Lymphokine activation of J774G8 cells and mouse peritoneal macrophages challenged with Toxoplasma gondii

An Extracellular Redox Signal Triggers Calcium Release and Impacts the Asexual Development of Toxoplasma gondii

The ability of an organism to sense and respond to environmental redox fluctuations relies on a signaling network that is incompletely understood in apicomplexan parasites such as Toxoplasma gondii. The impact of changes in redox upon the development of this intracellular parasite is not known. Here, we provide a revised collection of 58 genes containing domains related to canonical antioxidant function, with their encoded proteins widely dispersed throughout different cellular compartments. We demonstrate that addition of exogenous H₂O₂ to human fibroblasts infected with T. gondii triggers a Ca²⁺ flux in the cytosol of intracellular parasites that can induce egress. In line with existing models, egress triggered by exogenous H₂O₂ is reliant upon both Calcium-Dependent Protein Kinase 3 and diacylglycerol kinases. Finally, we show that the overexpression a glutaredoxin-roGFP2 redox sensor fusion protein in the parasitophorous vacuole severely impacts parasite replication. These data highlight the rich redox network that exists in T. gondii, evidencing a link between extracellular redox and intracellular Ca²⁺ signaling that can culminate in parasite egress. Our findings also indicate that the redox potential of the intracellular environment contributes to normal parasite growth. Combined, our findings highlight the important role of redox as an unexplored regulator of parasite biology.

Serological evidence of Toxoplasma gondii infection as potential risk for the development of lepromatous leprosy in an endemic area for both neglected tropical diseases in Brazil

Background

Mycobacterium leprae and Toxoplasma gondii infections are both neglected tropical diseases highly prevalent in Brazil. Infection with certain parasite species can significantly alter susceptibility to other important pathogens, and/or influence the development of pathology. Here we investigated the possible influence of M. leprae/T. gondii co-parasitism on the manifestation of leprosy and its clinical forms.

Methods

Participants (n = 291) were recruited in Campos dos Goytacazes city, Rio de Janeiro state, southeast Brazil, from August 2015 to December 2019 and clinically diagnosed for leprosy. Participants were selected based on the presence (patients) or absence (healthy controls) of the leprosy disease. Contacts of patients were also recruited for this study. Serum samples from patients (n = 199) with leprosy, contacts (n = 40) and healthy controls (n = 52) were investigated for levels of IgM and IgG anti-phenolic glycolipid-1 (PGL-1) by ELISA. Additionally, IgG antibody against soluble Toxoplasma antigen (STAg) was measured in sera samples from leprosy patients, contacts and healthy controls for Toxoplasma gondii serology by ELISA. Anti-PGL-1 IgG and IgM levels were compared using one-way ANOVA Kruskal-Wallis or Mann-Whitney, while Spearman test was used to correlate levels of IgG anti-STAg and IgM/IgG anti-PGL-1 from seropositive and seronegative individuals for T. gondii infection. The risk of T. gondii infection for leprosy disease was assessed using Fisher’s test.

Results

Levels of IgM anti-PGL-1 antibodies were significantly higher in multibacillary (MB) patients compared to paucibacillary (PB) patients (P = 0.0068). Higher IgM and IgG levels anti-PGL-1 were detected in patients with the lepromatous forms. The serologic prevalence for T. gondii infection was 74.9%. We detected increased anti-STAg antibody levels in leprosy patients (79.4%), reaching 88.8% within those with lepromatous form of this disease. The leprosy risk increase in T. gondii seropositive individuals was two-fold (odds ratio [OR] = 2.055; 95% confidence intervals [95% CI]: 1.18–3.51) higher than those seronegative, and considering the lepromatous leprosy risk this increase was even dramatic (OR = 4.33; 95% CI: 1.76–9.69) in T. gondii seropositive individuals. Moreover the leprosy risk in T. gondii seropositive individuals was weakly correlated to the levels of IgG anti-STAg and IgM/IgG anti-PGL-1.

Conclusions

Altogether, our results suggest that T. gondii infection may exert immunomodulatory properties that influence to the susceptibility of leprosy, mainly on its more severe clinical form. A better understanding of parasite immunomodulation can ultimately contribute to the development of medical applications.

