The document discusses immunologic tolerance, a specific unresponsiveness to antigens induced by exposure to those antigens. It details the mechanisms and types of tolerance (central and peripheral), the processes involved in self-tolerance, and potential applications in clinical medicine, including organ transplantation and treating autoimmune diseases. Key concepts include the induction of tolerance in lymphocytes, the role of tolerogens, and the distinctions between various forms of tolerance.

1. Immunologic Tolerance
prof.dr.ihsan edan alsaimary
department of microbiology – college of medicine –
university of basrah

2. Contents
• Part Ⅰ Introduction
• Part Ⅱ Development of Immunologic Tolerance
• Part Ⅲ Mechanism of Immunologic Tolerance
• Part Ⅳ Immunologic Tolerance and Clinic Medicine

3. • Definition: A type of specific unresponsiveness to
an antigen induced by the exposure of specific
lymphocytes to that antigen, but response to other
antigens normally.
• Tolerogens: antigens that induce tolerance
Part Ⅰ Introduction

4. General features of Immunologic tolerance
• Tolerance is antigenic specific and results from the
recognition of antigens by specific lymphocytes.
• Normal individuals are tolerant of their own
antigens(self antigen)----- Self-tolerance.
• Foreign antigens may be administered in ways that
preferentially inhibit immune response by inducing
tolerance in specific lymphocytes---antigen induction.

5. Immunologic features of tolerance
 It is an antigen-induced, active process
 Like immunologic memory, it is antigen specific
 Like immunologic memory, it can exist in B cells,
T cells or both
 Like immunologic memory, its easier to induce
and lasts longer in T cells than in B cell

6. Difference of Immuologic tolerance &
immunodeficiency, immunosuppression
Immunodeficiency: any condition in which there
is deficiency in the production of humoral and /or
cell-mediated immunity---non-specificity to Ag
Immunosuppression: The suppression of
immune responses to antigens. This can be
achieved by various means, including physical,
chemical----non-specificity to Ag

7. BASIC FACTS ABOUT TOLERANCE
• Tolerance – a state of
unresponsiveness specific for a given
antigen
• It is specific (negative) immune
response
• It is induced by prior exposure to that
antigen

8. BASIC FACTS ABOUT TOLERANCE-2
• Self tolerance – prevents the body to
elicit an immune attack against its own
tissues
• Mechanisms of active tolerance
prevent inflammatory reactions to many
innocuous airborne and food antigens
found at mucosal surfaces

9. Features of self-tolerance
• Self-non-self discrimination is learned
during development
• Tolerance is NOT genetically programmed
• The time of first encounter is critical in
determining responsiveness

10. FACTORS IMPORTANT IN THE
INDUCTION OF TOLERANCE
1. The stage of differentiation of lymphocytes
at the time of antigen confrontation
2. The site of encounter
3. The nature of cells presenting antigenic
epitopes
4. The number of lymphocytes able to respond
5. Microenvironment of encounter (expression
of cell adhesion molecules, influence of
cytokines etc.)

11. TOLERANCE – GENERAL PROPERTIES
1. Immature or developing lymphocyte is more
susceptible to tolerance induction than
mature one
2. Tolerance to foreign antigens is induced
even in mature lymphocytes under special
conditions
3. Tolerance of T lymphocytes is a particularly
effective for maintaining long-lived
unresponsiveness to self antigens

12. POSSIBLE WAYS OF PREVENTION OF
SELF-REACTIVITY
• Clonal deletion – physical elimination of cells
from the repertoire during their lifespan
• Clonal anergy – downregulating the intrinsic
mechanism of the immune response such as
lack of costimulatory molecules or insufficient
second signal for cell activation
• Suppression – inhibition of cellular activation
by interaction with other cells:
(Treg – CD4+, CD25+ T lymphocytes)

13. DIVISION OF TOLERANCE
Central
• The site for T cells is
the thymus
• The site for B cells
is the bone marrow
• The mechanism –
clonal deletion
Peripheral
• The site – everywhere
in the body
• Cells – both T and B
• Mechanisms – anergy,
cell death, immune
deviation

14. IMPORTANT OPPOSED TERMS
• Central tolerance (self-tolerance)
versus peripheral tolerance
• Active tolerance vs. passive tolerance
(ignorance)
• T-cell tolerance vs. B-cell tolerance
• Natural vs. artificial tolerance
• Complete vs. incomplete tolerance

15. IMMUNOLOGICALLY PRIVILEGED SITES
• Sites in the body where foreign
antigens or tissue grafts do not elicit
immune responses
• These antigens do interact with T cells,
but instead of destructive IR they
induce tolerance or a response
innocent to the tissue

