Monocyte Respiratory Burst Research Articles

Oligomannan suppresses neutrophil and monocyte respiratory burst: A possible mechanism for granulomatous inflammation in Crohn's disease

Oligomannan suppresses neutrophil and monocyte respiratory burst: A possible mechanism for granulomatous inflammation in Crohn's disease

Tachykinin activation of human monocytes from patients with rheumatoid arthritis: in vitro and ex-vivo effects of cyclosporin A

Three types of tachykinin receptors, namely NK1, NK2and NK3, are known to preferentially interact with substance P (SP), neurokinin A (NKA) and neurokinin B (NKB), respectively. We previously demonstrated that NK1and NK2receptors are present on human monocytes, SP and NKA inducing superoxide anion production and tumor necrosis factor- alpha (TNF-α) mRNA expression. NK2receptor stimulation also triggered an enhanced respiratory burst in monocytes isolated from rheumatoid arthritis (RA) patients. This study was aimed to evaluate the in vitro and ex-vivo effects of cyclosporin A (CsA) on tachykinins-evoked TNF-α release from monocytes of healthy donors and RA patients. CsA (100ng/ml) potently inhibited phorbol ester- and tachykinin-evoked TNF-α secretion. In RA patients treated with CsA (SandimmunRNeoralR) 2.5mg/kg/die, a significant time-dependent reduction in TNF-α secretion from monocytes was measured. This may contribute to the CsA therapeutic activity in RA.

The phagocytosis-associated respiratory burst in human monocytes is associated with increased uptake of glutathione.

During the phagocytic respiratory burst, oxygen is converted to potent cytotoxic oxidants. Monocytes and macrophages are potentially long-lived, and we have hypothesized that protective mechanisms against oxidant stress are varied and fully expressed in these cells. We report here that the respiratory burst in monocytes is accompanied by an increase in the uptake of [35S]glutathione ([35S]GSH) after 20-30 min to levels up to 10-fold greater than those at baseline. By 30 min, 49% of the cell-associated radioactivity was in the cytosol, 41% was in membrane, and 10% was associated with the nuclear fraction. GSH uptake was inhibited by catalase, which removes hydrogen peroxide (H2O2), and micromolar H2O2 stimulated GSH uptake effectively in monocytes and also lymphocytes. Oxidation of GSH to glutathione disulfide with H2O2 and glutathione peroxidase prevented uptake. Acivicin, which inhibits GSH breakdown by gamma-glutamyl transpeptidase (GGT), had no effect on the enhanced uptake seen during the respiratory burst. Uptake of cysteine or cystine, possible products of GGT activity, stayed the same or decreased during the respiratory burst. These results suggest that a GGT-independent mechanism is responsible for the enhanced GSH uptake seen during the respiratory burst. We describe here a sodium-independent, methionine-inhibitable transport system with a Km (8.5 microM) for GSH approximating the plasma GSH concentration. These results suggest that monocytes have a specific GSH transporter that is triggered by the release of H2O2 during the respiratory burst and that induces the uptake of GSH into the cell. Such a mechanism has the potential to protect the phagocyte against oxidant damage.

Advanced Oxidation Protein Products as Novel Mediators of Inflammation and Monocyte Activation in Chronic Renal Failure1, 2

Abstract We previously demonstrated the presence of advanced oxidation protein products (AOPP), a novel marker of oxidative stress in the plasma of uremic patients receiving maintenance dialysis. The present study in a cohort of 162 uremic patients showed that plasma concentrations of AOPP increased with progression of chronic renal failure and were closely related to advanced glycation end products (AGE)-pentosidine (r = 0.52, p < 0.001), taken as a marker of AGE. In vivo, the relevance of AOPP and AGE-pentosidine in monocyte-mediated inflammatory syndrome associated with uremia was evidenced by close correlations between AOPP or AGE-pentosidine and monocyte activation markers, including neopterin, IL-1R antagonist, TNF-α, and TNF soluble receptors (TNF-sR55 and TNF-sR75). To determine the mechanisms by which AOPP and AGE could be directly involved in monocyte activation, AOPP-human serum albumin (HSA) and AGE-HSA were produced in vitro by treating HSA with oxidants or glucose, respectively. Spectroscopic analysis confirmed that AOPP-HSA contains carbonyls and dityrosine. Both AOPP-HSA and AGE-HSA, but not purified dityrosine, were capable of triggering the oxidative burst of human monocytes in cultures. The AOPP-HSA-induced respiratory burst was dependent on the chlorinated nature of the oxidant and on the molar ratio HSA/HOCl. Collectively, these data first demonstrate that AOPP act as a mediator of oxidative stress and monocyte respiratory burst, which points to monocytes as both target and actor in the immune dysregulation associated with chronic uremia.

