Mercury exposure, serum antinuclear/antinucleolar antibodies, and serum cytokine levels in mining populations in Amazonian Brazil: a cross-sectional study

Abstract

Mercury is an immunotoxic substance that has been shown to induce autoimmune disease in rodent models, characterized by lymphoproliferation, overproduction of immunoglobulin (IgG and IgE), and high circulating levels of auto-antibodies directed at antigens located in the nucleus (antinuclear auto-antibodies, or ANA) or the nucleolus (antinucleolar auto-antibodies, or ANoA). We have reported elevated levels of ANA and ANoA in human populations exposed to mercury in artisanal gold mining, though other confounding variables that may also modulate ANA/ANoA levels were not well controlled. The goal of this study is to specifically test whether occupational and environmental conditions (other than mercury exposure) that are associated with artisanal gold mining affect the prevalence of markers of autoimmune dysfunction. We measured ANA, ANoA, and cytokine concentrations in serum and compared results from mercury-exposed artisanal gold miners to those from diamond and emerald miners working under similar conditions and with similar socio-economic status and risks of infectious disease. Mercury-exposed gold miners had higher prevalence of detectable ANA and ANoA and higher titers of ANA and ANoA as compared to diamond and emerald miners with no occupational mercury exposure. Also, mercury-exposed gold miners with detectable ANA or ANoA in serum had significantly higher concentrations of pro-inflammatory cytokines IL-1beta, TNF-alpha, and IFN-gamma in serum as compared to the diamond and emerald miners. This study provides further evidence that mercury exposure may lead to autoimmune dysfunction and systemic inflammation in affected populations.

Figure 1. Map of study sites within Brazil

Gold miners were recruited at Rio-Rato, a gold mining camp located along Rio Tapajós in the state of Pará. Emerald and diamond miners were recruited from small mining camps located in the state of Goiás.

Figure 2. Biomarkers of mercury exposure in five different mining communities

A. In Rio-Rato, urine mercury levels (µg Hg/L) were measured for each participant. Data are plotted on the log scale, with each circle representing the urine mercury level for a single participant. The inter-quartile range and median of the data are also represented. B. In the emerald mining communities of Campo Verde, Itaobi (CVI) and Campo Verde, Vereador (CVV) and in the diamond mining communities of Davinopolis (DAV) and Sao Antonio Rio Verde (SARV), hair mercury levels (µg Hg/g hair) were measured for each participant. Data are represented on the log scale as described in A.

Figure 3. Detectable ANA and ANoA are more prevalent in a mercury exposed, gold mining community compared to emerald and diamond mining communities

Serum samples from gold miners (n=98, from Rio-Rato), diamond miners (n=57, from Sao Antonio Rio Verde and Davinopolis), and emerald miners (n=91, from Campo Verde, Vereador and Campo Verde, Itaobi) were serially diluted and assayed for ANA and ANoA. Results show the proportion of the population that has a detectable amount of ANA (A) or ANoA (B) at the inverse titer shown, with ND indicating that no ANA or ANoA was detectable at any dilution.

Figure 4. Predicted probabilities and observed proportions of ANA and ANoA in mining populations

The predicted probabilities obtained from fitting Model 3 to the data are plotted in dashed lines, and compared to the observed proportion of positive responses for ANA (A) and ANoA (B) for each population of miners.

Figure 5. Serum concentrations of the pro-inflammatory cytokine IL-1β are higher in occupationally Hg-exposed, ANA+ or ANoA+ individuals compared to non-Hg-exposed individuals, though serum concentrations of the anti-inflammatory cytokine IL-1Ra do not differ between groups

A. Log-transformed data are shown for the populations described in Table 2, with each circle representing the IL-1β level for a single participant. The inter-quartile range and median of the data are also represented. The mean of High Hg/AN(o)A+ group is significantly different from both the Low Hg/AN(o)A+ (p < 0.001) and Low Hg/AN(o)A− (p=0.003) groups. B. Data are shown for IL-1Ra as in A. The means of the four groups did not differ significantly (p=0.120).

Figure 6. Serum concentrations of the pro-inflammatory cytokines TNF-α and IFN-γ are higher in occupationally Hg-exposed, ANA+ or ANoA+ individuals compared to non-Hg-exposed individuals

A. Log-transformed data are shown for the populations described in Table 2, with each circle representing the TNF-α level for a single participant. The inter-quartile range and median of the data are also represented. The mean of the observed log-transformed TNF-α concentrations for the High Hg/AN(o)A+ group is significantly different from both the Low Hg/AN(o)A+ (p < 0.007) and Low Hg/AN(o)A−(p=0.027) groups. B. Data are shown for IFN-γ as in A. The mean of the observed log-transformed IFN-γ concentrations for the High Hg/AN(o)A+ group is significantly different from both the Low Hg/AN(o)A+ (p = 0.006) and Low Hg/AN(o)A− (p=0.010) groups.