Beyond the anti-PD-1/PD-L1 era: promising role of the BTLA/HVEM axis as a future target for cancer immunotherapy.

Recent introduction of monoclonal antibodies targeting immune checkpoints to harness antitumor immunity has revolutionized the cancer treatment landscape. The therapeutic success of immune checkpoint blockade (ICB)-based therapies mainly relies on PD-1/PD-L1 and CTLA-4 blockade. However, the limited overall responses and lack of reliable predictive biomarkers of patient´s response are major pitfalls limiting immunotherapy success. Hence, this reflects the compelling need of unveiling novel targets for immunotherapy that allow to expand the spectrum of ICB-based strategies to achieve optimal therapeutic efficacy and benefit for cancer patients. This review thoroughly dissects current molecular and functional knowledge of BTLA/HVEM axis and the future perspectives to become a target for cancer immunotherapy. BTLA/HVEM dysregulation is commonly found and linked to poor prognosis in solid and hematological malignancies. Moreover, circulating BTLA has been revealed as a blood-based predictive biomarker of immunotherapy response in various cancers. On this basis, BTLA/HVEM axis emerges as a novel promising target for cancer immunotherapy. This prompted rapid development and clinical testing of the anti-BTLA blocking antibody Tifcemalimab/icatolimab as the first BTLA-targeted therapy in various ongoing phase I clinical trials with encouraging results on preliminary efficacy and safety profile as monotherapy and combined with other anti-PD-1/PD-L1 therapies. Nevertheless, it is anticipated that the intricate signaling network constituted by BTLA/HVEM/CD160/LIGHT involved in immune response regulation, tumor development and tumor microenvironment could limit therapeutic success. Therefore, in-depth functional characterization in different cancer settings is highly recommended for adequate design and implementation of BTLA-targeted therapies to guarantee the best clinical outcomes to benefit cancer patients.

Chemo-Immunotherapy: A New Trend in Cancer Treatment.

Chemotherapy has been the basis of advanced cancer treatment for decades. This therapy has largely been considered immunosuppressive, yet accumulated preclinical and clinical evidence shows that certain chemotherapeutic drugs, under defined conditions, may stimulate antitumor immunity and potentiate immune checkpoint inhibitor (ICI)-based therapy. Its effectiveness has been highlighted by recent regulatory approvals of various combinations of chemotherapy with ICIs in several tumors, particularly in some difficult-to-treat cancers. This review discusses the immune modulatory properties of chemotherapy and how they may be harnessed to develop novel chemo-immunotherapy combinations. It also highlights the key determinants of the success of chemo-immunotherapy and provides an overview of the combined chemo-immunotherapies that have been clinically approved.

BTLA dysregulation correlates with poor outcome and diminished T cell-mediated antitumor responses in chronic lymphocytic leukemia.

Patients with chronic lymphocytic leukemia (CLL) progressively develop marked immunosuppression, dampening innate and adaptive-driven antitumor responses. However, the underlying mechanisms promoting immune exhaustion are largely unknown. Herein, we provide new insights into the role of BTLA/HVEM axis promoting defects in T cell-mediated responses against leukemic cells. Increased expression of BTLA, an inhibitory immune checkpoint, was detected on the surface of CD4 + and CD8 + T lymphocytes in patients with CLL. Moreover, high levels of BTLA on CD4 + T cells correlated with diminished time to treatment. Signaling through BTLA activation led to decreased IL-2 and IFN-γ production ex vivo, whereas BTLA/HVEM binding disruption enhanced IFN-γ + CD8 + T lymphocytes. Accordingly, BTLA blockade in combination with bispecific anti-CD3/anti-CD19 antibody promoted CD8 + T cell-mediated anti-leukemic responses. Finally, treatment with an anti-BLTA blocking monoclonal antibody alone or in combination with ibrutinib-induced leukemic cell depletion in vitro. Altogether, our data reveal that BTLA dysregulation has a prognostic role and is limiting T cell-driven antitumor responses, thus providing new insights about immune exhaustion in patients with CLL.

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.

Cystatin C-Based Equations Detect Hidden Kidney Disease and Poor Prognosis in Newly Diagnosed Patients with Multiple Myeloma.

