Medroxyprogesterone acetate enhances in vivo and in vitro antibody production.

In the present study we examine the effects of medroxyprogesterone acetate (MPA) on the specific antibody secretion to T-dependent antigens. Our results show that the in vivo administration of MPA to mice, 7 or 90 days before immunization with sheep red blood cells (SRBC), significantly enhanced both, primary and secondary antibody responses, without affecting delayed-type hypersensitivity (DTH). These effects could be counteracted by the anti-progestin onapristone or ZK 98299 (ZK) suggesting that MPA interacted with progesterone (PRG) receptors to increase B-cell response. To better understand the mechanisms involved in MPA activity we carried out cultures of splenocytes, bone marrow cells or lymph node cells from immunized mice in the presence of MPA, and evaluated the amount of antibody release to supernatants. We found that low doses of MPA (10(-9) M and 10(-10) M) significantly enhanced the in vitro production of specific immunoglobulin G (IgG) antibodies, an effect that appears to involve the interaction of the progestin with PRG receptors, as judged by the inhibition of MPA effects with ZK (10(-8) M) or RU486 (10(-9) M). These receptors were detected by flow cytometry analysis in a proportion of T lymphocytes. Because MPA did not increase the number of immunoglobulin-secreting cells, our findings suggest that MPA enhanced the capacity of individual cells to produce specific immunoglobulin.

Promotion of neutrophil apoptosis by TNF-alpha.

We examined the ability of TNF-alpha to modulate human neutrophil apoptosis. Neutrophils cultured with TNF-alpha alone undergo a low but significant increase in the number of apoptotic cells. More interestingly, when neutrophils were pretreated with TNF-alpha for 1-2 min at 37 degrees C and then were exposed to a variety of agents such as immobilized IgG, IgG-coated erythrocytes, complement-treated erythrocytes, zymosan, PMA, zymosan-activated serum, fMLP, Escherichia coli, and GM-CSF for 3 h at 37 degrees C, a marked stimulation of apoptosis was observed. Similar results were obtained in neutrophils pretreated with TNF-alpha for 30 min, 1 h, 3 h, and 18 h. Dose-dependent studies showed that TNF-alpha enhances neutrophil apoptosis at concentrations ranging from 1 to 100 ng/ml. In contrast to the observations made in neutrophils pretreated with TNF-alpha, there was no stimulation of apoptosis when TNF-alpha was added to neutrophils previously activated by conventional agonists. Experiments performed to establish the mechanism through which TNF-alpha promotes neutrophil apoptosis showed that neither reactive oxygen intermediates nor the Fas/Fas ligand system appear to be involved. Our results suggest that TNF-alpha plays a critical role in the control of neutrophil survival by virtue of its ability to induce an apoptotic death program which could be triggered by a variety of conventional agonists.

Acidic pH increases the avidity of FcgammaR for immune complexes.

The interaction of immunoglobulin G (IgG) antibodies with FcgammaR constitutes a critical mechanism through which IgG antibody effector functions are mediated. In the current work we have examined whether human neutrophil FcgammaR exhibit pH dependence in their association with IgG. Binding assays were performed in culture medium adjusted to different pH values. It was found that the binding of either heat-aggregated human IgG (AIgG), soluble immune complexes (sIC) or IgG-coated erythrocytes (IgG-E) was markedly higher at pH 6.5 than at pH 7.3. This effect was not observed when saturation of FcgammaR was achieved, suggesting that acidic pH increases the avidity of FcgammaR for IC without modifying the total binding capacity. Similar results were observed for the binding of AIgG to either monocytes, natural killer (NK) or K562 cells, suggesting that acidic pH increases the avidity of both, FcgammaRII and FcgammaRIII. Additional experiments were performed to analyse whether the binding of IgG to FcgammaRI also showed pH dependence. To this aim, we employed interferon-gamma-treated human neutrophils and mouse inflammatory macrophages, previously incubated with blocking antibodies directed to FcgammaRII and FcgammaRIII. Acidic pH did not enhance the binding of AIgG nor monomeric IgG under these experimental conditions. Further studies are required to determine whether the enhancement of FcgammaR avidity for IC could be attributed to titration of histidine(s) residues on the Fc fragment of IgG.

Immune complexes inhibit apoptosis of chronic lymphocytic leukaemia B cells.

