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

Antigen is bound on the surface of follicular dendritic cells (FDC)Antigen is bound on the surface of follicular dendritic cells (FDC)

FDCFDC FDC 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

On the surface of FDC immune complexes form the so-called iccosomes that can be released and taken up later by the surrounding germinal center B cells

FDC depends on the presence of FDC depends on the presence of TNF-TNF-αα, LT, LTαα, LT, LTββ

„„Dating” in the peripheral lymphoid organsDating” 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

IMMUNOLOGICAL MEMORY – B CELLS

Memory B cells• previously activated• passed affinity maturation• present in the circulation• rapidly proliferate and differentiate to plasma cell upon re-activation, or enter the GC reaction again

Plasma cells•provides serological memory: •pre-existing neutralizing Abs to pathogens or toxins

Germinal Centre reaction•proliferation•somatic hypermutation•affinity maturation

BT

BB

BBBB

BBB

B

BB

B

FDC

FDC T

B

B

plasmacell

Somatic Somatic hypermutationhypermutation

SelectionSelectionRepeated cycleRepeated cycle

T CELL DEPENDENT B CELL ACTIVATION IN LYMPHOID ORGANST CELL DEPENDENT B CELL ACTIVATION IN LYMPHOID ORGANS

IgM

IgGIgAIgE

BLIMP expresszió B-lymphocyte induced maturation protein

Regulation of memory vs plasma cell differentiationRegulation of memory vs plasma cell differentiation

Bcl-6 szuppresszor Bcl-6 szuppresszor BLIMP gátlóBLIMP gátló

B cellB cell

T cell - activatedT cell - activated

CD40 signalingCD40 signaling

B

B

Memory B cell

B

apoptosis

T

CD40

CD40L

Follicular dendritic cell (FDC)

FcR

CD21

AgFcR

NO Ag

DEVELOPMENT OF B CELL MEMORY IN THE FOLLICLE

Marginal zone

Arteriole

Germinal centerT cell area – PALS

paracortex

TB

CELL INTERACTIONS IN PERIPHERAL LYMPHOID TISSUES

DC – T cell contact

DC Proliferating B cellscentroblasts

B – T cell contactSomatic hypermutation

Additional gene rearrangementIsotype switch

Plasma cell differentiationAntibody production

Memory B cells

Follicular dendritic cells

B cell

SELECTION OF HIGH AFFINITY B CELLS UPON INTERACTION WITH FOLLICULAR DENDRITIC CELLS

VLA-4

LFA-1

VCAM-1

ICAM-1

BCR

CD21 C3d

Inhibition of apoptosis

Tight junction

FDC

GCT cell

INTERACTION OF ANTIGEN-SPECIFIC T AND B CELLS

CD40

FasB cell

differentiation

apoptosis

CD40L

FasL

B and T cells recognize the same antigen

Repeated activation with the antigen drives B cell activation and plasma cell differentiation

• role of follicular dendritic cells in antigen storage (months-years?)• Polio: reinfections with Sabin vaccine strain• subclinical infections (Diphteria in 10% of the population)• hidden antigens (Measles genes persist in neurons – can induce Subacute Sclerosing Panencephalitis)

Bystander help:Cross-reactive antigensTLR ligandsCytokines...

memory B cell plasma cell

How antigen-specific Ab production is maintained?

Memory B cells continuously differentiate into plasma cells

Long-lived plasma cells in the bone marrow

MODEL 1. MODEL 2.

MODEL 3.

