Human NK receptors: from the molecules to the therapy of high risk leukemias

Insights into the Roles of Natural Killer Cells in Osteoarthritis

Osteoarthritis (OA) is now widely acknowledged as a low-grade inflammatory condition, in which the intrinsic immune system plays a significant role in its pathogenesis. While the involvement of macrophages and T cells in the development of OA has been extensively reviewed, recent research has provided mounting evidence supporting the crucial contribution of NK cells in both the initiation and advancement of OA. Accumulated evidence has emerged in recent years indicating that NK cells play a critical role in OA development and progression. This review will outline the ongoing understanding of the utility of NK cells in the etiology of OA, focusing on how NK cells interact with chondrocytes, synoviocytes, osteoclasts, and other immune cells to influence the course of OA disease.

Adaptive NK cell response to human cytomegalovirus: Facts and open issues

Human cytomegalovirus (HCMV) infection exerts broad effects on the immune system. These include the differentiation and persistent expansion of a mature NK cell subset which displays a characteristic phenotypic and functional profile hallmarked by expression of the HLA-E-specific CD94/NKG2C activating receptor. Based on our experience and recent advances in the field, we overview the adaptive features of the NKG2C+ NK cell response, discussing observations and open questions on: (a) the mechanisms and influence of viral and host factors; (b) the existence of other NKG2C- NK cell subsets sharing adaptive features; (c) the development and role of adaptive NKG2C+ NK cells in the response to HCMV in hematopoietic and solid organ transplant patients; (d) their relation with other viral infections, mainly HIV-1; and (e) current perspectives for their use in adoptive immunotherapy of cancer.

Killer-Cell Immunoglobulin-like Receptor Diversity in an Admixed South American Population

Natural Killer (NK) cells are innate immune cells that mediate antiviral and antitumor responses. NK cell activation and induction of effector functions are tightly regulated by the integration of activating and inhibitory receptors such as killer immunoglobulin-like receptors (KIR). KIR genes are characterized by a high degree of diversity due to presence or absence, gene copy number and allelic polymorphism. The aim of this study was to establish the distribution of KIR genes and genotypes, to infer the most common haplotypes in an admixed Colombian population and to compare these KIR gene frequencies with some Central and South American populations and worldwide. A total of 161 individuals from Medellin, Colombia were included in the study. Genomic DNA was used for KIR and HLA genotyping. We analyzed only KIR gene-content (presence or absence) based on PCR-SSO. The KIR genotype, most common haplotypes and combinations of KIR and HLA ligands frequencies were estimated according to the presence or absence of KIR and HLA genes. Dendrograms, principal component (PC) analysis and Heatmap analysis based on genetic distance were constructed to compare KIR gene frequencies among Central and South American, worldwide and Amerindian populations. The 16 KIR genes analyzed were distributed in 37 different genotypes and the 7 most frequent KIR inferred haplotypes. Importantly, we found three new genotypes not previously reported in any other ethnic group. Our genetic distance, PC and Heatmap analysis revealed marked differences in the distribution of KIR gene frequencies in the Medellin population compared to worldwide populations. These differences occurred mainly in the activating KIR isoforms, which are more frequent in our population, particularly KIR3DS1. Finally, we observed unique structural patterns of genotypes, which evidences the potential diversity and variability of this gene family in our population, and the need for exhaustive genetic studies to expand our understanding of the KIR gene complex in Colombian populations.

Impact of an Immune Modulator Mycobacterium-w on Adaptive Natural Killer Cells and Protection Against COVID-19

The kinetics of NKG2C+ adaptive natural killer (ANK) cells and NKG2A+inhibitory NK (iNK) cells with respect to the incidence of SARS-CoV-2 infection were studied for 6 months in a cohort of healthcare workers following the administration of the heat-killed Mycobacterium w (Mw group) in comparison to a control group. In both groups, corona virus disease 2019 (COVID-19) correlated with lower NKG2C+ANK cells at baseline. There was a significant upregulation of NKG2C expression and IFN-γ release in the Mw group (p=0.0009), particularly in those with a lower baseline NKG2C expression, along with the downregulation of iNK cells (p<0.0001). This translated to a significant reduction in the incidence and severity of COVID-19 in the Mw group (incidence risk ratio-0.15, p=0.0004). RNA-seq analysis at 6 months showed an upregulation of the ANK pathway genes and an enhanced ANK-mediated antibody-dependent cellular cytotoxicity (ADCC) signature. Thus, Mw was observed to have a salutary impact on the ANK cell profile and a long-term upregulation of ANK-ADCC pathways, which could have provided protection against COVID-19 in a non-immune high-risk population.

