LTB4 and BLT1 in inflammatory arthritis

Inflammatory arthritis, including rheumatoid arthritis (RA), is characterized by infiltration of inflammatory cells into the joints. Biological agents targeting TNF-α and IL-6 dramatically improve RA. However, some RA patients do not respond to current treatments and these broadly active upstream biological agents increase the risk of severe infection. Therefore, there remains a need for other effective and safe treatments for RA. Many studies have implicated that blockade of leukotriene B4 (LTB₄) and its high affinity receptor BLT1 dramatically suppress arthritis in animal models. In addition, levels of LTB₄ in serum, synovial fluid and synovial tissue are increased in RA patients compared to healthy donors or osteoarthritis patients. These data suggest that LTB₄ and BLT1 likely contribute to the pathogenesis of human RA. However, several clinical trials inhibiting BLT1 in RA were not successful. Our recent data revealed that LTB₄ is a key mediator in a complement, lipid, cytokine and chemokine cascade that first initiates and then sustains neutrophilic inflammation in inflammatory arthritis. These new mechanistic studies suggest novel ways to target the LTB₄-BLT1 pathway for the treatment of RA and other inflammatory diseases.     

Eicosanoid production in rheumatoid synovitis

Leukotriene B₄ (LTB₄) synthesis in rheumatoid synovitis was studied using peripheral and synovial fluid polymorphonuclear leukocytes (PMNs) and rheumatic synovial lining cells. No differences were found in LTB₄ synthesis between peripheral PMNs from healthy volunteers and rheumatoid arthritis patients. When peripheral and synovial PMNs from the same RA patient were compared, arachidonic acidinduced LTB₄ synthesis in synovial fluid PMNs was increased 1.7–7.2 fold, whereas the response to Ca ionophore A23187 stimulation was similar. This suggests 5-lipoxygenase stimulating factor(s) in inflamed joints. Rheumatic synovial lining cells in a primary cell culture produced small amounts of LTB₄, the concentrations being less than 0.1 per cent of those of prostaglandin E₂ (PGE₂). PGE₂ synthesis in synovial cells was increased when arachidonic acid or interleukin-1 was added to the culture, whereas LTB₄ production remained unaltered. The present results suggest that in inflamed joints LTB₄ originates mainly from PMNs whereas synovial lining cells are the source for PGE₂.     

Peripheral blood neutrophil leukotriene B4 release and migration in rheumatoid arthritis

The present study was designed to compare peripheral blood neutrophil migration and leukotriene (LT) release between patients with rheumatoid arthritis (RA) and healthy controls and to correlate the neutrophil functions with clinical disease activity. Nineteen patients with moderately active RA and 19 age and sex matched healthy volunteers participated in this study. Isolated peripheral blood neutrophils from RA patients released equal amounts of LTB₄ but their random migration was enhanced as compared with neutrophils from healthy controls. LTB₄ release in whole blood was significantly lower in samples from RA patients than in those from the healthy volunteers (13.5±1.4 and 19.1±1.4 ng/10⁶ neutrophils respectively; P<0.001). LTB₄ release from isolated RA neutrophils correlated with the levels of C-reactive protein, duration of morning stiffness and Ritchie articular swelling index. Concentrations of hyaluronate, cyclic AMP and 13,14-dihydro-15-keto-prostaglandin were not different between patients with RA and healthy volunteers. Neither was there any difference in TXB₂ production by platelets during blood clotting. In conclusion, peripheral blood neutrophils of RA patients seem to be primed and/or activated as their random migration is enhanced as compared with those of healthy volunteers. In RA, LTB₄ release from peripheral blood neutrophils seems to reflect the clinical activity of the disease. However, RA neutrophils released smaller (in whole blood) or equal (isolated cells) amounts of LTB₄ as compared with the respective controls. These contradictory findings suggest that LTB₄ release from peripheral blood neutrophils has no major role in the regulation of disease activity in rheumatoid arthritis.     

