Lec 05- Innate Immunity (Part 2) Flashcards
What are the 3 pathways of initiation of complement activation?
- alternative pathway
- classical pathway
- lectin pathway
What are the 3 effector junctions of the complement system?
- C3a: Inflammation
- C3b: Opsonization and phagocytosis
- C5a: Inflammation
What is the complement system?
consists of several plasma proteins that work together to:
- opsonize microbes
- promote recruitment of phagocytes to infection site
- directly kill the microbes (sometimes)
What is the 2nd step in the complement pathway?
production of C3b»_space; formation of C3 convertase
What happens when C3b initiates the activation of C5?
formation of C5 convertase
What is the last step in the complement pathway?
formation of membrane attack complex
What does the membrane attack complex do?
creates holes in plasma membranes
What kind of cascades does the complement activation involve?
proteolytic cascades
What does the proteolytic cascade do?
alters a zymogen (inactive precursor enzyme) to become an active protease
What does an active protease do in complement system activation?
cleaves the next complement protein in the proteolytic cascade
What is the result of enzymatic cascades in the complement system?
tremendous amplification of the amount of proteolytic products that are generated
What other medically important proteolytic cascades does the complement system work in cooperation with?
- blood coagulation pathways
- kinin-kallikrein system
What does the kinin-kallikrein system do?
regulates vascular permeability
What does C3 convertase do?
cleaves the central protein (C3) of the complement system into C3a and C3b
Which is a chemoattractant, C3a or C3b?
C3a
Which fragment (C3a or C3b) becomes covalently attached to the microbial surface?
C3b
Which fragment (C3a or C3b) serves as an opsonin to promote phagocytosis of the microbes?
C3b
What happens when the C3a fragment is released?
stimulates inflammation by acting as a chemoattractant for neutrophils
What does C5 convertase do?
cleaves C5 forming C5a and C5b which remains attached to the microbial cell membranes
C5a is a very potent chemoattactant that also ________
induces changes in the permeability of blood vessels
What does C5b initiate?
formation of membrane attack complex (MAC)
What is MAC made of?
Complement proteins: C6 C7 C8 C9
What does MAC do?
causes lysis of the cells where the complement is activated
What is the serum concentration of C3?
1000-1200
What is the function of C3b?
- binds to microbe surface to function as oposin
- component of C3 convertase
- component of C5 convertase
What is the function of C3a?
stimulates inflammation
What is the serum concentration of Factor B?
200
What is the function of Factor B?
Bb= serine protease
-the active enzyme of C3 and C5 convertases
What is the serum concentration of Factor D?
1-2
What is the function of Factor D?
Plasma serine protease
-cleaves factor B when it is bound to C3b
What is the serum concentration of Properdin?
25
What is the function of Properdin?
stabilizes C3 convertase (C2bBb) on microbial surfaces
What is the serum concentration of C4?
300-600
What is the function of C4a?
Stimulates inflammation
What is the function of C4b?
- covalently binds to surface of microbe or cell where antibody is bound and where complement is activated
- binds to C2 for cleavage by C1s
What is the serum concentration of C2?
20
What is the function of C2?
C2a = serine protease
active enzyme of C3 and C5 convertases
What is the serum concentration of Mannose-binding lectin (MBL)?
0.8 - 1
What is the function of MBL?
- initiates the lectin pathway
- binds to terminal mannose residues of microbial carbohydrates
What does an MBL-associated protease do?
activates C4 and C2, as in the classical pathway
What plasma protein does the classical pathway use?
C1q
What does C1q do?
detects antibodies bound to the surface of a microbe or other structure
What happens once C1q binds to the Fc portion of the antibodies?
C1r and C1s become activated
What do C1r and C1s do?
initiate a proteolytic cascade involving C2 and C4
What triggers the Lectin pathway?
Mannose-binding lectin (MBL)
What is MBL?
a plasma protein
What does MBL recognize?
terminal mannose residues on microbial glycoproteins and glycolipids
(similar to mannose receptor on phagocyte membranes)
What family does MBL belong to?
collectin family
What is MBL’s structure?
hexametric structure
similar to C1q component of complement system
What happens after MBL binds to microbes?
