Lecture 14 - Immunity to Bacterial I Flashcards
Describe the generic structure of the bacterial cell
Cell wall
Cell membrane
+/- Capsule
Intracellular constituents:
• DNA
• Ribosomes
• RNA
+/- Flagella
+/- Pilli
Compare Gram + and - bacteria
Gram +
• One plasma membrane
• Thick outer wall of peptidoglycan
Gram -
• Inner and outer plasma membranes
• Thin layer of peptidoglycan in between membranes
• LPS on outer membrane
Describe the components of bacterial cell walls
Peptidoglycan
Teichoic acid
What is another name for pilli?
Fimbriae
Made of protein
Describe the structure of LPS
Where is it found?
(Lipopolysaccharide) aka endotoxin
Present on the outer membrane of Gram -ve bacteria
Core glycolipid
• Outer and inner oligosaccharides
• Lipid A (anchors LPS in the lipid membrane)
O-specific polysaccharide chain
• Variable length
• Repeats
• Part that induces Ab responses
List some general features of immune responses to microbes
- Mediated by innate and adaptive immunity
- Distinct and specialised responses to different microbes
- Pathogenicity and survival of microbes influence by ability to evade immune system
- Latency can occur
- Tissue injury and disease can result from the host response against bacteria
Compare the features of extracellular and intracellular bacterial pathogens
Where do each reside?
Give an example each
Extracellular: • Evade phagocytosis • Reside in interstitial spaces, blood, lymph, or on epithelial surfaces • Non-invasive, toxin producers • e.g. E. coli, V. cholerae
Intracellular:
• Resist intracellular killing
• Reside in either cytoplasm or vesicles
• e.g. Salmonella typhinurium
List the general mechanisms of Ab against extracellular bacteria
• Neutralisation
- Preventing replication
- Preventing adhesion to / invasion of host cells
- Opsonisation and FcR-mediated phagocytosis
- C’ activation
- Phagocytosis of C3b coated bacteria
- Inflammation
- Lysis (MAC formation)
List some of the ligands on bacteria that trigger the C’ cascade
Which molecules are binding to these ligands on bacteria?
Gram -ve:
• LPS
• Proteins
• PS
Gram +ve:
• Phosphocholine
• Many proteins
Triggering molecules:
• C1q
• MBL
Give an overview of the pathways of C’ activation
- Classical
• Ab binds surface of pathogen and triggers C’ activation - Alternative
• C’ proteins bind directly to molecules on the pathogen - Lectin binding
• MBL binds sugars on the surface of microbes
Describe T cell responses to extracellular bacteria
CD4+ ‘helper’ T cell responses are most important
- Inflammation
• IL-17, TNF and other cytokine production - Macrophage activation
• IFN-γ production
• Increased phagocytosis and intracellular killing - Antibody response
• Help for B cells
Which components of the immune system fight against extracellular bacteria?
- Antibodies
- C’ cascade
- CD4+ ‘helper’ T cells
Which infections do people w/o Th17 responses suffer from?
Bacterial and fungal infections
Th17 produce IL-17 which is important for immunity against bacteria
Describe the roles of the various compartments of immunity against intracellular pathogens
- Innate immunity
• Stems spread of infection in early stages
a. Neutrophils
• IL-12 for NK cell activation
b. NK cells
• IFN-γ production for macrophage stimulation
c. Macrophages
• Phagocytosis of bacteria
• IFN-γ signalling from NK cells is crucial
- Adaptive immunity
• Needed for eradication of infection
a. Th1
• Help macrophages to clear the bacteria
b. CTLs
• Especially important against bacteria that escape into the cytoplasm
• eg. Listeria infections
• Perforin and granzyme dependent killing
Compare macrophages and neutrophils
Macrophages:
• Present in tissues
• Long lived
• Phagocytose bacteria
Neutrophils:
• Short lived
• Present in circulation
• Phagocytose bacteria
Are macrophages present in the spleen?
Yes, they are very abundant in the red pulp
How do macrophages recognise bacteria?
Various receptors: • C' receptors • C1q receptor • FcR • Mannose receptor • Scavenger receptors • PRRs
Describe the process of opsonisation and phagocytosis
- Opsonised bacterium:
• C3b and Abs on surface - Macrophage recognises opsonised bacterium
• C3b binds CR1
• Ab Fc region binds FcR - Triggering of phagocytosis
- Opsonised bacterium taken up into phagosome
- Lysosomes fuse with the phagosome to produce a phagolysosome, in which the bacteria are killed and degraded
Why don’t macrophages constantly bind Ab in the serum?
