Compensation between CSF1R+ macrophages and Foxp3+ Treg cells drives resistance to tumor immunotherapy

Defining mesenchymal stem/stromal cell-induced myeloid-derived suppressor cells using single-cell transcriptomics

Mesenchymal stem/stromal cells (MSCs) modulate the immune response through interactions with innate immune cells. We previously demonstrated that MSCs alleviate ocular autoimmune inflammation by directing bone marrow cell differentiation from pro-inflammatory CD11bhiLy6ChiLy6Glo cells into immunosuppressive CD11bmidLy6CmidLy6Glo cells. Herein, we analyzed MSC-induced CD11bmidLy6Cmid cells using single-cell RNA sequencing and compared them with CD11bhiLy6Chi cells. Our investigation revealed seven distinct immune cell types including myeloid-derived suppressor cells (MDSCs) in the CD11bmidLy6Cmid cells, while CD11bhiLy6Chi cells included mostly monocytes/macrophages with a small cluster of neutrophils. These MSC-induced MDSCs highly expressed Retnlg, Cxcl3, Cxcl2, Mmp8, Cd14, and Csf1r as well as Arg1. Comparative analyses of CSF-1RhiCD11bmidLy6Cmid and CSF-1RloCD11bmidLy6Cmid cells demonstrated that the former had a homogeneous monocyte morphology and produced elevated levels of interleukin-10. Functionally, these CSF-1RhiCD11bmidLy6Cmid cells, compared with the CSF-1RloCD11bmidLy6Cmid cells, inhibited CD4+ T cell proliferation and promoted CD4+CD25+Foxp3+ Treg expansion in culture and in a mouse model of experimental autoimmune uveoretinitis. Resistin-like molecule (RELM)-γ encoded by Retnlg, one of the highly upregulated genes in MSC-induced MDSCs, had no direct effects on T cell proliferation, Treg expansion, or splenocyte activation. Together, our study revealed a distinct transcriptional profile of MSC-induced MDSCs and identified CSF-1R as a key cell-surface marker for detection and therapeutic enrichment of MDSCs.

The ins and outs of microglial cells in brain health and disease

Microglia are the brain's resident macrophages that play pivotal roles in immune surveillance and maintaining homeostasis of the Central Nervous System (CNS). Microglia are functionally implicated in various cerebrovascular diseases, including stroke, aneurysm, and tumorigenesis as they regulate neuroinflammatory responses and tissue repair processes. Here, we review the manifold functions of microglia in the brain under physiological and pathological conditions, primarily focusing on the implication of microglia in glioma propagation and progression. We further review the current status of therapies targeting microglial cells, including their re-education, depletion, and re-population approaches as therapeutic options to improve patient outcomes for various neurological and neuroinflammatory disorders, including cancer.

CSF1R inhibition reprograms tumor-associated macrophages to potentiate anti-PD-1 therapy efficacy against colorectal cancer

PD-1 blockade therapy has made great breakthroughs in treatment of multiple solid tumors. However, patients with microsatellite-stable (MSS) colorectal cancer (CRC) respond poorly to anti-PD-1 immunotherapy. Although CRC patients with microstatellite instability (MSI) or microsatellite instability-high (MSI-H) can benefit from PD-1 blockade therapy, there are still some problems such as tumor recurrence. Tumor-associated macrophages (TAMs), most abundant immune components in tumor microenvironment (TME), largely limit the therapeutic efficacy of anti-PD-1 against CRC. The CSF1/CSF1R pathway plays a key role in regulating macrophage polarization, and blocking CSF1R signaling transduction may be a potential strategy to effectively reprogram macrophages and remodel TME. Here, we found that increasing expression of CSF1R in macrophages predicted poor prognosis in CRC cohort. Furthermore, we discovered a novel potent CSF1R inhibitor, PXB17, which significantly reprogramed M2 macrophages to M1 phenotype. Mechanically, PXB17 significantly blocked activation of PI3K/AKT/mTORC1 signaling, resulting in inhibition of cholesterol biosynthesis. Results from 3D co-culture system suggested that PXB17-repolarized macrophages could induce infiltration of CD8+ T lymphocytes in tumors and improve the immunosuppressive microenvironment. In vivo, PXB17 significantly halted CRC growth, with a stronger effect than PLX3397. In particular, PXB17 potently enhanced therapeutic activity of PD-1 mAb in CT-26 (MSS) model and prevented tumor recurrence in MC-38 (MSI-H) model by promoting formation of long-term memory immunity. Our study opens a new avenue for CSF1R in tumor innate and adaptive anti-tumor immunomodulatory activity and suggests that PXB17 is a promising immunotherapy molecule for enhancing the efficacy of PD-1 mAb or reducing tumor recurrence of CRC.

ICOS costimulation in combination with CTLA-4 blockade remodels tumor-associated macrophages toward an antitumor phenotype

We have previously demonstrated synergy between ICOS costimulation (IVAX; ICOSL-transduced B16-F10 cellular vaccine) and CTLA-4 blockade in antitumor therapy. In this study, we employed CyTOF and single-cell RNA sequencing and observed significant remodeling of the lymphoid and myeloid compartments in combination therapy. Compared with anti-CTLA-4 monotherapy, the combination therapy enriched Th1 CD4 T cells, effector CD8 T cells, and M1-like antitumor proinflammatory macrophages. These macrophages were critical to the therapeutic efficacy of anti-CTLA-4 combined with IVAX or anti-PD-1. Macrophage depletion with clodronate reduced the tumor-infiltrating effector CD4 and CD8 T cells, impairing their antitumor functions. Furthermore, the recruitment and polarization of M1-like macrophages required IFN-γ. Therefore, in this study, we show that there is a positive feedback loop between intratumoral effector T cells and tumor-associated macrophages (TAMs), in which the IFN-γ produced by the T cells polarizes the TAMs into M1-like phenotype, and the TAMs, in turn, reshape the tumor microenvironment to facilitate T cell infiltration, immune function, and tumor rejection.

Knockout cancer by nano-delivered immunotherapy using perfusion-aided scaffold-based tumor-on-a-chip

Cancer is a multifactorial disease produced by mutations in the oncogenes and tumor suppressor genes, which result in uncontrolled cell proliferation and resistance to cell death. Cancer progresses due to the escape of altered cells from immune monitoring, which is facilitated by the tumor's mutual interaction with its microenvironment. Understanding the mechanisms involved in immune surveillance evasion and the significance of the tumor microenvironment might thus aid in developing improved therapies. Although in vivo models are commonly utilized, they could be better for time, cost, and ethical concerns. As a result, it is critical to replicate an in vivo model and recreate the cellular and tissue-level functionalities. A 3D cell culture, which gives a 3D architecture similar to that found in vivo, is an appropriate model. Furthermore, numerous cell types can be cocultured, establishing cellular interactions between TME and tumor cells. Moreover, microfluidics perfusion can provide precision flow rates, thus simulating tissue/organ function. Immunotherapy can be used with the perfused 3D cell culture technique to help develop successful therapeutics. Immunotherapy employing nano delivery can target the spot and silence the responsible genes, ensuring treatment effectiveness while minimizing adverse effects. This study focuses on the importance of 3D cell culture in understanding the pathophysiology of 3D tumors and TME, the function of TME in drug resistance, tumor progression, and the development of advanced anticancer therapies for high-throughput drug screening.

De novo steroidogenesis in tumor cells drives bone metastasis and osteoclastogenesis

Osteoclasts play a central role in cancer-cell-induced osteolysis, but the molecular mechanisms of osteoclast activation during bone metastasis formation are incompletely understood. By performing RNA sequencing on a mouse breast carcinoma cell line with higher bone-metastatic potential, here we identify the enzyme CYP11A1 strongly upregulated in osteotropic tumor cells. Genetic deletion of Cyp11a1 in tumor cells leads to a decreased number of bone metastases but does not alter primary tumor growth and lung metastasis formation in mice. The product of CYP11A1 activity, pregnenolone, increases the number and function of mouse and human osteoclasts in vitro but does not alter osteoclast-specific gene expression. Instead, tumor-derived pregnenolone strongly enhances the fusion of pre-osteoclasts via prolyl 4-hydroxylase subunit beta (P4HB), identified as a potential interaction partner of pregnenolone. Taken together, our results demonstrate that Cyp11a1-expressing tumor cells produce pregnenolone, which is capable of promoting bone metastasis formation and osteoclast development via P4HB.

Advanced insights on tumor-associated macrophages revealed by single-cell RNA sequencing: The intratumor heterogeneity, functional phenotypes, and cellular interactions

Single-cell RNA sequencing (scRNA-seq) is an emerging technology used for cellular transcriptome analysis. The application of scRNA-seq has led to profoundly advanced oncology research, continuously optimizing novel therapeutic strategies. Intratumor heterogeneity extensively consists of all tumor components, contributing to different tumor behaviors and treatment responses. Tumor-associated macrophages (TAMs), the core immune cells linking innate and adaptive immunity, play significant roles in tumor progression and resistance to therapies. Moreover, dynamic changes occur in TAM phenotypes and functions subject to the regulation of the tumor microenvironment. The heterogeneity of TAMs corresponding to the state of the tumor microenvironment has been comprehensively recognized using scRNA-seq. Herein, we reviewed recent research and summarized variations in TAM phenotypes and functions from a developmental perspective to better understand the significance of TAMs in the tumor microenvironment.