Orally administered tolfenamic acid inhibits leukotriene synthesis in isolated human peripheral polymorphonuclear leukocytes

Special interest has been focused on the development of dual inhibitors of the cyclo-oxygenase and lipoxygenase pathways of arachidonic acid metabolism. In contrast to other classic NSAIDs, some fenamates in clinically achievable concentrations have been shown to inhibit synthesis of 5-lipoxygenase productsin vitro. In the present work, we studied the effect of orally administered tolfenamic acid (600 mg) on Ca ionophore A 23187-induced leukotriene synthesis in isolated human polymorphonuclear leukocytes. Leukotriene production was reduced in all 14 subjects studied, the mean inhibition of LTB₄ synthesis being 16±3% and that of LTC₄ 33±7%. The inhibition correlated positively with serum tolfenamic acid concentrations. We suggest that inhibition of leukotriene synthesis is an additional mechanism of the anti-inflammatory, antimigraine and antidysmenorrhoeic effects of tolfenamic acid, and a possible explanation for its rare gastric and bronchoconstrictive side-effects.     

Modulation of arachidonic acid metabolism by orally administered morniflumate in man

Unlike other classic NSAIDs, some fenamates given at therapeutic concentrations, have been shown to inhibit, bothin vitro andin vivo, the 5-lipoxygenase pathway of arachidonic acid cascade as well as the synthesis of cyclooxygenase products. This dual inhibitory property might represent an improvement in anti-inflammatory therapy. The aim of this work was to characterize the effect of morniflumate, admistered at therapeutic dosages to normal human volunteers, on leukotriene B₄ (LTB₄) and thromboxane (TXB₂) synthesis, both in purified PMNs and in whole blood. PMNs, isolated two hours after a single oral administration of morniflumate and at steady-state condition, fully retain their capacity to release LTB₄ and TXB₂. Since intracellular concentrations of the drug were undetectable, in spite of its elevated concentrations in platelet poor plasma, the results obtained using PMNs suggest a drug loss during the cells purification procedure. In whole blood experiments, morniflumate reduced blood LTB₄ synthesis induced by Ca-ionophore A23187 Bx approximately 50%, both after single dose and at steady state; the degree of inhibition showed a pattern similar to the plasma levels of the bioactive metabolite of morniflumate (M1). The inhibition of serum TXB₂ levels was higher than 85%. Hence, morniflumate is capable of reducing arachidonic acid metabolism acting both on cyclooxygenase and 5-lipoxygenase. This characteristic might provide a better approach in anti-inflammatory therapy.     

Therapeutic interventions in gastrointestinal disease based on an understanding of inflammatory mediators

Whatever initiates inflammation, the final message mediating cellular invasion is chemical. This consideration allows rational development of anti-inflammatory treatments. Two main classes of chemotactic mediator are recognised. Water-soluble peptides, e.g. cytokines derived from macrophages and other cells, play an important integrating part in the early recruitment of neutrophils and mononuclear cells, and in the amplification of immune responses. Lipid-soluble mediators, of which leukotriene B₄ is the most highly chemotactic for neutrophils, are important in secondary amplification.In inflammatory bowel disease, we have shown evidence of increased synthesis of cytokines interleukin 1, 6 and 8. These are associated with activation of circulating monocytes in active Crohn's disease, of lamina propria macrophages in relapse of both ulcerative colitis and Crohn's disease, and development of adhesion molecules on vascular endothelium. Our studies show that interleukin 6 is selectively increased in Crohn's disease, whilst preliminary findings suggest that enhanced synthesis of interleukin 8 is particularly characteristic of ulcerative colitis.Patterns of cytokine synthesis may, therefore, be of diagnostic value. They also offer the potential for therapeutic strategies since cytokine antagonists are becoming available.We have also demonstrated increased synthesis of leukotrienes in active inflammatory bowel disease. Since leukotriene B₄ is quantitatively the main chemotactic signal in the mucosa in inflammatory bowel disease during relapse, we investigated the therapeutic effect of suppressing leukotriene B₄ synthesis by treating patients with fish oil (as Hi-EPA), giving 4.5 g daily of eicosapentaenoic acid. This competes for the 5-lipoxygenase enzymes, inhibiting leukotriene B₄ and promoting synthesis of the less chemotactic product, LTB₄. Ninety-six patients were treated for one year in a study structured to investigate both relapse and remission. Hi-EPA was well tolerated and compliance was good, since increased local tissue contractions of EPA were sustained whilst leukotriene synthesis of LTB₅ rose and of LTB₄ fell, by approximately 50%. Although fish oil had on overall effect on the number of days in remission, there was a significant steroidsparing effect during relapse. Recent studies with 5-lipoxygenase inhibitors also support the therapeutic value of modulating leukotriene synthesis in relapse.     

