (DAMPs), and tumor microenvironment (TME) remodeling; therefore, it will be interesting to investigate the relationship between chemotherapy-induced TME changes and TNBC immunomodulation. In this review, we focus on the immunosuppressive and immunoreactive role of TME in TNBC immunomodulation and the contribution of TME constituents to TNBC subtype classification. Further, we also discuss the role of chemotherapy-induced TME remodeling in modulating TNBC immune response and tumor progression with emphasis on DAMPs-associated molecules including high mobility group box1 (HMGB1), exosomes, and sphingosine-1-phosphate receptor 1 (S1PR1), which may provide us with new clues to explore effective combined treatment options for TNBC.

Circulating and Tumor-Infiltrating NK Cells From Clear Cell Renal Cell Carcinoma Patients Exhibit a Predominantly Inhibitory Phenotype Characterized by Overexpression of CD85j, CD45, CD48 and PD-1

Although natural killer (NK) cells infiltrate clear cell renal cell carcinomas (ccRCC), the most frequent malignancy of the kidney, tumor progression suggests that they become dysfunctional. As ccRCC-driven subversion of NK cell effector functions is usually accompanied by phenotypic changes, analysis of such alterations might lead to the identification of novel biomarkers and/or targets in immuno-oncology. Consequently, we performed a phenotypic analysis of peripheral blood NK cells (PBNK) and tumor-infiltrating NK cells (TINK) from ccRCC patients. Compared to HD, PBNK from ccRCC patients exhibited features of activated cells as shown by CD25, CD69 and CD62L expression. They also displayed increased expression of DNAM-1, CD48, CD45, MHC-I, reduced expression of NKG2D, and higher frequencies of CD85j⁺ and PD-1⁺ cells. In addition, compared to PBNK from ccRCC patients, TINK exhibited higher expression of activation markers, tissue residency features and decreased expression of the activating receptors DNAM-1, NKp30, NKp46, NKp80 and CD16, suggesting a more inhibitory phenotype. Analysis of The Cancer Genome Atlas (TCGA) revealed that CD48, CD45, CD85j and PD-1 are significantly overexpressed in ccRCC and that their expression is associated with an NK cell infiltration signature. Calculation of z-scores revealed that their expression on PBNK, alone or combined, distinguished ccRCC patients from HD. Therefore, these molecules emerge as novel potential biomarkers and our results suggest that they might constitute possible targets for immunotherapy in ccRCC patients.

Natural Killer cells and monoclonal antibodies: Two partners for successful antibody dependent cytotoxicity against tumor cells

Monoclonal antibodies targeting tumors are one of the most important discoveries in the field of cancer. Although several effective antibodies have been developed, a relapse may occur. One of their mechanisms of action is Antibody Dependent Cell Cytotoxicity (ADCC), by engaging the Fc γ receptor CD16 expressing Natural Killer cells, innate lymphoid cells involved in cancer immunosurveillance and able to kill tumor cells. A lack of NK cells observed in many cancers may therefore be a cause of the low efficacy of antibodies observed in some clinical situations. Here we review clear evidences of the essential partnership between NK cells and antibodies showed in vitro, in vivo, and in clinical trials in different indications, describe the hurdles and ways to enhance ADCC and the evolution of monoclonal antibody therapy. NK cell adoptive immunotherapy combined with monoclonal antibodies may overcome the resistance to the treatment and enhance their efficacy.

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.

Immunosuppressive receptor LILRB1 acts as a potential regulator in hepatocellular carcinoma by integrating with SHP1

BACKGROUND

Immunosuppressive receptor LILRB1 regulates tumors progression by transducing immune inhibitory signals via intracellular immunoreceptor tyrosine-based inhibitory motifs. However, its role in Hepatocellular Carcinoma (HCC) remains vague.

OBJECTIVE

This study is aimed to disclose the association between LILRB1 and HCC.

METHODS

Immunoblotting and qRT-PCR were employed to evaluate the level of LILRB1 in hepatocarcinoma cells. LILRB1-positive cells in tissue array were measured using immunohistochemistry staining. The relation among LILRB1, SHP1 and SHP2 and survival rates were analyzed using Gene Expression Profiling Interactive Analysis (GEPIA) and Oncomine database.

RESULTS

LILRB1 was robustly reduced in hepatocarcinoma cells compared to normal cells. Clinically, LILRB1 was significantly higher in 49 of 75 (65%) paired paracarcinoma tissues than that in paired HCC samples. 48 of 75 (64%) HCC subjects in tissue microarray showed low level of LILRB1, compared to 25 of 75 (33%) in paired-adjacent tissues. Oncomine database and GEPIA analysis confirmed that LILRB1 was lower in HCC than normal tissues. Additionally, lowLILRB1 had a significant association with clinicopathological characteristics and Disease Free Survival, but no association with Overall Survival in HCC patients. Mechanismly, positive correlation between LILRB1 and SHP1, but not SHP2 was observed in HCC.

