Abstract:
A method for providing activated natural killer (NK) cells comprising the steps of administering to a population of cells which includes lymphocytes and monocytes, an effective amount of an NK cell activating cytokine or a NK cell activating flavonoid, wherein said NK cell activating cytokine is not IL-2 or IFN-α; and administering a compound effective to inhibit the production or release of hydrogen peroxide selected from the group consisting of histamine, other H 2  receptor agonists, and serotonin.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application is a continuation of application Ser. No. 09/516,641, filed on Mar. 1, 2000, which is a continuation of application Ser. No. 08/681,108, filed Jul. 22, 1996, now U.S. Pat. No. 6,071,509, which is a continuation of application Ser. No. 08/287,200, filed Aug. 8, 1994, now abandoned.  
       This application is related to U.S. application Ser. No. 07/843,052, filed Mar. 2, 1992. 
    
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to methods for the enhanced activation of natural killer (NK) cells, useful for example, in the treatment of cancer and viral infection. More specifically, the present invention relates to the activation of NK cells using a combination of a natural killer cell activator and a hydrogen peroxide inhibiting compound or scavenger. It also relates to the prevention of inactivation of NK cells.  
         BACKGROUND OF THE INVENTION  
       DESCRIPTION OF THE RELATED ART  
         [0003]    Natural killer (NK) cells are a subset of spontaneously cytotoxic lymphocytes that lytically destroy tumor cells without apparent antigen specificity or restriction by histocompatibility molecules. Lymphokines are lymphocyte-derived peptides that modulate immunologic and inflammatory responses by regulating the activity, growth and differentiation of a wide variety of leukocyte and nonleukocyte target cells. Similar factors produced by a variety of cell types, together with lymphokines, are known as cytokines. Several cytokines are known to stimulate proliferation of NK cells and to enhance their cytotoxic activity.  
           [0004]    Interleukin-2 (IL-2), formerly T-cell growth factor (TCGF), is a T-cell-derived cytokine. Since 1985, IL-2 has been used in the treatment of human neoplasia, mainly in patients with metastasizing solid tumors, such as malignant melanoma and renal cell carcinoma (Rosenberg et al.,  N. Engl. J. Med.  316:889-897 (1987); Bukowski et al.,  J. Clin. Oncol.  7:477-485 (1989)), but more recently also in acute myelogenous leukemia (AML) (Foa et al.,  Br. J. Haematol.  77:491-496.3 (1991)). In the initial studies, IL-2 was administered together with autologous lymphocytes that had been treated with IL-2 in vitro, but in recent years IL-2 has more frequently been administered as a single agent.  
           [0005]    The high expectations for the treatment of human cancer using IL-2 were based on the findings that treatment with IL-2 can induce the regression of established tumors in several animal tumor models in vivo (Rosenberg et al.,  J. Exp. Med.  161:1169-1188 (1985); Lotre and Rosenberg, in Interleukin-2, K. A. Smith, ed., Academic Press, San Diego, pp. 237-294 (1988)). The mechanism underlying this anti-tumor effect of IL-2 has been much debated, but accumulating evidence points to the anti-tumor effector cell as the natural killer (NK)-cell. Depletion of NK cells from experimental animals eliminates the anti-tumor effect of IL-2 in many experimental models for tumor growth and metastasis (Mule et al.,  J. Immunol.  139:285 (1987)). Further, the only subset of resting human peripheral blood lymphocytes that carry transducing receptors for IL-2 (IL-2R) on the cell surface are NK cells (Caliguri et al.,  J. Clin. Invest.  91:123-132 (1993)).  
           [0006]    IL-2 activates many NK-cell functions, including baseline or “natural” anti-tumor cytotoxicity, antibody-dependent cellular cytotoxicity (ADCC), proliferation, and cytokine production (Trinchieri,  Adv. Immunol.  47:187-376 (1989)). Also, IL-2-activated NK cells, frequently referred to as lymphokine-activated killer (LAK) cells, display a broader spectrum of reactivity against human and murine tumor target cells. Thus, NK cells activated by IL-2 not only kill NK cell-sensitive tumor cells more efficiently, but also kill tumor cells that are insensitive to the constitutive cytotoxic activity mediated by NK cells.  
           [0007]    Recent studies have also shown that IL-2, when combined with histamine or serotonin, augments NK cell cytotoxicity in the presence of monocytes in vitro (Hellstrand et al.,  J. Immunol.  145(12):4365-4370 (1990) and Hellstrand et al.,  Scand. J. Immunol.  32(2):183-192 (1990)). These studies suggest an interaction between monocytes and NK cells that is subject to regulation by these biogenic amines (Hellstrand et al.,  J. Interferon Rsch.  12:199-206 (1992). These NK cell regulating mechanisms are thus believed to be of importance to the NK cell mediated response to metastatic tumors in vivo.  
           [0008]    Despite the beneficial effects obtained with IL-2 therapy in experimental animals and despite the remarkable effects of IL-2 on the killing activity of human NK cells in vitro, the results of the clinical trials of IL-2 in human cancer have, as yet, been disappointing. Only a small fraction of patients with metastatic melanoma or renal cell carcinoma show objective regression of tumor burden after treatment with very high doses of IL-2 (Bukowski et al.,  J. Clin. Oncol.  7:477-485 (1989); Whitehead et al.,  J. Natl. Cancer Inst.  83:1250-1253 (1991)). In addition, IL-2 produces severe side effects, including hypotension, fluid retention (“capillary leak syndrome”), fever, lethargy and nausea.  
