Immune System: Phagocytosis and Defensins

Cathelicidins and Defensins

• Cathelicidins are stored in neutrophil granules and can be released within tissues in response to inflammation
• They can kill microbes extracellularly when released when neutrophils die during inflammation
• They can also kill microbes intracellularly after a cell (e.g. neutrophil) phagocytoses a pathogen

Phagocytosis and Phagocytes

• Phagocytosis is a key role in innate immunity as they can recognize, ingest, and destroy many pathogens without aid of an adaptive immune response
• Phagocytosis can also occur after an antibody has bound to an antigen – the antibody can act as a “signal” that triggers efficient phagocytosis
• Macrophages and neutrophils are the major phagocytes in the body

Phagocytic Cells

• Monocytes and macrophages: pro-monocytes (BM) → monocyte (blood) → macrophage/macrophage-like cells (tissues)
• Long-lived cells resident within the tissues
• Neutrophils: derived from hematopoietic precursors in the BM, non-dividing, short-lived cell type in blood (dominant WBC)

Pattern Recognition

• Evolutionarily conserved mechanism for recognizing common, conserved ‘signs’ of microbial infection, physiological stress, or other damage
• Recognition is immediate, does not require prior recognition, and activates several arms of the innate (and adaptive) immune response
• Responses are elicited via the engagement of Pattern Recognition Receptors (PRRs) found on phagocytes, in response to:
  • Pathogen Associated Molecular Patterns (PAMPs)
  • Danger Associated Molecular Patterns (DAMPs)

Pattern Recognition Receptors

• Examples of PRRs:
  • Toll-like receptors
  • Nod-like receptors
  • Lectins
• Elicit responses such as:
  • Phagocytosis
  • Cytokine secretion

Phagocytosis – the Process

• 1. A pattern-recognition-receptor (PRR) binds to a microbe or bit of debris, OR an opsonin created by another cell binds to the microbe
• 2. The microbe is engulfed – the PRR receptors signal the cell membrane to approach, coat, and then surround the sites where the receptor is bound
• 3 & 4. The phagosome is formed, mediated by intracellular signaling events and actin polymerization

Acute Inflammation

• Steps of Acute Inflammation: A. Alteration of vascular caliber - vasodilation B. Enhancement of vascular permeability
• Vasodilation and fluid loss (due to increased permeability) lead to slower blood flow, known as vascular congestion
• This helps with margination of leukocytes

Prostaglandins and Leukotrienes

• Prostaglandins and leukotrienes are produced when PLA2 generates arachidonic acid from membrane phospholipids
• Different types of cyclooxygenases produce different types of prostaglandins from arachidonic acid
• Important prostaglandins – PGE2, PGD2, and PGI2 cause vasodilation and increase vascular permeability, important acute inflammatory mediators
• Lipoxins are generated from arachidonic acid by 12-lipoxygenase – they decrease inflammation

Vascular Permeability

• Increased vascular permeability is due to contraction of endothelial cells
• Occurs mainly in venules, often short-lived
• Endothelial damage can also cause increased permeability

Leukocyte Migration

• Steps: a. Margination – leukocytes migrate towards vessel wall b. Rolling – formation & dissociation of adhesion bonds between leukocytes and endothelial cells
• Activation by chemokines presented on endothelial cells is required before the leukocyte can form stable adhesion bonds

Innate Immunity

• Innate immune defenses exist in all individuals and act within minutes to hours after encountering an infectious agent.
• Innate immunity discriminates effectively between host cells and pathogens.

Barriers

• Chemical barriers:
  • Lysozyme: present in secretions (mucus, tears, milk, saliva), breaks apart the peptidoglycan wall of bacterial cell walls through hydrolysis.
  • Defensins: small, heterogeneous, cationic peptides that kill Gram-negative and Gram-positive bacteria, some enveloped viruses, and fungi through multiple antimicrobial effects (destabilizing membranes, proteolytic degradation of bacterial proteins, inhibiting viral binding and entry, and inhibiting virus particle assembly).

Pattern Recognition Receptors

• Examples of pattern recognition receptors (PRRs):
  • Lectins
  • Toll-like receptors (TLRs)
  • Nod-like receptors
• PRRs elicit responses such as phagocytosis and cytokine secretion.

Toll-Like Receptors

• TLRs are a family of 10 cell membrane receptors with variable specificity for a range of pathogens.
• Ligands can include LPS, dsRNA, ssRNA, DNA, and flagellin.
• Cytokines secreted in response to TLRs include inflammatory cytokines (IL-1b, IL-6, CXCL8, IL-12, TNFa) and interferons (IFNa, IFNb, IFNl).

Nod-Like Receptors

• Nod-like receptors are intracellular receptors that detect products derived from the intracellular degradation of phagocytosed pathogens and recognize damage-associated molecular patterns (DAMPs) associated with cellular stress.
• They activate the expression of inflammatory cytokines.

Acute Inflammation

• Steps of acute inflammation:
  1. Alteration of vascular caliber - vasodilation
  2. Increased permeability and fluid loss
  3. Emigration and activation of leukocytes
  4. Chemotaxis of leukocytes to sites of injury or infection

Leukocyte Migration

• Leukocyte migration involves:
  1. Rolling: selectin expression by endothelial cells
  2. Tight adhesion: ICAM expression by endothelial cells and integrin affinity
  3. Diapedesis/transmigration: leukocyte migration through the endothelium
  4. Chemotaxis: migration to sites of injury or infection in response to chemotactic agents (leukotriene B4, bacterial products, activated complement, and chemokines)

Chemokines

• Chemokines are structurally-related cytokines that:
  • Bind to cell-surface receptors (usually leukocytes)
  • Induce movement of leukocytes along the chemokine concentration gradient
  • Mediate adhesion of leukocytes for the purposes of differentiation, inflammation, and migration
• There are two major chemokine families: CXC and CC.