DP

1
DP
2

• - keeps going until positively selected.
• many more J segments than IgL, so very likely to
  get functional a chain.
• not allelically excluded (as for IgL), but only
  one functional ab specificity emerges from
  positive selection (rearrangement stops when cell
  is positively selected)

3
N-nucleotides in both b- and a-chains
transcription factor networks program cell fate
4
Thymocytes become committed to the T cell lineage
(lineage restriction)
(receives signal from stroma)
Notch
B
DN1
NB this is depicting lineage potentials, not
actual fates
NK
DN2
DC
NKT
DN3
gd T
DP
SP
DN4
TCRgd
TCRb
TCRab
TCRab
(adapted from Mak, T.W. and Saunders, M. The
Immune Response)
5
ab vs gd T cells an either-or choice
6
Double positive thymocytes undergo two different
kinds of selection based on their interaction
with self-MHC/self-peptide complexes
Positive Selection
Negative Selection
affinity
Thymocytes that do not interact sufficiently with
self MHC/peptide complexes die death by neglect
Thymocytes that strongly interact with
self-peptide/MHC complexes die
survival
death by neglect
death by negative selection
Presumed function? To delete cells that would
not be useful in immune responses (because they
do not recognize MHC they are not
MHC-restricted)
Presumed function? To delete cells that would
be potentially dangerous (autoreactive)
7
Positive selection MHC restriction of T cells is
determined by MHC on radioresistant
(non-hematopoietic) cells
X
MHCb
MHCa
(In a radiation chimera experiment, all
hematopoietic cells in the irradiated host arise
from donor bone marrow)
8
Positive selection MHC restriction of T cells is
determined by MHC of thymic stromal cells
X
MHCb
MHCa
irradiated
(nude or thymectomized)
MHCa
thymic graft
9
Expression of transgenic TCR directly
demonstrates positive selection
(Transgenice mice expressing already rearranged
TCR of known specificity (in this case known to
be restricted to MHCa, a class I MHC allele)
10
Positive selection determines coreceptor
expression
11
Th-POK
One model continuous signaling when CD8 drops -gt
cells become CD4 disrupted signaling when
CD8 drops - gt cells become CD8
12
Positive selection MHC restriction of T cells is
determined by MHC of thymic epithelial cells
13
Negative selection Removal of T cells that
respond too strongly to self MHC/peptide
combinations (such that they could be
autoreactive in the periphery)
14
Demonstration of peptide-induced negative
selection
(e.g. TCR that recognizes flu peptide)
15
Bone marrow derived cells are the most important
cells driving negative selection
MHCb
skin graft
MHCa
rejection (due to lack of negative selection of
MHCa-reactive T cells i.e. alloreactivity)
Bone-marrow derived cells (the only ones in the
mouse that carry MHCb) must mediate negative
selection of the MHCb reactive T cells (which
would otherwise cause rejection)
16
What about tissue-specific antigens?
AIRE-mediated expression of tissue-specific
antigens in the thymus
Absence of AIRE in humans -gt Autoimmune
polyendocrinopathy-candidiasis-ectodermal
dystrophy (APECED) (with various autoimmune
manifestations) Absence of AIRE in mouse -gt also
multi-organ autoimmunity
17
There must be a difference in the signals that
induce positive and negative selection
Strength of signal? Quality of signal?
18
mature but naïve
Multiple encounters with MHC-peptide over T cell
life span - during thymic selection (and on an
ongoing basis in periphery tickling) -
activation in peripheral lymphoid organs - as
effector cells in periphery
19
Dendritic cells carry antigen from tissues to
present to naïve T cells in lymphoid organs
immature DCs in periphery
mature DCs
green MHC Class II red lysosomal protein
20
B7.1/B7.2 (also known as CD80, CD86) are
costimulatory molecules
21
Recognition of Self vs. Non-Self
ADAPTIVE
INNATE

• T cells, B cells
• Rearrangement of VDJ segments
• 400 million years old
• delayed response clonal expansion
• Memory

• Macrophages, Dendritic Cells, Neutrophils, Mast
  Cells, NK cells.
• Germline encoded receptors
• Evolutionarily ancient
• Immediate response
• No memory

Innate immune cells use germline-encoded pattern
recognition receptors to identify conserved
molecular patterns characteristic of
microbes. E.g. Toll-like Receptors Ligands
bacterial cell wall components
(lipopolysaccharide, lipoteichoic acid,
lipopeptides) flagellin (component of
bacterial flagella) dsRNA (characteristic of
some viruses) others
22
Innate recognition of microbial substances
induces maturation of DCs
Coupling upregulation of costimulatory molecules
on APCs to innate recognition -gt peripheral
tolerance (as opposed to central tolerance
established in thymus by negative selection)
23
(No Transcript)
24
T cell effector functions
IFN-g