           [0009]    Other interleukins are also known to stimulate NK cell activity. For example, IL-12, also known as natural killer cell stimulatory factor (NKSF), is a recently discovered cytokine which has also been reported to increase NK cell and cytotoxic T lymphocyte activity, T cell proliferation, and the production of interferon-γ. It has been found to enhance the spontaneous cytotoxic activity of peripheral blood lymphocytes against a variety of tumor-derived target cell lines (Chehimi et al.,  J Exp. Med.  175:789-796 (1992)). IL-1 is another cytokine known to enhance NK cell cytotoxicity.  
           [0010]    The interferons consist of a family of secreted proteins with potent antiproliferative and immunomodulatory activities. These immunomodulatory effects include activation of macrophages, augmentation of cellular and humoral immune responses, and enhancement of NK-cell activity. All three major subtypes of human interferon, i.e., interferon-α (IFN-α), interferon-β (IFN-β) and interferon-γ (IFN-γ), are known to enhance NK cell cytotoxicity. Interferon-α (IFN-α) is a major regulatory factor for NK cells. It has been found to stimulate NK cells (Silva et al.,  J. Immunol.  125:479-484 (1980)) and augment NK cell cytotoxicity both in vitro and in vivo (Trinchieri,  Adv. Immunol.  47:187-376 (1989)). Although IFN-α has been shown to be effective with some neoplasias, the overall results of therapy with high doses of IFN-α have been disappointing. In addition, patients treated with IFN-α often have acute toxic reactions including fever, chills, myalgias, anorexia, fatigue, headache, nausea and vomiting.  
           [0011]    Other known stimulators of NK cell activity include certain flavonoids. The flavonoids are a group of low molecular weight polyphenolic secondary plant metabolites. Flavone-8-acetic acid has been found to potently augment NK activity in the spleen, liver, lungs, and peritoneum (Wiltrout et al.,  J. Immunol.  140(9):3261-3265 (1988)). Xanthenone-4-acetic acid (XAA), an analog of FAA, and its methyl-substituted derivatives, have also been found to induce NK activity in vitro (Ching et al.,  Eur. J. Cancer  27(1):79-83 (1991)). Clinical trials of FAA have been disappointing, however, due to non-linear pharmokinetics, low dose potency and problems of drug precipitation.  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention provides a novel method for the activation of NK cells and the prevention of inactivation of these cells by monocytes, using a combination of a lymphokine or other NK cell activator and a peroxide reducing or scavenging compound. The present invention is especially useful in the treatment of solid tumors and viral infection.  
           [0013]    In accordance with one aspect of the present invention, there is provided a method for providing activated natural killer cells comprising the steps of administering to a population of cells which includes lymphocytes and monocytes, an effective amount of an NK cell activating compound and a compound effective to inhibit the production or release of intracellular hydrogen peroxide, provided that when said NK cell activating compound is IL-2 or IFN-α, said compound effective to inhibit the production or release of intracellular hydrogen peroxide is not histamine, an H 2  receptor agonist or serotonin.  
           [0014]    In a preferred embodiment of the present invention, the compound effective to inhibit the production or release of intracellular hydrogen peroxide is histamine, an H 2  receptor agonist or serotonin, and the NK cell activating compound is a cytokine or a flavonoid. In another preferred embodiment, the population of cells is located in vivo. In still another preferred embodiment, the administration of said NK cell activating compound and said compound effective to inhibit the production or release of intracellular hydrogen peroxide is performed simultaneously. Alternatively, the administration of said NK cell activating compound and said compound effective to inhibit the production or release of intracellular hydrogen peroxide is performed within 24 hours.  
           [0015]    In another preferred embodiment of the present invention, the NK cell activating compound is a cytokine, which is administered in a dose of from about 1,000 to about 300,000 U/kg/day. In the preferred embodiment wherein the NK cell activating compound is a flavonoid, the flavonoid is administered in a dose of from about 1 to about 100,000 mg/day. In still another preferred embodiment, the compound effective to inhibit the production or release of intracellular hydrogen peroxide is administered in a dose of from about 0.1 to about 10 mg/day.  
           [0016]    In accordance with another aspect of the present invention, there is provided a method for providing activated natural killer cells comprising the steps of administering to a population of cells which includes lymphocytes and monocytes, an effective amount of an NK cell activating compound and administering a hydrogen peroxide scavenger. In a preferred embodiment, the hydrogen peroxide scavenger catalyzes the decomposition of hydrogen peroxide. In a preferred embodiment, the hydrogen peroxide scavenger is catalase, glutathione peroxidase, or ascorbate peroxidase. In another preferred embodiment, the NK cell activating compound is a cytokine or a flavonoid. In still another preferred embodiment, the population of cells is located in vivo.  
           [0017]    The administration of said NK cell activating compound and said hydrogen peroxide scavenger is preferably performed simultaneously. Alternatively, the administration of said NK cell activating compound and said hydrogen peroxide scavenger is performed within 24 hours. In a preferred embodiment, the NK cell activating compound is a cytokine, and the cytokine is administered in a dose of from about 1,000 to about 300,000 U/kg/day. In the preferred embodiment wherein the NK cell activating compound is a flavonoid, the flavonoid is administered in a dose of from about 1 to about 100,000 mg/day. Preferably, the hydrogen peroxide scavenger is administered in a dose of from about 0.1 to about 10 mg/day. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 graphically depicts the activation of NK cells by catalase and synergy with IL-2. Culture medium (control; open bars) or IL-2 (10 U/ml; filled bars) was added to enriched human NK cells alone (FIG. 1A) or NK cells admixed with 30% monocytes (FIG. 1B) in the presence of catalase at the indicated final concentrations. The bars indicate NK cell-mediated killing of target tumor cells (cell lysis % “s.e.m. of sextuplicate determination).  
