May 2018 | Volume 9 | A icle 8471
Re iew
published: 04 May 2018
doi: 10.3389/ immu.2018.00847
F on ie s in Immunology | www. on ie sin.o g
Edi ed by:
Fab izio Ma ei,
Is i u o Supe io e di Sani à,
I aly
Re iewed by:
Alexand e Co hay,
Oslo Uni e si y Hospi al,
No way
Ch is ine Susanne Falk,
Hanno e Medical School,
Ge many
*Co espondence:
Ma ia José Oli ei a
[email p o ec ed]
Special y sec ion:
This a icle was submi ed
o Cance Immuni y
and Immuno he apy,
a sec ion o he jou nal
F on ie s in Immunology
Recei ed: 06Decembe 2017
Accep ed: 05Ap il2018
Published: 04May2018
Ci a ion:
Cas oF, Ca dosoAP,
Gonçal esRM, Se eK and
Oli ei aMJ (2018) In e e on-Gamma
a he C oss oads o Tumo Immune
Su eillance o E asion.
F on . Immunol. 9:847.
doi: 10.3389/ immu.2018.00847
in e e on-Gamma a he C oss oads
o Tumo immune Su eillance o
e asion
Flá ia Cas o1,2,3, Ana Pa ícia Ca doso1,2, Raquel Madei a Gonçal es1,2,3, Ka ine Se e4
and Ma ia José Oli ei a1,2,5*
1 i3S – Ins i u o de In es igação e Ino ação em Saúde, Uni e sidade do Po o, Po o, Po ugal, 2 INEB – Ins i u o de
Engenha ia Biomédica, Uni e sidade do Po o, Po o, Po ugal, 3 ICBAS – Ins i u o de Ciências Biomédicas Abel Salaza ,
Uni e sidade do Po o, Po o, Po ugal, 4 IMM – Ins i u o de Medicina Molecula João Lobo An unes, Faculdade de
Medicina, Uni e sidade de Lisboa, Lisbon, Po ugal, 5 Depa amen o de Pa ologia e Oncologia, Faculdade de Medicina,
Uni e sidade do Po o, Po o, Po ugal
In e e on-gamma (IFN-γ) is a pleio opic molecule wi h associa ed an ip oli e a i e,
p o-apop o ic and an i umo mechanisms. This e ec o cy okine, o en conside ed as a
majo e ec o o immuni y, has been used in he ea men o se e al diseases, despi e
i s ad e se e ec s. Al hough b oad e idence implica ing IFN-γ in umo immune su -
eillance, IFN-γ-based he apies unde going clinical ials ha e been o limi ed success.
In ac , ecen epo s sugges ed ha i may also play a p o umo igenic ole, namely,
h ough IFN-γ signaling insensi i i y, down egula ion o majo his ocompa ibili y com-
plexes, and up egula ion o indoleamine 2,3-dioxygenase and o checkpoin inhibi o s,
as p og ammed cell-dea h ligand 1. Howe e , he IFN-γ-media ed esponses a e s ill
posi i ely associa ed wi h pa ien ’s su i al in se e al cance s. Consequen ly, majo
esea ch e o s a e equi ed o unde s and he immune con ex u e in which IFN-γ
induces i s in ica e and highly egula ed e ec s in he umo mic oen i onmen . This
e iew discusses he cu en knowledge on he p o- and an i umo igenic e ec s o IFN-γ
as pa o he complex immune esponse o cance , highligh ing he ele ance o iden i y
IFN-γ esponsi e pa ien s o he imp o emen o he apies ha exploi associa ed sig-
naling pa hways.
Keywo ds: ype ii in e e on, immuno egula ion, cance mic oen i onmen , immuno he apy, immune con ex u e
iNTRODUCTiON
In e e ons (IFNs) a e pleio opic cy okines wi h an i i al, an i umo and immunomodula o y
p ope ies, being cen al coo dina o s o he immune esponse (1). The e m “in e e ons” comes
om he desc ip ion o molecules p o ec ing cells by “in e e ing” wi h i al in ec ion (2, 3). Th ee
majo ypes o IFNs a e dis inguished by hei sequence iden i y, gene ic loci, cell o o igin, na u e,
and dis ibu ion o hei ecep o s and esul ing s imuli (Table1).
The human ype I IFN amily comp ises 17 dis inc p o eins, mainly ep esen ed by IFN-α
and IFN-β, which a e ubiqui ously exp essed and signal h ough hei cogna e ecep o , composed
by IFNαR1 and IFNαR2 subuni s [ e iewed in Re . (4)]. IFN-γ is he lone membe o ype II IFN
amily. I is mo e es ic i ely exp essed and is s uc u ally and unc ionally di e en om he o he
ypes o IFNs. Mos ecen ly, a ype III IFN amily was desc ibed o be composed o ou homologous
p o eins (IFNλ1–4), which bind he IFNλR1 and in e leukin (IL)-10Rβ he e odime ic ecep o
[ e iewed in Re . (8)]. To da e, ype I and ype III IFNs ha e been mainly in ol ed in hos –pa hogen
Table 1 | Compa ison o human ype I, ype II, and ype III IFN p oduc ion and signaling.
P ope ies Type i iFN (iFN-α, iFN-β) Type ii iFN (iFN-γ) Type iii iFN (iFN-λ)
Membe s 17 p o eins: 13 IFN-α, IFN-β, IFN-ε, IFN-κ,
IFN-ω
1 p o ein: IFN-γ4 p o eins: IFN-λ1, IFN-λ2, IFN-λ3,
IFN-λ4
IFN-p oducing cells All nuclea ed cells Tcells, B cells, NKcells, NKTcells, and APCs All nuclea ed cells, mainly mDCs, pDCs,
and epi helial cells
IFN- esponding cells All nuclea ed cells All nuclea ed cells Lung, in es ine, and li e epi helial cells
S imuli DAMPs and PAMPs IL-12, IL-15, IL-18, ype I IFN, and PAMPs DAMPs and PAMPs
IFN ecep o IFN ype I ecep o (IFNαR): IFNαR1 and
IFNαR2 subuni s
IFN ype II ecep o (IFNγR): IFNγR1 and IFNγR2
subuni s
IFN ype III ecep o (IFNλR): IFNλR1
and IL10Rβ
Signaling molecules TYK2, JAK1, all STATs, CRKL, and IRS JAK1, JAK2, STAT1, and STAT3 TYK2, JAK1, STAT1, STAT2, and IRF9
T ansc ip ion ac o binding
si es
ISRE (canonical)
GAS (non-canonical)
GAS (canonical)
ISRE (non-canonical)
ISRE
Func ions An i i al, an ip oli e a i e esponse,
egula ion o cell su i al/apop osis, and
immuno egula ion
An i i al, an ip oli e a i e, immunomodula o y, and
an i umo esponse
An i i al esponse, mucosal immuni y
Re e ence (4, 5) (6, 7) (8)
APCs, an igen-p esen ing cells; CRKL, CT10 egula o o kinase-like; DAMPs, damage-associa ed molecula pa e ns; GAS, gamma-ac i a ed si e; IFN, in e e on; IFN-γ, in e e on-
gamma; IFNαR1–2, ype I ecep o ; IFNγR, ype II ecep o ; IFNλR, ype III ecep o ; IL, in e leukin; IRF, in e e on- egula o y ac o ; IRS, insulin ecep o subs a e; ISRE, in e e on-
sensi i e esponse elemen ; JAK, Janus kinase; mDCs, myeloid dend i ic cells; NK, na u al kille ; NKT, na u al kille Tcells; PAMPs, pa hogen-associa ed molecula pa e ns; pDCs,
plasmacy oid dend i ic cells; STAT, signal ansduce and ac i a o o ansc ip ion; TYK, y osine kinase.
