Identification of a CD11b+/Gr-1+/CD31+ myeloid progenitor capable of activating or suppressing CD8+T cells

New insights on extramedullary granulopoiesis and neutrophil heterogeneity in the spleen and its importance in disease

Neutrophils are traditionally viewed as uncomplicated exterminators that arrive quickly at sites of infection, kill pathogens, and then expire. However, recent studies employing modern transcriptomics coupled with novel imaging modalities have discovered that neutrophils exhibit significant heterogeneity within organs and have complex functional roles ranging from tissue homeostasis to cancer and chronic pathologies. This has revised the view that neutrophils are simplistic butchers, and there has been a resurgent interest in neutrophils. The spleen was described as a granulopoietic organ more than 4 decades ago, and studies indicate that neutrophils are briefly retained in the spleen before returning to circulation after proliferation. Transcriptomic studies have discovered that splenic neutrophils are heterogeneous and distinct compared with those in blood. This suggests that a unique hematopoietic niche exists in the splenic microenvironment, i.e., capable of programming neutrophils in the spleen. During severe systemic inflammation with an increased need of neutrophils, the spleen can adapt by producing neutrophils through emergency granulopoiesis. In this review, we describe the structure and microanatomy of the spleen and examine how cells within the splenic microenvironment help to regulate splenic granulopoiesis. A focus is placed on exploring the increase in splenic granulopoiesis to meet host needs during infection and inflammation. Emerging technologies such as single-cell RNA sequencing, which provide valuable insight into splenic neutrophil development and heterogeneity, are also discussed. Finally, we examine how tumors subvert this natural pathway in the spleen to generate granulocytic suppressor cells to promote tumor growth.

Myeloid-derived suppressor cells in influenza virus-induced asthma exacerbation

Myeloid-derived suppressor cells (MDSCs) are a phenotypically heterogenous group of cells that potently suppress the immune response. A growing body of evidence supports the important role of MDSCs in a variety of lung diseases, such as asthma. However, the role of MDSCs in asthma exacerbation has so far not been investigated. Here, we studied the role of MDSCs in a murine model of influenza virus-induced asthma exacerbation. BALB/c mice were exposed to house dust mite (HDM) three times a week for a total of five weeks to induce a chronic asthmatic phenotype, which was exacerbated by additional exposure to the A/Hamburg/5/2009 hemagglutinin 1 neuraminidase 1 (H1N1) influenza virus. Induction of lung inflammatory features, production of T helper (Th) 1- and Th2- associated inflammatory cytokines in the lavage fluid and an increased airway hyper-responsiveness were observed, establishing the asthma exacerbation model. The number and activity of pulmonary M-MDSCs increased in exacerbated asthmatic mice compared to non-exacerbated asthmatic mice. Furthermore, depletion of MDSCs aggravated airway hyper-responsiveness in exacerbated asthmatic mice. These findings further denote the role of MDSCs in asthma and provide some of the first evidence supporting a potential important role of MDSCs in asthma exacerbation.

Dysregulated Immunometabolism Is Associated with the Generation of Myeloid-Derived Suppressor Cells in Staphylococcus aureus Chronic Infection

Myeloid-derived suppressor cells (MDSCs) are a compendium of immature myeloid cells that exhibit potent T-cell suppressive capacity and expand during pathological conditions such as cancer and chronic infections. Although well-characterized in cancer, the physiology of MDSCs in the infection setting remains enigmatic. Here, we integrated single-cell RNA sequencing (scRNA-seq) and functional metabolic profiling to gain deeper insights into the factors governing the generation and maintenance of MDSCs in chronic Staphylococcus aureus infection. We found that MDSCs originate not only in the bone marrow but also at extramedullary sites in S. aureus-infected mice. scRNA-seq showed that infection-driven MDSCs encompass a spectrum of myeloid precursors in different stages of differentiation, ranging from promyelocytes to mature neutrophils. Furthermore, the scRNA-seq analysis has also uncovered valuable phenotypic markers to distinguish mature myeloid cells from immature MDSCs. Metabolic profiling indicates that MDSCs exhibit high glycolytic activity and high glucose consumption rates, which are required for undergoing terminal maturation. However, rapid glucose consumption by MDSCs added to infection-induced perturbations in the glucose supplies in infected mice hinders the terminal maturation of MDSCs and promotes their accumulation in an immature stage. In a proof-of-concept in vivo experiment, we demonstrate the beneficial effect of increasing glucose availability in promoting MDSC terminal differentiation in infected mice. Our results provide valuable information of how metabolic alterations induced by infection influence reprogramming and differentiation of MDSCs.

