Background
The occurrence of triple-negative breast cancer (TNBC) is significant, with an estimated
40,000 cases diagnosed in US women each year (American Cancer Society, 2018). TNBC
is a vastly heterogeneous disease that are grouped together histologically since they
lack hormone and Her-2 receptors. However, TNBC is best considered as an umbrella
term, encompassing a wide spectrum of entities with distinct genetic, transcriptional,
histological and clinical differences.1–3 As a group, TNBC is associated with high
proliferation, early recurrence and poor survival rates.2 4 This aggressive disease
is resistant to widely used targeted therapies such as trastuzumab and endocrine therapies,
which have been effective at reducing breast cancer mortality. The best chance for
survival is early detection, followed by neoadjuvant chemotherapy (NAC) and surgical
resection.5 Patients with early-stage TNBC have increased rates of pathologic complete
response (pCR) after NAC compared with other breast cancer subtypes. Indeed, the best
prognostic factor for TNBC is the patient’s response to NAC. However, the increased
pCR rates, but worse survival observed in TNBC—termed the triple negative paradox—appears
to be driven by higher relapse rates among those patients whose tumors are not eradicated
by chemotherapy.2 5 There are very limited and often ineffective treatment options
for patients with poor prognosis (chemoresistant and late-stage/metastatic) TNBC.2
Recent large-scale gene expression profiling and immunohistochemistry (IHC) analyses
reveal that TNBC may be the most regulated by tumor-infiltrating lymphocytes (TILs)
and thus the most responsive to immunotherapies compared with other breast cancer
subtypes.6 7 In addition, accumulating data suggest that certain chemotherapeutic
drugs such as anthracyclines mediate their anti-cancer activity through direct cytotoxic
effects and also through activation of TIL responses.8 The presence of TILs suggests
an immune response to tumor-associated antigens (TAAs). Several studies have evaluated
TILs in breast cancer specimens and a higher level of TILs has been reported in TNBCs
compared with other breast cancer subtypes.9 Furthermore, TNBC is characterized by
genomic instability and high rates of genetic mutations, which implicate the production
of more neoantigens and increased immunogenicity.10 11 Pivotal gene expression profiling
studies carried out by Lehmann et al
12–14 revealed the existence of six subtypes of TNBC, namely basal-like 1, basal-like
2, immunomodulatory, mesenchymal, mesenchymal stem-like and luminal androgen receptor.
While these first-generation signatures revealed the transcriptional heterogeneity
of TNBC, second-generation prognostic signatures based on immune response–related
genes allowed for the stratification of patients with TNBC according to overall and
relapse-free survival. Recently, a gene expression analysis of immune activating and
suppressive factors in TNBC of the GeparSixto study showed three subgroups of tumors,
immune group A, B and C with low, intermediate and high immune gene expression levels,
respectively. Importantly, immune group C tumors had a higher extent of TIL infiltrate
and better pCR rates compared with tumors from immune groups A and B. Furthermore,
the extent of TILs in residual TNBC following NAC has been associated with improved
overall and relapse-free survival, highlighting the potential of TIL as a biomarker
in the post-adjuvant setting to identify patients at risk of relapse.8 15 16 Taken
together, these studies provide strong rationale to therapeutically target poor prognosis
TNBC with novel immunotherapies, such as immunogenic tumor cell vaccines.
Infusion of inactivated whole tumor cells, extracted from a patient’s own tumor (autologous),
represents a truly personalized treatment option for malignancies with no targeted
therapies, such as TNBC. Currently, many immunotherapeutic strategies designed to
activate TNBC TILs are being tested in preclinical and clinical studies. Among them
are the use of vaccines against cancer testis antigens (MAGE-A, NY-ESO-1) and other
TAAs. Recent IHC and gene expression data show more frequent expression of these TAAs
in TNBC versus other subtypes.6 16 Given that TNBC is a highly heterogeneous disease,
it logically follows that not all patients will benefit from a vaccine strategy targeting
a single TAA. On the other hand, treatment with autologous tumor cells will expose
a patient with TNBC to their complete and individualized TAA repertoire. Importantly,
the multiple personalized epitopes contained within the autologous vaccine can potentiate
a polyclonal immune response capable of eliminating a diverse population of heterogeneous
TNBC tumors.17 However, whole tumor cell vaccines have demonstrated limited success
in clinical trials, mainly due to lack of immunogenicity.17 18 Furthermore, many tumors
downregulate their expression of MHC or costimulatory molecules, which are both needed
to generate a robust adaptive immune response.
We and others have endeavored to improve on the whole tumor cell vaccination paradigm
by infecting harvested tumor cells ex vivo with engineered oncolytic viruses (OVs)
followed by direct delivery into the tumor microenvironment.19–22 This approach avoids
the major barriers to systemic OV delivery and allows further ex vivo modification
of the tumor cells to improve immunogenicity.16 17 Recombinant Vesicular Stomatitis
Virus (VSVd51) is a genetically modified rhabdovirus with a point mutation in the matrix
protein, which expands its tropism for diverse cancer types, breast cancer included,
and greatly attenuates its replication in healthy tissue when compared with other
OVs.23 Specifically, the VSV M protein mutation improves its therapeutic index by
preventing viral blockade of host cell transcription and nucleocytoplasmic transport,
including interferon (IFN) production.24–26 Following infection with VSVd51, healthy
cells with intact IFN signaling pathways are protected, while malignant cells, which
have lost the ability to mount an IFN response, remain susceptible.24 27 28 Importantly,
the lack of pre-existing neutralizing antibodies in human populations, a major hurdle
that impedes the in vivo delivery of many other OVs (herpes, vaccinia, measles), warrants
the development of oncolytic rhabdoviruses for clinical applications.29 30 Schirrmacher’s
group provided the first proof of concept for an ex vivo infected tumor cell vaccine
approach using oncolytic Newcastle Disease Virus to effectively treat colorectal tumors,31
32 while Conrad et al
33 and Lemay et al
22 demonstrated efficacy in L1210 leukemic and B16 melanoma models, respectively.
