Effect of dietary alpha-linolenate on platelet-activating factor production in rat peritoneal polymorphonuclear leukocytes.

The effects of dietary alpha-linolenate (18:3, n-3) and linoleate (18:2, n-6) on platelet-activating factor (PAF) production were examined. Rats were fed an alpha-linolenic acid-rich (perilla oil) diet or a linoleic acid-rich (safflower oil) diet for 6 wk, and polymorphonuclear leukocytes (PMN) were elicited by peritoneal injection of casein. The overall phospholipid content and composition as well as the subclass distribution of choline and ethanolamine glycerophospholipids in PMN were not altered by these diets. However, with the perilla oil diet their content of a putative precursor of PAF, 1-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine was approximately 50% of that with safflower oil diet. On exposure to various concentrations of FMLP, PAF formation by PMN in the perilla oil group was less than 50% of that by PMN in the safflower oil group. A larger difference in PAF productions by PMN in the two dietary groups was observed on their stimulation with calcium ionophore A23187. These results demonstrate that PAF production is modulated in some as yet unknown way by changing the alpha-linolenate/linoleate balance of the diet.     

Ether lysophospholipid-induced production of platelet-activating factor in human polymorphonuclear leukocytes

Human polymorphonuclear leukocytes (PMN) produced considerable amounts of platelet-activating factor (PAF) when exposed to various concentrations of lyso-PAF, especially in the absence of albumin. The amount of produced PAF in the presence of 5 microM lyso-PAF (without albumin) was 1.1 pmol/10 min per 2.5 X 10(6) cells, which was close to the level in the case of opsonized zymosan stimulation. We found that the activity of neither acetyltransferase nor acetylhydrolase was affected markedly by the treatment of cells with lyso-PAF, suggesting that the increased availability of lyso-PAF could be responsible for the induction of PAF synthesis. We also found that PAF synthesis was induced not only by lyso-PAF but also by ether-containing ethanolamine lysophospholipids, 1-alkenyl(alkyl)-sn-glycero-3-phosphoethanolamine (GPE). The addition of 1-alkenyl(alkyl)-GPE caused the degradation of pre-existing 1-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine (GPC) and an increased level of lyso-PAF, followed by the formation of PAF. By contrast, 1-acyl-GPC and 1-acyl-GPE failed to induce PAF production. These results suggest a possible key role of the availability of lyso-PAF in triggering the biosynthesis of PAF in human PMN.     

The GM2 activator protein, a novel inhibitor of platelet-activating factor.

The GM2 activator protein is required as a substrate-specific cofactor for beta-hexosaminidase A to hydrolyze GM2 ganglioside. The GM2 activator protein reversibly binds and solubilizes individual GM2 ganglioside molecules, making them available as substrate. Although GM2 ganglioside is the strongest binding ligand for the activator protein, it can also bind and transport between membranes a series of other glycolipids, even at neutral pH. Biosynthetic studies have shown that a large portion of newly synthesized GM2 activator molecules are not targeted to the lysosome, but are secreted and can then be recaptured by other cells through a carbohydrate independent mechanism. Thus, the GM2 activator protein may have other in vivo functions. We found that the GM2 activator protein can inhibit, through specific binding, the ability of platelet activating factor (PAF) to stimulate the release of intracellular Ca2+ pools by human neutrophils. PAF is a biologically potent phosphoacylglycerol. Inhibitors for PAF's role in the pathogenesis of inflammatory bowel disease and asthma have been sought as potential therapeutic agents. The inherent stability and protease resistance of the small, monomeric GM2 activator protein, coupled with the ability to produce large quantities of the functional protein in transformed bacteria, suggest it may serve as such an agent.     

Augmented production of platelet-activating factor in human polymorphonuclear leukocytes by ketone bodies

The production of platelet-activating factor (PAF) in A23187-stimulated human polymorphonuclear leukocytes was markedly increased in the presence of 5 mM acetoacetate and beta-hydroxybutyrate. Such an augmentation was observed even at 500 microM but not at 50 microM. The augmented production of PAF by acetoacetate was also observed in the presence of autologous serum and was most prominent in the case of opsonized zymosan-stimulation rather than A23187-stimulation. These observations suggest that increased levels of acetoacetate and beta-hydroxybutyrate in blood may lead to the augmented production of PAF, which would amplify the various PAF-mediated biological reactions.     