16. IMMUNOLOGICALLY PRIVILEGED SITES - 2
• Immunosuppressive cytokines such as
TGF-beta seem to be resposible for
such unusual response
• The sites include: brain, eye, testis,
uterus (fetus)

17. IMPORTANT OPPOSED TERMS
• Central tolerance (self-tolerance)
versus peripheral tolerance
• Active tolerance vs. passive tolerance (ignorance)
• T-cell tolerance vs. B-cell tolerance
• Natural vs. artificial tolerance

18. IGNORANCE OF SELF ANTIGENS
• It is a passive form of immunological
tolerance
• It occurs when:
1. T cells can not contact with self –
antigens,
2. if self antigen is present in too low an
amount to be detected,
3. if it is present on cells with few or no MHC
molecules
4. if there are not enough T cells to respond
5. if there is the absence of co-stimulation

19. FEATURES OF B-CELL TOLERANCE
1. Binding soluble self antigens is tolerogenic
for B cells
2. Immature B cells that encounter self antigen
undergo clonal abortion
3. The fate of self-reactive B cells depends on
the affinity of the B cell antigen receptor and
the nature of the antigen
4. Receptor editing - autoreactive B cells
escape anergy or deletion by further
rearranging their immunoglobulin genes

20. IMPORTANT OPPOSED TERMS
• Central tolerance (self-tolerance)
versus peripheral tolerance
• Active tolerance vs. passive tolerance (ignorance)
• T-cell tolerance vs. B-cell tolerance
• Natural vs. artificial tolerance

21. ARTIFICIAL INDUCTION OF TOLERANCE
1. By chimerism in immunocompromized hosts
2. By antibodies to T cell receptors
3. By injection of soluble antigens

22. ARTIFICIAL INDUCTION OF TOLERANCE - 2
4. By oral administration of antigens
5. By clonal exhaustion via extensive clonal
proliferation after repeated antigenic
challenge
6. Anti-idiotypic antibody – partial result, affects
B cells only

23. FUTURE APPLICATIONS OF
TOLERANCE
• To promote tolerance to foreign tissue
grafts
• To control the damaging immune responses
in hypersensitivity states and autoimmune
diseases

24. --{ THANK YOU }—
and be tolerant to our
imperfections!

25. Part Ⅱ Development of Immunologic
Tolerance

26. • Owen first observed
immunologic
tolerance to
allogenic antigen in
fetal period in 1945
1. Induction of immunologic tolerance to
antigen in fetal period and neonate period

27. cattle of dizygotic twin

28. Experiment of Medawar on immunologic tolerance

30. 2. Induction of immunologic tolerance to
antigen in adult
Antigen and immunologic tolerance:
• Concentration of antigens
• Type of antigen: monomer, aggregates
• Pathway of antigen entering body
• Features of determinant: tolerogenic epitope
• Variation of antigen

31. High and low dose tolerance

32. Tolerance in T and B cells

33. Factors affecting tolerance
role of antigen
Factors which
affect response
Favor immune
response
Favor tolerance
 Physical form of
antigen
 Route of injection
 Dose of antigen
 Large, aggregated,
complex molecules,
properly processed
 Subcutaneous or
intramuscular
 Optimal dose
 soluble, aggregate-free,
simple small molecules,
not processed
 Oral or, sometimes,
intravenous
 Very large or very small
dose

34. Individual and immunologic tolerance:
Heredity, Age, Gender, Health

35. Factors affecting tolerance
the role of host
Factors that
affect response
Favor immune
response
Favor tolerance
 Age of responding
animal
 Differentiation
state of cells
 Fully differentiated;
memory T & B cells
 Older, immuno-
logically mature
 Newborn (mice), immuno-
logically immature
 Relative undifferentiated B
cell with only IgM, T cells
in the thymic cortex

36. Host age and antigen dose affect
tolerance
newborn adult

37. Part Ⅲ Mechanism of Immunologic
Tolerance

38. 1. Central tolerance:
Central tolerance occurs in the central
lymphoid organs as a consequence of
immature self-reactive lymphocytes
recognizing ubiquitous self-antigen.
2. Peripheral tolerance:
tolerance was induced in peripheral organs
as a result of mature self-reactive
lymphocytes encountering tissue-specific self
antigens under particular conditions

40. 1. Central tolerance
Clonal deletion (apoptotic cell death)
During maturation of lymphocytes in the
thymus for T cell or in the bone marrow for B
maturation, immature lymphocytes that
recognize ubiquitous self-antigen with high
affinity are deleted by negative selection

41. Clonal deletion:
negative selection of T cells in the thymus

42. Central Tolerance

43. Negative selection of B cells in
bone marrow

44. 2. Peripheral tolerance
① clonal deletion and clonal ignorance:
large tissue specific antigen delete specific T cells.
self-reactive lymphocytes remain viable and
functional but do not react to the self antigens in
any detectable way.
② Clonal anergy and inactivation:
functional inactivation without cell death: lack co-
stimulatory signal

