Myeloid Cells In Tumor Microenvironment Research Articles

AN0025 in combination with definitive chemoradiotherapy (dCRT) in unresectable locally advanced or locally recurrent esophageal cancer (EC): A single-arm, open-label, multicenter, phase Ib study.

e16076 Background: AN0025 is a selective EP4 inhibitor that demonstrates antitumor activity by modulating the accumulation and function of macrophages and immunosuppressive myeloid cells in tumor microenvironment. The combination of AN0025 with CRT as neoadjuvant therapy has shown synergistic antitumor efficacy in locally advanced rectal cancer in a prior clinical trial (NCT03152370). Methods: This Phase Ib study includes a dose escalation phase followed by an expansion phase to evaluate the safety, tolerability, and feasibility of AN0025 plus dCRT for unresectable locally advanced or locally recurrent EC or esophagogastric junction cancer. Antitumor efficacy was assessed per RECIST v1.1. The eligible patients (pts) are candidates for dCRT, who received RT (2.0 Gy/fraction, totally 50-60 Gy) with concurrent weekly paclitaxel and carboplatin for 4 weeks, followed by two doses of consolidation chemotherapy on Week 8 and Week 12. AN0025 (250 mg or 500 mg QD) was administered orally for 15 weeks concurrently with dCRT. At the beginning of Week 17, pts would start AN0025 monotherapy in the maintenance phase. Results: As of Jan 12, 2024, 12 pts (11 males and 1 female, median age 62) with histologically confirmed esophageal squamous cell carcinoma (ESCC) were enrolled and received the study treatment, 5 at the 250 mg dose level and 7 at the 500 mg dose level. Among those 12 pts, 11 were locally advanced ESCCs and 1 was locally recurrent ESCC. The most common AN0025-related adverse events (AEs) were weight decreased (33%), anemia (33%), and white blood cell count decreased (25%). Two pts (17%) experienced Grade ≥3 AN0025-related AEs (one Grade 3 weight decreased and one Grade 3 esophageal fistula). No dose-limiting toxicity occurred at either dose level, and the maximum tolerated dose was not reached. The median follow-up was 14.2 months, with 8 pts remaining on study treatment and a maximum time on treatment of 2 years. The disease control rate (DCR) was 92% (11/12). The median progression-free survival (PFS) was not reached, with 6-month and 1-year PFS rates of 91% and 73%, respectively. The 1-year overall survival (OS) rate was 82%. Additionally, among the 8 pts with measurable lesion(s) at baseline, 1 (13%) achieved confirmed complete response (CR), and 6 achieved partial response (PR), including 5 confirmed PRs, giving an unconfirmed overall response rate (ORR) of 88% (7/8) and a confirmed ORR of 75% (6/8). AN0025 PK approached linearity at both 250 mg and 500 mg QD. AN0025 PK in combination with dCRT showed no interactions. Conclusions: This Phase Ib study suggests that AN0025 plus dCRT exhibits favorable safety and encouraging preliminary efficacy in unresectable locally advanced or locally recurrent EC, warranting further investigation. Clinical trial information: NCT05191667 .

Abstract 2129: Predictive response biomarkers from Phase I clinical trial of a SIRPalpha inhibitor BI765063, stand-alone and in combination with ezabenlimab, a PD1 inhibitor, in patients with advanced solid tumors

