The heightened importance of the microbiome in cancer immunotherapy.

The human microbiome is recognized as a key factor in health and disease. This has been further corroborated by identifying changes in microbiome composition and function as a novel hallmark in cancer. These effects are exerted through microbiome interactions with host cells, impacting a wide variety of developmental and physiological processes. In this review, we discuss some of the latest findings on how the bacterial component of the microbiome can influence outcomes for different cancer immunotherapy modalities, highlighting identified mechanisms of action. We also address the clinical efforts to utilize this knowledge to achieve better responses to immunotherapy. A refined understanding of microbiome variations in patients and microbiome-host interactions with cancer therapies is essential to realize optimal clinical responses.

Naturally produced type I IFNs enhance human myeloid dendritic cell maturation and IL-12p70 production and mediate elevated effector functions in innate and adaptive immune cells.

There has recently been a paradigm shift in the field of dendritic cell (DC)-based immunotherapy, where several clinical studies have confirmed the feasibility and advantageousness of using directly isolated human blood-derived DCs over in vitro differentiated subsets. There are two major DC subsets found in blood; plasmacytoid DCs (pDCs) and myeloid DCs (mDCs), and both have been tested clinically. CD1c+ mDCs are highly efficient antigen-presenting cells that have the ability to secrete IL-12p70, while pDCs are professional IFN-α-secreting cells that are shown to induce innate immune responses in melanoma patients. Hence, combining mDCs and pDCs poses as an attractive, multi-functional vaccine approach. However, type I IFNs have been reported to inhibit IL-12p70 production and mDC-induced T-cell activation. In this study, we investigate the effect of IFN-α on mDC maturation and function. We demonstrate that both recombinant IFN-α and activated pDCs strongly enhance mDC maturation and increase IL-12p70 production. Co-cultured mDCs and pDCs additionally have beneficial effect on NK and NKT-cell activation and also enhances IFN-γ production by allogeneic T cells. In contrast, the presence of type I IFNs reduces the proliferative T-cell response. The mere presence of a small fraction of activated pDCs is sufficient for these effects and the required ratio between the subsets is non-stringent. Taken together, these results support the usage of mDCs and pDCs combined into one immunotherapeutic vaccine with broad immunostimulatory features.

Type I IFN-mediated synergistic activation of mouse and human DC subsets by TLR agonists.

Novel approaches of dendritic cell (DC) based cancer immunotherapy aim at harnessing the unique attributes of different DC subsets. Classical monocyte-derived DC vaccines are currently being replaced by either applying primary DCs or specifically targeting antigens and adjuvants to these subsets in vivo. Appropriate DC activation in both strategies is essential for optimal effect. For this purpose TLR agonists are favorable adjuvant choices, with TLR7 triggering being essential for inducing strong Th1 responses. However, mouse CD8α(+) DCs, considered to be the major cross-presenting subset, lack TLR7 expression. Interestingly, this DC subset can respond to TLR7 ligand upon concurrent TLR3 triggering. Nevertheless, the mechanism underlying this synergy remains obscure. We now show that TLR3 ligation results in the production of IFN-α, which rapidly induces the expression of TLR7, resulting in synergistic activation. Moreover, we demonstrate that this mechanism conversely holds for plasmacytoid DCs that respond to TLR3 ligation when TLR7 pathway is mobilized. We further demonstrate that this mechanism of sharpening DC senses is also conserved in human BDCA1(+) DCs and plasmacytoid DCs. These findings have important implications for future clinical trials as it suggests that combinations of TLR ligands should be applied irrespective of initial TLR expression profiles on natural DC subsets for optimal stimulation.

A Comparative Study of the T Cell Stimulatory and Polarizing Capacity of Human Primary Blood Dendritic Cell Subsets.

