10. How bacteria evade the human immune system to cause disease

Question 1

What is the red queen hypothesis?

Answer 1

1. A term coined in evolutionary biology to explain the evolution of interacting species for survival in a common environment.
2. Species must adapt and evolve not solely for reproductive advantage but also for survival against competing organisms who are also evolving.
3. This continued development is needed to maintain relative fitness to the system it is co-evolving with.
4. This was compared to the bit in Alice in Wonderland where the queen and Alice are running but not moving.

Question 2

How does the Red Queen Hypothesis apply to host pathogen interactions?

Answer 2

1. Pathogens are constantly encountered by the host and are detrimental to their hosts, thus pathogens exert high selection pressure on their host.
2. Hosts can also exert similar levels of selection on the pathogen to prevent infection. This results in ongoing cycles of reciprocal adaptation between the 2 organisms.
3. Involves host developing immunity to a pathogen and the pathogen then evolving mechanisms to evade host immunity and then the host to counter evolved and so on…

Question 3

What are the main aspects of the human innate immune response?

Answer 3

1. Physical barriers.
2. Chemical and enzymes barriers
3. Innate response like inflammasome, complement, antimicrobial peptides, cytokines and innate cells.

Question 4

What are the main aspects of the human adaptive immune response?

Answer 4

1. Cellular and humoral immunity
2. APC activate a naive CD4 T cell
3. These drive B cell and T cell responses.
4. cytokines are important for determining function of immune cells and communication.

Question 5

What are the 3 main classes of bacterial immune evasion?

Answer 5

1. Manipulation of innate immune responses
2. Disruption of adaptive immune responses.
3. Bacterial life cycle adaption

Question 6

How can bacteria manipulate the innate immune response?

Answer 6

1. Complement evasion
2. Disruption of phagocyte recruitment
3. Disruption of phagocytosis and NETosis.
4. Evasion of bactericidal innate immune molecules.
5. Targeted destruction of immune cells.

Question 7

How can bacteria disrupt adaptive immune responses?

Answer 7

1. Targeting B cells for destruction and/or manipulation of antibody production.
2. Manipulation of T cell responses like superantigens.

Question 8

How can bacterial change their lifecycle to evade immune responses?

Answer 8

1. Invasion/internalisation of host cells.
2. Biofilm development
3. Formation of persisters like spores

Question 9

What are the main examples of immune evasion included in this lecture?

Answer 9

1. Evasion of the complement system.
2. Protection against phagocytes
3. Manipulation of T cell responses.
4. Adopting an intracellular lifecycle

Question 10

What are the 3 complement pathways?

Answer 10

1. The classical pathway
2. The lectin pathway
3. The alternative pathway

Question 11

How is the classical complement pathway activated?

Answer 11

1. IgG bound to bacteria interacts with the C1q complex.
2. This causes C1q to undergo a conformational change.
3. This activates serine proteases.
4. This causes C4 and C2 to cleaved and C4b and C2a forms a C3 convertase.

Question 12

How is the lectin complement pathway activated?

Answer 12

1. Mannose binding lectin (MBL) recognises mannose and other sugars on the bacterial surface.
2. This causes MBL to undergo a conformational change.
3. This activates and releases serine proteases.
4. The causes C4 and C2 to be cleaved and C4b and C2a to form a C3 convertase.

Question 13

How is the alternative pathway different to the other complement pathways?

Answer 13

1. It is always on
2. Controlled by spontaneous hydrolysis of C3

Question 14

How is cleaved C3 generated in the alternative complement pathway?

Answer 14

1. C3 undergoes spontaneous hydrolysis into C3-water.
2. This binds to Factor B.
3. The C3-water factor B complex is a substrate for factor D.
4. This leads to the cleavage of C3 to make C3b.

Question 15

What is Properdin?

Answer 15

1. A positive regulator of the alternative complement pathway.
2. It increases C3 cleavage under the right conditions.

Question 16

What is the central complement component?

Answer 16

C3

Question 17

How do the classical and lectin complement pathways generate C3b?

Answer 17

By generating C3 convertases made up of C4b and C2a

Question 18

How does C3 orchestrate the complement response?

Answer 18

1. C3 is cleaved to C3b which is deposited on the cell surface.
2. C3a is also generated and it is an anaphylatoxin.
3. Other C3 convertases lead to a massive build up of C3b.
4. C3b opsonised bacteria are recognised by phagocytes that bind C3b.
5. Then more build up of C3b causes C5 convertase formation and activation.
6. C5a is another anaphylatoxin that recruits neutrophils.
   7.C5b binds to the bacterial surface.
7. This recruits C6, C7 and C8 and multiple C9 fragments.
8. This leads to the formation of the membrane attack complex that punctures holes in the gram negative membrane to lyse cells.
9. gram positives are not susceptible to MAC but can be opsonised.