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in e ac ions, and hei exp ession is ac i a ed h ough immune
sys em sen inel ecep o s, such as pa e n ecogni ion ecep o s.
Despi e he simila unc ion o ype I and III on an i i al in ec ions,
i is he i al opism ha dic a es he ela i e con ibu ion o each
IFN (9). Mo eo e , whe eas almos all nuclea ed cells espond o
ype I IFN, ype III IFNs esponse is es ic ed o issues wi h a
high isk o i al exposu e and in ec ion, as he mucosal su aces.
The ole o ype II IFN in p omo ing hos immune esponse o
mic oo ganisms is simila ly well documen ed. No ably, i is also
known o play a pi o al unc ion on cance immune su eillance,
s imula ing an i umo immuni y and p omo ing umo ecog-
ni ion and elimina ion (10–16).
This e iew ocuses on ype II IFN signaling, cellula unc-
ions, and di ec ed he apies and was encou aged by no el
indings e ealing egula o y mechanisms o IFN-γ and i s p og-
nos ic as well as he apeu ic po en ial. In ac , since Wheelock
who epo ed ha IFN-γ inhibi ed i al eplica ion in 1965 (17),
i ook a ound 30yea s o en isage his cy okine as a a ge o
an i umo immuni y (18).
In e e on-gamma is a homodime o med by he non-
co alen associa ion o wo 17kDa polypep ide subuni s. Du ing
syn hesis, a e mul iple N-glycosyla ion, bo h subuni s bind in
an an ipa allel manne , cons i u ing a ma u e 50kDa molecule
(19, 20). No ably, he IFN-γ symme y sugges s ha a single
molecule can bind simul aneously o wo ecep o s, ampli ying
he unde lying esponses. Cellula esponses induced by IFN-γ
may also in ol e c oss-communica ion wi h IFN-α/β ecep o s,
ampli ying IFN-γ signaling and i s e ec s (21, 22).
In e e on-gamma is sec e ed p edominan ly by ac i a ed
lymphocy es such as CD4 T helpe ype 1 (Th1) cells and CD8
cy o oxic Tcells (23–26), γδ Tcells (27–33), and na u al kille
(NK) cells (34, 35) and, o a less ex en , by na u al kille Tcells
(NKT), B cells (36–39), and p o essional an igen-p esen ing
cells (APCs) (40–42). I s exp ession is induced by mi ogens and
cy okines, such as IL-12 (43, 44), IL-15 (45), IL-18 (46, 47), and
ype I IFN (48, 49). IFN-γ pleio opic unc ions a e media ed by
cell-speci ic exp ession o hund eds o IFN-γ- egula ed genes
ha encompass in lamma o y signaling molecules, apop osis
and cell cycle egula o s, and ansc ip ional ac i a o s (50).
Au oc ine IFN-γ p oduced by APCs can ac locally and con ib-
u e o sus ain sel and neighbo cell ac i a ion (51–53), c ucial
o ea ly con ol o pa hogen sp eading, while Tlymphocy es a e
he majo pa ac ine sou ce o IFN-γ in adap i e immuni y. Unde
physiological condi ions, he cons i u i e exp ession o ype I and
II IFNs is igh ly con olled, emaining localized o issues, wi h-
ou sys emic e ec s (54–56). Fo ins ance, cons i u i e exp ession
o endogenous IFN-γ con ibu es o he homeos asis o immune
cell unc ions (57), main enance o he hema opoie ic s em cell
niche (58), and bone o ma ion (59). Combina ion app oaches o
boos inna e immune ac i a ion ha e been explo ed o con e ge
on o IFN pa hways. Howe e , IFN-γ- ela ed signaling can also
ha e supp essi e immuno egula o y e ec s on an i i al (60, 61),
au oimmune (62, 63), as well as on an i umo esponses (64, 65).
Un eiling cellula a ge s o IFN-γ is c i ically impo an o i s
he apeu ic applica ion, o p edic pa ien esponses, pa icula ly
in cance s whe e his cy okine can exe p o umo igenic e ec s.
The e o e, he cellula and molecula e ec s o IFN-γ, wi h pa -
icula emphasis on i s dual ole on umo immuni y and how o
o e come i s limi a ions, will be he majo ocus o his e iew.
CaNONiCal SiGNaliNG aND
ReGUlaTORY MeCHaNiSMS
The iFN-γ Recep o
The IFN-γ ecep o is composed o wo ligand-binding IFNγR1
chains associa ed wi h wo signal- ansducing IFNγR2 chains,
which a e esponsible o connec ing o he cy oplasmic ans-
duc ion machine y (see Figu e 1). The IFNGR1 and IFNGR2
a e localized in ch omosome 6 and 21, espec i ely, and hei
FiGURe 1 | In e e on-gamma (IFN-γ) canonical signaling pa hway. Upon ligand binding, IFNγR1 and IFNγR2 oligome ize and ansphospho yla e, ac i a ing Janus
ac i a ed kinase (JAK) 1 and JAK2. These, in u n, phospho yla e IFNγR1, c ea ing a docking si e o he signal ansduce and ac i a o o ansc ip ion (STAT) 1.
Phospho yla ed STAT1 homodime izes in an an ipa allel con igu a ion, o ming a complex gamma-ac i a ed ac o (GAF), which ansloca es o he nucleus and binds
o gamma-ac i a ed si e (GAS), loca ed a he p omo e s o p ima y esponse genes, inc easing hei ansc ip ion. Upon induc ion, ansc ip ion ac o in e e on-
egula o y ac o 1 (IRF1) binds o in e e on-s imula ed esponse elemen (ISRE) and enhances he ansc ip ion o se e al seconda y esponse genes esponsible
o se e al immunomodula o y unc ions. Supp esso o cy okine signaling (SOCS) p o eins nega i ely egula e he IFN-γ pa hway by inhibi ing JAKs and STAT1
phospho yla ion. Th ough dephospho yla ion and deace yla ion, he con igu a ion o STAT1 homodime s e e s o pa allel, igge ing hei exi om he nucleus.
