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Review Article
Oncol Res Treat 2017;40:294–297
DOI: 10.1159/000464353
PD-1/PD-L1 Pathway in Breast Cancer
Florian Schütz Stefan Stefanovic Luisa Mayer Alexandra von Au Christoph Domschke
Christof Sohn
Interdisziplinäres Brustzentrum, Universitätsfrauenklinik Heidelberg, Heidelberg, Germany
Immune Checkpoints
T-cells are activated by foreign antigens presented on major his-
tocompatibility complex and the co-expression of T-cell receptor
(TCR) on the one hand, and by a concurrent co-activation of co-
stimulatory and/or co-inhibitory signals on the other hand (fig.1).
The latter include members of the CD28/B7 family, and are known
as ‘immune checkpoints’ [1, 2].
Immune checkpoints are involved in T-cell tolerance as well as
activation. They play a crucial role in maintaining self-tolerance
and immune homeostasis under physiological conditions, thereby
protecting tissues from unnecessary damage when the immune
system has efficiently cleared the pathogen [3]. Even maternal im
-
mune tolerance towards the fetus is in part regulated by checkpoint
inhibitors [4].
Tumors may express immune inhibitory signals resulting in an at-
tenuated immune reaction against the pathologic antigens [5]. Cyto-
toxic T-lymphocyte-associated antigen-4 (CTLA-4), the pro-
grammed cell death-1 and its ligands (PD-1/PD-L1/2) axis, lympho-
cyte activation gene-3 (LAG-3), and T-cell immunoglobulin mucin-3
(TIM-3) are negative signals inhibiting T-cell immune response. In
the context of tumor immunology, CTLA-4 signaling is more in
-
volved in limiting the initiation of a T-cell response in the lymph
nodes, while PD-1 features more prominently later on in the process
and serves to limit T-cell activity in the tumor microenvironment [6].
After TCR engagement, CTLA-4 is upregulated to attenuate T-cell
responses and prevent expansion of autoreactive T-cells, primarily
during the priming phase within the lymph nodes. Anti-CTLA anti
-
bodies like ipilimumab and tremelimumab were both tested in solid
tumors including breast cancer with limited efficacy [7, 8].
PD-1/PD-L1 Pathway
PD-1 is an inhibitory immune checkpoint inhibitor which is ex-
pressed on the surface of T-cells, B-cells, natural killer T-cells,
Keywords
PD1/PD-L1 · Breast cancer · Immune checkpoints ·
Immunotherapy
Summary
The programmed cell death-1 receptor (PD-1) is an im-
mune checkpoint inhibitor which is expressed on the
surface of immune effector cells. It is activated mainly
by PD-L1 which can be expressed by all human cells.
The PD-1/PD-L1 pathway plays a subtle role in main-
taining peripheral T-lymphocyte tolerance and regulat-
ing inflammation. In cancer, the expression of PD-L1
seems to be one of the major immune escape mecha-
nisms. Many studies have shown efficacy of blocking
PD-1 or PD-L1 with specific antibodies like pembroli-
zumab or atezulizumab. In breast cancer, potential re-
sponse was demonstrated in metastatic triple-negative
breast cancers.
© 2017 S. Karger GmbH, Freiburg
Introduction
Solid tumors are initiated by a combination of mutations within
their genetic information. Mutations may not only induce prolif
-
eration and invasion but also de novo antigens by changing DNA
read-out. Every foreign antigen alerts the immune system, leading
to an immune reaction directed against it. Evidently, in existing
solid tumors, this antigen-directed immune reaction is initially
ineffective or disabled or both, a phenomenon called immune
escape.
Received: October 04, 2016
Accepted: February 21, 2017
Published online: March 27, 2017
Prof. Dr. med. Florian Schütz
Interdisziplinäres Brustzentrum
Universitätsfrauenklinik Heidelberg
Im Neuenheimer Feld 440, 69120 Heidelberg, Germany
florian_schuetz@med.uni-heidelberg.de
© 2017 S. Karger GmbH, Freiburg
PD-1/PD-L1 Pathway in Breast Cancer Oncol Res Treat 2017;40:294–297
295
monocytes, and dendritic cells, but not resting T-cells (fig.2). The
PD-1 pathway plays a subtle role in maintaining peripheral T-lym
-
phocyte tolerance and regulating inflammation [9].
