11 - B Cell Activation, Differentiation, and Memory Generation Flashcards
T-dependent B cell response overview
1) Signal 1: B cell binds Ag via Ig receptors.
some of the bound Ag is interalized in specialized vesicles, where they are processed and re-expressed in a peptide-MHC complex
2) Signal 2: an activated T cell binds to the B cell both via its Ag receptor and via an interaction between CD40 (B cell) and CD40L (TH cell).
3) Signal 3: Upon binding to the b cell, the T cell releases its activating cytokines.
The signals from the T cell stimulate the B cell to differentiate along one of three pathways:
1) it can proliferate to form a “primary focus” of Ab-secreting plasma cells that provide the initial IgM Abs of the primary response
2) it can develop directly into an IgM-bearing memory cell
3) it can enter the germinal center (GC) and undertake one of the most extraordinary differerntiation programs in all of biology
Naïve B cells encounter Ag in the lymph nodes and spleen
Some low-molecular-weigth antigens enter the lymph nodes via a leaky network of conduits that are sampled by the follicular B cells.
Higher molecular Ags are taken up first by Fc or complement receptors on sucapsular sinus macrophages or by similar receptors on B cells, DCs, and circulating macrophages, and subsequently passed on to the B cells.
Fc receptors = receptors that bind to the non-antigen-binding-regions of antibodies.
Follicular DCs (FDCs) = studded with Ab-Ag complexes, retained on the surface of the FDC through interactions with Fc or complement receptors.
B cell recognition of cell bound Ags culminates in …
the formation of immunological synapses
when the BCR recognizes its cognate Ag, the receptors on the B cell membrane briefly spread over the surface of the Ag-presenting membrane (to increase the number of interactions between the B cell and the Ag-bearing cell), and then contract, resulting in B cell receptor clustering. This represents the earliest phase of B cell activation.
Receptor clustering leads to the formation of an immunological synaps between the B cell and its Ag. In the B cell surface, an inner ring of BCRs forms the central part of the supramolecular activating cluster, or cSMAC, which is surrounded by a ring of adhesion molecules (pSMAC) encircled by a distal ring of polymerized actin (dSMAC).
BCR-Ag binding leads to the activation of a signal transduction cascade within the B cell
Ag stimulation eventually leads to the activation of several TFs, incl NF-κB, NFAT, Egr-1, Elk1, which together with other factors act on the cells trc-program. Other pathways trigger changes in membrane mobility, in te expression of adhesion molecules and chemokine receptors, and in the production of anti-apoptotic molecules.
Phosphorylation of tyrosine residues in the ITAMs of Igα and Igβ by Src family kinases. This phosphorylation initiates a response leading to the formation of a cytoplasmic signal-transducing complex called a signalsome (!!)
Multiple outcomes aer possible following BCR/Ag binding, which depend on the strength and duration of Ag binding and are further regulated by interactions between the BCR and other cell receptors, including CD21, CD40, IL-4R, IL-21R, and BAFF-R.
B cells coreceptors
Following Ag binding, the Ig receptor is noncovalently associated with three transmembrane molecules; CD19, CD21, CD81.
Sometimes the Ag is already bound to C3d. CD21 (aka CR2) binds to C3d on Ags. this coengagement of the BCR and coreceptor enhances the binding and brings the receptor in close contact with the coreceptor.
When this happens, tyrosine residues on the cytoplasmic face of the CD19 coreceptor become phosphorylated by the activated Scr family kinases, providing attachment sites for PI3 kinase. Localization of PI3 kinase to the coreceptor enhances cell survival, and results in trc changes.
How does the B cell acquire the Ag from an APC?
can be cleaved from the surface of the APC by B cell derived lysosomal proteases, or tugged from the surface by binding to the BCR.
In the former case, the lysosomes spill their contents into the immunological synaps when they reach it, thus acidifying the junction ans allowing the proteolytic enzymes to cleave the bonds between the Ag and the APC.
In the latter case, actomyosin fibres within the B cell haps pull the Ag bu exerting force on the BCR.
What happens once the B cell acquires the Ag?
BCR mediated endocytosis transports the Ag into vesicles, where it is broken down into peptides. These peptides are subsequently loaded onto MHC II and returned to the B cell surface
Ag engagement results in up-regulation of the expression of CD40, CD80 and CD86. These molecules bind with coreceptors on T cells, further facilitating productive B cell interactions with cognate T cells
The early phases of the T dependent response are charecterized by chemokine-directed B cell migration
Ag-binding B cells move under the instruction of three chemokine receptors (CXCR5, EBI2, CCR7) to the boundary of the B- and T cell zones in the lymph nodes. this happens ca 6h after stimulation. By this time, the B cell has internalized and processed its Ag, and is ready to present to the T cells.
the Ag-stimulated B cell makes contact with an AG-specific, activated T cell, and they engage for extended periods of time, in which the TCR and its microtubule-organizing center reorient to the immunological synapse, and the T cell begins to secrete Ils (IL-4 and IL-21) that enable the differentiation of the B cell. Activated B cells upregulate their receptors for these ligands.
