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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
2012; 188:4721-4729; ;
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The Journal of Immunology
by guest on June 13, 2013
Mammalian Target of Rapamycin Integrates Diverse
Inputs To Guide the Outcome of Antigen Recognition in
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 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 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.
of Immunology, 2012, 188: 4721–4729.
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
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
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 mammalian
target of rapamycin (mTOR).
In this Brief 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 immune 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,
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: email@example.com
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; PIP3, 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
by guest on June 13, 2013
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 (PIP3), 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. Powell, 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 strongly
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
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
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.
mTOR signaling. The figure depicts a
4722 BRIEF REVIEWS: mTOR IN T CELLS
by guest on June 13, 2013
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 reveals 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 to the spontaneous generation of Tregs
following activation under non-Treg culture conditions (as
was seen from mTOR null T cells lacking both mTORC1
and mTORC2). These observations support the view that
inhibition of both mTORC1 and mTORC2 is necessary to
promote generation of Foxp3+T regulatory cells. Such data
suggest that the new class of mTOR kinase inhibitors (that
simultaneously inhibit mTORC1 and mTORC2 activation)
might prove to be potent immunosuppressive agents (37).
These data lead us to propose a model in which mTOR
integrates diverse inputs to coordinate the downstream sig-
outcome of Ag recognition. For example, in addition to di-
rectly regulating IL-12–induced STAT4 activation, mTORC1
also regulates the activity of the glycolytic machinery (38).
Normal T cell activation has been shown to rely heavily on
oxidative glycolysis (39, 40). By standing at the crossroads of
these many critical signals for the activated T cell, mTOR
may serve as a biochemical traffic cop to coordinate the de-
velopment of effector T cells.
Inhibition of mTOR regulates CD8+memory T cell development.
CD8+T cell Ag recognition leads to a marked increase in pro-
liferation along with a switch from catabolism to anabolism and
CD8+effector generation requires increased protein synthesis;
thus, it is not surprising that Ag recognition in CD8+T cells
leads to both mTOR- and MAPK signaling-induced S6 phos-
phorylation (42). If this is blocked by inhibition of mTOR, the
consequence is actually promotion of memory CD8+T cell
generation. In a lymphocytic choriomeningitis virus model, it
was shown that low-dose rapamycin treatment during infection
promotes the generation of memory T cells (43). Similarly, long-
lived memory cells could be generated by culturing lymphocytic
choriomeningitis virus-specific T cells with rapamycin and then
adoptively transferring them into mice (44). Rao and colleagues
(45) were able to demonstrate that treating CD8+T cells with
rapamycin promoted memory generation in part by inhibiting
Likewise, in a model of homeostatic proliferation-induced mem-
ory, this group was able to show that blocking mTOR with
rapamycin abrogated the need for IL-15 signaling in upregu-
lating eomesodermin and thus promoting memory (46).
Metabolically, rapamycin-treated CD8+T cells demonstrate
an increase in oxidative phosphorylation (44). Along these
lines, Pearce et al. (47) observed that when TNFR-associated
factor 6 was specifically deleted in T cells, CD8+memory cell
generation was markedly impaired. The failure of the effector
cells to transition into memory cells was associated with an
inability to switch to catabolism relating to fatty acid oxida-
tion. Based on these observations, they went on to show that
activating AMP-activated protein kinase (AMPK) with met-
formin or inhibiting mTOR with rapamycin led to an in-
crease in fatty acid oxidation and a consequent increase in
mTOR regulates T cell trafficking. The ability of naive T cells
to circulate through secondary lymphoid tissue is facilitated
by the expression of a number of cell-surface receptors,
including CD62L and the chemokine receptor CCR7 (48).
Mechanistically, the expression of CD62L, CCR7, and the
memory marker IL-7Rb (CD127) has been linked to the
FOXO family of transcription factors and Kruppel-like fac-
tor 2 (KLF2) (49, 50). mTORC2 activation of Akt, inhibits
activation of the FOXOs, leading to decreased KLF2 ex-
pression (51, 52). Because KLF2 positively regulates the tran-
scription of these trafficking molecules, the expression of
CD62L and CCR7 declines upon mTOR and Akt activa-
tion. In this regard, a critical role for Akt in the regulation
of CD8+T cell trafficking has been described (17). Likewise,
the G protein-coupled receptor sphingosine 1-phosphate
receptor 1 (S1P1) is also regulated by KLF2 (53). S1P1
plays a critical role in promoting T cell egress from lymph
nodes (54). Mechanistically, the regulation of these homing
molecules by mTOR serves to coordinate activation status
with trafficking out of lymphoid tissues.
