Immunology of tropical infectious diseases
Written exam and a presentation. Every line on the overview slides can be an exam question. 5 points for the presentation and
5 points for the discussion. Counts for 20% of both exams. The score can be different for immunology and pathogenesis.
INTRODUCTION
Objective of the lectures:
1. Learn the basic principles of the innate immune system of the arthropod vector
2. Refresh basic knowledge about innate and adaptive immunity of the vertebrate host
3. Apply knowledge of innate and adaptive immunity to host-pathogen interactions
4. Understand the basis of successful infections through the skin and mucosal barriers
5. Understand the concept and examples of immune privilege
6. Learn about examples of immune escape
7. Relevance of the microbiome during host-pathogen interactions
1. INNATE IMMUNITY OF THE INVERTEBRATE HOST
Objectives:
1. Learn the basic principles of the innate immune system of the arthropod vector
2. What are major pathogens transmitted by arthropods
3. What are the physiological barriers
4. What are the immunological barriers
5. What are the cells of the innate immune system
6. What are the major immune recognition pathways
7. What are the major immune effectors
8. Which opportunities exist to target the arthropod and the pathogens they transmit
Immunology of tropical infectious diseases 1
,INVERTEBRATES AS PARASITE HOSTS AND DISEASE VECTORS
Various pathogens transmitted by blood feeding arthropods:
- Mites: Rickettsia spp. (gram neg IC bacteria)
- Ticks: Lyme disease, babesiosis, theileriosis, TBEV fe Ixodes ricinus
o babesia —> mainly associated with a missing spleen after fe car accident
- Lice: Epidemic typhus (Rickettsia), trench fever, recurrent fever
- Mosquitoes:
o Protozoa: Malaria
o Arboviruses: yellow fever, dengue, chikungunya, zika, WNV (lymphatic filarids)
o Nematodes: Wuchereria, Brugia
- Reduviid bugs: Chagas disease
- Dipters: Leishmaniasis and human African trypanosomiasis
virus = replication and parasite = proliferation (may need moulting, attachment, etc.)
When taking a blood meal, the arthropods can take up a pathogen or transmit a pathogen to another vector. The pathogen
has to enter the arthropod through the blood meal and in the end reach a place where it can be transmitted again. Parasite
life cycles depend on specific interactions with the vector:
- before it enters into midgut first needs to reach salivary gland —> needs to cross multiple physiological barriers
Besides parasites, insects harbour a microbiome, including bacterial symbionts in the midgut. This is interesting because you
can modify the microbiome of insects and render them for example resistant to certain pathogens.
ESTABLISHMENT IN THE ARTHROPOD GUT
Transmission relates to uptake of a blood meal. There are typical physiological barriers / triggers during the migration of the
pathogen from a human environment to the midgut of an insect. It has to colonize the arthropod midgut. Malpighian tubules
are primitive kidneys that help in the dehydration of the blood meal, to make sure you have fast removal of the water of the
blood meal.
The pathogen must overcome the environment of the midgut:
• Change of temperature
• Blood bolus moves to digestive part: high pH, proteolytic activity & gut microbiota
• The PM represents a mechanical barrier (peritrophic matrix)
o PM consists of chitin & proteoglycans: make mesh through which pathogens
cannot migrate but nutrients can diffuse.
o Ectoperitrophic space: between the PM and the epithelium
▪ Interesting for pathogens to invade this
There is a layer of chitin (carbohydrate) that coats the inside and outside of the GIT of the insect. It is also covered with a layer
of lipid, which prevents an insect of desiccation. The entire GIT is chitin-based except for the midgut where you have high
digestion of proteins.
Immunology of tropical infectious diseases 2
,ARTHROPOD IMMUNE SYSTEM: CELLS
If pathogens are able to cross the barrier, they end up in the insect. The body cavity of the
insect is called the hemocoel, and it contains hemocytes in its ‘blood’ or haemolymph. These
are cells of the immune system of the insect. They also function as blood cells to transport
oxygen. There are both circulating / patrolling hemocytes, and sessile hemocytes. They can be
sessile = located at strategic places and will stay there. hemocyanin coordinates copper
instead of iron // cannot induce immune memory.
