Humoral adaptive immunology Flashcards
Lectures: -Week 1, day 3, lecture 2: Adaptive immune response - humoral -Week 3, day 3, lecture 2: Developmental aspects - B-cell development
What do we mean by humoral immunity?
Humoral immunity refers to soluble macromolecules that are directed against pathogens
What are the three main groups of macromolecules involved humoral immunity? Are they innate or adaptive?
- Antimicrobial peptides, innate
- Complement proteins, mostly innate (can also be activated by antibodies)
- Secreted antibodies, adaptive
Which two phases can be distinguished in B-cell development?
- Antigen-independent B-cell differentiation in the bone marrow
- Antigen-dependent B-cell differentiation in perhipheral blood
What are the three main functions of antibodies?
- Neutralization of particles
- Opsonization: improve uptake by phagocytes
- Complement activation (classical pathway)
Where does gene rearrangement of B-cells take place?
Bone marrow
What is the structure of an antibody?
2 identical heavy chains + 2 identical light chains (either κ or λ), connected by disulfide bonds
How do antibodies promote phagocytosis?
By FcR-mediated endocytosis
Which complement pathway is activated by antibodies?
Classical pathway
Which domain of the antibody determines its antigen specificity?
Variable domain
Which domain of the antibody determines its biological function?
Constant domain
What is the difference between a B-cell receptor (BCR) and an antibody?
Antibodies are the BCR without a membrane anchor, allowing them to be secreted
These are created from the same gene through alternative splicing on RNA-level
Which 3 mechanisms add to antibody diversity?
- VDJ-recombination
- Combination of different heavy and light chains
- Junctional diversity: p- and n-nucleotide addition
What does the membrane anchor of a BCR consist of?
~25 hydrophobic amino acids
How is made sure that only one of the heavy and one of the light chain gene is expressed?
Allelic exclusion -> when a succesfull rearrangement is made and the (pre)receptor is functional, the other allele is shut down
What happens when the first rearrangement of the heavy chain is not succesful?
The B-cell will try to produce another succesfull arrangement with the other heavy chain allele
What is a pre-B-cell receptor (pre-BCR)? What is its function?
The product of the combination of a rearranged heavy chain with a surrogate light chain. Is used to test the heavy chain.
Can a pre-BCR bind antigen?
The pre-BCR cannot bind antigen
Where does negative selection of B-cells take place?
Bone marrow; it should not react to any self-antigen present there
What options are there when a B-cell reacts to self-antigen in the bone marrow?
- When binding a multivalent self-molecule: clonal deletion or receptor editing
- When binding a soluble self molecule: migration to periphery where the B-cell enters a state of anergy
- When binding a low-affinity non-cross-linking self molecule: migration to periphery, where the B-cell becomes clonally ignorant
What is receptor editing?
The B-cell will do another light chain arrangement to try to produce a BCR that is not self-responsive
What is an epitope?
The part of an antigen that is recognized by a BCR/TCR
What kind of interaction does an antibody have with an epitope/antibody?
Non-covalent binding
Which two signals are required for B-cell activation?
- BCR activation
- Encounter of repetitive structures (thymus-independent) or stimulation by Th-cells (thymus-dependent)
What are thymus-independent antigens?
Antigens that can activate a B-cell without the need for a Th-cell; these require repetitive structures on the surface of pathogens because this allows multiple BCR’s to activate simultaneously and crosslink, resulting in a strong signal
Example: LPS
What are thymus-dependent antigens?
Antigens that require Th-stimulation in order to activate B-cells
What is the difference between B-cells that can be activated by thymus-independent antigens and B-cells that are activated by thymus-dependent antigens?
B-cells that are activated by thymus-independent antigens don’t undergo a germinal centre reaction
What class of antibodies is secreted by B-cells that are activated by thymus-independent antigens, and why this class?
IgM, B-cells that are activated by thymus-independent antigens don’t undergo a germinal centre reaction, and therefore do not undergo class switch recombination
What is the process of thymus-dependent B-cell activation?
- B-cell takes up the antigen and presents it in MHCII to a Th-cell
- If the Th-cell recognizes the same antigen, it will provide costimulation to the B-cell, activating it
True or false: a Th-cell recognizing antigen presented by a B-cell in MHCII has to be specific for the same epitope as the B-cell
False -> they don’t have to be specific for the same epitope, as long as they recognize the same antigen
What is an additional requirement for Th-cells to be able to be activated by B-cells and in turn activate the B-cells?
The T-cell has to be primed by a DC (presenting the same antigen)
Which receptor-ligand interaction takes place between the Th-cell and the B-cell in order to activate the B-cell?
CD40 (B-cell)/CD40L (T-cell)
Why does thymus-dependent B-cell activation cause a stronger immune response? (2)
- Leads to long lived plasma cells (thymus-independent activation does not)
- Induces somatic hypermutation and class switch recombination
What is linked recognition?
A T-cell and B-cell recognizing the same antigen, and thus being able to activate eachother (not the same epitope!)
What is the process of B-cell activation in the lymph node?
- DC loaded with antigen enters the lymph node through afferent lymphatics and activates a Th-cell
- Th-cell activates B-cells -> begin to divide
- Some of these B-cells will encounter antigen in the lymphatic fluid in the T-cell zone, causing their BCR to be activated. These cells will form a primary focus.
- Some proliferating B-cells migrate to a follicle to form a germinal centre, others leave the lymph node as plasma cells
Which two processes take place in the germinal centre reaction?
- Somatic hypermutation
- Class switch recombination
In which part of the follicle does somatic hypermutation take place?
