CCL5 production in lung cancer cells leads to an altered immune microenvironment and promotes tumor development

Current immunotherapies for lung cancer are only effective in a subset of patients. Identifying tumor-derived factors that facilitate immunosuppression offers the opportunity to develop novel strategies to supplement and improve current therapeutics. We sought to determine whether expression of driver oncogenes in lung cancer cells affects cytokine secretion, alters the local immune environment, and influences lung tumor progression. We demonstrate that oncogenic EGFR and KRAS mutations, which are early events in lung tumourigenesis, can drive cytokine and chemokine production by cancer cells. One of the most prominent changes was in CCL5, which was rapidly induced by KRAS^G12V or EGFR^L858R expression, through MAPK activation. Immunocompetent mice implanted with syngeneic KRAS-mutant lung cancer cells deficient in CCL5 have decreased regulatory T cells (T_regs), evidence of T cell exhaustion, and reduced lung tumor burden, indicating tumor-cell CCL5 production contributes to an immune suppressive environment in the lungs. Furthermore, high CCL5 expression correlates with poor prognosis, immunosuppressive regulatory T cells, and alteration to CD8 effector function in lung adenocarcinoma patients. Our data support targeting CCL5 or CCL5 receptors on immune suppressive cells to prevent formation of an immune suppressive tumor microenvironment that promotes lung cancer progression and immunotherapy insensitivity.

Abstract 1737: Understanding the geometry and valency of bispecific antibodies in the optimization of tumor-dependent activation of 4-1BB

4-1BB is a TNF family receptor expressed on the surface of tumor-infiltrating T cells. Activation of 4-1BB enhances the activation, metabolism and function of tumor-infiltrating T cells and promotes tumor regression. Several anti-4-1BB antibodies have entered the clinic and have suffered from liver toxicity or lack of activity. To address the clinical liabilities with 4-1BB targeting, we designed bispecific 4-1BB x tumor associated antigen (TAA) antibodies to selectively activate T cells within the tumor microenvironment.

To understand the impact of antibody format on potency of T cell activation, we generated a panel of 4-1BBxTAA bispecific antibodies in different formats with the AzymetricTM and EFECTTM platforms. We compared potency and maximal activity of these constructs in co-culture assays with tumor cells and T or Jurkat reporter cells, in comparison to urelumab as a clinical benchmark. The AzymetricTM platform allowed the production and evaluation of unique antibody formats from which an extensive structure activity relationship analysis was performed. We also examined the accessible epitope space by sampling the possible paratope binding conformations of the molecule. From this we determined the geometric limits of antigen engagement.

Bispecific antibody constructs utilizing monovalent 4-1BB targeting showed inferior activity than urelumab in co-culture assays with TAAhi tumor cells. Activity greater than urelumab was seen with constructs containing two 4-1BB arms with one or two TAA binding scFv on the C-terminus of the Fc. Bivalent 4-1BB antibodies with an anti-TAA scFv fused to the N-terminus of the 4-1BB Fab showed decreased activity, potentially due to reduced ability to engage 4-1BB in a bivalent manner. Critically, the activity seen was transferable across multiple TAAs. The activity of the 4-1BB bispecific constructs was dependent on the expression level of the TAA. Without the presence of tumor cells, or with constructs where the TAA scFv was replaced by an irrelevant scFv, minimal activity was seen.

The modelling of the space accessible to the 4-1BB paratopes showed that the constructs with which we saw the most activity had a distance between 4-1BB and TAA binding paratopes similar to that seen between T and APC membranes as part of an immune synapse, suggesting that this may be a key characteristic for the development of conditional T cell agonist bispecifics.

This work represents an investigation of 4-1BBxTAA formats which would be difficult to make without an efficient bispecific technology. We identified a series of 4-1BBxTAA bispecific agonist antibody formats which were transferable between multiple TAAs. These formats also allowed optimization of activity and selectivity to promote maximal therapeutic index and efficacy, key factors which are potentially able to contribute to improved clinical outcomes.

Citation Format: Anna von Rossum, Harsh Pratap, Lisa Newhook, Elizabeth C. Halvorsen, Lee Freiburger, Renee Duan, Charles Stevens, Dimitri Tcaciuc, Gesa Volkers, Duncan Browman, Brandon Clavette, David Mills, Thomas Spreter, Daniel T. Patton. Understanding the geometry and valency of bispecific antibodies in the optimization of tumor-dependent activation of 4-1BB [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 1737.