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exp ession di e s signi ican ly. While IFNγR1 is cons i u i ely
exp essed a mode a e le els on he su ace o almos all cells,
IFNγR2 is cons i u i ely exp essed a low le els, and i s exp ession
is igh ly egula ed, acco ding o he s a e o cellula di e en ia-
ion o ac i a ion (66). Fo example, CD4 T helpe cell subse s
di e in hei abili y o espond o IFN-γ (67, 68). Rema kably,
IFN-γ ac i a es he signal ansduce and ac i a o o ansc ip-
ion (STAT) 1 ha main ains he exp ession o T-be , he mas e
ansc ip ion ac o ha con ols IFN-γ exp ession in Tcells (69).
This signaling cons i u es a posi i e eedback loop ha maximizes
Th1 immuni y (70–72). No ably, Th1cells a e mo e esis an o
he an ip oli e a i e e ec s o IFN-γ han Th2 cells. This is likely
due o lowe le els o exp ession o he IFNγR2 subuni ha
allows Th1cells o con inue o p oli e a e du ing IFN-γ signaling.
By con as , Th2 cells ha do no p oduce IFN-γ exp ess highe
le els o he IFNγR2 subuni , ende ing hem pa icula ly sus-
cep ible o he p esence o IFN-γ ha inhibi s hei p oli e a ion
(67, 68, 73). Ne e heless, IFNγR2 down egula ion may be also
induced in Th2 cells when hey a e exposed o IFN-γ (68). Thus,
IFN-γ appea s o egula e he exp ession o i s own ecep o
on speci ic cell ypes, ep esen ing a egula o y mechanism o
cellula desensi iza ion in esponse o cy okines p esen a he
local mic oen i onmen . As a esul , IFNγR2 exp ession can be a
limi ing ac o in IFN-γ esponsi eness and unc ional ou come
ha can dic a e he Th1–Th2 pheno ype swi ch and modula e he
subsequen immune esponse.
JaK/STaT Signaling Pa hway
The biological e ec s o IFN-γ a e elici ed h ough ac i a ion o
in acellula molecula signaling ne wo ks, mainly ia he JAK/
STAT pa hway, which modula es he ansc ip ion o hund eds
o genes and media es di e se biological esponses (50, 74–76).
Upon IFN-γ binding, he in acellula domains o IFNγR2
oligome ize and ansphospho yla e wi h IFNγR1, ac i a ing
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he downs eam signaling componen s, JAK1 and JAK2. The
ac i a ed JAKs phospho yla e he in acellula domain o he
ecep o ( y osine 440 on human IFNγR1), c ea ing binding si es
o STAT1 (77). STAT1 is hen phospho yla ed in he C- e minus
on y osine Y701 esidues by JAK, esul ing in he o ma ion
o STAT1 homodime s complexes, known as gamma-ac i a ed
ac o s (GAFs), which ansloca e o he nucleus and egula e
gene exp ession h ough binding o gamma-ac i a ed si e (GAS)
elemen s in he p omo e s o in e e on-s imula ed genes (ISGs)
(78). One o he majo p ima y esponse genes induced by STAT1
signaling is he ansc ip ion ac o in e e on- egula o y ac o 1
(IRF1), a membe o he IFN egula o y ansc ip ion ac o am-
ily (79). IRF1 unc ions as a ansc ip ion ac i a o o in e e on-
s imula ed esponse elemen s (ISRE), leading o he ansc ip ion
o a la ge numbe o seconda y esponse genes (Figu e1). Fo
ins ance in b eas cance cells, a genome-wide iden i ica ion
o IFN-γ-induced IRF1 ac i a ion e eals o e 17,000 binding
si es, wi h “apop osis” o “cell dea h” as he mos en iched a ge
p ocesses unde lying he di ec umo icidal p ope y o he
cy okine (80). Howe e , umo cells also de elop esis ance o
IFN-γ h ough di e en ial IRF1 esponsi eness, poin ing ou ha
he JAK/STAT signaling pa hway needs o be igh ly egula ed
o a oid de imen al consequences o excessi e s imula ion and
highligh ing i s ole on immune esponses and umo igenesis
(81). STAT1 a ge s o he IFN-γ-media ed signaling also include
he SMAD amily membe 7 (SMAD7), and p o eins in ol ed
in cell cycle egula ion, such as c-Myc and he cyclin-dependen
kinase inhibi o 1A (82–84).
The JAK/STAT signaling pa hway is egula ed a se e al le els
by posi i e and nega i e mechanisms. In pa icula , de egula ion
o inhibi ion o he JAK/STAT pa hway leads o lowe ed immu-
ni y and is o en associa ed wi h inc eased umo igenesis (85, 86)
o me as a ic dissemina ion (87). STATs a e also in ol ed in he
de elopmen and unc ion o he immune sys em and play a ole
in main aining umo su eillance [ e iewed in Re . (88)]. STAT1,
as a umo supp esso , is deduc ed o i s exp ession in umo
cells, modula es hei immunological s a us and consequen ly
hei esponse o an i umo immune esponses. Indeed, STAT1-
de icien umo cells we e mo e suscep ible o NK cells while
STAT1-p o icien umo cells we e mo e sensi i e o CD8+ Tcells
(89). In he same way, STAT1-de icien mice ha a e impai ed
in Th1cell pola iza ion, exhibi ed educed IFN-γ exp ession and
comp omised cy oly ic and NK ly ic ac i i y, ailing o con ol
umo g ow h in con as wi h wild- ype mice (90). In addi ion,
cell-au onomous umo -supp esso unc ions o STAT1 ha e also
been epo ed in b eas cance (91). Howe e , he e is g owing
e idence ha STAT1 also ac s as a umo p omo e (92–94)
since i can enhance esis ance o chemo he apeu ic agen s and
adia ion in ca cinoma (95). Impo an ly, STAT1 also pa ici-
pa es in he signaling om di e en cy okines, including IL-21,
IL-27, and IL-35. These cy okines ha e been p oposed o limi
an i umo immuni y in speci ic cellula , molecula , and mic o-
en i onmen al con ex s (96–101). Thus, STAT1 phospho yla ion
e lec s no only he h eshold and magni ude o IFN-γ esponse
bu also o o he immune media o s, highligh ing he impo -
ance o he egula ion o STAT1 phospho yla ion. One o he
mos impo an nega i e egula o s o he JAK/STAT signaling
pa hway is he supp esso o cy okine signaling (SOCS) p o eins,
which exp ession is inc eased in esponse o IFN-γ signaling
h ough IRF1 (102, 103). SOCS blocks he ac i i y o JAKs by a
nega i e eedback loop, bu also egula es o he cy okines down-
s eam signaling. SH2 domains in SOCS p o eins di ec ly bind
o phospho yla ed y osine esidues o ac i a ed JAKs, blocking
he ec ui men o signal ansduce adap o s, such as STATs, and
JAK ac i i y (102). Fu he mo e, SOCS p omo e in e ac ions ha
lead o ubiqui ina ion and p o easome deg ada ion o compo-
nen s o he JAK/STAT signaling (104, 105). SOCS1 e en p e en s
egula o y T (T eg) cells om p oducing IFN-γ by supp ession
o STAT1, a oiding he con e sion o T eg cells in o e ec o cells
(106). In addi ion, SOCS2-de icien mice showed a educ ion in
lung me as ases and an inc ease in su i al ollowing melanoma
challenge (107).
