Immune Receptors and Signal Transduction Part II Flashcards
What does the BCR complex consists of?
The Ag receptor in association with two other polypeptides, Igα and Igβ (CD79a and CD79b)
What are Igα and Igβ?
They are signaling molecules for the BCR and are also required for assembly and expression of Ig receptor
What are the B cell co-receptors?
CD21, CD32, CD19
What do the B cell co-receptors associate with?
The BCR complex especially when both the BCR and one or more of the co-receptors are linked through an antigen-complement/antibody complex
Depending on which molecules are ligated, signaling by what is enhanced or inhibited?
The Ig-Igα/Igβ complex
Binding of the B lymphocyte to an Ag via receptor provides what signal?
Signal 1 which isd not sufficient enough and may lead to anergy
What are required for cell-cell interaction and the signal transduction events leading to activation? what signal does this provide?
Accessory and costimulatory molecules on the surface of B cells are required
This give signal 2
What is B cell signaling initiated through and results in?
Initiated through the Igα/Igβ complex associated with the BCR and results in phosphoylation of tyrosine motifs (ITAMs)
What is followed by the phosphoylation of tyrosine motifs of the BCR signaling cascade?
By an ordered activation of kinases and phosphatases
What is the BCR cascade modulated by?
Signals from co-receptors
What doe second messengers of BCR signaling lead to?
Activation of trascription factors followed by activation of the effector function
What is BCR?
A transmembrane form of an Ab molecule associated with two signaling chains
What Abs are on the surface of mature B cells that associate with the invariant Igα/Igβ molecules?
IgM and IgD
BCR complexes in class-switched B cells and membory B cells contain what?
Membrane immunoglobulins that maybe of the IgG, IgA,or IgE classes
How are Igα/Igβ linked to one another and to the BCR it associates with?
They are disulfide linked to one another and non-covalently associated with the BCR
What doe Igα/Igβ contain in their cytoplasmic tails?
ITAMs
What doe ITAMs mediate?
Signaling functions
What does Src-Syk activation lead to?
Leads to activation of PLCγ and Ras and Rac small GTPase
What is the activation of PLCγ, Ras, and Rac small GTPase followed by?
The activation of several transcription factors including NFAT, NF-kB, and AP-1
When PLCγ and Ras are activated, what do they do?
Turn on genes involved in cell growth, differentiation and survival
What does signal initiation by Ag occur by? How is it facilitated?
Cross-linking of the BCR and is facilitated by the coreceptor for the BCR
Ag binding facilitates a conformational change in BCR-associated ITAMs making the accessible to what?
Src kinases such as Lyn, Fyn, and Blk
How are Lyn, Fyn, and Blk linked?
By lipid anchors to the inside of the plasma membrane
What does cross-linking by multivalent Ags bring together and activate?
Src kinases which phosphorylate the ITAMs of Igα and IGβ
What do cross-linked BRCs enter?
Lipid rafts where many adaptors and signaling molecules are concentrated
What do phosphorylated ITAMs of Igα and Igβ provide?
A docking site for the tandem SH2 domains of the Syk tyrosine kinase
What do Lyn, Fyn, and Blk kinases activate?
Syk kinase-associated with ITAMs
Ag-dependent cross-linking of the BCR or the activation of BCR by a coreceptor-dependent mechanism results in what?
ITAM phosphorylationand recruitment of Syk to the ITAM
What does activated Syk phosphorylates?
critical tyrosine residues on BLNK (B cell linker protein) and other adaptor protein
Once BLNK and other adaptor proteins are activated what follows?
The recruitment of other enzymes which activate Ras and Rac, PLCγ2, and the Btk tyrosine kinase
Recruitment facilitates the activation of downstream effectors each generally contributing to the activation of a distinct signaling pathway
What are the distinct signaling pathways downstream of the BCR?