The fat body in the insect is like a primitive liver. It is a lipid tissue composed primarily of storage cells and is important in
triggering immune responses. It is an important source of antimicrobial peptides.
- very short life span = isn’t the same as us because we need long for bone marrow etc.
These cells are produced during larval development by a lymphoid organ in the thorax of the insect. When the insect becomes
a mature adult, this larval lymphoid organ disappears and no news immune cells are created. The only way that insects can
respond to an infection is by proliferation of the cells.
If you take haemolymph of an insect you find:
1. Plasmatocytes: phagocytosis, encapsulation, AMPs (similar to macrophages, etc.)
1. AMP=antimicrobialpeptides
2. Lamellocytes: encapsulation >, melanization
1. Encapsulation: for pathogens larger than their own cell size
2. Melanization: to encapsulate pathogens & make a black colour around pathogen
3. Can really COAT
3. Crystal cells or oenocytoids: melanization (can create nodules, can coat, etc. this is important for innate immunity)
1. Harbour crystals: enzymes (prophenoloxidase at very high conc crystallize IC)
2. needs cleavage to be active —> will crystilize inside the cell
➔ Can all occur as sessile or circulating cells
slide 5 = example
looking at ontogeny of immune cells —> whole pathway of the
promenocyt?
different populations show plasmotocytes, indicates an important
population
look at the insect —> to challenge it with a parasitic wasp —> lay egg
inside the insect (model for infection in Drosophilla)
unwounded = not challenged —> shows the different subsets
wound = cells get triggered to proliferate in all subsets increase
wasp = consumed “used” to fight the infection (especially in
encapsulation)
Immunology of tropical infectious diseases 3
,CIRCULATING HEMOCYTES
An insect has a very primitive heart, which is a muscular tube, that contracts. It is positioned in an
open circulatory system which means the haemolymph passes through the tube and is just pushed
out by contraction.
You have 7 abdominal segments, and for each segment there is an opening in the heart / tube. This
means the haemolymph can enter there and upon contraction, it is pushed out. The haemolymph
is pumped a few times in the anterograde way and a few times in the retrograde way, so there is
good circulation in the body cavity.
- for each opening = ostia = lays periostial cells = allows kind of transport —> wait to
capture pathogens circulating
If there is an infection with bacteria, there is proliferation and the circulating bacteria will be taken
out. Predominantly, uptake of the bacteria happens at the openings of the heart: sessile hemocytes
are located at these openings (ostia), where all the pathogens have to pass.
Exam question: what are the immune cells in an insect and their major functions.
WORKING MECHANISMS OF THE ARTHROPOD IMMUNE SYSTEM
Basic innate immunity is largely the same as in vertebrate immunity, with the recognition of pathogens or PAMs by pathogen
recognition receptors. There’s 3 major pathways in the insect:
1) Toll pathway: mainly recognizes fungi & G+ bacteria, can also respond to G- (toll like receptors recognize PAMP usually,
however here → recognizes spatzle, there are still PAMPs which trigger activation of spatzle)
a) Bind to the receptor
b) Triggers Myd88 and activates NFkB-like transcription factor: Rel1, migration to the nucleus and transcription of
effector genes (AMPs)
i) Will phosphorylate Rel1 → will release cactus
ii) This will alleviate inhibition of cactus (activate within a few min) → can start transcribing effector genes
c) Very tightly regulated
d) Here: toll receptor is not a PAMPs receptor: recognizes spätzle: a cytokine that is activated in response to an infection
Immunology of tropical infectious diseases 4
, e) Antimicrobial peptides are important to fight of infection
2) Imd pathway: immunodeficiency pathway. It mainly recognises G- bacteria & protozoa.
a) Receptor = PGRP-LC: peptidoglycan recognition protein: mainly recognizes bacterial cell wall components
(peptidoglycans), & also has recognitional domains on protozoa.
