Three Classes of T Cells

CD8 T Cells: Virus Infected Cells

CD4(TH1) T Cells: Intraceluular Pathogens

CD4(TH2) T Cells: Extracellular Pathogens

Helper T cells (TH) and cytotoxic T cells (Tc) are defined by the surface molecules CD4 and CD8

CD4 facilitates TCR interaction with MHC class II molecules.

CD8 facilitates TCR interaction with MHC class I molecules.

In contrast to antibody.......

, T cells only see antigen in complex with host MHC molecules – thus, T cells respond to infected cells or professional antigen presenting cells (not to free pathogens).

  CD8 T cells :  antigen + MHC class I  (infected cells)

  CD4 T cells :  antigen + MHC class II  (professional APCs)

Inflammatory response influences the adaptive immune response

• iDCs continually sample extracellular material around them via macropinocytosis.
• When an iDC ingests a pathogen in infected tissue, it becomes activated and then migrates to nearby lymphoid organs.

MHC Class I

Tissue Distribution

Function

MHC Class II

Tissue Distribution

Function

    to CD4 T cells

MHC Class I Proteins

MHC Class II Proteins

Lymphocytes move from circulation into the lymph node......

via extravasation in the post-capillary venules.

Clonal Expansion of T Cells

Naïve T cells that DO encounter antigen that binds to their specific TCR (a rare event) become activated, proliferate, and differentiate into effector T cells.  This process takes 4 to 5 days.

The result :  a population of armed T cells specific for the pathogen.

Full T Cell Activation Requires Two Signals

    1.  epitope specific :           MHC+antigen / TCR

    2.  co-stimulatory signal :           B7 / CD28  

IL-2

is the third signal required for T cell proliferation 

Upregulation of IL-2 and

IL-2R is required for autocrine

stimulation and induction of proliferation.

Naïve CD4 T cells differentiate along two pathways

IFN-Γ: Cell Mediated Response (TH1)

-activates macrophages

enhances inflammation

activates cytotoxic T cells

helps with IgG response

- IL-4: Humoral Response (TH2)

co-activates B cells

helps with IgE response