IL-2 Receptor

12 Pages·0.112 MB·English

by  Smith K.A.

IL-2 Receptor
Kendall A. Smith
*
Division of Immunology, Cornell University Medical College, 525 East 68th Street Box 41,
New York, NY 10021, USA
*corresponding author tel: [phone], fax: [phone], e-mail: [email protected]
DOI: 10.1006/rwcy.2000.14001.
SUMMARY TcellspasttherestrictionpointintheG phaseofthe
1
cell cycle, which ultimately depends upon the
The interleukin 2 receptor (IL-2R) was the first concentrations of second messengers generated at
cytokine receptor to be discovered, characterized and thecytoplasmicdomainsofthe(cid:12) and(cid:13) chains.There
cloned. Therefore, the IL-2R has served as a are three known second messenger pathways acti-
prototypic cytokine receptor, with which all subse- vated: the JAK/STAT, the Ras/Raf/MAPK, and the
quent cytokine receptors have been compared and PI-3kinase/Aktpathways.Ultimatelythesepathways
contrasted. The discovery of the IL-2R depended convergeontranscriptionfactors,whichstimulatethe
upon demonstrating that purified radiolabeled IL-2 expression of specific genes that mediate the char-
binds to IL-2-responsive cells with all of the char- acteristic biological responses of cell proliferation,
acteristics of true hormone receptors, i.e. high survival, differentiation and activation-induced cell
affinity, specificity, and saturability. As well, the death.Someofthesegenesareknown,butmanyhave
IL-2 concentrations that bind to the receptor are yet to be discovered.
identical to the IL-2 concentrations that promote the The main physiologic roles of the IL-2Rs are to
characteristic IL-2 response, i.e. T cell proliferation. promote the proliferative expansion of T cells and
The high-affinity IL-2R, which is expressed by NK cells upon activation. In particular, the CD8+
antigen-activated T cells, binds IL-2 with an T cells are very dependent upon signals from the
equilibrium dissociation constant (K ) of 10 pM, IL-2R to be able to respond to antigenic stimulation
d
and comprises three separate noncovalently linked maximally. In addition, the IL-2R is responsible for
type I transmembrane proteins, designated (cid:11), (cid:12), and conveying survival signals to antigen-activated cells,
(cid:13). The intermediate-affinity IL-2R (K =1 nM) thereby promoting the persistence of antigen-selected
d
comprises only the (cid:12) and (cid:13) chains, and is expressed memory T cells. Accordingly, upon disappearance of
by 90% of natural killer (NK) cells. The low-affinity antigen, and the consequent diminution of the IL-2
IL-2R(K =10 nM)iscomposedofisolated(cid:11)chains, concentration available, cells that were activated
d
and is not found on any normal cells, but has been undergo cytokine withdrawal cell death. There are
detected on some human leukemia cells. The (cid:12) and (cid:13) alsoimportantnegativefeedbacksignalsimpartedvia
chains cooperate to signal the interior of the cell by the IL-2R that function to promote the return of
activating two tyrosine-specific kinases termed JAK1 activatedcellstoaquiescentstate,therebypromoting
and JAK3. In turn, these kinases phosphorylate homeostasis of the immune system after it has
specific tyrosines on the (cid:12) and (cid:13) chains of the IL-2R, successfully responded to an antigen. Because of
which then serve as docking sites for downstream these critical functions of the IL-2/IL-2R system, if
effector molecules. The receptor functions as an ‘on– there is a deficiency of signaling via the IL-2R, either
off’ switch which is regulated by IL-2 binding. produced pharmacologically or genetically, there are
Subsequent to IL-2 binding the receptor is switched profound phenotypes generated. Deficiency of the (cid:11)
‘on’. Upon dissociation of IL-2 from the receptor the and (cid:12) chains result in normal numbers of T cells in
signal to the JAKS is extinguished. the thymus and in the periphery, but there are
There are a finite number of IL-2/IL-2R interac- activation defects, so that immunodeficiency results
tions that are necessary to promote movement of the early, followed by the accumulation of cells with an
1460 Kendall A. Smith
activated phenotype and autoantibodies later. wastargetcellspecificity,inthatonlyIL-2-responsive
Deficiency of the (cid:13) chain results in severe combined cells, e.g. mitogen/antigen-activated T cells, had
immunodeficiency. Given these findings, it is not adsorptive capacity. In particular, resting T cells,
surprising that agents that block the IL-2/IL-2R and lipopolysaccharide (LPS)-activated B cells did
interaction are potent immunosuppressives. not adsorb IL-2 (Smith, 1980).