Dendritic cells (DCs) are central players of immune responses; they become activated upon infection or inflammation and migrate to lymph nodes, where they can initiate an antigen-specific immune response by activating naive T cells. Two major types of naturally occurring DCs circulate in peripheral blood, namely, myeloid and plasmacytoid DCs (pDCs). Myeloid DCs (mDCs) can be subdivided based on the expression of either CD1c or CD141. These human DC subsets differ in surface marker expression, Toll-like receptor (TLR) repertoire, and transcriptional profile, suggesting functional differences between them. Here, we directly compared the capacity of human blood mDCs and pDCs to activate and polarize CD4(+) T cells. CD141(+) mDCs show an overall more mature phenotype over CD1c(+) mDC and pDCs; they produce less IL-10 and more IL-12 than CD1c(+) mDCs. Despite these differences, all subsets can induce the production of IFN-γ in naive CD4(+) T cells. CD1c(+) and CD141(+) mDCs especially induce a strong T helper 1 profile. Importantly, naive CD4(+) T cells are not polarized towards regulatory T cells by any subset. These findings further establish all three human blood DCs-despite their differences-as promising candidates for immunostimulatory effectors in cancer immunotherapy.

Protamine-stabilized RNA as an ex vivo stimulant of primary human dendritic cell subsets.

Dendritic cells (DCs) are key in connecting innate and adaptive immunity. Their potential in inducing specific immune responses has made them interesting targets for immunotherapeutic approaches. Our research group was the first to exploit the naturally occurring myeloid DCs (mDCs) and plasmacytoid DCs (pDCs) in therapeutic vaccination trials against melanoma. To develop primary DC subsets as an optimal vaccine, the identification of a clinically applicable adjuvant activating both subsets is required. Although the expression of pathogen recognition receptors differs distinctly between the DC subsets, both pDCs and mDCs can respond to single-stranded RNA (ssRNA) via Toll-like receptors 7 and 8, respectively. Since ssRNA is easily degraded by RNases, we stabilized anionic RNA by complexing it with the positively charged protein protamine. This leads to the formation of protamine-RNA complexes with varying features depending on ionic content. We subsequently investigated the immunostimulatory effect of complexes that formed various salt concentrations on purified DC subsets. Both mDCs and pDCs upregulated maturation markers and produced pro-inflammatory cytokines in a dose-dependent way to the protamine-RNA complexes. This was dependent on endosomal acidification and correlated partly with the uptake of protamine-RNA complexes. Furthermore, both DC subsets induced T cell proliferation and IFN gamma secretion in a beneficial ratio to IL-10. These results indicate that protamine-RNA complexes can be used to stimulate human mDC and pDC ex vivo for use in immunotherapeutic settings.

Border Control: The Role of the Microbiome in Regulating Epithelial Barrier Function.

The gut mucosal epithelium is one of the largest organs in the body and plays a critical role in regulating the crosstalk between the resident microbiome and the host. To this effect, the tight control of what is permitted through this barrier is of high importance. There should be restricted passage of harmful microorganisms and antigens while at the same time allowing the absorption of nutrients and water. An increased gut permeability, or "leaky gut", has been associated with a variety of diseases ranging from infections, metabolic diseases, and inflammatory and autoimmune diseases to neurological conditions. Several factors can affect gut permeability, including cytokines, dietary components, and the gut microbiome. Here, we discuss how the gut microbiome impacts the permeability of the gut epithelial barrier and how this can be harnessed for therapeutic purposes.

Inhibition of CSF-1R and IL-6R prevents conversion of cDC2s into immune incompetent tumor-induced DC3s boosting DC-driven therapy potential.