PD-1 was originally isolated from a T-cell hybridoma undergo
-
ing T-cell receptor activation-induced cell death, hence its name,
programmed cell death-1 [10]. Despite its name, PD-1 does not in
-
duce cell death directly but reduces cell growth factors as well as
survival signals. PD-1 binds 2 ligands, PD-L1 (B7-H1) and PD-L2
(B7-DC) [11, 12]. Activation of PD-1 by PD-L1 or -L2 induces
downregulation of T-cell activity, reduced cytokine production, T-
cell lysis, and induction of tolerance to antigens [13–16]. In vitro
blockade of PD-1 with monoclonal antibodies led to a 2-fold in
-
crease in cytokine production [17]. However, the in vivo activity
also depends on T-cell motility as well as the duration of the inter
-
action with antigen-presenting cells and target cells [18]. When T-
cells have been activated by their TCR, PD-1 is expressed simulta-
neously to offer the attacked cell a way of escaping the immune re-
action. PD-1 decreases once the immune response has eliminated
the pathologic antigen [19]. An important role of the PD1/PD-L1
pathway has been shown in diabetes [20], cardiomyopathy [21],
human immunodeficiency virus infection [22], lupus [23] and
other autoimmune diseases [24], as well as in solid tumors.
Indeed, the blockade of immune checkpoints using respective
monoclonal antibodies has been shown to trigger efficient antitu
-
mor responses not only in classical ‘immunogenic’ tumor types
such as melanoma and renal cell carcinoma, but also in many other
solid tumors including lung, colorectal, ovarian, esophageal, blad
-
der, and breast cancer.
PD-1/PD-L1 Pathway in Solid Tumors
In solid tumors, the PD-1/PD-L1 inhibitory pathway can be
(mis-)used to silence the immune system by increasing the expres
-
sion of PD-L1 on the tumor cell surface [25]. PD-L1 expression has
been associated with large tumor size, high grade, high prolifera
-
tion, estrogen receptor-negative status, and HER2-positive status
[26], and it is inversely correlated with survival in ovarian [27, 28]
and breast cancer [29, 30]. PD-L1 is expressed in 20% of triple-neg
-
ative breast cancers (TNBCs) [31]. This indicates that although an-
titumor immunity is elicited against many solid tumors, it is coun-
terbalanced by immunosuppressive factors. It was shown in vivo
that PD-L1 increases tumorigenesis and invasiveness and makes
tumor cells less susceptible to specific CD8+ T-cells [32]. Mela
-
noma tumor growth is widely suppressed in PD-1 knockout mice.
Furthermore, it was shown in vivo that a blockade of the PD-1/
PD-L1 pathway using specific antibodies leads to stronger tumor
regression in cellular immunotherapies [33].
Translational research indicates that interferon-gamma, se
-
creted by tumor-infiltrating cytotoxic T-cells, leads to an upregula-
tion of PD-L1 on the surface of melanoma cells that activates the
PD-1 receptor to prevent immune recognition and destruction of
melanoma cells.
The goal of immune checkpoint inhibitors such as anti-CTLA-4
and anti-PD-1/anti-PD-L1 is to ‘release the brakes’ and enhance T-
cell activation by blocking negative pathways.
PD-1-Directed Treatment
The first PD-1 antibody examined in humans was nivolumab.
In 2 phase I trials with multiple tumor entities it was shown that
despite the fact that only a low number of patients responded to
the treatment, those few responders had an impressive long-lasting
Activating
interaction
Inhibiting
interaction
Fig. 1. Co-stimulatory and -inhibitory receptors expressed by T-cells (green)
and target cells (rose).
Fig. 2. Blockade of
CTLA-4 or PD-1 sign
-
aling in anticancer
immunotherapy [43].
Schütz/Stefanovic/Mayer/von Au/Domschke/
Sohn
Oncol Res Treat 2017;40:294–297
296
effect in terms of tumor remission [34]. Especially melanoma, non-
small cell lung cancer, and kidney cancer patients showed promis-
ing results with response rates of up to 33%. However, even in
these early studies, immune-related adverse events grade 3/4 were
shown in 5% of the patients, which is now known as immune-re
-
lated toxicity. These events included pneumonitis, vitiligo, colitis,
hepatitis, hypophysitis, and thyroiditis, and were not correlated to
dose. Even lethal events due to toxicity were reported. Hence, it is
mandatory for clinicians to be able to deal with this new toxicity
profile [35].
Another antibody – later named pembrolizumab – was tested in
multiple phase I–III trials in solid tumors (mainly melanoma and
lung cancer), showing similar results in terms of the number of re
-
sponding patients, duration of response, efficacy, and toxicity com-
pared to nivolumab.
