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Axis in Controlling Tissue Parasitism and Inflammation in the Heart and Central Nervous System during Trypanosoma cruzi Infection
From the Department of Biochemistry and Immunology,*
Institute of Biological Sciences, Federal University of Minas Gerais,
Belo Horizonte; the Laboratory of
Immunopathology,
René Rachov Research
Center-Oswaldo Cruz Foundation, Belo Horizonte; and the Department of
Immunology,
Institute of Oswaldo Cruz-Oswaldo
Cruz Foundation, Rio de Janeiro, Brazil
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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/iNOS axis and the antagonistic
effect of IL-4 in controlling heart tissue parasitism,
inflammation, and host resistance to acute infection with
T. cruzi. Further, the heart and central nervous
system were shown the main sites of reactivation of T.
cruzi infection in mice lacking functional genes for IFN-
and IL-12, respectively. Our results also show that in contrast
to IFN- knockout (KO) mice, splenocytes from IL-12 KO mice
infected with T. cruzi produced low levels of IFN-
upon stimulation with antigen. Consistently, high levels of
anti-T. cruzi IgG2a antibodies were detected in the sera
from IL-12 KO, but not from IFN- KO mice, infected
with the Colombian strain of T. cruzi.
Thus, our results suggest that the level of IFN- deficiency
is a major determinant of the site of reactivation of T.
cruzi infection in immunocompromised host.
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Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The symptoms presented by patients with
either acute or chronic Chagas’ disease are highly variable, and
thought to be dependent on both parasite and host-related
factors.3-14
Similarly, the pathology observed in mice
infected with T. cruzi is highly dependent on parasite
strain,15-17
size of parasite inoculum,18
host genetic background,19,20
and the action of various
components of host immune system.10,14
Different studies
indicate the crucial role of cytokines, such as interferon
(IFN)-,21
tumor necrosis factor-
,22
and
interleukin (IL)-1223
as well as nitric
oxide24-26
in host resistance to infection with T.
cruzi. Soon after the first rounds of replication in the
vertebrate host tissues, T. cruzi parasites are thought to
trigger the synthesis of proinflammatory cytokines that initiate the
production of IFN- by natural killer (NK) cells,27
CD4+CD8-ß+
and
CD4-CD8+ß+
T lymphocytes.28
IFN- combined with tumor necrosis
factor- will activate macrophages to produce high levels of nitric
oxide that are primarily responsible for control of parasite
replication during the acute phase of infection.22
However, scanty information is available regarding the role of
cytokines controlling parasite replication and pathology in the heart
tissue and CNS, the main sites of parasite replication and pathology
during Chagas’ disease in immunocompetent and immunodeficient hosts,
respectively. The lack of such information is mainly because various
studies using the experimental murine model of Chagas’ disease used
parasite strains with low tropism to the cardiac tissue and CNS.
