Monocyte-induced down-modulation of CD16 and CD56 antigens on human natural killer cells and its regulation by histamine H2-receptors

The CD16−/CD56brightSubset of NK Cells Is Resistant to Oxidant-Induced Cell Death

Phagocyte-derived reactive oxygen species ("oxygen radicals") have been ascribed a suppressive role in immunoregulation by inducing dysfunction and apoptotic cell death in lymphocytes. Earlier studies show that human NK cells are exceptionally sensitive to oxygen radical-induced apoptosis and functional inhibition. Two subsets of human CD56(+) NK cells have been identified: the highly cytotoxic CD56(dim) cells which constitute >90% of NK cells in peripheral blood, and the less cytotoxic but efficiently cytokine-producing CD56(bright) cells. In this study, we demonstrate that the CD56(bright) subset of NK cells, in contrast to CD56(dim) cells, remains viable and functionally intact after exposure to phagocyte-derived or exogenously added oxygen radicals. The resistance of CD56(bright) cells to oxidative stress was accompanied by a high capacity of neutralizing exogenous hydrogen peroxide, and by a high cell-surface expression of antioxidative thiols. Our results imply that CD56(bright) NK cells are endowed with an efficient antioxidative defense system that protects them from oxygen radical-induced inactivation.

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.

Cancer immunotherapy with natural killer cells

Natural killer (NK) cells were first identified for their ability to kill tumor cells of different origins. They were subsequently found to be able to kill some normal cells, especially those infected by certain viruses. Natural killer cells are a distinct lymphocytic population, with the morphology of large granular lymphocytes, and they lack surface Ig or T-cell markers. Natural killer cytotoxicity is enhanced in vitro in the presence of cytokines such as interleukin-2 and interferon-alpha. However, when used in immunotherapy, these cytokines have not consistently augmented NK activity or shown antitumor effects. One explanation for this divergence is that NK cell activity is suppressed in tumor tissues by various factors, including reactive oxygen species produced by infiltrating monocytes and macrophages. Agents that inhibit the generation of reactive oxygen species, such as histamine, may abrogate the suppression exerted on NK cells by monocytes/macrophages, therefore offering potential therapeutic benefit as immunoadjuvants.

Histamine as an adjunct to immunotherapy

Interleukin-2 and interferon-alpha have been used as therapeutic options in the treatment of certain malignancies such as metastatic malignant melanoma, acute myelogenous leukemia, and renal cell carcinoma. However, the outcome with these agents has been less than optimal. While experiments in vitro would lead one to believe that these agents would be useful therapeutic alternatives, the situation in vivo is confounded by the fact that the microenvironments of the tumor and surrounding tissue are infiltrated with monocytes and macrophages, which suppress the cytotoxic activity of T cells and natural killer cells. The mechanism by which this occurs is through the generation of reactive oxygen species that are responsible for apoptosis by both T cells and natural killer cells. Histamine abrogates this suppression, thus restoring the cytotoxicity of these cells. Therefore, the addition of histamine to regimens containing cytokines is expected to optimize cytokine therapy. Clinical trials with these regimens are under way in the treatment of metastatic malignant melanoma, acute myelogenous leukemia, and renal cell carcinoma. Results published thus far indicate that the addition of histamine to cytokine therapy is both safe and efficacious in the treatment of these diseases.

Histamine in cancer immunotherapy: a preclinical background

Natural killer (NK) cells and T cells are the primary targets of interleukin-2 (IL-2) and other cytokines used in cancer immunotherapy. However, these tumoricidal lymphocytes are frequently dysfunctional or apoptotic when residing within melanomas and other solid cancers. This phenomenon--tumor-induced immunosuppression--is poorly understood and conceivably limits the efficiency of strategies aiming at activating lymphocyte-mediated antitumor immunity. Recent studies imply that reactive oxygen species (oxygen radicals), produced by tumor-infiltrating monocyte/macrophages, may contribute to the state of lymphocyte inhibition in neoplastic tissue. Histamine, acting via H2-type histamine receptors on monocyte/macrophages, suppresses the activity of a key enzyme in oxygen radical formation, the NADPH oxidase. By this mechanism, histamine protects NK cells and T cells against oxygen radical-induced dysfunction and apoptosis, and also maintains their activation by IL-2 and other lymphocyte activators. In this review, these properties of histamine are discussed in relation to the current use of histamine as an adjunct to IL-2 in metastatic melanoma and other malignant diseases.

