L6- lymphoyte acitvation 2- B cells

What do overactive B cells do

Make antibodies against themselves

What does recognition of self Ag in bone marrow induce

Clonal deletion

Where do B cells go one they leave th. Bone marrow

Will enter blood and lymphatics

When do b cells get the survival signal

I F they bind to pathogen

Antibody functions (3)

• neutralisation → prevents bacterial adherence

• Opsonisation → promotes phagocytosis

• Complement activation → enhances opsonisation and lyses some bacteria

why do. B cells need several signals to become activated

As you dont want to make antibodies unnecessarily

What do naive b cells express

Membrane Ig/BCR (IgM(+Igd))

What happens once b cells leave bone marrow

Can enter secondary lymphoid tissues and encounter non self Ag

Waht provides signal 1 to the B cell and what happens

Binding of Ag to BCR → is a phosphorylation singla that leads to transcription factors being upregulated cuasing gene expression in b cell triggering cell division

What does cross linking of BCRs activate

Intracellular kinases

What are BCR associated polypeptides Invovled in

Signalling → Igalpha and Igbeta stay with th BCR whilst it on the cell sruface

What do the Igalpha and Igbeta attatched to the heavy chain of the BCR have wihtin the cytoplasmic domain

ITAM→ immunoreceptor tyrosine based activation motif

How can signal 1 be increased/ enhanced (4)

• if antigen has activated complement cascade

• Lots of C3b

• Complement receptor 2 (CR2) on B cells surface (CD21)

• CR2/CD19/Cd81 from the BCR co receptor complex

How can Ag bidning to BCR give increased signal 1

If the BCR is coated ith C then it can also bind CR2 on B cells which increases signal 1

CR2 (4)

• complement factor 2

• Boudn to atnigen

• Gives multiple signals

• Icnreases singal 1

What detemermines how the B cells receives signal 2

Depends on the type of Ag thy bind

Types of antigen b cell can recieve signal 2 from (2)

• thymus independent Ag (TI)

• Thymus dependent Ag (TD)

What is singal 2 provided by when thymus independent ag binds to B cell

Either the antigen itself or by extensive cross linking of the BCR

What provides signal 2 when the thymus dependent ag binds to the B cell

CD4+ T helper cells → releases cytokines and CD40L, CD154 and CD40 to help deliver signal 2

What does the thymus depdent antigen bind to

BCR to give signal 1

What does thymus in depdent Ag bidnign lead to

Antibody production (only IgM) that has no requirement for T cell involvement

What are the 2 types of antobody that have no requirement for T cell involvement

• TI-1Ag

• TI-2Ag

What does TI-1 Ag do

Binds to BCR and ther recpetors on al B cells providing signal 2 → in high concentrations act as polyclonal activators for B cells

What does polyclonal activators (Mitogens) for b cells mean TI-1 ag can do

Will activate many b cells irrespective of thei different BCRs

Exampels of Ti-1 Ag

Lipopolysaccharides from gram neg bacteria binds to TLR 4 expressed by B cells (linked with innate immunity) → the 2 singals , first from BCR and second from TLR lead to B cells which increases activation, proliferation and antibody secretion

Feautres of TI-2 Ag (4)

• contain repeated epitopes

  • Often polysaccharides

  • Imporant in some bacterial infections

• Will therefore cross link many BCR molecules on the same B cell surface

• Takes longer (more Ag required) to induce B cell activation

• Antibody responses to TI-2 ag dont usually develop until >5 years in humans

What can TI-2 antinges do alone

Can signla B cells to produce IgM antibody → gives one big signal 1 so antigen doesnt need T cell

Thymus dependent (TD) Ag

Antibodies to TD Ag, require presence of CD4+ T cells → are absent in the absence of a thymus

What are the antibody responses seen t TD Ag like compared to TI Ag

Are much better → are classic aquired responses, give bets immunity against pathogen

Stages of TD Ag antibody production (4)

• T cell activated by MHC/peptide on APC

• BCR binds antigne giving signal 1

• B cell internalised Ag, process and presents it to CD4+ T cells giving signal 2 Via CD40/CD40-L interactions

• Cytokines secreted by T cells which increases ahelp B cell to class switch → all classes of antibodies can be produced to TD Ag

How are antibodies against viral coat proteins produced (4)

• b cell binds virus through viral coat protein giving signal 1

• Virus particles is internalised and degraded and boudn to MHC Class II

• Peptides from internal prteins of the virus are presented ot the T cell whcih activated the b cell thourgh singal 2 with help from CD40 and CD40-L + other cytokines

• Activated b cell produces antibody against viral coat pathogen

How do b cells act as APCs for TD aAg (2)

• epitope recognised by antobody and T cell must be physically linked either from diffent parts of the smae moelcuel or from different molecuels of a comeplx eg viral proteins

