Host-Parasite Interactions

Question 1

Give some info on African trypanosomes.

Answer 1

• cause of trypanosomiasis
• single extracellular parasite
• spread by tsetse flies
• able to maintain themselves in the body for months or years due to its antigenic variation

Question 2

How does the variant surface glycoprotein allow trypanosomes to live in the body for so long?

Answer 2

• covers flagella, a major PAMP
• VSG proteins arranged as dimers, v tightly on top of membrane so any PAMPs are covered
• has specific disulphide bridges that allow it to fold into conserved shape but N and C terminals are v distinct

Question 3

How are the VSG genes structured?

Answer 3

• 10-20% of genome is dedicated to VSGs
• several hundred chromosomes, 11 megabase have housekeeping genes but have VSG genes towards the end (subtelomeric) but cannot be expressed as they are not in expression site
• VSG genes located at expression sites towards end of telomeres so can be expressed
• only 1 expression site is active at a time so 1 VSG gene is active
• there is a lot of RNA that need tio be made so transcribed by RNApol I which is highly processive
• each VSG gene has associated 70bp repeats which ensures that it is copied into expression site after previous one has been deleted = duplicative translocation

Question 4

How do the structure of VSG genes increase number of variants?

Answer 4

• all VSG genes are pseudogenes, meaning it cannot transcribe for a functional protein
• they must be recombined w each other to make a complete gene that can encode for a functional protein
• increases number of variants it can express, potentially 100 000s

Question 5

How is their a hierarchy of VSG expression site activations?

Answer 5

• early on, switching is dependent on 70bp repeats
• later on, switching is dependent on homology w other parts of other VSG genes so are therefore harder to activate
• it is more predictable what gene will eb activated later on as it will be related to one that has been activated before (still not v predictable)

Question 6

How does VSG switching occur? (trypanosomes)

Answer 6

switching is not activated by the host immune system, it is happening at a background level within the parasite regardless

Question 7

How do trypanosomes know when to change forms?

Answer 7

Stumpy Induction Factor (SIF)
a signal that tells it how many parasites are in the blood
works like bacterial quorum sensing
–> (concept as it is unidentified)

Trypanosomes possess peptidases that degrade proteins to generate oligopeptides
these may tell parasite the density of the population

Question 7

What cytokines are important for clearing Leishmania?

Answer 7

IL-17

IFNgamma

Question 7

Give a description of the Plasmodium life cycle.

Answer 7

1. Mosquito Stage:
   - The life cycle begins when an infected female Anopheles mosquito takes a blood meal from a human host, injecting sporozoites into the bloodstream.
2. Liver Stage:
   - Sporozoites: Sporozoites travel to the liver and infect hepatocytes, where they multiply asexually, forming thousands of merozoites.

• No Symptoms: This stage is asymptomatic, and infected individuals do not show any malaria symptoms.

3. Blood Stage:
   - Merozoites Release: Merozoites are released from the infected liver cells into the bloodstream.

• Red Blood Cell Infection: Merozoites invade red blood cells (RBCs) and undergo further multiplication.
• Cycle of Infection: Inside the RBCs, the merozoites develop into ring forms, then trophozoites, and finally schizonts, which rupture the RBCs, releasing new merozoites to infect more RBCs.
• Symptomatic Phase: The blood stage is responsible for the clinical symptoms of malaria, including fever, chills, fatigue, and anemia.

4. Mosquito Infection:
   - Gametocytes Formation: Some merozoites develop into sexual forms called gametocytes, which circulate in the bloodstream and can be ingested by a feeding mosquito.

• Mosquito Ingestion: When a female Anopheles mosquito feeds on an infected human host, it ingests the gametocytes.

Question 7

What are the different forms of African trypanosomes?

Answer 7

slender = proliferative,non-transmissable
stumpy = non-proliferative and is transmissible

stumpy for allows host to stay alive, causes chronic infection
stumpy form are adapted to continue life cycle within tsetse fly, while slender forms die within the fly

Question 7

Why is the stumpy form important for infection?

Answer 7

• prolongs host survival: too many slenders and host dies
• sustains transmission: too little and there wont be enough to be transmitted
• ensure strict use of the antigen repertoire

Question 8

Where can trypanosomes proliferate?

