Macrophage complement and lectin-like receptors bind Leishmania in the absence of serum.

We have examined the relative roles of the macrophage (M phi) plasma membrane receptor for the cleaved third complement component (iC3b, CR3) and of the mannosyl/fucosyl receptor (MFR) in binding and ingestion of Leishmania donovani. In the absence of exogenous complement, the binding and ingestion of promastigotes, which are good activators of the alternative complement pathway, were inhibited by the anti-CR3 monoclonal antibody M1/70, by the Fab portion of an anti-C3 antibody, or by the nucleophile, sodium salicyl hydroxamate, an inhibitor of C3 fixation. This provides strong evidence that M phi-derived, cleaved C3 (iC3b) present on the promastigote surface mediates binding to CR3. Equivalent inhibition of promastigote binding and ingestion was also observed using the soluble inhibitors of MFR activity, mannan or ribonuclease B. No additive effect for blocking the two M phi receptors simultaneously was observed. For amastigotes, which are poor activators of the alternative pathway, a lesser but nevertheless equivalent effect was observed for the three soluble inhibitors of CR3-mediated binding vs. the two soluble inhibitors of MFR-mediated binding. Modulation experiments in which either CR3 or MFR had been rendered inaccessible demonstrated that both receptors must be present on the segment of M phi membrane to which the parasite binds. The combined function of these two distinct M phi receptors may provide a general mechanism for recognition and ingestion of other pathogenic protozoa known to activate the alternative pathway.

Anionic phospholipids stimulate an arachidonoyl-hydrolyzing phospholipase A2 from macrophages and reduce the calcium requirement for activity.

Mechanisms involved in regulating the activity of intracellular phospholipase A2 enzymes that function in eicosanoid and platelet-activating factor production are poorly understood. The properties of the substrate in the membrane may play a role in modulating phospholipase A2 activity. In this study, the effect of anionic phospholipids, diacylglycerol (DAG) and phosphatidylethanolamine (PE) on the activity of a partially purified, intracellular, arachidonoyl-hydrolyzing phospholipase A2 from the macrophage cell line, RAW 264.7 was studied. For these experiments phospholipase A2 activity was assayed in the presence of 1 microM calcium by measuring the hydrolysis of [3H]arachidonic acid from sonicated dispersions of the ether-linked substrate, 1-O-hexadecyl-2[3H]arachidonoylglycerophosphocholine. All the anionic phospholipids tested, including phosphatidylserine (PS), phosphatidic acid (PA), phosphatidylinositol (PI) and phosphatidylinositol-4,5-bisphosphate (PIP2), stimulated phospholipase A2 activity. At the lowest concentration of anionic phospholipids tested. PIP2 was the most stimulatory, resulting in a 7-fold increase in phospholipase A2 activity at 1 mol%. Co-dispersion of either DAG or PE with the substrate also induced a dose-dependent increase in phospholipase A2 activity, whereas sphingomyelin was inhibitory suggesting that the phospholipase A2 more readily hydrolyzed the ether-linked substrate when there was a decrease in the packing density of the bilayer. PIP2, together with either DAG or PE, synergistically stimulated phospholipase A2 activity by about 20-fold, and dramatically decreased the calcium concentration (from mM to nM) required for full activity of the enzyme. The results of this study demonstrate that the presence of anionic phospholipids and the packing characteristics of the bilayer can have pronounced effects on the activity and calcium requirement of an intracellular, arachidonoyl-hydrolyzing phospholipase A2 from macrophages.

Properties and purification of an arachidonoyl-hydrolyzing phospholipase A2 from a macrophage cell line, RAW 264.7.