Regulation of oscillations in filamentous actin content in polymorphonuclear leukocytes stimulated with leukotriene B(4) and platelet-activating factor.

Stimulation of neutrophils with LTB(4) or PAF results in the production of a rapidly oscillating actin polymerization/depolymerization response. Treatment of neutrophils with inhibitors of PKC prior to stimulation with ligand resulted in a masking of the F-actin oscillations. Because myosin has been shown to be a substrate for neutrophil PKC, this protein was investigated as a potential downstream mediator of F-actin oscillations. Stimulation of neutrophils with LTB(4) resulted in myosin light chain being serine phosphorylated in a PKC-dependent manner. This phosphorylation was shown to occur in a manner that is kinetically distinct from the myosin phosphorylation induced by FMLP, a potent activator of actin polymerization that alone does not induce F-actin oscillations. Additionally, disruption of intracellular actin-myosin interactions resulted in inhibition of LTB(4)- as well as PAF-induced F-actin oscillations. These data suggest that PKC and downstream phosphorylation of myosin as well as actin-myosin interaction may play roles in mediating the production of neutrophil F-actin oscillations.     

Production of platelet-activating factor by stimulated human polymorphonuclear leukocytes. Correlation of synthesis with release, functional events, and leukotriene B4 metabolism

Platelet-activating factor (PAF) is a phospholipid mediator of inflammation that is synthesized by several human cell types including polymorphonuclear leukocytes (PMN). We examined the synthesis and release of PAF by stimulated human PMN under several conditions, assayed by the incorporation of [3H]acetate into PAF and by bioassay. PAF synthesis was induced by calcium ionophore A23187 (IoA), opsonized zymosan (OpsZ), and N-formyl-methionyl-leucyl-phenylalanine (FMLP) with the relative order of potency IoA much greater than OpsZ greater than FMLP. A variety of other agonists, including phorbol myristate acetate, an activator of protein kinase C and of PMN functional responses, did not stimulate PAF synthesis. PAF synthesis by PMN in response to IoA, OpsZ, and FMLP was concentration- and time-dependent but release of the phospholipid was not: little PAF (1 to 10%) was released from PMN in suspension regardless of the total amount produced, the agonist, its concentration, the time of incubation, or the concentration of extracellular albumin. This was also the case with functionally altered neutrophils that had been "primed" with cytochalasin B or lipopolysaccharide or that had adhered to surfaces. PAF synthesis was tightly coupled with leukotriene B4 production by adherent PMN as well as by neutrophils in suspension, supporting the hypothesis that the two lipid autacoids may be derived from a common precursor. However, PAF synthesis could be dissociated from aggregation and surface adhesion, indicating that it is not absolutely required for these responses of activated PMN. The total amount of PAF that accumulated, but not the percentage that was released, was altered in adherent PMN compared to cells in suspension. These experiments demonstrate that PAF production and its subsequent processing by human neutrophils are highly regulated events. PAF synthesis is associated with PMN activation, but it is not a requisite for early adhesive responses of neutrophils. Because little of the PAF produced by stimulated PMN is released from the cells, it appears that PAF has an intracellular role in PMN function and/or that it may have novel intercellular effects that do not require release into the fluid phase.     

Occurrence of an endogenous inhibitor of platelet-activating factor in rat liver

Hepatocytes obtained from livers derived from fed rats perfused with a collagenase-containing mixture were found to contain significant levels of platelet-activating factor activity as isolated by Silica Gel G thin layer chromatography. However, when soybean trypsin inhibitor was included in the collagenase-containing perfusion medium for hepatocyte preparation, platelet-activating factor activity could not be detected on Silica Gel G chromatograms. Examination of the lipids extracted from freeze clamped perfused rat livers revealed low, but detectable, levels of platelet-activating factor. Further investigation of these observations indicated that a lipid-like inhibitor was present in freeze-clamped perfused livers as well as in hepatocytes isolated in the presence of soybean trypsin inhibitor. In each instance platelet-activating factor and this newly discovered inhibitor, which comigrated at the same RF value on Silica Gel G thin layer chromatography plates, could be separated by further chromatography on high performance thin layer plates. The present study shows that platelet-activating factor is present in unstimulated liver and that its detection is masked by an endogenous lipid-like inhibitor.     

