MDI Initiates Cell Death and Inflammation in Human PDL Fibroblasts

Objectives: As Invisalign appliances are gaining popularity in the dental field to correct malocclusion, it is important to identify potential cytotoxic effects of its unpolymerized monomers on biological systems. The purpose of this study was to delineate any potential noxious effects of methylene dipheny diisocyanate (MDI), a monomer used in the fabrication of aligners, on human periodontal ligament (PDL) and bronchial fibroblasts.
Methods: Human PDL and bronchial fibroblasts (ScienCell Research Laboratories, Carlsbad, CA) were exposed to 0, 1.25, 2.5, 5.0, 10.0, or 20.0 μg/ml MDI in 0.5% dimethyl sulfoxide (DMSO) for 24 hours. A colorimetric MTT assay (Sigma Aldrich) was used to assess cell viability following the MDI treatment. IL-8 concentrations in culture medium were measured via ELISA (RnD Systems) as an indicator of inflammation.
Results: PDL fibroblasts displayed a significant decrease in viability with exposure to MDI concentrations higher than 2.5 μg/ml (p < 0.05 compared to control). Human bronchial fibroblasts demonstrated a reduction in cell viability as MDI concentration increased; at 20.0 μg/ml MDI cell viability was reduced to 74%. PDL fibroblasts displayed a significantly increased level of IL-8 at 2.5 μg/ml MDI (1386 pg/ml) compared to control (823 pg/ml, p < 0.05). Bronchial fibroblasts secreted significantly higher IL-8 at 1 μg/ml MDI (1758 pg/ml) compared to control (582 pg/ml, p < 0.05). However, the IL-8 concentrations were decreased at higher MDI concentration for both cell lines; this likely corresponded to increased cell death noted by the MTT assay.
Conclusions: Exposure to MDI, a monomer used in the fabrication of plastic aligners, altered cell viability and increased secretion of IL-8 in both human PDL and bronchial fibroblasts. Further study is required to confirm and explore changes in cell viability and IL-8 levels in cell lines and its implication for oral health.