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Infection and Immunity, August 2000, p. 4653-4657, Vol. 68, No. 8
Laboratoire de Parasitologie, Université Bordeaux II,
Bordeaux,1 Laboratoire PIOM, UMR 5501,
CNRS, ENSCPB, Talence,2 UMR 8619,
CNRS, Université Paris-Sud, Orsay,3
Laboratoire de Chimie et Biochimie Pharmacologiques, UMR
8601 CNRS, Université René Descartes,
Paris,4 Institut d'Epidémiologie
Neurologique et de Neurologie Tropicale, Faculté de
Médecine, Limoges,5 Unité
Physiologie de la Vigilance, CRSSA, La
Tronche,6 and Département de
Médecine Expérimentale, INSERM U52,
Lyon,7 France
Received 14 February 2000/Returned for modification 24 March
2000/Accepted 19 May 2000
Nitric oxide (NO) is an important effector molecule of the immune
system in eliminating numerous pathogens. Peritoneal macrophages from
Trypanosoma brucei brucei-infected mice express type II NO synthase (NOS-II), produce NO, and kill parasites in the presence of
L-arginine in vitro. Nevertheless, parasites proliferate in the vicinity of these macrophages in vivo. The present study shows that
L-arginine availability modulates NO production.
Trypanosomes use L-arginine for polyamine synthesis,
required for DNA and trypanothione synthesis. Moreover, arginase
activity is up-regulated in macrophages from infected mice from the
first days of infection. Arginase competes with NOS-II for their common
substrate, L-arginine. In vitro, arginase inhibitors
decreased urea production, increased macrophage nitrite production, and
restored trypanosome killing. In vivo, a dramatic decrease in
L-arginine concentration was observed in plasma from
infected mice. In situ restoration of NO production and trypanosome
killing were observed when excess L-arginine, but not
D-arginine or L-arginine plus
N Activated macrophages have been
shown previously to express arginase isoforms and nitric oxide (NO)
synthase type II (NOS-II) (14, 22, 38). The common substrate
used by these enzymes is L-arginine, which can be
hydrolyzed by arginase to ornithine and urea or oxidized by NOS-II to
L-citrulline and NO. Arginase, a suppressor of
macrophage-derived cytotoxicity (8, 19), has also been shown
in vitro to modulate NO production by macrophages through competition
for intracellular L-arginine (6, 12, 14, 44).
Arginase and NOS-II activities are induced in rodent macrophages by
lipopolysaccharides (6, 20, 35, 44). In elicited
macrophages, substrate availability relies mainly on exogenous
concentration, since recycling of L-arginine from
L-citrulline or L-ornithine occurs at a low
rate (15). The NOS-II-arginase balance is competitively
regulated by the Th1-Th2 cytokine balance (27).
NO synthesis must be accurately regulated, as NO is implicated in the
pathophysiology of inflammatory diseases, septic shock, and
immunosuppression (24, 36, 41). NOS-II and arginase pathways
have opposite biological effects (13). However, it has been
proposed that their successive induction plays an important role in
wound healing (37). The expression of NOS may be important in favoring microbiostasis-microbial killing and vasodilatation in the
early stages, whereas the expression of arginase, favoring polyamine
biosynthesis (5), fibroblast replication, and collagen induction (39), may be important in the later stages. These data suggest that a modification in the intensity, location, and chronology of these events may have negative effects during infectious diseases, by favoring microbial growth and reducing host effector mechanisms.
