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B CELL DEVELOPMENT IN THE BONE MARROW. a. a. O RDERED B -CELL DEVELOPMENT. immature B cell. pre B cell. ANTIGEN RECOGNIZING RECEPTOR H 2 L 2. pro B cell. H-chain + surrogate L-chain SIGNALING RECEPTOR. NO ANTIGEN RECOGNIZING RECEPTOR. B. B. Stromal cell. - PowerPoint PPT Presentation
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B CELL DEVELOPMENT IN THE BONE MARROW
OORDERED RDERED BB-CELL DEVELOPMENT-CELL DEVELOPMENT
pre B cellpro B cell
immature B cell
NO ANTIGEN RECOGNIZING RECEPTOR
H-chain + surrogate L-chainSIGNALING RECEPTOR
ANTIGEN RECOGNIZING RECEPTOR
H2L2
B
B
Stromal cell
SecretedFactors - CYTOKINES
2. Secretion of cytokines by stromal cells
B
Bone marrow stromal cells nurture developing B cells
Types of cytokines and cell-cell contacts needed at each stage of differentiation are different
Stromal cell
1. Specific cell-cell contacts between stromal cells and developing B cells
Cell-cell contact
Early pro-B
c-KitReceptor Tyrosinekinase
Stem cell factor
Cell-bound growthfactor
VLA-4(Integrin)
Stem
Cytokines and cell-cell contacts at each stage of differentiation are different
Stromal cell
Cell adhesionmolecules
VCAM-1(Ig superfamily)
D-J rearranged
Interleukin-7receptor
Stromal cell
Late pro-B Pre-B
Interleukin-7Growth factor
Cytokines and cell-cell contacts at each stage of differentiation are different
Early pro-B
VLA-4(Integrin)
VCAM-1(Ig superfamily)
V-D-J rearranged
µ-chain made
B cell receptor
Transiently expressed when VHDHJH CH is productively rearranged
VpreB/5 - the surrogate light chain (SLC), is required for surface expression
Ig & Ig signaltransductionmolecules
CH
Heavy chainVHDHJH
V-preB
5
Ligand for the pre-B cell receptor is unknown
Pre-
Ligation of the pre-B cell receptor
1. Ensures only one specificty ofAb expressed per cell
LargePre-B
Stromal cell
Unknown ligand of pre-B cell receptor
2. Triggers entry into cell cycle
ALLELIC EXCLUSION
1. Suppresses further H chain rearrangement
2. Expands only the pre-Bcells with in frame VHDHJH joins
LargePre-B
LargePre-B
LargePre-B
LargePre-B Large
Pre-BLargePre-B Large
Pre-BLargePre-B Large
Pre-BLargePre-B
Proliferation
Y ImmatureB cell
Light chain expressedIgM displayed on surface
IgM
Ligation of the pre-B cell receptor triggers entry into the cell cycle
Largepre-B
Many large pre-B cells with identical pre-B receptors
Large pre-B
Intracellular VDJCH chainVL-JL rearranges
Proliferation stops
Pre-receptor not
displayed
Small pre-B
V-J light chainrearranged V-J light chain expression is quite
efficient with an 85% success rateFrom a single µ chain 85 receptor!!!
100X expansion
B cell receptor
Ig & Ig signaltransductionmolecules
CH
Heavy chainVHDHJH
Light chainVLJLCL
L chain is rearranged
RECEPTOR EXPRESSION DURING B-CELL DEVELOPMENT
even though every B cell possesses a maternal and paternal locus of both genes, B cells express a single heavy and light chain. Does this „crippled” expression serve a purpose? Many of the genes (not all) are expressed co-dominantly, how could B cells manage to silence their other BCR-coding allels?
ALLELIC EXCLUSION
Allelic exclusion
Evidence for allelic exclusion
Allotypes can be identified by staining B cell surface Ig with antibodies
a/a b/b a/b
YBb YBa YBb
Y
YB ab
YBa AND
ALLOTYPE- a polymorphism in the Heavy chain C region of Ig
Suppression of H chain rearrangement by pre-B cell receptor prevents expression of two
specificities of antibody per cell
Allelic exclusion is needed for efficient clonal selection
All daughter cells must express the same Ig specificityotherwise the efficiency of the response would be compromised
Suppression of H chain gene rearrangement helps to prevent the emergence ofnew daughter specificities during proliferation after clonal selection
S. typhi
Antibody
S. typhi
YY
Y Y
Suppression of H chain gene rearrangementensures only one specificty of Ab expressed per cell.
