Production and properties of immune interferon from spleen cell cultures of Toxoplasma-infected mice

Over-expression screen of interferon-stimulated genes identifies RARRES3 as a restrictor of Toxoplasma gondii infection

Toxoplasma gondii is an important human pathogen infecting an estimated one in three people worldwide. The cytokine interferon gamma (IFNγ) is induced during infection and is critical for restricting T. gondii growth in human cells. Growth restriction is presumed to be due to the induction of interferon-stimulated genes (ISGs) that are upregulated to protect the host from infection. Although there are hundreds of ISGs induced by IFNγ, their individual roles in restricting parasite growth in human cells remain somewhat elusive. To address this deficiency, we screened a library of 414 IFNγ induced ISGs to identify factors that impact T. gondii infection in human cells. In addition to IRF1, which likely acts through the induction of numerous downstream genes, we identified RARRES3 as a single factor that restricts T. gondii infection by inducing premature egress of the parasite in multiple human cell lines. Overall, while we successfully identified a novel IFNγ induced factor restricting T. gondii infection, the limited number of ISGs capable of restricting T. gondii infection when individually expressed suggests that IFNγ-mediated immunity to T. gondii infection is a complex, multifactorial process.

Nanos gigantium humeris insidentes: old papers informing new research into Toxoplasma gondii

Since Nicolle, Manceaux and Splendore first described Toxoplasma gondii as a parasite of rodents and rabbits in the early 20th century, a diverse and vigorous research community has been built around studying this fascinating intracellular parasite. In addition to its importance as a pathogen of humans, livestock and wildlife, modern researchers are attracted to T. gondii as a facile experimental system to study many aspects of evolutionary biology, cellular biology, host-microbe interactions, and host immunity. For new researchers entering the field, the extensive literature describing the biology of the parasite, and the interactions with its host, can be daunting. In this review, we examine four foundational studies that describe various aspects of T. gondii biology, presenting a 'journal club'-style analysis of each. We have chosen a paper that established the beguiling life cycle of the parasite (Hutchison et al., 1971), a paper that described key features of its cellular biology that the parasite shares with related organisms (Gustafson et al., 1954), a paper that characterised the origin of the unique compartment in which the parasite resides within host cells (Jones and Hirsch, 1972), and a paper that established a key mechanism in the host immune response to parasite infection (Pfefferkorn, 1984). These interesting and far-reaching studies set the stage for subsequent research into numerous facets of parasite biology. As well as providing new researchers with an entry point into the literature surrounding the parasite, revisiting these studies can remind us of the roots of our discipline, how far we have come, and the new directions in which we might head.

Cell-autonomous immunity to Toxoplasma gondii in mouse and man

The protozoan, Toxoplasma gondii, is a natural pathogen of mouse and a zoonosis of man. Immunity against the pathogen is largely mediated by interferon-stimulated cell-autonomous mechanisms that are strikingly different between man and mouse. There are many poorly understood host and pathogen variables that affect the outcome of infection.

Enhancement by recombinant human interleukin 2 of host resistance to Toxoplasma gondii infection in pregnant mice

The lymphokine production by pregnant mice infected with a lethal dose of Toxoplasma gondii was evaluated in comparison with that by virgin mice infected with a sublethal dose of this protozoan parasite. Splenocytes taken from mice before and on the day after infection produced considerable amounts of IL-2 in response to concanavalin A (Con A) stimulation, but the titers rapidly declined in both pregnant and virgin mice as infection progressed. A trace amount or undetectable level of IL-2 was produced by splenocytes from acutely infected mice when stimulated with Toxoplasma lysate antigen (TLA). In contrast to the kinetics of IL-2 production, the levels of IFN-gamma produced by splenocytes cultured with Con A or TLA increased steadily in the later stage of infection in both pregnant and virgin mice. Thus, the response to Con A or TLA of splenocytes to produce IL-2 and IFN-gamma differed strikingly in acute toxoplasmosis in mice. The administration of rHuIL-2 resulted in a significant decrease in the mortality of pregnant mice infected with a lethal dose of Toxoplasma. The combination of rHuIL-2 and rMuIFN-gamma increased the survival rate slightly but not significantly compared with pregnant mice receiving either rHuIL-2 or rMuIFN-gamma. Moreover, exogenously administered rHuIL-2 enhanced the production of both IFN-alpha and IFN-gamma in the bloodstreams of pregnant mice, in accordance with the decreased mortality. These results indicate that IL-2 may play a significant role in modulating the host defense against Toxoplasma infection in pregnant mice.

