Download Models of infectious diseases in the fruit fly Drosophila melanogaster

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Disease Models & Mechanisms 1, 43-49 (2008) doi:10.1242/dmm.000307
Models of infectious diseases in the fruit fly
Drosophila melanogaster
Disease Models & Mechanisms DMM
Marc S. Dionne1 and David S. Schneider2,*
We examined the immune response of a fly as physicians
might, by looking at the genesis of diseases caused by
microorganisms. Fly infections are complex and there are
few simple rules that can predict how an infected fly
might fare. As we observed the finer details of the
infections, we found that almost every microbe caused a
different type of pathology in the fly. Two pattern
recognition pathways, Toll and immune deficiency (Imd),
were found to detect, and respond to, infections. The
physiological response of the fly was modified further by
Eiger, insulin, Wnt inhibitor of dorsal (WntD) and nitric
oxide (NO) signaling. As in humans, some of the damage
that occurred during the fly immune response was
caused by an over-aggressive response rather than by
the microbes themselves. When looking at the matrix of
signaling pathways and the microbes being tested, it was
immediately obvious that most of the pathways would
need to be studied in more detail before defining the
rules that govern their role in pathogenesis. This detailed
analysis of signaling and pathogenesis has the potential
to allow the fly to be used as a model patient instead of
as simply an innate immune system model.
Two features of Drosophila melanogaster make it particularly
valuable as a model organism for modern science. Firstly, the tools
available for working with Drosophila are very accommodating for
the study of basic biology. Secondly, Drosophila is an excellent
model for studying human disease because any findings translate
well into medicine. Evidence for both of these attributes can be
seen in the history of Toll signaling. Toll was first identified for its
participation in the maternal control of dorsal-ventral pattern
formation and more than a decade of subsequent Drosophila work
helped delineate the signaling pathway (Anderson et al., 1985;
Morisato and Anderson, 1995). The simultaneous study of
antimicrobial peptides and their regulation suggested that the
transcription of these peptides was controlled by NFκB family
members, which could be targets of Toll signaling (Boman et al.,
1972; Engstrom et al., 1993; Ip et al., 1993; Samakovlis et al., 1990).
1
Departments of Craniofacial Development and Microbiology, King’s College
London, London, SE1 9RT, UK
Department of Microbiology & Immunology, 299 Campus Drive, Stanford
University, Stanford, CA 94305-5124, USA
*Author for correspondence (e-mail: [email protected])
2
After proving the link between NFκB and Toll in Drosophila, the
Toll signaling pathway developed into a touchstone of innate
immunity (Lemaitre et al., 1996; Rosetto et al., 1995). The yearly
publications on Toll-like receptors (TLRs) in vertebrates now
outnumber those on the Drosophila world by at least 10 to 1,
illustrating the conservation of fundamental immune signaling
pathways and the importance of initial findings in Drosophila to
vertebrate organisms.
As our understanding of Toll signaling deepened, it became
apparent that some things work differently in flies versus humans.
For example, TLRs in vertebrates appear to bind many of their
microbial ligands directly; however, in the fly, pattern recognition
occurs upstream through peptidoglycan binding proteins. The
variability in some of the fine mechanics of signaling between flies
and humans highlights the need to identify pathway components
that are organism specific. However, the conservation of key
mechanisms regulating central physiological functions
demonstrates the usefulness of the fly as a research tool.
The utility of the fly extends to the study of infectious disease;
Drosophila is used to dissect host interactions with known insect
pathogens and is a tractable model system for human disease.
Studies with insect pathogens teach us general lessons about the
pathology of infection that are useful even if the microbes are not
primary human pathogens. Although fly models of human disease
cannot replicate the entire disease process that occurs in the human,
they possess rapid and simple genetics enabling studies to be carried
out that would not be possible in larger animals.
