Histamine-induced suppressor factor inhibition of NK cells: reversal with interferon and interleukin 2

Culture supernatants of lymphocytes stimulated with 10(-3) to 10(-8) M histamine contain histamine-induced soluble suppressor factor (HISSF) that significantly inhibits the natural killer (NK) cell functions of allogeneic lymphocytes. Lymphocytes precultured with increasing concentrations of HISSF showed a dose-dependent suppressive effect on their NK activity. HISSF was not cytotoxic itself and produced suppressive effects on PBL, NK-enriched large granular lymphocytes (LGL), and isolated T cells. Suppression was evident throughout a range of effector:target cell ratios. Production of HISSF was specifically blocked by the H2 antagonist cimetidine, but not by the H1 antagonist clemastine fumarate. Furthermore, H1 and H2 antagonists themselves do not induce production of HISSF. Although HISSF could inhibit the cytotoxicity of LGL, LGL themselves do not produce HISSF. HISSF inhibition of NK activity could be completely reversed by treating effector lymphocytes with recombinant interferon-alpha (IFN) for 1 or 2 hr or culturing them with purified interleukin 2(IL 2) for 36 hr. Our data suggest that exogenous IFN and IL 2 may have therapeutic potential in the treatment of immunological diseases associated with histamine-induced suppressor cell activity.     

Mechanism of cell-mediated cytotoxicity at the single-cell level: VII. Trigger of the lethal hit event is distinct for NK/K and LDCC effector cells as measured in the two-target conjugate assay

Normal human peripheral blood lymphocytes (PBL) express several in vitro cytotoxic functions, among which are natural killer (NK), antibody-dependent cellular cytotoxicity (ADCC), and lectin-dependent cellular cytotoxicity (LDCC). The relationship of these various cytotoxic functions and the identity of cells involved has been a subject of controversy. Recently it was reported that NK and K for ADCC can be mediated by the same cell, suggesting that they constitute in large part a single subpopulation with multiple cytotoxic functions. The ability of this NK/K effector cell to mediate LDCC was examined here using the two target conjugate assay. The effector cells were Ficoll-Hypaque PBL or LGL-enriched fractions. The targets used were K562 or MOLT for NK, RAJI coated with antibody for ADCC, and RAJI coated with PHA or Con A or modified by NaIO₄ for LDCC. In the two-target conjugate assay, one of the targets is fluorescein labeled for identification. The results show that (a) LDCC copurifies with NK/K and is enriched in the LGL fraction, as measured in both the ⁵¹Cr-release assay and the single-cell assay for cytotoxicity; (b) single effector cells simultaneously bind to NK or ADCC and LDCC targets, revealing that single cells bear binding receptors for all targets; and (c) single lymphocytes were not able to kill both bound NK/K and LDCC targets. However, significant two-target killing was obtained when both targets were NK targets, ADCC targets, LDCC targets, or one NK and one ADCC target. These results demonstrate that the NK and LDCC effector cells are distinct subpopulations copurified in the LGL fraction. In addition, the results show that lectin is unable to trigger globally an NK effector cell to mediate cytotoxicity against a bound NK insensitive target. Thus, although both NK and LDCC effector cells are present in the LGL fraction and can bind to both types of targets, the trigger of the lethal hit event is the function of specialized effector cells.     

Activation of human B lymphocytes. XVII. Synergy between nonspecific and specific signals in the antigen-specific responses of human B cells

The incubation of human peripheral blood lymphocytes (PBL) with the natural killer (NK)-sensitive MOLT-4 cell line results in PBL-target cell conjugate formation by certain lymphocyte subpopulations. Following velocity sedimentation, the PBL depleted of these conjugate-forming subpopulations are markedly diminished in the ability to mediate either antibody-dependent cellular cytotoxicity (ADCC) or NK activity. The immediate testing of highly pure PBL subpopulations isolated from the NK target conjugates does not reveal the expected recovery of augmented ADCC or NK levels. Following in vitro incubation, however, the PBL NK target-binding subpopulations do manifest augmented levels of both NK and ADCC, whereas the depleted PBL continue to display diminished NK and ADCC levels. In addition, the degree of augmented NK and ADCC levels recovered by the NK target-binding PBL subpopulations appears dependent on both the time and the temperature of in vitro incubation. Moreover, the ADCC recovery patterns are identical to those observed for NK activity regardless of the time and temperature of in vitro incubation. These results directly demonstrate that the PBL subpopulations isolated from certain NK target cells are functionally enriched in the ability to mediate from ADCC and NK activity.     

