6. Cell-mediated immune responses
can be divided into two major
categories:
1. Effector cells that have direct
cytotoxic activity .
2. Effector CD4 T-cells that mediate
Delayed-type hypersensitivity.
7. The activation requirements of
T-cells.
• The activation of naïve T-cells &
their subsequent :
1.proliferation.
and
2.differntiation.
into effector T-cells require.
8. Two signals :
• 1.Primary signal, when TCR interact
with peptide-MHC complex (antigen ).
• 2.Co-stimulatory signal, delivered by
antigen presenting cell (APC ).
9. Critical Role of Th Cells in
Specific Immunity
• Select effector mechanisms.
• Induce proliferation in appropriate effectors.
• Enhance functional activities of effectors.
APC
Th
cell
B cell
Tc
cell
NK
Ag
Ag
Ag
NK cell
Cytokines
Granulocyte Macrophage
Antigen-presenting cell
10. Th1 cells & Th2 cells.
• Depending on :
1. nature of antigen.
2. cytokines.
Th 0 cells ( naïve cells, uncommitted)
Take the Th1 pathway & produce
cellular immunity.
OR : the Th2 pathway & produce
humoral immunity.
11. Naïve Th Cells Can Differentiate Into Th1 or Th2
Cells
ThP
cell
ThO
cell
Th1
cell
Th2
cell
ThM
cell
IL-2 IFNγ
IL-2
IL-4
IL-5
IL-10
IL-4
IL-5
IL-6
IL-10
IL-2
IFNγ
IL-2
Naive Th cells Short-term
stimulation
Chronic
stimulation
Long term
Memory cells
12. Functions of Th1 and Th2 Cells
Th1
cell
Th2
cell
Macrophage B cell
IFNγ
Activates
IL-4 IL-5
IL-10
Activates
Inhibits production
Mast cell
Eosinophil
Antibodies (including IgE)
13. Cell-adhesion molecules
facilitate TCR interactions :
• CD2 & the integrin LFA-1
( on the surface of T-cells)
bind respectively to :
LFA-3 & ICAMs (intracellular)
( on antigen presenting cells )
14. Effector T-cells express a
variety of molecules :
• 1.Membrane- bound molecules include:
-Fas ligand on CD8 CTLs.
-TNF-B (tumor necrosis factor)
on Th1 cells .
-CD 40 ligand on Th2 cells.
15. 2. Soluble effector molecules include :
- CTLs secrete cytotoxins ( perforins
and granzymes ) .
also secrete : IFN-gamma & TNF-B.
17. Cytotoxic T-cells
• Cytotoxic T-cells (or CTL) are
CD8+ cells & are therefore:
class 1 MHC restricted .
( in rare instances CD4+ class11
restricted T-cells have been shown to
function as CTLs )
18. Cytolytic T (Tc) Cells
• Tc exiting the thymus are pre-Tc cells, i.e.
have TCR that can recognize antigen,
but are not mature and cannot kill until
“armed”.
• To become armed requires two signals:
1. Recognition by TCR of specific
antigen associated with class I MHC,
and
2. Exposure to cytokines (IL-2 and IFN-γ).
19. Role of Th1 cells in generation of
CTLs.
The interaction of Th 1 cells with
APC
result in production of IL -2 .
* IL-2 act on CTL precursor cells
and cause them to proliferate and
differentiate into active CTLs.
20. Features of cytolytic T- cell
Killing.
• Antigen - specific.
• Requires cell- cell contact.
• Each Tc capable of killing
many target cells.
21. Main Mechanism of Tc Killing
• Tc granules contain perforin and granzymes.
( fragmentins )
• Upon contact with target cell, granule contents
released, perforin polymerizes and forms
channel in target cell membrane.
• Granzymes (serine proteases) enter target cell
through channel, activate caspases and
nucleases, lead to apoptosis of target cell.
22. Steps in Tc Killing
Tc cell
1. Tc recognizes antigen on
target cell
Target cell
Tc cell
2. A lethal hit is delivered by
the Tc using agents such as
perforin or granzyme B
Target cell
Tc cell
3. The Tc detaches
from the target cell
Target cell
4. Target cell dies
by apoptosis
Target cell
24. Mechanisms of Tc cytotoxicity
• Tc kill using a variety of mechanisms:
–Direct cell-cell signaling via
surface molecules .
–Indirect signaling via cytokines.
25. Mechanisms of Tc cytotoxicity
(continued)
• Tc can kill via Fas or tumor
necrosis factor (TNF) receptor on a
target cell
(Signaling through Fas or TNF R.)
• Tc express Fas ligand (FasL) on their
surface.
• Fas ligand interact with Fas (on target
cells) and trigger:
apoptosis.
