Phagocytosis

Question 1

Neutrophils

Answer 1

• first leukocyte recruited to site of infection
• kills intracellularly or extracellularly
• cannot renew their lysosome and die after digesting few microbes

Question 2

Monocytes/Macrophages

Answer 2

• can process and present antigen to T lymphocytes - can return to secondary lympoid organs to “show off” what was internalized
• act more slowly than neutrophils but have longer life span

Question 3

Dendritic cells

Answer 3

• found in potential sites of pathogen entry (skin, lungs, GI tract)
• have branchlike cytoplasmic projections
• use projections to encircle and engulf fluid and extracellular pathogens
• important antigen-presenting cell to T cells

Question 4

Dendritic cells
important for phagocytosis

Answer 4

do NOT actively induce generation of reactive oxygen species and/or other toxic products that induce local inflammation

Question 5

Chemotaxis and movement of phagocytic cells to site of infection

Answer 5

• directed movement of cells through chemical gradient in response to endogenous factors: C5a, fibrinopeptide B and kinin products, IL-1, TNFalpha, chemokines

Question 6

mechanisms of internalizations: endocytosis
Pinocytosis

Answer 6

nonspecific “cell drinking” of soluble molecules
- nonspecific invaginations in cell membrane with coincidental internalization of accompanying extracellular molecules

Question 7

mechanisms of internalizations: endocytosis
Phagocytosis

Answer 7

process whereby cells ingest and destroy insoluble particles (bacteria, viruses, fungi, cells)

Question 8

receptor-mediated endocytosis

Answer 8

selective binding of macromolecules to membrane-bound receptors triggers internalization

Question 9

Steps of Phagocytosis

Answer 9

1. recognition and attachment of microbes
2. ingestion of microbes and other material
3. destruction of ingested microbes or other products
4. secretion of effector molecules

Question 10

Phagocytosis
1. recognition and attachment of microbes

Answer 10

• once at the site of injury, phagocytes must bind to the particle
• receptors on surface of phagocytes adhere to particles with help of opsonins (C3b and IgG)
• PRR also recognizes PAMPs
• there are specific receptors on surface of phagocytes for C3 (CR1-4) which binds to C3b and increase phagocytosis by macrophages, monocytes, and neutrophils
• IgG can be bound at the Fc portion of the Ab molecule by Fc receptors to stimulate phagocytosis

Question 11

Phagocytosis
2. ingestion of microbes and other materials

Answer 11

• after attachment, the microorganism is engulfed by extensions of the cytoplasm and cell membrane
• once internalized, a vesicle is formed (phagosome) which activates the phagocyte to increase in size, become more phagocytic, and initiate production of molecules that destroy the engulfed microorganism
• production of capsules that impede phagocytosis is common

Question 12

Phagocytosis
3. destruction of ingested microbes or other products
oxidative burst

Answer 12

• phagosome fuses with lysosomes in cytoplasm to form phagolysosome
• within phagolysosome, multiple products are produced that attack and destroy the ingested pathogen
• oxidative burst chemical rxn creates superoxide hydroxyl radicals and hypochlorite
• the hydrogen peroxide rx with granule enzyme, myeloperoxidase to form hypohalide ion which is highly reactive and kills pathogens
• the hydrogen perox

Question 13

Phagocytosis
3. destruction of ingested microbes or other products
toxic nitrogen oxides

Answer 13

• inducible nitric oxide synthase (iNOS) is induced by cytokines such as IFN-gamma and TNF-alpha
• iNOS modifies arginine and ultimately makes NO
• NO inhibits iron/sulfur-dependent enzymes; damages DNA and oxidizes membrane lipids

Question 14

Phagocytosis
3. destruction of ingested microbes or other products
other microbial products that contribute to killing

Answer 14

• lysozyme: attacks bacterial wall
• lactoferrin: chelates iron, which is necessary for normal microbial metabolism
• defensins: small antimicrobial peptides that disrupt membrane function and can induce osmotic lysis

Question 15

Phagocytosis
4. Secretion of effector molecules

Answer 15

not only are molecules produced in the phagolysosome to kill ingested organisms, activated phagocytes produced secreted molecules to alert and activate the rest of the immune response:
- chemokines to recruit additional cell types
- cytokines to activate infiltrating cells
- nitrous oxide can be secreted
- degradative enzymes

Question 16

Chronic Granulomatous Disease

Answer 16

• defect in enzymes that are important for respiratory burst
  nitrous oxide can phagocytose pathogen, but not kill it
  defect of intracellular killing
  thus nitrous oxide become a “trojan horse”
  manifests within first two years of life
  granulomatous lesions found in various organs
  defects in: cytochrome b, G-6-PDH, myeloperoxidase

Question 17

Mechanisms that control phagocyte activation:
neutrophils

Answer 17

inherently dont require significant stimulus to phagocytose and kill
- certain stimuli have been shown to enhance neutrophil activation, phagocytosis, and killing:
- stimulation of PRRs on neutrophils
- complement can increase oxidative burst in neutrophils
- cytokines can also help

Question 18

Mechanisms that control phagocyte activation:
monocytes/macrophages and dendritic cells

Answer 18

• stimulation of PRRs
• engagement of complement receptors and Fc receptors in the surface
• cytokines, like IFN-gamma, can sig. impact monocyte differentiation into macrophages and activation to kill
• IFN-gamma can also increase the ability of both macrophages and dendritic cells to present antigen

Question 19

What happens after pathogen is phagocytosed and destroyed?

Answer 19

• nucleic acids and amino acids are re-used
• proteins are broken down into peptide fragments that are “presented” to T lymphocytes to activate T cells (only macrophages and dendritic cells) in a process called antigen presentation