Suppression of human natural and antibody-dependent cytotoxicity by soluble factors from unstimulated normal lymphocytes

Serum-free culture supernatants of unstimulated normal human peripheral blood mononuclear cells contain soluble suppressor factor(s) (SSF) that significantly inhibit natural (NK) and antibody-dependent cellular cytotoxic (ADCC) activities of allogenic lymphocytes against a variety of target cells. Lymphocytes precultured with increasing concentrations of SSF showed a dose-dependent suppressive effect on these cytotoxic functions that was optimal at a concentration of 20% volume/volume. Adherent cells were not required for the production of SSF. Suppression was evident even at higher effector: target cell ratios and the inhibition was not reversed by washing lymphocytes. SSF was not itself cytotoxic, was stable at 56 degrees C, and its suppressive effect was maximal after 72 hr of incubation with effector lymphocytes. Initial estimate of the molecular weight of SSF by ultra-filtration was less than 20,000 daltons. Gel filtration of SSF on Sephacryl S-200 resulted in the elution of two peaks of activity; one in the region between markers of 13,700 and 25,000 daltons, and the other less than 13,700 daltons. Both fractions demonstrated significant suppressive activity on NK and ADCC functions of allogenic lymphocytes. SSF inhibition of NK activity could be partially reversed by incubating lymphocytes for 1 hr with human leukocyte interferon (IF) and almost completely reversed after 24 hr of IF treatment. A few selected monosaccharides (alpha-methyl-D-mannoside, L-fucose and L-rhamnose) showed a dose-dependent blocking effect on SSF activity, which suggests that SSF may act via receptor sites recognized by these sugars. As demonstrated for other lymphocyte functions, NK and ADCC activities may also be modulated by SSF elaborated by normal PBL.     

Immunomodulatory effects of corticosteroids on natural killer and antibody-dependent cellular cytotoxic activities of human lymphocytes

The in vitro effect of prednisolone (PRD) on NK and ADCC activities of human lymphocytes was investigated. PRD at concentrations ranging from 7.5 X 10(-3) to 1 X 10(-5) M significantly inhibited NK activity, while concentrations of 7.5 X 10(-3) to 1 X 10(-4) M inhibited ADCC activities of PBL when added directly to the mixture of effector and target cells. Lymphocytes pre-cultured for 24 hr with PRD at concentrations ranging from 1 X 10(-4) M to 1 X 10(-6) M showed significant suppression of their NK activity. Inhibition was proportional to the concentration of the drug, and was observed at as early as 1 hr of incubation at various effector to target cell ratios with several targets. PRD also inhibited NK and ADCC activities of purified T cells, non-T cells, and NK-enriched effector cells. In target-binding assays, PRD decreased the target-binding capacity of effector lymphocytes in a dose-dependent manner. PRD-induced inhibition could be reversed by incubating lymphocytes for 1 hr with interferon or IL 2. Pretreatment of targets with PRD for 4 hr did not affect cytotoxic activity. Inhibition of cytotoxicity was not due to direct toxicity to effector cells because lymphocytes treated with PRD showed normal spontaneous 51Cr release, and their viability after 24 hr of pre-culture with PRD was comparable to that of untreated control cells. These results demonstrate that PRD has significant immunomodulatory effects on human NK and ADCC activities that may be of clinical relevance.     

Mechanism of defective NK cell activity in patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex. II. Normal antibody-dependent cellular cytotoxicity (ADCC) mediated by effector cells defective in natural killer (NK) cytotoxicity