         [0019]    [0019]FIG. 2 graphically depicts the activation of NK cell-mediated clearance of YAC-1 lymphoma cells in vivo by catalase. Seventy-five thousand YAC-1 cells labeled with  51 Cr were injected intravenously into male or female 4-6-week-old Swiss Albino mice, together with vehicle (control; open bars) or catalase (100 U/kg; filled bars). Two hours after the inoculation of tumor cells, the mice were sacrificed by cervical dislocation. The results show retained radioactivity in lung tissue (% of radioactivity retained in lungs at t=0 after injection of labeled tumor cells). Results from 4 separate experiments are shown. Each bar represents the mean “s.e.m. of 3-5 animals.  
         [0020]    [0020]FIG. 3 shows the inhibition of IL-2-induced NK cell proliferation and cytotoxicity by monocytes and its reversal by histamine and catalase. Culture medium (open bars), histamine (hatched bars), and catalase (filled bars) were added to enriched NK cells (NK) or a mixture of NK cells and monocytes (NK+MO). FIG. 3A shows NK cell proliferation, and FIG. 3B shows cytotoxicity against K562 target cells.  
         [0021]    [0021]FIG. 4 illustrates the suppression of NK cell cytotoxicity by hydrogen peroxide and the role of myeloperoxidase. Open symbols represent the cytotoxicity of cells treated with hydrogen peroxide. Filled symbols represent corresponding cells treated with myeloperoxidase.  
         [0022]    [0022]FIG. 5 illustrates the kinetics of monocyte-induced inhibition of NK cell cytotoxicity. A mixture of NK cells and monocytes were treated with culture medium (open bars), histamine (hatched bars) or catalase (filled bars) at the indicated time points after the start of the microtoxicity assay.  
         [0023]    [0023]FIG. 6 shows that histamine inhibits the generation of hydrogen peroxide in monocytes. The luminol-enhanced chemiluminescence response of monocytes treated with culture medium (solid line) or histamine (dotted line) is shown in FIG. 6A. FIG. 6B shows the response of monocytes treated with sodium azide (control, solid line), or histamine plus sodium azide (dotted line).  
         [0024]    [0024]FIG. 7 shows that histamine H 2 -type receptors transduce the effects of histamine on the respiratory burst of monocytes. Monocytes were treated with histamine plus ranitidine (open circles) or AH20239AA (filled circles). 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0025]    The present invention relates to a method for activating NK cells in the presence of monocytes by using a hydrogen peroxide inhibiting or scavenging compound in combination with a cytokine or other NK cell activator. The method of the present invention is useful, for example, as a method of inhibiting tumor growth and the formation of metastases of malignant tumor cells, and in the treatment of viral infection.  
         [0026]    In the monovalent pathway of oxygen reduction, superoxide anion (O 2 ) is produced first, followed by the formation of hydrogen peroxide (H 2 O 2 ). Superoxide anion can react with hydrogen peroxide to form hydroxyl radical (OH). These reactive oxygen intermediates (ROI) are produced by phagocytes such as monocytes and polymorphonuclear neutrophils (PMNs). Hydrogen peroxide produced by monocytes has been found to suppress NK cell mediated cytotoxicity. This NK cell cytotoxicity plays an important role in a host&#39;s defenses against arising neoplasms and metastatic tumor cells in vivo. It has now been discovered that monocytes suppress NK cell cytotoxicity and that this monocyte derived suppressive signal effectively down-regulates the cytotoxic and proliferative activities of NK cells. Suppression of NK cells has been found to be halted in the presence of the biogenic amines histamine and serotonin (Hellstrand et al.,  J. Immunol.  145:4365-4370 (1990)).  
         [0027]    It is one of the surprising discoveries of the present invention that compounds which reduce the amount of hydrogen peroxide, when administered in combination with a cytokine or other compound known to stimulate NK cell activity, act to synergistically stimulate NK cell cytotoxicity in the presence of monocytes; thus, the administration of scavengers of peroxide, or compounds which inhibit the production or release of intracellular peroxide, in combination with a cytokine or other NK cell activator, has been found effective in the treatment of solid tumors and viral infection.  
         [0028]    Known scavengers of hydrogen peroxide include the enzymes catalase, glutathione peroxidase and ascorbate peroxidase. Compounds which inhibit the production or the release of intracellular peroxide are also effective in enhancing NK cell activity when administered together with an NK cell activator. Such compounds include serotonin, histamine, and H 2  receptor agonists such as dimaprit.  
         [0029]    The present invention therefore provides an effective method for preventing the inactivation of NK cells and for activation these cells. It also provides a method for treatment of tumors and viral infection, through the administration of compounds which reduce the amount of hydrogen peroxide, in combination with a cytokine or other compound known to stimulate NK cell activity. It is intended that the present invention cover the administration of the compounds listed and those compounds with similar activity, with the understanding that if the cytokine administered is IL-2 or IFN-α, the inhibitor is not histamine, an H 2  receptor agonist, or serotonin.  