In the present study we examined the effect of immune complexes (IC) on the survival of chronic lymphocytic leukaemia (B-CLL) B cells. Our results showed that either precipitating IC (pIC), Ab-coated erythrocytes (E-IgG) or heat-aggregated IgG (aIgG) significantly inhibited spontaneous apoptosis of B-CLL cells, as well as that induced by fludarabine, chlorambucil or dexamethasone. After depletion of T lymphocytes, monocytes and NK cells, incubation with IC was no longer able to delay B-CLL cells apoptosis, suggesting that prevention of apoptosis depends on IC interaction with accessory leucocytes. The release of IFNgamma by non-malignant cells upon activation with IC was responsible, to some extent, for IC effects as shown by the fact that neutralizing anti-IFNgamma MoAb partially prevented their ability to inhibit B-CLL cells apoptosis. The observation that treatment with IC resulted in increased expression of HLA-DR on B-CLL cells suggests that inhibition of apoptosis is associated with cellular activation.

Extracellular acidification induces human neutrophil activation.

In the current work, we evaluated the effect of extracellular acidification on neutrophil physiology. Neutrophils suspended in bicarbonate-buffered RPMI 1640 medium adjusted to acidic pH values (pH 6.5-7.0) underwent: 1) a rapid transient increase in intracellular free calcium concentration levels; 2) an increase in the forward light scattering properties; and 3) the up-regulation of surface expression of CD18. By contrast, extracellular acidosis was unable to induce neither the production of H2O2 nor the release of myeloperoxidase. Acidic extracellular pH also modulated the functional profile of neutrophils in response to conventional agonists such as FMLP, precipiting immune complexes, and opsonized zymosan. It was found that not only calcium mobilization, shape change response, and up-regulation of CD18 expression but also production of H2O2 and release of myeloperoxidase were markedly enhanced in neutrophils stimulated in acidic pH medium. Moreover, extracellular acidosis significantly delayed neutrophil apoptosis and concomitantly extended neutrophil functional lifespan. Extracellular acidification induced an immediate and abrupt fall in the intracellular pH, which persisted over the 240-s analyzed. A similar abrupt drop in the intracellular pH was detected in cells suspended in bicarbonate-supplemented PBS but not in those suspended in bicarbonate-free PBS. A role for intracellular acidification in neutrophil activation is suggested by the fact that only neutrophils suspended in bicarbonate-buffered media (i.e., RPMI 1640 and bicarbonate-supplemented PBS) underwent significant shape changes in response to extracellular acidification. Together, our results support the notion that extracellular acidosis may intensify acute inflammatory responses by inducing neutrophil activation as well as by delaying spontaneous apoptosis and extending neutrophil functional lifespan.

Effect of nitric oxide donors on oxygen-dependent cytotoxic responses mediated by neutrophils.

We analyzed the effect of nitric oxide (NO) on oxygen-dependent cytotoxic responses mediated by neutrophils against unopsonized erythrocytes using three NO donors: S-nitrosoglutathione (GSNO), S-nitroso-N-acetylpenicillamine (SNAP), and sodium nitroprusside (SNP). Neutrophils were treated with these compounds for 1-2 min at 37 degrees C and cytotoxicity was then triggered in the presence of NO donors by precipitating immune complexes, aggregated IgG, the chemotactic peptide FMLP, or opsonized zymosan. GSNO induced, in all cases, a marked increase in cytotoxic responses, while SNAP moderately increased cytotoxicity triggered by immune complexes, aggregated IgG, or Z, opsonized zymosen, without modifying those responses induced by FMLP. By contrast, SNP dramatically suppressed cytotoxicity triggered by all of the stimuli assessed. The enhancing effects mediated by GSNO and SNAP did not depend on the stimulation of guanylyl cyclase and were prevented by the NO scavengers hemoglobin and PTIO (2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl 3-oxide). The inhibitory activity of SNP, on the other hand, was not prevented by NO scavengers, suggesting that it cannot be ascribed to the release of NO. In another set of experiments, neutrophils were pretreated with GSNO or SNAP for different times. Then cells were washed to remove NO donors from the culture medium, and cytotoxicity was triggered by different stimuli. It was found that neutrophils must be pretreated with NO donors for at least 4 h to increase cytotoxic responses, and pretreatment for longer periods (i.e., 8 or 18 h) further increased cytotoxicity. Not only cytotoxic responses, but also the production of O2- and H2O2, and the release of myeloperoxidase were increased under these conditions.