Correlation between increased susceptibility to primary Toxoplasma gondii infection and depressed production of gamma interferon in pregnant mice

To explore a possible mechanism of pregnancy-associated suppression of T cell-mediated immunity to Toxoplasma gondii, acquired resistance and gamma interferone (IFN-gamma) production in pregnant mice were compared with those in virgin mice after infection with the S-273 strain of this protozoan parasite. The 50% lethal dose of this strain was less than 200 tachyzoites for pregnant mice and 2,800 organisms for virgin controls. Toxoplasma-induced production of both IFN-alpha and IFN-gamma in the bloodstream of pregnant mice was significantly depressed as compared with that in virgin controls. The administration of recombinant murine IFN-gamma (rMuIFN-gamma) resulted in a significant decrease of mortality and parasitic growth in the organs of pregnant mice infected with a lethal dose of S-273 strain tachyzoites. Thus, the impairment of T cell-mediated immune responses was evident in pregnant mice from the impaired IFN-gamma-generating capacity and poor survival rate after primary infection with Toxoplasma. When mice with chronic Toxoplasma infection were injected with specific antigen, the resultant production of IFN-gamma was also significantly suppressed during pregnancy. However, there was no direct correlation between the serum levels of IFN-gamma and susceptibility to reinfection, since the mortality rate of chronically infected pregnant mice after the challenge with the high virulent RH strain was not significantly higher than that of virgin controls.

Role of beta interferon in resistance to Toxoplasma gondii infection

The role of recombinant murine beta interferon (rMuIFN-beta) and recombinant human IFN-beta (rHuIFN-beta) in resistance to Toxoplasma gondii was examined. rMuIFN-beta protected mice against a lethal infection with the parasite. The protective effect appeared to depend on the concomitant release of gamma interferon. rMuIFN-beta did not activate murine peritoneal macrophages to inhibit or kill T. gondii whether used alone or in combination with lipopolysaccharide (LPS). rHuIFN-beta did not activate human monocyte-derived macrophages to inhibit or kill T. gondii when 5-day-old monocyte-derived macrophages were used. In contrast, significant killing of T. gondii was noted when 10-day-old monocyte-derived macrophages were used. The addition of LPS enhanced this effect. These results revealed a role for IFN-beta in the mechanisms of defense against T. gondii and suggest its potential use in the treatment of toxoplasmosis in humans.

CD8+ T cells are the major lymphocyte subpopulation involved in the protective immune response to Toxoplasma gondii in mice

The ability of the major T cell subsets to adoptively transfer resistance to T. gondii infection was studied. Spleen cells harvested from mice with a 3-month T. gondii infection and cells from uninfected mice were enriched for T cells by nylon/wool purification. Adoptive transfer of these cells from both groups of donor mice led to a significant increase in the survival of syngeneic recipient mice infected intraperitoneally with 20 T. gondii cysts. Increased survival was mediated particularly by CD4-depleted but also, to a lesser extent, CD8-depleted subpopulations. These results were confirmed in T cell reconstituted athymic nude mice. Unfractionated T cells from chronically infected donors produced a significant inhibition of cyst formation in the brains of recipient mice measured 10 weeks after infection compared with control mice. The inhibition of cyst formation was ablated by pretreating T cells with anti-CD8 antibody and complement, but not anti-CD4 antibody and complement. Mice receiving cells from infected donors produced an early increase in their IgG1 and IgG2a antibody titres compared with mice given cells from uninfected animals. The depletion of either CD8+ or CD4+ immune cells appeared to have little effect on the antibody responses in recipient mice and there was no correlation between antibody levels and immunity. The results indicate that CD8+ T lymphocytes from convalescent T. gondii-infected BALB/c mice are the principal mediators of resistance to T. gondii, although CD4+ T cells appear to be involved during the acute phase of infection.

Role of gamma interferon in Toxoplasma gondii infection

Toxoplasma gondii has emerged as an important pathogen in the ever increasing numbers of patients with disorders of the immune system. Better understanding of the mechanisms of resistance of the host against this protozoan is important for development of safe, effective alternative treatment regimens for toxoplasmosis. Gamma interferon is the cytokine that plays a central role in protection against Toxoplasma gondii. The purpose of this review is to highlight the current knowledge of the role of gamma interferon in Toxoplasma gondii infection.

Protection of nude rats against Toxoplasma infection by excreted-secreted antigen-specific helper T cells

In the present work we demonstrate the implication of excreted-secreted antigens in eliciting the protective cell-mediated immunity developed by rats toward Toxoplasma gondii. We first showed that 10(4) specific T cells from T. gondii-infected rats conferred to nude rats the ability to resist an infection by the highly virulent RH strain of T. gondii. In a second series of experiments, the role of excreted-secreted antigens in this protection was demonstrated. After the adoptive transfer to nude rats of various doses (10(3), 10(4), 10(5)) of excreted-secreted antigen-specific helper T cells (propagated in vitro during one month), significant protection toward T. gondii was induced. Moreover, these cells were responsible for a specific antibody response in nude rats, which are normally unable to develop any specific humoral response. The specificity of these antibodies was directed toward different molecules with molecular masses of 104, 97, 57, 39, 30, 21, and 18 kilodaltons; some of these have been previously characterized as major excreted-secreted antigens.

