Myeloid-derived suppressor cells coming of age. Nat Immunol. 2018;19:108-119. [PMID: 29348500 DOI: 10.1038/s41590-017-0022-x]

The hallmarks of successful anticancer immunotherapy

Immunotherapy is revolutionizing the clinical management of multiple tumors. However, only a fraction of patients with cancer responds to immunotherapy, and currently available immunotherapeutic agents are expensive and generally associated with considerable toxicity, calling for the identification of robust predictive biomarkers. The overall genomic configuration of malignant cells, potentially favoring the emergence of immunogenic tumor neoantigens, as well as specific mutations that compromise the ability of the immune system to recognize or eradicate the disease have been associated with differential sensitivity to immunotherapy in preclinical and clinical settings. Along similar lines, the type, density, localization, and functional orientation of the immune infiltrate have a prominent impact on anticancer immunity, as do features of the tumor microenvironment linked to the vasculature and stroma, and systemic factors including the composition of the gut microbiota. On the basis of these considerations, we outline the hallmarks of successful anticancer immunotherapy.

Myeloid-derived suppressor cells in non-neoplastic inflamed organs

BACKGROUND

Myeloid-derived suppressor cells (MDSCs) are a highly heterogeneous population of immature myeloid cells with immunosuppressive function. Although their function in tumor-bearing conditions is well studied, less is known about the role of MDSCs in various organs under non-neoplastic inflammatory conditions.

MAIN BODY

MDSCs are divided into two subpopulations, G-MDSCs and M-MDSCs, and their distribution varies between organs. MDSCs negatively control inflammation in inflamed organs such as the lungs, joints, liver, kidneys, intestines, central nervous system (CNS), and eyes by suppressing T cells and myeloid cells. MDSCs also regulate fibrosis in the lungs, liver, and kidneys and help repair CNS injuries. MDSCs in organs are plastic and can differentiate into osteoclasts and tolerogenic dendritic cells according to the microenvironment under non-neoplastic inflammatory conditions.

CONCLUSION

This article summarizes recent findings about MDSCs under inflammatory conditions, especially with respect to their function and differentiation in specific organs.

Extracellular vesicles in cancer immune responses: roles of purinergic receptors

Extracellular vesicles (EVs) are nano- to micro-scale membrane-enclosed vesicles that are released from presumably all cell types. Tumor cells and immune cells are prodigious generators of EVs often with competing phenotypes in terms of immune suppression versus immune stimulation. Purinergic receptors, proteins that bind diverse purine nucleotides and nucleosides (ATP, ADP, AMP, adenosine), are widely expressed across tissues and cell types, and are prominent players in immune and tumor cell nucleotide metabolism. The effects of purinergic receptor stimulation or agonism tend to produce inflammatory responses that may aid immune stimulation but may also provoke various immune suppression mechanisms, particularly in the tumor microenvironment. EVs released by cells following receptor stimulation are frequently pro-inflammatory, but often also pro-thrombolytic; these EVs may generate an environment that favors tumor progression at the cost of an effective immune response. Purinergic signaling pathways are becoming more recognized as valuable targets in various therapeutic scenarios, including cancer. It is possible that some of those clinically relevant compounds might also impact EV secretion and/or phenotype, which would hopefully capitalize on the immune stimulatory properties of purinergic signaling while minimizing the immune suppressive consequences. This review covers a relatively understudied area in EV biology, but even so, focuses almost exclusively on the purinergic receptors in a very limited capacity. There is much more to evaluate and incorporate into our understanding of extracellular nucleotides in EV biology, and we hope this work prompts further discovery.

Reasoning the effect of immunotherapy after chemoradiation in the PACIFIC trial

Durvalumab consolidation after chemoradiation has been a giant leap in the treatment of stage III non-small-cell lung cancer with an unprecedented 16.8-month median progression-free survival. PACIFIC trial is a new foray into chemoimmunotherapy trials where we apply our knowledge of 'immunogenic cell death' and 'Abscopal' effect of radiation in the clinic. Our understanding of immunotherapy after chemoradiation treatment and application of immunogenic cell death biomarkers in future trials may be the approach we need to maximize benefit of these treatments in the appropriate patients.

