Antigen Recognition by T lymphocytes (Chapter 5)

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Antigen Recognition by T lymphocytes(Chapter 5)
B cells T cells
Similar structure Similar structure
Produced by gene rearrangement Produced by gene rearrangement
Each clone expresses a single species of antigen receptor Each clone expresses a single species of antigen receptor
Express bound and soluble receptors Express only membrane-bound receptors (TCR)
Receptors bind intact molecules Receptors bind denatured peptides bound to MHC
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Antigen recognition by B-cells and T-cells
Antigen processing
B-cells
T-cells
Major Histocompatibility Complex (MHC) assembly
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Antigen processing and presentation
T-cell receptor can recognize antigen only in the
form of denatured peptide bound to an MHC
molecule ? pathogen-derived proteins must be
first degraded (antigen processing), and then
assembled into peptideMHC complex and presented
to T-cells (antigen presentation).
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Major Histocompatibility Complex (MHC) molecules

• Glycoproteins expressed almost on all cells of
  the body
• There are two main types of MHC molecules Type I
  and Type II
• Type I are expressed on all cells of human body,
  except red blood cells, brain and kidney
• Type II are expressed only on three cell types,
  co called Professional Antigen Presenting cells
  (APC) dendritic cells, macrophages, and B cells
• MHC (HLA) molecules exhibit significant variation
  in human population ? the primary cause of graft
  rejection in transplantations. The reason that
  the first successful organ transplantation was
  kidney transplantation is that kidney express
  very low levels of MHC molecules, and thus they
  did not interfere during transplantation

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Figure 32.1
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T cells

• T cells come in two different types, helper cells
  and cytotoxic (killer) cells. They are named
  after the thymus, an organ situated under the
  breastbone. T cells are produced in the bone
  marrow and later move to the thymus where they
  mature. 
• Thymus is essential for development of T cells.
  In mice (known as nude mice, since they also lack
  hair) and humans with diGeorge's syndrome, the
  thymus fails to form fully and the result is a
  lack of T cells and a consequent severe
  immunodeficiency.

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Nude mice cannot reject tumors and have been thus
used to test new anti-cancer therapies

• The nude mice have a dysfunctional immune system,
  and can only live in a sterile environment.
• They cannot reject any transplanted tissue,
  including tumors.
• Nude mice are very useful in cancer research
  because injected human cancer cells can grow into
  tumors allowing new ways to test cancer
  therapies.

Nude mouse with transplanted rabbit skin
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T cells express specific T cell receptors (TCR)

• In addition to TCR, each T cell expresses on its
  surface a glycoprotein either CD4 or CD8.
• CD4 and CD8 T cells have different functions.
• The function of CD8 T cells is to kill
  virus-infected cells.
• The function of CD4 T cells is to activate other
  immune cells.

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Helper T cells (CD4 cells)

• The main regulators of the immune defense. Their
  primary function is to release cytokines, which
  then activate other immune cells.
• The primary task of CD4 T cells is to activate
  other immune cells. However, the helper T cells
  themselves must be first activated. This happens
  when a dendritic cell, macrophage or B cells
  present the antigen assembled into MHC class II
  complex to CD4 T cell this is called antigen
  presentation.
• When the TCR of a helper T cell recognizes the
  antigen, the T cell is activated. Once activated,
  helper T cells start to divide and produce
  cytokines that activate B and T cells as well as
  other immune cells.

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Killer T cells (CD8 cells)

• Specialized in attacking cells of the body
  infected by viruses. It can also attack cancer
  cells. Their main function is to kill the
  infected cell.
• The cytotoxic killer CD8 T cell has receptors
  that are used to search each cell that it meets.
  CD8 T cells recognize antigens assembled into MHC
  class I complexes. MHC class I are expressed on
  all cells of human body, except for the brain. If
  a cell is infected, it is swiftly killed by CD8
  (cytotoxic, killer) T cells.

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T Cell Receptor (TCR)

• The absence of a secreted form of T Cell Receptor
  (TCR), and the requirement for TCR recognition of
  both peptide and MHC, made its isolation and
  characterization much more difficult than that of
  BCR. Thus, the structure of the T cell receptor
  was not elucidated until the 1980s.
• As T cells develop in the bone marrow, they
  rearrange TCR gene segments to produce a unique
  TCR.
• Immature T cells then migrate to the thymus,
  where they mature and complete their development.
  In the absence of thymus, T cells could not be
  produced, and this would result in a severe
  immune deficiency (that would resemble AIDS,
  since the HIV virus specifically attacks and
  destroys helper T cells).