Need cross-linking of FcR for activation of the macrophage
This only happens when Ab are aggregated on a bacterium
Outline how macrophages and neutrophils kill ingested organisms
- Acidification
• pH 3.5-4.0 in lysosomes
• This is bactericidal or bacteriostatic - Toxic oxygen-derived products
• Superoxide, 02-
• Hydrogen peroxide, H2O2
• Hydroxyl radical, OH- - Toxic nitrogen oxides
• NO - Antimicrobial proteins
• Cathelicidin in macrophages and neutrophils
• α and β defensins in neutrophils - Enzymes
• Lysozyme
• Acid hydrolases (e.g. elastase) - Competitors
• Lactoferrin in neutrophils
Describe the action of lysozyme
Digests cell walls of some Gram positive bacteria
Describe the mechanism of respiratory burst
Neutrophils have receptors that recognise peptides (fMLP) that are only produced by bacteria (prokaryotic cells)
- fMLP on bacteria activates receptor, resulting in activation of Rac2
- Rac2 is instrumental in the formation of the NADPH oxidase complex
- NADPH oxidase complex on membrane of secondary granules in neutrophils
- NADPH oxidase complex generates oxygen radicals
• Superoxide
• Hydrogen peroxide - Granules fuse with phagosomes (containing pathogen) and lysosomes
- Bacteria killed
Describe how neutrophils can kill extracellular bacteria
Neutrophil Extracellular Traps (NETs)
“NETosis”
Composition: • DNA • Anti-bacterial proteins: - Myeloperoxidase - Serine proteases - Elastase
Function:
• NETs are exocytosed
• Helps to immobilise and ultimately kill the bacteria
What molecules are present in the granules of neutrophils?
ROS
• OH-
• O2-
• H2O2
Defensins
Cathelicidins
Myeloperoxidase
Which cytokines and chemokines do macrophages produce?
- IL-1β
- TNF-α
- IL-6
- CXCL8
- IL-12
What is the role of IL-1β from macrophages?
Activates vascular endothelium
Activates lymphocytes
Local tissue destruction → increased access for effector cells
Systemic effects:
• Fever
• IL-6 production
What is the role of IL-6 from macrophages?
Lymphocyte activation (Th17)
Increased Ab production
Systemic effects:
• Fever
• Induces acute phase response; CRP production in liver
What is the role of TNF from macrophages?
Activation of vascular endothelium
Increased vascular permeability
Increased entry of IgG, C’, and effector cells
Systemic effects:
• Fever
• Shock
What is the role of CXCL8 from macrophages?
Recruits:
• Neutrophils
• Basophils
• T cells
to site of infection
What is the role of IL-12 from macrophages?
Activation of NK cells
Induces differentiation down the Th1 lineage
Which T cells play a role in anti-bacterial immunity?
CTLs Th1 Th2 Th17 Treg
Describe the role of CTLs in anti-bacterial immunity
Cytotoxicity against infected cells: • Perforin • Granzymes • Granulysin • FasL
Cytokine production:
• IFN-γ
• LT-α
• TNF-α
Describe the role of Th1 in anti-bacterial immunity
Activation of macrophages:
• IFN-γ
• CD40L
Killing of chronically infected cells
• FasL
• LT-β
Stimulation of monocytes in BM:
• IL-3
• GM-CSF
Activation of endothelium
• TNF
• LT-α
Accumulation of macrophages at site of infection:
• CXCL2
Stimulation of T cells:
• IL-2
Describe the role of Th2 in anti-bacterial immunity
Stimulate B cell responses & barrier immunity: • IL-4 • IL-5 • IL-13 • CD40L
Describe the role of Th17 in anti-bacterial immunity
Neutrophil recruitment:
• IL-17
• IL-6
Describe the role of Tregs in anti-bacterial immunity
Immune suppression:
• IL-10
• TGF-β
• GM-CSF
Compare different diseases caused by Mycobacterium leprae
M. leprae can cause two different types of diseases based on the host immune response (esp. Th response)
- Lepromatous disease:
• Destructive lesions in skin and underlying tissue
• Large n° of bacteria growing in macrophages
• Inadequate macrophage activation (due to their infection)
• High Ab titres
- the bacterium is intracellular, so Ab responses aren’t helping to clear the infection
• Weak cell-mediated immunity
• Th2 response predominates
2. Tuberculoid disease: • Granulomas; less tissue destruction • Low n° of bacteria • Low Ab titres • Strong cell-mediated immunity • Th1 response predominates
Describe the growth of the microbe in the following situations:
• Lacking innate immunity
• Lacking adaptive immunity
• Normal (immunocompetent)
Lacking innate immunity:
• Rapid growth of infection in early stages
• Infection not controlled
Lacking adaptive immunity:
• Innate immunity stems early growth
• Infection can not be cleared, and persists
Normal, immunocompetent:
• Growth of infection in early stages, but immune system eventually clears the bacteria
Describe the effect of the following conditions on anti-bacterial immunity: • Neutropaenia • MBL deficiency • Perforin • IL-12R deficiency
Also: intra- / extracellular infection
*
Perforin:
• Lacking CTL responses
• Susceptible to intracellular bacterial infections (esp. Salmonella typhimurium)
IL-12 deficiency:
• NK cells respond to IL-12
• Suffer from intracellular bacterial infections
Which bacteria commonly causes intracellular (cytosolic) infections?