The Biological Role and Therapeutic Potential of NK Cells in Hematological and Solid Tumors

NK cells are an attractive target for cancer immunotherapy due to their potent antitumor activity. The main advantage of using NK cells as cytotoxic effectors over T cells is a reduced risk of graft versus host disease. At present, several variants of NK-cell-based therapies are undergoing clinical trials and show considerable effectiveness for hematological tumors. In these types of cancers, the immune cells themselves often undergo malignant transformation, which determines the features of the disease. In contrast, the current use of NK cells as therapeutic agents for the treatment of solid tumors is much less promising. Most studies are at the stage of preclinical investigation, but few progress to clinical trials. Low efficiency of NK cell migration and functional activity in the tumor environment are currently considered the major barriers to NK cell anti-tumor therapies. Various therapeutic combinations, genetic engineering methods, alternative sources for obtaining NK cells, and other techniques are aiming at the development of promising NK cell anticancer therapies, regardless of tumorigenesis. In this review, we compare the role of NK cells in the pathogenesis of hematological and solid tumors and discuss current prospects of NK-cell-based therapy for hematological and solid tumors.

"Ex-vivo" T-cell depletion in allogeneic hematopoietic stem cell transplantation: New clinical approaches for old challenges

Allogeneic transplantation still remains as standard of care for patients with high-risk hematological malignancies at diagnosis or after relapse. However, GvHD remains yet as the most relevant clinical complication in the early post-transplant period. TCD allogeneic transplant is now considered a valid option to reduce severe GvHD and to provide a platform for cellular therapy to prevent relapse disease or to treat opportunistic infections.

NK cells and ILCs in tumor immunotherapy

Cells of the innate immunity play an important role in tumor immunotherapy. Thus, NK cells can control tumor growth and metastatic spread. Thanks to their strong cytolytic activity against tumors, different approaches have been developed for exploiting/harnessing their function in patients with leukemia or solid tumors. Pioneering trials were based on the adoptive transfer of autologous NK cell-enriched cell populations that were expanded in vitro and co-infused with IL-2. Although relevant results were obtained in patients with advanced melanoma, the effect was mostly limited to certain metastatic localizations, particularly to the lung. In addition, the severe IL-2-related toxicity and the preferential IL-2-induced expansion of Treg limited this type of approach. This limitation may be overcome by the use of IL-15, particularly of modified IL-15 molecules to improve its half-life and optimize the biological effects. Other approaches to harness NK cell function include stimulation via TLR, the use of bi- and tri-specific NK cell engagers (BiKE and TriKE) linking activating NK receptors (e.g. CD16) to tumor-associated antigens and even incorporating an IL-15 moiety (TriKE). As recently shown, in tumor patients, NK cells may also express inhibitory checkpoints, primarily PD-1. Accordingly, the therapeutic use of checkpoint inhibitors may unleash NK cells against PD-L1⁺ tumors. This effect may be predominant and crucial in tumors that have lost HLA cl-I expression, thus resulting "invisible" to T lymphocytes. Additional approaches in which NK cells may represent an important tool for cancer therapy, are to exploit the unique properties of the "adaptive" NK cells. These CD57⁺ NKG2C⁺ cells, despite their mature stage and a potent cytolytic activity, maintain a strong proliferating capacity. This property revealed to be crucial in hematopoietic stem cell transplantation (HSCT), particularly in the haplo-HSCT setting, to cure high-risk leukemias. T depleted haplo-HSCT (e.g. from one of the parents) allowed to save the life of thousands of patients lacking a HLA-compatible donor. In this setting, NK cells have been shown to play an essential role against leukemia cells and infections. Another major advance is represented by chimeric antigen receptor (CAR)-engineered NK cells. CAR-NK, different from CAR-T cells, may be obtained from allogeneic donors since they do not cause GvHD. Accordingly, they may represent "off-the-shelf" products to promptly treat tumor patients, with affordable costs. Different from NK cells, helper ILC (ILC1, ILC2 and ILC3), the innate counterpart of T helper cell subsets, remain rather ambiguous with respect to their anti-tumor activity. A possible exception is represented by a subset of ILC3: their frequency in peri-tumoral tissues in patients with NSCLC directly correlates with a better prognosis, possibly reflecting their ability to contribute to the organization of tertiary lymphoid structures, an important site of T cell-mediated anti-tumor responses. It is conceivable that innate immunity may significantly contribute to the major advances that immunotherapy has ensured and will continue to ensure to the cure of cancer.

