Alteration of leukotriene release by macrophages ingesting Toxoplasma gondii

Role of leukotrienes on protozoan and helminth infections

Leukotrienes (LTs), formed by the 5-lipoxygenase-(5-LO-) catalyzed oxidation of arachidonic acid, are lipid mediators that have potent proinflammatory activities. Pharmacologic or genetic inhibition of 5-LO biosynthesis in animals is associated with increased mortality and impaired clearance of bacteria, fungi, and parasites. LTs play a role in the control of helminth and protozoan infections by modulating the immune system and/or through direct cytotoxicity to parasites; however, LTs may also be associated with pathogenesis, such as in cerebral malaria and schistosomal granuloma. Interestingly, some proteins from the saliva of insect vectors that transmit protozoans and secreted protein from helminth could bind LTs and may consequently modulate the course of infection or pathogenesis. In addition, the decreased production of LTs in immunocompromised individuals might modulate the pathophysiology of helminth and protozoan infections. Herein, in this paper, we showed the immunomodulatory and pathogenic roles of LTs during the helminth and protozoan infections.

12/15-lipoxygenase-dependent myeloid production of interleukin-12 is essential for resistance to chronic toxoplasmosis

Interleukin-12 (IL-12) is critical for resistance to Toxoplasma gondii during both the acute and chronic stages of infection. However, the cellular and molecular pathways that regulate IL-12 production during chronic toxoplasmosis are incompletely defined. We recently discovered that 12/15-lipoxygenase (12/15-LOX), which oxidizes unsaturated lipids in macrophages, is a novel and selective regulator of IL-12 production. We now demonstrate the essential role of this enzyme in the chronic phase of toxoplasmosis. Although 12/15-LOX-deficient mice were resistant to acute T. gondii infection, 80% of 12/15-LOX-deficient mice died during chronic toxoplasmosis, compared to no deaths in wild-type controls. The morbidity of chronically infected 12/15-LOX mice was associated with an increase in brain inflammation and parasite burden. These data suggest that the evolution of the immune response to T. gondii is accompanied by an increasing requirement for 12/15-LOX-mediated signaling. Consistent with this conclusion, 12/15-LOX activity was enhanced during chronic, but not acute, toxoplasmosis. Furthermore, the enhanced susceptibility of 12/15-LOX-deficient mice to chronic toxoplasmosis was associated with reduced production of IL-12 and gamma interferon (IFN-gamma) that was not evident during acute infection. Importantly, ex vivo IFN-gamma production by 12/15-LOX-deficient splenocytes could be rescued by the addition of recombinant IL-12. These data establish that 12/15-LOX is a critical mediator of the chronic type 1 inflammatory response and that immune mediators can be subject to distinct cellular and/or molecular mechanisms of regulation at different stages of inflammation.

Leukotrienes: underappreciated mediators of innate immune responses

Leukotrienes are bronchoconstrictor and vasoactive lipid mediators that are targets in the treatment of asthma. Although they are increasingly recognized to exert broad proinflammatory effects, their role in innate immune responses is less well appreciated. These molecules are indeed synthesized by resident and recruited leukocytes during infection. Acting via cell surface G protein-coupled receptors and subsequent intracellular signaling events, they enhance leukocyte accumulation, phagocyte capacity for microbial ingestion and killing, and generation of other proinflammatory mediators. Interestingly, a variety of acquired states of immunodeficiency, such as HIV infection and malnutrition, are characterized by a relative deficiency of leukotriene synthesis. The data reviewed herein point to leukotrienes as underappreciated yet highly relevant mediators of innate immunity.

