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*
Department of Immunology and Infectious Diseases, Harvard School of Public Health, Boston, MA 02115; and
Department of Host Defense, Osaka University, Osaka, Japan
| Abstract |
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, those from IL-18-/- mice produced
significantly more IL-12 and IL-4. By 10 wk after infection, both
IL-18+/+ and IL-18-/- mice had resolved
L. major infection. At this time, lymph node cells from
both IL-18+/+ and IL-18-/- mice produced
IL-12 and IFN-
but no IL-4. Furthermore, administration of
anti-IFN-
Abs to IL-18-/- mice rendered them
susceptible to L. major. These results indicate that
despite the role IL-18 may play in early control of cutaneous L.
major lesion growth, this cytokine is not critical for
development of protective Th1 response and resolution of L.
major infection. | Introduction |
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(2, 3, 4). In contrast, susceptible BALB/c mice
develop large nonhealing lesions after L. major infection
and mount a Th2-type response that is associated with the production of
the cytokines IL-4 and IL-10 (2, 5).
IL-18 is a recently discovered cytokine that is produced by activated
macrophages (6). This cytokine has been shown to play a
critical role in the development of protective immunity against
intracellular pathogens including Mycobacterium tuberculosis,
Cryptococcus neoformans, Yersinia enterocolitica, and acute HSV
type 1 (7, 8, 9, 10). The protective role of IL-18 in these
infections has been attributed to its ability to activate NK cells,
enhance proliferation of activated T cells, and induce IFN-
production. Moreover, one study using
IL-12/IL-18-/- (double mutant) mice has
demonstrated that both IL-12 and IL-18 act in synergy to activate NK
cells and induce Th1 development in vivo (11).
Several studies have demonstrated that IL-12 is indispensable for the development of protective immunity against L. major (12, 13). A recent study also found that susceptible BALB/c mice treated with recombinant IL-18 required only small quantities of IL-12 to control cutaneous L. major infection (14). Although these results demonstrate that exogenously administered IL-18 may act synergistically with IL-12 to induce protection against L. major infection in susceptible BALB/c mice, it is not clear whether endogenous IL-18 plays a similar role in the development of protective immunity against L. major in resistant mice. Therefore, we examined the development of Th1 response and cutaneous growth of L. major in resistant C57BL/6 mice lacking the IL-18 gene. Our results suggest that IL-18 may be involved in controlling early lesion growth but that it is not required for the development of a Th1 response and the resolution of L. major infection.
| Materials and Methods |
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IL-18 gene-deficient C57BL/6 mice were generated as described previously (11) and were kindly provided by Dr. S. Akira (Osaka University, Osaka, Japan). The mice were bred and maintained in the facility at the Harvard School of Public Health (Boston, MA) according to the guidelines for animal research. Wild-type C57BL/6 and IL-12-/- C57BL/6 mice of the same sex and age were purchased from The Jackson Laboratory (Bar Harbor, ME) and were used as controls in all experiments.
Parasites and infection protocols
L. major LV39 was maintained by serial passage of amastigotes inoculated s.c. into the shaven rumps of BALB/c mice as described previously (15). Groups of C57BL/6 IL-18 gene-deficient and wild-type C57BL/6 mice were infected in the right hind footpad with 2 x 106 stationary-phase promastigotes of L. major (LV39). Lesion development was measured using a dial-gauge micrometer (Mitutoyo, Kanagawa, Japan) at weekly intervals up to 10 wk after infection. The increase in the thickness of the right hind footpad was compared with the uninfected left hind footpad.
T cell proliferation assay and cytokine analysis
The draining popliteal lymph nodes were removed from L.
major-infected mice at 2 and 10 wk after infection. T cell
proliferation assays were performed as previously described
(15). To the wells of a 96-well flat-bottom
tissue culture plate (Costar, Cambridge, MA), 3
x 106 lymph node cells were added. Cells were
stimulated with 20 µg/ml of freeze-thawed L. major Ag
(LmAg)3 or supplemented medium as a negative control. After
incubation at 37°C for 72 h in 5% CO2,
supernatants were collected from parallel cultures for ELISA
quantification of cytokine production as described previously
(15). Cultures were analyzed for production of IFN-
(reagents purchased from PharMingen, San Diego, CA; detection limit, 20
pg/ml), IL-12 (PharMingen; detection limit, 20 pg/ml), and IL-4
(PharMingen; detection limit, 3 pg/ml).
Leishmania-specific ELISA
Peripheral blood was collected at 3-wk intervals from L. major-infected IL-18+/+ and IL-18-/- mice. Serum was analyzed for Th2-associated Abs, IgG1, and for Th1-associated Abs, IgG2a. Specific levels of these Abs were measured using ELISA as described previously (16).
