# Latest Advances in Targeting the Tumor Microenvironment for Tumor Suppression

Bettina Couderc, Marcin Domagala, Augustin Le Naour, Chloé Laplagne, Christophe Quemerais, Dimitri Hamel, Jean-Jacques Fournié, Corinne Bousquet, Audrey Ferrand, Mary Poupot

► To cite this version:

Bettina Couderc, Marcin Domagala, Augustin Le Naour, Chloé Laplagne, Christophe Quemerais, et al. Latest Advances in Targeting the Tumor Microenvironment for Tumor Suppression. International Journal of Molecular Sciences, 2019, 20 (19), pp.4719. 10.3390/ijms20194719. hal-02347360

HAL Id: hal-02347360

https://hal.science/hal-02347360

Submitted on 26 May 2020

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

Review

Latest Advances in Targeting the Tumor
Microenvironment for Tumor Suppression

Chloé Laplagne <fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Marcin Domagala <fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Augustin Le Naour <fnref n="1" />,<fnref n="2" />,<fnref n="4" />, Christophe Quemerais <fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Dimitri Hamel <fnref n="2" />,<fnref n="5" />, Jean-Jacques Fournié <fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Bettina Couderc <fnref n="1" />,<fnref n="2" />,<fnref n="4" /> ID, Corinne Bousquet <fnref n="1" />,<fnref n="2" />,<fnref n="3" /> ID,
Audrey Ferrand <fnref n="2" />,<fnref n="5" /> and Mary Poupot <fnref n="1" />,<fnref n="2" />,<fnref n="3" />,*

Received: 29 July 2019; Accepted: 19 September 2019; Published: 23 September 2019

<figure><img src="image_3.png" /></figure>

check for
updates

**Abstract:** The tumor bulk is composed of a highly heterogeneous population of cancer cells, as well as a large variety of resident and infiltrating host cells, extracellular matrix proteins, and secreted proteins, collectively known as the tumor microenvironment (TME). The TME is essential for driving tumor development by promoting cancer cell survival, migration, metastasis, chemoresistance, and the ability to evade the immune system responses. Therapeutically targeting tumor-associated macrophages (TAMs), cancer-associated fibroblasts (CAFs), regulatory T-cells (T-regs), and mesenchymal stromal/stem cells (MSCs) is likely to have an impact in cancer treatment. In this review, we focus on describing the normal physiological functions of each of these cell types and their behavior in the cancer setting. Relying on the specific surface markers and secreted molecules in this context, we review the potential targeting of these cells inducing their depletion, reprogramming, or differentiation, or inhibiting their pro-tumor functions or recruitment. Different approaches were developed for this targeting, namely, immunotherapies, vaccines, small interfering RNA, or small molecules.

**Keywords:** tumor microenvironment; resistance; TAMs; CAFs; T-regs; MSCs

**1. Introduction**

Resistance to cancer treatment is not only related to intrinsic properties of tumor cells but also to other cellular or acellular parameters of the tumor microenvironment (TME) [1]. In general, the TME is a highly heterogeneous and dynamic network, composed of normal and cancerous tissue-resident cells, a large proportion of recruited immune cells, and extracellular matrix components. After the initial state of the tumor, the TME (in particular, the immune TME) is modified to support and promote the tumor development while suppressing immune cell-mediated cytotoxicity [2]. The cellular composition of the TME is highly variable and could include almost all immune cell types such as CD8, CD4, and γδ T-lymphocytes, macrophages, natural killer (NK) cells, dendritic cells (DC), B-cells, and mast cells, as well as fibroblasts, neuroendocrine cells, adipose cells, endothelial cells, and mesenchymal cells [3]. All these cells, including tumor cells, take part in the tumor progression by promoting tumor growth, tumor dormancy, tumor invasion, and metastasis [4,5]. Tumor cells are able to release pro-tumor

<footnote n="1">¹ Centre de Recherches en Cancérologie de Toulouse, Inserm UMR1037, 31037 Toulouse, France; chloe.laplagne@inserm.fr (C.L.); marcin.domagala@inserm.fr (M.D.); augustin.lenaour@inserm.fr (A.L.N.); christophe.quemerais@inserm.fr (C.Q.); jean-jacques.fournie@inserm.fr (J.-J.F.); bettina.couderc@inserm.fr (B.C.); corinne.bousquet@inserm.fr (C.B.)</footnote>

<footnote n="2">² Université Toulouse III Paul-Sabatier, 31400 Toulouse, France; dimitri.hamel@inserm.fr (D.H.); audrey.ferrand@inserm.fr (A.F.)</footnote>

<footnote n="3">³ ERL 5294 CNRS, 31037 Toulouse, France</footnote>

<footnote n="4">⁴ Institut Claudius Regaud, IUCT-Oncopole, 31000 Toulouse, France</footnote>

<footnote n="5">⁵ Institut de Recherche en Santé Digestive, Inserm U1220, INRA, ENVT, 31024 Toulouse, France</footnote>

<footnote>* Correspondence: mary.poupot@inserm.fr</footnote>
factors directly or indirectly by inducing hypoxia or necrosis, and by modifying other actors of the TME
in its favor. Tumor-infiltrating immune cells (T-cells and macrophages), fibroblasts, and mesenchymal
cells drive tumor progression. Depending on the tumor type, infiltrated immune cells can be found in
variable proportions and display heterogeneous phenotypes with either pro- or anti-inflammatory
properties [6]. In the latter case, despite their dual role in cancer, regulatory T-cells (T-regs) are a subset
of immunosuppressive infiltrated T-cells mostly involved in the immune escape [7]. Tumor-associated
macrophages (TAMs) or cancer-associated fibroblasts (CAFs) can also produce a favorable environment
for cancer cells, as well as mesenchymal stromal/stem cells (MSCs), as shown in Figure 1.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** Representation of the tumor microenvironment (TME) with tumor-associated macrophages (TAMs), mesenchymal stromal/stem cells (MSCs), regulatory T-cells (T-regs), and cancer-associated fibroblasts (CAFs) infiltrating the tumor.</figcaption></figure>

These cells are considered as targets in cancer in order to improve classical therapies or to find new
strategies to reverse treatment resistance. In this review, we focus on the latest advances in targeting of
TAMs, T-regs, MSCs, and CAFs which could provide new therapeutic approaches in cancer.

## 2. TAMs

### 2.1. Overview on Normal Macrophages and Their Physiological Functions

In normal tissue, resident macrophages have site-specific phenotypes, for example, microglia in
the brain, Kupffer cells in the liver, alveolar macrophages in the lung, and peritoneal macrophages
in the gut. These tissue-resident macrophages likely arise from embryonic precursors and are
maintained by self-renewal [8]. However, all these macrophages display a common and principal
role in maintaining steady-state homeostasis [9]. On the other hand, monocytes from the blood are
recruited and transformed into mature macrophages in specific tissues, in response to cytokines and
chemokines released during inflammation. Depending on the microenvironment, macrophages are
differentiated into either classical activated (M1/Th1-activated) macrophages or into alternatively
activated (M2/Th2-activated) macrophages [10].

M1 macrophages display pro-inflammatory properties involved in the resolution of infections and exert anti-tumor activities. Indeed, macrophages recognize pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharides through Toll-like receptors (TLRs) at their surface, activating transcription factors (e.g., interferon regulatory factors and nuclear factor kappa B) and initiating
the inflammatory response [11]. These M1-oriented macrophages, which could also be induced by cytokines secreted by other Th1 cells (e.g., IFN-γ), release pro-inflammatory cytokines (e.g., IL-1β, IL-6, IL-12, IL-23, and TNF-α) and type-1 cell-attracting chemokines (e.g., CXCL9 and CXCL10), favoring the recruitment of more macrophages and leucocytes to eliminate pathogens, characterized by the production of cytotoxic nitric oxide (NO) derived from the metabolism of arginine by inducible nitric oxide synthase (iNOS) [12]. There are no characteristic receptors at the surface of M1 macrophages, but they express CD16, CD86, CD80, IL-1RI, major histocompatibility complex class II (MHC-II), TLR2, and TLR4.

On the contrary, M2-oriented macrophages are involved in the wound-healing process and allergies, and they display pro-tumor activities [10,13]. M2 polarization is induced by the release of IL-4, IL-13, or IL-10 by damaged cells, resulting in activation of the STAT6 transcription factor through the JAK-STAT pathway [14]. M2 macrophages are characterized by many specific receptors, e.g., IL-1R II, the scavenging receptor I (CD204), the mannose receptor (CD206), and the hemoglobin scavenger receptor (CD163) [15]. These cells produce large amounts of anti-inflammatory cytokines such as IL-10 and chemokines such as CCL17, CCL22 and CCL24 [12,16]. However, all these macrophages are never in a completely blocked phenotype due to their plasticity and ability to switch between phenotypes depending on microenvironment cues. Wound sites can also be sensitive to pathogen attacks, and both macrophage types exist within this type of environment, essential for wound resolution [17].

## 2.2. Tumor-Associated Macrophages

Within tumors, macrophages can be in between or off the spectrum of the M1/M2 states depending on the tumor stage [18]. These TAMs can be derived from resident macrophages [19] or attracted from bone marrow and spleen to the tumor site thanks to the CCL2 (MCP-1) and CCL5 (RANTES) chemokines produced by the tumors, fibroblasts, endothelial cells, and even by the TAMs themselves [20]. Colony-stimulating factor-1 (CSF-1) is also a monocyte/macrophage attractant, as well as a macrophage survival and polarization signal, which is highly important in tumors [21]. Early in tumor development, M1 macrophages are able to kill and remove tumor cells, and to produce cytokines for the recruitment and activation of other immune effector cells [22]. Nevertheless, the evolution of the TME, depending on the stage of the tumor and on the type of cancer, can revert this anti-tumor program and favor a switch of infiltrated macrophages into an M2 phenotype with pro-tumor and immune-suppressive functions [23]. Pro-tumor functions of TAMs are the result of a macrophage polarization with a particular cytokine profile (macrophage colony-stimulating factor (M-CSF), IL-4, IL-13, IL-10, Prostaglandin E2 (PGE2)) of the TME [24]. However, chemokines and signals in the TME originating from tumor cells, B-cells, and stromal cells (e.g., CSFs, TGF-β, IL-1) can deviate macrophage functions and phenotype, astray from the classical M1/M2 polarized cells. However, in general, TAMs stimulate tumor cell proliferation, migration, and genetic instability, and can protect them from apoptosis or therapy. TAMs are, thus, able to promote tumor development at primary sites, invasion, metastasis, angiogenesis, and lymphangiogenesis [25]. These cells express at their surface the scavenger receptor, CD163, and the mannose receptor, CD206, and also have immunosuppressive functions neutralizing recruitment and functions of cytotoxic CD8 T-cells and natural killer cells through the secretion of IL-10 and TGF-β in the TME [26,27]. TAMs also represent a strong source of iron, which is essential to tumor development [28]. Interestingly, several meta-analyses of more than 55 studies of different cancers showed that high infiltration of TAMs correlated with poor overall survival in the majority of cancers, indicating that infiltrating TAMs preferentially display a pro-tumor phenotype [29]. Through their pro-tumor functions, TAMs are also able to promote the recovery of tumors in mouse models following chemotherapies, radiotherapies, or immunotherapies [30].

For all these reasons, TAMs are good candidates to target in TME, and several therapy strategies modulating TAM functions, infiltration, and number are emerging. Figure 2 summarizes the different drugs targeting TAMs addressed in this review.
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Targeting strategies to reprogram, eliminate, and inhibit TAM recruitment. Antibodies or molecules available to target surface, intracellular or soluble molecules involved in the phenotype, functions, and recruitment of TAMs in the TME (as outlined in the text).</figcaption></figure>

## 2.3. TAM Targeting

### 2.3.1. Differentiation and Depletion

TAMs can be depleted by blocking CSF-1 receptor (CSF-1R), a tyrosine kinase receptor whose binding with CSF-1 or IL-34 leads to a signaling cascade promoting proliferation, function, and survival of macrophages [31]. Several antagonists and antibodies to CSF-1R were developed for the therapeutic modulation of infiltrating TAMs and tested in preclinical models. In a preclinical breast cancer model, anti-mouse CSF-1R antibodies were shown to deplete macrophages and to reduce tumor size [32]. Treatment with RG71551, a humanized monoclonal antibody, was associated with a reduction in tumor-infiltrating TAMs and an increase in infiltration of effector T-cells in a fibrosarcoma mouse model, as well as in colorectal cancer and cancer patients with advanced tenosynovial giant cell tumors [31].

Alongside antibodies blocking CSF-1R, small-molecule antagonists to CSF-1R, capable of penetrating the blood–brain barrier, were been developed. For example, PLX3397 (pexidartinib) was associated with a reduction in the number of tumor-associated microglia and glioblastoma
invasions in glioma mouse models [33]. In an ovarian cancer model, GW2580 induced a reduction of ascite volume and a decrease in the number of infiltrating macrophages [34]. In a phase II clinical trial, patients with recurrent glioblastoma displayed a good tolerance to GW2580, but only 8% of 37 patients showed a six-month progression-free survival [35].

To enhance the potential of these inhibitors, combination strategies with conventional chemotherapy were proposed. The efficacy of paclitaxel in a mammary adenocarcinoma mouse model was enhanced by the blockade of CSF-1R/CSF-1, with paclitaxel upregulating the production of CSF-1 [36]. However, a recent phase I study using a combination of a monoclonal anti-CSF-1R with paclitaxel in patients with advanced and metastatic solid tumors revealed TAM depletion but no clinically relevant anti-tumor activity [37]. In the case of gemcitabine, its activity was enhanced by its combination with GW2580 in the treatment of mice with pancreatic adenocarcinoma, through the upregulation of cytidine deaminase causing resistance to gemcitabine [38]. More recently, combination of anti-CSF-1R with docetaxel (microtubule-stabilizing agent) revealed TAM depletion in a murine epithelial ovarian cancer model, in correlation with the anti-tumor effect of docetaxel [39]. Moreover, PLX3397 was able to potentiate the therapeutic effects of radiotherapy in a preclinical mouse xenograft model with intracranial human glioblastoma through the neutralization of CSF-1, which was upregulated by radiotherapy [40].

Finally, it was shown that CSF-1R targeting by the BLZ945 inhibitor could lead to the switch of the TAM phenotype toward an anti-tumor phenotype in a glioma preclinical model thanks to GM-CSF and IFN-γ produced by glioma cells concomitantly to TAM depletion [21].

Another way to deplete TAMs is to use bisphosphonates (BP), which induce apoptosis of myeloid cells by inhibiting the essential farnesyl diphosphate synthase, a key enzyme for cholesterol synthesis and protein prenylation. These compounds have a high affinity for bone hydroxyapatite, where they are internalized by bone osteoclasts leading to their apoptosis, and they are, thus, indicated as treatment in pathologies with abnormal bone resorption such as osteoporosis or multiple myeloma [41]. However, all tissue-resident macrophages, including TAMs, can be targeted by BPs such as clodronate in a liposomal formulation with an increase in survival in some preclinical cancer models correlated with macrophage depletion [42]. In a murine xenograft model for cutaneous T-cell lymphoma, the reduction of tumor growth was associated with a decrease in pSTAT3 and macrophage number [43]; the same association was observed in other preclinical mouse models of bone metastasis from lung cancer [44], lung metastasis from breast cancer [45], or melanoma xenograft models [46]. At the clinical level, clodronate was shown to reduce the incidence of bone and visceral metastasis in human mammary carcinomas treated with a combination of clodronate/chemo- or hormonal therapy [47], and, in patients with prostate cancer, another BP reduced skeletal-related events and improved progression-free survival time [48].

Finally, trabectedin, a tetrahydroisoquinoline alkaloid with anti-neoplastic effects, was used for the treatment of advanced-stage tissue sarcoma and relapsed ovarian cancer and was shown to cause a partial depletion of circulating monocytes and TAMs [49]. Trabectedin activates a TRAIL-dependent pathway of apoptosis, and myeloid cells expressing very low levels of TRAIL are insensitive to TRAIL triggering of apoptosis [50].

### 2.3.2. Reprogramming

Even if TAMs generally display a pro-tumor role, they can be converted into anti-tumor cells and activate the immune system depending on the context. Regarding this plasticity, TAMs can be reprogrammed toward a tumoricidal phenotype to restore their anti-tumor properties thanks to the targeting of surface markers or signaling molecules, or to modifications in the metabolism.

Several markers at the surface of TAMs can be targeted to switch their phenotype such as the scavenger receptor MARCO, TLRs, CD40, or CCR5 [51–54]. The pattern recognition scavenger receptor MARCO was linked to clinical outcome [51]. An anti-MARCO monoclonal antibody was, thus, developed and was shown as having an anti-tumor activity in breast and colon carcinoma, and in
melanoma models through reprogramming of TAMs populations to a pro-inflammatory phenotype and increasing tumor immunogenicity. This was shown as being dependent on the inhibitory Fc-receptor, FcγRIIB [51]. The pro-inflammatory phenotype can also be activated by specific ligands of TLRs or of CD40. Some TLR synthetic ligands were tested in cancer, such as the TLR3 agonist Poly(I:C) which activates the NF-κB pathway, leading to pro-inflammatory M1 polarization with production of type I IFN [55]. Intratumoral delivery of a TLR9 agonist, such as cytosine-phosphate-guanine oligodeoxynucleotides (CpG ODN), or a TLR7/8 agonist (R848) showed a tumoricidal activity in mouse models of melanoma and breast cancer [52,56]. Clinical trials with IMQ and a DNA-based TLR9 immunomodulator in metastatic cancer also showed histological tumor regression and an increase in lymphoid immune infiltration [57]. TLRs can also be activated by sensing bacterial ligands such as the attenuated ΔactA/ΔinlB strain of *Listeria monocytogenes*, which, when introduced into the aggressive ID8-Defb29/Vegf-A murine ovarian carcinoma, is preferentially phagocytosed by TAMs and reprograms the population from one of suppression to immunostimulation [58].

CD40 is also an important receptor at the TAM surface, as its ligation with the CD40 ligand at the T-cell surface stimulates T-cell-based anti-tumor responses. In preclinical studies, a rat anti-mouse CD40 antibody showed remarkable therapeutic activity in the treatment of CD40⁺ B-cell lymphomas [59]. The first class of modified anti-CD40 antibody, presenting five point mutations in the Fc domain (CP-870,893), showed efficient immunostimulation of effector T-cells in mouse models but little clinical effect in advanced cancer patients [60]. Recently, the same CD40 agonist was shown to increase pancreatic carcinoma sensitivity to chemotherapy in a PDAC mouse model associated with a switch of infiltrated macrophages due to an increase in CCL2 expression and IFNγ in these mice [54]. This last study opened promising perspectives for combination therapies. CCR5 antagonists such as maraviroc provided an anti-tumor effect in a phase I trial in patients with liver metastasis of advanced refractory colorectal cancer correlated with a macrophage repolarization [53].

Furthermore, a majority of cancer cells express a high amount of CD47 at their surface, which interacts with signal regulatory protein alpha SIRPα on myeloid cells such as macrophages to transmit the “do not eat me” signal. This inhibition of phagocytosis of autologous cells exists in homeostatic conditions by the prevention of myosin IIA accumulation at the phagocytic synapse. Anti-CD47 antibodies or engineered SIRPα-Fc fusion were shown to restore macrophage ability to phagocytose cancer cells and prime cytotoxic CD8 T-cells, leading to the tumor regression in several tumor models and in clinical trials. Two anti-CD47 antibodies (Hu5F9-G4 and CC-90002) and one soluble recombinant SIRPα-crystallizable fragment (Fc) fusion protein (TTI-621) are currently being tested in phase I clinical trials in AML and other hematological malignancies, pediatric brain tumors, and some multiple solid tumors (trial numbers NCT02678338 and NCT03957096). To minimize the off-target toxicity (transient anemia), bispecific antibodies targeting both CD47 and tumor-associated antigens were recently developed and obtained promising results [61].

