Lecture 15

Question 1

What is hematopoiesis?

Answer 1

Production of all immune cells

Question 2

What is lymphopoisesis?

Answer 2

Production of new lymphocytes

Question 3

what are hematopoietic stem cells?

Answer 3

common lymphoid progenitors

Question 4

what are the goals of lymphoid progenitor differentiation?

Answer 4

generate diverse BCR/TCR on circulating B and T cells, respond to wide range of pathogens
once receptor formed, remove those that are bad at binding/self bind

Question 5

Where does hematopoiesis occur?

Answer 5

fetal liver, usually populate mucosal & barrier tissues

Question 6

Where do hematopoietic stem cells migrate after birth?

Answer 6

bone marrow

Question 7

where does hematopoiesis occur after birth?

Answer 7

bone marrow

Question 8

What is a difference between human and mouse T cells? Similarity?

Answer 8

human T are functional at birth, mouse are immature and unfunctional at birth
both slow down with age

Question 9

Why does the location of hematopoiesis change?

Answer 9

fetal liver allows rapid expansion, but bone marrow is optimized for long-term stem cell maintenance, keeps stem cell at quiescence and renewal

Question 10

What is the life cycle of a B cell

Answer 10

opportunity for rearrangement to increase likelihood of expressing functional antigen receptor, checkpoints for requirement that each B cell expresses just one receptor specificity

Question 11

What is a stromal cell? What does it do?

Answer 11

bone-making osteoblasts and lipid-accumulating adipocytes
1. form specific adhesive contacts w developing lymphocytes (ligand-receptor)
2. provide soluble and membrane-bound cytokines and chemokines that control lymphocytes

Question 12

How do stromal cells help B cell development? Where are these stromal cells from?

Answer 12

bone marrow stromal cells
1. FLT3 – FLT3 ligand (tyrosine kinase) for differentiation, retained by CXCL12
2. IL-7, survival signal and adhesion molecule VCAM-1
3. CD117 (tyrosine kinase) binds to stem cell factor - proliferate

Question 13

Why is a B cell eliminated if it cannot make a functional heavy chain?

Answer 13

out of frame recombination, V gene segment repertoire contains pseudogenes, so about 45% of pro-B cells are lost

Question 14

How do FOX01 and PAX5 help B cell development?

Answer 14

FOX01 and E2A promote early B cell factor (EBF)
upregulated PAX5 commits to B cell lineage –> cannot become T cell anymore

Question 15

What is the difference between Early and Late Pro-B cells?

Answer 15

Early: display B cell surface markers (CD19, CD45R) but not yet VDJ gene rearrangement
Late: when VH and DJH joining occur

Question 16

What are large pre-B cell characteristics?

Answer 16

start to express pre-BCR, rearranged heavy chain, surrogate light chain

Question 17

What is the Quality Check for Large Pre-B cells? What are the two options after QC1?

Answer 17

heavy chain is correctly folded, which will trigger survival signals
if pre-BCR signaling is successful –> RAG gene turned off –> proliferation
if dysfunction in heavy chain –> apoptosis

Question 18

Why does Pre-BCR not need antigen?

Answer 18

self aggregation: pre-BCR can form dimers/clusters due to interactions between VpreB and λ5 domains
basal signaling: activates downstream survival pathways, ensuring only functional B continue developing

Question 19

What are characteristics of Small Pre-B cells?

Answer 19

right after proliferation ends, light chain gene rearrangement occurs –> formation of complete BCR

Question 20

What do immature B cells express?

Answer 20

surface IgM

Question 21

How do immature B cells become mature if they do not encounter antigen?

Answer 21

migrate from bone marrow to peripheral lymphoid tissues, become mature recirculating B cells bearing both IgM and IgD on surface

Question 22

What happens when immature B cells express receptors that recognize multivalent ligands? What are diseases associated with a problem with this?

Answer 22

receptors are eliminated from repertoire, receptor editing or apoptosis
called “central tolerance”
may lead to rheumatoid arthritis, lupus

Question 23

How do self-reactive B cells get rescued?

Answer 23

Receptor editing!! like dis:
B cell expresses antigen receptors strongly cross-linked by multivalent self antigens –> development arrested –> decreased IgM –> continued RAG synthesis –> continued light-chain gene rearrangement –> new productive rearrangement –> new light chain –> new receptor that might work

Question 24

What is peripheral tolerance? What antigens trigger this?

Answer 24

emigrated self-reactive B cells in periphery must be eliminated
some antigens tissue specific (ex thyroglobulin)

Question 25

What are the two fates of transitional B cells?

Answer 25

B that encounter a multivalent self antigen: strong BCR signal, cell death B that don't have strong self reaction: maturation, upregulated surface IgD (sIgD) --> in B-cell follicles of the spleen

Question 26

What necessary maturation and survival signals do transitional B cells get? Where?

Answer 26

in B cell follicles of spleen - expression of BAFF (B cell activating factor) from follicular dendritic cells to promote B cell survival - "weak" BCR signaling, low affinity self ligands

Question 27

How are follicular vs marginal zone B cells differentiated?

