B cell Development Flashcards
Which cells produce antibodies?
Plasma cells (B cells)
What is the structure of the antibody?
It has 2 heavy chains and 2 light chains, they are held together by disulfide bonds. It has 2 antigen binding sites.
B cells
- They produce antibodies
- There is a lot of them and each B cell is unique and recognises different antigen
- Each B cell expresses many copies of B cell receptor (BCR) and each BCR on that cell recognises the same antigen
- Foetal B cells are made in the liver and after birth they are made in the bone marrow
- Majority B cells will die but if they engage their target they can differentiate either in plasma cell or memory cell
- poor control of B cell responses result in autoimmunity
- defects in antibody production lead to high susceptibility to infection and very early death
- Naive B cells express shared molecular signatures (CD19, CD20) but they differ in BCR.
What’s the key aim of vaccination?
To induce antibody production
Overview of possible life of a B cell
- B cell precursor rearranges its immunoglobulin genes in the bone marrow
- If immature B cell binds to self antigen it is removed from the repertoire (negative selection in the marrow)
- Mature B cell if binds its antigen becomes activated, it migrates to the peripheral lymphoid organs
- Activated B cell give rise to plasma cells and memory cells
How are B cells made?
- B cells arise from stem cells
- Firstly, rearrangement of heavy chain (firstly D-J then V-DJ)
- B cell with rearranged heavy chain has a pre-B receptor on their surface
- Because they don’t have a light chain yet, they use surrogate light chain to check if the heavy chain is functional
- Now rearrangement of light chain (V-J)
- Now you have immature B cells with IgM immunoglobulin on the cell surface
- B cell leaves the bone marrow and finishes its maturation. The mature B cell expresses IgM and IgD on the surface
What RAG genes do?
They are expressed on immature T and B cells. They are needed for the rearrangement of TCR and BCR.
What is allelic exclusion?
It’s a process to ensure only one allele of the gene is expressed while the other is silenced.
When it comes to BCR rearrangement:
- Rearrangement can happen from either chromosome, but only one at a time. If the gene rearrangement from the first chromosome was successful, it will stop the rearrangement from the other chromosome but if was not successful then the rearrangement from other chromosome happens
- There are 2 light chains kappa and lambda
- 2 kappa genes are used first and then 2 lambda genes (if not successful)
- This ensures that one B cell will produce an antibody of one specificity
What happens if during development B cell recognises self antigen?
It gets removed or edited. There is no AIRE in B cell development, so the antigens these B cells during development can encounter are extracellular antigens and not intracellular. Hence, the negative control for B cells is good but not perfect, and many reactive B cells get through, and they recognise dsDNA or other intracellular antigens.
Reasons for B cells to be deleted during development?
- Multivalent self molecule - if antigens are spaced closely to each other and B cell binds multiple BCRs at the same time to multiple antigens, it causes very strong signal, then its deleted or the receptor is edited,
- Soluble self molecule - strength of the signal is weaker, usually happens in periphery and it leads to cell anergy.
What happens if B cell does not recognise self?
It migrates to the periphery and matures to naive B cell
Summary of development
- Pro-B cells - heavy chain gene rearrangement
- Pre-B cells - light chain gene rearrangement
- Immature B cells - express surface IgM and exit bone marrow but if strongly binds antigen is eliminated (negative selection)
- Mature B cells - express IgM and IgD on their surface, reside in the follicles of lymph nodes and the spleen and circulate between different lymph nodes
What type of cells can naive B cells develop into after encountering their antigens?
- plasma cells - secreting antibodies
- memory B cells - do not secrete antibodies unless antigen is re-encountered
What are different types of antibodies?
- IgM - on the surface of naive B cells, are first to be released when antigen is encountered, pentamer
- IgD - on the surface of naive B cells, low level in serum
- IgG - secreted by memory B cells, there are 4 subtypes (IgG1, IgG2, IgG3, IgG4), the most numerous in serum
- IgE - are on the surface of Mast cells, low number in serum
- IgA - found in mucosa serum, there are 2 subtypes (IgA1 and IgA2)
Resting/Naive B cell
- High surface Ig as didn’t encounter its antigen yet
- High expression of MHC class 2 to communicate with CD4 T cells
- Does not secrete antibodies
- Growth
- Somatic hypermutation
- Isotype switch
Plasma cell
- Low surface Ig as it already encountered its antigen
- No MHC class 2 expression
- High rate of Ig secretion
What are 2 main pathways for generating antibody responces?