Modulation of mTOR activity in T cells
As shown above, a critical role is emerging for mTOR in in-
tegrating signals and regulating the outcome of Ag recognition
in T cells. In the second part of this review, we highlight the
diverse array of environmental cues that can regulate mTOR in
T cells. By examining these inputs, summarized in Table I, two
interesting themes emerge. First, it is clear that a number
of well-established immunologic mediators, CD28 and pro-
grammed death-1 (PD-1), for example, exert their effects in
part by regulating mTOR activity. Second, there is mounting
evidence that nutrient availability and metabolic regulators
play a critical role in directing T cell differentiation and func-
tion in part by their ability to regulate mTOR (51, 55).
Surface receptors and ligands. The specificity of the TCR cannot
distinguish between self- or pathogen-associated peptides.
However, the concentration of the peptide presented by an
APC, as well as the affinity of the peptide–TCR interaction,
can convey biochemical information that can influence the
outcome of Ag recognition. For example, lower affinity or
altered peptide ligands can lead to the induction of T cell
anergy (56). Likewise, it has been shown that low doses of
peptide can promote Th2 responses in the absence of skewing
cytokines and very low doses of peptide can promote the
generation of Foxp3+Tregs (57, 58).
PI3K activation is downstream of TCR engagement, and
thus, Ag recognition can in fact lead to mTOR activation (59).
However, when compared with mTOR activation induced by
CD28 engagement, TCR-induced mTOR activity is relatively
weak and short-lived. Nonetheless, the modulation of PI3K,
and hence mTOR via the strength of TCR stimulation can
The Journal of Immunology4723
by guest on June 13, 2013
result in functional consequences. For example, the ability of
low-dose Ag to induce Foxp3+T cells has been attributed in
part to weak TCR-induced mTOR activity (35, 58). This is
particularly prominent when immature dendritic cells are
used as APCs (60). Similarly, it has been shown that pre-
mature termination of TCR engagement promotes Foxp3+
expression due to antagonized PI3K–mTOR signaling (34).
Katzman et al. (61) have been able to correlate the duration of
TCR signaling with the induction of T cell activation or
tolerance. In their model, short-lived T cell–APC interactions
leading to tolerance are correlated with decreased mTOR
Although it is now clear that the Signal 2 is in reality com-
prised of multiple ligand receptor interactions, perhaps the
best-described costimulatory signal on T cells is the interaction
between CD28 and its two known ligands B7.1 and B7.2.
CD28 facilitates the nuclear translocation of NF-kB and en-
hances transcription and translation of IL-2 (62). Thus, one
means CD28 ligation can promote mTOR activity is in an
autocrine fashion through IL-2 signaling. The ligation of
CD28 on an activated T cell can also directly activate PI3K.
PI3K binds the phosphorylated cytoplasmic tail of CD28 at
a conserved YMNM motif and mediates Akt activation (63).
Ab-mediated ligation of CD28 can induce Akt activity in-
dependently of TCR stimulation (64), and constitutively ac-
tive Akt can overcome the inability of CD28-deficient cells to
secrete IL-2 but cannot restore their proliferative capacity
The sustained activation of PI3K and mTOR resulting from
CD28 activation has been shown to promote proliferation in
T cells independently of IL-2 production (65). This is the
consequence of optimal expression of cyclin D3 and down-
regulation of the cell-cycle inhibitor p27 (66). In addition
to T cell activation, CD28-mediated costimulation plays an
important role in enhancing glycolysis and glucose uptake
(67). This process has been shown to be dependent on PI3K/
Akt signaling and involves the rapid upregulation in expres-
sion of the Glut1 glucose transporter (67).
