Download Evasion of innate immunity by parasitic protozoa

R EVIEW
© 2002 Nature Publishing Group http://www.nature.com/natureimmunology
Evasion of innate immunity by
parasitic protozoa
David Sacks and Alan Sher
Parasitic protozoa are a major cause of global
infectious disease. These eukaryotic pathogens
have evolved with the vertebrate immune system
and typically produce long-lasting chronic infections. A critical step in their host interaction is
the evasion of innate immune defenses. The ability to avoid attack by humoral effector mechanisms, such as complement lysis, is of particular
importance to extracellular parasites, whereas
intracellular protozoa must resist killing by lysosomal enzymes and toxic metabolites.They do so
by remodeling the phagosomal compartments in
which they reside and by interfering with signaling pathways that lead to cellular activation. In
addition, there is growing evidence that protozoan pathogens modify the antigen-presenting
and immunoregulatory functions of dendritic
cells, a process that facilitates their evasion of
both innate and adaptive immunity.
Parasitic protozoa are unicellular eukaryotic pathogens that dwell
inside host cells and/or in extracellular fluids. These organisms typically produce long-lasting chronic infections in order to maximize their
opportunities for successful transmission by contact with their intermediate hosts or release into the environment. Their success as parasites
depends on a series of intricate and highly evolved host adaptations that
enable them to evade destruction by the immune system.
Protozoan infections are a major global health problem, affecting
over half a billion people world wide. Several of the diseases they
induce (such as malaria, African trypanosomiasis and visceral leishmaniasis) represent major causes of mortality and morbidity in tropical
countries and, as such, are important impediments to economic development. No standardized vaccines exist for preventing any of the protozoan infections of humans, a situation attributed both to the complexity of the pathogens involved and their sophisticated strategies for
evading host immune responses1.
Protozoan parasites are highly adept at escaping the effects of adaptive humoral and cellular immunity. Perhaps the simplest solution for
evading antibody responses is the adoption of an intracellular life style,
as is done by Leishmania, Trypanosoma cruzi, Toxoplasma gondii and
the liver and blood stages of malaria. Another major strategy—antigenic variation—protects extracellular protozoa such as African try-
panosomes2 and Giardia3 and even malaria parasites4, which express
their antigens on the surface of infected erythrocytes, from immune
recognition. In addition, there is abundant evidence that protozoan
infections actively regulate adaptive T cell responses, resulting in suppressed effector functions1. A striking example of this phenomenon is
the recent demonstration that Leishmania major actively induces interleukin 10 (IL-10)–producing CD25+ T regulatory cells to prevent complete clearance of the parasite5.
Although parasitic protozoa have provided some of the best studied paradigms of evasion of antibody- and T cell–mediated immunity
by pathogens, a series of equally important adaptations occur during
the initial establishment of infection, when parasitic invaders confront
the innate immune system. The innate host defenses that the major
blood and tissue protozoan pathogens of humans must penetrate,
modify or avoid vary according to their mode of entry and the primary host tissue encountered (Table 1). These innate defenses include
the epithelial barrier of the skin, the alternative complement cascade
and other lytic serum components, lysosomal hydrolases and toxic
oxygen metabolites of mononuclear phagocytes and the antigen-presentation and immunoregulatory functions of dendritic cells (DCs),
which provide a crucial link with the adaptive immune response6.
That protozoa have evolved specific mechanisms to evade these
defenses is dramatically shown by the rapid destruction of preinfective vector-derived life cycle stages when artificially inoculated into
mammalian hosts. These developmental adaptations, which enable
blood and tissue protozoa to “run the gauntlet” of the vertebrate
innate response, are the focus of our review.
Evasion of humoral innate defenses
Innate resistance to protozoa is mediated, in part, by pre-existing soluble factors that can potentially recognize and destroy invading parasites or target them for killing by effector cells. The alternative pathway of complement activation provides a first line of defense against
extracellular parasites that must be subverted for infection to proceed.
For example, whereas the epimastigote stage of T. cruzi found in
insect vectors is susceptible to the alternative complement pathway,
infective metacyclic and blood-stream trypomastigotes are resistant7.