Question 19

How does the body control the complement?

Answer 19

1. It dampens the complement to reduce by stander tissue damage.
2. Plasma proteins like factor H
3. Cell bound regulators like CD59

Question 20

What are C3 convertases important for?

Answer 20

1. C3b deposition on the bacterial surface.
2. Flagging to the immune system

Question 21

How does S. aureus evade the compliment?

Answer 21

1. It secretes a number of different complement evasins.
2. Extracellular fibrinogen binding protein (Efb) that targets C3d.
3. Extracellular complement binding protein (Ecb) that targets C3d.
4. Staphylococcal complement inhibitor (SCIN) that targets C3b

Question 22

Where does SCIN bind to C3 convertases?

Answer 22

1. Its N-terminal tail interacts with MG6 and MG7 and Bb.
2. This prevents C3 convertase interactions with C3.
3. This results in no C3 cleavage.

Question 23

Where are S. aureus C3 convertase inhibitors encoded?

Answer 23

1. On Immune evasion clusters.
2. These are mobile and can undergo horizontal gene transfer.
3. IEC genes are specific for the human complement suggesting they have a role in colonisation and infection

Question 24

What mechanism do lots of pathogens use to evade the complement?

Answer 24

1. Proteases
2. Molecules that bind and block complement proteins.
3. Conversion of plasminogen to plasmin via streptokinase.
4. Inhibition of membrane attack complex formation.

Question 25

What is streptokinase?

Answer 25

1. A bacterial proteins that converts host plasminogen to plasmin. 2. Plasmin can cleave C3b on the surface of bacteria.

Question 26

What is factor H?

Answer 26

1. A 155kDa plasma glycoprotein. 2. It helps down regulate the complement by inhibiting the alternative pathway. 3. made up of 20 homologous complement control protein domains (CCP) 4. It is produced in the liver and locally in certain cells. 5. Reverse recognition as it binds to initial deposits of C3b on cells.

Question 27

What is the function of factor H?

Answer 27

1. It accelerates the decay of alternative pathway C3 convertases. 2. Acts as a co-factors for Factor I to cleave and inactivate C3b. 3. Compete with factor B for interaction with C3b. 4. Promotes C3bBb disruption.

Question 28

How do microbes take advantage of factor H?

Answer 28

1. They can actively recruit factor H 2. This prevents complement activation in response to the microbe.

Question 29

What is C4b binding protein?

Answer 29

1. A 500 kDa plasma glycoprotein 2. Major soluble inhibitor of the classical and lectin pathway. 3. Contain seven identical chains and one ß chain that interacts with the anticoagulant protein S.

Question 30

What is the function of C4-binding protein?

Answer 30

1. It binds and limits the function of activated C4b. 2. It acts as a cofactor for Factor I and inactivates soluble and cell bound C4b and prevents C4b2a convertase. 3. Can accelerate the natural decay of the classical convertase. 4. Acts as a cofactor to factor I to cleave C3b in the fluid phase which effects the alternative pathway.

Question 31

How do microbes take advantage of C4b-binding protein?

Answer 31

1. They can actively recruit C4b-binding protein. 2. This prevents complement activation in response to the microbe.

Question 32

Why do most soluble complement inhibitors bind to specific CCP domains?

Answer 32

1. They can interact with C3b and leads to factor H activity. 2. This prevents C3b deposition on the bacterial surface.

Question 33

What is the process of complement mediated phagocytosis?

Answer 33

1. C3b is deposited on the surface of the bacteria. 2. This is detected by CR1 on the phagocytes. 3. This enhances phagocytosis.

Question 34

What is the process of antibody-mediated phagocytosis?

Answer 34

1. The Fab antibody region binds to the pathogen. 2. The Fc region binds to the FcR on the macrophage to trigger phagocytosis

Question 35

What bacterial mechanisms can block phagocytosis?

Answer 35

1. Complement inhibition 2. Capsule formation 3. IgG binding proteins

Question 36

How does inhibiting complement inhibit phagocytosis?

Answer 36

1. Complement mediated phagocytosis is blocked 2. This is because C3b is not deposited on the bacterial surface.

Question 37

What is a bacterial capsule?