Generation of Myeloid Cells in Cancer: The Spleen Matters

Myeloid cells are key components of the tumor microenvironment and critical regulators of disease progression. These innate immune cells are usually short-lived and require constant replenishment. Emerging evidence indicates that tumors alter the host hematopoietic system and induce the biased differentiation of myeloid cells to tip the balance of the systemic immune activities toward tumor-promoting functions. Altered myelopoiesis is not restricted to the bone marrow and also occurs in extramedullary organs. In this review, we outline the recent advances in the field of cancer-associated myelopoiesis, with a focus on the spleen, the major site of extramedullary hematopoiesis in the cancer setting. We discuss the functional specialization, distinct mechanisms, and clinical relevance of cancer-associated myeloid cell generation from early progenitors in the spleen and its potential as a novel therapeutic target.

Tracking of intertissue migration reveals the origins of tumor-infiltrating monocytes

Myeloid cells such as monocytes and monocyte-derived macrophages promote tumor progression. Recent reports suggest that extramedullary hematopoiesis sustains a sizable reservoir of tumor-infiltrating monocytes in the spleen. However, the influence of the spleen on tumor development and the extent to which spleen monocytes populate the tumor relative to bone marrow (BM) monocytes remain controversial. Here, we used mice expressing the photoconvertible protein Kikume Green-Red to track the redistribution of monocytes from the BM and spleen, and mice expressing fluorescent ubiquitination-based cell-cycle indicator proteins to monitor active hematopoiesis in these tissues. In mice bearing late-stage tumors, the BM, besides being the major site of monocyte production, supplied the expansion of the spleen reservoir, replacing 9% of spleen monocytes every hour. Deployment of monocytes was equally rapid from the BM and the spleen. However, BM monocytes were younger than those in the spleen and were 2.7 times more likely to migrate into the tumor from the circulation. Partly as a result of this intrinsic difference in migration potential, spleen monocytes made only a minor contribution to the tumor-infiltrating monocyte population. At least 27% of tumor monocytes had traveled from the BM in the last 24 h, compared with only 2% from the spleen. These observations highlight the importance of the BM as the primary hematopoietic tissue and monocyte reservoir in tumor-bearing mice, despite the changes that occur in the spleen monocyte reservoir during tumor development.

Matrix metalloproteinase 12 overexpression in lung epithelial cells plays a key role in emphysema to lung bronchioalveolar adenocarcinoma transition

Chronic obstructive pulmonary disease (COPD) and lung cancer are two diseases that are related to smoking in humans. The molecular mechanism linking these two diseases is poorly understood. Matrix metalloproteinase 12 (MMP12) is a member of the MMP family, which can be induced by smoking. Because MMP12 overexpression in epithelial cells has been reported in inflammation-triggered lung remodeling, a murine CCSP-rtTA/(tetO)(7)-MMP12 bitransgenic model was created. In this model, MMP12-Flag fusion protein overexpression and its increased enzymatic activity were observed in the lung in an inducible manner, which led to inflammatory cell infiltration and increased epithelial growth. In sequential events, spontaneous emphysema and bronchioalveolar adenocarcinoma were developed as a result of MMP12 overexpression. During this process, the concentration of interleukin-6 was steadily increased in bronchioalveolar lavage fluid, which activated the oncogenic signal transducer and activator of transcription 3 (Stat3) in alveolar type II epithelial cells. Expression of Stat3 downstream genes that are known to stimulate inflammation and tumor formation was significantly increased in the lung. When tested in humans, MMP12 up-regulation was highly associated with COPD and lung cancer in patients. Together, these studies support that MMP12 is a potent proinflammatory and oncogenic molecule. MMP12 up-regulation plays a critical role in emphysema to lung cancer transition that is facilitated by inflammation.

The liver is a site for tumor-induced myeloid-derived suppressor cell accumulation and immunosuppression

Tumor-induced immunosuppression plays a key role in tumor evasion of the immune system. A key cell population recognized as myeloid-derived suppressor cells (MDSC) contributes and helps orchestrate this immunosuppression. MDSC can interact with T cells, macrophages, and natural killer cells to create an environment favorable for tumor progression. In various tumor models, their presence at high levels has been reported in the bone marrow, blood, spleen, and tumor. We report for the first time that MDSC accumulate and home to the liver in addition to the other organs. Liver MDSC suppress T cells and accumulate to levels comparable with splenic MDSC. Additionally, hematopoiesis in the liver contributes to the dramatic expansion of MDSC in this organ. Furthermore, MDSC in the liver interact with macrophages, also known as Kupffer cells, and cause their up-regulation of PD-L1, a negative T-cell costimulatory molecule. The liver is thus an organ in which MDSC accumulate and can contribute to immunosuppression directly and indirectly. MDSC play a role in various pathologic states in addition to cancer, and these results contribute to our understanding of their biology and interactions with immune-related cells.

