Clinical perspectives on targeting of myeloid derived suppressor cells in the treatment of cancer

Myeloid derived suppressor cells are present at high frequency in neonates and suppress in vitro T cell responses

Over 4 million infants die each year from infections, many of which are vaccine-preventable. Young infants respond relatively poorly to many infections and vaccines, but the basis of reduced immunity in infants is ill defined. We sought to investigate whether myeloid-derived suppressor cells (MDSC) represent one potential impediment to protective immunity in early life, which may help inform strategies for effective vaccination prior to pathogen exposure. We enrolled healthy neonates and children in the first 2 years of life along with healthy adult controls to examine the frequency and function of MDSC, a cell population able to potently suppress T cell responses. We found that MDSC, which are rarely seen in healthy adults, are present in high numbers in neonates and their frequency rapidly decreases during the first months of life. We determined that these neonatal MDSC are of granulocytic origin (G-MDSC), and suppress both CD4+ and CD8+ T cell proliferative responses in a contact-dependent manner and gamma interferon production. Understanding the role G-MDSC play in infant immunity could improve vaccine responsiveness in newborns and reduce mortality due to early-life infections.

MicroRNA-155 deficiency enhances the recruitment and functions of myeloid-derived suppressor cells in tumor microenvironment and promotes solid tumor growth

Immune cells in tumor microenvironment play a prominent role in tumor progression and metastasis. MicroRNA-155 (miR-155) represents an important player in innate and adaptive immunity by regulating differentiation, maturation and activation of macrophages, dendritic cells, B cells and T cells. However, the role of miR-155 expression in immune cells in solid tumor development is less elucidated. Our current study showed that both B16-F10 melanoma and Lewis lung carcinoma tumors grew much faster in bic/miR-155 knockout (miR-155(-/-) ) mice along with an increase of myeloid-derived suppressor cells (MDSCs) accumulation in tumors, compared to that in wild-type mice. Bone marrow transplantation study showed that bone marrow miR-155 deficiency could replicate the above tumor-promoting phenotype. In vitro study demonstrated that tumor-infiltrating miR-155(-/-) MDSCs showed greater migration ability and expressed higher level of multiple chemokines. Furthermore, we found that the level of HIF-1α, a direct target of miR-155, was increased in miR-155 deficient MDSCs, and that the increased HIF-1α upregulated CXCL1, CXCL3 and CXCL8 expression in MDSCs, contributing to the enhanced recruitment of miR-155(-/-) MDSCs to the tumors. Moreover, miR-155(-/-) MDSCs showed enhanced immunosuppressive and pro-angiogenic capacities. Taken together, our study, for the first time, demonstrated that miR-155 deficiency promoted solid tumor growth through increasing the recruitment of MDSCs to tumor microenvironment and enhancing the tumor-promoting functions of the recruited MDSCs. Thus, upregulating miR-155 expression in MDSCs may be developed as a therapeutic approach to halt tumor development.

CpG-mediated modulation of MDSC contributes to the efficacy of Ad5-TRAIL therapy against renal cell carcinoma

Tumor progression occurs through the modulation of a number of physiological parameters, including the development of immunosuppressive mechanisms to prevent immune detection and response. Among these immune evasion mechanisms, the mobilization of myeloid-derived suppressor cells (MDSC) is a major contributor to the suppression of antitumor T-cell immunity. Patients with renal cell carcinoma (RCC) show increased MDSC, and methods are being explored clinically to reduce the prevalence of MDSC and/or inhibit their function. In the present study, we investigated the relationship between MDSC and the therapeutic potential of a TRAIL-encoding recombinant adenovirus (Ad5-TRAIL) in combination with CpG-containing oligodeoxynucleotides (Ad5-TRAIL/CpG) in an orthotopic mouse model of RCC. This immunotherapy effectively clears renal (Renca) tumors and enhances survival, despite the presence of a high frequency of MDSC in the spleens and primary tumor-bearing kidneys at the time of treatment. Subsequent analyses revealed that the CpG component of the immunotherapy was responsible for decreasing the frequency of MDSC in Renca-bearing mice; further, treatment with CpG modulated the phenotype and function of MDSC that remained after immunotherapy and correlated with an increased T-cell response. Interestingly, the CpG-dependent alterations in MDSC frequency and function did not occur in tumor-bearing mice complicated with diet-induced obesity. Collectively, these data suggest that in addition to its adjuvant properties, CpG also enhances antitumor responses by altering the number and function of MDSC.

