Cytofluorometric characterization of the myeloid compartment of irradiated mouse tumors.

The use of ionizing radiation (IR) is a cornerstone for the treatment of cancer and radiotherapy (RT) is used in roughly 50% of cancer patients. It is now well established that RT exerts widespread effects on the tumor stroma, including the immune environment. Together with its deeply characterized effects on the lymphoid compartment, RT also deeply affects the myeloid cell compartment. Fluorescence-activated flow cytometry is one of the most widely used technologies in immunology, allowing the multiparametric analysis of cells on a cell-by-cell basis. Here, we provide a detailed flow cytometry protocol to analyze the myeloid cell populations of human papillomavirus (HPV)-positive TC1/Luc tumors engrafted in the oral mucosa of immunocompetent mice, and to evaluate their modulations in response to RT. The same method, with slight modifications, can be used to study the tumor myeloid cells from a variety of other mouse tumors.

Clinical and prognostic significance of CD14 (+) HLA-DR (-/low) myeloid-derived suppressor cells in patients with hepatocellular carcinoma received transarterial radioembolization with Yttrium-90.

Hepatocellular carcinoma (HCC) is the fifth most common cancer worldwide. For unresectable HCC, transarterial radioembolization (TARE) with Yttrium-90 is a widely used treatment. The aim of this study was to investigate whether monocytic myeloid-derived suppressor cells (M-MDSC) and CD39+ T cells can be non-invasive predictive biomarkers of radiological response and prognosis in patients with HCC treated with TARE. This study was conducted on 39 patients with HCC who were treated with TARE between August 2018 and December 2019 and the control group consisted of 23 healthy volunteers. CD4+, CD8+, CD39+ T cells, Natural killer (NK) cells, myeloid cells (MC) and M-MDSC parameters are examined in the course of TARE treatment with student t test and Kaplan-Meier method. There were statistically significant differences in M-MDSC, CD39+ T cells and MC values between healthy controls and HCC patients. A statistically significant difference was found in M-MDSC and CD4+ T cells values in the HCC patient group who responded to the treatment compared to those who did not. Survival analysis found that patients with lower frequencies (under 3.81%) of M-MDSC showed more prominent differences of overall survival (OS) compared to patients with all high groups. We found that M-MDSC in the peripheral blood might be a useful non-invasive biomarker to predict OS. We have shown for the first time that M-MDSC is correlated with treatment response in HCC patients treated with TARE. Additionally, we have found that the percentage of CD39+ T cells is high in HCC patients and these cells are positively correlated with M-MDSC.

Effects of long-term and brain-wide colonization of peripheral bone marrow-derived myeloid cells in the CNS.

BACKGROUND: Microglia, the primary resident myeloid cells of the brain, play critical roles in immune defense by maintaining tissue homeostasis and responding to injury or disease. However, microglial activation and dysfunction has been implicated in a number of central nervous system (CNS) disorders, thus developing tools to manipulate and replace these myeloid cells in the CNS is of therapeutic interest. METHODS: Using whole body irradiation, bone marrow transplant, and colony-stimulating factor 1 receptor inhibition, we achieve long-term and brain-wide (~ 80%) engraftment and colonization of peripheral bone marrow-derived myeloid cells (i.e., monocytes) in the brain parenchyma and evaluated the long-term effects of their colonization in the CNS. RESULTS: Here, we identify a monocyte signature that includes an upregulation in Ccr1, Ms4a6b, Ms4a6c, Ms4a7, Apobec1, Lyz2, Mrc1, Tmem221, Tlr8, Lilrb4a, Msr1, Nnt, and Wdfy1 and a downregulation of Siglech, Slc2a5, and Ccl21a/b. We demonstrate that irradiation and long-term (~ 6 months) engraftment of the CNS by monocytes induces brain region-dependent alterations in transcription profiles, astrocytes, neuronal structures, including synaptic components, and cognition. Although our results show that microglial replacement with peripherally derived myeloid cells is feasible and that irradiation-induced changes can be reversed by the replacement of microglia with monocytes in the hippocampus, we also observe that brain-wide engraftment of peripheral myeloid cells (relying on irradiation) can result in cognitive and synaptic deficits. CONCLUSIONS: These findings provide insight into better understanding the role and complexity of myeloid cells in the brain, including their regulation of other CNS cells and functional outcomes.

