The PLC 1 to NF- B pathway

In essence this process involves the formation of an E3 ubiquitin–ligase complex followed by K63-type ubiquitylation and recruitment of components that result in the activation of IKKand - (see page 460 onwards)· This leads to destruction of the inhibitor I B and hence nuclear translocation of NF- B. The assembly process involves the CARD domain, first discerned in the recruitment and activation of caspases (proteases involved in apoptosis). All of this depends on the activation of PKC (see Fig 9.8, page 254) by diacylglycerol in the lipid raft supporting the TCR-CD3 complex (see page 522). Activated PKCphosphorylates the membrane bound adaptor CARMA1 which causes it to unfold, resulting in the attachment of the adaptor Cbl-10, through a CARD-CARD domain interaction.

Cbl-10 is associated with the adaptor Malt1, which in turn binds to the TRAF6 E3-ubiquitin ligase complex (comprising TRAF6, Ubc13, and Mms2) (Figure 17.7). K63-type ubiquitylation of TRAF6 ensues and this recruits NEMO/IKK / IKK , as well as TAB1, -2, -3/TAK123 (see page 460). The recruitment of TAK1 allows phosphorylation and activation of IKKand this phosphorylates I B rendering the inhibitor susceptible to ubiquitylation (K48 type) by the SCF E3ubiquitin ligase complex. Ubiquitylated I B is destroyed by the proteasome (see Figure 15.8, page 462). NEMO is also ubiquitylated, allowing further recruitment of TAB/TAK1 complexes and enhancing the signal.24 The liberated NF- B now exposes its nuclear localization signal and translocates to the nucleus where it participates in the induction of interleukins together with NF-AT and AP-1.

As already mentioned, full T cell activation requires a second, costimulatory signal communicated through a separate receptor. For some helper T cells,

this is provided by CD28 which binds to the B7 molecule on antigen-presenting cells. The signalling pathway activated by CD28 involves PI 3-kinase and mTOR, and it acts to regulate the expression of cyclin E necessary for T cell proliferation. In fact, prolonged occupation of both TCR and CD28 reduces the requirement for an autocrine IL-2 signal.25 The involvement of mTOR in the costimulatory signal explains why rapamycin, an inhibitor of mTOR, acts as a potent inhibitor of T cell function (immunosuppressant)26 (see page 565).

Down-regulation of the TCR response

The strength of the TCR signal depends on the strength of the interaction between receptors and the antigen in the context of the MHC class II. It also depends on the number of TCRs that contribute to the formation of microclusters. This requires efficient recycling because as much as 1% of cell surface TCR-CD3 complexes are internalized every minute. Binding of the

adaptor SLAP prevents recycling. It binds to the chain of the TCR and recruits

The Cbl family. Cbl was discovered as an oncogenic protein v-Cbl, carried by the Cas NS-1 retrovirus, cause of B cell lymphomas in mice.21 It acts as an E3-ubiquitin ligase and plays a role in the down-regulation of tyrosine kinase-induced signalling pathways. It works by ubquitylating activated kinases. In the case of the EGFR and the TCR, this has the effect of directing the receptor to late endosomes so

preventing their recycling to the outer membrane. Cbls also down-regulates other effectors such as tyrosine phosphorylated Vav (a RhoGEF), tyrosine phosphorylated STAT5, or the Fc RI in mast cells. The oncogenic form, v-Cbl, may act as an inhibitor of endogenous Cbl activity. Cbl members also act as adaptors for numerous proteins such as PI 3-kinase, c-Src, tubulin, or yet another adaptor protein SLAP.22