Tachykinin receptors on human monocytes: their involvement in rheumatoid arthritis

Three types of tachykinin receptors, namely NK1, NK2 and NK3, are known to preferentially interact with substance P (SP), neurokinin A (NKA) and neurokinin B (NKB), respectively. Experimental evidence indicates that SP and NKA modulate the activity of inflammatory and immune cells, including mononuclear ones. This study evaluated the effects of mammalian tachykinins and selective tachykinin agonists and antagonists on human monocytes isolated from healthy donors: SP, NKA and NKB all evoked a dose-dependent superoxide anion (O2-) production and the NK2 selective agonist [beta-Ala8]-NKA(4-10) induced a full response. The NK3 selective agonist senktide was inactive, while the NK1 selective agonists septide and [Sar9Met(O2)11]SP displayed some effects. These results indicate that NK2 and also some NK1 receptors are present in monocytes isolated from healthy donors. The role of tachykinin receptor activation in rheumatoid arthritis was also investigated, by measuring O2- production and TNF-alpha mRNA expression in monocytes isolated from rheumatoid patients. Tachykinins enhanced the expression of this cytokine in both control and rheumatoid monocytes and NK2 receptor stimulation was shown to trigger an enhanced respiratory burst in monocytes from rheumatoid patients. In conclusion, these results indicate that NK2 and NK1 receptors are present on human monocytes, the former being preferentially involved in rheumatoid arthritis.

The Effect of Exercise on Lymphocyte Redistribution and Leucocyte Function in Asymptomatic HIV-Infected Subjects

This study was undertaken to examine the responsiveness of circulating leucocyte and lymphocyte populations to the physiological demands of exercise in asymptomatic HIV-infected subjects with CD4+ counts greater than 500/μl. Thirteen subjects infected with HIV and 14 control subjects underwent 20 min of defined moderate exercise at estimated 65% of their maximal ventilatory capacity on a bicycle ergometer. Blood samples were obtained for serum cortisol, norepinephrine, lymphocyte subsets (CD4, CD8, CD19, CD16–CD56), and phagocytic function at rest immediately after exercise and 20 min following the cessation of the exercise. The HIV-infected subjects had increased circulating concentrations of CD8 cells (p= .007) and CD16–CD56+ NK cells (p= .02) in response to the exercise, whereas the control group did not. There was a greater increase in monocyte respiratory burst activity following recovery from exercise in the control subjects (p= .016) but not in the HIV-infected subjects. The control subjects experienced an increase in serum cortisol in response to the exercise (p= .006), but the HIV-infected subjects did not. Our results show that the changes in the distribution and function of circulating leucocytes and adrenal neuroendocrine responses to moderate exercise differ in asymptomatic HIV-infected and control subjects.

Priming of monocyte respiratory burst by β-amyloid fragment (25–35)

In the present study we have examined the ability of the β-amyloid peptide (Aβ(25–35)) to modulate the respiratory burst activity of human monocytes in vitro. Incubation of the cells for 24 h with Aβ(25–35) as well as with Aβ(1–42) resulted in an enhanced production of reactive oxygen radicals (ROI) in response to phorbol 12-myristate 13-acetate (PMA). Such effect was additively increased by coincubation with interferon-γ (IFNγ), and was paralleled by modulation of gene and protein expression of some components of the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase system. Since the effects of Aβ(25–35) were also reproduced in primary rat microglia, our findings indicate that Aβ(25–35) can potentiate the ability of mononuclear phagocytes to produce ROI, and add further insights into its biological effects.