Interactions between CD40L (T cells) and CD40 (B cells) are important for B cell proliferation and differentiation. Interactions between CD28 (T cell) with B cell CD80/86 are also important.
After the T/B cell communication is over, some B cells downregulate CCR7 & ENI2, enter the interior regions of the B cell follicle, and start making germinal centres. Other B cells retain EBI2 expression but decrease CXCR5 and increase CXCR4, before moving to the splenic red pulp/lymph node medullary cords and forming primary foci of proliferating B cells, that rapidly differentiate into plasmablasts (secrete Abs, can still proliferate, still bear BCRs). These cells will later become nonproliferating plasma cells that secrete IgM.
Specification of the stimulated B cell fate depends on TF expression
Following stimulation of primary B cells at the Tcell/Bcell border within the lymph node, some B cells differentiate quickly into the plasma cells that form primary foci and secrete an initial wave of IgM Abs. This requires the upregulation of the plasma cell TFs IRF-4 and BLIMP-1.
Other B cells from the Ag-stimulated clones migrate to the primary follicles and form germinal centers, an action that requires TF Blc-6 and low levels of IRF-4.
what happens to B cells upon T cell stimulation?
Following T dependent stimulation by Ag, B cells enter the follicles and divide rapidly. The B cells, together with associated THF (T folliculat helper) cells and follicular DCs, form germinal centres.
Germinal centres are made up of dark zones (in which B cells divide rapidly and undergo somatic hypermutation) and light zones (where the B cells interact with TFH and follicular DCs, and B cells bearing high-affinity, mutated receptors are selected.
B cells in the dark zone = centroblasts
B cells in the light zone = centrocytes
Movement between the two zones is orchestrated by modifications in cell surface chemokine receptors.
germinal center B cells are prone to apoptosis. Survival requires productive interactions with TFH cells as well as with follicular DCs.
These are interactions between CD40L (T cell) and CD40 (B cell). Follicular DCs can hold Ags on their surface in Ag/Ab complexes, or via complement proteins on Ab and their corresponding receptors on FDC. Either way, B cells interacting with FDC-bound Ag recieve survival signals.
High-affinity B cells take up and present higher levels of Ag to TFH cells and thus recieve a greater quantity of survival signals.
What do plasma cells of the primary focus do?
secrete large quantities of IgM and IgG Abs that provide early, protective humoral immunity and help to drive affinity maturation
Somatic hypermutations
happens in germinal centers.
induced increase in mutatuions (10^3 to 10^6-fold over background rate) in the regions in and arround rearranged Ig-genes.
happens after Ag contact in mice and humans.
Initiated by a cytidine deamination (catalyzed by AID enzyme). DNA repair mechanisms then cause alterations of the sequence of the variable region of immunoglobulin genes.
AID activity is targeted to transcribed sequences that have high levels of specific targeting sequences. These targeting sequences are found at significant frequencies in areas of the Ig genes involved in SHM and CSR.
Class switch recombination results in the replacement of an Ab Heavy chain C-region, with a different C-region originally located 3’ to the first. Regulation of which Heavy chain C-regions will be targeted is mediated by cytokines secreted either by T cells or other immune cells.
SMH only occurs in the germinal centers, after T dependent Ag stimulation. CSR may occur in the GCs, but can happen elsewhere. can happen with/without T cell help.
Memory responses. Differences in IgG and IgM. memory cells vs naïve cells. plasma cells.
After initial AG exposure to a T dependent Ag, secondary and later encounters with the same Ag will give rise to faster and stronger recall respnses, referred to as “memory” responses
IgM bearing memory B cells are generated early in the primary response, prior to the onset of SHM, and secrete nonmutated Abs with lower affinity.
IgG-bearing memory cells generated later in the primary response, have mutated and selected receptors and yield recall responses that deliver high-affinity Abs.
Memory cells differ from naïve cells in their dependence on other cells for stimulation, their expression of cell-surface Ags and chemokine receptors, their metabolism, their concentrations of anti-apoptotic molecules, and the speed with which they can enter the cell cycle on Ag stimulation.
Long-lived plasma cells are generated late in a primary response, and survive for very long periods in the bone marrow, the mucosa, and other locations.
what happens to the newly generated B cells at the end of an immune response?
most newly generated B cells die by apoptosis.
T independent B cell responses : what Ags do they recognize?
Some microbes have polyvalent, rpeating determinants (shared among many species), sort of like PAMPs. They are recognized by B-1a and B-1b cells, as well as MZ B cells.
B-1a and B-1b mainly secrete IgM Abs (not yet subjected to SHM).
Two types: TI-1 ans TI-2 Ags (TI = T independent).
