Parasitology - Lecture 5 - 8/11 - Survival strategies of parasites

Information about parasites

• eukaryotes

• complicated cellular composition

• large genomes → can express many proteins → ability to present in different forms in different environments (needed for different stages in complex life cycles)

• multiple developmental stages in multiple hosts

• parasites can perform special tricks that are scientific / clinical interest

Trypanosome Brucei (african sleeping sickness)

• parasitic protozoa

• vector borne disease (Tsetse fly)

• symptoms;

  • 1st phase; blood phase, fever, headache

  • 2nd phase; migrates over BBB, neurological phase → mentally disturbed, coma, death

    • no effective therapy stage 2 (only arsenicum, very toxic)

Interesting features of trypanosomatids

• Clinical an important pathogen:

  • Sleeping sickness in humans.

  • Nagana in cattle.

• Alternating proliferation and differentiation.

• Single mitochondrion per cell (per parasite).

  • In humans: many mitochondria in one cell.

• Kinetoplast (mini and maxi circle DNA, these are interconnected).

  • Harbors the DNA of the mitochondrion.

• Glycosomes.

  • Peroxisomal origin.

  • No catalase.

  • Pyrophosphate metabolism.

  • Compartmentation of:

    • Glycolysis. (in glycosomes)

    • Nucleotide biosynthesis.

Mitochondrial RNA editing

• post-transcriptional modification of RNA sequences

• guide RNAs that match part of the mitochondrial sequence

• Allows for deletion/insertion of nucleotides → allow modification to make it encode for proper protein

• Target for therapy

• DNA sequence isnt equal to mRNA sequence

• DNA sequence doesnt encode protein sequence

Glycosomes

• membrane-enclosed organelle that contains the glycolytic enzymes

• part of glycolysis happens here

• Peroxisomal origin.

• No catalase.

• Pyrophosphate metabolism.

• Compartmentation of:

  • Glycolysis (in glycosomes)

  • Nucleotide biosynthesis.

Parasitemia during sleeping sickness

• waves of trypanosomes for weeks (sometimes a lot, sometimes low amount)

  • Variant Surface Glycoprotein (VSG) on membrane. It lays anchored in the membrane.

  • antibodies are made. The Abs activate the complement and enhance the function of innate immune cells, hereby destroying the parasites.

  • Trypanosomes can make new variants of VSG, so constantly new antibodies have to be made against parasites with new VSG coats. These ‘new’ parasites will replicate again.

  • do this via antigenic variation; multiple ways to do this

    • gene conversion

    • telomeric exchange

    • in situ switching

Antigenic variation

• GPI anchored, present on all eukaryotes

• involved in cellular processes; membrane fission / fusion

• this started a whole new field in research

• the 3 different ways is; telomere exchange, gene conversion, in situ switching

  • occur at different rates

  • switch occurring to a pattern

Ascaris lumbricoides

• roundworm (nematode), intestinal

• length 15-20 cm

• worm load determinant for pathology

• know lifecycle! → direct

  • Eggs in feces.

  • Soil stage important for embryo development (1-2 weeks).

  • Oral intake of an egg.

  • Egg hatches in small intestine (trigger is bile acids).

  • Larvae migrates through tissues to lung, is coughed up and ingested again (larvae are very small).

    • Often an immune response that results in fever, coughing.

  • Larvae mature in the intestine in a male or female worms.

    • A few worms remain unnoticed.

  • Sexual reproduction: Results in eggs, which are secreted in the feces.

    • Eggs don’t cause any symptoms in the host.

• life cycle can be maintained by laying many, many eggs

  • stay alive for very long! and infect the host for very long → can’t kill host and should withstand immune reactions

difference roundworm and flatworm

• roundworm → mostly direct life cycle

• very far apart in phylogenetic tree, roundworms more related to arthropoda than flat worms

• treatment against roundworm will not work in a flat worm because they are so different!

fasciola hepatica

• platyhelminth (flat worm) → liver flukes

• present in bile ducts (anaerobic) of the liver

• cattle / sheep biggest host → humans can also be infected

• life cycle

  • Cercariae attach to plants and stay there (resting phase) until they’re eating up by cows.

  • They will pass the stomach. Juvenile flukes enter the bloodstream through the intestinal wall, through which they enter the liver.