G-CSF-initiated myeloid cell mobilization and angiogenesis mediate tumor refractoriness to anti-VEGF therapy in mouse models

Recent studies suggest that tumor-associated CD11b(+)Gr1(+) myeloid cells contribute to refractoriness to antiangiogenic therapy with an anti-VEGF-A antibody. However, the mechanisms of peripheral mobilization and tumor-homing of CD11b(+)Gr1(+) cells are unclear. Here, we show that, compared with other cytokines [granulocyte-macrophage colony stimulating factor (GM-CSF), stromal derived factor 1alpha, and placenta growth factor], G-CSF and the G-CSF-induced Bv8 protein have preferential expression in refractory tumors. Treatment of refractory tumors with the combination of anti-VEGF and anti-G-CSF (or anti-Bv8) reduced tumor growth compared with anti-VEGF-A monotherapy. Anti-G-CSF treatment dramatically suppressed circulating or tumor-associated CD11b(+)Gr1(+) cells, reduced Bv8 levels, and affected the tumor vasculature. Conversely, G-CSF delivery to animals bearing anti-VEGF sensitive tumors resulted in reduced responsiveness to anti-VEGF-A treatment through induction of Bv8-dependent angiogenesis. We conclude that, at least in the models examined, G-CSF expression by tumor or stromal cells is a determinant of refractoriness to anti-VEGF-A treatment.

Arginase I-producing myeloid-derived suppressor cells in renal cell carcinoma are a subpopulation of activated granulocytes

Myeloid-derived suppressor cells (MDSC) producing arginase I are increased in the peripheral blood of patients with renal cell carcinoma (RCC). MDSC inhibit T-cell function by reducing the availability of L-arginine and are therefore considered an important tumor escape mechanism. We aimed to determine the origin of arginase I-producing MDSC in RCC patients and to identify the mechanisms used to deplete extracellular L-arginine. The results show that human MDSC are a subpopulation of activated polymorphonuclear (PMN) cells expressing high levels of CD66b, CD11b, and VEGFR1 and low levels of CD62L and CD16. In contrast to murine MDSC, human MDSC do not deplete L-arginine by increasing its uptake but instead release arginase I into the circulation. Activation of normal PMN induces phenotypic and functional changes similar to MDSC and also promotes the release of arginase I from intracellular granules. Interestingly, although activation of normal PMN usually ends with apoptosis, MDSC showed no increase in apoptosis compared with autologous PMN or PMN obtained from normal controls. High levels of VEGF have been shown to increase suppressor immature myeloid dendritic cells in cancer patients. Treatment of RCC patients with anti-VEGF antibody bevacizumab, however, did not reduce the accumulation of MDSC in peripheral blood. In contrast, the addition of interleukin-2 to the treatment increased the number of MDSC in peripheral blood and the plasma levels of arginase I. These results may provide new insights on the mechanisms of tumor-induced anergy/tolerance and may help explain why some immunotherapies fail to induce an antitumor response.

Reversal of myeloid cell-mediated immunosuppression in patients with metastatic renal cell carcinoma

PURPOSE

Tumor-induced immunosuppression remains a significant obstacle that limits the efficacy of biological therapy for renal cell carcinoma. Here we evaluate the role of CD33 myeloid-derived suppressor cells (MDSC) in the regulation of T-cell responses in renal cell carcinoma patients. We also examine effect of all-trans-retinoic acid (ATRA) on MDSC-mediated immune suppression.

EXPERIMENTAL DESIGN

CD33-positive myeloid cells were isolated from the peripheral blood of renal cell carcinoma patients with magnetic beads and tested in vitro for their ability to inhibit T-cell responses. T-cell function was evaluated using ELISPOT and CTL assays.

RESULTS

MDSC isolated from renal cell carcinoma patients, but not from healthy donors, were capable of suppressing antigen-specific T-cell responses in vitro through the secretion of reactive oxygen species and nitric oxide upon interaction with CTL. MDSC-mediated immune suppression and IFN-gamma down-regulation was reversible in vitro by exposing cells to the reactive oxygen species inhibitors. Moreover, ATRA was capable of abrogating MDSC-mediated immunosuppression and improving T-cell function by direct differentiation into antigen-presenting cell precursors.

CONCLUSIONS

These results may have significant implications regarding the future design of active immunotherapy protocols that may include differentiation agents as part of a multimodal approach to renal cell carcinoma immunotherapy.

