Pharmacological Analysis of NLRP3 Inflammasome Inhibitor Sodium [(1,2,3,5,6,7-Hexahydro-s-indacen-4-yl)carbamoyl][(1-methyl-1H-pyrazol-4-yl)({[(2S)-oxolan-2-yl]methyl})sulfamoyl]azanide in Cellular and Mouse Models of Inflammation Provides a Translational Framework.

Interleukin (IL)-1ß is an apex proinflammatory cytokine produced in response to tissue injury and infection. The output of IL-1ß from monocytes and macrophages is regulated not only by transcription and translation but also post-translationally. Release of the active cytokine requires activation of inflammasomes, which couple IL-1ß post-translational proteolysis with pyroptosis. Among inflammasome platforms, NOD-like receptor pyrin domain-containing protein 3 (NLRP3) is implicated in the pathogenesis of numerous human disorders in which disease-specific danger-associated molecular patterns (DAMPS) are positioned to drive its activation. As a promising therapeutic target, numerous candidate NLRP3-targeting therapeutics have been described and demonstrated to provide benefits in the context of animal disease models. While showing benefits, published preclinical studies have not explored dose-response relationships within the context of the models. Here, the preclinical pharmacology of a new chemical entity, [(1,2,3,5,6,7-hexahydro-s-indacen-4-yl)carbamoyl][(1-methyl-1H-pyrazol-4-yl)({[(2S)-oxolan-2-yl]methyl})sulfamoyl]azanide (NT-0249), is detailed, establishing its potency and selectivity as an NLRP3 inhibitor. NT-0249 also is evaluated in two acute in vivo mouse challenge models where pharmacodynamic/pharmacokinetic relationships align well with in vitro blood potency assessments. The therapeutic utility of NT-0249 is established in a mouse model of cryopyrin-associated periodic syndrome (CAPS). In this model, mice express a human gain-of-function NLRP3 allele and develop chronic and progressive IL-1ß-dependent autoinflammatory disease. NT-0249 dose-dependently reduced multiple inflammatory biomarkers in this model. Significantly, NT-0249 decreased mature IL-1ß levels in tissue homogenates, confirming in vivo target engagement. Our findings highlight not only the pharmacological attributes of NT-0249 but also provide insight into the extent of target suppression that will be required to achieve clinical benefit.

Species-specific NLRP3 regulation and its role in CNS autoinflammatory diseases.

The NLRP3 inflammasome is essential for caspase-1 activation and the release of interleukin (IL)-1ß, IL-18, and gasdermin-D in myeloid cells. However, research on species-specific NLRP3's physiological impact is limited. We engineer mice with the human NLRP3 gene, driven by either the human or mouse promoter, via syntenic replacement at the mouse Nlrp3 locus. Both promoters facilitate hNLRP3 expression in myeloid cells, but the mouse promoter responds more robustly to LPS. Investigating the disease impact of differential NLRP3 regulation, we introduce the D305N gain-of-function mutation into both humanized lines. Chronic inflammation is evident with both promoters; however, CNS outcomes vary significantly. Despite poor response to LPS, the human promoter results in D305N-associated aseptic meningitis, mirroring human pathology. The mouse promoter, although leading to increased CNS expression post-LPS, does not induce meningitis in D305N mutants. Therefore, human-like NLRP3 expression may be crucial for accurate modeling of its role in disease pathogenesis.

Target Cell Activation of a Structurally Novel NOD-Like Receptor Pyrin Domain-Containing Protein 3 Inhibitor NT-0796 Enhances Potency.

