Transcriptional profiling of human skin-resident Langerhans cells and CD1a+ dermal dendritic cells:: differential activation states suggest distinct functions

In human skin, two main populations of dendritic cells (DC) can be discriminated: dermal DC (DDC) and epidermal Langerhans cells (LC). Although extensively studied, most of the knowledge about DDC and LC phenotype and function is obtained from studying DDC and LC cultured in vitro or DDC and LC migrated from skin explants. These studies have left the exact relationship between steady-state human LC and DDC unclear: in particular, whether CD1a(+) DDC represent migrated LC or whether they constitute a separate subset. To gain further insight in the kinship between skin-resident CD1a(+) DDC and LC, we analyzed CD1a(+) DDC and LC, isolated from steadystate skin samples, by high-density microarray analysis. Results show that the CD1a(+) DDC specifically express markers associated with DDC phenotype, such as the macrophage mannose receptor, DC-specific ICAM-grabbing nonintegrin, the scavenger receptor CD36, coagulation factor XIIIa, and chemokine receptor CCR5, whereas LC specifically express Langerin, membrane ATPase (CD39), and CCR6, all hallmarks of the LC lineage. In addition, under steady-state conditions, both DC subsets display a strikingly different activation status, indicative of distinct functional properties. CD1a(+) DDC exhibit a more activated, proinflammatory, migratory, and T cell-stimulatory profile, as compared with LC, whereas LC mainly express molecules involved in cell adhesion and DC retention in the epidermis. In conclusion, transcriptional profiling is consistent with the notion that CD1a(+) DDC and LC represent two distinct DC subsets but also that under steady-state conditions, CD1a(+) DDC and epidermal LC represent opposites of the DC activation spectrum.