Ras-MAP kinase pathway
PLC pathway
PKC-β pathway
Describe the Ras MAP kinase activation pathway in BCR signaling
- GTP/GDP exchange factor SOS is recruited to BLNK
- Ras is then converted by SOS from an inactive GDP-bound form to an active GTP-bound form
- Ras activates Rac that may activate the MAP kinase pathway, once activated, Ras activates an enzymatic cascade of MAP kinases
What does the Ras MAP kinase activation result in?
Potent changes in the cell, such as the alteration of key proteins and changes in gene transcription
How is calcium signaling in B cells initiated?
- Ag recognition by the BCR activates different protein kinases, such as Lyn, BLNK, and Syk which activate PLCγ2
- PLCγ2 hydrolyzes membrane bound PIP2 into small amounts of DAG and IP3
- IP3 binds to its receptor IP3R located in the ER membrane and allows the release of stored calcium
To facilitate the extension of cellular responses, another route of entry of calcium through what?
Ca2+ release-activated Ca2+ channels (CRAC) is activated
What does DAG activate in the calcium signaling pathway in B cells?
Protein kinase C (PKC) and the Ras-MAP kinases which ultimately activate transcription factors AP-1 (a transcriptional complex formed by c-Fos and c-Jun) and NF-kB
Once the increase in intracytoplasmic Ca2+ levels is achieved, it activates what?
Calmodulin-calcineurin pathwya, with the final activation of NFAT
What do B cells express a complex of?
The CR2 complement receptor, CD19, and CD81
Microbial Ags opsonized by C3d can simultaneously engage what?
Both the CR2 molecule and BCR
What do the signaling cascades from both the BCR complex and the CR2 complex greatly enhance?
B cell activation as compared with the response to Ag alone
What is the role oof Cr2/CD21?
The complement coreceptor complex enhances activation of B cells
What does the cleavage of C3 results in?
The production of C3b that binds covalently to the microbe or Ag-Ab complex
What is C3b is further degraded into?
A fragment called C3d, which remains bound to the microbial surface or on the Ag-Ab complex
What do B cells express that is called the type 2 complement receptor?
a receptor for C3d
CR2 or CD21
The complex of C3d and Ag binds to B cells, what does the BCR and the CR2 recognize?
BCR: recognizing Ag
CR2: recognizing the micro-bound C3d
CR2 is expressed on mature B cells as a complex with two other membrane proteins called what?
CD19 and CD81 (also called TAPA-1)
What is the CR2-CD19-CD28 complex often called?
The B cell coreceptor complex because CR2 binds to Ags through attached C3d at the same time that BCR binds directly to the Ag
C3d binding to CR2 brings what in proximity to BCR-associated kinases?
CD19: this causes the cytoplamic tail of CD19 to become rapidly tyrosine phosphorylated
The recruitment of Lyn kinase can amplify what?
BCR signaling by greatly enhancing the phosphorylation of ITAM tyrosines in Igα and Igβ
What does phosphorylated CD19 also activate?
PI3-kinase, which in turn further augment signaling initiated by Ag binding to BCR
What is required for the activation of Btk and PLCγ2 because these enzymes must bind to PIP3 on the inner leaflet of the plasma membrane to be fully activated
PI3-kinase
What is the net result of coreceptor activation?
That the response of the Ag-stimulated B cell is greatly enhanced
The balance between stimulatory and inhibitory signals regulates what?
The functional responses of all cells
Activation of T and B lymphoyctes and NK cells is tightly controlled in order to limit what?
Immune responses against microbes in order to avoid collateral damage to host tissues
Also needs mechanisms that will prevent reactions against self antigens
Attenuation of signaling is essential to prevent what?
Uncontrolled inflammation and lymphoproliferation
Inhibitory signaling in lymphocytes is mediated primarily by what?
Inhibitory receptors and also by enzymes known as E3 ubiquitin ligases
Inhibitory receptors typically recruit and activate what?
phosphatases that encounter signaling events induced by Ag receptors
What do inhibitory receptors consists of?
