Harnessing innate immunity in cancer therapy

Prévia do material em texto

Review
https://doi.org/10.1038/s41586-019-1593-5
Harnessing innate immunity in cancer 
therapy
Olivier Demaria1, Stéphanie Cornen1, Marc Daëron2,3, Yannis Morel1, Ruslan Medzhitov4 & eric vivier1,2,5*
New therapies that promote antitumour immunity have been recently developed. Most of these immunomodulatory 
approaches have focused on enhancing T-cell responses, either by targeting inhibitory pathways with immune checkpoint 
inhibitors, or by targeting activating pathways, as with chimeric antigen receptor T cells or bispecific antibodies. 
Although these therapies have led to unprecedented successes, only a minority of patients with cancer benefit from 
these treatments, highlighting the need to identify new cells and molecules that could be exploited in the next generation 
of immunotherapy. Given the crucial role of innate immune responses in immunity, harnessing these responses opens 
up new possibilities for long-lasting, multilayered tumour control.
i mmuno-oncology is emerging as a revolution in cancer treatment. Immune checkpoint inhibitors, such as therapeutic monoclonal anti-bodies directed against the PD-1–PD-L1 (programmed cell death 
protein 1–programmed cell death ligand 1) pathway, have been approved 
for use as monotherapies or in combination treatments for several indi-
cations1. One of the main goals of immune checkpoint inhibitors is to 
unleash effector T cells. The key role of T cells in tumour immunity has 
been demonstrated by the positive correlation between T-cell infiltration 
at the tumour bed and prognosis2, and the clinical success of chimeric 
antigen receptor (CAR) T-cell infusions in some haematologic malig-
nancies3. These clinical successes have led to a T-cell-centred view of 
tumour immunity4–6. However, T cells are not autonomous in their effec-
tor functions. The onset and maintenance of T-cell responses and the 
development of long-lasting protective memory T cells depend on innate 
immune responses7 (Fig. 1). Innate immunity involves various types of 
cells of the myeloid lineage, including dendritic cells (DCs), monocytes, 
macrophages, polymorphonuclear cells, mast cells, and innate lymphoid 
cells (ILCs), such as natural killer (NK) cells7,8. Throughout the body, 
innate immune cells detect molecular alterations, such as changes induced 
by microbial infections7,9. These cells then launch adaptive immune 
responses while mounting their own effector responses, such as phago-
cytosis for macrophages and polymorphonuclear cells, and natural cyto-
toxicity for NK cells7, 8. Through the expression of Fc receptors (FcRs) for 
antibodies, innate immune cells also participate in effector responses after 
the induction of antibodies, via antibody-dependent cellular phagocytosis 
(ADCP) or antibody-dependent cell cytotoxicity (ADCC).
The interplay between innate and adaptive immunity in cancer can be 
illustrated by the key role of antigen-presenting cells (APCs) in eliciting 
the function of tumour-specific CD8 T cells in many tumour conditions 
in mice and humans10. The uptake of tumour antigens by APCs leads 
to cross-presentation—the presentation of exogenous antigens by major 
histocompatibility class I (MHC-I) molecules—and this induces prim-
ing of tumour-specific CD8 T cells and the expansion of this cell pop-
ulation. A specific subset of DCs specializes in cross-presentation11. In 
humans, these cells express XCR1, CLEC9A and CD141 and are referred 
to as cDC1s, whereas in mice they are CD8α+ DCs12 that express XCR113 
and CLEC9A14. No tumour-specific CD8 T cells are generated in mice 
that lack CD8α+ dendritic cells15, showing that these APCs are required 
for T-cell activity against tumours11. cDC1s also produce CXCL9 and 
CXCL10, which recruit tumour-specific T cells to the tumour16 (Fig. 2). 
In humans, a cDC1 signature is a strong prognostic factor associated 
with better survival10 and responsiveness to anti-PD-1 therapy17. 
Dysfunctional states have been described for T cells in tumours, but 
the APCs infiltrating the tumour microenvironment are also thought 
to have defective stimulatory functions. These defects may contribute to 
the impairment of T-cell activation and efficacy despite the availability of 
tumour neoantigens that can drive specific antitumour T-cell responses18.
NK cells are another type of innate immune cell that is involved in anti-
tumour immunity. NK cells are ILCs equipped with a panel of activating 
receptors, such as NKG2D and the NKP46, NKP30 and NKP44 natural 
cytotoxicity receptors (NCRs), which can detect cell-surface and soluble 
markers of stressed cells, such as tumour cells. In particular, DNA damage 
can generate NK ligands on tumour cells19. The activity of NK cells can be 
hampered by the engagement of inhibitory cell surface receptors, such as 
killer cell immunoglobulin-like receptors (KIRs) or NKG2A, which con-
tain intracytoplasmic immunoreceptor tyrosine-based inhibition motifs 
(ITIMs) and recognize MHC-I molecules expressed on tumour cells20. 
Upon interaction with diverse tumour types, NK cells can kill their targets 
and secrete cytokines, such as interferon-γ (IFNγ) and tumour necrosis 
factor α (TNF), and growth factors, such as granulocyte–macrophage 
colony-stimulating factor (GM-CSF)20. NK cells can also contribute to 
infiltration of cDC1s into tumours, through the production of CCL5, 
XCL1 and FLT3LG17,21. Activation of NK cells at the tumour site can thus 
contribute to antitumour immunity through a combination of direct cyto-
toxic activities and the promotion of adaptive responses through IFNγ 
secretion and cDC1 regulation (Fig. 2).
Here, we review the roles of innate immunity in antitumour responses, 
by highlighting the mechanisms by which innate immune cells can detect 
tumours, induce and amplify adaptive immune responses, and exert 
direct effector responses, and the mechanisms by which these responses 
are suppressed at the tumour bed. Multiple attempts have been made to 
manipulate innate immune responses in cancer. We limit our discussion 
to molecules that have led to strong preclinical data or promising signals 
in early clinical trials (Table 1, Supplementary Table 1).
Tumour recognition by innate immune cells
Tumour cells are highly heterogeneous and, despite the known role of 
somatic DNA mutations in eliciting the transformation of a normal cell 
1Innate Pharma, Marseille, France. 2Aix Marseille Université, INSERM, CNRS, Centre d’Immunologie de Marseille-Luminy, Marseille, France. 3Institut Pasteur, Paris, France. 4Howard Hughes Medical 
Institute and Department of Immunobiology, Yale University School of Medicine, New Haven, CT, USA. 5Service d’Immunologie, Marseille Immunopole, Hôpital de la Timone, Assistance Publique-
Hôpitaux de Marseille, Marseille, France. *e-mail: vivier@ciml.univ-mrs.fr
Corrected: Author Correction
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https://doi.org/10.1038/s41586-019-1593-5
mailto:vivier@ciml.univ-mrs.fr
https://doi.org/10.1038/s41586-019-1758-2
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into a malignant cell, attempts to identify molecular pathways that are 
common to all tumours have been unsuccessful. There is also no evi-
dence that common determinants or patterns expressed by tumour cells 
can be recognized by the immune system. Instead, immune cells can 
sense several types of modification associated with various tumours, but 
not specific to these tumours. These modifications include changes in 
cell metabolism22, tissue metabolism (such as hypoxia), or tissue anat-
omy (such as organ lesions) that may also occur in other conditions. 
The therapeutic manipulation of the immune pathways that detect these 
modifications offers new possibilities to enhance the immune control 
of tumours.
Antimicrobial immunity
Pathways that were initially shown to contribute to the detection of 
viral or bacterial infections have also been found to participate in the 
detection of tumour cells. On the basis of current knowledge, there-
fore, tumoursensing does not appear to involve specific machinery. 
