Review Article
Breast Cancer Immunotherapy: An Update
Mohammad Atiq1, Ahmed Alwbari1, Thomas Kieber-Emmons3 and Issam Makhoul1*
1Department of Internal Medicine, University of Arkansas for Medical Sciences, Little Rock, USA
2Department of Pathology, University of Arkansas for Medical Sciences, Little Rock, USA
*Corresponding author: Issam Makhoul, Department of Internal Medicine, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205, USA
Published: 25 Apr, 2017
Cite this article as: Atiq M, Alwbari A, Kieber-Emmons
T, Makhoul I. Breast Cancer
Immunotherapy: An Update. Clin Surg.
2017; 2: 1429.
Abstract
The immune system plays a major role in cancer surveillance. Harnessing its power to treat many cancers is now a reality that has led to cures in hopeless situations where no other solutions were available from traditional anticancer drugs. These spectacular achievements rekindled the oncology community’s interest in extending the benefits to all cancers including breast cancer. The first section of this article reviews the biological foundations of the immune response to different subtypes of breast cancer and the ways cancer may overcome the immune attack leading to cancerdisease. The second section is dedicated to the actual immune treatments including breast cancer vaccines, checkpoint inhibitors, monoclonal antibodies and the “unconventional” immune role of chemotherapy.
Introduction
Breast cancer is the most common cancer in females with an estimated 249, 260 new cases in the
United States in 2016 [1]. It is also the second leading cause of cancer death in women. Fortunately,
with advances in detection and treatment, death rates from breast cancer are declining. More recent
advancements in breast cancer therapy utilizing novel mechanisms involving actionable cancer
mutations and the body’s immune system have opened up new avenues for reducing the death rate
further. Many of the obvious successes in immunotherapy have been in the field of melanoma, renal
cancer, lung cancer and others that have traditionally been known to be immunogenic. However,
these are not the only cancers in which strides in immunotherapy are being made. Breast cancer is
one cancer that, while not originally thought to be immunogenic, has had many encouraging results
in the past few years. We aim to provide a succinct overview of breast cancer immunotherapy as well
as possible future directions.
The basis for immunotherapy in cancer has revolved around the concept of immunogenicity. For
a long time, breast cancer has been considered non-immunogenic. However, the role of the immune
system in the emergence of breast cancer has been firmly established [2,8]. Random or inherited
genetic and epigenetic abnormalities confer proliferative and/or survival advantages on certain cells.
These incipient cancer cells face internal and external control mechanisms including those from the
immune system. By targeting the new antigens created by these genetic changes, the immune system
plays a central role in cancer control that can be host-protective or tumor-promoting. A mutated
gene leads to the production of a neo-antigen when it is transcribed then translated, highlighting the
auto-antigenicity of self antigens as observed in model protein antigens [9].
Epitopes from the neo-antigen are presented after processing by the mammary epithelial cells in
association with MHC class I (MHC-I) on their surface. When an Antigen-Presenting Cell (APC)
encounters a neo-antigen released from debris of cancer cells or secreted in the environment, it
internalizes it via several mechanisms including endocytosis. The antigen resurfaces again after
processing on the MHC class II (MHC-II) receptors and can be recognized by T- Helper Cell
Receptors (TCR). T-Helpers (Th) stimulate and drive cytotoxic T-Cells (Tc) and B-cells to further
maturation. Tc maturation, proliferation, and survival require co-stimulatory signals from APCs
that are antigen independent. If the co-stimulatory signal is lacking then the process of activation
will be ineffective and may lead to Tc anergy. Once activated, Tc can attack the target cell by several
mechanisms, including TCR-MHC-I recognition and binding. This leads to secretion of cytotoxic
granules including perforin that result in cell lyses and demise [10]. Another mechanism by which
Tc can attack target cells is via FAS receptors on Tc that bind FASL on the target cell leading to
caspase 3 and 8 activation in the target cell and eventually apoptosis [11]. To ensure effective
immune regulation, the very same APC that sends a co-stimulatory signal (B7 family receptors on
APC bind the CD28 surface protein on T-cells) to intensify the activation of naïve T-cells also sends
inhibitory signals (B7 receptors bind CLTA4 on T-cell) to the already
activated T-cell when the immune response has to wind down. The
activated T-cell starts synthesizing CTLA4, which has higher affinity
to B7 and competes with the stimulatory B7-CD28 binding [12]. This
mechanism prevents overstimulation by transient T-cell activation.