Low-Level Ionizing Radiation Induces Selective Killing of HIV-1-Infected Cells with Reversal of Cytokine Induction Using mTOR Inhibitors.

HIV-1 infects 39.5 million people worldwide, and cART is effective in preventing viral spread by reducing HIV-1 plasma viral loads to undetectable levels. However, viral reservoirs persist by mechanisms, including the inhibition of autophagy by HIV-1 proteins (i.e., Nef and Tat). HIV-1 reservoirs can be targeted by the "shock and kill" strategy, which utilizes latency-reversing agents (LRAs) to activate latent proviruses and immunotarget the virus-producing cells. Yet, limitations include reduced LRA permeability across anatomical barriers and immune hyper-activation. Ionizing radiation (IR) induces effective viral activation across anatomical barriers. Like other LRAs, IR may cause inflammation and modulate the secretion of extracellular vesicles (EVs). We and others have shown that cells may secrete cytokines and viral proteins in EVs and, therefore, LRAs may contribute to inflammatory EVs. In the present study, we mitigated the effects of IR-induced inflammatory EVs (i.e., TNF-α), through the use of mTOR inhibitors (mTORi; Rapamycin and INK128). Further, mTORi were found to enhance the selective killing of HIV-1-infected myeloid and T-cell reservoirs at the exclusion of uninfected cells, potentially via inhibition of viral transcription/translation and induction of autophagy. Collectively, the proposed regimen using cART, IR, and mTORi presents a novel approach allowing for the targeting of viral reservoirs, prevention of immune hyper-activation, and selectively killing latently infected HIV-1 cells.

Therapeutic targeting of tumor-associated myeloid cells synergizes with radiation therapy for glioblastoma.

Tumor-associated myeloid cells (TAMCs) are key drivers of immunosuppression in the tumor microenvironment, which profoundly impedes the clinical response to immune-dependent and conventional therapeutic modalities. As a hallmark of glioblastoma (GBM), TAMCs are massively recruited to reach up to 50% of the brain tumor mass. Therefore, they have recently been recognized as an appealing therapeutic target to blunt immunosuppression in GBM with the hope of maximizing the clinical outcome of antitumor therapies. Here we report a nano-immunotherapy approach capable of actively targeting TAMCs in vivo. As we found that programmed death-ligand 1 (PD-L1) is highly expressed on glioma-associated TAMCs, we rationally designed a lipid nanoparticle (LNP) formulation surface-functionalized with an anti-PD-L1 therapeutic antibody (αPD-L1). We demonstrated that this system (αPD-L1-LNP) enabled effective and specific delivery of therapeutic payload to TAMCs. Specifically, encapsulation of dinaciclib, a cyclin-dependent kinase inhibitor, into PD-L1-targeted LNPs led to a robust depletion of TAMCs and an attenuation of their immunosuppressive functions. Importantly, the delivery efficiency of PD-L1-targeted LNPs was robustly enhanced in the context of radiation therapy (RT) owing to the RT-induced up-regulation of PD-L1 on glioma-infiltrating TAMCs. Accordingly, RT combined with our nano-immunotherapy led to dramatically extended survival of mice in 2 syngeneic glioma models, GL261 and CT2A. The high targeting efficiency of αPD-L1-LNP to human TAMCs from GBM patients further validated the clinical relevance. Thus, this study establishes a therapeutic approach with immense potential to improve the clinical response in the treatment of GBM and warrants a rapid translation into clinical practice.

The immunological consequences of radiation-induced DNA damage.

