Tolleranza delle cellule T nei tessuti linfoidi periferici
Vigorous reactivity against
pathogens
A lack of overt reactivity to self
Both are best achieved when lymphocytes express diverse,
clonally distributed antigen specific receptors (clonal
selection theory)
As the organism cannot predict the precise pathogen-derived
antigens that will be encountered, the immune system relies on
the generation and maintenance of a diverse T-cell receptor
(TCR) repertoire
GOD, generation of immune-receptor
diversity
Concept of Immune Regulation
• Immune responses are tightly regulated by complex interactions of
cells & mediators, and by mechanisms to prevent anti-self reactivity
• Failure of regulatory control can occur…
– Enhancement of immune responses or infection can generate autoimmune
reactions (loss of self–tolerance)
– Decrease of immune responses may lead to an immunodeficiency state
– Shift in immune responses can lead to allergy
Immunological Tolerance
History
- Ehrlich, Owen, Burnet
- Billingham, Brent and Medawar
Owen’s
observation
1945
Burnet’s Clonal Selection Model:
Central Tolerance
DEVELOPMENT
MATURITY
Clonal
Deletion
Anti-self
Self Ag
Lymphocyte
Differentiation
Anti-non-self
Lymphocyte
Activation
Foreign Ag
Burnet postulated that there was a temporal window of tolerance such
that antigens encountered while the immune system was immature
tolerized the relevant lymphocytes.
Medawar’s
experiment
demonstrating
neonatal
tolerance
induction (Nobel
Prize)
Medewar subsequently investigated the effects of transferring
hemopoietic cells from histoincompatible mice at different times
after birth. He found that if the cells were transferred in the first
few days of life (but not later) the recipient mouse acquired
lifelong tolerance to the antigens of the donor.
Immunological Tolerance
• Definition and Properties
– Specific unresponsive state induced by exposure to
antigenic epitopes
– Tolerance to self is initially induced during
embryonic life, and is maintained by antigen
– Tolerance occurs in both T and B cells
– Multiple mechanisms of tolerance exist
Tolerance
• Central Tolerance - this occurs during
lymphocyte development.
• Peripheral Tolerance - occurs after
lymphocytes leave the primary lymphoid
organs.
Central Tolerance
The thymus provides a fundamental initial step for
the elimination of potentially dangerous self-specific
T cells
Peripheral
tolerance
Acquisition and maintenance of peripheral
tolerance
Different hypotheses:
•Self-nonself discrimination (Bretscher, Cohn)
•INS model (Janeway)
•Danger model (Matzinger)
•Localization dose and time model (Zinkernagel)
Self nonself
Bretscher: two step, two-signal model
Signal two is given by a T cell
Cohn: associative
recognition of antigen
Infectious nonself and non-infectious self model
No peripheral tolerance only central tolerance
Recognition of a pathogen by PRRs
induces a set of endogenous signals,
including costimulatory molecules (B7),
inflammatory and effector cytokines,
and CD1d molecules. Additionally,
some cells can phagocytose the
pathogen (not shown), process its protein
constituents, and present its peptides
to T cells. Recognition of peptides
derived from a pathogen, along with
costimulatory molecules induced by the
pathogen, results in T cell activation.
Effector cytokines induced by the
pathogen instruct the activated T cells
to differentiate into a particular effector
cell type (T1 or T2). Activated T cells
then deliver an inducible signal (usually
a member of the TNF family, such as
CD40L or FasL) to the target cell in an
antigen-specific manner: the target cells
of T1 and T2 being macrophages and B
cells, respectively. L, ligand.
Danger model
Danger model
Absence of signal two induces tolerance
Danger Model
Anti-DEC205
Steady state DC
Come viene indotta la toleranza allo steady state?
Topi transgenici
esprimenti OVA sotto il
cotrollo di RIP
Localization dose and time model
Localization dose and time model
TCR tg anti-gp33 x RIP-gp33
ignoranza
20
30
50
40
30
20
10
0
60
20
0
0
10
20
30
100
80
60
40
20
0,1
Bpep mg/ml
1,0
CD44
10
30
10
20
30
40
high frequency
IFNg
production
80
70
60
50
40
30
20
10
0
0,0
0,01
Transient peptide
presentation by B
cells
40
CD69
0
Days
low frequency
40
20
CD69
upregulation
% CD69 high
% TCR high
TCR
downregulation
120
0,01
e
CD25
BALB/c
0,0
40
Anti-IgG2ab TCR
transgenic mice
IFNg ng/ml
% of CD25+
100
10
100
80
60
40
20
0
0
% of CD69+
0
0
c
proliferation
50
40
30
20
10
0
% of CD44high
%Vb14+ on CD4+
Localization dose and time model
0,1
Bpep mg/ml
1,0
8
7
6
5
4
3
2
1
0
naive
High
frequency
Low
frequency
0
10-5 10-4
10-3 10-2
Bpep mg/ml
10-1 1
Chronic peptide
presentation by B
cells
Organi immunoprivilegiati
Nel 1989 sono stati isolati anticorpi monoclonali citolitici
per diverse linee cellulari
La proteina riconosciuta da questi anticorpi è stata
denominata FAS
L’espressione costitutiva di FAS in trasfettanti porta alla
morte cellulare in seguito ad incubazione con anticorpi antiFAS
FAS trasduce un segnale apoptotico
FAS espresso in : TIMO, FEGATO, CUORE, POLMONI, RENI,
OVAIO
Timo: FAS espresso in tutte le popolazioni ma non nei doppi
negativi
Periferia: FAS espresso nei T maturi attivati
FASL: 40000 D, induce morte cellulare se interagisce con
linee cellulari esprimenti FAS
FASL: espresso dalle cellule T attivate, espresso nel
TESTICOLO, CORNEA, TIMO (organi
immunoprivilegiati)
FASL esiste anche solubile in forma trimerica
Per indurre morte cellulare è necessario il cross-linking di
FAS
Gli organi immunoprivilegiati esprimono il ligando di FAS
Se cellule T attivate arrivano agli organi immunoprivilegiati
sono uccise mediante interazione FAS-FASL
Dimostrato direttamente per la cornea
Walker and Abbas 2002, 2
Walker and Abbas 2002, 2
Tolerance and Regulatory T cells
Patrolling and Keeping the periphery in check.
•
Regulatory T cells patrol periphery and maintain tolerance :
• Self-reactive T cells are under the constant control of regulatory T cells.
•
Variety of regulatory or “suppressor” T cells:
•
Late 70’s - Early 80’s  Present
Regulatory T Cells
• CD25+CD4+ thymic origin
• Th3 - Induced by oral feeding of antigen
• TGF-b1 producing (IL-10).
• TGF-b1 dependent
• Tr1 - Induced
• Antigen administration in the presence of IL-10
• IL-10 dependent
• All induced by Ag X, thus known specificity.
Tregs
The reincarnation of the suppressor T cell
Number of publications
More questions than answers.
n
1
*
Questions
?
1995 - Present
Answers
CD4+CD25+ immunoregulatory T cells
The rebirth and revival…mid 1990’s.
 Naturally-occurring or “professional” regulatory T cells.
 Immunosuppressive in vivo :

