Histamine and interleukin-2 in acute myelogenous leukemia

Histamine in cancer immunology and immunotherapy. Current status and new perspectives

Cancer is the second leading cause of death globally and its incidence and mortality are rapidly increasing worldwide. The dynamic interaction of immune cells and tumor cells determines the clinical outcome of cancer. Immunotherapy comes to the forefront of cancer treatments, resulting in impressive and durable responses but only in a fraction of patients. Thus, understanding the characteristics and profiles of immune cells in the tumor microenvironment (TME) is a necessary step to move forward in the design of new immunomodulatory strategies that can boost the immune system to fight cancer. Histamine produces a complex and fine-tuned regulation of the phenotype and functions of the different immune cells, participating in multiple regulatory responses of the innate and adaptive immunity. Considering the important actions of histamine-producing immune cells in the TME, in this review we first address the most important immunomodulatory roles of histamine and histamine receptors in the context of cancer development and progression. In addition, this review highlights the current progress and foundational developments in the field of cancer immunotherapy in combination with histamine and pharmacological compounds targeting histamine receptors.

Cell-based and antibody-mediated immunotherapies directed against leukemic stem cells in acute myeloid leukemia: Perspectives and open issues

Despite new insights in molecular features of leukemic cells and the availability of novel treatment approaches and drugs, acute myeloid leukemia (AML) remains a major clinical challenge. In fact, many patients with AML relapse after standard therapy and eventually die from progressive disease. The basic concept of leukemic stem cells (LSC) has been coined with the goal to decipher clonal architectures in various leukemia-models and to develop curative drug therapies by eliminating LSC. Indeed, during the past few years, various immunotherapies have been tested in AML, and several of these therapies follow the strategy to eliminate relevant leukemic subclones by introducing LSC-targeting antibodies or LSC-targeting immune cells. These therapies include, among others, new generations of LSC-eliminating antibody-constructs, checkpoint-targeting antibodies, bi-specific antibodies, and CAR-T or CAR-NK cell-based strategies. However, responses are often limited and/or transient which may be due to LSC resistance. Indeed, AML LSC exhibit multiple forms of resistance against various drugs and immunotherapies. An additional problems are treatment-induced myelotoxicity and other side effects. The current article provides a short overview of immunological targets expressed on LSC in AML. Moreover, cell-based therapies and immunotherapies tested in AML are discussed. Finally, the article provides an overview about LSC resistance and strategies to overcome resistance.

Immunotherapy-Based Targeting and Elimination of Leukemic Stem Cells in AML and CML

The concept of leukemic stem cells (LSC) has been developed with the idea to explain the clonal hierarchies and architectures in leukemia, and the more or less curative anti-neoplastic effects of various targeted drugs. It is now widely accepted that curative therapies must have the potential to eliminate or completely suppress LSC, as only these cells can restore and propagate the malignancy for unlimited time periods. Since LSC represent a minor cell fraction in the leukemic clone, little is known about their properties and target expression profiles. Over the past few years, several cell-specific immunotherapy concepts have been developed, including new generations of cell-targeting antibodies, antibody-toxin conjugates, bispecific antibodies, and CAR-T cell-based strategies. Whereas such concepts have been translated and may improve outcomes of therapy in certain lymphoid neoplasms and a few other malignancies, only little is known about immunological targets that are clinically relevant and can be employed to establish such therapies in myeloid neoplasms. In the current article, we provide an overview of the immunologically relevant molecular targets expressed on LSC in patients with acute myeloid leukemia (AML) and chronic myeloid leukemia (CML). In addition, we discuss the current status of antibody-based therapies in these malignancies, their mode of action, and successful examples from the field.

Histamine dihydrochloride for maintaining remission in acute myeloid leukemia

SUMMARY 

Maintaining complete remission in patients with acute myeloid leukemia (AML) is a great challenge. Targeting the remaining malignant clones, using different regimens of chemotherapeutic agents did not prove to be fruitful. Host residual leukemia stem cells may be eradicated by donor immune cells in case of allogeneic stem cell transplantation. However, only a minority of AML patients may benefit from this procedure. IL-2 is very well known in its ability to enhance activation and proliferation of natural killer and T cells, albeit in the clinical setting this effect is suppressed by myeloid cells derived oxygen species. Histamine dihydrochloride can inhibit myeloid cells activity, therefore reaugment antileukemic natural killer/T-cell cytotoxic activity. This review summarizes different strategies to maintain complete remission in AML patients focusing on the use of histamine dihydrochloride/IL-2 treatment.

