Download Activation and Function

Activation and Function Of T and B cells
-
-
Activation of T and B cells expressing Ag-specific
receptors results in:
o proliferation.
o Differentiation into effector cells and memory cells.
The activation of T cells leads to the synthesis and
secretion of cytokines that affect many different cell types.
The activation of B cells results in Ab production.
Activation and Function of T and B cells
APCs
- dendritic cells
- Macrophages
- B cells
APCs take up Ag, process and present it to T cells and provide
costimulator signals that activate naive T cells.
Dendritic cells express high levels of MHC class II and class I.
They are motile moving from sites where they are exposed to Ag to
lymph nodes, where they can interact with T cells.
-1-
Fig. shows that the interaction of Ag with dendritic cells in tissues
results in the maturation of the dendritic cell, ultimately leading to the
Ag-bearing cell moving out of the tissue and to the lymph node
draining this tissue site.
In the fig.
the Ag "G-bacteria" is taken up by immature
dendritric cell, Ag interact with toll-like receptors "TLRs" expressed
on dendritric cells.
Activation of TLRs by microbial components leads to activation
of both branches of immunity "innate and adaptive" through the
production of cytokines
*Toll-like receptors (TLR) are considered as part of
“pattern
recognition molecules” .
They fit with a distinct pattern of corresponding piece of a pathogen.
They serve as flags alerting responding cells to the presence of
invading microorganism.
They facilitate phagocytosis.
TLR activate phagocytes and tissue dendritic cells in response to
pathogens.
They are large family of receptors each of which recognizes specific
microbial molecular patterns.
Activation of these cells facilitates initiation of acquired immune
response due to the production of proinflammatory cytokines “ this
illustrates the important relationship between the innate and acquired
immune system”
Coating microbes with complement makes phagocytosis
easy”opsonization”
,
Foreign Ags induce high levels of costimulatory molecules
while self Ags do not induce high levels of costimulatory molecules,
thus naive T cells are not activated.
-2-
Interactions at the surface of APC and CD4+ T cells
-3-
(1)
First signal for T cell activation
– Interaction between MHC class II molecules on APC +
peptide and "V + V" of TCR on T cell.
– This signal is necessary but is not enough for T cell
activation.
(2)
– Interaction of MHC class II with the co-receptor CD4
which
makes
T
cell
100 fold more responsive to Ag.
– CD4 is moved closer to TCR after binding of peptideMHC to TCR, a process enhancing signal transduction.
(3) Costimulator pairs "B7-CD28, CD152/CD40, CD154".
Costimulator molecules are required for activation of naive T
cells.
CD40-CD154 interaction increases the expression of B7 on
APC and enhances B7-CD28 interaction between the APC
and the T cell.
(4) Adhesion molecules.
CD54 + ICAM-1 "intercellular adhesion molecule "on
APC and CD11a/CD18 = LFA-1 "leukocyte functionassociated antigen" on T cell.
Adhesive interactions slow down the movement apart of
the APC and T cell.
(5) The Immunological Synapse.
– The area of contact between APC and T cell.
– The synapse is required for sustained intracellular
signaling, lasting until the two cells split apart after 8
hours of contact.
(6) T cells membrane lipids "cholesterol, glycosphingolipids"
are mobilized to the synapse and draw with them
intracellular signaling.
-4-
-5-
Intracellular Events in CD4+ T cell Activation
1- Binding of MHC II peptide to TCR transmits a signal via the
tightly associated CD3 and Zeta molecule into the interior of
the T cell.
The nature of the signal across the membrane may involve
aggregation of multiple TCR into the cell membrane or a
conformational change in the transmembrane region of the
TCR chain.
2- Activation of TKs which then activate proteins by adding P
group to tyrosine residues.
3- TK associated with CD3-zeta = Fyn - Src family
TK associated with CD4-zeta = Lck - Sark family
-6-
4- Fyn, Lck activated  they cluster with region of CD3 and zeta
chains that contain *ITAMS and phosphorylate them. They act
as docking sites for another TK "ZAP 70".
5- Lck activated ZAP 70 which phosphorylates adaptor molecules
proteins "LAT, SLP-76"  on the cell membrane forming
larger complex of signal transduction molecules at the
immuno-logical synapse.
Activation of Intracellular Signaling
-
Activated adaptor molecules activate intracellular signaling
pathways.
They bind PLC- "phosphorylated by ZAP-70" PIP2
"phosphitidyl inosital biphosphate" DAG "Diacyl glyceral"
which activates transcription factor NF-KB.
* immunoreceptor tyrosine-based activation motif.
Activates membrane-associated protein kinase, activate
transcription factor NF-KB and IP3 "inositol trip" which increases
intracellular Ca++ which activates Calcineurin which activates
transcription factor NF-AT.
This pathway is clinically significant because cyclosporine-A "a
drug preventing graft rejection" binds to calcineurin and inhibits T
cell activation.
Ras & Rac "guanosine-nucleotide binding proteins"
activate cytoplasmic cascade of mitogen.
Activated protein "MAP" kinase leading to activation of
Transcription factor Ap-1.
Activation of genes  enlargement of T cells  IL-2
which is growth factor for T cells & binds to high affinity IL-2
receptor on the same or on different T cell
-7-
Other ways of T cell Activation
1- Superantigens.
2- Mitogens
 Concanavalin A "Con A" ……… lectin
 Phytohemagglutinin "PHA" ……… lectin
 Pokeweed mitogen
……… lectin
3- Anti CD3 Abs.
-8-
CD4+ T cell subsets
TH1 and TH2 are generated from the Ag driven differentiation of
THO cells. TH1 cells synthesize IL-2, IFN and TNF-β(lymphotoxin).
Cytokines synthesized by TH1 cells activate cells involved
in CMI: CD8+ T cells, NK cells and M.