•
•
Day 3 thymectomy
Depletion of CD4+CD25+ T cells
Irradiation
 Gastritis, oophritis, orchitis,
 thyroiditis, pancreatitis, colitis
 Co-transfer with CD4+CD25- T cells suppresses
autoimmune disease.
 Absence of CD4+CD25+ T cells  Increased immunoreactivity to antigens.
CD4+CD25+ regulatory T cells
General characteristics

Secondary
lymphoid tissues

Originate from thymus. Role of periphery ?

“Partially activated” phenotype :



5-10%

CD4
Unique lineage of CD25+ (IL-2R) T cells

No other marker for regulatory T cells.
Altered negative selection?
TCR specificity is unknown :

CD25
Differ from conventional, activated CD25+ T cell.
Thymic developmental pathway is unclear:


CD62Lhigh, CD69low, CTLA-4, GITR and CD45Rblow
Diverse T cell repertoire.

Self-specific

Cross-reactive to foreign?
Hyporesponsive (anergic) :


Unresponsive to TCR stimulation : restored by IL-2.
Do not produce IL-2.
T cell activation induces
expression of functional T cell
surface molecules
MHC/peptide
Activated
CD4+ T cell
CD40L
TCR
APC
TCR
Resting
CD4+ T cell
CD25
CD69
CD44
CD25 is not a marker of Treg after T cell activation.
Only in the naïve T cell repertoire.
CD4+CD25+ regulatory T cells suppress
T cell proliferation and cytokine production
Proliferation
Cytokine Production
77.6
CD4+
CD8+
and
T cells
IL-2
CD8+
69
CD8+ /
CD4+CD25+
CD25
Suppress the induction of IL-2 mRNA
# CD4+CD25- (  ) or CD4+CD25+ ( ) T cells
IFN-g
IFN-g (ng/ml)
CPM
IL-2
CD8+
CD8+/-
CD8+/+
CD4+CD25+ CELLS REQUIRE ACTIVATION
TCR ENGAGEMENT FOR Treg ACIVITY
DO11.10 CD4+ Tg
WT CD4+CD25+
CD4+CD25+ T cell mediated suppression
Unknown mechanism.
•
•
•
•
•
CD4+CD25+ T cells inhibit the IL-2/IL-2R system in T cells
Do not suppress by consuming IL-2.
Suppressor molecules are not defined.
Not directly cytolytic.
Cell-contact dependent and cytokine independent.
Treg effector functions
CD4+CD25+ mediated suppression
Requirement for a T:T cell interaction
APC
CD4+
CD25+
CD8+
Activation
CD4+
CD25+
Suppression
CD8+
Direct evidence
for T:T conjugates ?
Role for cytokines?
CD4+
CD25+
X
T cell
Responder
IL-10 & TGF-b1
Immunosuppressive effects on APC and T cells
But the role of differentiated Tregs is not abrogated by inhibiting
IL-10 and TGF-b1
Tissue-specific CD4+CD25+ mediated disease
protection in the absence of IL-10.
CD4+CD25CD4+CD25+
CD4+CD25-
CD4+CD25IL-10-/- CD4+CD25+
Nude
Gastritis
IBD
No
Gastritis
No
IBD
No Gastritis
IBD develops !
Altered peptide ligands o
peptidi antagonisti