201 Immunology: goals

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In unimmunized mice 1 in 26,300 spleen B cells
could make anti-SRC IgM no detectable (lt1 in a
million) B cells that could make anti-SRC
IgG (Martínez-Maza, et al. Scandinavian J.
Immunol 17251, 1983) In immunized mice 1 in
219 B cells could make anti-SRC IgM (5d
post-immunization) 1 in 112 B cells could make
anti-SRC IgG (12d) 1 in 3,030 B cells could make
anti-SRC IgG (180d) (Martínez-Maza, et al.
Scandinavian J. Immunol 17345, 1983)
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T CELL DEVELOPMENT AND ACTIVATION

• There are a lot of similarities between T and B
  cells, in their development
• arise from hematopoietic precursors that are
  generated in the bone marrow
• undergo similar DNA rearrangements to generate
  the genes for their antigen receptor molecules
• have the capacity to respond to nearly any
  antigen
• the initial stages of development are
  antigen-independent, with final differentiation
  occurring after exposure to antigen
• cells that express antigen-receptors that react
  with self are eliminated

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• However, there are some significant differences
• since the T cell receptor can interact with
  antigen only when it is presented in association
  with self-MHC molecules, T cells need to be able
  to bind to a complex of self MHC Ag peptide
• in addition to this (perhaps because of this) T
  cells do not develop in the bone marrow, they
  undergo development in a specialized organ, the
  thymus.

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• T lymphocytes or T cells got their name from
  original observations that indicated that they
  were thymus-derived lymphocytes.
• T cell precursors travel from the bone marrow to
  the thymus

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• Following development into mature,
  antigen-responsive T cells, these T cells emerge
  from the thymus and migrate to secondary lymphoid
  tissues, where they interact with antigen,
  antigen-presenting cells, and other lymphocytes
   

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• The importance of the thymus in T cell
  development is demonstrated by inherited immune
  deficiencies people that do not have a thymus
  (DiGeorges syndrome, aka Thymic Aplasia) do not
  develop functional T cells.
• DiGeorges syndrome results from a developmental
  defect the failure of the third and fourth
  pharyngeal pouches to develop, which results not
  just in thymic defects, but also in absent
  parathyroids and in aortic arch defects.
• Thymectomy early in life reduces the ability to
  produce T cells.
• Thymectomy later in life does not markedly impair
  T cell number.
• In fact, the thymus decreases in size with age.
• However, the thymus can still produce new T cells
  up to middle-age, especially in situations where
  there is loss of T cells (HIV/AIDS).  

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• While in the thymus, immature T cells, or
  thymocytes, undergo several changes that allow
  them to develop into mature T cells, ready for
  contact with antigen.
• Thymocytes interact with thymic epithelial cells
  and various other cells while in the thymus.

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• The thymus is composed of several lobes, each of
  which has cortical and medullary regions

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• The cortex contains immature thymocytes in close
  contact with thymic epithelial cells.
• Medullary areas contain more mature thymocytes,
  epithelial cells, and dendritic cells and
  macrophages

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• During thymic differentiation, the great
  majority of thymocytes die by apoptosis, and are
  ingested by macrophages.
• Only a small minority of these T cell progenitors
  make it out as mature T cells

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• Thymic development occurs in two phases
• production of T cell receptors for antigen, by
• rearrangement of the TCR genes
• 2) selection of T cells that can interact
  effectively with self-MHC
•  

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• Changes in the expression of cell-surface
  molecules accompany the thymic differentiation of
  T cells
• entering thymocytes are TCR, CD3, CD4, and
  CD8-negative
• as thymocytes mature, and undergo rearrangement
  of their TCR genes to generate a functional TCR,
  they begin to express CD3, CD4, and CD8
• mature T cells ready to go to the periphery are
  TCR/CD3, and either CD4 or CD8 positive

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First phase of thymic development rearrangement
of TCR genes to produce a functional TCR

• Progenitor T cells enter the thymus
  (sub-capsular region of the outer cortex).
• These cells do not have rearranged TCR genes and
  lack expression of characteristic T cell surface
  molecules.
• Interaction with thymic stromal cells induces
  these progenitor T cells to proliferate.
• These immature thymocytes do not yet express CD4
  or CD8, molecules that are expressed by mature T
  cells double-negative thymocytes.
•  
•  

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• There are two types of T cell receptors gd and
  ab
•  
• ab TCR T cells are the most abundant, by far
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(or g d chain)
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• Unlike B cells, in which the genes that encode
  the BCR rearrange in a set order, the TCR b, g,
  and d genes start to rearrange at about the same
  time.
•  
• If a productive g or d rearrangement occurs
  first, the T cell is committed to that lineage,
  and stops further rearrangement of the b TCR
  gene.
•  

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However, if b is rearranged first, then the T
cell continues to proliferate, and undergoes
further rearrangements. This results either in
rearranged a TCR gene, yielding an ab TCR lineage
cell, or rearranging g and d genes, resulting in
a gd TCR cell.  
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• Rearrangements that lead to an ab T cell begin
  the rearrangement of the b TCR gene.
• The first step is D-J joining, followed by VDJ
  rearrangement.
• Expression of b chain stops further b chain
  rearrangements.
•  

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• b chain is then expressed on the surface of the
  thymocyte in association with a surrogate a chain
  (pTa).
•  
• Following this, there is rearrangement of the a
  TCR gene, resulting in a functional a chain, and
  in the expression of surface TCR, in association
  with other T cell-associated cell surface
  molecules.  