The NOD-like receptor pyrin domain-containing protein 3 (NLRP3) inflammasome is a central regulator of innate immunity, essential for processing and release of interleukin-1ß and pyroptotic cell death. As endogenous NLRP3 activating triggers are hallmarks of many human chronic inflammatory diseases, inhibition of NLRP3 has emerged as a therapeutic target. Here we identify NDT-19795 as a novel carboxylic acid-containing NLRP3 activation inhibitor in both human and mouse monocytes and macrophages. Remarkably, conversion of the carboxylate to an isopropyl-ester (NT-0796) greatly enhances NLRP3 inhibitory potency in human monocytes. This increase is attributed to the ester-containing pharmacophore being more cell-penetrant than the acid species and, once internalized, the ester being metabolized to NDT-19795 by carboxylesterase-1 (CES-1). Mouse macrophages do not express CES-1, and NT-0796 is ineffective in these cells. Mice also contain plasma esterase (Ces1c) activity which is absent in humans. To create a more human-like model, we generated a mouse line in which the genome was modified, removing Ces1c and replacing this segment of DNA with the human CES-1 gene driven by a mononuclear phagocyte-specific promoter. We show human CES-1 presence in monocytes/macrophages increases the ability of NT-0796 to inhibit NLRP3 activation both in vitro and in vivo. As NLRP3 is widely expressed by monocytes/macrophages, the co-existence of CES-1 in these same cells affords a unique opportunity to direct ester-containing NLRP3 inhibitors precisely to target cells of interest. Profiling NT-0796 in mice humanized with respect to CES-1 biology enables critical modeling of the pharmacokinetics and pharmacodynamics of this novel therapeutic candidate. SIGNIFICANCE STATEMENT: Inhibition of NLRP3 represents a desirable therapeutic strategy for the treatment of multiple human disorders. In this study pharmacological properties of a structurally-novel, ester-containing NLRP3 inhibitor NT-0796 are characterized. To study pharmacodynamics of NT-0796 in vivo, a mouse line was engineered possessing more human-like traits with respect to carboxylesterase biology. In the context of these hCES-1 mice, NT-0796 serves as a more effective inhibitor of NLRP3 activation than the corresponding acid, highlighting the full translational potential of the ester strategy.

Elarekibep (PRS-060/AZD1402), a new class of inhaled Anticalin medicine targeting IL-4Ra for type 2 endotype asthma.

BACKGROUND: Type 2 endotype asthma is driven by IL-4 and IL-13 signaling via IL-4Ra, which is highly expressed on airway epithelium, airway smooth muscle, and immunocytes in the respiratory mucosa, suggesting potential advantages of an inhalable antagonist. Lipocalin 1 (Lcn1), a 16 kDa protein abundant in human periciliary fluid, has a robust drug-like structure well suited to protein engineering, but it has never been used to make an inhaled Anticalin protein therapeutic. OBJECTIVES: We sought to reengineer Lcn1 into an inhalable IL-4Ra antagonist and assess its pharmacodynamic/kinetic profile. METHODS: Lcn1 was systematically modified by directed protein mutagenesis yielding a high-affinity, slowly dissociating, long-acting full antagonist of IL-4Ra designated PRS-060 with properties analogous to dupilumab, competitively antagonizing IL-4Ra-dependent cell proliferation, mucus induction, and eotaxin expression in vitro. Because PRS-060 displayed exquisite specificity for human IL-4Ra, with no cross-reactivity to rodents or higher primates, we created a new triple-humanized mouse model substituting human IL-4Ra, IL-4, and IL-13 at their correct syntenic murine loci to model clinical dosing. RESULTS: Inhaled PRS-060 strongly suppressed acute allergic inflammation indexes in triple-humanized mice with a duration of action longer than its bulk clearance, suggesting that it may act locally in the lung. CONCLUSION: Lcn1 can be reengineered into the Anticalin antagonist PRS-060 (elarekibep), exemplifying a new class of inhaled topical, long-acting therapeutic drugs with the potential to treat type 2 endotype asthma.

Beyond detoxification: Pleiotropic functions of multiple glutathione S-transferase isoforms protect mice against a toxic electrophile.

Environmental and endogenous electrophiles cause tissue damage through their high reactivity with endogenous nucleophiles such as DNA, proteins, and lipids. Protection against damage is mediated by glutathione (GSH) conjugation, which can occur spontaneously or be facilitated by the glutathione S-transferase (GST) enzymes. To determine the role of GST enzymes in protection against electrophiles as well as the role of specific GST families in mediating this protection, we exposed mutant mouse lines lacking the GSTP, GSTM, and/or GSTT enzyme families to the model electrophile acrylamide, a ubiquitous dietary contaminant known to cause adverse effects in humans. An analysis of urinary metabolites after acute acrylamide exposure identified the GSTM family as the primary mediator of GSH conjugation to acrylamide. However, surprisingly, mice lacking only this enzyme family did not show increased toxicity after an acute acrylamide exposure. Therefore, GSH conjugation is not the sole mechanism by which GSTs protect against the toxicity of this substrate. Given the prevalence of null GST polymorphisms in the human population (approximately 50% for GSTM1 and 20-50% for GSTT1), a substantial portion of the population may also have impaired acrylamide metabolism. However, our study also defines a role for GSTP and/or GSTT in protection against acrylamide mediated toxicity. Thus, while the canonical detoxification function of GSTs may be impaired in GSTM null individuals, disease risk secondary to acrylamide exposure may be mitigated through non-canonical pathways involving members of the GSTP and/or GSTT families.