Abstract Background: Signal Regulatory Protein α [SIRPα] is an inhibitory membrane receptor expressed by myeloid cells (macrophages and myeloid-derived suppressor cells, MDSCs) and specifically binds to CD47. BI765063 is a selective anti-SIRPα monoclonal antibody acting as a checkpoint inhibitor of SIRPα/CD47 axis, promoting anti-tumor immunity through increase of dendritic cells antigen presentation, enabling MDSC differentiation, potentiating macrophage phagocytic/inflammatory properties and reinstating myeloid cell chemokine secretion and human T cell migration1. The escalation phase I study in advanced solid tumor patients (pts) showed preliminary efficacy as monotherapy (MONO), and in combination with PD-1 inhibitor ezabenlimab (COMBO). Our goal was to investigate BI765063 predictive response markers. Methods: A total of 68 patients (50 in MONO arm, 18 in COMBO arm) have been enrolled. Receptor occupancy (RO) was determined on peripheral CD14+ monocytes. Tumor biopsies were collected before treatment from 44 pts (62%) representative of overall population. Tumor microenvironment (TME) was analysed using a Brightplex® IHC panel comprised of CD68+ macrophages, CD11b+myeloid cells, SIRPα, and CD47. NanoString tumor profiling used PanCancer IO360 panel. Results: One partial response (PR) in MONO (Hepatocellular carcinoma HCC), and 4 PRs in COMBO (2 endometrial cancer, 1 HCC, 1 iRecist PR in MSS CRC) were observed2. BI 765063 full RO saturation was achieved in V1/V1 pts treated with doses of 6 mg/kg and higher, while V1/V2 patients showed a more heterogeneous RO ranging between 40-80%, reaching an apparent saturation at doses of 12 mg/kg and higher. Comparison of pts by best response showed that baseline tumor SIRPα+ expression in CD68+ macrophages and in CD11b+ myeloid cells were higher in PR versus PD pts. High percentage of CD11b+SIRPα+ myeloid cells at baseline significantly correlated with better OS (p=0.023), while CD68+SIRPα+ macrophages high expression in TME showed a trend for a better OS but did not reach statistical significance. The CD47 tumor expression did not correlate with the OS. Responder signature based on the top deregulated genes in responders versus non-responders at baseline was devised to sort relevant TCGA cohorts and showed strong positive correlation with TME MDSC infiltration (p≤0.0001). Conclusions: High levels of CD11b+SIRPα+ myeloid cells in TME at baseline, but not CD47 tumor expression, correlates with longer survival while MDSC signature in TME at baseline correlates with clinical response. Thus, suggesting that MDSCs expressing SIRPα in TME could represent a predictive efficacy biomarker.

Abstract 4082: Impact of immunosuppressive monocytes on CART19 cell effector functions and outcomes

Abstract CD19 directed chimeric antigen receptor T (CART19) cell therapy has resulted in remarkable outcomes in B cell malignancies and was FDA approved in multiple indications. However, durable remissions are limited to 40% of treated patients. Inhibitory myeloid cells in tumor microenvironment have been found to suppress T cell expansion and contribute to failure of CART19 cell therapy. In this study, we aimed to unravel the interactions between monocytes, CART19 cells and tumor cells to understand how monocytes-CART19 cell interactions impact CART19 cell effector functions and clinical outcomes. Two sets of experiments were conducted, 1) use of healthy CART19 cells, CD19+ tumor cells, and healthy monocytes; 2) use of brexu-cel products from ZUMA-2 clinical trial treating mantle cell lymphoma (MCL), patient-matched monocytes and circulating MCL tumor cells (n = 11; 6 durable responders, 2 relapsed after initial response and 3 non-responders). CD28 costimulated CART19 (CART19-28ζ) cells generated in the lab from healthy donors were co-cultured with donor freshly isolated monocytes in the presence of Jeko-1 cells (a CD19+ MCL cell line). CART19 antigen specific proliferation was not inhibited by freshly isolated monocytes. When monocytes were co-cultured with CART19 and tumor cells, higher levels of eotaxin, GRO, MCP-3 and IL-7 were detected. When CART19 cells were co-cultured with the CD19+ JeKo-1 cells in the presence of ex vivo M2 polarized macrophages, CART19 antigen specific proliferation was inhibited (p=0.0045). Transwell experiments demonstrate that M2-induced CART19 inhibition is not contact dependent. Cytokine profile analysis indicated increased level of IL-1ra, IP-10 and MCP-1 and decreased level of IL-17A, sCD40L, IL-9 and MIP-1α when M2 macrophages were co-cultured with CART19 and tumor cells compared to co-cultures of tumor cells and CART19. Then we conducted ex vivo co-cultures of brexu-cel products, autologous monocytes and circulating MCL tumor cells from MCL patients (ZUMA-2) collected prior to CART19 cell infusion. Here we observed trends of elevation of IL-13 and IL-5 and reduction of GRO, MCP-3, MIP-1β and IL-8 in non-responders, compared to responders (durable responses or relapsed patients). Our results support that monocyte- and macrophage-dependent cytokine release could modulate CART19 effector and trafficking functions, and thus CART19 clinical outcomes. This warrants further investigation around strategies to improve durable responses to CART cell therapy. Citation Format: Kun Yun, Reona Leo Sakemura, Michelle J. Cox, Truc Huynh, Claudia Manriquez-Roman, Olivia Sirpilla, Carli M. Stewart, James H. Girsch, Ekene J. Ogbodo, Ismail Can, Brooke Kimball, Lionel A. Kankeu Fonkoua, Mehrdad Hefazi, Michael W. Ruff, Elizabeth L. Siegler, Mike Mattie, Sao-Mai Nguyen-Mau, Simone Filosto, Saad S. Kenderian. Impact of immunosuppressive monocytes on CART19 cell effector functions and outcomes. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 4082.