Turn Back the TIMe: Targeting Tumor Infiltrating Myeloid Cells to Revert Cancer Progression

Tumor cells frequently produce soluble factors that favor myelopoiesis and recruitment of myeloid cells to the tumor microenvironment (TME). Consequently, the TME of many cancer types is characterized by high infiltration of monocytes, macrophages, dendritic cells and granulocytes. Experimental and clinical studies show that most myeloid cells are kept in an immature state in the TME. These studies further show that tumor-derived factors mold these myeloid cells into cells that support cancer initiation and progression, amongst others by enabling immune evasion, tumor cell survival, proliferation, migration and metastasis. The key role of myeloid cells in cancer is further evidenced by the fact that they negatively impact on virtually all types of cancer therapy. Therefore, tumor-associated myeloid cells have been designated as the culprits in cancer. We review myeloid cells in the TME with a focus on the mechanisms they exploit to support cancer cells. In addition, we provide an overview of approaches that are under investigation to deplete myeloid cells or redirect their function, as these hold promise to overcome resistance to current cancer therapies.

Serum amyloid A - a review

Serum amyloid A (SAA) proteins were isolated and named over 50 years ago. They are small (104 amino acids) and have a striking relationship to the acute phase response with serum levels rising as much as 1000-fold in 24 hours. SAA proteins are encoded in a family of closely-related genes and have been remarkably conserved throughout vertebrate evolution. Amino-terminal fragments of SAA can form highly organized, insoluble fibrils that accumulate in “secondary” amyloid disease. Despite their evolutionary preservation and dynamic synthesis pattern SAA proteins have lacked well-defined physiologic roles. However, considering an array of many, often unrelated, reports now permits a more coordinated perspective. Protein studies have elucidated basic SAA structure and fibril formation. Appreciating SAA’s lipophilicity helps relate it to lipid transport and metabolism as well as atherosclerosis. SAA’s function as a cytokine-like protein has become recognized in cell-cell communication as well as feedback in inflammatory, immunologic, neoplastic and protective pathways. SAA likely has a critical role in control and possibly propagation of the primordial acute phase response. Appreciating the many cellular and molecular interactions for SAA suggests possibilities for improved understanding of pathophysiology as well as treatment and disease prevention.

Packed red blood cell donor age affects overall survival in transfused patients undergoing hepatectomy for non-hepatocellular malignancy

BACKGROUND

Patients undergoing hepatectomy often require packed red blood cell (PRBC) transfusion, which has been associated with worse oncologic outcomes. However, limited data exist regarding the impact of PRBC donor factors. We hypothesized that PRBC donor age impacts survival after hepatectomy for non-hepatocellular malignancies.

METHODS

Patients who underwent hepatectomy for non-hepatocellular malignancy from 2005 to 2014 were retrospectively evaluated. Impact of clinicopathologic and PRBC factors on oncologic outcomes were assessed.

RESULTS

Of 149 identified patients, 76 received a perioperative PRBC transfusion (median 2 units). Transfusion was associated with increased median length of stay (8 vs. 6 days; p < 0.01) and median operative blood loss (700 vs. 350 mL; p < 0.01) versus non-transfused, respectively. In transfused patients, receipt of PRBC from older donors compared to younger resulted in decreased RFS (0.94 vs. 2.63 years, respectively; p = 0.02) and OS (1.94 vs. 3.44 years, respectively; p = 0.6). The PRBC donor age was an independent predictor of decreased recurrence free survival on multivariate analysis (HR 2.5, p = 0.04).

CONCLUSIONS

In patients undergoing hepatectomy for non-hepatocellular malignancies and receiving perioperative transfusion, PRBC donor age may impact survival and warrants further investigation.

Cross-talk between T Cells and Hematopoietic Stem Cells during Adoptive Cellular Therapy for Malignant Glioma