Answer 8

• bloodstream
• adipose tissue (host fat): can contain more parasites than in the blood
• skin: less readily recognised by the immune system and more readily picked up by tsetse fly

Question 8

What is the Leishmania life cycle?

Answer 8

1. Sandfly Stage:
   - The life cycle begins when an infected female sandfly (genus Phlebotomus or Lutzomyia) bites a mammalian host, injecting promastigote forms of Leishmania parasites into the skin.
2. Mammalian Host Stage:
   - Promastigotes: Inside the mammalian host, the promastigote forms are engulfed by macrophages and other immune cells.

• Transformation: In the acidic environment of the phagolysosome, promastigotes transform into amastigotes, which are the intracellular replicative forms of the parasite.
• Multiplication: Amastigotes multiply within the host cells, causing localized tissue damage and inflammation, leading to the clinical manifestations of the disease.

3. Bloodfeeding Stage:
   - When another female sandfly feeds on the infected mammalian host, it ingests the amastigote-infected macrophages, completing the life cycle.

• Transformation in Sandfly: Inside the sandfly, amastigotes transform back into promastigotes, which migrate to the midgut and multiply.

4. Transmission to Mammalian Host:
   - During a subsequent blood meal, the infected sandfly injects the promastigote forms into a new mammalian host, perpetuating the cycle of infection.

Question 8

What are the 3 types of Leishmaniasis infection?

Answer 8

cutaneous: local and found in exposed body parts, lesions found on skin
- chronic relapsing
- localised

mucocutaneous: caused by L. viannia, causes nasal erosion
- lesions appear within 2 years
- most individuals heal naturally

visceral: causes enlargement of liver and spleen, months after initial infection
- infected macrophages move to spleen and liver
- no treatment = death

Question 8

What role does sandfly saliva play in Leishmania infection?

Answer 8

• injection of salivary gland material increases the lesion size
• promotes Th1 responses
  –> Th2 is important for parasite clearance

Question 8

Can stumpy trypanosomes undergo antigenic variation?

Answer 8

No, they are committed to cell cycle arrest
slender forms are here to undergo antigenic variation and to keep parasite levels up

Question 9

How do Leishmania cells enter mammalian cells?

Answer 9

promastigotes:
- have GP63 on surface which converts C3 into C3b which binds to CR1 (on parasite)
- along with factor i, CR1 cleaves C3b into iC3b which facilitates binding to CR3
- can parasitise neutrophils via CR3 and enter macrophages or DCs while inside short lived granulocytes

amastigotes: these have exited a macrophage so have less GP63 and LOG on their surface

Question 10

What is the role of the Circumsporozoite Protein (CSP) in Plasmodium infection?

Answer 10

• expressed during the sporozoite stage
• facilitates migration to salivary glands in mosquitoes
• facilitates attachment and invasion of liver hepatocytes by binding to heparan sulphate proteoglycans (HSPGs)

Question 11

When does disease occur in Plasmodium infection?

Answer 11

• during the asexual blood stage cycle
• rapid amplification of parasite numbers
• multiplies 10-20x every 48 hours
  –> results in cyclical fevers

Question 12

How do merozoites invade RBCs?

Answer 12

• adhesins on their surface allow attachment and invasion
• adhesins are polymorphic
• surface proteins shed during this process thought to act as a decoy that antibodies attack
• egression into blood cell is v quick - “the golden minute”

Question 13

What is cytoadherence?

Answer 13

• remodelling on RBC membrane by the parasite results in increased cell rigidity
• the spleen removes rigid red cells

Cytoadherence is mediated by P. falciparum PfEMP1, which adheres cells to the epithelium of blood vessels, preventing them from being removed
–> extreme antigenic variability, as it is exposed to the immune system

• causes obstruction and localised acidosis
  –> coma and death

Question 14

In the context of Plasmodium infection, what is gametogenesis?

Answer 14

• a minority of blood stage forms commit to gametocytogenesis
• schizonts transform into either male of female gametocytes
• Plasmodium don’t have sex chromosomes but external signals can induce variable morphologies

male gametocytes undergo exflagellation
- 3 rounds of mitosis
- 8 microgametes formed
fastest known genome replication in biology

Question 15

How does sexual Plasmodium reproduction occur?