The costimulatory signal provided by CD28 ligation on
naive T cells is important for the initiation of a T cell response,
but additional receptor–ligand interactions can also provide
a costimulatory signal and fine-tune the T cell activation
profile at the time of initial activation. The ICOS/ICOS li-
gand interaction is a potent inducer of PI3K activation. In
Table I.Immunological and environmental signals known to modulate mTOR activity
Input Mode of ActionReferences
TCR Activates PI3K in a dose-dependent fashion, leading to T cell activation and modulating naive
T cell differentiation
CD28 Activates PI3K to a much greater degree than TCR stimulation alone, facilitating T cell
proliferation and cytokine production
64, 65, 66, 67
ICOS Robustly activates PI3K via cytoplasmic tail68
OX40Facilitates T cell memory generation and cytokine secretion by activating PI3K and AKT 70, 71, 72, 73
CTLA-4 Ligation inhibits mTOR activity via PP2a-dependent dephosphorylation of AKT; alternatively,
has been shown to directly activate PI3K
74, 75, 76
PD-1 Induces expression of the phosphatase PTEN, facilitating the degradation of PIP3
74, 77, 78, 81
IL-1 Induces mTOR activity, facilitating the development of Th17 CD4 T cells91
IL-2Activates PI3K, facilitating cell-cycle progression and proliferation, while inhibiting the
induction of T cell anergy
IL-7Regulates T cell size and metabolism downstream of PI3K/AKT/mTOR 89, 90
IL-12 Prolongs TCR-induced mTOR activity, resulting in sustained T-bet expression45
Type I IFNsActivates PI3K following the IRS1/2-dependent recruitment of the p85 PI3K regulatory subunit 92, 93
Type II IFNs Induces mTOR activity by activating PI3K and downstream AKT94
G-protein coupled receptor-dependent activation of PI3K 98, 99, 100
LeptinReceptor stimulation induces PI3K activity, providing an antiapoptotic signal and inducing
effector T cell proliferation and Th1/Th17 cytokine production; plays an oscillatory role in
regulating Foxp3+Treg function
101, 102, 103, 104
S1P1 Receptor stimulation induces mTOR activity, facilitating Th1 CD4 T cell differentiation while
inhibiting Foxp3+Treg development
WNTInteraction of WNT with its receptor inhibits GSK3, thereby inhibiting the GAP activity of
TSC1/2 and leading to mTORC1 activation via GTP-bound Rheb
Low glucoseLeads to a decrease in the intracellular ATP/AMP ratio, thereby activating AMPK and resulting
in the activation of TSC1/2, the phosphorylation of Raptor, and inhibition of mTOR kinase
activity; by inhibiting mTOR activity, low levels of glucose promote T cell anergy
109, 112, 113
Leads to the activation of REDD1, thereby stabilizing TSC1/2 and inhibiting Rheb-dependent
115, 116, 117
Low amino acidDecreases mTORC1 activity by inhibiting Rheb localization; by inhibiting mTOR activity, low
levels of amino acids can induces T cell anergy
109, 122, 123, 124
IRS, Insulin receptor substrate; PTEN, phosphatase and tensin homolog.
4724 BRIEF REVIEWS: mTOR IN T CELLS
by guest on June 13, 2013
fact, studies suggest that the direct binding of PI3K to the
conserved YMFM motif on the ICOS cytoplasmic tail leads
to more robust activation than that induced by CD28 en-
gagement (68). Detailed studies examining the role of ICOS
on regulating mTOR activity in T cells have yet to be per-
formed. However, given the prominent role that ICOS plays
in PI3K activation in T cells, one would predict that ICOS
will also play an important role in regulating mTOR.
The surface receptor OX40 (CD134) has recently gained
recognition as a potent costimulatory molecule that comple-
ments the activity of CD28 and ICOS. A member of the
TNF-a receptor superfamily, OX40 expression is strongly,
though transiently, induced following TCR stimulation in
both CD4 and CD8 T cells, peaking in expression 48 h after
stimulation and returning to baseline by 120 h (69). Ligation
of this receptor, either through interaction with its APC-
restricted ligand OX40L (CD252) or by Ab-mediated cross-
linking, facilitates increased clonal proliferation, cell survival,
cytokine secretion, and memory generation (70). In part,
these effects are mediated by the ability of OX40 to stimulate
the activity of PI3K activity, thereby promoting AKT activity
upstream of mTOR (71–73). Interestingly, ligation of OX40
on the surface of naive T cells facilitates the generation and
proliferation of Foxp3+Tregs. However, regulatory cells
generated under OX40 stimulation are poorly suppressive and
display an exhausted phenotype, which can be reversed with
IL-2 treatment (71).