Toproceedbeyondthesedescriptiveexperiments,it
was necessary to purify and radiolabel IL-2. Initially,
we accomplished this painstakingly using standard
BACKGROUND
biochemical techniques, such as gel filtration and
isoelectricfocusing(Robbetal.,1981).Subsequently,
Discovery
we developed the first IL-2-reactive monoclonal
antibodies that could be used as affinity adsorbents,
Like the discovery of interleukin 2 (IL-2), the which enabled the purification of large amounts, i.e.
discovery of the IL-2 receptor (IL-2R) proceeded in milligrams,ofhomogeneousIL-2(Smithetal.,1983).
three phases: the discovery of the activity, the To ensure that the molecules were not denatured
discovery of the molecules, and the discovery of the during the radiolabeling procedures we first used
genes encoding the receptor molecules. radiolabeled amino acids, such as [35S]Met, [3H]Lys,
and[3H]Leu.Thus,biosyntheticlabelingensuredthat
the molecules were not damaged in the labeling
IL-2R Activity
process. Later, we switched to external labeling with
Classic receptors described by endocrinologists and 125I,whichfacilitatedthebindingexperiments,inthat
pharmacologists have two distinct activities: (1) they ithasahigherenergysothatahigherspecificactivity
bind the appropriate ligand with high specificity and could be attained.
affinity in a saturable fashion, and (2) they signal the Using techniques that we had already perfected to
cellortissueatligandconcentrationsthatarerelevant perform radiolabeled glucocorticoid (Smith et al.,
physiologically. 1977)andFcreceptor-bindingassays(Crabtreeetal.,
At the time that IL-2 activity was first described in 1979), the very first experiments we performed with
the mid-1970s, only a few classic receptors had been radiolabeled IL-2 were definitive. IL-2 binds with
discovered. Thus, the receptors for insulin, epidermal high affinity (K =5–10pM) in a saturable fashion to
d
growth factor (EGF), and nerve growth factor had IL-2-responsive cells, whether the cytolytic T lym-
been identified and characterized as cell surface phocyte lines (CTLLs), or later mitogen/antigen-
molecules that were capable of binding their radio- activatednormalTcells(Robbetal.,1981).Justasin
labeledligands.Aswell,thesteroidhormonereceptors, our adsorptive experiments, IL-2-unresponsive cells,
e.g. for estrogen and glucocorticoids, had been suchasrestingTcellsorLPS-activatedBcellshadno
identified via similar methods, but were found to be detectable IL-2 binding. Most important from the
intracellular receptors. standpoint of receptor definitions, the IL-2 concen-
Theseexperimentsestablishedtheprinciplesforthe trations that bound to the cells were identical to the
recognition of receptors, and also established the concentrationsthatpromotedTcellproliferation,i.e.
methods of radiolabeled ligand-binding assays neces- 50% effective concentration (EC )=5–10pM. Thus,
50
sarytodemonstratetruereceptoractivities.However, the IL-2 binding and biological response curves are
before one could contemplate these approaches coincident, and there are no ‘spare receptors’.
applied to IL-2, it was necessary to obtain pure, These experiments established the validity of true
homogeneous, native IL-2 and to radiolabel the hormone-like receptors as responsible for mediating
molecules so that binding assays could be performed. cytokine effects, and the IL-2-binding assay became
Before we had purified IL-2, the IL-2 bioassay prototypic for the cytokine field.
(Gillis et al., 1978) was the only method we had
available to detect it. Therefore, to provide initial
IL-2R Molecules: the (cid:11) Chain, the (cid:12) Chain, and
data regarding the IL-2R, we performed adsorption
the (cid:13) Chain
experiments that were patterned after adsorption
experiments that we had performed previously with Some receptor molecules are responsible for both
erythropoietin and erythropoietin-responsive cells receptoractivities,i.e.theligand-bindingactivitiesand
(Fredrickson et al., 1977). Using the IL-2-dependent the signaling activities. In the case of the IL-2R, and
T cell clones (Baker et al., 1979), we found that IL-2 most of the cytokine receptors, separate molecules
activity was adsorbed in a time-, temperature-, and function to bind the ligands versus those that signal
cellconcentration-dependentmanner.Moreover,there the cell. Consequently, the discovery of all of these
IL-2 Receptor 1461
molecules has taken a long time, more than 15 years, that IL-2 has a very rapid association rate with the
and has involved many investigators. Soon after we (cid:11) chain, approximately 100-fold faster than IL-2
reported the discovery of IL-2R activity, Warren bindingtothe(cid:12) chain.However,thedissociationrate
Leonard, Warner Greene, and Tom Waldmann of IL-2 from the (cid:11) chain is also very fast, while its
contacted us. Takashi Uchiyama, working in their dissociationfromthe(cid:12)chainisslow.Therefore,when
laboratory, had generated monoclonal antibodies (cid:11) and (cid:12) chains are expressed together on the cell
(mAbs) that reacted with leukemic cells derived from surface a very efficient receptor is created, with a fast
patients with adult T cell leukemia (ATL) (Uchiyama association rate contributed by the (cid:11) chain, and a
et al., 1981). We had already found that these slow dissociation rate contributed by the (cid:12) chain.
