Cancer immunotherapy utilizing checkpoint blockade antibodies or adoptive cellular therapy (ACT) with tumor-specific T cells has led to unprecedented clinical responses in patients with cancer and has been considered one of the most significant breakthroughs in cancer treatment in the past decade. Nevertheless, many cancers remain refractory to these therapies due to the presence of an immunosuppressive tumor microenvironment. This has led to the innovative idea of combining ACT with checkpoint inhibition. A landmark 2004 study by Blank and colleagues published in Cancer Research was one of the original demonstrations that adoptive transfer of T cells lacking the negative T-cell regulator, PD-1, was able to restore functional T-cell antitumor activity, resulting in rapid regression of established tumors in a preclinical model. This work was instrumental in not only driving clinical studies utilizing checkpoint inhibition but also a new wave of recent trials involving checkpoint blockade in the setting of ACT.

See related article by Blank and colleagues, Cancer Res 2004;64:1140–5

The advent of cancer immunotherapy involving activation of a patient's immune system has recently led to unprecedented responses in a number of patients with cancer and has since become the fourth pillar of treatment alongside surgery, chemotherapy, and radiotherapy. Although MacFarlane Burnet first proposed his theory of ‘tumor immune surveillance' in the 1950s, this was met with initial skepticism and took a further 50 years for his ideas to be accepted. The pivotal discovery of PD-1 and CTLA4 as negative regulators of T-cell function by Tasuku Honjo and James Allison led to development of antibodies to block the interaction of these T-cell–inhibitory receptors with their respective ligands (PD1/PD-L1 and PD-L2; CTLA4/B7.1 and B7.2). Known as “checkpoint blockade,” these interventions have been shown to reinvigorate antitumor responses in preclinical models and most importantly, in patients with cancer. These investigators were awarded the Nobel Prize in 2018 for their revolutionary discovery following the FDA approval of a number of checkpoint blockade antibody drugs, namely Nivolumab (anti–PD-1) and Ipilimumab (anti–CTLA4) for treating cancers such as melanoma. Despite this remarkable progress, overall responses have only been observed in approximately 20% to 30% of cancers, an effect that is postulated to be linked to the low frequency of T cells present in the majority of tumors.

Adoptive cellular therapy (ACT) is another form of cancer immunotherapy that involves the infusion of autologous T cells to treat patients. These can come in the form of ex vivo expanded tumor-infiltrating lymphocytes, or T cells further engineered with either a tumor-specific T-cell receptor (TCR) or a chimeric antigen receptor (CAR). However, clinical outcome using ACT in solid cancers has not met with great success, predominantly due to a hostile immunosuppressive microenvironment where the transferred T cells are rapidly inhibited by the presence of PD-L1. This ultimately led to an innovative area of research focusing on the augmentation of antitumor responses in solid cancers by combining ACT with checkpoint inhibition.

One of the pioneering studies to support this idea was the landmark paper by Blank and colleagues published in Cancer Research (1). They were one of the first to demonstrate that the lack of PD-1 signaling in adoptively transferred CD8+ T cells restored their antitumor function in vivo. In their model, the authors used the poorly immunogenic B16-F10 melanoma cell line transduced with the Kb binding peptide SIYRYYGL (B16.SIY), which can be recognized by high affinity 2C CD8+ TCR transgenic T cells. They found that these tumor cells were poorly recognized by primed 2C transgenic T cells despite their high tumor antigen expression. Given this lack of T-cell effector function, the authors screened the tumor cells for possible expression of inhibitory candidates following stimulation with IFNγ. This analysis revealed that while inhibitory candidates PD-L2, B7–1, and B7–2 remained unchanged in their expression, elevated levels of PD-L1 (B7H-1) were observed following IFNγ treatment.

To investigate the impact of the PD-1/PD-L1 interactions, the authors first compared the in vitro effector function of primed 2C TCR transgenic T cells lacking in PD-1 (2C/PD-1−/−) with wild-type 2C T cells. PD-1–deficient T cells exhibited superior functional activity through enhanced production of Th1 cytokines including IFNγ and IL2, as well as increased proliferation and cytolytic activity against B16.SIY tumor cells compared with the wild-type 2C T cells. Most importantly, the authors elegantly demonstrated that the adoptive transfer of 2C/PD-1−/− CD8+ T cells mediated effective rejection of established B16.SIY tumors compared with wild-type 2C T cells or wild-type 2C T cells lacking the CTLA4 receptor (2C/CTLA4−/−). These experiments unequivocally demonstrated for the first time that PD-1/PD-L1 engagement has the ability to trigger an inhibitory signal during the effector phase of CD8+ T-cell function and that targeting these interactions can enhance the cytolytic activity of adoptively transferred T cells against tumors. Blank and colleagues further highlighted that this inhibition was due to PD-1 as they found no increased antitumor effects by T cells lacking in CTLA4. However, several groups have since shown that CTLA4 blockade can enhance antitumor effects with ACT in other cancer models.

