Tumor-infiltrating M2 Macrophages Research Articles

IL-33/NF-κB/ST2L/Rab37 positive-feedback loop promotes M2 macrophage to limit chemotherapeutic efficacy in lung cancer

IL-33 is a danger signal that binds to its receptor ST2L to promote tumor progression. This study identifies the IL-33/ST2L positive-feedback loop and the trafficking of ST2L membrane presentation in macrophages that contribute to lung tumor progression. Mechanistically, IL-33 induces ST2L upregulation by activating NF-κB, which binds to the promoter region of the ST2L gene. Moreover, Rab37, a small GTPase involved in membrane trafficking, mediates ST2L trafficking to the plasma membrane of M2 macrophages. This IL-33/NF-κB/ST2L/Rab37 axis promotes positive-feedback loops that enhance ST2L expression and membrane trafficking in M2 macrophages. Notably, neutralizing antibodies against IL-33 or ST2L block NF-κB activity, suppress M2 macrophage polarization, and synergistically inhibit tumor growth when combined with cisplatin treatment in vitro/vivo. Clinically, Rab37+/ST2L+/CD206+ tumor-infiltrating M2 macrophages correlate with advanced-stage lung cancer patients with poor response to chemotherapy. These findings unveil a positive-feedback mechanism and provide a basis for IL-33/ST2L-targeting therapy for cancer.

Abstract 2662: Deciphering the cellular and molecular determinants of immunotherapy resistance in NASH-associated hepatocellular carcinoma by single-cell analysis

Abstract Introduction: Non-alcoholic steatohepatitis (NASH) has emerged as a prominent risk and the most rapidly increasing aetiology of hepatocellular carcinoma (HCC). Despite the effectiveness of immune checkpoint blockade (ICB), like anti-programmed cell death-ligand 1 (anti-PD-L1) antibodies, in a subset of HCC patients, NASH has been shown to impair the anti-tumor immunity in ICB-treated HCC. We aim to elucidate the mechanisms responsible for ICB resistance in NASH-HCC at single-cell resolution. Methods: Orthotopic NASH-HCC mouse models were generated by orthotopically injecting liver cancer cell line into NASH liver induced by methionine-and choline-deficient diet (MCD) or choline-deficient, L-amino acid-defined, high-fat diet (CDAHFD). Multi-color flow cytometry and single-cell RNA-sequencing (scRNA-seq) were utilized to dissect the liver and tumor immune microenvironment upon anti-PD-L1 treatment. Results: Anti-PD-L1 treatment failed to alleviate tumor burden in NASH mice, indicating that orthotopic NASH-HCC mouse models were resistant to ICB therapy. Compared to the control liver, multi-color flow cytometry demonstrated that CD11b+F4/80+CD206+M2 macrophages accumulated in NASH liver and further increased in liver and tumor upon anti-PD-L1 treatment. Tumor-infiltrating M2 macrophages exhibited the most significant and robust correlation with tumor weight among all immune cell types. Moreover, PD-1 expression on tumor-infiltrating CD8+ T cells positively correlated with tumor weight and intratumoral M2 macrophages. Furthermore, scRNA-seq analysis identified a NASH-specific tumor-infiltrating monocyte subcluster, which may differentiate into an ‘M2-like’ macrophage subtype upon anti-PD-L1 treatment as demonstrated by cell trajectory analysis. Conclusion: Our study provides insights into the significance of monocyte/macrophage reprogramming in ICB resistance in NASH-HCC. Further investigation is conducted to identify the molecular mechanisms during monocyte-macrophage differentiation and explore potential targeted strategies to overcome ICB resistance in NASH-HCC. Acknowledgment: This study is supported by the General Research Fund (14120621 and 14119023), the Collaborative Research Fund (C4045-18W), the Li Ka Shing Foundation and CUHK Strategic Seed Funding for Collaborative Research Scheme. Keywords: hepatocellular carcinoma, single-cell RNA-sequencing, anti-PD-L1, macrophages. Citation Format: Lingyun Zhang, Yiling Zhang, Wenshu Tang, Zhewen Xiong, Xiaoyu Liu, Zhixian Liang, Yuk Wah Tsang, Weiqin Yang, Xiaohang Long, Alfred Sze Lok Cheng. Deciphering the cellular and molecular determinants of immunotherapy resistance in NASH-associated hepatocellular carcinoma by single-cell analysis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 2662.

