Wk 1 Immunology - Innate Flashcards
Hematopoietic immune cell lineages
Key differences b/w innate and adaptive immunity
What does complement recognize?
- direct recognition of bacteria
2.
What does complement cause once activated?
inflammation
pathogen destruction
What happens when innate immune cells recognize bacteria?
- Secretion of cytokines, chemokines, inflammation
- Innate effector functions: phagocytosis, degranulation, and killing of infected cells
What is complement?
Group of > 30 proteins found constitutively in plasma and interstitial fluid
-proteases in an inactive form
-become activated when exposed to pathogen surface directly or a pathogen surface bound by antibodies -> proteolytic cascade -> destruction of pathogens, innate cell function, inflammation
What cells typically produce cytokines and chemokines?
Cells of the innate immune system
What are cytokines?
Small proteins that control growth, function and/or activation of immune cells expressing the appropriate cytokine receptor
-constitutive or inducible
-innate and adaptive immune response
-communication b/w cells of immune system
What are chemokines?
A subset of cytokines that induce chemotaxis (cell movement toward high chemokine concentration)
-induce inflammation response by attracting immune cells to site of injury or infection
What are 3 key effector functions of the innate immune system?
- phagocytosis by macrophages, neutrophils and eosinophils
- degranulation by neutrophils, mast cells, basophils, eosinophils
- extraceullular DNA traps (release DNA, which is sticky to foreign antigens)
- direct cytotoxicity, NK cells release cytotoxic granules
What cells are important for anti-bacterial, anti-fungal and anti-parasitic activity?
neutrophils
macrophages (tissue only)
What cell is significant for being anti-viral?
NK cells
What cell types are anti-helminth?
eosinophils, basophils, and mast cells (tissue only)
-also allergic response induction
What is the role of dendritic cells?
- ingest foreign pathogens
- migrate to draining lymph nodes
- activate T cells in lymph nodes
Where are lymphocytes activated?
In secondary lymphoid organs - lymph nodes, spleen
What happens when CD4+ T cells are activated?
They differentiate into T helper cells (Th) and:
1. secrete cytokines
2. activate innate cells
3. activate B cells
4. anti-bacterial, -viral, fungal, parasitic
What happens when CD8+ T cells are activated?
They differentiate into cytotoxic T lymphocytes (CTL) and:
1. secrete cytokines
2. kill infected cells
3. anti-viral
Compare CD4+ and CD8+ T cell functions
How are B cells activated?
By exposure to free antigens - can be soluble proteins or surface antigens on viruses or bacteria or other pathogens
What happens when B cells are activated?
They can differentiate into plasma cells that secrete antibodies
What do antibodies secreted from plasma cells do?
- neutralize (prevent infection)
- activate innate cells
- activate complement
How do adaptive immune responses form memory?
Some activated T and B cells can persist for years as memory cells
They undergo robust and rapid re-activation upon 2nd exposure
-size and quality of adaptive response increases upon 2nd exposure
What are 2 aspects of the innate immune system that allows for immediate pathogen recognition?
- complement
- pattern recognition receptors
What do activated complement components induce?
- lysis of pathogens directly
- enhance **phagocytosis ** of pathogens by opsonization
- induce vasodilation to increase inflammation
How does complement enhance phagocytosis?
- complement proteins undergo autocleavage when recognize bacteria
- some cleavage products covalently attach to bacteria’s surface, acting as opsonins (enhance phagocyte’s abilitiy to recognize and phagocytose the bacteria) - neutrophils and macrophages have surface complement receptors for this to happen
- the opsonization recruits more complement components -> lysis of bacteria
- complement fragments can also act on local capillaries, induce vasodilation and enhance inflammation
Summarize complement activation induction
- direct recognition of pathogen surfaces
- recognition of pathogen surfaces bound by antibodies
4 ways complement mediates opsonization
- fragments (opsonins) coat bacteria
- opsonins are recognized by receptors on phagocytes such as neutrophils and macrophages
- opsonins enhance phagocytosis
- some types of antibodies are also opsonins
How do PRRs work?
