Synthesis of platelet-activating factor by human monocytes stimulated by platelet-activating factor

The capacity of platelet-activating factor (PAF) to stimulate its own synthesis by human monocytes was examined. Adherent human monocytes of greater than 85% purity were incubated with 100 fM to 10 nM of PAF in the presence of 20 microCi of [3H]acetic acid to radiolabel newly synthesized PAF. After incubation for 15 minutes to 15 hours, PAF was purified by high-performance liquid chromatography, and newly synthesized PAF was quantified by its radioactivity. PAF stimulated its own synthesis in a dose-related manner with a maximal twofold to threefold increase in synthesis at 10 pM to 100 pM of PAF. Maximal PAF synthesis occurred after incubation for 6 to 8 hours. There was a good correlation (r = 0.95) between PAF quantified by [3H]acetic acid incorporation and by rabbit platelet aggregation bioassay, indicating that the radioactive material is PAF. The protein synthesis inhibitor, cycloheximide, did not inhibit delayed PAF synthesis, indicating that delayed PAF synthesis does not require protein synthesis. PAF is metabolized rapidly in vivo. The capacity of PAF to stimulate its own synthesis would result in a prolonged effective half-life in vivo. This prolonged half-life could contribute to the capacity of PAF to induce prolonged inflammatory reactions in vivo.     

Danazol suppresses the production of interleukin-1 beta and tumor necrosis factor by human monocytes

The effects of estradiol (E2), progesterone (P), and danazol on the production of interleukin-1 beta (IL-1 beta) and tumor necrosis factor (TNF) by OK-432 (a streptococcal preparation)-stimulated monocytes were examined. E2 and P at physiologic concentrations enhanced IL-1 beta and TNF production by monocytes from donors with lower control levels (without steroids added) of IL-1 beta and TNF. However, E2 and P at physiologic concentrations did not affect IL-1 beta and TNF production by monocytes from donors with higher control levels of IL-1 beta and TNF. Danazol inhibited IL-1 beta and TNF production by monocytes in a dose-dependent manner from not only donors with lower control levels of IL-1 beta and TNF but also donors with higher control levels of IL-1 beta and TNF. Danazol at a concentration of 10(-6) M significantly suppressed IL-1 beta and TNF production in the presence of E2 and/or P at concentrations giving peak responses of IL-1 beta production. These findings suggest possible new mechanisms of action for danazol in the treatment of endometriosis and infertility associated with immune abnormalities.     

Chlamydia trachomatis-induced production of interleukin-1 by human monocytes.

Human diseases caused by the intracellular bacterium Chlamydia trachomatis include genital tract infections and blinding trachoma. Chlamydial infections are characterized by chronic inflammation and scarring, and development of such complications is thought to be immunologically mediated. In this study, we show that coculture of C. trachomatis (serovar L2) with human blood monocytes induced the production of interleukin-1 (IL-1), an important mediator of inflammation, tissue remodeling, and scarring. IL-1 was produced in response to UV-inactivated elementary bodies containing from 0.1 to 50 micrograms of protein per ml, with a maximal response at 5 to 10 micrograms/ml. IL-1 activity was detected by 6 h of incubation and was maximal by 24 h. Peak levels were maintained throughout 96 h of incubation. Rabbit antibody to human IL-1(alpha + beta) effectively neutralized the thymocyte-stimulating activity of the supernatants. The apparent molecular weight of chlamydia-induced IL-1 was 16,000, as determined by gel filtration on a Bio-Gel P-60 column. Isoelectric focusing yielded two peaks of activity, with pIs of 5.5 and 6.9. Neutralization studies with antisera against human IL-1 alpha and IL-1 beta showed that the acidic and neutral peaks corresponded to IL-1 alpha and IL-1 beta, respectively, with IL-1 beta predominating. Heat-killed chlamydiae, which are not internalized by monocytes, were effective IL-1 inducers, indicating that phagocytosis was not required for IL-1 induction. Purified C. trachomatis lipopolysaccharide was also an effective IL-1 inducer, suggesting that the response to intact organisms may be largely a response to chlamydial lipopolysaccharide. Finally, purified chlamydial major outer membrane protein induced low but detectable IL-1 activity.     