Advances in anti-inflammatory therapy

Our goal was to evaluate the state of nonsteroidal anti-inflammatory drugs (NSAIDs) in the treatment of rheumatoid arthritis (RA), before the introduction of the coxibs. The prerequisite for inclusion was the presence of RA (ACR criteria) plus therapy with an NSAID with or without a disease modifying antirheumatic drug (DMARD). A total of 368 consecutive RA patients (81% women) from the outpatient clinic at the Vienna General Hospital were included. Rheumatoid factor was positive in 62%, the patients' mean age was 60 +/- 14 years. The period of observation was 1972-1998. Seventy-seven per cent of the patients had DMARD and NSAID therapy. NSAID therapy was dominated by diclofenac, accounting for 60% of all therapies. Eighteen other substances were applied more rarely. All NSAIDs together were given for 768 patient years (with a mean duration of therapy of 17 years +/- 21 months). Seventy-two per cent of the patients received GI-protective therapy mainly with histamine antagonists and sucralfate while on nonsteroidal therapy. NSAID toxicity mostly affected the GI tract. There was a similar incidence of GI-related adverse events between patients with and patients without GI protection, mainly dyspepsia and nausea. NSAIDs have the potential to cause adverse events in the GI tract. Therapy with histamine antagonists or sucralfate did not reduce the patients' rate of gastrointestinal adverse events.     

Therapeutic interventions in gastrointestinal disease based on an understanding of inflammatory mediators

Whatever initiates inflammation, the final message mediating cellular invasion is chemical. This consideration allows rational development of anti-inflammatory treatments. Two main classes of chemotactic mediator are recognised. Water-soluble peptides, e.g. cytokines derived from macrophages and other cells, play an important integrating part in the early recruitment of neutrophils and mononuclear cells, and in the amplification of immune responses. Lipid-soluble mediators, of which leukotriene B₄ is the most highly chemotactic for neutrophils, are important in secondary amplification.In inflammatory bowel disease, we have shown evidence of increased synthesis of cytokines interleukin 1, 6 and 8. These are associated with activation of circulating monocytes in active Crohn's disease, of lamina propria macrophages in relapse of both ulcerative colitis and Crohn's disease, and development of adhesion molecules on vascular endothelium. Our studies show that interleukin 6 is selectively increased in Crohn's disease, whilst preliminary findings suggest that enhanced synthesis of interleukin 8 is particularly characteristic of ulcerative colitis.Patterns of cytokine synthesis may, therefore, be of diagnostic value. They also offer the potential for therapeutic strategies since cytokine antagonists are becoming available.We have also demonstrated increased synthesis of leukotrienes in active inflammatory bowel disease. Since leukotriene B₄ is quantitatively the main chemotactic signal in the mucosa in inflammatory bowel disease during relapse, we investigated the therapeutic effect of suppressing leukotriene B₄ synthesis by treating patients with fish oil (as Hi-EPA), giving 4.5 g daily of eicosapentaenoic acid. This competes for the 5-lipoxygenase enzymes, inhibiting leukotriene B₄ and promoting synthesis of the less chemotactic product, LTB₄. Ninety-six patients were treated for one year in a study structured to investigate both relapse and remission. Hi-EPA was well tolerated and compliance was good, since increased local tissue contractions of EPA were sustained whilst leukotriene synthesis of LTB₅ rose and of LTB₄ fell, by approximately 50%. Although fish oil had on overall effect on the number of days in remission, there was a significant steroidsparing effect during relapse. Recent studies with 5-lipoxygenase inhibitors also support the therapeutic value of modulating leukotriene synthesis in relapse.     

Pharmacological investigation of the role of leukotrienes in the pathogenesis of experimental NSAID gastropathy