Exogenous pathogen and plant 15-lipoxygenase initiate endogenous lipoxin A4 biosynthesis

Lipoxin A4 (LXA4) is a potent endogenous lipoxygenase-derived eicosanoid with antiinflammatory and proresolving properties. Supraphysiological levels of LXA4 are generated during infection by Toxoplasma gondii, which in turn reduces interleukin (IL) 12 production by dendritic cells, thus dampening Th1-type cell-mediated immune responses and host immunopathology. In the present work, we sought evidence for the structural basis of T. gondii's ability to activate LXA4 biosynthesis. Proteomic analysis of T. gondii extract (soluble tachyzoite antigen [STAg]), which preserves the immunosuppressive and antiinflammatory activity of the parasite, yielded several peptide matches to known plant lipoxygenases. Hence, we incubated STAg itself with arachidonic acid and found using LC-UV-MS-MS-based lipidomics that STAg produced both 15-HETE and 5,15-diHETE, indicating that T. gondii carries 15-lipoxygenase activity. In addition, T. gondii tachyzoites (the rapidly multiplying and invasive stage of the parasite) generated LXA4 when provided with arachidonic acid. Local administration of a plant (soybean) lipoxygenase itself reduced neutrophilic infiltration in murine peritonitis, demonstrating that 15-lipoxygenase possesses antiinflammatory properties. Administration of plant 15-lipoxygenase generated endogenous LXA4 and mimicked the suppression of IL-12 production by splenic dendritic cells observed after T. gondii infection or STAg administration. Together, these results indicate that 15-lipoxygenase expressed by a pathogen as well as exogenously administered 15-lipoxygenase can interact with host biosynthetic circuits for endogenous "stop signals" that divert the host immune response and limit acute inflammation.

Regulation and function of T-cell-mediated immunity during Toxoplasma gondii infection

The intracellular protozoan Toxoplasma gondii is a widespread opportunistic parasite of humans and animals. Normally, T. gondii establishes itself within brain and skeletal muscle tissues, persisting for the life of the host. Initiating and sustaining strong T-cell-mediated immunity is crucial in preventing the emergence of T. gondii as a serious pathogen. The parasite induces high levels of gamma interferon (IFN-gamma) during initial infection as a result of early T-cell as well as natural killer (NK) cell activation. Induction of interleukin-12 by macrophages is a major mechanism driving early IFN-gamma synthesis. The latter cytokine, in addition to promoting the differentiation of Th1 effectors, is important in macrophage activation and acquisition of microbicidal functions, such as nitric oxide release. During chronic infection, parasite-specific T lymphocytes release high levels of IFN-gamma, which is required to prevent cyst reactivation. T-cell-mediated cytolytic activity against infected cells, while easily demonstrable, plays a secondary role to inflammatory cytokine production. While part of the clinical manifestations of toxoplasmosis results from direct tissue destruction by the parasite, inflammatory cytokine-mediated immunopathologic changes may also contribute to disease progression.

Candida albicans induces the release of inflammatory mediators from human peripheral blood monocytes

Candida albicans (C. albicans) is a major nosocomial pathogen. We examined arachidonic acid (AA) and cytokine production by monocytes stimulated with C. albicans. [14C]-AA labeled monocytes released 8.9 +/- 2.3% of the incorporated AA following stimulation with live C. albicans (C. albicans: monocyte of 16:1) (P = 0.0002). Prior studies indicate that soluble alpha-mannans and beta-glucans antagonize mannose and beta-glucan receptors, respectively. Preincubation of monocytes with alpha-mannan (100 micrograms/ml) caused 45.8 +/- 5.7% inhibition of [14C]-AA release, whereas beta-glucan (100 micrograms/ml) yielded 43.7 +/- 6.0% inhibition (P < 0.05 for each compared to control). Additionally, monocytes stimulated with C. albicans also released interleukin-1 beta (IL-1 beta), tumor necrosis factor-alpha (TNF alpha), interleukin-6 (IL-6) and interleukin-8 (IL-8). However, alpha-mannan or beta-glucan failed to inhibit IL-1 beta release. These data indicate that C. albicans induces monocytes to release AA and inflammatory cytokines. Furthermore, AA, but not cytokine liberation, is partially mediated by alpha-mannan and beta-glucan components of the fungus.