Purpose: Adoptive T-cell immunotherapy (ACT) has emerged as a viable therapeutic for peripheral and central nervous system (CNS) tumors. In peripheral cancers, optimal efficacy of ACT is reliant on dendritic cells (DCs) in the tumor microenvironment. However, the CNS is largely devoid of resident migratory DCs to function as antigen-presenting cells during immunotherapy. Herein, we demonstrate that cellular interactions between adoptively transferred tumor-reactive T cells and bone marrow-derived hematopoietic stem and progenitor cells (HSPCs) lead to the generation of potent intratumoral DCs within the CNS compartment.Experimental Design: We evaluated HSPC differentiation during ACT in vivo in glioma-bearing hosts and HSPC proliferation and differentiation in vitro using a T-cell coculture system. We utilized FACS, ELISAs, and gene expression profiling to study the phenotype and function of HSPC-derived cells ex vivo and in vivo To demonstrate the impact of HSPC differentiation and function on antitumor efficacy, we performed survival experiments.Results: Transfer of HSPCs with concomitant ACT led to the production of activated CD86+CD11c+MHCII+ cells consistent with DC phenotype and function within the brain tumor microenvironment. These intratumoral DCs largely supplanted abundant host myeloid-derived suppressor cells. We determined that during ACT, HSPC-derived cells in gliomas rely on T-cell-released IFNγ to differentiate into DCs, activate T cells, and reject intracranial tumors.Conclusions: Our data support the use of HSPCs as a novel cellular therapy. Although DC vaccines induce robust immune responses in the periphery, our data demonstrate that HSPC transfer uniquely generates intratumoral DCs that potentiate T-cell responses and promote glioma rejection in situClin Cancer Res; 24(16); 3955-66. ©2018 AACR.

Chemical Tools for Targeted Amplification of Reactive Oxygen Species in Neutrophils

A number of chemical compounds are known, which amplify the availability of reactive oxygen species (ROS) in neutrophils both in vitro and in vivo. They can be roughly classified into NADPH oxidase 2 (NOX2)-dependent and NOX2-independent reagents. NOX2 activation is triggered by protein kinase C agonists (e.g., phorbol esters, transition metal ions), redox mediators (e.g., paraquat) or formyl peptide receptor (FPR) agonists (e.g., aromatic hydrazine derivatives). NOX2-independent mechanisms are realized by reagents affecting glutathione homeostasis (e.g., l-buthionine sulfoximine), modulators of the mitochondrial respiratory chain (e.g., ionophores, inositol mimics, and agonists of peroxisome proliferator-activated receptor γ) and chemical ROS amplifiers [e.g., aminoferrocene-based prodrugs (ABPs)]. Since a number of inflammatory and autoimmune diseases, as well as cancer and bacterial infections, are triggered or enhanced by aberrant ROS production in neutrophils, it is tempting to use ROS amplifiers as drugs for the treatment of these diseases. However, since the known reagents are not cell specific, their application for treatment likely causes systemic enhancement of oxidative stress, leading to severe side effects. Cell-targeted ROS enhancement can be achieved either by using conjugates of ROS amplifiers with ligands binding to receptors expressed on neutrophils (e.g., the GPI-anchored myeloid differentiation marker Ly6G or FPR) or by designing reagents activated by neutrophil function [e.g., phagocytic activity or enzymatic activity of neutrophil elastase (NE)]. Since binding of an artificial ligand to a receptor may trigger or inhibit priming of neutrophils the latter approach has a smaller potential for severe side effects and is probably better suitable for therapy. Here, we review current approaches for the use of ROS amplifiers and discuss their applicability for treatment. As an example, we suggest a possible design of neutrophil-specific ROS amplifiers, which are based on NE-activated ABPs.

Myeloid-Derived Suppressor Cells and Pulmonary Hypertension

Myeloid–derived suppressor cells (MDSCs) comprised a heterogeneous subset of bone marrow–derived myeloid cells, best studied in cancer research, that are increasingly implicated in the pathogenesis of pulmonary vascular remodeling and the development of pulmonary hypertension. Stem cell transplantation represents one extreme interventional strategy for ablating the myeloid compartment but poses a number of translational challenges. There remains an outstanding need for additional therapeutic targets to impact MDSC function, including the potential to alter interactions with innate and adaptive immune subsets, or alternatively, alter trafficking receptors, metabolic pathways, and transcription factor signaling with readily available and safe drugs. In this review, we summarize the current literature on the role of myeloid cells in the development of pulmonary hypertension, first in pulmonary circulation changes associated with myelodysplastic syndromes, and then by examining intrinsic myeloid cell changes that contribute to disease progression in pulmonary hypertension. We then outline several tractable targets and pathways relevant to pulmonary hypertension via MDSC regulation. Identifying these MDSC-regulated effectors is part of an ongoing effort to impact the field of pulmonary hypertension research through identification of myeloid compartment-specific therapeutic applications in the treatment of pulmonary vasculopathies.