Monocyte-macrophage system as targets for immunomodulation by intravenous immunoglobulin

Pooled human intravenous immunoglobulin (IVIg) has been used successfully to treat or ameliorate the clinical manifestations of humoral immune deficiencies, haematological disorders, HIV infection and many other diseases states. However, the mechanism of action of IVIg remains unclear. Several mechanisms of action of IVIg have been proposed. These include Fcy receptor blockade, accelerated clearance of endogenous pathogenic auto-antibodies, inhibition of components of the complement cascade, neutralization of super-antigens and bacterial toxins as well as anti-cytokine and anti-idiotype effects. A major contributor to host immunity and immune surveillance against infection, tissue or cell damage and malignancy is the monocyte/macrophage system. Monocyte-directed inflammation is a desirable consequence of microbiological or malignant challenge. However, monocyte hyperactivity may contribute to certain pathological conditions. These include the systemic inflammatory response syndrome (SIRS), septic shock, other dysregulated inflammatory disorders and auto-immunity. Novel therapies that can suppress the hyperactive state or correct monocyte/macrophage dysfunction without compromising normal host cell-mediated immunity are desirable. In this review, we discuss the immunomodulatory effects of IVIg focussing particularly upon the monocyte/macrophage system in pertinent disease states.

Histamine and interleukin-2 in acute myelogenous leukemia

Interleukin-2 (IL-2) activates natural killer (NK)-cells to destroy leukemic blasts from patients with acute myelogenous leukemia (AML), but even aggressive regimens of IL-2 fail to prevent relapse or prolong remission time in AML. Results obtained in studies of NK-cell-mediated killing of AML blasts show that monocytes inhibit IL-2-induced lysis of AML blasts in vitro. Histamine, a biogenic amine, prevents the monocyte-derived, inhibitory signal; thereby, histamine and IL-2 synergize to induce killing of AML blasts. Here we present updated results of a post-consolidation trial in which histamine (0.5-0.7 mg s.c. bid) has been administered together with IL-2 (1 micro/kg s.c. bid) to 22 AML patients (aged 29-79, mean 59) in repeated courses of three weeks, continued until relapse or until a disease-free remission of 24 months. Low-dose therapy with cytarabine and thioguanine was given between the initial courses of histamine/IL-2. In 13 patients, treatment according to this protocol was started in first complete remission (CR1). The mean remission time in CR1 patients is 19 (median 14) months, and 9/13 remain in CR. Nine patients have entered the protocol in CR2 (n=6), CR3 (n=2), or CR4 (n=1). The mean remission time in CR2-4 is 19 (median 21) months, and 6/9 patients remain in CR. Seven out of seven evaluable patients have achieved a duration of CR which exceeds that of the foregoing remission. Histamine has been well tolerated, and 21/22 CR patients have treated themselves at home throughout the trial. We conclude that the putative benefit of histamine treatment in AML should be the focus of a randomized trial.

Histamine in cancer immunotherapy

A novel strategy for enhancing the efficacy of immunotherapy with interleukin-2 (IL-2) and interferon-alpha (IFN-alpha) in human neoplasia is presented. IL-2 and IFN-alpha are potent activators of the antitumour activity of natural killer (NK) cells but only rarely reduce the tumour burden in treated patients. Recent studies suggest that a reason why these cytokines are insufficiently effective in human cancer is that phagocytes inhibit the tumour-killing activity of NK cells at the site of the tumour. Histamine prevents the phagocyte-induced, NK cell-inhibiting signal; thus, histamine and IL-2 or histamine and IFN-alpha synergize to induce NK cell-mediated killing of human tumour cells in vitro. Further, treatment of tumour-bearing mice with histamine enhances IL-2- and IFN-alpha-induced destruction of NK cell-sensitive tumour cells in vivo. More than 50 patients with neoplastic disease have been treated with histamine, given in subcutaneous injections, together with IL-2 or IFN-alpha. The results of two pilot trials in metastatic melanoma suggest that the addition of histamine to IL-2 and IFN-alpha prolongs survival time and induces regression of tumours, such as liver melanoma, which are otherwise considered refractory to immunotherapy. The results of a trial in acute myelogenous leukaemia (AML) suggest that histamine and IL-2 protects AML patients against relapse of leukaemic disease. Histamine is well tolerated: for example, AML patients in remission have treated themselves with histamine at home without supervision for a total of > 300 weeks with only a handful of therapy-related hospital contacts. Controlled trials in melanoma and AML are under way to further investigate the putative benefit of histamine in neoplastic disease.