In this case, evasion of complement appears to be due to trypomastigote expression of a 160-kD glycoprotein (gp160), which is a
homolog of the host complement-regulatory protein decay-accelerating factor (DAF)8. Like DAF, gp160 can bind to C3b and C4b and
inhibit the uptake of subsequent members of the complement cascade,
thus preventing convertase formation and lysis of the parasite.
Importantly, whereas complement-sensitive epimastigotes fail to
express gp160, epimastigotes transfected with gp160 are resistant to
complement-mediated lysis9.
Another interesting strategy is deployed by Leishmania species, for
which the infective insect stage is transiently exposed to potentially
Laboratory of Parasitic Diseases, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
Correspondence should be addressed to D. S. ([email protected]).
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© 2002 Nature Publishing Group http://www.nature.com/natureimmunology
Table 1.The major protozoan pathogens of humans
Parasite
Vertebrate host habitat
Mode of transmission
Primary site of host invasion
Malaria
Leishmania
Toxoplasma
Intracellular (erythrocytes + hepatocytes)
Intracellular (macrophages)
Intracellular (macrophages, other
nucleated cells)
Intracellular (macrophages, muscle cells,
other nucleated cells) and extracellular
Extracellular
Vector-born (mosquitoes)
Vector-born (sand flies)
Ingestion of parasite cysts
Skin, blood (sporozoites)
Skin
Intestinal epithelial cells
Vector-born (reduviid bugs)
Skin, conjunctival mucosa
Vector-born (tse-tse flies)
Blood
T. cruzi
African trypanosomes
lethal serum components after inoculation by pool-feeding phlebotomine vectors. Leishmania evade complement-mediated lysis
while using complement activation as a mechanism for targeting host
cells. When insect-stage procyclic promastigotes develop into infective metacyclic forms, their membrane is altered to prevent insertion
of the lytic C5b-C9 membrane attack complex (MAC)10. This correlates with their expression of a modified surface lipophosphoglycan
(LPG) that is approximately twice as long as the form on procyclic
promastigotes, and which may act as a barrier for insertion of the
MAC into the parasite surface membrane11. A further developmental
change that occurs during generation of metacyclics is enhanced synthesis of the surface proteinase gp63, which can cleave C3b to the
inactive iC3b form, thus preventing deposition of the lytic C5b-C9
complex12. However, iC3b also opsonizes the parasites for phagocytosis through the complement receptors CR3 and CR1, thereby targeting
the parasite to the macrophage, its host cell of choice13. Conclusive
evidence that LPG and gp63 are virulence factors has emerged from
studies with L. major null-mutants that do not express these molecules; in each case these mutants were highly sensitive to complementmediated lysis and showed reduced virulence in BALB/c mice14,15.
In addition to complement, protozoa must evade other soluble
mediators of innate immunity in order to establish a foothold in the
host. Perhaps the best studied are the primate-specific trypanosome
lysis factors (TLFs)16, which contribute to the resistance of humans to
infection with Trypanosoma brucei, an important pathogen of livestock. Biochemical analysis of the activity present in human serum
revealed that high-density lipoproteins are part of the substance that
mediates cytolysis of the parasite, and initial studies demonstrated
that this complex, TLF1, is composed of several common apolipoproteins as well as a haptoglobin-related protein (Hpr)17,18. A second
cytolytic complex, TLF2, has also been identified that shares many of
the components of TLF1 but contains a distinct immunoglobulin M
component and a lower lipid content19. Although the final lytic event
has not been delineated, TLF must undergo receptor-mediated uptake
and enter an intracellular acidic compartment for cytotoxicity to
occur18. Whereas T. brucei is sensitive to TLF-mediated killing, the
species that infect humans—T. brucei gambiense and T. brucei rhodesiense—are both refractory. This resistance is associated with a block
in TLF endocytosis and retention of the complex in a structure known
as the flagellar pocket16. The refractoriness of certain T. brucei rhodensiese strains to lysis also correlates with the expression of a serum
resistance–associated (SRA) gene that is equivalent to truncated variant surface glycoprotein20. Importantly, transfection of SRA from T.
brucei rhodesiense into T. brucei confers resistance to lysis by human
serum; this argues that its expression may have been a critical step in
the adaptation of an ancestral parasite for infection of primates21.