Targeting T Cell Metabolism for Improvement of Cancer Immunotherapy

There has been significant progress in utilizing our immune system against cancer, mainly by checkpoint blockade and T cell-mediated therapies. The field of cancer immunotherapy is growing rapidly but durable clinical benefits occur only in a small subset of responding patients. It is currently recognized that cancer creates a suppressive metabolic microenvironment, which contributes to ineffective immune function. Metabolism is a common cellular feature, and although there has been significant progress in understanding the detrimental role of metabolic changes of the tumor microenvironment (TEM) in immune cells, there is still much to be learned regarding unique targetable pathways. Elucidation of cancer and immune cell metabolic profiles is critical for identifying mechanisms that regulate metabolic reprogramming within the TEM. Metabolic targets that mediate immunosuppression and are fundamental in sustaining tumor growth can be exploited therapeutically for the development of approaches to increase the efficacy of immunotherapies. Here, we will highlight the importance of metabolism on the function of tumor-associated immune cells and will address the role of key metabolic determinants that might be targets of therapeutic intervention for improvement of tumor immunotherapies.

CD45+CD33lowCD11bdim myeloid-derived suppressor cells suppress CD8+ T cell activity via the IL-6/IL-8-arginase I axis in human gastric cancer

Myeloid-derived suppressor cells (MDSCs) are a prominent component of the pro-tumoral response. The phenotype of and mechanisms used by MDSCs is heterogeneous and requires more precise characterization in gastric cancer (GC) patients. Here, we have identified a novel subset of CD45⁺CD33^lowCD11b^dim MDSCs in the peripheral blood of GC patients compared to healthy individuals. CD45⁺CD33^lowCD11b^dim MDSCs morphologically resembled neutrophils and expressed high levels of the neutrophil marker CD66b. Circulating CD45⁺CD33^lowCD11b^dim MDSCs effectively suppressed CD8⁺ T cells activity through the inhibition of CD8⁺ T cell proliferation and interferon-γ (IFN-γ) and granzyme B (GrB) production. The proportion of CD45⁺CD33^lowCD11b^dim MDSCs also negatively correlated with the proportion of IFN-γ⁺CD8⁺ T cell in the peripheral blood of GC patients. GC patient serum-derived IL-6 and IL-8 activated and induced CD45⁺CD33^lowCD11b^dim MDSCs to express arginase I via the PI3K-AKT signaling pathway. This pathway contributed to CD8⁺ T cell suppression as it was partially rescued by the blockade of the IL-6/IL-8-arginase I axis. Peripheral blood CD45⁺CD33^lowCD11b^dim MDSCs, as well as IL-6, IL-8, and arginase I serum levels, positively correlated with GC progression and negatively correlated with overall patient survival. Altogether, our results highlight that a subset of neutrophilic CD45⁺CD33^lowCD11b^dim MDSCs is functionally immunosuppressive and activated via the IL-6/IL-8-arginase I axis in GC patients.

The Effect of Immunosuppressive Drugs on MDSCs in Transplantation

Myeloid-derived suppressor cells (MDSCs) are a group of innate immune cells that regulates both innate and adaptive immune responses. In recent years, MDSCs were shown to play an important negative regulatory role in transplant immunology even upstream of regulatory T cells. In certain cases, MDSCs are closely involved in transplantation immune tolerance induction and maintenance. It is known that some immunosuppressant drugs negatively regulate MDSCs but others have positive effects on MDSCs in different transplant cases. We herein summarized our recent insights into the regulatory roles of MDSCs in transplantation specially focusing on the effects of immunosuppressive drugs on MDSCs and their mechanisms of action. Studies on the effects of immunosuppressive drugs on MDSCs will significantly expand our understanding of immunosuppressive drugs on immune regulatory cells in transplantation and offer new insights into transplant tolerance. We hope to emphasize our concern for the negative effects of immunosuppressive agents on MDSCs, which may potentially attenuate the immune tolerance induction in transplanted recipients.

Mechanisms of tumor immunotherapy, with a focus on thoracic cancers

If immunotherapy is currently considered as a valid treatment strategy in oncology, the concept that cancer could be cured by the mere manipulation of the immune system was almost inconceivable until few years ago, particularly in lung cancer. The use of immune checkpoint inhibitors has instead demonstrated to mediate significant long-term disease control so to rapidly enter clinical practice and represent the basis for most of the combination approaches under development. In light of the revolutionary results achieved through the pivotal clinical trials and the large expectations about the possibility to further improve clinical benefit and discover novel therapeutic targets, it is becoming nowadays mandatory to increase our knowledge on the basics of immunology in lung cancer. Defining the pathways that rule the interactions between tumor and immune cells and the requirements to achieve full-fledged immune responses able to mediate meaningful antitumor activity are present goals of the research ongoing worldwide. This knowledge would not only foster a more scientifically-based clinical development of novel drugs and combinations, but also provide valid biomarkers for patient selection and monitoring. In the present review we will address the available information about the immunological features of lung cancer, the backgrounds to the use of immunotherapeutics, the possible mechanisms underlying resistance and the strategies to improve immune-mediated tumor control. In doing this, we will be following the path traced in melanoma, the tumor histotype that taught us most of what we know about cancer immunotherapy.