FOXP3-regulated lncRNA NONHSAT136151 promotes colorectal cancer progression by disrupting QKI interaction with target mRNAs

The role of lncRNAs in the pathogenesis of cancer, including colorectal cancer (CRC), has repeatedly been demonstrated. However, very few lncRNAs have been well annotated functionally. Our study identified a novel lncRNA upregulated in CRC, NONHSAT136151, which was correlated with clinical progression. In functional assays, NONHSAT136151 significantly enhanced CRC cell proliferation, migration and invasion. Mechanistically, NONHSAT136151 interacted with RNA-binding protein (RBP) QKI (Quaking) to interfere with QKI binding to target mRNAs and regulate their expression. As well, FOXP3 may be causally related to the dysregulation of NONHSAT136151 in CRC cells through its transcriptional activity. In conclusion, our findings identified a novel lncRNA regulated by FOXP3 participates in CRC progression through interacting with QKI, indicating a novel lncRNA-RBP interaction mechanism is involved in CRC pathogenesis.

Targeting Myeloid-Derived Suppressor Cell Trafficking as a Novel Immunotherapeutic Approach in Microsatellite Stable Colorectal Cancer

Myeloid-derived suppressor cells (MDSCs) are a unique subset of immune cells that promote an immunosuppressive phenotype due to their impacts on CD8 and regulatory T cell function. The inhibition of MDSC trafficking to the tumor microenvironment (TME) may represent a novel target in microsatellite stable (MSS) colorectal cancer with the potential to reprogram the immune system. Here, we review the rationale of inhibiting myeloid suppressor cell trafficking in treatment-refractory MSS colorectal cancer and circulating tumor DNA (ctDNA) positive settings to determine whether this approach can serve as a backbone for promoting immunotherapy response in this difficult-to-treat disease.

A bedside to bench study of anti-PD-1, anti-CD40, and anti-CSF1R indicates that more is not necessarily better

BACKGROUND

Stimulating inflammatory tumor associated macrophages can overcome resistance to PD-(L)1 blockade. We previously conducted a phase I trial of cabiralizumab (anti-CSF1R), sotigalimab (CD40-agonist) and nivolumab. Our current purpose was to study the activity and cellular effects of this three-drug regimen in anti-PD-1-resistant melanoma.

METHODS

We employed a Simon's two-stage design and analyzed circulating immune cells from patients treated with this regimen for treatment-related changes. We assessed various dose levels of anti-CSF1R in murine melanoma models and studied the cellular and molecular effects.

RESULTS

Thirteen patients were enrolled in the first stage. We observed one (7.7%) confirmed and one (7.7%) unconfirmed partial response, 5 patients had stable disease (38.5%) and 6 disease progression (42.6%). We elected not to proceed to the second stage. CyTOF analysis revealed a reduction in non-classical monocytes. Patients with prolonged stable disease or partial response who remained on study for longer had increased markers of antigen presentation after treatment compared to patients whose disease progressed rapidly. In a murine model, higher anti-CSF1R doses resulted in increased tumor growth and worse survival. Using single-cell RNA-sequencing, we identified a suppressive monocyte/macrophage population in murine tumors exposed to higher doses.

CONCLUSIONS

Higher anti-CSF1R doses are inferior to lower doses in a preclinical model, inducing a suppressive macrophage population, and potentially explaining the disappointing results observed in patients. While it is impossible to directly infer human doses from murine studies, careful intra-species evaluation can provide important insight. Cabiralizumab dose optimization is necessary for this patient population with limited treatment options.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT03502330.

Tumor Antigen-Primed Dendritic Cell-Derived Exosome Synergizes with Colony Stimulating Factor-1 Receptor Inhibitor by Modulating the Tumor Microenvironment and Systemic Immunity

Dendritic cell-derived exosomes (Dex) have overcome the disadvantages associated with dendritic cell (DC) vaccines, such as cost effectiveness, stability, and sensitivity to the systemic microenvironment. However, in clinical trials, Dex failed to provide satisfactory results because of many reasons, including inadequate maturation of DC as well as the immunosuppressive tumor microenvironment (TME). Hence, culturing DCs in the presence of a maturation cocktail showed an induced expression of MHCs and co-stimulatory molecules. Additionally, targeting the colony stimulating factor-1 (CSF-1)/CSF-1 receptor (CSF-1R) signaling pathway by a CSF-1R inhibitor could deplete tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) which are responsible for immunosuppressive TME. Hence, in this study, mDexTA were isolated from bone marrow-derived DC cultured in the presence of a novel maturation cocktail and tumor antigen. mDexTA showed elevated expression of major histocompatibility complexes (MHCs) and co-stimulatory molecules and was found capable of activating naïve DC and T cells in vitro more efficiently when compared to imDexTA isolated from immature DCs. In addition, PLX-3397, a small molecule inhibitor of CSF-1/CSF-1R, was used in combination to enhance the antitumor efficacy of mDexTA. PLX-3397 showed dose-dependent toxicity against bone marrow-derived macrophages (BMDMs). In the B16-F10 murine melanoma model, we found that the combination treatment delayed tumor growth and improved survival compared to the mice treated with mDexTA alone by enhancing the CD8 T cells infiltration in TME. mDexTA when combined with PLX-3397 modulated the TME by shifting the Th1/Th2 toward a dominant Th1 population and depleting the TAMs and MDSCs. Interestingly, PLX-3397-induced FoxP3 expression was diminished when it was used in combination with mDexTA. Combination treatment also induced favorable systemic antitumor immunity in the spleen and lymph node. In conclusion, our findings provide insights into the synergy between mDexTA-based immunotherapy and PLX-3397 as the combination overcame the disadvantages associated with monotherapy and offer a therapeutic strategy for the treatment of solid tumors including melanoma.

Combinatorial Immunotherapy with Agonistic CD40 Activates Dendritic Cells to Express IL12 and Overcomes PD-1 Resistance

Checkpoint inhibitors have revolutionized cancer treatment, but resistance remains a significant clinical challenge. Myeloid cells within the tumor microenvironment can modulate checkpoint resistance by either supporting or suppressing adaptive immune responses. Using an anti-PD-1-resistant mouse melanoma model, we show that targeting the myeloid compartment via CD40 activation and CSF1R blockade in combination with anti-PD-1 results in complete tumor regression in a majority of mice. This triple therapy combination was primarily CD40 agonist-driven in the first 24 hours after therapy and showed a similar systemic cytokine profile in human patients as was seen in mice. Functional single-cell cytokine secretion profiling of dendritic cells (DC) using a novel microwell assay identified a CCL22+CCL5+ IL12-secreting DC subset as important early-stage effectors of triple therapy. CD4+ and CD8+ T cells are both critical effectors of treatment, and systems analysis of single-cell RNA sequencing data supported a role for DC-secreted IL12 in priming T-cell activation and recruitment. Finally, we showed that treatment with a novel IL12 mRNA therapeutic alone was sufficient to overcome PD-1 resistance and cause tumor regression. Overall, we conclude that combining myeloid-based innate immune activation and enhancement of adaptive immunity is a viable strategy to overcome anti-PD-1 resistance.

Subverted macrophages in the triple-negative breast cancer ecosystem

Tumor-associated macrophages (TAMs) are the most critical effector cells of innate immunity and the most abundant tumor-infiltrating immune cells. They play a key role in the clearance of apoptotic bodies, regulation of inflammation, and tissue repair to maintain homeostasis in vivo. With the progression of triple-negative breast cancer（TNBC）, TAMs are "subverted" from tumor-promoting immune cells to tumor-promoting immune suppressor cells, which play a significant role in tumor development and are considered potential targets for cancer therapy. Here, we explored how macrophages, as the most important part of the TNBC ecosystem, are "subverted" to drive cancer evolution and the uniqueness of TAMs in TNBC progression and metastasis. Similarly, we discuss the rationale and available evidence for TAMs as potential targets for TNBC therapy.

Polyoxazoline-Based Nanovaccine Synergizes with Tumor-Associated Macrophage Targeting and Anti-PD-1 Immunotherapy against Solid Tumors

Immune checkpoint blockade reaches remarkable clinical responses. However, even in the most favorable cases, half of these patients do not benefit from these therapies in the long term. It is hypothesized that the activation of host immunity by co-delivering peptide antigens, adjuvants, and regulators of the transforming growth factor (TGF)-β expression using a polyoxazoline (POx)-poly(lactic-co-glycolic) acid (PLGA) nanovaccine, while modulating the tumor-associated macrophages (TAM) function within the tumor microenvironment (TME) and blocking the anti-programmed cell death protein 1 (PD-1) can constitute an alternative approach for cancer immunotherapy. POx-Mannose (Man) nanovaccines generate antigen-specific T-cell responses that control tumor growth to a higher extent than poly(ethylene glycol) (PEG)-Man nanovaccines. This anti-tumor effect induced by the POx-Man nanovaccines is mediated by a CD8+ -T cell-dependent mechanism, in contrast to the PEG-Man nanovaccines. POx-Man nanovaccine combines with pexidartinib, a modulator of the TAM function, restricts the MC38 tumor growth, and synergizes with PD-1 blockade, controlling MC38 and CT26 tumor growth and survival. This data is further validated in the highly aggressive and poorly immunogenic B16F10 melanoma mouse model. Therefore, the synergistic anti-tumor effect induced by the combination of nanovaccines with the inhibition of both TAM- and PD-1-inducing immunosuppression, holds great potential for improving immunotherapy outcomes in solid cancer patients.

