Human keratinocytes and monocytes co-culture cell system: An important contribution for the study of moderate and weak sensitizers.

In vitro approaches to address key events in chemical-induced skin sensitization have been developed, but there is still uncertain how they will be useful to predict the potency for an effective risk assessment. Keratinocytes (KCs) play a key role in all phases of skin sensitization. Dendritic cells (DCs) activation and maturation require the binding of cytokines produced by KC as a result of initial chemical exposure. We previously identified interleukin-18 (IL-18) as useful marker for determination of skin sensitization potential of chemicals. The aim of this paper was to mimic the interaction between KCs and DCs using a co-culture of NCTC 2544 and THP-1 cells. Three selected contact allergens of different potency (Bandrowski's base, diethyl maleate, and imidazolidinyl urea) were tested in time-course experiments (24, 48 and 72 h). Cell surface markers expression (CD80, CD86, and HLA-DR) was determined by flow cytometry analysis while IL-18 production was evaluated with specific sandwich ELISA. Results obtained from this simple in vitro co-culture system show the possibility to study the contribution of KCs in DCs activation through the analysis of HLA-DR expression. Results obtained demonstrate the ability of the KCs to favor the full maturation of the DCs in the presence of moderate and weak allergens, while the extreme allergen induced a complete maturation of DC alone without the need of KCs.