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Cell Mediated Immunity
W. Robert Fleischmann, Ph.D.
Department of Urologic Surgery
University of Minnesota Medical School
[email protected]
(612) 626-5034
Objectives
• To understand the role of cell-mediated
immunity
• To understand the identity, mechanism of
activation, and method of killing by cytotoxic T
cells
• To understand the identity, mechanism of
activation, and method of killing by natural
killer (NK) cells
• To understand the method of killing by
antibody-mediated cellular cytotoxicity
(ADCC)
Role of Cell-Mediated Immunity
• Humoral immunity mediated by antibody is responsible for
the protection of the extra-cellular environment from
pathogens and toxins.
• The principal roles of cell-mediated immunity are to
recognize and destroy pathogen-infected cells and cells
that have undergone genetic alterations (tumor cells).
• The importance of cell-mediated immunity can be seen in
patients who lack a thymus (DiGeorge Syndrome).
– Although antibodies are not made, these patients deal relatively
well with extracellular bacteria.
• DiGeorge patients do suffer repeated infections because no memory.
– Infections with viruses, intracellular bacteria, and fungi are difficult
to clear and become life-threatening in DiGeorge patients.
The Mediators of CellMediated Immunity
• Antigen-nonspecific effector cell
– Natural Killer cells
– Macrophages
– Neutrophils
– Eosinophils
• Antigen-specific effector cells
– CD4+ T cells (aka, helper T cells, Th cells)
– CD8+ T cells (aka, cytotoxic T cells, CTLs,
Tc cells)
Different Effector T Cells Can Be
Recognized by the Effector Molecules
They Produce
Cytotoxic T Cells
Cytotoxic T Cells
• Naïve CTL cells are called CTL precursors (CTL-P) to
indicate their immature state.
• A CTL-P cell matures only after being activated by
interaction with a Th1 cell.
• Maturation requires three sequential signals.
– Antigen-specific signal is transmitted by the TCR upon
recognition of peptide:Class I antigen presented by a
professional antigen presenting cell (licensed antigenpresenting cell) or by a tissue cell.
– Costimulatory signal is transmitted by CD28:B7 interaction
between the CTL-P cell and the licensed antigen-presenting
cell.
– A signal induced by IL-2 secreted by a Th1 cell, results in
proliferation and differentiation of the antigen-activated CTLP cell to a CTL.
•
Before an APC can
activate a CTL-P
cell by peptide:MHC
class I presentation,
it must be “licensed”
to do so.
Activation of
Effector CTLs from
Naïve CTL-Ps
– APC must present
peptide via MHC
Class II to a Th1
cell.
– Somehow, this
interaction
licenses the APC
to present peptide
via MHC Class I
antigen to the
CTL-P cell.
•
It is not known
whether the Th1 cell
and the CTL-P cell
are bound to the
APC at the same
time.
Note: Naïve CTL-P = naïve CTL precursor
Importance of Licensing of APC
• Licensing of APC requires interaction of the
APC with the Th1 cell.
• Thus, presentation of antigen to the Th1 cell via
MHC class II and to the CTL-P cell via MHC
class I acts a control to prevent recognition of
self antigen (antigen must be a phagocytosed
antigen to be expressed on Class II).
– This is a function best carried out by a dendritic cell
acting as an APC.
– Infection of a dendritic cell by a virus assists in the
presentation of antigen associated with MHC class I.
Activation of Memory CTLs
• Th1 cell not required to provide IL-2.
• Autocrine IL-2 is sufficient.
Naïve CTL-P Versus Mature CTL
• Naïve CTL-P
– Does not express IL-2 or IL-2R and only begins to express them
after it begins to be activated.
– Expresses the CD45RA isoform of CD45.
– Expresses a low level of cell adhesion molecules CD2 and LFA-1.
– Does not exhibit cytotoxic activity.
• Mature CTL
– Expresses high affinity IL-2 receptor and requires high levels of IL2 to proliferate.
– Synthesizes low levels of IL-2.
– Expresses the CD45RO isoform of CD45.
– Expresses a high level of cell adhesion molecules CD2 and LFA-1
– Exhibits cytotoxicity.
• Memory CTL cells may not require Th1 help.
– Requires only low levels of IL-2 (produced by activated CTL-Ps)
for memory CTLs to become mature effector CTLs.
Overview of CTL Killing
•
Primary events
– Conjugate formation
– Reorientation of cytoplasmic
granules in CTL
– Granule exocytosis
– Dissociation
– Target cell death by apoptosis
Pictures of
CTL Killing
Note that the T cell approaches
the target cell, forms a close
binding to the target cell, and
reorients its granules (small
arrow) toward the target cell.
CTL Killing: Binding of Target Cell
• The TCR-CD3 membrane complex on the CTL
recognizes peptide bound to MHC class I on a
target cell.
• LFA-1 on CTL binds to ICAMs on target cell.
• Antigen activation converts LFA-1 from a lowaffinity state to a high-affinity state, resulting in
the formation of the conjugate.
