9 - B Cell Development Flashcards
B cell development in the bone marrow
the bone marrow contains microenvironments (niches) populated by hematopoietic cells and various bone marrow stromal cells. The stromal cells express proteins (including cell surface ligand, cytokines, and chemokines) that support the long-term survival and division of HSCs and the developmental pathways leading to the formation of mature blood cells.
At various points in their development, the precursors of B cells must interact with stromal cells expressing particular proteins that induce the developing cells to differentiate and move in an orderlu progression from location to location in the bone marrow.
Changes in Cell surface markers, gene expression, and immunoglobulin gene rearrangements define the stages of B cell development
cells at different stages of development can be characterized by their surface proteins (incl chemo- and cytokine receptors, adhesion molecules, ++), expression of specific TFs, and the rearrangement status of Ig genes.
Beginning around the time of birth and extending through adult life, hematopoiesis, including B cell development, occurs in the bone marrow and is influenced by various niches established by stromal cells.
The stages of B cell devekopment are controlled by networks of TFs and by epigenetic changes that influence the expression of key genes. The cells become increasingly committed to becoming B lymphocytes.
The earliest steps in lymphocyte differentiation culminate in the generation of a common lymphoid progenitor
HSC to CLP (common lymphoid progenitor)
HSCs express a unique set of surface proteins, one of which (c-kit) is the receptor for stem cell factor (SCF); their interaction triggers key signals that help induce the differentiation into multipotent progenitor cells (MPPs). HSCs also express stem cell-associated antigen-1 (Sca-1). Both SCF and Sca-1 are expressed in parallel on early progenitor cells, but their levels of expressoin drop as the cells commit to the lymphoid cell lineage.
MPPs lose the extensive self-renewal capacity, but retain their ability to differentiate into several different hematopoietic cells.
Important TFs and their functions:
- Ikaros : recruits chromatin-remodeling complexes to particular regions of DNA to ensure the accessibility of genes necessary for B cell development.
- Purine box factor 1 (PU.1) (ja, punktum) :
the levels of PU.1 determine lymphoid vs myeliod differentiation: low levels favor lymphoids. The level of PU.1 is regulated by Gfi1. PU.1 acts in part by initiating nucleosome remodeling, followed by specific histone modification of genomic regions associated with gene expression. - 2EA = induced by Ikaros + PU.1, plays critical roles in subsequent stages of B cell development.
the progenitors also express FLT-3, which binds to its ligand and signals for the synthetis of IL-7 receptor α chain, which pairs with the common γ chain found in receptors for various class 1 cytokines.
When the cells starts to prepare for the rearrangement of genes, they express RAG1/2 and TdT, defininf the cell as early lymphoid progenitor cell (ELPs). Some of these ELPs migrate out of the bone marrow toward the thymus (T cell precursors). As the levels of IL-7 receptor increase, expression of c-kit and Sca-1 decreases, and the ELP is now a CLP.
At the CLP stage, the cell cannot become myeliod, but can still become T, NK, and dendritic cells. The expression of EBF1 (early B cell factor 1), activated by signalling through IL-7 receptor (JAK/STAT pathway) and TFs E2A and Foxo1, is important for later steps in B cell differentiation.
The later stages of B cell development result in commitment to the B cell phenotype and the stepwise rearrangement of Ig genes
1) Pre-pro-B cells:
this stage starts with increasing levels of EBF1 and the acquisition of CD45R (B220). The trc of the Ebf1 gene (encoding EBF1) is under the control of multiple TFs (STAT5, E2A, Foxo1, Runc1).
EBF1 and E2A bind teh Ig heavy chain locus, promoteing accessibility of the D-JH gene segments and preparing the cells for the first step of Ig gene recombination. Epigenetic chromatin modifications also play a role, as RAG2 must rec methylated H3 in order for the RAG1/2 complex to bind to the RSS.