Secondly, most studies using genetically engineered immunodeficient
mice infected with T. cruzi, have limited their evaluation
to measurements of parasitemia and mortality rates. The present study
was undertaken to determine the role of different cytokines on the
kinetics of tissue parasitism and inflammatory cell infiltrates in the
heart tissue and CNS of mice infected with the Colombian strain, a
T. cruzi strain with high tropism for the cardiomyocytes and
previously shown to induce encephalitis in susceptible
mice.29-31
Our results show the essential role of
IL-12/IFN-/inducible nitric oxide synthase (iNOS) axis, and the
antagonistic effect of IL-4, in regulating parasite replication in the
heart tissue and host resistance to T. cruzi during acute
phase of infection. Moreover, we demonstrate that the deficiency of
endogenous IL-12 and IFN- is determinant of the reactivation of
T. cruzi infection in CNS and heart tissues, respectively.
Finally, this study provides the first experimental model that
resembles the parasite-elicited mass encephalitis, which is often found
in chronic chagasic patients with AIDS, and contributes with new
understanding of the pathogenesis of T. cruzi infection in
immunodeficient hosts.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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Female BALB/c and C57BL/6 mice were obtained from the Oswaldo Cruz
Foundation-FIOCRUZ (Rio de Janeiro, Brazil). The IL-4-KO in the BALB/c
genetic background, IFN--KO, IL-12-KO, and iNOS-KO mice all in the
C57BL/6 genetic background were provided by the Laboratory of
Gnotobiology, ICB-UFMG (Belo Horizonte, Brazil). Female mice, 6 to 8
weeks of age, were maintained under standard conditions with
environmental barriers in the animal house of the Centro de Pesquisas
René Rachou, FIOCRUZ (Belo Horizonte, Brazil), and used for the
experiments described bellow.
Parasites and Experimental Infection
The Colombian strain of T. cruzi was isolated by Frederici and colleagues31 and maintained by serial passages from mouse to mouse in the Laboratory of Chagas’ Disease, CPqRR-FIOCRUZ (Belo Horizonte, Brazil). BALB/c, C57BL/6, and different KO mice were infected intraperitoneally with 5000 blood trypomastigote forms of the Colombian strain of T. cruzi. In some experiments, the animals were treated with a subcurative dose of benznidazole (100 mg/kg/day) from day 10 to day 17 after infection. The levels of parasitemia were evaluated using 5 µl of blood in an optical microscope as previously described.32 All experiments were repeated three times and conducted according to the institutional guidelines for animal ethics of the Oswaldo Cruz Foundation.
Histological Evaluation
The myocardium and CNS were fixed in neutral 10% formalin, embedded in paraffin, sectioned, stained with hematoxylin and eosin (H&E), and examined by light microscopy. Tissue parasitism was scored by counting the total number of amastigote nests in 25 microscope fields (1 x 200 magnification) per histopathological section. Four sections were counted for each animal and the individual data determined as the mean result of the four sections.
An inflammatory infiltrate was considered when we detected 50 leukocytes or more in each inflammatory area. The inflammatory infiltrate was subdivided into focal and diffuse, depending on how close the inflammatory cells were associated. Most cells from the focal inflammatory infiltrate were in direct contact with each other, forming a continuous site of inflammation. In contrast, the diffuse inflammatory infiltrate was defined as a high density of mononuclear cells scattered throughout the cardiac our nervous tissue or composed of one or more small inflammatory foci containing less than 50 inflammatory cells. Considering that we found a high positive correlation between the intensities of focal and diffuse inflammatory infiltrates, for the sake of simplicity the results are presented as the average of the intensity of focal and diffuse inflammatory infiltrates. For the inflammatory infiltrate score, the total numbers of focal or diffuse inflammatory foci were counted in 25 microscope fields (1 x 200 magnification) per tissue section. Four sections were counted for each animal and individual data determined as the mean result of the four sections.
For both tissue parasitism and inflammation scores, the quantification was performed in four noncontiguous sections (40-µm distance between them) in 25 fields (1 x 200) magnification in a blind manner by three researchers.
Parasite-Specific Immunocytochemistry in the Cardiac Tissue and CNS from Infected Mice
For immunocytochemistry, deparaffinized sections were incubated for 30 minutes at 37°C in 1% bovine serum albumin to reduce nonspecific binding and then incubated in immune anti-T. cruzi mouse serum diluted 1:300 at 4°C overnight. Secondary biotinylated antibodies were goat anti-mouse antibodies. The sensitivity was improved with the avidin-biotin technique (ABC kit, PK-4000; Vector Laboratories, Inc., Burlingame, CA). The reaction was visualized by incubating the section with 3,3'-diaminobenzidine tetrahydrochloride (Amresco, Solon, OH).
Parasite Antigen Preparations
The epimastigote forms of the Colombian strain of T. cruzi were grown at 28°C in cell-free liver infusion tryptose medium supplemented with 10% fetal calf serum. Live parasites were harvested, washed three times in phosphate-buffered saline (PBS), resuspended at a final concentration of 109 parasites/ml and then submitted into three cycles of freeze and thawing at -70°C and 37°C, respectively, followed by five 30-second rounds of sonication. The resulting extract was centrifuged at 10,000 x g for 30 minutes and the supernatant, named soluble T. cruzi antigens (TcAg), frozen at -70°C until use in the stimulation of cytokine synthesis by spleen cells or enzyme-linked immunosorbent assay assays to measure serum levels of anti-T. cruzi antibodies.
