Exam 1: T Cell Activation and Effector Functions

Question 1

Antigen Sources

Answer 1

• Tissue antigens
  
  • Immature APCs phagocytize Ag
    
    • Langerhans cells
    • Resident macrophages
  • Carries Ag via lymph to lymph node
• Intersitial fluid antigens
  
  • Draining fluids carry Ag to lymph node
  • Ag activates APCs with LN
    
    • Dendritic cells
    • Macrophages
    • B-cells

Question 2

Professional APCs

Answer 2

1. Macrophages
   
   • Exogenous Ag from phagocytosis
2. B cells
   
   • Ag binds BCR
   • Exogenous Ag from Clathrin-coated vesicle endocytosis
     
     • Very homologous Ag due to BCR specificity
3. Dendritic cells
   
   • Most potent APC for naïve T cells
   • Bone marrow derived
   • Found in virtually all tissues
     
     • Langerhan cells ⇒ immature dendritic cell in skin
   • Constantly phagocytize until “danger signal” occurs
     
     • Skin damage or infection
     • PAMPs or DAMPS : PRR
   • Travels tissue ⇒ lymph node
     
     • Maturation ⇒ endocytose or pinocytose but no longer phagocytic
     • ↑ MHC I and II expression
     • ↑ B7 expression

Question 3

Naïve T-cell

Circulation

Answer 3

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

Question 4

Naïve T-cells

Lymph Node Recruitment

Answer 4

1. Rolling and sampling
   
   • L-selectins (T-cell) : CD34 (HEV)
     
     • L-selectin only expressed on naive T-cells
     • Targets them to LN
2. Stimulation
   
   • Chemokine ⇒ conformational ∆ of LFA-1 ⇒ ↑ affinity for ICAM-1
3. Firm attachment and pavementing
   
   • LFA-1 (T-cells) : ICAM-1 (HEV)
4. Diapedesis

Question 5

Naïve T-cell

Activation

Answer 5

• Naïve T cells “sample” MHC+peptide
  
  • Low affinity interactions via adhesion molecules
  • LFA-1 : ICAM-1
• TCR engagement ⇒ ↑ binding affinity of adhesion molecules
• Requires two signals for activation:
  
  1. TCR engagement ⇒ TCR : peptide+self-MHC
  2. Co-stimulatory signal ⇒ CD28 (T-cell) : B7 (APC)
• Only TCR engagement ⇒ anergy
  
  • Facilitates extra-thymic self & tumor tolerance

Question 6

T-Cell

Signaling

Answer 6

Common to CD4⁺ and CD8⁺ T-Cells:

1. TCR engagement ⇒ clustering of TCRs, CD4/CD8, CD28, and other cell surface receptors
2. Transphosphorylation of Lck and Fyn (protein tyrosine kinases)
3. Activated Lck & Fyn phosphorylate ITAMS on CD3 and ζ-chains
4. ZAP70 (Zeta-associated protein tyrosine kinase-70) docks via ITAM-℗
5. Lck phosphorylates ZAP70 ⇒ activation
6. ZAP 70 phosphorylates numerous proteins
   
   • Phospholipase Cγ (PLCγ)
   • Guanine nucleotide exchange facors (GEFs)
7. PKCγ and GEFs initiate cascades ⇒ transcription factors
   
   • NFAT, NF𝛋B, and AP-1
   • Cyclosporin and FK506 inhibit calcinerin ⇒ ⊗ transcription factors ⇒ immunosuppresion
8. Induces transcription of many genes
   
   • IL-2 and IL-2 receptor α-chain
     
     • Required for T cell proliferation and differentiation
     • T-cells say IL-2, I love you

Question 7

T-Cell

Proliferation

Answer 7

Clonal proliferation ⇒ 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 constitutively expressed
  • α-chain expressed on activated T-cells only
• Trimeric complex needed for IL-2 binding and signaling at physiological levels
• IL-2 : IL-2R ⇒ T cell proliferation

Question 8

Toxic Shock Syndrome

Answer 8

• Superantigen activates polycolonal T-cells in Ag-independent manner
• Causes a cytokine storm ⇒ massive release of IL-2

Question 9

Effector and Memory T-Cell

Activation

Answer 9

Effector and memory T cells ony requires TCR enagement for activation.

TCR : peptide+self-MHC

Still prefers CD28 : B7 interaction also.

Question 10

Effector T-cell

Peripheral Tissue Targeting

Answer 10

Effector T-cells no longer express L-selectin

Other molecules target it to site of infection/inflammation:

1. Rolling and sampling
   
   • Mediated by P and E-selectins on activated endothelium
2. Stimulation
   
   • Chemokine ⇒ conformation ∆ of LFA-1 and VLA-4 ⇒ ↑ affinity for ligands
3. Firm attachment
   
   • LFA-1 (T-cell) : ICAM1 (endothelium)
   • VLA-4 (T-cell) : VCAM-1 (endothelium)
4. Diapedesis
5. Chemotaxis

Question 11

Effector T_H Cell

Functions

Answer 11

“CD4⁺ T-cells”

• Primarily involved in cytokine production
• Effects:
  
  • CD8⁺ cell proliferation → defense against viruses & malignant cells
  • Macrophage activation → intracellular bacteria
  • NK cell stimulation → viruses, malignant cells
  • B cell proliferation / isotype switching → viruses (extracellular stages), bacteria (extracellular), helminthic parasites

Question 12

Effector T_C Cell

Functions

Answer 12

“CD8⁺ T-cells”

• Kills virally infected, malignant, or foreign cells
  
  • Up to 1,000 cells per NK cell via apoptotis

Question 13

T-cell Lifecycle

Summary

Answer 13

Question 14

Immune Response

Down Regulation

Answer 14

T-cell activation and proliferation dependent on antigen stimulation:

• Clearance of threat
  
  • Loss of T-cell activation signals
  • Stop 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 15

Immunological Memory

Answer 15

Memory ⇒ provide protective immunity so subsequent exposure results in an absent, subclinical, or mild infection.

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 Ab titers
  • Lower Ag titer required for activation

Question 16

Life of a T-cell

Summary

Answer 16

• Naïve T-cells wait in the periphery for specific Ag stimulation
• Naïve T-cells can recirculate for ~ 1 year
• Actual fate of these cells unknown
• Unclear if body produces subclinical Ag levels so memory cells stimulated and life extended