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Overproduction and High Level Apoptosis Are Associated with High but Not Low Virulence Toxoplasma gondii Infection1
Department of Microbiology and Immunology, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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and lack of NO production were found during
RH, in contrast to ME49 infection. These data demonstrate that
Toxoplasma strain characteristics exert a profound
effect on the host immune response and that the latter itself is a
crucial determinant in parasite virulence. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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are essential for protective immunity to
T. gondii (8, 9, 10). However, under certain
conditions, infection may lead to proinflammatory cytokine
overproduction resulting in pathology and death
(11, 12, 13, 14). T. gondii can be grouped into three clonal lineages (15). Thus, the type I lineage, typified by the RH strain, is extremely lethal in mice, displaying an LD50 < 10 during the acute phase of infection. In contrast, type II (e.g., ME49) and type III strains display lower virulence (LD50 > 100) and infections usually progress to the chronic phase. The acute virulence phenotype has been genetically linked to a specific region on parasite chromosome VIII (16). The strains also display unique patterns of epidemiological occurrence. Type II strains are frequently found in AIDS patients with reactivating toxoplasmosis or cases of human congenital disease, whereas type III strains are mostly found in animals (15). It is also known that different Toxoplasma strains express unique Ags (17, 18). Previous studies have suggested that the Toxoplasma strain can determine the severity of toxoplasmic encephalitis which may develop during chronic infection (15, 19, 20, 21). However, despite multiple genetic, biochemical, and epidemiological evidence for Toxoplasma strain variation, few studies have directly addressed the extent to which this variation exerts an effect on the host response during acute stage disease.
In the present study, we systematically compared the acute immune
response to infection with tachyzoites from the high virulence type I
strain RH, relative to infection with low virulence type II ME49 strain
tachyzoites. Our results show that RH infection leads to widespread
parasite dissemination and rapid death of mice, whereas after ME49
infection, death was delayed or absent and tachyzoite dissemination
much less extensive. Furthermore, RH infection triggers massive spleen
cell apoptotic death, disintegration of the splenic architecture, and
hyperinduction of IFN-, but fails to elicit NO production. In
contrast, during ME49 infection, there was dramatically less apoptosis
and lower amounts of IFN-. In addition, spleen structure was
maintained and NO levels were highly elevated during infection with
this low virulence strain. Together, our results demonstrate profound
differences in the immunity induced by high and low virulence strains
of T. gondii, suggesting that the host immune response
itself may play a role in determining the virulence characteristics of
the parasite.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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C57BL/6 female mice, 4–6 wk of age, were obtained from The Jackson Laboratory (Bar Harbor, ME). Until their use at 6–8 wk of age, mice were housed under specific pathogen-free conditions in the Animal Facility of the College of Veterinary Medicine at Cornell University, which is accredited by the American Association for Accreditation of Laboratory Animal Care.
Parasites and infection
Tachyzoites of the RH strain were maintained by biweekly passage on human foreskin fibroblast monolayers in fibroblast medium composed of DMEM (Life Technologies, Gaithersburg, MD) supplemented with 1% FCS (HyClone, Logan, UT), 100 U/ml penicillin, and 0.1 mg/ml streptomycin (Life Technologies). ME49 tachyzoites were initially obtained by inoculating brain homogenate containing ME49 cysts from Swiss Webster mice infected 1 mo earlier onto fibroblast monolayers. For initial passages, fibroblast monolayers were detached by scraping, and cells were forced through a syringe equipped with a 27-gauge needle to release the intracellular parasites. After approximately 4 weekly passages in this manner, ME49 could be maintained as for the RH culture. Before infection, parasites were washed in sterile PBS, and 103 or 102 tachyzoites of either the ME49 or the RH strain were inoculated i.p. into mice. The viability of parasite preparations and the initial rate of infection were controlled in vitro and were equivalent for both strains.
Spleen cell culture
Spleens were homogenized, RBC lysed (Red Blood Cell Lysis buffer; Sigma, St. Louis, MO), and splenocytes were resuspended in complete DMEM consisting of DMEM supplemented with 10% FCS, 1 mM sodium pyruvate (Life Technologies), 0.1 mM nonessential amino acids (Life Technologies), 30 mM HEPES (Life Technologies), 100 U/ml penicillin, and 0.1 mg/ml streptomycin and 50 µM 2-ME. Cells were cultured at 2 x 105 cells/well in 96-well flat-bottom tissue culture plates in the presence of medium alone or 25 µg/ml soluble tachyzoite Ag (prepared as described in Ref. 22) for 72 h at 37°C in 5% CO2. Cell-free supernatants were harvested and stored at -20°C until assayed for cytokines and NO.
