Regulatory dendritic cells in the tumor immunoenvironment

Activation of immunosuppressive network in the aging process

Chronic low-grade inflammation has a key role in the aging process, a state called inflammaging. It is known that the chronic inflammatory condition generates counteracting immunosuppressive state in many diseases. Inflammaging is also associated with an immune deficiency; generally termed as immunosenescence, although it is not known whether it represents the senescence of immune cells or the active remodeling of immune system. Evidence has accumulated since the 1970's indicating that immunosenescence might be caused by an increased activity of immunosuppressive cells rather than cellular senescence. Immune cells display remarkable plasticity; many of these cells can express both proinflammatory and immunosuppressive phenotypes in a context-dependent manner. The immunosuppressive network involves the regulatory subtypes of T (Treg) and B (Breg) cells as well as regulatory phenotypes of macrophages (Mreg), dendritic (DCreg), natural killer (NKreg), and type II natural killer T (NKT) cells. The immunosuppressive network also includes monocytic (M-MDSC) and polymorphonuclear (PMN-MDSC) myeloid-derived suppressor cells which are immature myeloid cells induced by inflammatory mediators. This co-operative network is stimulated in chronic inflammatory conditions preventing excessive inflammatory responses but at the same time they exert harmful effects on the immune system and tissue homeostasis. Recent studies have revealed that the aging process is associated with the activation of immunosuppressive network, especially the functions of MDSCs, Tregs, and Mregs are increased. I will briefly review the properties of the regulatory phenotypes of immune cells and examine in detail the evidences for an activation of immunosuppressive network with aging.

Therapeutic Monoclonal Antibodies Targeting Immune Checkpoints for the Treatment of Solid Tumors

Recently, modulation of immune checkpoints has risen to prominence as a means to treat a number of solid malignancies, given the durable response seen in many patients and improved side effect profile compared to conventional chemotherapeutic agents. Several classes of immune checkpoint modulators have been developed. Here, we review current monoclonal antibodies directed against immune checkpoints that are employed in practice today. We discuss the history, mechanism, indications, and clinical data for each class of therapies. Furthermore, we review the challenges to durable tumor responses that are seen in some patients and discuss possible interventions to circumvent these barriers.

In situ Vaccination by Direct Dendritic Cell Inoculation: The Coming of Age of an Old Idea?

For more than 25 years, dendritic cell (DC) based vaccination has flashily held promises to represent a therapeutic approach for cancer treatment. While the vast majority of studies has focused on the use of antigen loaded DC, the intratumoral delivery of unloaded DC aiming at in situ vaccination has gained much less attention. Such approach grounds on the ability of inoculated DC to internalize and process antigens directly released by tumor (usually in combination with cell-death-inducing agents) to activate broad patient-specific antitumor T cell response. In this review, we highlight the recent studies in both solid and hematological tumors showing promising clinical results and discuss the main pitfalls and advantages of this approach for endogenous cancer vaccination. Lastly, we discuss how in situ vaccination by DC inoculation may fit with current immunotherapy approaches to expand and prolong patient response.

Tumor-derived factors affecting immune cells

Tumor progression is accompanied by the production of a wide array of immunosuppressive factors by tumor and non-tumor cells forming the tumor microenvironment. These factors belonging to cytokines, growth factors, metabolites, glycan-binding proteins and glycoproteins are responsible for the establishment of immunosuppressive networks leading towards tumor promotion, invasion and metastasis. In pre-clinical tumor models, the inactivation of some of these suppressive networks reprograms the phenotypic and functional features of tumor-infiltrating immune cells, ultimately favoring effective anti-tumor immune responses. We will discuss factors and mechanisms identified in both mouse and human tumors, and the possibility to associate drugs inhibiting these mechanisms with new immunotherapy strategies already entered in the clinical practice.

STAT3 and NF-κB are Simultaneously Suppressed in Dendritic Cells in Lung Cancer

Tumour-induced dendritic cell (DC) dysfunction plays an important role in cancer immune escape. However, the underlying mechanisms are not yet fully understood, reflecting the lack of appropriate experimental models both in vivo and in vitro. In the present study, an in vitro study model for tumour-induced DC dysfunction was established by culturing DCs with pooled sera from multiple non-small cell lung cancer (NSCLC) patients. The results demonstrated that tumour-induced human monocyte-derived DCs exhibited systematic functional deficiencies. Transcriptomics analysis revealed that the expression of major functional cluster genes, including the MHC class II family, cytokines, chemokines, and co-stimulatory molecules, was significantly altered in tumour-induced DCs compared to that in control cells. Further examination confirmed that both NF-κB and STAT3 signalling pathways were simultaneously repressed by cancer sera, suggesting that the attenuated NF-κB and STAT3 signalling could be the leading cause of DC dysfunction in cancer. Furthermore, reversing the deactivated NF-κB and STAT3 signalling could be a strategy for cancer immunotherapy.

