Indeed, tracking of bar-coded progenitors transferred into irradiated Metformin in vivo mice indicates that lineage divergence among myeloid cell types might occur as early as a stage upstream of MDPs known as the lymphoid-primed multipotent progenitor (LMPP) [ 20••]. Mouse MDPs and CDPs exhibit substantial phenotypic overlap . They both lack lineage specifying markers, express CD115 and CD135 in addition to CX3CR1 and can only be distinguished
by the fact that CDPs express lower levels of CD117 (c-kit) than MDPs [27, 28, 29, 30 and 31]. We have recently demonstrated that DNGR-1 (encoded by the Clec9a gene and also known as CLEC9A) marks cells resembling CDPs but not MDPs. DNGR-1+ CDPs exhibit cDC-restricted differentiation potential and do not generate pDCs after adoptive
transfer [ 21••] or in vitro culture with Flt3L (BUS and CRS, unpublished observations). DNGR-1+ CDP express CD115, consistent with the recent demonstration that CD115+ CDPs exhibit a strong clonal bias to generate cDCs, whereas pDCs arise predominantly from CD115 negative CDPs [ 19•]. Thus, cDCs and pDCs appear to have distinct immediate progenitors, which can be distinguished by expression of CD115 [ 19•] and DNGR-1 [ 21••]. Some CD115+ CDP, which presumably express DNGR-1 [ 21••], have combined cDC and pDC potential in clonal assays [ 19•, 30 and 31], although the interpretation of such experiments might be marred by the reported in vitro developmental plasticity of differentiating DCs [ 35]. Altogether, these data can be integrated into a revised map of DC differentiation that takes into account the fact that cDCs, pDCs and monocytes develop Cetuximab in vitro as distinct lineages although the exact developmental Erastin intermediates and branching points remain to be clarified and may display considerable
plasticity ( Figure 1). Dependence on FLT3L is sometimes used as evidence that a given leukocyte should be considered a member of the DC lineage [36, 37 and 38]. This is because FLT3L strongly expands pDCs and cDCs in vivo [ 28, 39 and 40] and can be used to generate all functional subsets of DCs in vitro [ 41]. Conversely, mice lacking Flt3L display a severe deficiency in DCs, which is also apparent, although to a lesser extent, in mice lacking its receptor CD135 (Flt3) [ 42] or treated with CD135 inhibitors [ 43 and 44]. GM-CSF, on the other hand, is extensively used to differentiate monocytes into cells resembling DCs in vitro [ 45] but mice lacking GM-CSF or its receptor have normal development of monocyte-derived cells [ 46•] as well as lymphoid tissue DCs [ 28 and 47]. Instead, they exhibit a specific reduction of cDCs in many, but not all, non-lymphoid tissues [ 46•, 48, 49 and 50]. The GM-CSF dependence of CD103+ cDCs is stronger than that of CD11b+ cDCs [ 46•] although the extent of reduction relates to the markers used for cell identification [ 46• and 49], possibly because GM-CSF regulates CD103 expression [ 51].