BLD434 Section 3 Flashcards
Immunology
Pre-BCR
Receptor on the pre-BCR cell that allows for signaling to cause burst of proliferation of pre-B cell
- Consists of rearranged mu heavy chain, surrogate light chain (VpreB and lambda-5), and Ig-a and Ig-B
Surrogate light chain
(On pre-B cells) Consists of VpreB and lambda-5
- Allows small amounts of IgM to make it to the surface of pre-B cell
RAG-1/RAG-2
RAG complex; Recognizes RSS and cuts dsDNA for somatic recombination
- Transiently expressed only in pro-B cells for heavy chain recombination and pre-B cells for light chain recombination
- Gets turned off after recombination is achieved
CAM
Cell Adhesion Molecule
Proteins on the cell surface involved in cell-cell adhesion and communication. Keeps the developing B cell in contact with the stromal cell.
PAX-5
Main B cell transcription factor that locks the pre-B cell into B lymphocyte development. Once PAX-5 is turned on,
BTK
Bruton’s Tyrosine Kinase
- An enzyme essential for B cell signaling and development. If mutated, B cells will be stalled in the pre-B cell stage (XLA)
Polyspecific antibody
Abs capable of binding to multiple antigens due to their broad specificity, which can occur in certain situations such as during immune responses to diverse pathogens
Negative Selection
The process by which autoreactive (self-reactive) lymphocytes are eliminated during development to prevent autoimmunity.
Clonal deletion
The removal of autoreactive lymphocytes via apoptosis during negative selection.
Occurs if the lymphocyte’s receptor is crosslinked by self-antigen and cannot remove the self-reactivity by receptor editing.
Clonal anergy
A state of functional unresponsiveness to Ag activation in lymphocytes induced by exposure to soluble self-antigen, preventing activation of autoreactive cells.
Anergic
Self-reactive lymphocytes bind to self-Ag become inactivated and unresponsive to their specific antigens; developmental arrest.
Self-tolerance
The immune system’s ability to recognize ‘self’ and not react or attack it.
LT
Leukotoxin - Produced by B cells to provide maintenance (life) signals for FDC
BAFF
B cell-Activating Factor - Produced by FDC to provide life-saving and maturation signals to B cell
Primary lymphoid follicle
Where immature B cells congregate to interact with FDC to gain life-saving and maturation signal. Cell will die if it doesn’t get the signal in 3 days.
Secondary lymphoid follicle
A structure within lymphoid organs containing germinal centers where T cells and B cells migrate and (Ag-mediated) B cell activation, proliferation, and differentiation occur.
Germinal center
A cluster of developing B and T cells in the secondary lymphoid follicle. Here, B cells undergo proliferation, somatic hypermutation, and isotype switching in response to Ag.
Plasma cell
Terminally differentiated B cells that produce and secrete large amounts of antibodies to combat infections.
Centroblast
Large rapidly proliferating B cells in the germinal center (dark zone)
Centrocyte
Matured B cells (centroblasts) that have developed the highest affinity for Ag, located in the light zone of germinal centers
- Have undergone somatic hypermutation and isotype switching
Stages of B cell development
1) Stem cell
2) Pro-B cell
3) Pre-B cell
4) Immature B cell
5) Naive mature B cell
Stem cell stage
Stage 1
Undifferentiated precursor cell capable of self-renewal and giving rise to multiple cell lineages. Expresses CD34 protein.
Pro-B cell stage
Stage 2
Earliest B lineage cell. Heavy chain somatic recombination, then Mu heavy chain is expressed. Expression of surrogate light chain.
Pre-B cell stage
Stage 3
Transient expression of pre-BCR. Rearranged Ig heavy chain, begins V-J rearrangement of light chain, expresses surrogate light chain (VpreB, 5) and Ig, and Ig proteins
Immature B cell
Stage 4
B cell expressing IgM (mu chain) on the surface. Has mature BCR.
Naive mature B cell stage
Stage 5 (final)
Fully mature B cell presenting IgM and IgD on the surface; ready to encounter Ag
Why do pre-B cells undergo a burst of proliferation after signaling through the pre-BCR?
Amplification of the population of pre-BCR with successful heavy chain rearrangements.
- Beneficial because they can skip mu heavy chain rearrangement and increases the chance of getting a viable B cell after light chain rearrangement
When are RAG-1 and RAG-2 expressed during B cell development?
During the early stages - during somatic recombination of Ig genes in pro-B (for heavy chains) and pre-B cells (for light chains)
What percent of developing B lymphocytes make it through heavy and light chain rearrangement of somatic recombination?
Heavy = 66%
Light = 85%
Why does Ig light chain rearrangement have a higher success rate than heavy chain?
- Smaller gene segment pool (only V and J), less cuts required for recombination
- Light chain has 4 chances for successful rearrangement, while the heavy chain only has 2
What CD markers are used to identify B-1 lymphocytes?