Historically, our understanding of the cytotoxicity of radiation has centred on tumour cell-autonomous mechanisms of cell death. Here, tumour cell death occurs when a threshold number of radiation-induced non-reparable double-stranded DNA breaks is exceeded. However, in recent years, the importance of immune mechanisms of cell death has been increasingly recognised, as well as the impact of radiotherapy on non-malignant cellular components of the tumour microenvironment. Conserved antiviral pathways that detect foreign nucleic acid in the cytosol and drive downstream interferon (IFN) responses via the cyclic guanosine monophosphate-adenosine monophosphate synthase/stimulator of IFN genes (cGAS/STING) pathway are key components of the immune response to radiation-induced DNA damage. In preclinical models, acute induction of a type 1 IFN response is important for both direct and abscopal tumour responses to radiation. Inhibitors of the DNA damage response show promise in augmenting this inflammatory IFN response. However, a substantial proportion of tumours show chronic IFN signalling prior to radiotherapy, which paradoxically drives immunosuppression. This chronic IFN signalling leads to treatment resistance, and heterotypic interactions between stromal fibroblasts and tumour cells contribute to an aggressive tumour phenotype. The effect of radiotherapy on myeloid cell populations, particularly tumour-associated macrophages, has an additional impact on the immune tumour microenvironment. It is not yet clear how the above preclinical findings translate into a human context. Human tumours show greater intratumoural genomic heterogeneity and more variable levels of chromosomal instability than experimental murine models. High-quality translational studies of immunological changes occurring during radiotherapy that incorporate intrinsic tumour biology will enable a better understanding of the immunological consequences of radiation-induced DNA damage in patients. Copyright © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Targeting the CXCL12/CXCR4 pathway and myeloid cells to improve radiation treatment of locally advanced cervical cancer.

Cervical cancer is the fourth most commonly diagnosed cancer and the fourth leading cause of cancer death in women worldwide. Approximately half of cervical cancer patients present with locally advanced disease, for which surgery is not an option. These cases are nonetheless potentially curable with radiotherapy and cisplatin chemotherapy. Unfortunately, some tumours are resistant to treatment, and lymph node and distant recurrences are major problems in patients with advanced disease at diagnosis. New targeted treatments that can overcome treatment resistance and reduce metastases are urgently needed. The CXCL12/CXCR4 chemokine pathway is ubiquitously expressed in many normal tissues and cancers, including cervical cancer. Emerging evidence indicates that it plays a central role in cervical cancer pathogenesis, malignant progression, the development of metastases and radiation treatment response. Pre-clinical studies of standard-of-care fractionated radiotherapy and concurrent weekly cisplatin plus the CXCR4 inhibitor Plerixafor (AMD3100) in patient-derived orthotopic cervical cancer xenografts have shown improved primary tumour response and reduced lymph node metastases with no increase in early or late side effects. These studies have pointed the way forward to future clinical trials of radiotherapy/cisplatin plus Plerixafor or other newly emerging CXCL12 or CXCR4 inhibitors in women with cervical cancer.

4-(Nitrophenylsulfonyl)piperazines mitigate radiation damage to multiple tissues.

Our ability to use ionizing radiation as an energy source, as a therapeutic agent, and, unfortunately, as a weapon, has evolved tremendously over the past 120 years, yet our tool box to handle the consequences of accidental and unwanted radiation exposure remains very limited. We have identified a novel group of small molecule compounds with a 4-nitrophenylsulfonamide (NPS) backbone in common that dramatically decrease mortality from the hematopoietic acute radiation syndrome (hARS). The group emerged from an in vitro high throughput screen (HTS) for inhibitors of radiation-induced apoptosis. The lead compound also mitigates against death after local abdominal irradiation and after local thoracic irradiation (LTI) in models of subacute radiation pneumonitis and late radiation fibrosis. Mitigation of hARS is through activation of radiation-induced CD11b+Ly6G+Ly6C+ immature myeloid cells. This is consistent with the notion that myeloerythroid-restricted progenitors protect against WBI-induced lethality and extends the possible involvement of the myeloid lineage in radiation effects. The lead compound was active if given to mice before or after WBI and had some anti-tumor action, suggesting that these compounds may find broader applications to cancer radiation therapy.

An oral HemokineTM, α-methylhydrocinnamate, enhances myeloid and neutrophil recovery following irradiation in vivo.