DC

T cell

B cell

CELL INTERACTIONS IN THE PARACORTEX

CD40

CD40

CD40L

CD40LB7

B7

CD28

CD28

MHC

MHC

TCR

TCR

Antigen recognition by B and T lymphocytes

LigandLigand

SIGNALSIGNAL

Cross - linkingCross - linking

LigandLigand

CConformational changeonformational change

SIGNALSIGNAL

RECEPTOR MEDIATED CELL ACTIVATION

ligandligand

kinase activationkinase activation

phosphorylationphosphorylation

rrecruitmentecruitment oof adaptorsf adaptors

CROSS – LINKING OF THE RECEPTOR INITIATES A SIGNALING CASCADE

SIGNALSIGNAL

Gene transcriptionActivation of

transcription factors

antigantigeen n bindingbinding

mIg moleculemIg molecule

HH HH

LL LL

VV VV VV VV

Ig-Ig-/Ig-/Ig-heterodimerheterodimer

THE IgM B-CELL RECEPTOR

Signal transductionSignal transduction

Ly

n

KinKinasesases

SykSyk

BtkBtk

SHP-1PhosphatasesPhosphatases

SLP-65/BLNKSLP-65/BLNKPLCPLC

HS1HS1

VavVavAdaptors +Adaptors +substratessubstrates

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

Ag

RECENT MODEL OF B-CELL RECEPTOR MEDIATED RECENT MODEL OF B-CELL RECEPTOR MEDIATED SIGNALINGSIGNALING

Subsequent activation of 2 Subsequent activation of 2 kkinasesinases

= ITAM= ITAM

1. Cross-linking1. Cross-linking

Ly

n

Ly

n2. Src-family kinase activation2. Src-family kinase activation

4. 4. SLP phosphorylationSLP phosphorylation ++ Ca releaseCa release

SLPPP Calcium releaseCalcium release

PPPP

PPPP

and ITAM phosphorylationand ITAM phosphorylation

3. Syk recruitment and activation3. Syk recruitment and activation

Sy

kS

yk

Sy

kS

yk

PP PP

Main steps of B-cell signal transduction

Activation of B-cells by receptor crosslinkingActivation of B-cells by receptor crosslinking

AntigAntigenicenicdeterminantdeterminant

C3C3dd

THE THE CO-STIMULATORYCO-STIMULATORY ROLE OF ROLE OF CR2 (CD21) CR2 (CD21) COMPLEMENT RECEPTOR IN B – LYMPHOCYTESCOMPLEMENT RECEPTOR IN B – LYMPHOCYTES

ANTIGEN

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

KINETICS OF LYMPHOCYTE ACTIVATION

ANTIGEN SIGNAL1.

Ko-receptorAdhesion molecule

Cytokines SIGNAL2.

Resting lymphocyte G0PTK activation RNA synthesis Free Ca++ Protein synthesis Protein phosphorylation DNA synthesis

Lymphoblast

0 10sec 1min 5min 1hr 6 hrs 12 hrs 24 hrs

Nyugvó limfocita G0

G1

G2

M

Ssejtosztódás

DNA synthesis

Effector cell Memory cell

Transport Membrane changeRNA and protein synthesis

Resting lymphocyte G0

ANTIBODY – METIDATED EFFECTOR FUNCTIONS

• Neutralization – binding of the antibody inhibits the adhesion of the pathogen, its entry or multipolication

• Opsonization – binding of antibodies induces complement activation and promotes binding to immun cells through complement (CR1) and immunoglobulin binding (FcR) receptors

• Antibody isotypes differ in their complement activating and FcR binding capabilities

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

30 strongly neutralising McAb

60 strongly neutralising McAb Fab regions 60 weakly neutralising McAb Fab regions

Human Rhinovirus 14- a common cold virus

30nm

Models of Human Rhinovirus 14 neutralised by monoclonal antibodies

Electron micrographs of Antibodies and complement opsonising Epstein Barr Virus (EBV)

Negatively stained EBV

EBV coated with a corona ofanti-EBV antibodies

EBV coated with antibodies and activated complement components

PLASMA CELL

ANTIGEN

CYTOKINES

B -CELL

T – CELLS PROMOTE B – CELL DIFFERENTIATION

ISOTYPE SWITCH AND AFFINITY MATURATION OCCURS IN ISOTYPE SWITCH AND AFFINITY MATURATION OCCURS IN COLLABORATION WITH T – CELLS ONLYCOLLABORATION WITH T – CELLS ONLY

HOW T – CELLS RECOGNIZE ANTIGENS?