Macrophage Interactions

Question 1

Two groups of bacteria based on location where they cause disease

Answer 1

1. Extracellular bacteria: live outside of host cells and cause disease
2. Intracellular bacteria: cause disease from inside of host cells (can be cytoplasmic or phagosomal)

Question 2

Strategies used by extracellular pathogens to resist killing by phagocytes

Answer 2

• Avoid recognition by phagocytes: invade regions ont accessible to phagocytes
• Inhibit phagocyte engulfment: Produce capsule or other anti-phagocytic determinants on surface
• Kill or damage phagocytes: secrete extracellular enzymes or toxins that damage host cell

Question 3

Strategies used by intracellular pathogens to resist killing by phagocytes?

Answer 3

• Inhibit phagosyme-lysosyme fusion: secrete proteins that block phagosome maturation pathway
• Survive inside phagolysosome: resist degradation by acidic pH, produce enzymes able to counteact ROI and RNI radicals
• Escape from phagosome: secrete enzyme that degrade phagosomal membrane

Question 4

5 functional steps of phagocytosis

Answer 4

1. Recognition
2. Uptake
3. Maturation
4. Killing
5. Antigen presentation

Question 5

Bacterial recognition

Answer 5

• Phagocytes recognize
  
  • Components of bacterial cell wall or membrane
  • Components of immune system following opsonization
• Bacteria can be recognized by host receptors and specific interaction can define subsequent events in steps of phagocytosis

Question 6

Macrophage receptors

Answer 6

• PRRs recognize LPS (gram -) and LTA (gram +)
• Receptors that recognize plasma-derived molecules deposited on bacterial surface
• Receptors that recognize other signature elements on bacteria

* receptor engagement generates different host cell responses depending on receptor

Question 7

Bacterial uptake

Answer 7

• Recognition by receptor initiates cascade-signal transduction
• Surface structure remodeled by depolymerizing and repolymerizing actin and other cytoskeletal parts
• Membrane bound phagosome
• Usually passive internalization (some bacteria actively influence internalization)

* Some bacteria employ specialized entry mechanisms to prevent phagosome maturation

Question 8

Types of specialized phagocytosis

Answer 8

1. Looping phagocytosis (Francisella)
2. Coiling phagocytosis (Legionella)

Question 9

Entry into non-phagocytic cells

• Trigger mechanism (major membrane perturbations)
• Zipper mechanism (minor membrane perturbations)

Answer 9

Trigger mechanism

• Salmonella
• Major cytoskeleton remodeling; promoted by bacterial effector proteins injected into host cell
• promoted by bacterial effector proteins injected into the host cells

Zipper mechanism

• Listeria
• Minimal cytoskeleton remodeling; Bacteria “slide” into cell - involves bacterial proteins
• minor membrane perbutations

Question 10

Maturation of bacteria-containing phagosomes

Answer 10

1. Fusion with endocytic vesicles
2. Classified as either early endosomes or late endosomes (depending on proteins on surface)
3. Lysosome fuses with endosme = phagolysosome

Question 11

Microtubules

Answer 11

Cytoskeletal components that allow maturation of the phagosome from the periphery to perinuclear region

Question 12

Macrophage activation reduces ability of bacteria to _____ _______

Answer 12

Alter maturation

Question 13

How do bacteria alter phagosome trafficking? (4)

Answer 13

1. Survive and replicate in phagolysosome
2. Escape and replicate in cytosol
3. Modulate endocytic pathway
4. Alternative trafficking pathway

Question 14

Bacterial killing by lysosomes

Answer 14

• Oxygen dependent killing
  
  • generate nitric oxide
  • NADPH oxidase - generate super oxide
• Oxygen independent killing
  
  • Lower pH (phagolysosome)

Question 15

Processing of bacterial antigens

Answer 15

• Antigens degraded into oligopeptides (13-18 AA)
• Binding to class I or II MHC
• Antigen presentation on surface
• Stimulation of T-Cell

Question 16

Benefits of bactera internalization

Answer 16

• Induce cytokine production
• Mediate lymphocyte recruitment and macrophage activation
• Mediate cross-activation of other immune cells
• Increase vascular permeability

Question 17

Negative outcomes of bacteria ingestion by macrophages

Answer 17

• Tissue injury and disease upon continuous stimulation
• Hydrolytic enzymes lead to tissue damage
• Fever, wasting, septic shock
• Chronic inflammation
• Autoimmune diseases
• “Trojan Horse”