Targeting CD47-SIRPa axis shows potent preclinical anti-tumor activity as monotherapy and synergizes with PARP inhibition

The objective was to correlate CD47 gene expression with resistance to immune checkpoint inhibitors (ICI) in tumor tissue of gynecological cancer (GC). Further, we sought to assess the efficacy of targeting CD47 pathway alone and in combination in pre-clinical ovarian cancer (OC) models. We performed transcriptomic analyses in GC treated with ICI. Signaling pathway enrichment analysis was performed using Ingenuity Pathway Analysis. Immune cell abundance was estimated. CD47 expression was correlated with other pathways, objective response, and progression-free survival (PFS). Anti-tumor efficacy of anti-CD47 therapy alone and in combination was investigated both in-vitro and in-vivo using cell-line derived xenograft (CDX) and patient-derived xenograft (PDX) models. High CD47 expression associated with lower response to ICI and trended toward lower PFS in GC patients. Higher CD47 associated negatively with PDL1 and CTLA4 expression, as well as cytotoxic T-cells and dendritic cells but positively with TGF-β, BRD4 and CXCR4/CXCL12 expression. Anti-CD47 significantly enhanced macrophage-mediated phagocytosis of OC cells in-vitro and exhibited potent anti-tumor activity in-vivo in OC CDX and PDX models. In-vitro treatment with PARPi increased CD47 expression. Anti-CD47 led to significantly enhanced in-vitro phagocytosis, enhanced STING pathway and synergized in-vivo when combined with PARP inhibitors in BRCA-deficient OC models. This study provides insight on the potential role of CD47 in mediating immunotherapy resistance and its association with higher TGF-β, BRD4 and CXCR4/CXCL12 expression. Anti-CD47 showed potent anti-tumor activity and synergized with PARPi in OC models. These data support clinical development of anti-CD47 therapy with PARPi in OC.

Conserved transcriptional connectivity of regulatory T cells in the tumor microenvironment informs new combination cancer therapy strategies

While regulatory T (Treg) cells are traditionally viewed as professional suppressors of antigen presenting cells and effector T cells in both autoimmunity and cancer, recent findings of distinct Treg cell functions in tissue maintenance suggest that their regulatory purview extends to a wider range of cells and is broader than previously assumed. To elucidate tumoral Treg cell 'connectivity' to diverse tumor-supporting accessory cell types, we explored immediate early changes in their single-cell transcriptomes upon punctual Treg cell depletion in experimental lung cancer and injury-induced inflammation. Before any notable T cell activation and inflammation, fibroblasts, endothelial and myeloid cells exhibited pronounced changes in their gene expression in both cancer and injury settings. Factor analysis revealed shared Treg cell-dependent gene programs, foremost, prominent upregulation of VEGF and CCR2 signaling-related genes upon Treg cell deprivation in either setting, as well as in Treg cell-poor versus Treg cell-rich human lung adenocarcinomas. Accordingly, punctual Treg cell depletion combined with short-term VEGF blockade showed markedly improved control of PD-1 blockade-resistant lung adenocarcinoma progression in mice compared to the corresponding monotherapies, highlighting a promising factor-based querying approach to elucidating new rational combination treatments of solid organ cancers.

Spatiotemporal Characterization of Human Early Intervertebral Disc Formation at Single-Cell Resolution

The intervertebral disc (IVD) acts as a fibrocartilaginous joint to anchor adjacent vertebrae. Although several studies have demonstrated the cellular heterogeneity of adult mature IVDs, a single-cell transcriptomic atlas mapping early IVD formation is still lacking. Here, the authors generate a spatiotemporal and single cell-based transcriptomic atlas of human IVD formation at the embryonic stage and a comparative mouse transcript landscape. They identify two novel human notochord (NC)/nucleus pulposus (NP) clusters, SRY-box transcription factor 10 (SOX10)⁺ and cathepsin K (CTSK)⁺ , that are distributed in the early and late stages of IVD formation and they are validated by lineage tracing experiments in mice. Matrisome NC/NP clusters, T-box transcription factor T (TBXT)⁺ and CTSK⁺ , are responsible for the extracellular matrix homeostasis. The IVD atlas suggests that a subcluster of the vertebral chondrocyte subcluster might give rise to an inner annulus fibrosus of chondrogenic origin, while the fibroblastic outer annulus fibrosus preferentially expresseds transgelin and fibromodulin . Through analyzing intercellular crosstalk, the authors further find that notochordal secreted phosphoprotein 1 (SPP1) is a novel cue in the IVD microenvironment, and it is associated with IVD development and degeneration. In conclusion, the single-cell transcriptomic atlas will be leveraged to develop preventative and regenerative strategies for IVD degeneration.

Role of tumor-associated macrophages in common digestive system malignant tumors

Many digestive system malignant tumors are characterized by high incidence and mortality rate. Increasing evidence has revealed that the tumor microenvironment (TME) is involved in cancer initiation and tumor progression. Tumor-associated macrophages (TAMs) are a predominant constituent of the TME, and participate in the regulation of various biological behaviors and influence the prognosis of digestive system cancer. TAMs can be mainly classified into the antitumor M1 phenotype and protumor M2 phenotype. The latter especially are crucial drivers of tumor invasion, growth, angiogenesis, metastasis, immunosuppression, and resistance to therapy. TAMs are of importance in the occurrence, development, diagnosis, prognosis, and treatment of common digestive system malignant tumors. In this review, we summarize the role of TAMs in common digestive system malignant tumors, including esophageal, gastric, colorectal, pancreatic and liver cancers. How TAMs promote the development of tumors, and how they act as potential therapeutic targets and their clinical applications are also described.

Therapeutic targeting of tumour myeloid cells

Myeloid cells are pivotal within the immunosuppressive tumour microenvironment. The accumulation of tumour-modified myeloid cells derived from monocytes or neutrophils - termed 'myeloid-derived suppressor cells' - and tumour-associated macrophages is associated with poor outcome and resistance to treatments such as chemotherapy and immune checkpoint inhibitors. Unfortunately, there has been little success in large-scale clinical trials of myeloid cell modulators, and only a few distinct strategies have been used to target suppressive myeloid cells clinically so far. Preclinical and translational studies have now elucidated specific functions for different myeloid cell subpopulations within the tumour microenvironment, revealing context-specific roles of different myeloid cell populations in disease progression and influencing response to therapy. To improve the success of myeloid cell-targeted therapies, it will be important to target tumour types and patient subsets in which myeloid cells represent the dominant driver of therapy resistance, as well as to determine the most efficacious treatment regimens and combination partners. This Review discusses what we can learn from work with the first generation of myeloid modulators and highlights recent developments in modelling context-specific roles for different myeloid cell subtypes, which can ultimately inform how to drive more successful clinical trials.

Tumor-associated macrophages employ immunoediting mechanisms in colorectal tumor progression: Current research in Macrophage repolarization immunotherapy

Tumor-associated macrophages (TAMs) constitute the most prolific resident of the tumor microenvironment (TME) that regulate its TME into tumor suppressive or progressive milieu by utilizing immunoediting machinery. Here, the tumor cells construct an immunosuppressive microenvironment that educates TAMs to polarize from anti-tumor TAM-M1 to pro-tumor TAM-M2 phenotype consequently contributing to tumor progression. In colorectal cancer (CRC), the TME displays a prominent pro-tumorigenic immune profile with elevated expression of immune-checkpoint molecules notably PD-1, CTLA4, etc., in both MSI and ultra-mutated MSS tumors. This authenticated immune-checkpoint inhibition (ICI) immunotherapy as a pre-requisite for clinical benefit in CRC. However, in response to ICI, specifically, the MSI^hi tumors evolved to produce novel immune escape variants thus undermining ICI. Lately, TAM-directed therapies extending from macrophage depletion to repolarization have enabled TME alteration. While TAM accrual implicates clinical benefit in CRC, sustained inflammatory insult may program TAMs to shift from M1 to M2 phenotype. Their ability to oscillate on both facets of the spectrum represents macrophage repolarization immunotherapy as an effective approach to treating CRC. In this review, we briefly discuss the differentiation heterogeneity of colonic macrophages that partake in macrophage-directed immunoediting mechanisms in CRC progression and its employment in macrophage re-polarization immunotherapy.