The human dendritic cell (DC) family has recently been expanded by CD1c+CD14+CD163+ DCs, introduced as DC3s. DC3s are found in tumors and peripheral blood of cancer patients. Here, we report elevated frequencies of CD14+ cDC2s, which restore to normal frequencies after tumor resection, in non-small cell lung cancer patients. These CD14+ cDC2s phenotypically resemble DC3s and exhibit increased PD-L1, MERTK, IL-10, and IDO expression, consistent with inferior T cell activation ability compared with CD14- cDC2s. In melanoma patients undergoing CD1c+ DC vaccinations, increased CD1c+CD14+ DC frequencies correlate with reduced survival. We demonstrate conversion of CD5+/-CD1c+CD14- cDC2s to CD14+ cDC2s by tumor-associated factors, whereas monocytes failed to express CD1c under similar conditions. Targeted proteomics identified IL-6 and M-CSF as dominant drivers, and we show that IL-6R and CSF1R inhibition prevents tumor-induced CD14+ cDC2s. Together, this indicates cDC2s as direct pre-cursors of DC3-like CD1c+CD14+ DCs and provides insights into the importance and modulation of CD14+ DC3s in anti-tumor immune responses.

Vitamin D3 targets epidermal and dermal dendritic cells for induction of distinct regulatory T cells.

BACKGROUND: The vitamin D metabolite 1,25(OH)2D3 (VitD3) is a potent immunosuppressive drug and, among others, is used for topical treatment of psoriasis. A proposed mechanism of VitD3-mediated suppression is priming of dendritic cells (DCs) to induce regulatory T (Treg) cells. OBJECTIVE: Currently, there is confusion about the phenotype of VitD3-induced Treg cells and the DC-derived molecules driving their development. We investigated Treg cell induction after VitD3 priming of 2 distinct skin DC subsets: Langerhans cells (LCs) and dermal dendritic cells (DDCs). METHODS: LCs and DDCs primed with VitD3 were cocultured with allogeneic naive T cells. The phenotype and function of the DCs and induced T cells were analyzed. RESULTS: Both VitD3-primed DC subtypes induced T cells with regulatory activity. Unexpectedly, whereas the Treg cell populations generated by VitD3-primed LCs were CD25(hi)CD127(lo) forkhead box protein 3 (Foxp3)-positive cells, which meet the criteria of classical inducible Treg cells, the T cells developing in response to VitD3-primed DDCs were Foxp3(-) T(R)1 cells expressing IL-10. Inhibition experiments revealed that LC-derived TGF-ß is a key factor in the induction of Foxp3(+) Treg cells, whereas DDC-derived IL-10 is important for the induction of IL-10(+) T(R)1 cells. CONCLUSION: Thus we report the novel finding that distinct but closely related DC subsets are differentially programmed by VitD3 to support development of either TGF-ß-dependent Foxp3(+) Treg cells or IL-10-dependent IL-10(+) Treg cells.

BDCA1+CD14+ Immunosuppressive Cells in Cancer, a Potential Target?

Dendritic cell (DC) vaccines show promising effects in cancer immunotherapy. However, their efficacy is affected by a number of factors, including (1) the quality of the DC vaccine and (2) tumor immune evasion. The recently characterized BDCA1+CD14+ immunosuppressive cells combine both aspects; their presence in DC vaccines may directly hamper vaccine efficacy, whereas, in patients, BDCA1+CD14+ cells may suppress the induced immune response in an antigen-specific manner systemically and at the tumor site. We hypothesize that BDCA1+CD14+ cells are present in a broad spectrum of cancers and demand further investigation to reveal treatment opportunities and/or improvement for DC vaccines. In this review, we summarize the findings on BDCA1+CD14+ cells in solid cancers. In addition, we evaluate the presence of BDCA1+CD14+ cells in leukemic cancers. Preliminary results suggest that the presence of BDCA1+CD14+ cells correlates with clinical features of acute and chronic myeloid leukemia. Future research focusing on the differentiation from monocytes towards BDCA1+CD14+ cells could reveal more about their cell biology and clinical significance. Targeting these cells in cancer patients may improve the outcome of cancer immunotherapy.

Isolation of Mononuclear Cell Populations from Ovarian Carcinoma Ascites.