Spotlight on histamine dihydrochloride in acute myeloid leukaemia

Histamine dihydrochloride (Ceplene®) is a synthetic derivative of the biogenic amine histamine. Histamine dihydrochloride inhibits the formation of reactive oxygen species that suppress the activation of T cells and natural killer (NK) cells. When given in addition to the cytokine interleukin (IL)-2, histamine dihydrochloride enables the activation of T cells and NK cells by IL-2, resulting in the killing of cancer cells, including those of acute myeloid leukaemia (AML). In a large, 3-year, randomized, open-label, multicentre, phase III trial in adult patients with AML in first or subsequent remission, those who received subcutaneous histamine dihydrochloride and concomitant subcutaneous IL-2 as maintenance therapy had a significantly longer leukaemia-free survival (LFS; primary endpoint) than patients receiving no treatment. This difference was also shown for the subgroup of patients in first remission. The between-group difference in overall survival (OS) was not significant, although this trial was not powered to detect such a difference. Histamine dihydrochloride and IL-2 therapy had an acceptable tolerability profile in patients in the phase III trial. The majority of reported adverse events were of grade 1 or 2 severity. The most commonly reported grade 3 adverse events with active treatment were thrombocytopenia, headache, neutropenia, pyrexia, eosinophilia and diarrhoea; grade 4 adverse events were thrombocytopenia and leukopenia not otherwise specified. Serious adverse events were mostly relapse related. Histamine dihydrochloride and IL-2 as maintenance therapy significantly prolonged LFS compared with no treatment and had an acceptable tolerability profile in a large phase III trial in patients with AML. Although some issues remain to be addressed, most notably the effects of therapy on OS and the efficacy of treatment in older patients (who represent the majority of AML patients), histamine dihydrochloride in addition to IL-2 appears to be a useful maintenance therapy option for adult patients with AML in remission.

Individual patient data meta-analysis of randomized trials evaluating IL-2 monotherapy as remission maintenance therapy in acute myeloid leukemia

IL-2 is a natural, T cell-derived cytokine that stimulates the cytotoxic functions of T and natural killer cells. IL-2 monotherapy has been evaluated in several randomized clinical trials (RCTs) for remission maintenance in patients with acute myeloid leukemia (AML) in first complete remission (CR1), and none demonstrated a significant benefit of IL-2 monotherapy. The objective of this meta-analysis was to reliably determine IL-2 efficacy by combining all available individual patient data (IPD) from 5 RCTs (N = 905) and summary data from a sixth RCT (N = 550). Hazard ratios (HRs) were estimated using Cox regression models stratified by trial, with HR < 1 indicating treatment benefit. Combined IPD showed no benefit of IL-2 over no treatment in terms of leukemia-free survival (HR = 0.97; P = .74) or overall survival (HR = 1.08; P = .39). Analyses including the sixth RCT yielded qualitatively identical results (leukemia-free survival HR = 0.96, P = .52; overall survival HR = 1.06; P = .46). No significant heterogeneity was found between the trials. Prespecified subset analyses showed no interaction between the lack of IL-2 effect and any factor, including age, sex, baseline performance status, karyotype, AML subtype, and time from achievement of CR1 to initiation of maintenance therapy. We conclude that IL-2 alone is not an effective remission maintenance therapy for AML patients in CR1.

Histamine dihydrochloride and low-dose interleukin-2 as post-consolidation immunotherapy in acute myeloid leukemia

Acute myeloid leukemia (AML) is the most common acute leukemia in adults. Although most patients achieve complete remission (CR) after chemotherapy, the majority suffer from subsequent leukemic relapse, which is associated with poor long-term survival. Thus, new therapies to maintain CR are highly warranted. After the completion of chemotherapy, AML patients have a minimal burden of leukemic cells, which are reportedly susceptible to cytotoxic lymphocytes such as NK cells and T cells. A therapy that boosts the function of these effector cells therefore has the potential to eradicate the malignant clone in AML and prevent relapse, Here, we briefly review the literature on the role of the immune system in AML and introduce the rationale for the use of histamine dihydrochloride (HDC) in conjuction with low-dose IL-2 as relapse-preventive immunotherapy for this disease.

