IMMUNOLOGICAL TOLERANCE AND

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IMMUNOLOGICAL TOLERANCE AND IMMUNOREGULATION
Yufang Shi shiyu_at_umdnj.edu
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• Basic Concepts
• Self
• Recognized but not responded to.
• Immunological tolerance
• Lack of response to a given antigen, i.e.,
  specific immunological unresponsiveness
• Self tolerance
• Individuals normally do not respond to their
  "self" antigens, i.e., they are "tolerant" to
  self. This is the basis for self - non self
  discrimination by the immune system. Loss of
  self tolerance can lead to autoimmunity.
• Immunodeficiency
• Inability of the immune system to respond to a
  broad range of antigens.
• Immunosuppression
• A disorder or condition where the immune
  response is reduced or absent.
• Goal
• Sufficient for protection prompt
  downregulation does not persist after antigenic
  challenge resolved. Keep memory, eliminate
  effectors.

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History

• Paul Ehrlich (horror autotoxicus)
• The fear of self-poisoning
• First postulated the existence of autoimmune
  diseases in 1901.
• 80 autoimmune diseases are known (multiple
  sclerosis,
• type-I diabetes, rheumatoid arthritis,
  inflammatory
• bowel disease, and lupus)
• 2. Owen (dizygotic cattle twins, 1945)
• Sir Frank Macfarlane Burnet (The Function of
  Antibodies, 1949)
• Embryonic Ag exposure suppress future response
• 4. Peter Medawar
• (experimentally acquired tolerance, 1953)

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Timing
Cattle Placenta
Freemartins Sterile XX calves whose external
genitalia appear to be masculinized due to the
hormones from the XY twin Found when the
affected cow is one of a pair of twins, the other
twin being XY. Lillie, F. R. 1917. J. Exp. Zool.
23 371 - 452
Institut national de recherche pédagogique
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Ray Owen, 1945 Dizygotic cattle twins that
shared the same placenta were blood cell
chimaeras, ie contained blood cells of two
different haplotypes in adults and, - these two
haplotypes co-existed in the adults without
mutual rejection. Such chimaeric cattle will
exchange skin grafts without rejection.
Normally, if cells of one of the two haplotypes
were transferred to an adult cattle of the other
haplotype, these were promptly rejected. Thus,
tolerance in the dizygotic twins resulted from
the sharing of the placenta in fetal life, ie
from exposure to the antigen during the
development of the immune system.
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Peter Medawar (shared a Nobel Prize with Burnet.
1960)

• Autografts succeed, but allografts failed after
  an initial take.
• Second-set grafts underwent accelerated
  rejection.
• The destruction of the foreign epidermis was
  brought about by a mechanism of active
  immunization

Newborn mice of CBA strain received splenocytes
from A strain. Once immunological mature, they
were unable to reject a graft from the A strain
but remained perfectly able to reject an
equivalent graft from a third strain.
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Medawars experiment demonstrating neonatal
tolerance induction (Nobel Prize)
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Brent, Human Immunology Volume 52, Issue 2
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Self tolerance- A Learning Experience How
does the immune system learn to discriminate
between self and non-self during development ?
The primary repertoire of T (and B) cells is
enormous as a result of combinatorial diversity
and imprecisional joining of VDJ or VJ segments.
This repertoire contains self-reactive BCRs and
TCRs and yet a normal immune system does not
exhibit self-reactivity. In the lecture on T
cell development mechanisms, we have been
explained T cell tolerance for self antigen by
negative selection. Through positive and negative
selection, the resulting repertoire is therefore
one of T cells that react with non-self antigens
in association with self-MHC (slightly react to
self-antigen plus self-MHC). However, negative
selection is not complete and other mechanisms
are needed in the peripheral.
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Types of Tolerance
Only antigen receptor-bearing cells can be
tolerized