Objectives: The aim of this study was to compare the creatinine equations with cystatin C (CysC) equations to define renal impairment (RI) in newly diagnosed multiple myeloma (MM) patients and to analyse the equation that allows for identifying patients with more and worse prognostic factors. Methods: Renal function was evaluated prospectively in 61 patients with newly diagnosed untreated MM employing CKD-EPI and CAPA equations. The comparison was conducted using Bland-Altman graphics and Cohen's Kappa statistic. Mann-Whitney T and Chi-square tests were used, and univariate and multivariate analyses were carried out. Results: According to the IMWG criteria, 26% of patients showed RI (3 women/13 men) whilst the use of CysC equations allowed us to identify up to 39% of patients (7 women/17 men). The CAPA equation was less biased and dispersed and more sensitive than CKD-EPI-creatinine. Furthermore, univariate analysis unveiled an association between decreased CKD-EPI-CysC and poor prognosis based on R-ISS-3. Conclusions: The IMWG criteria may underestimate kidney disease, mostly in women, which could affect the dose received as well as its toxicity. Altogether, our data suggest that equations that include CysC are more accurate to detect hidden kidney disease, as well as patients with more and worse prognostic factors, in newly diagnosed MM.

Involvement of CD4+ and CD8+ T-lymphocytes in the modulation of nociceptive processing evoked by CCL4 in mice.

AIMS: To explore the mechanisms involved in the transformation of analgesia produced by low doses of CCL4 (pg/kg) to hyperalgesia when higher doses (ng/kg) are administered to mice. MAIN METHODS: The unilateral hot plate test was used to assess thermal nociception. CD3+, CD4+ or CD8+ blood cells were depleted with selective antibodies. Expression of CCR5 and IL-16 in lymphocytes was studied by flow cytometry and IL-16 blood levels were measured by ELISA. IL-16 and CD8 were detected by immunofluorescence. KEY FINDINGS: IL-16 and CCR5 expression were demonstrated in CD4+ and CD8+ T-lymphocytes by flow cytometry. Furthermore, CCL4-induced hyperalgesia was abolished by reducing circulating T-lymphocyte levels or by selectively depleting CD4+ lymphocytes. In contrast, when the anti-CD4 antibody was acutely administered, CCL4 induced analgesia instead of hyperalgesia. A similar response was obtained when administering A-770041, that prevents CD4-mediated CCR5 desensitization by inhibiting p56lck kinase. As occurred with the analgesic effect evoked by low doses of CCL4, analgesia evoked by combining CCL4 and A-770041 was reverted by naloxone, naltrindole or an anti-met-enk antibody. Interestingly, flow cytometry assays showed that the number of CD8+, but not CD4+, T-cells expressing IL-16 is reduced after the acute administration of CCL4, a result compatible with the description that CD8+-lymphocytes can rapidly release preformed IL-16. Accordingly, the rise in IL-16 blood concentration evoked by CCL4 was prevented after CD8+ lymphocyte depletion. SIGNIFICANCE: CCL4-evoked hyperalgesia is related to the desensitization of CCR5 in CD4+ T-cells and to the release of IL-16 from CD8+ lymphocytes.

BTLA/HVEM Axis Induces NK Cell Immunosuppression and Poor Outcome in Chronic Lymphocytic Leukemia.

Chronic lymphocytic leukemia (CLL) is characterized by progressive immunosuppression and diminished cancer immunosurveillance. Immune checkpoint blockade (ICB)-based therapies, a major breakthrough against cancer, have emerged as a powerful tool to reinvigorate antitumor responses. Herein, we analyzed the role of the novel inhibitory checkpoint BTLA and its ligand, HVEM, in the regulation of leukemic and natural killer (NK) cells in CLL. Flow cytometry analyses showed that BTLA expression is upregulated on leukemic cells and NK cells from patients with CLL, whereas HVEM is downregulated only in leukemic cells, especially in patients with advanced Rai-Binet stage. In silico analysis revealed that increased HVEM, but not BTLA, mRNA expression in leukemic cells correlated with diminished overall survival. Further, soluble BTLA (sBTLA) was found to be increased in the sera of patients with CLL and highly correlated with poor prognostic markers and shorter time to treatment. BTLA blockade with an anti-BTLA monoclonal antibody depleted leukemic cells and boosted NK cell-mediated responses ex vivo by increasing their IFN-γ production, cytotoxic capability, and antibody-dependent cytotoxicity (ADCC). In agreement with an inhibitory role of BTLA in NK cells, surface BTLA expression on NK cells was associated with poor outcome in patients with CLL. Overall, this study is the first to bring to light a role of BTLA/HVEM in the suppression of NK cell-mediated immune responses in CLL and its impact on patient's prognosis, suggesting that BTLA/HVEM axis may be a potential therapeutic target in this disease.

