Unit 2 Flashcards
What are the two components of the B cell receptor complex?
- two molecules Igɑ and Igꞵ 2. associated with membrane Ig
What are the first three steps that begin the signaling cascade?
- Cross-linking of BCR complex OR B cell receptor + CR2 receptor 2. Phosphorylation of tyrosine residues on the ITAMs of Igɑ and Igꞵ 3. Triggers downstream signaling events
What are the co-receptors for B cells and T cells
Co-receptors for T cells are CD28 and CTLA4. Co-receptors for B cells are CR2 that bind to complement
How does ubiquitin regulate B cell activation?
a protein gets recruited to TCR complex and can ubiquitinize CD3, ZAP70, Igɑ, and Igꞵ
What T cell receptor is most like the B cell Ig ɑ and Ig ꞵ chains?
CD3 and zeta chains
B cells can respond to multivalent antigens via BCR crosslinking, or to antigen+ coreceptor binding to complement proteins. But can B cells respond to small monovalent antigens?
By themselves, they cannot respond to small monovalent antigens
What does ubiquitination mediate?
Signal inhibition and signal generation
What are the first step and end outcome of cytokine receptors, TNF and JAK-STAT?
the first step is cytokine receptors contain inactive JAK enzymes and the end outcome is to activate gene transcription. All these receptors bind to different cytokines and have specific responses
CD4 only binds to
MHC
T cell receptor binds to
MHC + antigen
2 antibodies cross-linking can activate
a B cell
The purpose of inhibition of immune receptor signaling is?
control and prevent overstimulation of producing antibodies
What helps form protein complexes?
adaptor proteins
What drives differentiation of B cells and T cells?
Cytokines
What are the five steps of signal transduction?
- Binding of receptor to ligand 2. Crosslinking/dimerization of receptors 3. Phosphorylation of ITAM motifs on cytoplasmic domains 4. Activation/recruitment of other proteins in a signaling cascade 5. Activate transcription factors which alter gene regulation to alter cell behavior
What is one example of how binding of a cytokine receptor might change the “behavior” of a cell through cell signaling?
binding of a TNF receptor to TNF alpha would cause a cascade of activated protein complexes leading to apoptosis
What are the steps of VDJ combination?
synapsis → cleavage → hairpin opening and end-processing → joining
What is the difference between heavy and light chains?
light chains have a V and a J whereas heavy light chains have a V, D, and J
What are all the things that contribute to binding site diversity?
Recombination between the V and J on a light chain and the recombination between the V, D, and J on a heavy chain
What are the similarities and differences between B and T cell receptor development (where does it happen? What is the same/different?)?
Between B cells and T cells, they undergo VDJ recombination in the same way but happen in different places. For B cells, it happens in the bone marrow. For T cells, they rearrange in the thymus. B cells rearrange their heavy chain first and then their light chain after the second selection process. For T cells, it starts with the ꞵ chain (VDJ) and followed by the ɑ chain (VJ)
What is the 12/23 rule of recombination?
A rule in which 12NT spacers can only join with 23NT spacers
At what stage of T and B cell development is foreign antigen binding necessary?
foreign antigen is not required for B and T cell development
What would be the result of a RAG1 knockout mouse?
no mature B or T cells
What are two similarities between B and T cell receptor development?
B and T lymphocytes both have 2 main checkpoints and are both generated by the rearrangement in individual lymphocytes of different VDJ (variable diversity joining) gene segments
What is V(D)J recombination?
a specialized process of site-specific gene rearrangement
What role(s) do cytokines play in B and T cell development?
Cytokines are involved in the proliferation and differentiation of antigen-stimulated T and B cells and in the effector functions (autocrine + paracrine actions) of T cells
How is diversity generated in B and T cells?
rearrangement and expression of antigen receptor genes
Describe the process of V(D)J recombination
involves rearrangement of one V gene segment, one D segment, and one J segment in each lymphocyte to form a single V(D)J exon that will code for the variable region of an antigen receptor protein
What are the consequences of no V(D)J recombination?