An oral therapeutic which reduces duration of cytopenias and is active following accidental radiation exposures is an unmet need in radiation countermeasures. Alpha methylhydrocinnamate (ST7) prolongs STAT-5 phosphorylation, reduces growth-factor dependency of multi-lineage cell lines, and stimulates erythropoiesis. Here, ST7 and its isomers were studied for their effects on myeloid progenitors and hematopoietic stem cells (HSCs) following radiation, in nonhuman primates, and murine irradiation models. Addition of ST7 or ST7-S increased CFU-GM production by 1.7-fold (p<0.001), reduced neutrophil apoptosis comparable to G-CSF, and enhanced HSC survival post-radiation by 2-fold, (p=0.028). ST7 and ST7-S administered in normal baboons increased ANC and platelet counts by 50-400%. In sub-lethally-irradiated mice, ANC nadir remained >200/mm3 and neutropenia recovered in 6days with ST7 treatment and 18days in controls (p<0.05). In lethally-irradiated mice, marrow pathology at 15days was hypocellular (10% cellularity) in controls, but normal (55-75% cellularity) with complete neutrophil maturation with ST7-S treatment. Following lethal irradiation, ST7, given orally for 4days, reduced mortality, with 30% survival in ST7-animals vs 8% in controls, (p<0.05). Collectively, the studies indicate that ST7 and ST7-S enhance myeloid recovery post-radiation and merit further evaluation to accelerate hematologic recovery in conditions of radiation-related and other marrow hypoplasias.

M2-polarized macrophages contribute to neovasculogenesis, leading to relapse of oral cancer following radiation.

Despite the fact that radiation is one of the standard therapies in the treatment of patients with oral cancer, tumours can recur even in the early stages of the disease, negatively impacting prognosis and quality of life. We previously found that CD11b(+) bone marrow-derived cells (BMDCs) were recruited into human glioblastoma multiforme (GBM), leading to re-organization of the vasculature and tumour regrowth. However, it is not yet known how these cells contribute to tumour vascularization. In the present study, we investigated the role of infiltrating CD11b(+) myeloid cells in the vascularization and recurrence of oral squamous cell carcinoma (OSCC). In a xenograft mouse model, local irradiation caused vascular damage and hypoxia in the tumour and increased infiltration of CD11b(+) myeloid cells. These infiltrating cells showed characteristics of M2 macrophages (M2Mφs) and are associated with the promotion of vascularization. M2Mφs promoted tumour progression in recurrence after irradiation compared to non-irradiated tumours. In addition, we found that CD11b(+) myeloid cells, as well as CD206(+) M2Mφs, are increased during recurrence after radiotherapy in human OSCC specimens. Our findings may lead to the development of potential clinical biomarkers or treatment targets in irradiated OSCC patients.

Modeling Hematopoiesis and Responses to Radiation Countermeasures in a Bone Marrow-on-a-Chip.

Studies on hematopoiesis currently rely on animal models because in vitro culture methods do not accurately recapitulate complex bone marrow physiology. We recently described a bone marrow-on-a-chip microfluidic device that enables the culture of living hematopoietic bone marrow and mimics radiation toxicity in vitro. In the present study, we used this microdevice to demonstrate continuous blood cell production in vitro and model bone marrow responses to potential radiation countermeasure drugs. The device maintained mouse hematopoietic stem and progenitor cells in normal proportions for at least 2 weeks in culture. Increases in the number of leukocytes and red blood cells into the microfluidic circulation also could be detected over time, and addition of erythropoietin induced a significant increase in erythrocyte production. Exposure of the bone marrow chip to gamma radiation resulted in reduction of leukocyte production, and treatment of the chips with two potential therapeutics, granulocyte-colony stimulating factor or bactericidal/permeability-increasing protein (BPI), induced significant increases in the number of hematopoietic stem cells and myeloid cells in the fluidic outflow. In contrast, BPI was not found to have any effect when analyzed using static marrow cultures, even though it has been previously shown to accelerate recovery from radiation-induced toxicity in vivo. These findings demonstrate the potential value of the bone marrow-on-a-chip for modeling blood cell production, monitoring responses to hematopoiesis-modulating drugs, and testing radiation countermeasures in vitro.