Specific pathways involving STAT3 or STAT6 characteristic of an anti-inflammatory response can be targeted to modify M2 characteristics of TAMs. For instance, through its ligation to the IL-10 receptor, IL-10 and IL-6 induce the activation of STAT3 necessary to drive an anti-inflammatory effect. Formerly, microtubule-stabilizing agents such as paclitaxel were shown to promote polarization of MDSC to macrophages with the M1 phenotype via suppression of STAT3 phosphorylation [62]. The STAT3 axis was then more specifically targeted with an intravenous injection of a STAT3-small interfering RNA (siRNA)-CpG conjugate which was shown to silence the immune suppressor gene *STAT3*, reprogramming the functions of TAMs correlated with an immune-mediated tumor rejection [63]. A recent in vitro study showed that corosolic acid, packaged within long-circulating liposomes and coupled to an anti-CD163 antibody, inhibited the activation of STAT3 in human M2 type macrophages and their IL-10-induced gene expression [16]. In the same way, STAT6, one of the major signal transducers activated by IL-13, involved in M2 polarization, can be inhibited by different synthetic molecules (AS1517499, TMC-264, A771726), leading to an inhibition of tumor growth in the 4T1 mammary tumor model and modification of genetic markers for TAM infiltration [64].
Furthermore, microRNA (miRNA)-155 is best characterized as a pro-inflammatory miRNA since it enhances M1-like macrophage activation by decreasing inhibitors of pro-inflammatory responses and targets the IL-13 receptor to reduce alternative macrophage activation [65]. Currently, several miRNAs are implicated in the regulation of macrophage activation and polarization in cancer [66], and developments of pharmacological formulations that either suppress or enhance the activity of selected miRNAs to reprogram the TAM phenotype are emerging. For instance, targeting of miRNA-processing enzyme DICER in macrophages with lysozyme-M-driven Cre prompts M1-like TAM reprogramming in lung carcinoma and MC38 tumors [67].

The switch of M2 TAMs toward an M1-like phenotype can also be driven by rapamycin, which is an mTORC1-specific inhibitor, inducing an anti-tumor effect in a hepatocarcinoma mouse model [68]. Moreover, inhibitors of PI3kγ, a molecule upstream of mTOR, are able to promote TAM-immunostimulatory responses in several cancer models [69]. Activation of MAPKs via production of ROS, thanks to a copper chelator, CuNG (N-(2-hydroxy acetophenone) glycinate), can convert suppressive TAMs toward a pro-inflammatory phenotype [70]. M1 genes such as *iNOS* and *CXCL9* can also be upregulated concomitantly with the downregulated expression of M2 genes, *arginase-1* and *CD206*, thanks to the inhibition of the CYP4X1 monooxygenase or the activation of the autophagy-induced ReIB/p52 by flavonoids in glioma or hepatocellular carcinoma models [71,72]. Several synthetic molecules can also actively reverse the M2 phenotype of TAMs such as TMP195, an HDAC inhibitor, in breast cancer mouse models, or paclitaxel, an anti-microtubule agent, in patients with ovarian carcinoma [73,74].

Combination strategies were also reported to have a good effect on TAMs reprogramming. Anti-CD40 was combined with anti-CSF-1R to create a pro-inflammatory environment eliciting T-cell responses before depletion of TAMs [75] or with chemotherapies such as imatinib (tyrosine kinase inhibitor) with strong tumor regression in a gastrointestinal tumor mouse model [76]. A TLR9 agonist (IMO-2055) combined with chemotherapies and anti-VEGF showed anti-tumor activity in advanced or metastatic lung cancer [77]. Moreover, TLR9 agonists were also able to enhance radiofrequency-induced CTL responses, potentiating the inhibition of primary tumor growth and lung metastasis [78].

Regulating macrophage mitochondrial function is another way to activate the reprogramming of TAMs. Indeed, downregulation of the gene encoding pyruvate dehydrogenase is avoided by dampening the nuclear respiratory factor 1 degradation under hypoxia, which minimizes the Warburg effect and promotes M1 polarization of TAMs [79]. Moreover, cancer cells secrete lots of lactate through glycolysis, which is recognized by Gpr132 at the surface of TAMs and promotes M2 polarization. PPARγ agonists, which are suppressive for the Gpr132 axis, or siRNA silencing of Gpr132, was successfully used to desensitize TAMs to lactate stimulation in a breast tumor model [80,81].

Finally, recent studies identified novel drug targets for further investigation in the future. For instance, loss of *Gadd45b*, a gene involved in the regulation of growth and apoptosis in myeloid cells, restored activation of pro-inflammatory functions of TAMs and intra-tumor immune infiltration in a fibrosarcoma mouse model [82]. Interfering with angiopoietin family molecules might be an effective strategy for reprogramming TAM polarization in NSCLC [83]. CaMKK2 was defined as a new myeloid-selective checkpoint and could be blocked to facilitate a favorable reprogramming of immune cells in the TME [84]. Several studies also proposed to reprogram the energetic metabolism in TAMs, in which fatty-acid oxidation is preponderant, decreasing lipid and cholesterol intake and metabolism [85,86].

### 2.3.3. Inhibition of Recruitment

Modulation of the recruitment of monocytes and macrophages to the tumor site in order to disrupt TAM populations represents a promising complementary therapeutic strategy. Inhibition of CCR2/CCL2- and CSF-1R-dependent signaling pathways is a major focus in this field, as direct targeting of CCR2/CCL2 showed positive outcomes in various experimental cancer models [87–91]. However, clinical translation proved to be problematic. Clinical trials with agents depleting CCL2
(carlumab or CNT0888) or blocking CCR2 (MLN1202) showed poor therapeutic responses when used as monotherapies [92,93]. In fact, administration of an anti-CCL2 antibody initially showed decreased levels of CCL2 but led to elevated levels of this chemokine in the long term. Moreover, additional precaution must be taken, as discontinuation of CCL2 therapy in murine models of breast cancer showed increased monocyte mobilization and tumor progression [92]. Phase I and II clinical trials with carlumab were carried out in patients with different type of tumors and indicated that CCL2 levels were only partially suppressed associated with no therapeutic efficacy [94,95]. The blockade of CCR2 by PF-04136309 was assessed in a phase Ib trial in patients with pancreatic cancer in combination with folfirinox chemotherapy. Compared to no response with the monotherapy, 97% of patients with the combination showed an objective tumor response and others showed a local tumor response [96]. These results showed high complexity and mechanistic redundancy within the CCR2/CCL2 axis, which still needs to be addressed. An alternative approach could be the indirect inhibition of the CCR2/CCL2 pathway. It was observed that, in a mouse model of glioblastoma, TAMs express aryl hydrocarbon receptor (AHR), which, upon activation by tumor released kynurenine (KYN), upregulates CCR2 expression. Blocking the AHR led to decreased TAM infiltration and tumor growth with concomitant increased survival of the animals [26].

Targeting of CSF-1R also showed a decrease in macrophage/monocyte recruitment [97]. In spite of this, numerous clinical trials showed a limited therapeutic effect when CSF-1R was targeted alone [98], with the exception of benign tenosynovial giant cell tumors, where a decrease in TAM infiltration and disease stabilization was achieved [99]. Efficacy of CSF-1R inhibitors is still being tested in anticancer therapies, also in the form of broad-spectrum kinase inhibitors such as chiauranib (CSF-1R, VEGF1-3, PDGFRα, c-Kit, Aurora B inhibitor) [100] and pexidartinib (CSF-1R, KIT, FTL3 inhibitor), with potential anticancer results [101].

Research on a mouse model of lung adenocarcinoma showed that FGF released by CAF induced TAM recruitment. Blocking of FGF receptor (FGF-R) with the Fgfr tyrosine kinase family inhibitor AZD4547 showed strong TAM abolishment and tumor regression, making this receptor a potential therapeutic target [102]. In addition, targeting other important molecules for monocyte/macrophage recruitment in cancer, i.e., catecholamines by 6-hydroxydopamine [103] and complement components such as C5a, may also be promising in cancer treatment [104].

## 3. T-Regs

### 3.1. Overview on T-Regs and Their Physiological Functions

T-regs are a lymphoid cell population with a key function in modulating an immune-suppressed TME. T-regs are a subset of CD4⁺ T-cells identified by expression of transcription factor forkhead box P3 (FOXP3) and can be defined as CD4⁺ CD25⁺ FOXP3⁺ T-cells [105]. Compared to naïve T-regs, activated T-regs express higher levels of CC chemokine receptor 4 (CCR4) [106], CD28 [107], glucocorticoid-induced TNFR family related gene (GITR) [108], cytotoxic T-lymphocyte antigen-4 (CTLA-4) [109], lymphocyte-activation gene-3 (LAG-3, also known as CD223) [110], neuropilin 1 (Nrp-1) [111], programmed cell death-1 (PD-1) [112], T-cell immunoglobulin and mucin-domain containing-3 (TIM-3) [113], T-cell immunoreceptor with Ig and ITIM domains (TIGIT) [114], and V-domain Ig suppressor of T-cell activation (VISTA) [115]. T-regs are essential to maintain immunity equilibrium (i.e., self-tolerance) and resolve inflammation [116]; depletion of T-regs leads to autoimmune diseases in murine models [117]. T-regs regulate immune responses through four major mechanisms [118]: (i) secretion of inhibitory cytokines such as IL-10, IL-35, and TGF-β; (ii) direct cytolysis of target cells by the secretion granzyme-A, granzyme-B, and perforin [119]; (iii) IL-2 deprivation due to a high-affinity for CD25 (IL-2 receptor) [120]; and (iv) modulation of maturation and/or function of DC which permits activation of effector T-cells. Moreover, it was shown that LAG-3, a high-affinity binder of the MHC class II molecule required for maximal T-reg suppression,
could abort DC maturation through induction of an immune receptor tyrosine-based activation motif (ITAM)-mediated inhibitory signaling pathway [110].

## 3.2. Tumor-Infiltrating T-Regs

T-regs play a pernicious role in a tumor context by suppressing anti-tumor responses which limit the efficacy of cancer immunotherapy [111,121]. T-regs could infiltrate many solid human tumors since there are several studies which report an increased proportion of T-regs in the TME of melanoma patients [121], as well as lymphoma [122], breast and pancreatic cancers [123], and ovarian and lung [124] cancers. An increase in the proportion of tumor-infiltrating T-regs (TI T-regs) is often linked to a poor survival rate in many cancers. Despite the development of immunotherapies showing spectacular clinical responses, some patients do not respond to these therapies. As T-regs are mainly present in tumor tissues, they constitute an attractive target in T-cell infiltrating tumors. Therefore, depleting effector T-regs would modify the equilibrium in TME from immune suppression to active anti-tumor activity, while preserving a continual supply of effector T-regs from intact naïve T-regs.

The latest therapeutic advances targeting T-regs by using antibodies, vaccines, or small molecules specific for T-reg features are summarized in Figure 3.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Targeting strategies to eliminate or modulate T-reg functions. Available antibodies, small molecules, or vaccines specific for different cell surface or intracellular targets (as outlined in the text).</figcaption></figure>

## 3.3. T-Reg Targeting

### 3.3.1. Antibodies Depleting Tumor-Infiltrating T-Regs

Several antibodies were found to deplete T-reg cells including basiliximab, a chimeric anti-CD25 monoclonal antibody, which was first tested in kidney transplant patients and showed a transitional loss of FOXP3⁺ and FOXP3⁻ CD25⁺ T-cells in the circulation [125]. Daclizumab, an FDA-approved monoclonal antibody against the CD25 receptor, showed a durable decrease of circulating T-regs
without impairing vaccine-induced T-cell responses, reducing CD25 and FOXP3 expression of T-regs
and increasing their secretion of IFN-γ [126].

The anti-CCR4 antibody displayed an effective role in specifically depleting effector T-regs and increasing the induction of tumor antigen-specific CD4 and CD8 T-cells in vivo [106]. Indeed, antibodies against CCR4 decrease the proportion of TI T-regs in murine tumors [127], and a humanized anti-CCR4 antibody, KW-0761, is being tested in early clinical trials in patients [128]. Recent studies showed that targeting the CCL22-CCR4 axis could also deplete T-regs and block their interaction with DC, allowing a stronger immune response as DCs regulate adaptive immunity through the constitutive expression of CCL22 [129]. Interestingly, treating T-cells with the CCR4 ligand, CCL17, can disable the production of IFN-γ, thus suggesting chemokines as factors that can directly reprogram the functionality of T-cells [127].

An agonistic anti-GITR antibody (non-depleting) can reduce the T-reg cell-mediated suppressive function and increase the effector function of conventional T-cells in mouse models [108]. The agonistic antibody for GITR is already being tested on melanomas and other advanced solid tumor patients. Recently, this antibody was shown to lead to an important reduction of TI T-regs in mice [130] and could overcome resistance to immunotherapy in human solid cancers [131]. This study showed a decrease of peripheral and TI T-regs after anti-GITR antibody TRX518 (NCT01239134) treatment, providing an easily assessable biomarker of anti-GITR activity. Nonetheless, these results do not reveal a major clinical response despite T-reg depletion and increased T-effector:T-reg ratios in patients and in advanced tumor-bearing mice. Yet, combination with PD-1 blockade could overcome resistance of advanced tumors to anti-GITR monotherapy. Furthermore, antibodies against GITR, while capable of reprogramming T-regs, can also enhance the proliferation and cytokine production of intra-tumor CD8 T-cells [132].

CD28 is a co-stimulatory receptor that represents the strongest secondary stimulus for T-reg functionality. Indeed, blocking CD28 signaling using anti-CD28 antibodies completely inhibits their stability and function. Furthermore, using anti-CD28 in cancer impairs TI T-reg differentiation and function, reducing their capacity to suppress anti-tumor immune responses and promoting tumor control [107]. CD28 binding is opposed by CTLA-4, an inhibitory receptor that competes for binding to the same ligands, B7-1 and B7-2, on antigen-presenting cells [133].

Immune checkpoint blockade antibodies, such as anti-CTLA-4, anti-PD-1, or anti-TIGIT, were tested to deplete T-regs since they are upregulated both on CD8 T-cells and T-regs in tumors, enhancing anti-tumor T-cell functions while simultaneously disabling T-regs to potently enhance the liver cancer immune response [134]. Using an experimental model of Fc-receptor deficient mice, the anti-tumor effect of the anti-CTLA4 antibody was shown to be dependent on antibody-dependent cellular cytotoxicity of TI T-regs instead of re-activating conventional T-cells [135].

In cancer patients, strong correlations were reported between the clinical efficacy of ipilimumab and decrease in TI T-reg proportion. Some T-regs express PD-1, but possible effects of PD-1 blockade on PD-1-expressing T-regs in tumor tissues remain unknown [136]. A recent study showed that deletion or blockade of PD-1 increases proliferation and immunosuppressive activity of PD-1⁺ T-regs in humans and mice [137]. Apparently, PD-1⁺ effector T-regs could display a major role in hyper-progressive disease (HPD) development (i.e., rapid progression of the cancer instead of its regression in certain cancer patients treated with anti-PD-1 antibody), indicating that deletion of T-regs in tumor tissues could be effective in treating HPD during anti-PD-1 mAb therapy. CD28 blocking antibody can also be used to reprogram T-regs influencing effector T-cell activation, acquisition of the glycolytic metabolism essential for effector functions, and the effectiveness of PD-1 checkpoint blockade treatment. In tumor-bearing mice, B7 co-stimulation is required for successful PD-1 therapy. Moreover patients treated by PD-1 therapy exhibited CD8 T-cells expressing CD28 [138].

LAG-3 expression on T-regs was also shown to be required for maximal suppressive activity, as the blockade abrogates T-reg function in in vitro proliferation assays [110]. Additionally, transfection of LAG-3 in non-T-reg CD4 T-cells resulted in the acquisition of a regulatory phenotype, with reduced
proliferation of co-cultured responder T-cells. More recent studies showed that LAG-3 promotes T-reg differentiation, while LAG-3 blockade inhibits T-reg induction [139]. This study also showed that blockade or genetic deletion of LAG-3 maintained CD4 T-cells in a Th1 phenotype, with LAG-3 restricting IL-2 and STAT5 signaling, adjusting their capacity to be suppressed by T-regs.

However, unlike PD-1+/hi Tregs, TIM-3⁺ TI T-regs displayed higher a suppressive capacity due to a stronger expression of CTLA-4 and CD39. Increased ingress of TIM-3⁺ CD4 T-cells or TIM-3⁺ T-regs is associated with a poor prognosis in patients with various malignancies including non-small-cell lung cancer [140].

Several teams showed that TIGIT⁺ T-regs display higher immunosuppressive activity than TIGIT⁻ T-regs [141]. Nonetheless, the effect of the anti-TIGIT antibody on established tumors remains to be determined, especially if it acts on T-regs, CD8 T-cells, or both.

The VISTA monoclonal antibody increases the proportion of tumor-specific T-cells in the periphery, leading to ingress, spread, and effector function of TI T-cells within the TME. VISTA blockade changes the suppressive function of the TME by decreasing the proportion of MDSC, increasing the rate of activated dendritic cells within the TME, and reducing the appearance of tumor-specific T-regs [142]. Another study showed that VISTA monotherapy did not avoid recruitment of T-regs in the TME [143]. Indeed, VISTA blockade leads to transition of CD8 T-cells into functional effector T-cells, but is not sufficient to shrink tumor growth due to weak T-reg suppression in the TME. Nonetheless, combination of VISTA with CTLA-4 blocking antibodies efficiently inhibited T-reg recruitment and increased the ratios of both CD8 T-cells/T-regs and CD4 conventional T-cells/T-regs in the TME, which will be more effective than combined PD-1 and VISTA blockade for tumors in which T-reg-mediated immune regulation is dominant (such as head and neck squamous cell carcinoma).

Denileukin difitox (Ontak®) is a recombinant fusion protein product of the diphtheria toxin and IL-2 that selectively binds to the IL-2 receptor of cells and inhibits protein synthesis after its internalization [144]. Recently, a second-generation diphtheria-toxin-based fusion protein associated with a better tolerance (i.e., fewer vascular leak adverse events) showed interesting results both alone and in association with PD-1 blockade [145].

Depleting Nrp-1 in TME-infiltrating T-regs in a melanoma mouse model increases their ability to produce IFN-γ, which reinforces CD8 T-cell responses within the TME in a paracrine fashion and promotes the reprogramming of other TI T-regs into IFN-γ-producing cells [111].

### 3.3.2. Tumor-Infiltrating T-Reg Vaccine Approaches

As ectopic expression of FOXP3 in conventional T-cells confers an immunosuppressive phenotype, a strategy for targeting FOXP3 was developed to counteract imperfect immune responses against tumor cells. A recent study showed delayed tumor growth, decreased production of IL-10, IL-2, and TGF-β, and increased survival of mice after inoculation of the FOXP3-silenced B16F10 melanoma cell line compared to mice injected with the wild-type cell line [146]. Moreover, a tumor cell vaccine associated with FOXP3 gene silencing can improve the efficacy of therapeutic anti-tumor vaccination [147]. Furthermore, a recent study showed that vaccination of mice with Fox-Fc DNA vaccine/recombinant FOXP3-Fc fusion protein induced a CTL response against FOXP3⁺ T-regs, which decreased tumor growth and prolonged survival rates [148]. These results showed that the FOXP3 vaccine displays an immune response against tumors by targeting both T-regs and MDSC, which could be used as a potential immunotherapy approach [149].

### 3.3.3. Small Molecules for T-Reg Depletion or Functional Modulation

Repeated exposition of high-dose chemotherapy, cyclophosphamide, an alkylating molecule which interferes with DNA replication, kills proliferating cells and impacts all T-cells. Low-dose administration of cyclophosphamide over a long period was shown to selectively deplete highly proliferating T-regs in tumor tissues, and enhance anti-tumor immune responses in humans and rodents [150,151]. Low doses
of cyclophosphamide deplete TI T-regs in metastatic colorectal cancer patients [152]. Several studies combined chemotherapeutic agents such as cyclophosphamide with other drugs targeting T-regs [153].

TCR signaling molecules which are differentially controlled in T-regs in comparison with conventional T-cells can also be targeted. Indeed, ZAP-70, which is specifically repressed in T-regs upon TCR activation, can be targeted to abrogate TCR signaling by interfering with TCR proximal signaling molecules, resulting in selective death of T-regs, in particular effector T-regs [154].

Moreover, anti-tumor immune responses were increased by an inactivating mutation (D910A mutation) in phosphatidylinositol-3-kinase (PI3K) p110δ or a knockout of PI3K in T-regs in mice, without autoimmunity in the mutant mice [155]. However, PI3K activity seems to be essential for T-reg survival and function. Indeed, genetic deletion or pharmacological inhibition of the PI3K subunit p110δ selectively impairs TI T-reg function and favors anti-tumor immune responses [156].