26. Mechanism of Arming Tc Cells
Pre-Tc cell
Tc cell
T helper cell
Class I
MHC
Class II MHC
APC
1. Cell expressing class I MHC
presents antigen ( )
to a pre-Tc cell
IFN
IL-2
2. Antigen-presenting
cell presents antigen in
association with
class II MHC to Th cell
3. Th cell
makes cytokines
4. Pre-Tc cell
differentiates to
functional Tc cell
5. Tc recognizes antigen on
class I MHC-expressing target cell
6. Target cell
is killed
27. Fas/FasL and TNF killing by CTLs
• Binding of ligand to receptor leads to
induction of apoptosis through death
domains (dd).
• Receptors without dd do not lead to
apoptosis.
28. Ligation of receptor leads to trimerization of
the receptor and initiation of the caspase
cascade
Apoptosis =
Programmed cell death =
cell suicide.
29. Macrophage Activation
Two major mechanisms activate
macrophages :
• IFN-γ produced by Th or Th1 cells
plus bacterial endotoxin (LPS).
• IFN-γ produced by Th or Th1 cells
plus TNF-α.
31. Mechanism of Macrophage Activation.
Macrophage Activated
Macrophage
Bacterial endotoxin
(lipopolysaccharide)
triggers cytokine
production
Th1
cell
1
IFN gamma
IFN gamma
2
Macrophage
Activated
Macrophage
TNF alpha
Various
products
32. Detailed Functions of Macrophages
Inflammation – Fever, Production
of: IL-6, TNF-alpha, IL-1 – act as
pyrogen
Immunity
Selection of lymphocytes to be
activated:
IL-12 results in Th1 activation
IL-4 results in Th2 activation
Activation of lymphocytes:
Production of IL-1
Processing and presentation of
antigen
Reorganization of tissues,
Secretion of a variety of factors:
Degradative enzymes (elastase,
hyaluronidase, collagenase)
Fibroblast stimulation factors
Stimulation of angiogenesis
Damage to tissues
Hydrolases, Hydrogen peroxide production
Complement C3a
TNF alpha production
Antimicrobial action
O2–dependent production of:
hydrogen peroxide, superoxide,
hydroxyl radical, hypochlorous acid
O2-independent production of:
acid hydrolases, cationic proteins,
lysozyme
Anti-tumor activity produced by:
Toxic factors
Hydrogen peroxide
Complement C3a
Proteases, Arginase
Nitric oxide
TNF alpha
33. Natural Killer (NK) Cells .
• Derived from bone marrow.
• Lack most markers for T and B cells (do
not have TCR).
• Do not undergo thymic maturation.
• Express CD56, a specific NK marker.
• Express a receptor for Fc portion of
IgG, called FcRIII (CD16).
• Cytokines (IL-2) promote differentiation
into lymphokine – activated killer (LAK)
cells.
34. NK- Cell Effector Mechanisms.
• Mechanism of killing similar to those of Tc cells
• Not MHC-restricted.
Susceptibility of target cell to killing is
inversely proportional to expression of
class I MHC (killer inhibitory receptors , KIR)
on NK cells recognize class I MHC and
prevent killing .
35. NK Effector Mechanisms
(continued)
• IgG-coated target cells recognized by
FcRIII (CD16) are killed by antibody-
dependent cell-mediated cytotoxicity
(ADCC).
• Lymphokine-activated killer cells (LAK)
kill broader range of cells than do NK
cells.
36. Regulation of Immune Responses.
• Magnitude determined by balance between
the extent of lymphocyte activation and
tolerance induced by an antigen.
• Nature determined by specificities and
functional classes of lymphocytes activated.
• Regulatory mechanisms may act at :
the recognition, activation, or effector
phases of an immune response.
37. Cytokines Regulate Ig Class Switching
• Fc region of antibodies determines
effector function in different anatomical
locations
• Class (isotype) switching produces
class or subclass of antibody most
effective in host defense
• Cytokines acting alone or in
combination regulate class switching
38. T regulatory cells.
• CD4+CD25+ population is generated in the
thymus. In addition, CD4+ T regs can be
induced from non-regulatory T cells in the
periphery.
• T regs do not prevent initial T cell
activation, they inhibit a sustained
response and prevent chronic and
potentially damaging responses.
40. T regulatory cell (continued)
• T regulatory cells are anergic (non-responsive)
in vitro
• They do not have characteristics of Th1 or Th2
CD4+ cells
• T regulatory cells express Foxp3.
• Mutations in Foxp3 cause immune dysregulation
(polyendocrinopathy enteropathy, X-linked
syndrome). Leads to increased autoimmune and
inflammatory disease.
• T regs can suppress both the Th1 and Th2
responses
41. Possible CD25+CD4+ Treg suppression
mechanisms in vivo.
• Tregs may act in a cell contact–dependent manner by
competing directly for stimulatory ligands on the
APC
• By competing for essential growth factors such as IL-
2
• Directly transmitting an as-yet uncharacterized
negative signal.
• Secretion of suppressive cytokines such as IL-10 and
TGF-β.
• CD25+CD4+ Tregs may act through the APC
1. generation of immunosuppressive metabolites,
2. by perturbing the APC’s presenting capacity.
• Such mechanisms are not necessarily mutually
exclusive, and more than 1 might operate in tandem.
