Human monocyte antibody-dependent cell-mediated cytotoxicity to tumor cells

Small CD4 mimetics sensitize HIV-1-infected macrophages to antibody-dependent cellular cytotoxicity

HIV-1 envelope (Env) conformation determines the susceptibility of infected CD4+ T cells to antibody-dependent cellular cytotoxicity (ADCC). Upon interaction with CD4, Env adopts more "open" conformations, exposing ADCC epitopes. HIV-1 limits Env-CD4 interaction and protects infected cells against ADCC by downregulating CD4 via Nef, Vpu, and Env. Limited data exist, however, of the role of these proteins in downmodulating CD4 on infected macrophages and how this impacts Env conformation. While Nef, Vpu, and Env are all required to efficiently downregulate CD4 on infected CD4+ T cells, we show here that any one of these proteins is sufficient to downmodulate most CD4 from the surface of infected macrophages. Consistent with this finding, Nef and Vpu have a lesser impact on Env conformation and ADCC sensitivity in infected macrophages compared with CD4+ T cells. However, treatment of infected macrophages with small CD4 mimetics exposes vulnerable CD4-induced Env epitopes and sensitizes them to ADCC.

CD16-158-valine chimeric receptor T cells overcome the resistance of KRAS-mutated colorectal carcinoma cells to cetuximab

KRAS mutations hinder therapeutic efficacy of epidermal growth factor receptor (EGFR)-specific monoclonal antibodies cetuximab and panitumumab-based immunotherapy of EGFR+ cancers. Although cetuximab inhibits KRAS-mutated cancer cell growth in vitro by natural killer (NK) cell-mediated antibody-dependent cellular cytotoxicity (ADCC), KRAS-mutated colorectal carcinoma (CRC) cells escape NK cell immunosurveillance in vivo. To overcome this limitation, we used cetuximab and panitumumab to redirect Fcγ chimeric receptor (CR) T cells against KRAS-mutated HCT116 colorectal cancer (CRC) cells. We compared four polymorphic Fcγ-CR constructs including CD16158F -CR, CD16158V -CR, CD32131H -CR, and CD32131R -CR transduced into T cells by retroviral vectors. Percentages of transduced T cells expressing CD32131H -CR (83.5 ± 9.5) and CD32131R -CR (77.7 ± 13.2) were significantly higher than those expressing with CD16158F -CR (30.3 ± 10.2) and CD16158V -CR (51.7 ± 13.7) (p < 0.003). CD32131R -CR T cells specifically bound soluble cetuximab and panitumumab. However, only CD16158V -CR T cells released high levels of interferon gamma (IFNγ = 1,145.5 pg/ml ±16.5 pg/ml, p < 0.001) and tumor necrosis factor alpha (TNFα = 614 pg/ml ± 21 pg/ml, p < 0.001) upon incubation with cetuximab-opsonized HCT116 cells. Moreover, only CD16158V -CR T cells combined with cetuximab killed HCT116 cells and A549 KRAS-mutated cells in vitro. CD16158V -CR T cells also effectively controlled subcutaneous growth of HCT116 cells in CB17-SCID mice in vivo. Thus, CD16158V -CR T cells combined with cetuximab represent useful reagents to develop innovative EGFR+KRAS-mutated CRC immunotherapies.

CAPD and Bone Marrow Cell-Cell Interaction Abnormality in end Stage Renal Disease

In this research the early erythroid progenitor (BFU-E) growth has been observed in patients with chronic renal failure by studing in «in vitro» cultures the number of BFU-E developed per plate under various experimental conditions. Our results demonstrate that the BFU-E growth of uremic patients in the predialysis phase or on hemodialysis is suppressed both in basal conditions and with the addition of lympho-monocytes of normal subjects or CAPD patients to the «in vitro» cultures. In CAPD patients, on the contrary, the BFU-E growth appears near normal levels in basal conditions and is further more enhanced by adding the normal lympho-monocytes to the «in vitro» cultures. The above mentioned facts signify that in uremic patients both the abnormality of bone marrow and immunocompetent cells may be responsable for the suppressed BFU-E growth, while with CAPD the improvement of both systems allow a normalisation of erythropoiesis. Such a recovery with CAPD demonstrates the presence of an inhibitor material in uremia which is better depurated by this technique.

Downregulation of neuropilin-1 on macrophages modulates antibody-mediated tumoricidal activity

Neuropilin-1 (NRP-1)-expressing macrophages are engaged in antitumor immune functions via various mechanisms. In this study, we investigated the role of NRP-1 on macrophages in antibody-mediated tumoricidal activity. Treatment of macrophages with NRP-1 knockdown or an anti-NRP-1-neutralizing antibody significantly suppressed antibody-dependent cellular cytotoxicity and modulated cytokine secretion from macrophages in vitro. Furthermore, in vivo studies using a humanized mouse model bearing human epidermal growth factor receptor-2 (HER2)-positive breast cancer xenografts showed that antibody-mediated antitumor activity and tumor infiltration of CD4+ T lymphocytes were significantly downregulated when peripheral blood mononuclear cells in which NRP-1 was knocked down were co-administered with an anti-HER2 antibody. These results revealed that NRP-1 expressed on macrophages plays an important role in antibody-mediated antitumor immunity. Taken together, the induction of NRP-1 on macrophages may be a therapeutic indicator for antibody treatments that exert antibody-dependent cellular cytotoxicity activity, although further studies are needed in order to support this hypothesis.

Natural Killer (NK) Cell Education Differentially Influences HIV Antibody-Dependent NK Cell Activation and Antibody-Dependent Cellular Cytotoxicity