The role of myeloid-derived suppressor cells in immune ontogeny

Myeloid-derived suppressor cells (MDSC) are a heterogeneous population of granulocytic or monocytic cells that suppress innate as well as adaptive immune responses. In healthy adults, immature myeloid cells differentiate into macrophages, dendritic cells, and granulocytes in the bone marrow and MDSC are rarely detected in peripheral blood. However, in certain pathologies, in particular malignancies and chronic infection, differentiation of these cells is altered resulting in accumulation of circulating suppressive myeloid cells. MDSC express suppressive factors such as arginase-1, reactive oxygen species, and inducible nitric oxide synthase, which have the ability to inhibit T cell proliferation and cytoxicity, induce the expansion of regulatory T cells, and block natural killer cell activation. It is increasingly recognized that MDSC alter the immune response to several cancers, and perhaps chronic viral infections, in clinically important ways. In this review, we outline the potential contribution of MDSC to the generation of feto-maternal tolerance and to the ineffective immune responses to many infections and vaccines observed in early post-natal life. Granulocytic MDSC are present in large numbers in pregnant women and in cord blood, and wane rapidly during infancy. Furthermore, cord blood MDSC suppress in vitro T cell and NK responses, suggesting that they may play a significant role in human immune ontogeny. However, there are currently no data that demonstrate in vivo effects of MDSC on feto-maternal tolerance or immune ontogeny. Studies are ongoing to evaluate the functional importance of MDSC, including their effects on control of infection and response to vaccination in infancy. Importantly, several pharmacologic interventions have the potential to reverse MDSC function. Understanding the role of MDSC in infant ontogeny and their mechanisms of action could lead to interventions that reduce mortality due to early-life infections.

Clinical use of dendritic cells for cancer therapy

Since the mid-1990s, dendritic cells have been used in clinical trials as cellular mediators for therapeutic vaccination of patients with cancer. Dendritic cell-based immunotherapy is safe and can induce antitumour immunity, even in patients with advanced disease. However, clinical responses have been disappointing, with classic objective tumour response rates rarely exceeding 15%. Paradoxically, findings from emerging research indicate that dendritic cell-based vaccination might improve survival, advocating implementation of alternative endpoints to assess the true clinical potency of dendritic cell-based vaccination. We review the clinical effectiveness of dendritic cell-based vaccine therapy in melanoma, prostate cancer, malignant glioma, and renal cell carcinoma, and summarise the most important lessons from almost two decades of clinical studies of dendritic cell-based immunotherapy in these malignant disorders. We also address how the specialty is evolving, and which new therapeutic concepts are being translated into clinical trials to leverage the clinical effectiveness of dendritic cell-based cancer immunotherapy. Specifically, we discuss two main trends: the implementation of the next-generation dendritic cell vaccines that have improved immunogenicity, and the emerging paradigm of combination of dendritic cell vaccination with other cancer therapies.

Dissecting the role of bone marrow stromal cells on bone metastases

Tumor-induced bone disease is a dynamic process that involves interactions with many cell types. Once metastatic cancer cells reach the bone, they are in contact with many different cell types that are present in the cell-rich bone marrow. These cells include the immune cells, myeloid cells, fibroblasts, osteoblasts, osteoclasts, and mesenchymal stem cells. Each of these cell populations can influence the behavior or gene expression of both the tumor cells and the bone microenvironment. Additionally, the tumor itself can alter the behavior of these bone marrow cells which further alters both the microenvironment and the tumor cells. While many groups focus on studying these interactions, much remains unknown. A better understanding of the interactions between the tumor cells and the bone microenvironment will improve our knowledge on how tumors establish in bone and may lead to improvements in diagnosing and treating bone metastases. This review details our current knowledge on the interactions between tumor cells that reside in bone and their microenvironment.