LAG-3 Blockade with Relatlimab (BMS-986016) Restores Anti-Leukemic Responses in Chronic Lymphocytic Leukemia.

The inclusion of monoclonal antibodies targeting immune checkpoints such PD-1/PD-L1 or CTLA-4 has revolutionized the landscape of anti-cancer therapy. However, PD-1 and CTLA-4 blockade failed to achieve clinical benefit in CLL, thus attention has been focused on emerging checkpoints in this malignancy. LAG-3 is an immune checkpoint receptor that negatively regulates T cell-mediated responses by inducing an hyporesponsive state, thus promoting tumor escape. Patients with chronic lymphocytic leukemia (CLL) develop a profound immune suppression that leads to lessened immunosurveillance and increased risk of developing a secondary neoplasia. In the study herein, we report the profound dysregulation of LAG-3 on leukemic cells in CLL. Likewise, natural killer (NK) and T cells showed increased LAG-3 expression, hence suggesting a role for this checkpoint in CLL-associated immunosuppression. High LAG-3 expression, as well as high levels of soluble LAG-3 (sLAG-3), correlated with adverse cytogenetics and poor outcome in patients with CLL, highlighting the clinical relevance of this immune checkpoint. Treatment of peripheral blood mononuclear cells (PBMCs) from patients with CLL with relatlimab, a new anti-LAG-3 blocking antibody currently evaluated in numerous clinical trials, depleted leukemic cells and restored NK cell- and T cell-mediated responses. Moreover, combination of LAG-3 with the immunomodulatory drug (IMiD) lenalidomide significantly increased IL-2 production by T cells and antibody-dependent cytotoxicity (ADCC) mediated by NK cells. Altogether, these data provide new insights into the potential anti-leukemic effects of relatlimab, currently in clinical trials in CLL, and provides the rationale to further investigate its combination with IMiDs for the management of hematological malignancies.

Lectin-Like Transcript 1 (LLT1) Checkpoint: A Novel Independent Prognostic Factor in HPV-Negative Oropharyngeal Squamous Cell Carcinoma.

Lectin-like transcript 1 (LLT1) expression by tumor cells contributes to immune evasion, thereby emerging as a natural killer (NK) cell-mediated immunotherapeutic target. This study is the first to investigate LLT1 expression (encoded by CLEC2D gene) in head and neck cancers to ascertain its impact on patient prognosis. LLT1 expression was analyzed by immunohistochemistry in a homogeneous cohort of human papillomavirus (HPV)-negative oropharyngeal squamous cell carcinomas (OPSCC), and correlated with clinical data. Results were further validated using transcriptomic data from the TCGA database. Tumoral LLT1 expression was detected in 190/221 (86%) OPSCC specimens, whereas normal pharyngeal epithelium was negative. Patients harboring LLT1-positive tumors showed significantly lower disease-specific (DSS) and overall survival (OS) (p = 0.049 and p = 0.036, respectively, log-rank test). High density of LLT1-positive tumor-infiltrating lymphocytes (TIL) was also frequently detected in 160 (73%) OPSCC samples, and significantly associated with better DSS and OS (p < 0.001 and p = 0.007, respectively). Multivariate Cox analysis further revealed that tumoral LLT1 expression and infiltration of LLT1-positive TIL were independent prognostic factors for DSS and OS. CLEC2D mRNA levels are also significantly increased in primary tumors compared to normal tissue. Strikingly, the prognostic impact of CLEC2D mRNA levels varied depending on HPV status in OPSCC, and among distinct cancer types. CLEC2D expression was significantly correlated with NK cell infiltration using the MCP-counter model. These findings uncover LLT1/CLEC2D as an independent prognostic factor in HPV-negative OPSCC, and a potential novel target for immunotherapy.

