Cell-Mediated Immunity

1
Cell-Mediated Immunity

• An adaptive immune response mediated by specific
  cells of the immune system
• Primarily T lymphocytes (T cells), but also
  macrophages and NK cells.
• Formally defined as immunity that can be
  transferred from one organism to another by
  lymphoid cells, but not by serum antibody.
• T cells are the main agents of cellular immunity

2
T cells

• Main coordinators and effectors of cellular
  immunity
• Defined by their development in the thymus and
  the presence of a T-cell receptor (TCR) complex

A CD8 cytotoxic T cell killing a tumor cell
3
T cells, continued

• Two main types
• 1. CD4 Stimulate other immune cells.
• 2. CD8 Cytotoxic T cells Kill
  intracellularly-infected
• cells.
• Two major types of CD4 T cells
• 1. TH1 Inflammatory T cells -- Stimulate
  macrophages
• and promote inflammatory responses.
• 2. TH2 Helper T cells -- Stimulate B-cells to
  produce
• antibodies.
• (A third type, TH3, has recently been shown to
  promote IgA production.)

4
T cell development
(See Figure 13-1)
T cell development in the thymus
Immature double-negative T cells (CD8-, CD4-)
Positive selection/ negative selection
Immature double-positive T cells (CD8, CD4)
Cortex
Medulla
CD8 T cells
CD4 T cells
Mature T cells
5
The T cell Receptor

• Similar in structure to Immunoglobulins (similar
  to a single Fab fragment.
• Composed of two glycoprotein chains (?/? or ?/?).
  Most mature T cells have TCRs composed of an ?
  chain and a ? chain (they are called ?/? T
  cells).
• Each chain has a constant region and a variable
  region, similar to an antibody light chain.
• A TCR recognizes a small
• (8-13 aa) peptide epitope
• displayed on MHC

6
TCR compared to Immunoglobulins

• Similarities
• Both have specific Antigen-binding region created
  by the variable regions of two polypeptide
  chains.
• Both display great potential for diversity via
  genetic recombination at the genome level
• Differences
• A TCR is monovalent (has one binding site). An
  Ig is bivalent (has two binding sites).
• The TCR has no secreted form. It is always
  membrane-bound.
• The TCR does not recognize free antigen. Antigen
  must be presented to a T cell on an MHC molecule
  (next week).
• There is no class switching for the TCR. Once
  made, the TCR does not change.

Epitope-binding site
? chain
? chain
Variable region
Constant region
Transmembrane region
Immunoglobulin
T cell Receptor
7
The T cell Receptor, cont.

• The TCR only recognizes specific peptide/MHC
  complexes expressed on the surfaces of cells
• A TCR complex is composed of one heterodimeric
  TCR (ususally ?/?), plus a 5-polypeptide CD3
  complex which is involved in cell signalling for
  T cell activation.
• Each TCR is produced through genetic
  recombination and recognizes one small peptide
  epitope (about 8-13 amino acids).
• One T cell expresses only one specific type of
  TCR.

CD3 is the activation complex for the TCR
Binding of antigen/MHC to the TCR stimulates CD3.
CD3 then sends an activation signal to the
inside of the T cell.
8
TCR genetics Similar to Ig genetics
(numbers of segments in book is off, just like
for Ig genes)
? chain
? chain
See figure 13-4 in book -- the numbers of
segments differ, but the organization is the same
9
Responses to infection -- T cell component
(chapter 14) This chart is not intended to be
memorized
Recognition by pre-formed, non- specific effectors
Removal of infectious agent
Innate immunity (0-4 hours)
Infection
Recognition and activation of effector cells
Early induced response (4-96 hours)
Removal of infectious agent
Recruitment of effector cells
Infection
Clonal expansion and differentiation to effector
cells
Late adaptive response gt96 hours)
Transport of antigen to lymphoid organs
Recognition by naïve B and T cells
Removal of infectious agent
Infection
Recognition by pre-formed, Ab and T cells
Protective immunity
Removal of infectious agent
Infection
Rapid expansion and differentiation to effector
cells
Recognition by memory B cells and T cells
Immunological memory
Removal of infectious agent
Infection
The adaptive immune response involving
antigen-specific T cells and B cells is only one
part of the immune response and is required to
protect against pathogens. A pathogen is by
definition an organism that can cause disease.
In other words, a pathogen is an organism that
can bypass innate immunity and requires an
adaptive immune response for clearance.
10
Generation of an adaptive immune response