Answer 37

An extracellular physical barrier that is made from polysaccharides.

Question 38

How does a capsule block phagocytosis?

Answer 38

1. It blocks the access of complement activation fragments to the bacterial surface. 2. Prevents the membrane attack complex reaching the bacterial surface. 3. It can block binding between ligand and complement receptors to prevent opsonisation. 4. It can block antibodies from binding to their antigens. 5. Capsule production can be induced by human stimuli.

Question 39

What bacterial can make polysaccharides capsules?

Answer 39

1. S. aureus 2. E. coli 3. Pneumococcus

Question 40

What does making a capsule require?

Answer 40

Lots of energy

Question 41

What are IgG binding proteins?

Answer 41

1. Proteins that bind to IgG and IgM to prevent proper antibody-antigen recognition. 2. They can be soluble or membrane-bound.

Question 42

How do IgG binding proteins prevent phagocytosis?

Answer 42

1. They prevent antibody antigen recognition 2. They bind the antibody in the wrong orientation to prevent recruitment of phagocytes. 3. They bind to the Fc region of antibodies. 4. They can be soluble and bind to serum antibodies to sequester them.

Question 43

What else can IgG binding proteins do?

Answer 43

1. They can act as super antigens and cross-link IgM on the B cell. 2. This causes B cell apoptosis. 3. The reduces antibody production and dampens immunity.

Question 44

What are CHIPS?

Answer 44

Chemotaxis inhibitory proteins

Question 45

What does CHIPS do?

Answer 45

1. It binds to chemokine receptors like C5aR and formyl peptide receptor. 2. This prevents cells like neutrophils from detecting the chemotaxis gradient. 3. This prevents the migration of immune cells. 4.CHIPS prevents C5a binding to the C5aR. 5. It also prevents F-MP to the formyl peptide receptor.

Question 46

What are NETs?

Answer 46

1. Neutrophil extracellular traps 2. They are stimulated by a variety of triggers like pathogens. 3. NETs are composed of DNA and coated with histones. 4. They trap microbes as they are a concentration of antimicrobial peptides

Question 47

What are the 2 pathways that form NETs?

Answer 47

1. NETosis 2. Non-lytic NETosis

Question 48

What is NETosis?

Answer 48

1. Intracellular rearrangement and disassembly of the nuclear envelope. 2. Cellular depolarisation and chromatin decondensation. 3. Plasma membrane rupture and release of NETs.

Question 49

What is non-lytic NETosis

Answer 49

1. Degranulation and explusion of chromatin. 2. Extracellular assembly of NET 3. Bacterial cell death

Question 50

What are the 3 ways bacteria can evade NETs?

Answer 50

1. They have molecules that mimic sialic acids to dampen down ROS generation and reduce NETosis. 2. The production of capsules that defend against antimicrobial peptides produces during degranulation. 3. Secretion of endonucleases that degrade NETs. 4. Some bacteria can secrete specialised molecules that convert NET products into toxic components that kill immune cells

Question 51

How does S. aureus evade NETs?

Answer 51

1. Uses nuclease and adenosine synthase A. 2. These convert NETs to 2'-deoxyadenosine which triggers macrophage apoptosis. 3. This is an amazing evolutionary feat

Question 52

What are the phagocytic killing mechanisms?

Answer 52

1. Oxygen dependent methods involve the assembly of membrane-bound NADPH-dependent oxidase, which generates ROS. 2. Both include degrative enzymes and cationic peptides within granules that are trafficked to and fuses with phagosomes. 3. Combine oxygen dependent and independent methods contribute to the elimination of bacterial pathogens.

Question 53

How do bacteria survive in the phagosome?

Answer 53

1. They can neutralise ROS via superoxide dismutases, catalases and alkyl hyperperoxide. 2. They can mitigate antimicrobial peptides. eg staphyloxanthin 3. Incorporation of D-ala into teichoic acids and L-lysine into phosphatidylglycerol to reduce the negative charge on the bacterial membrane to limit the action of antimicrobial peptides. 4. O-acetylation of peptidoglycan by OatA to change the structure of peptidoglycan to resist lysozymes.

Question 54

What can trigger different mechanisms of evading phagocytosis?

Answer 54

1. Cell stress. 2. Presence of human factors like blood

Question 55

How can bacteria manipulate T cell responses?

Answer 55

Through superantigens.

Question 56

How do bacterial T cell superantigens work?

