Section F – The T cell response – cell-mediated immunity
region domain of class II MHC molecules thereby strengthening the association of the TCR with peptide-class II MHC molecules. Likewise, CD8 binds to class I MHC molecules to strengthen the association of the TCR with class I MHC molecules. In addition to engagement of these ligand–receptor pairs (Fig. 1), additional adhesion molecules, integrins, become engaged. These include inter- cellular adhesion molecules (ICAMs) and lymphocyte function-associated anti- gens (LFAs). Some of these accessory molecules are also important in regulating early activation events through signaling, e.g. CD4 and CD8, and are sometimes termed co-receptors.
Ligation of the TCR on its own does not stimulate T cell clonal expansion or lymphokine production. The full activation of antigen-specific T cells requires two signals. Signal one is provided by the engagement of the T cell antigen receptor and signal two is provided by engagement of a co-stimulatory mole- cule. The best-characterized co-stimulatory molecule is B7, which is on many APCs and binds to CD28 on the T cell. Signals emanating from the TCR and CD28 synergize to induce T cell lymphokine production and T cell proliferation.
If the T cell receives signal 1 (TCR binding) and not signal 2 (co-stimulation) the T cell is turned off (Fig. 2; Topic G2).
Precursors of CD8+cytotoxic T cells also need to be activated to develop into mature CD8 effector T cells containing granzymes and perforin (Topic F5). This requires attachment of their TCR to MHC class I–peptide complexes on APCs (signal 1). In addition, a second co-stimulatory signal involving binding of B7 to Co-stimulatory
molecules: two signals required for T cell activation
128 Section F – The T cell response – cell mediated immunity
MHC ICAM TCR
CD2 LFA-3
VLA-4 V-CAM
LFA-1
CD4 or CD8
T Cell APC/target cell
Fig. 1. Pairs of molecules which strengthen the association of T cells with antigen presenting and target cells.
MHC class II (αβ) TCR
CD4
B7 (CD80/CD86) CD28
Signal 1 antigen specific recognition
Signal 2 co-stimulation 1
2
Fig. 2. The role of co-stimulation in T cell activation.
CD28 on the CTL is required. Cytokines produced by Th cells and APCs and ligation of APC CD40 are important for enhancing expression of the co-stimula- tory molecules. Of note, although other Th-cell-conditioned APCs may be able to provide the necessary signals for Tc (CTL) activation, dendritic cells are the only cells which have a significant cross-over of processed antigen between exogenous and endogenous pathways (Topics F2 and F6). Moreover, they are, in general, the most efficient of the antigen-presenting cells.
Once activated, mature CD8+cytotoxic T cells do not, in most instances, need to be further activated to release granzyme and perforin. Thus, when activated CTLs come into contact with virus-infected cells, they appear to need, at least initially, only the first signal provided by TCR recognition of viral peptide plus MHC class I, although the interaction of LFA-1 on the cytotoxic cell with ICAM- I on the target cell is also important.
Some protein products of bacteria and viruses produce proteins known as superantigens that bind simultaneously to lateral surfaces of the MHC class II molecules (not in the peptide-binding groove) and the V region of the βsubunit of the TCR. Superantigens are not processed into peptides as conventional anti- gens, but are able to bind to a specific family of TCR. In a sense they ‘glue’ T cells to APC (Fig. 3) and cause stimulation of the T cell. However, these T cells are not specific for the pathogen that produced the superantigen, since all members of a particular family of TCR are activated. The consequence of bind- ing to a large percentage of the T cells is the massive production of cytokines leading, in some cases, to lymphokine-induced vascular leakage and shock.
Among the bacterial superantigens are the staphylococcal enterotoxins (SE) that cause common food poisoning and the toxic shock syndrome toxin (TSST).
Th activation through superantigens
F4 – T cell activation 129
Fig. 3. Superantigen activation of T cells by bridging TCR and MHC class II.