Pathogens have unique structures that are recognized by the immune system: PAMPs
What are PAMPs?
=pathogen-associated molecular patterns
What are 9 PAMPs?
- LPS
- lipoproteins
- lipoteichoic acid
- peptidoglycan
- flagellin
- fungal sugars
- unmethylated CpG DNA
- dsRNA
- viral ssRNA (ie uncapped)
Where are PRRs found?
- cell surface
- cytoplasmic
- endosomal
What cells express PRRs?
All WBCs, sometimes other cell types
Are PRRs inherited?
germline encoded, broad specificity (ie. any LPS can trigger a PRR, not just a bacterial LPS)
What happens when PRR recognizes PAMP?
Induction of inflammatory cytokines and chemokines:
endothelial cell activation
vasodilation
inflammation
What is the result of activation of inflammatory cytokines and chemokines?
Transmigration of neutrophils from activated capillary into infected tissue, following the chemokine concentration
-other leukocytes (lymphocytes, monocytes, eosinophils, etc) use similar migration mechanisms but the chemokines may differ
What are the hallmarks of inflammation?
- Rubor = redness due to the increased blood perfusion from vasodilation
- Tumor = swelling due to increased plasma leakage into tissue due to vasodilation, leukocyte recruitment
- Calor = heat from increased vasodilation, perfusion, and metabolic activity due to leukocyte infiltrate
- Dolor = pain from the binding of inflammatory cytokines and granule components to nocireceptors, localized pain
What are 4 local effects of inflammation?
- activation of local immune cells
- activation and vasodilation of endothelial cells for leukocyte trafficking
- chemotaxis of leukocytes (esp. neutrophils) to pathogen site
- macrophages (tissue residents) are the early warning system, neutrophils are the first responders
5 systemic effects of inflammation
- prolonged cytokine production -> systemic availability
- increased neutrophils in circulation b/c of BM release
- increased hematopoiesis (left shift)
- fever, rashes due to vasodilation
- sepsis, septic shock
What are the 3 pathways of complement activation?
- Alternative
- Lectin
- Classical
What is the alternative pathway?
- Activated by
exposure to pathogen membranes ->activation of Factor D - First to act- immediate
*
What is the lectin pathway?
Delayed response:
* Activated by
mannose-binding
lectin (MBL)
* MBL produced by liver during systemic inflammation (24-48 hours after infection)
* MBL binds to pathogen surfaces -> activates MASP proteases
What is the classical pathway?
Late response:
* 1st to be discovered * Activated by
IgM and IgG
* 5-7 days after 1st exposure
* Immediately after 2nd exposure (pre- existing antibodies)
* antibody binding to pathogen surfaces activates C1 proteases
What is the fxn of C3 convertase?
Catalyzes cleavage of C3 to C3a and C3b
C3b - opsonizes pathogens (“bathes”)
C3a - activates inflammatory response
What is the fxn of C5 convertase?
Cleaves C5 to C5a and C5b
C5b binds to pathogen surface, creates platform for C6-9 to form MAC
What are 2 common opsonins?
C3b and IgG
What are the fxns of C3a and C5a?
regulate vasodilation, increase the permeability of small blood vessels, and induce contraction of smooth muscles
What is the role of Factor D?
Cleaves C3 into 2 fragments (C3a and C3b)
-C3b binds to bacterial surface
Factor D also cleaves Factor B to form Bb fragment
-C3b and Bb form a C3 convertase (C3bBb)
Fxn of MASP proteases
Cleave C2 and C4, inducing formation of a C3 convertase (C4bC2b)
-lectin pathway
What regulates complement?
- Decay acceleration by inhibitors
- enzymatic inactivation of C4b and C3b
What are LPSs also known as?
endotoxins
-they are a major PAMP expressed by Gram-negative bacteria
What recognize PAMPs?
Toll-like receptors (TLRs)
-need to know TLR4, which pairs with CD14
What do TLRs induce?
