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Cancer immunotherapy is based on the fact that cancer
cells are immunogenic, and the aim of immunotherapy is to
strengthen the endogenous antitumor response via immu-
nologic interventions
[15]. Tumors develop in an immune-
suppressed environment in which cytotoxic CD8
+
T cells
and NK cells are repressed by inhibitory factors expressed
by tumor cells, Tregs, and MDSCs, and in which MDSCs and
type M2 macrophages instead promote tumor growth via
secretion of factors that stimulate angiogenesis and tumor
cell invasion
[15,16]. Most cancer immunotherapies are
developed to strengthen cytotoxic T and NK cell activity via
tumor vaccination or to inhibit immune checkpoint path-
ways such as the CTLA-4 or PDCD1/PD-L1 pathways
[15].
Very little is known about the immune cell profile in PC
[18_TD$DIFF]
metastases. In primary PC, low tumor infiltration of T cells,
B cells, and monocytes has been observed in advanced
disease and associated with poor prognosis
[17] ,although
recent studies highlight tumor infiltration of specific
lymphocyte/monocyte subtypes, such as FoxP3
+
Tregs,
CD163
+
M2 macrophages, and S100A9-positive inflamma-
tory cells, in lethal PC
[18–22] .High blood fractions of Tregs
and MDSC have been related to poor prognosis in patients
with CRPC
[23], as have a whole-blood–based mRNA profile
mirroring high monocyte/low lymphocyte numbers
[24]. Overall, this points to the rationale for using
immunotherapy for treatment of PC. Immunotherapies
that are being tested in the clinic for treatment of PC include
sipuleucel-T (dendritic cell–based vaccine using prostatic
acid phosphatase as antigen), Prostvac (viral-based vaccine
using PSA as antigen), GVAX (whole-cell–based vaccine),
tasquinimod (inhibitor of S100A9 and MDSC), and immune
checkpoint inhibitors such as ipilimumab (inhibitor of
CTLA-4) and pembrolizumab (inhibitor of PD-1)
[25]. Results from the present study highlight heterogene-
ities among CRPC bone metastases that might be important
to consider when choosing immunotherapy for individual
PC patients. For instance, the inverse correlation between
expression of MHC class I and AR-regulated genes probably
diminishes response to antigen-directed vaccines targeting
AR-stimulated genes (ACPP and PSA) in the majority of CRPC
cases with high AR activity (Supplementary Figure 4).
Instead, the high MHC class I expression, immune cell
infiltration, and levels of CTLA4, PDCD1, and S100A9
observed in non–AR-driven metastases suggest testing of
immune checkpoint inhibitors and Tasquinimod specifical-
ly in this subgroup of patients. However, the current study
includes a limited number of clinical CRPC bone metastases,
so the subgroup of 20% non–AR-driven cases is particularly
small. The results need to be verified in larger cohorts,
preferably including patients in trials for evaluation of
immune-strengthening therapies. Therapy-predicting mar-
kers in addition to low serum PSA levels could be MHC class
I expression in tumor cells and the immune cell profile in
metastasis tissue and blood. In patients with multimeta-
static disease, several metastases should be studied for
optimal information.
The molecular drivers behind the subgroups of CRPC
bone metastases observed are not known and need to
be examined further. We observed high levels of the AR
co-regulators FOXA1 and HOXB13, which might be
responsible for programming the AR cistrome in AR-driven
bone metastases
[26], while the function of the prostate-
derived Ets factor SPDEF in PC is more controversial
[27,28]. The low immune-cell infiltration observed in AR-
driven metastases might be explained in part by low levels
of LYVE1 (Supplementary Table 4) and thus low predicted
numbers of lymphatic vessels, recently demonstrated as a
critical determinant of the metastatic process in colorectal
cancer through reduced immune cytotoxicity
[29] .Low levels of the monocyte/lymphocyte chemoattractant
CCL5 and predicted low levels of pro-inflammatory
cytokines such as IFNG, TNF, CSF2, NFKB, and IL4
could obviously contribute as well. In non–AR-driven
metastases, the predicted activity of TGF
b
1, IL5, and other
anti-inflammatory factors might inhibit T-cell activity,
possibly via activation of Tregs and MDSC as discussed
above (Supplementary Table 4). The reduced expression of
MHC class I antigen–processing molecules in clinical PC
might be caused by structural defects, or possibly by
epigenetic, transcriptional, or post-transcriptional regula-
tion
[30]. If so, there might be a possibility of restoringMHC
class I expression with IFNG or drugs inhibiting methyl-
ation or histone deacetylation
[11,31,32] .The inverse
correlation observed between MHC class I expression and
AR activity is in line with previous results showing
increased lymphocyte density in human prostate after
ADT
[33,34]and with the general effects of androgens in
suppressing both adaptive and innate immune responses
[35]. Taken together, these findings support the rationale
for treating PC patients with combinations of ADT and
immunotherapy
[36].
5.
Conclusions
In conclusion, the majority of CRPC bone metastases show
high AR activity, high metabolic activity, low MHC class I
expression and lownumbers of infiltrating immune cells. By
contrast, a subgroup of metastases shows low AR and
metabolic activity, but high MHC class I expression and
immune cell infiltration. Targeted therapies for these two
CRPC subgroups should be explored.
Author contributions:
Pernilla Wikstro¨m had full access to all the data in
the study and takes responsibility for the integrity of the data and the
accuracy of the data analysis.
Study concept and design:
Wikstro¨m.
Acquisition of data:
Bovinder Ylitalo, Egevad, Bergh, Wikstro¨m.
Analysis and interpretation of data:
Bovinder Ylitalo, Thysell, Lundholm,
Wikstro¨m.
Drafting of the manuscript:
Wikstro¨m.
Critical revision of the manuscript for important intellectual content:
Bovinder Ylitalo, Thysell, Jernberg, Lundholm, Crnalic, Egevad, Stattin,
Widmark, Bergh.
Statistical analysis:
Thysell, Wikstro¨m.
Obtaining funding:
Wikstro¨m.
Administrative, technical, or material support:
Jernberg, Crnalic, Stattin,
Widmark, Bergh.
Supervision:
None.
Other:
None.
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