Al e na i ely, he ansc ip ional ac i i y o STAT1 can be
posi i ely egula ed by o he signaling cascades igge ed by
IFN-γ binding, such as he mi ogen-ac i a ed p o ein kinase
pa hway, p o ein kinase C, and PI3K/AKT, which phospho yl-
a e STAT1 in i s ansac i a ion domain (108). Adding o he
complexi y, unde ce ain ci cums ances, IFN-γ also can ac i a e
STAT1-independen pa hways h ough o he ansc ip ion ac-
o s, namely STAT3 (109), STAT5 (110), nuclea ac o -kappa
B (NF-κB) (111), and ac i a o p o ein 1 (112). In conclusion,
he p ima y esponse o IFN-γ is media ed by GAF ha ac s on
genes wi h GAS binding sequence in hei p omo e , while he
p ima y esponse o ype I IFNs is media ed by ISGF3 (STAT1/
STAT2/IRF9 complex) ha induces genes ha ha e ISRE in hei
p omo e . Thus, some o he ISGs a e egula ed by bo h ypes o
IFNs, whe eas o he s a e selec i ely egula ed by each ype o IFN,
consequen ly po en ia ing he di e si y o biological esponses.
biOlOGiCal FUNCTiONS
iFN-γ ac ions on immune Cells
In e e on-gamma signaling pa hway coo dina es se e al bio-
logical esponses, p ima ily in ol ed in hos de ense and immune
su eillance bu also in he es ablishmen o adap i e immuni y
(Figu e2) and in he egula ion o in lamma ion, apop osis and
cell cycle. One o he i s desc ibed biological e ec s o IFNs was
he up egula ion o he majo his ocompa ibili y complex (MHC)
molecules (113, 114) as well as he up egula ion o he whole
MHC I and II an igen p ocessing and p esen a ion machine y
including anspo e associa ed wi h an igen p ocessing (TAP)
1/2, in a ian chain, and he exp ession and ac i i y o he
p o easome (115–122). Fu he mo e, in some umo ypes,
such as mul iple myeloma and melanoma cells, IFN-γ can also
up egula e he MHC class II ansac i a o (CIITA) ha leads
o MHC class II exp ession (123, 124). Thus, IFN-γ ini ia es an
immune-an igenic exposu e p og am in he a ge cells, and his
ensu es he apid ecogni ion o s essed issues. IFN-γ is a majo
p oduc o Th1-media ed immune esponse and o ches a es
Th1 e ec o mechanisms, as u he ac i a ion o inna e immu-
ni y (mac ophages and NK cells) in a posi i e eedback loop.
Up egula ion o cell su ace MHC class I by IFN-γ is c ucial o
hos esponse o in acellula pa hogens and umo cells, due o
cy o oxic Tcell ac i a ion, p omo ing cell-media ed immuni y.
FiGURe 2 | Immunomodula o y e ec s o in e e on-gamma (IFN-γ). IFN-γ p oduced by immune cells a ec s he beha io o dis inc immune cells wi hin he umo
mic oen i onmen . Speci ically, IFN-γ plays a majo ole in ac i a ing an icance immuni y, by p omo ing he ac i i y o CD4 T helpe ype 1 cells, CD8 cy o oxic
T lymphocy e (CTL), na u al kille (NK) cells, dend i ic cells (DCs), and mac ophages, p omo ing he an igen p esen a ion. Addi ionally, IFN-γ ac i a es mac ophages
owa ds a mo e p o-in lamma o y and umo icidal pheno ype (M1-like). Al e na i ely, IFN-γ inhibi s egula o y T (T eg) cells, Th2 and Th17 di e en ia ion and unc ions.
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IFN-γ di ec ly ac s as a cy o oxic CD8 Tcell di e en ia ion signal,
and i is essen ial o he induc ion o cy o oxic Tcell p ecu -
so p oli e a ion (125, 126). IFN-γ also up egula es cell su ace
MHC class II on APCs, hus p omo ing pep ide-speci ic ac i a-
ion o CD4 Tcells (25, 127–129). In addi ion, IFN-γ ac i a es
mac ophages owa d a p o-in lamma o y p o ile, exhibi ing an
inc eased phagocy ic abili y as well as enhanced mic obial kill-
ing ac i i y (130). In ac , IFN-γ was ini ially shown o induce
“classical” ac i a ion o mac ophages and pola iza ion owa d a
umo icidal pheno ype (131). In e es ingly, he o iginal name
o IFN-γ was mac ophage ac i a ion ac o (132, 133). IFN-γ
con ols speci ic gene exp ession p og ams in ol ing mo e han
290 genes ela ed o cy okine and chemokine ecep o s, cell
ac i a ion ma ke s, cellula adhesion p o eins, MHC p o eins,
p o easome o ma ion, p o ein u no e , and signaling media-
o s and egula o s (134). The abili y o IFN-γ o induce umo
cell killing includes he ac i a ion o he NADPH-dependen
phagocy e oxidase sys em, ni ic oxide p oduc ion, yp ophan
deple ion and up egula ion o lysosomal enzymes (121, 135,
136). These e en s esul in ec ui men o e ec o cells o help
in he in lamma ion esolu ion p ocess (137, 138). In addi ion,
as a majo cy okine o Th1cells, IFN-γ main ains Th1 lineage
commi men h ough a posi i e eedback loop ha s abilizes
he Th cell pheno ype (72, 139–141) and c oss-inhibi s he di -
e en ia ion o o he Th cell subse s (Figu e2). Indeed, IFN-γ
inhibi s Th2 cell di e en ia ion (142, 143) and consequen ly IL-4
p oduc ion. This egula ion in ol es he inhibi ion o he IL-4/
STAT6 pa hway, equi ed o Th2 cell di e en ia ion, and i is
media ed a leas by IFN-γ-induced SOCS1 ha inhibi s IL-4R
signaling (144, 145). Fu he mo e, IFN-γ-induced T-be inhibi s
Th2 cell di e en ia ion by di ec ly in e e ing wi h he ac i i y o
Th2 cell-speci ic ansc ip ion ac o , GATA-3 (146). Hö e and
colleagues, using ma hema ical models, p oposed ha IL-4 also
ac s o p opaga e Th2 cell di e en ia ion (147). A high IL-4 le el
p omo es inc eased GATA-3 exp ession ha u he enhances
GATA-3 ansc ip ional imp in ing o Th2 di e en ia ion (147,
148). This model p oposed ha high exp ession s a e o GATA-3
can be supp essed by s ong inhibi ion o au oac i a ion, as
obse ed in he p esence o Th1-pola izing condi ions (147, 149).
IFN-γ was also desc ibed o down egula e he IL-4-inducible
gene exp ession (150). The c oss- egula ion o Th1 and Th2 cells
was also demons a ed in STAT6-de icien mice, which lack Th2
pheno ype and associa ed immune esponses. These animals
displayed augmen ed umo -speci ic IFN-γ p oduc ion and
cy o oxic Tcell ac i i y and, consequen ly ejec ed he umo cell
line ha g ew p og essi ely in he wild- ype con ol (151).