Collagen scaffold microenvironments modulate cell lineage commitment for differentiation of bone marrow cells into regulatory dendritic cells

The microenvironment plays a pivotal role for cell survival and functional regulation, and directs the cell fate determination. The biological functions of DCs have been extensively investigated to date. However, the influences of the microenvironment on the differentiation of bone marrow cells (BMCs) into dendritic cells (DCs) are not well defined. Here, we established a 3D collagen scaffold microenvironment to investigate whether such 3D collagen scaffolds could provide a favourable niche for BMCs to differentiate into specialised DCs. We found that BMCs embedded in the 3D collagen scaffold differentiated into a distinct subset of DC, exhibiting high expression of CD11b and low expression of CD11c, co-stimulator (CD40, CD80, CD83, and CD86) and MHC-II molecules compared to those grown in 2D culture. DCs cultured in the 3D collagen scaffold possessed weak antigen uptake ability and inhibited T-cell proliferation in vitro; in addition, they exhibited potent immunoregulatory function to alleviate allo-delay type hypersensitivity when transferred in vivo. Thus, DCs differentiated in the 3D collagen scaffold were defined as regulatory DCs, indicating that collagen scaffold microenvironments probably play an important role in modulating the lineage commitment of DCs and therefore might be applied as a promising tool for generation of specialised DCs.

Tumor-derived factors modulating dendritic cell function

Dendritic cells (DC) play unique and diverse roles in the tumor occurrence, development, progression and response to therapy. First of all, DC can actively uptake tumor-associated antigens, process them and present antigenic peptides to T cells inducing and maintaining tumor-specific T cell responses. DC interaction with different immune effector cells may also support innate antitumor immunity, as well as humoral responses also known to inhibit tumor development in certain cases. On the other hand, DC are recruited to the tumor site by specific tumor-derived and stroma-derived factors, which may also impair DC maturation, differentiation and function, thus resulting in the deficient formation of antitumor immune response or development of DC-mediated tolerance and immune suppression. Identification of DC-stimulating and DC-suppressing/polarizing factors in the tumor environment and the mechanism of DC modulation are important for designing effective DC-based vaccines and for recovery of immunodeficient resident DC responsible for maintenance of clinically relevant antitumor immunity in patients with cancer. DC-targeting tumor-derived factors and their effects on resident and administered DC in the tumor milieu are described and discussed in this review.

The emerging role of myeloid-derived suppressor cells in lung diseases

Myeloid-derived suppressor cells (MDSCs) are innate immune cells characterised by their potential to control T-cell responses and to dampen inflammation. While the role of MDSCs in cancer has been studied in depth, our understanding of their relevance for infectious and inflammatory disease conditions has just begun to evolve. Recent studies highlight an emerging and complex role for MDSCs in pulmonary diseases. In this review, we discuss the potential contribution of MDSCs as biomarkers and therapeutic targets in lung diseases, particularly lung cancer, tuberculosis, chronic obstructive pulmonary disease, asthma and cystic fibrosis.

Intratumoral injection of IFN-alpha dendritic cells after dacarbazine activates anti-tumor immunity: results from a phase I trial in advanced melanoma

Background

Advanced melanoma patients have an extremely poor long term prognosis and are in strong need of new therapies. The recently developed targeted therapies have resulted in a marked antitumor effect, but most responses are partial and some degree of toxicity remain the major concerns.

Dendritic cells play a key role in the activation of the immune system and have been typically used as ex vivo antigen-loaded cell drugs for cancer immunotherapy.

Another approach consists in intratumoral injection of unloaded DCs that can exploit the uptake of a wider array of tumor-specific and individual unique antigens. However, intratumoral immunization requires DCs endowed at the same time with properties typically belonging to both immature and mature DCs (i.e. antigen uptake and T cell priming). DCs generated in presence of interferon-alpha (IFN-DCs), due to their features of partially mature DCs, capable of efficiently up-taking, processing and cross-presenting antigens to T cells, could successfully carry out this task. Combining intratumoral immunization with tumor-destructing therapies can induce antigen release in situ, facilitating the injected DCs in triggering an antitumor immune response.

Methods

We tested in a phase I clinical study in advanced melanoma a chemo-immunotherapy approach based on unloaded IFN-DCs injected intratumorally one day after administration of dacarbazine. Primary endpoint of the study was treatment safety and tolerability. Secondary endpoints were immune and clinical responses of patients.

Results

Six patients were enrolled, and only three completed the treatment. The chemo-immunotherapy was well tolerated with no major side effects. Three patients showed temporary disease stabilization and two of them showed induction of T cells specific for tyrosinase, NY-ESO-1 and gp100. Of interest, one patient showing a remarkable long-term disease stabilization kept showing presence of tyrosinase specific T cells in PBMC and high infiltration of memory T cells in the tumor lesion at 21 months.

