The clinical application of cancer immunotherapy based on naturally circulating dendritic cells

Abstract

Dendritic cells (DCs) can initiate and direct adaptive immune responses. This ability is exploitable in DC vaccination strategies, in which DCs are educated ex vivo to present tumor antigens and are administered into the patient with the aim to induce a tumor-specific immune response. DC vaccination remains a promising approach with the potential to further improve cancer immunotherapy with little or no evidence of treatment-limiting toxicity. However, evidence for objective clinical antitumor activity of DC vaccination is currently limited, hampering the clinical implementation. One possible explanation for this is that the most commonly used monocyte-derived DCs may not be the best source for DC-based immunotherapy. The novel approach to use naturally circulating DCs may be an attractive alternative. In contrast to monocyte-derived DCs, naturally circulating DCs are relatively scarce but do not require extensive culture periods. Thereby, their functional capabilities are preserved, the reproducibility of clinical applications is increased, and the cells are not dysfunctional before injection. In human blood, at least three DC subsets can be distinguished, plasmacytoid DCs, CD141⁺ and CD1c⁺ myeloid/conventional DCs, each with distinct functional characteristics. In completed clinical trials, either CD1c⁺ myeloid DCs or plasmacytoid DCs were administered and showed encouraging immunological and clinical outcomes. Currently, also the combination of CD1c⁺ myeloid and plasmacytoid DCs as well as the intratumoral use of CD1c⁺ myeloid DCs is under investigation in the clinic. Isolation and culture strategies for CD141⁺ myeloid DCs are being developed. Here, we summarize and discuss recent clinical developments and future prospects of natural DC-based immunotherapy.

Keywords: Cancer; Conventional dendritic cells; Cross-presenting dendritic cells; Dendritic cells; Immunotherapy; Myeloid dendritic cells; Natural dendritic cells; Plasmacytoid dendritic cells; Vaccination.

Fig. 1

Dendritic cell subsets. Dendritic cells can be differentiated from monocytes (moDC), which are often used in clinical trials because of their high yield. The naturally circulating dendritic cells can now also be enriched by immunomagnetic isolation. The naturally circulating dendritic cells can further be divided in myeloid (CD141⁺ and CD1c⁺ mDC) and plasmacytoid dendritic cells (pDC). The subsets differ in function, localization, phenotype and cytokine production

Fig. 2

Production protocols for naturally circulating dendritic cells. Schematic overview of the (a) CD1c⁺ myeloid dendritic cell (mDC) and (b) plasmacytoid dendritic cell (pDC) production protocols and vaccination strategy of the various clinical trials