Instead, it makes use of pathogen and damage receptors. The detection 
of nucleic acids is a prime example of the use of infection detection 
pathways to drive the recognition of growing tumours and elicit a 
tumour-specific immune response (Box 1). Nucleic-acid sensing 
involves the endosomal Toll-like receptors (TLRs), and cytosolic nucleic 
acid sensors, such as STING (stimulator of interferon gene, which is 
also known as transmembrane protein 173: TMEM173) and RIG-I-like 
receptors (RLRs). Synthetic molecules resembling those induced by 
infection have been generated to stimulate a ‘pathogen-induced-like’ 
innate immune response at the tumour site for therapeutic purposes. 
Injection of these immunostimulatory products into tumours would 
be expected to increase their bioavailability and decrease their toxicity, 
by preventing the induction of the inflammatory syndromes associated 
with their systemic administration. In intratumoral immunotherapy, 
the tumour acts as its own vaccine, eliciting immune responses against 
cancer cell antigens and the generation of polyclonal antitumour 
immunity23. This strategy differs from other strategies that require 
prior identification of tumour antigens (for example, vaccination and 
CAR T cells), because it makes use of the endogenous antigen reper-
toire that is present within the tumour. The synthetic targeting of TLRs, 
STING and RLRs has yielded promising results in preclinical models, 
reducing tumour size at both the injection site and more distal sites (an 
abscopal effect). These activities are mediated by multiple processes, 
including the direct induction of tumour cell death, the production of 
type I IFNs, proinflammatory cytokines and T cell-tropic chemokines, 
NK cell cytotoxicity, phagocytosis, DC maturation and the promotion 
of tumour-specific CD8 T cells, thereby ensuring long-term systemic 
protection. The efficacy of intratumoral agonists is further increased 
by immune checkpoint blockade and depletion of regulatory T (Treg) 
cells24–26. In patients, the intratumoral administration of a TLR9 ago-
nist in combination with treatment with a PD-1 inhibitor has bene-
ficial effects on the tumour microenvironment and increases type I 
IFN production and infiltration of the tumour by CD8+ T cells. These 
effects are correlated with durable tumour responses27, with an over-
all response rate (ORR, n = 45) of 71% in metastatic melanoma28. 
Intratumoral injections of STING agonists have been shown to be 
highly effective for controlling tumour growth in mice through the 
induction of CD8+ T-cell responses and the promotion of responses 
to immune checkpoint blockers29,30. However, the efficacy of STING 
agonists may be limited by the dose injected intratumorally, as high 
doses of agonist may reduce the formation of an immunological mem-
ory and systemic efficacy31. In humans, STING agonists have been 
tested in early clinical trials. No complete or partial responses have 
been reported for a STING agonist alone, but combined treatment of 
advanced solid tumours or lymphomas with PD-1 blockade produced 
a trend towards a possible positive signal32. RIG-I agonists have also 
recently reached clinical trials. Preliminary interim data for a RIG-I 
agonist as a single agent have revealed signs of clinical efficacy, yielding 
disease stabilization in a few patients33. However, exploratory clini-
cal trials often include limited numbers of patients, with sometimes 
heterogeneous patient populations. Efficacy data are therefore difficult 
to analyse and cross-trial comparisons are very speculative. Further 
controlled clinical studies are required to determine the importance of 
each of the innate immune pathways targeted by intratumoral injec-
tions in cancer therapies more accurately.
Intrinsic differences34, including the clonal heterogeneity of tumour 
cells, molecular divergence, variations of antigen repertoire and in the 
immune infiltrate35, have been described between primary tumours 
and growing metastases within the same individual. Therefore, in 
addition to the lack of feasibility of injection directly into the lesion 
for some tumours, differences in distal lesions rendering them untarg-
etable by the immune responses induced in the treated lesion may limit 
the efficacy of intratumoral approaches for inducing abscopal effects. 
One strategy for overcoming the problem of heterogeneity between 
lesions within a given individual would be the systemic administration 
of agonists that target all of the different tumour sites. However, other 
approaches are currently favoured because of the toxicity of systemic 
agonist administration. Oncolytic viruses are genetically modified 
replication-competent viruses that can be administered systemically 
and selectively infect or replicate within malignant cells, inducing the 
specific killing of tumour cells while sparing the surrounding normal 
cells. Another possible approach involves the use of antibody adjuvant 
conjugates—immune agonists bound to an antibody directed against 
a broadly represented tumour antigen—making it possible to deliver 
the adjuvant specifically to all tumour sites. This strategy is currently 
being tested in a clinic trial to target TLR7 in HER2-positive solid 
tumours (NCT03696771).
Stress responses
Chemotherapy and radiotherapy can render tumour cells immunogenic 
through a process known as immunogenic cell death (ICD). ICD has 
been described in preclinical models, but it has not been shown to be 
induced or to promote antitumour immunity in treated patients. It has 
been suggested that several features of dying tumour cells known as 
damage-associated molecular patterns (DAMPs) trigger an immune 
response36. DAMPs include ‘eat-me’ signals, such as the cell-surface 
exposure of calreticulin, protein disulfide isomerase family A member 
3 (PDIA3), and heat shock proteins HSP70 and HSP90, which promote 
the uptake of cells or cell debris by APCs. ICD also induces the secre-
tion of immunostimulatory factors, such as ATP, IL-1β, type I IFNs 
and, subsequently, CXCL10, annexin A1 and high-mobility group 
protein B1 (HMGB1). These molecules can modulate the composition 
Table 1 | Current clinical landscape of drugs targeting innate 
immunity
Capitalizing on antimicrobial immunity for tumour control
TLR9, TLR7, TLR8, RIG-I, STING, TLR4, NLRP3
Induction and amplification of the immune response
APC activation, expansion and/or 
differentiation
FLT3L, GM-CSF, CD40
Costimulatory signals 4-1BB, OX40, ICOS, CD27, GITR
Cytokines IFNα2a, IFNα2b, IL-2, IL-15, IL-12, IL-10
Promotion of the effector responses of innate immunity
Broad spectrum immune 
checkpoint inhibitors
NKG2A, LAG-3, TIGIT, TIM-3, VISTA, 
CD32b, CD47, SIRPα, LILRB2, PVRIG
Relieving immune suppression at the tumour bed
Immunosuppressive factors A2AR, CD73, CD39, IDO, TGFβ, EP4, 
Arginase
Suppressive cell recruitment, 
depletion and polarization
C5aR1, IL-1β, IL-1α, CXCR2, CCR2, 
CCR5, ApoE, STAT3, PI3Kγ, CSF-1R
A summary of selected clinical programs targeting innate immunity in cancer therapy active 
as of September 2019. The table is organized according to four modes of action: drugs that 
capitalize on antimicrobial immunity for tumour control; drugs that induce or amplify the innate 
immune response; drugs that promote the effector responses of innate immunity; and drugs that 
relieve immune suppression at the tumour bed. A more detailed list of the selected active clinical 
programs, including target, drug name, current clinical phase, developer and drug format, is 
provided in Supplementary Table 1.
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and function of both innate and adaptive immune cells in the tumour 
microenvironment. Thus, liposomal doxorubicin, which induces ICD, 
was found to be effective when tumours were grown in immunocom-
petent mice, but not in immunocompromised nudemice, highlighting 
the importance of adaptive immunity for the efficacy of this treatment37. 
In vitro studies have also shown that various types of anticancer treat-
ment can upregulate stress-inducible ligands on cancer cells, render-
ing these cells susceptible to NK cell cytolysis. Indeed, the treatment 
of human cell lines derived from multiple myelomas with epigenetic 
drugs, such as BET (bromodomain and extra-terminal motif) inhibi-
tors, enhances the expression of MICA, a ligand of NKG2D, and thereby 
increases the efficiency of activation of NK cell cytotoxicity by myeloma 
cells38. Furthermore, the treatment of tumour cells with a large panel 
of standard antitumour drugs can upregulate expression of B7-H6 on 
tumour cells, turning these cells into NK cell targets39. Chemotherapy 
can also induce senescence, a non-lethal form of growth arrest, which 
alerts the innate immune system. For instance, the treatment of tumour 
cells in vivo with MEK and CDK4/6 inhibitors induces senescence and 
selectively triggers the antitumour functions of NK cells40. Additional 
investigations are required to dissect the complex roles of chemother-
apies in the stimulation or inhibition of innate and adaptive immunity.
