Gut-derived intraepithelial lymphocytes induce long term immunity against Toxoplasma gondii

Intraepithelial lymphocytes (IEL) of the intestine represent an important barrier in the prevention of infection against orally acquired pathogens. Adoptive transfer of Ag-primed IEL into a naive host can protect against challenge. Using a murine model, we demonstrate in two genetically distinct mouse strains (C57BL/6 and CBA/J) that protective IEL can be isolated at specific times after oral infection with cysts containing bradyzoites. Adoptive transfer of IEL obtained from the intestine of infected mice at these specific times can provide long term protection, as determined by mortality and cyst number against challenge. The protective IEL appear to be CD8+, TCR-alpha/beta and are at least partially dependent upon the presence of TCR-gamma/delta T cells in the host. Endogenous production of the pivotal cytokine, IFN-gamma, is essential for host immunity. These findings demonstrate that gut-derived IEL represent a potentially important mechanism to provide long term immunity to the host.

Inducible nitric oxide synthase is not required for long-term vaccine-based immunity against Toxoplasma gondii

Induction of reactive nitrogen intermediates by IFN-gamma is presumed an important mechanism of host resistance against acute and chronic infection with Toxoplasma gondii. Although nitric oxide (NO) has been shown to be important in the control of parasite replication in vivo, the role of this molecule in vaccine-based immunity against T. gondii is unknown. Mice with a targeted disruption of inducible NO synthase (iNOS) were immunized with an avirulent temperature-sensitive strain of this parasite (ts-4). Both the parental C57BL/6 and the iNOS(-/-) mice survived infection with the ts-4 mutant. Oral challenge of the vaccinated mice with a lethal dose of cysts containing bradyzoites resulted in reduced parasite burden and increased survival compared with nonvaccinated control mice. Host immunity in the iNOS(-/-) mice, similar to that observed in the parental strain, appears dependent upon both IFN-gamma and CD8+ T cells. These findings suggest that although vaccine-based long-term immunity against T. gondii is dependent upon the induction of IFN-gamma, it does not rely upon the anti-microbial effect of NO.

Inducible Nitric Oxide Synthase Is Not Required for Long-Term Vaccine-Based Immunity Against Toxoplasma gondii

Induction of reactive nitrogen intermediates by IFN-γ is presumed an important mechanism of host resistance against acute and chronic infection with Toxoplasma gondii. Although nitric oxide (NO) has been shown to be important in the control of parasite replication in vivo, the role of this molecule in vaccine-based immunity against T. gondii is unknown. Mice with a targeted disruption of inducible NO synthase (iNOS) were immunized with an avirulent temperature-sensitive strain of this parasite (ts-4). Both the parental C57BL/6 and the iNOS−/− mice survived infection with the ts-4 mutant. Oral challenge of the vaccinated mice with a lethal dose of cysts containing bradyzoites resulted in reduced parasite burden and increased survival compared with nonvaccinated control mice. Host immunity in the iNOS−/− mice, similar to that observed in the parental strain, appears dependent upon both IFN-γ and CD8+ T cells. These findings suggest that although vaccine-based long-term immunity against T. gondii is dependent upon the induction of IFN-γ, it does not rely upon the anti-microbial effect of NO.

Expression of Toxoplasma gondii-specific heat shock protein 70 during In vivo conversion of bradyzoites to tachyzoites

Stage conversion between bradyzoites and tachyzoites was investigated in C57BL/6 mice chronically infected with the ME-49 strain of Toxoplasma gondii. In order to promote bradyzoite-tachyzoite conversion, mice were treated in vivo with neutralizing doses of anti-gamma interferon (IFN-gamma) or anti-tumor necrosis factor alpha (TNF-alpha) antibodies. Expression of parasite-specific antigens SAG-1, SAG-2, and heat shock protein 70 (Hsp-70) was visualized in the central nervous system by immunocytochemistry and measured by photometric assay. The immunosuppressive effect of anti-IFN-gamma or anti-TNF-alpha treatment was immediate, leading to parasite stage conversion as indicated by the increased expression of tachyzoite-specific antigens (SAG-1 and SAG-2) and by rapid parasite replication. We also observed expression of high levels of Hsp-70 during a short period of conversion of bradyzoites to tachyzoites. Our data suggest that Hsp-70 may have an important role in the process of bradyzoite-tachyzoite conversion during the reactivation of chronic toxoplasmosis.

Antigen-specific CD8+ T cells protect against lethal toxoplasmosis in mice infected with Neospora caninum

Neospora caninum is a coccidial protozoan parasite that appears morphologically indistinguishable from Toxoplasma gondii and that infects a large range of mammals. Both inbred and outbred strains of mice exhibit a high degree of resistance to infection with N. caninum. Three inbred strains of mice (A/J, BALB/c, and C57BL/6) that were infected intraperitoneally with N. caninum were protected against a lethal challenge from T. gondii. Vaccine-induced protection was Neospora dose dependent. A rise in the CD8+ T-cell population in mice that had been vaccinated with N. caninum and challenged with T. gondii was observed. Adoptive transfer of CD8+ T-cell splenocytes from N. caninum-infected mice was protective against challenge with Toxoplasma. The CD8+ T cells from Neospora-infected mice proliferate to both Neospora and Toxoplasma antigens in vitro and secrete substantial quantities of gamma interferon when pulsed with the parasite antigen. These observations demonstrate that N. caninum protects against lethal T. gondii infection by the induction of CD8+ T cells that are immunoreactive to both parasites.

Molecular analysis and characterization of a protein involved in the replication of intracellular Toxoplasma gondii

Previous studies in our laboratory have identified a cytoplasmic protein (p97) of T. gondii that is involved in the process of intracellular parasite replication. Monoclonal antibody inhibits parasite replication in vitro and recognizes a protein of approximate 97 kDa by Western blot analysis. Using biotinylation, we demonstrate that p97 is not expressed on the surface of the tachyzoite. Polyclonal sera raised against the purified native protein was used to isolate a cDNA of 3.3 kb from a library. The product of this gene expresses a protein of approximate Mr 97 kDa that is reactive to the antibody (1B8) raised against the native antigen. The protein sequence of this product suggests that it is within the cytoplasm as suggested by the lack of a signal sequence or hydrophobic trans-membrane domain. This protein fails to dissociate into a monomer in the presence of non-ionic detergents as shown by gel filtration and density gradient. Southern blot analysis demonstrates a homologous gene sequence in two closely related Apicomplexa, Neospora caninum and Besnoitia jellisoni suggesting this protein is conserved among certain species of the Sarcocystidae.