• Central Tolerance happens during lymphocyte
  development.
• Negative selection or clonal deletion of T and B
  cells that have receptors for self-antigens. 90
  of immature lymphocyte die in thymus/bone marrow
• a. TCR/BCR gene rearrangement fails
• b. Not selected on thymic epithelial MHC
• c. Self antigen reactive
• Autoimmune regulator (AIRE) gene in thymic
  epithelial cells
• Peripheral Tolerance - occurs in the periphery
  after lymphocyte development.
• Ignorance low dose or immunologically
  privileged sites Self B cells w/o T cells no
  response
• Clonal contraction and AICD LPR Fas deficient,
  Gld FasL deficient. Both have autoimmune
  phenotypes. Cannot regulate duration of
  response
• Suppression specific immune regulation through
  regulatory T cells, TGFb, or IL-10
• Anergy Lack of co-stimulatory molecules during
  T cell activation solubable or low dose
  antigen
• Consequences apoptosis, anergy, inactivation

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Tolerance to Self Antigen

• Antigen sequestration (lens of eye,
  spermatozoa). Immunologically privileged sites

• Low MHC expression (i.e. hepatocytes)

Immune Response to Foreign Antigens

• Influence of Antigen

Dose (low zone, high zone) timing/duration of
exposure routes, nature of antigen, protein gt
CHO gtgt lipids, presence of adjuvants

• Influence of Antibody

feedback inhibition (IgG inhibits IgM),
differential antigen binding affinity

• Factors favoring tolerance
• Age, neurological and endocrine factors,
  Nutritional status, MHC
• Haplotypes

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Antigen Properties

• Molecular weight

• smaller favors tolerance

• adjuvant, aggregation favors immunogenicity

• Dosage

• small, large favors tolerance

• intermediate favors immunogenicity

• Routes of Exposure

• IV exposure tolerizes naïve but not memory T
  cells

• Rapid transit to spleen

• Binds to resting B cells

• Bypasses MHC and costimulatory molecule

• Result is anergy

• Oral, intratracheal, obital exposure

• Can activate mucosal T cells to secrete TGF?.
  special DC

• Cryptic epitopes

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• Regulation by antibody (end-product inhibition).
• Passively administered antibody can prevent an
  antibody response (von Behring). This principle
  is utilized in the prevention of erythroblastosis
  fetalis by RHOGAM (Many women are given RhoGAM
  around the 28th week of pregnancy)
• 1. Passive antibody inhibits primary response gt
  secondary response.
• 2. High affinity antibody of IgG subclass is
  best. Subunits (Fab) are far less effective.
  The effect is specific and is primarily on B
  cells helper T cells are relatively resistant.
• 3. Antibody made in response to an antigen may
  homeostatically regulate its own level.
• -Antibody may act by limiting the amount of
  antigen (afferent masking),
• - by inactivating specific lymphocytes (as
  central action immune complexes) or
• -by blocking immune effector function (efferent
  inhibition e.g. in human neoplasia or
  enhancement of transplanted allografts).
• Antibodies directed at idiotypes and
  anti-idiotypic suppressor cells also play a role
  in regulating responsiveness.
• Epitope on Ig tip

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• Mechanisms of Intravenous immunoglobulin (IVIg)
• Modulation of complement activation products
• Suppressing idiotypic antibody
• Saturating Fc receptors on macrophages
• Suppressing various inflammatory mediators
  including
• cytokines, chemokines, and metalloproteinases.

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Acquired Tolerance

• In many cases, experimental unresponsiveness may
  be mediated by suppressor (T) cells, which
  actively prevent an immune response.
• Original experiments of Gershon and Kondo (1970)
  showed that T cells were required for tolerance
  induction. Moreover, T cells from tolerant mice
  suppressed B cells from normal mice.
• Active suppression by T cells also seen in some
  responses under Ir gene control and in the
  regulation of IgE responses (Tada).
• Suppression commonly induced by systemic
  administration of antigen-coupled to self cells
  route of injection is important (i.v. favors
  intradermal gives contact sensitization).
• Suppressor T cells produce factors. These
  factors may act directly on T/B targets.
  However, until recently, there has been some
  evidence that these factors are encoded by TCRa
  and b chain loci.
• Can be adaptively transferred.