In e e on-gamma p oduced by Th1 cells also coun e ac s
Th17cell de elopmen and hei e ec o unc ions (152–154).
Se e al mechanisms can be conside ed as he inhibi ion o
molecules in ol ed in he Th17 di e en ia ion (155, 156), he
inhibi ion o STAT3 by STAT1 (157) and ecen ly, T-be was
demons a ed o p e en di e en ia ion o Th p ecu so s in o
Th17cells by blocking he exp ession o he Th17cell lineage-
speci ic ansc ip ion ac o , RORγ (158). Fu he mo e, IFN-γ
also exe s egula o y unc ions o limi issue damage associa ed
wi h in lamma ion (63, 159–162) (Figu e 2). IFN-γ has been
classically conside ed as a p o-in lamma o y cy okine, in ol ed
in he egula ion o an i-in lamma o y esponses, by an agoniz-
ing he IL-10 (157, 163) and TGF-be a (164) signaling pa hways.
Consequen ly, IFN-γ inhibi s T eg cell di e en ia ion and unc-
ions (165, 166). Howe e , in some ch onic in lamma ion condi-
ions, IFN-γ plays a c ucial ole in a enua ing issue des uc ion.
In his case, IFN-γ migh be p o ec i e (62, 167) by p omo ing he
numbe and unc ion o T eg cells (168–170). In addi ion, IFN-γ
p oduc ion by T eg cells hemsel es was shown o be a key ea u e
o he T eg cells ha a e capable o dampening Th1cell esponses
(171–174). Thus, IFN-γ dic a es he di e en ia ion o specialized
Foxp3+T-be + T eg cells ha selec i ely supp ess Th1cells, and
cons i u e a nega i e eedback loop o minimize he de imen al
e ec o IFN-γ. IFN-γ also p omo es he di e en ia ion o

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myeloid-de i ed supp esso cells (MDSCs) ha es ain o e ac-
i a ion o e ec o Tcells, main aining issue homeos asis (175,
176). O he egula o y mechanisms in ol ing IFN-γ signaling
ha dampen he magni ude o he immune esponse ha e been
epo ed, as he induc ion o indoleamine 2,3-dioxygenase (IDO)
by T eg cells, monocy es and s omal cells (177–180), and o he
p og ammed cell dea h 1 (PD-1) ligand (PD-L1) on immune and
ans o med cells, inhibi ing Tcell esponses (181–183).
iFN-γ ac ions on T ans o med Cells
and on he Tumo Mic oen i onmen
In e e on-gamma is in ol ed in an ip oli e a i e (18), an i-
angiogenic (184) and p o-apop o ic e ec s es ablished agains
neoplas ic cells. How IFN-γ induces he signaling pa hways
ini ia ing and p opaga ing he apop o ic cascade emains o be
elucida ed. The le el o complexi y is demons a ed by he ac
ha he mechanism migh depend on he umo cells hemsel es.
Fo example, while in a glioblas oma cell line he induc ion o
apop osis was due o supp ession o he PI3K/AKT pa hway, in
ano he glioblas oma cell line apop osis occu ed independen ly
o he PI3K/AKT pa hway bu equi ed NF-κB (185). I was also
shown ha IFN-γ induces apop osis o human panc ea ic ca -
cinoma cells in a caspase-1-dependen manne (186). A e iew
co e ed in de ail he mechanism o induc ion o p og ammed
cell dea h (187). So a , he known biological unc ions o IFN-γ
indica e ha , al hough i can ac as a po en induce o an i umo
immuni y, i ac ually has a dual ole and may also a o umo
immune e asion.
iFN-γ iN CaNCeR
The i s epo s poin ing o he ele ance o IFN-γ in an i umo
immuni y came om s udies wi h he ib osa coma (Me h A) cell
line, e ac o y o IFN-γ signaling, since i lacks he exp ession
o he IFNγR1 subuni . IFN-γ-insensi i e Me h A cells displayed
enhanced umo igenici y compa ed wi h con ol cells and we e
no ejec ed in syngeneic umo mice models, sugges ing ha
IFN-γ plays an impo an ole in umo cell elimina ion (18).
This inding was u he suppo ed by expe imen s using 129/
SV IFN-γ insensi i e mice, lacking he IFNγR1 subuni o
STAT1, which de eloped 3-me hylcholan h ene (MCA)-induced
sa comas mo e apidly and mo e equen ly han hei wild- ype
coun e pa s (12). Simila ly, hese IFN-γ-insensi i e mice lacking
he umo -supp esso p o ein p53 o med spon aneous umo s
mo e apidly han IFN-γ-sensi i e p53-de icien mouse (12).
In addi ion, C57BL/6 mice ha lack he gene encoding IFN-γ
also displayed highe suscep ibili y o expe imen al (B6, RM-1
p os a e ca cinoma) and spon aneous (BALB/c, DA3 mamma y
ca cinoma) models o p ima y and me as a ic umo s (13, 14).
No ably, u he s udies desc ibed ha IFN-γ may coope a e
wi h o he molecules o p e en umo o ma ion. Mice de icien
in bo h g anulocy e/mac ophage colony-s imula ing ac o
(GM-CSF) and IFN-γ de eloped lymphoma and non-lymphoid
solid umo s a a highe a e han did mice de icien in GM-CSF
o IFN-γ alone (15). Addi ional s udies e ealed ha mice insen-
si i e o IFN-γ, o ha lack he ecombina ion ac i a ing gene
(RAG) p o ein ( ailing o p oduce ma u e B and Tlymphocy es),
o ha lack bo h, showed simila incidence o MCA-induced
sa comas, sugges ing ha he Tcell–IFN-γ axis is in ol ed in
immune su eillance (10).
The ole o IFN-γ on cance immunoedi ing eme ged om
s udies assessing he immunogenici y o umo s om immuno-
compe en e sus immunode icien mice. Kaplan e al. showed
ha MCA-induced sa coma cells om IFNγR1-de icien mice
(un esponsi e o IFN-γ signaling) g ow as agg essi ely in immu-
nocompe en as in IFNγR1-de icien mice. Howe e , when IFN-γ
esponsi eness was con e ed on he umo cells by in oducing
he IFNγR1 subuni , hey became mo e immunogenic and we e
ejec ed h ough a Tcell-dependen manne (12). This cons i-
u es he i s demons a ion ha IFN-γ sensi i i y o he umo
is undamen al o an e icien an i umo esponse. O he s udies
e ealed ha wild- ype hos s ejec ed 40% o MCA-induced
sa comas de i ed om RAG2-de icien mice, showing ha hese
umo s we e mo e immunogenic han hose om wild- ype
mice (10). In addi ion, human umo s we e e alua ed o hei
abili y o up egula e MHC I exp ession in esponse o IFN-γ
s imula ion. These s udies e ealed ha 33% o 33 melanoma
umo cell lines showed a educ ion in IFN-γ sensi i i y while 4
o 17 lung adenoca cinoma cell lines we e o ally un esponsi e o
IFN-γ (12). This lack o esponse esul ed om cellula de ec s
on IFNγR1 and o JAK p o eins and may explain he abili y o
many umo cells o e ade he immune esponse. Recen ly,
JAK1/2 de iciency was demons a ed o p o ec melanoma cells
om an i umo IFN-γ ac i i y and esul s in T-cell- esis an
melanoma lesions (188). O he s epo ed he lack o STAT1 in
melanoma cell lines and in some ch onic myeloid leukemia cells
(189). Fu he mo e, DNA me hyla ion ha selec i ely ep esses
CIITA, in colo ec al and gas ic cance cell lines, was associa ed
wi h he absence o IFN-γ-induced HLA-DR, sugges ing ha
his epigene ic al e a ion o CIITA enables some gas oin es inal
cance cells o e ade he immune sys em (190). Concomi an ly,
epigene ic al e a ions ep essing MHC2TA we e desc ibed in
Tcell leukemias, Bcell lymphomas, and in se e al cance cells,
such as small cell lung cance and neu oblas oma cells ha we e
unable o exp ess MHC II upon IFN-γ s imula ion (191–194).
