Fall 1997 Immunology Exam #2 - Chapters 6 - 9
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HEV - high endothelial venules, special venules in secondary lymphoid organs that express special surface proteins that allow naive cells to bind and home to the secondary lymphoid organs (such as lymph noides). Lymphocytes will bind to and cross this endothelial lining to enter the lymph node.
selectin - this is a class of plasma membrane proteins that bind to select sugars on glycoproteins. One place you see selectins is on lymphocytes which helps initiate rolling in blood vessels which is a requirement for leaving the blood and entering a tissue.
linked recognition - activation of B cells by T cells that recognize the same antigen but not necessarily the same peptide/epitope.
IL-2 - after a T cell receives costimulation, the T cell begins to make IL-2, a cytokine, that stimulates cell division and then differentiation of the same T cell that made the IL-2.
Fas Ligand - a plasma membrane protein that is expressed by CD8+ cells that binds to Fas, which is expressed on every cell. This ligand can bind to Fas and initiate apoptosis, programed cell death, of the targeted cell.
perforin - secreted by CD8+ cells, it makes large holes in the plasma membrane of target cells which helps facilitate another way for CD8+ cells to kill other cells.
CD40 ligand - expressed by CD4+ cells, this plasma membrane protein binds to CD40 on B cells to induce the expression of the costimulatory molecule B7. This will enable the B cell to become an efficient APC. It can also induce isotype switching in B cells and macrophages to make TNF - a and to be more sensitive to activation.
C3 convertase - made of two subunits, this functional unit cleaves the complement protein C3 into C3a and C3b. C3b forms the basis for full complement activation while C3a is an inflammatory molecule.
diapedesis - directed movement of leukocytes through the wall
of a blood vessel. It is a part of a bigger process called extravasation
which includes the targeting of the tissue to be entered and homing
in on the site of infection.
1) Describe two ways in which a NK cell can recognize a cell that it will destroy.
NK cells can recognize their targets by 1) binding to antibodies via an Fc receptor. The antibody would be bound to the surface of cells to be destroyed; or 2) NK cells have selectin-like receptors (NKR - P1) that can bind to unusual glycoproteins (signs of infection or tumor formation). However, NK cells also have receptors (KIR) that can recognize MHCI moelcules which can prevent a NK cell from killing the potentially targeted cell. This second method uses two opposing receptors, one initiates killing while the second blocks the initiation of killing.
2) LIST the major steps involved in T cell receptor (a/b) formation. Start with germline DNA and include any other molecules (in addition to a and b) that arrive at the cell surface during this time and play a role in T cell development.
a) the progenitor T cell has no somatic recombination occur until after it reaches the thymus, then
b) D-J gene segment rearrangement of the beta gene for the T cell receptor (TCR)
c) V-DJ gene segment rearrangement of the beta gene
d) transcription and translation of the beta gene to form a variable (VDJ) and a constant region, recombination stops.
e) CD3, CD4, CD8, pTa, and the beta chain all arrive on the cell surface. A pre-T cell receptor is formed between the beta chain and the pTa.
f) if the step above is successful, a signal is transduced and these molecules are internalized and the alpha chain undergoes somatic recombination. V-J segments are joined (deleting the delta TCR gene). Both alleles of the alpha chain undergo recombination at the same time.
g) CD3, CD4, CD8, alpha and beta chains all arrive on the cell
3) Descrbie what happens during negative selection of T cells, what the cell that has survived negative selection looks like (list the surface markers), and why negative selection is a necessary step.
a) After positive selection (see below), a T cell has a TCR (alpha/beta) that is restricted to MHC alleles expressed by the thymic cortical epithelial cells. Negative selection is carried out predominantly by macrophages and dendritic cells of bone marrow origin. If a TCR binds to one of these APC that is presenting a peptide, the T cell will be stimulated to undergo apoptosis.
b) T cells that survive negative selection have all the surface markers of a mature, naive T cell. On the surface, one would find a TCR that is restricted to either MHCI ro MHCII, and either CD8 or CD4 respectively, and all the components of CD3. Since either CD4 or CD8 are expressed, these cells are called single positive thymocytes.
c) Negative selection is necessary to ensure that a TCR cannot
recognize a self-peptide which should be the only peptides
presented by these APC in the thymus.
4) Describe what happens during positive selection of T cells, what the cell that has survived positive selection looks like (list the surface markers), and why positive selection is a necessary step.
a) During positive selection, a T cell will have successfully arranged a beta chain and productively recombined one or two alpha chains. Therefore, this T cell will have one or two TCR on its surface. If one of these TCR can bind to the MHC molecules displayed on a thymic cortical epithelial cell, then this T cell will be given a signal that blocks apoptosis. Therefore, the only T cells to survive positive selection have been restricted to the organism's MHC haplotype.
b) These cells are called double positive in that they express both CD4 and CD8, as well as CD3, alpha and beta chains of a TCR.
c) This step is necessary to ensure that all of the T cells
express TCR that can recognize the MHC molecules expressed in
the organism. If the TCR is not able to bind to those MHC molecules,
the somatic recombination can continue for about 2 days which
gives each T cell multiple tries at producing a TCR that will
be functional in the organism.
