B cell activation Flashcards
What are the components of the clonal selection hypothesis?
This hypothesis states that antigen interacting with a receptor on a lymphocyte induces division and differentiation of that lymphocyte to form a clone of identical daughter cells. All daughter cells will bear the same receptor as the stimulated cell, and antibodies produced by B cells stimulated in this way will share the antigen-binding site with the membrane receptor of the stimulated cell. Following antigen elimination, representatives of the stimulated clone remain in the host as a source of immunological memory. Those clones of B cells that meet antigen at an immature stage of development will be eliminated from the repertoire.
What are the two major types of B-cell responses?
There are two major types of B-cell responses, which are elicited by structurally distinct types of antigens.
The first type of response is generated following recognition of protein antigens and requires the participation of CD4 helper T cells; this class of B-cell response is therefore known as a T dependent (TD) response. It is mediated by B-2 B cells binding to TD antigens.
The second type of response is directed toward multivalent or highly polymerized antigens and does not require T-cell help. This type of response is referred to as a T-independent response, and the antigens that elicit such responses are T independent
(TI) antigens.
TI-2 antigens require the presence of multiple repeating epitopes (usually at least two) to cross-link B cell receptors and activate signaling pathways that lead to B cell activation and differentiation.
How did we determine B/T cells are required for T-dependent responses? (experiment)
What two things can occur to B cells once mature and where does this occur?
Following the completion of their maturation program, B cells migrate to the lymphoid follicles, where one of two things can occur. The B cell can interact with antigen and become activated. Or, in the absence of immediate antigen stimulation, the B cell recirculates through the blood and lymphatic systems and back to the lymphoid follicles. This latter process can recur many times.
What are the three signals B-cells receive during activation? (overview)
At the start of a T-dependent B-cell response, the B cell binds antigen via its Ig receptors (signal 1).
Some of the bound antigen is internalized into specialized vesicles, where it is processed and re-expressed in the form of peptides in the antigen-binding groove of MHC class II molecules. Signal 2 is provided by an activated T cell, which binds to the B cell
both through its antigen receptor and via a separate interaction between CD40 on the B cell and CD40L (CD154) on the activated T cell.
On binding to the B cell, the T cell releases its activating cytokines (signal 3) directly into the T-cell/B-cell interface as described in Chapter 3. The nature of the response is also affected by cytokines released by other cells in the vicinity of the antigen encounter, as described later in this section.
What are the Alternative Fates of B Cells following T-Dependent Antigen Stimulation
It can proliferate to form a “primary focus” of antibody-secreting plasma cells that provide the initial IgM antibodies of the primary response;
It can develop directly into an IgM-bearing memory cell,
Or it can enter the germinal center (GC) and undertake one of the most extraordinary differentiation programs in all of biology.
What can occur to the antigen before binding during T-dependent B-cell response?
The mechanism of B-cell antigen acquisition varies according to the size of the antigen. Soluble antigens picked up by the afferent lymphatic vessels flow into the subcapsular sinus cavity of the lymph node.
From there, antigens with a molecular weight less than 70 kDa enter a system of conduits that originate in the base of the subcapsular sinus (SCS). These conduits are produced by fibroblasts and consist of highly organized bundles of collagen fibers, ensheathed by a basement membrane and surrounded by fibroblast reticular cells in the T-cell zone. (These reticular cells may be replaced by follicular dendritic cells in the B-cell follicles during lymph node development.) Since the cellular sheaths are somewhat leaky, dendritic cells, macrophages, and B cells can gain access to the antigens carried in these conduits by extending processes through the basement membrane.
Larger, more complex antigens take a different route into the lymph node. The subcapsular sinus macrophages (SCSMs), which lie within the layer of endothelial cells lining the subcapsular sinus, are a distinctive subpopulation of macrophages with limited phagocytic ability. They express high levels of cell-surface molecules able to bind and retain unprocessed antigen
What initially occurs when a B-cell touches an antigen during T-dependent responses?
