5. Development of B Cells

Hematopoiesis

Process in red bone marrow where new B cells are produced

What happens once B cells are produced?

1) Immature B cells produce BCR then go through further training in bone marrow

2) Once released into circulation, immature B cells enter secondary lymphatic tissue (usually lymph node) where they mature

3) Mature B cells remain naive until they encounter an antigen

List and describe the phases that happen in bone marrow.

Phase 1: Repertoire assembly; generation of diverse and clonally expressed BCRs in the bone marrow

Phase 2: Negative Selection; alteration, elimination, or inactivation of BCRs that bind to components of the human body

Phase 3: Positive Selection; Promotion of a fraction of immature B cells to become mature B cells in the 2° lymphoid tissues

(Phase 3 is more in-between)

List and describe the phases that happen in secondary lymphatic tissues or blood.

Phase 4: Searching for infection; Recirculation of mature B cells between lymph, blood, and secondary lymphoid tissues

Phase 5: Finding infection; Activation and clonal expansion of B cells by pathogen derived antigens in 2° lymphoid tissues 

Phase 6: Attacking infection; Differentiation to antibody-secreting plasma cells and memory B cells in 2° lymphoid tissues 

What is the pathway of the lineage to B cells?

Pluripotent hematopoietic stem cell → common lymphoid progenitor → B cell precursor → pro-B cell

What is the hallmark of pro-B cells?

Somatic recombination of BCR heavy chain genes

What are nonproductive rearrangements?

Occur when a nonfunctional heavy chain sequence is produced (this is what mostly happens in the somatic recombination due to the amount of potential gene combinations thus very inefficient)

What are productive rearrangements?

Maintain a proper heavy chain reading frame; these VH’s can associate with Iga+B and attach to VLs

How is repertoire assembly usually checked in late pro-B cells?

After the V-DJ attachment stage, a rearrangement is made for a retry

• If successful, it is signaled to survive to become pre-B cells (50%)

• If not, signaled to die by apoptosis (50%)

What is the hallmark of the pre-B cells stage?

Somatic recombination of BCR light chain genes, less genes to combine usually more efficient

What are the checks in Pre-B cells?

κ is rearranged first

• If not, tried again on 2nd chromosome

λ gene is rearranged first

• f not, tried again on 2nd chromosome

If can’t do any, apoptosis

What rearrangements occur in the immature B cell?

Rearrangement ceases, and cell either expresses μ and κ or μ and λ 

What is the function of nonlymphoid stromal cells?

Guides B cell development in the bone marrow

• Helps trigger recombination with specialized microenvironments

• Maintain contact with developing B cells through cell adhesion

• Produce growth factors that promote B cell development

When does a cell move on to negative selection?

When the immature B cell has a functional BCR (IgM) and can be tested if they can bind to healthy self-antigens

• If they don’t recognize/react to self-antigen, they are self-tolerant and pass

• If they are reactive/binds to antigen, they don’t pass, they will be retained in bone marrow

What happens to those immature B cells that are self-reactive in negative selection?

3 fates:

1) Survive by receptor editing

2) Apoptosis

3) Anergic

• pass (unofficially) but they become inactivated by being signaled to make IgD and soon die

How does a B cell travel out of the bone marrow to secondary lymphoid tissues?

If they are in a blood vessel, leaves high endothelial venules (HEV) via chemotaxis then enters primary lymphoid follicles

What is the function of FDCs?

Follicular dendritic cells; makes chemical gradient that guides from HEV to follicles, then secretes signals induces B cell maturation

Describe the steps of positive selection and after

1) Chemokine CCL21 attracts immature B cells to HEV

2) Chemokines CCL21 and CCL19 attract B cells into lymph node

3) Chemokine CXCL13 attracts B cells into the primary follicle

4) Interactions with follicular dendritic cells drive B cell maturation

5) Mature B cells recirculate between lymph, blood, and secondary lymphoid tissues

6) Moves onto Phase 4 of surveying for pathogens

What happens once the naive B cells finds an antigen?

Usually found in the primary follicle; it moves to the boundary of the T cell area (it has phagocytized, processed, and now displaying it on the MHC II)

• Waits until they find TH with same antigen to form cognate pair

• Goes back into primary follicle to form germinal centers (allows clonal expansion/selection)

Describe the cell pathway involved in Phase 5

1) Activated B cells (is bound to an antigen)

2) Once it finds its cognate pair, it goes to germinal center to form lympho/centro-blasts that are large and proliferative

3) Eventually turns into centrocytes that will be guided by TH cell to go through somatic hypermutation and choose the most efficient to go through isotype switching (see which antigen is most efficient to bind to)

4) Once they finish those processes, they become plasma cells that move to the heart of the lymph node and secrete antibodies to circulate

What are the 3 fates involved in attacking infection?

1) Some plasma stay in secondary lymphoid follicles then migrate to other lymphoid tissues

2) Some plasma cells enter bone marrow

3) Some become memory B cells and are retained

Describe the lifespan of 1) anergic immature B cells, 2) mature naive B cells, and memory B cells

1) Half life ~1-5 days

2) ~40 days

3) long-term

T/F? Plasma cells and memory B cells are produced by different sets of activated germinal center B cells

True

Immature B cells complete maturation in secondary lymphatic tissue when they receive signals from __.

Follicular dendritic cells

B cell development in the bone marrow is guided by __.

Stromal cell

During B cell repertoire assemble there are two checkpoints:

The 1st checkpoint occurs after the _(1)_ stage, assessing for productive HC arrangements

The 2nd checkpoint occurs after the _(2)_ stage, assessing for productive LC arrangements

1) Pro-B cell

2) Pre-B cell

T/F? The human body makes all possible combinations of unique BCRs, allowing circulating B cells to express every possible BCR.

False

(Quiz) Define Phase 1: repertoire assembly

creating B cells with diverse receptors

(Quiz) Define Phase 2: negative selection

inactivating or eliminating self-reactive B cells

(Quiz) Define Phase 3: positive selection

promoting maturation of a small group of immature B cells

(Quiz) Define Phase 4: searching for infection

mature naive B cells recirculate between blood and lymph

(Quiz) Define Phase 5: finding infection

antigen binding to B cell receptors activates B cells, inducing clonal expansion and clonal selection

(Quiz) Define Phase 6: attacking infection

differentiated plasma cells release antibodies with high-binding affinity for an antigen, memory B cells are retained in circulation

_(1)_ selection occurs in the bone marrow. This process removes _(2)_ B cells, leaving only _(3)_ B cells to leave the blood

1) Negative

2) Self-reactive

3) Self-tolerant

When activated in a lymph node, activated B cells develop into large proliferative _(1)_ which then become _(2)_ that undergo somatic hypermutation and isotype switching to become _(3)_.

A separate subset of activated B cells in the germinal center differentiate into _(4)_.

1) lymphoblasts

2) centrocytes

3) plasma cells

4) memory B cells