Induction of antigen-specific human cytotoxic T cells by Toxoplasma gondii

To further the understanding of the role of T cells in immunity to the parasite Toxoplasma gondii, antigen-specific T cell clones were generated using peripheral blood mononuclear cells from seropositive individuals. Whole parasites were used to stimulate a proliferative expansion of antigen-reactive cells, followed by limiting dilution cloning in the presence of irradiated, autologous PBMC and rIL-2. Three parasite antigen-specific T cell clones expressing the CD3+ phenotype were selected for further characterization. Phenotypic analysis with monoclonal antibodies revealed two clones reactive with CD8 (RTg1 and RTg3) while the other (RTg2) phenotyped as CD4+, CD8-. When tested in a proliferation assay using a panel of different T. gondii proteins, clone RTg1 reacted with a single large protein (Mr greater than 180,000) as well as smaller components (less than 12,000), clone RTg2 reacted with a protein of Mr = 28,000 and clone RTg3 reacted with a protein of 116,000 plus smaller components (less than 12,000). Only the 28,000 = Mr antigen recognized by RTg2 was reactive on Western blot with autologous donor antisera. All three clones produced IFN-gamma and IL-2 in varying amounts upon antigenic stimulation in the presence of irradiated APC. Moreover, one clone RTg1, exhibited direct parasite cytotoxicity, inhibiting extracellular T. gondii by greater than 70% when incubated at an effector/target ratio of 40:1. This clone was alpha, beta TCR heterodimer positive and exerted its cytotoxic parasiticidal activity in the apparent absence of MHC restriction. The results provide evidence for the existence of circulating antigen-specific cytotoxic T cells in normal humans who are toxoplasma antibody seropositive.

Studies on ovine efferent lymph following infection with Toxoplasma gondii

Cell output was monitored in efferent popliteal lymphatics of sheep following subcutaneous injection into the lateral tarsus of either 100 T. gondii tissue cysts or control inoculum. Sheep undergoing a primary infection developed a substantially increased lymphoblast output which reached a peak of more than 50 per cent of all cells. The majority of lymphoblasts did not contain Ig and may have been T cells. IgG antibody against T. gondii was detected in lymph by day 9. Animals with naturally acquired antibody to T. gondii developed a more rapid lymphoblast response to the parasite which, at its peak, accounted for 27.5 per cent of all cells. Many of these lymphoblasts contained Ig, presumably specific antibody, which, by increasing phagocytosis and intracellular killing of the T. gondii, would prevent multiplication and dissemination of the parasite.

Specific and nonspecific mediation of protective immunity to Toxoplasma gondii

We studied the specificity of protection conferred by Toxoplasma gondii immune lymphocytes and their supernatants on infected hamster kidney cells, using Besnoitia jellisoni immune lymphocytes as a nonspecific control. The intracellular growth of the organisms was measured by [3H]uracil incorporation, and inhibition of multiplication was used as a measurement of immunity. Although the immune lymphocytes restricted principally the multiplication of homologous organisms, partial protection, expressed against the heterologous organism, was found. This was true for either parasite with intact lymphocytes or their supernatants. Exposure of immune lymphocytes to antigen for 18 to 24 h and treatment of kidney cells with supernatant fluids for 18 to 24 h were required for maximal protection. The specific protective mediator in supernatants of immune lymphocytes was characterized by dialysis as having a molecular weight between 3,000 and 12,000 and was found in the 3,000 to 5,000 peak after Sephadex G-50 chromatography. Nonspecific protective activity was greater than 12,000 by dialysis; it chromatographed in the excluded peak, measuring over 43,000, and was destroyed by exposure to pH 2. In vitro production of lymphokines from toxoplasma immune lymphocytes was first detected 7 to 10 days after vaccination of hamsters. At about the same time, hamsters began to resist challenge infection with the pathogenic RH strain of T. gondii and were able to prevent its multiplication in lungs, liver, spleen, and the subcutaneous infection site. The expression of tissue immunity and the production of toxoplasma-immune lymphokines appear to be time-related events.

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.