b) It triggers Imd and there is activation of Rel2: NFkB-like transcription factor. This will activate a number of AMPs
i) Here regulating = caspar
ii) Differentiating between Rel1 and 2 allows finetuning
c) Recognizes G- & protozoa: identify highly immunogenic bacteria -> introduce in population of insects -> stimulate
Imd pathway -> resistant to protozoa infecion
3) Jak/Stat pathway: janus-activated kinase pathway. It’s a kinase that is activated by cytokines: Upd (unpaired), or vago.
a) Phosphorylation -> transcription factor activated -> signal transduction -> effector genes
b) Stat is the transcription factor
The effector genes resulting from these pathways are almost all AMPs, depending on what type of AMP it can target more G+
or G- bacteria, protozoa... will cleave viral RNA as a result = small enzymatic method → replication of the virus will be affected.
1 of the major effector pathways in transmission of viruses is the RNAi pathway. There is infection with a +ssRNA virus, so the
host cell will make the – complementary strand, so you get the ds intermediate. This is recognized by R2D2 and Dicer, which
will cleave the dsRNA into small interfering RNA’s. These are recognized by the RISC complex that removed the sense strand:
degradation of viral RNA. Recognition by Dicer also triggers the cytokine Vago, causing prod. of AMP via the Jak/Stat pathway.
Upstream, Spätzle is a cytokine that is activated upon recognition of a pathogen.
This is because fungi produce proteases that cleave a number of proteins. There is a
number of PAMPs that bind fungal components, to become activated: serine
protease activity. Spätzle is activated by proteolytic cleavage that circulates in the
haemolymph and will bind to the Toll-receptor.
HOW CAN WE USE THIS?
In biologicals and other molecules, we have to be careful for endotoxins such as LPS.
These are very harmful for humans so these cannot be injected in more than 0,5
ng/kg body weight to prevent endotoxic reactions. We can detect LPS in a sample
by using the LAL assay: limulus amebocyte lysate test. By using the haemolymph
from the horseshoe crab that contains hemocytes, the lysate can be collected. In the
lysate, a soluble protein is present that can recognize a bacterial component, in this case LPS. This causes enzymatic activation
upon recognition of LPS: quantitative assay with substrate that is detectable → pNA is a chromogenic substrate, if the enzyme
cuts there is colour.
- Factor C is present → protease will cause binding because of LPS → factor C will become active
- If we cut peptide => yellow colour (due to release of pNA)
ARTHROPOD IMMUNE SYSTEM: EFFECTORS
Can introduce changes → results in different types of lytic mechanisms.
AMPs most of the time have a positive charge and the bacterial cell wall has a negative charge: bind by electrostatic interaction
& conformational change. This disrupts the structure of the microbiotic membrane in various ways. Mostly, it happens by the
formation of a helix that enters into the bacterial membrane.
Immunology of tropical infectious diseases 5
, ROS and RNS are also typically present upon an infection with a bacterium. There is metabolic
activation and respiratory burst, which results in many cases in killing of the pathogens.
The major response in the insect happens at the level of the epithelium. The midgut epithelium is
very important in inducing ROS and RNS to control the bacterial population. It is based on
degradation of arginine into citrulline using NADPH as an electron donor & producing NO. NO is
mainly affecting protein structures. Locally produced NO can also migrate towards membranes
through the body of the insect: intertissue communication.
ROS production is most of the time based on NADPH oxidase that uses NADPH as an electron
donor and it produces H2O2 which is highly reactive. In insects and esp. at the epithelial surface,
there is present of DUOX.
Peroxidase domain: this is a very potent oxidative enzyme that doesn’t only produce H2O2, but it also has a
peroxidase domain that can convert peroxide to hypochloride (bleach) that has potent antimicrobial activity.
Immune reactive catalase can degrade H2 allowing it to be neutralized.
Selectivity of insects can also be important → makes it more attractive to mosquitos.