Allelic exclusion prevents unwanted responses
BSelf antigenexpressed by
e.g. Liver cells
S. aureusY Y
YYYB
S. aureus
YY
Y
YY
Y
Y
AntiS. aureus
Antibodies
Y
Y Y
Y Y
YAntiLiver cell
Abs
One Ag receptor per cell IF there were two Ag receptors per cell
Y
Y Y
Y
YY Y
AntiS. aureus
Antibodies
Prevents induction of unwanted responses by pathogens
Allelic exclusion is needed to prevent holes in the repertoire
Exclusion of anti-brain B cells i.e. self tolerance
YYBB
One specificity of Agreceptor per cell
S. aureus
Anti-brain IgAND
anti-S. Aureus IgYYYBB
IF there were two specificitiesof Ag receptor per cell
Anti-brain Ig
BB
Deletion Anergy
OR
anti S.Aureus B cells will be excluded leaving a “hole in the repertoire”
BUT
YYYBB
1. Combination of gene segments results in a huge number of various variable regions of the heavy and light chains expressed by different B-cells
SOMATIC GENE REARRANGEMENT
2. Successful somatic rearrangement in one chromosome inhibits gene rearrangement in the other chromosome
ALLELIC EXCLUSION
3. One B-cell produces only one type of heavy and one type of light chain
COMMITMENT TO ONE TYPE OF ANTIGEN BINDING SITE
4. The B-cell pool consist of B-cells with differently rearranged immunoglobulin genes
THE RESULT OF SOMATIC GENE REARRANGEMENTS
INDEPENDENT OF ANTIGEN
OCCURS DURING B-CELL DEVELOPMENT IN THE BONE MARROW
Allelic exclusion helps diagnose and monitor lymphoma:
Due to clonal expansion of a single cell that contains a unique rearrangement the amount of cancer cells in blood or in bone
marrow can be determined
Can be used to monitor residual tumor cells upon treatment
Peripheral
Stages of B cell development
Stem Cell Early pro-B cell Late pro-B cell Large pre-B cell
Small pre-B cell Immature B cell Mature B cell
Each stage of development is defined by IgH and IgL chain genes, expression of adhesion molecules and cytokine receptors
Y
ReceptorH+L
YReceptor
H+L
SYNTHESIS OF IMMUNOGLOBULINS
ER
Golgi
mRNA
Ribosome
Leader sequence
Membrane Ig Secreted Ig
H and L chains are synthesized on separated
ribosomes
CHAPERONES
B
B
B
B
c-kit/CD44
RAG-1/RAG-2
Limphoid precursor
H átrendeződésH rearrrangement
Surrogate L
L rearrangement
Selectionclonal deletion
DEVELOPMENT OF B-LYMPHOCYTES IN THE BONE MARROW
PERIPHERAL LYMPHOID TISSUES
B cells recognizing self structures
Cell surface moleculesMHC proteinsCommon molecules of haemopoetic cells
apoptosis, clonal deletion
Soluble moleculesHouse keeping genesMetabolites
functional unresponsiveness
anergia
Other specificites
n3 42 51BONE
MARROW
Potential B-cell repertoire
Self structure
Self recognitionClonal deletion
PERIPHERAL LYMPHOID ORGANS
Available B-cell repertoireForeign antigen independent
About 30 billion mature naive B cells leave the bone marrow per day to circulate in blood
Negative selection of immatureB-cells in the bone marrow
RNA editing
Immature B cells with specificity for multivalent self antigens are retained in the bone marrow.
Receptor Editing of Immature B cells with self-reactive BCR (Bone Marrow)
Immature B cells specific for monovalent self antigens develop a state of anergy.