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.

Haploidentical Stem Cell Transplantation in Children With Hematological Malignancies Using αβ+ T-Cell Receptor and CD19+ Cell Depleted Grafts: High CD56dim/CD56bright NK Cell Ratio Early Following Transplantation Is Associated With Lower Relapse Incidence and Better Outcome

We prospectively analyzed outcomes of haploidentical hematopoietic stem cell transplantation using αβ⁺ T-cell receptor/CD19⁺ depleted grafts. Sixty-three transplantations were performed in 60 patients. Twenty-eight patients were diagnosed with acute lymphoblastic leukemia (ALL), 27 patients were diagnosed with acute myelogenous leukemia, and in eight other hematological malignancies were diagnosed. Twenty-three were in first complete remission (CR), 20 in second CR, 20 beyond second CR. Four patients developed graft failure. Median time to neutrophil and platelet recovery was 14 (range 9–25) and 10 days (range 7–30), respectively. The probability of non-relapse mortality (NRM) by day +100 after transplantation was 10 ± 4%. With a median follow-up of 28 months, the probability of relapse was 32 ± 6% and disease-free survival was 52 ± 6%. Immune reconstitution was leaded by NK cells. As such, a high CD56^dim/CD56^bright NK cell ratio early after transplantation was associated with better disease-free survival (DFS) (≥3.5; 77 ± 8% vs. <3.5; 28 ± 5%; p = 0.001) due to lower relapse incidence (≥3.5; 15 ± 7% vs. <3.5; 37 ± 9%; p = 0.04). T-cell reconstitution was delayed and associated with severe infections after transplant. Viral reactivation/disease and presence of venooclusive disease of liver in the non-caucasian population had a significant impact on NRM. αβ⁺ T-cell receptor/CD19⁺ cell-depleted haploidentical transplant is associated with good outcomes especially in patients in early phase of disease. A rapid expansion of “mature” natural killer cells early after transplantation resulted on lower probability of relapse, suggesting a graft vs. leukemia effect independent from graft-vs.-host reactions.

NK Cell-Based Immunotherapy for Hematological Malignancies

Natural killer (NK) lymphocytes are an integral component of the innate immune system and represent important effector cells in cancer immunotherapy, particularly in the control of hematological malignancies. Refined knowledge of NK cellular and molecular biology has fueled the interest in NK cell-based antitumor therapies, and recent efforts have been made to exploit the high potential of these cells in clinical practice. Infusion of high numbers of mature NK cells through the novel graft manipulation based on the selective depletion of T cells and CD19+ B cells has resulted into an improved outcome in children with acute leukemia given human leucocyte antigen (HLA)-haploidentical hematopoietic transplantation. Likewise, adoptive transfer of purified third-party NK cells showed promising results in patients with myeloid malignancies. Strategies based on the use of cytokines or monoclonal antibodies able to induce and optimize NK cell activation, persistence, and expansion also represent a novel field of investigation with remarkable perspectives of favorably impacting on outcome of patients with hematological neoplasia. In addition, preliminary results suggest that engineering of mature NK cells through chimeric antigen receptor (CAR) constructs deserve further investigation, with the goal of obtaining an "off-the-shelf" NK cell bank that may serve many different recipients for granting an efficient antileukemia activity.

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.

Targeting NKG2A to elucidate natural killer cell ontogenesis and to develop novel immune-therapeutic strategies in cancer therapy

Natural Killer (NK) cells are innate immune cells with a primary role in the immune surveillance against non-self-cells. NK cell recognition of "self" relies on the surface expression on autologous cells of MHC class I (MHC-I) molecules. Either the absence or the down-modulation of MHC-I on target cells "license" NK cells to kill threatening tumor-transformed or virally infected cells. This phenomenon is controlled by a limited repertoire of activating and inhibitory NK receptors (aNKRs and iNKRs) that tunes NK cell activation and effector functions. Hence, the calibration of NK cell alloreactivity depends on the ability of iNKRs to bind MHC-I complex and these interactions are key in regulating both NK cell differentiation and effector functions. Indeed, the presence of iNKRs specific for self-MHC haplotypes (i) plays a role in the "licensing/education" process that controls the responsiveness of mature NK cells and prevents their activation against the "self" and (ii) is exploited by tumor cells to escape from NK cell cytotoxicity. Herein, we review our current knowledge on function and clinical application of NKG2A, a C-type lectin iNKR that binds specific haplotypes of human leukocyte antigens early during the NK cell maturation process, thus contributing to modulate the terminal maturation of NK cells as potent effectors against cancers cells. These NKG2A-mediated mechanisms are currently being exploited for developing promising immune-therapeutic strategies to improve the prognosis of solid and blood tumors and to ameliorate the clinical outcome of patients undergone allogeneic hematopoietic stem cell transplantation to treat high-risk hematologic malignancies.