The role of leukotrienes in the pathogenesis of acute gastric ulceration induced by nonsteroidal antiinflammatory drugs was investigated using a rat model. One part of the study involved oral pretreatment with a leukotriene synthesis inhibitor 1 h prior to administration of indomethacin (20 mg/kg per os). Three hours after indomethacin, the extent of macroscopically visible gastric damage was determined, and gastric LTB₄ synthesis was determined. The compounds tested were PF-5901, A-64077, nordihydroguaiaretic acid, and L-698,037. Each compound produced dose-related inhibition of gastric LTB₄ synthesis and a parallel reduction in the severity of indomethacin-inducec damage. The antioxidant properties of these compounds was assessed using an in vitro assay. There was no correlation between the antioxidant properties of the compounds and their ability to reduce the severity of indomethacin-induced gastric damage. In the second part of the study, the effects of intravenous administration of LTD₄ and LTB₄ receptor antagonists on indomethacin-induced gastric epithelial damage (measured by permeability to [⁵¹Cr]EDTA) were assessed. The two LTD₄ receptor antagonists (MK-571 and ICI-204,219) significantly reduced the permeability changes induced by indomethacin, while the two LTB₄ antagonists (SC-41930 and LY-255,283) were without significant effect. Despite the reduction of gastric epithelial injury, blockade of LTD₄ receptors did not markedly affect the extent of macroscopically visibls injury. These data are consistent with the hypothesis that leukotrienes contribute to the epithelial injury and macroscopically visible damage induced by NSAIDs. However, it remains unclear to what extent leukotrienes are involved in the initiation of the injury, as opposed to its amplification.     

5-Lipoxygenase Reaction Products Modulate Alveolar Macrophage Phagocytosis of Klebsiella pneumoniae

The leukotrienes are potent lipid mediators of inflammation formed by the 5-lipoxygenase-catalyzed oxidation of arachidonic acid. Although the effects of leukotrienes on neutrophil chemotaxis and activation have been established, their role in modulating innate host defense mechanisms is poorly understood. In a previous study (M. Bailie, T. Standiford, L. Laichalk, M. Coffey, R. Strieter, and M. Peters-Golden, J. Immunol. 157:5221–5224, 1996), we used 5-lipoxygenase knockout mice to establish a critical role for endogenous leukotrienes in pulmonary clearance and alveolar macrophage phagocytosis of Klebsiella pneumoniae. In the present study, we investigated the role of specific endogenous leukotrienes in phagocytosis of K. pneumoniae and explored the possibility that exogenous leukotrienes could restore phagocytosis in alveolar macrophages with endogenous leukotriene synthesis inhibition and enhance this process in leukotriene-competent cells. Rat alveolar macrophages produced leukotriene B₄ (LTB₄), LTC₄, and 5-hydoxyeicosatetraenoic acid (5-HETE) during the process of phagocytosis, and the inhibition of endogenous leukotriene synthesis with zileuton and MK-886 dramatically attenuated phagocytosis. We also observed a reduction in phagocytosis when we treated alveolar macrophages with antagonists to the plasma membrane receptors for either LTB₄, cysteinyl-leukotrienes, or both. In leukotriene-competent cells, LTC₄ augmented phagocytosis to the greatest extent, followed by 5-HETE and LTB₄. These 5-lipoxygenase reaction products demonstrated similar relative abilities to reconstitute phagocytosis in zileuton-treated rat alveolar macrophages and in alveolar macrophages from 5-lipoxygenase knockout mice. We conclude that endogenous synthesis of all major 5-lipoxygenase reaction products plays an essential role in phagocytosis. The restorative and pharmacologic effects of LTC₄, LTB₄, and 5-HETE may provide a basis for their exogenous administration as an adjunctive treatment for patients with gram-negative bacterial pneumonia.     

Effect of exogenous leukotriene B4 (LTB4) on BALB/c mice splenocyte production of Th1 and Th2 lymphokines

The effect of exogenous leukotriene B₄ (LTB₄) on the production of cytokines typical of Th₁ (interleukin-2 and interferon-γ) and Th₂ (interleukin-4 and interleukin-10) lymphocytes was studied. Splenocytes were stimulated with concanavalin A (ConA) with or without different concentrations of LTB₄ (3 × 10⁻¹⁰ to 3 × 10⁻⁷M) for various times in the presence of BW 755C to inhibit the endogenous synthesis of eicosanoids. LTB₄ was not able to induce cytokine secretion by itself. However, LTB₄ augmented ConA spleen cell production of interleukin-2 (IL-2) and interferon-γ (IFN-γ) from Th₁ cells and interleukin-4 (IL-4) and interleukin-10 (IL-10) from Th₂ cells more than the controls treated with ConA alone. The pre-exposition of splenocytes to LTB₄ for 3 h made these cells more sensitive to ConA in terms of IL-2 and IL-10 production than those treated with LTB₄ at the onset of the incubation and maintained during the whole culture period. The results suggest that LTB₄ may participate as a component of the signal transduction process for ConA-induced Th₁ and Th₂ cytokine production in a timedependent manner.     