Quantification of Parasite-Specific IgG1 and IgG2a
A Maxsorp enzyme-linked immunosorbent assay plates (Nalge Nunc International, Rochester, NY) were covered overnight at 4°C with 10 µg/ml of TcAg in carbonate-bicarbonate buffer (0.1 mol/L, pH 9.6). Serum from individual animals were diluted 1:40 in PBS-0.05% Tween 20 (PBS-Tween) and incubated for 1 hour at 37°C after blocking with 1% albumin in carbonate-bicarbonate buffer for 2 hours at 37°C. One hundred µl of 1:1000 of nonconjugated goat anti-mouse IgG1 or IgG2a (Southern Biotechnology Associates, Birmingham, AL) diluted in PBS-Tween was added to each well and the plate incubated for 1 hour at 37°C. For the detection of anti-IgG1 and IgG2a antibodies, a peroxidase-conjugated anti-goat IgG (1:5000) was incubated for 1 hour at 37°C. The assay was developed using the 2,2'-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) substrate (Sigma Chemical Co., St. Louis, MO), and the reaction was stopped with 20 µl of 20% sulfuric acid solution.
Spleen Cell Preparations and IFN- Assay
Spleens were removed aseptically and single-cell suspensions were
prepared in complete medium after lysis of red cells. Splenocytes were
then cultured in the presence of medium alone, with 20 µg of TcAg, or
5 µg/ml Con-A. After incubation for 72 hours, supernatants were
removed and assayed for IFN-.29
IFN- was assayed in
a two-site enzyme-linked immunosorbent assay using a rat anti-IFN-
mAb R46A2 (ATCC, Rockville, MD) and a polyclonal rabbit serum specific
for the cytokine. IFN- levels were calculated by reference to a
standard curve constructed with recombinant cytokine (Genzyme,
Cambridge, MA). Sensitivity of this method was 100 pg/ml.
Statistical Analysis
Arithmetic means (parasitemia, amastigote nests, inflammatory foci, and antibody levels) and standard deviations of the means were calculated. The Student’s t-test was used to determine the statistical significance of the differences observed. Differences were considered statistically significant when P < 0.05. The Kruskal-Wallis test was used to compare the mouse survival rates, and differences were considered statistically significant when P < 0.05.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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The tissue parasitism (Figure 1a
, and Figure 2, D
and E), parasitemia
(Figure 1c
), and rate of mortality (Figure 1d
) were all
higher in the BALB/c as compared to C57BL/6 mice infected with T.
cruzi. In contrast, the intensity and the kinetics of diffuse and
focal inflammatory infiltrates were similar in the heart tissue of both
isogenic mouse strains infected with T. cruzi parasites
(Figure 1b
and Figure 2, D
and E). Very rare isolated
amastigote nests and no or only mild inflammation were found in the CNS
of either BALB/c or C57BL/6 mice infected with the Colombian strain of
T. cruzi (data not shown). The serum levels of both
parasite-specific IgG1 and IgG2a isotypes were higher in BALB/c (Figure 1e
) than in C57BL/6 (Figure 1f
) mice. Further, the IgG1/IgG2a
ratio was also higher in BALB/c as compared to C57BL/6 mice infected
with the Colombian strain of T. cruzi.