         [0030]    Administration of NK Cell Activator and Hydrogen Peroxide Scavenger or Inhibitor  
         [0031]    The administration of the cytokine or other compound known to enhance NK cell activity, together with the inhibiting or scavenging compounds discussed above, can be by any of a number of methods well known to those of skill in the art. Such methods include the parenteral delivery through intravenous, intraperitoneal, or intramuscular injection. The NK cell activity enhancer and the hydrogen peroxide scavenger can be administered separately or as a single composition. When administered separately, it is contemplated that the NK cell activity enhancer may be administered either first or last.  
         [0032]    The compounds of the present invention may be administered in water with or without a surfactant such as hydroxypropyl cellulose. Dispersions are also contemplated, such as those utilizing glycerol, liquid polyethylene glycols, and oils. Antimicrobial compounds may also be added to the preparations. Injectable preparations may include sterile aqueous solutions or dispersions and powders which may be diluted or suspended in a sterile environment prior to use. Carriers such as solvents or dispersion media contain water, ethanol polyols, vegetable oils and the like may also be added to the compounds of the present invention. Coatings such as lecithins and surfactants may be used to maintain the proper fluidity of the composition. Isotonic agents such as sugars or sodium chloride may be added, as well as products intended to delay absorption of the active compounds such as aluminum monostearate and gelatin. Sterile injectable solutions are prepared according to methods well known to those of skill in the art and can be filtered prior to storage and/or use. Sterile powders may be vacuum or freeze dried from a solution or suspension them. Sustained-release preparations and formulations are also contemplated by the present invention. Any material used in the composition of the present invention should be pharmaceutically acceptable and substantially non-toxic in the amounts employed.  
         [0033]    All preparations may be provided in dosage unit forms for uniform dosage and ease of administration. Each dosage unit form contains a predetermined quantity of active ingredient calculated to produce a desired effect in association with an amount of pharmaceutically acceptable carrier.  
         [0034]    Although in the Examples which follow the compounds are administered as a single dose, it should be understood that the compounds may be administered for prolonged periods of time. Typically, the treatment may be administered for periods up to about one week, and even for periods longer than one month. In some instances, the treatment may be discontinued and then resumed at a later time. A daily dose may be administered as a single dose, or it can be divided into several doses, especially if negative effects are observed. In addition, the compounds of the present invention can be administered as a single composition, or separately. If administered separately, the compounds should be given on the same day, such that the activation of NK cells by the lymphokine or other compound is enhanced.  
         [0035]    Preferred dosage range can be determined using techniques known to those having ordinary skill in the art. IL-1, IL-2 or IL-12 can be administered in an amount of from about 1,000 to about 300,000 U/kg/day; more preferable, the amount is from about 3,000 to about 100,000 U/kg/day, and even more preferably, the amount is from about 5,000 to about 20,000 U/kg/day.  
         [0036]    IFN-α, IFN-β, and IFN-γ can also be administered in an amount of from about 1,000 to about 300,000 U/kg/day; more preferable, the amount is from about 3,000 to about 100,000 U/kg/day, and even more preferably, the amount is from about 10,000 to about 50,000 U/kg/day.  
         [0037]    Flavonoid compounds can be administered in an amount of from about 1 to about 100,000 mg/day; more preferable, the amount is from about 5 to about 10,000 mg/day, and even more preferably, the amount is from about 50 to about 1,000 mg/day.  
         [0038]    Compounds which inhibit the release or formation of intracellular hydrogen peroxide, or scavengers of hydrogen peroxide, can be administered in an amount of from about 0.1 to about 10 mg/day; more preferable, the amount is from about 0.5 to about 8 mg/day, and even more preferably, the amount is from about 1 to about 5 mg/day. However, in each case, the dose depends on the activity of the administered compound. The foregoing doses are appropriate for histamine, catalase and for H 2  receptor agonists. Appropriate doses for any particular host can be readily determined by empirical techniques well known to those of ordinary skill in the art.  
         [0039]    The method of the present invention may be utilized alone or in combination with other anti-cancer therapies, as determined by the practitioner.  
         [0040]    Monocyte-Induced Inhibition of NK Cells: Reversal by Catalase and the Role of Reactive Oxygen and Nitrogen Species  
         [0041]    To investigate the effects of hydrogen peroxide scavengers on the activation of NK cells by cytokines, we studied the effects of catalase, a heme containing enzyme that catabolizes hydrogen peroxide (H 2 O 2 ) to oxygen and water, on human NK cell-mediated killing of tumor cells in vitro and on NK cell function in mice in vivo. These experiments are described below in Examples 1 and 2. The following examples are merely illustrative of the present invention, and are not intended to limit the invention in any way.  
       EXAMPLE 1  
       [0042]    Using blood obtained from a healthy human blood donor, we studied the effects of catalase on NK cell-mediated killing of tumor cells. K562 cells, from an NK sensitive erythroleukemic cell line, were used as target cells in all experiments. Washed cells (10×10 6  cells/ml) were incubated with  51 Cr (Amersham) at a concentration of 150 μCi/ml cell suspension for 2-4 hours. After centrifugation and resuspension in cell culture medium, 10 4  cells in 50 μl portions were added to the effector cells in microplate wells.  
         [0043]    In the first set of experiments, human NK cells alone (1.5×10 5  cells/well) were added to the K562 target cells. The combined cells were then exposed to culture medium (control) or human recombinant IL-2 (EuroCetus, Amsterdam, The Netherlands) at a final concentration of 10 U/ml. The cells were then exposed to catalase (Boehringer-Mannheim) at concentrations of 0 to 100 U/ml. These same conditions were repeated using human NK cells admixed with 30% human peripheral blood monocytes, recovered by centrifugal elutriation, added to the target K562 cells.  
