Lecture 6 - The Development and Survival of Lymphocytes Flashcards
Are the early stages of B and T cell development similar?
YUP
Aim of early stages of B and T cell development?
- Ensure diversity of the antigen-binding receptor repertoire
- Purge autoreactive cells
What is a critical developmental checkpoint of B and T cells?
Expression of an antigen receptor on the cell surface
How is the strength of the signal of antigen binding important in lymphocyte development?
Very
- A strong signal with a T cell will cause the cell to undergo apoptosis
- Valency binding is important for B cells
When are autoreactive lymphocytes eliminated?
Before they become fully mature
What is clonal selection of B and T cells?
- B cells stay in the bone marrow while T cell progenitors migrate to the thymus
- A single progenitor cell gives rise to a large number of lymphocytes, each with a different specificity
- Removal of potentially self-reactive immature lymphocytes by clonal deletion
- Left with a pool of mature lymphocytes
- Proliferation and differentiation of activated specific lymphocytes to form a clone of effector cells to eliminate the antigen
How is antigen specificity of B and T cells maintained?
Maintained as the progeny proliferate and differentiate into effector cells
Origin of the progenitor cell that gives rise to lymphocytes?
Pluripotent stem cells
Mechanism of T cell migration from the bone marrow to the thymus? What does this also signal?
Notch signaling (transcription factor) via stromal cells
Also signals thymocytes to commit to the T-cell lineage
Where do the T cell progenitors become committed to becoming T cells?
Thymus
Effect of knock down of Notch transcription factor on lymphocyte development?
The progenitor cells that migrate to the thymus become B cells
Describe the steps necessary for the maturation and activation of B cells.
- Progenitor B cells in the bone marrow rearrange their immunoglobulin genes to generate immature B cells with cell surface IgM
- Self-reactive B cells are eliminated
- Surviving immature B cells emerge into the periphery and mature to express IgD as well as IgM => they can now be activated by encounter with their specific foreign antigen in a peripheral lymphoid organ
- Activated B cells proliferate, and differentiate into antibody-secreting plasma cells and long-lived memory cells
What is the generation of BCRs in the bone marrow dependent on?
Dependent on interactions with bone marrow stromal cells
How are self-reacting B cells eliminated?
Negative selection ONLY
Where do the B cells migrate during development?
Spleen first in the lymphoid follicles
Describe the steps of Ig gene rearrangement? Where are most cells lost?
- Early pro-B cell rearranges D-J on the heavy chain on both chromosomes
- Late pro-B cell rearranges V-DJ on the heavy chain on the first chromosome:
- if successful => heavy chain is paired with a surrogate light chain along with the Ig alpha and beta in the cytoplasm to be inserted in cell membrane
- if unsuccessful => rearrangement done on second chromosome => if also unsuccessful => cell loss - Several rounds of cell division
4a. Pre-B cell rearranges kappa gene on first chromosome of light chain
4b. If step 4a unsuccessful, pre-B cell rearranges kappa gene on second chromosome of light chain
4c. If step 4b unsuccessful, pre-B cell rearranges lambda gene on first chromosome of light chain
4d. If step 4c unsuccessful, pre-B cell rearranges lambda gene on second chromosome of light chain
4e. If step 4d unsuccessful => cell loss - Immature B cells formed have the same heavy chain and different light chains
The scope for repeated rearrangements is greater at the light-chain loci, so that fewer cells are lost between the pre-B and immature B-cell stages than in the pro-B to pre-B transition
How many late pro-B cells survive?
1 in 3
What fraction of heavy chain rearrangements give rise to a heavy chain?
4 in 9
How can some immature B cells escape negative selection in the bone marrow via apoptosis? Which cells can do this? What is the fate of these cells?
B cells responsive to multivalent self-molecule (strong BCR signal) => receptor editing: self-reactive receptor specificity is deleted as Rag genes associated with V-DJ recombination are reactivated so the cell can rearrange its light chain gene segments
Fate: if successful, B cells released from bone marrow
What happens to B cells that are responsive to soluble self-antigens in the bone marrow?
These cells exhibit low valency => low signal strength => rendered unresponsive to the antigen (anergic) and bear little surface IgM => migrate to the periphery, where they express IgD but remain anergic => if in competition with other B cells in the periphery, they are rapidly lost
What happens to B cells that are responsive to self-antigens with low-affinity without any cross-linking OR to self-antigens in low concentrations/not present in the blood in the bone marrow? What do we call these?