Prophenoloxidase is important for the melanization pathway
Melanization was a response of an insect to cope with an infection, sometimes of
rel. large pathogens by the production of melanin. Melanin is highly insoluble and
will coat the surface of the pathogen. If you inject a bacterium into a mosquito larva,
there will start to form aggregates or conglomeratos of lamelocytes, plasmocytes
and melanin: dark pigmented spots. This is part of the innate immune system.
- Indole-5,6-quinone = melanin
- Can see a lot of aggregates of melanin → part of the immune response of the innate immune
system
It starts with the recognition of a pathogen, and PAMPs that recognize the pathogen. The trigger is
also serine protease (like in Toll pathway). You also need activation of prophenol oxidase: PO
enzyme secreted by the crystal cells. PO is activated by PPAE: prophenol oxidase activating enzyme.
This enzyme is not present as an activated enzyme but as pro-PPAE. This is activated by itself by
the serine protease! PO is an oxidizing enzyme. Will result in a proteolytical active enzyme = serine
protease. Allowing it to become a phenoloxidase = active enzyme.
The pathway completely relies on the amino acid tyrosine. Many close analogues of tyrosine are also recognized by PO: dopa,
dopaquinone & dopamine. The end-point is the local production of indole-5,6-quinone. This is melanin and will bind to the
pathogen surface: the pathogen can no longer take up nutrients, will create ROS and intoxicate itself: self-containment in the
capsule.
Pathogen is killed locally by the melanization pathway.
SUMMARY OF THE IMMUNE SYSTEM: CASE EXAMPLE OF PLASMODIUM
When you have a Plasmodium infection in mosquitoes, there is quite a lot of variability.
Some mosquitoes are fully resistant or very susceptible. Essential is the surface
molecule Plasmodium falciparum Pfs47 that determines if the Plasmodium strain is
able to infect or not. It determines the efficiency of transmission.
Ookinete enters in the epithelium → NO will nitrozilate protein targets on the pathogen
but also the basal lamina.
The mosquito molecule TEP1 is a complement factor. Complement is also one of the
effector molecules besides ROS, melanisation,... especially used
for Plasmodium infections. Plasmodium infection in mosquitoes: gametocytes →
Immunology of tropical infectious diseases 6
Written exam and a presentation. Every line on the overview slides can be an exam question. 5 points for the presentation and
5 points for the discussion. Counts for 20% of both exams. The score can be different for immunology and pathogenesis.
INTRODUCTION
Objective of the lectures:
1. Learn the basic principles of the innate immune system of the arthropod vector
2. Refresh basic knowledge about innate and adaptive immunity of the vertebrate host
3. Apply knowledge of innate and adaptive immunity to host-pathogen interactions
4. Understand the basis of successful infections through the skin and mucosal barriers
5. Understand the concept and examples of immune privilege
6. Learn about examples of immune escape
7. Relevance of the microbiome during host-pathogen interactions
1. INNATE IMMUNITY OF THE INVERTEBRATE HOST
Objectives:
1. Learn the basic principles of the innate immune system of the arthropod vector
2. What are major pathogens transmitted by arthropods
3. What are the physiological barriers
4. What are the immunological barriers
5. What are the cells of the innate immune system
6. What are the major immune recognition pathways
7. What are the major immune effectors
8. Which opportunities exist to target the arthropod and the pathogens they transmit
Immunology of tropical infectious diseases 1
,INVERTEBRATES AS PARASITE HOSTS AND DISEASE VECTORS
Various pathogens transmitted by blood feeding arthropods:
- Mites: Rickettsia spp. (gram neg IC bacteria)
- Ticks: Lyme disease, babesiosis, theileriosis, TBEV fe Ixodes ricinus
o babesia —> mainly associated with a missing spleen after fe car accident
- Lice: Epidemic typhus (Rickettsia), trench fever, recurrent fever
- Mosquitoes:
o Protozoa: Malaria
o Arboviruses: yellow fever, dengue, chikungunya, zika, WNV (lymphatic filarids)
o Nematodes: Wuchereria, Brugia
- Reduviid bugs: Chagas disease
- Dipters: Leishmaniasis and human African trypanosomiasis
virus = replication and parasite = proliferation (may need moulting, attachment, etc.)