Answer 27

upregulated IgD --> follicular weak self-reaction and high levels of complement receptor CD21 --> marginal zone

Question 28

What are differences between follicular vs marginal B cells

Answer 28

follicular: circulate between secondary lymphoid organs, T-cell dependent activation, give rise to long-lived plasma and memory cells marginal: do not circulate in same way, rapid response to blood-borne pathogens, do not require T cell help, respond to polysaccharides/lipid

Question 29

How can immunity be sustained once the T cell repertoire is established?

Answer 29

do not need significant additional production, pool of peripheral T maintained by long-lived T cells and division of some mature T

Question 30

How do stromal cells provide signals to T cell development? Where?

Answer 30

in thymic stroma (epithelia network) there are many developing T-cell precursors embedded --> unique microenvironment for T-cell development

Question 31

How is the human thymus organized?

Answer 31

cortex: immature thymocytes, branched cortical epithelial cells, scattered macrophages (clear apoptotic thymocytes) medulla: mature thymocytes, medullary epithelial cells, macrophages + dendritic cells of bone marrow origin

Question 32

How does the thymus cortex "filter" precursors to turn into mature cells?

Answer 32

precursors arrive from bone marrow, apoptosis cells recognized/ingested by macrophages, screening for ability to recognize self peptide:self MHC complexes and for self-tolerance

Question 33

How do "double negative" thymocytes turn into differentiated T cell types?

Answer 33

changes in cell surface molecules. enter thymus as CD3-4-8-, majority give rise to αβ T, some γδ T

Question 34

How do αβ T cells differentiate?

Answer 34

double negative progress to double positive, co express CD4 and CD8 (so become CD3+αβ+4+8+) and generate either single positive CD4 or CD8

Question 35

How does Notch signaling induce T cell commitment in Pro-T cells?

Answer 35

triggers early thymic progenitor (ETP) committed to T lineage, induce transcription factors TCF1 and GATA3 --> initiate T lineage genes like CD3 and RAG1 Third TF (Bcl11b) required so that precursors do not adopt other fates Notch also induces IL7R expression

Question 36

How do small pre-T cells turn into large Pre-T cells?

Answer 36

rearrange β chain, paired with surrogate TCRα to express pre-TCRβ associated with CD3 signaling to stop rearrangement and rapid proliferation

Question 37

What are the double negative (DN) stages of T cells? What are the rearrangements of CD44/CD25 for eACh?

Answer 37

DN1: ETP, CD44+CD25- DN2: pro-T, CD44+CD25+ DN3: small pre-T, CD44lowCD25+ DN4: large pre-T, CD44-CD25-

Question 38

What is the movement process of each DN cell type?

Answer 38

1. DN1 enter thymus from blood stream near cortico-medullary junction 2. DN2 migrate to cortex 3. DN3 reside near subcapsular region where they proliferate 4. DN4 in area as progenitor matures further to CD4+CD8+ double positive stage

Question 39

What is the life span of a double positive cell? What can keep them alive>

Answer 39

3-4 days unless rescue through TCR engagement --> maturation into CD4/CD8 single positive cells

Question 40

How do double-positive T cells form?

Answer 40

end of proliferation --> RAG1/RAG2 re express --> every cell with a successfully rearranged β-chain gene gives rise to CD4+CD8+ --> once stop dividing, can rearrange α chain genes so a single functional β chain can be associated with many different α chains in progeny

Question 41

How does positive selection of T cells work?

Answer 41

only keep T cells that recognize self MHC ++ move into thymic cortex, interaction of TCR with MHC:peptide complex presented by thymic cortical epithelial cells decides ++ cell fate

Question 42

What are the fates of a double positive T cell in the thymic cortex?

Answer 42

if they can recognize MHCII and receive survival/maturation signals --> stop expressing CD8 but maintain CD4 --> ThPOK drive commitment to CD4 T cells If they can bind to MHCI and receive survival/maturation signals, stop CD4 but maintain CD8 --> Runx3 reinforces commitment to CD8 T cells If u cant bind to either u die. if u bind to both u die!!

Question 43

Where do mature single positive T cells migrate? What are the two fates there?

Answer 43

thymic medulla 1: if strong reactivity to self-antigen in medulla --> apoptosis (negative selection) 2: weaker level of self reactivity --> induced to alt lineages (Treg, NKT, MAIT)

Question 44

How do single positive (SP) T cells undergo negative selection?

Answer 44

interact with medullary (m) TECs and mDCs --> each SP thymocyte can interact w multiple mDCs that present self peptides

Question 45

What cells express Tissue-Restricted Antigens (TRAs)? What do TRAs do?

Answer 45

mDCs and mTECS promote expression of tissue-specific antigens in thymic medullary epithelial cells

Question 46

What drives TRA expression?

Answer 46

AIRE (autoimmune regulator) expressed by medullary stromal cells

Question 47

How is thymocyte emigration induced?

Answer 47

signaling through sphingosine 1-phosphate receptor, S1PR1 emigration requires recognition of lipid molecule S1P also express CD62L (L-selection): lymph node homing receptor