Our studies and other investigations have shown that NK effector cells can also mediate antibody-dependent cellular cytotoxicity (ADCC) through the use of the Fc gamma receptor on the NK cell membrane. Peripheral blood lymphocytes (PBL) derived from patients with acquired immunodeficiency syndrome (AIDS) and AIDS-related complex exhibit a poor NK activity due to a defective "trigger" required for activation in the lethal hit stage of the NK lytic pathway. Consequently, it was important to delineate whether the defect in AIDS NK cells affected the ADCC function. By using the 51Cr-release assay, the ADCC cytotoxic activity of AIDS PBL was found to be within the normal range, despite the absence of significant NK activity. Several experiments corroborated that the same effector cells mediate both NK CMC and ADCC. Depletion of Fc gamma R-bearing cells resulted in elimination of both the ADCC and NK cytotoxic functions. Single cell analyses, using one- and two-target cell conjugates, revealed that the frequency of ADCC effector:target conjugates and the frequency of killer cells from AIDS PBL were comparable to the frequencies seen in the normal controls. However, when mixtures of NK and ADCC targets were used to form mixed two-target conjugates, the AIDS effector cells lysed only the bound ADCC target, whereas the normal effector cells lysed both the bound NK and ADCC targets. These results demonstrate clearly that the same NK/K effector cells from AIDS PBL, defective in NK activity, are not impaired in mediating ADCC activity. These findings were supported by the demonstration that AIDS PBL stimulated with ADCC targets, but not with NK targets, released NK cytotoxic factors, postulated mediators of the NK CMC reaction. These findings indicate that the NK/K cells in AIDS are triggered normally for ADCC activity but are not triggered for NK activity. Furthermore, the results indicate that the lytic machinery is not impaired in the AIDS NK/K cells.     

Lymphocyte-mediated cytotoxicity to autologous cells infected with measles virus. II. Specificity of the cytotoxic reaction and characterization of the effector cells involved.

The mechanism of lymphocyte-mediated cytotoxicity to cells infected with measles virus was investigated. Cytotoxicity was measured in a direct assay, immediately after the isolation of lymphocytes from human peripheral blood; mononuclear leukocytes, infected with measles virus in vitro, served as autologous target cells. Virus-specific cytotoxicity required the presence of both IgG antibodies against measles virus and of effector lymphocytes. The effector lymphocytes had Fc receptors and were mainly present in a fraction of non-T lymphocytes. Monocytes were not cytotoxic but rather inhibitory. These results indicate that lysis of virus-infected cells in this direct assay is due to antibody-dependent cellular cytotoxicity (ADCC), caused by K cells. Control experiments showed that the virus-infected target cells were sensitive to incubation with human serum or IgG, resulting in a nonspecific increase of ⁵¹Cr release; however, this did not affect the results of K-cell cytotoxicity. Maximal virus-specific lysis by ADCC did not reach the level obtained by complement-dependent cytotoxicity. Possible explanations for this difference are discussed.     

Virus-mediated induction in human lymphocytes of antibody-independent cytotoxicity (VDCC) and enhancement of antibody-dependent cytotoxicity (ADCC) against natural killer-resistant tumor target cells

The effect of Parotis virus on the in vitro cytotoxicity of human lymphocytes against NK-resistant mouse mastocytoma cells was studied. In the ⁵¹Cr-release assay, treatment of lymphocytes with virus induced a rapid cytotoxicity in the absence of anti-P8 15 antibody (virus-dependent cellular Cytotoxicity, VDCC) and strongly enhanced antibody-dependent cytotoxicity (ADCC). At the effector cell level, virus treatment was found to increase the frequency of target-binding cells (TBC) as well as the proportion thereof mediating VDCC and/ or ADCC, indicating recruitment of active effector cells. The recruited cells were heterogeneous but contained a major fraction bearing the T-cell-associated antigen T3. Virus was found to decrease rather than to increase the recycling capacity of the cytotoxic lymphocytes, suggesting that VDCC induction and ADCC enhancement were due to a virus-mediated improvement of effector cell-target cell interactions. VDCC and ADCC enhancement may be of protective importance in early phases of virus infection as well as for the production of nonspecific tissue injuries associated with viral disease.     

Differential effects of ouabain on human cell-mediated cytotoxicity. II. Stimulation of monocyte-mediated, spontaneous cytotoxicity against chicken red cell targets.

We have previously shown that ouabain inhibits mitogen-induced cellular cytotoxicity (MICC) and antibody-dependent cellular cytotoxicity (ADCC) against chicken red cell (CRC) targets. We now report that ouabain increases spontaneous killing of CRC targets in the absence of mitogen or antibody. Spontaneous cytotoxicity by fresh mononuclear leukocytes (MNL) was enhanced by ouabain in a dose-dependent fashion and was maximal at a ouabain concentration of 5 × 10^?5M. Removal of phagocytic cells from the MNL effector cell population abrogated ouabain-induced spontaneous cytotoxicity, suggesting that the effector cell activated by ouabain was a monocyte. Ouabain-induced spontaneous cytotoxicity was relatively inefficient compared to MICC or ADCC and was only demonstrated consistently at effector:target cell ratios higher than those routinely employed for MICC and ADCC. Very low concentrations of ouabain (5 × 10^?9M) also enhanced spontaneous cytotoxicity of MNL precultured for 7 days, when added at either Day 0 or Day 6 of preculture. The cell effecting spontaneous cytotoxicity after 7 days of culture has been previously shown to be a monocyte. Thus, ouabain has opposing effects on cell-mediated cytotoxic functions: it inhibits MICC and ADCC against CRC targets, but stimulates spontaneous, monocyte-mediated cytotoxicity against the same targets.     