CTLA-4 is an inhibitory member of the CD28 receptor
inhibiting IL-2 production and hence autocrine IL-2–induced
mTOR activity. From a signaling perspective, the mechanism
by which CTLA-4 ligation inhibits T cell activation is complex
and incompletely understood. Ligation of CTLA-4 on the
surface of T cells following TCR/CD28 stimulation does not
result in a reduction in PI3K activity, but does reduce Akt
phosphorylation in a process that appears to be dependent on
the phosphatase PP2a (74, 75). However, other studies have
shown that CTLA-4 ligation induces PI3K and Akt activation
that in turn inhibits apoptosis and thus sustains T cell anergy
while simultaneously preventing cell death (76).
The surface receptor PD-1, and its associated ligands pro-
grammed death ligand-1 (PD-L1; B7-H1) and PD-L2 (B7-
DC), provide another inhibitory counterbalance to the co-
stimulatory signals induced by the interaction of CD28 and
its ligands B7.1/B7.2 or ICOS with ICOS ligand (77, 78).
As is the case for CTLA-4, the mechanism by which PD-1
modulates T cell activation, effector differentiation, and the
development of Tregs is multifaceted and incompletely un-
derstood. Association of SHP-1 and/or -2 to the immuno-
receptor tyrosine-based switch motif of the cytoplasmic tails
of PD-1 can directly antagonize TCR-induced phosphoryla-
tion of ZAP70 (79, 80). The exposure of CD4+T cells to PD-
L1–coated microbeads has been shown to result in an increase
in expression of the phosphatase PTEN, which antagonizes
PI3K/mTOR function by facilitating the degradation of PIP3
(81, 82). PD-1 can inhibit the PI3K–Akt axis by preventing
CD28-mediated activation of PI3K (74). Additionally, it has
been shown that the ability of PD-1/PD-L1 interaction to
promote the development, maintenance, and function of in-
ducible Tregs is dependent upon the inhibition of mTOR
(81). That is, the ability of PD-L1 to promote inducible Tregs
is mediated through the downregulation of the Akt–mTOR
Cytokines/IFNs/chemokines. mTOR signaling plays a role in
regulating the downstream consequences of a number of
immunologically relevant cytokines. Early studies identified
mTOR activity as being increased upon IL-2–induced stim-
ulation (83). IL-2–induced mTOR activation was shown to
be important for facilitating cell-cycle progression and prolif-
eration (83). These observations led to a series of studies
examining the ability of mTOR to regulate T cell anergy
(65, 84–86). It has been shown that the ability of IL-2 to
both prevent and reverse T cell anergy is dependent upon
mTOR activation (85, 87). Other common g-chain cytokine
receptors also activate mTOR. Like IL-2, IL-4R signaling is
another potent inducer of T cell proliferation but has the
added ability to skew naive CD4 T cell to a Th2 phenotype.
The cytoplasmic tail of the IL-4R possesses five evolutionarily
conserved tyrosine residues that have been shown to differ-
entially regulate STAT5 and PI3K activity (88). The loss of
the Y1 residue inhibits the ability of IL-4 treatment to induce
PI3K activity and downstream mTOR activation, but leaves
intact the ability of IL-4 to induce STAT5 and STAT6 phos-
phorylation (88). The ability of the IL-4R to induce STAT5/6
activity appears to be dependent on the Y2-4 residues on
the cytoplasmic tail and acts independently of PI3K/mTOR
The IL-7R also activates the PI3K/Akt/mTOR axis (89).
IL-7 plays an important role in maintaining T cell metabo-
lism and survival. Interestingly, it has been shown that the
ability of IL-7 to promote Bcl2 expression is mTOR inde-
pendent (90). In contrast, IL-7R–induced increases in size
and glucose metabolism are dependent on mTOR signaling.