Consistent with the notion of therapeutic ICD induction, clinical 
studies have been launched to test the hypothesis that chemotherapy 
or targeted therapy can influence immune responses and sensitize 
tumours to checkpoint blockade41. Indeed, combinations of chemo-
therapy and/or targeted therapy with immune checkpoint inhibitor 
immunotherapies have been approved as first-line treatments for 
several conditions, including lung cancer and advanced renal cell car-
cinoma. Immune checkpoint inhibitors have yielded disappointing 
results for the treatment of breast cancer so far, but a phase III trial has 
shown that the addition of immunotherapy to standard chemother-
apy increases progression-free survival in metastatic triple-negative 
breast cancer, particularly in patients with PD-L1-positive tumours42. 
Similarly, radiotherapy induces ICD markers, such as the surface expo-
sure of calreticulin43 and the secretion of HMGB144. It also induces the 
release of tumour DNA, which can activate STING in innate immune 
cells in the tumour microenvironment45. On the basis of this ration-
ale, clinical studies have tested the addition of immunotherapy to 
Lymph node
Blood
vessel
Tumour
Priming and activation
(APCs and T cells)
Traf�cking of
T cells to
tumours
Infiltration
of T cells
into tumours
Effector innate immune
response (phagocytosis,
cytotoxicity)
Cancer antigen
presentation (DCs)
Ampli�cation of the 
innate immune response 
(macrophages, NK, DCs)
Release
of DAMPs
Release of
tumour
antigens
Killing of
cancer cells
Recognition of
cancer cells by T cells
Tumour
detection
Fig. 1 | The cancer–innate immunity cycle. The cancer–immunity cycle 
has conceptualized the mechanisms that drive the T-cell response against 
tumours4. Innate immunity is essential to the onset and maintenance 
of adaptive immunity and fully integrates the cancer–immunity cycle. 
Tumour detection induces activation of innate immune cells, which 
promotes effector functions and tumour cell destruction. Tumour 
destruction eventually generates more detection signals, propagating the 
response. In addition to their direct tumoricidal effect, tumour-activated 
innate immune cells participate in all steps of T-cell generation and 
activity against cancer cells, by participating in tumour-specific T-cell 
priming, expansion and infiltration at the tumour site. The cancer–innate 
immunity cycle is also regulated by inhibitory factors that limit innate 
immune cell functions, with direct consequences for their tumour-killing 
capacities and T-cell protective immunity. Steps of the cancer–immunity 
cycle that require innate immune cells are highlighed in bold.
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radiotherapy and have reported enhancement of clinical activity46,47. 
However, preclinical data have identified a mechanism of resistance 
to the radiation-induced immune stimulation mediated by the three-
prime repair exonuclease 1 (TREX1). TREX1 is a DNA exonuclease 
that is induced by high doses of radiation and degrades cytosolic DNA, 
preventing activation of STING and subsequent immune stimulation48.
Induction and amplification of the immune response
Given the key role of innate immunity in promoting T-cell effector func-
tions, many attempts have been made to boost this effect (Fig. 1, Table 1, 
Supplementary Table 1, Box 2). Most of these approaches have yet to 
be validated clinically, but interesting preclinical results have been 
obtained for the use of cytokines, such as FLT3L (Fms-like tyrosine 
kinase 3 ligand), GM-CSF, type I IFNs, IL-2 and IL-15, to induce APC 
proliferation and maturation or NK cell activation.
FLT3L is a cytokine that promotes haematopoietic progenitor com-
mitment to the DC lineage and enhances DC survival and prolifer-
ation. The injection of FLT3L into mice promotes the expansion of 
cross-presenting DC populations and enhances the response to intra-
tumoral TLR3 agonists by increasing the number of tumour-infiltrating 
antigen-specific CD8 T cells and increasing the ratio of CD8 T cells 
to Treg cells in the tumour49. In humans, studies with an intratumoral 
vaccine that combined FLT3L with a TLR3 agonist and localized radi-
otherapy have shown that the manipulation of cross-presenting DCs, 
by increasing their number and activation at the tumour site, can pro-
mote tumour-specific CD8 T-cell priming, induce abscopal effects and 
improve immune checkpoint blockade efficacy50.
GM-CSF also has an important role in the differentiation and recruit-
ment of DCs. GM-CSF mediates antitumour effects by recruiting NK 
cells and myeloid cells, including DCs, and thereby promoting the 
Box 1 
Tumour recognition by nucleic 
acid sensors
STING is an endoplasmic reticulum-resident transmembrane 
protein that drives the recognition of intracellular DNA173,174. 
STING is essential for inducing immune responses to transplanted 
tumours. Indeed, STING-deficient mice fail to generate tumour-
specific T cells and to control tumour growth175. Activation of 
STING triggers production of type I IFNs in cancer cells176, but 
its activation outside the tumour cell compartment is required to 
induce a T-cell response. STING-deficient animals are unable to 
mount a specific CD8 T-cell response even if a functional STING 
protein is expressed in tumour cells175. Activation of STING in 
APCs175, macrophages176,177 and endothelial cells29, has been 
shown to drive antitumour T-cell responses (Fig. 2). STING does 
not sense cytosolic DNA directly, but instead recognizes cyclic 
dinucleotides178.
cGAS is the cytosolic DNA sensor that catalyses the formation 
of cyclic dinucleotide guanosine monophosphate-adenine 
monophosphate (cGAMP)179, which activates STING to induce 
immune responses180,181. It was initially thought that the transfer 
of tumour DNA from tumour cells might activate cGAS and STING 
in tumour-infiltrating APCs175. However, it has also been suggested 
that cGAMP might be transferred to the surrounding cells182 to 
induce immune activation at the tumour site172. Indeed, cytosolic 
DNA is particularly abundant in malignant cells176, in the form of 
micronuclei that arise as a result of genomic instability or genotoxic 
stress during the intense cell division process183. The cGAS present 
in tumour cells senses these cytosolic micronuclei and induces 
the production of cGAMP183,184, which, in turn, activates STING, 
promoting an inflammatory response185. As chromosome and 
genome instability are key drivers in tumorigenesis186, the process 
that leads to tumour cell formation also leads to the recognition of 
these cells by the immune system.
In addition to the detection of cytosolic tumour DNA, the 
machinery for RNA detection in the cytoplasm,which was also 
initially described in antiviral immunity, has been investigated for 
its role in the promotion of spontaneous antitumour immunity. 
Mitochondrial antiviral-signalling protein (MAVS) is an adaptor 
molecule downstream from the cytosolic RNA sensors RIG-I, LGP2 
and MDA5 that drives the signalling cascade after the activation of 
these sensors187. As MAVS-deficient mice are not impaired in the 
generation of tumour-specific T cells175, the contribution of these 
cytosolic RNA receptors in host cells to the generation of tumour 
immunity remains to be specified. However, in the context of the 
loss of ADAR1188, an RNA-editing enzyme that prevents aberrant 
stimulation by self-RNA189, the tumour cell-intrinsic activation of 
MDA5 and PKR (another RNA sensor) induces an inflammatory 
response190 and sensitization to immunotherapy188. Thus, in 
addition to the recognition of cytosolic tumour DNA by STING in 
host innate immune cells, detection of cytosolic tumour RNA within 
tumour cells may participate in the immune sensing of tumours.
TLRs also trigger immune responses by detecting nucleic acids 
from pathogens. MyD88 and TRIF are adaptor molecules that drive 
signalling downstream from all TLRs. Tumours grow normally in 
mice lacking MyD88 or TRIF, suggesting that these receptors may 
not be essential for tumour recognition in mouse models175.