Myeloid-derived suppressor cells (MDSC): an important partner in cellular/tissue senescence

The aging process is associated with a low-grade chronic inflammation and the accumulation of senescent cells into tissues. Diverse stresses can trigger cellular senescence, a cell fate characterized by cell-cycle arrest and flat morphology. Oncogenic signaling can also induce cellular senescence which has been termed oncogene-induced senescence (OIS). Senescent cells display a pro-inflammatory phenotype which has been called the senescence-associated secretory phenotype (SASP). The secretomes associated with SASP contain colony-stimulating factors and chemokines which stimulate the generation of myeloid-derived suppressor cells (MDSC) by enhancing myelopoiesis in bone marrow and spleen. Enhanced myelopoiesis and increased level of MDSCs have been observed in bone marrow, spleen, and blood in both tumor-bearing and aged mice. Immunosuppressive MDSCs are recruited via chemotaxis into inflamed tissues where they proliferate and consequently suppress acute inflammatory reactions by inhibiting the functions of distinct components of innate and adaptive immunity. For instance, MDSCs stimulate the activity of immunosuppressive regulatory T-cells (Tregs). They also increase the expression of amino acid catabolizing enzymes and the secretion of anti-inflammatory cytokines, e.g. IL-10 and TGF-β, and reactive oxygen species (ROS). On the other hand, the accumulation of MDSCs into tissues exerts harmful effects in chronic pathological disorders, e.g. tumors and many age-related diseases, since the immunosuppression induced by MDSCs impairs the clearance of senescent and cancer cells and also disturbs the maintenance of energy metabolism and tissue proteostasis. The co-operation between senescent cells and immunosuppressive MDSCs regulates not only tumorigenesis and chronic inflammatory disorders but it also might promote inflammaging during the aging process.

Myeloid-Derived Suppressor Cells Induce Podocyte Injury Through Increasing Reactive Oxygen Species in Lupus Nephritis

The expansion of myeloid-derived suppressor cells (MDSCs) has been documented in murine models and patients with lupus nephritis (LN), but the exact role of MDSCs in this process remains largely unknown. In this study, we investigated whether MDSCs are involved in the process of podocyte injury in the development of LN. In toll-like receptor-7 (TLR-7) agonist imiquimod-induced lupus mice, we found the severe podocyte injury in glomeruli of lupus mice and significant expansion of MDSCs in spleens and kidneys of lupus mice. The function of TLR-7 activated MDSCs was enhanced including the increased generation of reactive oxygen species (ROS) in vivo and in vitro. Moreover, the ROS production of MDSCs induced podocyte injury through activating the p-38MAPK and NF-kB signaling. Furthermore, we verified that podocyte injury was indeed correlated with expansion of MDSCs and their ROS secretion in LN of pristane-induced lupus mice. These findings first indicate that the podocyte injury in LN was associated with the increased MDSCs in kidney and MDSCs may be a promising therapeutic target of LN in the future.

Blood-based analyses of cancer: Circulating myeloid-derived suppressor cells - is a new era coming?

Progress in cancer treatment made by the beginning of the 21st century has shifted the paradigm from one-size-fits-all to tailor-made treatment. The popular vision, to study solid tumors through the relatively noninvasive sampling of blood, is one of the most thrilling and rapidly advancing fields in global cancer diagnostics. From this perspective, immune-cell analysis in cancer could play a pivotal role in oncology practice. This approach is driven both by rapid technological developments, including the analysis of circulating myeloid-derived suppressor cells (cMDSCs), and by the increasing application of (immune) therapies, the success or failure of which may depend on effective and timely measurements of relevant biomarkers. Although the implementation of these powerful noninvasive diagnostic capabilities in guiding precision cancer treatment is poised to change the ways in which we select and monitor cancer therapy, challenges remain. Here, we discuss the challenges associated with the analysis and clinical aspects of cMDSCs and assess whether the problems in implementing tumor-evolution monitoring as a global tool in personalized oncology can be overcome.

