Notes 4.12

I. Activation of T-cells
>vast majority (99.9%) of T-cells ever made are killed in thymusbinding of self
antigens
a) Co-receptors
*strengthen interaction btw T-cell and dendritic cell
*induce signal transduction, leading to activation of T-cell
i. CD4 and CD8
*interact directly with MHC
ii. CD28 and B7
*B7—expressed by mature, activated dendritic cells; interacts with CD28 on T-cells
*CD28 binding to B7signal transductionactivation of T-cells
1. NF-kappa-B signaling
*activated by TCR-MHC interaction and by CD28-B7
binding
b) IL-2
*NF-kappa –B activates transcription/translation of IL-2 and IL-2
receptor
*IL-2 major growth factor for T-cells (thisclonal expansion, IL-2
stimulates T-cell division)
c) Homing to secondary lymphoid tissue
*CCR7 on activated T-cell surface interacts w/ CCL2 in lymph nodes
endothelial cells
*this allows T-cell diapedesis through lymph node venule to get to interior
of lymph node
d) Proliferation
*CD4+ T-cells multiply to ~10000 times starting # in 24 hr after activation
*CD8+ T-cells ~50000x
*CTLA4 expression turned on after IL-2, IL-2 R expression
-counter growth factor, stopping clonal expansion in T-cells
-turned on ~24 hrs

I. Differentiation of T-cell subtypes
*CD8+ T-cell development a result of interactions w/ CD4+ T-cells and IL-12
*all differentiation in T-cells (even irreversible changes) from changes in gene
expression, not change to DNA sequence of cell
II. CD4+ T-cells
a) TH1
*secreted IFN-gamma
*IFN-gamma leads to the activation of natural killer cells, CD8+ toxic
T-cells, macrophages; can also induce B-cells to make pro-phagocytic
antibodies (IgG)
—kind of antibody that can stimulate classical complement pathway
b) TH2
*secrete cytokines that:
—activate mast cells , basophils, eosinophils
*expulsive responses
*release antimicrobial content of granules
—activate B-cells to release “barrier antibodies” (IgE)
not good opsonins, but cover microbe, disable function of
microb
c) TFH
*interact w/ B-cells in secondary lymphoid tissue
d) TH17
*secretes IL-17
*a cytokine which activates neutraphils, attracts to site of inflammation
III. CD8+ T-cells
*=cytotoxic T-cells
*mechanisms of toxicity to intracellular-pathogen infected cells:
1-perforins and granzymes = cytotoxic chemicals stored in granules in CD8+
T-cells
*contents released on TCR-MHC1-CD8+ binding perforines, granzymes
lyse target cell
2- Fas-L on T-cell interacts w/ Fas-R on target cell
*apoptosis
*CD8+ cell protects self from own cytotoxic chemicals by lining granules w/
cathepsins, which block perforin, granzyme effects
IV. T-memory cells
*naïve T-cells (those never stimulated) maintained by IL-7
—over time, ability to produce IL-7-R is reduced, so fewer and fewer naïve
T-cells over time
*memory T-cells also maintained by IL-7
a) CD4+
*maintained by IL-7 + antigen-MHC-TCR interactions
*survive for ~10 yrs
b) CD8+
*live ~30 yrs without stimulation by antigen
*maintained by IL-7
*important for defense against very mutation-prone viruses (like flu)
*can be stimulated by re-exposure to the same antigen, and then go back
through a clonal expansion (this especially happens w/ latent viruses)
*latent viruses: hang out in cells, typically asymptomatic
*
over time, entire population of CD8+ T-memory cells just against
latent viruses you’re infected with

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