Fly immunity is a multilayered system
The fly has a multilayered immune system consisting of at least
seven defensive mechanisms that might be encountered by a
pathogen as it tries to enter the body of the fly (Fig. 1). Moving
from the outside of the body to the inside these are: (1) regulation
of the native microbiota in the gut through antimicrobial peptides
(AMPs) and reactive oxygen species (Ryu et al., 2008); (2) the
barrier epithelial response, which can recognize infections and
wounds, produce local AMPs and send signals to the rest of the
body (Tzou et al., 2000); (3) the clotting response, which not only
seals wounds and prevents bleeding, but can physically trap
bacteria (Scherfer et al., 2006); (4) the phenoloxidase response,
which deposits melanin at the site of an immune reaction, releasing
potentially antimicrobial reactive oxygen species (Sugumaran,
2002); (5) the phagocytic response, through which phagocytes can
kill microbes directly by either encapsulation or phagocytosis, or
indirectly by releasing systemic signals (Agaisse et al., 2003;
Dijkers and O’Farrell, 2007; Elrod-Erickson et al., 2000; Kocks et
al., 2005); (6) the systemic AMP response, which involves the
release of massive quantities of AMPs from the fat body (the liver
analog) into the circulation (Meister et al., 1997); and (7) the RNAi
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Disease Models & Mechanisms DMM
COMMENTARY
The Drosophila immune system
response, which is required to fight viral infections (GalianaArnoux et al., 2006; Wang et al., 2006; Zambon et al., 2005). The
majority of the work in flies concentrates on the immune response
in the fat body. The transcriptional response of the fat body is so
large that 1000-fold inductions can be measured even when whole
fly homogenates are assayed. This transcriptional response is
primarily under the control of three Rel-related transcription
factors, Dif, Dorsal and Relish; these transcription factors are
themselves regulated by two cross-reacting pathways, Toll and
immune deficiency (Imd). Many recent reviews describe various
aspects of this biology (Brennan and Anderson, 2004; Ferrandon
et al., 2007; Lemaitre and Hoffmann, 2007; Royet and Dziarski,
2007; Schneider, 2007).
In order to demonstrate how basic Drosophila studies on
infection and immunity apply to medicine, we have written this
review from the viewpoint of a ‘doctor of fly medicine’ treating ‘fly
patients’ with primary defects in the immune system. As ‘fly
physicians’, our primary concern is correlating genetic defects with
susceptibility to pathogens. For example, fly patients may want to
know, ‘If I have a mutation in the Imd signaling pathway, will I be
more susceptible to Streptococcus pneumoniae?’ The mechanismbased answer is that there is no evidence that Imd is involved in
this process because S. pneumoniae doesn’t induce transcriptional
changes that are typically dependent on Imd. However, this
overlooks important information. The patient would benefit if it
learned that the loss of Imd would make them more susceptible to
S. pneumoniae, even though the exact mechanism is not yet
recognized. Therefore, this review concentrates on the survival
outcomes of infections of various mutant fly lines with pathogens
rather than the transcriptional responses induced by microbial
elicitors.
The information in Fig. 2 focuses on infection experiments in
which microbes were injected into the circulation of adult flies,
because this is where the largest number of microbes have been
tested to date, and it highlights the largest variety of microbes while
covering the smallest number of responses. The one exception to
this, is the inclusion of nitric oxide (NO) signaling as a host
response to infection. In this case, a pathogenic infection of the
gut leads to the production of NO, which acts on hemocytes (fly
phagocytes) (Dijkers and O’Farrell, 2007). The hemocytes produce
an unknown signal, which then induces AMP production in the
fat body. This was included as an intriguing example of the
integration of responses in multiple organs, involving other
pathways that are already discussed in the literature (Dijkers and
O’Farrell, 2007).
The essential message from this analysis is that there is no simple
set of rules that can be used to describe infections in the fly. The
table in Fig. 2 shows that there are many classes of pathogens and
each causes a different type of infection. Since this is consistently
true in all other systems that have been examined in depth, from
plants to humans, it is not particularly surprising that it holds true
in the fly.
Are there simple rules to control Toll and Imd signaling?