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Because the BALB/c mice are normally high producers of IL-4, which
has been shown to antagonize various IFN--mediated protective immune
responses in various systems,33
we evaluated the course of
infection with the Colombian strain of T. cruzi in IL-4-KO
BALB/c mice. Accordingly, IL-4 KO mice were more resistant to T.
cruzi infection as indicated by heart tissue parasitism (Figure 2, E
and F, and Figure 3a
),
parasitemia (Figure 3c
), and lower mortality ratio (Figure 3d
). No major differences were observed in the intensity of
diffuse and focal inflammation in the heart tissue when comparing IL-4
KO and wild-type (WT) animals (Figure 2, E
and F, and Figure 3b
). The histopathology sections presented in Figure 2, E
and
F, show an intense inflammatory reaction in the heart tissue of a
BALB/c and an IL-4 KO mouse at 30 days after infection containing
various and few amastigote nests, respectively. Figure 3e
shows
that the levels of anti-T. cruzi-specific IgG1 antibodies
were reduced in sera of IL-4 KO mice infected with T. cruzi.
In contrast, no alteration in the serum levels of parasite-specific
IgG2a antibodies was observed when comparing IL-4 KO (Figure 3e
)
and WT BALB/c (Figure 3f
) mice infected with T. cruzi.
|
KO, or iNOS KO Are Highly Susceptible to
Infection with the Colombian Strain of T. cruzi
We also infected IL-12 and IFN- C57BL/6 KO mice with the
Colombian strain of T. cruzi. Our results show that both
IFN- KO and IL-12 KO were highly susceptible to infection displaying
a high number of amastigote nests (Figure 4a
), parasitemia (Figure 4c
),
and 100% of mortality by 20 days after infection (Figure 4d
). In
contrast to the intensity of tissue parasitism, we observed limited
inflammatory reaction in the heart tissue from IFN- KO and IL-12 KO,
at 15 days after infection (Figure 4b
). No enhanced tissue
parasitism and/or inflammation in the CNS was observed, when comparing
IFN- KO and IL-12 KO to WT mice acutely infected with the Colombian
strain of T. cruzi (data not shown).
|
), parasitemia
(Figure 4g
), and accelerated mortality (Figure 4h
) as
compared to C57BL/6 mice. These results differed from those obtained
from IL-12 KO and IFN- KO, in that the iNOS KO survived more than 30
to 40 days of infection presenting the peak of heart tissue parasitism
(Figure 4e
) and parasitemia (Figure 4g
) at day 30 after
infection. Differences were observed in the intensity of diffuse and
focal inflammation in the cardiac tissue, when comparing iNOS KO and WT
other KO mice at 15 days after infection, but not at the latter time
point.
The histopathology sections show one amastigote nest in the cardiac
tissue from C57BL/6 mice (Figure 2A)
contrasting with multiple
amastigote nests in the heart from IFN- KO (Figure 2B)
and iNOS KO
(Figure 2C)
mice, all at 15 days after infection. Note mild
inflammation observed in IFN- KO mice, contrasting with more intense
inflammation in iNOS KO mice (Figure 2C)
.
Heart Tissue and CNS as the Major Sites of Reactivation of
T. cruzi Infection in IFN- KO and IL-12 KO Mice
We next tested the role of endogenous IFN-, IL-12, and iNOS in
the control of tissue parasitism, parasitemia, and mortality in mice
that have latent infection with the Colombian strain of T.
cruzi. WT and the various KO mice were infected and submitted to
treatment with benznidazole, beginning at 10 days after infection, soon
after parasitemia become patent. The mice received 100 mg/kg/day of
benznidazole from days 10 to 17 after infection. This therapeutic
protocol, although highly efficient in controlling tissue parasitism
and parasitemia, is not curative for mice infected with the Colombian
strain of T. cruzi. After cessation of chemotherapy, the
mice were followed for tissue parasitism, parasitemia, and mortality.
