Tumor-Associated Macrophages as Treatment Targets in
Oncology
Alberto Mantovani
1,2
, Federica Marchesi
1,3
, Alberto Malesci
1,3
, Luigi Laghi
1
, and Paola
Allavena
1,2
1
Istituto Clinico Humanitas IRCCS, via Manzoni 113, 20089 Rozzano, Italy
2
Humanitas University, via Manzoni 113, 20089 Rozzano, Italy
3
Dept. Medical Biotechnology and Translational Medicine, University of Milan, Milan, Italy
Abstract
Macrophages are crucial drivers of tumor-promoting inflammation. Tumor-associated
macrophages (TAM) contribute to tumor progression at different levels, including promoting
genetic instability, nurturing cancer stem cells, paving the way to metastasis, taming protective
adaptive immunity. TAM can exert a dual, yin yang influence on the effectiveness of cytoreductive
therapies (chemotherapy and radiotherapy), antagonizing antitumor activity by orchestrating a
tumor-promoting, tissue repair response or, instead, contributing to their ultimate antineoplastic
efficacy. TAM express triggers of checkpoints of T cell activation and are targets of checkpoint
blockade immunotherapy. Macrophage-centered therapeutic approaches include: strategies to
block recruitment and survival in tumors; functional reeducation to an antitumor, M1-like mode;
tumor-directed monoclonal antibodies which elicit extracellular killing or phagocytosis of cancer
cells. We surmise that TAM can provide tools to tailor cytoreductive therapies and immunotherapy
and that TAM-centered therapeutic strategies have the potential to complement and synergize with
chemotherapy and immunotherapy.
1 Introduction
Inflammatory cells are a key component of the ecological niche of cancer 1–4. The
formation of an inflammatory microenvironment in tumors is driven by genetic events which
cause cancer (oncogenes and oncosuppressor genes) or by chronic non-resolving
inflammatory conditions such as inflammatory bowel disease, which increase the risk of
developing cancer 1. In general, cancer-associated inflammation is characterized by being
non-resolving 5.
Macrophages are a major component of the leukocyte infiltrate present in widely different
amounts in all tumors 6. Tumor-associated macrophages (TAM) have served as a paradigm
for leukocytes and inflammatory mediators present in the tumor context and play a dominant
role as orchestrators of cancer-related inflammation (CRI). CRI is considerably diverse in
Correspondence to: Alberto Mantovani, MD, Istituto Clinico Humanitas IRCCS, via Manzoni 113, 20089 Rozzano, Italy;
alberto.mantovani@humanitasresearch.it.
AM, FM and PA equally contributed to this review.
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Published in final edited form as:
Nat Rev Clin Oncol
. 2017 July ; 14(7): 399–416. doi:10.1038/nrclinonc.2016.217.
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tumors arising in different tissues 2, 7. However, though cellular components of CRI differ
in quality and quantity and mediators which orchestrate macrophage function can differ
considerably in different cancers, TAM represent a final common pathway driving CRI 8.
In the ’70 it was realized that macrophages activated by bacterial products and cytokines
acquired the capacity to kill tumor cells 9–11. On the other hand it was soon realized that
TAM from malignant metastatic tumors promoted tumor growth and metastasis 12. Thus,
early on evidence suggested that macrophages could engage in a dual yin yang relationship
with cancer.
Here we review current understanding of the role of TAM in different cancer treatment
modalities as well as emerging macrophage targeting therapeutic strategies. As a premise, a
concise overview of the role of macrophages in tumor initiation and progression will be
provided. Previous reviews on CRI and specifically on myeloid cells in tumors provide the
background of the present essay 1–3, 6, 13–15.
2 Role in Tumor Progression
Fig. 1 provides a schematic representation of the origin and function of TAM and a general
framework for subsequent sections focused on therapy (see also Box 1). It has long been
held that TAM originate from the blood compartment and that chemotactic signals
originating from tumor cells or from normal cells present in the cancer microenvironment
recruit monocytic precursors at the primary and metastatic tumor sites 11, 15–19. However,
recent evidence raises questions as to this long held view. In the mouse, resident
macrophages in some tissues (e.g microglia in the brain) originate from precursors seeding
there during fetal and embryonal life rather than from circulating monocytes (Box 1) 20,21.