Abstract 6258: Development of humanized patient-derived xenograft models for triple negative breast cancer

Abstract As a new promising in vivo platform for personalized treatment in the field of cancer research, the most improved way has been adopted by far is grafting patient-derived xenograft (PDX) to humanized mouse models which represent the unique features of interaction with human immune system and tumor microenvironment (TME). Here, we engineered triple negative breast cancer (TNBC) humanized PDX mouse (Hu-PDX) model to observe human immune system and tumor growth in TME. In addition, we analyzed gene ontology of PDX isolated from hu or non-humanized mouse model. Method: Human CD34+hematopoietic stem cells (HSCs) derived from cord blood is reconstructed into mature human leukocytes (hCD45+) in NOD.Cg-PrkdcscidIL2γgtm1 Sug (NOG) mice. TNBC PDX tumors are established it with unmatched human leukocyte antigen (HLA) genotypes, confirmed by that hCD45+ engraftment levels did not exceed 20-30% in the peripheral blood of mice. To be repopulated functional human T cells in xenograft models with severely depleted thymus, hCD34-PBMC cells are directly intravenously injected into tail vein of humanized mice followed by hCD34+cells injected without the onset of graft-versus-host disease (GVHD). The growth and gene expression of TNBC PDX tumors, as well as engrafted human immune population including T, B, natural killer (NK) cells and myeloid cells in TME, are monitored and investigated in both hu and non-hu-mice model. Results: Human immune reconstruction of cells, of the lymphoid as well as myeloid lineages, has been successfully accomplished in peripheral blood, spleen, and bone marrow over time in TME. Although the number of human mature leukocytes was gradually decreased in mice by 17 weeks, humanized mice have enabled studies of human immunity and TME in limitation period. Also, one of remaining limitations is unable to positive and negative selection of human T cells in the humanized mouse thymus and required to human major histocompatibility complex (MHC) molecules in humanized mice. To overcome this barrier, additional stimulation factor for generating functional T cells have successfully engrafted by transferring hCD34-PBMC cells. Moreover, tumor infiltrating human lymphocytes (TILs) have obviously detected in Hu-PDX model, although the differences tumor growth rate was not significantly monitored, compared with non-humanized mice. Interestingly, the analysis of gene ontology; biological process, cellular component and molecular function, ribosomal relevant proteins, not immune related genes, were highly up-regulated in TNBC PDX humanized in comparison to non-humanized mouse model. Conclusion: Humanized PDX model will contribute to studies more efficiently for the machinery underlying immunological response in breast tumor microenvironment. Furthermore, it might be an important unknown mechanism that the effect of elevated cancer-related ribosomal proteins expression within hu immune-TME. Citation Format: Yujeong Her, Hye Yeon Son, Ju Hee Kim, Woo Hang Heo, Mingji Quan, Songbin Li, Hyeong Gon Moon. Development of humanized patient-derived xenograft models for triple negative breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 6258.

Combined Effects of Myeloid Cells in the Neuroblastoma Tumor Microenvironment.