Conclusion

We tested a chemo-immunotherapeutic approach based on IFN-DCs injected intratumorally one day after DTIC in advanced melanoma. The treatment was well tolerated, and clinical and immunological responses, including development of vitiligo, were observed, therefore warranting additional clinical studies aimed at evaluating efficacy of this approach.

Trial registration

Trial Registration Number not publicly available due to EudraCT regulations: https://www.clinicaltrialsregister.eu/doc/EU_CTR_FAQ.pdf

Incorporation of porcine adenovirus 4 fiber protein enhances infectivity of adenovirus vector on dendritic cells: implications for immune-mediated cancer therapy

One strategy in cancer immunotherapy is to capitalize on the key immunoregulatory and antigen presenting capabilities of dendritic cells (DCs). This approach is dependent on efficient delivery of tumor specific antigens to DCs, which subsequently induce an anti-tumor T-cell mediated immune response. Human adenovirus serotype 5 (HAdV5) has been used in human studies for gene delivery, but has limited infection in DCs, which lack the proper receptors. Addition of the porcine fiber knob (PK) from porcine adenovirus type 4 to HAdV5 allows the virus to deliver genetic material via binding to glycosylated surface proteins and bypasses the coxsackie-and-adenovirus receptor required by wild-type HAdV5. In this study we explored the potential therapeutic applications of an adenovirus with PK-based tropism against cancers expressing mesothelin. Infectivity and gene transfer assays were used to compare Ad5-PK to wild-type HAdV5. Mouse models were used to demonstrate peptide specificity and T-cell responses. We show that the PK modification highly augmented infection of DCs, including the CD141+ DC subset, a key subset for activation of naïve CD8+ T-cells. We also show that Ad5-PK increases DC infectivity and tumor specific antigen expression. Finally, vaccination of mice with the Ad5-PK vector resulted in enhanced T-cell-mediated interferon gamma (IFN-γ) release in response to both mesothelin peptide and a tumor line expressing mesothelin. Ad5-PK is a promising tool for cancer immunotherapy as it improves infectivity, gene transfer, protein expression, and subsequent T-cell activation in DCs compared to wild-type HAdV5 viruses.

Combined Tumor Cell-Based Vaccination and Interleukin-12 Gene Therapy Polarizes the Tumor Microenvironment in Mice

Tumor progression depends on tumor milieu, which influences neovasculature formation and immunosuppression. Combining immunotherapy with antiangiogenic/antivascular therapy might be an effective therapeutic approach. The aim of our study was to elaborate an anticancer therapeutic strategy based on the induction of immune response which leads to polarization of tumor milieu. To achieve this, we developed a tumor cell-based vaccine. CAMEL peptide was used as a B16-F10 cell death-inducing agent. The lysates were used as a vaccine to immunize mice bearing B16-F10 melanoma tumors. To further improve the therapeutic effect of the vaccine, we combined it with interleukin (IL)-12 gene therapy. IL-12, a cytokine with antiangiogenic properties, activates nonspecific and specific immune responses. We observed that combined therapy is significantly more effective (as compared with monotherapies) in inhibiting tumor growth. Furthermore, the tested combination polarizes the tumor microenvironment, which results in a switch from a proangiogenic/immunosuppressive to an antiangiogenic/immunostimulatory one. The switch manifests itself as a decreased number of tumor blood vessels, increased levels of tumor-infiltrating CD4⁺, CD8⁺ and NK cells, as well as lower level of suppressor lymphocytes (Treg). Our results suggest that polarizing tumor milieu by such combined therapy does inhibit tumor growth and seems to be a promising therapeutic strategy.

Nano-gold corking and enzymatic uncorking of carbon nanotube cups

Because of their unique stacked, cup-shaped, hollow compartments, nitrogen-doped carbon nanotube cups (NCNCs) have promising potential as nanoscale containers. Individual NCNCs are isolated from their stacked structure through acid oxidation and subsequent probe-tip sonication. The NCNCs are then effectively corked with gold nanoparticles (GNPs) by sodium citrate reduction with chloroauric acid, forming graphitic nanocapsules with significant surface-enhanced Raman signature. Mechanistically, the growth of the GNP corks starts from the nucleation and welding of gold seeds on the open rims of NCNCs enriched with nitrogen functionalities, as confirmed by density functional theory calculations. A potent oxidizing enzyme of neutrophils, myeloperoxidase (MPO), can effectively open the corked NCNCs through GNP detachment, with subsequent complete enzymatic degradation of the graphitic shells. This controlled opening and degradation was further carried out in vitro with human neutrophils. Furthermore, the GNP-corked NCNCs were demonstrated to function as novel drug delivery carriers, capable of effective (i) delivery of paclitaxel to tumor-associated myeloid-derived suppressor cells (MDSC), (ii) MPO-regulated release, and (iii) blockade of MDSC immunosuppressive potential.

