Introduction
Resin-based biomaterials used in dentistry consist of methacrylate monomers that are polymerized in situ. The conversion to polymer is never complete and this causes leakage and patient exposure to electrophile monomers such as 2-hydroxyethyl methacrylate (HEMA). In addition, dental personnel that handle these materials are exposed on a daily basis. Lipopolysaccharide (LPS) is commonly used in studies investigating inflammatory
macrophage responses to bacteria. HEMA has been shown to decrease LPS-induced cytokine release of interleukin (IL)-1β and tumor necrosis factor (TNF)-α in murine macrophages (RAW 264.7). In this study, we aim to investigate if HEMA reduces live bacteria-induced cytokine release in human macrophages. In addition, the effects of HEMA on phagocytosis of live bacteria were investigated to study macrophage functionality.

Methods
The human monocytic cell line THP-1 was differentiated to macrophages by phorbol myristate acetate (PMA). THP-1 macrophages were subsequently exposed to HEMA and LPS from E. coli or to HEMA and live S. aureus bacteria. S. aureus was chosen as a cytokine inducer as it is present in the oral cavity and known to induce a strong IL-1β release. Cytokine levels of IL-1β, IL-8 and TNF-α were measured by enzyme-linked immunosorbent
assay (ELISA) in the experiments involving live S. aureus bacteria, while only IL-1β was measured in the LPS experiments. In addition, the effect of HEMA on THP-1 macrophages ability to phagocytose live S. aureus was investigated.

Results
HEMA reduced the LPS-induced cytokine release of IL-1β, as previously shown in RAW 264.7 macrophages. Furthermore, HEMA similarly reduced the S. aureus-induced release of IL-8 and TNF-α. Phagocytosis is important for elimination of bacteria. In contrast to the reduced cytokine release, THP-1 macrophages exposed to HEMA increased phagocytosis of S. aureus bacteria.

Conclusion
HEMA reduced LPS and S. aureus-induced release of IL-1β from human macrophages, while increasing phagocytosis of S. aureus bacteria. Consequently it appears that HEMA may affect the macrophage immune response.

Authors:
Bergitte P Olderbø¹, Håkon Valen¹, Jan Tore Samuelsen¹, Anette K Bølling¹²
¹ Nordic Institute of Dental Materials, Oslo, Norway
² Norwegian Institute of Public Health, Oslo, Norway

Presented at: The Norwegian Society of Pharmacology and Toxicology (NSFT) 47th annual winter meeting at Beitostølen, Norway, 24-27 January 2019.

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