Is Infant Immunity Actively Suppressed or Immature?

Almost 7 million children under the age 5 die each year, and most of these deaths are attributable to vaccine-preventable infections. Young infants respond poorly to infections and vaccines. In particular, dendritic cells secrete less IL-12 and IL-18, CD8^pos T cells and NK cells have defective cytolysis and cytokine production, and CD4^pos T cell responses tend to bias towards a Th2 phenotype and promotion of regulatory T cells (T_regs). The basis for these differences is not well understood and may be in part explained by epigenetic differences, as well as immaturity of the infant's immune system. Here we present a third possibility, which involves active suppression by immune regulatory cells and place in context the immune suppressive pathways of mesenchymal stromal cells (MSC), myeloid-derived suppressor cells (MDSC), CD5^pos B cells, and T_regs. The immune pathways that these immune regulatory cells inhibit are similar to those that are defective in the infant. Therefore, the immune deficiencies seen in infants could be explained, in part, by active suppressive cells, indicating potential new avenues for intervention.

Modification of the tumor microenvironment as a novel target of renal cell carcinoma therapeutics

To move forward with immunotherapy, it is important to understand how the tumor microenvironment generates systemic immunosuppression in patients with renal cell carcinoma (RCC) as well as in patients with other types of solid tumors. Even though antigen discovery in RCC has lagged behind melanoma, recent clinical trials have finally authenticated that RCC is susceptible to vaccine-based therapy. Furthermore, judicious coadministration of cytokines and chemotherapy can potentiate therapeutic responses to vaccine in RCC and prolong survival, as has already proved possible for melanoma. Although high-dose interleukin 2 immunotherapy has been superseded as first-line therapy for RCC by promiscuous receptor tyrosine kinase inhibitors (rTKIs) such as sunitinib, sunitinib itself is a potent immunoadjunct in animal tumor models. A reasonable therapeutic goal is to unite antiangiogenic strategies with immunotherapy as first-line therapy for RCC. This strategy is equally appropriate for testing in all solid tumors in which the microenvironment generates immunosuppression. A common element of RCC and pancreatic, colon, breast, and other solid tumors is large numbers of circulating myeloid-derived suppressor cells (MDSCs), and because MDSCs elicit regulatory T cells rather than vice versa, gaining control over MDSCs is an important initial step in any immunotherapy. Although rTKIs like sunitinib have a remarkable capacity to deplete MDSCs and restore normal T-cell function in peripheral body compartments such as the bloodstream and the spleen, such rTKIs are effective only against MDSCs, which are engaged in phospho-STAT3-dependent programming (pSTAT3+). Unfortunately, rTKI-resistant pSTAT3- MDSCs are especially apt to arise within the tumor microenvironment itself, necessitating strategies that do not rely exclusively on STAT3 disruption. The most utilitarian strategy to gain control of both pSTAT3+ and pSTAT3- MDSCs may be to exploit the natural differentiation pathway, which permits MDSCs to mature into tumoricidal macrophages (TM1) via such stimuli as Toll-like receptor agonists, interferon γ, and CD40 ligation. Overall, this review highlights the mechanisms of immune suppression used by the different regulatory cell types operative in RCC as well as other tumors. It also describes the different therapeutic strategies to overcome the suppressive nature of the tumor microenvironment.