A dichotomous role for nitric oxide during acute Toxoplasma gondii infection in mice

Production of nitric oxide by macrophages is believed to be an important microbicidal mechanism for a variety of intracellular pathogens, including Toxoplasma gondii. Mice with a targeted disruption of the inducible nitric oxide synthase gene (iNOS) were infected orally with T. gondii tissue cysts. Time to death was prolonged compared with parental controls. Histologic analysis of tissue from infected mice showed scattered small foci of inflammation with parasites in various tissues of iNOS-/- mice, whereas tissue from the parental C57BL/6 mice had more extensive tissue inflammation with few visible parasites. In particular, extensive ulceration and necrosis of distal small intestine and fatty degeneration of the liver was seen in the parental mice at day 7 postinfection, as compared with the iNOS-/- mice where these tissues appeared normal. Serum interferon gamma and tumor necrosis factor alpha levels postinfection were equally elevated in both mouse strains. Treatment of the parental mice with a NO synthase inhibitor, aminoguanidine, prevented early death in these mice as well as the hepatic degeneration and small bowel necrosis seen in acutely infected control parentals. These findings indicate that NO production during acute infection with T. gondii can kill intracellular parasites but can be detrimental, even lethal, to the host.

Toxoplasma gondii: dithiol-induced Ca2+ flux causes egress of parasites from the parasitophorous vacuole

Ca2+ is an essential activator of motility in the obligate intracellular parasite Toxoplasma gondii. Ca2+ ionophore A23187 and intracellular microinjection of Ca2+ initiate motility of parasites residing in parasitophorous vacuoles (PV). The source of Ca2+ and the mechanism by which it activates motility in vivo remain uncertain. Exposure of the parasites to dithiothreitol (DTT) can activate egress of previously nonmotile intravacuolar parasites within 60 sec. DTT is also known to activate both isoforms of the highly concentrated nucleoside triphosphate hydrolase (NTPase) produced by T. gondii. Using an adherent cell analysis system (ACAS) for Ca2+ imaging, a brief 15-50% increase in intra-PV fluorescence ratio was observed after exposure of infected fibroblasts to 5 mM DTT. Chelation of intracellular Ca2+ with BAPTA-AM and extracellular Ca2+ with EGTA blocked the DTT effect; however, this chelation did not prevent the activation of parasites nor the Ca2+ response to the Ca2+ ionophore ionomycin, suggesting that the Ca2+ that activates motility may reside near or within the parasite itself. This result demonstrates that an increase in Ca2+ within the vacuole precedes the onset of motility and the correlation of the DTT effect on motility and tachyzoite NTPase suggests that NTPase activation may be involved in the Ca2+ flux.

Adoptive transfer of gut intraepithelial lymphocytes protects against murine infection with Toxoplasma gondii

Intraepithelial lymphocytes (IEL) of the gut represent a primary immune barrier against infection by orally acquired pathogens. Naturally acquired infection with Toxoplasma gondii induces the proliferation of CD8+ T cells in both the gut and spleen. Gut-derived CD8alpha/beta+ IEL exhibit MHC-restricted cytotoxicity against parasite-infected enterocytes and macrophages. In a murine model, we demonstrate that the adoptive transfer of IEL obtained from inbred mice at day 11 postinfection is able to protect against a virulent challenge in syngenic recipients. In CBA mice, the parasite cyst load within the brain of the recipients receiving primed IEL was reduced by 90%. In BALB/c and C57BL/6 mice, a 50% decrease in mortality was observed following adoptive transfer of primed IEL. To determine the T cell subset responsible for protective immunity, a purified CD8alpha/beta+ IEL population was isolated from infected mice at day 11 postinfection. These cells were able to protect naive mice by adoptive transfer against a lethal parasite challenge. RNA analysis by reverse-transcriptase PCR revealed that primed CD8alpha/beta+ IEL produce significant message for IFN-gamma, an essential cytokine for host protection against toxoplasmosis. Administration of anti-IFN-gamma at the time of adoptive transfer of primed IEL abrogated protection. The adoptive transfer of these protective IEL was not restricted to the Ld class I locus. These data demonstrate that IFN-gamma-producing IEL may be an important primary barrier against acute and perhaps recurrent infection with T. gondii.

Adoptive transfer of gut intraepithelial lymphocytes protects against murine infection with Toxoplasma gondii

Intraepithelial lymphocytes (IEL) of the gut represent a primary immune barrier against infection by orally acquired pathogens. Naturally acquired infection with Toxoplasma gondii induces the proliferation of CD8+ T cells in both the gut and spleen. Gut-derived CD8alpha/beta+ IEL exhibit MHC-restricted cytotoxicity against parasite-infected enterocytes and macrophages. In a murine model, we demonstrate that the adoptive transfer of IEL obtained from inbred mice at day 11 postinfection is able to protect against a virulent challenge in syngenic recipients. In CBA mice, the parasite cyst load within the brain of the recipients receiving primed IEL was reduced by 90%. In BALB/c and C57BL/6 mice, a 50% decrease in mortality was observed following adoptive transfer of primed IEL. To determine the T cell subset responsible for protective immunity, a purified CD8alpha/beta+ IEL population was isolated from infected mice at day 11 postinfection. These cells were able to protect naive mice by adoptive transfer against a lethal parasite challenge. RNA analysis by reverse-transcriptase PCR revealed that primed CD8alpha/beta+ IEL produce significant message for IFN-gamma, an essential cytokine for host protection against toxoplasmosis. Administration of anti-IFN-gamma at the time of adoptive transfer of primed IEL abrogated protection. The adoptive transfer of these protective IEL was not restricted to the Ld class I locus. These data demonstrate that IFN-gamma-producing IEL may be an important primary barrier against acute and perhaps recurrent infection with T. gondii.

Neospora caninum: role for immune cytokines in host immunity

Neospora caninum is a coccidial protozoan parasite that infects a large range of mammals including dogs, cats, mice, and cattle. Morphologically, N. caninum appears indistinguishable from Toxoplasma gondii, although they are genetically distinct. To date there have been no reported cases of this infection in humans, although nonhuman primates may be susceptible to infection. Inbred A/J mice develop no clinical and little histologic evidence of infection in spite of a high-dose inoculum of N. caninum. Splenocytes obtained from infected mice proliferate in vitro in response to both N. caninum and T. gondii-soluble antigen. A transient state of T cell hyporesponsiveness to parasite antigen and mitogen was observed at Day 7 p.i. This downregulatory response could be partially reversed by the addition of the nitric oxide antagonist LNMMA, but not antibody to IL-10. Mice infected with N. caninum produce significant quantities of IL-12 and IFN gamma, most evident shortly after infection. In vivo, antibody to IL-12 is able to neutralize immune resistance to the parasite. Moreover, in vivo depletion of IFN gamma with antibody renders the mice susceptible to infection. These observations suggest that N. caninum induces a T cell immune response in the infected host that is at least partially mediated by IL-12 and IFN gamma.