Simple SummaryNeuroblastoma is a type of childhood solid cancer often characterized by resistance to treatment and with poor prognosis. The tumors contain a mixture of cell types as well as cancer cells, and one of the most common is a type of blood cells called myeloid cells. In this review article we present a summary of what is known about the function of myeloid cells within neuroblastoma tumors.Despite multimodal treatment, survival chances for high-risk neuroblastoma patients remain poor. Immunotherapeutic approaches focusing on the activation and/or modification of host immunity for eliminating tumor cells, such as chimeric antigen receptor (CAR) T cells, are currently in development, however clinical trials have failed to reproduce the preclinical results. The tumor microenvironment is emerging as a major contributor to immune suppression and tumor evasion in solid cancers and thus has to be overcome for therapies relying on a functional immune response. Among the cellular components of the neuroblastoma tumor microenvironment, suppressive myeloid cells have been described as key players in inhibition of antitumor immune responses and have been shown to positively correlate with more aggressive disease, resistance to treatments, and overall poor prognosis. This review article summarizes how neuroblastoma-driven inflammation induces suppressive myeloid cells in the tumor microenvironment and how they in turn sustain the tumor niche through suppressor functions, such as nutrient depletion and generation of oxidative stress. Numerous preclinical studies have suggested a range of drug and cellular therapy approaches to overcome myeloid-derived suppression in neuroblastoma that warrant evaluation in future clinical studies.

Immunoscore Signature Predicts Postoperative Survival and Adjuvant Chemotherapeutic Benefits in Esophageal Squamous Cell Carcinoma.

ObjectiveThe aim of this study was to construct the immunoscore (IS) to facilitate the prediction of postoperative survival and benefit from adjuvant chemotherapy (ACT) in esophageal squamous cell carcinoma (ESCC).MethodsA total of 249 patients who received radical esophagectomy at Fudan University Shanghai Cancer Center were divided into training set and testing set. Eighty-nine patients with ESCC from TCGA database were enrolled into the validation set. Myeloid cells in tumor microenvironment were evaluated by immunohistochemistry or CIBERSORT, and then were included into a LASSO Cox regression model to construct the immunoscore. The predictive value of the immunoscore for prognosis after surgery or ACT was analyzed.ResultsThe immunoscore was constructed by four types of myeloid cells including macrophages, neutrophils, mast cells, and dendritic cells and was demonstrated as IS=2^(0.527719*Mφ −0.2604269*MC-0.4812935*DC-0.4519706*Neu). The overall survival was significantly different between two immunotypes, which were divided according to the immunoscore, in all sets (P<0.001, P=0.005, and P=0.002, respectively). Immunotype A was identified as an independent predictor for survival benefit in all three sets (HR=2.068, P=0.005; HR=2.028, P=0.007; HR=6.474, P=0.007; respectively). In patients who received ACT, immunotype A was significantly related to longer overall survival both in the training set (P<0.001) and in the testing set (P=0.011). The nomogram based on immunotype and other clinicopathological factors showed good efficiency of predicting response to ACT. Finally, several important cytokines and pathways were highly enriched in immunoscore A subgroup.ConclusionThe immunoscore was an effective prognostic predictor in ESCC for patients undergoing surgical resection and receiving ACT.

ARG1 functions as a tumor suppressor in breast cancer.

Arginase I (ARG1) is a cytosolic enzyme that catalyzes the hydrolysis of L-arginine to L-ornithine and urea. The association of ARG1 with cancer has mostly been focused on the ARG1 released by tumor-associated myeloid cells in tumor microenvironment. However, the role of ARG1 expressed in cancer cells is unclear. Here, we showed that the expression of ARG1 in human breast cancer (BC) is related to a good prognosis in BC patients. Overexpression of ARG1 suppresses BC cell proliferation and migration in vitro and xenograft tumor growth and development in mouse models. Furthermore, ARG1 expression down-regulates the expression of p-AKT, leading to the de-activation of AKT signal pathway in BC cells. Thus, our results established that in contrast to the role of ARG1 released from tumor-associated myeloid cells in tumor microenvironment that promotes tumor immune escape, ARG1 expressed in BC cells suppresses AKT signaling pathway and functions as a tumor suppressor.