Immunotherapy using broadly neutralizing antibodies (bNAbs) endowed with Fc-mediated effector functions has been shown to be critical for protecting or controlling viral replication in animal models. In human, the RV144 Thai trial was the first trial to demonstrate a significant protection against HIV infection following vaccination. Analysis of the correlates of immune protection in this trial identified an association between the presence of antibody-dependent cellular cytotoxicity (ADCC) mediated by immunoglobulin G (IgG) antibodies (Abs) to HIV envelope (Env) V1/V2 loop structures and protection from infection, provided IgA Abs with competing specificity were not present. Systems serology analyses implicated a broader range of Ab-dependent functions in protection from HIV infection, including but not limited to ADCC and Ab-dependent NK cell activation (ADNKA) for secretion of IFN-γ and CCL4 and expression of the degranulation marker CD107a. The existence of such correlations in the absence of bNAbs in the RV144 trial suggest that NK cells could be instrumental in protecting against HIV infection by limiting viral spread through Fc-mediated functions such as ADCC and the production of antiviral cytokines/chemokines. Beside the engagement of FcγRIIIa or CD16 by the Fc portion of anti-Env IgG1 and IgG3 Abs, natural killer (NK) cells are also able to directly kill infected cells and produce cytokines/chemokines in an Ab-independent manner. Responsiveness of NK cells depends on the integration of activating and inhibitory signals through NK receptors, which is determined by a process during their development known as education. NK cell education requires the engagement of inhibitory NK receptors by their human leukocyte antigen ligands to establish tolerance to self while allowing NK cells to respond to self cells altered by virus infection, transformation, stress, and to allogeneic cells. Here, we review recent findings regarding the impact of inter-individual differences in NK cell education on Ab-dependent functions such as ADCC and ADNKA, including what is known about the HIV Env epitope specificity of ADCC competent Abs and the conformation of HIV Env on target cells used for ADCC assays.

NK Cell-Mediated Antitumor Effects of a Folate-Conjugated Immunoglobulin Are Enhanced by Cytokines

Optimally effective antitumor therapies would not only activate immune effector cells but also engage them at the tumor. Folate conjugated to immunoglobulin (F-IgG) could direct innate immune cells with Fc receptors to folate receptor-expressing cancer cells. F-IgG bound to human KB and HeLa cells, as well as murine L1210JF, a folate receptor (FR)-overexpressing cancer cell line, as determined by flow cytometry. Recognition of F-IgG by natural killer (NK) cell Fc receptors led to phosphorylation of the ERK transcription factor and increased NK cell expression of CD69. Lysis of KB tumor cells by NK cells increased by about 5-fold after treatment with F-IgG, an effect synergistically enhanced by treatment with IL2, IL12, IL15, or IL21 (P< 0.001). F-IgG also enhanced the lysis of chronic lymphocytic leukemia cells by autologous NK cells. NK cells significantly increased production of IFNγ, MIP-1α, and RANTES in response to F-IgG-coated KB target cells in the presence of the NK cell-activating cytokine IL12, and these coculture supernatants induced significant T-cell chemotaxis (P< 0.001). F-IgG-coated targets also stimulated FcR-mediated monocyte effector functions. Studies in a murine leukemia model confirmed the intratumoral localization and antitumor activity of F-IgG, as well as enhancement of its effects by IL12 (P =0.05). The antitumor effect of this combination was dependent on NK cells and led to decreased tumor cell proliferation in vivo Thus, F-IgG can induce an immune response against FR-positive tumor cells that is mediated by NK cells and can be augmented by cytokine therapy.

Fcγ receptor-induced soluble vascular endothelial growth factor receptor-1 (VEGFR-1) production inhibits angiogenesis and enhances efficacy of anti-tumor antibodies

Monocytes/macrophages are potent mediators of antitumor antibody therapy, where they engage target cells via Fcγ receptors (FcγR). Binding of these cells to opsonized tumor targets elicits cytokine production, phagocytosis, and antibody-mediated cellular cytotoxicity. Here we show for the first time that activation of monocyte FcγR results in the secretion of soluble vascular endothelial growth factor receptor-1 (VEGFR-1/sFlt-1), which serves to antagonize VEGF-mediated angiogenesis and tumor growth. Consistent with this, using a murine solid tumor model of antibody therapy, we show that sFlt-1 is involved in restricting tumor growth. Analyzing the mechanism of induction of sFlt-1, we found that the Erk and PI3K pathways were required for transcription, and NF-κB was required for translation. Upon closer examination of the role of NF-κB, we found that a microRNA, miR181a, negatively regulates FcγR-mediated sFlt-1 production and that NF-κB serves to antagonize this microRNA. Taken together, these results demonstrate a novel and biologically important function of monocytes and macrophages during antibody therapy.

Role of monocytes in mediating HIV-specific antibody-dependent cellular cytotoxicity

Antibodies (Abs) that mediate antibody-dependent cellular cytotoxicity (ADCC) activity against HIV-1 are of major interest. A widely used method to measure ADCC Abs is the rapid and fluorometric antibody-dependent cellular cytotoxicity (RFADCC) assay. Antibody-dependent killing of a labelled target cell line by PBMC is assessed by loss of intracellular CFSE but retention of membrane dye PKH26 (CFSE-PKH26+). Cells of this phenotype are assumed to be derived from CFSE+PKH26+ target cells killed by NK cells. We assessed the effector cells that mediate ADCC in this assay. Backgating analysis and phenotyping of CFSE-PKH26+ revealed that the RFADCC assay's readout mainly represents CD3-CD14+ monocytes taking up the PKH26 dye. This was confirmed for 53 HIV+plasma-purified IgG samples when co-cultured with PBMC from three separate healthy donors. Emergence of the CFSE-PKH26+ monocyte population was observed upon co-culture of targets with purified monocytes but not with purified NK cells. Image flow cytometry and microscopy showed a monocyte-specific interaction with target cells without typical morphological changes associated with phagocytosis, suggesting a monocyte-mediated ADCC process. We conclude that the RFADCC assay primarily reflects Ab-mediated monocyte function. Further studies on the immunological importance of HIV-specific monocyte-mediated ADCC are warranted.

Tumour cell lines HT-29 and FaDu produce proinflammatory cytokines and activate neutrophils in vitro: possible applications for neutrophil-based antitumour treatment

There is evidence that polymorphonuclear neutrophils (PMNs) can exert severe antineoplastic effects. Cross-talk between tumour cells and endothelial cells (ECs) is necessary for the accumulation of PMN around a tumour. This work reports the ability of two PMN-sensitive, human, permanent cell lines—colorectal adenocarcinoma (HT-29) and pharyngeal squamous-cell carcinoma (FaDu) cells—to act as inflammatory foci. PMNs were cytotoxic to both lines, the adhesion of the PMNs to the tumour cells being important in this effect. The tumour cells released appreciable amounts of IL-8 and GROα, and induced the transmigration of PMN through human microvascular-EC monolayers. Conditioning media associated with both lines induced the adhesion of PMN and the surface expression of ICAM-1 in microvascular-EC. In addition, FaDu-conditioning-medium strongly induced the production of proinflammatory cytokines by microvascular-EC. These results support the idea that tumour cells might normally induce a potent acute inflammatory response, leading to their own destruction.