Dendritic cells as effector cells: gamma interferon activation of murine dendritic cells triggers oxygen-dependent inhibition of Toxoplasma gondii replication

Toxoplasma gondii is an obligate intracellular parasite that infects a wide variety of nucleated cells in its numerous intermediate hosts, including humans. Much interest has focused on the ability of gamma interferon (IFN-gamma)-activated macrophages to prevent intracellular replication, but some other cells (e.g., fibroblasts, endothelial cells, microglial cells, astrocytes, enterocytes and retinal pigment cells) can also be activated to induce this inhibition of proliferation. Dendritic cells are generally known to be involved in the induction of immune responses, but no previous study had investigated the possibility that dendritic cells may act as effector cells of this system. Our results show that IFN-gamma-activation inhibits the replication of T. gondii in dendritic cells, with the inhibition being dose dependent. Neither nitrogen derivatives nor tryptophan starvation appears to be involved in the inhibition of parasite replication by IFN-gamma. Experiments with oxygen scavengers indicate that intracellular T. gondii replication is oxygen dependent. Our findings suggest that, in addition to their essential role in stimulating the immune system, dendritic cells probably act as effector cells in the first line of defense against pathogen invasion.

Lytic cycle of Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular pathogen within the phylum Apicomplexa. This protozoan parasite is one of the most widespread, with a broad host range including many birds and mammals and a geographic range that is nearly worldwide. While infection of healthy adults is usually relatively mild, serious disease can result in utero or when the host is immunocompromised. This sophisticated eukaryote has many specialized features that make it well suited to its intracellular lifestyle. In this review, we describe the current knowledge of how the asexual tachyzoite stage of Toxoplasma attaches to, invades, replicates in, and exits the host cell. Since this process is closely analogous to the way in which viruses reproduce, we refer to it as the Toxoplasma "lytic cycle."

Invasion of vertebrate cells by Toxoplasma gondii

Toxoplasma gondii actively invades its host cell rather than entering by conventional phagocytosis. Invasion is powered by a microfilament-based motility system in the parasite that leads to invagination of the host cell plasma membrane to form a novel intracellular vacuole. This vacuole resists fusion with the endocytic network of the host cell and provides a protective environment for replication of the parasite.

Human indoleamine 2,3-dioxygenase inhibits Toxoplasma gondii growth in fibroblast cells

Interferon-gamma (IFN-gamma) is known to inhibit the growth of Toxoplasma gondii both in vivo and in vitro. The IFN-gamma induced anti-toxoplasma activity in human cells is strongly correlated with the degradation of the essential amino acid L-tryptophan in vitro. Destruction of L-tryptophan is due to an increased activity of indoleamine 2,3-dioxygenase (IDO), which is transcriptionally activated by IFN-gamma. To determine if indoleamine 2,3-dioxygenase alone is sufficient to block the T. gondii growth, we transfected human fibroblast cells with an IDO cDNA expression plasmid using a metallothionein-inducible promoter. We showed that IDO mRNA and its enzymatic activity are inducible in fibroblast cells transfected with right-orientation IDO cDNA upon addition of CdCl2 to culture medium. The elevated IDO enzyme activity is strongly correlated with an inhibition of T. gondii growth. No IDO mRNA nor enzyme activity is induced by CdCl2 in reverse orientation transfected cells, and no adverse effects were observed on T. gondii growth in cells transfected with the reverse IDO-construct or in control parent cells with or without supplementation of CdCl2. Our observations along with the recent report by Habara-Ohkubo et al. (Infect. Immun. 61, 1810-1813, 1993) suggest that IFN-gamma-induced antitoxoplasma activity is due at least in part to the activation of IDO gene.