CXCL9,
CXCL10
cGAMP
cGAMP
cGAS STING
cGAS STING
cDC1 cell
Killing
Killing
RLR
DNA cGAMP
Tumour cell
NK cell
DC/macrophage/
endothelial cell
Type I IFNs
Tumouourur
microeroenvironnmemenenm t
XCL1,
CCL5,
FLT3LGCXCL9,
CXCL10
Tumour-speci�c
CD8 T cell
Fig. 2 | The role of type I IFNs in the articulation between innate 
immunity and T-cell antitumour immunity. Type I IFNs are produced in 
the tumour microenvironment after the activation of nucleic-acid-sensing 
cytosolic receptors (cGAS–STING or RLRs). Type I IFNs promote the 
recruitment and activation of inflammatory cells, including NK cells, at the 
tumour site172. Ligands expressed by transformed cancer cells activate NK 
cells and induce both cytotoxicity and the production of XCL1, CCL521 
or FLT3LG17—chemoattractants that promote the recruitment of cDC1s, 
which are specialized in cross-presentation. After tumour antigen uptake, 
cDC1s migrate to secondary lymphoid organs to cross-present the tumour 
antigens and prime tumour-specific CD8 T cells. In addition, cDC1s in 
the tumour produce CXCL9 and CXCL10, two chemoattractants that are 
essential for infiltration of tumour-specific CD8 T cells into the tumour 
and killing activity15. CXCL9 and CXCL10 also induce the recruitment of 
NK cells, which can sustain T-cell responses.
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presentation of tumour antigens. Injection of GM-CSF at the tumour 
site has been shown to stimulate immunity, generating long-lasting 
antitumour responses in mice51,52. In patients with melanoma, intra-
tumoral injection of recombinant GM-CSF has been shown to increase 
the number of tumour-infiltrating DCs53. GM-CSF is currently being 
tested in various cancer vaccine approaches, in association with tumour 
peptide vaccine or in the formulations of vaccine platforms, such as 
GVAX (a vaccine strategy involving the use of irradiated allogeneic 
cancer cells modified to express GM-CSF) and T-VEC (talimogene 
laherparepvec, an engineered oncolytic virus with the gene that encodes 
GM-CSF). In patients with advanced melanoma, intratumoral T-VEC 
has been shown to be superior to subcutaneous GM-CSF54 and has 
been approved by the FDA for the local treatment of unresectable 
melanoma that recurs after initial surgery. The efficacy of T-VEC is 
associated with an increase in the number of tumour-specific T cells 
in regressing metastases and a decrease in the number of Treg cells in 
treated lesions, consistent with systemic antitumour immunity54. The 
combination of T-VEC with approved immune checkpoint block-
ers in a phase Ib study has revealed the potentially greater efficacy 
of such combinations over the expected responses for each of these 
treatments separately55,56. An ongoing phase III study of the combina-
tion of T-VEC and pembrolizumab is currently seeking to confirm the 
efficacy signal obtained in the phase Ib trial.
It is still unclear whether the above approaches to promote APC 
numbers and activity may generate new neoantigen specific T cells or 
only expand the pool of existing tumour-reactive T cells. The latter case 
may represent a limitation to the efficacy of these strategies.
Type I IFNs have pleiotropic effects on innate immune cells (Fig. 2). 
Type I IFNs act on APCs by promoting cross-priming and increasing 
the ability of these cells to present dead cell-associated antigens and to 
migrate to draining lymph nodes57. Studies in mice have shown that 
type I IFNs are essential for the spontaneous priming of tumour-spe-
cific CD8 T cells, thereby contributing to tumour control14,58,59 and 
immunoediting59. Type I IFNs are involved in the efficacy of intra-
tumoral injections of TLR760, TLR9, and STING29,30 agonists and in 
the response to PD-1 blockade61. Type I IFNs also influence activity 
and attraction to the tumour site of NK cells by promoting the pro-
duction of CXCL9 and CXCL1062. Mice that lack IFNAR1, IFNβ or 
downstream components of the type I IFN pathway display low lev-
els of NK cell cytotoxicity and impaired NK cell-mediated tumour 
surveillance63. IFNα2a and IFNα2b have been generated as either 
unmodified recombinant proteins or their pegylated IFN variants for 
the treatment of multiple cancer indications. However, these drugs are 
not widely used, mostly owing to toxicity and conflicting data regard-
ing overall survival benefit. Type I IFN-based antibody–cytokine 
conjugates generated to deliver type I IFNs to the tumour site have 
yielded encouraging preclinical results64–66. However, owing to the 
broad expression of type I IFN receptors, injected engineered type 
I IFN molecules can disappear in the bloodstream before they reach 
their targets. Low-affinity type I IFNs have been developed to over-
come this problem. Interestingly, the delivery of such low-affinity type 
I IFNs to cross-presenting DCs by an antibody–cytokine conjugate 
targeting CLEC9A has been shown to result in antitumour activity 
with no obvious signs of toxicity67.
IL-2 induces the proliferation and activation of T cells, resulting in 
the generation of effector and memory T cells. IL-2 also stimulates 
other cells, including NK cells and ILC2s68,69. IL-2 binds with low affin-
ity to CD25, with intermediate affinity to the CD122–CD132 complex 
that is expressed on resting NK cells and CD8 T cells, and with high 
affinity to the CD25–CD122–CD132 complex that is expressed on 
activated lymphocytes. Treg cells constitutively express the high-affinity 
CD25–CD122–CD132 receptor, and are highly sensitive to IL-2. 
Treatment with recombinant IL-2 leads to a substantial regression of 
established tumour metastases in mouse models. However, IL-2 has 
proved effective in only a small fraction of patients with advanced 
melanoma or renal cell cancer, probably because it induces expansion 
of the Treg cell population in non-responders70,71. The short half-life 
of IL-2 is another concern, necessitating the administration of high 
doses that result in substantial toxicity due to cytokine outburst and 
capillary leak syndrome. Engineered forms of IL-2 are currently being 
developed to overcome these problems, and various strategies, includ-
ing PEGylation, genetic variants, antibody–cytokine conjugates and 
fusion proteins, are being considered to extend the half-life of this 
molecule and to prevent Treg cell activation by limiting binding to 
CD25. NKTR-214, a pegylated IL-2, yielded clinical signs of efficacy 
in combination with PD-1 blockade in first-line metastatic melanoma 
and urothelial carcinoma72. Phase III randomized studies of NKTR-
214 plusnivolumab have been launched in patients with previously 
untreated advanced renal cell carcinoma (NCT03729245)73 and 
patients with previously untreated, unresectable or metastatic mela-
noma (NCT03635983)74.
IL-15 signals through the heterodimeric CD122–CD132 recep-
tor. Thus, like IL-2, IL-15 promotes the activation and expansion of 
CD8 T cells, NK cells, γδ T cells and natural killer T cells (NKT cells). 
Haematopoietic and non-haematopoietic cells that express IL-15Rα+ 
must trans-present IL-15 to CD122–CD132-expressing cells to achieve 
full potency. Treatment with IL-15 induces the proliferation and sur-
vival of T cells, promotes the proliferation and differentiation of NK 
cells, and induces the generation of cytotoxic lymphocytes75. Preclinical 
studies in mice have suggested that IL-15 has robust antitumour activity 
with a more favourable toxicity profile than IL-2. IL-15 and IL-2 have 
similar immunity-enhancing properties, but IL-15 does not interact 
with CD25 or stimulate Treg cell expansion and does not, therefore, 
have immunosuppressive activity or cause capillary leak syndrome. 
The stronger biological activity of trans-presented IL-15 has led to 
the development of methods that mimic trans-presentation in vivo to 
achieve super-agonist effects.
The effector responses of innate immunity
In addition of their essential role in promoting T-cell activity, innate 
immune cells also exert antitumour effector functions that can be 
manipulated, as for T cells, either by targeting activating receptors or 
by blocking inhibitory pathways.
Box 2 
Costimulatory signals
Naive T cells require both antigen receptor engagement and 
costimulatory signals delivered by innate immune cells, especially 
APCs, for optimal activation, proliferation, and differentiation. Two 
strategies for potentiating this effect are currently being tested.