Trypanosomes of the brucei group, the causative agents of
human and animal African trypanosomiasis, are extracellular parasites highly susceptible to NO (43). A large expansion of
macrophages in the liver, spleen, and bone marrow has been observed for
Trypanosoma brucei brucei-infected mice (29). The
presence of NOS-II was demonstrated in these cells (36). The
antitrypanosome effects of NO synthesized by macrophages from infected
mice have been analyzed in vitro (10, 25). Nevertheless, in
infected mice, parasites proliferate in the vicinity of macrophages in
the peritoneal cavity, suggesting a reduced efficiency of NO-dependent
cytotoxicity in vivo. NO production is dependent on
L-arginine concentration (15). Complex, strong
interactions between the L-arginine-metabolizing enzymes
have been demonstrated elsewhere (3, 4). These interactions are even more complex in infected mice, as trypanosomes use
L-arginine for the polyamine synthesis required for the
synthesis of DNA and trypanothione, the equivalent of glutathione in
mammals (9). Arginase activity may be involved in NOS
activity impairment (6). In this study, an increase in
arginase activity was observed in peritoneal macrophages from the first
days of infection. An induction of NOS-II in macrophages and a decrease
in plasma L-arginine concentration were observed later.
Intraperitoneal NO production and NO-dependent parasite killing were
restored by injection of L-arginine.
Reagents.
L-Arginine, D-arginine,
N Parasites and mice.
The Antat 1.1 E clone of T. b.
brucei was obtained from the Institute of Tropical Medicine in
Antwerp, Belgium. Trypanosomes were kept frozen in liquid nitrogen as
stabilates (10). Female Swiss mice (Iffa Credo,
Saint-Germain-sur-l'Arbresle, France) were subcutaneously infected
with 104 parasites per mouse. All animals were housed under
conventional conditions, given water and chow ad libitum, and infected
at 10 to 15 weeks of age. The use of the animals conformed to
institutional guidelines.
Cells.
Peritoneal cells from control or infected
ether-anesthetized mice were collected in Hanks' buffered salt
solution (HBSS) free of Ca2+, Mg2+, and phenol
red (Life Technologies, Paisley, Scotland), supplemented with 20 mM
HEPES, 2 mM sodium pyruvate, and 10 µg of gentamicin per ml. Cells
(8 × 105/well) were layered on 24-well plates (Nunc
Inc., Naperville, Ill.), and macrophages were enriched by washing off
nonadherent cells after a 1-h incubation and were counted.
L-Arginine or L-[14C]arginine was
used as a substrate for NOS-II and arginase.
0019-9567/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
L-Arginine Availability Modulates Local Nitric Oxide
Production and Parasite Killing in Experimental
Trypanosomiasis
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ABSTRACT
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
-nitro-L-arginine (a NOS
inhibitor), was injected into the peritoneum of infected mice. These
data indicate the role of L-arginine depletion, induced by
arginase and parasites, in modulating the L-arginine-NO pathway under pathophysiological conditions.
![]()
INTRODUCTION
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
![]()
MATERIALS AND METHODS
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
-hydroxy-L-arginine (NOHA), and
N
-nitro-L-arginine
(L-NA) were obtained from Alexis (San Diego, Calif.). N
-Hydroxynor-L-arginine
(nor-NOHA) was prepared as previously described (26).
L-[Guanido-14C]arginine
(L-[14C]arginine) was purchased from NEN
(Cambridge, Mass.). All other chemicals were from Sigma (St. Louis,
Mo.).
Determination of arginase activity.
Two methods were used.
(i) Arginase activity was evaluated in ex vivo macrophages from
uninfected mice or from mice at days 2, 4, 6, 8, 10, 12, and 14 postinfection using the micromethod described by Corraliza et al.
(7). Briefly, 105 cells were treated with 0.1%
Triton X-100-0.01% pepstatin-0.01% aprotinin-0.01% antipain.
MnCl2 (10 mM) and L-arginine (0.5 M) were
successively added to the supernatants. The reaction was stopped by the
addition of an acid solution, and the urea formed by the arginase was
analyzed by adding
-isonitrosopropiophenone. The colored product was
quantified by absorption at 540 nm. (ii) Arginase activity was measured
in cultures by the conversion of L-[14C]arginine to [14C]urea
(34). Briefly, macrophages (4 × 105/well)
were cultured in supplemented HBSS containing 2 mM
L-arginine and 0.1 µM
L-[14C]arginine (specific activity, 51.5 mCi/mmol), with or without L-norvaline (10 mM), nor-NOHA
(0.1 mM), or NOHA (1 mM). The same media were incubated without cells
as a negative control. At the indicated times, 150 µl of supernatant
was removed and added to 800 µl of 250 mM acetic acid solution, pH
4.5, containing 100 mM urea and 10 mM L-arginine. After the
addition of Dowex resin (HCR-W2; Sigma), the tubes were centrifuged at
120 × g for 5 min. Supernatants (500 µl) containing
[14C]urea were removed and added to 3 ml of scintillation
fluid in counting vials. The percentage of
L-[14C]arginine converted to
[14C]urea was calculated as previously described
(34).
