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

Human cDC2, traditionally known as CD1c<sup>+</sup> or BDCA1<sup>+</sup> DCs, constitute ~1% 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> DCs 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

isoforms of Clec10A have been found in mice and should be carefully considered when using it across species [27]. Heterogeneity within the human cDC2 subset has been identified using CD5 or CD32B versus CD163 and CD36. The CD5lo or CD163<sup>+</sup> CD36<sup>+</sup> 'cDC2' are transcriptionally more related to monocytes than the other cDC2 subset (CD5hi or CD32B+ ) [8, 28]. Like cDC1, CD1c<sup>+</sup> cDC2s require FL, but also rely on transcription factors *IRF4* and *IRF8*, for development [20*,* 29].

The cDC2 DCs highly express TLR2 and also express a range of cytosolic viral RNA sensors such as RIG-I [30, 31]. Different proposed cDC2 subsets also seem to have different PRR expression patterns. For example, CD5hi cDC2 express high levels of TLR7 and 8 compared to CD5lo cDC2 and CD32B+ cDC2 express higher levels of *TMEM173* (also known as STING) in comparison to CD163+ CD36+ cDC2 subset [8, 28].

Activated cDC2s can drive Th17 immune response and can also produce high levels of IL-12p70, potentially inducing Th1 differentiation [2, 29]*.* However, current data suggests Th17 versus Th1 driven responses may be independently driven by CD5+ versus CD5lo cDC2 subsets, respectively [8, 28].

Human cDC2s are able to cross-present *soluble* Ag to naïve and memory CD8<sup>+</sup> T cells at comparable levels with cDC1s [32–35]. However, the mechanism of crosspresentation differs between both subsets [35] and cDC2 do not possess the potent ability to cross-present Ags from dead cells. Human cDC2 are also potent stimulators of CD4+ T cells [8, 10, 16].
