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CUTTING EDGE |
Department of Immunology, University of Glasgow, Western Infirmary, Glasgow, G11 6NT United Kingdom.
| Abstract |
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but less IL-4
and IL-5 than cells from MRL-+/+ mice. Peritoneal macrophages from the
mutant mice also produced more IL-12 and NO after stimulation with LPS.
Thus, Fas expression is essential for resistance against leishmaniasis,
and Fas-mediated apoptosis may form an integral part of the
Th1-mediated microbicidal function. | Introduction |
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Cutaneous leishmaniasis is the most polarized example of the
differential roles of Th1 and Th2 subsets of CD4+ T cells.
An impressive range of clinical and experimental evidence supports the
host-protective role of Th1 cells that produce IFN-
, which activates
macrophages to produce NO, which kills the intracellular parasite
(8, 9, 10). IL-12 is the major inducer of Th1 cell differentiation (11).
In contrast, Th2 cells driven by IL-4 and also producing IL-4 are
disease promoting, via the inhibition of the expression of inducible
nitric oxide synthase (iNOS)5
by IL-4 (12, 13).
We report here that contrary to expectation, MRL/lpr are highly susceptible to L. major infection. While all the L. major-infected wild-type MRL-+/+ mice achieved spontaneous lesion healing, the MRL/lpr mice were unable to restrict the disease development. This was so despite the fact that the infected mutant mice produced more IL-12, developed elevated Th1 responses, and synthesized more NO than the wild-type mice. These data therefore demonstrate that Fas expression is required in the resistance against leishmaniasis. This requirement can override the strong presence of IL-12, Th1, and NO activities, suggesting that the Fas-Fas ligand (FasL) pathway is an integral part of the Th1 microbicidal function.
| Materials and Methods |
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Female MRL/lpr and age- and sex-matched control
MRL/MP-+/+ (MRL-+/+) mice were obtained from Harlan U.K. (Bicester,
U.K.). Some of the mice were bred in the animal facilities, University
of Glasgow, from pairs obtained from Harlan U.K. They were housed under
virus-free environment. The iNOS-deficient mice were derived as
described previously (8). Disruption of the murine iNOS gene was
achieved by homologous recombination in 129sv embryonic stem cells. The
recombinant allele was passed through the germline following mating of
embryonic stem cell chimeras with 129sv (Harlan U.K.). All the mice
used were from littermate matings and were kept in a
specific-pathogen-free environment. Extensive experiments using a
variety of parameters demonstrated phenotypic similarity between the
heterozygous and wild-type littermates. Peritoneal cells from the
mutant mice did not produce iNOS protein following activation with
IFN-
and LPS in vitro, as judged by Western blot. They produced only
background levels of nitrite up to 48 h of culture with IFN-
and LPS.
Parasite and infection
The L. major isolate MRHO/SU/59/P, also known as LV39, was used throughout. The maintenance, cultivation, and isolation of the parasites have been described in detail elsewhere (14). Mice were injected s.c. in the right hind footpad with 1 x 106 stationary phase promastigotes, and the lesion development was measured at regular intervals (14). At the end of the experiments, mice were killed by cervical dislocation, and serum, spleen, and footpads were collected. Parasite loads in the infected footpad and draining lymph nodes were estimated by limiting dilution (15). Soluble antigen was prepared from promastigotes by five cycles of freezing and thawing (2 x 108 organisms/ml in PBS), followed by centrifugation at 8000 x g for 10 min in 4°C. The supernatant was filtered (0.45-µm pore size filter) and stored at -70°C.
Cell culture
Resident peritoneal cells (2 x 105
cell/well) in 200 µl were cultured in 96-well culture plates (Nunc,
Roskilde, Denmark) in complete culture medium (RPMI 1640 (Life
Technologies, Paisley, U.K.) supplemented with 10% heat-inactivated
FCS, 50 U/ml penicillin, 50 µg/ml streptomycin, and 50 µM
2-mercaptoethanol) at 37°C and 5% CO2 for up to 6
d. To stimulate for IL-12 and NO production, LPS (100 ng/ml,
Salmonella enteritidis, Sigma, Poole, U.K.) and IFN-
(50
U/ml, a kind gift of Dr. G. Adolf, Bender, Vienna, Austria) were added.
To stimulate for IFN-
, IL-4, and IL-5 production, spleen cells
(2 x 105 cells/well) were cultured with soluble
leishmanial antigen (5 x 106 organism equivalent/ml)
or Con A (2.5 µg/ml). Culture supernatants were collected at regular
intervals and stored at -70°C. T cell proliferation was determined
by [3H]TdR incorporation, and results were expressed as
mean ± SD in triplicate cultures.