A Mutation in the Borcs7 Subunit of the Lysosome Regulatory BORC Complex Results in Motor Deficits and Dystrophic Axonopathy in Mice.

Lysosomes play a critical role in maintenance of the integrity of neuronal function, and mutations in genes that contribute to lysosome formation, transport, and activity are associated with neurodegenerative disorders. Recently, the multisubunit complex, BLOC-one-related complex (BORC), has been shown to be involved in positioning lysosomes within the cytoplasm, although the consequences of altered BORC function in adult animals have not been established. We show that a spontaneous truncation mutation in the mouse Borcs7 gene, identified through whole-genome sequencing followed by genetic complementation, results in progressive axonal dystrophy with dramatic impairment of motor function. Furthermore, mice homozygous for deletion of the entire Borcs7 coding sequence die shortly after birth, and neurons cultured from these animals show impaired centrifugal transport of lysosomes. This identifies BORCS7 as a central factor in axonal transport of lysosomes and a possible target for improving disease-related disturbances in this important function.

An NLRP3 Mutation Causes Arthropathy and Osteoporosis in Humanized Mice.

The NLRP3 inflammasome plays a critical role in host defense by facilitating caspase I activation and maturation of IL-1ß and IL-18, whereas dysregulation of inflammasome activity results in autoinflammatory disease. Factors regulating human NLRP3 activity that contribute to the phenotypic heterogeneity of NLRP3-related diseases have largely been inferred from the study of Nlrp3 mutant mice. By generating a mouse line in which the NLRP3 locus is humanized by syntenic replacement, we show the functioning of the human NLRP3 proteins in vivo, demonstrating the ability of the human inflammasome to orchestrate immune reactions in response to innate stimuli. Humanized mice expressing disease-associated mutations develop normally but display acute sensitivity to endotoxin and develop progressive and debilitating arthritis characterized by granulocytic infiltrates, elevated cytokines, erosion of bones, and osteoporosis. This NLRP3-dependent arthritis model provides a platform for testing therapeutic reagents targeting the human inflammasome.

Mice lacking three Loci encoding 14 glutathione transferase genes: a novel tool for assigning function to the GSTP, GSTM, and GSTT families.

Glutathione S-transferases (GSTs) form a superfamily defined by their ability to catalyze the conjugation of glutathione with electrophilic substrates. These enzymes are proposed to play a critical role in protection of cellular components from damage mediated by reactive metabolites. Twenty-two cytosolic GSTs, grouped into seven families, are recognized in mice. This complexity hinders the assignment of function to a subset or family of these genes. We report generation of a mouse line in which the locus encoding three GST gene families is deleted. This includes the four Gstt genes spanning 65 kb on chromosome 10 and the seven Gstm genes found on a 150 kb segment of DNA chromosome 3. In addition, we delete two Gstp genes on chromosome 19 as well as a third related gene located 15 kb telomeric to Gstp1 and Gstp2, which we identify as a potential new member of this gene family. We show that, despite the loss of up to 75% of total GST activity in some tissues from these animals, the mice are healthy and fertile, with normal life expectancy. The normal development and health of these animals make them an appropriate model for defining the role of these families in redox homeostasis and metabolism of drugs and environmental pollutants.

Genetic loss of murine pyrin, the Familial Mediterranean Fever protein, increases interleukin-1ß levels.

Familial Mediterranean Fever (FMF) is an inherited autoinflammatory disorder characterized by unprovoked episodes of fever and inflammation. The associated gene, MEFV (Mediterranean Fever), is expressed primarily by cells of myeloid lineage and encodes the protein pyrin/TRIM20/Marenostrin. The mechanism by which mutations in pyrin alter protein function to cause episodic inflammation is controversial. To address this question, we have generated a mouse line lacking the Mefv gene by removing a 21 kb fragment containing the entire Mefv locus. While the development of immune cell populations appears normal in these animals, we show enhanced interleukin (IL) 1ß release by Mefv(-/-) macrophages in response to a spectrum of inflammatory stimuli, including stimuli dependent on IL-1ß processing by the NLRP1b, NLRP3 and NLRC4 inflammasomes. Caspase-1 activity, however, did not change under identical conditions. These results are consistent with a model in which pyrin acts to limit the release of IL-1ß generated by activation and assembly of inflammasomes in response to subclinical immune challenges.