Effects of cytokines in the activation of peritoneal macrophages from mice infected with Toxoplasma gondii

The present study was undertaken to assess the role of cytokines in the activation of peritoneal macrophages from Toxoplasma-infected mice. Peritoneal macrophages from Toxoplasma-infected mice (10 cysts of Beverley strain/mouse) were harvested 8 weeks after infection, and incubated with the mitogen-induced lymphokine, recombinant mouse interferon-gamma (IFN-gamma), recombinant mouse tumor necrosis factor-alpha (TNF-alpha) alone or in combination with IFN-gamma (IFN-gamma/TNF-alpha) for 24 hr at 37 degrees C, 5% CO2. Macrophage activation was measured by the amount of H2O2 and NO2- production, and anti-Toxoplasma activities of macrophages. IFN-gamma or IFN-gamma/TNF-alpha-treated macrophages from Toxoplasma-infected mice revealed significantly higher H2O2 production than resident macrophages from Toxoplasma-infected mice. The production of NO2- by TNF-alpha-, IFN-gamma- or IFN-gamma/TNF-alpha-treated macrophages from Toxoplasma-infected mice were significantly higher than that by resident macrophages, whereas lymphokine-treated group produced similar amount as that produced by resident macrophages. Anti-Toxoplasma activities of cytokine-treated macrophages from Toxoplasma-infected mice were significantly higher than those of resident macrophages. IFN-gamma-treated macrophages were significantly increased production of H2O2 and NO2-, and anti-Toxoplasma activities of macrophages between normal and Toxoplasma-infected mice, whereas the other cytokine-treated groups were not significant differences between them. These data suggested that IFN-gamma was the only one of cytokines capable of significantly activating the peritoneal macrophages from Toxoplasma-infected mice.

Regulation of murine macrophage effector functions by lipoarabinomannan from mycobacterial strains with different degrees of virulence

Lipoarabinomannan (LAM) is the major arabinose- and mannose-containing phosphorylated lipopolysaccharide (LPS) in mycobacterial cell walls. LAM preparations from a virulent strain (Erdman) (LAM(Erdman)) and an attenuated strain (H37Ra) (LAMH37Ra) of Mycobacterium tuberculosis, as well as from M. leprae (a virulent mycobacterium), were analyzed for their effects on various macrophage (M phi) effector functions. LAMH37Ra, like gram-negative LPS, exhibited a dose-dependent ability to induce tumor necrosis factor alpha (TNF-alpha) production in normal M phi, and gamma interferon (IFN-gamma) priming of the M phi greatly augmented the levels of TNF-alpha. However, the effects of LAMH37Ra were unaffected by polymyxin B, which totally abrogated the effects of LPS. LAM(Erdman) and LAM from M. leprae, on the other hand, induced virtually no TNF-alpha production. Analysis of M phi mRNA by reverse transcription-polymerase chain reaction revealed that the levels of production. Analysis of M phi mRNA by reverse transcription-polymerase chain reaction revealed that the levels of TNF-alpha mRNA induced by the various preparations correlated with the levels of TNF-alpha protein detected. Interestingly, both LAMH37Ra and LAM(Erdman) could block subsequent IFN-gamma- and LPS-induced M phi activation, a previously reported measure of the potent ability of LAM to down-regulate M phi effector functions. Two lines of evidence suggested, however, that M phi cyclooxygenase products did not play a role in this down-regulation. LAMH37Ra and LPS could induce the production of NO2- in both normal and IFN-gamma-primed M phi, whereas LAM(Erdman) could stimulate NO2- production only in primed M phi. Both LAMH37Ra and LAM(Erdman) could substitute for LPS as a triggering signal for IFN-gamma-primed M phi in a toxoplasma killing assay. The triggering ability of LAM(Erdman), however, was abrogated by an anti-TNF-alpha antibody, suggesting that sufficient TNF-alpha production was stimulated by LAM(Erdman) to drive a M phi function relevant in host resistance. Thus, mycobacterial LAM is a potent regulator of M phi functions, a fact that may have important consequences in mycobacterial disease.