Resveratrol blocks interleukin-1beta-induced activation of the nuclear transcription factor NF-kappaB, inhibits proliferation, causes S-phase arrest, and induces apoptosis of acute myeloid leukemia cells

Resveratrol, an edible polyphenolic stilbene, has been reported to possess substantial antileukemic activities in different leukemia cell lines. We investigated whether resveratrol is active against fresh acute myeloid leukemia (AML) cells and its mechanism of action. Because interleukin 1beta(IL-1beta) plays a key role in proliferation of AML cells, we first tested the effect of resveratrol on the AML cell lines OCIM2 and OCI/AML3, both of which produce IL-1beta and proliferate in response to it. Resveratrol inhibited proliferation of both cell lines in a dose-dependent fashion (5-75 microM) by arresting the cells at S phase, thus preventing their progression through the cell cycle; IL-1beta partially reversed this inhibitory effect. Resveratrol significantly reduced production of IL-1beta in OCIM2 cells. It also suppressed the IL-1beta-induced activation of transcription factor nuclear factor kappaB (NF-kappaB), which modulates an array of signals controlling cellular survival, proliferation, and cytokine production. Indeed, incubation of OCIM2 cells with resveratrol resulted in apoptotic cell death. Because caspase inhibitors Ac-DEVD-CHO or z-DEVD-FMK partially reversed the antiproliferative effect of resveratrol, we tested its effect on the caspase pathway and found that resveratrol induced the activation of the cysteine protease caspase 3 and subsequent cleavage of the DNA repair enzyme poly (adenosine diphosphate [ADP]-ribose) polymerase. Finally, resveratrol suppressed colony-forming cell proliferation of fresh AML marrow cells from 5 patients with newly diagnosed AML in a dose-dependent fashion. Taken together, our data showing that resveratrol is an effective in vitro inhibitor of AML cells suggest that this compound may have a role in future therapies for AML.

N-(4-Hydroxylphenyl)retinamide (fenretinide, 4-HPR), a retinoid compound with antileukemic and proapoptotic activity in acute lymphoblastic leukemia (ALL)

BACKGROUND

Retinoids have been shown to regulate vital cellular processes including cell proliferation, differentiation and apoptosis. N-(4-Hydroxyphenyl)-all-trans-retinamide (fenretinide, 4-HPR) is a synthetic ATRA derivative with chemopreventive and cytotoxic activity against various cancer cell lines including myeloid leukemia. Although several modes of action have been postulated, its mechanism of action in hematologic malignancies remains unclear. Furthermore, only limited information exists as to its activity in lymphoid malignancies.

METHODS AND RESULTS

To test whether 4-HPR has activity in acute lymphoblastic leukemia (ALL), we first analyzed its antiproliferative effect in five ALL (Z-33, Z-138, Z-119, Z-181, and Jurkat) cell lines. We found that 4-HPR inhibited the proliferation of all cell lines in a dose-dependent manner at concentrations ranging from 1 to 10 microM. We further demonstrated by cell cycle analysis that 5 microM of 4-HPR blocked Z-119 cells in S phase thus preventing their progression through the cycle. Next we tested whether 4-HPR activated the caspase pathway and induced apoptotic cell death. We found that 4-HPR induced apoptosis in Z-119 cells through the activation of caspase-3 and subsequent cleavage of its substrate poly(ADP-ribose) polymerase (PARP). We then asked whether 4-HPR could affect fresh ALL progenitor cells. Therefore, we obtained bone marrow and peripheral blood cells from five patients with newly diagnosed ALL and tested the effect of 4-HPR using the ALL blast colony culture assay. To supplement our results, we also performed the ALL blast assay on one ALL cell line (ALL-1). We found that 4-HPR significantly inhibited ALL colony-forming cell proliferation in a dose-dependent manner.

CONCLUSIONS

Our data show that 4-HPR is a potent inhibitor of ALL cell proliferation and that it induces in vitro apoptotic cell death in ALL blasts. Further studies are warranted to establish the in vivo effect of 4-HPR particularly in patients with ALL.

Interleukin-1 beta enhances and interferon-gamma suppresses activin A actions by reciprocally regulating activin A and follistatin secretion from bone marrow stromal fibroblasts

Activin A is a multi-functional cytokine with a potent stimulation on erythroid cell differentiation in the bone marrow. The actions of activin A are determined by a balance of the levels of activin A and its inhibitor, follistatin (FS). However, the regulation of its actions in the bone marrow has been unclear. Here we show that bone marrow-derived stromal fibroblasts are the major source of activin A and FS in the bone marrow, and that the production of activin A is enhanced by interleukin-1beta (IL-1beta) and lipopolysaccharide (LPS), whereas interferon-gamma (IFN-gamma) inhibits the secretion of activin A by stromal fibroblasts. Concomitantly, IL-1beta as well as LPS inhibits and IFN-gamma stimulates FS secretion from stromal fibroblasts. Thus, these cytokines potently regulate activin A actions by reciprocal modulation of activin A and FS secretion from stromal fibroblasts. Because activin A exhibits anti-inflammatory effects in various tissues, up-regulation of activin A actions by IL-1beta and endotoxin in the bone marrow may play a protective role against inflammatory processes as well as anaemia. The present results also suggest that the inhibitory effect of IFN-gamma on erythropoiesis is mediated at least in part by a suppression of activin A actions in bone marrow.

Phenylarsine Oxide Blocks Interleukin-1β–Induced Activation of the Nuclear Transcription Factor NF-κB, Inhibits Proliferation, and Induces Apoptosis of Acute Myelogenous Leukemia Cells