CONCLUSIONS

LILRB1 possibly plays an antitumor effect in hepatocarcinoma cells by integrating SHP1, providing evidence that LILRB1 might be involved in the pathologic progression of HCC.

Natural Killer Cell Defects in Breast Cancer: A Key Pathway for Tumor Evasion

As the most important innate immune component cancers invader, natural killer (NK) cells have a magnificent role in antitumor immunity without any prior sensitization. Different subsets of NK cells have distinct responses during tumor cell exposure, according to their phenotypes and environments. Their function is induced mainly by the activity of both inhibitory and activating receptors against cancerous cells. Since the immunosuppression in the tumor microenvironment of breast cancer patients has directly deteriorated the phenotype and disturbed the function of NK cells, recruiting compensatory mechanisms indicate promising outcomes for immunotherapeutic approaches. These evidences accentuate the importance of NK cell distinct features in protection against breast tumors. In this review, we discuss the several mechanisms involved in NK cells suppression which consequently promote tumor progression and disease recurrence in patients with breast cancer.

Leukocyte immunoglobulin-like receptor subfamily B member 1 potentially acts as a diagnostic and prognostic target in certain subtypes of adenocarcinoma

BACKGROUND

Leukocyte immunoglobulin (Ig)-like receptor subfamily B member 1 (LILRB1) involves in the occurrence and development of various tumors through transmitting immune inhibitory signals. However, the regulatory mechanism of LILRB1 underlying the disease progression of adenocarcinoma remains vague. This study is aimed to disclose the expression pattern of LILRB1 on adenocarcinoma and its indicative roles on the diagnosis and prognosis of adenocarcinoma patients.

METHODS

LILRB1 level in microarray was measured using immunohistochemistry (IHC) staining. Expression analysis of LILRB1 gene were based on the Gene Expression Profiling Interactive Analysis 2.0 (GEPIA2) and Oncomine databases. Survival and correlation analyses were analyzed using The Cancer Genome Atlas (TCGA) database (Breastinvasivecarcinoma, TCGA-BRCA).

RESULTS

The IHC results showed that the number of LILRB1-positive cells were robustly elevated in some common subtypes of adenocarcinoma including thyroid gland papillary carcinoma, gastric mixed adenocarcinoma, colon and rectal mucinous adenocarcinoma, pancreatic ductal adenocarcinoma and invasive ductal breast carcinoma compared to their corresponding para-carcinoma. Although the enhancement of LILRB1 expression was only observed in pancreaticadenocarcinoma (PAAD) by using GEPIA2, its expression presented a significant increase in the above subtypes of adenocarcinoma by analyzing using Oncomine database. Besides, there had a significant positive association between LILRB1 expression status and pathological stages, and a negative association between LILRB1 status and Overall Survival (OS) probability in the above certain subtypes of adenocarcinoma.

CONCLUSION

LILRB1 is abnormally upregulated in certain subtypes of adenocarcinoma. Patients with low LILRB1 possibly portend a good prognosis in adenocarcinoma. These findings imply that LILRB1 may act as a diagnostic and prognostic target in some subtypes of adenocarcinoma.

The MHC class I-LILRB1 signalling axis as a promising target in cancer therapy

Immune checkpoint inhibitors are among the newest, cutting-edge methods for the treatment of cancer. Currently, they primarily influence T cell adaptive immunotherapy targeting the PD-1/PD-L1 and CTLA-4/B7 signalling pathways. These inhibitors fight cancer by reactivating the patient's own adaptive immune system, with good results in many cancers. With the discovery of the "Don't Eat Me" molecule, CD47, antibody-based drugs that target the macrophage-related innate immunosuppressive signalling pathway, CD47-SIRPα, have been developed and have achieved stunning results in the laboratory and the clinic, but there remain unexplained instances of tumour immune escape. While investigating the immunological tolerance of cancer to anti-CD47 antibodies, a second "Don't Eat Me" molecule on tumour cells, beta 2 microglobulin (β2m), a component of MHC class I, was described. Some tumour cells reduce their surface expression of MHC class I to escape T cell recognition. However, other tumour cells highly express β2m complexed with the MHC class I heavy chain to send a "Don't Eat Me" signal by binding to leucocyte immunoglobulin-like receptor family B, member 1 (LILRB1) on macrophages, leading to a loss of immune surveillance. Investigating the mechanisms underlying this immunosuppressive MHC class I-LILRB1 signalling axis in tumour-associated macrophages will be useful in developing therapies to restore macrophage function and control MHC class I signalling in patient tumours. The goal is to promote adaptive immunity while suppressing the innate immune response to tumours. This work will identify new therapeutic targets for the development of pharmaceutical-based tumour immunotherapy.