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.

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.

Attachment of Toxoplasma gondii to host cells is host cell cycle dependent

The initial attachment of Toxoplasma tachyzoites to target host cells is an important event in the life cycle of the parasite and hence critical in the pathogenesis of this infection. The efficiency of Toxoplasma attachment to synchronized populations of Chinese hamster ovary cells and bovine kidney cells was investigated by using a glutaraldehyde-fixed host cell assay system. For both cell lines, parasite attachment increased as the synchronized host cells proceeded from the G1 phase to the mid-S phase and then decreased as the cells entered the G2-M boundary. Postulating that these differences in attachment reflect the upregulation of a specific receptor, polyclonal antibodies were generated against whole MDBK antigen at 0 and 4 h into the S phase. Both antisera were shown to inhibit parasite attachment to both synchronous and asynchronous host cell populations. However, the attachment blockade observed with the 4-h antiserum was significantly greater than that with the 0-h antiserum, completely abolishing the cell cycle-dependent increase in attachment found in control samples. These findings suggest that Toxoplasma tachyzoites bind specifically to a host cell receptor which is upregulated in the mid-S phase of the cell cycle.

IL-15 augments CD8+ T cell-mediated immunity against Toxoplasma gondii infection in mice

Cytokines of the Th1 profile are important mediators of protective host immunity against Toxoplasma gondii infection in mice. In this study we describe the effect of the recently identified cytokine, IL-15, on prevention of murine infection with T. gondii. Administration of exogenous rIL-15 with soluble Toxoplasma lysate Ag (TLA) provides complete protection against a lethal parasite challenge, whereas treatment with either rIL-15 or TLA alone is not protective. Following immunization with TLA/rIL-15, there is a significant proliferation of splenocytes expressing the CD8+ phenotype in response to TLA. A significant rise in the level of serum IFN gamma was observed in vaccinated mice. Adoptive transfer of CD8+ T cells, but not CD4+ T cells, from TLA/rIL-15-vaccinated mice protects naive mice from a lethal parasite challenge. These CD8+ T cells exhibit enhanced CTL activity against target macrophages infected with T. gondii. Mice that have been immunized are protected against lethal parasite challenge for at least 1 mo postvaccination. These observations demonstrate that TLA when administered with exogenous rIL-15 generates toxoplasmacidal Ag-specific CD8+ T cells. These T cells proliferate upon exposure to parasite Ag, exhibit long term memory CTL against infected target cells, and may be involved in host immune memory to this parasite.

IL-15 augments CD8+ T cell-mediated immunity against Toxoplasma gondii infection in mice

Cytokines of the Th1 profile are important mediators of protective host immunity against Toxoplasma gondii infection in mice. In this study we describe the effect of the recently identified cytokine, IL-15, on prevention of murine infection with T. gondii. Administration of exogenous rIL-15 with soluble Toxoplasma lysate Ag (TLA) provides complete protection against a lethal parasite challenge, whereas treatment with either rIL-15 or TLA alone is not protective. Following immunization with TLA/rIL-15, there is a significant proliferation of splenocytes expressing the CD8+ phenotype in response to TLA. A significant rise in the level of serum IFN gamma was observed in vaccinated mice. Adoptive transfer of CD8+ T cells, but not CD4+ T cells, from TLA/rIL-15-vaccinated mice protects naive mice from a lethal parasite challenge. These CD8+ T cells exhibit enhanced CTL activity against target macrophages infected with T. gondii. Mice that have been immunized are protected against lethal parasite challenge for at least 1 mo postvaccination. These observations demonstrate that TLA when administered with exogenous rIL-15 generates toxoplasmacidal Ag-specific CD8+ T cells. These T cells proliferate upon exposure to parasite Ag, exhibit long term memory CTL against infected target cells, and may be involved in host immune memory to this parasite.

In vivo treatment with interleukin 2 reduces parasitemia and restores IFN-gamma gene expression and T-cell proliferation during acute murine malaria

In this study, we describe the functional alterations in the host immune system that occur following acute infection with Plasmodium yoelii. Further, we have addressed the question whether the transient condition of altered immune responsiveness can be restored by a cytokine therapy. The lymphoproliferative response towards concanavalin A (Con A) or to cross-linked anti-CD3 mAb was significantly diminished in acutely infected mice compared to immune and normal animals. This condition was associated with poor production of IL-2. In vivo treatment with recombinant IL-2 (rIL-2) resulted in marked diminution of parasitemia (from 24% +/- 6% to 8% +/- 3%) in mice during the acute phase of infection. Despite this diminution in parasitemia, 70% of the IL-2 treated mice died by day 17 post infection. In vivo treatment with rIL-2 led to a partial but significant restoration in lymphoproliferative response to TCR-mediated (cross-linked anti-CD3 mAb) or to Con A-induced stimulation in acutely infected mice. The transcripts for IL-4, IL-5, GM-CSF, and TNF-alpha were expressed in the splenocytes from acutely infected mice not treated with rIL-2. mRNAs for IL-2, IFN-gamma, IL-6, IL-10 which were not detected in acutely infected mice could be reversed by administration of rIL-2 in vivo. We suggest that some of the hyporesponsive T-cells in the acute phase of infection have the potential to be reversed, and this reversal is manifested also at the level of cytokine gene expression.

Activation-mediated CD4+ T cell unresponsiveness during acute Toxoplasma gondii infection in mice

Infection of mice with Toxoplasma gondii has been shown to induce a transient state of immune down-regulation. Earlier reports have demonstrated the role of cytokines, in particular IL-10, in this host response. Here evidence is presented that T. gondii, a major opportunistic pathogen of the newborn and those with AIDS, is able to induce CD4(+) T cell apoptosis in the infected murine host. We have examined the changes in the CD4(+) T cell population that occur during acute infection in an experimental mouse model. Seventy-six percent of the CD4(+) T cell population increased in volume by day 7 post-infection and expressed T cell maturation markers (CD44(hi), IL-2Rhi, Mel-14(lo)). Further noted was a clonal activation of several CD4(+) T cell to mitogen or parasite antigen stimulation was observed, in particular Vbeta5 T cells. Addition of rIL-2 partially restored the CD4(+) T cell proliferative response in vitro. The T cell activation marker CTLA-4 could not be detected and the co-stimulatory molecule, CD28, was down-regulated. Electrophoretic and morphologic analysis of these cells post-culture demonstrated a DNA fragmentation pattern and cell death consistent with apoptosis. These studies demonstrate for the first time in a protozoan parasite that activation induced CD4(+) T cell unresponsiveness occurs during acute T. gondii infection in mice, and may be important in immune down-regulation and parasite persistence in the infected host.

