Article

Mammalian Target of Rapamycin Integrates Diverse Inputs To Guide the Outcome of Antigen Recognition in T Cells

Department of Oncology, Sidney Kimmel Comprehensive Cancer Center, The Johns Hopkins University School of Medicine, Baltimore, MD 21231, USA.
The Journal of Immunology (Impact Factor: 4.92). 05/2012; 188(10):4721-9. DOI: 10.4049/jimmunol.1103143
Source: PubMed
ABSTRACT
T cells must integrate a diverse array of intrinsic and extrinsic signals upon Ag recognition. Although these signals have canonically been categorized into three distinct events--Signal 1 (TCR engagement), Signal 2 (costimulation or inhibition), and Signal 3 (cytokine exposure)--it is now appreciated that many other environmental cues also dictate the outcome of T cell activation. These include nutrient availability, the presence of growth factors and stress signals, as well as chemokine exposure. Although all of these distinct inputs initiate unique signaling cascades, they also modulate the activity of the evolutionarily conserved serine/threonine kinase mammalian target of rapamycin (mTOR). Indeed, mTOR serves to integrate these diverse environmental inputs, ultimately transmitting a signaling program that determines the fate of newly activated T cells. In this review, we highlight how diverse signals from the immune microenvironment can guide the outcome of TCR activation through the activation of the mTOR pathway.

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Antigen Recognition in T Cells
Diverse Inputs To Guide the Outcome of
Mammalian Target of Rapamycin Integrates
Adam T. Waickman and Jonathan D. Powell
http://www.jimmunol.org/content/188/10/4721
doi: 10.4049/jimmunol.1103143
2012; 188:4721-4729; ;J Immunol
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Mammalian Target of Rapamycin Integrates Diverse
Inputs To Guide the Outcome of Antigen Recognition in
T Cells
Adam T. Waickman and Jonathan D. Powell
T cells must integrate a diverse array of intrinsic and
extrinsic signals upon Ag recognition. Although these
signals have canonically been categorized into three dis-
tinct events—Signal 1 (TCR engagement), Signal 2
(costimulation or inhibition), and Signal 3 (cytokine
exposure)—it is now appreciated that many other en-
vironmental cues also dictate the outcome of T cell
activation. These include nutrient availability, the pres-
ence of growth factors and stress signals, as well as
chemokine ex posure . Althoug h all of these dist inct
inputs initiate unique signaling cascades, t hey also
modulate the activity of the evolutionarily conserved
serine/threonine kinase mammalian target of rapamy-
cin (mTOR). Indeed, mTOR serves to integrate these
diverse environmental inputs, ultimately transmitting
a signaling program that determines the fate of newly
activated T cells. In this review, we highlight how
diverse signals from the immune microenvironment
can guide the outcome of TCR activation through
the activation of the mTOR pathway. The Journal
of Immunology, 2012, 188: 4721–4729.
T
he two-signal model of TCR stimulation as Signal 1
and costimulation via CD28 and other receptors as
Signal 2 has provided a useful paradigm for dissecting
the differences in stimuli leading to T cell activation versus
tolerance. Over the past two decades, it has become apparent
that the outcome of Ag recognition is not merely determined
by activation or tolerance; rather, there is plasticity of Th cells
such that TCR engagement can lead to a variety of different
CD4
+
effector phenotypes, depending on the environmental
milieu (1–5). In this regard, some have referred to cytokine
exposure as Signal 3 (6). More recently, it has become ap-
parent that other environmental cues such as nutrient avail-
ability, oxygen, growth factors, and chemokines can all make
significant contributions to molding the outcome of TCR
engagement. Although this broad range of signals can activate
a complex array of signaling pathways, one common feature
they share is an ability to modulate the activity of the evo-
lutionarily conserved serine/threonine kinase mam malian
target of rapamycin (mTOR).
In this Br ief Review, we highlight the diverse inputs that can
modulate mTOR activity in T cells and how this can subse-
quently guide the outcome of TCR engagement. In the first
part of this review, we provide a general overview of mTOR
signaling and the emerging role of mTOR in regulating T cell
activation, differentiation, and trafficking. As there have been
a number of in-depth reviews on this topic, our goal is not to
exhaustively catalog these pathways (7, 8). Rather, we hope to
provide a framework for the second part of this review that
seeks to explore the diverse inputs that can modulate mTOR
in T cells. In doing so we hope to demonstrate how: 1) known
immunologic signals mediate their effects in part by regulat-
ing the mTOR pathway; and 2) environmental cues not
previously associated with regulating T cell function may
change the outcome of Ag recognition in part through their
ability to regulate mTOR.