Cytokine ELISA
Quantitation of IL-12p40, IFN-, and TNF-
in the splenocyte
culture supernatants and on plasma was accomplished using a two-site
ELISA as previously described (23, 24). At the time of
euthanasia, mice were bled by cardiac puncture, blood was centrifuged,
plasma was collected, and equivalent quantities from five mice per
group were pooled and stored at -20°C for cytokine assays. NO was
measured in the cell supernatant by the Greiss reaction
(25).
Flow cytometry
For intracellular parasite detection, freshly isolated splenocytes were washed, Fc receptors were blocked with 10% normal mouse serum (Jackson ImmunoResearch Laboratories, West Grove, PA), and cells were stained for surface markers with monoclonal PE-conjugated Abs directed against CD3, B220, or Gr-1 (Ly-6G) (BD PharMingen, San Diego, CA). Cells were then further fixed and permeabilized with a commercially obtained kit (Cytofix/Cytoperm; BD PharMingen), incubated for 1 h on ice with a purified polyclonal rabbit anti-Toxoplasma Ab (BioGenex Laboratories, San Ramon, CA), washed again, and incubated for another hour with a goat anti-rabbit FITC-conjugated Ab (Jackson ImmunoResearch Laboratories).
For intracellular cytokine detection, freshly isolated cells were
stimulated for 4 h in complete DMEM in the presence of 5 ng/ml PMA
and 500 ng/ml ionomycin (Sigma). Brefeldin A (GolgiPlug; BD PharMingen)
was added for the last 2 h. Cells were then washed, blocked, and
surface stained with FITC-conjugated mAb directed against CD4, CD8,
NK1.1, Gr-1 (BD PharMingen), and B220 (Caltag, Burlingame, CA).
Splenocytes were subsequently permeabilized as described above and
further stained with a PE-conjugated anti-mouse IFN- mAb (BD
PharMingen).
All flow cytometric data were acquired on a FACSCalibur flow cytometer and analyzed with CellQuest software (BD Immunocytometry Systems, San Jose, CA). To obtain absolute cell numbers, percentages obtained after flow analysis were multiplied by the total number of cells in the spleen.
Parasite burden determination
Tissue samples of spleen, liver, brain, pancreas, gut, and lung were homogenized in RNA Stat60 (Tel-Test, Friendswood, TX) and mRNA was isolated according to the manufacturer’s instructions. Equal amounts of mRNA from two mice per group were pooled and assayed for mRNA transcripts encoding the SAG-2 (p22) protein (a tachyzoite stage-specific surface protein, ref. 26) and hypoxanthine phosphoribosyltransferase (host-cell endogenous control), using a one-step RT-PCR kit (TaqMan; Applied Biosystems, Foster City, CA). Real-time PCR was conducted on an Applied Biosystems 7700 Sequence Detector (PE Biosystems, Foster City, CA), using fluorescent-labeled internal probes for SAG-2 and hypoxanthine phosphoribosyltransferase, for the purpose of comparison between organs and parasite strains. We chose to normalize the data to the lowest amount of SAG-2 mRNA detectable, found in samples from the gut of ME49-infected mice. No SAG-2 message was detected in samples from noninfected mice.
RNase protection assay
mRNA from 2 x 107 splenocytes was
isolated as described above, and 5 µg was subjected to RNase
protection assay
(RPA)3
using the mAPO3 template kit and a custom made template commercially
purchased (BD PharMingen), according to the manufacturer’s
instructions. Briefly, antisense mRNA probes labeled with
[-ortho-32P]UTP (NEN Life
Sciences Products, Boston, MA) were hybridized to sample RNA, incubated
with RNase H for ssRNA digestion, and electrophoresed on a QuickPoint
sequencing gel (NOVEX, San Diego, CA). The gel was exposed to a
phosphor imaging screen and read on a STORM 860 phosphor imaging
system. Band intensities were quantitated using the ImageQuant
software (Molecular Dynamics, Sunnyvale, CA).