Interestingly, antibodies raised against a homolog of SRA localize to
the flagellar pocket, suggesting a possible role for the protein in TLF
retention22.
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Remodeling host-cell compartments
Parasitic protozoa that are adapted to an intracellular lifestyle must
resist the antimicrobial mechanisms that can be induced in phagocytic
and even nonphagoctyic host cells. Insofar as the acidified hydrolytic
environment of host cell lysosomes represents the heart of the defensive
machinery of many nucleated cells, the ability of acid-labile proteasesensitive parasites to avoid or modify this compartment is likely to be
one key to their survival (Fig. 1). T. gondii resides in a phagosome that
restricts its fusion with host endosomes and lysosomes. Toxoplasma
actively penetrates both phagocytic and nonphagocytic cells, propelled
by an actin-myosin–dependent gliding motility23. In the process, it
establishes a nonfusigenic compartment, called the parasitophorous
vacuole (PV), that lacks integral membrane proteins of host cell origin,
but is extensively modified by secreted parasite proteins24,25. This
remodeling seems to be crucial to the inhibition of PV acidification and
lysosome fusion, as these events proceed normally after uptake of dead
or opsonized parasites, which are internalized via classical receptormediated phagocytosis.
T. cruzi trypomastigotes also actively invade mammalian cells but,
unlike T. gondii, their cell entry is dependent not on the propulsive
properties of the parasite, but on their ability to subvert a Ca2+-regulated lysosomal exocytic pathway23. The membrane of the parasitophorous vacuole is, therefore, derived from the membrane of lysosomes, and the vacuole itself remains acidic and potentially fusigenic
with other lysosomes. T. cruzi growth and development cannot be sustained within this compartment; it depends instead on the ability of the
parasite to escape rapidly into the cytosol. Exit from the vacuole is
mediated by a parasite-secreted molecule, Tc-TOX, which has membrane pore-forming activity at acidic pH and is facilitated by a transsialidase present on the trypomastigote surface26,27. Interestingly,
T. cruzi is unable to invade cells lacking transforming growth factor-β
(TGF-β) receptors I or II28; this suggests that triggering this signaling
pathway, perhaps by some parasite TNF-β homolog, might be needed
to deactivate host cells during the early stages of infection.
Leishmania parasites lack the machinery necessary for active invasion and are confined instead to professional phagocytes, mainly
macrophages, with some exceptions (for example, fibroblasts, DCs and
neutrophils)29. The uptake of Leishmania by macrophages proceeds via
conventional receptor-mediated phagocytosis that involves a diversity
of opsonic or pattern-recognition receptors (for example, CR3, CR1
and mannose fucose receptors)30 that are used depending on the species
and stage of parasite and the presence or absence of fresh serum.
Leishmania metacyclic promastigotes and amastigotes do not seem to
remodel the phagosome in a major way because it rapidly fuses (within 30 min) with late endosomes or lysosomes31, generating a parasitophorous vacuole that maintains an acidic pH and hydrolytic activity. Using macrophage cell lines and unselected promastigotes,
researchers have shown that LPG-repeating units can transiently inhibit phagosome maturation and that this delay may be necessary to allow
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R EVIEW
sufficient time for promastigotes
competitive inhibitor of the PKC
Phagosome
to differentiate into more hydroactivator DAG and/or by altering
lase-resistant
amastigotes32,33.
the physical properties of the
bilayer to inhibit PKC membrane
These findings are consistent with
T. cruzi
trypomastigote
translocation. These findings are
the attenuation of L. major proagain consistent with impaired
mastigote survival within macrointracellular survival of the L.
phages after infection with targetmajor LPG–deficient mutant
ed null mutants lacking LPG14.