The Toll pathway is activated when the Toll receptor binds its ligand
Spaetzle. Spaetzle is secreted as a pro-protein and must be
proteolytically activated through the action of a protease cascade
before it can activate Toll. Microbial elicitors such as Lys-type
peptidoglycan from some Gram-positive bacteria and glucan from
fungi can induce this pathway. In addition, a fungal protease is
sufficient, although not necessary, to activate Toll (Gottar et al.,
2006). This leaves open the possibilities that either the fly is
monitoring the presence of fungal virulence factors, as described
in the guard model for plant immunity, or that the fungus is
intentionally activating Toll. The final output of Toll is the activation
of two transcription factors, Dif and Dorsal. In summary, the
majority of the literature states that Toll responds to Lys-type
peptidoglycan and fungal glucan, and can be activated by a fungal
Fig. 1. Fly immunity is controlled by a multilayered
system.
2. Barrier epithelia can
produce AMPs
3. Clots entrap bacteria
1. Gut regulates growth of
native microbiota and
invading pathogens with
reactive oxygen and AMPs
4. Melanization
5. Phagocytosis
The fly immune response
requires the cooperation of
many tissues and involves
many mechanisms
6. The fat body produces different
constellations of AMPs depending
upon the elicitors
7. Virus infected tissues
are defended by RNAi
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COMMENTARY
The Drosophila immune system
Pattern and pathogenesis
recognition
Pathogenesis modulation
Upd1,2,3
Eiger
Spz
Domeless
wgn
Toll
Pathogenesis modulation
Insulin-like
peptides
WntD
Nitric oxide
(NO)
?
Imd
Calcineurin
Nucleus
Insulin
receptor
Legend
Immune induced ligands
A. fumigatus
C. albicans
E. coli
S. typhimurium
B. cepacia
M. marinum
B. bassiana
S. pneumoniae
E. faecalis
S. aureus
L. monocytogenes
P. aeruginosa
S. marcesens
DXV
Mutant fly survives better
Insulin
WntD
Toll
Imd
Inhibitory signals
Eiger
Challenge
JAK/STAT
Intracellular
Extracellular
Gram+ bacterium
Gram– bacterium
Fungus
Virus
Disease Models & Mechanisms DMM
Signaling to nucleus
Mutant fly survives worse
No change in survival
Simple rules? A subset of microbes gives the
impression that Toll and Imd signaling are highly
microbe-specific. This subset suggests that Toll is
required to fight fungi and Gram-positive bacteria,
whereas Imd is required to fight Gram-negative bacteria.
Pathogens reveal complexity. As more microbes are
added to this list and the microbes are categorized
depending upon their pathogenicity, rather than their ability
to induce a set of transcripts, a new picture emerges. Toll
and Imd signaling remain critical for fighting microbes;
however, no simple rules emerge.
Insulin signaling alters resistance to infection.
Constitutive insulin signaling caused by the loss of
FoxO makes flies resistant to M. marinum infections, not
by enabling the flies to fight the microbe, but by
causing them to resist pathological wasting. Loss of
insulin signaling leads to resistance of E. faecalis or
P. aeruginosa by an unknown mechanism.
Too much immune activation can be a bad thing. WntD is a
negative feedback regulator of Toll. WntD transcription is increased by
Toll activity. WntD reduces Toll signaling by preventing the nuclear
translocation of the transcription factor Dorsal. WntD mutants
hyperactivate Toll during an infection and this increases sensitivity to
Listeria infections.
DCV
JAK/STAT signaling is required to fight a virus.
This pathway is regulated by the Unpaired ligands
and the receptor Domeless, and may inhibit the
transcriptional response of the Imd pathway.
Mutations in this pathway have not been widely
tested for their effects on infection.
Eiger, a TNF family member, is required to fight extracellular microbes but causes
harm when fighting some intracellular microbes. Loss of Eiger function increases
the sensitivity of flies to extracellular infections by reducing the ability to control
microbial growth. In contrast, Eiger mutations make flies resistant to S. typhimurium
by allowing the fly to tolerate the microbe without affecting microbial growth.
Fig. 2. A summary of the interactions between the fly immune system and a variety of pathogens.