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The T-cell receptor resembles a single
antigen-binding arm (Fab) of Ig

• T-cell receptor is composed of two different
  peptide chains, and has one antigen binding site.
• It is always membrane bound.
• Each chain has a variable region that binds
  antigen, and a constant region.
• During T-cell development, the variability in the
  V region is produced by gene rearrangement.

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Structure of T-cell receptor

• T-cell receptor consists of two different
  polypeptide chains
• T-cell receptor a chain (TCRa), and T-cell
  receptor b chain (TCRb).
• a and b chains are organized into variable
  regions (V regions) and constant regions (C
  regions).
• T-cell receptor is anchored in the membrane by a
  hydrophobic cytoplasmic tail.
• Va and Vb form the antigen-recognition site each
  T-cell receptor possesses only one
  antigen-binding site.

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Three-dimensional structure of the T-cell
receptor showing the antigen complementarity
determining regions (CDRs)

• The sequence variation in the a and b chains is
  clustered into regions of hypervariability.
• These loops form the antigen binding site, and
  are called complementarity determining regions
  (CDRs).
• The T-cell receptor V domains have three CDR
  loops, CDR1, CDR2 and CDR3.

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Figure 3-3
T-cell receptor rearrangement occurring during
T-cell development in the thymus
a-chain locus Similar to the Ig light chain
contains V and J
b-chain locus Similar to the Ig heavy chain
contains V, J and D
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There are two classes of T-cell receptor
ab gd

• T-cells express either ab, or gd, but NEVER
  both.
• T-cells bearing gd receptors represent only 1-5
  of the T-cells.
• gd T-cells do not require MHC molecules.
• The biological function of gd receptors is not
  well understood.

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MHC molecules present the antigen to T cells
Helper T cells
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CD4 and CD8 Molecules

• T helper cells express CD4 but not CD8
• T cytotoxic cells express CD8 but not CD4
• CD4 and CD8 are associated in the membrane with
  the TCR.
• Both CD4 and CD8 function as adhesion molecules
  CD4 binds to Class II MHC molecules and CD8
  binds to Class I MHC molecules.
• Binding of the TCR to the peptide/MHC complex is
  greatly augmented if CD4 or CD8 are assisting.
  Their cytoplasmic domains may also allow for
  signal transduction to occur.
• The signal-transduction property of CD4 and CD8
  is mediated through their cytoplasmic domains.

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CD proteins (Clusters of differentiation)

• CD proteins are glycoproteins that are expressed
  on human leukocytes there are more than 250
  different CD proteins expressed on human white
  blood cells.
• Each type of leukocytes expresses different CD
  protein(s) on its surface ? CD proteins are used
  as markers for different types of leukocytes.
• CD3 all T cells (helper and killer T cells)
• CD4 Helper T cells (specifically destroyed by
  HIV virus)
• CD8 Killer T cells
• CD25 Regulatory (helper, CD4) T cells
• CD56 NK cells
• CD66 neutrophils
• CD proteins are recognized by specific monoclonal
  antibodies and are used to classify different
  types of leukemia. 

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There are two classes of MHC molecules MHC class
I Presents antigen of intracellular origin
(viruses) to CD8 T cells MHC class II Presents
antigen of extracellular origin (bacteria) to CD4
T cells

• Molecular basis of the interactions specific
  protein-protein interactions between CD8
  glycoprotein and MHC class I, and between CD4 and
  MHC class II.
• CD8 and CD4 molecules are also called T-cell
  co-receptors.

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Two classes of T cells are specialized to respond
to intracellular (viral) and extracellular
(bacterial) infection
Intracellular (viruses) CD8 cytotoxic T
cells Extracellular (bacteria) CD4 helper T cells
INFECTION
TH1 cells activate macrophages to kill the
bacteria and produce cytokines
Helper CD4 T Cells Main function is to help
other immune cells to respond to extracellular
infection (bacteria)
TH2 cells Stimulate B cells to make antibodies
Cytotoxic CD8 T cells Main function is to kill
the cells infected with the virus
CD4 and CD8 molecules are glycoproteins
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3
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Generation of peptide antigens that are presented
by MHC molecules