47. Clonal anergy in T cells

48. Clonal anergy in B cells

49. ③ Action of Suppressor lymphocyte (Ts)
④ Action of cytokines: TGF- , IL-10
⑤ Holdback in signal tranduction
⑥ Immunologically privileged sites
anatomic barrier: clonal ignorance

50. Fas
FasL
cytokines
Apoptosis
Inhibition of
proliferation &
effector action
Activated
T cells
Normal
Response
CD28 B7
Proliferation & differentiation
Antigen Recognition
without co-stimulation
Anergy
CTLA4 B7
Functionally
Unresponsive
CTL4-B7 interaction
Fas-FasL interaction
Cytokine-mediated suppression
Activation
induced
cell death
Cytokine
regulation
Pathways to Peripheral Tolerance

51. The Two Signal Hypothesis for T-cell
Activation
Mature
Dendritic
cell
APC
TH
cell
CD28
B7
MHC II
TCR
Signal 2
Signal 1
Activated
TH cell

52. Hypothetical mechanism of tolerance in
mature T cells
CD28
Resting
B-cell
APC
TH0 cell
Tolerance (anergy or apoptosis)
from lack of signal 2
Signal 1
Tolerant
T cell

53. Normal
Response
CD28 B7
Proliferation &
differentiation
Summary: Lack of co-stimulation can
lead to tolerance (anergy)
Antigen Recognition
without co-stimulation
Anergy

54. Regulation by CTLA-4
CTLA4
B7
Functionally
Unresponsive (Anergic) T cell
CTLA4-B7 interaction
Activated T cell

55. Regulatory T cells
Functionally
Unresponsive T cell
Production of IL-10 or TGF-
Regulatory
T cell

56. Pathways to Peripheral
Tolerance

57. Inhibition by Antibody Feedback
• Passively administered antibody can prevent an
antibody response
• Antibody produced during an immune responses
leads to elimination of antigen (stimulus)
–Less antigen available to stimulate specific
cells
–Immune complexes can bind to inhibitory
receptors
Application: RhoGam for Erythroblastosis
Fetalis

58. Major Immune Inhibitory
Receptors
• B cells
– FcgRII
• T cells
– CTLA4
• NK cells
– KIR (killer cell Ig-like receptors),

59. Anti-Idiotypes and Immune
Regulation
• Definition
– anti-idiotype response-antibody produced
against immunoglobulin or TCR idiotypes
that serve to down-regulate immune
response
– The epitope for an responsive anti-idiotype
molecule (antibody, BCR, or TCR) is the
internal image formed by the CDR region
of the respective epitopes antigen receptor

60. Idiotype/Anti-idiotype network

61. Part Ⅳ Immunologic Tolerance and
Clinic Medicine

62. • Prevent the rejection of organ allografts
and xenografts
• Treat autoimmune diseases
• Treat allergic diseases
1. To induce immunologic tolerance

63. 2. To terminate immunologic
tolerance
• To treat tumor:
enhance first signal or second signal
• To treat infection diseases

64. And now for a clinical case….

66. • 6 year old male, ER with unexplained
bruising associated with minor trauma
• Patient has minimal clotting activity
• FVIII levels <1% of normal
• Patient given i.v. FVIII concentrate i.v.
and released but returns in two weeks
with same problem
• Repeated FVIII treatment
• However, FVIII is ineffective.
Patient Presentation

67. Issues
• Coagulation factor inhibitors (anti-FVIII
activity)
• Basis?
• Lack of tolerance. Why?
• Prevalence/impact
• 20-30% FVIII, less FIX
• Treatment/problems
• FVIII concentrate or rFVIII
• Inhibitors develop that neutralize FVIII
• Therapy?
• Porcine FVIII with less cross-reactivity
• Tolerance (high dose)
• Gene therapy

68. What are Inhibitors?
• IgG; commonly subclass 4, mixed 1 & 4
• Occur in
• Congenital factor deficiency = alloimmune
• Previously unaffected = autoimmune
• Associated with pregnancy, autoimmunity,
malignancy, multi-transfusion, advanced age etc.

69. Summary
Definition of immunologic tolerance
Features of immunologic tolerance
Induction of immunologic tolerance
Mechanism of immunologic tolerance
Clinical application of immunologic tolerance

70. Diagnostic Immunology
Topic: Immunological Tolerance
Objectives:
 Define Immunological tolerance
 Discuss mechanism of tolerance induction
 Know types of tolerance
 Central thymic
 Peripheral post thymic
 Explain B cell tolerance to self antigens
 Discuss artificial induction of tolerance
 Know therapeutic applications of tolerance

71. Immune tolerance or
immunological tolerance
 is the process by which the immune system
does not attack an antigen.
 It can be either :
1) Natural' or 'self tolerance', where the body
does not mount an immune response to self
antigens.
2) Induced tolerance', where tolerance to
external antigens can be created by
manipulating the immune system.
 It occurs in three forms: central tolerance,
peripheral tolerance and acquired tolerance.