Histopathology
Infected footpads from IL-18-/- and IL-18+/+ mice were removed and fixed in decalcifying solution F (Stephens Lab, Riverdale, NJ) for 7 days. The tissues were processed and embedded in paraffin, and 4- to 8-µm sections were cut. The sections were hydrated and stained by routine hematoxylin and eosin staining.
Anti-IFN-
neutralizing Ab treatment
Rat anti-mouse IFN-
(clone, XMG 1.2) neutralizing mAb was
kindly provided by Dr. Mary Russell. IL-18-/-
mice were treated by i.p. administration of 1 mg anti-IFN-
neutralizing Ab or control Ab 1 day before L.
major infection and a weekly dose of 1 mg/mouse thereafter for
7 wk.
Statistical significance
Students unpaired t test was used to determine statistical significance of values obtained. Differences in Ab endpoint titers were determined using the Mann-Whitney U prime test.
| Results and Discussion |
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.
Previous studies have demonstrated that IL-12 plays a critical role in
mediating protective immunity against L. major (14, 17, 18). The protective role of IL-12 in murine L.
major infection has been attributed to its ability to activate NK
cells and induce IFN-
production required to develop a Th1 cell
response (19). However, we have recently demonstrated that
endogenous IL-12 can directly induce protective Th1-like response in
the absence of NK cells and can control L. major infection
(15). IL-18 is a recently discovered cytokine that is
produced by activated macrophages (6, 20). This cytokine
shares immunoregulatory functions with IL-12 and plays a critical role
in the host defense against several pathogens (8, 9, 19, 21, 22, 23). A recent study has demonstrated that the treatment of
susceptible BALB/c mice with exogenous IL-18 together with small
quantities of IL-12 significantly enhanced their resistance to
cutaneous L. major infection, indicating that IL-18 may play
a role in the development of protective immunity against L.
major (14). In the present study, we found that
IL-18-/- C57BL/6 mice, although they
occasionally developed significantly larger lesions during the early
course of infection (Fig. 1
B),
eventually resolved their lesions (Fig. 1
, AC)
and controlled parasite replication (Fig. 2
). Furthermore, infected footpads from
both IL-18+/+ and
IL-18-/- mice displayed preserved skin and an
inflammatory infiltrate comprised of lymphocytes and macrophages with
few or no parasites. In contrast, in an another replicate experiment,
concomitantly infected IL-12-/- C57BL/6 mice
developed large nonhealing lesions by week 6 after infection (Fig. 1
C). The lesions from IL-12-/- mice
showed significant ulceration of skin with necrosis and inflammatory
infiltrate comprised primarily of heavily parasitized macrophages,
neutrophils, and eosinophils. Our findings contradict a recent study
that demonstrated that IL-18-/- mice on CD1
genetic background are highly susceptible to L. major and
develop large nonhealing lesions by day 40 after infection
(24). Although different results observed in our study and
that by Wei et al. (24) can be attributed to the
differences in genetic backgrounds of the strains used, we also
monitored disease progression for a longer duration. As reported by Wei
et al., L. major-infected IL-18-/-
mice did develop larger lesions than infected
IL-18+/+ mice did during early course of
infection but eventually resolved them (Fig. 1
, AC). Moreover, the difference in early lesion
sizes between IL-18-/- and
IL-18+/+ mice was statistically significant in
only one of the three experiments (Fig. 1
B).
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early in L. major infection that is
required for the development of a Th1 response in resistant mice
(19). Recent studies have demonstrated that NK cells and
the Th1 subset of CD4+ T cells express IL-18R
(25). Furthermore, NK cell activity is also significantly
impaired in IL-18-/- mice (11).
Therefore, we measured IL-12 and IFN-
production by LmAg-stimulated
lymph node cells and also determined the serum levels of Th1-associated
LmAg-specific IgG2a Abs in IL-18+/+ and
IL-18-/- mice at 2 and 10 wk after L.
major infection. At both of these time points, LmAg-stimulated
lymph node cells from both IL-18+/+ and
IL-18-/- mice produced comparable levels of
IFN-
(Fig. 3
neutralizing Ab developed significantly
larger lesions compared with those on similarly infected
IL-18-/- mice treated with control Ab (Fig. 5
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In conclusion, IL-18-/- mice from a resistant C57BL/6 genetic background may develop larger lesions during early course of L. major infection but eventually will resolve them efficiently. Moreover, the resolution of L. major infection by IL-18-/- mice is associated with the development of an efficient Th1 response.
| Footnotes |
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2 Address correspondence and reprint requests to Dr. Abhay R. Satoskar, Department of Immunology and Infectious Diseases, Building 1, Room 804, Harvard School of Public Health, 665 Huntington Avenue, Boston, MA 02115. ![]()
3 Abbreviation used in this paper: LmAg, freeze-thawed L. major Ag. ![]()
Received for publication November 15, 1999. Accepted for publication March 21, 2000.
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