Ovarian cancer is one of the most fatal tumors in women. Due to a lack of symptoms and adequate screening methods, patients are diagnosed at advanced stages with extensive tumor burden (Jelovac and Armstrong, 2011). Interestingly, ovarian cancer metastasis is generally found within the peritoneal cavity rather than other tissues (Lengyel, 2010; Tan et al., 2006 ). The reason behind this tissue tropism of the peritoneal cavity remains elusive. A prominent feature of this selectivity is ascites, the accumulation of fluid within the peritoneal cavity, containing, amongst others, immune cells, tumor cells and various soluble factors that can be involved in the progression of ovarian cancer ( Kipps et al., 2013 ). The protocol described here is used to isolate mononuclear cells from ascites to study the functionality of the immune system within the peritoneal cavity.

Harnessing RNA sequencing for global, unbiased evaluation of two new adjuvants for dendritic-cell immunotherapy.

Effective stimulation of immune cells is crucial for the success of cancer immunotherapies. Current approaches to evaluate the efficiency of stimuli are mainly defined by known flow cytometry-based cell activation or cell maturation markers. This method however does not give a complete overview of the achieved activation state and may leave important side effects unnoticed. Here, we used an unbiased RNA sequencing (RNA-seq)-based approach to compare the capacity of four clinical-grade dendritic cell (DC) activation stimuli used to prepare DC-vaccines composed of various types of DC subsets; the already clinically applied GM-CSF and Frühsommer meningoencephalitis (FSME) prophylactic vaccine and the novel clinical grade adjuvants protamine-RNA complexes (pRNA) and CpG-P. We found that GM-CSF and pRNA had similar effects on their target cells, whereas pRNA and CpG-P induced stronger type I interferon (IFN) expression than FSME. In general, the pathways most affected by all stimuli were related to immune activity and cell migration. GM-CSF stimulation, however, also induced a significant increase of genes related to nonsense-mediated decay, indicating a possible deleterious effect of this stimulus. Taken together, the two novel stimuli appear to be promising alternatives. Our study demonstrates how RNA-seq based investigation of changes in a large number of genes and gene groups can be exploited for fast and unbiased, global evaluation of clinical-grade stimuli, as opposed to the general limited evaluation of a pre-specified set of genes, by which one might miss important biological effects that are detrimental for vaccine efficacy.

Innate Lymphoid Cells in Tumor Immunity.

Innate lymphoid cells (ILCs) are a group of immune cells of the lymphoid lineage that do not possess antigen specificity. The group includes natural killer (NK) cells, lymphoid tissue inducer (LTi) cells and the recently identified ILC1s, ILC2s and ILC3s. Although the role of NK cells in the context of cancer has been well established, the involvement of other ILC subsets in cancer progression and resistance is just emerging. Here, we review the literature on the role of the different ILC subsets in tumor immunity and discuss its implications for cancer treatment and monitoring.

Human blood myeloid and plasmacytoid dendritic cells cross activate each other and synergize in inducing NK cell cytotoxicity.

Human blood dendritic cells (DCs) hold great potential for use in anticancer immunotherapies. CD1c+ myeloid DCs and plasmacytoid DCs (pDCs) have been successfully utilized in clinical vaccination trials against melanoma. We hypothesize that combining both DC subsets in a single vaccine can further improve vaccine efficacy. Here, we have determined the potential synergy between the two subsets in vitro on the level of maturation, cytokine expression, and effector cell induction. Toll-like receptor (TLR) stimulation of CD1c+ DCs induced cross-activation of immature pDCs and vice versa. When both subsets were stimulated together using TLR agonists, CD86 expression on pDCs was increased and higher levels of interferon (IFN)-α were produced by DC co-cultures. Although the two subsets did not display any synergistic effect on naive CD4+ and CD8+ T cell polarization, CD1c+ DCs and pDCs were able to complement each other's induction of other immune effector cells. The mere presence of pDCs in DC co-cultures promoted plasma cell differentiation from activated autologous B cells. Similarly, CD1c+ DCs, alone or in co-cultures, induced high levels of IFN-γ from allogeneic peripheral blood lymphocytes or activated autologous natural killer (NK) cells. Both CD1c+ DCs and pDCs could enhance NK cell cytotoxicity, and interestingly DC co-cultures further enhanced NK cell-mediated killing of an NK-resistant tumor cell line. These results indicate that co-application of human blood DC subsets could render DC-based anticancer vaccines more efficacious.