Histology and immunofluorescence
Immediately following euthanasia, spleen, liver, brain, pancreas, gut, and lung samples were fixed in 10% (w/v) buffered formaldehyde, paraffin embedded, cut in 5-µm-thick sections, and stained with H&E by the Cornell University College of Veterinary Medicine Histology Laboratory. Spleens were also frozen as previously described (27), cut in 5-µm-thick sections, acetone fixed, blocked in 5% normal goat serum (Sigma), and stained with a rat anti-mouse IgD Ab, then a goat anti-rat FITC-conjugated secondary Ab. Fluorescence images were acquired with a Zeiss Axioscop 2 Plus equipped with an Axiocam and an Axiovison 2.0.5 software (Zeiss, Thornwood, NY).
DNA laddering
Detection of DNA laddering was accomplished by isolating DNA from spleen cells using a commercially available kit (Promega, Madison, WI) and running 6 µg/lane on a 2% agarose gel, which was subsequently stained with ethidium bromide.
In situ cell death detection
Frozen spleen sections were fixed in 3.7% Formalin, permeabilized, and subjected to TdT-mediated labeling of nick-end DNA strands with FITC-conjugated dUTP (TUNEL), using a commercially available kit and following the manufacturer’s protocol (Boehringer Mannheim, Indianapolis, IN).
Statistical analysis
A Student’s t test was used for comparison of spleen cell numbers at 8 days postinfection (dpi) between noninfected and RH-infected, and ME49 and RH-infected groups.
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Results |
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The T. gondii strain RH is well known for its high
virulence, whereas the ME49 strain is equally well noted for its low
virulence in mice (22, 28). Nevertheless, while ME49
infection is most commonly initiated by cyst administration, tachyzoite
injection is used for RH infection. Thus, direct comparison of the
virulence characteristics of the strains is problematic. We now show
(Fig. 1
) that mice inoculated i.p. with RH tachyzoites uniformly succumbed to
infection, but 80% of the animals survived an equivalent dose of ME49
tachyzoites.
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Parasites were detected by flow cytometry, real-time RT-PCR, and
histology at 8 dpi with ME49 and RH strain tachyzoites (Fig. 2). Numerous clusters of intracellular tachyzoites of the RH strain were
readily detected in the spleen (Fig. 2A), whereas flow
cytometry revealed 2% of the splenocytes being positive for
intracellular parasites (Fig. 2B). Moreover, 50% of the
cells containing intracellular T. gondii tachyzoites
expressed the Gr-1 (Ly-6G) marker, while no infected cells were
detected in the CD3+ or
B220+ populations (data not shown). We assume the
infected Gr-1- cells to be macrophages. No
parasites were detected by these methods in spleen samples of ME49
strain-infected animals. We could also detect parasite SAG-2 message
(encoding a major tachyzoite surface protein) in the pancreas, gut,
liver, lung, and brain of RH-infected mice at 8 dpi, but not in the
liver and brain samples collected from ME49-infected mice. RH message
was always at least a log higher than the ME49 message (Fig. 2C). This could not be attributed to inherent invasive
properties of the parasite, since in vitro infection rates were
equivalent for both strains (data not shown). At 4 dpi with the RH
strain, we found extensive pyogranulomatous inflammation accompanied by
tissue damage in the pancreas, mild vacuolization of hepatocytes, and
interstitial lymphocyte infiltration in the lung. At the same time
point, there was little evidence of pathology in the ME49-infected
animals. At 8 dpi, pathology in the RH-infected mice was greatly
increased and organs of ME49-infected animals closely resembled those
of RH-infected mice at 4 dpi (data not shown).
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After an initial increase in spleen cell numbers at 4 dpi with
tachyzoites of both ME49 and RH strains, the splenocyte population of
RH-infected mice dramatically decreased 4 days later, to a level below
those of noninfected controls (Fig. 3A, p < 0.001). This was associated with an
increase in nonviable cell recovery at 8 dpi. We stained splenocytes
for surface markers, and found at day 4 an equal initial increase in
CD4+, CD8+,
NK1.1+, and B220+
populations in both infections. However, by day 8 postinfection,
CD4+ and CD8+ T cell
populations were greatly reduced in RH-infected mice (Fig. 3, B and C), while the B220+
and NK1.1+ populations decreased to noninfected
levels (Fig. 3, D and E). These losses were
present, but not as pronounced in ME49-infected animals.