Standardization of a Flow Cytometric Assay for Phagocyte Respiratory Burst Activity

The authors optimized the flow cytometric dichlorofluorescin (DCFH)-oxidation assay for buffy coat neutrophil and monocyte respiratory burst activity. Sample handlings were minimized, monocytes identified with a CD14 antibody, and viability evaluated with propidium iodide. Sodium citrate was a better anticoagulant than heparin, with a more intense Yersinia enterocolitica (YER)-induced dichlorofluorescein (DCF)-fluorescence intensity and a higher proportion of DCF-positive cells. EDTA was unsuitable as an anticoagulant with reduced cell viability and poor DCF response. Exposure of cells to YER, phorbol myristate acetate (PMA) or N-formyl-methionyl-leucyl-phenylalanine (FMLP) elicited two neutrophil subpopulations, one with low and the other with high forward light scattering properties. FMLP induced only a marginal DCF response, but after YER or PMA, virtually all neutrophils responded with an increased DCF production. During optimal conditions, the resulting DCF- fluorescence histogram was two-peaked, and the subset of cells with increased forward light scattering properties corresponded to the cells with intense DCF-fluorescence. A similar heterogeneity was frequently but not always observed amongst monocytes. The results indicate that in the peripheral blood there are at least two neutrophil and monocyte populations. One is an effective responder to stimuli, the other exhibiting a moderate response only. Properly optimized, the DCFH-oxidation assay may be used for evaluating neutrophil and monocyte subsets in a clinical setting.

Administration of rHuGM-CSF activates monocyte reactive oxygen species secretion and adhesion molecule expression in vivo in patients following high-dose chemotherapy.

Microbicidal and cytocidal products of the respiratory burst and integrin adhesion molecule expression have been studied in monocytes from patients who received rHuGM-CSF during regeneration after high-dose chemotherapy. In this study, administration of rHuGM-CSF after high-dose chemotherapy significantly augmented the secretion of inducible products of the monocyte respiratory burst. Monocyte activation persisted for several weeks after the cessation of GM-CSF therapy. Under in vitro conditions that mimicked gram-negative (LPS) and gram-positive (opsonized Staphylococcus aureus) sepsis, the monocyte responded to such stimulation by exhibiting an enhanced release of hydrogen peroxide at both regeneration and several weeks later (P < 0.001). Similarly, GM-CSF administration significantly augmented the phenotypic expression of the beta 2-integrin adhesion molecules and allowed the leucocyte-specific selectin, LAM-1, and the beta 2-integrins to respond normally to inflammatory stimulation by LPS. We further present evidence that GM-CSF therapy restored the otherwise refractory status of monocytes to inflammatory stimulation that existed in those patients given chemotherapy alone. The restoration of monocyte responsiveness by GM-CSF following high-dose chemotherapy could be a potentially valuable and hitherto not described action of rHuGM-CSF on monocyte function. We conclude that administration of GM-CSF may have the potential for restoring as well as augmenting the anti-microbial and anti-tumour function of the monocyte after high-dose chemotherapy.

Interleukin-3 administration enhances human monocyte function in vivo.

In addition to its haemopoietic effects, interleukin-3 (IL-3) enhances leucocyte function in vitro. In this study we examined the effects on haematological variables and monocyte function of a single IL-3 infusion in five haematologically normal individuals. There was a rapid fall in circulating monocyte (to 24 +/- 6% of pre-infusion value) and eosinophil numbers (to 3 +/- 2%) with a nadir at 30 min and gradual return to baseline over 6 h. No significant changes in monocyte expression of the adhesion molecules CD11b or L-selectin or of monocyte respiratory burst activity were detected. There was a significant increase in monocyte phagocytosis and killing of Candida after IL-3 infusion: the percentage of monocytes which had ingested Candida increased from 39 +/- 10% to 62 +/- 12% and the total number of Candida killed per 100 monocytes increased from 63 +/- 34 to 210 +/- 59 (P < 0.05 and P < 0.01 respectively). There was no inhibition of neutrophil migration into a 'skin window' site and monocyte migration was moderately enhanced (peak increase of 260 +/- 47%). These results show that IL-3 has significant effects on monocyte function in vivo and could be of use in augmenting host defence mechanisms in immunocompromised patients.

S-thiolation of glyceraldehyde-3-phosphate dehydrogenase induced by the phagocytosis-associated respiratory burst in blood monocytes.