They do not receive any signal and mature normally => potentially self-reactive and are said to be clonally ignorant because their ligand is present but is unable to activate them
Can the bone marrow express all epitopes that the B cell maturing might experience?
NOPE
What signals immature B cells to travel to the spleen? What do we call them?
Immature B cells express the receptor S1PR1 that binds high concentrations of SIP that exist in the blood enticing them to leave the bood marrow and travel to the spleen
Transitional B cells in circulatio
What happens to self-responsive B cells in the spleen? 4 types.
- Responsive to multivalent self-molecule => strong BCR signal => apoptosis
- Responsive to a soluble self-molecule => rendered anergic => die within a few days due to being excluded from the B-cell follicles in the spleen
- Responsive with NO cross-linking => remain clonally ignorant of the self antigen and continue their maturation
- Response with low affinity => marginal zone B cells
What do the final stages of B cell maturation in the spleen involve? Where does this occur? What do they become after this?
- Upregulation of surface IgD
- Low level of signaling through BCR and antigens presented on follicular DCs
- Expression of BAFF, a TNF-family member, on FDCs => stimulates the BAFF-R on transitional B cells, promoting B-cell survival
Takes place in the B-cell follicles in the spleen
Become long-lived B cells
What surface proteins do newly emigrated transitional B cells (T1) exhibit? What do stage 2 transitional cells exhibit?
- Surface IgM
- Little IgD
- BAFF-R
- CD5
In the B-cell follicles, these cells upregulate CD21 to become transitional stage 2 B cells (T2)
What are the 2 different types of long-lived B cells? Which include most of them?
- ***Recirculating B cells
2. Marginal zone B-cells
Other name for recirculating B cells?
Follicular B cells
What happens to transitional T1 B cells that are excluded from the follicles?
They fail to receive maturation and survival signals and will die within 2–3 days of leaving the bone marrow
Describe marginal zone B cells. Purpose?
Weakly self-reactive and express very high levels of the complement receptor CD21 => migrate to the marginal zones of the splenic white pulp => poised to make rapid responses to blood-borne antigens or pathogens in the early adaptive immunity response (esp. encapsulated bacteria)
Where is the marginal zone of the spleen?
Area at the white pulp/red pulp junctions
Are follicular dendritic cells are the same are regulare DCs?
NOPE - these reside in the B cell follicles, do not undergo licensing and phagocytosis (only role is to present antigens)
What is CD21? What cells express it? Purpose?
Part of a trimolecular co-receptor for the BCR => makes the B cell more sensitive to signaling
T2 B cells express it
When first produced?
- B1 cells
- B2 cells
- Marginal zone B cells
- Fetus liver and omentum
- After birth
- After birth
Why do B1 cells have a restricted V-region repertoire?
- Because they are partly activated selfrenewing pool of lymphocytes that are selected by ubiquitous self and foreign antigens
- Because the cells are produced early in life
Where are B1 cells the major population of B cells? Why?
In certain body cavities (peritoneal, pleural), most probably because of exposure at these sites to antigens that drive B-1 cell proliferation
Non-templated nucleotide insertion (aka N-regions) at the VDJ junctions?
- B1 cells
- B2 cells
- Marginal zone B cells
- Few
- Extensive
- Yes
Primary location?
- B1 cells
- B2 cells
- Marginal zone B cells
- Body cavities
- Secondary lymphoid organs
- Spleen
Mode of renewal?
- B1 cells
- B2 cells
- Marginal zone B cells
- Self-renewing
- Replaced from bone-marrow
- Long-lived
Spontaneous production of Ig?
- B1 cells
- B2 cells
- Marginal zone B cells
- High
- Low
- Low
Ig isotypes secreted?
- B1 cells
- B2 cells
- Marginal zone B cells
- IgM»_space; IgG
- IgG > IgM
- IgM > IgG
Response to protein antigen?
- B1 cells
- B2 cells
- Marginal zone B cells
- Maybe
- Yes
- Yes
Do they have a requirement for T-cell help?
- B1 cells
- B2 cells
- Marginal zone B cells
- No
- Yes => T-dependent
- Sometimes
Somatic hypermutation?
- B1 cells
- B2 cells
- Marginal zone B cells
- Low to none
- High
- ?
Memory development?
- B1 cells
- B2 cells
- Marginal zone B cells
- No
- Yes
- ?
What antigens are marginal B cells responsive to?
Restricted repertoire that may be selected by a set of antigens similar to those that select B-1 cells
Which cells contribute much of the IgM that circulates in the blood?
B1 cells
How is the immune response of B1 cells and marginal B cells different than that of B2 cells? Why? What can they be compared to?