Arsenic compounds have recently been shown to induce high rates of complete remission in patients with acute promyelocytic leukemia (APL). One of these compounds, As2O3, induces apoptosis in APL cells via a mechanism independent of the retinoic acid pathway. To test the hypothesis that arsenic compounds may be effective against other forms of acute myelogenous leukemia (AML), we studied the membrane-permeable arsenic compound phenylarsine oxide (PAO). Because interleukin-1β (IL-1β) plays a key role in AML cell proliferation, we first tested the effect of PAO on OCIM2 and OCI/AML3 AML cell lines, both of which produce IL-1β and proliferate in response to it. We found that PAO inhibited the proliferation of both OCIM2 and OCI/AML3 cells in a dose-dependent fashion (0.01 to 0.1 μmol/L) and that IL-1β partially reversed this inhibitory effect. We then measured IL-1β levels in these cells by using an enzyme-linked immunosorbent assay and Western immunoblotting and found that PAO almost completely abolished the production of IL-1β in these AML cells, whereas it did not affect the production of IL-1 receptor antagonist. Because PAO inhibits activation of the transcription factor NF-κB and because NF-κB modulates an array of signals controlling cellular survival, proliferation, and cytokine production, we also studied the effect of PAO on NF-κB activation in AML cells and found that PAO suppressed the IL-1β–induced activation of NF-κB. Because inhibition of NF-κB may result in cellular apoptosis, we also tested whether PAO may induce apoptotic cell death in AML cells. We found that PAO induced apoptosis in OCIM2 cells through activation of the cystein protease caspase 3 and subsequent cleavage of its substrate, the DNA repair enzyme poly (ADP-ribose) polymerase. The PAO-induced apoptosis was caspase dependent, because it was completely blocked by the caspase inhibitor Z-DEVD-FMK. Finally, we tested the effect of PAO on fresh AML marrow cells from 7 patients with newly diagnosed AML and found that PAO suppressed AML colony-forming cell proliferation in a dose-dependent fashion. Taken together, our data showing that PAO is an effective in vitro inhibitor of AML cells suggest that this compound may have a role in future therapies for AML.

Thrombopoietin stimulates myelodysplastic syndrome granulocyte-macrophage and erythroid progenitor proliferation

Thrombopoietin (TPO) has been successfully used to stimulate megakaryocyte progenitor proliferation and platelet production both in vitro and in vivo. We and other investigators have found that TPO also stimulates normal marrow colony-forming unit granulocyte-macrophage (CFU-GM) and burst-forming unit-erythroid (BFU-E) growth. In contrast to its effect on normal marrow precursors, TPO stimulates acute myelogenous leukemia (AML) progenitor proliferation in only 25% of the cases. Because the hematopoietic cells in Myelodysplastic syndrome (MDS) originate from both the normal and leukemic clones, we hypothesized that TPO may be a useful therapeutic agent for MDS. To test this hypothesis, we used fresh marrow samples taken from 14 MDS patients. We found that in the presence of fetal calf serum (FCS) and erythropoietin (EPO) TPO (5 to 40 ng/ml) MDS CFU-GM and BFU-E colony-forming cell proliferation were stimulated in a dose-dependent fashion by up to 103% and 93% respectively. This effect was similar to the stimulation obtained with optimal concentrations of granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage CSF (GM-CSF), or interleukin-3 (IL-3). Furthermore, TPO increased the colony-stimulatory effects of G-CSF, GM-CSF, IL-3, and stem cell factor (SCF) on MDS marrow cells. However, depletion of either T lymphocytes or adherent cells abrogated the effect of TPO, suggesting that the effect is not a direct one but is mediated through interaction with cytokines produced by accessory cells. Taken together, our data suggest that the therapeutic role of TPO in the management of MDS warrants further investigation.

1,25-Dihydroxyvitamin D3 and its analogues inhibit acute myelogenous leukemia progenitor proliferation by suppressing interleukin-1beta production

We hypothesized that 1,25-dihydroxyvitamin D3 (1,25D3) and its analogues may inhibit acute myelogenous leukemia (AML) proliferation by interrupting IL-1beta-mediated growth-stimulatory signals. The incubation of the IL-1beta- responsive AML cell line OCIM2 with 10 nM 1,25D3 reduced growth 80% in liquid culture, and a 100-1000-fold lower concentration of 20-epi analogues (MC1288 and MC1301) was sufficient to achieve similar growth inhibition. The growth inhibition was associated with a rapid but transient downregulation of IL-1beta and IL-1beta-converting enzyme (ICE) mRNAs in 1,25D3- and 20-epi analogue- treated cells, and the 20-epi analogue was more effective than 1,25D3 in repressing ICE expression. An examination of long-term changes in the levels of mature IL-1beta and its precursor revealed that 24-h incubation of OCIM2 with either 1,25D3 or its 20-epi analogues abolished the production of mature IL-1beta. The effect of 1,25D3 and its analogues on growth of fresh bone marrow cells from seven AML patients was tested by a clonogenic assay. Growth inhibition of 60% was reached in only one of seven 1,25D3-treated samples, but all seven samples were inhibited 60-90% by the 20-epi analogue MC1301. Growth inhibition by 1,25D3 and the analogue was reversible by addition of IL-1beta. These results suggest that 1,25D3 and its 20-epi analogues interrupt IL-1beta autocrine growth regulation by inhibiting IL-1beta production and processing but not the response to IL-1beta.

Growth factors controlling interleukin-4 action on hematopoietic progenitors

We investigated the effect of interleukin-4 (IL-4) on human hematopoietic progenitors using low-density bone marrow cells from 29 hematologically normal donors. We found that IL-4 could either inhibit or stimulate cell growth, depending upon the other constituents of the culture medium. At concentrations ranging from 0.1 to 10.0 micrograms/ml, it significantly inhibited colony-forming units granulocyte-macrophage (CFU-GM) in the presence of either fetal calf serum alone, erythropoietin, leukocyte-conditioned medium prepared with phytohemagglutinin, granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), or stem cell factor (SCF), in a dose-dependent fashion. In contrast, IL-4 stimulated CFU-GM colony multiplication in the presence of granulocyte colony-stimulating factor (G-CSF). Similar but less significant inhibitory effects were exerted by IL-4 on burst-forming units-erythroid (BFU-E). The growth-suppressive effect of IL-4 was partially reversed by IL-1 beta, and to a lesser extent by IL-6. When tested by enzyme-linked immunosorbent assay (ELISA), IL-4 suppressed cellular IL-1 beta production, and, similar to IL-4, anti-IL-1 beta-neutralizing antibodies inhibited CFU-GM colony growth, suggesting that the inhibition of endogenous IL-1 beta is a factor in regulating the IL-4 effect. Furthermore, in the absence of exogenous growth factors, IL-4 inhibited CFU-GM colony growth when anti-G-CSF neutralizing antibodies were also present. Therefore, we tested the effect of IL-4 on G-CSF receptors and found that 6- or 24-h incubation of low-density marrow cells with 1.0 microgram/ml IL-4 resulted in up-regulation of G-CSF receptors. Taken together, these results suggest that IL-4 possesses a dual modulatory role in the hematopoietic system via interaction with various cytokines.

