T CELL ACTIVATION AND TERMINATION

1. Rakesh S.P.

2. General features of T cell activation
on recognition of peptide-MHC by TCR , T cell gets activated ,becomes
proliferated, makes clonal expantion and differentiates into
memory and effector T cells.
T cells recognize antigens in lymphoid organs and ,in peripheral nonlymphoid
tissues actively perform their effector functions.
The activation of T cells requires recognition of antigens displayed on APCs,
costimulators and cytokines produced by the APCs and by the T cells
themselves.
Naive T cells require activation by dendritic cells or other professional APC
such as Macrophages or B cells.

3. Two signals are necessary for full T cell activation:
• Signal 1: generated by interaction of MHCpeptide
with the TCR-CD3 complex
• Signal 2: generated by interaction of CD28
on the T cells and members of the B7 family on the
APC, it is also called co-stimulatory signal
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4. Important properties of the major accessory molecules

5. Naive T cells: They become activated on antigen recognition which
is presented by APC (e.g. dendritic cells).
Effector cells: cells which having short life with special functions such as
cytokine secretion , helps B-cell and cytotoxic killing activity.
Effector cells are derived from naïve or memory cells after
antigen activation (e.g. CTLs)
Memory cells: long-lived resting cells that are derived from naïve
and effector cells.
They respond faster and stronger to a subsequent challenge
with the same antigen.
Regulatory T cells: cells that can inhibit the proliferation of other T cell
population which cause problem to host body .
(e.g. CD4,CD25)

6. Interaction of T cells and APC
Interaction between TCR and Ag/MHC
alone is not strong enough to sustain the
contact between T cells and APC
Integrin and their receptors on T and APC
strengthen the interaction so that TCR and
CD28 receptors on T cells can receive
prolonged and stable signals.
Signals through integrin on T cells also
enhance T cell activatioTnCR. -Ag/MHC

7. The role of costimulation in T cell activation

8. Role of costimulation and helper T cells in the differentiation of CD8 T cells

9. Superantigens induce T-cell activation by binding the TCR and MHC
simultaneously
Superantigens (SAgs) are a class of antigens
which cause non-specific activation of
T-cells resulting in polyclonal T cell
activation and massive cytokine release.
Superantigens (viral or bacterial protein)
can be produced by pathogenic microbes
as a defense mechanism against the
immune system.
Anti-CD3 and Anti-CD28 Antibodies (CD28-
SuperMAB) have also shown to be highly
potent superantigens (and can activate up to
100% of T cells).

10. Formation of the immunological synapse
This schematic diagram illustrates the
steps in the formation of the
immunological synapse.
Before antigen recognition, various
receptors on T cells and their ligands on
APCs are dispersed in the plasma
membranes of the two cells.
When the T cell recognizes antigen
presented by the antigen-presenting cell
(APC), selected receptors on the T cell and
their respective ligands are redistributed
to a defined area of cell-cell contact,
forming the synapse.
The molecules in the central portion of the
synapse form the central supramolecular
activation cluster (cSMAC), and the
molecules in the periphery form the
peripheral supramolecular activation
cluster (pSMAC)

11. CD8+ T cells
Naive CD8+ T cell differentiate into cytotoxic T cell lymphocytes (CTLs) which
kill target APCs processing the correct MHC+ peptide.
The mechanism involved are:
Perforins:
which makes the pores in membrane
Granzymes:
serine proteases that are capable of activating caspases.
Fas-Fasl signalling resulting in apoptosis

12. Mechanisms of killing of infected cells by CD8+ CTLs

13. Different phases of T cell response

15. Activation of T cells generate effector and memory T cells

16. Activation of transcription factors in T cells.

17. Early tyrosin kinase signalling pathway
Invariant components of TCR: the γ, δ and ε chains
(Collectively known as CD3 complex) and ζ chains .
As TCR and MHC peptide binds, src family tyrosine kinases lck are activated.
They phosphorylates specific motifs called Immunoreceptor Tyrosine based
Activation Motifs (ITAMs) present on the ζ chain and CD3 subunits of TCR complex.
Phosphorylation of these motifs promots recrutement and activation of zap-70
tyrosine kinase,which inturn activates several target adaptor proteins such as LAT
and SLP-76.
Phosphorylation of LAT and SLP-76 further activates phosholipase c –γ1 (plc-γ1)
enzyme by tyrosine phosphorylation.
Plc-γ1 catalyzes the formation of second messengers,inositol 1,4,5- trisphosphate
(IP3) and diacyle glycerol (DAG) , which respectively triggers Ca2+ flux and contribute
to protein kinase C (PKC) and Ras activation.
This ultimately activates several transcription factors such as NF-AT ,NF-kB and AP1
which direct the transcription of new genes needed for Tcell response.