Amino-biphosphonate-mediated MMP-9 inhibition breaks the tumor-bone marrow axis responsible for myeloid-derived suppressor cell expansion and macrophage infiltration in tumor stroma

BALB-neuT mice expressing an activated rat c-erbB-2/neu transgene under the mouse mammary tumor virus long terminal repeat show enhanced hematopoiesis with hyperproduction of myeloid-derived suppressor cells (MDSC) because of vascular endothelial growth factor (VEGF) secreted by the tumor. Here, we show that both tumor and stromal cells express matrix metalloproteinase-9 (MMP-9), thereby increasing the levels of pro-MMP-9 in the sera of tumor-bearing mice. Treatment with amino-biphosphonates impaired tumor growth, significantly decreased MMP-9 expression and the number of macrophages in tumor stroma, and reduced MDSC expansion both in bone marrow and peripheral blood by dropping serum pro-MMP-9 and VEGF. We dissected the role of tumor-derived MMP-9 from that secreted by stromal leukocytes by transplanting bone marrow from MMP-9 knockout mice into BALB-neuT mice. Although bone marrow progenitor-derived MMP-9 had a major role in driving MDSC expansion, amino-biphosphonate treatment of bone marrow chimeras further reduced both myelopoiesis and the supportive tumor stroma, thus enhancing tumor necrosis. Moreover, by reducing MDSC, amino-biphosphonates overcome the tumor-induced immune suppression and improved the generation and maintenance of antitumor immune response induced by immunization against the p185/HER-2. Our data reveal that suppression of MMP-9 activity breaks the vicious loop linking tumor growth and myeloid cell expansion, thus reducing immunosuppression. Amino-biphosphonates disclose a specific MMP-9 inhibitory activity that may broaden their application above their current usage.

Toll-like receptors in tumor immunotherapy

Lymphodepletion with chemotherapeutic agents or total body irradiation (TBI) before adoptive transfer of tumor-specific T cells is a critical advancement in the treatment of patients with melanoma. More than 50% of patients that are refractory to other treatments experience an objective or curative response with this approach. Emerging data indicate that the key mechanisms underlying how TBI augments the functions of adoptively transferred T cells include (a) the depletion of regulatory T cells (T(reg)) and myeloid-derived suppressor cells that limit the function and proliferation of adoptively transferred cells; (b) the removal of immune cells that act as "sinks" for homeostatic cytokines, whose levels increase after lymphodepletion; and (c) the activation of the innate immune system via Toll-like receptor 4 signaling, which is engaged by microbial lipopolysaccharide that translocated across the radiation-injured gut. Here, we review these mechanisms and focus on the effect of Toll-like receptor agonists in adoptive immunotherapy. We also discuss alternate regimens to chemotherapy or TBI, which might be used to safely treat patients with advanced disease and promote tumor regression.

Pathways mediating the expansion and immunosuppressive activity of myeloid-derived suppressor cells and their relevance to cancer therapy

Cancer immunotherapy has focused on inducing and expanding CTLs and improving the immune recognition of weak antigenic determinants expressed by tumors. However, few positive clinical outcomes have been reported due, in part, to tumor-associated immunologic tolerance, supporting the need for an emphasis on overcoming immunosuppression. Systemic immunosuppression is associated with abnormal myelopoiesis secondary to tumor growth, myelosuppressive therapy, and growth factor administration and subsequent expansion/mobilization of bone marrow-derived immunosuppressive cells. These myeloid-derived suppressor cells (MDSC) reduce activated T-cell number and inhibit their function by multiple mechanisms, including depletion of l-arginine by arginase-1 (ARG1) production of nitric oxide, reactive oxygen species, and reactive nitrogen oxide species by inducible nitric oxide synthase. Increased numbers of MDSCs are associated with neoplastic, inflammatory, infectious, and graft-versus-host diseases where they restrain exuberant or novel T-cell responses. In this review, we discuss critical components of MDSC-mediated suppression of T-cell function, including cellular expansion and activation-induced secretion of immunosuppressive mediators. Both components of MDSC bioactivity are amenable to pharmacologic intervention as discussed herein. We also focus on the relationship between MDSCs, tumor growth, therapeutic responses, and the mechanisms of cellular expansion, activation, and immunosuppression.

Prostaglandin E2 promotes tumor progression by inducing myeloid-derived suppressor cells

A causative relationship between chronic inflammation and cancer has been postulated for many years, and clinical observations and laboratory experiments support the hypothesis that inflammation contributes to tumor onset and progression. However, the precise mechanisms underlying the relationship are not known. We recently reported that the proinflammatory cytokine, interleukin-1beta, induces the accumulation and retention of myeloid-derived suppressor cells (MDSC), which are commonly found in many patients and experimental animals with cancer and are potent suppressors of adaptive and innate immunity. This finding led us to hypothesize that inflammation leads to cancer through the induction of MDSC, which inhibit immunosurveillance and thereby allow the unchecked persistence and proliferation of premalignant and malignant cells. We now report that host MDSC have receptors for prostaglandin E2 (PGE2) and that E-prostanoid receptor agonists, including PGE2, induce the differentiation of Gr1(+)CD11b(+) MDSC from bone marrow stem cells, whereas receptor antagonists block differentiation. BALB/c EP2 knockout mice inoculated with the spontaneously metastatic BALB/c-derived 4T1 mammary carcinoma have delayed tumor growth and reduced numbers of MDSC relative to wild-type mice, suggesting that PGE2 partially mediates MDSC induction through the EP2 receptor. Treatment of 4T1-tumor-bearing wild-type mice with the cyclooxygenase 2 inhibitor, SC58236, delays primary tumor growth and reduces MDSC accumulation, further showing that PGE2 induces MDSC and providing a therapeutic approach for reducing this tumor-promoting cell population.