Anergic B cells have a halflife of 4-5 days (10% that of regular B cells)
How can mature B-cells express surface IgM and IgD
Co-Expression of cell surface IgM and IgGOn Mature B-cells is controlled by alternative
RNA processing
1. Somatic rearrangement of Ig gene segments in a highly controlled manner
2. Single B-cells become committed to the synthesis of one unique H-chain and one unique L-chain variable domain, which determine their specificities
3. In one individual a large B-cell repertoire is generated consisting of B-cell clones with different H- and L-chain variable domains
4. This potential B-cell repertoire is able to recognize a wide array of various antigens
5. Immature B-cells express IgM and IgD surface Ig with the same variable domains
RESULT OF SOMATIC GENE REARRANGEMENT AND ALLELIC EXCLUSION
B – CELL ACTIVATION
Where and how do all these things take place?
B-cell recycling in the absence of antigen (lymph node)
B cells in blood
Efferenslymph
T cell area
B cell area
Antigen entersnode in afferent
lymphatic
Y
Y
Y
Y
Y
YY
Y
Y
Y
Y
Y
Y
Y
YY
Y
YB cells leave blood & enter lymph node via
high endothelial venulesB cellsproliferate
rapidly
GERMINAL CENTRETransient structure ofIntense proliferation
Germinal centrereleases B cellsthat differentiateinto plasma cells
Recirculating B cells are trapped by foreign antigens in lymphoid organs
Germinal Center Reaction
„Dating” in the peripheral lymphoid organs
The structure of the germinal centre
Somatic hypermutation
FDC
Somatic hypermutation
DZ
LZ
LZ: light zoneDZ: dark zoneFDC: follicular dendritic cell
Antigen is bound on the surface of follicular dendritic cells (FDC)
FDC FDC-s bind immune complexes (Ag-Ab ) Ag detectable for 12 months following immunization A single cell binds various antigens
B cells recognize Ag on the surface of FDC
Fig. 9.15. On the surface of FDC-s immune complexes form the so-called iccosomes,that can be released and taken up later by the surrounding germinal center B cells
T CELL DEPENDENT B CELL ACTIVATION IN LYMPHOID ORGANS
IgM
IgGIgAIgE
Ig-Ig-/CD79a/CD79a Ig-Ig-/CD79b/CD79b
ITAM: ITAM: IImmunoreceptor mmunoreceptor TTyrosineyrosine--based based AActivation ctivation MMotifotif
Y
Y
Y
YITAMITAM ITAMITAM
Ig domain + CHOIg domain + CHO
SIGNALING UNITS OF THE B-CELL RECEPTOR
ITAM:ITAM: Y YxxxxLL x7x7 YYxxxxII
Main steps of B-cell signal transduction
AntigAntigenicenicdeterminantdeterminant
C3C3dd
THE THE CO-STIMULATORYCO-STIMULATORY ROLE OF ROLE OF CR2 (CD21) CR2 (CD21) COMPLEMENT RECEPTOR IN B – LYMPHOCYTESCOMPLEMENT RECEPTOR IN B – LYMPHOCYTES
ANTIGÉN
CD21CD21/CR2/CR2
CD19CD19
YY
TAPA=CD81TAPA=CD81
Enhanced B-cell activation
BB-CELL-CELL
THE NEURAMIC ACID RECEPTOR CD22 INHIBITS THE NEURAMIC ACID RECEPTOR CD22 INHIBITS ACTIVATION THROUGH THE A B-CELL RECEPTOR ACTIVATION THROUGH THE A B-CELL RECEPTOR
B B CellCellAntigAntigeenn
Tissue cells
BaBacctteeriumrium
MannMannoseose
CD22CD22
Neuraminic acid
Inhibited B cell activation
EFFECTOR FUNCTIONS OF ANTIBODIES
PLASMA CELL
NEUTRALIZATION
Small proportion of antibodies
INHIBITIONBinding of bacteria to
epithelial cellsBinding of viruses to
receptorBinding of bacterial toxins to target cells
OPSONIZATION
Binding of antibody increases phagocytosis
FcR
FcR
FcR CR1
ComplementC3b
COMPLEMENT ACTIVATION
Opsonization by C3b
PHAGOCYTES
ENGULFMENT, DEGRADATION