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). When frozen sections were stained for surface IgD to detect mature B
cells in germinal centers, this pattern was confirmed, with loss of
cellular architecture at 8 dpi with RH (Fig. 4). At higher
magnification, multiple clusters of dead cells, clearly distinct from
tachyzoite nests, were visible in the spleens of RH-infected, but not
ME49-infected, mice (Fig. 5, A and B). Nevertheless, the data formally do not
rule out the possibility that an earlier parasite infection at this
site would give rise to the dead cell clusters seen in Fig. 5B. To assess whether this phenomenon was specific to the
high virulence strain, we also examined spleen samples from moribund
ME49-infected mice (day 13 postinfection). Although we could indeed see
loss of global architecture and multiple, randomly distributed pyknotic
nuclei, there was a complete absence of dead cell clusters, and
parasites nests were not visible (data not shown).
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Given the high parasite loads in RH-infected mice, along with the
dramatic reduction of T lymphocyte subsets, we hypothesized that the
inability of animals to control infection might be the result of a
defective type 1 response during infection with this strain. We
therefore determined the type 1 cytokine response of splenocytes from
infected mice and were surprised to find the contrary (Fig. 6). At 8 dpi with RH, flow cytometry revealed a major population of
IFN--positive cells (15.7%), most of which were
CD4+ (55.4%). Further analysis showed that
58.9% of the CD4+ splenocytes from RH-infected
mice were positive for IFN-. In contrast, only 1.2% of the
splenocytes from ME49-infected mice were positive for IFN-, and only
5.5% of their CD4+ cells were positive for the
cytokine (Fig. 6A). Our results demonstrate that during in
vivo infection with T. gondii, most of the IFN- derives
from CD4+ T lymphocytes, as has previously been
shown to be the case during in vitro restimulation assays
(22). When percentages of IFN-+
cells were converted to absolute cell numbers, we found a similar
RH-induced increase, which failed to occur in ME49 infection (Fig. 6B). In vitro cultures confirmed the FACS analysis. Thus,
splenocytes from RH-infected mice constitutively produced five times
more IFN- at day 8 postinfection than identical cultures from
ME49-infected animals (Fig. 6C).
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production in the same cultures.
Splenocyte IL-12 production was high at day 4 after RH infection, then
decreased to background levels at day 8. Secretion of this cytokine was
low at day 4, then slightly increased at day 8 after ME49 infection
(Fig. 7
A).
Parasite-induced TNF- production, which could not be detected at day
4 postinfection, was present at similar levels at day 8 (Fig. 7B).
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C), despite the
presence of IFN-, IL-12, and TNF- (Figs. 6C and 7,
A and B, respectively). The lack of NO production
could be traced to a failure to up-regulate inducible NO
synthase (iNOS) mRNA transcription after RH, but not ME49
infection, as shown by spectrophotometric analysis of RPA products of
splenic RNA from infected mice (Fig. 6D). Assessment of apoptosis during T. gondii infection
Our histological data suggested that spleen cell death was
associated with T. gondii infection, particularly for the
case of the virulent RH strain. Therefore, we examined whether
induction of apoptosis might accompany parasite infection. Numerous
clusters of apoptotic cells were detected by TUNEL at day 8
postinfection in the spleens of RH-infected mice (Fig. 5D).
In spleens from ME49-infected animals, TUNEL-positive cells were
strikingly less frequent and randomly distributed (Fig. 5C).
As shown in Fig. 8A, 200-bp laddering, indicative of programmed cell death, was
induced by both RH and ME49 infections. This response peaked at day 4
and for the case of RH (lane 2) was apparent as soon
as 2 days after infection. For RH infection, the laddering became
inapparent at day 8 postinfection, possibly as a result of further DNA
degradation. Because apoptosis can be triggered independently by
extracellular signals involving TNFRp55 and Fas (Apo-1/CD95) engagement
(29), we used RPA analysis to determine whether
transcription of messages for one or the other of these pathways was
up-regulated during infection. Fig. 8, B and C,
shows the respective changes in Fas ligand (FasL) and TNFRp55
mRNA levels during infection. Transcripts for these apoptosis-related
genes were greatly elevated at 4 dpi with either ME49 or RH. However,
we found earlier (2 dpi) and more sustained (8 dpi) induction of these
genes after RH infection (Fig. 8, B and C). We
found a comparable pattern of transcriptional activation when genes for
apoptosis-related molecules lying downstream of FasL and TNFRp55 were
similarly analyzed (data not shown). In summary, triggering of
programmed cell death is associated with both high and low virulence
T. gondii parasite infections. Nevertheless, the response is
greatly enhanced in animals undergoing infection with the high
virulence RH strain.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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production was greatly elevated
in animals undergoing RH infection, and the source of most of this
cytokine was traced to CD4+ T cells. Despite high
levels of proinflammatory cytokines, production of NO was much lower in
these cultures than in comparable cultures derived from ME49-infected
animals.