Reversal of tumor-mediated immunosuppression

Therapeutic cancer vaccines, one form of active immunotherapy, have long been under investigation; consequently, several vaccine-based strategies have now moved from the bench to the clinical arena. Despite their tremendous promise, current vaccine strategies have shown only limited success in clinical settings, even in renal cell carcinoma (RCC), a prototypical malignancy for the application of immunotherapy. There is ample evidence that, especially in RCC, multiple immunosuppressive mechanisms exist that considerably dampen antitumor responses and weaken the activity of current immunotherapeutic regimens. Therefore, it will be necessary to reverse tumor-mediated immunosuppression before immunotherapies can successfully be applied. Recent insights into the nature and characteristics of the regulatory elements of the immune system have provided new opportunities to enhance vaccine-mediated antitumor immunity and, thereby, increase the chance for improving patient outcome. These new insights represent important considerations for the future design and application of more effective cancer vaccines against RCC and other cancers.

All-trans-retinoic acid improves differentiation of myeloid cells and immune response in cancer patients

Abnormal dendritic cell differentiation and accumulation of immature myeloid suppressor cells (ImC) is one of the major mechanisms of tumor escape. We tested the possibility of pharmacologic regulation of myeloid cell differentiation using all-trans-retinoic acid (ATRA). Eighteen patients with metastatic renal cell carcinoma were treated with ATRA followed by s.c. interleukin 2 (IL-2). Eight healthy individuals comprised a control group. As expected, the cancer patients had substantially elevated levels of ImC. We observed that ATRA dramatically reduced the number of ImC. This effect was observed only in patients with high plasma concentration of ATRA (>150 ng/mL), but not in patients with lower ATRA concentrations (<135 ng/mL). Effects of ATRA on the proportions of different dendritic cell populations were minor. However, ATRA significantly improved myeloid/lymphoid dendritic cell ratio and the ability of patients' mononuclear cells to stimulate allogeneic T cells. This effect was associated with significant improvement of tetanus-toxoid-specific T-cell response. During the IL-2 treatment, the ATRA effect was completely eliminated. To assess the role of IL-2, specimens from 15 patients with metastatic renal cell carcinoma who had been treated with i.v. IL-2 alone were analyzed. In this group also, IL-2 significantly reduced the number and function of dendritic cells as well as T-cell function. These data indicate that ATRA at effective concentrations eliminated ImC, improved myeloid/lymphoid dendritic cell ratio, dendritic cell function, and antigen-specific T-cell response. ATRA treatment did not result in significant toxicity and it could be tested in therapeutic combination with cancer vaccines.

Cationic amino acid transporter 2 regulates inflammatory homeostasis in the lung

Arginine is an amino acid that serves as a substrate for nitric oxide synthase and arginase. As such, arginine has the potential to influence diverse fundamental processes in the lung. Here we report that the arginine transport protein, cationic amino acid transporter (CAT)2, has a critical role in regulating lung inflammatory responses. Analysis of CAT2-deficient mice revealed spontaneous inflammation in the lung. Marked eosinophilia, associated with up-regulation of eotaxin-1, was present in the bronchoalveolar lavage fluid of 3-week-old CAT2-deficient mice. The eosinophilia was gradually replaced by neutrophilia in adult mice, while eotaxin-1 levels decreased and GRO-alpha levels increased. Despite the presence of activated alveolar macrophages in CAT2-deficient mice, NO production was compromised in these cells. Examination of dendritic cell activation, which can be affected by NO release, indicated increased dendritic cell activation in the lungs of CAT2-deficient mice. This process was accompanied by an increase in the number of memory T cells. Thus, our data suggest that CAT2 regulates anti-inflammatory processes in the lungs via regulation of dendritic cell activation and subsequent T cell responses.