MHC class II TCR
Superantigen Pathogen Antigen
nonspecific T-cell activation
The early signaling events are complex and therefore only a simplified outline will be presented. The area of contact between the TCR accessory and co-recep- tor molecules on the T cell surface with molecules and ligands on the APC is called the ‘immunological synapse’. This is the specialized membrane region that conveys a signal from the T cell surface via the cytosol into the nucleus to give rise to specific gene transcription. This signal transduction is mediated by TCR molecules, co-receptors and enzymes that lie in cholesterol-rich areas of the membrane called ‘lipid rafts’ (Fig. 4).
Early signaling events through co-receptors
130 Section F – The T cell response – cell mediated immunity
Antigen presenting cell MHC class II CD45L
CD45
CD4
TCR
ζ and CD3 ε chains
CD28
Immunological synapse B7 (CD80/86)
Cell surface
Cytosol 2nd signal ITAMS
Phosphatase Lck Fyn
ZAP70
PLC-γ GEFS
(splits) PIP2
DAG IP3
PKC
NFkB Transcription
factors
Nucleus Ca2++
Calcineurin
NFAT Fos (part of
AP-1) Ras
* Via SLP-76
*
*
*
*
MAP kinase cascade
*
Fig. 4. Early biochemical events leading to T cell activation. The ligation of the TCR results in the initiation of signaling pathways through the action of CD45, the kinases lck and Fyn and other kinases including ZAP70. These enzymes ‘activate’ their target molecules by removing or adding phosphates. Ligation of CD45 induces activation of lck and Fyn which these phosphorylate ITAMs on the ζchain and CD3. Once phosphorylated ZAP70 binds to the ITAMs and is activated by lck to initiate two main biochemical pathways. One is via the phosphatidyl inositol pathway where phospholipase C-γ(PLC-γ) cleaves phosphatidylinositol bisphosphate (PIP2) to produce diacyl glycerol (DAG) and inositol trisphosphate (IP3). DAG activates protein kinase C (PKC) which activates NFκB that translocates to the nucleus. IP3 increases intracellular Ca2+activating the phosphatase calcineurin that, in turn, activates NFAT (nuclear factor of activated T cells) and causes it to translocate to the nucleus. The second pathway involves the MAPkinase cascade initiated by RAS through GEFS activated via binding of SL76. This cascade leads to activation of Fos, a component of the AP-1 transcription factor. The second signal delivered via T cell CD28 interaction with CD80/CD86 on the antigen-presenting cell is required to activate the cell. It is thought that CD28 ligation (and indeed ligation of CTLA4) with CD80/CD86 modulates these biochemical pathways.
Neither of the two chains of the TCR have intracytoplasmic tails of sufficient length or amino acid composition to act as signaling molecules. Therefore, T cell signaling is initiated through the longer tails of the ζchains, and εchains of the associated CD3 molecule that have sets of tyrosine molecules called ITAMS (immunoreceptor tyrosine activation motifs). On ligation of the TCR, CD45 (an endogenous phosphatase) activates (by removal of phosphates) two enzymes lck and Fyn that phosphorylate the ITAMs of the ζchains. ZAP70 then ‘docks’
with the phosphorylated ITAMs and itself becomes activated leading to further phosphorylation events. Activation of phospholipase C-γ via the phosphatidyl inositol pathway leads to activation of the transcription factors NFAT (nuclear factor of activated T cells) and NFκB and their translocation into the nucleus.
Another consequence of the phosphorylation mediated by ZAP70 is the activa- tion of the MAP kinase cascade via SLP-76, Guanine nucleoside exchange factors (GEFS) and Ras which finally leads to activation of Fos – a component of the AP-1 transcription factor. This whole process is very rapid and the multiple phosphorylation and dephosphorylation events in the membrane take place within seconds of ligation of the TCR. The initial signal occurring within the lipid rafts is amplified via the molecules of the different biochemical pathways (‘second messengers’) leading finally to transcription of effector molecules, e.g.
cytokines (IL-2, IL-4 and IFNγ) and cell cycle proteins (cyclins), required for clonal expansion.
F4 – T cell activation 131
Section F – The T cell response – cell-mediated immunity