Inhibition of human natural killer (NK) activity and antibody dependent cellular cytotoxicity (ADCC) by lipomodulin, a phospholipase inhibitory protein

A highly purified preparation of lipomodulin, a phospholipase-inhibitory protein from rabbit neutrophils treated with glucocorticoids, inhibited NK and antibody-dependent cellular cytotoxicity (ADCC) activities of human peripheral blood lymphocytes in a dose-dependent manner. The presence of lipomodulin during the early period of the cytotoxicity assay was necessary to obtain maximal inhibition. The inhibition of NK or ADCC activity by lipomodulin was greater when effector cells were treated with lipomodulin than when target cells were incubated with lipomodulin. As lipomodulin did not block binding of effector cells to target cells, our results suggest that lipomodulin inhibits the cytolytic phase of NK and ADCC activities after binding to target cells, and imply that phospholipase(s) may be involved in NK and ADCC activities.     

Effects of carrageenan on spontaneous and antibody-dependent cell-mediated cytotoxicity

We examined the effect of carrageenan on in vitro antibody-dependent cell-mediated cytolysis (ADCC) and spontaneous cell-mediated cytolysis (SCMC) in cultures of human peripheral blood mononuclear cells (PBL). Carrageenan, when present in the assay, nonspecifically reduced ADCC and SCMC against both Chang and chicken erythrocyte (CRBC) target cells. This reduction in cytotoxicity could not be attributed entirely to the macrophage toxic and complement-inhibitory properties of carrageenan because neither removal of complement nor macrophage depletion prevented the dose-dependent inhibition. In contrast, pretreatment of effector PBL, with carrageenan followed by removal of Carrageenan by washing did not alter ADCC or SCMC against Chang cells, which are mediated by nonphagocytic cells, but reduced both ADCC and SCMC activity against CRBC targets, which are mediated in part by macrophages. Thus, Carrageenan, when present in in vitro cell-mediated cytotoxicity assays, causes a nonspecific impairment of cytotoxicity that is independent of its anticomplement or macrophage-toxic properties.     

Polyadnylic acid sequences in RNA able to transfer specific delayed type hypersensitivity in vitro.

Antibody-depedent cell-mediated cytotoxicity (ADCC) could be initiated without protein synthesis [human peripheral blood lymphocytes as effector cells incubated with 10^?3M cycloheximide, (Cy)], although the reaction did not achieve its full lytic ability. This partial inhibition of ADCC was dependent on the dose of Cy. Both ADCC and protein synthesis returned to normal values after removal of the inhibitor. The kinetics of the reaction carried out by Cy-treated effector cells for short periods was similar to that of controls. After this time, the percentage of lysed target cells increased continuously in controls, while the cytotoxiciy of Cy-treated effector cells reached a plateau. When effector cells carried out ADCC in the presence of Cy, their lytic mechanism was “wasted,” and it could be recovered only by removal of the inhibitor. Our results indicate that effector cells have a preformed lytic mechanism operative in ADCC. This lytic mechanism is consumed during the reaction and its recovery requires protein synthesis.     

Feline cytotoxic immune mechanisms against virus-associated leukemia and fibrosarcoma