Abstract 2746: CD276 in tumor cells contributes to the M2 macrophage-rich immunosuppressive tumor microenvironment via upregulating the CCL2-CCR2 axis in ovarian cancer

Abstract [Objective] Increased infiltration of CD8+ T cells and decreased infiltration of M2 macrophages are favorable prognostic factors in high-grade serous ovarian cancer (HGSOC). CD276 (B7H3) is a transmembrane co-inhibitory protein belonging to the B7 family. CD276 expression is suppressed in CD8+ T cell-rich, PD-L1-high immune reactive tumors. This suggests that CD276 plays immunosuppressive roles in addition to acting as a molecular brake for countering the T cell activation. In this study, we aimed to elucidate the role of CD276 in HGSOC microenvironment toward M2 macrophages. [Methods] CD276 expression was evaluated in HGSOC clinical cases (n=62) by immunohistochemistry tests. The correlation between survival and the presence of tumor-infiltrating immune cells, including M2 macrophages, was assessed. CD276 knockout murine ovarian cancer cell lines (HM1 and ID8) were established and used for experiments. [Results] In HGSOC patients, higher CD276 expression associated with poor prognosis (p<0.001). CD276 expression positively correlated with the number of CD206+ M2 macrophages (Pearson's r=0.29, p=0.02), but not with that of CD8+ T cells. Although CD276 knockout did not affect in vitro proliferation of HM1 and ID8 cell lines, CD276 knockout tumors were less progressive than control tumors in immunocompetent mouse models (p<0.001). Lower infiltration of CCR2+/CD206+ M2 macrophages and higher infiltration of IFN-γ+ CD8+ T cells was observed in the knockout tumors (p<0.001 and P=0.02, respectively). The difference in progression between CD276 knockout tumors and control tumors was negated when tumor cells were inoculated into immunodeficient nude mice. RNA sequencing and protein analyses revealed decreased levels of CCL2, the ligand of CCR2, in CD276 knockout cells and tumors. In vitro, monocyte chemotaxis was reduced by CD276 knockout cells' supernatant (p<0.001), and the difference was partially negated with CCR2 antagonist pretreatment. Furthermore, CCR2 blocking in vivo reduced tumor-infiltrating M2 macrophages and tumor volume in the CD276 control (p=0.048 and p=0.02, respectively), but not in the CD276 knockout group, suggesting that CCL2-CCR2 axis-induced M2 macrophages partially contribute to CD276-mediated tumor progression. [Conclusion] CD276 contributes to an unfavorable immunosuppressive tumor microenvironment driven by M2 macrophages via the CCL2-CCR2 axis in HGSOC. Citation Format: Taito Miyamoto, Ryusuke Murakami, Masayo Ukita, Mana Taki, Koji Yamanoi, Kaoru Abiko, Ken Yamaguchi, Junzo Hamanishi, Takashi Kobayashi, Masaki Mandai. CD276 in tumor cells contributes to the M2 macrophage-rich immunosuppressive tumor microenvironment via upregulating the CCL2-CCR2 axis in ovarian cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2746.

Effective Predictor of Colorectal Cancer Survival Based on Exclusive Expression Pattern Among Different Immune Cell Infiltration.

Tumor-infiltrating immune/inflammatory cells, the important components of the tumor microenvironment (TME), remarkably affect the progression of human cancers. To understand the actual conditions within the TME of colorectal cancer (CRC), the interrelationship among tumor-infiltrating neutrophils, M2 macrophages, and regulatory T-cells (Tregs) was systematically analyzed. The infiltration conditions of CD66b+ neutrophils, CD163+ M2 macrophages, and FOXP3+ Tregs in tissue microarrays including 1021 cases of CRC were determined by immunohistochemical analysis. The prediction power of these immune cells for CRC prognosis was evaluated by subgroup analysis of the CRC cohort. Results revealed the existence pattern of infiltrating neutrophils, and Tregs/M2 macrophages fulfilled a "X-low implies Y-high" Boolean relationship, indicative of a mutually exclusive correlation between neutrophils and M2 macrophages, and between neutrophils and Tregs in the TME of CRC. What's more, the tumor-infiltrating M2 macrophages and Tregs were associated with adverse prognostic factors, whereas neutrophils were corelated with favorable factors. The high infiltration of neutrophils predicted longer survival and better chemotherapeutic response. Nonetheless, high infiltration of M2 macrophages and Tregs predicted poor prognosis. The combination of these tumor-infiltrating immune cells can serve as an effective predictor for the survival of CRC and for the chemotherapeutic outcomes of stage II-III patients. .