Toxoplasma gondii alters eicosanoid release by human mononuclear phagocytes: role of leukotrienes in interferon gamma-induced antitoxoplasma activity

Toxoplasma gondii tachyzoites markedly alter the profile of eicosanoids released by human mononuclear phagocytes. Freshly isolated, 2-h adherent human monocytes release both cyclooxygenase (e.g., thromboxane [TX] B2, prostaglandin [PG] E2) and 5-lipoxygenase (e.g., leukotriene [LT] B4, LTC4) products of arachidonic acid metabolism after stimulation by the calcium ionophore A23187 or ingestion of opsonized zymosan particles or heat-killed T. gondii. However, after incubation with viable T. gondii, normal and chronic granulomatous disease monocytes release only the cyclooxygenase products TXB2 and PGE2 and fail to form LTB4, LTC4, or other 5-lipoxygenase products. Monocytes maintained in culture for 5 d lose this capacity to release TXB2 and PGE2 after incubation with T. gondii. T. gondii significantly inhibit calcium ionophore A23187-induced LTB4 release by monocyte-derived macrophages; heat-killed organisms do not affect this calcium ionophore A23187-induced release of LTB4. T. gondii-induced inhibition of LTB4 release by calcium ionophore A23187-stimulated monocyte-derived macrophage is reversed by interferon (IFN)-gamma treatment of the monolayers. LTB4 induced extensive damage to the cellular membranes and cytoplasmic contents of the organisms as observed by transmission electron microscopy. Exogenous LTB4 (10(-6) M) induced intracellular killing of ingested T. gondii by non-IFN-gamma-treated monocyte-derived macrophages. IFN-gamma-induced antitoxoplasma activity in monocyte-derived macrophages was inhibited by the selective 5-lipoxygenase inhibitor zileuton but not by the cyclooxygenase inhibitor indomethacin. These findings suggest a novel role for 5-lipoxygenase arachidonic acid products in human macrophage IFN-gamma-induced antitoxoplasma activity.

Influence of dietary (n-3)-polyunsaturated fatty acids on leukotriene B4 and prostaglandin E2 synthesis and course of experimental tuberculosis in guinea pigs

In the present study eicosanoid synthesis was studied in macrophages of guinea pigs fed different amounts of (n-6)- and (n-3)-polyunsaturated fatty acids (PUFA). Three groups of weanling guinea pigs were fed by isocaloric diets differing only in their contents of PUFA: controls with 2.8 Cal% of linoleic acid (LA; 18:2(n-6)); (n-6)-rich fed animals with 15.4 Cal% of LA; and (n-3)-rich fed animals with 10.1 Cal% of LA, 1.4 Cal% of eicosapentaenoic acid (20:5(n-3)) and docosahexaenoic acid (22:6(n-3)). After 13 weeks half the number of animals from each group was infected i.m. by 180 colony forming units of Mycobacterium tuberculosis strain H37Rv. Seven weeks after infection the release of leukotriene (LT)B4 and prostaglandin (PG)E2 was quantified in calcium ionophore stimulated whole blood, peritoneal macrophage cultures and alveolar macrophages by immunoassays after high performance liquid chromatography. Synthesis of LTB4 and PGE2 was found to be reduced in (n-3)-rich fed guinea pigs (p < 0.05), and equivalent between controls and (n-6)-rich fed animals. Controls and (n-6)-rich fed animals showed the same mycobacterial counts in the spleen whereas (n-3)-rich fed guinea pigs demonstrated an increased number of mycobacteria (p < 0.05). Our results demonstrate that an increased dietary intake of (n-3)-PUFA suppress LTB4 and PGE2 synthesis. The increased number of M. tuberculosis found in the spleens of (n-3)-rich fed animals could represent persistence of the experimental infection. It may be speculated that a functional relationship exists between the two findings.