They carry out a more primitive, less adaptive, immune response than conventional B cells:
- Limited diversity of repertoire
- Propensity of these cells to react with common bacterial carbohydrate antigens
=> comparable to γ:δ T cells
Why is it important marginal B cells can respond to carb antigens?
Major component of membrane of encapsulated bacteria
What is the primary source of B cell differentiation before the bone marrow in the fetus?
Liver
Describe the 4 steps of T cell development in the thymus.
- T cell precursor rearranges its TCR genes => α:β T-cell receptors that have low-affinity binding with self-MHC molecules transmit a survival signal on interacting with thymic epithelium => positive selection of the cells that bear them
- Self-reactive receptors and those that have high-affinity binding (meaning it is binding a peptide in the groove of MHC) with self-MHC molecules transmit a signal that leads to cell death, and cells bearing them are removed from the repertoire = negative selection
- T cells that survive selection mature and leave the thymus to circulate in the periphery => they repeatedly leave the blood to migrate through the peripheral lymphoid organs, where they may encounter their specific foreign antigen and become activated
- Activated T cells => lose their ability to exit from the node and become activated to proliferate through clonal expansion and to differentiate into effector T cells => after several days, they regain the expression of receptors needed to exit from the node, leave via the efferent lymphatics, and enter the circulation in greatly increased numbers =>
a. Become an effector T cell => migrate into peripheral sites to eliminate infection (kill or activate macrophages) OR migrate to B-cell areas to activate the antibody response
b. Become a memory T cell
c. Become a follicular helper T cell
Where is the thymus located?
Midline of the body, above the heart
Thymus structure?
Several lobules, each of which contains discrete cortical (outer) and medullary (central) regions
What does the thymus cortex consist of?
- Immature thymocytes: outer ones are proliferating immature cells and deeper ones are mainly immature T cells undergoing thymic selection
- Branched cortical epithelial cells (APCs) with which the immature cortical thymocytes are closely associated
- Scattered macrophages, which are involved in clearing apoptotic thymocytes
What does the thymus medulla consist of?
- Mature thymocytes
- Medullary epithelial cells (APCs)
- Macrophages (APCs)
- Dendritic cells of bone marrow origin (APCs)
- Hassall’s corpuscles
Purpose of Hassall’s corpuscles?
Sites of cell destruction in the thymus medulla
Where does positive selection of T cells take place in the thymus?
Cortex
Where does negative selection of T cells take place in the thymus?
Medulla
List the 6 stages of α:β T-cell development. Describe these.
- Double-negative 1 (DN1): CD44+, CD25-
- Double-negative 2 (DN2): CD44+, CD25+> beta chain rearrangement
- Double-negative 3 (DN3) => pre-TCR receptor paired with surrogate alpha chain => cell membrane => beta chain rearrangement stops
- Double-negative 4 (DN4) => CD44-, CD25- => proliferation
- Double-positive => CD4+, CD8+ => rearrangement at the α-chain locus + CD3+
- Single-positive => TCR with either CD4+ OR CD8+
At what stage do T cells become irreversibly committed to the T cell lineage?
DN2
Surface markers on single-positive stage T cells?
Either CD4 or CD8
Describe the pre-TCR.
Beta is paired with a surrogate alpha chain
What are the steps of Ig gene rearrangement in B-cells comparable to?
Gene rearrangement to form the TCR
Describe the positive selection of T cells? What is their fate after that?
Random aka stochastic:
- Double-positive thymocytes attempt to bind with MHC I or II on cortical epithelial cells
2a. DP thymocytes that bind MHC II cease CD8 expression
2b. DP thymocytes that bind MHC I cease CD4 expression
Fate: single-positive thymocytes migrate to the thymic medulla
What is negative selection driven most efficiently by?
APCs (dendritic cells and macrophages) in the thymus
What happens to thymocytes that fail to bind to either MHC I or II?
Either apoptosis or are salvaged by receptor editing (rearranging the alpha chain)
Can receptor editing of thymocytes occur during both positive and negative selection? Why?
NOPE - only positive selection
Reason: if it were done during negative selection, it might compromise the ability of the TCR to recognize MHC
How does the thymus try to display every conceivable antigen in the body to maturing thymocytes to avoid self-reactivity? Explain how this works. What to note?
It expresses AIRE = autoimmune regulator: causes transcription of a wide selection of organ-specific genes that encode proteins that are usually only expressed in peripheral tissues
Note: this occurs in the medulla in a subset of epithelial-like cells
How do negative and positive selection differ?