Characteristics of Tumor-Infiltrating Lymphocytes Prior to and During Immune Checkpoint Inhibitor Therapy.

The tumor immune contexture plays a major role for the clinical outcome of patients. High densities of CD45RO+ T helper 1 cells and CD8+ T cells are associated with improved survival of patients with various cancer entities. In contrast, a higher frequency of tumor-infiltrating M2 macrophages is correlated with poor prognosis. Recent studies provide evidence that the tumor immune architecture also essentially contributes to the clinical efficacy of immune checkpoint inhibitor (CPI) therapy in patients. Pretreatment melanoma samples from patients who experienced a clinical response to anti-programmed cell death protein 1 (PD-1) treatment show higher densities of infiltrating CD8+ T cells compared to samples from patients that progressed during therapy. Anti-PD-1 therapy results in an increased density of tumor-infiltrating T lymphocytes in treatment responders. In addition, elevated frequencies of melanoma-infiltrating TCF7+CD8+ T cells are correlated with beneficial clinical outcome of anti-PD-1-treated patients. In contrast, a high density of tumor-infiltrating, dysfunctional PD-1+CD38hi CD8+ cells in melanoma patients is associated with anti-PD-1 resistance. Such findings indicate that comprehensive tumor immune contexture profiling prior to and during CPI therapy may lead to the identification of underlying mechanisms for treatment response or resistance, and the design of improved immunotherapeutic strategies. Here, we focus on studies exploring the impact of intratumoral T and B cells at baseline on the clinical outcome of CPI-treated cancer patients. In addition, recent findings demonstrating the influence of CPIs on tumor-infiltrating lymphocytes are summarized.

Tumor infiltrating M2 macrophages could predict biochemical recurrence of localized prostate cancer after radical prostatectomy

Given the critical role of the tumor microenvironment in PCa progression, we aimed to assess the prognostic effect of tumor infiltrating M2 macrophages (TIMMs) on biochemical recurrence (BCR) in patients with localized prostate cancer (PCa) after radical prostatectomy. A total of 127 localized PCa patients from GSE116918, 268 patients from TCGA database and 77 patients from GSE70770 were enrolled in our study. TIMMs were evaluated by the CIBERSORT method. Patients with high TIMMs had a significantly poorer recurrence free survival (RFS) (P = 0.017, P = 0.0063 and P = 0.001) in the three sets. In the multivariate analysis, the presence of high TIMMs (HR = 3.026, P = 0.023; HR = 2.679, P = 0.017; HR = 2.648, P = 0.005) was identified as an independent prognostic factor for RFS in the three sets. Harrell's Concordance index (C-index) increased in all three sets after combining TIMMs with traditional risk factors (PSA, clinical stage(T) and Gleason score). Gene Set Enrichment Analysis showed that T cell receptor signaling pathway, B cell receptor signaling pathway and primary immunodeficiency were significantly enriched in the low TIMMs group. TIMMs could serve as an independent prognostic factor for BCR in localized PCa patients after RP. Incorporation of TIMMs into traditional risk classification might further stratify patients with different prognosis.

Optimization of the process for purifying icariin from Herba Epimedii by macroporous resin and the regulatory role of icariin in the tumor immune microenvironment

Pancreatic cancer is a digestive tract malignancy that poses a serious threat to human health. Compounds derived from traditional Chinese medicines have been an important source of anticancer drugs and adjuvant agents to regulate the tumor immune microenvironment in patients with pancreatic cancer. In this study, icariin was purified from Herba Epimedii using macropores, and its bioactivity against pancreatic cancer was also investigated. We found that icariin has direct inhibitory and immunomodulatory effects on tumor cells. In vitro experiments showed that icariin can inhibit the migration and proliferation of Panc02 pancreatic cancer cells and induce apoptosis. Our in vivo experiments show that icariin inhibits the development of mouse pancreatic cancer by inhibiting tumor-infiltrating M2 macrophages and polymorphonuclear myeloid-derived suppressor cells (MDSCs) (PMN-MDSCs). In addition, icariin inhibits the polarization of RAW 264.7 cells into M2 macrophages by inhibiting the expression of ARG1 and MRC1 and downregulating the IL4-STAT6 signaling pathway. In conclusion, the inhibitory effect of icariin on pancreatic cancer can not only directly affect tumor cells but also inhibit tumor development by regulating the tumor immune microenvironment.