Arginase-1 production by myeloid cells in tumor microenvironment induce inflammasome activation and activate PI3K pathway in tumor-associated macrophages in head and neck cancer

Abstract Tumor-associated macrophages (TAMs) express cytokines and mediators that can suppress T cell recruitment and activation in the tumor microenvironment, support tumor growth, and promote resistance to therapy. We found that Arginase-1 is produced in high quantities by TAMs and tumor cells in head and neck cancer tumors with different background mutations. High levels of Arginase-1 were found in tumor samples and the circulation of patients and animal models of head and neck cancer. Arginase-1 is released into the cellular microenvironment and packed into the exosomes in a metabolically active form. Mechanistically, Arginase-1 further propagated the anti-inflammatory macrophage phenotype in the tumor microenvironment and induced the epithelial-mesenchymal transition (EMT) by activation of STAT3 and beta-catenin. Arginase-1 released from the tumor further induced activation of the autophagy cascade and the PI3K pathway in TAMs. Mechanistic results showed that STAT3 activation by Arginase-1 containing exosomes has a positive feedback effect on the expression of Arginase-1, Il-1β, and lead to inflammasome activation. The silencing of STAT3 negated the effect of Arginase-1 containing exosomes and led to modulation of the inflammasome activation in the head and neck cancer tumor microenvironment. These results indicate a mechanistic role of TAMs and Arginase-1 containing exosomes in the tumor microenvironment by activation of autophagy and inflammasome, and induction of the EMT. The mechanistic links presented in this study regarding the role of arginase in head and neck cancer pathogenesis could be targeted as therapeutic strategies.

Inhibition of NLRP3 inflammasome in tumor microenvironment leads to suppression of metastatic potential of cancer cells

Tumor microenvironment favors tumor cells to promote their growth and metastasis such as migration, invasion, and angiogenesis. IL-1β, one of the inflammatory cytokines released from myeloid cells in tumor microenvironment, plays an important role in development and progress of tumor. The activation of inflammasome is a critical step to secrete mature IL-1β through stepwise reactions to activate capspase-1. Therefore, we investigated whether the inhibition of NACHT, LRR and PYD domains-containing protein 3 (NLRP3) inflammasome in macrophages regulated the metastatic potential of tumor cells. NLRP3 inflammasome was activated by ATP in bone marrow-derived primary mouse macrophages. The metastatic potential of mouse melanoma cell line (B16F10) was determined by migration and invasion assays with transwell system. ATP-treated wild-type macrophages increased the migration and invasion of melanoma cells. However, NLRP3- or caspase-1-knockout macrophages exhibited greatly diminished ability to promote the migration and invasion of melanoma cells. In addition, treatment with celastrol, an inhibitor of NLRP3 inflammasome, reduced the potency of macrophages to stimulate migration and invasion of melanoma cells. The results demonstrate that inhibition of the NLRP3 inflammasome in macrophages by genetic deficiency or a pharmacological inhibitor is linked to suppression of the metastatic potential of tumor cells. The results would provide a novel anti-cancer strategy to modulate tumor microenvironment by suppressing NLRP3 inflammasome and consequently reducing IL-1β production.

Abstract 69: Chemotherapy-induced cancer cell death diminishes therapeutic efficacy through heme oxygenase-1-mediated inactivation of M1-like tumor associated macrophages

