Inhibition of clonogenic growth of fresh leukemia cells by unstimulated and IL-2 stimulated NK cells of normal donors

Natural Killer Cells in the Malignant Niche of Multiple Myeloma

Natural killer (NK) cells represent a subset of CD3- CD7+ CD56+/dim lymphocytes with cytotoxic and suppressor activity against virus-infected cells and cancer cells. The overall potential of NK cells has brought them to the spotlight of targeted immunotherapy in solid and hematological malignancies, including multiple myeloma (MM). Nonetheless, NK cells are subjected to a variety of cancer defense mechanisms, leading to impaired maturation, chemotaxis, target recognition, and killing. This review aims to summarize the available and most current knowledge about cancer-related impairment of NK cell function occurring in MM.

MAIT cells numbers and frequencies in patients with acute myeloid leukemia at diagnosis: association with cytogenetic profile and gene mutations

Harnessing or monitoring immune cells is actually a major topic in pre-clinical and clinical studies in acute myeloid leukemia (AML). Mucosal-Associated Invariant T cells (MAIT) constitute one of the largest subset of innate-like, cytotoxic T cell subsets in humans. Despite some papers suggesting a role for MAIT cells in cancer, their specific involvement remains unclear, especially in myeloid malignancies. This prospective monocentric study included 216 patients with a newly diagnosed AML. Circulating MAIT cells were quantified by flow cytometry at diagnosis and during intensive chemotherapy. We observed that circulating MAIT cells show a specific decline in AML patients at diagnosis compared to healthy donors. Post-induction monitored patients presented with a drastic drop in MAIT cell numbers, with recovery after one month. We also found correlation between decrease in MAIT cells number and adverse cytogenetic profile. FLT3-ITD and IDH ½ mutations were associated with higher MAIT cell numbers. Patients with high level of activated MAIT cells are under-represented within patients with a favorable cytogenetic profile, and over-represented among patients with IDH1 mutations or bi-allelic CEBPA mutations. We show for the first time that circulating MAIT cells are affected in newly diagnosed AML patients, suggesting a link between MAIT cells and AML progression. Our work fosters new studies to deepen our knowledge about the role of MAIT cells in cancer.

NK Cells in Myeloproliferative Neoplasms (MPN)

Simple Summary

NK cells are important innate immune effectors that contribute substantially to tumor control, however the role of NK cells in haematological cancers is not as well understood. The aim of this review is to highlight the importance of the role of NK cells in the management of Ph+ Myeloproliferative Neoplasms, and emphasize the need and possible benefits of a more in-depth investigation into their role in classical MPNs and show potential strategies to harness the anti-tumoral capacities of NK cells.

Abstract

Myeloproliferative neoplasms (MPNs) comprise a heterogenous group of hematologic neoplasms which are divided into Philadelphia positive (Ph+), and Philadelphia negative (Ph−) or classical MPNs. A variety of immunological factors including inflammatory, as well as immunomodulatory processes, closely interact with the disease phenotypes in MPNs. NK cells are important innate immune effectors and substantially contribute to tumor control. Changes to the absolute and proportionate numbers of NK cell, as well as phenotypical and functional alterations are seen in MPNs. In addition to the disease itself, a variety of therapeutic options in MPNs may modify NK cell characteristics. Reports of suppressive effects of MPN treatment strategies on NK cell activity have led to intensive investigations into the respective compounds, to elucidate the possible negative effects of MPN therapy on control of the leukemic clones. We hereby review the available literature on NK cells in Ph+ and Ph− MPNs and summarize today’s knowledge on disease-related alterations in this cell compartment with particular focus on known therapy-associated changes. Furthermore, we critically evaluate conflicting data with possible implications for future projects. We also aim to highlight the relevance of full NK cell functionality for disease control in MPNs and the importance of considering specific changes related to therapy in order to avoid suppressive effects on immune surveillance.

The role of KIR positive NK cells in diseases and its importance in clinical intervention

Natural killer (NK) cells are essential for the elimination of the transformed and cancerous cells. Killer cell immunoglobulin-like receptors (KIRs) which expressed by T and NK cells, are key regulator of NK cell function. The KIR and their ligands, MHC class I (HLA-A, B and C) molecules, are highly polymorphic and their related genes are located on 19 q13.4 and 6 q21.3 chromosomes, respectively. It is clear that particular interaction between the KIRs and their related ligands can influence on the prevalence, progression and outcome of several diseases, like complications of pregnancy, viral infection, autoimmune diseases, and hematological malignancies. The mechanisms of immune signaling in particular NK cells involvement in causing pathological conditions are not completely understood yet. Therefore, better understanding of the molecular mechanism of KIR-MHC class I interaction could facilitate the treatment strategy of diseases. The present review focused on the main characteristics and functional details of various KIR and their combination with related ligands in diseases and also highlights ongoing efforts to manipulate the key checkpoints in NK cell-based immunotherapy.

NK and NKT Cell-Mediated Immune Surveillance against Hematological Malignancies

Recent cancer treatment modalities have been intensively focused on immunotherapy. The success of chimeric antigen receptor T cell therapy for treatment of refractory B cell acute lymphoblastic leukemia has pushed forward research on hematological malignancies. Among the effector types of innate lymphocytes, natural killer (NK) cells show great importance in immune surveillance against infectious and tumor diseases. Particularly, the role of NK cells has been argued in either elimination of target tumor cells or escape of tumor cells from immune surveillance. Therefore, an NK cell activation approach has been explored. Recent findings demonstrate that invariant natural killer T (iNKT) cells capable of producing IFN-γ when optimally activated can promptly trigger NK cells. Here, we review the role of NKT and/or NK cells and their interaction in anti-tumor responses by highlighting how innate immune cells recognize tumors, exert effector functions, and amplify adaptive immune responses. In addition, we discuss these innate lymphocytes in hematological disorders, particularly multiple myeloma and acute myeloid leukemia. The immune balance at different stages of both diseases is explored in light of disease progression. Various types of innate immunity-mediated therapeutic approaches, recent advances in clinical immunotherapies, and iNKT-mediated cancer immunotherapy as next-generation immunotherapy are then discussed.

