Dendritic cells

Three subsets of DCs can be identified in peripheral blood: plasmacytoid DCs (pDCs) and myeloid DCs (mDCs) or conventional DC (cDCs), which are further subdivided into two subsets based on the expression of the surface markers CD1c (BDCA-1) and CD141 (BDCA-3).

pDCs produce large amounts of type I IFNs, IFN-α, and IFN-β in response to binding nucleic acids typical of viruses and bacteria on toll-like receptors (TLR). TLR7 recognizes single-stranded RNA, and TLR9 recognizes CpG DNA. IFNs have pleiotropic effects on several immune cells including T cells, NK cells, and mDCs, which makes pDCs critical responders to viral infections. However, the antigen uptake capacity of pDCs is inferior to mDCs and at steady-state, pDCs can induce tolerance rather than immune response (PMID: 28536413, 25852695).

Human pDCs are defined as CD303 (BDCA-2)⁺, CD304 (BDCA-4/Neuropilin-1)⁺, CD123⁺, CD4⁺, CD45RA⁺, CD141 (BDCA-3)^dim, CD1c (BDCA-1)^–, and CD2^–. They lack expression of lineage markers (CD3, CD14, CD16, CD19, CD20, CD56) and express neither myeloid markers, such as CD13 and CD33, nor Fc receptors, such as CD16, CD64, or FcεRI (PMID: 16920966, 11602645, 11913066).

CD1c⁺ mDCs, also called cDC2 or MDC2, account for 0.6% of PBMCs. They produce IL-12 upon TLR3 or TLR8 stimulation with poly (I:C) or R848, which leads to polarization of T_H1 CD4⁺ T cells and priming of naive CD8⁺ T cells. The CD1c (BDCA-1) antigen is a member of the CD1 protein family that are structurally related to MHC class I proteins and mediate the presentation of non-peptide antigens to T cells.

The CD1c antigen is specifically expressed on DCs that are CD11c^high CD123^low. CD1c⁺ mDCs have monocytic morphology and express myeloid markers such as CD13 and CD33, as well as Fc receptors, such as CD32, CD64, and FcεRI. Furthermore, they are Lin (CD3, CD16, CD19, CD20, CD56)^–, CD2⁺, CD45RO⁺, CD141 (BDCA-3)^low, CD303 (BDCA-2)^–, and CD304 (BDCA-4/Neuropilin-1)^–. A minor proportion of CD1c⁺ mDCs expresses CD14 and CD11b. CD1c is also expressed by circulating B cells and by CD1a⁺ DCs generated ex vivo from monocytes or hematopoietic precursor cells.

CD141⁺ mDCs, sometimes also called cDC1 or MDC1, are a very rare subset of blood DCs representing less than 0.05% of total PBMCs. They share several functional and phenotypical features with CD1c⁺ mDCs, such as IL-12 secretion and TLR8 expression, but are also characterized by IFN-λ secretion upon activation. CD141⁺ mDCs exhibit better cross-presentation of antigens derived from dead cells thanks to expression of the necrotic receptor Clec9a. This receptor and the chemokine (C motif) receptor 1 (XCR1) are exclusive markers of this subset. XCR1 is also expressed by the corresponding mouse subset CD8α⁺ DCs.

Miltenyi Biotec has created dedicated application protocols for working with human DCs.

• Generation of Mo-DCs

Although primary DCs open new exciting avenues to investigate DCs for basic and clinical applications, for decades human DCs have been generated in vitro from peripheral blood monocytes using GM-CSF and IL-4 to drive differentiation over 5–7 days. For products for the isolation of monocytes, see cell separation reagents.

These monocyte-derived DCs (Mo-DCs) have the characteristics of immature DCs and can be matured by culturing them in the presence of microbial, pro-inflammatory, or T cell–derived stimuli. Mo-DCs have been tested in a broad range of immunotherapy-based protocols, primarily for cancer research.