Cytokine assays
These were conducted by ELISA in 96-well plates (Immulon 4,
Dynatech, Billinghurst, U.K.). For IL-12, the capture Abs were C15.1.2
and C15.6 (kind gifts of the Genetic Institute, Boston, MA), and the
detection Ab was a rabbit anti-IL-12 Ab (Rab 74.6). For IFN-
,
IL-4, and IL-5, paired mAbs from PharMingen (Cambridge Bioscience,
Cambridge, U.K.) were used. Recombinant cytokines (IL-12, Genetic
Institute; IFN-
, Bender; IL-4/IL-5, Genzyme, West Malling, U.K.)
were used as standards.
Assays for NO production
Total nitrate and nitrite concentration in serum was determined by the conversion of nitrate into nitrite following deproteination as described previously (16). Total nitrite content was then measured by the Greiss reaction (17), using NaNO2 as standard with detection limit of 1 µM.
Leishmanicidal assay
This was conducted as described previously (18). Briefly, cells (5 x 105/well; 24-well plate) were washed with prewarmed (37°C) DMEM (Life Technologies) before addition of stationary phase promastigotes (1 x 107/ml) at a promastigote-macrophage ratio of 10:1 over an infection period of 18 h. Infection rate was estimated in Lab-Tek (Nunc, Roskilde, Denmark) incubation slides by May-Grünwald-Giemsa staining. Approximately 50% of the macrophages contained at least one parasite. Medium containing nonphagocytosed parasites was gently removed by washing. Cultures were then stimulated with LPS (500 ng/ml) alone or LPS plus L-NG-monomethylarginine (L-NMMA) (10 mM). After 72 h, cells were washed with prewarmed DMEM and lysed using 0.01% SDS in 100 µl of prewarmed FCS-free DMEM for up to 30 min. This was assisted by pipetting the cells 10 times followed by 3 passages through a 26-gauge needle. Released amastigotes were resuspended in a total of 600 µl/well Schneiders Drosophila medium (Life Technologies) containing 30% FCS and cultured for a further 72 h. Aliquots of this culture (150 µl) were then transferred to quadruplicate wells of a 96-well plate for each sample and pulsed with [methyl-3H]TdR (1 µCi/well) for a further 18 h. The cultures were then harvested and counted in a beta counter (Betaplate, LKB, Uppsala, Sweden).
Statistical analysis
Statistical significance (p value) was calculated by Students t test.
| Results and Discussion |
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and LPS in vitro (Fig. 2
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than cells from the wild-type mice when
stimulated with either soluble leishmanial antigen or Con A (Fig. 3
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in vitro (Fig. 4
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Thus, resistant strains of mice preferentially produce IL-12 (23),
which induces Th1 cell differentiation (11). Th1 cells mediate
resistance to leishmanial infection, at least, via the following two
pathways: 1) induction of macrophage apoptosis by the FasL-Fas
pathway; and 2) production of IFN-
, which activates macrophages to
produce NO that kills the parasites directly. Also NO can induce
macrophage apoptosis (24). We have also found that NO inhibits
the expression of Bcl2 (F.-P. Huang and F. Y. Liew, unpublished
observations), an inhibitor of apoptosis (25). Therefore, NO
may contribute to the host resistance via the induction of macrophage
apoptosis.
In conclusion, we demonstrate here the crucial role of Fas expression in resistance to leishmanial infection. Furthermore, the interaction of NO and the Fas-FasL pathway of apoptosis may be required for the effective resistance against intracellular pathogens.
Note added in proof.
Recently, Fatima Conceicao-Silva et al. (1998, Eur. J. Immunol. 28:237) also reported that the resolution of lesions induced by L. major in mice requires a functional Fas (APO-1, CD95) pathway of cytotoxicity.
| Footnotes |
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2 Current address: Sir William Dunn School of Pathology, University of Oxford, South Park Road, Oxford, U.K. ![]()
3 Current address: Department of Biomedical Sciences, University of St. Andrews, St. Andrews, Scotland, U.K. ![]()
4 Address correspondence and reprint requests to Dr. F. Y. Liew, Department of Immunology, University of Glasgow, Glasgow G11 6NT, U.K. E-mail address: ![]()
5 Abbreviations used in this paper: iNOS, nitric oxide synthase; FasL, Fas ligand; L-NMMA, L-NG-monomethylarginine. ![]()
Received for publication January 13, 1998. Accepted for publication February 23, 1998.
| References |
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