Natural Killer Cells in Myeloid Malignancies: Immune Surveillance, NK Cell Dysfunction, and Pharmacological Opportunities to Bolster the Endogenous NK Cells

Natural killer (NK) cells are large granular lymphocytes involved in our defense against certain virus-infected and malignant cells. In contrast to T cells, NK cells elicit rapid anti-tumor responses based on signals from activating and inhibitory cell surface receptors. They also lyse target cells via antibody-dependent cellular cytotoxicity, a critical mode of action of several therapeutic antibodies used to treat cancer. A body of evidence shows that NK cells can exhibit potent anti-tumor activity against chronic myeloid leukemia (CML), acute myeloid leukemia (AML), and myelodysplastic syndromes (MDS). However, disease-associated mechanisms often restrain the proper functions of endogenous NK cells, leading to inadequate tumor control and risk for disease progression. Although allogeneic NK cells can prevent leukemia relapse in certain settings of stem cell transplantation, not all patients are eligible for this type of therapy. Moreover, remissions induced by adoptively infused NK cells are only transient and require subsequent therapy to maintain durable responses. Hence, new strategies are needed to trigger full and durable anti-leukemia responses by NK cells in patients with myeloid malignancies. To achieve this, we need to better understand the interplay between the malignant cells, their microenvironment, and the NK cells. This review focuses on mechanisms that are involved in suppressing NK cells in patients with myeloid leukemia and MDS, and means to restore their full anti-tumor potential. It also discusses novel molecular targets and approaches, such as bi- and tri-specific antibodies and immune checkpoint inhibitors, to redirect and/or unleash the NK cells against the leukemic cells.

Flip-flops of natural killer cells in autoimmune diseases versus cancers: Immunologic axis

Natural killer (NK) cells play an essential role in the immune response to infections, inflammations, and malignancies. Recent studies suggest that NK cell surface receptors and cytokines are the key points of the disease development and protection. We hypothesized that the interactions between NK cell receptors and targeted cells construct an eventual niche, and this niche has an eventual profile in various autoimmune diseases and cancers. The NK cells preactivated with cytokines, such as interleukin-2 (IL-2), IL-12, IL-15, and IL-18 can have higher cytotoxicity; however, the toxic side effect of IL-2 should be considered. The vicissitudes of NK cell profile and its receptors obey the environmental communications and cell interactions. Our vision around the NK cells as an immune axis remained dual, and we still cannot judge the immune responses based on the NK cell flip-flop. A design of eventual niche to monitor the NK cell and targeted cell interaction is needed to strengthen our ability in diagnosis and treatment approaches based on the NK cells. Here, we have reviewed the shifts in the NK cells and their surface receptors in autoimmune diseases, solid tumors, and leukemia, and also discussed the effective chemokines that affect NK cell activation and proliferation. The main aim of this review is to present a broader vision of the NK cell changes in autoimmune disease and cancers.

A Pilot Trial of Lirilumab With or Without Azacitidine for Patients With Myelodysplastic Syndrome

BACKGROUND

Enhancement of natural killer cell activity by blocking interactions between killer immunoglobulin (Ig)-like receptors (KIRs) and human leukocyte antigen-C (HLA-C) molecules can improve outcomes in myeloid malignancies. Lirilumab is a human IgG4 monoclonal antibody that blocks KIR/HLA-C interaction. We designed a study to evaluate the safety and efficacy of lirilumab as a single agent and in combination with azacitidine in patients with myelodysplastic syndrome (MDS).

PATIENTS AND METHODS

Adult patients with MDS who had not received previous hypomethylating agents were included. Lower-risk MDS patients received single-agent lirilumab (3 mg/kg); higher-risk patients received azacitidine (75 mg/m²/day for 7 days) in combination with lirilumab (3 mg/kg, on day 7), in a 28-day cycle. Responses were evaluated according to 2006 International Working Group criteria.

RESULTS

A total of 10 patients including 8 with higher and 2 with lower-risk enrolled. The median age was 70 (range, 50-84) years and 4 (40%) had complex cytogenetics. Baseline molecular mutations included TP53 (n = 5), TET2 (n = 3), and NRAS (n = 2). Patients received a median of 4 (range, 2-13) and 9 (range, 5-14) cycles of treatment with azacitidine with lirilumab and single-agent lirilumab, respectively. Two patients achieved complete remission (CR), 5 marrow CR, and 3 had stable disease. The median event-free survival for the entire cohort was 8 months (95% confidence interval, 4 months to not reached), and the median overall survival has not yet been reached. Five patients experienced 8 episodes of Grade ≥3 adverse events attributable to study drug, with the most frequent being infection or neutropenic fever (75%).

CONCLUSION

Lirilumab either as a single agent as well as used in combination with azacitidine has clinical activity in patients with MDS. Further studies are needed to confirm our findings.

Engineering Natural Killer Cells for Cancer Immunotherapy

The past several years have seen tremendous advances in the engineering of immune effector cells as therapy for cancer. While chimeric antigen receptors (CARs) have been used extensively to redirect the specificity of autologous T cells against hematological malignancies with striking clinical results, studies of CAR-modified natural killer (NK) cells have been largely preclinical. In this review, we focus on recent advances in NK cell engineering, particularly on preclinical evidence suggesting that NK cells may be as effective as T cells in recognizing and killing targets after genetic modification. We will discuss strategies to introduce CARs into both primary NK cells and NK cell lines in an effort to provide antigen specificity, the challenges of manufacturing engineered NK cells, and evidence supporting the effectiveness of this approach from preclinical and early-phase clinical studies using CAR-engineered NK cells. CAR-NK cells hold great promise as a novel cellular immunotherapy against refractory malignancies. Notably, NK cells can provide an "off-the-shelf" product, eliminating the need for a personalized and patient-specific product that plagues current CAR-T cell therapies. The ability to more potently direct NK cell-mediated cytotoxicity against refractory tumors through the expression of CAR is likely to contribute to the recent paradigm shift in cancer treatment.