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• During this process, a cell that makes an
  unproductive a chain rearrangement can try again
  until gets a good a chain, or it exhausts its
  possibilities

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• Thymocytes that have a functional b
  rearrangement, and express ab or b the
  surrogate a chain (pTa) are induced to express
  both CD4 and CD8 simultaneously these are
  called double-positive cells.
•  
• Immature T cells that do not undergo a productive
  rearrangement die by apoptosis.

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Second phase of thymic development selection
of T cells that can interact with self MHC and
antigen

• This applies only to ab TCR-bearing cells (gt95
  of T cells).
• gd T cells are not restricted to interactions
  with MHC class I or class II molecules
• This phase of T cell development consists of two
  steps
• positive selection (TCR that can interact with
  self-MHC)
• negative selection (eliminate self-reactive cells
  that are stimulated by MHC self)
•  

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Positive Selection

• Positive selection refers to the selection of
  thymocytes that are able to bind to, and interact
  with, self-MHC molecules
• In positive selection developing thymocytes
  continue to live if they bind MHC well enough to
  receive a signal through their TCR.
• This signal is mediated by the interactions of
  these cells with MHC-expressing thymic cortical
  epithelial cells.
• The 95 of thymocytes that do not receive this
  signal undergo apoptosis.

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Positive selection takes place in the cortex of
the thymus lobules
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• These CD4 CD8 TCR thymocytes interact with
  thymic epithelial cells that express both MHC
  class I and MHC class II molecules, complexed
  with self-peptides.
• Thymocytes that bind MHC survive those that
  dont bind to self-MHC die.
•  
• TCR a chain rearrangements can continue during
  positive selection, allowing cells to explore
  alternative a chains for MHC binding.
• Once a T cell is positively selected, TCR
  rearrangement stops.
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• The expression of either CD4 or CD8 by a given T
  cell is determined during positive selection,
  leading to single-positive cells (CD4 or
  CD8-positive).
• Those cells that have a TCR that binds to MHC
  class II end up as CD4 single-positive cells
• Those that bind MHC class I as CD8 positive
  cells
•  

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Negative Selection

• Negative selection refers to the elimination of
  those thymocytes that bind to self-MHC molecules
  self with high affinity.
• In negative selection developing thymocytes die
  if they bind MHC self peptides too well
  (strongly enough so that they would be activated
  by this interaction, via signaling through their
  TCR).

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• Thymocytes undergo negative selection in the
  medullary region

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• There, they interact with antigen-presenting
  cells (dendritic cells, macrophages) that express
  self-antigens MHC class I or MHC class II
  molecules.
• Thymocytes that bind to self MHC too strongly
  are eliminated as possibly self-reactive cells,
  and undergo apoptosis.
• If self-reactive T cells were allowed to exit the
  thymus, such cells would mediate autoimmune
  disease.

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• Some T cells are reactive with self molecules
  that are not expressed in the thymus
• such cells can be eliminated in peripheral
  lymphoid tissues by the induction of anergy
• signal 1 only - incomplete stimulation via their
  TCR)

thymocyte
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• T cells that exit the thymus have undergone a
  series of changes that allow them to
• develop a functional TCR
• interact with self-MHC
• while eliminating self-reactive T cells

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The specificity or affinity of positive selection
must differ from that of negative selection
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Antigen-driven T cell Differentiation in
Secondary Lymphoid Organs

• Mature T cells leave the thymus and migrate to
  secondary lymphoid tissues (lymph nodes, spleen,
  mucosa-associated lymphoid tissue), recirculating
  via the blood and lymph, just like mature B cells
  do.
• Mature T cells are longer lived than mature B
  cells, and can survive for years without
  antigenic stimulation.
•  
•  

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• Unlike B cells, which have just one type of
  terminally-differentiated cell (plasma cell),
  there are various types of effector T cells
• CD8 T cells, which can differentiate into
  cytotoxic T cells
• CD4 T cells, which can become either TH1 or TH2
  helper cells.

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T cells interact with antigen in the T cell-rich
areas of peripheral lymphoid tissues
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T cells (and B cells) are targeted to, and enter,
secondary lymphoid organs by their expression of
various adhesion molecules. These molecules
interact with ligands expressed on endothelial
cells, allowing these lymphocytes to bind and
enter these lymphoid organs
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There, they can interact with antigen-presenting
cells (dendritic cells, macrophages, B cells) and
be stimulated on encounter with an appropriate
antigen, and function as helper T cells,
interacting with B cells and other lymphocytes.
 
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• Ligation of the T cells receptor for antigen
  results in an initial activation signal (first
  signal), as is true for B cells.
•  
• Again, as with B cells, this first signal is not
  sufficient to activate the cell
• second signals (co-stimulatory signals) are
  necessary for activation
•  
• The principal co-stimulatory signal for T cells
  is delivered via ligation of CD28 by B7 on the
  APC

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• Ligation of the TCR without co-stimulation
  results in T cells becoming non-responsive or
  apoptotic

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Activation, proliferation, survival
modified from Laâbi, Y. and A. Strasser.
Science 289883, 2000
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• T cell signaling occurs via the cytoplasmic tails
  of the molecules that make up the CD3 complex,
  which is associated with the TCR.

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• These associate with protein tyrosine kinases and
  initiate intracellular signaling that results in
  altered gene expression

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• Encounter with antigen can result in the
  formation of memory T cells.
• Some immunologists have claimed that continuing
  re-contact with antigen may be important for the
  survival of these memory T cells.
• One significant differences between memory T
  cells and memory B cells is that the TCR does not
  undergo isotype switching or affinity maturation
  by somatic mutation, unlike the BCR.
• However, it is clear that there are long-lived
  CD4 and CD8 cells that are rapidly activated on
  contact with antigen.

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• Memory T cells can be defined by a change in the
  expression of certain surface molecules