If there is no V(D)J recombination, there would be no functional B and T cells as all of them die. The immune system would have no adaptive immunity.
Order of Lymphocyte Development
Commitment → proliferation → sequential and ordered rearrangement of antigen receptor genes → selection events → differentiation of B and T cells
What molecules are key to triggering committed progenitor cells to proliferate?
cytokines
What three signals are required for fully functional precursor proliferation?
a signal through the cytokine, a signal through the pre B/T cell receptor, and activation of cytokines
What is negative selection?
if cells are too specific for cell antigens, then the cells are eliminated
What are nonamers?
tells enzymes where to bind on the genes
How does a VDJ form?
has repeating v’s, j’s, and d’s. Always end up with one V, one D, and one J that together form the variable region of the antigen. DJ (DJ starts the party) forms first followed by V
What is combinatorial diversity?
Random assortment of one V, one D, and one J
What is junctional diversity?
non homologous end joining joins the nucleotides mediated by terminal deoxynucleotidyl transferase (TdT) creating the same variable region that is different from any other cell because it is changed at DNA level
What are the components of a B cell and a T cell to form a single VDJ exon?
In B cells, the components are a light chain and heavy chain. In T cells, the components are an alpha chain and a beta chain
What recombination occurs to ensure one V, D, and J?
Recombination between VDJ and constant region
Compare and contrast TCR and BCR complexes (check w/ Dr. Mills Ko)
CD3 and ζ form the TCR complex whereas the Igɑ and Igꞵ form the BCR complex. Both complexes contain ITAMs in their cytoplasmic tails that mediate signaling functions
What are the early steps of complement pathway activation?
The alternative pathway is activated by C3b binding to various activating surfaces; the classical pathway is initiated by C1 binding to antigen-antibody complexes; the lectin pathway is activated by binding of a plasma lectin to microbes. C3b generated by action of C3 convertase binds to microbial cell surface and becomes a component of enzyme that cleaves C5 to initiate the late steps of complement activation
How can BCR complex activation demonstrate a link between innate and adaptive immune responses?
Activation of B cells is enhanced by signals that are provided by complement proteins and the CD21 coreceptor complex, which link innate immunity to the adaptive humoral immune response (p. 166 of txtbk)
What signaling pathways are downstream of B cell receptor activation?
Ras-MAP kinase, phosphatidylinositol-specific phospholipase C activation, PKC-ꞵ activation, and activation of P13-kinase (p. 168-169)
How do signaling pathways compare between naive and mature B cells? (check w/ Dr. Mills Ko)
The same signaling pathways are used by membrane IgM and IgD on both naive and mature B cells
Why does activation of lymphocytes need to be tightly controlled?
Activation of lymphocytes has to be tightly controlled to limit immune responses against foreign antigens in order to avoid collateral damage to host tissues (p. 169 of txtbk)
What mediates inhibitory signaling for lymphocytes?
inhibitory receptors and by enzymes known as E3 ubiquitin ligases that mark certain signaling molecules for degradation
Most cytokine receptors are activated by what property?
by ligand-induced receptor clustering, bringing together the cytoplasmic proteins of two or more receptor molecules, and thus inducing the activity of unique non-receptor tyrosine kinases (p. 170)
Discuss the downstream consequences of signaling mutations/dysfunction and their implications in immunological disorders
activation of the NF-ƙB pathway and the JNK MAP kinase pathway or the induction of apoptotic death
What are the consequences of no V(D)J recombination?
we would not have a huge diversity of antigen receptors for antigens to bind to, preventing many T and B lymphocytes from being matched and activated with a specific antigen
State the steps of VDJ Recombination
- DJ starts the party (randomly select one J & select one D) 2. Enzyme connects 2 pieces & a 12 spacer RSS on both sides 3. 23 is looking for a 12 on the V side
Random nucleotides stick at the bottom (like glue) to combine two blunt ends for
creating junctional diversity
What is purpose of VDJ?