• Proteins derived from intracellular (viruses)
  and extracellular (bacteria) pathogens are
  generated in different cellular compartments.
• Peptides derived from intracellular pathogens
  presented by MHC I class Formed in the cytosol
  (cytoplasm) by proteasome degradation, and
  delivered to the endoplasmic reticulum, where
  they bind the MHC class I. Then they are
  presented to CD8 cytotoxic cells.
• Peptides derived from extracellular pathogens
  presented by MHC II class Taken up by
  phagocytosis and degraded by proteases in the
  lysosomes. The antigenic peptides produced
  associate with MHC class II in the vesicular
  system (lysosomes), and are then presented to CD4
  helper cells.

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Formation and transport of peptides that bind to
MHC class I
Pathogen-derived intracellular proteins in
infected cells are degraded by proteasome, a
multi-protein complex with protease
activities. The generated peptides are
transported into the endoplasmic reticulum (ER),
which is accomplished by a protein called
transporter associated with antigen processing
(TAP). TAP is essential for export of the peptide
from cytosol to ER. In patient with Bare
Lymphocyte Syndrome, TAP is not functional ? no
peptide is transported to ER ? MHC class I are
not expressed ? high susceptibility to viral
infections. In the endoplasmic reticulum, the
generated peptides are bound to newly synthesized
and folded MHC class I molecules, which is aided
by chaperones. The MHC class I-peptide complex is
transported through Golgi to the plasma membrane.
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Proteasome Selective protein degradation occurs
in the proteasome, a large protein complex in the
nucleus cytosol of eukaryotic cells.
In 2004, Irwin Rose from the University of
California, received the Nobel Prize for
discovery of the function of proteasome.
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Formation and transport of peptides that bind to
MHC class II
Extracellular pathogens (bacteria) are engulfed
by endocytosis (smaller antigens) or by
phagocytosis (larger pathogens) by neutrophils
and macrophages. The engulfed proteins are
degraded by proteases in the phagolysosomes,
special vesicles formed by fusion of phagocytic
vesicles with lysosomes. In the phagolysosomes,
the generated peptides are bound to newly
synthesized and folded MHC class II
molecules. The MHC class II-peptide complex is
transported to the plasma membrane in outgoing
vesicles.
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Comparison of antigen processing and presentation
by MHC class I and II molecules
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Expression of MHC molecules on human cells

• MHC class I
• Expressed by almost all human cells
  constitutively (all the time).
• Since all human cells are susceptible to viral
  infections, this enables comprehensive
  surveillance by CD8 T cells. The lowest levels
  occur on kidney cells and in the brain. This may
  be one reason why kidney transplants are among
  the most successful organ transplants, and that
  herpes virus can successfully hide in the brain
  without being recognized by the cells of the
  immune system.
• MHC class II
• Constitutively expressed on only a few cell
  types, which are cells specialized for the uptake
  and presentation of antigens (professional
  antigen-presenting cells dendritic cells,
  macrophages and B cells). Expression of MHC class
  II can be increased during immune response by
  cytokines and interferons. MHC class II are
  recognized by CD4 cells.

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Human Leukocyte Antigen Complex

• Antibodies used to identify the MHC molecules
  react white leukocytes (white blood cells) but
  not with erythrocytes (red blood cells), which
  lack the MHC molecules
• MHC molecules are also called Human Leukocyte
  Antigen complex (HLA)

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MHC and Transplantation

• The cause of transplant rejection is recognition
  of foreign MHC antigens by T cells and activation
  of those T cells to become cytotoxic or helper T
  cells.
• Tissue matching involves identifying MHC antigens
  on both donor and recipient cells and using donor
  cells with as many MHC alleles identical to those
  of the recipient as possible.
• HLA matching is done by serological assays that
  use antibodies to HLA alleles to type donor and
  recipient cells.
• HLA matching improves graft survival but does not
  prevent rejection, even in MHC-identical siblings
  (except for identical twins).
• Improved success in transplantation is due to
  increasing technical expertise, the availability
  of transplant centers to do HLA matching and
  minimize organ delivery time, and the
  availability of immunosuppressive drugs that
  block T cell activation.
• The fetus is an almost perfect allograft (
  transplanted organ). We do not understand why
  most pregnancies are not rejected even though
  half of the baby's antigens are foreign to the
  mother.