CD5 (B-1 only) and CD19/CD20 (all B cells)
B-1 cells
- produced in fetus
- self-renewal capabilities
- HIGH spontaneous Ig production
- IgM»_space; IgG
- Low/No somatic hypermutation
- respond to carbohydrate Ag
- doesn’t require help from T cells
B-2 cells
- Produced continuously after birth
- No self-renewal – replaced from bone marrow
- LOW spontaneous Ig production
- IgG > IgM
- High somatic hypermutation
- require activation for Ab secretion
- Respond to carbohydrate and protein Ag
- Requires help from T cells
Receptor editing
B cells that express self-reactivity can undergo successive light chain gene rearrangements to edit their light chains to avoid negative selection.
B lymphocyte half-life after it exits the bone marrow and enters circulation
100 days
Path of B lymphocyte from entry to exit of a lymph node (assuming it’s not stimulated by Ag in the LN)
B cell enters via afferent lymph vessel, then through the HEV valve into the T cell area (primary follicle), then primary follicle in B cell area, then leaves via efferent lymph vessel
Follicular Dendritic Cells (FDC)
Located in the primary lymph follicle where they secrete BAFF to induce a survival/maturation signal to naive B lymphocyte.
What interaction triggers isotype switching and somatic hypermutation?
T cell interaction where Helper T cells present Ag to a B cell to stimulate it to mature
Plasma cell (function & 4 locations)
To produce and secrete large amounts of Ab
May reside in red pulp of spleen, bone marrow, lymph node medulla, GALT
3 main options for what a B lymphocyte can do functionally after Ag activation
- Antigen-activated B lymphoblast (alternative splicing to secrete Ig. Isotype switching and somatic hypermutation)
- Plasma cell secreting IgM or IgG (fighting the current infection)
- Memory cell (preparing for future infection)
Stages of thymocytes
Double-negative thymocyte (Pro-T cell)
- missing CD4 and CD8
Double-positive thymocyte (Pre-T cell)
- express both CD4 and CD8
Single-positive T cell (Mature T cell)
- express CD4 or CD8
pTa
surrogate TCR alpha chain (part of the Pre-TCR)
What triggers commitment of uncommitted lymphoid progenitor to the T lymphocyte lineage and where does this interaction occur?
Notch 1 receptor on LP binding to delta-1 ligand in the cortex of the thymus.
Medulla of the thymus
inner part of the thymus lobule (lighter areas) – medullary epithelial cells, macrophages, dendritic cells, (and mature thymocytes).
thymic involution
Thymic involution is the gradual decline in thymus (TECs) and function that occurs as early as age 1.
Cortex of the thymus
the outer part of the thymus lobule (darker area) – cortical epithelial cells, thymocytes (developing T cells), macrophages.
List the proteins that compose the pre-TCR
TCR-beta, pTa, CD3 complex, zeta chain, and CD and CD8
Describe somatic recombination of the TCR gene loci during thymocyte development
- B, G, D TCR simultaneously rearrange, then it’s a race:
- If TCR G, D recomb. first, then it commits to GD T cell lineage.
- If TCR B recomb. first, then burst of prolif. so the daughter cells do a second round of somatic recomb. of the a, G, D genes.
3a. Step 2 can happen
3b. If TCRa undergoes somatic
recomb., then T cell commits to aB T cell lineage and expresses aBTCR, CD4 and CD8
Normal proportional of aB to GD T cells produced after birth
90% are alpha-beta T cells
What cells undergo positive vs. negative selection?
Positive selection = aB T cells
Negative selection = B cells and aB T cells
***GD T cells DON’T undergo any selection
Positive selection in aB T cells
(in thymus) Testing whether the TCR can bind to our own MHC and interact with MHC
- Determines if it will be CD4+ or CD8+
- Carried out by cortical epithelial cells
Negative selection: B cells vs aB T cells
B – To make sure they aren’t self-reactive
aB T – To remove the self-reactive cells (Will kill any thymocyte that bind with moderate to high affinity to our own MHC I or II expressed on Thymic DC, which is what carries this selection out)
Function of dendritic cells in the thymus
- express naive and memory cells
- present Ag to T cells
- promote T-cell tolerance
Macrophages
- eat T cells that don’t pass the positive selection
in the cortex - can only activate memory cells
- present Ag to T cells at a lower level
AIRE
Autoimmune Regulatory Protein
- Expressed primarily in medullary thymic epithelial cells
- Turns on low level of expression of every gene in the thymus so they can be presented in self MHC for negative selection
Strength of TCR binding in selection
In positive selection - want affinity to be low
In negative selection - if there is high affinity for self-Ag (MHC), the cell will undergo apoptosis.
Central Tolerance
Deletion of self-reactive T lymphocytes in the thymus by negative selection; induced by apoptosis.
