10 - T Cell Activation, Helper Subset Differentiation, and Memory Flashcards
T cell activation and the Two-signal hypothesis
The T cells that leave thymus are naïve, as they have not encountered an Ag. their chromatin is condensed, they have very little cytoplasm, and they exhibit little trc activity, though they are mobile. Migrates from tissue to tissue until it encounters an MHC-peptide complex for which is has high affinity.
two-signal hypothisis = needs two signals to activate a naïve T cell.
Signal 1 is triggered by TCR engagement and Signal 2 by engagement of costimulatory molecules, like CD28. It actually also requires a third signal, provided by local cytokines, that directs the differentiation of T cells into distinct effector cell types.
A successful T cell-APC interaction results in the stable organization of signaling molecules into an immune synapse. The TCR/MHC complex aggregate in the central part of this synapse (central supramolecuar activating complex, cSMAC). The affinity in the complex is quite low, but stabilized by other molecules, together increasing avidity. The coreceptors CD4 and CD8 (also in the cSMAC) are key participants and bind to MHCs.
In contrast, adhesion molecules and their lgands (LFA1/ICAM1 and LFA-2/CD2) organize themselves around the perimeter of the aggregate, forming the peripheral SMAC, pSMAC.
However, the increased avidity is not enough, and the second signal is still needed.
Signal 1
1) the activation of Lck.
TCR signaling begins with the activation of the tyrosine kinase Lck. CD4 and CD8 stabilize the TCR/MHC complex, but also have cytoplasmic tails that associate with Lck. Once Lck is close to the TCR/CD3 complex, it can phosphorylate the tyrosines in the ITAMs in the CD3 tail.
2) the phosphorylation of ITAMs generates a new docking site for proteins with SH2 domains, includig ZAP-70 (T cell specific tyrosine kinase), which joins the signaling complex. Here, it is phosphorylated (and thus activated) by Lck. Important substrates: LAT (linker protein of activated T cells) and SLP-76 (associate of LAT).
3) ZAP-70 phosphorylates LAT and SLP-76. this generates docking sites, permitting the assembly of new protein complexes.
3a) PLCγ signaling and activation of TFs NF-κB and NFAT.
phospholipase C γ (PLCγ) binds P-LAT (phosphorylated LAT).
PLCγ is now close to its substrates, phopholipids in the cell membrane. It splits the heads from the tails, generating two new signaling molecules (soluble IP3 and membrane-bound DAG).
IP3 induces the release of Ca2+, binding calmodulin and activating calcineurin (a phosphatase), which in turn dephosphorylates and activates NFAT (nuclear factor of activated T cells).
DAG binds PKC-θ. This cascade culminates in the degradation of inhibitors for NF-kB, which moves into the nucleus.
3b) Ras-ERK signaling and the activation of TF AP-1
P-LAT also associates with Grb2, an adaptor molecule that recruits components of the Ras pathway to the signaling complex. In T cells, this triggers activation of MAPKs, which phosphorylate Ser-residues, and include RAF, MEK and ERK. ERK has many targets, but is particularly important for the activation of TF AP-1.
Due to costimulatory molecules, only a few TCR-MHC interactions are needed to trigger a signaling cascade that culminates in cell survival, poroliferation and differentiation.
Cosignals
anergy = T cell nonresponsiveness in the absence of costimulation from APCs, even if TRC/MHC interaction.
Upon activation, T cells release cytokines that enhance entry into the cell cycle and proliferation.
coinhibitory receptors:
1) maintaining peripheral T cell tolerance
2) reduce inflammation after natural infection and during responses to chronic infection.
CD28
expressed by all naïve and activated human CD4+ T cells, and in 50% of human CD8+ T cells.
coreceptor, enhances proliferation and survival by secreting IL-2 and Bcl-x_L (respectively). Also strengthens TCR signal, and assembles a unique group of signaling molecules, like PI3 kinase (activates many kinases and regulate cell metabolism, survival and division)
ligands: CD80 and CD86.
Most cells do not express these ligands, only pAPCs do (DC constitutively, macros and B cells upregulate upon Ag-encounter)
Costimulatory receptors
ICOS = inducible costimulator.
Binds ICOS-L (ICOS ligand), also expressed on a subset of activated APCs.
not expressed on naïve T cells, but on memory and effector T cells.
CD28 plays key costimulatory role during initiation of activation, ICOS plays key role in maintaining the activity of already differentiated effector and memory T cells.