Because RH strain tachyzoites reached higher levels and become more
widely disseminated than those of the ME49 strain, it is tempting to
speculate that differences in disease pathogenesis induced by the two
T. gondii strains relates to an ability of RH to replicate
faster than ME49 strain parasites. Indeed, RH tachyzoites multiply more
rapidly than those of the ME49 strain during in vitro culture on
fibroblasts (our unpublished observations). Nevertheless, the
data do not allow us to rule out the possibility that the high
virulence of RH is attributable to factors independent of the
replication rate itself, but nevertheless directly proportional to the
antigenic load. In this regard, the extensive cell death and spleen
dissociation during RH infection is unlikely to result from direct
damage as the parasite replicates and eventually ruptures host cells,
since the level of intracellular parasites detected was relatively low
(2% infected cells). In addition, at the time of splenic architecture
disruption, the abundant dead cells foci were clearly distinct from the
less frequent tachyzoite nests found within the spleen. Infection with
RH tachyzoites led to remarkably high levels of IFN- production in
the spleen, which was also a site of widespread tissue destruction and
apoptotic cell death. Numerous studies have unequivocally demonstrated
the protective role of IFN- in surviving acute and chronic
toxoplasmosis (8, 9, 22, 28, 30, 31). Nevertheless, it is
also clear that T. gondii-induced overproduction of this and
other proinflammatory cytokines may result in pathology and death. For
example, oral infection of the susceptible C57BL/6 mouse strain leads
to lethal inflammatory gut necrosis (12, 13, 14) and apoptotic
cell death in Peyer’s patches (32). Both of these effects
are mediated by proinflammatory cytokines such as IFN-. In another
situation, Toxoplasma infection of IL-10 knockout mice leads
to early death resulting from dysregulated type 1 cytokine production
(11, 14, 33). We hypothesize, as have others
(34), that RH-induced overproduction of IFN- may
contribute to the high virulence of this parasite strain relative to
the low virulence of ME49 tachyzoites.
In this study, we found that almost all of the RH-induced IFN-
derived from CD4+ T cells. Furthermore, 
50%
of the splenic CD4+ cell population displayed
intracellular IFN- by 8 days after infection. We do not at present
know whether this population is composed entirely of activated MHC
class II-restricted, Ag-specific Th1 cells. However, previous studies
have shown that T. gondii infection leads to an early
expansion of T cells bearing the V
5 chain of the mouse TCR
(35, 36). Current studies are direct toward examining the
TCR specificity of the IFN-+
CD4+ cells preferentially induced by infection
with RH strain parasites.
Paradoxically, despite high levels of macrophage-activating cytokines,
we found defective production of NO and low levels of iNOS gene
induction associated with RH, but not ME49, infection. Although we do
not for the moment understand the reasons for this phenomenon, one
possibility is that RH infection induces a population of the recently
described "alternatively activated" or "M-2" macrophages.
Rather than producing proinflammatory mediators, the latter cells
produce anti-inflammatory mediators such as IL-10 and IL-1R
antagonist, and are hypothesized to play a role in down-regulating the
immune response (37, 38, 39). Of possible relevance to our
studies, M-2 macrophages do not up-regulate iNOS gene transcription,
but rather display up-regulated arginase activity (40).
The latter enzyme converts L-arginine to
L-ornithine and urea, thereby removing the substrate for
iNOS which results in down-regulated NO production. A related
hypothesis would be that parasite replication could be promoted by
TGF- release by macrophages engulfing apoptotic cells, as has been
shown to occur during Trypanosoma cruzi infection
(41).
Although NO production was defective during acute RH, relative to ME49, infection, we do not believe that lack of this microbicidal molecule accounts for the inability of mice to control virulent parasite infection. Thus, previous studies have demonstrated that iNOS knockout mice survive acute infection with ME49, although eventually succumbing during later stages of disease (42). Although NO is well known to be effective during in vitro macrophage-mediated tachyzoite killing assays (43, 44), in the setting of in vivo infection this mediator appears more important in preventing toxoplasmic encephalitis rather than controlling acute stage disease.