Chronic intracerebroventricular administration of beta-endorphin augments natural killer cell cytotoxicity in rats

We have studied the effect of chronic intracerebroventricular (i.c.v.) infusion of different opioid peptides on natural killer (NK) cell mediated cytotoxicity in vivo in the spontaneously hypertensive rat (SHR). The in vivo NK cell activity was measured as the clearance of 51Cr-labelled YAC-l lymphoma cells from the lung tissues. Further, the phenotype of lymphocytes in spleen and peripheral blood was analysed by flow cytometry (FACS). All opioid drugs were administered i.c.v. for 6 days with osmotic minipumps releasing 1.0 microliter/h. beta-Endorphin (10 or 20 micrograms/rat per day) significantly increased NK cell cytotoxicity in vivo. The opioid receptor antagonist naloxone (10 mg/kg, i.p.) given immediately before the injection of YAC-lymphoma cells, completely abolished the effects of i.c.v. administered beta-endorphin. Corresponding doses of beta-endorphin administered subcutaneously (s.c.) with minipumps for 6 days did not significantly affect NK cell cytotoxicity. Neither Leu- or Met-enkephalin (20 micrograms/rat per day) nor dynorphin (20 micrograms/rat per day) administered i.c.v. had any significant effects on NK cell activity. In beta-endorphin treated SHR, the percentage of cells with NK cell phenotype (OX52+/CD5-) in peripheral blood was not significantly different from that of controls, while the percentage of cells with T cell phenotype (CD5+/OX52-) was significantly decreased. The percentage of splenic NK cells (OX52+/CD5-) and T cells (CD5+/OX52-) was also unchanged by beta-endorphin treatment i.c.v. These results suggest that of the opioid peptides administered i.c.v., only beta-endorphin augments in vivo NK cell mediated cytotoxicity. We thus conclude that these effects most probably are centrally and opioid receptor mediated effects, since beta-endorphin in the same dose administered peripherally does not influence in vivo NK cell cytotoxicity.

Granulocyte-macrophage colony-stimulating factor (GM-CSF) counteracts the inhibiting effect of monocytes on natural killer (NK) cells

GM-CSF is known to accelerate haematopoietic recovery following allogeneic bone marrow transplantation (BMT). In addition, it may restore and enhance both granulocyte and monocyte functions. Stimulation of monocyte functions may induce a direct or an indirect anti-leukaemic activity due to an increase of cellular cytotoxicity and production of cytokines which may result in a reduction of the relapse rate after BMT. NK cells may play a crucial role in this activity. Therefore we studied the influence of monocytes on NK activity in combination with GM-CSF. Lymphocytes and monocytes were isolated from buffy coats of healthy individuals by counterflow centrifugation elutriation (CCE). NK activity was exerted by CD3-CD56+ cell populations and could be enhanced by IL-2 incubation overnight. Incubation of CD3-CD56+ cells with GM-CSF in the presence or absence of IL-2 hardly influenced NK activity of the lymphocyte population. Low amounts of monocytes enhanced NK activity. NK activity in lymphocyte population in the presence of equivalent numbers of monocytes with or without IL-2 was strongly decreased irrespective of the effector:target ratio (ETR). This appeared not to result from sterical hindrance effects of the present number of cells. However, addition of GM-CSF abrogated the inhibition of NK activity by monocytes in the presence of IL-2. In monocyte fractions neither IL-2 nor GM-CSF yielded NK activity. Our findings indicate that GM-CSF can affect NK activity by counteracting the suppressing effects of monocytes, and hence may improve the outcome after BMT.