         [0044]    After incubation at 37° C. for 16 hours, supernatant fluids were collected by a tissue collecting system (Amersham) and assayed for radioactivity in a gamma counter. Maximum  51 Cr release was determined in target cell cultures treated with Triton-X. NK cell cytotoxicity was calculated as cell lysis % according to the following formula:  
         cell                 lysis                 %     =     100   ×               experimental                 release     -               spontaneous                 release               maximum                 release     -     spontaneous                 release                                 
 
         [0045]    The results of these studies are illustrated in FIG. 1. FIG. 1A shows that catalase does not affect the level of NK-cell-mediated killing of tumor cells in the absence of monocytes, regardless of whether the NK cells are unactivated or activated by IL-2. FIG. 1B shows that the anti-tumor activity of unactivated NK cells is suppressed by the presence of monocytes. Further, catalase, at final concentrations exceeding 10 U/ml, reverses this suppressive signal. FIG. 1A further shows that IL-2 does not significantly activate NK-cell cytotoxicity against tumor cells in the presence of monocytes unless catalase is present.  
         [0046]    Effects of Catalase and Other Scavengers  
         [0047]    To study the relationship between the monocyte-derived inhibitory signal and the respiratory burst activity of monocytes, we added catalase and various other scavengers of reactive oxygen metabolites to NK cells, alone or admixed with monocytes. We then measured the cytotoxicity of the NK cells against NK cell sensitive K562 target cells as described above. The results of this testing are shown in Table I.  
                                                   TABLE I                           Effect of scavengers of reactive oxygen and nitrogen metabolites       on monocyte-induced inhibition of NK-cell cytotoxicity.            exp.   cell lysis % a              no.   treatment   conc.   MO   medium   histamine               1   catalase   0   −   55 ″ 3   53 ″ 2           ″    20 U/ml   −   60 ″ 3   52 ″ 4           ″   0   +   19 ″ 2   57 ″ 1           ″    2.5 U/ml   +   21 ″ 1   55 ″ 2           ″    5 U/ml   +   34 ″ 2   55 ″ 2           ″    10 U/ml   +   49 ″ 1   52 ″ 2           ″    20 U/ml   +   57 ″ 2   50 ″ 4           SOD   200 U/ml   −   52 ″ 2   54 ″ 2           ″   200 U/ml   +   1.3 ″ 2    54 ″ 2       2   taurin   0   −   72 ″ 2   70 ″ 3           ″   10 −3 M   −   70 ″ 4   75 ″ 4           ″   0   +    6 ″ 2   63 ″ 4           ″   10 −3 M   +    6 ″ 1   64 ″ 3       3   deferox   0   −   79 ″ 4   78 ″ 2           ″   10 −4 M   −   81 ″ 2   74 ″ 5           ″   0   +    9 ″ 3   62 ″ 2           ″   10 −4 M   +   12 ″ 3   60 ″ 2       4   mannitol   0   −   77 ″ 4   69 ″ 2           ″   3 × 10  −4 M   −   73 ″ 3   68 ″ 3           ″   0   +   19 ″ 2   64 ″ 4           ″   3 × 10 −4 M   +   20 ″ 1   67 ″ 2       5   L-NMMA   0   −   58 ″ 1   51 ″ 2           ″   2 × 10 −4 M   −   61 ″ 3   54 ″ 2           ″   0   +   12 ″ 1   45 ″ 2           ″   2 × 10 −4 M   +   11 ″ 1   45 ″ 3                          
 
         [0048]    Catalase, which effectively degrades H 2 O 2 , had no effect on the cytotoxicity of NK cells in the absence of monocytes but was found to completely abrogate the monocyte-induced inhibition of baseline NK cell cytotoxicity. Catalase was effective at concentrations exceeding 5 U/ml. Histamine (histamine dihydrochloride; Sigma) at concentrations exceeding 10 −7  M abrogated the monocyte induced suppression of NK cells but was ineffective in the presence of catalase.  
         [0049]    It was also discovered that superoxide dismutase (SOD), a scavenger of O 2   −  did not alter the suppressive effect of monocytes on NK cells over a wide range of concentrations. Similarly, taurin, a scavenger of HOCl − , and scavengers of OH such as mannitol and deferoxamine, were ineffective at reducing the suppressive effects of monocytes on NK cells.  
         [0050]    Further, monocytes and macrophages produce reactive nitrogen intermediates of which nitric oxide (NO) is the ultimate effector molecule. To study whether NO induction in monocytes contributed to NK cell inhibition, we used a NO synthetase inhibitor, N-monomethyl-L-arginine (L-NMMA). This compound, used at concentrations sufficient to inhibit induction of NO in monocytes, did not alter the suppression of NK cell function by monocytes. These results are also shown in Table I.  
         [0051]    Thus, we have concluded that NK cell-mediated cytotoxicity is suppressed by H 2 O 2  produced by monocytes. This suppression of NK cell-mediated cytotoxicity induced by H 2 O 2  is abrogated by the presence of catalase or histamine. In addition, it was discovered that human NK cells do not respond to IL-2 unless the monocyte-derived H 2 O 2  is scavenged by catalase or by some other scavenger.  
         [0052]    In vivo Effects of Hydrogen Peroxide Scavengers  
         [0053]    To study whether the regulatory effects on human NK cell function induced by catalase in vitro are of importance for NK cell-mediated killing of tumor cells in vivo, experiments were performed in which catalase was injected intravenously to mice shortly before intravenous inoculation of NK cell-sensitive tumor cells. These experiments are described below in Example 2.  