ATL cells had high levels of IL-2Rs. Moreover, the These experiments appeared to solidify the struc-
mAbs recognized activated T cells but not resting T ture of the IL-2R as a heterodimer. However, when
cells,hencethename,anti-Tac,todesignateactivated the cDNA encoding the (cid:12) chain was isolated a few
T cells. The very first experiments that we performed years later (Hatakeyama et al., 1989) and cells were
with these mAbs were clear-cut. Anti-Tac competed constructed to express both the (cid:11) and (cid:12) chains, it
for IL-2 binding and IL-2-promoted T cell prolifera- became obvious that these two chains formed a
tion in a concentration-dependent fashion (Leonard receptor that only had a ‘pseudo high-affinity’
et al., 1982). binding site for IL-2. Thus, the K =100 pM rather
d
Anti-Tac precipitated a 55kDa molecule from the than 10 pM. Accordingly, the search was on for a
surfaceoftheATLcellsandfromactivatednormalT putative third IL-2R chain.
cells.However,soonthereafter,RichardRobb(Robb Kazuo Sugamura and his group from Sendai won
et al., 1984) found evidence for two distinct IL-2- the race to identify the (cid:13) chain of the IL-2R
bindingsitesonnormalTcells,andwenotedthatnot (Takeshita et al., 1992b). They did so by careful
all anti-Tac+ cells could bind radiolabeled IL-2 with biochemical approaches aided by a panel of mono-
the same affinity. Some cell lines appeared to have clonal antibodies reactive with the (cid:12) chain that they
two distinct binding sites, one with a high affinity had generated. They found that they were able to
similar to activated normal T cells, and another precipitate a (cid:24)65 kDa chain together with the
binding moiety that had a 100-fold lower affinity. 55 kDa (cid:11) chain and the 75 kDa (cid:12) chain from the
Subsequently, we identified cell lines that were cell surface, provided that they first chemically
capable of binding IL-2, but did not react with anti- crosslinked IL-2 to the IL-2R. Because the three
Tac. Therefore, it appeared that there might be chains are so similar in size, the only way that the
another molecule capable of binding IL-2. 65 kDa (cid:13) chain could be separated and distinguished
Since mAbs that reacted with this putative second fromthe55 kDa(cid:11)chainandthe75 kDa(cid:12) chainwas
chain had yet to be discovered, we performed experi- by two-dimensional SDS-PAGE.
ments to chemically cross-link radiolabeled IL-2 to
various cells. We found that a natural killer (NK)
The Genes Encoding the IL-2R Chains
leukemic cell line (YT) that did not react with anti-
TacexpressedanIL-2-bindingchainthatwas75kDa, Once the IL-2R binding proteins were identified, it
and thus distinct from the anti-Tac reactive protein was fairly straightforward to clone the cDNAs
that was 55 kDa (Teshigawara et al., 1987). Tom encoding each of the proteins. However, in each
Waldmann’s group also identified a molecule of instance, the key to success proved to be the
similar size on a leukemic cell line derived from a identification of cell lines that expressed high levels
gibbon ape (MLA-144) (Tsudo et al., 1986), and ofeachofthechains,andtothegenerationofspecific
Warren Leonard’s group produced evidence that mAbs reactive with the respective chains.
normalTcellsexpressedasimilarchain(Sharonetal., In the case of the (cid:11) chain, the anti-Tac mAbs were
1986). used by Warren Leonard and his coworkers Tom
With the availability of leukemic cell lines that Waldmann, Warner Greene, and Gerald Crabtree
expressed solely the (cid:11) chain, the (cid:12) chain or both (Leonard et al., 1984), and also by Toshio Nikaido
together, we performed kinetic and equilibrium IL-2- andhiscoworkersTakashiUchiyama,TasukaHonjo
binding experiments (Wang and Smith, 1987). These and Junji Yodoi (Nikaido et al., 1984), to simulta-
experimentsshowedthatthe(cid:11)chainboundIL-2with neouslyclonethe(cid:11)chaincDNA usingATLcell lines
alowaffinity,(cid:24)10 nM,andthe(cid:12) chainbyitselfalso thatexpressedhighlevelsofthe(cid:11)chain.Inthecaseof
had a low affinity for IL-2 binding ((cid:24)1nM). Only the (cid:12) chain, Mitsuro Tsudo developed the first mAbs
whenboth(cid:11)and(cid:12)chainswereexpressedonthesame reactive with this chain, and in collaboration with
cell, did we detect high-affinity binding (5–10 pM). Hatekeyama and Taniguchi the (cid:12) chain was cloned
In addition, kinetic binding experiments revealed from the YT clones that we had generated
1462 Kendall A. Smith
(Hatakeyamaetal.,1989).The(cid:13) chainwasclonedby subsequently found to be a part of the receptors for
Kazuo Sugamura and his group (Takeshita et al., IL-4, IL-7, IL-9, IL-13, and IL-15. The (cid:12) chain is
1992a), after they had generated the first mAbs sometimes referred to as the common (cid:12) chain,
reactive with the (cid:13) chain. because it is shared by the IL-15R.
IL-2R Signaling Molecules
Structure
WhiletheIL-2R-bindingchainswerebeingidentified,
and mAbs as well as the cDNAs were under
The high-affinity IL-2R is composed of three
investigation during the 1980s and early 1990s, the
noncovalently linked chains: (cid:11) (p55), (cid:12) (p75), and (cid:13)
molecules responsible for IL-2R signaling remained
(p65). When expressed on activated T cells there is a
enigmatic. As early as 1990, it was reported that
10–20-fold excess of (cid:11) chains as compared with (cid:12)(cid:13)
within a few minutes of IL-2 binding to cells that
chains. Thus, there are (cid:24)1000 high-affinity IL-2Rs
tyrosine-specific phosphorylation of several cytoplas-
on activated T cells and an excess of (cid:24)10,000–20,000
mic proteins could be identified. However, as the IL-
(cid:11)chains.TheIL-2Rhasyettobecrystallized,sothat
2R-bindingproteinswereidentifiedonebyone,itwas
thethree-dimensionalstructureisonlyaconjectureat
realized that these proteins themselves were probably
this time, based upon the structure of the human
notresponsibleforthesephosphorylationevents.The
growth hormone receptor. However, it is clear that
IL-2R-bindingchainssimplydidnotcontainprimary
the (cid:11) chain binds to different residues on the IL-2
sequences that could be protein tyrosine kinases
moleculecomparedwiththe(cid:12)(cid:13) dimer.Consequently,
(PTK).