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Cells Currently Known to Bear the Capacity to
Suppress Immune Responses
T Suppressor cells (CD8), Discovered by Richard
Gershon CD4CD25 Treg Veto
cells CD4-CD8- Th3
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• Five Nos of Suppressor T Cells
• No identifiable cell surface marker
• No clone or cell lines with suppressor activity
• T hybridomas producing antigen specific
  suppressor factors
• has no TCR rearrangement
• No I-J gene exist
• No antigen specific suppressor factor gene
  identified

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Ts Cells are Back
Nature Immunology 5, 469 - 471 (2004)
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• Regulatory T Cells (CD4CD25)
• Naïve T cells to syngeneic lymphocyte deficient
  mice develop multiorgan autoimmunity. Plus
  activated T cells, prevent
• CD4CD25GITRCD45RBlow FOXP3
• Produce TGF?, IL-10.
• Active in vitro and in vivo
• Suppress proliferation of responding CD4 and
  CD8 T cells by inhibiting the production of
  IL-2, as well as inhibiting the upregulation of
  IL-2 receptors
• Suppress immune response to tumor antigens
• Depletion of CD25 cells, enhances tumor
  rejection.

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Veto Cells

• The veto cell can be a CD8 CTL.
• For the veto function, the TCR is not required.
• The veto function is a backward action of CTL.
• Only self-reactive CTL are suppressed by veto
  cells.

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Upon recognition of class I on the veto cell by
TCR on the effector T cell, Fas is up-regulated
on effector cells, allowing for FasL on the veto
CTL to induce apoptosis
Reich-Zeliger et al. 2004 J. Immunol. 1736660
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Th3
From Weiner, Microbes and Infection , 3947
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Idiotypic Regulation The idiotypes expressed
by T and B cells can be recognized by the immune
system. 1. Anti-idiotypic responses
may serve to down-regulate an ongoing immune
response. 2. Since some anti-idiotypes (Ab2)
may look like antigen, they can be used as
"vaccines 3. V region genes code for idiotype
4. Anti-idiotype response can initiate
control loops How important is the idiotypic
network?
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T Helper Cell Differentiation and Function
Antigen

APC
IL-12
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A Model Depicting the Role of Apoptosis in Th1
and Th2 Balance
DC1
DC2
TCR
?
MHCAg
MHCAg
IL-12
Thp
IL-4
Stat 4 ERM T-bet
Stat 6 GATA-3 c-Maf IRS
Th2
Th1
FLIP
Suicide Fratricide
TRAIL
Fratricide
CD95L
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• Tolerance can be broken
• New clones of T and B cells appear in the absence
  of antigen
• Viral infection can create a cytokine milieu to
  turn on quiescent (anergic) cells
• New epitopes are introduced either by viral
  modification or iatrogenically

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• Summary
• DEFINITION Tolerance may be defined as a
  specific unresponsive state induced by prior
  exposure to an antigen. Therefore, tolerance is a
  learned process and is specific.
• 1. Tolerance is manifested in both T and B cells.
• 2. The kinetics of tolerance differ in B vs T
  cells.
• a. T cells exhibit more rapid tolerance
  induction than B cells.
• b. Tolerance is lost in B cells more
  rapidly than in T cells.
• c. T cells are tolerized by lower doses of
  antigen (tolerogen) than B cells.
• Waning reflects the differentiation of new T and
  B cells in the absence of
• sufficient tolerogen, and can be
  prevented with more antigen.
• 4. Tolerance can be broken
• a. New clones of T and B cells appear in the
  absence of antigen
• b. Viral infection can create a cytokine
  milieu to turn on quiescent (anergic) cells
• c. New epitopes are introduced either by
  viral modification or iatrogenically
• d. Cross-reactive or modified antigens. New
  epitopes are introduced either by viral

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Factors Affecting Tolerance Induction A.
Age Young immunologically immature animals show
tolerance antigen exposure. 1. Immature B
cells lack surface IgD and fail to resynthesize
IgM receptors after capping. 2. Antigen is
poorly localized and presented in immature
animals B. Route of exposure i.v. or
oral exposure favors tolerance. S.c. or
intradermal favors immunity. Intramuscular
favors Th2. C. Dose of antigen High
doses favor tolerance however, repeated low
doses can also cause tolerance D.
Associated antigens Coupling of antigens to
self Ig or self cells enhances tolerogenicity.
1. Coupling nucleosides to self
carriers can prevent anti-DNA in genetically
autoimmune mice.
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Immunosuppressive cytokines, factors and
drugs TGF?, IL-10 Corticosteroids,
prostaglandins, Neuropeptides, FasL, TRAIL,
TNF CTLA4 Cyclosporin, rapamycin,
FK506 Apoptotic Cells???
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Cell Death and Immune Responses