Elevated Expressions of 15-Lipoxygenase and Lipoxin A4 in Children with Acute Poststreptococcal Glomerulonephritis

Anti-inflammatory effects of the 15-lipoxygenase (15-LO) derivatives lipoxin A₄ (LXA₄) and 15-S-hydroxyeicosatetraenoic acid (15-S-HETE) have been documented in many experimental models of acute inflammation. However, the expression levels of 15-LO and its products in human renal diseases remain unknown. This study investigated the expression levels of LXA₄, leukotriene B₄ (LTB₄), and 15-LO in leukocytes and glomeruli obtained from 22 children with acute poststreptococcal glomerulonephritis (APSGN), and determined the modulatory effects of both 15-S-HETE and LXA₄ on LTB₄ synthesis in leukocytes and LTB₄-evoked chemotaxis of polymorphonuclear leukocytes (PMNs) obtained from children during the first 3 days after onset of APSGN. Expression levels of both LXA₄ and 15-LO in leukocytes and glomeruli were up-regulated during the acute phase of disease, further peaking between days 10 and 14, and remained increased after 6 to 8 weeks of APSGN onset. In contrast, blood and urinary levels of LTB₄ as well as the number of glomerular PMNs peaked during the acute phase of disease and then decreased during the resolution phase. Administration of both 15-S-HETE and LXA₄ in vitro inhibited LTB₄-induced chemotaxis of PMNs and production of LTB₄ from leukocytes obtained from patients with APSGN. The current study provides further support for an anti-inflammatory role for 15-LO products in human nephritis through both antagonism and inhibition of leukotriene synthesis and its biological activity.     

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and opsonization synergistically enhance leukotriene B4 (LTB4) synthesis induced by phagocytosis in human neutrophils

Normal human blood neutrophils were studied for their capacity to synthesize leukotriene B₄ (LTB₄) and its -oxidized metabolites after phagocytosis of zymosan. Phagocytosis of serum-opsonized particles led to a higher release of LTs than did unopsonized zymosan. The most stricking effect of phagocytosis was observed when neutrophils were primed with granulocyte-macrophage colony-stimulating factor (GM-CSF): opsonization and GM-CSF synergistically increased LTB₄ synthesis by neutrophils.     

Effects of antirheumatic drugs on leukotriene B4 and prostanoid synthesis in human polymorphonuclear leukocytesin vitro

The effects ofd-penicillamine, sodium aurothiomalate, indomethacin, timegadine and tolfenamic acid on the lipoxygenase and cyclo-oxygenase pathways of arachidonic acid metabolism were studied in human polymorphonuclear leukocytes (PMNs)in vitro. In short-term incubations,d-penicillamine and aurothiomalate did not affect leukotriene B₄ (LTB₄), prostaglandin E₂ (PGE₂) or thromboxane B₂ (TXB₂) production. Each of the three non-steroidal anti-inflammatory drugs (NSAIDs) used were potent inhibitors of prostanoid synthesis. In higher concentrations they also reduced LTB₄ production; timegadine and tolfenamic acid were effective in concentrations comparable to those measured in plasma during drug therapy, whereas indomethacin was needed in ten times higher concentrations. The different effects of NSAIDs on 5-lipoxygenase activity may be of importance in their therapeutic actions as well as in the appearance of some side-effects, e.g. gastric irritation and aspirin-induced asthma.     

Tolfenamic acid inhibits leukotriene B4-induced chemotaxis of polymorphonuclear leukocytes in vitro

Inhibition of prostanoid synthesis is usually regarded as the mode of action of nonsteroidal antiinflammatory drugs (NSAIDs). In addition, some NSAIDs have been reported to have prostanoid-independent inhibitory effects on neutrophil functions. In the present study, we examined the effects of acetylsalicylic acid, diclofenac, indomethacin, ketoprofen, piroxicam and tolfenamic acid on leukotriene B₄ (LTB₄)-induced chemotaxis of human polymorphonuclear leukocytes (PMNs) in vitro. Tolfenamic acid inhibited LTB₄-induced chemotaxis (IC₅₀ 59M), whereas the other compounds were ineffective. Tolfenamic acid inhibited also FMLP-induced chemotaxis at the same concentration range (IC₅₀ 46M). About 25% reduction in the chemotactic response was achieved with therapeutic concentrations of tolfenamic acid. We suggest that the inhibition of PMN chemotaxis is an additional mechanism in the antiinflammatory action of tolfenamic acid and that this action is not ligand specific.     