NK Cell-Based Immunotherapy in Cancer Metastasis

Metastasis represents the leading cause of cancer-related death mainly owing to the limited efficacy of current anticancer therapies on advanced malignancies. Although immunotherapy is rendering promising results in the treatment of cancer, many adverse events and factors hampering therapeutic efficacy, especially in solid tumors and metastases, still need to be solved. Moreover, immunotherapeutic strategies have mainly focused on modulating the activity of T cells, while Natural Killer (NK) cells have only recently been taken into consideration. NK cells represent an attractive target for cancer immunotherapy owing to their innate capacity to eliminate malignant tumors in a non-Major Histocompatibility Complex (MHC) and non-tumor antigen-restricted manner. In this review, we analyze the mechanisms and efficacy of NK cells in the control of metastasis and we detail the immunosubversive strategies developed by metastatic cells to evade NK cell-mediated immunosurveillance. We also share current and cutting-edge clinical approaches aimed at unleashing the full anti-metastatic potential of NK cells, including the adoptive transfer of NK cells, boosting of NK cell activity, redirecting NK cell activity against metastatic cells and the release of evasion mechanisms dampening NK cell immunosurveillance.

High-Risk Leukemia: Past, Present, and Future Role of NK Cells

Natural killer (NK) cells are a population of cytotoxic innate lymphocytes that evolved prior to their adaptive counterparts and constitute one of the first lines of defense against infected/mutated cells. Several studies have shown that in patients with acute leukemia given haploidentical hematopoietic stem cell transplantation, donor-derived NK cells play a key role in the eradication of cancer cells. The antileukemic effect is mostly related to the presence of “alloreactive” NK cells, that is, mature KIR+ NK cells that express inhibitory KIR mismatched with HLA class I (KIR-L) of the patient. A genotypic analysis detecting KIR B haplotype and the relative B content is an additional donor selection criterion. These data provided the rationale for implementing phase I/II clinical trials of adoptive infusion of either selected or ex vivo-activated NK cells, often from an HLA-mismatched donor. In this review, we provide a historical perspective on the role played by NK cells in patients with acute leukemia, focusing also on the various approaches to adoptive NK cell therapy and the unresolved issues therein. In addition, we outline new methods to enhance NK activity, including anti-KIR monoclonal antibody, bi-/trispecific antibodies linking NK cells to cytokines and/or target antigens, and CAR-engineered NK cells.

Cytokines can counteract the inhibitory effect of MEK-i on NK-cell function

Oncogene-targeted therapies based on mutated BRAF- and/or MEK-specific inhibitors have been developed for melanoma treatment. Although these drugs induce tumor regression in a high percentage of patients, clinical responses are frequently limited in time and tumors often recur. Recent studies suggested that the combination of BRAF/MEK inhibition with immunotherapy could represent a promising strategy for the cure of melanoma. NK cells are suitable effectors for tumor immunotherapy. Here we show that PLX4032 (a mutant BRAF^V600 inhibitor) had no effect on the functional properties of NK cells cultured in the presence of IL-2 or IL-15. In contrast, PD0325901 (a MEK inhibitor) induced the down-regulation of the main activating NK receptors and inhibited NK cell function. Importantly, PD0325901 did not affect the anti-tumor activity of NK cells that had been exposed to a combination of IL-15 and IL-18. In addition, both PLX4032 and PD0325901 did not exert any inhibitory effect on in vitro IL-2 or IL-15 pre-activated NK cells.

Our data may provide a rationale for future clinical protocols that combine IL-15/IL-18 cytokine administration with MEK inhibitors. In addition, they suggest that oncogene-targeting drugs are compatible with NK-based adoptive therapy.

