B CELL Public Health MSc 6th week, 2014. DEFINITIONS Antigen (Ag) - any substance, which is recognized by the mature immune system of a given organism.

B CELL

Public Health MSc

6th week, 2014

DEFINITIONS

• Antigen (Ag) - any substance, which is recognized by the mature immune system of a given organism

– antigenicity - specific reactivity with cells or molecules of the immune system (weak antigen vs. strong antigen)

– immunogenicity - capability to elicit an immune response

– tolerogenicity - capability to induce immunological tolerance

part of the antigen that directly interacts with the antigen-specific receptors of lymphocytes (TCR or BCR/antibody)

ANTIGENIC DETERMINANT (=EPITOPE)

B cell epitope(recognized by B cells)

T cell epitope(recognized by T cells)

proteinspolysaccharideslipidsDNAsteroidsetc. (even artificial molecules)

cell or matrix associated or soluble

proteins mainly (8-23 amino acids)

requires processing and presentation by APCs

Several epitops of one microbes can be recognized by different B cells

approx. 10 – 1000 million (107 - 9) different antigen receptors, unique specificity of B cells

approx. 10 – 1000 million (107 - 9) different antigen receptors, unique specificity of T cells

ADAPTIVE IMMUNE SYSTEM

Diversity of receptor strucure

How can the antigen receptors of lymphocytes recognize extremly diverse antigens

Random hands, millions of variations

Random selection of gene segments ensures millions of different receptors (variable domains)

Happens during the maturation of B cells in the red bone marrow

VH D JH

VL JL

V-Domains

C-Domains

VH-D-JH VL-JL

VARIABILITY OF B-CELL ANTIGEN RECEPTORS AND ANTIBODIES

B cells of one individual 1 2 3 4

Estimates of combinatorial diversity

Taking account of functional V D and J genes:

65 VH x 27 DH x 6JH = 10,530 combinations

40 V k x 5 Jk = 200combinations30 V l x 4 Jl = 120 combinations

= 320 different light chains

If H and L chains pair randomly as H2L2 i.e. 10,530x 320 = 3,369,600 possibilities Due only to COMBINATORIAL diversity

In practice, some H + L combinations do not occur as they are unstableCertain V and J genes are also used more frequently than others.

There are other mechanisms that add diversity at the junctions between genes - JUNCTIONAL diversity

GENERATES A POTENTIAL B-CELL REPERTOIRE

Several antibodies are expressed on B cells (arround 100.000) but all of them has the same specificity

Forms of immunoglobulins:• membrane-bound (expressed as BCR on the surface of B cells)• soluble (secreted by plasma cells [antibody])

Membrane bound and soluble Igs recognize the same antigen when originated from the same B cell

Differentiation

Plazma cellSecreted

antibodies

B – CELL ACTIVATION

Where and how do all these things take place?

SECONDARY LYMPHOID ORGANS/TISSUES

• LYMPH NODES

• SPLEEN

• TONSILS (Waldeyer’s ring)

• Diffuse lymphoid layers under the

epithelial barriers:

– SALT (skin-associated lymphoid

tissue)

– MALT (mucosa-associated lymphoid

tissue)

• BALT (bronchus-associated lymphoid tissue)

• GALT (gut-associated lymphoid tissue)

Sites of lymphocyte activation and terminal differentiation

B-cell recycling in the absence of antigen (lymph node)

B cells in blood

Efferentlymph

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

when B cells recognize their antigens originated from the afferent lymphatics, they start to migrate to the boarder of the B cell zone for the help of helper T cells

After helper T cells become activated by APCs (mostly DCs) in the T cell zone, and they differentiate into effector cells, they start to migrate to the boarder of the T cell zone to help the activation of B cells

B-se jt

c itokinek

C D4TC R

MHC II+ pep tid

T-se jt

2

1

only works in the presence of pathogenic proteins!

T-DEPENDENT ACTIVATION OF B CELLS

B CELLT CELL

cytokines

MHC-II +

peptide

T-INDEPENDENT ACTIVATION OF B CELLS

aggregation of multiple BCRs cross-phosphorylation signaling

GC reaction:

• proliferation (clonal expansion) of activated B cells

• affinity maturation (stronger binding to epitopes)

• isotype switch (different effector functions)

• memory B cell formation (from improved clones)

Only by the help of Th cells!

AFFINITY MATURATION

B cells compete for the antigen

High affinity B cells can grab the antigen and get survival signals while low affinity cells will lack those and undergo

apoptosis selection of high affinity clones

ActivationClonal expansionDifferencaition

Plasma cells Antibody production

Memory B cells

CirculationRestricted lifespan

(few days)Apoptosis

Specific B cells Non-specific B cells

Antigen recognition by specific BCR induces clonal expansion and differentiation of the sepcific B cells.

Antigen

Activation of specific B cells

1. Clonal expansion

Antigen Antigen

2.Differentiation

Plasma cells, antibody production

MEMORY B CELLS

Antigen

Antigen

Antigen recognition by specific BCR induces clonal expansion and differentiation of the sepcific B cells.

Antigen

ActivationClonal expansion

B cell

Antigen receptor, BCR

Ag

Clonal antigen receptors are expressed exclusively on T- and B lymphfocyties.

Antigen

Antigen

Antigen

Antigen recognition by specific BCR induces clonal expansion of the sepcific B cells.

Ag

Ag

B cell repertoire

Specific, activated B cells

Plasma cells

Antigen specific antibodies

POLYCLONAL RESPONSE

EFFECTOR FUNCTIONS OF ANTIBODIES

Antibody-mediated immune responses

• NEUTRALIZATION

• OPSONIZATION

• opsonized phagocytosis (IgG)

• ADCC (NK cell-mediated killing) (IgG)

• mast cell degranulation (IgE)

• COMPLEMENT ACTIVATION

Antigen

Activation1. Clonal expansion

Antigen Antigen

2.Differentiation

Plasma cells

MEMORY B cells

Antigen recognition by specific BCR induces clonal expansion and differentiation of the sepcific B cells.

Antigen

Antigen