Modulation of human neutrophil apoptosis by immune complexes.

In the present study we examined whether immune complexes (IC) are able to modulate human neutrophil apoptosis. We observed different effects depending on the type of IC employed. Precipitating IC (pIC) and Ab-coated erythrocytes (E-IgG) triggered a marked stimulation of apoptosis, while heat-aggregated IgG and soluble IC, significantly delayed spontaneous apoptosis. Blocking Abs directed to Fcgamma receptor type II (FcgammaRII), but not to FcgammaRIII, markedly diminished the acceleration of apoptosis triggered by either pIC or E-IgG, supporting a critical role for FcgammaRII in apoptosis stimulation. This phenomenon, on the other hand, does not appear to involve IC phagocytosis or the participation of CR3. Acceleration of neutrophil apoptosis triggered by either pIC or E-IgG seems to require the activation of the respiratory burst, as suggested by 1) the ability of catalase to prevent apoptosis stimulation; 2) the effect of azide, an heme enzyme inhibitor, which dramatically enhanced apoptosis induced by pIC or E-IgG; and 3) the inability of pIC or E-IgG to accelerate apoptosis of neutrophils isolated from CGD patients. It is well established that IC affect the course of inflammation by inducing the release of inflammatory cytokines, proteolytic enzymes, oxidative agents, and other toxic molecules. Our results suggest that IC may also affect the course of inflammation by virtue of their ability to modulate neutrophil apoptosis.

Losartan, a selective inhibitor of subtype AT1 receptors for angiotensin II, inhibits the binding of N-formylmethionyl-leucyl-phenylalanine to neutrophil receptors.

Losartan, a selective antagonist of AT1 receptors for angiotensin II, is widely used clinically to manage hypertension. We report here that losartan markedly inhibits neutrophil shape change, adherence and chemiluminescence responses triggered by N-formylmethionyl-leucyl-phenylalanine (fMLP), without affecting responses induced by immune complexes, zymosan or concanavalin A. Neither saralasin, another antagonist of angiotensin II receptors, nor captopril, an angiotensin-converting enzyme inhibitor, reproduced the effects of losartan. It was also observed that neutrophil responses triggered by fMLP were not affected by exogenously added angiotensin II. The effect of losartan on the binding of fMLP was measured using [3H]fMLP. It was found that losartan inhibits the binding of [3H]fMLP to neutrophil receptors. As observed for neutrophils, studies performed with monocytes showed that losartan inhibits chemiluminescence emission triggered by fMLP, without affecting chemiluminescence responses triggered by immune complexes, zymosan or concanavalin A.

Inhibition of human neutrophil apoptosis by platelets.

In the absence of appropriate stimuli, polymorphonuclear neutrophils rapidly undergo characteristic changes indicative of programmed cell death or apoptosis. We report here that neutrophils cultured in the presence of platelets (neutrophil:platelet ratios of 1:50, 1:25, and 1:10) show a dramatic inhibition of apoptosis compared with neutrophils cultured alone. Similar degrees of apoptosis delay were induced by viable unstimulated platelets, fixed unstimulated platelets, or fixed activated (1 U/ml thrombin) platelets. Inhibition of apoptosis was associated with prolongation of the functional lifespan of the neutrophil, as indicated by the higher capacity of platelet-treated neutrophils to display chemiluminescence responses triggered by FMLP, immune complexes, and zymosan. The mechanism responsible for the inhibition of neutrophil apoptosis by platelets has not yet been defined. However, it seems that classical recognition systems such as those mediated by the interaction between platelet P-selectin (CD62) or glycoprotein IIb/IIIa complex and their counter-receptors expressed by neutrophils are not involved.

Nitric oxide synthase inhibitors enhance plasma levels of corticosterone and ACTH.

The role of nitric oxide in the regulation of adrenal steroidogenesis was examined in BALB/c mice by employing the nitric oxide synthase inhibitors NG-nitro L-arginine methyl ester (L-NAME) and NG-nitro L-arginine (L-NNA). The administration of a single dose of nitric oxide inhibitors (50 mg kg-1 body wt. i.p.) induced a fourfold increase in plasma corticosterone. Treatment with L-arginine (750 mg kg-1 body wt. s.c.), but not D-arginine, completely prevented corticosterone increases induced by L-NAME. To analyse whether the activation of adrenal steroidogenesis induced by nitric oxide synthase inhibitors involved the stimulation of the hypothalamo-pituitary-adrenal axis. ACTH levels were assessed. It was found that L-NAME significantly enhanced plasma ACTH concentrations. Genetic variations in this regulatory pathway are suggested by the fact that L-NAME increased corticosterone levels in BALB/c. C3H/He and DBA-2 mice, but not in C57BI/c mice, a strain characterized by a low steroid response to stress.