Analysis of the effects of tumor necrosis factor inhibitors on human hematopoiesis

Truncated soluble fragments of tumor necrosis factor (TNF) receptors have recently been isolated from human serum and urine. These shed forms of TNF receptors bind TNF-alpha and lymphotoxin and inhibit various effects of TNF in culture. In this study, we evaluated the role of these molecules in the hematopoietic system. TNF-alpha and lymphotoxin inhibited colony forming units granulocyte-macrophage (CFU-GM) and burst forming units-erythroid (BFU-E) in a dose-dependent fashion at concentrations ranging from 1 to 5,000 U/ml and 25 to 250 U/ml, respectively. TNF-alpha exerted a similar dose-dependent inhibitory effect on a CD34 enriched marrow cell population, suggesting that its effect is not mediated through CD34 accessory cells. Its suppressive effect was partially reversed by anti-TNF-alpha neutralizing antibodies, thus proving its specificity. Two shed forms of TNF receptors, TNF binding protein (TNF-bp) and TNF receptor fusion protein (TNFR-fc), had no significant effect on CFU-GM proliferation. Both molecules, however, significantly reversed the inhibitory effect of TNF-alpha (p < 0.015 and p < 0.03, respectively), whereas they had no effect on the lymphotoxin-induced CFU-GM growth inhibition. These results indicate that TNF-bp and TNFR-fc may modulate the inhibitory effects of TNF-alpha in the hematopoietic system.

Complete nucleotide sequence, genome organization, and biological properties of human immunodeficiency virus type 1 in vivo: evidence for limited defectiveness and complementation

Previous studies of the genetic and biologic characteristics of human immunodeficiency virus type 1 (HIV-1) have by necessity used tissue culture-derived virus. We recently reported the molecular cloning of four full-length HIV-1 genomes directly from uncultured human brain tissue (Y. Li, J. C. Kappes, J. A. Conway, R. W. Price, G. M. Shaw, and B. H. Hahn, J. Virol. 65:3973-3985, 1991). In this report, we describe the biologic properties of these four clones and the complete nucleotide sequences and genome organization of two of them. Clones HIV-1YU-2 and HIV-1YU-10 were 9,174 and 9,176 nucleotides in length, differed by 0.26% in nucleotide sequence, and except for a frameshift mutation in the pol gene in HIV-1YU-10, contained open reading frames corresponding to 5'-gag-pol-vif-vpr-tat-rev-vpu-env-nef-3' flanked by long terminal repeats. HIV-1YU-2 was fully replication competent, while HIV-1YU-10 and two other clones, HIV-1YU-21 and HIV-1YU-32, were defective. All three defective clones, however, when transfected into Cos-1 cells in any pairwise combination, yielded virions that were replication competent and transmissible by cell-free passage. The cellular host range of HIV-1YU-2 was strictly limited to primary T lymphocytes and monocyte-macrophages, a property conferred by its external envelope glycoprotein. Phylogenetic analyses of HIV-1YU-2 gene sequences revealed this virus to be a member of the North American/European HIV-1 subgroup, with specific similarity to other monocyte-tropic viruses in its V3 envelope amino acid sequence. These results indicate that HIV-1 infection of brain is characterized by the persistence of mixtures of fully competent, minimally defective, and more substantially altered viral forms and that complementation among them is readily attainable. In addition, the limited degree of genotypic heterogeneity observed among HIV-1YU and other brain-derived viruses and their preferential tropism for monocyte-macrophages suggest that viral replication within the central nervous system may differ from that within the peripheral lymphoid compartment in significant and clinically important ways. The availability of genetically and biologically well characterized HIV-1 clones from uncultured human tissue should facilitate future studies of virus-cell interactions relevant to viral pathogenesis and drug and vaccine development.

Granulocyte-macrophage colony-stimulating factor and interleukin-3 in combination: a potent and consistent myelodysplastic syndrome bone marrow stimulant in vitro

In an effort to overcome bone marrow failure in myelodysplastic syndrome (MDS), we have investigated recombinant human granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-3 (IL-3) in phase I-II clinical trials. Although these agents partially increased peripheral blood granulocyte counts, their effect on other hematopoietic lineages was generally sporadic. Since in vitro analysis and in vivo studies in primates indicate that GM-CSF and IL-3 synergistically enhance hematopoietic stem cell proliferation, we evaluated their combined effect on marrow progenitors obtained from ten MDS patients. When used singly, each growth factor stimulated replication of granulocyte-macrophage (CFU-GM) and erythroid (BFU-E) colony-forming cells in a dose-dependent fashion. When colony-stimulating activity was compared at concentrations that maximally amplified individual MDS patients' colony numbers, IL-3 was a more potent stimulant in some patients and GM-CSF in others. When used in combination, IL-3 plus GM-CSF was more effective than each growth factor by itself in five of six patients. Our data indicate that the MDS hematopoietic progenitor stimulatory effect of these growth factors varies from patient to patient. However, the combination of GM-CSF and IL-3 appears to be more potent than the individual molecules in the majority of patients.

Modulation of human marrow haematopoiesis by leucotrienes in vitro

The influence of products derived from the arachidonic acid 5-lipoxygenase (5-LO) pathway on normal human marrow haematopoiesis was studied using clonogenic assays. The leucotrienes (LT) B4 and C4 caused a reduction in granulocyte-macrophage (CFU-GM) as well as erythroid (CFU-E and BFU-E) colony numbers in a dose-dependent manner. Conversely, nordihydroguaiaretic acid (NDGA), a LT synthesis inhibitor, promoted proliferation of these colonies. The inhibitory effect of LTC4 but not that of LTB4 was abolished by FPL-55712, a LT antagonist; the enhanced colony growth induced by NDGA was reversed by both LTB4 and LTC4. LTD4 had no significant effect on colony growth despite its structural similarity to LTC4, which implies that the haematopoietic suppression by LTB4 and LTC4 is specific for these compounds. Depletion of marrow T-cells or removal of adherent cells, or both, did not alter the response to LTB4 and LTC4, suggesting that LT action is exerted directly on progenitor cells and probably is not mediated by other cell populations. Our studies show that the 5-LO pathway is functional in these culture systems and yields products with inhibitory properties. The 5-LO pathway may have an important regulatory function in normal marrow haematopoiesis.