Dark zone of the follicle
Which process takes place in the light zone of the follicle?
The light zone contains follicular dendritic cells that present antigen to B-cells that have undergone affinity maturation; B-cells that have a high affinity get selected to proliferate, B-cells that have lower/no affinity get selected out and undergo apoptosis
Where does apoptosis of B-cells that have a low/no affinity after somatic hypermutation take place?
The mantle zone of the follicle
How long after antigen stimulation does a germinal centre reaction take place?
~1 week after first stimulation
True or false: follicular dendritic cells are not of haematopoietic origin
True: these cells are not of haematopoietic origin, but are part of the stromal cells of the lymph node
How do follicular dendritic cells bind antigen and present it to B-cells? What is this process like?
Follicular DC’s bind and present antigen in Fc-receptors and complement receptors (CR’s). B-cells take up this antigen and process it to present it on MHCII to T-follicle helper cells (Tfh-cells) -> these provide survival signal to the B-cells
How do cutting enzymes identify where to make DNA-breaks for class switch recombination?
Switch regions: repetitive sequences of DNA that allow for recombination
Why can cells only undergo class-switching to a class that is downstream of their current class?
In class switch recombination, all upstream genes are deleted
How do Th-cells direct class switch recombination?
Different subsets of Th-cells produce different cytokines, that induce class switching to different classes (e.g. TH1 = IFN-γ -> switch to IgG2/IgG3, whereas Th2 = IL-4 -> class switch to IgG1/IgE or IL-6 -> class switch to IgA)
Which enzyme is involved to generate gene mutations in somatic hypermutation and class switch recombination? How does the same enzyme induce different effects?
AID
In somatic hypermutation, AID introduces a low density of AID-dependent lesions -> leads to mutations
In class switching, AID introduces a high density of AID-dependent lesions -> ssDNA-breaks that allows for the occurrence of dsDNA breaks, which get repaired by non-homologous end-joining or alternative end-joining
What is the ‘default’ class of Ig of B-cells before class switching?
IgM/IgD
How can a B-cells that has not undergone class switch recombination produce IgM antibodies and IgM/IgD BCR’s?
Alternative splicing of the same primary transcript to a Fc-domain (IgM antibody), μ-domain (IgM BCR) or δ-domain (IgD BCR)
True or false: in class switch recombinations, frameshifts can occur
False: the switch regions ensure recombination without frameshifts
What is the structural makeup of IgM?
Pentamer bound together by J-chains
What is the structural makeup of IgA?
Dimer bound together by J-chains
What is the advantage of multimerization of antibodies in IgA and IgM?
Allows for higher avidity -> low affinity (mainly IgM) compensated by the fact that there are many binding sites in the antibody multimer
What is the avidity of an antibody? When is a high avidity especially important?
Affinity*number of antigen binding sites
A high avidity is important to bind pathogens with repetitive structures on their surface
Where in the body is IgM mainly found, and why?
In the blood, because the high molecular weight of the pentamer makes it diffucult to penetrate into the tissue
Why is the affinity of IgM low?
B-cells that produce IgM have not undergone a germinal centre reaction, and therefore also no somatic hypermutation
In which of the three functions of antibodies (neutralization, opsonization, compliment activation) is IgM specialized?
IgM mainly induces a strong complement activation
Which isotype of antibody is most abundant in serum?
IgG
Which types of IgG can be identified?
IgG1, IgG2a, IgG2b, IgG3, IgG4
Which isotype of antibody can be moved through the placenta?
IgG
Which of the three functions of antibodies (neutralization, opsonization, complement activation) are carried out by the different IgG’s?
IgG1 = neutralization, opsonization, complement activation (+NK-activation)
IgG2 = neutralization, complement activation (weak)
IgG3 = neutralization, opsonization, complement activation (strong)
IgG4 = neutralization, opsonization (weak)
Where in the body is IgA found?
Mucosal tissues and secreted fluids
Where are IgA-secreting plasma cells located?
Lamina propria of mucosal tissues
What is the name of the process that allows IgA to cross the epithelium? What are the steps of this process? (4)
Transcytosis:
1. Binding to poly-Ig-receptor
2. Endocytosis
3. Vesicular transport
4. Exocytosis
How does IgA protect the mucosal tissues from pathogens?
Prevens attachments of pathogens to the epithelium (neutralization)
How can maternal IgA be transferred to a child?
IgA is secreted in milk, and can be taken up into the blood in the gut
The serum concentration of IgE is [high/low]
Low
Where can IgE be found?
IgE can be found bound to high affinity Fc-ε receptors on mast cells and basophils
What is the effect of an antigen binding to IgE bound to mast cells/basophils? What is released, and what does this often result in?
Degranulation, releasing histamine, prostaglandins and leukotrienes -> allergic reactions
Where are mast cells located?
In the mucosal tissues of the gut, in the skin and in the lungs
Where are basophils located?
Basophils that have not bound IgE circulate in the blood; upon binding IgE they move into tissues
What is the original immunological function of IgE and mast cells/basophils?
Immune responses against parasitic worms
What is the function of IgD?
The function of IgD is mostly unkown
What are the characteristics of plasma cells? (4)
- Secretion of large amounts of antibodies
- Large cytoplasm and high density of RER
- No expression of membrane-bound Ig or MHCII
- Lifespan: several weeks up to years
What are the characteristics of memory B-cells? (3)
- Only membrane-bound Ig
- Rapidly reactivated upon re-encounter with antigen
- Can survive for many years