Pneumocystis carinii induces the release of arachidonic acid and its metabolites from alveolar macrophages

Pneumocystis carinii is an opportunistic organism that causes severe lung injury in immunocompromised hosts. Macrophage responses to P. carinii are poorly defined. Arachidonic acid (AA) and its metabolites are potent mediators of inflammation and have been implicated in host response to microorganisms. We therefore examined the production of eicosanoids from rat and rabbit alveolar macrophages stimulated with purified P. carinii. [14C]AA-labeled rabbit macrophages released 8.50 +/- 1.33% of the incorporated [14C]AA after 90 min in response to P. carinii (P = 0.0001 compared with unstimulated controls). In contrast, a similar number of rat alveolar macrophages exhibited a smaller but significant response to P. carinii, releasing 3.84 +/- 1.54% of their [14C]AA after 90 min (P = 0.001 compared with control). We further determined that P. carinii stimulated substantial production of prostaglandin E2 and concurrently a small amount of leukotriene B4 release from alveolar macrophages. To further investigate whether serum opsonization of P. carinii enhances these alterations in AA metabolism, we assessed the effect of P. carinii immune serum on P. carinii-induced AA release. P. carinii opsonized with this antiserum caused significantly greater AA release from rat alveolar macrophages than either unopsonized P. carinii or organisms opsonized with nonimmune serum. Previous studies suggest that P. carinii interacts with macrophage beta-glucan and mannose receptors. However, incubation of macrophages with P. carinii in the presence of either soluble beta-glucan or alpha-mannan failed to alter the release of AA from macrophages in response to P. carinii. Macrophage release of eicosanoids represents a potentially important host inflammatory response to P. carinii infection.

Eicosanoid production by mouse peritoneal macrophages during Toxoplasma gondii penetration: role of parasite and host cell phospholipases

The metabolism of endogenous arachidonic acid by mouse resident peritoneal macrophages infected in vitro with Toxoplasma gondii was studied. Prelabeling of macrophages with [5,6,8,9,11,12,14,15-3H]arachidonic acid and challenge with tachyzoites for 15 min resulted in a high mobilization of free labeled arachidonic acid (178%) in the culture medium. The parasites also triggered the synthesis of 6-keto-prostaglandin F1 alpha (47%), prostaglandin E2 (44%), leukotrienes C4 and D4 (33%) and 5-, 12-hydroxyeicosatetraenoic acids (155%). The study indicated that during the intracellular development phase of the parasites, 6-keto-prostaglandin F1 alpha (38%), prostaglandin E2 (31%) leukotrienes C4 and D4 (15%), hydroxyeicosatetraenoic acids (43%), and free arachidonic acid (110%) were secreted into the culture medium. Pretreatment of tachyzoites with phospholipase A2 inhibitors (4-p-bromophenacyl bromide and quinacrine) and no calcium in the culture medium resulted in inhibition of tachyzoite penetration into the macrophages and a decrease of the arachidonic acid metabolism. The triggering of the arachidonic acid cascade by T. gondii was dependent on the active penetration of the parasites into the macrophages, whereas preincubation of the macrophages with phospholipase A2 inhibitors did not affect penetration or free arachidonic acid release, thereby supporting a role for parasite phospholipase in the penetration process and in arachidonic acid mobilization from macrophage membrane phospholipids. Moreover, treatment of macrophages with phospholipase A2 inhibitors decreased the activities of the cyclooxygenase and lipoxygenase pathways, also suggesting an activation of host cell phospholipase A2 by the parasite.

Cytotoxic activity of 13-hydroxyoctadecadienoic acid against Toxoplasma gondii

Recent data indicate that platelets may play an important role in the host defence against Toxoplasma gondii infections. T. gondii-stimulated human platelets release thromboxane A2 (TXA2) and 12-hydroxyeicosatetraenoic acid (12-HETE) from arachidonic acid and 13-hydroxyoctadecadienoic acid (13-HODE) from linoleic acid (Yong et al. 1991; Henderson et al. 1992). We have previously demonstrated that the eicosanoid TXA2 has potent cytotoxic activity against T. gondii trophozoites (Yong et al. 1991). In this study, we examined whether 12-HETE, 13-HODE, and linoleic acid also have toxoplasmacidal activity. 13-HODE at concentrations > or = 10(-8) M rapidly induced cytotoxic changes in T. gondii. Ultrastructural changes induced by 13-HODE in T. gondii included an initial leakage of cytoplasmic contents into a space between the inner and outer parasite bilayer membrane units which was followed by intracellular vacuolation and loss of cytoplasmic contents. In contrast, linoleic acid and 12-HETE lacked toxoplasmacidal activity at 10(-10)-10(-6) M concentrations. These data indicate that 13-HODE, a product of linoleic acid metabolism, has potent cytotoxic activity against T. gondii; this toxoplasmacidal activity may be important in the inflammatory response to this pathogen.