Post-consolidation immunotherapy with histamine dihydrochloride and interleukin-2 in AML

The initial chemotherapy in acute myeloid leukaemia (AML) comprises a first phase of induction and a second phase of consolidation. In the majority of patients, the induction treatment leads to complete remission (CR), defined as microscopic disappearance of leukaemic disease along with the return of normal haematopoiesis. However, despite the introduction of more efficacious consolidation regimens, a worryingly large proportion of AML patients in CR will subsequently experience relapses with poor prospects of long-term survival. A relapse is assumed to be the result of expansion of residual leukaemic cells that have escaped the initial chemotherapy. The anti-leukaemic functions of T cells and natural killer (NK) cells has formed the background to the use of interleukin-2 (IL-2), a T- and NK cell-activating cytokine, with the aim to eliminate residual leukaemia and hence reduce the relapse rate in AML, but the clinical trials using IL-2 monotherapy have yielded disappointment. A recent phase III study has demonstrated that post-consolidation treatment with the combination of histamine dihydrochloride (HDC) and IL-2 significantly prevents relapse in AML patients. Here we account for the preclinical background to the use of HDC/IL-2 in AML along with a review of clinical results.

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.

Histamine dihydrochloride for the treatment of acute myeloid leukemia, malignant melanoma and renal cell carcinoma

Histamine dihydrochloride is a vasoactive biogenic amine. It inhibits the reactive oxygen species formation in monocytes via histamine H2 receptors and protects natural killer and T cells from oxidative damage. Histamine has the potential to optimize cytokine-induced activation of T cells and natural killer cells; therefore, the addition of histamine to cytokine treatment may improve treatment efficacy. Clinical trials in solid tumors and in acute myeloid leukemia have demonstrated the potential to improve treatment outcome when histamine dihydrochloride is combined with immunotherapy. In patients with metastatic malignant melanoma, this strategy improved remission rates and increased survival. On the other hand, less promising results were reported for histamine dihydrochloride added to cytokines in patients with other solid tumors, especially in advanced renal cell carcinoma. A recent international Phase III trial performed in 320 patients showed that maintenance therapy with histamine dihydrochloride and IL-2 was able to improve leukemia-free survival in patients with acute myeloid leukemia, without an effect on overall survival. The combination of histamine dihydrochloride with IL-2 potentially offers an efficacious and tolerable maintenance strategy for patients with acute myeloid leukemia; however, its impact on survival remains to be explored.

NKp46 and NKG2D receptor expression in NK cells with CD56dim and CD56bright phenotype: regulation by histamine and reactive oxygen species

The cytotoxicity of natural killer (NK) cells is dependent on the interaction between target cell ligands and a series of stimulatory receptors on NK cells. Two of these triggering receptors, the NKp46 natural cytotoxicity receptor (NKp46) and the major histocompatibility complex (MHC) class I-interactive NKG2D receptor, are deficiently expressed by NK cells recovered from patients with acute myeloid leukaemia (AML), but little is known regarding the regulation of NKp46 and NKG2D expression. Here we report that mononuclear and polymorphonuclear phagocytes downregulate the cell surface density of NKp46 and NKG2D on NK cells with CD56(dim) phenotype in vitro by a mechanism that is dependent on the availability of phagocyte-derived reactive oxygen species (ROS). Histamine maintained NKp46 and NKG2D expression despite the presence of inhibitory phagocytes by targeting an H2 receptor on phagocytes. By contrast, NKp46 and NKG2D expression by the CD56(bright) subset of NK cells was resistant to inhibition by phagocytes. Our findings are suggestive of a novel mechanism of relevance to the regulation of NKp46/NKG2D receptor expression. Moreover, our findings suggest that the previously reported action of histamine on NK cell-mediated killing of leukaemic cells may be related to the preservation of activatory NK-cell receptors.

