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Slides Explanation
Slide 3:
Before we go on to see development of B-Cell we need ti understand the VDJ recombination with occurs in B cells (Immunoglobulin gene) as well as T cells (T cell Receptor Gene)
V(D)J recombination is the unique mechanism of genetic recombination that occurs only in developing lymphocytes during the early stages of T and B cell maturation. It involves somatic recombination, and results in the highly diverse repertoire of antibodies/immunoglobulins (Igs) and T cell receptors (TCRs) found on B cells and T cells, respectively. The process is a defining feature of the adaptive immune system.
V(D)J recombination occurs in the primary lymphoid organs (bone marrow for B cells and thymus for T cells) and in a nearly random fashion rearranges variable (V), joining (J), and in some cases, diversity (D) gene segments.
The image here shows VDJ recombination in Ig gene (Heavy chain).
In the developing B cell, the first recombination event to occur is between one D and one J gene segment of the heavy chain locus. Any DNA between these two gene segments is deleted. This D-J recombination is followed by the joining of one V gene segment, from a region upstream of the newly formed DJ complex, forming a rearranged VDJ gene segment. All other gene segments between V and D segments are now deleted from the cell’s genome.
The kappa (κ) and lambda (λ) chains of the immunoglobulin light chain loci rearrange in a very similar way, except that the light chains lack a D segment. In other words, the first step of recombination for the light chains involves the joining of the V and J chains to give a VJ complex before the addition of the constant chain gene during primary transcription.
Slide 9:
Simplified theory that a Multpotent progenitor (MPP) gives rise to mutualy exclusive Common lymphoid progenitor (CLP) and Common myeloid progenitor (CMP) is now refuted based on finding of various progenitors with overlapping differentiation potential based on phenotypic studies.
For example as Shown in slide as the MPP goes on to become CLP the intermediate stages have potential to give rise to myeloid precursors also.
Slide 15 and 16:
The cells undergo D-J joining on the H chain chromosome to become early pro-B cells and also begin expressing CD45 (B220) and Class II MHC. Joining of a V segment to the D-JH completes the late pro-B cell stage.
Pro-B cells become pre-B cells when they express membrane μ chains with surrogate light chains in the pre-B receptor. Surrogate L chains resemble actual L chains but are the same on every pre-B cell. The cell halts recombination of H chain and proliferates into a clone of B cells all producing the same m chain. Since dividing cells are larger than resting cells, this stage is called the large pre-B cell.
Following proliferation, small pre-B cells (no longer dividing) undergo V-J joining on one L chain chromosome. Once L chain has been successfully synthesized, it is expressed with μ chain on the cell membrane and the cell is called an immature B cell.
Immature B cells are very sensitive to antigen binding, so if they bind self antigen in the bone marrow they die. B cells that do not bind self antigen express d chain and membrane IgD with their IgM about the time they leave the marrow and become mature naive (resting) B cells.
Slide 34:
Image here shows Normal B-cell differentiation and its relationship to major B-cell neoplasms.
B-cell neoplasms correspond to various stages of normal B-cell maturation, although the normal cell counterparts are unknown in some instances. Precursor B cells, which mature in the bone marrow, may undergo apoptosis or develop into mature naive B cells, which, following exposure to antigen (AG) and blast transformation, may develop into short-lived plasma cells or enter the germinal centre (GC), where somatic hypermutation and heavychain class switching occur.
Centroblasts, the transformed cells of the GC, either undergo apoptosis or develop into centrocytes.
Post-GC cells include both long-lived plasma cells and memory/marginal-zone B cells. Most B cells are activated within the GC, but T-cell-independent activation can take place outside the GC and probably also leads to memory-type B cells.
Slide 35:
Transformed cells derived from naive B cells that have encountered antigen may mature directly into plasma cells that produce the early lgM antibody response to antigen. This T-cell independent maturation can take place
outside the germinal centre.
Other antigen-exposed B cells migrate into the centre of a primary follicle, proliferate, and fill the follicular dendritic cell meshwork, forming a germinal centre.
Slide 36:
Within the germinal center, Ig genes undergo class switching: the µ constant regions replaced by other constant regions and the variable region is subject to somatic hypermutation.
Mutated variable region subject to antigen-mediated selection.
Low affinity and autoreactive B cells die while those with improved affinity leave the germinal centers.
Antibodies with mutations in the variable region appear in the circulation within 6-10 days.
Germinal centre centroblasts express low levels of slg and switch off expression of BCL2; therefore, they and their progeny are susceptible to apoptosis. Centroblasts express CD10 as well as BCL6, a nuclear transcription factor also expressed by centrocytes. BCL-6 is most specific marker for Germinal Centre cells and important transcription factor for all the processes going in germinal centre cells.
Slide 37& 38:
5-15 days after initial antigen exposure, a portion of the germinal center B cells will upregulate IRF-4 (critical for plasma cell differentiation)
IRF-4 induces BLIMP-1 expression which leads to downregulation of genes important for B cell proliferation, class switching and somatic hypermutation while upregulating synthetic rate of Ig synthesis and secretion.
Downregulation of CXCR5 (has kept the B cell in the germinal center), upregulation of CXCR4 leads to signal that leave the lymph node.
References:
- WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues 2017 Revised 4th
- Color Atlas of Immunology (Thieme, 2003)
- Kelley’s Textbook of Rheumatology 9th edition
- Kubys Immunology 8ed
- Identification of Flt3+ lympho-myeloid stem cells lacking erythro-megakaryocytic potential a revised road map for adult blood lineage commitment.
Cell. 2005; 121: 295-306 - Kondo M, Scherer DC, Miyamoto T, King AG, Akashi K, Sugamura K, et al. Cell-fate conversion of lymphoid-committed progenitors by instructive actions of cytokines. Nature. 2000 Sep 21;407(6802):383–6.
- Some of the slides is inspired/clipped by Presentation by Sarah Holstein, MD, PhD
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