L-Arginine-dependent killing of intraperitoneal trypanosomes. HBSS (0.5 ml) alone or containing 0.1 M L-arginine, 0.1 M D-arginine, or 0.1 M L-arginine plus 0.1 M L-NA was intraperitoneally injected into 10-day-infected mice (i.e., the quantity injected per mouse was 8.7 mg of L-arginine or D-arginine or 10.9 mg of L-NA).
The injection of the arginase inhibitor nor-NOHA was not used because it was available in only limited quantities, insufficient for in vivo experiments. L-NA was chosen as an NOS inhibitor as it does not modify trypanosome and cell cultures (10). The peritoneal content of each mouse was collected by an injection of HBSS (4 ml) 24 h later to measure nitrite concentration and number of parasites.Measurement of NO2
concentration.
The induction of NOS-II activity was assessed by measuring
NO2
concentration in supernatants of
macrophages cultured for 24 h in HBSS supplemented with an excess
of L-arginine (1 mM).
, a stable oxidized
derivative of NO in cell cultures (23), was determined
spectrophotometrically at 540 nm in each cell culture supernatant or in
peritoneal liquid, as previously described.
Measurement of plasma L-arginine concentration. Plasma L-arginine was quantified on an automatic amino acid analyzer (Beckman 6300; Beckman, London, England) using a high-resolution method for amino acid analysis involving the use of lithium buffers and a high-performance chromatography lithium ion-exchange column (16).
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RESULTS |
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Time course of NOS-II and arginase activities in macrophages from
T. b. brucei-infected mice.
In the peritoneal fluid
from 8-day-infected mice, macrophages had the morphological
characteristics of activated cells (enlargement and numerous
cytoplasmic vacuoles) and trypanosomes were also observed (ranging from
107 to 108 per mouse). Arginase and NOS
activities were measured in macrophages from mice at days 2, 4, 6, 8, 10, 12, and 14 postinfection. Day 0 represents arginase and nitrite
production of macrophages from uninfected mice (control macrophages).
An increase in macrophage arginase activity was observed after 2 days
of infection, whereas significant NO2
production occurred after 6 days (Fig.
1). Arginase and nitrite production
remained high during the disease until the animals died (mean survival
time, 14 days).
|
L-Arginine degradation by macrophage arginase from
infected mice.
The increase in arginase activity in macrophages
from infected mice was confirmed by the generation of
[14C]urea from
L-[14C]arginine in culture. A time-dependent
consumption of L-[14C]arginine by macrophages
from 10-day-infected mice was observed (Fig.
2). After 12 and 24 h of culture, 32 and 65% of the L-arginine had been metabolized into urea,
respectively. Urea synthesis by macrophages from infected animals was
inhibited by using NOHA or nor-NOHA in cultures.
L-Norvaline was a less effective arginase inhibitor. Basal
arginase activity was detected in supernatants from control macrophages
(5% of conversion after a 24-h incubation).
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The effect of arginase inhibition on NO2
production and trypanosome growth in vitro.
The inhibition of
macrophage arginase on parasite growth and nitrite production was
assessed in vitro. nor-NOHA, the most efficient arginase inhibitor, was
used for in vitro experiments. Macrophages from 10-day-infected mice
were cocultured with parasites in a medium containing increasing
concentrations of L-arginine, with or without 500 µM
nor-NOHA. Nitrite production and trypanosome counts were assessed 3 days later (Fig. 3). Parasite killing and nitrite production were maximal at L-arginine
concentrations greater than 0.3 mM. At physiological concentrations of
L-arginine (0.1 to 0.2 mM), the presence of 500 µM
nor-NOHA enhanced nitrite production and trypanosome killing (Fig. 3).