Interaction of BCRs with multivalent, cell-bound antigens induces a rather spectacular response of the B-cell membrane. First, a few BCRs and their cognate antigens interactat the initial site of contact. Changes in the submembrane network that anchors the cell-surface
receptors and coreceptors then allow the formation of microclusters of 50 to 100 BCRs with
associated coreceptors and signaling molecules.
Following this successful microcluster formation, the B-cell membrane rapidly spreads over the target membrane. This membrane-spreading response is quite dramatic and serves to increase the number of molecular interactions between the B cell and the antigen-bearing cell.
Figure 11-7 shows an experiment in which the antigen was presented on an artificial lipid membrane. This spreading reaction peaked around 2 minutes after antigen contact. After maximal spreading, the area of contact between the cell and the artificial lipid membrane began to contract, and by approximately 10 minutes after antigen contact, the antigen-receptor complex was gathered into a central, defined cluster with an area of approximately 16 μm
Once the antigen has been recognized and the B-cell BCR begin to do what regarding the assembly of signaling? (Large reason)
- Prior to antigen contact, the majority of B-cell receptors (BCRs) are expressed on the B-cell surface in tiny nanoclusters.
- Interaction of BCRs with antigens induces a rather spectacular response of the B-cell membrane. First, a few BCRs and their cognate antigens interact at the initial site of contact. Changes in the submembrane network that anchors the cell-surface receptors and coreceptors then allow the formation of microclusters of 50 to 100 BCRs with associated coreceptors and signaling molecules.
- Following this successful microcluster formation, the B-cell membrane rapidly spreads over the target membrane. This membrane-spreading response is quite dramatic and serves to increase the number of molecular interactions between the B cell and the antigen-bearing cell.
- During this antigen-induced oligomerization of receptor molecules, the BCR complex moves transiently into parts of the membrane designated as lipid rafts. Association of the BCR with lipid rafts brings the immunoreceptor tyrosine-based activation motifs (ITAMs) of the Igα and Igβ components of the BCR into close apposition with the raft-tethered, Src-family member tyrosine kinase, Lyn, and allows for initiation of the BCR signaling cascade These rafts are specialized regions on the membrane that are enriched in certain lipid and proteins, including Lyn, a tyrosine kinase that is essential for BCR signaling.
- By the end of the contraction phase of the membrane response, the BCR microclusters have collapsed into a single central cluster of receptors. This cluster of BCRs (the central supramolecular activation cluster, or cSMAC) is surrounded by a ring of adhesion molecules, including the integrin LFA-1, which is referred to as the peripheral supramolecular activation cluster, or pSMAC. The pSMAC is in turn encircled by an actin ring forming the distal, or dSMAC. The integrins promote adhesion of the B cells to the antigen-presenting cells, lowering the threshold of antigen-binding affinity required for B-cell activation. This arrangement corresponds to that formed on T cells following recognition of antigen-presenting cells and is known as an “immunological synapse.”
During this antigen-induced oligomerization of receptor molecules, the BCR complex moves transiently into parts of the membrane characterized as lipid rafts.
Association of the BCR with lipid rafts brings the immunoreceptor tyrosine-based activation motifs (ITAMs) of the Igα and Igβ components of the BCR into close apposition with the raft-tethered, Src-family member tyrosine kinase, Lyn, and allows for initiation of the BCR signaling cascade.
What cells are required for T-dependent responses: signal transduction?
Describe the whole process from antigen recognition to antigen internalization
- Encounter with Antigen: A B-cell can encounter an antigen through various mechanisms, such as by direct contact with a pathogen or by encountering soluble antigens that have been released by infected cells.
- Binding of Antigen to BCR: When an antigen binds to a BCR on the surface of a B-cell, it triggers a conformational change in the BCR that leads to the clustering of multiple BCRs around the bound antigen. This clustering can be facilitated by the concentration of BCRs within lipid rafts on the surface of the B-cell.