Our data show that within 2 days of RH infection, substantial levels of apoptotic cell death occur in the spleen, and this is likely to contribute to the structural breakdown of this organ as seen at later stages of acute infection. ME49 infection also initiated programmed cell death in the spleen, but the levels detected were generally lower. Apoptosis during Toxoplasma infection has been observed previously by other investigators. Thus, splenic CD4+ T cells were reported to undergo programmed cell death during infection, and in a model of ocular toxoplasmosis, inflammatory cell apoptosis was implicated in disease pathogenesis (45, 46). T cell apoptosis in the Peyer’s patches was also observed accompanying intestinal necrosis during oral T. gondii infection (32). Our study is the first to suggest that induction of apoptosis may be preferentially associated with the virulence characteristics of the parasite.
Three known pathways lead to apoptotic cell death. Two of the pathways
are triggered by ligation of cell surface receptors with extracellular
mediators (TNF-/TNFR I; Fas/FasL) (47), whereas another
is linked to oxidation events initiated within the mitochondria
(48, 49). Programmed cell death is a normal circumstance
following clonal T cell proliferation and is thought to be an important
immunoregulatory event involved in terminating the immune response
after resolution of infection (48, 50, 51). The TNF-
and Fas pathways of apoptosis have been shown to be important in
resolution of inflammation after certain intracellular protozoan
infections (52, 53). However, it is also clear that
abnormally high levels of apoptosis in both T and non-T cells are
associated with many microbial infections (54, 55, 56, 57, 58, 59, 60). Our
data provide evidence that both TNF-/TNFR I and Fas/FasL
interactions may be involved in parasite-triggered apoptosis, since
both TNFR I and FasL up-regulation accompany appearance of DNA
degradation. We are currently examining whether the
mitochondria-dependent pathway leading to apoptosis is also involved.
Because IFN- can trigger programmed cell death (32, 61), we are also determining whether this cytokine is involved
in apoptotic spleen cell destruction associated, in particular, with RH
infection. Whether induction of apoptosis represents a microbial
strategy to evade the immune response or, for the case of T cells,
reflects high levels of activation in response to infection is not
clear.
In conclusion, our study suggests that virulence of a specific T. gondii strain relates to the inherent ability of the tachyzoite stage to rapidly replicate and disseminate, and to the immune response subsequently generated within the host. Further studies are required to determine the degree to which these two variables are linked.
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Acknowledgments |
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Footnotes |
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2 Address correspondence and reprint requests to Dr. Eric Y. Denkers, Department of Microbiology and Immunology, College of Veterinary Medicine, Cornell University, Ithaca, NY 14853-6401. E-mail address: eyd1{at}cornell.edu
3 Abbreviations used in this paper: RPA, RNase protection assay; iNOS, inducible NO synthase; dpi, days postinfection; FasL, Fas ligand.
Received for publication February 21, 2001. Accepted for publication May 14, 2001.
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References |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
interferon-treated mouse macrophages. Infect. Immun. 61:1141. for the prevention of toxoplasmic encephalitis in mice. J. Immunol. 143:2045.[Abstract]
mice develop unimpaired IL-12 responses to Toxoplasma gondii while failing to control acute infection. J. Immunol. 157:4045.[Abstract]
-activated intestinal epithelial cells. Immunol. Cell Biol. 75:511.[Medline]
, and TNF-. J. Immunol. 157:798.[Abstract]
-mediated necrosis of the small intestine with genetic susceptibility of mice to peroral infection with Toxoplasma gondii. J. Exp. Med. 184:597., NO and IFN- are all critical for development of necrosis in the small intestine and early mortality in genetically susceptible mice infected perorally with Toxoplasma gondii. Parasite Immunol. 21:365.[Medline]
. Parasitol. Res. 80:125.[Medline]
production and protective immunity induced by an attenuated T. gondii vaccine. J. Immunol. 146:286.[Abstract]