Abstract A50: Interleukin-33 increases ST2+ regulatory T cells and promotes metastatic tumor growth in the lungs in an amphiregulin-dependent manner

Regulatory T cells (Tregs) can facilitate primary and metastatic tumor growth through the suppression of antitumor immunity. Emerging evidence suggests a distinct role for Tregs in tissue repair and tissue barrier integrity by IL-33 mediated activation of the IL-33 receptor (ST2), causing the production of the epidermal growth factor receptor (EGFR) ligand amphiregulin (AREG). Treg-derived AREG is critical for the repair of infection-induced damage in the lungs, and AREG may induce tumor cell proliferation, invasion, migration or resistance to apoptosis by signaling through EGFR. We have previously shown that immune-suppressive myeloid-derived suppressor cells (MDSCs), macrophages, and Tregs are recruited to the lungs of mice bearing metastatic mammary tumors and promote metastatic growth in the lungs. We now show that IL-33 is dramatically increased in and around metastatic tumor foci in the lungs of mice bearing metastatic murine mammary tumors. We have also found that the majority of Tregs in the lungs of metastatic tumor-bearing mice express ST2, that Tregs express significantly more ST2 than conventional T cells, and that ST2+ Tregs produce significantly more AREG than ST2- Tregs. The intranasal administration of recombinant IL-33 increased the proportion of AREG producing ST2+ Tregs and enhanced the level of phosphorylated EGFR (pEGFR) in the metastatic lungs. While recombinant AREG did not impact the proliferation of mammary tumor cells in vitro despite inducing a dose-dependent increase in pEGFR, intranasal administration of recombinant AREG resulted in a ten-fold increase in pulmonary metastatic tumor burden in vivo. Further, the intranasal administration of recombinant IL-33 significantly increased metastatic tumor burden in the lungs in an AREG-dependent manner. These data identify ST2+ Tregs as a microenvironmental source of AREG in the lungs of metastatic mammary tumor-bearing mice and show that inhibition of AREG can reduce IL33-mediated increases in metastatic tumor growth. Our findings highlight an important role for IL33 and AREG in promoting metastatic growth and support the development of therapeutic strategies to inhibit AREG to reduce tumor metastases in the lungs.

Citation Format: Elizabeth C. Halvorsen, S. Elizabeth Franks, Brennan J. Wadsworth, Bryant T. Harbourne, Rachel A. Cederberg, Catherine A. Steer, Itziar Martinez-Gonzalez, Jack Calder, William W. Lockwood, Kevin L. Bennewith. Interleukin-33 increases ST2+ regulatory T cells and promotes metastatic tumor growth in the lungs in an amphiregulin-dependent manner [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr A50.

Targeting myeloid-derived suppressor cells in combination with primary mammary tumor resection reduces metastatic growth in the lungs

BACKGROUND

Solid tumors produce proteins that can induce the accumulation of bone marrow-derived cells in various tissues, and these cells can enhance metastatic tumor growth by several mechanisms. 4T1 murine mammary tumors are known to produce granulocyte colony-stimulating factor (G-CSF) and increase the numbers of immunosuppressive CD11b⁺Gr1⁺ myeloid-derived suppressor cells (MDSCs) in tissues such as the spleen and lungs of tumor-bearing mice. While surgical resection of primary tumors decreases MDSC levels in the spleen, the longevity and impact of MDSCs and other immune cells in the lungs after tumor resection have been less studied.

METHODS

We used mass cytometry time of flight (CyTOF) and flow cytometry to quantify MDSCs in the spleen, peripheral blood, and lungs of mice bearing orthotopic murine mammary tumors. We also tested the effect of primary tumor resection and/or gemcitabine treatment on the levels of MDSCs, other immune suppressor and effector cells, and metastatic tumor cells in the lungs.

RESULTS

We have found that, similar to mice with 4T1 tumors, mice bearing metastatic 4T07 tumors also exhibit accumulation of CD11b⁺Gr1⁺ MDSCs in the spleen and lungs, while tissues of mice with non-metastatic 67NR tumors do not contain MDSCs. Mice with orthotopically implanted 4T1 tumors have increased granulocytic (G-) MDSCs, monocytic (M-) MDSCs, macrophages, eosinophils, and NK cells in the lungs. Resection of primary 4T1 tumors decreases G-MDSCs, M-MDSCs, and macrophages in the lungs within 48 h, but significant numbers of functional immunosuppressive G-MDSCs persist in the lungs for 2 weeks after tumor resection, indicative of an environment that can promote metastatic tumor growth. The chemotherapeutic agent gemcitabine depletes G-MDSCs, M-MDSCs, macrophages, and eosinophils in the lungs of 4T1 tumor-bearing mice, and we found that treating mice with gemcitabine after primary tumor resection decreases residual G-MDSCs in the lungs and decreases subsequent metastatic growth.

