Transcript Immunological Tolerance PP - The University of Arizona

T-cells &
Immunological Tolerance
Chapter 10.
Tolerance
• Our own bodies produce some 100,000
•
different proteins and one of the longstanding
conundrums of immunology has been to
understand how the immune system
produces a virtual repertoire against
pathogens while at the same time avoiding
reacting to self.
The strict definition of immunological
tolerance occurs when an immunocompetent
host fails to respond to an immunogenic
challenge with a specific antigen.
Tolerance
• The mechanisms the immune system uses
•
•
to ensure the absence of self-reactivity
(autoimmunity) include:
Central Tolerance - this occurs during
lymphocyte development.
Peripheral Tolerance - occurs after
lymphocytes leave the primary lymphoid
organs.
Central Tolerance
Peripheral Tolerance
TOLERANCE
Introduction
• Tolerance refers to the specific
immunological non-reactivity to an antigen
resulting from a previous exposure to the
same antigen.
• While the most important form of tolerance
is non-reactivity to self antigens, it is
possible to induce tolerance to non-self
antigens. When an antigen induces
tolerance, it is termed tolerogen.
TOLERANCE
•
•
Tolerance is different from non-specific
immunosuppression and immunodeficiency.
It is an active antigen-dependent process in
response to the antigen.
Like immune response, tolerance is specific
and like immunological memory, it can exist
in T-cells, B cells or both and like
immunological memory, tolerance at the T
cell level is longer lasting than tolerance at
the B cell level.
Tolerance to tissues and cells
•
•
Tolerance to tissue and cell antigens can be
induced by injection of hemopoietic (stem) cells
in neonatal or severely immunocompromised (by
lethal irradiation or drug treatment) animals.
Also, grafting of allogeneic bone marrow or
thymus in early life results in tolerance to the
donor type cells and tissues. Such animals are
known as chimeras. These findings are of
significant practical application in bone marrow
grafting.
Tolerance to soluble antigens
• A state of tolerance to a variety of T-
•
dependent and T-independent antigens
has been achieved in various experimental
models.
Based on these observations it is clear
that a number of factors determine
whether an antigen will stimulate an
immune response or tolerance
Tolerance
Also see Table 10-1 of text
Tolerance
• Induction of tolerance in T cells is easier
•
•
and requires relatively smaller amounts of
tolerogen than tolerance in B cells.
Maintenance of immunological tolerance
requires persistence of antigen.
Tolerance can be broken naturally (as in
autoimmune diseases) or artificially (as
shown in experimental animals, by xirradiation, certain drug treatments and by
exposure to cross reactive antigens).
Ignorance
• It can be shown that there are T cells and B
•
cells specific for auto-antigens present in
circulation.
These cells are quite capable of making a
response but are unaware of the presence of
their auto-antigen. This arises for 2 reasons.
Ignorance
• The first is that the antigen may simply be
present in too low concentration. Since all
lymphocytes have a threshold for receptor
occupancy which is required to trigger a
response then very low concentrations of
antigen (in the case of T cells these are very
low, see below) will not be sensed.
Ignorance
•
•
•
The second possibility is a more interesting one.
Some antigens are sequestered from the
immune system in locations which are not freely
exposed to surveillance.
These are termed immunologically privileged
sites. Examples of such sites are the eye, CNS
and testis.
Pathologically mediated disruption of these
privileged sites may expose the sequestered
antigens leading to an autoimmune response.
Mechanism of tolerance induction
Clonal deletion:
• Functionally immature cells of a clone
encountering antigen undergo a programmed
cell death, as auto-reactive T-cells are
eliminated in the thymus following interaction
with self antigen during their differentiation
(negative selection).
Mechanism of tolerance induction
Clonal deletion:
• Likewise, differentiating early B cells become
•
tolerant when they encounter cell-associated
or soluble self antigen.
Clonal deletion has been shown to occur
also in the periphery.
Mechanism of tolerance induction
Clonal anergy:
• Auto-reactive T cells, when exposed to
antigenic peptides which do not possess costimulatory molecules (B7-1 or B7-2),
become anergic to the antigen.