- T-independent antibody responses - extrafollicular, B cells differentiate to become IgM and IgG plasma cells producing antibody of low affinity.
- T dependent antibody responses - germinal centre responses, selective process to generate plasma cells of high affinity, class switch, leads to production of antibodies and memory B cells.
T independent responses
- purified bacterial capsular polysaccharide
- no involvement of T cells
- IgM and IgG, low affinity
T dependent responses
- Protein
- Involvement of CD4 T cells
- IgM, IgG, IgA, IgE, high affinity
- Memory
How do adaptive responses develop?
- DC recognises a foreign antigen, presents it via MHC class 2 to a CD4 T cell,
- This T cell interacts with the antigen specific B cell
- This B cell can then migrate to the red pulp of spleen or medulla of lymph nodes to form short-lived plasma cells
- Or it can migrate to the follicles and germinal centres together with T cell and it produces long-lived plasma cells and memory cells
What 2 signals are required for T-dependent antibody responses?
- Direct, physical contact between CD4 T cell and B cell
- Cytokines (IL-4 and IL-21)
What are Tfh cells?
- CD4 T follicular helper cells
- Essential for the generation of T-dependent antibody responses
- Recognise peptides through MHC class 2
- They get activated by DCs, and they recognise signals from MHC class 2 from B cells
- Co-stimulation of CD40 and CD40L
- Co-stimulation with cytokines (IL-4 and IL-21)
What happens in the absence of CD40/CD40L?
Hyper IgM syndrome where patient does not produce productive antibody responses, no class switch, greater risk of infections
T independent response
- TI-2 antigens e.g. purified bacterial capsular polysaccharide, which is a multivalent antigen (repeats over and over and over on the surface)
- not a protein, so no T cell involvement
- very important for vaccines
- Responses are rapid and peak within days, no switching to IgG
- negligible memory and no boosting effect, second vaccine will show slower response rather than faster (hyporesponsiveness)
- short lived responses, typically around 2 years
- TI-2 vaccines not effective in infants under 5 years old because of small range of B cells
Conjugate vaccines
Conjugate purified capsular polysaccharide with a protein to involve T cells in the response and provide memory. These vaccines are effective in infants, create memory and save lives.
Germinal centres
- necessary for the production of long-lived plasma cells and memory
- class switch, high-affinity antibodies
- B cells need to undergo further recombination of their Ig V genes
- needs to be regulated to avoid autoimmunity and cancer
The process of forming germinal centres
- DCs pick up antigen
- B cells pick up antigen
- they travel to the T zone
- DC activates the T cell
- T cell interacts with B cell
- Some T cells and B cells migrate to the follicles and develop germinal centres
Players in germinal centres
- Centroblasts - proliferating B cells that undergo somatic mutation - dark zone
- Centrocytes - B cells that have undergone affinity maturation and are out of cell cycle - light zone
- Follicular Dendritic Cells - different to dendritic cells, they have antigen in its naive form bound to their surface - light zone
- Follicular T helper cells - germinal centre T cells which give survival signals to centrocytes after they come out of the dark zone - light zone
AID
Essential for class switch and IgV hypermutation
BCL-6
master transcription factor for the commitment of germinal centre B cells and is required for the generation of Tfh cells
What are somatic hypermutations and affinity maturation for?
To help antibody to increase its affinity. It happens in the germinal centre
germinal centre steps
- centroblast undergoes somatic hypermutations in the dark zone
- centroblasts leave the dark zone and become centrocytes in the light zone
- centrocytes compete for the antigen presented by follicular dendritic cells
- the centrocytes with high affinity get survival signals from Tfh cells, the rest die of neglect
- surviving cells can differentiate into plasma cells and memory B cells