Disease Models & Mechanisms
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Disease Models & Mechanisms DMM
protease (Brennan and Anderson, 2004; Ferrandon et al., 2007;
Lemaitre and Hoffmann, 2007; Royet and Dziarski, 2007; Schneider,
2007).
The Imd pathway is activated by peptidoglycan binding
proteins that are found both on the membrane and within cells.
Biochemical work supports the hypothesis that this pathway is
activated by DAP-type peptidoglycan. Recent work shows
that tissue damage may also be a method of activating Imd
through the cleavage of a transmembrane peptidoglycan
recognition protein (PGRP) PGRP-LC (Schmidt et al., 2008). The
final output of this pathway is activation of the transcription
factor Relish. In summary, signaling papers conclude that the Imd
pathway is activated by DAP-type peptidoglycan and proteolytic
cleavage (Brennan and Anderson, 2004; Ferrandon et al., 2007;
Lemaitre and Hoffmann, 2007; Royet and Dziarski, 2007;
Schneider, 2007).
Exceptions to the rules for Toll and Imd signaling
Many experiments measuring AMP induction support the status
quo for Toll and Imd signaling: Toll is activated by glucan and Lystype peptidoglycan, whereas Imd is activated by DAP-type
peptidoglycan. However, the literature contains a growing number
of apparent violations of these simple rules. These experiments
consistently demonstrate that flies carrying mutations blocking
either the Toll or Imd pathways are unexpectedly sensitive to
microbial infection. Mutations in Toll pathway components can
alter resistance to Listeria monocytogenes, Pseudomonas aeruginosa
and Serratia marscescens – all of which produce DAP-type
peptidoglycan (Lau et al., 2003; Lazzaro et al., 2004; Mansfield et
al., 2003). In addition, Imd pathway mutants die faster than wildtype flies when infected with a variety of fungi and Lys-type
peptidoglycan-containing bacteria (De Gregorio et al., 2002;
Hedengren-Olcott et al., 2004; Pham et al., 2007). Given the recent
work on tolerance and the uncoupling of resistance from survival,
as shown below, it is important to consider these outliers because
they can point to previously overlooked physiologies. Of course,
another explanation is that all of these outlier experiments are
simply wrong; we took the view that, as they are all peer-reviewed
publications, these observations warrant discussion and might
teach us something new.
Flies with mutations in the Toll pathway are unexpectedly
sensitive to L. monocytogenes and P. aeruginosa, as measured by
survival (Lau et al., 2003; Mansfield et al., 2003). Although we
typically measure the output of Toll signaling by looking at the
expression of the AMP drosomycin from the fat body, it is also
clear that Toll somehow regulates hemocyte activation (Zettervall
et al., 2004). Given that L. monocytogenes infects these cells, it
would not be surprising if Toll had an effect on Listeria infections.
Further evidence for the participation of Toll in ‘off target’
infections comes from ecological immunity studies (Lazzaro et
al., 2004). The analysis of genetically diverse strains of Drosophila
has demonstrated that their abilities to resist infection, as
measured by bacterial growth, can differ greatly. Candidate
polymorphic traits were followed and the authors found that
polymorphisms in the Toll pathway were correlated with altered
resistance to the DAP-containing bacteria S. marscescens. A
recent structural study suggests that one of the pattern
recognition molecules upstream of Toll binds to DAP-type
46
The Drosophila immune system
peptidoglycan, providing a biochemical explanation for how Toll
mutants might be sensitive to DAP-containing microbes (Leone
et al., 2008).
In S. marscescens, Toll and IMD signaling appears to be complex;
one strain of S. marscescens kills both wild-type flies and flies with
mutations in Imd or Toll extremely rapidly (Nehme et al., 2007).
One interpretation of this is that the microbe is completely resistant
to the fly’s immune response. A second interpretation is that the
flies die too rapidly to determine whether the mutations have an
effect on survival. We marked S. marscescens as showing no change
in killing rates between wild-type and Toll or Imd mutant flies. As
mentioned above, there is evidence that Toll, as well as Imd,
signaling is used to suppress the growth of another strain of S.
marscescens in a similar injection model.