T-regs regulate immune responses trough the secretion of inhibitory cytokines such as TGF-β, IL-10, and IL-35. Their increase in tumors is associated with a poor prognosis in various cancer types. TGF-β promotes the differentiation of induced T-regs in vitro [157]. Deletion of IL-10 in T-regs induces spontaneous colitis, highlighting the physiological importance of T-reg-derived IL-10 [158]. T-reg-derived IL-10 alters the myeloid compartment in the TME, indirectly providing regulation of T-cell-mediated anti-tumor immune responses through upregulation of T-cell stimulatory molecules such as major histocompatibility complex class II and CD80 on intra-tumor DCs [159].

Finally, the next challenge in T-reg targeting will be to use optimized antibodies specific for TI T-regs or engineered IL-2 molecules which do not bind T-regs [160]. Future generations of T-reg-based immunotherapies must consider (i) a suitable combination of targets to promote effector responses, (ii) abolishing specific TME T-reg infiltration or function, and (iii) determining the appropriate timeline of therapeutic administration leading to a better benefit/risk ratio.

# 4. MSCs

## 4.1. Overview on Normal MSCs and Their Physiological Functions

Mesenchymal progenitor cells were firstly isolated three decades ago from bone marrow (BM-MSC). Since this first characterization, it was shown that MSCs can be isolated from most tissues including fat tissue (adipocyte-derived mesenchymal stem cells), skin, heart, kidney, etc., or from perivascular space (pericyte-derived MSCs) [161,162]. They are capable of differentiating into fibroblasts, adipocytes, osteoblasts, chondroblasts, vascular and perivascular structures, etc. They could be isolated on the basis of their ability to adhere to the plastic and for the expression of CD73, CD90, and CD105 markers. They do not express CD45, CD34, CD14, CD19, and human leucocyte antigen DR (HLA-DR) [161]. MSCs possess hallmark characteristics of stem cells or at least progenitor cells with regard to their self-renewal and differentiation properties [162]. MSCs could be used as therapeutic agents for regenerative medicine as they could contribute to tissue healing, mainly through the secretion of paracrine factors such as cellular adhesion molecules (including VCAM-1 and ICAM-1), growth factors (including TFG-β, EGF, HGF), cytokines (such as IL-1α, IL-1β, IL-6, and IL-8), angiogenic factors (such as VEGF and PDGF), and immunomodulatory molecules (PGE2). In addition, MSCs play a role in immune tolerance, through their immunosuppressive effects on T- and B-lymphocytes, and their influence on macrophage and dendritic cell polarization, thus preventing immunoreaction in both physiological and pathological conditions [163,164].

## 4.2. MSCs in Cancer

Although MSCs can halt cancer progression by inducing apoptosis, suppressing signaling pathways, initiating cell-cycle arrest, and increasing infiltration of monocytes and granulocytes [165,166], the majority of publications report pro-tumor activities. MSCs are able to stimulate tumor growth, increase angiogenesis, and favor metastasis development mainly through the release of activating
factors such as cytokines and growth factors including IL-6 [167] and GDF15 in hematological malignancies [168].

In solid cancers, following recruitment to the tumor site, MSCs as CA-MSCs (cancer-associated MSCs) are distinct from the original cells. These infiltrated MSCs were described as having a role to play in ovarian cancer progression by releasing IL-6, CXCR1/2 ligands (CXCL1, CXCL2, and IL-8), and related cytokine leukemia inhibitory factor (LIF) [167,169]. MSCs are also able to promote tumor growth and metastasize through direct cell-to-cell contact or through their secreted exosomes, which are able to carry different molecules such as IL-6, TFG-β1, CCL2, and fibronectin, or miR-375, miR340 and miR-155 [162]. These exosomes also participate in tumor homing in vivo, indicating that MSCs play a role in metastasis [170]. The mechanisms via which exosomes promote cell proliferation, increase cell survival, activate the AKT and the MAPK pathways, and also inhibit p38, p53, and JNK pathways are being studied.

They also were described to enhance vascular endothelial growth factor (VEGF) expression in tumor cells by activating the extracellular signal-regulated kinase 1/2 (ERK1/2) pathway [171]. Moreover, MSCs can inhibit the immune response mediated by NK cells [172], T-lymphocytes, or macrophages [173]. They are able to inhibit T-cell proliferation through the secretion of TGF-β and HGF, which induce T-cell proliferation arrest in the G1 phase and apoptosis of activated T-cells [172]. MSCs are also able to alter the activation and the differentiation process of T-cells and promote the generation of T-regs. In addition, MSCs could suppress the proliferation, differentiation, and activation of B-cells [172]. Through their inhibition of the T/B-cell immune response, MSCs are involved in tumor progression as tumor cells will not be recognized by the cellular and humoral effectors of the immune response.

MSCs could also inhibit the activation of dendritic cells, downregulating their endocytotic and IL-12 secreting activities [172] and activating the generation of TAMs through the release of CXCR1/2 ligands [174]. Finally, MSCs are also implicated in the acquisition of chemoresistance via i) the release of soluble factors by MSCs (IL-6, IL-8, VEGF), ii) an exchange of membrane proteins between MSCs and tumor cells, iii) the release of exosomes, or iv) through the direct interaction within tumor cells [175].

### 4.3. MSC Targeting

#### 4.3.1. Inhibition of Recruitment

MSCs are recruited to the tumor site from the bone marrow in response to soluble factors produced by tumor cells, such as IL-1, SDF-1, CCL5, and more recently the aspartic acid protease cathepsin D [176–178]. They can also migrate with the tumor cells from the primary tumor to another site and promote the growth of distant metastasis [179]. In the hypothesis that the use of antibodies targeting these chemoattractive molecules could be a way to limit MSC recruitment to the tumor sites, no results on in vivo models are reported in the literature.

#### 4.3.2. Reprogramming

CA-MSCs closely associated with both solid and hematologic malignancies were genetically and phenotypically analyzed [180]. As an example in myeloma patients, CA-MSCs which express alternative cytokines (IL-6, GDF15, IL-8) have an abnormal proliferative capacity, and present distinctive gene expression profiles [181]. In other hematological malignancies, it was shown that tumor cells activate the AKT/mTOR pathway in MSC and elicit VEGF and HIF-1 production. With regard to solid cancers, Rafii et al. [175] and more recently Mishra and Banergee isolated mesothelial-like stem cells (which presented a typical fibroblast-like phenotype) and identified the expression of several membrane markers that could be targeted such as CD9, CD10, CD29, CD73, CD146, and CD166 [182]. Finally, Vishnubalaji showed that MSCs could even transform into sarcoma cells and become neoplastic [180].

Taken together, all these observations led to the affirmation that CA-MSCs acquired different potencies when they were activated by the tumor microenvironment. They express several proteins
that in theory could be targeted in order to eliminate CA-MSCs without affecting the other MSCs even if, to our knowledge, it is yet to be performed.

#### 4.3.3. Blocking Functions

Pro-tumor molecules such as IL-6, IL-8, and HGF secreted by CA-MSCs or their receptors can be blocked to circumvent the pro-tumor effects of the MSC. Pasquier et al. showed that tocilizumab (anti-IL-6R therapy) in association with chemotherapy significantly reduced ovarian cancer progression induced by MSC recruitment in an in vivo model [183].

Le Naour et al. showed that ovarian CA-MSCs overexpress CXCR1/2 ligands, which are involved in ovarian cancer progression and the acquisition of chemoresistance. They used a CXCR1/2 inhibitor to sensitize ovarian tumor cells to carboplatin and circumvent the pro-tumor effects of CA-MSCs. They showed that CXCR1/2 inhibition could be a potential therapeutic strategy to revert carboplatin resistance mediated by MSCs [184].

Another strategy could be to inhibit the effects of the MSC-released molecules. Castells et al. showed that MSCs secreted molecules able to activate the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway, inducing the phosphorylation of the X-linked inhibitor of apoptosis protein (XIAP; caspase inhibitor from inhibitor of apoptosis protein (IAP) family). This IAP activation leads to a chemoresistance acquisition of the ovarian cancer cells (inhibition of apoptosis). Targeting XIAP in ovarian cancer could be of particular interest to circumvent pro-tumor effects of MSCs. SMAC mimetics such as DEBIO1143 could inhibit cIAP proteins. This compound was evaluated in both hematological or solid malignancies for its capacities to inhibit cIAP and promote the sensitization of tumor cells to chemotherapies both in mice models [185] and in clinical trials [186]. SMAC mimetics potentiate the effect of chemotherapy such as platinum salt. They induce tumor stabilization and regression. They are currently being studied in phase III clinical trials.

#### 4.3.4. Blocking the Cell-to-Cell Contact

MSCs could act on tumor cells through a direct cell-to-cell contact. BM-MSCs may affect developing myeloma cell lines via direct cell-to-cell interactions. Breast and ovarian cancer cells at least transiently acquire new functional properties following interaction with MSCs via gap junctional intercellular communication or Notch signaling in vitro and in vivo. Several molecules and receptors are involved in the interactions between cells, such as ICAM-1 expressed on MSC and mucin-1 (MUC-1) on the cell surface of breast cancer cells [187]. Blocking Eph/Ephrin signaling affects cell attachment in vitro leading to cell death [188]. Cell-to cell contact and fusion are involved in chemoresistance acquisition mediated by MSCs, as this contact can induce membrane exchanges between the cells and the acquisition of new properties by the tumor cells (such as MDR proteins). Oncological trogocytosis and a fusion between the cells (hybrid) were described. The tetraspanin CD9 could be involved in the chemoresistance acquisition of the hybrid cells [175,189]. Targeting this molecule and inhibiting MSC/tumor cell fusion could be a way of inhibiting MSC pro-tumor activities.

#### 4.3.5. Inhibition of the MSC Immune Suppressor Capacities

MSCs are known to inhibit the anti-tumor immune response through their action on T- and B-lymphocyte activation and M2 macrophage polarization. Recent work showed that the CXCR1/2 blockade could prevent M2 macrophage polarization mediated by the CXCL1/2 ligands secreted by MSCs. The macrophages retained an M1 polarization, allowing tumor cell phagocytosis and tumor regression [184].

To conclude on MSCs, it remains difficult to establish a therapeutic strategy that specifically targets the CA-MSCs and abolishes their pro-tumor effects. However, numerous studies enabled the description of MSC biology, particularly in a tumor context, such as recruitment to the tumor sites, activations, and secreted molecules. Therefore, it is possible to consider in the short term the establishment of new therapies that would target signaling pathways activated in cancer cells and
stromal cells by CA-MSCs. We can also decide to not eliminate MSCs but to use them as therapeutic agents. MSCs present well-established tumor-homing properties. They could, therefore, serve as promising systemic delivery tools. This idea was first suggested by Studeny et al. who showed that injected genetically engineered MSC cells (MSC-IFN-β) suppress the growth of pulmonary metastases, through the local production of IFN-β in the tumor microenvironment [190]. Since 2004, many researchers developed other genetically engineered MSCs producing TRAIL, IL-12, HSV-Tk, or sodium iodide symporter (NIS) and even a hepatocellular carcinoma-specific oncolytic adenovirus [191]. They both showed anti-tumor effects following injection of the genetically engineered MSCs to tumor-bearing animals. Taken together, all these studies revealed that, despite the fact that MSCs could promote tumor growth, they can serve as vehicles to specifically target anti-tumor molecules to tumor sites. This led to the proposal of clinical trials. Two trials are ongoing for ovarian cancer patients. The first involves the injection of genetically engineered MSCs expressing IFNβ, and the other involves oncolytic virus-loaded MSCs [192]. Finally, exosomes participate in tumor cell-to-MSC communication and are implicated in cancer pathogenesis. MSC exosomes could even display anti-tumor activities. They can act alone or as valuable vehicles for drug delivery. Exosomes derived from MSC enhance radiation effects observed in the control of metastatic spread of melanoma cells [193]. MSC-derived exosomes were shown to suppress oral cancer cell proliferation, invasion, and migration through microRNA-101-3p-targeting [194]. Lastly, in vivo taxol-loaded MSC-derived exosomes not only reduce the subcutaneous primary tumor but also prevent metastasis [195].

Therefore, if we cannot specifically circumvent CA-MSC pro-tumor effects, they can be used to target tumors.

## 5. CAFs

### 5.1. Overview on Normal Fibroblasts and Their Physiological Functions

Under physiological conditions, fibroblasts are encased within the interstitial extracellular matrix, representing the main component of the connective stromal tissue. Amongst stromal cells, fibroblasts represent the major cell type and contribute to the main functions of the connective tissue. Indeed, by producing extracellular matrix (ECM) proteins and ensuring their reorganization, as well as secreting several cytokines, chemokines, growth factors, and metabolites, they shape the tissue architecture while providing mechanical and metabolic support to the other cell types, and particularly epithelia. Fibroblasts also contribute to the regulation of the tissue immune response by regulating the recruitment of immune cells and sensitizing them to bacterial lipopolysaccharide. Thus, fibroblasts play crucial roles in tissue development, homeostasis, and repair [196].

Tissue-resident fibroblasts remain quiescent and display low proliferative and secretive rates. To date, the molecular signature of this quiescent state remains largely incomplete due to the lack of specific markers. Fibroblast-specific protein 1 (FSP1), also named S100A4, was routinely used until it was shown to also be expressed by macrophages and cancer cells [197]. Interestingly, a definite interplay between fibroblasts and their tissue of residency was highlighted [198]. Indeed, depending on their organ and site of origin, fibroblasts display differential transcriptional programs under the control of specific epigenetic alterations, demonstrating the tissue-specific plasticity of these cells and explaining the ECM particularities depending on the organs [199]. The ECM is mainly composed of collagens (type I, III, and V), elastin, fibrin, fibronectins, proteoglycans, and glycosaminoglycans, creating a three-dimensional network sheltering other stromal cells (immune, vascular, and glial cells, and neurons) [200]. In fact, ECM production, degradation, and interactions are most certainly the main fibroblast functions, making them essential players in tissue homeostasis regulation and remodeling. Therefore, as described for fibroblast types, ECM composition and organization also varies depending on the organ and localization within the tissue.

However, in response to injury, leading to wound healing, or under pathological conditions such as chronic inflammation, epithelial and immune cells secrete several cytokines (IFN-γ, TNF-α,
IL-1, IL-6, and IL-8) and growth factors (TGF-β, PDGF (platelet-derived growth factor), and FGF2) activating fibroblasts, which changes their morphology from an elongated and fusiform shape to a wide cruciform shape, mainly due to the expression of αSMA (smooth muscle actin). These now-called myofibroblasts display increased migratory, secretive, and proliferative capacities but are, in turn, also more susceptible to epigenetic alterations [201]. In wounds, these activated fibroblasts actively remodel the ECM via the secretion of several ECM components and matrix-degrading proteases, including metalloproteinases (MMPs), cathepsins, urokinase–plasminogen system proteins, tissue inhibitors of MMPs (TIMPs), and aggrecanases, as well as protease inhibitors, facilitating the repair process [202]. In a physiological context, such as wound healing, fibroblast activation is reversible. Indeed, in response to other cell signals, they also express an intrinsic program aiming to control ECM remodeling and prevent any excessive scarring process, which could lead to fibrosis and dysfunctional regeneration, harmful for the tissue and the organ [203]. Epigenetic alterations, such as RASAL1 promoter hypermethylation leading to Ras-GTP activity in renal fibrosis, seems to contribute to the mechanism underlying excessive fibroblast activation [204]. Aging and fibroblast senescence are also known to participate in tissue-resident fibroblast activation [205], and may play a role in malignancy.

## 5.2. Cancer-Associated Fibroblasts

CAFs are a complex of dynamically heterogeneous population of mesodermal cells in solid tumors, and represent a dominant cell type. CAF functions are different from those of tissue-resident fibroblasts, since these fibroblastic populations, with different origins, irreversibly develop an activated phenotype following chronic tumor-derived stimulation. This conversion stage is most likely epigenetic, as reported for skin fibroblasts upon leukemia-inhibitory factor chronic autocrine stimulation [206]. Intriguingly, global hypomethylation of the CAF genome was also reported, possibly involved in the upregulation of genes encoding for pro-tumorigenic secreted proteins [207]. Hence, CAFs are recognized as the main producers within the tumor of cytokines, chemokines, metabolites, and extracellular matrix-modifying enzymes such as matrix metalloproteases, which fuel tumor progression (growth and metastasis). Nevertheless, discrepancies regarding the pro- or anti-tumoral role of CAFs emerged and could be explained by either their different origins or their dynamic phenotypic evolution during tumor progression, implying heterogeneity in markers and in functions.

CAFs present in the tumor bed are a mix of mesodermal cells, mainly originating from the activation of tissue-resident quiescent fibroblasts, which expand in the host tissue upon chronic stress induced by the developing tumor. CAFs may also originate from distant sources such as the bone marrow, thereby forming a functionally distinct stromal cell population as reported in breast cancer [208]. Trans-differentiation of local stromal cells was also described to give rise to CAF-like populations, through program switches called endothelial-to-mesenchymal transition for pericytes or endothelial cells [207], and epithelial-to-mesenchymal transition (EMT) for epithelial cells [207]. Since CAFs display normal allelotypes, at least in pancreatic cancer [209], EMT-derived CAFs may not arise from tumor cells directly but from normal epithelial cells, which receive paracrine signals that induce alterations in their epithelial programs from the adjacent tumor, (i.e., cell polarity, cell-to-cell and cell-to-ECM junctions). Alongside the generation of CAFs, this EMT program may dampen the so-called epithelial intercellular surveillance, thereby fostering tumor progression [210]. Nevertheless, the biological origin of the different hypothetic CAF populations is still under debate due to the lack of specific markers for each population. Transcriptional analyses at the single-cell level in tumors will soon lead to the identification of molecular signatures of the different stromal cell populations and specific markers, which are necessary to clearly identify their biological origins (for example, through genetic lineage tracing) and respective functions.

Numerous non-specific markers for CAFs were reported due to the different cell origins. Tissue-resident fibroblasts exhibit different organ-specific transcriptomic profiles [198], implying organ-specific markers for CAFs derived in the respective tumors. For example, αSMA⁺ CAFs are predominant in pancreatic cancer arising from resident fibroblastic stellate cell activation [211],
whereas PDGFRα⁺ CAFs reflect in melanoma the activation and expansion from dermal fibroblasts [212]. Lack of specific markers for each hypothetical CAF population implies the overlap of different markers, such as FSP1/S100A4 and αSMA markers in pancreatic or breast CAFs [207]. Based on the relative co-expression of six fibroblastic markers (αSMA, FAPα (fibroblastic-activated protein α), integrin β1/CD29, S100-A4/FSP1, PDGFRβ, and CAV1 (caveolin 1), Costa et al. identified, by fluorescent analysis cell-sorting, four distinct CAF subsets in human breast cancer that accumulate differentially in tumors and juxta-tumors, one of these being functionally endowed with immunosuppressive properties [213].

Neuzillet et al. proposed the “pCAF assigner” classification to segregate pancreatic patient tumors into prognosis groups depending on expression levels of three different markers, i.e., periostin, MYH11 (myosin-11), and podoplanin [214]. Prognostic signatures of stroma based on the identification of different marker sets (gene expression studies) were uncovered in patients to predict survival, recurrence, or resistance to neoadjuvant chemotherapies, in several cancers; however, the lack of uniformity between tumors, probably reflecting CAF heterogeneity, will require the design of specific signatures for each pathology [215].

Markers most commonly used to identify CAFs relate to the main biological functions of CAFs, as the main producers of ECM components (collagens I and II, fibronectin, tenascin-C, and periostin), remodeling enzymes (lysyl oxidases, matrix metalloproteases, and their inhibitors), secreted growth factors (e.g., TGFβ, VEGF, PDGFs, EGF, FGFs, CTGF (connective growth factor), WNTs), and cytokines (IL-6, CXCL12), and as contractile cells (cytoskeletal components such as αSMA). Intriguingly, distinct spatial tissue distributions in pancreatic cancer between different reported CAF populations expressing specific markers may result from different dialogues with tumor cells; inflammatory IL-6⁺ CAFs, distant from tumor cells, receive an IL-1 signal which triggers the JAK-STAT pathway activation and promotes a CAF inflammatory state, whereas CAFs residing in the vicinity of tumor glands mainly receive a TGFβ signal that favors a myofibroblastic SMA⁺ state by antagonizing the IL-1 signal [216,217]. This example provides a mechanism through which distinct spatial fibroblast niches are established in the tumor microenvironment, and also illuminates the importance of studying CAF functional heterogeneity by considering their spatial distribution [218]. Very recently, single-cell RNA sequencing to pancreatic tumor tissue identified distinct CAF subsets, confirming the existence of distinct myofibroblastic and inflammatory CAF subsets, and providing novel marker genes for these cells (TNC, TGFB1, THY1, TAGLN, COL12A1, and PDGFRB for myofibroblastic CAFs, or CLEC3B, COL14A1, GSN, LY6C1, and CXCL12 for inflammatory CAFs). This study also identified a new CAF subtype that expresses MHC class II-related genes and specific markers (SLPI, SAA3, CD74, H2-AB1, NKAIN4, IRF5), and induces T-cell receptor ligation in CD4⁺ T-cells in an antigen-dependent manner [219]. Since these CAFs lack the co-stimulatory molecules required to induce T-cell proliferation, they are hypothesized to inhibit optimal T-cell response.

