7. Phagocytosis

Ways pathogens avoid phagocytosis?

Avoiding Contact: grow where phagocytes cant reach, suppress inflammatory response, inhibit chemotaxis, and tricking phagocytes into thinking they are self cells. Avoiding Engulfment using sugar capsules. Surviving within phagocyte: preventing phagolysosome fusion, creating a protective vacuole within phagocyte, and producing catalase and SOD to break down ROS Killing Phagocytes: cytolysins poke holes in phagocytes Disruption of Cell Signaling.

Which cells typically perform phagocytosis?

Cells that have toll-like receptors (tlr's) that bind to pamp's to phagocytose harmful pathogens. examples: neutrophils, monos/macs/dendritic cells, mast cells, and other granulocytes to a lesser extent (basophils, eosinophils)

Types of Endocytosis

Clathrin-mediated endocytosis (micropinocytosis) Caveolae (another form of micropinocytosis) Macropinocytosis Phagocytosis

Antigen processing/presenting

Done by macrophages, dendritic cells and kind of by B cells. Phagocytose foreign particle and digest into small pieces and present those pieces to helper T cells in lymph nodes to upregulate adaptive immunity. Sometimes test T cells to make sure they're unreactive to self-cells by presenting self antigens.

Non-professional phagocytes

Dying cells/pathogens can be consumed by other cells than the professional phagocytes. More limited in the type of particles they can pick up and phagocytosis is not their primary function. Lack efficient phagocytic receptors (opsonins). Most do not produce reactive oxygen containing molecules. examples: Fibroblasts, epithelial cells, endothelial cells, mesenchymal cells, immature cell types.

Exocytosis

Exporting material from cell to extracellular space.

Pinocytosis

Form of endocytosis. Means drinking fluid into cell. 2 forms: micropinocytosis (sipping) and macropinocytosis (gulping) Prater used analogy of sipping wine vs. chugging beer.

Phagocytosis

Form of endocytosis. Means eating solid material into cell.

Neutrophils

Formed in bone marrow. Usually don't leave marrow until mature, but sometimes in overwhelming infection there's a shift to immaturity. Neutrophils live for 5 days. The older they are, the more segmented their nucleus is. Can reach infection in 30 minutes. Die in tissues. Don't return to blood.

Endocytosis

Importing material into cell. Broad term meaning engulfing material from extracellular space to within cell. Plasma membrane invaginates to form an endosome. Performed by all nucleated cells to engult polar molecules that otherwise couldn't pass thru hydrophobic membrane.

How does iron enter the cell?

Iron uses transferrin and is transported by receptor-mediated endocytosis.

How do lipids enter the cell?

LDL's enter using the LDL receptor and are transported by receptor-mediated endocytosis. Apolipoprotein B is the primary protein of LDL (bad cholesterol). There are 2 isoforms apoB48 (produced by small intestine) and apoB100 (produced by the liver). Increased apoB100 can cause hypercholesteremia, atherosclerosis, and heart disease.

Oxygen-independent killing

Less effective. 4 methods: 1. Electrically charged proteins damage bacterial membrane. 2. Lysozyme to break down cell wall 3. Lactoferrins from neutrophil granules remove essential iron from bacteria to keep it from proliferating 4. Proteases and hydolytic enzymes digest bacterial proteins.

Opsonins

Like ketchup of phagocytosis. Placed on pathogens to make them more tasty to cell. They can be antibodies or breakdown product of complement system.

Fibroblasts

Long, spindle-shaped in extracellular matrix. Role in wound healing. Local tissue damage stimulates fibroblasts. Produces 2 substances to help extracellular matrix heal wounds: ground susbstance (GAG's) and fibers (reticular, collagen, elastin). Non-professional phagocyte at the end of inflammation during wound healing.

Endothelial Cells

Look like fried eggs. Line vessels and keep plasma inside. When they are disruped, underlying collagen is exposed which intiates inflammation. Damaged endothelial cells secrete prostaglandins (vasodilation) and adhesion molecules (extravasation).