Overview of mTOR signaling
mTOR is a large (289 kDa), highly conserved serine/threonine
kinase initially defined as the mammalian target of the natu-
ral macrolide rapamycin (9). Although initially developed as
an antifungal antibiotic, rapamycin is a potent immunosup-
pressive agent, has been employed clinically in a wide range of
transplantation procedures, and has shown great promise in
several experimental models of autoimmunity (10–12). The
exact mechanism by which rapamycin facilitates systemic
immunosuppression is still an area of active investigation, but
the compound has been shown to influence cellular prolifer-
ation, differentiation, and cytokine secretion of cells belong-
ing to both the innate and adaptive im mune systems (7).
Department of Oncology, Sidney Kimmel Comprehensive Cancer Center, The Johns
Hopkins University School of Medicine, Baltimore, MD 21231
Received for publication November 28, 2011. Accepted for publication February 28,
2012.
This work was supported by National Institute of Allergy and Infectious Diseases Grants
R01AI077610 and R01 AI091481-01.
Address correspondence and reprint requests to Dr. Jonathan D. Powell, The Johns
Hopkins University School of Medicine, 1650 Orleans Street, CRB1 Room 443, Balti-
more, MD 21231. E-mail address: poweljo@jhmi.edu
Abbreviations used in this article: AMPK, AMP-activated protein kinase; 2-DG, 2-
deoxyglucose; GSK3b, glycogen synthetase kinase-3b; KLF2, Kruppel-like factor 2;
mTOR, mammalian target of rapamycin; mTORC, mammalian target of rapamycin
complex; PD-1, programmed death-1; PDK1, phosphoinositide-dependent kinase-1;
PD-L1, programmed death ligand-1; PIP
3
, phosphatidylinositol 3,4,5-triphosphate;
Raptor, regulatory-associated protein of mammalian target of rapamycin; REDD, reg-
ulated in the development of DNA damage response 1; Rheb, Ras homolog enriched in
brain; S1P1, sphingosine 1-phosphate receptor 1; Treg, regulatory T cell; TSC, tuberous
sclerosis complex.
Copyright Ó 2012 by The American Association of Immunologists, Inc. 0022-1767/12/$16.00
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In mammalian cells, mTOR exists as one gene but forms two
distinct protein complexes: mTOR complex (mTORC) 1 and
mTORC2, which differ in their inputs and substrates (Fig.
1) (13). mTORC1 consists of the regulatory-associated pro-
tein of mTOR (Raptor), mLST8, PRAS40, and DEPTOR.
mLST8 and DEPTOR are also found in the mTORC2
complex, with the addition of RICTOR, mSIN1 proteins,
and PROTOR (13). Upstream of the mTORC1 complex is
the small activating GTPase Ras homolog enriched in brain
(Rheb), the function of which is regulated by the GAP activity
of tuberous sclerosis complex 1 (TSC-1) and TSC-2 (14, 15).
The GAP activity of TSC-1/2 can be inhibited via phosphor-
ylation by the kinase Akt, thereby permitting the GTP-bound
form of Rheb to activate mTOR (16). The activation of Akt
is facilitated by receptor-mediated activation of PI3K, which,
through the production of phosphatidylinositol 3,4,5-triphos-
phate (PIP
3
), activates phosphoinositide-dependent kinase-1
(PDK1), which in turn activates Akt. Although the activa-
tion of AKT by PDK1 has long been thought to be critical to
the activation of mTORC-1, recent evidence has suggested
that mTORC1 can be activated in T cells independently
of AKT (17) (J.D. Powe ll, unpublished observations). Addi-
tionally, AKT-mediated inhibition of PRAS40 has been
shown the promote mTORC1 activity independently of
TSC-1/2 (18). The activity of mTORC1 is commonly as-
sessed by measuring the phosphorylation of its substrates p70
S6-kinase and 4E-BP1 (19). mTORC1 plays a critical role in
regulating mRNA translation, glucose and lipid metabolism,
mitochondrial biosynthesis, and autophagy (20–23).