Neutrophil apoptosis induced by proteolytic enzymes.

In this study, we show that three proteolytic enzymes of different specificity-pronase, chymotrypsin, and trypsin-induced a dramatic stimulation of neutrophil apoptosis as shown by morphologic characteristics, analysis of cell DNA content, and presence of a characteristic "ladder" pattern of DNA fragmentation. The action of either chymotrypsin or trypsin was completely prevented by the serine protease inhibitor aprotinin, indicating that the proteolytic activity of the enzymes accounts for apoptosis induction. Stimulation of neutrophil apoptosis by proteases was observed in culture medium supplemented with either inactivated fetal calf serum (0.1-50%), autologous serum (0.1-50%), bovine serum albumin (0.1%), or in protein-free medium. Other cell types such as human peripheral blood monocytes and lymphocytes, human leukemic cells from THP-1, HL-60 and K562 lines, murine L929 fibroblasts, and unstimulated murine macrophages harvested from the peritoneal cavity were not induced to undergo apoptosis after the treatment with proteases. In an attempt to determine whether neutrophil serine proteases could induce apoptosis as chymotrypsin and trypsin do, the effect of elastase was assessed. A significant increase in the percentage of apoptotic cells was observed in elastase-treated neutrophils. We propose that the selective stimulation of neutrophil apoptosis by proteolytic enzymes may play an important role in the normal resolution of inflammation by limiting the autotoxic potential of the neutrophil.

Activation of human neutrophils induced by immune complexes prepared with cationic and anionic fractions of normal IgG antibodies.

The authors have recently shown that the ability of immune complexes (IC) to trigger Fc gamma R-dependent cell responses can be dramatically enhanced when the isoelectric point (pI) of normal IgG antibodies is increased from 5.8-8.5 to 8.5-9.8 by treatment with 1-ethyl-3-2(3-dimethylaminopropyl) carbodiimide HCl and ethylene diamine. In the current work the authors analyse whether differences in the charge of normal IgG antibodies may also affect IC activity. Soluble IC (sIC) were prepared with (a) rabbit IgG antibodies to human IgG and anionic or cationic fractions of human IgG; and (b) bovine serum albumin (BSA) and anionic or cationic fractions of rabbit IgG anti-BSA antibodies. Similar abilities to bind to neutrophil surface were observed for sIC prepared with both anionic (anIC) and cationic fractions of IgG (catIC). Moreover, no differences were found when neutrophil shape change, chemiluminescence (CL) emission and elastase release were induced by either anIC or catIC. As in the case of sIC, particulate IC prepared with erythrocytes (E) and anionic or cationic fractions of specific IgG antibodies (IgG-E) showed no differences in their abilities to trigger either CL emission or ADCC. Taken together, these results suggest that the pI of normal IgG antibodies do not affect the ability of IC to trigger neutrophil responses mediated by receptors for the Fc portion of IgG antibodies (Fc gamma R).

Involvement of nitric oxide in the regulation of peripheral blood leukocyte counts.

A role for nitric oxide (NO) in the regulation of blood leukocyte numbers was examined in BALB/c mice by employing the NO synthase inhibitor NG-nitro L-arginine methyl ester (L-NAME). Treatment of animals with a single dose of 50 mg/kg body wt caused a dramatic increase in the number of circulating neutrophils and a moderate decrease in the number of circulating lymphocytes. These effects were partially reversed by the simultaneous inoculation of L-arginine (250 mg/kg body wt.) but not by D-arginine. A second NO synthase inhibitor, NG-nitro L-arginine, induced changes comparable to those elicited by L-NAME. Because catecholamines and glucocorticoids are well-known modulators of blood leukocyte counts, experiments were carried out in adrenalectomized mice. It was found that adrenalectomy did not modify the increase in the number of circulating neutrophils induced by L-NAME but completely prevented the decrease of circulating lymphocytes. Taken together, these findings support the hypothesis that NO plays an important role in the regulation of the peripheral blood number of neutrophils and lymphocytes, and that this function involves, in each case, the participation of different mechanisms.