Regulation of platelet-activating factor synthesis in human monocytes by dipalmitoyl phosphatidylcholine

Platelet-activating factor (PAF) has a major role in inflammatory responses within the lung. This study investigates the effect of pulmonary surfactant on the synthesis of PAF in human monocytic cells. The pulmonary surfactant preparation Curosurf significantly inhibited lipopolysaccharide (LPS)-stimulated PAF biosynthesis (P<0.01) in a human monocytic cell line, Mono mac-6 (MM6), as determined by (3)H PAF scintillation-proximity assay. The inhibitory properties of surfactant were determined to be associated, at least in part, with the 1,2-dipalmitoyl phosphatidylcholine (DPPC) component of surfactant. DPPC alone also inhibited LPS-stimulated PAF biosynthesis in human peripheral blood monocytes. DPPC treatment did not affect LPS-stimulated phospholipase A(2) activity in MM6 cell lysates. However, DPPC significantly inhibited LPS-stimulated coenzyme A (CoA)-independent transacylase and acetyl CoA:lyso-PAF acetyltransferase activity. DPPC treatment of MM6 cells decreased plasma membrane fluidity as demonstrated by electron paramagnetic resonance spectroscopy coupled with spin labeling. Taken together, these findings indicate that pulmonary surfactant, particularly the DPPC component, can inhibit LPS-stimulated PAF production via perturbation of the cell membrane, which inhibits the activity of specific membrane-associated enzymes involved in PAF biosynthesis.     

Paf-acether (platelet-activating factor) and interleukin-1-like cytokine production by lipopolysaccharide-stimulated glomeruli

The production of inflammatory mediators by glomerular cells may be instrumental in the development of pathophysiological alterations during glomerulonephritis. Since bacterial lipopolysaccharide (LPS) is a naturally occurring immunological stimulus, we studied its inflammatory effects on isolated renal glomeruli. LPS stimulation of human and rat isolated glomeruli resulted in a dose- and time-dependent platelet-activating factor (paf-acether) production. Maximal paf-acether generation (1.04 to 1.50 ng/mg protein) (n = 18) was obtained when glomeruli were stimulated for periods of 1 to 4 hr and with 1-2 micrograms/ml LPS. Paf-acether derived from human and rat glomeruli exhibited identical biological and physicochemical characteristics. In addition, rat glomeruli stimulated with doses of LPS from 100 ng to 50 micrograms/ml released an Interleukin-1 (IL-1)-like cytokine differing in part from that described in cultured mesangial cells. Maximal release of IL-1-like activity by rat glomeruli was obtained after 24 to 48 hr incubation in the presence of LPS. After gel chromatography resolution, the glomerular cytokine presented IL-1-like activity in fractions corresponding to molecular weights of 15-35 and 4-8 kDa. The latter compounds could represent metabolites similar to those described in normal urine. Thus the local release of paf-acether and IL-1-like cytokine by glomeruli in response to bacterial stimuli may represent a prominent feature of glomerular inflammation.     

Glomerular basement membrane-containing immune complexes stimulate tumor necrosis factor and interleukin-1 production by human monocytes.

The ability of human peripheral blood monocytes to produce tumor necrosis factor (TNF) and interleukin-1 (IL-1) in an in vitro model of immune complex-mediated glomerulonephritis was investigated. When isolated monocytes were incubated with human glomerular basement membrane (GBM) containing anti-GBM immune complexes, both TNF and IL-1 were produced and secreted into the medium. The time course of secretion differed, with IL-1 production being maximal after approximately 8 hours, whereas TNF levels continued to rise for 30 hours. The activities of the monocyte-derived TNF and IL-1 were inhibitable by specific antibodies. No effect was seen when monocytes were incubated separately with either GBM alone or anti-GBM IgG. The levels of TNF and IL-1 released were comparable with those induced by high concentrations of LPS, indicating that production was close to the maximal levels reported for these cells. High levels of TNF and IL-1 also were produced in response to soluble immune complexes. The results show that monocytes can produce significant levels of TNF and IL-1 in response to both surface-bound and soluble immune complexes and provide support for the participation of these monokines in glomerulonephritis.     

Protein kinase C regulates the synthesis of platelet-activating factor by human monocytes.