Consis en ly, IFN-γ up egula es CIITA exp ession on mul iple
myeloma and melanoma cells inc easing hei MHC II exp es-
sion (123, 124). These indings indica e ha IFN-γ ac s on umo
cells, enhancing hei ecogni ion by CD8 Tcells as well as by
CD4 Tcells, and un eiling a key ole in he p omo ion o umo
immunogenici y. Al oge he , hese wo ks pa e he way o he
elabo a ion o he s epping-s one concep o immunoedi ing
p omo ed by IFN-γ (195, 196).
iFN-γ-Media ed Mechanisms Unde lying
an i umo igenic e ec s
As desc ibed ea lie , he mechanisms by which IFN-γ exe s
i s an i umo e ec s depend on mul iple p ocesses. IFN-γ is
desc ibed as an an ip oli e a i e agen ha egula es he exp es-
sion o cyclin-dependen kinase inhibi o 1 (p21) h ough STAT1
ac i a ion in umo cells (84, 197). Mo eo e , IFN-γ is able o
p omo e umo cells apop osis by up egula ing he exp ession o
caspase-1, -3, -8 (198, 199) and by enhancing he sec e ion o
FAS and FAS ligand (200) and TNF- ela ed apop osis-inducing
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ligand (201, 202). Recen s udies showed ha IFN-γ also induces
i s umo icidal e ec s h ough a o m o egula ed nec o ic dea h
(also named as nec op osis) ha elies on he ac i i y o he
se ine– h eonine kinase RIP1 (203). Impo an ly, IFN-γ is also
in ol ed in he inhibi ion o angiogenesis, impai ing he p oli -
e a ion and su i al o endo helial cells, inducing ischemia in he
umo s oma (184, 204, 205). In pa icula , IFNγR is exp essed
on blood endo helial cells and engagemen o he ecep o esul s
in blood essel des uc ion and nec osis, an impo an mecha-
nism ha leads o umo ejec ion (206).
Conside ing he e ec o IFN-γ on he hos immune cells
p esen a he umo mic oen i onmen , majo e o s ha e
been made o he de elopmen and es ablishmen o combined
clinical he apeu ic applica ions (90, 151, 207). IFN-γ is c i ical
o Tcell, NK and NKTcell a icking in o he umo s h ough
CXCL9, CXCL10, and CXCL11 chemokine induc ion (208, 209).
Acco dingly, Tcells ail o mig a e o umo si e in IFNγ-de icien
mice (65). In commi men , dipep idylpep idase 4 inhibi ion, a
p o ease ha inac i a es hese chemokines, enhanced umo
ejec ion by inc easing lymphocy es a icking in o he umo
(210). La ely, galec in-3 sec e ed by se e al umo s was dem-
ons a ed o bind glycosyla ed IFN-γ a he umo ex acellula
ma ix, a oiding IFN-γ di usion and he o ma ion o an IFN-
γ-induced chemokine g adien equi ed o Tcell ec ui men
and in il a ion (211). In addi ion, CXCL10 also p e en s umo
angiogenesis by blocking endo helial cell p oli e a ion (212)
and consequen ly a dec ease in mic o essel densi y as obse ed
in melanoma umo xenog a s (213). Apop osis o endo helial
cells by IFNs causes es ic ion o blood low wi hin he umo
ascula u e, leading o umo sh inkage (214). This is an e ec o
IFN-γ, no di ec ly a ge ed o he umo cell, bu o he umo
ascula u e, wi h d as ic and desi able e ec s on umo g ow h. A
ecen epo also showed ha IFN-γ was essen ial o he ini ial
p iming and di e en ia ion o cy o oxic Tcells esiding in he
pe iphe y o he eye, con ibu ing o he eg ession o in aocula
umo s (215). Suppo ing da a om he apy models showed ha
IFN-γ induces su i in and i i202, wo genes in ol ed in Tcell
ma u a ion, su i al, and p oli e a ion, in umo -speci ic Tcells
(216). O e all, hese s udies demons a ed he ele ance o IFN-γ
on Tcell-media ed an i umo immuni y.
In e e on-gamma is also in ol ed in mac ophages umo i-
cidal ac i i y (217). This cy okine suppo s a CD4 T cell/
mac ophage e ec o axis which ac s as immune su eillance
mechanism o MHC II-nega i e cance cells (25). Indeed, upon
ecogni ion o umo an igens p esen in he con ex o MHC II
by mac ophages, CD4 Tcells sec e e IFN-γ ha u he ac i a es
mac ophages in he umo , leading o umo g ow h inhibi ion
(25). This collabo a ion be ween CD4 Tcells and mac ophages
was also essen ial o success ul cance immune su eillance in
non-solid cance s, as myeloma and B-cell lymphoma. Indeed,
Th1-sec e ed IFN-γ was shown o igge a cy o oxic ac i i y
o umo -associa ed mac ophages (TAMs) and also induces
CXCL9/MIG and CXCL10/IP-10 sec e ion by mac ophages,
which may a ec he umo p og ession by angiogenesis
inhibi ion (129). IFN-γ-ac i a ed mac ophages also acqui e a
umo icidal pheno ype wi h he up egula ion o cy o oxici y-
associa ed ma ke s including g anzyme A/B, and NKG2D (129).
In addi ion, in STAT6-de icien mice, ha display inc eased
le els o IFN-γ, ejec ion o me as a ic disease a e emo al o
he p ima y umo in ol ed he gene a ion o p o-in lamma o y
mac ophages, also e med M1-like mac ophages, and a dec ease
in MSDCs ha accumula ed du ing p ima y umo o ma ion
(218). S udies om APCMin/+ mice ( ha a e highly suscep ible
o spon aneous in es inal adenoma o ma ion) lacking IFN-γ
signaling showed an accumula ion o TAMs, mo e p one owa ds
p o umo al (M2-like) pola iza ion, and up egula ion o ma ix
me allop o eases. These esul s sugges ha IFN-γ un esponsi e-
ness con ibu es o he c ea ion o an an i-in lamma o y mic o-
en i onmen , a o able o in es inal umo igenesis (219). The
p ope ies o IFN-γ o e e se he myeloid immunosupp essi e
unc ions we e also demons a ed in p o umo ole o human
o a ian TAMs (220) and human M2-like mac ophages (221).