We recently demonstrated that the intratumoral delivery of autologous colon cancer
cells infected with an attenuated rhabdovirus (MG1) provided a significant therapeutic
benefit to normally resistant mouse models of established peritoneal disease.19 A
whole cell vaccine infected with MG1 was well tolerated by mice while inducing a robust
recruitment of cytotoxic natural killer (NK) and T cells to the peritoneal cavity
that was associated with robust long-term survival.19 However, in the setting of developing
human TNBC that have already escaped the protective immune response, it is unlikely
that ICV alone will be effective. A combination of surgery, chemotherapy along with
novel small molecule inhibitors and monoclonal antibodies are currently administered
to patients with TNBC in clinical trials with the goal of eradicating cancer and eliminating
recurrence due to resistant or mutated TNBC cells.2 34 In the phase III Impassion
130 trial, a positive median overall survival (OS) was observed in patients with TNBC
receiving anti-PD-L1 atezolizumab with nab-paclitaxel, compared with patients receiving
nab-paclitaxel plus placebo.1 35 Given the importance of activating TILs in the immunosuppressive
TNBC microenvironment and the high therapeutic potential of immunogenic cancer vaccines,
we propose to develop and characterize an oncolytic VSVd51-based cancer vaccine in
combination with anti-PD-1 checkpoint inhibitor to treat poor prognosis TNBC in the
postoperative setting.
Methods
Cell lines and viruses
4T1 and MDA-MB-231 were maintained in DMEM; BT-549 in RPMI, all supplemented with
10% heat inactivated FBS+100 U/mL penicillin and 100 µg/mL streptomycin (P/S). All
cell lines were purchased from ATCC in the past year and were verified to be mycoplasma
free and show appropriate microscopic morphology at time of use. VSVd51 expressing
GFP was propagated on Vero cells and purified using Opti-Prep purification methods.
Viral titers were determined by a standard plaque assay as previously published.19
Viral cytotoxicity was assessed on the indicated cell lines, and cell viability was
carried out as described previously.19
Mice
Female BALB/c mice (6–8 weeks old, 20–25 g) were purchased from Charles Rivers (Quebec).
Animals were housed in pathogen-free conditions at the Central Animal Facility of
the Université de Sherbrooke with access to food/water ad libitum. Animals were euthanized
by cervical dislocation under anesthesia. All studies were conducted in accordance
with university guidelines and the Canadian Council on Animal Care.
4T1 syngeneic mouse model with resection and vaccination
We have previously developed and validated a mouse model of spontaneous metastasis
and surgical resection of primary TNBC.36 37 At day 0, 1×105 4T1 cells in 100 µL (>98%
viability) were injected orthotopically into the fourth mammary fat pad of BALB/c
mice. Following tumor implantation, mice were monitored daily by palpation of the
injection site, the volume of palpable tumors were measured by a Vernier caliper and
the tumor volume was calculated via the equation (width2×length)/2. At days 12–14,
a complete resection of the primary tumor was performed (tumor volume=75–80 mm3).
During surgery, all mice were kept under anesthesia (3% induction, 1.5% maintenance
of isoflurane with 2% O2). For perioperative pain management, all mice were injected
with 0.05 mg/kg of buprenorphine 1 hour before and 4 hours following surgery. Mice
were randomized into different cohorts for treatment. At 1 day and 3 days following
surgery, irradiated 4T1 cells, infected cell vaccine (4T1-ICV), VSVd51 alone or sterile
PBS was injected subcutaneously into the cleared mammary fat pad. For the in vivo
depletion of immune cell populations, 6 doses of depletion antibodies (1 dose 24 hours
before surgery, followed by 5 additional doses 2–3 days apart) were administered by
intraperitoneal injection at 20 µg/dose for anti-Asialo (GM1; Life Technologies) and
250 µg/dose for anti-CD8α (53-6.7; BioXCell). For anti-PD-1 treatment, mice were injected
intraperitoneally with 1 dose of anti-PD-1 (RMP1-14; BioXCell) at 100 µg/dose, followed
by 1 additional dose 3 days later.
TNBC patient tumor dissociation
TNBC tissue from patients were collected after surgery and placed in DMEM supplemented
with 10% heat inactivated FBS+P/S. Tumors were dissociated using the human tumor dissociation
kit (Miltenyi biotec) according to the manufacturer’s recommendations. Briefly, tumors
were cut into small pieces (<2 mm3), then treated with enzymes and placed into the
gentle MACS OctoDissociator (Miltenyi biotec). Macroscopic pieces were removed using
70 µm cell strainers. Tumor cells were washed twice in DMEM. Cells were viably frozen
down or freshly used for downstream experiments.
ICV preparation
ICV using VSVd51 was prepared as previously published.19 Viable single cell suspensions
(5×106) of 4T1 primary tumors were γ-irradiated at 50 Gy. This range of cell number
and irradiation has been previously determined to create a non-proliferating but intact
whole-cell vaccine.22 VSVd51 was added to the cells at 5×107 plaque-forming units
(PFU) and further incubated at 37°C for 24 hours. This preparation was injected subcutaneously
in mice at 100 µL, giving each mouse 1:10 γ-irradiated cells to virus per dose (10
multiplicity of infection, MOI).