CD80 in immune suppression by mouse ovarian carcinoma-associated Gr-1+CD11b+ myeloid cells

An elevated number of Gr-1+CD11b+ myeloid cells has been described in mice bearing transplantable tumors, and has been associated with immune suppression. We examined the role of such myeloid suppressor cells in mice bearing the spontaneously transformed syngeneic mouse ovarian surface epithelial cell line, 1D8. We observed high levels of CD80 expression by Gr-1+CD11b+ cells from spleen, ascites, and tumor tissue of mice bearing 1D8 ovarian carcinoma, whereas CD40 and CD86 were absent. CD80 expression was not detected on Gr-1+CD11b+ cells from naïve mice. However, the expression of CD80 by Gr-1+CD11b+ cells from naïve mice was promoted by coculture with 1D8 cells. Because irradiated 1D8 cells, but not 1D8-conditioned medium, up-regulate CD80 expression by Gr-1+CD11b+ cells, this phenomenon likely requires direct interaction. Gr-1+CD11b+ cells derived from 1D8 tumor-bearing mice provided significant suppression of antigen-specific immune responses, but Gr-1+CD11b+ cells from naïve mice did not. Both short interfering RNA-mediated knockdown and genetic knockout of CD80 expression by Gr-1+CD11b+ cells of 1D8 tumor-bearing mice alleviated the suppression of antigen-specific immune responses. Suppression via CD80 on Gr-1+CD11b+ myeloid cells was mediated by CD4+CD25+ T regulatory cells and required CD152. CD80 knockout or antibody blockade of either CD80 or CD152 retarded the growth of 1D8 tumor in mice, suggesting that expression of CD80 on Gr-1+CD11b+ myeloid cells triggered by 1D8 ovarian carcinoma suppresses antigen-specific immunity via CD152 signaling and CD4+CD25+ T regulatory cells. Thus, CD80-dependent responses to myeloid suppressor cells may contribute to tumor tolerance and the progression of ovarian carcinoma.

Interleukin-13-regulated M2 macrophages in combination with myeloid suppressor cells block immune surveillance against metastasis

CD1-deficient mice reject established, disseminated 4T1 metastatic mammary cancer and survive indefinitely if their primary mammary tumors are surgically removed. This highly effective immune surveillance is due to three interacting mechanisms: (a) the generation of inducible nitric oxide synthase (iNOS)-producing M1 macrophages that are tumoricidal for 4T1 tumor cells; (b) a rapid decrease in myeloid-derived Gr1(+)CD11b(+) suppressor cells that are elevated and down-regulate the CD3zeta chain when primary tumor is present and that suppress T cells by producing arginase; and (c) production of activated lymphocytes. Macrophages from wild-type BALB/c mice are polarized by interleukin-13 (IL-13) towards a tumor-promoting M2 phenotype, thereby inhibiting the generation of tumoricidal M1 macrophages. In contrast, CD1(-/-) mice, which are deficient for IL-13 because they lack IL-13-producting NKT cells, generate M1 macrophages that are cytotoxic for 4T1 via the production of nitric oxide. Although tumoricidal macrophages are a necessary component of immune surveillance in CD1(-/-) mice, they alone are not sufficient for tumor resistance because IL-4Ralpha(-/-) mice have M1 macrophages and retain high levels of myeloid suppressor cells after surgery; in addition, they are susceptible to 4T1 metastatic disease. These results show that effective immune surveillance against established metastatic disease is negatively regulated by IL-13 and requires the induction of tumoricidal M1 macrophages and lymphocytes combined with a reduction in tumor-induced myeloid suppressor cells.

Gemcitabine selectively eliminates splenic Gr-1+/CD11b+ myeloid suppressor cells in tumor-bearing animals and enhances antitumor immune activity

PURPOSE

Myeloid suppressor (Gr-1(+)/CD11b(+)) cells accumulate in the spleens of tumor-bearing mice where they contribute to immunosuppression by inhibiting the function of CD8(+) T cells and by promoting tumor angiogenesis. Elimination of these myeloid suppressor cells may thus significantly improve antitumor responses and enhance effects of cancer immunotherapy, although to date few practical options exist.

EXPERIMENTAL DESIGN

The effect of the chemotherapy drug gemcitabine on the number of (Gr-1(+)/CD11b(+)) cells in the spleens of animals bearing large tumors derived from five cancer lines grown in both C57Bl/6 and BALB/c mice was analyzed. Suppressive activity of splenocytes from gemcitabine-treated and control animals was measured in natural killer (NK) cell lysis and Winn assays. The impact of myeloid suppressor cell activity was determined in an immunogene therapy model using an adenovirus expressing IFN-beta.

RESULTS

This study shows that the chemotherapeutic drug gemcitabine, given at a dose similar to the equivalent dose used in patients, was able to dramatically and specifically reduce the number of myeloid suppressor cells found in the spleens of animals bearing large tumors with no significant reductions in CD4(+) T cells, CD8(+) T cells, NK cells, macrophages, or B cells. The loss of myeloid suppressor cells was accompanied by an increase in the antitumor activity of CD8(+) T cells and activated NK cells. Combining gemcitabine with cytokine immunogene therapy using IFN-beta markedly enhanced antitumor efficacy.

