Lecture 1-3: Immune Response to and Immune Evasion by Bacteria Flashcards
What is a pathogen ?
A microbe that can cause disease in its host
What are the goals of pathogens ?
- Invade host and gain a foothold
- Multiply and persist
- Disseminate through the body
- Cause disease and transmit to new hosts
What are the functions of the immune system ?
- Recognition of bacteria
- Clearance of bacteria
- Resolution of inflammation
What does microbial invasion initially encounter ?
Innate immune defences
What have pathogens evolved to overcome innate immunity ?
Mechanisms
What is adaptive immunity required for ?
Clearance of pathogens and prevention of re-infection
What have successful pathogens evolved ?
A range of anti-immune strategies to overcome both innate an acquired immunity
What does immunity of bacteria depend on ?
The nature of bacteria and the mechanism of pathogenicity
What are some examples of the nature of bacteria ?
- Intracellular verses extracellular
- Gram positive or negatiive
- Toxicity or invasiveness
What is virulence factor ?
A quantative measure of pathogenicity
What are some virulence factors ?
- Colonisation of a niche in host
- Immune evasion
- Entry and exit out of cells
- Obtain nutrient from host
Where do extra-cellular bacteria live ?
Outside cell
How do extracellular bacteria replicate ?
Quickly to avoid elimination
What are extracellular bacterias immune response targeted to ?
Eliminate bacteria and neutralise toxins
What are some examples of extra-cellular bacteria ?
- Straphylococcus aureus
- E. coli
- Streptococcus pneumoniae
- ” pyrogenes
- Pseudomonas aeruginosa
What are the immune responses in extra-cellular bacteria ?
- Complement
- Anti-microbial peptides
- Phagocytes
- Antibodies
- T-helper cell
Where do intracellular bacteria live ?
Lives inside host cells
What are intra-cellular bacteria capable of ?
Avoiding many host immune strategies
How do intracellular bacteria enter cells ?
Phagocytosis
What must intracellular bacteria evade ?
Lysosomal killing
What are common host cells of intracellular bacteria ?
Macrophages and epithelial cells
What are some examples of intracellular bacteria ?
- M. Tuberculosis
- Listeria
- Salmonella enterica
- ” typhimurium
- ” trachomatis
What are the immune responses of intracellular bacteria ?
- Phagocytes
- Cytotoxic T cells
- NK cells
- T helper cells
What type of immunity are extracellular bacteria ?
Humoral
What type of immunity is intracellular bacteria ?
Cellular immunity
What is the bacterial cell wall PAMPs ?
Bacterial surface that the host immune system sees
What are appendages ?
Proteins attached to cell surface
What is a capsule ?
Carbohydrate coating
What is the structure of bacteria ?
- Bacteria cell wall PAMPs
- Appendages
- Capsule
What is gram negative target for ?
Outer lipid membrane target for membrane lysis eg. complement
What does gram positive bacteria require ?
Killing requires uptake by phagocytes
What are the pathogen recognition receptors ?
- TLRS
- NLRS
- CLRS
- DNA sensors
What do CLRs do ?
Bind carbohydrates in a ca+2 dependent manner
What does TLR5 recognise ?
Flagella
What are exotoxins ?
Proteins secreted by bacteria, act at sites removed from bacteria
What are endotoxins ?
Cell associated moleucles, structural molecules of the bacteria. Maybe released from growing bacteria or cells that are lysed
Where do endotoxins act in the vicinity of ?
Bacterial growth or presence
What can inhibit opsonisation ?
Carbohydrate coating
What are the functions of endotoxins ?
- Protective permeability barrier
- Barrier to lyozyme and antimicrobial agents
- Impedes destruction of the bacterial cells by serum components and phagocytic cells
- Role as an adhesin
- Variations in LPS result in antigenic variation
What are the most powerful human poisons ?
Bacterial toxins
Where does enterotoxin target ?
Intestines
Where does Neurotoxin target ?
Neurons
Where does leukocidin target ?
Leukocytes
Where does hemolysin target ?
Red blood cells
What are the exotoxins mechanisms of action ?