Impo an ly, IFN-γ has also a key ole on IL-12 p oduc ion,
suppo ing he ac i i y o his la e cy okine in cance immune
su eillance (222–225). Indeed, exogenous IL-12 adminis a ion
in o ib osa coma-bea ing mice esul ed in a comple e umo
eg ession (222). This obse a ion was ex ended o p ima y
umo igenesis models ea ed wi h exogenous IL-12 (226, 227).
Consis en wi h his, chime ic an igen ecep o - edi ec ed Tcells
enginee ed o p oduce IL-12 whe e ound o sec e e inc eased IFN-
γ le els and o display enhanced an i umo cell ac i i y (228–230).
Rega ding he impo ance o IFN-γ in cance diagnos ics, IFN-
γ-associa ed signa u es ha e a p edic i e alue in cance immune
pheno ypes (81, 231, 232). In addi ion, IFN- ela ed gene signa-
u e is a p edic i e ma ke o chemo he apy and adio he apy
e iciency o b eas cance (94) as well as o PD-1 o cy o oxic
Tlymphocy e an igen-4 (CTLA-4) blockade in a ious ypes o
malignancies (233–235). Consis en ly, immuno he apy using
immune checkpoin blocke s (an i-CTLA-4 and/o an i-PD-1)
combined wi h an icance accines, clea ly associa e inhibi ion
o umo g ow h wi h inc eased p opo ion o IFN-γ-p oducing
e ec o Tcells (236, 237). This is also e i ied in clinical ials,
h ough which he an i-CTLA-4 he apy was associa ed wi h an
inc ease o IFN-γ-p oducing ICOS+ (inducible cos imula o y)
CD4 Tcells and o T e ec o /T eg cell a io in bladde cance
samples (238). In addi ion, PD-1 blockade was demons a ed
o enhance T cell in il a ion by p omo ing IFN-γ-inducible
chemokines (239). In o he way, i was ecen ly shown ha
IFN-γ-induced T eg cell agili y (loss o supp essi e unc ion) is
equi ed o esponse o an i-PD-1 he apy (240).
Al oge he , he e sa ili y o IFN-γ and i s ine- uned biologi-
cal e ec s highligh i s ele ance o he apeu ic applica ions,
and some clinical ials ha e al eady encou aging esul s. In ac ,
75% o me as a ic melanoma pa ien s we e non- esponde s o
an i-CTLA-4 he apy, and his was associa ed wi h genomic
de ec s o IFN-γ signaling genes on umo s (241). Recen ly,
apelin ecep o (APLNR) was desc ibed o egula e JAK/
STAT signaling, modula ing IFN-γ esponses. Mul iple loss-
o - unc ion mu a ions in APLNR we e iden i ied in pa ien
umo s e ac o y o immuno he apy (242). The inclusion o
IFN-γ in he i s -line ea men o o a ian cance esul ed in
bene i ega ding p og ession- ee su i al, wi h accep able ox-
ici y (243). IFN-γ ea men also appea s o be e ec i e agains
bladde umo s by ec ui men and ac i a ion o in a umo al
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leukocy es (244). In a phase I clinical ial, which combined
adop i e T cell he apy wi h in alesional adminis a ion o
adeno i us exp essing IFN-γ in me as a ic melanoma, 38.5% o
he pa ien s had an o e all objec i e esponse and 46% we e able
o con ol he disease (245).
iFN-γ-Media ed Mechanisms Unde lying
P o umo igenic e ec s
I is becoming inc easingly clea ha IFN-γ can exe ce ain
e ec s suppo ing umo igenesis. Immune e asion can ope a e
h ough umo cells losing esponsi e o IFN-γ signaling o
a oid i s an ip oli e a i e, p o-apop o ic, and immuno egula-
o y ac ions. This has been demons a ed wi h he umo cells
losing he ecep o o IFN-γ o a componen o JAK/STAT
signaling (12, 18). In addi ion, cons i u i e ac i a ion o inhibi-
o y molecules o his pa hway, as SOCS1 and SOCS3, limi s he
ac ions o IFNs on human melanoma cells (246) and a o s he
ac i a ion o al e na i e signaling pa hways, as STAT3, which
is associa ed wi h umo p og ession (247). These e idences
sugges ha umo cells de elop IFN-γ-dependen s a egies
o e ade he immune sys em, leading o he eme gence o e y
agg essi e umo s, which a e on he basis o immunoedi ing. In
2011, Zaidi and Me lino p oposed ha IFN-γ ac ions migh play
a physiological ole in p o ec ing cells om damage in a se ing o
issue emodeling and epai , while on cells ha bo ing oncogenic
mu a ions, he same mechanisms may p e en cell des uc ion
and allow comple e ans o ma ion (248). Consis en wi h his,
NF-κB in umo cells was shown o ac as a p o ec i e mechanism
agains IFN-γ-induced nec op osis (203).
Indeed, he e a e signi ican e idences ha umo cells can
ake he ad an age o IFN-γ as an induce o an i-in lamma o y
esponses and p o umo e ec s. The i s epo o he nega i e
po en ial e ec s was in 1987 by Taniguchi and colleagues who
p oposed ha IFN-γ changes he me as a ic abili y o he B16
melanoma cells in a cell-au onomous manne (249). Da a om
expe imen s using he CT26 colon ca cinoma model showed
ha IFN-γ p omo es umo escape h ough he down egula ion
o he endogenous umo an igen gp70 (250). IFN-γ exp ession
by human melanoma samples was associa ed wi h enhanced
exp ession o MHC class II molecules and he acquisi ion o a
mo e agg essi e pheno ype (251, 252).
One o he p incipal mechanisms o umo immune escape
is he supp ession o cy o oxic Tcells and o NKcell-media ed
immune esponses. B ody and colleagues showed ha IFN-γ
up egula es IDO in melanoma cells and ec ui s T eg cells o
a oid immune ecogni ion (253). Cu iously, IFN-γ induced IDO
compe ence on human monocy e-de i ed DCs bu had no e ec
on p o-in lamma o y cy okine elease, sugges ing ha IFN-γ
igge s IDO ac i i y and p o-in lamma o y cy okine elease
as dis inc cellula p og ams. In addi ion, IDO-compe en DCs
induced egula o y ac i i y on allogeneic Tcells (179). IFN-γ was
also desc ibed o be in ol ed in he accumula ion o MDSCs in
in lamed li e , which leads o Tcell supp ession (254). MDSCs
p oducing ni ic oxide dec eased IFN-γ esponsi eness o
immune cells, such as T and NKcells (255).