Structures of the four Ig-like domain LILRB2 and the four-domain LILRB1 and HLA-G1 complex

Leukocyte immunoglobulin (Ig)-like receptors (LILRs), also known as CD85 and immunoglobulin-like transcripts (ILTs), play pivotal roles in regulating immune responses. These receptors define an immune checkpoint that immune therapy can target. Through cis or trans interactions with human leukocyte antigen (HLA)-G, the two most abundantly expressed inhibitory LILRs, LILRB1, and LILRB2 (LILRB1/2, also known as CD85j/d and ILT2/4), are involved in immunotolerance in pregnancy and transplantation, autoimmune diseases, and immune evasion by tumors. Although the discrete domains of LILRB1/2 are clear, the assembly mode of the four extracellular Ig-like domains (D1, D2, D3, and D4) remains unknown. Previous data indicate that D1D2 is responsible for binding to HLA class I (HLA-I), but the roles of D3D4 are still unclear. Here, we determined the crystal structure of the four Ig-like domain LILRB2 and four-domain LILRB1 in complex with HLA-G1. The angles between adjacent domains and the staggered assembly of the four domains suggest limited flexibility and limited plasticity of the receptors during ligand binding. The complex structure of four-domain LILRB1 and HLA-G1 supports the model that D1D2 is responsible for HLA-I binding, while D3D4 acts as a scaffold. Accordingly, cis and trans binding models for HLA-I binding to LILRB1/2 are proposed. The geometries of LILRB1/2 in complex with dimeric and monomeric HLA-G1 suggest the accessibility of the dimeric receptor, which in turn, transduces more inhibitory signals. The assembly of LILRB1/2 and its binding to HLA-G1 could aid in the design of immune regulators and benefit immune interference.

High neutrophil to lymphocyte ratio and decreased CD69+NK cells represent a phenotype of high risk in early-stage breast cancer patients

Purpose

Breast cancer (BC) is a highly heterogeneous disease presenting a broad range of clinical and molecular characteristics. In the past years, a growing body of evidence demonstrated that immune response plays a significant role in cancer outcome. However, immune prognostic markers are not completely validated in clinical practice in BC patients.

Materials and methods

With the aim to characterize immune features, several parameters were analyzed in peripheral blood at diagnosis of 85 nonmetastatic BC patients between April 2011 and July 2014.

Results

With a median follow-up of 38.6 months, peripheral blood analysis of BC patients (stages I, II, and III) showed that total lymphocyte and T lymphocyte counts were augmented in nonrelapsed patients. Also, a higher neutrophil-to-lymphocytes ratio was associated with prolonged disease-free survival. Natural killer cell receptor analysis revealed that early activation receptor CD69 was associated with a better outcome.

Conclusion

This preliminary evidence is in accordance with the concept of immune surveillance. We suggest an “immune phenotype” that provides relevant prognostic information in early-stage BC patients and which could be useful in the decision-making process.

In situ delivery of allogeneic natural killer cell (NK) combined with Cetuximab in liver metastases of gastrointestinal carcinoma: A phase I clinical trial

Despite successful introduction of NK-based cellular therapy in the treatment of myeloid leukemia, the potential use of NK alloreactivity in solid malignancies is still elusive. We performed a phase I clinical trial to assess the safety and efficacy of in situ delivery of allogeneic NK cells combined with cetuximab in liver metastasis of gastrointestinal origin. The conditioning chemotherapy was administrated before the allogeneic NK cells injection via hepatic artery. Three escalating doses were tested (3.10⁶, 8.10⁶ and 12.10⁶ NK cells/kg) following by a high-dose interleukin-2 (IL-2). Cetuximab was administered intravenously every week for 7 weeks. Nine patients with liver metastases of colorectal or pancreatic cancers were included, three per dose level. Hepatic artery injection was successfully performed in all patients with no report of dose-limiting toxicity. Two patients had febrile aplasia requiring a short-term antibiotherapy. Grade 3/4 anemia and thrombopenia were also observed related to the chemotherapy. Objective clinical responses were documented in 3 patients and among them 2 occurred in patients injected with cell products harboring two KIR ligand mismatches and one in a patient with one KIR ligand mismatch. Immune monitoring revealed that most patients presented an increase but transient of IL-15 and IL-7 cytokines levels one week after chemotherapy. Furthermore, a high expansion of FoxP3⁺regulatory T cells and PD-1⁺ T cells was observed in all patients, related to IL-2 administration. Our results demonstrated that combining allogeneic NK cells transfer via intra-hepatic artery, cetuximab and a high-dose IL-2 is feasible, well tolerated and may result in clinical responses.