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.

Production of nitric oxide (NO) is not essential for protection against acute Toxoplasma gondii infection in IRF-1-/- mice

Production of nitric oxide (NO) by macrophages is important for the killing of intracellular pathogens. IFN-gamma and LPS stimulate NO production by transcriptional up-regulation of inducible nitric oxide synthetase (iNOS). In the present study we used mice with a targeted disruption of the IFN regulatory factor-1 gene (IRF-1-/-) to investigate the importance of NO in the host immune response against Toxoplasma gondii, a major cause of infection in newborns and those with AIDS. IRF-1-/- mice were more susceptible to acute Toxoplasma infection, and treatment with either exogenous IFN-gamma or in vivo neutralization of endogenous IFN-gamma had little effect on their susceptibility to infection. However, administration of exogenous IL-12 was able to prolong survival even when IFN-gamma was depleted. An in vivo depletion study suggested that the mechanism of this protective response is mediated in part by CD4+ T cells. The administration of IL-12 could not overcome the inhibition of lymphoproliferative response in T. gondii-infected mice and treatment with N-monomethyl-L-arginine (L-NMMA), a nitric oxide synthase (iNOS) antagonist in vitro was unable to reverse the immunosuppression. In response to Toxoplasma infection, splenocytes from IRF-1-/- mice exhibited increased production of IL-10 as well as a 30-fold increase in its message expression. These studies indicate that NO may not be essential for host immunity to the parasite, and moreover that IL-12 appears to induce an IFN-gamma-independent mechanism of protection against this opportunistic pathogen.

Production of nitric oxide (NO) is not essential for protection against acute Toxoplasma gondii infection in IRF-1-/- mice

Production of nitric oxide (NO) by macrophages is important for the killing of intracellular pathogens. IFN-gamma and LPS stimulate NO production by transcriptional up-regulation of inducible nitric oxide synthetase (iNOS). In the present study we used mice with a targeted disruption of the IFN regulatory factor-1 gene (IRF-1-/-) to investigate the importance of NO in the host immune response against Toxoplasma gondii, a major cause of infection in newborns and those with AIDS. IRF-1-/- mice were more susceptible to acute Toxoplasma infection, and treatment with either exogenous IFN-gamma or in vivo neutralization of endogenous IFN-gamma had little effect on their susceptibility to infection. However, administration of exogenous IL-12 was able to prolong survival even when IFN-gamma was depleted. An in vivo depletion study suggested that the mechanism of this protective response is mediated in part by CD4+ T cells. The administration of IL-12 could not overcome the inhibition of lymphoproliferative response in T. gondii-infected mice and treatment with N-monomethyl-L-arginine (L-NMMA), a nitric oxide synthase (iNOS) antagonist in vitro was unable to reverse the immunosuppression. In response to Toxoplasma infection, splenocytes from IRF-1-/- mice exhibited increased production of IL-10 as well as a 30-fold increase in its message expression. These studies indicate that NO may not be essential for host immunity to the parasite, and moreover that IL-12 appears to induce an IFN-gamma-independent mechanism of protection against this opportunistic pathogen.

IL-7 stimulates protective immunity in mice against the intracellular pathogen, Toxoplasma gondii

Cytokines, in particular IFN-gamma and IL-12, are important in host protection against infection with Toxoplasma gondii. This parasite is a major cause of congenital infection and morbidity in immunosuppressed persons, especially those with AIDS. IL-7, a monomeric protein produced by bone marrow stromal cells and fetal thymus, is able to induce the proliferation of pro-B cells and CD4+ and CD8+ T cells, and to enhance cytotoxicity of CTL and NK cells. Inbred mice were infected with a lethal dose of T. gondii and given IL-7 twice daily. Mice treated with IL-7 beginning at the time of infection survived, whereas mice either treated after infection or not treated died. Phenotypic analysis of splenocytes identified an expansion of NK (asialo GM1+) cells and CD8+ T cell populations. In vivo depletion of NK (asialo GM1+) and CD8+ T cells showed that cells expressing these phenotypes were important for maintaining protection against the parasite. IFN-gamma depletion resulted in complete reversal of the protective effect of IL-7 administration. In vivo depletion of endogenous IL-7 enhanced susceptibility to infection. Cytokine analysis by semiquantitative reverse-transcriptase PCR showed that IL-7 enhances the IFN-gamma response and furthermore reverses the parasite-mediated down-regulatory response on IL-2. These observations indicate that exogenous administration of human rIL-7 is able to protect mice against acute parasite challenge by stimulating IFN-gamma production and augmenting the CD8+ T cell-mediated CTL response.

IL-7 stimulates protective immunity in mice against the intracellular pathogen, Toxoplasma gondii

Cytokines, in particular IFN-gamma and IL-12, are important in host protection against infection with Toxoplasma gondii. This parasite is a major cause of congenital infection and morbidity in immunosuppressed persons, especially those with AIDS. IL-7, a monomeric protein produced by bone marrow stromal cells and fetal thymus, is able to induce the proliferation of pro-B cells and CD4+ and CD8+ T cells, and to enhance cytotoxicity of CTL and NK cells. Inbred mice were infected with a lethal dose of T. gondii and given IL-7 twice daily. Mice treated with IL-7 beginning at the time of infection survived, whereas mice either treated after infection or not treated died. Phenotypic analysis of splenocytes identified an expansion of NK (asialo GM1+) cells and CD8+ T cell populations. In vivo depletion of NK (asialo GM1+) and CD8+ T cells showed that cells expressing these phenotypes were important for maintaining protection against the parasite. IFN-gamma depletion resulted in complete reversal of the protective effect of IL-7 administration. In vivo depletion of endogenous IL-7 enhanced susceptibility to infection. Cytokine analysis by semiquantitative reverse-transcriptase PCR showed that IL-7 enhances the IFN-gamma response and furthermore reverses the parasite-mediated down-regulatory response on IL-2. These observations indicate that exogenous administration of human rIL-7 is able to protect mice against acute parasite challenge by stimulating IFN-gamma production and augmenting the CD8+ T cell-mediated CTL response.

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.