It has an extracellular ligand-binding domain and a cytosolic ITIM motif
Involving inhibitory receptors, ligand binding results in what?
Phosphorylation of the ITIM tyrosine by a Src family kinase
Involving inhibitory receptors, the phosphorylated ITMs (P-ITMs) recruit what?
Recruit SH2 domain containing tyrosine phosphatase that can attenuate immune receptor signaling
The key inhibitory receptors in B cell include:
FcγRIIB and CD22
Define FcγRIIB
Is an important attenuator of singaling in activated B cells, DCs, and Mo
Define CD22
An inhibitory receptor on B cells only
The ITIMs of inhibitory receptors can be phosphorylated by what?
Src family kinases linked to lymphocyte activation
What do P-ITMs recruit?
SH2 domain that contains tyrosine phosphatases, such as SHP-1 and SHP-2, and an SH2 domain-containing inositol phosphatase callled SHIP
SHIP remove what?
Phosphate moieties from PIP3
Removing phosphate moieties from PIP3 results in what?
Inhibits PI3-kinase activity in lymphocytes, NK cells, and innate immune cells
What is the function of SHP-1 and SHP-2
Attenuate tyrosine kinase-initiated signaling from activating receptors in NK cells as well as from the BCR and TCR in B and T cells, respectively
Describe ubiquitin
A 76-amino acid protein that is transferred to lysine residues on specific substrates that are recognized by specific E3 ubiquitiin ligases
How can a polyubiquitin chain be generated?
By attachment of another ubiquitin molecules
The shape of the ubiquitin chain is important and depends on what?
On which lysine resiude on the preceding ubiquitin molecule in the chain is used
If lysine in position 48 of the first ibiquitin is used, what will be generated?
A lysine-48 type of ubiquitin chain will be generated and the protein will be targeted for degredation in the proteasome
Some E3 ligases generate a different type of polyubiquitin chain called a lysine-63 type: what does this type of chain do? How is this important?
I DOES NOT target proteins for degradation
Instead it generates a structure for latching the marked proteins onto other specific proteins
This mechanism is important in non-canonical NF=kB signaling
Describe canonical vs non-canonical NF-kB signaling pathways
Conical:Noncanonical
Rapid and Transient : Slow and persistent
Independent of protein synthesis : Dependent on p synth
Respond to numerous stimuli : Respond to a subset of TNFR signals
Diverse functions : Specific functions
What is the cononical pathway used for?
Inflammation, survival, proliferation
What is the noncanonical pathway used for?
Chemomkines, lymphoid stroma, survival
What does NF-kB form?
A family of transcription factors that participates in various biological process, including immune response and inflammation
What is the NF-kB family composed of?
Five members including: RelA (also named p65) Rel B c-Rel NFk-B1 p50 and NF-kB2 p52
What does the NF-kB family members form?
Various dimeric complexes that transactivate numerous target genes via binding to the kB enhancer
Where are NF-kB proteins normally sequestered in?
The cytoplasm by IkBα
What is the sequestering of NF-kB proteins triggered by?
Numerous signals, including those mediated by innate and adaptive immune receptors
What does canonical NF-kB signaling involve and results in?
It involves the activation of IKK complex by Tak1, IKK-mediated IkBα phosphorylation and subsequent degradation
Results in rapid and transient nuclear translocation of the protoypical NF-kB heterodimer RelA/p50
What does Non-canonical NF-kB signaling rely on?
Relies on phosphorylation-induced p100 processing, which is triggered by signaling from subset of TNFR members
What does non-canonical signaling depend on and what does it mediate?
Dependent on NIK and IKKα, but not on the trimeric IKK complex, and mediates the persisten activation of RelB/p52 complex
How are NF-kB1 and NF-kB2 translated?
Translated as precursor proteins, p105 and p100, which contain an IkB-like C-terminal portion and functions as NF-kB inhibitors
What does proteasome-mediated processing of p105 and p100 produce?