Expression and function of immune ligand-receptor pairs in NK cells and cancer stem cells: therapeutic implications

BACKGROUND

The interplay between the immune system and cancer cells has come to the forefront of cancer therapeutics, with novel immune blockade inhibitors being approved for the treatment of an increasing list of cancers. However, the majority of cancer patients still display or develop resistance to these promising drugs. It is possible that cancer stem cells (CSCs) are contributing to this therapeutic resistance. Although CSCs usually represent a small percentage of the total number of cancer cells, they are endowed with the ability of self-renewal and to produce differentiated progeny. Additionally, they have shown the capacity to establish tumors after transplantation to animals, even in small numbers. CSCs have also been found to be resistant to various anti-cancer therapies, including chemotherapy, radiation therapy and, more recently, immunotherapy. This is true despite the sensitivity of CSCs to lysis in vitro by natural killer (NK) cells, the main effector cells of the innate immune system. In this paper the expression of ligands specific for NK cells on CSCs, the intracellular network responsible for the expression of the NK cytotoxicity receptors, and the status of activation of NK cells in the tumor micro-environment are reviewed. The aim of this review is to highlight potential strategies for overcoming CSC immune resistance, thereby enhancing the efficacy of current and future anti-cancer therapies.

THERAPEUTIC IMPLICATIONS

NK cell activation in the tumor micro-environment through drugs neutralizing inhibitory immune receptors, and combined with other drugs harnessing the potential of the adaptive immune system, could be the most effective approach for attacking both stem cell and non-stem cell cancer populations.

An improved method to quantify human NK cell-mediated antibody-dependent cell-mediated cytotoxicity (ADCC) per IgG FcR-positive NK cell without purification of NK cells

Natural killer (NK) lymphocyte ADCC supports anti-viral protection and monoclonal antibody (mAb) anti-tumor therapies. To predict in vivo ADCC therapeutic responses of different individuals, measurement of both ADCC cellular lytic capacity and their NK cellular receptor recognition of antibodies on 'target' cells are needed, using clinically available amounts of blood. Twenty ml of blood provides sufficient peripheral blood mononuclear cells (PBMCs) for the new assay for lytic capacity described here and for an antibody EC₅₀ assay for Fc-receptor recognition. For the lytic capacity assay, we employed flow cytometry to quantify the CD16A IgG Fc-receptor positive NK effector cells from PBMCs to avoid loss of NKs during isolation. Targets were ⁵¹Cr-labeled Daudi B cells pretreated with excess obinutuzumab type 2 anti-CD20 mAb and washed; remaining free mAb was insufficient to convert B cells in the PBMCs into 'targets'. We calculated: the percentage Daudis killed at a 1:1 ratio of CD16A-positive NK cells to Daudis (CX1:1); lytic slopes; and ADCC₅₀ lytic units. Among 27 donors, we detected wide ranges in CX1:1 (16-73% targets killed) and in lytic slopes. Slope variations prevented application of lytic units. We recommend CX1:1 to compare individuals' ADCC capacity. CX1:1 was similar for purified NK cells vs. PBMCs and independent of CD16A V & F genotypes and antibody EC₅₀s. With high mAb bound onto targets and the high affinity of obinutuzumab Fc for CD16A, CX1:1 measurements discern ADCC lytic capacity rather than antibody recognition. This assay allows ADCC to be quantified without NK cell isolation and avoids distortion associated with lytic units.

Natural killer cell activity for IFN-gamma production as a supportive diagnostic marker for gastric cancer

BACKGROUND/AIM

Decreased Natural killer cell activity (NKA) for interferon-gamma production (NKA-IFNγ) has been reported in cancer patients. The aim of this study was to determine the diagnostic performance of NKA-IFNγ for gastric cancer (GC).

RESULTS

NKA-IFNγ levels were decreased in 261 GC patients with all stages of tumor compared to those in 48 healthy donors (P < 0.001), and lower levels of NKA-IFNγ were associated with higher GC stages. NKA-IFNγ levels were also associated with clinicopathological parameters including tumor size, depth of invasion, and lymph node metastasis. NKA-INFγ assay had better diagnostic value (AUC = 0.822) compared to serum CEA (0.624) or CA19-9 assay (0.566) (P < 0.001). Using different cut-off levels, serum CEA and CA19-9 showed sensitivities of 6.1-14.2% and 4.2-28.0%, respectively, which were much lower than that of NKA-IFNγ (55.6-66.7%).