Histamine improves survival and protects against interleukin-2-induced pulmonary vascular leak syndrome in mice

The therapeutic efficacy in the treatment of metastatic cancer with high doses of interleukin-2 (IL-2) has been limited by the onset of vascular leak syndrome (VLS) and related toxicities. VLS is characterized by an increase in vascular permeability and severe hypotension resulting in interstitial edema and organ failure. This study explores the protective effects of histamine dihydrochloride (HDC) against IL-2-induced toxicities in mice. Treatment with HDC administered before or after IL-2 (1.25 x 10(6) IU, BID) was shown to protect mice from VLS-related toxicities and mortality in a dose-dependent manner. Survival rates when HDC was added were 56, 75 and 81% at doses of 0.47, 4.7 and 47.0 mg/kg, respectively, compared to 42% survival with IL-2 alone. HDC protected against IL-2-induced macroscopic pulmonary lesions, reduced edema (up to 62% reduction in lung wet/dry weight ratio) and reduced capillary leakage into the lungs as measured by a reduction in Evans Blue dye content. In addition, the systemic effect on serum cytokine levels showed that HDC only moderately lowered IL-2 induced IFN-gamma, IL-6, IL-10, IL-18 and TNF-alpha. Serum levels of IL-1beta, IL-4 and IL-12 were not measurably induced by IL-2 treatment. HDC modulates many cellular functions including regulating cytokines and blocking immune-suppression caused by reactive oxygen species (ROS) generated by the NADPH oxidase. However, the protective effect of HDC on alleviating IL-2-induced pulmonary edema was not related to ROS inhibition. Our data indicate that HDC treatment improves survival and protects against IL-2 induced VLS independent of ROS regulation in mice.

Addition of Histamine to Interleukin 2 Treatment Augments Type 1 T-Cell Responses in Patients with Melanoma In vivo: Immunologic Results from a Randomized Clinical Trial of Interleukin 2 with or without Histamine (MP 104)

Purpose: Preclinical investigations suggest that histamine dihydrochloride (HDC) protects T cells and natural killer cells from inhibition by monocyte-derived reactive oxygen metabolites and synergizes with interleukin (IL) 2 in inducing T-cell activation. Here, we investigate whether this mechanism is operational in patients with melanoma treated with HDC as an adjunct to IL-2.

Experimental Design: Melanoma patients having liver metastases were treated with IL-2 with or without HDC within a randomized, multicenter, phase III trial. The effect of HDC on type 1 and type 2 T-cell cytokine production was investigated in peripheral blood samples from 19 patients with the use of intracellular cytokine flow cytometry. Melanoma-specific T-cell responses were analyzed in eight HLA-A2–positive patients.

Results: Frequencies of CD3+ T cells producing IFN-γ (type 1 T cells) in response to phorbol myristate acetate/ionomycin increased (median, 1.8-fold) in patients receiving IL-2 plus HDC but not in those receiving IL-2 alone (P &lt; 0.01 for comparison between arms). In contrast, the number of IL-13-producing type 2 T cells that increased in patients after treatment with IL-2 was not modulated by HDC. Melanoma- and tyrosinase-specific IFN-γ and IL-13-producing T cells were detected in two of four HLA-A2–positive patients with melanoma following treatment with HDC + IL-2.

Conclusions: Treatment of patients with stage IV melanoma with HDC in combination with IL-2 increases type 1 T-cell responses and may promote induction of melanoma-specific T cells. These effects are of relevance for tumor immunotherapy and provide a potential mechanism for the clinical efficacy of HDC added to IL-2.

Toxicology and toxicokinetics of acute and subchronic administration of histamine dihydrochloride in rats

