The THP-1 cell toolbox: a new concept integrating the key events of skin sensitization

According to the current scientific consensus, one in vitro test is insufficient to cover the key events (KE) defined by the adverse outcome pathway (AOP) for skin sensitization. To address this issue we combined different end points in the same cell line to cover all KEs defined by the skin sensitization AOP. Since dendritic cells (DC) play a key role in the sensitization phase leading to the development of allergic contact dermatitis (ACD), we used THP-1 cells as a surrogate for DC. We measured ROS production and GSH depletion for KE1 (binding to proteins), Nrf2 activation pathway and gene expressions for KE2 (keratinocyte response), phenotype modifications using cell-surface markers and cytokine production for KE3 (DC activation), and T-cell proliferation for KE4 (T-cell activation). These measurements were performed using the THP-1 cell line and an original THP-1/T-cell co-culture system following exposure to a variety of chemicals, including irritant, non-sensitizers, and chemicals sensitizers (pro/prehaptens). Results showed that treatment with sensitizers such as cinnamaldehyde (100 µM) or methylisothiazolinone (150 µM) was able to trigger the three main key events (KE1, KE2, and KE3) of the sensitization phase of ACD in THP-1 cells. In addition, all sensitizers were able to induce T lymphocyte proliferation (KE4), while non-sensitizers and irritants did not. Our study shows for the first time that addressing the four main KE of skin sensitization AOP in a single cell line is an achievable task.

Lymphocyte surface markers and cytokines are suitable for detection and potency assessment of skin-sensitizing chemicals in an in vitro model of allergic contact dermatitis: the LCSA-ly

Allergic contact dermatitis is a widespread health disorder and occupational skin disease. Hence, screening for contact-sensitizing chemicals is highly relevant to toxicology, dermatology, and occupational medicine. The use of animal tests for this purpose is constrained by ethical considerations, need for high-throughput screening, and legislation (e.g., for cosmetics in the European Union). T cell activation is the final and most specific key event of the "adverse outcome pathway" for skin sensitization and therefore a promising target for the development of in vitro sensitization assays. We present a novel in vitro sensitization assay with a lymphocyte endpoint as an add-on to the loose-fit coculture-based sensitization assay (LCSA): the LCSA-ly. While the LCSA measures dendritic cell activation, the LCSA-ly offers the option for an additional lymphocyte endpoint which can be measured concurrently. We incorporated lymphocytes in our previously established coculture of primary human keratinocytes and monocyte-derived dendritic cells and tested nine substances: five sensitizers [2,4-dinitrochlorobenzene (DNCB) 1.25-15 µmol/l, p-phenylenediamine (PPD) 15.6-125 µmol/l, 2-mercaptobenzothiazole (MBT) 50-1000 µmol/l, coumarin, and resorcinol (both: 250-1500 µmol/l)] and four non-sensitizers (monochlorobenzene, caprylic acid, glycerol, and salicylic acid (all: 125-1000 µmol/l)]. DNCB and MBT increased a subset of IL-23 receptor+/IFN-γ receptor 1 (CD119)+ lymphocytes. DNCB, PPD, and MBT enhanced a subunit of the IL-4 receptor (CD124) and a memory marker (CD44) on lymphocytes. Remarkably, DNCB, PPD, and MBT raised IL-4 concentrations in coculture supernatants while IFN-γ levels decreased, which might point to Th2 activation in vitro. Coumarin, resorcinol, and non-sensitizers did not alter any of the tested surface markers or cytokines. IL-17 was not affected by any of the substances. Relative strength of sensitizers according to lymphocyte markers was DNCB > PPD > MBT, which corresponds to earlier results from the LCSA without lymphocyte endpoint, the murine local lymph node assay, and human data. This study is the first to prove the suitability of lymphocyte surface markers for sensitization testing and potency assessment.