The lipid mediators, platelet activating factor (PAF) and the eicosanoids, can be coordinately produced from the common phospholipid precursor, 1-O-alkyl-2-arachidonoylglycerophosphocholine (1-O-alkyl-2-arachidonoyl-GPC), through the initial action of a phospholipase A2 that cleaves arachidonic acid from the sn-2 position. The mouse macrophage cell line RAW 264.7, which was used as a model macrophage system to study the arachidonoyl-hydrolyzing phospholipase A2 enzyme(s), could be induced to release arachidonic acid in response to inflammatory stimuli. A phospholipase A2 that hydrolyzed 1-O-hexadecyl-2-[3H]arachidonoyl-GPC was identified in the cytosolic fraction of these macrophages. This phospholipase activity was optimal at pH 8 and dependent on calcium. Enzyme activity could be stimulated 3-fold by heparin, suggesting the presence of phospholipase inhibitory proteins in the macrophage cytosol. Compared to 1-alkyl-2-arachidonoyl-GPC, the enzyme hydrolyzed 1-acyl-2-arachidonoylglycerophosphoethanolamine (1-acyl-2-arachidonoyl-GPE) with similar activity but showed slightly greater activity against 1-acyl-2-arachidonoyl-GPC, suggesting no specificity for the sn-1 linkage or the phospholipid base group. Although comparable activity against 1-acyl-2-arachidonoylglycerophosphoinositol (1-acyl-2-arachidonoyl-GPI) could be achieved, the enzyme exhibited much lower affinity for the inositol-containing substrate. The enzyme did, however, show apparent specificity for arachidonic acid at the sn-2 position, since much lower activity was observed against choline-containing substrates with either linoleic or oleic acids at the sn-2 position. The cytosolic phospholipase A2 was purified by first precipitating the enzyme with ammonium sulfate followed by chromatography over Sephadex G150, where the phospholipase A2 eluted between molecular weight markers of 67,000 and 150,000. The active peak was then chromatographed over DEAE-cellulose, phenyl-Sepharose, Q-Sepharose, Sephadex G150 and finally hydroxylapatite. The purification scheme has resulted in over a 1000-fold increase in specific activity (2 mumol/min per mg protein). Under non-reducing conditions, a major band on SDS-polyacrylamide gels at 70 kDa was observed, which shifted to a lower molecular weight, 60,000, under reducing conditions. The properties of the purified enzyme including the specificity for sn-2-arachidonoyl-containing phospholipids was similar to that observed for the crude enzyme. The results demonstrate the presence of a phospholipase A2 in the macrophage cell line. RAW 264.7, that preferentially hydrolyzes arachidonoyl-containing phospholipid substrates.

Zymosan-stimulated production of phosphatidic acid by macrophages: relationship to release of superoxide anion and inhibition by agents that increase intracellular cyclic AMP.

Murine peritoneal macrophages (m phi) respond to unopsonized zymosan with the production of superoxide anion (O2-). We investigated the involvement of phospholipid turnover in the transduction mechanism for this phenomenon. Zymosan-stimulated m phi produced increased amounts of phosphatidic acid (PA); the increase was first detected at 1.5 min and continued for 10 min of incubation. Production of O2- was not detected until between 2 to 4 min after stimulation, and continued to increase through 60 min. Inhibition experiments suggested that these two processes were linked. Theophylline (theo)/dibutyrylcyclic AMP (dbcAMP) and theo/prostaglandin E2 (PGE2) inhibited O2- production at every time point (79% and 80% inhibition, respectively, at 4 min). Corresponding inhibition of PA production was also achieved at every time point (85% by theo/dbcAMP; 67% by theo/PGE2 at 4 min). These results are compatible with a role for phospholipid remodeling in the transduction process associated with the respiratory burst. Results suggest that the phospholipid species could be phosphatidylcholine (PC) as well as phosphatidylinositol (PI).

Secretion of cytokines and chemokines by polarized human epithelial cells from the female reproductive tract.

BACKGROUND: Pro-inflammatory chemokines that attract and cytokines that activate immune cells contribute to normal physiological homeostasis in the female reproductive tract, and are needed to deal effectively with potential pathogenic microbes. Mucosal epithelial cells are capable of producing these factors that communicate with cells of the innate and adaptive immune systems. METHODS: Epithelial cells from Fallopian tube, endometrium and endocervix were isolated and grown to high transepithelial resistance in cell inserts from seven patients who had hysterectomies. Interleukin (IL)-8, IL-6, granulocyte colony-stimulating factor (G-CSF), monocyte chemoattractant protein-1 (MCP-1), granulocyte-macrophage colony-stimulating factor (GM-CSF), tumour necrosis factor-alpha (TNF-alpha) and macrophage inflammatory peptide-1beta (MIP-1beta) were assessed by Luminex bead analysis or enzyme-linked immunosorbent assay (ELISA) in epithelial cell conditioned media from the apical and basolateral compartments. RESULTS: With the exception of MCP-1, the seven chemokines/cytokines constitutively produced by the polarized epithelial cells were preferentially secreted apically. A concentration pattern was found in all cases, with IL-8 and IL-6 produced in the greatest quantity. CONCLUSIONS: The concentrations of IL-8, IL-6, G-CSF and MCP-1 are similar to the levels found in reproductive tract fluids of patients with infection. The constitutive secretion and compartmentalization of large quantities of bioactive chemokines and cytokines provide additional evidence for the role of epithelial cells as gatekeepers of innate immune protection in the female reproductive tract.

