Macrophage Inflammatory Protein 3α Is Involved in the Constitutive Trafficking of Epidermal Langerhans Cells

Certain types of dendritic cells (DCs) appear in inflammatory lesions of various etiologies, whereas other DCs, e.g., Langerhans cells (LCs), populate peripheral organs constitutively. Until now, the molecular mechanism behind such differential behavior has not been elucidated. Here, we show that CD1a+ LC precursors respond selectively and specifically to the CC chemokine macrophage inflammatory protein (MIP)-3α. In contrast, CD14+ precursors of DC and monocytes are not attracted by MIP-3α. LCs lose the migratory responsiveness to MIP-3α during their maturation, and non-LC DCs do not acquire MIP-3α sensitivity. The notion that MIP-3α may be responsible for selective LC recruitment into the epidermis is further supported by the following observations: (a) MIP-3α is expressed by keratinocytes and venular endothelial cells in clinically normal appearing human skin; (b) LCs express CC chemokine receptor (CCR)6, the sole MIP-3α receptor both in situ and in vitro; and (c) non-LC DCs that are not found in normal epidermis lack CCR6. The mature forms of LCs and non-LC DCs display comparable sensitivity for MIP-3β, a CCR7 ligand, suggesting that DC subtype–specific chemokine responses are r