Myeloid human cell lines lack functional regulation of aryl hydrocarbon receptor-dependent phase I genes.

Primary dendritic cells and myeloid cell lines are used to assess the skin sensitization hazard in in vitro approaches. The aryl hydrocarbon receptor (AhR) modulates expression of CYP enzymes which play a significant role in the bioactivation of various xenobiotics. These studies revealed a strong constitutive expression of the AhR in primary human monocytes, monocyte-derived immature dendritic cells (iDC) and cord blood-derived Langerhans cells (LC). In contrast, mRNA and protein expression of AhR was hardly detectable in the cell lines THP-1 and MUTZ-3. U937 cells and MUTZ-3-derived dendritic (MUTZ-DC) or Langerhans cells (MUTZ-LC) showed about half the expression of AhR compared to iDC. Incubation of cells with the specific AhR-inducer benzo[a]anthracene resulted in an upregulation of CYP and IL-1β mRNA expression in primary monocytes and iDC. CYP1A1 but not CYP1B1 and IL-1β expression was increased by benzo[a]anthracene in these cell lines except for U937 cells. AhR-independent CYP genes were not regulated by benzo[a]anthracene. Constitutive mRNA expression of other non AhR-dependent CYP enzymes was higher in some of the cell lines compared to the corresponding primary cells. This study demonstrates significant differences in expression and regulation of phase I genes in cell lines currently used for in vitro skin sensitization hazard assessment compared to primary cells. Additional studies are required regarding the combination of cutaneous xenobiotic metabolizing enzymes and APC-sensitization for the development of valid in vitro models for skin sensitization assessment.