L#7 & #8: B-Cell Differentiation and Activation
L#7 & #8: B-Cell Differentiation and Activation 0530
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This 3 page Class Notes was uploaded by Denise Croote on Saturday January 23, 2016. The Class Notes belongs to 0530 at Brown University taught by Dr. Richard Bungiro in Fall 2013. Since its upload, it has received 21 views. For similar materials see Principles of Immunology in Biology at Brown University.
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Date Created: 01/23/16
Lecture Seven & Eight: B Cell Activation and Differentiation All of the lg receptors on a single B cell’s surface have identical specificity and any clones of that B cell will have the same specificity and will produce Ab with the same antigen specificity B cells with receptors for self Ag will be deleted during development At the close of the immune response, B cells with specificity for the Ag will stick around and become memory B cells Naïve B cells have IgM and IgD. When they come into contact with a microbe they become activated. They clonally expand and from there they can….. o Differentiate into effector cells and secrete Ab o Class switch to express IgG or another isotype o Affinity mature into a higher affinity IgG o Become a memory B cell During the secondary response if a memory B cells is activated you still get the clonal expansion but you do not get as much class switching or affinity maturation because the B cell has already undergone those specifications Types of B cell antigens: o Thymus dependent (TD) – antigens require interaction of B and T cells for optimal antibody production o Thymus Independent (TI) – antigens can induce Ab production without the presence of T cells They can be mitogens that activate TLR regardless of Ig specificity They can have repeating epitopes that extensively cross link with proteins on the B cell An example is LPS – antigen sends a very strong signal B cells responses to TI antigens do not involve isotype switching, affinity maturation, or memory b cells Polyclonal activation refers to how TI antigen can activate the B cell regardless of BCR specificity Once they have matured and survived negative selection B cells will migrate to the lymph or re- circulate in the blood looking to bind to Ag. If they do not contact an Ag after a while they can become apoptotic or anergic. In order to become activated a naïve B cell must: o Have receptor engagement by antigen o Interact will a T helper cell activated by the same antigen B cell must ingest the protein, break it up, and present its pieces on the MHC to the helper T cell In order to proliferate a B cell must receive cytokine driven progression signals from T helper cells. A BCR receptor has the Ag binging Ig but it also has ITAMS – ITAMS are signal transducing molecules that communicate with the inside of the cell. A BCR also has C3d receptors, C3d is part of the complement system and is good at increasing the likelihood the B cell gets activated ITIMS are inhibitory signaling receptors and they down regulate the activity of the BCR. ITIMS are always working to inhibit because you would not want your B cells constantly producing antibody. Therefore to begin producing Ab you need a very strong signal and constant antigen presence for activation Events in a sequence of Activation: o 1.) Ag binds to Ab Ig and the two form a cross linked complex. The complex is internalized, broken down, and presented on MHC Class II for helper T cells to recognize o 2.) The T helper cell that matches begins to express CD40L and this interacts with the CD40 on the B cell to serve as signal 2. o 3.) B cells begin to produce receptors for cytokines and the T helper cells begin to release cytokines that trigger the B cell to proliferation (synthesize DNA ect.) When the B cell binds the antigen, the initial contact is small but then a conjugate forms with extensive contact between the antigen and the B cell Peripheral Tolerance- not all of the self antigens out there are present in the primary lymphoid organs, therefore B cells specific for self Ag may escape into the blood stream. These could be Ag on the peripheral organs. The self reactive cells must be tolerized in the periphery and this may occur by clonal anergy or deletion via apoptosis. In a secondary humoral response, the memory B cells activate more quickly, have a peak response sooner, have a higher affinity for the Ag, and are longer lived. IgM predominates in the primary response and IgG predominates in the secondary response Haptens: small organic molecules that are antigenic but not immunogenic, they contain B cell epitopes but not T cell epitopes so they need to be coupled with a carrier that is capable of activating T cells For a primary antihapten response – you need B cells specific to the hapten and T cells specific to the carrier For a secondary response you need hapten specific memory B cells and carrier specific memory T cells o DNP- BSA primary and secondary immunization will give you a response o DNP+BSA primary and DNP-BSA secondary will not give you a response because you will not generate memory B cells for the hapten because it is not conjugated to a carrier protein and does not have a molecule that helps recognize it o DNP-BSA primary and DNP+BSA secondary – here you will not get a response even though you create memory B and T cells because there is nothing to recall the memory cells in the secondary response. Hapten is not coupled so it cannot be recognized. o DNP-BSA primary and DNP-BGG secondary will not give you a response because there are no memory BGG cells. This shows the carrier effect. You must have seen the carrier before (conjugated or not conjugated) for this to work o DNP-BSA + BGG primary and DNP-BGG secondary you will get a response because here you have memory B cells for DNP and memory T cells for BGG so you are fine. Secondary Mouse Experiment: injecting DNP-BSA and BGG from two mice into one mouse will give you a DNP-BGG response. Knocking out the thymus of the mouse supplying BGG gives no DNP-BGG response (because no memory T cells) and knocking out the thymus of the mouse that provides the DNP-BSA will still give a DNP-BGG response because it still has memory DNP cells and it has the BGG cells from the other mouse. Hapten Vaccinations: some bacteria have peptides on their structure that we can create vaccines for even though these peptides are TI antigens. o Children under 2 years old mount poor TI antibody responses but if you couple the TI antigen to a protein carrier you can increase immunogenicity o B cells will bind the hapten and internalize the hapten and its carrier. They will then present it to a T cell and you will get memory hapten and memory T cells. You can produce antibodies. o If you were to encounter the real pathogen you would not recall memory cells because the real pathogen is not coupled to a carrier protein. o The point of the vaccine is to amount enough Ab in your blood to counter the pathogen when it arrives. Catching the antigen occurs in the secondary lymphoid organs. Ag is carried via the lymphatic’s to the lymph nodes. DCs in the lymph nodes grab the Ag and present it to T cells. T helper cells then interact with B cells that have interacted with the same antigen, providing the second signal B cells need for activation B cells that have bound to the antigen and T cells that have bound the antigen will migrate from their respective areas (b cells zone and the paracortex) to each other Germinal centers are clusters of activated B cells that are all competing for resources. Those with the highest affinity for the antigen will win, and the others will apoptosis. Follicular dendritic cells bind Ag-Ab complexes and allow B cells to interact with them Isotype switching (switching the constant region) and affinity maturation occur here These B cells are competing to interact with the T cells nearby Some B cells become short lived plasma cells that secrete antibodies, while others (from TD antigens only) become memory cells and recirculate in the blood for up to 90 years. Once you go from a naïve B cell (that can undergo class switching and growth) to a plasma cell you can no longer undergo class switching, or growth, and because you are secreting Ab you have low surface Ab) Class switching is when the constant regions of H chains are rearranged, leading to a variety of activated B cells expressing different isotypes (IgG, IgE, IgA ect) – this change does not affect the variable region so the antigen specificity stays the same The process of class switching is mediated by cytokines like IL4 Somatic hypermutation is the generation of point mutations at a high rate in the V regions of the H and L chains of B cells. This much mutation would be lethal in non-lg genes o A few of these mutant B cells will have a higher affinity for the Ag and are selected for additional proliferation but other will undergo apoptosis from lost affinity o Affinity maturation assures that the memory B cells that persist have the highest capability for Ag binding. o As you go from a primary, to a secondary, to a tertiary response you will notice that the amount of DNA mutation increases drastically, leading to a higher affinity B cell.
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