Lecture 2 - B cells Flashcards
Development and activation of B cells
- The development of B-cells begins in the bone marrow and is driven by IL-7 produced by marrow stromal cells.
- Cells are activated by encountering their cognate antigen
- Activated B cells secrete antibodies
- Activated B-cells develop into long-lived plasma and memory cells which persist in lymph nodes or return to the bone marrow
Precursors of B1 and B2 cells
- Fetal liver–derived HSCs are the precursors of B-1 cells.
- These respond to T-independent antigens and make natural antibodies reactive with polysaccharides.
- Bone marrow–derived HSCs give rise to the majority of B cells (B-2).
- These cells develop into marginal zone B cells or into follicular B cells
Stem cell (development and response to antigen)
Anatomic site: Bone marrow
Ig DNA, RNA: Unrecombined/germline DNA
Ig expression: None
Surface markers: CD43+
Response to antigen: None
Pro-B cell (development and response to antigen)
Anatomic site: Bone marrow
Ig DNA, RNA: Unrecombined/germline DNA
Ig expression: None
Surface markers: CD43+, CD19+, CD10+
Response to antigen: None
Pre-B cell (development and response to antigen)
Anatomic site: Bone marrow
Ig DNA, RNA: Recombined H chain gene (VDJ); u mRNA
Ig expression: Cytoplasmic u and pre B-receptor associated u
Surface markers: B220lo, CD43+
Response to antigen: None
Immature B cell (development and response to antigen)
Anatomic site: Periphery
Ig DNA, RNA: Recombined H chain gene (VDJ), k or λ chain genes, u or k or λ mRNA
Ig expression: Membrane IgM (u + k or λ light chain)
Surface markers: IgMlo, CD43-
Response to antigen: Negative selection (depletion), receptor editing
Mature B cell (development and response to antigen)
Anatomic site: Periphery
Ig DNA, RNA: Alternative splicing of VDJ-C RNA (primary transcript) to form Cu and Cδ mRNA
Ig expression: Membrane IgM and IgD
Surface markers: IgMhi
Response to antigen: Activation (proliferation and differentiation)
Functional changes after B cell activation
- Clonal expansion
- Antibody secretion
- Isotype switching
- Affinity maturation
- Memory B cells forming
Role of heavy chains in antibodies
- Regulate the formation of pentamers (IgM) and dimers (IgA)
- Determine binding to cells via Fc receptors.
- Determine functional properties (eg: C’ fixation )
Role of light chains in antibodies
- Important for antigen specificity and binding
- labelled as either k or λ
Weight of heavy and light chains
Heavy chain: 50kDa
Light chain: 25 kDa
Full Ig: 150kDa
Role of IgA
- Found of mucosal surfaces and in many bodily secretions
- Important first line of defence against antigens
- about 13% of total serum
- are usually monomers or dimer structures
Role of IgG
- Important for memory/ long term immune response
- Binds many pathogens to control infections
- about 80% of total serum
- monomer structure
Role of IgM
- Important for initial response to a pathogen/infection (short term immune response
- controls B cell activation
- about 6% of total serum
- pentamer structure
Role of IgE
- Important for allergic responses and responding to intestinal parasites
- less than 0.1% of total serum
- monomer structure
Role of IgD
- Important as a B cell surface receptor
- Signals B cells to be activated and then secrete antibodies
- about 0.2% of total serum
- monomer structure
Variable and constant regions
- The genes encoding heavy and light chains have variable and constant regions, which are shuffled (somatic recombination) to yield multiple antigen specificities
- This occurs in the bone marrow & is followed by a phase in which autoreactive cells are purged.
Which immunoglobulins serve as antigen receptors on B cells?
Membrane IgM (u chain) and IgD (𝛿 chain)
Isotope switching (B and T cells)
At the primary follicle (spleen) there is:
- Activation and migration of T & B cells in LN
- T-B cell interactions
- B cell differentiation, Ig secretion, isotype switching
- Germinal Centre reaction (contains follicular dendtritic cells), affinity maturation, memory B cells
What happens (with regards to immunoglobulin) when B cells are activated?
IgM: complement activation
IgG subclasses: Fc receptor-dependent phagocyte responses; complement activation; neonatal immunity (placental transfer of IgG)
IgE: Immunity against helminths; mast cell degranulation (immediate hypersensitivity)
IgA: Musocal immunity (transfer of IgA through epithelium)
Main enzyme needed for isotope switching
The key enzyme required for isotype switching and affinity maturation is activation-induced deaminase (AID) - activated mainly by CD40 signals from TFH cells
How isotope switching works
- AID converts cytosine (C) residues to uracil (U) residues.
- Switch regions are rich in G and C bases, and switch region transcripts tend to form stable DNA-RNA
hybrids involving the coding strand of DNA - this frees up the bottom or non-template strand, which forms an open single stranded DNA loop called an R-loop.
- The double-stranded breaks in the two switch regions are joined together
- DNA between the two switch regions is deleted,
- the original rearranged V region becomes adjacent to a new constant region.
How affinity maturation works
- A B cell is activated by a protein antigen and helper T cells
- The B cell migrates into the germinal centre
- In the germinal centre there are B cells with somatically mutated Ig V genes and Igs with varing affinities for the antigen
- Only B cells with high affinity membrane Ig bind the antigen on follicular dendritic cells
- B cells that encounter the antigen on follicular dendritic cells are selected to survive, and other B cells die
Cellular components of the germinal center
- Mantle zone (outermost)
- Light zone (middle)
- Dark zone (innermost)
- After activation by helper T cells at the edge of a primary follicle, B-cells migrate into the follicle
and proliferate, forming the dark zone. - Somatic mutations of Ig V genes occur in these B cells, and they migrate into the light zone where they encounter follicular dendritic cells displaying
antigen. - B-cells with the highest affinity Ig receptors are selected to survive - they differentiate
into antibody-secreting or memory B cells.
Histology of a secondary follicle with a germinal center in a lymph node
The germinal center is contained within the follicle and includes a basal dark zone and an adjacent light zone. The mantle zone is the parent follicle within which the germinal center has formed
FDCs with immunohistochemistry
- Immunohistochemical stain shows follicular dendritic cells (FDC, stained brown with a specific enzyme-labeled antibody) in a follicle of the spleen. B-cells in the follicle are stained blue.
- FDC are HLA class II-negative and do not arise in the bone marrow.
Role of Fc receptors
involved in B-cell regulation and antibody function
Opsonisation and phagocytosis steps
- Opsonistaion of the microbe by IgG
- Binding of opsonised microbes to phagocyte Fc receptors (FcγRI)
- Fc receptor signals activate phagocyte
- Phagocytosis and killing of ingested microbe