Histamine dihydrochloride is currently being evaluated as an adjuvant to immunotherapy regimens in neoplastic and infectious diseases. The no-observed-effect-level (NOEL), no-observable-adverse-effect-level (NOAEL), and pharmacokinetics of subcutaneously administered histamine dihydrochloride were determined via 5 and 28 day repeated dose studies in Sprague-Dawley rats. In the five day study, male rats received 0 (vehicle), 5, 30, 500, or 1000 mg/kg BID. Acute tissue damage was observed at one or more injection sites in the two highest dose groups after 24 h. At five days, animals in these groups displayed indications of pathological inflammation at the injection sites. In the 28 day study, male and female rats received 0 (vehicle), 0.5, 5, or 100 mg/kg BID. The most significant treatment-related pathological findings were signs of inflammation at the injection sites for animals in the 100 mg/kg BID group. Hematology and clinical chemistry changes in the highest dose groups in both studies were consistent with inflammation and anemia but were found to be reversible following a 14-day recovery. Plasma histamine levels were quantified from male and female animals receiving 0.5, 5, and 100 mg/kg injections on Day 1 and 28 of the twenty-eight day study. Cmax was achieved within 0.25 h and was dose-proportional. The elimination half-life and tmax were longer at the 100 mg/kg dose than the lower doses. No marked differences between genders or between Day 1 and 28 were found. Based on these findings, the NOEL and NOAEL were established at 0.5 mg/kg BID and 5 mg/kg BID, respectively. When converted to human equivalent dose, the NOAEL is 0.81 mg/kg which is 54 times the intended human dose. These studies support a wide safety margin for histamine dihydrochloride.

Histamine dihydrochloride and interleukin-2 in the treatment of acute myeloid leukemia

Despite successful chemotherapeutic remission induction, most patients with acute myeloid leukemia still are destined to die from recurrent and refractory disease. Strategies to prolong remission and improve survival include high-dose chemotherapy, autologous or allogeneic stem cell transplantation, and other immunotherapeutic approaches. The focus on interleukin-2 (IL-2) has arisen from in vitro demonstration of enhancement of tumor-specific cytotoxic T-lymphocyte and natural killer-cell activity after cytokine activation. The local secretion of inhibitory substances and oxidative stress by monocyte/macrophage populations may inhibit the cytotoxic capacity of these effector cells. Recently, agents capable of modulating cytotoxic effector function have been identified, including histamine dihydrochloride, which has been shown to act as an inhibitor of cellular-mediated free-radical production. In the presence of histamine, cytotoxic effector function may be restored. The addition of histamine to IL-2-based cytokine therapies has resulted in enhanced in vitro cytotoxic T-cell and natural killer-cell function and has allowed for the development of lower, and thus, less toxic, IL-2 regimens. Preliminary studies of this combination in patients with acute myeloid leukemia in remission suggest improved survival with tolerable toxicity. The strategy for implementation of IL-2-based immunotherapy in patients with acute myeloid leukemia is reviewed.

Results from a randomized phase III study comparing combined treatment with histamine dihydrochloride plus interleukin-2 versus interleukin-2 alone in patients with metastatic melanoma

PURPOSE

Reactive oxidative species (ROS) produced by phagocytic cells have been ascribed a role in the localized suppression of lymphocyte function within malignant tumors. Histamine has been shown to inhibit ROS formation and possibly synergize with cytokines to permit activation of natural killer cells and T cells. This study was designed to determine whether the addition of histamine to a subcutaneous (SC) regimen of interleukin-2 (IL-2) would improve the survival of metastatic melanoma patients.

PATIENTS AND METHODS

A phase III, multicenter, randomized, parallel group study comparing IL-2 plus histamine with IL-2 alone was conducted in 305 patients with advanced metastatic melanoma. Patients were randomized to IL-2 (9 MIU/m(2) bid SC on days 1 to 2 of weeks 1 and 3, and 2 MIU/m(2) bid SC on days 1 to 5 of weeks 2 and 4) with or without histamine (1.0 mg bid SC days 1 to 5, weeks 1 to 4). The primary end point, survival, was prospectively applied to all randomized patients (intent-to-treat-overall population, ITT-OA) and all patients having liver metastases at randomization (ITT-LM population). Secondary end points included safety of the combined treatment, time to disease progression, and response rate.

RESULTS

Combined treatment with histamine plus IL-2 significantly improved overall survival in the ITT-LM population (P =.004) and showed a trend for improved survival in the ITT population (P =.125). Grade 3 and 4 adverse events were comparable in the two arms.

CONCLUSION

Use of histamine as an adjunct to IL-2 is safe, well tolerated, and associated with a statistically significant prolongation of survival compared with IL-2 alone in metastatic melanoma patients with liver involvement. Further trials to confirm and understand the role of histamine in this combination treatment are underway.