the three chains cooperate to form the high-affinity
In the early and mid-1990s, as receptor-signaling
IL-2R in much the way that the heavy and light
molecules were identified in other receptor systems,
chains cooperate to form the antigen-binding region
many investigators attempted to ascribe IL-2 signal-
of the antibody molecule.
ingofPTKactivitytoeachofthenewmolecules.For
example, when the PTKs of the src family were the
only PTKs known, these molecules were reported to
Main activities and
be involved in IL-2 signaling. However, it was not
pathophysiological roles
until the discovery of the Janus kinase (JAK) family
that definitive experiments revealed that both JAK1
and JAK3 were involved in the early events of IL-2R Functional IL-2Rs are only expressed transiently on
signaling (Miyazaki et al., 1994). Both of these PTKs antigen-activated T cells and B cells (Smith, 1989).
are already associated with the IL-2R prior to ligand Accordingly, IL-2Rs only function during the
binding.Apparently,afterIL-2bindstothecombined adaptive immune response briefly after antigen
(cid:11)(cid:12)(cid:13) heterotrimer, the receptor complex is stabilized. activation, and when antigen is cleared via the
Then, JAK1 in association with the (cid:12) chain and reticuloendothelial system the IL-2R expression
JAK3inassociationwiththe(cid:13) chainarebroughtinto disappears. By comparison, NK cells express IL-2Rs
close enough proximity so that they begin to constitutively. Approximately 10% of NK cells
phosphorylate one another, as well as both of the express trimeric high-affinity IL-2Rs, while (cid:24)90%
receptor chains. Specific tyrosine residues are phos- only express intermediate affinity IL-2Rs, which are
phorylated on each of these chains, and these phos- comprised of (cid:12)(cid:13) dimers (Caligiuri et al., 1990). It is
photyrosine residues then serve as docking sites for unlikely that this intermediate-affinity IL-2R plays
the downstream effector molecules. The IL-2R is any role in immune responses, because IL-2
knowntoactivatethreesignalingpathways:theJAK/ concentrations high enough to bind to this receptor
STAT pathway, the Ras/Raf/MAPK pathway, and are never generated in vivo.
the PI-3 kinase/Akt pathway. In essence, all three of IL-2Rs promote four distinct cellular changes that
thesepathwaysdependupontheinitialPTKactivities are fundamental to the generation of an effective
of the JAKs. immuneresponse.First,theIL-2/IL-2Rinteractionis
responsible for mediating cell cycle progression
from early G to the G /S phase interface, thereby
Alternative names 1 1
accountingfortheproliferativeexpansionofantigen-
selected clones (Cantrell and Smith, 1984). Second,
TheonlyalternativenamesfortheIL-2Rrelatetothe the IL-2/IL-2R interaction is crucial in imparting
namesgiventothe(cid:11)chain,astheTacantigen,andto survival signals to the cell (Gillis et al., 1978), such
the (cid:13) chain as the common (cid:13) chain. This chain was that if IL-2 is withdrawn prematurely after antigen
IL-2 Receptor 1463
activation, apoptosis occurs rapidly via the ‘cytokine diminution of CTL activity as detected by the 51Cr-
withdrawal’ pathways. Third, the IL-2/IL-2R inter- release assay, it was concluded that IL-2 must not be
action activates and potentiates cellular differentia- absolutely obligatory for the generation of cytolytic
tion programs within the target cells, which permits cells. Also, the fact that CTL activity was detectable
effector mechanisms to respond to the antigenic at all was interpreted as evidence that cellular
stimulation (Le Gros et al., 1990; Seder et al., 1994; proliferation was intact and relatively normal in
Swain, 1994). Finally, the IL-2/IL-2R interaction these animals. Therefore, it was interpreted that
primes the cell for ‘activation-induced cell death’ in vivo there had to be other cytokines that could
(AICD), a phenomenon that may be one mechanism substituteforIL-2,eventhoughinvitroonlyIL-2was
operating to limit the duration and magnitude of the capable of correcting the defect in proliferation
immune response, and that may play a role in the observed upon polyclonal activation of the IL-2
generation and maintenance of peripheral tolerance knockout cells.
