Mercury induces an unopposed inflammatory response in human peripheral blood mononuclear cells in vitro

Abstract

Background: The human immune response to mercury is not well characterized despite the body of evidence that suggests that Hg can modulate immune responses, including the induction of autoimmune disease in some mouse models. Dysregulation of cytokine signaling appears to play an important role in the etiology of Hg-induced autoimmunity in animal models.

Objectives: In this study, we systematically investigated the human immune response to Hg in vitro in terms of cytokine release.

Methods: Human peripheral blood mononuclear cells (PBMCs) were isolated from 20 volunteers who donated blood six separate times. PBMCs were cultured with lipopolysaccharide and concentrations of mercuric chloride (HgCl(2)) up to 200 nM. Seven cytokines representing important pathways in physiologic and pathologic immune responses were measured in supernatants. We used multilevel models to account for the intrinsic clustering in the cytokine data due to experimental design.

Results: We found a consistent increase in the release of the proinflammatory cytokines interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha, and concurrent decrease in release of the antiinflammatory cytokines interleukin 1-receptor antagonist (IL-1Ra) and IL-10 in human PBMCs treated with subcytotoxic concentrations of HgCl(2). IL-4, IL-17, and interferon-gamma increased in a concentration-response manner. These results were replicated in a second, independently recruited population of 20 different volunteers.

Conclusions: Low concentrations of HgCl(2) affect immune function in human cells by dysregulation of cytokine signaling pathways, with the potential to influence diverse health outcomes such as susceptibility to infectious disease or risk of autoimmunity.

Keywords: immunotoxicity; inflammation; mercury; multilevel modeling.

Figure 1

Model estimates [median (95% CI)] derived from phase 1 volunteers predict the response to LPS and HgCl₂ in phase 2 volunteers. Model 2 estimates for concentration–response curves for each of the seven different cytokines derived from the log-transformed data from 111 visits of the 20 phase 1 volunteers are compared with model 3 estimates derived from the single visit of the 20 phase 2 volunteers. β₀ is the estimate for the intercept of the HgCl₂ concentration–response curve; β₁ is the estimate for slope (log change in pg/mL of cytokine per 1-nM increase in HgCl₂); and R_b0 is the proportion of variation in the data set that is explained by variation in the baseline response to LPS. The vertical dashed lines for β₀ and β₁ indicate zero.

Figure 2

Subject- and visit-specific variables may affect response to LPS and HgCl₂. AI, autoimmune. Total data for TNF-α release for all subjects (i = 40) and all visits (j = 131) were evaluated with model 6. The effect of each covariate was tested on both the intercept (β₀) and the slope (β₁) using concentration–response data from the specified subpopulations (see Table 1 for covariate descriptions). The estimated median and 95% CI for each model parameter were compared with the median model estimates from a model with no covariates (model 2), represented by the dashed vertical lines.