Chemoattractant-induced NADPH oxidase activity in human monocytes is terminated without any association of receptor-ligand complex to cytoskeleton

When the chemotactic peptide formylmethionyl-leucyl-phenylalanine binds to its cell surface receptor, a transmembrane signal is generated that activates the superoxide-producing NADPH oxidase of human phagocytes. Comparing monocytes and neutrophils with regard to the production of superoxide anion induced by the peptide, we found a similar time-course for both types of cells. In neutrophils, ligand binding induced a conversion of the receptor to a high-affinity form, a change suggested to be due to an association of the receptor-ligand complex to the Triton X-100-insoluble cytoskeleton. This event has been hypothesized to terminate the signal that activates the NADPH oxidase and thereby results in cessation of the cellular production of superoxide anion. Neutrophils preincubated with the cytoskeleton-disrupting drug cytochalasin B showed an increased and prolonged superoxide anion production after activation with the peptide, thus indicating that the cytoskeleton is involved in terminating this response. Formylmethionyl-leucyl-phenylalanine was also found to induce polymerization of actin in monocytes; however, cytochalasin B had no effect on the peptide-induced generation of superoxide anion in these cells. Furthermore, also in monocytes, ligand binding induced a conversion of the receptor to a high-affinity form; however, the receptor-ligand complex did not coisolate with the Triton X-100-insoluble cytoskeleton. These results indicate that, in monocytes, the NADPH oxidase activating pathway is terminated without any association of the receptor-ligand complex to the Triton X-100-insoluble cytoskeleton.

Serotonergic 5-HT1A receptors regulate a cell contact-mediated interaction between natural killer cells and monocytes

Autologous monocytes irreversibly suppressed functions of human natural killer (NK) cells including baseline and lymphokine-induced cytotoxicity, antibody-dependent cellular cytotoxicity (ADCC), and interleukin-2 (IL-2)-induced proliferation. The suppression of these NK-cell functions was cell contact-dependent and could be evoked only by purified monocytes, recovered directly from peripheral blood by countercurrent centrifugal elutriation (CCE). The presence of monocytes also induced the disappearance of CD16 and CD56 antigen on CD3- NK cells (CD3-/16+/56+-->CD3-/16-/56-). By contrast, T-cell proliferation and the expression of CD3 on CD56- T cells were not susceptible to cell contact-mediated suppression by monocytes. The biogenic amine serotonin abrogated monocyte-induced suppression of NK-cell functions as well as down-modulation of CD16/56 NK-cell antigen. Serotonin thus markedly augmented baseline and lymphokine-induced NK-cell cytotoxicity, ADCC, and NK-cell proliferation, and maintained the expression of NK-cell surface antigens in the presence of elutriated monocytes. The effect of serotonin was mediated by 5-HT1A-type serotonin receptors (5-HT1AR) as indicated by mimicry exerted by 5-HT1AR agonists such as 8-OH-DPAT and (+)-ALK, partial antagonism by the 5-HT1AR antagonists pindolol and cyproheptadine, and lack of antagonism by the 5-HT2R antagonist ketanserin or the 5-HT3R antagonist ondansetron. Our data are suggestive of a cell-to-cell-mediated mechanism by which monocytes down-modulate NK-cell function and phenotype and its serotonergic regulation.

Regulation of the natural killer cell response to interferon-alpha by biogenic amines

Monocytes, recovered from human peripheral blood by counter-current centrifugal elutriation (CCE), suppressed baseline natural killer (NK) cell cytotoxicity (NKCC) and rendered NK cells resistant to activation of cytotoxicity by human recombinant interferon-alpha (IFN-alpha) by a cell contact-dependent mechanism. Monocyte-induced suppression of resting and IFN-activated NK cells was abrogated by the biogenic amines histamine [via H2-type receptors (H2R)] and serotonin [via 5-HT1A-type receptors (5-HT1AR)]. Our data are suggestive of a monocyte/NK cell interaction that is subject to regulation by biogenic amines.