Immunology - 7 - T cells

Question 1

Describe the T lymphocyte receptor for antigen and its accessory molecules.

Answer 1

Receptors of CD4+ helper T lymphocyte

• Signal transduction
  
  • CD4
  • CD28 –> binds to B7-1/7-2 (APC)
• Ag Recognition
  
  • TCR –> binds to MHC II (APC)
• Adhesion
  
  • LFA-1 –> ICAM-1 (APC)

Question 2

Describe the process of antigen recognition by T lymphocytes.

Answer 2

Ag recognition

• APC shows Ag (Lymphoid organs)
  
  • naive CD4+ T cell –>
    
    • cytokines (IL-2) bid to IL-2R –> (clonal expansion)
      
      • Effector CD4 T cells
      • Memory CD4+ T cells
    • Activation of macrophages, B cells, etc
  • naive CD8+ T cell
    
    • Effector CD8+ T cell (CTL)
    • Memory CD8+ T cell
  • Killing of infected target cell; macrophage activation

Question 3

Compare and contrast Th1 and Th2 T immune responses.

Answer 3

TH1 Cells

• (bacteria) ACP bind to Naive T cells
  
  • TH1 cells
    
    • IFN-y –>
      
      • Macrophages
        
        • classic macrophage activation
        • enhanced microbial killing
      • B cells
        
        • complement binding & opsonizing IgG Abs
        • Fc recepot

TH2 Cells

• (helminth + protein Ag) APC binds to Naive CD4+ T cell
  
  • Proliferation + differentiation of TH2
    
    • IL-4 –> B cells
      
      • IgG –> Ab production
      • IgE –> Mast cell degranulation
    • IL-4 / IL-13 –>
      
      • Intestinal mucus secretion & peristalsis
    • IL-5 –> Eosinophil activation
      
      • Fight helminths
    • IL-4 / IL-13 –> Macrophage activation
      
      • enhanced fibrosis/tissue repair

Question 4

Outline the process of T lymphocyte maturation and activation.

Answer 4

• Stem Cell
• Double (-) CD4- / CD8- pro-T cell
• Double (+) CD4+ / CD8+ immature T cell
  
  • Weak recognition of MHC II peptide = positive selection
    
    • –> mature CD4+ T cell
  • Weak recognition of MHC I peptide = positive selection
    
    • —> mature CD8+ T cel;
  • No MHC recognition
    
    • Apoptosis - failire for positive selection (death by neglect)
  • Strong recognition of MHC I or II
    
    • Apoptosis - NEGATIVE selection

Question 5

Describe how T lymphocytes help other immune cells.

Answer 5

• APC or macrophage w/ ingested microbe
  
  • releases IL-12
    
    • activates naive CD4+ T cell
    • becomes an effector T cell (TH1)
• Effector T cell
  
  • releases IFN-y
    
    • activation of macrophages
      
      • killing of microbes

Question 6

Describe how cytotoxic T lymphocytes kill target cells.

Answer 6

Cytotoxic T Lyphocytes

• Perforin/granzyme-mediated cell killing
  
  • CD8+ CTL binds to target cell –>
  • apoptosis of target cell
  • Granzymes enter target cell cytosol
    
    • activate caspases
• Fas/FasL- mediated cell killing
  
  • CD8+ CTL binds to target cell –>
  • FASL on CTL interacts w/ Fas on target cell
  • Apoptosis of target cell