Spleen and lymph node cell populations, in vitro cell proliferation and interferon-gamma production during the primary immune response to Toxoplasma gondii

An animal model for the study of transient lymphadenopathy-splenomegaly during toxoplasmosis is presented. Injection of CBA/J mice with the low virulent, cyst-forming strain of Toxoplasma gondii (Pe strain) induces a three to four fold increase in weight and cellularity of spleen and lymph nodes with peak changes at 30-50 days after infection. The spleen displays marked haemopoiesis, a 30 fold increase in mononuclear phagocytes, and a two fold increase in Lyt2+ lymphocytes. Lymph nodes show a five fold increase in mononuclear phagocytes and a four and a half fold increase in Lyt2+ T cells. The increase in mononuclear phagocytes significantly alters T cell/macrophage ratios and this is associated with decreases in in vitro cell proliferation to mitogen and toxoplasma antigen. The relationship between alterations in cell balance of mononuclear phagocytes and T cell subsets and the expression of transient immune dysfunction can now be examined by modulating changes in these cell types.

Decreased production of interferon-gamma by human neonatal cells. Intrinsic and regulatory deficiencies

Human neonatal lymphocytes produced little macrophage activation factor in response to mitogens. This correlated with decreased production of interferon-gamma (IFN gamma): adult lymphokines contained 894.2 +/- 177.1 U/ml, whereas neonatal cord and peripheral lymphokines contained 66.9 +/- 17.0 and 116.7 +/- 29.6 U/ml by bioassay. Results by radioimmunoassay (RIA) for IFN gamma were similar. In contrast, the interleukin 2 content of cord lymphokines was greater (P less than 0.01) and that of neonatal peripheral blood lymphokines similar to that of adults. Interleukin 1 production and interleukin 2 receptor expression and affinity were similar for adult and neonatal cells. Interleukins 1 and 2 in amounts comparable to those in adult lymphokines did not increase production of macrophage activation factor or IFN gamma by neonatal cells. Neonatal cells did not contain intracellular IFN or degrade exogenous IFN. Excess suppressor activity was not found in neonatal cultures. Addition of IFN alpha, 10,000-50,000 U/ml of interleukin 2 or phorbol myristate acetate (PMA) to cord mononuclear cells or of adult monocytes or PMA to cord T cells increased IFN gamma production compared to cells stimulated with concanavalin A (ConA) alone. Nevertheless, under optimal conditions (T cells + PMA + Con A), adult cells produced much more IFN gamma (1,360 +/- 261 U/ml by RIA) than cord cells (122 +/- 37 U/ml). Staphylococcal enterotoxin A (SEA) stimulated cord cell IFN gamma production at low cell densities; nevertheless, adult cells produced more IFN in response to SEA 1,341 +/- 350 U/ml) than cord cells (350 +/- 33 U/ml). Decreased production of IFN gamma by neonatal cells appears to be due both to differences in their intrinsic capacity to produce IFN gamma and to differences in regulatory mechanisms.

Immunologic basis for increased susceptibility of the neonate to infection

Neonatal infection with the GBS occurs in a small fraction of exposed infants who lack specific antibody. Diminished influx of PMNs to sites of infection as a result of abnormalities in chemotaxis, bone marrow exhaustion, and to a lesser degree relative complement deficiency and decreased microbicidal activity of PMNs may be additional predisposing factors. Infection with HSV occurs more often in infants born to mothers with primary rather than secondary infection; the lack of passively acquired antibody in such infants is a possible but unproved susceptibility factor. The failure of neonates to control HSV may also be related to decreased production of or response to interferon or to decreased activity of nonimmune and immune cellular cytotoxic mechanisms. Similarly, infection with Toxoplasma and intracellular bacterial pathogens, such as Listeria, may be more severe because of the decreased generation of lymphokines and interleukins, which attract macrophages to the site of infection and enable them to kill these organisms. Much of this analysis based on in vitro and animal studies summarizes current information in a rapidly changing field rather than stating established fact. The precise age at which most of the immune functions discussed reach maturity is unknown. However, the risk of severe infection with these pathogens appears to wane by 2 to 3 months of age. Although this may partly reflect decreased exposure, we might hypothesize that immune functions that are mature by this age are those most critical for protection. Future studies focusing on changes in immune function during the first months of life may provide useful insights into the immunobiology of these diseases and direct attention to the most fruitful areas for immunologic intervention.