Histamine H2 Receptor in Blood Cells: A Suitable Target for the Treatment of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) consists in a cancer of early hematopoietic cells arising in the bone marrow, most often of those cells that would turn into white blood cells (except lymphocytes). Chemotherapy is the treatment of choice for AML but one of the major complications is that current drugs are highly toxic and poorly tolerated. In general, treatment for AML consists of induction chemotherapy and post-remission therapy. If no further post-remission is given, almost all patients will eventually relapse. Histamine, acting at histamine type-2 (H₂) receptors on phagocytes and AML blast cells, helps prevent the production and release of oxygen-free radicals, thereby protecting NK and cytotoxic T cells. This protection allows immune-stimulating agents, such as interleukin-2 (IL-2), to activate cytotoxic cells more effectively, enhancing the killing of tumor cells. Based on this mechanism, post-remission therapy with histamine and IL-2 was found to significantly prevent relapse of AML. Alternatively, another potentially less toxic approach to treat AML employs drugs to induce differentiation of malignant cells. It is based on the assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment results in tumor reprogramming and the induction of terminal differentiation. There are promissory results showing that an elevated and sustained signaling through H₂ receptors is able to differentiate leukemia-derived cell lines, opening the door for the use of H₂ agonists for specific differentiation therapies. In both situations, histamine acting through H₂ receptors constitutes an eligible treatment to induce leukemic cell differentiation, improving combined therapies.

Killer-cell immunoglobulin-like receptor genes and ligands and their role in hematologic malignancies

Natural killer (NK) cells are considered crucial for the elimination of emerging tumor cells. Effector NK-cell functions are controlled by interactions of inhibitory and activating killer-cell immunoglobulin-like receptors (KIRs) on NK cells with human leukocyte antigen (HLA) class I ligands on target cells. KIR and HLA are highly polymorphic genetic systems segregating independently, creating a great diversity in KIR/HLA gene profiles in different individuals. There is an increasing evidence supporting the relevance of KIR and HLA ligand gene background for the occurrence and outcome of certain cancers. However, the data are still controversial and the mechanisms of receptor-ligand mediated NK-cell action remain unclear. Here, the main characteristics and functions of KIRs and their HLA class I ligands are reviewed. In addition, we review the HLA and KIR correlations with different hematological malignancies and discuss our current understanding of the biological significance and mechanisms underlying these associations.

The TGF-β/SMAD pathway is an important mechanism for NK cell immune evasion in childhood B-acute lymphoblastic leukemia

Natural killer (NK) cells are key components of the innate immune system, providing potent antitumor immunity. Here, we show that the tumor growth factor-β (TGF-β)/SMAD signaling pathway is an important mechanism for NK cell immune evasion in childhood B-acute lymphoblastic leukemia (ALL). We characterized NK cells in 50 consecutive children with B-ALL at diagnosis, end induction and during maintenance therapy compared with age-matched controls. ALL-NK cells at diagnosis had an inhibitory phenotype associated with impaired function, most notably interferon-γ production and cytotoxicity. By maintenance therapy, these phenotypic and functional abnormalities partially normalized; however, cytotoxicity against autologous blasts remained impaired. We identified ALL-derived TGF-β1 to be an important mediator of leukemia-induced NK cell dysfunction. The TGF-β/SMAD signaling pathway was constitutively activated in ALL-NK cells at diagnosis and end induction when compared with healthy controls and patients during maintenance therapy. Culture of ALL blasts with healthy NK cells induced NK dysfunction and an inhibitory phenotype, mediated by activation of the TGF-β/SMAD signaling pathway, and abrogated by blocking TGF-β. These data indicate that by regulating the TGF-β/SMAD pathway, ALL blasts induce changes in NK cells to evade innate immune surveillance, thus highlighting the importance of developing novel therapies to target this inhibitory pathway and restore antileukemic cytotoxicity.

The Application of Natural Killer Cell Immunotherapy for the Treatment of Cancer

Natural killer (NK) cells are essential components of the innate immune system and play a critical role in host immunity against cancer. Recent progress in our understanding of NK cell immunobiology has paved the way for novel NK cell-based therapeutic strategies for the treatment of cancer. In this review, we will focus on recent advances in the field of NK cell immunotherapy, including augmentation of antibody-dependent cellular cytotoxicity, manipulation of receptor-mediated activation, and adoptive immunotherapy with ex vivo-expanded, chimeric antigen receptor (CAR)-engineered, or engager-modified NK cells. In contrast to T lymphocytes, donor NK cells do not attack non-hematopoietic tissues, suggesting that an NK-mediated antitumor effect can be achieved in the absence of graft-vs.-host disease. Despite reports of clinical efficacy, a number of factors limit the application of NK cell immunotherapy for the treatment of cancer, such as the failure of infused NK cells to expand and persist in vivo. Therefore, efforts to enhance the therapeutic benefit of NK cell-based immunotherapy by developing strategies to manipulate the NK cell product, host factors, and tumor targets are the subject of intense research. In the preclinical setting, genetic engineering of NK cells to express CARs to redirect their antitumor specificity has shown significant promise. Given the short lifespan and potent cytolytic function of mature NK cells, they are attractive candidate effector cells to express CARs for adoptive immunotherapies. Another innovative approach to redirect NK cytotoxicity towards tumor cells is to create either bispecific or trispecific antibodies, thus augmenting cytotoxicity against tumor-associated antigens. These are exciting times for the study of NK cells; with recent advances in the field of NK cell biology and translational research, it is likely that NK cell immunotherapy will move to the forefront of cancer immunotherapy over the next few years.

Leukemia-induced phenotypic and functional defects in natural killer cells predict failure to achieve remission in acute myeloid leukemia

The majority of patients with acute myeloid leukemia will relapse, and older patients often fail to achieve remission with induction chemotherapy. We explored the possibility that leukemic suppression of innate immunity might contribute to treatment failure. Natural killer cell phenotype and function was measured in 32 consecutive acute myeloid leukemia patients at presentation, including 12 achieving complete remission. Compared to 15 healthy age-matched controls, natural killer cells from acute myeloid leukemia patients were abnormal at presentation, with downregulation of the activating receptor NKp46 (P=0.007) and upregulation of the inhibitory receptor NKG2A (P=0.04). Natural killer cells from acute myeloid leukemia patients had impaired effector function against autologous blasts and K562 targets, with significantly reduced CD107a degranulation, TNF-α and IFN-γ production. Failure to achieve remission was associated with NKG2A overexpression and reduced TNF-α production. These phenotypic and functional abnormalities were partially restored in the 12 patients achieving remission. In vitro co-incubation of acute myeloid leukemia blasts with natural killer cells from healthy donors induced significant impairment in natural killer cell TNF-α and IFN-γ production (P=0.02 and P=0.01, respectively) against K562 targets and a trend to reduced CD107a degranulation (P=0.07). Under transwell conditions, the inhibitory effect of AML blasts on NK cytotoxicity and effector function was still present, and this inhibitory effect was primarily mediated by IL-10. These results suggest that acute myeloid leukemia blasts induce long-lasting changes in natural killer cells, impairing their effector function and reducing the competence of the innate immune system, favoring leukemia survival.