Negative consequence of phagocytosis?

Neutrophils are messy eaters and can spill their ROS's into local tissue causing damage of healthy self-cells. Must be cleaned up by macrophages.

Granulocytes

Neutrophils, Eosinophils, and Basophils. Bacteria and resident macrophages near the site of infection release chemotactic signals. Neutrophils follow the signal, and migrate into tissues. They use their many tlr's to bind to pathogens. They perform phagocytosis, and the endosome fuses with lysosome for digestion.

Caveolae

No clathrin is used. Lipid rafts in endothelial cells, adipocytes. They contain protein, cholesterol, and are important in signal transduction and turnover of adhesive complexes. Important for endothelium where there's damage underneath and need to put out adhesion molecules. When we don't want them to be exposed on the exterior of cells this is how we bring those adhesion molecules back within the cell.

Oxygen-dependent killing

O2 consumption increases when cells use this method (respiratory burst). Reactive oxygen species (ROS) are toxic to pathogen and host so neutrophil dies quickly. 1st type of oxy-dependent production is superoxide. 2nd type uses enzyme myeloperoxidase. Uses H2O2 and chlorine to create hypochloride (bleach). Myeloperoxidase is why pus is green (contains heme pigment)

2 ways to digest endosomes in phagocytosis

Oxygen-dependent vs. Oxygen-independent Oxy-dependent uses NADPH to mediate the production of reactive oxygen species (H2O2, myeloperoxidase) Oxy-independent releases granules containing proteolytic enzymes (defensins, lysozyme, cationic proteins.

Macropinocytosis

Ruffle/invagination of cell membrane w/ large volume of extracellular fluid coming into cell. (non-selective) Fuses with lysosome for digestion. May facilitate antigen presentation. May be a means for pathogens to enter the cell. This would be like chugging a beer in her example of chugging beer vs sipping wine.

Monocytes/Macrophages

Similar TLR-mediated phagocytosis and intracellular killing as neutrophils. Arrive at infection in about 30 hours. Monocytes in blood, Macrophages in tissues. No granules, but they do have lysosomes. More efficient at respiratory bursts than neutrophils. Found in chronic inflammatory conditions. Also serve as antigen-presenting cells. Activated by helper T cells secreting IFN-gamm (interferon-gamma), and secrete pro-inflammatory cytokines for adaptive immunity. Role in tumor destruction by producing TNF-alpha, IFN-gamma, NO, ROS's, cationic proteins, and hydrolytic enzymes

Clathrin-mediated endocytosis (micropinocytosis)

Small vesicles containing protein. Vesicles form "clathrin-coated pits" with receptors for endocytosis of LDL, transferrin, growth factors, antibodies, and nutrient uptake. Used by most all nucleated cells. Uses receptors specific to molecules being internalized.

Phagocytosis

Specialized cell binds and engulfs particles (dust, debris, mircoorganisms, apoptotic bodies, etc) Receptor mediated. Produces membrane bound phagosome that fuses with lysosome in cytoplasm. Engulfment is facilitated by actin/myosin contraction. Can use opsonins (ketchup) to make the particles more tasty to the cell and encourage phagocytosis.

Caveolae (micropinocytosis)

Very small pits in membrane with no clathrin. Abundant in muscle, lung, fat, endothelium, and fibroblasts. Used for uptake of extracellular molecules, using receptors. Important in signal transduction. Used by some pathogens to gain entry into cell. Happens continuously.

Mast Cells

have TLR's and interact with dendritic cells, B cells, and T cells to mediate adaptive immune system. Can do antigen presentation but this isn't well understood. Can consume/kill gram - bacteria. Really good at processing fimbrial proteins on bacteria surface which help them stick to host tissues. Produce cytokines that induce inflammatory response which attract other phagocytes to site of infection.