Interaction of tumor‑associated macrophages with stromal and immune components in solid tumors: Research progress (Review)

Tumor‑associated macrophages (TAMs) are crucial cells of the tumor microenvironment (TME), which belong to the innate immune system and regulate primary tumor growth, immunosuppression, angiogenesis, extracellular matrix remodeling and metastasis. The review discusses current knowledge of essential cell‑cell interactions of TAMs within the TME of solid tumors. It summarizes the mechanisms of stromal cell (including cancer‑associated fibroblasts and endothelial cells)‑mediated monocyte recruitment and regulation of differentiation, as well as pro‑tumor and antitumor polarization of TAMs. Additionally, it focuses on the perivascular TAM subpopulations that regulate angiogenesis and lymphangiogenesis. It describes the possible mechanisms of reciprocal interactions of TAMs with other immune cells responsible for immunosuppression. Finally, it highlights the perspectives for novel therapeutic approaches to use combined cellular targets that include TAMs and other stromal and immune cells in the TME. The collected data demonstrated the importance of understanding cell‑cell interactions in the TME to prevent distant metastasis and reduce the risk of tumor recurrence.

Myeloid cell-targeted therapies for solid tumours

Myeloid cells are the most abundant immune components of the tumour microenvironment, where they have a variety of functions, ranging from immunosuppressive to immunostimulatory roles. The myeloid cell compartment comprises many different cell types, including monocytes, macrophages, dendritic cells and granulocytes, that are highly plastic and can differentiate into diverse phenotypes depending on cues received from their microenvironment. In the past few decades, we have gained a better appreciation of the complexity of myeloid cell subsets and how they are involved in tumour progression and resistance to cancer therapies, including immunotherapy. In this Review, we highlight key features of monocyte and macrophage biology that are being explored as potential targets for cancer therapies and what aspects of myeloid cells need a deeper understanding to identify rational combinatorial strategies to improve clinical outcomes of patients with cancer. We discuss therapies that aim to modulate the functional activities of myeloid cell populations, impacting their recruitment, survival and activity in the tumour microenvironment, acting at the level of cell surface receptors, signalling pathways, epigenetic machinery and metabolic regulators. We also describe advances in the development of genetically engineered myeloid cells for cancer therapy.

Interleukin-34 and immune checkpoint inhibitors: Unified weapons against cancer

Interleukin-34 (IL-34) is a cytokine that is involved in the regulation of immune cells, including macrophages, in the tumor microenvironment (TME). Macrophages are a type of immune cell that can be found in large numbers within the TME and have been shown to have a role in the suppression of immune responses in cancer. This mmune suppression can contribute to cancer development and tumors' ability to evade the immune system. Immune checkpoint inhibitors (ICIs) are a type of cancer treatment that target proteins on immune cells that act as "checkpoints," regulating the activity of the immune system. Examples of these proteins include programmed cell death protein 1 (PD-1) and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4). ICIs work by blocking the activity of these proteins, allowing the immune system to mount a stronger response against cancer cells. The combination of IL-34 inhibition with ICIs has been proposed as a potential treatment option for cancer due to the role of IL-34 in the TME and its potential involvement in resistance to ICIs. Inhibiting the activity of IL-34 or targeting its signaling pathways may help to overcome resistance to ICIs and improve the effectiveness of these therapies. This review summarizes the current state of knowledge concerning the involvement of IL-34-mediated regulation of TME and the promotion of ICI resistance. Besides, this work may shed light on whether targeting IL-34 might be exploited as a potential treatment option for cancer patients in the future. However, further research is needed to fully understand the mechanisms underlying the role of IL-34 in TME and to determine the safety and efficacy of this approach in cancer patients.

Mannosylated-serum albumin nanoparticle imaging to monitor tumor-associated macrophages under anti-PD1 treatment

BACKGROUND

Immune checkpoint inhibitors such as anti-programmed cell death protein 1 (PD1) block tumor growth by reinvigorating the immune system; however, determining their efficacy only by the changes in tumor size may prove inaccurate. As the immune cells including macrophages in the tumor microenvironment (TME) are associated with the response to anti-PD1 therapy, tumor-associated macrophages (TAMs) imaging using nanoparticles can noninvasively provide the immune enrichment status of TME. Herein, the mannosylated-serum albumin (MSA) nanoparticle was labeled with radioactive isotope ⁶⁸Ga to target the mannose receptors on macrophages for noninvasive monitoring of the TME according to anti-PD1 therapy.

RESULTS

B16F10-Luc and MC38-Luc tumor-bearing mice were treated with anti-PD1, and the response to anti-PD1 was determined by the tumor volume. According to the flow cytometry, the responders to anti-PD1 showed an increased proportion of TAMs, as well as lymphocytes, and the most enriched immune cell population in the TME was also TAMs. For noninvasive imaging of TAMs as a surrogate of immune cell augmentation in the TME via anti-PD1, we acquired [⁶⁸Ga] Ga-MSA positron emission tomography. According to the imaging study, an increased number of TAMs in responders at the early phase of anti-PD1 treatment was observed in both B16F10-Luc and MC38-Luc tumor-bearing mice models.

CONCLUSION

As representative immune cells in the TME, non-invasive imaging of TAMs using MSA nanoparticles can reflect the immune cell enrichment status in the TME closely associated with the response to anti-PD1. As non-invasive imaging using MSA nanoparticles, this approach shows a potential to monitor and evaluate anti-tumor response to immune checkpoint inhibitors.

The Tumor Immune Microenvironment in Primary CNS Neoplasms: A Review of Current Knowledge and Therapeutic Approaches

Primary CNS neoplasms are responsible for considerable mortality and morbidity, and many therapies directed at primary brain tumors have proven unsuccessful despite their success in preclinical studies. Recently, the tumor immune microenvironment has emerged as a critical aspect of primary CNS neoplasms that may affect their malignancy, prognosis, and response to therapy across patients and tumor grades. This review covers the tumor microenvironment of various primary CNS neoplasms, with a focus on glioblastoma and meningioma. Additionally, current therapeutic strategies based on elements of the tumor microenvironment, including checkpoint inhibitor therapy and immunotherapeutic vaccines, are discussed.

Safety and efficacy of dual PI3K-δ, γ inhibitor, duvelisib in patients with relapsed or refractory lymphoid neoplasms: A systematic review and meta-analysis of prospective clinical trials

Background

Duvelisib is the first FDA-approved oral dual inhibitor of phosphatidylinositol-3-kinase PI3K-delta (PI3K-δ) and PI3K-gamma (PI3K-γ). Although many clinical studies support the efficacy of duvelisib, the safety of duvelisib remains with great attention. This systematic review and meta-analysis aimed to evaluate the safety and efficacy of duvelisib in treating different relapsed or refractory (RR) lymphoid neoplasm types.

Methods

We searched prospective clinical trials from PUBMED, EMBASE, Cochrane Library, and ClinicalTrials.gov. For efficacy analysis, Overall response rate (ORR), complete response rate (CR), partial response rate (PR), rate of stable disease (SDR), rate of progressive disease (PDR), median progression-free survival (mPFS), 12-/24-month PFS, and 12-month overall survival (OS) were assessed. For safety analysis, the incidences of any grade and grade ≥3 adverse events (AEs), serious AEs, and treatment-related discontinuation and death were evaluated. Subgroup analysis based on the disease type was performed.

Results

We included 11 studies and 683 patients, including 305 chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL), 187 B-cell indolent non-Hodgkin lymphoma (iNHL), 39 B-cell aggressive non-Hodgkin lymphoma (aNHL), and 152 T-cell non-Hodgkin lymphoma (T-NHL) patients. The pooled ORR in CLL/SLL, iNHL, aNHL and T-NHL was 70%, 70%, 28% and 47%, respectively. Additionally, the pooled ORR in CLL/SLL patients with or without TP53 mutation/17p-deletion (62% vs. 74%, p=0.45) and in follicular lymphoma (FL) or other iNHL (69% vs. 57%, p=0.38) had no significant differences. Mantle cell lymphoma (MCL) patients had higher pooled ORR than other aNHL (68% vs. 17%, p=0.04). Angioimmunoblastic TCL (AITL) patients had higher pooled ORR than other PTCL patients (67% vs. 42%, p=0.01). The pooled incidence of any grade, grade ≥3, serious AEs, treatment-related discontinuation and death was 99%, 79%, 63%, 33% and 3%, respectively. The most frequent any-grade AEs were diarrhea (47%), ALT/AST increase (39%), and neutropenia (38%). The most frequent grade ≥3 AEs were neutropenia (25%), ALT/AST increased (16%), diarrhea (12%), and anemia (12%).

Conclusion

Generally, duvelisib could offer favorable efficacy in patients with RR CLL/SLL, iNHL, MCL, and AITL. Risk and severity in duvelisib treatment may be mitigated through proper identification and management.

