Lecture B6 - Gut Parasites and Surface Antigen Variations in Giardia

Question 1

What are 3 common gut parasites?

Answer 1

Cryptosporidium (oocysts) - small intestine
Entamoeba (cysts) - colon
Giardia (cysts) - small intestine

Question 2

How can some parasites avoid being eradicated?

Answer 2

Resistant forms contaminate the environment and infect the host and can replicate within the host. Some can then differentiate into the cyst form and avoid the immune response and antibiotics as they are resistant.

Question 3

What are some examples of extracellular parasites and where do they sit?

Answer 3

Entamoeba and giardia, sit on the mucosa.

Question 4

What is an example of an intracellular parasite and where does it sit?

Answer 4

Cryptosporidium, goes into the epithelium.

Question 5

Describe the immunopathogenesis of gut parasites.

Answer 5

Indirectly or directly interact with epithelial cells which triggers stimulation of neutrophils. Changes to the epithelium by either killing the cells or changing the properties, leading to diarrhoea.

Question 6

What are the two main species of cryptosporidium?

Answer 6

Responsible for 90% of infections.
C. parvum (cattle and other mammals).
C. hominis (only/mainly humans).

Question 7

What are some other species of cryptosporidium?

Answer 7

C. meleagridis
C. cuniculus
C. felis
C. canis

Question 8

What does cryptosporidium cause?

Answer 8

Self-limiting diarrhoea associated with the young, elderly and HIV/AIDs patients. Can be life threatening.

Question 9

What does cryptosporidium lack?

Answer 9

Gene families that could mediate surface antigenic variation.

Question 10

Describe CD4+ T cells in HIV infection.

Answer 10

Initial collapse of CD4 cells but they go up again, Once the virus has established itself the CD4 cells collapse again and there is no effective adaptive immune response. Hard to then get a relevant immune response to any invading pathogen which is dangerous.

Question 11

Describe Entamoeba histolytica.

Answer 11

Endemic in regions with poor sanitation.
Amebiasis - often asymptomatic, abdominal disease and diarrhoea, chronic conditions, invasive infections, amebic dysentry and liver and other tissues can get infected.
Fecal-oral life cycle.
No current vaccines.
Interacts with epithelial cells, parasite limited to the lumen of the gut.

Question 12

What are the four major processes involved in Entamoeba histolytica and microbiota interactions?

Answer 12

Modulation of Entamoeba virulence.
Entamoeba induces dysbiosis .
Translocation through the mucosa into the portal vein might be facilitated by the microbiota and include some members of the microbiota contributing damaging the gut and liver.
Alterations of both the mucosal and systemic immune systems could contribute to an excessive inflammatory response.

Question 13

Why does Entamoeba histolytica produce no symptoms?

Answer 13

Located in the lumen. Problem starts when the pathogen goes across the epithelium leading to diarrhoea and bloody mucus, becoming chronic and can cause severe damage to the intestine.

Question 14

What are some key virulence factors of Entamoeba?

Answer 14

Phagocytosis of microbiota.
Dysbiosis of the microbiota.
Secreted proteases and hydrolyses - degradation of mucus, extracellular matrix proteins.
Contact independent processes - tight junction integrity and ion absorption.
Adherence to IEC - contact dependent cytotoxicity.
Trogocytosis and phagocytosis of human cells.

Question 15

What is the worst scenario of Entamoeba?

Answer 15

Gets into the portal vein and can end up in the liver.

Question 16

What is the invasive stage of Entamoeba?

Answer 16

Formation of flask shaped lesions as trophpzoites cross damaged epithelium and enter the submucosa. This can cause necrosis and bloody stools as well as secondary bacterial infections.

Question 17

Describe Giardia lamblia?

Answer 17

Higher prevalence in developing countries.
Most common protozoa in human stools - 200 million reported clinical cases a year.
Giardiasis is often asymptomatic, acute or chronic diarrhoea, non-invasive and limited to the lumen of the gut.
Fecal-oral life cycle.
Zoonotic origin.

Question 18

What are some clinical features and symptoms of Giardiasis?

Answer 18

Acute - sudden, explosive, watery diarrhoea, upper gastro-intestinal uneasiness, bloating, flatulence, belching, cramps, nausea, vomiting, anorexia. Usually clears spontaneous but can persist and become chronic.
Sub-acute/chronic - recurrent diarrhoeal episodes, cramps, sulphuric belching, anorexia, frequent nausea.

Question 19

Where does Giardia sit on top of and replicate?

Answer 19

Epithelial cells, has an adhesive disk to mediate contact.

Question 20

How can Giardia avoid IgA clearance?

Answer 20

VSP on the trophozoite surface switches to avoid the IgA directed clearance.

Question 21

What are the other major virulence factors of Giardia?

Answer 21

Attachment - ventral adhesive disc and surface lectins enable attachment to and colonisation of the intestinal endothelium.
Circumvention of the natural factors of the intestinal lumen - flagellar motility enable delocalisation to new endothelial cells during colonisation and VSPs potentially help to protect against luminal proteases, oxygen and free radicals.
Alteration of host innate defences - released arginine deiminase and other Giardia products downregulateepithelial production of NO.
Anti-inflammatory modifications - unknown trophozoite products have anti-inflammatory roles.
Survival in stomach acid and the external environment - differentiation into cysts.

Question 22

What is surface antigen variation?

Answer 22

Surface antigen variation is a major mechanism to evade mammalian adaptive immune responses.
Required to maintain chronic infections and permit reinfections.
Mediated through genetic or epigenetic mechanisms.

Question 23

What are the 3 surface antigen variation requirements?

Answer 23

A family of homologous genes encoding immunodominant, antigenically different surface molecules.
A mechanism(s) that guarantees the mutually exclusive expression of only one/few antigens at a time.
A mechanism for reversibility switching the expression of these molecules in individual cells.

Question 24

What are the characteristics of antigenic variation in Giardia lamblia?

Answer 24

VSPs cover the entire surface of the trophozoites.
VSPs form an important dimension of the host-parasite interface.
VSPs trigger a strong cellular and humeral response.
Only one or a few VSP expressed at a time.
Frequency of switching varies between 6 and 18 generations.
Anti-VSP monoclonal antibodies are cytotoxic.
Giardia can produce persistent and recurrent infections because of this.

Question 24

What are the characteristics of the Giardia lamblia VSPs?

Answer 24

Family of cysteine rich proteins.
Molecular mass varies between 33 and 200 kDa.
Conserved C-terminal domain.
Variable amino-terminal region rich in cysteine, commonly as CXXC.
Binds Fe and Zn.
Protect the parasite from the action of intestinal proteases.

Question 25

What chromosome is the most densely populated with VSP genes?

Answer 25

Chromosome 4

Question 26

What is the RNA interference pathway?

Answer 26

Regulates the amount of RNA present for a given gene and can be used to attack RNA viruses or regulate expression of genes in our own cells.

Question 27

Describe Giardia VSP genes genomic organisation and regulation of expression.

Answer 27

Most Giardia VSP genes are not subtelomeric, with only three that are located at ends of chromosomes.
No requirement for subtelomeric location for VSP expression.
73 VSP share 2 motifs essential and sufficient for membrane location.
microRNAs shown to repress expression of VSP mRNA by binding throughout the ORFs.