Page 208 - Clinical Immunology_ Principles and Practice ( PDFDrive )

188 PART ONE Principles of Immune Response

alterations in thymocyte development and demonstrate a partially Several hematopoietic-specific adaptors play essential roles
stimulated state in the absence of antigen binding. in T-cell development, in coordinating the signals necessary for
TCR stimulation also triggers a cascade headed by phospha- mature T-cell activation, and in the process of terminating T-cell
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tidylinositol 3′-hydroxyl kinase (PI3K) (see Fig. 12.4). PI3Ks responses. Two examples are linker of activated T cells (LAT)
are composed of two noncovalently bound subunits; the p85 and SH2 domain-containing leukocyte phosphoprotein of 76 kDa
regulatory subunit activates the kinase activity of the p110 catalytic (SLP-76). Both LAT and SLP-76 were identified during efforts
subunit. PI3K phosphorylates phosphoinositides, which play an to characterize the substrates of the PTKs stimulated by TCR
important role in the regulation of several downstream serine/ engagement.
threonine kinases, including protein kinase B (PKB) (Akt). One LAT is a 36-kDa integral membrane protein that contains
central function of Akt is promotion of the NFκB pathway numerous tyrosine residues within its cytoplasmic tail (see Fig.
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downstream of PLCγ1- and DAG-driven PKC. Although TCR 12.5A). Tyrosines exist within specific sequence motifs needed
engagement alone can stimulate some degree of PI3K function, to bind the SH2 domains of other T-cell signaling molecules.
full activity of the lipid kinase requires costimulation of the T In stimulated T cells, LAT inducibly associates with the SH2
cell through receptors, such as CD28 (see below). The outcomes domains of GRB2, GADS (Grb2 family member), PLCγ1, and
of PI3K and Akt signaling play essential diverse roles in T-cell the regulatory (p85) subunit of PI3K. It is likely that these induced
development, differentiation, and effector function, most of which intermolecular interactions are critical for communicating TCR
maximize effector T-cell activation responses. These include engagement to downstream second-messenger cascades. The
suppressing the induction of Foxp3-expressing regulatory T cells importance of LAT for T-cell activation was suggested by the
(Tregs); suppressing expression of proapoptotic molecules, signaling phenotype of LAT-deficient variants of the Jurkat T-cell
including Bim or Bad, and of cell cycle inhibitors; and promoting leukemic line. Engagement of the TCR on these cells fails to
T-cell survival via metabolic processes, including glucose uptake result in signaling events downstream of ZAP-70 activation.
and glycolysis. The importance of this pathway is suggested by Furthermore, Lat plays an essential role in T-cell development,
the efficacy of PI3K inhibitors in animal models of T cell–driven as Lat-deficient mice have significantly decreased numbers of
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autoimmunity and in current clinical trials in human hematologic thymocytes. Thymocytes that remain in Lat mice are arrested
malignancies. 13 at an early stage of development and cannot give rise to any
TCR cross-linking also leads to activation of the serine- peripheral T cells.
threonine kinase PKC-θ. PKC activation ultimately engenders Using similar approaches, it was shown that the adapter SLP-76
nuclear translocation of members of the nuclear factor-κB (NFκB) is absolutely required for both T-cell development and signaling
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family of transcription factors (see Fig. 12.4). In the basal state, via the mature TCR. SLP-76 contains an amino-terminal region
NFκB family members are sequestered in the T-cell cytoplasm with tyrosine phosphorylation sites, a central proline-rich domain,
through interaction with inhibitors of NFκB (IκB). TCR stimula- and a carboxyl-terminal SH2 domain. Unlike LAT, SLP-76 does
tion results in the formation of a multimolecular activating not possess a transmembrane domain. By means of its proline-rich
complex composed of CARMA1, BCL-10, and MALT1 (CBM). region, SLP-76 constitutively associates with the adaptor GADS.
The assembly of the complex lies upstream of signals leading to Through its SH2 domain, SLP-76 can inducibly interact with
activation of IκB kinases (IKKs), which phosphorylate and degrade other tyrosine-phosphorylated adaptors, such as HPK-1 and
IκB. Transient IκB degradation frees NFκB for translocation to ADAP. Following tyrosine phosphorylation, SLP-76 inducibly
the nucleus. Defects in T-cell activation and survival result from binds other SH2-domain-containing proteins (e.g., VAV, an
deficiencies in CBM proteins. exchange factor for the RAC GTP-binding protein; NCK, an
adaptor protein; and the Tec family kinase ITK). In mutant T-cell
INTEGRATION OF SECOND-MESSENGER lines lacking SLP-76, the PLCγ1 and RAS/MAPK signaling cascades
PATHWAYS BY ADAPTOR PROTEINS are not activated by TCR ligation, despite normal TCR-stimulated
activation of Zap-70. Mice deficient in SLP-76 show a complete
Identification and characterization of adapter proteins has block in early thymocyte development. Each of the SLP-76
provided insight into how intracellular signaling pathways are domains appears critical for its function, as the overexpression
initiated by TCR cross-linking. 15,16 Adaptors lack enzymatic or of mutant variants of SLP-76 is unable to restore TCR signaling
transcriptional regulatory activity. Instead, they possess modular efficiency fully in either Jurkat T cells or SLP-76-deficient mice.
domains responsible for subcellular relocalization and intermo- One model for how SLP-76 and LAT functionally interact
lecular interactions. Both constitutive and induced intermolecular holds that each contributes multiple molecular interactions to
interactions mediated by adaptor molecules can promote TCR a larger “signalosome” complex at the plasma membrane (see
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signal transduction. Fig. 12.5B). Following TCR engagement, the two adaptors
Adaptor proteins commonly contain modular domains that associate with each other, bridged by GADS and PLCγ1. Formation
exhibit affinity for phosphorylated tyrosine residues (Fig. 12.5A). of this complex brings Itk into proximity of PLCγ1, resulting in
Such regions include the SH2 and phosphotyrosine-binding its phosphorylation and activation and leading to the generation
(PTB) domains, which recognize phosphorylated tyrosine residues of IP3 and DAG, as described above.
within particular sequence contexts. PTB domains obtain their Other adapter proteins play important roles in restraining
specificity based on residues amino-terminal to the key phos- TCR-mediated signal transduction events, frequently by dictating
photyrosine, whereas SH2 domains recognize sequence motifs subcellular localization of regulatory enzymes. For example, in
carboxyl-terminal to phosphotyrosine. Other adaptor domains the absence of TCR stimulation, the transmembrane adaptor
include SH3 modules, which bind proline-rich regions, WW PAG binds to the cytoplasmic PTK CSK, bringing it to the plasma
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regions responsible for interactions with proline/tyrosine or membrane. Membrane-localized CSK phosphorylates the
proline/leucine motifs, and PH domains that have specificity for regulatory tyrosine of LCK, thus keeping LCK inactive in resting
phospholipids. T cells.