Flow cytometry
Antibodies are listed in online supplementary table S1. To analyze mouse spleen and
blood lymphocyte populations, an initial incubation was done in ACK lysis buffer for
5 min to lyse red blood cells. A total of 1×106 splenocytes or blood cells were then
added to each tube. Fc block was added prior to antibody staining for 20 min at 4°C.
Samples were washed twice with flow cytometry buffer (PBS+2% FBS) and acquired on
a CytoFLEX 30 (Beckman Coulter). Data were analyzed with CytExpert software. For assessment
of NK and T-cell functionality, fresh blood or spleen lymphocytes were cultured with
VSVd51-infected 4T1 cell lysates for 4 hours in the presence of brefeldin A (1 µL/mL)
at 37°C. After 4 hours, cells were washed twice with PBS, and then stained for NK
and T-cell markers (online supplementary table S1). Cells were then fixed and permeabilized
using BD Cytofix/Cytoperm kit, according to the manufacturer’s protocol, and intracellular
staining for granzyme B and IFNγ was performed. For CD107a, the antibody was added
to cells in the beginning for cell-surface staining of CD107a on degranulation, as
described previously.
10.1136/jitc-2019-000465.supp1
Supplementary data
Immunogenic cell death (ICD) assays
Conditioned media (CM) was obtained by seeding 5×105 cells in 24-well plates in their
corresponding media for 24 hours followed by infection with VSVd51 at the indicated
PFU for the indicated time points. Flow cytometry: infected cells were harvested and
processed as described above. Antibodies are listed in online supplementary table
S1. Bioimaging was performed using a fluorescence microscope (Leica). Western blot:
proteins from CM (HMGB1) or cell lysates (for other proteins) were resolved by SDS-PAGE
and transferred to Immun-Blot-PVDF membranes (BioRad) for immunoblotting. Protein
expression was detected using specific primary antibodies (1:1000) and corresponding
HRP-conjugated secondary antibodies (1:10,000). Protein expression was visualized
by chemiluminescence detection (ChemiDoc; BioRad). All antibodies are listed in online
supplementary table S1. For ATP detection, the relative concentration of ATP in the
CM was measured with the ENLITEN-ATP kit (Promega). Briefly, 100 µL of CM was transferred
to 96-well opaque plates. Then 100 µL of reconstituted rLuciferase/Luciferin reagent
was added to each well followed by measurement of luciferase using a luminescence
microplate reader (Fusion V.3.0).
Transmission electron microscopy (EM)
Seventy-two hours following infection with VSVd51, TNBC cells were harvested and provided
to the histological platform (Université de Sherbrooke) for processing and imaging.
Briefly, cells were fixed in 2.5% cacodylate-buffered glutaraldehyde for a minimum
of 2 hours. Cells were then centrifuged and the pellet resuspended in sodium cacodylate
buffer (pH 7.2), post-fixed in 2% osmium tetroxide for 2 hours and dehydrated up to
absolute ethanol concentrations. A final dehydration in pure acetone was followed
by three changes in Spurr’s resin and a final embedding at 65°C. Then 80 nm sections
were cut using a Leica Ultracut UCT and stained with uranyl acetate and lead citrate.
Grids were screened on a Hitachi 7500 Transmission EM and the images were digitally
captured.
Quantitative PCR (qPCR)
Total RNA from infected mouse and human TNBC cell lines and single cell suspensions
of patient TNBC tissue were extracted using Trizol (Invitrogen) and cDNA was prepared
using the QuantiTect Reverse Transcription Kit (Qiagen) from 1.2 µg of total RNA in
the presence of 20 IU of RNAseOUT (Invitrogen). Target transcripts were quantified
by qPCR by cDNA amplification with Taq Polymerase (Feldan Bio) and SYBR Green I (Millipore
Sigma) according to the manufacturer’s instructions. All primers were purchased from
Integrated DNA Technologies and are listed in online supplementary table S2. Target
gene expression was normalized to housekeeping genes 18S ribosomal proteins for human
cell lines and tissue samples or to Rplp0 ribosomal protein for mouse cell lines.
Transcript levels were calculated using the ΔΔCT (cycle threshold) method and results
were displayed as fold change of infected samples relative to uninfected samples.
Genes were additionally visualized on 2% agarose gel to confirm their expression if
basal levels were not detected.
10.1136/jitc-2019-000465.supp2
Supplementary data
Enzyme-linked immunosorbent assays
Culture supernatants were diluted fivefold. ELISA kits for detecting mouse CXCL10,
CCL5, CCL2 and CCL4 were purchased from Peprotech and performed according to the manufacturer’s
instructions.
Human polarization, proliferation and migration assays
Polarization: human monocytes were isolated from peripheral blood (Human CD14+ isolation
kit; Stemcell). Then 5×105 monocytes were seeded in 24-well plates in complete RPMI
and incubated overnight at 37°C and 5% CO2. Twenty-four hours later, the monocyte
media was replaced with the CM of infected human cell lines. For controls, monocytes
were co-cultured with recombinant human IL-10, IL-4 and TGFβ (BioBasic) all at a final
concentration of 20 ng/mL for differentiation to M2-like macrophages; and with lipopolysaccharides
(50 ng/mL) (Millipore Sigma) and recombinant human IFNγ (20 ng/mL) (BioBasic) for
M1-like macrophages. Undifferentiated monocytes remained in complete media as M0.
