4/5 Innate Immunity and Complement Flashcards
Innate immune system function
First line of defense: identify/ initiate inflammation, eliminate and repair, and activate and regulate adaptive immunity.
Characteristics of innate immunity
No memory or specificity, (no protective immunity) rapid response
3 barrier types of the innate immune system:
Anatomical barriers (skin and mucous membranes)
Physiological barriers: chemical-mediated, involves microflora and the enzymes they produce
Phagocytic barriers: cell-mediated, using macrophages and neutrophils
Chemical defensive barriers
Lysozyme, stomach acid, defensins (microbes can kill organisms like them who compete for resources) and lactoferring (prevents iron-scavenging of infectious bodies)
Types of phagocytic barriers
Monocytes (bloodstream) and macrophages (all connective tissue and organs)
Neutrophils (polymorphonuclear leukocytes/ PMNs)
Recognition of foreign particles
- Pattern recognition receptors
—> conserved, found on PHAGOCYTIC cells including antigen-presenting dendritic cells, recognize PAMPs and DAMPs
Function of phagocytosis
Uptake of particulate material from the local environment by specialized cells (Macrophages, neutrophils, and dendritic/ antigen-presenting cells)
—> engulfment and internalization of materials for their clearance and destruction
Steps of the phagocytic process
- Antigen binds to PRRs on membrane evaginations called pseudopodia
- Antigen is ingested, forming phagosome
- Phagosome fuses with lysosome
- Antigen killed and digested by low pH-activated lysosomal enzymes (lysozymes) which include reactive oxygen and nitrogen species
- Digestion products are released from the cell
What are PAMPs? Two examples:
Pathogen associated molecular patterns (epitopes not present on host cells). These target cells for clearance and shortens response time.
Examples: Viral RNA, or bacterial cell wall peptidoglycan
What are PRRs?
Pattern recognition receptors: found on phagocytic cells and recognize PAMPs and DAMPs
Four most common types of PRRs
Toll-Like Receptors (TLRs)
C-type lectin receptor (CLR)
RIG-I-like receptor (RLR)
NOD-like receptors (NLR)
Toll-Like Receptor; location on the cell, and to what types of molecules do they bind?
Found on surface and within cells (to recognize both extracellular and intracellular PAMPs)
Different TLRs recognize different PAMPs
(Extracellular: Gram + and - Bacteria, parasites, fungi, flagella)
(Intracellular: viral components and some bacterial DNA)
TLR activation pathways
Activates transcription factors to stimulate expression of cytokines, enzymes etc
C-type lectin receptors: location and recognition types
Extracellular pathogens (located on membrane) recognizes carbohydrate components of fungi, viruses, mycobacterium, parasites and allergens. Transcription factors activated promote expression of proinflammatory cytokines (IL-1B, TNF, IL-23)
NOD-like Receptors: cell location and primary targets
Located intracellularly, and recognize components of cells walls (muramic acid), NOD1 and NOD2 bind to these PAMPs, while NLRP-3 binds microbial products and DAMPs. Induces expression of antimicrobial proteins and peptides, and can initiate autophagy by forming autophagosomes that fuse with lysosomes to kill bacteria.
RIG-I-like receptors, cell location and primary targets
Work in the cytosol and recognize viral double-stranded RNAs, which is bound by the RLR helicase domain. Triggers the pathway to activated interferons to trigger antiviral interferon responses
Killing Mechanism of phagocytosis
Kill with enzymes: lysozyme and acid hydrolase’s to dissolve/ digest, lactoferrin and Vitamin B12-binding protein to sequester nutrients pathogen would need to grow, and defensins and cationic proteins to direct antimicrobials to site of infection.
Also use reactive oxygen and nitrogen species that are directly toxic to bacteria
When phagocytic cells encounter invading microbes, name one transcription factor that is activated
NF-kB
5 signs of inflammation, and what is responsible for each sign
Redness, pain, swelling, warmth (fever), loss of function
Vasodilation causes local edema (swelling) and increases the temperature, and causes redness
Bradykinin triggers release of prostaglandins which bind to free nerve endings, causing pain (and edema broadly)
Swelling can cut off mobility and function to a region
Step 1 of inflammation
Tissue damage and pathogen introduction begins the release of inflammatory mediators.
- Damage cells and local immune cells release chemokines that activate capillary dilation and attract more phagocytes
- Bradykinin is one of these chemokines and also causes cascade leading to swelling, rise in temperature, and pain
Stage 2 of Inflammation
Phagocytes normally roll casually down the vessel endothelium using L-selectins. Cytokines produced by macrophages lead epithelium to display E-selectins which bind to phagocyte’s integrin receptors, causing rolling to stop at “door” of infection. Phagocytes squeeze between endothelial cells into interstitial fluid (diapedesis). Phagocytes then migrate to the site of injury, moving along a chemokine gradient. When arrived, phagocytes destroy pathogen or cause of tissue damage, and removes dead and damaged host cells.