A Toxoplasma gondii-derived factor(s) stimulates immune downregulation: an in vitro model

Suppression of the T-cell lymphoproliferative response and downregulation of interleukin 2 (IL-2) production by Toxoplasma gondii has been observed following in vivo infection. In this study, an experimental in vitro murine system was developed to evaluate the kinetics of these responses. Normal splenocytes from uninfected mice were stimulated with either concanavalin A or an anti-CD3 monoclonal antibody and cocultured with Toxoplasma tachyzoites either directly or separated by a transwell. A progressive decline in the lymphoproliferative response was observed as the concentration of parasites in culture increased. Neither heat-killed nor formaldehyde-fixed parasites stimulated this downregulatory response by the splenocytes. A decline in IL-2 production was associated with the decrease in lymphocyte proliferation. The addition of an antibody to IL-10 or heat-inactivated anti-Toxoplasma sera to the culture supernatant partially neutralized the inhibitory effect on lymphocyte proliferation. Cytokine analysis of the responder splenocytes demonstrated a decrease in the message for IL-2 and IL-2 receptor and an increase in IL-10. Together, these observations suggest that during in vitro culture in a murine system, parasite antigens that stimulate the release of a soluble factor(s), such as IL-10, that inhibits proliferation of mitogen-stimulated T cells are expressed.

IL-10 mediates immunosuppression following primary infection with Toxoplasma gondii in mice

Suppression of the host immune response by Toxoplasma gondii has been observed in both human and experimental murine infection. In this study, inbred mice were infected with T. gondii. At day 7 post-infection, the lymphoproliferative response to both mitogen and superantigen as well as parasite antigen were found to be significantly depressed. Using a transwell system, it was determined that the reduced proliferative response was due to soluble factor(s) being expressed by splenocytes from the infected mice. Isolation of the splenocytes into an adherent and nonadherent population suggested that both macrophages and T cells were able to produce at least one soluble factor. Tissue culture supernatant derived from the splenocytes of the infected mice contain increased levels of IL-10, whereas measurable IL-2 levels could not be quantitated. At day 7 post-infection, both a biologic assay for IFN-gamma in culture supernatant and the expression of IFN-gamma mRNA in the splenocytes were reduced. Antibody to IL-10 was able to partially neutralize (almost 50%) the in vitro immune downregulation of the tissue culture supernatant. Anti-IL-10 in combination with a nitric oxide (NO) antagonist was able to reverse the inhibitory activity of the culture supernatant by 85%. Since IL-10 is a potent antagonist of IFN-gamma, it may represent a critical cytokine involved in mediating T. gondii induced immunosuppression in the infected host.

Toxoplasma gondii: a monoclonal antibody that inhibits intracellular replication

During its intracellular life cycle within the infected host cell, Toxoplasma gondii is able to undergo rapid asexual replication. Neither the mechanism by which the parasite initiates this process nor the requirements for maintaining it are understood. We produced a monoclonal antibody, 1B8, that identifies a parasite antigen of approximate M(r) 97 kDa as determined by SDS-PAGE. The epitope recognized by mAb 1B8 appears as a collection of vesicular structures scattered throughout the cell cytoplasm. When RH strain parasites are incubated with mAb 1B8 in the absence of serum complement, parasite growth is inhibited by > 90% as determined by radioisotope incorporation. Both attachment and invasion assays show that neither of these parasite-host cell interactions are inhibited by the mAb. However, a marked reduction in the number of intracellular rosettes was observed following mAb treatment of the parasites. Viable extracellular parasites are able to endocytose mAb 1B8. Once within the parasite cytosol the antibody recognizes the vesicular structures similar to those observed with fixed parasites. Immunofluorescence assays with Besnoitia jellisoni and Plasmodium falciparum show that the epitope recognized by mAb 1B8 is conserved among Coccidiae but not the kinetoplastid Leishmania.

Attachment of Toxoplasma gondii to host cells involves major surface protein, SAG-1 (P30)

Previous observations have demonstrated that monoclonal and polyclonal antibodies directed at SAG-1, the major surface protein of Toxoplasma gondii, decreased the number of T. gondii that infected fibroblast monolayers. Direct evaluation of parasite-host cell attachment using glutaraldehyde-fixed human fibroblasts and live tachyzoites was performed to determine whether SAG-1 was a ligand for the host cell receptor. The interaction between the fixed cells and T. gondii was specific and saturable as determined by a radioisotope competitive binding assay. Moreover, the specificity of this interaction was confirmed by comparison to another member of the Apicomplexa, Besnoitia jellisoni. Treatment of fresh extracellular T. gondii with rabbit polyclonal anti-SAG-1 serum inhibited parasite attachment to host cells by 71%. A monoclonal antibody (6A8) directed at SAG-1 was able to inhibit parasite binding to fixed host cells by 65%. Other mAb's directed at SAG-1 failed to inhibit parasite attachment in this assay. Fab derived from 6A8 mAb showed dose-dependent inhibition of parasite attachment. At an Fab concentration of 25 micrograms/ml, 47% inhibition was observed. Attachment assays using mutant parasites with defective SAG-1 (PTgA and PTgC) showed significantly reduced binding (26 and 39%) when compared to wild-type (SAG-1+) parentals. Taken together, these observations suggest that SAG-1 is an important parasite ligand that binds to the host cell in the process of T. gondii invasion.

Interleukin-12 enhances murine survival against acute toxoplasmosis

Protective immunity against Toxoplasma gondii is mediated by the host cellular immune response. Interleukin-12 (IL-12), a recently described cytokine that stimulates NK cells to produce gamma interferon (IFN-gamma), is able to enhance host protection against this parasite in SCID mice. Administration of IL-12 to A/J mice significantly increased survival over that of control mice when IL-12 was delivered early in the course of acute infection. If it was administered at day 3 or thereafter, there was no observed difference in mortality between treated and control mice. Antibody depletion of IL-12 increased susceptibility to infection, as measured by mortality, only when the IL-12 was administered before day 3 postinfection. Mice treated with IL-12 at day 0 postinfection exhibited a significant rise above the control in both IL-2 and IFN-gamma production. Once infection has been established in the host (3 days), administration of exogenous IL-12 is unable to alter parasite-induced downregulation of IFN-gamma production. Thus, IL-12 appears to play an important, but transitory, role in protection against acute infection with T. gondii in the normal murine host.