A direct toxic effect of nor-NOHA on trypanosomes, independent of NO
generation, was ruled out. In cocultures of control macrophages and
trypanosomes, parasite growth was not affected by the presence of
nor-NOHA (data not shown).
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Measuring plasma L-arginine concentration. The L-arginine concentration in the plasma of control mice was 189 ± 20 µM (mean ± standard error from eight animals). A decrease in L-arginine concentration was observed in the plasma of 6-, 10-, and 13-day-infected mice (96 ± 15, 42 ± 13, and 39 ± 16 µM, respectively; mean ± standard error from eight animals).
L-Arginine-dependent killing of intraperitoneal
trypanosomes.
HBSS (0.5 ml) alone or containing 0.1 M
L-arginine, 0.1 M D-arginine, or 0.1 M
L-arginine plus 0.1 M L-NA was
intraperitoneally injected into 10-day-infected mice. The arginase
inhibitor nor-NOHA was not used as it was available in only limited
quantities, insufficient for in vivo experiments. Peritoneal content
was collected by washing with 4 ml of HBSS 24 h later. When
L-arginine was injected, an increase in
[NO2
] and a dramatic decrease in the number
of peritoneal parasites were observed (Fig.
4). Compared to the injection of HBSS
alone, the injection of L-arginine plus L-NA or
D-arginine had no effect on NO2
concentrations and parasite counts (Fig. 4). No significant differences in peritoneal macrophage numbers were observed in any of the mice. The
number of macrophages and the parasite/macrophage ratio were (9.2 ± 0.4) × 106 and 7.8 ± 3.5 (HBSS-injected
mice), (9.4 ± 0.5) × 106 and (5.6 ± 3.9) × 10
4(L-arginine-injected mice),
(9.6 ± 0.3) × 106 and 12.1 ± 4.2 (D-arginine-injected mice), and (9.4 ± 0.5) × 106 and 9.1 ± 3.1 (L-arginine-plus-L-NA-injected mice),
respectively. Trypanosome growth in culture was not affected by using
L-arginine, D-arginine, L-NA, or
L-arginine plus L-NA, as previously indicated (10).
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DISCUSSION |
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This study highlights the major role of L-arginine availability in NO production and parasite growth control by macrophages in vivo. Arginase activity increased in macrophages from T. b. brucei-infected mice. The use of L-[14C]arginine showed that, in these macrophages, the majority of L-arginine was metabolized by arginase. Moreover, parasites use L-arginine in their polyamine metabolism. All these elements contribute to a decrease in plasma L-arginine in T. b. brucei-infected mice. This decrease in L-arginine, previously reported for Trypanosoma brucei gambiense-infected voles (30), may reduce NO production and alter parasite killing. The restoration of NO production and trypanosome killing were observed in vitro when an arginase inhibitor was added to a culture medium containing physiological concentrations of L-arginine and in vivo in the peritoneum of infected mice when an excess of L-arginine was intraperitoneally injected. These data suggest an in situ regulation of NO production through substrate depletion.
In T. b. brucei-infected mice, an increase in serum nitrate is followed by a decrease (21). Although the expression of NOS-II in the presence of sufficient L-arginine is able to exert a trypanocidal activity, reduced availability of L-arginine may reduce nitrite production and trypanosome killing. Besides consumption by parasites and insufficient NO production for parasite killing, L-arginine availability may be reduced by the activity of arginase isoforms, the hepatic type (arginase I) and extrahepatic type (arginase II). The genes for the two arginase isoforms are regulated differentially. Both arginase I and arginase II are induced in activated mouse peritoneal macrophages (20, 35). Arginase I is induced by Th2 cytokines in bone marrow-derived macrophages (28). Endotoxin and circulating Th1 and Th2 cytokine levels are increased in trypanosomiasis (1, 32, 33).