CONCLUSIONS

Our data support the development of therapeutic strategies to target MDSCs and to monitor MDSC levels before and after primary tumor resection to enhance the effectiveness of immune-based therapies and improve the treatment of metastatic breast cancer in the clinic.

IL-33 increases ST2+ Tregs and promotes metastatic tumour growth in the lungs in an amphiregulin-dependent manner

Regulatory T cells (Tregs) facilitate primary and metastatic tumour growth through the suppression of anti-tumour immunity. Emerging evidence suggests a distinct role for Tregs in mediating tissue repair and barrier integrity in the lungs by IL-33 mediated production of the growth factor amphiregulin (AREG). Dependent on the type of cancer and local microenvironment, AREG may induce tumour cell proliferation, invasion, migration or resistance to apoptosis by signaling through the epidermal growth factor receptor (EGFR). We have found that IL-33 is dramatically increased in and around metastatic tumour foci in the lungs of mice bearing orthotopic murine mammary tumours. We observed that Tregs express significantly more of the IL-33 receptor, ST2, relative to conventional T cells, that ST2⁺ Tregs accumulate in the lungs of metastatic tumour-bearing mice, and that ST2⁺ Tregs produce significantly more AREG than ST2^- Tregs. The intranasal administration of recombinant IL-33 increased the proportion of AREG producing ST2⁺ Tregs and enhanced the level of phosphorylated EGFR in the metastatic lungs. While recombinant AREG did not impact mammary tumour cell proliferation in vitro despite inducing a dose-dependent increase in phosphorylated EGFR, intranasal administration of AREG resulted in a ten-fold increase in pulmonary metastatic tumour burden in vivo. Further, the intranasal administration of recombinant IL-33 significantly increased metastatic tumour burden in the lungs in an amphiregulin-dependent manner. These data identify ST2⁺ Tregs as a microenvironmental source of AREG in the lungs of mice with orthotopic metastatic mammary tumours and highlight an important role for AREG in promoting metastatic tumour growth in the lungs.

Abstract A02: Oncogenic drivers of lung cancer induce production of CCL5 and recruitment of regulatory T-cells

Lung cancer development is driven by the expression of mutant oncogenes, with EGFR and KRAS being the most frequent mutations in lung adenocarcinoma. However, additional factors may influence lung tumor development and progression, including the balance of antitumor immune effector cells and pro-tumorigenic immune suppressor cells within the lung and lung tumor microenvironment. Tumor cells can evade immune attack by producing cytokines that recruit immune modulatory cells, such as regulatory T cells (Tregs), that promote a localized immune suppressive environment We hypothesized that oncogene signaling regulates the production of cytokines by tumor cells at the earliest stages of transformation that can recruit immune suppressive cells and promote lung tumour development. We used CIBERSORT-based analysis of gene expression data to quantify 22 different immune cell types from over 300 human lung adenocarcinomas and 100 matched normal lung tissues. We found that Tregs were significantly enriched in early-stage lung adenocarcinoma tumors compared to matched normal tissue from the same patient, and validated these findings with immunohistochemistry staining of lung sections. To identify cytokines that could recruit Tregs early in lung tumorigenesis, we used normal cells expressing doxycycline-inducible wild-type EGFR, mutant EGFRL858R or mutant KRASG12V. Secreted cytokines were quantified using a multiplex LUMINEX assay with subsequent validation by ELISA. Induction of EGFRL858R and KRASG12V expression in normal cells rapidly increased the production of CCL5 (RANTES), as did expression of wild-type EGFR in the presence of exogenous EGF. To elucidate the mechanism of oncogene-driven CCL5 secretion, we treated lung cancer cells harboring EGFR or KRAS mutations with a MEK inhibitor (trametinib) to disrupt oncogenic signaling downstream of EGFR and KRAS. In KRAS mutant lung cancer cells, treatment with trametinib decreased CCL5 production and inhibited both ERK and AKT signaling. To determine if oncogene-driven cytokines could induce migration of Tregs ex vivo, we used a trans-well assay with conditioned media from cells expressing doxycycline inducible EGFRL858R or KRASG12V. Conditioned media from EGFRL858R and KRASG12V-expressing cells induced Treg migration, which was mitigated by the addition of an anti-CCL5 antibody. These data indicate that oncogenic EGFR and KRAS signaling regulates expression of CCL5 in lung tumor cells, and that CCL5-mediated Treg recruitment to lung tumors may occur in early stages of lung tumor development. Therefore, targeted inhibition of CCL5, Tregs, and/or oncogenic EGFR and KRAS signaling may represent therapeutic strategies to block recruitment and function of immunosuppressive Tregs during lung tumor development.