Following overnight incubation, cells were harvested and processed for flow cytometry
as described above. Antibodies are listed in online supplementary table S1. Proliferation:
monocytes from the polarization assay were further incubated with carboxyfluorescein
succinimidyl ester (CFSE)–labeled (3 µM; BD Horizon) human peripheral blood mononuclear
cells (PBMCs) for 5 days in the presence of plate-bound anti-CD3 (OKT3, 5 µg/mL) and
anti-CD28 (CD28.2, 2 µg/mL). Cells were harvested and stained with anti-CD3 (HIT3a),
anti-CD8 (SK1) for flow cytometric analysis of CFSE dilution. Migration: 200 µL of
CM was placed in the lower well of Boyden chambers, separated from the top well by
a 5 mm pore polycarbonic membrane (Neuro Probe). Then 6×105 human PBMCs was added
to the top chamber, followed by incubation at 37°C, 5% CO2 for 45 min. Next, the media
in the top of the chamber was aspirated and the membrane removed with forceps. This
was followed by harvesting of media in the bottom chamber and quantification of migrated
cells by Trypan Blue exclusion. The cells were stained and acquired by flow cytometry
as described above.
Histological analysis
Lungs from treated mice were harvested and fixed in 4% formaldehyde for 24 hours and
kept in ethanol for analysis. Lungs were then embedded in paraffin and 5 µm sections
were used for immunofluorescence staining using CD3, IFNγ and granzyme B. All antibodies
are listed in online supplementary table S1. Slides were scanned using a digital slide
scanner (Nanozoomer-XR C12000; Hamamatsu) provided by the Histology Platform (Université
de Sherbrooke). Percentage staining of marker-positive areas were quantified using
ImageJ software (NIH).
Statistical analysis
All statistical analyses were generated using Prism V.7 (GraphPad). Unpaired two-tailed
t-tests were used for comparing uninfected or infected cells or differentially treated
mice. Survival differences of tumor-bearing and treated mice were assessed using Kaplan-Meier
curves and analyzed by log-rank testing. P value <0.05 was considered as statistically
significant.
Results
Necrotic phenotype accompanies TNBC cell death following infection with VSVd51
We previously demonstrated that an autologous rhabdovirus ICV elicited profound anti-tumor
immune responses in B16 melanoma and CT26 peritoneal carcinomatosis preclinical models.19
Given the lack of therapeutic options for poor-prognosis TNBC, we proposed to develop
an adjuvant ICV to prevent relapse and reduce metastases in this aggressive disease.
We used rhabdoviral VSVd51 expressing enhanced green fluorescence protein (GFP) and
first assessed its cytotoxic activity in mouse and human TNBC cells. VSVd51 was able
to infect mouse 4T1 and human MDA-MB-231 and BT-549 cells as shown by GFP expression
following 72 hours of infection with 10 MOI (figure 1A) and induce cellular cytotoxicity
over a range of increasing MOI as measured by a MTT assay (figure 1C). Given the importance
of the mode of tumor cell death in initiating anti-tumor immune responses,38 39 we
investigated cell death features following infection of TNBC cells with VSVd51. We
first examined cellular morphology using transmission EM (figure 1B). Condensed nuclear
structures, cytoplasmic vacuoles and ruptured cellular membranes were observed. Next,
we detected high mobility group box 1 (HMGB1) protein (figure 1D) and ATP (figure
1E) in the supernatant of VSVd51-infected cells at various time points post-infection,
suggesting passive release from necrotic cells. Another feature of necrosis is the
presence of cell surface externalized calreticulin. Following VSVd51 infection, we
observed an increase in the percentage of necrotic (calreticulin+/DAPI+) cells in
all tested cell lines at 48 and 72 hours post-infection (figure 1F). Together, the
presence of these danger-associated molecular patterns (DAMPs) suggest a necrosis-like
phenotype of TNBC cells following VSVd51 infection. Features of classical apoptosis
(Annexin V+/DAPI−, Caspase-3 and PARP cleavage) were minimally or not observed (online
supplementary figure 1A, B). In addition, the autophagic flux was blocked by bafilomycin
treatment and no differences in the conversion of LC3-I to LC3-II was observed following
VSVd51 infection in all cell lines tested. This suggests that VSVd51 infection of
TNBC cells does not lead to autophagic cell death (online supplementary figure 1C).
By comparison, treatment of TNBC cells with doxorubicin, a clinically relevant neoadjuvant
chemotherapeutic for TNBC, revealed that VSVd51 induced greater release of HMGB1 and
calreticulin exposure (online supplementary figure 1D, E).
10.1136/jitc-2019-000465.supp3
Supplementary data
Figure 1
Necrotic phenotype accompanies triple-negative breast cancer (TNBC) cell death following
infection with VSVd51. (A) Light microscopy images of TNBC cell lines infected with
10 multiplicity of infection (MOI) of VSVd51 for 24 hours. (B) Electron microscopy
images of TNBC cell lines infected with 10 MOI of VSVd51 for 72 hours. (C) Cell viability
assay, (D) Western blot analysis of HMGB1 from cell-free supernatants, (E) luminometry
measurement of relative ATP from cell-free supernatants and (F) measurement of cell
surface calreticulin of TNBC cell lines infected with VSVd51 at indicated MOI and
following indicated time points. All data are representative of at least three similar
experiments where n=3 for technical replicates, *p<0.05; **p<0.01; ***p<0.001; ****p<0.0001;
n.s., not significant.