compared to the mutant in which
The general significance of the
Leishmania
amastigote
LPG expression was restored,
transient inhibition of phagoalthough effects on specific sigsome-endosome fusion has been
Parasitophorous
naling events were not directly
questioned, however, for two reavacuoles
compared14.
sons. First, Leishmania mexicana
Leishmania
promastigote mutants deficient in
In addition to their innate
promastigote
Toxoplasma gondii
LPG or other phosphoglycan-conmicrobicidal responses, macrotachyzoite
Phagolysosome
taining molecules survive equally
phages can initiate the host actiLysosomes
well as wild-type organisms withvation cascade by presenting antiin macrophages34, and second,
gens and costimulatory molecules
Figure 1. Remodeling of macrophage intracellular compartments by
and by providing regulatory
there is an absence of direct data
parasitic protozoa. T. cruzi trypomastigotes enter the macrophage by induccytokines to T cells. A number of
to indicate that mature promastiging the recruitment of lysosomes to the plasma membrane; they only transientstudies suggest mechanisms by
otes are any less hydrolase-resisly reside in the parasitophorous vacuole before escape into the cytoplasm via
which intracellular protozoa can
tant than amastigotes. That
secretion of a pore-forming molecule, termed Tc-TOX (yellow). T. gondii tachyzoites actively invade the cell and remodel a parasitophorous vacuole membrane
interfere with the immune-initiaamastigotes are remarkably robust
(blue) that contains secreted parasite proteins but excludes host proteins that
tion functions of macrophages.
is clear: the parasite does not
would normally promote phagosome maturation, thereby preventing lysosome
One of the more consistent and
escape from the vacuole, but manfusion. Leishmania metacyclic promastigotes are taken up by receptor-mediated
striking dysfunctions observed in
ages to withstand the low pH and
phagocytosis; phagosome maturation may be transiently inhibited by LPG
macrophages infected with protoonslaught of acid hydrolases, pre(green), which becomes incorporated into the phagosome membrane.The replicating amastigote stage ultimately resides within a phagolysosome where they
zoan parasites is their inability to
sumably by producing an abunsurvive via production of cell-surface and secreted glycoconjugates, including
produce IL-12, which—as the
dance of cell surface and secreted
GIPLS and proteophosphoglycan (PPG) (green).
main physiological inducer of
glyconjugates that protect the cell
interferon-γ (IFN-γ) and T helper
from proteolytic damage. As the
L. mexicana phosphoglycan–deficient mutants were able to survive type 1 (TH1) cell differentiation—is an essential cytokine for the develnormally, it is likely that this species displays different or redundant opment of acquired resistance to most intracellular pathogens. In addivirulence determinants. It is possible, for example, that the huge para- tion, excess IL-12 production can lead to severe tissue damage and even
sitophorous vacuole that is formed after infection with parasites of the mortality, a consequence that may also be detrimental to the parasite in
L. mexicana complex might effectively dilute the hydrolases to which blocking its transmission cycle. It is no wonder then that protozoa have
the parasite is exposed within the vacuole and obviate the need for cer- evolved a series of intricate and often redundant mechanisms for regutain surface or secreted glycans.
lating IL-12 production by antigen-presenting cells, especially
macrophages, which are the major potential source of the cytokine
stimulated during early infection.
Inhibition of host cell signaling pathways
Infectious stages of Leishmania do not merely avoid IL-12 inducMacrophages possess primary defense mechanisms—including activation of macrophage oxidative metabolism and synthesis and release of tion, they actively and selectively inhibit it, leaving other pro-inflamarachidonic acid metabolites—that are induced by the attachment and matory cytokine or chemokine response pathways relatively intact41.