Benznidazole therapy resulted in 100% and 80% of survival (and no
measurable parasitemia) in WT and iNOS KO mice, respectively. In
contrast, greater parasitemia (Figure 5a
) and accelerated mortality
(Figure 5b
) became apparent in the IFN- KO and IL-12 KO mice
were after cessation of chemotherapy. Although increased, the
parasitemia (Figure 5a
) and heart tissue parasitism (Figure 5c
and
Figure 2G and 2
hours) of IL-12 KO was relatively small when compared
to that observed in the IFN- KO mice. One hundred percent mortality
was observed at 40 and 55 days after infection of IFN- KO and IL-12
KO mice, respectively (Figure 5b
). The small increase in tissue
parasitism observed in IL-12 KO mice was accompanied by a dramatic
increase in heart tissue inflammation at day 40 after infection and
after (Figure 5e
). In contrast, a weak inflammatory process was
observed in the heavily parasitized heart tissue from IFN- KO mice
at 40 days after infection or earlier (Figure 5e
). The WT
and iNOS KO mice were relatively resistant to reactivation of infection
up to 100 days after infection (Figure 5, a
and b). These
findings are in agreement with those published by Saeftel and
colleagues34
showing that mouse treatment with iNOS
inhibitors does not change the course of late acute and chronic
infection with T. cruzi.
|
and
Figure 6, E
and F),
inflammation (Figures 5f
and 6E), and lesions in the
brain (Figures 6, E
and G, and 6 hours) of IL-12 KO
progressively increased from day 40 to 55 after infection, as indicated
by analysis at days 40, 45 (not shown), 50, and 55 (not shown) days
after infection. The inflammatory reaction in the IL-12 KO mice was
characterized by glial nodules and necrotic lesions (Figure 6E
),
vascular cuffing by lymphocytes (Figure 6G
), and meningitis
(Figure 6H
). An intense tissue parasitism accompanying the
inflammatory reaction in the brain of IL-12 KO mice at 50 to 55 days
after infection, can be observed in either histopathological section
(Figure 6E
) or immunocytochemistry (Figure 6F
). Although less
intense then in IL-12 KO mice, a gradual increased tissue parasitism
and inflammation in the brain was also observed during reactivation of
infection in IFN- KO (Figure 5d
and Figure 6, C
and
D) mice, at 30, 35 (not shown), and 40 days after infection.
Inflammation (Figures 5e
and 6A) and amastigote nests
(Figures 5d
and 6B) were a rare finding in the brain of WT
mice infected with Colombian strain of T. cruzi. It is
noteworthy that under a similar protocol, infection with Y strain of
T. cruzi resulted in reactivation of infection in the
IFN- KO but not in the IL-12 KO mice (data not shown), indicating a
parasite strain dependency for the reactivation of T. cruzi
in the CNS.
|
in the in the inflammatory processes found
in the heart from IL-12 KO mice at 45 to 55 days after infection.
IFN--producing cells were a rare finding in the CNS of IL-12 KO mice
at 45 to 55 days after infection with the Colombian strain of T.
cruzi (data not shown).
We also measured the levels of anti-T. cruzi IgG1 and IgG2a
antibodies in the sera of mice infected with Colombian strain and
treated with benznidazole. Our results show that the parasite-specific
IgG1 (Figure 5g
) and IgG2a (Figure 5h
) levels were higher in
IFN- KO and IL-12 KO, respectively. In agreement with these results,
we found that when stimulated with TcAg, spleen cells from IL-12
KO mice produced 
20% of the total IFN- produced by splenocytes
from WT animals (Figure 5i
). Together these results indicate that
an IL-12-independent pathway for IFN- synthesis is operating during
T. cruzi infection.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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In the present study, we used the Colombian strain of T.
cruzi that causes an intense cardiomyopathy29,31,35
as well as encephalitis30
during experimental infection in
mice. Our previous study showed that inflammation, elicited by the
infection with Colombian strain in mice, is accompanied by enhanced
expression of a mixed profile of type 1 (ie, IL-12 and IFN-) as well
as type 2 cytokine (ie, IL-4 and IL-10) mRNAs in the
heart.29
Herein, by using isogenic as well as cytokine and
iNOS KO mice, we investigated the role of different cytokines on
control of tissue parasitism and inflammation during infection with the
Colombian strain of T. cruzi in mice.