One impo an aspec is he abili y o IFN-γ o induce PD-L1
exp ession in cance , s omal and myeloid cells o impai e ec o
umo immuni y (181). Abiko and colleagues demons a ed ha
he con ac be ween umo cells and CD8 Tcells is necessa y o
he induc ion o PD-L1, unde lying he impo ance o pa ac ine
exposu e o IFN-γ (256). Recen epo s sugges ha loss o IFN-γ
pa hway genes, such as JAK1 and JAK2, is associa ed wi h esis -
ance o an i-PD-1 he apy (257, 258). P olonged IFN-γ signaling
in umo s was also shown o coo dina e PD-L1-dependen and
PD-L1-independen esis ance o immune checkpoin blockade
and o o he he apeu ic combina ions, such as adia ion and
an i-CTLA-4, h ough a mul igenic esis ance p og am (259).
In addi ion, o he inhibi o y pa hways a e ein o ced by IFN-γ,
including CTLA-4 and CD86/CD80 in e ac ion (260).
In e e on-gamma was used in clinical ials o melanoma
bu no signi ican imp o emen o pa ien s was obse ed
(261–264). In ac , IFN-γ ea men had no con ibu ion o
he ou comes o pa ien s wi h me as a ic enal cell ca cinomas
(265), leukemia (266), panc ea ic ca cinoma (267), b eas can-
ce (268), o in o he pos ope a i e su gical he apy o colon
cance (269). Fu he mo e, a phase 3 ial o IFN-γ plus s and-
a d ea men wi h ca bopla in/pacli axel e sus ca bopla in o
pacli axel alone, o ea ed ad anced o a ian umo s, was ea ly
e mina ed due o a highe incidence o se ious hema ological
oxici ies in pa ien s ecei ing combined he apy compa ed wi h
chemo he apy alone (270). The ailed a emp s o ea cance
pa ien s wi h exogenous IFN-γ aised se e al conce ns: he
absence o umo immunogenici y, he lack o IFN-γ-signaling
componen s, he up egula ion o IFN-γ signaling inhibi o s,
he immunosupp essi e umo mic oen i onmen , he lack o
e ec o Tcells, o p esence o ane gic Tcells and, in some cases
oxici y. These accumula ing e idences ein o ce he impo ance
o de e mine he g ade o pa ien s’ IFN-γ- esponsi eness. Fo
example, in cases wi h low IFN-γ ac ions, ac i e immuniza ion
ei he ia IFN-γ ea men o ia adju an s o he immune
sys em, as oll-like ecep o ligands, should be conside ed, as
demons a ed ecen ly by using bac e ial ou e memb ane esi-
cles ha e adica e es ablished umo s in an IFN-γ-dependen
mechanism (271). The combina ion wi h adio- and chemo-
he apy is expec ed o be use ul h ough immunogenic cell dea h
ha also elici s he inna e immune sys em. P omising esul s
we e ob ained wi h combina ion o low-dose 5- luo ou acil
wi h ecombinan in e e on-gamma (IFN-γ) in pa ien s wi h
ad anced hepa ocellula ca cinoma (272). In cases wi h high
le els o IFN-γ signaling, he he apy wi h an i-PD-1/an i-PD-
L1 is expec ed o be impo an .
O e all, hese indings indica e ha he local immune
mic oen i onmen o umo s is complex and a iable and ha
o an e ec i e he apy i is essen ial o e alua e, indi idually,
he immune p o ile o pa ien s o immune con ex u e [ e iewed
in Re . (232, 273)], aking in o accoun ha i may e ol e and
modi y h oughou he an icance he apy (Figu e3).
iFN-γ iN THeRaPY—wHeRe aRe
we aND wHeRe aRe we GOiNG?
In e e on-gamma he apy has ensued in clinical applica ions
app o ed by he Food and D ug Adminis a ion in he ea -
men o ch onic g anuloma ous disease, in 1999 and se e e
FiGURe 3 | Dual ace o in e e on-gamma (IFN-γ) in umo immuni y. IFN-γ can display bo h an i umo and p o umo ac i i ies. Unde bo h ci cums ances, IFN-γ
in luences umo cells di ec ly and indi ec ly, by ac i a ion o immune cells. The an i umo e ec s comp ise he de elopmen , ec ui men , and ac i a ion o inna e
immune cells as well as he ac i a ion and main enance o e ec o Tcells. The an i umo e ec s o IFN-γ esul in di ec inhibi ion o umo p oli e a ion, ecogni ion,
and elimina ion. In o he way, he p o umo igenic ole o IFN-γ in ol es p oli e a i e and an iapop o ic signals, as well as escape o he umo cells om ecogni ion
and cy olysis by cy o oxic Tlymphocy es (CTLs) and na u al kille (NK) cells. The b oad ange o IFN-γ ac ions depends on he con ex o umo speci ici y,
IFN-γ-signaling in ensi y, and o he mic oen i onmen condi ions.
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F on ie s in Immunology | www. on ie sin.o g May 2018 | Volume 9 | A icle 847
malignan os eope osis, in 2000. Despi e he p omising
he apeu ic applica ions o IFN-γ in se e al se ings, i s limi ed
success in cance -immuno he apy ials migh be due o cance
cell un esponsi eness o his cy okine, he ailu e o deli e i
locally o wi h he adequa e pe iodici y o achie e a he apeu ic
e ec . Mo eo e , IFN-γ clinical use has also been es ic ed
due o se e al limi a ions inhe en o i s molecula p ope ies.
Essen ially, hese include s abili y p oblems, such as acid deg-
ada ion, and also he endency o agg ega e i e e sibly unde
mild dena u ing condi ions, wi h subsequen loss o biological
ac i i y [ he pha macological aspec o IFN-γ is e iewed in
Re . (274, 275)]. Fu he mo e, IFN-γ is apidly clea ed om he
blood when adminis e ed in a enously (276), equi ing equen
e-adminis a ions o high cy okine concen a ions, o elici an
e ec i e esponse a he a ge si e, leading o sys emic oxici y
and side e ec s, such as e e , a igue, nausea, omi ing, dia hea,
neu o oxici y, and leukopenia (277). These ad e se e ec s a e
caused mainly by high se um concen a ion o he p o ein, due
o an unequal dis ibu ion be ween body luids and issues (276)
and, addi ionally, o he ubiqui y o ecep o s which a e exp essed
a he memb ane o he majo i y o human cells (278, 279) and
also o he exis ence o a ci cula ing soluble o m (which unc ion
emains elusi e) (280).
These cons ain s in he clinical use o IFN-γ ha e encou aged
he de elopmen o al e na i e deli e y me hods wi h he pu pose
o achie ing highe he apeu ic ou comes and, simul aneously,
weaken i s oxici y. Nume ous epo s ha e ocused mainly on
e icien ou es o deli e y a he han on sys emic applica ions
(281–287). In ac , IFN-γ is na u ally p oduced in a pa ac ine
manne , wi h local sec e ion and di usion o he su ounding
cells and mic oen i onmen h oughou he ex acellula luids
(288). The e o e, a localized deli e y o his cy okine has been
16
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