Neutrophil chemotaxis induced by immune complexes.

In the current work we have analyzed the ability of different soluble immune complexes (IC) prepared with IgG antibodies to induce neutrophil chemotactic responses in vitro. While, in all cases, IC were able to induce neutrophil migration in a concentration-dependent fashion, IgG antibodies alone were completely unable to induce locomotor responses. Checkerboard analysis indicated the chemotactic nature of motility. On the other hand, chemotaxis induced by IC was markedly inhibited by IV. 3, a monoclonal antibody (mAb) to Fc gamma RII, slightly reduced by 3G8 F(ab')2, a mAb to Fc gamma RIII, and nearly abrogated by both mAbs. The impact of IC on neutrophil migration induced by FMLP was also studied. We found that when a suboptimal concentration of FMLP was employed, the simultaneous addition of IC increased the migration acting in additive form. The significance of these results is discussed.

Effect of proteolytic enzymes on neutrophil Fc gamma RII activity.

Previous studies have demonstrated that the treatment of neutrophils with proteolytic enzymes markedly reduces the expression of receptor III for the Fc portion of IgG (Fc gamma RIII), but it does not affect the number of Fc gamma RII on the cell surface. In the present study, we analysed the effect of proteolytic enzymes on functional responses of neutrophils induced by immune complexes (IC). Our results showed that treatment with pronase or chymotrypsin markedly increased the binding of IgG-coated erythrocytes (IgG-E) to neutrophils, as well as their capability to display IgG-mediated functions such as antibody-dependent cellular cytotoxicity (ADCC) and chemiluminescence (CL) induced by IgG-E, responses that have been shown to be completely dependent on Fc gamma RII. A similar enhancing effect was observed, in all cases, after neutrophil treatment with neuroaminidase. We also studied the effect of proteolysis on neutrophil activation induced by other types of IC. It was found that pronase and chymotrypsin significantly enhanced CL responses induced by soluble IC (sIC) but did not modify the responses induced by either precipitating IC (pIC) or soluble IC prepared with cationized antibodies (catIC). On the other hand, neuroaminidase significantly enhanced CL induced by either sIC, pIC or catIC. Taken together, our data suggest that the activity of Fc gamma RII can be up-regulated by proteolysis. However, this effect appears to be strongly dependent on the characteristics of the IC employed as stimulus.

Activation of human platelets by immune complexes prepared with cationized human IgG.

The present study shows that the ability of soluble immune complexes (IC), prepared with human IgG and rabbit IgG antibodies against human IgG, to trigger platelet activation was markedly higher for IC prepared with cationized human IgG (catIC) compared with those prepared with untreated human IgG (cIC). CatIC induced platelet aggregation and adenosine triphosphate release in washed platelets (WP), gel-filtered platelets (GFP), or platelet-rich plasma (PRP) at physiologic concentrations of platelets (3 x 10(8)/mL) and at low concentrations of catIC (1 to 30 micrograms/mL). On the contrary, under similar experimental conditions, cIC did not induce aggregation in PRP, WP, or GFP. Low aggregation responses were only observed using high concentrations of both WP (9 x 10(8)/mL) and cIC (500 micrograms/mL). Interestingly, catIC were also able to induce platelet activation under nonaggregating conditions, as evidenced by P-selectin expression. Cationized human IgG alone did not induce platelet aggregation in PRP but triggered either WP or GFP aggregation. However, the concentration needed to induce these responses, was about eightfold higher than those required for catIC. The responses induced either by catIC or cationized human IgG were completely inhibited by treatment with heparin, dextran sulphate, EDTA, prostaglandin E1, or IV3, a monoclonal antibody against the receptor II for the Fc portion of IgG (Fc gamma RII). The data presented in this study suggest that IgG charge constitutes a critical property that conditions the ability of IC to trigger platelet activation.

Activation of human neutrophils and monocytes induced by immune complexes prepared with cationized antibodies or antigens.

