Jurjen Tel

Toll-like receptor expression and function in human dendritic cell subsets: implications for dendritic cell-based anti-cancer immunotherapy

Dendritic cells (DCs) are central players of the immune response. To date, DC-based immunotherapy is explored worldwide in clinical vaccination trials with cancer patients, predominantly with ex vivo-cultured monocyte-derived DCs (moDCs). However, the extensive culture period and compounds required to differentiate them into DCs may negatively affect their immunological potential. Therefore, it is attractive to consider alternative DC sources, such as blood DCs. Two major types of naturally occurring DCs circulate in peripheral blood, myeloid DCs (mDCs) and plasmacytoid (pDCs). These DC subsets express different surface molecules and are suggested to have distinct functions. Besides scavenging pathogens and presenting antigens, DCs secrete cytokines, all of which is vital for both the acquired and the innate immune system. These immunological functions relate to Toll-like receptors (TLRs) expressed by DCs. TLRs recognize pathogen-derived products and subsequently provoke DC maturation, antigen presentation and cytokine secretion. However, not every TLR is expressed on each DC subset nor causes the same effects when activated. Considering the large amount of clinical trials using DC-based immunotherapy for cancer patients and the decisive role of TLRs in DC maturation, this review summarizes TLR expression in different DC subsets in relation to their function. Emphasis will be given to the therapeutic potential of TLR-matured DC subsets for DC-based immunotherapy.

Natural Human Plasmacytoid Dendritic Cells Induce Antigen-Specific T-Cell Responses in Melanoma Patients

Vaccination against cancer by using dendritic cells has for more than a decade been based on dendritic cells generated ex vivo from monocytes or CD34(+) progenitors. Here, we report on the first clinical study of therapeutic vaccination against cancer using naturally occurring plasmacytoid dendritic cells (pDC). Fifteen patients with metastatic melanoma received intranodal injections of pDCs activated and loaded with tumor antigen-associated peptides ex vivo. In vivo imaging showed that administered pDCs migrated and distributed over multiple lymph nodes. Several patients mounted antivaccine CD4(+) and CD8(+) T-cell responses. Despite the limited number of administered pDCs, an IFN signature was observed after each vaccination. These results indicate that vaccination with naturally occurring pDC is feasible with minimal toxicity and that in patients with metastatic melanoma, it induces favorable immune responses.

The C type lectin receptor CLEC9A mediates antigen uptake and (cross-)presentation by human blood BDCA3+ myeloid dendritic cells

CLEC9A is a recently discovered C-type lectin receptor involved in sensing necrotic cells. In humans, this receptor is selectively expressed by BDCA3(+) myeloid dendritic cells (mDCs), which have been proposed to be the main human cross-presenting mDCs and may represent the human homologue of murine CD8(+) DCs. In mice, it was demonstrated that antigens delivered with antibodies to CLEC9A are presented by CD8(+) DCs to both CD4(+) and CD8(+) T cells and induce antitumor immunity in a melanoma model. Here we assessed the ability of CLEC9A to mediate antigen presentation by human BDCA3(+) mDCs, which represent < 0.05% of peripheral blood leukocytes. We demonstrate that CLEC9A is only expressed on immature BDCA3(+) mDCs and that cell surface expression is lost after TLR-mediated maturation. CLEC9A triggering via antibody binding rapidly induces receptor internalization but does not affect TLR-induced cytokine production or expression of costimulatory molecules. More importantly, antigens delivered via CLEC9A antibodies to BDCA3(+) mDCs are presented by both MHC class I (cross-presentation) and MHC class II to antigen-specific T cells. We conclude that CLEC9A is a promising target for in vivo antigen delivery in humans to increase the efficiency of vaccines against infectious or malignant diseases.