Citation Format: Elizabeth Franks, Elizabeth C. Halvorsen, Etienne Melese, Unni Arun, Jenna L. Collier, Bryant T. Harbourne, Min Hee Oh, Lam Vivian, Gerry Krystal, John C. English, Wan L. Lam, Stephen Lam, Ninan Abraham, Kevin L. Bennewith, William W. Lockwood. Oncogenic drivers of lung cancer induce production of CCL5 and recruitment of regulatory T-cells [abstract]. In: Proceedings of the Fifth AACR-IASLC International Joint Conference: Lung Cancer Translational Science from the Bench to the Clinic; Jan 8-11, 2018; San Diego, CA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(17_Suppl):Abstract nr A02.

SHIP represses lung inflammation and inhibits mammary tumor metastasis in BALB/c mice

SH2-containing-inositol-5'-phosphatase (SHIP) is a negative regulator of the phosphatidylinositol-3-kinase pathway in hematopoietic cells and limits the development of leukemias and lymphomas. The potential role of SHIP in solid tumor development and metastasis remains unknown. While SHIP restricts the aberrant development of myeloid cells in C57BL/6 mice, there are conflicting reports regarding the effect of SHIP deletion in BALB/c mice with important consequences for determining the influence of SHIP in different model tumor systems. We generated SHIP-/- BALB/c mice and challenged them with syngeneic non-metastatic 67NR or metastatic 4T1 mammary tumors. We demonstrate that SHIP restricts the development, alternative-activation, and immunosuppressive function of myeloid cells in tumor-free and tumor-bearing BALB/c mice. Tumor-free SHIP-/- BALB/c mice exhibited pulmonary inflammation, myeloid hyperplasia, and M2-polarized macrophages and this phenotype was greatly exacerbated by 4T1, but not 67NR, tumors. 4T1-bearing SHIP-/- mice rapidly lost weight and died from necrohemorrhagic inflammatory pulmonary disease, characterized by massive infiltration of pulmonary macrophages and myeloid-derived suppressor cells that were more M2-polarized and immunosuppressive than wild-type cells. Importantly, while SHIP loss did not affect primary tumor growth, 4T1-bearing SHIP-/- mice had 7.5-fold more metastatic tumor cells in their lungs than wild-type mice, consistent with the influence of immunosuppressive myeloid cells on metastatic growth. Our findings identify the hematopoietic cell-restricted protein SHIP as an intriguing target to influence the development of solid tumor metastases, and support development of SHIP agonists to prevent the accumulation of immunosuppressive myeloid cells and tumor metastases in the lungs to improve treatment of metastatic breast cancer.

Emerging roles of T helper 17 and regulatory T cells in lung cancer progression and metastasis

Lung cancer is a leading cause of cancer-related deaths worldwide. Lung cancer risk factors, including smoking and exposure to environmental carcinogens, have been linked to chronic inflammation. An integral feature of inflammation is the activation, expansion and infiltration of diverse immune cell types, including CD4⁺ T cells. Within this T cell subset are immunosuppressive regulatory T (Treg) cells and pro-inflammatory T helper 17 (Th17) cells that act in a fine balance to regulate appropriate adaptive immune responses.In the context of lung cancer, evidence suggests that Tregs promote metastasis and metastatic tumor foci development. Additionally, Th17 cells have been shown to be an integral component of the inflammatory milieu in the tumor microenvironment, and potentially involved in promoting distinct lung tumor phenotypes. Studies have shown that the composition of Tregs and Th17 cells are altered in the tumor microenvironment, and that these two CD4⁺ T cell subsets play active roles in promoting lung cancer progression and metastasis.We review current knowledge on the influence of Treg and Th17 cells on lung cancer tumorigenesis, progression, metastasis and prognosis. Furthermore, we discuss the potential biological and clinical implications of the balance among Treg/Th17 cells in the context of the lung tumor microenvironment and highlight the potential prognostic function and relationship to metastasis in lung cancer.

