T Cell Activation and Effector Functions

Question 1

T-Cell Activation

Answer 1

Naïve T_c and T_H cells require two signs for activation:

• Naïve T cells “sample” MHC/peptide combinations through low affinity interactions mediated by adhesion molecules.
• Engagement of the TCR results in increased binding affinity of adhesion molecules
• Naïve T cells need two signals for activation:
  
  1. TCR engagement – TCR: peptide + self-MHC
  2. Costimulatory signal – CD28 (T-cell) : B7 (APC)
• Cells that receive only signal #1 become anergized.
  
  • Facilitates extra-thymic self (& tumor) tolerance

Effector and memory T cells require only signal #1 for activation.

• Only requires TCR: peptide/MHC interaction but still prefers CD28:B7 interaction as well

Question 2

Antigen Sources

Answer 2

• Immature dendritic cells and macrophages phagocytize the Ag in the tissue and carry it to the draining lymph node.
• Draining interstitial fluids carry Ag to dendritic cells, macrophages, and B-cells in the lymph node.

Question 3

Professional APC’s

Answer 3

1. Macrophages
   
   • exogenous Ag from phagocytosis
2. B-cells
   
   • Ag binds Ab (BCR)
   • Clathrin coated vesicle endocytosed
   • Exogenous Ag ⇒ very homologous Ag due to BCR specificity
3. Dendritic cells
   
   • Most potent APC for naïve T cells
   • Bone marrow derived
   • Found in virtuall ALL tissues
     
     • Langerhan cells = immature dendritic cell in skin
   • Constantly phagocytose until a “danger signal” occurs
     
     • Skin damage or infection
     • PAMPs or DAMPS : PRR
   • Danger signal causes cell to leave tissue and enter lymph node
     
     • Matures ⇒ can endocytose or pinocytose but no longer phagocytic
     • Increases MHC I and II expression
     • Increases B7 expression

Question 4

T-Cell Signaling

Answer 4

Common to both CD4 and CD8 T-Cells

1. T cell receptor engagement results in clustering of multiple TCRs (#30-200), CD4/CD8 molecules, CD28, and other cell surface receptors.
2. Transphosphorylation/activation of Lck and Fyn (protein tyrosine kinases)
3. Activated Lck/Fyn phosphorylate ITAMS on CD3 and ζ (zeta) chains.
4. ZAP70 (Zeta-associated protein tyrosine kinase-70) docks via ITAM-P.
5. Lck phosphorylates ZAP70 leading to activation.
6. ZAP 70 phosphorylates numerous proteins including phospholipase Cγ (PLCγ) and guanine nucleotide exchange facors (GEFs)
7. PKCγ and GEFs both initiate a series of cascades leading to production of transcription factors (NFAT, NF𝛋B, and AP-1).
   
   • Cyclosporin and FK506 block production of transcription factors by blocking calcinerin function = immunosuppresion.
8. Induces transcription of many genes including IL-2 and IL-2 receptor α-chain.
9. IL-2:IL-2 receptor activation key for T cell proliferation and differentiation.
   
   • IL-2 and receptor alpha-subunit made within several hours after activation.
   • Takes ~ 24 hours for each round of T cell division.
   • T-cells say IL-2, I love you.

Question 5

T-Cell

Clonal Proliferation

Answer 5

Fevery few T-cells will react against a single Ag, therefore, clonal proliferation must occur before there is a significant T cell response.

Mechanism of antigen-specific T cell amplification:

• IL-2 Receptor is a trimeric protein (α, β, and γ)
  
  • β and γ chains are constitutively expressed on mature T cells.
  • α-chain expressed on activated but not resting T-cells
• Trimeric complex needed for IL-2 binding and signaling at physiological cytokine levels.
  
  • low affinity receptor = α
  • intermediate affinity receptor = βγ
  • high affinity receptor = αβγ
• IL-2 binding and IL-2R signaling leads to T cell proliferation.
  
  • This significantly increases the number of T cells reactive to the threat/pathogen.
  • Allows for greater effector activity.