MASP1 and MASP2 are activated
What are MASP1 and MASP2?
mannose-associated serene protease
or
mannan-binding lectin-associated serene protease
What is the function of MASP1 and MASP2?
initiate downstream proteolytic steps
similar to C1r and C1s in classical pathway
What are the 3 homologous hexameric proteins?
C1q
MBL
Ficolin
What do the homologous hexametric proteins do?
initiate complement activation
What does C1q do?
- bonds to Fc regions of IgM
- activates the serene protease activity of C1r and C1s
What does MBL do?
- binds mannose on microbe surface
- activates MASP1 and MASP2 on the mannose-binding lectin
What does Ficolin do?
- binds N-acetylglucosamine on microbe surface
- activates MASP1 and MASP2 associated with Ficolin
What are collectins?
- a family of trimeric proteins with lectin head
- soluble effector molecules in the innate immune system
What are the 3 collecins?
- MBL
- SP-A (pulmonary surfactant protein)
- SP-D (pulmonary surfactant protein)
What is a soluble PRR that binds carbohydrates with terminal mannose and fructose to enhance phagocytosis of microbes?
MBL
What are low MBL levels associated with?
increased susceptibility to a variety of infections
Which collections have lipophilic properties?
SP-A
SP-D
What are the major functions of SP-A and SP-D?
- maintain ability of lungs to expand
- mediators of innate immune response
- act as oposins
- facilitate phagocytosis by alveolar macrophages
What do SP-A and SP-D bind to?
microorganisms
What do SP-A and SP-D directly do?
- inhibit bacterial growth
- activate macrophages
What are Ficolins?
- plasma proteins
- humoral molecules of the innate immune system
Ficolins are structurally similar to collectins except that they have a ________________
fibrinogen-type carbohydrate recognition domain
What have ficolins been shown to do?
bind several species of bacteria, opsonizing them and activating complement in a manner similar to MBL
How are ficolins similar to MBL?
- their involvement in an opsonin-dependent phagocytosis thus limiting the infection and concurrently orchestrating the subsequent adaptive clonal immune response
- initiate the lectin pathway of complement activation through MASPs
What triggers the start of the alternative pathway?
when C3 directly recognizes certain microbial surface structures (bacterial LPS)
Why is the alternative pathway not activated on the surface of mammalian cells?
mammals have a regulatory protein that inhibits the activation of the alternative pathway
What 3 things dissociate the complement C3 convertase (C3bBb) from the host cell?
- Decay-accelerating factor (DAF)
- Complement receptor 1 (CR1)
- Factor 1
What happens when C5b binds to C6 and C7?
forms C5b67 which binds to the membrane via C7
What does C8 bind to?
C8 binds to C5b67 and inserts into the cell membrane
What happens when 16 molecules of C9 bind?
They bind and polymerize to form a pore
Phagocytosis and intracellular killing steps
1- Microbes bind to phagocyte receptors
2- Phagocyte membrane zips up around microbe
3- Microbe ingested in phagosome
4- Fusion of phagosome with lysosome forms phagolysosomes
5- Activation of phagocyte
6- Killing of microbes
What are the 3 ways of killing microbes in phagosomes?
- ROS
- NO
- Lysosomal enzymes
What are macrophages activated by?
- microbial products (LPS)
- natural killer cell-derived IFN-y
- stimuli from T lymphocytes (CD40 ligand and INF-y)
What does macrophage activation lead to?
- activation of transcription factors
- transcription of various genes
- synthesis of proteins
Macrophages may also be alternatively-activated by other signals to promote ________
tissue repair and fibrosis
What is the principal cell source of TNF?
- macrophages
- T cells
What is the principal cell source of IL-1?
- macrophages
- endothelial cells
- some epithelial cells
What is the principal cell source of chemokines?
- macrophages
- dendritic cells
- endothelial cells
- T lymphocytes
- fibroblasts
- platelets
What is the principal cell source of IL-12?
- macrophages
- dendritic cells
What is the principal cell source of IFN-y?
- NK cells
- T lymphocytes
What is the principal cell source of Type I IFN-a?
- macrophages
- dendritic cells
What is the principal cell source of Type I IFN-b?
-fibroblasts
What is the principal cell source of IL-10?
- macrophages
- dendritic cells
- T cells
What is the principal cell source of IL-6?
- macrophages
- endothelial cells
- T cells
What is the principal cell source of IL-15?