Naturally occurring immunoglobulin M antibodies: enhancement of phagocytic and microbicidal activities of human neutrophils against Toxoplasma gondii

Naturally occurring immunoglobulin M antibodies to Toxoplasma gondii in human sera were examined for their ability to enhance toxoplasmacidal activities of human neutrophils to show a role of natural antibodies in immunity to the early stage of Toxoplasma infection in humans. Neutrophils were mixed with tachyzoites at a ratio of 1:5 in the presence or absence of antibodies and were cultured for 1-18 h for microscopical examination. The count of tachyzoites phagocytosed in 200 neutrophils within 1 h was significantly higher in the presence of sera with natural IgM antibody levels of greater than 0.8 than those of less than 0.1, with 2.4- to 2.9-fold differences (P less than 0.02). The total tachyzoite counts at 18 h decreased to 26-39% of those at 1 h in the presence of natural IgM antibody levels of greater than 0.8 (P less than 0.01), while the counts at 1 h consistently increased by 18 h in the absence of natural IgM antibodies. These results indicate that relatively high levels of natural IgM antibodies enhanced phagocytic and microbicidal activities of neutrophils against Toxoplasma. The enhancement was dose dependent, and was also weaker than that obtained with the same dilution of sera from individuals with chronic or acute infections. Live and dead tachyzoite counts in infected neutrophils suggested a quicker effect of natural IgM antibodies than IgG antibodies elicited by infection.

Activation of human macrophages for the killing of intracellular Trypanosoma cruzi by TNF-alpha and IFN-gamma through a nitric oxide-dependent mechanism

The protozoan parasite Trypanosoma cruzi is able to replicate in the cytoplasm of primary resident macrophages, but is killed by activated macrophages. Pretreatment of human macrophages with recombinant IFN-gamma and to a lesser extent with TNF-alpha, induced a significant trypanocidal activity. Furthermore, TNF-alpha had a synergistic effect with IFN-gamma on macrophage activation in T. cruzi killing. Similarly, IFN-gamma triggered the production of nitric oxide (NO) by macrophages, whereas TNF-alpha was less effective, although it was also synergistic with IFN-gamma. Both NO production and trypanocidal activity, but not superoxide (O2-) generation, induced in macrophages by TNF-alpha or IFN-gamma alone or in combination, were inhibited by N-monomethyl-L-arginine (N-MMLA), a competitive inhibitor of NO synthase activity. Furthermore, a strong correlation was found between the levels of NO production and trypanocidal activity induced by different lymphokine preparations. These results suggest that IFN-gamma and TNF-alpha are involved in the activation of the trypanocidal activity of human macrophages through a NO-dependent mechanism.

Synergism between tumor necrosis factor-alpha and interferon-gamma on macrophage activation for the killing of intracellular Trypanosoma cruzi through a nitric oxide-dependent mechanism

Intracellular replication of the protozoan parasite Trypanosoma cruzi inside macrophages is essential for the production of the disease and the development of the parasite. Two CD4+ T cell lines, A10 and A28, were established from T. cruzi-infected BALB/c mice which specifically proliferated to parasite antigens. The trypanocidal activity of BALB/c macrophages was induced upon culture with the A10, but not with the A28 T cell line. The cell-free supernatant from this A10 line, as well as from immune spleen cells stimulated with specific antigen or concanavalin A, but not from the A28 T cell line also activated the trypanocidal activity of peritoneal macrophages or of the J774 macrophage-like cell line. when the lymphokine content of the supernatants from both cell lines was analyzed, it was found that the A10 T cell line secreted interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha and interleukin 2, whereas the A28 line did not secrete IFN-gamma upon stimulation. Furthermore, the trypanocidal-inducing ability of A10 supernatant was completely abrogated by neutralizing anti-IFN-gamma antibodies and partially abrogated by neutralizing anti-TNF-alpha antibodies. When recombinant cytokines were added to J774 cells, IFN-gamma was able to induce significant trypanocidal activity whereas TNF-alpha was almost ineffective. However, TNF-alpha or lipopolysaccharide (LPS) showed a synergistic effect with IFN-gamma on macrophage activation. IFN-gamma triggered nitric oxide (NO) synthesis by J774 cells whereas TNF-alpha was almost ineffective. TNF-alpha and LPS were also synergistic with IFN-gamma in the NO production. Both the NO production and the trypanocidal activity in J774 cells induced by T cell supernatants or lymphokine combinations were inhibited by N-monomethyl-L-arginine, a competitive inhibitor of NO synthase activity. A good correlation between the levels of NO production and trypanocidal activity induced by different lymphokine preparations was found. Those results suggest that IFN-gamma and TNF-alpha, secreted by T. cruzi-immune T cells, are involved in the activation of the trypanocidal activity of mouse macrophages through an NO-dependent mechanism.