We have recently showed that soluble immune complexes (IC) prepared with cationic antibodies (catIC) induce high levels of neutrophil-mediated cytotoxicity against nonsensitized target cells. In the present work we extended our previous findings by studying the ability of catIC to induce different responses mediated by monocytes and/or neutrophils: monocyte cytotoxicity against nonsensitized target cells, chemiluminescence emission by monocytes and neutrophils, and elastase release from neutrophils. Our results showed that, in all cases, cell responses induced by catIC were markedly higher than those induced by control IC, indicating that cationized antibodies enhance IC ability to trigger phagocytic cell activation. A second aim of the present study was to analyze the effect of antigen cationization on IC properties. Interestingly, we found that all the phagocytic cell responses induced by IC prepared with cationized ovalbumin (OA) were significantly higher than those induced by IC prepared with untreated OA. Our results suggest that the charge of antibody and/or antigen constitutes a critical property that conditions the biological activity of IC. Furthermore, these findings support an important role of cationic antibodies and antigens in the development of inflammatory events associated with certain IC-induced diseases.

Neutrophil cytotoxicity induced by immune complexes prepared with cationized antibodies.

Here we analyse the ability of soluble immune complexes (IC) prepared with cationized antibodies to induce cytotoxic responses mediated by neutrophils. While cationized IC induced high levels of cytotoxicity, control IC induced very low levels of response. Inhibition of cytotoxicity by catalase but not by three haemenzyme inhibitors suggests that oxygen-dependent but myeloperoxidase-independent mechanisms are responsible for cytolysis. While the response induced by control IC was enhanced by cytochalasin B and was not modified by colchicine, that induced by cationized IC was markedly inhibited by cytochalasin B and significantly enhanced by colchicine. Cytotoxicity induced by cationized IC was completely abrogated by monoclonal antibodies to Fc gamma RII. Using control IC, a partial inhibition was observed employing either anti-Fc gamma RII or anti-Fc gamma RIII monoclonal antibodies. Treatment of neutrophils with chemotrypsin or pronase significantly enhanced cytotoxicity induced by cationized IC but not by control IC. We also found that non-specific absorptive mechanisms appear to play an important role in the binding of cationized IC, but not control IC, to the neutrophil surface. The significance of these results is discussed.

Extracellular acidic pH modulates oxygen-dependent cytotoxic responses mediated by polymorphonuclear leucocytes and monocytes.

In the present study, we compared the ability of human neutrophils and monocytes to display oxygen-dependent cytotoxic responses at pH 7.4 and 6.2. Our results show that cytotoxicity induced by immune complexes (IC), zymosan, N-formyl-methionyl-leucyl-phenylalanine (FMLP) and concanavalin A (Con A) were markedly increased when they were carried out at pH 6.2 instead of pH 7.4. Cytotoxicity induced by phorbol myristate acetate (PMA), on the contrary, was significantly decreased at pH 6.2. It is noteworthy that cytotoxic responses induced by IC, zymosan and Con A were also increased when, 2 h after effector cell stimulation at pH 6.2, cytotoxicity was measured at pH 7.4. Finally, when we examined possible mechanisms involved in the augmentation of cytotoxicity, we observed that the oxidative response of IC-stimulated neutrophils, measured as chemiluminescence emission, was not increased at pH 6.2, on the contrary, it was significantly decreased. The relevance of these results is discussed.

Interferon-gamma is unable to increase monocyte and neutrophil-mediated nonspecific cytotoxicity induced by immune complexes.

We have previously demonstrated that normal human neutrophils and monocytes triggered by immune complexes (IC) are able to destroy non-sensitized target cells through the activation of a nonspecific cytotoxic mechanism (NSC), that is dependent on the generation of reactive oxygen intermediates (IRO). In the present study, we analyze the ability of interferon-gamma (IFN-gamma) to modulate NSC. Our results indicate that, despite the ability of IFN-gamma to increase both the generation of superoxide anion and hydrogen peroxide by phagocytic cells and the expression of the high-affinity 72-kDa Fc gamma RI, it is completely unable to increase NSC mediated either by neutrophils or monocytes. These data suggest that there is no correlation between cytotoxicity and the ability of phagocytic cells to release superoxide anion and/or hydrogen peroxide. They also indicate that Fc gamma RI is not involved in the induction of NSC. To further analyze this point, we studied the ability of two monoclonal antibodies (mAb), specific for different epitopes of Fc gamma RI, to inhibit NSC. These mAb strongly inhibited ADCC mediated by untreated monocytes or IFN-gamma treated monocytes and neutrophils. On the other hand, they were completely unable to inhibit NSC mediated by untreated or IFN-gamma treated cells.