• During an adaptive immune response,T cells which
  recognize specific antigen(s) are selected for
  differentiation into armed effector cells which
  undergo clonal expansion to produce a battery of
  antigen-specific cells.
• Clonal expansion refers to the process by which
  antigen-specific T cells or B cells are
  stimulated to reproduce clones of themselves to
  increase the systems repertoire of
  antigen-specific effectors.
• Activation of antigen-specific T cells (the
  initiation of the adaptive response) occurs in
  the secondary lymph tissues (lymph nodes and
  spleen).
• This activation depends upon antigen presentation
  by a professional antigen presenting cell (APC)
  along with simultaneous co-stimulation. (eg., B7
  on the APC, CD28 on the T cell).

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Initiation of the adaptive immune response

• The first step is the draining of antigen into
  the lymph node(s).
• In the lymph node(s) (or spleen), antigens are
  trapped by professional APCs which display them
  to T cells.

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The professional Antigen Presenting Cells (APCs)

• Three types of APC are found in the lymph nodes
• Dendritic cells -- constitutively express MHC I
  and MHC II (can stimulate both CD4 and CD8 T
  cells) as well as B7 (the co-stimulatory signal).
  Antigen presentation appears to be the sole
  purpose of dendritic cells, and these cells can
  be infected by a wide variety of viruses.
  Dendritic cells are not phagocytic. They can
  present some viral peptides on their MHC II, and
  contribute to the induction of antibody against
  viruses. They are very efficient at stimulation
  of cytotoxic responses.
• Macrophages -- Resting macrophages express little
  MHC II or B7, but have receptors for bacterial
  cell wall components which, upon binding,
  activate the macrophage to express high levels of
  B7 and MHC II. Once activated, macrophages are
  efficient at stimulating CD4 T cells, both for
  inflammatory responses and helper (antibody)
  responses.
• B cells -- B cells express high levels of MHC II,
  but not B7. Microbial cell wall components can
  induce B7 expression by B cells (like
  macrophages). Once induced to express B7, B
  cells can activate helper T cells. B cells can
  take up soluble antigen through their Ig
  receptors (unlike dendritic cells or
  macrophages).

13
The antigen presenting cells, continued
Note this B cell is not a plasma cell -- a
plasma cell is shown above. Plasma cells do not
present antigen. They simply pump out antibody
for a few days then die.
Dendritic Cell
Macrophage
B cell
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Capture of circulating T cells in lymph nodes
15
T cells continuously circulate via the blood and
lymph through different lymph nodes until they
either find presented antigen or eventually die

• When a T cell encounters an APC displaying
  antigen to which it can bind, it stops migrating
  and binds strongly to the APC.
• Within about 2 days (48 hours), most
  antigen-specific T cells have been trapped by
  antigen and within about 4 to5 days armed
  effector T cells are migrating
• out of the lymph node.

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Review -- Cytokines produced early in response to
infection influence the future functions of
activated CD4 cells

• Cytokines produced by TH1 cells inhibit TH2 cells
• Cytokines produced by TH2 cells inhibit TH1 cells
• An immune response is often dominated by a
  cell-mediated response or an antibody response.
• Some pathogens have evolved strategies to shift
  the immune response toward the less effective
  type for that pathogen.