Since this seminal paper by Blank and colleagues, there have been many reports that have emanated from this study using checkpoint blockade in the context of adoptive T-cell immunotherapy. For example, additive antitumor effects were observed following anti-CTLA4 or anti–PD-1 mAb blockade, with the adoptive transfer of antigen-specific pMel CD8+ T cells targeting the gp100 antigen (2). More recent work has also suggested that particular T-cell populations can have different synergistic responses when combined with anti–PD-1 immunotherapy, which has led to studies investigating the impact of in vitro manipulation of T cells to enrich for specific subsets used for ACT (Fig. 1). Moreover, administration of an anti-PD1 mAb was also found to significantly improve the antitumor response of adoptively transferred murine T cells expressing an anti-Her2 CAR (3) as well as human CAR T cells targeting NY-ESO-1 (4). Furthermore, blocking both CTLA4 and PD-1 was reported to significantly augment responses following adoptive transfer of CD8+ T cells compared with blockade of either pathway alone (5). The impact of these findings is exemplified by the clinical trial development of combinatory approaches involving checkpoint blockade antibodies and ACT (Fig. 1). To date, there have been encouraging antitumor responses reported in patients following CAR T-cell therapy in combination with anti-PD1 therapy (6).

Figure 1.

Strategies to target the PD-1/PD-L1 axis during adoptive T-cell therapy. Antibody-based immunotherapy directed at blocking the interaction between PD-1 and PD-L1 is being used in conjunction with the adoptive transfer of CAR T cells or tumor-infiltrating lymphocytes in a number of clinical trials. Gene therapy involving the deletion of the PDCD1 locus in T cells is currently being evaluated against multiple cancer types. Preclinical studies are underway to enhance the efficacy of ACT through the in vitro manipulation of T cells to enrich for specific subtypes, or via further genetic modification to modify PD-1 signaling [e.g., overexpression of a PD-1 dominant negative receptor (DNR)] as well as localized secretion of anti–PD-1 blocking agents.

Figure 1.

Strategies to target the PD-1/PD-L1 axis during adoptive T-cell therapy. Antibody-based immunotherapy directed at blocking the interaction between PD-1 and PD-L1 is being used in conjunction with the adoptive transfer of CAR T cells or tumor-infiltrating lymphocytes in a number of clinical trials. Gene therapy involving the deletion of the PDCD1 locus in T cells is currently being evaluated against multiple cancer types. Preclinical studies are underway to enhance the efficacy of ACT through the in vitro manipulation of T cells to enrich for specific subtypes, or via further genetic modification to modify PD-1 signaling [e.g., overexpression of a PD-1 dominant negative receptor (DNR)] as well as localized secretion of anti–PD-1 blocking agents.

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Given that antibody checkpoint blockade has resulted in severe toxicity in some patients, more recent studies have developed intrinsic approaches to interfere with the PD-1/PD-L1 inhibitory pathway. This includes localized delivery of anti–PD-1 blocking single-chain Fab variable (ScFv; ref. 7) and receptor switching to either eliminate PD-1 signaling or reprogramming of negative PD-1 signaling into downstream activating signals (8). The emergence of cutting-edge gene-editing strategies such as CRISPR/Cas9 has further allowed more precise deletion of the PD-1 gene in CAR T cells. In a preclinical model of EGFRvIII+ human glioblastoma, adoptive transfer of PD-1 gene-edited CAR T cells led to increased antitumor activity (9). These types of approaches are now being tested in phase I clinical trials using PD-1 and TCR-deficient CAR T cells (Fig. 1; ref. 10).