Immunogenic gene signature is detected on TNBC cells after infection with VSVd51
Next, we sought to determine if VSVd51-induced necrosis is immunogenic in nature.
To accomplish this, we examined a panel of genes related to pro-inflammatory, anti-inflammatory,
antigen presentation and immune differentiation markers by qPCR. Following overnight
infection with VSVd51, we detected a general upregulation of genes related to immune
cell recruitment and activation in mouse and human TNBC cell lines tested. Notably,
mouse CCL2, CCL4, CCL5, CXCL10, IL-6 and MHC-I related genes showed an increase in
expression in 4T1 cells following infection compared with non-infected controls (figure
2A). The expression of several top immunogenic genes (CCL2, CCL4, CCL5 and CXCL10)
at the protein level were measured by ELISA in 4T1 cells following infection to support
the gene expression data (online supplementary figure 2A). In human MDA-MB-231 and
BT-549 cell lines, CCL5, CXCL2, IRF-1 and MHC-I genes were upregulated (figure 2B,
C). Genes were additionally visualized on 2% agarose gel to confirm their expression
if basal levels were not detected. Notably, IFNγ, IL-2 and PD1 genes were induced
following infection in 4T1 cells (online supplementary figure 2B), while CSF-1, CCL4
and CXCL10 were induced in BT-549 and MDA-MB-231 infected cells (online supplementary
figure 2C, D). These data suggest that an immunogenic gene signature is present in
TNBC cells following VSVd51-induced necrotic cell death.
10.1136/jitc-2019-000465.supp4
Supplementary data
Figure 2
Immunogenic gene signature is detected on triple-negative breast cancer (TNBC) cells
after infection with VSVd51. Fold change in gene expression of (A) mouse 4T1, human
(B) MDA-MB-231 and (C) BT-549 TNBC cells following infection with VSVd51 at 10 multiplicity
of infection for 24 hours. Quantitative PCR was performed using mRNA pooled from three
independent experiments.
Enhanced innate and adaptive immune cell activation by ICV
To determine if the observed in vitro ICD features (figure 1) and gene signatures
(figure 2) translate to enhanced immune function in vivo, we tested the ICV in the
adjuvant setting in BALB/c mice bearing orthotopic 4T1 tumors following primary tumor
resection (figure 3A, timeline). This model makes use of an aggressive mouse stage
IV TNBC from the BALB/c strain that spontaneously metastasizes from the mammary glands
to multiple distant sites, in particular the lungs. We included the following treatment
cohorts (PBS, irradiated 4T1 cells, VSVd51 alone and ICV) to delineate the role of
the vaccine’s constituent parts. At early and late time points following vaccination,
we observed that postoperative vaccination of mice with 2 doses of ICV significantly
enhanced the proportion of blood IFNγ+, granzyme B+ (cytotoxicity) and CD107a+ (degranulation)
NK cells compared with administration of virus alone, non-infected cells or PBS (figure
3B). Similar results were observed in CD11c+ conventional dendritic cells (DCs) in
terms of their overall proportion and activation status (CD86+) (figure 3C). Analysis
of both blood and lung CD3+/CD8+ T cells showed enhanced IFNγ, granzyme B and CD107a
degranulation in ICV-treated mice over controls (figure 3D, E). Immunofluorescence
staining of mice lungs bearing metastatic 4T1 tumors treated with ICV showed increased
presence of CD3+ T cells, granzyme B and IFNγ expression compared with lungs from
mice treated with irradiated 4T1 cells (figure 3F, G). By comparison with ICV, the
addition of a treatment cohort receiving systemic doxorubicin injection, a known ICD
inducer, resulted in decreased CD8+ T-cell CD107a degranulation and IFNγ production
compared with ICV treatment. Importantly, survival of ICV-treated mice significantly
surpassed those cohorts treated with irradiated 4T1 cells or doxorubicin (online supplementary
figure 3A–C). Taken together, these in vivo data demonstrate the innate and adaptive
immune activating capacity of the ICV approach.
10.1136/jitc-2019-000465.supp5
Supplementary data
Figure 3
Enhanced innate and adaptive immune cell activation by infected cell vaccine (ICV).
(A) Timeline of in vivo BALB/c-4T1 experiment. BALB/c mice were orthotopically implanted
with 1×105 4T1 cells followed by a complete primary tumor resection on day 12. On
days 14 and 16, mice received two doses subcutaneously of either virus alone (VSVd51,
MOI=1×106 PFU/mL), irradiated cells alone (irr4T1, 5×106), an ICV (5×106 infected
cells) or left untreated (1× PBS). (B–E) Immune cell suspensions from the peripheral
blood (B–D) or lungs (E) of mice following indicated treatments were stained with
(B) NK cell markers (CD122+, CD3−, IFNγ+, granzyme B+, CD107a+), (C) DC markers (CD11c+,
CD86+), (D, E) T-cell markers (CD3+, CD8+, IFNγ+, granzyme B+, CD107a+) and analyzed
by flow cytometry. (F) Representative immunofluorescent images and (G) quantification
of % expression of CD3+, IFNγ+ and granzyme B+ positive lung area in mice treated
with irradiated cells or ICV. Scale: top panel, 2 mm; bottom panel, 0.5 mm. All data
are representative of at least three similar experiments where n=3–6 mice/treatment.
*p<0.05; **p<0.01; ***p<0.001; n.s., not significant. DC, dendritic cell; MOI, multiplicity
of infection; PFU, plaque-forming unit.