Tumor necrosis factor-α promotes survival and phenotypic maturation of poly(I:C)-treated dendritic cells but impairs their Th1 and Th17 polarizing capability

BACKGROUND AIMS

Toll-like receptor (TLR)-3 synthetic agonist polyinosinic-polycytidylic acid (poly(I:C)) is a promising agent for dendritic cell (DC)-based anti-tumor vaccines because of its ability to induce a strong maturation of DCs, but such an effect is followed by stimulation of DC apoptosis. Tumor necrosis factor (TNF)-α may promote the survival of poly(I:C)-stimulated DCs, but it is not known in detail how this combination affects the maturation and polarization capacity of monocyte-derived (Mo)DCs.

METHODS

Immature MoDCs, generated from human monocytes, were treated with different concentrations of poly(I:C) combined with TNF-α, and the effect on survival, phenotype, production of cytokines, allostimulatory and Th polarization capacity was assessed after 24 and 48 h.

RESULTS

We showed that TNF-α inhibited the dose-dependent pro-apoptotic effect of poly(I:C). However, TNF-α also decreased poly(I:C)-induced production of interleukin (IL)-12 and IL-23 by MoDCs, which correlated with their diminished capacity to stimulate cellular proliferation, interferon-γ and IL-17 production by allogeneic CD4(+)T cells in co-culture. Such an effect was more pronounced after 24 h and could not be restored by CD40 ligation. In the presence of CD40L, TNF-α even stimulated IL-10 production and immunoglobulin-like transcript 3 expression by poly(I:C)-matured DCs, which correlated with their increased capacity to induce IL-10 production by CD4(+)T cells.

CONCLUSION

Even though TNF-α could promote the survival of poly(I:C)-matured MoDCs, it also suppresses key anti-tumor functions of these cells, which could have important implications when considering this, already suggested, protocol for the DC-based anti-tumor therapy.

Immunological Mechanisms of Low and Ultra-Low Dose Cancer Chemotherapy

Traditionally, anticancer chemotherapy has been generally considered to be strongly immunosuppressive. However, increasing evidence suggests that certain chemotherapeutic agents rely on the induction of antitumor immune responses, in both experimental animal models and patients with cancer. Many of these chemotherapeutic agents exert immunogenic effects via the induction and release of immunostimulatory "danger" signals from dying cancerous cells when used in low doses. New data suggests that several common chemotherapeutic agents may also display direct stimulating effects on immune cells even when applied in ultra-low concentrations (chemoimmunomodulation). Importantly, it is becoming clear that both immune effector cells and immune regulatory cells can be targeted by various chemotherapeutic agents to produce favorable antitumor immune responses. Therefore, utilizing cancer drugs to enhance host antitumor immunity should be considered a feasible therapeutic approach; and recent characterization of the immunomodulatory mechanisms of anticancer chemotherapy using both new and traditional cytotoxic agents suggests that combinations of these approaches with "classical" immunomodulatory agents could lead to a viable new therapeutic paradigm for the treatment of cancer.

Antitumor effects of recombinant antivascular protein ABRaA-VEGF121 combined with IL-12 gene therapy

Development and neoplastic progression strongly rely on tumor microenvironment cells. Various kinds of cells that form such tumor milieu play substantial roles in angiogenesis and immunosuppression. Attempts to inhibit tumor vascularization alter tumor milieu and enhance immune response against the tumor. Anticancer therapeutic strategy bringing together antiangiogenic and immunostimulating agents has emerged as a promising approach. We here investigated whether therapy directed against preexisting vessels, combined with an immunomodulatory factor would be equally effective in arresting tumor growth. To this goal, we investigated the effectiveness of ABRaA-vascular endothelial growth factor isoform 121 (VEGF₁₂₁), an antivascular drug constructed by us. It is a fusion protein composed of VEGF₁₂₁, and abrin A chain (translation-inhibiting toxin). We used it in combination with interleukin (IL-12) gene therapy and tried to inhibit B16-F10 melanoma tumor growth. ABRaA-VEGF₁₂₁ is a chimeric recombinant protein capable of destroying tumor vasculature and triggering necrosis in the vicinity of damaged vessels. IL-12 cytokine, in turn, activates both specific and non-specific immune responses. Our results demonstrate that combination of ABRaA-VEGF₁₂₁ antivascular agent with immunostimulatory cytokine IL-12 indeed inhibits tumor growth more effectively than either agent alone, leading to complete cure of ca. 20 % mice. Post-therapeutic analysis of tumors excised from mice treated with combination therapy showed decreased numbers of blood microvessels in the tumor microenvironment, lowered numbers of regulatory T lymphocytes, as well as showed higher levels of CD4⁺ and CD8⁺ as compared to control mice. It seems that bringing together antivascular strategy and the action of immunostimulating agents indeed inhibits growth of tumors.