Not only does it produce the mature NF-kB1 and NF-kB2 proteins (p50 and p52) but also results in disruption of the IkB-like function of these precursor proteins
Cytokines IL-2, TGFβ + Naive CD4 T cell creates an induced T reg cell that has what effector functions?
Uses effector mediator IL-10 to regulate, and suppress inflammatory responses
Cytokines IL-6, TGFβ + Naive CD4 T cells creates a Th17 cell that has what effector functions?
Uses effector mediators IL-17A, IL-17F, and IL-22 to create inflammation
IL-4 + CD4 Tcell = Th2 cell + ______?
IL-4, IL-5, IL-13 for allergic and helminth responses
IL-6, IL21 + CD4 T cell = Tfh cell + ____?
IL-4, IL-21 for germinal centre help
Il-12, IFNγ + CD4 T cell = Th1 cell + ______?
IFNγ, TNF for macrophage activation, inflammation
IL-2 + CD4 T cell = Cytolytic CD4 T cell + ______?
Granzyme B, perforin, FASL for killing of infected cells
The antibody transmembrane molecules on B cells switch to what with the help of T helper cells: CD40L, and cytokine IFN-γ?
IgG subclasses (IgG1 and IgG3) for Fc receptor-dependent phagocyte responses; complement activation; neonatal immunity (placental transfer)
The antibody transmembrane molecules on B cells switch to what with the help of T helper cells: CD40L and cytokine IL-4?
IgE
Immunity against helminths
Mast cell degranulation (immediate hypersensitivity)
The antibody transmembrane molecules on B cells switch to what with the help of T helper cells: CD40L and cytokines produced in mucosal tissues (TGF-β, BAFF, others)?
IgA Mucosal immunity (transport of IgA through epithelia)
What do B cells express without T cells or cytokines present?
IgM for complement activation
Receptors for different cytokines are classified into families on the basis of what?
Conserved extracellular domain structures and signaling mechanisms
What are the 5 main classes of cytokine receptors
Type 1 cytokine (hemopoietin) receptors Type II cytokine receptors TNF receptor family IL-1 receptor family Seven transmembrane G-protein-coupled receptors
What do all cytokine receptors consist one or more of?
Transmembrane proteins
What are the extracellular domains of cytokine receptors responsible for?
Cytokine binding
What are the cytoplasmic tails of cytokine receptors responsible for?
For initiation of intracellular signaling pathways
What are the signaling pathways of cytokine receptors activated by?
Ligand-induced receptor clustering
What do cytokine binding and clustering activate?
Unique non-receptor tyrosine kinases
What is the classification of cytokine receptors based on/
On structural homologies of the extracellular cytokine-binding domains and shared intracellular signaling mechanisms
What do four out of the five receptors families include?
Single-pass transmembrane proteins whose extracellular moieties have complex mulit-domain structures and are all beta proteins (the secondary structure is composed entirely of β-sheets)
What does the Type I cytokine receptor family typically consists of?
Unique ligand-binding chains and one or more signal transducing chains, which are often shared by receptors for different cytokines
What do the chains of the Type I cytokine receptor family contain?
One or two domains with a conserved pair of cysteine residues
And a tryptophan-serine-X-tryptophan-serine (WSXWS) motif
**X is any aa
What does X aa determine in Type I cytokine receptor chains?
Determines the specificity for an individual cytokine
What can type I cytokine receptor family be divided into?
Subgroups based on structural homologies or the use of shared signaling polypeptides
What do ALL of the type I cytokine receptors engage?
JAK_STAT signaling pathways
What are the key players in the Type I and II cytokine receptor signaling?
The signal transducers and activators of transcription (STATS) and Janus kinases (JAKs)
What does JAKs phosphorylate?
Tyrosines on STATs which are latent transcription factors
After STATs are phosphorylated, what happens?