Human plasmacytoid dendritic cells efficiently cross-present exogenous Ags to CD8+ T cells despite lower Ag uptake than myeloid dendritic cell subsets

In human peripheral blood, 4 populations of dendritic cells (DCs) can be distinguished, plasmacytoid dendritic cells (pDCs) and CD16(+), CD1c(+), and BDCA-3(+) myeloid DCs (mDCs), each with distinct functional characteristics. DCs have the unique capacity to cross-present exogenously encountered antigens (Ags) to CD8(+) T cells. Here we studied the ability of all 4 blood DC subsets to take up, process, and present tumor Ags to T cells. Although pDCs take up less Ags than CD1c(+) and BDCA3(+) mDCs, pDCs induce potent Ag-specific CD4(+) and CD8(+) T-cell responses. We show that pDCs can preserve Ags for prolonged periods of time and on stimulation show strong induction of both MHC class I and II, which explains their efficient activation of both CD4(+) and CD8(+) T cells. Furthermore, pDCs cross-present soluble and cell-associated tumor Ags to cytotoxic T lymphocytes equally well as BDCA3(+) mDCs. These findings, and the fact that pDCs outnumber BDCA3(+) mDCs, both in peripheral blood and lymph nodes, together with their potent IFN-I production, known to activate both components of the innate and adaptive immune system, put human pDCs forward as potent activators of CD8(+) T cells in antitumor responses. Our findings may therefore have important consequences for the development of antitumor immunotherapy.

Targeting Uptake Receptors on Human Plasmacytoid Dendritic Cells Triggers Antigen Cross-Presentation and Robust Type I IFN Secretion

Plasmacytoid dendritic cells (pDCs) play a crucial role in initiating immune responses by secreting large amounts of type I IFNs. Currently, the role for human pDCs as professional APCs in the cross-presentation of exogenous Ags is being re-evaluated. Human pDCs are equipped with a broad repertoire of Ag uptake receptors and an efficient Ag-processing machinery. In this study, we set out to investigate which receptor can best be deployed to deliver Ag to pDCs for Ag (cross-)presentation. We show that targeting nanoparticles to pDCs via the C-type lectins DEC-205, DC immunoreceptor, blood DC Ag-2, or the FcR CD32 led to uptake, processing, and (cross-) presentation of encapsulated Ag to both CD4+ and CD8+ T cells. This makes these receptors good candidates for potential in vivo targeting of pDCs by nanocarriers. Notably, the coencapsulated TLR7 agonist R848 efficiently activated pDCs, resulting in phenotypical maturation as well as robust IFN-α and TNF-α production. Taken together, their cross-presentation capacity and type I IFN production to further activate components of both the innate and adaptive immune system mark pDCs as inducers of potent antitumor responses. These findings pave the way to actively recruit human pDCs for cellular cancer immunotherapy.

Antigen cross-presentation by dendritic cell subsets: one general or all sergeants?

Antigen cross-presentation describes the process through which dendritic cells (DCs) acquire exogenous antigens for presentation on MHC class I molecules. The ability to cross-present has been thought of as a feature of specialized DC subsets. Emerging data, however, suggest that the cross-presenting ability of each DC subset is tuned by and dependent on several factors, such as DC location and activation status, and the type of antigen and inflammatory signals. Thus, we argue that capacity of cross-presentation is not an exclusive trait of one or several distinct DC subtypes, but rather a common feature of the DC family in both mice and humans. Understanding DC subset activation and antigen-presentation pathways might yield improved tools and targets to exploit the unique cross-presenting capacity of DCs in immunotherapy.