(Lenardo,1991;SingerandAbbas,1994;Zhengetal., Subsequent experiments where the proliferation of
1995). the CD8+ T cells was monitored directly after
Early inour investigationofIL-2Rs,it wasevident experimental infection with lymphocytic choriomenin-
that although both CD4+ T cells and CD8+ T cells gitis virus (LCMV), revealed that >90% of the
express IL-2Rs after antigen activation, the prolif- proliferation detectable in IL-2 knockout mice was
erative response on the part of the CD8+ T cells is attenuated (Cousens et al., 1995). This is extremely
more long-lasting (Gullberg and Smith, 1986). Thus, importantforourviewoftheimportanceoftheIL-2/
in vitro, CD4+ T cells cease proliferating after (cid:24)7 IL-2R interaction for the generation of immune
days, while CD8+ T cells proliferate exponentially responses. It indicates that in vivo, as in vitro, IL-2 is
for several more days. Consequently, CD8+ T cells theprincipalTcellgrowthfactor,andthattheroleof
will predominate after 10–14 days of culture. This IL-2 in the immune response is not redundant.
same phenomenon is observed in vivo after antigen Although other cytokines, such as IL-4, IL-7, IL-9,
activation.CD8+Tcellsexpandmassively,whilethere IL-13, and IL-15 are all capable of promoting T cell
is a much more modest increase in CD4+ T cells. cycle progression in vitro, only IL-2 is produced in
Until recently, the contribution of the expansion of sufficient amounts in vivo in response to antigenic
the number of antigen-reactive T cells, especially of stimulation to mediate the rapid clonal expansion
CD8+Tcells,wasnotappreciated.Thiswasduetoa necessary to effect an adequate immune response.
technicaldifficultyindetectingantigen-specificTcells OncetheIL-2-responsivecellshaveexpanded,their
in vivo after the injection of antigen. It has been longevityisdependentonacontinuedsupplyofIL-2.
appreciated for over 20 years that there is a transient In a self-limited infectionwhen the antigen is cleared,
expansion of T cells, but the vast majority of the the antigen/TCR-dependent triggering of IL-2 pro-
expandedcells,i.e.>90%,werethoughttobeantigen duction ceases. Subsequently, the antigen-activated,
nonreactive, so-called ‘innocent bystanders’. This IL-2-responsive cells undergo cytokine withdrawal
view originated in experiments that attempted to apoptosis. In large part, the IL-2-induced survival is
quantify antigen-specific cells via limiting dilution attributable to the induction of survival genes of the
analysis (LDA), using the 51Cr-release cytotoxicity Bcl-2 family (Haldar et al., 1990). However, there are
assay. However, now that it is possible to enumerate most likely other such survival genes that are also
antigen-specificcellsdirectlyusingtheMHCtetramer inducedbyIL-2,sothat uponwithdrawalofIL-2the
assay, it has become appreciated that all of the cessation of their expression contributes to the
expanded cells are actually antigen-specific (Murali- initiation of the apoptotic pathways.
Krishna et al., 1998). Thus, in experimental viral in- IL-2-induced differentiation influences the secre-
fections in the mouse, antigen-specific CD8+ T cells tion of cytokines by both CD4+ and CD8+ target
havebeenfoundtoexpandasmuchas100,000-foldin cells, and also the secretion of cytolytic molecules
only 8 days. Accordingly, the calculated doubling such as perforin, the granzymes, and the Fas/FasL
times are extremely rapid, (cid:24)6 hours. pathway. In fact, the differentiation of T helper cells
IthasnotbeengenerallyappreciatedthattheIL-2/ into the TH1 and TH2 pathways is dependent on IL-
IL-2R interaction is responsible for this massive 2. If IL-2 is excluded from the cultures, neither TH1
proliferativeclonalexpansion.Soonafterthecreation nor TH2 CD4+ T cells differentiate. Likewise, the
of the first IL-2 knockout mouse by Ivan Horak differentiation of CTL is dependent upon IL-2.
(Schorleetal.,1991),experimentsweredonetoassess Accordingly, IL-2 is not only obligatory for the
theeffectoftheIL-2genedeletionontheresponseof expansionofthenumberofantigen-selectedcells,itis
these mice to experimental viral infections (Kundig also required for their survival as well as their
et al., 1993). Although there was a (cid:24)10-fold differentiated effector functions.
1464 Kendall A. Smith
GENE and lymphoma cells, especially from ATL patients.
The (cid:12) chain is expressed in antigen-activated T cells
Accession numbers and B cells, and NK cells, as well as leukemic cells
and cell lines from ATL patients and NK cell leu-
kemia patients. The (cid:13) chain is expressed in all major
IL-2R (cid:11) chain: gene M10322, cDNA X01057
lymphocyte subsets.
IL-2R (cid:12) chain: gene X53093, cDNA M26062
IL-2R (cid:13) chain: gene AH002843, cDNA NM_00026
Regulation of receptor expression
PROTEIN
The (cid:11) chain is expressed in response to activation via
the TCR-induced activation of NF(cid:20)B/Rel, and by
Accession numbers
signals from the IL-2R itself via the JAK/STAT
pathway. The (cid:12) chain was reported initially to be
IL-2R (cid:11) chain: P01589 expressed constitutively by T cells. However, this
IL-2R (cid:12) chain: P14784 resultwasbaseduponcrosslinkageexperiments,using
IL-2R (cid:13) chain: P31785 radiolabeledIL-2andcell populationsthatcontained
NKcells.Subsequently,itwasshownthatTcellsonly
Description of protein express the (cid:12) chain when activated via the TCR. By
comparison, NK cells express the (cid:12) chain constitu-
tively. The (cid:13) chain is constitutively expressed by
The(cid:11)chain,the(cid:12)chain,andthe(cid:13)chainarealltypeI
T cells, B cells, and NK cells, and does not appear to
transmembrane proteins. There are only 13 amino
be regulated by the TCR.
acid residues in the intracellular domain of the (cid:11)
chain; the (cid:12) chain intracellular domain contains 286
amino acid residues; and the (cid:13) chain intracellular Release of soluble receptors
domain is composed of 86 amino acid residues.
