B-Cell and T-Cell Maturation Dr. Diebel 5/13/14 Flashcards
(38 cards)
B-Cell Development
Start out as stem cells w/ IL-3 guiding them toward the lymphoid progenitor line. Bone marrow stromal cells and signals that guide the B-cell development. Starts out w/ early proB where DJ H chain recombination occurs and the start of V DJ H chain recombination. Next is proB or preB and here V DJ H chain recombination completes, clonal expansion and VJ L chain recombination occurs. Finally immature B when negative selection occurs. After this they now have the ability to leave the bone marrow, go through transitional phases 1,2, etc and become mature B cells w/ IgM and IgD.
B cell development occurs in the bone marrow and does not require contact w/ antigen. The initial BCR complex includes: mIgM, and the signaling chains Ig alpha and Ig beta (CD79a and CD79b).
If the cells don’t encounter antigen within a few weeks they will die by apoptosis.
proB Cells
Earliest stage of antigen-independent B cell development and can be divided into three groups based on the expression of TdT and B220 (CD45R)(receptor for growth and differentiation).
- Early proB express TdT alone
- Intermediate express both TdT and B220
- Late express B220 (remains expressed on the surface throughout the remainder of B cell ontogeny) and have down regulated TdT
As the cells progress through the proB stage they rearrange their heavy chains and begin to express CD43 (leukosialin), CD19 (BCR co-receptor, works w/ CD21 and CD81), RAG 1 and 2.
proB also express c-kit which binds to stem cell factor expressed on bone marrow stromal cells which induces the proB cells to proliferate and differentiate into preB cells.
As proB enters preB they down regulate TdT, RAG 1, RAG 2, and CD43. (c-kit and CD34 are basically gone in late preB which allows the B cell to be released from the stromal cells)
preB Cells
Divided into two groups.
- Large mitotically active preB cells
- Small non-dividing preB cells
Both groups of cells express IgM heavy chains in the cytoplasm and preB BCR complex on their surface.
The large preB cells have successfully rearranged their Ig Heavy chains and as they progress to the small preB group they being to rearrange their Ig light chain genes and up regulate RAG 1 and RAG 2.
preB cells express IL-7R and are stimulated to divide and differentiate using IL-7 (growth function released from stromal cells).
Immature B Cells
Final stage of B cell development. At this point the B cells have successfully rearranged their light chain genes and express IgM. The RAG 1 and RAG 2 expression has been down regulated. As the immature B cells develop even more they start to express both IgM and IgD on their surface (gradient).
After exiting the bone marrow B cells go through a phase known as transition phase.
Cytokines required for B cell development
Common lymphoid progenitors (CLP) are responsive to IL-7 which promotes B cell lineage development at the proB cell stage.
Blys signaling through its receptor BR3 is important for the survival of pre-immune B-cell stages from transition stage onwards.
IL-4, IL-3 and low molecular weight B growth factor (L-BCGF) are important in initiating the process of B cell differentiation.
E2A and EBF
Transcription factors in B cell development that coordinately activate early B-cell genes
Pax5
Transcription factors in B cell development that ensures development to B-cell lineages by restricting transcription of lineage inappropriate genes and activating expression of B lineage signaling molecules
Sox4 and LEF1
Transcription factors in B cell development that promote the survival and proliferation of proB cells
IRF4 & IRF8
Transcription factors in B cell development that terminate pre-BCR signaling by IRF4 and promote differentiation to small pre-B cells
Bcl-6
Transcription factors in B cell development which is required for germinal B cell differentiation and generation of memory B cells
Blimp-1
Suppresses Bcl-6 expression and is required for development of Ig secreting cells and maintenance of long lived plasma cells.
Surface Ig receptor expression is present during what stages?
No expression is present during pre-pro, early pro, or late pro.
Pre-BCR is present during large pre cell stage.
Decreasing levels of pre-BCR is found during small pre cell stage.
IgM is present during immature B stage.
c-kit is expressed during what cell stages?
Low amounts of c-kit are found during early and late pro cell stages.
CD25 is expressed during what cell stages?
+/- during late pro and is present from then on.
CD19 is expressed during what cell stages?
Every stage except pre-pro.
B220 (CD45R) is expressed during what cell stages?
Expressed during every stage.
Immunodeficiency XLA
Leads to a block at the proB cell to large preB cell transition. The defect is mostly caused by a mutation in the gene encoding the enzyme Bruton’s tyrosine kinase (Btk) which is a key enzyme involved in signal transduction downstream of the pre-BCR and BCR. In these patients there is very few circulating B cells and negligible serum immunoglobulin.
Common variable immunodeficiency (CVID)
Impacts the later stages of B cell development and manifests itself in reduced serum Ig, memory B-cells, class switch recombination and B cell activation. Mutations in CD40 ligand on T cells, B cell surface receptor CD19, activated T cell costimulatory molecules ICOS and TACI (another receptor for Blys) have all been identified in CVID patients.
Negative selection
Occurs in the bone marrow (clonal deletion). If immature B cells that express mIgM recognize self-antigen:
1. Some of the immature B cells undergo apoptosis.
2. Some of the immature B cells undergo editing of light chain genes to produce a different light chain that does not recognize self-antigen.
Not all self-reactive B cells are eliminated in the bone marrow because not every self-anitgen is expressed in the bone marrow.
Transitional Immature B cells
T1 expresses IgM only. Negative selection in PALS.
T2 expresses both IgM and IgD. Positive selection in follicle.
The three major populations of B cell in the periphery
- Follicular (B-2) B cells
These are the cells when we normally think of B cells. They are found in the secondary lymphoid organs. Formed from precursors in bone marrow, somatic hypermutation, requires T cell help, produces high level of IgG, surface IgD found on naive B cells.
- B-1 B cells
These are found in the peritoneal and pleural cavities and self-renewing. Somatic hypermutation does not occur, no T cell help is needed, express high levels of IgM, little or no memory, little or no surface IgD. CD5 is found on the surface.
- Marginal zone B cells
Found in the marginal zones of the spleen and are long lived or might be self-renewing. Primarily IgM and some IgG. In general not much is known about these.
T-independent B cell activation
A small number of antigens can activate B cells w/o MHC II restricted T cell help. These can be divided into two groups TI-1 and TI-2 based on the manner in which they activate B cells.
TI-1 predominately bacterial cell wall components (LPS of gram (-) is a major stimulant).
TI-2 predominately large polysaccharide molecules w/ repeating antigenic determinants (e.g. Ficoll, dextran, polymeric bacterial flagellin, and poliomyelitis virus).
Many TI antigens are PAMPS that can be recognized by TLR
CD5 receptor
TI antigen produce IgM about a day earlier than TD IgM.
B cell response to type 1 TI antigen
B1 B cells can bind to LPS either through TLR4 or BCR
TLR4=nonspecific (polyclonal activation) and can stimulate both mature and immature cells.
BCR=specific (clonal)
Only IgM is produced in response and no memory is formed.
The B cell-co-receptor complex consists of CD21, CD19, and CD81 binding to complement. (C3D)
B cell response to type 2 TI antigen
Can only stimulate mature B cells through cross linking of BCR (specific clonal activation). Mostly IgM is produced in response to this situation.
CD4 + Th2 T cells can be involved to produce cytokine for a full B cell response that can include class-switching. (very low level of IgD produced)
