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) proteins polysaccharides lipids DNA steroids etc. (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 ( ) different antigen receptors, unique specificity of B cells approx. 10 – 1000 million ( ) 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 VLJL V-Domains C-Domains VH-D-JH VL-JL VARIABILITY OF B-CELL ANTIGEN RECEPTORS AND ANTIBODIES B cells of one individual

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  x 5 J  = 200combinations 30 V  x 4 J = 120 combinations = 320 different light chains If H and L chains pair randomly as H 2 L 2 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 unstable Certain 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 ) 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 cell Secreted 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 Efferent lymph T cell area B cell area

Antigen enters node in afferent lymphatic Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y Y B cells leave blood & enter lymph node via high endothelial venules B cells proliferate rapidly GERMINAL CENTRE Transient structure of Intense proliferation Germinal centre releases B cells that differentiate into 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

only works in the presence of pathogenic proteins! T-DEPENDENT ACTIVATION OF B CELLS B CELL T 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

Activation Clonal expansion Differencaition Plasma cells Antibody production Memory B cells Circulation Restricted lifespan (few days) Apoptosis Specific B cellsNon-specific B cells Antigen recognition by specific BCR induces clonal expansion and differentiation of the sepcific B cells.

Activation of specific B cells 1. Clonal expansion 2.Differen tiation Plasma cells, antibody production MEMORY B CELLS Antigen recognition by specific BCR induces clonal expansion and differentiation of the sepcific B cells.

Activation Clonal expansion B cell Antigen receptor, BCR Ag Clonal antigen receptors are expressed exclusively on T- and B lymphfocyties. Antigen recognition by specific BCR induces clonal expansion of the sepcific B cells.

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

Activation 1. Clonal expansion 2.Differen tiation Plasma cells MEMORY B cells Antigen recognition by specific BCR induces clonal expansion and differentiation of the sepcific B cells.