When taking a blood meal, the arthropods can take up a pathogen or transmit a pathogen to another vector. The pathogen
has to enter the arthropod through the blood meal and in the end reach a place where it can be transmitted again. Parasite
life cycles depend on specific interactions with the vector:
- before it enters into midgut first needs to reach salivary gland —> needs to cross multiple physiological barriers
Besides parasites, insects harbour a microbiome, including bacterial symbionts in the midgut. This is interesting because you
can modify the microbiome of insects and render them for example resistant to certain pathogens.
ESTABLISHMENT IN THE ARTHROPOD GUT
Transmission relates to uptake of a blood meal. There are typical physiological barriers / triggers during the migration of the
pathogen from a human environment to the midgut of an insect. It has to colonize the arthropod midgut. Malpighian tubules
are primitive kidneys that help in the dehydration of the blood meal, to make sure you have fast removal of the water of the
blood meal.
The pathogen must overcome the environment of the midgut:
• Change of temperature
• Blood bolus moves to digestive part: high pH, proteolytic activity & gut microbiota
• The PM represents a mechanical barrier (peritrophic matrix)
o PM consists of chitin & proteoglycans: make mesh through which pathogens
cannot migrate but nutrients can diffuse.
o Ectoperitrophic space: between the PM and the epithelium
▪ Interesting for pathogens to invade this
There is a layer of chitin (carbohydrate) that coats the inside and outside of the GIT of the insect. It is also covered with a layer
of lipid, which prevents an insect of desiccation. The entire GIT is chitin-based except for the midgut where you have high
digestion of proteins.
Immunology of tropical infectious diseases 2
,ARTHROPOD IMMUNE SYSTEM: CELLS
If pathogens are able to cross the barrier, they end up in the insect. The body cavity of the
insect is called the hemocoel, and it contains hemocytes in its ‘blood’ or haemolymph. These
are cells of the immune system of the insect. They also function as blood cells to transport
oxygen. There are both circulating / patrolling hemocytes, and sessile hemocytes. They can be
sessile = located at strategic places and will stay there. hemocyanin coordinates copper
instead of iron // cannot induce immune memory.
The fat body in the insect is like a primitive liver. It is a lipid tissue composed primarily of storage cells and is important in
triggering immune responses. It is an important source of antimicrobial peptides.
- very short life span = isn’t the same as us because we need long for bone marrow etc.
These cells are produced during larval development by a lymphoid organ in the thorax of the insect. When the insect becomes
a mature adult, this larval lymphoid organ disappears and no news immune cells are created. The only way that insects can
respond to an infection is by proliferation of the cells.
If you take haemolymph of an insect you find:
1. Plasmatocytes: phagocytosis, encapsulation, AMPs (similar to macrophages, etc.)
1. AMP=antimicrobialpeptides
2. Lamellocytes: encapsulation >, melanization
1. Encapsulation: for pathogens larger than their own cell size
2. Melanization: to encapsulate pathogens & make a black colour around pathogen
3. Can really COAT
3. Crystal cells or oenocytoids: melanization (can create nodules, can coat, etc. this is important for innate immunity)
1. Harbour crystals: enzymes (prophenoloxidase at very high conc crystallize IC)
2. needs cleavage to be active —> will crystilize inside the cell
➔ Can all occur as sessile or circulating cells
slide 5 = example
looking at ontogeny of immune cells —> whole pathway of the
promenocyt?
different populations show plasmotocytes, indicates an important
population
look at the insect —> to challenge it with a parasitic wasp —> lay egg
inside the insect (model for infection in Drosophilla)
unwounded = not challenged —> shows the different subsets
wound = cells get triggered to proliferate in all subsets increase
wasp = consumed “used” to fight the infection (especially in
encapsulation)
Immunology of tropical infectious diseases 3
,CIRCULATING HEMOCYTES
An insect has a very primitive heart, which is a muscular tube, that contracts. It is positioned in an
open circulatory system which means the haemolymph passes through the tube and is just pushed
out by contraction.