Myeloid-Derived Suppressor Cells Hinder the Anti-Cancer Activity of Immune Checkpoint Inhibitors

Immune checkpoint inhibitors (ICI) used for cancer immunotherapy were shown to boost the existing anti-tumor immune response by preventing the inhibition of T cells by tumor cells. Antibodies targeting two negative immune checkpoint pathways, namely cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), programmed cell death protein 1 (PD-1), and programmed cell death-ligand 1 (PD-L1), have been approved first for patients with melanoma, squamous non-small cell lung cancer (NSCLC), and renal cell carcinoma. Clinical trials are ongoing to verify the efficiency of these antibodies for other cancer types and to evaluate strategies to block other checkpoint molecules. However, a number of patients do not respond to this treatment possibly due to profound immunosuppression, which is mediated partly by myeloid-derived suppressor cells (MDSC). This heterogeneous population of immature myeloid cells can strongly inhibit anti-tumor activities of T and NK cells and stimulate regulatory T cells (Treg), leading to tumor progression. Moreover, MDSC can contribute to patient resistance to immune checkpoint inhibition. Accumulating evidence demonstrates that the frequency and immunosuppressive function of MDSC in cancer patients can be used as a predictive marker for therapy response. This review focuses on the role of MDSC in immune checkpoint inhibition and provides an analysis of combination strategies for MDSC targeting together with ICI to improve their therapeutic efficiency in cancer patients.

Vaccination of Urological Cancer Patients With WT1 Peptide-Pulsed Dendritic Cells in Combination With Molecular Targeted Therapy or Conventional Chemotherapy Induces Immunological and Clinical Responses

The prognosis of metastatic or relapsed renal cell carcinoma (RCC) or bladder cancer (BC) remains poor despite the introduction of immune checkpoint blockade agents. We aimed to investigate the safety and the feasibility of a vaccination with WT1 peptide-loaded dendritic cells (DCs) and OK-432 adjuvant combined with molecular targeted therapy or conventional chemotherapy. Five eligible patients with metastatic or relapsed RCC and five eligible patients with BC were enrolled. No severe adverse events related to a vaccination were observed. Seven patients with RCC or non-muscle invasive BC had durable stable disease and three other patients had disease progression after DC vaccination. DC vaccination augmented WT1 specific immunity and the reduction of regulatory T cells which might be related to clinical outcome. These results indicate that DC-based immunotherapy combined with a molecular targeted therapy or a conventional chemotherapy is safe and feasible for patients in advanced stage of RCC or BC.

Emergence of immunotherapy as a novel way to treat hepatocellular carcinoma

Tumor immunity proceeds through multiple processes, which consist of antigen presentation by antigen presenting cells (APCs) to educate effector cells and destruction by the effector cytotoxic cells. However, tumor immunity is frequently repressed at tumor sites. Malignantly transformed cells rarely survive the attack by the immune system, but cells that do survive change their phenotypes to reduce their immunogenicity. The resultant cells evade the attack by the immune system and form clinically discernible tumors. Tumor microenvironments simultaneously contain a wide variety of immune suppressive molecules and cells to dampen tumor immunity. Moreover, the liver microenvironment exhibits immune tolerance to reduce aberrant immune responses to massively-exposed antigens via the portal vein, and immune dysfunction is frequently associated with liver cirrhosis, which is widespread in hepatocellular carcinoma (HCC) patients. Immune therapy aims to reduce tumor burden, but it is also expected to prevent non-cancerous liver lesions from progressing to HCC, because HCC develops or recurs from non-cancerous liver lesions with chronic inflammatory states and/or cirrhosis and these lesions cannot be cured and/or eradicated by local and/or systemic therapies. Nevertheless, cancer immune therapy should augment specific tumor immunity by using two distinct measures: enhancing the effector cell functions such as antigen presentation capacity of APCs and tumor cell killing capacity of cytotoxic cells, and reactivating the immune system in immune-suppressive tumor microenvironments. Here, we will summarize the current status and discuss the future perspective on immune therapy for HCC.

