MIB- lecture 5 - Virulence factors immune system

Receptors of innate and adaptive immune response;

• T cell receptors recognize mainly proteins

• Formylated-peptide receptor (FPR1 and FPR2, both found on neutrophils) → N-formlylmethione is recognized in by this receptor → it has a different group compared to methionine → this is a start codon

  • In bacteria the methionine is formylated to N-formylmethionine

  • All bacteria start with this ‘start’ codon

  • So the receptor can easily recognize the bacteria

• Toll-like receptor: detect and bind lipoproteints and also free DNA and RNA

• Lectins and NOD-like receptors: recognize polysaccharides or glycans, saccharides

name different ways that immune cells combat virulence factors

• Granulocytes contain lactoferrin/transferrin, involved in iron uptake

• Antimicrobial peptides → kills bacteria by forming holes in the membrane

• Lysozymes → can specifically cleave (peptidoglycan of bacteria) or hydrolyze bacteria and initiate cell dead

Effectors innate immune system

• phagocytosis

• antimicrobial peptides / lysozyme

• reactive oxygen / nitrogen species

• complement activation

• iron / nutrient withholding

• neutrophil granules

• neutrophil extracellular traps; takes chromosome and throws it on bacteria and traps it (NETosis)

s. aureus, how has this pathogen developed specific mechanism to cope with immune defense system against it, explain CHIPS

• They body protects against the S. aureus by

  • Coagulation: trapping pathogen and limiting spread

  • secretion antimicrobial peptides → lead to pores in bacterial membrane

  • Complements system activated

  • Phagocytosis by neutrophils and macrophages → neutrophils actively migrates to pathogen, where the concentration is the highest

  • all these immune cells know where to go because of chemotaxis and the FPR1/2 receptors

• S. aureus block the migration of the neutrophils to protect itself from the immune system → produces CHIPS (chemotaxic inhibitory protein of S. aureus).

  • CHIPS (protein): binds to the formylpeptide receptor on the neutrophils → the neutrophils do not recognize it anymore and the migration is blocked

  • CHIPS bind to C5A receptor (complement system)

Explain the toxins S. aureus produces to kill immune cells

• A-hemolysis (Cytotoxin) recognizes ADAM10 → Monomer binds to this in the membrane, this attracts all the other a-hemolysis monomer → create hole that lyse red bloods cells

• LukED recognizes the receptor CCR5 → kills T cells, DC and macrophages

• PVL (cytotoxin) → receptor is unknown → but does kill cells

is PVL a virulence factor for S. aureus? and how does PVL work

• PVL forms pores in the membranes of white blood cells resulting in cell lysis, releasing inflammatory mediators

• Patients infected with PVL+ strains have a worse survival rate than people without PVL → does impact virulence

• mouse study → no different between PVL +/- → does not have affect on virulence in mouse model

• PVL binds C5a receptor + leukocytes of humans → makes a hole in the membrane

  • thus virulence factors are also host specific not only cell specific

S. aureus and the complement system

• different methods to evade complement system

• Protein A SAK: binds to C1 so the antibody cannot bind, therefore the cascade of the classical pathway can not start. can also bind to C3B, stops opsonization pathway

• SCIN: Directly binds to C2 and B (complement system) and inactivates these so the alternative and the classical pathway stops

• CHIPS binds to C5a on the neutrophils

• Understand: high amount of redundancy → multiple mechanisms that perform similar function

Facts about the protein being secreted by s. aureus interacting with complement;

• small

• have similar folding

• homologous

Viruses also block complement

• Influenza and astroviruses inhibit the binding of C1, the cascade of the complement can not start

• Other viruses interfere with the formation of the membrane attack complex

herpesvirus adaptive immune response

• Herpesviruses inhibit proteasome, so it cannot be degraded

  • With Us11, EBNA-1 and LANA-1

• Herpesviruses inhibit TAP complex → prevent the transport from the degraded proteins to the chaperones

  • By Us6, BNFL2a, ICP47 and UL49.5

• all results in the protein not being loaded on MHC class 1

anitgen variation / african sleeping sickness

• Caused by protozoa of the species Trypanosoma brucei and transmitted by tsetse fly