increases diversity of different antigens we can recognize
Name the steps of lymphocyte development of B cells in order
- Rearrangement and expression of Ig genes in a precise order 2. Selection and proliferation of developing B cells at the preantigen receptor checkpoint 3. Selection of the mature B cell repertoire
Name the steps of lymphocyte development of T cells in order
Stem cell → Pro-T → Pre-T → Double positive → Single positive (immature T cell) → Mature T cell
What are some similarities and differences between B and T cell development?
In B cells, the heavy chain is rearranged while in T cells, the beta chain is rearranged. Also, in B cells, the light chain is rearranged while in T cells, the alpha chain is rearranged
How does V(D)J rearrangement connect to the checkpoints and steps of lymphocyte development?
First recombination is on heavy chain. VDJ joins. RNA processing connected to constant region and translates heavy chain protein. Checks against self-antigen and signal transductions tell us it worked triggering proliferation and recombination of light chain to make a fully functional receptor and see that it binds appropriately
What chains recombine first?
Heavy chains (B cells) & beta chains (T cells)
How does selection occur?
Through the expression of one chain. Double negative (no CD4 and CD8) becomes a double positive (has BOTH CD4 and CD8) by binding to either MHC I or MHC II
How does selection stimulate the next phase of recombination/proliferation?
In weak recognition (what we want), positive selection occurs stimulating cell survival. In strong recognition, negative selection occurs triggering apoptosis and delete any cell reactive B cells
How does the Pre-BCR regulate arrangement of the Ig genes?
- Rearranged heavy chain & a “placeholder” light chain
- Tells cell if 1st rearrangement is successful or not (if not, apoptosis; if yes, cell proliferates & light chain rearranges)
What is the difference between a Pre-B cell and an Immature B cell?
A Pre-B cell only has a rearranged heavy chain; an immature B cell has both a rearranged heavy chain and rearranged light chain
How does expression of co-receptors CD4 and CD8 influence T cell maturation?
CD4 & CD8 come as double negative and become double positive. T cell will pick either CD4 or CD8
What is the purpose of AIRE (autoimmune regulator) in lymphocyte maturation?
- Gene expressed in thymus cortical epithelial cells that present APCs to T cells
- Select out any T cells that bind to self-antigen
Compare and contrast the structure of the Pre-T Cell Receptor of immature T cells to the TCR of mature T cells
T cells get to decide what kinds of T cells they want to be. Environment created in thymus influences how T cells are developed
What does RAG expression do?
enzyme that does recombination (for B & T cells)
What is the purpose of checkpoints?
Checkpoints make sure recombinant events work and does not cause any autoimmunity
A weak binding
serves as a functional test to show that receptor works
What are the 2 checkpoints in the development of the B lymphocyte and what is their function?
After recombination, first checkpoint checks to see if the heavy chain binds to anything in large Pre-B cell. If heavy chain binds weakly, another checkpoint checks to see if light chain works and ensures weak binding worked in mature B
What are the 2 checkpoints in the development of the T lymphocyte and what is their function?
After recombination of beta chain, first checkpoint checks to see if recombination of beta chain works. If first checkpoint works, the T lymphocyte will proliferate and alpha chain recombines. Second checkpoint checks to see if alpha chain is still functioning and if it binds anything too strongly
If T cell receptor binds to MHC I and antigen (selection process),
signal transduction tells cells to become a CD8 T cells; don’t make any more CD4 T cells
If T cell receptor binds to MHC II,
signal transduction tells cells to become a CD4 T cells; don’t make any more CD8 T cells
If antigen does not bind to MHC I or II,
apoptosis is activated and cell dies
How does the process of T cell maturation happen in the thymus?
double negative → double positive → single positive (CD4) or single positive (CD8)