IL-1R–dependent mTOR activation has recently been
shown to be indispensible for the generation and prolifera-
tion of Th17 CD4 T cells (91). Gulen et al. (91) have dem-
onstrated that Th17 differentiation induces the expression
of SIGIRR, a negative regulator of IL-1 signaling that acts
as a damper to continued IL-17 secretion. The deletion
of SIGIRR results in an increase in IL-17 production under
Th17 culture conditions and a corresponding increase in
mTOR activity. Importantly, the T cell-specific deletion of
mTOR negates the ability of IL-1 treatment to enhance Th17
proliferation. With regard to CD8+effector generation, IL-12
has been shown to prolong mTOR activation upon stimula-
tion (45). This in turn leads to an increase in T-bet expres-
sion. Likewise, both type I and type II IFNs have been shown
to induce mTOR activity via PI3K activation (92–94). Stim-
ulation of type I IFN receptors results in the rapid phosphor-
ylation of insulin receptor substrates 1/2, resulting in the re-
cruitment of the p85 regulatory subunit of PI3K and the
induction of downstream Akt and mTOR activity.
Chemokine receptors regulate cellular migration primarily
through the b/g subunits of the G-protein coupled receptor’s
activation of PLCg2/g3 and PI3K (95, 96). The link to
mTOR was made by the observation that the addition of
rapamycin can inhibit the migration of neutrophils in re-
sponse to GM-CSF, as well as smooth muscle cells in response
to fibronectin (97, 98). Subsequently, it has been shown that
many G-protein coupled chemokine receptors rely on mTOR
signaling for at least some aspects of their migratory effects.
Naive T cells use mTOR signaling to respond to CXCL12
The Journal of Immunology4725
by guest on June 13, 2013
stimulation (99). For activated Th1/Th2 CD4 T cells,
mTOR activity is required for CCR5/CCL5 (RANTES)-
mediated migration that is dependent upon 4EBP-mediated
translation (100). However, not all chemokines depend on
mTOR activation. For example, mTOR signaling is dis-
pensable for CCL19 (Mip3b)-mediated migration (99).
Although best known for regulating appetite and energy
expenditure, the adipokine leptin also plays a significant role
in regulating the functionality and proliferative capacity of
T cells through its ability to stimulate mTOR activity (101–
103). In the absence of leptin receptor stimulation, autore-
active CD4+T cells exhibit decreased expression of the anti-
apoptotic factor Bcl2, an impaired ability to skew to a Th1/
Th17 phenotype, and a failure to upregulate mTOR activity
(103). Further, leptin acting via the mTOR signaling pathway
has been shown to provide a link between energy status and
Treg function (104).
The lysophospholipid S1P is another potent inducer of
mTOR activity in T cells via its G-protein coupled receptor
S1P1 (105, 106). Although S1P1 signaling is able to induce
mTOR activity in T cells, the receptor facilitates its own
downregulation due to the ability of mTORC2 activity to
suppress the activity of the transcription factor KLF2 (48,
107). S1P1 signaling has canonically been thought to regulate
T cell migration from the thymus and secondary lymphoid
organs (54). However, it has recently been recognized that
S1P1-dependent modulation of mTOR activity plays a criti-
cal role in regulating CD4+T cell differentiation and the
functionality of Tregs (105, 106). Overexpression of S1P1 in
CD4+T cells facilitates the development of Th1-polarized
cells while inhibiting Foxp3+Treg development in an
mTOR-dependent process. Conversely, the deletion of the
S1P1 receptor facilitates Treg development and enhances their
suppressive capacity (105).
Regulation of mTOR by nutrients, energy, and stress. Lack of
nutrients or oxygen deprivation all lead to the inhibition of
mTOR activity (23). A cell normally maintains a very high
intracellular ATP to AMP ratio. Increased AMP activates
increasing its GAP activity and decreasing Rheb-dependent
mTORC-1 activity. In addition, activation of AMPK can
result in the direct phosphorylation of Raptor, inhibiting
mTORC1 activity in a TSC1/2-independent fashion (108).
