Rakesh S.P.

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 APCsuch as Macrophages or B cells.

Two signals are necessary for full T cell activation:

• Signal 1: generated by interaction of MHCpeptidewith the TCR-CD3 complex

• Signal 2: generated by interaction of CD28 on the T cells and members of the B7 family on theAPC, it is also called co-stimulatory signal

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Important properties of the major accessory molecules

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)

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 activation.TCR-Ag/MHC

The role of costimulation in T cell activation

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

Superantigens induce T-cell activation by binding the TCR and MHC simultaneously

Superantigens (SAgs) are a class of antigenswhich cause non-specific activation ofT-cells resulting in polyclonal T cell activation and massive cytokine release.

Superantigens (viral or bacterial protein) can be produced by pathogenic microbesas 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).

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)

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

Mechanisms of killing of infected cells by CD8+ CTLs

Different phases of T cell response

Activation of T cells generate effector and memory T cells

Activation of transcription factors in T cells.

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 AP1which direct the transcription of new genes needed for Tcell response.

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

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

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

There is need to down modulate T cell response

otherwice patient may die due to uncontrolled

immune responses ( immunopathology )

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.

CTLA-4 ( Cytotoxic T-Lymphocyte Antigen 4 )

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

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.

Cbl protein

Mutations to this gene have been implicated in a number of human cancers,particularly acute myeloid leukaemia.

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.

Mechanisms of action of the cbl-b

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.

T cell activation In Brief…

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 , andnuclear transcription of genes.

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

Cont…