The interaction between the immune system and incipient
cancer cells, also called immunoediting, goes through three phases:
elimination, equilibrium and escape [13-15]. Elimination is supported
by a wealth of experimental evidence in animals and humans. The
innate and adaptive arms of the immune system recognize incipient
cancer cells by the new antigens (resulting from mutations or
translocations) presented on their surface in association with MHC-I
or by the distress signals usually expressed by transformed cells that
have undergone chromosomal changes (aneuploidy or hyperploidy)
[16,17] and eliminate them. Equilibrium is reached when the immune
system fails to eliminate the transformed cells but stops them from
progressing further. This can be conceived as the dormancy phase of
cancer development. This phase is mediated by equilibrium between
cells and cytokines that promote elimination (IL-12, IFNγ, TNFα,
CD4 Th1, CD8+ T cells, NK cells, γδT cells) and those that promote
persistence of the nascent tumor (IL-23, IL-6, IL10, TGFβ, NKT cells,
CD4 Th2, Foxp3+ Treg cells, and MDSCs) [18-20]. Monocytes play
an important role in this process. Under the influence of the tumor
microenvironment they may differentiate into pro-inflammatory M1
or anti-inflammatory M2 types [21,22]. Immune escape of cancer cells
occurs by different mechanisms. In HR positive breast cancer, the
absence of strong tumor antigens and low expression of MHC-I allow
the tumor to progress unnoticed by the immune system [23]. Estrogen
plays an immunosuppressive role in the tumor microenvironment
that promotes tolerance of the weakly immunogenic cancer. Most
immune cells including macrophages, T- and B-lymphocytes and NK
cells express ER [24]. In presence of estrogen, the immune response
is polarized to Th2- rather than Th1-effector immune response [25].
In HER2 cancer cells, MHC-I presentation is inversely correlated
with HER2 expression [26]. Triple Negative Breast Cancers (TNBC)
exhibit a spectrum of MHC-I presentation and strong tumor antigen
expression but immune escape in this subtype is mostly related to the
development of the immunosuppressive tumor microenvironment
(Tregs, MDSCs, PD1/PD-L1).
However, it is still unclear how the balance established during the
equilibrium phase gets tilted towards tumor progression. The answer
to this question is very likely multifactorial. Aging is associated with
reduced production of new B and T lymphocytes in the bone marrow
and the thymus, respectively and with decreased function of the
existing mature lymphocytes [27]. Systemic inflammation associated
with aging and the local pro-inflammatory microenvironment in the
breast are incriminated in promoting the cancerous transformation
of mammary stem cells that have been primed by losing tumor
suppressor genes [28,29]. Pro-inflammatory cytokines (TNFα and
IL-6) are associated with overexpression of COX2 and the aromatase
enzyme [30], which lead to increased local concentrations of
estrogens. Estrogens induce the expansion of Tregs and MDSCs, as
well as the inhibition of antigen presenting cells [31-34]. In addition
to the gradual decline of the immune system, dietary, commensal
microbiota, use of antibiotics, procreational and hormonal factors,
all play some role of variable importance in tilting the balance from
equilibrium to escape [35-38].
Assessment of Breast Cancer Immunogenicity
Traditional pathology and immunohistochemistry, gene
expression profiling, RNA sequencing and combined scores have
been used to assess the immunogenicity of breast cancer. Traditional
pathology tools allow the assessment of breast cancer immunogenicity
by studying the presence of tumor-Infiltrating Lymphocytes (TILs)
and assessing their types and correlation with survival and recurrence.
While tumor-Infiltrating Lymphocytes (TILs) were not found to have
a prognostic value in the overall breast cancer population or estrogen
receptor positive/human epidermal growth factor receptor 2 negative
(ER+/HER2-) patients, TILs were found to have a prognostic value for
Disease-Free Survival (DFS) and Overall Survival (OS) in TNBC [39].