Chemical oxidants can induce the covalent binding of low molecular weight thiols to reactive sulfhydryls on proteins (S-thiolation). We found that stimulation of the respiratory burst of human blood monocytes resulted in S-thiolation of several proteins, most prominently one of 38 kDa. This purified protein was identified as glyceraldehyde-3-phosphate dehydrogenase (GAPDH) by enzyme activity, immunoblotting, and amino acid analysis. After stimulation of the respiratory burst, S-thiolation of GAPDH gradually increased, and cytosol GAPDH activity decreased; so that at 60 min, GAPDH activity was reduced by approximately 40%. Activity was restored by the addition of the sulfhydryl-reducing agent dithioerythritol. H2O2 appeared to be particularly important in mediating S-thiolation during the respiratory burst. Exposure of monocytes to H2O2 induced concentration-dependent S-thiolation of GAPDH and a concomitant decrease in enzyme activity. The addition of respiratory burst stimuli to lymphocytes, which lack a full respiratory burst, had no effect on GAPDH S-thiolation or activity; but H2O2 induced S-thiolation of lymphocyte GAPDH and inhibition of enzyme activity. Stimulation of monocytes from three patients with chronic granulomatous disease resulted in no respiratory burst, S-thiolation of GAPDH, or inactivation of GAPDH activity. The thiols covalently bound to purified S-thiolated GAPDH were removed by dithioerythritol and were identified as glutathione and cysteine; glutathione was predominant. These results indicate that during the respiratory burst in monocytes, low molecular weight thiols can bind to specific cytosolic proteins, including GAPDH. It is possible that S-thiolation of cytosolic proteins serves to modulate cellular metabolic events during phagocytosis.

Production of tumor necrosis factor and other proinflammatory cytokines by human mononuclear phagocytes stimulated with myelin P2 protein.

In this study we examined the effect of myelin P2 protein on some proinflammatory functions exerted by human mononuclear phagocytes. Northern blot analysis demonstrated that P2 protein selectively induced in monocytes and macrophages mRNA accumulation of tumor necrosis factor (TNF), interleukin 1 beta (IL-1 beta), and interleukin 8 (IL-8) in a time-dependent manner. Natural killer stimulating factor (IL-12) mRNA and protein secretion was strongly induced by lipopolysaccharide but not by P2 protein. Supernatants harvested from P2-stimulated monocytes contained significant amounts of TNF, IL-1 beta, and IL-8, whereas those from macrophages contained only TNF and IL-8. The effect of the P2 protein on TNF and IL-8 mRNA accumulation and secretion was not affected by polymyxin B, which, on the other hand, almost completely abolished the effect of lipopolysaccharide. Finally, P2 protein did not directly trigger hydrogen peroxide release but, through the induced release of TNF, potentiated monocyte respiratory burst capability. Since P2 protein is the antigen responsible for the induction of experimental allergic neuritis, these findings identify a potential mechanism involved in the inflammatory reaction and myelin damage during experimental allergic neuritis.

Chemiluminescence of human bloodstream monocytes and neutrophils: an unusual oxidant(s) generated by monocytes during the respiratory burst.

Maximal rates of O2- and H2O2 production by human bloodstream monocytes activated during the respiratory burst by phorbol ester were only about 10% of those of neutrophils. Furthermore, monocytes possess only about 5% of the myeloperoxidase activity of neutrophils and so can only produce low levels of HOCl and related compounds. These combined reductions in O2- generating ability and lower myeloperoxidase levels result in low levels of luminol chemiluminescence stimulated during the respiratory burst of monocytes. However, although monocytes generate much lower levels of O2- and H2O2 than neutrophils, these cells produce comparable rates of PMA-stimulated lucigenin chemiluminescence. Hence, this assay does not accurately reflect the production of either of these two oxidants by activated phagocytes, and further lucigenin must react with some other oxidant(s) via a process which leads to photon emission. This oxidant(s) is not O2-, H2O2, .OH, 1O2 or NO, but is derived from O2- generated during the respiratory burst and is generated in greater quantities by activated monocytes compared with neutrophils. Thus, lucigenin chemiluminescence is an indirect measure of superoxide release.

Effect of 1,25-dihydroxyvitamin D3, lipopolysaccharide, or lipoteichoic acid on the expression of NADPH oxidase components in cultured human monocytes.

