1. Immune Regulation and Tolerance

2. Immunoregulation: A balance between activation and suppression of effector cells to achieve an efficient immune response without damaging the host.
Activation (immunity)
Suppression (tolerance)
autoimmunity
immunodeficiency
Significance: The induction of tolerance may be exploited to prevent graft rejection, to treat autoimmune and allergic diseases, and to prevent immune responses in gene therapy.

3. Important features of immunoregulation:
Antigen specific; affects T or B lymphocytes
Tolerance vs. activation? Determined by the nature of antigen and associated stimuli, and when and where the antigen is encountered

4. Immunological Ignorance
Mechanisms of unresponsiveness:
Immunological Ignorance
Normal response
Proliferation and
differentiation
Mechanisms of
unresponsiveness
Antigen/lymphocyte barrier
Mechanisms of
autoimmunity
Tissue abnormalities contributing to release and
presentation of self antigens.
Disease models
Sympathetic ophthalmia, experimental allergic
encephalomyelitis (EAE)

5. Central tolerance in B and T cells (I): Clonal Deletion
Mechanisms of unresponsiveness:
Central tolerance in B and T cells (I): Clonal Deletion
Self antigen
presented in generative lymphoid organs
Deletion of immature lymphocytes strongly recognizing self antigens present in generative organs
Lymphoid
precursor
Survival of clones which are only moderately responsive to self antigens present in generative organs; forms T cell repertoire

7. AIRE: Autoimmune regulator.
Transcription factor.
Expressed at a high level by thymic medullar epithelium
cells.
Autosomal recessive mutation leads to autoimmune
polyendocrine syndrom - type 1 (APS-1)
Inactivation of aire abolishes expression of tissue specific
genes in thymic medulla.

8. aire -/- WT

9. Central tolerance in B cells (II):Receptor editing
Mechanisms of unresponsiveness:
Central tolerance in B cells (II):Receptor editing
pre-B
bone
marrow B
rearrangement
10%
Maturation of clones:
Non-reactive to soluble-
low-affinity to soluble-
self-reactive to monovalent-
antigens in bone marrow
further
rearrangement
immature B

10. B220 Moma-1 TCR B follicle T cell 2% zone Spleen artery Self antigens
90%

11. Mechanisms of unresponsiveness: Peripheral tolerance in B cells (I): Follicular
exclusion
- B cells binding to autoantigens in the periphery
may be excluded from follicles
- Excluded B cells undergo apoptosis
mechanisms for elimination is independent of Fas, T cells.
- Rapid elimination depends on the presence of a
normal repertoire of B cells
competition between B cells (for limited survival factors?)

13. BAFF: TNF family cytokine, critical B cell survival factor.
BAFF reduction leads to a decrease in peripheral B cell numbers;
Over-expression of BAFF results in autoimmune diseases (SLE).
BAFF receptor: expressed on mature B cells.

14. Mechanisms of unresponsiveness: Peripheral tolerance in B cells (II): Anergy
Immunogenic signaling
Tolerogenic signaling
Acute
antigens
BCR
CD40
CD40L
TLR4
LPS
NFkB
Ca++
Growth genes
Chronic
antigens
BCR
CD40
TLR4
NFkB
Ca++
Inhibitory genes
Fcg2b

15. Anergic B cells can respond to “Stronger” antigens
Remains anergic
Oligovalent self antigens
Constitutively exposed
Activated
Multivalent foreign antigens
Acutely exposed

16. The two-signal requirement for T cell activation
Microbial antigen
presented by APC
TCR
MHC
Signal 1
APC
Signal 2 B7
Costimulatory
Receptor (CD28)

17. Innate immune response
The role of co-stimulation in T cell activation
Antigen recognition
T cell response
CD28
Resting APC:
(costimulator-
deficient)
No response
Activation of APC
Innate immune response
Activated APC:
increased
expression of
costimulators,
secretion of
cytokines
CD28 B7
T cell proliferation
And differentiation

19. Mechanisms of unresponsiveness: Peripheral tolerance in T cells (II): Anergy
Pretreatment
of T cells
Stimulation
with antigens
T cell
response
CD28
CD28 B7
18-24 hr
CD28 B7
CD28 B7
18-24 hr