• After 5-10 minutes, the LFA-1 returns to a lowaffinity state, resulting in dissociation of the
CTL from the target cell.
Bound CTLs Kill Targets by
Two* Mechanisms
• Perforin/Granzyme are secreted from granules
– Perforin molecules form pore
– Granzyme molecules activate apoptosis by
cleavage of caspases
• FasLigand protein on cell membrane surface
– Membrane-bound FasL binds to Fas on the
membrane of the target cell to initiate cell killing
– Activates apoptosis by cleavage of caspases
* CTLs can also kill by TNF production and
secretion
CTL Killing: Degranulation Events
• The CTLs (but not naïve CTL-Ps) contain intracellular granules.
– Contain monomers of perforin
• Perforin monomers polymerize to form pore in the target cell
• Perforin has some homology with C9 and forms pores similar to complement pores
– Contain serine proteases called granzymes or fragmentins
• Granzymes taken up by receptor mediated endocytosis (granzyme binds to
mannose 6-phosphate receptor) or through perforin pore (perforin-assisted
pathway)
• Granzymes induce apoptosis
Apoptosis Induction
by FasL and
Granzyme
• FasL on the CTL binds to Fas
(TNF family receptor) on the
target cell, activating its death
domains to bind to FADD (FASassociated Death Domain) that,
in turn, activates caspase-8
• Granzymes initiate apoptosis by
activating caspase-8
Natural Killer Cells
Natural Killer Cells (NK Cells)
• NK cells make up 5-10% of the population of
circulating lymphocytes.
• NK cells play major roles in killing virusinfected cells, intracellular pathogen-infected
cells, and tumor cells.
• NK cells produce a number of important
cytokines, including IFN-.
– IFN- can tilt the immune response toward Th1
cells by inhibiting Th2 and by inducing IL-12
production by macrophages and dendritic cells.
– IFN- can activate macrophages and NK cells.
• NK cell activity is stimulated by IFN-, IFN-,
IFN-, TNF-, IL-12 and IL-15.
NK Cells Are an Early
Defense Against Viruses
NK Cells versus CTLs
• NK cells express CD16, FcRIII.
• NK cells do not need to be educated by the thymus.
• NK cells do not undergo rearrangement of receptor
genes and, thus, do not express T cell receptors or
CD3.
• NK cell killing is not MHC restricted (identical killing
levels are seen for allogeneic and syngenic tumor
cells).
• NK cells do not show immunologic memory that can
be primed by re-exposure to antigen.
• NK cell killing is similar to CTL killing
– FasL expressed on surface and can kill cells expressing Fas
– Perforin and granzyme released from granules
– TNF expressed on surface and secreted
NK Cell Receptors
• NK cells have both activation and inhibition
receptors.
• NK cell receptors fall into two categories but
members of each group can be activation or
inhibition receptors.
– Lectin-like receptors
• Bind proteins rather than polysaccharides
• Can be activation or inhibition receptors
– Immunoglobulin-like receptors (aka, KIR = killer
cell immunoglobulin-like receptors)
• Bind to most class I MHC molecules
• Can be activation or inhibition receptors, but most are
inhibition receptors
Opposing Signals Model of
NK Cell Killing
• If an antigen recognition
signal is given to the NK
cell and the level of MHC
class I is high, no killing
occurs.
• If an antigen recognition
signal is given to the NK
cell and the level of MHC
class I is low, killing
occurs.
NK Cells and CTL Have
Complementary Activities
NK Cells
• Kill cells that mask
the presence of
foreign antigen on
MHC class I
CTLs
• Kill cells that
express foreign
antigen on MHC
class I
NKT Cells
NKT Cells
• NK cells express lectin-like receptors and KIR.
• CTL express T cell receptor and CD3.
• NKT cells express T cell receptor but differs from CTL.
– Maturation is in the thymus.
– The T cell receptor is invariant (specific gene segments encode
TCR and TCR chains).
– The T cell receptor does not recognize MHC-bound peptides but
does recognize a glycolipid presented by CD1d.
– NKT cells do not exhibit memory.
– NKT cells express cell surface markers characteristic of NK cells.
– NKT cells may play a role in killing bacteria and tumor cells but
precise role is uncertain.
• Bacteria killing via expression of glycosyl ceramides (glycolipid)?
• Tumor cell killing via expression of glycolipids that are specific to
tumor cells?
Antibody-Dependent
Cell-Mediated Cytotoxicity
(ADCC)
ADCC Killing
• Effector cells associated with
ADCC
–
–
–
–
–
NK cells
Macrophages
Monocytes
Neutrophils
Eosinophils
• Effector cells bind antigen via
antibody bound to Fc receptor
• Killing is mediated by
– Cytolytic enzyme release by
macrophages,neutrophils,and
eosinophils
– TNF release by NK cells,
monocytes, and macrophages
– Perforin release by NK cells and
eosinophils
– Granzyme release by NK cells