Different requirements for the induction of antibody-dependent and immune complexes triggered cytotoxicity mediated by monocytes

We have previously shown that immune complexes triggered nonspecific cytotoxicity (NSC) towards nonsensitized target cells and antibody-dependent cellular cytotoxicity (ADCC), two functions mediated through monocyte Fc gamma receptors, employing different lytic mechanisms [Geffner, J. R., et al. (1986) Clin. Exp. Immunol. 67, 646]. In this report, we analyze some of the metabolic requirements involved in the induction of monocyte NSC and ADCC. The results showed NSC to be dependent on: (1) metabolic energy derived from glycolysis, (2) availability of external Ca2+, (3) calmodulin activity, (4) integrity of microtubules, but not the microfilament system, and (5) activation of phospholipase(s) and lipoxygenase. On the other hand, ADCC was not impaired by: (1) inhibition of glycolysis, (2) Ca2+ chelation, (3) disruption of microtubules, or (4) inhibition of calmodulin or lipoxygenase. It is concluded that monocyte NSC and ADCC are regulated by different endogenous signals.

The role of reactive oxygen intermediates in nonspecific monocyte cytotoxicity induced by immune complexes

Normal human monocytes were induced to lyse nonsensitized target cells when triggered by precipitating immune complexes (IC) or soluble heat-aggregated IgG (HAIgG). Catalase, azide, cyanide and three aminoacids employed as quenchers of ClO, significantly inhibited this nonspecific cytotoxicity (NSC), suggesting an important role for the myeloperoxidase (MPO) system. However, HO and/or 1O2 may also be involved in the lysis, since certain scavengers of these species such as mannitol, benzoate, ethanol and histidine, as well as superoxide dismutase (SOD), partially inhibited NSC. Moreover, cyanide and azide were unable to completely abrogate this lytic activity. When NSC was compared to antibody dependent cellular cytotoxicity (ADCC), it was found that neither catalase nor oxygen-species scavengers affected ADCC while azide and cyanide significantly enhanced it. Antibody-coated target cells were also destroyed by IC-triggered monocytes. However, kinetic analysis and studies on the capacity of catalase to inhibit the lysis demonstrated that it was mediated through a NSC-like mechanism. The cytotoxic system described in this report offers a suitable model to study in vitro alternative lytic mechanisms triggered through monocyte receptors for the Fc portion of IgG (Fc gamma R).

Detection of residual acute lymphoblastic leukemia cells in cultures of bone marrow obtained during remission

We used a semisolid culture assay to quantitate leukemia cells in the bone marrow of patients with childhood acute lymphoblastic leukemia (ALL). In bone marrow cultures from 40 patients with newly diagnosed disease, the colonies that developed in vitro consisted of lymphoblasts with the same surface markers and abnormal karyotype as the original diagnostic marrow specimens. We also studied marrow cultures from 13 patients in chemotherapy-induced remission; 6 of these, including 1 obtained from a patient during successful engraftment after marrow transplantation, also yielded lymphoblast colonies in culture, with the same immunologic phenotype or abnormal karyotype as the original leukemic marrow. Four of these patients, including the one who underwent marrow transplantation, relapsed within 2 to 30 months of the abnormal cultures; the other two are still in remission, one of them 30 months after diagnosis. Bone marrow cultures from eight normal controls and from the other seven patients in remission did not yield lymphoblast colonies; all seven of the latter are still in remission. This assay appears to allow detection of small numbers of residual leukemic cells. We conclude that the technique will be valuable in monitoring the efficacy of chemotherapy and allogeneic bone marrow transplantation in acute lymphoblastic leukemia, as well as in evaluating the quality of purged marrow for autologous marrow transplantation.

Involvement of macrophages in the pathology of toxic epidermal necrolysis

In toxic epidermal necrolysis (TEN), as in the 'epidermal type' of erythema multiforme, the necrotic epidermis is infiltrated with mononuclear cells. We studied the epidermal infiltrate in seven cases of TEN. About half the cells obtained from pieces of cleaved epidermis dissociated by trypsin were non-epithelial. On cytologic analysis, 80% of these foreign cells exhibited markers of macrophages, 15% were granulocytes and only 5% were lymphocytes (almost exclusively OKT8 T lymphocytes). Semi-thin sections of early prenecrotic lesions showed exocytosis of mononuclear cells within the epidermis with features of satellite cell necrosis and formation of colloid bodies. Almost all these mononuclear cells were macrophages as evidenced by endogenous peroxidase-positive granules. These findings suggest that some kind of macrophage-mediated cytotoxicity may play a role in the necrosis of epidermal cells during TEN.

Erythroid progenitor growth in erythrocytosic transplanted patients

In 55 patients who underwent renal transplantation, erythrocytosis (E) was observed in 6. In all patients, the growth of erythroid progenitors [burst-forming unit erythroid (BFU-e)], using mononuclear blood cells, was evaluated in in vitro cultures. The results showed a significant increase of serum erythropoietin (s-Ep) in erythrocytosis and in nonerythrocytosis patients with a subsequent decrease when hematocrit (Hct) and hemoglobin (Hb) reached high levels, showing a recovery of a feedback control system. Notwithstanding the diminution of s-Ep, six patients demonstrated further progressive rise of the Hct and Hb to a final state of E. The in vitro BFU-e cultures from these patients showed a noticeable sensitivity to progressively reduced doses of Ep and also the capacity to develop a few colonies when the medium was Ep-free. These results were not verified using peripheral blood depleted of monocyte and/or T lymphocyte cells. Therefore, in transplanted patients with erythrocytosis, it is possible that particular cellular interactions stimulate an early hyperproliferation of BFU-e with a greater Ep sensitivity and at least partly the capacity to grow also in the absence of Ep.