You have 7 abdominal segments, and for each segment there is an opening in the heart / tube. This
means the haemolymph can enter there and upon contraction, it is pushed out. The haemolymph
is pumped a few times in the anterograde way and a few times in the retrograde way, so there is
good circulation in the body cavity.
- for each opening = ostia = lays periostial cells = allows kind of transport —> wait to
capture pathogens circulating
If there is an infection with bacteria, there is proliferation and the circulating bacteria will be taken
out. Predominantly, uptake of the bacteria happens at the openings of the heart: sessile hemocytes
are located at these openings (ostia), where all the pathogens have to pass.
Exam question: what are the immune cells in an insect and their major functions.
WORKING MECHANISMS OF THE ARTHROPOD IMMUNE SYSTEM
Basic innate immunity is largely the same as in vertebrate immunity, with the recognition of pathogens or PAMs by pathogen
recognition receptors. There’s 3 major pathways in the insect:
1) Toll pathway: mainly recognizes fungi & G+ bacteria, can also respond to G- (toll like receptors recognize PAMP usually,
however here → recognizes spatzle, there are still PAMPs which trigger activation of spatzle)
a) Bind to the receptor
b) Triggers Myd88 and activates NFkB-like transcription factor: Rel1, migration to the nucleus and transcription of
effector genes (AMPs)
i) Will phosphorylate Rel1 → will release cactus
ii) This will alleviate inhibition of cactus (activate within a few min) → can start transcribing effector genes
c) Very tightly regulated
d) Here: toll receptor is not a PAMPs receptor: recognizes spätzle: a cytokine that is activated in response to an infection
Immunology of tropical infectious diseases 4
, e) Antimicrobial peptides are important to fight of infection
2) Imd pathway: immunodeficiency pathway. It mainly recognises G- bacteria & protozoa.
a) Receptor = PGRP-LC: peptidoglycan recognition protein: mainly recognizes bacterial cell wall components
(peptidoglycans), & also has recognitional domains on protozoa.
b) It triggers Imd and there is activation of Rel2: NFkB-like transcription factor. This will activate a number of AMPs
i) Here regulating = caspar
ii) Differentiating between Rel1 and 2 allows finetuning
c) Recognizes G- & protozoa: identify highly immunogenic bacteria -> introduce in population of insects -> stimulate
Imd pathway -> resistant to protozoa infecion
3) Jak/Stat pathway: janus-activated kinase pathway. It’s a kinase that is activated by cytokines: Upd (unpaired), or vago.
a) Phosphorylation -> transcription factor activated -> signal transduction -> effector genes
b) Stat is the transcription factor
The effector genes resulting from these pathways are almost all AMPs, depending on what type of AMP it can target more G+
or G- bacteria, protozoa... will cleave viral RNA as a result = small enzymatic method → replication of the virus will be affected.
1 of the major effector pathways in transmission of viruses is the RNAi pathway. There is infection with a +ssRNA virus, so the
host cell will make the – complementary strand, so you get the ds intermediate. This is recognized by R2D2 and Dicer, which
will cleave the dsRNA into small interfering RNA’s. These are recognized by the RISC complex that removed the sense strand:
degradation of viral RNA. Recognition by Dicer also triggers the cytokine Vago, causing prod. of AMP via the Jak/Stat pathway.
Upstream, Spätzle is a cytokine that is activated upon recognition of a pathogen.
This is because fungi produce proteases that cleave a number of proteins. There is a
number of PAMPs that bind fungal components, to become activated: serine
protease activity. Spätzle is activated by proteolytic cleavage that circulates in the
haemolymph and will bind to the Toll-receptor.
HOW CAN WE USE THIS?
In biologicals and other molecules, we have to be careful for endotoxins such as LPS.
These are very harmful for humans so these cannot be injected in more than 0,5
ng/kg body weight to prevent endotoxic reactions. We can detect LPS in a sample
by using the LAL assay: limulus amebocyte lysate test. By using the haemolymph
from the horseshoe crab that contains hemocytes, the lysate can be collected. In the
lysate, a soluble protein is present that can recognize a bacterial component, in this case LPS. This causes enzymatic activation
upon recognition of LPS: quantitative assay with substrate that is detectable → pNA is a chromogenic substrate, if the enzyme
cuts there is colour.
