**1. Introduction to dendritic cells**

Dendritic cells (DCs) are professional antigen presenting cells (APCs), the only cells capable of specifically activating naïve T cells and are key orchestrators of an immune response. They are a rare, heterogeneous population of haematopoietic cells that are equipped to capture, process and present antigen (Ag) to the adaptive immune system.

In a non-inflamed or steady state setting, DCs constantly sample the local environment for Ags and have the potential to induce peripheral tolerance via T cell anergy or deletion [1]. DCs recognise danger via pattern recognition receptors (PRR) on their cell surface, the cytoplasm and within cellular organelles [2]. Ligation of PRRs by pathogen associated molecular patterns (PAMPs) or damage associated molecular patterns (DAMPs), activates DC and licences DC to upregulate co-stimulatory marker expression such as CD86 and CD80 on their cell surface and initiate immunogenic T cell priming.

DCs situated in non-lymphoid tissues, also known as migratory DCs, constantly migrate to draining lymph nodes (LNs), maturing during this process, to present Ag to naïve T cells. Resident DCs in lymphoid organs are immature and maintain tolerance during steady state, but can stimulate naïve T cells when activated *in situ.* The DC maturation process not only involves morphological changes into their characteristic stellate shape with dendritic cytoplasmic processes and increased expression of MHC and co-stimulatory markers, but their Ag acquisition and sampling capabilities are initially upregulated and then rapidly shut down while MHCII expression on the cell's surface is increased due to the simultaneous up- and down-regulation of MHCII synthesis and turnover events respectively [3]. This allows mature DCs to present a snapshot of the Ag profile in its local environment prior to migration and/ or activation. Furthermore, activated DCs produce a combination of cytokines that modulate an immune response that is specific to the initial danger signals.

In humans, the majority of DC characterisation studies are of DCs isolated from the blood due to the rarity of the cell type and limited access to human tissue samples, although more investigations on non-lymphoid DCs in the skin, lung and liver have recently emerged [4–7]. DCs in the blood comprise ~1% of total peripheral blood mononuclear cells (PBMCs) and are traditionally identified by the high expression of MHCII (HLA-DR) and the lack of lineage markers CD3, CD14, CD15, CD19, CD20 and CD56, although the latter marker has recently been shown to be expressed on gut and other non-lymphoid DCs [6].

Human blood DCs can be divided into conventional DCs (cDCs) and plasmacytoid DCs (pDCs), which are HLA-DRhiCD11c+ 123<sup>−</sup> and HLA-DRhiCD11c−123+ respectively. Human blood cDCs are further categorised into cDC1 and cDC2 subsets. Additionally, there are monocyte-derived DCs that originate separately from cDCs and pDC precursors. The recent use of whole population and single cell sequencing techniques has been instrumental in elucidating transcription factors and surface markers that are unique to each DC subset, which has helped identify


*\* Previous Ag presentation abilities by pDCs are now suggested to be contributed by contaminating AXL+ Siglec6+ (AS) DCs.*

**103**

*Dendritic Cells and Their Roles in Anti-Tumour Immunity*

DC functional analyses [6–9], summarised in **Table 1**.

typic, transcriptional and functional assays, these CD141+

HLA-DR+

DCs [4, 21], firmly establishing CD141+

ability to cross-present Ags from dead or necrotic cells to CD8+

**3. Conventional dendritic cells 2 (cDC2)**

Human cDC2, traditionally known as CD1c<sup>+</sup>

**2. Conventional dendritic cells 1 (cDC1)**

characterised as CD11c<sup>+</sup>

The dependence of CD141<sup>+</sup>

tional profile clustering between CD141+

as well as between human blood CD141+

[12–16].

CD103+

CD8+

as IFN-lambda (λ).

relationships between DC subsets across species and tissues as well as corroborate

cDC1s constitute ~0.03% of PBMCs and are found in the blood, tonsil, spleen and non-lymphoid tissues such as the skin. They were classically defined by the high expression of CD141 (blood DC antigen 3 (BDCA3) or thrombomodulin) [10]. However, CD141 is not a completely specific marker for cDC1 as it is also expressed on endothelial cells, monocytes and other DC subsets [8]. Using pheno-

lack monocytic markers CD14 and CD16 [4, 11] identifying them as human cDC1

mental factor, has been demonstrated *in vitro* and *in vivo* [11, 17–19] and transcription factor *BATF3* is required *in vitro* but not *in vivo* [15]. Another cDC1-defining transcription factor, *IRF8*, is also highly expressed in human cDC1, although patients harbouring mutations in *IRF8* did not exhibit cDC1 deficiencies, suggesting the involvement of other transcription factors as well [6, 20]. Furthermore, genome wide expression profiling and microarray analyses have revealed transcrip-

PRRs expressed by human cDC1s are predominantly Toll-like receptor (TLR) 3, located in endosomes and which recognises double-stranded RNA and TLR8, also located in endosomes and which recognises bacterial ssRNA and mammalian mitochondrial RNA [10, 22]. In response to TLR3 signals [23] and also HCV *in vivo* [23, 24], the cDC1 produce large amounts of type III interferon (IFN), also known

The cDC1s are superior to other DC subsets in their ability to present ex*ogenous* Ag on MHCI, a process known as cross-presentation [2] and the activation of cytotoxic

T cells, crucial for anti-tumour responses. In particular, they have a specialised