In patients with TNBC who had residual disease after neoadjuvant
chemotherapy, the presence of TILs was found to be associated with
better OS as well as with metastasis-free survival [40]. In ER negative
breast cancers, TILs, specifically CD8+ lymphocytes, were associated
with better breast cancer specific survival [39,41]. The presence of
CD8+ lymphocytes in patients with ER negative breast cancers was
also related to longer DFS [41]. In general, the presence of TILs was
positively correlated with MHC-I expression and inversely correlated
with ER expression. The more immunogenic the breast cancer, the
higher the concentration of TILs will be. Hence, it is not surprising
that HR positive breast cancer is considered the least immunogenic.
Recent advances in genomics and proteomics allow the detection
of neo-antigens that underlie immunogenicity in breast cancer and
shed light on possible targets for therapy [42,43]. Immunogenicity
of a tumor is evaluated by the assessment of its antigenicity and the
latter is evaluated by assessing its mutagenicity. Mutational load, the
average number of somatic mutations per cancer cell, is associated
with antigenicity and is, in general, lower in breast cancer compared
with other tumors such as melanoma or lung cancer. However, major
differences exist between different subtypes of breast cancer; TNBC
has the highest mutational load compared with HR positive breast
cancers [44,45] and high mutational load is associated with better
prognosis in TNBC and HER2+ compared with low mutational load
in the same types of breast cancer (see below). Conversely, higher
mutational load is associated with higher concentrations of TILs
and with poor prognosis in HR positive breast cancer. Mutational
load continues increasing in metastatic breast cancer but TILs,
PD-1 and PDL-1 expression decreases, very likely as a result of
immune exhaustion and not because of decreased immunogenicity
in advanced disease as suggested by Luen “et al.” [46]. Some specific
mutations in DNA repair mechanisms such as those in the BRCA1/2
and MMR genes are associated with high mutational loads that can
be localized (kataegis) or generalized [47,48]. High mutational load
is associated with high rates of neo-antigens, which predict overall
survival and response to check point inhibitors [42,43,49-51].
In assessing response to neoadjuvant treatment, the benefit of the
presence of TILs can be seen here as well. Breast cancers with higher
levels of TILs have better responses to neoadjuvant chemotherapy
[7]. In patients with HER2+ or TNBC, those with >60% TILs treated
with an anthracycline plus taxane combination were more likely
to have a pathologic complete response and the rates of pathologic
complete response were even higher when carboplatin was added
to the treatment regimen [8]. ER negative breast cancers that are
lymphocyte-rich have far greater pathologic complete response rates
when treated with neoadjuvant anthracycline-based chemotherapy
compared to patients with lymphocyte-poor ER- breast cancers [52].
HER2+ breast cancers with TILs were associated with better diseasefree
survival as well as overall survival in response to treatment with
anthracyclines [2]. There was a significantly associated decreasing
risk of distant recurrence in patients being treated with adjuvant
chemotherapy simultaneously with trastuzumab in HER2+ breast
cancer for every 10% increase in TILs [3]. Moreover, irrespective of
whether or not a patient received systemic adjuvant chemotherapy,
TILs and immune signatures were associated with better prognosis
in HER2+ breast cancer [53]. In patients with HER2 overexpression,
a higher CD8+ infiltrate was seen after chemotherapy and this was
associated with improved relapse-free survival [54].
Strategies to Harness the Power of the Immune System
Several strategies have been used to harness the power of the
immune system and redirect it to eradicate breast cancer or to induce
immune dormancy.
1. Breast cancer vaccines
2. Monoclonal antibodies
3. Checkpoint inhibitors
4. Enhance the immune-mediated effect of chemotherapy
Breast Cancer Vaccines
Breast cancer vaccines are used for primary or secondary
prevention and some are therapeutic. Several strategies have been
used including peptide vaccines, recombinant protein vaccines,
dendritic cell vaccines, whole tumor cell vaccines, DNA vaccines, and
recombinant viral vectors vaccines.
They are all designed to stimulate an intrinsic antitumor response
targeting Tumor-Associated Antigens (TAAs). TAAs that are
specifically recognized by T cells include HER2, mucin 1 (MUC-1),
carcinoembryonic antigen (CEA), sialyl-Tn (STn), human telomerase
reverse transcriptase (hTERT), Wilms’ Tumor gene (WT1) and
Tumor Associated Carbohydrate Antigens (TACAs) [55]. The
antigens where current studies are primarily focused around include
HER2, MUC-1, and TACAs.
As for the use of HER2 in vaccine developments, there have
been a few attempts involving the E75, GP2, and AE37 peptides.