METHODS

This study included 261 patients with newly diagnosed GC and 48 healthy donors. NKA for IFNγ was determined by enzyme immunoassay after incubation of whole blood, and diagnostic performance was evaluated.

CONCLUSIONS

NK cell activities for IFNγ production could be used as a supportive non-invasive tumor marker for GC diagnosis.

Immune Checkpoint Function of CD85j in CD8 T Cell Differentiation and Aging

Aging is associated with an increased susceptibility to infection and a failure to control latent viruses thought to be driven, at least in part, by alterations in CD8 T cell function. The aging T cell repertoire is characterized by an accumulation of effector CD8 T cells, many of which express the negative regulatory receptor CD85j. To define the biological significance of CD85j expression on CD8 T cells and to address the question whether presence of CD85j in older individuals is beneficial or detrimental for immune function, we examined the specific attributes of CD8 T cells expressing CD85j as well as the functional role of CD85j in antigen-specific CD8 T cell responses during immune aging. Here, we show that CD85j is mainly expressed by terminally differentiated effector (TEMRAs) CD8 T cells, which increase with age, in cytomegalovirus (CMV) infection and in males. CD85j⁺ CMV-specific cells demonstrate clonal expansion. However, TCR diversity is similar between CD85j⁺ and CD85j⁻ compartments, suggesting that CD85j does not directly impact the repertoire of antigen-specific cells. Further phenotypic and functional analyses revealed that CD85j identifies a specific subset of CMV-responsive CD8 T cells that coexpress a marker of senescence (CD57) but retain polyfunctional cytokine production and expression of cytotoxic mediators. Blocking CD85j binding enhanced proliferation of CMV-specific CD8 T cells upon antigen stimulation but did not alter polyfunctional cytokine production. Taken together, these data demonstrate that CD85j characterizes a population of “senescent,” but not exhausted antigen-specific effector CD8 T cells and indicates that CD85j is an important checkpoint regulator controlling expansion of virus-specific T cells during aging. Inhibition of CD85j activity may be a mechanism to promote stronger CD8 T cell effector responses during immune aging.

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.

Phenotypic and Functional Dysregulated Blood NK Cells in Colorectal Cancer Patients Can Be Activated by Cetuximab Plus IL-2 or IL-15

The clinical outcome of colorectal cancer (CRC) is associated with the immune response; thus, these tumors could be responsive to different immune therapy approaches. Natural killer (NK) cells are key antitumor primary effectors that can eliminate CRC cells without prior immunization. We previously determined that NK cells from the local tumor environment of CRC tumors display a profoundly altered phenotype compared with circulating NK cells from healthy donors (HD). In this study, we evaluated peripheral blood NK cells from untreated patients and their possible role in metastasis progression. We observed profound deregulation in receptor expression even in early stages of disease compared with HD. CRC-NK cells displayed underexpression of CD16, NKG2D, DNAM-1, CD161, NKp46, and NKp30 activating receptors, while inhibitory receptors CD85j and NKG2A were overexpressed. This inhibited phenotype affected cytotoxic functionality against CRC cells and interferon-γ production. We also determined that NKp30 and NKp46 are the key receptors involved in detriment of CRC-NK cells’ antitumor activity. Moreover, NKp46 expression correlated with relapse-free survival of CRC patients with a maximum follow-up of 71 months. CRC-NK cells also exhibited altered antibody-dependent cellular cytotoxicity function responding poorly to cetuximab. IL-2 and IL-15 in combination with cetuximab stimulated NK cell, improving cytotoxicity. These results show potential strategies to enhance CRC-NK cell activity.

The Expanding Spectrum of Ligands for Leukocyte Ig-like Receptors

The human leukocyte Ig-like receptor family is part of the paired receptor system. The receptors are widely expressed by various immune cells, and new functions continue to emerge. Understanding the range of functions of the receptors is of general interest because several types of pathogens exploit the receptors and genetic diversity of the receptors has been linked to various autoimmune diseases. Class I major histocompatibility molecules were the first ligands appreciated for these receptors, but the types of ligands identified over the last several years are quite diverse, including intact pathogens, immune-modulatory proteins, and molecules normally found within the CNS. This review focuses on the types of ligands described to date, how the individual receptors bind to several distinct types of ligands, and the known functional consequences of those interactions.