- macrophages
- others
What is the principal cell source of IL-18?
-macrophages
What is the principal cell source of TGF-b?
-many cell types
What are the principal cellular targets and biological effects of TNF?
- Endothelial cells: activation (inflammation, coagulation)
- Neutrophils: activation
- Hypothalamus: fever
- Liver: synthesis of acute-phase
- Muscle, fat: catabolism
- Many cell types: apoptosis
What are the principal cellular targets and biological effects of IL-1?
- Endothelial cells: activation (inflammation, coagulation)
- Hypothalamus: fever
- Liver: synthesis of acute-phase
- T cells: Th17 differentiation
What are the principal cellular targets and biological effects of chemokines?
Leukocytes:
- increased integrin activity
- chemotaxis
- activation
What are the principal cellular targets and biological effects of IL-12?
- NK and T cells: IFN-y production, increased cytotoxic activity
- T cells: Th1 differentiation
What are the principal cellular targets and biological effects of Type I IFNs?
- All cells: antiviral state, increased class I MHC expression
- NK cells: activation
What are the principal cellular targets and biological effects of IL-10?
Macrophages and Dendritic cells:
- inhibition of IL-12 production
- reduced expression of costimulators
- reduced expression of class II MHC molecules
What are the principal cellular targets and biological effects of IL-6?
- Liver: synthesis of acute-phase
- B cells: proliferation of antibody producing cells
What are the principal cellular targets and biological effects of IL-15?
- NK cells: proliferation
- T cells: proliferation
What are the principal cellular targets and biological effects of IL-18?
-NK and T cells: IFN-y synthesis
What are the principal cellular targets and biological effects of TGF-b?
- inhibition of inflammation
- T cells: Th17 differentiation, differentiation of regulatory T cells
What is the major cytokine related to septic shock in patients?
TNF
Which cytokine is the major activator of fibroblasts which causes increased amounts of connective tissue thus causing scarring?
IL-18
All cytokines of innate immunity have macrophages as a principal cell source except which?
IFN-y
Which cytokine is the most important when it comes to inducing fever?
IL-1
What 3 cytokines have multiple local and systemic inflammatory effects?
TNF
IL-1
IL-6
What 2 cytokines act locally on leukocytes and endothelium to induce acute inflammation and induce the expression of IL-6?
TNF
IL-1
What 3 cytokines mediate protective systemic effects of inflammation including:
- induction of fever
- acute-phase protein synthesis
- increased production of leukocytes by bone marrow?
TNF
IL-1
IL-6
What 3 pathologic abnormalities can systemic TNF cause that lead to septic shock?
- decreased cardiac function
- thrombosis and capillary leak
- metabolic abnormalities due to insulin resistance
Plasma proteins that recognize microbial structures and participate in innate immunity belong to which family?
pentraxin family
What are the 2 major acute-phase proteins from the pentraxin family?
C-reactive protein (CRP)
Serum amyloid P (SAP)
How much can plasma concentrations of CRP and SAP increase to during infections and in response to inflammatory stimuli?
up to 1000 fold
How are CRP and SAP levels increased?
1- Innate immune response
2- Phagocytes are produced
3- IL-1 and IL-6 induce increased synthesis of CRP and SAP by the liver
What does CRP recognize?
phosphorylcholine
found on bacteria and apoptotic cells
What does SAP recognize?
phosphatidylethanolamine
found on bacteria and apoptotic cells
How can CRP and SAP activate complement?
by binding to C1q, initiating the classical pathway
What produces Type I IFNs?
virus-infected cells
What triggers the production of Type I IFNs?
response to:
- intracellular TLR signaling
- other sensors of viral RNA
What is the function of Type I IFNs?
- bind to receptors on neighboring uninfected cells
- activate JAK-STAT signaling pathway
What happens when JAK-STAT pathway is activated by Type I IFNs?
- induces expression of genes
- gene products interfere with viral replication
What happens when Type I IFNs also bind to receptors on infected cells?
induce expression of genes whose products enhance the cell’s susceptibility to CTL-mediated killing
What does Ag recognition by B cells and T lymphocytes provide?
provides signal 1 for the activation of lymphocytes
What provides signal 2 in the interface between Adaptive and Innate immunity?
molecules induced by innate immunity responses to microbes