       EXAMPLE 2  
       [0054]    Seventy-five thousand  51 Cr-labeled, NK cell-sensitive YAC-1 mouse lymphoma cells were injected intravenously into male or female 4 to 6 week old Swiss Albino mice, together with vehicle (control) or 100 U/kg catalase. Two hours after inoculation with the tumor cells, the mice were sacrificed by cervical dislocation. The lungs were removed and placed in test tubes in a gamma-counter, and the radioactivity in the lung tissue was measured. The radioactivity in lung tissue is an inverse measure of NK cell-mediated killing of tumor cells in vivo (see Hanna et al.,  JNCI  65:801 (1980)), and is expressed as a percent of the  51 Cr that is retained in lungs immediately after inoculation of radiolabeled tumor cells.  
         [0055]    The results of this testing are shown in FIG. 2. The data in FIG. 2 represent four separate experiments. Each bar represents the retained radioactivity in lung pairs (mean “s.e.m. of 3-5 animals). Consistently, it was found that treatment with catalase augmented the NK cell-mediated killing of YAC-1 lymphoma cells in vivo.  
         [0056]    To determine whether the NK cell-mediated killing of tumor cells is enhanced using other NK cell activators and other peroxide scavengers, the experiments described below are performed.  
       EXAMPLE 3  
       [0057]    The experiment described in Example 1 is repeated using IL-12 as the NK cell activator. Similar results are obtained.  
       EXAMPLE 4  
       [0058]    The experiment described in Example 1 is repeated using IFN-α as the NK cell activator. Similar results are obtained.  
       EXAMPLE 5  
       [0059]    The experiment described in Example 1 is repeated using FAA as the NK cell activator. Similar results are obtained.  
       EXAMPLE 6  
       [0060]    The experiment described in Example 2 is repeated using glutathione peroxidase as the hydrogen peroxide scavenger. Similar results are obtained.  
       EXAMPLE 7  
       [0061]    The experiment described in Example 2 is repeated using ascorbate peroxidase as the hydrogen peroxide scavenger. Similar results are obtained.  
         [0062]    Inhibition of IL-2 Induced NK Cell Functions by Monocytes: Reversal by Catalase and Histamine  
         [0063]    IL-2 activates NK cell mediated cytotoxicity and induces proliferation of the resting population of NK cells. Elutriated monocytes effectively inhibit the IL-2 induced proliferation of enriched NK cells as well as the activation of NK cell cytotoxicity. To show that histamine, a compound we have discovered to suppress the generation of H 2 O 2  in monocytes, and catalase, a scavenger of H 2 O 2 , reverse the monocyte-induced inhibition, the following experiments were performed.  
       EXAMPLE 8  
       [0064]    Cell culture medium (control), histamine (10 −4  M), or catalase (20 U/ml), was added to either enriched NK cells alone or a mixture of NK cells and monocytes in microplates (1.5×10 5  cells/well). Each group of cells then received 50 U/ml human recombinant IL-2 and were allowed to incubate for 48 hours. During the last 8 hours of incubation, cells were pulsed with  3 H-methyl-thymidine (specific activity 2 Ci/mole; New England Nuclear Corp.; 1 μCi/2×10 5  cells). Following incubation, the cells were collected on glass fiber filters with an automatic cell harvester and cell-incorporated  3 H-methyl-thymidine was estimated by liquid scintillography.  
         [0065]    The results are shown in FIG. 3A, which illustrates NK cell proliferation, as reflected by  3 H-methyl-thymidine incorporation after treatment with IL-2. The bars represent cpm×10 3  (proliferation)±s.e.m. of sextuplicates. The results show that monocytes inhibit the proliferation of NK cells induced by IL-2. Both histamine and catalase effectively reverse this monocyte-induced inhibition.  
         [0066]    To show the inhibitory effect of monocytes on IL-2 induced NK cell cytotoxicity, and its reversal by histamine and catalase, the following experiment was performed.  
       EXAMPLE 9  
       [0067]    Cell culture medium (control), histamine (10 −4  M), or catalase (20 U/ml), was added to either enriched NK cells alone or a mixture of NK cells and monocytes (1.5×10 5  cells/well). These mixtures were then incubated with K562 target cells in sextuplicate in microplates in a total volume of 200 μl and assayed for microcytotoxicity. After incubation at 37° C. for 16 hours in the presence of culture medium (control) or IL-2 (50 U/ml), supernatant fluids were collected and assayed for radioactivity as described above in connection with Example 1. The results are illustrated in FIG. 3B.  
         [0068]    [0068]FIG. 3B shows the cytotoxicity of the respective cell mixtures against K562 target cells. The bars represent percent cell lysis±s.e.m. of sextuplicates. Again, it is clear from this data that monocytes inhibit the cytotoxicity of NK cells induced by IL-2. Both histamine and catalase effectively reverse this monocyte-induced inhibition.  
         [0069]    Reconstitution of Monocyte Induced Inhibition by H 2 O 2    
         [0070]    The finding that catalase, but not scavengers of O 2  or of OH; reversed the suppression of NK cells by monocytes suggests that H 2 O 2 , or metabolites of this compound, is essential for expression of the inhibitory signal. We therefore studied whether hydrogen peroxide could reconstitute the inhibitory effects of monocytes on NK cells.  