Flies lacking the imd gene are unexpectedly sensitive to the
Gram-positive bacterium S. pneumoniae and the fungus Beauveria
bassiana (De Gregorio et al., 2002; Hedengren-Olcott et al., 2004;
Pham et al., 2007). These microbes cause pathology in the fly,
suggesting that a PGRP-LC-mediated response to tissue damage
may be an important mediator of antimicrobial defenses in the fly.
Further exceptions can be found by looking at mycobacteria and
at viruses. Injection of Mycobacterium marinum into the fly
produces an early transcriptional response similar to medium alone
(Dionne et al., 2003; Dionne et al., 2006). Neither the Imd nor the
Toll pathway appears able to detect this particular microbe, even
though its peptidoglycan is similar to the DAP type that is an
optimal agonist for the Imd pathway. Toll mutant flies are also more
sensitive to the virus Drosophila X Virus (DXV) (Zambon et al.,
2005). In this case, flies carrying an activated allele of Toll are
sensitive to the virus and have an increased titer. The same is true
for loss-of-function mutants in the transcription factor Dif, which
blocks Toll signaling. As discussed below, there are now several
examples where the immune response of the fly is finely balanced
and that over- or under-activation of the pathway can lead to
pathology during an infection. The mechanisms behind this
increased sensitivity are unknown.
The point of all of this is not to undercut the importance of the
original papers that demonstrated the importance and the
discriminatory power of the Imd and Toll pathways. We believe
these papers are rightly regarded as the cornerstones of modern
Drosophila immunology; however, from the standpoint of a ‘fly
physician’, it is important for us to recognize where these rules break
down so as to better understand the diseases and physiology of the
patient. Perhaps the way to deal with this in the future is to consider
immunity to be a quantitative trait and to report the statistically
significant changes in survival, rather than simplifying phenotypes
so that they appear to fall into only two classes.
Why does an infected fly die?
As fly doctors, we need to be concerned with the cause of death
in our patients. The most commonly studied fly deaths caused by
infections are cases where there is a mutation in the Toll or Imd
signaling pathway and the mutant flies die faster than wild-type
flies. Studies have found that when the mutant flies have died faster
there is an enormous increase in the amount of bacterial growth
in these mutants. Given that these pathways are responsible for
inducing AMP transcription and that mutation of these pathways
leads to microbial growth and host death, AMP transcription is
dmm.biologists.org
Disease Models & Mechanisms DMM
The Drosophila immune system
often used as a proxy for resistance to infection. The assumption
is that these AMPs are the main weapons used to fight infection
and that these block bacterial reproduction.
Several recent papers have uncoupled bacterial growth and, by
extension, AMP transcription, from survival. Wild-type and
genetically diverse strains of flies infected with P. aeruginosa all
die, but there is no correlation between the number of bacteria
growing in these flies and their mean time of death (Corby-Harris
et al., 2007). A screen for mutant flies with increased sensitivity to
L. monocytogenes revealed that one-third of the 12 mutants isolated
died rapidly upon infection, but showed no significant change in
bacterial growth as compared with wild-type flies (Ayres et al.,
2008). Work on Eiger and insulin signaling (shown below) produced
similar results (Dionne et al., 2006; Schneider et al., 2007). These
results show the limitation of using AMPs as a proxy for the defense
against infection; expression studies teach us important lessons
about the requirements for AMP induction but miss important
immune responses.
Evolutionary ecologists suggest that when a host’s fitness is
limited by infection, there are two evolutionary routes to fight this
(Raberg et al., 2007). The first is resistance, which directly reduces
the number of pathogens preying on the host. The second is
tolerance, in which the host evolves mechanisms to limit the
damage created by the pathogen. This experimental uncoupling of
bacterial growth and survival suggests that tolerance mechanisms
play an important role in fighting infections in the fly. The majority
of work to date has not measured survival and bacterial growth
but has relied on proxies. Fly patients are dying to know the
mechanisms behind tolerance.
