Antibodies, B cells, Tcells, MHCs

4 lectures!!!

CHmu and CHdelta → IgM and IgD

immunoglobins

RNA splicing

class switching

mu → delta → gamma → epsilon → alpha

IgM

agglutination

pentameric - monomers joined by J chain

easy for C1q to bind to Fc region → MAC (complement activation)

main role = complement activator

not in tissue fluid

monomeric - B cell surveillance

serum - 5-10%

IgG

all roles (+neutralisation and complement)

monomeric

best → opsinisation (Fcgamma region binding to pahgocytic cells

IgE

FCepislon receptor → bound to mast cells and eosinophils

crosslinking with helminth

mass degranulisation

IgA

dimer + J chain + secretory component

neutralisation and mucosal immunity

B cell intracellular motif

CD19

• phosphorylated following Ig epitope bind antigen

T cell intracellular motif

CD3 phosphorylated

when MHC class II:peptide complex binds to CD4 complex occurs (binding to TCR)

• combined with danger signal (interdigitating dendritic cell )

• activation of calcineurin

• increases intracellular calcium (from inner stores)

• T cell activation

• Th2 or TFHs → recognise B cells APCS → clonal expansion → differentiation into plasma cells

  • lymphoid follicle →medullary chords

  • or migrate in blood to none marrow

Areas of the lymph node

Paracortex - T cells

Follicles - B cells

medullary cords communicate with efferent lymph

What happens in the lymph nodes - cells

• recruitment of lymphocytes from blood → lymph via High Endothelial Venules

• interdigitating dendritic cells are present in paracortex

• follicular dendritic cells present in primary follicle

Processes in the lymph node- B cell maturation

1. Interdigitating dendritic cells endocytose, process and present antigens in lymph nodes

2. Naive T cells in paracortex activated: interdigitating dendritic cells present MHC2:peptide complexes + CD80/86 to TCR + CD28

3. T cell clonal expansion and differentiation, partly into TH2s

4. B cell enters lymph node via HEV, detecting antigen passively swept into lymph node → present on MHC2

   • Antigens also deposited on follicular dendritic cells

5. TH2 B cell interaction → B cell presents to TH2, B cell activation → clonal expansion (primary focus - still in paracortex)

6. Proliferating B cells enter primary follicle

7. B cells interact with follicular dendritic cells and CD4 T follicular helper cells

8. Primary → secondary follicle with germinal centre

9. somatic hypermutation and affinity maturation occurs

10. Class switching occurs

    • Continuating activation via cytokine release, cell-cell contact, CD40/CD40L contact → class switching of IgM → IgG

11. B cells differentiate into plasma cells within secondary follicle

12. Plasma cells migrate to medullary cords

Somatic hypermutation

B cells make point mutations in variable region DNA → changes affinity with target antigen via changing Fab (fragment of antigen binding)

Affinity maturation

Preferential selection after somatic hypermutation of Fab with higher affinity to target antigen

• After successive generations → accumulate mutations to have affinity to antigenic epitope

When does class-switching of B cells occur

Class switching happens in the germinal center, before differentiation into plasma or memory B cells.

• dark zone, after B cells undergo proliferation and somatic hypermutation

• Class switching does not occur after B cells have become plasma cells and migrated to the medullary cords.

  • Plasma cells are terminally differentiated and no longer undergo class switching.

Antibody pathway

• Secreted into efferent lymph

• Thoracic or tracheal duct

Normal MHC Class 1 pathway

1. INTRACELLULAR antigen degraded by proteasome (enzyme complex) into peptide fragment

2. Fragment transported to ER via TAP transporter (transporter of antigen processing

3. MHC molecules synthesisd by host cell is assembled in ER using chaperone proteins

4. Peptides fragment loades onto empty MHC molecule

5. MHC class 1:peptide complex → Golgi → host cell surface

6. MHC class 1:peptide complex detected by CD8+

Normal MHC Class 1 pathway INHIBITION

• MHC inhibition reduces adaptive immune susceptibility

• BUT INCREASES NK cell response

• decreased MHC class 1 → seen as foreign kill

1. INTRACELLULAR antigen degraded by proteasome (enzyme complex) into peptide fragment

2. Fragment transported to ER via TAP transporter (transporter of antigen processing

   • Viral proteins block TAP transporters → peptide not in ER

3. Peptide fragement and MHC bind in ER

4. MHC class 1:peptide complex → Golgi → host cell surface

   • Viral proteins ‘glue’ MHC1:peptide complex to ER → no movement to Golgi → CSM

   • Viral protein causes complex → cytoplasm instead of CSM → degraded by proteosome

5. MHC class 1:peptide complex detected by CD8+ → immune response

   • retrovirus → superantigen production → nonspecific MHC and TCR binding → immune response to MULTIPLE ANTIGENS → dilutes the effective virus neutralising response