Humoral and cellular cytotoxic immune mechanisms of cats were compared against feline leukemia virus (FeLV)- and feline sarcoma virus (FeSV)-transformed cells. The groups of animals studied were nonexposed control cats; FeLV-infected immune or viremic tumor-bearing cats; FeSV-inoculated tumor progressor or regressor cats, and cats immunized with FeSV-transformed autochthonous fibroblasts (ATF). Sera containing complement-dependent antibodies (CDA), which lysed FeLV-producer lymphoma lines, had no cytotoxic effects when tested against FeLV-producer FeSV-transformed fibroblasts. Sera with lytic CDA activity were also tested for antibody-dependent cellular cytotoxic (ADCC) effects with peripheral blood lymphocytes (PBL) from nonimmune cats. No ADCC activity was detected against either lymphoid or fibroblast target lines. To demonstrate that cat PBL contained ADCC effector cells, antibody-coated murine target cells were employed and positive results obtained. Natural killer (NK) assays were performed using PBL from normal and tumor-bearing cats. Cytotoxic effects were only detectable to FeLV-producer lymphomas, and comparable levels of NK activity were found in normal and lymphoid tumor-bearing animals. In cats immunized with ATF, a population of effector cells was found in peripheral blood which had functional characteristics of cytotoxic T lymphocytes (CTL). The killing of ATF by CTL-like cells was not inhibited by FeLV/FeSV immune sera or by sera from autochthonous immune cats. The comparative importance of humoral and cellular cytotoxic mechanisms against FeLV- and FeSV-induced tumors is discussed.     

Cytotoxicity from cultured cells: analysis of precursors involved in generation of human cells mediating natural and antibody-dependent cell-mediated cytotoxicity.

Natural cell-mediated cytotoxicity of human peripheral blood lymphocytes natural killer (NK) against K-562 and antibody-dependent cellular cytotoxicity (ADCC) against Chang cells, as measured in a 4-hr ⁵¹Cr release assay, were both completely removed by depletion of Fc receptor-positive (FcR+) cells. After in vitro culture for 7 days, however, NK- and ADCC-like activities spontaneously regenerated. The nature of precursor cells was studied by examination of lymphocyte subpopulations required for generation of this cytotoxicity. After depletion of FcR+ cells from PBL, the following subpopulations were prepared: sheep erythrocyte rosette-forming cells (E+), surface membrane immunoglobulin-positive cells (SmIg+), and null cells (lacking E+, SmIg+, or FcR+ markers). Separate cell types or mixtures were cultured in vitro in medium containing 10% fetal calf serum for 7 days and then tested for NK and ADCC. Whereas unseparated FcR-depleted cells developed substantial cytotoxic activity, each of the subpopulations cultured alone was negative or had low activity. Addition of SmIg+ cells to other cell types had no effect; however, mixture of 80% E+ and 20% null cells resulted in optimal NK and ADCC. It is not presently clear which population the precursors were in. However, the requirement for proliferation by the null cell population but not by the E+ cells (as indicated by sensitivity to radiation and mitomycin C) suggested that the precursors for NK cells may be null cells.     

H1-histaminergic activation stimulates inositol-1-phosphate accumulation in chromaffin cells

Adrenal medullary chromaffin cells maintained in vitro were prelabeled with [3H]inositol and the accumulation of [3H]inositol-1-phosphate, was determined following stimulation with a variety of pharmacological agents. Carbachol, bradykinin, and histamine produced significantly greater accumulation of [3H] inositol-1-phosphate over basal levels, with histamine producing the greatest effect. H1-histamine receptor antagonists, mepyramine, pyrilamine, tripelennamine and clemastine were all able to reduce or completely block the histamine response. The two specific H2-histamine receptor antagonists, cimetidine and ranitidine, had no effect on this response. Histamine dose-response characteristics in the presence of mepyramine and clemastine suggest the H1 antagonism to be competitive in nature.     

Pharmacological characterization of the histamine uptake system in HL-60 human promyelocytic leukemia cells

Serotonin and histamine H₁, H₂ receptor agonists or antagonists inhibited [³H]histamine uptake by HL-60 cells, according to the following order of potency: impromidine >4-MH>histamine>AET>PEA and: cimetidine, histamine>diphenhydramine, serotonin. It is concluded that histamine uptake by HL-60 cells was specifically controlled by the H₂ type histamine receptor and that this active process might be involved in pathophysiological regulations in leukemic and normal granulocytic precursors and in the control of histamine levels in peripheral blood and tissues in man.     

Localization of histamine and histamine H2-receptor antagonists in the gastric mucosa.