Expansion of a BDCA1+CD14+ Myeloid Cell Population in Melanoma Patients May Attenuate the Efficacy of Dendritic Cell Vaccines.

The tumor microenvironment is characterized by regulatory T cells, type II macrophages, myeloid-derived suppressor cells, and other immunosuppressive cells that promote malignant progression. Here we report the identification of a novel BDCA1(+)CD14(+) population of immunosuppressive myeloid cells that are expanded in melanoma patients and are present in dendritic cell-based vaccines, where they suppress CD4(+) T cells in an antigen-specific manner. Mechanistic investigations showed that BDCA1(+)CD14(+) cells expressed high levels of the immune checkpoint molecule PD-L1 to hinder T-cell proliferation. While this BDCA1(+)CD14(+) cell population expressed markers of both BDCA1(+) dendritic cells and monocytes, analyses of function, transcriptome, and proteome established their unique nature as exploited by tumors for immune escape. We propose that targeting these cells may improve the efficacy of cancer immunotherapy. Cancer Res; 76(15); 4332-46. ©2016 AACR.

Crosstalk between dendritic cell subsets and implications for dendritic cell-based anticancer immunotherapy.

Dendritic cells (DCs) are a family of professional antigen-presenting cells that have an indispensable role in the initiation of innate and adaptive immune responses against pathogens and tumor cells. The DC family is very heterogeneous. Two main types of naturally occurring DCs circulate in peripheral blood, each with its unique phenotypic and functional characteristics: myeloid DCs and plasmacytoid. There is an ample number of studies that have focused on the bi-directional crosstalk between DCs and natural killer cells or T cells. However, the crosstalk among the different DC subsets, in the context of infectious diseases and cancer, has until now not received much attention. Here, we review all available literature that has dealt with the crosstalk between plasmacytoid and myeloid DCs and the potential mode of action. Emphasis will be given to the therapeutic potential of the combination of DC subsets for DC-based immunotherapy.

Vitamin D3 metabolite calcidiol primes human dendritic cells to promote the development of immunomodulatory IL-10-producing T cells.

Vitamin D is recognized as a potent immunosuppressive drug. The suppressive effects of vitamin D are attributed to its physiologically active metabolite 1,25 dihydroxy vitamin D3 (calcitriol), which was shown, to prime dendritic cells (DCs) to promote the development of regulatory T (Treg) cells. Despite the potential benefit in treating autoimmune diseases, clinical application of calcitriol is hindered by deleterious side effects manifested by hypercalcemia and hypercalciuria. Conversely, the physiological precursors of calcitriol, vitamin D3 (cholecalciferol) and its first metabolite 25-hydroxy vitamin D3 (calcidiol) are widely applied in the clinic due to their low calcimic burden. However, the mechanisms by which cholecalciferol and calcidiol may modulate adaptive immunity remain elusive. This prompted us to unravel the immunosuppressive capacity of these precursors by assessing their influence on DC functions and the subsequent polarization of naïve CD4(+) T cells. In this study we show that, whereas cholecalciferol has insignificant effects on DC maturation and cytokine production, it only weakly primed DCs to induce suppressive T cells. However, like calcitriol, calcidiol not only exerted an inhibitory effect on DC maturation and cytokine production, and primed DCs to promote the development of suppressive IL-10-producing Treg cells. Strikingly, in contrast to the population of IL-10-producing Treg cells induced by calcitriol-primed DCs, the IL-10-producing Treg cells induced by calcidiol-primed DCs exhibited sustained IFN-γ production in face of their suppressive capacity. Experiments with the steroid synthesis inhibitor ketoconazole indicated that the immunomodulatory features of the precursors are dependent on their conversion into calcitriol. Collectively, calcidiol is a potent immune modulator, which may be more adequate than calcitriol for the treatment of chronic inflammatory diseases, since it is less hypercalcimic. This may be of particular interest for the treatment of allergic disease, where concurrent suppression and sustained IFN-γ production by Treg cells effectively counterbalance the Th2-dominated immune responses.