In the first, agonistic antibodies acting directly on T cells have 
been developed, which induce costimulatory signals normally 
provided by APCs. The TNFR–TNF superfamily members OX40, 
4-1BB and GITR are expressed on T cells and their respective 
ligands are strongly expressed on APCs, in addition to other cell 
types. As TNFRs can be activated by receptor clustering, agonistic 
antibodies that trigger receptor clustering can lead to a cellular 
response. Studies in preclinical mouse models have shown that 
agonistic antibodies against OX40, 4-1BB and GITR can promote 
cytotoxic T-cell and antitumour responses.
In the second strategy, monoclonal antibodies that promote 
activation of APCs have been generated to enhance the ability of 
APCs to activate T cells. CD40 is another member of the TNFR 
superfamily that is expressed on APCs and B cells. Activation 
of CD40 stimulates APC functions and promotes antitumour 
responses in preclinical models of cancer. The effects of anti-CD40 
monoclonal antibodies depend on the location of the recognized 
epitope and the interaction of the Fc region of the monoclonal 
antibody with FcRs191,192. Many antibodies are currently in 
clinical development, and promising data have been obtained for 
APX005M, for example193 (NCT03214250).
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Fc receptors
FcRs enable various innate immune cells lacking antigen receptors to 
act specifically on cancer cells. Indeed, on binding to FcRs, antitumour 
antibodies provide most myeloid cells and some innate lymphoid cells 
with bona fide tumour antigen-specific receptors. Thus, therapeutic 
antibodies against tumour antigens can use cytotoxic (for example, NK 
cells) and/or phagocytic cells (for example, monocytes or macrophages) 
that express immunoreceptor tyrosine-based activation motif (ITAM)-
containing activating FcRs to destroy cancer cells.
Most tumour-targeting therapeutic antibodies are human IgG1 
antibodies that bind to FcRs for IgG (FcγRs). Activating FcγRs 
include the single-chain receptors FcγRIIA (also known as CD32A) 
and FcγRIIC (also known as CD32C), and the multi-chain receptors 
FcγRIIIA (also known as CD16A) and FcγRI (also known as CD64). 
These FcγRs have different tissue distributions, and, therefore, dif-
ferent biological effects. FcγRIIAs are expressed by monocytes, mac-
rophages, polymorphonuclear cells, mast cells and platelets; FcγRIICs 
are expressed by NK cells in 20% of the human population; FcγRIIIAs 
are expressed by monocytes, macrophages, DCs and NK cells; and 
FcγRIs are expressed by monocytes, macrophages and DCs. Cell 
types, FcγRs and mechanisms that account for the therapeutic effect 
may differ between antibodies. A historical example is provided 
by the anti-CD20 monoclonal antibody rituximab. This antibody, 
used to treat non-Hodgkin lymphoma and chronic B-lymphocyte 
leukaemia, destroys malignant B lymphocytes by engaging ITAM-
containing FcγRs on monocytes and macrophages76. A new gener-
ation of therapeutic monoclonal antibodies has been engineered to 
improve preferential binding to activating FcγRs of various types77–79. 
The impact of FcγRs on the efficacy of antibody therapy is currently 
being evaluated in a prospective clinical study comparing mar-
getuximab, an Fc-engineered anti-Her2 antibody and trastuzumab 
(Herceptin) (NCT02492711 phase III SOPHIA clinical trial). FcγR 
polymorphisms influence immune responses, and the Fc moiety of 
margetuximab was engineered to enhance its affinity for the activat-
ing CD16A and to decrease affinity for the inhibitory FcγRIIB(also 
known as CD32B). According to the preliminary results of this trial, 
margetuximab improves progression-free survival over that obtained 
with trastuzumab in patients with pretreated HER2-positive meta-
static breast cancer. This clinical trial also suggests that ADCC or 
ADCP mechanisms could be manipulated in solid tumours, as they 
already are in haematological malignancies.
Both experimental and spontaneous tumours elicit antitumour 
antibodies, in mice and humans, respectively80–82. These antibodies 
can have two effects: they may protect against cancer, like passively 
administered therapeutic antibodies, but they may also enhance 
tumour growth. Protection probably results primarily from the engage-
ment of activating FcRs on innate cells. However, most myeloid cells 
co-express activating and inhibitory FcγRs. By contrast, FcγRIIBs are 
inhibitory receptors. They have the same specificity as FcγRIIAs and 
FcγRIIIAs for human IgG1 and IgG3. These single-chain receptors 
have an ITIM, but no ITAM. ITIM-containing receptors inhibit cell 
activation when co-engaged with ITAM-containing receptors on the 
same cell83. FcγRIIBs and FcγRIIAs have similar tissue distributions. 
FcγRIIBs are therefore thought to attenuate the FcγRIIA-mediated pro-
tection induced by antitumour antibodies. FcγRIIBs may also account 
for the antibody-induced enhancement of experimental tumour 
growth84.
FcγRIIB appears to be an immune checkpoint in tumour immu-
nity85. Anti-FcγRIIB antibodies that prevent the interaction of IgG 
antibodies with FcγRIIBs, but not with FcγRIIAs, would be expected 
to enhance the protective effect of antitumour antibodies86. Indeed, 
such antibodies have been shown to abolish the inhibitory effects of 
FcγRIIBs in several cell types. These antibodies may represent a new 
class of potential immune checkpoint inhibitors that could be used to 
enhance the efficacy of both endogenous protective antitumour anti-
bodies involved in immunosurveillance and exogenous antitumour 
antibodies used in cancer immunotherapy.
Broad-spectrum immune checkpoint inhibitors
Broad-spectrum immune checkpoint inhibitors have been developed to 
target NK cells and/or myeloid cells in addition to or instead of T cells. 
The rationale of these innovative treatments was initially based on two 
concepts: release of the multiple brakes on T cells, and the beneficial 
effects of unleashing both innate immunity and T-cell functions.
NKG2A is an inhibitory receptor expressed by NK cells andT cells 
that recognizes HLA-E. Unlike classical MHC class I molecules, HLA-E 
is expressed normally or more strongly in tumours. Monalizumab is 
an IgG4-blocking monoclonal antibody against NKG2A. It promotes 
effector T-cell responses in combination with anti-PD-1 antibody, and 
it enhances NK cell effector functions and ADCC. As such, monal-
izumab is the lead example of an immune checkpoint inhibitor that 
can unleash both NK and T-cell responses87. The antibody-mediated 
blockade of NKG2A has also been shown to potentiate the CD8+ T-cell 
responses induced by a peptide-based cancer vaccine in a mouse model 
of HPV16-induced carcinoma88. Several studies are currently investi-
gating the clinical potential of monalizumab. Attention has focused 
particularly on its use in combination with durvalumab, an anti-PD-L1 
antibody, for the treatment of solid tumours, and in combination with 
the anti-EGFR antibody cetuximab for the treatment of head and neck 
cancers, a context in which signs of efficacy have been demonstrated 
in a phase II clinical trial with an ORR of 31%, much greater than the 
ORR of 13% previously reported for cetuximab alone87.
TIGIT (T-cell immunoglobulin and ITIM domain) is an inhibitory 
receptor that is expressed on NK cells and subsets of T cells89,90. The 
interaction of TIGIT with CD155 (also known as PVR) and CD112 
(also known as PVRL2, nectin-2), which are expressed on APCs, T cells, 
and various types of non-haematopoietic cells, including tumour cells, 
downregulates antitumour responses. TIGIT-deficient mice display 
significantly slower tumour growth than wild-type mice89. According 
to preclinical data demonstrating the antitumour efficacy of blocking 
TIGIT and PD-L191, the blockade of both TIGIT and PD-1 in CD8 
T cells from patients with melanoma resulted in additive improve-
ments in terms of cell proliferation, cytokine production, and degran-
ulation92,93. MK-7684, an anti-TIGIT antibody, has been evaluated in 
a phase I trial (NCT02964013), in monotherapy and in combination 
with anti-PD-1, in 68 patients with advanced solid tumours for whom 
standard treatment options had failed. Preliminary data showed a man-
ageable safety profile and clinical responses were observed, but it is too 
early to draw conclusions on this pathway.