Nelipepimut-S (Neu-Vax) is a combination of E75, a peptide from
the extracellular domain of HER2 and GM-CSF; it stimulates
cytotoxic T lymphocytes and CD8+ memory cells with high affinity
for HLA-A2/A3. However, the immunity induced by the E75 vaccine
waned after six months from initial vaccination requiring a booster
given at six months from completion of the primary vaccination [56].
NeuVax was tested in a phase I/II trial and showed improvement of
disease-free survival in HER2 positive breast cancer patients [57]. The
study enrolled 187 early-stage breast cancer patients deemed at high
risk for recurrence. Patients received six injections of NeuVax after
tumor resection with standard of care (chemo or RT) as indicated.
The 5-year DFS was 89.7% for the vaccinated group vs. 80.2% for the
controls (P=0.08). When the optimally dosed cohort was considered,
DFS was increased to 94.8% vs. 80.2% (P=0.05). Apparently, the
induction of cytotoxic T lymphocytes was crucial for the response to
NeuVax as only 1 recurrence was observed in 30 patients (3%) who
achieved cytotoxic T lymphocytes above the mean, compared with 8
of 56 (14%) for patients with levels of cytotoxic T lymphocytes below
the mean [58]. A phase III registration PRESENT trial is evaluating
E75 in 758 early-stage, node-positive HLA-A2/A3 patients with low
to intermediate HER2 expression with no evidence of disease after
standard treatment. Patients are randomized to GM-CSF with E75
or GM-CSF with placebo, receiving six monthly injections, followed
by a booster vaccination every 6 months for 3 years. The primary
endpoint is disease-free survival at 3 years [59].
Work with the GP2 peptide is currently ongoing in a phase II
clinical trial where vaccines containing GP2, a class I epitope derived
from the HER2 transmembrane domain, is combined with GM-CSF
and then compared to treatment of patients with GM-CSF only.
Interim analysis presented in 2009 was already showing a decreased
recurrence rate at 17.9 months in a group of patients treated with
GP2 and GM-CSF (VG) versus GM-CSF alone (CG), 7.4% (2/27)
compared to 13% (3/23), respectively (p=0.65) [60]. At 34 (1-60)
month median follow-up, DFS was compared in the intent to treat
(ITT) (85% VG v 81% CG, p=0.57) and per-treatment (PT) (94% VG v
85% CG, p=0.17) populations. In patients with HER2 overexpression
(51 VG and 50 CG) DFS was 94% VG v 89% CG, p=0.86 (ITT) and
100% VG v 89% CG, p = 0.08 (PT) [61].
The premise behind the AE37 vaccine is that it stimulates a
CD4+ T lymphocyte response that could potentially result in a more
sustained immune response. The current data from clinical trials
does suggest that this vaccine has an effect on the risk of recurrence
[62]. The trial enrolled 298 patients; 153 received AE37+GM-CSF
and 145 received GM-CSF alone. At the time of the primary analysis,
the recurrence rate in the vaccinated group was 12.4% versus 13.8%
in the control group [relative risk reduction 12%, HR 0.885, 95%
Confidence Interval (CI) 0.472–1.659, P=0.70]. The Kaplan–Meier
estimated 5-year DFS rate was 80.8% in vaccinated versus 79.5% in
control patients. In planned subset analyses of patients with IHC
1+/2+ HER2-expressing tumors, 5-year DFS was 77.2% in vaccinated
patients (n=76) versus 65.7% in control patients (n=78) (P=0.21). In
patients with triple-negative breast cancer (HER2 IHC 1+/2+ and
hormone receptor negative) DFS was 77.7% in vaccinated patients
(n=25) versus 49.0% in control patients (n=25) (P=0.12) [63].
Although the trial was negative for the whole population, the results
in the triple negative subset of patients were encouraging and warrant
further investigation.
The presence of high levels of antibodies to specific glycoforms of
the MUC-1 antigen has been shown to be associated with reduced rates
and delay to metastasis in patients who have early stage breast cancer
[64]. One of these particular glycoforms, STnMUC1, has already
been used in a phase III trial in the form of the vaccine Theratope
(STnMUC1, keyhole limpet hemocyanin, and the adjuvant Detox B).