Synthesis of platelet-activating factor by polymorphonuclear neutrophils stimulated with interleukin-8.

Human interleukin-8 (IL-8) was evaluated for its capability to induce the synthesis and release of platelet-activating factor (PAF) from human polymorphonuclear neutrophils (PMN). IL-8 promotes in a dose-dependent fashion (1-100 ng/ml) a rapid synthesis of PAF, which is only partially released. The synthesis of PAF is preceded by the activation of acetyl-CoA: 1-alkyl-2-lyso-sn-glycero-3-phosphocholine acetyl-transferase, suggesting that IL-8 activates the "remodeling pathway" of PAF synthesis. By thin layer chromatography and reverse-phase high pressure liquid chromatography, we demonstrated that PAF synthesized by human PMN stimulated with IL-8 is heterogeneous: the 2-acetylated phospholipids having the biological and physicochemical characteristics of PAF include the 1-O-alkyl form, which is produced in large extent (51%), and the 1-acyl form (20%). The analysis of the individual molecular species of radyl chain indicated nine peaks, 16:0 and 18:0 being the predominant forms. These results identify PAF as a direct product of IL-8 stimulation in PMN.     

Quantitative analysis of platelet-activating factor in rat brain.

Age-related decrease of the platelet-activating factor (PAF) content in rat brain was shown by a convenient method consisting of solid extraction of lipids with a Sep-Pak C-18 cartridge, lipid separation by HPLC and bioassay on rabbit platelets. This method was sufficiently sensitive to allow measurement of PAF in a single brain, and the recovery of PAF was quite high throughout the procedure.     

Triacsin C: A differential inhibitor of arachidonoyl-CoA synthetase and nospecific long chain acyl-CoA synthetase,

Triacsins A,B,C, and D are newly discovered compounds isolated from the culture filtrate of streptomyces which are known to inhibit nonspecific long chain acyl-CoA synthetase (EC 6.2.1.3.). These inhibitors have not been previously studied with regard to their effects on arachidonoyl-CoA synthetase, an enzyme which specifically utilizes arachidonate and other icosanoid precursor fatty acids. To explore his question, we used triacsin C, a potent inhibitor of the nonspecific acyl-CoA synthetase. Triacsin C was found to inhibit the action of arachidonoyl-CoA synthetase and the nonspecific enzyme in sonicates of HSDM₁C₁ mouse fibrosarcoma cells. Importantly, however, the triacsin concentration and length of pre-incubation with the enzymes could be adjusted to almost completely inhibit (>80%) the nonspecific long chain acyl CoA-synthetase, with less than 20% inhibition of arachidonoyl-CoA synthetase. Using intact cultured cells exposed to 1 ug/ml traicsin for up to 15 minutes, we unexpectedly observed preferential inhibition of arachidonoyl-CoA synthetase activity. In intact cell studies, arachidonoyl-CoA synthetase was inhibited > 90%, with 55–60% inhibition of the nonspecific acyl-CoA synthetase. As additional evidence of its inhibition of acyl-CoA synthetase enzymes in intact cells, triacsin c inhibited both fatty acid uptake into cells and icosanoid production, metabolic processes which in certain cell types appear to be dependent on acyl-CoA synthetase activity. Thus, triacsin C is a novel inhibitor which can alter the fatty metabolism of intact cells. This compound can be of significant value in determining the specific cellular functions of the two acyl-CoA synthetase enzymes.     

Prolonged pulmonary hypertension caused by platelet-activating factor and leukotriene C4 in the rat lung.