The (cid:11) chain can be found in tissue culture media of
Relevant homologies and species
IL-2R+ cells, and in the serum of experimental
differences animals and humans undergoing an immune
response. The (cid:11) chain is cleaved from the cell surface
via nonspecific proteolysis. There is no alternatively
The(cid:11)chainishomologouswiththe(cid:11)chainoftheIL-
spliced mRNA accounting for a secreted versus a
15R. The (cid:12) chain is homologous with the receptor
membranous form.
chains of the interleukin/hematopoietic cytokine
family, especially the external domains, including
receptors of IL-3, IL-4, IL-5, IL-6, IL-7, IL-9, IL-12,
SIGNAL TRANSDUCTION
IL-13, IL-15, erythropoietin, G-CSF, GM-CSF,
prolactin, and human growth hormone. The (cid:13) chain
Associated or intrinsic kinases
is homologous with the IL-2R (cid:12) chain, and the other
members of the interleukin/hematopoietic receptor
family. Members of the Janus family of tyrosine-specific
kinases are the initiators of IL-2R signal transduc-
tion.JAK1isassociatedwiththe(cid:12)chainandJAK3is
Affinity for ligand(s)
associatedwiththe(cid:13) chain.JAK3expressionisunder
the regulatory control of the TCR, so that resting
IL-2R (cid:11) chain: 10 nM T cells are unresponsive to IL-2 because they lack
IL-2R (cid:12) chain: 1 nM expression of both the (cid:11) and (cid:12) chains of the IL-2R,
IL-2R (cid:13) chain: immeasurable and they lack the signaling molecule associated with
the (cid:13) chain.
Cell types and tissues expressing the
receptor Cytoplasmic signaling cascades
The (cid:11)chain is expressed in recently antigen-activated IL-2Rsignalingoccursoncetyrosineresiduesonboth
T cells and B cells and (cid:24)10% of NK cells, leukemia the (cid:12) chain and (cid:13) chain become phosphorylated.
IL-2 Receptor 1465
Three cytoplasmic signaling cascades become acti- early on, and found to be expressed after stimulation
vated: (1) STAT5a and STAT5b, (2) Ras/Raf/ with phytohemagglutinin (PHA), and augmented by
MAPK, and (3) PI-3 kinase/Akt. IL-2, were the tumor suppressor p53, the proto-
oncogene N-Ras, and the transferrin receptor. By
comparison, the proto-oncogenes c-Myc and c-Fos
areexpressedbothafterTCRtriggeringandafterIL-
DOWNSTREAM GENE
2 triggering.
ACTIVATION
Proceedingbeyondsimplyscreeningfortheexpres-
sion of known genes after IL-2 triggering proved
Transcription factors activated more difficult. First, a system had to be developed to
trigger the IL-2R independently of the TCR/CD28
The JAKs phosphorylate tyrosine residues and complex. This was accomplished in my laboratory
thereby activate STAT5a and STAT5b. Initially it based upon the work of Doreen Cantrell (Cantrell
wasreportedthatthesetranscriptionfactorswerenot and Smith, 1984). Using human T cells grown for
required for the IL-2-induced proliferative responses. 10–14daysinIL-2,thenremovedfromIL-2for36–48
However, this interpretation was based upon compli- hoursbeforerestimulation,itwaspossibletopromote
cated experiments with IL-2R mutants that were a semi-synchronous entry of all of the cells into the
subject to alternative interpretations. Recently, using cell cycle. Next, Carol Beading developed a method
genetic approaches of gene deletion of both STAT5a using thiol-derivatized uridine to label newly synthe-
and STAT5b, more definitive data indicate that sized mRNA, which could then be affinity purified
STAT5 activation is obligatory for the proliferative using a mercury-Sepharose column (Beadling et al.,
response (Beadling et al., 1994; Moriggl et al., 1993).
1999a,b). Using this technique, called SLAP (for sulfhydryl
The Ras/Raf/MAPK pathway eventually activates labeling and affinity purification), eight cytokine
theAP-1familyoftranscriptionfactors,whilethePI- response (CR) genes were identified, only one of
3K pathway has been reported to eventually lead to which was also expressed after TCR triggering. CR1
the activation of the E2F family of transcription encodes a regulator of G protein signaling (RGS),
factors. which is a GTPase-activating protein (GAP) for the
heterotrimericGproteins(Beadlingetal.,1999).CR2
is novel and remains unknown. CR3 encodes the
prostaglandin E receptor, while CR4 encodes a
Genes induced 2
matrix-associated region (MAR) protein. CR5 was
also cloned by Miyajima and coworkers and termed
Earlyreportsoftheinductionofnewgeneexpression CIS, for cytokine inhibitor of signaling. This gene
by activated T cells did not distinguish carefully product was found to be a member of a larger family
between TCR signals and IL-2 signals. Therefore, ofSH2-containingproteinsthatappeartofunctionas
fromtheliterature,oftenitisdifficulttobeabsolutely feedback inhibitors of cytokine receptor signaling.