Human peripheral blood monocytes synthesize the potent lipid autacoid platelet-activating factor (PAF) following appropriate stimulation. We examined the role of protein kinase C (PKC) in regulating the synthesis of PAF by stimulated monocytes. 4 beta-phorbol 12-myristate 13-acetate (PMA) and 1,2-dioctanoyl-sn-glycerol, which directly activate PKC, stimulated the synthesis of PAF. Sphingosine, a long-chain amine that inhibits PKC, blocked both the binding of phorbol esters to monocytes and the synthesis of PAF in response to PMA (half-maximal inhibition at 5 to 10 microM and complete inhibition at 10 to 30 microM sphingosine). Thus, the activation of PKC was necessary and sufficient for PAF synthesis in response to phorbol ester. Sphingosine also blocked PAF synthesis in response to the calcium ionophore A23187 and opsonized zymosan particles by specific inhibition of PKC. Two other PKC inhibitors, stearylamine and staurosporine, also blocked PAF synthesis following A23187 or opsonized zymosan stimulation. These experiments demonstrated that PKC activation was required for PAF synthesis in response to the calcium signal generated by A23187 or a receptor-mediated agonist, opsonized zymosan. The synthesis of PAF and leukotriene B4 were temporally coupled following cell stimulation. Further, production of these two lipid mediators, and the release of arachidonic acid, were inhibited in parallel by sphingosine. Thus, PKC regulate the synthesis of both PAF and leukotriene B4 at a common step, probably phospholipase A2.     

Modulatory effect of interleukin-10 on the production of platelet-activating factor and superoxide anions by human leucocytes.

We observed that human monocytes (MO) and polymorphonuclear neutrophils (PMN) stimulated by lipopolysaccharide (LPS) produce platelet-activating factor (PAF) in a pattern characterized by an early and a delayed peak of synthesis. The early peak of PAF synthesis was due to a direct stimulation of these cells through mCD14 receptor as it was inhibited by anti-CD14 monoclonal antibody. The delayed and sustained peak of PAF synthesis was dependent on protein synthesis and cytokine production as shown by the inhibitory effect of cycloheximide on both MO and PMN, and of anti-tumour necrosis factor-alpha (anti-TNF-alpha) and of anti-interleukin-8 (anti-IL-8) neutralizing antibodies on MO and PMN respectively. IL-10 completely prevented this second, cytokine-dependent peak of PAF synthesis. In contrast, IL-10 markedly enhanced the first peak of PAF synthesis both in MO and PMN. Moreover, IL-10 was shown to modulate the production of superoxide anions (O2-) on both MO and PMN. As suggested by previous studies, IL-10 inhibited the delayed production of O2-. In the present study, we observed that IL-10 directly stimulated an early production of O2-. In addition, IL-10 enhanced the synthesis of O2- by MO and PMN challenged with LPS. The IL-10-induced O2- production was dependent, at least in part, from its effect on PAF synthesis, as it was inhibited by the PAF receptor antagonist WEB 2170. These results suggest that IL-10 may upregulate the early synthesis of PAF and O2- triggered by direct LPS stimulation, whereas it may downregulate the delayed production of these mediators.     

Correlation of leukocyte interleukin-1 production with the stimulation of prostaglandin and tissue factor synthesis by human umbilical vein endothelial cells

Human leukocyte suspensions (neutrophils 80–85%, monocyte 15–20%) were incubated alone or with cultured human umbilical vein endothelial cells. Leukocytes were either directly added to the endothelial cell cultures or separated from them by a 0.4 micron insert filter. Supernatants or cell lysates were obtained at 0.5, 1, 2, and 4 hours of incubation. Supernatants were assayed for the prostacyclin (PGI₂) metabolite 6-keto prostaglandin F₁ and prostaglandin E₂ (PGE₂) by radioimmunoassay and for interleukin-1 (IL-1) by the thymocyte co-mitogen assay. Cell lysates were analyzed for cell-associated procoagulant activity (PCA). Co-incubation of endothelial cells with leukocytes stimulated the synthesis of PGI₂, PGE₂, and PCA. These biochemical changes correlated partially with the release of IL-1 beta. The results suggest that IL-1 released in monocyte/neutrophil co-cultures can produce prothrombotic (increased PCA expression) and inflammatory changes (increased synthesis of vasodilatory and permeability enhancing PGI₂ and PGE₂) in endothelial cells. Neutrophils may represent a source of the released IL-1 and/or may act to stimulate monocyte release of this cytokine and thus play an important role in vascular pathology by a mechanism unrelated to their more direct cytotoxic activity.     

Increased production of interleukin-1 and tumor necrosis factor by human monocytes treated in vitro with cisplatin or other biological response modifiers.

Supernatants collected from cisplatin, lipopolysaccharide (LPS), muramyl dipeptide (MDP) or interferon-gamma (IFN-gamma) treated human monocytes enhance the thymocyte proliferation by a submitogenic concentration of concanavalin A. Also supernatants collected from cisplatin or IFN-gamma treated monocytes demonstrated enhanced cytotoxicity against actinomycin-D treated L 929 cells, suggesting that cisplatin or rIFN-gamma treated monocytes release tumor necrosis factor (TNF) into the culture medium. The supernatant collected from untreated monocytes showed only little IL-1 and TNF activity.     