β-catenin-mediated inhibition of cross-priming: A new mechanism for tumors to evade immunosurveillance

Cross-priming plays a major role in generating CD8⁺ T cell-dependent antitumor immunity through cross-presentation. However, the cross-presentation of tumor-associated antigens by dendritic cells often promotes tolerance rather than CD8⁺ T-cell immunity. We have now identified a β-catenin-dependent pathway of cross-priming inhibition as a novel and potentially broad mechanism whereby neoplastic cells promote immunosuppression.

Imaging aspects of the tumor stroma with therapeutic implications

Cancer cells rely on extensive support from the stroma in order to survive, proliferate and invade. The tumor stroma is thus an important potential target for anti-cancer therapy. Typical changes in the stroma include a shift from the quiescence promoting-antiangiogenic extracellular matrix to a provisional matrix that promotes invasion and angiogenesis. These changes in the extracellular matrix are induced by changes in the secretion of extracellular matrix proteins and glucose amino glycans, extravasation of plasma proteins from hyperpermeable vessels and release of matrix modifying enzymes resulting in cleavage and cross-linking of matrix macromolecules. These in turn alter the rigidity of the matrix and the exposure and release of cytokines. Changes in matrix rigidity and vessel permeability affect drug delivery and mediate resistance to cytotoxic therapy. These stroma changes are brought about not only by the cancer cells, but also through the action of many cell types that are recruited by tumors including immune cells, fibroblasts and endothelial cells. Within the tumor, these normal host cells are activated resulting in loss of inhibitory and induction of cancer promoting activities. Key to the development of stroma-targeted therapies, selective biomarkers were developed for specific imaging of key aspects of the tumor stroma.

Wnt5a inhibits human monocyte-derived myeloid dendritic cell generation

Wnt5a is a non-canonical Wnt protein that is expressed at elevated levels in inflammatory conditions. Its role in inflammation remains unclear, although it is known that Wnt5a is expressed at a higher level in monocyte-derived myeloid dendritic cells (Mo-mDCs) than in monocytes and macrophages. The function of Wnt5a in dendritic cells (DCs) remains relatively unexplored. Here, we found that under Mo-mDC culture conditions, Wnt5a inhibited the generation of CD14(⁺/low) Mo-mDCs while promoting the generation of CD14⁺/⁺⁺ CD16⁺ monocytes. We could further show that stimulation of monocytes with rWnt5a induced a rapid IL-6 production and that the rWnt5a treated Mo-mDC differentiation was restored upon blocking of IL-6. Also, conditioned media from Wnt5a stimulated human breast cancer cells producing IL-6, specifically inhibited Mo-mDC differentiation. These observations are strengthened by our finding that patients with sepsis, a disease involving elevated Wnt5a and IL-6 levels, also showed a significant increase in the CD14⁺ CD16⁺⁺/CD14⁺/⁺⁺ CD16⁺ monocyte populations, which was accompanied by a significant decrease in circulating mDCs. We finally show that under typical Mo-mDC culture conditions, monocytes isolated from patients with sepsis as compared to healthy controls, preferentially differentiated into CD14CD14⁺/⁺⁺ HLA-DR⁺⁺ cells. We suggest that Wnt5a is a possible candidate mediator for the CD14⁺/⁺⁺ CD16⁺ monocyte accumulation seen in patients with infectious disease and cancer.

The chemotherapeutic agent topotecan differentially modulates the phenotype and function of dendritic cells

The camptothecin analogue topotecan (TPT) induces tumor cell apoptosis due to interference with topoisomerase I and is clinically used as a second-line chemotherapeutic in the treatment for metastasizing ovarian and small cell lung carcinoma. Based on the more recent finding of TPT-mediated inhibition of the transcription factor hypoxia-induced factor-1α, a hallmark of solid tumors, TPT, is currently tested in clinical trials for its suitability as a first-line chemotherapeutic for the treatment for various types of tumors. Due to the gained clinical interest in TPT and in light of its modulatory effect on signaling pathways, which are also of importance for immune cell functions, we asked for potential effects of TPT on dendritic cells (DCs), the main antigen-presenting cell population of the immune system. Here, we show that TPT at a therapeutically relevant dose partially activated monocyte-derived DCs as reflected by enhanced migratory activity, elevated expression of HLA-DR and costimulatory/maturation markers, and accordingly an increased allogenic CD4(+) T cell stimulation. In marked contrast, TPT prevented full maturation of DCs stimulated with a cocktail of proinflammatory mediators, accompanied by somewhat lower upregulation of NF-κB factors p65 and RelB.