Salmonella typhimurium
Which injuries can occur from immune response to bacteria (intra- and extracellular)
– Extracellular bacteria –
1. Inflammation
• Temporary
• Chronic (H. pylori)
- Septic shock
• aka Endotoxic shock - Superantigen-induced shock
- Disease producing Abs mediated
- Intracellular bacteria –
1. Delayed type hypersensitivity
2. Granuloma formation
- Intracellular bacteria –
Describe how bacteria can cause chronic inflammation
Helicobacter pylori
Resides in the mucous lining the epithelium of the stomach
Bind to host cells triggers a host Th1 response
Th1 produce IFN-γ, which recruits many macrophages
Macrophages can not eliminate the bacteria
IFN-γ results in chronic gastritis
Describe how septic shock occurs
In response to LPS, hence endotoxic shock
Large amounts of LPS in the blood stream
Normally:
1. LPS released when bacteria are lysed
- LPS mopped up by LBP (LPS-binding protein)
- LBP/LPS complex binds CD14:
• Soluble
• Bound to macrophages and PMNs - mCD14-LBP/LPS complex associates with TLR4
5. Signal-transduction, resulting in release of: • Cytokines: - IL-1 - IL-6 - TNF • Tissue factor
- When there is a large amount of LPS, this happens at a great level, resulting in vast amounts of these cytokines being produced
(LPS can also act directly on endothelial cells, resulting in cytokine production)
- Inflammatory response brought about by the cytokines
• Neutrophil recruitment
• Superoxide production
• Degranulation - Tissue factor triggers coagulation cascade:
• Thrombin activation
• Fibrin activation
• Formation of a thrombotic and fibrinolytic response
9. Damage: • Endothelial damage • Loss of fluid in tissues • Drop in BP • Circulatory collapse
- Multiple organ failure
• Nearly always fatal
• V. difficult to reverse
What is mCD14?
Protein bound to surface of macrophages and PMNs that binds LBP/LPS
What are superantigens?
Which bacteria produce them?
Describe how they induce damage
Ags that bind MHC class II molecules and TCR
Do not require processing
- Binds to:
• Conserved regions of Vβ region of TCR
• Conserved residues on MHC II - Results in activation of many T cells (not just those specific for a particular epitope)
- T cells and macrophages produce vast amounts of cytokines
• TNF - Results in:
• Disseminated vascular coagulation
• Cardiovascular shock
Bacteria:
• Staph. aureus produced Staphylococcal enterotoxins
Describe what is happening during the late stages of super antigen induced shock
Monocytes release cytokines that act on endothelial cells
Intravascular co-agulation of blood
Loss of fluid from vasculature into tissues (through leaky endothelium)
Give an example of how antibodies can produce disease
e. g. Streptococcus pyogenes (Group A strep.)
1. Group A strep infection in throat or skin
- Cross-reactivity of anti-bacterium antibodies with ‘self’
• anti-M protein Abs react w/ Myocardial proteins - Antibodies bind to proteins in heart, triggering C’
- Inflammation and carditis
- Immune complexes can also deposit in the kidney, causing nephritis
• Immune complexes formed during an infection, if not properly removed
What is the Mantoux test?
Test to determine if an individual has been exposed to M. tuberculosis
- Tuberculin (purified protein derivate) injected
If there has been prior exposure:
2. Formation of a DTH response:
• Tuberculin antigen is taken up, processed by DCs and presented to T cells
- Presence of CD4+ T cells in skin where injected
Size of the reaction is measures
Describe granuloma formation
What causes this?
Caused by intracellular bacteria that resist killing
- Intracellular bacterial infection resists killing, persists
- Chronic Ag stimulation of T cells and macrophages
- Macrophages form giant multinucleate cells containing bacteria
- Influex of activated macrophages and surrounded by T cells
- May result in tissue necrosis and fibrosis
- Localises infections and prevents spread of microbes
Which pathogen classically causes granuloma formation?
M. tuberculosis
How is it possible that superantigens can activate 2% of T cells?
Superantigens bind conserved regions between the β chains of TCRs
2% of T cells will share certain regions in the Vβ domain
e.g. Staphylococcal enterotoxin B binds Vβ3, which is present in roughly 2% of TCRs
Describe the structure of a granuloma
Centrally:
• Giant multinucleate macrophages containing the bacteria
• Activated macrophages
• CD4+ T cells around the outside
Outline the various roles of macrophages in anti-bacterial immunity
- Release of cytokines and chemokines
2. Phagocytosis of pathogens
Which cytokine is vitally important for NK cell activation?
Where does this cytokine come from?
IL-12
Released by macrophages that have sensed pathogens