Abstract One of the most common treatment options for cancer is a chemotherapy. After chemotherapy, however, unwanted host effects provoke tumor recurrence and aggressiveness of cancer cells, which often arise as a consequence of disruption in the patient's immune system. Lymphocytes, such as CD8+ cytotoxic T lymphocytes (CTLs), which have capability of suppressing cancer progression, are depleted following chemotherapy. Conventional chemotherapeutic agents kill cancer cells, which boosts recruitment of myeloid cells. Tumor-associated macrophages (TAMs), emerging as abundant tumor-infiltrating myeloid cells in tumor microenvironment, play an important role in immunosuppression caused by conventional chemotherapy. Chemotherapy-induced dying cancer cells could hijack accumulated TAMs, provoking tumor recurrence. Therefore, reprogramming of TAMs to maximize the chemotherapeutic efficacy is considered a promising novel anticancer strategy. In this study, we investigated whether dying cancer cells after chemotherapy could reduce therapeutic efficacy by modulating tumor-infiltrating macrophage activity. In a 4T1 syngeneic mouse breast cancer model, there was a marked decrease in the proportion of M1-like TAMs expressing CD86 with tumor-suppressive activity and reduced tumor infiltration of CTLs following treatment with paclitaxel. Paclitaxel treatment also resulted in an enhancement of heme oxygenase-1 (HO-1) mRNA levels in tumor-infiltrating myeloid cells engulfing dying cancer cells. Consistent with the in vivo profile of TAMs, bone marrow-derived macrophages (BMDMs) phagocytosing dying breast cancer cells exhibited significant upregulation of HO-1 expression. HO-1 induction in BMDMs engulfing dying breast cancer cells facilitated M2 polarization. In contrast, abrogation of HO-1 activity sustained M1 activation of BMDMs co-cultured with dying breast cancer cells. Pharmacologic inhibition of HO-1 activity with zinc protoporphyin IX augmented paclitaxel efficacy by stimulating tumor infiltration of CTLs in a 4T1 breast cancer model. Furthermore, blockade of HO-1 in breast tumor-bearing mice induced increased M1-like TAMs expressing CD86 that plays an important role in stimulating antitumor immune response. Taken together, HO-1 induction by the chemotherapy-derived dying cancer cells in TAMs dampens antitumor effects of chemotherapy by manipulating innate and adaptive immunity. Our results demonstrate that targeting overexpressed HO-1 in the breast tumor microenvironment can control the immune system to potentiate the efficacy of chemotherapy. Citation Format: Seung Hyeon Kim, Xiancai Zhong, Shin-Young Gwak, Ishrat Aklima Muna, Sin-Aye Park, Young-Nam Cha, Young-Joon Surh. Chemotherapy-induced cancer cell death diminishes therapeutic efficacy through heme oxygenase-1-mediated inactivation of M1-like tumor associated macrophages [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 69.

Abstract BS1-2: Myeloid-derived suppressor cells: many facets of regulation of tumor progression

Abstract Tumor associated macrophages (TAM), myeloid derived suppressor cells (MDSC), represented by PMN-MDSC and M-MDSC, and dendritic cells (DC) are major components of tumor microenvironment playing critical role in tumor progression. There are now compelling evidence linking MDSC with negative clinical outcomes in many types of cancer. Therefore, these cells are attractive therapeutic targets. Recently, several therapeutic strategies have been developed that target MDSC in mouse animal models and in first clinical trials. Recent findings of selective targeting of MDSC in cancer patients using agonistic TRAIL receptor DR5 antibody will be discussed. In 16 patients with advanced solid tumors this antibody demonstrated selective depletion of MDSC without affecting populations of neutrophils, monocytes, or lymphocytes. Another therapeutic strategy involves histone deacetilase inhibitor – entinostat. Recent study in tumor-bearing mice revealed that entinostat slightly increased expansion and accumulation of granulocytic cells in tumors. However, in contrast to untreated mice, these cells lacked suppressive activity. This effect was associated with potent antitumor activity of combination of entinostat with check-point inhibitors. By studying the effect of CSF1R inhibitor in tumor-bearing mice we identified the complex network regulating different groups of myeloid cells in tumor microenvironment. This network involves tumor cells and cancer associated fibroblast and protect tumors from loss of individual component of myeloid cell compartment. We determined that this network is operational not only in mice but also in cancer patients. Targeting of this network dramatically enhance the effect of cancer therapy. Citation Format: Gabrilovich DI. Myeloid-derived suppressor cells: many facets of regulation of tumor progression [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr BS1-2.