The STATS either homo- or heterodimerize and translocate to the nucleus to induce the expression of genes
What does the STAT1-STAT2 heterodimer complexes associate with?
What does this new complex bind to?
Associates with nuclear protein p48
This complex binds to gene promoters
What does the STAT1 and STAT3 homo- and heterodimers bind directly to?
The promoter sequence
During JAK-STAT signaling, what pathways are also activated?
MAPK pathways are also activated which phosphorylates serine residues of STAT1 and STAT3 and further enhancing their transcriptional activity
What are the Type II cytokine receptors also called?
Interferon receptor family
What do Type II cytokine receptors not contain?
Do not contain the WSXWS motifs but have two extracellular domains with conserved cysteines
What do Type II cytokine receptors consist of?
One ligand-binding polypeptide chain and one signal-trasducing chain
What do ALL of the type II cytokine receptors engage?
JAK-STAT signaling pathways
What does the type II cytokine receptor family include receptors for?
IFNα/β or, IFN-γ, IL-10, IL-20, IL-22
What is the TNF receptors a part of?
A large family of preformed trimers
What do TNF receptors have?
A conserved cysteine-rich extracellular domains and shared intracellular signaling mechanisms that typically stimulate gene expression
Signaling of some TNF receptors may lead to what?
apoptosis
What are the most important TNF receptors?
TNFRI and TNFRII, the CD40 protein, Fas, the TNF-β (lymphotoxin) receptor, and the B cell-activating factor (BAFF) receptor
The ligands for the TNF receptors form _____
trimers
Some of the ligands for TNF receptors are _______, whereas others are _______
Membrane bound
Soluble
What can TNF receptor signaling result in?
NF-kB and MAP kinase activation or in the induction of apoptosis
What does the ligation of the TNF-RI result in?
The recruitment of an adaptor protein TRADD which in turn can activate TRAF molecules (E3 ubiquitin ligases) and the RIP1 kinases
What are the downstream consequences of TNF receptor signaling?
Includes the activation of the NF-kB pathway and the JNK MAP kinase pathway or the induction of apoptotic death
What do the members of the IL-1 receptor family (TLRs) share?
A conserved cytosolic sequence, called the Toll-like/IL-1 receptor (TIR) domain
What does the engagement of the IL-1R or TLRs result in?
Results in receptor dimerization and the recruitment of one or more of four known TIR domain-containing adaptors
What do adaptors link TLRs/IL-1R to?
Different members of the IRAK (IL-1R-associated kinase) family
What do IRAKs linked to TLRs/IL-1R in turn link adaptors to?
to TRAF6, which is an E3 ubiquitin ligase required for NF-kB activation
What does IL-1R signaling include?
MAP kinase activation and the phosphorylation of IRF3 and IRF7, inducers of type I interferon transcription
What does the binding of chemokine to its receptor induce?
Many different cellular responses like: Integrin upregulation and activation Actin cytoskeleton reorganization Firm adhesion Cellular shape changes Migration Chemotaxis
What does the engagement of a GPCR by a chemoattractant results in what?
The activation of the associated G protein
What does the G protein dissociate into?
The GTP-bound Gα-subunit and the Gβγ-complex
In chemokine receptor signaling, what does the Gα-subunit inhibit?
Inhibits some adenylyl cyclase leading to a decrease of intracellular cAMP-levels and cAMP-dependent protein kinase activity
In chemokine receptor signaling, what does the βγ-complex activate?
Activates Ras that induces the activation of phophatidylinositol-3-kinase (PI3Kγ)
And phopholipase C (PLC)
What does PLC do in chemokine receptor signaling?
Hydrolyzes phophatidylinositol-bisphophate to generate inositol trisphosphate (IP3).
In chemokine receptor signaling, what does IP3 mobilize?
Mobilizes calcium from non-mitochondrial stores
What does diacylglycerol (DAG) activate in the chemokine receptor signaling cascade?
Activates Ca2+-independent and Ca2+-dependent protein kinase C