Lymphokine-induced inhibition of growth of Eimeria bovis and Eimeria papillata (Apicomplexa) in cultured bovine monocytes

Sporozoites of Eimeria bovis penetrated and developed normally to first-generation meronts in bovine monocytes (BM) and Madin-Darby bovine kidney (MDBK) cells that had been pretreated with culture medium (CM) or supernatant (NS) from nonstimulated bovine T cells. At 240 h after sporozoite inoculation (ASI), the mean percent development (meronts/[sporozoites + meronts]) in CM- and NS-pretreated BM was 52 and 28%, respectively; values for MDBK cells were 36 and 35%, respectively. Pretreatment of BM and MDBK cells with supernatant (ConAS) from concanavalin A-stimulated bovine T cells had no effect on the ability of sporozoites to penetrate cells; however, at 240 h ASI, only 1% of the sporozoites in ConAS-pretreated BM cultures had developed to meronts. In contrast, ConAS had no adverse effect on the ability of E. bovis sporozoites to develop to first-generation meronts in MDBK cells. At 240 h ASI, E. bovis meronts in ConAS-pretreated BM were abnormal in appearance and retarded in development, whereas sporozoites appeared structurally normal by light microscopy. Pretreatment of BM with ConAS had no effect on the ability of sporozoites of Eimeria papillata (Apicomplexa) to penetrate cells. Sporozoites of E. papillata did not develop to meronts in ConAS-pretreated BM and, in contrast to E. bovis, most sporozoites were destroyed intracellularly.

Contribution of immune interferon (IFN-gamma) in lymphokine-induced anti-toxoplasma activity: studies with recombinant murine IFN-gamma

Recombinant E. coli-derived murine interferon gamma (cDNA IFN-gamma) per se induced resident mouse peritoneal macrophages (MPM) and mouse embryo cells to exert marked antitoxoplasma activity. This capacity of cDNA IFN-gamma was abrogated by a specific antiserum to cDNA IFN-gamma which could only neutralize the antiviral activity mediated by this product, whereas a rabbit antiserum directed against murine IFN-alpha/beta proved ineffective in neutralizing these functions. It has been found that rabbit antiserum to cDNA IFN-gamma could also neutralize IFN-gamma-mediated antiviral activity present in crude lymphokine-enriched supernatants of antigen-stimulated toxoplasma-sensitized spleen cells (Toxo-LK) but proved ineffective in abolishing the capacity of Toxo-LK to trigger macrophage anti-toxoplasma activity. The data obtained suggest that macrophage anti-toxoplasma activity induced by Toxo-LK may be an interplay of multiple factor(s) and that Toxo-LK preparations contain soluble factor(s) other than IFN-gamma, which can induce macrophages to kill intracellular Toxoplasma. Experiments in which crude Toxo-LK preparations were incubated with lectin concanavalin A (Con A) showed that this treatment resulted in a block of anti-toxoplasma arming factor(s) activity, as well as a significant reduction of IFN-gamma-mediated antiviral activity present in Toxo-LK. By contrast, no significant difference was observed in the macrophage anti-toxoplasma activity mediated by Con A or untreated cDNA-IFN-gamma.

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

In vitro activation of macrophage cell line J774G8 and mouse peritoneal macrophages resulted in oxygen-dependent and oxygen-independent killing of intracellular Toxoplasma gondii. Activation was characterized by oxygen-dependent killing detectable by enhanced lysosome fusion and digestion of T. gondii. The toxoplasmacidal activity of activated J774G8 cells and peritoneal macrophages was prevented by adding the oxygen intermediate scavengers catalase or superoxide dismutase during culture. Activated J774G8 cells and peritoneal macrophages also inhibited replication of those Toxoplasma organisms which survived the initial microbicidal activity. The inhibition of Toxoplasma replication was not significantly affected by exogenous catalase or superoxide dismutase. Peritoneal macrophages from Toxoplasma-immune mice showed similar microbicidal and inhibitory responses, supporting the model that activation leads to destruction of intracellular parasites by two different mechanisms.

Efficient activation of human blood monocytes to a tumoricidal state by liposomes containing human recombinant gamma interferon

Human recombinant gamma interferon (INF-gamma) activated human peripheral blood monocytes to a cytotoxic state capable of lysing adherent tumorigenic cells without harming normal cells. The efficiency of INF-gamma activation of monocytes is enhanced by encapsulating INF-gamma within liposomes: The minimum effective dose (MED) of free INF-gamma for monocyte activation was found to be 1-10 U/ml, per 10(5) monocytes, whereas the minimum dose for IFN-gamma encapsulated in liposomes was less than 0.0025 U. Monocytes treated with liposome-encapsulated INF-gamma retained their cytotoxic phenotype for much longer than do monocytes treated with free INF-gamma. Since liposomes can be passively targeted to cells of the reticuloendothelial system following IV administration, these findings suggest that liposome-encapsulated INF-gamma may have therapeutic potential that should be evaluated in vivo.