Antigen-specific CD8+ T cell clone protects against acute Toxoplasma gondii infection in mice

Encephalitis caused by the obligate intracellular protozoan, Toxoplasma gondii, is a major cause of morbidity and mortality in those afflicted with AIDS. The cell-mediated response is critical for host immunity against this parasite. A panel of T cell clones that proliferate in response to a major parasite protein, p30 (SAG-1) were isolated from mice. One of these clones (C3) bearing the CD8+ phenotype was able to induce nearly 100% protection against acute infection when adoptively transferred into naive mice. In contrast, an Ag-specific CD4+ T cell clone (C2) failed to show significant protection in spite of its ability to produce similar quantities of IFN-gamma. Depletion of host CD4+ T cells with mAb had no effect on CD8(+)-mediated adoptive protection, whereas treatment with anti-IFN-gamma completely abrogated this protection. These results indicate that Ag-specific CD8+ T cells in combination with IFN-gamma are able to induce significant protection against acute toxoplasmosis in the experimental murine model.

Antigen-specific CD8+ T cell clone protects against acute Toxoplasma gondii infection in mice

Encephalitis caused by the obligate intracellular protozoan, Toxoplasma gondii, is a major cause of morbidity and mortality in those afflicted with AIDS. The cell-mediated response is critical for host immunity against this parasite. A panel of T cell clones that proliferate in response to a major parasite protein, p30 (SAG-1) were isolated from mice. One of these clones (C3) bearing the CD8+ phenotype was able to induce nearly 100% protection against acute infection when adoptively transferred into naive mice. In contrast, an Ag-specific CD4+ T cell clone (C2) failed to show significant protection in spite of its ability to produce similar quantities of IFN-gamma. Depletion of host CD4+ T cells with mAb had no effect on CD8(+)-mediated adoptive protection, whereas treatment with anti-IFN-gamma completely abrogated this protection. These results indicate that Ag-specific CD8+ T cells in combination with IFN-gamma are able to induce significant protection against acute toxoplasmosis in the experimental murine model.

Antibodies to Toxoplasma gondii major surface protein (SAG-1, P30) inhibit infection of host cells and are produced in murine intestine after peroral infection

Monoclonal and polyclonal, monospecific antibodies to the major surface antigen of Toxoplasma gondii (SAG-1, P30) inhibit infection of human fibroblasts and murine enterocytes. Fab prepared from polyclonal, monospecific antibody to P30 also have this inhibitory effect on invasion, which indicates that this antibody directly blocks parasite infection of host cells rather agglutinating the parasite. Antibodies to another surface protein (P22) did not alter in vitro infection. If the inhibitory effect of antibody to P30 was due to steric hindrance or complexing of surface epitopes contiguous to P30, antibodies to other surface epitopes would also be inhibitory and they are not. Urea treatment of antibody (which permits discrimination of high and low avidity antibody) did not alter the effect of anti-P30 antibody. This observation indicates that the effect of the antibody to P30 was not an artifact of differences in the avidity of the antibody to P22 and P30. Heat inactivated antisera from mice infected with either RH or PTg strain T. gondii (P30+) inhibit infection of fibroblasts when challenged with autologous wild-type parasites by 87 and 40%, respectively. In contrast, these antisera have little inhibitory effect (13 and 19%, respectively) against infection of human fibroblasts by a P30-deficient mutant (PTgB). Antisera raised to the P30-deficient mutant had no significant effect on infection of cells by wild-type strains that have surface P30. The neoglycoprotein, BSA-glucosamide, competitively blocks infection of human fibroblasts by P30+ tachyzoites with surface P30 in higher level than those without surface P30. This observation indicates that there is likely to be a glycosylated host cell receptor to which T. gondii's major surface Ag SAG-1 (P30) binds. Mice infected perorally develop intestinal IgA antibody to the major 30-kDa epitope of T. gondii. Thus, the major surface epitope of T. gondii, SAG-1 (P30), has an important, functional role in infection of host cells by T. gondii and elicits an intestinal antibody response after peroral infection.

Antibodies to Toxoplasma gondii major surface protein (SAG-1, P30) inhibit infection of host cells and are produced in murine intestine after peroral infection

Monoclonal and polyclonal, monospecific antibodies to the major surface antigen of Toxoplasma gondii (SAG-1, P30) inhibit infection of human fibroblasts and murine enterocytes. Fab prepared from polyclonal, monospecific antibody to P30 also have this inhibitory effect on invasion, which indicates that this antibody directly blocks parasite infection of host cells rather agglutinating the parasite. Antibodies to another surface protein (P22) did not alter in vitro infection. If the inhibitory effect of antibody to P30 was due to steric hindrance or complexing of surface epitopes contiguous to P30, antibodies to other surface epitopes would also be inhibitory and they are not. Urea treatment of antibody (which permits discrimination of high and low avidity antibody) did not alter the effect of anti-P30 antibody. This observation indicates that the effect of the antibody to P30 was not an artifact of differences in the avidity of the antibody to P22 and P30. Heat inactivated antisera from mice infected with either RH or PTg strain T. gondii (P30+) inhibit infection of fibroblasts when challenged with autologous wild-type parasites by 87 and 40%, respectively. In contrast, these antisera have little inhibitory effect (13 and 19%, respectively) against infection of human fibroblasts by a P30-deficient mutant (PTgB). Antisera raised to the P30-deficient mutant had no significant effect on infection of cells by wild-type strains that have surface P30. The neoglycoprotein, BSA-glucosamide, competitively blocks infection of human fibroblasts by P30+ tachyzoites with surface P30 in higher level than those without surface P30. This observation indicates that there is likely to be a glycosylated host cell receptor to which T. gondii's major surface Ag SAG-1 (P30) binds. Mice infected perorally develop intestinal IgA antibody to the major 30-kDa epitope of T. gondii. Thus, the major surface epitope of T. gondii, SAG-1 (P30), has an important, functional role in infection of host cells by T. gondii and elicits an intestinal antibody response after peroral infection.

Antigen-specific (p30) mouse CD8+ T cells are cytotoxic against Toxoplasma gondii-infected peritoneal macrophages

The importance of CD8+ T cells in immunity against Toxoplasma gondii is now well recognized. The mechanism by which these CD8+ T cells are able to confer this immunity is not yet understood. To examine the Ag specificity of this response, immune splenocytes from mice immunized with p30, a major surface parasite Ag, were evaluated for their ability to lyse peritoneal macrophages infected with three different strains of T. gondii. Macrophages infected with either the RH or P wild-type strain tachyzoites were lysed at varying E:T ratios by nylon wool nonadherent immune splenocytes whereas macrophages infected with a p30-deficient mutant (B mutant) of the P strain were not. The gene encoding p30 for the wild type and B mutant were amplified by the polymerase chain reaction. This revealed a nonsense mutation in the B mutant such that its primary translation product is predicted to be about two-thirds the size of the wild-type p30 molecule. mAb depletion studies indicate that the cytotoxic effect of the immune splenocytes is mediated by the CD8+ T cell population. Peritoneal macrophages infected with the three different strains (RH, P wild type, B mutant) from mice genetically restricted were not lysed by the immune CD8+ effector cell population. A cloned line (C3) of p30 Ag-specific CD8+ T cells exhibited significant cytotoxicity against syngeneic peritoneal macrophages infected with either the RH or P strain tachyzoites. There was no macrophage lysis observed by these CD8+ effector cells of either syngeneic macrophages infected with the B mutant or nonsyngeneic macrophages infected with the three different tachyzoite strains.

