Deibel- B cell and T cell maturation Flashcards by Sarah Carlson

Where does B cell development occur in adults? Does it require contact with an antigen?

Bone marrow

Does NOT require contact w/ an antigen

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Describe B cell generation in the bone marrow.

B cells rearrange their genes for H or L chains and synthesize cell surface IgM which acts as their initial BCR.

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What are the components of the BCR complex?

Membrane bound mIgM

Signaling chains Ig alpha and Ig beta (CD79a,b,)

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Where do B cells go after development in the bone marrow?

Enter the periphery where they continue to mature in secondary lymphoid organs (spleen)

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What happens to B cells that don’t encounter antigens in secondary lymphoid organs?

APOPTOSIS

they die w/in a few weeks

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What happens to B cells if they DO find specific antigen in the secondary lymphoid organs?

Activation>
proliferation >
differentiation >
generation of plasma cells and memory B cells

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What are progenitor B cells?

They are the earliest stage of antigen-independent B cell development

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B cells can be divided into three groups based on expression of TdT and B220 (CD45R). What are they?

Early- Tdt alone

Intermediate- both TdT and B220

Late- B220 and have downregulated Tdt

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What is B220 (CD45R receptor)?

Receptor for cell growth and differentiation that remains expressed on the cells surface throughout B cell ontogeny

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The rearrangement of heavy chain genes in the pro-B cel stage leads to the expression of …..

CD43- leukosialin
CD 19
RAG 1 and 2

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What is CD 19?

A BCR co receptor that works w/ CD21 and CD81

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What is down regulated as late pro-B cells pass into the pre- B cell stage?

TdT
RAG 1 and 2
CD43

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What is cKIT?

A molecule expressed by pro-B cells

It binds to stem cell factor expressed on bone marrow stromal cells and induces pro- B to prolif and differentiate into precursor B cells (pre-B)

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What is on the surface of pre-B cells?

Igu heavy chains
a pre-B cell receptor complex
Surrogate light chain–holds complex stable while it’s going through recombination

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What cytokine do Pre-B cells express and what does it do?

IL-7R

Stimulate to divide and differentiate using IL-7 (hematopoietic GF secreted by bone marrow stromal cells)

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What are the 6 stages of B cells seen in B cell maturation?

Stem cell
Pro B cell
Pre B cell
Immature B cell
Naive B cell
Mature B cell

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What is the final stage of B cell development in the bone marrow?

Immature B cell stage

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What is observed on the the surface of immature B cells?

IgM

They have successfully rearranged light chain genes

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In immature B cells RAG 1 and 2 are….

downregulated

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What happens to immature B cells as they develop into mature B cells?

They begin to express IgM and IgD on their surface.

This means they are free to exit the bone marrow and can enter the transition phase.

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Several cytokines affect B-cell development. What does IL-7 do?

Promotes B lineage development

*Mice that don’t ahve IL-7 have an early arrest in the pro-B cell stage

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What does Blys do?

It’s important for the survival of pre-immune B cell stages from transition stage onwards.

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What cytokines are important in initiating the process of B cell differentiation?

IL-4, IL-3, L-BCGF

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Transcription factors also help to regulate B cell generation. What do E2A and EBF do?

activate early B cell genes

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What does Pax5 do?

Ensures development to B cell lineages Restricts transcription of lineage inappropriate genes and activates expression of B lineage signaling molecules

What do Sox4 and LEFI do?

Promote the survival and proliferatoin of pro-B cells

What do IRF4 and IRF8 do?

terminate pre-BCR signaling by IRF4 and promote differentiation to small pre-B cells

Bcl-6 exp is req for....?

Germinal B cell differentiation and generation of memory B cells

What TF suppresses Bcl-6 expression and is required for development of Ig secreting cells and maintenance of long lived plasma cells?

Blimp 1

Immnodeficiency XLA in humans affects what stage of B cell development?

It leads to a block at the pro-B cell to large pre-B cell transition in the bone marrow

What enzyme is linked to XLA? What does it do?

Bruton's tyrosine kinase (BTK) Key enzyme involved in signal transduction downstream of the pre-BCR and

XLA pts have_______circulating B cells and _______serum immunoglobin.

few | negligible

Agammaglobulinemia is most often linked to what genetic defect?

XLA

How does CVID differ from XLA?

CVID affects the later stages of B cell development while XLA affects the earlier stages.

What are the characteristics of what CVID?

Reduced serum Ig Reduced memory B cells Reduced class switch recombination and B cell activation

Mutations in: CD40 ligand on T cells B cell surface receptor CD19 Costimulatory molecules ICOS and TACI are all identified in pts with what disease?

CVID

Where does negative selection of self-reactive B cells occur and what does it do?

Bone marrow (occasionally in periphery after somatic hypermutation) Limits the development of antibody-mediated autoimmunity

What are the two things that can happen if immature B cells expressing mIgM recognize self-antigen?