The specificity/affinity of positive selection differs from that of negative selection:
- Low affinity: no selection => death by neglect
- Affinity sufficient to generate T-cell receptor-dependent survival signals => positive selection
- Excessively high affinity => negative selection by clonal deletion
Fate of T-cells that have an affinity for MHC:self-peptide complexes that is higher than normal but not enough to cause apoptosis? What is this called?
Central regulatory T cells => agonist selection
What determines whether a T cell will be α:β or γ:δ?
During the development of T cells in the thymus, double-negative (DN) thymocytes begin to rearrange the γ, δ, and β T-cell receptor loci simultaneously =>
- If a complete γ:δ T-cell receptor is formed before a successful β-chain gene rearrangement has led to the production of the pre-TCR, the thymocyte receives signals through the γ:δ receptor, which shuts off further rearrangement of the β-chain gene => strong activation of Erk => commits the cell to the γ:δ lineage
- If a functional β chain is formed before a complete γ:δ receptor, it pairs with the pTα to generate a pre-TCR => developing thymocyte receives a signal through the pre-TCR that shuts off rearrangements of the γ and δ loci (The δ locus is contained within the α locus so that α locus rearrangement eliminates the δ coding sequences) => weaker Erk activation => commitment to the α:β lineage
What are γ:δ T cells? What to note?
Unique and primitive cells in the epithelia of the mucosal immune system expressing a gamma-delta TCR and respond to self-molecules that signal potential danger or cellular stress
Note: various subsets of γ:δ T cells have shown antitumor and immunoregulatory activities
How does the rearrangement of TCR γ and δ genes proceed? What does this mean?
In waves of cells expressing different Vγ and Vδ gene segments and each wave is destined for a different location: skin, uterus, lungs, spleen, LNs
Means the cells have very low TCR specificity diversity because they express the same δ-chain gene
How are autoreactive cells that escape the thymus eliminated or controlled?
- Autoreactive T cells may be ignorant of their antigen (tissue-specific peptide-MHC [pMHC]) by being physically separated from it because naive T cells are excluded from nonlymphoid peripheral tissues aka sequestered
- In the absence of inflammation lymph node and spleen resident DCs (unlicenced) induce tolerance in naive autoreactive T cells but they will not display co-stimulatory molecules
- Autoreactive T cells rely on their own expression of counter-regulatory receptors such as CTLA-4 and PD-1 to develop and maintain, respectively, a state of functional unresponsiveness to peripheral self pMHC presentation
How many T cells leave the mouse thymus each day?
About 1-2 million
What do T-cells that have left the thymus require for survival? Where does this occur? Effect?
- IL-7 from lymphoid stromal cells
- Contact with self-peptide:MHC (weak binding) from resident DCs in T cell zones of secondary lymphoid tissues with low co-stimulatory potential
In the LNs
=> exit the lymph node through the cortical sinuses (lymphatics) to return to the circulation
How often do T-cells divide?
Infrequently
What happens to antigen-experienced T cells?
They naturally adopt an alternative pattern of circulation that allows steady state trafficking through most tissues of the body, preferential homing to local sites of inflammation, retention at sites of pMHC accumulation, and eventual return to the blood by way of the tissue-draining lymph
Mechanism of γ:δ T cell?
- Infection of epithelia signals the synthesis of stress-induced proteins
- Infected epithelial cell is induced to express MHC I-like molecules = MICs (stress protein) and TLs
- γ:δ T cells (with the CD8α:α homodimer) + NKCs (with NKG2D receptor) bind to MIC
- Infected cell dies by apoptosis (Fas pathway) and γ:δ T cell helps repair the wound by secreting cytokines and growth factor that prompt repair
- Dead cells are replaced by adjacent healthy cells and the damaged epithelium is repaired
Why are B1 cells considered part of the innate immune system?
Because they produce the majority of “natural” immunoglobulin M (IgM) and IgA, which is largely encoded by germline immunoglobulin genes
Origin of B1 cells?
- B cell precursor
OR - Distinct B cell precursor
Do γ:δ T cells recognize peptides?
NOPE
Are γ:δ T cells limited to using their TCR?
NOPE
Do γ:δ T cells express CD4, CD8, or CD28?
NOPE - but can express CD8αα
What are the γ:δ T cells receptors similar to?
NKC receptors
Are γ:δ T cells considered innate or adaptive?
Innate
How is the diversity of MHC different from that of Ig/TCR?
Ig and TCR generate their diversity differently by using multiple gene segments that rearrange and Ig has somatic mutation
MHC just has many alleles and types