NLRP1-dependent pyroptosis leads to acute lung injury and morbidity in mice.

Acute inflammation in response to both exogenous and endogenous danger signals can lead to the assembly of cytoplasmic inflammasomes that stimulate the activation of caspase-1. Subsequently, caspase-1 facilitates the maturation and release of cytokines and also, under some circumstances, the induction of cell death by pyroptosis. Using a mouse line lacking expression of NLRP1, we show that assembly of this inflammasome in cells is triggered by a toxin from anthrax and that it initiates caspase-1 activation and release of IL-1ß. Furthermore, NLRP1 inflammasome activation also leads to cell death, which escalates over 3 d following exposure to the toxin and culminates in acute lung injury and death of the mice. We show that these events are not dependent on production of IL-1ß by the inflammasome but are dependent on caspase-1 expression. In contrast, muramyl dipeptide-mediated inflammasome formation is not dependent on NLRP1 but NLRP3. Taken together, our findings show that assembly of the NLRP1 inflammasome is sufficient to initiate pyroptosis, which subsequently leads to a self-amplifying cascade of cell injury within the lung from which the lung cannot recover, eventually resulting in catastrophic consequences for the organism.

PGE2 through the EP4 receptor controls smooth muscle gene expression patterns in the ductus arteriosus critical for remodeling at birth.

The ductus arteriosus (DA) is a fetal shunt that directs right ventricular outflow away from pulmonary circulation and into the aorta. Critical roles for prostaglandin E(2) (PGE(2)) and the EP4 receptor (EP4) have been established in maintaining both the patency of the vessel in utero and in its closure at birth. Here we have generated mice in which loss of EP4 expression is limited to either the smooth muscle (SMC) or endothelial cells and demonstrated that SMC, but not endothelial cell expression of EP4 is required for DA closure. The genome wide expression analysis of full term wild type and EP4(-/-) DA indicates that PGE(2)/EP4 signaling modulates expression of a number of unique pathways, including those involved in SMC proliferation, cell migration, and vascular tone. Together this supports a mechanism by which maturation and increased contractility of the vessel is coupled to the potent smooth muscle dilatory actions of PGE(2).

ONZIN deficiency attenuates contact hypersensitivity responses in mice.

ONZIN is abundantly expressed in immune cells of both the myeloid and lymphoid lineage. Expression by lymphoid cells has been reported to further increase after cutaneous exposure of mice to antigens and haptens capable of inducing contact hypersensitivity (CHS), suggesting that ONZIN has a critical role in this response. Here, we report that indeed ONZIN-deficient mice develop attenuated CHS to a number of different haptens. Dampened CHS responses correlated with a significant reduction in pro-inflammatory IL-6 at the challenge site in ONZIN-deficient animals, compared with wild-type controls. Together the study of these animals indicates that loss of ONZIN impacts the effector phase of the CHS response through the regulation of pro-inflammatory factors.

A major role for the EP4 receptor in regulation of renin.

Prostaglandins have been implicated as paracrine regulators of renin secretion, but the specific pathways and receptor(s) carrying out these functions have not been fully elucidated. To examine the contributions of prostanoid synthetic pathways and receptors to regulation of renin in the intact animal, we used a panel of mice with targeted disruption of several key genes: cyclooxygenase-2 (COX-2), microsomal PGE synthases 1 and 2 (mPGES1, mPGES2), EP2 and EP4 receptors for PGE(2), and the IP receptor for PGI(2). To activate the macula densa signal for renin stimulation, mice were treated with furosemide over 5 days and renin mRNA levels were determined by real-time RT-PCR. At baseline, there were no differences in renin mRNA levels between wild-type and the various strains of mutant mice. Furosemide caused marked stimulation of renin mRNA expression across all groups of wild-type control mice. This response was completely abrogated in the absence of COX-2, but was unaffected in mice lacking mPGES1 or mPGES2. The absence of G(s)/cAMP-linked EP2 receptors had no effect on stimulation of renin by furosemide and there was only a modest, insignificant reduction in renin responses in mice lacking the IP receptor. By contrast, renin stimulation in EP4(-/-) mice was significantly reduced by ∼70% compared with wild-type controls. These data suggest that stimulation of renin by the macula densa mechanism is mediated by PGE(2) through a pathway requiring COX-2 and the EP4 receptor, but not EP2 or IP receptors. Surprisingly, mPGES1 or mPGES2 are not required, suggesting other alternative mechanisms for generating PGE(2) in response to macula densa stimulation.