Toll-like receptors in immunity and inflammatory diseases: Past, present, and future

The immune system is a very diverse system of the host that evolved during evolution to cope with various pathogens present in the vicinity of environmental surroundings inhabited by multicellular organisms ranging from achordates to chordates (including humans). For example, cells of immune system express various pattern recognition receptors (PRRs) that detect danger via recognizing specific pathogen-associated molecular patterns (PAMPs) and mount a specific immune response. Toll-like receptors (TLRs) are one of these PRRs expressed by various immune cells. However, they were first discovered in the Drosophila melanogaster (common fruit fly) as genes/proteins important in embryonic development and dorso-ventral body patterning/polarity. Till date, 13 different types of TLRs (TLR1-TLR13) have been discovered and described in mammals since the first discovery of TLR4 in humans in late 1997. This discovery of TLR4 in humans revolutionized the field of innate immunity and thus the immunology and host-pathogen interaction. Since then TLRs are found to be expressed on various immune cells and have been targeted for therapeutic drug development for various infectious and inflammatory diseases including cancer. Even, Single nucleotide polymorphisms (SNPs) among various TLR genes have been identified among the different human population and their association with susceptibility/resistance to certain infections and other inflammatory diseases. Thus, in the present review the current and future importance of TLRs in immunity, their pattern of expression among various immune cells along with TLR based therapeutic approach is reviewed.

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TLRs are first described PRRs that revolutionized the biology of host-pathogen interaction and immune response

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The discovery of different TLRs in humans proved milestone in the field of innate immunity and inflammation

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The pattern of expression of all the TLRs expressed by human immune cells

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An association of various TLR SNPs with different inflammatory diseases

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Currently available drugs or vaccines based on TLRs and their future in drug targeting along with the role in reproduction, and regeneration

Brazilian propolis ethanol extract and its component kaempferol induce myeloid-derived suppressor cells from macrophages of mice in vivo and in vitro

Background

Brazilian green propolis is produced by mixing secretions from Africanized honey bees with exudate, mainly from Baccharis dracunculifolia. Brazilian propolis is especially rich in flavonoids and cinammic acid derivatives, and it has been widely used in folk medicine owing to its anti-inflammatory, anti-viral, tumoricidal, and analgesic effects. Moreover, it is applied to prevent metabolic disorders, such as type 2 diabetes and arteriosclerosis. Previously, we demonstrated that propolis ethanol extract ameliorated type 2 diabetes in a mouse model through the resolution of adipose tissue inflammation. The aims of this study were to identify the immunosuppressive cells directly elicited by propolis extract and to evaluate the flavonoids that induce such cells.

Methods

Ethanol extract of Brazilian propolis (PEE; 100 mg/kg i.p., twice a week) was injected into lean or high fat-fed obese C57BL/6 mice or C57BL/6 ob/ob mice for one month. Subsequently, immune cells in visceral adipose tissue and the peritoneal cavity were monitored using FACS analysis. Isolated macrophages and the macrophage-like cell line J774.1 were treated with PEE and its constituent components, and the expression of immune suppressive myeloid markers were evaluated. Finally, we injected one of the identified compounds, kaempferol, into C57BL/6 mice and performed FACS analysis on the adipose tissue.

Results

Intraperitoneal treatment of PEE induces CD11b⁺, Gr-1⁺ myeloid-derived suppressor cells (MDSCs) in visceral adipose tissue and the peritoneal cavity of lean and obese mice. PEE directly stimulates cultured M1 macrophages to transdifferentiate into MDSCs. Among twelve compounds isolated from PEE, kaempferol has an exclusive effect on MDSCs induction in vitro. Accordingly, intraperitoneal injection of kaempferol causes accumulation of MDSCs in the visceral adipose tissue of mice.

Conclusion

Brazilian PEE and its compound kaempferol strongly induce MDSCs in visceral adipose tissue at a relatively early phase of inflammation. Given the strong anti-inflammatory action of MDSCs, the induction of MDSCs by PEE and kaempferol is expected to be useful for anti-diabetic and anti-inflammatory therapies.

Graphical Abstract

Electronic supplementary material

The online version of this article (10.1186/s12906-018-2198-5) contains supplementary material, which is available to authorized users.

Monocytic myeloid-derived suppressor cells generated from rhesus macaque bone marrow enrich for regulatory T cells

Putative monocytic myeloid-derived suppressor cells (mMDSC; lineage^-HLA-DR^-/lo) were generated in 7-day cultures from normal rhesus macaque bone marrow (BM) cells in GM-CSF and IL-6. Three subsets were identified based on their differential expression of CD14, CD33, CD34 and CD11b. Following flow sorting, assessment of the capacity of these subsets to suppress anti-CD3/CD28-stimulated CD4 and CD8 T cell proliferation revealed that the most potent population was CD14^hiCD33^-/loCD34^loCD11b^hi. These BM-derived mMDSC markedly increased the incidence of CD4⁺CD25⁺CD127^-Foxp3⁺ regulatory T cells in responder T cell populations. They offer potential value in testing the therapeutic efficacy of immunoregulatory mMDSC for the promotion of tolerance in nonhuman primate transplant models.