Peripheral K-lymphocyte population in head and neck cancer

Peripheral K-lymphocytes in head and neck cancer were measured by assay of plaque forming-cells. In the cancer patients, the percentage (4.37 +/- 0.87%, mean +/- SD, N = 42, P less than 0.01) and absolute counts (68 +/- 28/mm3, P less than 0.01) of K-lymphocytes were significantly lower than those in normal controls (8.04 +/- 1.41%; 175 +/- 53/mm3, N = 29). The untreated group showed decreased K-lymphocyte counts (63 +/- 22/mm3, N = 13) as compared with the treated, disease-free group (83 +/- 27/mm3, N = 13, P less than 0.05). There were significant correlations between absolute counts of K-lymphocytes and T-cells or B-cells within the untreated group (r = 0.79 in T-cells, P less than 0.01; r = 0.64 in B cells, P less than 0.01). Moreover, absolute counts and percentage of K-lymphocytes in the patients having regional lymph node metastasis (58 +/- 25/mm3; 4.14 +/- 0.77%, N = 22) were significantly lower than those in the negative node group (80 +/- 26/mm3, P less than 0.01; 4.63 +/- 0.91%, P less than 0.05; N = 20). In a total of 25 patients with squamous cell carcinoma who were grouped into grade I, II, and III according to classification of the histologic differentiation of the World Health Organization, the absolute counts and percentage of K-lymphocytes in the grade I group (93 +/- 33/mm3; 5.14 +/- 1.08%, N = 7) showed significant increases in comparison to those in the grade II group (62 +/- 19/mm3, P less than 0.02; 4.06 +/- 0.62%, P less than 0.02, N = 14). Moreover, the change of the K-lymphocyte population in the treated, disease-free eight patients revealed a gradual increase of K-lymphocytes. These results led us to suggest that the measurement of peripheral K-lymphocytes is a useful method of characterizing host defense in head and neck cancer.

Mitogen-induced stimulation and suppression of erythroid burst promoting activity production by human mononuclear cells

Exposure of human peripheral blood mononuclear cells or highly enriched monocytes to various plant lectins substantially alters their production of erythroid burst promoting activity (BPA). Neither unstimulated, nor mitogen stimulated, enriched T lymphocytes produced demonstrable BPA. Each of the lectins tested resulted in a different pattern of alteration of BPA production by mononuclear cells. Increasing concentrations of phytohaemagglutinin (PHA) caused a progressive increase in BPA production up to a plateau level at concentrations above 0.25-0.5 microliter/ml. Concanavalin A (Con A) at concentrations of 0.05-0.1 micrograms/ml stimulated BPA production, but Con A concentrations greater than 1 microgram/ml never augmented BPA production by mononuclear cells. Pokeweed mitogen inhibited BPA production by mononuclear cells in a concentration-dependent manner. Since PHA and Con A can bind to and stimulate both monocytes/macrophages and T lymphocytes, some production of BPA by stimulated T cells in the presence of monocytes cannot be ruled out. Earlier studies demonstrated that T cells augment monocyte production of BPA. Thus, monocyte-T cell interactions, as well as activation of monocytes and perhaps lymphocytes, play an important role in regulation of BPA production in vitro.

Activation of monocyte and granulocyte antibody-dependent cytotoxicity by phorbol myristate acetate

We have characterized the effects of phorbol myristate acetate (PMA) on human monocyte and neutrophil oxidative metabolism and antibody-dependent cytotoxicity toward anti-D sensitized human erythrocytes (RBC) and a human lymphoblastoid cell line (CEM). Hexose monophosphate shunt activity was measured by [1-(14)C]glucose oxidation and target lysis by (51)Cr release. PMA produced a dose-dependent stimulation of hexose monophosphate shunt activity. Neutrophils responded with higher hexose monophosphate shunt activity and at a lower PMA concentration than did monocytes. PMA increased monocyte lysis of antibody-sensitized RBC by two-thirds, but did not affect lysis of CEM targets. Neutrophils were unable to lyse either antibody-sensitized or nonsensitized RBC without the addition of PMA. When PMA was added, lysis of both targets increased markedly. Neutrophils without PMA were able to lyse a small number of both antibody-sensitized and nonsensitized CEM targets. PMA also increased neutrophil lysis of these targets. Target lysis by neutrophils from a patient with chronic granulomatous disease, cells unable to produce reactive oxygen species, was not increased by PMA. Chronic granulomatous disease monocytes, however, responded to PMA by more than doubling lysis of antibody-sensitized RBC. Hypoxia inhibited PMA augmentation of antibody-sensitized RBC lysis by neutrophils, but not by monocytes. Generation of reactive oxygen species by the xanthine-xanthine oxidase system inhibited CEM growth, but did not cause lysis, indicating that in some cases oxidative injury may be nonlytic. We suggest that PMA augments neutrophil cytotoxicity to tumor and RBC targets by stimulating reactive oxygen species-mediated lysis, but in monocytes augmentation of lysis is due to activation of a nonoxidative mechanism of lysis.

Monocyte and granulocyte-mediated tumor cell destruction. A role for the hydrogen peroxide-myeloperoxidase-chloride system

Human monocytes stimulated with phorbol myristate acetate were able to destroy a T lymphoblast cell target (CEM). Stimulated human granulocytes were also capable of mediating CEM cytotoxicity to a comparable degree as the monocyte. CEM destruction was dependent on the pH and the effector cell number. Both monocyte or granulocyte mediated cytotoxicity were inhibited by the addition of catalase, whereas superoxide dismutase had no inhibitory effect. In addition, CEM were protected from cytolysis by the effector cells by the myeloperoxidase inhibitors, azide and cyanide, or by performing the experiment under halide-free conditions. Glucose oxidase, an enzyme system capable of generating hydrogen peroxide, did not mediate CEM cytotoxicity, while the addition of purified myeloperoxidase dramatically enhanced cytolysis. Hypochlorous acid scavengers prevented CEM destruction by the glucose oxidase-myeloperoxidase-chloride system but neither hydroxyl radical nor singlet oxygen scavengers had any protective effect. These hypochlorous acid scavengers were also successful in inhibiting monocyte or granulocyte-mediated CEM cytotoxicity. Based on these observations we propose that human monocytes or granulocytes can utilize the hydrogen peroxide-myeloperoxidase-chloride system to generate hypochlorous acid or species of similar reactivity as a potential mediator of CEM destruction.