Can exercise-related improvements in immunity influence cancer prevention and prognosis in the elderly?

Cancer incidence increases with advancing age. Over 60% of new cancers and 70% of cancer deaths occur in individuals aged 65 years or older. One factor that may contribute to this is immunosenescence - a canopy term that is used to describe age-related declines in the normal functioning of the immune system. There are multiple age-related deficits in both the innate and adaptive systems that may play a role in the increased incidence of cancer. These include decreased NK-cell function, impaired antigen uptake and presentation by monocytes and dendritic cells, an increase in 'inflammaging', a decline in the number of naïve T-cells able to respond to evolving tumor cells, and an increase in functionally exhausted senescent cells. There is consensus that habitual physical exercise can offer protection against certain types of cancer; however the evidence linking immunological mechanisms, exercise, and reduced cancer risk remain tentative. Multiple studies published over the last two decades suggest that exercise can mitigate the deleterious effects of age on immune function, thus increasing anti-cancer immunity. The potential ameliorative effect of exercise on these mechanisms include evidence that physical activity is able to stimulate greater NK-cell activity, enhance antigen-presentation, reduce inflammation, and prevent senescent cell accumulation in the elderly. Here we discuss the role played by the immune system in preventing and controlling cancer and how aging may retard these anti-cancer mechanisms. We also propose a pathway by which exercise-induced alterations in immunosenescence may decrease the incidence of cancer and help improve prognosis in cancer patients.

Immunosuppressive mechanisms of regulatory dendritic cells in cancer

Three major functional subsets of dendritic cells (DCs) have been described in the tumor microenvironment in patients with cancer and tumor-bearing animals: (i) conventional DCs with intact antigen-presenting capabilities, (ii) functionally defective DCs with decreased motility and low ability to uptake, process and present antigens or produce cytokines and (iii) regulatory DCs with high capacity to suppress T cell proliferation, induce differentiation of regulatory T cells or support immune tolerance. Phenotypic characteristics of regulatory DCs (regDCs), as well as the mechanisms of T cell inhibition, vary in different experimental conditions and environments, suggesting high level of their plasticity and probably different origin. Although new data demonstrate that regDCs may play an important role at early stages of tumor development, functional differences and clinical significance of emergence of different myeloid regulatory cells (MDSCs, regDCs, M2 macrophages, N2 neutrophils, mast cells) in cancer remain to be determined.

Carbon nanotubes enhance metastatic growth of lung carcinoma via up-regulation of myeloid-derived suppressor cells

Metastatic establishment and growth of Lewis lung carcinoma is promoted by single-walled carbon nanotubes (SWCNT) in C57BL6/J mice. The effect is mediated by increased local and systemic accumulation of myeloid-derived suppressor cells (MDSC), as their depletion abrogated pro-tumor activity in vivo. These data are important for the design of novel theranostics platforms with modules capable of depleting or functionally suppressing MDSC to ensure effective immunosurveillance in the tumor microenvironment.

Antisense oligonucleotide treatment enhances the recovery of acute lung injury through IL-10-secreting M2-like macrophage-induced expansion of CD4+ regulatory T cells

MicroRNAs (miRNAs) have been shown as an important regulator in the pathologies of acute lung injury (ALI). However, the potential effect of miRNA-based therapeutic studies in ALI remains poorly understood. We assessed the effect of antisense oligonucleotides (ASOs) against miR-155 on the development of ALI using a murine ALI model. We found that miR-155 ASO treatment could enhance the recovery of ALI as evidenced by accelerated body weight back, reduced level of bronchoalveolar lavage (BAL) protein and proinflammatory cytokines, and reduced number of BAL cells. Adoptive cell transfer assay in RAG1(-/-) mice showed that CD4(+)CD25(+) regulatory T cells (Tregs) mediated the enhanced recovery of ALI. Mechanistic evidence showed that enhanced expansion of Tregs in vivo, dominantly induced by IL-10-secreting M2-like macrophages, was critical for their elevated proportion in miR-155 ASO-treated ALI mice. Finally, we report that C/EBPβ, a target molecule of miR-155, was upregulated and associated with IL-10 secretion and M2-like phenotype of macrophages. These data provided a previously unknown mechanism for miRNA-based therapy against ALI, which could ultimately aid the understanding of recovery of ALI and the development of new therapeutic strategies against clinical inflammatory lung disease.