Answer 15

occurs in mosquito gut, induced by temperature change (human –> mosquito) and gametocyte activating factor, xanthurenic acid

Question 16

How can the immune system protect against Plasmodium infection?

Answer 16

innate immune responses
- balance between parasite killing vs inflammation

adaptive immunity
- antibodies against sporozoites
- antibodies against surface of infected erythrocytes
- antibodies against extracellular merozoite

eg inhibiting sequestration, inhibiting invasion, phagocytosis

Question 17

What are helminths?

Answer 17

parasites from three taxonomic categories
- nematodes (roundworms)
eg roundworm, hookworm, whipworm

• cestodes (tapeworms)
• trematodes (flukes)
  eg Schistosoma spp.
• most common parasite infections
• most common are the soil transmitted helminths (STH)

Question 18

Male and female worms are normally required for fertilisation. What is the exception to this?

Answer 18

tapeworms, which are hermaphrodites

Question 19

What are the characteristics of a direct helminth lifecycle?

Answer 19

• do not require vectors but do require the environment for transmission
• cannot be transmitted person-person

eg Ascaris lumbricodes (roundworm)
- parasite can develop within egg in the external environment
- hard egg casing dissolves in the stomach and can get into bloodstream –> lungs –> trachea –> intestines

Question 20

What are the characteristics of an indirect helminth lifecycle?

Answer 20

require vectors to transmit

eg Filarioidea
- live within lymphatics, most commonly in the egs
- can cause blockage if there are enough
- moves from lymph to bloodstream where it can be picked up by a mosquito

Question 21

How to control helminth infections?

Answer 21

• stopping transmission
• using drugs

eg - providing shoes to stop hookworm transmission
- preventing contamination of environment

chemotherapy is an option, but it only provides short term treatment
–> re-infection can occur in as little as 2 months

drugs for STHs are cheap, safe, and easy to administer

Question 22

Give a description of the Schistosoma life cycle?

Answer 22

1. Eggs in Water:
   - Adult female Schistosoma parasites reside in the blood vessels of the human host and produce eggs.
   - Eggs are released into freshwater environments through the host’s feces or urine.
2. Miracidium Stage:
   - Upon reaching freshwater, the eggs hatch, releasing free-swimming larvae called miracidia.
   - Miracidia penetrate specific freshwater snail intermediate hosts and transform into another larval stage.

3.Sporocyst and Cercariae Stages:
- Sporocysts: Inside the snail, miracidia develop into sporocysts, which produce asexually reproducing larvae called cercariae.
- Cercariae: Cercariae are released from the snail into the water, where they actively swim and seek out human hosts for infection.

4. Human Infection:
   - Skin Penetration: Cercariae penetrate the intact human skin upon contact with contaminated freshwater during activities such as swimming, bathing, or washing.
   - Transformation: Once inside the human host, cercariae shed their tails and transform into schistosomulae, which enter the bloodstream and migrate to the blood vessels of the liver and intestines.
5. Adult Worms and Egg Production:
   - Maturity and Pairing: Schistosomulae mature into adult male and female worms, which pair up and migrate to the mesenteric veins (intestines) or the venous plexus of the bladder (urinary system).
   - Egg Production: Adult female worms produce eggs, which are released into the bloodstream and eventually reach the intestines or bladder.
6. Egg Excretion and Transmission:
   - Fecal or Urinary Excretion: Eggs are excreted in the host’s feces (intestinal schistosomiasis) or urine (urinary schistosomiasis), contaminating freshwater and perpetuating the transmission cycle.
   - Contact with Snails: Eggs that reach freshwater can infect snails, completing the life cycle when they release new cercariae.

Question 23

How does Schistosoma cause clinical disease?

Answer 23

• disease is associated with eggs rather than the adult worms
• granulomas can form around eggs that have been swept up into the bloodstream which can end up anywhere
  –> if these form next to an organ this can cause major problems
• neurological problems can arise by lodging of schistosome eggs in the brain