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Infection and Immunity, January 2005, p. 617-621, Vol. 73, No. 1
0019-9567/05/$08.00+0 doi:10.1128/IAI.73.1.617-621.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Interleukin-17/Interleukin-17 Receptor-Mediated Signaling Is Important for Generation of an Optimal Polymorphonuclear Response against Toxoplasma gondii Infection
Michelle N. Kelly,1
Jay K. Kolls,2
Kyle Happel,3
Joseph D. Schwartzman,4
Paul Schwarzenberger,5
Crescent Combe,1
Magali Moretto,1 and
Imtiaz A. Khan1*
Department of Microbiology, Immunology and Parasitology,1
Department of Pulmonary Medicine,3
Division of Gene Therapy, Louisiana State University Health Sciences Center, New Orleans, Louisiana,5
Department of Pediatrics, University of Pittsburgh, Pittsburgh, Pennsylvania,2
Department of Pathology, Dartmouth Medical School, Lebanon, New Hampshire4
Received 15 April 2004/
Returned for modification 7 June 2004/
Accepted 8 September 2004

ABSTRACT
We investigated the role of interleukin-17 (IL-17)/IL-17 receptor
(IL-17R)-mediated signaling in the protective immunity against
Toxoplasma gondii. IL-17R
/ mice developed a normal
adaptive immunity against the parasite. However, increased mortality
in the knockout animals can be attributed to a defect in the
migration of polymorphonuclear leukocytes to infected sites
during early infection.