engulfment of microbial agents. The major source of reactive oxygen Whereas the glycophosphatidylinositol (GPI) anchors of many paraintermediates (ROIs) in macrophages is the NADPH oxidase, a multi- sites are potent macrophage activators, Leishmania LPG and glycomponent enzyme that catalyzes the transfer of electrons from coinositol phospholipid (GIPL) inhibit IL-12p40 transcription while
NADPH to molecular oxygen, resulting in the production of superox- failing to suppress TNF gene expression42. Although the receptors and
ide and hydrogen peroxide35. It is generally believed that activation of signaling pathways involved in this selective inhibition of IL-12 synprotein kinase C (PKC) and protein tyrosine kinases (PTKs) are the two thesis have yet to be identified, suppression of NF-κB activity does not
critical events involved in regulating phagocyte functions in response to appear to be involved. Because many IL-12 agonists that are inhibited
a variety of extracellular stimuli. In vitro studies involving human cells in Leishmania-infected macrophages—including lipopolysaccharide
have shown that macrophage functions, including phagocytosis and (LPS), CD40 ligand (CD40L) and especially IFN-γ—signal primarily
ROI generation, are severely impaired after uptake of an insoluble through protein-tyrosine kinases, the observation that Janus kinase–sigdegraded host hemoglobin, called hemozoin, generated during blood- nal transducers and activators of transcription (Jak-STAT) signaling
stage malarial infection36 (Fig. 2). These dysfunctions were attributed pathways are also inhibited in Leishmania-infected cells seems espeto inhibition of PKC translocation and activation in the hemozoin- cially relevant to the defective IL-12 response43. Defective phosphoryloaded cells37. Early studies suggested that Leishmania parasites also lation of Jak2 is attributed to rapid activation of a cytoplasmic protein
avoid triggering the oxidative burst by actively inhibiting PKC activa- tyrosine phosphatase (PTP), SHP-144 (Fig. 2). SHP-1 is necessary for
tion in macrophages38,39. Inhibition of PKC-mediated protein phospho- L. major survival insofar as SHP-1–deficient motheaten mice do not
rylation was observed with purified LPG40, which might act either as produce lesions and their macrophages fail to support infection in
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LPS
TLR
β
Cytokine
© 2002 Nature Publishing Group http://www.nature.com/natureimmunology
α
Jak
cytb558
Hemozoin
PKC
RhoGDI
Rac
GDP
O2
p67phox
SHP1
MyD88
STAT
IRAK
?
IRAK
IKK1 IKK2
p47phox
p40phox
Jak
YP
Y
Y
P
CIS–
P
γ
Leishmania
TRAF6
STAT
Y
P
P
Y
STAT
MAP
3K
Figure 2. Inhibition of macrophage signaling
pathways. Inhibition of pathways by malaria-generated hemozoin or infection with Leishmania or T.
gondii. Hemozoin and Leishmania impair the oxidative
burst associated with phagocytosis by inhibiting the
PKC activation required for assembly of the NADPH
oxidase complex in its active form. Leishmania parasites also inhibit IFN-γ– or CD40L-induced PTKdependent signaling involved in IL-12 production by
activation of the cellular phosphatase SHP-1 that
inhibits Jak2 and STAT1 phosphorylation (P).
Toxoplasma inhibits LPS-induced cytokine responses
by inhibiting nuclear translocation of NF-κB and possibly phosphorylated STAT1.
Toxoplasma
O2–
Y
P
STAT
STAT
Degradation
of IκB
Nucleus
P
Y
IκB
NF-κB
Cytoplasm
Induction of inflammatory and
immune-response genes
vitro45. Because specific ligation of the macrophage receptors CR1 and
CR3 also leads to selective inhibition of IL-12 production—which, at
least in the case of CR3, is associated with impaired tyrosine phosphorylation of STAT146—it is likely that host cell PTPs are rapidly induced
by CR3 ligation during attachment and uptake of serum-exposed C3opsonized parasites.
