B cell development and activation 9/5 Flashcards
Mature B cells
- Exit the bone marrow and migrate to secondary primary lymph organs (lymph nodes and spleen)
- They enter the tissue by specialized endothelium (HEV)
- Circulate for weeks and if not activated are apoptosed
Resting Mature B cells express:
- BCR: mIG, Igalpha and Igbeta
- Co-BCR: CD21, (CD19, CD81, CR2)
- HLA-DR (class II)
- CD40
- CD45
B Cell Subsets
B-1 cells: B cells that develop from fetal liver-derived stem cells differentiate into B-1 cells
- B-1 cells respond to non-protein Ag in the mucosa (have CD5+ marker). Found in GALT, MALT, BALT
- Mainly IgM
- short-lived plasma cells
B cells that develop from BM progenitors after birth differentiate into B-2 cells….. they can be divided into two major groups……
Follicular B cells are re-circulating B cells: Majority
- Isotype-switched, high affinity Abs (IgG, IgA, IgE)
- long lived plasma cells
Marginal B cells reside in the spleen: blood-borne polysaccharide Ags – do not recirculate
- mainly IgM
- short-lived plasma cells
Antigen Dependent Phase
- Response initiated by recognition of antigen (epitope) by B cell specific for that antigen (idiotope).
- Antigen binds to sIg on naïve cells and activates these cells.
- Activation can occur in a T-dependent or T-independent manner
Naive B cells in circulation
- travel throughout secondary lymphoid tissue (SLT)
- spleen
- lymph nodes: enter via lymphatic system
- lymph nodes have primary lymphoid follicles, which contain follicular dendritic cells (FDC’s) - this is located in the cortex B cell area of the lymph node
- passage through SLT:
- Naive B cells enter through HEV
- if they don’t bind an Ag they migrate to the primary follicle - where they receive a signal to survive by the FDC’s
- exit through efferent lymphatic vessel
- if there is too many B cells, there will not be enough FDC’s to provide survival signals for all naive B cells. This will result in naive B cell dying within weeks in absence of Ag.
B cell Homing
(B cell and T cell homing of naive cells utilize the same basic mechanisms)
–> Naive B cells express L-selectin, CCR7, LFA1 and CXCR4 which bind to Pnad, CXCL19, CXCL21, ICAM-1 and CXC12 respectively on lymph node HEV’s
- Binding of the chemokines in the node activates integrins and follicular migration occurs through the mediation of CXCL13.
1. Stromal cells and high endothelial venules (HEV) of lymph nodes secrete the chemokine CCL21
2. Dendritic Cells express a receptor for CCL21 and migrate into the developing lyymph node via the lymphatics
3. Dendritic cells secrete CCL18 and CCL19 which attract T cells to the developing lymph node
4. B cells are initially attracted into the developing lymph node by the same chemokines
5. B cells induce follicular dendritic cells, which in turn secrete the chemokine CXCL13 to attract more B cells
B cell Activation: First signals (2 different ones)
- Activation occurs in the node, and requires 2 signals:
- First Signal #1:
- Ag recognition by membrane Igs - must crosslink 2 or more BCR
- signalling occurs through the Igalph and IgBeta cytoplasmic tailes
- This results in minmal activation as the B cell waits for sencond interaction signal.
OR. First signal #2:
- Ag has a bound C3d, which is recognized by mIgs and CR2
- CR2 provides cross-linkage for signaling (signaling occurs through Igalph and Igbeta, CR2 and CD19 cytoplasmic tails) - you don’t need the crosslinking of alph and beta tails if C3D is involved
- If C3D is attached to protein Ag the Ag is 1000x more immunogenic
- this prepares cell for interaction with 2nd signal.
Lymphocyte migration (b and T cells) to Lymph Node
- Activated B cells: alter chemokine receptor expression to migrate into interface zone
- increase CCR7, decrease CXCR5
- Activated T cells: alter chemokine receptor expression to migrate into interface zone
- increase CXCR5, decrease CCR7
–> this allows for migration of B’s and T’s into interface zone to interact for second signal
Second signal to activate B cells
- B cell is activated by binding of Ag
- results in expression of B7 on B cell which binds to CD28 on T cell
- Activated T cells (activated from TCR/HLA I interaction) express CD40L which binds CD40
- CD40 provides the second signal for B cell –> resulting in B cell activation and differentiation and class switching
*** important concept: Because the expression of CD40L (on T cell) and B7 (on B cell) are dependent on antigen stimulation, only lymphocytes specifically interacting with antigenic epitope will be activated. This maintains the specificity of the immune response.
Class switching in the germinal center
Cytokines released by Th cells promote two general functions.
The first is to induce H chain class switching.
- IL-4 promotes switch to IgE
- IFNgamma induces switch to IgG2a
- TGF-beta and IL-5 lead switch to IgA
The second function is to augment B
cell differentiation and proliferation = clonal expansion
There is great redundancy in this
system as many cytokines have
overlapping functions. Each cytokine
has multiple effects and acts on multiple
cell types.
Switch Recombination
CD40:CD40L and cytokines trigger isotype switching by increasing the accessibility of the DNA at a specific C region.
Rearranged VDJ gene segment recombines with a downstream C region gene and the intervening DNA is deleted.
Involve switch regions modulated by cytokines
CD40:CD40L required for isotype switching: T-dependent Ag (proteins)
Key enzyme: activation-induced deaminase (AID)
Expressed by CD40 signals
Somatic Hypermutation (affinity maturation)
- Random Introduction of point mutations in the V regions of the Ig genes resulting in an expansion of the antibody repertoire in hopes to generate high-affinity antigen-specific antibodies.
- 103 to 104 times higher than normal spontaneous mutation rates
- CD40:CD40L required for affinity maturation: T-dependent Ag
- Key enzyme: AID, coverts Cs to Us
- Only B cells with high affinity antigen receptors encounter the antigen on the FDCs and present the antigen to Th cells. If this interaction does not occur the cell will die. ONly B cells with high-affinity antigen receptors are selected to survive
- see the most somatic mutations when class switching occurs in the end of the primary response, and mostly in the secondary and tertiary responses. This results in higher affinity Abs coming out in the later responses
Plasma Cells/Memory cells
- Plasma Cells: A terminally differentiated Ab secreting B cell. Surface markers: Lose CD19 & 20. Identified by flow by CD27++. Secrete at rates ranging from hundreds to thousands of antibodies per second per cell.= workhorse of immune response
- Memory Cells: Survive for long periods of time without additional Ag stimulation. Capable of mounting a rapid response to subsequent exposure. Responsible for Secondary immune response.
Thymus Dependent/Thymus Independent Ags
T Dependent Antigens Require T Cell ‘Help’ - Contact Dependent (these are proteins only, needed for isotype switching, affinity maturation and imm memory)
T Independent Antigens Do Not Require T Cell ‘Help’ – Mitogens (bacterial cellular components, no switching, no memory)
The nature of the B cell response to T independent antigens differs from that to T dependent antigens
Antibody Feedback
Control mechanism triggered by secreted Ab that blocks further Ab production.
–> this is IgG only.
–> a mechanism that tells the body there is too much Ab, and that it needs to be shut down
- the secreted Ab forms a complex with the Ag. the Ag-Ab complex binds to B cell Ig and Fc receptor. The Fc receptor-associated phosphatase removes phosphataes in B0cell receptor complex and thereby blocks the signalling through the alpha/beta complexes. This blocks B cell receptor signalling.
Natural Abs
- Igm
- produced by B-1 and marginal zone B cells
- specific for the bacteria in the area
- cross-reacts with blood alloantigens