• Endemic in some sub-Saharan contries

• two different stages of disease

  • Haemolymphopatic phase → fever, headache, joints pains

  • Neurological phase → confusion, reduced coordination

• Infection cycle → disease diagnosed by looking at the blood where you see the worm (microscopy) → does not hide because it has VSG

How the VSG coat helps Trypanosoma Brucei

• VSG (variant surface glycoprotein) coat covers entire surface of parasite

  • highly variable surface glycoproteins

  • can be targeted by specific antibodies → pathogen can be cleared

  • The proteins switch, to avoid being seen → different waves of serotypes

• VSG expression:

  • Pol I → transcribes active VSG gene

  • allelic exclusion: only one allel is expressed the other is silenced

  • telomere exchange → active VSG gene swapped with VSG gene on different chromosome (Gene not deleted!)

  • Gene conversion → silent VSG gene is copied and replaces the active VSG gene (Gene deleted!)

Red Queen Hypothesis, and question if the adaptive immune system is superior

• evolutionary change gives a temporary advantage. but after adaptation of the pathogen the end result will be similar

• Superiority of the adaptive immune system

  • Invertebrates can live very long without having an adaptive immune system → they get bigger than humans

  • Non-vertebrates have much more TLRs than humans → Their TLRs are much more specific than those of humans

• drawback adaptive immune system

  • Molecular mimicry → antigens produced are very similar to humans → this causes autoimmunity

Innate immune response - Iron

• pathogens need iron as nutrient

• host ensures that the amount of available iron is extremely low → the pathogens can counteract this → red-queen hypothesis

interplay between host and pathogen - iron

• Human body full of iron (in red blood cells) →

  • a-haemolysin kills red blood cells by binding to ADAM10 → this way the iron of the red blood cells is flushed out → the host produces proteins to stop this

• The human body releases haemopexin (HPX) bind to free haem and haptoglobin (HP) binds to free haemoglobin → no longer available to pathogen

• Free iron (Fe 3+) is bound by transferrin (more in the blood) or lactoferrin (more on the mucosal surface)

• The macrophages keep the pathogens in the phagosome to stop them from getting iron → NRAMP1 (iron pump, pumps it out of phagosome)

Effect of iron overload

• storage disease- hemochromatosis. These patients have more iron in the blood

• more prone to have infections of opportunistic pathogens

bacterial response to iron withdrawal

• Transferrin → binds free iron

  • Neisseria meningitidis, produces protein TbpB that captures transferrin, takes up iron from the host

  • Disadvantage for pathogen: Binding of Tf/Lf by TbpB is highly species specific (might bind human but not other vertebrates)

• production of siderophore to capture iron

  • Siderophore is iron carrier (binds free iron) → the pathogen has a specific receptor so that the iron can be taken up again (against concentration gradient)

enterobactin and aerobactin

• siderophores produced by E. coli

• Enterobactin is strongest siderophore → removes iron bound to transferrin

• aerobactin (pathogenic strain), binds iron less strong than enterobactin

• Why would pathogenic strain have a less effective siderophore?

  • human cells produce lipocalin-2 (binds enterobactin, preventing E. coli from using it)

  • The purification of lipocalin-2 always resulted in a brown product → it was contaminated by enterobactin bound to iron

  • KO of lipocalin 2 → severe infection of E. coli cells in mice

  • Aerobactin is not bound by lipocalin 2 allowing e. coli to bypass this immune defense

• → host is expressing lipocalin, so enterobactin is then useless. So the bacteria has developed a new strategy to capture iron (aerobactin), that is not countered by the host yet

• → But tear lipocalin (in tear liquid) is binding a wide range of siderophores → the new counteract of the host

counteract of the body against lentiviruses

• APOBEC3G (in non permissive cells) packaged in new virus cells, deanimates cytosine → converting it to uracil. viral DNA unreadable, no viable virus produced

• Vif (HIV gene) binds to APOBEC3G flags ubiquitin, is degraded

Host pathogen interaction in mammalian evolution

• genes related to host-pathogen interaction have changed

  • LCT → protein against digesting milk.

  • SLC45A2 → important in skin colour, less sunlight in the north

  • TLR genes

  • MHC genes

  • Autophagy regulation (ability to remove internal infection in a cell)