Contrasting effects of interferon-gamma and interleukin-4 on the interleukin-6 activity of stimulated human monocytes.

Stimulated human monocytes/macrophages are a source of interleukin-6 (IL-6), which is a likely mediator involved in immune and inflammatory reactions. The means to control production of IL-6 by these cells could therefore have therapeutic applications. We report here, for lipopolysaccharide (LPS)-stimulated human monocytes in vitro, that the lymphokine, interferon-gamma (IFN-gamma) (100 U/ml), enhanced the level of IL-6 activity, whereas another lymphokine, interleukin-4 (IL-4) (greater than or equal to 0.1 U/ml; greater than or equal to 1.2 x 10(-11) M), suppressed it. The effects of the two lymphokines were manifested at the level of mRNA. The action of the IL-4 was similar to that of the glucocorticoid, dexamethasone, but observed at a lower molar concentration. Such regulation of monocyte IL-6 activity is similar to that found previously for interleukin-1 (IL-1) and tumour necrosis factor-alpha (TNF-alpha) synthesis.     

Mycoplasma arginini TUH-14 membrane lipoproteins induce production of interleukin-1, interleukin-6, and tumor necrosis factor alpha by human monocytes.

Mycoplasma arginini TUH-14 partially purified membrane lipoproteins (TUH-14-pp) directly induce secretion of the cytokines involved in the inflammatory response, namely, interleukin 1 (IL-1), tumor necrosis factor alpha, and IL-6, by human monocytes cultured in the absence of serum. The biological activity of each cytokine correlates with its immunoreactivity. Upon stimulation with either TUH-14-pp or lipopolysaccharide, most tumor necrosis factor alpha and IL-6 is secreted in the extracellular compartment, whereas a significant amount of IL-1 remains cell associated. Finally, polymyxin B does not affect secretion of cytokines induced by TUH-14-pp, indicating that mycoplasma lipopolysaccharide does not account for their effects on monocytes. Altogether, our data show that direct interaction of mycoplasma membrane components with human blood monocytes induces secretion of high levels of cytokines known to trigger inflammatory responses. This new concept of membrane-bound active components of mycoplasma may explain its ability to efficiently initiate inflammatory reactions.     

Platelet-activating factor (PAF-acether) enhances the concomitant production of tumour necrosis factor-alpha and interleukin-1 by subsets of human monocytes.

The production of the cytokines tumour necrosis factor (TNF) and interleukin-1 (IL-1) by human monocytes was analysed following their stimulation with muramyl dipeptide (MDP; 1 microgram/ml), in the absence or presence of graded concentrations of platelet-activating factor (PAF). Significantly enhanced production of both TNF and IL-1 was observed at two concentration ranges of PAF: a major enhancement was observed at 10(-8)-10(-6) M and this was blocked by the PAF antagonist BN 52021 (10(-4) M). A second enhancement was observed at 10(-15)-10(-14) M PAF, which was not blocked by BN 52021. Monocytes isolated either by adherence or counterflow elutriation had similar responses to PAF. The biologically inactive precursor-metabolite, lyso-PAF, had no effect on cytokine production. PAF was shown to augment the production of both bioactive TNF and IL-1 and immunoreactive TNF-alpha and IL-1 alpha and beta. Fractionation of monocytes on a discontinuous Percoll gradient yielded a denser subpopulation, which responded preferentially to higher PAF concentrations, while the less dense subpopulation responded to both concentration ranges. These data indicate that PAF can modulate monocyte functions as related to cytokine production, and may thus contribute to amplification of inflammatory reactions and regulation of immune responses by interacting with subsets of human monocytes.     

Transforming growth factor-beta1 suppresses interleukin-15-mediated interferon-gamma production in human T lymphocytes