Pro- and anti-tumoral functions were attributed to CAFs in cancers, most certainly due to their different origins, and phenotypic evolution during tumor progression. The two-step activation model differentiating normal active fibroblasts (part I) from CAFs further supports the concept of the fibroblast's dual behavior against or supporting tumor cell growth during cancer initiation or progression, respectively. If one considers tumors as a wound that neverheals, it is conceivable that acutely (reversibly) activated fibroblasts will restrain tumor initiation (such as in wound-healing process), whereas chronically (irreversibly) activated fibroblasts will sustain tumor progression [220].

Studies describing pro-tumorigenic functions for CAFs are more numerous than those attributing restraining activities. This may result from CAF heterogeneity within a same tumor and from ad-mixing experiments using ex vivo primary CAF cultures established from patient tumors; a possible enrichment in these cultures of tumor-promoting CAFs (at the expense of tumor-restraining CAFs) may result from the secretion by these CAFs of pro-survival factors that promote not only tumor cell growth and invasion but also autocrine growth stimulating loops in CAFs [221].

CAF pro-tumorigenic functions are mainly triggered by their altered secretomes, whereby paracrine signaling from CAFs boosts tumor progression by positively impacting on cancer cell survival,
proliferation, stemness, metabolism, metastasis-initiating capacity, and resistance to therapies (reviewed in Reference [221]). Soluble factors reported to be involved in such dialogues largely comprise soluble (growth factors, cytokines, and chemokines) and insoluble proteins (extracellular matrix (ECM)), such as CXCL12, TGFβ, FGFs, HGF, IL-6, or periostin [222–227]. These proteins act on tumor cells, but also indirectly stimulate tumor growth by impacting other stromal cells, such as endothelial cells, thereby remodeling the tumor vasculature, or immune cells, favoring the specific recruitment or intra-tumor polarization of pro-tumorigenic inflammatory cells (reviewed in Reference [228]). An explanation for altered protein secretomes in pancreatic CAFs was reported to depend on exacerbated mTOR-induced protein synthesis, the targeting of which revealed a therapeutic benefit in vivo, restraining tumor growth and metastasis, as well as re-sensitizing cancer cells to chemotherapy [229,230]. Other paracrine factors secreted by CAFs that support tumor growth by improving tumor energy synthesis include lipids, such as recently demonstrated in pancreatic tumors, where CAFs undergo a dramatic lipid metabolic shift [231], or metabolites such as lactate or ketone bodies [232].

ECM proteins and ECM-remodeling enzymes secreted by CAFs not only support cancer cell survival by stimulating integrin receptor signaling in cancer cells or by releasing ECM-sequestered growth factors, respectively (reviewed in Reference [233]), but compose a crosslinked collagen fiber network organized into a parallel orientation forming stiff “migration highways” that promote the directionality and velocity of cancer cells (reviewed in Reference [234]), through cooperative collective invasion with CAFs [235]. Communication between CAFs and cancer cells also involve extracellular microvesicles that transmit biological information by carrying microRNAs, long non-coding RNAs (lncRNAs), proteins, or metabolites, both locally and at distance, thereby involved in preparing distant pre-metastatic niches (reviewed in Reference [236]). Such cargo functions for extracellular microvesicles may in the future have clinical utility.

Whether CAF populations involved in restraining tumor progression correspond to fibroblasts resistant to irreversible conversion into CAFs, or to distinct CAF subsets remains unknown. There is a growing body of evidence suggesting that cancer cells influence normal fibroblasts to suppress their tumor-suppressing activity. In a normal setting, stromal cells secrete TGF-β inhibitors [237] or the β-catenin-signaling inhibiting factor SLIT2 [238], or express safeguard checkpoints (e.g., p53) [239], which, upon epithelial stress (e.g., carcinogenesis initiation), will restrain epithelial cell proliferation. Inactivation of these stromal checkpoints was correlated with tumor progression. Tumor-suppressing functions of CAFs were reported in pancreatic cancer, in which targeting the believed pro-tumor hedgehog signaling pathway in CAFs, or genetic depletion of αSMA⁺ CAFs from tumors unexpectedly enhanced tumor progression. These results, therefore, suggest that Hedgehog-responsive and myofibroblastic αSMA⁺ CAFs may comprise two tumor-suppressive subsets. Interestingly, these two subsets may belong to the same CAF population since it was shown, using a Hedgehog pathway reporter allele, that nearly all αSMA⁺ fibroblasts are Hedgehog-responsive. Further supporting the tumor restraining effect of the Hedgehog pathway, a clinical trial that combined chemotherapy with IPI-926, a small-molecule inhibitor of the Hedgehog pathway, was aborted due to a shortened patient overall survival (NCT01130142) [240].

### 5.3. CAFs Targeting

Therapeutically targeting CAFs in cancer may have its use, as they are (i) the most numerous cell type in many fibrotic aggressive tumors, e.g., pancreatic cancer, (ii) endowed with tumor-growth features, and (iii) genetically stable and consequently less likely than tumor cells to acquire therapeutic resistance. Nevertheless, until CAF tumor-suppressive subsets are clearly identified and characterized, such targeting strategies may not be as promising as expected.
### 5.3.1. Depletion

Strategies attempting to ablate CAFs from tumors using cell subset markers provided promising therapeutic benefits when targeting the FAP$\alpha^+$ fibroblasts in pancreatic carcinoma [241], but not the $\alpha$SMA$^+$ subsets [242], further strengthening the functional diversity of these distinct CAF populations. Preclinical studies targeting FAP$\alpha^+$ fibroblasts with either a DNA vaccine against FAP$\alpha$, an FAP$\alpha$ enzymatic inhibitor, an FAP$\alpha$ antibody, or chimeric antigen receptor T-cells targeting FAP$\alpha$, also provided promising preclinical results in mice [243], but failed in clinical trials (phase II in metastatic colorectal cancer using a humanized anti-FAP$\alpha$ antibody or the inhibitor PT-100) [243,244]. Outcomes of a novel strategy targeting FAP$\alpha^+$ cells using a recombinant fusion protein that targets an engineered variant form of IL-2 to human FAP$\alpha^+$ cells, to stimulate a local immune response, are awaited (phase I and II clinical trials in combination with EGFR inhibitor (NCT02627274) or anti-PD-L1 antibody (NCT03386721)). Another CAF subset (CD10$^+$ GPR77$^+$) was identified in chemotherapy-resistant breast tumors, the targeting of which using a neutralizing anti-GPR77 antibody abolished tumor formation and restored tumor chemosensitivity of patient-derived xenografts [245]. CAFs from pancreatic cancer were described to express the somatostatin receptor sst1, whose activation using an sst1 agonist (SOM230, pasireotide) inhibited the mTOR protein synthesis pathway in CAFs, thereby dramatically decreasing the secretion of pro-tumor secreted proteins, including IL-6, inhibiting tumor growth and metastasis, and restoring chemosensitivity in preclinical mouse models including PDX [229,230]. Clinical phase I trials using pasireotide with the chemotherapies, gemcitabine or FOLFIRI, showed manageable safety profiles in patients with gastrointestinal malignancies and provided preliminary signals of activity. Larger phase II trials are, therefore, warranted. This therapeutic approach using SOM230 targeting CAF protein synthesis of secreted factors sounds wiser than drugs targeting single secreted proteins, since compensatory functions between the large panel of CAF secreted factors may occur. An example of such a strategy targets the HGF-cMet signal using a neutralizing anti-HGF antibody or cMet inhibitor, demonstrating early signs of anti-tumor activity in a phase I clinical trial [246,247].

### 5.3.2. Reprogramming

Strategies to reprogram CAFs to re-normalize tumor stroma targeted the epigenetic machinery, or pathways involved in CAF activation, e.g., TGF-$\beta$-mediated Smad2/3 signaling, using the viTAMsin-D derivative or the bioactive lipid lipoxin A4 [248,249], providing encouraging growth suppression in preclinical models.

Strategies using CAF's enzymatic features as a means of drug delivery are being developed, such as a prodrug or micelles encapsulating a chemotherapeutic drug, designed to be cleaved using the protease activity of CAF-expressed FAP$\alpha$ or MMPs, respectively [250,251]. LRRC15$^+$ (leucine-rich-repeat-containing) CAFs were targeted using an LRRC15-antibody drug conjugate (phase I ABBV-085) to deliver an anti-mitotic drug. Recent approaches which may delay peritoneal metastasis were designed to use CAFs as ecological traps that attract disseminated cancer cells, by encapsulating live CAFs into microparticles that are intraperitoneally injected and then retrieved using the microparticle's magnetic properties [252].

## 6. Conclusions

In this review, we highlighted cellular components of the TME, notably TAMs, T-regs, MSCs, and CAFs, which can be pro-tumorigenic by promoting cancer cell survival, immune system evasion, chemoresistance, migration, and metastasis. Interestingly, these infiltrated cells share the ability to produce the same cytokines involved in all these pro-tumorigenic mechanisms, including, for instance, TGF-$\beta$, one of the key mechanisms suppressing immune effectors including NK cells [253]. Therapeutically targeting these cells or the soluble factors they secrete may have great benefits in cancer. The most promising preliminary results were obtained by exhausting TAMs with anti-CSF-1R
or abrogating T-reg functions with anti-PD-1. Even if no real response was achieved in vivo with monotherapy, combination strategies, in particular anti-CSF-1R/chemotherapy, showed a good tumor regression in patients. Targeting CAFs or MSCs could also be an interesting breakthrough. However, strategies available to date are not as promising as expected for targeting CAFs and not specific enough for targeting MSCs. Development of combination therapies acting both to suppress TME barriers and stimulate immune responses remains the most promising for patient survival. Many advances are yet to come with a better understanding of tumor immunology, identification of new targets, and optimization of immunotherapy protocols.

**Acknowledgments:** This work was supported by institutional grants from the Institut National de la Santé et de la Recherche Médicale (INSERM), the Université Toulouse 3, and the Centre National de la Recherche Scientifique (CNRS).

**Conflicts of Interest:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as potential conflicts of interest.

## References

1. Roma-Rodrigues, C.; Mendes, R.; Baptista, P.V.; Fernandes, A.R. Targeting Tumor Microenvironment for Cancer Therapy. *Int. J. Mol. Sci.* **2019**, *20*, 840. [CrossRef]

2. Hanahan, D.; Coussens, L.M. Accessories to the Crime: Functions of Cells Recruited to the Tumor Microenvironment. *Cancer Cell* **2012**, *21*, 309–322. [CrossRef]

3. Wang, M.; Zhao, J.; Zhang, L.; Wei, F.; Lian, Y.; Wu, Y.; Gong, Z.; Zhang, S.; Zhou, J.; Cao, K.; et al. Role of tumor microenvironment in tumorigenesis. *J. Cancer* **2017**, *8*, 761–773. [CrossRef]

4. Kim, K.; Marquez-Palencia, M.; Malladi, S. Metastatic Latency, a Veiled Threat. *Front. Immunol.* **2019**, *10*, 1836. [CrossRef] [PubMed]

5. Joyce, J.A.; Pollard, J.W. Microenvironmental regulation of metastasis. *Nat. Rev. Cancer* **2009**, *9*, 239–252. [CrossRef] [PubMed]

6. Pagès, F.; Mlecnik, B.; Marliot, F.; Bindea, G.; Ou, F.-S.; Bifulco, C.; Lugli, A.; Zlobec, I.; Rau, T.T.; Berger, M.D.; et al. International validation of the consensus Immunoscore for the classification of colon cancer: A prognostic and accuracy study. *Lancet* **2018**, *391*, 2128–2139. [CrossRef]

7. Frydrychowicz, M.; Boruczkowski, M.; Kolecka-Bednarczyk, A.; Dworacki, G. The Dual Role of Treg in Cancer. *Scand. J. Immunol.* **2017**, *86*, 436–443. [CrossRef] [PubMed]

8. Hoeffel, G.; Ginhoux, F. Ontogeny of Tissue-Resident Macrophages. *Front. Immunol.* **2015**, *6*, 486. [CrossRef] [PubMed]

9. Okabe, Y.; Medzhitov, R. Tissue-specific signals control reversible program of localization and functional polarization of macrophages. *Cell* **2014**, *157*, 832–844. [CrossRef] [PubMed]

10. Sica, A.; Schioppa, T.; Mantovani, A.; Allavena, P. Tumour-associated macrophages are a distinct M2 polarised population promoting tumour progression: Potential targets of anti-cancer therapy. *Eur. J. Cancer* **2006**, *42*, 717–727. [CrossRef] [PubMed]

11. Kawai, T.; Akira, S. The role of pattern-recognition receptors in innate immunity: Update on Toll-like receptors. *Nat. Immunol.* **2010**, *11*, 373–384. [CrossRef]

12. Mantovani, A.; Sica, A.; Sozzani, S.; Allavena, P.; Vecchi, A.; Locati, M. The chemokine system in diverse forms of macrophage activation and polarization. *Trends Immunol.* **2004**, *25*, 677–686. [CrossRef]

13. Murray, P.J.; Allen, J.E.; Biswas, S.K.; Fisher, E.A.; Gilroy, D.W.; Goerdt, S.; Gordon, S.; Hamilton, J.A.; Ivashkiv, L.B.; Lawrence, T.; et al. Macrophage activation and polarization: Nomenclature and experimental guidelines. *Immunity* **2014**, *41*, 14–20. [CrossRef]

14. Takeda, K.; Tanaka, T.; Shi, W.; Matsumoto, M.; Minami, M.; Kashiwamura, S.; Nakanishi, K.; Yoshida, N.; Kishimoto, T.; Akira, S. Essential role of Stat6 in IL-4 signalling. *Nature* **1996**, *380*, 627–630. [CrossRef]

15. Pilling, D.; Fan, T.; Huang, D.; Kaul, B.; Gomer, R.H. Identification of markers that distinguish monocyte-derived fibrocytes from monocytes, macrophages, and fibroblasts. *PLoS ONE* **2009**, *4*, e7475. [CrossRef]
16. Andersen, M.N.; Etzerodt, A.; Graversen, J.H.; Holthof, L.C.; Moestrup, S.K.; Hokland, M.; Møller, H.J. STAT3 inhibition specifically in human monocytes and macrophages by CD163-targeted corosolic acid-containing liposomes. *Cancer Immunol. Immunother.* **2019**, *68*, 489–502. [CrossRef]

17. Martinez, F.O.; Gordon, S. The M1 and M2 paradigm of macrophage activation: Time for reassessment. *F1000Prime Rep.* **2014**, *6*, 13. [CrossRef]

18. Tamura, R.; Tanaka, T.; Yamamoto, Y.; Akasaki, Y.; Sasaki, H. Dual role of macrophage in tumor immunity. *Immunotherapy* **2018**, *10*, 899–909. [CrossRef]

19. Zhu, Y.; Herndon, J.M.; Sojka, D.K.; Kim, K.-W.; Knolhoff, B.L.; Zuo, C.; Cullinan, D.R.; Luo, J.; Bearden, A.R.; Lavine, K.J.; et al. Tissue-Resident Macrophages in Pancreatic Ductal Adenocarcinoma Originate from Embryonic Hematopoiesis and Promote Tumor Progression. *Immunity* **2017**, *47*, 597. [CrossRef]

20. Qian, B.-Z.; Li, J.; Zhang, H.; Kitamura, T.; Zhang, J.; Campion, L.R.; Kaiser, E.A.; Snyder, L.A.; Pollard, J.W. CCL2 recruits inflammatory monocytes to facilitate breast-tumour metastasis. *Nature* **2011**, *475*, 222–225. [CrossRef]

21. Pyonteck, S.M.; Akkari, L.; Schuhmacher, A.J.; Bowman, R.L.; Sevenich, L.; Quail, D.F.; Olson, O.C.; Quick, M.L.; Huse, J.T.; Teijeiro, V.; et al. CSF-1R inhibition alters macrophage polarization and blocks glioma progression. *Nat. Med.* **2013**, *19*, 1264–1272. [CrossRef]

22. Evans, R.; Alexander, P. Cooperation of immune lymphoid cells with macrophages in tumour immunity. *Nature* **1970**, *228*, 620–622. [CrossRef]

23. Cassetta, L.; Pollard, J.W. Targeting macrophages: Therapeutic approaches in cancer. *Nat. Rev. Drug Discov.* **2018**, *17*, 887–904. [CrossRef]

24. Goswami, K.K.; Ghosh, T.; Ghosh, S.; Sarkar, M.; Bose, A.; Baral, R. Tumor promoting role of anti-tumor macrophages in tumor microenvironment. *Cell. Immunol.* **2017**, *316*, 1–10. [CrossRef]

25. Quail, D.F.; Joyce, J.A. Microenvironmental regulation of tumor progression and metastasis. *Nat. Med.* **2013**, *19*, 1423–1437. [CrossRef]

26. Takenaka, M.C.; Gabriely, G.; Rothhammer, V.; Mascanfroni, I.D.; Wheeler, M.A.; Chao, C.-C.; Gutiérrez-Vázquez, C.; Kenison, J.; Tjon, E.C.; Barroso, A.; et al. Control of tumor-associated macrophages and T cells in glioblastoma via AHR and CD39. *Nat. Neurosci.* **2019**, *22*, 729. [CrossRef]

27. Kato, Y.; Tabata, K.; Kimura, T.; Yachie-Kinoshita, A.; Ozawa, Y.; Yamada, K.; Ito, J.; Tachino, S.; Hori, Y.; Matsuki, M.; et al. Lenvatinib plus anti-PD-1 antibody combination treatment activates CD8⁺ T cells through reduction of tumor-associated macrophage and activation of the interferon pathway. *PLoS ONE* **2019**, *14*, e0212513. [CrossRef]

28. Jung, M.; Mertens, C.; Tomat, E.; Brüne, B. Iron as a Central Player and Promising Target in Cancer Progression. *Int. J. Mol. Sci.* **2019**, *20*, 273. [CrossRef]

29. Zhang, Q.; Liu, L.; Gong, C.; Shi, H.; Zeng, Y.; Wang, X.; Zhao, Y.; Wei, Y. Prognostic significance of tumor-associated macrophages in solid tumor: A meta-analysis of the literature. *PLoS ONE* **2012**, *7*, e50946. [CrossRef]

30. Ruffell, B.; Coussens, L.M. Macrophages and therapeutic resistance in cancer. *Cancer Cell* **2015**, *27*, 462–472. [CrossRef]

31. Ries, C.H.; Cannarile, M.A.; Hoves, S.; Benz, J.; Wartha, K.; Runza, V.; Rey-Giraud, F.; Pradel, L.P.; Feuerhake, F.; Klaman, I.; et al. Targeting tumor-associated macrophages with anti-CSF-1R antibody reveals a strategy for cancer therapy. *Cancer Cell* **2014**, *25*, 846–859. [CrossRef]

32. Hume, D.A.; MacDonald, K.P.A. Therapeutic applications of macrophage colony-stimulating factor-1 (CSF-1) and antagonists of CSF-1 receptor (CSF-1R) signaling. *Blood* **2012**, *119*, 1810–1820. [CrossRef]

33. Yan, D.; Kowal, J.; Akkari, L.; Schuhmacher, A.J.; Huse, J.T.; West, B.L.; Joyce, J.A. Inhibition of colony stimulating factor-1 receptor abrogates microenvironment-mediated therapeutic resistance in gliomas. *Oncogene* **2017**, *36*, 6049–6058. [CrossRef]

