T-cells

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T-cells Immunological Tolerance
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Chapter 10.
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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.

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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.

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Central Tolerance
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Peripheral Tolerance
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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.

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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.

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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.

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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

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Tolerance
Also see Table 10-1 of text
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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 x-irradiation, certain
  drug treatments and by exposure to cross reactive
  antigens).

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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.

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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.

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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.

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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).

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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.

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Mechanism of tolerance induction

• Clonal anergy
• Auto-reactive T cells, when exposed to antigenic
  peptides which do not possess co-stimulatory
  molecules (B7-1 or B7-2), become anergic to the
  antigen.

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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.

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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.

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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)

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RAG-1/RAG-2
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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.

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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.

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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.

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Regulatory T cells
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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 IL-10.

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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.

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Fig 10-10
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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.

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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.

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Oral tolerance
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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.

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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.

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T regulatory cell family

• After subsequent in vitro studies by several
  groups, this population is now referred to as
  CD4CD25 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.

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T regulatory cell family

• They represent 510 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.

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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.

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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.

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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 a4b1CD25
  Tregs migrate in vivo to inflamed tissues where
  they can inhibit effector T cell responses.
• a4b7CD25 Tregs are specialized to migrate
  to mucosal tissues, to counteract autoreactive T
  cells, thereby preventing chronic mucosal
  inflammations.

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T regulatory cell family
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History of Tolerance
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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.

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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.

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History
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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.

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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).

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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).

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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.

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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.