Brain radiation injury leads to a dose- and time-dependent recruitment of peripheral myeloid cells that depends on CCR2 signaling.

BACKGROUND: Cranial radiotherapy is used to treat tumors of the central nervous system (CNS), as well as non-neoplastic conditions such as arterio-venous malformations; however, its use is limited by the tolerance of adjacent normal CNS tissue, which can lead to devastating long-term sequelae for patients. Despite decades of research, the underlying mechanisms by which radiation induces CNS tissue injury remain unclear. Neuroinflammation and immune cell infiltration are a recognized component of the CNS radiation response; however, the extent and mechanisms by which bone marrow-derived (BMD) immune cells participate in late radiation injury is unknown. Thus, we set out to better characterize the response and tested the hypothesis that C-C chemokine receptor type 2 (CCR2) signaling was required for myeloid cell recruitment following brain irradiation. METHODS: We used young adult C57BL/6 male bone marrow chimeric mice created with donor mice that constitutively express enhanced green fluorescent protein (eGFP). The head was shielded to avoid brain radiation exposure during chimera construction. Radiation dose and time response studies were conducted in wild-type chimeras, and additional experiments were performed with chimeras created using donor marrow from CCR2 deficient, eGFP-expressing mice. Infiltrating eGFP+ cells were identified and quantified using immunofluorescent microscopy. RESULTS: Brain irradiation resulted in a dose- and time-dependent infiltration of BMD immune cells (predominately myeloid) that began at 1 month and persisted until 6 months following ≥15 Gy brain irradiation. Infiltration was limited to areas that were directly exposed to radiation. CCR2 signaling loss resulted in decreased numbers of infiltrating cells at 6 months that appeared to be restricted to cells also expressing major histocompatibility complex class II molecules. CONCLUSIONS: The potential roles played by infiltrating immune cells are of current importance due to increasing interest in immunotherapeutic approaches for cancer treatment and a growing clinical interest in survivorship and quality of life issues. Our findings demonstrate that injury from brain radiation facilitates a dose- and time-dependent recruitment of BMD cells that persists for at least 6 months and, in the case of myeloid cells, is dependent on CCR2 signaling.

Lymphoid and Myeloid Recovery in Rhesus Macaques Following Total Body X-Irradiation.

Recovery from severe immunosuppression requires hematopoietic stem cell reconstitution and effective thymopoiesis to restore a functional immune cell repertoire. Herein, a model of immune cell reconstitution consequent to potentially lethal doses of irradiation is described, which may be valuable in evaluating potential medical countermeasures. Male rhesus macaques were total body irradiated by exposure to 6.00 Gy 250 kVp x-radiation (midline tissue dose, 0.13 Gy min), resulting in an approximate LD10/60 (n = 5/59). Animals received medical management, and hematopoietic and immune cell recovery was assessed (n ≤ 14) through 370 d post exposure. A subset of animals (n ≤ 8) was examined through 700 d. Myeloid recovery was assessed by neutrophil and platelet-related parameters. Lymphoid recovery was assessed by the absolute lymphocyte count and FACS-based phenotyping of B- and T-cell subsets. Recent thymic emigrants were identified by T cell receptor excision circle quantification. Severe neutropenia, lymphopenia, and thrombocytopenia resolved within 30 d. Total CD3+ cells µL required 60 d to reach values 60% of normal, followed by subsequent slow recovery to approximately normal by 180 d post irradiation. Recovery of CD3+4+ and CD3+8+ cell memory and naïve subsets were markedly different. Memory populations were ≥ 100% of normal by day 60, whereas naïve populations were only 57% normal at 180 d and never fully recovered to baseline post irradiation. Total (CD20+) B cells µL were within normal levels by 77 d post exposure. This animal model elucidates the variable T- and B-cell subset recovery kinetics after a potentially lethal dose of total-body irradiation that are dependent on marrow-derived stem and progenitor cell recovery, peripheral homeostatic expansion, and thymopoiesis.