Pharmacologic activation of AMPK by AICAR inhibits T cell
function and has been shown to block the induction of experi-
mental autoimmune encephalomyelitis and promote anergy by
inhibiting mTOR (109–111). Likewise, activation of AMPK,
by the glucose analog 2-deoxyglucose (2-DG), leads to the
inhibition of mTOR (109, 112, 113). 2-DG is readily taken
up by T cells via the GLUT-1 transporter; however, 2-DG–6-
phosphate cannot be processed further by the cellular glycolytic
machinery and therefore competitively inhibits the process
of glycolysis. Given the well-defined role for mTOR inhib-
ition in facilitating the development of memory T cells, one
might hypothesize that many of the clinically approved AMPK
agonists, such as metformin, may turn out to facilitate the
generation of memory T cells.
The phosphorylation and activation of the GAP activity of
TSC can also be promoted by GSK-3b (114). This is a mech-
anism of action analogous to that observed for AMPK-mediated
mTOR inhibition, and it appears that the phosphorylation of
TSC2 by GSK-3b is dependent on prior phosphorylation of
the substrate by AMPK at the S1345 residue (114). The in-
teraction of Wnt with its receptor on the plasma membrane of
most mammalian cells inhibits the activity of GSK-3b. As such,
Wnt signaling can promote mTORC1 activity.
Low oxygen tension (as might be experienced in a tumor
microenvironment) can also regulate mTOR activity. It has
been shown that in the setting of low oxygen, the hypoxia-
induced factor protein regulated in the development of
DNA damage response 1 (REDD1) can inhibit mTOR by
promoting the assembly and activation of TSC (115). Cells
lacking REDD1 show continued mTOR activity even under
conditions of nutrient withdrawal (116), whereas hypoxia
facilitates the REDD1-mediated activation of the TSC1/2 by
facilitating the stabilization of TSC1/2 by 14-3-3 protein
(117). Although hypoxia is a potent regulator of mTOR ac-
tivity, mTORC-1 regulates the expression of the canonical
hypoxia response element hypoxia inducible factor-1 (118,
119). Hypoxia inducible factor-1 expression has recently been
shown to facilitate the development of Th17 CD4 T cells via
the formation transcriptionally active complex with RORgT
and the induction of a highly glycolytic metabolic phenotype
while simultaneously inhibiting the development of Tregs by
facilitating the degradation of Foxp3 (120, 121).
Availability of amino acids also regulates mTOR activity.
Specifically, branch chain amino acids such as leucine promote
mTOR activity (122). This is accomplished by promoting
the interaction between Rheb and mTORC1. The ability
of branch chain amino acids to activate mTOR has immu-
nologic consequences. For example, Tregs can facilitate the
generation of infectious tolerance in part by depleting branch
chain amino acids, leading to mTOR inhibition and further
Treg generation (123). Likewise, it has been shown that the
leucine analog NALA can inhibit T cell function, and TCR
engagement in the presence of NALA promotes T cell anergy
by inhibiting mTOR (109, 124)
Although the two-signal model provides a framework for
understanding the generation of the adaptive immune re-
sponse, it is clear that the inputs that influence the outcome of
Ag recognition are varied and complex. Likewise, there is a
greater appreciation for the diversity of outcomes upon TCR
engagement. In this regard, mTOR has emerged as a critical
integrator of environmental cues in T cells. Concomitant with
our increasing appreciation for mTOR to influence T cell
activation, differentiation, and tolerance is a greater appreci-
ation for the diversity of environmental inputs that can in-
fluence these processes by regulating mTOR. In a number of
cases (for example CTLA-4), a connection between receptor
ligand interaction and PI3K signaling has been made but the
precise connection to downstream mTOR signaling has yet to
be defined. Nonetheless, the role of a diversity of inputs in
regulating mTOR and the increasing role of mTOR in reg-
ulating T cell function suggest that these pathways may prove
and enhancing T cell responses.
We thank Emily Heikamp, Sam Collins, and Kristen Pollizzi for suggestions
and Christopher Gamper for invaluable editorial assistance.
4726 BRIEF REVIEWS: mTOR IN T CELLS
by guest on June 13, 2013
The authors have no financial conflicts of interest.
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