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Functions of the different T cell types

• CD8 cells Kill virally infected cells
• CD4 cells
• TH1 Activate macrophages to aggressively ingest
  antigen and to kill ingested microbes.
• TH2 Stimulate B cells to differentiate into
  antibody-producing plasma cells. B cells will
  only undergo isotype switching after receiving T
  cell help. The Ig class that a B cell switches
  to is specified by the types and balance of
  cytokines secreted by the helper T cell. Most
  plasma cells migrate to the bone marrow where
  they live out the rest of their lives.

18
One cytotoxic T cell can kill multiple targets
Micrographs Left healthy cell. Middle lower
right cell is in beginning stage of
apoptosis Right small cell in middle is in
advanced apoptosis. Its nucleus is highly
condensed and it has shed much of its cytoplasm.

• A cytotoxic T cell causes its target to undergo
  apoptosis (cell suicide) by the focussed
  secretion of vesicles carrying cytotoxins.
• The T cell binds to its target, delivers its
  cytotoxins, and moves on before it has a chance
  to be hurt itself (one T cell can kill another,
  so a T cell is not immune to the cytotoxins).

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Immunological memory

• When B cells are activated to reproduce, some
  differentiate into plasma cells and some become
  long-term memory cells.
• An adaptive immune response also produces T cell
  memory, but the nature of memory T cells is
  unknown. Two possibilities exist. Memory T
  cells probably originate from either
• 1. A long-lived subset of effector T cells that
  differentiates into memory T cells -- like memory
  B cells.
• 2. The continuous low-level activation of naïve
  T cells by specific antigen that is retained in
  the lymph nodes after an infection. This
  mechanism would suggest that APCs in the lymph
  node hold on to antigen on a long-term basis
  after an infection and continuously stimulate T
  cells at a low level so there is always a small
  effector population ready to go.

20
MHC classes I and II

• Functions
• class I MHC
• Displays peptides derived from antigen
  originating inside the cell (endogenous antigen).
• Important in cytotoxic responses (eg,
  CD8-killing of virus-infected cells).
• Class II MHC
• Displays antigen derived from ingested antigens
  (exogenous antigen).
• Important in humoral (antibody) responses as well
  in fighting as some intracellular parasites (eg.
  Mycobacterium tuberculosis and M. leprae)
• Locations
• Class I MHC found on all nucleated cells (all
  cells need to be prepared to be killed in case of
  a viral take-over or tumorigenic transformation).
• Class II MHC found only on antigen presenting
  cells (cells that present antigen to CD4 T
  cells --gt Macrophages, activated B-cells,
  dendritic cells.

21
Antigen Presentation to T cells MHC
See chapter 13 pp 103-107

• Antigens are presented to T cells as short
  peptide fragments bound to Major
  Histocompatibility (MHC) molecules.
• Two types of MHC in humans and mice
• MHC I presents an 8-10 amino acid peptide to
  CD8 T cells.
• MHC II presents a longer peptide (13 aa or more)
  to CD4 T cells.

22
MHC structure
See figure 13-5 in book

• MHC classes I and II have an almost identical 3-D
  structure.
• Both classes of MHC are polygenic (each cell has
  many MHC genes) and polymorphic (there are many
  alleles for each locus), but the MHC genes do not
  undergo recombination.
• Note Human MHC are called HLA (human leukocyte
  antigen).

23
MHC / T cell interactions
Look at figures 13-2, 13-8 and 13-10 in book
Class II MHC
Class I MHC
target cell
Antigen presenting cell
CD8
CD4
CD4 T cell
CD8 T cell
TCR complex
TCR complex

• The MCH/peptide-TCR interaction is facilitated by
  the CD4 or CD8 co-receptor.

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Antigen processing Endogenous pathway

• All nucleated cells can process endogenous
  proteins and present fragments on their class I
  MHC.

Display of MHC I peptide on cell surface
degradation
Vesicle carrying MHC I-peptide
Cytoplasmic proteins
Processing in E.R. and complexing with MHC I
Endoplasmic reticulum
Nucleus
25
Antigen processing Exogenous pathway

• Professional antigen presenting cells ingest
  microbes and free particles, degrade them in
  lysozomes, and present fragments to CD4 T cells
  on MHC II.