Peripheral myeloid-derived suppressor and T regulatory PD-1 positive cells predict response to neoadjuvant short-course radiotherapy in rectal cancer patients.

Short-course preoperative radiotherapy (SC-RT) followed by total mesorectal excision (TME) is one therapeutic option for locally advanced rectal cancer (LARC) patients. Since radio-induced DNA damage may affect tumor immunogenicity, Myeloid-derived suppressor cells (MDSCs) and T regulatory cells (Tregs) were evaluated in 13 patients undergoing SC-RT and TME for LARC. Peripheral Granulocytic-MDSCs (G-MDSC) [LIN-/HLA-DR-/CD11b+/CD14-/CD15+/CD33+], Monocytic (M-MDSC) [CD14+/HLA-DR-/lowCD11b+/CD33+] and Tregs [CD4+/CD25hi+/FOXP3+- CTLA-4/PD1] basal value was significantly higher in LARC patients compared to healthy donors (HD). Peripheral MDSC and Tregs were evaluated at time 0 (T0), after 2 and 5 weeks (T2-T5) from radiotherapy; before surgery (T8) and 6-12 months after surgery (T9, T10). G-MDSC decreased at T5 and further at T8 while M-MDSC cells decreased at T5; Tregs reached the lowest value at T5. LARC poor responder patients displayed a major decrease in M-MDSC after SC-RT and an increase of Treg-PD-1. In this pilot study MDSCs and Tregs decrease during the SC-RT treatment could represent a biomarker of response in LARC patients. Further studies are needed to confirm that the deepest M-MDSC reduction and increase in Treg-PD1 cells within 5-8 weeks from the beginning of treatment could discriminate LARC patients poor responding to SC-RT.

Ablative Tumor Radiation Can Change the Tumor Immune Cell Microenvironment to Induce Durable Complete Remissions.

PURPOSE: The goals of the study were to elucidate the immune mechanisms that contribute to desirable complete remissions of murine colon tumors treated with single radiation dose of 30 Gy. This dose is at the upper end of the ablative range used clinically to treat advanced or metastatic colorectal, liver, and non-small cell lung tumors. EXPERIMENTAL DESIGN: Changes in the tumor immune microenvironment of single tumor nodules exposed to radiation were studied using 21-day (>1 cm in diameter) CT26 and MC38 colon tumors. These are well-characterized weakly immunogenic tumors. RESULTS: We found that the high-dose radiation transformed the immunosuppressive tumor microenvironment resulting in an intense CD8(+) T-cell tumor infiltrate, and a loss of myeloid-derived suppressor cells (MDSC). The change was dependent on antigen cross-presenting CD8(+) dendritic cells, secretion of IFNγ, and CD4(+)T cells expressing CD40L. Antitumor CD8(+) T cells entered tumors shortly after radiotherapy, reversed MDSC infiltration, and mediated durable remissions in an IFNγ-dependent manner. Interestingly, extended fractionated radiation regimen did not result in robust CD8(+) T-cell infiltration. CONCLUSIONS: For immunologically sensitive tumors, these results indicate that remissions induced by a short course of high-dose radiotherapy depend on the development of antitumor immunity that is reflected by the nature and kinetics of changes induced in the tumor cell microenvironment. These results suggest that systematic examination of the tumor immune microenvironment may help in optimizing the radiation regimen used to treat tumors by adding a robust immune response.

A study of the effect of sequential injection of 5-androstenediol on irradiation-induced myelosuppression in mice.