5) If MHCaxb bone marrow were grafted into both MHCa and MHCb mice used for these experiments (all recipients of marrow were depleted of their marrow cells and cells derived from marrow), what would happen with the following skin grafts? Explain your answers.
haplotype of wild-type donor haplotype of recipient: recipients are bone marrow chimeras
a) MHCa MHCa
b) MHCa MHCb
c) MHCb MHCa
d) MHCb MHCb
In all cases, the skin grafts would be accepted as "self" because the APC cells used in the bone marrow grafts express both haplotypes. Therefore, both MHCa and MHCb molecules would be seen as self and since APCs do the negative selection, they would have deleted any T cells that had TCRs which bound to and recognized the MHC molecules. Skin cells of MHCa or MHCb would be seen as "self". (See figure 6.20).
6) Describe the steps involved in getting a leukocyte from a blood vessel into an inflamed tissue.
There are four major steps in this process called extravasation:
1) rolling - selectins binding to certain sugars on addressins, this allows the leukocyte to slow down and move along the wall of the vessel so that it has better chance to interact with additional surface proteins.
2) binding - cells stop moving and adhere tightly to endothelial cells via integrins and chemokine receptors.
3) diapedesis - the movement of a leukocyte through the vessel wall. It involves proteins at the tight junctions and on the leukocyte called PECAM.
4) the final step is migration of the leukocyte to the
site of infection. Chemokines are expressed in the area of infection
so the leukocyte follows the concentration gradient of chemokines
to the source of the infection.
7) List all the plasma membrane molecules on both APC and T cells which are involved in the initiation of clonal expansion of a naive T cell. Give a brief explanation of the role for each molecule you list. Finally, explain how this assits in self-tolerance.
T cell: CD3, CD4 or 8, alpha and beta chains of a TCR interact with
APC: MHC II or I with a peptide.
These molecules interact to provide the T cell with the first of two signals for activation. This ensures that the T cell is recognizing a particular peptide within the context of a self-MHC molecule.
T cell: CD28 interacts with
This second complex is the co-stimulatory mechanism that is required for activation of a naive T cell.
This need for co-stimulation prevents T cells that were not
killed during negative selection from being activated by self-peptides
in MHC molecules since only APC can express B7 and APC only express
B7 under limited conditions.
8) What are plasmablasts?
Where will they eventually settle?
What is the term associated with this specific tissue relocalization?
Plasmablasts are B cells that have been activated to become antibody secreting plasma cells. They are called plasmablasts after being in the germinal center and after affinity maturation.
They will eventually settle in either the bone marrow (if activated in lymph nodes) or under the skin and in the gut areas (if activated in Peyer's patches).
Plasmablasts will HOME to their proper location.
9) LIST 5 reasons (related to the immune system) why your children should be breast fed instead of formula fed.
There are many but some include:
providing antibodies between 3 months and 1 year when the infant has few circulating immunoglobulins.
milk contains agents that stimulate the growth of "friendly" bacteria in the gut
milk contains agents that inhibit the growth of other bacteria (by limiting the amount of iron)
milk contains lymphocytes that are primed for recognizing antigens in the mother's environment
milk contains lytic molecules like defensins that can kill
10) Describe where affinity maturation takes place, and how it happens.
Affinity maturation takes place in B cells within germinal centers of the secondary lymphoid organs. These B cells have already been activated by T helper cells and are able to undergo isotype switching as well. Affinity maturation is the somatic hypermutation that happens in rapidly dividing B cells such that point mutations occur frequently within the DNA that encodes the variable region of the antibody. Some B cells will form idiotypes that will have a higher affinity for the antigen, while others will form idiotypes with lower affinities for the antigen. The result is a mixed population with varying affinities for the original antigen. A competition of sorts follows such that those B cells with higher affinities can out compete those with lower affinity and bind to antigen displayed on follicular dendritic cells. The winners of this competition are stimulated to survive and divide (eventually forming plasma cells and memory cells) while the losers will die.
11) Where are IgA (list 3 places) and IgE (list two places) molecules found that do not have any antigen bound to them?
IgA - tears, milk, and in the gut.
IgE - on mast cells and in the tissue
12) Go to this URL <www.bio.davidson.edu/biology/courses/immunolgy/test2fig1.html>.
a) What is going on in this photo?
b) Which antibody isotype or isotypes are involved?
c) What leukocytes are involved in this process?
Eosinophils are attacking a large paratsite. The worm is coated
with IgE and eosinophils are bound to the parasite via their Fce
receptors. The eosinophils are secreting the contents of their
lysosomes to kill the parasite.
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