34. Moughon, D.L.; He, H.; Schokrpur, S.; Jiang, Z.K.; Yaqoob, M.; David, J.; Lin, C.; Iruela-Arispe, M.L.; Dorigo, O.; Wu, L. Macrophage Blockade Using CSF1R Inhibitors Reverses the Vascular Leakage Underlying Malignant Ascites in Late-Stage Epithelial Ovarian Cancer. *Cancer Res.* **2015**, *75*, 4742–4752. [CrossRef]

35. Butowski, N.; Colman, H.; De Groot, J.F.; Omuro, A.M.; Nayak, L.; Wen, P.Y.; Cloughesy, T.F.; Marimuthu, A.; Haidar, S.; Perry, A.; et al. Orally administered colony stimulating factor 1 receptor inhibitor PLX3397 in recurrent glioblastoma: An Ivy Foundation Early Phase Clinical Trials Consortium phase II study. *Neuro-Oncology* **2016**, *18*, 557–564. [CrossRef]
36. DeNardo, D.G.; Brennan, D.J.; Rexhepaj, E.; Ruffell, B.; Shiao, S.L.; Madden, S.F.; Gallagher, W.M.; Wadhwani, N.; Keil, S.D.; Junaid, S.A.; et al. Leukocyte complexity predicts breast cancer survival and functionally regulates response to chemotherapy. *Cancer Discov.* **2011**, *1*, 54–67. [CrossRef]

37. Gomez-Roca, C.A.; Italiano, A.; Le Tourneau, C.; Cassier, P.A.; Toulmonde, M.; D'Angelo, S.P.; Campone, M.; Weber, K.L.; Loirat, D.; Cannarile, M.A.; et al. Phase I Study of Emactuzumab Single Agent or in Combination with Paclitaxel in Patients with Advanced/Metastatic Solid Tumors Reveals Depletion of Immunosuppressive M2-like Macrophages. *Ann. Oncol.* **2019**, *30*, 1381–1392. [CrossRef]

38. Weizman, N.; Krelin, Y.; Shabtay-Orbach, A.; Amit, M.; Binenbaum, Y.; Wong, R.J.; Gil, Z. Macrophages mediate gemcitabine resistance of pancreatic adenocarcinoma by upregulating cytidine deaminase. *Oncogene* **2014**, *33*, 3812–3819. [CrossRef]

39. Lu, X.; Meng, T. Depletion of tumor-associated macrophages enhances the anti-tumor effect of docetaxel in a murine epithelial ovarian cancer. *Immunobiology* **2019**, *224*, 355–361. [CrossRef]

40. Stafford, J.H.; Hirai, T.; Deng, L.; Chernikova, S.B.; Urata, K.; West, B.L.; Brown, J.M. Colony stimulating factor 1 receptor inhibition delays recurrence of glioblastoma after radiation by altering myeloid cell recruitment and polarization. *Neuro-Oncology* **2016**, *18*, 797–806. [CrossRef]

41. Michaelson, M.D.; Smith, M.R. Bisphosphonates for treatment and prevention of bone metastases. *J. Clin. Oncol.* **2005**, *23*, 8219–8224. [CrossRef] [PubMed]

42. Zeisberger, S.M.; Odermatt, B.; Marty, C.; Zehnder-Fjällman, A.H.M.; Ballmer-Hofer, K.; Schwendener, R.A. Clodronate-liposome-mediated depletion of tumour-associated macrophages: A new and highly effective antiangiogenic therapy approach. *Br. J. Cancer* **2006**, *95*, 272–281. [CrossRef] [PubMed]

43. Wu, X.; Schulte, B.C.; Zhou, Y.; Haribhai, D.; Mackinnon, A.C.; Plaza, J.A.; Williams, C.B.; Hwang, S.T. Depletion of M2-like tumor-associated macrophages delays cutaneous T-cell lymphoma development in vivo. *J. Invest. Dermatol.* **2014**, *134*, 2814–2822. [CrossRef] [PubMed]

44. Hiraoka, K.; Zenmyo, M.; Watari, K.; Iguchi, H.; Fotovati, A.; Kimura, Y.N.; Hosoi, F.; Shoda, T.; Nagata, K.; Osada, H.; et al. Inhibition of bone and muscle metastases of lung cancer cells by a decrease in the number of monocytes/macrophages. *Cancer Sci.* **2008**, *99*, 1595–1602. [CrossRef] [PubMed]

45. Qian, B.; Deng, Y.; Im, J.H.; Muschel, R.J.; Zou, Y.; Li, J.; Lang, R.A.; Pollard, J.W. A distinct macrophage population mediates metastatic breast cancer cell extravasation, establishment and growth. *PLoS ONE* **2009**, *4*, e6562. [CrossRef] [PubMed]

46. Gazzaniga, S.; Bravo, A.I.; Guglielmotti, A.; van Rooijen, N.; Maschi, F.; Vecchi, A.; Mantovani, A.; Mordoh, J.; Wainstok, R. Targeting tumor-associated macrophages and inhibition of MCP-1 reduce angiogenesis and tumor growth in a human melanoma xenograft. *J. Invest. Dermatol.* **2007**, *127*, 2031–2041. [CrossRef]

47. Diel, I.J.; Solomayer, E.F.; Costa, S.D.; Gollan, C.; Goerner, R.; Wallwiener, D.; Kaufmann, M.; Bastert, G. Reduction in new metastases in breast cancer with adjuvant clodronate treatment. *N. Engl. J. Med.* **1998**, *339*, 357–363. [CrossRef]

48. Van Acker, H.H.; Anguille, S.; Willemen, Y.; Smits, E.L.; Van Tendeloo, V.F. Bisphosphonates for cancer treatment: Mechanisms of action and lessons from clinical trials. *Pharmacol. Ther.* **2016**, *158*, 24–40. [CrossRef]

49. Germano, G.; Frapolli, R.; Belgiovine, C.; Anselmo, A.; Pesce, S.; Liguori, M.; Erba, E.; Uboldi, S.; Zucchetti, M.; Pasqualini, F.; et al. Role of macrophage targeting in the antitumor activity of trabectedin. *Cancer Cell* **2013**, *23*, 249–262. [CrossRef]

50. Liguori, M.; Buracchi, C.; Pasqualini, F.; Bergomas, F.; Pesce, S.; Sironi, M.; Grizzi, F.; Mantovani, A.; Belgiovine, C.; Allavena, P. Functional TRAIL receptors in monocytes and tumor-associated macrophages: A possible targeting pathway in the tumor microenvironment. *Oncotarget* **2016**, *7*, 41662–41676. [CrossRef]

51. Georgoudaki, A.-M.; Prokopec, K.E.; Boura, V.F.; Hellqvist, E.; Sohn, S.; Östling, J.; Dahan, R.; Harris, R.A.; Rantalainen, M.; Klevebring, D.; et al. Reprogramming Tumor-Associated Macrophages by Antibody Targeting Inhibits Cancer Progression and Metastasis. *Cell Rep.* **2016**, *15*, 2000–2011. [CrossRef]

52. Sommariva, M.; Le Noci, V.; Storti, C.; Bianchi, F.; Tagliabue, E.; Balsari, A.; Sfondrini, L. Activation of NK cell cytotoxicity by aerosolized CpG-ODN/poly(I:C) against lung melanoma metastases is mediated by alveolar macrophages. *Cell. Immunol.* **2017**, *313*, 52–58. [CrossRef]

53. Halama, N.; Zoernig, I.; Berthel, A.; Kahlert, C.; Klupp, F.; Suarez-Carmona, M.; Suetterlin, T.; Brand, K.; Krauss, J.; Lasitschka, F.; et al. Tumoral Immune Cell Exploitation in Colorectal Cancer Metastases Can Be Targeted Effectively by Anti-CCR5 Therapy in Cancer Patients. *Cancer Cell* **2016**, *29*, 587–601. [CrossRef]
54. Long, K.B.; Gladney, W.L.; Tooker, G.M.; Graham, K.; Fraietta, J.A.; Beatty, G.L. IFNγ and CCL2 Cooperate to Redirect Tumor-Infiltrating Monocytes to Degrade Fibrosis and Enhance Chemotherapy Efficacy in Pancreatic Carcinoma. *Cancer Discov.* **2016**, *6*, 400–413. [CrossRef]

55. Maeda, A.; Digifico, E.; Andon, F.T.; Mantovani, A.; Allavena, P. Poly(I:C) stimulation is superior than Imiquimod to induce the antitumoral functional profile of tumor-conditioned macrophages. *Eur. J. Immunol.* **2019**, *49*, 801–811. [CrossRef]

56. Yin, T.; He, S.; Wang, Y. Toll-like receptor 7/8 agonist, R848, exhibits antitumoral effects in a breast cancer model. *Mol. Med. Rep.* **2015**, *12*, 3515–3520. [CrossRef]

57. Weihrauch, M.R.; Richly, H.; von Bergwelt-Baildon, M.S.; Becker, H.J.; Schmidt, M.; Hacker, U.T.; Shimabukuro-Vornhagen, A.; Holtick, U.; Nokay, B.; Schroff, M.; et al. Phase I clinical study of the toll-like receptor 9 agonist MGN1703 in patients with metastatic solid tumours. *Eur. J. Cancer* **2015**, *51*, 146–156. [CrossRef]

58. Lizotte, P.H.; Baird, J.R.; Stevens, C.A.; Lauer, P.; Green, W.R.; Brockstedt, D.G.; Fiering, S.N. Attenuated Listeria monocytogenes reprograms M2-polarized tumor-associated macrophages in ovarian cancer leading to iNOS-mediated tumor cell lysis. *Oncoimmunology* **2014**, *3*, e28926. [CrossRef]

59. Tutt, A.L.; O'Brien, L.; Hussain, A.; Crowther, G.R.; French, R.R.; Glennie, M.J. T Cell Immunity to Lymphoma Following Treatment with Anti-CD40 Monoclonal Antibody. *J. Immunol.* **2002**, *168*, 2720–2728. [CrossRef]

60. Rüter, J.; Antonia, S.J.; Burris, H.A.; Huhn, R.D.; Vonderheide, R.H. Immune modulation with weekly dosing of an agonist CD40 antibody in a phase I study of patients with advanced solid tumors. *Cancer Biol. Ther.* **2010**, *10*, 983–993. [CrossRef]

61. Dheilly, E.; Moine, V.; Broyer, L.; Salgado-Pires, S.; Johnson, Z.; Papaioannou, A.; Cons, L.; Calloud, S.; Majocchi, S.; Nelson, R.; et al. Selective Blockade of the Ubiquitous Checkpoint Receptor CD47 Is Enabled by Dual-Targeting Bispecific Antibodies. *Mol. Ther.* **2017**, *25*, 523–533. [CrossRef]

62. Mullins, D.W.; Burger, C.J.; Elgert, K.D. Paclitaxel enhances macrophage IL-12 production in tumor-bearing hosts through nitric oxide. *J. Immunol.* **1999**, *162*, 6811–6818.

63. Kortylewski, M.; Swiderski, P.; Herrmann, A.; Wang, L.; Kowolik, C.; Kujawski, M.; Lee, H.; Scuto, A.; Liu, Y.; Yang, C.; et al. In vivo delivery of siRNA to immune cells by conjugation to a TLR9 agonist enhances antitumor immune responses. *Nat. Biotechnol.* **2009**, *27*, 925–932. [CrossRef]

64. Binnemars-Postma, K.; Bansal, R.; Storm, G.; Prakash, J. Targeting the Stat6 pathway in tumor-associated macrophages reduces tumor growth and metastatic niche formation in breast cancer. *FASEB J.* **2018**, *32*, 969–978. [CrossRef]

65. Squadrito, M.L.; Etzrodt, M.; De Palma, M.; Pittet, M.J. MicroRNA-mediated control of macrophages and its implications for cancer. *Trends Immunol.* **2013**, *34*, 350–359. [CrossRef]

66. Sonda, N.; Simonato, F.; Peranzoni, E.; Cali, B.; Bortoluzzi, S.; Bisognin, A.; Wang, E.; Marincola, F.M.; Naldini, L.; Gentner, B.; et al. miR-142-3p prevents macrophage differentiation during cancer-induced myelopoiesis. *Immunity* **2013**, *38*, 1236–1249. [CrossRef]

67. Baer, C.; Squadrito, M.L.; Laoui, D.; Thompson, D.; Hansen, S.K.; Kiialainen, A.; Hoves, S.; Ries, C.H.; Ooi, C.-H.; De Palma, M. Suppression of microRNA activity amplifies IFN-γ-induced macrophage activation and promotes anti-tumour immunity. *Nat. Cell Biol.* **2016**, *18*, 790–802. [CrossRef]

68. Chen, W.; Ma, T.; Shen, X.; Xia, X.; Xu, G.; Bai, X.; Liang, T. Macrophage-induced tumor angiogenesis is regulated by the TSC2-mTOR pathway. *Cancer Res.* **2012**, *72*, 1363–1372. [CrossRef]

69. Kaneda, M.M.; Messer, K.S.; Ralainirina, N.; Li, H.; Leem, C.J.; Gorjestani, S.; Woo, G.; Nguyen, A.V.; Figueiredo, C.C.; Foubert, P.; et al. PI3Kγ is a molecular switch that controls immune suppression. *Nature* **2016**, *539*, 437–442. [CrossRef]

70. Chakraborty, P.; Chatterjee, S.; Ganguly, A.; Saha, P.; Adhikary, A.; Das, T.; Chatterjee, M.; Choudhuri, S.K. Reprogramming of TAM toward proimmunogenic type through regulation of MAP kinases using a redox-active copper chelate. *J. Leukoc. Biol.* **2012**, *91*, 609–619. [CrossRef]

71. Wang, C.; Li, Y.; Chen, H.; Huang, K.; Liu, X.; Qiu, M.; Liu, Y.; Yang, Y.; Yang, J. CYP4 × 1 Inhibition by Flavonoid CH625 Normalizes Glioma Vasculature through Reprogramming TAMs via CB2 and EGFR-STAT3 Axis. *J. Pharmacol. Exp. Ther.* **2018**, *365*, 72–83. [CrossRef]

72. Tan, H.-Y.; Wang, N.; Man, K.; Tsao, S.-W.; Che, C.-M.; Feng, Y. Autophagy-induced RelB/p52 activation mediates tumour-associated macrophage repolarisation and suppression of hepatocellular carcinoma by natural compound baicalin. *Cell Death Dis.* **2015**, *6*, e1942. [CrossRef]
73. Guerriero, J.L.; Sotayo, A.; Ponichtera, H.E.; Castrillon, J.A.; Pourzia, A.L.; Schad, S.; Johnson, S.F.; Carrasco, R.D.; Lazo, S.; Bronson, R.T.; et al. Class IIa HDAC inhibition reduces breast tumours and metastases through anti-tumour macrophages. *Nature* **2017**, *543*, 428–432. [CrossRef]

74. Wanderley, C.W.; Colón, D.F.; Luiz, J.P.M.; Oliveira, F.F.; Viacava, P.R.; Leite, C.A.; Pereira, J.A.; Silva, C.M.; Silva, C.R.; Silva, R.L.; et al. Paclitaxel Reduces Tumor Growth by Reprogramming Tumor-Associated Macrophages to an M1 Profile in a TLR4-Dependent Manner. *Cancer Res.* **2018**, *78*, 5891–5900. [CrossRef]

75. Perry, C.J.; Muñoz-Rojas, A.R.; Meeth, K.M.; Kellman, L.N.; Amezquita, R.A.; Thakral, D.; Du, V.Y.; Wang, J.X.; Damsky, W.; Kuhlmann, A.L.; et al. Myeloid-targeted immunotherapies act in synergy to induce inflammation and antitumor immunity. *J. Exp. Med.* **2018**, *215*, 877–893. [CrossRef]

76. Zhang, J.Q.; Zeng, S.; Vitiello, G.A.; Seifert, A.M.; Medina, B.D.; Beckman, M.J.; Loo, J.K.; Santamaria-Barria, J.; Maltbaek, J.H.; Param, N.J.; et al. Macrophages and CD8⁺ T Cells Mediate the Antitumor Efficacy of Combined CD40 Ligation and Imatinib Therapy in Gastrointestinal Stromal Tumors. *Cancer Immunol. Res.* **2018**, *6*, 434–447. [CrossRef]

77. Smith, D.A.; Conkling, P.; Richards, D.A.; Nemunaitis, J.J.; Boyd, T.E.; Mita, A.C.; de La Bourdonnaye, G.; Wages, D.; Bexon, A.S. Antitumor activity and safety of combination therapy with the Toll-like receptor 9 agonist IMO-2055, erlotinib, and bevacizumab in advanced or metastatic non-small cell lung cancer patients who have progressed following chemotherapy. *Cancer Immunol. Immunother.* **2014**, *63*, 787–796. [CrossRef]

78. Xu, A.; Zhang, L.; Yuan, J.; Babikr, F.; Freywald, A.; Chibbar, R.; Moser, M.; Zhang, W.; Zhang, B.; Fu, Z.; et al. TLR9 agonist enhances radiofrequency ablation-induced CTL responses, leading to the potent inhibition of primary tumor growth and lung metastasis. *Cell. Mol. Immunol.* **2018**. [CrossRef]

79. Ma, B.; Cheng, H.; Mu, C.; Geng, G.; Zhao, T.; Luo, Q.; Ma, K.; Chang, R.; Liu, Q.; Gao, R.; et al. The SIAH2-NRF1 axis spatially regulates tumor microenvironment remodeling for tumor progression. *Nat. Commun.* **2019**, *10*, 1034. [CrossRef]

80. Colegio, O.R.; Chu, N.-Q.; Szabo, A.L.; Chu, T.; Rhebergen, A.M.; Jairam, V.; Cyrus, N.; Brokowski, C.E.; Eisenbarth, S.C.; Phillips, G.M.; et al. Functional polarization of tumour-associated macrophages by tumour-derived lactic acid. *Nature* **2014**, *513*, 559–563. [CrossRef]

81. Chen, P.; Zuo, H.; Xiong, H.; Kolar, M.J.; Chu, Q.; Saghatelian, A.; Siegwart, D.J.; Wan, Y. Gpr132 sensing of lactate mediates tumor-macrophage interplay to promote breast cancer metastasis. *Proc. Natl. Acad. Sci. USA* **2017**, *114*, 580–585. [CrossRef]

82. Verzella, D.; Bennett, J.; Fischietti, M.; Thotakura, A.K.; Recordati, C.; Pasqualini, F.; Capece, D.; Vecchiotti, D.; D'Andrea, D.; Di Francesco, B.; et al. GADD45β Loss Ablates Innate Immunosuppression in Cancer. *Cancer Res.* **2018**, *78*, 1275–1292. [CrossRef]

83. Wei, X.; Nie, S.; Liu, H.; Sun, J.; Liu, J.; Li, J.; Li, S.; Wang, S.; Han, S.; Wang, J.; et al. Angiopoietin-like protein 2 facilitates non-small cell lung cancer progression by promoting the polarization of M2 tumor-associated macrophages. *Am. J. Cancer Res.* **2017**, *7*, 2220–2233.

