Week 5- Immune Evasion

Question 1

What does the body detect in gram positive bacteria?

Answer 1

LTA

Question 2

What does the body detect in gram negative bacteria?

Answer 2

LPS

Question 3

Describe the process of how a neutrophil reaches the stage of phagocytosis

Answer 3

1. Bacteria are opsonized by antibodies
2. They produce C3a or C5a protein in a gradient
3. Blood vessel endothelial cells are upregulated
4. Neutrophils roll along them down the gradient (chemotaxis)
5. Once they reach the phagocyte they perform phagocytosis

Question 4

What are the main methods by which bacteria evade antibodies?

Answer 4

1. Capsule prevents detection of antigens
2. Inhibit antibody opsonization: they produce spa/sbi proteins that bind to the antibody Fc region so the antibody binds back to front on the pathogen and is undetected
3. Inhibit antibody detection: SSL10 and Sak proteins bind to the antibody once on the antigen so it can’t be detected
4. Synthesis of proteases that cleave the antibody in half

Question 5

What are the 3 ways the complement cascade can be activated and generally what happens during activation?

Answer 5

Classical pathway: antibody deposition causes CIQRs to be formed p, activating C3 convertase
MDL pathway: Activation of C2b/C4b convertase
Alternative pathway: C3 convertase is deposited on microbes

Generally: Activation of C3 convertase activates C5 convertase leading to formation of a membrane attack complex (MAC)

Question 6

What are the main methods by which bacteria can evade activation of the complement cascade?

Answer 6

Inhibit convertases
Inhibit C3 processing
Make proteases that cleave complements
Inhibit formation of a MAC

Question 7

In the immune system, what do activatory receptors do?

Answer 7

Enhance immune response eg by increasing synthesis of chemokines/chemoattractants

Question 8

In the immune system, what do inhibitory receptors do?

Answer 8

Prevent neutrophil activation at the rwong place or wrong time

Question 9

What are the 2 main ways that neutrophil function is evaded?

Answer 9

1. Chemotaxis is inhibited: bacteria makes CHIPs which bind to receptors that detect C5a and fMLP preventing chemotaxis
2. Direct evasion: producing toxins that directly bind to and kill neutrophils or receptor agonists that inhibit activatory receptors by binding to them

Question 10

What are the 2 main CHIPs bacteria produce when aiming to inhibit chemotaxis? What receptor do each of them inhibit? When antibody does this prevent binding to the receptors?

Answer 10

SSL5 inhibits Fc alpha, prevents IgA binding

FLIPr inhibits Fc gamma, prevents IgG binding