TIM-3 (T-cell immunoglobulin and mucin-domain containing-3) 
is expressed on T cells and other immune cells, including NK and 
NKT cells, DCs and macrophages. TIM-3 has been shown to recog-
nize several different ligands: galectin-9, HMGB1, carcinoembryonic 
antigen cell adhesion molecule 1 (CEACAM1) and phosphatidylser-
ine expressed on the surface of apoptotic cells. TIM-3 is an inhibitory 
receptor that regulates the functions of T and NK cells and is associated 
with their exhaustion status and with the immunosuppressive capacity 
of Treg cells and tumour-associated macrophages94. TIM-3 also prevents 
tumour detection by DCs, by inhibiting tumour nucleic acid sens-
ing95. Through these various mechanisms, TIM-3 inhibits antitumour 
immune responses and promotes tumour tolerance. The monoclonal 
antibody-mediated targeting of TIM-3 has proved effective against 
tumours in several preclinical tumour models96. Preliminary data 
from studies evaluating TIM-3-targeting antibodies in solid tumours 
were recently released (TSR-022 phase I NCT02817633 and MBG453 
Phase I-Ib/II NCT02608268). Both studies reported a manageable 
safety profile, consistent with the safety profiles of other checkpoint 
inhibitors, and revealed early signs of activity (even in patients already 
treated with PD-1 or PD-L1 inhibitors). However, further studies will 
be required to understand the activity of molecules that target this 
pathway.
Lymphocyte activation gene-3 (LAG-3) is an inhibitory receptor 
first described as suppressing T-cell activation and cytokine secre-
tion97. LAG-3 inhibits cellular functions following the recognition 
of several molecules expressed by cancer cells and present in the 
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tumour microenvironment: MHC-II98, galectin-399, LSECtin100 and 
fibrinogen-like protein 1 (FGL1)101. LAG-3 is expressed on CD4 T cells, 
including Treg cells, and CD8+ tumour-infiltrating lymphocytes (TILs), 
but it is also found on B cells, NK cells, NKT cells, and plasmacytoid 
DCs, in which it downregulates effector functions. Clinical studies have 
identified LAG-3 as a promising new checkpoint. Indeed, several mol-
ecules that target the LAG-3 pathway, including blocking anti-LAG-3 
antibodies (relatlimab) and soluble LAG-3–immunoglobulin  fusion 
proteins, have shown signs of activity in early-stage clinical trials, in 
monotherapy or in combination (NCT02720068; NCT01968109; 
NCT02676869), and a randomized phase II/III study comparing relat-
limab combined with nivolumab to nivolumab in patients with previ-
ously untreated metastatic or unresectable melanoma (NCT03470922) 
is currently underway.
Signal-regulatory protein-α (SIRPα) is an ITIM-bearing inhibitory 
receptor expressed on myeloid cells, including macrophages, DCs, mast 
cells and neutrophils; it recognizes CD47, a receptor that is often over-
expressed on cancer cells102. Interactions between CD47 and SIRPα 
prevent cancer cells from undergoing phagocytosis, and the inhibition 
of CD47–SIRPα interactions by anti-CD47 antibodies enhances the 
ability of macrophages to engulf tumour cells. Anti-CD47 antibod-
ies also act in synergy with therapeutic antibodies in inducing ADCC 
to promote tumour elimination103. A phase I trial of an anti-CD47 
antibody, in patients with advanced solid tumours and lymphomas, 
demonstrated a safe toxicity profile and objective tumour responses in 
previously heavily pretreated patients104. Other trials of combination 
with other anticancer therapies are currently ongoing105.
Relieving immune suppression at the tumour bed
Many mechanisms of resistance to immunotherapy have been 
described. The immunosuppressive microenvironment at the tumour 
site contributes to the lack of efficacy of checkpoint blockades in 
non-responsive patients. Immunosuppressive factors produced 
in tumours can directly alter T-cell effector functions, but they can 
also inhibit innate immune cells, preventing them from sustaining 
efficient antitumour immune responses. For instance, the infiltration 
into the tumour of myeloid cells, such as myeloid-derived suppressor 
cells (MDSCs) or tumour-associated macrophages (TAMs), protects 
cancer cells106 by generating a tolerant environment that suppresses 
antitumour immune activity. TAMs or MDSCs isolated from human 
tumours can have immunosuppressive functions and, accordingly, high 
levels of circulating MDSCs and tumour-infiltrating MDSCs or TAMs 
are associated with a poor prognosis in many cancers107,108. Several 
pathways have been shown to control immune suppression, either 
by directly inhibiting effector cell activity or by promoting suppres-
sive cell expansion, infiltration and activity. The targeting of some of 
these pathways is currently being evaluated in clinical trials (Table 1, 
Supplementary Table 1, Box 3).
Increased activity in the adenosinergic pathway is associated with a 
poor prognosis in multiple types of cancer. Adenosine is generated in 
hypoxic areas of the tumour microenvironment, through the degra-
dation of ATP by ectonucleotidases. ATP is sequentially broken down 
into ADP and AMP by CD39, and AMP is then dephosphorylated 
by CD73 to generate adenosine. The engagement of the adenosine 
receptors A2AR and A2BR decreases the proliferation and effector 
functions of NK cells109 and CD8 T cells110. In addition, signalling via 
A2AR or A2BR in myeloid cells, including TAMs, MDSCs and tumour- 
associated DCs, promotes the establishment of an immunosuppres-
sive environment by increasing the expression of the anti-inflamma-
tory cytokine IL-10, and by controlling the numbers and activation of 
T cells and NK cells at thetumour site111. Accordingly, deletion of the 
genes that encode CD39 or CD73 in mice abolishes adenosine synthe-
sis and induces tumour rejection in several syngeneic models112,113. 
CD39 can also attenuate immune activation by degrading extracellular 
ATP. ATP stimulates immune responses via P2X receptors, affecting 
DC cross-priming in particular. Indeed, the CD39-mediated hydroly-
sis of ATP reduces the activation of DCs and T cells114. Expression of 
CD39 by Treg cells also inhibits NK cell function, promoting tumour 
growth115. Several molecules that target the adenosinergic pathway are 
currently in clinical development113. No clinical data are yet available 
for the targeting of CD39, whereas a combination of anti-CD73 and 
anti-PD1 treatments has been tested in patients with advanced solid 
tumours; preliminary antitumour activity was reported116,117, sup-
porting further evaluation. Nevertheless, it remains to be determined 
whether the joint blockade of CD39, CD73, and/or A2AR is redun-
dant or could be used in combination to improve antitumour immune 
responses further.
Transforming growth factor-β (TGFβ) is a well-studied cytokine 
that is secreted by many cells, including immune and tumour cells. 
Production of TGFβ may be regulated at many steps, including tran-
scription, translation, secretion, and activation in the extracellular envi-
ronment. TGFβ has pleiotropic activity on most cell types and has a 
dual action in cancer, as a tumour suppressor and a tumour promoter. 
It can affect both the adaptive and innate immune systems and con-
tributes to the evasion of immune surveillance. TGFβ directly inhibits 
CD8 T-cell cytotoxicity118, stimulates the generation of Treg cells and 
contributes to exclusion of T cells from the tumour core119,120. TGFβ 
targets the innate immune system by decreasing NK cell proliferation 
and cytotoxic functions121 through repression of the mTOR path-
way122. It can convert NK cells with potent antitumour activity into 
less functional ILC1 subsets123. TGFβ affects myeloid cells, including 
tumour-infiltrating macrophages and neutrophils, by increasing their 
immunosuppressive activity124. TGFβ signalling can also regulate the 
adaptive immune response by affecting DC activation and decreasing 
the ability of these cells to cross-present antigens to CD8 T cells124. 