Therapeutic prospects of targeting myeloid-derived suppressor cells and immune checkpoints in cancer

Immune evasion is a characteristic of most human malignancies and is induced via various mechanisms. Immunosuppressive cells, including myeloid-derived suppressor cells (MDSC) and regulatory T cells (Treg), are key mediators in assisting tumors to escape immune surveillance. Expansion of MDSC, Treg and elevated levels of immune checkpoints (IC) are frequently detected in the tumor microenvironment and periphery of cancer patients. Various therapeutic agents have been shown to target MDSC and to block IC for inducing anti-tumor immunity and reversal of tumor immune escape. Importantly, some recent studies have shown that MDSC targeting improves the efficacy of IC blockade in cancer therapy. However, there is a pressing need to improve our understanding of the distinct role of these cells to develop combination therapy that attacks tumor cells from all frontiers to improve cancer therapeutics. Herein, we discuss the role of MDSC in cancer progression, interactions with IC in the context of anti-cancer immunity and the current therapeutic strategies to target MDSC and block IC in cancer.

Targeting Tumor-Associated Macrophages as a Potential Strategy to Enhance the Response to Immune Checkpoint Inhibitors

Inhibition of immune checkpoint pathways in CD8⁺ T cell is a promising therapeutic strategy for the treatment of solid tumors that has shown significant anti-tumor effects and is now approved by the FDA to treat patients with melanoma and lung cancer. However the response to this therapy is limited to a certain fraction of patients and tumor types, for reasons still unknown. To ensure success of this treatment, CD8⁺ T cells, the main target of the checkpoint inhibitors, should exert full cytotoxicity against tumor cells. However recent studies show that tumor-associated macrophages (TAM) can impede this process by different mechanisms. In this mini-review we will summarize recent studies showing the effect of TAM targeting on immune checkpoint inhibitors efficacy. We will also discuss on the limitations of the current strategies as well on the future scientific challenges for the progress of the tumor immunology field.

Therapeutic Potential of Invariant Natural Killer T Cells in Autoimmunity

Tolerance against self-antigens is regulated by a variety of cell types with immunoregulatory properties, such as CD1d-restricted invariant natural killer T (iNKT) cells. In many experimental models of autoimmunity, iNKT cells promote self-tolerance and protect against autoimmunity. These findings are supported by studies with patients suffering from autoimmune diseases. Based on these studies, the therapeutic potential of iNKT cells in autoimmunity has been explored. Many of these studies have been performed with the potent iNKT cell agonist KRN7000 or its structural variants. These findings have generated promising results in several autoimmune diseases, although mechanisms by which iNKT cells modulate autoimmunity remain incompletely understood. Here, we will review these preclinical studies and discuss the prospects for translating their findings to patients suffering from autoimmune diseases.

Transitory presence of myeloid-derived suppressor cells in neonates is critical for control of inflammation

Myeloid-derived suppressor cells (MDSC) are pathologically activated and relatively immature myeloid cells, which are implicated in the immune regulation of many pathologic conditions^1,2. Phenotypically and morphologically MDSC are similar to neutrophils (PMN-MDSC) and monocytes (M-MDSC). However, they have potent suppressive activity, a distinct gene expression profile, and biochemical characteristics³. None or very few MDSC are observed in steady state physiological conditions. Therefore, until recently, accumulation of MDSC was considered as a consequence of pathological process or pregnancy. Here, we report that MDSC with a potent ability to suppress T cells are present during the first weeks of life in mice and humans. MDSC suppressive activity was triggered by lactoferrin and mediated by nitric oxide, PGE2, and S100A9/A8 proteins. Newborn MDSC had a transcriptome similar to that of tumor MDSC, but with a strong up-regulation of an antimicrobial gene network and had potent antibacterial activity. MDSC played a critical role in control of experimental necrotizing enterocolitis (NEC) in newborn mice. MDSC in infants with very low-weight, which are prone to the development of NEC, had lower MDSC levels and suppressive activity than infants with normal weight. Thus, the transitory presence of MDSC may be critical for regulation of inflammation in newborns.