Human blood monocytes in culture gradually lose their capability to produce superoxide when stimulated. Addition of 10(-8) M 1,25-dihydroxyvitamin D3 [1, 25(OH)2D3], 2.5 ng/ml LPS, or 25 ng/ml lipotechoic acid (LTA) prevented this decrease in monocyte respiratory burst when added at initiation of culture. Monocytes cultured for 4 days in the presence of 1,25 (OH)2D3, LPS, or LTA retained the ability to produce superoxide and 1,25 (OH)2D3 actually increased the oxidative capacity compared with fresh monocytes. Furthermore, addition of 1,25(OH)2D3, LPS, or LTA to monocytes at day 4 of culture restored the oxidase activity by day 7 to the levels seen with fresh monocytes. Cytosol or membrane fractions from monocytes cultured with or without the agents indicated above were tested in a cell-free assay of superoxide production by mixing with fresh monocyte membrane or cytosol fractions, respectively. Membrane oxidase activity from these cultured monocytes showed only minimal changes regardless of the agent present or absent from the culture. However, the activity of cytosol fractions from these same cultured monocytes showed substantial differences depending upon the culture conditions and these changes correlated closely with changes in activity of the intact monocytes from which the cytosols were derived. Immunoblot analysis of monocyte membranes and cytosol was used to assess the amount of the 91-kDa and 22-kDa subunits of membrane oxidase cytochrome 558 (gp91 and p22), and the 47-kDa and 67-kDa cytosol oxidase factors (p47 and p67) in the different culture conditions. The gp91 and p22 decreased during culture of monocytes, although the decrease was only apparent by 7 days in culture. When 1,25 (OH)2D3, LPS, or LTA were present at the initiation of culture, then an increase in gp91 and p22 was seen as compared with fresh monocytes at day 4. When these agents were added at day 4 of culture, the levels of gp91 and p22 at day 7 were similar to fresh monocytes. The p67 cytosol oxidase component showed little change in amount by immunoblot regardless of the time in culture or whether 1,25(OH)2D3, LPS, or LTA were present. In contrast, the p47 cytosol oxidase factor decreased by day 4 of culture and was markedly depressed by day 7. Monocytes cultured in the presence of either 1,25(OH)2D3, LPS, or LTA preserved the expression of p47, whereas addition of these agents to monocytes at day 4 of culture restored the expression of p47 by day 7 to the level of fresh monocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo effects of macrophage colony‐stimulating factor on human monocyte function

Macrophage colony-stimulating factor (M-CSF) is reported to enhance a variety of functions of mature monocyte/macrophages in vitro. We have examined the effects of a 2 h intravenous infusion of M-CSF obtained from human urine (hM-CSF) on haematological parameters and selected monocyte functions. There was a rapid, small, but consistent reduction in Hb concentration (mean 6.5 +/- 2.3%, P less than 0.0005 by paired t test) by the completion of the hM-CSF infusion and small, transient falls in platelet, monocyte and neutrophil counts were noted in the 2 h following the end of the infusion. No effect on monocyte or neutrophil CD11b cellular adhesion molecule expression was detected. Exposure to hM-CSF in vivo did not directly stimulate the monocyte respiratory burst, but increased the percentage of monocytes responding to f-met-leu-phe from 9.8 +/- 2.5 to 16.6 +/- 4.2 (P less than 0.01). The number of candida ingested and degraded per 100 monocytes increased from 101 +/- 14 pre-infusion to 160 +/- 22 post-infusion (P less than 0.01). There was a rapid increase in the numbers of monocytes entering a skin window membrane from a mean of 226 +/- 71 pre-infusion to 1064 +/- 404 at the end of the infusion, with no effect on neutrophil migration. These data show that the administration of hM-CSF enhances several of the functions of peripheral blood monocytes in vivo, and this may be of benefit in the treatment of selected infections.

Molecular characterization and functional analysis of the leukocyte surface protein CD31.

The CD31 Ag is a surface glycoprotein of 130 kDa with a broad cellular distribution. We show that among peripheral human blood cells, it is expressed on monocytes, granulocytes, platelets, and a subpopulation of lymphocytes. Activation of granulocytes leads to down-regulation of CD31 molecule expression. Sequence analysis and quantitative measurements of the relatedness of the CD31 molecule to other known proteins demonstrate that it consists of six Ig constant domains and that each domain bears substantial similarity to Fc gamma R domains. We find, however, that the CD31 molecule does not bind Ig Fc domains. On human monocytes we demonstrate that CD31 mAb recognizing certain epitopes of the CD31 molecule induce the generation of reactive oxygen metabolites. No such effect was seen with human granulocytes. By using two CD31 mAb, termed 1B5 and 7E4, we analyzed the requirements for activation of the monocyte respiratory burst via CD31 Ag in more detail. We show that signal transduction occurs via formation of a CD31 Ag-mAb-Fc gamma R complex involving either Fc gamma RI (CD64) or Fc gamma RII (CDw32) molecules.