Platelet-activating factor (PAF) and leukotrienes (LTs) are potent pulmonary hypertensive and inflammatory mediators produced by the lung. Previously we showed that a rapid injection of PAF into the pulmonary artery of an isolated rat lung produced an extended elevation in mean pulmonary arterial pressure (PAP). The objective of the present study was to determine whether the extended pressor response induced by PAF was caused by prolonged activation of the 5-lipoxygenase pathway or slow clearance of LTs from the lung parenchyma. Rat lungs were perfused with a nonrecirculating physiological salt solution that contained indomethacin and albumin. Five minutes after a rapid injection of PAF into the pulmonary artery catheter, the following elevations (mean % above baseline) were observed: PAP (83%), LTB4 (3,260%), LTC4 (1,490%), LTD4 (970%), and LTE4 (1,500%). At 20 min these levels declined but were still significantly elevated above baseline. The 5-lipoxygenase inhibitor diethylcarbamazine (DEC), administered before the PAF injection, inhibited the elevations of PAP and all LTs. DEC administration that began 5 min after PAF reduced PAP and only LTC4 levels at 20 min in comparison to lungs with no DEC. The 5-lipoxygenase-activating protein inhibitor MK886, administered orally 2-6 h before perfusion, also inhibited the pressor response to PAF as well as LT production, as did DEC. We conclude that 1) the extended pulmonary hypertension induced by PAF was caused mainly by prolonged activation of 5-lipoxygenase with LTC4 production, 2) the relative overall lung clearance of LTB4, LTD4, and LTE4 was slower than that of LTC4, and 3) LTB4, LTD4, and LTE4 had no appreciable pressor effect.     

Release of eicosanoids in rat peritoneal cavity stimulated with platelet-activating factor (PAF). Effect of the PAF-antagonist BN-52021

Platelet activating factor (PAF; 10 micrograms) was injected in the peritoneal cavity of rats in the absence or presence of the PAF antagonist BN-52021 (5 mg/kg). Thirty min later, the peritoneal cavity was washed with 3 ml of saline, the fluid was collected and the concentrations of selected eicosanoids were measured using novel enzyme immunoassays. PAF increased by 2.9, 2.8 and 1.7 fold the levels of thromboxane B2, prostaglandin E2 and leukotriene B4 respectively in the peritoneal fluid. The stimulatory effects of PAF was reduced by 42, 51, and 86% for thromboxane B2, prostaglandin E2 and leukotriene B4 respectively by the specific PAF antagonist. These results confirm the presence of specific PAF receptors in tissues and/or cells of rat peritoneal cavity and underline the complex interactions between PAF and eicosanoids.     

The regulation of platelet-activating factor production in endothelial cells. The role of calcium and protein kinase C

Endothelial cells (EC) synthesize platelet-activating factor (PAF) when stimulated with agonists that bind to cell-surface receptors. We examined events that link receptor binding to synthesis of PAF by EC. Bovine EC stimulated with agonists that interact with specific cell-surface receptors accumulated PAF only in the presence of extracellular calcium. Hormonal stimulation of EC resulted in Ca2+ entry characteristic of that seen with receptor-operated calcium channels; Indo-1 measurements demonstrated that this inward flux of Ca2+ caused prolonged elevated levels of intracellular Ca2+. EC were exposed to melittin or theta toxin from Clostridium perfringens (pore-forming peptides that increase the permeability of the plasma membrane for small molecules) resulting in an inward flux of Ca2+ and accumulation of PAF. Ca2+ appears to be regulatory for PAF production at the level of phospholipase A2-mediated production of the PAF precursor 1-O-alkyl-2-lyso-sn-glycero-3-phosphocholine, as Ca2+ was required for the stimulated hydrolysis of 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine. PAF accumulation in EC is also regulated by protein kinase C. Pretreatment of EC with phorbol esters that activate protein kinase C or with dioctanoylglycerol, followed by stimulation, resulted in a 2-fold increase in stimulated PAF production. The regulatory effect of protein kinase C also appears to be at a phospholipase A2-mediated hydrolysis of 1-O-alkyl-2-acyl-sn-glycero-3-phosphocholine.     

Ethanol increases superoxide anion production stimulated with 4beta-phorbol 12-myristate 13-acetate in human polymorphonuclear leukocytes. Involvement of protein kinase C.

Stimulation of human polymorphonuclear leukocytes (PMNs) with PMA initiates a cascade of events leading to the production and release of superoxide anion (O-2), a major component in anti-bacterial defense. Generation of O-2 by PMA-stimulated PMNs occurs through the translocation and activation of protein kinase C (PKC). In this study, using freshly isolated PMNs, we examined the effect of ethanol on this response to PMA. Our results show that the basal production of O-2 was not affected by ethanol. In contrast, the response induced by PMA was potentiated by ethanol. This potentiation was observed even at high doses of PMA (200 nM) which alone had stimulated the O-2 response maximally. This enhanced response was not due to an increase of PMA uptake by PMNs. The maximal effect was obtained when the cells were preincubated with 80 mM of ethanol before PMA stimulation. Measurement of PKC activity in the cytosolic and membrane fractions showed that pretreatment of PMNs with ethanol increased twofold the PMA-stimulated PKC activity in the membrane fraction. Furthermore, Western blot analysis verified that this increase in PKC activity in the membrane fraction was linked to an increase in the translocation of PKC-alpha and -beta isoforms to the membrane. These results suggest that ethanol potentiates PMA-induced O-2 production through increasing PKC translocation and activity in PMNs.     