Maraviroc decreases CCL8-mediated migration of CCR5(+) regulatory T cells and reduces metastatic tumor growth in the lungs

Regulatory T cells (Tregs) play a crucial physiological role in the regulation of immune homeostasis, although recent data suggest Tregs can contribute to primary tumor growth by suppressing antitumor immune responses. Tregs may also influence the development of tumor metastases, although there is a paucity of information regarding the phenotype and function of Tregs in metastatic target organs. Herein, we demonstrate that orthotopically implanted metastatic mammary tumors induce significant Treg accumulation in the lungs, which is a site of mammary tumor metastasis. Tregs in the primary tumor and metastatic lungs express high levels of C-C chemokine receptor type 5 (CCR5) relative to Tregs in the mammary fat pad and lungs of tumor-free mice, and Tregs in the metastatic lungs are enriched for CCR5 expression in comparison to other immune cell populations. We also identify that C-C chemokine ligand 8 (CCL8), an endogenous ligand of CCR5, is produced by F4/80(+) macrophages in the lungs of mice with metastatic primary tumors. Migration of Tregs toward CCL8 ex vivo is reduced in the presence of the CCR5 inhibitor Maraviroc. Importantly, treatment of mice with Maraviroc (MVC) reduces the level of CCR5(+) Tregs and metastatic tumor burden in the lungs. This work provides evidence of a CCL8/CCR5 signaling axis driving Treg recruitment to the lungs of mice bearing metastatic primary tumors, representing a potential therapeutic target to decrease Treg accumulation and metastatic tumor growth.

Emerging roles of regulatory T cells in tumour progression and metastasis

The metastasis of cancer is a complex and life-threatening process that is only partially understood. Immune suppressive cells are recognized as important contributors to tumour progression and may also promote the development and growth of tumour metastases. Specifically, regulatory T cells (Tregs) have been found to promote primary tumour progression, and emerging pre-clinical data suggests that Tregs may promote metastasis and metastatic tumour growth. While the precise role that Tregs play in metastatic progression is understudied, recent findings have indicated that by suppressing innate and adaptive anti-tumour immunity, Tregs may shield tumour cells from immune detection, and thereby allow tumour cells to survive, proliferate and acquire characteristics that facilitate dissemination. This review will highlight our current understanding of Tregs in metastasis, including an overview of pre-clinical findings and discussion of clinical data regarding Tregs and therapeutic outcome. Evolving strategies to directly ablate Tregs or to inhibit their function will also be discussed. Improving our understanding of how Tregs may influence tumour metastasis may lead to novel treatments for metastatic cancer.

Macrophages are more potent immune suppressors ex vivo than immature myeloid-derived suppressor cells induced by metastatic murine mammary carcinomas

Myeloid-derived suppressor cells (MDSCs) are emerging as potential promoters of metastatic tumor growth, and there is interest in targeting immature MDSCs by inducing their differentiation into more mature myeloid cells. We used all-trans retinoic acid (ATRA) to differentiate MDSCs in mice bearing metastatic 4T1 or 4TO7 murine mammary tumors, and assessed the immune-suppressive mechanisms and potencies of different myeloid cell subpopulations. Metastatic mammary tumors induced the accumulation of distinct populations of immature CD11b(+)Gr1(+)F4/80(-)Ly6C(mid)Ly6G(+) MDSCs ("Gr1(+) cells") and mature CD11b(+)Gr1(-)F4/80(+) cells ("F4/80(+) cells") in metastatic target organs. ATRA triggered the differentiation of Gr1(+) cells into F4/80(+) cells in the lungs and, unexpectedly, enhanced pulmonary metastatic tumor growth. We found that F4/80(+)Ly6C(-)Ly6G(-) mature macrophages (Ms) were up to 30-fold more potent immune suppressors than Gr1(+) cells on a per-cell basis, which we postulate may contribute to the increased metastatic growth observed with ATRA treatment. F4/80(+) cells and Gr1(+) cells used different reactive oxygen species (ROS)-mediated mechanisms of immunosuppression ex vivo, with F4/80(+) cells producing higher levels of ROS, which is consistent with their superior immunosuppressive abilities. These data highlight the potent immunosuppressive functions of Ms, reveal that Ms can suppress T cell responses via ROS production, and suggest that ROS inhibitors may be useful in promoting antitumor immune responses. Our findings also caution against using ATRA to modulate myeloid cell differentiation and function to treat breast cancer metastases in the lung, and support the development of therapeutic strategies to enhance antitumor immunity by targeting myeloid cells as a collective group.