Alterations in murine macrophage arachidonic acid metabolism following ingestion of nonviable Histoplasma capsulatum

The effect of ingestion of heat-killed Histoplasma capsulatum yeast cells on the metabolism of arachidonic acid (20:4) to prostenoids and leukotrienes was examined in murine peritoneal macrophages (M phi s). H. capsulatum-containing M phi s exhibited a metabolite profile similar to that of zymosan-challenged phagocytes; however, there were differences with respect to the relative and total amounts of products produced. While proteose peptone-elicited M phi s exposed to H. capsulatum released quantitatively less prostaglandin E2 (PGE2) and leukotriene C4 than zymosan-treated M phi s, they metabolized a greater percentage of total product to prostenoids. In addition, whereas in vitro priming with gamma interferon increased both the PGE2 and leukotriene C4 contents of zymosan-stimulated M phi supernatants, similarly primed M phi s challenged with H. capsulatum selectively increased only PGE2 production. The immunosuppressive effect of a relative excess of prostenoids in H. capsulatum-containing M phi s may contribute to the overall disturbance in cell-mediated immunity characteristic of disseminated histoplasmosis.

Toxoplasma gondii stimulates the release of 13- and 9-hydroxyoctadecadienoic acids by human platelets

We have recently demonstrated a novel cytotoxic effect of human platelets against Toxoplasma gondii and a role for thromboxane (TX) in this process (Yong et al., 1991). We now report on the spectrum of lipid mediators released by human platelets after interaction with T. gondii. In addition to TXB2, human platelets after incubation with T. gondii for 90 min released 12-hydroxyheptadecatrienoic acid (12-HHT), 12-hydroxyeicosatetraenoic acid (12-HETE), and an unidentified peak (UVmax 234 nm) as determined by reverse-phase high-performance liquid chromatography. Thermospray-liquid chromatography/mass spectrometry analysis and straight-phase HPLC identified the unknown peak as a mixture of 13-hydroxyoctadecadienoic acid (HODE) and 9-HODE. Radiolabeling studies with [14C]linoleic acid indicated that the platelets were the cellular source of the octadecanoids with 13-HODE (87.7%) greater than 9-HODE (12.3%). Inhibitor studies with indomethacin indicated that 13-HODE was a lipoxygenase product and 9-HODE was a cyclooxygenase product of linoleic acid. Thus, Toxoplasma-stimulated platelets release oxygenated products of both arachidonic acid and linoleic acid which may be important in the host response to T. gondii infection.

Bacterial metabolism of human polymorphonuclear leukocyte-derived arachidonic acid

