Immunology

1. T-Cell Maturation, Activation,
and Differentiation

2. • Progenitor T cells
migrate from bone
marrow to thymus
• T cells can be grown
in vitro in absence of
thymic fragments
– Grown on bone
marrow stem cells
with Notch protein
– Notch protein is key
in determining T-
lineage specification

3. • Progenitor T cells migrate to thymus
• At about 8th
or 9th
week of gestation in humans
• T cell maturation involves rearrangements
of the germ-line TCR genes
• In thymus, thymocytes proliferate and
differentiate

4. • Selection process in thymus
– Positive selection
• Survival of only T cells whose TCRs recognize
self-MHC molecules
– Negative selection
• Eliminates T cells that react too strongly with self
MHC or MHC with self-peptides

5. T-cell Development
• Begins with arrival of small numbers of
lymphoid precursors migrating from blood
to thymus
• When they do arrive in thymus, T-cell precursors
don’t express signature surface markers (CD3,
CD4, and CD8)
• Do not express RAG-1 or RAG-2 that are
necessary for gene rearrangement

6. T-cell Development
• During 3 week development, differentiating
T cells pass through stages of
development based on surface
phenotypes

7. DN = Double
negative
CD4- and
CD8-
DP = Double
positive
CD4+ and
CD8+
C-kit –
receptor for
stem cell
growth factor
CD44 – an
adhesion
molecule
CD25 - alpha
chain of IL-2
receptor

8. • T cell development is expensive for host
• 98% of all thymocytes do not mature, die by
apoptosis within thymus

13. Insertion of rearranged
TCR genes suppress
other gene
rearrangements in these
mice

14. T cell Activation
• Initiated by TCR-CD3 complex with
processed antigen on MHC molecule
• CD8+ cells with Class I
• CD4+ cells with Class II
• Initiates cascade of biochemical events
• Inducing resting T cell to enter cell cycle,
proliferate, differentiate into memory and effector T
cells

15. T cell Activation
• Cascade of biochemical events leading to
gene expression:
– Interaction of signal and molecule (example:
TCR + MHC and antigen)
– Generation of “second messenger” that
diffuses to other areas of cell
– Protein kinases and protein phosphatases are
activated or inhibitied
– Signals are amplified by enzyme cascades

16. T cell Activation
• Gene products after activation
• Immediate genes (1/2 hour of recognition)
– Transcription factors (c-Myc, NFAT, NF-κB)
• Early genes (1-2 hours from recognition)
– IL-2, IL2R, IL-6, IFN-γ
• Late genes (more than 2 days later)
– Encode adhesion molecules

22. “Signal 1 and 2”- TCR activation
isn’t the whole story
• TCR activation is necessary, but not
sufficient, to produce activation. It is called
“signal 1”.
• The T cell also needs “signal 2”- CD28-B7
interaction.
• Its absence produces clonal anergy

23. Control! CTLA 4 acts to
prevent overstimulation!

25. Superantigens
• Bind to BOTH the TCR and
MHC
• Can cause over-activation
– Overproduction of TH-cell
cytokines, leading to
systemic toxicity
• Exogeneous
– Variety of exotoxins
secreted by some Gram+
bacteria
• Endogeneous
– Cell membrane proteins
encoded by viruses

27. T-Cell Differentiation
• CD4+ and CD8+ cells leave thymus and enter
circulation in G0 phase
• Naïve cells (condensed chromatin, little cytoplasm)
• About twice as many CD4+
• Naïve cell recognized MHC-antigen complex
• Initiated primary response
• After 48 hours, enlarges into blast cell and undergoes
repeated rounds of cell division
• Differentiate into:
» Effector cells – cytokine secretion, B-cell help
» Memory cells – long lived, respond with
heightened activity (secondary response)

28. Treg Cells
• Shown to inhibit proliferation of other T
cells in vitro
• CD4+CD25+
• Shown to inhibit development of
autoimmune diseases

29. Cell Death and T Cell Populations
• Apoptosis plays critical role
– Deletion of potentially autoreactive
thymocytes
– Deletion of T cell populations after activation
• Fas and FasL pathway to induce self death