Toxoplasma gondii-induced immune suppression by human peripheral blood monocytes: role of gamma interferon.

The ability of Toxoplasma gondii to evade the host immune response during primary infection in humans is poorly understood. In murine toxoplasmosis, infected spleen macrophages release soluble factors that mediate a transient immunosuppression, which may allow the parasite to become established. When an enriched population of human monocytes from seronegative individuals was incubated with toxoplasmas in vitro, soluble factors that mediated market suppression of mitogen-induced lymphocyte DNA synthesis were released. Irradiated tachyzoites that do not undergo replication were sufficient stimuli for near-maximal soluble factor release. Up to 50% of the soluble factor-mediated suppression is attributable to a gamma interferon (IFN-gamma)-dependent pathway, and the mediator of the remaining inhibition is neither interleukin-10, transforming growth factor beta, prostaglandin E2, lipoxygenase products, nitric oxide, nor tumor necrosis factor alpha-induced mitochondrial cell-derived reactive oxygen intermediates. IFN-gamma also mediates the up-regulation of an antigen-presenting cell phenotype by both infected and uninfected macrophages. However, IFN-gamma does not activate macrophages to become toxoplasmacidal; instead, intracellular toxoplasmas replicate and reinfect, eventually lysing the macrophage population. These results suggest that T. gondii is able to evade the naive host immune response by induction of soluble immunosuppressive factors that allow the parasite to become established during an acute infection.

A study of the sensitivity of Leishmania donovani promastigotes and amastigotes to hydrogen peroxide. I. Differences in sensitivity correlate with parasite-mediated removal of hydrogen peroxide.

The sensitivities of promastigotes and amastigotes of Leishmania donovani to reagent or glucose oxidase-generated hydrogen peroxide (H2O2) were examined in a phagocyte-free system and compared with direct measurements of loss of H2O2 due to reaction with the parasite. Using a combined fluorescence dye uptake/dye exclusion viability assay in conjunction with motility and transformation data it was shown that log-phase promastigotes harvested from recently transformed cultures were intermediate in their H2O2 sensitivity between amastigotes and log-phase promastigotes harvested from long-term subcultures. It was also observed that, while promastigotes are equally sensitive to either form of H2O2 stress, amastigotes are more resistant to single larger amounts of reagent H2O2 than to equivalent amounts of H2O2 generated over a 1 h period. In each case the respective LD50 values obtained for each form of the parasite under each type of H2O2 stress correlated with saturation of their ability to remove H2O2 from the phagocyte-free system. For both promastigotes and amastigotes there was always a time delay after removal of either form of H2O2 stress before H2O2-mediated damage to membranes became apparent. The results suggest that the differential responses of promastigotes and amastigotes to different forms of H2O2 stress may depend upon different H2O2 scavenging mechanisms examined in more detail in the accompanying paper.

A study of the sensitivity of Leishmania donovani promastigotes and amastigotes to hydrogen peroxide. II. Possible mechanisms involved in protective H2O2 scavenging.

Different hydrogen peroxide (H2O2)-scavenging mechanisms, and the conditions under which they operate, have been examined in promastigotes and amastigotes of Leishmania donovani. For promastigotes, the ability of the parasite to remove H2O2 was completely ablated by sonication whereas for sonicated amastigotes substantial loss of H2O2 from the phagocyte-free test system still occurred. In direct contrast, the ability of amastigotes, but not promastigotes, to remove H2O2 was markedly inhibited by aminotriazole or sodium azide. This suggested a role for haem-containing enzymes, catalase or peroxidases, as a protective H2O2-scavenging mechanism and was consistent with detection of catalase in amastigotes but not promastigotes using a spectrophotometric assay. Both forms of the parasite did, however, show reduced ability to remove H2O2 at 5-7 degrees C indicating that additional enzymatic scavenging mechanisms may operate. Glutathione peroxidase activity was undetectable in either form of the parasite. The total thiol sink, glutathione (GSH) plus protein thiols, was greater in promastigotes but the ability to regenerate GSH via glutathione reductase was equivalent for promastigotes and amastigotes. Less temperature-dependent non-enzymatic mechanisms (e.g. an unsaturated lipid sink) also appear to contribute to removal of H2O2 by both promastigotes and amastigotes. It seems likely, nevertheless, that the difference in H2O2 sensitivity between the two forms of the parasite relates to the activity of the direct H2O2-scavenging enzyme, catalase, which appears to operate more efficiently against a bolus of reagent H2O2.