Although the upstream signals that regulate mTORC1 ac-
tivity have been very well defined, identification of the precise
signals regulating mTORC2 is still an active area of investi-
gation. Recent studies have shown that mTORC2 is stron gly
and specifically activated following association with ribosomes,
whereas its kinase activity is inhibited by endoplasmic retic-
ulum stress and the glycogen synthetase kinase-3b (GSK-3b)
(24, 25). Downstream targets of mTORC2 include Akt, se-
rum and glucocorticoid-inducible kinase 1, and protein kinase
C (26, 27). It should be noted that Akt acts as both an up-
stream regulator of mTORC1 activity (as indicated by the
PI3K/PDK1-dependent phosphorylation at the T308 residue)
as well as a downstream target of mTORC2 (as indicated
by phosphorylation at S473 residue). Akt-dependent inhibi-
tion of TSC2 (upstream of mTORC1) does not require
mTORC2 (27–29).
mTOR signaling guides CD4
+
T cell fate and function. To spe-
cifically address the potential role of mTOR in CD4
+
T cell
differentiation, our group selectively knocked out mTOR in
T cells (30). Interestingly, CD4
+
T cells lacking mTOR fail to
differentiate into Th1, Th2, or Th17 effector cells when cul-
tured in appropriate conditions in vitro. Rather, the mTOR
null T cells become Foxp3
+
regulatory T cells (Tregs). The
inability of mTOR-deficient CD4
+
T cells to differentiate
toward an effector phenotype is accompanied by decreased
STAT4, STAT3, and STAT6 phosphorylation in response to
IL-12, IL-6, and IL-4, respectively (30). Pharmacological in-
hibition of mTOR signaling in naive CD4 T cells by rapa-
mycin treatment also facilitates the development of Foxp3
+
Tregs, and Foxp3
+
CD4 T cells exhibit lower levels of
mTOR activity than their effector counterparts (31–34). In-
terestingly, although genetic deletion and pharmacological
inhibition of mTOR signaling can result in the induction of
a large population of Foxp3
+
regulatory CD4 T cells in the
absence of high concentrations of exogenous cytokines, this
process is still dependent on the low levels of TGF-b found
in serum-containing media (35).
Rapamycin has classically been held to be a selective in-
hibitor of mTORC1 signaling due to its avidity in a complex
with FKBP12 for the Raptor component of mTORC1.
However, recent data indicate that prolonged exposure to
higher doses leads to inhibition of mTORC2 signaling as well
(28, 36). Therefore, it has taken recent genetic approaches
to clarify precise roles of mTORC1 and mTORC2 signaling
FIGURE 1. mTOR signaling. The figure depicts a
generalized scheme of mTOR signaling for reference.
Environmental cues, such as TCR stimulation, cytokine
signaling and nutrient availability, stimulate the activ-
ity of PI3K, inducing the phosphorylation of Akt at the
T308 residue and leading to the subsequent inhibi-
tion of TSC1/2. This results in the activation of the
small GTPase Rheb, which promotes the activation of
mTORC1 and the downstream phosphorylation of S6-
kinase and 4E-BP1. In most cell types examined, acti-
vation of these factors results in the enhancement of
protein synthesis, mitochondria biogenesis, and glucose/
lipid metabolism. The events leading to the activation of
mTORC2 have yet to be precisely determined, although
recent work suggests that association with ribosomes
promotes activation. Downstream, mTORC2 signaling
phosphorylates Akt at the S473 residue as well as serum
glucocorticoid kinase-1 and protein kinase C. mTORC2
activation has been shown to play role in promoting
transcription and regulating cell survival and actin reor-
ganization. Green arrows, activation; red lines, inhibition.
4722 BRIEF REVIEWS: mTOR IN T CELLS
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in T cell effector function. Selectively deletion of Rheb in
T cells specifically inhibits mTORC1 activity but maintains
mTORC2 activity (28). As was the case with the mTOR
null T cells, Rheb null T cells fail to become Th1 and Th17
cells when activated under appropriate culture conditions.
However, somewhat surprisingly, the Rheb null T cells still
maintain the ability to differentiate into Th2 cells. Con-
versely, examination of T cells lacking mTORC2 activity via
selective deletion of Rictor reve als that Rictor null T cells fail
to become Th2 cells in response to IL-4 but, unlike the Rheb
null T cells, Rictor null T cells still maintain the ability to
become Th1 and Th17 cells. Another group has also condi-
tionally deleted Rictor in T cells using a different Cre trans-
gene and likewise observed these cells fail to become Th2
cells, but interestingly, this was accompanied by a decrease in
Th1 differentiation as well in this system (29). Importantly,
elimination of either mTORC1 or mTORC2 signaling alone
in T cells did not lead