Myeloid-derived suppressor cells in cancer: therapeutic, predictive, and prognostic implications

Immune evasion is a hallmark of cancer. While there are multiple different mechanisms that cancer cells employ, myeloid-derived suppressor cells (MDSCs) are one of the key drivers of tumor-mediated immune evasion. MDSCs begin as myeloid cells recruited to the tumor microenvironment, where they are transformed into potent immunosuppressive cells. However, our understanding of the clinical relevance of MDSCs in cancer patients has significantly lagged behind the preclinical literature in part due to the absence of a cognate molecule present in mice, as well as to the considerable heterogeneity of MDSCs. However, if one evaluates the clinical literature through the filter of clinically robust endpoints, such as overall survival, three important phenotypes emerge: promyelocytic, monocytic, and granulocytic. Based on these studies, MDSCs have clear prognostic importance in multiple solid tumors, and emerging data support the utility of circulating MDSCs as a predictive marker for cancer immunotherapy, and even as an early leading marker for predicting clinical response to systemic chemotherapy in patients with advanced solid tumors. More recent preclinical data in immunosuppressed murine models suggest that MDSCs play an important role in tumor progression and the metastatic process that is independent of their immunosuppressive properties. Consequently, targeting MDSCs either in combination with cancer immunotherapy or independently as part of an approach to inhibit the metastatic process appears to be a very clinically promising strategy. We review different approaches to target MDSCs that could potentially be tested in future clinical trials in cancer patients.

Immune-based mechanisms of cytotoxic chemotherapy: implications for the design of novel and rationale-based combined treatments against cancer

Conventional anticancer chemotherapy has been historically thought to act through direct killing of tumor cells. This concept stems from the fact that cytotoxic drugs interfere with DNA synthesis and replication. Accumulating evidence, however, indicates that the antitumor activities of chemotherapy also rely on several off-target effects, especially directed to the host immune system, that cooperate for successful tumor eradication. Chemotherapeutic agents stimulate both the innate and adaptive arms of the immune system through several modalities: (i) by promoting specific rearrangements on dying tumor cells, which render them visible to the immune system; (ii) by influencing the homeostasis of the hematopoietic compartment through transient lymphodepletion followed by rebound replenishment of immune cell pools; (iii) by subverting tumor-induced immunosuppressive mechanisms and (iv) by exerting direct or indirect stimulatory effects on immune effectors. Among the indirect ways of immune cell stimulation, some cytotoxic drugs have been shown to induce an immunogenic type of cell death in tumor cells, resulting in the emission of specific signals that trigger phagocytosis of cell debris and promote the maturation of dendritic cells, ultimately resulting in the induction of potent antitumor responses. Here, we provide an extensive overview of the multiple immune-based mechanisms exploited by the most commonly employed cytotoxic drugs, with the final aim of identifying prerequisites for optimal combination with immunotherapy strategies for the development of more effective treatments against cancer.

Chronic inflammation and cancer: suppressing the suppressors

Chronic inflammation typical to various chronic diseases is associated with immunosuppression, mediated primarily by immature myeloid-derived suppressor cells (MDSCs). A variety of factors induce MDSC differentiation arrest, thus manipulating the host's immune function and suppressing the innate and adaptive immune systems, as reflected by their impaired status associated with down-regulated expression of the CD247 molecule. Such chronic inflammation-induced immunosuppressive features are also found in many tumors, generating tumor micro- and macro-environments that act as critical barriers to effective anti-tumor responses and therapies. This knowledge offers new and novel candidate immune targets for therapeutic interventions, in combination with more conventional approaches as chemotherapy, radiotherapy, and cancer cell targeted therapy. Therapeutic manipulation of chronic inflammation during cancer development is likely to enhance efficacy of treatments such as vaccinations, and adoptive T cell transfer, thus switching the chronic pro-cancer inflammatory environments into an anti-cancer milieu. Based on the functional relevance of immune networking in tumors, it is advantageous to merge monitoring immune biomarkers into the traditional patient's categorization and treatment regiments, which will provide new prognostic and/or predictive tools to clinical practice. A better identification of environmental and tumor-specific inflammatory mechanisms will allow directing the clinical management of cancer toward a more personalized medicine.