sure whether a gene expressed after T cell activation CR6 encodes a new member of a three-gene family
is due to IL-2 or due to the TCR. In addition, that appear to function as regulators of both G and
1
experimental results have often been interpreted as G phases of the cell cycle (Fan et al., 1999). CR7
2
indicative of activation via the so-called ‘second encodesthecellularproto-oncogenec-Pim,whichisa
signals’ generated by activation of the accessory serine/threonine kinase. The v-Pim counterpart is the
moleculeCD28.However,activationviathisreceptor oncogene of Moloney leukemia virus, which causes
markedly potentiates the production of IL-2 and Tcellleukemias.CR8encodesanewbasichelix-loop-
other cytokines as well. Therefore, unless the experi- helix (bHLH)-containing protein that functions to
ments investigating TCR(cid:6)CD28 triggering were regulate the length of G , and inhibits apoptosis.
1
done in the presence of a protein synthesis inhibitor, Thus,allofthesenewlydiscoveredCRgenespromise
such as cycloheximide, it is impossible to accurately to give us insight as to how IL-2 promotes its effects
ascribe new gene expression to the triggering of the on its target cells.
TCR, or the CD28 molecule, or IL-2, or even one of OthersaccomplishedtheseparationofTCRsignals
the other cytokines released early on after TCR from IL-2R signals by using IL-2-dependent T cell
activation. clones. In particular, Prystowsky’s group was one of
c-Myb was the first gene found to be expressed by the first to report the isolation of IL-2-induced genes
IL-2R signaling, distinct from the TCR and/or (Sabath et al., 1990). However, these investigators
CD28 (Stern and Smith, 1986). Other genes tested focused on genes expressed 24 hours after activation
1466 Kendall A. Smith
with IL-2, after the early events of IL-2R triggering BIOLOGICAL CONSEQUENCES
were already completed. Consequently, these investi-
OF ACTIVATING OR INHIBITING
gators identified many structural genes of the cyto-
RECEPTOR AND
skeleton, as well as genes involved in the metabolic
pathwaysinoxidationandenergyproductionthatare PATHOPHYSIOLOGY
expressed in response to IL-2 stimulation.
More recently, Jacque Theze and coworkers Unique biological effects of
(Herblot et al., 1999) have focused on IL-2R-induced
activating the receptors
genes that are distinct from IL-4R-induced genes.
Usingarepresentationaldisplaysubtractiveapproach,
thisgrouphasisolated66IL-2-inducedgenes,only16 OneofthemostimportantaspectsofIL-2Rsignaling,
of which correspond to already known genes. The andofsignalingofcellsurfacereceptorsingeneral,is
knowngenesincludecytoskeletonproteins,transcrip- thewayinwhichthesignalisreceivedbythecell,such
tionfactors,nuclearproteins,ribosomalproteins,and that it is able to make the all-or-none decision to
ion transporters. respond. Thus, binding of only one IL-2 molecule to
James Ihle’s group (Moriggl et al., 1999a,b) one IL-2R molecule is insufficient to trigger a
recently reported an extremely important contribu- response. There are a finite number of IL-2/IL-2R
tion to our knowledge of IL-2-induced gene expres- interactionsthatmustoccurbeforethecellmakesthe
sioninvolvedinsignalingcellcycleprogression.They irrevocable decision to respond (Cantrell and Smith,
found that the STAT5a/b double knockout mouse is 1984; Smith, 1989, 1995). For the most part, this
unable to mount a proliferative response to poly- quantalnatureofIL-2Rsignalinghasbeenstudiedby
clonal TCR activators, and that the defect is second- monitoring the proliferative response to IL-2, but the
arytothelackofsignalsthatnormallyemanatefrom otherbiologicalresponses directedbyIL-2 behaveby
the IL-2R. In particular, the expression of the genes the same rules. As well, there is a growing awareness
encoding cyclins D2, D3, E, A, and cdk6, were all that the TCR behaves in the same quantal fashion
either absent or deficient in cells from the double when triggering of the expression of cytokine genes,
knockoutmice.Bycomparison,theexpressionofcdk4 such as the IL-2 genes or the IFN(cid:13) genes, are
andcdk2werenotfoundtobedependentonSTAT5, monitored. Thus, there is a threshold of activation
nor was the G degradation of the cyclin-dependent thatmustbesurpassedbeforeacellcanrespond,and
1
kinase inhibitor p27. a population of cells that differ by the number of IL-
All of these data support the interpretation that 2Rs will display a heterogeneous response over time
T cell proliferation after TCR activation is actually when exposed to a receptor saturating IL-2 concen-
mediatedbyIL-2anditsreceptor,andthatthesignals tration. Cells with a low number of IL-2Rs will take
and genes activated by the TCR and its accessory longer to reach the critical threshold compared with
molecule CD28 are not involved in promoting G cells that express high levels of IL-2Rs. Accordingly,
1
progression or S phase transition. normal cells are under the control of this ligand/
receptor threshold, while abnormal cells result when
this threshold is no longer operative.