Given as a single agent, Theratope did not show any improvement in
survival. However, when given along with endocrine therapy, there
was a demonstrated improvement in time to progression and overall
survival [65]. The reactivity of antibodies to MUC1 glycoforms might
still be deceptive and can be related to an artifact rather than a true
immune response to MUC1. The example of anti-Gal alpha (1,3) Gal
antibodies is instructive. These antibodies are observed to react with
mucin 1 (MUC1) found on the surface of human breast cancer cells
[66]. Natural occurring anti-Gal alpha (1,3) Gal antibodies found in
all human serum can react with self peptides (MUC1) expressed in
large amounts on the surface of tumor cells, but not on normal cells.
These findings are of interest and serve to explain reported findings
that human cells can, at times, express Gal alpha (1,3) Gal; in reality,
such expression is suggested as an artifact in that anti-Gal alpha
(1,3) Gal antibodies react with mucin peptides [66]. However, some
antibodies display exquisite specificity, like those directed toward the
Thomsen-Friedenreich (TF) antigen [67]. TF antibodies may arise
in the postpartum period against carbohydrate structures expressed
on the cell walls of the gastrointestinal flora and, presumably, may
provide an early barrier against TF-carrying tumor cells.
The widely used regimen of neoadjuvant chemotherapy
is demonstrated to stimulate the immune response to Tumor
Associated Carbohydrate Antigens (TACA) in some patients [68].
Small retrospective studies have suggested that post-chemotherapy
lymphocyte infiltrates could be associated with better outcomes in
patients who did not reach pathologic complete response [68]. The
high levels of anti-TF antibody before surgery is another example in
which antibody targeting is associated with a better survival of stage
II breast cancer patients [69]. This may indicate that the selection of
immunopotentiating regimens of neoadjuvant chemotherapy might
be beneficial for the host in conjunction with the functional activity
of natural anti-cancer antibodies.
Since tumor tissue rejection is the goal of cancer immunotherapies,
broad-spectrum tumor associated antigens, like TACAs, are plausible
targets once the problem of their low immunogenicity is solved
[70]. The fact that multiple proteins and lipids on the cancer cell are
modified with the same carbohydrate structure creates a powerful
advantage for TACAs as cancer targets in immunotherapy strategies.
Thus, targeting TACAs has the potential to broaden the spectrum
of target pathways recognized by the immune response, thereby
lowering the risk of developing escape variants due to the loss of
a given protein or carbohydrate antigen. While TACAs are poor
immunogens, certain investigators succeeded in eliciting cytotoxic
antibodies reactive with naturally occurring forms of TACA using
molecular mimicry to generate peptide mimotopes of TACA
(carbohydrate mimetic peptides - CMPs). Vaccination of mice with
TACA peptide mimotopes reduced tumor growth and prolonged
host survival in a murine tumor model [71]. The first reports of this
strategy in humans are promising and trials exploring their role in
different types of breast cancer are underway [72].
Multivalent vaccines comprised of two or more candidate
proteins are considered to substantially enhance the efficacy of
vaccination against breast tumors. The enhancement in anti-tumor
effect by using a multivalent vaccination approach would be achieved
on two levels: 1) by increasing the strength of immune response
against arising tumor due to activation of a larger T cell repertoire
comprised of multiple T cell lineages reactive to more than one tumor
specific target; 2) by covering a broader range of tumors, including
those that do not express the target protein by a univalent vaccination
approach such as HER2 or MUC1. In addition, a multivalent vaccine
will have the potential to target tumors that have lost or downregulated
expression of one or more proteins or acquired expression
of alternate proteins due to transcriptional dysregulation during
their evolution from normal to dysplastic, to carcinoma in situ, to
invasive, and to metastatic stages of breast tumor evolution. In other
words, a multivalent vaccine approach could apply greater multitarget
immunological pressure both on early and evolving tumors.
It will thereby cover a larger tumor variety and increase efficacy of
prevention as well as provide more effective therapy by lowering
the probability of tumor escape and generation of resistance to the
vaccine. Such approaches are heading to the clinic.