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SUMMARY Chapter 5Antigen Recognition by T
Lymphocytes

• T cells recognize antigens through T cell
  receptors (TCR).
• T cell receptors are glycoproteins composed of V
  and C domains, similarly as immunoglobulins
  (Igs), and a membrane-spanning region.
• There are two types of T-cell receptors one made
  up of a and b chains (expressed on most T cells),
  and another made up of g and d chains.
• All four types of T-cell receptor chains (a, b, g
  and d) require DNA rearrangements in order to be
  expressed, similarly as Igs.
• T-cell receptors differ from Igs in that they are
  expressed only as cell surface receptors and are
  not secreted as soluble proteins with effector
  functions (as Igs).

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SUMMARY-continued

• T cells expressing ab receptors recognize
  peptides presented by MHC molecules, which have
  degenerate peptide binding sites.
• Cytotoxic CD8 T cells recognize peptides
  presented by MHC class I, while the helper CD4 T
  cells recognize peptides presented by MHC class
  II.
• The role of CD8 cytotoxic cells is to kill cells
  that have become infected with a virus or some
  other intracellular pathogen.
• The role of CD4 helper cells is to help other
  immune cells to respond to extracellular
  infection
• TH2 cells stimulate B cells to make
  antibodies,
• TH1 cells activate macrophages to kill the
  pathogen.

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Summary-continued

• Protein antigens from intracellular and
  extracellular sources are processed into peptides
  by two different pathways.
• Peptides generated by proteasome from
  intracellular pathogens (viruses) enter ER where
  they bind MHC class I, and are then recognized by
  CD8 cytotoxic T cells. MHC class I are expressed
  by all cell types except for erythrocytes and the
  brain.
• Extracellular pathogens are taken up by
  phagocytosis and degraded into peptides in the
  lysosomes, where they also bind MHC class II
  molecules. MHC class II molecules are expressed
  only on professional antigen presenting cells
  (APC) dendritic cells, macrophages, and B cells.
  They activate CD4 helper T cells.

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Development of T Lymphocytes

• (Chapter 7)
• Development of T cells in bone marrow by gene
  rearrangement
• Positive and negative selection of T cells in the
  thymus (analogy of the phase II in B cells
  elimination of self-reactive cells)

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Development of T Lymphocytes

• Similarly as B cells, T cells (lymphocytes)
  originate from bone marrow stem cells.
• Another similarity with B cells is the gene
  rearrangement in order to produce antigen
  receptors.
• However, in contrast to B cells that rearrange
  the Ig genes in bone marrow, T cells have to
  leave the bone marrow, and enter another primary
  lymphoid organ the thymus.
• Another difference between B and T cells is that
  in T cells, there are no changes in TCR structure
  after activation with antigen (no somatic
  hypermutation or isotype switching)
• Two lineages of T lymphocytes (T cells) develop
  in the thymus ab T cells
  (majority)
• gd T cells (1-5 of all T cells dont need
  MHC molecules to recognize antigen)

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T cells develop in the thymus

• T cells develop in bone marrow, but in order to
  mature, they need to migrate to thymus
  thymus-(T)-dependent lymphocytes
• Thymus is a primary lymphoid organ located in the
  upper thorax (chest), just above the heart.
• Progenitor (precursor) T cells ( thymocytes)
  enter the thymus, where they mature. Mature T
  cells leave the thymus and enter the secondary
  lymphoid tissues, where they become activated
  after exposure to antigen, and differentiate into
  effector T cells.

Mature T cells enter the blood stream, and after
infection accumulate in the secondary lymphoid
tissues where they are activated.
T cells develop in bone marrow and then migrate
to thymus where they mature
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Thymus

• Essential organ for development of T cells. In
  mice (known as nude, since they also lack hair)
  and humans with diGeorge's syndrome, the thymus
  fails to form fully and the result is a lack of T
  cells and a consequent severe immunodeficiency.
• The key function of the thymus is the selection
  of the T cell repertoire that the immune system
  uses to combat infections.
• This involves selection of T cells that are
  functional (positive selection), and elimination
  of T cells that are auto-reactive (negative
  selection).
• Cells that pass both levels of selection are
  released into the bloodstream to perform vital
  immune functions.
• If the negative selection fails, auto-immune
  diseases may arise.
• Thymus is most active in babies and children, and
  then starts gradually shrinking. This results in
  the decreased production of new T cells, and in
  the increased susceptibility to infection in
  older people.