Specific mediation of cellular immunity to Toxoplasma gondii in somatic cells of mice

Lymphocytes from mice immunized against Toxoplasma gondii protected T. gondii-infected macrophage and kidney cell cultures. After contact with antigens, supernatants of such immune lymphocytes, also contained a factor protective for T. gondii-infected macrophages and kidney cells. Supernatants were protective only when the lymphocytes and kidneys cells were isogeneic. Protection was specific in that supernatants from only T. gondii-immune, but not Besnoitia jellisoni-immune, lymphocytes provided protection against toxoplasmosis. Sixteen to 24 h were required for an appreciable amount of protective factor to be secreted; a similar absorption time was necessary for kidney cells to be protected. Peritoneal lymphocyte lysates, prepared as transfer factor, contained protective substances with a potency similar to that of lymphocyte supernatants, which were also strain restricted in their effect.

Interferon-gamma inhibits growth of Coxiella burnetii in mouse fibroblasts

We studied the effects of various mouse interferon preparations on the growth of Coxiella burnetii in mouse fibroblasts. The addition of both recombinant interferon-gamma and a crude lymphokine preparation that contained interferon-gamma to infected L929 cells inhibited the growth of C. burnetii, whereas the addition of a crude preparation of type I interferons did not. Cycloheximide suppressed the inhibitory effects of recombinant interferon-gamma and crude lymphokines.

Inhibition of growth of Toxoplasma gondii in cultured fibroblasts by human recombinant gamma interferon

The growth of Toxoplasma gondii in cultured human fibroblasts was inhibited by recombinant human gamma interferon at concentrations of 8 to 16 U/ml. The interferon was titrated by observing a total inhibition of parasite plaque formation 7 days after infection. Inhibition of the growth of T. gondii in the early days after infection was measured by marked reductions in the incorporation of radioactive uracil, a precursor that can only be used by the parasites. This assay showed that when cells were pretreated with gamma interferon for 1 day and then infected, inhibition of T. gondii growth could be readily detected 1 or 2 days after infection. When the pretreatment was omitted and parasites and gamma interferon were added at the same time, no inhibition of parasite growth could be detected 1 day later, although it was apparent after 2 days. Cultures from which the gamma interferon had been removed by washing after a 1-day treatment showed inhibition of T. gondii growth. Gamma interferon had no effect on the viability of extracellular parasites, but it did inhibit the synthesis of host cell RNA and protein by ca. 50% 3 days after treatment. This degree of inhibition is unlikely, of itself, to compromise the growth of T. gondii. Recombinant alpha and beta interferons had no effect on the growth of T. gondii.

Analysis of the roles of immune interferon (IFN-gamma) and colony-stimulating factor(s) in the induction of macrophage anti-toxoplasma activity

Lymphokine-enriched, cell-free supernatants from specific antigen-stimulated spleen cells of toxoplasma-immune mice lacking detectable anti-toxoplasma antibody could generate effective anti-toxoplasma activity within normal (non-immune) murine peritoneal macrophages. Such supernatants also contained high levels of IFN-gamma as well as Ia-antigen(s) inducing activity. Supernatants from ConA stimulated normal (non-immune) spleen cells with high IFN-gamma, as well as CSF-containing supernatants from lung explants, lacked the capacity to induce anti-toxoplasma activity within peritoneal macrophages. ConA-stimulated spleen cell supernatants with high IFN-gamma titers but not CSF-enriched lung explant supernatants could induce the expression of macrophage cell surface Ia-antigen(s). Based on the results of our experiments, we have been able to eliminate the direct (by itself) role of IFN-gamma or CSF in generating macrophage anti-toxoplasma activity. However, the possibility that molecules like IFN-gamma or CSF synergize together with other immune spleen cell-derived factor(s) in the generation of effective macrophage anti-toxoplasma activity has not been ruled out.

Interferon gamma blocks the growth of Toxoplasma gondii in human fibroblasts by inducing the host cells to degrade tryptophan

Treatment of human fibroblasts with human recombinant gamma interferon blocked the growth of Toxoplasma gondii, an obligate intracellular protozoan parasite. Growth of the parasite was measured by a plaque assay 7 days after infection or by the incorporation of [3H]uracil 1 or 2 days after infection. The antitoxoplasma activity induced in the host cells by gamma interferon was strongly dependent upon the tryptophan concentration of the medium. Progressively higher minimal inhibitory concentrations of gamma interferon were observed as the tryptophan concentration in the culture medium was increased. Treatment with gamma interferon did not make the cells impermeable to tryptophan. The kinetics of [3H]tryptophan uptake into the acid-soluble pools of control and gamma interferon-treated cultures were identical during the first 48 sec. Thereafter uptake of [3H]tryptophan into the acid-soluble pool of control fibroblasts reached the expected plateau after 96 sec. In contrast, uptake of [3H]tryptophan continued for at least 12 min in the gamma interferon-treated cultures. At that time, the acid-soluble pool of the gamma interferon-treated cultures contained 8 times the radioactivity of the control cultures. This continued accumulation was the result of rapid intracellular degradation of [3H]tryptophan into kynurenine and N-formylkynurenine that leaked slowly from the cells. These two metabolites were also recovered from the medium of cultures treated for 1 or 2 days with gamma interferon. Human recombinant alpha and beta interferons, which have no antitoxoplasma activity, did not induce any detectable degradation of tryptophan. Several hypotheses are presented to explain how the intracellular degradation of tryptophan induced by gamma interferon could restrict the growth of an obligate intracellular parasite.