CONCLUSIONS

These results suggest that gemcitabine may be a practical strategy for the reduction of myeloid suppressor cells and should be evaluated in conjunction with a variety of immunotherapy approaches.

Nitroaspirin corrects immune dysfunction in tumor-bearing hosts and promotes tumor eradication by cancer vaccination

Active suppression of tumor-specific T lymphocytes can limit the immune-mediated destruction of cancer cells. Of the various strategies used by tumors to counteract immune attacks, myeloid suppressors recruited by growing cancers are particularly efficient, often resulting in the induction of systemic T lymphocyte dysfunction. We have previously shown that the mechanism by which myeloid cells from tumor-bearing hosts block immune defense strategies involves two enzymes that metabolize L-arginine: arginase and nitric oxide (NO) synthase. NO-releasing aspirin is a classic aspirin molecule covalently linked to a NO donor group. NO aspirin does not possess direct antitumor activity. However, by interfering with the inhibitory enzymatic activities of myeloid cells, orally administered NO aspirin normalized the immune status of tumor-bearing hosts, increased the number and function of tumor-antigen-specific T lymphocytes, and enhanced the preventive and therapeutic effectiveness of the antitumor immunity elicited by cancer vaccination. Because cancer vaccines and NO aspirin are currently being investigated in independent phase I/II clinical trials, these findings offer a rationale to combine these treatments in subjects with advanced neoplastic diseases.

Arginase I production in the tumor microenvironment by mature myeloid cells inhibits T-cell receptor expression and antigen-specific T-cell responses

T cells infiltrating tumors have a decreased expression of signal transduction proteins, a diminished ability to proliferate, and a decreased production of cytokines. The mechanisms causing these changes have remained unclear. We demonstrated recently that peritoneal macrophages stimulated with interleukin 4 + interleukin 13 produce arginase I, which decreases the expression of the T-cell receptor CD3zeta chain and impairs T-cell responses. Using a 3LL murine lung carcinoma model we tested whether arginase I was produced in the tumor microenvironment and could decrease CD3zeta expression and impair T-cell function. The results show that a subpopulation of mature tumor-associated myeloid cells express high levels of arginase I, whereas tumor cells and infiltrating lymphocytes do not. Arginase I expression in the tumor was seen on day 7 after tumor injection. Tumor-associated myeloid cells also expressed high levels of cationic amino acid transporter 2B, which allowed them to rapidly incorporate L-Arginine (L-Arg) and deplete extracellular L-Arg in vitro. L-Arg depletion by tumor-associated myeloid cells blocked the re-expression of CD3zeta in stimulated T cells and inhibited antigen-specific proliferation of OT-1 and OT-2 cells. The injection of the arginase inhibitor N-hydroxy-nor-L-Arg blocked growth of s.c. 3LL lung carcinoma in mice. High levels of arginase I were also found in tumor samples of patients with non-small cell carcinoma. Therefore, arginase I production by mature myeloid cells in the tumor microenvironment may be a central mechanism for tumor evasion and may represent a target for new therapies.

High-dose granulocyte-macrophage colony-stimulating factor-producing vaccines impair the immune response through the recruitment of myeloid suppressor cells

Tumor vaccines have shown promise in early clinical trials. Among them, tumor cells genetically engineered to secrete biologically active granulocyte-macrophage colony-stimulating factor (GM-CSF) can generate a systemic antitumor immune response. Although the minimal required GM-CSF dose produced by modified tumor cells to achieve a measurable antitumor effect is well known, no data examined whether an upper therapeutic limit may exist for this vaccination strategy. Because recent data demonstrate an immunosuppressive effect of GM-CSF produced by growing tumors, we thus sought to determine whether high GM-CSF doses administered in a vaccine formulation could impair antitumor immunity. Using a vaccine strategy involving a GM-CSF-producing bystander cell line (B78H1-GM) admixed with autologous tumor, we assessed the impact of varying doses of GM-CSF while maintaining a constant antigen dose. Our results defined a threshold above which a GM-CSF-based vaccine not only lost its efficacy, but more importantly for its clinical implications resulted in substantial immunosuppression in vivo. Above this threshold, GM-CSF induced Gr1+/CD11b+ myeloid suppressor cells that substantially impaired antigen-specific T-cell responses and adversely affected antitumor immune responses in vivo. The dual effects of GM-CSF are mediated by the systemic and not local concentration of this cytokine. Myeloid suppressor cell-induced immunosuppression is mediated by nitric oxide production via inducible nitric oxide synthase (iNOS) because the specific iNOS inhibitor, l-NMMA, restored antigen-specific T-cell responsiveness in vitro. Taken together, our data demonstrated the negative impact of supra-therapeutic vaccine doses of GM-CSF and underscored the importance of identifying these critical variables in an effort to increase the therapeutic efficacy of tumor vaccines.