Abstract 1197: Role of NOS2-COX2 inhibition in radiation-induced tumor growth delay and immuno-modulation in the tumor micro-environment

Abstract Triple negative breast cancer (TNBC) is associated with lack of expression of human epidermal growth factor receptor 2 (HER2), estrogen receptor (ER) and progesterone receptor (PR), and do not respond to hormonal therapy. It is one of the most aggressive breast cancer phenotypes and remains a major health hazard among women with drug resistance being a limiting factor in treatment. Inflammation is a key driver of poor survival among TNBC patients through increase in metastasis and chemo-resistance. We recently demonstrated that co-expression of pro-inflammatory enzymes nitric oxide synthase2 (NOS2) and cycloxygenase2 (COX2) is a powerful prognostic marker of poor outcome (HR=21) among ER(-) patients where we showed that inflammatory loops involving these proteins globally drive major oncogenic pathways [1]. Apart from intramural signaling, the crosstalk of tumor cells with immune cells is a key driver of immuno-suppression. Tumor progression is associated with tumor infiltrating M2 macrophages and Th2 cells leading to immuno-suppression, aberrant activation of cytokines, chemokines and growth factors thus creating a conducive environment for tumor growth and metastasis. Our goal is to modulate the tumor micro-environment (TME) to increase efficacy of current radiation- and immunotherapy. Radiation therapy is a commonly used treatment option in different types of cancer including breast cancer. Focal radiation limits systemic side effects commonly associated with chemotherapy. It also activates the immune system. A key component of the immune system mediated tumor clearance is cytotoxic CD8 T cells. More recently a study found that increased CD8 cells and Th17 cells are specifically associated with TNBC patients [2]. However, they undergo functional reprogramming in the TME evident from decreased cytotoxic (IFN-γ) and proliferation marker (granzyme B). We used confocal microscopy and flow-cytometry techniques to investigate the role of NOS2 and COX2 in radiation induced tumor growth delay and metastasis. We also examined the ability of NOS2 and COX2 in regulation of the immune profile of the TME, thus emphasizing their importance in tumor growth and immune-surveillance. Lastly, we evaluated the role of COX2 and NOS2 inhibition using commercially available inhibitors on radiation induced tumor growth delay in murine models of ER- breast cancer. [1] Basudhar, D. et al, Proceedings of the National Academy of Sciences of the United States of America 2017, 114 (49), 13030-13035. [2] Gil Del Alcazar, et al., Cancer discovery 2017, 7 (10), 1098-1115. Note: This abstract was not presented at the meeting. Citation Format: Debashree Basudhar, Veena Somasundaram, David A. Scheiblin, Robert Y. Cheng, Stephen J. Lockett, David Wink, Lisa A. Ridnour. Role of NOS2-COX2 inhibition in radiation-induced tumor growth delay and immuno-modulation in the tumor micro-environment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1197.

Osthole inhibits pancreatic cancer progression by directly exerting negative effects on cancer cells and attenuating tumor-infiltrating M2 macrophages

Pancreatic cancer has remained a major cause of cancer-related deaths. A hallmark of pancreatic cancer is extensive stromal reactions, resulting in a unique tumor microenvironment, especially the involvement of macrophages. These tumor-educated cells limit the efficacy of chemotherapy. Therefore, it is necessary to identify an effective treatment strategy. In this study, we aimed to explore the anti-tumor and immunomodulatory effects of osthole on pancreatic cancer. We found that osthole suppressed Panc 02 cell migration and proliferation and induced apoptosis as shown in vitro. Osthole also attenuated the development of pancreatic cancer in mice by inhibiting tumor-infiltrating M2 macrophages in our study. Additionally, osthole inhibited the polarization of primary bone marrow cells into M2 macrophages and inhibited the expression of MRC1, CCL22 and TGF-β in the M2 polarization process in vitro. Detection of the related signaling pathways revealed that osthole exerted immunomodulatory effects on M2 macrophages by down-regulating p-STAT6 and the p-ERK1/2-C/EBP β axis. These results indicated that osthole has effective anti-tumor and immunomodulatory effects on pancreatic cancer.

