B-cell (Development and Activation)

B-CELL AND HUMORAL IMMUNITY Santosh Yadav M.Sc. Clinical Microbiology Dept. of MicrobiologyInstitute of MedicineTribhuvan Univarsity Teaching Hospital, Nepal

Santosh YadavB-cell development and maturation

Santosh Yadav Introduction

Derives name from the organ it matures (bursa of fabricus in birds)In human it matures in bone marrow.

The lymphocyte which arise and develop in bone marrow is called B-lymphocytes.

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The discovery of B cell immunity1954 - Bruce Glick

Studies on the function of the bursa of Fabricius, a lymphoid organ in the cloacal region of the chicken

Bursectomised chickens were later used in experiments to raise antibodies to Salmonella antigens None of the bursectomised chickens made anti-Salmonella antibodiesBursa was later found to be the organ in which antibody producing cells developed antibody producing cells were thereafter called B cells

Mammals do not have a bursa of Fabricius

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Generation begins in the embryo and continues throughout lifeBefore birth- Yolk sacFetal liver Fetal bone marrow After birthBone marrow

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A mature and immunocompetent B-cell have three uniqueproperties:-

It is specific only to one particular antigen.It produces antibodies specific to the antigen to which it was primed, andIt generates memory for any similar antigenic attack in future.

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B-cell Carry many surface marker:CD19CD21CD22 Membranous immunoglobulinIg and Ig heterodimer

Santosh YadavStages of B cell DevelopmentStem cell Progenitor B cell (Pro B cell)Precursor B cell (Pre B cell)Immature B cellMature B cell Each stage characterized by distinct cell surface markers and a specific pattern of Ig gene expression.

Santosh Yadav Pro -B-cellIs the earliest recognisable B-cell stage.It has following surface marker:-CD -45Ig and Ig heterodimer :-part of B-cell membrane receptor in later stage.CD-19CD-24CD-43C-kitThe heavy chain gene rearrangement begins in this stage and its completion signals the end of this stage.

Santosh Yadav Pre-B-cellCharacterized by the beginning of the translation of heavy chain hene.Essentially require bone marrow stromal cell.Mu chain is usually first synthesized.No light chains are synthesized.Small peptide called Surrogate light chain are synthesized( not true light chain)They complex with mu chain and this complex is expressed on the cell surface.The mu chain and surrogate light chain complex associate with Ig/Ig hetrodimer to form pre-B-cell receptor.

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Early pro-B

Interleukin-7receptor

Late pro-B

Pre-B

Interleukin-7Growth factor

Stromal cell

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Santosh Yadav Immature -B-cellBegins with light chain gene.

Only one type of light chain is expressed(kappa or lambda)

Light chain associates with mu chain and so a true IgM expression occurs on the B-cell Surface.

This also results in B-cell commited to one particular antigenic specificity as determined by binding site of surface IgM.

True BCR appears.

Santosh Yadav Mature B-CellCharacterized by simultaneous expression of IgM and Igd on B-cell surface.The mature B-cell then leaves the bone marrow and go to the peripheral lymphoid tissue where they get activated on encountering the antigen and produce two types of effector cells:-a)Plasma cell:- produce antibody, and b) Memory cell:- produce secondary immune response after re-encounter with the same antigen.If the mature B-cell cannot encounter antigen, they die within few days by apoptosis.

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So the production of mature B-cell in bone marrow is an antigen independent phase while activation and effector cell production is an antigen dependent phase.

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Santosh Yadav Negative selection of B-cellsOn an average, only 10 percent of total newly cells produced(5 x 10 cells) i.e. 5 x 10 cells are recruited into circulation everyday.Rest of 90 percent of newly produced B-cells die.This loss is due to the negative selection of those developing B-cells which express self reactive mIgM on their cell surface.Such cells are eliminated by programmed apoptosis. This process is called negative selection.

Santosh YadavDevelopment of Humoral immune response

Humoral immune response refers to the production of secreted antibodies by plasma cells following antigenic stimulation of B-cells.

Involves activation of B-cells by antigen followed by their proliferation and differentiation in to plasma cells and memory cell.