Ecological niches for colorectal cancer stem cell survival and thrival

To date, colorectal cancer is still ranking top three cancer types severely threatening lives. According to cancer stem cell hypothesis, malignant colorectal lumps are cultivated by a set of abnormal epithelial cells with stem cell-like characteristics. These vicious stem cells are derived from intestinal epithelial stem cells or transformed by terminally differentiated epithelial cells when they accumulate an array of transforming genomic alterations. Colorectal cancer stem cells, whatever cell-of-origin, give rise to all morphologically and functionally heterogenous tumor daughter cells, conferring them with overwhelming resilience to intrinsic and extrinsic stresses. On the other hand, colorectal cancer stem cells and their daughter cells continuously participate in constructing ecological niches for their survival and thrival by communicating with adjacent stromal cells and circulating immune guardians. In this review, we first provide an overview of the normal cell-of-origin populations contributing to colorectal cancer stem cell reservoirs and the niche architecture which cancer stem cells depend on at early stage. Then we survey recent advances on how these aberrant niches are fostered by cancer stem cells and their neighbors. We also discuss recent research on how niche microenvironment affects colorectal cancer stem cell behaviors such as plasticity, metabolism, escape of immune surveillance as well as resistance to clinical therapies, therefore endowing them with competitive advantages compared to their normal partners. In the end, we explore therapeutic strategies available to target malignant stem cells.

Pexidartinib synergize PD-1 antibody through inhibiting treg infiltration by reducing TAM-derived CCL22 in lung adenocarcinoma

There is a crosstalk between Tumor-associated macrophages (TAM) and tumor-infiltrating T cells in tumor environment. TAM could inhibit the activity of cytotoxic T cells; TAM could also regulate the composition of T cells in tumor immune environment. The combination therapy for TAM and tumor infiltrated T cells has been widely noticed, but the crosstalk between TAM and tumor infiltrated T cells remains unclear in the process of combination therapy. We treated lung adenocarcinoma tumor models with pexidartinib, which targets macrophage colony stimulating factor receptor (M-CSFR) and c-kit tyrosine kinase, to inhibited TAM. Pexidartinib inhibited the ratio of macrophages in the tumor and also altered macrophage polarization. In addition to reprogram TAM, pexidartinib also changed the composition of tumor-invasive T cells. After pexidartinib treatment, the total number of T cells, CD8⁺ T cells and Treg cells were all decreased, the ratio of CD8+T/Treg increased significantly. According to the analysis of cytokines and chemokines during the treatment of pexidartinib, CCL22, as a chemokine for Treg recruitment, significantly decreased after the treatment of pexidartinib. Base on the above observation, the combination of pexidartinib and PD-1 antibody were used in the treatment of lung adenocarcinoma subcutaneous tumor model, the combination therapy has significantly improved the efficacy of tumor treatment compared with the monotherapy. Meanwhile, compared with pexidartinib monotherapy, the combination treatment further switches the polarization status of tumor-associated macrophages. In summary, our results showed that the combination of pexidartinib and PD-1 antibody showed a synergy and significantly improved the anti-tumor efficacy, through pexidartinib increasing CD8T/Treg ratio by reducing TAM-derived CCL22.

Silence of a dependence receptor CSF1R in colorectal cancer cells activates tumor-associated macrophages

BACKGROUND

Colony-stimulating factor 1 receptor (CSF1R), a classic tyrosine kinase receptor, has been identified as a proto-oncogene in multiple cancers. The CSF1/CSF1R axis is essential for the survival and differentiation of M2-phenotype tumor-associated macrophages (M2 TAMs). However, we found here that the CSF1R expression was abnormally down-regulated in colorectal cancer (CRC), and its biological functions and underlying mechanisms have become elusive in CRC progression.

METHODS

The expression of class III receptor tyrosine kinases in CRC and normal intestinal mucosa was accessed using The Cancer Genome Atlas and Gene Expression Omnibus datasets and was further validated by our tested cohort. CSF1R was reconstructed in CRC cells to identify its biological functions in vitro and in vivo. We compared CSF1R expression and methylation differences between CRC cells and macrophages. Furthermore, a co-culture system was used to mimic a competitive mechanism between CSF1R-overexpressed CRC cells and M2-like macrophages. We utilized a CSF1R inhibitor PLX3397 to ablate M2 TAMs and evaluated its efficacy on CRC treatment in animal models.

RESULTS

We found here that the CSF1R is silenced in CRC, and the reintroduced expression of the receptor in CRC cells can be cleaved by caspases and constrain tumor growth in vitro and in vivo, functioning as a tumor suppressor gene. We further identified CSF1R as a novel dependence receptor, which has the potential to act as either a tumor suppressor gene or an oncogene, depending on its activated state. In CRC tumors, CSF1R expression is enriched in TAMs, and its expression is associated with poor prognosis in patients ith CRC. In a co-culture system, CRC cells expressing CSF1R compete with M2-like macrophages for CSF1R ligands, resulting in a decrease in CSF1R activation and cell proliferation in macrophages. Blocking CSF1R by PLX3397 could deplete M2 TAMs and augments CD8⁺ T cell infiltration, effectively inhibiting tumor growth and metastasis and improving responses to chemotherapy and immunotherapy.

CONCLUSION

Our findings revealed that CSF1R is a novel identified dependence receptor silenced in CRC. The silence abalienates its ligands to stimulate CSF1R expressed on M2 TAMs, which is an appealing therapeutic target for M2 TAM depletion and CRC treatment.

DKK1 Promotes Tumor Immune Evasion and Impedes Anti-PD-1 Treatment by Inducing Immunosuppressive Macrophages in Gastric Cancer

Tumor-associated macrophages (TAM) have key functions in promoting a suppressive tumor immune microenvironment (TIME) and immune evasion, which largely limit treatment effects of immune-checkpoint inhibitors (ICI) in different cancers, including gastric cancer. Dickkopf-1 (DKK1) is associated with tumor progression and has been shown to negatively regulate antitumor immunity, but the impact of DKK1 on the TIME remains incompletely understood. Here, we found that tumoral DKK1 expression is closely associated with worse survival and a suppressive TIME in gastric cancer patients. Results from in vitro coculture assays suggested that DKK1 induces macrophages to become immunosuppressive, thereby inhibiting antitumor responses of CD8+ T cells and natural killer (NK) cells. In vivo DKK1 blockade in syngeneic gastric cancer mouse models reprogramed TAMs to restore the immune activity in the TIME and triggered significant tumor regression. DKK1 blockade also directly reduced the growth of human gastric cancer tumors with high DKK1 expression in a xenograft model. Mechanistically, DKK1 interacted with cytoskeleton-associated protein 4 (CKAP4) on the macrophage surface and activated downstream PI3K-AKT signaling, which contributed to immune suppression. TAM reprogramming by DKK1 blockade also augmented the efficacy of programmed cell death protein-1 (PD-1) blockade in gastric cancer models. Therefore, our study provides novel insights into the role of DKK1 on tumor-intrinsic, innate, and adaptive antitumor immunity modulation and suggests that DKK1 is a promising immunotherapeutic target for enhanced PD-1 blockade therapy in gastric cancer.

Treg-driven tumour control by PI3Kδ inhibition limits myeloid-derived suppressor cell expansion

BACKGROUND

Recent studies have demonstrated that blocking the PI3Kδ signalling enzyme (by administering a small molecule inhibitor, PI-3065) can potently improve the anti-tumour T-cell response through direct inhibition of Tregs. This treatment also has a negative impact on MDSC numbers but the primary mechanism driving this effect has remained unclear.

METHODS

The 4T1 breast cancer mouse model was used in combination with PI-3065 to gain insights into the effect of PI3Kδ inhibition on MDSCs.

RESULTS

PI-3065 treatment resulted in a concomitant reduction in MDSC expansion and tumour size. However, targeting Tregs independent of PI-3065 was also associated with reduced tumour volume and MDSC numbers. Surgical removal of tumours resulted in a rapid and significant decline in MDSC numbers, whilst ex vivo studies using cells from PI-3065-treated mice demonstrated no direct effect of the inhibitor on MDSC activity.

CONCLUSIONS

Our data suggest that MDSCs are not inhibited directly by PI-3065 treatment but that their reduced recruitment and immunosuppression within the tumour microenvironment is an indirect consequence of PI3Kδ-inhibition-driven tumour control. This indicates that PI3Kδ inhibition drives tumour immunity by breaking down multiple immunosuppressive pathways through both direct mechanisms (on Treg) and indirect mechanisms, secondary to tumour control (on MDSCs).

Tumor Cell-Autonomous SHP2 Contributes to Immune Suppression in Metastatic Breast Cancer

SH2 containing protein tyrosine phosphatase-2 (SHP2) is recognized as a druggable oncogenic phosphatase that is expressed in both tumor cells and immune cells. How tumor cell-autonomous SHP2 contributes to an immunosuppressive tumor microenvironment (TME) and therapeutic failure of immune checkpoint blockades in metastatic breast cancer (MBC) is not fully understood. Herein, we utilized systemic SHP2 inhibition and inducible genetic depletion of SHP2 to investigate immune reprogramming during SHP2 targeting. Pharmacologic inhibition of SHP2 sensitized MBC cells growing in the lung to α-programmed death ligand 1 (α-PD-L1) antibody treatment via relieving T-cell exhaustion induced by checkpoint blockade. Tumor cell-specific depletion of SHP2 similarly reduced pulmonary metastasis and also relieved exhaustion markers on CD8+ and CD4+ cells. Both systemic SHP2 inhibition and tumor cell-autonomous SHP2 depletion reduced tumor-infiltrated CD4+ T cells and M2-polarized tumor-associated macrophages. Analysis of TCGA datasets revealed that phosphorylation of SHP2 is important for immune-cell infiltration, T-cell activation and antigen presentation. To investigate this mechanistically, we conducted in vitro T-cell killing assays, which demonstrated that pretreatment of tumor cells with FGF2 and PDGF reduced the cytotoxicity of CD8+ T cells in a SHP2-dependent manner. Both growth factor receptor signaling and three-dimensional culture conditions transcriptionally induced PD-L1 via SHP2. Finally, SHP2 inhibition reduced MAPK signaling and enhanced STAT1 signaling, preventing growth factor-mediated suppression of MHC class I. Overall, our findings support the conclusion that tumor cell-autonomous SHP2 is a key signaling node utilized by MBC cells to engage immune-suppressive mechanisms in response to diverse signaling inputs from TME.