The role of TLR4 in the paclitaxel effects on neuronal growth in vitro

Paclitaxel (Pac) is an antitumor agent that is widely used for treatment of solid cancers. While being effective as a chemotherapeutic agent, Pac in high doses is neurotoxic, specifically targeting sensory innervations. In view of these toxic effects associated with conventional chemotherapy, decreasing the dose of Pac has been recently suggested as an alternative approach, which might limit neurotoxicity and immunosuppression. However, it remains unclear if low doses of Pac retain its neurotoxic properties or might exhibit unusual effects on neuronal cells. The goal of this study was to analyze the concentration-dependent effect of Pac on isolated and cultured DRG neuronal cells from wild-type and TLR4 knockout mice. Three different morphological parameters were analyzed: the number of neurons which developed neurites, the number of neurites per cell and the total length of neurites per cell. Our data demonstrate that low concentrations of Pac (0.1 nM and 0.5 nM) do not influence the neuronal growth in cultures in both wild type and TLR4 knockout mice. Higher concentrations of Pac (1–100 nM) had a significant effect on DRG neurons from wild type mice, affecting the number of neurons which developed neurites, number of neurites per cell, and the length of neurites. In DRG from TLR4 knockout mice high concentrations of Pac showed a similar effect on the number of neurons which developed neurites and the length of neurites. At the same time, the number of neurites per cell, indicating the process of growth cone initiation, was not affected by high concentrations of Pac. Thus, our data showed that Pac in high concentrations has a significant damaging effect on axonal growth and that this effect is partially mediated through TLR4 pathways. Low doses of Pac are devoid of neuronal toxicity and thus can be safely used in a chemomodulation mode.

Dendritic cells in the cancer microenvironment

The complexity of the tumor immunoenvironment is underscored by the emergence and discovery of different subsets of immune effectors and regulatory cells. Tumor-induced polarization of immune cell differentiation and function makes this unique environment even more intricate and variable. Dendritic cells (DCs) represent a special group of cells that display different phenotype and activity at the tumor site and exhibit differential pro-tumorigenic and anti-tumorigenic functions. DCs play a key role in inducing and maintaining the antitumor immunity, but in the tumor environment their antigen-presenting function may be lost or inefficient. DCs might be also polarized into immunosuppressive/tolerogenic regulatory DCs, which limit activity of effector T cells and support tumor growth and progression. Although various factors and signaling pathways have been described to be responsible for abnormal functioning of DCs in cancer, there are still no feasible therapeutic modalities available for preventing or reversing DC malfunction in tumor-bearing hosts. Thus, better understanding of DC immunobiology in cancer is pivotal for designing novel or improved therapeutic approaches that will allow proper functioning of DCs in patients with cancer.

Instruction of myeloid cells by the tumor microenvironment: Open questions on the dynamics and plasticity of different tumor-associated myeloid cell populations

The versatility and plasticity of myeloid cell polarization/differentiation has turned out to be crucial in health and disease, and has become the subject of intense investigation during the last years. On one hand, myeloid cells provide a critical contribution to tissue homeostasis and repair. On the other hand, myeloid cells not only play an important role as first line defense against pathogens but also they are involved in a broad array of inflammation-related diseases such as cancer. Recent studies show that macrophages can exist in different activation states within the same tumor, underlining their plasticity and heterogeneity. In this review, we will discuss recent evidence on how the tumor microenvironment, as it evolves, shapes the recruitment, function, polarization and differentiation of the myeloid cell compartment, leading to the selection of myeloid cells with immunosuppressive and angiogenic functions that facilitate tumor progression and dissemination.

Regulatory dendritic cells: there is more than just immune activation

The immune system exists in a delicate equilibrium between inflammatory responses and tolerance. This unique feature allows the immune system to recognize and respond to potential threats in a controlled but normally limited fashion thereby preventing a destructive overreaction against healthy tissues. While the adaptive immune system was the major research focus concerning activation vs. tolerance in the immune system more recent findings suggest that cells of the innate immune system are important players in the decision between effective immunity and induction of tolerance or immune inhibition. Among immune cells of the innate immune system dendritic cells (DCs) have a special function linking innate immune functions with the induction of adaptive immunity. DCs are the primary professional antigen presenting cells (APCs) initiating adaptive immune responses. They belong to the hematopoietic system and arise from CD34(+) stem cells in the bone marrow. Particularly in the murine system two major subgroups of DCs, namely myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) can be distinguished. DCs are important mediators of innate and adaptive immunity mostly due to their remarkable capacity to present processed antigens via major histocompatibility complexes (MHC) to T cells and B cells in secondary lymphoid organs. A large body of literature has been accumulated during the last two decades describing which role DCs play during activation of T cell responses but also during the establishment and maintenance of central tolerance (Steinman et al., 2003). While the concept of peripheral tolerance has been clearly established during the last years, the role of different sets of DCs and their particular molecular mechanisms of immune deviation has not yet fully been appreciated. In this review we summarize accumulating evidence about the role of regulatory DCs in situations where the balance between tolerance and immunogenicity has been altered leading to pathologic conditions such as chronic inflammation or malignancies.