Synopsis Histamine stimulates acid secretion by the parietal cell and this secretion is inhibited by the histamine H₂-receptor antagonists. Whole body autoradiography showed that radioactivity from¹⁴C-histamine was localized in the artery walls of the stomach and in the muscularis mucosae, but that the level in the fundic mucosa was the same as the blood.When the H₂-receptor antagonists burimamide, metiamide and cimetidine were labelled with³⁵S,¹⁴C or³H and dosed to rats, whole body autoradiography showed that the stomach was predominantly labelled in the glandular mucosa from 5 to 120 min after administration. Microautoradiography in the rat and dog after intravenous injection of [³H] metiamide or [³H] cimetidine demonstrated an uptake of tritium in the parietal cell cytoplasm that was 3- to 4-times greater than that found in adjacent peptic cells or areas of muscularis mucosa. The preferential labelling persisted at a low level up to 6 h after injection in the rat. The localization of radioactivity from the H₂-antagonists in the parietal cell cytoplasm correlates well with their pharmacological activity in preventing acid secretion from this cell.     

Immunomodulatory activity of commonly used drugs on Fc-receptor-mediated human natural killer cell activation

Natural killer (NK) cells mediate defense against neoplastic as well as infected cells. Yet, how their effector functions are affected by the large variety of pharmacological compounds commonly in use has not been investigated systematically. Here, we screened 1,200 in-use or previously approved drugs for their biological effect on freshly isolated human peripheral blood-derived NK cells. Mimicking antibody-dependent cellular cytotoxicity (ADCC), known to be important in antibody-based immunotherapies against, e.g., human malignancies, the cells were stimulated by Fc-receptor (CD16) engagement. Cellular responses were assessed by flow cytometry. Fifty-six compounds that significantly inhibited and twelve that enhanced one or more of the readouts of adhesion, exocytosis, and chemokine production were identified and confirmed as hits. Among the confirmed inhibitors, 80 % could be assigned to one of seven major pharmacological classes. These classes were β2-adrenergic agonists, prostaglandins, phosphodiesterase-4 inhibitors, Ca²⁺-channel blockers, histamine H1-receptor antagonists, serotonin/dopamine receptor antagonists, and topoisomerase inhibitors that displayed distinct inhibitory patterns on NK cell responses. Among observed enhancers, interestingly, two ergosterol synthesis inhibitors were identified that specifically promoted exocytosis. In summary, these results provide a comprehensive knowledge base of the effect known drugs have on NK cells. More specifically, they provide an overview of drugs that may modulate NK cell-mediated ADCC in the context of clinical immunotherapies.     

Cytotoxic effector cell function in organized gut-associated lymphoid tissue (GALT).

Lymphocytes from the organized gut-associated lymphoid tissues (GALT) of adult guinea pigs were examined for surface markers characteristic of T and B lymphocytes and for their capacity to function as effector cells in mitogen-induced cellular cytotoxicity (MICC) and antibody-dependent cellular cytotoxicity (ADCC) reactions. GALT lymphocytes formed rosettes with rabbit erythrocytes, a T-cell marker, and underwent proliferative responses in vitro in the presence of phytohemagglutinin (PHA), concanavalin A (Con A), and pokeweed mitogen (PWM). GALT lymphocytes were cytotoxic in vitro for erythrocyte and DBA mastocytoma targets in the presence of PHA. A population of GALT lymphocytes bound aggregated γ-globulin; however, they functioned poorly in ADCC reactions. Thus, organized GALT in the guinea pig contains lymphocytes capable of functioning in T-cell-dependent MICC reactions but either lacks the effector cell population which mediates ADCC or contains an effector cell which functions poorly in ADCC.     

Effect of human recombinant interferon on cytotoxic activity of natural killer (NK) cells and monocytes

Studies have been performed on the in vitro immunologic effects of homogeneous recombinant human leukocyte interferon, IFLrA. Large granular lymphocytes, enriched for natural killer (NK) cell activity, were pretreated wtih IFLrA or natural interferon preparations and then tested for augmentation of NK activity and of antibody-dependent cell-mediated cytoxicity (ADCC). Monocytes were tested for cytolytic and cytostatic activity in 48–72 hr radioisotopic assays performed in the presence or absence of interferon. Treatment with IFLrA caused significant augmentation of NK, ADCC, and monocyte-mediated cytotoxic activities. Even 10 units of IFLrA induced augmentation of NK activity, and 100 units or more boosted monocyte-mediated activity. The effects in each of these assays were species-specific, with no detectable effects on the activity of mouse effector cells. These results indicate that homogeneous recombinant interferon has potent in vitro immunomodulating effects and thus provide a basis for carefully examining the in vivo effects of this protein on host defenses in forthcoming clinical trials with cancer patients.