Given that Jak-STAT signaling pathways are involved in a broad
range of macrophage responses, including induction of most proinflammatory cytokines, it is difficult to understand how Leishmania
infection maintains such a selective effect on IL-12. One possible
explanation lies in two observations: first, STAT1 regulates IFN consensus-binding protein (ICSBP) induction and—of the cytokines
assayed—only activation of the IL-12p40 promoter appears to require
ICSBP, and second, ICSBP knock-out mice (on resistant background)
are susceptible to L. major infection47. The involvement of redundant
STAT1-independent induction pathways for tumor necrosis factor-α
(TNF-α), IL-1β and inducible nitric oxide synthase (iNOS) has been
described (see below) and suggests a basis for the maintenance of these
responses in Leishmania-infected cells. A similar mechanism may
account for the down-regulation of IL-12p40 gene expression in mouse
macrophages after uptake of Plasmodium berghei–infected erythrocytes48, which appear to be selective for IL-12 also and do not appear to
be due to impaired NF-κB activation.
The NF-κB family of transcription factors is an evolutionarily conserved group of proteins that are important in the regulation of numerous
genes involved in innate and adaptive immunity—including those encoding IL-12, IFN-γ, TNF-α, iNOS and adhesion molecules—as well as
those involved in cell proliferation and survival. Studies in T. gondii49,50
have demonstrated the ability of intracellular protozoa to actively interfere
with the NF-κB–activation pathway in macrophages. In T. gondii–infected cells, despite rapid IκB phosphorylation and degradation, NF-κB fails
to translocate to the nucleus. In contrast to Leishmania, T. gondii–induced
response defects are more generalized and include both IL-12 and TNFα. With the use of virulent strains of T. gondii, the inhibition of NF-κB
translocation in macrophages and its effect on iNOS expression was
shown to be dependent on HS70 expression by the parasite that might
impede nuclear transport by competing for access to nuclear pore complexes51. Disrupted nuclear transport of phosphorylated STATα has also
been reported for T. gondii–infected macrophages and is thought to inhibit Jak-STAT–dependent MHC class II expression51 (Fig. 2).
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In addition to being directly suppressed by the parasite within infected cells, macrophage IL-12 production and effector functions can also
be held in check by the down-modulatory cytokines IL-10 and TGF-β,
which can themselves be up-regulated by the parasites or their products. Striking evidence in support of the importance of IL-10–dependent down-regulation of IL-12 production for both host and parasite
comes from studies in IL-10–deficient mice infected with either T.
gondii or T. cruzi. These animals display dysregulated IL-12 production
and die of cytokine-associated tissue damage while showing decreased
parasite burdens52,53. Similarly, low amounts of active TGF-β production by splenic mononuclear cells are associated with a lethal outcome
in rodent malaria, and anti–TGF-β treatment transformed a normally
resolving infection into a lethal one due to overproduction of pathogenic pro-inflammatory cytokines54. With respect to Leishmania, there
is as yet no evidence that deficient production of these anti-inflammatory mediators is responsible for severe clinical outcomes. In contrast,
there is ample evidence to suggest that overproduction of these
cytokines contribute to uncontrolled parasite growth and nonhealing
infections. Both IL-10 and TGF-β are produced by murine
macrophages after Leishmania infection in vitro, and both promote
intramacrophage replication and are important factors for determining
in vivo susceptibility55,56.
Finally, parasitic protozoa have evolved strategies to down-modulate
signaling pathways leading to host cell apoptosis, thereby prolonging
the life of the host cell and their own intracellular survival57. Induced
apoptotic pathways in macrophages infected with either Leishmania
donovani promastigotes58 or T. gondii tachyzoites59 were strongly inhibited, perhaps by parasite induced up-regulation of Bcl-2 homologs.
Interestingly, the pro-apoptotic effects that T. cruzi infection has on
both CD4+ and CD8+ T cells indirectly enhanced parasite growth, as the
binding of apoptotic lymphocytes to vitronectin receptors on
macrophages triggered TGF-β production and a burst of parasite replication in infected cells60.