Human Plasmacytoid Dendritic Cells Phagocytose, Process, and Present Exogenous Particulate Antigen

Plasmacytoid dendritic cells (pDCs) play a major role in shaping both innate and adaptive immune responses, mainly via their production of large amounts of type I IFNs. pDCs are considered to primarily present endogenous Ags and are thought not to participate in the uptake and presentation of Ags from the extracellular environment, in contrast to their myeloid counterparts, which efficiently endocytose extracellular particulates. In this study, we show that human pDCs are able to phagocytose and process particulate forms of Ag entrapped in poly(lactic-coglycolic acid) microparticles. Furthermore, pDCs were also able to sense TLR ligands (TLR-Ls) incorporated in these particles, resulting in rapid pDC activation and high IFN-α secretion. Combining a tetanus toxoid peptide and TLR-Ls (CpG C and R848) in these microparticles resulted in efficient pDC activation and concomitant Ag-specific T cell stimulation. Moreover, particulate Ag was phagocytosed and presented more efficiently than soluble Ag, indicating that microparticles can be exploited to facilitate efficient delivery of antigenic cargo and immunostimulatory molecules to pDCs. Together, our results show that in addition to their potency to stimulate innate immunity, pDCs can polarize adaptive immune responses against exogenous particulate Ag. These results may have important consequences for the development of new immunotherapeutic strategies exploiting Ag and TLR-Ls encapsulated in microparticles to target APC subsets.

Human plasmacytoid dendritic cells are equipped with antigen-presenting and tumoricidal capacities

Human plasmacytoid dendritic cells (pDCs) represent a highly specialized naturally occurring dendritic-cell subset and are the main producers of type I interferons (IFNs) in response to viral infections. We show that human pDCs activated by the preventive vaccine FSME specifically up-regulate CD56 on their surface, a marker that was thought to be specific for NK cells and associated with cytolytic effector functions. We observed that FSME-activated pDCs specifically lysed NK target cells and expressed cytotoxic molecules, such as tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and granzyme B. Elevated levels of these molecules coincided with the expression of CD56, indicative for skewing human pDCs toward an interferon-producing killer DC subset. Detailed phenotypical and functional analysis revealed that pDCs attained a mature phenotype, secreted proinflammatory cytokines, and had the capacity to present antigens and stimulate T cells. Here, we report on the generation of CD56(+) human interferon producing killer pDCs with the capacity to present antigens. These findings aid in deciphering the role for pDCs in antitumor immunity and present a promising prospect of developing antitumor therapy using pDCs.

Towards efficient cancer immunotherapy: advances in developing artificial antigen-presenting cells

Highlights • Active immunotherapy is promising for the development of potent cancer therapeutics.• Various types of artificial antigen-presenting cells (aAPCs) may be used as ‘off-the-shelf’ products to induce antigen-specific T cell activation both ex vivo and in vivo.• Size, shape, cytokine delivery mechanism, ligand composition, ligand mobility, and ligand positioning on aAPCs all have significant effects on T cell activation, and therefore should be taken into account when designing novel constructs.

Effective Clinical Responses in Metastatic Melanoma Patients after Vaccination with Primary Myeloid Dendritic Cells

Purpose: Thus far, dendritic cell (DC)-based immunotherapy of cancer was primarily based on in vitro–generated monocyte-derived DCs, which require extensive in vitro manipulation. Here, we report on a clinical study exploiting primary CD1c+ myeloid DCs, naturally circulating in the blood. Experimental Design: Fourteen stage IV melanoma patients, without previous systemic treatment for metastatic disease, received autologous CD1c+ myeloid DCs, activated by only brief (16 hours) ex vivo culture and loaded with tumor-associated antigens of tyrosinase and gp100. Results: Our results show that therapeutic vaccination against melanoma with small amounts (3–10 × 106) of myeloid DCs is feasible and without substantial toxicity. Four of 14 patients showed long-term progression-free survival (12–35 months), which directly correlated with the development of multifunctional CD8+ T-cell responses in three of these patients. In particular, high CD107a expression, indicative for cytolytic activity, and IFNγ as well as TNFα and CCL4 production was observed. Apparently, these T-cell responses are essential to induce tumor regression and promote long-term survival by stalling tumor growth. Conclusions: We show that vaccination of metastatic melanoma patients with primary myeloid DCs is feasible and safe and results in induction of effective antitumor immune responses that coincide with improved progression-free survival. Clin Cancer Res; 22(9); 2155–66. ©2015 AACR.