Emerging role of tumor-associated macrophages as therapeutic targets in patients with metastatic renal cell carcinoma

Tumor-associated macrophages (TAMs) derived from peripheral blood monocytes recruited into the renal cell carcinoma (RCC) microenvironment. In response to inflammatory stimuli, macrophages undergo M1 (classical) or M2 (alternative) activation. M1 cells produce high levels of inflammatory cytokines, such as tumor necrosis factor-α, interleukin (IL)-12, IL-23 and IL-6, while M2 cells produce anti-inflammatory cytokines, such as IL-10, thus contributing to RCC-related immune dysfunction. The presence of extensive TAM infiltration in RCC microenvironment contributes to cancer progression and metastasis by stimulating angiogenesis, tumor growth, and cellular migration and invasion. Moreover, TAMs are involved in epithelial-mesenchymal transition of RCC cancer cells and in the development of tumor resistance to targeted agents. Interestingly, macrophage autophagy seems to play an important role in RCC. Based on this scenario, TAMs represent a promising and effective target for cancer therapy in RCC. Several strategies have been proposed to suppress TAM recruitment, to deplete their number, to switch M2 TAMs into antitumor M1 phenotype and to inhibit TAM-associated molecules. In this review, we summarize current data on the essential role of TAMs in RCC angiogenesis, invasion, impaired anti-tumor immune response and development of drug resistance, thus describing the emerging TAM-centered therapies for RCC patients.

Immune-checkpoint blockade and active immunotherapy for glioma

Cancer immunotherapy has made tremendous progress, including promising results in patients with malignant gliomas. Nonetheless, the immunological microenvironment of the brain and tumors arising therein is still believed to be suboptimal for sufficient antitumor immune responses for a variety of reasons, including the operation of “immune-checkpoint” mechanisms. While these mechanisms prevent autoimmunity in physiological conditions, malignant tumors, including brain tumors, actively employ these mechanisms to evade from immunological attacks. Development of agents designed to unblock these checkpoint steps is currently one of the most active areas of cancer research. In this review, we summarize recent progresses in the field of brain tumor immunology with particular foci in the area of immune-checkpoint mechanisms and development of active immunotherapy strategies. In the last decade, a number of specific monoclonal antibodies designed to block immune-checkpoint mechanisms have been developed and show efficacy in other cancers, such as melanoma. On the other hand, active immunotherapy approaches, such as vaccines, have shown encouraging outcomes. We believe that development of effective immunotherapy approaches should ultimately integrate those checkpoint-blockade agents to enhance the efficacy of therapeutic approaches. With these agents available, it is going to be quite an exciting time in the field. The eventual success of immunotherapies for brain tumors will be dependent upon not only an in-depth understanding of immunology behind the brain and brain tumors, but also collaboration and teamwork for the development of novel trials that address multiple layers of immunological challenges in gliomas.

Clinical evaluation of systemic and local immune responses in cancer: time for integration

The immune system has a dual role in cancer development and progression. On the one hand, it can eradicate emerging malignant cells, but on the other hand, it can actively promote growth of malignant cells, their invasive capacities and their ability to metastasize. Immune cells with predominantly anti-tumor functionality include cells of the innate immune system, such as natural killer cells, and cells of adaptive immunity, such as conventional dendritic cells and cytotoxic T lymphocytes. Immune cells with predominantly pro-tumor functionality include a broad spectrum of cells of the innate and adaptive immune system, such as type 2 neutrophils and macrophages, plasmacytoid DC, myeloid-derived suppressor cells and regulatory T lymphocytes. The presence of immune cells with tumor-suppressive and tumor-promoting activity in the cancer microenvironment and in peripheral blood is usually associated with good clinical outcomes and poor clinical outcomes, respectively. Significant advances in experimental and clinical oncoimmunology achieved in the last decade open an opportunity for the use of modern morphologic, flow cytometric and functional tests in clinical practice. In this review, we describe an integrated approach to clinical evaluation of the immune status of cancer patients for diagnostic purposes, prognostic/predictive purposes (evaluation of patient prognosis and response to treatment) and for therapeutic purposes.