Mechanism of tolerance induction
Clonal anergy:
• Also, B cells when exposed to large amounts
of soluble antigen down regulate their
surface IgM and become anergic. These
cells also up-regulate the Fas molecules on
their surface. An interaction of these B cells
with Fas-ligand-bearing cells results in their
death via apoptosis.
Mechanism of tolerance induction
Receptor editing:
• B cells which encounter large amounts of
soluble antigen, as they do in the body, and
bind to this antigen with very low affinity
become activated to re-express their RAG-1
and RAG-2 genes.
• These genes cause them to undergo DNA
recombination and change their antigen
specificity.
What are RAG-1 RAG-2?
• Recombination signal sequences
•
•
(RAG).
RAG-1 is a specific endonuclease and is
only active when complexed with RAG-2.
Specific DNA sequences (heptamers)
found adjacent to the V, D, and J
segments in the antigen receptor loci and
recognized by the RAG-1/RAG-2
component of the V(D)J recombinase.
(see figure 7-11)
RAG-1/RAG-2
Mechanism of tolerance induction
Anti-idiotype antibody:
• Anti-idiotype antibodies produced experimentally
have been demonstrated to inhibit immune
response to specific antigens.
• Anti-idiotype antibodies are produced during the
process of tolerization.
• Such antibodies may respond to the unique
receptors of other lymphocytes and serve to shut
off antigen specific responses.
• Therefore, these antibodies prevent the receptor
from combining with antigen.
Mechanism of tolerance induction
Termination of tolerance
• Experimentally induced tolerance can be
terminated by prolonged absence of exposure to
the tolerogen, by treatments which severely
damage the immune system (x-irradiation) or by
immunization with cross reactive antigens.
• These observations are of significance in the
conceptualization of autoimmune diseases.
Dendritic cells: regulators of alloimmunity and
opportunities for tolerance induction
• Dendritic cells (DCs) are uniquely well-
•
equipped antigen-presenting cells (APCs)
regarded classically as sentinels of the
immune response, which induce and
regulate T-cell reactivity.
They play critical roles in central tolerance
and in the maintenance of peripheral
tolerance in the normal steady state.
Regulatory T cells
Mechanism of tolerance induction
Suppressor cells:
• Both low and high doses of antigen may
induce suppressor T cells (Regulatory T
cells) which can specifically suppress
immune responses of both B and T cells,
either directly or by production of
cytokines, most importantly, TGF-b and IL10.
Regulatory T cells
• CD4+ T lymphocytes that express high
•
•
levels of IL-2r a chain (CD25) but not other
markers of activation.
Regulatory T cells may be generated by
self antigen recognition in the thymus or in
the periphery.
These cells induce immunosuppression by
secreting TGF-b and IL-10 and thereby
inhibit Mf function and IFN-g activity.
Fig 10-10
Oral tolerance
•
•
•
•
The gastrointestinal tract is the largest
immunologic organ in the body.
It is constantly bombarded by a myriad of dietary
proteins.
Despite the extent of protein exposure, very few
patients have food allergies because of
development of oral tolerance to these antigens.
Once proteins contact the intestinal surface, they
are sampled by different cells and, depending on
their characteristics, result in different responses.
•
•
•
•
Oral tolerance
Antigens might be taken up by Microfold cells
overlying Peyer's patches, dendritic cells, or
epithelial cells.
Different cells of the immune system participate
in oral tolerance induction, with regulatory T
cells being the most important.
Several factors can influence tolerance
induction.
Some are antigen related, and others are
inherent to the host. Disturbances at different
steps in the path to oral tolerance have been
described in food hypersensitivity.
Oral tolerance
T regulatory cell family
• The idea of specific suppressor T cell
•
populations that counteract harmful
autoaggressive immune responses in the
periphery was first described in the 1970s
by Gershon et al.
However, at that time neither the cells nor
the hypothetical soluble suppressor factors
responsible for the observed effects could
be identified.
T regulatory cell family
• In 1995 Sakaguchi et al. described for the
first time a subpopulation of CD4+ T helper
cells, characterized by a constitutive
expression of the IL-2 receptor a-chain
(CD25), that is essential to control
autoaggressive immune responses in mice.