In gliomas, tumor-associated macrophages constitute a mixed population that includes
resident brain microglia, infiltrating blood monocytes, and macrophages. The relative
contribution of these cells has been investigated in a genetically engineered mouse model:
accumulation of Ly-6C
hi
circulating “inflammatory” monocytes into tumor tissue was
responsible for the increased tumor incidence and shorter survival times, with no
contribution of microglial cells 22. In the perspective of macrophage function in the tumor
microenvironment, it is noteworthy that recent results support that in the mouse the
ontogenetic origin does not have an appreciable impact on the macrophage phenotype in
response to tissue-derived cues 23. Whether embryo-derived tissue macrophages contribute
to the number, location and diversity of TAM remains an open question 24. TAM
proliferation has been observed in murine and human sarcomas and murine breast
carcinomas but this does not appear to be a general mechanism sustaining TAM numbers in
the face of growing tumors 25, 26, 27. Circulating precursors that are recruited into tumor
tissues and there differentiate into TAM include conventional inflammatory monocytes and
Mo-MDSC (see Box 1). Down regulation of the transcription factor STAT3 plays a key role
in the differentiation of Mo-MDSCs into mature TAM28. Inflammatory monocytes, defined
in the mouse as Ly6C
+
/CCR2
+
cells have been shown to contribute to TAM accumulation
and maintenance in a mouse mammary tumor model 27 and pulmonary metastases of
murine and human breast cancer cells 19. The process of differentiation of mouse
inflammatory monocytes into TAM was dependent on the transcriptional regulator RBPJ and
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its genetic deletion in TAM resulted into reduced tumor burden, confirming a non-redundant
function of monocyte-derived TAM in tumor growth 27. In contrast, protective function has
been shown for CX3CR1
+
/Ly6C
-
“nonclassical” patrolling mouse monocytes, which depend
on the N4a1 transcription factor and patrol the lung microvasculature in steady state
conditions. These cells, which rarely extravasate into tissues and differentiate into
macrophages, rapidly accumulate into lung metastases and inhibit tumor cell seeding and
growth in mouse models 29. Their antitumor function includes scavenging of tumor debris
and recruitment and activation of natural killer cells 29.
Chemoattractants involved in monocyte recruitment include chemokines (e.g. CCL2,
CCL5), colony-stimulating factor-1 (CSF-1), and members of the VEGF family. TAM
themselves can be a source of CCL2 in cancer. Recently, genetic evidence in the mouse
suggested that Complement components, C5a in particular, play an important role in
recruitment and functional polarization of TAM 30. Chemotactic factors are more than
attractants in that they activate transcriptional programs in macrophages and contribute to
functional skewing (e.g. 31). CSF-1 in particular is a monocyte attractant as well as a
macrophage survival and polarization signal 6, 32. Unlike M-CSF, GM-CSF activates
macrophage functions related to anti-tumor activity 33.
Signals originating from tumor cells, T and B lymphocytes and stroma orchestrate TAM
function and their diversity. Classically activated macrophages (M1, Box 1) can kill tumor
cells extracellularly and mediate tissue destructive reactions centered on the vessel wall
(hemorrhagic necrosis) 15,9,10,11. Accordingly, there is evidence that macrophages
contribute to the early elimination phase of nascent tumors orchestrated by T cells and
interferons 34 . Tumor progression is associated with skewing and subversion of
macrophage function. In established progressing tumors such as murine and human breast
and pancreatic cancer, IL-4 and IL-13 derived from TH2 cells 35–37, eosinophils 38 or
basophils 39 elicit alternative M2 activation of TAM (Box 1, Fig. 1). In addition, signals
originating from tumor cells (e.g. chemokines, CSFs, TGFβ), B cells (immune complexes)
and stromal cells (e.g. IL-1) can act as drivers to skew macrophage function to diverse
phenotypes (for a recent summary, 8) which do not fit with classic M1/M2 polarized cells
(Box 1). In general, in spite of intra and inter-tumor diversity, TAM in progressing
neoplasms have surface molecules (e.g. the scavenger receptor CD163; mannose receptor
CD206) and properties related to angiogenesis, suppression of specific immunity and
promotion of cancer growth and metastasis. For convenience, we and others refer to these
diverse populations as M
2
-like. In line with a consensus recommendation 33 (Box 1) we use
M2 when IL-4 or IL-13 are major drivers of polarization.
There is evidence that the relative importance of distinct pathways varies in different tumors,
resulting in heterogeneous phenotypes 2, 7. However as diverse as CRI can be in different
cancers, TAM polarization appears to represent a final common pathway. Within a given
human or mouse tumor, TAM exhibit different phenotypes, influenced for instance by access
to oxygen and activation of the HIF pathway 40, 41 42. Dissecting TAM diversity at the
single cell level and integrating information represent current challenges.
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TAM influence tumor cell intrinsic properties as well as the tumor microenvironment (Fig.
1). TAM can stimulate tumor cell proliferation, migration and genetic instability. Acting at
the primary tumor site or at sites of secondary localization they promote invasion and
metastasis (Fig. 1). TAM promote angiogenesis and lymphoangiogenesis as well as tissue
remodeling with fibrous tissue deposition.
Myelomonocytic cell contribute to suppression of effective adaptive immunity, a key feature
of cancer 3, at various levels and through multiple mechanisms. MDSC, and in particular
MoDSCs, suppress immunity in lymphoid organs. MoMDSC are recruited in tumors
contributing to an immunosuppressive microenvironment and here they undergo
differentiation into TAM 28 (Box 1). TAM can promote T regulatory cell (Treg) in a
bidirectional interaction 15. Immunosuppressive cytokines (IL-10 and TGFβ) are produced
by macrophages in the tumor context. Aminoacid metabolism in polarized macrophages and
TAM results in metabolic starving of T cells and production of immunosuppressive
metabolites via the indoleamine 2,3 dioxygenase (IDO) pathway 33. Prostaglandins
produced from the arachidonic acid metabolism have suppressive functions. Finally, TAM
express PD-L1 and PD-L2 which trigger checkpoint blockade in T cells, as well as B7H
4
43
and VISTA 44 which may exert similar functions.