Distribution of M1 and M2 macrophages in tumor islets and stroma in relation to prognosis of non-small cell lung cancer

BackgroundNon-small cell lung cancer (NSCLC) remains the most common cause of cancer related death worldwide. Tumor-infiltrating macrophages are believed to play an important role in growth, progression, and metastasis of tumors. In NSCLC, the role of macrophages remains controversial; therefore, we aimed to evaluate the distribution of macrophages (M1 and M2) in tumor islets and stroma and to analyze their relations to patients’ survival.MethodsLung tissue specimens from 80 NSCLC patients who underwent surgical resection for NSCLC (pathological stage I-III) and 16 control group subjects who underwent surgery because of recurrent spontaneous pneumothorax were analyzed. Immunohistochemical double staining of CD68/iNOS (markers for M1 macrophages) and CD68/CD163 (markers for M2 macrophages) was performed and evaluated in a blinded manner. The numbers of M1 and M2 macrophages in tumor islets and stroma were counted manually.ResultsPredominant infiltration of M1 and M2 macrophages was observed in the tumor stroma compared with the tumor islets. M2 macrophages predominated over M1 macrophages in the tumor tissue. Tumor islets-infiltrating M1 macrophages and the number of total tumor-infiltrating M2 macrophages were independent predictors of patients survival: high infiltration of M1 macrophages in tumor islets was associated with increased overall survival in NSCLC (P < 0.05); high infiltration of total M2 macrophages in tumor (islets and stroma) was associated with reduced overall survival in NSCLC (P < 0.05).ConclusionsThis study demonstrated that high infiltration of M1 macrophages in the tumor islets and low infiltration of total tumor-infiltrating M2 macrophages were associated with improved NSCLC patients’ survival.Trial registrationClinicalTrials.gov NCT01955343, registered on September 27, 2013

Abstract 4005: HDAC inhibitors regulate M2 tumor-associated macrophage function through histone acetylation

Abstract Macrophage tumor infiltration in metastatic prostate cancer is a predictor of patient prognosis. Macrophages influence tumors in contrasting ways depending on their polarization: M1 macrophages have anti-tumor functions while M2 macrophages are pro-tumor. The external stimuli influencing macrophage differentiation and M2 polarization have been largely elucidated but the resulting downstream changes remain unknown. There are a number of epigenetic differences between M1 and M2 macrophages that act as important functional determinants. Recent work studying these epigenetic changes has implicated histone deacetylases (HDACs), as critical regulators in macrophage differentiation and in maintaining M1 or M2 function. Pan-HDAC inhibitors (HDIs) such as the clinically utilized suberanilohydroxamic acid (SAHA) target a wide range of HDACs and provide a means for manipulating macrophage histone acetylation. Though HDIs have been found to attenuate inflammatory functions in M1 macrophages, the field has yet to explore effects of histone acetylation in M2 macrophages. Using M2 macrophages derived from human CD14+ monocytes, we found that SAHA regulates M2 macrophage polarization and function through alteration of histone acetylation. Work is ongoing to test expression levels of canonical M2 markers such as the mannose receptor CD206 and scavenger receptor CD163 in M2 macrophages exposed to SAHA either during or after M2 polarization. Additionally, investigation into the ability of these populations to secrete M2 cytokines such as IL-10 and VEG-FA and induce EMT in prostate cancer cells is also being pursued. These results contribute to scientific knowledge of epigenetic regulation in macrophage function and elucidate strategies to decrease the pro-tumor function of tumor-infiltrating M2 macrophages. This work lays the foundation for using epigenetic regulators to modulate the tumor microenvironment and advance cancer medicine. Citation Format: Amber E. de Groot, Jelani C. Zarif, Kenneth J. Pienta. HDAC inhibitors regulate M2 tumor-associated macrophage function through histone acetylation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4005. doi:10.1158/1538-7445.AM2017-4005

Abstract 4126: CSF-1R inhibition blocks rhabdomyoscarcoma metastasis by polarizing macrophage differentiation