Margination
The process by which phagocytes stick to the lining of blood vessels at site of infection using E-selectins, as opposed to rolling casually using endothelial L-selectins
Diapedesis
Cytoskeleton of phagocytes reorganizes and flattens out along endothelium, allowing the phagocyte to squeeze through the endothelial cells into the interstitial fluid
Phagocytic extravasion
How phagocytes migrate to the site of injury. When in the interstitial fluid, they move along a chemokine gradient, like a hound dog following a stronger and stronger scent.
Step 3 of inflammation
Tissue repair: fibrin clot is formed to prevent spread of pathogen, dead and damaged cells are replaced, cytokines production and activity is turned off to prevent chronic inflammation.
Acute Phase response of inflammation
Increased synthesis/ secretion of antimicrobial proteins by the liver. These proteins activate other processes to help eliminate pathogens. APR is induced by proinflammatory cytokines (IL-1, TNF-a, IL-6)
Acute Phase Response Proteins
- Mannose binding lectin (MBL) responsible for complement activation
- C reactive protein (CRP) which promotes opsonization and complement activation
- components of complements
Natural Killer Cells
Lymphocytes that function within the INNATE immune system. Found in blood an lymphatic system, and kill cells that are struggling to produce self-proteins due to infection, malignant transformation, or other stressors
Natural Killer Cell recognition mechanism
Major histocompatibility complex 1 (MHC1) is expressed on all nucleated host cells. If they are not expressed to a certain density (due to infection or malignancy etc) the negative signal will not be stimulated (as cell has many positive signals that need to be canceled out by a certain density of these negative MHC1 signals) and NK cells will kill it.
How NK cells kill cells
- Kill the altered self-cell by releasing performing and granzymes to induce apoptosis
- Produce cytokines that induce adaptive responses against altered self-cell
Function of histamine in inflammation
Histamine causes capillary epithelial cells to open wider, leading to edema and increased extravasion
Prostaglandins function in inflammation
Prostaglandin promotes edema as well, and binds to free nerve endings causing pain
What is the complement system?
Group of ~30 serum proteins which circulate in inactive form and are activated to become major body defense system
Function of complement system
- Controls inflammatory reaction (through chemoattractants and promoting phagocytosis)
- Clear immune complexes (antibody+antigen) so doesn’t block the kidneys
- Activates other immune cells and antimicrobial defenses
- Can self-regulate/ degrade other complements to prevent chronic inflammatory response
Complement activating pathways
There are three: classical, alternative, and lectin pathway.
How is classical complement pathway activated
(Works in adaptive immunity)
IgM or IgG binds to a multitalented antigen, and the complement binds to the antibody, causing a conformational change in and activation of the complement. This activated complement and then trigger a cascade.
At the end of this cascade, C5 convertase is produced
how Lectin complement pathway is activated
Lectins bind to components of microbial cell surfaces, allowing lectin-associated serine proteases to dock there. These proteases can then cleave the first complement products and begin the cascade. Eventually this leads to C5 convertase
Alternative complement pathway activation
- Tickover pathway: small amounts of C3 are always produced/ cleaved, but will die if doesn’t interact with non-self epitope.
- Properdin: binds directly to pathogen-cell surface to begin complement cascade
- If clotting is initiated due to break in skin, thrombin is produced and thrombin can cleave complement components, leading to cascade. Platelet activation also releases components that maintain C3b in the fluid phase (free-floating)
What is a respiratory burst?
To produce the reactive oxygen and nitrogen species necessary for phagocytes to kill ‘swallowed’ microbes, there is a sharp uptake of oxygen by the sell, which NADPH converts into a superoxide ion/ radical.
When does a respiratory burst occur?
NADPH oxidase enzyme complex is activated when microbes bind to phagocytic receptors. This triggers a sharp increase in cellular oxygen uptake.
TNF-å is produced by what cells?
Monocytes, macrophages, dendritic cells, mast cells, NK cells, epithelial cells.
What is the role of TNF-å?
- Activates monocytes into macrophages
- Activates vascular endothelium to increase vascular permeability, fluid loss, and local blood clotting
- Induces acute phase response from the liver
- Induces fever response in the hypothalamus
- Cytotoxic for many tumor cells
What is opsonization?
Greek: to make tasty; it is antibodies or complements attaching to an antigen and promoting its phagocytosis
Which complement proteins serve as opsonins?
C3b, C1q, and MBL (mannose binding lectin)
Does activation of the transcription factor NF-kB lead to the expression of cytokines?
Yes,
What are two ways microbes evade the complement system?
- Microbial proteases destroy complement proteins
2. Some microbes mimic or bind complement regulatory proteins, down-regulating their effect
What are ways by which the complement system is regulated (turned off)?
- If C3b isn’t stabilized by properdin, it has a short half-life and will fall apart
- C1 (which is suppressed by immune response/ inflammatory components( is activated as the immune response dies down, and it furthers this process by blocking the classical and lectin pathways of complement activation