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Positive and negative selection of the ab T-cell
repertoire

• During the first phase of T-cell development,
  thymus produces millions of TCR regardless of
  their antigen specificity only small portion of
  these TCR can interact with the MHC isoforms
  expressed by an individual ? will be able to
  respond to antigens presented by these MHC
  molecules
• The second phase of T-cell development involves a
  critical examination of the receptors produced,
  and selection of those that can work effectively
  with the individuals own MHC molecules.
• These selection processes involve only ab T
  cells, since gd cells do not require the MHC
  proteins.
• The ab double-positive thymocytes undergo first
  a positive selection, to ensure selection of
  T-cells that recognize peptides presented by
  self-MHC molecules, and then a negative
  selection, to eliminate the T-cells that bind to
  self-peptides and self-MHC molecules too
  strongly, and could potentially be auto-reactive.

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Positive selection

• After T-cells are produced in bone marrow, they
  migrate to the thymus, where they undergo first
  the positive selection.
• During the positive selection, only the
  thymocytes ( immature T cells) that can interact
  (bind) with self-MHC molecules of the individual
  are selected. The rest (95 of thymocytes)
  undergoes apoptosis, and is removed
  (phagocytosed) by macrophages present in the
  thymus.

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Negative selection

• Eliminates T cells that bind too strongly to the
  complexes of self-peptides and self-MHC molecules
  presented by the cells in the thymus. If not
  eliminated, these cells could cause autoimmune
  diseases and tissue damage.
• The cells that interact with MHC molecules too
  strongly undergo apoptosis, and are engulfed by
  macrophages present in the thymus.
• In contrast to the positive selection (that is
  mediated by the epithelial cells of the thymus),
  the negative selection is mediated by
  bone-marrow-derived dendritic cells and
  macrophages present in the thymus.
• After the negative selection, the mature
  single-positive T cells leave the thymus and
  enter the blood circulation.

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Positive and negative T cell selection
Only about 2 of all thymocytes ( immature T
cells in the thymus) survive the dual strictures
of positive and negative selection. One
prominent immunologist has described the role of
thymocyte selection as "preventing the harmful
and rejecting the useless" and it may help to
view it in this light. Negative selection helps
prevent autoimmunity, positive selection ensures
that the peripheral T cells will be useful (
will bind the self-MHC molecules).
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T cells undergo activation and differentiation
into effector T cells in secondary lymphoid
tissues after encounter with antigen

• Only a small fraction of naive T cells (mature T
  cells before they encounter antigen) survives the
  positive and negative selection, and leaves the
  thymus.
• Mature naive T cells can re-circulate between
  blood and lymphoid tissues for many years (in
  contrast to B cells, which have shorter life
  span).
• In secondary lymphoid tissues, T cells accumulate
  in T cell areas, where they become activated by
  their specific antigens.
• Encounter with antigen induces the final stage of
  T cell development their differentiation into
  effector T cells. Some effector T cells stay in
  the lymphoid tissues (CD4-TH2 cells), while
  others migrate to site of infection (CD8TH1
  cells).

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Figure 1-18
In lymphoid tissues, T cells accumulate in T cell
areas
Antigen
T cells
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Effector T cells

• In contrast to terminally differentiated B cells
  (plasma cells), there are several types of
  terminally differentiated effector T cells.
• CD8 T cells Cytotoxic T cells
  (recognize MHC class I molecules)
• CD4 T cells

Activation (cytokines)
TH1 helper cells (activate macrophages) TH2
helper cells (induce differentiation of B cells
into plasma cells and production of antibodies)
(recognize MHC II molecules)
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Summary

• T cells develop in bone marrow, where they
  produce TCR (ab cells).
• After they are produced in bone marrow, immature
  T cells, thymocytes, migrate to the thymus, where
  they complete their maturation, and undergo
  positive and negative selection.
• During positive selection, only thymocytes that
  can interact with self-MHC molecules are
  selected. The rest undergoes apoptosis.
• During negative selection, thymocytes that
  interact with self-MHC complexes too strongly are
  eliminated. After negative selection, T cells
  leave the thymus, and circulate in blood for
  decades.
• T cells become activated in T cell areas of the
  secondary lymphoid tissues
• Activated T cells differentiate into effector T
  cells (CD8 ? cytotoxic T cells CD4 ? helper T
  cells.