Cell cycle-dependent entry of Toxoplasma gondii into synchronized HL-60 cells

The degree of attraction of Toxoplasma gondii to vertebrate cells varies with cell type and cell phase. Human promyelocytic leukemia cells, HL-60, were synchronized by double thymidine block method and co-cultured with Toxoplasma for 1 hr at each cell stage to investigate the cell cycle specific susceptibility of parasites to host cells. For 30 hr the average number of Toxoplasma that invaded was a little changed except at 3 hr from G1/S phase boundary which concurred with the peak point of DNA synthesis. At 3 hr which is a relatively short interval compared to whole S phase, modification of cells by parasitic invasion was most remarkable. The number of Toxoplasma that penetrated was increased to more than six times. The shape of the cells became sludgy and almost indiscernible by strong accessibility of parasites only for an hour of mid-S phase. The same fluctuation was also observed at the second peak of S phase but weakly. This suggests that there be surface molecules concerning with the attachment of Toxoplasma to the host cells, which is expressed at special point of S phase. Further studies on the specific protein or similar molecules related could be carried out using synchronized HL-60 cells.

Strategies of obligate intracellular parasites for evading host defences

During the course of establishing infection in a susceptible host, obligate intracellular parasites evade host defence mechanisms before, during and after entry into host cells. Before entry they circumvent the lytic activity of the complement cascade, during cell entry they avoid being killed by toxic oxygen metabolites and after entry they escape nonoxidative killing mechanisms such as degradation by lysosomal hydrolases. Different intracellular parasites, exemplified here by Leishmania spp, Trypanosoma cruzi and Toxoplasma gondii, undermine host defences at each step by various strategies that ultimately ensure their targeting to, and survival in, an appropriate intracellular compartment.

Toxoplasma gondii: fusion competence of parasitophorous vacuoles in Fc receptor-transfected fibroblasts

After actively entering its host cells, the protozoan parasite Toxoplasma gondii resides in an intracellular vacuole that is completely unable to fuse with other endocytic or biosynthetic organelles. The fusion blocking requires entry of viable organisms but is irreversible: fusion competence of the vacuole is not restored if the parasite is killed after entry. The fusion block can be overcome, however, by altering the parasite's route of entry. Thus, phagocytosis of viable antibody-coated T. gondii by Chinese hamster ovary cells transfected with macrophage-lymphocyte Fc receptors results in the formation of vacuoles that are capable of both fusion and acidification. Phagocytosis and fusion appear to involve a domain of the Fc receptor cytoplasmic tail distinct from that required for localization at clathrin-coated pits. These results suggest that the mechanism of fusion inhibition is likely to reflect a modification of the vacuole membrane at the time of its formation, as opposed to the secretion of a soluble inhibitor by the parasite.

Macrophage oxidative metabolism and intracellular Toxoplasma gondii

We explored the mechanisms by which Toxoplasma gondii avoids destruction by the oxidative metabolism of normal macrophages. Unelicited murine peritoneal macrophages were infected with T. gondii and exposed to different experimental conditions. As endpoints we used measurement of hydrogen peroxide (H2O2) release and intracellular reduction of nitroblue tetrazolium dye (NBT). Three main observations were made. Firstly, different T. gondii preparations (live or dead, opsonized or not) failed to trigger the respiratory burst. Combined challenges also showed that a primary T. gondii infection was able to block H2O2 release triggered by heat-killed (HK)-Candida albicans. The H2O2 release, however, once triggered by HK-C. albicans, was not inhibited by a subsequent challenge with T. gondii. Secondly, when a respiratory burst was obtained in T. gondii-infected macrophages--for instance by stimulation with phorbol myristate acetate (PMA)--the toxic oxygen metabolites (as determined by the NBT reduction test) did not seem to reach the vacuoles containing the parasite. Thirdly, when a respiratory burst occurred in T. gondii-infected macrophages, the intracellular development of T. gondii did not seem to be affected. In conclusion, we hypothesize that T. gondii is not damaged by the macrophage oxidative metabolism because the parasite fails to encounter toxic oxygen metabolites. The killing of intracellular T. gondii, as it is commonly observed in activated macrophages, does not appear oxygen-dependent.

