Monocyte-derived Dendritic cells (DCs) have successfully been employed to induce immune responses against tumor-associated antigens in patients with various cancer entities. However, objective clinical responses have only been achieved in a minority of patients. Additionally, generation of GMP-compliant DCs requires time- and labor-intensive cell differentiation. In contrast, Blood DCs (BDCs) require only minimal ex vivo handling, as differentiation occurs in vivo resulting in potentially better functional capacities and survival. We aimed to identify a protocol for optimal in vitro activation of BDCs including the three subsets pDCs, cDC1s, and cDC2s. We evaluated several TLR ligand combinations and demonstrated that polyinosinic:polycytidylic acid [poly(I:C)] and R848, ligands for TLR3 and TLR7/8, respectively, constituted the optimal combination for inducing a positive co-stimulatory profile in all BDC subsets. In addition, TLR3 and TLR7/8 activation led to high secretion of IFN-α and IL-12p70. Simultaneous as opposed to separate tailored activation of pDCs and cDCs increased immunostimulatory capacities, suggesting that BDC subsets engage in synergistic cross-talk during activation. Stimulation of BDCs with this protocol resulted in enhanced migration, high NK-cell activation, and potent antigen-specific T-cell induction.We conclude that simultaneous activation of all BDC subsets with a combination of R848 + poly(I:C) generates highly immunostimulatory DCs. These results support further investigation and clinical testing, as standalone or in conjunction with other immunotherapeutic strategies including adoptive T-cell transfer and checkpoint inhibition.

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Despite their role in the induction of adaptive immune responses, plasmacytoid dendritic cells (pDC) play also a crucial role in innate immune defense by recognizing pathogen derived molecular patterns. Unmethylated CpG motifs of bacterial DNA bind specifically endosomal and lysosomal TLR9 in pDCs, thereby inducing secretion of type I IFN or proinflammatory cytokines and classical DC maturation, respectively. However, TLR9 also recognizes CpG motifs in vertebrate DNA of damaged cells as danger-associated molecular patterns (DAMPs) that activate pDCs. For in-vitro research pDCs may be isolated by indirect magnetic cell separation using either Miltenyi Biotec’s column-based or a column-free system from other suppliers. Here we describe the influence of the isolation method on the functionality of the purified pDCs. In addition to the significantly better separation performance, our column-based method enriched 10- and 30-fold less DNA-containing dead cells and debris, respectively. Although both methods yielded pDCs with a comparable phenotype instantly after isolation, after 24h of culture without stimulation pDCs isolated with the column-free system appeared pre-activated as demonstrated by elevated expression of CD80, −83, −86 and HLA-DR and by significantly higher basal IFN-α secretion. Moreover, upon stimulation with CpG-B pDCs isolated with the column-free method induced extraordinary high amounts of IFN-α, whereas pDCs isolated with our system exhibited normal responses to CpG-A and CpG-B. These results clearly demonstrate that unspecific enrichment of dead cells and debris may significantly influence the activation status of the isolated pDCs and therefore affect the results of downstream applications.