Inhibition of leukotriene B4 in neutrophils by lipoxins A4 and B4

Lipoxins are derived from the oxygenation products of arachidonic acid in human leukocytes. They have exhibited selective biological effects different from those of other eicosanoids. We have examined the effect of lipoxin A₄ and B₄ (LXA₄, LXB₄) on the production of leukotriene B₄ (LTB₄) in human neutrophils. Cultured human polymorphonuclear leukocytes were preincubated with LXA and B and their ability to inhibit LTB₄ generation was assessed after incubation with calcium ionophore A23187. We found that the pretreatment of neutrophils with lipoxins inhibit the release of LTB₄ by A23187 stimulated PMNs. Our data suggests that LXA₄ and B₄ can contribute to immunosuppression in an inflammatory state via the inhibition of LTB₄ synthesis.     

Leukotriene B4 metabolism in human leukocytes: Fact or artefact?

The object of this study was to investigate the importance of omega oxidation in regulating leukotriene B₄ (LTB₄) levels in man. In human polymorphonuclear leukocytes metabolism of LTB₄ was rapid but was critically dependent on PMN number: greater than 1.5×10⁶ PMN/ml were required. Metabolism of LTB₄ was blocked in the presence of plasma. In whole blood and in PMN-rich rheumatoid synovial fluids no significant metabolism of LTB₄ was detected within 30 min at 37°C. We conclude that LTB₄ metabolism at inflamed sites will be regulated both by cellular content and the degree of plasma exudation. In most pathological conditions rapid exchange with the micro-vasculature will be more important than metabolism in limiting LTB₄ levels.     

Interactions between synthesis of platelet-activating factor and leukotriene B4 in isolated human polymorphonuclear leukocytes

The aim of the present work was to study interactions between the synthesis of platelet-activating factor (PAF) and leukotriene B₄ (LTB₄) in human polymorphonuclear leukocytes (PMNs) in vitro. PAF, at nanomolar concentrations, stimulated calcium ionophore A23187-activated PMNs to release LTB₄ and 5-hydroxyeicosatetraenoic acid (5-HETE). This seems to be a receptor-mediated process as it was blocked by a PAF receptor antagonist WEB 2086 (IC₅₀ 6.6±3.9M). Moreover, LTB₄ stimulated the formation of PAF in activated PMNs. WEB 2086 did not, however, alter PMN migration towards either LTB₄ or the chemotactic peptide FMLP. This suggests that the enhancement of PAF synthesis in response to LTB₄ is a concomitant event rather than a mediating process in LTB₄-induced chemotactic movement of PMNs. These effects are implicated in the complex network of interactions between inflammatory mediators that results accumulation and activation of PMNs in the exacerbation of inflammatory processes.     

Eicosanoid production by density-defined human peritoneal macrophages during inflammation

Density-defined macrophages isolated from fluids of patients with liver cirrhosis mainly generated the 5-lipoxygenase products leukotriene B₄ (LTB₄, 16%) and 5-hydroxy-eicosatetraenoic acid (5-HETE, 24%) and the cyclooxygenase products 12-hydroxyheptadecatrienoic acid (HHT, 22%) and thromboxane B₂ (TXB₂, 18%). The synthesis of eicosanoids was linear with the maturity of the macrophage subpopulations, suggesting that eicosanoid production is increased inin-vivo activated macrophages.     

Eicosanoid production by density-defined human peritoneal macrophages during inflammation

Density-defined macrophages isolated from fluids of patients with liver cirrhosis mainly generated the 5-lipoxygenase products leukotriene B₄ (LTB₄, 16%) and 5-hydroxy-eicosatetraenoic acid (5-HETE, 24%) and the cyclooxygenase products 12-hydroxyheptadecatrienoic acid (HHT, 22%) and thromboxane B₂ (TXB₂, 18%). The synthesis of eicosanoids was linear with the maturity of the macrophage subpopulations, suggesting that eicosanoid production is increased inin-vivo activated macrophages.