Emerging natural killer cell immunotherapies: large-scale ex vivo production of highly potent anticancer effectors

Natural killer (NK) cell therapies are emerging worldwide as promising anticancer treatments, exploiting the fast cytolytic action of NK effectors and their potentially broad applicability against a wide range of malignancies. Until recently, clinical protocols have mainly involved freshly isolated NK cells or short- term activated NK cells or lymphokine-activated killer (LAK) cells. However, overall effector numbers and their anticancer potencies remained restricted, which poses a limiting factor to clinical efficacy. Recent developments in the field aim to improve clinical trial designs by increasing effector to target cell ratios in vivo and by application of superior cytotoxic NK effectors. Large-scale production of clinical grade NK cells through long-term activation in ex vivo cultures are another novel means in achieving these goals. However, such procedures require compliance with the strict Good Manufacturing Practice (GMP) regulations to ensure quality and safety of the NK cell product. Although the overall number of new protocols still remains comparably low, some of the protocols are already translated into clinical use. Also striking is the diversity of the different protocols proposed. We highlight in this review the most recent developments in the NK cell field with a focus on long-term NK cell expansion. Critical issues relating to this novel and promising type of therapy are highlighted and discussed.

Primary human tumor cells expressing CD155 impair tumor targeting by down-regulating DNAM-1 on NK cells

The activating NK cell receptor DNAX accessory molecule-1 (DNAM-1) contributes to tumor immune surveillance and plays a crucial role in NK cell-mediated recognition of several types of human tumors, including ovarian carcinoma. Here, we have analyzed the receptor repertoire and functional integrity of NK cells in peritoneal effusions from patients with ovarian carcinoma. Relative to autologous peripheral blood NK cells, tumor-associated NK cells expressed reduced levels of the DNAM-1, 2B4, and CD16 receptors and were hyporesponsive to HLA class I-deficient K562 cells and to coactivation via DNAM-1 and 2B4. Moreover, tumor-associated NK cells were also refractory to CD16 receptor stimulation, resulting in diminished Ab-dependent cellular cytotoxicity against autologous tumor cells. Coincubation of NK cells with ovarian carcinoma cells expressing the DNAM-1 ligand CD155 led to reduction of DNAM-1 expression. Therefore, NK cell-mediated rejection of ovarian carcinoma may be limited by perturbed DNAM-1 expression on tumor-associated NK cells induced by chronic ligand exposure. Thus, these data support the notion that tumor-induced alterations of activating NK cell receptor expression may hamper immune surveillance and promote tumor progression.

Natural killer cell-based immunotherapy in cancer: current insights and future prospects

As our understanding of the molecular mechanisms governing natural killer (NK) cell activity increases, their potential in cancer immunotherapy is growing increasingly prominent. This review analyses the currently available preclinical and clinical data regarding NK cell-based immunotherapeutic approaches in cancer starting from a historical background and an overview of molecular mechanisms taking part in NK cell responses. The status of NK cells in cancer patients, currently investigated clinical applications such as in vivo modulation of NK cell activity, ex vivo purification/expansion and adoptive transfer as well as future possibilities such as genetic modifications are discussed in detail.

Low dose interleukin-2 following intensification therapy with high dose cytarabine for acute myelogenous leukemia in first complete remission

The most important problem in the therapy of patients with acute myeloid leukemia (AML) is relapse after intensive therapy. We sought to determine if interleukin-2 (low-dose with intermittent boluses) administration could be feasibly administered after standard therapy to potentiate anti-tumor immunity in a fashion analogous to the post-allogeneic stem cell transplant "graft-vs-leukemic" effect. Adults with de novo AML received daunorubicin and cytosine arabinoside induction therapy. Patients achieving complete remission received high dose ara-C (HIDAC) for three courses followed by low dose rIL-2 (Amgen), administered by continuous infusion (450,000 U/m(2)/day) for 10 weeks with intermittent boluses (500,000/U/m(2) over 2 hr) given in weekly intervals starting on Week 4. Of the 32 enrolled patients, 27 achieved CR; 8/11 who received rIL-2 completed therapy. 6/11 are long term survivors (median follow-up, 139 months). rIL-2 was well tolerated and associated with a 5-fold increase in circulating NK-lymphocytes and a 3-fold increase in circulating T-cells. Mononuclear cells from patients receiving rIL-2 exhibited enhanced cytolytic activity in vitro against cryopreserved autologous leukemia cells. This study supports further investigation of immunotherapy in the post-intensive chemotherapy setting in the management of patients with AML.

Low-dose interleukin-2 immunotherapy does not improve outcome of patients age 60 years and older with acute myeloid leukemia in first complete remission: Cancer and Leukemia Group B Study 9720

PURPOSE

Cancer and Leukemia Group B (CALGB) 9720 evaluated subcutaneous low-dose recombinant interleukin-2 (rIL-2) maintenance immunotherapy as a strategy for prolonging remission in older patients with acute myeloid leukemia (AML).

PATIENTS AND METHODS

AML patients age 60 years and older in first complete remission after induction and consolidation chemotherapy were randomly assigned to no further therapy or a 90-day regimen of 14-day cycles of low-dose rIL-2, aimed at expanding natural killer (NK) cells, followed by 3-day higher doses aimed at activating cytotoxicity of expanded NK cells to lyse residual AML cells. All randomly assigned patients were included in an intention-to-treat analysis.

RESULTS

A total of 163 (64%) of 254 patients who completed induction and consolidation chemotherapy on CALGB 9720 were randomly assigned to rIL-2 (n = 81) or no further therapy (n = 82); the most common reasons for lack of random assignment were patient refusal and relapse. Fifteen patients randomly assigned to rIL-2 never initiated it because of refusal, intercurrent medical problems, or relapse, and 24 patients initiated rIL-2 but stopped early because of toxicity or relapse. Grade 4 toxicities during rIL-2 therapy included thrombocytopenia (65%) and neutropenia (64%), and grade 3 toxicities included anemia (33%), infection (24%) and malaise/fatigue (14%). Forty-two patients (52%) randomly assigned to rIL-2 completed the full 90-day course. Patients in both arms had similar distributions of both disease-free (combined median = 6.1 months; P = .47) and overall survival (combined median = 14.7 months; P = .61) after random assignment. Moreover, the 42 patients who completed all planned therapy did not show prolongation of disease-free or overall survival.