Initially, we used the C57BL/6 and BALB/c isogenic mice strains that,
on various stimuli or microbial infections, have been shown to develop
a dominant type 1 and type 2 immune response,
respectively.36,37
Consistent, with studies performed with
other T. cruzi strains,19,20
BALB/c mice were
shown to be more susceptible than C57BL/6 when infected with the
Colombian strain. The enhanced susceptibility of BALB/c mice to
T. cruzi infection was associated with enhanced production
of IL-4 (data not shown) as also indicated by the higher
parasite-specific IgG1/IgG2a ratios, when compared to C57BL/6 mice.
More importantly, we could identify IL-4 as a major immunological
factor mediating susceptibility of BALB/c mice, as suggested by the
lower number of amastigote nests, parasitemia, and lower rate of
mortality of IL-4 KO BALB/c mice acutely infected with the Colombian
strain of T. cruzi. In contrast to our results, early
reports demonstrate no differences of resistance in IL-4 KO or STAT6 KO
as compared to WT mice infected with other T. cruzi strains
(ie, Y and Brazil), that present low tropism for the cardiac
tissue.38,39
Indeed, we have previously observed that in
contrast to the Colombian strain, the Y strain of T. cruzi
induces lower levels of IL-4 synthesis and a more dominant IL-12 and
IFN- immune response during infection in the murine
model.29,32
In the conditions used here, the levels of cardiac inflammation were similar when comparing BALB/c and C57BL/6 or IL-4 BALB/c KO mice. Other studies have demonstrated the important anti-inflammatory role of IL-440 and IL-1041,42 in preventing excessive inflammation in animals infected with T. cruzi. The anti-inflammatory effect of IL-10 during infection with T. cruzi also seems to be dependent on parasite strain, because IL-10 does not influence the course of infection with the Y strain of T. cruzi.43 Nevertheless, these studies suggest that IL-4 and IL-10 may act in synergy to control myocarditis elicited during infection with T. cruzi. However, as shown here, if the immunoregulatory role of IL-4 is dominant it will favor parasite replication leading to enhanced host susceptibility to infection. It is noteworthy that, a study performed by Reis and colleagues44 show a close association of IL-4-producing T cells and the presence of amastigote nests in the myocarditis elicited by T. cruzi parasites in humans.
Different studies have demonstrated the importance of
IL-12/IFN-/iNOS axis in resistance to T. cruzi
infection.21-26
Therefore, our next set of experiments
evaluated the role of endogenous IL-12, IFN-, and iNOS on host
resistance to infection with the Colombian strain of T.
cruzi. Our results indicate that either IL-12 or IFN- are
essential for host resistance to acute infection with this parasite.
According to a previous study,26
our experiments show that
iNOS is also essential to control parasite replication in the cardiac
tissue and host resistance to infection with T. cruzi. It is
noteworthy that the iNOS KO mice were somewhat more resistant than
either IL-12 KO or IFN- KO mice, indicating that an additional
IFN--dependent mechanism, but iNOS-independent may also contribute
to the control of parasite replication in heart tissue and host
resistance to the acute infection with the Colombian strain of T.
cruzi.
Regarding the reactivation of T. cruzi infection in IFN-
KO mice was characterized by extremely high levels of parasitemia and
cardiac tissue parasitism, and relatively low levels of inflammation.
In contrast, a small increase in parasitemia and cardiac tissue
parasitism was associated to a dramatic increase in inflammation in
cardiac tissue of the IL-12 KO mice after cessation of chemotherapy.
Interestingly, similar findings are observed in heart tissue of AIDS
patients during reactivation of chronic T. cruzi
infection.45
More importantly, reactivation of T.
cruzi infection in IL-12 KO mice was associated with signs of
neurological disturbances, encephalitis, and areas of the CNS with
intense parasite replication, inflammation, and tissue damage. The
essential role of endogenous IL-12 on control of Toxoplasma
gondii and Cryptococcus neoformans infection in the CNS
from the murine model has also been reported.46,47
Based on parasite-elicited IFN- production by spleen cells and
parasite-specific IgG isotype measurements in sera from WT, IL-12 KO,
and IFN- KO mice, we favor the hypothesis that the level of
endogenous IFN- is a major determinant of the site of parasite
replication during reactivation of T. cruzi infection.
