Downregulation of mac-1 expression in monocytes by surface-bound IgG.

Physical and functional association between the beta2-integrin Mac-1 (CD11b/CD18) and receptors of immunoglobulin G (IgG) (FcgammaRs) has been previously reported. In this study, we examined the modulation of Mac-1 expression by IgG in different leucocyte populations. Our data show that human monocytes, but not neutrophils, macrophages, dendritic or natural killer cells, downregulate the expression of Mac-1 after overnight exposure to surface-bound IgG. This effect, which requires at least 6 h of incubation, is not associated with a general downmodulation of membrane antigens, and is selectively induced by immobilized IgG (iIgG), as the stimulation of monocytes with N-formyl-methionyl-leucyl-phenylalanine, lipopolysaccharide, tumour necrosis factor-alpha (TNF-alpha) or soluble IgG did not modify the Mac-1 expression after 18 h in culture. The loss of Mac-1 was completely prevented by blocking antibodies (Abs) directed to FcgammaRII (CD32) or CD18. On the other hand, the serine protease inhibitor, phenyl methyl sulphonyl fluoride, but not inhibitors of cysteine proteases or neutral endopeptidases, partially prevented the downregulation of Mac-1 by iIgG. Monocytes cultured overnight on iIgG exhibited a dramatic decrease in their capacity to ingest zymosan particles that could be attributed to the reduced expression of Mac-1. However, there was no inhibition of TNF-alpha production induced by zymosan, suggesting that Mac-1-dependent responses require different levels of its expression to be fully activated.

Non-malignant leukocytes delay spontaneous B-CLL cell apoptosis.

Malignant B cells from chronic lymphocytic leukemia (B-CLL) patients have a long survival in vivo, although, in culture, they spontaneously die by apoptosis. Here, we analyzed the capacity of accessory leukocytes to modulate apoptosis of B-CLL cells in vitro. To this end, we performed long-term cultures using total mononuclear cells (TMC) from B-CLL patients and TMC depleted from monocytes, NK cells and T lymphocytes (B-CLL cells). In all the patients studied (n = 25) the presence of accessory leukocytes markedly prolonged the survival of B-CLL cells. The anti-apoptotic effect was exerted by monocytes and, to a lesser degree, NK cells, partially through the release of soluble factors. Indeed, accessory leukocytes separated from leukemic cells by semipermeable membranes were still able to prolong B-CLL cell survival. By flow cytometric analysis we found that the protective effect of non-malignant cells was associated with delayed down-regulation of Bcl-2 expression on leukemic cells. By contrast, the expression of Fas and Fas ligand proteins was unchanged in most samples. Our findings suggest that monocytes and NK cells, by delaying leukemic cell apoptosis, may play a role in B-CLL cell accumulation in vivo.

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.

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.