CONCLUSION

Low-dose rIL-2 maintenance immunotherapy is not a successful strategy in older AML patients.

Role of cytokines in the treatment of acute leukemias: a review

Myeloid growth factors, such as granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor, have been used to decrease the duration of chemotherapy-induced neutropenia and thereby reduce the incidence and severity of infections in various regimens used to treat acute myeloid leukemia and acute lymphoblastic leukemia. These growth factors have also been used to recruit dormant myeloid leukemia cells into the S phase of cell cycle in order to increase their susceptibility to the antileukemic effects of agents such as cytarabine. Multiple prospective randomized trials have examined the benefit and safety of the addition of growth factors before, during, and after chemotherapy. A reduction in the duration of neutropenia has been the most consistent finding; this has not been associated with stimulation of leukemia cells, the main concern of using this strategy. Unfortunately, few studies have reported a benefit in prolonging the duration of disease-free survival or overall survival. Other cytokines, including interleukins and thrombopoietin, have also been evaluated for their theoretical ability to recruit immune mechanisms to eradicate residual leukemia burden after chemotherapy, and to stimulate platelet production. In this review, we summarize the clinical experience with these growth factors in treating acute leukemias.

Identification of natural killer cell receptor phenotypes associated with leukemia

Natural killer (NK) cells play a key role in defense against tumor cells that have the capacity to downregulate human leukocyte antigen (HLA) class I expression. It has been reported that leukemic cells can have downregulated expression of HLA class I molecules. The polymorphic nature of NK cell receptor (NKR) genes generates diverse repertoires in the human population, which display specificity in the innate immune response. In the present study, 11 KIR and two CD94/NKG2 receptors were genotyped by PCR-SSP in 96 leukemic patients and 148 healthy Caucasians. Here, we report a significant increased frequency of the more inhibitory AB killer cell immunoglobulin-like receptor (KIR) phenotype in leukemic patients compared to the controls (31.1% in healthy controls vs 51.0% in leukemic patients, Pc=0.002), which is related to the high prevalence of the inhibitory KIR2DL2 in this population (Pc=0.007). Moreover, two specific KIR phenotypes AB1 and AB9, including all inhibitory KIRs, were significantly associated with leukemic patients. Our study suggests that an important percentage of leukemic patients express a KIR phenotype in favor of escape from NK cell immunity.

Functional expression and release of ligands for the activating immunoreceptor NKG2D in leukemia

NKG2D ligands (NKG2DLs) mark malignant cells for recognition by natural killer (NK) cells and cytotoxic T lymphocytes via the activating immunoreceptor NKG2D. This led to the hypothesis that NKG2DLs play a critical role in tumor immune surveillance. The human NKG2DLs MICA and MICB are expressed on tumors of epithelial origin in vivo. For the other recently described set of human NKG2DLs, the UL16-binding proteins (ULBPs), expression in vivo is as yet undefined. In this study we investigated expression and function of NKG2DLs in leukemia using a panel of newly generated NKG2DL-specific monoclonal antibodies. We report that leukemia cells from patients variously express MIC and ULBP molecules on the cell surface with MICA most frequently detected. Patient leukemia cells expressing MICA were lysed by NK cells in an NKG2D-dependent fashion. Sera of patients, but not of healthy donors, contained elevated levels of soluble MICA (sMICA). We also detected increased sMICB levels in patient sera using a newly established MICB-specific enzyme-linked immunosorbent assay. Reduction of leukemia MIC surface expression by shedding may impair NKG2D-mediated immune surveillance of leukemias. In addition, determination of sMICA and sMICB levels may be implemented as a prognostic parameter in patients with hematopoietic malignancies.

Anti-myeloma activity of natural killer lymphocytes

Natural killer (NK) cells are assumed to contribute to a graft-versus-leukaemia effect. In vitro experiments have shown that many leukaemic cells are NK-cell sensitive. Nevertheless, no data concerning the influence of purified NK cells on malignant myeloma (MM) cells exist. We co-incubated NK cells with three different MM cell lines and fresh bone marrow samples of nine MM patients. The proportion of vital MM cells was determined before and after co-cultivation by a flow-cytometry-based assay. All MM cells tested, with the exception of one cell line (NCI H929), were susceptible to a NK-cell attack even without exogenous interleukin 2 (IL-2). The mean killing of the native MM samples was 23.1 +/- 5.4% and 34.5 +/- 6.5% at 10:1 and 20:1 effector:target ratio respectively, This corresponded to about 2/3 of those values obtained with the highly sensitive line K562. In contrast, CD34-positive haematopoietic stem cells as well as peripheral mononuclear cells were completely resistant under similar experimental conditions (1.3% killing). To elucidate the underlying triggering mechanisms, we measured human leucocyte antigen (HLA)-class I expression of the MM cells. No evidence for HLA loss, which could have explained the NK-cell recognition if it occurred, was demonstrated. These findings may contribute to the understanding of in vivo NK-cell activation and encourage clinical applications of NK cells for MM patients.

NK cell-mediated lysis of autologous human oligodendrocytes

Although considered an autoimmune disease, the mechanisms underlying oligodendrocyte (OL)/myelin injury in multiple sclerosis (MS) remain to be established. We utilized in vitro assays to demonstrate that human OLs, as well as other glial elements (astrocytes, microglia), were susceptible to injury mediated by peripheral blood-derived mononuclear cell preparations (MNCs) enriched for natural killer (NK cells) by depleting CD3(+) +/- CD19(+) cells through use of either magnetic beads or cell sorting. Cytotoxic effects of the NK cell-enriched effectors were dependent on pre-exposure of these cells to IL-2. Furthermore, we found that autologous OLs were as susceptible to injury mediated by IL-2 activated NK cells as were heterologous OLs. In context of the tissue injury that occurs in MS, our results suggest that the inflammatory milieu in MS lesions could provide conditions required for NK cell activation and that such effector cells can bypass the putative protective effects of self-MHC class I molecules that may be expressed on OLs.

Innovative therapy for chronic myelogenous leukemia

Innovative therapies for chronic myelogenous leukemia (CML) have focused mainly on combining autologous transplantation with another modality of therapy for purging of the graft or treatment of the patient after transplant. Of the three categories of innovative therapies, two are based on studies that demonstrate the bcr/abl gene rearrangement in the pathogenesis of CML, whereas the third is based on the observation that allogeneic disparity is important to maintain remissions in CML. The rationale and data supporting these innovative approaches are reviewed in this article and future strategies are discussed.