Cancer as an immune-mediated disease

The link between oncology and immunology has a long history and its development is forced by the necessity to develop innovative and highly efficient modalities for immunological destruction of malignant cells. The limited efficacy of surgery, chemotherapy and radiation also exemplify these issues, as these treatments do not eliminate all cancerous cells, do not address the immunosuppressive nature of the disease and can further impair the patient's immune response weakening patient's resistance to the cancer. Multidisciplinary analysis of the interaction between the immune system and cancer in preclinical and clinical settings suggests that the immune system is closely intertwined with both cancer pathogenesis and treatment. On the one hand, cancer is a manifestation of malfunctions in immunity, as malignant cells manage to escape recognition and elimination by the immune system. Chronic infections and inflammation associated with limited or polarized immune responses also contribute to carcinogenesis and tumor progression. The tumor immunoenvironment represents specific conditions and elements that support cancerous cell survival, proliferation and spreading. On the other hand, the specificity and strength of antitumor immunity is a powerful and efficient tool that can be used to recognize and destroy neoplastic cells or their supporting microenvironment. Understanding the role of the immune system in controlling and supporting tumor initiation, formation, growth and progression has crucial implications for cancer therapy and will therefore guide the future development of cancer immunotherapy and its combination with conventional therapies to achieve optimal antitumor effects in patients with different types of cancer.

Tumor microenvironment complexity: emerging roles in cancer therapy

The tumor microenvironment (TME) consists of cells, soluble factors, signaling molecules, extracellular matrix, and mechanical cues that can promote neoplastic transformation, support tumor growth and invasion, protect the tumor from host immunity, foster therapeutic resistance, and provide niches for dormant metastases to thrive. An American Association for Cancer Research (AACR) special conference held on November 3-6, 2011, addressed five emerging concepts in our understanding of the TME: its dynamic evolution, how it is educated by tumor cells, pathways of communication between stromal and tumor cells, immunomodulatory roles of the lymphatic system, and contribution of the intestinal microbiota. These discussions raised critical questions on how to include the analysis of the TME in personalized cancer diagnosis and treatment.

Tumor associated regulatory dendritic cells

Immune effector and regulatory cells in the tumor microenvironment are key factors in tumor development and progression as the pathogenesis of cancer vitally depends on the multifaceted interactions between various microenvironmental stimuli provided by tumor-associated immune cells. Immune regulatory cells participate in all stages of cancer development from the induction of genomic instability to the maintenance of intratumoral angiogenesis, proliferation and spreading of malignant cells, and formation of premetastatic niches in distal tissues. Dendritic cells in the tumor microenvironment serve as a double-edged sword and, in addition to initiating potent anti-tumor immune responses, may mediate genomic damage, support neovascularization, block anti-tumor immunity and stimulate cancerous cell growth and spreading. Regulatory dendritic cells in cancer may directly and indirectly maintain antigen-specific and non-specific T cell unresponsiveness by controlling T cell polarization, MDSC and Treg differentiation and activity, and affecting specific microenvironmental conditions in premalignant niches. Understanding the mechanisms involved in regulatory dendritic cell polarization and operation and revealing pharmacological means for harnessing these pathways will provide additional opportunities for modifying the tumor microenvironment and improving the efficacy of different therapeutic approaches to cancer.

Paclitaxel promotes differentiation of myeloid-derived suppressor cells into dendritic cells in vitro in a TLR4-independent manner

Myeloid cells play a key role in the outcome of anti-tumor immunity and response to anti-cancer therapy, since in the tumor microenvironment they may exert both stimulatory and inhibitory pressures on the proliferative, angiogenic, metastatic, and immunomodulating potential of tumor cells. Therefore, understanding the mechanisms of myeloid regulatory cell differentiation is critical for developing strategies for the therapeutic reversal of myeloid derived suppressor cell (MDSC) accumulation in the tumor-bearing hosts. Here, using an in vitro model system, several potential mechanisms of the direct effect of paclitaxel on MDSC were tested, which might be responsible for the anti-tumor potential of low-dose paclitaxel therapy in mice. It was hypothesized that a decreased level of MDSC in vivo after paclitaxel administration might be due to (i) the blockage of MDSC generation, (ii) an induction of MDSC apoptosis, or (iii) the stimulation of MDSC differentiation. The results revealed that paclitaxel in ultra-low concentrations neither increased MDSC apoptosis nor blocked MDSC generation, but stimulated MDSC differentiation towards dendritic cells. This effect of paclitaxel was TLR4-independent since it was not diminished in cell cultures originated from TLR4-/- mice. These results support a new concept that certain chemotherapeutic agents in ultra-low non-cytotoxic doses may suppress tumor progression by targeting several cell populations in the tumor microenvironment, including MDSC.