Display of MHC II peptide on cell surface
Ingestion of microbe
Vesicle fusion, assembly of peptide/MHC II
Vesicle carrying MHC II
Degradtion in lysozome
MHC II is assembled in ER
Endoplasmic reticulum
Nucleus
26
CD4 T cell activation

• T cells require co-stimulation for activation --
  binding of the TCR to MHC/peptide is not enough
  to activate a T cell by itself.
• B7 on an APC binds to CD28 on the T cell to
  deliver a co-stimulatory signal. (see figure
  13-8).
• Activation by peptide/MHC-TCR binding plus a
  co-stimulatory signal leads to Interleukin-2
  (IL-2) release and up-regulation of the IL-2
  receptor on the T cell.
• IL-2 stimulates growth and proliferation of T
  cells.

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CD8 T cell activation

• A naïve circulating CD8 T cell also requires
  co-stimulation to become an armed effector
  cell.
• A CD8 T cell can be activated by an APC
  displaying MHC I/peptide along with B7 (CD8
  cells also have CD28).
• Activation of the CD8 cell causes upregulation
  of the IL-2 receptor and production of IL-2,
  leading to growth and proliferation.
• An activated CD8 T cell can sustain itself on
  its own IL-2 production, once activated.

28
TCR genetics Similar to Ig genetics
(numbers of segments in book is off, just like
for Ig genes)
? chain
? chain
See figure 13-4 in book -- the numbers of
segments differ, but the organization is the same
29
Mechanism of TCR (or Ig) gene rearrangement.
This DNA is lost forever
30
Mechanism of TCR (or Ig) gene rearrangement.
31
Mechanism of TCR (or Ig) gene rearrangement.
32
Mechanism of TCR (or Ig) gene rearrangement.
This DNA is lost forever
Rearranged ? chain
33
Mechanism of TCR (or Ig) gene rearrangement.
34
Mechanism of TCR (or Ig) gene rearrangement.
35
Mechanism of TCR (or Ig) gene rearrangement.
36
Mechanism of TCR (or Ig) gene rearrangement.
37
Mechanism of TCR (or Ig) gene rearrangement.
38
Mechanism of TCR (or Ig) gene rearrangement.
39
Mechanism of TCR (or Ig) gene rearrangement.
40
Mechanism of TCR (or Ig) gene rearrangement.
eg., ? chain rearrangement
41
Mechanism of TCR (or Ig) gene rearrangement.
42
T cells develop in the thymusand undergo
positive and negative selection

• Positive selection T cells which can react to
  self MHC (major histocompatability complex)
  carrying peptides are allowed to live. Those
  that cannot undergo apoptosis (suicide).
• Negative selection T cells that react strongly
  to self-antigens on MHC are eliminated.
• Only those T cells that can react to MHC, but do
  not bind strongly to self-antigens emerge as
  mature T cells from the thymus.
• Only about 2 of immature T cells make it through
  positive and negative selection.

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• Next Week
• Introduction to MHC
• Antigen Presentation
• T cell functions
• Cytotoxic T cell functions
• Inflammatory T cell responses
• Helper T cell responses
• Today
• The T cell receptor (TCR) Structure and
  function
• TCR expression
• Genetic organization
• Gene rearrangement

44
TCR gene rearrangement, continued

• The ? chain rearrangement occurs before ? chain
  rearrangement.
• If a functional ? chain is produced, the ? chain
  gene is rearranged.
• If a functional ? chain is not produced, the
  pre-T cell dies.
• The mechanism of rearrangement is basically the
  same as for B cells -- the same enzymes are even
  used.
• Note ? chain rearrangement can occur several
  times, so once a functional ? chain is produced,
  a functional TCR will most likely be produced.
  (Both TCR ? chain rearrangements and Ig heavy and
  light chain rearrangements generally only happen
  once for each chromosome in each cell).