Influence of malignant cell clonogenic capacities and position along the maturation pathway on their susceptibility to lymphokine-activated killer cell cytotoxicity

In order to investigate the sensitivity of malignant target cells to lysis by LAK cells according to their clonogenic capacities and their position along the maturation pathway, we compared clonogenic and chromium release cytotoxicity assays performed on human hematopoietic cell lines using Effector: Target ratios of 1:1, 3:1, 6:1, 12:1, 24:1, 48:1 and 96:1, and studied the sensitivity of HL-60 and U937 human cell lines after exposure to different factors including GM-CSF, SCF, IFN, Retinoic acid (RA), DMSO, and TPA which are able to recruit cells into the cell cycle or to induce cell differentiation. There was a good correlation between the lysis of the target cells using 51Cr release and the growth inhibition in semisolid medium. The degree of inhibition was significantly higher using the colony growth assay (p = 0.006). Regarding the effects of culturing cell lines with proliferating and differentiating agents on the sensitivity of these cell lines to LAK cytolysis, a correlation was noted between the proliferative response of the U937 cell line and susceptibility to LAK cell lysis (p = 0.01), while results appeared close to significance with HL-60. The most significant effects were a decreased sensitivity of HL-60 to LAK lysis with RA (p < 0.001) and TPA (p < 0.001), and an increased susceptibility of U937 to LAK lysis with GM-CSF (p < 0.0001). In studies planned to investigate whether susceptibility of treated cells to LAK activity was a consequence of a downregulation of adhesion molecules expressed on target cell surface, the proportion of cells expressing adhesion molecules was not significantly changed, except for CD54 expression on HL-60 cells which showed a higher expression, after cells were treated with RA or DMSO. We conclude that clonogenic cells are more sensitive to LAK cell lysis and that cell line sensitivity to LAK cytolysis can be modulated by a variety of agents of potential therapeutic use. The poor correlation between adhesion molecules expression and sensitivity to LAK lysis suggests that molecules other than CD54, CD56, CD58, and CD106 may possibly be more central to the processes involved.

The role of autologous natural killer cells in chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is a lethal disease of the hematopoietic stem cell. Bone marrow transplantation has highlighted the importance of allogeneic disparity in maintaining remissions in CML. However, it has been unclear whether the immune effect against CML is mediated by T cells, natural killer cells (NK) or a combination of both. We have previously demonstrated that autologous activated NK are capable of selectively lysing malignant CML progenitors while sparing benign progenitors. NK effectors may play an important role in CML since NK lytic function, clonogenic frequency and proliferative capacity decrease as CML progresses from chronic phase to advanced phase and blast crisis. Incubation of CML NK with IL-2 is capable of restoring cytolytic activity to normal levels. We hypothesize that activated NK represent a potential therapy against CML to maintain remissions in a minimal residual disease setting induced by autologous transplantation. Clinical trials are in progress to test whether IL-2 based immunotherapy and activated cell infusions play a therapeutic role in CML.

Cellular immunotherapy and autologous transplantation for hematologic malignancy

The success of allogeneic transplantation is in part due to the immunotherapeutic effect mediated by the graft. Autologous transplantation is hampered by the absence of this effect, leading to a higher relapse rate. We have conducted a series of studies designed to augment the immunologic activity of the graft after autologous transplant with a view towards introducing an autologous graft-versus-tumor effect that could decrease the rate of relapse after autologous transplant. These studies have included IL-2 activation of marrow followed by post-transplant infusional IL-2, the development of a novel protocol for the generation of highly efficient cytotoxic effector cells, termed cytokine-induced killer (CIK) cells, with broad and potent antitumor activity. In order to determine the potential for generating peptide-specific cytolytic T cells, studies have been conducted upon transducing antigen-presenting cells (APC) with AAV vector-mediated gene transfer, a vector capable of transducing non-proliferating target cells. Transduction of human monocytes and macrophages resulted in high expression of the transduced gene. This latter study forms the basis for determining whether genetic modification of APC can potentiate specific immune responses to tumor-specific gene products. Taken together, these strategies will hopefully increase the therapeutic efficacy of autologous transplantation.

Interleukin-2 for the treatment of advanced acute myelogenous leukemia patients with limited disease: updated experience with 20 cases

Since 1988 we have treated a first group of 14 patients with recombinant interleukin-2 (rIL-2), which was previously published, and 6 other consecutive patients affected by refractory or relapsed acute myelogenous leukemia (AML) with >5% and < or = 30% bone marrow blasts, but not suitable for further chemotherapy. The rIL-2 schedule consisted of four 5-day high-dose cycles administered by continuous infusion with a 72-hour rest period between each cycle. Patients who achieved a response received a lower dose of subcutaneous rIL-2 maintenance treatment administered for 5 days every month. Following high-dose rIL-2, 11/20 patients (55%) obtained a complete remission (CR). Six remain in persistent CR after a median follow-up time of 50 months (9, 33, 49, 51, 52, 87 months, respectively); the length of remission is the longest in the natural history of the disease for each individual patient. One patient with stable disease at the end of rIL-2 induction is alive and well, with a stable number of blasts in the bone marrow, 18 months later. These 7 patients continue maintenance treatment with subcutaneous rIL-2. Close clinical and laboratory monitoring reveal that side effects are acceptable and no toxic deaths have been recorded. This update confirms the feasibility and antileukemic activity of high dose rIL-2 in advanced AML patients with limited disease, and suggests a potential clinical role of prolonged rIL-2 maintenance treatment.