Antigen-specific (p30) mouse CD8+ T cells are cytotoxic against Toxoplasma gondii-infected peritoneal macrophages

The importance of CD8+ T cells in immunity against Toxoplasma gondii is now well recognized. The mechanism by which these CD8+ T cells are able to confer this immunity is not yet understood. To examine the Ag specificity of this response, immune splenocytes from mice immunized with p30, a major surface parasite Ag, were evaluated for their ability to lyse peritoneal macrophages infected with three different strains of T. gondii. Macrophages infected with either the RH or P wild-type strain tachyzoites were lysed at varying E:T ratios by nylon wool nonadherent immune splenocytes whereas macrophages infected with a p30-deficient mutant (B mutant) of the P strain were not. The gene encoding p30 for the wild type and B mutant were amplified by the polymerase chain reaction. This revealed a nonsense mutation in the B mutant such that its primary translation product is predicted to be about two-thirds the size of the wild-type p30 molecule. mAb depletion studies indicate that the cytotoxic effect of the immune splenocytes is mediated by the CD8+ T cell population. Peritoneal macrophages infected with the three different strains (RH, P wild type, B mutant) from mice genetically restricted were not lysed by the immune CD8+ effector cell population. A cloned line (C3) of p30 Ag-specific CD8+ T cells exhibited significant cytotoxicity against syngeneic peritoneal macrophages infected with either the RH or P strain tachyzoites. There was no macrophage lysis observed by these CD8+ effector cells of either syngeneic macrophages infected with the B mutant or nonsyngeneic macrophages infected with the three different tachyzoite strains.

A purified parasite antigen (p30) mediates CD8+ T cell immunity against fatal Toxoplasma gondii infection in mice

Induction of protective immunity against acute and chronic toxoplasmosis can be achieved using p30, the major membrane and excreted/secreted protein of Toxoplasma gondii. This protein, when administered to outbred mice in the presence of the saponin Quil A, is able to induce almost 100% protection against acute infection without evidence of intracerebral cyst development. Adoptive transfer of immune splenocytes from immunized inbred A/J mice conferred a significant level (p less than 0.001) of protection against subsequent challenge. Phenotypic analysis in outbred as well as two different strains of inbred mice (A/J and C57BL/6) demonstrated that CD8+ T cells are selectively stimulated by this immunization protocol. T cell depletion studies using specific mAb directed at either CD3+ or CD8+ T cell phenotype, followed by adoptive transfer, failed to confer protective immunity, whereas CD4+ depletion had no effect. These cytotoxic CD8+ T cells produced high titers of both IFN-gamma and IL-2. Moreover, these CD8+ T cells were directly parasiticidal against radiolabeled extracellular T. gondii, further supporting the critical immune function of these p30 Ag-specific CD8+ T cells in host immunity against T. gondii infection.

A purified parasite antigen (p30) mediates CD8+ T cell immunity against fatal Toxoplasma gondii infection in mice

Induction of protective immunity against acute and chronic toxoplasmosis can be achieved using p30, the major membrane and excreted/secreted protein of Toxoplasma gondii. This protein, when administered to outbred mice in the presence of the saponin Quil A, is able to induce almost 100% protection against acute infection without evidence of intracerebral cyst development. Adoptive transfer of immune splenocytes from immunized inbred A/J mice conferred a significant level (p less than 0.001) of protection against subsequent challenge. Phenotypic analysis in outbred as well as two different strains of inbred mice (A/J and C57BL/6) demonstrated that CD8+ T cells are selectively stimulated by this immunization protocol. T cell depletion studies using specific mAb directed at either CD3+ or CD8+ T cell phenotype, followed by adoptive transfer, failed to confer protective immunity, whereas CD4+ depletion had no effect. These cytotoxic CD8+ T cells produced high titers of both IFN-gamma and IL-2. Moreover, these CD8+ T cells were directly parasiticidal against radiolabeled extracellular T. gondii, further supporting the critical immune function of these p30 Ag-specific CD8+ T cells in host immunity against T. gondii infection.

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.

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.

Identification of stage-specific antigens of Toxoplasma gondii

An immunologic evaluation of the surface antigens of the three major life-cycle stages of Toxoplasma gondii was performed. Mouse antisera were raised against these stages, which included the oocyst-sporozoite (feline-excreted stage), bradyzoite (chronic tissue cyst stage), and tachyzoite (invasive stage). The antisera were used in an enzyme-linked immunosorbent assay and Western blot (immunoblot) analysis to demonstrate the presence of stage-specific antigens. These antigens were of various molecular weights and were specific to each stage investigated. Cross-reaction studies showed that the mouse antisera recognized commonly shared antigens to at least two of the three stages. A panel of monoclonal antibodies identified specific immune epitopes unique to each of the stages investigated. These studies further support the hypothesis that stage-specific antigens are present in T. gondii.

Molecular analysis of the gene encoding the major surface antigen of Toxoplasma gondii

The complete sequence of P30, the major surface Ag of the protozoan parasite, Toxoplasma gondii, has been deduced through the cloning and analysis of its gene. Using polyclonal serum specific for P30, we have isolated a P30 cDNA clone from a lambda gt11 cDNA expression library derived from tachyzoites of T. gondii (RH strain). This clone produces a beta-galactosidase fusion protein which reacts with several anti-P30 mAb. In addition, polyclonal anti-serum raised to the fusion protein reacts with purified P30 protein and exclusively with P30 in a whole cell lysate of T. gondii. This cDNA clone was used to isolate near full-length cDNA molecules and a cosmid clone containing the P30 gene. Sequence analysis of the cDNA reveals a single open reading frame with coding capacity for 34.7 kDa of primary translation product (consistent with the apparent Mr of P30 on SDS-acrylamide gels) including a presumptive hydrophobic signal sequence. The P30 primary translation product also has a carboxy-terminal hydrophobic tail which is predictive of a posttranslational cleavage and modification with a glycolipid anchor. We have identified the apparent 5' and 3' ends of the P30 mRNA transcript which is extremely abundant, 1500 nucleotides in length, and polyadenylated. The P30 gene is single copy and contains no introns.

