Lecture 9 - Immune Receptors and Signal Transduction Flashcards
What is the structure and function of BCR
in signaling pathways?
L9 S3
Consists of:
- membrane bound IgM or IgD
- Igα/Igβ heterodimer
Igα/Igβ heterodimer is associated with membrane bound Ig molecule and act as signal transducer with ITAM domain
Binding of Ag to BCR provides signal 1 which is necessary, but not sufficient, for B cell activation
What are the coreceptors of the BCR?
L9 S4;11
CD19:
-signaling component
CD21 (complement receptor 2; CR2):
- Ag binding
- binds Ag bound C3d
CD81:
- links CD19 and CD21
- anchors complex to membrane
CD21:CD19:CD81 complex:
- positively regulates B cell activation
- lowers Ag threshold
- forms signal 2
CD32 (FcγRIIB):
- contains ITIM
- negatively regulates B cell activation
Describe the steps in the BCR cascade.
L9 S6-7
- Ag binds BCR causing conformational change in ITAMs of Igα/Igβ
- ITAMs are phosphorylated by membrane bound Src kinases (Lyn, Fyn, and Blk)
- phosphorylated ITAMs act as binding sit for SH2 domains of Syk tyrosine kinase
- Syk kinase is activated by Src kinases
- Syk kinase phosphorylates BLNK (B cell linker)
- BLNK recruits proteins that activate PLC-γ and ras/rac
- PLC-γ cleaves PIP2 to form IP3 and DAG
- DAG + calcium activates PKC/IKK (protein kinase C); PKC phosphorylates IκΒ targeting it for ubiquitination causing NFκB to migrate into nucleus
- IP3 + calcium activates calcinurin which activates NFAT (nuclear factor of activated T-cells)
-ras/rac cause phosphorylation cascade which activates AP-1 family transcription factor that migrate into the nucleus
What is the CR2 signaling pathway?
L9 S10-11
C3b binds antigen and is later degraded into C3d.
C3d is the ligand for CD21 (CR2) in CD21/19 complex.
Because the Ag can also be bound by BCR, when it is bound by CD21, CD19 is brought into close proximity to
- BCR associated kinases phosphorylated ITAMs of CD19 which activates PI3-kinase
- PI3-kinase activates Btk and PLCγ2
- B cell activation is greatly enhanced
What are the main inhibitory receptors of B cells and by what mechanism do they inhibit B cells?
L9 S16
FcγRIIB (CD32):
-also found on DCs and Mφ’s
CD22 (Sialic acid binding Ig like lectins; SIGLEC):
-unique to B cells
The inhibitors are activated by binding of ligand and their cytosolic ITIMs are phosphorylated.
- phosphorylated ITIMs bind SH2 domains of tyrosine phosphatases such as SHP and SHIP
- SHP and SHIP remove phosphates from PIP3 which inhibits PI3-kinase cascade
How does E3 ubiquitin ligase affect B cell signaling?
L9 S17
E3 ubiquitin ligase ubiquitinates lysine residue of certain substrates.
The ubiquitin chain can then be further ubiquitinated from one of two lysine residues in the ubiquitin molecule.
Lysine-48 Type ubiquitin chain:
-targets proteins for degradation by proteasome
Lysine-63 Type ubiquitin chain:
-proteins are not targeted for degradation
How do B cells interact with CD4+ helper T cells?
L9 S18
MHC class II: -peptide-MHC class II complex of B cell complexes with TCR:CD3:CD4 complex of helper T cell
CD28:CD80/86:
-CD28 of helper T cell binds CD80/86 of B cell; results in secretion of IL-4 (B cell activating cytokine)
CD40:CD40L:
-CD40 of B cell binds CD40L (CD154) of helper T cell stimulating IL-4R (IL-4 receptor) on B cell
What different pathways of naïve CD4+ helper T cell activation are there?
What cytokines stimulate each pathway, what cytokines are produced by each pathway, and what is the function of each pathway?
