0428 - B cell development - BX

Question 1

What are the characteristics of Adaptive immunity?

Answer 1

• slow, specific to antigens, memory (elevated response to re-introduced antigens)
• diverse, self vs non-self distinction, only in vertebrates

Question 2

What is Clonal selection?

Answer 2

• refers to specific antigens activating a specific lineage of B cells, in T cells a specific MHC type with their peptide presentation will activate them
• self-antigenicity is eliminated during maturation process

Question 3

Describe the regions of an Immunoglobulin

Answer 3

• Variable region (V region) antigen recognition region from both light and heavy chains
• Constant region anchors T and B-cell surface immunoglobulins
• constant region of effector B-cells have variable constant regions (they have signaling functions as these Ig’s are floating)
• The two arms of the heavy chains are joined by disulfide bond

Question 4

What are the 5 types of immunoglobulins and what are the 3 ways they function?

Answer 4

• 5 types: G A M E D (distinguished by constant regions)
• 3 modes of action, neutralization, opsonization, complement activation

Question 5

Where are B cells produced and matured?

Answer 5

• produced in primary lymphoid tissues (bone marrow, fetal liver, thymus, neonatal spleen)
• migration to secondary lymphoid organs (lymph nodes, spleen, mucosal lymphoid tissue)

Question 6

Outline the process of lymphocyte development (6 steps)

Answer 6

• progeintor cell commitment to either T or B cell
• proliferation of progenitors and immature committed cells
• genome rearrangement for antigen recognition
• selection for succesful receptor production
• elimination or self antigen recognition
• differentiation and maturation of lymphocytes in secondary lymphoid tissues, functional diversity emerges

Question 7

Name the stages of B cell development and their corresponding milestones

Answer 7

Pro-B cells

• product of pluripotent hematopoietic stem cells
• somatic recombination of the heavy chain happens in this stage
• early pro-B = DJ recombination
• late pro-B = VDJ recombination

Pre-B cells (large then small)

• functional mu chain (heavy chain) is produced (large pre-b) and the surrogate chains will express them as receptors on cell surface for further signaling (pre-B cell receptor)
• receptors on cell surface signals termination of heavy chain recombination
• cell division happens several times, giving rise to small pre-B cells
  - small pre-B cells is stage where light chain recombination begins

Immature B-cell

• heavy and light chain recombination both finished, immunoglobulin production begins
• Ig’s expressed on cell surface
• begins to undergo self-antigen selection and survivability in periphery

Mature B-cell

• completed self-selection and survivability tests
• further differentiates and migrates to periphery to become naïve B cells
• expression of IgD and IgM immunoglobulins

Question 8

What is the role of thymic stromal cells in B-cell development?

Answer 8

• cell adhesion molecules allow attachment for developing B-cells to progress
• provides growth factors to stimulate differentiation
  - some growth factors include SCF (stem cell factor)
  - later stages include IL-7, SDF-1 and PBSF (pre-B cell growth stimulating factor)

Question 9

Describe the process of somatic recombination in chronological order.

Answer 9

1. recombination of heavy chain happens first
   - if a productive rearrangement has occurred, functional heavy chains will be expressed and triggers signaling processes that begin light chain rearrangement
2. surrogate light chains can bind to heavy chains and present them to the surface of the cell
   - association with Ig-alpha and Ig-beta facilitates signal transduction for next stage
3. heavy chain rearrangement stops by a decreased level of RAG 1 and RAG 2 proteins
4. light chain rearrangement
   - if light chain rearrangement is successful, a full antibody is produced at the surface to check for self-antigen reactiveness
5. allelic exclusion shuts down one of the genes, giving only one functional allele on one chromosome
6. failure to create both chains leads to either continued rearrangement until successful product is made, or if there are no more segments to rearrange (cell death)

Question 10

What are the gene segments in the heavy chain? In what order is recombination achieved?

Answer 10

• Heavy Chain segment of V region
  - V region has 3 segments
  - V, D, and J, D and J join first and V region joins after
• heavy chain has one of 5 C regions for the different types of Ig’s present in our body

Question 11

What are the gene segments in the light chain? In what order is recombination achieved?

Answer 11

There are three gene loci for the variable region of the Ig

           - V J and C (no D region as in heavy chain)
           - V joins with J
           - only 1 C segment present (no recombination)
           - kappa, lamda (separate chromosomal location) are for light                   chain V-regions, each has only one C region segment
           - light chain exclusion (only kappa or lamda locus is used)

Question 12

What are RAG proteins?

Answer 12

• RAG1 and RAG2 proteins are required for the initiation of recombination
• only exist in the lymphoid cells destined to become lymphocytes
• activate other somatic proteins that are responsible for DNA repair and modification.
  extra: TdT proteins add diversity to B cell genome by adding random nucleotides between gene segments

Question 13

Describe the process of central tolerance. (i.e. Self-antigenicity elimination)

Answer 13

four major outcomes

• apoptosis, receptor editing, induction of anergic state, clonal ignorance
• apoptosis happens when multiple self-antigens are recognized, or when receptor editing has failed
  - receptor editing: RAG 1 and RAG 2 levels re-stimulated, light-chain rearrangement continues
• anergic receptor: self-antigens such as small soluble proteins, permanent inactive state, allowed to travel to periphery, but soon dies due to competition, cannot be activated by T-cells
  - few surface receptors present, mostly in cytoplasm
• clonal ignorance at the bone marrow: self-antigen affinity weak, or no affinity at all present but does no signaling response. (or self-antigen is not present at bone marrow)