Th cell Activation and differentiation

• 1. Activation– TCR coupled signaling pathways– Co-stimulatory signal-Differences amongst APC’s– Super antigen induced Tcell activation

• 2. Differentiation– Generation of Effector and Memory cells– Cell death of Tcell population– Function of peripheral gamma, delta T cells

Concept

• Interaction of TCR CD3 complex

+• Processed Antigenic peptide bound MHC I or II• Signals• Cascade of Events• Proliferation• Differentiation: Memory cell

Effector cells

Gene Products• Immediate genes:30 min

Transcription factors: cFos, cMyc,NF-AT

• Early Genes:1-2hrs

Various Cytokines: Il-2,IL-2R,IL-3 etc

• Late Genes-2 days

Adhesion molecules VLA-4,HLA-DM

Common themes in signal Transduction: Environmental Stimuli

• Receptor: On cell surface• Intracellular- hydrophobic ex: steroids• Generation of second messenger:diffuse into other sites of cell

eg:cAMP,cGMP,Ca,DAG,IP3• Protein Kinases :add Phosphate• Protein Phosphatases Remove Phosphate tyrosine, serine or

threonine residues• Induced Assembly: Adaptor proteins sites for receptor and

cytoplasmic molecule• Large and small G proteins :GTP activates GDP inactivates• Cascade of Events• Default setting is Off

TCR coupled signaling pathways

• Fyn protein tyrosine kinase: near tail of TCR CD3 complex• Translocation of Co receptors near TCR• p56Lck protein tyrosine kinase (bought near TCR CD3 tail)• Phosphorylate Immunoreceptor Tyrosine Based Activation Motif

ITAM (CD3)• Creates Docking site for SH2 site of ZAP-70• This activates Many pathways• Phospholipase C gamma splits phosphatidylinositol 4,5-biphosphate

into Inositol 1,4,5-Triphosphate and DiacylGlycerol • IP3- increases Ca+2 --- activates Calmodulin dependent

Phosphatase called Calcineurin—dephosphorylates inactive form NF-AT

• DAG activates Protein Kinase PKC which phosphorylates and releases Nuclear factor Kappa B

Co-stimulatory signal-Differences amongst APC’s

• Signal 1: TCR+ Ag peptide signal cascade• Signal 2:Ag non specific

– Interaction b/w CD28(t cell) & B7 fly(APC)– B7-1,B7-2 Ig superfly– V & c like domain ,cytosolic domains diff

– Constitutively expressed : Dc– Induced on activated macrophages,B cells

– Ligand for B7:CD28» CTLA-4 Cytotoxic T Lymphocyte AssociatedProtein

4

Lymphocyte Homeostasis

• CD28: +• T cell proliferation• P on resting

&activated cells .• Peaks high • Less affinity a• As its level increases

CTLA level go up• Regulatory feedback

• CTLA-4: negative• Downregulatory• P on activated cells• Detected 24hrs• Peak 2-3 days• Higher affinity• In Knockout mice: • Lymphadenopathy• splenomegaly

Super antigen induced Tactivation• Viral/bacterial Ag• Vbeta domain of TCR & alpha chain of MHC II• Exogenous: Staphylococcal enterotoxin

Toxic shock syndrome• Endogenous: cm ptns. Retroviral ptns called Minor

lymphocyte stimulating determinants produced my Mouse Mammary Tumor virus.

• Any T cell with particular Vbeta domain: polyclonal activation: over production of Cytokines

• Influence T cell maturation in Thymus : T cells lost by negative selection

Generation of Effectors and Memory cells

• CD4/8 (naïve cells) leave thymus: enter circulation(G0):recirculate blood lymph node till it encounters Ag:12-24hrs

• After encounters Ag• TCR CD3 coupled signal• Co stimulatory signal• Blast cell• G1 phase • IL-2 production 100 fold• Activated cells divide 2-3 for 4-5 days• Differentiate into memory effectors cells

Death of Tcell : APOPTOSIS

• Programmed Cell death • Lymphocyte Homeostasis• Deletion of autoreactive thymocytes in Negative

selection• Proteases called “CASPASES”• 14 mammals ,highly conserved• Every cell posses it• Caenorhabditis elegans• Cysteine residues specificity for asparatic acid

residues

Two Pathways

• Ag stimulation increases• Fas• FasL• FADD • Procaspase 8• Caspase 8• Procaspase 3• Caspase 3• Proteolysis essential

molecules• AICD Activation Induced

Cell Death

• Negative Selection• Cytochrome c leaks from

mitochondria• Binds Apaf-1 Apoptopic

Protease Activating factor• Procaspase 9• Cyt3+Apaf-

1+Procaspase1: APOPTOSOME

• Caspase 9• Caspase 3• Bid 3: Activates

Mitochondrial pathway

Function of peripheral gamma, delta T cells

• 5% Skin, intestinal• pulmonary epithelium• Some express only CD3• Some CD3+CD8• No recirculation• React with Ag found at

these sites.• Binds : ptn lik Abs

Not commonly bound to APCs

• Imp role in immune response: location commonly encounters Ag

• Primary line of defense• Non peptide Ag• Conserved epitopes on

Ag• Clearance damaged cells • No clear cut • Tumor cell lysis