The nature of activatory and tolerogenic dendritic cell-derived signal II.

Dendritic cells (DCs) are central in maintaining the intricate balance between immunity and tolerance by orchestrating adaptive immune responses. Being the most potent antigen presenting cells, DCs are capable of educating naïve T cells into a wide variety of effector cells ranging from immunogenic CD4(+) T helper cells and cytotoxic CD8(+) T cells to tolerogenic regulatory T cells. This education is based on three fundamental signals. Signal I, which is mediated by antigen/major histocompatibility complexes binding to antigen-specific T cell receptors, guarantees antigen specificity. The co-stimulatory signal II, mediated by B7 family molecules, is crucial for the expansion of the antigen-specific T cells. The final step is T cell polarization by signal III, which is conveyed by DC-derived cytokines and determines the effector functions of the emerging T cell. Although co-stimulation is widely recognized to result from the engagement of T cell-derived CD28 with DC-expressed B7 molecules (CD80/CD86), other co-stimulatory pathways have been identified. These pathways can be divided into two groups based on their impact on primed T cells. Whereas pathways delivering activatory signals to T cells are termed co-stimulatory pathways, pathways delivering tolerogenic signals to T cells are termed co-inhibitory pathways. In this review, we discuss how the nature of DC-derived signal II determines the quality of ensuing T cell responses and eventually promoting either immunity or tolerance. A thorough understanding of this process is instrumental in determining the underlying mechanism of disorders demonstrating distorted immunity/tolerance balance, and would help innovating new therapeutic approaches for such disorders.

Intradermal application of vitamin D3 increases migration of CD14+ dermal dendritic cells and promotes the development of Foxp3+ regulatory T cells.

The active form of vitamin D3 (VitD) is a potent immunosuppressive drug. Its effects are mediated in part through dendritic cells (DCs) that promote the development of regulatory T cells (Tregs). However, it remains elusive how VitD would influence the different human skin DC subsets, e.g., CD1a(+)/langerin(+) Langerhans cells, CD14(+) DDCs and CD1a(+) DDCs upon administration through the skin route in their natural environment. We addressed this issue by intradermal (ID) administration of VitD in a human skin explant system that closely resembles physiological conditions. ID injection of VitD selectively enhanced the migration of CD14(+) DDCs, a subset known for the induction of tolerance. Moreover, ID injection of VitD repressed the LPS-induced T cell stimulatory capacity of migrating DCs. These migrating DCs collectively induced T cells with suppressive activity and abolished IFN-γ productivity. Those induced T cells were characterized by the expression of Foxp3. Thus, we report the novel finding that ID injection of VitD not only modifies skin DC migration, but also programs these DCs in their natural milieu to promote the development of Foxp3(+) Tregs.

Viral dsRNA-activated human dendritic cells produce IL-27, which selectively promotes cytotoxicity in naive CD8+ T cells.

Viral recognition programs DCs to express Signal 3 molecules that promote the differentiation of effector CD8(+) T cells. Besides IL-12, another DC-derived IL-12 family member, IL-27, has been reported to contribute herein, but its specific role is not well understood. Here, we show that whereas IL-12 potently induces inflammatory cytokines (i.e., IFN-γ and TNF-α, but not IL-2), IL-27 excels in inducing proliferation and a cytotoxic profile (GrB, cytotoxicity of target cells) in human naïve CD8(+) T cells. Compared with bacterial cell-wall peptidoglycan, viral dsRNA-mimic poly (I:C) is superior in priming human BDCA1(+) peripheral blood DCs to produce IL-12 and IL-27, which promote inflammatory cytokines and a cytotoxic profile in differentiating CD8(+) T cells, respectively. These data support the concept that viral dsRNA-activated human DCs produce IL-27 to act as a specialized procytotoxic, antiviral cytokine in the development of effector CD8(+) T cells.