- Formation of the Immune Synapse: The B-cell can then spread out and form an immune synapse, which is a specialized structure that allows for the concentration and clustering of BCRs around the bound antigen. This involves the recruitment of various signaling molecules and cytoskeletal proteins to the site of BCR clustering, which can be facilitated by the coalescence of lipid rafts.
4.Contraction of the B-cell: Once the BCRs have formed a tight cluster around the bound antigen, the B-cell can contract around the antigen-BCR complex. This helps to create a tight, enclosed space that facilitates efficient signaling and processing of the antigen, and may be aided by the organization of lipid rafts within the plasma membrane.
- Receptor-Mediated Endocytosis: The B-cell can then internalize the antigen-BCR complex through a process called receptor-mediated endocytosis. This involves the formation of a clathrin-coated pit around the antigen-BCR complex, which is then internalized into the B-cell as a vesicle. The clustering of BCRs within lipid rafts may play a role in the recruitment of clathrin and other endocytic machinery to the site of BCR clustering.
- Processing and Presentation of Antigen: Once the antigen-BCR complex has been internalized into the B-cell, the antigen can be further processed and presented to other immune cells, such as T-cells. This helps to activate an adaptive immune response against the pathogen or foreign substance. The organization of lipid rafts within the plasma membrane may also be important for the efficient presentation of antigen to other immune cells.
Overall, lipid rafts play an important role in the organization and clustering of signaling molecules, including BCRs, during B-cell activation and antigen recognition. They are thought to help facilitate the formation of the immune synapse, the contraction of the B-cell around the antigen-BCR complex, and the internalization of the complex through receptor-mediated endocytosis. The organization of lipid rafts within the plasma membrane may also play a role in the efficient presentation of antigen to other immune cells.
What are the two nonexclusive methods for B cells to internalize a cell bound antigen?
What happens when a B-cell internalizes an antigen?
How has B cell chemokine direction migration been studied and why does it do this?
After antigen processing, what do B cells express, what is that for?
What does the above cause?
What is the germinal center response?
Following activation by antigen in the presence of T cells, B cells can differentiate into plasma cells, memory cells, or activated germinal center B cells that secrete the high-affinity antibodies of the late primary response.
Interaction between CD40L (CD154) on T cells and CD40 on B cells is key to continued B-cell proliferation and differentiation.
Those stimulated B cells that entered the follicles following an encounter with antigen begin to
divide rapidly and undergo further differentiation, resulting in the formation of specialized
structures called germinal centers (GCs). Although GCs consist primarily of rapidly dividing B cells, they also contain follicular dendritic cells (FDCs), T follicular helper (T ) cells, and macrophages.
What are germinal center dark and light zones?
Draw a picture of the light zone and dark zone. Label what is happening
What is somatic hypermutation and affinity selection?
Where does this occur?
What helps somatic hypermutation?
AID-induced formation of deoxyuridine, creates a U-G mismatch in the double-stranded DNA. Several alternative mechanisms then come into play that participate in the resolution of the original mismatch, and lead to the creation of shorter or longer stretches of mutated DNA.
Draw the different sections of a lymph node
Where (motifs) does somatic hypermutations occur?
Careful analysis of germ line Ig variable region sequences revealed that some sequence motifs were more likely than others to be targeted by the AID mutational apparatus. These sequences are referred to as mutational hot spots. Early experiments suggested that mutations preferentially accumulated at the G and C residues embedded within the sequence motif AGCT. Further analysis expanded this motif to the sequence
DGYW (Do good, you win)
HCRW (how can RNA work)
Where does switch recombination occur?
Describe the processes and features?
What signals are required for switch recombination to occur?
What are the molecular mechanisms for switch recombination? (5)
- Activation-induced cytidine deaminase (AID) is produced in B cells in response to T cell-dependent signals.
- AID binds to switch (S) regions upstream of each constant (C) region gene in the B cell’s DNA.
- AID then deaminates cytidines in the DNA to uridines, creating abasic sites (sites without a base).