72. Definitions:
 Tolerance is a state of unresponsiveness that is
specific for a particular antigen
 Self tolerance is the mechanisms by which the
body is prevented from mounting an immune
response against its own tissues.
 Self reactivity is prevented by processes during
development rather than being programmed.

73. Factors influencing tolerance
 Molecule structure
 Stage of differentiation when
lymphocyte first encounter the epitopes
 The site of the encounter
 The nature of the cell presenting the
epitopes
 Number of responding lymphocytes
Q. What is an epitope?

74. History
 Trub (1938):
Inutero mice against choriomeningitis virus
 Medawar (1953)
Induced immunological tolerance to skin allograft
 Burnet (1957)
Clonal selection theory: Particular immunocyte is
selected by an antigen and then divides to give rise to
clone of daughter cells with the same specificity

75. Leaderburg (1959)
Modification of Clonal selection theory ( Stage of cell
maturation)
Key discoveries of (1960)
a) Role of thymus in development of the immune system
a) Existence of two interacting subsets of lymphocytes

78. Types of Tolerance
 Central Tolerance :
It occurs during lymphocyte development.
[Thymus or Bone marrow]
 Peripheral Tolerance :
Occurs after lymphocytes leave the primary
organs.

79. Central thymic Tolerance to Self Antigens
 The immune system generates a vast array of
TCRs
 T cells are not only effector cells but are also
regulators of the immune system
 T cells become “educated” in the thymus
 They become dependant on self MHC for
survival

83. III-T cell development is subjected to several
checkpoints
 β selection checkpoint:
Only cells with a rearranged β chain mature from
double negative to double positive cells.
Independent on MHC
 α selection checkpoint:
Cells expressing an αβ complex must interact with
MHC molecules to survive

84.  Lineage commitment checkpoint:
Cells are instructed to repress expression of
either CD4 or CD8 and to develop into
single positive cells
 Negative selection checkpoint:
Cells that interact strongly with MHC and
antigen in the thymus are deleted

85. Peripheral or Post-Thymic Tolerance to Self
Antigens
Many auto-reactive T cells escape central tolerance
due to:
a) Antigens are absent
b) Antigens are insufficient
However, tolerance is maintained by mechanisms
in peripheral lymphoid organs

86. 1-Sequestration of antigens in some tissues:
Many self antigens are hidden in tissues that are anatomically
located away from T lymphocytes
2- Privileged sites are protected by regulatory
mechanisms:
Privileged sites include brain, testes and anterior champers of
the eye
In these sites lymphocytes are controlled by apoptosis or
cytokines such as TNFβ and IL10
These mechanisms include:

87. 3- T cell death can be induced by persistent activation
or neglect:
-Cells possess Fas receptor , when it cross-links with its ligand
FasL it promote apoptopic cell death.
- Activated lymphocyte will die by passive cell death (PCD)
when deprived from its antigen
4- Regulation by suppressive cytokines
IL-2 and IL-2 receptor regulate sensitivity to Fas mediated
apoptosis

88. Summary of peripheral post thymic mechanisms of
tolerance:
Thymus
Deletion Escape
Immune
regulation
By
Suppressive
cytokines
Deletion
By
activation-
induced
Cell death
Sequestration
Antigen
hidden from
immune
system
Privileged
sites
Prevention
by Fas
and
cytokines

89. B- Cell Tolerance to Self Antigens
 High affinity IgG production is T cell dependent
 Lack of T cell leads to non-self reactivity by B
cells
 Self reactive B cells either:
a) Edit their receptors to become non-reactive
b) Die by the process of apoptosis
 In peripheral B cell tolerance, self reactive cells
are removed by negative selection in the
spleen in a process that is similar to T cell
removal in the thymus.

90. B cell tolerance in the BM
During B cell development in the bone marrow:
 The complete antigen receptor (IgM) is first expressed
on 'immature' B cells
 If those cells encounter their target antigen in a form
which can cross-link their IgM then such cells are
programmed to die
 The requirement for cross-linking means that the antigen
has to be polyvalent

92. Potential clinical applications for tolerance:
 Understanding of tolerance is valuable in many
ways:
 Promote tolerance to tissue graft
 Control damaging immune response in
hypersensitivity
 Control damaging in autoimmune disease
 Limit tumor growth