Induction of antigen-specific parasiticidal cytotoxic T cell splenocytes by a major membrane protein (P30) of Toxoplasma gondii

Infection with Toxoplasma gondii has become a major cause of morbidity in patients with AIDS. To investigate the mechanisms responsible for immune responses to toxoplasma Ag we used a highly purified membrane protein (P30) of T. gondii to stimulate an in vitro Ag-specific cytotoxic T cell response. P30 immune mouse splenocytes reduced extracellular T. gondii plaque-forming units by more than 50% when incubated at an E/T ratio of 10:1 or greater. By using a [3H]uracil radioisotope release assay, the effect of the immune splenocytes was determined to be a direct parasite lytic mechanism. The immune splenocytes were P30 Ag specific and of the Thy 1.2, Lyt2,3+ (CD4-, CD8+) phenotype, specific for mouse cytotoxic T cells. Opsonization of the parasites with monoclonal P30-reactive mAb did not enhance parasiticidal activity. Culture supernatants obtained during the 2-h cytotoxic assay were not parasiticidal, and anti-asialo-GM1 antibody plus C did not destroy the parasiticidal activity of the P30 responder cells. Accordingly, we have identified an Ag-specific subset of CD4-, CD8+, P30 responder T cells that are directly parasiticidal to extracellular T. gondii, and that exhibit cytotoxicity independent of antibody opsonization, lymphokine secretion, NK cell activity, and, apparently, MHC involvement as well.

Induction of antigen-specific parasiticidal cytotoxic T cell splenocytes by a major membrane protein (P30) of Toxoplasma gondii

Infection with Toxoplasma gondii has become a major cause of morbidity in patients with AIDS. To investigate the mechanisms responsible for immune responses to toxoplasma Ag we used a highly purified membrane protein (P30) of T. gondii to stimulate an in vitro Ag-specific cytotoxic T cell response. P30 immune mouse splenocytes reduced extracellular T. gondii plaque-forming units by more than 50% when incubated at an E/T ratio of 10:1 or greater. By using a [3H]uracil radioisotope release assay, the effect of the immune splenocytes was determined to be a direct parasite lytic mechanism. The immune splenocytes were P30 Ag specific and of the Thy 1.2, Lyt2,3+ (CD4-, CD8+) phenotype, specific for mouse cytotoxic T cells. Opsonization of the parasites with monoclonal P30-reactive mAb did not enhance parasiticidal activity. Culture supernatants obtained during the 2-h cytotoxic assay were not parasiticidal, and anti-asialo-GM1 antibody plus C did not destroy the parasiticidal activity of the P30 responder cells. Accordingly, we have identified an Ag-specific subset of CD4-, CD8+, P30 responder T cells that are directly parasiticidal to extracellular T. gondii, and that exhibit cytotoxicity independent of antibody opsonization, lymphokine secretion, NK cell activity, and, apparently, MHC involvement as well.

Molecular analysis of the gene encoding the major surface antigen of Toxoplasma gondii

The complete sequence of P30, the major surface Ag of the protozoan parasite, Toxoplasma gondii, has been deduced through the cloning and analysis of its gene. Using polyclonal serum specific for P30, we have isolated a P30 cDNA clone from a lambda gt11 cDNA expression library derived from tachyzoites of T. gondii (RH strain). This clone produces a beta-galactosidase fusion protein which reacts with several anti-P30 mAb. In addition, polyclonal anti-serum raised to the fusion protein reacts with purified P30 protein and exclusively with P30 in a whole cell lysate of T. gondii. This cDNA clone was used to isolate near full-length cDNA molecules and a cosmid clone containing the P30 gene. Sequence analysis of the cDNA reveals a single open reading frame with coding capacity for 34.7 kDa of primary translation product (consistent with the apparent Mr of P30 on SDS-acrylamide gels) including a presumptive hydrophobic signal sequence. The P30 primary translation product also has a carboxy-terminal hydrophobic tail which is predictive of a posttranslational cleavage and modification with a glycolipid anchor. We have identified the apparent 5' and 3' ends of the P30 mRNA transcript which is extremely abundant, 1500 nucleotides in length, and polyadenylated. The P30 gene is single copy and contains no introns.

Absence of a role for natural killer cells in the control of acute infection by Toxoplasma gondii oocysts

The active phase of primary and challenge oral infections of Toxoplasma gondii was investigated with respect to natural killer (NK) activity against YAC-1 tumour cell targets in vitro and serum interferon (IFN) titres. Primary (non-lethal) oral infection of BALB/c mice with Me49 oocysts resulted in a rapid increase of serum IFN titres, followed by augmented NK activity. NK levels became depressed, rising again by 15 days after infection to normal levels, again preceded by elevated IFN titres. In challenge infections NK was not augmented and IFN titres rose only if a high dose of oocysts was given. IFN activity was pH2-labile in all cases and considered to be due to IFN-gamma. Cold target inhibition studies indicated that T. gondii did not bind to NK cells. A bioassay for the effects of NK cells on T. gondii tachyzoites was developed and there was no evidence of killing in vitro by cells with NK function; T. gondii survived better when cultured with NK cells than when cultured alone. Studies using C57BL/6bg/bg,bg/+ and +/+ mice showed that there was no difference in mean time to death after administration of a lethal ME49 oocyst infection by mouth. Cytotoxicity against YAC-1 in both spleen and mesenteric lymph node (MLN) cell populations was highly augmented in bg/+ and +/+, but not in bg/bg mice. Genetic deficiency of NK activity had no effect on survival of mice after infection. Therefore NK has at best a minimal role to play in protection during the acute phase of Toxoplasma infection.

Production of gamma interferon by cultured human lymphocytes stimulated with a purified membrane protein (P30) from Toxoplasma gondii

Purified P30, the principal iodinatable membrane protein of Toxoplasma gondii, induced proliferation of peripheral blood mononuclear cells from seropositive individuals but not from seronegative individuals. Culture supernatants from stimulated cells of seropositive individuals blocked the growth of T. gondii in human fibroblasts, whereas those from antibody-negative individuals failed to do so. The anti-toxoplasmic effect of culture supernatants correlated with the induction of indoleamine 2,3-dioxygenase and the destruction of tryptophan, as previously described for fibroblasts treated with recombinant gamma interferon (IFN-gamma). The anti-toxoplasmic effect was blocked by monoclonal antibody to IFN-gamma. The protective effect correlated with the amount of IFN-gamma in the culture supernatant, as measured by inhibition of viral CPE. Thus, the level of IFN-gamma appears to be an important immune factor in protection against toxoplasmosis in humans.