Manipulation of DC function
To the extent that pathogenic protozoa have learned to condition their
initial encounter with the innate immune system as a means of manipulating the subsequent adaptive immune response, it has been important to extend these observations to effects on “professional” antigenpresenting cells. In contrast to primary targets of infection, such as
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Infection of monocytederived human DCs with T.
cruzi trypomastigotes also
Parasite
Inhibitory signal
Immunologic effects
inhibits LPS-induced secreLeishmania
Live infection (L. amazonensis)
Inhibition of DC IL-12 production with turn on of IL-476
tion of pro-inflammatory
Parasite culture media or LPG
Inhibition of DC or LC migration73,74
T. cruzi
Live infection GIPL (or ceramide portion)
Suppression of LPS-induced IL-12, TNF-α, HLA-DR and
cytokines—including IL-12
CD40 expression in human DCs71,72
and TNF-α—as well as HLAT. gondii
Parasite induction of host LXA4 in vivo
Inhibition of CCR5-dependent IL-12 production by DCs80,81
DR and CD40 up-regulation71.
P. falciparum
Ligation of CD36 (or CD51) by parasitized
Suppression of DC maturation and function67,68
Similar effects were observed
erythrocytes
when human DCs were
exposed to T. cruzi–derived
macrophages, DCs may be temporally removed from encounter with GIPL or to only the ceramide portion of the compound72, although
parasites or their products and may respond distinctively. For example, nonphysiological high concentrations of both molecules were used in
whereas IL-12 production is actively suppressed in macrophages this in vitro study.
infected with L. major or T. gondii, DCs actively produce IL-12p40 in
With respect to Leishmania, the spontaneous migration of murine
response to the same parasites, both in vitro and in vivo61–65. It should be splenic DCs or Langerhans cells (LCs) was inhibited by L. major pronoted that in each case the parasite infection alone was not sufficient for mastigote–conditioned media or L. major LPGs, respectively; this sughigh IL-12p70 production in vitro and that endogenous agonists, such gested that their ability to transport antigen to or within lymphoid tisas CD40L or IFN-γ, were necessary as costimuli. The different out- sue might be impaired during infection73,74. These effects need to be
comes of protozoan encounter on IL-12 production by macrophages interpreted, however, in the context of other studies (referred to above).
versus DCs is consistent with the delayed, albeit effective, immune These studies involved infection of murine DCs with viable L. major
response to these infections that is ultimately achieved. Apart from the promastigotes or amastigotes; the infected cells up-regulated MHC
obvious parasite advantage that delayed onset of immunity brings to class II and costimulatory molecules, produced IL-12p40 and down
bear, suppression of IL-12 production by infected macrophages may regulated E-cadherin, which indicated that they were competent for
again represent a coadaptation that dampens their local activation while antigen transport and presentation61,75. Uptake of Leishmania amazoprotecting the host from cytokine shock.
nensis amastigotes or metacyclic promastigotes by mouse bone marFor those parasitic infections associated with T cell control mecha- row–derived DCs (BMDCs) also induced up-regulated expression of
nisms that are not simply delayed, but are persistently compromised, MHC class II, CD40, CD80 and CD86 but, in contrast to L. major, the
impaired maturation and function of appropriate DC subsets might be infected DCs produced IL-4 and no IL-1276. This suggested that the
more expected (Table 2). A particularly interesting example is the inhi- DCs were conditioned by the parasite to prime the pathogenic TH2 cells
bition of DC maturation by the malaria parasite Plasmodium falci- that dominate the response in vivo.
parum66. Malaria-infected erythrocytes bind to the surface of myeloid
Studies involving yet other Leishmania species suggest a more pasDCs in vitro and markedly suppress the normal up-regulation of major sive strategy for immune evasion, in which their interactions with antihistocompatibility complex (MHC) class II molecules, adhesion mole- gen-presenting cells appear to proceed in a relatively silent fashion.
