Immune Evasion By Microbes (19)

Question 1

What is opsonisation?

Answer 1

the coating of microbial cell surfaces with substances that are recognised by phagocytic cells, enhancing their uptake

Question 2

What can act as opsonins?

Answer 2

IgG (antibody) and complement components- e.g. C3b, as well as products of C3b breakdown (iC3b, C3dg, and C3d), as all are recognised by Complement Receptors on phagocytes

Question 3

What are the 3 complement pathways?

Answer 3

classical, lectin (MBL) and alternative

Question 4

Key steps in antibody-mediated activation of the classical complement pathway *(extra detail)

Answer 4

• IgM or IgG antibody, bound to antigen, will link to 1st molecule: C1q–>then C1r cleaves C1s
• C4 binds to and is cleaved by C1s, revealing a thiolester bond in C4b–> which complexes with C2
• C4b2a produced has C3 convertase activity
• proteolysis of many C3 molecules–> C3b, which adds to C4b2a to make a C5 convertase, which generates C5a
• C5a is chemotactic and C5b forms first component of MAC

Question 5

Compare the classical, lectin and alternative complement pathways

Answer 5

• the classical pathway is activated by antibody, whereas the alternative and lectin pathways are not
• key central event for all 3 is cleavage of C3 by C3 convertase (C4b2a for classical and lectin; C3bBb for alternative)

Question 6

What are the key steps of complement cascades?

Answer 6

1. Initiation
2. C3 convertase formation
3. C5 convertase formation
4. MAC formation (deposition of C6-C9)

Question 7

What does the MAC do?

Answer 7

produce a pore in the bacterial cell membrane, resulting in lysis

Question 8

How does S.aureus evade complement opsonisation? N.B. detail SCIN protein

Answer 8

SCIN binds to C3bBb (C3 convertase)

• -> preventing production of C3b in cascade, so no deposition onto surface of bacterial cell or formation of MAC
• -> prevents formation of C3a and C5a (important immunostimulants bc inflammation chemoattractants)

Question 9

How does S.aureus evade complement opsonisation?
N.B. detail Efb surface protein

(similar mechanism used by M.catarrhalis w/ UspAs)

Answer 9

Efb binds to C3, preventing it from being cleaved into C3b–> so no formation of C3 convertase (C3bBb )

• -> prevents binding of factor B to C3 (so C3b not protected from cleavage by factor H)
• -> prevents C3dg from binding to CR2 (complement receptor 2)

Question 10

*How do bacterial proteins evade complement opsonisation?

Answer 10

1. inhibit C3/C5 convertases
2. bind to complement factors and prevent their processing
3. recruit regulators of complement (e.g. factor H) to surface of bacteria, so C3b inactivated and deposition= inefficient
4. proteases cleave complement factors into non-functional forms

Question 11

What components make up the Membrane Attack Complex?

Answer 11

C5b, C6, C7, C8 and C9

Question 12

By what process do neutrophils migrate to a site of infection?

Answer 12

chemotaxis

Question 13

What do pathogen recognition receptors (PRRs) do?

Answer 13

directly detect microbes/microbial products

expressed on neutrophils that are primed/activated

Question 14

What do CLEC receptors recognise?

Answer 14

microbial carbohydrates- sugar modifications on surface of pathogen

Question 15

What do TLR receptors recognise?

Answer 15

conserved microbial structures

e.g. LPS in gram -ve cell wall

Question 16

What do FPR receptors recognise?

Answer 16

formylated peptides released by bacteria

Question 17

What immune receptors indirectly detect bacteria?

Answer 17

• Fc receptors (antibody opsonised bacteria)

- Complement receptors

Question 18

What are the 2 important chemotactic receptors that are expressed by neutrophils?

Answer 18

C5aR- detects C5a

and FPR1- detects formylated peptides

Question 19

What happens when S.aureus expresses CHIPs protein?

Answer 19

inhibits chemotaxis
CHIPs binds with a higher affinity to C5aR and FPR1 than their agonists, so prevents their binding–> neutrophils don’t recognise stimulants–> no migration to site of infection

Question 20

What happens when S.aureus expresses FLIPr/SSL5 protein?

Answer 20

inhibits detection of antibody-opsonised bacteria by neutrophils (and so no phagocytosis)
FLIPr binds to and inhibits Fc y receptors (preventing detection of IgG-opsonised bacteria)
and SSL5 inhibits Fc a receptors (^^IgA)

Question 21

*How does S.aureus evade neutrophils?

Answer 21

1. Inhibit chemotaxis * detailed
2. Inhibit detection of bacteria * detailed
3. Kill neutrophils with toxins
4. Bind to and stimulate inhibitory receptors on neutrophils
5. Disrupt intracellular signalling

Question 22

Why must neutrophil responses be balanced?

Answer 22

to prevent infection, but also damage to host by inflammation

Question 23

What is the purpose of antibody opsonisation?

Answer 23

Antibodies bind to antigens so that 1. complement can be deposited for the classical pathway and 2. neutrophils and other phagocytes can detect invading microbes

Question 24

*How do bacteria evade antibody opsonisation?

Answer 24

• capsule- hides antigens on surface of bacteria- so cannot be detected by complement/antibodies etc.
• SpA surface protein binds antibodies via their Fc region, instead of Fab region–> prevents normal opsonisation
• SSL10 protein secreted by bacteria binds to Fc region of IgG–> preventing detection by neutrophil Fc receptors
• IdeS protease cleaves antibodies
• antigenic variation e.g. Opa - switch expression of antigens under certain stimuli–> lowers amount of opsonisation

Many bacteria use multiple strategies simultaneously…