18. Function of IL-2
IL2 is autocrine T cell growth factor so it produced by T cell and helps them
to proliferate

19. Major events involved in T-cell activation
Event Example
Cell-cell interaction T cell - APC
CTL - target cell
Receptor - ligand binding TCR - antigen/MHC
Transmembrane signal transduction Activation of Lck and zap-70
Generation of second messengers 1,4,5-IP3 and DAG
Second-messenger effects Ca2+ mobilization
Protein kinase C activation
Biochemical pathways Phosphatidylinositol pathway
Ras pathway
Cellular events Secretion of cytolytic granules
Early gene activation c-Myc, c-Fos
Intermediate gene activation Lymphokines, lymphokine
receptors, nutrient receptors
Late gene activation Genes involved in cell proliferation
Il-2, IL-2Ra, VLA-2 etc

20. Importance of T cell activation study :
Abnormal enhanced T cell function : observed in autoimmune conditions
such as multiple sclerosis , insulin dependent diabetes mellitus etc.
Reduced T cell activation : leads to increesed susceptibility of the host to
infectious microbes and tumors, e.g. AIDS,HIV etc.
Importantly suppression of T cell activation is required for organ transplants
to be successful
(MHC as well as suppression of endogenous T cell - do not reject graft)
on the other hand, adjuvants which enhance T cell function are required for
successful vaccination

21. There is need to down modulate T cell response
otherwice patient may die due to uncontrolled
immune responses ( immunopathology )

22. CTLA-4 ( Cytotoxic T-Lymphocyte Antigen 4 )
CTLA-4 is found on the surface of T cells which is also
known as CD152 (Cluster of differentiation 152).
It is a member of the immunoglobulin superfamily, which
is expressed on the surface of T cells and transmits an
inhibitory signal to T cells.
Function: CTLA-4 is a protein receptor that downregulates
the immune response.
CTLA-4 is similar to the T-cell co-stimulatory protein.
The T cell attack can be turned on by stimulating
the CD28 receptor on the T cell.
The T cell attack can be turned off by stimulating the
CTLA-4 receptor, which acts as an "off" switch.
Mutation in CTLA-4 is associated with different
autoimmune diseases.

23. Mechanisms of
action of the
inhibitory receptor
CTLA-4(CD 152)

25. Cbl protein
Cbl protein (982 amino acids long) is an E3 ubiquitin-protein ligase involved in cell
signalling and protein ubiquitination.
E3 ubiquitin-protein ligase is a negative regulator of protein tyrosin kinase signaling.
Functions Ubiquitin ligase
Ubiquitination is the process of chemically attaching ubiquitin monomers to a
protein, thereby targeting it for degradation.
Cbl functions as an E3 ligase, and therefore is able to catalyse the formation of a
covalent bond between ubiquitin and Cbl's protein substrate - typically a receptor
tyrosine kinase (eg. Zap-70).
The stepwise attachment of ubiquitin to the substrate receptor tyrosine kinase can
lead to its removal from the plasma membrane.
Mutations to this gene have been implicated in a number of human cancers,
particularly acute myeloid leukaemia.

26. Mechanisms of action of the cbl-b

27. T cell activation In Brief…
T cell responses are initiated by signals provided by clustering of TCR
complexes through recognition of antigen on the surface of an APC and
through signals provided by costimulators expressed on APCs.
The response of the T cell varies with the nature of the antigen, the APC that
presents the antigen, and the stage of maturation and differentiation of the T
cells.
The best defined costimulators for T cells are the B7 proteins, which are
recognized by CD28 on T cells.
B7 molecules are expressed on professional APCs, and their expression is
enhanced by microbes and by cytokines produced during innate immune
reactions to microbes.

28. Cont…
The requirement for costimulation, especially for activation of naive T cells,
ensures that T cell responses are induced in lymphoid organs, where professional
APCs are concentrated, and against microbes and microbial products
T cell responses to antigen and costimulators include synthesis of cytokines ,
cellular proliferation, differentiation into effector and memory cells, and
performance of effector functions.
Clustering of TCRs on antigen recognition triggers intracellular signaling pathways
that result in the production of transcription factors, which activate a variety of
genes in T cells.
Intracellular signaling may be divided into
membrane events ,
cytoplasmic signaling pathways , and
nuclear transcription of genes.

29. Cont…
Membrane events include the recruitment and activation of protein tyrosine
kinases into the TCR complex ; the phosphorylation of TCR complex constituents
(e.g., the ζ chains) ; and the recruitment of protein tyrosine kinases, especially
ZAP-70 , and adapter proteins.
Cytoplasmic signaling pathways lead to the activation of effector enzymes, such
as the kinases ERK, JNK, and PKC, and the phosphatase calcineurin.
These enzymes contribute to the activation of transcription factors such as NF-AT,
AP-1, and NF-κb, which function to enhance gene expression in antigen-stimulated
T cells.
Some peptides in which the TCR contact residues are altered may induce partial T
cell responses or inhibit T cell activation by poorly understood biochemical
mechanisms