Answer 56

1. They alter the immune response by binding directly to MHC class 2 and the TCR as the same time. 2. This activates T cells without antigen recognition basically bypassing the TCR. 3. This causes the uncontrolled activation of large numbers of conventional CD4+ and CD8+ T cells. 4. This plays are major role in proinflammatory diseases like scarlet fever and toxic shock syndrome.

Question 57

What mechanism do superantigens use to activate T cells?

Answer 57

1. Superantigens link the TCR and MHC2. 2. This activates the Lck tyrosine kinase pathway. 3. After several rounds of phosphorylation, phospholipase C is activated. 4. This hydrolyses PIP2 into IP3 and DAG. 5. This downstream signalling causes the 3 T cell nuclear transcription factors: NFAT, AP-1 and NF-kB. 6. This triggers gene expression that causes T cell activation, proliferation and cytokine production.

Question 58

How do superantigens reduce immune responses?

Answer 58

1. They reduce TCR recognition of the antigenic peptide. 2. They prevent the mounting of an antigen-specific immune response to the bacteria.

Question 59

What T cell responses can superantigens activate?

Answer 59

1. CD4 and CD8 T cells 2. iNKT cells and MAIT cells. 3. Skew T cell responses to Th1, Th2, Th17, or Treg depending which can play a role in specific clinical manifestations.

Question 60

What can low concentrations of superantigen induce?

Answer 60

1. Immunosuppression 2. Promotes a Treg response with IL-10

Question 61

Why is manipulating the T cell response so effective for bacterial evasion?

Answer 61

1. It is an important and central part of the immune system. 2. Preventing this prevents a proper antigen-specific response to bacteria

Question 62

How can superantigens promote abscess formation?

Answer 62

1. In invasive infection, superantigens can promote abscess formation due to excessive IFNy production. 2. Excessive and rapid IFNy production can inhibit immune response due to broad-spectrum recruitment of immune and inflammatory cells 3. These cells, along with high IFNy, impede the clearance of intracellular pathogens like S. aureus. 4. This can spread to the liver and cause abscess formation as the bacteria replicate inside the macrophages.

Question 63

What is listeria monocytogenes?

Answer 63

1. A gram positive pathogen. 2. Causes the food borne disease listeriosis. 3. Opportunistic infections mainly occur in young, old, pregnant and immunocompromised patients. 4. Listeria creates an intracellular niche in susceptible cells to evade immune detection.

Question 64

How does listeria monocytogenes adopt an intracellular lifestyle?

Answer 64

1. It invades host cells via interaction of surface internalins which target host cell receptors E-cadherin and C-Met. 2. Listeria becomes endocytosed but escapes from phagosome before fusion with the lysosome. 3. This escape is mediated by the secretion of the pore-forming toxin LLO and PLC. 4. It can replicate in the cytosol.

Question 65

How does Listeria spread between cells?

Answer 65

1. Via actin polymerisation 2. ActA drives bacterial propulsion by inducing actin polymerisation on the bacterial surface, permitting intracellular motility and cell to cell spreading. 3. Rupture of the plasma membrane is also mediated by LLO and PLC

Question 66

How does listeria evade intracellular immunity?

Answer 66

1. Normally Listeria is detected by TLR2 or NOD like receptors. 2. It can remodel its cell wall 3. It can also dampen host inflammatory responses.

Question 67

How does listeria remodel its cell wall to evade immunity?

Answer 67

1. It uses peptidoglycan N-deacetylation to avoid PRR. 2. It uses O-acetyltransferase to change peptidoglycan to increase resistance to lysozymes.

Question 68

How does listeria dampen host pro-inflammatory responses to evade immunity?

Answer 68

1. It secretes InLC to inhibit NF-kB. 2. InLC directly interacts with IKKa kinase to prevent phosphorylation of the inhibitor of NF-kB (IkB). 3. IkB is no longer degraded in response to pro-inflammatory signals. 4. This sequesters NF-kB in the cytosol and reduces pro-inflammatory signalling.

Question 69

How can Listeria block cell death?

Answer 69

1. InLB blocks Fas mediated cell death. 2. This protects the host cells from CD8+ T cell mediated killing via Fas. 3. This allows cells to survive longer in the blood and results in listeria transfer to the brain and intestinal tissue. 4. This releases listeria in the faeces and the environment. 5. This means bacterial survival and spread.

Question 70

How does listeria's InLB inhibit fas mediated cell death?

Answer 70

1. InLB upregulates FLIP. 2. FLIP is a competitive inhibitor of procaspase 8. 3. This competitively inhibits cleavage of caspase 8 which blocks FasL-Fas mediated cell death.