       EXAMPLE 10  
       [0071]    Culture medium (control) or H 2 O 2  at concentrations between 0-10 μM was added to NK cell enriched lymphocytes for assay of cytotoxicity against  51 Cr K562 target cells as described above. Addition of H 2 O 2  to enriched NK cells effectively suppressed NK cell cytotoxicity. The results of this testing are shown in FIG. 4, which shows the cell lysis %±s.e.m. of sextuplicates. The ED 50  of H 2 O 2  was approximately 2×10 −6  M, as seen in FIG. 4.  
         [0072]    It was also discovered that catalase (20 U/ml), but not histamine, completely reversed the inhibition of NK cells induced by H 2 O 2  (data not shown).  
         [0073]    Role of MPO  
         [0074]    To study whether H 2 O 2  alone or its reactive metabolites mediated the inhibitory effect of exogenous H 2 O 2  on NK cells, myeloperoxidase (MPO), a monocyte-derived enzyme that forms toxic hypohalous acids such as HOCI from H 2 O 2 , and halides and OH −  from H 2 H 2  and ferrous iron, was added to enriched NK cells, alone or together with H 2 O 2 . If radicals such as hypohalous acids contributed to the NK cell inhibitory signal, it was expected that MPO would potentiate the suppressive effect of H 2 O 2  on enriched NK cells. This testing is described below in Example 11.  
       EXAMPLE 11  
       [0075]    MPO (100 U/ml) and H 2 O 2  at concentrations between 0-10 μM were added to NK cell enriched lymphocytes for assay of cytotoxicity against  51 Cr K562 target cells as described above. The results of this testing are shown in FIG. 4, which shows the cell lysis %±s.e.m. of sextuplicates.  
         [0076]    MPO did not potentiate the suppressive effect of H 2 O 2  on NK cells. It was found that addition of MPO slightly but significantly scavenged H 2 O 2  in these experiments.  
         [0077]    These results, along with the finding the mannitol, taurin and deferoxamine, all of which are scavengers of MPO catalyzed products, did not affect the inhibition of NK cells by monocytes suggested that the inhibitory signal is independent of MPO activity.  
         [0078]    Kinetics of the Monocyte Derived NK Cell Inhibitory Signal  
         [0079]    To assess when the inhibitory signal is conveyed from monocytes to NK cells, experiments were performed in which catalase or histamine were added to mixtures of monocytes and NK cells at various time points after the beginning of the microcytotoxicity assay against K562 target cells.  
       EXAMPLE 12  
       [0080]    A mixture of enriched NK cells and monocytes were treated with culture medium (control), histamine (10 −4  M), or catalase (20 U/ml) and assayed for microcytotoxicity against target K562 cells as described above in connection with Example 1. It was found that catalase and histamine were effective in inhibiting the NK cell suppressive signal only when these compounds were added within the first hour of incubation of monocytes with NK cells. The results of this testing are shown in FIG. 5. The data shown is cell lysis %±s.e.m. of sextuplicates.  
         [0081]    In additional experiments, NK cell enriched lymphocytes were pretreated in petri dishes with culture medium (control), catalase or histamine, in the concentrations indicated below in Table II, in the presence or absence of monocytes. After 1 hour incubation, nonadherent lymphocytes were recovered, washed twice, and assayed for cytotoxicity as described above. We wished to determine whether the NK cell inhibitory signal was reversible by removal of monocytes and removal of monocyte-derived products. It was found that lymphocytes recovered from monocyte/NK cell mixtures pretreated with catalase or histamine were more cytotoxic against K562 target cells than control mixtures pretreated with medium only. The results of this testing are shown below in Table II. Data are cell lysis %±s.e.m. of sextuplicates and are the results of 2 separate experiments.  
                                     TABLE II                           Irreversible inhibition of NK-cells by monocytes and H 2 O 2 .            exp. no.   pretreatment   conc.   MO   cell lysis % a                 1   medium       −   66″3           histamine   10 −5  M   −   59″3           catalase   20 U/ml   −   67″2           medium       +   14″1           histamine   10 −5  M   +   51″2           catalase   20 U/ml   +   48″3       2   medium       −   59″3           H 2 O 2     1.5 × 10 −6 M   −   26″2           ″     3 × 10 −6 M   −   12″1           ″     6 × 10 −6 M   −    2″1                          
 
         [0082]    These data show that the inhibition of NK cells is evoked within the first hour of incubation with monocytes and that the inhibition is not reversible by removal of monocytes or monocyte derived factors. To confirm this finding, enriched NK cells were treated with H 2 O 2  for 20 minutes followed by extensive washing and assay for cytotoxicity. Pretreatment with H 2 O 2  at micromolar concentrations was sufficient to effectively inhibit NK cell cytotoxicity. The results of this testing are shown in Table II.  
         [0083]    Histaminergic Regulation of the Respiratory Burst of Monocytes  
         [0084]    Histamine has been reported to affect several functions ascribed to monocytes and macrophages, but effects of histamine on the respiratory burst of monocytes have remained unknown. To determine these effects, we first tested whether histamine could act as a scavenger of H 2 O 2  or its radical metabolites in a cell free system. We then studied the effects of histamine and H 2 R-interactive compounds on the respiratory burst activity of monocytes. These experiments are described in the following Examples.  