Imatinib and Nilotinib Off-Target Effects on Human NK Cells, Monocytes, and M2 Macrophages

Tyrosine kinase inhibitors (TKIs) are used in the clinical management of hematological neoplasms. Moreover, in solid tumors such as stage 4 neuroblastomas (NB), imatinib showed benefits that might depend on both on-target and immunological off-target effects. We investigated the effects of imatinib and nilotinib on human NK cells, monocytes, and macrophages. High numbers of monocytes died upon exposure to TKI concentrations similar to those achieved in patients. Conversely, NK cells were highly resistant to the TKI cytotoxic effect, were properly activated by immunostimulatory cytokines, and degranulated in the presence of NB cells. In NB, neither drug reduced the expression of ligands for activating NK receptors or upregulated that of HLA class I, B7-H3, PD-L1, and PD-L2, molecules that might limit NK cell function. Interestingly, TKIs modulated the chemokine receptor repertoire of immune cells. Acting at the transcriptional level, they increased the surface expression of CXCR4, an effect observed also in NK cells and monocytes of patients receiving imatinib for chronic myeloid leukemia. Moreover, TKIs reduced the expression of CXCR3 (in NK cells) and CCR1 (in monocytes). Monocytes also decreased the expression of M-CSFR, and low numbers of cells underwent differentiation toward macrophages. M0 and M2 macrophages were highly resistant to TKIs and maintained their phenotypic and functional characteristics. Importantly, also in the presence of TKIs, the M2 immunosuppressive polarization was reverted by TLR engagement, and M1-oriented macrophages fully activated autologous NK cells. Our results contribute to better interpreting the off-target efficacy of TKIs in tumors and to envisaging strategies aimed at facilitating antitumor immune responses.

The impact of KIR2DS4 gene on clinical outcome after hematopoietic stem cell transplantation

Killer cell immunoglobulin-like receptors (KIR) are a family of inhibitory/activating receptors expressed on NK cells. Interactions of KIR receptors with KIR ligands have been shown to modify hematopoietic stem cell transplantation (HSCT) outcome. The aim of this research was to determine the KIR2DS4 allele variants distribution among 111 patients with different hematological malignancy who underwent HSCT and their donors, and to evaluate KIR2DS4 alleles' impact on HSCT outcome. The KIR gene frequency analysis showed a significantly higher incidence of full-length KIR2DS4 alleles among patients. The impact of KIR2DS4 alleles on transplantation outcomes revealed that donors' full-length KIR2DS4 alleles is associated with lower overall survival rates, higher risk of GVHD and higher relapse incidence. The expression of full-length KIR2DS4 allele variants may contribute to a worse clinical outcome after HSCT. KIR typing for KIR2DS4 could be used as an additional criterion for selecting suitable donors in cases when more than one HLA identical donor is identified for a specific patient.

An Fc-optimized CD133 antibody for induction of NK cell reactivity against myeloid leukemia

Antibody-dependent cellular cytotoxicity (ADCC) of natural killer (NK) cells largely contributes to the success of monoclonal antibody (mAb) treatment in cancer. As no antibodies are clinically available for immunotherapy of myeloid leukemias (MLs), we aimed to develop an Fc-optimized CD133 mAb for induction of NK ADCC against MLs. When comparing different available CD133 mAbs, no difference was observed with regard to binding to primary chronic myeloid leukemia cells. However, clone 293C3 recognized acute myeloid leukemia (AML) cells in a substantially higher percentage of patient cases and was thus chosen to generate chimeric mAbs with either wild-type Fc part (293C3-WT) or a variant containing amino-acid exchanges (S239D/I332E) to enhance affinity to CD16 on NK cells (293C3-SDIE). In vitro, treatment with 293C3-SDIE significantly enhanced activation, degranulation and lysis of primary CD133-positive AML cells by allogeneic and autologous NK cells as compared with its wild-type counterpart. In line with the observed lower expression levels of CD133 on healthy cells compared with malignant hematopoietic cells, 293C3-SDIE caused no relevant toxicity towards committed hematopoietic progenitor cells. In a NOD.Cg-PrkdcscidIL2rgtmWjl/Sz xenotransplantation model, 293C3-SDIE facilitated elimination of patient AML cells by human NK cells. Thus, 293C3-SDIE constitutes an attractive immunotherapeutic compound, in particular for elimination of minimal residual disease in the context of allogeneic stem cell transplantation in AML.

Progress toward curing HIV infections with hematopoietic stem cell transplantation

Combination antiretroviral therapy can suppress human immunodeficiency virus (HIV) infection but cannot completely eradicate the virus. A major obstacle in the quest for a cure is the difficulty in targeting and measuring latently infected cells. To date, a single person seems to have been cured of HIV. Hematopoietic stem cell transplantation (HSCT) preceded this cancer patient's long-term sustained HIV remission, but researchers have been unable to replicate this cure, and the mechanisms that led to HIV remission remain to be established. In February 2014, the National Institute of Allergy and Infectious Diseases sponsored a workshop that provided a venue for in-depth discussion of whether HSCT could be exploited to cure HIV in cancer patients requiring such procedures. Participants also discussed how HSCT might be applied to a broader community of HIV-infected persons in whom the risks of HSCT currently outweigh the likelihood and benefits of HIV cure.