Santosh Yadav B-cell Activation

Occurs in the peripheral lymphoid organAntigen-driven activation and clonal selection of naive B cells Generation of plasma cells and memory B cells In absence of antigen-induced activation, naive B cells die within few weeks by apoptosis

Santosh Yadav 2 types of Antigens

Thymus cell independent antigen(TI Ag)Do not require direct interaction of Ag specific helper T lymphocytes.Require cytokines secreted by TH cell.Multivalent nonprotein Ags Polysaccharides, lipids, nucleic acids

Thymus cell dependent antigen (TD Ag) Require direct interaction with TH cells Proteins antigens

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Santosh YadavMultivalent Ag composed of repeated identical antigenic epitopes

Maximal cross-linking of the BCR complex on specific B-cells

Sufficient signalling for activation.

Bcell co-receptor complex, further augment signaling

T CELL INDEPENDENT RESPONSE

Santosh YadavB cell co-receptor complex3 proteinCD19CR-2/CD 21TAPA-1(Target of antiproliferative antibody-1)

Santosh Yadav T cell dependent antigen response

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Memory Cells26

Santosh YadavChanges in germinal centres somatic hypermutationSelectionClass switching

Santosh YadavSOMATIC HYPERMUTATION

B cells undergo rapid cell divisionMutation of genes occurs randomlyAg binding affinity of mIg on B cells mayincreases Decrease No affinity

Santosh YadavSelection

Centrocytes compete with each other to bind Ag on follicular dendritic cells B cells bearing high-affinity mIg are most likely to compete Those that fail die by apoptosis Signal from TH cells for further survival Differentiate into Memory B cells or Plasma cells

Santosh YadavClonal selection of B cellsGenerates a clone of short-lived activated effector cells and a clone of long-lived memory cells

Antigen moleculesAntigenreceptor

B cells thatdiffer inantigenspecificity Antibodymolecules

Clone of memory cellsClone of plasma cellsAntigen moleculesbind to the antigenreceptors of only oneof the three B cellsshown.

The selected B cellproliferates, forminga clone of identicalcells bearingreceptors for theselecting antigen.

Some proliferatingcells develop intoshort-lived plasmacells that secreteantibodies specificfor the antigen.

Some proliferating cellsdevelop into long-livedmemory cells that canrespond rapidly uponsubsequent exposureto the same antigen.

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Santosh YadavClass switching or Isotype switching Process by which B cell changes class of IG produced while preserving antigenic specificity

Allows any given VH domain to associate with the constant region of any isotype Enables antibody specificity to remain constant while the biological effector activities of the molecule varyInteraction between CD40 on the B cell and CD40L on the TH cell is essential Occurs only during active immune response

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Clonal SelectionOnly one type of antibodyand one type of B cellresponds to the antigenic determinantThat cell type then produces a large number of clones32

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Santosh YadavPlasma cell

Membrane form Ig changes to secreted formThe rate of transcription of heavy and light-chain genes significantly greater

Santosh YadavMemory cell Seen in T-dependent immune responsesSurvive for long periodsHigh-affinity antigen receptorsCapable of mounting rapid responses to subsequent introduction of AgRemain in the lymphoid organ where generated or recirculates

Santosh YadavPrimary and secondary immune response Primary immune responseThe first contact of an exogenous antigen with an individual generates a primary humoral response, characterized by the production of antibody-secreting plasma cells and memory B cells.The kinetics of the primary response, as measured by serum antibody level, depend on the nature of the antigen, the route of antigen administration, the presence or absence of adjuvants, and the species or strain being immunized.Gradual rise in antibody production taking days to weeks Antibody level declines

Santosh Yadav Secondary immune responseActivation of memory cells by antigen results in a secondary antibody response that can be distinguished from the primary response in several ways. The secondary response has a shorter lag period, reaches a greater magnitude, and lasts longer. The secondary response is also characterized by secretion of antibody with a higher affinity for the antigen,And isotypes other than IgM predominateSecond exposure to same antigen.Recognition of antigen is immediate.Results in immediate production of protective antibody, mainly IgG but may see some IgM

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