One of the most remarkable means by which tumour cells manage to evade recognition and elimination by the immune system is the release of immunosuppressive mediators, such as interleukin (IL)-10 or transforming growth factor-beta (TGF-beta). For antitumour immunotherapies to reach their full potential, cytokine cocktails will have to be custom-tailored to the tumour's individual cytokine microenvironment. One of the components of such a cytokine cocktail may be interleukin (IL)-15, which has demonstrated an excellent stimulatory potential of antitumour immunity. In an in vitro model, we have previously been able to show that the negative effects of IL-10 on IL-15-mediated cytotoxic T-cell activation can be outweighed by the addition of interleukin (IL)-12. The mechanism by which TGF-beta may influence the effect of IL-15 remains poorly understood, however. We have therefore taken our T-cell model further and have studied the effect of TGF-beta on IL-15-mediated interferon-gamma (IFN-gamma) production. In activated, IL-15-stimulated peripheral blood T lymphocytes, TGF-beta suppressed IFN-gamma mRNA and protein levels by approximately 75%. This effect was likewise observed on both CD4+ and CD8+ T cells and, in contrast to the effect of IL-10 in this system, could not be neutralized by the addition of IL-12. Thus, immunotherapy for TGF-beta-producing tumours may benefit from the addition of TGF-neutralizing activity rather than IL-12.     

Increased C3 production in human monocytes after stimulation with Candida albicans is suppressed by granulocyte-macrophage colony-stimulating factor.

Activation of the complement system is an important part of host resistance against fungal infections. When human monocytes, cultured for 2 days or more, were treated in vitro with Candida albicans for 24 h, an enhancement of their biosynthesis of the complement components C3 and factor B was found. However, when C. albicans was administered to freshly isolated monocytes, a consistent stimulation of factor B biosynthesis occurred, while the C3 production was increased in about 50% of the donors. C. albicans also induced the release of granulocyte-macrophage colony-stimulating factor (GM-CSF) from the cultured cells, apparently in larger amounts in the donors in whom no stimulation of C3 production was found. An antibody to GM-CSF administered with the yeast at the initiation of the monocyte culture caused an increase in the C3 production. Furthermore, when monocytes were treated with recombinant human GM-CSF either at the same time as or 4 days prior to the addition of C. albicans, the increase in C3 production was suppressed or neutralized, while factor B biosynthesis was unaffected. Taken together, these results indicate that monocytes respond to C. albicans with an increased production of complement factors. This may be an important mechanism both for opsonization of the fungus and for initiation of an inflammatory reaction. At an inflammatory site, this complement response may be suppressed by locally produced GM-CSF.     

Dissociation between allogeneic T cell stimulation and interleukin-1 or tumor necrosis factor production by human lung dendritic cells

A small portion of human lung mononuclear cells are very potent stimulators of allogeneic resting T cells. Although several-fold more effective than phagocytic alveolar macrophages (AM) and blood monocytes (Mo), they do not produce more of the lymphocyte co-stimulators interleukin-1-alpha (IL-1 alpha), interleukin-1-beta (IL-1 beta), or tumor necrosis factor-alpha (TNF-alpha) than did Mo. Blocking antibodies against IL-1 alpha, IL-1 beta, TNF-alpha, and IL-6 did not reduce T cell proliferation. These potent antigen-presenting cells (APC) are loosely adherent and do not have phagocytic inclusions. Most of them have the marker RFD1 of dendritic cells (DC) rarely present on Mo or AM and have a strong tendency to form clusters with T cells like murine DC. Thus, we demonstrate an example in the human system of a dissociation between T cell activation and IL-1 or TNF-alpha production by DC or Mo, implying a major role for other "co-stimulating signals" by lung APC with dendritic features. The presence of different APC with various co-stimulating signals may be of importance for T cell subsets modulation.     

Acetyltransferase activity and production of platelet-activating factor by human neutrophils activated with various stimuli

The acetyltransferase activity and biosynthesis of platelet-activating factor (PAF) were assessed in human neutrophils activated by 4 microM A23187, 20 ng/ml phorbol myristate acetate (PMA), and 10(-6) M n-formyl-methionyl-leucyl-phenylalanine (fMLP). All three agents elevated the acetyltransferase activity dose-dependently. There were no significant differences in the Km values for acetyl CoA between non-stimulated and stimulated cells. All three stimuli gave a similar Vmax value of acetyltransferase determined 10 min after stimulation, being more than twice as high as the control value. By contrast, the amount of PAF produced by the neutrophils differed with the stimuli, A23187 being by far the most potent. The time course of PAF synthesis, particularly when activated by PMA, did not parallel that of acetyltransferase activity; PMA-induced PAF production was negligible for the first 20 min and gradually increased to reach its plateau 60 min after stimulation, while enzyme activity was at its highest level 5 min after stimulation. These results suggest that the different stimuli activated the same acetyltransferase, and that there was an increase in the number of the enzyme molecules in the activated state. It is unlikely that a change in the biological activity of a preexisting enzyme without a change in the number of active enzyme molecules could be the cause. Certain factors other than acetyltransferase may also regulate the PAF biosynthesis induced by these stimuli.