Markedly enhanced production of gamma interferon in murine T lymphocytes treated with lentil lectin and the diterpene ester, mezerein

Gamma interferon (IFN-gamma) was induced in murine splenocytes first stimulated to grow by concanavalin A (Con A) and subsequently treated for 3 h with the diterpene ester, mezerein (MZN) and then with lectin from Lens culinaris for 24 h. Yields as high as 60,000 u/ml were obtained in cells from either male or female, random-bred, white Swiss mice or inbred C67B1/6 mice. Antibody to Thy 1.2 surface antigen completely obliterated the mouse gamma interferon (MuIFN-gamma) response, whereas anti-Lyt 1.2 and anti-Lyt 2.2 each destroyed a portion of the lymphocyte population responsible for MuIFN-gamma production. Kinetic analysis of production and release showed that IFN was detectable in culture fluids within 4 h after treatment with very little IFN remaining cell-associated (less than 10%). A simple, rapid, and economical two-step purification procedure involving ammonium sulfate fractionation and yeast RNA affinity chromatography resulted in as much as 770-fold purification to achieve specific activities greater than 10(7) u/mg protein. The purified MuIFN-gamma was shown to be predominantly acid-labile, inactivated by sodium dodecyl sulfate (SDS), and neutralized by antiserum to MuIFN-gamma. Approximately 10% of the MuIFN-gamma was acid-stable and SDS-resistant, but was still neutralized by anti-MuIFN-gamma serum. Two molecular weight peaks of about 40 and 20 kD were demonstrated by SDS-polyacrylamide gel electrophoresis. Isoelectric focusing in polyacrylamide slab gels gave a relatively heterogeneous band of activity between pH 5.5 and 6.5. The mechanism by which the combination treatment described enhances MuIFN-gamma production so markedly remains unknown, but the degree of enhancement is greater than additive.

Species and strain differences in sensitivity to Toxoplasma infection among laboratory rodents

Different susceptibilities to Toxoplasma infection were shown among various inbred and outbred laboratory rodents. On the basis of several criteria, the animals can be divided into three groups: (i) highly sensitive (S) mice, Syrian hamsters and Chinese hamsters; (ii) moderately sensitive (M) Mongolian gerbils and mastomyses ; and (iii) resistant (R) rats and Korean striped field mice. S and M hosts supported intraperitoneal growth of the parasites and died of acute infection. M hosts showed prolonged time to death and low fatality as compared with S hosts. Some R hosts died only when they had received an extremely large size of inoculum, accompanying poor intraperitoneal proliferation of the parasite. WM/Ms and WKY rats markedly resisted the infection. These hosts were considered to be useful models for studying infection of and immunity to toxoplasmosis.

Comparison of the properties of antirickettsial activity and interferon in mouse lymphokines

Certain properties of the antirickettsial activity and interferon in lymphokine preparations obtained from concanavalin A-stimulated mouse spleen cells were compared. Both the antirickettsial activity and interferon were relatively stable to heating at 56 degrees C, whereas both activities were destroyed by trypsin, by heating at 80 degrees C, or by exposure to pH 2 for 24 h. Both activities were likewise inhibited after incubation with rabbit antisera to partially purified murine interferon-gamma. In contrast to the mouse lymphokine preparations, which contained both interferon-gamma and antirickettsial activity, a preparation of virus-induced interferons (type I) had no detectable antirickettsial activity. Human foreskin fibroblasts, which were not sensitive to the antirickettsial activity in mouse lymphokines, acquired the ability to inhibit rickettsial growth when they were cocultured with sensitive mouse L929 cells treated with mouse lymphokines. These results are consistent with the idea that the antirickettsial activity in mouse lymphokines is due to interferon-gamma.