Herein, we aimed at examining the therapeutic effects of 5-androstenediol (5-AED), a natural hormone produced in the adrenal cortex, on radiation-induced myelosuppression in C3H/HeN mice. The mice were subjected to whole-body irradiation with a sublethal dose of 5 Gy gamma-irradiation to induce severe myelosuppression, and 5-AED (50 mg/kg) was administered subcutaneously. 5-AED was administrated 1 day before irradiation (pre-treatment) or twice weekly for 3 weeks starting from 1 h after irradiation (post-treatment). Treatment with 5-AED significantly ameliorated the decrease in the peripheral blood neutrophil and platelet populations in irradiated myelosuppressive mice, but had no effect on the lymphocyte population. It also ameliorated hypocellularity and disruption of bone marrow induced by irradiation and led to rapid recovery of myeloid cells. Further, it attenuated the decrease in spleen weight and megakaryocyte and myeloid cell populations in the spleen and promoted multilineage hematopoietic recovery. We found that a single injection of 5-AED produced only a temporary therapeutic effect, while sequential injection of 5-AED after irradiation had a more pronounced and prolonged therapeutic effect and reduced myelosuppression by irradiation. Thus, sequential injection of 5-AED after irradiation has therapeutic potential for radiation-induced myelosuppression when administered continuously and can be a significant therapeutic candidate for the management of acute radiation syndrome, particularly in a mass casualty scenario where rapid and economic intervention is important.

Myeloid-derived cells in tumors: effects of radiation.

The discrepancy between the in vitro and in vivo response to radiation is readily explained by the fact that tumors do not exist independently of the host organism; cancer cells grow in the context of a complex microenvironment composed of stromal cells, vasculature, and elements of the immune system. As the antitumor effect of radiotherapy depends in part on the immune system, and myeloid-derived cells in the tumor microenvironment modulate the immune response to tumors, it follows that understanding the effect of radiation on myeloid cells in the tumor is likely to be essential for comprehending the antitumor effects of radiotherapy. In this review, we describe the phenotype and function of these myeloid-derived cells, and stress the complexity of studying this important cell compartment owing to its intrinsic plasticity. With regard to the response to radiation of myeloid cells in the tumor, evidence has emerged demonstrating that it is both model and dose dependent. Deciphering the effects of myeloid-derived cells in tumors, particularly in irradiated tumors, is key for attempting to pharmacologically modulate their actions in the clinic as part of cancer therapy.

Expression of arginase I in myeloid cells limits control of residual disease after radiation therapy of tumors in mice.

An accumulating body of evidence demonstrates that radiation therapy can generate adaptive immune responses that contribute to tumor control. However, in the absence of additional immune therapy, the adaptive immune response is insufficient to prevent tumor recurrence or affect distant disease. It has been shown in multiple models that tumor-infiltrating myeloid cells exhibit alternative activation phenotypes and are able to suppress adaptive immune responses, and recent data suggests that the myeloid response in tumors treated with cytotoxic therapy limits tumor control. We hypothesized that tumor myeloid cells inhibit the adaptive immune response after radiation therapy through expression of the enzyme arginase I. Using a myeloid cell-specific deletion of arginase I in mice, we demonstrate an improved tumor control after radiation therapy. However, tumors still recurred despite the conditional knockdown of arginase I. Since multiple alternative factors may combine to inhibit adaptive immunity, we propose that targeting macrophage differentiation may be a more effective strategy than targeting individual suppressive pathways.

Gr-1+CD11b+ cells facilitate Lewis lung cancer recurrence by enhancing neovasculature after local irradiation.

Studies have shown that bone marrow-derived cells play an important role in tumor recurrence after chemotherapy and radiotherapy. In this study, we examined the relationship between the accumulation of Gr-1+CD11b+ cells and tumor recurrence after irradiation in tumor-bearing mice. By transplanting bone marrow cells into whole body-irradiated mice depleted of bone marrow, we assessed the role of Gr-1+CD11b+ cells in lung carcinoma models after local irradiation (LI). 20 Gy local irradiation could recruit CD11b+CXCR4+ cells into the irradiated tissues, and the recruited CD11b+CXCR4+ cells could promote tumor recurrence. Further 6 Gy whole body irradiation (WBI6Gy) could decrease tumor recurrence by inhibiting the accumulation of Gr-1+CD11b+ cells and then suppressing tumor vasculogenesis and angiogenesis. Our results suggest that the accumulation of CD11b+Gr-1+ cells promote tumor re-growth after local irradiation by enhancing tumor neovascularization, and low dose of whole body irradiation or irradiation of enlarged spleen may provide a new alternative for anti-angiogenesis therapies.