B Cells 2 PDF

B Cells 2 Emily Gwyer Findlay –  Outline of lectures 1.Overview of B cells 2.Structure and diversity of the B cell receptor and antibodies 3.B cell development and activation 4.Antibody subclasses and their functions 5.Plasma cells and memory cells  Maturation process...

B Cells 2 Emily Gwyer Findlay –  Outline of lectures 1.Overview of B cells 2.Structure and diversity of the B cell receptor and antibodies 3.B cell development and activation 4.Antibody subclasses and their functions 5.Plasma cells and memory cells  Maturation process heavy chain recombination light chain recombination heavy chain recombination BONE MARROW PERIPHERY  What do B cells do? They release ANTIBODIES One (mathematical) model suggests we may each be able to make one quintillion unique antibodies, and have up to one trillion at any one time Which type is induced depends upon the challenge and the antigen that each B cell has seen  T cell help B cells respond to antigen in a thymus dependent or thymus independent manner Thymus dependent – The B cell requires T helper cells Thymus independent – No T helper cells required  Thymus independent antigens The part of an antigen which is seen by an antibody or TCR Typically polysaccharides with repeating sequences of sugars (therefore multiple repeats of the same antigens) These sugar epitopes bind surface IgM and IgD. These cluster and a strong activatory signal is generated. The B cells release IgM  T cell help ANTIGEN DEPENDENT maturation requires the help of T cells which recognize the same antigen. This is called LINKED RECOGNITION. Like two factor authentication for B cells T cell help  T cell help Th cells express CD40L – stimulates B cells to proliferate and become plasma cells IL4 drives proliferation IL5 and IL6 drive plasma cells  B cell proliferation occurs in germinal centres o B cells become temporarily trapped in the T cell zone to come into contact with antigen and T helper cells o The germinal center is made up of proliferating B cells. It forms 7-10 days after an infection o The mantle zone is made up of resting B cells displaced by activated B cells o In the germinal centre B cells undergo somatic hypermutation, affinity maturation and class switching  Somatic hypermutation Some of the new CDRs bind antigen better than the original, and these can be preferentially chosen to mature into plasma cells. This is called affinity maturation This allows for B cells to develop receptors with a hugely increased ability to recognise antigens  Somatic hypermutation Further diversifies antibodies to generate a more specific antigen response Introduces point mutations in the V region of the light and heavy chain AID is a cytidine deaminase that introduces nicks in the DNA that are ‘repaired’ Requires a single strand of DNA – Targets DNA that is being actively transcribed e.g. Ig genes  Affinity maturation As antibody V regions are mutated the B cells are selected based on their ability to bind antigen As somatic hypermutation progresses the antibodies become more specific for antigen – Affinity maturation  Vaccination As the humoral immune response progresses, antibodies with higher affinity are produced Vaccination leads to higher affinity antibodies  Class switching Early in an immune response B cells produce IgM Class switching occurs through DNA recombination Changes to IgG, IgA or IgE during the immune response after T cell help Allows a different C region to be used in an antibody that has a specific antigen binding region Class switching o The variable region (VDJ) stays the same but the constant region is changed o Therefore antigen specificity does not change o Instead, the constant region changes so the OUTCOME of antibody binding is altered, as different subclasses have different effector functions Wikipedia  Generation of diversity BONE MARROW Antigen independent: Pairing of different heavy and light chain Recombination Variability on the joins of the recombined gene segments P- and N region nucleotide addition PERIPHER Y Antigen dependent: Somatic hypermutation Class switching and affinity maturation What do antibodies do?  