Phagocytosis

1. chemotaxis and adherence of microbe to phagocyte

2. ingestion of microbe by phagocyte

3. formation of phagosome

4. phagosome + lysosome = phagolysosome

5. digestion of ingested microbe by enzymes

6. ingestion of residual body containing ingestible material

7. discharge of waste material

• Recognition = receptor binds to pathogen

• Internalization (endocytosis) = pathogen in cell into phagosome

• Phagosome-lysosome fusion= forms phagolysosome with enzymes inside

• Destruction (exocytosis) = enzymes destroys pathogen

Receptors for Recognition

1. dectin-1

• binds to glucan

2. mannose receptor

• binds to mannose

• clears host glycoprotein

3. scavenger receptors

• binds to anionic (-) polymers and acetylated low density lipoproteins

• shielded by sialic acid

• becomes uncovered as cell dies → activates phagocytosis

4. lipid receptor

• binds to lipids

5. fMLF receptor

• bind to fMLF (protein)

  • activates second messenger = chemoattraction

  • production of reactive oxygen species (ROS) in phagolysosome

6. TLR

• recognizes peptidoglycan, LPS, lipoteichoic acids, flagella, lipoarabinomannan, double stranded RNA, and particular antigens from parasites and fungi

• binding on surface = phagocytosis

• binding anywhere = signal cell to produce chemokines and cytokines

• binding in professional phagocytes = release of cytokines

Internalization

• when a molecule binds to receptor, internal signal is produced

• internal signal triggers cell to internalize the bound pathogen

• phagocytosis = endocytosis

• process requires ATP

Phagosome-lysosome Fusion

• phagosome + lysosome = phagolysosome

• acidic enzymes help kill pathogens

Mechanisms of Killing Pathogens

1. Acidification

• proton pump @ phagolysosome

• increase protons = decrease pH (acidic)

2. Antimicrobial peptides and other enzymes from lysosome

• lysosome carries many antimicrobial peptides in phagolysosome

3. Respiratory Burst

• phagocyte activated (increase oxygen) = respiratory burst

• phagocyte can be activated by:

  • cytokines

  • binding to TLR, fMLF receptor, Fc receptor, and complement receptor for C3

• toxic ROS:

  • activate phagocyte = NADPH oxidase

  • phagolysosome → phagocytosis → binding to fMLF, C5a, or cytokine = activates Rac2

• NADPH oxidase produces superoxide

• superoxide dismutase (SOD) = O2 → H2O2

• myeloperoxide = H2O2 → OCl- or OBr- or OH-

4. Toxic nitrogen derived products

   a.) bind to TLR with IFNy = phagocytes produces NOS

   • nNOS or NOS1 = in neuronal tissue

   • iNOS or NOS2 = phagocytes

   • eNOS or NOS3 = endothelial cells

   b.) NOS = arginine → NOHLA → citulline and nitric oxide

   c.) nitric oxide diffuses into phagolysosome

   d.) superoxide produces peroxynitrite radicals = kills microbes