Significance

Findings present inhibition of SHP2 as a therapeutic option to limit breast cancer metastasis by promoting antitumor immunity.

Regulatory T Cells in Pancreatic Cancer: Of Mice and Men

Simple Summary

Regulatory T cells (Treg) are a major immunosuppressive cell subset in the pancreatic tumor microenvironment. Tregs influence tumor growth by acting either directly on cancer cells or via the inhibition of effector immune cells. Treg cells form a partially redundant network with other immunosuppressive cells such as myeloid-derived suppressor cells (MDSC) that confer robustness to tumor immunosuppression and resistance to immunotherapy. The results obtained in preclinical studies, whereupon Treg depletion, MDSCs concomitantly decreased in early tumors whereas an inverse association was seen in advanced PCa, urge a comprehensive analysis of the immunosuppressive profile of PCa throughout tumorigenesis. One relevant context to analyse these compensatory mechanisms may be patients with locally advanced PCa undergoing neoadjuvant therapy (neoTx). In order to understand these dynamics and to uncover stage-specific actional strategies involving Tregs, pre-clinical models that allow the administration of neoTx to different stages of PCa may be a very useful platform.

Abstract

Regulatory T cells (Treg) are one of the major immunosuppressive cell subsets in the pancreatic tumor microenvironment. Tregs influence tumor growth by acting either directly on cancer cells or via the inhibition of effector immune cells. Treg cells mechanisms form a partially redundant network with other immunosuppressive cells such as myeloid-derived suppressor cells (MDSC) that confer robustness to tumor immunosuppression and resistance to immunotherapy. The results obtained in preclinical studies where after Treg depletion, MDSCs concomitantly decreased in early tumors whereas an inverse association was seen in advanced PCa, urge a comprehensive analysis of the immunosuppressive profile of PCa throughout tumorigenesis. One relevant context to analyse these complex compensatory mechanisms may be the tumors of patients who underwent neoTx. Here, we observed a parallel decrease in the numbers of both intratumoral Tregs and MDSC after neoTx even in locally advanced PCa. NeoTx also led to decreased amounts of αSMA⁺ myofibroblastic cancer-associated fibroblasts (myCAF) and increased proportions of CD8⁺ cytotoxic T lymphocytes in the tumor. In order to understand these dynamics and to uncover stage-specific actional strategies involving Tregs, pre-clinical models that allow the administration of neoTx to different stages of PCa may be a very useful platform.

Targeting Cellular Components of the Tumor Microenvironment in Solid Malignancies

Simple Summary

The tumor microenvironment comprises numerous different cellular players that engage transformed cancer cells and may exert pro- vs anti-tumor functions. Their crosstalk, functionality, as well as cell recruitment to cancer lesions, is chiefly dictated by cytokines/chemokines-receptor pairs and these axes can represent therapeutic vulnerabilities—as for immune checkpoint blockers. In this review, we recapitulate the main drivers of cellular TME dynamics, interactions, and functionality, mainly focusing on T lymphocytes, macrophages and cancer associated fibroblasts, also providing an outlook on state-of-the-art TME-targeting agents.

Abstract

Cancers are composed of transformed cells, characterized by aberrant growth and invasiveness, in close relationship with non-transformed healthy cells and stromal tissue. The latter two comprise the so-called tumor microenvironment (TME), which plays a key role in tumorigenesis, cancer progression, metastatic seeding, and therapy resistance. In these regards, cancer-TME interactions are complex and dynamic, with malignant cells actively imposing an immune-suppressive and tumor-promoting state on surrounding, non-transformed, cells. Immune cells (both lymphoid and myeloid) can be recruited from the circulation and/or bone marrow by means of chemotactic signals, and their functionality is hijacked upon arrival at tumor sites. Molecular characterization of tumor-TME interactions led to the introduction of novel anti-cancer therapies targeting specific components of the TME, such as immune checkpoint blockers (ICB) (i.e., anti-programmed death 1, anti-PD1; anti-Cytotoxic T-Lymphocyte Antigen 4, anti-CTLA4). However, ICB resistance often develops and, despite the introduction of newer technologies able to study the TME at the single-cell level, a detailed understanding of all tumor-TME connections is still largely lacking. In this work, we highlight the main cellular and extracellular components of the TME, discuss their dynamics and functionality, and provide an outlook on the most relevant clinical data obtained with novel TME-targeting agents, with a focus on T lymphocytes, macrophages, and cancer-associated fibroblasts.

CAR T-Cell Targeting of Macrophage Colony-Stimulating Factor Receptor

Macrophage colony-stimulating factor receptor (M-CSFR) is found in cells of the mononuclear phagocyte lineage and is aberrantly expressed in a range of tumours, in addition to tumour-associated macrophages. Consequently, a variety of cancer therapies directed against M-CSFR are under development. We set out to engineer chimeric antigen receptors (CARs) that employ the natural ligands of this receptor, namely M-CSF or interleukin (IL)-34, to achieve specificity for M-CSFR-expressing target cells. Both M-CSF and IL-34 bind to overlapping regions of M-CSFR, although affinity of IL-34 is significantly greater than that of M-CSF. Matched second- and third-generation CARs targeted using M-CSF or IL-34 were expressed in human T-cells using the SFG retroviral vector. We found that both M-CSF- and IL-34-containing CARs enable T-cells to mediate selective destruction of tumour cells that express enforced or endogenous M-CSFR, accompanied by production of both IL-2 and interferon (IFN)-γ. Although they contain an additional co-stimulatory module, third-generation CARs did not outperform second-generation CARs. M-CSF-containing CARs mediated enhanced cytokine production and cytolytic activity compared to IL-34-containing CARs. These data demonstrate the feasibility of targeting M-CSFR using ligand-based CARs and raise the possibility that the low picomolar affinity of IL-34 for M-CSFR is detrimental to CAR function.

Assessment of the TGFB1 gene expression and methylation status of the promoter region in patients with colorectal cancer

The aim of this study was to evaluate the expression of the TGFB1 gene encoding the TGF-β1 cytokine in 64 patients, and then to compare it with clinico-pathological features. The study also investigated whether the regulation of the gene expression is caused by methylation of the promoter region between − 235 and + 22 nucleotide from the start of transcription. The dependence of the relative level of the TGFB1 gene expression on the clinical advancement according to the TNM classifications was shown. Additionally, the individual grades of the T and M features of the TNM classification differed in the relative transcript levels of the TGFB1 gene. Moreover, the higher relative expression level of the studied gene was associated with a lack of vascular invasion by cancer cells and presence of lymphocytes in the neoplastic tissue. The obtained results may indicate a possible impact of the gene on the process of carcinogenesis in colorectal cancer and reduction of its expression level may be one of the factors contributing to progression of the disease.

Targeting IL-34/MCSF-1R Axis in Colon Cancer

Colorectal carcinoma (CRC) is one of the most common neoplasias in the Western world and it is still one of the most deadly cancers worldwide mainly due to the fact that metastatic CRC is not responsive to current pharmacologic treatment. Identification of pathways that sustain CRC cell behaviour could help develop effective therapeutic compounds. A large body of evidence indicates that colon carcinogenesis is a dynamic process in which multiple cell types present in the tumor microenvironment either stimulate or suppress CRC cell growth, survival, and diffusion mainly via the production of cytokines. Interleukin-34 (IL-34), a cytokine initially known for its ability to regulate monocyte/macrophage survival and function, is highly produced in human CRC by both cancer cells and non-tumoral cells. IL-34 function is mainly mediated by interaction with the macrophage colony-stimulating factor-1 receptor (MCSF-1R), which is also over-expressed by CRC cells as well as by tumour-associated macrophages (TAMs) and cancer-associated fibroblasts. IL-34-driven MCSF-1R activation triggers several pro-tumoral functions in the colon. In this article, we review the current understanding of the involvement of IL-34 and its receptor in CRC, with particular attention to the available evidence about the IL-34/MCSF-1R axis-mediated regulation of TAMs and the role of IL-34 and MCSF-1R in promoting cancer resistance to chemotherapy and immunotherapy