Role of immature myeloid Gr-1+ cells in the development of antitumor immunity

One of the mechanisms by which tumor cells evade the immune system is the lack of proper antigen-presenting cells. Improvement in host immunity against tumor cells can be achieved by promoting the differentiation of dendritic cells (DCs) from immature myeloid cells (Gr-1(+)Ly-6C(+)F4/80(+)) that accumulate in the bone marrow and lymphoid organs of mice with large tumor burdens. The enriched immature myeloid cells inhibit T-cell proliferation and tumor-specific T-cell response, which can be reversed by the differentiation of immature myeloid cells or depletion of F4/80(+) cells. Sorted Gr-1(+)/F4/80(+) immature myeloid cells differentiated into CD11c(+) cells that express CD80 and I-A/I-E (MHC class II) in the presence of recombinant murine granulocyte macrophage colony-stimulating factor (GM-CSF). Furthermore, intratumoral gene delivery of GM-CSF not only promoted the differentiation of carboxyfluoroscein succinimidyl ester-labeled immature myeloid cells into CD11c(+) cells with the characteristics of mature DCs (CD80(+), I-A/I-E(+)) but also enhanced innate natural killer and adaptive cytolytic T-cell activities in mice treated with interleukin (IL)-12 and anti-4-1BB combination therapy. More importantly, intratumoral delivery of GM-CSF and IL-12 genes in combination with 4-1BB costimulation greatly improved the long-term survival rate of mice bearing large tumors and eradicated the untreated existing hepatic tumor. The results suggest that inducing the maturation of immature myeloid cells, thus preventing their inhibitory activity and enhancing their antigen-presenting capability, by GM-CSF gene therapy is a critically important step in the development of effective antitumor responses in hosts with advanced tumors.

Depletion of neutrophils blocks the recruitment of antigen-nonspecific cells into the liver without affecting the antiviral activity of hepatitis B virus-specific cytotoxic T lymphocytes

Using transgenic mice that replicate hepatitis B virus (HBV) in their livers, we previously showed that passively transferred HBV-specific cytotoxic T cells (CTLs) recruit antigen-nonspecific lymphomononuclear and polymorphonuclear inflammatory cells that contribute to the pathogenesis of liver disease. This process is chemokine-dependent, because we recently showed that blocking the chemokines CXCL9 and CXCL10 reduces the recruitment of antigen-nonspecific lymphomononuclear cells and the severity of liver disease after CTL injection. In the current study we show that the severity of the CTL-initiated liver disease is also ameliorated by the depletion of neutrophils. Interestingly, depletion of neutrophils does not affect the intrahepatic migration or antiviral activity of CTLs, but it profoundly inhibits the recruitment of all antigen-nonspecific cells into the liver. This effect occurs in face of high intrahepatic levels of chemokine gene expression, suggesting that neutrophil-dependent functions other than chemokine induction are necessary for the recruitment process to occur. The notion that depletion of neutrophils is associated with maintenance of antiviral effects but diminished tissue damage may be significant for the development of immunotherapeutic approaches for the treatment of chronic HBV infection.

Immortalized myeloid suppressor cells trigger apoptosis in antigen-activated T lymphocytes

We described a generalized suppression of CTL anamnestic responses that occurred in mice bearing large tumor nodules or immunized with powerful recombinant viral immunogens. Immune suppression entirely depended on GM-CSF-driven accumulation of CD11b(+)/Gr-1(+) myeloid suppressor cells (MSC) in secondary lymphoid organs. To further investigate the nature and properties of MSC, we immortalized CD11b(+)/Gr-1(+) cells isolated from the spleens of immunosuppressed mice, using a retrovirus encoding the v-myc and v-raf oncogenes. Immortalized cells expressed monocyte/macrophage markers (CD11b, F4/80, CD86, CD11c), but they differed from previously characterized macrophage lines in their capacities to inhibit T lymphocyte activation. Two MSC lines, MSC-1 and MSC-2, were selected based upon their abilities to inhibit Ag-specific proliferative and functional CTL responses. MSC-1 line was constitutively inhibitory, while suppressive functions of MSC-2 line were stimulated by exposure to the cytokine IL-4. Both MSC lines triggered the apoptotic cascade in Ag-activated T lymphocytes by a mechanism requiring cell-cell contact. Some well-known membrane molecules involved in the activation of apoptotic pathways (e.g., TNF-related apoptosis-inducing ligand, Fas ligand, TNF-alpha) were ruled out as candidate effectors for the suppression mechanism. The immortalized myeloid lines represent a novel, useful tool to shed light on the molecules involved in the differentiation of myeloid-related suppressors as well as in the inhibitory pathway they use to control T lymphocyte activation.