Mice lacking NKCC1 are protected from development of bacteremia and hypothermic sepsis secondary to bacterial pneumonia.

The contribution of the Na(+)-K(+)-Cl(-) transporter (NKCC1) to fluid in ion transport and fluid secretion in the lung and in other secretory epithelia has been well established. Far less is known concerning the role of this cotransporter in the physiological response of the pulmonary system during acute inflammation. Here we show that mice lacking this transporter are protected against hypothermic sepsis and bacteremia developing as a result of Klebsiella pneumoniae infection in the lung. In contrast, this protection was not observed in NKCC1(-/-) mice with K. pneumoniae-induced peritonitis. Although overall recruitment of cells to the lungs was not altered, the number of cells present in the airways was increased in the NKCC1(-/-) animals. Despite this robust inflammatory response, the increase in vascular permeability observed in this acute inflammatory model was attenuated in the NKCC1(-/-) animals. Our studies suggest that NKCC1 plays a unique and untoward unrecognized role in acute inflammatory responses in the lung and that specific inhibition of this NKCC isoform could be beneficial in treatment of sepsis.

Enhanced mast cell activation in mice deficient in the A2b adenosine receptor.

Antigen-mediated cross-linking of IgE bound to mast cells via the high affinity receptor for IgE triggers a signaling cascade that results in the release of intracellular calcium stores, followed by an influx of extracellular calcium. The collective increase in intracellular calcium is critical to the release of the granular contents of the mast cell, which include the mediators of acute anaphylaxis. We show that the sensitivity of the mast cell to antigen-mediated degranulation through this pathway can be dramatically influenced by the A2b adenosine receptor. Loss of this Gs-coupled receptor on mouse bone marrow-derived mast cells results in decreased basal levels of cyclic AMP and an excessive influx of extracellular calcium through store-operated calcium channels following antigen activation. Mice lacking the A2b receptor display increased sensitivity to IgE-mediated anaphylaxis. Collectively, these findings show that the A2b adenosine receptor functions as a critical regulator of signaling pathways within the mast cell, which act in concert to limit the magnitude of mast cell responsiveness when antigen is encountered.

Age-induced reprogramming of mast cell degranulation.

Mast cell degranulation can initiate an acute inflammatory response and contribute to the progression of chronic diseases. Alteration in the cellular programs that determine the requirement for mast cell degranulation would therefore have the potential to dramatically impact disease severity. Mast cells are exposed to increased levels of PGE2 during inflammation. We show that although PGE2 does not trigger the degranulation of dermal mast cells of young animals, in older mice, PGE2 is a potent mast cell stimulator. Intradermal administration of PGE2 leads to an EP3 receptor-dependent degranulation of mast cells, with the number of degranulated cells approaching levels observed in IgE- and Ag-treated controls. Taken together, these studies suggest that the ability of PGE2 to initiate mast cell degranulation changes in the aging animal. Therefore, elevated PGE2 levels might provide an important pathway by which mast cells are engaged to participate in inflammatory responses in the elderly patient.

E-prostanoid-3 receptors mediate the proinflammatory actions of prostaglandin E2 in acute cutaneous inflammation.

PGs are derived from arachidonic acid by PG-endoperoxide synthase (PTGS)-1 and PTGS2. Although enhanced levels of PGs are present during acute and chronic inflammation, a functional role for prostanoids in inflammation has not been clearly defined. Using a series of genetically engineered mice, we find that PTGS1 has the capacity to induce acute inflammation, but PTGS2 has negligible effects on the initiation of this response. Furthermore, we show that the contribution of PTGS1 is mediated by PGE(2) acting through the E-prostanoid (EP)3 receptor. Moreover, in the absence of EP3 receptors, inflammation is markedly attenuated, and the addition of nonsteroidal anti-inflammatory agents does not further impair the response. These studies demonstrate that PGE(2) promotes acute inflammation by activating EP3 receptors and suggest that EP3 receptors may be useful targets for anti-inflammatory therapy.