Many TGFβ pathway inhibitors have been investigated, but the dual 
role of TGFβ in cancer, its pleiotropic activity, the biological differences 
between TGFβ1, TGFβ2 and TGFβ3 and the multiple levels of regu-
lation of this molecule make it a challenging target. Some of the most 
advanced TGFβ pathway-targeting therapies in clinical development 
include blocking antibodies, small-molecule inhibitors and TGFβ trap-
ping mediated by soluble forms of its receptor ectodomain. Trapping 
Box 3 
indoleamine 2,3-dioxygenase
Indoleamine 2, 3-dioxygenase (IDO) is an enzyme, produced 
in the tumour microenvironment, that catalyses tryptophan 
degradation, resulting in the local depletion of this amino acid and 
the production of kynurenine. IDO can be expressed by tumour 
cells and by host myeloid cells, including APCs infiltrating the 
tumour microenvironment. Both tryptophan deprivation and 
kynurenine production contribute to the suppression of effector 
T cells and the promotion of differentiation and activation of Treg 
cells, thereby inducing tumour tolerance194,195. Kynurenine also 
directly suppresses NK- and T-cell functions196 and proliferation 
by inducing cell death197. IDO promotes tumour infiltration by 
immunosuppressive myeloid cells194 and mediates resistance 
to anti-CTLA4 and anti-PD-1 therapies198. However, despite the 
encouraging findings obtained in multiple phase I/II trials that 
have tested IDO inhibitors in combination with CTLA4199 or PD-1 
blockade200, the first phase III trial in melanoma (ECHO-301) 
revealed no synergy with anti-PD-1 agents and no benefit of IDO 
inhibition for the improvement of clinical outcome201. This failure 
has raised many questions beyond IDO, regarding the strategy 
to be followed during clinical development. With the exception of 
the proof-of-concept of biological activity in vivo, the relevance for 
translation into humans of the data collected from mouse tumour 
models during preclinical studies is questionable. In addition, it 
remains unclear how best to interpret the results of early-stage 
clinical trials, which typically include very small numbers of 
patients and lack appropriate control arms.
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approaches are currently under extensive evaluation in clinical trials 
with M7824, a bifunctional immunotherapy combining a TGFβ trap 
with an anti-PD-L1 antibody. Following a report of promising results in 
second-line treatment for patients with advanced non-small-cell lung 
cancer in a phase I study (NCT02517398), a phase II study has been 
initiated to evaluate M7824 in monotherapy versus pembrolizumab as 
a first-line treatment for patients with advanced non-small-cell lung 
cancer and high levels of tumour PD-L1 expression (NCT03631706).
Cyclooxygenase-2 (COX-2) is an inducible enzyme that catalyses 
the metabolism of arachidonic acid to generate prostanoids, including 
PGE2, which activates specific prostaglandin receptors (EP1 to EP4) 
expressed on immune cells. COX-2 is overexpressed in most solid 
tumours and participates in immune evasion by suppressing adaptive 
and innate immune responses and creating a tolerant immune environ-
ment. The COX-2–PGE2 pathway inhibits the maturation of DCs125, 
and the activation and proliferation of T and NK cells126,127, while 
enhancing the differentiation of immunosuppressive M2 macrophages 
and Treg cells. In a mouse model of melanoma, cyclooxygenase deletion 
in tumour cells promoted CD8 T cell-mediated tumour rejection by 
increasing the activation and accumulation of cDC1s in tumours128. 
Prostanoids produced by tumour cells also regulate the recruitment 
of NK cells, which promote cDC1 infiltration in the tumour21. These 
two studies suggest that the NK–cDC1 innate immune axis is essen-
tial for the establishment of an efficient T-cell response, providing an 
illustration of the need for intact innate immune activation to drive a 
fully competent antitumour response.
The anaphylatoxin C5a is a potent chemoattractant produced during 
the complement cascade. It binds to its high-affinity receptor C5aR1, 
which is expressed essentially on myeloid cells. C5a was initially shown 
to enhance the recruitment of myeloid suppressive cells to the tumour 
microenvironment, thereby inhibiting CD8+ T-cell antitumour activity 
and promoting tumour growth129,130. C5a activates protumorigenic and 
immunosuppressive functions in myeloid cells infiltrating tumours 
and promotes cancer-associated inflammation and neoplastic progres-
sion131. In several mouse models of cancer, the pharmacological block-
ade of C5a or the genetic deletion of C5ar1 results in tumours being 
infiltrated with markedly fewer MDSCs, leading to the production of 
smaller amounts of immunosuppressive cytokines, with a lower likeli-
hood that NK and T cells will be suppressed131. Consequently, blockade 
of C5a–C5aR1 signalling delays tumour growth in preclinical models 
and acts in synergy with immune checkpoint therapies132.
The cytokine colony-stimulating factor 1 (CSF-1) is frequently found 
in solid tumours. Its binding to CSF-1R increases the proliferation of 
myeloid cells and their differentiation into immunosuppressive MDSCs 
and M2 macrophages. The targeting of CSF-1R has thus emerged as a 
strategy for altering the numbers and immunosuppressive functions of 
MDSCs and TAMs at the tumour site133,134. Antibody-mediated block-
ade of CSF-1R modifies the tumour immune infiltrate, inverting the 
ratio of myeloid cells to T cells or NK cells, and slows primary tumour 
growth135. TAMs inhibit T-cell activity but also prevent T-cell infiltra-tion into tumours. The CSF-1R-mediated depletion of TAM increases 
CD8 T-cell infiltration136 and increases the efficacy of immune check-
point therapies137. CSF-1R blockade also increases the anticancer 
efficacy of platinum-based chemotherapy agents by modulating intra-
tumoral type I IFNs138. Diffuse-type tenosynovial giant cell tumour of 
the soft tissue (D-TGCT) is a rare disease characterized by the over-
expression of CSF-1 and massive infiltration by CSF-1R-expressing 
mononuclear cells. In this condition, in which CSF-1 is a driver of the 
disease, CSF-1R blockade has proved highly clinically effective, with 
an ORR of 86% (n = 24)139. Compared with the findings for D-TGCT, 
anti-CSF-1R monotherapy in solid tumours has proved to have limited 
efficacy, with stable disease as the best response in 32% of patients 
(n = 8)140. In another trial (NCT02265536), stable disease was the best 
response observed in 5 out of 22 patients with metastatic breast cancer 
and in 3 out of 7 patients with evaluable metastatic castration-resistant 
prostate cancer141. Combinations of CSF-1R blockade with immune 
checkpoint blockers are currently being evaluated. Several mechanisms 
of resistance to CSF-1R targeting may decrease the efficacy of this 
approach. It has been suggested that decreases in TAM levels medi-
ated by CSF-1R blockade in mice may be compensated by an increase 
in FOXP3+ Treg cells, limiting antitumour efficacy142. Furthermore, the 
CSF-1R-dependent loss of myeloid cells may also decrease the number 
of cells capable of IL-15 trans-presentation, which is required for effec-
tive NK cell cytotoxicity, potentially promoting metastasis143.
IL-1 is a potent pro-inflammatory cytokine targeted in the treatment 
of multiple chronic inflammatory diseases, such as rheumatoid arthri-
tis, cryopyrin-associated periodic syndromes and gout. IL-1α and IL-1β 
signal through the same receptor and induce the same downstream 
events after receptor activation144. The IL-1α precursor is constitutively 
present in epithelial cells, endothelial cells, astrocytes and macrophages. 