L9 S20
Treg cell:
- induced by IL-2 and TGFβ
- produces IL-10
- regulates and suppresses inflammation
Th17 cell:
- induced by IL-6 and TGFβ
- produces IL-17A/F and IL-22
- stimulates inflammation
Th2 cell:
- induced by IL-4
- produces IL-4, IL-5, and IL-13
- stimulates allergic and parasitic response
Tfh (follicle) cell:
- induced by IL-6 and IL-21
- produces IL-4 and IL-21
- germinal center of lymph node help
Th1 cell:
- induced by IL-12 and IFNγ
- produces IFNγ and TNF
- activates macrophages and stimulates inflammation
For each Ig isotype, are helper T cells needed for isotype switching and what cytokine stimulate this?
L9 S21
IgM:
-no helper T cell
IgG (IgG1 and IgG3):
-stimulated by IFN-γ from helper T cell
IgE:
-stimulated by IL-4 from helper T cell
IgA:
-simulated by cytokines in mucosal tissues (TGF-β and BAFF)
What are the different families of cytokine receptors?
L9 S23
Type I cytokine receptors
Type II cytokine receptors
TNF receptors
IL-1 receptor/TLR
G-protien-coupled receptors
What are the cytokines recognized by type I cytokine receptors?
What is the structure of the receptor?
By what mechanism do they generate a signal?
L9 S25
Cytokines:
-IL-2, IL-4, IL-7, IL-9, IL-15, and IL-21
Structure:
-transmembrane receptor that forms dimers upon cytokine binding
Signal via JAK-STAT pathway:
- cytokine receptors dimerize
- cytosolic, receptor bound JAK phosphorylate tyrosine on receptors
- cytosolic STAT binds phosphorylated receptor
- tyrosine on STAT is phosphorylated and STAT dissociates and dimerizes
- STAT dimers enter nucleus and bind promoter
What are the cytokines recognized by type II cytokine receptors?
What is the structure of the receptor?
By what mechanism do they generate a signal?
L9 S29
Cytokines:
-IFN-α/β/γ and IL-10
Structure:
-transmembrane dimer consisting of ligand binding and signal transducing chains
Signaling through JAK-STAT or JAK-STAT and Tyk-STAT pathways
What are the cytokines recognized by TNF family receptors?
What is the structure of the receptor?
By what mechanism do they generate a signal?
L9 S31-32
Cytokines:
-TNF-α/β, LT, CD40, and FasL
Structure:
-trimer containing extracellular cysteine-rich domains and intracellular death domains
Binding of ligand leads to ligation of receptor which I turn recruits TRADD (TNFRSF1-associated via death domain). TRADD activates TRAF which activates the JNK MAP kinase pathway and ubiquitination of IκB which leads to activation of AP-1 and IκB respectively.
What are the cytokines recognized by IL-1/TLR family receptors?
What is the structure of the receptor?
By what mechanism do they generate a signal?
L9 S34-35
Cytokines:
-IL-1 and IL-18 (possibles TLR-4 ligand, LPS?)
Structure:
-conserved cytosolic Toll-like/IL-1 receptor domain (TIR)
Activation of IL-1R/TLR causes dimerization of TIR domain with TIR domain of MyD88. MyD88 attracts IRAK (IL-1R-associated kinase) family proteins. IRAKs link TRAF6 which ubiquitinates IκB, activating NF-κB.
Also activates IRF3/7 and MAP kinase pathway that produces JNK and p38 (leads to AP-1)
What are the cytokines recognized by GPCR/chemokine receptor family?
What is the structure of the receptor?
By what mechanism do they generate a signal?
L9 S39-40
Cytokine:
-chemokines
Structure:
- 7 trans membrane domains
- cytosolic G-protein consisting of α,β, and γ subunits
Chemokine binding causes dissociation of G-protein into α-subunit and βγ-complex. α-subunit deactivates adenylyl cyclase reducing cAMP level and associated MAP activity.
βγ-complex activates PLC which cleaves IP2 into IP3 and DAG causing the release of calcium and activation of PKC. βγ-complex also activates Ras which activates PI3K