Tracking in vivo dynamics of NK cells transferred in patients undergoing stem cell transplantation

Haploidentical stem cell transplantation (haploSCT) offers an alternative treatment option for advanced leukemia patients lacking a HLA-compatible donor. Transfer of NK cells represents a promising therapeutic option in combination with SCT, as NK cells can promote graft versus leukemia with low risk of GVH disease. In this study, we show results from a phase I/II trial in which 24 acute myeloid leukemia patients underwent haploSCT in combination with early transfer of unmodified NK cells and observed a promising 2-year overall survival rate of 37%. By performing immunomonitoring and subsequent principal component analysis, we tracked donor NK-cell dynamics in the patients and distinguished between NK cells reconstituting from CD34(+) precursors, giving rise over time to a continuum of multiple differentiation stages, and adoptively transferred NK cells. Transferred NK cells displayed a mature phenotype and proliferated in vivo during the early days after haploSCT even in the absence of exogenous IL-2 administration. Moreover, we identified the NK-cell phenotype associated with in vivo expansion. Thus, our study indicates a promising path for adoptive transfer of unmodified NK cells in the treatment of high-risk acute myeloid leukemia.

TLR activation of tumor-associated macrophages from ovarian cancer patients triggers cytolytic activity of NK cells

We analyzed the functional outcome of the interaction between tumor-associated macrophages (TAMs) and natural killer (NK) cells. TAMs from ascites of ovarian cancer patients displayed an alternatively activated functional phenotype (M2) characterized by a remarkably high frequency and surface density of membrane-bound IL-18. Upon TLR engagement, TAMs acquired a classically activated functional phenotype (M1), released immunostimulatory cytokines (IL-12, soluble IL-18), and efficiently triggered the cytolytic activity of NK cells. TAMs also induced the release of IFN-γ from NK cells, which however was significantly lower compared with that induced by in vitro-polarized M2 cells. Most tumor-associated NK cells displayed a CD56(bright) , CD16(neg) or CD56(bright) , CD16(dim) phenotype, and very poor cytolytic activities, despite an increased expression of the activation marker CD69. They also showed downregulation of DNAM-1, 2B4, and NTB-A activating receptors, and an altered chemokine receptor repertoire. Importantly however, when appropriately stimulated, NK cells from the patients, including those cells isolated from ascites, efficiently killed autologous TAMs that expressed low, "nonprotective" levels of HLA class I molecules. Overall, our data show the existence of a complex tumor microenvironment in which poorly cytolytic/immature NK cells deal with immunosuppressive tumor-educated macrophages.

Natural killer cells and neuroblastoma: tumor recognition, escape mechanisms, and possible novel immunotherapeutic approaches

Neuroblastoma (NB) is the most common extra-cranial solid tumor of childhood and arises from developing sympathetic nervous system. Most primary tumors localize in the abdomen, the adrenal gland, or lumbar sympathetic ganglia. Amplification in tumor cells of MYCN, the major oncogenic driver, patients' age over 18 months, and the presence at diagnosis of a metastatic disease (stage IV, M) identify NB at high risk of treatment failure. Conventional therapies did not significantly improve the overall survival of these patients. Moreover, the limited landscape of somatic mutations detected in NB is hampering the development of novel pharmacological approaches. Major efforts aim to identify novel NB-associated surface molecules that activate immune responses and/or direct drugs to tumor cells and tumor-associated vessels. PVR (Poliovirus Receptor) and B7-H3 are promising targets, since they are expressed by most high-risk NB, are upregulated in tumor vasculature and are essential for tumor survival/invasiveness. PVR is a ligand of DNAM-1 activating receptor that triggers the cytolytic activity of natural killer (NK) cells against NB. In animal models, targeting of PVR with an attenuated oncolytic poliovirus induced tumor regression and elimination. Also B7-H3 was successfully targeted in preclinical studies and is now being tested in phase I/II clinical trials. B7-H3 down-regulates NK cytotoxicity, providing NB with a mechanism of escape from immune response. The immunosuppressive potential of NB can be enhanced by the release of soluble factors that impair NK cell function and/or recruitment. Among these, TGF-β1 modulates the cytotoxicity receptors and the chemokine receptor repertoire of NK cells. Here, we summarize the current knowledge on the main cell surface molecules and soluble mediators that modulate the function of NK cells in NB, considering the pros and cons that must be taken into account in the design of novel NK cell-based immunotherapeutic approaches.