Insulin signaling alters pathogenesis
Insulin signaling status is an important predictor of survival for
Drosophila infections. M. marinum infections induce a pathological
state in which insulin signaling is reduced and the flies appear to
die from a wasting condition in which they consume all of their
fat and glycogen stores (Dionne et al., 2006). The lifespan of the
fly can be extended by forcing the induction of the insulin signaling
pathway. This resembles the situation in humans, where patients
entering an emergency room and suffering from septic shock do
better if their insulin levels are carefully controlled (van den Berghe
et al., 2001). In flies, this insulin effect appears to be a tolerance
effect and not a resistance effect because changes in insulin
signaling do not alter M. marinum numbers but do alter survival.
The fly insulin signaling story is complicated; inactivation of the
insulin signaling pathway by mutation of the chico gene makes flies
resistant to Enterococcus faecalis and P. aeruginosa infections
(Libert et al., 2008). Bacterial loads were not determined in these
experiments and thus we cannot distinguish between endurance
and tolerance effects.
The message for fly physicians here is that it is important to
understand the nature of the infection before manipulating insulin
signaling. Not many organisms have been tested here and there is
currently no set of rules. Future experiments will involve testing a
broad array of organisms and uncovering the molecular mechanism
behind insulin’s affects on immunity. The M. marinum story
provides one explanation for pathology, but insulin has pleiotropic
effects, altering autophagy, energy storage and feeding, and could
affect different microbes in different ways.
Disease Models & Mechanisms
COMMENTARY
Too much signaling is a bad thing
The fly community is just starting to determine the pathological
causes of death in infected flies. In humans, it is clear that damage
caused by an infection can come from our immune responses and
that appropriate negative regulation is critical. Experiments in the
fly suggest that the situation is similar and that the community is
beginning to determine how misregulation of our immune response
can cause harm.
An example of the importance of negative regulation in surviving
infections comes from the study of the signaling molecule, Wnt
inhibitor of dorsal (WntD) (Ganguly et al., 2005; Gordon et al.,
2005). Two papers have identified the Wnt family member, WntD,
as a negative regulator of Toll signaling in the maternal control of
dorsal-ventral pattern formation. Studies have shown that WntD
is induced by Toll signaling and that it blocks the activation of Toll
signaling by preventing the activation of the transcription factor
Dorsal. WntD flies were found to be significantly more sensitive
to infection with L. monocytogenes and the hypothesis for this was
that this sensitivity was due to increased signaling by Toll through
Dorsal (Gordon et al., 2005). To test this hypothesis, the authors
measured the survival of flies with 0, 1 or 2 copies of Dorsal in a
WntD background. A reduction in the number of Dorsal copies
was shown to reduce the severity of the phenotype, thereby
supporting the hypothesis. We note that Dorsal is not typically
considered to be a major regulator of the massive fat body
transcriptional response. The mechanism behind the pathology of
induced Dorsal activity is not yet known.
Eiger, a member of the tumour necrosis factor (TNF) ligand family
is involved in the immune response
The TNF family plays an important role during infections in
humans. The fly has a single member of this family, Eiger. Eiger
plays a complicated role in fly immunity, just as TNF does in
humans. Eiger mutants die faster when infected by extracellular
microbes, regardless of whether they are Gram-positive or Gramnegative bacteria, or fungi (Brandt et al., 2004; Schneider et al.,
2007). This is a resistance phenotype because the increase in death
rate is associated with an increase in microbial load. In contrast to
the results with extracellular infections, infection of Eiger mutants
with intracellular microbes leads to either increased survival or no
change in microbial load. Thus Eiger has a tolerance effect on
intracellular microbes such as Salmonella typhimurium.
Eiger mutations have been shown to affect three immune
responses in the fly. Eiger mutants do not show a normal melanization
response in larvae (Bidla et al., 2007), but do display a reduction in
the phagocytosis of labeled, dead Staphylococcus aureus in adult flies.