       EXAMPLE 13  
       [0085]    Chemiluminescence (CL) of cells was recorded at 37° C. in a 6-channel Biolumat LB 9505 (Berthold Co., Wildbad, Germany) using 4 ml polypropylene tubes as described by Lock et al.  Anal. Biochem.  173:450 (1988). The reaction mixture contained 0.8 ml elutriated monocytes (5×10 6  cells/ml). The tubes were allowed to equilibrate for 5 minutes at 37° C. before formylmethionyl-leucyl-phenylalanine (fMLP; Sigma; 10 −7  M final concentration) and luminol (Sigma; 10 −6  M) were added and light emission recorded. Formylmethionyl-leucyl-phenylalanine was dissolved to 10 −2  M in dimethyl sulfoxide and subsequently diluted in Krebs-Ringer phosphate buffer supplemented with glucose (10 mM), Ca 2+  (1 mM), and Mg 2+  (1.5 mM). Luminol was dissolved in 0.1 mM NaOH to 5×10 −2  M and further diluted in Krebs-Ringer phosphate buffer.  
         [0086]    The CL recorded with H 2 O 2  and/or MPO (10 Fg/ml) or H 2 O 2  and horseradish peroxidase (HRP; Calbiochem, La Jolla, Calif.) was unchanged by histamine (10 −4  M). Further, we used an assay system in which lysis of elutriated,  51 Cr-labelled RBC was measured in microplates. Addition of H 2 O 2  (5×10 −5  M) to 10 5  RBC induced lysis of approximately 50% of RBC. Histamine (10 −4  M) did not alter the level of RBC killing induced by H 2 O 2 . It is therefore concluded that histamine is not a scavenger of H 2 O 2  or its radical metabolites.  
       EXAMPLE 14  
       [0087]    In a second set of experiments, effects of histamine and H 2 R-interactive compounds on the respiratory burst activity, as measured by the luminol-enhanced CL response of enriched, elutriated monocytes, were studied. Monocytes were treated with culture medium (control), histamine (10 −5  M), sodium azide (10 −5  M), or histamine and sodium azide as described below. Emission of CL was recorded after addition of fMLP at time=0.  
         [0088]    It was found that histamine effectively inhibited both the burst activity of unstimulated monocytes and the induction of burst by fMLP. The results of this are shown in FIG. 6A. The inhibitory effect of histamine was dose dependent at final histamine concentrations of 10 −4 −10 −7  M.  
         [0089]    To assess whether histamine acted by inhibiting the generation of H 2 O 2  or by reducing the availability of peroxidase, we next studied the effects of histamine in monocytes treated with sodium azide to inhibit endogenous myeloperoxidase (MPO) and with exogenous, azide-insensitive peroxidase (HRP) in excess. Histamine inhibited the fMLP induced CL response also in this type of assay, showing that histamine specifically inhibits the formation of H 2 O 2  in monocytes. The results of this testing are shown in FIG. 6B.  
         [0090]    Dimaprit, (SK&amp;F, Hertfordshire, England), a specific H 2 R agonist, mimicked the effect of histamine on the respiratory burst of monocytes. In contrast, nor-dimaprit, (SK&amp;F, Hertfordshire, England), an H 2 R inactive structural analog of dimaprit, was ineffective. A striking difference between histamine and dimaprit was that whereas histamine blocked respiratory burst activity within seconds, the effect of dimaprit was not maximal until after 10-15 minutes of incubation (data not shown).  
         [0091]    [0091]FIG. 7 shows that the effects of histamine were entirely blocked by simultaneous treatment with the specific H 2 R antagonist ranitidine (Glaxo). To exclude non-specific effects of ranitidine, we used a ranitidine analog (AH20239AA; C 13 H 22 O 4 ; Glaxo) in which the thioether of ranitidine is replaced by an ether, thereby strongly reducing its H 2 R antagonist properties. In these experiments, monocytes were treated with histamine (10 −5  M) together with ranitidine or AH20239AA at final concentrations indicated in FIG. 7. All cells were treated with fMLP at time=0. Peak CL recorded in untreated monocytes (control) was 2.5×10 7  cpm.  
         [0092]    The chemical control to ranitidine, AH20239AA, was more than 100-fold less potent than ranitidine in antagonizing the effects of histamine on the suppression of NK cell function by monocytes, as well as the inhibition of respiratory burst activity by histamine, as shown in FIG. 7. The effects of histamine on the respiratory burst activity of monocytes, therefore, are specifically transduced by H 2 R.  
         [0093]    Conclusion  
         [0094]    We have discovered that hydrogen peroxide is a pivotal mediator of monocyte-derived, NK cell suppressive signal. The inhibitory effective of hydrogen peroxide on NK cells was not catalyzed by the addition of MPO, thus demonstrating that the MPO activity is not required to mediate NK cell inhibitory signals. Further, scavengers of MPO catalyzed radicals do not affect the inhibition of NK cell function induced by monocytes.  
         [0095]    It is clear from our results that histamine, serotonin, or other H 2  receptor agonists, acting via monocyte H 2  receptors, inhibit the generation of reactive oxygen products by monocytes, and thereby inhibit the NK cell suppressive signal. It is clear that scavengers of hydrogen peroxide also act to inhibit the NK cell suppressive signal.  
         [0096]    We have thus shown that treatment with a combination of an NK cell activating cytokine or other compound and a hydrogen peroxide scavenger or inhibiting compound in the presence of monocytes prevents the inactivation of NK cells and enhances NK cell cytotoxicity against tumor cells. These are unexpectedly superior results, since under similar circumstances, NK cell activators alone had no such beneficial effect. Of particular importance is that the potentiation of the anti-tumor effect of the NK cell activators induced by the concomitant treatment with a peroxide scavenger or inhibiting compound permits a reduction in the high doses of lymphokines which are used in cancer therapy. Advantageously, high dose treatments of lymphokines and the accompanying serious side effects can be eliminated by the method of the present invention.