These interesting results naturally raise the question of whether
PD-1/PD-L1 pathway-directed therapy can also be of benefit in
breast cancer patients. Breast cancer has several subtypes that can
be analyzed by immunohistochemistry or with gene expression
profiles. Especially in the estrogen-, progesterone-, and HER2-neg
-
ative subtype (TNBC), in which many mutations occur that may
give rise to neoantigens, we see immunogenic potential. The pres
-
ence of tumor-infiltrating lymphocytes within the tumor tissue
[36] as well as the prognostic value of immunity-related gene sig
-
natures in TNBC are proof of this hypothesis [37]. Especially in
TNBC with its immune gene signature, PD-L1 expression can be
detected in a higher proportion of tumors [38].
Single-agent pembrolizumab was tested in 27 heavily pretreated
patients with metastatic PD-L1-positive TNBC within the phase Ib
study, KEYNOTE-012. The antibody pembrolizumab was given in
-
travenously at 10 mg/kg every 2 weeks. Tumor samples were
screened for PD-L1 expression using a prototype immunohisto
-
chemistry assay. Patients with distinctive stromal or ˰1% tumor
cell nest PD-L1 staining were eligible because some data suggested
that patients with PD-L1 overexpressing tumors have improved
clinical outcomes with anti-PD-1-directed therapy. There was a
clinical benefit rate of approximately 20%: 1 complete response
(CR), 4 partial responses (PR), and 7 cases of stable disease (SD); 3
patients remained on pembrolizumab for at least 11 months. 1
treatment-related death due to disseminated intravascular coagula
-
tion was reported. Again, patients who experienced a benefit with
pembrolizumab showed long duration of response (median 17.9
weeks (range 7.3–32.4 weeks)) [39]. In this small cohort, the re
-
ported side effects were comparable to those reported from mela-
noma and lung cancer trials. In the future, a combination with cy-
totoxic therapies (e.g., chemotherapy or radiation) might prove
more effective than PD-1 inhibition alone. However, to date, there
are only preliminary data with no efficacy data available as yet.
Atezolizumab is a human anti-PD-L1 antibody with a modified
Fc region to avoid antibody-dependent cytotoxicity or comple
-
ment-dependent cytotoxicity induction [40]. A total of 54 TNBC
patients treated within a phase I study reached a 19% objective
response rate. The duration of response ranged from 0.1 to >41.6
weeks.
Furthermore, the anti-PD-1 nivolumab (BMS-936558/MDX-
1106) and the anti-PD-L1 durvalumab (MEDI4736) are currently
under investigation in breast cancer.
Is PD-L1 Expression in Tumors a Predictive
Biomarker?
Since only a minority of cancer patients showed clinically rele-
vant tumor remission after anti-PD-1 treatment, a predictive factor
is required to determine patients who will derive a benefit from a
checkpoint inhibitor strategy. There are several unresolved issues
regarding PD-L1 analysis: variable detection antibodies, differing
immunohistochemistry cut-offs, tissue preparation, processing
variability, primary versus metastatic biopsies, oncogenic versus
induced PD-L1 expression, and staining of tumor/stroma versus
immune cells. However, despite the problems of different assays
and undefined standards, it was shown in vivo that responses to
antibody therapy were greater in tumors with high PD-L1 expres
-
sion [41]. Trials conducted in melanoma patients reported that pa-
tients with high PD-L1 expression had a greater chance of response
than those with low expression. However, there was a remarkable
number of false-negative results in tumors with low PD-L1 expres
-
sion even with low cut-offs; furthermore, even if high PD-L1 ex-
pression was found, 70% of melanoma tumors did not respond to
anti-PD-L1 treatment.
Hence, PD-L1 expression does not seem to be a reliable predic
-
tive marker as it would exclude patients from receiving an effective
treatment they may in fact respond to.
Conclusion
Immunomodulation seems to be a promising strategy in solid
tumors. High immunogenicity has been described in breast cancer
subtypes with a high proliferation index (TNBC, HER2). Immune
checkpoints are one of the major mechanisms of immune escape.
Expression of PD-L1 on tumor cells leads to lower activity of CD8+
T-cells. Antibodies against PD-1 or PD-L1 are being investigated in
clinical trials. First results are promising but only a subset of pa
-
tients (20%) respond to immune checkpoint inhibitory treatment.
Predictive markers are urgently needed to select those patients with
the best chance for an effective treatment [42].
Disclosure Statement
The authors did not provide a disclosure statement.
PD-1/PD-L1 Pathway in Breast Cancer Oncol Res Treat 2017;40:294–297
297
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