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