Interaction of acute leukemia cells with the bone marrow microenvironment: implications for control of minimal residual disease

There is increasing evidence for an interaction between acute leukemia cells and the microenvironment of the bone marrow. Blast cells from cases of acute myeloid leukemia (AML) and acute lymphoblastic leukemia (ALL) bind to cellular and extracellular matrix components of the bone marrow stroma. In AML, adhesion to stroma is mediated by the combined action of beta 1 (principally VLA-4) and beta 2 integrins, while in precursor-B ALL VLA-4 and VLA-5 integrins play a major role. Adhesion molecules such as CD31, CD44, non-beta 1, beta 2 integrins, growth factor receptors such as c-kit, and other molecules are also likely to play a role. Binding of acute leukemia blasts to ligands on stroma has several pathophysiological consequences. Stromal contact is able to inhibit programmed cell death (apoptosis) in a proportion of cases of both AML and ALL. In ALL, diffusible molecules derived from stroma appear to contribute. Marrow stroma also plays a part in regulating leukemic cell proliferation. While this is partly due to stromal production of hemopoietic growth factors, in soluble or transmembrane form or bound to extracellular matrix, signalling mediated directly by binding of adhesion molecules on leukemic cells may also have a role. Contact of ALL blasts with marrow fibroblasts is followed by migration of leukemic cells, utilizing VLA-4 and VLA-5 integrins, potentially allowing homing of blasts to favourable microenvironmental sites, or controlling egress into the circulation. AML cells compete for stromal binding sites with natural killer cells and cytotoxic lymphocytes, which are known to inhibit their clonogenic growth. We speculate that these complex interactions between leukemic blasts, cellular and matrix components of stroma, and cytotoxic lymphocytes, play a critical role in determining the fate of small numbers of leukemic cells surviving after cytotoxic chemotherapy.

Is there a place for immunotherapy with interleukin-2 to prevent relapse after autologous stem cell transplantation for acute leukemia?

Chemotherapy-resistant cells cause disease recurrence in a significant proportion of patients with acute leukemia treated with autologous stem cell transplantation due to the lack of immune-mediated effects which contribute significantly to the prevention of post-treatment disease recurrence. This conclusion is based on the observation that relapse after high dose chemotherapy supported by a stem cell transplant from a twin donor is 3-4 times higher than after transplant from an allogeneic donor. This anti-leukemic mechanism of transplanted donor cells has been termed graft-versus-leukemia (GVL) effect, and efforts are being directed toward utilizing such an immune-mechanism after autologous transplantation. Since interleukin-2 (IL-2) can induce remissions in selected patients with advanced leukemia, it has become a candidate cytokine to be used in attempts to introduce GVL after autologous stem cell transplantation. Here we review the available clinical data with IL-2 and critically evaluate whether IL-2 has a place as adjunct treatment to prevent relapse after autologous transplantation for acute leukemia.

Interleukin-2 and gene therapy in the management of acute lymphoblastic leukaemia

The overall unfavourable prognosis of adult acute leukaemia patients has prompted the search for alternative therapeutic strategies. Probably the most sought challenge, which over the years has been met by consistent disillusion, has been immunotherapy. With little doubt the goal of stimulating the immune system of the host in the hope of controlling or eradicating residual disease following more conventional ablative regimens, remains conceptually a highly desirable approach. During the last few years an innovative strategy, based on the in vitro demonstration that IL2 is capable of inducing a previously unrecognized cytotoxic function directed against primary tumours and named LAK, has been applied with some success in solid tumour patients. Here, we shall review the pre-clinical data which indicate that IL2-based immunotherapy may be employed also in the management of patients with acute leukaemia. Clinical data which support a possible in vivo antileukaemic effect of IL2 are presented. The clinicohaematological modifications, as well as the biological modulations induced in the patients following the administration of IL2 are also discussed. In view of the recent demonstration that the IL2 gene can be successfully transduced into human neoplastic cells, we finally discuss the rationale of gene transfer approaches in an attempt to overcome some of the limitations associated with the administration of high doses of exogenous IL2.

Expansion and activation of human natural killer cells for autologous therapy

Ex vivo depletion of lymphocytes has been used to decrease the incidence of acute graft-versus-host disease following the allogeneic bone marrow transplantation. Many depletion techniques also remove natural killer (NK) cells that may mediate a beneficial graft-versus-leukemia effect. In this paper we review our studies on ex vivo expansion and activation of NK cells for use in the therapy of chronic myelogenous leukemia.

Induction of cytotoxic lymphocyte subsets against leukemia by stimulation with AML blasts

Although the application of interleukin-2 (IL-2) activated lymphocytes in immunotherapy of acute myelogenous leukemia (AML) is of therapeutic interest, the high resistance of AML blasts to lymphocyte lysis may represent an obstacle to this type of therapy. However, our data shows that the leukemia resistance can be conquered by concomitant culture of lymphocytes with IL-2 and AML blasts. This approach induces not only leukemia-directed cytotoxic cells, but also promotes their growth. Additionally, multiple cytotoxic lymphocyte populations with leukemia lytic activity are induced in AML/IL-2 cultures. These include natural killer (NK) cells and subsets of T cells with both the major histocompatibility complex (MHC)-restricted and MHC-nonrestricted cytotoxic function. Thus, this protocol, which is conducive to general stimulation of cellular immune responses against leukemia, may enhance the benefits of lymphocyte therapy.

Endogenous lymphokine activated killer cell activity and cytogenetic response in chronic myelogenous leukaemia treated with alpha-interferon

The capacity of alpha-interferon (alpha-IFN) to induce lymphokine activated killer (LAK) cytotoxicity in the absence of interleukin-2 (IL2) has prompted us to test whether or not its ability to reduce dramatically the number of Ph1+ clones in chronic myelogenous leukaemia (CML) patients is in part mediated through the generation of natural killer (NK) or LAK activity. The latter were tested using NK-sensitive (K562) and NK-resistant (Raji) cell lines in a target-cell colony-growth inhibition assay. Effector cells (E) were patient blood mononuclear cells (MC) without in vitro activation prior to their coculture with targets (T). Out of 16 patients tested so far, three failed to undergo cytogenetic remission under alpha-IFN therapy. No NK nor LAK cells could be detected in the MC from two of them while the other displayed NK activity within upper normal limits. 13 patients underwent complete (eight) or partial (five) cytogenetic remission together with significantly high NK and/or LAK activity as compared to normal controls. These observations could favour the hypothesis of an indirect effect of alpha-IFN on leukaemic cells, mediated by cells involved in immune surveillance.