T regulatory cell family
• After subsequent in vitro studies by several
•
groups, this population is now referred to as
CD4+CD25+ T regulatory cells (Tregs).
This distinct T cell population was originally
described in mice. However, comparable T
cell suppressor populations, with identical
phenotype and functional activities have
been defined more recently in rats and
humans.
T regulatory cell family
• They represent 5–10% of all peripheral
•
CD4+ T cells.
Freshly isolated CD25+ Tregs do not
proliferate after allogeneic or polyclonal
activation in vitro, but Tregs suppress the
activation and cytokine release of CD4+ and
CD8+ T cells in an antigen-nonspecific and
cell contact-dependent manner.
T regulatory cell family
• However, it should be mentioned that the
•
•
activation of Tregs is also antigen-specific.
The main mechanism of suppression
seems to be the inhibition of IL-2
transcription in the responder T cell
population.
Nevertheless, the molecules involved in this
cell contact-dependent suppression are still
largely unknown.
T regulatory cell family
•
•
•
In the human immune system, two distinct
subsets of resident CD25+ Tregs can be
distinguished based on the expression of distinct
integrins.
Tregs expressing the a4 b7 integrin can convert
CD4+ T cells into IL-10-producing Tr1-like cells,
whereas a4 b1+ Tregs induce TGF-b -producing
Th3-like cells.
The integrins a4 b1 and a4b7 are homing
receptors for cellular migration of T lymphocytes
to inflamed tissues and to mucosal sites,
respectively.
T regulatory cell family
•
•
The a4b1-integrin binds to VCAM1 (vascular cell
adhesion molecule-1), which is induced on the
endothelium of inflamed tissues, whereas the
a4b7-integrin binds to vascular addressins,
selectively expressed by venules in mucosal
tissues.
Therefore, it can be postulated that a4b1+CD25+
Tregs migrate in vivo to inflamed tissues where
they can inhibit effector T cell responses.
a4b7+CD25+ Tregs are specialized to migrate to
mucosal tissues, to counteract autoreactive T
cells, thereby preventing chronic mucosal
inflammations.
T regulatory cell family
History of Tolerance
History of Tolerance
Timing
• Some 50 years ago Owen observed two
types of non-identical twin cattle, those
that had shared a hemopoietic system in
utero were tolerant of blood cells from
each other and those who had not, were
not cross-tolerant.
History
•
•
Burnet postulated that there was a temporal
window of tolerance such that antigens
encountered while the immune system was
immature tolerized the relevant lymphocytes.
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.
History
History
The Danger Hypothesis
• Matzinger versus Medewar?
• Matzinger has proposed that there is not a
special window for tolerance during
neonatal life but that whether encounter
with an antigen results in tolerance or an
immune response is determined by
whether the prevailing host environment
promotes a response via nonspecific cues
'sensing' danger.
History
•
•
Polly has further suggested that the controlled
death process of apoptosis is critical in
preventing autoimmunity when old or surplus
cells are disposed.
The notion that the normal, default pathway of
the immune system is tolerance rather than
response is not a new idea to immunologists antigens usually fail to elicit a response unless
given with adjuvants, whose purpose is probably
to generate stimulatory cues (cytokines).
History
•
•
Polly has further suggested that the controlled
death process of apoptosis is critical in
preventing autoimmunity when old or surplus
cells are disposed.
The notion that the normal, default pathway of
the immune system is tolerance rather than
response is not a new idea to immunologists antigens usually fail to elicit a response unless
given with adjuvants, whose purpose is probably
to generate stimulatory cues (cytokines).
History
• Recent experiments have shown that not
only can adults be tolerize under certain
circumstances, but that neonates can
make effective immune responses if the
antigen is presented in sufficiently
immunogenic form.
History
•
•
•
I believe that the supposed conflict between
Matzinger and Medewar is rather 'hyped up' and
essentially a matter of detail.
Neonatal T cells are not intrinsically tolerizable
but the systemic neonatal environment does
predispose to tolerance.
Nevertheless, I think that her hypothesis has
drawn the attention of a wider audience to
current ideas about tolerance induction and the
factors determining immune responsiveness.