cules (for example, ICAM-1) and costimulatory molecules (CD83 and Uptake of L. mexicana by mouse BMDCs was not sufficient to activate
CD86) on the cells after stimulation with LPS. The resulting DCs were them in vitro, although their maturation response to other stimuli was
severely impaired in their capacity to induce allogenic as well as anti- not impaired77. A similar species restriction in Leishmania-induced DC
gen-specific primary and secondary T cell responses67. The receptors on activation was observed after uptake of metacyclic promastigotes by
the surface of DCs mediating this inhibitory effect are CD36 and CD51, human myeloid DCs, which were efficiently primed for CD40Land their artificial ligation mimicked the suppression induced by infect- induced ILp70 secretion by L. major, but not by L. tropica or L. donoed erythrocytes68. Interestingly, the same receptors are also involved in vani78. These observations may be significant because whereas L. major
the recognition of apoptotic cells and, when incubated with DCs, such produces self-limiting infections that are controlled in the skin, the
cells trigger reduced secretion of IL-12 and increased production of IL- other species mentioned are capable of disseminating to the viscera or
10 by these antigen-presenting cells. The major parasite ligand for to other cutaneous sites. Given the well described polymorphisms in the
CD36 binding in infected erythrocytes is thought to be a conserved surface and secreted glycan structures that are expressed by different
domain of P. falciparum erythrocyte membrane protein 1 (PfEMP1), a Leishmania species79, it will be interesting to determine whether they
molecule that undergoes antigenic variation. Thus, a nonadherent para- account—at least in part—for the differences in innate response patsite line, which does not express PfEMP1 antigens on the red cell sur- terns and the spectrum of clinical disease.
face, failed to inhibit DC maturation.
From the above discussion it would appear that parasites carefully
The relevance of these observations to acute disease is indicated by regulate the induction of IL-12 from DCs during early infection as a
the finding that the percentage of HLA-DR+ DCs was significantly means of determining the character of the adaptive immune response
lower in children with severe or mild malaria compared to healthy con- that eventually decides their fate in the host. In addition, there is emergtrols68. At present there is no direct evidence that impaired DC matura- ing evidence for the existence of distinct parasite-triggered pathways
tion in children with acute malaria is CD36-mediated or that this event for regulating DC IL-12 production once it has been initiated. One such
is at all necessary for successful malaria infection. Nevertheless, it is mechanism has been identified by studies of IL-12 synthesis by murine
intriguing that infected erythrocytes with high CD36 binding affinity splenic DCs after injection of a soluble extract of T. gondii tachyzoites
are more frequently observed in isolates from patients with mild as (STAg). The response was short-lived and could not be recalled by a
opposed to severe disease69,70. It suggests that adhesion of infected ery- second injection of STAg for a period of 1 week after initial priming80.
throcytes to CD36 may suppress the pro-inflammatory response to the This “paralysis” of the DC IL-12 response induced by STAg does not
parasite, an outcome that would favor both host and parasite.
require IL-10 but instead appears to depend on the induction of lipoxin
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Table 2. Impairment of DC function by parasitic protozoa
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A4 (LXA4), a product of arachadonic metabolism81. This eicosonoid
appears to function by suppressing CCR5 expression, a chemokine
receptor involved in IL-12 stimulation by STAg. The same mechanism
of IL-12 suppression is likely to operate during natural infection
because T. gondii–infected mice with a defect in an enzyme (5-LO)
required for LXA4 show excess IL-12 production and, in common with
infected IL-10–deficient mice53, succumb to cytokine shock and show
decreased parasite burdens (J. Aliberti et al., unpublished data). Taken
together, these findings suggest that once IL-12 production by DCs has
been induced and TH1 responses initiated, synthesis of the cytokine is
actively down-regulated, probably to the joint benefit of both host and
parasite. Whether analogous mechanisms suppress the IL-12 responses
of DCs to other protozoa remains to be determined.
Concluding comments
In learning to evade innate host defenses, protozoan parasites appear to
have mastered the intricacies of both cell biology and cellular
immunology. As such, they can teach us important lessons about both
fields of biology as well as their points of intersection. In particular, the
strategies used by these pathogens for remodeling cellular compartments offer insights into the dynamics of host membranes and intracellular trafficking. At the same time there is much to be learned about
host-signaling pathways and immune-response initiation and polarization by studying the mechanisms used by protozoa to suppress or divert
immune responses. Because innate defenses (for example, lysis by
complement or TLF) have the potential to block infection, the mechanisms used to evade them are potential “Achilles’ heels”, and offer
newly identified and largely unexploited targets for vaccine and/or
pharmacologic intervention.
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