Adaptive resistance mediated by inhibitory ligands such as PD-L1 has emerged as an
important mechanism of malignant cell survival and spurred the development of new
agents that disrupt the PD-L1/PD-1 immune checkpoint.
1
Analysis of patient specimens from clinical trials of novel immune checkpoint inhibitors
indicates that high basal expression of PD-L1 on tumor cells may predict sensitivity
to and be necessary to elicit significant clinical benefit from this drug class.
2, 3
These data suggest that strategies that increase PD-L1 levels could potentially prime
malignant cells with low PD-L1 expression and render them sensitive to anti-PD-1/PD-L1
blockade. In order to investigate this possibility, we first conducted an analysis
of basal PD-L1 transcript levels in multiple myeloma (MM) patients (n = 351) from
the total therapy 2 (TT2) trial. These analyses demonstrated that expression is very
heterogeneous amongst MM patients and that MM patients as a group do not significantly
overexpress PD-L1 compared to normal plasma cells (Supplementary Figure 1). Consistent
with the low basal PD-L1 expression we identified, significant clinical responses
were not observed in any MM patients in a phase I study of nivolumab in patients with
relapsed/refractory lymphoid malignancies.
4
The oncolytic reovirus-based anticancer agent Reolysin is known to have significant
immunomodulatory effects and has demonstrated promising preclinical efficacy in MM
models and favorable safety and tolerability in early MM clinical trials.
5–7
We hypothesized that Reolysin treatment could be used as a precision priming strategy
to potentiate the anti-MM efficacy of PD-1/PD-L1 targeted therapy by promoting myeloma
immune recognition and PD-L1 upregulation. To test this possibility, we infected a
panel of MM cell lines and normal B, plasma, and activated T cells with 30 PFU/cell
Reolysin for 48h. Reolysin treatment produced significant and selective reovirus replication
in MM cell lines, but not in normal B cells (Supplementary Figure 2 and Supplementary
Methods). Expression profiling analyses of U266 and RPMI-8226 cells demonstrated that
Reolysin treatment induced a dramatic increase in CD274 (PD-L1) levels (Figure 1A).
Quantitative RT-PCR (qRT-PCR) analyses of MM cell lines, normal B, plasma, and activated
T cells treated with Reolysin revealed a significant and selective induction of PD-L1
in MM cell lines (Figure 1B). The results were confirmed by immunoblotting in MM cell
lines (Figure 1B). Reolysin also upregulated PD-L2 expression in a manner that mirrored
PD-L1, albeit to a lesser extent than PD-L1 induction (Supplementary Figures 3A–B).
Importantly, flow cytometric analysis showed that Reolysin significantly increased
the cell surface expression of PD-L1 in both MM cell lines and primary CD138+ cells
from MM patients (Figure 1C, Supplementary Figures 4A–B, and Supplementary Table 1).
Since reovirus failed to replicate and induce PD-L1 expression in normal B cells,
we investigated whether active reovirus replication was essential for PD-L1 upregulation.
A comparison of the anti-MM effects achieved by live reovirus versus UV-inactivated
reovirus demonstrated that live reovirus is required to decrease MM cell viability
(Supplementary Figure 5A) and upregulate PD-L1 expression (Supplementary Figures 5B–C).
PD-L1 levels can be induced by interferon (IFN) exposure. To determine if Reolysin-induced
PD-L1 expression was mediated by IFN, we blocked IFNγ using a neutralizing antibody,
which significantly reduced Reolysin-mediated PD-L1 upregulation (Supplementary Figure
6). This study reveals that the IFNγ/JAK/STAT axis primarily regulates PD-L1 expression
in response to Reolysin. Collectively, our data demonstrate proof of concept that
reovirus infection and replication in MM cells can efficiently and selectively upregulate
PD-L1 levels in malignant cells with low target expression.
We next conducted a xenograft study to investigate the therapeutic potential of Reolysin-based
PD-L1 expression priming in vivo. Reolysin treatment produced significant anti-myeloma
activity against RPMI-8226 xenografts (Supplementary Figure 7A) and promoted in vivo
viral replication in tumors (Supplementary Figure 7B). Consistent with our in vitro
findings, reovirus infection induced a significant increase in PD-L1 levels in RPMI-8226
xenografts (Supplementary Figure 7C). To further evaluate the immune priming effects
of Reolysin in an immunocompetent system, we utilized the 5TGM1-luc syngeneic MM bone
disease mouse model, which produces lesions throughout the mouse skeleton, replicates
many other clinical, cytogenetic, histologic, and molecular features of human MM,
and secretes IgG2bκ immunoglobulin.
