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CHAPTER 12 T-Cell Activation and Tolerance 187

TCR/CD3

PIP2
P13K PLCγ1
IP3 DAG RASGRP RAS
PTKs
Ca ++ PKC RAF

Calcineurin CARMA MAPKK (MEK)
NFAT~P MALT1
BCL-10
~P
IKK MAPK (ERK)
IκBα/NFκB

Nucleus NFAT NFκB AP-1

Transcriptional activation

FIG 12.4 Signaling Pathways Activated by T-Cell Receptor (TCR) Engagement. TCR ligation
results in activation of protein tyrosine kinases (PTKs), such as LCK and ZAP-70. Phospholipase
Cγ1 (PLCγ1) becomes phosphorylated and activated by PTKs, including ITK. Hydrolysis of phosphatidyl
inositol bisphosphate (PIP 2 ) by PLCγ1 releases diacylglycerol (DAG) and inositol trisphosphate
(IP 3 ). IP 3 stimulates an increase in intracellular calcium concentration, which activates the phosphatase
calcineurin. Calcineurin dephosphorylates NFAT, thereby signaling NFAT translocation to the
nucleus. The formation of DAG leads to activation of RAS-GRP1 GEF activity and RAS activation.
Active RAS binds and stimulates the kinase RAF1, initiating a cascade of serine/threonine kinases
(MAPK cascade) leading to phosphorylation and nuclear translocation of the ERK kinases. DAG
formation also results in activation of the CARMA/BCL-1/MALT1 complex leading to phosphorylation
of IκB kinase (IKK). Active IKK phosphorylates IκB-α, leading to IκB-α degradation and release
of NF-κB to the nucleus. TCR-activated PI3K catalyzes formation of PIP3 from membrane-associated
PIP 2; the phosphatase PTEN antagonizes PIP 3 formation. PIP 3 binds to the Akt lipid-binding
pleckstrin homology domain, a required element in Akt activation. Active Akt both promotes the
PKC-CARMA-NFκB pathway and blocks FoxO-dependent transcriptional regulation. Transcription
factors NFAT, NFκB, and those activated by the MAPK pathway cooperate to upregulate transcription
of genes, such as IL-2, critical for T-cell activation.

binding capacity and clustering of integrins, a family of molecules regulated. RAS activity may be curbed through the hydrolysis
that mediate TCR signal-augmented cell binding to adhesion of RAS-bound GTP to GDP, either through the intrinsic guanosine
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molecules on APCs. RasGRP mediates activation of the Ras triphosphatase (GTPase) activity of RAS or through the recruit-
cascade. Deletion of RasGRP1 leads to severe defects in thymocyte ment of GTPase-activating proteins (GAPs) to the active RAS
development in mice, highlighting the importance of the RasGRP molecule.
pathway in TCR signaling. PKC is essential for full activation of Active RAS recruits the serine-threonine kinase RAF to the
the Ras/ERK and NF-κB cascades (see below) that are required plasma membrane, initiating a phosphorylation cascade. Active
to mount a TCR-stimulated gene transcriptional program and RAF, in turn, phosphorylates MEK, which phosphorylates
drive cellular activation. extracellular signal-regulated kinase (ERK), which undergoes
In T lymphocytes, RasGRP1 is one of two distinct guanine nuclear translocation. Upon entry into the nucleus, ERK can
exchange factors (GEFs) that mediate exchange of guanosine phosphorylate and activate several transcription factors that are
triphosphate (GTP) for guanosine diphosphate (GDP) bound critical for TCR-induced transactivation of cytokines and other
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to RAS. The other, parallel-acting factor, Son of Sevenless (SOS), activation genes. Studies in cell lines and in genetically altered
is recruited to the plasma membrane through its interaction mice attest to the central importance of RAS activation for T-cell
with the GRB2 adaptor protein. Activation via the GRB2-SOS function. Jurkat T cells expressing inhibitory mutant forms of
pathway acts as a digital “on–off” switch, whereas RasGRP1- RAS fail to produce interleukin-2 (IL-2) following TCR engage-
mediated activation can be envisioned as an analog rheostat, ment. In contrast, Jurkat cells expressing an activated form of
leading to varying degrees of downstream signal, depending on RAS produce IL-2 much more readily than do wild-type cells.
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the strength of upstream stimulus. RAS activity is tightly Similarly, mice transgenic for activating Ras mutants show