Figure 1.

Proposed model for migration of human monocytes and dendritic cell (DC) progenitors into tissues in steady-state and inflammatory conditions. After differentiation in the bone barrow, precursors of DCs and monocytes enter the blood stream and are distributed to lymphoid organs [through high endothelial venules (HEV)] and to various peripheral tissues. In steady state, non-classical monocytes are preferentially recruited into the resting vasculature, where they patrol the endothelium and may contribute to the maintenance of tissue-resident macrophage and DC populations. Conventional DCs (cDCs) recirculate between peripheral tissues and lymphoid organs (migratory cDCs), participating in the induction of peripheral tolerance, or reside in the lymphoid organs (lymphoid-resident cDCs). By contrast, plasmacytoid DCs (pDCs) mostly populate lymphoid tissues (lymphoid-resident pDCs) and lack migratory ability under steady-state conditions. Upon inflammation, classical monocytes, cDCs, and pDCs are recruited to affected tissues. After antigen uptake and differentiation into fully functional mature DCs, monocyte-derived DCs (moDCs), and cDCs enter draining lymph nodes via afferent lymphatics. pDCs can only access reactive lymph nodes from the blood stream via HEVs.