Trypanosomes release a T-lymphocyte-triggering factor that induces the
secretion of gamma interferon (IFN-
) (31). This production of IFN-
during experimental trypanosomiasis is probably responsible for NOS-II induction in macrophages. However, for cattle
infected with Trypanosoma congolense, reduced secretion of
NO by IFN-
-activated monocytes and increased transcription of
interleukin-10 have been reported previously (40).
T-lymphocyte-triggering factor may also be involved in the production
of interleukin-4 and transforming growth factor
, a potent
stimulator of arginase activity, by CD8+ T cells (1,
5). All these data suggest that the induction of NOS-II and
arginase may reflect the marked dysregulation of the cytokine network
in trypanosomiasis and/or be involved in several distinct phenomena at
various stages in the disease. Induction of arginases I and II in
various tissues in infected mice may contribute to the decreased
availability of L-arginine and impair NO-dependent
mechanisms. As Th1 and Th2 cytokines regulate the NOS-II-arginase
balance in macrophages, the production and time course of these
cytokines in trypanosomiasis deserve further investigation.
The production of NO, assessed by measuring plasma nitrate concentrations, has been evidenced in response to T. b. brucei infection in mice (21). At different times and locations in trypanosome infections, NO-dependent mechanisms may have opposite effects on the host-parasite relationship. As NO and S-nitroso proteins are involved in the cytostatic-cytotoxic activity of macrophages (10, 25), an impairment of NO production may be correlated with trypanosome growth and disease severity at the onset of infection, as trypanoresistant mice produce more nitrites than do susceptible ones (18). It has also been speculated that antitrypanosome antibodies mediate the attachment of trypanosomes to activated macrophages and that NO-derived reactive species may affect the juxtaposed parasites (18). However, as NO is cytotoxic for several cell types, such as neurons or lymphocytes, arginase activity may be the result of a regulation mechanism limiting NO synthesis in infected mice. Inappropriate, persistent NO production may also be involved in immunosuppressive mechanisms (36) and the pathophysiology of the meningoencephalitis observed during trypanosomiasis. In vivo, the systemic inhibition of NOS attenuates immunosuppressive phenomena in T. brucei-infected mice (39). The conditions required for NO synthesis, the quantities produced, and the timing of these phenomena may be critical for this molecule's role in trypanosomiasis. NO may enhance trypanosome resistance in certain tissues and trypanosome susceptibility in others, as has been suggested for experimental malaria (17). The local restoration of NO production by administration of L-arginine to infected mice has been correlated with a decrease in parasite burden in the peritoneum. The restoration of NO production is likely to be ineffective in other locations. In blood, due to the excess of NO scavengers, the role of other elements, such as antibodies or tumor necrosis factor alpha, may be predominant.
Trypanosomes have developed several strategies for bypassing the immune system, including evasion of the specific immune response via the antigenic variation phenomenon (2). Arginase also promotes the synthesis of L-ornithine, a precursor of the polyamines involved in DNA and trypanothione synthesis (9). Difluoromethyl ornithine, an inhibitor of ornithine decarboxylase, a key enzyme in polyamine synthesis, is now used in the treatment of human sleeping sickness (42). The early increase in arginase production in trypanosomiasis may represent a way for parasites to avoid the antimicrobial effect of reactive nitrogen intermediates and benefit from larger quantities of L-ornithine. Arginase inhibitors are currently being developed and may represent good candidates for controlling microbial proliferation.
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ACKNOWLEDGMENT |
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This research was supported by grants from the Conseil Régional d'Aquitaine.
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FOOTNOTES |
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* Corresponding author. Mailing address: Laboratoire de Parasitologie, Université de Bordeaux II, Bât 1B, 146 rue Léo Saignat, 33076 Bordeaux Cedex, France. Phone: 33-557-57-17-73. Fax: 33-556-84-66-29. E-mail: Philippe.Vincendeau{at}parasito.u-bordeaux2.fr.
Editor: S. H. E. Kaufmann
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