20. Molecular basis of anergy in T lymphocytes
TCR
CD3z P
PROXIMAL
EVENTS:
- Reduced tyrosine
phosphorylation
- Reduced Ca++ influx
Zap 70
GROWTH
FACTORS
NUCLEAR EVENTS:
- no induction of NFkB
JNK activities
CELL
CYCLE

21. Co-stimulatory pathways
CD28 interacts with CD80 (B7-1) CD86 (B7-2) to
initiate T cell responses.
Preferentially expressed in naive T cells
ICOS (CD28 homolog) stimulate effector T cell responses.
Preferentially expressed in activated T cells
CTLA-4 and PD-1 negatively regulate T cell activation

22. Mechanisms of unresponsiveness: Peripheral tolerance in T cells (II): Anergy
Pretreatment
of T cells
Stimulation
with antigens
T cell
response
18-24 hr
CD28 B7
primed
CD28
18-24 hr B7
tolerated

23. CTLA-4-/- T cells resist tolerance induction
Primed
Tolerated
Days

24. How do T cells choose between CD28 and CTLA-4?
Level of B7 expression on APCs: low levels favor CTLA-4 engagement (high affinity receptor)
Kinetics: B7 on APCs engages CD28 early, CTLA-4 late in T cell responses

25. Regulation of T cell homeostasis during immune responses
Anergy
T cell
expansion B7
CTLA-4
Magnitude of T cell response
Activated T cells
express CTLA-4
Apoptosis
Surviving
memory cells B7
CD28
Activated T cells are
deprived of antigen
and other stimuli
T cell
activation
Time after antigen exposure

26. Pathways of apoptosis in T cells
Release of
mitochondrial
cytochrome c,
activation of
caspase-9
Elimination
of Antigen and
other signals
“Passive”
cell death
(death by
neglect)
CD28 B7
IL-2
T cell proliferation
FasL
Persistence
of antigen,
repeated
stimulation
Fas
Activation of
caspase-8
Activation
induced
cell death

27. T cell mediated suppression
MBP-TCR
In Rag-/-
MBP
MBP-TCR
In wildtype
MBP
MBP-TCR
In Rag-/-
MBP
CD4+CD25+ T cells

28. Science (2003) 299:1057

29. Foxp3 is expressed specifically in TR
Expression of Foxp3 converts
naïve CD4+ T cells to TR

30. Regulatory T cells (TR) in self tolerance
Phenotype and ontogeny:
CD4+cells (most are CD25+, some are CD25-) , develop in the thymus
Recognize self antigens?
Other populations of regulatory T cells exist (including CD8+)
Mechanisms of action:
Antigen-induced suppression; secrete immunosuppressive cytokines,
trigger inhibitory cell surface molecules (CTLA-4 expressed on TR).
Prevent the activation of T cells; suppress cell proliferation and IL-2
production

31. Role of cytokines in suppression of cell-mediated immune responses
Antigen
recognition
T cell proliferation
and differentiation
Effector functions
of T cells
IL-12
Effector T cells
(TH1)
Activated
macrophages
APC
Naïve T cell
IFN-
Cytokines
produced by
suppressor
T cells
IL-10 inhibits
Functions of APCs:
IL-12 secretion, B7
expression
TGF- inhibits
T cell
proliferation
IL-4 inhibits
action of
IFN-
IL-10, TGF-
inhibit
macrophage
activation
Suppressor T cells

32. Suppressor T cells - unresolved issues
How many types of suppressor T cells  cellular markers?
How do they develop  positive/negative selection?
What are their physiological ligands?
What are their target cells?
What is their mechanisms of actions?
Are they beneficial (for the prevention of autoimmunity, allergy and graft rejection)?
Are they harmful (in terms of their effects on tumor immunity, immune response to chronic infections and weak vaccines)?

33. Conclusions: Tolerance vs. Immunity
Immune responses are the outcome of a balance between the need to make a protective response and the need to maintain self-tolerance
Mechanisms of unresponsiveness:
Central tolerance: Deletion; Receptor editing
Peripheral tolerance: Clonal ignorance; Clonal deletion; Anergy; Suppression