5, 8
In agreement with our human MM cell line data, in vitro treatment of murine 5TGM1
cells with Reolysin robustly increased PD-L1 expression (Supplementary Figure 8A).
As expected, IV injection of 5TGM1-luc cells efficiently homed to the skeletons of
mice (Figure 2A, far left). Preliminary therapeutic scheduling studies revealed that
pre-treatment with Reolysin followed by anti-PD-L1 therapy demonstrated superior efficacy
compared to simultaneous co-administration. The pre-treatment with Reolysin is required
in order to allow sufficient time for oncolytic reovirus to replicate inside MM cells
and subsequently increase PD-L1 expression. We therefore treated mice once a week
with Reolysin (5 x 108 TCID50, IV) for 3 weeks (D1, D7, and D14) followed by anti-PD-L1
antibody (200 μg/mouse, IP) every other day for 8 treatments beginning on D15. Single
agent therapy with Reolysin or murine anti-PD-L1 antibody significantly reduced skeletal
tumor burden (Figure 2A). Notably, Reolysin pre-treatment strongly improved the anti-myeloma
efficacy of anti-PD-L1 therapy as bioluminescent quantification of MM disease showed
near complete resolution of the skeletal tumor burden in mice treated with the combination
of Reolysin and anti-PD-L1 antibody therapy (Figure 2A, far right). In addition, measurement
of IgG2bκ levels in mouse serum by ELISA demonstrated a potent (>95%) reduction in
M protein levels by combination therapy further illustrating the promising activity
of this therapeutic approach (Figure 2B). Analysis of bone marrow samples collected
from Reolysin treated mice by qRT-PCR detected a significant increase in CD274 (PD-L1)
expression (Supplementary Figure 8B). Importantly, dual treatment with Reolysin and
anti-PD-L1 antibody resulted in a significant increase in overall animal survival
including several mice with sustained disease regression (Figure 2C). In agreement
with several clinical trials that have reported no serious adverse toxicities associated
with Reolysin alone or in combination with chemotherapy, no significant signs of toxicity
or animal weight loss were observed in any of the treatment groups (Supplementary
Figure 9). Our results show for the first time that the combination of Reolysin and
anti-PD-L1 antibody therapy is highly effective in a syngeneic mouse model of MM that
mimics many of the hallmark features of MM pathogenesis.
Preliminary results of a phase I study of nivolumab as a single agent in patients
with relapsed/refractory lymphoid malignancies did not demonstrate significant efficacy
in patients with MM.
4
However, preclinical and phase 1 studies targeting PD-1 in combination with lenalidomide
have shown significant activity.
9–12
These findings suggest that immune checkpoint blockade in MM may be most effective
in combination with agents that increase MM tumor immunogenicity. Measurement of the
levels of PD-L1 in MM patients has produced varied results with some studies finding
significant expression and others finding low levels.
13–15
In our study, we found that PD-L1 was expressed on MM cells, but was not significantly
upregulated compared to normal plasma cells, albeit with considerable variability
among patients. Since PD-L1 expression has been identified as a potential predictive
biomarker of PD-L1/PD-1 therapy, many MM patients with low PD-L1 expression may not
benefit from targeting the PD-L1/PD-1 axis with monotherapy.
With the use of the fully immunocompetent 5TGM1-luc MM bone disease mouse model, we
demonstrated that anti-PD-L1 antibody therapy displayed very modest single agent activity.
It is notable that 5TGM1 MM cells express low but detectable basal levels of PD-L1,
which may explain the partial response in the animal study. Importantly, reovirus
infection was able to dramatically upregulate PD-L1 expression and lead to significantly
greater efficacy of anti-PD-L1 therapy in this model including several mice with complete
responses.
While we show that Reolysin strongly upregulates PD-L1 expression in MM cell lines
and primary patient specimens, it is important to note that Reolysin is also able
to increase PD-L1 expression in other hematological and solid tumor types that are
susceptible to oncolytic reovirus infection and replication (data not shown). Therefore,
this therapeutic approach could have immediate and broad clinical applications against
a diverse array of malignancies. Collectively, we provide the framework for clinical
testing of the combination of Reolysin and PD-1/PD-L1 targeted therapy for patients
with MM. These findings demonstrate proof of concept that oncolytic reovirus immune
priming is a novel precision approach to selectively induce upregulation of PD-L1
in malignant cells and sensitize cancer cells with low basal PD-L1 expression to anti-PD-1/PD-L1
antibody therapy.
Supplementary Material
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