Abstract A9: Immune suppressive myeloid cells induced by hypoxic mammary tumor cells persist after primary tumor resection and promote metastatic growth

A role for bone marrow-derived cells (BMDCs) in promoting metastatic tumor growth is emerging, with important implications for therapeutic strategies to decrease tumor metastases. While previous work has shown accumulation of CD11b+ BMDCs in the lungs of mice bearing metastatic breast tumors, questions remain about the precise identity of these cells, the factors that regulate CD11b+ cell accumulation, and the potential long-term influence of CD11b+ cells on metastatic growth. We used transplantable (4T1, 4TO7, 67NR) and spontaneous (polyomavirus middle-T; PyVmT) mammary tumor models to study the identity, induction, longevity, and function of CD11b+ BMDCs in tissues. Using flow cytometry and ex vivo immune suppression assays, we established that metastatic mammary tumors induce splenic expansion and pulmonary accumulation of functional CD11b+Gr1+ myeloid-derived suppressor cells (MDSCs) and CD11b+F4/80+ macrophages (Mφs). MDSCs suppress T cell-mediated immune responses against tumor cells and are thought to promote tumor cell survival in vivo, while both Mφs and MDSCs are known to have a variety of other functions that promote primary tumor growth and metastasis.

Interestingly, we found that not all tumors induce MDSC accumulation. Rather, MDSC expansion in the spleen and accumulation in the lungs is dependent on poorly oxygenated (hypoxic) cells in the primary tumor. MDSCs did not accumulate in mice bearing well-oxygenated 67NR tumors or in mice with spontaneous PyVmT tumors until hypoxia had developed in the primary PyVmT tumors. Further, specifically targeting hypoxic 4T1 tumor cells in vivo with the hypoxia-activated cytotoxin tirapazamine significantly decreased MDSC levels in the spleen and lungs. Since proteins secreted by tumor cells are capable of inducing BMDC mobilization and accumulation in tissues, we used proteomics-based analyses of medium conditioned by hypoxic or normoxic tumor cells in vitro to identify and quantify several hypoxia-induced secreted proteins that are involved in promoting myeloid cell accumulation in tissues. We found that continued secretion of proteins by primary tumors is required to maintain high levels of splenic MDSC expansion, since surgical resection of metastatic primary tumors rapidly decreased MDSC expansion in the spleen and accumulation in the lungs. Interestingly, functional immune suppressive MDSCs remained elevated in the lungs for several weeks after resection of metastatic primary tumors, and these myeloid cells were associated with enhanced subsequent pulmonary metastatic tumor growth.

In order to reduce the immune suppressive environment in the lungs, we used all-trans retinoic acid (ATRA) to induce the differentiation of immature MDSCs into mature Mφs. While ATRA treatment decreased MDSC levels and increased Mφs in the lungs, ATRA-treated mice displayed enhanced metastatic tumor growth compared to untreated mice. Consistent with these data, we found that Mφs were 30-fold more potent suppressors of T cell-mediated immune responses than MDSCs on a per cell basis. These data highlight the influence of immune suppressive myeloid cell sub-types on tumor metastasis, and identify immune suppressive Mφs as potential therapeutic targets.

Taken together, these data suggest that hypoxia-mediated accumulation of myeloid cells in the lungs can have long-lasting implications for the growth of secondary metastatic tumors after treatment. Our data support the development of therapeutic strategies to target hypoxic tumor cells and immune suppressive myeloid cells to treat metastatic breast cancer.

Citation Format: Melisa J. Hamilton, Momir Bosiljcic, Bryant T. Harbourne, Nancy E. LePard, Elizabeth C. Halvorsen, Ada Y. Kim, Judit P. Banath, Gerald Krystal, Kevin L. Bennewith. Immune suppressive myeloid cells induced by hypoxic mammary tumor cells persist after primary tumor resection and promote metastatic growth. [abstract]. In: Proceedings of the AACR Special Conference on Tumor Invasion and Metastasis; Jan 20-23, 2013; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2013;73(3 Suppl):Abstract nr A9.