Coinhibitory receptors
CTLA-4 and PD-1 and BTLA
1) CTLA-4 (CD28 family, but inhibitor). Also binds CD80/86.
not expressed constitutively on resting T cells, rather induced 24h after activation of a naïve T cell.
Even though peak CTLA-4 is lower than peak CD28, it binds the ligands with higher affinity, and can thus compete.
Upregulated in response to upregulation of CD28, to make sure the immune response it not too great.
2) PD-1 = programmed cell death 1. Coinhibitory receptor expressed by both B and T cells.
2 ligands: PD-L1 and PD-L2.
PD-L2 is expressed predominantly on APCs, but PD-L1 is expressed more broadly and may help mediate T cell tolerance in nonlymphoid tissues.
3) BTLA (B and T lymphocyte attenuator).
expressed on TH, γδ and regulatory T cells, NK, some macrophages and DCs, and most highly on B cells.
ligand: HVEM (herpes virus entry mediator), expressed on many cell types.
downregulate inflammatory and autoimmune responses
naïve T cells do not express coinhibitory receptors, so they can be activated in secondary pymphoid organs without an immune response.
Clonal anergy
if a naïve T cell recieves Signal 1 but not Signal 2, it becomes unresponsive to subsequent stimulation (= clonal anergy).
Decreases the possibility of autoreactive T cells becoming dangerous. insulin is only made in pancreas, if the T cell has not been exposed to it before it will rec it as foreign, but the pancreatic cells cannot express costimulatory ligands, anf there will therefore not be an attack.
coinhibitory receptor can also cause clonal anergy in activated T cells to stop the immune response.
Signal 3
provided by cytokines.
IL-2 very important in T cell activation. Plays key role in optimal T cell proliferation, esp if Ag/costimulatory ligands are limiting.
also includes polarizing cytokines, produced in a variety of cell types, incl APCs, T and ILCs. play central roles in determining what type of effector cells naïve T cells will become.
APCs role in activation of T cells (general)
APCs: DCs, macrophages, B cells
DCs most potent. pAPCs differ by location and migratory behaviour. Some circulate, others reside.
MHC I and II, and costimulatory ligands. Can give Signal 1, 2, 3.
Must be activated by PRRs, which upregulated MHC and costim. ligands.
DCs role in T cell activation
Different types.
1) conventional DCs: circulate, are activated by Ags in peripheral tissues and travel to lymph nodes to the T cells. Also some conventional DCs that remain in the lymph node, processing Ags from the blood or from other APCs.
2) Langerhans. Migratory. Exclusively on skin. also travel to a lymph node (near or far) in response to Ag.
B cell role in T cell activation
can activate late in immune response.
1) bind Ag
2) upregulate MHC II and CD80/86
3) present Ag to activated CD4+ T cell.
happens at junction between T anc B cell zones, allowing T cells to help B cell proliferation, differentiation and memory generation.
Superantigens
special class of T cell activators.
Examples: endotoxins, minor lymphocyte-stimulating determinants (Mls)
products of bacteria and viruses that activat eT cells in an Ag-nonspecific manner. Still needs APCs as Signal 2.
bind directly to select TCR β chains, activating all T cells expressing these chains, regardless of what α chain they have. This large-scale T cell activation can lead to inflammation that can cause disease, like toxic shoxk syndrome.
Helper CD4+ T cell differentiation
1-2 days after DC engagement in secondary lymph organ, naïve T cells enlarge into blast cells, and divides 2-3x /day, 4-5 days.
several possible fates:
- cytotoxic T cell - circulate to site of infection, bind and kill infected cells.
- helper T cells: secrete cytokines (auto- or paracrine) that actovate several other cell types (incl B cells, macrophages, other T cells). Some (esp those who activate B cells) stay in the lymphoid tissue. Others go to site of infection to enhance macrophages and cytotoxic cells. Required for development of B cells and CD8+ T cell memory.
Helper T cells: Type 1 and Type 2 responses
Type 1: triggered by viral and many bacterial infections. Polarize CD4+ T cells to the TH1 and TH17 subsets. Work with other immune cells (ILC1s and ILC3s) to generate cytotoxic responses.
Type 2: triggered by larger parasites, incl worms, protoza, and allergens. Polarize CD4+ T cells to TH2 and TH9 helper subsets, which work with other immune cells (incl ILC2s) to generate an IgE and eosiniphilic response.
Treg
IL-2 and TGF-β
master transcriptional regulator: FoxP3
secrete IL-10 and TGF-β
function: supress immune responses.
disease: inhibits antitumor response in disease.