Vip Inhibition of Monocyte Respiratory Burst Ex Vivo During Prolonged Strain and Energy Deficiency

Vip Inhibition of Monocyte Respiratory Burst Ex Vivo During Prolonged Strain and Energy Deficiency

Vasoactive intestinal peptide inhibits the respiratory burst in human monocytes by a cyclic AMP-mediated mechanism

The neuropeptide vasoactive intestinal peptide (VIP) was shown to inhibit the production of reactive oxygen compounds (respiratory burst) in monocytes activated by serum opsonized zymosan. Reactive oxygen compounds are of importance for host defence against micro-organisms and cancer, but normal tissues are also susceptible to damage from these reactive substances. Maximum inhibition of respiratory burst was 40% by 0.1 μM VIP (ID 100), while ID 50 for the VIP effect was 0.36 nM VIP. PHM-27, closely related to VIP on the basis of the amino acid sequence, inhibited the respiratory burst with much lower potency ( ID 50 = 60 nM , ID 100 = 1 μ M ). Secretin, related to VIP and PHM-27, produced no effect on the respiratory burst in monocytes. VIP was also shown to stimulate the cyclic AMP production in monocytes in a dose dependent manner. IBMX and forskolin, as well as the cyclic AMP analogue butyryl cyclic AMP were shown to produce an inhibition of the respiratory burst. In conclusion, this study showed that VIP inhibited the respiratory burst in monocytes by a cyclic AMP-mediated mechanism, and serves to establish still another role for VIP as a mediator in the neuro-immune axis.

Changes in respiratory burst activity during human monocyte differentiation in suspension culture.

Monocytes undergo a process of differentiation following their accumulation into extravascular spaces. This process has been examined previously by culturing monocytes and identifying changes in cell morphology, metabolism, and function over time. The present study was designed to characterize mononuclear phagocyte respiratory burst activity as related to differentiation by measuring chemiluminescence and superoxide anion generation in cultured human monocytes. Monocytes maintained in Teflon vials for up to 12 days increased in size, were positive for nonspecific esterase, and retained the ability to ingest latex particles. During culture, however, cells progressively lost their peroxidase-positive granules. When monocytes were cultured for one or five days, they elicited less than 50% of the luminol-enhanced chemiluminescence produced by fresh monocytes following PMA stimulation. By day 7, less than 20% of day 0 PMA-elicited chemiluminescence was observed. A comparable loss of serum-opsonized zymosan-induced chemiluminescence occurred during monocyte culture. Since it is recognized that luminol-enhanced chemiluminescence is, in large part, dependent upon myeloperoxidase and since differentiated mononuclear phagocytes are only minimally peroxidase-positive, cultured monocyte respiratory burst activity was also assessed by directly quantifying superoxide anion generation. When monocytes were cultured for three or five days, they elicited 38% more superoxide anion than did fresh monocytes following PMA stimulation. At day 7, PMA-induced superoxide anion release was comparable to day 0 levels. These data indicate that monocytes allowed to differentiate under nonadherent conditions maintain the ability to undergo a respiratory burst response as measured by superoxide anion release, but they concomitantly lose peroxidase-dependent luminol-enhanced chemiluminescence. In this regard, monocytes cultured in suspension metabolically resemble macrophages that have undergone differentiation within sites of inflammation.

Opioid-mediated suppression of cultured peripheral blood mononuclear cell respiratory burst activity.

Opiate addiction and stress have been associated with altered immune responses. In this study, we evaluated the influence of morphine and the stress responsive opioid peptide beta-endorphin (beta-END) on O-2 and H2O2 production by cultured human peripheral blood mononuclear cells. Exposure of these cells during 48 hr of culture to morphine and beta-END at pharmacologically (10(-8) M) and physiologically (10(-12) M) relevant concentrations, respectively, markedly suppressed peripheral blood mononuclear cell O-2 and H2O2 release in response to the respiratory burst stimuli opsonized zymosan and phorbol myristate acetate. Both opioids also induced a minimal, but statistically significant, increase in resting O-2 and H2O2 generation. The modulatory effects of morphine and beta-END on peripheral blood mononuclear cell oxygen metabolism appeared to involve a classical opioid receptor, because opioid activity was blocked by naloxone and was not observed with N-acetylated-beta-END. Using purified lymphocyte and monocyte preparations, we determined that although opioids directly increase monocyte-resting oxygen metabolism, lymphocytes are the primary target cell in opioid-mediated suppression of monocyte respiratory burst activity. The release of a suppressive product from opioid-triggered lymphocytes was inhibited by cyclosporine. Based on the results of this study, we propose that opioid-mediated suppression of mononuclear phagocyte respiratory burst activity is another factor to be considered in the immunodeficiency of opiate addiction and stress.