IN VIVO SYNGENEIC TUMOR MODELS WITH ACQUIRED RESISTANCE TO ANTI-PD-1/PD-L1 THERAPIES

Antibodies targeting PD-1 and PD-L1 have produced durable responses in a subset of cancer patients. However, a majority of these patients will ultimately relapse due to acquired resistance. To explore the underlying mechanisms of this secondary resistance, we developed five syngeneic murine tumor variants with acquired resistance to anti-PD-1 and/or PD-L1 antibodies in vivo. Resistant in vivo models were obtained by serial treatment/reimplantation cycles of the MC38 colorectal, MB49 and MBT2 bladder, TyrNras melanoma and RENCA kidney models. Tumor immune infiltrates were characterized for wild type and resistant tumors using spectral cytometry and their molecular alterations analyzed using RNA-seq analyses. Alterations in the tumor immune microenvironment were strongly heterogeneous amongst resistant models, involving select lymphoid and/or myeloid subpopulations. Molecular alterations in resistant models included previously identified pathways as well as novel candidate genes found to be deregulated in several resistant models. Among these, Serpinf1, coding for Pigment Epithelial Derived Factor was further explored in the MC38 and the MBT2 models. Overexpression of Serpinf1 induced resistance to anti-PD-1 antibodies in the MC38 model, whereas knock-down of Serpinf1 sensitized this model as well as the primarily resistant MBT2 model. Serpinf1 overexpression was associated with increased production of free fatty acids and reduced activation of CD8+ cells, while orlistat, a compound that reduces the production of free fatty acids, reversed resistance to anti-PD-1 therapy. Our results suggest that a panel of syngeneic resistant models constitutes a useful tool to model the heterogeneity of resistance mechanisms encountered in the clinic.

STING agonism reprograms tumor-associated macrophages and overcomes resistance to PARP inhibition in BRCA1-deficient models of breast cancer

PARP inhibitors (PARPi) have drastically changed the treatment landscape of advanced ovarian tumors with BRCA mutations. However, the impact of this class of inhibitors in patients with advanced BRCA-mutant breast cancer is relatively modest. Using a syngeneic genetically-engineered mouse model of breast tumor driven by Brca1 deficiency, we show that tumor-associated macrophages (TAMs) blunt PARPi efficacy both in vivo and in vitro. Mechanistically, BRCA1-deficient breast tumor cells induce pro-tumor polarization of TAMs, which in turn suppress PARPi-elicited DNA damage in tumor cells, leading to reduced production of dsDNA fragments and synthetic lethality, hence impairing STING-dependent anti-tumor immunity. STING agonists reprogram M2-like pro-tumor macrophages into an M1-like anti-tumor state in a macrophage STING-dependent manner. Systemic administration of a STING agonist breaches multiple layers of tumor cell-mediated suppression of immune cells, and synergizes with PARPi to suppress tumor growth. The therapeutic benefits of this combination require host STING and are mediated by a type I IFN response and CD8+ T cells, but do not rely on tumor cell-intrinsic STING. Our data illustrate the importance of targeting innate immune suppression to facilitate PARPi-mediated engagement of anti-tumor immunity in breast cancer.

Spatial transcriptomics of macrophage infiltration in non-small cell lung cancer reveals determinants of sensitivity and resistance to anti-PD1/PD-L1 antibodies

BACKGROUND

Tumor-associated macrophages (TAMs) having immunosuppressive properties are one of the most abundant immune cells in the tumor microenvironment (TME). Preclinical studies have highlighted the potential role of TAMs in resistance to immune checkpoint blockers (ICBs). Here, we investigated the predictive value of TAM infiltration in patients with non-small cell lung cancer (NSCLC) treated with ICBs and characterized their transcriptomic profiles.

METHODS

Tumor samples were collected from 152 patients with NSCLC before ICB treatment onset. After immunohistochemical staining and image analysis, the correlation between CD163+ cell infiltration and survival was analyzed. Spatial transcriptomic analyses were performed using the NanoString GeoMx Immune Pathways assay to compare the gene expression profile of tumors with high or low levels of CD163+ cell infiltration and to identify determinants of response to ICBs in tumors with high CD163+ infiltration.

RESULTS

Low intratumoral CD163+ cell infiltration was associated with longer progression-free survival (PFS; HR 0.61, 95% CI 0.40 to 0.94, p=0.023) and overall survival (OS; HR 0.48, 95% CI 0.28 to 0.80, p=0.004) under ICB treatment. Spatial transcriptomic profiles of 16 tumors revealed the upregulation of ITGAM, CD27, and CCL5 in tumors with high CD163+ cell infiltration. Moreover, in tumors with high macrophage infiltration, the upregulation of genes associated with the interferon-γ signaling pathway and the M1 phenotype was associated with better responses under immunotherapy. Surprisingly, we found also a significantly higher expression of CSF1R in the tumors of responders. Analysis of three independent data sets confirmed that high CSF1R expression was associated with an increased durable clinical benefit rate (47% vs 6%, p=0.004), PFS (median 10.89 months vs 1.67 months, p=0.001), and OS (median 23.11 months vs 2.66 months, p<0.001) under ICB treatment.

CONCLUSIONS

Enrichment of TAMs in the TME of NSCLC is associated with resistance to immunotherapy regardless of the programmed death ligand 1 status and is driven by upregulation of CD27, ITGAM, and CCL5 gene expression within the tumor compartment. Our transcriptomic analyses identify new potential targets to alter TAM recruitment/polarization and highlight the complexity of the CSF1R pathway, which may not be a suitable target to improve ICB efficacy.

Aging Leads to Increased Monocytes and Macrophages With Altered CSF-1 Receptor Expression and Earlier Tumor-Associated Macrophage Expansion in Murine Mesothelioma

Increased cancer incidence occurs with the emergence of immunosenescence, highlighting the indispensability of the immune system in preventing cancer and its dysregulation with aging. Tumor-associated macrophages (TAMs) are often present in high numbers and are associated with poor clinical outcomes in solid cancers, including mesothelioma. Monocytes and macrophages from the bone marrow and spleen can respond to tumor-derived factors, such as CSF-1, and initiation of the CSF-1R signaling cascade results in their proliferation, differentiation, and migration to the tumor. Age-related changes occur in monocytes and macrophages in terms of numbers and function, which in turn can impact tumor initiation and progression. Whether this is due to changes in CSF-1R expression with aging is currently unknown and was investigated in this study. We examined monocytes and macrophages in the bone marrow and spleen during healthy aging in young (3–4 months) and elderly (20–24 months) female C57BL/6J mice. Additionally, changes to these tissues and in TAMs were examined during AE17 mesothelioma tumor growth. Healthy aging resulted in an expansion of Ly6C^high monocytes and macrophages in the bone marrow and spleen. CSF-1R expression levels were reduced in elderly splenic macrophages only, suggesting differences in CSF-1R signaling between both cell type and tissue site. In tumor-bearing mice, Ly6C^high monocytes increased with tumor growth in the spleen in the elderly and increased intracellular CSF-1R expression occurred in bone marrow Ly6C^high monocytes in elderly mice bearing large tumors. Age-related changes to bone marrow and splenic Ly6C^high monocytes were reflected in the tumor, where we observed increased Ly6C^high TAMs earlier and expansion of Ly6C^low TAMs later during AE17 tumor growth in the elderly compared to young mice. F4/80^high TAMs increased with tumor growth in both young and elderly mice and were the largest subset of TAMs in the tumor. Together, this suggests there may be a faster transition of Ly6C^high towards F4/80^high TAMs with aging. Amongst TAM subsets, expression of CSF-1R was lowest in F4/80^high TAMs, however Ly6C^low TAMs had higher intracellular CSF-1R expression. This suggests downstream CSF-1R signaling may vary between macrophage subsets, which can have implications towards CSF-1R blockade therapies targeting macrophages in cancer.

Role of macrophages in tumor progression and therapy (Review)

The number and phenotype of macrophages are closely related to tumor growth and prognosis. Macrophages are recruited to (and polarized at) the tumor site thereby promoting tumor growth, stimulating tumor angiogenesis, facilitating tumor cell migration, and creating a favorable environment for subsequent colonization by (and survival of) tumor cells. These phenomena contribute to the formation of an immunosuppressive tumor microenvironment (TME) and therefore speed up tumor cell proliferation and metastasis and reduce the efficacy of antitumor factors and therapies. The ability of macrophages to remodel the TME through interactions with other cells and corresponding changes in their number, activity, and phenotype during conventional therapies, as well as the association between these changes and drug resistance, make tumor-associated macrophages a new target for antitumor therapies. In this review, advantages and limitations of the existing antitumor strategies targeting macrophages in Traditional Chinese and Western medicine were analyzed, starting with the effect of macrophages on tumors and their interactions with other cells and then the role of macrophages in conventional treatments was explored. Possible directions of future developments in this field from an all-around multitarget standpoint were also examined.