Energy metabolism of parasites (fasciola hepatica) in humans

• environmental conditions change during life cycle;

  • free living stages → oxygen but no food (aerobic)

  • parasitic stage (in host) → food but no oxygen

In humans there is glycolysis and anaerobic glycolysis

• anaerobic glycolysis in humans only limited amount of time because lactate is acidic.

Helminths (fasciola hepatica) have slightly different anaerobic metabolism → do not convert pyruvate to lactate but

• PEP to malate

• more efficient than glycolysis in humans → more ATP generated

• Conversion to malate and conversion of malate into acetate and propionate takes places in the mitochondria of the helminths, which is interesting as in humans the conversion to lactate takes place in the cytoplasm.

• anaerobically functioning mitochondria!! Able to produce ATP without need for oxygen.

• this is able to happen because mitochondria are different among eukaryotes

All parasites have deviant energy metabolism

• fermentative metabolism → even when they use oxygen

• metabolic adaptations

• differences with their host (target)

Difference in progression of an infection by a parasitic protozoa versus a helminth

• Protozoa replicate within their host and have multiple survival mechanisms:

  • Antigenic variation (Trypanosoma, Plasmodium)

  • Intra-cellular location (they are small): hidden from the immune system (Plasmodium (smart to go in RBC, no MHC), Toxoplasma, Leishmania).

  • Manipulation of host immune system (schistosome)

• Helminths do not replicate in their host. They only produce offspring to infect another host. They need to survive long-term to generate offspring and therefore they preferably don’t cause mortality:

  • Not so challenging environment (the gut).

  • Manipulation of the host (very complex interactions).

Schistosomes causing pathology

• helminth

• flatworm

• also called bilharzia

• live in blood vessels

• Know lifecycle!!

• don’t get sick from the adults in blood vessels but from the eggs

  • immune response → granuloma formation , this is main cause of pathology

  • when eggs reach liver; first enlarged liver + spleen (because there is damage) → liver fibrosis, failure

  • 50% of the eggs get stuck in host tissue (because they are excreted in the blood vessel and have to travel to the liver)

  • important to treat schistosomiasis before liver damage

Host response → how adult schistosomas survive for years

Immune response against invasive helminths;

• A Th2 response induced by egg excretion factors (T2 ribonuclease (omega-1)).

• Th2 → IL4, -5 and -13.

• These cytokines activate eosinophils and mast cells.

  • normally allergic reactions.

  • when infected with helminths less allergic reactions. (allergy hyper Th2 response against non-pathogenic foreign molecules)

• Mast cells and eosinophils degranulate upon binding to the helminths with the IgE antibodies that are bound to these cells. In case of schistosomiasis, the Th2 type cytokines attract cells such as eosinophils to the eggs to form granuloma’s.

• Then, a Treg response is induced to prevent liver fibrosis. A Th1 response during the worm stage is probably beneficial to prevent an attack of eosinophils to the worms

  • IL-4: Stimulates Th2-response.

  • IL-4 & -13: IgE synthesis by B cells.

  • IL-5: Stimulates proliferation eosinophils.

survival strategies by adult worms

• adult worm big for blood vessel → blood flow disturbed, no thrombotic complications

• patients suffer from spontaneous bleeding. schistosomes prevent blood clotting because of tegument

• outer surface S. mansoni (tegument) → continuous layer of fused cells. MAC pore (complement) does nothing → new membrane will be formed

Tegumental membrane

• saturated in phospholipids

• high content lyso-phospholipids

• rich in unusual fatty acids → contribute to host resistance

  • Glycolipids stimulate IL-10, IL-16 and TNFα production.

  • Lyso-phosphatidylserine (PS) stimulates Toll-like receptor 2 (so a Th2-reponse), IL-10 production and reduces the immune response.

    • So, the worms also protect themselves by dampening the immune response

• Interaction with platelets (thrombocytes):

  • Thrombocytes (platelets) bind to S. mansoni eggs → anchoring eggs to endothelial wall of blood vessel → tissue overgrows eggs, protected against environment

  • Platelets don’t attach to the adult worms.

• Main survival mechanisms of schistosome in blood vessel

  • Tegument surface structure → double membrane

  • mimicry → binding of host components on cell surface (e.g. LDL particles)

  • immune modulation by excreted factors → Th2 (omega 1), Treg (lyso-phospholipids)

  • anti-thrombotic activity → inhibition of platelet activation