Driver Mutations and Single Copy Number Abnormalities Identify Binet Stage A Patients with Chronic Lymphocytic Leukemia with Aggressive Progression.

The correlation between progression and the genetic characteristics of Binet stage A patients with chronic lymphocytic leukemia (CLL) detected by whole exome sequencing (WES) was analyzed in 55 patients. The median follow-up for the patients was 102 months. During the follow-up, 24 patients (43%) progressed. Univariate Cox analysis showed that the presence of driver mutations, the accumulation of two or more mutations, the presence of adverse mutations, immunoglobulin heavy chain genes (IGHV) mutation status and unfavorable single copy number abnormalities (SCNAs) were associated with a higher risk of progression. Particularly, the occurrence of an adverse mutation and unfavorable SCNAs increased the risk of progression nine-fold and five-fold, respectively. Nevertheless, only the occurrence of adverse mutations retained statistical significance in the multivariate analysis. All patients carrying an unfavorable mutation progressed with a median progression-free survival (PFS) of 29 months. The accumulation of two or more mutations also increased the risk of progression with a median PFS of 29 months. The median PFS of patients with unfavorable SCNAs was 38 months. Combining mutations and SCNAs, patients may be stratified into three groups with different prognostic outcomes: adverse (17% probability of five-year PFS), protective (86% probability of five-year PFS) and neither (62% probability of five-year PFS, p < 0.001). Overall, the analysis of the mutational status of patients with CLL at an early stage of the disease may allow the identification of patients with a high risk of progression. The feasibility of an early therapeutic intervention in these particular patients requires further investigation.

Mechanisms of Apoptosis Resistance to NK Cell-Mediated Cytotoxicity in Cancer.

Natural killer (NK) cells are major contributors to immunosurveillance and control of tumor development by inducing apoptosis of malignant cells. Among the main mechanisms involved in NK cell-mediated cytotoxicity, the death receptor pathway and the release of granules containing perforin/granzymes stand out due to their efficacy in eliminating tumor cells. However, accumulated evidence suggest a profound immune suppression in the context of tumor progression affecting effector cells, such as NK cells, leading to decreased cytotoxicity. This diminished capability, together with the development of resistance to apoptosis by cancer cells, favor the loss of immunogenicity and promote immunosuppression, thus partially inducing NK cell-mediated killing resistance. Altered expression patterns of pro- and anti-apoptotic proteins along with genetic background comprise the main mechanisms of resistance to NK cell-related apoptosis. Herein, we summarize the main effector cytotoxic mechanisms against tumor cells, as well as the major resistance strategies acquired by tumor cells that hamper the extrinsic and intrinsic apoptotic pathways related to NK cell-mediated killing.

Mechanisms of Resistance to NK Cell Immunotherapy.

Immunotherapy has recently been a major breakthrough in cancer treatment. Natural killer (NK) cells are suitable targets for immunotherapy owing to their potent cytotoxic activity that may target cancer cells in a major histocompatibility complex (MHC) and antigen-unrestricted manner. Current therapies targeting NK cells include monoclonal antibodies that promote NK cell antibody-dependent cell-mediated cytotoxicity (ADCC), hematopoietic stem cell transplantation (HSCT), the adoptive transfer of NK cells, the redirection of NK cells using chimeric antigen receptor (CAR)-NK cells and the use of cytokines and immunostimulatory drugs to boost the anti-tumor activity of NK cells. Despite some encouraging clinical results, patients receiving these therapies frequently develop resistance, and a myriad of mechanisms of resistance affecting both the immune system and cancer cells have been reported. A first contributing factor that modulates the efficacy of the NK cell therapy is the genetic profile of the individual, which regulates all aspects of NK cell biology. Additionally, the resistance of cancer cells to apoptosis and the immunoediting of cancer cells, a process that decreases their immunogenicity and promotes immunosuppression, are major determinants of the resistance to NK cell therapy. Consequently, the efficacy of NK cell anti-tumor therapy is specific to each patient and disease. The elucidation of such immunosubversive mechanisms is crucial to developing new procedures and therapeutic strategies to fully harness the anti-tumor potential of NK cells.