In contrast to a multivalent approach, a pan-immunogen that
elicits responses to several antigens but as a univalent vaccine can
achieve the same end as a multivalent vaccine. TACAs are among
the most challenging of clinical targets for cancer immunotherapy,
but this difficulty can be overcome by CMPs. CMPs are sufficiently
potent to activate broad-spectrum anti-tumor reactivity. However,
the activation of immune responses against terminal mono- and
disaccharide constituents of TACA raises concerns regarding the
balance between “tumor destruction” and “tissue damage”, as monoand
disaccharides are also expressed on normal tissue. To support the
development of CMPs for clinical trial testing, we have demonstrated
in preclinical safety assessment studies in mice that vaccination with
CMPs can enhance responses to TACAs without mediating tissue
damage to normal cells expressing TACA [73] and are pursuing such
an approach in multiple Phase II trials. Particularly important is that
these CMP-induced antibodies can overcome resistance to anoikis
and drug resistance against breast cancer and enhance the efficacy
of taxanes. This aspect might suggest that immunization with such
CMPs can change the clinical paradigm in the neoadjuvant/adjuvant
setting.
Monoclonal Antibodies
Monoclonal antibodies are an integral part of our armamentarium
in the fight against cancer. They can be divided into those that target
the immune system (check point inhibitors) and those that target
oncogenic membrane receptors (HER2) or other surface molecules
of unknown function (CD20). Trastuzumab is a standard component
of the treatment of HER2-positive breast cancer. Its development
in the 1990’s was considered a landmark achievement in the field of
targeted therapy. When combined with chemotherapy it improves
progression free survival and OS in metastatic HER2-positive breast
cancer and DFS and OS in early stage HER2-positive breast cancer.
Trastuzumab’s mechanism of action remains elusive. It targets
HER2 and leads to its internalization and degradation. It inhibits
downstream signaling pathways leading to decreased proliferation
and increased apoptosis of cancer cells. Recently, its role in activating
the immune system against tumor cells emerged as the main
mechanism of action. The FinHer investigators found that every 10%
increase in TILs was associated with decreased distant recurrence [89]
and other studies found that TILs had a prognostic and predictive
value as their presence predicted for higher pCR to trastuzumabcontaining
chemotherapy and better DFS [19,90]. A meta-analysis of
neoadjuvant RCTs showed that the pCR rate was significantly higher
in patients with lymphocyte predominant breast cancer (LPBC) in
HER2-positive BC settings, with an absolute difference of 33.3% (95%
CI, 23.6%–42.7%) [91].
The nature of tumor infiltrating immune cells is more important
than the mere presence or absence of TILs. Using CIBERSORT
(leukocyte gene matrix LM22) to characterize immune cell
composition of 7270 unrelated breast cancer samples from their gene
expression profiles, Bense “et al.” [92] showed that the composition of
the immune cell types differed per breast cancer subtype and interacted
with the treatment. Increased fraction of regulatory T-cells in HER2–
positive tumors was associated with a lower pCR rate (OR=0.15) as
well as shorter DFS (HR=3.13) and OS (HR=7.69). Increased fraction
of γδT-cells in all breast cancer patients was associated with a higher
pCR rate (OR=1.55), prolonged DFS (HR=0.68), and, in HER2-
positive tumors, with prolonged OS (HR=0.27). A higher fraction of
activated mast cells was associated with worse DFS (HR=5.85) and
OS (HR=5.33) in HER2-positive tumors. Furthermore, a high CD8+
T-cell exhaustion signature score was associated with shortened
DFS in patients with ER-positive tumors regardless of HER2 status
(HR=1.80) [92].
The implications of these findings are substantial. Sorting out the
anti-oncogenic from the immune stimulating roles of trastuzumab
may be very difficult. However, the available data from the ALTTO
study suggest that interrupting HER2 downstream signaling using
lapatinib does not add any benefit in early stage breast cancer [93].
It is not clear whether all TKIs will behave like lapatinib but if this
observation is confirmed other TKIs may not add more benefit either.
The challenge for future development of novel drugs is to capitalize
on the immune mechanism.