This precursor is fully active and can be secreted constitutively or upon 
cell damage, or expressed directly on the cell surface144. By contrast, 
functional IL-1β is produced essentially by myeloid cells and must be 
cleaved from the IL-1β precursor by caspase-1144. IL-1α and IL-1β 
have been associated with the promotion of cancer, as they provide an 
inflammatory microenvironment that promotes carcinogenesis, cancer 
invasion and metastasis144,145. Both are also involved in the generation 
of an immunosuppressive environment at the tumour site through the 
enhanced recruitment of suppressive myeloid cells, such as MDSCs 
or TAMs146. IL-1α produced by tumour cells contributes to the estab-
lishment of a tumour-promoting microenvironment by stimulating 
infiltrating myeloid cells to produce factors sustaining tumour cell sur-
vival147. In humans, a large study of participants treated with anti-IL-1β 
therapy to prevent thrombosis associated with a persistent inflamma-
tory response (CANTOS, NCT01327846) reported a significantly lower 
total cancer incidence in the group treated with anti-IL-1β monoclonal 
antibodies before diagnosis than in the placebo group148. In a thera-
peutic context, MABp1, a monoclonal antibody targeting IL-1α, was 
designed to prevent tumour-related inflammation. Treatment with 
MABp1 showed signs of efficacy in a phase I study in patients with 
refractory cancers149. In a phase III study testing MABp1 in patients 
with advanced colorectal cancer, MABp1 provided clinical benefit for 
patients, including an increase in lean body mass and improvements 
in pain, fatigue and anorexia symptoms150. Further clinical studies that 
analyse overall survival as a primary end point will shed light on the 
antitumour efficacy of IL-1α targeting in humans. Studies of combi-
nations with immune checkpoint blockade will also highlight the role 
of this agent in attenuating the immune response.
Apolipoprotein E (ApoE) is a secreted protein involved in lipopro-
tein metabolism that can help to combat metastatic progression151. 
Expression of ApoE is driven by several transcription factors, including 
liver X receptors (LXRa and LXRb). The therapeutic activation of LXRb 
with a specific agonist, GW3965, has been shown to suppress melanoma 
tumour progression and metastatic colonization152. LXRb agonism also 
has an immune effect against tumours. The ApoE generated by LXRb 
agonists suppresses MDSC survival and enhances the infiltration of 
activated T cells into the tumour153. Finally, translational data have 
shown that RGX-104, a first-in-class LXR agonist, strongly depletes 
MDSCs and activates CD8 T cells in human patients with cancer153.
Next-generation immunotherapies
In addition to immune checkpoint inhibitors and recent advances in 
cancer vaccines154, the development of innovative treatments based on 
the infusion of T cells expressing a CAR has excited considerable inter-
est155. CARs are generated by fusing a monoclonal antibody fragment 
that recognizes a tumour antigen with T-cell receptor (TCR) modules 
that transduce activation signals in T cells3. CAR T cells targeting 
CD19 (tisagenlecleucel and axicabtagene ciloleucel) have proved to be 
remarkably effective against several B-cell leukaemias and lympho-
mas. However, the use of CAR T cells is currently limited by severe 
toxicity156 (including life-threatening cytokine release syndrome) and 
neurotoxicity, and by their lack of efficacy in solid tumours157. Several 
studies are currently underway using CAR non-T cells. In particular, 
given the absence of graft-versus-host disease following the injection of 
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allogenic NK cells, infusions of off-the-shelf cord-blood-derived CAR 
NK cells are being tested in clinical trials against several types of leu-
kaemia after chemotherapy158. In addition, CAR NK cells derived from 
human induced pluripotent stem cells have been generated; these cells 
displayed antitumour activity at least as high as that of CAR T cells, but 
with lower toxicity, in preclinical models159. Finally, CAR macrophages 
are also being generated, based on the rationale that monocytes and 
macrophages are actively recruited to solid tumours, and that engi-
neered CAR macrophages can be polarized towards an antitumour 
macrophage phenotype (M1), enhancing the activation and recruitment 
of immune cells, such as T cells (https://carismatx.com).
Multifunctional molecules formed by the assembly of various com-
ponents of monoclonal antibodies also appear to be promising tools for 
manipulating the immune system in patients with cancer. In parallel 
to bispecific T-cell engagers (BiTEs), which engage all T cells via the 
antigen receptor complex, BiKEs (bispecific killer cell engagers), which 
engage CD16, and TriKEs (trispecific killer cell engagers), which engage 
CD16 and contain IL-15, have also been developed to target antigens 
expressed on solid tumours, including EpCAM and/or CD133160. BiKEs 
and TriKEs are highly effective both in vitro and in preclinical models. 
Trifunctional NK cell engagers (NKCEs) that co-engage NKp46 and 
CD16 on NK cells together with a tumour antigen have recently been 
developed and shown to have stronger antitumour activity in preclini-
cal situations than approved monoclonal antibodies, such as rituximab, 
obinituzumab and cetuximab161. Activating receptors, such as CD16, are 
downregulated in solid tumours, and it remains unclear whether mul-
tifunctional CD16 engager antibodies can activate TILs with low levels 
of CD16 expression162. By contrast, expression of NKp46 remains intact 
in many tumours and NKp46 engagement thus provides an opportu-
nity to promote NK cell function efficiently in solid tumours. These 
results support the clinical development of NKCEs for cancer immu-
notherapy, as a complement to existingimmuno-oncology approaches. 
Moreover, the downregulation of HLA class I molecules and the loss 
of β2-microglobulin expression, two inhibitory signals for NK cells, 
have been described as mechanisms of resistance to T cell-mediated 
lysis after immune checkpoint inhibitor therapies163, suggesting that 
tumours displaying such features would be ideal targets for therapies 
engaging NK cells.
It has been known for decades that glycosylation is altered in cancer 
cells, but only limited progress has been made towards the development 
of glycosylation-targeting treatment strategies against cancer. Sialylation 
is one of the most widely occurring changes to glycosylation; it is asso-
ciated with poor clinical outcomes in several types of cancer164. Siglecs 
(sialic acid–binding immunoglobulin-like lectins) bind sialic acids in 
a specific manner. The Siglec family has 16 known members, 10 of 
which have an ITIM and generate inhibitory signals. These molecules 
are mostly expressed on immune cells, and this distribution, together 
with the inhibitory signalling of these molecules, has suggested that 
sialylation might be involved in immune escape. The targeting of Siglec 
family members in patients with cancer might, therefore, provide an 
opportunity to inhibit tumour-promoting cancer-associated changes to 
glycosylation. For instance, Siglec-15 is upregulated in myeloid cells and 
macrophages that infiltrate tumours and has potent inhibitory effects on 
T cell-mediated antitumour efficacy, through the induction of an immu-
nosuppressing environment in tumours165. Accordingly, the monoclo-
nal antibody-mediated blockade of Siglec-15 abolishes this suppressive 
activity and promotes antitumour responses165. Siglec-7 and Siglec-9 are 
also of particular interest, given the broad range of cells on which they 
are expressed, including myeloid cells and NK cells, and their inhibitory 
function166–168.
Other types of innate immune system lymphocytes, such as ILC1s, 
ILC2s and ILC3s8,169, or lymphocytes with features intermediate 
between innate and adaptive immunity, such as mucosal associated 
invariant T cells (MAIT cells), NKT cells and γδ T cells170, have also 
been identified at the tumour bed. The dissection of the involvement 
of these cells in tumour immunity may open up new opportunities 
for their clinical manipulation in innovative cancer treatments.
Many attempts have been made to improve the capacity of innate 
immunity to sustain and increase tumour-specific T effector cell num-
bers and functions (Table 1, Supplementary Table 1). However, it is 
unclear whether the strategies tested to date also induce the generation 
of new neoantigen-specific T cells. Further efforts to develop therapies 
by enlarging the neoantigen landscape recognized by T cells would, in 
theory, greatly increase the efficacy of the current immunotherapies.
Finally, future studies will need to address several conceptual gaps 
in our understanding of antitumour immunity: in the absence of any 
known specific cancer-sensing mechanism, what factors determine the 
outcome of tumour–immune interactions? Why does the inflammation 
induced by cell death in some cases lead to tissue repair responses, 
which are generally tumour-promoting, whereas in others it leads to 
T-cell priming and antitumour responses? Does the outcome depend 
on the tissue of origin, driver mutations, or currently undefined aspects 
of the tumour microenvironment?
In addition, many new immunotherapeutic strategies have been 
tested in early trials that are not sufficiently controlled to allow a clear-
cut go/no go decision regarding the clinical activity of the molecule. 
Introducing randomization early in clinical trials could be a way to 
identify more efficiently promising treatments171.
Received: 14 May 2019; Accepted: 21 August 2019; 
Published online 2 October 2019.
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