- Enzyme activity
- Pore forming
- Super antigens
What do invasins do ?
Degrae tissue
What can intracellular enymati activity ie needs to be taken up into cell occur by ?
- Type 3 secretion system
- Receptor mediated endocytosis
How does Pertussis toxin work ?
Inhibits G protein coupling thus interfering with adenylate cyclase activity resulting in build up in intracellular cAMP which interferes with cell signalling pathways that regulate eg. phagocytosis
What happens during pore forming ?
Inserts a transmembrane pore into cell membrane disrupting influx and efflux of ions across the membranes
What is an example of bacteria which work by enyzymatic activity ?
- Staphylococcus aureus
- Streptococcus pyogenes
What bacteria pore forms ?
- Streptolysim
- alpha toxin- s.aureus
- Listeriolysin - lysteria
How do superantigens work ?
Hyper stimulation of the immune system - bind MHCII directly resulting in massive T cell proliferation and associated cytokine production
Example of bacteria with superantigens ?
Pyrogenic exotoxins produced by staphlocooci and streptococci cause high fever and rash associated with toxic shock
What is the complement system ?
An important humoral component of the innate immune system
What does the complement system help do ?
Helps phagocytes and antibodies to clear infection
What does the complement system consist of ?
Small proteins found in blood, generated in the liver
How is the amplified enzyme cascade activated ?
Proteases activated by cleavage by other proteases in the system
What are the functions of complement system ?
- Opsonisation - enhacing phagocytosis of antigens
- Chemotaxis - attracting phagocytes
- Lysis of bacteria - rupturing membranes of foreign cells
What are the three pathways of the complement system ?
- Classical
- Mannose binding lectin
- Alternative
What are the components involved in the classical pathway ?
C1, C2, C4
What is the classical pathway triggered by ?
Antibody antigen complexes binding to C1 (C1Qq, C1r, C1s)
What does the classical pathway form ?
C3 convertase (C4bC2a)
How is the mannose binding lectin pathway activated ?
MBL binding directly to mannose resiues on the bacterial surface
What does the binding of MBL to mannose activate ?
MBL associated serine proteases MASP-1 and MASP-2 which activates C4 and C2
How does the alternative pathway work ?
Involves various factors B, D, H, I which interact with C3B to form an alternative C3 convertase. Functions to amplify pathway
What is the lytic pathway ?
Initiated by the splitting of C5 and attachment of C5b to C6, C7, C8 and C9 to form the membrane attack complex, which inserts into the outer membrane of some bacteria causing lysis
What are antimicrobial peptides ?
Cationic host defense peptides
What type of molecule are antimicrobial peptides ?
Small positively charges amphipathic molecules
How long are antimicrobial peptides ?
6 to 100 aa
How are antimicrobial peptides grouped ?
Secondary protein structures
What can antimicrobial peptides target ?
Bacteria, viruses, fungi and even cancerous cells
What are the most prominent antimicrobial peptides ?
- Cathelicidians
- Defensins
What are the antimicrobial mechanisms of action ?
- Selectively target bacterial cell membranes
- Destruction of target cell
What do defensins and cathelicidins do ?
Form pores which promote lysis
How does lysozyme work ?
Disrupts bacterial membranes through enzymatic digestion which targets peptidoglycan backbone
What is LL37 ?
Cathelicidins
What does LL37 do ?
Chemotactic activity that can attract neutrophils, monocytes, mast cells and T cells
What can LL37 supress ?
TLR signalling by binding LPS or LTA
How can LL37 promote inflammation ?
By binding nucleic acids to facilitate recognition of self DNA, self RNA, DSRNA
What are the roles of phagocytes ?
- Recruitment to site of infection
- Recognition of infecting organism
- Ingestion of invading organisms
- Killing of ingested organism
- Clearance of activated phagocyte from site of infection
- Activation of adaptive immune response
- Presentation of antigen
What are pathogens recognised by on the surface of phagocyte ?
Receptor
How can optimal recognition by phagocytes occur ?
Opsonisation
What is opsonin independent recognition ?
Direct recognition of targets on microbe
What is the function of PRRs ?
