Immunotoxicity Evaluation of Trihalophenolic Disinfection By-Products in Mouse and Human Mononuclear Macrophage Systems: The Role of RNA Epitranscriptomic Modification in Mammalian Immunity.

BACKGROUND

2,4,6-Trichlorophenol (TCP), 2,4,6-tribromophenol (TBP) and 2,4,6-triiodophenol (TIP) are three widely detected trihalophenolic disinfection by-products (DBPs). Previous studies have mainly focused on the carcinogenic risk and developmental toxicity of 2,4,6-trihalophenols. Very little is known about their immunotoxicity in mammals.

OBJECTIVES

We investigated the effects of 2,4,6-trihalophenols on mammalian immunity using a mouse macrophage model infected with bacteria or intracellular parasites and aimed to elucidate the underlying mechanisms from an epitranscriptomic perspective. The identified mechanisms were further validated in human peripheral blood mononuclear cells (PBMCs).

METHODS

The mouse macrophage cell line RAW264.7 and primary mouse peritoneal macrophages were exposed to different concentrations of TCP, TBP, and TIP. The pro-inflammatory marker Ly6C, the survival of the bacterium Escherichia coli (E. coli), and the parasite burden of Toxoplasma gondii (T. gondii) were assessed. Furthermore, the global gene expression profiling of macrophages following exposure to 2,4,6-trihalophenols was obtained through RNA-sequencing (RNA-seq). The effects of 2,4,6-trihalophenols on RNA N6-methyladenosine (m6A) methyltransferases and total RNA m6A levels were evaluated using Western blotting and dot blot, respectively. Transcriptome-wide m6A methylome was analyzed by m6A-seq. In addition, expression of m6A regulators and total RNA m6A levels in human PBMCs exposed to 2,4,6-trihalophenols were detected using quantitative reverse transcriptase polymerase chain reaction and dot blot, respectively.

RESULTS

Mouse macrophages exposed to TCP, TBP, or TIP had lower expression of the pro-inflammatory marker Ly6C, with a greater difference from control observed for TIP-exposed cells. Consistently, macrophages exposed to such DBPs, especially TIP, were susceptible to infection with the bacterium E. coli and the intracellular parasite T. gondii, indicating a compromised ability of macrophages to defend against pathogens. Intriguingly, macrophages exposed to TIP had significantly greater m6A levels, which correlated with the greater expression levels of m6A methyltransferases. Macrophages exposed to each of the three 2,4,6-trihalophenols exhibited transcriptome-wide redistribution of m6A. In particular, the m6A peaks in genes associated with immune-related pathways were altered after exposure. In addition, differences in m6A were also observed in human PBMCs after exposure to 2,4,6-trihalophenols.

DISCUSSION: These findings suggest that 2,4,6-trihalophenol exposure impaired the ability of macrophages to defend against pathogens. This response might be associated with notable differences in m6A after exposure. To the best of our knowledge, this study presents the first m6A landscape across the transcriptome of immune cells exposed to pollutants. However, significant challenges remain in elucidating the mechanisms by which m6A mediates immune dysregulation in infected macrophages after 2,4,6-trihalophenol exposure. https://doi.org/10.1289/EHP11329.