Peripheral Tolerance
Control of T lymphocyte activation in the periphery; induced by anergy or by control of Treg cells
Treg
Regulatory T cells, of the CD4 subset.
Suppress the T activation of CD4+ cells in the periphery by self-Ag to make sure there’s no self-tissue attacking.
Allow for peripheral tolerance.
Two-signal hypothesis
- TCR binding to MHC+peptide
- B7 binds to CD28 on naive CD4+ or CD8+
CTLA-4
Cytotoxic T-Lymphocyte Associated-protein 4
Receptor on Treg that binds to B7 with 20-fold strength of CD28 (blocks CD28 from binding).
This inhibits the activation and proliferation of T cells, leads to apoptosis of effector cell
2 ways foreign Ag can reach the secondary lymphoid tissues to activate an adaptive immune response
- Being carried by DC
- Through lymph fluid
Selectins and Integrins
selectins provide low affinity adhesion (cell rolling on HEV)
integrins provide high-affinity adhesion between cells (firm attachment to HEV and diapedesis)
3 functions of leukocyte adhesion molecules prior to naive T cell activation by APC
- Naive T cell rolls through secondary lymphoid tissue venules
- Tight binding and extravasation by diapedesis
- T cell and APC interaction in secondary lymphoid tissues (adhesion) in the LN
Importance of IL-2 in T cell responses and how do immunosuppressive drugs (Cyclosporin A and Rapamycin) short-circuit this signal?
IL-2 is an autocrine growth factor secreted by Th1 cells.
What 3 factors can influence the choice of a naive CD4 T cell to a functional subset choice and which is dominant?
- (Dominant) Tissue origin of activating DC (mucosal tissue, skin tissue, etc.)
- Cytokines induced by innate response to infection
- Types of infectious agents that will activate pattern recognition receptors
Th1
T-bet (transcription factor) activates secretion of IL-2, IFN-Y, and LT
Th2
GATA3 (transcription factor) activates the secretion of IL-4 and IL-5
Th17
RORYT (transcription factor) activates the secretion of IL-17 and IL-22
TFH
Bcel-6 (transcription factor) activates the secretion of IL-21
Treg
FoxP3 (transcription factor) activates the secretion of TGF-B and IL-10
What cytokine is needed in high concentration to activate a naive CD8+ T cell to a Tc? What aid the APC in this activation process?
IL-2 and MHC
After activation in the secondary lymphoid tissue, where do TFH, Th1, Th2, and Tc reside?
TFH - germinal center
Th1, Th2, Tc - body tissues (infection site)
Adhesion (homing) receptors on effector T vs naive T lymphocytes
Effector T cells
- Lose L-selectin expression
- Gain VLA-4 to home to inflamed BV endothelium
CD40 ligand function (related to effector T cell function)
CD40L on a TFH binds to CD40 on the B cell responding to Ag, activating the B cell
IL-2
Cytokine secreted by Tc and Th1
- Keeps T cells active
IL-4
Cytokine secreted by Th2 and TFH
- Isotype switch factor for IgE production
- Autocrine growth factor for Th2 to keep them active
IL-5
Cytokine secreted by Th2
- Eosinophil growth factor
IL-10 & TGF-B
Cytokine secreted by Th2 and Treg
- Suppress activation of Th1, Th2, Th17
- Prevent autoimmune diseases and allergies to harmless environmental Ag
IL-21
Cytokine secreted by Th17 and TFH
- Autocrine support of effector function
- Recruit TFH cells to germinal center
IFN-Y
Cytokine secreted by Tc, Th1, TFH
- Activate macrophages & carry out cell-mediated immunity
GM-CSF
Cytokine secreted by GM-CSF
- Stimulate WBC production (neutrophils, eosinophils, basophils)
- Increases myelopoiesis
TNF-a
Inflammatory cytokine secreted by Th1
- Macrophage activation, viral infections, cellular cytotoxicity
LT
Cytokine secreted by Tc and Th1
- Activate macrophages & carry out cell-mediated immunity
Linked recognition
T-dependent antibody responses require the activation of B cells by helper T cells that respond to the same antigen.
Goal = make sure the T cell help provided during an immune response to a pathogen is directed at antigen-specific B cells responding to that same pathogen
- Cells involved: TFH, MHC II, Naive B cell, CD40L, and CD40
- If process fails, the B cell will not undergo isotype switching or somatic hypermutation
B cell co-receptor complex
CD19 (transduce signal from CD21)
CD21 (bind to opsonized C3d, activates complex)
CD81 (adapter protein that holds the B cell co-receptor complex to the BCR)
T-dependent B lymphocyte Ab response
- majority of Ab from B cells are T-dependent
- produces IgG (high affinity Ab)
- Ag-specific memory response
T-independent B lymphocyte Ab response