With the development of these new strategies targeting a variety of antigens and tumor types, evaluation of therapeutic efficacy will have to be balanced with the careful monitoring of potential autoimmune effects. Other key considerations would include the optimal dose of CAR T cells required, as well as whether these novel approaches may lead to transformation of the adoptively transferred T cells. This could be obviated by the inclusion of suicide genes within the gene-modified T cells. It would also be interesting in future studies to examine the impact of specific T-cell subsets, in particular, the T-cell differentiation status and the role of CD4+ T cells in combination therapy involving ACT and checkpoint blockade. The groundbreaking research by Blank and colleagues has been the inspiration for the remarkable effects we are now witnessing with checkpoint blockade in patients and new combination immunotherapies involving checkpoint inhibition in the context of ACT.

No disclosures were reported.

The authors wish to acknowledge funding support from the National Health and Medical Research Council (NHMRC) of Australia, the National Breast Cancer Foundation, Cancer Council of Victoria. J. Lai is supported by a Cancer Research Irvington postdoctoral fellowship (award no. 3530). P.A. Beavis is supported by a fellowship from the Victorian Cancer Agency (MCRF20011). P.K. Darcy is supported by an NHMRC Senior Research Fellowship (APP1136680).

1.
Blank
C
,
Brown
I
,
Peterson
AC
,
Spiotto
M
,
Iwai
Y
,
Honjo
T
, et al
PD-L1/B7H-1 inhibits the effector phase of tumor rejection by T cell receptor (TCR) transgenic CD8+ T cells
.
Cancer Res
2004
;
64
:
1140
5
.
2.
Peng
W
,
Liu
C
,
Xu
C
,
Lou
Y
,
Chen
J
,
Yang
Y
, et al
PD-1 blockade enhances T-cell migration to tumors by elevating IFN-gamma inducible chemokines
.
Cancer Res
2012
;
72
:
5209
18
.
3.
John
LB
,
Devaud
C
,
Duong
CPM
,
Yong
CS
,
Beavis
PA
,
Haynes
NM
, et al
Anti-PD-1 antibody therapy potently enhances the eradication of established tumors by gene-modified T cells
.
Clin Cancer Res
2013
;
19
:
5636
46
.
4.
Moon
EK
,
Ranganathan
R
,
Eruslanov
E
,
Kim
S
,
Newick
K
,
O'Brien
S
, et al
Blockade of programmed death 1 augments the ability of human T cells engineered to target NY-ESO-1 to control tumor growth after adoptive transfer
.
Clin Cancer Res
2016
;
22
:
436
47
.
5.
Shi
LZ
,
Goswami
S
,
Fu
T
,
Guan
B
,
Chen
J
,
Xiong
L
, et al
Blockade of CTLA-4 and PD-1 enhances adoptive T-cell therapy efficacy in an ICOS-mediated manner
.
Cancer Immunol Res
2019
;
7
:
1803
12
.
6.
Adusumilli
PS
,
Zauderer
MG
,
Rivière
I
,
Solomon
SB
,
Rusch
VW
,
O'Cearbhaill
RE
, et al
A phase I trial of regional mesothelin-targeted CAR T-cell therapy in patients with malignant pleural disease, in combination with the anti-PD-1 agent pembrolizumab
.
Cancer Discov
2021 Jul 15 [Epub ahead of print]
.
7.
Rafiq
S
,
Yeku
OO
,
Jackson
HJ
,
Purdon
TJ
,
van Leeuwen
DG
,
Drakes
DJ
, et al
Targeted delivery of a PD-1-blocking scFv by CAR-T cells enhances anti-tumor efficacy in vivo
.
Nat Biotechnol
2018
;
36
:
847
56
.
8.
Liu
X
,
Ranganathan
R
,
Jiang
S
,
Fang
C
,
Sun
J
,
Kim
S
, et al
A chimeric switch-receptor targeting PD1 augments the efficacy of second-generation CAR T cells in advanced solid tumors
.
Cancer Res
2016
;
76
:
1578
90
.
9.
Choi
BD
,
Yu
X
,
Castano
AP
,
Darr
H
,
Henderson
DB
,
Bouffard
AA
, et al
CRISPR-Cas9 disruption of PD-1 enhances activity of universal EGFRvIII CAR T cells in a preclinical model of human glioblastoma
.
J Immunother Cancer
2019
;
7
:
304
.
10.
Wang
Z
,
Li
N
,
Feng
K
,
Chen
M
,
Zhang
Y
,
Liu
Y
, et al
Phase I study of CAR-T cells with PD-1 and TCR disruption in mesothelin-positive solid tumors
.
Cell Mol Immunol
2021
;
18
:
2188
98
.