A calcium-dependent mechanism for associating a soluble arachidonoyl-hydrolyzing phospholipase A2 with membrane in the macrophage cell line RAW 264.7.

Arachidonoyl-hydrolyzing phospholipase A2 plays a central role in providing substrate for the synthesis of the potent lipid mediators of inflammation, the eicosanoids, and platelet-activating factor. Although Ca2+ is required for arachidonic acid release in vivo and most phospholipase A2 enzymes require Ca2+ for activity in vitro, the role of Ca2+ in phospholipase A2 activation is not understood. We have found that an arachidonoyl-hydrolyzing phospholipase A2 from the macrophage-like cell line, RAW 264.7, exhibits Ca2(+)-dependent association with membrane. The intracellular distribution of the enzyme was studied as a function of the Ca2+ concentration present in homogenization buffer. The enzyme was found almost completely in the 100,000 x g soluble fraction when cells were homogenized in the presence of Ca2+ chelators and there was a slight decrease in soluble fraction activity when cells were homogenized at the level of Ca2+ in an unstimulated cell (80 nM). When cells were homogenized at Ca2+ concentrations expected in stimulated cells (230-450 nM), 60-70% of the phospholipase A2 activity was lost from the soluble fraction and became associated with the particulate fraction in a manner that was partly reversible with EGTA. Membrane-associated phospholipase A2 activity was demonstrated by [3H]arachidonic acid release both from exogenous liposomes and from radiolabeled membranes. With radiolabeled particulate fraction as substrate, this enzyme hydrolyzed arachidonic acid but not oleic acid from membrane phospholipid, and [3H]arachidonic acid was derived from phosphatidylcholine, phosphatidylethanolamine, and phosphatidylinositol/phosphatidylserine. We suggest a mechanism in which the activity of phospholipase A2 is regulated by Ca2+: in an unstimulated cell phospholipase A2 is found in the cytosol; upon receptor ligation the cytosolic Ca2+ concentration increases, and the enzyme becomes membrane-associated which facilitates arachidonic acid hydrolysis.

A study of the differential respiratory burst activity elicited by promastigotes and amastigotes of Leishmania donovani in murine resident peritoneal macrophages.

Acridine orange and ethidium bromide and a combination of fluorescent and transmitted light microscopy used in conjunction with the qualitative nitroblue tetrazolium assay for superoxide anion (O2-) release demonstrated dramatic differences in the binding of and respiratory burst (RB) activity elicited by promastigotes and amastigotes of Leishmania donovani in resident peritoneal macrophages (M phi) from C57BL/10ScSn mice. When amastigotes were incubated with M phi for 30 min the number of parasites per 100 M phi was 2-4-fold higher, a higher proportion of M phi became infected and the mean number of parasites per infected M phi was higher than in promastigote infections. RB activity was higher for promastigotes than amastigotes both in terms of the percentage of infected M phi containing formazan positive parasites and the percentage of individual formazan positive parasites. In an attempt to explain the differential response to promastigotes and amastigotes, RB activity was examined for sodium azide-treated, glutaraldehyde-fixed and heat-killed parasites and for various transformation intermediates between amastigotes and promastigotes. Binding and RB activity were also examined in conjunction with competitive binding assays designed to determine the specific receptors involved in ligand binding of both forms of the parasite to the M phi. The results indicate that, while amastigotes may possess an azide-sensitive mechanism which either competes for O2- produced or causes localized inactivation of RB activity, this cannot account for the full magnitude of the difference between the two forms of the parasite. The transformation and competitive binding studies suggest that the more likely explanation lies in both qualitative and quantitative differences in the distribution of surface ligands involved in binding the parasite to the M phi plasma membrane and that the well characterized mannose/fucose receptor may be important in promastigote, but not amastigote, binding and RB activity.

Differential infectivity and division of Toxoplasma gondii in human peripheral blood leukocytes.

When tachyzoites were incubated with human peripheral blood leukocytes in vitro, more monocytes and dendritic cells than neutrophils or lymphocytes were infected. Although tachyzoites were able to divide in each of these cell types, monocytes and dendritic cells were more permissive to rapid tachyzoite division than neutrophils or lymphocytes.

Survival of immunoglobulin G-opsonized Toxoplasma gondii in nonadherent human monocytes.