Indeed, previous studies have suggested that the induction of IFN-
during T. cruzi infection may also occur via an
IL-12-independent manner.23,48
Thus, in the IFN- KO
mice, where virtually no IFN- is produced, the reactivation of
infection occurred in peripheral organs mainly in the heart tissue. In
contrast, in the IL-12 KO mice, the levels of IFN- and
IFN--dependent effector mechanisms, although lower than in the WT
mice, are sufficient to control parasite replication in the peripheral
organs. However, the IL-12-independent IFN--based immunity seems not
to be sufficient to control parasite replication in the CNS. Our
studies with iNOS KO mice indicate that the main mechanism responsible
for the control of reactivation of T. cruzi infection is
dependent on IFN-, but relatively independent of iNOS.
Importantly, during reactivation of infection the inflammatory
processes in the CNS from IL-12 KO mice were rich in polymorphic
nuclear cells and mostly devoid of CD8+ T cells.
Further, the CD4:CD8 ratio in the CNS and heart from
benznidazole-treated IL-12 KO mice, at 50 days after infection, were
3.7: 1 and 1:3.4, respectively. Consistent with the hypothesis that
CD8+ T lymphocytes are major sources of IFN-
during T. cruzi infection, we found IFN--producing cells
in the heart, but not in the CNS from these animals. Thus, considering
the important role of CD8+ T lymphocytes in
resistance to T. cruzi,28,49
the increased
susceptibility to parasite replication and mass encephalitis could be
explained by the lack of CD8+ T lymphocyte
migration and local production of IFN- in the CNS from IL-12 KO
mice. The parasite-specific IgG2a antibodies could be an alternative
IFN--inducible effector mechanism responsible for controlling
parasite replication and preventing reactivation of T. cruzi
infection. However, we found that in contrast to IFN- KO, the IL-12
KO mice produce high levels of parasite-specific IgG2a when infected
with T. cruzi. Thus, it is possible that parasite-specific
antibodies from IgG2a isotype play an important role in controlling
parasitemia and tissue parasitism in the peripheral organs but not in
the CNS.
Finally, this study provides for the first time an experimental model
and new basis to understand the pathogenesis of the T.
cruzi-induced mass encephalitis in immunodeficient hosts. Thus,
our results indicate that host resistance to T. cruzi
replication in the CNS (an immunologically privileged site) as compared
to peripheral organs, requires higher levels of endogenous IFN-
and/or IFN--mediated effector functions. Considering the gradual
decline in IL-12 and IFN- production in HIV-1-infected
patients,50-52
these findings may explain, in part, why
the CNS is the main site of reactivation of infection with T.
cruzi12,13
in AIDS patients.
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Acknowledgements |
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Footnotes |
|---|
Supported in part by the FAPEMIG (CBB 916/96), PAPES-FIOCRUZ (no. 2), and CNPq (522.056/95-4). R. T. G., L. Q. V., and J. L. V. are research fellows from CNPq. V. M. and N. M. S. are graduate students with scholarships from FIOCRUZ and CNPq, respectively.
Accepted for publication July 23, 2001.
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References |
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H. C. Santiago, C. G. Feng, A. Bafica, E. Roffe, R. M. Arantes, A. Cheever, G. Taylor, L. Q. Vierira, J. Aliberti, R. T. Gazzinelli, et al. Mice Deficient in LRG-47 Display Enhanced Susceptibility to Trypanosoma cruzi Infection Associated with Defective Hemopoiesis and Intracellular Control of Parasite Growth J. Immunol., December 15, 2005; 175(12): 8165 - 8172. [Abstract] [Full Text] [PDF]
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) or BALB/c
(
) mice at different
times after infection. b: Number of inflammatory infiltrates
present in 25 histopathological fields (original
magnification, 1 x 200) of cardiac
sections obtained from C57BL/6
(