CD45 phosphatase regulates the fate of myeloid cells in tumor microenvironment by inhibiting STAT3 activity

Abstract The abnormal differentiation of myeloid cells is one of the key features of altered immune response in cancer. Myeloid derived suppressor cells (MDSC) and tumor associated macrophages (TAM) are the major components of immune suppressive network. MDSC accumulate in large numbers in peripheral lymphoid organs and inside tumors. However, in contrast to peripheral lymphoid organs such as spleen, inside tumors, monocytic MDSC rapidly differentiate to TAM with potent immune suppressive activity. We have demonstrated that this phenomenon is mediated by dramatic down-regulation of STAT3 activity in MDSC in tumor site. In cancer patients, MDSC in tumor site also had substantially lower level of pSTAT3 than MDSC in peripheral blood. These results were unexpected, since high level of STAT3 activity is a hallmark of MDSC present in blood and lymphoid organs. The mice with constitutively active STAT3 had shown the expansion of MDSC and down-regulation of macrophage population in tumor microenvironment, emphasizing the crucial role of STAT3 in myeloid cell differentiation. Hypoxia is an intricate part of tumor microenvironment. The decreased STAT3 activity was caused by hypoxia and affected all myeloid cells but was not observed in tumor cells. We further demonstrate that the up-regulation of CD45 tyrosine phosphatase activity in MDSC exposed to hypoxia in tumor site was responsible for down-regulation of STAT3 activity. Moreover, the positive regulation of CD45 activity in hypoxia was mediated by the disruption of CD45 protein dimerization via sialylation. This suggests a novel role of STAT3 in the regulation of myeloid cells in tumors with potential implications for therapeutic targeting.

Abstract A74: Spatial organization of myeloid cells in tumor microenvironment

Abstract Myeloid cells infiltrating tumor microenvironment orchestrate immune response to induce cancer progression. From tumor cell proliferation to angiogenesis, invasion, and metastasis, every step of cancer development is controlled by distinct types of immune cells. Tumor-associated macrophages (TAMs), myeloid derived suppressor cells (MDSCs), TIE2-expressing monocytes (TEMs), tumor associated neutrophils (TANs), and sub-population of dendritic cells are examples of tumor promoting myeloid cells. These numerous types of myeloid cells have been identified and characterized but their spatial organization within the tumor microenvironment have not been analyzed thoroughly. To understand their actions and mechanisms on cancer promotion, understanding their organization is required. By using EL4 thymoma syngenic graft model, we analyzed composition of myeloid cells infiltrating EL4 tumor microenvironment and revealed spatial organization of each type of myeloid cells. Dozens of immune cell surface markers were used to comprehensively define and characterize tumor infiltrating myeloid cells. Most of CD11b positive myeloid cells were localized at surrounding matrix of tumor mass. In particular, immunosuppressive immune cells like MDSCs were localized surrounding matrix of tumor and barely infiltrating actual tumor mass. In contrast, TAMs and dendritic cells are integrated with tumor cells inside the tumor mass. Dendritic cells can be subdivided based on CD11b expression level and those two distinct subsets are localized at distinct area of tumor mass. This analysis of spatial localization of immune cells implies division of labor among the cancer promoting myeloid cells. Tumor mass integrated myeloid cells suspected to promote angiogenesis and tumor cell proliferation. Tumor surrounding matrix localized myeloid cells suspected to promote immune suppression, invasion and metastasis. Citation Format: Minhyeok Kim, Seon Woo Lee, Suk-Jo Kang, Changwon Kang. Spatial organization of myeloid cells in tumor microenvironment. [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy: A New Chapter; December 1-4, 2014; Orlando, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2015;3(10 Suppl):Abstract nr A74.

Targeting tumor-associated myeloid cells for cancer immunotherapy

Tumor-associated myeloid cells undermine the therapeutic efficacy of cancer immunotherapy by their inhibitory properties on immune effector cells. Development of therapeutic agents to deplete suppressive myeloid cells in tumor microenvironment requires identification of cell-specific targets. A competitive phage display technique on live cells paves the way to discovery of such a target.