CD8+ cytotoxic T cells are critical for ICV efficacy and combination treatment with
anti-PD1 checkpoint inhibitor improves survival in the BALB/c-4T1 model
To further investigate the critical role of NK and CD8+ T cells after ICV administration,
we monitored for survival in ICV-treated mice that were pharmacologically depleted
singly or of both immune cell populations (figure 4A, B). In support of the in vivo
data showing enhanced NK and CD8+ T-cell function (figure 3), the protective effect
of vaccinated mice with ICV was partially abrogated on depletion of NK cells, but
completely abrogated on depletion of CD8+ T cells or combination of NK and CD8+ T
cells (figure 4B). These results suggest that the therapeutic benefit of this treatment
strategy is dependent on both NK and CD8+ T-cell recruitment, but likely more dependent
on CD8+ T cells. Given the importance of CD8+ T cells and their role in mediating
the response to ICV treatment, we examined cell surface expression of exhaustion markers
on CD8+ T cells at day 9 following ICV treatment and observed augmented levels of
PD-1, but not TIM-3 or LAG-3 (figure 4C). In addition, we observed upregulation of
PD-L1 expression levels on 4T1 cells following infection with VSVd51 in vitro (figure
4D). These data suggest that the adaptive T-cell response could be modulated to override
exhaustion. Therefore, to improve the immune response and survival of vaccinated mice,
we combined ICV with anti-PD1 checkpoint inhibitor treatment (figure 4E, F). We observed
that combination therapy prolonged survival compared with either monotherapy ICV or
anti-PD-1 alone. These preclinical results demonstrate the therapeutic potential of
ICV in combination with checkpoint inhibitors to treat TNBC.
Figure 4
CD8+ cytotoxic T cells are critical for infected cell vaccine (ICV) efficacy and combination
treatment with anti-PD1 checkpoint inhibitor improves survival in BALB/c-4T1 model.
(A) Timeline of immune cell depletion in the BALB/c-4T1 in vivo model. One day before
surgical resection, NK cells, CD8+ T cells and NK+CD8+ T cells were depleted using
antibodies to GM1, CD8 and GM1+CD8, respectively, and continued every 3–4 days for
a total of 6 doses. On days 14 and 16, mice received 2 doses of ICV. Blood droplet
denotes verification of in vivo depletion by flow cytometry. (B) Kaplan-Meier survival
analysis of BALB/c mice bearing intramammary 4T1 tumors and receiving ICV and antibody
depletion. n=10–12 mice/group. *p<0.05; n.s., not significant, log-rank test. (C)
Single cell suspensions from the peripheral blood of mice following indicated treatments
were stained with exhaustion markers on CD8+ T cells (PD1, Tim3, LAG3) and analyzed
by flow cytometry. All data are representative of three similar experiments where
n=3–5 mice/treatment. *p<0.05; n.s., no significance. (D) Cell surface staining of
PD-L1 on 4T1 cells following infection with VSVd51 in the presence or absence of IFNγ
and analyzed by flow cytometry. (E) Timeline of combination therapy ICV+αPD1 in the
BALB/c-4T1 in vivo model. Two days after vaccination, mice received 2 doses of anti-PD1
intraperitoneally 3 days apart. (F) Kaplan-Meier survival analysis of BALB/c mice
bearing intramammary 4T1 tumors and receiving ICV and anti-PD-1. n=10–12 mice/group.
*p<0.05; n.s., not significant, log-rank test.
Polarization of human monocytes to M1 phenotype and enhanced migration and proliferation
of human CD8+ T cells following exposure to ICV
To improve the translational potential of our work, we examined the effect of ICV
on human primary antigen-presenting cells. In ex vivo co-culture experiments with
CD14+ human monocytes incubated with cell-free lysates derived from infected human
TNBC cells, we observed polarization of monocytes toward an M1-like phenotype that
have been previously suggested40 41 to promote anti-tumor immune responses (figure
5A). We additionally examined the activation status of human DC treated ex vivo with
the same cell-free lysates (online supplementary figure 4A). ICV-lysate treated DC
displayed a more mature phenotype compared with controls. To examine the consequences
of ICV-induced M1-like monocytes on effector immune cells, we measured human NK and
CD8+ T-cell migration and CD8+ T-cell proliferation in the ex vivo setting. We observed
increased migration of NK cells and increased migration and proliferation (CFSE dilution)
of CD3+/CD8+ T cells in co-cultures with ICV-lysate treated M1 monocytes (figure 5B,
C). Taken together, these data using human primary immune cells and human TNBC cell
lines demonstrate the immune activating potential of ICV.
10.1136/jitc-2019-000465.supp6
Supplementary data
Figure 5
Polarization of human monocytes to M1 phenotype, increased migration of NK and CD8+
T cells and increased proliferation of CD8+ T cells following exposure to infected
cell vaccine (ICV). (A) Polarization of purified human monocytes in the presence of
conditioned media (CM) from human triple-negative breast cancer (TNBC) cell lines
infected with VSVd51 (10 multiplicity of infection, 24 hours). Monocytes exposed to
cytokines for control polarization as indicated. (B) Migration assay of purified human
CD3+/CD8+ T cells and CD3−/CD56+ NK cells following exposure to CM of infected TNBC
cells or controls as indicated. (C) CFSE-based proliferation assay of CD3+/CD8+ T
cells following co-culture with human monocytes treated with CM or controls as indicated.
All data are representative of at least three similar experiments where n=3 for technical
replicates. *p<0.05; **p<0.01; ***p<0.001; n.s., not significant. MFI, mean fluorescence
intensity.