1. undergo apoptosis | 2. edit light chain genes to produce a different light chain that when combined w/ heavy doesn't recognize self antigen

What is a t-dependent antigen?

Immune response to most antigens depends on both T cells and B cells recognizing antigen in a linked fashion

What are TI angiens?

A small number of antigens that can activate B cells w/out MCH II restricted T cell help

What TI antigen are predominately bacterial cell wall components, like LPS from Gram - bacteria?

TI-1

What TI antigens are predominantley large polysaccharide molecules w/ repeating antigenic determinants?

TI-2 | ficoll, dextran, polymeric bacterial flagellin

TI antigens can also be ________recognized by _______.

PAMPS | TLRs

What do TI antigens activate?

B1 B cells w/ CD5 receptors

How does a B1 B cell bind to LPS and what are the characteristics of these receptors?

TLR4- non specific--> stimulate both immature and mature B cells--> polyclonal activation or BCR= specific--> clonal activation

What is produced in response to TLR4 and BCR stimulation?

ONLY IgM

How do B1 B cells bind to TI-2 antigens and what does this cause?

Cross-linking BCR--> stimulates mature B cells ONLY--> clonal activation

What is produced by TI-2 activation?

Mostly IgM, but it can involve class switching w/ help of Th2 produced cytokines

How do TI and TD relate to memory cells?

TI produces poor memory, antigens come a day sooner and you only get IgM TD requires CD4 T cells, IgM early, IgG late, and can be rechallenged in the future

Describe the intracellular process that leads to B cell activation.

1. BCR receptor plus something is required. 2. IgM or IgD bind. 3. If cross linked, then CD79a/b activate signaling through ITAM repeats 4. Activation of NFkB and G proteins that activate TFs Rho, Rac and Ras

What IL's does Th2 make? What do these IL's do?

IL-2,4,5 Push B cell proliferation and differentiation.

Describe the primary Ab response.

``` Naive B cell lag 4-7 days Primarily IgM Ab affinity low Short lived cells ```

Describe the secondary Ab response

``` Memory B cell Lag 1-3 days Primarily IgG Ab affinity high d/t somatic hypermutation long lived cells mag of response greater ```

What is the main difference between the primary and secondary immune responses?

Secondary is faster better bigger

When do T cells mature and where do B cells mature?

B cells are mature when they leave the bone marrow, where as T cells require further education in the thymus.

What is required to commit T cells to lineage differentiation?

Notch

How many weeks does it take for T cells to pass through the thymus and what percent make it out?

3 | 2%

Describe the progression of T cell maturation in the thymus:

``` HSC HPC T cell precursor DN1 DN2 DN3 DN4 Double positive (CD4,8) Single positive (CD8 OR CD4) ```

What is DN?

Double negative | NO CD4 or CD8

What is DP?

Double positive | both CD4 and CD8

What cellular markers are used to determine the stage of T cell development?

CD44 Ckit CD25

CD44 is required for...

relocalization to the thymus

Ckit is required for...

replication

CD25 is required for....

IL-2 driven replication

What is positive selection?

Restriction T cells must recognize self MHC molecules w/ intermediate binding affinity

What is negative selection?

Self Tolerance T cells CANNOT respond to self antigens.

What happens to T cells once they enter circulation?

1. Naive CD4 or CD 8 recognize MCHI/II presented by APC. 2. Antigen binding leads to gene activation 3. Activation 4. Clonal expansion 5. Differentiation 6. Effector functions (activation of macrophages and B cells; killing of targeted cells)

What genes are activated immediately upon antigen binding?

Fos, Myc, Jun, NFAT, NFkB

What genes are activated early upon antigen binding?

IL2, IL2R, IL3, IL6, IFNy

What genes are activated late upon antigen binding?

adhesion molecules

What are the requirements for T cell activation?

TCR binds to MHC-ag of APC Appropriate costimulatory molecules cytokines

Activation of T cells leads to the expression of what type of receptor on the T cell?

High affinity IL-2R *IL-2 synthesis increases by 100x

IL2 binding to IL-2R leads to

clonal expansion generation of effector and memory cells **after expansion most T cells die

What is clonal anergy?

When APC MHC binds to a T cell but no CD28/B7 binding occurs--> desensitized cell that can't be activated--> cell dies in a few days

What is a superantigen?

Can bind both the MHC nad TCR outside of the antigen groove of the complexes. It initiates non-specific interactions to stimulate many T cells of different antigenic specifications. Activation of many T cells > over induction of IFNy and TNFalpha> toxic shock.

What mediates the polarization of CD4 into Th1, TH17 and Th2?

Different cytokines released by APCs **Different types of pathogens influence cytokines released so most appropriate T cell can be recruited to the site of injury