Facilitate recognition and enhance this process by activation of signalling pathways in phagocyte
What does activation of signalling in phagocytosis do ?
Leads to actin polymerisation and cytoskeleton rearrangement
What is the fundamental process of phagocytosis ?
Rearrangement of actin cytoskeleton
What are the intracellular signalling pathways triggered upon receptor activation ?
- Src family kinases
- SYK tyrosine kinases
What are Rho GTPases ?
Binding proteins - molecular switches which control various cellular processes
When does cell shape change ?
- Migration
- Phagocytosis
- Using pseudopods
What are endosomes ?
Intracellular membrane mound storage organelles
What is endocytosis ?
Uptake and transport of extracellular material through the formation of membrane bound vesicles
How can intracellular killing by phagocytes occur
- Reactive oxygen species
- Reactive nitrogen species
What do reactive oxygen species do ?
Oxidising agents adversely effect cellular structures including membranes and nucleic acids
What do reactive nitrogen species do ?
React with various biological targets and components of the cell including lipids, thiols, amino acid residues
What does phagocytosis of apoptic neutrophils result in ?
- Increase TGF-beta, IL10
- Decrease in TNF-alpha, IL23
How can antimicrobial peptides be blocked ?
- Proteolytic cleavage
- Alteration of cell surface net charge
What does proteolytic cleavage result in ?
Bacteria secrete proteases and peptidases
How are alterations of cell surface net charge occur ?
Modification of membrane structures to reduce anionic charge
How is s. aureus modified to block antimicrobial peptides ?
- LTA modified by D-alanine
- Phospholipids with L-lysine
How is s.entricas modified to block antimicrobial peptides ?
Modifies lipid A of LPS by deacetylation
What are the targets of bacterial evasion of complement cascade ?
- Inhibition of C1q binidng to Ig molecules on bacteria
- Modulation of C3 convertase
- Targeting C3b
- Targeting C5a
- Inhibition of membrane attack complex
How does C1q binding to Ig molecules be inhibited ?
Molecules such as protein A (s.aureus) and M.protein (Group A streptococcus) binding to Ig
What happens in modulation of C3 convertase ?
Molecules such as staphylococcal complement inhibitor can target C3 convertase
How does targeting C3b work ?
Proteases expressed by bacteria cleave C3b from surface of bacteria therefore preventing opsononophagocytosis
What does targeting C5a do ?
Inhibit neutrophil recruitment
What does inhibition of membrane attack complex ?
Prevents lysis
What do all forms of phagocytosis require ?
F-actin to be recruited beneath the surface bound particle into 3d structures which then resolve to allow membrane extension
What is actin re-organisation controlled by ?
Rho GTPase
What does RhoA stimulate ?
Acto-mysoin contraction and F-actin stabilisation and formation of stress fibres
What does Rac1 promote ?
Assembly of branched F-actin network and formation of lamellipodia
What does Cdc42 promote ?
Short actin filaments
What is bacterial evasion of phagocytic killing ?
- Inhibiton of phagosomal maturation
- Inhibition of killing mechanisms
- Escape from phagosome
How does m.tuberculosis inhibit phagolysosome maturation ?
- Ingested into phagosome but maturation to phagolysosome fails
- No accumulation of EEA1 or hVPS34
- Effect dependent upon component of cell wall phosphoinositide lipoarabinomannan which prevents increase in CA2+
How does m.tuberculosis inhibit intracellular killing by counteract acid accumulation?
- Stimulation of macrophages with IFN-Y promotes killing by overcoming block on phagosome maturation
- m.tuberculosis expresses thick waxy cell wall to resist low pH
- rV3571c gene encoding an enzyme important in peptidoglycan synthesis - absence bacteria becomes susceptible to acidification
4.
What does listeriolysin O do ?
Create pores in phagosome membrane
What is L.monocytogenes effect restricted to phagosome ?
pH dependent
How do bacteria become motile ?
Hijack cells actin cytoskeletal machinery
What does actin assembly inducing protein which is produced by L.monocytogenes do ?
Mimics host protein WASP involved in initiating actin polymerisation
