Lecture 17 Flashcards
What is the T cell antigen receptor complex?
This consists of a hypervariable antigen receptor which exists as either an alpha/beta or gamma/delta and will only recognize MHC molecules as well as associated CD3 complex which has 4 chains which can be phosphorylated
There is either the C4 or CD8 coreceptor which initiate activation by binding to MHC and using their non-covalently bound to their cytoplasmic domains
There is also the CD28 molecule which can proved the second signal to the T cell by bind B7
There are important adhesion molecules such as LFA1-ICAM-I, CD2-LFA3
What are supramolecular activation clusters?
When a T or B cell is activated the critical molecules are found co-located on the membrane in clusters including TcR, CD4/8, CD3 costimulatory molecules, kinases and adhesion molecules
These first form as micro clusters which congeal into one large cluster (a SMAC)
What are the CD3 chains?
Gamma- type 1 Ig-like membrane protein with a short cytoplasmic tail and 1 ITAM
Delta- Type Ig-Like membrane protein with short cytoplasmic tail and –ve charge in the transmembrane region containing 1 ITAM
Eta- type 1 Ig like membrane protein with short cytoplasmic tail and negative charge in the transmembrane region, contains 1 ITAM
Zeta- only has a 9 aa extracellular domain with the remainder being cytoplasmic, homodimer contain two sets of 3 ITAMs and is phosphorylated by Lck associated with CD4/8
What is believed to be the structure of the TcR CD3 complex?
This CD3 chains and Tcr are unusual in that they all have charged amino acids residues in their transmembrane regions making them form a complex of dimers with the important zeta-zeta dimer believed to be in the middle of this dimer
What are the early events in T cell activation?
CD45 is a phosphatase that primes lck by removing phosphate comprises 1% of membrane protein
CD4/8 brings lck/fyn/yes proximal to the CD3 zeta-chain
At least 2 ITAMs phosphorylated on CD3 zeta-zeta dimer
Syk/ZAP70 kinase binds phosphorylated ITAM on zeta chain via its SH2 domain
Phosphorylation of many adaptor proteins including LAT and SLP-76
Phospholipase C-gamma docks onto phosphorylation LAT-SLP-76
PLC-Gamma cleaves PIP3 to IP3 and DAG
DAG activates PKC-0 and the RasGEP-Ras pathway
Increase in Ca2+ and activation of the Ser-Thr phosphatase calcineurin
What is ITAM?
This is an Immunoreceptor tyrosine based activation motif
The cytoplasmic domain of these membrane glycoproteins have motifs that regulate activity by phosphorylation
These domains are found on all the CD3 chains with CD3 zeta containing 3 of them
When these domains are phosphorylated they bind ZAP70 which initiates the signal cascade
What is an ITIM?
Immunoreceptor tyrosine based Inhibitory motif these motifs bind a phosphatase, examplse include CTLA-4 which cometes with CD28 for B7 receptors, this receptor is a negative regulator of T cell activation
What is the process through which ITIMs act?
ITIMs bind SHP-1 and SHP-2 phosphatases via an SH2 domain
They also bind SHIP- ionosotide phosphatase degrades IP3
Dephosphorylates ITAM associated with ITAMs
CTLA4 on T cells directly opposes CD 28
What are the downstream events in T cell activation?
After PIP2 is cleaved to DAG and IP3 by PLC gamma there are 3 main signalling cascades activated, namely generation of NF-AT, Activation of Ras mediated MAP kinase cascade and activation of PKC-theta
How does generation of nuclear NF-AT occur?
IP3 increases Ca flux which binds calmodulin activating the serine threonine phosphatase calcineurin
Cacineruin dephosphorylates a nuclear transcription factor NF-ATc
NF-AT translocates to the nucleus
How does activation of Ras mediated MAP kinase cascade occur?
Activation of guanine exchange factors bind to Ras displacing GDP allowing active GTPase
Activation of MAP kinase pathway leading to translocation elk, fos and jun transcription factors
Binding of AP-1 (fos-jun) to T cell specifc genes to activate transcription and translation
How does activation of PKC-theta occur?
DAG activates protein kinase C – theta resulting in activation of the NFkappaB transcription factor translocating to the nucleus upregulating proinflammatory genes
What is the role of costimulation in T cell activation?
A second signal is required to activate a naïve T cell, this is supplied by CD 28 without this signal T cell will remain unresponsive and anergic
CD28 binds B7.1 and B7.2 on the APC which are only expressed on the APC in the presence of pathogens
Activation of CD28 results in activation of a different but complementary MAP kinase pathway with the difference being whether Vav or SOS is activated at the head of the cascade with co-stimulation seeming to prolong or amplify gene transcription
CTLA4 is another molecule expressed after T cell activation which bind B7 molecules with a 20 times higher affinity than CD28 as well as containing an ITIM domain
What cytokines can influence the development of Th cells into their various subtypes??
IFN-gamma drives them to become Th1
IL-4 drives them to become TH2
TGFbeta and IL-6 drives them to become Th17
TGFbeta alone drives them to become Trags
What is the role of cytokines in CD8 T cell development?
These cells develop into precytotoxic T lymphocytes through exposure to IL-2 and after exposure to IL-12, IFN-Gamma and IL6 from activated APC these cells will produce cytotoxic granules and become “armed”