Clec9a on cDC1 binding to actin filaments exposed on dead and dying cells [25]. The cDC1 are superior at inducing Th1 differentiation of CD4 helper T cells [11, 16].

of PBMCs and can be identified by the expression of CD11c, CD11b, CD13, CD33, CD172a, HLA-DR and CD45RO [2, 10, 26]. The phenotypic similarities between these DCs and moDCs, as well as the expression of CD1c on B cells and other DC subsets, have made the precise segregation of this subset quite difficult. Although previous studies have used CD64 to exclude monocytes from bonafide CD1c<sup>+</sup>

in the blood, cDCs express low levels of this marker and cannot be definitively used to separate the cell populations [6, 7]. More recently, the use of single cell RNA sequencing techniques has identified additional surface phenotypic markers, such as *CLEC10A*, *FCGR2B*, *FCER1A*, to distinguish human cDC2 subsets [7, 8]. In particular, *CLEC10A* protein has been proposed as the cDC1 *CLEC9A*equivalent marker for cDC2s in different species and tissues. However, different

CD11b<sup>−</sup>CD172a<sup>−</sup> CLEC9a+

DCs have been further

Necl2+

and migratory

T cells, enhanced by

DCs, constitute ~1%

DCs

cells that

XCR1+

DCs in blood and non-lymphoid tissues,

DCs on Flt3 ligand (FL), an important DC develop-

DCs and murine CD8a<sup>+</sup>

cDC as cDC1.

or BDCA1<sup>+</sup>

*DOI: http://dx.doi.org/10.5772/intechopen.91692*

#### **Table 1.** *Key features of human DC subsets.*

*Current Cancer Treatment*

DCs situated in non-lymphoid tissues, also known as migratory DCs, constantly migrate to draining lymph nodes (LNs), maturing during this process, to present Ag to naïve T cells. Resident DCs in lymphoid organs are immature and maintain tolerance during steady state, but can stimulate naïve T cells when activated *in situ.* The DC maturation process not only involves morphological changes into their characteristic stellate shape with dendritic cytoplasmic processes and increased expression of MHC and co-stimulatory markers, but their Ag acquisition and sampling capabilities are initially upregulated and then rapidly shut down while MHCII expression on the cell's surface is increased due to the simultaneous up- and down-regulation of MHCII synthesis and turnover events respectively [3]. This allows mature DCs to present a snapshot of the Ag profile in its local environment prior to migration and/ or activation. Furthermore, activated DCs produce a combination of cytokines that

modulate an immune response that is specific to the initial danger signals.

expressed on gut and other non-lymphoid DCs [6].

cytoid DCs (pDCs), which are HLA-DRhiCD11c+

**DC subsets**

HLA-DR+ CD123<sup>−</sup>CLEC9A+ XCR1+

Necl2+

Cross-presentation of cellular Ag

Potent producer of Type III IFN (after TLR3 stimulation), CTL priming, Th1 response

CD11c+

CD141+ CD11b<sup>−</sup>CD172α−

In humans, the majority of DC characterisation studies are of DCs isolated from the blood due to the rarity of the cell type and limited access to human tissue samples, although more investigations on non-lymphoid DCs in the skin, lung and liver have recently emerged [4–7]. DCs in the blood comprise ~1% of total peripheral blood mononuclear cells (PBMCs) and are traditionally identified by the high expression of MHCII (HLA-DR) and the lack of lineage markers CD3, CD14, CD15, CD19, CD20 and CD56, although the latter marker has recently been shown to be

Human blood DCs can be divided into conventional DCs (cDCs) and plasma-

**cDC1 cDC2 pDC**

CD11b+

subdivision based on CD5hiCD32B+

CD5loCD32B−CD163+

HLA-DR+

CD123<sup>−</sup>

CD163<sup>−</sup>CD36<sup>−</sup> or

CD36+

Cross-presentation of soluble Ag CD4+

Th1, Th17 response Potent producers of

CD172α<sup>+</sup>

*BATF3*, *IRF8 IRF4*, *IRF8 TCF4*, *SPIB*, *ZEB2*,

with further

respectively. Human blood cDCs are further categorised into cDC1 and cDC2 subsets. Additionally, there are monocyte-derived DCs that originate separately from cDCs and pDC precursors. The recent use of whole population and single cell sequencing techniques has been instrumental in elucidating transcription factors and surface markers that are unique to each DC subset, which has helped identify

CD11c+

CD1c+

CLEC10A+

TLR3, 8 TLR2, cytosolic RNA sensors (RIG-I, MDA-5), STING

*Previous Ag presentation abilities by pDCs are now suggested to be contributed by contaminating AXL+*

123<sup>−</sup> and HLA-DRhiCD11c−123+

CD11c<sup>−</sup>HLA-DR+ CD123+

*IRF4*, *IRF8*, *IRF7*

TLR7, 9, STING

priming\*

and CD8+

Type I and III IFN and mediating antiviral immunity

*Siglec6+*

T cell

CD304+

CD2+/<sup>−</sup>

CD303+

CD45RA+

**102**

*\**

*(AS) DCs.*

*Key features of human DC subsets.*

**Table 1.**

Surface phenotype

Transcription factors

PRR expression

Ag presentation

Roles in immunity relationships between DC subsets across species and tissues as well as corroborate DC functional analyses [6–9], summarised in **Table 1**.