84. Racioppi, L.; Nelson, E.R.; Huang, W.; Mukherjee, D.; Lawrence, S.A.; Lento, W.; Masci, A.M.; Jiao, Y.; Park, S.; York, B.; et al. CaMKK2 in myeloid cells is a key regulator of the immune-suppressive microenvironment in breast cancer. *Nat. Commun.* **2019**, *10*, 2450. [CrossRef]

85. Zhang, Q.; Wang, H.; Mao, C.; Sun, M.; Dominah, G.; Chen, L.; Zhuang, Z. Fatty acid oxidation contributes to IL-1β secretion in M2 macrophages and promotes macrophage-mediated tumor cell migration. *Mol. Immunol.* **2018**, *94*, 27–35. [CrossRef]

86. Goossens, P.; Rodriguez-Vita, J.; Etzerodt, A.; Masse, M.; Rastoin, O.; Gouirand, V.; Ulas, T.; Papantonopoulou, O.; Van Eck, M.; Auphan-Anezin, N.; et al. Membrane Cholesterol Efflux Drives Tumor-Associated Macrophage Reprogramming and Tumor Progression. *Cell Metab.* **2019**, *29*, 1376–1389. [CrossRef]

87. Bartneck, M.; Schrammen, P.L.; Möckel, D.; Govaere, O.; Liepelt, A.; Krenkel, O.; Ergen, C.; McCain, M.V.; Eulberg, D.; Luedde, T.; et al. The CCR2⁺ Macrophage Subset Promotes Pathogenic Angiogenesis for Tumor Vascularization in Fibrotic Livers. *Cell. Mol. Gastroenterol. Hepatol.* **2019**, *7*, 371–390. [CrossRef]

88. Gui, P.; Ben-Neji, M.; Belozertseva, E.; Dalenc, F.; Franchet, C.; Gilhodes, J.; Labrousse, A.; Bellard, E.; Golzio, M.; Poincloux, R.; et al. The Protease-Dependent Mesenchymal Migration of Tumor-Associated Macrophages as a Target in Cancer Immunotherapy. *Cancer Immunol. Res.* **2018**, *6*, 1337–1351. [CrossRef]
89. Li, F.; Kitajima, S.; Kohno, S.; Yoshida, A.; Tange, S.; Sasaki, S.; Okada, N.; Nishimoto, Y.; Muranaka, H.; Nagatani, N.; et al. Retinoblastoma inactivation induces a protumoral microenvironment via enhanced CCL2 secretion. *Cancer Res.* **2019**, *79*, 3903–3915. [CrossRef]

90. Nywening, T.M.; Belt, B.A.; Cullinan, D.R.; Panni, R.Z.; Han, B.J.; Sanford, D.E.; Jacobs, R.C.; Ye, J.; Patel, A.A.; Gillanders, W.E.; et al. Targeting both tumour-associated CXCR2⁺ neutrophils and CCR2⁺ macrophages disrupts myeloid recruitment and improves chemotherapeutic responses in pancreatic ductal adenocarcinoma. *Gut* **2018**, *67*, 1112–1123. [CrossRef]

91. Yang, Z.; Li, H.; Wang, W.; Zhang, J.; Jia, S.; Wang, J.; Wei, J.; Lei, D.; Hu, K.; Yang, X. CCL2/CCR2 Axis Promotes the Progression of Salivary Adenoid Cystic Carcinoma via Recruiting and Reprogramming the Tumor-Associated Macrophages. *Front. Oncol.* **2019**, *9*, 231. [CrossRef]

92. Bonapace, L.; Coissieux, M.-M.; Wyckoff, J.; Mertz, K.D.; Varga, Z.; Junt, T.; Bentires-Alj, M. Cessation of CCL2 inhibition accelerates breast cancer metastasis by promoting angiogenesis. *Nature* **2014**, *515*, 130–133. [CrossRef]

93. Vela, M.; Aris, M.; Llorente, M.; Garcia-Sanz, J.A.; Kremer, L. Chemokine receptor-specific antibodies in cancer immunotherapy: Achievements and challenges. *Front. Immunol.* **2015**, *6*, 12. [CrossRef]

94. Sandhu, S.K.; Papadopoulos, K.; Fong, P.C.; Patnaik, A.; Messiou, C.; Olmos, D.; Wang, G.; Tromp, B.J.; Puchalski, T.A.; Balkwill, F.; et al. A first-in-human, first-in-class, phase I study of carlumab (CNT0 888), a human monoclonal antibody against CC-chemokine ligand 2 in patients with solid tumors. *Cancer Chemother. Pharmacol.* **2013**, *71*, 1041–1050. [CrossRef]

95. Pienta, K.J.; Machiels, J.-P.; Schrijvers, D.; Alekseev, B.; Shkolnik, M.; Crabb, S.J.; Li, S.; Seetharam, S.; Puchalski, T.A.; Takimoto, C.; et al. Phase 2 study of carlumab (CNT0 888), a human monoclonal antibody against CC-chemokine ligand 2 (CCL2), in metastatic castration-resistant prostate cancer. *Invest. New Drugs* **2013**, *31*, 760–768. [CrossRef]

96. Nywening, T.M.; Wang-Gillam, A.; Sanford, D.E.; Belt, B.A.; Panni, R.Z.; Cusworth, B.M.; Toriola, A.T.; Nieman, R.K.; Worley, L.A.; Yano, M.; et al. Targeting tumour-associated macrophages with CCR2 inhibition in combination with FOLFIRINOX in patients with borderline resectable and locally advanced pancreatic cancer: A single-centre, open-label, dose-finding, non-randomised, phase 1b trial. *Lancet Oncol.* **2016**, *17*, 651–662. [CrossRef]

97. Leftin, A.; Ben-Chetrit, N.; Joyce, J.A.; Koutcher, J.A. Imaging endogenous macrophage iron deposits reveals a metabolic biomarker of polarized tumor macrophage infiltration and response to CSF1R breast cancer immunotherapy. *Sci. Rep.* **2019**, *9*, 857. [CrossRef]

98. Papadopoulos, K.P.; Gluck, L.; Martin, L.P.; Olszanski, A.J.; Tolcher, A.W.; Ngarmchamnanrith, G.; Rasmussen, E.; Amore, B.M.; Nagorsen, D.; Hill, J.S.; et al. First-in-Human Study of AMG 820, a Monoclonal Anti-Colony-Stimulating Factor 1 Receptor Antibody, in Patients with Advanced Solid Tumors. *Clin. Cancer Res.* **2017**, *23*, 5703–5710. [CrossRef]

99. Cassier, P.A.; Italiano, A.; Gomez-Roca, C.A.; Le Tourneau, C.; Toulmonde, M.; Cannarile, M.A.; Ries, C.; Brillouet, A.; Müller, C.; Jegg, A.M.; et al. CSF1R inhibition with emactuzumab in locally advanced diffuse-type tenosynovial giant cell tumours of the soft tissue: A dose-escalation and dose-expansion phase 1 study. *Lancet Oncol.* **2015**, *16*, 949–956. [CrossRef]

100. Sun, Y.; Yang, L.; Hao, X.; Liu, Y.; Zhang, J.; Ning, Z.; Shi, Y. Phase I dose-escalation study of chiauranib, a novel angiogenic, mitotic, and chronic inflammation inhibitor, in patients with advanced solid tumors. *J. Hematol. Oncol.* **2019**, *12*, 9. [CrossRef]

101. Rosenbaum, E.; Kelly, C.; D'Angelo, S.P.; Dickson, M.A.; Gounder, M.; Keohan, M.L.; Movva, S.; Condy, M.; Adamson, T.; Mcfadyen, C.R.; et al. A Phase I Study of Binimetinib (MEK162) Combined with Pexidartinib (PLX3397) in Patients with Advanced Gastrointestinal Stromal Tumor. *Oncologist* **2019**, *24*, 1–8. [CrossRef]

102. Hegab, A.E.; Ozaki, M.; Kameyama, N.; Gao, J.; Kagawa, S.; Yasuda, H.; Soejima, K.; Yin, Y.; Guzy, R.D.; Nakamura, Y.; et al. Effect of FGF/FGFR pathway blocking on lung adenocarcinoma and its cancer-associated fibroblasts. *J. Pathol.* **2019**. [CrossRef]

103. Xia, Y.; Wei, Y.; Li, Z.-Y.; Cai, X.-Y.; Zhang, L.-L.; Dong, X.-R.; Zhang, S.; Zhang, R.-G.; Meng, R.; Zhu, F.; et al. Catecholamines contribute to the neovascularization of lung cancer via tumor-associated macrophages. *Brain Behav. Immun.* **2019**, *81*, 111–121. [CrossRef]
104. Piao, C.; Zhang, W.-M.; Li, T.-T.; Zhang, C.-C.; Qiu, S.; Liu, Y.; Liu, S.; Jin, M.; Jia, L.-X.; Song, W.-C.; et al. Complement 5a stimulates macrophage polarization and contributes to tumor metastases of colon cancer. *Exp. Cell Res.* **2018**, *366*, 127–138. [CrossRef]

105. Hori, S.; Nomura, T.; Sakaguchi, S. Control of regulatory T cell development by the transcription factor Foxp3. *Science* **2003**, *299*, 1057–1061. [CrossRef]

106. Sugiyama, D.; Nishikawa, H.; Maeda, Y.; Nishioka, M.; Tanemura, A.; Katayama, I.; Ezoe, S.; Kanakura, Y.; Sato, E.; Fukumori, Y.; et al. Anti-CCR4 mAb selectively depletes effector-type FoxP3<sup>+</sup>CD4<sup>+</sup> regulatory T cells, evoking antitumor immune responses in humans. *Proc. Natl. Acad. Sci. USA* **2013**, *110*, 17945–17950. [CrossRef]

107. Tang, Q.; Henriksen, K.J.; Boden, E.K.; Tooley, A.J.; Ye, J.; Subudhi, S.K.; Zheng, X.X.; Strom, T.B.; Bluestone, J.A. Cutting edge: CD28 controls peripheral homeostasis of CD4<sup>+</sup>CD25<sup>+</sup> regulatory T cells. *J. Immunol.* **2003**, *171*, 3348–3352. [CrossRef]

108. Shimizu, J.; Yamazaki, S.; Takahashi, T.; Ishida, Y.; Sakaguchi, S. Stimulation of CD25<sup>+</sup>CD4<sup>+</sup> regulatory T cells through GITR breaks immunological self-tolerance. *Nat. Immunol.* **2002**, *3*, 135–142. [CrossRef]

109. Read, S.; Malmström, V.; Powrie, F. Cytotoxic T lymphocyte-associated antigen 4 plays an essential role in the function of CD25<sup>+</sup>CD4<sup>+</sup> regulatory cells that control intestinal inflammation. *J. Exp. Med.* **2000**, *192*, 295–302. [CrossRef]

110. Huang, C.-T.; Workman, C.J.; Flies, D.; Pan, X.; Marson, A.L.; Zhou, G.; Hipkiss, E.L.; Ravi, S.; Kowalski, J.; Levitsky, H.I.; et al. Role of LAG-3 in regulatory T cells. *Immunity* **2004**, *21*, 503–513. [CrossRef]

111. Overacre-Delgoffe, A.E.; Chikina, M.; Dadey, R.E.; Yano, H.; Brunazzi, E.A.; Shayan, G.; Horne, W.; Moskovitz, J.M.; Kolls, J.K.; Sander, C.; et al. Interferon-γ Drives Treg Fragility to Promote Anti-tumor Immunity. *Cell* **2017**, *169*, 1130–1141. [CrossRef]

112. Asano, T.; Meguri, Y.; Yoshioka, T.; Kishi, Y.; Iwamoto, M.; Nakamura, M.; Sando, Y.; Yagita, H.; Koreth, J.; Kim, H.T.; et al. PD-1 modulates regulatory T-cell homeostasis during low-dose interleukin-2 therapy. *Blood* **2017**, *129*, 2186–2197. [CrossRef]

113. Liu, Z.; McMichael, E.L.; Shayan, G.; Li, J.; Chen, K.; Srivastava, R.; Kane, L.P.; Lu, B.; Ferris, R.L. Novel Effector Phenotype of Tim-3<sup>+</sup> Regulatory T Cells Leads to Enhanced Suppressive Function in Head and Neck Cancer Patients. *Clin. Cancer Res.* **2018**, *24*, 4529–4538. [CrossRef]

114. Joller, N.; Lozano, E.; Burkett, P.R.; Patel, B.; Xiao, S.; Zhu, C.; Xia, J.; Tan, T.G.; Sefik, E.; Yajnik, V.; et al. Treg cells expressing the coinhibitory molecule TIGIT selectively inhibit proinflammatory Th1 and Th17 cell responses. *Immunity* **2014**, *40*, 569–581. [CrossRef]

115. Wang, L.; Rubinstein, R.; Lines, J.L.; Wasiuk, A.; Ahonen, C.; Guo, Y.; Lu, L.-F.; Gondek, D.; Wang, Y.; Fava, R.A.; et al. VISTA, a novel mouse Ig superfamily ligand that negatively regulates T cell responses. *J. Exp. Med.* **2011**, *208*, 577–592. [CrossRef]

116. Vignali, D.A.A.; Collison, L.W.; Workman, C.J. How regulatory T cells work. *Nat. Rev. Immunol.* **2008**, *8*, 523–532. [CrossRef]

117. Sakaguchi, S. Naturally arising CD4<sup>+</sup> regulatory T cells for immunologic self-tolerance and negative control of immune responses. *Annu. Rev. Immunol.* **2004**, *22*, 531–562. [CrossRef]

118. Cao, X.; Cai, S.F.; Fehniger, T.A.; Song, J.; Collins, L.I.; Piwnica-Worms, D.R.; Ley, T.J. Granzyme B and perforin are important for regulatory T cell-mediated suppression of tumor clearance. *Immunity* **2007**, *27*, 635–646. [CrossRef]

119. Pandiyan, P.; Zheng, L.; Ishihara, S.; Reed, J.; Lenardo, M.J. CD4<sup>+</sup>CD25<sup>+</sup>Foxp3<sup>+</sup> regulatory T cells induce cytokine deprivation-mediated apoptosis of effector CD4<sup>+</sup> T cells. *Nat. Immunol.* **2007**, *8*, 1353–1362. [CrossRef]

120. Saito, T.; Nishikawa, H.; Wada, H.; Nagano, Y.; Sugiyama, D.; Atarashi, K.; Maeda, Y.; Hamaguchi, M.; Ohkura, N.; Sato, E.; et al. Two FOXP3<sup>+</sup>CD4<sup>+</sup> T cell subpopulations distinctly control the prognosis of colorectal cancers. *Nat. Med.* **2016**, *22*, 679–684. [CrossRef]

121. Viguier, M.; Lemaître, F.; Verola, O.; Cho, M.-S.; Gorochov, G.; Dubertret, L.; Bachelez, H.; Kourilsky, P.; Ferradini, L. Foxp3 expressing CD4<sup>+</sup>CD25(high) regulatory T cells are overrepresented in human metastatic melanoma lymph nodes and inhibit the function of infiltrating T cells. *J. Immunol.* **2004**, *173*, 1444–1453. [CrossRef]
122. Yang, Z.-Z.; Novak, A.J.; Stenson, M.J.; Witzig, T.E.; Ansell, S.M. Intratumoral CD4⁺CD25⁺ regulatory T-cell-mediated suppression of infiltrating CD4⁺ T cells in B-cell non-Hodgkin lymphoma. *Blood* **2006**, *107*, 3639–3646. [CrossRef]

123. Liyanage, U.K.; Moore, T.T.; Joo, H.-G.; Tanaka, Y.; Herrmann, V.; Doherty, G.; Drebin, J.A.; Strasberg, S.M.; Eberlein, T.J.; Goedegebuure, P.S.; et al. Prevalence of regulatory T cells is increased in peripheral blood and tumor microenvironment of patients with pancreas or breast adenocarcinoma. *J. Immunol.* **2002**, *169*, 2756–2761. [CrossRef]

124. Woo, E.Y.; Chu, C.S.; Goletz, T.J.; Schlienger, K.; Yeh, H.; Coukos, G.; Rubin, S.C.; Kaiser, L.R.; June, C.H. Regulatory CD4⁺CD25⁺ T cells in tumors from patients with early-stage non-small cell lung cancer and late-stage ovarian cancer. *Cancer Res.* **2001**, *61*, 4766–4772.

125. Bluestone, J.A.; Liu, W.; Yabu, J.M.; Laszik, Z.G.; Putnam, A.; Belingheri, M.; Gross, D.M.; Townsend, R.M.; Vincenti, F. The effect of costimulatory and interleukin 2 receptor blockade on regulatory T cells in renal transplantation. *Am. J. Transplant.* **2008**, *8*, 2086–2096. [CrossRef]

126. Rech, A.J.; Mick, R.; Martin, S.; Recio, A.; Aqui, N.A.; Powell, D.J.; Colligon, T.A.; Trosko, J.A.; Leinbach, L.I.; Pletcher, C.H.; et al. CD25 blockade depletes and selectively reprograms regulatory T cells in concert with immunotherapy in cancer patients. *Sci. Transl. Med.* **2012**, *4*, 134ra62. [CrossRef]

127. Berlato, C.; Khan, M.N.; Schioppa, T.; Thompson, R.; Maniati, E.; Montfort, A.; Jangani, M.; Canosa, M.; Kulbe, H.; Hagemann, U.B.; et al. A CCR4 antagonist reverses the tumor-promoting microenvironment of renal cancer. *J. Clin. Invest.* **2017**, *127*, 801–813. [CrossRef]

128. Kurose, K.; Ohue, Y.; Wada, H.; Iida, S.; Ishida, T.; Kojima, T.; Doi, T.; Suzuki, S.; Isobe, M.; Funakoshi, T.; et al. Phase Ia Study of FoxP3⁺ CD4 Treg Depletion by Infusion of a Humanized Anti-CCR4 Antibody, KW-0761, in Cancer Patients. *Clin. Cancer Res.* **2015**, *21*, 4327–4336. [CrossRef]

129. Rapp, M.; Wintergerst, M.W.M.; Kunz, W.G.; Vetter, V.K.; Knott, M.M.L.; Lisowski, D.; Haubner, S.; Moder, S.; Thaler, R.; Eiber, S.; et al. CCL22 controls immunity by promoting regulatory T cell communication with dendritic cells in lymph nodes. *J. Exp. Med.* **2019**, *216*, 1170–1181. [CrossRef]

130. Schoenhals, J.E.; Cushman, T.R.; Barsoumian, H.B.; Li, A.; Cadena, A.P.; Niknam, S.; Younes, A.I.; Caetano, M.D.S.; Cortez, M.A.; Welsh, J.W. Anti-glucocorticoid-induced Tumor Necrosis Factor-Related Protein (GITR) Therapy Overcomes Radiation-Induced Treg Immunosuppression and Drives Abscopal Effects. *Front. Immunol.* **2018**, *9*, 2170. [CrossRef]

131. Zappasodi, R.; Sirard, C.; Li, Y.; Budhu, S.; Abu-Akeel, M.; Liu, C.; Yang, X.; Zhong, H.; Newman, W.; Qi, J.; et al. Rational design of anti-GITR-based combination immunotherapy. *Nat. Med.* **2019**, *25*, 759–766. [CrossRef]

132. Sabharwal, S.S.; Rosen, D.B.; Grein, J.; Tedesco, D.; Joyce-Shaikh, B.; Ueda, R.; Semana, M.; Bauer, M.; Bang, K.; Stevenson, C.; et al. GITR Agonism Enhances Cellular Metabolism to Support CD8⁺ T-cell Proliferation and Effector Cytokine Production in a Mouse Tumor Model. *Cancer Immunol. Res.* **2018**, *6*, 1199–1211. [CrossRef]

133. Marangoni, F.; Zhang, R.; Mani, V.; Thelen, M.; Ali Akbar, N.J.; Warner, R.D.; Äijö, T.; Zappulli, V.; Martinez, G.J.; Turka, L.A.; et al. Tumor Tolerance-Promoting Function of Regulatory T Cells Is Optimized by CD28, but Strictly Dependent on Calcineurin. *J. Immunol.* **2018**, *200*, 3647–3661. [CrossRef]

134. Zheng, C.; Zheng, L.; Yoo, J.-K.; Guo, H.; Zhang, Y.; Guo, X.; Kang, B.; Hu, R.; Huang, J.Y.; Zhang, Q.; et al. Landscape of Infiltrating T Cells in Liver Cancer Revealed by Single-Cell Sequencing. *Cell* **2017**, *169*, 1342–1356.e16. [CrossRef]

135. Simpson, T.R.; Li, F.; Montalvo-Ortiz, W.; Sepulveda, M.A.; Bergerhoff, K.; Arce, F.; Roddie, C.; Henry, J.Y.; Yagita, H.; Wolchok, J.D.; et al. Fc-dependent depletion of tumor-infiltrating regulatory T cells co-defines the efficacy of anti-CTLA-4 therapy against melanoma. *J. Exp. Med.* **2013**, *210*, 1695–1710. [CrossRef]

136. Zou, W.; Wolchok, J.D.; Chen, L. PD-L1 (B7-H1) and PD-1 pathway blockade for cancer therapy: Mechanisms, response biomarkers, and combinations. *Sci. Transl. Med.* **2016**, *8*, 328rv4. [CrossRef]

137. Kamada, T.; Togashi, Y.; Tay, C.; Ha, D.; Sasaki, A.; Nakamura, Y.; Sato, E.; Fukuoka, S.; Tada, Y.; Tanaka, A.; et al. PD-1⁺ regulatory T cells amplified by PD-1 blockade promote hyperprogression of cancer. *Proc. Natl. Acad. Sci. USA* **2019**, *116*, 9999–10008. [CrossRef]