MHC class II

• exogenous pathway

• APCs: dendritic cells, macrophages, B cells

• all nucleated cells have MHC class 1

1. Foreign antigen endocytosed

   • lysosomes fuse with primary endosomes → protein antigens digested into peptides

2. MHC class II synthesised in ER + invarient chain attached (chaperone protein - prevents a different fragment from binding)

3. entered the secretory pathway via the Golgi

4. MHC and endocytic pathway fuse → form MIIC compartment

5. Invarient chain degraded leaving CLIP peptide blocking peptide-binding groove

6. Then CLIP is replaced by foreign peptide

7. MHC class II is transported to cell surface and detected by CD4 T cells

Signal 3 cytokine Th1

IL-12

Signal 3 cytokine Th2

IL-4

Relationship NFAT and IL-2

• Nuclear factor of activated T-cells

After costimulation present and T cell activated…

NFAT ⇒ transcription factor → activate immune response genes for…

→ interleukin 2 (IL-2) (autocrine

• Drives clonal expansion

IL-1, IL-3, IL-6

Encourages proliferation of the neutrophil pool

• probably in response to GM-CSF

• granulocyte-macrophage cell stimulating factor

T cell signal 2

TCR → APC (interdigitating dendritic) ligand

CD 28 → CD 80/86

B cell signal 2

B cell Receptor → T cell (Th2 or Tfh) ligand

CD 40 → CD 40L

• CD40L → CD40 (B cell) can contribute to upregulation of IL-4 receptor expression,

• CD40-CD40L interaction itself does not directly trigger the JAK/STAT pathway; rather, it primes the B cell to respond to cytokines like IL-4, which then activate JAK/STAT.

• IL-4–JAK/STAT pathway: drive B cell activation, proliferation, and class switching

T cell activation intracellular motif cascade

(signal 1)

binding to APC

phosphorylation CD3

activation of calcineurin

intracellular Ca2+ increases

Ciclosporin blocks calcineurin activation

• via BLOCKING phosphorylation of intracellular motif of CD3

B cell activation (signal 1)

antigen epitope binding surface immunoglobulin (IgD/M)

• phosphorulation of CD19 intracellular motif

• naive B cell → APC

• binding of antigen to surface IgM and IgD on a naïve B cell (BCR engagement) initiates signaling that upregulates expression of CD40 on the B cell surface.

B cell activation signal 3

• part of T cell dependent response of B cell activation

T cell releases IL-4 → B cell growth factor

T cell independent response

• IgM produced by naive B cell

• No T cell help

• Non proteinaceous antigens

• TI antigens → Polysaccharides, long chain lipids

• Cause multiple crosslinks on B cells to trigger response

• Only transient IgM production, no memory cells produced

  • Plasma cell apoptoses

Describe cytokine receptor JAK/STAT pathway

cytokine binds to receptor

Jak activated

Stat dimerises

Stat homodimer acts as TF

Oclacitinib [JAK inhibitor]→ binds to jak to prevent stat activaion stat of IL-31 receptor

Iunocitinib → nonselective cytokine/JAK inhibitor

Respiratory burst

TH1 releases IFN-gamma → activates macrophages

Respiratory burst upregulated

Respiratory burst upregulated

• Increased oxygen free-radicals

• Increased nitric oxide production

• Increased anti-microbial peptides and proteases

• Enhanced killing ability

Increased lysosomal digestion (better digestion of pathogen)

• Increased fusion of lysosomes with endosomes (phagosome) results in increased digestive ability

MHC diversity

polygenic → many genes for MHCs

polymorphic → many alleles for each genes

Lokivetmab

Monoclonal antibody

binds directly to itchy IL-31 cytokine

Prevents receptor binding

Dendritic Cell maturation/migration

(mission impossible)

• monocytes travel in the blood → reach tissues through vascular endothelium

• Differentiate into immature dendritic cells 

• Migrate to epithelial surfaces → where encounter and capture a foreign antigen 

• Migrate from tissues using lymphatic system via afferent lymphatic vessel

• Enter nearest lymph node (paracortex)

• Antigen processed and presented → APCs!!(dendritic cell becomes antigen presenting cell)

  • present in lymph node

  • processes on the way

• Interdigitating dendritic cells in LN paracortex with antigen fragments displayed

• Recognised by naive T cells that express TCRs (T cell receptors) -[T cells  enter via HEVS]

Peptide binding groove

antigen fragments binds to MHC

anchor residues

peptide → MHC in binding groove

contact residues

peptide → TCR

B cell dev

T cell dev

B cell → bone marrow ( + bursa of Fabricius) plus naive T cells

T cells → differentiation in thymus (CD4 and CD8)

What differentiates in secondary lymphoid tissue?

Thelper

Tregs

Tkiller

Plasma cells

spleen and lymph nodes and MALTS