Graft-versus-leukemia activity after bone marrow transplantation does not require graft-versus-host disease

Clinical data have suggested that graft-versus-host disease (GVHD) plays a crucial role in the antileukemic effects of bone marrow grafts. We investigated (a) whether bone marrow cells unable to induce GVHD can effect graft-versus-leukemia (GVL) activity and (b) whether such antileukemic capacity depends on the presence of T lymphocytes in the graft. Balb/c mice were inoculated with A20 cells, a B-cell lymphoma/leukemia of Balb/c origin. Four weeks after tumor inoculation the animals were lethally irradiated and received a bone marrow graft. Cells from (Balb/c x C57) F1 or (C3H x Balb/c) F1 hybrids were transplanted into parental-strain Balb/c mice. Since lymphocytes from F1 hybrids are unable to cause graft-versus-host reactivity against a parental-strain animal, we used this experimental setting to explore GVL effects in a GVHD-free system. In vitro incubation with monoclonal anti-Thy-1.2 antibody plus complement was used to eliminate Thy-1+ cells. After syngeneic transplantation, the death rate due to leukemia remained unchanged (91%) compared with that among untreated animals (86%). Following transplantation of F1 marrow cells of either (C57 x Balb/c) F1 or (C3H x Balb/c) F1 origin, death rates of 40% and 50% were observed; these were significantly lower. Depletion of Thy 1+ cells from bone marrow graft caused only a slight increase in the leukemic death rate after transplantation of bone marrow of (C57 x Balb/c) F1 hybrid origin (50%), but a high leukemic death rate was seen after transplantation of (C3H x Balb/c) F1 bone marrow (100%). Additional experiments with fully allogeneic, T-cell-depleted C57 bone marrow transplantation suggest an antileukemic effect that is comparable to that seen after transplantation of unmanipulated F1 bone marrow. Taken together, our results indicate that GVL activity can be dissociated from graft-versus-host reaction.

Role of interleukin-2 activated MHC-nonrestricted lymphocytes in antileukemia activity and therapy

Although the application of biological therapy for solid tumors with cytokines and adoptively transferred activated lymphocytes has received substantial attention, this approach has not been actively explored in treatment of hematopoietic neoplasms. This review will address the possibilities of interleukin-2 (IL-2) and IL-2 activated natural killer (NK) cells and T cells in antileukemia reactivity and therapy. The new approaches to optimal activation and generation of oncolytic cells, selective propagation of lymphocyte subsets, and the role of adhesion molecules in antileukemia cytotoxicity will also be addressed. We trust that this article will be conducive to the development of new directions in leukemia research and treatment.

Treatment of acute myeloid leukaemia patients with recombinant interleukin 2: a pilot study

Twelve patients with acute myeloid leukaemia (AML) with evidence of resistant disease were treated with recombinant interleukin 2 (rIL2) given intravenously by continuous infusion. No objective response to rIL2 alone was documented in the seven patients with advanced disease (20-90% resistant blasts in the marrow), except for a partial response to rIL2 plus chemotherapy in one. Of the five patients with limited disease (8-15% marrow blasts), three obtained a complete disappearance of the blasts following two to four 5d courses of rIL2 alone. One patient persists in fourth complete remission (CR) 30 months later, another obtained a third CR for 4 months, and the last remained in third CR for 9 months before relapsing. This latter patient achieved a fourth CR with low-dose cytarabine. The remissions have been maintained with low-dose monthly courses of rIL2 given on an out-patient basis. Two AML did not respond to rIL2 alone; one, however, obtained a fourth CR with chemotherapy and rIL2. Administration of rIL2 was accompanied by organomegaly and leucocytosis, with a frequent lymphocytosis and increase in eosinophils and large granular lymphocytes, both in the blood and in the marrow. Side effects, though often severe, were controllable using a daily dose escalating protocol and never required intensive care treatment. The results of this pilot study indicate that treatment of AML patients with rIL2 is feasible and may result in the disappearance of chemotherapy-resistant blasts in patients with limited but detectable disease. Further controlled trials in AML in CR appear warranted.

Inducibility of lymphokine activated killer (LAK) cells in patients with acute myelogenous leukaemia in complete remission and its clinical relevance

Peripheral blood mononuclear cells (PBMC) from 42 patients with acute myelogenous leukaemia (AML) in complete remission (CR) and from normal donors were activated into LAK cells in the presence of 1000 U/ml of recombinant interleukin-2 (rIL-2). Cytotoxicity of LAK cells was assayed against K562, Daudi, and Raji cell lines, and autologous and/or allogeneic thawed leukaemic blasts. Fresh unactivated PBMC from normal donors and AML patients served as controls. Mean +/- standard deviation (SD) percentage lysis of the different targets by patient LAK cells were: K562 61 +/- 20%, Daudi 62 +/- 23%, Raji 48 +/- 24%, autologous blast cells 12 +/- 16% and allogeneic blast cells 13 +/- 10%. Lysis of the different targets by LAK cells from normal donors was similar to that achieved with LAK cells from AML patients. Overall there was a good correlation between the lysis of the different targets. There was no significant difference between the percentage lysis of autologous and allogeneic thawed blast cells, although LAK cells from seven out of the 18 patients tested were unable to lyse autologous leukaemic cells. Activity of patient LAK cells did not correlate with the initial characteristics of the patient nor with the time spent in CR before harvesting PBMC for activation. At the time of analysis, 32 patients were in continuing CR and 10 had relapsed. Multivariant analysis for prognostic factors showed that patients whose LAK cells had more lytic activity on K562 (P = 0.005) and fresh blast cell (P = 0.02) targets had significantly less risk of relapse than patients with little inducible LAK cell activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Production of tumour-derived suppressor factor in patients with acute myeloid leukaemia

In an attempt to investigate the underlying cause of impaired cellular cytotoxic functions in patients with acute myeloid leukaemia and the relative ineffectiveness of immunotherapy with recombinant interleukin 2 (IL-2), normal donor lymphocytes were incubated in AML sera and in supernatant of myeloblasts. There was significant inhibition of both the natural killer activity and the lectin dependent cellular cytotoxicity of the normal donor lymphocytes compared to when incubation took place in autologous or normal allogeneic sera or marrow supernatant. This inhibition was time-related and partially reversible by washing of the normal lymphocytes immediately before the cytotoxicity assay. The suppressor factor, however, did not inhibit the IL-2 induced lymphocyte proliferation or affect the cytotoxicity-linked cytoplasmic serine esterase expression in the normal lymphocytes. This suppressor phenomenon was of myeloblast origin. Chronic exposure to the tumour-derived suppressor factor may be responsible for the impaired cellular cytotoxic functions observed in patients with acute myeloid leukaemia. It may also suppress the in vivo cytotoxic functions of IL-2 activated lymphocytes in patients treated with recombinant IL-2, hence leading to the disappointing results of immunotherapy so often encountered in clinical setting.