Checkpoint Inhibitors
Targeting programmed death-1 and programmed death-ligand
1 (PD-1/PD-L1) in breast cancer appears increasingly appealing
after the success of such an approach in other cancers. The PD-1
receptor inhibits innate and adaptive immunity when upregulated on
immune cells and engaged by its ligand, PD-L1 [94]. Cancers take
advantage of this mechanism to induce a local immunosuppression
by overexpressing PD-L1. The prognostic significance of PD-L1 is
still unclear, as some studies have described its value as a positive and
other as negative prognostic factor [75,76]. Regardless, the concept of
inhibiting the PD-1/PD-L1 pathway is based on the idea of “inhibiting
the inhibition” of the immune system. The agents being tried in
breast cancer draw from those already being used in melanoma
and other malignancies including Nivolumab and Pembrolizumab
(anti-PD-1 antibodies). Currently, results from a phase I study in
heavily pretreated TNBC patients who received Pembrolizumab
demonstrated an acceptable toxicity and good safety profile and it is
now in a phase II study [77]. More trials using PD-1/PD-L1 inhibitors
are being planned in TNBC as this is the breast cancer subtype in
which PD-1+ TILs and PD-L1+ cancer cells are more commonly seen
[78]. A randomized, phase III trial to evaluate the efficacy and safety
of Pembrolizumab as adjuvant therapy for triple negative breast
cancer with ≥1 cm residual invasive cancer or positive lymph nodes
(ypN+) after neoadjuvant chemotherapy started accruing patients in
November 2016 [79].
CTLA-4 is another immune checkpoint that is being targeted in
breast cancer. Similar to the PD-1/PD-L1 inhibitors, most ongoing
clinical trials involving CTLA-4 generally revolve around melanoma.
Ipilimumab is a CTLA-4 monoclonal antibody FDA-approved for
the treatment of unresectable melanoma [80]. It is currently being
used in a phase I study examining its safety in combination with a
new anti-B7-H3 mAb, Enoblituzumab, to patients with multiple
refractory cancers, including triple-negative breast cancer [81].
Ipilimumab is also being combined with Entinostat and Nivolumab
in a phase I study for metastatic HER2-negative breast cancer as well
as with just Nivolumab in a phase II study for patients with recurrent
Stage IV HER2-negative breast cancer [82]. There are other ongoing
trials evaluating the combination of a CTLA-4 inhibitor, with
additional treatments. There is a phase II study of tremelimumab
(CTLA-4 inhibitor) with a PD-L1 inhibitor, MEDI4736, in patients
with HER2-negative breast cancer to look for the safety and efficacy
of this regimen [83]. A phase I study has already been completed
with the combination of tremelimumab and exemestane in patients
with hormone-responsive advanced breast cancer [84]. Besides
demonstrating that this treatment regimen is tolerable, the study
showed that there was an associated increase in T cells with inducible
costiumulators (ICOS) and that more of the patients with stable
disease tended to express higher levels of ICOS+ T cells versus the
patients with progressive disease [84]. CTLA-4 inhibitors have been
evaluated in combination with other interventions as well. A phase I
trial evaluating preoperative intervention in the form of ipilimumab
and/or cryoablation in early stage breast cancer showed these
treatments to be safe and tolerable and plans are being made for a
phase II trial with this regimen [85].
Future development of these treatments should balance their
benefit with their potential toxicity. CTLA-4 mAbs have been shown
to have immune-related adverse events mostly affecting the skin
and gastrointestinal tract [80]. Other toxicities include hepatitis,
thyroiditis, colitis, and hypophysitis [86]. Compared to treatments
targeting CTLA-4, therapy targeting PD-1/PD-L1 appears to have
a lower frequency of immune-related adverse events [87]. The
combinations of anti-PD-1/PD-L1 mAbs and anti-CTLA-4 mAbs
are more effective than single agents but they may be associated with
increased incidence of pneumonitis that responds to holding the drug
and/or using immunosuppressive agents; the rate of pneumonitis was
5% in one study [88].
The Immune-mediated Effect of Chemotherapy
Traditionally, the effect of chemotherapy has been explained by the
induction of apoptosis of cancer cells after interrupting their cell cycle
apparatus. However, alternative mechanisms involving the immune
system have been recently invoked [94,95]. Taxanes, doxorubicin,
and cyclophosphamide, which are standard chemotherapeutic agents
in the treatment of breast cancer, are known to have major effects
on the immune system in animals and human experiments [95-100].
For example, taxanes, as a class, increase serum IFN-gamma, IL-2,
IL-6, and GM-CSF levels as well as reducing the levels of IL-1 and
TNF-alpha [101]. Paclitaxel given neoadjuvantly increases the levels
of tumor-infiltrating lymphocytes within the tumor itself [102].