ICV enhances immune signature and biomarkers of ICD in human TNBC patient tissue
To investigate whether the ICV could elicit an immunogenic signature in human TNBC
patient tissue, patients with TNBC were enrolled in the VACS study as part of the
Sherbrooke Gynecologic Biobank (Ethics no. 2018-2414). Dissociated breast tumor tissue
was obtained from two patients with TNBC (BRC1762, BRC1756). The cells were infected
with VSVd51 overnight and qPCR for gene expression analysis and assays to measure
biomarkers of ICD were conducted. Patient BRC1762 displayed an immunogenic gene expression
pattern with enhanced expression of multiple immune genes, notably CCL5, CCL2, CXCL9,
CXCL11, CCL3, TGFb, CSF-2, TAP1 and TAP2 (figure 6A). In addition, the genes CCL20,
IFNα, IFNβ and GRA that were not basally expressed in uninfected samples showed induced
expression following infection with VSVd51. In patient BRC1756, CCL2, CCL5, CXCL2,
CCL20, IRF1, TAP1 and TAP2 gene expression were also increased (figure 6B) and the
genes CCL20, CTLA-4, CCL3 and CCL4 were induced following infection (online supplementary
figure 4B). Biomarkers of ICD including calreticulin cell surface expression (for
patient BRC1762) (figure 6C), ATP (figure 6D) and HMGB1 (figure 6E) release for both
patients were detected at higher levels in VSVd51-infected cells compared with uninfected
controls. These human data demonstrate that an ICD gene signature is present in patient
TNBC cells following VSVd51 infection, and this phenotype has the potential of recruiting
and activating important immune cells in vivo. Taken together, our translational data
highlight the clinical potential of using ICV as adjuvant vaccine to treat patients
with TNBC.
Figure 6
Infected cell vaccine (ICV) enhances immune signature and biomarkers of immunogenic
cell death in human triple-negative breast cancer (TNBC) patient tissue. Fold change
in gene expression from human TNBC patient tissue (A) BRC1762 and (B) BRC1756 following
infection with VSVd51 at 10 multiplicity of infection (MOI) for 24 hours. (C) Measurements
of cell surface calreticulin, (D) luminometry measurement of relative ATP and (E)
Western blot analysis of HMGB1 from cell-free supernatants from TNBC patient tissue
following infection with VSVd51 after 24 hours and at indicated MOI. Data are pooled
from technical replicates, n=3, *p<0.05; **p<0.01; n.s., not significant.
Discussion
Given the lack of effective treatments in TNBC, several efforts over the last few
years have been made to improve therapeutic opportunities for patients with TNBC,
especially for those patients who do not achieve pCR after NAC. In the phase III Impassion
130 trial, a significantly improved progression-free survival (PFS) and a positive
median OS was observed in patients with TNBC receiving anti-PD-L1 atezolizumab with
nab-paclitaxel, compared with patients receiving nab-paclitaxel plus placebo.1 35
Preliminary data from the phase Ib/II KEYNOTE-150 trial investigating the combination
of anti-PD-1 pembrolizumab with eribulin (microtubule inhibitor) demonstrated substantial
benefits in both PFS and OS in the combination treatment arm.1 42 In addition to combination
immune checkpoint and chemotherapy trials, PARP inhibitors are also undergoing early
phase trials for the treatment of TNBC, especially when associated with homologous
recombination deficiency.43 Despite these new treatments, early data show modest improvements
in survival, underscoring the need to improve therapeutic outcome for patients with
TNBC.
Autologous tumor cell vaccines are an antigen agnostic form of personalized immunotherapy.
Unlike single tumor antigen-targeted vaccines (pre-defined antigens), treatment with
autologous tumor cell vaccines exposes a patient with cancer to their complete and
individualized TAA repertoire, therefore reducing the likelihood of tumor escape due
to tumor heterogeneity and eliminating the need to sequence the tumor a priori, saving
both time and money.18–20 The combination of cytokine delivery with whole tumor cells
is capable of significantly delaying tumor growth through the creation of a pro-inflammatory
environment to enhance immune system activation against TAAs.44 Existing data suggest
that disease recurrence is significantly delayed when patients successfully mount an
immune response against the tumor, as evidenced by a delayed-type hypersensitivity
response.45 Clinical studies have consistently shown that survival is significantly
better in patients who mount an immune response against their tumor cells.21 22 The
strong immunological rationale for cytokine-based whole cell vaccines continues to
drive the clinical development of this novel approach.23–26 Unfortunately, the majority
of patients do not mount such a response, either because the tumor cell vaccine and
cytokine combination are not immunogenic enough or because the host immune system
is suppressed in response to the cancer. The FANG vaccine, which is composed of granulocyte
macrophage colony-stimulating factor/shRNAi furin vector-transfected autologous tumor
cells, was designed to improve immunogenicity and dampen immune suppression.46 Treatment
with this vaccine was associated with a high rate of T-cell activation and prolonged
recurrence-free survival in patients with stage III/IV ovarian cancer,47 demonstrating
the clinical potential of immunogenic autologous tumor vaccines.