Activation of NFkB and cytokine production
LPS activating pathway
LPS -> TLR -> NFkB -> cytokine response
What cytokines do we need to know?
IL-1
IL-6
TNF
What 4 things do cytokines mediate?
- innate immunity and inflammation
- adaptive immunity
- cellular trafficking (chemokines)
- hematopoiesis
Effects of IL-6
Fever, induces acute-phase protein production by hepatocytes
TNF-alpha effects
Activates vascular endothelium, increases vascular permeability -> increased entry of complement and cells to tissues
Systemically -> fever, mobilization of metabolites, shock
Effects of IL-1beta
Activates vascular endothelium, lymphocytes, local tissues destruction, increases access of effector cells
systemically -> fever, production of IL-6
What happens with endothelial cell activation?
Increased leukocyte recruitment from the blood
Local vs systemic effects of inflammatory cytokines
What are 2 common tests for systemic inflammation?
- C-reactive protein (CRP) -
CRP and Fibrinogen are produced by the liver as part of the acute phase response. - Erythrocyte sedimentation rate (ESR) - * faster during systemic inflammation due to fibrinogen-
induced RBC coagulation
How is a fever caused?
IL-1 acts on the hypothalamus -> increased COX-1 & 2 -> conversion of arachidonic acid to prostaglandins -> fever
3 Fxns of prostaglandins
- act on temp control center of hypothalamus
- stim sympathetic system -> non-shivering thermogenesis to produce body heat
- induce vasoconstriction to decrease heat loss from body surface
What can TNF induce?
septic shock
What is the anti-viral cytokine we need to know?
IFNalpha/beta (Type I interferon)
-induced by viral PAMPs
-produced by most cells
-DCs can make a lot
activates Natural Killer cells
What are NK cells?
Natural Killer cells
-migrate from blood vessels and patrol tissues
-activated by IFNalpha/beta and IL-12
-directly kill infected cells (anti-viral)
-early source of IFN gamma, which enhances macrophage activation and can directly kill virally infected cells
What are 4 types of adhesion molecules?
- vascular addressin (CD34)
- selectin (L-selectin)
- integrin (LFA-1) = leukocyte adhesion factor-1 (binds ICAM) required by all - expression causes rolling of leukocyte to stop
- ICAM-1
What cells are the first responders?
Mostly neutrophils, some monocytes, which differentiate into tissue macrophages once they migrate into sites of inflammation
What are 3 specialized macrophages?
- Kupffer cells (liver)
- Osteoclasts (bone)
- Microglia (brain)
What are 6 features of phagolysosomes?
- acidification
- oxidative burst (NADPH oxidase)
- Nitrosative burst - expression of NO synthase, production of nitric oxide (free radical)
- Enzymatic degradation
- Defensins & other anti-microbial peptides
- Phagolysosome fxn enhanced in presence of interferon gamma
What are 5 enzymes involved in degradation?
- lysozyme -bacterial wall degradation
- MPO - acidification, free radicals
- proteases
- lipases
- nucleases
What is the oxidative burst w/in phagolysosomes?
enzymatic rxns involving superoxide dismutase to make hydrogen peroxide and NADPH oxidase to form superoxide
-catalase converts H2O2 (hydrogen peroxide) to H2O + O2
*many bacteria have their own form of catalase to counterbalance the effects of oxidative burst
What do do granules of activated neutrophils contain?
- Lysozyme
- Myeloperoxidase
- Reactive oxygen species
- Defensins
- Leukotrienes
- Histamines
Granule contents have anti-microbial functions (i.e. enzymes, ROS, defensins) and are highly vasoactive (histamine, leukotrienes)
Note: The toxic effects of oxidative burst and degranulation means that activated neutrophils typically die within 24 hours. “Pus” is mostly dead neutrophils.
* etc.
What is NETosis?
=neutrophil extracellular trap
- Release of DNA and histones from dying PMNs, but loss of chromatin structure
- Results in sticky matrix rich in antimicrobial effectors (i.e. defensins)
- Neutrophils die in the process (they only live 12-24 hours after activation anyway).