The immune effects of chemotherapy may be summarized by: 1)
rendering dying cancer cells more visible to the immune system by
exposing their TAAs; 2) stimulating the innate immune system; 3)
stimulating T cell differentiation; 4) promoting a cytokine profile that
increases the likelihood of Th1 polarization; 5) inhibition of myeloidderived
suppressor cells and M2 macrophages and 6) suppression of
FOXP3+ regulatory T cells [99]. Acknowledging these mechanisms is
of major importance to optimize their benefit and minimize toxicity
to the immune system that becomes an important executioner of
chemotherapy effect. Furthermore, integrating chemotherapy with
vaccines or checkpoint inhibitors is promising [103,104].
Conclusion and Future Directions
Immunogenicity of breast cancer is subtype-dependent with
a spectrum that spans from the most immunogenic to the nonimmunogenic
subtypes. On one end, TNBC is the most immunogenic
with high mutation and neo-antigen load and high MHC-I expression.
The immune system is already activated against the cancer as attested
by the high TILs, but the cancer is counterattacking by creating an
immune suppressive environment (Tregs, MDSCs) or expressing
checkpoint immune inhibitory molecules (CTLA4, PD-1/PD-L1). On
the other end, Luminal A is the least immunogenic with the lowest
mutation and neo-antigen load and the loss or down regulation of
the expression of TAAs. MHC-I expression is significantly reduced
or absent. Hence, infiltration with TILs is minimal if any. High local
concentrations of estrogen stimulate growth and maintain a local
immune suppression by attracting Tregs and MDSCs. The other
breast cancer subtypes fall in between these two extremes.
The overall goal of cancer immunotherapy is the activation of
the immune system against the cancer. Vaccination has traditionally
been to boost the latent immune response to tumor-specific
antigens. Approaches have included cell-based protocols involving
immunization with whole autologous or allogeneic tumors, as well
as antigen-based strategies involving immunization with proteins
or peptides overexpressed in tumors and under expressed in normal
tissues. HER2 and MUC1 are the predominant antigens used in
human breast cancer vaccine trials. Although vaccination using these
antigens may demonstrate tumor-reducing effects, neither antigen
provides any tissue or tumor specificity since both are expressed
in a variety of normal tissues and tumors raising concerns about
the possibility of off target-damage if a robust immune response is
developed. However, despite the lack of inherent tissue specificity
of HER2 and MUC1, these concerns about systemic autoimmune
sequelae have not been substantiated so far. TACAs are panimmunogens
that elicit responses to several antigens, thus achieving
the same goal as a multivalent vaccine. To overcome their low
immunogenicity, investigators have used CMPs that seem to elicit
a broad-spectrum anti-tumor reactivity. Here again, the activation
of immune responses against TACAs raises concerns regarding
the balance between “tumor destruction” and “tissue damage”, as
TACAs are also expressed on normal tissues. The evidence gleaned
from phase I and II trials is reassuring. It is not clear which subtype of
breast cancer would benefit from this approach.
Monoclonal antibodies are an integral part of our armamentarium
in the fight against cancer. They can be divided into those that target
the immune system and those that target oncogenic membrane
receptors (HER2) or other surface molecules of unknown function
(CD20). Anti-HER2 antibodies have changed the outlook of this
disease. The failures of small molecules that inhibit the oncogenic
stimulation of HER2 and the lack or minimal response to these
antibodies in tumors that lack TILs suggest that their action is more
immune-mediated than oncogenic-mediated.
Monoclonal antibodies that inhibit checkpoints (checkpoint
inhibitors) are changing the paradigm of care in many solid tumors.
The first results of their use in breast cancer suggest that they are
the most effective in TNBC. Their use is being investigated in the
other subtypes. Due to the low immunogenicity of luminal A and B
breast cancers, a combination strategy using vaccines to stimulate the
immune response followed by checkpoint inhibitors is rational but its
clinical usefulness remains to be proven.
Finally, the immune mechanism of chemotherapy is being
increasingly recognized. Its contribution in the total effect of
chemotherapy relative to the direct cytotoxic effect is not known. Any
further development of chemotherapy in the future should take this
aspect into consideration to maximize the immune stimulatory effect
and minimize the immune suppressive effect of chemotherapy.
Funding
This work was partially supported by funds from the Laura F. Hutchins, M.D. Distinguished Chair for Hematology and Oncology.
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