• Act as APCs to get signal 2 and be activated

• Eg in viral coat proteins Antibody formation

What is a way of improving the efficiency of a vaccine against pathogens with T1 antigens

Converting a TI Ag to a TD Ag

Example of a conjugate vaccine (3 +3)

• Haemophilus influenzae type B

  • Protective repsonse requires antibodies to capsular polysaccharide which is a TI Ag

  • Coupling this to a protein such as tetanus toxoid converts to a TD Ag

  • Allows young children to be immunised and protected

• MenC

• Pneumococcal conjugate vaccine

Process of converting TI Ag to TD Ag with tetanus exmaples (4)

• B cells binds bacterial polysaccharide epitope linked to tetanus toxoid protein

• Antigen is internalised and processed

• Peptides from the protein complement are presented to the T cell

• activated b cell produces antobody against polysaccharide antigen on the surface of the bacterium

What are very improtant for good antobody response adn why (4) (conjugates)

• B/CD4+ T cells

• b cell eneter lymph node from blood

• If comes into contact with specific Ag can be activated

• If Ag is TD, the B cell presents the peptide from the Ag to CD4+ TH cells at the boundary of teh T/B areas within the lymph node to form B/T cell conjugate

How is CD40 ligand (CD40-l) expressed

B cell binds Ag via BCR and present peptide on MHC class II from Ag to activated CD4+ th cell → causes T cell to express CD40 ligand

What does CD40-L do (2)

• Secreted cytokines

• binds with CD40 and with cytokines from T cell binding receptor gives B cell signal 2 causing b cell proliferation

What does CD40 signal do

Induces activation induced deaminase (AID) which is required for class swithicng and somatic hypermutation (SHM)

What happens to the b cell follicle with proliferation

Gets bigger and bigger

Where does T cell activation happen

In T cell zone below b cell zone

Where do conjugates of b lymphoblasts and T cells move to and what do they form

Pirmary follicles ( b cell areas) → form germinal centres (GC) within a b cell follicle in secondary lymphoid tissues

What do b cells rapidly divide to and undergo (2)

• Divide to centroblasts and undergo SHM and isotype swithicng

• - differentiate into non dividing centrocytes which are smaller

In good antibody response, what leads to GC fromation

B/CD4+ t cells binds bacterial interaction

What happens to b cells once they are in germinal centres (GC) (3)

• can either

• Differentiate into plasma cells and secrete various isotypes or hihg affinity antibody that is somatically mutated

• Form long live memory cells for re ciruclation

• Die within lymphoid tissue if BCR no longer binds antigen as a result of unsuccesful V reigon or somatic mutation

What dos SHM do

Introduced point mutations into V regions of Ig → approv one mutation/V region/cell division using primarily activation induced deaminase (AID) enyzme and DNA repair genes

What other cells are present in germinal centres

Follicular dendritic cells

Follicular dendritic cells (FDCs) (2)

• capture intact Ag for centrocytes to bind via BCR and present

• Not bone marrow derived

How do FDCs capture Ag

Via FcR and Cr as immune complexes

What do FDCs oversee

B cell affinity maturation

What do centrocytes that have undergone SHM express

Mutated BCR on surface

What do centrocytes compete with each other from

Ag on FDC and for signals from Tfh cell

What happens if the mutated BCD that centrocytes express on teh surface binds Ag on FDC better than unmutated BCR

Will present more efficiently and recieve CD40 singal from the Tfh cell

What happens to the centrocytes that ahve generated higher affinity BCR

Survive to differentiate into plasma cells

Tfh cell features (4)

• recently defined as CD4+ th subset foudn predominantly in teh b cell follciles of the lymph nodes

• Specialised to provide help to b cells

• Secrete either Th1 or Th2 type cytokines

• Can be identifies iwth speicfic markers tha differ from. Other subsets of CD4 Th cels

What happens do mutated BCR wiht a lower affinitiy for Ag

Is not cross linked and cant present atnigen to T cell so dies by apoptosis

What happens to the mutated BCR with a high affinity for Ag

T cell helps nd b cell receptor cross linking sustain b cell proliferation adn maturation into memory b cells and plasma cells

What is the role of CD40

Singal via CD40-L expressed on Tfh to protect centrocytes from apoptosis and induces isotype swithicng

What happens to people who don’t have the CD40 ligand

Suffers from CD40L deficiency - hyper IgM syndrome where they can only make IgM and IgD and cant drive class swithicng so are extremely l immuno deficient

What does AID do

Induces DNA breakage new constant reigon of antibody joined without affecting exisitng VDJ region

Control o isotope swithicng (4)

• different Ag induce different isotypes

  • Polysaccharides: IgM (TI)

  • Protiens: IgG1 and IgG3 or IgG4 (TD)

• Ag at mucosal surfaces induce IgA

• Some Ag elicit IgE

• Cytokines are iportnant eg IL-4 imporant for IgE switch