Progress has been made defining the molecular pathways responsible for the orchestration of
macrophage function in tumors, including members of the STAT and NFkB family 8.
Among these, Myc is interesting in that it acts both in cancer cells and macrophages. The
Myc oncogene orchestrates approximately 40% of the transcriptional fingerprint of human
M2 activation and is overexpressed in human TAM 45. On the cancer cell side, the
Myc
oncogene was found to induce expression of CD47 and PD-L1 46. CD47 is a “don’t eat me”
signal (see below). Thus, the oncogene Myc appears to tame innate immunity in the form of
macrophage-mediated phagocytosis through CD47 as well as activation of effective adaptive
antitumor immunity through induction of PD-L1.
3 Prognostic or Predictive Biomarkers
Studies on the prognostic significance of TAM have relied on a variety of methodological
approaches, ranging from morphological identification in early efforts 11 to gene expression
profiling 47. The most extensively used human macrophage marker is CD68, a pan
macrophage marker. However, CD68 can occasionally be expressed in stromal cells as well
as in cancer cells themselves. Therefore its use requires careful assessment 48. In many
studies, CD163, a scavenger receptor associated with M2-like polarization, CD204 and
CD206 (the mannose receptor, induced by IL-4) were used, with overall results comparable
to usage of CD68. In addition a range of molecules have been used to characterize TAM.
These include membrane molecules (e.g. stabilin-1 49, expressed in M2 polarized
macrophages and TAM). Chemokines and chemokine receptors (e.g. CCL17), cytokines and
cytokine receptors (e.g. IL-10 and IL-12 50). M1-like macrophages polarized with IFNγ and
with antitumor activity usually present with high levels of HLA-DR 51, although this marker
is widely expressed in other leukocyte populations. Different approaches are used for
identification of macrophage precursor monocytes and monocyte-myeloid-derived
suppressor cells (MoMDSCs) in cancer, as these circulating cells are commonly investigated
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by multicolor flow cytometry. Monocytes are referred to as “inflammatory”, when CD14
+
/
CD16
-
20, or “patrolling” when CD14
dim
/CD16
+
20. As to MDSCs, a recent consensus has
been reached on MoMSCs as CD11b
+
/CD14
+
/HLA-DR
low
//CD15
-
/CD33
+
, with CD33
being highly expressed only on MoMDSC (compared to NeuMDSC) 52. Macrophages
infiltrating mouse and human tumors show considerable diversity within a given cancer
depending on their microanatomical localization 41, 49. Hypoxia is a major driver of
macrophage diversity within tumors13, 53. Therefore, an inherent limitation of available
information is that it does not take into account intratumor diversity of TAM.
It has long been known that in many human solid tumors high macrophage infiltration is
associated with poor prognosis 54. These observations represented a pillar for the now
largely accepted view that TAM promote tumor progression as discussed above. In breast
carcinoma macrophage infiltration was associated with grade, lack of hormone receptors,
basal like type and outcome 26. TAM were correlated with more advanced stage in breast
and bladder cancer 55, 56 but the reverse was true in ovarian and gastric cancer 57, 58. The
negative prognostic significance of TAM infiltration has been confirmed in a recent meta-
analysis including all available data 59.
In apparent contrast with the above results in selected human tumors (non-small cell lung
cancer, prostate and colorectal cancer, CRC) high macrophage infiltration has been
associated with better prognosis. This observation in CRC has stood up in the meta-analysis
conducted by Zhang et al 59 and were confirmed in an analysis of 209 CRC patients in our
Institution (
Malesci et al., unpublished data
). Interestingly, in CRC also neutrophil
infiltration was found to be associated with better survival 60. As discussed below, the
favorable prognostic significance of macrophage infiltration is related to the impact of TAM
on response to chemotherapy.
As discussed above, it has long been held that TAM originate from blood monocytes.
However, in selected murine tumors 25 TAM proliferation was observed though this was not
clearly shown in human tumors. In a recent study, proliferating macrophages were identified
as PCNA+ cells in human breast carcinoma. Proliferating TAM were associated with
hormone receptor negativity, basal-like phenotype and worse outcome 26. It will be
important to assess the presence of PCNA+ TAM in other tumors and assess their
significance.
In classic Hodgkin’s lymphoma (CHL), a gene signature of TAM and an increased number
of CD68+ cells were associated with shortened survival in patients treated with
chemotherapy regimens, so that TAMs have been proposed as a biomarker for risk
stratification 61 (Table 1). High CD68 or CD163 expression were later confirmed to be
significant independent predictors of worse survival in a multicenter randomized controlled
clinical trial, reinforcing the prognostic significance of TAMs in chemotherapy-treated
patients with locally extensive and advanced-stage CHL 62 (Table 1).
Previously, high TAM (CD68+) content had been found associated with unfavorable
outcome also in patients with follicular lymphoma (FL) treated with multiagent
chemotherapy 63, 64, a prognostic association reversed in CHOP (cyclophosphamide,
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