Histamine dihydrochloride: inhibiting oxidants and synergising IL-2-mediated immune activation in the tumour microenvironment

The potential role of histamine in cancer immunotherapy has been a subject of interest for more than a decade. A significant body of research has elucidated the action of histamine in a model system that mimics the tumour microenvironment. In vitro evidence indicates that histamine inhibits the generation and release of reactive oxygen species (ROS) by monocytes/macrophages (MO) during respiratory burst. Since ROS have been shown to abrogate peritumoural and intratumoural cytokine activation of natural killer (NK) and T-cells and induce apoptosis of these cells in vitro, inhibition of ROS may enable cytokines to activate NK and T-cells and restore their antineoplastic, cytotoxic capabilities. Experimental data indicate that histamine and interleukin-2 (IL-2) act synergistically to activate NK cell cytotoxicity (NKCC). Although IL-2, a regulator of immune responses, has been shown to promote NKCC in monotherapy for metastatic melanoma (MM), renal cell carcinoma (RCC) and acute myeloid leukaemia (AML), objective responses occur in a minority of patients and survival is not significantly extended, except for a minority of patients with MM using high-dose regimens which have not been widely adopted. In vitro findings suggest that the addition of histamine to IL-2 therapy might improve response rates and disease-free survival by protecting the cells of the immune system from oxidative stress and inducing natural endogenous immune cytotoxicity. An IL-2/histamine Phase III trial is in progress in a population of AML patients. A recently completed Phase III trial of IL-2 vs. IL-2/histamine in patients with MM demonstrated a trend towards a superior survival benefit from IL-2/histamine for all patients entered, and a statistically significant survival benefit for patients with hepatic metastases.

Remission inversion and no transplant-related mortality--a single centre experience of autologous stem cell transplantation in malignant lymphoma

Between 1989 and 1998 93 patients with malignant lymphoma were treated, in our centre, with high-dose chemotherapy and autologous stem cell transplantation. Diagnosis according to the REAL-classification were: 38 patients with high-grade lymphoma (diffuse large B-cell lymphoma (DLCL) (n = 26), anaplastic T-cell (n = 5), lymfoblastic (n = 3) and others (n = 4)), 31 patients with low-grade lymphoma (follicular (n = 18), mantle cell (n = 4), B-CLL (n = 3) and others (n = 6)) and, finally, 24 patients with Hodgkin's disease. The source of stem cells was bone marrow (14 patients), peripheral blood stem cells (64 patients) or a combination of both sources (15 patients). There was no early ( < 100 days) transplant-related mortality. One patient died 11 months post-transplant in unexplained liver failure and all other causes of death were related to relapse of lymphoma. So far, no case of myelodysplastic syndromes or secondary acute leukacmia's has occurred. Overall survival (OS) and progression-free survival (PFS) are: (a) DLCL (26 patients, 4-year probability) OS 40%. PFS 33%; (b) follicular (18 patients, 3-year probability) OS 79%, PFS 52%; (c) Hodgkin's lymphoma (24 patients, 5-year probability) OS 65%, PFS 55%. Out of 52 evaluable patients, 34 (65%) have reached remission inversion. The most important findings are no early transplantation-related mortality, remission inversion in a majority of patients, and so far no cases of secondary myelodysplastic syndromes (MDS) acute myelogenous leukaemias (AML). Concerning OS and PFS, our results seem to be in accordance with other centres.

Alleviating oxidative stress in cancer immunotherapy: a role for histamine?