138. Kamphorst, A.O.; Wieland, A.; Nasti, T.; Yang, S.; Zhang, R.; Barber, D.L.; Konieczny, B.T.; Daugherty, C.Z.; Koenig, L.; Yu, K.; et al. Rescue of exhausted CD8 T cells by PD-1-targeted therapies is CD28-dependent. *Science* **2017**, *355*, 1423–1427. [CrossRef]
139. Durham, N.M.; Nirschl, C.J.; Jackson, C.M.; Elias, J.; Kochel, C.M.; Anders, R.A.; Drake, C.G. Lymphocyte Activation Gene 3 (LAG-3) modulates the ability of CD4 T-cells to be suppressed in vivo. *PLoS ONE* **2014**, *9*, e109080. [CrossRef]

140. Du, W.; Yang, M.; Turner, A.; Xu, C.; Ferris, R.L.; Huang, J.; Kane, L.P.; Lu, B. TIM-3 as a Target for Cancer Immunotherapy and Mechanisms of Action. *Int. J. Mol. Sci.* **2017**, *18*, 645. [CrossRef]

141. Fuhrman, C.A.; Yeh, W.-I.; Seay, H.R.; Saikumar Lakshmi, P.; Chopra, G.; Zhang, L.; Perry, D.J.; McClymont, S.A.; Yadav, M.; Lopez, M.-C.; et al. Divergent Phenotypes of Human Regulatory T Cells Expressing the Receptors TIGIT and CD226. *J. Immunol.* **2015**, *195*, 145–155. [CrossRef]

142. Le Mercier, I.; Chen, W.; Lines, J.L.; Day, M.; Li, J.; Sergent, P.; Noelle, R.J.; Wang, L. VISTA Regulates the Development of Protective Antitumor Immunity. *Cancer Res.* **2014**, *74*, 1933–1944. [CrossRef]

143. Kondo, Y.; Ohno, T.; Nishii, N.; Harada, K.; Yagita, H.; Azuma, M. Differential contribution of three immune checkpoint (VISTA, CTLA-4, PD-1) pathways to antitumor responses against squamous cell carcinoma. *Oral Oncol.* **2016**, *57*, 54–60. [CrossRef]

144. Foss, F.M. DAB(389)IL-2 (denileukin diftitox, ONTAK): A new fusion protein technology. *Clin Lymphoma* **2000**, *1* (Suppl. 1), S27–S31. [CrossRef]

145. Cheung, L.S.; Fu, J.; Kumar, P.; Kumar, A.; Urbanowski, M.E.; Ihms, E.A.; Parveen, S.; Bullen, C.K.; Patrick, G.J.; Harrison, R.; et al. Second-generation IL-2 receptor-targeted diphtheria fusion toxin exhibits antitumor activity and synergy with anti-PD-1 in melanoma. *Proc. Natl. Acad. Sci. USA* **2019**, *116*, 3100–3105. [CrossRef]

146. Franco-Molina, M.A.; Miranda-Hernández, D.F.; Mendoza-Gamboa, E.; Zapata-Benavides, P.; Coronado-Cerda, E.E.; Sierra-Rivera, C.A.; Saavedra-Alonso, S.; Taméz-Guerra, R.S.; Rodríguez-Padilla, C. Silencing of Foxp3 delays the growth of murine melanomas and modifies the tumor immunosuppressive environment. *Onco Targets Ther.* **2016**, *9*, 243–253. [CrossRef]

147. Miguel, A.; Sendra, L.; Noé, V.; Ciudad, C.J.; Dasí, F.; Hervas, D.; Herrero, M.J.; Aliño, S.F. Silencing of Foxp3 enhances the antitumor efficacy of GM-CSF genetically modified tumor cell vaccine against B16 melanoma. *Onco Targets Ther.* **2017**, *10*, 503–514. [CrossRef]

148. Mousavi Niri, N.; Memarnejadian, A.; Pilehvar-Soltanahmadi, Y.; Agha Sadeghi, M.; Mahdavi, M.; Kheshtchin, N.; Arab, S.; Namdar, A.; Jadidi, F.; Zarghami, N.; et al. Improved Anti-Treg Vaccination Targeting Foxp3 Efficiently Decreases Regulatory T Cells in Mice. *J. Immunother.* **2016**, *39*, 269–275. [CrossRef]

149. Namdar, A.; Mirzaei, R.; Memarnejadian, A.; Boghosian, R.; Samadi, M.; Mirzaei, H.R.; Farajifard, H.; Zavar, M.; Azadmanesh, K.; Elahi, S.; et al. Prophylactic DNA vaccine targeting Foxp3⁺ regulatory T cells depletes myeloid-derived suppressor cells and improves anti-melanoma immune responses in a murine model. *Cancer Immunol. Immunother.* **2018**, *67*, 367–379. [CrossRef]

150. Ghiringhelli, F.; Larmonier, N.; Schmitt, E.; Parcellier, A.; Cathelin, D.; Garrido, C.; Chauffert, B.; Solary, E.; Bonnotte, B.; Martin, F. CD4⁺CD25⁺ regulatory T cells suppress tumor immunity but are sensitive to cyclophosphamide which allows immunotherapy of established tumors to be curative. *Eur. J. Immunol.* **2004**, *34*, 336–344. [CrossRef]

151. Ge, Y.; Domschke, C.; Stoiber, N.; Schott, S.; Heil, J.; Rom, J.; Blumenstein, M.; Thum, J.; Sohn, C.; Schneeweiss, A.; et al. Metronomic cyclophosphamide treatment in metastasized breast cancer patients: immunological effects and clinical outcome. *Cancer Immunol. Immunother.* **2012**, *61*, 353–362. [CrossRef]

152. Scurr, M.; Pembroke, T.; Bloom, A.; Roberts, D.; Thomson, A.; Smart, K.; Bridgeman, H.; Adams, R.; Brewster, A.; Jones, R.; et al. Low-Dose Cyclophosphamide Induces Antitumor T-Cell Responses, which Associate with Survival in Metastatic Colorectal Cancer. *Clin. Cancer Res.* **2017**, *23*, 6771–6780. [CrossRef]

153. Le, D.T.; Jaffee, E.M. Regulatory T-cell modulation using cyclophosphamide in vaccine approaches: A current perspective. *Cancer Res.* **2012**, *72*, 3439–3444. [CrossRef]

154. Ohkura, N.; Hamaguchi, M.; Morikawa, H.; Sugimura, K.; Tanaka, A.; Ito, Y.; Osaki, M.; Tanaka, Y.; Yamashita, R.; Nakano, N.; et al. T cell receptor stimulation-induced epigenetic changes and Foxp3 expression are independent and complementary events required for Treg cell development. *Immunity* **2012**, *37*, 785–799. [CrossRef]

155. Ali, K.; Soond, D.R.; Pineiro, R.; Hagemann, T.; Pearce, W.; Lim, E.L.; Bouabe, H.; Scudamore, C.L.; Hancox, T.; Maecker, H.; et al. Inactivation of PI(3)K p110δ breaks regulatory T-cell-mediated immune tolerance to cancer. *Nature* **2014**, *510*, 407–411. [CrossRef]
156. Ahmad, S.; Abu-Eid, R.; Shrimali, R.; Webb, M.; Verma, V.; Doroodchi, A.; Berrong, Z.; Samara, R.; Rodriguez, P.C.; Mkrtichyan, M.; et al. Differential PI3Kδ Signaling in CD4⁺ T-cell Subsets Enables Selective Targeting of T Regulatory Cells to Enhance Cancer Immunotherapy. *Cancer Res.* **2017**, *77*, 1892–1904. [CrossRef]

157. Li, M.O.; Flavell, R.A. TGF-beta: A master of all T cell trades. *Cell* **2008**, *134*, 392–404. [CrossRef]

158. Wei, X.; Zhang, J.; Gu, Q.; Huang, M.; Zhang, W.; Guo, J.; Zhou, X. Reciprocal Expression of IL-35 and IL-10 Defines Two Distinct Effector Treg Subsets that Are Required for Maintenance of Immune Tolerance. *Cell Rep.* **2017**, *21*, 1853–1869. [CrossRef]

159. Sawant, D.V.; Yano, H.; Chikina, M.; Zhang, Q.; Liao, M.; Liu, C.; Callahan, D.J.; Sun, Z.; Sun, T.; Tabib, T.; et al. Adaptive plasticity of IL-10⁺ and IL-35⁺ Treg cells cooperatively promotes tumor T cell exhaustion. *Nat. Immunol.* **2019**, *20*, 724–735. [CrossRef]

160. Arce Vargas, F.; Furness, A.J.S.; Solomon, I.; Joshi, K.; Mekkaoui, L.; Lesko, M.H.; Miranda Rota, E.; Dahan, R.; Georgiou, A.; Sledzinska, A.; et al. Fc-Optimized Anti-CD25 Depletes Tumor-Infiltrating Regulatory T Cells and Synergizes with PD-1 Blockade to Eradicate Established Tumors. *Immunity* **2017**, *46*, 577–586. [CrossRef]

161. Pittenger, M.F.; Mackay, A.M.; Beck, S.C.; Jaiswal, R.K.; Douglas, R.; Mosca, J.D.; Moorman, M.A.; Simonetti, D.W.; Craig, S.; Marshak, D.R. Multilineage potential of adult human mesenchymal stem cells. *Science* **1999**, *284*, 143–147. [CrossRef]

162. Cominal, J.G.; da Costa Cacemiro, M.; Pinto-Simões, B.; Kolb, H.-J.; Malmegrim, K.C.R.; de Castro, F.A. Emerging Role of Mesenchymal Stromal Cell-Derived Extracellular Vesicles in Pathogenesis of Haematological Malignancies. *Stem Cells Int.* **2019**, *2019*, 6854080. [CrossRef]

163. He, S.; Gleason, J.; Fik-Rymarkiewicz, E.; DiFiglia, A.; Bharathan, M.; Morschauser, A.; Djuretic, I.; Xu, Y.; Krakovsky, M.; Jankovic, V.; et al. Human Placenta-Derived Mesenchymal Stromal-Like Cells Enhance Angiogenesis via T Cell-Dependent Reprogramming of Macrophage Differentiation. *Stem Cells* **2017**, *35*, 1603–1613. [CrossRef]

164. Najar, M.; Raicevic, G.; Fayyad-Kazan, H.; Bron, D.; Toungouz, M.; Lagneaux, L. Mesenchymal stromal cells and immunomodulation: A gathering of regulatory immune cells. *Cytotherapy* **2016**, *18*, 160–171. [CrossRef]

165. Khakoo, A.Y.; Pati, S.; Anderson, S.A.; Reid, W.; Elshal, M.F.; Rovira, I.I.; Nguyen, A.T.; Malide, D.; Combs, C.A.; Hall, G.; et al. Human mesenchymal stem cells exert potent antitumorigenic effects in a model of Kaposi's sarcoma. *J. Exp. Med.* **2006**, *203*, 1235–1247. [CrossRef]

166. Qiao, L.; Xu, Z.-L.; Zhao, T.-J.; Ye, L.-H.; Zhang, X.-D. Dkk-1 secreted by mesenchymal stem cells inhibits growth of breast cancer cells via depression of Wnt signalling. *Cancer Lett.* **2008**, *269*, 67–77. [CrossRef]

167. Castells, M.; Thibault, B.; Mery, E.; Golzio, M.; Pasquet, M.; Hennebelle, I.; Bourin, P.; Mirshahi, M.; Delord, J.P.; Querleu, D.; et al. Ovarian ascites-derived Hospicells promote angiogenesis via activation of macrophages. *Cancer Lett.* **2012**, *326*, 59–68. [CrossRef]

168. Corre, J.; Hébraud, B.; Bourin, P. Concise review: Growth differentiation factor 15 in pathology: A clinical role? *Stem Cells Transl. Med.* **2013**, *2*, 946–952. [CrossRef]

169. McLean, K.; Tan, L.; Bolland, D.E.; Coffman, L.G.; Peterson, L.F.; Talpaz, M.; Neamati, N.; Buckanovich, R.J. Leukemia inhibitory factor functions in parallel with interleukin-6 to promote ovarian cancer growth. *Oncogene* **2019**, *38*, 1576–1584. [CrossRef]

170. Roccaro, A.M.; Sacco, A.; Maiso, P.; Azab, A.K.; Tai, Y.-T.; Reagan, M.; Azab, F.; Flores, L.M.; Campigotto, F.; Weller, E.; et al. BM mesenchymal stromal cell-derived exosomes facilitate multiple myeloma progression. *J. Clin. Invest.* **2013**, *123*, 1542–1555. [CrossRef]

171. Zhu, W.; Huang, L.; Li, Y.; Zhang, X.; Gu, J.; Yan, Y.; Xu, X.; Wang, M.; Qian, H.; Xu, W. Exosomes derived from human bone marrow mesenchymal stem cells promote tumor growth in vivo. *Cancer Lett.* **2012**, *315*, 28–37. [CrossRef]

172. Li, N.; Hua, J. Interactions between mesenchymal stem cells and the immune system. *Cell. Mol. Life Sci.* **2017**, *74*, 2345–2360. [CrossRef]

173. Castells, M.; Milhas, D.; Gandy, C.; Thibault, B.; Rafii, A.; Delord, J.-P.; Couderc, B. Microenvironment mesenchymal cells protect ovarian cancer cell lines from apoptosis by inhibiting XIAP inactivation. *Cell Death Dis.* **2013**, *4*, e887. [CrossRef]

174. Le Naour, A.; Couderc, B. Role of MSCs in antitumor drug resistance. In *Mesenchymal Stromal Cells as Tumor Stromal Modulators*; Bolontrade, M., García, M., Eds.; Academic Press: Cambridge, MA, USA, 2016; pp. 295–333.
175. Rafii, A.; Mirshahi, P.; Poupot, M.; Faussat, A.-M.; Simon, A.; Ducros, E.; Mery, E.; Couderc, B.; Lis, R.; Capdet, J.; et al. Oncologic trogocytosis of an original stromal cells induces chemoresistance of ovarian tumours. *PLoS ONE* **2008**, *3*, e3894. [CrossRef]

176. Madar, S.; Goldstein, I.; Rotter, V. 'Cancer associated fibroblasts'—more than meets the eye. *Trends Mol. Med.* **2013**, *19*, 447–453. [CrossRef]

177. Karnoub, A.E.; Dash, A.B.; Vo, A.P.; Sullivan, A.; Brooks, M.W.; Bell, G.W.; Richardson, A.L.; Polyak, K.; Tubo, R.; Weinberg, R.A. Mesenchymal stem cells within tumour stroma promote breast cancer metastasis. *Nature* **2007**, *449*, 557–563. [CrossRef]

178. Vangala, G.; Imhoff, F.M.; Squires, C.M.L.; Cridge, A.G.; Baird, S.K. Mesenchymal stem cell homing towards cancer cells is increased by enzyme activity of cathepsin D. *Exp. Cell Res.* **2019**, *383*, 111494. [CrossRef]

179. Gonzalez, M.E.; Martin, E.E.; Anwar, T.; Arellano-Garcia, C.; Medhora, N.; Lama, A.; Chen, Y.-C.; Tanager, K.S.; Yoon, E.; Kidwell, K.M.; et al. Mesenchymal Stem Cell-Induced DDR2 Mediates Stromal-Breast Cancer Interactions and Metastasis Growth. *Cell Rep.* **2017**, *18*, 1215–1228. [CrossRef]

180. Vishnubalaji, R.; Elango, R.; Al-Toub, M.; Manikandan, M.; Al-Rikabi, A.; Harkness, L.; Ditzel, N.; Atteya, M.; Hamam, R.; Alfayez, M.; et al. Neoplastic Transformation of Human Mesenchymal Stromal Cells Mediated via LIN28B. *Sci. Rep.* **2019**, *9*, 8101. [CrossRef]

181. Favreau, M.; Menu, E.; Gaublomme, D.; Vanderkerken, K.; Faict, S.; Maes, K.; De Bruyne, E.; Govindarajan, S.; Drennan, M.; Van Calenbergh, S.; et al. Leptin receptor antagonism of iNKT cell function: A novel strategy to combat multiple myeloma. *Leukemia* **2017**, *31*, 2678–2685. [CrossRef]

182. Mishra, P.J.; Banerjee, D. Activation and Differentiation of Mesenchymal Stem Cells. *Methods Mol. Biol.* **2017**, *1554*, 201–209.

183. Pasquier, J.; Gosset, M.; Geyl, C.; Hoarau-Véchot, J.; Chevrot, A.; Pocard, M.; Mirshahi, M.; Lis, R.; Rafii, A.; Touboul, C. CCL2/CCL5 secreted by the stroma induce IL-6/PYK2 dependent chemoresistance in ovarian cancer. *Mol. Cancer* **2018**, *17*, 47–51. [CrossRef]

184. Naour, A.L.; Prat, M.; Thibault, B.; Mével, R.; Lemaitre, L.; Leray, H.; Joubert, M.-V.; Coulson, K.; Golzio, M.; Lefevre, L.; et al. Tumor cells educate mesenchymal stromal cells to release chemoprotective and immunomodulatory factors. *J. Mol. Cell Biol.* **2019**. [CrossRef]

185. Thibault, B.; Genre, L.; Le Naour, A.; Broca, C.; Mery, E.; Vuagniaux, G.; Delord, J.P.; Wiedemann, N.; Couderc, B. DEBIO 1143, an IAP inhibitor, reverses carboplatin resistance in ovarian cancer cells and triggers apoptotic or necroptotic cell death. *Sci. Rep.* **2018**, *8*, 17862. [CrossRef]

186. DiPersio, J.F.; Erba, H.P.; Larson, R.A.; Luger, S.M.; Tallman, M.S.; Brill, J.M.; Vuagniaux, G.; Rouits, E.; Sorensen, J.M.; Zanna, C. Oral Debio1143 (AT406), an antagonist of inhibitor of apoptosis proteins, combined with daunorubicin and cytarabine in patients with poor-risk acute myeloid leukemia–results of a phase I dose-escalation study. *Clin. Lymphoma Myeloma Leuk* **2015**, *15*, 443–449. [CrossRef]

187. Melzer, C.; von der Ohe, J.; Hass, R. Enhanced metastatic capacity of breast cancer cells after interaction and hybrid formation with mesenchymal stroma/stem cells (MSC). *Cell Commun. Signal* **2018**, *16*, 2. [CrossRef]

188. Alfaro, D.; Zapata, A.G. Eph/Ephrin-mediated stimulation of human bone marrow mesenchymal stromal cells correlates with changes in cell adherence and increased cell death. *Stem Cell Res. Ther.* **2018**, *9*, 172. [CrossRef]

189. Ullah, M.; Akbar, A.; Ng, N.N.; Concepcion, W.; Thakor, A.S. Mesenchymal stem cells confer chemoresistance in breast cancer via a CD9 dependent mechanism. *Oncotarget* **2019**, *10*, 3435–3450. [CrossRef]

190. Studeny, M.; Marini, F.C.; Dembinski, J.L.; Zompetta, C.; Cabreira-Hansen, M.; Bekele, B.N.; Champlin, R.E.; Andreeff, M. Mesenchymal stem cells: Potential precursors for tumor stroma and targeted-delivery vehicles for anticancer agents. *J. Natl. Cancer Inst.* **2004**, *96*, 1593–1603. [CrossRef]

191. Yoon, A.-R.; Hong, J.; Li, Y.; Shin, H.C.; Lee, H.; Kim, H.S.; Yun, C.-O. Mesenchymal stem cell-mediated delivery of an oncolytic adenovirus enhances antitumor efficacy in hepatocellular carcinoma. *Cancer Res.* **2019**, *79*, 4503–4514. [CrossRef]

192. Yin, Z.; Jiang, K.; Li, R.; Dong, C.; Wang, L. Multipotent mesenchymal stromal cells play critical roles in hepatocellular carcinoma initiation, progression and therapy. *Mol. Cancer* **2018**, *17*, 178. [CrossRef]

193. de Araujo Farias, V.; O'Valle, F.; Serrano-Saenz, S.; Anderson, P.; Andrés, E.; López-Peñalver, J.; Tovar, I.; Nieto, A.; Santos, A.; Martín, F.; et al. Exosomes derived from mesenchymal stem cells enhance radiotherapy-induced cell death in tumor and metastatic tumor foci. *Mol. Cancer* **2018**, *17*, 122. [CrossRef]
194. Xie, C.; Du, L.-Y.; Guo, F.; Li, X.; Cheng, B. Exosomes derived from microRNA-101-3p-overexpressing human bone marrow mesenchymal stem cells suppress oral cancer cell proliferation, invasion, and migration. *Mol. Cell. Biochem.* **2019**, *458*, 11–26. [CrossRef]