Toxic shock syndrome

• ​Causes a cytokine storm resulting in a massive release of IL-2
• Able to active T-cells in an Ag-independent manner.

Question 6

Naïve T-cell

Circulation

Answer 6

1. Enter the lymph node via peripheral blood through high endothelial venules (HEVs)
2. Search for its MHC+peptide match in LN
3. Exit via lymphatics → circulation → another LN to maximize opportunity for Ag+MHC recognition.

Question 7

Lymph Node

Recruitment of Naïve T-cells

Answer 7

1. Rolling is mediated by L-selectins on T-cell that bind to special siayl-Lewis^x glycoprotein ligands (CD34) on HEV in LN.
   
   • The L-selectin:CD34 ligand interaction targets naïve T-cells to LN
2. Chemokine stimulation of T-cell → conformational change of LFA-1 (integrin) molecule → increased affinity of LFA-1 for its ligand ICAM-1
3. LFA-1 on T-cells binds ICAM-1 on HEV leading to firm attachment → diapedesis

Question 8

Inflammatory/Infectious Focus

Recruitment of Activated T-Cells

Answer 8

Following activation, proliferation, and differentiation - effector T-cells are free to migrate out of the lymph node.

Effector T-cells no longer express L-selectin and up-regulate other homing molecules to promote migration to site of infection.

1. Effector T-cell rolling mediated by P and E-selectins on the activated endothelium.
2. Chemokine stimulation of T-cell → conformation change of LFA-1 and VLA-4 molecules on T cell → increased affinity for ligands ICAM-1 and VCAM-1 on endothelium
3. LFA-1/VLA-4 on T-cells bind to ICAM1/VCAM-1 on endothelium promoting firm attachment → diapedesis.

Question 9

Effector T-Cell

Stimulation and Effector Functions

Answer 9

Effector T-cells smaple cells for MHC+peptide match.

Signal #1 through TCR leads to activation and effector functions.

(Effector T cells only require signal #1 for activation)

CD4 T-cells

• Cytokine production leads to:
  
  • CD8 cell proliferation → defense against viruses & malignant cells
  • Macrophage activation → intracellular bacteria
  • NK cell function → viruses, malignant cells
  • B cell proliferation / isotype switching → viruses (extracellular stages), bacteria (extracellular, helminthic parasites

CD8 T-cells

• Killing
  
  • Serial killers (up to 1,000) cells via apoptotic death

Question 10

T-cell Lifecycle

Summary

Answer 10

Question 11

Immune Response

Down Regulation

Answer 11

T-cell activation and proliferation is dependent upon antigen stimulation.

• Clearance of threat
  
  • loss of T cell activation
  • stop of IL-2 and IL-2R synthesis
  • loss of survival factors
  • death by apoptosis
• Down modulatory cytokines
  
  • TGF-β made by regulatory T-cells
    
    • Inhibits the immune response

Within 1-2 weeks of clearance, the only remaining component of the immune response is a pool of memory cells.

Question 12

Immunological Memory

Answer 12

Memory allows the immune system to respond more rapidly and effectively to previously encountered threats or pathogens.

Goal: provide protective immunity (immunity to reinfection) so that an absent, subclinical, or mild infection occurs upon subsequent exposure.

Mechanisms:

• Pre-existence of a clonally expanded population.
• Memory cell fuctionally different
  
  • Faster response
    
    • naïve cells take 4-5 days
    • memory cells take 1-2 days
  • Better response
    
    • T-cells: more complex cytokine profile
    • B-cells:
      
      • different isotypes: IgG, IgA
      • higher affinity
      • greater quantities
  • Lower Ag titer required for activation

Question 13

Life of a T-cell

Summary

Answer 13

Naïve T-cells wait in the periphery for specific Ag stimulation.

Naïve T-cells can recirculate for a year or so waiting for activation.

Actual fate of these cells is unknown.

Unclear if body produces subclinical Ag levels so memory cells stimulated and life extended.