- Trap and kill bacteria even after neutrophil death
What are alternative pathway deficiencies associated with?
Susceptibility to Neisseria meningitidis infection (and Streptococcus pneumoniae and Haemophilus influenzae to a lesser extent). All encapsulated.
What is Factor H/I definiciency?
Loss of negative regulators of complement leads to constant over-activation of complement, over-consumption of C3 and impairment of normal complement function. Therefore, these deficiencies also lead to increased susceptibility to Neisseria meningitidis infection.
What are classical pathway deficiencies?
- Neisseria meningitidis
- Deficiencies in C1, C2, C4
Hereditary angioedema: HAE
- Hereditary angioedema (HAE) is caused by loss, depression or mis-functioning C1 inhibitor (C1 INH), leading to over-production of bradykinin.
- Rapid swelling of the hands, feet, limbs, face, intestinal tract, larynx or trachea, due to overproduction of bradykinin.
- Can become more severe in late childhood and adolescence.
Paroxysmal nocturnal hemoglobinuria
- PNH is a complement-induced intravascular hemolytic anemia (RBC), dark urine (especially in the morning) and thrombosis.
- The enzyme phosphatidylinositol glycan A (PIGA) is needed to make glycosylphosphatidylinositol (GPI) anchors and is encoded on the X chromosome
- CD59 and DAF are held to membrane with a glycolipid tail (GPI). Lack of PIGA leads to a lack of surface CD59 and DAF, over-activation of the MAC and periodic lysis of RBCs.
- Also heightened susceptibility to Neisseria meningitidis. Constant over-activation of complement leads to over-consumption of C3 and impairment of normal complement function.
- Acquired mutation, not inherited.
Paroxysmal nocturnal hemoglobinuria
- PNH is a complement-induced intravascular hemolytic anemia (RBC), dark urine (especially in the morning) and thrombosis.
- The enzyme phosphatidylinositol glycan A (PIGA) is needed to make glycosylphosphatidylinositol (GPI) anchors and is encoded on the X chromosome
- CD59 and DAF are held to membrane with a glycolipid tail (GPI). Lack of PIGA leads to a lack of surface CD59 and DAF, over-activation of the MAC and periodic lysis of RBCs.
- Also heightened susceptibility to Neisseria meningitidis. Constant over-activation of complement leads to over-consumption of C3 and impairment of normal complement function.
- Acquired mutation, not inherited.
Chronic Granulomatous Disease
-a redox defect - defective NADPH oxidase
Leukocyte adhesion deficiency
Loss of LFA-1->
* Failure of leukocytes to traffic into tissues
* Increased susceptibility to many infections
What are the unique enzymes for each complement pathway?
Factor D - alternative
MASP proteases - lectin
C1 proteases - classical
Unique components of each complement pathway
Factor D, Factor B - alternative
Mannose-binding lectin, C2, C4 - lectin
C1, C2, C4 - classical
What do all 3 complement pathways induce?
C3 convertase
What does C3 convertase do?
- catalyzes cleavage of C3 to C3a and C3b
- induces formation of C5 convertase
- induces generation of C5b
- recruits the MAC (C6-C9)
What are the 3 functions of complement?
- pathogen lysis
- opsonization
- inflammation
What 2 molecules activate endothelial cells and induce inflammation?
C3a and C5a
(cytokines do too, esp IL-1 and TNF)
What are 6 complement regulators?
- C1 INH (destabilizes C1)
- Factor H (prevent C3 convertase activity on host cell surface)
- Factor I (prevent C3 convertase activity on host cell surface)
- DAF (prevent C3 convertase activity on host cell surface)
- MCP (prevent C3 convertase activity on host cell surface)
- CD59 (inhibits MAC formation on host cell surface)
What chemokine do endothelial cells secrete?
IL-8
What is diapedesis?
The transmigration between tight junctions of firmly adherent neutrophils