TheIL-2Rshareswithothercytokinereceptorsthe
Promoter regions involved
capability of promoting cell cycle progression,
survival, and differentiation. Accordingly, the most
Only a few of the promoter regions of the known IL- uniqueeffect ofIL-2 signaling is its ability toprovide
2R-induced genes have been characterized thus far. negative feedback signals that seem to be so
One of the first to be investigated was the IL-2R important for the normal functioning of the immune
(cid:11) chain promoter (Nakajima et al., 1997), which was system (Parijs et al., 1999). Thus, as discussed below,
of interest because it seemed to be regulated by both the phenotype produced when either IL-2 or its
the TCR and the IL-2R (Smith and Cantrell, 1985). receptors are deleted indicate that the IL-2/IL-2R
Also, it was somewhat paradoxical that a hormone system is very important for maintaining the inte-
would upregulate its own receptor. It has been grity of the system as a whole. There are a few
found that there are canonical NF(cid:20)B/Rel sites reports that focus on the possible molecular mechan-
immediately upstream of the transcriptional start isms that could be responsible for the negative
site.Theseresponseelementsareunderthecontrolof feedback effect. However, we still do not have a very
theTCR.Bycomparison,therearecanonicalSTAT5 complete picture as to exactly what is responsible,
sites much further upstream that account for the nor how the apparent paradoxical effects of
observed IL-2 regulation of (cid:11) chain expression. pro-survival and growth/differentiation are regulated
IL-2 Receptor 1467
in relationship with the pro-apoptotic effects of the proliferative responses, underscoring that the TCR
IL-2R. cannot generate signals that are capable of moving
the cell through the cell cycle.
The phenotype of the (cid:13) chain knockout mice is
Phenotypes of receptor knockouts distinctly different from the phenotypes of the
knockouts discussed thus far (Leonard et al., 1995).
and receptor overexpression mice
These mice suffer from severe combined immunodefi-
ciency (SCID). The discovery that the (cid:13) chain gene is
The phenotype of the IL-2 knockout was a paradox on the X chromosome led to the discovery that
when first uncovered (Schorle et al., 1991). Thus, (cid:24)50%ofX-linkedSCIDisattributabletomutations
instead of immunodeficiency, IL-2 knockout mice of the (cid:13) chain gene. Subsequently, it was shown that
develop an accumulation of T cells with an activated defects of IL-7 or the IL-7R result in a similar
surface phenotype, and they develop an autoimmune phenotype. Therefore, the lack of the common (cid:13)
hemolytic anemia as well as a diffuse ulcerative colitis chain prevents the IL-7-dependent proliferation of
(Sadlack et al., 1993). The etiology of this syndrome both B cell and T cell progenitors.
remains obscure, but a similar phenotype results
when the IL-2R genes are deleted.
Deletion of the IL-2R (cid:11) chain does not influence
Human abnormalities
lymphocyte development, so that at birth, normal
numbers and proportions of the major lymphocyte
HumanswithX-SCIDdueto(cid:13) chainmutationshave
subsets are present in both primary and secondary
a somewhat different phenotype compared with (cid:13)
lymphoid organs (Willerford et al., 1995). However,
chain knockout mice (Leonard et al., 1995). Thus,
with aging, these mice develop progressive enlarge-
they are markedly deficient in T cells, but have
ment of peripheral lymphoid organs associated with
normal numbers of B cells. By comparison, (cid:13) chain
polyclonal T and B cell expansion. Older (cid:11) chain
knockout mice accumulate T cells over time, but
knockout mice also develop autoimmune disorders,
almost entirely lack B cells.
including hemolytic anemia and inflammatory bowel
disease. The (cid:11) chain of the IL-2R is not involved in
signaling, and only imparts its rapid association rate
to the high-affinity heterotrimeric IL-2R. Therefore, THERAPEUTIC UTILITY
thefactthatthephenotypeofthe(cid:11)chainknockoutis
identical to the IL-2 knockout indicates that levels of Effects of inhibitors (antibodies) to
IL-2 high enough to bind to the intermediate affinity
receptors
IL-2R are not produced in vivo.
Deletion of the (cid:12) chain results in a syndrome
very similar to that seen in (cid:11) chain knockout mice The original monoclonal antibody found to be
(Suzuki et al., 1995). There is normal T and B cell reactive with the (cid:11) chain, anti-Tac, has now been
development at birth, but thereafter there is pro- ‘humanized’ and has been used in clinical trials as an
gressive accumulation of activated T cells and B cells immune suppressant for patients who have had a
and autoimmune phenomena, including diffuse renal allograft (Waldmann and O’Shea, 1998).
hypergammaglobulinemia, autoimmune hemolytic ane- Compared with pan-reactive T cell monoclonal anti-
mia, and inflammatory bowel disease, leading to bodies, the anti-Tac appears to be just as effective in
premature death. However, these mice suffer from preventing graft rejection. The obvious advantage of
immunodeficiencies before this autoimmune pheno- the IL-2R (cid:11) chain mAb versus a pan T cell reactive
type becomes evident. Thus, when young 3-week-old mAb is the selectivity of the anti-Tac mAb for
mice are infected with vesicular stomatitis virus recently antigen-activated T cells.
(VSV), they fail to make either IgM or IgG anti-
bodies. Also, when infected with lymphocyte chor-
iomeningitis virus (LCMV), these mice fail to mount
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