EBF1 also facilitates the activation of B-lineage genes (previosly silenced), through interactions with SWI/SNF, and the commitment by inhibiting Notch1 and GATA-3 (support T cell development) and ID2 (promotes NK/ILC development).
Pre-pro B cells remain in contact with CXCL12-secreting stromal cells in the bone marrow.
2) pro-B cells (progenitor B cells)
D-to-JH recombination is completed, cell begins to prepare for VH-DJH joining. needs the TF PAX5 to do so. PAX5 will be expressed until the B cell encounters Ag to differentiate into Ab secreting plasma cell. Pax5 gene controlled by E2A, EBF1, Ikaros, PU.1. PAX5 reinforces EBF1 expression (positive feedback loop). This is commitment, so non-B-cell genes are not expressed anymore. PAX5 can also block Notch1 gene.
other important genes turned on by PAX5 and othr TFs:
Igα & Igβ (the signalling chains of the membrna-Ig BCRs) & CD19 (one component of the B cell coreceptor).
PAX5 also promotes the VH-DJH recombination by “contracting” the Ig_H locus.
3) Pre B cells
PAX5 and EBF1 also turn on genes encoding VpreB and λ5. Together these comprise the surrogate light chain (SLC). Two of these, along with two μ heavy chains form the preBCR. The structure of VpreB and λ5 covers the heavy-chain complementaroty-derermining region 3 (CDR3), preventing the pre-BCR from recognizing Ags.
The pre-BCRs self aggrigate due to interactions in the oppositely charged tails of VpreB and λ5, initiating signaling through Igα/Igβ signaling chains, with contributions from the CD19 coreceptor, and initiates event crucial for B cell development. 5 things are triggered:
1 - survival and proliferation (2-7 rounds)
2 - RAG1/2 downregulated (trc stop and
degradation of existing)
VH-DJH also downregulated (success)
3 - PreBCR and IL-7 signals downregulated
IL-7 down = VH, D, JH locus apart in
unarranged locus.
proliferation downregulated
4 - RAG1/2 upregulated
5 - light chain VlJL gene rearrangement
upregulated.
The signalling through pre-BCR induce 2-7 rounds of proliferation. the size of the cells proliferating is large, therefore called large pre-B cells. If the μ chain has not been made correctly, the cell instead undergoes apoptosis, in the forct B cell checkpoint.
The daughter cells from this proliferation will have the same heavy chains, but the light chain stuff happens later and thus they will have unique BCRs.
Once the cells stop proliferating, they enter the small pre-B cell stage, and then commence light chain recombination. once light chain successful, IgM BCR is expressed in the surface and signals the cell (apparently spontaneously/without binding) to terminate further attempts. It is now an IMMATURE B CELL (defined by IgM expression). This is the second checkpoint, if neither κ nor λ was successful, the cell dies.
Immature B cells in the bone marrow are sensitive to tolerance induction through the elimination of self-reactive cells
immature B cells express IgM (not IgD or any other Ig yet, as they do on mature naïve B cells), but they continue to express B220 and CD19.
Autoreactive B cells are bad - three fates;
1) lost from repertoire before leaving bone marrow by the BCR mediated induction of apoptosis
2) reactivate RAG genes to initiate editing of light chain receptor
3) escape but become unresponsive (anric) to further Ag stimuli
1) and 2) are referred to as central tolerance. immature B cells are susceptible to apoptosis as they have low levels of anti-apoptic proteins.
Completion of B cell development in the spleen
S1P receptor rec the chemoattractant S1P, inducing the immature B cell to leave the bone marrow and migrate to the spleen to complete their development.
Cells derived from immature B cells are divided into T1 and T2 (transitional B cells). These differentiate in a series of steps into fully mature B cells. T2 B cells have higher levels of mIgD and express CD21 and CD23, and the BAFF receptor (BAFF-R, B cell activating factor beloning to the tunor necrosis factor family). As T2 transition to full maturity, mIgD expression increases and mIgM decreases. Mature B cells also express CD24 and CD93.