A cytofluorimetric assay to evaluate intracellular cytokine production by NK cells.

Natural killer (NK) cells are innate lymphoid cells that, together with CD8+ T lymphocytes, are tightly correlated with immunesurveillance and the elimination of transforming and malignant cells both during early steps of tumorigenesis and metastasis. This capacity is due, but not limited to, their great cytotoxic capacity upon recognition of stress-related activating ligands of NK cells on the surface of tumor cells. Due to the emerging role of NK cells in the response to treatment with immune checkpoint blockers (ICBs) and other immunotherapies, it has become essential to deeply study this immune subset. As mentioned above, NK cell antitumor responses not only rely on their high cytotoxic capacity, cytokine production by this immune subset, such as interferon gamma (IFN-γ) or tumor necrosis factor (TNF-α), also plays a key role on NK cell function. In this chapter, we provide a detailed protocol to measure the intracellular expression of cytokines produced by NK cells upon stimulation. A step-by-step guide to isolate peripheral blood mononuclear cells, purify NK cells from whole blood, co-culture with tumor cell lines and evaluate their cytokine production capacity by flow cytometry is here provided.

Evaluation of NK cell cytotoxic activity against malignant cells by the calcein assay.

The immune system exerts a tight cancer surveillance to eradicate nascent and developing tumors. Natural killer (NK) cells rely on their cytotoxic activity to eliminate abnormal or stressed cells, including those undergoing malignant transformation. Thereupon, NK cells, along with CD8+ T cells, are effector immune cells with a pivotal role in cancer immunosurveillance. Due to the outstanding effectivity of immunotherapy in cancer management, strategies aimed at boosting NK cell-mediated tumor recognition and elimination are being thoroughly studied, such as approaches to induce tumor surface expression of NK cell activating ligands. Additionally, the development of methods to evaluate NK cell cytotoxic responses towards malignant cells with distinct immunogenic profiles is of paramount relevance. In this chapter, we detail a simple but highly sensitive and reproducible methodological approach to quantify in vitro NK cell-mediated lysis of a given target cell, e.g. a tumor cell, based on calcein-AM staining and fluorescence detection. Further, this method can be employed to determine the impact of chemical modulators or antibodies blocking cell surface proteins on the antitumor capacity of NK cells, with the aim to improve anticancer immune responses and elucidate the underlying mechanisms regulating NK cell activity.

The Mithralog EC-7072 Induces Chronic Lymphocytic Leukemia Cell Death by Targeting Tonic B-Cell Receptor Signaling.

B-cell receptor (BCR)-dependent signaling is central for leukemia B-cell homeostasis, as underscored by the promising clinical results obtained in patients with chronic lymphocytic leukemia (CLL) treated with novel agents targeting components of this pathway. Herein, we demonstrate that the mithralog EC-7072 displays high ex vivo cytotoxic activity against leukemia cells from CLL patients independently from high-risk prognostic markers and IGHV mutational status. EC-7072 was significantly less toxic against T cells and NK cells and did not alter the production of the immune effector molecules IFN-γ and perforin. EC-7072 directly triggered caspase-3-dependent CLL cell apoptosis, which was not abrogated by microenvironment-derived factors that sustain leukemia cell survival. RNA-sequencing analyses revealed a dramatic EC-7072-driven reprograming of the transcriptome of CLL cells, including a wide downregulation of multiple components and targets of the BCR signaling pathway. Accordingly, we found decreased levels of phosphorylated signaling nodes downstream of the BCR. Crosslinking-mediated BCR activation antagonized CLL cell death triggered by EC-7072, increased the phosphorylation levels of the abovementioned signaling nodes and upregulated BCL2 expression, suggesting that the mithralog disrupts CLL cell viability by targeting the BCR signaling axis at multiple levels. EC-7072 exerted similar or higher antileukemic activity than that of several available CLL therapies and displayed additive or synergistic interaction with these drugs in killing CLL cells. Overall, our findings provide rationale for future investigation to test whether EC-7072 may be a potential therapeutic option for patients with CLL and other B-cell malignancies.