Killing of intracellular Leishmania donovani by lymphokine-stimulated human mononuclear phagocytes. Evidence that interferon-gamma is the activating lymphokine

We have found that the crude lymphokines, which prime the human monocyte-derived macrophage to generate H2O2 and exert microbicidal activity against intracellular Leishmania donovani, are rich in interferon (IFN)-gamma (600-3,000 U/ml). To determine the role of this specific lymphocyte product in macrophage activation, lymphokines were pretreated with a monoclonal antibody that neutralizes human IFN-gamma. Antibody exposure completely abolished the capacity of both mitogen- and antigen-stimulated lymphokines to either enhance macrophage H2O2 release or induce leishmanicidal activity. In addition, partially purified and pure recombinant human IFN-gamma were as effective as crude lymphokines in activating macrophages, and 3 d of treatment with 300 U/ml resulted in a seven- to eightfold increase in H2O2 generation and the intracellular killing of both L. donovani promastigotes and amastigotes. The ability of crude lymphokines to induce monocytes and macrophages from a patient with chronic granulomatous disease to kill L. donovani promastigotes was similarly abrogated by anti-IFN-gamma antibody, and could also be achieved by IFN-gamma alone. These results suggest that IFN-gamma is the key macrophage-activating molecule present within human lymphokines, and indicate that IFN-gamma can enhance both the oxygen-dependent and -independent antiprotozoal mechanisms of human mononuclear phagocytes.

Identification of interferon-gamma as the lymphokine that activates human macrophage oxidative metabolism and antimicrobial activity

Human blood mononuclear leukocytes stimulated with toxoplasma antigen, concanavalin A, mezerein plus lentil lectin, or staphylococcal enterotoxin A secreted a factor (macrophage-activating factor, or MAF) that enhanced the capacity of human macrophages to release H2O2 and to kill toxoplasmas. The same lymphoid supernatants contained IFN gamma but not IFN alpha or IFN beta. The MAF activity of six of seven unfractionated supernatants was completely eliminated by a monoclonal antibody that neutralizes IFN gamma, and MAF in the remaining supernatant was almost completely neutralized. Native IFN gamma partially purified by two independent protocols to specific activities of 1 X 10(6) and 10(7) U/mg protein was enriched in MAF activity at least as much as in antiviral activity. The capacity of macrophages to secrete H2O2 after incubation in partially purified native IFN gamma (mean peak stimulation, 8.8-fold) was greater than with unpurified lymphokines (3.8-fold) and sometimes equaled or exceeded the capacity of freshly harvested monocytes. The MAF activity of the partially purified native IFN gamma preparations was abolished by monoclonal anti-IFN gamma. Finally, IFN gamma of greater than 99% estimated purity was isolated (at Genentech, Inc.) from bacteria transformed with the cloned human gene for this lymphokine. Recombinant IFN gamma had potent MAF activity, stimulating the peroxide-releasing capacity of macrophages an average of 19.8-fold at peak response and enhancing their ability to kill toxoplasmas from 2.6 +/- 1.3% for untreated cells to 54 +/- 0.4% for treated cells. Attainment of 50% of the maximal elevation in peroxide-releasing capacity required a geometric mean concentration of 0.1 antiviral U/ml of recombinant IFN gamma, which is estimated to be approximately 6 picomolar for this preparation. Peroxide secretory capacity and toxoplasmacidal activity of macrophages peaked 2-4 d after exposure to IFN gamma. Peroxide-secretory capacity remained elevated during at least 6 d of continuous exposure, but the effect of IFN gamma was reversed within about 3 d of its removal. Activation was usually but not invariably accompanied by characteristic changes in cell morphology. Thus, IFN gamma activates human macrophage oxidative metabolism and antimicrobial activity, and appeared to be the only factor consistently capable of doing so in the diverse LK preparations tested.

Inhibition of the growth of Rickettsia prowazekii in cultured fibroblasts by lymphokines

The effect of lymphokine treatment of mouse and human fibroblast cell lines on the growth of Rickettsia prowazekii within the fibroblasts was studied. Treatment of mouse L929 cells with concanavalin A- or antigen-induced mouse lymphokines both before and after infection with R. prowazekii led to clearance of the rickettsiae from a substantial proportion of the cells and suppression of rickettsial growth in those cells which remained infected. Similar but less dramatic anti-rickettsial effects were observed in L929 cells treated with mouse lymphokines either only before or after infection with rickettsiae. Mouse lymphokine treatment of L929 cells had similar anti-rickettsial effects on the avirulent E strain and the virulent Breinl strain of R. prowazekii. Addition of cycloheximide or emetine to L929 cells at the same time as the lymphokines markedly suppressed the inhibition of rickettsial growth by the lymphokines. Mouse lymphokine treatment inhibited rickettsial survival and growth in mouse 3T3-A31 cells as well as in mouse L929 cells, but had no effect on rickettsial survival and growth in human foreskin fibroblasts. Conversely, concanavalin A-induced human lymphokines inhibited rickettsial survival and growth in human foreskin fibroblasts but had no effect on rickettsial survival and growth in mouse L929 cells. The rickettsia inhibitory activity in concanavalin A-induced mouse lymphokines was destroyed by heating the lymphokines at 80 degrees C for 10 min or by holding the lymphokines at pH 2 for 24 h but was retained after heating at 56 degrees C for 30 min.