TEXT
While the induction of a cell-mediated response is essential
for protection against
Toxoplasma gondii, the initial innate
immune response led by neutrophils has also been reported to
be critical for successful resolution of the infection (
1,
6,
13). Depletion studies have shown that their loss leads to exacerbation
of infection (
3,
13). The factors involved in the development
of this neutrophil response against infection have not been
well studied. Recently, interleukin 17 (IL-17) has been shown
to be one of the major cytokines involved in the development
and recruitment of neutrophils (
18). In the present study, the
role of IL-17 receptor (IL-17R)-mediated signaling in the generation
of protective immunity against an intracellular parasite was
evaluated for the first time.
IL-17R/ mice are more susceptible to T. gondii than are parental wild types.
To determine if a lack of IL-17 signaling can alter the susceptibility to T. gondii infection, 5- to 6-week-old female IL-17R/ mice (Amgen Inc., Thousand Oaks, Calif.) and parental C57BL/6 mice (Jackson Laboratories, Bar Harbor, Maine) were challenged perorally with 15 cysts of the 76K strain of T. gondii. As seen in Fig. 1A, nearly 80% of the knockout mice succumbed to infection by day 22 postinfection (p.i.). Conversely, none of the wild-type mice died or exhibited any signs of clinical sickness, and they survived until the termination of the experiment. When the inoculum was increased to 30 cysts, parental control animals survived longer (P < 0.001) but ultimately succumbed to the infection (Fig. 1B).
To determine if the susceptibility of IL-17R
/ mice was due to increased parasite burden, tissues from both
knockout and parental mice were analyzed by quantitative PCR
as already described (
4). The relative abundance of the B1 gene,
a genetic marker for
T. gondii, was determined at day 14 p.i.
in the animals receiving the low dose (15 cysts) and at day
7 p.i. in those infected with a higher dose (30 cysts). As shown
in Fig.
1C, a low-dose infection resulted in an approximately
2 log increase of parasite burden in the tissues (spleen, liver,
gut, and brain) of knockout animals compared to wild-type mice
(
P < 0.05). The levels in the parental controls were close
to uninfected levels, indicating that they were able to resolve
the infection. Similarly, when the mice were infected with a
higher parasite dose, tissues of the knockout mice had a significantly
higher parasite burden than the parental strain, with the liver
displaying the largest difference (
P < 0.05) (Fig.
1D).
IL-17R/ mice display less severe tissue damage in response to T. gondii infection.
Previous studies have reported that C57BL/6 mice infected orally with a high dose of T. gondii cysts develop a severe inflammatory response (8, 15). To determine if lack of IL-17R can alter the inflammatory response, tissues from mice at day 7 p.i were subjected to histopathological analysis as previously described (4). As expected, the liver from parental C57BL/6 mice showed extensive fatty change in hepatocytes (Fig. 2A), while IL-17R-knockout mice had slightly less severe changes (Fig. 2B). As shown in Fig. 2C, the small intestine of the wild-type animals exhibited extensive necrosis and hemorrhage as previously described (10), but the IL-17R/ animals showed much less mucosal damage (Fig. 2D). T. gondii tachyzoites were evident in the lamina propria of the IL-17R/ mice (Fig. 2, insert), while the multiple fields from the wild-type mice did not show any presence of parasites.
PMN influx into tissues is inhibited in IL-17R/ mice.
Our present studies demonstrated that although IL-17R
/ mice exhibit increased mortality from toxoplasma infection,
they develop a normal antigen-specific T-cell immunity and NK-cell
response against the parasite (data not shown). As neutrophils
are important in host defense to
T. gondii (
1,
13) and IL-17
has been implicated in polymorphonuclear leukocytes (PMN) granulopoeisis
and recruitment (
9,
11,
18), we were interested in evaluating
the trafficking of PMNs to the tissues of animals infected with
30 cysts of the 76K strain of
T. gondii. Hematoxylin and eosin-stained
sections of liver and intestine were examined, and relative
PMN indices were determined by examination under an Olympus
Van Ox microscope. Multiple sections of each tissue were counted
in a blinded manner (
7). At days 2 and 4 p.i., a decreased PMN
index was observed in both tissues in the IL-17R
/ mice (Table
1). This difference was more pronounced in the intestine,
but the number of PMNs began to increase in both tissues by
day 6 p.i.
Neutrophil migration into the peritoneal cavity is significantly decreased in IL-17R/ mice after T. gondii infection.
In order to assess the ability of IL-17R
/ mice
to recruit neutrophils, wild-type and knockout animals were
injected intraperitoneally with 2
x 10
6 RH strain tachyzoites.
Four hours later, peritoneal exudate cells were collected (2
x 10
5/sample) and cytospun (700 rpm for 5 min) onto glass microscope
slides (Fisher Scientific, Pittsburgh, Pa.) pretreated with
Vectabond (Vector Lab, Burlingame, Calif.) by using cytofunnels
(Thermo Shandon, Pittsburgh, Pa.). To determine the number of
neutrophils, differential counts (300 cells/slide) were performed
using a Diff-Quick stain (IMEB, San Marco, Calif.) (
5). Previous
studies (
2) have shown that intraperitoneal infection of mice
with tachyzoites of
T. gondii results in a rapid recruitment
of PMNs into the peritoneal cavity. As shown in Fig.
3, the
number of neutrophils in the peritoneum of the wild-type mice
as a result of
T. gondii infection was significantly higher
than that of the IL-17R
/ mice 4 h p.i. (
P <
0.05).
To determine if a defect in neutrophil recruitment is accompanied
by decreased macrophage inflammatory protein 2 (MIP-2) levels,
serum from the infected animals was evaluated for the chemokine
at days 0, 2, 4, and 6 p.i. by using an enzyme-linked immunosorbent
assay kit (R&D Systems) according to the manufacturer's
protocol (
14,
17). As shown in Fig.
4, there was no difference
between control and knockout mice in the initial chemokine level
(day 0). However, at days 2 and 4 p.i., levels of MIP-2 in the
control animals increased significantly (
P < 0.02) over those
in the knockout mice, which remained at concentrations similar
to those of uninfected animals (Fig.
4). By day 6, the level
of MIP-2 in the knockout mice began to increase but was still
significantly less than in the parental control animals (
P <
0.05).
In summary, as stated above, PMNs appear to play an important
role during early
T. gondii infection (
5). The mechanism of
rapid induction of neutrophil response during
T. gondii infection
has not been described. The present study demonstrates for the
first time that early neutrophil induction during
T. gondii infection is dependent on IL-17-mediated signaling. A diminished
response in IL-17R
/ mice was associated with failure
to produce the chemokine MIP-2 early in infection. Based on
our findings, we hypothesize that
T. gondii infection induces
a strong and early neutrophil response. The neutrophils clear
the parasites during initial stages of infection so that adaptive
immunity, which is induced later, is not overburdened. In the
presence of suboptimal levels of neutrophils, parasite load
is not efficiently reduced and adaptive immunity is unable to
handle this elevated burden, which ultimately leads to increased
mortality.

ACKNOWLEDGMENTS
We are thankful to Joseph Chaiban for his help in phenotypic
studies.
This work was supported by National Institutes of Health grant A133325 awarded to I.A.K.

FOOTNOTES
* Corresponding author. Mailing address: Department of Microbiology, Immunology and Parasitology, Louisiana State University Health Sciences Center, New Orleans, LA 70112. Phone: (504) 568-6116. Fax: (504) 568-2918. E-mail:
ikhan{at}lsuhsc.edu.

Editor: S. H. E. Kaufmann

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Infection and Immunity, January 2005, p. 617-621, Vol. 73, No. 1
0019-9567/05/$08.00+0 doi:10.1128/IAI.73.1.617-621.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
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