Killer immunoglobulin-like receptors and tumor immunity

were originally named for their capacity to elicit potent cytotoxicity against tumor cells independent of prior sensitization or gene rearrangement. This process is facilitated through the expression of activating and inhibitory receptors that provide for NK cell "education" and a subsequent ability to survey, recognize, and lyse infected or transformed cells, especially those lacking or possessing mutated MHC class I expression. Since these original observations were made, how NK cells recognize candidate target cells continues to be the topic of ongoing investigation. It is now appreciated that NK cells express a diverse repertoire of activating and inhibitory receptors of which killer immunoglobulin-like receptors (KIR) appear to play a critical role in mediating self-tolerance as well as facilitating cytotoxicity against infected or transformed cells. In addition, in the presence of an activating signal, the absence or mismatch of MHC class I molecules on such targets (which serve as inhibitory KIR ligands) promotes NK cell–mediated lysis. An increasing understanding of the complexities of KIR biology has provided recent opportunities to leverage the NK cell versus tumor effect as a novel avenue of immunotherapy for cancer. The present review summarizes the current understanding of KIR expression and function and highlights ongoing efforts to translate these discoveries into novel NK cell–mediated immunotherapies for cancer.

Baseline and Dynamic Expression of Activating NK Cell Receptors in the Control of Chronic Viral Infections: The Paradigm of HIV-1 and HCV

Natural killer (NK) cell function is regulated by a balance between the triggering of activating and inhibitory receptors expressed on their surface. A relevant effort has been focused so far on the study of KIR carriage/expression setting the basis for NK cell education and self-tolerance. Focus on the evolution and regulation of activating NK receptors has lagged behind so far. Our understanding of activating receptor expression and regulation has recently improved by evidences derived from in vitro and in vivo studies. Virus infection - either acute or chronic - determines preferential expansion of NK cells with specific phenotype, activating receptors, and with recall-like functional activity. Studies on patients with viral infection (HIV and HCV) and specific diverging clinical courses confirm that inter-individual differences may exist in baseline expression of natural cytotoxicity receptors (NKp46 and NKp30). The findings that patients with divergent clinical courses have different kinetics of activating receptor density expression upon NK cell activation in vitro provide an additional, time-dependent, functional parameter. Kinetic changes in receptor expression thus represent an additional parameter to basal receptor density expression. Different expression and inducibilities of activating receptors on NK cells contribute to the high diversity of NK cell populations and may help our understanding of the inter-individual differences in innate responses that underlie divergent disease courses.

At the Bench: Preclinical rationale for exploiting NK cells and γδ T lymphocytes for the treatment of high-risk leukemias

NK cells and γδ T lymphocytes display potent cytolytic activity against leukemias and CMV-infected cells and are thus, promising immune effector cells in the context of allo-HSCT. NK cells express HLA class I-specific inhibitory receptors and preferentially kill HLA class I(low) tumors or virus-infected cells. Killing occurs upon engagement of activating NKRs with ligands that are up-regulated on tumors and infected cells. A similar activating receptor/ligand interaction strategy is used by γδ T cells, which in addition, use their TCRs for recognition of phosphorylated antigens and still largely undefined ligands on tumor cells. In the haploidentical allo-HSCT setting, alloreactive NK cells, derived from donor HSCs, can exert potent antileukemia activity and kill residual patient DCs and T cells, thus preventing GvHD and graft rejection. However, generation of KIR(+) alloreactive NK cells from HSCs requires many weeks, during which leukemia relapses, and life-threatening infections may occur. Importantly, mature NK cells and γδ T cells can control certain infectious agents efficiently, in particular, limit CMV reactivation, and infusion of such donor cells at the time of HSCT has been implemented. Development of novel, cell-based immunotherapies, allowing improved trafficking and better targeting, will endow NK cells and γδ T lymphocytes with enhanced anti-tumor activity, also making them key reagents for therapies against solid tumors. The clinical aspects of using NK cells and γδ T lymphocytes against hematological malignancies, including the allo-HSCT context, are reviewed in the related side-by-side paper by Locatelli and colleagues [1].

KIR gene mismatching and KIR/C ligands in liver transplantation: consequences for short-term liver allograft injury

BACKGROUND

Killer immunoglobulin-like receptors (KIRs) bind human leukocyte antigen (HLA) class-I (HLA-I) ligands and regulate functions of natural killer cells and subsets of T cells. KIR/HLA-I interactions allow predicting natural killer cell alloreactivity in hematopoietic stem cell transplantation and in HLA-compatible kidney transplants, but its meaning in liver transplantation remains controversial.