Evidence for transcellular bacterial metabolism of phagocyte-derived arachidonic acid was sought by exposing human blood polymorphonuclear leukocytes, prelabelled with [3H]arachidonic acid, to opsonized, stationary-phase Pseudomonas aeruginosa (bacteria-to-phagocyte ratio of 50:1) for 90 min at 37 degrees C. Control leukocytes were stimulated with the calcium ionophore A23187 (5 microM) for 5 min. Radiochromatograms of arachidonic acid metabolites, extracted from A23187-stimulated cultures and then separated by reverse-phase high-performance liquid chromatography, revealed leukotriene B4, its omega-oxidation products, and 5-hydroxy-eicosatetraenoic acid. In contrast, two major metabolite peaks, distinct from known polymorphonuclear leukocyte arachidonic acid products by high-performance liquid chromatography or by thin-layer chromatography, were identified in cultures of P. aeruginosa with [3H]arachidonic acid-labelled polymorphonuclear leukocytes. Respective chromatographic characteristics of these novel products were identical to those of two major metabolite peaks produced by incubation of stationary-phase P. aeruginosa with [3H]arachidonic acid. Production of the metabolites was dependent upon pseudomonal viability. UV spectral data were consistent with a conjugated diene structure. Metabolism of arachidonic acid by P. aeruginosa was not influenced by the presence of catalase, superoxide dismutase, nordihydroguaiaretic acid, ethanol, dimethyl sulfoxide, or ferrous ions but was inhibited by carbon monoxide, ketoconazole, and 1,2-epoxy-3,3,3-trichloropropane. Our data suggest that pseudomonal metabolism of polymorphonuclear leukocyte-derived arachidonic acid occurs during phagocytosis, probably by enzymatic epoxidation and hydroxylation via an oxygenase. By this means, potential proinflammatory effects of arachidonic acid or its metabolites may be modulated by P. aeruginosa at sites of infection in vivo.

Human platelet-mediated cytotoxicity against Toxoplasma gondii: role of thromboxane

Human platelets, in the absence of antibody, are cytotoxic to tachyzoites of Toxoplasma gondii as determined by vital staining, transmission electron microscopy, and the failure of Toxoplasma to survive and replicate in mice after in vitro interaction of the organisms with platelets. Platelet to T. gondii ratios as low as 1:3 were toxic to the organisms with direct cell-cell contact essential for platelet-mediated cytotoxicity. Adherence of platelets to T. gondii and disruption of surface membranes and cytoplasmic contents of the organisms were observed ultrastructurally. Reactive oxygen species were not implicated in the platelet-mediated toxicity. The interaction of T. gondii with platelets resulted in a marked increase in thromboxane B2 (TXB2) production compared with that by unstimulated platelets. The cyclooxygenase inhibitors acetylsalicylic acid and indomethacin inhibited platelet-mediated cytolytic activity as did the selective TXA2 synthetase inhibitor dazmegrel, indicating a role for thromboxane in the platelet-induced cytotoxicity. Further, toxoplasmacidal activity was retained in the TXA2 synthetase-containing microsomal fractions of platelets disrupted by freezing and thawing; cytolytic activity was absent in microsome-depleted platelet supernatant fractions. Both the TXA2-generating platelet microsome system and a stable TXA2 analogue induced damage to the cellular membranes of the Toxoplasma as noted by transmission electron microscopy. These findings suggest that platelets may play a role in the host defense against Toxoplasma and that release of thromboxane may be important in this cytolytic process.

Effects of Histoplasma capsulatum on murine macrophage functions: inhibition of macrophage priming, oxidative burst, and antifungal activities

Histoplasma capsulatum yeast cells fail to trigger an oxidative burst response in normal murine macrophages. The results of this study, in which an in vitro assay of macrophage antifungal effects was used, extend these findings. During 18 h of incubation, unprimed elicited murine macrophages inhibited H. capsulatum growth only when macrophages were present in great excess. Gamma interferon (IFN-gamma)-primed macrophages showed enhanced fungal growth inhibition but a similar requirement for an excess of phagocytes. Macrophages containing heat-killed H. capsulatum exhibited diminished antifungal effects toward viable H. capsulatum and Saccharomyces cerevisiae cells. Parallel experiments showed no comparable effect of ingested latex particles on macrophage antifungal activity. Using chemiluminescence as a measure of the oxidative burst, we found that macrophages primed in vitro with IFN-gamma alone failed to exhibit a significant response to triggering by H. capsulatum yeast cells unless a second priming agent (tumor necrosis factor alpha or bacterial lipopolysaccharide) was added to IFN-gamma. Furthermore, macrophage priming with single agents was blocked by the prior ingestion of heat-killed H. capsulatum. These studies provide evidence that ingestion of H. capsulatum yeast cells can induce a prompt and enduring deactivation of murine macrophages.