-independent IL-12. J. Immunol. 163:2081.: the major mediator of resistance against Toxoplasma gondii. Science 240:516.-Interferon induces Fas-dependent apoptosis of Peyer’s patch T cells in mice following peroral infection with Toxoplasma gondii. Infect. Immun. 65:4682.[Abstract]
5 chain of the mouse T cell receptor. J. Immunol. 156:1089.[Abstract]
activated macrophages. J. Immunol. 148:1792.[Abstract]
-Interferon modulates CD95 (Fas) and CD95 ligand (Fas-L) expression and NO-induced apoptosis during the acute phase of Trypanosoma cruzi infection: a possible role in immune response control. Infect. Immun. 67:3864. and tumor necrosis factor- secretion. Blood 84:2622.This article has been cited by other articles:
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|
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|
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|
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J. P. J. Saeij, J. P. Boyle, M. E. Grigg, G. Arrizabalaga, and J. C. Boothroyd Bioluminescence Imaging of Toxoplasma gondii Infection in Living Mice Reveals Dramatic Differences between Strains Infect. Immun., February 1, 2005; 73(2): 695 - 702. [Abstract] [Full Text] [PDF]
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N. Rachinel, D. Buzoni-Gatel, C. Dutta, F. J. D. Mennechet, S. Luangsay, L. A. Minns, M. E. Grigg, S. Tomavo, J. C. Boothroyd, and L. H. Kasper The Induction of Acute Ileitis by a Single Microbial Antigen of Toxoplasma gondii J. Immunol., August 15, 2004; 173(4): 2725 - 2735. [Abstract] [Full Text] [PDF]
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L. Del Rio, B. A. Butcher, S. Bennouna, S. Hieny, A. Sher, and E. Y. Denkers Toxoplasma gondii Triggers Myeloid Differentiation Factor 88-Dependent IL-12 and Chemokine Ligand 2 (Monocyte Chemoattractant Protein 1) Responses Using Distinct Parasite Molecules and Host Receptors J. Immunol., June 1, 2004; 172(11): 6954 - 6960. [Abstract] [Full Text] [PDF]
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P. M. Robben, D. G. Mordue, S. M. Truscott, K. Takeda, S. Akira, and L. D. Sibley Production of IL-12 by Macrophages Infected with Toxoplasma gondii Depends on the Parasite Genotype J. Immunol., March 15, 2004; 172(6): 3686 - 3694. [Abstract] [Full Text] [PDF]
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L. C. Gavrilescu, B. A. Butcher, L. Del Rio, G. A. Taylor, and E. Y. Denkers STAT1 Is Essential for Antimicrobial Effector Function but Dispensable for Gamma Interferon Production during Toxoplasma gondii Infection Infect. Immun., March 1, 2004; 72(3): 1257 - 1264. [Abstract] [Full Text] [PDF]
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L. C. Gavrilescu and E. Y. Denkers Apoptosis and the Balance of Homeostatic and Pathologic Responses to Protozoan Infection Infect. Immun., November 1, 2003; 71(11): 6109 - 6115. [Full Text] [PDF]
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L. C. Gavrilescu and E. Y. Denkers Interleukin-12 p40- and Fas Ligand-Dependent Apoptotic Pathways Involving STAT-1 Phosphorylation Are Triggered during Infection with a Virulent Strain of Toxoplasma gondii Infect. Immun., May 1, 2003; 71(5): 2577 - 2583. [Abstract] [Full Text] [PDF]
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 C. Su, D. K. Howe, J. P. Dubey, J. W. Ajioka, and L. D. Sibley Identification of quantitative trait loci controlling acute virulence in Toxoplasma gondii PNAS, August 6, 2002; 99(16): 10753 - 10758. [Abstract] [Full Text] [PDF]
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 A. Barragan and L. D. Sibley Transepithelial Migration of Toxoplasma gondii Is Linked to Parasite Motility and Virulence J. Exp. Med., June 17, 2002; 195(12): 1625 - 1633. [Abstract] [Full Text] [PDF]
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L. L. Johnson and P. C. Sayles Deficient Humoral Responses Underlie Susceptibility to Toxoplasma gondii in CD4-Deficient Mice Infect. Immun., January 1, 2002; 70(1): 185 - 191. [Abstract] [Full Text] [PDF]
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D. G. Mordue, F. Monroy, M. La Regina, C. A. Dinarello, and L. D. Sibley Acute Toxoplasmosis Leads to Lethal Overproduction of Th1 Cytokines J. Immunol., October 15, 2001; 167(8): 4574 - 4584. [Abstract] [Full Text] [PDF]
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, Noninfected
controls; 
, ME49-infected mice; 
, RH-infected mice.