Receptors and pathways mediating the effects of prostaglandin E2 on airway tone.

Prostaglandin E(2) (PGE(2)) has complex effects on airway tone, and the existence of four PGE(2) [E-prostanoid (EP)] receptors, each with distinct signaling characteristics, has provided a possible explanation for the seemingly contradictory actions of this lipid mediator. To identify the receptors mediating the actions of PGE(2) on bronchomotor tone, we examined its effects on the airways of wild-type and EP receptor-deficient mice. In conscious mice the administration of PGE(2) increased airway responsiveness primarily through the EP1 receptor, although on certain genetic backgrounds a contribution of the EP3 receptor was detected. These effects of PGE(2) were eliminated by pretreatment with either atropine or bupivacaine and were undetectable in anesthetized mice or in denervated tracheal rings, where only EP2-mediated relaxation of airway smooth muscle was observed. Together, our findings are consistent with a model in which PGE(2) modulates airway tone by activating multiple receptors expressed on various cell populations and in which the relative contribution of these receptors might depend on the expression of modifier alleles. PGE(2)/EP1/EP3-induced airway constriction occurs indirectly through activation of neural pathways, whereas PGE(2)-induced bronchodilation results from direct activation of EP2 receptors on airway smooth muscle. This segregation of EP receptor function within the airway suggests that PGE(2) analogs that selectively activate the EP2 receptor without activating the EP1/EP3 receptors might prove useful in the treatment of asthma.

Receptors and signaling mechanisms required for prostaglandin E2-mediated regulation of mast cell degranulation and IL-6 production.

Mast cells are implicated in the pathogenesis of a broad spectrum of immunological disorders. These cells release inflammatory mediators in response to a number of stimuli, including IgE-Ag complexes. The degranulation of mast cells is modified by PGs. To begin to delineate the pathway(s) used by PGs to regulate mast cell function, we examined bone marrow-derived mast cells (BMMC) cultured from mice deficient in the EP(1), EP(2), EP(3), and EP(4) receptors for PGE(2). Although BMMCs express all four of these PGE(2) receptors, potentiation of Ag-stimulated degranulation and IL-6 cytokine production by PGE(2) is dependent on the EP(3) receptor. Consistent with the coupling of this receptor to G(alphai), PGE(2) activation of the EP(3) receptor leads to both inhibition of adenylate cyclase and increased intracellular Ca(2+). The magnitude of increase in intracellular Ca(2+) induced by EP(3) activation is similar to that observed after activation of cells with IgE and Ag. Although PGE alone is not sufficient to initiate BMMC degranulation, stimulation of cells with PGE along with PMA induces degranulation. These actions are mediated by the EP(3) receptor through signals involving Ca(2+) mobilization and/or decreased cAMP levels. Accordingly, these studies identify PGE(2)/EP(3) as a proinflammatory signaling pathway that promotes mast cell activation.

Transcellular biosynthesis contributes to the production of leukotrienes during inflammatory responses in vivo.

Leukotrienes are lipid mediators that evoke primarily proinflammatory responses by activating receptors present on virtually all cells. The production of leukotrienes is tightly regulated, and expression of 5-lipoxygenase, the enzyme required for the first step in leukotriene synthesis, is generally restricted to leukocytes. Arachidonic acid released from the cell membrane of activated leukocytes is rapidly converted to LTA(4) by 5-lipoxygenase. LTA(4) is further metabolized to either LTC(4) or LTB(4) by the enzyme LTC(4) synthase or LTA(4) hydrolase, respectively. Unlike 5-lipoxygenase, these enzymes are expressed in most tissues. This observation previously has led to the suggestion that LTA(4) produced by leukocytes may, in some cases, be delivered to other cell types before being converted into LTC(4) or LTB(4). While in vitro studies indicate that this process, termed transcellular biosynthesis, can lead to the production of leukotrienes, it has not been possible to determine the significance of this pathway in vivo. Using a series of bone marrow chimeras generated from 5-lipoxygenase- and LTA(4) hydrolase-deficient mice, we show here that transcellular biosynthesis contributes to the production of leukotrienes in vivo and that leukotrienes produced by this pathway are sufficient to contribute significantly to the physiological changes that characterize an ongoing inflammatory response.