Interleukin-2 is a remarkable activator of lymphocytes with anti-neoplastic properties such as T-cells or natural killer cells, but tumor regression only rarely occurs in interleukin-2-treated cancer patients. In this review, we focus on interactions between monocytes/macrophages and T-cells/natural killer-cells, and in particular the role of such interactions for the outcome of cancer immunotherapy with interleukin-2. We propose that interleukin-2 therapy should be supplemented with compounds that alleviate toxicity inflicted by monocyte/macrophage-derived reactive oxygen metabolites within and around tumors. The hypothesis is founded on data demonstrating that (i) functions of intratumoral lymphocytes in many human malignant tumors are inhibited by reactive oxygen metabolites, generated by neighboring monocytes/macrophages, (ii) interleukin-2 only weakly activates T-cells or natural killer cells in an environment of oxidative stress, and (iii) inhibitors of the formation of reactive oxygen metabolites or scavengers of reactive oxygen metabolites synergize with interleukin-2 to activate these lymphocyte subsets. We also review the preclinical background to the use of histamine dihydrochloride, an inhibitor of reactive oxygen metabolite formation in monocytes/macrophages, as a supplement to cancer immunotherapy with interleukin-2.

Histamine: a novel approach to cancer immunotherapy

The functions of intratumoral lymphocytes in many human malignant tumors are inhibited by reactive oxygen species (ROS), generated by adjacent monocytes/macrophages (MO). In vitro data suggest that immunotherapeutic cytokines such as interleukin-2 (IL-2) or interferon-alpha (IFN-alpha) only weakly activate T cells or natural killer (NK) cells in a reconstituted environment of oxidative stress and that inhibitors of the formation of ROS or scavengers of ROS synergize with IL-2 and IFN-alpha to activate T cells and NK cells. In this review, we focus on the immunoenhancing properties of histamine, a biogenic amine. Histamine inhibits ROS formation in MO via H2-receptors; thereby, histamine protects NK cells from MO-mediated inhibition and synergizes with IL-2 and IFN-alpha to induce killing of NK cell-sensitive human tumor cells in vitro. Histamine also optimizes cytokine-induced activation of several subsets of T cells by affording protection against MO-inflicted oxidative inhibition. The putative clinical benefit of histamine as an adjunct to immunotherapy with IL-2 and/or IFN-alpha is currently evaluated in clinical trials in metastatic malignant melanoma and acute myelogenous leukemia.

Treatment of acute myelogenous leukemia in older adults

The overall strategy for the treatment of older adults is summarized in Table 8. Soon after the birth of effective chemotherapy for acute leukemia, the perspective for all patients was summarized as follows: 'With all humility it may be claimed that there are, at least, grounds for hope and encouragement in this recently acquired ability occasionally to halt for a while the formerly unrelenting malignant process known as acute leukemia'. In reviewing the overall survival data for older adults one may feel that we are at a similar juncture in assessing the outcome for this particular population. It is hoped that some of the potential advances may provide greater hope and improved results over the next decade.

Adjuvant histamine in cancer immunotherapy

Interleukin-2 (IL-2) is an effective activator of lymphocytes with anti-neoplastic properties such as T-cells or natural killer cells, and this property of IL-2 has formed the basis for its widespread used as an immunotherapeutic agent in human neoplastic disease. In recent years, IL-2 therapy for solid neoplastic diseases and hematopoietic cancers has been supplemented with histamine dihydrochloride with the aim of counteracting immunosuppressive signals from monocytes/macrophages. Here we review the preclinical basis for the use of histamine as an adjunct to IL-2 in cancer immunotherapy.

Immunotherapy of AML: future directions

Immunotherapy in the form of allogeneic GvL has been curing AML patients for nearly 30 years but our understanding of the mechanisms has been poor. Our rapidly evolving understanding of the human immune system and the concomitant technical developments in ex vivo cell manipulation, the vision of funding bodies, the dedication of clinical and research staff, and above all the commitment of our patients, promise substantial progress in the treatment of this disease in the year 2000 and beyond.

Histamine protects T cells and natural killer cells against oxidative stress

Oxidative stress inflicted by monocytes/macrophages (MO) is recognized as an important immunosuppressive mechanism in human neoplastic disease. We report that two types of lymphocytes of relevance for protection against malignant cells, T cells and natural killer (NK) cells, became anergic to the T cell and NK cell activator interleukin-2 (IL-2) after exposure to MO-derived reactive oxygen metabolites and subsequently acquired features characteristic of apoptosis. The MO-induced anergy and apoptosis in T cells and NK cells were reversed by histamine, an inhibitor of reactive oxygen metabolite synthesis in MO. We propose that strategies to circumvent oxidative inhibition of lymphocytes may be of benefit in immunotherapy of neoplastic disease.