Estrogen effects on platelet-activating factor and platelet-activating factor acetylhydrolase activity in rat uterus during the late stages of pregnancy.

The platelet-activating factor (PAF) concentration of the uterus spontaneously increased during pregnancy. When 17alpha-ethynylestradiol (0.25 mg/kg) was administered subcutaneously to pregnant rats for 3 days starting on Day 17 of pregnancy, some rats delivered prematurely on Day 20. However, none of the vehicle-treated (80% dimethylsulfoxide and 20% ethanol) pregnant rats delivered prematurely. The PAF concentration of the uterus in pregnant rats treated with 17alpha-ethynylestradiol was significantly higher than in those treated with vehicle on Days 19 and 20. On the other hand, the specific activity of uterine PAF-acetylhydrolase (PAF-AH) in pregnant rats treated with 17alpha-ethynylestradiol was significantly lower than in those treated with vehicle on Days 19 and 20, and the plasma PAF-AH activity in pregnant rats treated with estrogen was also significantly lower than in treated with vehicle on Days 18, 19, and 20. These findings indicate that estrogen increases PAF concentrations in the rat uterus, and this was correlated with a decrease in PAF-AH in the uterus and plasma. The increase in PAF concentrations in the uterus may be related to premature delivery and labor caused by PAF's known effect on myometrial contraction.     

1-O-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine. A common source of platelet-activating factor and arachidonate in human polymorphonuclear leukocytes

1-O-[3H]Alkyl-2-lyso-sn-glycero-3-phosphocholine (1-O-[3H]alkyl-2-lyso-GPC) incubated with human polymorphonuclear leukocytes (PMN) for 30 min is metabolized to 1-O-alkyl-2-acyl-GPC containing greater than 80% arachidonate at the 2 position (Chilton, F. H., O'Flaherty, J. T., Ellis, J. M., Swendsen, C. L., and Wykle, R. L. (1983) J. Biol. Chem. 258, 7268-7271). PMN containing 1-O-[3H]alkyl-2-arachidonoyl-GPC incorporated into their cellular phospholipids in this manner were stimulated with Ca2+ ionophore (A23187). Within 5 min after stimulation, 14%, 7%, and 7% of the total 1-O-[3H]alkyl-2-arachidonoyl-GPC in the cells had been converted to 1-O-[3H]alkyl-2-acetyl-GPC (platelet-activating factor), 1-O-[3H]alkyl-2-lyso-GPC, and 3H-labeled neutral lipid, respectively. Stimulation by opsonized zymosan yielded similar results. In related studies, cells were labeled with 1-O-hexadecyl-2-arachidonoyl-GPC containing a [methyl-14C] choline moiety. The nature of the long-chain acyl residues in the sn-2 position of the labeled 1-O-hexadecyl-2-acyl-GPC remaining after stimulation with A23187 was examined. Analysis by high-performance liquid chromatography using synthetic 1-O-hexadecyl-2-acyl-GPC standards indicated there is a time-dependent loss of arachidonate from the 2 position of the labeled 1-O-hexadecyl-2-arachidonoyl-GPC followed by reacylation by other fatty acids (primarily linoleic and oleic). This shift in the acylation pattern exhibited after Ca2+ ionophore stimulation was further examined in PMN preincubated with A23187 and subsequently incubated with labeled 1-O-alkyl-2-lyso-GPC; the stimulated cells produced 1-O-[3H]alkyl-2-acetyl-GPC (greater than 15% of total label) and 1-O-[3H]alkyl-2-acyl-GPC containing linoleic acid and oleic acid, rather than arachidonic acid in the sn-2 position. The findings demonstrate that upon stimulation of PMN, 1-O-alkyl-2-arachidonoyl-GPC can yield arachidonate and 1-O-alkyl-2-lyso-GPC; the 1-O-alkyl-2-lyso-GPC formed may be acetylated producing platelet-activating factor or reacylated with fatty acyl residues other than arachidonate.     