- Factor C is present → protease will cause binding because of LPS → factor C will become active
- If we cut peptide => yellow colour (due to release of pNA)
ARTHROPOD IMMUNE SYSTEM: EFFECTORS
Can introduce changes → results in different types of lytic mechanisms.
AMPs most of the time have a positive charge and the bacterial cell wall has a negative charge: bind by electrostatic interaction
& conformational change. This disrupts the structure of the microbiotic membrane in various ways. Mostly, it happens by the
formation of a helix that enters into the bacterial membrane.
Immunology of tropical infectious diseases 5
, ROS and RNS are also typically present upon an infection with a bacterium. There is metabolic
activation and respiratory burst, which results in many cases in killing of the pathogens.
The major response in the insect happens at the level of the epithelium. The midgut epithelium is
very important in inducing ROS and RNS to control the bacterial population. It is based on
degradation of arginine into citrulline using NADPH as an electron donor & producing NO. NO is
mainly affecting protein structures. Locally produced NO can also migrate towards membranes
through the body of the insect: intertissue communication.
ROS production is most of the time based on NADPH oxidase that uses NADPH as an electron
donor and it produces H2O2 which is highly reactive. In insects and esp. at the epithelial surface,
there is present of DUOX.
Peroxidase domain: this is a very potent oxidative enzyme that doesn’t only produce H2O2, but it also has a
peroxidase domain that can convert peroxide to hypochloride (bleach) that has potent antimicrobial activity.
Immune reactive catalase can degrade H2 allowing it to be neutralized.
Selectivity of insects can also be important → makes it more attractive to mosquitos.
Prophenoloxidase is important for the melanization pathway
Melanization was a response of an insect to cope with an infection, sometimes of
rel. large pathogens by the production of melanin. Melanin is highly insoluble and
will coat the surface of the pathogen. If you inject a bacterium into a mosquito larva,
there will start to form aggregates or conglomeratos of lamelocytes, plasmocytes
and melanin: dark pigmented spots. This is part of the innate immune system.
- Indole-5,6-quinone = melanin
- Can see a lot of aggregates of melanin → part of the immune response of the innate immune
system
It starts with the recognition of a pathogen, and PAMPs that recognize the pathogen. The trigger is
also serine protease (like in Toll pathway). You also need activation of prophenol oxidase: PO
enzyme secreted by the crystal cells. PO is activated by PPAE: prophenol oxidase activating enzyme.
This enzyme is not present as an activated enzyme but as pro-PPAE. This is activated by itself by
the serine protease! PO is an oxidizing enzyme. Will result in a proteolytical active enzyme = serine
protease. Allowing it to become a phenoloxidase = active enzyme.
The pathway completely relies on the amino acid tyrosine. Many close analogues of tyrosine are also recognized by PO: dopa,
dopaquinone & dopamine. The end-point is the local production of indole-5,6-quinone. This is melanin and will bind to the
pathogen surface: the pathogen can no longer take up nutrients, will create ROS and intoxicate itself: self-containment in the
capsule.
Pathogen is killed locally by the melanization pathway.
SUMMARY OF THE IMMUNE SYSTEM: CASE EXAMPLE OF PLASMODIUM
When you have a Plasmodium infection in mosquitoes, there is quite a lot of variability.
Some mosquitoes are fully resistant or very susceptible. Essential is the surface
molecule Plasmodium falciparum Pfs47 that determines if the Plasmodium strain is
able to infect or not. It determines the efficiency of transmission.
Ookinete enters in the epithelium → NO will nitrozilate protein targets on the pathogen
but also the basal lamina.
The mosquito molecule TEP1 is a complement factor. Complement is also one of the
effector molecules besides ROS, melanisation,... especially used
for Plasmodium infections. Plasmodium infection in mosquitoes: gametocytes →
Immunology of tropical infectious diseases 6