Induction of unresponsiveness to gamma interferon in macrophages infected with Mycobacterium leprae

We have previously demonstrated that Mycobacterium leprae-burdened granuloma macrophages isolated from infected nude mice are refractory to activation by gamma interferon (IFN-gamma). To explore further both the afferent and efferent functional capacity of M. leprae-infected macrophages, we examined the IFN-gamma-mediated activation of resident mouse peritoneal macrophages infected in vitro with live or dead M. leprae. When IFN-gamma was administered within 24 h of M. leprae infection, macrophages were fully activated. However, defective activation was evident at 3 to 5 days postinfection in macrophages that were heavily burdened with viable M. leprae. This defect was evident by four parameters of activation in which IFN-gamma failed to stimulate the enhancement of microbicidal activity, cytotoxicity for tumor target cells, O2- production, and surface Ia antigen expression. The development of defective activation closely followed an increase in macrophage production of prostaglandin E2. Defective activation of M. leprae-burdened macrophages was reversible by indomethacin, and a similar block in IFN-gamma activation was observed in three of these four parameters in normal macrophages treated with exogenous prostaglandin E2. Thus, infection of mouse macrophages with M. leprae appears to restrict IFN-gamma-mediated activation at least in part by induction of inhibitory levels of prostaglandin E2.

Nonoxidative microbicidal activity in normal human alveolar and peritoneal macrophages

Although Toxoplasma gondii multiplies within normal murine alveolar and peritoneal macrophages, it is killed by normal rat alveolar and peritoneal macrophages. The killing by rat macrophages is by a nonoxidative mechanism. Studies on normal human alveolar macrophages have reported disparate results in regard to their ability to inhibit or kill T. gondii. We considered it of interest to explore further the effect of normal human alveolar and peritoneal macrophages on T. gondii. Unstimulated alveolar macrophages from each of seven individuals demonstrated a marked ability to kill or inhibit multiplication of T. gondii in vitro (e.g., the number of parasites per 100 alveolar macrophages was 31 at time zero and 2 at 18 h, whereas this value increased from 37 at time zero to 183 at 18 h in murine macrophages assayed in parallel). In quantitative assays of superoxide, alveolar macrophages released a substantial amount of superoxide when exposed to phorbol myristate acetate or to candidae. In contrast, alveolar macrophages incubated with T. gondii released no more superoxide than when in medium alone. Scavengers of superoxide anions, hydrogen peroxide, singlet oxygen, and hydroxyl radicals failed to inhibit killing of T. gondii by alveolar macrophages. Peritoneal macrophages from each of six normal women undergoing laparoscopy killed T. gondii in vitro; results of quantitative superoxide assays and scavenger experiments demonstrated that no oxidative burst was triggered in these macrophages by exposure to T. gondii. These data indicate that normal human alveolar and peritoneal macrophages can kill an intracellular parasite by nonoxidative mechanisms and suggest that these mechanisms are important in inhibition or killing of other opportunistic intracellular pathogens.

Inhibition of the intracellular growth of Histoplasma capsulatum by recombinant murine gamma interferon

Recombinant murine gamma interferon as well as lymphokines prepared from immune splenocytes and concanavalin A-stimulated T-cell hybridoma activated normal mouse peritoneal macrophages to inhibit the intracellular growth of Histoplasma capsulatum. The activities of the lymphokine from immune splenocytes and of recombinant murine gamma interferon were neutralized by rabbit anti-murine gamma interferon antibody. The intracellular yeasts were not killed by the interaction even though growth was completely inhibited.