Toxoplasma gondii is a protozoan parasite that is able to penetrate human monocytes by either passive uptake during phagocytosis or active penetration. It is expected that immunoglobulin G (IgG) opsonization will target the parasite to macrophage Fc gamma receptors for phagocytic processing and subsequent degradation. Antibody-opsonized T. gondii tachyzoites were used to infect nonadherent and adherent human monocytes obtained from the peripheral blood of seronegative individuals. The infected monocytes were evaluated for the presence of intracellular parasites and the degree of parasiticidal activity. A marked difference in both the numbers of infected macrophages and numbers of parasites per 100 macrophages was observed in the nonadherent cells when compared with those of the adherent cell population. When macrophage Fc gamma receptors were down-modulated, opsonized tachyzoites retained their ability to penetrate the host cell at a rate similar to that observed for unopsonized parasites. These results suggest that antibody opsonization of T. gondii does not prevent active penetration of human monocytes by the parasite and, furthermore, has little effect on intracellular replication of the parasite.

Attachment ligands of viable Toxoplasma gondii induce soluble immunosuppressive factors in human monocytes.

Previous studies have demonstrated that surface antigen proteins, in particular SAG-1, of Toxoplasma gondii are important to this parasite as attachment ligands for the host cell. An in vitro assay was developed to test whether these ligands and other secretory proteins are involved in the immune response of human cells to toxoplasma. Human monocytes were infected with tachyzoites in the presence of antiparasite antibodies, and their effect on mitogen-induced lymphoproliferation was examined. The presence of antibody to either parasite-excreted proteins (MIC-1 and MIC-2) or surface proteins (SAG-1 and SAG-2) during infection neutralized the marked decrease seen in mitogen-induced lymphoproliferation in the presence of infected monocytes. Conversely, antibodies to other secreted proteins (ROP-1) and cytoplasmic molecules had no effect on parasite-induced, monocyte-mediated downregulation. Fluorescence microscope analysis detected microneme and surface antigen proteins on the monocyte cell surface during infection. These results suggest that microneme and surface antigen proteins trigger monocytes to downregulate mitogen-induced lymphoproliferation.

Induction of gammadelta T cells during acute murine infection with Toxoplasma gondii.

The importance of the host gammadelta T cell response to microbial pathogens is not yet fully understood. We report that inbred mice infected with T. gondii developed a gammadelta T cell proliferative response to parasite Ag. Mice depleted of either the alphabeta or gammadelta TCR were found to be significantly more susceptible to infection than the parent mouse strain. Proliferation of gammadelta T cells was observed in mice deficient in the TCR-alphabeta in response to UV-irradiated parasites. These T cells lyse parasite-infected syngenic macrophages. Adoptive transfer of these gammadelta T cells into beta2 microglobulin-deficient mice that have been depleted of both CD4+ and NK cells prolongs survival against acute parasite challenge when compared with nontransferred controls. The gammadelta T cells isolated from infected alpha -/- mice express a 10-fold increase in mRNA and produce high titers of IFN-gamma. These data suggest that gammadelta T cells may play an important role in the early host response to infection with T. gondii.

Functional and quantitative analysis of splenic T cell immune responses following oral toxoplasma gondii infection in mice.

Functional and quantitative analysis of splenic T cell immune responses following oral Toxoplasma gondii infection in mice. Experimental Parasitology 91, 212-221. Immunity to Toxoplasma gondii is mediated primarily by the host T cell response. Although there is considerable information regarding host immunity following intraperitoneal infection with tachyzoites, little information is available regarding naturally acquired infection following peroral infection with bradyzoites. In this study, a sequential quantitative analysis of the cell-mediated immune response was performed at the single cell level. To assess the kinetics of this response and parasitic loads, inbred mice were orally infected with the 76K strain bradyzoites of T. gondii. Within 24 h of infection, follicular hyperplasia followed by infiltration with histiocytes, macrophages, and apoptotic bodies was observed in the spleens of infected mice. T. gondii were detected from day 1, and counts increased gradually during the experimental period. Splenocyte DNA synthesis to antigen and mitogen was severely suppressed at days 7 and 10. The percentages of NK1.1(+) or delta gamma T cells were increased from day 1, whereas CD4(+) and CD8alpha+ T cells were signficantly increased after day 7 postinfection. CD25 expression and intracellular IFN-gamma production increased in NK1.1(+) cells on day 1 and by all other T cell subsets after day 4. Intracellular IL-4 did not increase until day 7, and IL-10 production was increased in all T cell subsets after day 4. Together, these findings indicate that oral infection with T. gondii stimulates a strong cellular immune response that appears to polarize toward an early Th1 response. However, within 7 days, a strong immune Th2 regulatory response as well as high parasitic loads can be observed, with a reduction in lymphoproliferation to mitogen stimulation, increased production of IL-4 and IL-10, and evidence of T cell apoptosis in the splenic immune compartment.