Abstract Colony Stimulating Factor 1 (CSF-1) is overexpressed by many tumors and predicts poor clinical outcome. CSF-1 expression results in the recruitment of CSF-1R (Fms)-expressing cells, which have been shown to suppress tumor immunity, thereby promoting cancer progression. PLX3397 is a novel, orally active small molecule inhibitor that selectively targets CSF-1R, KIT, and oncogenic FIT3. CSF-1R and KIT regulate the tumor, tumor microenvironment, and the pre-metastatic niche. However, it is unknown if targeting the myeloid CSF-1R expressing component of the tumor microenvironment regulates metastasis of pediatric tumors, such as rhabdomyosarcoma. Using the M3-9-M murine syngeneic orthotopic rhabdomyosarcoma model, we evaluated the effects of PLX3397 on primary tumor growth and metastatic progression. This highly metastatic rhabdomyosarcoma model has high infiltration of CSF-1R-expressing bone marrow-derived cells, which includes inflammatory monocytes, M2 macrophages, and myeloid-derived suppressor cells. Therefore, we hypothesized PLX3397 would have a direct effect on CSF-1R expressing M2 macrophages and inflammatory monocytes, resulting in decreased metastasis. Using both in vitro and in vivo models, we found that PLX3397 treatment decreased tumor-infiltrating M2 macrophages and skewed the differentiation of inflammatory monocytes toward M1 macrophages and development of immunocompetent mature dendritic cells. This resulted in impairment of spontaneous M3-9-M metastasis to secondary sites. In addition, we have found that the polarization of inflammatory monocytes reversed the T cell suppressive capability of tumor-derived myeloid cells. In conclusion, CSF-1R blockage with PLX3397 inhibited metastatic progression by decreasing the infiltration of M2 macrophages in the primary tumor and pre-metastatic niche. The polarization of inflammatory monocytes to dendritic cells restored the immune recognition and decreased metastasis. This data supports our rationale for testing PLX3397 in patients with high M2 infiltration and suggests value in this agent as an adjuvant setting for metastasis prevention in high-risk patients. Note: This abstract was not presented at the meeting. Citation Format: Justin Evans, Amber Giles, Caitlin Reid, Rosandra Kaplan. CSF-1R inhibition blocks rhabdomyoscarcoma metastasis by polarizing macrophage differentiation. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4126. doi:10.1158/1538-7445.AM2015-4126

CSF1R-Inhibition and Reduction of Macrophages Delays Multiple Myeloma Growth in a Non-T-Cell-Dependent Manner

Macrophages are important tumor-promoting cells, and can constitute up to half of the tumor mass, orchestrating remodeling, inducing angiogenesis and suppressing the immune system to terminate local inflammatory responses. Colony stimulating factor (CSF-1), also known as macrophage colony-stimulating factor (M-CSF), is the most important cytokine involved in survival, proliferation and differentiation of tissue macrophages and their precursors. Previous experiments from our group has shown that amount of CSF-1 in the tumor bed correlates with successful rejection of cancer cells in a TCR-transgenic model where CD4+ T cells recognize a tumor specific antigen produced by multiple myeloma cells. We therefore tested if reduced signaling via CSF-1R, and thereby a decrease in tumor infiltrating macrophages, would results in decreased growth of multiple myeloma cells. .PLX3397, a tyrosine kinase inhibitor with specificity for CSF1R and KIT, has been demonstrated to delay tumor growth in a CD8+ T-cell-dependent manner after chemotherapeutic treatment in a murine model of mammary carcinogenesis. Here, we show that MOPC315 multiple myeloma cells implanted subcutaneously in matrigel plugs in SCID mice become infiltrated with M2 phenotype macrophages that enhances their growth. When given chow containing PLX3397, recruitment of macrophages is strikingly reduced, with significant delay in tumor growth. Importantly, this delay in tumor development is not T cell dependent, since the anti-tumor effect is seen in T cell deficient SCID-mice as well as in BALB/c mice. Importantly, treatment with PLX3397 does not abrogate efficient tumor killing, and does not affect survival, when tested in a TCR-transgenic model where CD4+ T cells via induction of M1 macrophages reject multiple myeloma.In conclusion, PLX3397 delays tumor growth by reducing amounts of tumor infiltrating M2 macrophages, while rendering effective tumor killing by M1 macrophages uninhibited. Thus, drugs that inhibit signaling of CSF-1 could be of value in treatment of multiple myeloma. DisclosuresNo relevant conflicts of interest to declare.