Platelet-activating factor acetylhydrolase increases during macrophage differentiation. A novel mechanism that regulates accumulation of platelet-activating factor

Monocytes and macrophages produce bioactive lipids, such as platelet-activating factor (1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine, PAF), that mediate inflammation. These cells synthesize PAF following their activation, but not constitutively. Previous studies have demonstrated that PAF accumulation is regulated by the activity of the synthetic enzymes. We observed that the accumulation of PAF in stimulated human monocytes decreased by 90% as they differentiated into macrophages. There was no decrease in the activities of the synthetic enzymes; however, the activity of the degradative enzyme, PAF acetylhydrolase, increased 260-fold. The increase in PAF acetylhydrolase activity appeared to result from a net increase in the synthesis of a new enzyme. These studies demonstrate a novel mechanism in which an increase of the degradative enzyme regulates the accumulation of PAF. This may be an important mechanism by which macrophages modulate inflammatory responses.     

Endothelin inhibits the atrial natriuretic factor stimulated cGMP production by activating the protein kinase C in rat aortic smooth muscle cells.

Preincubation of rat thoracic aortic smooth muscle cells with endothelin inhibits the atrial natriuretic factor (ANF)-induced cGMP accumulation in these cells in a concentration dependent manner. The maximal inhibition of 64% was afforded by 1 x 10(-6) M endothelin and the half maximal inhibition (IC50) was achieved with 1 x 10(-9) M endothelin. Endothelin (1 x 10(-6) M) also increased the plasma membrane bound protein kinase C (PKC) activity by 4 fold. Hormone-dependent increase in PKC activity was limited to plasma membranes only and some decrease in cytosolic PKC activity was observed. However, phorbol 12-myristate 13-acetate (PMA) (1 x 10(-6)M) provoked a total loss of cytosolic PKC activity and a net gain in membranous PKC activity indicative of the translocation of the enzyme. Pretreatment of these cells with H-7, a PKC inhibitor, released the endothelin and PMA-mediated attenuation of ANF-stimulated cGMP formation. These results suggest that PKC is involved in the regulation of ANF-induced cGMP accumulation and that the vasoconstrictor activity of endothelin might involve inhibition of the vasorelaxant activity of ANF through the inhibition of cGMP accumulation in smooth muscle cells (SMCs) of the rat aorta.     

Effect of long-term treatment with platelet-activating factor on IL-1 and IL-2 production by rat spleen cells

To study the effect of the in vivo administration of platelet-activating factor (PAF) on cytokine production, alzet minipumps loaded with the mediator or solvent alone were connected to the jugular vein and placed under the skin of Sprague-Dawley rats. Over 7 days the animals received total doses of 0.5, 1, 4.5, 9, or 28 micrograms PAF or the solvent alone. The spleen mononuclear cells isolated from Ficoll gradients and the adherent cell fraction were separated before determination of basal and mitogen-stimulated IL-1 and IL-2 production, respectively. Adherent splenocytes from rats having received 28 micrograms PAF exhibited a decreased capability to produce IL-1, as compared to those from vehicle-treated animals. In contrast, adherent splenocytes from rats having received 9 and 4.5 micrograms PAF yielded higher amounts of released and cell-associated IL-1 activity upon LPS stimulation, as compared to those from solvent-treated animals. The PAF antagonist, BN 52021, given orally (5 mg/kg, twice a day throughout the experiments) inhibited the in vivo effect of 28 micrograms PAF. Statistically significant 144 +/- 43% (p less than 0.001, n = 5) and 73 +/- 33%, (p less than 0.01, n = 3) increases in IL-2 production were observed when whole spleen mononuclear cells from rats administered with 1 and 4.5 micrograms PAF, respectively, were stimulated with Con A. BN 52021 markedly inhibited the in vivo effect of 1 microgram PAF on the IL-2 release. Our study demonstrates that PAF can modulate immune functions in vivo and suggests that the specific PAF antagonist, BN 52021, may be used as an immunomodulatory agent.