195. Melzer, C.; Rehn, V.; Yang, Y.; Bähre, H.; von der Ohe, J.; Hass, R. Taxol-Loaded MSC-Derived Exosomes Provide a Therapeutic Vehicle to Target Metastatic Breast Cancer and Other Carcinoma Cells. *Cancers* (Basel) **2019**, *11*, 798. [CrossRef]

196. Parsonage, G.; Filer, A.D.; Haworth, O.; Nash, G.B.; Rainger, G.E.; Salmon, M.; Buckley, C.D. A stromal address code defined by fibroblasts. *Trends Immunol.* **2005**, *26*, 150–156. [CrossRef]

197. Kikuchi, N.; Horiuchi, A.; Osada, R.; Imai, T.; Wang, C.; Chen, X.; Konishi, I. Nuclear expression of S100A4 is associated with aggressive behavior of epithelial ovarian carcinoma: An important autocrine/paracrine factor in tumor progression. *Cancer Sci.* **2006**, *97*, 1061–1069. [CrossRef]

198. Rinn, J.L.; Bondre, C.; Gladstone, H.B.; Brown, P.O.; Chang, H.Y. Anatomic demarcation by positional variation in fibroblast gene expression programs. *PLoS Genet.* **2006**, *2*, e119. [CrossRef]

199. Rinn, J.L.; Kertesz, M.; Wang, J.K.; Squazzo, S.L.; Xu, X.; Brugmann, S.A.; Goodnough, L.H.; Helms, J.A.; Farnham, P.J.; Segal, E.; et al. Functional demarcation of active and silent chromatin domains in human HOX loci by noncoding RNAs. *Cell* **2007**, *129*, 1311–1323. [CrossRef]

200. Frantz, C.; Stewart, K.M.; Weaver, V.M. The extracellular matrix at a glance. *J. Cell Sci.* **2010**, *123*, 4195–4200. [CrossRef]

201. Marsh, T.; Pietras, K.; McAllister, S.S. Fibroblasts as architects of cancer pathogenesis. *Biochim. Biophys. Acta* **2013**, *1832*, 1070–1078. [CrossRef]

202. Bainbridge, P. Wound healing and the role of fibroblasts. *J. Wound Care* **2013**, *22*, 407–408.

203. Duffield, J.S.; Lupher, M.; Thannickal, V.J.; Wynn, T.A. Host responses in tissue repair and fibrosis. *Annu. Rev. Pathol.* **2013**, *8*, 241–276. [CrossRef]

204. Bechtel, W.; McGoohan, S.; Zeisberg, E.M.; Muller, G.A.; Kalbacher, H.; Salant, D.J.; Muller, C.A.; Kalluri, R.; Zeisberg, M. Methylation determines fibroblast activation and fibrogenesis in the kidney. *Nat. Med.* **2010**, *16*, 544–550. [CrossRef]

205. Procopio, M.G.; Laszlo, C.; Al Labban, D.; Kim, D.E.; Bordignon, P.; Jo, S.H.; Goruppi, S.; Menietti, E.; Ostano, P.; Ala, U.; et al. Combined CSL and p53 downregulation promotes cancer-associated fibroblast activation. *Nat. Cell Biol.* **2015**, *17*, 1193–1204. [CrossRef]

206. Albrengues, J.; Bourget, I.; Pons, C.; Butet, V.; Hofman, P.; Tartare-Deckert, S.; Feral, C.C.; Meneguzzi, G.; Gaggioli, C. LIF mediates proinvasive activation of stromal fibroblasts in cancer. *Cell Rep.* **2014**, *7*, 1664–1678. [CrossRef]

207. LeBleu, V.S.; Kalluri, R. A peek into cancer-associated fibroblasts: Origins, functions and translational impact. *Dis. Model Mech.* **2018**, *11*. [CrossRef]

208. Raz, Y.; Cohen, N.; Shani, O.; Bell, R.E.; Novitskiy, S.V.; Abramovitz, L.; Levy, C.; Milyavsky, M.; Leider-Trejo, L.; Moses, H.L.; et al. Bone marrow-derived fibroblasts are a functionally distinct stromal cell population in breast cancer. *J. Exp. Med.* **2018**, *215*, 3075–3093. [CrossRef]

209. Walter, K.; Omura, N.; Hong, S.M.; Griffith, M.; Goggins, M. Pancreatic cancer associated fibroblasts display normal allelotypes. *Cancer Biol. Ther.* **2008**, *7*, 882–888. [CrossRef]

210. Klein, G. Toward a genetics of cancer resistance. *Proc. Natl. Acad. Sci. USA* **2009**, *106*, 859–863. [CrossRef]

211. Ferdek, P.E.; Jakubowska, M.A. Biology of pancreatic stellate cells-more than just pancreatic cancer. *Pflug. Arch.* **2017**, *469*, 1039–1050. [CrossRef]

212. Lynch, M.D.; Watt, F.M. Fibroblast heterogeneity: Implications for human disease. *J. Clin. Invest.* **2018**, *128*, 26–35. [CrossRef]

213. Costa, A.; Kieffer, Y.; Scholer-Dahirel, A.; Pelon, F.; Bourachot, B.; Cardon, M.; Sirven, P.; Magagna, I.; Fuhrmann, L.; Bernard, C.; et al. Fibroblast Heterogeneity and Immunosuppressive Environment in Human Breast Cancer. *Cancer Cell* **2018**, *33*, 463–479. [CrossRef]

214. Neuzillet, C.; Tijeras-Raballand, A.; Ragulan, C.; Cros, J.; Patil, Y.; Martinet, M.; Erkan, M.; Kleeff, J.; Wilson, J.; Apte, M.; et al. Inter- and intra-tumoural heterogeneity in cancer-associated fibroblasts of human pancreatic ductal adenocarcinoma. *J. Pathol.* **2019**, *248*, 51–65. [CrossRef]

215. Farmer, P.; Bonnefoi, H.; Anderle, P.; Cameron, D.; Wirapati, P.; Becette, V.; Andre, S.; Piccart, M.; Campone, M.; Brain, E.; et al. A stroma-related gene signature predicts resistance to neoadjuvant chemotherapy in breast cancer. *Nat. Med.* **2009**, *15*, 68–74. [CrossRef]
216. Ohlund, D.; Handly-Santana, A.; Biffi, G.; Elyada, E.; Almeida, A.S.; Ponz-Sarvise, M.; Corbo, V.; Oni, T.E.; Hearn, S.A.; Lee, E.J.; et al. Distinct populations of inflammatory fibroblasts and myofibroblasts in pancreatic cancer. *J. Exp. Med.* **2017**, *214*, 579–596.

217. Biffi, G.; Tuveson, D.A. A FATal Combination: Fibroblast-Derived Lipids and Cancer-Derived Autotaxin Promote Pancreatic Cancer Growth. *Cancer Discov.* **2019**, *9*, 578–580. [CrossRef]

218. Bartoschek, M.; Oskolkov, N.; Bocci, M.; Lovrot, J.; Larsson, C.; Sommarin, M.; Madsen, C.D.; Lindgren, D.; Pekar, G.; Karlsson, G.; et al. Spatially and functionally distinct subclasses of breast cancer-associated fibroblasts revealed by single cell RNA sequencing. *Nat. Commun.* **2018**, *9*, 5150. [CrossRef]

219. Elyada, E.; Bolisetty, M.; Laise, P.; Flynn, W.F.; Courtois, E.T.; Burkhart, R.A.; Teinor, J.A.; Belleau, P.; Biffi, G.; Lucito, M.S.; et al. Cross-Species Single-Cell Analysis of Pancreatic Ductal Adenocarcinoma Reveals Antigen-Presenting Cancer-Associated Fibroblasts. *Cancer Discov.* **2019**, *9*, 1102–1123. [CrossRef]

220. Sugimoto, H.; Mundel, T.M.; Kieran, M.W.; Kalluri, R. Identification of fibroblast heterogeneity in the tumor microenvironment. *Cancer Biol. Ther.* **2006**, *5*, 1640–1646. [CrossRef]

221. Kalluri, R. The biology and function of fibroblasts in cancer. *Nat. Rev. Cancer* **2016**, *16*, 582–598. [CrossRef]

222. De Wever, O.; Nguyen, Q.D.; Van Hoorde, L.; Bracke, M.; Bruyneel, E.; Gespach, C.; Mareel, M. Tenascin-C and SF/HGF produced by myofibroblasts in vitro provide convergent pro-invasive signals to human colon cancer cells through RhoA and Rac. *FASEB J.* **2004**, *18*, 1016–1018. [CrossRef] [PubMed]

223. Orimo, A.; Gupta, P.B.; Sgroi, D.C.; Arenzana-Seisdedos, F.; Delaunay, T.; Naeem, R.; Carey, V.J.; Richardson, A.L.; Weinberg, R.A. Stromal fibroblasts present in invasive human breast carcinomas promote tumor growth and angiogenesis through elevated SDF-1/CXCL12 secretion. *Cell* **2005**, *121*, 335–348. [CrossRef] [PubMed]

224. Erez, N.; Glanz, S.; Raz, Y.; Avivi, C.; Barshack, I. Cancer associated fibroblasts express pro-inflammatory factors in human breast and ovarian tumors. *Biochem. Biophys. Res. Commun.* **2013**, *437*, 397–402. [CrossRef] [PubMed]

225. Calon, A.; Tauriello, D.V.; Batlle, E. TGF-beta in CAF-mediated tumor growth and metastasis. *Semin. Cancer Biol.* **2014**, *25*, 15–22. [CrossRef] [PubMed]

226. Underwood, T.J.; Hayden, A.L.; Derouet, M.; Garcia, E.; Noble, F.; White, M.J.; Thirdborough, S.; Mead, A.; Clemons, N.; Mellone, M.; et al. Cancer-associated fibroblasts predict poor outcome and promote periostin-dependent invasion in oesophageal adenocarcinoma. *J. Pathol.* **2015**, *235*, 466–477. [CrossRef] [PubMed]

227. Subramaniam, K.S.; Omar, I.S.; Kwong, S.C.; Mohamed, Z.; Woo, Y.L.; Mat Adenan, N.A.; Chung, I. Cancer-associated fibroblasts promote endometrial cancer growth via activation of interleukin-6/STAT-3/c-Myc pathway. *Am. J. Cancer Res.* **2016**, *6*, 200–213. [PubMed]

228. Gascard, P.; Tlsty, T.D. Carcinoma-associated fibroblasts: Orchestrating the composition of malignancy. *Genes Dev.* **2016**, *30*, 1002–1019. [CrossRef]

229. Duluc, C.; Moatassim-Billah, S.; Chalabi-Dchar, M.; Perraud, A.; Samain, R.; Breibach, F.; Gayral, M.; Cordelier, P.; Delisle, M.B.; Bousquet-Dubouch, M.P.; et al. Pharmacological targeting of the protein synthesis mTOR/4E-BP1 pathway in cancer-associated fibroblasts abrogates pancreatic tumour chemoresistance. *EMBO Mol. Med.* **2015**, *7*, 735–753. [CrossRef]

230. Moatassim-Billah, S.; Duluc, C.; Samain, R.; Jean, C.; Perraud, A.; Decaup, E.; Cassant-Sourdy, S.; Bakri, Y.; Selves, J.; Schmid, H.; et al. Anti-metastatic potential of somatostatin analog SOM230: Indirect pharmacological targeting of pancreatic cancer-associated fibroblasts. *Oncotarget* **2016**, *7*, 41584–41598. [CrossRef]

231. Auciello, F.R.; Bulusu, V.; Oon, C.; Tait-Mulder, J.; Berry, M.; Bhattacharyya, S.; Tumanov, S.; Allen-Petersen, B.L.; Link, J.; Kendsersky, N.D.; et al. A Stromal Lysolipid-Autotaxin Signaling Axis Promotes Pancreatic Tumor Progression. *Cancer Discov.* **2019**, *9*, 617–627. [CrossRef]

232. Yang, L.; Achreja, A.; Yeung, T.L.; Mangala, L.S.; Jiang, D.; Han, C.; Baddour, J.; Marini, J.C.; Ni, J.; Nakahara, R.; et al. Targeting Stromal Glutamine Synthetase in Tumors Disrupts Tumor Microenvironment-Regulated Cancer Cell Growth. *Cell Metab.* **2016**, *24*, 685–700. [CrossRef] [PubMed]

233. Bonnans, C.; Chou, J.; Werb, Z. Remodelling the extracellular matrix in development and disease. *Nat. Rev. Mol. Cell Biol.* **2014**, *15*, 786–801. [CrossRef] [PubMed]

234. Kai, F.; Laklai, H.; Weaver, V.M. Force Matters: Biomechanical Regulation of Cell Invasion and Migration in Disease. *Trends Cell Biol.* **2016**, *26*, 486–497. [CrossRef] [PubMed]
235. Labernadie, A.; Kato, T.; Brugues, A.; Serra-Picamal, X.; Derzsi, S.; Arwert, E.; Weston, A.; Gonzalez-Tarrago, V.; Elosegui-Artola, A.; Albertazzi, L.; et al. A mechanically active heterotypic E-cadherin/N-cadherin adhesion enables fibroblasts to drive cancer cell invasion. *Nat. Cell Biol.* **2017**, *19*, 224–237. [CrossRef] [PubMed]

236. Wortzel, I.; Dror, S.; Kenific, C.M.; Lyden, D. Exosome-Mediated Metastasis: Communication from a Distance. *Dev. Cell* **2019**, *49*, 347–360. [CrossRef] [PubMed]

237. Maris, P.; Blomme, A.; Palacios, A.P.; Costanza, B.; Bellahcene, A.; Bianchi, E.; Gofflot, S.; Drion, P.; Trombino, G.E.; Di Valentin, E.; et al. Asporin Is a Fibroblast-Derived TGF-beta1 Inhibitor and a Tumor Suppressor Associated with Good Prognosis in Breast Cancer. *PLoS Med.* **2015**, *12*, e1001871. [CrossRef] [PubMed]

238. Chang, P.H.; Hwang-Verslues, W.W.; Chang, Y.C.; Chen, C.C.; Hsiao, M.; Jeng, Y.M.; Chang, K.J.; Lee, E.Y.; Shew, J.Y.; Lee, W.H. Activation of Robo1 signaling of breast cancer cells by Slit2 from stromal fibroblast restrains tumorigenesis via blocking PI3K/Akt/beta-catenin pathway. *Cancer Res.* **2012**, *72*, 4652–4661. [CrossRef]

239. Hill, R.; Song, Y.; Cardiff, R.D.; Van Dyke, T. Selective evolution of stromal mesenchyme with p53 loss in response to epithelial tumorigenesis. *Cell* **2005**, *123*, 1001–1011. [CrossRef]

240. Catenacci, D.V.; Junttila, M.R.; Karrison, T.; Bahary, N.; Horiba, M.N.; Nattam, S.R.; Marsh, R.; Wallace, J.; Kozloff, M.; Rajdev, L.; et al. Randomized Phase Ib/II Study of Gemcitabine Plus Placebo or Vismodegib, a Hedgehog Pathway Inhibitor, in Patients With Metastatic Pancreatic Cancer. *J. Clin. Oncol.* **2015**, *33*, 4284–4292. [CrossRef]

241. Lo, A.; Li, C.P.; Buza, E.L.; Blomberg, R.; Govindaraju, P.; Avery, D.; Monslow, J.; Hsiao, M.; Pure, E. Fibroblast activation protein augments progression and metastasis of pancreatic ductal adenocarcinoma. *JCI Insight* **2017**, *2*, 92232. [CrossRef]

242. Ozdemir, B.C.; Pentcheva-Hoang, T.; Carstens, J.L.; Zheng, X.; Wu, C.C.; Simpson, T.R.; Laklai, H.; Sugimoto, H.; Kahlert, C.; Novitskiy, S.V.; et al. Depletion of carcinoma-associated fibroblasts and fibrosis induces immunosuppression and accelerates pancreas cancer with reduced survival. *Cancer Cell* **2014**, *25*, 719–734. [CrossRef] [PubMed]

243. Hofheinz, R.D.; Weisser, A.; Willer, A.; Hehlmann, R.; Hochhaus, A. Treatment of a patient with advanced esophageal cancer with a combination of mitomycin C and capecitabine: Activation of the thymidine phosphorylase as active principle? *Onkologie* **2003**, *26*, 161–164. [CrossRef] [PubMed]

244. Narra, K.; Mullins, S.R.; Lee, H.O.; Strzemkowski-Brun, B.; Magalong, K.; Christiansen, V.J.; McKee, P.A.; Egleston, B.; Cohen, S.J.; Weiner, L.M.; et al. Phase II trial of single agent Val-boroPro (Talabostat) inhibiting Fibroblast Activation Protein in patients with metastatic colorectal cancer. *Cancer Biol. Ther.* **2007**, *6*, 1691–1699. [CrossRef] [PubMed]

245. Su, S.; Chen, J.; Yao, H.; Liu, J.; Yu, S.; Lao, L.; Wang, M.; Luo, M.; Xing, Y.; Chen, F.; et al. CD10<sup>+</sup>GPR77<sup>+</sup> Cancer-Associated Fibroblasts Promote Cancer Formation and Chemoresistance by Sustaining Cancer Stemness. *Cell* **2018**, *172*, 841–856. [CrossRef] [PubMed]

246. Pant, S.; Saleh, M.; Bendell, J.; Infante, J.R.; Jones, S.; Kurkjian, C.D.; Moore, K.M.; Kazakin, J.; Abbadessa, G.; Wang, Y.; et al. A phase I dose escalation study of oral c-MET inhibitor tivantinib (ARQ 197) in combination with gemcitabine in patients with solid tumors. *Ann. Oncol.* **2014**, *25*, 1416–1421. [CrossRef] [PubMed]

247. Parikh, P.K.; Ghate, M.D. Recent advances in the discovery of small molecule c-Met Kinase inhibitors. *Eur. J. Med. Chem.* **2018**, *143*, 1103–1138. [CrossRef] [PubMed]

248. Sherman, M.H.; Yu, R.T.; Engle, D.D.; Ding, N.; Atkins, A.R.; Tiriac, H.; Collisson, E.A.; Connor, F.; Van Dyke, T.; Kozlov, S.; et al. Vitamin D receptor-mediated stromal reprogramming suppresses pancreatitis and enhances pancreatic cancer therapy. *Cell* **2014**, *159*, 80–93. [CrossRef] [PubMed]

249. Schnittert, J.; Heinrich, M.A.; Kuninty, P.R.; Storm, G.; Prakash, J. Reprogramming tumor stroma using an endogenous lipid lipoxin A4 to treat pancreatic cancer. *Cancer Lett.* **2018**, *420*, 247–258. [CrossRef]

250. Brennen, W.N.; Rosen, D.M.; Wang, H.; Isaacs, J.T.; Denmeade, S.R. Targeting carcinoma-associated fibroblasts within the tumor stroma with a fibroblast activation protein-activated prodrug. *J. Natl. Cancer Inst.* **2012**, *104*, 1320–1334. [CrossRef]

251. Gao, W.; Xiang, B.; Meng, T.T.; Liu, F.; Qi, X.R. Chemotherapeutic drug delivery to cancer cells using a combination of folate targeting and tumor microenvironment-sensitive polypeptides. *Biomaterials* **2013**, *34*, 4137–4149. [CrossRef]
252. De Vlieghere, E.; Gremonprez, F.; Verset, L.; Marien, L.; Jones, C.J.; De Craene, B.; Berx, G.; Descamps, B.; Vanhove, C.; Remon, J.P.; et al. Tumor-environment biomimetics delay peritoneal metastasis formation by deceiving and redirecting disseminated cancer cells. *Biomaterials* **2015**, *54*, 148–157. [CrossRef] [PubMed]

253. Cortez, V.S.; Ulland, T.K.; Cervantes-Barragan, L.; Bando, J.K.; Robinette, M.L.; Wang, Q.; White, A.J.; Gilfillan, S.; Cella, M.; Colonna, M. SMAD4 impedes the conversion of NK cells into ILC1-like cells by curtailing non-canonical TGF-β signaling. *Nat. Immunol.* **2017**, *18*, 995–1003. [CrossRef] [PubMed]

<figure><img src="image_7.png" /></figure>

© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).