Our laboratory and others have endeavored to improve on the whole cell vaccination
paradigm by infecting tumor cells ex vivo with OV.19–22 As proof of concept for solid
tumors, we recently demonstrated that the intratumoral delivery of autologous colon
cancer cells infected with rhabodoviral MG1 provided a significant therapeutic benefit
to normally resistant mouse models of established peritoneal disease.19 Both NK and
T cells demonstrated enhanced recruitment to the peritoneal cavity following MG1-ICV
administration.19 From in vitro experiments in this study, we determined that infection
of mouse and human TNBC cells with rhabdoviral VSVd51 results in higher necrotic cell
death than in non-infected cells. We observed morphological features of necrosis by
transmission EM, enhanced release of intracellular HMGB1 and ATP and increased calreticulin+/DAPI+
populations (figure 1). Further, an immunogenic gene signature was detected in infected
TNBC cell lines. From in vivo experiments, we observed that postoperative vaccination
of mice with 2 doses of ICV significantly augmented both innate and adaptive immune
cell functionality. Both NK and CD8+ T cells were important in contributing toward
vaccine efficacy. However, CD8+ T cells appear to play a more important role in mediating
therapeutic efficacy as evidenced by shortened survival in CD8+ T-cell-depleted mice,
singly or in combination with NK cells.
In many cancer types, checkpoint blockade immunotherapy has been shown to provide
long-lasting survival benefit by re-invigorating immune cells within the tumor; however,
this occurs in only a small percentage of responding patients.48–50 Resistance to
checkpoint blockade therapy due to tumors evolving to escape immune attack further
detracts from the clinical utility of this ground-breaking immunotherapy. As immunotherapy
continues to reinforce itself at the forefront of oncology treatment, we strive to
take advantage of these promising therapies by extending their clinical utility to
immunogenically cold tumors such as TNBC. We propose to increase the recruitment of
TILs into the TNBC tumor microenvironment through the use of immune-stimulatory combination
immunotherapies. This could potentially be achieved through ICD-inducing chemotherapies
such as doxorubicin or anthracyclines. Our in vitro and in vivo data showed that ICV
is superior to drug treatment for enhancing ICD, immune recruitment and survival.
However, this does not preclude the possibility of drug treatment with ICV prior to
checkpoint blockade. Using ICD-inducing drug plus VSVd51-based ICV to initiate an
anti-tumorigenic inflammatory response in TNBC tumors prior to treatment with checkpoint
blockade has the potential to significantly improve patient prognosis by optimizing
the power of complementary immunotherapy strategies. We envisage a future clinical
trial to consist of an optimized adjuvant ICV-based strategy to initiate ICD and TAA
release to promote an anti-tumor immune response. This will be followed by checkpoint
inhibitor administration to further potentiate the anti-tumor activity of T cells
at the tumor site. Our in vivo studies in BALB/c-4T1 mice showed improved and prolonged
overall survival compared with monotherapy ICV or anti-PD1 alone. Immune profiling
of other exhaustion markers on both NK and CD8+ T cells following ICV treatment indicates
that other checkpoint blockades including LAG3 on NK cells (data not shown) could
be added to further improve the efficacy of this dual therapy. In a recent preclinical
study to mimic the treatment course for patients with newly diagnosed TNBC, neoadjuvant
OV was used to sensitize the tumor to checkpoint blockade therapy.51 A neoadjuvant
priming OV could potentially be administered prior to adjvuant ICV in a heterologous
prime-boost strategy to achieve synergistic long-term anti-tumor benefits.
In human studies, we demonstrated that an analogous mechanism of ICV-induced immune
activation is occurring in human TNBC cell lines and in human TNBC patient tumor samples.
Our human TNBC cell line data demonstrate that ICV lysate can polarize monocytes toward
an M1-like phenotype, induce maturation of DC (online supplementary figure 3A) and
lead to greater NK and T-cell migration and T-cell proliferation (figure 5). In addition,
qPCR data demonstrated upregulation of many genes that are involved in the immune
process and the release of immunogenic DAMPs following infection of TNBC cell lines
with VSVd51 (figure 2). In two TNBC patient samples, gene expression data revealed
an immunogenic gene signature, while evaluation of ICD biomarkers showed augmented
release of DAMPs following infection of patient tissue with VSVd51. Taken together,
these human results demonstrate the feasibility of developing a VSVd51-based immunogenic
vaccine to treat TNBC.
Conclusions
In summary, we characterized the mechanism and clinical potential of a VSVd51-based
cancer vaccine for treating TNBC. We demonstrated that both innate and adaptive immune
cells play mediating roles in the in vivo efficacy of ICV (figure 7). Further translational
testing in our laboratory will include identifying ICD pathways intrinsic to individual
TNBC patient tissue and their response to VSVd51 infection. This will allow us to
engineer precision ICV to ensure that ICD is present, regardless of tumor heterogeneity,
which is especially prevalent in the TNBC population. In addition, we aim to further
understand the molecular events unleashed by ICV in our validated mice models and
in patient samples that dictate immunogenicity and subsequent development of anti-tumor
immunity. These translational studies could lead to future clinical trials of ICV
monotherapy in TNBC or as potent anti-tumor immune response drivers in combination
with immune checkpoint blockade. Although these studies are being conducted in TNBC,
they have potential widespread implications across various solid tumor types.
Figure 7
Proposed immune mechanism of action for ICV. Adjuvant vaccination with ICV results
in the release of immunogenic cell death markers that recruit and activate innate
and adaptive immune cells. Notably, antigen-presenting cells such as monocytes and
dendritic cells (DCs) are primed to cross-present tumor-associated antigen to cytotoxic
CD8+ T cells. These adaptive immune T cells along with activated natural killer (NK)
cells unleash tumor-targeted cytokines and cell death–inducing granules to reduce
residual and metastatic triple-negative breast cancer (TNBC).