- Uracil DNA glycosylase (UNG) recognizes and cleaves the abasic sites, creating a single-strand break.
- Endonucleases are recruited to the site of the single-strand break and cut the DNA at the S region, creating a double-strand break.
- The broken ends of the DNA are processed by exonucleases to create 3’ single-stranded overhangs.
- The overhangs are then coated with the recombinase enzyme, activation-induced cytidine deaminase (AID), and other factors to form a nucleoprotein filament.
What transcription factors are required to either push for plasma B cells or to create a germinal center?
What are plasmoblasts vs plasma cells?
The transcription factors that control whether antigen-stimulated B cells differentiate along the plasma cell or germinal center route are linked in a mutually regulatory network. Pax-5 and Bcl-6, along with low levels of IRF-4, favor the generation of proliferating, germinal center cells. Conversely, the expression of BLIMP-1 and of high levels of IRF-4 support the generation of antibody-secreting cells.
Where are plasma cells made?
What are plasma cells?
What transcriptions factors are required for plasma cells?
What are LLPC?
Starting at around 10 days after encountering antigen, a significant fraction of germinal center B cells up-regulate the expression of transcription factors that drive plasma cell fate. Thus, LLPC differentiation in the GC begins after the bulk of conventional memory cells have already been generated. Interactions between the T PD-1 receptor and its ligands, PD-L1 and PD-L2, on GC B cells are important in the formation of LLPCs.
How are LLPC maintained and where?
Within the bone marrow, LLPCs receive cytokine signals provided by mesenchymal stromal cells (CXCL12) and by eosinophils and megakaryocytes (APRIL). These cytokines induce the up-regulation of anti-apoptotic molecules such as Mcl-1, that support long-term survival of the LLPCs. Plasma cell longevity also appears to depend on specialized cellular metabolism; for example, plasma cells engage in autophagy, thus enhancing their supply of nutrients.
What are memory B-cells and how do they differ from LLPCs?
What are the two different types of memory B cells?
Analysis of the recall responses of the IgM- versus IgG-bearing memory cell pools has revealed that they have different sensitivities to inhibition by serum antibody, which results in part from differences in the affinities of their respective receptor molecules—high-affinity, somatically hypermutated receptors will compete more effectively for antigen binding than will low-affinity
receptors (Figure 11-22). Antigen restimulation of IgM-bearing memory B cells is readily inhibited by the presence of antigen-specific antibodies and so, during a recall response, these low-affinity IgM memory cells tend not to respond until after the antibody levels decline. In contrast, IgG bearing memory cells are not susceptible to this inhibitory response. This suggests a biphasic recall response in which the IgG memory cells are stimulated very quickly on secondary antigen encounter, and the IgM-bearing memory cells respond later. Stimulation of IgM memory cells results in their entry (or re-entry) into the GCs, where they may undergo somatic hypermutation, generating new memory cells for subsequent responses. This division of labor between the two types of memory B cells decreases the tendency of the memory responses to be exhausted by frequent encounters with the same, or cross-reacting antigens. It further allows the lower affinity,
IgM-bearing memory cells to mutate in response to variants of the original antigen (e.g., mutated forms of viruses such as influenza, whose antigenic determinants may change over the course of a single infection), producing new, high-affinity, IgM- and IgG-bearing cells.
How is the adaptive immunity stopped?
What are T-Independent B-cell response?
What are the different types?
What B-cells mediates response to T-Independant antigens
What are MZ B cells
Marginal zone (MZ) B cells, which reside in the marginal zone of the spleen (Figure 11-24), also
fall into the “innate-like” category of B cells capable of responding to TI antigens. The unique
location of this subset gives it rapid exposure to antigens that are carried to the spleen via the
blood. Typically, antigens that are presented to MZ B cells are captured by metallophilic or
marginal zone macrophages in the marginal sinus, or alternatively are picked up by neutrophils or
dendritic cells in the circulation and hence brought into the MZ.