Antibody-dependent cell-mediated cytotoxicity in human cancer: characterization of patient leukocyte activity and treatment effects

Antibody-dependent cell-mediated cytotoxicity (ADCC), medicated by peripheral blood Hypaque-Ficoll separated mononuclear cells, was studied in humans using chicken erythrocytes (CRBC) incubated in a 1:1200 dilution of rabbit anti-CRBC and human B erythrocytes (HRBC) incubation in a 1:20 dilution of isoantibody. At the optimal target effector ratio of 3:1, ADCC to both CRBC and HRBC was significantly higher than normal in 27 lung cancer, 18 malignant melanoma, and seven colon cancer patients, but not in 20 breast cancer patients. Chemotherapy (single-agent or combination) in 12 patients did not effect ADCC in vitro but significantly suppressed ADCC to both targets after only four or five days of therapy in vivo (ADCC to CRBC, 47.4 to 24.1% lysis: ADCC to HRBC, 48.1 to 16.3% lysis). Immunotherapy with intravenous (IV) corynebacterium parvum or IV methanol extraction residue of BCG (MER) boosted ADCC to both targets within four to seven days of the first dose. It was found that ADCC to HRBC but not to CRBC was completely absent in three cases of active hairy cell leukemia but was present in two cases in remission. The ADCC to HRBC showed an age-dependent increase in both the 51 normal subject and the cancer patients. This was not observed for ADCC to CRBC. The ADCC to CRBC was mediated mainly by an Fc-receptor-positive, nonadherent, small lymphocyte, and ADCC to HRBC was mediated entirely by an adherent monocyte. The ADCC did not correlate significantly with the H3 thymidine incorporation of peripheral blood mononuclear cells, cultured without stimulation for either one or seven days. It also did not correlate with the number of residual granulocytes in the mononuclear cell suspensions. Measurement of ADCC is a useful method of characterizing host defense in malignant disease and its modification by therapy.

Deficient strongly adherent monocytes in the peripheral blood of cancer patients

The numbers of strongly adherent monocytes in the peripheral blood of normal subjects and cancer patients were determined. The method used was to place peripheral blood mononuclear cells in microwells and culture them for 1 week. At the end of that period, adherent macrophages were counted in the Coulter counter after release. Adherent cells per milliliter of blood, per total cells, and per mononuclear cells or monocytes plated were markedly diminished in the peripheral blood mononuclear cells of 44 melanoma, 23 breast cancer, 18 lung cancer, nine colon cancer, and 27 leukemia patients. Median values were 14.8 X 10(4) adherent cells per ml peripheral blood for 86 normal subjects, as against 2.5 X 10(4) per ml in the peripheral blood of the 125 patients (P less than 0.001). There was a poor correlation between the adherent cell numbers and the peripheral blood leukocyte counts, but an excellent correlation of the different adherent cell counts with each other. The number of adherent cells in the peripheral blood varied inversely with age in the cancer patients, but not in the normal subjects (r = 0.29, P less than 0.005). When patients under age 50 were compared to the controls, the deficiency of adherent cells was slightly more severe in patients with stage IV lung cancer than in those with stage III lung cancer. In contrast, there was no difference in the degree of deficiency between patients with stage III melanoma and no evident disease and patients with stage IV disseminated metastatic disease. The implications of these results are discussed.

Human erythroid burst-forming units. Growth in vitro is dependent on monocytes, but not T lymphocytes

The roles of monocytes and T lymphocytes in the regulation of human peripheral blood erythroid burst-forming units (BFU-E) were studied in erythropoietin-containing plasma clot cultures of subpopulations of human blood mononuclear cells. BFU-E growth was decreased significantly after depletion of monocytes alone (mean 11% of expected, range 0 to 42% of expected) or depletion of both monocytes and T cells (mean 6.5% of expected, range 0.5 to 12% of expected) from mononuclear cells. T cell depletion did not impair BFU-E growth in vitro. Using 10(5) monocyte- and T lymphocyte-depleted mononuclear cells as target cells (less than 1% monocytes, less than 5% T cells), BFU-E growth was restored to 40% of expected by addition of 10(4) monocytes, and to 96% of expected by 10(5) monocytes alone. Addition of as many as 2 X 10(5) T cells but no monocytes resulted in stimulation to only 34% of expected BFU-E growth. Addition of 2 X 10(4) T cells, which alone did not affect BFU-E growth, could augment significantly the stimulatory effect of 5-20 X 10(3) monocytes on BFU-E growth. Thus, monocytes alone appear to be capable of stimulating BFU-E growth in vitro in the presence of erythropoietin. T cells also may make small quantities of BFU-E stimulators. However, it seems more likely that the most important role of T lymphocytes in BFU-E regulation in vitro is a result of interactions with monocytes and augmentation of monocyte production of stimulators of BFU-E growth.

Increased IgE-dependent cytotoxicity by blood mononuclear cells of allergic patients

Peripheral blood mononuclear cells from 14 healthy donors and 22 allergic patients were incubated with 51Cr-labelled chicken erythrocytes coated with an IgE myeloma protein or rabbit IgG antibodies. Mononuclear cells from patients with severe atopic disorders released a significantly greater percentage of 51Cr (P less than 0.001) from IgE-coated target cells than mononuclear cells from healthy controls, patients with mild atopic disease, or patients with severe atopic disease taking oral prednisone. Specific 51Cr-release from IgE-coated target cells was directly correlated to the percentage of monocytes (latex-ingesting cells) with Fc receptors for IgE (r = 0.87, P less than 0.01) as detected by a rosette assay employing ox erythrocytes coated with IgE. Mononuclear cells from patients and normals released similar amounts of 51Cr from IgG-sensitized target cells. Depletion of monocytes from mononuclear cell preparations from two severe atopic patients decreased 51Cr-release from IgE-coated target cells to levels seen in healthy donors or patients with mild allergic disease. These results demonstrate that mononuclear cells from severely allergic patients have a significantly increased cytotoxicity toward IgE-coated targets coated target cells and that this cytotoxicity correlates highly with the percentage of monocytes with Fc receptors for IgE in these mononuclear preparations.

Oxidative mechanisms of monocyte-mediated cytotoxicity

Human monocytes stimulated with phorbol myristate acetate were able to rapidly destroy autologous erythrocyte targets. Monocyte-mediated cytotoxicity was related to phorbol myristate acetate concentration and monocyte number. Purified preparations of lymphocytes were incapable of mediating erythrocyte lysis in this system. The ability of phorbol myristate acetate-stimulated monocytes to lyse erythrocyte targets was markedly impaired by catalase or superoxide dismutase but not by heat-inactivated enzymes or albumin. Despite a simultaneous requirement for superoxide anion and hydrogen peroxide in the cytotoxic event, a variety of hydroxyl radical and singlet oxygen scavengers did not effect cytolysis. However, tryptophan significantly inhibited cytotoxicity. The myeloperoxidase inhibitor cyanide enhanced erythrocyte destruction, whereas azide reduced it modestly. The inability of cyanide to reduce cytotoxicity coupled with the protective effect of superoxide dismutase suggests that cytotoxicity is independent of the classic myeloperoxidase system. We conclude that monocytes, stimulated with phorbol myristate acetate, generate superoxide anion and hydrogen peroxide, which together play an integral role in this cytotoxic mechanism.