METHODS

KIR and HLA genotypes were studied in 402 liver transplants, using sequence-specific oligonucleotides and primer methods. Recipients and donor KIRs, HLA-C genotypes, KIR gene mismatches (MMs) between recipient-donor pairs, and KIR/HLA-ligand combinations were analyzed in overall transplantations, in the acute rejection (AR; n=110) and non-AR (n=292) groups.

RESULTS

KIR gene MMs between recipients and donors, mainly in activating KIRs, and KIR2DL3 and KIR2DS1 of recipients in the presence of donor C2 ligands, significantly enhanced early AR rate (P<0.05), with KIR2DL3 and KIR2DS1 exhibiting a synergic effect in dependence of the donor C2 ligand number (χ2=7.662, P=0.022). KIR2DL3, KIR2DS1, and also KIR2DS4 significantly influenced short-term graft survival, with a benefit for transplantations combining KIR2DL3 recipients and donors having C1 ligands (log rank, P<0.019 at 1 year; hazards ratio [HR], 0.321; 95% confidence interval [CI], 0.107-0.962; P=0.042), whereas KIR2DS1 and KIR2DS4 recipients combined with donors lacking C1 ligands (C2/C2) exhibited a worse graft survival (log rank, P=0.035 at 6 months; HR, 7.713; 95% CI, 2.156-27.369; P=0.002 for KIR2DS1; and log rank, P=0.006 at 1 year; HR, 3.794; 95% CI, 1.267-11.365; P=0.017 for KIR2DS4).

CONCLUSIONS

This study shows that KIR gene-gene MMs increase AR and that KIRs/C ligands associated to AR and KIR2DS4/C ligands also influence short-term graft survival.

Neuroblastoma-derived TGF-β1 modulates the chemokine receptor repertoire of human resting NK cells

In this study, we show that neuroblastoma (NB) cell conditioning affects the chemokine receptor repertoire of human resting NK cells. In particular, NB cells upregulated the expression of CXCR4 and CXCR3 in all NK cells and downregulated CX3CR1 in the CD56(dim) subset. On the contrary, the expression of CXCR1 and CCR7 remained unaltered. The phenomenon was dependent on the release by NB cells of TGF-β1, and rTGF-β1 induced a chemokine receptor repertoire identical to that of NB-conditioned NK cells. The immune modulatory role of TGF-β1 appears to be dose dependent because low amounts of the cytokine were sufficient to modulate CXCR4 and CX3CR1 expression, intermediate amounts modified that of CXCR3, and high amounts were necessary to downregulate the expression of the NKp30 activating receptor. Notably, a similar receptor modulation was observed in rTGF-β2-conditioned NK cells. Finally, the analysis of NK cells from patients with stage 4 NB suggests that NB conditioning could exert in vivo an immune modulatory effect resembling that emerged from in vitro experiments. Altogether our data propose a novel tumor escape-mechanism based on the modulation of chemokine receptors that play pivotal roles in NK cells bone marrow homing, egress, or recruitment into peripheral tissues.

Comprehensive analysis of NKG2D ligand expression and release in leukemia: implications for NKG2D-mediated NK cell responses

Ligands of the prototypical activating NK receptor NKG2D render cancer cells susceptible to NK cell-mediated cytolysis if expressed at sufficiently high levels. However, malignant cells employ mechanisms to evade NKG2D-mediated immunosurveillance, such as NKG2D ligand (NKG2DL) shedding resulting in reduced surface expression levels. In addition, systemic downregulation of NKG2D on NK cells of cancer patients has been observed in many studies and was attributed to soluble NKG2DL (sNKG2DL), although there also are conflicting data. Likewise, relevant expression of NKG2DL in leukemia has been reported by some, but not all studies. Hence, we comprehensively studied expression, release, and function of the NKG2D ligands MHC class I chain-related molecules A and B and UL16-binding proteins 1-3 in 205 leukemia patients. Leukemia cells of most patients (75%) expressed at least one NKG2DL at the surface, and all investigated patient sera contained elevated sNKG2DL levels. Besides correlating NKG2DL levels with clinical data and outcome, we demonstrate that sNKG2DL in patient sera reduce NKG2D expression on NK cells, resulting in impaired antileukemia reactivity, which also critically depends on number and levels of surface-expressed NKG2DL. Together, we provide comprehensive data on the relevance of NKG2D/NKG2DL expression, release, and function for NK reactivity in leukemia, which exemplifies the mechanisms underlying NKG2D-mediated tumor immunosurveillance and escape.