It has yet to be determined whether this is a direct immune
phenotype or whether there is a change in the development of the
phagocytes (Schneider et al., 2007). Lastly, Eiger mutants show
statistically significant increases in the transcription of diptericin, an
AMP typically used as a proxy for Imd signaling, suggesting that Eiger
could be an inhibitor of Imd signaling (Schneider et al., 2007). It is
unclear why the release of Imd from negative control should lead to
increased susceptibility to any microbes.
The implication for fly medicine here, is that manipulation of
the Eiger pathway in the fly should be done only with knowledge
of the type of infection, because an incorrect treatment will kill the
fly more rapidly.
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COMMENTARY
JAK/STAT signaling
Flies appear to lack interferons, but they nonetheless have a Janus
kinase (JAK)/signal transducer and activator of transcription
(STAT) signaling pathway that affects immunity. This pathway is
controlled by a single receptor, Domeless, and by three ligands,
Unpaired 1, 2 and 3. Studies of this system in vivo demonstrate
that immune activation of hemocytes can induce Unpaired 3
expression in hemocytes (Agaisse et al., 2003). This ligand acts on
the fat body and induces the production of a subset of genes that
are different from those typically induced by Imd activation. As
stated above, JAK/STAT activation is also expected to reduce Imd
signaling.
A full analysis of the contribution of this pathway to the survival
of infections in adult flies has been performed with only one virus,
Drosophila C Virus (DCV) (Dostert et al., 2005). This virus has
been shown to kill flies with an altered JAK/STAT signaling
pathway more rapidly than wild-type flies. These mutants also carry
a higher load of virus, which suggests a defect in resistance. The
mechanism governing JAK/STAT-regulated resistance to DCV is
unknown. Given that this pathway can negatively regulate the Imd
pathway, JAK/STAT signaling may help fight viral infections by
downregulating Imd during the response. Alternatively, JAK/STAT
signaling can induce its own unique immune response and this
could be required for immunity. More microbes should be tested
for changes in resistance and tolerance in JAK/STAT mutants.
Simplifications
All diagrams simplify and we omitted many immune processes in
making ours. Our purpose in making Fig. 2 and writing this review
is to highlight the variety of immune responses generated by
different pathogenic microbes in the fly. One big simplification is
that we drew our diagram as if all signaling was occurring in a
single cell. This is probably not true in all cases and in some cases
we know for certain that it is not (Dijkers and O’Farrell, 2007).
We focus almost entirely on injection experiments, ignoring
interesting and exciting work on gut infections and the interaction
of the fly’s immune system with its native microbiota. We also
omit work carried out using eukaryotic parasites such as Crithidia,
Plasmodium, parasitoid wasps and parasitic nematodes. Wounding
and the response to damaged tissue were omitted, as was a detailed
discussion on the activity of Drosophila phagocytes. A huge story
emerging in fly immunity is that the fly’s immune system integrates
the total physiology of the fly and its environment when
responding to an infection. For example, there are bidirectional
loops that have been identified between reproduction, circadian
rhythm, energy metabolism, aging and past immune responses.
None of these was integrated into this model. This research has
a large potential impact on models of the fly for disease as it shows
how the immune response affects, and is affected by, other
physiologies. None of these events is yet understood at a
mechanistic level.
We have presented a picture in which the Toll and Imd signaling
pathways are the primary sentinels of immunity and where their
activity is altered by a variety of feedback loops. It is clear that there
is more to surviving an infection than simply resisting the microbe.
The community has a reasonable understanding of the methods
that are required for resistance, including AMPs, clotting and
phagocytosis. In only one case do we have an explanation for
48
The Drosophila immune system
changes in tolerance – that of wasting in M. marinum infections.
On top of this, we have shown that fly disease is complicated; when
enough factors are studied, it appears that every microbe tested
provokes a different disease in the fly. This is good news because
otherwise the fly would serve as a poor model of human immunity
and infection. It also means that we cannot look at one or two
microbes and make sweeping conclusions about a large class of
microbes. We hope that this review will highlight rich areas for
future work in Drosophila infection and immunity, and will reveal
new possibilities for translation of this work into the realm of human
medicine.
COMPETING INTERESTS
The authors declare no competing financial interests.
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