NK Cells in the Treatment of Hematological Malignancies.

Natural killer (NK) cells have the innate ability to kill cancer cells, however, tumor cells may acquire the capability of evading the immune response, thereby leading to malignancies. Restoring or potentiation of this natural antitumor activity of NK cells has become a relevant therapeutic approach in cancer and, particularly, in hematological cancers. The use of tumor-specific antibodies that promote antibody-dependent cell-mediated cytotoxicity (ADCC) through the ligation of CD16 receptor on NK cells has become standard for many hematologic malignancies. Hematopoietic stem cell transplantation is another key therapeutic strategy that harnesses the alloreactivity of NK cells against cancer cells. This strategy may be refined by adoptive transfer of NK cells that may be previously expanded, activated, or redirected (chimeric antigen receptor (CAR)-NK cells) against cancer cells. The antitumor activity of NK cells can also be boosted by cytokines or immunostimulatory drugs such as lenalidomide or pomalidomide. Finally, targeting immunosubversive mechanisms developed by hematological cancers and, in particular, using antibodies that block NK cell inhibitory receptors and checkpoint proteins are novel promising therapeutic approaches in these malignant diseases.

Immunosurveillance of cancer cell stress.

Cancer development is tightly controlled by effector immune responses that recognize and eliminate malignantly transformed cells. Nonetheless, certain immune subsets, such as tumor-associated macrophages, have been described to promote tumor growth, unraveling a double-edge role of the immune system in cancer. Cell stress can modulate the crosstalk between immune cells and tumor cells, reshaping tumor immunogenicity and/or immune function and phenotype. Infiltrating immune cells are exposed to the challenging conditions typically present in the tumor microenvironment. In return, the myriad of signaling pathways activated in response to stress conditions may tip the balance toward stimulation of antitumor responses or immune-mediated tumor progression. Here, we explore how distinct situations of cellular stress influence innate and adaptive immunity and the consequent impact on cancer establishment and progression.

CD107a Degranulation Assay to Evaluate Immune Cell Antitumor Activity.

Cancer development is under surveillance by the immune system of the host. Tumor cells can be recognized and killed by cytotoxic lymphocytes- such as CD8+ T lymphocytes and natural killer (NK) cells-mainly through the immune secretion of lytic granules that kill target cells. This process involves the fusion of the granule membrane with the cytoplasmic membrane of the immune effector cell, resulting in surface exposure of lysosomal-associated proteins that are typically present on the lipid bilayer surrounding lytic granules, such as CD107a. Therefore, membrane expression of CD107a constitutes a marker of immune cell activation and cytotoxic degranulation. In this chapter, we detail the steps required to isolate peripheral blood mononuclear cells (PBMCs), coculture them with target tumor cell lines, and evaluate the cytotoxic immune function by means of flow cytometry evaluation of CD107a expression on the surface of NK cells.

A Flow Cytometric NK Cell-Mediated Cytotoxicity Assay to Evaluate Anticancer Immune Responses In Vitro.

The immune system is able to detect and eliminate nascent and developing tumors. Thus, T lymphocytes of the adaptive immunity recognize cancer cells by detecting tumor-associated antigens, whereas certain innate immune cells scan for molecules that are mainly overexpressed on malignant and infected cells. The best example of the latter is natural killer (NK) cells. The activity of these immune cells is strictly governed by a balance between positive and negative signals provided by stress-regulated molecules that bind NK cell membrane receptors, such as the activatory receptor NKG2D. Given the key role of NK cells in eradicating cancer cells, the development of assays to study the cellular and molecular determinants of NK cell antitumor activity are of great relevance. In this chapter, we describe a flow cytometric in vitro assay to evaluate the cytotoxic activity of NK cells against a given target cell, including tumor cells. Moreover, this method is highly versatile, as it can be complemented with the use of antibodies blocking cell surface proteins (receptors or ligands) or a variety of chemical modulators, which allows the elucidation of molecules and signaling pathways that regulate NK cell anticancer function.