Activation by synergism between endotoxin and lymphokines of the mouse macrophage cell line J774 against infection by Trypanosoma cruzi

The mouse macrophage cell line J774 was easily infected by T. cruzi epimastigotes which were transformed to amastigotes that multiplied inside the cells. Spleen-T-cells from T. cruzi immune mice stimulated with Concanavalin A or T. cruzi, but not with unrelated antigens, released lymphokines into the supernatants that when added to J774 cells were unable to induce complete trypanocidal activity, although they were able to delay the rate of infection by protecting the cells from being infected. Addition of bacterial lipopolysaccharide (LPS), although inactive by itself, acted synergistically with the supernatants in inducing complete trypanocidal activity without affecting the susceptibility of J774 cells to infection. Gamma-interferon (gamma-IFN) activity was detected in the supernatants, however, but was not solely responsible for the trypanocidal inducing activities, since: there was no correlation between the levels of gamma-IFN and macrophage activation; gamma-IFN alone was less effective than the supernatants alone; and two active fractions of 100,000-150,000 mol. wt and 30,000 mol. wt were separated by gel filtration chromatography of the lymphokine preparations. The latter, which showed the characteristics of gamma-IFN with respect to size, pH 2 sensitivity and antiviral activity, had some trypanocidal activity alone. However, the 100,000-150,000 mol. wt fraction was active only in the presence of LPS. Finally, this trypanocidal inducing activity of the supernatants was not due to the induction of synthesis of gamma-IFN by the J774 cells.

Intracellular fate of Mycobacterium leprae in normal and activated mouse macrophages

Mycobacterium leprae replicates within mononuclear phagocytes, reaching enormous numbers in the macrophage-rich granulomas of lepromatous leprosy. To examine the capability of macrophages to digest M. leprae, we studied the intracellular fate of M. leprae organisms in normal and activated mouse macrophages by using the electron-dense secondary lysosome tracer Thoria Sol. Intracellular M. leprae organisms, surrounded by a characteristic electron-transparent zone, were contained within phagosomal vacuoles of macrophages cultured in vitro for 1 to 6 days. In normal macrophages, a majority of phagosomes containing freshly isolated live M. leprae cells resisted fusion with Thoria Sol-labeled lysosomes. The extent of fusion was not significantly affected by pretreatment of M. leprae with human patient serum high in specific immunoglobulin G and M antibodies. In contrast, a majority of phagosomes containing gamma-irradiated M. leprae cells underwent lysosome fusion in normal macrophages. In addition, increased phagolysosome fusion was observed with live M. leprae-containing phagosomes in macrophages activated with gamma interferon. Increased fusion was associated with an increase in the number of fragmented and damaged bacilli, suggesting that increased digestion followed fusion. This study indicates that activated macrophages may have an increased capacity for clearance of normally resistant M. leprae.

Mycobacterium leprae-burdened macrophages are refractory to activation by gamma interferon

Mycobacterium leprae grows to enormous numbers in the nu/nu mouse footpad, producing granulomas resembling those of lepromatous leprosy in humans. Footpad granuloma cells gorged with M. leprae were established in primary cell culture to examine their functional capabilities. These cells were classified as macrophages by the following criteria: positive staining for nonspecific esterase, reduction of Nitro Blue Tetrazolium during phagocytosis of Candida albicans, possession of Fc receptors, and possession of Mac-1 antigen. Footpad macrophages also phagocytized and supported the intracellular growth of Toxoplasma gondii. However, unlike peritoneal macrophages, footpad macrophages could not be activated to kill or inhibit T. gondii by macrophage-activating factor produced by mitogen-stimulated spleen cells or by recombinant gamma interferon. Thus, although the lepromatous macrophages appeared to be normal in many of their functions, they were defective in response to macrophage-activating signals.

Strain differences of interferon-generating capacity and resistance in toxoplasma-infected mice

To explore a possible correlation between susceptibility to Toxoplasma and interferon (IFN)-generating capacity in mice, we compared the levels of serum IFN induced by stimulation with Toxoplasma lysate antigen (TLA) in different strains of Toxoplasma-infected and uninfected mice. Injection of TLA into five strains of mice with chronic Toxoplasma infection resulted in the release of considerable amounts of IFN into the circulation. Most of these IFN activities were acid labile and not neutralized by sheep antiserum against mouse IFN-alpha/beta, indicating that IFN-gamma was the dominant form produced in this system. In contrast, the majority of IFN induced in uninfected mice was characterized as IFN-alpha/beta by their acid stability and antigenicity. The response of IFN production in Toxoplasma-infected and uninfected mice varied quantitatively depending on the mouse strains examined. C57BL/6 mice were found to be the best producers of both IFN-alpha/beta and IFN-gamma, while BALB/c mice were consistently poor producers of both IFN populations. A/J, DBA/2, and C3H/He mice could be roughly classified as intermediate producers of both IFN populations. C57BL/6 and C3H/He mice showed a significant prolongation of mean survival time following primary or secondary infection with Toxoplasma compared to that of BALB/c mice. However, there was no direct correlation between the susceptibility to Toxoplasma and the levels of serum IFN.