TH2 cells synthesize IL-4, IL-5, and IL-13.
IL-4 and IL-13 influence b-cell class switch to IgE IgG4 in
humans and IL-5 activates eosinophils.
Viruses and bacteria favor the production of TH1 cells,
whereas allergens and parasites favor TH2 cell induction.
Cytokines produced by TH1 can inhibit the function of
TH2 and vice versa.
The recently characterized TH17 subset synthesizes IL-17
family cytokines, which induce pro-inflammatory responses in
many different cell types. TH17 cells also respond to fungi and
some extracellular bacteria.
Treg cells inhibit the function of the other subsets of CD4+
T cells by cell contact and by the synthesis of inhibitory cytokines
TGF- β and IL-10.
-9-
-
Cytokines Influence Differentiation into a particular
Subset of CD4+ T Cells(Fig. below).
Cytokines synthesized by cells of the innate immune system
influence the differentiation into specific subsets of CD4+ T cells,
particularly from dendritic cells. Thus, cells of the innate immune
system have a critical role in shaping the pattern of the adaptive
immune response. The critical role played by these cytokines is often
referred to as a (third signal) that works in conjunction of the first
(peptide-MHC-TCR) and second(costimulatory) signals in driving the
differentiation of activated CD4+ T cells.
- 10 -
T – B Cooperation
- 11 -
-
-
All proteins are thymus dependent "TD" Ags, because they
require CD4+ T cells to "help" or cooperate with B cells to
synthesize Abs.
The set of CD4+ cells that participate in Ab responses to
TD Ags is referred to as helper T cells (TH).
T & B cells must be specific for a particular Ag.
Epitopes must be part of the same protein sequence.
TH and B cells may respond to different epitopes in the
antigen; the TH to an internal epitope generated during the
processing of the antigen , the B cell to an external epitope.
- 12 -
-
T – B cooperation in the response to a TD Ag is also known as
linked recognition
B cells can act as APC for the CD4+ T cell "Fig. 10.6".
Fig10.7:
o Interactions between B and T cells are critical for the
mutual activation of T and B cells, as a result T cells
synthesize cytokines and the B cells synthesize Abs.
o CD40-CD40L interactions are required for T and B cell
activation and for the B cell to switch from IgM synthesis
to other isotypes.
Activation of CD8+ T cells
- 13 -
-
-
-
CD8+ T cells can kill cells infected with bacteria or viruses,
as well as transplanted foreign cells.
CD8+ T cells recognize a combination of peptide in
association with MHC class I molecules.
CD8+ T cells also synthesize cytokines "IFN-, TNF-,
IL-4.
Activation of CD8+ T cells involve first signal "peptideMHC-TCR", the second or costimulatory signals, in addition of
cytokine IL-12"( third signal), this cytokine is synthesized by cells
of the innate immune response, it is critical for activation of CD8.
The paired adhesion molecules expressed on the T-cell and
target cell surface help to maintain contact between the cells for
several hours.
Activated CD8+ T cells contains granules that contain
cytotoxic proteins and expresses CD178 "Fas ligand".
Killing by CD8+ T cells occurs by two pathways
1- Exocytosis onto target cells "perforin and granzymes".
 Perforin form ringlike transmembrane channels or
pores in the target cell membrane.
 Granzyme "serine esterase" passes into the cells and
induces apoptosis.
- 14 -
 Granulysin is a small protein which can kill
intracellular pathogens such as Listeria and
Mycobacteria that live inside macrophages and
dendritic cells.
2- Target cell killing via the interaction of the molecule
CD178 "Fas ligand" on CD8+ T cells with CD95 "Fas", a
surface molecule expressed on may cells.
3- T cell can kill itself via Fas FasL interactions.
4- Once T CD8+ cells finishes the killing process, it
detaches from the target cell to attack and kill additional
Target cells.
-
Harmless or noninfectious Ags "killed virus" do not
trigger CD8+ T cells responses.
-
-
- 15 -
Conjugate Vaccine
Conjugate vaccines utilize the principle of linked recognition to
generate TD response. Fig.10.11 shows that a purified polysaccharide
from the bacterium is conjugated i.e physically linked to a carrier
protein. Tetanus toxoid is used in the H.influenzae b conjugate
vaccine and many others. The carrier protein generates T-cell epitopes
that activate T helper cells which interact with B cells specific for the
polysaccharide. The resulting TD response involves switching to
isotypes such as IgG and the development of long term memory.
Notes:
Two antigen-specific cells , a CD4+ T helper and a B cell,
need to interact to generate Ab in response to TD Ags. The
epitopes the T helper and the B cell recognize must be part of the
structure of the same Ag(linked recognition).
T-B cell cooperation involves interactions between pairs of
molecules on the surface of the CD4+ T cell and the B cell: an Ag
specific first signal ( peptide-MHC class II expressed on the B cell
with the TCR) and critical second or costimulator signals, which
- 16 -
include CD40-CD40 ligand and ICOS-ICOS ligand. The T cell
and the B cell are mutually activated, the T cell to sythesize
cytokines and the B cell to synthesize Ab. The cytokine
synthesized by the T cell determines the isotype of the Ab
sythesized by the B cell.
Some Ags _such as polysaccharides that have many
repeating epitopes_ are thymus- independent-TI- and activate B
cell Ab synthesis without significant T help. Responses to TI Ags
are predominantly IgM and immunological memory does not
develop. People lacking a functional thymus or T cells make
responses to TI Ags.
The intracellular pathways involved in B cell activation are
similar to activation pathways of CD4+ and CD8+ T cells. The
signal through the Ag-specific BCR can be modulated positively
(via the B cell coreceptor) or negatively( for example via the low
affinity Fc receptor for IgG).
- 17 -