Fc receptors Fc receptors bind to the Fc of Ig molecules These receptors are specific for different antibody subclasses and isotypes Found on many immune cells – NK cells, mast cells, neutrophils, eosinophils  Fc receptors 1. Phagocytic cells can internalise antigen through Fc receptor binding 2. Natural Killer Cells will kill target cells through Antibody Dependent cell mediated cytotoxicity (ADCC)  Cytokine production B cells are much slower than T cells in cytokine production and it is less of a critical function for them (but cytokines are still produced) Naïve B cells do not make cytokines They produce chemokines to induce the migration of T cells, to assist with T cell help They can also produce cytokines which regulate the responses of immune cells around them, for example IL-2 which boosts T cell proliferation, of TNF which drives inflammatory responses  Antibody subclasses – IgM is the first antibody subclass expressed Expressed before somatic hypermutation - low affinity – IgD is expressed on activation and is a cell- surface antigen receptor – IgG opsonizes pathogens for phagocytosis. Most abundant, can cross the placenta – IgA functions on epithelial surfaces – neutralization. Mucosal antibody – IgE localises with mast cells on the mucosa  Subclasses IgM First antibody made by B cells Usually pentameric – five antibodies linked by disulphide bonds This means it has high avidity for antigens and activates complement strongly IgD Not released – bound to the B cell surface. It signals to mature and activate the B cell  Subclasses IgG The majority of antibodies in humans are IgG – about 75% in blood There are 4 subclasses of IgG (IgG1, 2, 3, 4), which have slightly different roles Monomeric and therefore small. Diffuses well out of tissues Triggers antibody-mediated cellular cytotoxicity The only antibody that can cross the placenta. IgG from the mother is key to providing immunity to pathogens in the first months of life  Subclasses IgA IgA is about 10% of all antibodies in humans. Common at mucosal sites – found in mucus, saliva, breast milk, and in the intestine. Key for defence in the gut IgE IgE is the least common antibody Main role is binding to mast cells and cross linking their receptors (Fc receptors). This induces degranulation of the mast cells – key for attack of pathogens especially parasites This is a key trigger of allergic responses  BCR –not secreted First one made Class switching produces these  Class switching What antibodies would you see in the plasma of a patient who had a deletion in the CD40L gene?  Further maturation Surviving B cells within the germinal center differentiate into plasma cells or memory cells – Controlled by BLIMP1 – switches off proliferation and affinity maturation Memory B cells have undergone affinity maturation and are long lived – They have a BCR that is highly specific for an antigen but do not secrete antibody  Plasma cells Can make 2000 antibodies / second Only live a few days. Do not proliferate or switch classes, no longer have MHC II  Memory B cells Antigen-specific memory B cells respond to antigen rapidly and proliferate much faster than naïve B cells The antibodies they make are of higher affinity and also continues to increase throughout the secondary response IgG is produced more quickly than in primary challenges, with less IgM, as the cells have already switched classes The course of an infection The antibody class switches during the course of an infection  Dysregulated B cell responses B cell responses sometimes go wrong The most common result is systemic lupus erthymatosus (SLE or lupus) In healthy people, B cells which make antibodies to SELF proteins are deleted. In SLE, this goes wrong and B cells survive which make antibodies to nuclear components such as DNA, RNA and histones Patients also have more B cells and more memory cells – these have a lower activation threshold = even more self-reactive antibodies  90% of SLE patients are women, commonly diagnosed in late teens / early 20s When the antibodies bind antigen, they are then deposited in either the kidneys or the joints. This causes serious problems – 20% of patients need a kidney transplant  Summary B cells express a B cell receptor that has the same structure as a secreted antibody An antibody is made of 2 heavy chains and 2 light chains that form a Variable region and Constant region Antibody diversity is formed through VDJ recombination B cells develop and undergo VDJ recombination in the bone marrow Activation of B cells occurs in the periphery, particularly in lymphoid tissues  Summary II B cells present antigen to T helper cells and receive positive cytokine signals In the germinal centers B cells undergo: – Somatic hypermutation – Affinity maturation – Class switching Antibodies function by: – Neutralization – Opsonization – Complement The memory immune response is due to higher affinity antibodies The classes of antibodies have different roles