Toxoplasma modifies macrophage phagosomes by secretion of a vesicular network rich in surface proteins

Modification of macrophage phagosomes begins shortly after formation as Toxoplasma cells secrete membranous vesicles that form a reticulate network within the vacuole. The Toxoplasma-modified compartments then resist normal endocytic processing and digestion. We have used the pronounced Ca++-dependent stability of the intraphagosomal membrane (IPM) network to purify and characterize the structural proteins of this assembly. In addition to the structural matrix, Toxoplasma secretes a discrete set of soluble proteins, including a newly described 22-kD calcium-binding protein. The IPM network adheres to intact Toxoplasma cells after host cell lysis in the presence of 1 mM Ca++; however, the network readily disperses in calcium-free buffer and was purified as vesicles that sedimented at 100,000 g. Purified IPM vesicles were specifically recognized by immune sera from mice with chronic Toxoplasma infection and consisted primarily of a 30-kD protein when analyzed by SDS PAGE. IPM network proteins share a major antigenic component located on the surface of extracellular Toxoplasma cells as shown by immunoperoxidase electron microscopy using a polyclonal antibody prepared against the IPM vesicles. Moreover, in Toxoplasma-infected macrophages, anti-IMP antibody confirmed that the extensive IPM array contains proteins also found on the Toxoplasma cell surface. Our results indicate the IMP network represents a unique structural modification of the phagosome comprised in part of Toxoplasma surface proteins.

Oxygen-independent killing by alveolar macrophages

We have found that normal alveolar macrophages can kill an intracellular parasite by a mechanism that does not involve toxic metabolites of oxygen. We studied the interaction between Toxoplasma gondii and rat alveolar macrophages in vitro. We were interested in Toxoplasma because it causes pneumonia in immunosuppressed patients but not in healthy individuals, and we chose the rat because it resembles immunocompetent human subjects in being resistant to T. gondii. Resident rat alveolar macrophages could kill large numbers of T. gondii. This occurred without a respiratory burst as judged by intracellular reduction of nitroblue tetrazolium and quantitative release of superoxide. Furthermore, scavengers of toxic oxygen metabolites had no effect on the toxoplasmacidal activity of the alveolar macrophages, nor did prior exhaustion of their respiratory burst with PMA. Whereas acid pH (e.g., 4.5-6.0) rapidly kills extracellular T. gondii, raising of the intralysosomal acid pH of rat alveolar macrophages by incubating them with weak bases did not inhibit their ability to kill T. gondii. Killing of Toxoplasma occurred within 1 h of initial exposure to the alveolar macrophages. However, there was no evidence that killing preceded ingestion; Toxoplasma attached to the surface of the cell appeared viable, and when phagocytosis was blocked with sodium fluoride the organisms survived. These results indicate that rat alveolar macrophages possess a powerful nonoxidative microbicidal mechanism, which is distinct from acidification of the phagolysosome but which probably involves phagosome formation. This mechanism may be clinically relevant, for we have recently observed that human alveolar macrophages also kill T. gondii by an oxygen-independent process.

Superoxide dismutase and catalase in Toxoplasma gondii

Catalase and superoxide dismutase detected in both RH and C strain Toxoplasma gondii tachyzoites were distinctly different in electrophoretic mobility from host cell enzymes. Catalase and superoxide dismutase activity levels were similar in both Toxoplasma strains and showed narrow pH optima around 8.0. Toxoplasma superoxide dismutase was resistant to cyanide but inhibited by azide or peroxide, consistent with an iron-containing enzyme typical of protozoan parasites. These enzymes may play a role in intracellular survival; however, they do not appear to be the basis for differences in virulence to mice.