CSF1/CSF1R signaling mediates malignant pleural effusion formation

Malignant pleural effusion (MPE) is an incurable common manifestation of many malignancies. Its formation is orchestrated by complex interactions among tumor cells, inflammatory cells, and the vasculature. Tumor-associated macrophages present the dominant inflammatory population of MPE, and M2 macrophage numbers account for dismal prognosis. M2 polarization is known to be triggered by CSF1/CSF1 receptor (CSF1R) signaling. We hypothesized that CSF1R⁺ M2 macrophages favor MPE formation and could be therapeutically targeted to limit MPE. We generated mice with CSF1R-deficient macrophages and induced lung and colon adenocarcinoma–associated MPE. We also examined the therapeutic potential of a clinically relevant CSF1R inhibitor (BLZ945) in lung and colon adenocarcinoma–induced experimental MPE. We showed that CSF1R⁺ macrophages promoted pleural fluid accumulation by enhancing vascular permeability, destabilizing tumor vessels, and favoring immune suppression. We also showed that CSF1R inhibition limited MPE in vivo by reducing vascular permeability and neoangiogenesis and impeding tumor progression. This was because apart from macrophages, CSF1R signals in cancer-associated fibroblasts leading to macrophage inflammatory protein 2 secretion triggered the manifestation of suppressive and angiogenic properties in macrophages upon CXCR2 paracrine activation. Pharmacological targeting of the CSF1/CSF1R axis can therefore be a vital strategy for limiting MPE.

MEK1/2 inhibition transiently alters the tumor immune microenvironment to enhance immunotherapy efficacy against head and neck cancer

Background

Although the mitogen-activated protein kinases (MAPK) pathway is hyperactive in head and neck cancer (HNC), inhibition of MEK1/2 in HNC patients has not shown clinically meaningful activity. Therefore, we aimed to characterize the effect of MEK1/2 inhibition on the tumor microenvironment (TME) of MAPK-driven HNC, elucidate tumor-host interaction mechanisms facilitating immune escape on treatment, and apply rationale-based therapy combination immunotherapy and MEK1/2 inhibitor to induce tumor clearance.

Methods

Mouse syngeneic tumors and xenografts experiments were used to analyze tumor growth in vivo. Single-cell cytometry by time of flight, flow cytometry, and tissue stainings were used to profile the TME in response to trametinib (MEK1/2 inhibitor). Co-culture of myeloid-derived suppressor cells (MDSC) with CD8⁺ T cells was used to measure immune suppression. Overexpression of colony-stimulating factor-1 (CSF-1) in tumor cells was used to show the effect of tumor-derived CSF-1 on sensitivity to trametinib and anti-programmed death- 1 (αPD-1) in mice. In HNC patients, the ratio between CSF-1 and CD8A was measured to test the association with clinical benefit to αPD-1 and αPD-L1 treatment.

Results

Using preclinical HNC models, we demonstrated that treatment with trametinib delays HNC initiation and progression by reducing tumor cell proliferation and enhancing the antitumor immunity of CD8⁺ T cells. Activation of CD8⁺ T cells by supplementation with αPD-1 antibody eliminated tumors and induced an immune memory in the cured mice. Mechanistically, an early response to trametinib treatment sensitized tumors to αPD-1-supplementation by attenuating the expression of tumor-derived CSF-1, which reduced the abundance of two CSF-1R⁺CD11c⁺ MDSC populations in the TME. In contrast, prolonged treatment with trametinib abolished the antitumor activity of αPD-1, because tumor cells undergoing the epithelial to mesenchymal transition in response to trametinib restored CSF-1 expression and recreated an immune-suppressive TME.

Conclusion

Our findings provide the rationale for testing the trametinib/αPD-1 combination in HNC and highlight the importance of sensitizing tumors to αPD-1 by using MEK1/2 to interfere with the tumor–host interaction. Moreover, we describe the concept that treatment of cancer with a targeted therapy transiently induces an immune-active microenvironment, and supplementation of immunotherapy during this time further activates the antitumor machinery to cause tumor elimination.

Tissue Resident Foxp3+ Regulatory T Cells: Sentinels and Saboteurs in Health and Disease

Foxp3⁺ regulatory T (Treg) cells are a CD4 T cell subset with unique immune regulatory function that are indispensable in immunity and tolerance. Their indisputable importance has been investigated in numerous disease settings and experimental models. Despite the extensive efforts in determining the cellular and molecular mechanisms operating their functions, our understanding their biology especially in vivo remains limited. There is emerging evidence that Treg cells resident in the non-lymphoid tissues play a central role in regulating tissue homeostasis, inflammation, and repair. Furthermore, tissue-specific properties of those Treg cells that allow them to express tissue specific functions have been explored. In this review, we will discuss the potential mechanisms and key cellular/molecular factors responsible for the homeostasis and functions of tissue resident Treg cells under steady-state and inflammatory conditions.

Therapeutic Approaches Targeting Proteins in Tumor-Associated Macrophages and Their Applications in Cancers

Tumor-associated macrophages (TAMs) promote tumor proliferation, invasion, angiogenesis, stemness, therapeutic resistance, and immune tolerance in a protein-dependent manner. Therefore, the traditional target paradigms are often insufficient to exterminate tumor cells. These pro-tumoral functions are mediated by the subsets of macrophages that exhibit canonical protein markers, while simultaneously having unique transcriptional features, which makes the proteins expressed on TAMs promising targets during anti-tumor therapy. Herein, TAM-associated protein-dependent target strategies were developed with the aim of either reducing the numbers of TAMs or inhibiting the pro-tumoral functions of TAMs. Furthermore, the recent advances in TAMs associated with tumor metabolism and immunity were extensively exploited to repolarize these TAMs to become anti-tumor elements and reverse the immunosuppressive tumor microenvironment. In this review, we systematically summarize these current studies to fully illustrate the TAM-associated protein targets and their inhibitors, and we highlight the potential clinical applications of targeting the crosstalk among TAMs, tumor cells, and immune cells in anti-tumor therapy.

Tumor-associated macrophages in cancer: recent advancements in cancer nanoimmunotherapies

Cancer immunotherapy has emerged as a novel cancer treatment, although recent immunotherapy trials have produced suboptimal outcomes, with durable responses seen only in a small number of patients. The tumor microenvironment (TME) has been shown to be responsible for tumor immune escape and therapy failure. The vital component of the TME is tumor-associated macrophages (TAMs), which are usually associated with poor prognosis and drug resistance, including immunotherapies, and have emerged as promising targets for cancer immunotherapy. Recently, nanoparticles, because of their unique physicochemical characteristics, have emerged as crucial translational moieties in tackling tumor-promoting TAMs that amplify immune responses and sensitize tumors to immunotherapies in a safe and effective manner. In this review, we mainly described the current potential nanomaterial-based therapeutic strategies that target TAMs, including restricting TAMs survival, inhibiting TAMs recruitment to tumors and functionally repolarizing tumor-supportive TAMs to antitumor type. The current understanding of the origin and polarization of TAMs, their crucial role in cancer progression and prognostic significance was also discussed in this review. We also highlighted the recent evolution of chimeric antigen receptor (CAR)-macrophage cell therapy.

Macrophages as a Therapeutic Target in Metastatic Prostate Cancer: A Way to Overcome Immunotherapy Resistance?

Prostate cancer (PC) is the most common malignancy and the fifth cause of cancer death in men. The treatment for localized or locally advanced stages offers a high probability of cure. Even though the therapeutic landscape has significantly improved over the last decade, metastatic PC (mPC) still has a poor prognosis mainly due to the development of therapy resistance. In this context, the use of immunotherapy alone or in combination with other drugs has been explored in recent years. However, T-cell directed immune checkpoint inhibitors (ICIs) have shown limited activity with inconclusive results in mPC patients, most likely due to the highly immunosuppressive PC tumor microenvironment (TME). In this scenario, targeting macrophages, a highly abundant immunosuppressive cell type in the TME, could offer a new therapeutic strategy to improve immunotherapy efficacy. In this review, we summarize the growing field of macrophage-directed immunotherapies and discuss how these could be applied in the treatment of mPC, focusing on their combination with ICIs.

PI3K Isoform Immunotherapy for Solid Tumours

Improving the anti-tumour T cell response as a consequence of immunotherapy can result in eradication of tumour burden, however, the majority of patients fail with current treatment regimens and so novel immunotherapies with greater efficacy and improved tolerability are needed. The phosphoinositide-3-kinase (PI3K) family members that are directly involved in cell signalling comprise PI3Kα, PI3Kβ, PI3Kδ and PI3Kγ, with the latter two isoforms expressed primarily by leukocytes. The survival and optimal function of regulatory T cells (Treg) and myeloid-derived suppressor cells (MDSCs) is dependent on PI3Kδ, whereas tumour-associated macrophages (TAMs), use PI3Kγ. Blocking these signalling isoforms can boost development of effective anti-cancer immune responses and result in control of tumour burden. The dependence on different PI3K isoforms in immune cells makes targeting this pathway an attractive approach for tumour immunotherapy. Herein, we discuss how inhibiting specific PI3K isoforms in pro-tumoural Tregs, MDSCS and TAMs can unleash a powerful anti-tumour immune response, driven by CD8+ T cells, capable of controlling tumour burden and consider how the immune response to therapy needs careful investigation, to identify both the correlates of successful treatment and those that impede the generation of robust anti-tumour responses. Furthermore, we review how combination immunotherapy approaches with both PI3K inhibitors and subsequent immune checkpoint blockade can potentiate the efficacy of monotherapy. Finally, we discuss the recent advances in the use of PI3K isoform-specific inhibitors as an immunotherapy for solid tumours in clinical trials.