Both T1 and T2 can differentiate into mature B cells. Most T1 B cells differentiate into T2 in the spleen, but some emerge from the bone marrow as T2 already. T2 cells can recirculate among blood, lymph nodes and spleen, and can enter B cell follicles in the lymph node and spleen.
Self reactive T1 B cells are eliminated by apoptosis in response to strong BCR signal. once it develops into T2, it becomes more resitant to apoptosis. BCR signals in T1 results in [calcium] but no diacylglycerol, and provides apoptotic signal. In T2, both calcium and diacylglycerol increase, delivering both maturational and survival signals to the cell. The difference might stem from differences in cell membrane (more cholesterol in T2)
Signals through BAFF binding to BAFF-R are necessary for the survival of transitional and mature B cells.
T2 B cells five rise to mature follicular B-2 B cells
fully mature and convensional B-2 cells (follicular B cells) express high levels of IgD and intermediate levels of IgM. They recirculate between blood and lymphoid organs, etering the B cell follicles in the spleen and lymph nodes, where they respond to Ag encounter with the help from TH cells.
T3 B cells
appears to consist of B cells anergized by exposure to weakly stimulating peripheral self antigens, such as soluble proteins
Anergic B cells have a shorter half-life than normal B cells, and appear to be excluded from the B cell follicles in the lymph nodes and spleen.
Function? one possibility is that they serve to absob excess self antigens that might otherwise be able to deliver activatinf signals to high affinity B cells, and thus lead to autoimmune responses. Another is that they represent cells destined for apoptosis that do not yet display the characteristic microanatomy of apoptotic cells. Yet another is that these cells will eventually develop into B regulatory cells.
The properties of B-1 and MZ B cells
In addition to predominant population of B-2/follicular B cells, B-1 and MZ B cells also arise through other developmental programs, and occupy distinct anatomical locations and have distinct functions.
B-1 B cells (B-1a and b) and MZ B cells differ from B-2 cells in surface markers, and they have a more limiter repertoire of Ab specificities, including cross-reactions between self- and microbial antigens. They spontaneously generate Ab responses to microbial infection without TH cells.
MZ B cells have high levels of IgM and the coreceptor CD21, but low levels of IgD and the Fc receptor CD23.
B-1a B cells are derived from a distinct developmental lineage
B-1a cells are derived from early embryonic precursors, distinct from and preceding HSCs, that seed out into the pleural and perironeal cavities and are capable of self-renewal, generating B-1a cells thoughout life. B-2 cells mus be constantly replenished
B-1b and MZ B cells seem to be derievd from HSCs or B2- lymphoid progenitors .MZ B cells inhabiting the splenic marginal zone arise from T2 B cells.
Comparison of B cell and T cell development
Both types of lymphocytes include two distinct cell lineages, with one (γδ and B-1) dispersing to its won particular peripheral niches during fetal development, there to function as a self-renewing population until the death of the host.
beginning at the time of birth and continuing through adult life, the development of B-2 and αβ cells commences in the bone marrow, starting with HSCs.
B and T cells also share the early phases of their developmental programs, as they pass through progressively more differentiated stages as MPPs, ELPs, and CLPs.
T cells leave the bone marrow at the ELP and CLP stages, migrating to the thymus to complete development, while B cells remain in the bone marrow.
Both B and T cells initiate V(D)J rearrangement for one of their chains, which are expressed with a surrogate second chain. The resulting pre-BCR or pre-TCR signals cells that the first chein rearranged productively, and to go ahead and rearrange the second receptor gene family.
B cells expressing mIgM BCR and T cells expressing αβ TCR must both pass through stages of positive selection, in which those cells capable of recieving survival signals are retained at the expense of those who cannot. The positive selection process is well understood for T cells, but not for B cells.
Conventional B and T cells must survive negative selection.
